Influence of gallium sources on carbon incorporation efficiency into InGaAs grown by metalorganic chemical vapor deposition

......... CRYSTAL GROWTH

ELSEVIER

Journal of Crystal Growth 165 (1996) 215-221

Influence of gallium sources on carbon incorporation efficiency into InGaAs grown by metalorganic chemical vapor deposition Hiroshi Ito *, Kenji Kurishima NTT LSI Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi. Kanagawa 243-01, Japan

Received 18 September 1995: accepted 24 November 1995

Abstract Carbon incorporation into InGaAs has been systematically investigated using trimethylgallium (TMGa) and triethylgallium (TEGa) as gallium sources. Although the hole concentrations decrease with increasing In composition, the hole concentrations in layers grown with TMGa are found to be several times higher than those grown with TEGa. From Hall and secondary ion mass spectroscopy measurements, the hole concentration decrease with increasing In composition is revealed to be due to a decrease in the efficiency of C incorporation into InGaAs. Based on a numerical analysis where it was assumed the In atoms enhance the C source dissociation from the substrate surface, the higher surface step density in the kinetically limited growth mode region is proposed as the essential reason for the higher C incorporation efficiency into InGaAs grown with TMGa. This model is experimentally confirmed by the atomic force microscope observation and comparing higher surface step density substrates.

1. Introduction

Recently, carbon (C) has been investigated extensively as a p-type dopant in molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), because it offers several advantages such as a very high maximum doping level of o v e r 10 21 cm -3 [1] and a much lower diffusion coefficient [2] than those of conventionally used p-type dopants (Be [3] and Zn [4]). These features are especially important for growing a heterojunction bipolar transistor (HBT) structure because a heavily doped base layer with a well-defined dopant profile is essential for achieving the ultra-high speed performance of HBTs. In addition, a practically important advantage of C as * Corresponding author.

a p-type dopant is that it can greatly improve the device lifetime of HBTs [5]. An InP/In0 53Ga047As system, lattice matched to InP substrate, is a promising material for high-speed device applications due to the high electron mobility and high peak electron drift velocity in In053Ga0.~7As. Although C can be used as a p-type dopant in InGaAs [6-12], several difficulties related to C doping in InGaAs have been pointed out: the doping efficiency is relatively low compared with that in GaAs and decreases with increasing In composition [6,7], the amphoteric behavior of C is enhanced by the increase in In composition [8,9] and the conduction type inversion from p- to n-type occurs at higher In compositions [7-9], and a significant free carrier concentration reduction due to the hydrogenation of C acceptors occurs depending on

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H. ho, K. Kurishima / Journal (~["Co,stal Growth 165 (1996) 215-221

the growth condition [6,10-12]. These unfavorable characteristics of C as a dopant make the control and the achievement of high free carrier concentration in InGaAs rather difficult. Therefore, an understanding of the basic C incorporation mechanism into InGaAs is important for practical purposes besides being of basic scientific interest. One of the common ways to elucidate the doping characteristics is to look at the carrier concentration dependence on growth conditions. However, in MOCVD growth, it is rather difficult to change a specific growth parameter without influencing the others because each is mutually related through the surface and gas-phase reactions of several source molecules and their fragments. The other way to change the growth environment is to use differently oriented substrates, which can modify the surface step density. This will change several factors such as the pyrolysis of the source molecules, the surface migration of decomposed atoms and molecules, and the growth site density on the substrate. In fact, the effectiveness of using differently oriented substrates in revealing the dopant incorporation mechanism has been demonstrated [13,14]. Both of these approaches attempt to change the parameters quantitatively. On the other hand, we can also expect that a qualitative change of parameters would also be effective in clarifying the dopant incorporation behavior. One way to do this is to use different source molecules for a specific element with different ligands, such as methyl, ethyl and butyl compounds. Actually, a qualitatively different behaviors in growth rate [ 15,16] and residual impurity incorporation [17] between layers grown with trimethylgallium (TMGa) and triethylgallium (TEGa) have been reported. Therefore, we should be able to modify the incorporation behavior of C into InGaAs by changing the source molecules and extract the essentials of its mechanism. This paper reports the influence of gallium sources (TMGa and TEGa) on C incorporation behavior in InGaAs grown by MOCVD. The In composition was varied across the entire compositional range of the material system. Free carrier concentration decreased substantially with increasing In composition in both epilayers grown with TMGa and TEGa. However, the C incorporation efficiency in layers grown with TMGa was several times higher than those with

TEGa. From Hall and secondary ion mass spectroscopy (SIMS) measurements together with the atomic force microscope (AFM) observations, a comprehensive interpretation for the decrease in free carrier concentration with increasing In composition and for the enhanced C incorporation rate in layers grown with TMGa as a gallium source are presented.

2. Experimental procedure Unless otherwise mentioned, the epilayers were grown on semi-insulating (100) oriented GaAs or InP substrates in a low-pressure (60 Torr) vertical MOCVD reactor (EMCORE GS3300). The GaAs substrates were used for the growth of In,Ga~ ,As with smaller In composition (x < 0.5), and InP substrates were used when x > 0.5. Sources were TMGa, TEGa, trimethylindium (TMIn) and 100% arsine. The dopant was CBr 4. The substrate temperature during growth was 500°C. Hydrogen was used as the carrier gas. The typical growth rate was 0.9 txm/h, and the V / I I I flow ratio was 4. The free carrier concentration and Hall mobility in the grown layers were evaluated using van der Pauw measurements. The atomic concentration was measured by SIMS. Indium composition in InGaAs layers were determined by Auger analysis and X-ray diffraction measurement. The microscopic surface structure was evaluated by air ambient AFM observation.

3. Results and discussion 3.1. C incorporation behaL,ior

The dependence of free carrier concentrations in InGaAs layers grown with TMGa and TEGa against the In composition are shown in Fig. 1. Here, the CBr 4 flow rate was kept constant at 4 × 10 -v mol/min. Free carrier concentration decreased gradually and then more rapidly when the In composition increased from 0 to 0.7 (with TMGa) or to 0.5 (with TEGa). In these regions, the conduction type was p. When the In composition was greater than 0.7 (with TMGa) or 0.5 (with TEGa), free carrier concentration increased and then saturated with an increase in the In composition. The conduction was n-type in

H. lto, K. Kurishima /Journal of Crystal Growth 165 (1996)215-221

wE

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J I TEGa ~.A TMGa

C-doped InGaAs

~'o 1019[ ~ T M

Ga

~ 10 3

10 ,

o v

102 O 0)

,~-

1016 1015

I ,

,

0.2

0.4

101

I I I I/ , 0.6 0.8

0

In Composition Fig. 1. Dependence of free carrier concentrations in C-doped InGaAs layers grown with TMGa and TEGa against In composition. Open marks are for the p-type conduction and solid marks are for the n-type conduction.

these regions. Such a free carrier concentration reduction and a conduction-type inversion in InGaAs with increasing In composition were also observed in MBE [7-9] and MOCVD [6] grown materials, where the carrier concentration decrease was attributed to several different mechanisms, such as a reduced rate of C incorporation into InGaAs [6,7], the self-compensation of C atoms [8,9,18], and the hydrogenation of C acceptors [6,10-12]. Here we found that the hole concentrations in the epilayers grown with TMGa were several times higher than those with TEGa when 0 < x < 0.5. This difference should be a reflection of the difference in the influence of source molecules on the mechanism of C incorporation into InGaAs. Fig. 2 shows the dependences of Hall mobilities in C-doped InGaAs grown with TMGa and TEGa against In composition. For both types of sources, the mobilities stayed almost constant at first and then showed a decreasing tendency especially for TMGa at around the conduction type inversion points as the In composition increased from 0 to 0.5 or to 0.7. When the conduction type changed from p- to n-type, mobilities increased with increasing In composition. Although the free carrier concentrations decrease considerably with increasing In composition as shown in Fig. 1, there is no obvious decrease in the mobilities in the p-type region except at around the conduction type inversion points. Thus, we speculate that the hole concentration reduction with increasing In

t i t t 0.2 0.4 0.6 0.8 In Composition

Fig. 2. Dependence of Hall mobilities in C-doped InGaAs layers grown with TMGa and TEGa against In composition. Open and solid marks are for the p- and n-type conduction samples, respectively.

composition is not due to an increase in the C self-compensation ratio, which was the case in MBE grown InGaAs layers [8,9]. It was reported that hydrogen-passivated C atoms can be effectively re-activated by annealing in N 2 ambient [19]. This technique can thus be used to evaluate the amount of hydrogenated C atoms in the epilayers. Fig. 3 shows the relationship between hole concentrations in (In)GaAs for several carrier concentrations before and after annealing in N 2 ambient at 500°C for 5 min. The effective removal of hydrogen atoms from epilayers with these annealing conditions was confirmed in advance of the experiment. In

-~

1020 [] TEGa zx TMGa

E ¢-

<

1019

[]/ /

<

9"

~ lO18 ~ o 0

o 0

-r

/

v

1017

J

L( /

in N2 at 500 cO for 5 min I I I 1016 1016 1017 1 0 1 6 1019 020 Hole Concentration Before Anneal (cm-3)

Fig. 3. Relationship between hole concentrations before and after annealing in C-doped ]nGaAs grown with TMGa and TEGa. The annealing was performed in N 2 ambient at 500°C for 5 min.

218

14. lto, K. Kurishima / JounTal of C(vstal Growth 165 (1996) 215-221

1020

SIMS

E

~

c o

1019

~ 1018 oto (..)

~~TMGa TEGa ~

g 1017

0

,\

1016 0

0.2 0.4 0.6 0.8 In Composition

Fig. 4. Dependence of C concentrations in C-doped InGaAs layers grown with TMGa and TEGa against In composition. Solid curves are the results fitted using Eq. (i).

Fig. 3, the In compositions were larger for the lower-hole-concentration samples. As seen in this figure, the increment of hole concentration by annealing is relatively small (10-30%). This indicates that hydrogenation is not the dominant factor in the reduction of free carrier concentration with In composition increase. To further confirm this, SIMS analysis was performed for some samples. The hydrogen concentration was found to be about 1//3 of the carbon concentration in each sample, indicating the influence of hydrogenation was relatively small. Fig. 4 shows the dependence of C atom concentrations in layers grown with TMGa and TEGa measured by SIMS against the In composition. The C concentration decreased monotonically with increasing In composition in spite of the constant CBr 4 flow rate. The C concentrations in InGaAs layers grown with TMGa were several times higher than those with TEGa. These results clearly indicate that the C incorporation rate into InGaAs itself decreases with increasing In composition. In addition, the C incorporation rates are distinctly different between the two groups of layers grown with different gallium sources. A comparison of these results with the free carrier concentration data shown in Fig. 1 reveals that the C activation rate is a little lower than unity for In compositions from 0 to 0.4-0.6, but becomes smaller at around the conduction-type inversion points. Usually, residual electron concentration in InGaAs with larger In composition is inevitable for

intrinsic [20] and extrinsic [21] reasons such as the existence of surface inversion layers and residual donors. Therefore, the conduction-type inversion at In composition of 0.5 or 0.7 can be attributed to the compensation of decreasing C acceptors and increasing residual donors with increasing In composition rather than the self-compensation of C atoms. This is consistent with the Hall mobility results shown in Fig. 2 where electron mobilities increase rapidly with increasing In composition, and the conduction-type inversion composition is larger for layers grown with TMGa, which have a higher C concentration for the same In composition. 3.2. C incorporation mechanism into hlGaAs The C incorporation rate variations in InGaAs with different In compositions and gallium sources can be explained by a simple model [22] as follows. The binding energy between Ga and C is regarded to be stronger than that between In and C [8,9]. Although the C source may be adsorbed on the substrate surface in the form of incompletely decomposed molecules, such as CBr and CBr 2, the above relationship should hold true for the binding energies between C sources and group III atoms. If we assume that the C sources will migrate on the substrate surface until they are incorporated into crystal or dissociate from the substrate surface, and the dissociation will be enhanced when a C source meets with an In atom during the migration due to its smaller binding energy, the C concentration ([C]) variation against the In composition ( x ) can be expressed as [C] = [ C 0 ] ( l - kx)".

(1)

Here, [C 0] is the C concentration in GaAs, k is the dissociation enhancement factor for In atoms, and n is the average number of times one C source meets with group III atoms on the substrate surface during the migration. By using the experimental [Co] values of 4.4 × 1019/cm 3 and 3.5 × 1019/cm 3 for layers grown with TMGa and TEGa, parameters k and n for both cases were determined by fitting the experimental values shown in Fig. 4. The results were k = 0 . 3 and n = 2 6 for TMGa, and k = 0 . 2 and n = 52 for TEGa. The curves calculated using Eq. (1) and the above obtained parameters are also plotted in Fig. 4. The agreement between the experi-

219

H. Ito, K. Kurishima / Journal of Crystal Growth 165 (1996)215-221

ments and calculations are reasonably good. These results indicate that, for TEGa, the average number of the reactions between C source and group III atom (proportional to the average migration length of the C source) is larger while the dissociation enhancement is smaller than those for TMGa. These features must reflect that the difference in the Ga-source molecules influenced the surface kinetics of the sources.

In the relatively low growth temperature region, the surface roughness of the substrate will generally increase with decreasing growth temperature [23], especially in the kinetically limited region, because the surface migration of source molecules is substantially suppressed. The critical temperature for the growth mode change from the mass-transport limited to kinetically limited regime depends on the kind of group III source used because the decomposition efficiencies for each molecule are different [16]. At the growth temperature of 500°C, the growth mode was in the kinetically limited region when TMGa was used while it was in the mass-transport limited region when TEGa was used. Therefore, at the growth temperature we used, it is expected that the surface step density is considerably higher for TMGa than for TEGa. When the density of the surface step, where the actual incorporation of source atoms occurs, is higher, the incorporation probability of C atoms could be higher because the stability of the C atom at the A-step edge (Ga or In single bonds arrangement) increases on arrival of the adjacent group III atom due to the increased number of bonds between C and the group III atom [24]. Moreover, the potential barriers at the surface steps that atoms must overcome to migrate on the substrate surface should be larger for the higher surface step density, because the surface steps are usually regarded to have a higher potential barrier than that on the (100) surface [25]. Therefore, the average migration length should be smaller for TMGa. Both of these factors will make the number of C source reactions with group III atoms smaller. This is actually seen in the results mentioned above, i.e., the n for TMGa is 1/2 that for TEGa. On the other hand, at the same time, the existence of a Ga or In single bond at the A-step can enhance the dissociation of C molecules from the substrate surface. This is because a Ga singlebond site is a weak bond site for atoms that will

(a)

(b) l#m

Fig. 5. AFM micrographson the surfaces of C-dopedGaAs layers grown with TMGa (a) and TEGa (b).

occupy As sites [14]. This explanation is consistent with the larger k value for TMGa than that for TEGa, as shown above. The smaller n value will result in a higher C incorporation rate while the larger k value will result in a lower C incorporation rate. As a result of the superposition of these two opposing factors, the higher step density in the TMGa case acted to enhance the rate of C incorporation into InGaAs for the growth conditions we used. This consistently explains the variation in the rate of C incorporation into InGaAs grown with TMGa and TEGa shown in Fig. 4. In order to verify the model stated above, the surface of the C-doped epilayers were characterized by AFM. Fig. 5 shows the surface AFM images of C-doped GaAs layers grown with (a) TMGa and (b) TEGa. Wide and smooth terrace arrays with monolayer step height were found to be formed on the epilayer surface when TEGa was used as the Ga source. This result is in contrast to the TMGa case, where, instead of a distinguishable terrace array, two-dimensional islands were formed. Similar tendencies between TEGa and TMGa cases were also found for the C-doped InGaAs layers though the clearness of the terrace array formation became poorer probably due to the increased strain caused by the lattice mismatch between the epilayer and the substrate [26]. These results solidly support the hypothesis presented above. To further confirm the influence of surface step density on the behavior of C incorporation into

H. ho, K. Kurishima /Journal of Crystal Growth 165 (1996)215-221

220 1 020

O;;og;O,oGaAs

+I g 1019

E

10181

o¢8

1017

"~ 0

1016

~

101~"

3)A

n

,11(,

,

0.2

0.4

0.6

0.8

In Composition

Fig. 6. Dependence of flee carrier concentrations in C-doped InGaAs layers grown with TEGa on (100) and (311)A substrates against In composition. Open marks are for the p-type conduction and solid marks are for the n-type conduction.

InGaAs, we attempted to obtain higher surface step density using (311)A GaAs substrates [22], which have been characterized as a surface with very high step density. Fig. 6 shows the dependences of free carrier concentrations in InGaAs layers grown with TEGa on (31 I)A and (100) substrates against the In composition. Here, the CBr 4 flow rate was kept constant at 4 × 10 7 tool/rain. Although the hole concentrations in epilayers grown on both kinds of substrates decreased with increasing In composition, the hole concentrations in the epilayers grown on (311)A substrates were found to be several times higher than those on (100) substrates. These results are also consistent with the C incorporation mechanism proposed above, i.e., higher surface step density enhances the rate of C incorporation into InGaAs. From the results we have obtained, we can conclude that the essential aspect for obtaining efficient C incorporation into InGaAs layers is to achieve higher surface step density by choosing appropriate growth conditions, especially lower growth temperatures, even if TEGa is used as a gallium source.

4. Conclusion We have systematically investigated the carbon incorporation into InGaAs layers grown with TMGa and TEGa as gallium sources in MOCVD growth.

The free carrier concentrations decreased with increasing In composition and the conduction-type inversion occurred regardless of the Ga source species. It was found that the free carrier concentration in layers grown with TMGa are relatively higher than those grown with TEGa. The influences of hydrogenation and C self-compensation on the free carrier concentration were found to be relatively small. From these results together with the SIMS analysis of C concentration for various In compositions, the decrease in the hole concentration with increasing In composition is confirmed to be due to a decrease in the efficiency of C incorporation into InGaAs. Based on a numerical analysis assuming In atoms act as dissociation enhancement sites, it was proposed that the higher C incorporation efficiency in layers grown with TMGa over TEGa is related to the higher surface step density in the kinetically limited growth mode, which has the effect of both surface migration suppression and the dissociation enhancement of C sources. As a result of the superposition of these two opposing factors, C incorporation into InGaAs is enhanced by using TMGa as a gallium source. These hypotheses were experimentally confirmed by AFM observations of the epilayer surface and doping characteristics comparison using (100) and (311)A oriented substrates.

Acknowledgements The authors wish to thank Y. Homma for the SIMS analysis, M. Suzuki for the Auger analysis, and M. Shinohara for the AFM observation. They are also grateful to Y. Imamura and Y. Ishii for their continuous encouragement.

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