Semi-insulating GaAs Substrates Research Articles

Invalidity of graded buffers for InAs grown on GaAs (0 0 1)—A comparison between direct and graded-buffer growth

We investigated the effects of InAlAs graded-buffer growth for InAs layers in association with a fundamental mechanism of reduction in threading dislocation (TD) density by the graded buffers. Four InAs layers were grown on semi-insulating GaAs (0 0 1) substrates by direct and graded-buffer growth using molecular beam epitaxy. The materials were characterized by using atomic force microscopy (AFM), plan-view transmission electron microscopy (PTEM), and X-ray diffraction (XRD). We found that the graded-buffer growth is ineffective for the InAs on GaAs; it is invalid to reduce the InAs near-surface TD density in comparison with the direct growth, despite its effectiveness for InGa(Al)As below the intermediate indium contents. This can be attributed to a driving force of upward moving of TDs above the intermediate indium contents due to alloy hardening, while the direction of the driving force is downward below the intermediate indium contents.

Ferromagnetism in laser-deposited GaMnAs layers

The properties of the GaMnAs layers grown by laser deposition on semi-insulating GaAs(100) substrates at temperatures from 300 to 650°C have been investigated. A strong anisotropy of the hysteretic curve of the angle of rotation of the plane of polarization with a change in the magnetic field direction in the sample plane was found during investigation of the magneto-optical Kerr effect (300 K). The domain structure in GaMnAs layers has been observed for the first time at room temperature by magnetic-force microscopy.

Influences of As flux on the lattice constants, magnetic and transport properties of (Ga, Mn)As epilayers

A series of (Ga, Mn)As epilayers have been prepared on semi-insulating GaAs (001) substrates at 230 ∘C by molecular-beam epitaxy under fixed temperatures of Ga and Mn cells and varied temperatures of the As cell. By systematically studying the lattice constants, magnetic and magneto-transport properties in a self-consistent manner, we find that the concentration of As antisites monotonically increases with increasing As flux, while the concentration of interstitial Mn defects decreases with it. Such a trend sensitively affects the properties of (Ga, Mn)As epilayers.

Hot Wall Epitaxy(HWE)법에 의한 AgGaSe2단결정 박막 성장과 가전자대 갈라짐에 대한 광전류 연구

Single crystal layers were grown on thoroughly etched semi-insulating GaAs(100) substrate at with hot wall epitaxy (HWE) system by evaporating source at . The crystalline structure of the single crystal thin films was investigated by the photoluminescence and double crystal X-ray diffraction (DCXD). The carrier density and mobility of single crystal thin films measured with Hall effect by van der Pauw method are , at 293 K, respectively. The temperature dependence of the energy band gap of the obtained from the absorption spectra was well described by the Varshni`s relation,

In As ∕ Al Sb high-electron-mobility transistors by molecular-beam epitaxy for low-power applications

In As ∕ Al Sb high-electron-mobility transistor technology has transitioned from research to development stages in recent years. Development efforts at Northrop Grumman Space Technology, in collaboration with the Naval Research Laboratory and the University of California, Los Angeles, have focused on X-band and W-band low-noise amplifier monolithic millimeter-wave integrated circuits fabricated for applications requiring ultralow-power dissipation. The materials for the circuits discussed in this article were grown at Northrop Grumman Space Technology on 3-in.-diameter semi-insulating GaAs substrates by molecular-beam epitaxy. Atomic-force microscopy of the as-grown surface on each wafer showed that the rms roughness for all of the wafers ranged between 0.5 and 3.5nm, and this range of roughness was fully compatible with the fabrication process. The high electron mobility that InAs can provide was achieved reproducibly in these materials. It was maintained almost always above 25000cm2V−1s−1, and in several cases even exceeded 30000cm2V−1s−1. The associated electron sheet concentration ranged between 1.2×1012 and 1.8×1012cm−2. These combined mobilities and sheet concentrations gave corresponding sheet resistances in the range of 170±40Ω∕sq, with nonuniformity below 6% over these 3-in.-diameter wafers. These materials characteristics enabled successful fabrication of several recently published X-band and W-band low-noise amplifier circuits, and figures of merit for the circuits that were made specifically from these materials are referenced in this article.

A comparison between contactless electroreflectance and photoreflectance spectra from n-doped GaAs on a semi-insulating GaAs substrate

Contactless electroreflectance (CER) and photoreflectance (PR) measurements have been performed on samples with the structure of an n-doped GaAs epitaxial layer on a semi-insulating GaAs substrate. Modulated reflectance signals from the n-GaAs surface and those from the n-GaAs/SI-GaAs interface are superposed in PR spectra. For the case of CER measurement, however, Franz–Keldysh oscillations (FKOs) from the interface, which are observed in PR spectra, cannot be detected. This discrepancy is attributed to different modulation mechanisms of CER and PR. In CER experiments, the electric field modulation cannot be added to the interfacial electric field because of the effective screening by the fast response of carriers across the interface. FKOs from the interface without any perturbation by the surface signals are extracted by subtracting CER spectra from PR spectra.

Technology and properties of a vector hall sensor

Symmetrical four-sided ∼12-μm-high pyramids with 30°-tilted sides were revealed by the etching of semi-insulating (1 0 0) GaAs substrates in 1H 3PO 4:×H 2O 2:8H 2O at ∼25 °C via sacrificial 〈0 0 1〉-oriented Ti/GaAs/AlAs (100/2000/100 nm) etching mask patterns. The pyramids, MOCVD overgrown with InGaP/AlGaAs/GaAs heterostructure pyramids, were used as the base for magnetic field vector sensors. Each sensor consisted of three Hall probes defined on the sides of a pyramid. The device processing was realized via AZ5214-E layers deposited conformally over the pyramids by draping from water surface. While the planar reference 5×5-μm 2-sized Hall probes exhibited a sensitivity of ∼930 VA −1T −1 at 298 K, the sensitivity of those on the 30°-tilted facets was impossible to determine because they had a resistance of ∼100 kΩ at 298 K. Further work is necessary to optimize the InGaP/AlGaAs/GaAs heterostructure growth and dopant incorporation on the 30°-tilted pyramidal facets.

4.9-GHz low-phase-noise transformer-based superharmonic-coupled GaInP/GaAs HBT QVCO

The low-phase-noise GaInP/GaAs heterojunction bipolar transistor (HBT) quadrature voltage controlled oscillator (QVCO) using transformer-based superharmonic coupling topology is demonstrated for the first time. The fully integrated QVCO at 4.87GHz has phase noise of -131dBc/Hz at 1-MHz offset frequency, output power of -4dBm and the figure of merit (FOM) -198dBc/Hz. The state-of-the-art phase noise FOM is attributed to the superior GaInP/GaAs HBT low-frequency device noise and the high quality transformer formed on the GaAs semi-insulating substrate.

Effect of substrate surface defects and Te dopant concentration on crystalline quality and electrical characteristics of AlGaAsSb epitaxial layers

The influence of GaSb substrate surface defects such as native oxides on the crystalline quality of epitaxial layers was investigated using transmission electron microscopy (TEM). Cross-sectional TEM imaging showed that there are discrete defects at the GaSb-substrate/epilayer interface. Secondary ion mass spectroscopy (SIMS) results revealed high oxygen concentration at the interface, indicating that the defects are likely oxides and presumed to be native oxides since other impurities were not detected. High-resolution TEM micrographs showed that the subsequent growth of the epilayer continues beyond the defects without any additional defect generation or propagation. Tellurium-doped AlGaAsSb epitaxial layers were grown lattice-matched on GaSb substrates and lattice-mismatched on semi-insulating GaAs substrates by organometallic vapor phase epitaxy. SIMS and Hall data showed that the ratio of carrier concentration to Te concentration decreases significantly when the carrier concentration increases from 2.5×10 17 to 6.5×10 17 cm −3. TEM imaging showed that the material with heavily doped Te generates a high density (about 10 8 cm 2) of planar defects (stacking fault) located on (1 1 1) planes. Most of the Te-related defects originate at the GaSb buffer layer/AlGaAsSb epilayer interface. In addition, discrete precipitates were observed in the heavily doped AlGaAsSb layer.

Tunneling through MnAs particles at a GaAs p+n+ junction

In this article we examine tunneling through MnAs particles at a GaAs p+n+ junction. We grew the device structures by molecular beam epitaxy on semi-insulating GaAs (001) substrates, with the n+(5×1018cm−3Si) and p+(2×1019cm−3Be) layers grown at 580°C. At the p+n+ junction, we grew a 30nm layer of random alloy Ga1−xMnxAs at 250°C. In situ annealing the Ga1−xMnxAs transforms to thermodynamically stable MnAs particles in a GaAs matrix. Magnetization measurements show that the MnAs particles are superparamagnetic with a distribution of blocking temperatures that depends on the annealing protocol. The MnAs particles at the interface are imaged using atomic force microscopy of selectively etched, MnAs-topped nanocolumns. Current-voltage (IV) scans show that the presence of particles increases the forward bias current density. Low-temperature current-voltage (IV) scans confirm an increase in the forward bias current density due to tunneling through MnAs particles.

Design and fabrication of InAs/GaAs quantum-dot resonant-cavity avalanche photodetectors

Abstract InAs/GaAs quantum dot (QD) resonant-cavity avalanche photodetectors (RC-APD) operating at 1 μm were designed and fabricated. The structure of RC-APD was designed such that it would resonate with the light incident from the top mesa. The RC-APD was grown by solid-source molecular-beam epitaxy on a semi-insulating GaAs substrate. The area of the top mesa was 10 × 10 μm2 and the absorbing region consisted of a 2.1 monolayer (ML) InAs QD layer sandwiched by two 75.5 nm-thick GaAs spacers. One of the RC-APDs exhibited a large gain factor M ∼100 and a low dark current of ∼10 nA at a breakdown voltage of ∼20 V.

Preparation and AFM characterization of self-ordered porous alumina films on semi-insulated gaas substrate

Self-ordered porous alumina films on a semi-insulated GaAs substrate were prepared in oxalic acid aqueous solutions by three-step anodization. The I–t curve of anodization process was recorded to observe time effects of anodization. Atomic force microscopy was used to investigate structure and morphology of alumina films. It was revealed that the case of oxalic acid resulted in a self-ordered porous structure, with the pore diameters of 60–70 nm, the pore density of the order of about 10 10 pore cm −2, and interpore distances of 95–100 nm. At the same time the pore size and shape change with the pore widening time. Field-enhanced dissolution model and theory of deformation relaxation combined were brought forward to be the cause of self-ordered pore structure according to I–t curve of anodization and structure characteristics of porous alumina films.

Magnetic and transport properties of Ge : Mn granular system

Ge : Mn granular thin films were fabricated on semi-insulating GaAs(001) substrates by molecular beam epitaxy. Transmission electron microscopy and Raman study showed that the sample has a granular structure consisting of GeMn crystallites in a Ge polycrystalline host matrix. A Curie temperature of ∼ 300 K was observed in the magnetization measurement, suggesting that the granules are Mn 5Ge 3. The granular nature of the material was also revealed clearly in the differential conductance vs. bias voltage curves. The unique conductance versus bias voltage curve suggests that the electrical transport is determined by Schottky barriers at the nanoparticle/host matrix interface. This type of material might be useful for studying spin-injections from metallic magnetic nanostructures to semiconductors.

Transport and magnetic properties of Mn- and Mg-implanted GaAs layers

Mn-doped GaAs layers were fabricated by direct ion implantation into semi-insulating GaAs (1 0 0) substrates. The implanted samples were annealed at temperatures T a = 700 – 800 ° C . At these temperatures MnAs clusters are formed in GaAs due to decay of the supersaturated solid solution of Mn in GaAs. Additional Mg ion implantation was used to provide an enhancement of p-type doping in (Ga,Mn)As layers. Temperature dependence of resistance was measured between 4.2 and 300 K, and the Hall effect was measured at temperatures of 4.2–200 K. Anomalous Hall effect and ferromagnetic behavior have been found for all samples. An enhanced positive magnetoresistance was also observed at T > 30 K .

Structural and optoelectrical properties of ZnO thin films deposited on GaAs substrate by metal-organic chemical vapour deposition (MOCVD)

ZnO thin films were deposited on semi-insulating (0 0 1) GaAs substrates at different growth conditions by metal-organic chemical vapour deposition. The effect of growth temperature and ambient oxygen partial pressure on the properties of the ZnO films was studied. It was found that these two important parameters affected the structure, crystallinity, surface morphology, and optical and electrical characteristics of the ZnO films. At the present growth conditions, the full width at half maximum of a ZnO (0 0 2) diffraction peak decreased to 0.185° at 610 °C under an oxygen partial pressure of 45 Pa. By increasing the oxygen partial pressure from 45 to 68 Pa, the surface of the ZnO films became smoother and the grain size smaller and more homogeneous. In addition, the near-band-edge emission and the deep-level emission in the photoluminescence spectra and the electrical properties of the ZnO thin films strongly depended on the ambient oxygen content.

High mobility NMOSFET structure with high-/spl kappa/ dielectric

High-/spl kappa/ NMOSFET structures designed for enhancement mode operation have been fabricated with mobilities exceeding 6000 cm/sup 2//Vs. The NMOSFET structures which have been grown by molecular beam epitaxy on 3-in semi-insulating GaAs substrate comprise a 10 nm strained InGaAs channel layer and a high-/spl kappa/ dielectric layer (/spl kappa//spl cong/20). Electron mobilities of >6000 and 3822 cm/sup 2//Vs have been measured for sheet carrier concentrations n/sub s/ of 2-3/spl times/10/sup 12/ and /spl cong/5.85/spl times/10/sup 12/ cm/sup -2/, respectively. Sheet resistivities as low as 280 /spl Omega//sq. have been obtained.

Activation energies for Te and Be in metamorphically grown AlSb and InxAl1−xSb layers

The activation energies of tellurium and beryllium in InxAl1−xSb grown by solid-source molecular beam epitaxy on semi-insulating GaAs substrates are reported. Temperature-dependent Hall measurements of carrier concentration show that while the activation energy of beryllium is relatively low for all compositions (EA<32meV), the activation energy of tellurium is highly dependent on composition and appears to form a deep level donor state (ED>100meV) in the composition range between approximately 0.1<x<0.4. For compositions outside this range, significantly reduced Te activation energies (ED<44meV) are obtained. The ability to dope InxAl1−xSb beyond the deep-donor range has been demonstrated by fabricating a p-i-n diode from In0.50Al0.50Sb, with a turn-on voltage of ∼0.4V and a forward series resistance of 0.64Ω.

Fabrication and characterization of InGaP/GaAs heterojunction bipolar transistors on GOI substrates

In this letter, we report the first demonstration of InGaP/GaAs heterojunction bipolar transistors (HBTs) on germanium-on-insulator (GOI) substrates. We have performed physical characterization of the epitaxial layers to verify the high quality of the III-V epitaxial material grown on the GOI substrates and performed dc characterization of large-area InGaP/GaAs HBTs fabricated on the substrates. The InGaP/GaAs HBTs realized on GOI substrates were compared with identical devices grown on bulk germanium substrates and similar devices on semi-insulating GaAs substrates.

Comparative study of InAs quantum dots grown on different GaAs substrates by MOCVD

We present a comparative study of InAs quantum dots grown on Si-doped GaAs (1 0 0) substrates, Si-doped GaAs (1 0 0) vicinal substrates, and semi-insulating GaAs (1 0 0) substrates. The density and size distribution of quantum dots varied greatly with the different substrates used. While dots on exact substrates showed only one dominant size, a clear bimodal size distribution of the InAs quantum dots was observed on GaAs vicinal substrates, which is attributed to the reduced surface diffusion due to the presence of multiatomic steps. The emission wavelength is blueshifted during the growth of GaAs cap layer with a significant narrowing of FWHM. We found that the blueshift is smaller for QDs grown on GaAs (1 0 0) vicinal substrates than that for dots on exact GaAs (1 0 0) substrates. This is attributed to the energy barrier formed at the multiatomic step kinks which prohibits the migration of In adatoms during the early stage of cap layer growth.

Fabry–Perot Gunn laser

A truly monopolar GaAs Fabry–Perot cavity Gunn laser is demonstrated. The device is grown by metalorganic chemical vapor deposition on a semi-insulating GaAs substrate and consists of an n=4.6×1017cm−3 doped GaAs active layer sandwiched between the AlxGa1−xAs(x=0.32) waveguiding layers. The operation of the device is based on the band to band recombination of impact-ionized nonequilibrium electron-hole pairs in the propagating high field domains in the Gunn diode, which is placed in a Fabry–Perot cavity, and biased above the threshold of negative differential resistance. Lasing from the device is observed at temperature T⩾95K. The maximum power emitted from the device is P=25.4μW at λ=840nm and T=95K.