Impurity levels and bandedge electronic structure in as-grown arsenic-doped HgCdTe by infrared photoreflectance spectroscopy

The nature of bandedge electronic structure represents a theoretically interesting and technologically important question in arsenic-doped narrow-gap HgCdTe. In this study, temperature-dependent (11-290 K) photoreflectance (PR) measurements were carried out in midinfrared spectral region on three as-grown arsenic (As)-doped narrow-gap Hg1-x Cdx Te (x=0.317, 0.287, and 0.310, respectively) samples prepared by molecular-beam epitaxy. Low-energy photomodulated Fabry-Pérot interference (FPI) and high-energy Franz-Keldysh oscillation (FKO) were analyzed and bandedge PR features were discriminated. Curve fittings of the bandedge PR features were performed, band-gap and below-gap PR processes were identified, and critical energies were determined. While no obvious FPI and FKO could be identified for the sample with a doping level of about 1018 cm-3, they were strong for the doping levels of about 1016 - 1017 cm-3, and got enhanced with temperature. Detailed analyses indicated that the below-gap PR features are linked to donor-acceptor, conduction-band-acceptor, and donor-valence-band transitions. The average energy levels of the donor and acceptor states were evaluated to be about 17±1meV and 27±3meV below conduction-band minimum, and 14±1meV above valence-band maximum at 77 K, and the origins were assigned to AsHg, TeHg, and VHg, respectively. The effect of possible deep-level impurities were also discussed, and a value of about 69 meV above the valence-band maximum was assumed preliminarily for the second level of VHg acceptor (V Hg II) and 42 meV below the conduction-band minimum for As tetramer [(AsHg -Asi) dimmer] 3. The evolution of PR critical energies with temperature was compared with an empirical formula for HgCdTe band-gap energy, and the implication of "effective band gap" was evidenced for the empirical formula. Finally, a schematic diagram was drawn for the bandedge electronic structure of the as-grown As-doped narrow-gap HgCdTe epilayers.