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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.

Investigation of excitonic lines in differential surface photovoltage (DSPV) spectra of p-type (Be) ‐-doped GaAs/AlAs multiple quantum well (MQW) structures is reported. From the lineshape analysis of the DSPV spectra, the energies and line broadening parameters for a large number of QW related excitonic transitions were determined. It is found that transition energies are in a good agreement with calculations carried out within the envelope function approximation taking into account the nonparabolicity of energy bands. Examining the dependence of the exciton linewidth on the QW thickness, the line-broadening mechanisms were revealed and interface roughness in the MQW structures was evaluated. An asymmetrical lineshape of certain excitonic transitions in SPV spectra of MQW structures was shown to be related to the Fano resonance.

Differential surface photovoltage (DSPV) spectra of Be δ-doped GaAs/AlAs multiple quantum wells (MQWs) with doping densities below (5 × 1010 cm–2) and near (5 × 1012 cm–2) a Mott transition were studied at 300 K and 90 K. From the line shape analysis of the DSPV spectra, exhibiting obvious doping and temperature dependences, an origin of optical transitions has been revealed. The spectra of lightly doped structures were accounted for by ground state heavy and light-hole related excitonic transitions. In highly doped GaAs/AlAs MQWs the heavy-hole excitons were found to be quenched while rather broadened light-hole related transitions still exhibited an excitonic character. The experimental observations suggest that with increasing doping level phase-space filling effects dominate over Coulomb screening. The renormalization effects due to many body interactions were found to be more pronounced for the lowest heavy-hole subband when compared to the light-hole one. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 present the photoluminescence properties of highly Be �-doped GaAs/AlAs multiple quantum wells at liquid nitrogen and room temperatures. Possible mechanisms of carrier recombination focusing on peculiarities of excitonic and free-carriers– acceptor photoluminescence are discussed. It is estimated that for Be �-doped GaAs/AlAs quantum wells (LW = 5 nm) the Mott transition should occur at acceptor concentration NBe some greater than 5 · 10 12 cm 2 .

We present a Raman scattering study of two GaN/AlxGa1−xN multiple quantum well (MQW) structures with x = 0.3 and x = 0.4 and well widths of 6 nm and 4 nm respectively, they were nominally undoped but are expected to contain a low-density, n-type, carrier population due to the residual donors in the barriers. Polarization dependent Raman scattering was performed at room temperature, strong scattering due to intersubband transitions (ISBT's) in the GaN quantum wells was observed from both the e1–e2 and e1–e3 transitions for both samples. The first sample has an e1–e2 transition at 300 meV and e1–e3 at 480 meV whilst the second sample has an e1–e2 transition at 360 meV and e1–e3 at 580 meV. The full width half maximum of the Raman peaks is lower than that reported in infra-red absorption for similar, heavily doped, MQW structures. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a combined analysis of scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELs) of silicon-rich-silicon-oxide (SRSO) thin film on silicon, grown by plasma enhanced chemical vapour deposition (PECVD). For un-doped samples, strong room temperature luminescence at ~1.6 eV (780 nm) is observed, which we ascribe, by way of plasmon intensity mapping and dasiachemical fingerprintingpsila to phase segregated, highly crystalline, silicon-rich nano-clusters embedded in an amorphous-silicon dioxide (a-SiO2) matrix. For samples doped with increasing concentrations of Er, a quenching of the 1.6 eV line, concurrent with the emergence of a second emission with increasing intensity at ~0.8 eV (1535 nm) is observed. This is attributed to a rapid and efficient, indirect nano-crystal mediated excitation of the Er.

Photoreflectance (PR) and differential surface photovoltage (DSPV) spectroscopies were employed to characterize optically Be and Si δ‐doped GaAs/AlAs multiple quantum wells. The surface electric field strength was estimated from the Franz‐Keldysh oscillations clearly visible in PR spectra. Line shape analysis of DSPV spectra allowed one to estimate interband excitonic transition energies and broadening parameters for a large number of QW‐related subbands; reasonable agreement was found between experimental and calculated transition energies. The interface quality and the main factors responsible for exciton line broadening were evaluated from spectroscopic data. These MQW structures were also studied as selective THz sensors by spectral photocurrent measurements at low temperatures.

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.

We have investigated the optical transitions in Be $\ensuremath{\delta}$-doped $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ multiple quantum wells with various width and doping levels. The fractional dimensionality model was extended to describe free-electron--acceptor (free hole-donor) transitions in a quantum well. The measured photoluminescence spectra from the samples were interpreted within the framework of this model, and acceptor-impurity induced effects in the photoluminescence line shapes from multiple quantum wells of different widths were demonstrated.

Experimental Study of Optical Transitions in Be-Doped GaAs/AlAs Multiple Quantum Wells J. Kundrotasa,e,∗, A. Cerskus, S. Asmontas, G. Valusis, B. Sherliker, M.-P. Halsall, P. Harrison and M.-J. Steer Semiconductor Physics Institute, A. Gostauto 11, 01108 Vilnius, Lithuania Department of Physics, UMIST, University of Manchester Manchester M60 1QD, United Kingdom IMP, School of Electronic and Electrical Engineering, University of Leeds Leeds LS2 9JT, United Kingdom Department of Electronic and Electrical Engineering, University of Sheffield Sheffield S1 3JJD, United Kingdom Vilnius Gediminas Technical University Saulėtekio al. 11, 10223 Vilnius, Lithuania

We present a study of silicon (Si) and erbium (Er) coimplanted silica (SiO2) in which we observe, by combining high resolution scanning transmission electron microscopy and selective electron energy loss spectroscopy (EELS), a high spatial correlation between silicon nanocrystals (Si-NCs), Er, and oxygen (O) after a single high temperature (1100 °C) anneal. The observation of a spatial overlap of the EELS chemical maps of dark field (DF) images at the Er N4,5, Si L2,3, and O K edges is concomitant with an intense room temperature infrared luminescence around 1534 nm. We suggest that these observations correspond to Er–O complexes within an amorphous silicon (a-Si) shell at the Si-NC/SiO2 interface. The presence of a crystalline phase at the Si-NC center, verified by high resolution electron micrographs and DF diffraction contrast images and the low solubility of Er in crystalline Si (c-Si) would tend to suggest a preferential Er agglomeration toward the Si-NC/SiO2 interface during formation, particularly when high concentrations of both Si and Er are obtained in a narrow region of the SiO2 after coimplantation. The absence of narrow Stark related features in the Er emission spectrum at low temperature and an inhomogeneous broadening with increasing temperature, which are characteristic of Er confined by an amorphous, rather than a crystalline host further support these hypotheses. After comparing the luminescence to that from a SiO2:Er control sample prepared in exactly the same manner but without Si-NCs, we find that, despite the observed spatial correlation, only a small fraction ( ∼ 7%) of the Er are sensitized by the Si-NCs. We ascribe this low fraction to a combination of low sensitizer (Si-NC) density and Auger-type losses arising principally from Er ion-ion interactions.

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.

Er-doped SiO2 and Si nano-crystals (NCs) embedded in a SiO2 matrix were produced by ion beam implantation of Si (100) substrates. After annealing Er ions agglomerate in different positions with different compositional properties in samples with and without Si implants. HAADF and EELS show that in the sample with Si implants the Si and Er distribution is identical and within a band of ~110nm width ~75nm below theSiO2 surface whereas in the sample with no excess Si, Er forms on average much larger, amorphous aggregates, presumably an Er-oxide, in the SiO2 matrix with tendency to move towards the surface of the SiO2 layer.

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.