Electrical properties, structural properties, and deep trap spectra of thin α-Ga2O3 films grown by halide vapor phase epitaxy on basal plane sapphire substrates

Abstract

Undoped epitaxial films of α-Ga2O3 were grown on basal plane sapphire substrates by halide vapor phase epitaxy (HVPE) in three different modes: standard HVPE, HVPE with constant flow of Ga and pulsed supply of O2 (O2-control growth regime), and with constant flow of O2 and pulsed delivery of Ga (Ga-control growth fashion). The best crystalline quality as judged by x-ray symmetric and asymmetric reflection half-widths and by atomic force microscopy morphology profiling was obtained with the O2-control deposition, and these results appear to be the best so far reported for α-Ga2O3 films. All grown α-Ga2O3 epilayers were high-resistivity n-type, with the Fermi level pinned near Ec − 1 eV deep traps. Photoinduced current transient spectra also showed the existence in standard HVPE samples and samples grown under the O2-control pulsed growth conditions of deep hole traps with levels near Ev + 1.4 eV whose density was suppressed in the Ga-control pulsed HVPE samples. The levels of the dominant deep traps in these α-Ga2O3 samples are close to the position of dominant electron and hole traps in well documented β-Ga2O3 crystals and films.

On the origin of dislocation generation and annihilation in α -Ga2O3 epilayers on sapphire

“…1(d). Within each individual domain, the atomic steps are straight and elongated along the [11][12][13][14][15][16][17][18][19][20] and no surface depressions within the terraces are observed. Existing theories developed for dislocation mediated surface morphologies have been employed to interpret the formation of pinned steps, spiral growth hillocks, and surface depression.…”

“…In recent years, high quality a-Ga 2 O 3 epilayers and heterostructures have been realized on sapphire substrates by mist-CVD 6 and halide vapor phase epitaxy (HVPE). [10][11][12] Consequently, vertical Schottky diodes with a low on-resistance of 0.4 mX cm 2 and a breakdown voltage of 855 V have been reported. 13 Due to the large in-plane lattice mismatch of about 4.81%, a-Ga 2 O 3 epilayers deposited on sapphire substrates typically result in a high density of dislocations of the order of 10 10 cm À2 .…”

“…From the Burgers vectors b e ¼ 1 3 h2110i and b s ¼ h0001i, an estimation of the screw and edge dislocation densities in this 2.5 lm-thick sample is 1.6 Â 10 6 and 5.0 Â 10 9 cm À2 , respectively, which is comparable to other results reported in the a-Ga 2 O 3 epilayers grown by mist-CVD and HVPE techniques. 14,16 Figures 3(a) and 3(b) show the XRC of (0006) and (10)(11)(12)(13)(14) planes for a series of samples with different thicknesses, respectively. The XRC width for the (0006) plane is almost independent of the thickness with an averaged value of 49 arc sec, while the width for the (10)(11)(12)(13)(14) plane decreases remarkably with the increasing thickness.…”

“…14,16 Figures 3(a) and 3(b) show the XRC of (0006) and (10)(11)(12)(13)(14) planes for a series of samples with different thicknesses, respectively. The XRC width for the (0006) plane is almost independent of the thickness with an averaged value of 49 arc sec, while the width for the (10)(11)(12)(13)(14) plane decreases remarkably with the increasing thickness. Following the same approach described above, the screw and edge dislocations in different thick layers have been calculated and are summarized in Fig.…”

“…3. (a) and (b) XRCs of (0006) and (10)(11)(12)(13)(14) planes for a-Ga 2 O 3 epilayers with different thicknesses, respectively, and (c) calculated edge and screw dislocation densities as functions of film thickness. The dashed line is a least-squares fit to the relationship of D e ¼ C=h.…”