Near Defect Free GaN Substrates
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crystallographic, optical and electric will be discussed. Subsequently the results of surface 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 MB3E will be discussed. PHYSICAL PROPERTIES OF Ga(l)-N 2-GaN SYSTEM AND GaN CRYSTAL GROWTH Thermodynamic and kinetic properties of Ga(l)-N 2-GaN(s) system are determined mostly by strong bonding both in N 2 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 N 2 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
- 2800 K [8]. The estimated N 2 equilibrium pressure for GaN is over 45 1750 1500 250 kbar [9]. Such pressures and 0,01501 ' ture, K temperatures are not accessible to tempera ture, K S tempera present day crystal growth 0,0125 apparatus making necessary use of lower temperatures and lower emper • 0,0100 nitrogen pressures. At present the large volume high gas pressure 2,0,0075 apparatus allows to obtain the nitrogen pressures up to 20 kbar. ;20,0050 It is believed that GaN grows from nitrogen solution in 0,0025 r liquid Ga. Thus the synthesis of GaN consists of several stages: 0,0000L 008 i/ adsorption of N2 on liquid Ga 0,0004 0,0005 0,0006 0,0007 surface 1 1/T, Kii/ dissolution of nitrogen in liquid Ga and its diffusion to the cold 2000 17.50 1500 50 zone innnnnm temperature,K iii/ growth from solution Recent quantum mechanical 20 kbar ---calculations have shown that adsorption of N2 on liquid Ga pressure limit L10000: of 20 kbar surface leads to dissociation of N 2 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 N 2 bonding energy. This indicates that the bonding between Ga and N Ga+1/2N • , , . , k2 atoms plays important role in the process. The estimate of the rate of 100' 0,0004 0,0005 0,0006 0,0007 008 the adsorption indicates that this 11T, 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 TMGaN
2500 21900
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 Ga, in the beginning of the process GaN polycrystalline crust is created on the Ga surface. Some of the crust GaN crystal
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