Very strong coefficients for spontaneous and piezoelectric polarizations have recently been predicted for III-V nitride semiconductors with natural wurtzite symmetry. Such polarizations influence significantly the mechanisms of radiative emissions in quantum-confinement heterostructures based on these materials. The photoluminescence decay time of excitons is used as a probe of internal electric fields in GaN-(Ga, Al)N quantum wells in various configurations of strain, well widths, and barrier widths. The measured decays are not only controlled by radiative lifetimes, which depend on the fields inside GaN wells but also on the nonradiative escape of carriers through Ga1 - xAlyN barriers, which depends on their widths and on the electric field in these layers. It is shown in particular that the magnitude of the held in the wells is not a simple function of the strain of these layers via the only piezoelectric effect, but rather the result of the interplay of spontaneous and piezoelectric polarizations in both well and barrier materials. [S0163-1829(99)02923-9].

Wide bandgap nitride semiconductors have recently attracted a great level of attention owing to their direct bandgaps in the visible to ultraviolet regions of the spectrum. A number of scientific challenges remain, however, including important issues such as the determination and control of film polarity and relative extent of various polarization effects. Polarization effects arise from two sources: 1) spontaneous polarization at heterointerfaces due to the ionic wurtzite nitride bonds, and 2) strain-induced piezoelectric polarization caused by lattice misfit or thermal strain. Polarization effects can impact heterojunction structures such as MODFETs and quantum wells. In quantum wells, the fundamental electron-heavy-hole transition in GaN/AlGaN quantum wells is observed to red-shift well below the GaN bulk gap for well widths larger than 3 nm for the specific quantum wells investigated. Polarization effects as pertained to nitride semiconductor heterostructures and devices will be treated.

Electrostatic effects which take place in group-m nitrides in their wurtzite crystallographic phase have important consequences on the optical properties of (Al,Ga)N/GaN multiple quantum wells. A low-temperature photoluminescence study shows that the behavior of the transition energy versus the barrier width is the opposite of that currently obtained for more usual m-V semiconductor compounds like arsenides: when decreasing the barrier thickness, a blueshift is observed. This behavior is attributed to a redistribution across the samples of the huge built-in electric field (several hundreds kV/cm) induced by the polarization difference between wells and barriers. The trend of the barrier-width dependence of the electric field is reproduced by using simple electrostatic arguments. However, the field magnitude is higher than that predicted laking account only piezoelectric effects. This result points out the role of the spontaneous polarization in wurtzite nitrides. [S0163-1829(99)13627-0].

Wide bandgap nitride semiconductors have recently attracted a great level of attention owing to their direct bandgaps in the visible to ultraviolet regions of the spectrum as emitters and detectors. Moreover, this material system with its favorable hetero-junctions and transport properties began to produce very respectable power levels in microwave amplifiers. If and when the breakdown fields achieved experimentally approach the predicted values, this material system would also be very attractive for power switching devices. In addition to the premature breakdown, high concentration of defects, and inhomogeneities, a number of scientific challenges remain including a clear experimental investigation of polarization effects. In this paper, following a succinct review of the progress that has been made, spontaneous and piezoelectric polarization effects and their impact on sample device-like hetero-structures will be treated.

Wide bandgap nitride semiconductors have recently attracted a great level of attention owing to their direct bandgaps in the visible to ultraviolet regions of the spectrum as emitters and detectors. Moreover, this material system with its favorable hetero-junctions and transport properties began to produce very respectable power levels in microwave amplifiers. If and when the breakdown fields achieved experimentally approach the predicted values, this material system would also be very attractive for power switching devices. In addition to the premature breakdown, and high concentration of defects and inhomogeneities, a number of scientific challenges remain including a clear experimental investigation of polarization effects. In this paper, following a succinct review of the progress that has been made, spontaneous and piezoelectric polarization effects and their impact on sample device like hetero-structures will be treated. (C) 1999 Elsevier Science Ltd. All rights reserved.

We combined the self-consistent procedure of solving the Schrodinger and Poisson equations for electron and hole wave functions with the variational calculation of exciton states in strained GaN/AlGaN quantum wells. The procedure accounted properly for the free-carrier effects on the excitonic wave function, namely, bleaching and quantum exclusion effects and allowed to quantify the dependence of the exciton energy and oscillator strength on the optical pumping density. The calculation revealed an interesting interplay between the screening of the polarisation fields which leads to the increase of the electron-hole overlap and the screening of the electron-hole interaction which affects the exciton Bohr radius. Peculiar non-monotonic behaviour of the exciton binding energy as function of the density of electron-hole plasma results from these effects.

We calculate the original properties of excitons in GaN-AlGaN quantum wells by a variational approach in the envelope function formalism. The separation of electrons and holes by huge internal electric fields induces an enhanced dependence of exciton binding energy and oscillator strength on the well width. We demonstrate the necessity,to perform excitonic calculations for obtaining, in particular, reliable values of oscillator strengths (thus radiative lifetimes), which are extremely sensitive to the well width.