S. Mazzucato

S-shaped behaviour of the temperature-dependent energy band gap in dilute nitrides

We have investigated the temperature dependence of the band gap energy in GaInNAs, GaNAs and InGaAs quantum wells. In the structures containing nitrogen the well-known S-shaped characteristic was observed at low temperatures. We explain this anomalous temperature behaviour by strong carrier localization in potential fluctuations at low temperatures. In the nitrogen free samples, there was no S-shaped behaviour and the empirical Varshni dependence was followed.

GaInNAs-based Hellish-vertical cavity semiconductor optical amplifier for 1.3 μm operation.

Hot electron light emission and lasing in semiconductor heterostructure (Hellish) devices are surface emitters the operation of which is based on the longitudinal injection of electrons and holes in the active region. These devices can be designed to be used as vertical cavity surface emitting laser or, as in this study, as a vertical cavity semiconductor optical amplifier (VCSOA). This study investigates the prospects for a Hellish VCSOA based on GaInNAs/GaAs material for operation in the 1.3-μm wavelength range. Hellish VCSOAs have increased functionality, and use undoped distributed Bragg reflectors; and this coupled with direct injection into the active region is expected to yield improvements in the gain and bandwidth. The design of the Hellish VCSOA is based on the transfer matrix method and the optical field distribution within the structure, where the determination of the position of quantum wells is crucial. A full assessment of Hellish VCSOAs has been performed in a device with eleven layers of Ga0.35In0.65N0.02As0.08/GaAs quantum wells (QWs) in the active region. It was characterised through I-V, L-V and by spectral photoluminescence, electroluminescence and electro-photoluminescence as a function of temperature and applied bias. Cavity resonance and gain peak curves have been calculated at different temperatures. Good agreement between experimental and theoretical results has been obtained.

In-plane photovoltage and photoluminescence studies in sequentially grown GaInNAs and GaInAs quantum wells

We have investigated in-plane photovoltage (IPV) and photoluminescence (PL) in sequentially grown Ga0.8In0.2As/GaAs and Ga0.8In0.2N0.015As0.985/GaAs quantum wells. Temperature, excitation intensity, spectral and time dependent study of the IPV, arising from Fermi level fluctuations along the layers of the double quantum well structure, gives valuable information about the nonradiative centers and hence about the optical quality of the GaInNAs quantum well. It also provides information about the radiative transition energies in all the layers. In order to obtain either the trap activation energies and the detrapping rates of photogenerated carriers in the GaInNAs the IPV results are analyzed in terms of a theoretical model based on random doping fluctuations in nominally undoped multilayer structures. The PL results are analyzed in terms of the band anticrossing model to obtain the electron effective mass from the coupling parameter CNM.

Photoconductivity and photoluminescence under bias in GaInNAs/GaAs MQW p-i-n structures

The low temperature photoluminescence under bias (PLb) and the photoconductivity (PC) of a p-i-n GaInNAs/GaAs multiple quantum well sample have been investigated. Under optical excitation with photons of energy greater than the GaAs bandgap, PC and PLb results show a number of step-like increases when the sample is reverse biased. The nature of these steps, which depends upon the temperature, exciting wavelength and intensity and the number of quantum wells (QWs) in the device, is explained in terms of thermionic emission and negative charge accumulation due to the low confinement of holes in GaInNAs QWs. At high temperature, thermal escape from the wells becomes much more dominant and the steps smear out.

Spectral photoconductivity and in-plane photovoltage studies of as-grown and annealed GaInNAs/GaAs and GaInAs/GaAs quantum well structures

We present an investigation of thermal annealing effects on spectral photoconductivity and in-plane photovoltage, at temperatures between 30 K and 300 K, in sequentially grown GaInNAs/GaAs and GaInAs/GaAs quantum well structures. Our results indicate that thermal annealing not only improves the sample quality but also causes the blueshift as commonly observed by other groups in optical studies. We show that the observed anneal-induced blueshift behaviour can be explained in terms of two competing mechanisms: the redistribution of nearest neighbour configuration and the change of quantum well profile. We also show that thermal annealing increases the intensity of photoconductivity signal but reduces the in-plane photovoltage signal drastically.

Hot electron capture and power loss in 2D GaN

We report the experimental studies of hot electron energy relaxation and capture in a low electron mobility GaN/ GaAlNHEMT structure grown with MBE on sapphire substrate. Pulsed IV characteristics indicate that at elevated electric fields, current pulse decays exponentially with time. The threshold electric field ðFthÞ above which the current pulse exhibits the decay, increases with temperature (EthB200 V/cm at T ¼ 77 K, EthB350 V/cm at T ¼ 300 K). The magnitude of the decay decreases with increasing temperature and increases with applied field. Time constant associated with this decay decreases rapidly with increasing applied field. These observations indicate the presence of hot electron capture over potential barriers. Electron temperature as a function of applied electric field is obtained by comparing the measured electric field dependence of the mobility mE at a low lattice temperature, with the lattice temperature dependence of the mobility at a low electric field. Electron energy loss rate is then determined from the electron temperature dependence of the power loss using the power balance equations. Our results show that the experimental power loss has an exponential factor, _o=k which is much smaller than the value expected from LO phonon emission. Furthermore, the magnitude of the experimental power loss is also much lower than the theoretical value. This observation is explained in terms of capture of hot electrons. r 2002 Elsevier Science B.V. All rights reserved.

Experimental investigation and numerical modelling of photocurrent oscillations in lattice matched Ga1−xInxNyAs1−y/GaAs quantum well p-i-n photodiodes

Photocurrent oscillations, observed at low temperatures in lattice-matched Ga1−xInxNyAs1−y/GaAs multiple quantum well (MQW) p-i-n samples, are investigated as a function of applied bias and excitation wavelength and are modelled with the aid of semiconductor simulation software. The oscillations appear only at low temperatures and have the highest amplitude when the optical excitation energy is in resonance with the GaInNAs bandgap. They are explained in terms of electron accumulation and the formation of high-field domains in the GaInNAs QWs as a result of the disparity between the photoexcited electron and hole escape rates from the QWs. The application of the external bias results in the motion of the high-field domain towards the anode where the excess charge dissipates from the well adjacent to anode via tunnelling.

Dilute nitride and GaAs n-i-p-i solar cells

We demonstrate for the first time the operation of GaInNAs and GaAs n-i-p-i doping solar cells with ion-implanted selective contacts. Multiple layers of alternate doping are grown by molecular beam epitaxy to form the n-i-p-i structure. After growth, vertical selective contacts are fabricated by Mg and Si ion implantation, followed by rapid thermal annealing treatment and fabrication into circular mesa cells. As means of characterisation, spectral response and illuminated current–voltage (I-V) were measured on the samples. The spectral response suggests that all horizontal layers are able to contribute to the photocurrent. Performance of the devices is discussed with interest in the n-i-p-i structure as a possible design for the GaInP/GaAs/GaInNAs tandem solar cell.

Optical properties of GaInNAs/GaAs quantum wells

Abstract We report the results of our studies of optical and electro-optic properties of GaInNAs/GaAs single quantum wells grown by chemical beam epitaxy. The quantum wells have been characterised by scanning transmission electron microscopy and energy dispersive X-ray analysis. Photoluminescence measurements from sequentially grown GaInAs and GaInNAs quantum wells were carried out between 4 K and room temperature. A significant difference in the temperature dependence of GaInNAs band gap compared to nitrogen-free GaInAs is observed. Photoluminescence results are used to determine the interband transition energies. The results are compared with the theoretical values obtained using the band-anticrossing model. When the device is illuminated with monochromatic light, a finite photovoltage develops in the plane of the quantum wells due to Fermi level fluctuations.

Medium energy ion scattering studies of as-grown and annealed GaInNAs/GaAs quantum well

The post-growth thermal annealing of GaInNAs quantum well (QW) based lasers has been widely employed to optimise the desired laser emission wavelength and efficiency although the mechanism for this process is uncertain. A number of factors have been implicated including the inter-diffusion of gallium and indium atoms; reduction of non-radiative centres; and the enhanced incorporation of nitrogen atoms into the GaInNAs lattice structure. This paper investigates the distribution of indium within as-grown and annealed GaInNAs QW’s characterised by medium energy ion scattering. Comparison of the channelled and randomly scattered Ga and In atomic distributions indicates the aggregation of indium in the as-grown QW’s occurs on the nanoscale. After annealing at temperatures up to 600 °C the indium concentration in the 〈100〉 blocking dips is decreased and the strain in the QW is marginally increased. At 640 °C surface degradation is observed in the sample and the GaInNAs QW decomposes.