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��Optical and Quantum Electronics 32: 227�248, 2000. � 2000 Kluwer Academic Publishers. Printed in the Netherlands. 227 Solutions for heteroepitaxial growth of GaN and their impact on devices MARKUS KAMP Department of Optoelectronics, University of Ulm, 89069 Ulm, Germany (E-mail: markus.kamp@e-technik.uni-ulm.de) Abstract. GaN technology relies on highly mismatched heteroepitaxial growth, mainly on sapphire or SiC substrates, and therefore su�ers from 109 to 1010 threading dislocations per cm2. The origin and the deteriorating in�uence of the extremely high dislocation densities are analyzed with regard to the speci�c circumstances of GaN technology. Various attempts to cope with heteroepitaxial growth are discussed, from the use of nucleation layers to the growth on GaN single bulk crystals. Special focus is put on the impact of the approaches on the device performance. Key words: dislocations, GaN, heteroepitaxy, laser, LEDs 1. Introduction GaN based materials are today's fastest developing III�V compound semi- conductor technology. The excellent optical and electrical properties, the wide direct bandgap, the thermal, mechanical, and chemical robustness make GaN based semiconductors the superior material system for optoelectronic devices (LEDs, laser, photodetectors) in the UV to visible range. Addition- ally, electronic devices such as GaN based �eld e�ect transistors (FETs) and heterobipolar transistors (HBTs) o�er new applications in high power, high frequency microelectronics. Various opto- and micro-electronic devices are either already established or approaching the markets. Despite the tremen- dous success this technology still su�ers mostly from the lack of a perfect substrate and therefore has to cope with strongly mismatched heteroepitaxial growth. Di�erences in lattice constants as well as thermal expansion coe�- cients result in about 109�1010 dislocations/cm2 thus limiting device perfor- mance and lifetime. This paper provides an introduction to general issues of heteroepitaxial growth and the generation of dislocations, both with special regard to GaN technology and the impact on device performance. Potential substrates as well as various techniques for the reduction of dislocation densities are elaborated. Di�erent approaches to improve the layer properties, from low temperature nucleation layers to homoepitaxial growth, are discussed. In particular the new results on homoepitaxial growth on GaN single bulk crystals provide new standards in GaN material quality. The exceptional 228 M. KAMP quality is determined by a reduction of the photoluminescence linewidth from 5 to 0.1 meV and a reduced XRD rocking curve width from 400 to 20 arcsec. The outstanding material quality provided new insights into fun- damental material parameters (e.g. lattice parameters, excitonic binding en- ergies, etc.) being not accessible by heteroepitaxial growth. 2. Potential substrates for GaN technology Despite the fast and outstanding achievements of GaN based optical and electrical devices, the technology still su�ers from strongly mismatched he- teroepitaxial growth. GaN substrates pulled from a melt are not available, yet. Predicted temperatures and pressures of about 2800 K and 45000 bar being mandatory for melt growth will inhibit these substrates for the fore- seeable future (Van Vechten 1973). All substrates other than GaN itself lead to heteroepitaxial growth, thus giving rise to a deterioration in epitaxial quality due to di�erences in lattice constants and thermal expansion coe�- cients between substrate and layer. For a comprehensive overview, potential substrates for GaN technology are listed in Table 1 together with their fundamental crystalline parameters. Except of LiGaO2 none of the potential substrates can provide lattice matching even close to the requirements of other III�V technologies. In Table 1. Potential substrates for GaN technology and their fundamental physical parameter Substrate Crystal structure Lattice mismatch Di�. in therm. Cleavage Stability for to a-GaN (%) Expansion coe�. plane MOVPE at 300 K to a-GaN (�10)6) process Si Diamond 20.1 )2.0 (111) Good GaAs Zincblende 25.3 0.4 (110) Su�cient GaP Zincblende 20.7 0.9 (110) Su�cient MgO Rocksalt )6.5 4.9 (100) Su�cient MnO Rocksalt )1.4 (100) Instable CoO Rocksalt )5.4 (100) Instable NiO Rocksalt )7.6 (100) Instable MgAl2O4 Spinel )10.3 1.9 (100) Good NdGaO3 Perovskite )1.2 1.9 Su�cient ZnO Wurtzite 2.0 )2.7 (1�100) Su�cient (11�20) (0001) 6H-SiC Zns 6H )3.4 )1.4 (1�100) Good (11�20) (0001) LiAlO2 b-NaFeO2 1.7 1.7 (001) Instable LiGaO2 b-NaFeO2 )0.1 1.9 (010) Instable Al2O3 Corundum 13.8 1.9 (1�102) Good LiNbO3 Ilmenite )6.7 9.9 (1�102) Instable LiTaO3 Ilmenite )6.8 10.6 (1�102) Su�cient SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 229 addition to above properties, the thermal and the electrical conductivity as well as price and availability have to be considered, leaving 6H-SiC and c-plane sapphire (Al2O3) as the only two substrate widely used in GaN technology. Heteroepitaxial growth of GaN results in about 109 threading dislocations (TD) per cm2 for the present GaN technology on sapphire or SiC substrates. Such high dislocation densities, being 5�6 orders of magnitude higher than in conventional III�V technologies are present even in state of the art device material. The deteriorating in�uence of the TD, however, is signi�cant lower than expected. Figure 1 shows the normalized e�ciency of LEDs versus the dislocation density of these structures for a variety of di�erent III�V semi- conductor systems including GaN (after Lester (1995)). On the �rst glance TD seem to be no problem to group III nitrides, as judged from their LED e�ciency. However, as we will see later in Section 5, `The Rule of Dislocations in GaN', TD severely hamper GaN based devices in many terms including lifetime and performance. 3. Heteroepitaxial growth Semiconductor technology requires epitaxial growth of an extremely high quality. Perfect crystal growth can only be attained using a substrate that is identical in crystal structure, lattice constant and thermal expansion coe�- cient. This is only guaranteed for homoepitaxy, where substrate and epitaxial Fig. 1. Normalized e�ciency of LEDs versus dislocation density of these structures, for a variety of di�erent III�V semiconductor systems. 230 M. KAMP layer consist of identical material. Under those circumstances, layer-by-layer growth can be obtained, resulting in two-dimensional growth without gen- eration of dislocations. If a homoepitaxial substrate is not available, above criteria should be matched as close as possible. Almost every semiconductor material system either group IV, III�V or II�VI is grown using homoepitaxial growth or at least a closely lattice matched substrate (Da=a 10 3). At growth temperature the overall mismatch between epitaxial �lm and substrate, resulting from di�erent thermal expansion coe�cients as well as lattice mismatch at room temperature, is relaxed under formation of dislocations. The lattice mismatch at growth temperature can be calculated according to Equation (1) Da aL�T � aS�T � aL�20 C��1 � aLDT � aS�20 C��1 � aSDT � �T � � � a aS�T � aS�20 C��1 � aSDT � �1� where aL and aS are the temperature dependent lattice constants of layer and substrate, respectively, aL and aS are their thermal expansion coe�cients, and DT is the di�erence between room and growth temperature. Upon cooling to room temperature di�erences in thermal expansion co- e�cients determine the residual biaxial stress in the epitaxial layer. The acting forces (P), the stress (r) and the curvature radius (R) can be calculated according to the two-dimensional elastic beam theory for isotropic materials. 2 3 P P dj di X DT j>i i Pi � Edi6 � dj�ai aj�7 �2� 4 5 2R d j>i Pi E di ri � � xi �3� di R 2 �di � dj�3 P P R � �4� 6DT didj�ai aj� i j E being Youngs moduli, di the thickness, and xi the distance as measured from the central axis of the layer i. The resulting strain, which can be up to 0.6 GPa for a 3 lm thick GaN layer grown on sapphire account for serious macroscopic e�ects such a sig- ni�cant curvature of the substrate/layer sandwich (Kozawa et al. 1995). However, the key to heteroepitaxial growth is the stress release on the mi- croscopic scale that can be approached considering the free energy of the growing surface. Naturally and in general an ideal growing surface endeavors SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 231 to minimize its free surface energy (c) which is usually achieved by a mini- mization of the surface area, thus suppressing surface steps. However, in case of a strained, and particularly of compressively strained, epitaxial �lms it can be energetically favorable to minimize the free energy by an undulation of the surface (see Fig. 2). Where amplitude t and period k of the roughness ful�ll the following unequation (Pidduck et al. 1993): E�Da=a�2 t=k < �5� 4cp2 Under increasing and strong strain, however, the minimization of the surface free energy eventually leads to the formation of a network of TD. 4. Generation of dislocations The accommodation of lattice mis�t across the interface between an epitaxial layer and its substrate was �rst considered by Frank and van der Merwe (1949). They showed that a mis�t smaller than about 7% can be accom- modated by biaxial elastic strain until a critical thickness of the epitaxial �lm is reached. Above a certain strain, relaxation takes place by formation of dislocations. For a given lattice mismatch, determined by the lattice pa- rameters, thermal expansion coe�cients and growth temperature, the stress is corresponding to a speci�c thickness, i.e., the so-called critical thickness (hi). The concept evolved by Matthews and Blakeslee (1974) presumes that below the critical thickness a dislocation-free, coherently strained interface is stable, whereas a mis�t dislocation structure, being semicoherently strained, would be stable for higher thickness. Fig. 2. In�uence of heteroepitaxially induced strain on the morphology and the formation of dislocations. 232 M. KAMP The critical thickness can then be calculated by considering the forces acting on a present TD. The force exerted on the given dislocation by the mis�t strain is given by 2G1�1 � m� Fmisfit � fbl cos k cos b �6� �1 m� The tension in the dislocation is given by G1G2 Flinetension � b2�1 m cos2 h��ln�h=b� �1��7� p�1 m��G1 � G2� G being the bulk moduli, m the Poisson ratio, f the mis�t at the hetero interface, b the length of the Burgers vector, l the length of the threading segment, h the layer thickness. k being the angle between the Burgers vector and the interfacial plane, b the angle between the normal of the slip plane and the interfacial plane, h the angle between the Burgers vector and the line direction of the mis�t segment. If Fmisfit exceeds Flinetension the dislocation will move within the interfacial plane and form a mis�t dislocation, thereby destroying the coherence of the interface. The Matthews�Blakeslee model is will established and successfully applied to most conventional III�V semiconductors. However, nitride semiconduc- tors crystallizing in the wurtzite structure, reveal some peculiarities. The low symmetry of the hexagonal system allows for multiple epitaxial orientations being very similar but not identical in terms of their free surface energy and chemical potential. The high c=a ratio of about 1.626, the narrow slip plane spacing (d) and the length of the Burgers vector b have an direct impact on the formation of mis�t TD according to the Matthews�Blakeslee model. As compared to zincblende structures, the wurtzite have extraordinary high Peierls forces (FPeierls) for c-type or screw dislocations (b ��= �0001�) and mixed c,a-type dislocations (b � 1=3 < 11�23 >), whereas the a-type or edge dislocations (b � 1=3 < 11�20 >) encounter only a small Peierls force (Jahnen et al. 1998). 1 m cos2 v d�1 m cos2 v� FPeierls � 2blG1 x exp 2p x ; 1 m b�1 m� 4 nkBT x � exp p2 �8� 5 G1V d being the spacing of the slip planes and v the angle between the Burgers vector and the line direction of the threading segment, n is the number of atoms per unit cell, kB is Boltzmann's constant and T the growth tem- perature. SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 233 The Peierls force counteracts the driving shear force (Fmisfit) in addition to the line tension force (Flinetension) thus giving a new equilibrium condition for the formation of mis�t TD. Fmisfit � Flinetension � FPeierls �9� For completion it should be noted that beside the Matthews�Blakeslee model, a second model for strain relaxation was developed in 1963 by Van der Merwe assuming that the interfacial energy between �lm and substrate is the minimum energy available for generation of mis�t dislocations (Van der Merwe 1963). By minimizing the total energy, strain can be calculated as function of layer thickness and the critical thickness is determined equating the two energies. 5. The rule of dislocations in GaN The generation of dislocations can hardly be avoided in lattice mismatched material systems. Therefore, their repercussions have to be discussed. Threading dislocations have strong in�uences on the semiconductor material, whereas most of the e�ects come along with serious limitations to device performances, too. Threading dislocations are scattering centers for light propagating within the crystal. They are therefore introducing losses, particularly deteriorating laser performance. The in�uence of threading dislocations has been inves- tigated by Liau et al. (1996) who calculated an absorption of 3 102 cm 1 for a dislocation density of 2 1010 cm 2 and decent assumptions for a laser geometry. Using nowadays data, for a more reasonable estimate, one would expect losses of about 1�10 cm 1 being in good agreement with previously reported losses of about 45 cm 1 (Nakamura 1997). Screw type TD, potentially having on open core in the center, can create nano-pipes with open diameters of 30�50 nm (Qian et al. 1995). Those holes deteriorate the electrical properties of layers and devices by providing low energy di�usion paths for contact metals, dopants and impurities (Osinski et al. 1996). Since solid state di�usion is very low in GaN based materials, di�usion along TD is supposed to be one of the major degra- dation mechanisms for devices. TD also act as vertical shortcuts. Using TD `free' Epitaxial Lateral Overgrown (ELOG) substrates, the reverse bias leakage current of pn-junction diodes has been dramatically reduced by a factor of 1000 (Kozodoy et al. 1998). Threading dislocation can be regarded as charged line defects being re- sponsible for the unexpected low mobilities observed in GaN technology. TD are long known as scattering center in semiconductors. B. Podor (1966) � 234 M. KAMP calculated the impact of TD on the electron mobilities in Ge crystals as early as 1966. He proposed the following dependence of the carrier mobility (l) on the TD density (N), Debye length (LD) and temperature (T ) �kBT �3=2 l / �10� LDN This is in good agreement with experimental electron mobilities falling signi�cantly short compared to mobilities above 1000 cm2/Vs as expected from Monte�Carlo simulations. Recently Weimann et al. (1998) propose that charged traps along dislocations, acting as scattering center for lateral currents, are the dominating scattering mechanisms in highly deteriorated GaN layers. Threading dislocations, being known for having a dislocation mobility 1010 times lower than that of GaAs, gain extraordinary mobility with increasing temperature (T 400 C). Thus for elevated temperatures, e.g. present in high temperature electronics, TD become increasingly important for device degradation (Sugiura 1997). 6. Concepts for dislocation reduction As described earlier the stress induced by di�erent lattice constants and thermal expansion coe�cients between layer and substrate, above a critical thickness, is reduced by generation of dislocations. Fig. 3 shows TEM micrographs of a MBE grown GaN layer deposited directly on Al2O3. The pictures clearly reveal a multi-crystalline layer with several epitaxial orien- tations not suitable for a device structure. Within III�V compound semi- conductor technology various concepts have been developed and successfully applied to overcome or reduce this limitation. 6.1. STRAINED LAYER SUPERLATTICES The introduction of strained layer superlattices (SLS) was, for example, successfully used in GaAs/Si technology where the dislocation density could be reduced from about 108 cm 2 to approx. 105 cm 2. The general concept is the bending of dislocations in the strain �eld of the heterojunction. The TD then can either follow the interface to the edge of the wafer or annihilate themselves (see Fig. 4). However, in GaN technology there are no reports on an e�cient reduction in dislocation density by SLS. The low e�ciency in dislocation reduction goes back to the high Peierls forces already discussed in Section 4. The low gliding plane distance again inhibits the gliding of the dislocations along the interface. However impurity gettering by SLS being SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 235 Fig. 3. TEM micrographs of a GaN layer grown directly on a sapphire surface. known from many �elds of III�V technology (Meier et al. 1994) is still an issue for SLS in GaN technology. 6.2. NUCLEATION LAYERS Low temperature nucleation layers (NL) have initially been introduced into GaN technology in 1986 by Amano et al. (1986). Thereby, for the �rst time, Fig. 4. Concept of a Strain Layer Superlattice (SLS) structure for the reduction of dislocations. 236 M. KAMP high quality GaN layers have been grown. The usage of NL provided a breakthrough in GaN technology. Today a variety of various types of nu- cleation layers is known from the literature, including GaN, AlN, GaN/AlN combination layers, etc. Several groups report the usage of an additional nitridation before NL growth, whereas other initiate growth directly after degasing of the sapphire. The huge number of free parameters (i.e. V/III ratio, temperature, thickness, growth rate, temperature ramps, crystallization temperature, etc.) make the optimization of the NL extremely di�cult and time consuming. Additionally, these parameters seem to depend strongly and non-linearly on each other. Optimized thickness and temperatures are sig- ni�cantly di�erent for GaN or AlN NL and strongly depending on V/III ratio, etc. Obviously, there is no converging into a particular set of param- eters within the published data. The one thing NL have in common is that they determine the defect structure of the subsequently grown GaN layers and thereby strongly in�u- ence the quality of that material. The ways NL improves the GaN material are as many and di�erent, as there are NL. They can for instance reduce the residual strain of layers grown on sapphire substrates by new elastic strain relaxation mechanisms (Albrecht et al. 1997). Figure 5 shows a GaN/Al2O3 interface grown by MBE (Mayer et al., unpublished) depicting the selfaligned periodic formation of grainlets with alternating compressive (13.8%) and tensile ( 25.8%) strain where islands with di�erent orientations are com- pressive and tensile strained. Probably, the most important aspect with NL is that they provide nucle- ation centers on the sapphire surface which form isolated island with facets di�erent from (0001), such as the (01 11) and the (01 12) facet for instance (Albrecht and Kamp, unpublished). If the growth rate of those facets is higher than the (0001) growth rate, the islands will coalesce under formation of low angle grain boundaries (see Fig. 6). This mechanism of preferential lateral growth has indeed some similarities to the extremely successful ELOG approach discussed later. The impact of the initial stages of growth on the optical properties of a 2 lm thick GaN layer grown by GSMBE under identical condition is de- picted in Fig. 7. 6.3. NITRIDATION Especially with sapphire substrates, one additional process step is often ap- plied for further improvement of the NL. The bare sapphire surface is exposed to the reactive nitrogen source at elevated temperatures. Depending on the growth technique this can be activated atomic nitrogen in plasma enhanced MBE (PEMBE) (Heinlein et al. 1997) or ammonia in reactive MBE (RMBE) SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 237 Fig. 5. Sketch and corresponding TEM micrograph of a newly found strain release mechanism by a periodic formation of grainlets with alternating compressive (13.8%) and tensile ( 25.8%) strain. (Grandjean et al. 1996) and MOVPE (Uchida et al. 1996). The intention is to exchange oxygen atoms from the surface layers against the supplied nitrogen atoms, thereby forming an AlxNy layer more suitable for epitaxial growth. The time (degree) of nitridation has to be controlled carefully, since an extended nitridation may eventually end in the formation of GaN whiskers. The in�uence of the nitridation on the density and kind of dislocations has been elaborated by a careful investigation of J. Specks group (Wu et al. 1998). They report that a short nitridation (60 s) reduces the TD density from 1 1010 to 2 108 cm2 compared to a long nitridation of 400 s. Whereas the short nitridation produces screw or mixed dislocations, the long nitridation is reported to yield mainly pure edge dislocations. The material quality achieved on the short nitridated layer is also signi�cantly improved. The e�ciency of the nitridation is observed by X-ray photoelectron spec- troscopy (XPS) and other techniques (Auger sputter pro�ling, re�ection high-energy electron di�raction, low energy electron di�raction) which re- ported various degrees of e�ciencies for the nitridation. ECR plasma sources are being more e�cient than RF sources. Ammonia is found suitable for nitridation in both RMBE and MOVPE. 238 M. KAMP Fig. 6. Nucleation and coalescence of a nucleation layer. The established facets have an increased lateral growth rate eventually yielding a closed and planar layer. Fig. 7. In�uence of nitridation, nucleation and their combination on the PL (20 K) of a 2 lm GaN layer deposited under otherwise identical conditions. SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 239 Only very recently, it has been reported that the nitridation can yield a very inhomogeneous surface. MOVPE overgrowth of those surfaces results into GaN-layers with irregular Ga- and N-terminated areas that are deteriorating the layer quality (Seelmann-Eggebert et al. 1997). 6.4. MULTIPLE NUCLEATION LAYERS The usage of repeated NL separeated by about 1 lm of high temperature GaN layers was initially proposed by Amano et al. (1999). Figure 8 shows the temperature and growth pro�le during the growth of a GaN layer using multiple NL. Repeating such a layer sequence up to 7 NL, the TD density can be reduced from initially 5 109 to 5 107 TD/cm2 (Amano et al. 1999). The in�uence of a second NL on the optical output power of an InGaN/GaN MQW LED is shown in Fig. 9 (Schwegler et al. unpublished). The second NL is grown identical to the initial one after deposition of 1 lm GaN at high temperature (1050 C). A signi�cant increase in the optical output power can be observed for the structure with two NL, indicating a reduced density of non-radiative recombination centers. 6.5. THICK LAYERS Another concept successfully employed in GaN technology is the growth of thick GaN layers, preferably by hydride vapor phase epitaxy (HVPE). As- suming a certain probability for the termination and annihilation of dislo- cations, the dislocation density can be reduced just by the growth of thick layers. The reduction takes place by annihilation of dislocations and by local abolition of the translation invariance of the crystal, by dislocations, strain �elds, or even point defects (Beneking et al. 1985). Films up to 300 lm thickness have been grown by HVPE. The epitaxial layers reveal a huge Fig. 8. Scheme of a growth sequence for use of a multiple NL layer. 240 M. KAMP Fig. 9. In�uence of a 2nd NL on the optical output power of an InGaN/GaN MQW LED. vertical inhomogeneity with a strong reduction in carrier density and strain within the �rst 30 lm above the interface (Siegle et al. 1999). At the surface the dislocation density is reduced down to 107 cm 2 and the free carrier concentration can be as low as 1 1017 cm 3. Here, the lattice constants are approximately the ones of GaN due to an almost complete relaxation. Once di�culties in crack formation and surface morphology are overcome by a careful optimization of the growth process, high quality layers can be achieved. Excellent PL data have been reported with clearly resolved A, B and C free excitons and D X linewidths as low as 0.8 meV (Meyer 1999). As soon as those layers are commercially available, they are promising for use as quasi-substrates in GaN technology. 6.6. EPITAXIALLY LATERAL OVERGROWTH As mentioned earlier, a low temperature nucleation layer grown under appropriate conditions makes use of a lateral growth rate being signi�- cantly higher than the vertical growth rate. The same physical e�ect is used in ELOG GaN. This technique principally known from GaAs growth on Si substrates was initially employed to GaN technology by Usui et al. (1997). First, a regular low temperature nucleation layer is deposited on sapphire by MOVPE. Subsequently, an approx. 2 lm thick MOVPE GaN-layer is deposited. The layer is then removed from the MOVPE system and about 0.1 lm thick SiO2 or SixNy masks are deposited on the surface preferably in h1�100i direction. The width of the mask stripes is approx. 5 lm at a distance of approximately 10 lm. After introduction of the masked layers into a SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 241 MOVPE or HVPE system, a GaN layer is epitaxially grown on top of the structure to a thickness of about 20 or 200 lm, respectively. As is indicated in Fig. 10 dislocations below the masked area cannot propagate into the layer above. Only in the non-masked area, dislocations will be able to continue into the upper layers. The lateral growth of the subsequently deposited thick GaN layer leads to an overgrowth of the masked area. Since the epitaxial information is from the sidewalls of the GaN growing in the non-masked regions, the epitaxial quality is extremely high with dislocation densities as low as 1 106 cm2 and 3 107 cm2 in masked and windowed region, respectively. Since lateral overgrowth naturally takes place from both sides of the mask, a single dislocation will occur in the middle of the mask where both regions meet. In addition to the dislocation generated at the concurrence of the low, but existing vertical growth rate, lead to voids close to the center of the masks (Fig. 10). The device quality can be signi�cantly improved as is shown by Nakamura et al. (1999), who could increase LD cw-lifetime from about 50 to 10,000 h introducing ELOG substrates and AlGaN/GaN modulation doped barriers. Increasing the thickness of the �nal GaN layer to about 200 lm, by means of HVPE, allows for the separation of the GaN layer from the sapphire substrate (Nakamura et al. 1998). The self-sustaining layers can be separated by either polishing or by laser induced thermal dissociation in a process similar to the one described in (Kelly et al. 1996). Laser diodes fabricated on such freestanding GaN �lms reveal a signi�cant increase in lifetime. Comparing LD with an identical threshold current density, devices on freestanding GaN �lms have a reported lifetime four times longer than their counterparts on sapphire (Nakamura, private commu- nication). The improved performance is attributed to a reduced thermal load of the devices and improved laser facets, which can be obtained by simple cleaving, since device structure and quasi-substrate have the same orientation. Fig. 10. Schematics of the ELOG substrates depicting the processing steps and the obtained distribution of the dislocation density. 242 M. KAMP 7. GaN homoepitaxy Finally, the use of GaN single crystal substrates shall be discussed. As stated earlier melt growth of GaN is impossible due to the extraordinary temper- atures and pressures required for this process. However, the Polish High Pressure Research Center (Unipress) succeeded in GaN growth by employing a high pressure, high temperature process. GaN is formed from atomic nitrogen dissolved in a Ga melt, a process requiring N2 pressures of about 15 kbar and temperatures of about 1400 C (Porowski 1999). At a growth rate of approximately 100 lmh 1 perpendicular to the c-plane, the wurtzite crystals are grown up to areas of some 100 mm2 at a thickness of about 200 lm. The crystal quality of the GaN substrates is excellent as indicated by X-ray rocking curve measurements. Using CuKa1 radiation, linewidths of 20 arcsec are obtained for the (0002) re�ex. The excellent structural properties are also pointed out by very low dislocation densities ranging from 103� 105 cm 2. The optical quality, however, is poor, near-bandgap excitonic transitions are not visible, weak PL at 380 nm and at 530 nm is observable at room temperature (RT). Undoped crystals reveal a �at (000 1) surface (i.e. N-polarity) and a slightly rough (0001) surface (i.e. Ga-polarity). Both orientations of the un- doped single crystal substrates have been investigated for growth under identical conditions. Whereas the material quality of the (000 1) surface is still good (PL linewidth is approx. 5 meV) compared to heteroepitaxial growth, the properties achieved on the (0001) surface are clearly superior. The di�erences of the both orientations can be traced back to the di�erent free surface energies of the orientations. From ab-initio calculations it is determined that the free surface energy of the (000 1) surface is signi�cantly higher than the one of the (0001) surface (Zywietz et al. 1998). From this point, the (0001) orientation provides a more stable surface with a lower probability of dopant incorporation (Leszczynski and Meyer, unpublished). Furthermore, both orientations have distinctly di�erent surface morpholo- gies requiring a di�erent treatment. The almost �at (000 1) surface can be mechano-chemically polished to achieve an atomically �at surface, whereas the rougher (0001) side is chemically inert and can be mechanically polished only. The latter process leaves behind subsurface damage, which can be re- moved by dry etching of about 300 nm. Fig. 11(a) shows a SEM micrograph of a homoepitaxial grown GaN layer where only top half of the substrate was dry etched before growth. The epitaxial layer on top of the dry etched part of the substrate reveals an improved surface topography with almost no visible scratches, trenches, or holes. Fig. 11(b) shows the corresponding CL intensity distribution of the same region of the sample. On the etched part, the in- tensity variation is almost negligible. In contrast, the are being not etched yields only weak CL signals (1000 times less intensity) which also �uctuate SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 243 Fig. 11. SEM image (11a), corresponding cathodoluminescence (CL) intensities (11b) and local CL spectra (11c) obtained from an epitaxial GaN layer grown on partially CAIBE treated (0001) � oriented GaN substrates. CL measurements by F. Bertram, T. Riemann, and J. Christen, University Magdeburg. locally. In addition to the improved intensity the pre-treated area of the epitaxial layer shows a ten times narrower linewidth in CL (FWHM < 2 meV, still resolution limited) [Fig. 11(c)]. Homoepitaxial GaN layers with outstanding properties have been achieved on (0001) surfaces using above described CAIBE technique. High resolution PL at 4.2 K reveals free excitons A, B, C as well as excited states of those excitons, where the identi�cation is veri�ed by re�ectance measurements also included in Fig. 12. The linewidth of the bound excep- tions (3.464�3.472 eV) is as low as 0.1 meV. Initial LED structures have been homotype pn-junction LEDs. Only substrates that underwent a CAIBE treatment yield functional devices, otherwise the metallization across the trenches causes a shortcut over the pn-junction. The EL of homoepitaxial GaN pn-junction LEDs is depicted in Fig. 13 for various current densities. The LEDs show an intense, single peak emission at about 425 nm wavelength with a linewidth of 60 nm for low 244 M. KAMP Fig. 12. Re�ectance (above, linear scale) and low temperature photoluminescence (below, log. scale) of a 1.5 lm thick GaN layer grown by MOVPE on GaN single bulk substrates. The outstanding material quality is express by the world record narrow linewidth and the observance of strong free exciton and their excited states. The linewidth of the bound exciton is as narrow as 0.1 meV. Measurements by K. Kornitzer, K. Thonke, and R. Sauer. currents. It is remarkable that the emission wavelength is at approx. 425 nm even for current densities up to 3 kA cm 2. As was initially pointed out by Nakamura et al. (1991) this is a clear indicative for the high quality of the p-type material obtainable by homoepitaxial growth. The EL obtained from Fig. 13. Electroluminescence of a GaN homojunction pn-LED grown on GaN substrate. Emission spectra are depicted at various current densities. At a given current density, the homoepitaxial devices are twice as bright as comparable LEDs grown on sapphire (dashed line). SOLUTIONS FOR HETEROEPITAXIAL GROWTH OF GaN 245 heteroepitaxial LEDs grown on sapphire under identical conditions is also included into Fig. 13 for comparison. The heteroepitaxial device reveals a clear shift towards shorter wavelength being attributed to an inferior quality of the p-material at the pn-junction. In addition, the data in Fig. 13 reveal that the homoepitaxial LED is approximately twice as bright as their counterpart on sapphire. In addition to above homojunction LEDs, �rst InGaN/GaN DH-LEDs have been fabricated using single bulk crystal substrates (Kamp et al. 1999). Compared to identical structures on sapphire substrates the homoepitaxial heterostructure LEDs reveal an improved performance in particular at low current densities, indicating a lower concentration of non-radiative recombination centers. However, further work, depending on the avail- ability of the substrates, has to be carried out to develop the homoepitaxial device to a point where they become fully competitive to commercial LEDs. 8. Summary The deteriorating in�uence of threading dislocations in GaN is signi�cantly smaller than in other semiconductor systems. However, with a dislocation density being 6�7 orders of magnitude higher than with other III�V semi- conductors, they still have a severe in�uence on device performance and lifetime. Within the paper, the origin of the formation of the dislocation has been elucidated with special respect to GaN. Low temperature nucleation layers can signi�cantly improve the material quality. Additional nucleation Fig. 14. Electroluminescence of an InGaN/GaN LED grown on GaN single bulk crystal substrates. Excellent EL is achieved at low current densities, revealing a low density of non-radiative defects. 246 M. KAMP layers have been investigated and a further reduction of the dislocation density is obtained using this moderate e�ort coming along with an in- creasing LED device performance. Thick HVPE layer can also reduce the dislocation density and in combination with ELOG can provide high quality quasi-substrates successfully employed for GaN laser diodes. Bulk GaN single crystal substrates, however, are the ultimate benchmark for GaN technology. With this substrates PL linewidths as narrow as 0.1 meV have been demonstrated (Fig. 14). First LED work on homojunction GaN and heterojunction InGaN LEDs on GaN substrates is promising, with homo- epitaxial LEDs being about twice as bright as their heteroepitaxial coun- terparts. Being not di�erent from other semiconductor systems in that point, growth of GaN on GaN (quasi-)substrates is clearly favorable over heteroepitaxy. Acknowledgements The author is indebted and grateful to C. Kirchner, A. Pelzmann, M. Mayer, V. Schwegler, and K.J. Ebeling from Department of Optoelectronics at University of Ulm for their valuable contributions, continuous support and helpful discussions. Without their work, this paper would never have been written. Several other researchers contributed to this work, including K. Kornitzer, K. Thonke, and R. Sauer from University Ulm, Department of Semiconductor Physics (high resolution PL measurements and re�ectance), F. Bertram, T. Riemann, and J. Christen from University Magdeburg (CL measurements), S. Christiansen, M. Albrecht, and H.P. Strunk from Uni- versity Erlangen-Nurnberg (TEM measurements). The outstanding GaN � bulk crystal substrates have kindly been supplied by the Polish High Pressure Research Center, namely by M. Leszcnynski, I. Grzegory, and S. Porowski. The author gratefully acknowledge their valuable contributions. 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