Near Defect Free GaN Substrates
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preparation techniques to GaN single crystals will be illustrated by results of microscopic and structural investigations. Finally the results of epitaxy both by MOCVD and MBE will be discussed. PHYSICAL PROPERTIES OF Ga(l)-N2-GaN SYSTEM AND GaN CRYSTAL GROWTH
pressure, bar
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Thermodynamic and kinetic properties of Ga(l)-N2-GaN(s) system are determined mostly by strong bonding both in N2 molecule and in GaN crystal. The transition between the initial state of liquid gallium and nitrogen N2 and GaN crystal requires breaking the extremely strong bonding in N2 molecule (bond energy 9.8 eV/molecule) and creation of GaN bonds. Strong bonding in GaN crystal (bond energy (9.32 eV/atom pair) leads to high melting temperature TMGaN ~ 2800 K [8]. The estimated N2 equilibrium pressure for GaN is over 45 1500 1250 kbar [9]. Such pressures and 2500 2000 1750 0,0150 temperatures are not accessible to temperature, K present day crystal growth 0,0125 apparatus making necessary use of lower temperatures and lower 0,0100 nitrogen pressures. At present the large volume high gas pressure 0,0075 apparatus allows to obtain the nitrogen pressures up to 20 kbar. 0,0050 It is believed that GaN grows from nitrogen solution in 0,0025 liquid Ga. Thus the synthesis of GaN consists of several stages: 0,0000 i/ adsorption of N2 on liquid Ga 0,0004 0,0005 0,0006 0,0007 0,0008 surface 1/T, K-1 ii/ dissolution of nitrogen in liquid Ga and its diffusion to the cold 2000 1750 1500 1250 zone 100000 temperature,K iii/ growth from solution Recent quantum mechanical 20 kbar calculations have shown that adsorption of N2 on liquid Ga pressure limit 10000 surface leads to dissociation of N2 of 20 kbar molecule [11]. The energy barrier for this process is of the order of 4.2 eV which is less than half of the 1000 GaN N2 bonding energy. This indicates that the bonding between Ga and N Ga+1/2N 2 atoms plays important role in the process. The estimate of the rate of 100 0,0004 0,0005 0,0006 0,0007 0,0008 the adsorption indicates that this 1/T, K-1 process is not the rate limiting Fig 1. N2(g) - Ga(l) - GaN(s) phase diagram: a - p-T coordinates process in GaN crystallization at [9]; b - x-T coordinates [10] high pressures [6]. As it can be seen from Figure 1 nitrogen pressure about 20 kbar corresponds to nitrogen solubility in liquid gallium of the order of 1%. Lower temperatures correspond to lower nitrogen solubility, therefore by controlling the temperature Downloaded from https://www.cambridge.org/core. IP address: 188.27.129.34, on 15 Jun 2018 at 10:16:08, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300002210
difference in the crucible we can control the difference of the nitrogen concentration and accordingly the supersaturation in the growth zone. Our growth results suggest that the nitrogen solubility and diffusion from the hot to cold zone of the gallium is the rate limiting step in the growth. Due to relatively rapid dissolution of nitrogen in liquid
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