CdS superlattices

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ELSEVIER

CRYSTAL GROWTH

Journal of Crystal Growth 138 (1994) 150-154

Strain relief and growth modes in wurtzite type epitaxial layers of CdSe and CdS and in CdSe/CdS superlattices M. Griin

,,a

M. Hetterich a, C. Klingshirn a, A. R o s e n a u e r b, j. Zweck W. G e b h a r d t b

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a Department of Physics, University of Kaiserslautern, D-67653 Kaiserslautern, Germany b Department of Physics, Unicersity of Regensburg, D-93053 Regensburg, Germany

Abstract

The relaxation of the mismatch-induced strain in (0001) wurtzite type epilayers of CdSe and CdS and of related superlattices on GaAs(111) is discussed. For CdSe/GaAs(111), high-resolution electron microscopy shows that the misfit dislocations are 60° dislocations, the glide of which is limited to the plane parallel to the interface. The epilayers are therefore free of in-grown threading arms. The thickness of that region in the layer, which is significantly strained, is found to be larger in case of a two-dimensional growth mode than in case of a three-dimensional one.

I. Introduction

In this article the relief of mismatch-induced strain in I I - V I epitaxial layers with the wurtzite type crystal structure is studied. The orientation of the primary glide planes relative to the interface is found to have significant influence on the dislocation structure induced by lattice mismatch. In (001) zinc-blende type epilayers, which are commonly used, the four {111} glide planes allow misfit segment formation by glide of existing dislocations or surface nucleated half loops. The line ends of the half loops extend into the growing layer and form the well-known network of threading arms [1], which degrade the electrical

* Corresponding author.

and optical epilayer properties. The dislocation density in the active layer of Z n C d S e / Z n S e based G R I N S C H single quantum well laser diodes, for instance, can exceed 1 x 107 c m - 2 because of these in-grown threading arms [2]. They originate in these devices mainly from misfit dislocations at the barrier-cladding interface. H e r e we report a study of (0001) wurtzite type I I - V I epitaxial layers by high-resolution electron microscopy ( H R E M ) and by reflective high-energy electron diffraction ( R H E E D ) . The only primary glide plane in these structures is situated parallel to the interface. We show that the formation of threading arms is then eliminated completely. The structures studied are CdSe and CdS layers and C d S e / C d S superlattices grown on G a A s ( l l l ) , which are highly mismatched. The (111) substrate orientation provides the required (0001) layer orientation.

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2. Experiment The wurtzite type epitaxial layers were grown by hot-wall epitaxy and hot-wall beam epitaxy on ( l l l ) B GaAs [3]. The substrates were etched in the standard manner and deoxidized in a hydrogen atmosphere. CdSe and CdS were evaporated from two parallel effusion cells and the substrate was moved to each of the desired cells. CdSe layers were grown at 470°C without a spacing between the substrate and the effusion cell, which is the usual hot-wall epitaxy (HWE). CdS layers and CdSe/CdS superlattices were grown at 430°C with a spacing of 10 mm, which is called hot-wall beam epitaxy (HWBE) [4]. In-situ RHEED was carried out at the CdS growth position in the HWBE mode. The CdSe/CdS strained layer superlattices (SLSs) were grown on thick CdS buffer layers and consist of 30 symmetric periods capped with thin CdS layers. Growth of CdSe/CdS SLSs by metalorganic chemical vapour deposition has already been reported by Halsall et al. [5]. HREM was performed using a Philips CM 30 microscope operated at 300 kV. The point resolution was 0.23 nm. Cross-sectional samples were prepared in a conventional manner using a twoside argon ion milling process at liquid nitrogen temperature for final thinning.

3. Results 3.1. H R E M

An example of single-beam bright field imaging of the C d S e / G a A s ( l l l ) interface is shown in Fig. 1. The photograph reveals only a few planar defects, but no further dislocations in the layer. The selected area diffraction pattern shows a nearly unstrained wurtzite type epilayer. Multibeam lattice images show a few stacking faults within the wurtzite type layer (Fig. 2a). Misfit dislocations were recognized by Fourier filtering of selected atomic netplanes in the high-resolution image (Fig. 2b). Dislocations are found within the first few CdSe bilayers at an averaged distance of 5.27 + 0.13 nm in the (ll2)OaAs direction. The observed dislocation distance corre-

Fig. 1. (a) Cross-sectional single-beam bright field TEM micrograph of a C d S e / G a A s ( l l l ) interface. Horizontal lines in the undamaged layer region are attributed to planar defects. (b) ('i120)caselI(li0)GaA~ selected area diffraction pattern with the (000) reflection marked.

sponds to a relief of 95 + 2% of the strain, which is induced by the mismatch of -6.9% at the growth temperature. The Burgers vectors of the misfit dislocations were determined from Burgers circuits. Fig. 3 shows a circuit around a dislocation. The Burgers vector is parallel to the interface plane with magnitude ~1a GaA~(ll0). It belongs to a full or possibly dissociated 60° dislocation, the glide of which is limited to the (0001) plane. This limitation of the glide motion parallel to the interface rules out the formation of threading arms, which produces a CdSe epilayer nearly free from in-grown dislocations. For CdS epilayers unfortunately low resolution single-beam images were not taken, but the absence of threading arms is similarly probable in this system. Multi-beam lattice images of the C d S / G a A s ( l l l ) interface show an interface region of about 8 nm width with a high stacking fault density. Misfit dislocations are spread within

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this region. A measurement of the amount of strain relief using the Fourier filtering technique failed. From these observations we suppose a significantly more sluggish strain relief in C d S / G a A s ( l l l ) than in C d S e / G a A s ( l l l ) . 3.2. R H E E D

The H W B E grown CdS layers always show streaky R H E E D patterns. The associated quasi-

Fig. 3. Burgers circuit around an interfacial misfit dislocation from an enlarged section of Fig. 2a. The black lines are guides to the eye in order to recognize more easily the dislocation site.

Fig. 2. (a) H R E M image of a C d S e / G a A s ( l l l ) interface in (110)GaAs projection. (b) Interface region after Fourier filtering: the black arrows indicate terminating {111} substrate planes.

two-dimensional (2D) growth mode is obtained throughout the whole growth cycle. Unfavourable growth conditions induce spotty R H E E D patterns, which give some evidence for cubic twinning during the initial growth stage. The H W E grown CdSe layers either on GaAs or on CdS buffer layers show a spotty R H E E D pattern, i.e. a three-dimensional (3D) growth mode. Possibly, this finding is due to the limited range of growth conditions available in our epitaxy apparatus or to the different lattice mismatches. Cubic twinning is not observed for CdSe. The R H E E D patterns of the C d S e / C d S SLSs show a transition from streaky patterns to spotty ones. Fig. 4 is a series of patterns obtained during growth of a superlattice with a period length of 8 rim. The streaky pattern is preserved up to about the 7th period corresponding to approximately 50 nm total SLS thickness (Figs. 4b and 4c). Then it

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Fig. 4. Set of RHEED patterns during growth of a 4/4 nm CdSe/CdS superlattice on a 700 nm CdS buffer layer on GaAs(lll): (a) CdS buffer layer; (b) 4th SLS period; (c) 7th SLS period; (d) 15th SLS period; (e) 20th SLS period.

gradually changes into a spotty pattern within the next 10 periods (Figs. 4d and 4e). The spotty pattern then remains unchanged during growth of the remaining SLS. It reveals some cubic twinning. During cap layer growth the streaky pattern is restored. The growth mode transition from quasi-2D to 3D is related to the relaxation of the initially pseudomorphic SLS.

4. Discussion The stacking fault region at the C d S / G a A s (111) interface observed in H R E M and R H E E D is ascribed to the presence of compressive strain. Compressive strain larger than about 1% should cause a loss of stability of the bulk-stable phase in layer growth via an increase of the actual bond length ratio of vertical to in-plane bonds over the

corresponding ratio of the metastable phase [6]. The observed stacking faults are assumed to be due to this effect or possibly due to stacking fault bands of strain relieving Shockley partials. This assumption allows a consistent interpretation of our H R E M and R H E E D results: the mismatchinduced strain in (0001) wurtzite type layers is relieved by full or dissociated 60° dislocations, the nucleation of which does not take place via a half-loop mechanism and is hampered by a 2D growth mode and facilitated by a 3D growth mode. The 3D growth of C d S e / G a A s ( l l l ) results in an efficient strain relief near the interface and subsequent growth with low stacking fault density. The 2D growth of C d S / G a A s ( l l l ) and the SLSs causes strained layer regions of noticeable thickness. The observed "critical thickness" referring here to the thickness of a significantly

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strained region is about 8 nm for C d S / G a A s ( l l l ) with - 3 . 1 % mismatch as deduced from H R E M . For the C d S e / C d S SLS the onset of a 3D growth mode above 50 nm total superlattice thickness allows formation of misfit dislocations, which shifts the SLS in-plane lattice constant from that of the buffer layer to that of the free-standing SLS [7]. The R H E E D deduced value of 50 nm represents therefore roughly a "critical thickness" of the SLS as a whole, which has a - 1 . 9 % mismatch to the CdS buffer layer. Possibly, the free-standing part is relaxed to a certain extent due to its 3D growth mode and only the pseudomorphic part is coherent. The cubic twinning in the SLS most likely originates from the compressively strained CdSe layers. From photoluminescence measurements, a "critical thickness" value of about 5 nm for the individual CdSe and CdS layers within the SLS is estimated ( - 3 . 8 % mismatch in the pseudomorphic case). A detailed discussion of this result is beyond this article. We state here only that the energy of the luminescent electron-hole transition in the coherently strained SLSs depends linearly on the period length due to strain-induced piezo-electric fields [3, 5]. Strain relaxation is thus indicated by a deviation from this linear period length dependence. The observed "critical thickness" values are a factor of 2 to 5 above the Matthews-Blakeslee critical thickness calculated for a (111) oriented zinc-blende type epilayer and 60 ° dislocations [1]. We believe that the delayed strain relaxation is caused mainly by the loss of the half-loop nucleation mechanism in the (0001) wurtzite structure. Nucleation of the observed 60° dislocations most probably requires at least a bilayer surface step, the density of which is low on smooth 2D surfaces. The details of the nucleation mechanism are still an open question. In case of 3D growth, the base edges of small islands are likely to serve as nucleation sites, as considered by Snyder et al. [8] for InGaAs/GaAs(001) and by Horn-von Hoegen et al. [9] for G e / S i ( l l l ) . The latter authors reported also misfit dislocations with Burgers vectors parallel to the interface. Rosenauer et al. [10] recently found strain relief by only this type

of dislocations in cubic Z n T e grown on G a A s ( l l l ) . These two results show that the suppression of threading arm formation in mismatched epitaxy is also possible in zincblende type epilayers, provided that a (111) interface is used and that only misfit dislocations are generated with glide planes parallel to the interface.

5. Conclusions

Wurtzite type epilayers of CdSe and CdS in (0001) orientation show no extension of mismatch-induced dislocations into the epilayer volume. The misfit dislocations in C d S e / G a A s ( l l l ) are full or dissociated 60 ° dislocations, the glide motion of which is limited to the interfacial (0001) plane. Threading arms are therefore not formed. R H E E D data of CdSe and CdS layers and of C d S e / C d S superlattices give evidence that the strain relief is speeded up by a 3D growth mode.

6. References [1] See, e.g., R. Hull, Crit. Rev. Solid State Mater. Sci. 17 (1992) 507. [2] C.T. Walker, J.M. DePuydt, M.A. Haase, J. Qiu and H. Cheng, Physica B 185 (1993) 27. [3] M. Griin, M. Hetterich, U. Becker, Th. Gilsdorf, H. Giessen, H. Zangerle, M. Miiller, J. Loidolt, F. Zhou and C. Klingshirn, in: The Physics of Semiconductors, Vol. 1, Proc. 21st Int. Conf. on Physics of Semiconductors, Beijing, August 1992, Eds. P. Jiang and H.-Z. Zheng (World Scientific, Singapore 1992) p. 574. [4] M.A. Herman and H. Sitter, Molecular Beam Epitaxy (Springer, Berlin, 1989) p. 108. [5] M.P. Halsall, J.E. Nicholls, J.J. Davies, B. Cockayne and P.J. Wright, J. Appl. Phys. 71 (1992) 907. [6] For a relation between the wurtzite parameter and the phase stability see, e.g., P. Lawaetz, Phys. Rev. B 5 (1972) 507. 17] E. Kasper in: Physics and Applications of Quantum Wells and Superlattices, Eds. E.E. Mendez and K. von Klitzing (Plenum, New York, 1987) p. 101. [8] C.W. Snyder, B.G. Orr, D. Kessler and L.M. Sander, Phys. Rev. Lett. 66 (1991) 3032. [9l M. Horn-von Hoegen, M. Pook, A. AI Falou, B.H. Miiller and M. Henzler, Surf. Sci. 284 (1993) 53. [10l A. Rosenauer, H. Stanzl, K. Wolf, S. Bauer, M. Kastner, M. Griln and W. Gebhardt, in: Proc. 17th Int. Conf. on Defects in Semiconductors, Gmunden, July 1993 (Trans Tech, Ziirich, in press).