Precipitation of Tellurium-Rich Phase in Heavily Diethyltellurium-Doped GaAs during Organometallic Vapor Phase Epitaxy

4294 J. Electrochem. Soc., Vol. 142, No. 12, December 1995 �9 The Electrochemical Society, Inc.

8. V. S. Wang and R. J. Matyi, J. Electron. Mater., 21, 23 (1992).

9. H. M. Hobgood, D. L. Barrett, J. P. McHugh, R. C. Clarke, S. Sriram, A. A. Burk, J. Greggi, C. D. Brandt,

R. H. Hopkins, and W. J. Choyke, J. Cryst. Growth, 137, 181 (1994).

10. J. F. Young and R. S. Shane, Materials and Processes, Part B, 3rd ed., Marcel Dekker Inc., New York (1985).

Precipitation of Tellurium-Rich Phase in Heavily Diethyltellurium-Doped GaAs during

Organometallic Vapor Phase Epitaxy Y. Park, W. Qian, and M. Skowronski

Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

ABSTRACT

GaAs layers were heavily doped with liquid source diethyltellurium (DETe) during organometallic vapor phase epitaxy. Samples were characterized by Hall effect measurements and transmission electron microscopy (TEM). The electron concentration was observed to saturate at 3 • i0 ~9 cm -3 for high DETe flows, which is independent of growth temperature in the investigated range from 475 to 609~ TEM examination revealed the presence of precipitates in layers doped above the saturation level. The precipitates were Te-rich by energy dispersive x-ray analysis. Electron diffraction study showed that they have the zinc-blende structure with a lattice constant of 6.5 A. The appearance of tellurides at high doping levels indicates that the formation of a second phase could be one of the main mechanisms responsible for the carrier saturation in Te-doped GaAs. The liquid source DETe causes persistent memory effects associated with the adsorption of DETe on the stainless steel tube walls of the gas distribution system.

Introduction Tellurium is an attractive n-type dopant for the fabrica-

tion of GaAs and AIGaAs optoelectronic devices because its diffusion coefficient is lower than those of other group IV and VI dopants such as St, Sn, S, and Se. i The low diffusion coefficient allows for sharp doping profiles during device fabrication processes. In addition to the low diffusion coefficient, Te has a lower activation energy than Si in AIGaAs (18 meV vs. 60 meV in Al0.3Ga0.TAs). This is of interest in modulation doped field effect transistors (MODFETs) because the smaller activation energy results in a larger sheet electron density in the MODFET two-di- mensional electron gas. 2 Although GaAs and AIGaAs op- toeleetronie devices are grown widely by organometallic vapor phase epitaxy (OMVPE), studies of Te doping of GaAs and AIGaAs during OMVPE 2,3 and the carrier saturation in the heavily doped materials 4-7 have been limited.

Heavily doped n-type GaAs and AIGaAs layers are im- portant for the fabrication of high speed electronic devices and low resistance ohmic contacts. However, the free-electron concentration obtainable in GaAs has been limited to less than 8 x 10 TM cm -3 in St-doped GaAs 8-10 and 1 • 1019 em 3 in Te-doped GaAs. TM Several different mechanisms have been proposed to explain the saturation of electron density. For St-doped GaAs, the proposed mechanisms are: site switching due to amphoterie characteristic of sili- con, 12'13 formation of neutral nearest neighbor pairs (SiGa- SiAs),14 occupation of localized negatively charged D• state of hydrogenic donors (SiGa), I~ formation of donor-vacancy complex ((SiGa-VGa)2-), 8,13 and precipitation of second phases and/or dopant clustering due to a dopant supersaturation. ~~ For Te-doped GaAs, defects such as DX center v and donor-vacancy complex 6 have been proposed to be responsible for the limit of electron concentration.

We describe here the doping of GaAs using the liquid source DETe during OMVPE and the investigation of the epilayers by Hall effect measurements and TEM.

Experimental The layers examined were grown by atmospheric pres-

sure OMVPE with a horizontal reactor heated by infrared radiation. Substrates were (100) undoped semi-insulating

GaAs wafers misoriented 2 ~ toward [110]. The substrates were used as received without cleaning or etching prior to growth. The oxide layer on the surface was removed by a thermal desorption at 750~ for 10 rain in an arsenic overpressure. Trimethylgallium (TMG) and tertiarybutyl- arsenic (TBA) were used as source materials for the growth of GaAs with hydrogen as a carrier gas. The mole fractions of TMG and TBA in a carrier gas flow of 4 slpm were maintained constant at 5 • 10 5 and 5.6 • 10 -4, respectively, resulting in growth rates of 1.5 ~m h 1 at 600~ 1.0 ~m h at 500~ and 0.6 ~m h 1 at 475~ Electronic grade liquid DETe was used as an n-type dopant source. The temperature of DETe bubbler was maintained at - 10~ which cor- responds to a DETe equilibrium vapor pressure of 1.08 Torr. Using the low bubbler temperature eliminated the possi- bility of DETe supersaturation and subsequent condensa- tion in the gas distribution lines.

The room temperature electron concentration was deter- mined by Hall effect measurements in the Van der Pauw configuration. The 1.5 ~m thick layers for Hall effect measurements were grown by adjusting growth time at different growth temperatures. Ohmic contacts were made by sinter- ing 50% In-50% Sn alloy on the layer at 350~ for 20 s in a flow of forming gas. Electron concentration profiles vs. the layer depth were obtained by the electrochemical C-V pro- filer using a Polaron system.

Plan-view TEM specimens were prepared by mounting the epilayer face down on a glass block and mechanically polishing it from the substrate side down to - 5 0 ~xm. The sample was then chemically polished to electron transpar- ency in a solution of 1 volume percent (v/o) bromine- methanol. TEM observations were carried out in a Philips EM 420-TEM operated at 120 keV and equipped with a Princeton Gamma-Tech energy dispersive x-ray unit.

Results and Discussion Hall-effect measurements.--Doping of GaAs layers with

the liquid source DETe exhibited severe memory effects. To reduce the memory effects inherent in DETe doping, samples for the Hall effect measurements were grown carefully with increasing DETe mole fraction in steps; at every incre- ment of the DETe mole fraction, the DETe line walls were exposed to a new DETe mole fraction for 2.5 h to obtain a

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.137.5.42Downloaded on 2014-11-13 to IP

http://ecsdl.org/site/terms_use

J. Electrochem. Soc., Vol. 142, No. 12, December 1995 �9 The Electrochemical Society, Inc. 4295

uniform electron concentration profile to the layer depth. This is discussed in detail in the section on Memory effects of DETe doping. Plots of electron concentrations at different growth temperatures and DETe mole fractions in the gas phase is shown in Fig. i. At low DETe mole fractions the electron concentration depends on the growth temperature. At 600 and 500~ the electron concentration is pro- portional to the DETe mole fraction in the gas phase. The samples grown at 500~ had a higher electron concentration than those at 600~ by a factor of approximately four. The decrease of carrier density with increasing growth temperature has been observed before and attributed to higher reevaporation rate of Te at elevated temperatures. 2 In contrast, the electron concentration increases superlin- early with increasing DETe mole fraction at 475~ the low- est growth temperature which gives a measurable growth rate in our growth system. A similar superlinear increase of electron concentration with increasing DETe mole fraction has been observed in DETe-doped InP. 17 At this point, the mechanism responsible for this phenomenon is not under- stood. In the lightly doped regime, the lower electron concentration at 475~ than at the higher temperatures seems to result from an incomplete decomposition of DETe. At high DETe mole fractions, the carrier density saturated at 3 • i0 ~9 cm -3. This is the highest electron concentration in OMVPE-grown GaAs reported so far. 2,3 The saturation value is independent of growth temperature in the investigated temperature range from 475 to 600~

Carrier saturation is common in heavily doped n-type GaAs and AIGaAs; most of the research examined the GaAs:Si system. Similar to the case of the self-compensa- tion in St-doped GaAs, DX center 7 and donor-vacancy complex ((TeA~-VG~)-) 6 have been suggested as the primary defects accounting for the carrier saturation in Te-doped GaAs. However, these interpretations are not without their weak points. DX states have been commonly observed in St, Sn, and Ge-doped GaAs under hydrostatic pressure, ~a8-2~ but not in GaAs:Te. 4.5 Sallese 4 reported that in GaAs:Te grown by OMVPE, DX states did not emerge under hydrostatic pressure up to 1.5 GPa at which the St- doped sample showed a significant decrease in electron concentration due to the DX- center. Suski et al. 5 also reported that although bulk GaAs doped with St, Sn, and S exhibited the effect of carrier freeze-out at pressures below 2.0 GPa, the electron concentration in Te-doped material was constant up to 2.5 GPa. The different behavior of Si and Te dopants in GaAs could be explained by the existence of a Te DX level farther above the conduction band- edge than those of other donors. 4'2~ The different chemical

1020 i

0

0

�9

' ' ' ' ' " q ' ' ' ' ' " q ' ' ' ' ' " ' 1 ' ' ' ' ' " '

, /

�9 475 ~

�9 500 ~

. 6 0 0 ~

1019

1018

1017 . . . . . . . . ,

10 -2 10 -1 10 ~ 101 10 2

DETe Mole Fraction (x 1 O -v)

Fig. 1. Electron concentration of DETe-doped GaAs as a function of DETe mole fraction at different growth temperatures.

Fig. 2. Nomarski contrast optical micrographs of surface morphologies of DETe-doped GaAs layers. The growth conditions are: (a) growth temperature fiG) = 600~ DETe mole fraction (XoE, J = 2 • 10 z; (b) T~ = 600~ XOET~ = 1.6 • 10-6; and (c) T~ = 500~ XDEr~ = 1.6 • 10 6. The markers correspond to 50 mm.

environment surrounding Si and Te in the GaAs lattice (St occupies Ga site as a donor and Te occupies As site) and the large difference in electronegativities of Si and Te may cause such a difference in the DX level position. 21 Thus, the above argument suggests that even if the DX center exists in Te-doped GaAs, it does not participate in carrier saturation in the same way as it does in St-doped GaAs.

Surface morphology.--Examination of surface morphology of the layers by a Nomarski contrast microscopy gave an insight into the carrier saturation in the heavily Te- doped GaAs. Figure 2 shows the surface morphologies of three DETe-doped layers deposited at 600 and 500~ It is apparent that the surface morphology degrades in the heavily doped layer grown at 600~ while mirrorlike surface morphologies are seen in the samples with Te concentrations below the saturation at 600~ and grown at the lower growth temperature. Houng and Low 2 suggested that surface morphology could be degraded by the strain in- duced by an increase of the lattice parameter of GaAs on addition of more than 2 • 1018 cm -3 tellurium. 22 However, the mirrorlike surface of the layer grown at 500~ (Fig. 2c) contradicts this interpretation. The expected Te concentra-



4296 J. Electrochem. Soc., Vol. 142, No. 12, December 1995 �9 The Electrochemical Society, Inc.

tion in the layer obtained by extrapolation to a given DETe mole fraction is higher at 500 than at 600~ Thus, the degradation of surface morphology is most likely caused by other mechanisms such as the precipitation of second phase during growth. The good surface morphologies of the layer grown at 500~ could be explained by the exceedingly small size of precipitates due to small surface diffusion length at low temperature. Bigger precipitates and more preferential growth around them due to a faster surface diffusion at 600~ are expected to increase the surface roughness.

Transmission electron microscopy.--Information about the microstructure of the heavily Te-doped GaAs layers was obtained by plan-view TEM examination. In agree- ment with the analysis of surface morphology, the most heavily doped layers grown at 500 and 600~ were found to contain small preci~tates. Figure 3a shows the bright-field micrograph (g = (022)) of the heavily doped layer grown at 500~ The presence of precipitates was demonstrated by the moir~ interference fringes (highlighted by arrows in Fig. 3a). These fringes result from the lattice constant difference and/or relative crystallographic misorientation between the precipitate and the GaAs matrix. 23 The distribution of precipitates is homogeneous throughout the layer and their average size depends on the growth temperature. The precipitates in the layers grown at 600~ have an average diameter of :--400 A, while those in the layer grown at 500~ are -140 A in diameter. This agrees with the surface morphology observations discussed in the previous section. Similar precipitates have been reported in Te-doped, melt- grown, bulk GaAs. 24

The selected area electron diffraction pattern obtained from the sample is shown in Fig. 3b. Since the sample volume on which the aperture was focused contained a num- ber of precipitates together with the GaAs matrix, the diffraction pattern consists of the pattern from the (100) GaAs

Fig. 3. TEM images of a GaAs layer doped with a DETe mole fraction of 1.6 • 10 -6 at 500~ (a) Bright field micrograph (g = 022) showing precipitates pointed by arrow; (b) the electron diffraction pattern obtained from both the precipitates and the matrix.

600

500- Ga Ka Te L~_~,

~ 4 0 0 - ~ I A i K~

u , 300- ~

IL 200- ~,

Cu Ka Mo Ka

1~176 o 0,0 4,0 8.0 12.0 16.0 20.0

Energy (keV) Fig. 4. Energy dispersive x-rtw spectrum taken from the precipi-

tates shown in Fig. 3a.

matrix and a superimposed weak ring pattern due to the precipitates. The presence of the ring pattern suggests that the precipitates are oriented randomly with respect to the GaAs matrix. By analyzing the ring pattern, the precipitates were found to have the zinc-blende structure with a lattice parameter of 6.5 A.

The composition of the precipitates was studied by an energy dispersive X-ray (EDX) analysis in the TEM. Fig- ure 4 shows an EDX spectrum taken from the precipitate. In addition to the gallium and arsenic peaks from the GaAs matrix, and weak peaks of copper and molybdenum from the specimen supporting grid and TEM specimen holder, very strong tellurium peaks were observed. Since the ex- trapolated atomic concentration of tellurium in our sample is less than 4 • 1020 cm -3, the EDX spectrum taken from a large sample area (not shown) contains only very weak tellurium peaks. Thus, the strong tellurium peaks in Fig. 4 clearly indicates that the precipitates are tellurium com- pounds. Precipitation of tellurides is expected to decrease the tellurium concentration in the matrix significantly. This suggests that the carrier saturation shown in Fig. 1 could be attr ibuted to the precipitation of tellurides. Among the possible tellurides of Ga or As, 2~ only Ga2Te3 has the zinc-blende structure. However, its lattice constant of 5.88 A is smaller than the value measured from the ring pattern. The origin of this discrepancy is under investigation.

Although the above examination strongly suggests that the precipitation of tellurides could be a main mechanism responsible for the carrier saturation in Te-doped GaAs grown by OMVPE, more investigation is necessary to con- firm it. We could not observe a strong evidence of the precipitation of tellurides in the layer with tellurium concentration just above the carrier saturation. This could be due to the small size of the precipitates and/or their small volume fraction. If the precipitation of tellurides is a main factor limiting carrier concentration, one expects to achieve higher electron concentration in layers deposited at temperatures lower than used here. Lower deposition temperature should reduce the diffusion length of atoms on the growth surface and result in the creation of a supersat- urated GaAs:Te solid solution. This could be accomplished using molecular beam epitaxy in which deposition temperature can be lowered to 200~ In our OMVPE system, the rapid decrease of growth rate at temperatures below 500~ due to incomplete decomposition of trimethylgal-



J. Electrochem. Soc., Vol. 142, No. 12, December 1995 �9 The Electrochemical Society, Inc. 4297

l ium limited the investigated temperature range to 475~ and higher.

Memory effects of DETe doping.--Doping with hydrogen-diluted DETe has been reported to exhibit a small memory effect due to its adsorption on the stainless steel tube walls of the OMVPE system. 2 The residual doping level of adsorbed DETe was on the order of 101~ cm -3. In our growth system, however, the memory effect of the liquid source DETe was much more pronounced for instance, an undoped GaAs layer grown at 600~ immediately after the growth of an intentionally doped layer with an electron concentration of 1.4 • 10 a9 cm -3 showed a background electron concentration of ~1 • 10 is cm -3. Part of the difference between our observations and that of Houng and Low could be associated with the high doping levels investigated in this work. Most of it, however, probably was due to the difference in form of a dopant (liquid vs. gaseous). Heating the DETe line to ~70~ and keeping it for 24 h at a pressure of 10 Torr reduced the background electron concentration to less than 1 • 10 ~6 cm -~.

Another result of the adsorption of DETe on the line walls during growth was a nonuniform electron concentration profile vs. the layer depth. Figure 5 shows the electron concentration profiles of two 1.5 ~m thick layers grown at 600~ The layer exhibiting the electron concentration profile of Fig. 5a was grown immediately after the DETe line was heated and pumped to minimize the memory effect as described above. It is clear that the electron concentration increases by more than an order of magnitude from the beginning to the end of the growth. The net amount of DETe flowing into the reactor increases during the growth at a given DETe mole fraction in the gas phase. This could be explained by adsorption and desorption of DETe onto and off the line walls. At the beginning of the growth, the adsorption of DETe onto the wails is expected to be domi- nant over its desorption off the walls because they are more or less free of DETe immediately after being heated and pumped. As the growth progresses, the line walls are cov- ered with more DETe and the adsorption rate decreases while the desorption rate increases, which results in the increase of the net flow of DETe into the reactor. At the end of the growth, the density of DETe on the line walls is in equil ibrium with a given DETe mole fraction in the gas phase, and both rates are in balance. This leads to a constant flow of DETe into the reactor, equivalent to the DETe mole fraction in the gas phase, and further growth shows a uniform electron concentration profile. Figure 4b demon- strates that the above analysis is correct. The concentration profile shown in Fig. 4b was obtained on the layer grown after the exposure of the line walls to the DETe flow for

0 . p . . . q

�9 L9

o

o

�9

1 0 1 9 , , , , ,

1 0 1 8

10IV

1014

lOiS

(b)

1 0 1 4

surface Depth

- I I I I I I I J

2 ~tm

Fig. 5. Electron concentration profiles of two DETe-doped GaAs layers grown at 600~ (a) The layer grown with DETe mole fraction of 6 • 10 - l~ cm 3 immediately after the DETe lines were purified as described in text, (b} the layer grown with DETe mole fraction of 2 • 10 -8 cm -s after the DETe line walls were saturated with DETe. Note that the depths down to 2 mm of the two layers are not to scale.

2.5 h. Because of the above persistent memory effect, when DETe is used as a dopant source during OMVPE growth, care must be taken.

Conclusion Te-doped GaAs by using liquid source DETe during

OMVPE has been investigated by Hal] effect measurements and TEM. The electron concentration saturated at 3 • i0 ~9 cm -3 for high DETe flows, which is independent of growth temperature in the range investigated from 475 to 600~ TEM images and EDX analysis demonstrated the existence of tellurium-rich precipitates in heavily doped layers. Selected area electron diffraction indicated that the precipitates have the -zine-blende structure with a lattice parameter of 6.5 ,~. The appearance of tellurides in heavily doped samples suggests that the precipitation of second phases could be responsible for limiting the electron concentration in Te-doped GaAs during OMVPE. Doping with the liquid source DETe produced long-term memory effects associated with adsorption of DETe on the stainless steel tubing of the gas distribution system.

Acknowledgment The authors thank Dr. R. Messham of Westinghouse Sci-

ence and Technology Center for C-V profiling. This project has been supported by NSF Grant No. DMR-9202683.

Manuscript submitted March 16, 1995; revised manuscript received Aug. 7, 1995.

Carnegie Mellon University assisted in meeting the pub- lication costs of this article.

REFERENCES 1. R. Sankaran, J. Cryst. Growth, 50, 859 (1980). 2. Y. Houng and T. S. Low, ibid., 77, 272 (1986). 3. S. Z. Sun, E. A. Armour, K. Zheng, and C. F. Schaus,

ibid., 113, 103 (1991). 4. J. M. Sallese, Phys. Status Solidi, 1 1 9 , K41 (1990). 5. T. Suski, R. Piotrzkowski, P Wisniewski, E. Litwin-

Staszewska, and L. Dmowski, Phys. Rev. B, 40, 10 (1989).

6. R. Krause-Rehberg, H. S. Leipner, A. Kupsch, A. Polity, and T. Drost, ibid., 49, 2385 (1994).

7. H. Bemelmans, G. Borghs, and G. Langouche, Phys. Rev. Lett., 72, 856 (1994).

8. J. K. Kung and W. G. Spitzer, J. Appl. Phys., 45, 4477 (1974).

9. Y. Okano, H. Seto, H. Katahama, S. Nishine, I. Fuji- moto, and T. Suzuki, Jpn. J. Appl. Phys., 28, L151 (1989).

10. Y. G. Chat, R. Chow, and C. E. C. Wood, Appl. Phys. Lett., 39, 80 (1981).

11. I.V. Mitchell, J. W. Mayer, J. K. Kung, andW. G. Spitzer, J. Appl. Phys., 42, 3982 (1971).

12. S. Schuppler, D. L. Adler, L. N. Pfeiffer, K. W. West, E. E. Chaban, and P. H. Citrin, Appl. Phys. Lett., 63,235 (1993).

13. J. E. Northrup and S. B. Zhang, Phys. Rev. B, 47 (1993). 14. A. Masuyama, M.-A. Nicolet, I. Golecki, J. L. Tandon,

D. K. Sadana, andJ . Washburn, Appl. Phys. Lett., 36, 749 (1980).

15. T. N. Theis, P. M. Mooney, and S. L. Wright, Phys. Rev. Lett., 60, 361 (1988).

16. I. Fujimoto, N. Kamata, K. Kobayashi, and T. Suzuki, Inst. Phys. Conf. Ser., 79, 199 (1985).

17. C. C. Hsu, J. S. Yuan, R. M. Cohen, and G. B. Strinfet- low, J. Cryst. Growth, 74, 535 (1986).

18. M. Mizuta, M. Tachikawa, H. Kukimoto, and S. Mi- nomura, Jpn. J. Appl. Phys., 24, L143 (1985).

19. D. K. Maude, J. C. Portal, L. Dmowski, T. Foster, L. Eaves, M. Nathan, M. Heiblum, J. J. Harris, and R. B. Beall, Phys. Rev. Lett., 59, 815 (1987).

20. M. Baj, L. H. Dmowski, and T. Slupinski, ibid., 71, 3529 (1993).

21. P M. Mooney, J. Appl. Phys., 67, R1 (1990). 22. D. T. J. Hur]e, J. Phys. Chem. Solids, 40, 613 (1979). 23. P. Hirsch, A. Howie, R. B. Nicholson, D. W. Pashley, and

M. J. Whelan, Electron Microscopy of Thin Crystals, Robert E. Krieger, Malabar (1977).

24. L. M. Morgulis, V. B. Osvenskii and M. G. Milvidskii, Sov. Phys. Solid State, 16, 142 (1974).

25. W. B. Pearson, in Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, New York (1967).



Documents

Precipitation of Tellurium-Rich Phase in Heavily Diethyltellurium-Doped GaAs during Organometallic Vapor Phase Epitaxy