Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer and with different numbers of GaN monolayers. The emission energies, E_PL, measured at ambient pressure, are close to the value of the band gap, E_g, in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dE_PL/dp, however, resembles that of the InN energy gap. Further, the magnitudes of both E_PL and dE_PL/dp are significantly higher than those obtained from ab-initio calculations for 1InN/nGaN superlattices. Some causes of these discrepancies are suggested.

The pressure-induced changes in the electronic band structures of In-containing nitride alloys, ${\text{In}}_x{\text{Ga}}_{1\text{-}x}\text{N}$ and ${\text{In}}_x{\text{Al}}_{1\text{-}x}\text{N}$ are examined experimentally as well as by ab initio calculations. It is found that the band gap pressure coefficients, $d{E}_g/dp$, exhibit very large bowing with $x$, and calculations with simulation of clustered distributions of the In atoms over the cation sites show a strong enhancement of this effect. This relates well to the experimental data obtained from pressure dependent photoluminescence measurements for ${\text{In}}_x{\text{Ga}}_{1\text{-}x}\text{N}$ and ${\text{In}}_x{\text{Al}}_{1\text{-}x}\text{N}$ layers, performed in this work, and combined with existing data for ${\text{In}}_x{\text{Ga}}_{1\text{-}x}\text{N}$ layers. We discuss possible explanations of the anomalously large magnitude of the $d{E}_g/dp$ bowing in these nitride alloys. DOI: http://dx.doi.org/10.1103/PhysRevB.81.235206 © 2010 The American Physical Society

Discussion of band gap behavior based on first principles calculations of the electronic band structures for several InN/GaN superlattices (SLs) (free-standing and pseudomorphic) grown along different directions (polar and nonpolar) is presented. Taking into account the dependence on internal strain and lattice geometry mainly two factors influence the dependence of the band gap, E _g on the layer thickness: the internal electric field and the hyb wells) is more important. We also consider mIn ridization of well and barrier wave functions. We illustrate their influence on the band gap engineering by calculating the strength of built-in electric field and the oscillator strength. It appears that there are two interesting ranges of layer thicknesses. In one the influence of the electric field on the gaps is dominant (wider wells), whereas in the other the wave function hybridization (narrow wells) is more important. We also consider mIn _0.33 Ga _0.67 N/nGaN SLs, which seem to be easier to fabricate than high In content quantum wells. The calculated band gaps are compared with recent experimental data. It is shown that for In(Ga)N/GaN superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary InGaN alloys.

The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6–13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.