Sublimation growth of AlN crystals: Growth mode and structure evolution

Transcription

1 Journal of Crystal Growth 281 (2005) Sublimation growth of AlN crystals: Growth mode and structure evolution R. Yakimova, A. Kakanakova-Georgieva, G.R. Yazdi, G.K. Gueorguiev, M. Syva ja rvi Department of Physics and Measurement Technology, Linköping University, SE Linköping, Sweden Available online 15 April 2005 Abstract The aim of this study has been to realize growth conditions suitable for seeded sublimation growth of AlN and to understand the relationship between external growth parameters and the initial stages of growth with respect to growth mode and structure evolution. Close space sublimation growth geometry has been used in a RF-heated furnace employing high-purity graphite coated by TaC with a possibility to change the growth environment from C- to Ta-rich. Influence of certain impurities on the initially formed crystallites with respect to their shape, size and population has been considered. It is shown that some impurity containing vapor molecules may act as transport agents and suppliers of nitrogen for the AlN growth. SiC seeds, both bare and with MOCVD AlN buffer, have been employed. By varying the process conditions we have grown crystals with different habits, e.g. from needles, columnar- and plate-like, to freestanding quasi-bulk material. The growth temperature ranged C whereas the optimal external nitrogen pressure varied from 200 to 700 mbar. There is a narrow parameter window in the relationship temperature pressure for the evolution of different structural forms. Growth modes with respect to process conditions are discussed. r 2005 Elsevier B.V. All rights reserved. PACS: Mg; a; w; 81.15Kk; Hd Keywords: A1. Crystal morphology and structure; A2. Growth from vapor; A3. Sublimation epitaxy; B1. Aluminium nitride 1. Introduction Aluminum nitride (AlN) substrates are becoming increasingly attractive for short-wavelength Corresponding author. Tel.: ; fax: address: roy@ifm.liu.se (R. Yakimova). LEDs and lasers, as well as for high-power and high-frequency electronics [1,2]. AlN is particularly interesting for deep UV optoelectronic devices due to its large bandgap (6.2 ev). At the present stage of development, the AlN substrates are limited in size and frequently contain macrodefects such as cracks and inclusions [3]. Besides, AlN is extremely perceptive to oxygen and easily /$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi: /j.jcrysgro

2 82 R. Yakimova et al. / Journal of Crystal Growth 281 (2005) forms Al-carbides, that is affecting not only the purity of the material but also the growth kinetics and mass transport [4,5]. It is worth noting that AlN tends to grow in preferential crystallographic directions (c-axis) resulting in crystals with high aspect ratio instead of desired layer-by-layer grown bulk crystals. It has been observed that some impurities (e.g. boron) are likely to promote AlN anisotropicgrowth [6]. During recent years, growth of AlN by seeded sublimation technique has been intensively studied [7 11]. Concomitantly, though 25 mm diameter boules of AlN have been reported, full control of the bulk growth process via sublimation is still a challenge. The quality and the yield of the grown material depend on many intrinsicand extrinsic factors that have been analyzed by a number of authors; however, nucleation and crystal habit are issues that remain to be resolved. These indicate an urgent need for deeper understanding of the role of growth parameters and residual impurities in growth of large-area AlN bulk crystals. The aim of this research has been to realize seeded sublimation growth of AlN by a simple and reliable set-up and to understand the relationship between external growth parameters and the initial stages of growth with respect to growth mode and structure evolution. 2. Experimental results and discussion 2.1. Growth approach Sublimation-based growth was used assuming the reaction 2Al(v)+N 2 (v)22aln(s), where v stands for vapor and s for solid. The growth arrangement is based on the close space sublimation method, with a variable spacing between the source and the seed, thus providing conditions to study the nucleation and crystal development at the initial stage of growth. Another advantage of the close space geometry is that the probability of parasitic reactions with crucible walls is much lower compared to a large crucible. SiC seeds were utilized, either bare or with MOCVD AlN buffer, which offers several potential benefits, both with respect to growth and device engineering. In addition to the smaller lattice mismatch between AlN and SiC (compared to sapphire substrates), the SiC seeds provide oxygen-free interface at the beginning of the growth. AlN ceramics served as a source of vapor species. The growth furnace was a RF-heated air-cooled vertical quartz chamber. The crucibles, which also worked as secondary heaters, were fabricated of high-purity graphite, coated with TaC. For some experiments, only graphite crucibles were employed while in other cases a Ta cap was inserted in the TaC-coated crucible, thus making it possible to change the ambience from C- to Ta-rich. This allowed us to observe influence of certain impurities on the initially formed crystallites with respect to their shape, size and population Effect of temperature and pressure on crystal shape and evolution Aluminum nitride growth Fig. 1 was performed by sublimation of AlN source, which supplies both Al- and N-containing species, and deposition on a single-crystal SiC substrate in the ranges of temperature C and external nitrogen pressure mbar. AlN is known to evaporate congruently at high temperatures providing nearly stoichiometric composition of the vapor phase. The pressure of Al and N 2 species over AlN surface reaches 1000 mbar at temperatures of C as it is shown in Ref. [12]. Based on this, and having in mind our aim to study initial stages of crystal formation, we choose AlN source temperature to be not higher than C. Under these conditions, the activation energy of the Fig. 1. Schematic of the AlN growth geometry. The distance between source and seed can be varied from 1 to 3 mm. Seed dimension is a quarter of 2 00 SiC wafers, Si-face up.

3 R. Yakimova et al. / Journal of Crystal Growth 281 (2005) growth as deduced from the Arrhenius plot of the growth rate is 550 kj/mol, which is close to the enthalpy of sublimation of AlN (630 kj/mol). External nitrogen pressure is usually used to support the growth and compensate for the lowsticking coefficient of nitrogen. Our experiments showed that if the growth is performed at temperatures higher than C, the growth rate increases with increasing the external nitrogen pressure while bellow that temperature the growth rate decreases with increasing nitrogen pressure. These observations indicate that in AlN growth there is an optimal external nitrogen gas pressure and that the surface mobility of the growth species should be considered when selecting growth conditions. By varying the growth conditions, crystals with different habits were prepared, e.g. from platelet-like, needles, and columnar to freestanding quasi-bulk material. Table 1 summarizes the observed growth modes depending on the temperature and nitrogen pressure, while Fig. 2(a) (e) visualizes obtained crystal forms. It appears that high temperature and medium pressure promote continuous growth, whereas with decreasing temperature and increasing pressure growth anisotropy becomes more pronounced. One can see that the parameter window in the relationship temperature pressure for the evolution of different structural forms is rather narrow. At low temperature (1600 1C) and high nitrogen pressure (700 mbar), AlN grows as thin single-crystal platelets, which nucleate presumably at the steps existing on off-oriented SiC substrates. The discrete crystallites are aligned in a similar way and with a preferential orientation (Fig. 2a). These features suggest an epitaxial growth via 2D nucleation, which in principle is a perfect start of good-quality crystals. However, due to the low supersaturation and relatively high gas pressure (see Table 1), the surface concentration and mobility of the growth species seem to be too low to allow complete coalescence of the nuclei. At higher temperature, e.g C, needle-like crystals typically grow (Fig. 2b). Similar results have been obtained by other researchers at around C [13]. Taking into account that temperature measurements differ form setup to setup, these findings are in a good agreement. As to the effect of the growth environment it is worth pointing out that needle formation was observed only in TaCcoated graphite crucibles. The needles have sixsided prismaticshape with a flat top with no indications of liquid droplets assisting the growth and therefore we believe that the growth mode does not involve the vapor liquid solid (VLS) mechanism, but rather direct feeding from the vapor phase. The needles nucleate from hexagonal hillocks initially formed on the seeding SiC wafer (Fig. 3a). Fig. 3b shows an AFM image of a hillock illustrating the hexagonal shape. The needles evolve from the apex of the hillocks. It was observed that the density of the nucleation sites and the needles depends not only on temperature but on pressure as well, i.e. with pressure decrease needle population becomes denser. The average number of the hillocks is cm 2, thus being very close to the dislocation density in SiC as revealed by KOH etching. One can speculate that the intersections of the treading dislocations (typically with a screw component) from the substrate with its surface are locations with higher strain energy and thus acting as a driving force for 3D nucleation of AlN. Ones the hillock is formed, the in-built screw dislocation Table 1 Summary of growth modes of AlN crystals vs. growth conditions Temperature 1C C-crucible N-Pressure (mbar) AlN-buffer Growth mode Single crystal, 2D growth, platelet-like Single crystal, needle-like Continuous, columnar background, some cracks Free-standing, polycrystal, no cracks Continuous, single crystal, 2D growth, some cracks If not mentioned TaC coated crucibles and bare SiC off oriented substrates were used.

4 84 R. Yakimova et al. / Journal of Crystal Growth 281 (2005) Fig. 2. Crystal appearance under different growth conditions: (a) platelets; (b) needles; (c) columnar continuous, 340 mm thick; (d) single crystal, 200 mm thick, diameter of 8 mm and (e) free-standing polycrystalline, 120 mm thick, no cracks, diameter of 9 mm. For details on growth conditions see Table 1. Fig. 3. (a) Optical microscopy image of hexagonal hillocks on SiC substrate, which are nucleation sites for needle growth and (b) an AFM image of a hillock illustrating the hexagonal shape. will promote fast growth in a preferable direction, e.g. c-axis. Unfortunately, at the present we do not have evidence whether the needles contain dislocations or they are dislocation-free. Needle formation is likely to occur at around C, that is the temperature when decomposition of SiC could be expected and that might result in decoration of the threading dislocations, this providing favorable nucleation sites. Fig. 4. CL spectrum taken from a needle at 4.7 K, with beam energy 20 KeV. In the inset the peak related to band gap emission is magnified. A CL spectrum taken from a needle confirms that it is AlN (Fig. 4). The spectrum exhibits a small-intensity band gap emission and two broad high-intensity peaks characteristic of impurity vacancy complexes.

5 R. Yakimova et al. / Journal of Crystal Growth 281 (2005) Fig. 5. Effect of the growth environment on size, shape and population of AlN crystallites grown under similar conditions (temperature of C, nitrogen pressure of 200 mbar, and growth time of 60 min), except for: (a) C-rich environment and (b) Ta-rich environment Effect of environment on growth kinetics In this study we had the possibility to change the growth environment by changing the crucible material, e.g. from C-rich to Ta-rich. This allowed us to observe influence of certain impurities on the initially formed crystallites in respect to their shape, size and population. We considered carbon and oxygen, since these are the most persistent contaminations in AlN growth by sublimation. By calculating the cohesive energy (CE) of possible molecular species in the vapor phase (for details see Ref. [14]), it was found that NO and CN molecules are readily present during growth and these species rather than N 2, which has higher CE, may take part in the following reactions: 2AlðvÞþNOðvÞ2AlNAlO N ðsþ; (1) 2AlðvÞþCNðvÞ2AlNAlC N ðsþ: (2) The first reaction results in O 2 incorporation while the second one yields C incorporation. Other vapor molecules may act as transport agents and suppliers of nitrogen for the AlN growth: AlðvÞþNO 2 ðvþ2alnðsþþo 2 ðvþ; (3) Al 2 CðvÞþ2NOðvÞ22AlNðsÞþCO 2 ðvþ: (4) In case of Ta-rich environment, Ta will attract C to form TaC with higher probability compared to Al 2 C, because of the higher CE, respectively 5.2 and 3.3 ev. For similar reason oxygen and nitrogen will be adsorbed by Ta. Consequently, the availability of NO, NO 2 and CN molecules will decrease resulting in less nitrogen supply. These results are supported by experimental observations suggesting that AlN crystal nucleation in Ta-rich environment becomes suppressed due to the scarcity of nitrogen. In Fig. 5 transition from C- to Ta-rich growth environment is illustrated. It is seen that the size and the shape of the AlN crystallites is influenced by the change of the vapor-phase composition due to different growth environments. We also believe that the presence of impurities may facilitate the kinetics of the overall growth process, for instance, the observed lower activation energy of the growth compared to the expected one for sublimation might be regarded as an indication of such an influence. 3. Conclusion The results of this study demonstrate that the close space sublimation technique is perfectly suited for studying nucleation and structure evolution of AlN crystals: it is easy to operate, does not require expensive procedures and is a prototype of seeded sublimation growth. There exists a narrow parameter window for the occurrence of crystals with different habits, suggesting that the growth mechanism is very sensitive to deviations from optimal process conditions. This is a peculiarity characteristic of AlN and has to be stressed prior to development of growth technology for large area crystals. Besides temperature and nitrogen pressure, presence of impurities in the growth chamber is critical not

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