Below bandgap photoluminescence of an AlN crystal: Co-existence of two different charging states of a defect center

Qin Zhou,Zhaofu Zhang,Jiannong Wang,Xi Tang,Hui Li,Sergii Golovynskyi,Honglei Wu,Baikui Li

doi:10.1063/5.0012685

Qin Zhou, Zhaofu Zhang + Show 6 more

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https://doi.org/10.1063/5.0012685

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Abstract

The below bandgap optical transitions of an aluminum nitride (AlN) crystal grown on a tungsten (W) substrate by physical vapor transport (PVT) are investigated by below-bandgap-excited photoluminescence (PL) spectroscopy and first-principles calculations. Oxygen (O) is the only impurity in the AlN-on-W crystal grown by PVT. By analyzing the excitation-power-, excitation-photon-energy-, and temperature-dependence of the PL spectra, the emission peaks of defect complexes involving aluminum vacancy (VAl) and substitutional oxygen (ON) with different spatial and atomic configurations, i.e., VAl–ON and VAl–2ON with ON featuring axial or basal configurations, are identified. It is revealed that two different charging states coexist in thermal equilibrium for each configuration of VAl–ON complexes. The optical transitions between the conduction band and (VAl–ON)2− and/or (VAl–2ON)1− contribute the UV emissions and those between the valence band and (VAl–ON)1− or (VAl–2ON)0 contribute the red emissions.

Highlights

We investigated the PL characteristics of an aluminum nitride (AlN) crystal grown on a W substrate by physical vapor transport (PVT)
Profile, respectively, of the AlN crystal grown on W by PVT
It is shown that O is the only bulk impurity that can be detected in the Al-on-W crystal grown by PVT

Summary

INTRODUCTION

Aluminum nitride (AlN) is a semiconductor with a direct bandgap of ∼6.0 eV, a thermal conductivity of 3.2 W cm−1 K−1, and a critical electric field of 12 MV/cm at room temperature, emerging as a promising candidate for deep ultraviolet (UV) optoelectronics and power electronics. The prime obstacle on the way to the development of AlN-based devices remains the difficulty to control the formation and concentration of various defect centers. The performances of either optoelectronic or power electronic devices are explicitly impacted by defects. Notably, the ultra-wide bandgap of AlN causes a wider accommodation energy range for defect levels, which can act as recombination or trap centers. Thereby, the in-depth exploration of the properties of defects is of high importance for the further progress in the material growth and device development. In AlN crystals grown either by physical vapor transport (PVT) using tantalum carbide (TaC) crucibles and/or silicon carbide (SiC) substrates, or AlN epilayers by hydride vapor phase epitaxy (HVPE) or by metal–organic chemical vapor deposition (MOCVD), multiple impurities, such as silicon (Si), carbon (C), and oxygen (O), are process-induced and coexist inevitably. This leads to additional complexity and difficulty or even contradiction to identify the optical properties of specific defect centers. It was further revealed that two different charging states coexist ft2toihOorenNtvsh)a1ebl−ee(ntcVwcoAenele–btrnaOinbtNduh)te(eoVcrtohB(neV)dUaAunlV–cdt2ie(OomVnNAis)bl–saciOoonnmdN.)p(T1C−lheBxeo)rocae(pnVntidtAcelar(–lsV2.tArOTal–hNnOe)s0ioNtcip)oo2tnn−icstaorblirebttr(uwaVtneeAsetlin–othe red emission

EXPERIMENT AND CALCULATION METHODS

RESULTS AND DISCUSSIONS

CONCLUSION