GaN Homoepitaxy for Device Applications
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simple facet cleaving [3] becomes feasible and the far field pattern will be improved due to the relative high index of refraction of the GaN substrates. These outstanding properties justify the enormous endeavor conducted at several laboratories on the growth of single crystal GaN substrates [4],[5],[6]. Even when the effort for those substrates is probably too high for mass products such as LEDs, it might be worth for high power laser applications. Experimental Substrate preparation The GaN substrates have been grown in a high-temperature high-pressure process where the GaN is formed at N2 pressures up to 20 kbar and at temperatures up to 1600°C from atomic nitrogen dissolved in a Ga melt. At a growth rate of approximately 100 µm/h perpendicular to the c-plane, the wurtzite crystals are grown up to areas of some 100 mm2 at a thickness of about 200 µm. Undoped crystals used in this study reveal an flat (000-1) surface (i.e. N-polarity) and a rough (0001) surface (i.e. Ga-polarity), following E. Hellman’s convention on the orientation [7]. The (000-1) surface can be mechano-chemically polished to achieve an atomically flat surface. The initial roughness of the (0001) side requires a polishing for device quality growth. Due to the chemically inertness of the (0001) surface this can only be achieved by mechanical polishing. The crystal quality of the GaN substrates is excellent as indicated by x-ray rocking curve measurements [8]. Using CuK α 1 radiation, linewidths of 20 arcsec are obtained for the (0002) reflex. The excellent structural properties are also pointed out by very low dislocation densities ranging from 103 - 105 cm-2. The optical quality however is poor, near-bandgap excitonic transitions are not visible, weak PL at 380 nm and at 530 nm is observed at room temperature (RT). Epitaxial growth The homoepitaxial growth of GaN is performed in a horizontal, RF heated, water cooled quartz MOVPE reactor (AIXTRON AIX 200 RF) operated at low pressures. Trimethylgallium (TMGa), Trimethylindium (TMIn), Trimethylaluminum (TMAl), ammonia (NH3), silan (SiH4) and bis-cyclopentadienylmagnesium (Cp2Mg) are used as precursors, hydrogen is used as carrier gas. The one side polished GaN substrates (approximately 8x6 mm2 in size) were heated to 1060°C under a steady flow of ammonia to avoid surface degradation. During growth, the flow rates of NH3 and TMGa were kept at 2.0 slm and 17 µmol/min, respectively. The Cp 2Mg flow rate was 90 nmol/min for p-type doping. After growth, thermal annealing under nitrogen ambient at 750°C was performed for 5 min to obtain p-type conductivity. Characterization Photoluminescence measurements at RT and 4 K are performed using a HeCd laser (λ = 325 nm) with an excitation density of approx. 10 mW/mm2. X-ray diffraction (XRD) with CuK α 1 radiation is used t
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