Incident wavelength and polarization dependence of spectral shifts in \u03b2-Ga2O3 UV photoluminescence

Yunshan Wang,Samuel Sprawls,Steve Blair,Joel B Varley,Michael Scarpulla,Berardi Sensale-Rodriguez,Xiaojuan Ni,Vincenzo Lordi,Emmanuel Lotubai,Peter T Dickens,Kelvin G Lynn,Sriram Krishnamoorthy,Feng Liu

doi:10.1038/s41598-018-36676-7

Abstract

We report polarization dependent photoluminescence studies on unintentionally-, Mg-, and Ca-doped β-Ga2O3 bulk crystals grown by the Czochralski method. In particular, we observe a wavelength shift of the highest-energy UV emission which is dependent on the pump photon energy and polarization. For 240 nm (5.17 eV) excitation almost no shift of the UV emission is observed between E||b and E||c, while a shift of the UV emission centroid is clearly observed for 266 nm (4.66 eV), a photon energy lying between the band absorption onsets for the two polarizations. These results are consistent with UV emission originating from transitions between conduction band electrons and two differentially-populated self-trapped hole (STH) states. Calcuations based on hybrid and self-interaction-corrected density functional theories further validate that the polarization dependence is consistent with the relative stability of two STHs. This observation implies that the STHs form primarily at the oxygen atoms involved in the original photon absorption event, thus providing the connection between incident polarization and emission wavelength. The data imposes a lower bound on the energy separation between the self-trapped hole states of ~70–160 meV, which is supported by the calculations.

Highlights

We report polarization dependent photoluminescence studies on unintentionally, Mg, and Ca-doped β-Ga2O3 bulk crystals grown by the Czochralski method
Glow discharge mass spectrometry (GDMS) analysis of the seed material was consistent with reports in ref
The average Mg concentration for the crystal grown for 0.25% doping is expected to be ~0.03%22 with variation of less than 0.002% between the top and bottom

Summary

Introduction

We report polarization dependent photoluminescence studies on unintentionally-, Mg-, and Ca-doped β-Ga2O3 bulk crystals grown by the Czochralski method. For 240 nm (5.17 eV) excitation almost no shift of the UV emission is observed between E||b and E||c, while a shift of the UV emission centroid is clearly observed for 266 nm (4.66 eV), a photon energy lying between the band absorption onsets for the two polarizations These results are consistent with UV emission originating from transitions between conduction band electrons and two differentially-populated selftrapped hole (STH) states. Monoclinic β-Ga2O3 is an ultra-wide bandgap oxide semiconductor exhibiting optical absorption onsets at 4.6– 4.9 eV depending on incident light direction and polarization[1]. It is of interest for optoelectronic applications such as UV-transparent electrodes and visible-blind photodetectors[2,3]. Our studies further elucidate the complexities of the optical processes in β-Ga2O3 and begin to illuminate the role of group-II acceptors such as Mg and Ca

Methods

Results

Conclusion