We review in this paper the application of ZnO/(Zn,Mg)O quantum wells to the photodetection of the polarization state of UV light. This photodetection is achieved by using the natural anisotropy that exists in non-polar ZnO/(Zn,Mg)O quantum wells, which separates the excitonic absorption from the three valence bands to the conduction band depending on the incident light polarization. The device structures covered here consist of Schottky photodiodes on a- and m-plane orientations, grown by molecular beam epitaxy on ZnO or sapphire substrates, and are analyzed as a function of the incident light polarization.
(Al,Ga)N light emitting diodes (LEDs), emitting over a large spectral range from 360 nm (GaN) down to 210 nm (AlN), have been successfully fabricated over the last decade. Clear advantages compared to the traditional mercury lamp technology (e.g. compactness, low-power operation, lifetime) have been demonstrated. However, LED efficiencies still need to be improved. The main problems are related to the structural quality and the p-type doping efficiency of (Al,Ga)N. Among the current approaches, GaN nanostructures, which confine carriers along both the growth direction and the growth plane, are seen as a solution for improving the radiative recombination efficiency by strongly reducing the impact of surrounding defects. Our approach, based on a 2D - 3D growth mode transition in molecular beam epitaxy, can lead to the spontaneous formation of GaN nanostructures on (Al,Ga)N over a broad range of Al compositions. Furthermore, the versatility of the process makes it possible to fabricate nanostructures on both (0001) oriented “polar” and (11 2 2) oriented “semipolar” materials. We show that the change in the crystal orientation has a strong impact on the morphological and optical properties of the nanostructures. The influence of growth conditions are also investigated by combining microscopy (SEM, TEM) and photoluminescence techniques. Finally, their potential as UV emitters will be discussed and the performances of GaN / (Al,Ga)N nanostructure-based LED demonstrators are presented.
A study of the optimisation of the detectivity of a mid infrared double heterostructures photovoltaic detector is proposed. Simple approximate analytic expressions for the dark current are compared with full numerical calculations, and give physical insight on the mechanisms dominating the dark current. The analysis is performed step by step in different structures, from a simple p-n junction to the full double heterostructures. The influence of temperature, barrier band gap energy and absorbing layer thickness in a double heterostructures, doping density in the active region on diffusion and generation-recombination mechanisms is analysed.
A third-order-mode-emitting laser diode is demonstrated. The AlGaAs/GaAs hetero-structure is engineered to emit a photon pair through intra-cavity modal phase-matched parametric down-conversion. Device operations and twin photon generation experimental issues are discussed.
We describe some key growth issues for Mid-Infrared electroluminescent devices based on a quantum-cascade design using InAs/AlSb heterostructures grown on GaSb substrates. Structural and optical properties of antimonide/arsenide interfaces are first investigated on InAs/AlSb multiple quantum well samples with different types of Sb-like interfaces and various InAs thicknesses. We show that X-ray reflectometry is a powerful complementary tool to High Resolution X-ray Diffraction (HRXRD) to extract both individual layer thicknesses and interface roughnesses using only electronic densities as input parameters. The good structural quality of samples is evidenced by the persistence of sharp high order satellite peaks on HRXRD spectra. The associated optical properties are studied by photo-induced intersubband absorption. Strong E12 p- polarized intersubband absorptions are observed with a full- width-at-half-maximum (FWHM) around 12 meV at 77 K showing good material quality. Absorption peak positions are compared to theoretical simulations based on a 2 X 9-band k.p calculation. These results allow us to properly design and fabricate InAs/AlSb quantum cascade light emitting devices in the 3 - 5 micrometers wavelength window taking into account the growth constraints. Well-resolved Mid-Infrared (3.7 - 5.3 micrometers ) electroluminescence peaks are observed up to 300 K with FWHM to emission energy ratio ((Delta) E/E) around 8%.
We describe a mid-IR photovoltaic detector using InAsSb as active material, grown by MBE on a GaSb substrate. The purpose of this study is to show that quantum detectors can offer an alternative to thermal detectors (pyroelectric or resistive bolometers) for high temperature (near room temperature) operation. With a 9% Sb content, InAsSb is lattice matched to GaSb and thus provides an excellent material quality, with Shockley-Read lifetimes of the order of 200 ns as measured by photoconductive gain measurements as well as time resolved photoconductivity experiments. The band gap of InAsSb corresponds to a wavelength of 5 microns at room temperature. This makes InAsSb an ideal candidate for room temperature detection in the 3-5 microns atmospheric window. Photovoltaic structures are characterized by current voltage characteristics as a function of temperature. Using the absorption value obtained on the test samples, a detectivity of 7X109 Jones at 3.5 micrometers is estimated at a temperature of 250 K, which can easily be reached with Peltier cooling. Considering the photovoltaic spectrum, this leads to a NETD lower than 80 mK.
We describe a mid-IR photovoltaic detector using InAsSb as active material, grown by MBE on a GaSb substrate. The purpose of this study is to show that quantum detectors can offer an alternative to thermal detectors for high temperature operation. With a 9 percent Sb content, InAsSb is lattice matched to GaSb and thus provides an excellent material quality, with Shokley-Read lifetimes of the order of 200 ns as measured by photoconductive gain measurements as well as time resolved photoconductivity experiments. The band gap of InAsSb corresponds to a wavelengths as well as time resolved photoconductivity experiments. The band gap of InAsSb corresponds to a wavelength of 5 microns at room temperature. This makes InAsSb an ideal candidate for rom temperature detection in the 3-5 microns atmospheric window. Photovoltaic structures are characterized by current voltage characteristics as a function of temperature. Using the absorption value obtained on the test samples, a detectivity of 7 by 109 Jones can be obtained at a temperature of 250 K, which can easily be reached with Peltier cooling. This leads to a NETD lower than 80 mK.
Quantum well infrared photodetectors (QWIPs) form a new generation of infrared detectors based on carrier confinement in ultrathin semiconductor heterostructures. The artificial energy levels in these wells can be tailored to match any optical transition in the 3 - 20 micrometer photon wavelength range by adjusting the quantum well width and the barrier composition. In this communication, we summarize our present understanding of the physics of QWIP detection: photoexcited carrier emission and capture probability, contact injection, and noise mechanisms. We also present the performances of optimized devices for the infrared Detection in the 3 - 5 micrometer and 8 - 12 micrometer wavelength ranges. We also illustrate the major advantages of this new technology for infrared staring arrays: (1) standard III-V substrates and technology, thermal stability, uniformity, large areas, low development costs, radiation hardness, (2) adjustability from 3 to 20 micrometer, (3) new functions: multispectrality, spectrophotometry, band switching, optical reading.