D. Martin

Exciton localization on basal stacking faults in a-plane epitaxial lateral overgrown GaN grown by hydride vapor phase epitaxy

We present a detailed study of the luminescence at 3.42 eV usually observed in a-plane epitaxial lateral overgrowth (ELO) GaN grown by hydride vapor phase epitaxy on r-plane sapphire. This band is related to radiative recombination of excitons in a commonly encountered extended defect of a-plane GaN: I1 basal stacking fault. Cathodoluminescence measurements show that these stacking faults are essentially located in the windows and the N-face wings of the ELO-GaN and that they can appear isolated as well as organized into bundles. Time-integrated and time-resolved photoluminescence, supported by a qualitative model, evidence not only the efficient trapping of free excitons (FXs) by basal plane stacking faults but also some localization inside I1 stacking faults themselves. Measurements at room temperature show that FXs recombine efficiently with rather long luminescence decay times (360 ps), comparable to those encountered in high-quality GaN epilayers. We discuss the possible role of I1 stacking faults in...

Indium Distribution in Ingaas Quantum Wires Observed with the Scanning Tunneling Microscope

The incorporation of In in the growth of crescent‐shaped In0.12Ga0.88As quantum wires embedded in (AlAs)4(GaAs)8 superlattice barriers is studied in atomic detail using cross‐sectional scanning tunneling microscopy. It is found that the In distribution in both the surface and the first subsurface layer can be atomically resolved in the empty‐ and filled‐state images, respectively. Strong In segregation is seen at the InGaAs/GaAs interfaces, but neither an expected enhancement of the In concentration at the center of the quantum wire compared to the planar quantum well nor In clustering beyond the statistical expectation is observed.

All-optical reflectivity tuning and logic gating in a GaAs/AlAs periodic layered structure

The optical nonlinearity of a GaAs/AlAs periodic layered structure was experimentally investigated for the first time. The shift of the reflectivity peak with increasing intensity was observed. A reflectivity contrast of about 10:1 was obtained by varying the incident intensity. Hysteresis loops due to the response delay of both the electronic and the thermal nonlinearity were observed. All‐optical logic operations were also demonstrated. A further improvement of the structure may lead to a new type of optical bistable device.

Study of the epitaxial relationships between III-nitrides and M-plane sapphire

GaN films epitaxially-grown on M-sapphire may have different orientations, either nonpolar and semipolar. In this paper, the different epitaxial relationships are investigated in details thanks to transmission electron microscopy. It is shown that these relationships drastically depend on the nitridation conditions. Mechanisms explaining the different epitaxial relationships are proposed. These mechanisms take into account the formation of a nitridation layer, the lattice mismatches between the growing films, and the substrate, and the surface energies. It is, moreover, shown that the difference of symmetries between the M-sapphire surface and the epitaxial films leads to the formation of growth twins for the (101¯0) and (101¯3¯) orientations.

Low-temperature time-resolved cathodoluminescence study of exciton dynamics involving basal stacking faults in a-plane GaN

Time-resolved cathodoluminescence at 27 K has been performed on a-plane GaN grown by epitaxial lateral overgrowth. We detail the relaxation and recombination mechanisms of excitons [free or bound to neutral donors, or bound to I1-type basal stacking faults (BSFs)] in relation to the local density in BSFs. We describe the slow exciton capture rate on isolated BSFs by a diffusion model involving donors via a hopping process. Where BSFs are organized into bundles, we relate the shorter rise time to intra-BSF localization processes and the multiexponential decay to the type-II band alignment of BSFs in wurtzite GaN.

Atomic structure and luminescence excitation of GaAs/(AlAs)n(GaAs)m quantum wires with the scanning tunneling microscope

We report on the imaging of a molecular beam epitaxially grown GaAs/(AlAs)n(GaAs)m quantum well‐wire array by means of cross‐sectional scanning tunneling microscopy (XSTM) and scanning tunneling‐induced luminescence (STL). XSTM provides atomically resolved cross‐sectional images of sets of quantum well wires with chemical sensitivity within the group III species and electrical sensitivity to single dopant atoms. This permits the precise observation of growth mechanisms and the identification of defects responsible for inhomogeneities in the growth morphology, as well as the determination of dopant incorporation throughout the structure. STL permits the relative quantum efficiency of individual quantum wires to be quantified.

All‐optical bistable switching and signal regeneration in a semiconductor layered distributed‐feedback/Fabry–Perot structure

We report the experimental observation of optical bistable switching in a combined distributed‐feedback/Fabry–Perot structure. The sample had a high switching contrast with almost zero reflectivity in the on‐state. A turnoff time as short as 4 ns was measured. The threshold power for bistability was about 2 mW. Bistable switching with the memory effect and a potential application for all‐optical signal regeneration were demonstrated.

InGaN based micro light emitting diodes featuring a buried GaN tunnel junction

GaN tunnel junctions (TJs) are grown by ammonia molecular beam epitaxy. High doping levels are achieved with a net acceptor concentration close to ∼1020 cm−3, thanks to the low growth temperature. This allows for the realization of p-n junctions with ultrathin depletion width enabling efficient interband tunneling. n-p-n structures featuring such a TJ exhibit low leakage current densities, e.g., <5 × 10−5 A cm−2 at reverse bias of 10 V. Under forward bias, the voltage is 3.3 V and 4.8 V for current densities of 20 A cm−2 and 2000 A cm−2, respectively. The specific series resistance of the whole device is 3.7 × 10−4 Ω cm2. Then micro-light emitting diodes (μ-LEDs) featuring buried TJs are fabricated. Excellent current confinement is demonstrated together with homogeneous electrical injection, as seen on electroluminescence mapping. Finally, the I-V characteristics of μ-LEDs with various diameters point out the role of the access resistance at the current aperture edge.

High quality thin GaN templates grown by hydride vapor phase epitaxy on sapphire substrates

The growth by hydride vapor phase epitaxy (HVPE) of high quality thin GaN layers (d=8μm) on c-plane sapphire substrates with dislocation densities lower than 2×108cm−2 is demonstrated using a two-step process similar to that of metal organic vapor phase epitaxy (MOVPE). Ex situ surface preparation and nucleation layer thickness are shown to be critical factors in achieving these high quality epilayers as they allow controlling the polarity and the dislocation density, respectively. Furthermore, we demonstrate that in situ reflectivity monitoring applied to HVPE is a powerful technique for rapidly optimizing the growth parameters. As a result, thin HVPE-grown GaN layers with state of the art MOVPE GaN quality are obtained as demonstrated through structural and optical characterizations.

Exciton recombination dynamics in a-plane (Al,Ga)N/GaN quantum wells probed by picosecond photo and cathodoluminescence

We present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluc...