Bernard Gil

High internal electric field in a graded-width InGaN/GaN quantum well: Accurate determination by time-resolved photoluminescence spectroscopy

Time-resolvedphotoluminescence (PL), at T=8 K, is used to study a graded-width InGaN/GaN quantum well. Across the sample, the well width continuously varies from ∼5.5 to 2.0 nm corresponding to PL peak energies varying between 2.0 and 2.9 eV and to PL decay rates covering four orders of magnitude. The plot of decay times versus PL energies is very well fitted by a calculation of the electron–hole recombination probability versus well width. The only fitting parameter is the electric field in the well, which we find equal to 2.45±0.25 MV/cm, in excellent agreement with experimental Stokes shifts for this type of samples.

Hexagonal boron nitride is an indirect bandgap semiconductor

Scientists resolve the long-debated issue of the nature and value of the bandgap in hexagonal boron nitride by providing evidence for an indirect bandgap at 5.955 eV and an exciton binding energy of about 130 meV by means of optical spectroscopy.

Optical properties of GaN epilayers on sapphire

The optical properties of GaN epilayers grown by metal‐organic vapor‐phase epitaxy on (0001)‐oriented sapphire are investigated by means of photoluminescence, reflectance, and differential spectroscopy. We obtain quantitative information about the intrinsic or extrinsic nature of the 2 and 300 K photoluminescence features. From detailed investigations of the reflectance properties of these layers we can quantify the residual strain field in these layers and determine the GaN deformation potentials. Comparison of these values with quantities measured on other semiconductors with wurtzite symmetry is also addressed. Last we utilize photoreflectance spectroscopy to measure exciton binding energies.

Polarization effects in nitride semiconductor device structures and performance of modulation doped field effect transistors

Abstract 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.

Giant exciton-light coupling in ZnO quantum dots

We investigate the strength of the coupling of the electronic states with the electromagnetic field in ZnO nanospheres, taking into account the retardation effect. We show that the coupling strength is particularly strong: the bulk properties are so enhanced that the radiative decay time can reach some 200 ps for quantum dot sizes of some 30 nm.

Barrier composition dependence of the internal electric field in ZnO∕Zn1−xMgxO quantum wells

Time-integrated photoluminescence experiments are used to study the excitonic optical recombinations in wurtzite ZnO∕Zn1−xMgxO single quantum wells of varying widths and magnesium compositions. By comparing experimental results with a variational calculation of excitonic energies, the authors determine the magnitude of the built-in electric field that is induced by both spontaneous and piezoelectric polarizations. It is found that the electric field varies linearly with magnesium composition. By taking into consideration the well-known distribution of electric field among the barrier and the well layers in multiquantum wells, the authors show that their results are fully consistent with previously reported data.

Large size dependence of exciton-longitudinal-optical-phonon coupling in nitride-based quantum wells and quantum boxes

We present an experimental and theoretical study of the size dependence of the coupling between electron–hole pairs and longitudinal-optical phonons in Ga1−xInxN/GaN-based quantum wells and quantum boxes. We found that the Huang–Rhys factor S, which determines the distribution of luminescence intensities between the phonon replicas and the zero-phonon peak, increases significantly when the vertical size of the boxes or the thickness of quantum well increases. We assign this variation to (1) the strong electric field present along the growth axis of the system, due to spontaneous and piezoelectric polarizations in these wurtzite materials, and (2) the localization on separate sites of electrons and holes in the plane of the wells or boxes, due to potential fluctuations in the ternary alloy. Indeed, envelope-function calculations for free or localized excitons, with electron–hole distance only controlled by Coulomb interaction, do not account quantitatively for the measured behavior of the S factor. In fact...

High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots

We compare the quality factor values of the whispering gallery modes of microdisks (μ-disks) incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor resonant modes on the whole spectrum, which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor, which reflects the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs-based μ-disks show very high Q values (Q>7000) whereas the Q factor is only up to 2000 in μ-disks embedding QDs grown on the AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation.

Effects of GaAlN barriers and of dimensionality on optical recombination processes in InGaN quantum wells and quantum boxes

We compare several InGaN-based low-dimensional systems, by time-resolved photoluminescence (PL), versus temperature (8<T<280 K). We investigate the influence of growing or not an AlGaN barrier on top of the active layer. We address the differences between quantum wells and quantum boxes 5–10 nm in diameter and 2 nm in height. Our results are consistent with carrier localization on potential fluctuations with spatial extension much smaller than the size of the quantum boxes. Growing an AlGaN barrier reduces the carrier mobility between fluctuations, thus maintaining an effective PL dominated by localized carriers up to room temperature.

Photoreflectance investigations of the bowing parameter in AlGaN alloys lattice-matched to GaN

Room temperature photoreflectance investigations have been performed on a series of AlGaN layers grown both by metalorganic vapor phase epitaxy and molecular beam epitaxy on c-plane sapphire substrates. The aluminum composition was ranging between 0% and 20%, and was determined independently in the different growth laboratories, by various methods. It is found that within the experimental uncertainty, there is no detectable bowing parameter in these alloys. This contradicts some previous experimental investigations and confirms other ones.