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Epitaxial metallic films grown on semiconductor substrates are most attractive when they consist of materials which are thermodynamically stable with respect to the underlying semiconductor. This allows, under the proper growth conditions, the overgrowth of epitaxial semiconductor layers, thus forming monocrystalline semiconductor/metal/semiconductor heterostructures with exciting physics and device possibilities. This paper discusses our recent work in the growth of stable and epitaxial metallic layers which meet these stringent criteria on III-V semiconductors. Work on layers sharing either a common group III element with the III-V, such as AlAs/NiAl/AlAs or a common group V, such as GaAs/ErAs/GaAs, will be presented. The potential applications are further enhanced when the metallic films display an added functionality such as ferromagnetic behavior, demonstrated in epitaxial MnAl/AlAs/GaAs heterostructures. Examples of new directions for III-V structures made possible by such heteroepitaxy, are discussed. They include tailoring of Schottky barrier heights, integration of metallic quantum wells into high-speed three-terminal devices, and integration of magnetic storage functionality with III-V electronics.

We have optically investigated quantum dots containing a few electrons each. In zero magnetic field, the photoluminescence (PL) should reflect the occupied density of states of such artificial atoms. However, the few-electron response to recombination of one of these electrons with a localized hole appears to be enhanced as the energy of the electrons approaches the highest occupied state. This phenomenon resembles the Fermi-edge singularity observed in the PL spectra from n-type two- and one-dimensional electron systems. Like in the case of doped n-type quantum wires, this singularity is seen to dominate the PL spectrum of the quantum dots at a low magnetic field with a sharp fall in intensity at a specific magnetic field.

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.

Photoluminescence spectroscopy has been used to probe the occupied electron states below the Fermi energy of zero-dimensional electron systems (0DESs) in both zero and finite magnetic fields. The arrays of modulation-doped quantum dots investigated were fabricated by both reactive-ion etching and strain-confining GaAs heterojunctions with a δ-layer of Be present in the GaAs, in order to improve luminescence efficiency. For the etched quantum dots we show that the low magnetic field dispersion (B

We have studied carrier transfer to strain confined dots from a host quantum well, using selectively excited time-resolved photoluminescence spectroscopy. We find that the photoluminescence decay rate after picosecond excitation is much slower when excitation is indirect, via transfer from the quantum well, then when it is directly into the dot, even though the decay of the quantum well photoluminescence is fast. This result is inconsistent with a simple three level kinetic model of the transfer process. We propose a tentative model involving an intermediate nonradiative state, possibly the charge separated state predicted theoretically.

By studying the temperature dependence of the photoluminescence intensity in strain-confined GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wires and quantum dots, we show that as dimensionality is reduced from two-dimensional (2D) through 1D to 0D, there is no reduction of luminescent efficiency at low temperature, and that high quantum efficiency persists to significantly higher temperature. There is efficient spatial energy transfer from the 2D region to the 1D or 0D region. This transfer increases with temperature, showing that there is a barrier to transfer a few meV high. This barrier is lower than theoretically predicted. For above band-gap excitation there is substantial ``capture transfer'' in which unthermalized carriers or excitons transfer even at very low temperature. Exciton localization due to the well-width fluctuations of the host quantum well also plays an important role in determining the temperature dependence of the exciton transfer in these structures.

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.

We have observed lateral exciton confinement at the GaAs/AlxGa1-xAs interface. The confinement is achieved in both the growth and lateral directions by the strain potential under an amorphous carbon stressor. The potential welt varies from 15 meV to 40 meV for different stressor sizes. We have also made transient measurements on this structure, which show efficient exciton transfer from the bulk GaAs to the confined region.

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.

New ultrathin epitaxial magnetic multilayers consisting of ferromagnetic τMnAl and nonmagnetic NiAl have been prepared on GaAs by molecular beam epitaxy. The magnetic and magneto‐optic properties of these multilayers and the effect of incorporating nonmagnetic NiAl into the structure have been investigated. Perpendicular magnetization was confirmed and the remnant magnetization and coercivity were improved compared to τMnAl thin films. In addition, by systematically changing the thickness of the NiAl layer the magnetic properties could be controlled. We discuss these issues and compare the magnetic and magneto‐optic properties of these structures with other epitaxial magnetic structures.

We have studied magnetic and magnetotransport properties of two kinds of new epitaxial magnetic superlattices, MnGa/NiGa and MnAl/NiAl, consisting of ferromagnetic tetragonal MnGa (MnAl) and non-magnetic CsCl-type NiGa (NiAl), grown by molecular beam epitaxy (MBE) on (001) GaAs substrates. Strong perpendicular magnetization is evidenced in both types of superlattices, with the values of remanent magnetization higher than those of MBE-grown MnGa and MnAl thin films reported previously. At room temperature, the hysteresis loops in the extraordinary Hall effect (EHE) measurements on the MnGa/NiGa superlattices exhibit evenly-spaced multiple steps, while the MnAl/NiAl superlattices show rectangular single-stepped EHE hysteresis loops with nearly perfect squareness. The capability of growing these kinds of new epitaxial magnetic superlattices gives a new degree of materials design of the magnetic thin films on GaAs substrates, offering the possibility of integrating magnetic devices with III–V semiconductor electronics and photonics.

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...

The asymmetry in the one-phonon Raman lines of a Si:GaAs δ-doped doping superlattice is interpreted as a quantum-mechanical interference process in which an incident photon is inelastically scattered by a resonance excitation composed of a bulk LO one-phonon state mixed to a continuum of electron intersubband transitions. The dependence of the line shape on the frequency as well as on the polarisations relative to the crystal axis of the incident and the inelastically scattered radiation is produced by the difference in Raman-scattering amplitudes associated with each component of the mixed excitation

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.

Some of the earliest theoretical speculation, stimulated by the growth of semiconductor superlattices, focused on novel devices based on vertical transport through engineered band structures; Esaki and Tsu promised Bloch oscillators in narrow mini-band systems and Kazarinov and Suris contemplated electrically stimulated intersubband transitions as sources of infrared radiation. Nearly twenty years later these material systems have been perfected, characterized and understood and experiments are emerging that test some of these original concepts for novel submillimetre wave electronics. The authors describe recent experiments on intersubband emission in quantum wells stimulated by resonant tunnelling currents. A critical issue at this time is devising a way to achieve population inversion. Other experiments explore 'saturation' effects in narrow miniband transport. Thermal saturation may be viewed as a precursor to Bloch oscillation if the same effects can be induced with an applied electric field.

Two different types of indirect two-dimensional exciton systems, a real-space indirect coupled quantum well and u new type of real- and k-space indirect GaAs/AlAs quantum structure, are studied hy steady state and time resoved optical spectroscopy. No clear evidence for exciton condensation is found. In the new structure dramatic narrowing of the exciton linewidth in the indirect regime is found and shown to originate from intrinsic confinement effects. A nonmonotonic behavior of the intensity of the indirect photoluminescence as a function of Γ-X separation is tentativily attributed to zone-edge phonon emission.

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.

A compact and ultrafast holographic memory using a recently developed surface-emitting microlaser diode array is described. The system does not require a bulky laser, spatial light modulators, or beam deflectors. The memory is capable of retrieving a high-resolution page (approximately 105 bits) in less than 1 nsec.

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.

We have created one-dimensional quantum wells (quantum wires) by laterally straining a GaAs quantum well with patterned carbon stressors 180 nm in width. We find four well-resolved one-dimensional subbands in the excitation spectra, whose constant spacing of 2.4 meV confirms quantitatively that there is quantum confinement in the parabolic well predicted by continuum elasticity theory. The lateral width of the electron ground state is 35 nm

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.

The dynamic, polarization, and transverse mode characteristics of strained InGaAs-GaAs quantum well vertical cavity surface emitting lasers (VCSELs) emitting at 0.98 mu m are investigated. The dynamic behavior of VCSELs with high and low operating voltages and series resistances is compared. A large wavelength chirp in the lasing spectrum was observed for the lasers with high voltage/resistance, even under low-duty-cycle pulse operation. This is thought to be due to resistive heating close to the laser junction. It is observed that the transverse mode structure of VCSELs and their dependence on laser dimensions and drive current are highly analogous to those of edge emitting lasers, whereas the polarization characteristics of the two types of lasers are significantly different. >

Techniques to achieve wavelength multiplexing and tuning capabilities in vertical-cavity surface-emitting lasers (VCSELs) are described, and experimental results are given. The authors obtained 140 unique, uniformly separated, single-mode wavelength emissions from a 7*20 VCSEL array. Large total wavelength span ( approximately 430 AA) and small wavelength separation ( approximately 3 AA) are obtained simultaneously with uncompromised laser performance. All 140 lasers have nearly the same threshold currents, voltages, and resistances. Wavelength tuning is obtained by using a three-mirror coupled-cavity configuration. The three-mirror laser is a two-terminal device and requires only one top contact. Discrete tuning with a range as large as 61 AA is achieved with a small change in drive current of only 10.5 mA. >

Epitaxial ferromagnetic films of τ-MnAl in thickness from 3 to 60 nm have been successfully grown on AlAs/GaAs heterostructures using the molecular beam epitaxy (MBE) growth technique. This magnetic phase is a non-equilibrium metastable structure, stabilized in part by its close epitaxial registry with the underlying III–V semiconductor. The growth window is very narrow due to the metastability of the τ-phase. Growth has been achieved through the use of predeposited MnAl template layers of 1–3 bilayers, which effectively separate the nucleation and growth steps. Template formation and subsequent metal growth are monitored by reflection high energy electron diffraction (RHEED). RHEED has also been used to monitor preliminary attempts at III–V semiconductor overgrowth on the magnetic MnAl films. The data indicate that the overgrowth proceeds in a three-dimensional island growth mode of single crystal III–V semiconductor.

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 two-dimensional (2-D) surface emitting laser array emitting 140 unique, nonredundant, uniformly separated, single-mode wavelengths in the 980-nm regime is described. The wavelength separation between neighboring lasers is as small as 0.3 nm. A large total wavelength span of 43 nm was obtained without compromising the performance of the lasers. All 140 lasers have nearly the same threshold currents, voltages, and resistances. The techniques used are generic and can be readily extended to both longer and shorter wavelength lasers. The authors also report the first wavelength division multiplexing system experiment using part of this laser array. A BER (bit-error ratio) of 10/sup -9/ at 155 Mb/s was obtained with simultaneous operation of four lasers at a wavelength separation of 1.5 nm. Negligible optical and electrical crosstalk was observed between the lasers. >

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

During the past decade, nonequilibrium techniques have been developed for the growth of epitaxial semiconductors, insulators, and metals which have led to new classes of artificially structured materials. Structures can now be grown which present the materials scientist with systems that exhibit new properties and demonstrate new physical concepts. For example, quantum-well structures with molecular dimensions give rise to new phenomena resulting from quantum mechanical effects. Layered structures with periodicity of a few atomic layers result in coherent behavior for long-range interactions such as magnetism in metallic systems. Metastable structures can be generated which possess important properties not present in equilibrium systems. Studies of these materials are leading to significant advances in our basic understanding of the physics of materials as well as to important new technologies. Despite the rate of progress and the large number of laboratories throughout the world with active programs in various aspects of epitaxial growth, our current understanding of the processes which control growth at a fundamental, atomic level is remarkably primitive. Much of the work to date has been driven by the motivation to produce high quality materials for high performance electronic devices. As a result, most of the effort in epitaxial materials hasmore » concentrated on semiconductors, particularly GaAs and related compounds.« less

We describe the molecular beam epitaxial growth of NiAl metallic quantum wells as thin as 1 nm buried in III–V semiconductor heterostructures with emphasis on the growth conditions that yield monocrystalline NiAl films with atomically abrupt interfaces. Cross-sectional transmission electron microscope images show that the AlAs/NiAl interfaces contain {100} terraces with ledges of height equal to the NiAl{100} lattice spacing, d, of ∼ 0.29 nm. On the basis of this observation it is argued that the local thickness of an ALAs-clad NiAl quantum well is quantized in units of d. Furthermore, since the de Broglie wavelength of an electron 1–2 eV above the Fermi level in NiAl is nearly commensurate with the lattice at λ = 4d 3 , the calculated subband energy minima, En, are evenly interleaved such that a composite energy level diagram for a range of well thicknesses displays characteristic energy gaps (for a given in-plane component of the crystal momentum) of ΔE n 2 . For a nominal well thickness of 3–3.5 nm, ΔE n 2 is ∼ consistent with our room-temperature observations of negative-differential-resistance regions in the current-voltage characteristics of AlAs-clad NiAl quantum wells with nominal thicknesses in this range. The large intersubband separation ( ∼ 1 eV) and large electron confinement energies (> 1 eV) of metallic quantum wells offer the possibility of quantum-effect electronic and photonic devices that operate at temperatures and wavelengths that are inaccessible to semiconductor quantum well devices.

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.

We describe a nonresonant tunneling structure that exhibits an intrinsic optoelectronic bistability. Switching in a closed hysteresis loop is controled either by optical excitation or by electrical field. The two states between which switching occurs involve different levels of charge accumulation in a quantum well. The internal positive feedback required for intrinsic bistability arises from a transition between low field vertical transport and hot carrier transport.

Long before the advent of nanofabrication and quantum-effect devices, the technological limitations imposed by polycrystalline, multiphase and thermally unstable contacts to III-V semiconductors were of concern to forward-looking materials scientists. In the early 1980s, efforts to elucidate the complex behaviour of reactive metal/III-V systems were initiated. These early efforts evolved slowly and culminated in the recent achievement of stable and epitaxial metallizations to III-V semiconductors. In this review, we first describe the criteria that must be met for the fabrication of metal/III-V heterostructures. Bulk phase equilibria are useful guides for selecting metal/semiconductor combinations which will not react during growth at moderate temperatures or during subsequent processing steps. We show, however, that phase stability is not sufficient for the fabrication of ultrathin metal overlayers or buried metal heterostructures. Growth conditions must be carefully optimized and combined with the appropriate selection of metallic phases with high melting points in order to suppress the strong tendency for island formation during growth and film agglomeration during overgrowth or processing. In our discussion of metal/semiconductor hetero-structures we highlight the relationship between symmetry differences and defects (domain boundaries) with particular emphasis on semiconductor overlayers grown on high-symmetry metals. Our work and that of others has shown that stable and epitaxial metallizations to III-V semiconductors as well as more complex metal/III-V heterostructures can be achieved with two classes of metallic materials; the transition-metal gallides and aluminides with the CsCl structure (TM-III) and the rare-earth monopnictides with the NaCl structure (RE-V). We discuss and compare the growth of these III-V/TM-III/III-V and III-V/RE-V/III-V heterostructures by molecular beam epitaxy, focusing special attention to the initial stages of growth of metallic films on III-V substrates and III-V overlayers on metallic films. Going beyond the strictly materials issues, we describe the electrical properties of such heterostructures, including stable enhanced-barrier Schottky contacts and semiconductor-clad metallic quantum wells, structures which may be the basis for exciting and novel electronic, photonic and magnetic devices.

We have studied the magnetic, magneto-optic, and structural properties of Mn1−xNixAs films grown by molecular beam epitaxy on GaAs(001). The compositions are x=0.1, 0.2, 0.3 with a thickness about 100 nm. In the magneto-optical Kerr effect, Mn0.9Ni0.1As displays distinct longitudinal and polar Kerr rotations. At x=0.2, the effects are smaller, and are undetectable at x=0.3. Magnetometry yields a linear dependence of magnetization on Ni concentration. X-ray diffraction shows a gradual transition from hexagonal to orthorhombic structure. In contrast, bulk Mn1−xNixAs alloys at 300 K are paramagnetic and orthorhombic for x>0.02 and transform to an hexagonal phase at x=0.22.

Effects on magnetic and optical properties of perpendicular electric fields up to 1 MV/m were studied on thin ferromagnetic films of MnAs grown epitaxially on GaAs(001) substrates. Film thicknesses were 20, 50, 100 and 200 nm. With positive electric field corresponding to positive polarity of the MnAs film with respect to the GaAs substrate, it was found that at a field magnitude of 1 MVlm: (a) the magnetizations of the 20 and 50 nm films decreased by 3.5 and 1 percent, respectively, under both positive and negative fields as measured by vibrating sample magnetometry; (b) the magnetooptical Kerr effect (MOKE) of the 20 nm samples decreased by 3.5 percent in negative field but was unchanged by positive field; (c) the index of refraction of the 20 nm films determined by ellipsometry increased by 1 percent in positive field but was unchanged by negative field. No effects were found in thicker films, indicating the observed effects arise from the region of MnAs near the substrate. The MOKE signal of the 20 nm films has a power law dependence on electric field with an exponent of 0.36 with a standard deviation of 0.04. The intensity of the (10-10) x-ray diffraction peak from the ferromagnetic, hexagonal MnAs phase of a 20 nm film decreased by 5 percent in both positive and negative fields of 1 MV/m; however, there was no corresponding increase in the nonmagnetic, orthorhombic phase.

We have reduced the voltage required for threshold in vertical cavity surface emitting lasers (VCSELs) to 1.7 V. Molecular beam epitaxy (MBE) was used to grow 30 pairs of n-doped Al_(0.15)GaAs/AlAs bottom mirrors and the active p-n junction with a 1 μm p-doped top contact. 12 pairs of alternating SiO_2/Si_3N_4 layers formed a high-reflectivity mirror which was used to complete the laser cavity. We have evaluated these reactive sputter-deposited mirrors by using finesse measurements in resonator structures, and obtain reflectivities of above 98% in 9.5 pairs. Individual laser elements were defined by ion etching through the p-n junction, followed by planarization with SiO_2 to define the current path. Then, Au-Zn p-contacts were deposited and alloyed for lateral current injection. Finally, another ion milling step was used to isolate individual contacts. Lasers with widths ranging from 7.5 μm to 25 μm were fabricated and measured.

Vertical‐cavity surface emitting lasers (VCSELs) were grown by molecular beam epitaxy over openings in a SiN4 mask, deposited on a GaAs wafer. We obtained ready to use lasers and laser arrays, laterally defined by the polycrystalline, highly resistive material grown over the SiN4 laser. The performance of the lasers (e.g., 20×20 μm2 by size) were similar to that of VCSELs defined by post growth processing methods. Special transverse modes characteristics and a well defined polarization were observed in square shaped lasers and it was attributed to selective growth induced anisotropy.