Search

We used a low temperature near-field scanning optical microscopy (NSOM) to study a formation of Wigner molecules (WMs) in the emission spectra of self-organized InP/GaInP QDs having up to seven electrons. We used a Schottky diode structure for the electrostatic control of the number of the electrons (N) in QD, and we observed the emission of the charged excitons (for N 2) in the regime of weak Wigner localization. We show, that a contribution of whispering gallery modes (WGMs) of the electrons and rearrangement of the electrons between the WM and WGM states in the QDs, can explain the anomalous dependence of NSOM image size on the quantum confinement observed for some dots.

We investigated structural and emission properties of self-organized InP/GaInP quantum dots (QD) grown by metal organic chemical vapor deposition using an amount of deposited In from 7 to 2 monolayers (ML). In the uncapped samples, using atomic force microscopy (AFM), we observed lateral sizes of 100–200 nm, together with a bimodal height distribution having maxima at ∼5 and ∼15 nm, which we denoted as QDs of type A and B, respectively; and reduction of the density of the type-B dots from 4.4 to 1.6 μm–2. The reduction of the density of B-type dots were observed also using transmission electron microscopy of the capped samples. Using single dot low-temperature photoluminescence (PL) spectroscopy we demonstrated effects of Wigner localization for the electrons accumulated in these dots.

The emissivity of unstrained quantum-dimensional InP/AlInAs nanostructures and their lasing properties in microdisk cavities prepared by wet etching have been studied. For as-prepared structures, it has been found that they radiate owing to quantum-dimensional InP islands 50–300 nm in diameter. At temperatures below 160 K, whispering gallery modes have been observed in the microdisks. Experimental data on the PL intensity for microcavity modes versus the pump power, which were obtained at liquid helium temperature, have made it possible to find the lasing threshold, 50 W/cm2. The half-width of the laser line at above-threshold powers equals 0.06 nm, which corresponds to a Q factor of 15 000.

Structural and emission properties of few-electron In(Ga)P/GaInP quantum dots (QDs) representing natural Wigner molecules (WM) and whispering gallery mode (WGM) electron ($e$) cavities have been investigated. QD structures were grown using self-organized metal-organic vapor phase epitaxy and deposition from $\ensuremath{\sim}3$ to 7 monolayers of InP at $700{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Using atomic force microscopy, transmission electron microscopy, near-field scanning optical microscopy (NSOM), and $\ensuremath{\mu}$-photoluminescence ($\ensuremath{\mu}$-PL) spectra we obtained In(Ga)P/GaInP QDs having lateral size 80--180 nm, height 5--30 nm, Ga content 0.0--0.4, density $2\ensuremath{-}10\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}$, and electron population up to 20 and demonstrated control of their density and size distribution. Using high-spatial-resolution low-temperature PL spectra, NSOM imaging, and calculations of charge density distributions we observed Wigner localization and $e$-cavity effects for a series of dots having quantum confinement $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}=0.5\ensuremath{-}6\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. We used these data together with time-resolved PL measurements to clarify the effect of Coulomb interaction and WM formation on emission spectra of few-electron QDs. We present direct observation of $2e$, $6e$, and $9e$ WMs; $2e$ and $4e$ WGMs; and Fabry-Perot $e$ modes and establish conditions of $e$-WGM-cavity formation in these QDs.

We used high-spatial-resolution, low-temperature near-field scanning optical microscopy (NSOM) operating at magnetic fields $B=0--10\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ to study the effects of Wigner localization (WL) on emission spectra of single self-organized InP/GaInP quantum dots (QDs) and investigate the stability of singly (trion) and doubly (tetron) charged exciton complexes in the weak quantum confinement regime. Using NSOM measurements together with configuration interaction calculations, we identify the dots having different electron population $\mathrm{N}(\mathrm{N}=1--12)$, quantum confinement $(\ensuremath{\hbar}{\ensuremath{\omega}}_{0}=0.6--8\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ and size $D(D=70--170\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$. For $\mathrm{N}=2$, we observed a magnetic-field-induced molecular-droplet transition, accompanied by the decomposition of the tetron into a Wigner molecule complex (WMC), and the activation of rotovibronic structure. For $\mathrm{N}=1$, unusually strong vibronic structure resulting from a trion-type WMC was observed. We have shown that magnetic-field-induced shifts of this structure allow measurement of single particle Fock-Darwin levels and angular momentum transitions of the WMC. In addition, we demonstrated the use of NSOM imaging to probe the charge density distribution and observed anomalous dependence of the image size on the quantum confinement, implying a pairing of electrons or formation of whispering gallery modes in the QD. We demonstrated that InP/GaInP QDs, provide a Wigner-Seitz radius $({r}_{s})$ up to 13, and that the measurements of NSOM magneto-optical spectroscopy using these dots makes it possible to study effects arising from strong Coulomb interaction of a few confined electrons (holes).

Surface photovoltage (SPV) spectra were measured for GaN 0.014 As 0.986 layers at room temperature and compared with room temperature photoreflectance (PR) and contactless electroreflectance (CER) measurements. Spectral features related to E − and E + transitions were clearly observed in SPV spectra at energies corresponding to PR and CER resonances. In this way it has been shown that SPV spectroscopy is an alternative absorption-like technique to study both the E − and E + transitions in dilute nitrides. The observation of E + transition in SPV spectra means that it is a direct optical transition at the Γ point of GaNAs band structure which can be explained by the band anticrossing interaction between the localized states of N and the extended conduction band states of the GaAs host.

AlInAs microdisk cavities having quality factor Q ~ 15 000 were fabricated from lattice-matched InP/AlInAs (interface/aggregation) quantum dot (QD) structures using wet chemical etching. The QD emission coupled to whispering gallery modes was observed at spectral range 920 - 1000 nm at temperatures of 10 - 160 K. The laser generation with threshold power density of 50 W/cm2 at T = 10 K was observed under optical pumping. It was found that the spontaneous emission coupling factor β equals 0.23 for these microdisks. The low temperature lasing operation and the small coupling factors observed suggest the existence of small QDs formed at the InP/InAlAs interface as the main active elements.

Properties of light-emitting structures with an InGaN/GaN active region emitting in a range of 500–550 nm are studied. Photoluminescence of the structures is studied at various values of external bias and temperature as well as with time resolution. With the reverse bias, a decrease in the carrier lifetime associated with tunneling exit of the carriers from the active region is found. The mechanism of tunneling leakage is simulated allowing for the Boltzmann distribution of carriers by energy; it is shown that the calculated and experimental dependences agree well. It is shown that the tunneling transport exerts a considerable effect on the characteristics of structures with an InGaN/GaN active region.

We have developed a nano-optical technique using diffraction-limited far-field fluorescence monitoring of near-field scanning optical microscopy (NSOM) experiments. The technique was combined with time-resolved (TR) spectroscopy and was used to study the emission properties of single colloidal CdSe nanowires (NWs) with spatial resolution ∼100 nm. The use of this novel technique reveals a critical role of organic suspensions in optical experiments with single colloidal nanostructures. We observed that organic suspensions form a viscous shell which acts as a nano-lens providing efficient coupling of excited and emitted photons to the NW. We demonstrate that physical distortion of the shell produced by the near-field probe affects the emission dynamics of single NWs. Temperature dependent measurements show that the emission decay of a colloidal NW is dominated by size-dependent non-radiative recombination.

We report the formation of CdTe self-assembled quantum dots (QDs) on high-quality ZnTe epilayers grown on a GaSb substrate by molecular-beam epitaxy. Reflection high-energy electron diffraction and atomic force micro- scopy measurements clearly show the formation of CdTe QDs. Photoluminescence (PL) measurements on ZnTe-capped CdTe QD samples show a wide distribution of dot size. The temperature dependence of PL spectra indicates much stronger exciton localization in QDs than that in quantum wells.

Using wafer bonding (WB) and wet oxidation (WO) techniques, GaInP microdisks having an asymmetric waveguide (diameters D = 1 – 3 μm ) with embedded InP quantum dots (size/density ∼100 nm/ ∼ 10 9 cm − 2 ) have been fabricated on Si and GaAs substrates, respectively. The TE m , l ( m = 28 – 12 , l = 1 , 2 ) and TM m , l ( m = 25 – 10 , l = 1 – 4 ) whispering gallery modes with quality factors Q ∼ 2 – 5 × 10 3 have been identified in photoluminescence spectra of these microdisks (MDs) in the spectral range 720–770 nm. Lasing thresholds of 6 (30) μW and mode coupling constants 0.9 (0.7) have been demonstrated for WO (WB) MDs.

We study the emission spectra of single self-organized InP/GaInP QDs (size 100-220 nm) using high-spatial-resolution, low-temperature (5K) near-field scanning optical microscope (NSOM) operating at magnetic field strength B=0-10T. The dots contain up to twenty electrons and represent natural Wigner molecules (WM). We observed vibronic-type shake-up structure in single electron QDs manifesting formation of two electron (2e) WM in photo-excited state. We found that relative intensities of the shake-up components described well by vibronic Frank-Condon factors giving for dots having parabolic confinement energy ħω0=1.2-4 meV molecule bond lengths 40-140 nm. We used measurements of magnetic-field-induced shifts to distinguish emission of 2e-WM and singly charged exciton (trion). We also observed magnetic-field-induced molecular-droplet transition for two electron dot, emitting through doubly charge exciton (tetron) at zero magnetic field.

InGaN quantum dot (QD) formation in a wide pressure range MOCVD reactor was studied. The existence of QDs and their lateral size (2–5 nm) were demonstrated using transmission electron microscopy and high spatial resolution (~ 100 nm) near-field magneto-photoluminescence spectroscopy. We found that an increase of the reactor pressure from 400 to 1000 mbar leads to an order of magnitude increase in light emission efficiency of the InGaN / GaN QDs accompanied by ~ 100 meV redshift of the emission wavelength. We explored stimulated phase separation (SPS) to control carrier localization and emission wavelength. The SPS was achieved by adding In in the matrix material. This leads to formation of extremely deep InGaN / InGaN QDs having energy localization up to ~ 0.8 eV, which was revealed from selectively excited far-field photoluminescence (PL) spectra. Without SPS the QD activation energy is found to be below 0.2 eV. A nonequilibrium carrier population strongly suppresses the temperature-induced shift of the PL emission in deep InGaN QDs.

Structural and optical properties of MOCVD grown InGaN quantum dot (QD) layers have been studied using transmission electron microscopy, X-ray diffraction, near-field spectroscopy and selectively excited photoluminescence. We have shown that the presence of Indium in the matrix strongly increases the activation energy of QD layers, due to stimulated phase separation. The increase of the activation energy is accompanied by an increase of the activation time, which was revealed from photocurrent spectroscopy. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we demonstrate how near-field scanning optical microscopy (NSOM) can be utilized for tuning the emission energy of a single quantum dot (QD) using nanoindentation with the tapered optical fiber probe. Thus the use of NSOM in the nanoindentation regime can provide spectral matching of QD excitons and localised photons in photonic crystal cavities. We present results of NSOM study of confined photon modes in triangular lattice photonic crystal microcavities containing InAs quantum dots with emission wavelength λ ∼ 1.2 µm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The authors present measurements of photoluminescence spectra and imaging of the sharp emission lines which result from the radiative recombination of excitons trapped in compositional fluctuations in dilute nitride epitaxial layers and quantum wells, using low-temperature near-field scanning optical microscopy (NSOM). Both weak and strong localisation (i.e. quantum-dot-like emission) are observed for samples of GaAsN and InGaAsN, with little difference depending on the presence or absence of In. Localisation energies are in the range 10–60 meV. Zeeman splitting and diamagnetic shifts are observed in the near-field magneto-photoluminescence, and from these effects the size (r=3–17 nm), composition fluctuations (Δy ∼0.5%) and density (100–1000 μm−3) of the compositional clusters are determined. Using two-dimensional NSOM images, the authors observe a strong lateral inhomogeniety for the GaAsN epilayers resulting from nitrogen clustering. A one-dimensional compositional modulation along the [110] direction is also observed for a GaAsSbN quantum well on the length scale of 100 nm. Transmission electron micrographs confirm the existence of these spatial nonuniformities. Time-resolved spectroscopy has also been carried out on one of the samples reported here (In0.08GaAsN0.03). Pump-and-probe photoluminescence experiments provide the following time constants: the exciton relaxation time to the ground states of the localisation regions is found to be ∼40–70 ps as the emission varies from high to low energy, and the time for depopulation of these localised states is 2–4 ns.

A discussion of the structural and luminescent characteristics of InGaP films lattice matched to GaAs substrates is given. The films were grown by the liquid phase epitaxy technique using a graphite boat in a conventional horizontal reactor. The crystal structure of the layer was characterized by high-resolution X-ray, TEM and EDXS measurements. In the [2 2 0] dark field image in plane view the structure showed a coarse tweed pattern, with a period of 0.2 μm, oriented along the [1 0 0] and [0 1 0] directions. Photoluminescence (PL) measurements were performed in wide temperature (4–250 K) and exciting power density (four orders of magnitude) ranges for [0 1 1] and [0 1 1] polarizations of the emitted radiation. The measured PL spectra showed the presence of deformation related to lattice mismatch and thermal strain. The difference between the spectral peak positions for different polarizations is attributed to the splitting of the valence-band into a heavy- and light-hole bands due to lattice mismatch strain.

We have studied the anisotropy of the photoluminescence (PL) of InGaP films grown by liquid phase epitaxy on GaAs substrates. The PL measurements were performed in a wide temperature range (4–250 K) and exciting power density range for polarizations of the emitted radiation along the [011] and [ ] directions. It was found that the donor–acceptor transition dominates at low temperature (4 K) while the band-to-band transition dominates at a higher temperature (250 K). The dependence of PL intensity on excitation intensity shows a characteristic behaviour for donor–acceptor recombination. The difference in the position of the spectral peak for [011] and [ ] polarizations is a result of the splitting of the valence-band into heavy- and light-hole bands due to strain. Moreover, the difference in the line shape of the spectra for different polarizations indicates the presence of anisotropy for different crystallographic directions.

Near-field magneto-photoluminescence scanning microscopy has been used to investigate the structural and optical properties of quantum-dot (QD)-like compositional fluctuations in GaAsN and InGaAsN alloys. Sharp spectral lines (halfwidth 0.5– 2 meV ) from these QDs are observed at T K , and their Zeeman splitting and diamagnetic shifts are used to determine the size (r∼6– 18 nm ), density (∼100 μm −3 ) , and nitrogen composition. Near-field scanning images reveal phase separation effects in the distribution of nitrogen, but little effect appears from the presence or absence of indium. Indium does have a strong effect on the exciton g-factor for observations in a magnetic field.

Nanocrystalline GaP and amorphous GaPN solid solution have been synthesized in voids of artificial opal. The opal-GaP and opal-GaPN composites obtained clearly demonstrate properties of photonic crystals. The reflection spectra of the opal-GaPN composite exhibit specific features related to multiple Bragg diffraction on two systems of {111} planes, parallel and nonparallel to the surface of the photonic crystal. The study of photoluminescence spectra revealed a considerable modification of the emission band of the opal-GaPN composite, which was attributed to the influence of the photonic band gap.

We make a comparative study of the optical and structural characteristics of two InxGa1−xP (x≈0.5) films nearly lattice matched to GaAs, here referred to as sample S1 and sample S2. The films were grown by liquid phase epitaxy (LPE). Photoluminescence (PL) measurements were performed in a wide temperature and exciting power density range for different polarizations (parallel to the [0 1 1] and [0 1 1] directions) of the emitted radiation. Observations suggest that the InxGa1−xP layer in one of the samples has the usual characteristics commonly obtained in an LPE grown material, while in the other sample, an anomalous growth took place. Our experimental observations show differences in the layer thickness, the surface morphology, as well as in the structural and optical characteristics such as the 4 K PL energy peak position.

The experimentally measured input-output characteristics of optically pumped semiconductor microcavities exhibits unexpected oscillations modifying the fundamentally linear slope in the excitation power regime below lasing. A systematic microscopic analysis reproduces these oscillations, identifying them as a genuine quantum-memory effect, i.e., a photon-density correlation accumulated during the excitation. With the use of projected quantum measurements, it is shown that the input-output oscillations can be controlled and enhanced by an order of magnitude when the quantum fluctuations of the pump are adjusted.

We make a comparative study of the optical and structural characteristics of two InxGa1-xP(x≈0.5 ) films nearly lattice matched to GaAs, (here referred to as disordered type 1 sample (S1) and ordered type 2 sample (S2)). The films were grown by liquid phase epitaxy (LPE). Photoluminescence (PL) measurements were performed in a wide temperature and exciting power density range for different polarization of the emitted radiation along the [110] and [1-10] directions. Observations suggest that the InxGa1-xP layer in sample 1 is disordered, as commonly obtained in LPE growth, while in sample 2 at least a partially spontaneously ordered layer was obtained. Moreover, the energy position of the 20 K PL peak is close to the predicted band gap energy for the InGaP 2 material ordered in the CuPt phase. [1].

We report the characteristics of InP self-assembled quantum dots embedded in In0.5Al0.5P on GaAs substrates grown by metalorganic chemical vapor deposition. The InP quantum dots show increased average dot sizes and decreased dot densities, as the growth temperature increases from 475°C to 600°C with constant growth time. Above the growth temperature of 600°C, however, dramatically smaller and densely distributed self-assembled InP quantum dots are formed. The small InP quantum dots grown at 650°C are dislocation-free “coherent” regions with an average size of ∼20 nm (height) and a density of ∼1.5 × 108 mm−2. These InP quantum dots have a broad range of luminescence corresponding to red or organge in the visible spectrum.

Whispering-gallery-mode (WGM) GaInP micro-disc (MD) and micro-ring (MR) resonators (diameters D=1-3 mm, quality factors Q up to 5×103) with embedded InP quantum dots (size/density ∼100 nm/∼109cm-2, emission range 720-770 nm) have been fabricated using a wet oxidation technique. Ultralow (sub-nanoWatt) lasing thresholds for a single InP quantum dot have been demonstrated for these resonators at 15 K under optical pumping. Observed thresholds are two orders of magnitude lower than for single InAs QD lasing (M. Nomura et al., Opt. Express 17, 15975 (2009) [1]). (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Time-resolved studies of the wetting layer photoluminescence is combined with state-filling spectroscopy of the quantum dot emission to obtain carrier transfer rates from the wetting layer to the quantum dot states. This method allows us to obtain the capture rates as a function of wetting layer carrier concentration, a result that cannot be obtained from a time-dependent measurement of the quantum dot emission itself. The results show unamibiguously that the capture efficiency increases significantly with the carrier concentration in the wetting layer, indicating the dominant role of Auger processes in the capture dynamics. In the analysis the concept of capture cross section per unit time is introduced to take into account possible changes in capture times for different dot areal densities.

Studies of the effects of reactive ion etching on molecular beam epitaxy grown CdxZn1−xSe/ZnSe strained quantum wells (QWs) using photoluminescence (PL) and time-resolved reflectivity measurements are reported. The shallow (50 nm cap layer) QW samples exhibit a blueshift in their PL peak position as a function of etch voltage up to a certain point, after which the blueshift is reduced. The reduction in the blueshift of the PL spectrum is strongly correlated with a reduction in the carrier lifetimes measured by transient reflectivity. From these experiments we suggest that the initial blueshift is a result of ion damage at the surface interacting with strain in the QW. On the other hand, the reduced carrier lifetime at higher voltages is a result of more severe structural damage in the QW.

An introduction to the operation of quantum-dot cellular automata (QCA) is presented, along with recent experimental results. QCA is a transistorless computation paradigm that addresses the issues of device density and interconnection. The basic building blocks of the QCA architecture, such as AND, OR, and NOT are presented. The experimental device is a four-dot QCA cell with two electrometers. The dots are metal islands, which are coupled by capacitors and tunnel junctions. An improved design of the cell is presented in which all four dots of the cell are coupled by tunnel junctions. A noninvasive electrometer is presented which improves the sensitivity and linearity of dot potential measurements. The operation of this basic cell is confirmed by an externally controlled polarization change of the cell.

We discuss novel nanoelectronic architecture paradigms based on cells composed of coupled quantum-dots. Boolean logic functions may be implemented in specific arrays of cells representing binary information, the so-called quantum-dot cellular automata (QCA). Cells may also be viewed as carrying analogue information and we outline a network-theoretic description of such quantum-dot nonlinear networks (Q-CNN). In addition, we discuss possible realizations of these structures in a variety of semiconductor systems (including GaAs/AlGaAs, Si/SiGe, and Si/SiO2), rings of metallic tunnel junctions, and candidates for molecular implementations. We report the experimental demonstration of all the necessary elements of a QCA cell, including direct measurement of the charge polarization of a double-dot system, and direct control of the polarization of those dots via single electron transitions in driver dots. Our experiments are the first demonstration of a single electron controlled by single electrons.

Studying a uniformly excited ensemble of InAs islands, the emission intensities of the allowed radiative transitions are modeled in the state-filling limit for pulsed and steady-state excitation as a function of the average number of electron-hole pairs per dot. The only unknown parameters included in the equations are the radiative recombination rates of each transition. For steady-state excitation, it is found that in the high excitation limit the emission intensity ratio of any two transitions tends towards a well-defined value proportional to the ratio of their recombination rates. The results of the model are compared with experiments for steady-state excitation and excited states radiative lifetimes are obtained experimentally.

We present microphotoluminescence measurements of self-assembled quantum dots subject to an electric field applied along the growth axis. The spectra reveal sharp peaks corresponding to a number of ``single-dot'' emission lines. An asymmetric Stark shift as a function of applied field is obtained; for positive fields (electron pushed towards the apex of the dot), an initial blueshift is measured, followed by saturation and eventually a small redshift for stronger positive fields, while a continuous redshift is observed for negative field values. High positive fields also give rise to structural changes in the emission spectra as lines are enhanced or quenched under the influence of the field. The field dependence of the emission lines are reproduced in our theoretical calculations, which show that the asymmetric Stark shift is caused by the combination of dot geometry and strong lateral confinement.

We report the systematic investigation of ripening of CdSe self-assembled quantum dots (QDs) on ZnSe. We investigate the size and density of the QDs as a function of time after deposition of CdSe has stopped. The dynamics of the ripening process is interpreted in terms of the theory of Ostwald ripening. Furthermore, the experimental results allow us to identify the growth mode of the QD formation process. [S0031-9007(98)07378-5]

A strong dependence of the intensity of the ordering-induced phonon bands on thermal treatment (rapid cooling and annealing) was observed in room temperature Raman spectra (RS) of -ordered alloys. The changes of the Raman intensity occur only in diagonal backscattering configuration: , where the polarization of light is parallel to the mirror planes of the ordered structure. The changes are absent in the configuration, where the polarization of light is perpendicular to the mirror planes. We found that there are at least two different intensity distributions of the optical phonon bands, the appearance of which in the RS of are determined by thermal treatment and by the alloy film thickness. A bond polarizability model analysis is presented here which shows that the observed RS behaviour can be described in terms of a martensitic transition involving trigonal lattice sites. This is the first observation of the martensitic transition in spontaneously ordered semiconductor alloys.

We present the experimental demonstration of a basic cell of quantum-dot cellular automata (QCA), a transistorless computation paradigm which addresses the issues of device density and interconnection. The device presented is a six-dot quantum-dot cellular system consisting of a four-dot QCA cell and two electrometer dots. The system is fabricated using metal dots which are connected by capacitors and tunnel junctions. The operation of a basic cell is confirmed by the externally controlled polarization change of the cell. The cell exhibits a bistable response, with more than 80% polarization of the charge within a cell.

We present experimental demonstration of a basic cell of Quantum-dot Cellular Automata, a transistorless computation paradigm which addresses the issues of device density and interconnection. The devices presented consist of four and six-dot quantum-dot cellular systems where the metal dots are connected by capacitors and tunnel junctions. The operation of a basic cell is confirmed by the externally controlled polarization change of the cell. The cell exhibits a bistable response and voltage gain. We present an experimental technique which cancels the parasitic cross-talk capacitors in the system.

We have theoretically and experimentally investigated the dynamics of the radiative recombination process in an n-type modulation doped GaAs/AlGaAs heterostructure. The dynamical reshaping of the potential profile across the heterojunction, and the decay of the spatial indirect radiative recombination between electrons in the two-dimensional electron gas (2DEG) and photocreated holes in the valence band, has been numerically simulated for various values of the electric field across the heterojunction. The optical matrix elements were deduced from a self-consistent solution of the coupled Schrodinger and Poisson equations at every discrete point of time in the numerical solution of the rate equation. The calculated recombination energies and integrated luminescence intensities were compared with experimental data from time-resolved photoluminescence (PL) measurements on an 800 A wide GaAs/AlGaAs heterostructure in the time range of 6.2 ns to 1.3 μs, under various applied gate voltages. The theoretical simulations reproduce the experimental observations; a slower radiative decay with increasing positive gate voltage and a smaller dynamical red shift of the 2DEG related luminescence with increasing gate voltage.

We discuss the formation of self-assembling II–VI quantum dots during MBE growth, with emphasis on CdSe dots grown on ZnSe. AFM measurements on specimens with uncapped dots show relatively narrow dot size distributions, with typical dot diameters of 35 ± 5nm. Uncapped CdSe dots are unstable with time, showing clear evidence of ripening. Photoluminescence from capped dots indicates strong exciton localization, as evidenced by very intense PL emission even at relatively high temperatures.

Raman spectra of coherently strained layers of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ grown on (001) GaAs with $x=0\char21{}0.05$ by metalorganic molecular-beam epitaxy are reported. The optical phonons of the GaAs and GaN types, as well as disorder-activated acoustical phonons, are observed. A strongly confined GaAs optical mode at $\ensuremath{\sim}255{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ indicating the ordering of As and N atoms, is also detected. The GaAs- and GaN-type optical phonons exhibit strong diagonal components, forbidden for the zinc-blende structure. A bond polarizability analysis of the Raman selection rules shows that these components are activated by the trigonal distortion of the alloy lattice. The trigonal distortion arises from the formation of ordered {111}-$(\mathrm{GaN}{)}_{m}(\mathrm{GaAs}{)}_{n}$ clusters with $n=m=1.$

An experimental study of the optical properties of acceptor-center doped quantum wells (QWs) is presented. We have studied the effects of acceptor doping at concentration levels varying from 1016 up to 1019 cm−3 using steady-state photoluminescence (PL) and photoluminescence excitation (PLE). The quenching of excitons has been investigated by comparing the results obtained for n-and p-type bulk and QW structures doped both in the well and in the barrier. At a doping concentration higher than 5 × 1016 cm−3, an additional excitonic component is observed on the low energy side of the principal neutral acceptor bound exciton (BE). The second feature is associated with BEs formed by excitons bound at interacting acceptors. Additional information is obtained from temperature, excitation intensity dependence and time resolved measurements. The effect of hydrogen passivation of the same samples is also studied.

The emission spectrum of self-assembled InGaAs/GaAs quantum dots filled with up to 10 excitions is measured in magnetic fields up to 13 Tesla. The spectrum shows a number of peaks which split and rearrange with the magnetic field. The behaviour of the spectrum with carrier density and magnetic field is compared with detailed calculations. The model allows us to interpret our results in terms of coherent many-exciton states and their destruction by the magnetic field. The level structure of the spectra is related to the shell structure and collective excitations of many-excition droplets in these artificial atoms.

We present selective photoluminescence and photoluminescence excitation spectroscopy on self-assembled quantum dots. These spectroscopic techniques reveal a large apparent `Stokes' shift in In(Ga)As-Ga(Al)As self-assembled quantum dots. We demonstrate that the first two excited states observed in PLE originate from the doubly-degenerate first excited state. The ground state is not observed in the quantum dot PLE due to the sharp d-funtion-like density of states of individual dots. Our experimental data agree with a recent theoretical calculation, suggesting that both the degeneracy-lifting and broadening of energy levels are caused by the random potential of compositional, size and strain fluctuations. Due to large energy separations in these quantum dots, we also demonstrate the importance of multi-phonon relaxation processes

Several practical issues in the development and operation of quantum-dot cellular automata (QCA) cells and systems are discussed. The need for adiabatic clocking of QCA systems and modeling of electrostatic confinement of quantum dots are presented. Experimental data on dot coupling and applications to QCA detectors in a 2-dimensional electron gas (2DEG) are presented. We report a charge detection scheme where we observe strong modulation in the detector signal, in addition to the detector exhibiting minimal effect on the dot being measured. With this investigation, we demonstrate these two key components required for QCA in AlGaAs/GaAs materials, namely dot coupling and charge-state detection.

We present radiative lifetime measurements of excited states in semiconductor self-assembled quantum dots. By increasing the photoexcitation intensity, excited-state interband transitions up to n=5 can be observed in photoluminescence. The dynamics of the interband transitions and the intersublevel relaxation in these zero-dimensional energy levels lead to state filling of the lower-energy states, allowing the Fermi level to be raised by more than 200 meV due to the combined large intersublevel spacing and the low density of states. The decay time of each energy level obtained under various excitation conditions is used to evaluate the intersublevel thermalization time. \textcopyright{} 1996 The American Physical Society.

Near-surface GaAs AlGaAs quantum wells, with and without a GaAs cap, have been used as effective probes of the electronic properties of oxidized GaAs surfaces and AlGaAs surfaces. We find that the variation of surface stoichiometry due to hydrogen ion treatments appears to be similar for both oxidized AlGaAs and GaAs surfaces, as examined by Auger electron spectroscopy. However, the composition of the oxide significantly affects both hydrogenation process and the efficacy of surface passivation.

The InAs/Ga1−xInxSb strained‐layer superlattice (SLS) holds promise as an alternative III–V semiconductor system for long wavelength infrared detectors. In this article, we present the first investigation, to the best of our knowledge, of heterojunction photodiodes using this new material. The devices were grown by molecular beam epitaxy on GaSb substrates, and are comprised of a 38 A InAs/16 A Ga0.64In0.36Sb SLS used in double heterojunctions with GaSb contact layers. The structures were designed to optimize the quantum efficiency while minimizing transport barriers at the heterointerfaces. The photodiodes are assessed through the correlation of their performance with the SLS material quality and the detector design. X‐ray diffraction, absorption, and Hall measurements are used to determine the SLS material properties. The electrical and optical properties of the photodiodes are determined using current–voltage and spectral responsivity measurements. At 78 K, these devices exhibit rectifying electrical behavior and photoresponse out to a wavelength of 10.6 μm corresponding to the SLS energy gap. The responsivity and resistance in these thin‐layered (0.75 μm), unpassivated photodiodes result in a detectivity of 1×1010 cm √Hz/W at 8.8 μm and 78 K. Based upon the performance of these devices, we conclude that high‐sensitivity operation of long‐wavelength photovoltaic detectors at temperatures well in excess of conventional III–V band gap‐engineered systems, and potentially in excess of HgCdTe, is feasible using this material system.