surface morphology of heavily carbon-doped gaas grown by solid source molecular beam epitaxy

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Journal of Crystal Growth 263 (2004) 105–113

*Corresp

E-mail

0022-0248/

doi:10.101

Surface morphology of heavily carbon-doped GaAs grown bysolid source molecular beam epitaxy

K.H. Tan, S.F. Yoon*, R. Zhang, Q.F. Huang, Z.Z. Sun

School of Electrical and Electronic Engineering (Block S1), Nanyang Technological University, Nanyang Avenue,

Singapore 639798, Singapore

Received 15 October 2003; accepted 18 November 2003

Communicated by M. Schieber

Abstract

This paper reports the surface morphology of carbon-doped GaAs (GaAs:C) samples grown by solid source

molecular beam epitaxy (SSMBE) using carbon tetrabromide (CBR4) as p-dopant source. Sample characterization was

carried out using Hall effect, X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. The results

show that samples grown at 650�C contain dicarbon defects and the surface morphology is rough. Further experiments

on two different sets of GaAs:C samples have shown the absence of bromine-related surface roughening reactions

during GaAs:C growth, and the presence of dicarbon defects contribute to the rough surface morphology. It is

suggested that the presence of dicarbon defects disrupt the two-dimensional (2-D) step flow growth mode and promote

three-dimensional (3-D) growth of GaAs:C through a step bunching mechanism.

r 2003 Elsevier B.V. All rights reserved.

PACS: 61.72.Vv; 61.72.JI

Keywords: A1. Characterization; A1. Doping; A3. Molecular beam epitaxy; B1. Carbon tetrabromide

1. Introduction

Carbon-doped gallium arsenide (GaAs:C) hasattracted considerable interest due to its applicationin high speed heterojunction bipolar transistor(HBT). The use of carbon as p-type dopant allowshigh base layer doping concentration (>1020 cm�3),leading to low base layer resistance. This contri-butes to increase in the cut-off frequency, fT andmaximum frequency of oscillation, fmax of the HBT.Furthermore, the low diffusion coefficient of

onding author. Fax: +65-7912-687.

address: [email protected] (S.F. Yoon).

$ - see front matter r 2003 Elsevier B.V. All rights reserve

6/j.jcrysgro.2003.11.079

carbon dopant in GaAs, compared to other p-typedopants, such as Be and Zn [1], enables theformation of more abrupt base–emitter or base–collector junctions, and thus enhances the DCperformance of the device. Apart from electricalproperties of GaAs:C, such as carrier mobility anddopant activation, structural properties such assurface morphology also play a significant role indevice performance. A smooth surface is beneficialto formation of abrupt heterojunctions and mini-mizes interface defect density. This contributes tooverall improvement in device performance.

GaAs:C grown by solid source molecular beamepitaxy (SSMBE) has several advantages over

d.

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0 5 10 15 20 25

4

8

12

16

20

grown at 650o C

grown at 600o C

Hol

e C

once

ntra

tion

( x

1019

cm

-3)

CBr4 Flux (x 10-8 Torr)

Fig. 1. Plot of hole concentration vs. CBr4 flux for GaAs:C

samples grown at 650�C and 600�C.

10

K.H. Tan et al. / Journal of Crystal Growth 263 (2004) 105–113106

those grown using metal-organic chemical vapordeposition (MOCVD) and metal-organic vapor-phase epitaxy (MOVPE) [2]. These include unitycarbon atom activation and absence of hydrogenrelated defects. The properties of GaAs:C grownby SSMBE, such as carrier mobility, dopantactivation and lattice mismatch have been studiedrather extensively [3,4], except for surface mor-phology for which existing reports are basedmainly on GaAs:C grown by MOVPE [5,6].

In this paper, we report the effects of substratetemperature and carbon tetrabromide (CBr4) fluxon surface morphology of GaAs:C grown bySSMBE. Sample characterization was carried outusing Hall effect, X-ray diffraction (XRD) andatomic force microscopy (AFM) measurements.Factors which affect the surface morphology ofGaAs:C are identified, and a kinetic mechanism ofthe process is described.

0 5 10 15 20 25

2

4

6

8

grown at 650o C

grown at 600o C

Surf

ace

Roughnes

s, r

ms

(nm

)

CBr4 Flux (x 10

-8Torr)

Fig. 2. Plot of rms surface roughness vs. CBr4 flux for GaAs:C

samples grown at 650�C and 600�C.

2. Experimental procedure

The GaAs:C samples were grown using SSMBEon GaAs(1 0 0) S.I. substrates. Carbon tetrabro-mide (CBr4) was used as carbon source withoutcarrier gas and contained in a sealed stainless-steelcylinder in a temperature-controlled immersionbath. The source was sublimed between 2�C and20�C to provide the desired CBr4 flux, rangingfrom 2� 10�8 to 2� 10�7 Torr. The flux magni-tude was regulated by a high precision leak valve.The CBr4 cylinder was connected into the MBEchamber by a stainless-steel line, which is heated to80�C to prevent condensation of CBr4 along theinner wall of the line.

The surface morphology of six GaAs:C samplesgrown at different CBr4 fluxes at 600�C and650�C, respectively, was examined. Hall effect,XRD, and AFM were used for sample character-ization. Furthermore, secondary ion mass spectro-scopy (SIMS) was used to measure the atomiccarbon concentration in the samples. The resultsof room temperature Hall effect and AFMmeasurements are shown in Figs. 1–3, respectively.Fig. 1 shows the hole concentrations in samplesgrown at 650�C are lower than those in samplesgrown at 600�C. Figs. 2 and 3 show lower rms

surface roughness in samples grown at 600�Ccompared to ones grown at 650�C. The rmssurface roughness of samples grown at 600�C isB0.3–0.5 nm, which is typical for two-dimensional(2-D) step flow growth mode in GaAs material.However, for the samples grown at 650�C, the rmssurface roughness increases to B3–10 nm, andincreases further following increase in CBR4 flux.

The lattice mismatch of GaAs:C samples areplotted against carbon concentration in Fig. 4. Thesolid line is theoretical curve of lattice mismatch,calculated using the following equation assuming

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600ºC 650ºC In

crea

se I

n C

Br 4

Flu

x

Fig. 3. AFM image for GaAs:C samples grown at 650�C and 600�C.

K.H. Tan et al. / Journal of Crystal Growth 263 (2004) 105–113 107

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4 8 12 16 20 24400

800

1200

1600

2000

2400

2800

grown at 600o C

grown at 650o C

Lat

tice

Mis

mat

ch (

ppm

)

Carbon Concentration (x 1019cm-3)

Fig. 4. Plot of lattice mismatch vs. atomic carbon concentra-

tion for GaAs:C samples grown at 650�C and 600oC. The solid

line represents the theoretical curve calculated using Eq. (1).


all carbon atoms are incorporated into arsenicsites [7]:

Da

a

� �>¼

c11 þ 2c12

c11

dCAs� d

d

NC

NAs;

dCAs¼ rC þ rGa;

d ¼ rAs þ rGa; ð1Þ

where, c11 and c12 are the stiffness coefficients ofthe crystal. For GaAs, c11 ¼ 11:88� 1011 dyn/cm2

and c12 ¼ 5:38� 1011 dyn/cm2, respectively. NC

and NAs are density of carbon atoms and arsenicatoms (2.21� 1022 cm�3), respectively. rc; rAs andrGa are radii of carbon, arsenic and gallium atoms,respectively. XRD measurements along the (1 1 5)and ð%1 %1 5Þ planes were performed, and the resultsshow that our samples can be regarded as fullystrained. Thus, the c11 þ 2c12=c11 term is includedin Eq. (1) to account for the fully strained natureof our samples.

Figs. 1 and 4 show concurrent reduction in holeconcentration and lattice mismatch in samplesgrown at 650�C. It was previously reported [8,9]that the effect of dicarbon defects is exhibited asconcurrent reduction in hole concentration andlattice mismatch. Dicarbon defects are formedwhen two carbon atoms share an arsenic site.Their lattice constant is larger than that of singlecarbon atom and behaves as a donor. Thus, the

presence of dicarbon defects will compensate thehole concentration and reduce the lattice mis-match. Figs. 2 and 3 show all samples grown at650�C have rough surface morphology. It is notedthat samples grown at 650�C have smaller latticemismatch and hole concentration than samplesgrown at 600�C, even though identical CBr4 fluxwas used in both cases. SIMS measurement [10]revealed the total carbon concentration remainedunchanged following increase in substrate tem-perature if the CBr4 flux is kept constant. Thus, theresults suggest that samples grown at 650�Ccontain dicarbon defects, which causes reductionin lattice mismatch and hole concentration.

The results so far have indicated that thepresence of dicarbon defects and/or high substratetemperature (650�C) has contributed to roughsurface morphology in GaAs:C, while othergrowth conditions such as V/III ratio and growthrate were kept identical for all samples. It isnoteworthy that the possible presence of bromineresidues in the growth chamber produced bydissociation of CBr4 may instigate some bro-mine-related surface roughening reactions at highsubstrate temperature of 650�C or above.

To further examine the reasons for poor surfacemorphology in GaAs:C, two sets of samples weregrown: (a) the first set was grown at 650�C withoutdicarbon defects. The purpose is to investigate theeffect of high substrate temperature alone, and (b)the second set was grown at the normal GaAsgrowth temperature of 600�C and containeddicarbon defects. The purpose is to investigateeffect of dicarbon defects alone. Comparing thetwo sets of samples will shed some insight intofactor(s) that contribute to rough surface mor-phology in GaAs:C.

It is known that GaAs:C samples withoutdicarbon defects can be grown at high substratetemperature (650�C) if the atomic carbon concen-tration is lower than 2� 1019 cm�3. Four GaAs:Csamples were grown at different substrate tem-peratures (560�C, 590�C, 620�C, 650�C) at CBr4flux of 2� 10�8 Torr. SIMS measurement showthe carbon concentration in GaAs:C samplesgrown at CBr4 flux of 2� 10�8 Torr to beB2� 1019 cm�3. Table 1 shows the hole concen-tration and XRD data of these samples. The hole

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Table 1

Experimental results of hole concentration, lattice mismatch and surface roughness for 4 samples grown from 560�C to 650�C at CBr4flux of 2� 10�8 Torr

Sample Substrate

temperature (�C)

Hole concentration

(� 1019 cm�3) 710%

Lattice mismatch

750 (ppm)

RMS surface

roughness 70.2 (nm)

1 560 2.2 B�321 0.33

2 590 2.4 B�269 0.28

3 620 2.2 B�300 0.57

4 650 2.5 B�290 0.43


concentration in samples grown at the fourdifferent temperatures was B2� 1019 cm�3. Forsamples with carbon concentration of2� 1019 cm�3, the lattice mismatch calculatedusing Eq. (1) is 290 ppm, assuming all carbonatoms were incorporated into arsenic sites andthere is absence of dicarbon defects. This calcu-lated lattice mismatch value is close to themeasured lattice mismatch in samples 1–4. Thehole concentration and XRD data in Table 1verified the carbon atoms in samples 1–4 ap-proached 100% electrical activation in absence ofdicarbon defects. AFM data in Table 1 and AFMimage in Fig. 5 show smooth 2D growth surfaceand indicate no significant surface roughnessincrease in the sample grown at 650�C (sample4), compared to those grown at lower temperature(samples 1–3). The average surface roughness ofB0.5 nm in these samples suggests a 2-D step flowgrowth mode.

The general reaction rate for bromine-relatedchemical reactions is

ratepexp�Ea

kT

� �½CBr4 ð2Þ

where, Ea; k; T and ½CBr4 denotes the activationenergy, Boltzmann constant, substrate tempera-ture and CBr4 flux, respectively. If bromine-relatedsurface roughening reactions had been presentduring GaAs:C growth at 650�C, some signs ofsurface roughness increase would be expected insample 4, even though the degree of roughnessincrease might not be as serious as that in samplesshown in Fig. 2, due to the usage of lower CBr4flux. However, the results in Table 1, which clearlyshow no surface morphology deterioration insample 4, suggest that bromine-related surfaceroughening reactions were insignificant at 650�C.

Therefore, high substrate temperature (650�C)growth is unlikely to be responsible for the highsurface roughness observed in the GaAs:C samplesgrown at 650�C as shown in Fig. 2. Furthermore,the absence of rough surface morphology in theGaAs:C sample grown at 650�C without dicarbondefects, strongly suggests that dicarbon defectsmay be responsible for the observed rough surfacemorphology in samples shown in Fig. 2.

For further confirmation, two more samples(samples 5 and 6) grown at the normal GaAsgrowth temperature of 600�C with dicarbondefects were examined. Dicarbon defects can beincorporated into GaAs:C grown at 600�C underhigh CBr4 flux (in our case, >4� 10�7 Torr).SIMS measurement shows the atomic carbonconcentrations in these samples are greater than2� 1020 cm�3. Hole concentration, XRD andAFM measurement data of these samples areshown in Table 2 and Fig. 6. From Eq. (1), asample with atomic carbon concentration exceed-ing 2� 1020 cm�3 and free from dicarbon defectswill have lattice mismatch greater than 2900 ppm.As discussed above, the low hole concentrationcompared to its atomic carbon concentration, andsmall lattice mismatch values in samples 5 and 6,clearly demonstrates the presence of dicarbondefects in these samples, which is estimated atB5� 1019 cm�3. AFM measurements show thepresence of very rough surface morphology inthese samples. Thus, the above comparison clearlyshows the presence of dicarbon defects in GaAs:Ccontributes to the rough surface morphology andpresence of 3-D bunches on the surface. In thefollowing section, the effect of dicarbon defects indisrupting the 2-D step flow growth mode ofGaAs:C, leading to surface morphology rough-ening is discussed.

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560ºC 590ºC

620ºC 650ºC

Fig. 5. AFM image for GaAs:C samples grown using 2� 10�8 Torr of CBr4 flux at 560�C, 590�C, 620�C and 650�C, respectively.


3. Discussion

In order to understand the effect of dicarbondefects on the step flow growth mode of GaAs:C, asimplified description of the step flow growthprocess is first given. The side view of the step flow

growth mode is illustrated schematically in Fig. 7.In step flow growth, the arriving atoms willimpinge on the step terraces. The width of thisterrace, wn ¼ xnþ1 � xn: Upon arrival at the stepterraces, the atoms will diffuse towards the stepedges and bind to favorable sites [11]. The number

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Table 2

Experimental results of hole concentration, lattice mismatch and surface roughness for 4 samples grown at 600�C at CBr4 flux of

B4� 10�7 Torr

Sample Substrate

temperature

(�C)

CBr4 flux

(� 10�8 Torr)

Hole concentration

(� 1019 cm�3)

710%

Lattice mismatch

750 (ppm)

RMS surface

roughness

70.2 (nm)

5 600 48 9.6 B�1768 67

6 600 47 11 B�2037 44

Fig. 6. AFM image for GaAs:C samples with presence of dicarbon defects.

Atom Binding

Atom migration on Terrace

Impinge of Atom

Step Edge

xn+2

xn+1

xn

Wn

Substrate Direction

Growth Direction

Step Edge

Fig. 7. Schematic representation of step flow growth mode.


of atoms that diffuse to the step edges depends onthe arrival rate of atoms at the step terraces. Thus,the velocity of step propagation across the surface,step velocity, dxn=dt is a function of wn [11,12]

dxn

dt¼ f ðwnÞ: ð3Þ

Kandel et al. [11] have reported that the 2-D stepflow growth mode is stable if slope df ðwnÞ=dwn ofthe velocity function f ðwnÞ is positive. In otherwords, the step flow growth mode and width ofstep terrace, wn will be unstable. This leads to stepbunching described in Frank’s impurity mechan-ism reported by Kandel et al. [11] as shown inFig. 8, if the increase in terrace width slows downthe step velocity. If the terrace is contaminated byimpurities with negligible migration length and few

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unsaturated bonds [12], slope df ðwnÞ=dwn of thevelocity function f ðwnÞ will be negative.

We suggest dicarbon defects could be one ofsuch impurities. With wider step terraces, it islikely more dicarbon defects will be exposed toarriving atoms. Since the energy of the Ga–C bond(247KJ/mol) is larger than that of Ga–As bond(71.4KJ/mol), it is expected that the carbon atomswill have shorter migration length compared toarsenic atoms. Furthermore, the Ga atoms areexpected to have shorter migration length at thegrowth surface in the presence of dicarbon defectscompared to a defect-free surface. This leads toreduction in arrival rate of Ga atoms at the stepedges. The dicarbon defects may also reduce thenumber of suitable sites at the step edges for Gaatoms to bond, due to them having fewerunsaturated bonds compared to As atoms [13].Hence, short migration length of Ga and C atoms,coupled with fewer unsaturated bonding sites atthe step edges will impede the movement of stepsand fulfill the requirements for step bunching

Table 3

Experimental results of carbon concentration and surface roughness

Low growth rate

Carbon concentration (� 1020 cm�3) RMS roughness (nm)

1 o1

1.3 7

1.5 15

All data are from Kuhl et al. [15].

Coalescence of Mutliple Steps

Step Bunch

Instability of Terrace Width

Substrate Direction

Growth Direction

Fig. 8. Schematic representation of step bunching described in

Frank’s impurity mechanism reported by Kandel et al. [11].

described in Frank’s impurity mechanism reportedby Kandel et al. [11]. Step bunching leads tocoalescence of different step edges, disruption ofstep flow growth mode and promotion of 3-Dgrowth as shown in Fig. 6, resulting in surfaceroughness increase, even to macroscopic levels [14].

We suggest the contribution of dicarbon defectsto surface morphology may not be limited only toGaAs:C. Kuhl et al. [15] has reported a study onsurface morphology of C-doped InGaAs (In-GaAs:C), where the samples were grown bychemical beam epitaxy (CBE) using CBr4 ascarbon source. Experimenting with two growthrates (0.6 and 1.3 mm/h), it was mentioned in thereport that samples grown at low growth rate havelower dopant activation rate (or hole/carbonconcentration ratio) compared to samples grownat high growth rate. This suggests a higherdicarbon defect concentration in low growth ratesamples, since hydrogen passivation was ruled outin their report. The surface morphology data ofInGaAs:C samples in Kuhl et al.’s report aresummarized in Table 3, which show higher surfaceroughness in samples with higher dicarbon defectconcentration. So far, apart from the report byKuhl et al. [15], no further literature is available onthe surface morphology of InGaAs:C. Therefore,further investigation is needed to provide moreevidence on the effect of dicarbon defects onInGaAs:C surface morphology.

4. Conclusion

This paper reports the surface morphology ofcarbon-doped GaAs (GaAs:C) samples grown bysolid source molecular beam epitaxy (SSMBE)using carbon tetrabromide (CBR4) as p-dopant

for low growth rate and high growth rate samples

High growth rate

Carbon concentration (� 1020 cm�3) RMS roughness (nm)

1.1 o1

1.5 B1

1.8 5

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source. The results show the presence of dicarbondefects has clearly contributed to increase insurface roughness in GaAs:C, and rules outcontribution from possible bromine-related sur-face roughening reactions. The presence of dicar-bon defects disrupts the 2-D step flow growthmode in GaAs:C, leading to surface morphologyroughening. The reduced migration length of Gaand C atoms, coupled with fewer unsaturatedbonding sites at the step edges of the growthsurface impede the movement of steps resulting instep bunching. This leads to coalescence ofdifferent step edges, disruption of step flow growthmode and promotion of 3-D growth in GaAs:C.

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surface morphology of heavily carbon-doped gaas grown by solid source molecular beam epitaxy

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