The influence of phosphorus, arsenic and antimony vapour ambients on the diffusion of zinc into gallium arsenide

ELSEVIER

Materials Chemistry

and Physics 42 (1995)

68-71

The influence of phosphorus, arsenic and antimony vapour ambients on the diffusion of zinc into gallium arsenide G. Bijsker a, H.-G. Hettwer a, A. Rucki b, N.A. Stolwijk a, H. Mehrer a, W. Jtiger b, K. Urban b aInstitutfi’ir b Institutfiir

Metallforschung,

Festkdrperforschug,

Universitiit Mtinster, D-48149 Forschungszentrum

Miinster, Germany

Jiilich, D-52425

Jiilich, Germany

Received 23 March 1995; revised 21 April 1995; accepted 27 April 1995

Abstract Diffusion of Zn at high concentrations into GaAs under As-deficient ambient conditions causes generation of crystal defects like dislocation loops, elongated dislocations and Ga precipitates decorated with voids throughout the diffusion zone. Similar treatments under As vapour lead to recovery from diffusion-induced damage in a region underneath the GaAs surface. This feature is accompanied by the appearance of two distinct steps in the Zn concentration profile. Previous experiments suggested these phenomena to be connected with out-diffusion of Ga from the precipitates towards the surface. The present work shows that the replacement of As by P or Sb in the diffusion environment produces similar recovery effects. This observation provides evidence that ambient Group V elements render near-surface GaAs to an effective sink for diffusion-induced excess Ga. Keywords:

Diffusion

profiles; Self-interstitials;

Kick-out mechanism;

Precipitates;

1. Introduction Beside technological applications, diffusion of Zn into GaAs may be used to gain information about native point

defects of the III-V semiconductor host. This relies on the property that fast migrating Zn interstitials change over to Ga-lattice sites ( Ga,aa) in order to attain the solubility limit of the preferred immobile ZnGa configuration. Previous investigations [l-4] have provided evidence for this changeover to proceed through the kick-out reaction: Zn,+Gao,,

+

Zno,+Ga,

(1)

generating excess self-interstitials Gq. Provision of a Zn diffusion source with sufficient thermodynamic activity promotes effective supply of Zn, which, in conjunction with the comparatively low transport capability of Ga, and a low density of appropriate sinks, may lead to Ga, supersaturation [ I]. Earlier experiments of the Jiilich-Mtinster collaboration [ 3,4] show that such supersaturations do not commonly persist but rather decay through agglomeration processes, resulting in the formation of dislocation loops and Ga precipitates at the diffusion front. These extended defects are subject to changes in shape, distribution and even character as Zn diffusion progresses, leading to a complicated defect structure in the diffusion region as revealed by transmission electron microscopy (TEM) [ 3,4]. The present paper focuses on the 0254-0584/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIO254-0584(95)01560-H

Voids; Point-defect

sinks

temporal evolution of one early formed, major defect type, i.e. the Ga precipitate, under various diffusion environments as well as on its connection with the shape of the Zn diffusion profile. First we recall earlier results described in detail in Refs. [ 3-51 referring to diffusion ambients containing either only Zn or both Zn and As. Then we present new data arising from similar experiments with As replaced by either P or Sb.

2. Experimental As sample material, semi-insulating GaAs with a dislocation density less than or equal to 10’ cm-* was used. This may be considered as virtually dislocation-free, since Zn penetration depths are smaller or comparable to the mean dislocation spacing. All diffusion treatments were performed in sealed quartz ampoules with about 8 cm3 inner volume inserted into a furnace resistance-heated at 906 “C. About 5 mg elemental Zn was enclosed as diffusion source. In pertinent cases the ampoule also included in elemental form either 12.4 mg As or equivalent mole fractions of P or Sb. These added amounts determine the GaAs thermal equilibrium state, the solubility of Zn and its diffusion behaviour [ 4,6,7]. Thermal equilibrium with respect to the gas-containing ambient is established first in a near-surface region. The depth of this region increases with diffusion time.

G. Biisker et al. /Materials

Chemistry and Physics 42 (1995) 68-71

Concentration-depth profiles were measured by spreading-resistance analysis (SRA) and electron microprobe analysis (EMPA) [ 81. The latter monitors the total Zn concentration, i.e. Zni, ZnGa and Zn incorporated in precipitates, whereas SRA is mainly sensitive to the predominant electrically active species ZnGa. Therefore, in sample regions where Zn precipitation has occurred or where electrical compensation plays a role (Zni+, Gq2+ versus Zn& [l] ) the SRA signal may fall below the EMPA signal. Zn-diffusion-induced defects were characterized in ( 110) cross-section samples by TEM in a JEOL 4000FX microscope at 400 kV. The composition of precipitates (diameter less than or equal to 50 nm) was determined by energydispersive X-ray spectroscopy (EDX) at 200 kV.

3. Results Experimental data emerging from our different diffusion environments are diplayed in Fig. 1. These comprise both SRA and EMPA profiles (left) as well as the corresponding precipitate/void morphology (right). As a common property, a sharp decrease of the Zn concentration at the diffusion front is shown both by SRA and EMPA. TEM bright-field images reveal beside dislocations (not discussed here) various other diffusion-induced structural defects (I, I”, II, III, see below). Their appearance turns out to be correlated with diffusion ambient and with distinct regions of the Zn penetration profile as indicated in Fig. 1. The bulk region beyond the diffusion front is virtually defect-free. The diffusioninduced defects under consideration are: (I) Ga precipitates with or without adjacent void volume, the latter type preferentially appearing closer to the Zn diffusion front. The precipitate/void agglomerates are mostly polyhedral in shape and may contain minor fractions of Zn dissolved in Ga. Close to the surface larger precipitate/void agglomerates are present in lower volume density (I’). These large precipitates which are only found after diffusion in pure Zn atmosphere contain substantial fractions of Zn. (II) Voids of polyhedral or tetrahedral shape without precipitate portion. The latter shape is found closer to the surface. (III) A polycrystalline zone containing P which extends from the surface to a depth of about 15 pm. This zone is formed when Zn diffusion proceeds under P-rich conditions. For the As- and Sb-rich conditions other morphologies are observed (see below). Notably, changes in the defect morphology when moving back from the diffusion front toward the surface reflect the temporal evolution upon progressing in-diffusion of Zn. The specific features of each experimental result will be pointed out in the following. Zn-only case: Exposing GaAs exclusively to vaporized Zn produces one-stage profiles like that in Fig. 1 (a). Ga precipitates without void portion, lo-30 nm in size, are observed near the diffusion front. Going back to the surface, the precipitate size gradually increases and a void fraction starts to

69

develop. Near the surface the precipitate/void agglomerates attain sizes of about 100 nm or even larger while their density decreases. Accordingly, the total precipitate/void volume remains approximately the same, as does the precipitate-tovoid-volume ratio. A 5 pm deep zone right underneath the surface is entirely depleted of precipitate/void agglomerates. Zn +As case: Addition of As to the diffusion atmosphere leads to Zn distributions with two distinct steps, as seen in Fig. 1 (b) . Compared to the case without As (Fig. 1 (a) ) , the maximum penetration depth has decreased from about 155 pm to roughly 90 pm, even though the diffusion duration was about five times longer. Possible reasons for this are discussed in Ref. [4]. In the profile front region, the defect structure closely resembles that in the front region of the Znonly case. However, in a narrow depth region located near the kink of the Zn profile(s) the precipitate portions of the void/precipitate agglomerates disappear, and the void fractions increase to 100%. Size and distribution of Ga precipitates (with small void portion) beyond the kink position and voids (without precipitate portion) below it are very similar to each other. This supports the view that Ga depletion of the precipitates has occurred. Close to the surface, voids are mainly of tetrahedral shape and their density decreases. A narrow zone right below the surface is free of voids. Details about the (equilibrium) shape of Ga precipitates and voids in Zn-diffused GaAs can be found in Ref. [ 51. On top of original sample surface areas not deteriorated by liquid phase formation epitaxial GaAs layers are formed. Comparing the data of both experiments gives rise to the idea that out-diffusion of Ga has been much more efficient in the As-rich ambient case than in the Zn-only case. This raises the question, how the potential ability of the GaAs surface region to act as sink for Ga atoms can be affected by the diffusion environment. In this respect it is noteworthy that the amount of Ga originally contained inside the voids in regime II of Fig. 1 (b) corresponds to roughly 10” atoms/ cm2 or 100 monolayers. To examine the role of ambient and surface further we performed Zn diffusion experiments under atmospheres containing either P or Sb. Temperature (906 “C) and time (435 min) of diffusion were chosen equal to the As-rich case. Zn + P case: The datadisplayed in Fig. 1 (d) originate from Zn diffusion under P-rich ambient. As indicated by SRA and EMPA, the two-stage Zn distribution has a near-surface stage which is about 20 pm wider than in the corresponding Asrich case (Fig. 1 (b)). TEM investigations show that this is related to the presence of a P-rich zone extending from the surface to a depth of about 15 pm. As illustrated in diagram III of Fig. 1, this zone consists of crystallites with typical dimensions of 5-10 pm. Fig. 1 (d) also shows the P profile as measured by EMPA. At zero depth the P concentration is estimated to be roughly 50% (omitting corrections for Pinduced changes of the mass density). EDX analysis shows that the decrease of the P concentration with penetration depth goes along with an increase of the As concentration. This points to substantial replacement of As by P. The defect

G. Eijsker

70

et al. /Materials

Chemistry and Physics 42 (1995) 68-71

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xbml IO**

cj-

02’

1 E % 1

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020

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Fig. 1. Experimental results from Zn diffusion into GaAs at 906 “C. Left: Zn concentration-depth profiles as measured by SRA (crosses) and EMPA (open squares) associated with four different ambient conditions indicated by (a)-(d). Added ambient elements and diffusion times are as foIlows: (a) no element beside Zn, 90 min; (b) Zn + As, 435 min; (c) Zn + Sb, 435 min; (d) Zn + P, 435 min. this diagram also contains an EMPA phosphorous profile (closed triangles). Right: TEM bright-tieId micrographs of microstructures connected with distinct regions in the Zn diffusion profiles. (I) Ga precipitates without or with adjacent void volume, the latter type appearing closer to the Zn diffusion front. (I’) Zn-rich precipitates near the surface. (II) Voids of polyhedral or tetrahedral shape without precipitate portion. The latter shape is found closer to the surface (III) A polycrystalline zone containing P.

G. Biisker et al. /Materials Chemistry and Physics 42 (1995) 68-71

morphology beyond the P-rich polycrystalline zone resembles that of the As-ambient case. Again, initially formed Ga precipitates become depleted near the transition from the deep-penetration to the near-surface profile stage. Zn+Sb case: Similar treatment of GaAs with Sb in the diffusion ambient yields the data plotted in Fig. 1 (c) . Also here the Zn profile has two-stage character but now the deeppenetration stage is wider than the near-surface stage, in contrast to the As- or P-ambient case. Remarkably, SRA and EMPA coincide throughout the diffusion zone. This points to predominant incorporation of Zn on Ga sublattice sites and absence of electrical compensation. These features may be related to the comparatively low boundary concentration of Zn which is probably due to Zn dissolution in molten Sb at the diffusion temperature. The precipitate/void structure reveals the same salient features as already described. In particular, we mention the correlation between the appearance of various defect types and characteristic locations in the Zn profile. Unlike the P-ambient case, large parts of the GaAs surface region were not substantially affected by Sb, consistent with the absence of Sb-related signals in the EMPA and EDX analysis (Sb detection limit: 5 X 10” cmd3).

71

in the P-ambient case there is evidence for recrystallization of near-surface GaAs to GaP,As,, with x values up to about 0.5. In Sb-rich ambients, the amount of Ga associated with depleted precipitates is relatively small (cf. width of regime II in Fig. 1 (b)-(d) ) . This feature combined with the low vapour pressure of Sb at 906 “C compared to that of P and As might explain why Sb could not be detected near wellretained GaAs surface areas. An alternative explanation based on a direct coupling between the in-diffusion of Group V elements and that of Zn appears to be very unlikely. This conclusion relies mainly on our EMPA depth profilings of P and Sb since diffusion data of Group V elements in highly p-doped GaAs are lacking in the literature. In Fig. 1 (d) the penetration range of P does not correlate with the near-surface profile stage of Zn. In the Sb-ambient case, it seems difficult to attribute substantial influence to Sb concentrations in GaAs below the detection limit. In addition, the diffusivity of this comparatively heavy Group V element is expected to be much smaller than that of P. Similarly, in As-rich environments, the estimated As penetration does not exceed a distance of about 5 pm.

5. Conclusions 4. Discussion The above findings can be rationalized as follows. In GaAs exclusively exposed to Zn vapour, the crystal surface area represents a less effective sink for Ga atoms generated in excess by in-diffusing Zn. Therefore, the precipitates initially formed at the diffusion front are essentially retained as diffusion proceeds. On the other hand, adding Group V elements (X = P, As, Sb) to the diffusion ambient allows for the reaction: 1 G& + - Xi n

Gao,XA,

(2)

where Xfi denotes the predominant X cluster in the gas phase. For P and As n = 4, while for Sb it = 1 applies. Reaction (2) may proceed directly at the GaAs surface or in the X-penetrated volume below the surface as net effect of the two following intermediate reactions: ix:n

x,,+v,,

Vci, + Ga, -

G%,

(3) (4)

where VGa stands for vacant sites on the Ga sublattice. If enough reaction partners (X) are available, then excess Ga can be effectively annihilated near the surface. This results in depletion of the Ga precipitates through out-diffusion of G% which dominates over VGa as a diffusion vehicle in ptype GaAs [ 1,9]. This view is also supported by the observation of epitaxial GaAs in the As-ambient case. Similarly,

The present experiments reveal how Zn diffusion into GaAs depends on the prevailing ambient conditions. Although this has been recognized earlier with respect to surface degradation and point-defect equilibrium, the present data underline the role of the GaAs surface region for the annihilation of diffusion-induced excess Ga.

Acknowledgements We acknowledge B. Lentfort for helpful cooperation concerning EMPA and the Freiberger Elektronikwerkstoffe GmbH for the donation of GaAs wafers.

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