GaN Crystals: Growth and Doping Under Pressure
- PDF / 1,803,537 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 77 Downloads / 235 Views
N 2 , becomes quite close to that of the crystal. It is illustrated in Fig. I where the free energy of GaN (1 mole) and the free energy of the system of its constituents (Ga+1/2N 2 ) is shown as a function of temperature and N2 pressure. temperature,K With increasing temperature, G(T) of 0 5oo 1000 1500 2000 2500 3000 3500 the constituents decreases faster than G(T) of 0Ga÷1-GaN the crystal and at higher temperatures, the ,t -50o G-H-TS nitride becomes thermodynamically unstable. ;,-0G Go (G, ,+12G,,) The crossing of G(T) curves determines the 0 10-in equi#ibrw G-0
equilibrium temperature where GaN coexists with its constituents at given N 2 pressure. The application of pressure increases the free A-250. GaN .kbar energy of the constituents in much higher degree than G(T) of the crystal. As a S-300 S_350 . consequence the equilibrium point shifts to higher temperatures and GaN stability range Fig.1 Gibbs free energy of GaN and its constituents extends. The equilibrium PN2 - T conditions for GaN have been determined experimentally by Karpinski et al. [8] and the curve following from these data is shown in Figure 2. Crystallization processes discussed in this paper have been carried-out at N 2 pressure up to 20 kbar which corresponds to GaN stability limit of 1960K. These conditions are marked in Fig.2 The extension of GaN stability range allows to grow GaN crystals from the solution in the liquid Ga. In Fig. 3 we have shown the N solubility data resulting from the annealing of Ga at the three phase equilibrium conditions. Even the highest available temperature of 1960 K is quite far from the melting temperature of GaN. According to the theoretical estimation of Van Vechten [9] it is close to 3000K. -150
S-00G
2000
a+I/2N2 1Ibar
1500 1250
1000 temperature,K
pressurelim.it of 20 kbar 10000
1800 r31
Wa 8.1rn
1700
1000
GaN
ai•
10
2
~~60
Ga+/2
150
100
1 0.0004
0.0006
14000.0008
0.0010
0.0012
0.001
Fig. 2 Equilibrium curve for GaN [8]
0.002
0.003
0.004
0.005
XN, at.fr.
lVT, K
Fig.3 Liquidus line for Ga-GaN system: the solid line was calculated in the ideal solution approximation.
Therefore the N concentrations are not high (below 1 at.%) and the growth experiments have to be long to get high quality crystals with dimensions appropriate for research and applications. Therefore the long time (>100h) of stable work is an additional requirement for the growth system.
16
The solid line in Fig. 3 is the liquidus line for Ga-GaN system calculated in ideal solution approximation with Van Vechten's melting temperature of 2790K. For this approximation the solubility can be expressed as follows: n = no exp-AHso1 kT
(1)
where AHsol is the heat of dissolution. For GaN AHsol=44.7kcal/mole=0.49 eV/bond and expresses the bonding energy in the crystal in relation to its mother phase - the solution. Apparatus and procedure 3 At present GaN is crystallized in gas pressure chambers with volume up to 1500 cm 3 allowing crucibles with the working volume of 50-100 cm . The high pressure - high temperature
Data Loading...
