Influence of H2 on electrical and optical properties of carbon-doped InP grown by MOMBE using tertiarybutylphosphine (TBP)

,. . . . . . . . C R Y S T A L GROWTH

ELSEVIER

Journal of Crystal Growth 164 (1996) 425-429

Influence of H 2 o n electrical and optical properties of carbon-doped InP grown by MOMBE using tertiarybutylphosphine (TBP) Je-Hwan Oh, Fumihiko Fukuchi, Ho-Cheol Kang, Makoto Konagai * Department of Electrical and Electronic Engineering, Toky'oInstitute of Technology, 2-12-10-okayama, Meguro-ku, Tokyo 152, Japan

Abstract

The influence of the introduction of H 2 during growth on electrical and optical properties of carbon-doped InP is discussed. The existence of H 2 in the cracker affects the tertiarybutylphosphine (TBP) cracking behavior. According to the initial mass spectroscopic study, it was found that the decomposition of carbon-containing species originating from TBP is enhanced when they are cracked in ambient H 2. As a result, the increase of the electron concentration of InP grown under the introduction of H 2 is presumably attributed to the increase of the incorporation of a carbon donor. It was also found that a phosphorus polymer, which contributes to the epitaxial growth of InP, in the direct beam cracked under H 2, is slightly decreased. This leads to the reduction of an effective V/Ill ratio on the growing surface and may be responsible for the drastic change in the PL characteristics.

1. I n t r o d u c t i o n

Recently, I n P / I n G a A s heterojunction bipolar transistors (HBTs) have attracted significant attention as a most promising candidate for ultra-high speed device applications, because of the superior transport properties of InGaAs compared to those of GaAs of conventional A I G a A s / G a A s HBTs [1,2]. Furthermore, carbon is very important as a well-behaved, nearly ideal, p-type dopant in the base material, such as GaAs and InGaAs, because of its low diffusivity and capability of heavy doping accompanied with high electrical activity [3,4]. Besides, it is believed that carbon behaves as a predominant am-

* Corresponding author. Fax: + 81 3 5734 2897; E-mail: [email protected].

photeric dopant in I I I - V compounds, i.e. as an acceptor in GaAs [3,4] and GaP [5], and as a donor in InP [6] and InAs [7], respectively. In fact, we have reported that heavily carbon-doped n-type InP (up to n = 1.8 × 1019 cm -3) was obtained by metalorganic molecular beam epitaxy (MOMBE) using tertiarybutylphosphine (TBP) [6]. From this point of view, if carbon-doped n-type InP can be used for the emitter material of carbon-doped base I n P / I n G a A s HBTs, further improved device performance is expected. There have been several studies on the growth of InP using MOMBE, CBE and MOVPE techniques with TBP as a replacement of phosphine (PH 3) [8-10]. In our previous paper, we have reported that TBP is believed to be a promising replacement of PH 3 and can be also used as a carbon auto-doping source for M O M B E growth of n-type InP [6]. Also, carbon-doping characteristics dependent on TBP

0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 0 1 5 - 2

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J.-H. Oh et al. / Journal of Cr3,stal Growth 164 (1996) 425-429

cracking behavior have been discussed in detailed. In this study, we discuss the influence of the introduction of H 2 during growth on electrical and optical properties of carbon-doped InP.

2. Experimental procedure

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The epitaxial growth of InP was performed with a modified VG V-80H MOMBE chamber. Elemental indium (In) and TBP were used as source materials of group III and V, respectively. All carbon-doped InP epilayers were grown on Fe-doped semi-insulating (SI) InP(100) substrates. TBP was introduced into the chamber without a carrier gas and controlled by a mass flow controller (MFC). For the efficient cracking of TBP, the newly designed cracking cell with tantalum (Ta) as a catalyst was employed [11]. H 2 was introduced into the chamber through the TBP cracking cell. The electrical and optical properties of carbon-doped InP epilayers were investigated by the Van der Pauw-Hall method and low temperature (4.2 K) photoluminescence (PL) measurements using the emission lines of Ar ÷ (514.5 nm) and He-Cd (325.0 nm) lasers as excitation sources. In order to characterize the low pressure thermal cracking behavior of TBP, mass spectroscopic measurements were carried out.

3. Results and discussion 3.1. Carbon-doping characteristics The growth and TBP cracking-temperature dependence of carbon doping characteristics are summarized in Fig. 1. Firstly, the growth temperature dependence of the electron concentration at a given TBP cracking temperature is closely related to the adsorption a n d / o r desorption process of the carboncontaining species originated from the decomposed TBP on the growing surface, as discussed in our previous report [6]. Secondly, the weaker growth temperature dependence of the electron concentration at a higher TBP cracking temperature indicates that the overall carbon incorporation is dominated by the simpler carbon-containing species, which can be easily adsorbed on the growing surface. Ultimately,

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this leads to an increase in the electron concentration of InP with an increase of the TBP cracking temperature. According to mass spectroscopic measurements, TBP at a cracking temperature of 700°C is already decomposed into a phosphorus polymer, mainly P and P2, but it is not sure whether P is entirely due to the cracking of P2 in the cracker. In addition, various types of carbon-containing species, mainly C H 3 , C 3 H 3, C 3 H 5 and C a l l 9, are also detected in the direct beam. Similar observation has been reported by Hincelin et al. [10]. As cracking temperature is increased, these carbon-containing species became more simpler-type ones. This leads to a weaker growth temperature dependence of the electron concentration at a higher TBP cracking temperature [6]. Fig. 2 shows PL spectra of carbon-doped InP grown at various growth temperatures. Mainly two distinct peaks are observed in the investigated wavelength range. This is very similar to what has been observed for the growth of InP using trimethylindium (TMI) and PH 3 [12]. At the highest growth temperature of 450°C, only the strong and sharp bound-exciton emission peak (FWHM = 2.59 meV) at 875 nm is observed, which can be assigned as a neutral-donor-bound-exciton peak (Do,X) [8,12]. This indicates that an InP epilayer grown by MOMBE using TBP at 450°C has a good optical property.

J.-H. Oh et al. / Journal of Co,stal Growth 164 (1996) 425-429

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Fig. 3. PL spectra (4.2 K) of the carbon-doped InP epilayer measured with various excitation intensities of the emission line of the Ar ÷ laser (514.5 nm).

Fig. 2. PL spectra (4.2 K) of carbon-doped InP epilayers grown at various growth temperatures. During the growth, TBP flow rate and cracking temperature were fixed at 0.3 sccm and 700°C, respectively. By Hall measurement (300 K), sample (a) showed high resistivity and samples (b), (c) and (d) were represented in n = 1.2)< 10 i7, 3.8× 10 ]7 and 3.8)< 10 TM cm -3, respectively.

Besides, the broad peak at 897.5 nm strongly depends on growth temperature. In order to examine the origin of this peak, excitation intensity depen-

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Mass Fig. 4. Relative intensities of decomposed species of TBP cracked at 700°C with and without the introduction of H 2. During the mass spectroscopic measurement, TBP and H 2 flow rates were 0.01 and 0.1 seem, respectively.

428

J.-H. Oh et al. / Journal of Crystal Growth 164 (1996) 425-429

dence measurements were carried out. As seen in Fig. 3, the peak shift to higher energy (the rate being about 1.5 meV per ten-fold change in excitation intensity) with the increase of excitation intensity is clearly observed. Thus, the peak at 897.5 nm, shown in Fig. 2, can be assigned as a donor-acceptor pair (DAP) emission peak, although the dominant acceptor impurity in the pair is not clear yet. It is also found that the DAP peak intensity increases as the growth temperature decreases. This reflects the increase of the carbon-donor incorporation and shows a good agreement with the growth temperature dependence of electron concentration, as evidenced by Fig. !.

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3.2. Influence of H 2 on electrical and optical properties of InP To investigate the change of the TBP cracking behavior due to the introduction of H 2, mass spectroscopic measurements were carried out. During the measurements, the background pressure inside the chamber was maintained at about 1 X 10 -5 mbar. The relative intensities of the decomposed species from TBP with and without the introduction of H 2 at a cracking temperature of 700°C are shown in Fig. 4. The lines through the data points are drawn as a

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Flow rate of H 2 ( sccm ) Fig. 5. Electron concentration (300 K) of carbon-doped InP epilayers grown under introduction of different H 2 supplies at a TBP cracking temperature of 700°C. During the growth, the TBP flow rate and In-flux were fixed at 0.3 sccm and 5 X 10 -7 mbar, respectively.

Fig. 6. PL spectra (4.2 K) of carbon-doped InP epilayers grown at 450°C without (a) or with (b and c) the introduction of H 2. The measured electron concentrations (300 K) of samples (a), (b) and (c) are 1.l X ]016, 3.3× 1016 and 1.7× 1017 cm -3, respectively.

guide to the eye. TBP signals in both cases are very weak, so an accurate comparison of the intensities is critical. However, a significant change of carbonand phosphorus-containing species signals is observed. That is, the increase in the overall carboncontaining species in the case of the introduction of H 2 is clearly shown. This reflects that the decomposition of carbon-containing species originating from TBP is enhanced when they are cracked in ambient H 2, although the reaction mechanism is not clear yet. In addition, phosphorus polymers are slightly decreased and PH, PH 2 and PH3 appear in the direct beam. This indicates a reduction of phosphorus, which contributes to the epitaxial growth of InP and leads to the reduction of the effective V / I l l ratio on the growing surface. Fig. 5 shows the electrical properties of carbondoped InP grown under introduction of H 2. The electron concentration of carbon-doped InP grown at 450°C strongly depends on the H 2 supply and is increased from 1.1 × 1016 to 1.7 × 1017 cm -3 as the H 2 supply is increased from 0 to 1.0 seem. In the case of a growth temperature of 230°C, however, no significant change is observed. This different behavior in the high and low growth temperature regions may be attributed to the distinct carbon incorporation

J.-H. Oh et al. /Journal of Co'stal Growth 164 (1996) 425-429

mechanism, as discussed in our previous report [6]. So, we restrict the discussion to the case of high growth temperature. As mentioned in Fig. 4, the presence of H 2 in the cracker enhances the decomposition of carbon-containing species originating from TBP. This may lead to the increase of the electron concentration of InP grown under introduction of H 2, due to more carbon-donor incorporation. Fig. 6 shows PL spectra of InP epilayers grown at 450°C with different H a supplies. As seen in this figure, the drastic change of the PL characteristics due to the introduction of H z is clearly observed. As H 2 supply is increased, the sub-peak at 892.7 nm becomes dominant compared to that at 897.5 nm, and its intensity is increased with the increase of electron concentration. Although the origin of this peak is not clear yet, similar observation has been reported by Kawaguchi et al. [13]. As mentioned in Fig. 4, the decomposition of TBP under introduction of H 2 causes the reduction of the phosphorus polymer in the direct beam, that is, the reduction of the effective V / I l l ratio on the growing surface. This may lead to a distinct surface reaction process during growth. Also, the reduction of the effective V / I l l ratio may induce a point defect, such as phosphorus vacancy, in the grown epilayer. Thus, it is likely that this unknown emission band (at 892.7 nm) shown in Fig. 6 is attributed to the defect-induced DAP transition, similar to that in the case of MBE-grown GaAs [14]. To examine whether these behaviors are attributed to hydrogen directly incorporated into the epilayer, we carried out low temperature annealing (450°C, 15 min) in the ambient of pure N 2 for the samples grown under introduction of H 2. However, no remarkable discrepancy of electrical and optical properties of the samples before and after annealing was observed. So, it is likely that there is no direct influence due to shallow impurity neutralization or other compensating defects created by atomic hydrogen [15]. Therefore, it is concluded that the change in electrical and optical properties of carbon-doped InP grown under introduction of H 2 seems to be closely related to the TBP cracking behavior.

429

4. Conclusion It was found that electrical and optical properties of carbon-doped InP was influenced by the introduction of H2 during growth. According to mass spectroscopic study, it was revealed that the decomposition of TBP under introduction of H 2 c a u s e s the enhanced decomposition of carbon-containing species originating from TBP and the reduction of the phosphorus polymer. This may lead to the increase of carbon incorporation into the InP layer and the reduction of the effective V / I I I ratio on the growing surface. Therefore, it is concluded that the change in electrical and optical properties of carbon-doped InP grown under introduction of H 2 seems to be closely related to the TBP cracking behavior.

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