Ultra-wide bandgap β-Ga2O3 films: Optical, phonon, and temperature response properties

Dinesh Thapa,Isiaka Lukman,Leah Bergman,Jeffrey Lapp

doi:10.1063/5.0074697

Abstract

Optical and phonon interactions of Ga2O3 thin films with nanocrystalline morphology were studied at extreme temperatures. The films were grown using a sputtering technique and analyzed via temperature response transmission, Raman scattering, and high-resolution deep-UV photoluminescence (PL). Raman modes indicated that the structure corresponds to the β-phase. The optical-gap at the range of 77–620 K exhibited a redshift of ∼200 meV, with a temperature coefficient of ∼0.4 meV/K. The optical-gap at room-temperature is 4.85 eV. The electron–phonon interaction model at that temperature range pointed to a low energy phonon, ∼31 meV, that is involved in the thermal properties of the optical-gap. Detailed Urbach energy analysis indicated that defects are the dominant mechanism controlling the band-edge characteristics even at an elevated temperature regime where phonon dominance is usually expected. Defects are attributed to the disordered forms of graphite that were detected via Raman scattering and to the granular morphology of the film. A deep-UV laser with an above-bandgap exaction line of 5.1 eV was employed to map the PL of the films. The highly resolved spectra, even at room-temperature, show a strong emission of ∼3.56 eV attributed to self-trapped holes (STHs). The STH is discussed and modeled in terms of the self-trapped exciton. Moreover, a very distinct but low-intensity emission was found at 4.85 eV that agrees with the value of the optical-gap and is attributed to bandgap recombination. The intensity ratio between the STH and that of the bandgap was found to be 6:1.

Highlights

Ga2O3 can crystallize in several polytypes, in which the most common are the α-phase and the β-phase.[11,12] The α-phase is the less stable one and typically can be formed at the low temperature regime, while the β-phase is stable up to high temperatures of ∼1800 ○C
The optical gap exhibited a shift of ∼200 meV
The optical gap at room temperature was found to be 4.85 eV, which is characteristic of thin films with a nanocrystalline morphology

Summary

INTRODUCTION

Β-Ga2O3 is a semiconductor with an ultra-wide bandgap in the deep UV range of ∼5 eV.[1]. The study that employed an above-bandgap laser excitation of 5.1 eV resulted in a weak PL at 4.85 eV attributed to a bandgap recombination of free electron–hole pairs This value agrees with the room-temperature optical gap of the film. The weak intensity is in accordance with the previous theoretical work that predicted that bandgap luminescence of β-Ga2O3 should be inhibited by the strong self-trapping of holes due to the large crystal distortion associated with this material.[1] On the other hand, a strong PL emission at ∼3.56 eV was present whose energy-range and behavior were found to be consistent with the emission from the self-trapped holes (STHs) At subbandgap exaction, this emission was totally suppressed, as expected, due to the lack of free electron–hole pairs needed for the STH luminescence process. The PL study indicated a large Stokes shift of ∼1.29 eV as is expected from materials with large phonon coupling.[1]

EXPERIMENT

RESULTS AND DISCUSSION

SUMMARY