E. Frayssinet

Micro Epitaxial lateral overgrowth of GaN/sapphire by Metal Organic Vapour Phase Epitaxy.

GaN/sapphire layers have been grown by Metal Organic Vapour Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using a SiH 4 /NH 3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early beginning of the growth, resulting in a kind of maskless ELO process with random opening sizes. This produces a significant decrease of the threading dislocation (TD) density compared to the best GaN/sapphire templates. Ultra Low Dislocation density (ULD) GaN layers were obtained with TD density as low as 7×10 7 cm −2 as measured by atomic force microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM). Time-resolved photoluminescence experiments show that the lifetime of the A free exciton is principally limited by capture onto residual donors, similar to the situation for nearly dislocation-free homoepitaxial layers.

Valence band offset of the ZnO/AlN heterojunction determined by x-ray photoemission spectroscopy

The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43±0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29±0.20 eV, this indicates that a type-II (staggered) band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values.

Evidence of free carrier concentration gradient along the c-axis for undoped GaN single crystals

Results of measurements of infrared reflectivity and micro-Raman scattering on the undoped GaN high pressure grown single crystals are reported. These crystals have usually a high electron concentration due to unintentional doping by oxygen. We show, by the shift of the plasma edge (infrared reflectivity measurements), that the free electron concentration is always higher on the (0 0 0 % 1)N face of the GaN single crystal than on the (0 0 0 1)Ga face. In order to determine the profile of the free carrier concentration, we performed transverse micro-Raman scattering measurements along the (0 0 0 1) c-axis of the crystal with spatial resolution of 1mm. Micro-Raman experiments give a quantitative information on the free carrier concentration via the longitudinal optical phonon–plasmon (LPP) coupling modes. Thus, by studying the behavior of the LPP mode along the c-axis, we found the presence of a gradient of free electrons. We suppose that this gradient of electrons is due to the gradient of the main electron donor, in undoped GaN single crystals, i.e. oxygen impurity. We propose a growth model which explains qualitatively the incorporation of oxygen during the growth of GaN crystal under high pressure of nitrogen. # 2001 Elsevier Science B.V. All rights reserved.

Demonstration of AlGaN/GaN High-Electron-Mobility Transistors Grown by Molecular Beam Epitaxy on Si(110)

The growth of AlGaN/GaN-based heterostructure on Si(110) substrates by molecular beam epitaxy using ammonia as the nitrogen precursor is reported. The structural, optical, and electrical properties of such heterostructure are assessed and are quite similar to the ones obtained on Si(111). A 2-D electron gas is formed at the Al0.3Ga0.7N/GaN interface with a sheet carrier density of 9.6 times 1012 cm-2 and a mobility of 1980 cm2/V middots at room temperature. Preliminary results concerning high-electron-mobility-transistor static characteristics are presented and compared with that of devices realized on other orientations of silicon.

Acoustic phonon scattering of two-dimensional electrons in GaN/AlGaN heterostructures

We have measured the temperature dependence of the mobility of the two-dimensional electron gas in AlGaN/GaN heterostructures grown on bulk GaN substrates. The linear dependence of the inverse mobility on temperature at temperatures below 50 K indicates the importance of acoustic phonon scattering in these high mobility heterostructures. Using the temperature dependence of the mobility at a range of carrier densities, we determined the GaN conduction band deformation potential to be a(c)=9.1+/-0.7 eV. This result provides a crucial parameter for accurate calculations of intrinsic mobility limits in AlGaN/GaN heterostructures

Experimental observation of strong light-matter coupling in ZnO microcavities: Influence of large excitonic absorption

We present experimental observation of the strong light-matter coupling regime in ZnO bulk microcavities grown on silicon. Angle resolved reflectivity measurements, corroborated by transfer-matrix simulations, show that Rabi splittings in the order of 70 meV are achieved even for low finesse cavities. The impact of the large excitonic absorption, which enables a ZnO bulk-like behavior to be observed even in the strong coupling regime, is illustrated both experimentally and theoretically by considering cavities with increasing thickness.

Effective g(*) factor of two-dimensional electrons in GaN/AlGaN heterojunctions

The conduction band parameters of two-dimensional (2D) electrons in high density GaN/AlGaN heterojunctions were studied using the cyclotron resonance and magnetotransport techniques in high magnetic fields (24 T) and low temperatures (300 mK). The Landau level splitting determined from the cyclotron resonance experiment yielded the effective mass of 2D carriers, m* = 0.242 +/- 0.002 m(0). The Lande g factor for the 2D electrons (g = 2.06 +/- 0.04) was determined from the angular dependence of the amplitude of Shubnikov-de-Haas oscillations experiments in tilted magnetic field

Relaxation and emission of Bragg-mode and cavity-mode polaritons in a ZnO microcavity at room temperature

The strong coupling regime in a ZnO microcavity is investigated through room temperature photoluminescence and reflectivity experiments. The simultaneous strong coupling of excitons to the cavity mode and the first Bragg mode is demonstrated at room temperature. The polariton relaxation is followed as a function of the excitation density. A relaxation bottleneck is evidenced in the Bragg-mode polariton branch. It is partly broken under strong excitation density, so that the emission from this branch dominates the one from cavity-mode polaritons.

Blue (Ga,In)N/GaN Light Emitting Diodes on Si(110) Substrate

We have fabricated and characterized blue (Ga,In)N/GaN multiple quantum well light emitting diodes grown on a Si(110) substrate by molecular beam epitaxy. For a 20 mA current, we have found that the operating voltage and the series resistance are as low as 3.5 V and 17 Ω, respectively. A maximum light output power of 72 µW is obtained as measured on the wafer. These characteristics are almost identical to those obtained on a reference sample grown on the commonly used Si(111) orientation.

High-Quality 2'' Bulk-Like Free-Standing GaN Grown by HydrideVapour Phase Epitaxy on a Si-doped Metal Organic Vapour Phase Epitaxial GaN Template with an Ultra Low Dislocation Density

High-quality 2? crack-free free-standing GaN has been attained by hydride vapour phase epitaxial growth on a Si-doped MOVPE GaN template with a low dislocation density and subsequent laser-induced ...