We report on the influence of a 2-D electron gas (2-D EG) on the electroreflectance (ER) spectra of an AlGaAs/GaAs/AlGaAs single quantum well. The ER and, for comparison, photoluminescence (PL) measurements were performed as a function of temperature and carrier density in a gated 2-D EG. For negative gate voltages the observed ER signals are due to the quantum confined Stark effect (QCSE). For forward biases and at a temperature of 10 K, the broadening of the PL line and the increase of the Stokes shift between the corresponding ER and PL transitions give clear evidence for the population of the first electron subband. Moreover, with the formation of the 2-D EG we found a strong enhancement of the magnitude as well as two additional phase inversions for the ER transitions between the ground heavy and light hole states and the first electron subband. These effects were seen to decrease with increasing temperature. The results are explained in terms of transformation of the excitonic absorption mechanism from a QCSE-related mechanism to one determined by the `Mahan exciton.'

We have studied the photoreflectance spectra at 300 K and 80 K related to the intersubband transitions from several (001) GaAs/GaAlAs structures fabricated by molecular beam epitaxy using the digital alloy compositional grading (DACG) method. These samples include an asymmetric triangular quantum well (ATQW) and three rectangular quantum wells (RQW) with different unit cells [50 angstrom (X 2), 25 angstrom (X 4) and 12.5 angstrom (X 8)]. For the ATQW comparison of the observed intersubband resonances with a theoretical calculation (envelope function method) provided a self-consistent verification that the DACG produced the intended result, i.e., an effective linearly graded profile. For the RQW materials the 12.5 angstrom unit cell sample essentially had the characteristics of the intended analog configuration while the other two samples displayed observable differences.

Abstract not available.

Normal incidence absorption between valence band states is demonstrated in high indium content InGaAs/AlGaAs quantum wells (QWs) grown on GaAs. We report absorption at energies up to 300 meV (4.13 micrometers ) and integrated absorption fractions as high as 56 mAbs-meV per QW. The dependence of the absorption strength and peak energy on well width, barrier width, doping concentration, and doping profile is explored. It is found that a stronger absorption is obtained with the same sheet doping density for structures where the doping is in the barriers. We also discuss the differences observed between transverse magnetic and transverse electric incident polarizations in the absorption spectra.

We present a theory for the exciton magnetoluminescence in GaAs/AlAs superlattices. We have found the energies of the exciton states, taking into account an additional exchange splitting, which has been found experimentally. This splitting is attributed to the symmetry reduction from D_2d to C_2v, and it should lead to the polarized luminescence under the appropriately polarized excitation. Instead, a non-polarized luminescence was observed. At present, there are two alternative models explaining this phenomena. We have calculated the effect of the magnetic field on the exciton levels splitting in GaAs/AlAs superlattices and demonstrated that the two models give very different results for the hole g-factor. We show that measurements of the luminescence polarization allow us to choose between the alternative models of this splitting. The exciton luminescence, while completely non-polarized at zero magnetic field, becomes partially polarized at some specific values of the magnetic field. At these points, a level crossing occurs and one of the splittings in the exciton multiplet becomes zero. We show that these values of the magnetic field are very different in the two models of the additional exchange splitting.

We present a photoluminescence and Raman study of a GaAs/AlAs multiple quantum well structure doped n-type in the AlAs barriers. Electrons transfer from the silicon donor states associated with the AlAs X-valley minima to the GaAs wells where they form a dense quasi two-dimensional electron gas. At zero magnetic field the luminescence spectrum is broad and featureless lacking any sharp excitonic features characteristic of undoped wells. In the presence of a magnetic field applied parallel to the growth axis the luminescence exhibits a number of distinct lines that are due to interband transitions between conduction and valence band Landau levels. The Raman spectra were excited in resonance with the e₃h₃ exciton. At zero field a feature associated with the e₁ yields e₂ intersubband transition is observed. When an external magnetic field is applied, this feature evolves into a combination mode between the e₁ yields e₂ transition and the electron cyclotron resonance.

The photoluminescent properties of GaAs/Al_xGa_{1 - x}As multiple quantum well structures grown on (100) GaAs substrates by molecular beam epitaxy have been investigated as a function of the AlAs mole fraction. It is found that as the AlAs mole fraction in the Al_xGa_{1 - x}As barriers is increased, the corresponding photoluminescent peak intensity of the quantum well structures increases and reaches a maximum at an AlAs mole fraction of about 45%. Further increases in the Al content of the barriers beyond 45% results into a decrease in the luminescence efficiency. At the maximum intensity, the peak luminescence from the quantum well structures is enhanced by about two orders of magnitude (at room temperature) when compared to the luminescence from such structures with an AlAs mole fraction of 20% in the barriers.

Photoluminescence (PL) and time-resolved PL measurements were used to study the exciton recombination processes in GaAs/AlGaAs, InGaAs/GaAs, and InGaAs/AlGaAs quantum wells (QWs). An increasing lifetime with decreasing well width has been observed in very narrow and high quality GaAs/AlGaAs samples, and attributed to the reduced overlap of the electron and hole wave functions and the increase of the exciton effective volume. In InGaAs/GaAs strained QWs the measured exciton lifetimes were found to be of In composition dependence: the more the indium composition, the shorter the lifetime. The mechanisms, including the random alloy disordering and the degeneration of quasi two-dimensional properties of excitons were inferred to explain the experimental results. The nonradiative recombination was stressed in our InGaAs/AlGaAs QWs. A combined analysis of cw PL and time-resolved PL measurements allows us to separate the radiative and nonradiative decay times in our sample. The observed dominant nonradiative recombination has been tentatively ascribed to the poor quality of AlGaAs.

Intermixing the wells and barriers of quantum well structures generally results in an increase in the bandgap and is accompanied by changes in the refractive index. A range of techniques, based on impurity diffusion, dielectric capping and laser annealing, have been developed to enhance the quantum well intermixing (QWI) rate in selected areas of a wafer -- such processes offer the prospect of a powerful and relatively simple fabrication route for integrating optoelectronic devices and for forming photonic integrated circuits (PICs). Recent progress in QWI techniques is reviewed, concentrating on processes that are compatible with PIC applications and illustrated with device demonstrators.

In this paper we report novel high contrast, high reflectivity n⁺-AlGaAs/GaAsAl/Ag asymmetric Fabry-Perot (ASFP) optical modulators and self-electro-optic devices (SEED) using the Franz-Keldysh (FK) electroabsorption in bulk GaAlAs layer. These modulators exhibit `normally off' and `normally on' optical modulation at and below the band edge with contrast ratios in the range of 25:1 to 200:1 and reflectivities of about 30% to 50%. We show that our experimental data is consistent with a model of electroabsorption that includes unbound excitons. The FK-SEED, exhibiting contrast rations of approximately 90:1 and reflectivities of 27% at 11 V, operates based on the combined effects of negative electroabsorption and the `normally off' properties of the device. In addition to these devices operating in the 10 nm vicinity of the GaAs band gap (872 nm), we also report a high contrast modulator with Ga_0.975Al_0.025As FK layer operating at the standard 850 nm wavelength. These devices demonstrate the feasibility of using the bulk Franz-Keldysh effect as an alternative to quantum confined stark effects (QCSE) for efficient optical switching and modulation for many applications.

We compare photoluminescence data collected in either a surface-normal configuration (NPL) or with the pump and collection paths perpendicular to a cross-section of the epitaxial layers (XPL) for various vertical-cavity surface-emitting lasers and distributed quantum well structures. We report the spatial resolution of the XPL technique, particularly as it applies to distinguishing features in complex multilayer structures. We assess a potential simulation method for transforming the perturbed NPL spectra into the unperturbed XPL spectra, taking into account a number of experimental and material parameters which may influence the lineshape. These factors include the pump field distribution and its influence on the weighting of the emitters, the collection optics, and the changes in the dispersive complex dielectric constant of the quantum wells. This information is of import not only to optimizing device manufacture, but to basic physical and materials research as well. Whereas the XPL technique is a relatively simple but destructive characterization tool, a complete understanding of NPL emission could be made to yield the same information via rapid, nondestructive means.

A novel technique for bringing the light- and heavy-hole valence bands in a quantum well, (QW), into approximate degeneracy is described and demonstrated. It utilizes pseudomorphic tensile strain in the barriers generated by lattice mismatch between the barrier and the substrate material. An important consequence of this strain is that the splitting of the light- and heavy-hole valence band energies at the Brillouin zone center, due to the quantum confinement effect, is approximately cancelled. Unlike a similar result in systems with tensily strained wells, this degeneracy is not sensitive to the exact QW width (for QW widths greater than 5 nm) or the precise strain present in the layer. It is thus more amenable to the growth and fabrication of devices which should simultaneously exhibit the polarization isotropy of bulk structures and the enhanced performance of QWs. The technique is demonstrated by an optical investigation of GaAs/GaAs_{1 - y}P_y quantum wells grown on GaAs substrates by metalorganic chemical vapor deposition.

We report on recent theoretical and experimental results on n-i-p-i based smart pixels. We present first results on dynamical switching in these devices. In particular, we present 1.9 ns switching times at an optical power of 880 (mu) W for the photoconductive switch. This corresponds to a total switching energy of 1.7 pJ or 2.4 fJ/micrometers ² relative to the device area. A contrast of the electronic output signal larger than 10⁷ and a maximum dc gain exceeding 10⁶ is achieved. For an optical NOR gate -- composed of a photoconductive switch and an electroabsorptive n-i-p-i modulator -- we were able to demonstrate operation at optical switching energies of 2 pJ and switching times of 1 microsecond(s) . This corresponds to an opto-optical gain (fan out) of 750. The switching contrast of the optical output signal is 4.5:1.

Quantum well intermixing has been performed using ion implantation techniques to increase the optical bandgap in a spatially selective manner. We show that there is a maximum single dose beyond which further intermixing of the QWs is impeded by damage to the semiconductor surface. We overcome this problem by using a series of implants and rapid thermal anneals, with each rapid thermal anneal repairing the crystal surface. Using this technique we have demonstrated shifts in optical bandgap for multiple implants greater than seven times that observed for a single implant.

This paper describes a 1.3-micrometers edge-emitting LED (light-emitting diode) with multiple quantum well structure. We introduce quantum well structure to the emission region in order to improve the thermal stability of the optical output power. The least temperature dependence and the highest optical output power are obtained at ten quantum wells. In the range from -30 degree(s)C to 85 degree(s)C, the temperature coefficient of the optical output power and the peak wavelength are - 0.87%/ degree(s)C, 0.52 nm/ degree(s)C, respectively. The LED yields a coupled power of 20 (mu) W into a single-mode fiber at a driving current of 100 mA and 25 degree(s)C.

The existence of equidistant smooth maxima series was found in the spontaneous emission spectra of stripe single-quantum-well 0.98 micrometers range lasers with a strained InGaAs active layer. Within the limits of a 200 angstrom wide general envelope the characteristic distance between maxima for different lasers varied from 20 to 40 angstrom, corresponding to 10 to 20 longitudinal Fabry-Perot cavity mode intervals. Possible physical models of observed maxima origin are considered: recombination of electrons and holes with selection rules (Delta) u equals 0, and (Delta) u equals 1, 2, ..., where u is the level number; resonant self-effect of Fabry-Perot cavity modes field via the volume of laser crystal, layers of which, including GaAs substrate, are transparent in the 0.98 micrometers wavelength range. The latter model was confirmed experimentally.

Abstract not available.

We review the progress in our physical understanding and modeling of integrated surface emitting harmonic generators and also present experimental results for the monolithic integration of active laser sources embedded with a quasi phased matched nonlinear heterostructure cavity for surface emission in the blue-green wavelength region.

The interband and intersubband transitions in amorphous Si/SiO₂ multiple quantum well structures, nonlinear effects, and relations between nonlinearity and electron recombination channels have been investigated. Three types of radiative transitions have been observed: subband-to-subband recombination, recombination between subbands of the well and the impurity states in the barrier layers, and the recombination related to impurity states in the barrier layers. A relaxation of excited carriers between the subbands within the conduction and valence bands has been found that allows one to observe hot luminescence caused by higher subbands of the quantum well. The dependencies of the luminescence intensity on the excitation intensity show that the recombination rate is dependent on the concentration of excited carriers. Intersubband absorption has been observed for the first time in undoped amorphous multiple quantum well structures under interband excitation. The transitions take place between the first and second subband of the conduction band involving nonequilibrium electrons excited in the first subband with optical pumping. The dispersive nonlinearity has been investigated in the Fabry-Perot formed by the top interface of the structure and the substrate. The refractive index changes obtained from intensity dependent reflection spectra depend on the excitation intensity in nonlinear manner and can be described by the model of saturating nonlinearity for lower pump intensity. The nonlinear refractive index reveals resonant behavior associated with the subband structure of the QWs.

We have performed second harmonic generation (SHG) measurements in the 3 - 5 micrometers region on p-type stepped quantum wells (QWs) using the tunable, high peak power pulses generated by a free electron laser. The samples were grown by MBE on (100) GaAs wafers. The asymmetric QWs are made of m monolayers of GaAs and n monolayers of Al_0.5Ga_0.5As sandwiched between AlAs barriers. The QWs were characterized by x-ray diffraction and room temperature photoluminescence (PL). We measured an order of magnitude enhancement of the second order susceptibility over bulk GaAs. In contrast to n-type QWs, the dominant component is the (Chi) _xyz⁽²) component. The results are explained by a full pseudopotential band structure calculation of (Chi) ⁽²).

Conduction band intersubband absorption and second harmonic generation (SHG) are demonstrated in doubly resonant asymmetric step high indium content quantum wells (QW) grown on a GaAs substrate. Intersubband absorption peaks at 5.51 micrometers and 3.05 micrometers corresponding to the 1 to 2 and 1 to 3 transitions are measured. The susceptibility of SHG from the QW, (Chi) _QW⁽²), is measured using a free electron laser by interference between the SHG fields generated from the QW and the GaAs substrate. A large asymmetry in the SHG power with rotation angle of the sample arising from (Chi) _QW⁽²) is observed. The magnitude and phase of (Chi) _QW⁽²) is measured in the 4.6 - 6.3 micrometers pump wavelength range. (Chi) _QW⁽²) is of maximum amplitude at 6.0 micrometers with a value of 145 +/- 20 nm/V. A change in the sign of the phase of (Chi) _QW⁽²) within the SHG resonance is demonstrated for the first time. Agreement of both the linear and nonlinear properties to a simple model assuming Lorentzian linewidths is discussed. SHG of 2.0 micrometers light is also demonstrated in coupled In_0.60Ga_0.40As/AlAs quantum wells.

A microscopic local-field calculation of the infrared second-harmonic generation associated with intersubband transitions in a single GaAs/AlGaAs quantum-well structure subjected to an applied electric field is presented. Taking as a starting point a fundamental selfconsistent integral equation for the local field, the p-polarized first-harmonic fields inside the quantum well are calculated exactly. The result for the local-field calculation at the first-harmonic frequency is used to calculate the p-polarized second-harmonic local field. The conversion efficiency of the second-harmonic generation from the quantum well is determined. Numerical calculations of the frequency spectra of the second-harmonic powers are presented for different applied fields. The numerical results show that strong second-harmonic generation occurs in the vicinity of the resonance frequencies for the first- and second-harmonic local field inside the quantum well. The influence of the applied field on the optical second- harmonic generation is investigated. It is demonstrated that the presence of the dc bias leads to a blue-shift of the resonance frequencies in the SH energy reflection spectra stemming from the quantum-confined Stark effect, and that the maximum value of the SH power decreases when the applied field is increased.

We report electrical transport measurements on gated nanostructures, quantum dots of various shapes and sizes in which magnetic focusing effects are remarkably simple. We compare these results to numerical simulations using a classical billiard ball approach, and investigate the fundamental properties of magnetic focusing of ballistic electrons in a confined geometry. With quantum dots attracting interest in studies ranging from Coulomb blockade to quantum chaos, the ability to determine fundamental properties of the geometry and material is becoming increasingly important. We illustrate the potential of magnetic focusing of ballistic electrons as a tool to probe a confined region for otherwise unobtainable parameters such as the local electron concentration in an dot and the geometry of a dot. An additional gate located in the center of a dot transforms it into a ring. Results and analysis of structures performed to date are given.

Abstract not available.

We have used the molecular beam growth technique we call cleaved edge overgrowth to fabricate highly efficient lasers, which operate in the 1D quantum limit. The active region of our laser consists of quantum wires that form at the T-shaped intersections of 7 nm wide GaAs quantum wells grown along the (001) and after an in situ cleave along the (110) crystal axis. These intersections are, in turn, embedded in a T-shaped dielectric waveguide, which confines the optical mode to the vicinity of the quantum wires. The precise control of the quantum wire dimensions achievable in this way allows the observation of stimulated optical emission from the lowest exciton state in optically pumped devices. The implied absence of band-gap renormalization effects suggests an enhanced stability of the exciton gas phase in 1D. This is consistent with the observed red-shift of the quantum wire photoluminescence signal with respect to the quantum well signal of about 17 meV which strongly exceeds the shift predicted by theory for free carrier transitions and indicates an enhancement of the 1D exciton binding energy by more than 50%.

Laser holographic lithography and selective chemical etching have been used to create GaAs/AlGaAs and InGaAs/GaAs quantum well wires. Strong differences are observed between the photoluminescence spectra from the GaAs and InGaAs quantum wires, with the former being dominated by spatially indirect transitions, and the latter by direct transitions. Detailed time resolved photoluminescence studies show a red shift in photoluminescence energy with increasing time delay after excitation for the indirect transition in the GaAs wires. This is accompanied by a non-exponential decay time which increases from 10 ns to greater than 150 ns as the delay time is increased (in a manner similar to that observed in nipi doping superlattices). The InGaAs wires show no such behavior, with a decay time constant of 320 ps at 1.8 K, independent of time delay after excitation, both indicative of a spatially direct transition.

We have investigated the excitonic lifetime and the carrier capture in InGaAs/GaAs quantum dots with geometrical widths down to 40 nm by time resolved photoluminescence spectroscopy. The excitonic lifetime decreases with decreasing dot size due to nonradiative recombination at the open sidewalls of the quantum dots. Using a model calculation to fit the experimentally observed width dependence of the lifetime the surface recombination velocity is determined to be about 7*10⁴ cm/s at T equals 60 K. Lateral carrier capture into the dots leads to an increase of the time integrated photoluminescence intensity above that of a 2D reference. The variation of the capture time as a function of the dot size could be modeled by taking into account both the vertical and the lateral carrier capture.

We have studied different strained InGaAs/GaAs ultrathin quantum wells grown on vicinal surfaces for various terrace lengths and In contents. From photoluminescence experiments we observe an enhancement of the continuum density of states of quantum wells wit large In content (x equals 0.35). We associate this behavior to the localization of carriers in regions of quasi one-dimensional confinement which are induced by fluctuations in the lateral periodicity of the strained layers. This assumption is supported by time resolved measurements and explained through theoretical calculations.

Using contactless photoreflectance at 300 K we have studied several GaAs/Ga_0.7Al_0.3As quantum dot arrays fabricated by reactive-ion etching using SiCl₄. The spectrum from a control sample that had no dots also was recorded. From the observed shifts of the fundamental conduction to heavy- and light-hole quantum transitions we have evaluated the magnitude and nature of the process-induced strain in the dots.

The time-resolved photoluminescence of semiconductor doped glasses was obtained as a function of laser excitation intensity at a low temperature. At laser intensities lower than 3.3 X 10⁵ W/cm², only the exciton recombination with a lifetime of 4 ns was obtained. When the laser intensity was increased, the biexciton recombination with a lifetime of about 1 ns was observed. At very high laser intensities, the free carrier recombination with a much shorter lifetime dominated. Relative intensities of photoluminescence contributed from excitons, biexcitons, and free carriers were estimated from the laser intensity distribution in the semiconductor doped glass and agree with experimental results.

The quantum-well emission originating from triple-barrier AlAs/GaAs resonant tunneling light emitting diodes has been investigated. In these devices, three barriers defined two asymmetric quantum wells. Two nominally identical structures were grown that had a different sequence of the two quantum wells. Depending on which well was the first well for the electron tunneling transport, double or triple resonances were observed. In the latter case, the two- dimensional accumulation layer is aligned with the subbands from both wells. The subband occupation and the charge distribution between the two wells is studied, showing that at any resonance the wider of the two wells will emit more luminescence but at a double resonance the narrower well can be the stronger light emitter. Bandstructure calculations under all bias conditions further confirm the occurrence of both kinds of resonances in these triple-barrier structures.

We introduce charge accumulation in quantum wells through the use of a nonlinear Schrodinger equation. Looking first at infinite and finite square wells allows us to calculate the new energy spectrum including the separate effects of a biasing electric field and charge accumulation. This gives us insight into the new resonant tunneling energies that arise due to the quasibound states being shifted by either the external field or the reaction field built up through the accumulation of charge. Using a double barrier potential, we calculate the transmission coefficient with and without the external bias field and then with charge accumulation. To study the tunneling dynamics, we first start with a single barrier in an infinite well and discover a fractal-like character to the probability for finding an electron wavepacket in one side of the structure. Finally we numerically integrate the full time- dependent nonlinear Schrodinger equation with various barrier potentials to obtain the dynamics of a wavepacket incident on the structures.

Special electronic transport properties in superlattices (SLs) and quantum wells (QWs) give rise to both new physical phenomena and useful devices. Studies of carrier transport in superlattice and multiple quantum well (MQW) structures therefore have practical implications. There are different regimes for SL and MQW carrier transport. In this paper, we report on our experimental study of current-voltage (I-V), capacitance-voltage (C-V), and intersubband photocurrent vs. voltage (I_p-V) characteristics in a MQW structure having a coupled double-well period. Extremely regular periodic peaks in I-V, C-V, and I_p-V are observed in the coupled double well MQW. Hysteresis is displayed in all the measurements with respect to the voltage scan direction. A phenomenological model is used to explain the observed results.

Excitonic optical spectra in quantum structures are sensitive to fluctuations of the well width (interface roughness). Since it is generally accepted that the exciton luminescence linewidth reflects the interface quality, a reliable theory is highly desirable. A Schrodinger equation for the exciton center-of-mass motion is derived which contains a disordered potential being Gauss distributed and correlated over distances of the exciton Bohr radius. The optical density (OD) as well as the excitonic density of states (DOS) are obtained by numerical methods. The OD determining the absorption lineshape shows a distinct asymmetry and reduced linewidth compared to the underlying potential distribution (motional narrowing). The luminescence lineshape is given by the product of the OD and the distribution function. Using a kinetic equation with generation, recombination, diffusion, and drift we show that at low temperatures the lineshape is determined by topology only as the excitons diffuse to the next local potential minimum before they recombine. Time-dependent simulations following a short broad-band excitation pulse show that the spectrum narrows and shifts to the red, from the absorption towards the topological lineshape.

Abstract not available.

Abstract not available.

We have fabricated mesa and ridge waveguide diodes from Si/Ge quantum well (QW) and short-period superlattice samples deposited by MBE on a (100) Si substrate. We have grown Si/Ge/Si_1-xGe_x QW samples consisting of thin Ge wells where 20 MLs of Si are embedded in-between and followed by a SiGe layer elastically strained on a Si substrate as well as symmetrically strained on a strain symmetrizing buffer layer. We have also grown Si_mGe_n short-period, strained layer superlattices consisting of N periods of (m + n) monolayers per period which are deposited on a strain adjusting buffer layer. The edge emitting ridge waveguide diodes fabricated from this material with standard semiconductor processing techniques were polished on the <110> side faces and etched to a height of roughly 1.0 micrometers with lateral dimensions of roughly 100 micrometers width X 3 mm length. Also circular mesa diodes were fabricated with the same height and varying diameters from 100 micrometers to 800 micrometers and were provided with a metal ring contact on top defining the illumination window. Photocurrent and electroluminescence signals at h(nu) approximately 0.8 eV were detected at room temperature from QW samples. The emission characteristics as a function of strain and composition of the SLS and QW layers are discussed.