Understanding and reducing deleterious defects in the metastable alloy GaAsBi

Abstract

Technological applications of novel metastable materials are frequently inhibited by abundant defects residing in these materials. Using first-principles methods, we investigate the defect thermodynamics and phase segregation in the technologically important metastable alloy GaAsBi. Our calculations predict defect energy levels in good agreement with those from numerous previous experiments and clarify the defect structures giving rise to these levels. We find that vacancies in some charge states become metastable or unstable with respect to antisite formation, and this instability is a general characteristic of zincblende semiconductors with small ionicity. The dominant point defects that degrade the electronic and optical performances are predicted to be As Ga , Bi Ga , As Ga +Bi As , Bi Ga +Bi As , V Ga and V Ga +Bi As , of which the first four and last two defects are minority-electron and minority-hole traps, respectively. V Ga is also observed to have a critical role in controlling metastable Bi supersaturation by mediating Bi diffusion and clustering. To reduce the influences of these deleterious defects, we suggest shifting the growth away from an As-rich condition and/or using hydrogen passivation to reduce the minority-carrier traps. We expect this work to aid in the applications of GaAsBi for novel electronic and optoelectronic devices and to illuminate the control of deleterious defects in other metastable materials.

Deep-level defects in n-type GaAsBi alloys grown by molecular beam epitaxy at low temperature and their influence on optical properties

“…According to these results, we find that the ET 0 and ET 1 levels revealed in the present work by DLTS, correspond reasonably well to those calculated in ref. 38 for Bi-related pair defects, i.e. (V Ga + Bi Ga ) −/2− and (As Ga + Bi Ga ) 0/1− , respectively; the calculated energy levels of these traps are 0.1 eV and 0.18 eV below the conduction band, thus very close to our experimentally obtained values.…”

“…Indeed, DFT calculations performed by Luo et al . 38 indicate the existence of another complex defect (V Ga + Bi As ) 2−/3− with an activation energy of 0.36 eV, which is very close to the ET 2 level. The predicted defect energy level of (V Ga + Bi As ) 2−/3− is also reasonably consistent with recent DLTS experiments which estimated the energy position of the majority-electron trap E3 at about 0.23–0.28 eV below the conduction band in n-type GaAs 1− x Bi x layers with 0.3% Bi, grown by MBE under intense UV illumination 44 .…”

“…In order to resolve the nature of the Bi-related ET 0 and ET 1 deep energy levels, we used the results of ab initio calculations reported recently by Luo et al . 38 , who investigated defect thermodynamics and Bi segregation in GaAsBi alloys using density functional theory (DFT). Their DFT calculations predicted defect energy levels, which are in good agreement with the measured deep energy levels of defects in n-type and p-type GaAs and GaAsBi alloys grown at low temperatures, reported recently by Mooney et al .…”

GaAs1-xBix growth on Ge: anti-phase domains, ordering, and exciton localization

“…Ge out-diffusion from the substrate is also noticeable in the EDS images, accompanied by reduced Ga and As concentrations. Due to large size of Bi atoms, vacancy-mediated hopping of Bi atoms has been suggested as the primary means of Bi diffusion and clustering 16 . The higher density of Ga and As vacancies at the anti-phase domains, as presented in the previous section, could explain the preferential Bi dissolution from the APD interiors during high-temperature annealing.…”

“…The bismide has presently emerged as a more promising candidate for photovoltaics (PV) since the trap states are formed near its VBE, whereas they occur near CBE in the nitride. High electron mobilities and lifetimes can thus be preserved in the former [14][15][16][17] . A lattice-matched 1.0 eV bandgap absorber material is highly desirable for III-V multi-junction (M-J) PV applications.…”