Borge Vinter

Benefits of homoepitaxy on the properties of nonpolar (Zn,Mg)O/ZnO quantum wells on a-plane ZnO substrates

We report on the properties of nonpolar (Zn,Mg)O/ZnO quantum wells (QW) homoepitaxially grown by molecular beam epitaxy on a-plane ZnO substrates. We demonstrate a drastic improvement of the structural properties. We compare the photoluminescence properties of nonpolar homoepitaxial QWs and nonpolar heteroepitaxial QWs grown on sapphire and show that the reduction in structural defects and the improvement of surface morphology are correlated with a strong enhancement of the photoluminescence properties: reduction in full width at half maximum, strong increase in the luminescence intensities and their thermal stability. The comparison convincingly demonstrates the interest of homoepitaxial nonpolar QWs for bright UV emission applications.

Room temperature InAsSb photovoltaic midinfrared detector

An InAs0.91Sb0.09 p-i-n photovoltaic midinfrared detector grown by molecular beam epitaxy and operating at room temperature is presented. An R0A of 1.05 Ω cm2 at 250 K and 0.12 Ω cm2 at 295 K has been achieved, resulting in a detectivity of 4.5×109 cmHz/W at 3.39 μm and 250 K. The quality of the active region material ensures a sufficiently low generation-recombination current. Room temperature performances are limited by the diffusion of holes from the active region through the confining barriers.

Tunneling assisted modulation of the intersubband absorption in double quantum wells

We present a new efficient modulator structure in which electron tunneling between asymmetric coupled quantum wells is used to populate or deplete the ground level for the infrared (IR) absorption, leading to externally controlled IR absorption at two different wavelengths (here 10.2 and 11.4 μm). The temperature dependence of the modulation depth is measured. We have used photoluminescence and photoluminescence excitation spectroscopies to analyze the space charge effects in the double quantum wells, and to estimate the carrier concentration in each well as a function of bias. The behavior of the device has been simulated using a self‐consistent model.

Residual and nitrogen doping of homoepitaxial nonpolar m-plane ZnO films grown by molecular beam epitaxy

We report the homoepitaxial growth by molecular beam epitaxy of high quality nonpolar m-plane ZnO and ZnO:N films over a large temperature range. The nonintentionally doped ZnO layers exhibit a residual doping as low as ∼1014 cm−3. Despite an effective incorporation of nitrogen, p-type doping was not achieved, ZnO:N films becoming insulating. The high purity of the layers and their low residual n-type doping evidence compensation mechanisms in ZnO:N films.

InAs/AlSb quantum-cascade light-emitting devices in the 3–5 μm wavelength region

Midinfrared (3.7–5.3 μm) electroluminescent devices based on a quantum-cascade (QC) design have been demonstrated using InAs/AlSb heterostructures, grown on GaSb substrates. The very high conduction band discontinuity (>2 eV) of this material system allows the design of QC devices at very short wavelengths. Well-resolved luminescence peaks were observed up to 300 K, with a full-width-at-half-maximum to peak wavelength ratio (Δλ/λ) of the order of 8%. The emission wavelengths are in good agreement with the results of our model. The emitted optical power is lower than that predicted, due to a nonoptimized electron injection into the active region.

Interface band gap engineering in InAsSb photodiodes

The optimization of an InAs0.91Sb0.09 based infrared detector has been performed. The importance of the interfaces between the active region and the contacts in generation recombination phenomena is demonstrated. The two sides of the active region are optimized independently through heterostructure band gap engineering. The use of an Al0.15In0.85As0.91Sb0.09 quaternary makes it possible reach a detectivity of 4.4×109cm√Hz∕W at 290 K and 1.4×1010cm√Hz∕W at 250 K at 3.39μm, offering the perspective of a noncryogenic infrared imaging in the 3–5μm band with quantum detectors.

Role of elastic scattering mechanisms in GaInAs/AlInAs quantum cascade lasers

Electron scattering spectroscopy has been performed on a GaInAs∕AlInAs midinfrared quantum cascade laser by applying a strong magnetic field along its growth axis. The interpretation of the experimental data, supported by our model of the electron lifetime in the presence of a strong magnetic field, very clearly demonstrates that the elastic contribution to the total scattering rate has a weight comparable to that of the optical phonons. The authors believe that this contribution has its origin in the alloy disorder, an efficient mechanism in this material system, which limits the lifetime of the excited subband of the laser transition.

Residual strain in nonpolar a-plane Zn1−xMgxO (0<x<0.55) and its effect on the band structure of (Zn,Mg)O/ZnO quantum wells

We investigate the dependence on Mg content of the lattice parameters and the surface morphology of nonpolar a-(112¯0) Zn1−xMgxO (x≤0.55) grown by molecular beam epitaxy. The anisotropy of the lattice parameters gives rise to an unusual in-plane strain state in the ZnO QWs: tensile strain along [11¯00] and compressive strain along [0001]. For a Zn0.6Mg0.4O barrier, the strain in a ZnO QW reaches −1.3% along [0001] and +0.3% along [11¯00]. This induces a strong blueshift of the excitonic transitions, in addition to the confinement effects, which we observe in photoluminescence excitation experiments.

Optical characterization and room temperature lifetime measurements of high quality MBE-grown InAsSb on GaSb

InAs0.91 Sb0.09 lattice matched to a GaSb substrate by molecular beam epitaxy (MBE) is characterized by photoluminescence and photoconductivity measurements. Absorption, photoluminescence and spectrally resolved dc photoconductivity measurements lead to a band gap expression for InAs1-x Sbx , as a function of the Sb content x , where 0.06<x <0.12 and the material is nearly lattice matched to GaSb. The expression is in good agreement with recent results focusing on this range of Sb content, but deviates significantly from the one known from the literature and covering the entire Sb composition range. The carrier lifetime was measured by stationary and time resolved photoconductivity with an excitation source at 3.39 µm. We report Shockley-Read limited lifetimes as long as 200 ns, which indicate the high quality of the material. The accuracy and consistency of these optical characterization techniques allow a reliable comparison of samples and provide a guide for the optimization of growth parameters.

Polarization-sensitive Schottky photodiodes based on a-plane ZnO/ZnMgO multiple quantum-wells

Light polarization-sensitive UV photodetectors (PSPDs) using non-polar a-plane ZnMgO/ZnO multiple quantum wells grown both on sapphire and ZnO substrates have been demonstrated. For the PSPDs grown on sapphire with anisotropic biaxial in-plain strain, the responsivity absorption edge shifts by ΔE ∼ 21 meV between light polarized perpendicular (⊥) and parallel (||) to the c-axis, and the maximum responsivity (R) contrast is (R⊥/R||)max ∼ 6. For the PSPDs grown on ZnO, with strain-free quantum wells, ΔE ∼ 40 meV and (R⊥/R||)max ∼ 5. These light polarization sensitivities have been explained in terms of the excitonic transitions between the conduction and the three valence bands.