Bismuth incorporation and the role of ordering in GaAsBi/GaAs structures

The structure and composition of single GaAsBi/GaAs epilayers grown by molecular beam epitaxy were investigated by optical and transmission electron microscopy techniques. Firstly, the GaAsBi layers exhibit two distinct regions and a varying Bi composition profile in the growth direction. In the lower (25 nm) region, the Bi content decays exponentially from an initial maximum value, while the upper region comprises an almost constant Bi content until the end of the layer. Secondly, despite the relatively low Bi content, CuPtB-type ordering was observed both in electron diffraction patterns and in fast Fourier transform reconstructions from high-resolution transmission electron microscopy images. The estimation of the long-range ordering parameter and the development of ordering maps by using geometrical phase algorithms indicate a direct connection between the solubility of Bi and the amount of ordering. The occurrence of both phase separation and atomic ordering has a significant effect on the optical properties of these layers

MBE grown GaAsBi/GaAs multiple quantum well structures: Structural and optical characterization

A series of GaAsBi/GaAs multiple quantum well p–i–n diodes were grown by molecular beam epitaxy. Nomarski images showed evidence of sub-surface damage in each diode, with an increase in the cross-hatching associated with strain relaxation for the diodes containing more than 40 quantum wells. X-ray diffraction ω–2θ scans of the (004) reflections showed that multiple quantum well regions with clearly defined well periodicities were grown. The superlattice peaks of the diodes containing more than 40 wells were much broader than those of the other diodes. The photoluminescence spectra showed a redshift of 56 meV and an attenuation of nearly two orders of magnitude for the 54 and 63 well diodes. Calculations of the quantum confinement and strain induced band gap modifications suggest that the wells in all diodes are thinner than their intended widths and that both loss of quantum confinement and strain probably contributed to the observed redshift and attenuation in the 54 and 63 well diodes. Comparison of this data with that gathered for InGaAs/GaAs multiple quantum wells, suggests that the onset of relaxation occurs at a similar average strain–thickness product for both systems. Given the rapid band gap reduction of GaAsBi with Bi incorporation, this data suggests that GaAsBi is a promising photovoltaic material candidate

Telecommunication wavelength GaAsBi light emitting diodes

GaAsBi light emitting diodes containing ∼6% Bi are grown on GaAs substrates. Good room-temperature electroluminescence spectra are obtained at current densities as low as 8 Acm − 2. Measurements of the integrated emitted luminescence suggest that there is a continuum of localised Bi states extending up to 75 meV into the bandgap, which is in good agreement with previous photoluminescence studies. X-ray diffraction analysis shows that strain relaxation has probably occurred in the thicker samples grown in this study

In Situ Surface Studies of III-V Semiconductor Compounds

Since its advent in the early 1980s, Scanning Tunnelling Microscopy (STM) has been used to advance the knowledge of semiconductor grow processes. Hybridisation of STM with other analytical methods and the Molecular Beam Epitaxy (MBE) growth technique allowed a flexible and diverse approach to growth front exploration. The first hybrid, limited the applicability of STM to in vacuo operation whereby the sample is rapidly cooled or “quenched” in an attempt to preserve the growing surface, before imaging can commence. This technique suffers dually from the unknown effects of the quenching procedure and the limiting ability to only capture frozen-in-time images of the surface. The ultimate evolution of STM would be to allow concurrent or in situ MBE and STM operation. The ability to perform concurrent MBE and STM requires three basic criteria: accurate and stable control of the sample temperature, reliable and maintainable STM tunnelling tip procedures and controlled, sustained emission from the MBE effusion cells within the STM chamber. Samples are slivers 8 x 1 mm2 to 12 x 4 mm2 of wafer mounted for either direct current heating or radiative pyrolytic boron nitride heating within the STM chamber. No direct temperature monitoring method is available and thus a myriad of techniques were employed to map the current-temperature response for samples including Reflection High Energy Electron Diffraction (RHEED), thermocouples and thermography, yielding a reliable heating profile. Tunnelling tip fabrication involves manufacturing an atomically sharp tip via a two-step electrochemical etching and annealing procedure. An extensive and exhaustive investigation sought to produce a quantitative method for tip identification and etching parameterisation based on the available variables of differential sensitivity, etching voltage, immersion depth and etchant concentration. An optimised tip type transfer diagram of tip fabrication resulted, after which, an anneal algorithm was formulated resulting in clean, sharp tips without the side effect of apex distortion and melting. Quality of the initial growth layer depends strongly on the clean-up conditions. As a prequel to growth, sample preparation methods are investigated via STM analysis to determine the best preparation conditions in order to achieve high quality MBE growth in the STM chamber. The final stage involves MBE source operation during STM. Initial investigation focused on flux alteration of surface reconstructions and allowed the effects of As4 on the STM stage to be investigated. This is the first documented case where an e-beam As4 source has been successfully operated within an STM system, during imaging. The inclusion of group III elements in the evaporation flux proves unequivocally that III-V Molecular Beam Scanning Tunnelling Microscopy (MBSTM) is a realisable investigatory technique. Simultaneous deposition of In and As whilst imaging allowed dynamic observation of the InAs/GaAs wetting layer evolution on GaAs(001)-(2 × 4). The experiment followed initial heteroepitaxial growth through wetting layer evolution to the onset of 3D growth

Interaction of Mn with GaAs and InSb : incorporation, surface reconstruction and nano-cluster formation

The deposition of Mn on to reconstructed InSb and GaAs surfaces, without coincident As or Sb flux, has been studied by reflection high energy electron diffraction, atomic force microscopy and scanning tunnelling microscopy. On both Ga- and As-terminated GaAs(0 0 1), (2 × n) Mn-induced reconstruction domains arise with n = 2 for the most well ordered reconstructions. On the Ga-terminated (4 × 6), the Mn-induced (2 × 2) persists up to around 0.5 ML Mn followed by Mn nano-cluster formation. For deposition on initially β2(2 × 4)-reconstructed GaAs(0 0 1), the characteristic trench structure of the reconstruction is partially preserved even beyond 1 monolayer Mn coverage. On both the β2(2 × 4) and c(4 × 4) surfaces, MnAs-like nano-clusters form alongside the reconstruction changes. In contrast, there are no new Mn-induced surface reconstructions on InSb. Instead, the Sb-terminated surfaces of InSb (0 0 1), (1 1 1)A and (1 1 1)B revert to reconstructions characteristic of clean In-rich surfaces after well defined coverages of Mn proportional to the Sb content of the starting reconstruction. These surfaces are decorated with self-assembled MnSb nanoclusters. These results are discussed in terms of basic thermodynamic quantities and the generalized electron counting rule

Polariton condensation in a strain-compensated planar microcavity with InGaAs quantum wells

The investigation of intrinsic interactions in polariton condensates is currently limited by the photonic disorder of semiconductor microcavity structures. Here, we use a strain compensated planar GaAs/AlAs0.98P0.02 microcavity with embedded InGaAs quantum wells having a reduced cross-hatch disorder to overcome this issue. Using real and reciprocal space spectroscopic imaging under non-resonant optical excitation, we observe polariton condensation and a second threshold marking the onset of photon lasing, i.e., the transition from the strong to the weak-coupling regime. Condensation in a structure with suppressed photonic disorder is a necessary step towards the implementation of periodic lattices of interacting condensates, providing a platform for on chip quantum simulations