51 research outputs found
Self-consistent model for ambipolar tunneling in quantum-well systems
We present a self-consistent approach to describe ambipolar tunneling in
asymmetrical double quantum wells under steady-state excitation and extend the
results to the case of tunneling from a near-surface quantum well to surface
states. The results of the model compare very well with the behavior observed
in photoluminescence experiments in asymmetric double quantum
wells and in near-surface single quantum wells.Comment: 10 pages, REVTeX 3.
Femtosecond pulse generation in passively mode locked InAs quantum dot lasers
Optical pulse durations of an InAs two-section passively mode-locked quantum dot laser with a proton bombarded absorber section reduce from 8.4 ps at 250K to 290 fs at 20 K, a factor of 29, with a corresponding increase in optical bandwidth. Rate equation analysis of gain and emission spectra using rate equations suggests this is due to the very low emission rate of carriers to the wetting layer in the low temperature, random population regime which enables dots across the whole inhomogeneous distribution to act as independent oscillators. (C) 2013 AIP Publishing LLC
Faraday-cage-assisted etching of suspended gallium nitride nanostructures
We have developed an inductively coupled plasma etching technique using a Faraday cage to create suspended gallium-nitride devices in a single step. The angle of the Faraday cage, gas mix, and chamber condition define the angle of the etch and the cross-sectional profile, which can feature undercut angles of up to 45°. We fabricate singly- and doubly-clamped cantilevers of a triangular cross section and show that they can support single optical modes in the telecom C-band
The molecular beam epitaxial growth of GaAs/GaAs(111)B: doping and growth temperature studies
A series of investigations are presented which address various aspects of the growth, by molecular beam epitaxy, of nâtype (Si doped) onâaxis GaAs/GaAs(111)B. Inâsitu characterization by reflection highâenergy electron diffraction has identified four surface phases on the static (zero growth rate) surface, and three reconstructions which occur, depending upon the substrate temperature, during growth. The nâtype doping properties of GaAs/GaAs(111)B epilayers have been compared with nâGaAs/GaAs(100) structures. Hall effect and lowâtemperature photoluminescence measurements have demonstrated that it is possible to dope GaAs/GaAs(111)B with Si in the 6Ă1014 to 1018 cmâ3 range. A variable growth temperature study is also presented which examines the surface structural, electrical, optical, and surface morphological properties of nâGaAs/GaAs(111)B grown in the 400 to 650â°C temperature range. The onset of electrical conduction, and optically active material, was found to be directly related to changes in the dynamic surface structure. The variable growth temperature study also revealed a temperature regime within which it was possible to significantly improve the surface morphology of onâaxis GaAs/GaAs(111)B structures whilst retaining good electrical and optical properties
Photoluminescence measurements for GaAs grown on Si(100) and Si(111) by molecular beam epitaxy
Photoluminescence measurements have been used to characterize Siâdoped GaAs layers, ranging in thickness from 1.1â8.1 ÎŒm, grown on Si(111) and misorientated Si(100) substrates by molecular beam epitaxy. 4.2 K PL spectra for GaAs/Si (100) show a strainâinduced splitting between the heavy and light hole valence bands which corresponds to a biaxial tensile stress of 2.8± 0.15 kbar acting on the GaAs layer. Similar measurements for GaAs/Si(111) indicate that the GaAs layer is subject to a biaxial tensile stress of 3.9±0.15 kbar at 4.2 K. Furthermore, the intensity and line shape of luminescence features for GaAs/Si(111) for the first time indicate a crystalline quality comparable with the best GaAs/Si(100) material
Observation of photoluminescence from InAs surface quantum wells grown on InP(100) by molecular beam epitaxy
Photoluminescence (PL) measurements are presented for thin epitaxial layers of InAs, 2.5 Ă
<d <36 Ă
, grown on InP(100) by molecular beam epitaxy. The combination of efficient carrier capture and PL redshift with increasing InAs thickness clearly indicate the formation of InAs quantum wells on the InP surface. Data are also presented for InAs/InP structures capped with strained layers of either GaAs or In0.5 Al0.5 As. Since radiative recombination within the InAs layers can be distinguished from PL arising from both bulk and surface defects, this system allows us to monitor the quality of both the InAs/InP and InAs/air interfaces via their influence on the InAs quantum well luminescence
Degradation of IIIâV Quantum Dot Lasers Grown Directly on Silicon Substrates
Initial age-related degradation mechanisms for InAs quantum dot lasers grown on silicon substrates emitting at 1.3 ÎŒm are investigated. The rate of degradation is observed to increase for devices operated at higher carrier densities and is therefore dependent on gain requirement or cavity length. While carrier localization in quantum dots minimizes degradation, an increase in the number of defects in the early stages of aging can increase the internal optical-loss that can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the two-dimensional states is considered the major factor for determining the rate of degradation, which can be significant for lasers requiring high threshold carrier densities. This is demonstrated by operating lasers of different cavity lengths with a constant current and measuring the change in threshold current at regular intervals. A segmented-contact device, which can be used to measure the modal absorption and also operate as a laser, is used to determine how the internal optical-loss changes in the early stages of degradation. Structures grown on silicon show an increase in internal optical loss, whereas the same structure grown on GaAs shows no signs of increase in internal optical loss when operated under the same conditions
Electrically Pumped Continuous-Wave III-V Quantum Dot Lasers Monolithically Grown On Silicon
We demonstrate electrically pumped continuous-wave InAs/GaAs quantum dot lasers monolithically grown on silicon substrates with a low threshold current density of 62.5 Acm -2 , a room temperature output exceeding 105 mW, operation up to 120 °C, and long extrapolated lifetime exceeding 100,000 h
Improvement of composition of CdTe thin films during heat treatment in the presence of CdCl2
CdCl2 treatment is a crucial step in development of CdS/CdTe solar cells. Although this rocessing step has been used over a period of three decades, full understanding is not yet achieved. This paper reports the experimental evidence for improvement of composition of CdTe layers during CdCl2 treatment. This investigation makes use of four selected analytical techniques; Photo-electro-chemical (PEC) cell, X-ray diffraction (XRD), Raman spectroscopy and Scanning electron microscopy (SEM). CdTe layers used were electroplated using three Cd precursors; CdSO4, Cd(NO3)2 and CdCl2. Results show the improvement of stoichiometry of CdTe layers during CdCl2 treatment through chemical reaction between Cd from CdCl2 and elemental Te that usually precipitate during CdTe growth, due to its natural
behaviour. XRD and SEM results show that the low-temperature (~85ÂșC) electroplated CdTe layers consist of ~(20-60) nm size crystallites, but after CdCl2 treatment, the layers show drastic recrystallisation with grains becoming a few microns in size. These CdCl2 treated
layers are then comparable to high temperature grown CdTe layers by the size of grains
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