Fabrication of GaAs hole array as a 2D-photonic crystal and their application to photonic bandgap waveguide

A two dimensional (2D) GaAs hole array with a high aspect ratio was successfully fabricated by dry etching techniques using a highly ordered alumina membrane as a mask. The reflection spectra of the GaAs hole array shows characteristics of the photonic bandgap (PBG) calculated by using the 2D triangular lattice structure. Various defect lines were formed etching in the GaAs hole array using focused ion beam. The defect-type PBG waveguide was experimentally demonstrated.     

Anisotropic positive magnetoresistance of a nonplanar 2D electron gas in a parallel pagnetic field

Magnetotransport properties of a 2D electron gas in narrow GaAs quantum wells with AlAs/GaAs superlattice barriers were studied. It is shown that the anisotropic positive magnetoresistance observed in selectively doped semiconductor structures in a parallel magnetic field is caused by the spatial modulation of the 2D electron gas.     

Microscopic view of a two-dimensional lattice-Gas ising system within the grand canonical ensemble

A reversible 2D critical transition is observed on the GaAs(001) surface and modeled as a lattice-gas Ising system. Without depositing any material, 2D GaAs islands spontaneously form. The order parameter, four critical exponents, and coupling energies are measured from scanning tunneling microscope images of the microscopic domain structure and correlation functions as a function of temperature and pressure. Unprecedented insight into the domain structure of a 2D Ising system through the critical point and a complete Hamiltonian for modeling the GaAs(001) surface are presented.     

Magnetophonon resonance in a GaAs quantum well with AlAs/GaAs superlattice barriers at high filling factors

The magnetotransport of a high-mobility 2D electron gas in single GaAs quantum wells with AlAs/GaAs superlattice barriers is studied at high filling factors. For the selectively doped structures under study in the temperature range from 10 to 25 K, magnetoresistance oscillations periodic in the inverse magnetic field are observed with their frequency being proportional to the Fermi wave vector of the 2D electron gas. The experimental results are explained by the interaction of the 2D electron gas with leaky interface acoustic phonons.     

Magnetotransport properties of a ballistic ring interferometer on the basis of a GaAs quantum well with a high concentration of 2D electron gas

Magnetotransport properties of ballistic ring interferometers made on the basis of 2D electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers are studied. An asymmetry of magnetoresistance and a phase reversal in h/e oscillations are observed when the bias voltage across the ring exceeds kT/e.     

Semiclassical negative magnetoresistance of a 2D electron gas caused by scattering by short-range and long-range potentials

The magnetoresistance of a 2D electron gas confined in narrow GaAs quantum wells with AlAs/GaAs superlattice barriers is studied in the classical range of magnetic fields. It is shown that the negative magnetoresistance observed in this kind of structures with nonplanar heterointerfaces is semiclassical and qualitatively agrees with the model of negative magnetoresistance due to the scattering of charge carriers by two types of random potential, namely, the short-range and long-range ones.     

Deuterium in In-based quantum wells

The photoluminescence (PL) of high quality InGaAs/GaAs typically shows one strong intrinsic band, due to the heavy-bole free-exciton (HHFE) recombination. After sample irradiation with deuterium, two bands appear at energies below that of HHFE: a deeper band, D, due to radiative recombination at a deuterium-related site, and a shallower band, (D,X), attributed to an exciton bound to the same state. A maximum of the binding energy has been observed as a function of the well width for both states. As the indium molar fraction x increases, the strength of the D band decreases, together with its binding energy, until the band becomes no more detectable at x=0.37. The (D,X) band cannot be resolved for x ≥ 0.19. Deuterium diffusion has no beneficial effect on the PL intensity of InGaAs/GaAs quantum wells. Different is the case of InAs/GaAs quantum wells having well width below 2 monolayers. Although no strain relaxation occurs in the samples, the PL of the virgin samples is typical of structures with a high number of defects. After deuterium diffusion, the PL intensity increases by one to three orders of magnitude, depending whether non-radiative centers or thermal escape of carriers from the well rule the PL efficiency.     

Stress evolution during growth of bilayer self-assembled InAs/GaAs quantum dots

We investigated the stress evolution during molecular-beam epitaxy of bilayer InAs/GaAs(001) quantum dot (QD) structures in real time and with sub-monolayer precision using an in-situ cantilever beam setup. During growth of the InAs at 470 °C a stress of 5.1 GPa develops in the wetting layer, in good agreement with the theoretical misfit stress. At a critical thickness of 1.5 monolayers the strain is relieved by the QD formation. In the case of InAs/GaAs bilayer structures, the second InAs layer grows identical to the first for GaAs spacer thicknesses exceeding ∼13 nm. For thinner spacers the critical thickness for the 2D/3D transition in the second layer decreases. The stress of the second InAs layer does not reach the value of the first, indicating that InAs QDs grow on partially strained areas due to the strain field of the previous InAs layer. PACS 68.35.-p; 68.35.Gy; 68.65.Hb; 81.07.Ta; 81.10.Aj     

Studies on the prospects of GaInAs and GaInAsP for double-drift region heterostructure IMPATTs

Double heterojunctions having the material combinations InP/GaInAs/InP, GaAs/GaInAs/GaAs and InP/GaInAsP/InP have been studied to assess their potential for double-drift region (DDR) IMPATT diodes. An accurate and realistic computer simulation program has been framed and used for the dc and high-frequency analysis of the DDRs. The analysis is carried out both in IMPATT (IMPact Avalanche Transit Time) and MITATT (MIxed Tunnelling Avalanche Transit Time) modes. Our results indicate that the GaAs/GaInAs/GaAs DDR would provide the best mm-wave performance up to sufficiently high frequencies. Further, the performance of the DDR diodes is observed to deteriorate at high frequency of operation due to phase distortion introduced by tunnel injected current, which is found to be the least in the case of GaAs/GaInAs/GaAs DDR leading to the best performance of this DDR amongst the three.     

Spin Hall effect in doped semiconductor structures

In this Letter we present a microscopic theory of the extrinsic spin Hall effect based on the diagrammatic perturbation theory. Side-jump and skew-scattering contributions are explicitly taken into account to calculate the spin Hall conductivity, and we show that their effects scale as sigma(xy)SJ/sigma(xy)SS approximately (h/tau)/epsilonF, with tau being the transport relaxation time. Motivated by recent experimental work we apply our theory to n- and p-doped 3D and 2D GaAs structures, obtaining sigma(s)/sigma(c) approximately 10(-3)-10(-4), where sigma(s(c)) is the spin Hall (charge) conductivity, which is in reasonable agreement with the recent experimental results of Kato et al. [Science 306, 1910 (2004)] in n-doped 3D GaAs system.     

Formation of AlxGa1−xAs periodic array of micro-hexagonal pillars and air holes by selective area MOVPE

A two-dimensional (2D) periodic array having air/semiconductor interfaces can be applied to photonic crystals (PCs), which are expected to control spontaneous emission and optical transports in the next-generation devices. In this paper, we report on the selective area metal-organic vapor phase epitaxial (SA-MOVPE) growth of a Al_xGa_1−xAs 2D periodic array on a GaAs (1 1 1)B substrate for application to 2DPCs having GaAs/AlGaAs heterostructures. Al_xGa_1−xAs (x=0, 0.25 and 0.50) growth was carried out on triangular lattice array of hexagonal GaAs openings and hexagonal SiN_x masks. A uniform Al_0.50Ga_0.50As hexagonal pillar array and a GaAs hexagonal air-hole array with a 1 μm-period were successfully obtained. The important growth parameter for uniform 2DPC structure formation by SA-MOVPE was clarified. Furthermore, we describe the successful demonstration of a 400 nm-period pillar array and an air-hole array, which corresponds to the optical communication wavelength λ=1.3–1.55 μm. The results indicate that SA-MOVPE method is very promising for the formation of uniform semiconductor 2DPCs without the occurrence of process-induced damages.     

Magnetophonon resonances of the two-dimensional electron gas in GaAs/AlGaAs heterostructures

We have studied magnetophonon resonances of the two-dimensional (2D) electron gas at the GaAs/AlGaAs interface in a single interface heterostructure and a superlattice. In magnetic fields up to 30T, one set of oscillations is observed, corresponding to the coupling of 2D electrons with LO phonons of GaAs. The effective mass obtained directly from the magnetic field position of the fundamental resonance, where the Landau splitting equals the bulk phonon energy, and of the next two harmonics is $\text{m}{}^1\text{= (0.071 ± 0.0015)m}{}_0$. A comparison with cyclotron resonance measurements on the same system and with bulk GaAs data gives an upper limit of about 2% for the mass corrections due to polaron effects and due to the confinement of the electron gas.     

Microwave-induced magnetic field state with zero conductivity in GaAs/AlAs Corbino disks and hall bars

The microwave photoconductivity of the 2D electron gas in GaAs/AlAs heterostructures has been investigated at a temperature of 4.2 K in magnetic fields up to 1.5 T. It has been found that the magnetic field state with zero conductivity appears in GaAs/AlAs Corbino disks irradiated by 130.70-GHz microwave radiation. This state was previously observed only in GaAs/AlGaAs Corbino disks with much higher electron mobility and lower density. It has been shown that the microwave photoconductivity measured in high magnetic fields on Corbino disks can significantly differ from the value calculated from the results of the measurements on Hall bars. This difference is explained by the fact that the conditions of the appearing magnetoplasmons that affect the magnitude and character of the microwave photoconductivity (photoresistance) in the Corbino disks are nonequivalent to those in the Hall bars.     

Strongly enhanced hole-phonon coupling in the metallic state of the dilute two-dimensional hole gas

We have studied the temperature dependent phonon emission rate P(T) of a strongly interacting (r(s) > or =22) dilute 2D GaAs hole system using a standard carrier heating technique. In the still poorly understood metallic state, we observe that P(T) changes from P(T) approximately T5 to P(T) approximately T7 above 100 mK, indicating a crossover from screened piezoelectric (PZ) coupling to screened deformation potential (DP) coupling for hole-phonon scattering. Quantitative comparison with theory shows that the long range PZ coupling between holes and phonons has the expected magnitude; however, in the metallic state, the short range DP coupling between holes and phonons is almost 20 times stronger than expected from theory. The density dependence of P(T) shows that it is easier to cool low-density 2D holes in GaAs than higher density 2D hole systems.     

Properties of vanadium in InP

Experiments are performed on InP/V single crystals extracted from a polycristalline ingot grown in a two zones gradient freeze furnace; in photoluminescence (PL) we observe a band with two zero phonon lines at 5692.5 cm^-1 and 5705.5 cm^-1. Photoluminescence excitation (P.L.E.) spectra have also been observed. This type of luminescence spectra has already been observed in V doped GaAs and GaP and attributed to an internal transition of V²⁺ (3 d³). D.L.T.S. and O.D.L.T.S. experiments do not reveal any deep level in the forbidden band gap. These two results seem to indicate a puzzling behaviour of V in InP as in GaAs which is discussed in the framework of a model proposed for GaAs/V : the V²⁺ ground state has a large relaxation energy and so, the excited states giving rise to the P.L. and P.L.E. spectra are not resonant with the conduction band.     

Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE

Self-assembled InAs quantum dots (QDs) with high-density were grown on GaAs(0 0 1) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many two-dimensional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.     

Excitons in GaAs quantum wells

This paper attempts to summarize some of the salient properties of excitons in GaAs quantum wells and in doing so it will emphasize work at AT&T Bell Labs with which the authors have been associated. Although the text relies heavily on published material, an effort has been made to stress new material, and where feasible, unpublished aspects, e.g., figures, related to earlier work. Topics discussed on the quasi-2D excitons in GaAs quantum well include: their inherent tendency for intrinsic free-exciton emission, exciton binding energies, bound and localized excitons including biexcitons and excitons bound to neutral impurities, effects of n- and p-type modulation and antimodulation doping, and the developments leading to a proposed set of quantum well parameters that results in acceptable fits to the observed exciton transitions for GaAs quantum wells with both square and parabolic potential profiles.     

Transformation of the dimensionality of excitonic states in quantum wells with asymmetric barriers in an electric field

A transformation of the dimensionality of excitonic states from 2D to 3D with increasing external electric field is observed in single GaAs/Al_xGa_1−x As quantum-well structures with asymmetric barriers. The binding energy of a 2D exciton remains constant over a wide range of variation of the field, since the decrease in the binding energy is compensated by increasingly larger penetration of the electronic wave function into the barrier layer, where the exciton binding energy is higher because the effective mass is larger and the dielectric constant of AlGaAs is lower than that of GaAs. When the maximum of the electron wave function is displaced into the barrier as the field increases, the exciton binding energy decreases. As the field increases further, a 2D exciton transforms into a quasi-3D exciton, with a heavy hole in the quantum well and an electron in a resonant above-barrier state. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 207–211 (10 February 1998)     

Optical transitions involving excitons in quantum wires

Il Nuovo Cimento D - We present a comprehensive discussion of the excitonic properties of V-shaped GaAs and InGaAs quantum wires grown on patterned substrates. Systematic linear and non-linear...     

The influence of local phonon modes in a wide-band matrix on the tunnel current-voltage characteristics of quasi-zero-dimensional structures

Experimental results on the visualization of the density of states in InAs/GaSa(001) quantum dots that were obtained by tunnel atomic-force microscopy in an ultrahigh vacuum are presented. A one-dimensional (1D) model of dissipative quantum tunneling is proposed for describing experimental current-voltage characteristics of a tunnel contact between an atomic force microscope probe and the surface of InAs/GaAs (001) quantum dots. It was found that the influence of two local modes of the wide-band matrix on the probability of 1D dissipative tunneling leads to the appearance of several randomly spaced peaks in the field dependence. It was shown that the theoretical dependence agrees qualitatively with experimental the current-voltage characteristic of the atomic force microscope tip and the surface of InAs/GaAs(001) quantum dots.