Defect Studies of GaN under Large Hydrostatic Pressure
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417 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
EXPERIMENTAL 10-HTVPE GaN
. Small GaN bulk crystals produced by two different T = 6K ,' growth techniques have been studied. Platelets of bulk GaN at dimensions of typically 150 x 100 0.5 x 50 [im 3 have been grown by an unconventional high temperature eA vapor phase technique [8,9]. -ILO eA From ammonia and elemental ......... -2LO gallium thick films of GaN where o . .. .. . .. ...
.
3.42eV
,
bn
eA
b) .............. deposited at 1240 oC onto i 0.0 various substrates. At high 3.5 3.4 3.3 3.2 3.1 growth rates of typically 200 p Energy (eV) m/h a spontaneous nucleation of small crystals can be found in Figure 1: Low temperaturePL of GaN. In addition to donor addition. On the other hand bound exciton and band acceptor transitions (spectrum a) crystals at dimensions of an intense contributionis found at 3.42 eV (spectrum b). approximately 500 x 300 x 20 ýt m3 have been grown by high pressure synthesis from molecular nitrogen dissolved in Ga [10]. Crystals from both techniques are optically clear and show no grain boundaries visible to the eye. While the samples from the high pressure synthesis show a brownish color, the vapor phase material studied here appears to be perfectly transparent. The c-axis is normal to the larger dimensions. All samples were nominally undoped and n-type. Hydrostatic pressure was applied by means of a diamond anvil cell using nitrogen as a pressure medium. The pressure was determined using the R1 ruby fluorescence. Photoluminescence (PL) was excited by the 325 nm line of a HeCd laser in an optical cryostat operating at a temperature of 6 K. The emission was dispersed by a 0.85 m double grating spectrometer and detected by a UV sensitive photomultiplier.
RESULTS 3.42 eV Luminescence Band Low temperature PL of two different samples of the HTVPE material is presented in Fig. 1. Common features are the donor bound exciton (D 0 ,X) at 3.472 eV [11]and the set of bandacceptor recombination at 3.27 eV [8] including several phonon replica at lower energies. While no luminescence is observed in the intermediate spectral range for sample a) a strong contribution is found for sample b). The rather broad peak at 3.42 eV has repeatedly been reported in PL studies and was assigned to an oxygen related defect recently [12]. It appears at an optical binding energy of-80 meV with respect to the low temperature bandgap of 3.504 eV. This is a very large value when compared to typical binding energies of excitons bound to shallow donors (-30 meV) or acceptors (-40 meV). On the other hand this value is too small to be caused by a donor-acceptor pair interaction where the smallest reported optical binding energies are about 260 meV [8]. Such an observation could indicate the participation of a strongly localized defect for which the above energies typically does not apply.
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In order to test for such a participation we applied hydrostatic pressure up to + T=6K 6 GPa. Several spectra are + 3.5 presented in Fig. 2. The shallow b
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