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Photoluminescence spectroscopy has been used to probe the occupied electron states below the Fermi energy of zero-dimensional electron systems in both zero and finite magnetic fields. The arrays of modulation-doped quantum dots investigated were fabricated by reactive-ion etching of Al0.3Ga0.7As/GaAs heterojunctions with a δ-layer of Be present in the GaAs at 200 A from the interface in order to improve luminescence efficiency. We show that the low magnetic field dispersion (B < 8 T) of the acceptor recombination line is directly related to the magnetic field dependence of the total ground state energy of interacting electrons in the quantum dots.

A model is proposed to rationalize the occurence of CuPt-type ordering in mixed III–V epitaxial layers grown on (001) substrates. It is invoked that 2× surface reconstruction occuring on group V terminated (001) surfaces produces dilated and compressed regions in sub-surface layers. The presence of these regions biases the occupation of certain sites during the growth of a layer consisting of atoms differing in their tetrahedral radii. By assuming that the reconstruction always occurs at the growing surface, it is possible to explain the evolution of CuPt-type ordering in epitaxial layers. Also, based on the model, experimental observations pertaining to the influence of growth conditions, surface misorientation and steps on ordering can be rationalized.

The low DC field conductance and the nonlinear I-V characteristics of GaAs/Al 0.70 Ga 0.30 As superlattices have been measured in the presence of intense electric fields produced by UCSB's free-electron lasers. The DC conductance exhibits oscillatory behavior as a function of terahertz field strength that resembles the zeroth-order Bessel function, J 0 (edE AC /ħω), where e, d, and E AC are the electron charge, superlattice period and AC field strength, respectively. The I-V characteristics, measured between 600 GHz and 3.1 THz, display new steps and plateaus. The dependence of the new structure of frequency suggests the presence of new conduction channels that are most naturally assigned to photon-mediated sequential tunneling

Surface reconstruction of GaAs (001) during organometallic chemical vapour deposition (OMCVD) growth has been investigated with reflectance-difference spectroscopy (RDS). RD spectra reveal that surface reconstructions similar or identical to (4 x 2), (2 x 4), and c (4 x 4) that occur on surface prepared by molecular beam epitaxy (MBE) in ultrahigh vacuum (UHV) occur even in atmospheric pressure OMCVD growth environments. Based on the RDS database we established on static surfaces in UHV , we studied the structure of surfaces under both static and dynamic conditions in non-UHV ambients. We find, in contrast to previous models, that the surfaces under various non-UHV conditions exhibit dimer formation. In addition, OMCVD growth and atomic layer epitaxy (ALE) typically occur under disordered c (4 x 4)[ d (4 x 4)]-like conditions where the surface is terminated by multilayers of As. When trimethylgallium (TMG) and arsine (AsH 3 ) are supplied simultaneously, the surface structure varies as a function of the supply rates of TMG and AsH 3 , and the substrate temperature.

The chemistry, overlayer morphology, and band bending occurring upon room temperature deposition of Ga and Pb on GaAs(100) are studied via high resolution photoemission spectroscopy, low energy electron diffraction and contact potential difference measurement. Both interfaces are abrupt and unreacted. At submonolayer coverage, Ga and Pb grow two dimensionally on GaAs(100). An ordered (1×2) superstructure is obtained for Pb(1/2 ML)/GaAs(100) by low temperature annealing of the interface (∼300 °C). Clustering is dominant at high coverage. With Ga, the Fermi level (EF) converges toward a position 0.68 eV above the valence band maximum (VBM), in agreement with the position obtained for the GaAs(110) surface. The room temperature deposition of Pb leads to EF stabilization at 0.57 eV above VBM, independent of the initial surface stoichiometry (Ga or As rich). The evolution of the band bending as a function of metal coverage emphasizes in both cases the importance of adatom‐induced states at low coverages and th...

We summarize recent applications of two real-time optical diagnostic techniques, reflectance difference spectroscopy (RDS) and spectroellipsometry (SE), to epitaxial growth on GaAs and atomic layer epitaxy (ALE) in particular. Using RDS, we obtain the first real-time spectroscopic data of the evolution of the (001) GaAs surface to cyclic and non-cycle exposures of atmospheric pressure H 2 , H 2 and trimethylgallium, and H 2 and arsine, which simulate growth by ALE. None of our observations is consistent with any previously proposed simple model, emphasizing the necessity of real-time measurements for the analysis of complex surface reactions. Using SE we have constructed a closed-loop system for controlling the compositions of Al x Ga 1− x As layers grown by chemical beam epitaxy. We have produced various graded-compositional structures, including parabolic quantum wells 200 A wide where the composition was controlled by analysis of the running outermost 3 A (about 1 monolayer) of depositing material.

We describe the coherent epitaxy of MnNiAl fenomagnetic thin films on AlAs/GaAs heterostructures using molecular beam epitaxy. The epitaxy helps to stabilize the τ phase, which is known to be a metastable ferromagnetic phase with a tetragonal lattice. The easy axis of the ferromagnetic thin film is oriented perpendicular to the surface as a consequence of the coherent epitaxy. The results of structural and magnetic analysis are given for thin MnxAl1-x films with 0.40 < x < 0.60. We have further assessed the change in structural and magnetic properties of the thin films when Ni is added as a substitution for Mn in Mn0.60-yNiyAl0.40, with 0 < y < 0.18. We find that coherent epitaxy is obtained over these wide compositional ranges and that the change in magnetic properties using various Ni additions adds a degree of freedom in the design of ferromagnetic thin layers on AlAs/GaAs for future device structures.

Reflectance-difference spectroscopy (RDS) is employed to study in situ the (4\ifmmode\times\else\texttimes\fi{}2), (1\ifmmode\times\else\texttimes\fi{}6), (4\ifmmode\times\else\texttimes\fi{}6), (3\ifmmode\times\else\texttimes\fi{}1), (2\ifmmode\times\else\texttimes\fi{}4)-\ensuremath{\alpha}, (2\ifmmode\times\else\texttimes\fi{}4)-\ensuremath{\beta}, (2\ifmmode\times\else\texttimes\fi{}4)-\ensuremath{\gamma}, c(4\ifmmode\times\else\texttimes\fi{}4), and d(4\ifmmode\times\else\texttimes\fi{}4) reconstructions of (001) GaAs surfaces prepared in ultrahigh vacuum (UHV) by molecular-beam epitaxy and simultaneously characterized by reflection high-energy electron diffraction (RHEED). Reproducibility of the data is excellent. With the aid of previous theoretical calculations, we interpret characteristic spectral features at 1.9, 2.6, and 4.2 eV in terms of electronic excitations involving surface dimers of Ga, As, and As, respectively. Because RD couples to local electronic structure rather than to long-range order, RD spectra not only determine surface reconstructions but also provide details not accessible by RHEED, such as the existence of As dimers in the (1\ifmmode\times\else\texttimes\fi{}6), (4\ifmmode\times\else\texttimes\fi{}6), and (3\ifmmode\times\else\texttimes\fi{}1) reconstructions and of the fractional coverage within a given reconstruction. Our data show that the (3\ifmmode\times\else\texttimes\fi{}1), (1\ifmmode\times\else\texttimes\fi{}6), and (4\ifmmode\times\else\texttimes\fi{}6) reconstructions are at least partly determined by kinetics, since they can only be obtained by following specific heating or cooling procedures under very low ${\mathrm{As}}_{4}$ flux. More generally, it is possible to employ this optical technique to determine surface atomic and electronic structure. Because RD spectra can be obtained with the surface in any transparent ambient, the database that we have established here provides a new approach for elucidating surface reconstructions of (001) GaAs and hence the dynamics of surface reactions in non-UHV environments.

We report the first direct observation of reconstructions of semiconductor surfaces in atmospheric pressure (AP) environments. We use reflectance difference spectroscopy (RDS), a surface-sensitive optical probe, to bridge the gap between ultrahigh vacuum (UHV) and AP, and show that the primary surface reconstructions that occur on (001)GaAs surfaces in UHV also occur in the AP environments used with organometallic chemical vapor deposition (OMCVD). Our results justify the applicability of the results of UHV surface science to understanding surfaces under non-UHV environments. Our results also show that during OMCVD growth conditions the surface is terminated with multilayers of As, contrary to generally accepted models. We also apply our newly developed approach, multi-transient spectroscopy (MTS), to the study of atomic layer epitaxy (ALE). Using MTS, we observe surface spectra within a time resolution of 100 ms during actual ALE growth cycles, thus allowing the dynamics of surface reactions to be investigated.

We use in situ photoluminescence (PL) to investigate recombination at (100)GaAs surfaces and GaAs/AlGaAs interfaces in a controlled crystal growth environment. PL was monitored for different GaAs surface reconstructions, after surface chemical modification, and during early stages of AlGaAs heteroepitaxy. Depositing ∼1 ML of Se to form a (2×1) surface increased the GaAs PL intensity 200 times. Surprisingly, it required 6 ML (15 A) of heteroepitaxial AlGaAs to achieve the same degree of surface passivation. We invoke lateral variations in interfacial AlGaAs composition to explain these results.

Enhancement of the Schottky barrier height was obtained in MBE grown (CoAl or NiAl)/ AlAs/GaAs heterostructures by incorporating ultra thin AlAs layers between the metal and GaAs. By varying the AlAs thickness from 0 to 10 nm the effective barrier height, determined by current-voltage and capacitance-voltage measurements, could be tailored from 0.76 eV to 1.1 eV. Internal photoemission measurements were performed to verify the 1.1 eV barrier height. The temperature dependence of the current-voltage characteristics showed that thermionic emission was the dominant transport mechanism for all thicknesses of AlAs. The results were consistant with transport through the X-band of AlAs.

We present a far-infrared-absorption study of electrons in lightly doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As superlattices. A grating coupler enables us to observe transitions that require an electric-field polarization along the superlattice axis. We investigate weakly and strongly coupled superlattices, and demonstrate the difference between intersubband transitions and transitions between extended minibands. The line shape of the interminiband absorption deviates considerably from the predictions of a simple single-particle model. We discuss possible reasons in terms of fluctuations and localization. At low temperatures, the absorption spectra are dominated by donor transitions. The transition from the donor ground state to the pure 2${\mathit{p}}_{\mathit{z}}$ state, which is associated with the second subband, is observed. In a variational calculation, we compute the four-lowest donor states for a variety of superlattice parameters. Excellent agreement between experiment and theory is achieved.

Reflectance difference spectroscopy (RDS) is a surface analysis technique that was invented in 1985 by Aspnes. Here, we give a summary of its application to crystal growth techniques which gave new and valuable real time information about the growth process. Also, this information was obtained in-situ, in molecular beam epitaxy (MBE) and organometallic chemical vapor deposition (OMCVD) crystal growing setups which are routinely used to produce high quality device structures. The application of RDS to OMCVD allowed us to develop a “textbook” like model of growth kinetics, which includes two independent microscopic mechanisms, i.e. adsorption (at -26 kcal/mol) of the reacting molecule (trimethylgallium (TMG) in the case of GaAs), followed by its decomposition (at 39 kcal/mol) on the growing GaAs surface. Our model includes an effect called steric hindrance, associated with the large size of the TMG molecule. This study represents the first direct quantitative evaluation of the catalytic effect of the GaAs surface for the decomposition of TMG. We discuss implications of the model both for growth in the ALE mode as well as for conventional OMCVD growth and comment on the relative importance of surface and gas phase reactions. The application of RDS to MBE revealed remarkable details about the complex intermediate steps that surfaces undergo during growth and enabled to extract directly surface dielectric functions. Finally, applications of the technique as well as results obtained in a number of laboratories where RDS is currently being developed are discussed.

A low‐resistance and nonspiking contact consisting of a layered structure of Si/Ni(Mg) on p‐GaAs is formed by solid‐phase regrowth. Backside secondary‐ion mass spectrometry and cross‐sectional transmission electron microscopy show an initial reaction between Ni and GaAs to form NixGaAs which is later decomposed to form NiSi by reacting with the Si overlayer. This reaction leads to the solid‐phase epitaxial regrowth of a p+ ‐GaAs layer doped with Mg. The total consumption of substrate is limited to a few hundred angstroms. The as‐formed ohmic contact structure is uniform and planar with an average specific contact resistivity of ∼7×10−7 Ω cm2 on substrates doped to 8×1018 cm−3. The thermal stability of this contact scheme is also reported.

A backside secondary ion mass spectrometry (SIMS) technique is employed to examine seven distinctive metallic contacts on GaAs at various stages of formation. The contacts are formed on multilayered GaAs/AlGaAs structures grown by molecular beam epitaxy (MBE), each containing an AlGaAs etch‐stop layer and AlGaAs marker layers for precise alignment of the SIMS depth profiles. After removal of the substrate, the contact structures are profiled from the backside to avoid depth resolution degradation, which results when sputtering through a nonuniform multilayered metallic contact. The seven contacts examined are TaSix, Au/Ge/Ni, Au/Ge/Pd, Ge/Pd, Si/Pd, In/Pd, and Si/Ni/Mg. The TaSix Schottky contact exhibits extensive interdiffusion. The Au/Ge/Ni contact reveals extensive GaAs consumption. Much less consumption occurs in the Au/Ge/Pd contact. The absence of Au in the Ge/Pd contact leads to an extremely abrupt contact interface, with the Ge concentration dropping by four orders of magnitude within ∼100 A of t...

On observe des excitons spatialement indirects ayant des durees de vie en exces de 200 nsec. Ceci represente deux ordres de grandeur de plus que la duree de vie de l'exciton spatialement direct

A backside secondary ion mass spectrometry technique is employed to examine elemental redistribution in the Ge/Pd/GaAs ohmic contact as a function of annealing conditions. Dilute Al containing marker layers (Ga1−x Alx As) in the GaAs permit precise calibration and alignment of the elemental depth profiles. Double etch‐stop thinning yields high depth resolution. The onset of ohmic behavior is found to occur when Ge is detected at the GaAs surface. Good ohmic behavior is observed when an interfacial layer of reacted Pd4GaAs is dispersed and complete coverage of Ge occurs. The Ge/GaAs interface is abrupt with the Ge concentration dropping by over three orders of magnitude within 100 A. About 40 A of GaAs is found to be consumed during the ohmic contact formation. Degradation of the ohmic contacts, as a result of further heat treatment, was found to correlate with Ge in‐diffusion into the GaAs. The results place strict limitations on doping and heterointerface models of ohmic behavior for this contact.

Summary form only given. Recent progress in semiconductor microlaser device performance and design, integration with electronic circuitry and micro-optics, and the applicability of microlaser arrays to optical interconnects, signal processing and displays will be presented. >

The design, fabrication, and characterization of an 8*8 and a 32*32 InGaAs/GaAs vertical cavity surface-emitting laser array, each incorporating a different on-chip electronic addressing scheme, are reported. In the 8*8 laser array, each laser is independently addressable through band pads located around the perimeter of the array. In the 32*32 laser array, the electrical contact geometry is chosen to allow for matrix addressing of the array. In this scheme, the (i,jth) laser in the array is activated by applying voltage between ith row and jth column. Matrix addressing is attractive for large arrays, requiring only 2N bond pads for an N*N array. These are believed to be the first such electronically addressable arrays of vertical cavity laser structures. >

Ferromagnetic Perpendicularly magnetized epitaxial thin films of tau (Mn,Ni)AI have been successfully grown on AlAs/GaAs heterostructures by molecular beam epitaxy. We have investigated the polar Kerr rotation and magnetization of tau MnAl and (Mn,Ni) Al as a function of Mn and Ni concentration. The largest polar Kerr rotation and remnant magnetization were obtained for Mn0.5Al0.5 thin films with values of 0.16-degrees and 224 emu/cm3, respectively. We observed that the Kerr rotation and magnetization remained constant with Ni additions up to about 12 at. % and subsequently decreased with further Ni additions. We discuss these results and one possible method of enhancing the Kerr rotation.

Arsenic dimers and multilayers are shown to exist on (001)GaAs surfaces under atmospheric pressure (AP) organometallic chemical vapor deposition (OMCVD) conditions. We obtained reflectance‐difference spectra from surfaces in AP H2 that are equivalent to those obtained from the (2×4) and disordered‐c(4×4) reconstructions prepared in ultrahigh vacuum by molecular beam epitaxy. Implications for models of OMCVD growth.

The authors have measured transport properties in an AlGaAs/AlxGa1-xAs triangular quantum well whose energy spectrum has been varied by means of a gate bias. They have observed several nonlinear effects in the lateral conductance arising at positive gate voltages as the increasing Fermi level is moved towards the lowering energy positions of the excited subbands in the quantum well. They interpret the results in terms of electron population of the excited subbands in which electrons possess low mobility. Finally, they find new features at high lateral voltages which are considered to be evidence of previously predicted electro-phonon resonance.

We use GaAs photoluminescence (PL) obtained in situ in an epitaxial growth chamber to measure substrate temperatures from 25 to 450 °C with an accuracy of ±8 °C. Since temperature is obtained directly from the measured GaAs band gap, the approach is free of the problems associated with thermal contacts and emissivity calibration, which are encountered when using thermocouples and pyrometers. The PL technique can be used for any type of sample mounting, and since absolute temperature is measured, it is unnecessary to track the thermal history of the sample.

We describe the interaction of light pulses with a GaAs/AlAs resonant tunneling structure. We demonstrate that light with an average power of less than 10 μW can induce switching, and show that switching is accompanied by a change in the optical absorption. These results suggest a number of new applications for the resonant tunneling structure, including light‐by‐light switching.

We have investigated the transport properties of two‐dimensional electron systems in strained InxGa1−xAs channels confined in a potential well formed by delta doping and GaAs barriers. The dependence of the transport parameters on the indium composition has been studied using Hall, Shubnikov–de Haas, and cyclotron resonance measurements. Experimental measurements of the effective mass have been compared with theoretical data obtained from self‐consistent calculations, which take account of effects due to biaxial strain and nonparabolicity on the band structure of InxGa1−xAs.

Positive and negative lithographic patterns have been produced in epitaxial layers of GaAs, achieved by electron beam writing and subsequent etching by Cl2 gas at elevated temperatures. A latent image is formed in the native oxide which is either less resistant to thermal Cl2 etching (positive pattern) or more resistant to thermal Cl2 etching (negative pattern), depending on the electron beam dose. The pattern is stable in air for at least several weeks. The tone of the pattern also depends on the electron beam accelerating voltage, the etching conditions, and the thickness and initial state of the native oxide. Significant changes in the resulting lithography are due to changes in an oxide only a few monolayers thick. Both positive and negative patterns can be produced in adjacent areas with high contrast by variation of the electron‐beam does. Initial Auger analysis suggests that chemical rearrangement of the native oxide occurs with electron beam exposure. The discovery that the native oxide on GaAs ac...

We describe a new semiconductor heterostructure, configured into a bistable device that can be switched either optically or electrically. The two states between which switching occurs involve very different levels of charge accumulation in a quantum well channel, affecting both the photoluminescence spectrum and the vertical photocurrent. The agents in the switching mechanism appear to be hot carriers.

The optical coupling of GaAs photodetectors integrated with LiNbO/sub 3/ waveguides using epitaxial liftoff is measured and compared to calculations based on a complex index model. The measured coupling is found to be comparable to that obtained in epitaxial semiconductor waveguide detectors, but it is lower than expected. Low coupling efficiency is attributed to the presence of a low index barrier layer, not present in semiconductor-based structures, at the GaAs-LiNbO/sub 3/ interface. A simple method of restoring the coupling to its original value without the need to eliminate the barrier layer is proposed. >

The transport properties of a two-dimensional electron gas in Al/sub 0.3/Ga/sub 0.7/As/GaAs quantum-well delta-doped heterostructures are studied. Electron energy subbands in the quantum well were calculated by a self-consistent method. The FETs having a gate length of 1.3 mu m showed a transconductance as high as 340 mS/mm. The FETs also showed a broad plateau of transconductance around its peak, which is not typical in MODFETs. >

We describe chemical modifications of GaAs surfaces which produce robust selenium layers that significantly enhance the electronic properties of the surface. The terminating layers are produced by depositing elemental selenium on GaAs surfaces under alkaline conditions followed by conversion to selenide and selenate using sodium sulfide. These selenium phases are more stable against photo‐oxidation than their sulfide counterparts. On the chemically modified surface, photoluminescence is enhanced 400× and Raman spectroscopy indicates that surface band bending has been reduced to ∼0.1 eV. X‐ray photoelectron spectroscopy reveals significant AsSe bond formation at the surface and a complicated interfacial structure with selenium present in oxidation states varying from 2− to 4+.

Strain modulation of a semiconductor band gap spatially confines excitons in a near-surface GaAs/AlGaAs quantum well. We report photoluminescence (PL) results of quantum dot arrays fabricated using tungsten stressors. Excitons are confined in a 35 meV potential well under 100 nm dot stressors, which is 2.4 times deeper than that observed using 100 nm carbon stressors. Such a confinement potential corresponds to quantum dot energy level splittings of approximately 7 meV. Arrays of dots are studied, each with individual stressors of a different size ranging from 100 to 800 nm. The lateral potential well depth, as measured by the red shift in PL signal, is dependent on the stressor width and thickness in a well-understood manner. The integrated PL signal from the dots increases with increasing temperature, indicating increased efficiency of trapping of excitons from the quantum well into the quantum dots. Interpretation of excitation spectra, and observation of higher energy levels, are complicated by the details of the strain pattern and the near-field coupling of radiation between the metallic stressors and the quantum well.

Ultrathin coherent epitaxial films of ferromagnetic tau(Mn,Ni)0.60Al0.40 have been grown by molecular beam epitaxy on GaAs substrates. X-ray scattering and cross-sectional transmission electron microscopy measurements confirm that the c axis of the tetragonal tau unit cell is aligned normal to the (001) GaAs substrate. Measurements of the extraordinary Hall effect (EHE) show that the films are perpendicularly magnetized, exhibiting EHE resistivities saturating in the range of 3.3-7.1 muOMEGA-cm at room temperature. These values of EHE resistivity correspond to signals as large as +7 and -7 mV for the two magnetic states of the film with a measurement current of 1 mA. Switching between the two magnetic states is found to occur at distinct field values that depend on the previously applied maximum field. These observations suggest that the films are magnetically uniform. As such, tau(Mn,Ni)Al films may be an excellent medium for high-density storage of binary information.

We report the first observation of reconstructions on semiconductor surfaces in atmospheric pressure (AP) environments. Using reflectance-difference spectroscopy we show that the primary reconstructions that occur on (001) GaAs in ultrahigh vacuum (UHV) also occur under AP H 2 , He, and N 2 . These results demonstrate that dimer formation is not restricted to surfaces in UHV and justify the use of UHV studies to determine (001) GaAs chemistry during AP organometallic chemical vapor deposition (OMCVD)

A far-infrared absorption study of electrons in lightly-doped GaAs/Al 0.3 Ga 0.7 As superlattices is presented. Both weakly and strongly coupled superlattices are investigated, and the difference between intersubband transitions and transitions between extended minibands is demonstrated. At low temperatures, the absorption spectra are dominated by donor transitions. The 1s-2p. transition, which is intimately related to the intersubband transition, is observed. All experimental data are compared to an envelope function calculation for the miniband structure and a variational calculation for the donor energies. Excellent agreement between experiment and theory is achieved.

Vertical-cavity surface-emitting lasers are generating much interest due to their geometric suitability for two-dimensional array fabrication and their potential for achieving ultra-low thresholds. Here we report on optically- and electrically-pumped microlaser devices. having transverse dimensions of a few microns and active material lengths of a few hundred A. The very small volumes are a key factor in achieving low thresholds. So far however surface recombination has prevented us from achieving thresholds much below 1 mA.

We have measured transport properties in an AlGaAs/AlxGa1−xAs, triangular quantum well whose energy spectrum has been varied by means of gate bias. We have observed several nonlinear effects in the lateral conductance arising at positive gate voltages as the increasing Fermi level is moved toward the lowering energy positions of the excited subbands in the quantum well. We interpret our results in terms of electron population of the excited subbands in which electrons possess low mobility. Finally, we find new features at high lateral voltages which are considered to be an evidence of previous predicted electrophonon resonance.

Transition metal aluminides and gallides and rare earth monopnictides have been grown as buried conducting layers in 111-v compound semiconductor heterostructures. These metallic and semi-metallic compounds have the CsC1 and NaC1 structures, respectively. The criteria for achieving (100) oriented epitaxial growth on (100)111-V semiconductor surfaces are different for each class of material. The methods used to achieve 111-V/metal/Ill-V heteroepitaxial structures are described. The different approaches needed for the aluminides or gallides and the monopnictides form the basis for a comparative study.

The performance of strained layer InGaAs/GaAs vertical cavity surface emitting lasers defined by ion implantation over a ∼75 o C temperature range is reported on. Maximum CW output levels for the temperature extremes of 10 and 86 o C are 7.5 mW and 200 μW, respectively, for 20×20 μm 2 devices. The temperature dependence of the CW threshold current exhibits exponential behaviour to 80 o C

It is shown that the waveguide losses in lattice-mismatched GaAs-on-InP structures can be significantly reduced using an appropriate buffer layer. An AlGaAs buffer layer sequence was used for this purpose. A thin (400 nm) layer of Al/sub 0.7/Ga/sub 0.3/As, with an index below that of InP, was placed adjacent to the GaAs guiding layer both to maximize optical confinement in the guide and to increase the allowable guide dimensions for a single planar waveguide mode. Additional separation between guide and mismatched interface was achieved by inserting an Al/sub 0.5/Ga/sub 0.5/As layer with an index nearly equal to that of InP between the low-index buffer and InP. The final waveguide structure also included a thin ( >

The work presented here shows a correlation between the position of Pb‐induced gap states, responsible for the initial movement of the Fermi level (EF) at the Pb–GaAs(100) interface, and the position of the Pb 5d core level measured by photoemission spectroscopy. The difference in Pb‐induced gap states is obtained by depositing submonolayer amounts of Pb on the As‐rich (2×4)–C(2×8) or the Ga‐rich (4×2)–C(8×2) GaAs(100) surfaces. These reconstructions are known to produce a large dipole‐induced difference in the work function of the clean surface. The correlation suggests that the reconstruction‐induced dipole is mostly preserved during the initial stage of Pb deposition and shifts the energy levels of the adsorbate.

We report the first evidence of dimer formation on semiconductor surfaces under atmospheric pressure (AP) conditions. We successfully employ reflectance‐difference spectroscopy to link structures of well‐defined (001) GaAs surfaces prepared in ultrahigh vacuum (UHV) by molecular‐beam epitaxy with structures of (001) GaAs surfaces prepared in AP by organometallic chemical vapor deposition (OMCVD). These measurements reveal that surface reconstructions also occur under AP‐OMCVD conditions, forming structures similar, possibly identical, to the (4×2), (2×4), c(4×4), and disordered c(4×4), that occur in UHV. Our results provide a justification for applying the results of UHV studies of these surfaces to non‐UHV environments and yield new insights on the mechanisms and kinetics of non‐UHV growth.

We present the first observation of resolved quantum dot levels produced by strain modulation of a semiconductor. Excitons are confined in lateral potential wells of up to 60 meV, the largest yet reported for strain‐induced quantum wire or dot structures. The dependence of the quantum dot level separation on dot size is in agreement with calculations of the strain‐induced band edge modulation. For 200 nm wide carbon dot stressors the level separations are approximately 2 meV. Frustration of acceptor recombination by three‐dimensional localization of carriers in the strain‐induced potential wells is clearly observed. Finally, we detect luminescence from a single quantum dot at low excitation intensity, a result of the frustration of nonradiative recombination by localization.

Reflectance‐difference (RD) studies were performed on variously reconstructed (001) GaAs and AlAs surfaces. The spectra of the (2×4) and (4×2) reconstructions on (001) GaAs show prominent features due to electronic transitions between lone‐pair orbitals and dimer states, as previously identified by theoretical calculations. The spectra of the c(4×4) reconstructions on (001) GaAs and AlAs show similar features that we also interpret in terms of surface dimer excitations. These dimer features provide a capability of obtaining real‐time, in situ information of dynamics on polar surfaces.

We have investigated a front-surface-emitting laser diode (FSELD) which has both the n and p electrodes on the top surface and emits laser light from the front surface of the wafer. Since its structure is similar to that of HBT, it was fabricated using the conventional HBT process and mask sets. The current flows bypassing the high-series-resistance DBR stacks, and the active layer is surrounded by an oxygen-implanted semi-insulating layer. It showed a low series resistance and low threshold current of 6 mA, for a 25 µm-diameter laser.

Vertical-cavity surface emitting lasers (VCSELs) are a new laser technology which has the potential to provide highspeed low-cost technology for optical interconnects in nextgeneration computing and switching systems. The use of a thermal chlorine etch to improve the surface condition of air-post vertical cavity surface emitting lasers (VCSELs) is reported. As much as a factor of three improvement in MBEgrown GaAs/InGaAs VCSEL thresholds is obtained. In 9μm diameter devices we achieve submilliamp thresholds, and milliwatt-level maximum output powers.

The fabrication of an 8*8 independently addressable InGaAs/GaAs vertical-cavity surface-emitting laser array based on planar ion-implantation processes is described. The uniformity of laser characteristics across the array under both pulsed and CW operation is discussed. Simultaneous addressing of multiple lasers is demonstrated.

The polarisation properties of quantum well vertical cavity surface emitting lasers are observed to be significantly different from their edge emitting counterparts. A near threshold, highly linearly polarised TEMoo mode whose direction varies randomly from one laser to the other is obtained. At higher currents, high order modes lase in a linear polarisation orthogonal to the fundamental mode. No polarisation selection rule is observed.

The design, fabrication and characterisation of 1024-element matrix addressable vertical cavity surface emitting laser (VCSEL) arrays are described. A strained InGaAs quantum well VCSEL structure was grown by molecular beam epitaxy and an array of 32×32 lasers was defined using a proton implantation process. A matrix addressing architecture was employed, which enables the individual addressing of each of the 1024 lasers using only 64 electrical contacts. All the lasers in the array, measured after the laser definition step, were operating with fairly homogeneous characteristics; threshold current of 6.8 mA and output quantum differential efficiency of about 8%.

Using the double‐modulation extension of reflectance‐difference spectroscopy, we obtain surface dielectric anisotropy spectra under steady‐state conditions for several reconstructions encountered in molecular beam epitaxy on (001) GaAs and follow their evolution with temperature. The changes observed with temperature are gradual, generally not exhibiting distinct boundaries. Marked changes in anisotropy are observed during thermal desorption of the native oxide, indicating that the process is not simple but involves surface roughening.