Optoelectronic and Structural Properties of High-Quality GaN Grown by Hydride Vapor Phase Epitaxy
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ABSTRACT
Gallium nitride (GaN) films have been grown by hydride vapor phase epitaxy (HVPE) in a vertical reactor design. We report on GaN growth directly on sapphire using a GaCI surface pretreatment. The electrical properties of these films compare favorably with the highest values reported in the literature for GaN. Specifically, a room temperature Hall mobility as high as 540 cm 2/V s, with a corresponding carrier concentration of 2xl017cm"3, have been attained. Additionally, the vertical reactor design has assisted in reducing nonuniformities in both film thickness as well as in transport properties due to depletion effects, as compared with horizontal designs. The dislocation density in these films has been determined by plan-view transmission electron microscopy to be -3x10 cm . Photoluminescence spectra obtained at 2 K show intense, sharp, near-bandedge emission with minimal deep level emissions. Stimulated emission has been observed in these films, utilizing2a nitrogen laser pump source (X=337.1 nm) with a threshold pump power of -0.5 MW/cm . These results suggest that HVPE is viable for the growth of high-quality nitride films, particularly for the subsequent homoepitaxial overgrowth of device structures by other growth methods such as OMVPE and MBE.
INTRODUCTION Recent progress in high-brightness, efficient nitride-based light emitting diodes (LED's) has strengthened the contention that short-wavelength injection lasers will be possible with improvements in material quality. Typical state-of-the-art material often contains relatively high densities of structural defects (10 -10' 0 cm 2 ) which do not appear to introduce non-radiative defect centers as in most other III-V semiconductor systems. This has allowed LED's with external quantum efficiencies as high as 9.2 %.1 It does appear, however, that when these defects are driven under higher current densities, which are required for injection lasers, these devices will degrade, likely through processes mediated by the structural defects. As it is generally agreed that these defects are a direct or indirect result of the lattice mismatch between the nitride epilayer and its foreign substrate, attention has recently focused on developing alternative substrate technologies. Due to the high equilibrium vapor pressure of N 2 over GaN, bulk growth has been carried out at extremely high pressures (-20 kbar) and has produced GaN platelets of limited dimensions (< 1 cm). Heteroepitaxial substrate materials with better lattice match to the nitrides, such as ZnO, LiGaO2 or LiA10 2, show promise for MBE growth, but their lack of thermal or chemical stability limits their suitability for the CVD growth of nitrides. In order to generate large areas of thick GaN films, several groups are pursuing hydride vapor phase epitaxy (HVPE) as a quasi-bulk technique.2'- This method has the advantages of high growth rate (typically >10 jim/h) and low cost. Significant drawbacks associated with this technique are the large background donor concentration usually found in these film
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