Effect of Extended Defects on Photoluminescence of Gallium Oxide and Aluminum Gallium Oxide Epitaxial Films

Abstract

Photoluminescence (PL) has been under intense scrutiny in an attempt to define the mechanisms that generate the emissions in gallium oxide (Ga,0.). In general, PL spectra is used to characterize the defects leading to radiative recombination processes within a specific material. In this regard, the PL spectra for B-Ga,0, has generally been deconvoluted in three emission peaks: UV, blue, and green. However, peak shapes from any point defect are expected to require more than a single Gaussian for a complex crystal structure like B-Ga,0., possibly having an asymmetric shape and requiring a more complex model, which takes into consideration the vibrational broadening of the PL spectrum due to electron-phonon coupling. The sum of many phonon replicas makes it challenging to fit the spectra uniquely. Adding energy level broadening from disorder or extended defects makes deconvolution of the B-Ga 793 PL spectra even more difficult. As such, there is little chance of being able to spectroscopically discern different types of defects in B-Ga,0, from luminescence as any defect considered can result in a very wide luminescence band, even near 0 Kelvin. This has resulted in an intense debate in defining the defects and phenomenological explanations of electronic processes that cause these particular emissions. In these debates, point defects have been the only explored and discussed potential source for the visible PL emission in B-Ga,0.. But, as far as can be determined, no previous works in the literature have discussed whether extended defects affect PL. Here, a systematic PL study on three series of B-Ga,0, /aluminum gallium oxide films and bulk single crystals is performed. The first series includes three film samples with varying Si-doping: UID (~10 ~cm_~),~10 cm, and ~10'8 om”, All samples in the series are (010) oriented Si-doped B-Ga,0, film grown on Fe-doped B-Ga,0.. The second series contains three samples and compares (-201) B-Ga,0.. This series includes a bulk crystal of (-201) UID B-Ga,0., a (-201) UID B-Ga,0, film grown on Fe-doped B-Ga,0., and a (-201) UID B-Ga,0, film grown on sapphire. The last series compares (010) oriented (Al Ga, _)903 films, i.e., AGO of varying Al concentrations (0%, 10%, and 25%), grown on Fe-doped B-Ga,,O.,,. These are compared to a (100) oriented bulk 10% AGO (B-Alp Ga 2-3 . O,,) crystal. A second part of the series compares (-201) oriented 0%, 2%, 10%, and 28% AGO films grown on 3 sapphire. The Si-doped series yields homogeneous crystalline films with a low density of extended defects [1] and a dominant UV emission in PL. In the (-201) series, the bulk (-201) B-Ga,0, sample also shows a dominant UV emission. On the other hand, Heteroepitaxial and homoepitaxial (-201) B-Ga,0, films show consistent PL features with dominant blue emission. The (-201) films’ poor quality [2, 3] is likely the reason for this shift in the dominant emission, as observed when comparing UID films grown on (010) and (-201) bulk B-Ga,0, crystals with comparable growth conditions. The AGO series shows consistent blue centered PL with no UV emission for both homoepitaxial and heteroepitaxial AGO samples. The AGO films show extended defects throughout a poor-quality film revealed using transmission electron microscopy (TEM) on the 25% AGO grown on (010) bulk B-Ga,0, crystal. Lastly, representative samples chosen from all three series were compared and it is shown that extended defects have an essential role in the PL emission of B-Ga,0., and AGO films. Homogeneous films with no extended defects or stacking faults and bulk crystals will yield a UV dominant PL emission. Samples of poor film quality that have stacking faults and extended defects will have a dominant blue PL emission. 1.8