Electrical and optical studies of dilute nitride and bismide compound semiconductors

Abstract

A few percent of nitrogen (N) or bimuth (Bi) incorporation in GaAs compound semiconductors have proved to lower significantly its bandgap. This unusual bandgap reduction is of interest for numerous applications such as long wave-length lasers, solar cells etc. However, the addition of these impurity atoms also introduces defect levels in the bandgap of the host materials. These can have severe implications on the material's quality, for example they can decrease the lifetime of the charge carriers and degrade the optical efficiency. In this work, deep levels traps were investigated in silicon-doped GaAsN epitaxial layers containing N concentrations from 0.2% to 1.2% grown by molecular beam epitaxy (MBE) on n+ GaAs substrates using DLTS and high resolution Laplace DLTS techniques. In addition, a further investigation was carried out to study the effect of annealing and hydrogenation treatments on the defects present in the as-grown layers. Several deep levels were detected in the as-grown GaAsN samples. These were identified with previously reported (SiGa-NAs), EL6 (Ga vacancies-related complex), (N-As)As, EL3 (off-centre substitutional oxygen in As sites) and EL2-like (anti site AsGa) defect levels. It was found that, depending upon the N concentration, heat treatment has a different effect on the traps. For samples with N = 0.2 - 0.4 %, some defects were annihilated and no generation of new defects was observed. In the case of samples with N = 0.8- 1.2 % the annealing results in both generation of new traps and elimination of some existing traps. In general, it was found that hydrogenation of the as-grown GaAsN epilayers passivates most of the deep levels. However, for the samples with N = 0.8%, although hydrogen passivates some of the defects and reduce the concentration of others it also creates new defects which are suspected to be hydrogen-related complexes. (100) and (311)B GaAsBi layers grown by molecular beam epitaxy under various arsenic overpressures have been investigated using optical and structural techniques. The optimised Bi incorporation was found to occur near stoichiometric conditions. The incorporation of Bi into the GaAsBi alloy, as determined by high resolution X-ray diffraction (HRXRD), is sizably larger in the (311)B epilayers than in (100) epilayers. HRXRD reveals 4% Bi-incorporation in (311)B and 3% in (100) GaAs orientations. The conventional optical transmission results confirmed that the bandgap of the (311)B epilayer is around 90 meV lower than that corresponding to (100) sample. This measurement provide further evidence that Bi incorporates more in (311)B than in (100) surfaces. The low temperature post-growth heat treatment of GaAsBi alloys reveals an improvement in the structural and optical properties of these materials. A substantial increase in photoluminescence signal infers a large reduction of defects