In the crystal growth process, the reaction AlN (s) ↔ Al (v) +1/2 N2 is driven in the forward direction at the source material, and in the reverse direction at the crystal growth zone by a slight reduction in temperature. The source material was AlN powder with elemental analysis of O less then 0.9 wt% and C less than 0.04 wt%. Tungsten crucible was used in tungsten furnace. All other candidate crucibles, including TaC, NbC, HPBN, and graphite, were used in graphite furnace.
The AlN crystals were grown between 1800 °C and 2200 °C. Ultra high purity nitrogen continuously flowed through the system at 800 torr to prevent the heating element from reacting with trace oxygen. The growth environment was always nitrogen-rich as the nitrogen pressure is orders of magnitude higher than the saturation vapor pressure of nitrogen over AlN, which is less than 9.9×10−3 atm at the growth temperature range [23]. The temperature difference between the source zone, where AlN source sublimes, and the growth zone, where Al and nitrogen recondense, was less than 10 °C as determined by the premeasured temperature profile.
Macrostructure features of the as-grown AlN crystals were characterized by optical microscopy at magnifications of 10-200X. Micro-Raman spectra were obtained using a Renishaw UV micro-Raman system with the 325 nm line of a HeCd laser as the excitation source. The spot size and the spectral resolution were 12 μm and 3–4 cm−1, respectively. X-ray topographies were taken at the Stony Brook Synchrotron Topography Station. Topographies were recorded on 8” ×10” Kodak Industrex SR-45-1 high-resolution X-ray film.
For the first time, TaC coated graphite crucible was employed in bulk AlN crystal growth. Amber colored AlN crystals were produced in a TaC coated graphite crucible at 2074 °C and 805 torr for 24 hours. The AlN crystals nucleated in high density on the TaC, and grew into a polycrystalline mass. The phonon frequency of E2(high) mode and A1(TO) mode from Raman characterization are 655 cm−1 and 612 cm−1, with line widths are 7 cm−1 and 15 cm−1 respectively, indicating the AlN crystal’s high quality. However, since the AlN wetted with TaC, it was very difficult to separate the crystals from the nucleation surface. A scanning Auger microscopy (SAM) line scan on the cross-sectional area of the TaC foil with AlN crystal shows that the TaC foil lose carbon and pick up small amount of nitrogen (figure 3). Although the TaC film has good process durability and good crystal nucleation characteristics, pinholes in the coating lead to the reactions between the Al vapor and underlying graphite, and cause the coating to spall off.
Figure 3Line scan of the cross-sectional area of TaC foil with AlN crystal showing N “segregation” and C “depletion”.
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Prism shaped AlN needles and hexagonal hillocks of millimeter size crystals were grown on the nucleation surface with high nucleation density. However, NbC coated graphite crucibles suffered from the same pinhole problem as TaC coated crucibles. SAM characterization shows the NbC coating is stable against Al, and nitrogen is detected from the damaged NbC coating, primarily at the grain boundaries and not in the bulk NbC. From selective etching analysis in 45 wt% KOH solution, we infer that freely nucleated crystals predominately have the nitrogen to aluminum direction pointing out from the nucleation surface, which means that the ends of the AlN crystals facing the source are aluminum polarity [33].
Typically, amber colored AlN crystals are produced in tungsten crucibles with a high nucleation density in a three-dimensional growth mode. Sliced and polished AlN crystals grown in W crucible have grain size about 0.5mm×0.5mm (Figure 4). The growth temperature was about 2000 °C and growth time was 14 hours. Due to a high nucleation density, it was hard to get single crystalline seeds from this kind of samples for the subsequent seeded growth. The crystals grown in tungsten crucible contain no intentional impurities, but W and O are inherited from the crucible and source material. Neutron activation analysis revealed 0.2 to 4.6 ppm wt W in the grown crystals, lower than the 7.9 ppm wt reported by Bickermann [8]. However, combustion analysis shows an average value of 500 ppm wt O in the grown crystals, which is higher than the 86 ppm wt from Bickermann’s result [8]. There is no reaction between the tungsten furnace fixtures and nitrogen under the growth condition, thus no tungsten nitride, which decomposes at temperature less than 800 °C [34], formed during AlN crystal growth.
Figure 4Sliced and polished AlN crystal grown in tungsten crucible.
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Growth in graphite crucibles consistently produces needles and kite-shaped twinned crystals bounded on edges along the c direction. These platelets are transparent and colorless m-plane (1100) crystals with very flat surface morphology. The corner with the smallest angle (60°) is where the crystal was in contact with the graphite crucible and hence is the initial point where the crystals began to grow. Dark particles are usually attached to this corner, and these m-plane crystals do not etch in molten KOH solution. A thorough discussion of this type of twin can be found in reference [18]. Graphite has good process durability below 2050 °C, but carbon causes the AlN crystals to twin. At the crystal growth temperature of 2100 °C and 1 atm of nitrogen, the calculated carbon vapor pressure is in the order of 10−8 atm. However, cyanogen (CN) has an even higher partial pressure of 10−5 atm, which will always be significant in a graphite furnace at the temperatures typical for AlN crystal growth. Thus, there is a high potential for carbon to incorporate into AlN crystals, and cyanogen is more likely the main transport agent for carbon than elemental carbon. In AlN crystals, C can substitute N and create specific defects, such as stacking faults and inversion domains. Al4C3 will also form from reaction between graphite and Al vapor, or Al2CO if there is any trace oxygen in the system. Figure 5 shows the needle and kite shape AlN crystals with graphite contamination grown in a graphite crucible under 1800 °C, 800 torr for 48 hours.
Figure 5Needle and kite-shape AlN crystals grown in graphite crucible.
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Hot pressed boron nitride (HPBN), pyrolytic boron nitride (pBN) and a titanium diboride/BN composite (60% TiB2, 40% BN), were investigated as crucible materials for bulk AlN crystal growth. Only AlN needles were obtained in TiB2/BN crucibles under the typical crystal growth condition. The pBN crucible began to form pits on the crucible wall at a temperature about 1800 ° C. At higher temperatures, the crucible wall began to peel until it finally failed. The HPBN crucible was the most durable crucible of these three boron-based crucibles. Although the HPBN crucible encountered mass loss each run, which is less than 0.30 wt% per hour, a crucible with 2mm wall thickness can survive several hundred of hours under the growth conditions.
Clear and colorless AlN platelets up to 60 mm2 were prepared in HPBN crucibles by sublimation-recondensation technique under 2100 °C, 800 torr for 48 hours (Figure 6). Crystals grown in HPBN crucibles typically form thin platelets with the fastest growth rate above 400 μm/hr occurring in the c-axis direction. In the a-direction, the growth rate is much lower, about 50 μm/hr under the same growth condition. Growth striations run the length of the crystals in the c-direction, the cause of which has not been identified. In contrast to the other crucible materials employed in this study, the AlN nucleation density was relatively low on HPBN, and the crystals did not agglomerate together to form a dense polycrystalline mass. Instead, it was highly porous. Figure 7 shows the Raman spectrum for AlN crystals grown in HPBN crucibles. There is no significant difference in phonon frequency between sample with and without striations. The phonon frequency of E2 (high) mode and A1 (TO) mode from Raman characterization for both samples are 655 cm−1 and 609 cm−1. Line widths for both modes are 6 cm−1, which indicates good crystal quality. X-ray topography reveals numerous crystallites concentrate in some regions on the sample surface (figure 8). Crystallites are also scattered in some other regions. The crystallites induce strains and sometimes cracking on the surface, as evident in the x-ray topography. A few inclusions (I) are also observed. This sample appears essentially dislocation-free. However, from thermodynamic calculations, vapor pressure of B over BN in graphite furnace is very high among all the candidate crucible materials under the crystal growth condition, which is 3.5×10−7 atm.
Figure 6AlN crystal produced in HPBN crucible up to 60 mm2.
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Figure 7Raman spectrum for AlN crystal as seen in figure 6.
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Figure 8Transmission topograph (g = 0002, λ = 0.69Å) showing inclusions (I). Arrow marks indicate contrast from surface artifacts such as ridges, scratches, strains, and cracks due to crystallites.
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