Exact determination of electrical properties of wurtzite Al1−xInxN/(AlN)/GaN heterostructures (0.07≤x≤0.21) by means of a detailed charge balance equation

This paper discusses the determination of key electrical parameters of AlInN/(AlN)/GaN heterostructures from capacitance-voltage (CV) measurements. These heterostructures gained recently importance since they allow for high electron mobility transistor (HEMT) devices with several remarkable records: densities of the 2D electron gas (2DEG) of 2.6×1013 cm−2 for lattice-matched (LM) heterostructures and barrier thickness of 14nm, beyond 2 A/mm saturation currents, above 100GHz operation for heterostructures grown on Si (111) with gate length of 0.1µm. Despite these striking experimental results, a consistent determination of the most important electrical parameters, namely polarization sheet charge density, surface potential, and dielectric constant of the alloy are still missing. By setting up the correct charge balance equation, these parameters can unambiguously be determined. For instance, in the case of nearly LM Al0.85In0.15N these parameters amount to σAl0.85In0.15N/GaN~3.7×1017 m−2, eΦS~3 eV and Al0.85In0.15N ~11.2, for the charge density, the surface barrier potential, and the dielectric constant, respectivel

Progress in Violet Light-Emitting Diodes Based on ZnO/GaN Heterojunction

Progress in light-emitting diodes (LEDs) based on ZnO/GaN heterojunctions has run into several obstacles during the last twenty years. While both the energy bandgap and lattice parameter of the two semiconductors are favorable to the development of such devices, other features related to the electrical and structural properties of the GaN layer prevent an efficient radiative recombination. This work illustrates some advances made on ZnO/GaN-based LEDs, by using high-thickness GaN layers for the p-region of the device and an ad hoc device topology. Heterojunction LEDs consist of a quasicoalesced non-intentionally doped ZnO nanorod layer deposited by chemical bath deposition onto a metal–organic vapor-phase epitaxy -grown epitaxial layer of p-doped GaN. Circular 200 μm-sized violet-emitting LEDs with a p-n contact distance as low as 3 μm exhibit a turn-on voltage of 3 V, and an emitting optical power at 395 nm of a few microwatts. Electroluminescence spectrum investigation shows that the emissive process can be ascribed to four different recombination transitions, dominated by the electron-hole recombinations on the ZnO side

Impact of biexcitons on the relaxation mechanisms of polaritons in III-nitride based multiple quantum well microcavities

We report on the direct observation of biexcitons in a III nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one.This work was supported by the NCCR Quantum Photonics, research instrument of the Swiss National Science Foundation, through Grant No. 129715 and Grant No. 200020-113542, and by the EU-project Clermont4 (Grant No. FP7-235114)

Hétéro-épitaxie de Nitrure de Gallium sur substrat de silicium (111) et applications

NICE-BU Sciences (060882101) / SudocSudocFranceF

Current Spreading Length and Injection Efficiency in ZnO/GaN-Based Light-Emitting Diodes

We report on carrier injection features in light-emitting diodes (LEDs) based on nonintentionally doped-ZnO/p-GaN heterostructures. These LEDs consist of a ZnO layer grown by chemical-bath deposition (CBD) onto a p-GaN template without using any seed layer. The ZnO layer (∼1-µm thickness) consists of a dense collection of partially coalesced ZnO nanorods, organized in wurtzite phase with marked vertical orientation, whose density depends on the concentration of the solution during the CBD process. Due to the limited conductivity of the p-GaN layer, the recombination in the n-region is strongly dependent on the spreading length of the holes, Lh, coming from the p-contact. Moreover, the evaluation of Lh is not easy and generally requires the design and the fabrication of several LED test patterns. We propose a simple and effective method to calculate Lh, just based on simple considerations on I–V characteristics, and a way to improve the injection efficiency in the n region based on a noncircular electrode geometry. In particular, an interdigitated electrode structure is proved to be more efficient in terms of hole injection from n- to p-region

Impact of saturation on the polariton renormalization in III-nitride based planar microcavities

It has been widely observed that an increasing carrier density in a strongly coupled semiconductor microcavity (MC) alters the dispersion of cavity polaritons, below and above the condensation threshold. The interacting nature of cavity polaritons stems from their excitonic fraction being intrinsically subject to Coulomb interactions and the Pauli-blocking principle at high carrier densities. By means of injection-dependent photoluminescence studies performed nonresonantly on a GaN-based MC at various temperatures, it is shown that already below the condensation threshold saturation effects generally dominate over any energy variation in the excitonic resonance. This observation is in sharp contrast to the usually assumed picture in strongly coupled semiconductor MCs, where the impact of saturation is widely neglected. These experimental findings are confirmed by tracking the exciton emission properties of the bare MC active medium and those of a high-quality single GaN quantum well up to the Mott density. The systematic investigation of renormalization up to the polariton condensation threshold as a function of lattice temperature and exciton-cavity photon detuning is strongly hampered by photonic disorder. However, when overcoming the latter by averaging over a larger spot size, a behavior in agreement with a saturation-dominated polariton renormalization is revealed. Finally, a comparison with other inorganic material systems suggests that for correctly reproducing polariton renormalization, exciton saturation effects should be taken into account systematically

InGaN/GaN quantum wells for polariton laser diodes: Role of inhomogeneous broadening

Contrary to the case of III-nitride based visible light-emitting diodes for which the inhomogeneous linewidth broadening characteristic of InGaN-based multiple quantum well (MQW) heterostructures does not appear as a detrimental parameter, such a broadening issue can prevent a microcavity (MC) system entering into the strong light-matter coupling regime (SCR). The impact of excitonic disorder in low indium content (x similar to 0.1) InxGa1-xN/GaN MQW active regions is therefore investigated for the subsequent realization of polariton laser diodes by considering both simulations and optical characterizations. It allows deriving the requirements for such MQWs in terms of absorption, emission linewidth, and Stokes shift. Systematic absorption-like and photoluminescence (PL) spectroscopy experiments are performed on single and multiple In0.1Ga0.9N/GaN quantum wells (QWs). Micro-PL mappings reveal a low temperature PL linewidth of similar to 30 meV, compatible with SCR requirements, for single QWs for which the microscopic origin responsible for this broadening is qualitatively discussed. When stacking several InGaN/GaN QWs, a departure from such a narrow linewidth value and an increase in the Stokes shift are observed. Various possible reasons for this degradation such as inhomogeneous built-in field distribution among the QWs are then identified. An alternative solution for the MC design to achieve the SCR with the InGaN alloy is briefly discussed. (C) 2014 AIP Publishing LLC