Reduction of dislocation density in heteroepitaxial GaN: role of SiH 4 treatment

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1 Journal of Crystal Growth 267 (2004) 1 7 Reduction of dislocation density in heteroepitaxial GaN: role of SiH 4 treatment K. Paku"a a, *, R. Bo zek a, J.M. Baranowski a, J. Jasinski b, Z. Liliental-Weber b a Institute of Experimental Physics, Warsaw University, Ho za 69, Warsaw, Poland b Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Received 4 February 2004; accepted 11 March 2004 Communicated by H. Ohno Abstract TEM and AFM data show that a significant reduction of threading dislocations in heteroepitaxial GaN/Al 2 O 3 grown by MOCVD has been achieved. The reduction has been obtained by growth interruption followed by annealing in silane (SiH 4 ). Density of threading dislocations in the GaNlayer above the silane-exposed surface decreased to cm 2 in comparison to 10 9 cm 2 in the layer below this surface. TEM data showed the existence of pyramidal pits at the silane-exposed surface. They were overgrown by the subsequent GaNlayer. The presence of these pits indicates that the GaNsurface was selectively etched during the silane flow. These pits were sites where dislocations drastically changed propagation direction from parallel to the c-axis to horizontal. Horizontal propagation of dislocations above the surface treated by silane (where formation of SiNwas expected) suggests that the GaNlayer in this region was grown in the lateral epitaxial overgrowth mode. EDX measurements performed at the interface between the SiH 4 -treated GaN layer and the subsequently grown GaNdid not show any presence of Si. Therefore, it is believed that the dislocation reduction is related to the lateral overgrowth above the pits and not to the formation of a SiNinterlayer. r 2004 Elsevier B.V. All rights reserved. Keywords: A1. Atomic force microscopy; A1. Dislocations; A1. Transmission electron microscopy; A2. Lateral growth; A3. Metalorganic vapor phase epitaxy; B1. Gallium nitride 1. Introduction *Corresponding author. Tel.: ; fax: address: krzysztof.pakula@fuw.edu.pl (K. Paku"a). The Rapid progress in the development of nitride based devices such as laser diodes, blue and ultra violet light emitting diodes, field effect transistors and UV detectors is based on improved heteroepitaxial growth. The development of heteroepitaxial GaNgrowth is connected with the lack of widely available large size GaNsubstrates. For that reason GaNepilayers have to be grown on substrates such as sapphire, silicon carbide or silicon. Due to considerable lattice mismatch between GaNand these substrates, a high density of dislocations characterizes the heteroepitaxial layers. Threading dislocation density in common heteroepitaxially grown GaNon sapphire or SiC substrates is in the range of cm 2. These dislocations affect performance of several devices /$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi: /j.jcrysgro

2 2 K.Paku!a et al. / Journal of Crystal Growth 267 (2004) 1 7 In particular they act as non-radiative recombination centers, reducing the efficiency of optoelectronic devices. It has been found recently that threading dislocations reduce the light output of UV LEDs much more than they do for blue GaN LED s [1 4]. Also for laser diodes a low density of dislocations is essential for extensions of their lifetime. This shows that threading dislocations play an important role and methods for their reduction need to be developed to ensure successful nitride optoelectronics One well-known technique to reduce dislocation density of heteroepitaxial GaNlayers is epitaxial lateral overgrowth (ELOG) [5,6]. This method consists of formation of SiO 2 strips on the surface of primary GaNepilayers. Continuation of GaN growth through openings between the strips leads to lateral growth above the SiO 2 strips and the bending of dislocations from the c-direction to the c-planes. Further growth leads to local elimination of dislocations along the c-direction in the areas above the SiO 2 masks. This technique has resulted in serious advances in the technology of GaN lasers. A similar technique is PENDO epitaxy [7,8]. However, a drawback of these methods is the high degree of complication arising from the necessity of ex situ lithography and the fact that low dislocation density material is obtained only in the form of microns size strips. In addition, some tilt occurs where two overgrown areas meet above a SiO 2 mask leading locally to high dislocation density to accommodate this tilt [9]. Another technique of dislocation reduction is treatment of the sapphire substrate with a mixture of silane and ammonia before formation of a low temperature (LT) GaNnucleation layer [10,11]. Such a mixture of silane and ammonia acts as an anti-surfactant which inhibits the GaNfilm from wetting and modifies the growth into the three-dimensional mode [10]. This technique was used by Haffouz [12] for GaNgrowth on sapphire. This process was also used for GaNgrowth on Si [13] and SiC [14] substrates. The Si x N y was formed directly on the substrate, prior to the LT buffer layer, in order to reduce the density of nucleation sites. In this work we report a method for reduction of threading dislocation density in GaNepilayers by in situ etching of the GaNsurface by SiH 4 at high temperature. 2. Experimental procedure The GaNgrowth on sapphire was performed in a horizontal, low pressure MOVPE reactor with precursors: trimethylgallium (TMG), ammonia (NH 3 ) and diluted silane (SiH 4 ) 100 ppm in H 2, and hydrogen as a carrier gas. In order to get insight into different stages of growth, we prepared a set of layers, whose growth was interrupted at crucial stages. Characterization of these layers at specific stages of growth was done by atomic force microscopy (AFM). Counting pits observed on the surface allows determination of dislocation densities. It was determined that dislocations with a screw component (both pure screw or mixed) are associated with larger pits (B50 nm) which are the origin of atomic steps. Pure edge type dislocations correspond to smaller pits (B20 nm) that are not related to atomic steps. Additionally, the intermediate layers with rough morphology were observed with scanning electron microscopy to check if the AFM images were not strongly influenced by the geometry of the tip. The layers were also investigated with high resolution X-ray diffraction (HRXD) and by transmission electron microscopy (TEM). The primary GaNlayer was grown in a standard way starting with a LT (500 C) GaN buffer layer, followed by a 3 mm layer grown at 1100 C under 400mbar pressure. The layer had smooth morphology. After completing the primary layer the TMG was switched off and without changing the temperature, the SiH 4 was switched on. The flow of silane was continued until the GaN surface was covered with coalescing hill-like growths shown in Fig. 1. The surface morphology change is probably due to etching of GaNby silane. In Figs. 2 and 3 two annealed surfaces of GaNare shown. The AFM picture shown in Fig. 2 shows a GaNsurface annealed for 10 min at 900 C in NH 3 atmosphere. Mixed and edge dislocations are seen as dark points. Fig. 3 shows the AFM image of a GaNsurface annealed under the same conditions but with SiH 4 additionally introduced.

3 K.Paku!a et al. / Journal of Crystal Growth 267 (2004) Fig. 1. AFM image of GaNsurface after 300 s in- situ treatment by silane at 1100 C Fig. 3. AFM image of the GaNepilayer after annealing for 600 s at the temperature of 900 C in the atmosphere of NH 3 + H 2 +SiH 4. Etched pits around mixed dislocations are visible. Fig. 2. AFM image of the reference GaNlayer grown without silane treatment. Pits related to pure edge and mixed dislocations are denoted by E and M, respectively. Edge dislocations often appear themselves in a form strings. It is seen that mixed dislocations are now strongly etched. At each mixed dislocation a hole of about 10 nm in depth and about 100 nm in diameter is formed. The edge dislocations, which are not associated with atomic steps on the surface, are not etched and appear as in Fig. 2. SiH 4 etches only the area around dislocations with a screw component. The etching of mixed dislocations at the growth temperature, close to 1100 C, should be even more extreme. This is probably the main reason of the rough GaNsurface observed after treatment with SiH 4. After this etching step growth is continued using TMG. At that stage the temperature is lowered to 1025 C. This temperature stimulates vertical growth and delays coalescing of the growth sites. It seems that an antisurfactant role of silane plays an important role. It may inhibit wetting of the rough surface of the GaNand stimulate threedimensional growth at nucleation sites. The surface morphology, revealed by AFM, is shown in Fig. 4. It can be seen that the secondary GaNlayer starts to grow from distinctly separated sites. After this GaNgrain nucleation the temperature is again increased to 1100 C to restore lateral growth. Quality of the layer grown at 1100 C depends on the time of the silane etching. Too short a treatment by silane does not lead to a noticeable improvement of the morphology of the GaNlayer.

4 4 K.Paku!a et al. / Journal of Crystal Growth 267 (2004) 1 7 Fig. 4. AFM image of separated GaNcrystallites after 50 s deposition on the GaNsurface. The three-dimensional crystallites are nucleated on the isolated spots. Fig. 5. AFM image of an optimized GaNlayer on sapphire grown after growth interruption and annealing in silane. Parallel ordering of the atomic steps is kept in the areas up to mm. Too long a silane treatment causes polycrystalline growth. The best results were obtained for about 300 s of silane flow. Comparison between the optimized layers (Fig. 5) and the reference layers (Fig. 2) shows noticeable reduction of dislocations. The total density of dislocations estimated from AFM data for epilayers grown with and without silane treatment were found to be approximately and cm 2, respectively. The main change was in the density of pure edge dislocations. AFM studies also showed a highly parallel ordering of atomic steps on the surface in the case of the layer grown on a silane-treated surface. Such an ordering was preserved over areas of the order 50 mm 50 mm i.e. much larger than shown in Fig. 5. The structural properties of layers grown with and without silane treatment have also been investigated with HRXD. It was found that the mosaic structure of GaNlayers grown on the surface annealed with silane clearly improved. Such treatment leads to an increase of coherent scattering areas, which indicates a significant increase of the subgrain size within the mosaic structure [15]. TEM was used to study threading dislocation density reduction after growth interruption and annealing in silane. For this study, a 6 mm-thick GaNlayer was deposited on top of the SiH 4 - treated, 3 mm-thick, primary GaNlayer. Crosssectional TEM specimens were prepared by a standard method of mechanical thinning followed by Ar ion milling. TEM observations were performed using a JEOL JEM3010 microscope operating at 300 kv. In addition, an energy dispersive X-ray spectroscopy (EDX) study of the interface region between the silane-treated primary GaNlayer and the GaNlayer grown on top of it, was carried out. A 200 kv Philips CM200 microscope operating in STEM mode with a nanoprobe of 1.2 nm in diameter was used for this study. A low-magnification bright-field TEM micrograph, showing the whole layer structure of the described sample, is presented in Fig. 6. This micrograph was taken under symmetrical multibeam diffraction conditions, so all threading dislocations were in contrast. One can notice a high density of threading dislocations present in the lower, 3 mm-thick, GaNlayer and a much lower density in the top, 6 mm-thick, GaNlayer,

5 K.Paku!a et al. / Journal of Crystal Growth 267 (2004) Fig. 6. Bright-field TEM micrograph (taken under symmetrical multi-beam conditions) showing the effect of growth interruption and annealing in silane on threading dislocation reduction in MOCVD GaNgrown on sapphire( ) substrate. deposited after the SiH 4 treatment. Based on TEM it was estimated that the density of threading dislocation was reduced from B cm 2 in the lower GaNlayer to B cm 2 in the top GaNlayer. This estimate correlates well with the one obtained from AFM results. In order to distinguish between different types of dislocations, weak-beam, dark-field TEM micrographs, recorded for g-vector parallel and perpendicular to the c-axis, were analyzed. A pair of such micrographs obtained from the same area is shown in Fig. 7. Dislocations with a screw component appear only in the micrograph recorded for g- vector parallel to the c-axis (Fig. 7a), whereas dislocations with edge component are visible only in the micrograph recorded for g-vector perpendicular to the c-axis (Fig. 7b). One can notice that Fig. 7. Weak-beam, dark-field TEM micrographs of the SiH 4 - treated interface. Dislocations with (a) screw and (b) edge component are visible. densities of edge and mixed dislocations were very similar and there were almost no pure screw dislocations present in this layer. This observation is also consistent with AFM results. It also can be noticed that, in the area of observation, the number of dislocations did not decrease substantially, above the SiH 4 treated

6 6 K.Paku!a et al. / Journal of Crystal Growth 267 (2004) 1 7 interface. However, dislocation lines changed their direction drastically. In the lower GaNlayer most of the dislocations were parallel to the c-axis, whereas above the interface they were inclined and often propagated horizontally. This will be discussed in Section Discussion TEM And AFM data show that a significant reduction of threading dislocations in heteroepitaxial GaNcan be achieved. What is the possible mechanism for this reduction of dislocation density? Results of TEM show a high density of overgrown pyramidal pits at the SiH 4 treated interface. Several such pits are clearly shown in the inset of Fig. 8a. The presence of these pits confirms that the GaNsurface was selectively etched during silane flow. The depth of these pits, about 40 nm, is in agreement with results of AFM. The pits decorate the whole interface. One may also notice that they serve as sites for dislocation bending. Horizontal propagation of dislocations above the SiH 4 treated interface suggests that the secondary GaNlayer grew in the lateral epitaxial overgrowth mode. This change of growth mode leads to bending of dislocations, and most likely is responsible for reduction of their density. However, some dislocations terminate within the interface region, indicating that they propagate horizontally within the interface and recombine with dislocations having opposite Burgers vector. The antisurfactant properties of silane may cause creation of open voids between the two layers of GaN. Evidence of such voids may be seen in Fig. 8b, where a number of pyramidal-like defects is visible at the interface, where the silane treatment was performed. EDX measurements show traces of Si inside these defects. However, a thick SiNlayer between the two GaNlayers has not been confirmed by TEM measurements. Also EDX line profiles measured across the SiH 4 treated interface did not show any presence of such a layer. Taking into account the conditions of this experiment it can be concluded that, either there is no SiNlayer formed at this interface or it is very thin (1 2 nm at most) which is hard to confirm using this method. Therefore, the dominant mechanism responsible for reduction of the density of threading dislocations appears to be lateral overgrowth of the silane-etched GaNsurface. Finally, it should be stressed that the described method of heteroepitaxy allows growing of epilayers with relatively low dislocation density over a large area of a substrate, with dislocations that are uniformly distributed without formation of subgrains with significant tilt as observed in ELOG [9] and PENDO [8] epitaxy. Acknowledgements Fig. 8. Weak-beam, dark-field TEM micrographs of the SiH 4 - treated interface recorded with g-vector parallel (a) and perpendicular (b) to the c-axis. Note that many dislocations bend at the interface within overgrown pyramidal pits formed due to selective etching of GaN. Inset in (a) shows several of such pits. In (b) pyramidal pits are out of contrast however, number of pyramidal-like defects is visible within the region of SiH 4 -treated interface (inset shows one of such defects). Work at LBNL was supported in part by AFOSR No. FQ , through the US DOE under Contract No. DE-AC03-76SF0098. The TEM group (J.J. and Z.L.-W.) would like to thank the National Center for Electron Microscopy at LBNL for the opportunity to use its facilities. Work at Warsaw University was

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