Dipankar Biswas

Calculations for the band lineup of strained InxGa1-xN/GaN quantum wells : Effects of strain on the band offsets

Band lineup is one of the most important parameters associated with carrier confinement in heterostructures. Relations for computing the band lineups of InxGa1−xN based heterostructures have been developed. The band positions for InxGa1−xN/GaN heterointerfaces are calculated from the equations developed, which directly corelate the positions of the bands with the band gap of InN and strain at the interface. The strains are calculated from the In mole fractions and lattice constants. The parameters implicitly involved are the elastic stiffness constants (C11 and C12), the hydrostatic deformation potential of the conduction band (a′), and the hydrostatic deformation potential (a) and shear deformation potential (b) for the valence band. Computations have been carried out for different reported band gaps of InN. The effects of strain become prominent as the mole fraction of In increases, changing the band offset ratio.

Effects of a Gaussian size distribution on the absorption spectra of III-V semiconductor quantum dots

The advancement in the fabrication of low-dimensional semiconductor structures has made it possible to grow zero-dimensional electron-hole systems called quantum dots (QDs). In recent years, there have been extensive studies on III-V semiconductor QDs. In this paper, we have formulated the absorption spectra of realistic QD systems with dot size distribution described by a Gaussian function. The dots were approximated as cubic boxes having finite potentials at the boundaries. The effects of size nonuniformity on the optical absorption spectra of few realistic QD systems were analyzed, and the results have been compared with ideal dots having infinite potentials at the boundaries.

Interdiffusion induced changes in the photoluminescence of InXGa1−XAs∕GaAs quantum dots interpreted

Interdiffusion in InXGa1−XAs∕GaAs quantum dots (QDs) may occur during high temperature growth and processing, which may create problems in the ultimate device performance. It is simulated through successive high temperature annealing, and the changes at each stage are studied through photoluminescence (PL). Significant changes are observed in the peak energy, linewidth, and intensity of the PL spectra. These have been attributed to relaxation of strain, changes in the composition of InXGa1−XAs, and size distribution of the QDs, which fail to establish proper understanding qualitatively and quantitatively. In this Communication we present appropriate interpretations of the changes in the observed PL through quantum mechanical concepts and computations.

Anomalous capacitance–voltage profiles in quantum wells explained by a quantum mechanical model

We have developed a quantum mechanical model for understanding and explaining the capacitance–voltage (C–V) carrier profiles observed in quantum wells (QW). The external field imposed on the QW during C–V profiling changes the carrier distribution of the system. This model considers the effects of field and quantum confinement of the carriers in the well. The results obtained by iterative solutions of Schrodinger’s and Poisson’s equations give a better understanding of the experiments than the previous models where quantum confinement is ignored.

Computations of the optical transitions and absorption spectra in a set of realistic, elongated InAs/GaAs quantum boxes having a Gaussian distribution

InAs/GaAs quantum dots (QDs) grown by various methods do not have the same dimensions in the three axes. This paper reports on expressions for computations of the optical transitions and absorption spectra of InAs/GaAs QDs that have a square base and the variation of the height is Gaussian. The dots were considered to be elongated quantum boxes with square bases having finite potentials at the boundaries. The results are in excellent agreement with reported experimental data of photoluminescence and absorption. The expressions could be successfully applied to short quantum wires.

Conspicuous Presence of Higher Order Transitions in the Photoluminescence of InxGa1-xN/GaN Quantum Wells

For successive annealing stages the photoluminescence (PL) peaks of InXGa1-XN/GaN quantum wells (QWs) shift initially towards red which is followed by a blue. This phenomenon contradicts the usual monotonic blueshift. We have found that the phenomena can be explained properly only if we consider recombinations from the higher sub-bands to be present in the PL of the InXGa1-XN/GaN QWs, which is not usual. When a strong piezoelectric field exists across a QW, as encountered in InXGa1-XN/GaN QWs, the probability of optical transitions from higher sub-bands of the QW become more probable. In this paper this theory has been established from experimental results.

Guidelines for the design of appropriate structures for proper capacitance-voltage measurements on III–V quantum wells

Errors encountered in the capacitance-voltage (C-V) measurements of quantum well (QW) structures are usually attributed to Debye smearing. The other sources from which errors may occur are not well discussed in literature. In this paper we have highlighted the limits of C-V measurements on QW structures. Simulations have been carried out through the self-consistent solutions of the Schrodinger and Poisson equations for various band offsets, dopings, and temperatures. This will provide guidelines for suitable design of quantum structures for proper C-V measurements.

Dependence of the photoluminescence of annealed III-V semiconductor quantum dots on their shape and dimension

Interdiffusion in III-V semiconductor quantum dots (QDs) may occur during growth and subsequent device processing steps. The photoluminescence (PL) spectra of InXGa1−XAs/GaAs and InXGa1−XN/GaN QDs change significantly on annealing. The size and shape of a QD dot are important parameters, which govern this change of the PL spectra. In this communication, we have investigated the effects of interdiffusion in realistic InXGa1−XAs/GaAs and InXGa1−XN/GaN QDs with various geometries which are of theoretical and practical interest such as pyramidal, truncated pyramidal, and lens shaped, through quantum mechanical computations.

Inconsistent temperature dependence in capacitance-voltage profiling of quantum wells

Carrier profiles of quantum wells, obtained from experimental capacitance-voltage (C-V) measurements match the theoretically simulated carrier profile quite closely but these are much different from the actual carrier profile. It is observed that the peaks of the experimental and simulated carrier profiles move in the opposite direction and at low temperature there nature changes drastically. These observations have been explained taking into account the two dimensional carrier confinement and its temperature dependence through self-consistent solutions of the Schrodinger and Poisson equations. The effect of the series resistance seems to be highly pronounced in experimental C-V measurements.

Dependence of the Absorption Spectra of III-V Semiconductor Quantum Dots on the Size Distribution

In recent years there have been extensive studies on III-V semiconductor quantum dots (QDs). In this paper we have formulated the absorption spectra of a realistic QD system with dot size distribution described by a Gaussian function. The dots were approximated as cubic boxes having finite potentials at the boundaries. The effects of size non uniformity on the optical absorption spectra of a realistic QD system was analyzed and the results have been compared with ideal dots having infinite potentials at the boundaries.