Thermal stability of low-temperature-grown GaAs

N.H CRYSTAL GROWTH ELSEVIER

Journal of Crystal Growth 143 (1994) 354-355

Letter to the Editors

Thermal stability of low-temperature-grown GaAs T.W. Fan

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J.B. Liang

a,

H.J. Deng

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R.G. Li

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Z.G. Wang

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W. Gen ~

Laboratory of Semiconductor Materials Science, Chinese Academy of Sciences, P.O. Box 9/2, Beijing 100053. People Is Republic of China Beijing Laboratory of Llcctron Microscopy, (‘hinese Academy of Sciences, P.O. Box 2724, Bet/big 100080, People’s Republic of (‘lana

Received 24 March 1994: manuscript received in final form 16 June 994

Abstract The thermal stability of GaAs sayers grown at low temperature by molecular beam epitaxy and subsequently annealed from 250 to 850°has been investigated by transmission electron microscopy and Hall measurements. The results show that the formation of arsenic precipitates in annealed GaAs epilayers grown at low temperature and their crystallographical configuration are highly dependent on the temperature and duration of the annealing. Thermal annealing at 850°for 30 mm causes arsenic precipitates to become amorphous and some traction of the arsenic precipitates to redissolve in the crystal matrix. An analysis of the dependence of resistivity on anneal temperature by Hall measurements gives an activation energy of about 1.6 eV for the lormation of arsenic precipitates. Our results are discussed taking regard of the role electrically active point defects during the prccipitat rig process.

In recent years. it has been demonstrated that MBE GaAs grown at low temperatures, such as 200°C (called LT GaAs) has unique properties and can lead to new and improved devices [1,21. LT GaAs materials have been shown to be very nonstoichiometric with up to I at% excess As and a large amount of ~ in the order of 10is_10~a 3 [31.Annealing at 600°Cchanged the resiscm tance from a low resistivity state to a high resistivity state and caused the formation of As precipitates [41.It has been shown that the final precipitate sizes and densities depend upon the temperature and duration of the anneal [5—71. It was

Corresponding author. Present address: Department of Electronic and Electrical

explained that arsenic precipitates act as internal Schottky barriers to control the electrical and optical properties of annealed LT GaAs [8.9]. Therefore, it is very useful to explore the As precipitate size and density and understand its nucleation and growth mechanism by the precipitateThe coarsening thermal process. stability of LT GaAs is an attractive subject too, because LT GaAs materials have to undergo heat treatments necessary’ for the growth of active layers at higher temperature, or the activation processes of ion-implanted atoms at more than 800°C. In this letter, we report the thermal stability of LT GaAs and attempt to better understand the role of electrically active point defects during the formation process of arsenic precipitates.

Engineering, Guildford. Surrey GU2 SXII. UK. 0022-0248/94/$07.0() 1994 Elsevier Science B.V. All rights reserved SSDJ 0022-0245(94 )t)0421 -I-I

Undoped semi-insulating GaAs substrates with

T WI Fan et al. /Journal of Crystal Growth 143 (1994) 354—358

a (100) axial orientation were used for the molecular beam epitaxy (MBE) growth. First, the surface was etched in a solution of 5 :1:1 (H2S04 H202 : H20) for 1 mm, followed by a deionized water rinse. The samples were then indiumbonded to molybdenum block. Before growth, the substrate was outgassed for 1 h in the chamber of the MBE system at 300°C. A GaAs buffer layer was first grown at 580°C and then the substrate temperature was lowered to 250°Cwhile continuing to grow a 2 ~im thick GaAs layer. The ternperature was measured by a calibrated substrate thermocouple. The growth rate was 1 tim/h under arsenic-rich conditions with an As4/Ga flux ratio of 20. The sample was cut into pieces and each piece was isothermally annealed at 450, 520, 600, 700 and 850°C,for 10 or 30 mm. In order to avoid arsenic out-diffusion during annealing, a

~

355

SI-GaAs wafer was employed to cover the sample surface. Plan-view samples for transmission electron microscopy (TEM) were prepared by mechnical thinning down to 30 j.tm, followed by ion milling at liquid nitrogen temperature from the back side. The influence of slight heating during the preparation upon the microstructure of the sample can be neglected. TEM experiments were carried out on Hitachi-9000 and JEM2O1O electron microscopes. The measurement of electrical resistivity was made using a standard high-impedance Hall-effect system with samples in the Van der Pauw geometry. TEM examinations on the samples annealed from 450 to 850°Cshow that there are no detectable arsenic precipitates until the annealing temperature rises to 600°C.Fig. 1 is a set of TEM bright field images of the samples annealed un-

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p

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Fig. I. PIan-vie~TEM bright images of LT GaAs samples annealed under different conditions, with the same reflection vector g220: (a) 600°,10 mm; (b) 700°C,10 mm; (c) 850°C,10 mm; (d) 850°C,30 mm.

356

‘I. IL Fan et al. /Journal of (‘rystal Growth /43 (/994) 354—358

der the specified conditions with g~70excitation. As seen from Figs. Ia to Ic, the sizes and the concentrations of the arsenic precipitates change dramatically with a rise in the annealing temperature. The structural characteristics of the arsenic precipitates are also strongly dependent on annealing conditions. In Fig. la, the precipitates were imaged with double-arc contrast having a line of zero contrast perpendicular to the reflection vector g. The strain field of the arsenic precipitates was identified as spherical because the line of zero contrast always keeps a direction normal to the g-vector for more than two reflection g-vectors when the sample was tilted from B [001] to B [1141 during the observation by TEM. When the annealing temperature was increased to 700°C, moire fringes were formed on all of the precipitates. Fig. ic shows the arsenic precipitates formed after an 850°C, 10 mm annealing; only a few moire fringes on the precipitates are seen (indicated by arrow). The interfaces between precipitate and matrix are becoming less clear. Therefore, it is believed that the high temperature annealing at 850°C has produced important effects on the crystallographic characteristics of the arsenic precipitates. It is well known that moire patterns are produced =

=

when two crystals which have differences in lattice parameter or orientation overlap. In our case. the presence or absence of moire fringes iridicates that there is or is not coherence between the As precipitates and the GaAs matrix. Therefore, it is probable that the disappearance of the moire fringes implies that As precipitates have changed their crystallographic characteristics after high temperature treatments. Fig. Id is an image of arsenic precipitates (shown by arrow) formed after an anneal at 850°Cfor 30 mm. From a comparison between these images, it has been found that (1) there are less moire fringes on the As precipitates in Fig. lc, (2) the precipitates in Fig. Id have a similar size, hut lower density, and (3) after annealing for a longer period, the smaller precipitates disappear hut the larger precipitates remain. The average sizes and densities of the arsenic precipitates were obtained from TEM micrographs taken from several regions of each sample. Assuming the precipitates being spherical, the volume fraction of them was calculated (Table I). from which three points can be drawn: (1) When the annealing temperature increases, the average size of the precipitates increases while the density decreases.

Fig. 2. High resolution image of an arsenic precipitate (shown by arrow) in an annealed (850°C’for 30 mm) LTC1aAs sample. The resolved plane is ((1(11) of GaAs crystal.

T W. Fan et a!. /Journal of Crystal Growth 143 (1994) 354—358

(2) Within experimental accuracy, there is no dif-

ference observed in the volume of the arsenic which has precipitated in the samples annealed at different temperatures. (3) However, after annealing at 850°Cfor 30 mm, a reduction of the volume fraction of the arsenic precipitates is found. This demonstrates that the annealing temperature of 850°Cis high enough to remelt the arsenic precipitates and for some fraction to be redissolved into the GaAs matrix. Fig. 2 shows a high-resolution electron microscopy (HREM) image taken from the sample annealed at 850°cfor 30 mm. The resolved plan is (001). It reveals that the arsenic precipitates are becoming amorphous, particularly in the middle of the precipitate. It seems that annealing for a longer duration at 850°C,which is higher than the melting potnt of arsentc, 810 C [101, results in a remelting and redissolution of the arsenic precipitates. Our results demonstrate that the formation and the coarsening of arsenic precipitates in LT GaAs are extremely sensitive to the thermal process. On the other hand, it is not excluded that the excess As can dissolve directly into matrix lattice or diffuse and evaporate from the surface, which is not avoided completely at higher temperature and the annealing environment we used. It is well documented that LT GaAs is As rich, The expected points defects are ASGa, As1 and VGS. The concentration of LT A5Ga is as highis asa 3 in as-grown GaAs. There 3smaller X 1020 concentration cm (1018 cm3) with the ar-

senic precipitates in annealed LT GaAs at 550— 600°C.Hence, it is important, not only for basic studies but also for practical needs in device

Table I The effect of annealling conditions on the average size, density and volume fraction of arsenic precipitates in annealed low temperature GaAs Annealing conditions 600°C,10 mm

Average size oo (A)

Density 3) (cm 1.7x

700°C,tO mm 850°C,10mm 850°C,30 mm

ioo

3.7X 10~

290 270

1.6X10’ 5.9 x 1014

10i7

Volume fraction (%) 1.9 1.9 2.0 0.6

357

1E+007 1E4-006

1o~om ioooo 1000 100

-

10 1

o.i

‘

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 1000/[T(K)I [Annealtemperature I Fig. 3. The dependence of the resistivity on anneal temperature within the range of 250—600°C;annealing time 15 mm.

fabrication, to understand which kind of point defect plays a role in the precipitating process and how. In order to examine the active species during the heating, isochronal annealing studies were carried out in the range of 250 to 600°C. The dependence of the resistivity on the annealing temperature is plotted in Fig. 3. According to our measurements, the resistivity p can be cxpressed as i/p (i/po) exp(—z.IE/kT), where k is Boltzmann’s constant and T is the annealing =

temperature. It shows that the resistivity increases rapidly as the temperature is increased. The slope corresponds to an activation energy ~~1E 1.6 eV. It was suggested by the mode of hopping mechanism via defect [11] that the increase of the resistivity of LT GaAs during the heat treatment has been attributed to the reduction of the concentration of As 05-related defects. It seems that the formation of As precipitates, which happens at the same time, is a results of the loss of ASGa. Therefore, it is reasonable that this activation energy for the change of resistivity can be assumed to be the formation energy of arsenic precipitates. Bliss [121 proposed VGa-assisted diffusion of A5Gaet asala possible mech=

anism of the formation of arsenic precipitates and reported an ASGa removal activation energy of 1.7 ±0.3 eV obtained from isothermal anneal.

.

.

.

ing. This value is in good agreement with that

358

11W Fan et al. /Jour,mal of Crystal Growth 143 (1994) 354—358

which we obtained from isochronal annealing, supporting the mechanism of V~~-assisted diffusion of ASGr From these and other results documented in the literature, we concluded that the increase in the resistivity of LT GaAs during the annealing process is closely related to the formation of arsenic precipitates and that the loss of As~~.1 leads directly to the formation of As precipitates. In summary, we have examined the thermal stability of LT MBE GaAs samples annealed by isothermal and isochronal processes by TEM and Hall measurements. TEM results indicate that the formation, the coarsening and the redissolving of the arsenic precipitates in annealed LT GaAs samples are highly sensitive to the thermal process. The lower thermal stability is probably due to the lower formation energy ~E of arsenic precipitates, which is estimated to be 1.6 eV from Hall measurements of isochronally annealed sampIes. Our results support the model of V6a-assisted diffusion of ASGa to arsenic precipitates and the fact that further studies regarding the role of A5~a in the formation mechanism of arsenic precipitates are necessary. The authors are grateful to Mr. Y.Q. Zhou of the Institute of Physics, Chinese Academy of

Sciences, for his experimental assistance in TEM observation. References [I] F.W. Smith. A.R. Calawa. CL. Chen. Mi. Manira and L.J. Mahoney, IEEE Electron Device Lett. EDL-9(1988) 77. [2] J.F Lin, C.P. Kocot. DL Mars and R Jaeger. IEEE Trans. Electron Devices ED-37 (1990) 45. 131 M. Kaminska. Z.L. Weher. E.R. Weher. J. George and J.B. Kortright, F.W. Smith. B.Y. Tsaur and AR. (‘al:msv:m. AppI. Phys. Lett. 54(1989)1881. [4] AC. Warren, J.M. Woodall. i.E. Freeouf. D. Grisehkowski, D.T. Mclntarff, M.R. Melloch and N. t)tsuka, AppI. Phys. Leit. 57(199(1)1331. [51 K. Mahalingam, N. Otsuka, M.R. Melloch. J.M. Woodall and A.C. Warren, J. Vac. Scm. Technol. B 9 (1991) 2328. [6] Z.L. Weber, Mater. Res. Soc. Symp. Proc. 241 (1992) 101 [7] Z.L. Weher. G. Cooper. R. Mariella. Jr and C. Kocot. Vac. Sci. Technol. B 9(1991)2323. [81 R.E. Viturro, MR. Melloch and J.M. Woodall. AppI. Phys. Lett. 60 (1992) 3007. [9] D.D. Nolte. R.M. Bruhaker. M.R. Melloch. J.M. Wood,mll and S.J. Ralph. AppI. Phys. Lett. 61(1992) 3))98. [10] K.L. Kavanagh, J C~ P Chang P D. Kirchner. A C Warren and J.M. Woodall. AppI. Phys. Lett. 62(1993) 286. [11] D.C. Look, D.C. Walters. MO. Mansreh, J.R. Sizelove, CE. Stutz and K.R. Evans. Phys. Rev. B 42 (199))) 3578. [12] D.E. Bliss, W. Walukiewmcz. J.W. Ager Ill. E.E. haIler. K.T. C’han and S. Tanigawa. 1. AppI. Phys. 71 (1992) 1699.