Roger Aulombard

Thermal stability of GaN investigated by Raman scattering

We have investigated the thermal stability of GaN using Raman scattering. Noninvasive optical monitoring of structural damage to GaN by high-temperature anneals in nitrogen ambient has been demonstrated. Characteristic features in the Raman spectrum identify three thermal stability regimes. Thermal damage between 900 and 1000 °C results in the appearance of a broad Raman peak between the E2 and A1(LO) phonon. For anneals at temperatures higher than 1000 °C emerging macroscopic disorder gives rise to distinct Raman modes at 630, 656, and 770 cm−1. Below 900 °C no thermal damage has been observed. The evolution of the Raman spectrum of GaN with increasing annealing temperature is discussed in terms of disorder-induced Raman scattering. We find clear indications for a reaction at the GaN/sapphire interface for anneals higher than 1000 °C.

Biosensing and protein fluorescence enhancement by functionalized porous silicon devices.

Porous silicon (PSi) is a promising biomaterial presenting the advantage of being biocompatible and bioresorbable. Due to the large specific surface area and unique optical features, these microporous structures are excellent candidates for biosensing applications. Investigating device functionality and developing simple Si-based transducers need to be addressed in novel biological detection. Our work demonstrates that, among the various PSi configurations for molecular detection, PSi microcavity structure demonstrates the best biosensing performance, reflected through the enhanced luminescence response and the changes in the refractive index. For successful immobilization, molecular infiltration and confinement are the two key factors that are controlled by the pore size distribution of the PSi microcavities and by the surface modification obtained by silane-glutaraldehyde chemistry. Enhancement of the fluorescence emission of confined fluorescent biomolecules in the active layer of PSi microcavities was observed for a nonlabeled protein with a natural green fluorescence, the glucose oxidase enzyme (GOX). An increase in the fluorescence emission was also observed when functionalized PSi material was used to detect specific binding between biotin and a low concentration of labeled streptavidin. Evidence for the enzymatic activity of GOX in its adsorbed form is also presented. Use of smart silicon devices, enabling enhancement of fluorescence emission of biomolecules, offers easy-to-use biosensing, based on the luminescence response of the molecules to be detected.

Properties of a photovoltaic detector based on an n-type GaN Schottky barrier

In this article, we report on the characterization of a photovoltaic detector based on an n-type GaN Schottky barrier. We first present the photovoltaic responsivity above the gap. Its spectrum is explained by the combined effects of absorption and diffusion. The hole diffusion length is estimated to be in the 0.1 μm range with a numerical model. The photoresponse below the gap is also investigated and it is shown that the current generated by the internal photoemission is the major contribution to the photocurrent at reverse biases at 80 K. At room temperature, an additional component to the photocurrent is clearly demonstrated and identified. This extra current stems from the existence of traps. Several spectroscopy techniques are used to characterize those traps. The supplementary current emitted from the traps in the depletion region accounts for the spectral and the temporal behavior of the Schottky photodetector at room temperature.

Coupling of GaN- and AlN-like longitudinal optic phonons in Ga1−xAlxN solid solutions

Long-wavelength optical phonons of Ga1−xAlxN solid solutions have been identified in a wide compositional range by Raman spectroscopy. The A1 and E1 polar phonon frequencies evolve continuously with x from one-member crystal to the other. The same behavior seems to hold true for the silent B1 mode, which manifests itself by an interference with an unidentified continuum. Coupling of the longitudinal-optic (LO) modes associated with the two types of bonds, via the macroscopic electric field, is treated by a generalization of the dielectric model [D. T. Hon and W. L. Faust, J. Appl. Phys. 1, 241 (1973)]. This approach accounts for the observed frequencies and supports the apparent one-mode behavior of the polar LO phonons.

Optimization of the MOVPE growth of GaN on sapphire

Abstract The low pressure MOVPE growth process of GaN deposited onto GaN buffer layers on sapphire substrates is studied in detail. The relevant growth parameters are identified, and their influences are studied. These include the influence of the sapphire nitridation, the growth of the GaN buffer layer and its subsequent thermal treatment, and the growth parameters of the GaN epilayer (growth temperature, precursor flow rates, V/III ratio, etc.). Low temperature photoluminescence was mainly used to investigate the layer quality and, as a result of this optimization study, we have been able to reproducibly grow layers which have a 2 K photoluminescence dominated by the free exciton.

Competitive adsorption effects in the metalorganic vapor phase epitaxy of GaN

The growth mechanisms of GaN grown on a GaN buffer deposited onto sapphire substrates are studied here. The growth rate was measured at different temperatures versus ammonia flow and was found to decrease with increasing NH3 flow. This surprising behavior is modeled in terms of competitive adsorption of species on the growing surface. Very good agreement is obtained between the model and the experimental data.

Realization and optical characterization of etched mirror facets in GaN cavities

We report on the realization of etched mirror facets in GaN cavities by chemically assisted ion-beam etching. The etching conditions are adjusted to obtain a high degree of verticality and smoothness. Optical pumping experiments and gain measurements are performed in etched GaN cavities of various geometries. Stimulated emission and lasing are observed. The study of the value of the gain at threshold as a function of the cavity length allows a determination of the reflection coefficient of the etched mirror. The measured value of 15% is in good agreement with the one expected for a perfect air–GaN interface. This demonstrates the high quality of the etched mirror facets.

Sensing by Means of Nonlinear Optics with Functionalized GaAs/AlGaAs Photonic Crystals

We report on specific functionalization of GaAs/AlGaAs photonic structures for molecular sensing via the optical second harmonic generation signal in the visible range exhibited by these nanostructures. Functionalization has been achieved by peptides selected by the phage display technology, revealing specific recognition for semiconducting surfaces. These small peptides when biotinylated serve for controlled placement of biotin onto the substrate to capture then streptavidin. Functionalization (with biotinylated peptide) and molecular recognition (of streptavidin) events both result in enhancing the nonlinear optical response of the samples. Adsorption and infiltration of biomolecules into the GaAs/AlGaAs photonic structure were monitored by atomic force and scanning electron microscopy combined with Energy Dispersive X-ray spectroscopy. We demonstrate that once functionalized with specific peptides, photonic structures could be used as miniature biosensors down to femtomolar detection sensitivity, by monitoring changes in the second harmonic signal when molecules are captured. Our results prove the outstanding sensitivity of the nonlinear approach in biosensing with photonic crystal waveguides as compared to linear absorption techniques on the same samples. The present work is expected to pioneer development of a new class of extremely small affinity-based biosensors with high sensitivity and demonstrates that photonic structures support device functionality that includes strongly confined and localized nonlinear radiation emission and detection processes.

Optimization and optical studies of ZnCdSeZnSe heterostructures grown by MOVPE

Graded index-separate confinement heterostructures (GRIN-SCH) based on ZnCdSe and ZnSe wide band gap semiconductors have been grown by low pressure MOVPE on (100) GaAs substrates. First, we have paid attention to the growth of thick ZnCdSe layers. A first study is carried out using selenium hydride, dimethyl cadmium and two dimethylzinc adducts as Se, Cd and Zn precursor respectively. This combination leads to intense prereactions and poor layer morphology. To limit this problem we have then used the tetrahydrothiophene:dimethylcadmium adduct. GRIN-SCH structures grown using the two Cd metalorganics are studied using photoluminescence. We have made a detailed study on the influence of the temperature on the carrier trapping and detrapping in the structures.

Positron annihilation in ZnSe layers grown on GaAs: Zinc vacancies and drift in the electric field at the ZnSe/GaAs interface

We used a slow positron beam to investigate the depth dependence of the positron–electron pair momentum distribution in ZnSe layers grown on a GaAs substrate. We report evidence that positrons annihilate in lattice in undoped ZnSe and at vacancies in heavily n-type ZnSe. It is also demonstrated that positrons in semi-insulating ZnSe are drifted to GaAs by fields of 1–3 kV/cm.