Formation Of Subbands Research Articles

GaAs/AlGaAs quantum wires fabricated by SiO2 capping-induced intermixing

We demonstrate that selective intermixing of GaAs/AlGaAs quantum well heterostructures induced by SiO2 capping and subsequent annealing can be spatially localized on a length scale compatible with the lateral confinement of carriers into quantum wires. Low temperature optical spectroscopy measurements including linear polarization anisotropy analysis show evidence of the formation of one-dimensional subbands. A mechanism involving the ability of the thermal stress field generated in the heterostructure by the patterned SiO2 film to pilot the diffusion of the excess Ga vacancies, which are responsible for the enhanced interdiffusion under SiO2 is suggested to account for the high lateral selectivity achievable with this novel process.

Magnetoresistance oscillations in 50-nm-wide GaAs/AlxGa1-xAs multiple-edge quantum wires.

We fabricated a 50-nm-wide GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum-wire structure with the cleaved-edge overgrowth technique and investigated the characteristics of its low-temperature magnetoresistances. A clear Shubnikov--de Haas oscillation was observed and found to contain unexpected oscillatory components. By analyzing the magnetoresistance and its temperature dependence, the formation of one-dimensional subbands was revealed. On the basis of a semiclassical model, the width of lateral confinement was estimated and found to be \ensuremath{\sim}50 nm.

Fabrication of quantum wires by selective intermixing induced in GaAs/AlGaAs quantum well heterostructures by SiO 2 capping and subsequent annealing

Results are presented demonstrating that selective intermixing of GaAs/AlGaAs quantum well heterostructures by SiO 2 capping and subsequent annealing can be spatially localized with a length scale compatible with the observation of lateral quantum confinement effects. Patterning of a 400 nm-thick SiO 2 encapsulation layer deposited by rapid thermal chemical vapor deposition into arrays of wires was performed using high resolution electron beam lithography and subsequent reactive ion etching. After high temperature (850°C) annealing, photoluminescence experiments indicate the creation of double barrier quantum wires when small trenches (< 100 nm) are etched in the SiO 2 film at a period greater than 800 nm. Signatures of the formation of one-dimensional subbands are observed both in photoluminescence excitation spectroscopy and linear polarization anisotropy analysis. A mechanism involving the ability of the stress field generated during annealing at the SiO 2 film edges to pilot the diffusion of the excess gallium vacancies which are responsible for the enhanced interdiffusion under SiO 2 is suggested to account for the high lateral selectivity achievable with this novel process.

Formation of subbands in delta -doped semiconductors.

The formation of the two-dimensional subbands in uncompensated \ensuremath{\delta}-doped semiconductors from single impurity bound states with increasing impurity concentration is calculated using multiple-scattering theory. Previous calculations treated the \ensuremath{\delta} plane as a metallic sheet by solving a one-dimensional Schr\"odinger equation in a self-consistently screened one-dimensional potential and could not reproduce single impurity bound states in the limit c\ensuremath{\rightarrow}0. In contrast to this approximation multiple-scattering theory can describe well both limits of small and large c. The cross over from impurity bands at low concentrations to subbands at large concentrations is discussed and a Mott transition within the \ensuremath{\delta} plane (defined as the vanishing of the gaps in the density of states) is found at ${\mathit{c}}_{\mathrm{cr}}$=9.95\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ in the case of GaAs:Si.

Inelastic light scattering by electrons in GaAs quantum wires: Spin-density, charge-density and single-particle excitations

In inelastic light scattering experiments we observe for the first time clearly resolved one dimensional (1D) intersubband spin-density excitations. Together with new structure at energies close to the 2D intersubband transitions, these observations display the formation of 1D subbands. The depolarization shift ( W dep) and the excitonic shift ( W xc) can be deduced approximately from our experiments. These shifts are of special interest because they are related to the direct and exchange-correlation terms of the electron-electron interaction. We find ratios of the shifts ( W xc/ W dep) of up to 55%.

Demonstration of Lateral p-n Subband Junctions in Si Delta-Doped Quantum Wells on (111)A Patterned Substrates

The lateral subband p-n junction (LSJ) is demonstrated through a Si delta-doped GaAs/AlGaAs multiple quantum well (MQW) structure on patterned (111)A GaAs substrates. The optical properties of the MQW on a (311)A sloped surface and a (111)A flat surface are evaluated by cathodoluminescence. We confirm the subband formation and lateral p-n junctions for Si delta-doped MQWs. The LSJ shows good current-voltage properties when forward currents are injected into it. A recombination emission (791 nm) with an energy level higher than the band gap of bulk GaAs is observed at 300 K; this clearly shows that the potential bending occurs between n-type subbands and p-type subbands. The rise time of light-emitting diodes (LED) with the LSJ (LSJ-LED) is below 5 ns (>200 MHz). The LSJ has a high potential for applications in optical devices.

Effects of band structure and spin in quantum dots.

Band structure and spin can modify the simple two-mode far-infrared spectra of quantum dots. In our investigation of these effects we start from a k\ensuremath{\cdot}p Hamiltonian for the conduction, valence, and spin-orbit split-off bands, with potentials for subband formation and harmonic lateral confinement. Taking into account the nonparabolicity of the conduction band and spin-orbit coupling due to the confining potentials, we calculate the dipole excitation spectra of quantum dots on InSb with one and two electrons. Spin-orbit coupling leads to a level anticrossing in the ${\mathrm{\ensuremath{\omega}}}_{\mathrm{\ensuremath{-}}}$ mode and nonparabolicity modifies the ${\mathrm{\ensuremath{\omega}}}_{+}$ mode demonstrating violation of Kohn's theorem.

X-ray emission study of bonding states in FeTiH x hydrides

Participation of iron and titanium valence states in covalent and ionic components of metal-hydrogen bonding in FeTiH x hydrides has been investigated by means of X-ray emission spectroscopy. Both iron and titanium valence p-states take an active part in the formation of hybrid sub band. At the same time X-ray emission spectra didn't show any significant participation of iron d states in metal-hydrogen bonding. Correlation between the electronic structure and technologically applicable properties of FeTiH x hydrides has been established.

Stepwise optical absorption in amorphous SeTe/CdSe superlattices

Optical absorption and photoconductivity of new chalcogenide amorphous SeTe/CdSe superlattices of pnpn type are investigated. The optical absorption coefficient α changes stepwise with the variation of the intensity and duration of the illumination. The analysis of the experimental results indicates the formation of subbands in the conduction and valence band. The essential role of the indirect transitions at sublayer thickness below 100 Å is discussed.

Optical transitions of a GaSb-AlSb superlattice in the 2–5 eV range

We report reflectance measurements in the range 2–5 eV of a GaSb-AlSb superlattice grown by molecular-beam epitaxy. In the region of the E1 and E1 + Δ1 transitions, the electronic properties can be related to the formation of electronic subbands of a type II superlattice at the L point.

Far-infrared spectroscopy of one- and zero-dimensional electronic systems

We review the far infrared (FIR) response of one- and zero-dimensional electronic systems (0DES and 1DES), quantum wires and quantum dots, respectively, which have been realized by ultrafine mesa etching of modulation-doped AlGaAs/GaAs heterostructures. From dc magnetotransport measurements on quantum wires the formation of 1D subbands with typical energy separations of 1 to 3 meV was found in electron channels of 400 to 150 nm width. However, in the far infrared (FIR) response resonances at significantly higher frequencies were observed as compared to the single particle energy level separation. We will discuss that the optical response exhibits a very complex behaviour which is dominated by collective effects. In the case of a purely parabolic external potential the FIR response can be easily understood as the excitation of a rigid oscillation of the whole electron distribution. Especially in our quantum dot structures we are able to resolve additional excitation modes and a resonant anticrossing, probing the internal motion of the electrons within a single dot. We will discuss these excitations starting from two different models, namely from a classical one, which is based on plasmonic excitations in 2DES of finite size, and, in more details, from a quantum mechanical model, which treats atom-like systems with discrete energy levels including collective corrections.

Quantum wires and quantum dots on indium antimonide

The authors report on the study of quantum wires and dots near the surface of InSb that contain quasi-one-dimensional and zero-dimensional, i.e. discrete, electron systems. Both types of samples are derived from metal-oxide-semiconductor devices and make it possible to tune the electron number by a gate voltage. The laterally confining potential leads to the formation of one-dimensional sub-bands and to discrete levels respectively. Both are examined by far-infrared resonance spectroscopy. In order to attain a sufficiently high absorptance they fabricated the wires and dots by holographic lithography in arrays on macroscopic areas. Resonance energies of about 10 meV due to the lateral quantisation are found to be comparatively large because of the low conduction band mass of InSb. In particular, the influence of the narrow-gap band structure of InSb and of the electron-electron interaction on the quantised levels is discussed.

Quantum interference devices fabricated using molecular-beam epitaxy and ultra-high-resolution electron-beam lithography

Summary form only given. The authors have fabricated various new lateral quantum interference devices (QIDs) using molecular-beam epitaxy and ultra-high-resolution electron-beam lithography and have observed conductance oscillations in these devices. These QIDs have a structure which is similar to that of a conventional MODFET, except for the gate(s), which consists of various nanometer patterns instead of a plain gate. The gate nanometer patterns create electrostatic potential barriers and well(s) in the channel. At low temperatures, as the electron mean-free path approaches or becomes longer than the gate length, quantum interference of the electron wave with the barriers and wells results in the formation of quasi-bound states or subbands, and, as a result, the conductance oscillates as either the gate voltage or the source-drain voltage is scanned. Several novel QIDs are presented with emphasis on a new lateral quantum box transistor (LQBFET), which has a railway track gate. >

CdTeCdMnTe superlattices

We describe the CdTeCdMnTe superlattice, the first and the most extensively studied heterostructure with a diluted magnetic semiconductor. The structures are fabricated by molecular beam epitaxy and evaluated by x-ray diffraction. Optical measurements illustrate the subband formation and the carrier-spin interactions, and magnetic measurements demonstrate the effect of dimensionality on the magnetic phase transition. Recent progress in conductivity control provides opportunities for transport experiments.

Electronic subbands of a delta doping layer in GaAs in a parallel magnetic field.

The ${B}_{?}$ magnetoconductivity in the V-shaped potential well of \ensuremath{\delta}-doped GaAs(Si) shows oscillatory behavior along the ${B}_{?}$ axis at constant ${N}_{s}$. At finite field ${B}_{?}$ the subband levels are raised by a diamagnetic energy shift and may be pushed over the Fermi level. Due to the formation of magnetoelectric subbands the E(k) dispersions are distorted. According to a self-consistent model calculation the features of \ensuremath{\sigma}(${B}_{?}$) are explained in terms of the oscillating density of states at the Fermi level.

Magneto-optical Kerr-effect of uranium compounds: A study of the electronic structure

A comprehensive study of the magneto-optical properties of metallic uranium compounds is presented in the 0.5–6 eV photon energy range. New results about the electronic and the magnetic structure of these materials are derived. The f states are best described in an itinerant model with an occupation of nearly 3 for all investigated materials except the Th 3P 4-structure compounds, although correlation effects are shown to play a dominant role in UTe. A spin-polarized band structure of f and d states in the system UTe-USb-YSb is derived and the competition of f-d and f-p hybridization in this system is discussed. The maintenance of a f 3−f 2 valence transition in the USb-YSb pseudo-binary system is challenged. The magnetic f-d exchange energy is established to be negative for all investigated materials i.e the f and d moments allign antiparallel upon magnetic ordering. This property manifests itself in two magneto-optical effects: Firstly, the conduction electron spin-polarization displays a negative sign and its size is observed to range up to – 100% for certain compounds, which is an extraordinary value for magnetic metals. Secondly, the f → d transition energy displays a magnetic red-shift in the order of 200 meV due to the formation of spin-polarized subbands of the d states which are energetically split by the exchange energy. The size of the magnetic shift is found to depend on the sublattice magnetization rather than on the net moment which results in similar magnitudes for some ferro- and antiferromagnets. This behavior completely differs from the one known up to now for antiferromagnetic semiconductors, but is well understood in terms of the magnetic structure of the uranium pnictides. For most of the materials, the value of the f polarization is calculated. The f moment itself is evidenced to consist of antiparallel spin and orbital contributions with a predominant orbital part in most of the investgated NaCl-structure compounds. Some of the investigated uranium systems seem to be very promising candidates for technical applications due to the by far largest Kerr-rotation ever observed for metals, reaching up to ≈ 9.2° at 65% reflectivity.

Hybrid cyclotron-intersubband resonance in thin InAs layers confined between GaSb

In a heterostructure of InAs confined between GaSb, measurements of far infrared transmission as a function of the magnetic field and its orientation demonstrate the formation of hybrid subbands due to electric and magnetic quantization. The observed energies of resonant transitions with the magnetic field parallel to the layer plane are in quantitative agreement with those calculated theoretically.

Cyclotron resonance and Shubnikov—de Haas experiments in an-InAs-GaSb superlattice

Cyclotron resonance is observed in a semiconducting $n$-InAs-GaSb superlattice by measuring the change of the far-infrared-radiation (FIR) transmission in a magnetic field at three different FIR frequencies. The angular dependence of the cyclotron resonance position is measured and shows a two-dimensional behavior. The resonance observed is attributed to an upwards shift of 145 meV of the InAs conduction band as a consequence of the superlattice subband formation. A simple calculation leads to a value of 195 meV for this shift.

Doping superlattices

The MBE technique has made feasible the growth of periodic semiconductor structures composed of ultrathin layers with alternating p- and n-doping. This kind of superlattice, in contrast to familiar heterostructure systems, does not contain any interfaces, and, therefore, does not show the problems related to those. Doping superlattices, on the contrary, are crystallographically only slightly perturbed by a relatively small amount of dopants (typically 1017 to 1019 cm−3 for the most interesting range). The periodic potential in this type of superlattice is due to the electrostatic potential of fixed ionized impurities, which may be partly compensated by mobile electrons and holes confined to the n- and p-doped layers, respectively. In addition to the features related to the subband formation as known from heterostructure superlattices, crystals with doping superstructure show intriguing pecularities which originate from the fact that electron and hole states are more or less separated from each other in local space. As a first consequence the electron and hole concentration in the crystal can be varied within a wide range either by optical excitation or by carrier injection or extraction via appropriate lateral electrodes. By the internal space charge fields the electrons and holes are always collected in the n- and p-layers, respectively. At sufficiently large superlattice period and low temperatures even large deviations from the equilibrium electron and hole concentration are quasistable. Secondly, the amplitude of the periodic potential, which may amount up to more than half of the energy gap of the pure semiconductor in the ground state, is gradually reduced with increasing carrier concentration by compensation of ionized impurity charges. As a consequence, a semiconductor with doping superstructure, in fact, represents a crystal with variable band gap and with variable carrier concentration. These unique features open a new field for both scientific research and device application. By appropriate choice of doping concentration and superlattice period, for instance, one may ’’design’’ crystals in which the conductivity parallel to the doping layers for both types of carriers may be modulated from zero up to typical values for low resistivity semiconductors (field effect transistor as a bulk device!) crystals with variable optical absorption coefficient for h/ω&amp;lt; and ≳Eg, or with variable photo- or electroluminescence frequency and intensity (tunable infrared laser with the whole bulk as active material and with low threshold intensity), and many kinds of other crystals with unusual properties.

Periodic doping structure in GaAs

A new type of superlattice consisting of a periodic sequence of ultrathin p- and n- doped semiconductor layers, possibly with intrinsic (i-) layers in between, is considered. In addition to the features related to the subband formation, as known from the study of the familiar heterostructure superlattices of the type GaxAl1−xAs, crystals with doping superstructure also show other intriguing pecularities. A semiconductor miconductor with doping superstructure, in fact, represents a crystal whose band gap and carrier concentration can be tuned within wide limits. In the first part of this review, some of the unique features of this class of materials are discussed. In the second part, approaches for the realization of doping structures and nipi superlattices in GaAs by the technique of molecular beam epitaxy are outlined.