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doi:10.1016/j.jcr

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Journal of Crystal Growth 275 (2005) 404–409

www.elsevier.com/locate/jcrysgro

Carbon-doped GaAs and InGaAs grown by solid sourcemolecular beam epitaxy and effect of III/V ratio on

their properties

K.H. Tan�, S.F. Yoon, R. Zhang, Z.Z. Sun

School of Electrical and Electronic Engineering (Block S1), Nanyang Technological University, Nanyang Avenue, Singapore 639798,

Republic of Singapore

Received 29 July 2004; accepted 3 December 2004

Communicated by M. Schieber

Available online 19 January 2005

Abstract

Carbon-doped gallium arsenide (GaAs:C) and indium gallium arsenide (InGaAs:C) samples were grown by solid

source molecular epitaxy using carbon tetrabromide (CBr4) as a carbon source. The samples were characterized using

Hall and photoluminescence measurements. For the purpose of investigation, GaAs:C and InGaAs:C samples were

grown using different arsenic to group III (V/III) ratio. This study showed that V/III ratio affects the formation of mid-

gap non-radiative recombination centers in GaAs:C and InGaAs:C. It is also found that the mid-gap recombination

centers were greatly suppressed when V/III ratio of 25 and 20 were used in growth of GaAs:C and InGaAs:C layers,

respectively. Furthermore, GaAs:C-based and InGaAs:C-based heterojunction bipolar transistors have been grown and

their DC performance characterized.

r 2004 Elsevier B.V. All rights reserved.

PACS: 61.72.Vv; 61.72.Ji

Keywords: A1. Characterization; A1. Defects; A1. Doping; A3. Molecular beam epitaxy

1. Introduction

The use of carbon as a p-type dopant for GaAsand InGaAs in heterojunction bipolar transistors

e front matter r 2004 Elsevier B.V. All rights reserve

ysgro.2004.12.008

ng author. Tel.: +656790 4528;

318.

ss: tkianhua1@yahoo.com (K.H. Tan).

(HBTs) has attracted great interest [1,2]. Com-pared to an alternative dopant, such as Be and Zn,carbon has a lower diffusivity [3] and is ableto achieve high doping concentration(41� 1020 cm�3 compared to �5� 1019 cm�3 ifusing Be) in both materials. Low diffusivity allowsthe formation of abrupt collection–base andemitter–base junctions, which is important for

d.

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K.H. Tan et al. / Journal of Crystal Growth 275 (2005) 404–409 405

HBT application. High base doping concentrationlowers the base resistance, RB, of the HBT, leadingto improvement in its high-speed performance.

This paper reports the properties of carbon-doped GaAs (GaAs:C) and InGaAs (InGaAs:C)grown by solid source molecular beam epitaxy(SSMBE) using carbon tetrabromide (CBr4) ascarbon source. The properties of carbon-dopedGaAs and InGaAs are characterized using Halleffect and X-ray diffraction. Furthermore, theeffects of III/V ratio on the properties of GaAs:Cand InGaAs:C are investigated. With an exceptionof V/III ratio, the effects of other growth condi-tions, such as substrate temperature and growthrate have been previously reported [1,2,4,5]. Thisreport will complete the study of important MBEgrowth parameters, and show that V/III ratio hasan important effect on the electrical properties ofthe material. Using optimal V/III ratio values,GaAs- and InGaAs-based HBTs with carbon-doped base layers have been grown and their DCperformance characterized.

12

16

m2 /V

s)

x 10

19 cm

-3)

70

75

2. Experimental procedure

GaAs:C and InGaAs:C samples were grownusing the SSMBE. CBr4 was introduced into thegrowth chamber via a precision leak valve. TheCBr4 flux was controlled by the leak valve open-ing. In our experimental setup, the CBr4 sourcewas stored in a stainless-steel cylinder maintainedunder 2 1C. Under such conditions, the CBr4source was able to provide up to 8� 10�8 Torr ofconstant CBr4 flux. The CBr4 source was con-nected to the precision leak valve through a seriesof evacuated ultra-clean gas lines. The gas line was

CBr4 Cylinder

MBE Growth

Chamber

N2 Purge & Exhaust

Pump system

Turbo Pump

Leak valve Bellow valve

Fig. 1. Schematic diagram of the CBr4 delivery system.

heated up to 80 1C to prevent the condensation ofCBr4 along its inner wall. The schematic diagramof the CBr4 delivery system is shown in Fig. 1.GaAs:C and InGaAs:C were grown at different

CBr4 flux at 600 and 450 1C, respectively. Thesamples have been characterized using Hall effectmeasurement to obtain its Hall concentration andmobility at room temperature. Furthermore,carbon-doped GaAs and InGaAs samples weregrown at different V/III ratios. For GaAs:Csamples, V/III ratios of 18, 25, 30 and 35 wereused, while for InGaAs:C samples, V/III ratios of15, 20 and 25 were used. All V/III ratios wereobtained by dividing the beam equivalent pressure(BEP) of the group V element by the total BEP ofthe group III elements. The samples grown atdifferent V/III ratios were characterized usingphotoluminescence (PL) at 4K.

3. Results and discussion

The Hall measurement data of the GaAs:C andInGaAs:C samples are shown in Figs. 2 and 3,respectively. It can be seen that the Hall mobilitiesof InGaAs:C are lower than those of GaAs:Csamples. The carrier mobility in a material isaffected by acoustic and nonpolar optical-phononscattering, polar optical-phonon scattering, io-nized-impurity scattering, piezoelectric scatteringand alloy scattering. In a ternary material system

0 5 10 15 20

4

8

Hal

l Mob

ility

(c

Hal

l Con

cent

ratio

n (

CBr4 flux ( x 10-8 Torr)

45

50

55

60

65

Fig. 2. Hall concentration and carrier mobility in carbon-

doped GaAs as function of CBr4 flux.

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1 3 4 51

2

3

4

5

6

Hal

l Mob

ility

(cm

2 /Vs)

Hal

l Con

cent

ratio

n (x

1019

cm

-3)

CBr4 flux ( x 10-8 Torr)

30

35

40

45

50

55

60

65

70

2

Fig. 3. Hall concentration and carrier mobility in carbon-

doped InGaAs as function of CBr4 flux.

1.42 1.44 1.46 1.48 1.50 1.52

-5

0

5

10

15

20

25

30

V/III = 30

V/III = 18

V/III = 35

V/III = 25

PL I

nten

sity

(a.

u.)

Bandgap Energy (eV)

Fig. 4. PL spectra of carbon-doped GaAs grown at V/III ratio

of 18, 25, 30 and 35.

0.70 0.75 0.80 0.85 0.900

200

400

600

800

1000

V/III = 25

V/III = 15

V/III = 20

PL I

nten

sity

(a.

u)

Bandgap Energy (eV)

Fig. 5. PL spectra of carbon-doped InGaAs grown at V/III

ratio of 15, 20 and 25.

2000 Å , n cap layer GaAs:Si , [Si]= 5×1018 cm-3

1000 Å , n GaAs:Si , [Si]= 5×1017 cm-3

1000 Å , p+ GaAs:C , [C]= 4×1019 cm-3

Semi-insulating GaAs Substrate

Fig. 6. Structure of p+–n diode used to investigate the effect of

V/III ratio on device performance.

K.H. Tan et al. / Journal of Crystal Growth 275 (2005) 404–409406

such as InxGa1�xAs, the main scattering mechan-ism that affects the carrier mobility is alloyscattering [6,7]. At x � 0:5; alloy scattering greatlyreduces the carrier mobility in a ternary material,leading to lower mobility compared to the binarymaterial system. This explains the lower carriermobility in the InGaAs:C (x ¼ 0:53) samples,compared to the GaAs:C (x ¼ 0) samples, asshown in Figs. 2 and 3.

The 4K PL spectra of the GaAs:C andInGaAs:C samples grown at different V/III ratiosare shown in Figs. 4 and 5, respectively. In Fig. 4,it can be seen that the GaAs:C sample grown at V/

III ¼ 25 has the highest PL intensity, compared toother samples. On the other hand, as shown inFig. 5, the InGaAs:C sample grown at V/III ¼ 20,has the highest PL intensity compared to othersamples. The above results show that changes inthe V/III ratio affect the PL intensity of thesamples. To further investigate the effect of V/IIIratio on device performance, a p+–n diodestructure as shown in Fig. 6 has been grown atdifferent V/III ratios. Figs. 7 and 8 show thecurrent–voltage (I2V ) curve and ideality factor ofthese p+–n diodes, respectively. The p+–n diodewith the lowest ideality factor and leakage currentwas grown at V/III ¼ 25, which is the same V/IIIratio which produced the highest PL intensity inthe GaAs:C sample.For a p+–n junction as shown in Fig. 6,

the forward biased current, JF can be dividedinto the diffusion and junction recombination

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-2 -1 0 1 2

10-1

10-3

10-5

10-7

10-9

10-11

V/III = 30V/III = 35

V/III = 18

V/III = 25

Cur

rent

(A

)

Voltage (V)

Fig. 7. Current–voltage characteristic curves of p+–n diodes

grown at V/III ratio of 18, 25, 30 and 35.

15 20 25 30 35 401.2

1.3

1.4

1.5

1.6

1.7

Idea

lity

Fact

or o

f PN

Jun

ctio

n

III / V Ratio

Fig. 8. Plot of p+–n diode ideality factor as function of V/III

ratio.

K.H. Tan et al. / Journal of Crystal Growth 275 (2005) 404–409 407

current [8] components:

JF ¼ q

ffiffiffiffiffiffiDp

tp

sn2i

NDeqV=kT þ

qWni

2treqV=2kT : (1)

The first and second terms on the right-handside of Eq. (1) are attributed to diffusion currentand recombination current components, respec-tively. q, Dp, tp, ND, ni are the electronic charge,hole diffusivity, hole lifetime, donor impurityconcentration and intrinsic carrier concentration,respectively. W, tr, V, k and T are the depletionregion width, effective recombination lifetime,

forward bias voltage, Boltzmann’s constant andtemperature, respectively. The diffusion currenthas an ideality factor of 1. The junction recombi-nation current gives an ideality factor of 2. As seenin the Eq. (1), the junction recombination currenthas low current increment following increase involtage, leading to deterioration in linearity andlow DC current gain in a HBT. The magnitude ofjunction recombination current is governed by thecarrier recombination rate U, which can beexpressed as [8]:

U ¼s0uthN tn

2i ðe

qV=kT � 1Þ

n þ p þ 2ni cos hðEi�EtÞ=kT; (2)

where, nth, s0; and nt are the carrier thermalvelocity, capture cross section and concentrationof recombination centers, respectively. p and n arethe hole and electron concentrations, respectively.From Eq. (2), it can be seen that the recombina-tion rate will be effective only if the recombinationcenter energy level Et is close to the intrinsic Fermilevel Ei. In other words, the junction recombina-tion current is mainly due to the existence of mid-gap recombination centers.From Fig. 8, it can be seen that changes in V/III

ratio affect the ideality factor. This suggests thatthe junction recombination current componentwas directly affected by the V/III ratio. Thus, thiscould also imply that the concentration of mid-gaprecombination centers was affected by the V/IIIratio. Furthermore, the fact that the GaAs:Csamples grown at V/III ¼ 25 has the lowestideality factor and highest PL intensity, couldsuggest that such mid-gap recombination centersare non-radiative in nature.As previously mentioned, in the case of the

InGaAs:C samples, the highest PL intensity wasobtained in the sample grown at V/III ¼ 20 (asshown in Fig. 5). Based on the relation between PLintensity and device ideality factor in GaAs:Csamples discussed above, the results from InGaAs:Csamples could suggest that a V/III ratio of 20 givesthe lowest non-radiative mid-gap recombinationcenter concentration in this material.Using optimal V/III ratio values of 25 and 20

established for GaAs:C and InGaAs:C, respec-tively, two HBT devices, as shown in Fig. 9 have

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200Å In0.2Ga0.8As:Si Cap, [Si]=1×1019 cm-3

1200Å GaAs:Si Contact, [Si]=4×1018 cm-3

500Å GaInP:Si Emitter, [Si]=3×1017 cm-3

600Å GaAs:C Base, [C]=2×1019 cm-3

7000Å GaAs:Si Collector I, [Si]=1×1016 cm-3

500Å GaAs:Si Sub-collector, [Si]=4×1018 cm-3

200Å GaInP:Si Etch stop, [Si]=1×1018 cm-3

5500Å GaAs:Si, Sub-collector II, [Si]=4×1018 cm-3

S.I. GaAs Substrate

1400Å In0.53Ga0.47As:Si Cap, [Si]=1×1019 cm-3

600Å InP:Si Contact, [Si]=1×1019 cm-3

900Å InP:Si Emitter, [Si]=3×1017 cm-3

500Å In0.53Ga0.47As:C Base, [C]=2×1019 cm-3

4000Å In0.53Ga0.47As:Si Collector, [Si]=1×1016 cm-3

4500Å In0.53Ga0.47As:Si Sub-collector, [Si]=5×1018 cm-3

S.I. InP Substrate

(a)

(b)

Fig. 9. Structures of: (a) carbon-doped GaAs, and (b) carbon-

doped InGaAs-based heterojunction bipolar transistor.

0.5 1.0 1.5 2.0

10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

ICIB

I B, I

C (

A)

VB (V)

nb = 1.30

nc = 1.01

Fig. 10. Gummel plot of carbon-doped GaAs-based hetero-

junction bipolar transistor.

0.0 0.4 0.8 1.2 1.6

10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

nC = 1.1

nB = 1.35

IB

IC

I B, I

C (

A)

VB (V)

Fig. 11. Gummel plot of carbon-doped InGaAs-based hetero-

junction bipolar transistor.

K.H. Tan et al. / Journal of Crystal Growth 275 (2005) 404–409408

been grown. For the HBT with GaAs:C as basematerial, DC gain of 100 and base current idealityfactor, Ib of 1.3 have been obtained. Fig. 10 showsthe Gummel plot for this device. For the HBTdevice with InGaAs:C as base material, the DCgain and Ib are 45 and 1.35, respectively. Fig. 11shows the device Gummel plot. Recombinationbase current with ideality factor of 2 is thedominant component in the small current region.Thus, HBTs with significant recombination basecurrent will not only show high base currentideality factor, but also a large base current (withideality factor of 2) compared to the collectorcurrent at small current region. From Gummelplots shown in Figs. 10 and 11, there is no

significant base current with an ideality factor of2 at small current region as the magnitude of basecurrent is smaller than collector current. Thus, itindicated that recombination base current hasbeen greatly suppressed in HBTs grown withoptimal V/III ratio. This observation agreed withdiscussion above that optimal V/III ratio hasgreatly reduce the concentration of non-radiativemid-gap recombination center in the base layer.

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K.H. Tan et al. / Journal of Crystal Growth 275 (2005) 404–409 409

4. Conclusions

GaAs:C and InGaAs:C samples have beengrown by SSMBE using CBr4 as carbon source.Sample characterization was carried out using Halleffect, XRD and PL measurements. The resultssuggest that V/III ratio has an effect on theformation of mid-gap non-radiative recombina-tion centers in GaAs:C and InGaAs:C. This couldlead to deterioration in the PL intensity andideality factor of the p–n junction. The optimalV/III ratios for GaAs:C and InGaAs:C have beendetermined to be 25 and 20, respectively. Usingthese optimal V/III ratios, GaAs:C-based andInGaAs:C-based HBTs have been grown and DCcharacteristics measured.

References

[1] W.E. Hoke, D.G. Weir, P.J. Lemonias, H.T. Hendriks,

Appl. Phys. Lett. 64 (1994) 202.

[2] W.Y. Hwang, D.L. Miller, Y.K. Chen, D.A. Humphrey,

J. Vac. Sci. B 12 (1994) 1193.

[3] N. Kobayashi, T. Makimoto, Y. Horikoshi, Appl. Phys.

Lett. 50 (1987) 1435.

[4] Kai Zhang, W.Y. Hwang, D.L. Miller, L.W. Kapitan, Appl.

Phys. Lett. 63 (1993) 2399.

[5] P.J. Lemonias, W.E. Hoke, D.G. Weir, H.T. Hendriks,

J. Vac. Sci. B 12 (1994) 1190.

[6] D.C. Look, D.K. Lorance, J.R. Sizelove, C.E. Stutz, K.R.

Evans, D.W. Whitson, J. Appl. Phys. 71 (1992) 260.

[7] A.H. Ramelan, E.M. Goldys, J. Appl. Phys. 92 (2002) 6051.

[8] S.M. Sze, Semiconductor Devices: Physics and Technology,

Wiley, New York, 1985, pp. 92–96.