I. Gorczyca

High pressure phase transition in aluminium nitride

Abstract Self-consistent Linear Muffin-Tin Orbital method is used to investigate the structural properties of AlN under high pressure. There is found the phase transition to the rocksalt structure at the pressure 16.6 GPa. We report a direct experimental observation of the phase transition in A1N. Our experiment locates the phase transition between 16 and 17 GPa which agrees perfectly with the above calculations.

Limitations to band gap tuning in nitride semiconductor alloys

Relations between the band gaps of nitride alloys and their lattice parameters are presented and limits to tuning of the fundamental gap in nitride semiconductors are set by combining a large number of experimental data with ab initio theoretical calculations. Large band gap bowings obtained theoretically for GaxAl1−xN, InxGa1−xN, and InxAl1−xN for uniform as well as clustered arrangements of the cation atoms are considered in the theoretical analysis. It is shown that indium plays a particular role in nitride alloys being responsible for most of the observed effects.

The discrepancies between theory and experiment in the optical emission of monolayer In(Ga)N quantum wells revisited by transmission electron microscopy

Quantitative high resolution transmission electron microscopy studies of intentionally grown 1InN/nGaN short-period superlattices (SLs) were performed. The structures were found to consist of an InxGa1−xN monolayer with an Indium content of x = 0.33 instead of the intended x = 1. Self-consistent calculations of the band structures of 1In0.33Ga0.67N/nGaN SLs were carried out, including a semi-empirical correction for the band gaps. The calculated band gap, Eg, as well as its pressure derivative, dEg/dp, are in very good agreement with the measured photoluminescence energy, EPL, and its pressure derivative, dEPL/dp, for a series of 1In0.33Ga0.67N/nGaN samples with n ranging from 2 to 40. This resolves a discrepancy found earlier between measured and calculated optical emission properties, as those calculations were made with the assumption of a 1InN/nGaN SL composition.

III-V Semiconducting Nitrides : Physical Properties under Pressure

Results of experimental and theoretical studies of the fundamental electronic properties of III-V nitrides are presented. Single crystals of nitrides grown by means of an equilibrium high-pressure technique have been used for the determination of optical and structural properties. The electronic band structure, its pressure dependence, and high-pressure phase transitions have been calculated using the density functional theory.

Band gap bowing in quaternary nitride semiconducting alloys

Structural properties of InxGayAl1−x−yN alloys are derived from total-energy minimization within the local-density approximation (LDA). The electronic properties are studied by band structure calculations including a semiempirical correction for the “LDA gap error.” The effects of varying the composition and atomic arrangements are examined using a supercell geometry. An analytical expression for the band gap is derived for the entire range of compositions. The range of (x, y) values for which InxGayAl1−x−yN is lattice matched to GaN, and the ensuing energy gaps, are given. This range of available gaps becomes smaller when In atoms form clusters. Comparison to experimental data is made.

Hydrostatic pressure and strain effects in short period InN/GaN superlattices

The electronic structures of short-period pseudomorphically grown superlattices (SLs) of the form mInN/nGaN are calculated and the band gap variation with the well and the barrier thicknesses is discussed including hydrostatic pressure effects. The calculated band gap shows a strong dependence on the superlattice geometry. The superlattice gap vanishes for n = m ≥ 4. These effects are related to the existence of the internal electric fields that strongly influence the valence- and conduction-band profiles and thus determine the effective band gap and emission energies. The electric field strength depends strongly on the strain conditions and SL geometry, but weakly on the applied external hydrostatic pressure.

Cubic InN inclusions : Proposed explanation for the small pressure-shift anomaly of the luminescence in InGaN-based quantum wells

Abstract We propose a new approach to explain the unusually low pressure coefficients of the luminescence peaks observed in single-quantum-well InGaN-based light emitting diodes manufactured by Nichia Chemical Industries. In view of the most recent first principles band structure calculations for InN under hydrostatic pressure, we find that it is possible to reproduce the measured low (12–16 meV/GPa) pressure coefficients of the luminescence by assuming the formation of zincblende InN inclusions in the InGaN quantum well layers. These cubic inclusions, surrounded by the usual wurtzite material, should act like quantum dots giving rise to enhanced electron localization. The pressure shift of the luminescence peaks in blue and green InGaN-based emitters predicted by this model is close to 14 meV/GPa, in good agreement with our experimental results. This explanation of the observed low pressure coefficients in these devices is consistent with recent independent evidence for InN inclusions in InGaN epilayers.

Deformation potential in high electron mobility GaAs/GaAsAs heterostructures

We have studied scattering mechanisms in high electron mobility GaAs/Ga0.7Al0.3 As heterostructures. We have performed measurements of two-dimensional electron gas concentration and mobility as functions of temperature and hydrostatic pressure. We focused our attention on two-dimensional scattering by acoustic phonons and we found that the magnitude of the deformation potential D for GaAs in the considered heterostructures is -12±1.0 eV. Contrary to the common assumptions about the pressure independence of D we have found that its absolute value decreases with applied hydrostatic pressure by about 10%/GPa.

Band gaps and internal electric fields in semipolar short period InN/GaN superlattices

The electronic structures and internal electric fields of semipolar short-period mInN/nGaN superlattices (SLs) have been calculated for several compositions (m, n). Two types of SL are considered, (112¯2) and (202¯1), corresponding to growth along the wurtzite s2 and s6 directions, respectively. The results are compared to similar calculations for polar SLs (grown in the c-direction) and nonpolar SLs (grown in the a- and m-directions). The calculated band gaps for the semipolar SLs lie between those calculated for the nonpolar and polar SLs: For s2-SLs they fall slightly below the band gaps of a-plane SLs, whereas for s6-SLs they are considerably smaller.