G. Neu

GaN–AlGaN heterostructure field-effect transistors over bulk GaN substrates

We report on AlGaN/GaN heterostructures and heterostructure field-effect transistors (HFETs) fabricated on high-pressure-grown bulk GaN substrates. The 2d electron gas channel exhibits excellent electronic properties with room-temperature electron Hall mobility as high as μ=1650 cm2/V s combined with a very large electron sheet density ns≈1.4×1013 cm−2. The HFET devices demonstrated better linearity of transconductance and low gate leakage, especially at elevated temperatures. We also present the comparative study of high-current AlGaN/GaN HFETs (nsμ>2×1016 V−1 s−1) grown on bulk GaN, sapphire, and SiC substrates under the same conditions. We demonstrate that in the high-power regime, the self-heating effects, and not a dislocation density, is the dominant factor determining the device behavior.

Growth by solid phase recrystallization and assessment of large ZnSe crystals of high purity and structural perfection

Some physical properties of ZnSe resulting from the ionicity of its chemical bond make it difficult its melt-growth, suggesting the use of low temperature growth. Among the classical techniques of low temperature growth, solution and vapor growth are shown to present some limitations. Solid state recrystallization is presented as an attractive alternative for the growth of large ZnSe crystals of high purity and structural perfection, as shown from photoluminescence and X-ray diffraction measurements.

Spectroscopy of excitons, bound excitons and impurities in h-ZnO epilayers

Photoluminescence (PL) and selective photoluminescence (SPL) experiments have been carried out to analyse the neutral donor bound exciton spectra of nominally undoped wurtzite ZnO epilayers and to study the electronic levels introduced by arsenic doping. The investigated samples were grown by molecular beam epitaxy on various substrates. The PL spectra of the undoped heterostructure layers exhibit little or no detectable deep emissions and are dominated by donor bound exciton recombination lines with distinct emission energies. The existence of residual strain distributions in the samples make possible the observation of resonant sharp lines due to the recombination of the bound excitons selectively created in their excited states. It is being shown that the first excited state spectra of the bound excitons consist of rotational states of the hole belonging to the (A) valence band. The PL spectra of ZnO: As samples present a new excitonic line as well as donor-acceptor pair emissions. The arsenic impurity is revealed to generate a shallow acceptor level in ZnO, an estimate of its binding energy is given.

Temperature effects on the photoluminescence of GaAs grown on Si

Photoluminescence properties of as‐grown and post‐growth annealed GaAs directly grown on Si substrates by metalorganic vapor phase epitaxy are studied at various temperatures. At low temperature, three extrinsic lines and an intrinsic exciton line split by residual biaxial tensile stress are observed. The extrinsic lines give evidence of growth‐induced defects. One of these lines, involving the presence of Si acceptors, appears after post‐growth annealing (10 min at 800 °C). The biaxial stress deduced from the intrinsic lines varies with temperature; extrapolation to zero stress results in a temperature slightly below the growth temperature.

Photoluminescence study of ZnSe single crystals grown by solid‐phase recrystallization

We investigate through low‐temperature photoluminescence (PL) and selective photoluminescence (SPL) spectroscopies, ZnSe single crystals grown by solid‐phase recrystallization. The PL spectra are dominated by the so‐called I1deep excitonic line, a neutral–acceptor bound–exciton line I1, the free‐exciton emission FX, and the n=2 excited state of FX. We identify the main residual impurities. Donor–acceptor pair bands are hardly detected. A major characteristic of these samples is the quasiabsence of any Cu‐related deep emission which generally plagues the PL spectra of bulk ZnSe. Consequently, I1deep is ascribed to an exciton bound to Zn‐vacancies related acceptors. Our results indicate that these ZnSe samples are of high quality and that solid‐phase recrystallization is a promising technique to prepare ZnSe epitaxial substrates.

Very high purity InP epilayer grown by metalorganic chemical vapor deposition

Very high purity InP epilayers have been grown by low‐pressure metalorganic chemical vapor deposition growth technique using trimethylindium as In source. Residual doping levels as low as 3×1013 cm−3 , with Hall mobility as high as 6000 cm2  V−1  s−1 at 300 K and 200 000 cm2  V−1  s−1 at 50 K have been measured. These results and photoluminescence at 2 K showed that it is the purest InP epilayer ever reported in the literature, with zero compensation ratio.

Heteroepitaxy of GaAs on Si: The effect of in situ thermal annealing under AsH3

This letter shows that an in situ thermal annealing step in AsH3/H2 during the metalorganic vapor phase epitaxy of GaAs on Si(001) improves the crystalline quality. The dislocation density is reduced (below 107 cm−2) without affecting the Si diffusion across the heterointerface or the strain level in the epilayer. The nature of the various near‐band‐gap recombinations present in the unannealed and annealed samples is discussed in light of selective photoluminescence experiments.

Optical properties of GaN epilayers and GaN/AlGaN quantum wells grown by molecular beam epitaxy on GaN(0001) single crystal substrate

GaN epilayers and GaN/AlGaN quantum wells (QWs) were grown by molecular beam epitaxy on GaN(0001) single crystal substrates. Transmission electron microscopy (TEM) was used to assess the crystal quality of the homoepitaxial layers. A dislocation density of less than 105 cm−2 is deduced from TEM imaging. Low temperature (1.8 K) photoluminescence (PL) of homoepitaxial GaN reveals PL linewidths as low as 0.3 meV for bound excitons. The PL integrated intensity variation between 10 and 300 K is compared to that observed on a typical heteroepitaxial GaN/Al2O3 layer. A 2 nm thick GaN/Al0.1Ga0.9N QW has been studied by time-resolved and continuous wave PL. The decay time is close to a purely radiative decay, as expected for a low defect density. Finally, the built-in polarization field measured in a homoepitaxial QW is shown to be comparable to that measured on heteroepitaxial QWs grown either on sapphire or silicon substrates.

Structural and optical characterization of ZnSe single crystals grown by solid‐phase recrystallization

We investigate by high‐resolution x‐ray diffraction (HRXRD), temperature‐dependent photoluminescence (PL) and reflectivity spectroscopies, and low‐temperature selective‐photoluminescence spectroscopy ZnSe single crystals grown by solid‐phase recrystallization. HRXRD reveals the high structural perfection of the samples which exhibit rocking‐curve linewidths in the 15–20 arcsec range. The low‐temperature PL spectra are dominated by the so‐called Ideep1 excitonic line, a neutral‐acceptor bound‐exciton line I1, the free‐exciton emission FX, and the n=2 excited state of FX. We identify the main residual impurities to be Li acceptors. Donor–acceptor pair bands are very hardly detected at low temperature which indicates a low donor content. A major characteristics of these samples is the quasi‐absence of any Cu‐related deep emission which generally plagues the PL spectra of bulk ZnSe. Consequently, Ideep1 is ascribed to Zn‐vacancy–donor complexes. Finally, from the temperature dependence of the PL emission and ...

Temperature Dependence of Exciton Lifetimes in GaAs/AlGaAs Quantum Well Structures

The photoluminescence decay times in GaAs/AlGaAs single quantum well structures with layer thickness between 20 A and 80 A have been investigated in the temperature range from 4 K up to room temperature. It is shown that the increase in the PL decay time usually ascribed to radiative free-excitons recombination is present even in conditions where other mechanisms such as recombination of localized excitons and free carriers, exciton ionization and thermal activation of nonradiative processes are effective.