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DEPOSITION OF DOPED HYDROGENATED

DIAMOND-LIKE AMORPHOUS CARBON FILMS USING

A DC SADDLE-FIELD GLOW-DISCHARGE SYSTEM

Wallace Chun Wai Chan

A thesis submitted in confonnity with the requiremenü

for the dcgree of Mûstcr of Applied Science

Graduate Depanment of Electricrl and Cornputer Engineering

University of Toronto

Q Copyright by Wallace Chun Wai Chan (1997)

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DEPOSITION OF DOPED HYDROGENATED DIAMOND-LIKE AMORPHOUS CARBON FILMS USlNC

A DC SADDLE-FIELD GLOW-DISCHARGE SYSTEM

Wdace Chun Wai Chan

A thesis submitted in confonnity with the requirements for the degree of Master of Applied Science

Graduate Deparuneni of Electrical and Cornputer Engineering University d Toronto

1997

Abstract

Several undoped. phosphonis-doped and horon-doped hydrogenatcd diamond-likc

amorphous carhon tïlms were deposited at suhstrace iempcraiurcs of 20O0C and 400°C

using thc DC saddle-tield glow-discharge system. The doped t h s wcre gruwn using male

î'raction of dopant gases (phosphine or dihonne) rdnging t'rom Sx 10-' to lx IO-'. The

sinicturd. optical and electncrl properties of the films were studied.

For the samples deposited at 2(K)OC. ihc struciural analyses show ihai the mosi

heavily boron-doped sample has a Iûrger mole fraction of hydrogen ;uid a larger fraction of

ieirihedrd bnding than oiher doped samples. The opticd energy gaps tOr dl smplcs are

similv (from 2.4eV to 2.SeV). The resistances of the sarnples are very high and lheir

conductivities could not be accumiely detennined.

For the samples deposited at 400°C. the conduc tivity measurements indicate that

boron-doping affects the electrical conductivity. Phosphorus-doping does not affect the

electrical conductivity.

Acknowledgments

1 would 1- to thank Prof. S. Zukotynski for his time. guidance and supervision

h r ihis work.

I dso wish to acknowledge the help provided by Dr. F. Caspari. Prof. P. Lim and

Prof. T. Bilgildeyeva. The support from R. Chan. F. Dadahhai. T. Kosteski. R. Shmayda.

L. Sidhu. V. Sivaninec. Y. Su, C. Wang are greatly appreciated. In addition, I want to

ihünk Dr. R. Sodhi and Dr. R. van der Heide for performing the XAES and SIMS

mcüsurements, respectively. The assistance of E. Moreno and Y. Su in ihc construction of

the conduçtivity measurement systerns and in the conduçtivity rneasurcments is ;ils«

appreciiited.

1 m grcrtly in deht to my whole h i l y for hcir support and encourügement

during the course «f this work.

Findly. the linuicid suppon of ihe NSERC postpradurte scholarship is grüiefully

acknowlcdged.

iii

Table of Contents

Abstrxt

Acknow ledgments

Table of Contents

List of Tables

List of Figures

Chapter 1 Introduction

Chapter 2 Deposition

2.1 Introduction

2.2 DC saddlc-field glow-discharge system

2.3 Prcpwation of Suhsuaies

2.4 Deposition procedure and deposition panmeten

2.5 Summary

Chapter 3 Structural proper0ies

3.1 Introduction

3.2 Thicknesses

3.2.1 Introduction

3.2.2 Mcasurements and results

3.2.3 Discussion of the resu1t.s

3.3 Compositions

3.3.1 Introduction

3.3.2 Mcasurements and ccsults

3.3.3 Discussion of the rcsults

3.4 Fractions of tetrahedntl and triponal bonds

3 -4.1 Introduction

3.4.2 Basic principles and measurements of XAES

3.4.3 Calculacion of the fractions of sp3 and sp2 bonds

3.4.4 Discussion of the results

3.5 Compilation of resulu and Summary

Chapter 4 Optical propdcs - emrgy gap

4.1 [ntroduçtion

4.2 Theory

4.3 Experimentd appariiitus

4.4 Meüsurements

4.5 Rcsults

4.6 Summary

Chopter 5 Elecbical pprpertis - conductivity

Introduction

Measuremcnt technique

Exprimenial setup

Mcasurements and resul~ for sümples deposited at 2(M) OC

Mcasurements and results for ssrrnplcs deposiied ai 4(WI O C

Discussion of rcsults

Summary

Chapter 6 Conciusions

References

Table 2.1 Deposition parameters for the samples deposited at 200 OC.

Table 2.2 The nûmes of the sûmples depositcd at 200 O C and the mole tixtion

of dopant gases in the p s mixtures.

Table 2.3 The names of the samplcs deposi ted ai 400 OC and the mole fraction

of dopant gases in the gÿs mixtures.

Table 3.1 The thiçknesses of the samples with Coming 7059 glass suhstaies.

Table 3.2 The Growth rates of thin films on a varicty of substatcs.

Table 3.3 The values of u mi b (as in eq. 3.3) obiained from the curve îïtting

for the doped samplcs.

Table 3.4 The valws of A (as in eq. 3.4) obtained from thecurvc titting for the

doped siunples.

Table 3.5 Dilferent kinds of .vp3 and ndP2 bonds that cün exist in the undoped and

doped simples.

Table S. 1 The values of E, and a, t'or ~he 4K samples.

List of Figures

Fig. 2.1 A schematic side view of the DC saddle-field glow-dischürge chmber P-4

and gas handling system (modified from [7] ).

Fig. 3.1 The thickness of r thin tïim can be measurcd using a pmfilometer. p. I O

Fig. 33 The procedures to create a step. p.11

Fig. 3.3 Thickness of the P-doped and B-doped samplcs (with Si suhstrates) vs p. 12

mole fraction of the dopant g i s in the bas mixiure.

Fig. 3.4 Thickness of the P-doped and 8-dopd samplcs (with fuscd silica

substrates) vs mole fraction of the dopmt gas in the gas mixture.

Fig. 3.5 Concentration of the eiements in the simple PI vs depth.

Fig. 3.6 Concentration of the clements in the P-doped and B-doped sarnples vs

mole hction of the dopant gas in the gas mixture.

Fig. 3.7 Fractional concentration of the elemcnts in the P-doped and B-doped

samples vs mole fraction of the dopant gas in the gas mixture.

Fig. 3.8 The process of KLL Auger electron emission for a mode! atom [ 131. p.2 1

Fig. 3.9 The raw. smoothed iuid tïrst-denvative C KLL specua for the sample Pl p.24

(the most heavily P-doped sample).

Fig. 3.10 The D parameter for the P-doped and B-doped samples vs

mole fraction of the dopant gas in the gas mixture.

Fig. 3.11 The %sp3 for the P-doped and B-doped samples vs

mole fraction of the dopant gas in the güs mixture.

Fig. 4 3 The setup of the Lmdba 18 spectrorneter used for measuting T and R.

Fil. 4.3 Trmsmittuice and reilec~nce vs wavclengih (for sample P 1-fs)

Fig. 4.5 The optical energy gap ( E, ) for the P-doped and B-doped samples vs

mole hction of the dopant pas in the gas mixture.

Fig. 5.1 An arbi~arily shaped sample with four coniacts (231.

Fig. 5.2 The van der Pauw correction facior F as a function of R, [23].

Fig. 5.3 The test configuration used for measuring ihe resistance of a standard

resistor.

Fig. 5.4 The schematic of the electrical contigurrtion used for dctcrrnining R,, -. ,, .

Fig. 5.5 Natural logailhm of conductivity vs inverse of temperature

(t'or the 400C samples)

viii

Chapter 1 Introduction

Amorphous semiconductors have ken the subject of interest for a long tirne [ 1).

In the 1930's. the study of chnlcogenides as a photoconductive matcrial w u stimulüted by

the invention of xerography. In the 1970's, the use of hydrogenated ümorphous silicon in

solar cells becarne a popular research topic as a resuli of the oil crises. More recendy,

interest in hydrogenoied diamond-like amorphous carbon (a-C:H) hûs grown due to some

of it.5 unique mechanical. structural and optoelectronic propenies.

Being relatively chemicdly inen and mechmicdly hard [ 2 ] . hydrogcnmd

morphous carbon films cm be used as excellent protective coûtings against corrosion and

Wear. As well. the structure of diamond-like çarbon füms is unique. It consisu of two

types of bonds, one king similür to ihose in crystalline diamond (tetrahedrd bonds) and

the other one king similar to those in crystalline graphite (trigona1 honds). In addition.

diümond-like carhon fdrns with a wide range of optical energy gaps (from leV to 4eV)

can he grown [3]. The tlexibüity to grow tïirns with different energy gaps pcrmi~s the use

of amorphous carbon tilms as optical coatings with varying spectril propenies. In

panicular, î lms with large energy gaps are transparent io visible light and can be used as

protective çoatings on optical devices such as 1en.w and rnirmrs. In elecuonic

applications, diarnond-likc carbon can be used in place of silicon dioxidc as an insulaior LS

i i s resistivity is very high. Furthemore. it may bc possible to make electronic deviccs out

of amorphous carbon through p-type and n-type doping.

In this research project, p-type and n-type doping of hydrogenated dimond-like

amorphous carbon füms were attempted by incorporating boron and phosphorus dopants.

respcctively. In order to investigate the possibilities of using nmorphous cuhon in

optoelectronic applications. the smctural. optical and electrical propercies of several films

were studied. As very littie data about diamond-like morphous carbon is available. it LS

hoped that the results of this resemh project will add to the datahase of knowledge in this

tield.

This thesis is orgmizd in the following format. In Chapter 2. the deposition

system and deposition parmeters used to grow the undoped and doped diamond-like

crrhon tlrns are disfussed. Chapter 3 focuses on the experimental results çonceming the

structural propenies of the samples. narnely the hickness. composition as well ns the

tiactions of tetrahedrd and trigond bonds. The measurements of the opticd energy grp

and elecuical conductivity are presented in Chapiers 4 and 5 , respectively. A conclusion is

then provided in Chapter 6.

Chapter 2 De position

2.1 Introduction

A vdeiy of deposition methods. such as riidio frequency (RF) glow-discharge and direct

curreni (DC) glow-discharge. have been reponed in the literature t'or the preparation of

amorphous semiconductors i4.5). In this research project. we employed the DC saddle-

field glow-discharge system 16). which was developed at the University of Toronto. tu

deposit dimond-like amorphous carbon films. In this chapter. the contïgur;ition of the

system. prepantion of substrates. as well as the deposition procedure and deposition

parmeters are discussed. A bnef summary then follows.

2.2 DC saddle-field glow-discharge system

A schematic of the DC saddle-field glow-discharge system used in this project is shown in

Fig. 2.1. The saddle-field system muily consists of a deposition chamher, a DC power

supply. a vacuum pumping system. a gas storage system and a gas mixing system.

The deposition chamber is a stainless steel cylinder with a diümeter of 1 0 inches

and a length of 12 inches. Since cesidual moisture may exist inside the chmher. sevenl

band heaters mounted outside the chamber are used to b&e the system pnor to a

deposition. Inside the chamber. as shown in Fig. 2.1. there are tive coplanar stainless steel

electrodes: one cenirai wire-mesh electrode, two middle wire-mesh elecvodes ruid two

ouier solid e1eçtrodes. The central electrode functions as an anode while the IWO middle

electrodes function as cathodes. One of the outer eleçtrodes is used as a suhstrate holder

vacuum pumplng

. A

m m

Fig. 2.1 A schemrtic side Mew of the DC saddle-tïeld glow-discharge chnmher and gas hmdling system (modifed from [7] ).

and the other one is connected through a smail pinhole (around 0.5mm in diameter) to a

mass spectrometer, which can be used to maly-re the composition and energy of the

plasma The substrate holder is electrically isolated from the chamber using a Viton 0-

ring. The potential of the substrate holder cm be set to any value hy using a DC voltage

source. In this research prognrn, the substrate potential was maintained at OV. A heater

and a thermocouple are also mounted inside the substrate holder so that the suhstrites cm

be heated during a deposition.

A Spellman SL6ûû DC power supply, which is connected hetween the anode and

the cathodes, is used to provide a constant discharge current for the hreakdown of g w s

into a plasma. The potentiais of the cathodes are maintained at OV. The voltage of the

anode is typically amund +400V.

The vacuum pumping sysicrn consists of a CTI CT-100 cryopump. r Bdïrrs

TPUllOH variable speed turbo pump and an Edwards mechmicd pump. Pnor to a

deposition, al1 thcsc pumps are activaied to maintain a hase pressure of approximateiy

10'' Torr insi& the deposition chamber. During the deposition. the cryopump is isolated

from the chamhcr iuid only the turbo and mcchanicd pumps are u.sed. A turbo pump

conuoller is used to r~gulate the speed of thc turbo pump so chat a constant dep«sition

pressure. which is typically around 50 mTorr. is maintained. The pressure inside the

çhamhcr is monitored by a MKS pressure transducer.

For the deposition of undoped and doped amorphous carhon films. three types OC

pases wcre used in this project: methme (CH,), phosphine ( PH,) and dihorane ( B,H, ).

Since phosphine and diborane are quite toxic. these gases were stored in diluted fom

rather than in pure Corn. in our gas storage system. we stored the gases in three sepante

hottles: melane. 9.68% phosphine in methane (by mole) and 9.83% dihorane in mehane

(by mole).

In order to deposit a diamond-like carbon T h with a parciculiu level of dopant

(phosphorus or boron). a gas mixture with an appropriate ratio of the dopant gas

(phosphine or diborane) to methane is required. A gas rnixing sysiem, which consisls of

several mixing bottles. was therefore designed to allow such mixing. The gas mixture is

prepared by tirsr admitting an appropriate amount of diluted dopant gas into the mking

hottles. An appropriate amount of pure methsne is then added into the hottles. Dunng the

deposition. this gas mixture is ûelivered to the deposition chamher at a constant llow rate

using a MKS mass flow çonuoUer.

2.3 Preparation of Substrates

In this project. three kinds of substrates. namely silicon. fused silica and Corning 7059

glus. were used for the dcposition of amorphous carhon films. Since the exisicncc of

contaminants. such as dust and tumgerprints. on the substrate surtacc would leüd t« poor

film adhesion. these substrates had to he thoroughly cleaned prior to dcposition. The tirst

cleaning procedure wcis to ultrasonicaily degrcase the suhsvûtes in tnchloroethylene for 20

minutes. The substrates were then ulvau>nicJly nnsed in acetone. meihünol and deionized

waier. Each nnse lasted for 20 minutes. Findly, dry compressed air w u used to hlow dry

the substrates. The substrates were then mounted on the substratc holder insidc the

deposition chamber.

2.4 Deposition procedure and deposition parameters

Before perfomiing a deposition. the band heaters were tumed on ovemight to hake the

system. As wel. the vacuum pumping system wiis activated to pump down the deposition

chamber to vacuum (=IO* Torr).

When perfonning a deposition. the tirst step w u to establish a constant suhstrate

temperüture by activating the substrate temperature controller. A gis mixture with the

itppropriate mole fraction of the dopant gas w u hen prepared. This gas mixture wss

delivered to the deposition chamber at a constant tlow rate. Next. the turbo pump

controller was activated io maintain a constant deposition pressure inside the chamher. To

start growing the film. the DC power supply was tumed on and rdjusted to ignite the glow

discharge and provide a constant discharge current. The gas mixture was hroken down in

the plasma and f i growth occurred. After the T i was allowed to grow for a pre-

detennined amount of time, the deposition was stopped by tuming off the DC power

supply. The substrate heater was thcn tumed off. Once the sample had cooled down to

room temperature, it could be taken out of the deposition chamber.

In this research program. two sels of diamond-like carbon tilms werc grown. In the

tïrst set. several undoped, phosphorus-doped (P-doped) and horon-doped (B-dopcd)

smples were deposited on different types of substrates for three hours each. Al1 ihcse

siimples were grown using the same deposition pitrameters as shown hclow:

Table 2.1 Deposition parameters for the samples deposited at 2OU OC.

For the deposition of doped tiims. mole fraction of dopant gases in the pas mixtures

riging fmm 5x10- to 1x10~ was used. The following table is a summary of the nmes

of the samples and the mole fraction of dopant gases used to grow the tïlms in this set:

Table 2 .Z The names of the samples deposited at 200 O C and the mole fraction of dopant gases in the gas mixtures.

undoped sample: P-doped samples:

mole jmcîàon of dopant gas O

Nume of sanrple Cl Pl P2 P3 P4 P5

B-doped samples:

For convenience. the suffixes "-tS" and "-cgw are added to the names of the smplcs for

those dcposited on fused silicü and Coming 7059 glass suhstrates, respçtively. For thosc

deposited on silicon suhstrates. no suffix is used. For this set of sarnples, the structural.

optical and electricd properties were studied.

B 1 B2 B3 84 BS

The second set of samples were deposited on Coming 7059 glass. Thesc .simples

were grown for five hours using the same deposition parimeters ü s s h o w in Table 2.1.

except for the substrate temperature. The substrate temperature used t'or growing this set

of simples was 400 OC. The niunes and the mole fraction of dopant gûses for thcsc

samples are listed in Table 2.2.

Table 2.3 The names of the sarnples deposited at 40 O C and the mole fraction of dopant gases in the gas mixtures.

For this set of samples. the elecvical pfoperties were investigated.

undoped sarnple: P-doped sample: 8-doped sample:

. Nana ofsample Cl 1 Pl 1 BI 1

nioie fmcrion of dopaiit gus ,

O 5x 10'' 5x 10"

In ihis chapter, the configuration of the DC saddle-field glow discharge systern employed

io deposit diarnond-like carbon 1Xms is described. A gas mixing system wüs üdded to the

deposition system so ihat tilms with various leveis of dopants could he grown. The

pmcedure For cleaning the substrates prior to deposition is provided. Finally. the

deposition procedure and deposition parameters used for the growth of the füms are

presented.

Chapter 3 Structural properties

3.1 Introduction

In bis chapter. the meaîurernenu of severd structural propenies of the dimond-likc

carbon thin f i will be presented. The structural properties studied are the ihiçkness. the

composition. as wel as the fractions of teuahedral and üigonal bonds. A compilation of

the results and a surnmary will then follow.

3.2 Thicknesses

3.2.1 Introduction

The thickness of Our thin füm sample is iypically a frxtion of ci micrometer. In ordcr io

measure such smdl thicknesses. speQal equipmcnt such as 3 Tençor Alpha-srp thin lilm

protilometer has to k used. A profilorneter has a dimond tipped stylus. Whcn thc siylus

moves across a step, the height of the step cm he obtained by meuunng the detlection of

the stylus. By knowing che height of the step. as the one shown in Fig. 3.1. onc ccin

dctcrmine the thickness of a thin tillm.

Fig. 3.1 The thickness of a thin fiim can be measured using a pmtilometer.

1 O

TO m a t e such a step. one cm use a clamp to cover up pan of the substrate hefore

the deposition. As a result. dunng the deposition. the T h will not grow on the area

covered hy the clamp. Whcn the deposition is finished. the clamp çan he rcmoved and a

step will be created. The thickness of the sample cm then be determined using a

protïlometer. Fig. 3.2 summarizes the procedures used to create a step.

mm cbmp thln fim

3.2.2 Measurements and results

In bis resemh program, a Tencor Instruments Alpha-step 200 profilorneter was used to

measure the thickncsses of the tilms. For each sample. live measuremcnt.. were performed

at dift'el'crent locations and an averige value was cdculated. These mecisurcmenls indicüted

that the thickncss of a I"h was unit'orm (f 5%) at diî'îèrent locations. The results for the

thiçknesses of the samples with Si and fused silica substrates are shown in Figs. 3.3 ünd

3.4. respectively. The thicknesscs of the samples on Coming 7059 g las substrates are

listcd in Table 3.1.

Table 3.1 The chichesses of the smples with Coming 7059 g1;ls.c su bstrates.

sample C 1-cg (undoped sample) smple P 1 -cg (heavil y P-doped sample) sample B 1-cg (heavily B-doped sample)

0.27M.02 ~ r n 0.3 lf0.02 pm 0.33I0.02 Hm

(a) P-doped samples (with Si substrates)

OW9 IIr

O 1 0 4 104 104 IV 10-1 mole fraction of phosphine

(b) 6-doped samples (with Si substrates)

O thicknesses for P-doped sam~les A thickness for an undoped sampie

O 10-5 104 io3 io2 10' mole fraction of diborane

O thicknesses for 6-doped samples A Ihickness for an undoped sarnple

Fig. 3.3 Thickness of the P-doped and B-doped samples (with Si subsuites) vs mole fraction of the dopant gas in the gas mixture. (For reference, the &ta for an undoped simple are also shown.)

12

(a) P-doped samples (with fused silica substrates)

0 thicknesses for P-doped samples 4 thickness for an unôoped sample

O 10-5 1 0 - ~ 1 0 - ~ IO* IO-1 mole fraction of phosphine

(b) 6-doped samples (with fused silica substrates)

O thicknesses for 6-doped sarnples A thickness for an undoped sample

O 104 104 104 IO* iol mole fraction of diborane

Fig. 3.4 Thickness of rhe P-doped and B-doped siunples (with tùsed silica suhsuaics) vs mole fraction of the dopant gas in the gas mixture (For reference. the &ta for an undoped simple are also shown.)

13

3.2.3 Discussion of the results

Fig. 3.3. Fig. 3.4 and Table 3.1 show thitt for al1 the samples with r pmicular kind of

substrate (Si. fused siiica or Coming glass), the hicknesses of the thin füms are sirnilar.

The gmwth rate cm be cdculated by using the formula

thickness gmwth rate =

deposition rime

The tellowing table summajizes the growth raies of the thin tïlrns on different types of

subs trates:

Table 3.2 The Growth rates of thin tïlms on a vluiety of suhstntes.

The resul~s show that for a panicular type of subsirate, the growth rates for dl ihc

smples are similar. in addition. it can he seen that the iilms' growth rates arc higher for

the rclatively conductive Si subsintes when compûred io those for the non-conduciivc

suhsuars. such as fused siüca and Coming 7059 glus.

1

samples on Si substrates samples on fuscd silica substrates samplcs on Coming 7059 glas substrates

3.3 Composition

growth rates (nmhaur) tiom 201 to 240 tkom 125 ta 152 from 90 to 1 1 0

One of the ways to detemine the composition of a .simple is to use secondüry ion mass

specmscopy (SIMS). In SIMS. a pnmary beam of ions. such as Cs'. is used to bombard

and etch. i.e.. sputter away. the top surface layer of a sarnple [8,9]. During this etching

process, secondary ions emitted fmm the sample are collected and mu-analyzed to

detemine the concentration of elements. This in turn provides information about the

composition of the sample. To prevent charging effecu, Le., the building up of charge on

surfaces which leads to the defocusing of the primary ion bearn, siunples used in S i M S

analysis should be relatively conductive. SlMS is a very sensitive technique and cm te

used to determine the concentration of dopants.

3.3.2 Measurements and results

In order io detemine the amount of dopants (phosphorus or boron) incorporated into the

doped samples. SIMS meaîurements were perfomed. Al1 the SIMS meüsurements in this

thcsis were perïormed in the Surfûce Sciences Lahoritory at the University of Wcsrm

Ontario using a Cameca IMS-3f SIMS instrument. The samples submitted for SIMS

anaiysis have the thin f h s grown on relativcly conductive silicon substrates to üvoid

charging efl'ects. As a representative example. the concentration of carhon. hydrogen and

phosphorous in sample P l (the most heavily P-doped sample) is plotted against depth in

Fig. 3.5. The concentration of the elemenu in the P-doped and B-doped samples is

summxized in Figs. 3.6(r) and (b), respectivcly. The data for an undoped smple arc ais«

includcd in thcsc plots for refcrence purposes.

The composition of a sample can he found by calculating the fractional

concentration of each element in the sample. The fractional concentration of an clernent LS

simply the proportion of the element (in concentration) piysent in the sunplc. For

exmple. the t'nctional concentration of carbon in a smple is detïned as

fractional concentrution of carbon = [cl rI+ [Hl + [dopont1 *

where [Cl, [Hl and [dopant] are the concentration of carbon. hydmgen and dopant.

respectively. In the P-doped and B-doped samples. [dopant] mfers to the concentration of

phosphorous and boron. respectively. In the undoped sample. [dopant] is equal to zero.

Sample P l (the most heavily P-doped sample)

,+, conc. of C 4 conc. of H -.@- conc. of P

Fig. 3.5 Concentration of theelements in thesample Pl vs depth The estimated error in concentration is &Y%.

(a) P-doped samples

iP4 IF O canc. of C in P-doped sanples conc. of H in P-doped sanples

O conc. of P in P-cbped samples A conc. of C in undoped saniple A conc. of H in undoped sarnple

mole fraction of phosphine

(b) B-doped samples

1 O conc. of C in 6-âoped samples conc. of H in 6-doped samples conc. of P in Bdoped samies

A conc. of C in undoped sarnple A conc. of H in undoped sample

O IO-= 104 to3 io2 IO-' mole fraction of diborane

Fig. 3.6 Concentration of the elements in the P-doped and B-doped simples vs mole fraction of the dopant gas in the $as mixture. The estimated error in concentration is %%. (For reference. the data for an undoped sarnple ;ire also shown.)

17

(a) P-doped sampies

O frac. conc. oi C in P-doped sanyies frac. conc. of H in Paoped sampbs

O frac. conc. of P in P-do@ samples il frac. conc. of C in undoped sampîe A frac. conc. of H in unûoped sample

mole fraction of phosphine

(b) B-doped sam ples

0 frac. conc. of C in 6-doped savies frac. conc. of H in B-doped sampies

Q frac. conc. of 0 in Bdoped samies A frac. conc. of C in undoped sample A frac. conc. of H in undoped sampie

O 10-5 iw 1 0 ~ IO-* tot mole fraction of diborane

F i . 3 7 Fractional concentration of the elemenu in P-doped and B-doped srmples vs mole fraction of the dopant gas in the gas mixture nie estimated error in fractional concentration is f 10%. (For reference. the data for an undoped sarnple are also shown.)

18

The composition of the P-doped and B-doped samples is summari?ed in Figs. 3.7(a) and

(b). For convenience. the symbols AC], flH]. nP] and AB] are used to denote the

fraçtionül concentration of carhon. hydmgen. phosphorus and boron. respectively.

3.3.3 Discussion of the results

%

Fig. 3.5 shows that the concentration of the elements in the tiim is independent of depth.

This confimis that the deposition puamecers are stable throughout ihe growth of the film.

From Figs. 3.7(a) and (b). it cm be seen that there exists a good correlation

hetween the hctional concentration of the dopants (phosphorus or huron) in the samples

and the corresponding mole fraction of the dopant gases (phosphine or dihome) in the

gas mixtures. A relation between two quantities X and Y which appears as a strüight line in

a loglog p p h cm he expressed by the formula

iogY =o+blogX . (3.3)

whcre o and b are constants. The experirnental data for the fractionai concentration of the

dopant (Y) and the corresponding mole t'rüction of the dopani pu ( X ) were îitted io ihe

ahovc cquation. The curve-fit results. as show in Figs. 3.7(r) and (h). show chai the

experirnental data fit quite well to eq. 3.3. 'The values of a and b detennined liom the

çurve titting for the doped samples are listed as follows:

Table 3.3 The values of a and b (as in eq. 3.3) obiained from the curve tïtting for the doped samples.

1 phosphorus-doped samples 1 0.19 f 0.02 1 0.97 f 0.09 1 1 horon-doped samples 1 4 . 2 4 f 0.02 1 0.95 I0.09 1

It cm he seen from Table 3.3 that the constant b is equal to 1 within experimenial emr.

Therefore, eq. 3.3 can be approximated as:

Y = A X , (3.4)

w h e ~ A = 10'. The values of A for the doped samples are given in the following table:

Table 3.4 The values of A (as in eq. 3.4) obtained from the curvc tïtting for the doped samples.

1 phosphorus-doped sarnples 1 1.5 f 0.2 1 1 horon-doped samples 1 0.58 I0.06 1

These rcsults indicate that various levcls of doping can be achieved prediclühly by us

the appropriate mole fraction of the dopant gases.

From Figs. 3.7(r) and (b). ii çan also be seen that the doped samples exhihit

lower hiactionai concentration of hydrogen than the undoped sample. This is truc for hoth

phosphorous-doped and boron-doped sarnples. even at very low doping levcls. ûnly the

rnost heavily horon-doped sample does not fit this trend. This cffeci should he cwefully

cxplored in future studies, as hydrogen is important in reducing the çonçcnuütion of gap

statcs [ 101. and is therefore very important in electricaily active substitutional doping.

3.4 Fractions of tetrahedral and trigonal bonds

3.4.1 Introduction

Diamond and graphite are the two crysiillline fonns of carbon. The bonds in dimond are

purely tetrahedral (sp3) while those in gnphite are purely trigond (sp2). Amorphous

carhon tïïms c m contain both tecrahednl and trigonal bonds. Robertson [ 1 11 suggested

that one crn chancterize arnorphous carbon tïïms hy detennining the fractions of

teirahedral and trigonal carbon bonds in the tilms. Lascoviçh et al. [ 121 showed that these

fractions cm be evaluated by using the C KLL (carbon KLL) spectra obtained from X-rdy

excited Auger Electron Spectroscopy (XAES).

In this section. we will discuss the basic principles and measurements of XAES.

The method for cûlculating the fractions of tetrahednl (sp3) and trigonal (spL) carhon

bonds in the Iilms will be presented. Discussion of our experimental resulrï will then

ti1llow.

3.4.2 Basic principles and measurements of XAES

KLL XAES measurements çan be performed by using an x-rüy photoelectron

spectromeier. Monochromatic x-rays are used to irradiate thc siunples. When an x-ray

photon interücts with û K-sheîl (inner shell) eleciron of an atom. a photdeciron LS

relcrised. An electmn h m the L-shell (outer shell) thcn tilLs the K-shcll vacÿncy. »lien

uansl'crring the excess energy to another electron in the L-shell. This process of direct

cncgy cxchanp between two elecuons in an atom is rekrred to as the Auger process md

the cmitted elcctron is referred to as an Auger electron. The Auger elccuons rclcüscd t'rom

the smple are collected by the spectrometer. Since one K-shell and two L-shell clccuons

are involvcd. the process is typiçdly known as KLL Auger elcctron cmission or KLL

Fig. 3.8 The process of KLL Auger electron emission for a mode1 atom [13]. There are severai types of KLL Auger clectron emission. In this figure. the prucess of KL, &, Auger electron emission is shown as a representative example.

2 1

Auger et'fect. KLL Auger elecuon emission is a collective term used to indude ail pssihle

cases. such as KL, 4 . Kh &, and K&, L?, Auger electron emissions. n i e process of

KL&, Auger electron emission for a mode1 atom is shown in Fig. 3.8 as a representativc

exmple. The KLL XAES specuum cm be obtained by plotting the intensity of Auger

signds. which is proportional to the nurnher of collected Auger electrons, against the

Auger eleçtron kineiic energy.

In this resemh program. all the C KLL (carbon KLL) XAES measurements were

perîbrmed at the Ontario Centcr for Materials Research using a Leybold MAX 2 0 x-riy

photocleciron spectrometer. To avoid charging et'tec~s as discusscd in section 3.3.1. ihc

thin Nms deposited on the relatively conductive silicon substrates were suhrniited for

XAES malysis. During the XAES me;LsurerncnLs. the spectrometer chiimbcr wüs

mainiliined at ulua high vacuum (approximatcly 10-" Torr). The sarnplcs werc irradiard

with monachromatic soft x-nys produced by a Mg Ka source (hv=1253.6 eV) [131. The

Auger clcctmns were then collected in the energy anülyzer. The intensity of ihc Augcr

signal, N(E). wüs recorded over a kinetic energy ring from 210eV to 290kV. The C KLL

XAES spcctra were then trmsferred to an [BM compatible PC for funher malysis. The

riw C KLL spectrum for the sample Pl (the mosi heavily P-doped sarnplc) is shown in

Fig. 3.9(a) as a reprcsentative example.

3.4.3 Calculation of the fractions of sp3 and sp2 bonds

Lascovich rt al. [12] suggested that the fractions of tetrahedd (sp3) and trigonal (.sP2)

cürhon bonds of amorphous carbon f h s can he obiained by linear inteipolation between

the D panmeters for natural diarnond and graphite. The D parameter. as shown in Fi$.

3.9(h). represenu the width of the dominant peak ai around 26OeV in the C KLL

specmm. This peak is quite broad. and therefore deiennining the D parameter from the C

KLL spectrum is not very reliable. uistead, the D parameter can he deterrnined more

conveniently and accuraiely from the first-derivative C KLL spectrum. In the first-

derivative C KLL speçuum, the D parameter is defined as the distance between the

maximum of the positive-going excursion and the minimum negative-going excursion. A

graphical illutration of how to detennine the D parameter is shown in Fig. 3.9(c).

In this thesis. the fractions of sp3 and sp2 bonds in the samples were determineci hy

pdtinning ihc hliowing sieps. Firs~. ihe C KLL s p ç i n ware smooihrd ÿnd

différentiated. Second, the D parameters were extrücted from the differentiatcd C KLL

spectra. Finally. the tiactions of sp3 and sp2 bonds in the sample were obtained by linev

interpolation bctween the D parameters for niiturd diamond and graphite.

Smooihing was prformed on the C KLL spectra to eliminüie noise introduced

during dru collection so chat differentiation, whiçh is a tlucturtion-sensitive opcriilion, cün

hc npplicd rncuiingfully. In this thesis, the C K U spectra werc smoothed using ihe

Wicncr mcthod providcd by an XAES malysis softwi~c package (141. The smoothed

spectra were ihen differentirted to gcnerate the tirsi-denvative C KLL spectra. As a

rcprcsentative cxmple, the raw. smoothcd md tint-derivative C KLL spectra for samplc

P l (the mosi heaviiy P-dopcd sunplc) arc shown in Figs. 3.9(a), (h) and (ç). rcspectively.

The D parameters for dl the samples, w hiçh are summvizcd in Figs. 3.10(a) and (h). wcrc

extractcd [rom the tirst-derivaiive C KLL spectrd.

The percentap of sp3 bonds (%sp3) for a sample cm be calculated hy lincar

interpolation between the D panmeters for naturd diamond and graphite:

where D ,amp,t. Dd-, and D ,,,, are the D parameters for the simple, naturd diamond

and graphite, respectively. The percentage of sp2 bonds (%sp2) cari d s o he deterrnined hy

using the fornula

%sp2 =lm - %sp3 ( 3 m

width of peak (= 0 parameter)

1 I I I D parametet

I 1 I I

220 240 260 280 kinetic energy (eV)

(a) taw C KLL specttum

(b) smwthed C KLL spectnim

(c) first-derivative C KLL spectrum

Fig. 3.9 The raw. smoothed and first-denvative C KLL spectra for the sample Pl (the most heavily Pdoped sample)

24

(a) P-doped sam ples

O D parameters for P-doped samples A D parameter for an undoped sample

(b) B-doped samples

1-

P E E

O DpammetersforB-dopedsamples A D parameter for an undoped sample

mole fraction of phosphine

O 105 O IO-^ io2 i lol mole fraction of diborane

Fig. 3.10 The D parameter for the P-doped and B-doped samples vs mole fractio;. of the dopant gas in the gas mixture (For rebrence, data for an undoped sample are Jso shwon)

25

(a) P-doped sam ples

O %sp3 for Pdoped samples A %sp3 for undoped sample

10-5 O 103 IO-* 10-1 mole fraction of phosphine

(b) 6-doped samp les

O 56sp3 for Bdoped samples %sp3 for undoped sanple

O 10-5 IV 104 IO-* iol mole fraction of diborane

F i 3 . The %sp3 for the P-doped and B-doped samples vs mole fraction of the dopant gas in the gas mixture (For derence. data for an undoped sample are also shown)

26

The values of Ddim,, and D,,,. which were determined by Lim [15) for the

spectrometer, used in this work are 13.3f0.2eV and 20.7I0.2eV. respectively. The

perfentages of sp3 bonds (%sp3) in natural diamond and graphite are, by detinition. lOo%

and 0%. respectively. The %sp3 t'or dl the smples were calculated and the values are

summaizcd in Figs. 3.1 l(a) and (b).

3.4.4 Discussion of the results

Figs. 3.1 l(a) ûnd (b) indicate that most of Our doped and undoped samples have a similu

%sp3 rmging frorn 47% to 53%. However. the heavily B-dopcd sample has a iargcr Oksp 3

equal to 61%. It .seems that the level of doping does not affect the percentages oî' sp'

honds in the samples. except in the most heavily B-doped one.

It should be noted that C KU. XAES can only provide information about the

percentages of sp3 and sp2 carbon bonds. Information about whiçh piuzicular kinds of .sp"

or sp2 honds ;ire present in the simples cannot be ohtained from the XAES rneüsurcmentx

The different types of sp3 and sp2 bonds that can exist in the undaped and doped sümples

are listed in the following table:

Table 3.5 Different kinds of sp3 and sp2 bonds that can exist in the undoped and doped samples.

If the various kinds of sp3 bonds (or sp' honds) are to be difterentiated from one another.

more powerful techniques. such as electron energy loss spectroscopy (EELS), must be

employed.

undopcd s~mplcs P-dom s m p l o s B-doped samples

spJ bonds C-C and C-H bonds C-C. C-H and C-P bonds C-C, C-H and C-B bonds

sp' bonds Cs honds

1

C=C and C=P bonds C=C bonds

3.5 Compilation of results and Swnmary

In this section, we will compile and summarize the results obtained from the measurements

of al1 the thrce structural properties.

The resdts of the thickness measurements indicate that when the s m c kind of

substrates üre used. both the undoped and doped siunples have similiv hicknes.ses and

growth rates. The resulis also show that the growth rates of tlms on the relütively

conductive silicon substrates are higher than those on the fused silica and Coming 7059

glaîs.

From the SIMS measuremcnts. we observe ihüt iri good correlûtion exis~s betwecn

the fr~ctional concentration of the dopants in the doped samples and the corrcsponding

mole fraction of the dopant gases in the gas mixtures. The measurements also show that

cven at vciy low doping. most of the doped simples exhibit a lower fractional

conceniration of hydrogen than the undoped simple. The only exception io this is the

most hcavily baron-doped sample. This suggcsts ihat the structure of the heüvily boron-

dopcd simple is somewhat dillèrent from thosc of the other doped mples . Additional

evidencc of this dit'térence can be seen from the results of the XAES measurements: the

frxtion of teuühedral bonds in the rnosc hcavily boron-doped sample is hiphcr than that in

any other doped sample.

Chapter 4 Optical properties energy gap

4.1 Introduction

The opticnl energy grp (E, ) is one oî' the physical properties used to charücterizc thin

lilm amorphous semiconductors. In this chaptcr. ihe theory used to deteminc E, liom

triinsmitmce (T) and retlecimce (R) measurements will first be discussed. Next, the

cxperimental ûppantus employed to mesure T and R will he prcsented. The

meüsunments, a discussion of the resulu and a summary then hllow.

4.2 Theory

k u c 116,171 sugpsied thrt for photon cneqics rbove E, . the ciptical absorption

çocftïcicnt (a) of an amorphous scrniconductor is relûtcd to the optical cnergy p p hy the

relation

&Ë = c o n w ( ~ - E ~ ) , (4.1)

when: E is the photon enegy. This energy gap cm thus be ohtained hom a gnph , such as

show in Fig. 4.1. of

J~;EvsE.

The t i n t step is to extrapolate the dope of the cuwe ahove E, , Le., in the extended states

region. Then the energy gap Fan ôe deiecmined from l e x-intercepi of the extnpolrted

suaight line.

Fig. 4.1 &Ë vs E

In order io deicrmine the absocpiion coefficient. the transmission and rellectiviiy of

ihe iilm musi be measured. Demichelis et al. [ ln ] showcd thai in ihc exicndcd sirtcs

rcgion. a good approximation to J s c a n bc cxpressed in tcrrns of the triinsmiii;incc (T)

and rctlectance (R) by the formula

T = (1 - R ) exp ( - a d ) . (4.2)

whcre d is the thiçkness of the film. Afkr re-arnnging the above equation. we gei

WC cm then multiply E o n both sides and idce the square rooi to pi

Now, instead OC using the graph of &Ë vs E . we fan determine the energy gap iiom

the plot of JadE vs E . This is because the multiplication of= hy the constant a will only change the scale of the y-mis but will not affect the value of the x-intercept.

By taking into account eq. 4.4. the graph of 4- vs E is quivalent to the

griph of (I- vu E . In other words, dter measuring the ~iinrrnittuiue and

retlectance. one can use the gnph of ln (y)- E ur E io determine the cnergy gap

of m amorphous semiconducting !Ym.

4.3 Experimental apparatus

The transmiitance (T) and retlectance (R) of a thin tilm cün he meüsured using m

ultraviolei/visible Light (UVNis) spectrometer. In bis resemh program, a Perkin Elmer

UVIVis Lamdha 18 spectrometer was used. This spectromeier hm a hem splittcr thrt

spli~s the parcnt photon b e m into two daughtcr barns. nmcly the samplc and rcfcrencc

beams. For bis thcsis, the simple hem is used to rncuurc T or R, whilc the rcfcrcnçc

k a m is monitored to account t'or any variations in the sample hem's intcnsity ovcr timc.

The setup used to measurc T is show in Fig. 4.2(a). Thc intcnsitics of the

rcfcrcnce and sample bems are first recorded as Ir&, ) and q, (t, ) respec.tively ai somc

time t, . Next, the thin T h sample of interest is placed in the path of the smple hem

show in the figure. The intensitics of the reference and smple k v n s are thcn recorded

as Ir,, ( t , ) and Tm (t, ) respectively ai time t , . If we neglect the vûnûtion in the .smple

hm's intensity between times t, and t2 . then the ansm mit tance (T) can simply hc

dcfined as:

However. if we take into account such variation, we should use the following formula

instead:

The reference and sarnple beams are both generated from the same source. i.e., the parent

hem. Therefore, if there is any variation in the parent beun's intensity, the refercnce and

smple heiuns will experience the same fractional variation. As a result. if we wrnt io

ascount for any tluctiiation in the sample heam's intensity, we Fan sirnply monitor the

ret'ercncc bearn9s intensity. In other words. we can use the ratio ['"('Xe, (?, )] , which

appears in the denominator in eq. 4.6. as a correction factor to takc into account the

variation in the sample beam's intensity between times c, and t, .

The setup used to measure R is shown in Fig. 4.2(h). In this contïguwiion, a

Perkin Elmer îïxed 6' specular retlectance accessory is placed in the path of the samplc

kam. This retlectance accessory has a small angle of incidence (only 6') from the normal.

Thus. the measured retlectance is vcry close to the retlectance at normal incidence. and the

crror so introduced is quite small [19]. As s h o w in the tigure. a high qurlity mirror is

plÿced on the top of the rrtlectancc acccssory. The iniensitics of the reference and .wmplc

hems are then recorded ûs Ir&) and R,, ( t , ) nispectivcly ri some time t, . The ncxt

step is to remove the mirror and place the simple (with the thin füm k i n g down) on the

iop of the retlectance accessory. The intensities of the refcrence and sample bems are

then rccorded as I&) and Rfih(t ,) respectively at time t ? . Similar to the way T is

ohi;iined. R can be determined using the formula:

ut time = t,:

ut time = t2:

Fig. 4.2 The setup of the Lamdba 18 spectrometer used for measuring T and R.

4.4 Measurements

The trmsmittances (T) and relectances (R) of the undoped and doped thin films deposited

on t'used silica were measured using photon bems with wavelengths nnging from 185nm

to SWXhim. The samples on fused silica subsuatcs were used in these measurements

k a u s e Cused silicr is transparent in this wavelength nnge. The T and R data for sample

Pl-Ss (ihr mosi bravily phosphonis-doprd sampb) are s h o w in Fig. 4.3 u r

repnseniative exmple.

The wavelength (A) crn be easily convcned to energy ( E ) using the equation

where h and c ûn: Planck's constant (4.135~10-" P V + S ) and thc speed ol' lipht

(3.0~10' m e s - ' ). respectivcly. The meaïured values of T and R are thcn used to plot the

gnph of

As an exampic, the data for sample P 14s arc plottcd in Fig. 4.4.

4.5 Results

The detcrmination of the energy Sap for sample Pl-fs is indicated in Fig. 4.4. It ean he

seen that t'rom approxirnrtely 3eV to approximately 6eV, the Tauc relation is sütist'ied

quite well.

There is considerable discussion in the literature regarding the vdidity of the T ~ u c

approach [20,21]. In kt denvation. Tauc assumed that in the exiended states region, the

density of states increases with energy as the square rooi of energy. as it does in

R ( rd lectance)

200 300 400 500 600 700 800 900 wavelength (nm)

Fig. 4.3 Transnittrince and relleçmce vs wavelcngth (for svnplc P I 4s)

Fig. 4.4 ,/ h (lP)* E vs E (for sample P l -fs)

(a) P-doped sam ples

mole fraction of phosphine

(b) B-doped samples

1-

O 105 1 103 1 0 ' ~ 10-1 mole fraction of diborane

O Eg for 6-doped samples A Eg for an undoped sample

Fig. 4.5 The optical engergy gap (Eg) for the P-doped and B-doped samples vs mole fraction of the dopant gas in the gas mixture (For reference. the data for m undoped sample are dso shown.)

crystalline semiconduçtors. However, it has ken shown that the lack of long range ordcr,

which is responsible t'or the creation of tail states in amorphous semiconduçtors. dso

modifies the density of states in the extended states region. The density of states in the

extended states region is r very complicated function of energy in amorphous

semiçonduc~ors. Furthemore. it depends on the degree of disorder present in the material

[ 2 11. The simpiicity of the Tauc approach makcs it useful in this work for the purposc of

companng samples and so we adopt it here. However. it must be understood thrt the

physicai meanhg of the ierm "encrgy gap" in the context of morphous .semiconductors is

somewhat ambiguous.

The energy güps for dl the undoped and doped smplcs are summuized in Figs.

4.5(r) and (b). The resulu show that l e energy Laps for the sarnples range from ?.&V to

2.5cV. The simildty of the energy gaps indicaies that ihc incorporation of the dopants

docs not affect the optical energy gap very much.

In this chüpter, we have discussed the determination of the enegy gap of an amorphous

1 - R thin film h m the graph of ,/ ln E vs E. The ciprimenul apparatus for

meuuring T and R was presented. By using the T and R data. the energy gaps for the

srrnples were âetemined. The results show thût the undoped and dopcd Nms have similv

optical energy gaps, indicating that doping hiis no significani effcct on the cnegy gap.

Chapter 5 Electrical properties conductivity

5.1 Introduction

In this chapter. we discuss one of the electncal properties. namely the conductivity, of

diamand-like carbon thin tilms. The rneasurement technique and experimentiil seiup are

tirst presenied. The measurements and results as well as a bnef summary thcn follow.

5.2 Measurement technique

The simples1 way to measurc the conductivity of an arbitrvily shüped siunple is to use the

van der Pauw method i22.231. The van der Pauw meihod. as shown in Fig. 5.1. uses a

four-point probe configuration. in which a currcni is camed hy two of the probes ÿnd

volirgcs arc measured ai the other two. This technique is rclativcly insensitive to whcther

the contacts are ohmic or rectifying. When using the van der Pauw method. smdl contacts

should he made at the periphery of the sample. As well. the tWh should k of uniform

thickness and free of isolated holes.

Fig. 5.1 An arbiuarily shaped sample with four contacts [23].

3 8

The conductivity ( O ) of an arbitrariiy shaped sample of thickness t is given by

oz- (~12.~4 + ~ 3 . 4 1 ) '

The quantity &,, is defined as

where I , , is the current that enters the sample through contact 1 and leaves through

contact 2, and b;, = V, - Y , is the voltage difference between the contacts 4 and 3 The

quantity R,,,,, is defined in a similar way. The correction factor F satisfies the relation

RI2.34 This correction factor, which depends implicitly on the ratio R, = - , can be 4 3 . 4 1

determined by solving the above equation numerkally. This i s available in the literature

and the results are shown in Fig. 5.2.

Fi8.5.2 The van der Pauw correction factor F as a function of 4 [23].

5.3 Experimentalsetup

In order to perform measurements on a smple of high resistivity. such as a diamond-like

carbon f i . high impedance voltmeters (i-e. electrometers) and current sources must he

employed. The impedances of these instruments should be much higher (eg. >l(M) times)

than ihat of the sample; othewise. significant mors will result. To minirnh cumnt

Icakagc. uiaxial cables arc used to mÿke elecuical connections bctwccn thcsc insuumcnls

and the sample. A triaxial cable consists of three concentric conductors. nmely the signal

(center wire). guard (inner shield) and ground (outer shield). The insulation between these

concentric conductors is made of Tetlon. which has an extremely high impdrncc

(>10" 0 ) [24] and cm therefore help prcvent current leakage. To liinher minimize

leakrge, each instrument sets the guard's voltage equd to the signal's voltage using its

huilt-in high impedance unity gain amplifier. Since the guard's voltage is the samc as ihc

signal's voltage. no current will lcak t'rom the signal to the guard conductor.

In this resemh prognm, a Keithley 220 current source, a Keithley 595 unmeter

and two Keithley 617 electrometers were used. The cunent source and the electromcters

have interna1 impedances of at least IO'% [25.26]. In order to verify whcther Our systcm

had signiticant current leakage paths. meûsurcments at room temperature were perlimncd

on r Victorcen standard resistor (with a resistance of 5x10"Qf 1 % ) as a test. The test

configuration is shown in Fig. 5.3. Dunng the test. the current source delivered a smdl

current of l x l 0 - ' ' ~ to the resistor and the ammeler was used to meüsure the üctual

cunent that went through the resistor. The results showed that almost al1 of the delivered

current ( I n ) actually passed through the resistor and that the calculated resistiuice

( Ra, = Va - Vh ) was very close to the specified resistance value (R= 5 x 1 0 " ~ i 1% ). LI, These results indicated that only mal1 fument leakage exisied in the measurement system.

Fig. 5.3 nie test contiguration uscd for measunng the resistance of a standard resistor.

To make electrical contacts, tour Teîlon-insulated copper wires were honded to

the corners of each of thc yuarc samplcs ( l m x km) using EPO-TEK H20E silvcr

pastc. These wires werc then connccted to triaxial rccepi;icles. whiçh wcrc in tum

connected to the electromcters using uiaxirl çûhles. Fig. 5.4 shows a schcmütic of the

electricrl configuration used for determining the quantity R,, -. ,, (as delincd in cq. 5.1 ). The

value of R?, , , waî obtained using a similar setup.

Fig. 5.4 The schematic of the electrical contiguration used for determining R,, ,, . -.

A dedicated chamber with temperature control capahility was huilt to measure the

conductivities of diamond-like carbon films at various temperatures. In the chamber. a thin

T h simple w u plüced on top of a copper plate. inside which a hcater and r temperature

sensor were mounted. The temperature of the sample was stabiliïed (within I 0 . 5 OC)

using a RMC 4075 temperature controller. The vacuum ( d l d Torr) inside the chamhcr

was maintained using a RMC LTS-22". 1CH cold head. a Balzers TPH035 turbo pump and

an Edwards mechanical pump.

5.4 Measurements and results for samples deposited at 200 OC

To avoid current leakage through l e suhstrates during the conductivity mcasurcrncnts.

suhstrates with relatively high resistivity must be used 10 grow the thin films. In this

research program. Coming 7059 glus substrates (wilh a resistivity of 1 . 3 ~ l O ~ ~ ~ + c m ai

250 OC [27] ) were uscd to dcposit diamond-like carbon Tdms at r suhstraic temperature

of 200 O C . Conductivity measurcmcnrs wcrc performed ai tempcraturcs ranging from

25 O C to 200 O C on thrce simples, nmcly the undoped, the most hcavily horon-doped md

the most heavily phosphorus-dopcd smples. It should he noted thût sincc the Iiims wcrc

grwn ai a suhstrûr temper;iiure equül to 200 OC. conductivity rneûsuremenrs could be

pcrt'omed at temperatures no higher than 2(IO OC. If higher temprrtures would hc uscd in

the mcüsurernents. structural changes could occur to the thin tïlms.

When the current source was set tu deliver a srna11 current of ~xlO-" A to ;uiy of

these smples at lemperatures from 25 O C to 200 OC. the mmeier indicated thüt thc

current passing through the simple w u close to îero. This ohsenmion showcd thai the

resistances of the thin t-hs (as seen by the current source) wcre much higher thün the

interna1 impedance of the current source. As a result, the delivered current did not cntcr

the samples but passed through the intemal resis~ncc of the curreni source. Therefore, the

conductivities of these samples could not be xcurately detemined.

5.5 Measurements and results for samples deposited at 400 OC

Sinçe the conductivities of those sarnples deposited üt 200 'C could not be studied. ;vi

additional set of three samples were grown. The deposition parameters used for depositing

these additional smples were very similar to those used for depositing those samples

studied in the previous section. The only diî'ference is that the new Films were grown at a

suhstrate temperature of 400 OC rather ihan üt 2(M) OC. For convenience, we will use the

tcrms "4oK samples" and "200C samples" to descrihe thin tlms deposited at the

suhstrate tempratures 400 OC and 200 OC. respectively. Since the 4MK: siunples were

grown at 400 OC, canductivity measurements on these smples could he performed nt

temperatures up io 400 OC without altering k i r structures. The thicknesses of thc

undoped. mosi heavily boron-doped and most hcüvily phosphorus-doped 40(K t h s wcrc

0.17kf0.02pm. O.17f0.02prn and 0.1 Hk0.02pm. respectively.

Since the appantus described in section 5.3 was designcd to opernte at

temperatures up to only 230 O C , a second systcm w u built with an openting temperature

of up to 380 OC. This new systern was desiped to operate in air and no purnps were

needed. The system was veritied to have very little current leakage using the s m e test

configuration indicated in Fig. 5.3. Meûsuremcnts of conductivity were pcrlonncd on the

4NC smples at tempratures ranging from 225 OC to 380 OC using the elmricül

configuration shown in Fig. 5.4. These 400C samples had the s m e size (Icm x km) as

the 2MK: samples.

The conductivity of an arnorphous semiconducior, such as r wide band grp

diümond-like carbon f i , is thermally activated and cm be expressed ~s r function of

temperature (7) as î'ollows [28,29] :

where a, . E, and k are the conductivity prektor, the activation energy and Boltzmann's

constant ( 8.62~10" e~ K-' ). respectively. By inking the natural logiuithm of hoth sides

of thc rhove equütion, wc see thlit

Assuming thrt E, is constant. Ina depnds linearly on T l . Thereforc, E, and a, cün

he dctermined tiom the gnph of ha versus T-' hy pertbrming r lineu curve tii on the

expcrimental data. Fig. 5.5 shows the resulis of the conductivity mcasurcmencs for the

4OK smples. The values of E, and a, detemined from the curve titting are

summarized in Table 5.1.

Table 5.1 The values of E, and a, for the 40W samples.

undoped sample

From Fig. 5.5, it c m he seen that the data for al1 the samples, exçept the horon-

doped one, fit very weU to single straight lines over the entire temperiture region

investigated. The data for the boron-doped sarnple. however. cm he fitted well to two

stmight lines with different slopes over two temprature regions. This indicates that for

P-doped sample

B-doped sample

Temperature ronge

for 225" C < T c 380" C

; L

for 225" C < T c 380" C

for 225" C < T c 340" C

for 340" C c T < 380° C

E, (eV) 0.96 k 0.04

a, ( R - c m )

1.70 f 0.07

0.88 * 0.04

0.72 * 0.03

1.88 f 0.08

7 . 0 ~ 10" f 0 . 3 ~ 10-1

5.1 x 10-' t 0 . 2 ~ 10-'

1 . 4 ~ 10' I 0 . 0 6 ~ 1 0 ~

B-doped sample 61 1 -cg P-doped sarnple Pl 1 -cg undoped sam ple C 1 1 -cg

Fig. 5.5 Natural logarithm of conductivity vs inverse of temperature (for the 400C samples).

the boron-doped sarnple, the dominant conduction rnechanisms at lower and higher

temperatures are different These dominant mechanisms are most k l y to te viuirhle-

range hopping conduction (between localized states) at lower temperatures and extended

state conduction at higher temperatures [30.31]. For the oiher two samples. the dominant

mechanism over the entire temperature region studied is most likely to be variable-ringe

hopping conduction [30].

It is known that for the variable-range hopping conduction mechanism. a change Ui

the activation eneqy indicates a shift of the Fermi cnergy [29j. As well, weak doping will

only shift the Fermi energy slighdy [301. Our resulu in Fig. 5.5 show that the activation

encrgics t'or the boron-doped and phosphorus doped samples devirte Irom thrt of the

undoped simple by 0.223).OSeV and 0.08fi).06eV. respectively. This indicates thai ihc

doping with boron (p-type doping) is more effective than the doping with phosphorus (n-

type doping) in the samples.

For extcnded state conduction in a diamond-like carbon Mm. the activation cncrgy

rppcars to be similv to the optical energy gap. The activation encgy for the horon-dopcd

smplc in extended state conduction was Iound to be 1.X8kf0.08eV. For cornparison

purposcs. the optical energy pap for a similar horon-doped siunple, which wlis grown

using the same deposition panmeters but on a tùsed silicü suhstratc. was detennined

employing the method described in Ch. 4. The optical energy gap of the smplc w u found

to be 1.8M. leV. which is vcry similar to the activation energy mentioned ahove.

Howevcr, as a mode! for exrnded state conduction in diamond-like morphous carbon

hu not k e n fuUy developed. further investigation is needed to çontinn the physicd

interpretation of thcse data.

5.6 Discussion of results

The results of the conductivity measurements on the 40(K3 smplcs indiçatc that horon-

doping is more electncally active thm phosphoms-doping. This tendency for the horon-

doped smple to behave differently than the phosphocus-doped one has ken ohservcd not

anly in the conductivity measurements of the 4M.K samples but also in the analyses o l the

sinicturd propercies of the 300C samples. These wdyses (in scctions 3.3 and 3.4) show

that the composition and the fraction of teuahedral bonds in the hcavily horon-doped

200K sample are somewhat different from those in the heavily phosphorus-doped 2OK

smple. In addition. we know that al! the deposition paramciers. exçept depmition

tcmpcriture. used for the growth of the 2 O K and 4oW samples were identical. From ail

the ahove infornation. it is quite reasonable io expect thüt thc conductivitcs of the dopcd

2OW smples exhibit temperature dependence similv to those of the doped 4OW

smplcs. In particular, wc expect that boron-doping is more clectricully active han

phosphorus-dopinp in the 2 ( K samples.

Two rneasurement systems. with openting temperatures up to 230 O C and 380 OC

respectively, were designed for conductivity measuremenu on diamond-like crrhon tilms.

Conductivity measurements at temperatures ranging from 25 OC to 310 OC wcrc

pcrtbrmed on samples deposited at 200 O C ( 2 0 C samples). The results indicate that the

mistances of these samples were very high and consequently their conductivities çould

not he iiccurately detennined.

Conductivity measurements were then performed from 225 O C to 380 O C on

smples prepared at a higher temperature of 400 OC ( 4 0 C samples). The results show

that for the boron-doped 400C sample, two diflerenc conduction mechanisms dominated

over two temperature regions. For the other two 400C simples. one conduction

mechünism was dominant over the entire temperature region studied. The most b l y

dominant electrical conduction mechanism for these smples is variable range hopping

conduction. For the boron-doped sample above 340 OC. extended state conduction

appears to be the dominant mechanism. The results dso indicate that boron-doping (p-

type doping) is more elecviçally active than phosphonis-doping (n-type doping) in the

4W)C smples.

Furthemore. relevant evidence was presented to argue that the conductivites or

the doped 2WC samples most iikely exhibit temperature dependence similu to ihosc of

the doped 400C siunples. In pimiçuliir, it is expcçted that horon-doping is more

electrically active than phosphorus-doping in the 2oOC samples.

Chapter 6 Conclusions

Wiih the addition of a gûs mixing system, the DC saddle-field glow discharge systcm wlis

uscd to deposit undoped and doped hydrogenated diamond-like amorphous carhon l h s

on silicon. hsed silica and Corning 7059 glass substrates. For the growth of phosphorus-

doped d horon-doped films, mole fraction of dopant gases (phosphinc or dihorine) in

the $as mixtures ranging from 5x 10" to lx 10" was used. The structural, optical and

elcctrical properties of undoped and doped tlms depositcd at a suhstrate temperature of

20() O C were studied. The clecvical propenies of undoped luid doped tlms grown at 40()

OC wcre also investigated.

The analyses of the structural propenies included thickness. composition and the

iiüction of tetrahednl bonds in the samples. Thc rcsulis of the thickness mcasurcmcnts

indicate that for a particular type of suhstrate, the growih rates for al1 thc undopcd iuid

dopcd samples arc similar. The compositional analysis shows that a good correlation cxists

hctwecn the fractional concentration of dopants in the doped samples and the

correspondhg mole fraction of dopant gases in the $as mixtures. This meÿns that .simples

with predictable doping levels of phosphoms and horon can he grown. It was also found

thüt the fractional concentration of hydrogen in the most heavily boron-doped samplc L!

higher than that in other doped samples. The structural analysis also shows that thc

fractions of tetrahedrai bonds for d l the samples are quite similar (t'rom 47% to 53%).

exccpt for the most heavily boron-doped one (61%). From d l the structurai analyses, it

w u concluded that the structure of the most heiwiiy boron-doped smple is somewhat

diî'ferent t'rom those of other doped samples.

From the studies of the optical properties, it was lound that al1 the undoped and

doped smples have simiiar optical energy gaps (from 2AeV to 2.5eV). This mems that

doping does not have any significant ellèct on the optical energy gap.

Two measurement systems were built for conductivity measurements on undoped

and dopcd films. The resistances of the samples deposited at 200 O C were very high and

their conductivities could not be accurately determined. Measurements were thcn

pcrformed from 225 "C to 380 O C on the sÿmples deposikd at 400 OC. Thc results show

thrt variable range hopping conduction appeus to be the dominant conduction mechanism

h r these smples. For the heavily boron-doped simple at temperatures ahove 340 O C .

cxtcnded strte conduction iippears 10 k the dominant conduction mechmism. The rcsults

also indicate that bomn-doping (p-type doping) is more elcctrically active than

p hosphoms-doping (n-type doing).

Morcovcr. evidence was prescnted to argue that the conductivitics of the doped

samples deposited at 200 OC most ükcly exhibit sirniliu tcmpcraturc dcpcndcncc as the

dopcd siunples deposited ai 400 OC. Ii is expectcd ihat horon-doping is more clecuiçülly

active than phosphorus-doping in the sarnples deposited at 200 OC.

To summarize, the analyses of the structural and electncal properties indiçate thot

horon-doping (p-type doing) is relaiively more suwessful than phosphorus-doping (n-type

doping) in the hydrogenated diamond-like amorphous carbon tilrns. The reason for this

appeûrs to he the incorporation of a Iarguger mole fraction of hydrogen and a larger fraction

of ietrdhedral bonding in the boron-doped film. Perhaps changing deposition parameters.

in panicular increasing the growth temperature and using oiher dopant gases. such as

nitmgen. will lead to bettet undoped and doped films in the future.

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