Structural characterization of heavily Zn-doped liquid encapsulated Czochralski InP

I~IAIVJIIIJUlS

ELS EVIER

Materials Science and Engineering B28 (1994) 120-125

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Structural characterization of heavily Zn-doped liquid encapsulated Czochralski InP C. Frigeri a, C. Ferrari a, R. Fornari a, J.L. Weyher a, L, F. Longo a, G.M. G u a d a l u p i b ~CNR-MASPEC Institute, Via Chiavari 18/A, 43100Parma, Italy b TEMA V, Via delle Industrie 39, 30175 Porto Marghera, Italy

Abstract A structural characterization of liquid encapsulated Czochralski InP heavily doped with Z n is presented. At a hole density as high as 3.0 x l()~s cm-3, corresponding to a Zn content of 1()~" atoms cm-~, the crystals are dislocation-free. They contain, however, a high density (ca. 7 x I0 ~ cm -3) of precipitates identified as Zn3P2 by electron diffraction. This supports the model in which Zn in excess of that occupying In sites as electrically active acceptor can react with the group V element to form precipitates. Other possible lattice locations of the excess Zn cannot be checked by our techniques. The Zn3P2 precipitates tend to disappear for a hole concentration of 2.6 x l 0 ~ cm 3, but dislocations are generated since the hardening effect associated with dopant atoms decreases. The majority of the dislocations have climbed, leaving behind a local high density of microdefects. The possible mechanisms for the generation of these microdefects are discussed.

Ke),words: Indium phosphide; Defect formation; Electron microscopy; Doping effects

1. Introduction p-Type doping of both bulk and epitaxial InP and GaAs is commonly achieved by adding zinc. The doping level is usually well in excess of 1 x 10 is cm 3 because p-type material is mostly employed for optoelectronic applications. Much has been done in the field of Zn-diffused InP for studying both the process of formation of p - n junctions and the basic mechanisms of the interaction of Zn with the host InP matrix (for example [1-8]). As regards bulk p-type InP, the necessity for high doping combines positively with the requirement for producing dislocation-free crystals since a high dopant density causes solution hardening of the lattice, thus preventing the generation of dislocations. The possibility of obtaining dislocation-free InP by this route was demonstrated long ago [9, 10]. However, it has been reported that high doping of Zn-diffused InP produces

t Present address: IFF, Forschungszentrum Jtilich, 52428 Jiilich, Germany. 0921-5107/94/$7.00 © 1994 - Elsevier Science S.A. All rights reserved SSDI 0921-5107(94)00804-3

precipitates, generally of the Z n 3 P 2 type [2, 3, 6]. In bulk liquid encapsulated Czochralski (LEC) InP, precipitates have been detected by transmission electron microscopy (TEM) [10] and by synchrotron X-ray topography [11], but not identified. A detailed analysis of precipitates of Z n - A s phases present in heavily Zndoped LEC GaAs has been published recently [12]. We present here a study of the nature of precipitates and of the transition from dislocation-free to dislocated crystals in heavily Zn-doped LEC InP.

2. Experimental details The investigated 2 in wafers were cut from the top part of two (100)-oriented InP crystals grown by the LEC method and Zn-doped by adding elemental zinc. The electrical properties were assessed by the Hall effect in the Van der Pauw configuration. The structural characterization was carried out by chemical etching, cathodoluminescence (CL), TEM and doublecrystal X-ray diffraction topography (DCT). Two types of chemical etching were used, namely photoetching

C. Frigeriet al. / MaterialsScience and Engineering B28 (1994) 120-125 (DSL method) [13] and B C A etching based on the HBr-K2Cr207-H20 system [14]. For photoetching the $1/2 composition for 10 min was used (notation $1/2 after [13]). For CL observations in a scanning electron microscope, a Si or Ge solid-state detector in the reflection geometry was employed. TEM investigations in the diffraction contrast mode were performed on plan-view specimens thinned to electron transparency by bombardment with Ar + ion beams at liquid nitrogen temperature. For DCT the Cu Ka~ wavelength and the (533) Bragg reflection were used.

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nm. An example of precipitate is shown in Fig. 3(a). TEM confirmed the absence of dislocations, at least within the detectability limit of the technique (ca. 5 × 104 cm-2). By means of electron diffraction patterns, such as that shown in Fig. 3(b), some lattice spacings of the precipitates could be measured and were found to correspond to those of Zn3P2, which belongs to the tetragonal system and has lattice constants a = 0.8113 nm and c = 1.147 nm, the space group being P 42/mmc [15]. To analyse Fig. 3(b) one has to consider that the larger bright diffraction spots are due to the InP matrix whereas some of the small ones, such as those labelled

3. Results

The mean values of the electrical parameters measured by the Hall effect are shown in Table 1. 3.1. Crystal l ( p = 3 . 0 × lOm cm -~) The most heavily doped sample (crystal 1) exhibited a very high density of shallow pits (S-pits), homogeneously distributed all over the wafer surface, after being submitted to chemical etching. Fig. 1 shows the surface of a wafer etched with the BCA 101 solution [14] for 1 min. The S-pits have no clear crystallographic symmetry and have to be ascribed to microdefects. No characteristic dislocation-related etch pits were detected. These results were also confirmed by photoetching. No dislocations were detected by DCT either. In only one wafer were some dislocations arranged in a few slip lines along (110) directions at the very wafer rim detected. However, inspection of the DCT images at high magnification always showed a spotty contrast that suggests the presence of a high density of microdefects (Fig. 2). DCT also clearly detected circular growth striations around the growth axis all over the wafer, indicating that the liquid-solid interface remained convex during growth. T E M observations showed that the crystal contains homogeneously distributed precipitates in a density as high as ca. 9 x 105 cm -2 or ca. 7 x 109 cm -3 (error ca. 50%, mostly due to uncertainty in the TEM specimen thickness). The size of the precipitates is ca. 150-200

Fig. 1. Crystal 1 (p = 3.0 x 10TM cm-3): etch pits on a wafer submitted to BCA 101 etching: Bar = 25 ~tm.

Table 1 Electrical parameters of the Zn-doped InP crystals Electrical parameter

Crystal l

Crystal 2

p (cm -3) /~ (cm2V-I s-~) p(g2 cm)

3.0 x 10 TM 47.8 4.22x 10 -~

2.6 x 1018 48.5 4.90x 10 -2

Fig. 2. Crystal 1 (p = 3.0 x 10~ cm--~): high-magnification print of a DCT image. Bar = 200 p.m.

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Fig. 4. Crystal 1 ( p = 3 . 0 x 10 js cm 3): cathodoluminescence image showing striations but no dislocations.

the photons emitted by the irradiated sample. It should be noted that two other factors can reduce the luminescence of the crystal, namely (i) the overall luminescence of p-type material is always lower than that of n-type material and (ii) similarly to the n-type case [16], the luminescence efficiency should further decrease significantly with increasing doping level when p-> 1 x 1018 cm-3 [10]. 3.2. Crystal 2 (p= 2.6 x 1Ca cm -~)

Fig. 3. Crystal 1 (p = 3.0 x 1() ~s cm-~): (a) transmission electron micrograph of a precipitate, g = [022], bar = 10(l nm: (b) electron diffraction pattern from the precipitate shown in (a).

p, are due to the precipitate and some others, such as those labelled d, are due to double diffraction, i.e. diffraction by the precipitate of the beams already diffracted by the InP matrix. By taking into consideration double diffraction, all the diffraction spots in Fig. 3(b) can be accounted for. only growth striations and no dislocations were detected by CL (Fig. 4). The luminescence efficiency of the crystal was very low, as the CL signal was only barely detectable even when the maximum beam energy was used. Such a low efficiency can certainly be ascribed to the presence of the precipitates that are expected to act as non-radiative centres and to absorb

The less-doped InP crystal (p = 2.6 x 10 ~s cm 3) is no longer dislocation-free. Only the centre part of the wafers is dislocation-free, as shown by the DCT map in Fig. 5. High-magnification inspection of the X-ray maps in the rim area shows that the dislocations exhibit an anomalous contrast, very likely because of decoration by some microdefects. Moreover, the contrast of several dislocations is elongated in the slip direction, suggesting movement of the dislocations themselves. Similar results were obtained by chemical etching. Fig. 6(a) shows the centre part of a wafer submitted to BCA 201 [14] for 30 s. It can be seen that the same type of etch pits as detected in crystal 1 are also present in crystal 2, although in a smaller density, suggesting the presence of precipitates. However, the wafer rim photoetching, and also BCA, produced etch features typical of dislocations, the majority of which had associated S-pits, probably due to microdefects (Fig. 6(b)). At the periphery isolated etch pits related to precipitates only were also detected. In the centre of the wafer, TEM detected Zn3P2 precipitates in a density of nearly one third of that measured in crystal 1. Very few Zn3P2 precipitates were also seen in the rim area, very often close to disloca-

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Fig. 5. Crystal 2 (p = 2.6 x 101~cm- 2): X-ray DCT of a quater of a wafer. S is a scratch produced while handling the wafer. Bar = 5 mm.

tions (Fig. 7(a)). The periphery region, however, is characterized by the presence of dislocations with a density in the range ca. 5 x 104-10 x 104 cm -2. In the majority of cases the dislocations were bent or formed helices with microdefects nearby (Fig. 7(a) and (b)). The helix or bent configuration of the dislocations suggests that they have moved by climbing. The microdefects were only seen in close spatial correlation to dislocations and confined to the regions through which the climbing dislocation appears to have passed. The microdefects exhibit double-arc contrast and a socalled line of no-contrast (Fig. 7(b)). For the smallest strain centres the line of no contrast was always perpendicular to the g vector, whichever g was used. This is typical of small spherical precipitates [17]. In contrast, the largest strain centres have a loop-like behaviour as their "line of no contrast" does not change direction on changing g and show inside-outside contrast by changing + g to - g . Analysis by the method of Dahmen [18] showed that they are of vacancy type.

4. Discussion

The Zn3P2 nature of the precipitates is in agreement with similar findings of previous workers [2, 3, 6] who studied precipitates in heavily doped Zn-diffused InP.

Fig. 6. Crystal 2 (p = 2.6 x 10 TM cm-3): (a) etch pits produced by BCA 201 etching in the centre of the wafer; (b) etch features produced by DSL photoetching in the rim region of the wafer. Bars = 25 ~tm.

More generally, our results also agree with the findings of Schlossmacher et al. [12], who identified as As2Zn 3 and As2Zn the precipitates present in heavily Zndoped GaAs. In fact, all this set of results, except those of Chan et al. [2], who also reported the presence of Z n precipitates besides Zn3P2, show that Z n prefers to precipitate compounded with the group V element of the hosting matrix rather than as elemental Zn. Moreover, it seems that the Zn-related precipitates form independently of the method used to introduce the dopant atoms, i.e. either atomic diffusion or incorporation from the melt. This suggests that precipitation takes place during the cooling of the L E C crystal.

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It has been reported that in InP a hole concentration p greater than ca. 3 x 10IS-5 x 10 is cm -3 cannot be achieved whatever the amount of Zn added to InP [1,2]. This occurs also in our case as the amount of Zn contained in the more heavily doped crystal (crystal 1, p = 3 x 10 ~s c m -3) is 500 ppm, as measured by glow discharge mass spectrometry, corresponding to ca. 1019 atoms c m -3. The massive presence of Zn3P: pre-

d

Fig. 7. Crystal 2 (p=2.6x 10 TM cm-3): transmission electron micrographs (g = [022]) from the rim region of the crystal of (a) Zn3P 2 precipitate and a dislocation with microdefects in the vicinity, bar = 0.5 ixm; and (b) dislocation helix with associated microdefects, bar = 0.2 ~tm.

cipitates in crystal 1 ( p = 3 x l 0 js cm ;). is clear evidence that the Zn atoms in excess of ca. 3 x 10 ~s cm -~ can form precipitates. The destination of the excess Zn is widely debated in the literature and models other than the formation of precipitates, namely (i) Zn atoms forming neutral complexes with phosphorus vacancies, Vv, (ii) Zn atoms in interstitial sites forming compensating donors and (iii) all Zn atoms acting as acceptors but being (partially) compensated by native defects, have also been suggested [2]. By our techniques it is not possible to check these models. It should be noted that according to Chan et al. [2], the upper density of 3 x 10 is cm -3 for the electrically active Zn as an acceptor is not limited by the low solubility of Zn but by the fact that accommodation of Zn is energetically more stable in interstitial sites rather than in substitutional sites when the hole concentration is very high. As the hole density decreases, Zn is substitutional sites is more favoured and the density of Zn interstitial decreases, thus reducing the probability of precipitate formation [2]. This would explain the onset of dislocation generation in crystal 2. The hardening effect, in fact, is expected to decrease as more Zn occupies substitutional sites, thus reducing the lattice parameter. The strengthening of the matrix due to the Zn3P2 precipitates also decreases by reducing their density [10]. Moreover, the locking stress exerted by the dopant impurities on the dislocations also decreases as the Zn concentration decreases and dislocations are more easily introduced [19]. The dislocations first appear at the periphery of the crystal because the thermal stresses are higher there. A possible model for the generation of microprecipitates and loop-like defects by climbing dislocations might be envisaged as follows. Let us assume that Zn is incorporated in the host InP lattice by the kick-out mechanism rather than the dissociative mechanism [20]. In i interstitials are therefore produced [20]. In i could promote dislocation climbing. As InP has the zinc blende structure, Pi are also necessary for the climbing. They could be produced directly at the core of the climbing dislocation by the Petroff-Kimerling (P-K) mechanism, provided that Vp are released into the lattice [21]. Such an excess of V v can then condense to form vacancy-type dislocation loops. For this to occur V~n are necessary. They could be generated at the core of the dislocation loop, again by the P-K mechanism, with the consequent production of In~ which may either give rise to the microprecipitates or contribute further to dislocation climbing. In the latter case the microprecipitates could be due to Zn, either alone or compounded with P, the precipitation of which should be favoured by the strain introduced by the loops [22]. High-resolution T E M work is in progress to identify the microprecipitates.

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Acknowledgment This w o r k was s u p p o r t e d by Progetto N a z i o n a l e Materiali Innovativi Avanzati (OPTEL).

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