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We describe the impact of non-radiative de-excitation mechanisms on the optical emission from silicon nanocrystals formed in SiO 2 . Auger excitation via free carriers deliberately introduced through phosphorus ion implantation, shows a monotonic increase with increasing phosphorus concentration which can be modelled adequately using a simple statistical approach. We also report a reduction in nanocrystal luminescence intensity with increasing exposure to UV radiation and suggest this phenomenon results from the introduction of non-radiative defects in the Si/SiO 2 network. The effect of UV radiation varies significantly depending on the sample preparation.

Experimental results showing room temperature normal incidence mid-infrared detection by AlGaN/GaN quantum well infrared photodetectors are presented. Designed structures have intersubband transitions corresponding to wavelengths in the region of 3 and 4 µm, where strong absorption in a sapphire substrate dominates. The intersubband spectra, therefore, were characterized by electronic Raman scattering and infrared photocurrent spectroscopy. The absorption spectra agree well with theoretical predictions. Details of device fabrication are presented with sensitivity estimates for the devices.

Ion implantation is a postgrowth processing technique which, when combined with annealing, can be used to tune the absorption wavelength of quantum well devices. We have implanted and annealed, three different quantum well infrared photodetector structures, and measured the absorption spectra of the samples by Fourier transform spectroscopy. The peak absorption wavelength shift of each structure has been calculated as a function of diffusion length by simulating the diffusion processes. We found different diffusion rates for the structures and attribute this to different numbers of as-grown defects. Our results indicate that agglomeration of single defects into defect clusters limits the ability of ion implantation to tune the wavelength of a structure with a higher number of as-grown defects. Thus, a structure with the lowest number of as-grown defects is most useful for fabricating a multi-color quantum well photodetector by ion implantation, because in this case ion implantation can enhance the diffusion rate considerably leading to large red- shift in peak absorption wavelength.

We report a microphotoluminescence and photocurrent study of individual InGaN quantum dots grown on GaN with surface densities of < 1 dot/μm 2 . Two samples were grown using standard metal-organic vapour phase epitaxy and consist of 1 μm GaN buffer layer followed by an InGaN epilayer grown with nominal thicknesses of 1.0 and 1.5 nm. Uncapped samples were studied by AFM and it was found that both samples showed a low surface density of quantum dots with average diameter of 30 nm and average height of 5 nm. Macrophotoluminescence studies of capped samples show two overlapping bands, which we assign to emission from the InGaN quantum well and dots. Simple mesa devices were fabricated from the samples consisting of a top Schottky contact and an ohmic ring base contact. Sub-micron aperture arrays were then prepared lithographically by local thinning of the top contact. Micro-photoluminescence conducted through these apertures shows fine line spectra that occur with the expected frequency for the density of dots observed in AFM. The application of a bias shifts and broadens the fine line structure. The observed line-width dependence on bias is interpreted in terms of spectral diffusion caused by the varying carrier populations in the vicinity of the dots.

We present comprehensive experimental study of p -type (Be) and n -type (Si) δ-doped GaAs/AlAs multiple quantum wells (QWs) intended to be used as selective sensors/emitters in terahertz (THz) range. The structures of various designs and doping levels were studied via different optical-photoreflectance-, surface photovoltage- and differential surface photovoltage. spectroscopies and a THz photocurrent technique using as THz emission source either free electron- or optically-pumped molecular THz laser within 4.300 K range of temperatures. Analysis of Franz-Keldysh oscillations in photoreflectance spectra and line shapes of the differential surface photovoltage spectra enabled to estimate built-in electric fields and excitonic parameters for a large number of QW subbands. The experimental interband transition energies were compared with calculations performed within the envelope function approximation taking into account non-parabolicity of the energy bands. The dominant exciton line broadening mechanisms were revealed, and the interface roughness was evaluated from analysis of the dependence of exciton linewidth broadening on the QW width. Terahertz spectroscopic measurements in p-type structures have indicated strong absorption around 55 μm wavelength due to intraband absorption of the bound holes, while increase in photocurrent in the structures below 80 μm wavelength is caused by photothermal ionization of Be acceptors.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have studied interband optical transitions, electronic structure and structural quality of p -type (Be) and n -type (Si) δ- doped GaAs/AlAs MQWs designed for selective THz sensing applying differential surface photovoltage (DSPV) spectroscopy. Sharp derivative-like features associated with excitonic optical transitions in GaAs/AlAs MQWs have been observed in the spectra at 300 K and 90 K temperature. The energies and line broadening parameters for a large number of QW related excitonic transitions were determined from the line-shape analysis of the DSPV spectra. The spectroscopic data of transition energies were found to be in a good agreement with calculations within the envelope function approximation which took into account the nonparabolicity of energy bands. Analysis of the dependence of the exciton linewidth broadening on the quantum subband number allowed evaluate line-broadening mechanisms and interface roughness in the MQW structures. It was determined that doping with Si broadens more effectively the optical spectra lines in comparison with the structures of the same design doped with Be.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

InGaN/GaN multilayer quantum dot structures produced by MOCVD techniques on c-plane sapphire were studied by transmission electron microscopy (TEM) and photoluminescence (PL) techniques. Indium fluctuations ranging from 1-4 nm were observed with both energy filtered TEM (EFTEM) and high angle annular dark field (HAADF) scanning TEM. The existence of V-shaped defects with nucleation centres at the termination of threading dislocation were observed in HAADF images. There was also evidence of the formation of large quantum dots at low densities from lattice HRTEM images. This was further confirmed by PL measurements through the observation of a single sharp line at low power with the typical saturation behaviour at higher power excitation.

An optical and structural study of InGaN/GaN quantum dots (QDs) is reported. With increasing InGaN deposition time, the dominant emission changes from wetting layer (WL) to QDs, and a strong redshift of the emission occurs. Emission from localized WL states is observed, with a density and nature very different to that due to the QDs. Structural measurements reveal a disordered WL, consistent with the form of the WL photoluminescence excitation spectra.

Beryllium and silicon δ-doped GaAs∕AlAs multiple quantum wells (MQWs) were designed and fabricated for selective sensing of terahertz radiation. Optical characterization of the structures by photoreflectance spectroscopy has indicated a presence of built-in electric fields–from 18 up to 49kV∕cm depending on the structure design–located mainly in the cap and buffer layers, but do not penetrating into the MQWs region. Terahertz sensing under normal incidence via impurity bound-to-unbound transitions is demonstrated in photocurrent experiments within 0.6–4.2THz in silicon-doped MQWs and 3.5–7.3THz range in beryllium-doped MQWs at low temperatures.

We report a photoluminescence (PL) study of the effects of carrier depletion on the electronic states of InGaN quantum dots. Samples were fabricated into mesa devices with top Schottky contacts and back ohmic contacts. Submicrometer apertures were created lithographically. Capacitance-voltage measurements of the devices suggest that the dots are fully depleted when they are unbiased. Micro-PL studies of individual dots show narrow linewidths under zero or reverse bias conditions. Forward biasing of the junction results in broadening of the dot levels due to the populating of nearby conduction band states in the underlying quantum well.

B. Cechavicius, J. Kavaliauskasa,∗, G. Krivaitė, D. Seliuta, E. Sirmulis, J. Devenson, G. Valusis, B. Sherliker, M.P. Halsall, M.J. Steer and P. Harrison Semiconductor Physics Institute, A. Gostauto 11, 01108, Vilnius, Lithuania Department of Physics, UMIST, Manchester M60 1QD, United Kingdom Department of Electronic and Electrical Engineering, University of Sheffield Sheffield S1 3JD, United Kingdom IMP, School of Electronic and Electrical Engineering, University of Leeds Leeds LS2 9JT, United Kingdom

D. SELIUTA, B. CECHAVICIUS, J. KAVALIAUSKAS, G. KRIVAITĖ, I. GRIGELIONIS, S. BALAKAUSKAS, G. VALUSIS, B. SHERLIKER, .M. P. HALSALL, M. LACHAB, S. P. KHANNA, P. HARRISON, AND E. H. LINFIELD a Semiconductor Physics Institute, A. Gostauto 11, LT-01108 Vilnius, Lithuania b School of Electronic and Electrical Engineering, University of Manchester Manchester, United Kingdom c School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom

We report the growth and characterization of AlGaN/GaN multiple quantum well structures designed to have intersubband transitions in the mid-infrared region of the spectrum. The samples were nominally undoped but were found to contain a high electron population in the wells induced by the local polarization fields. The sample was characterized by the use of the Raman spectroscopy and photocurrent spectroscopy. The Raman spectroscopy shows electronic Raman scattering from intersubband transitions in the AlGaN/GaN quantum wells. The e1-e2 and e1-e3 transitions of the confined 2d electron population in the wells can clearly be observed. A sample designed to absorb at 4μm was fabricated into mesa structures and the vertical photocurrent measured under normal incidence illumination from the free-electron laser FELIX. A wavelength and bias dependent photocurrent was observed in the mid-IR region of spectrum. The peak responsivity was of the order of 50μA/W at 4 K, the photocurrent still being measurable at room temperature.