Gas Heterostructures Research Articles

High temperature scanning Hall probe microscopy using AlGaN∕GaN two dimensional electron gas micro-Hall probes

Hall sensors fabricated using conventional narrow band gap semiconductors such as InSb and GaAs are unstable above room temperature due to the onset of intrinsic conduction and physical degradation of the semiconductor materials. Gallium nitride (GaN) based wide band gap semiconductors are stable at elevated temperatures and show potential for fabrication of high temperature electronic devices. Here, we incorporated high sensitivity AlGaN∕GaN two dimensional electron gas heterostructure micro-Hall probes (HPs) into a high temperature scanning Hall probe microscope and for magnetic imaging of domains in crystalline iron garnet thin films from room temperature to 100°C. The active area and Hall coefficient the HPs were 2×2μm2 and 0.01Ω∕G at 100°C, respectively. The evolution of the size of magnetic domains with increasing temperature under external magnetic fields is described.

Potassium selective chemically modified field effect transistors based on AlGaN/GaN two-dimensional electron gas heterostructures

We investigate the use of the AlGaN/GaN high electron mobility transistor (HEMT) as a novel transducer for the development of ion-selective chemically modified HEMT sensors (ChemHEMTs). For this, polyvinyl chloride (PVC) membrane doped with ion-selective ionophores is deposited onto the area of the gate for the chemical recognition step, while the AlGaN/GaN HEMT is used as the transducer. In particular, the use of a valinocycin doped membrane with thickness of 50μm generates a sensor with excellent analytical characteristics for the monitoring of K+. The K+-ChemHEMT has sensitivity of 52.4mV/pK+in the linear range of 10−5 to 10−2M, while the detection limit is in the order of 3.1×10−6M. Also, the sensor shows selectivity similar to valinomycin-based ISEs, while the signal stability over time and the measurement to measurement reproducibility are very good.

High sensitivity and quantitative magnetic field measurements at 600°C

High sensitivity micro-Hall sensors were fabricated using AlGaN∕GaN two-dimensional electron gas heterostructures grown by molecular beam epitaxy for high temperature operation. The room temperature electron mobility and sheet carrier concentration of the heterostructures were 850cm2∕Vs and 7.7×1012cm−2, respectively. The Hall voltage increased linearly with applied magnetic field up to 0.5T for all temperatures in the range of temperature from 20to600°C. A 100μm×100μm Hall device with a magnetic sensitivity of 0.35G∕(Hz)1∕2 at 600°C was used to measure the Curie temperature of NdFeB. The AlGaN∕GaN micro-Hall sensors will find a wide range of applications including high temperature scanning Hall microscopy, monitoring rotating parts in the engines of automobiles as well as space vehicles which are exposed to extreme changes in temperature and radiation.

Modulation of Noise in SubmicronGaAs/AlGaAsHall Devices by Gating

We present a systematic characterization of fluctuations in submicron Hall devices based on GaAs/AlGaAs two-dimensional electron gas heterostructures at temperatures between 1.5 to 60 K. A large variety of noise spectra, from 1/f to Lorentzian, are obtained by gating the Hall devices. The noise level can be reduced by up to several orders of magnitude with a moderate gate voltage of 0.2 V, whereas the carrier density increases less than 60% in the same range. The significant dependence of the Hall noise spectra on temperature and gate voltage is explained in terms of the switching processes related to impurities in n-AlGaAs.

CONTROLLABLE SPIN-FILTERING AND MAGNETOCONDUCTANCE IN A NOVEL HYBRID FM/2DEG/FM STRUCTURE

Spin-dependent ballistic transport is studied for a two-dimensional electron gas (2DEG) heterostructure, sandwiched between two ferromagnetic metal (FM) layers. The device exhibits a strong spin-filtering behavior when the magnetization directions of the FM layers are anti-parallel to one another. Additionally, a pronounced magnetoconductance (MC) effect is predicted when the relative magnetization direction of the FM layers is changed, e.g. by applying an external magnetic field sweep. Both the spin-filter and spin-valve (SV) functions can be tuned externally by adjusting the applied voltage and the fringe field strength from the FM layers. Compared to conventional all-metal SVs, the proposed 2DEG-based SV potentially has the advantages of a higher MC ratio (of as high as 90%), external tenability, and less susceptibility to interfacial imperfections.

Study of internal electric fields in AlGaAs/GaAs two-dimensional electron gas heterostructures

Modulation-doped GaAs/AlGaAs heterostructures have been studied by photoreflectance spectroscopy. The spectra at room temperature show Franz–Keldysh oscillations associated to the substrate–buffer layer interface. The built-in electric field magnitude calculated from these oscillations is related with the two-dimensional electron gas (2DEG) mobility. In addition we observed two signals associated to the GaAs capping layer and to the 2DEG, respectively.

Magnetism and interface properties of epitaxial Fe films on high-mobility GaAs/Al0.35Ga0.65As(001) two-dimensional electron gas heterostructures

An optimized heterostructure design and an optimized surface sputter-cleaning procedure allow the growth of high-quality epitaxial Fe(001) thin films at Ts<∼50 °C on selectively doped GaAs/Al0.35Ga0.65As heterostructures, while retaining the high quality transport property of the two-dimensional electron gas. Magneto-optic Kerr effect measurements and model calculations indicate a dominant uniaxial in-plane anisotropy (easy axis along [110], hard axis along [1−10]) and small coercivity (∼15 Oe). Interface sensitive Fe57 Mössbauer measurements prove the absence of both magnetic “dead layers” and “half-magnetization” phases (compared to pure Fe), and provide evidence for intermixing within a few monolayers, retaining, however, a metallic Fe state and high Fe magnetic moments at the interface.

Tunable spin‐injection and magnetoconductance in a novel 2DEG‐ferromagnet structure

AbstractIn this paper, we calculate the spin‐dependent ballistic transport through a two‐dimensional electron gas (2DEG) heterostructure sandwiched between two ferromagnetic metal (FM) layers. A strong spin‐injection effect is predicted when the magnetization directions of two FM layers are antiparallel. The spin‐injection rate can be tuned by both, applied voltage and magnetic field. The device also exhibits a significant magnetoconductance (MC) when the relative magnetization of FM layers is switched, which functions as a spin‐valve (SV). Using a GaAs system as a 2DEG material in our calculation, the MC can be tuned by both the applied voltage and the magnetic field, and reaches up to as high as 37.5%. The device's dual spin‐valve and spin‐injector characteristics open the possibility for spin electronic and sensor applications in semiconductor based systems.

High Performance HFET Devices on Sapphire and SiC: Passivation with AlN

ABSTRACTAlGaN/GaN two dimensional electron gas (2DEG) heterostructures were grown by ammonia-MBE on sapphire and SiC substrates. Devices fabricated from these optimized HFET layers, with optically defined gates showed excellent characteristics, e.g. a maximum drain current density of 1.3 A/mm, maximum transconductance of 220 mS/mm, fT of 15.6 GHz and fMAX of 58.1 GHz was measured for devices with 0.9 μm gate length and 40 μm gate width. Shorter gate length devices exhibited higher frequency responses: fT of 68 GHz and fMAX of 125 GHz for 0.25 μm gate length and fT of 103 GHz and fMAX of 170 GHz for 0.15 μm gate length. However, these devices showed “current collapse” when subjected to load pull measurements. Current collapse was also observed in sequentially repeated DC measurements in the dark, both on sapphire and SiC substrates, although the degree of collapse varied greatly from one wafer to another. One method of reducing the current collapse was to apply a thin (100 - 6000 Å) magnetron sputtered AlN passivation layer (over the gates) or a 500 Å layer under the gates so that MISFET devices were obtained. The electrical characteristics of the passivated and unpassivated devices are discussed.

The dependence of fractal conductance fluctuations on semiconductor billiard parameters

We present a comprehensive study of the dependence of fractal conductance fluctuations (FCF) on semiconductor billiard parameters. We find that the fractal dimension depends solely on an empirical parameter that quantifies the resolution of the discrete energy level spectrum of the billiard. We have also investigated the effect of profile softness using billiards formed in a double two-dimensional electron gas heterostructure and find that FCF are robust to the changes in profile softness achieved.

Ultra-sensitive photoresponse and persistentphotoconductivity in modulation doped Ge/SiGe and Si/SiGeheterostructures

We have studied the effect of incident light onGe/SiGe and Si/SiGe modulation doped heterostructures with bothinverted and normal types of doping. We report for the firsttime an extremely sensitive photoresponse in modulation dopedinverted Si/GexSi1-x/Ge/GexSi1-x two-dimensional hole gas (2DHG) heterostructures atlow temperatures (~10 K). The resistance of the 2DHG isfound to decrease significantly upon illumination with 640 nmradiation of incident powers as low as ~1 fW. Thechange in resistance persists after the illumination isswitched off, indicating the persistent photoconductivityeffect. The sensitivity of the sample to light exposure wasmeasured over several orders of incident powers from ~1 fWto 10 nW and found to increase with power. By comparing thesensitivity in different normal and inverted heterostructuresit was concluded that the high sensitivity of the invertedstructures is due to the high mobility of the photocarriersconfined in the channel closer to the top interface.

Growth of gallium nitride epitaxial layers and applications

The lack of appropriate substrates has delayed the realisation of devices based on IIInitrides. Currently, the heteroepitaxial growth of GaN by metal organic vapour phase epitaxy (MOVPE) produces GaN layers which, despite huge densities of dislocations, allow the fabrication of highly efficient optoelectronic devices. Henceforth, a new technology in heteroepitaxy of GaN, the epitaxial lateral overgrowth (ELO) has produced GaN layers in which the density of dislocations has been reduced by several orders of magnitude. With the ELO, nitride based laser diodes (LDs) working at room temperature in cw mode with a lifetime of 10,000 hours have been demonstrated by Nichia. In addition to LDs, IIInitrides presently offer a wide range of applications in optoelectronics (high brightness light emitting diodes (LEDs), from amber to UV, solar blind detectors); in electronics, high temperature/high power field effect transistors (FETs). The development of molecular beam epitaxy (MBE) of nitrides has been hindered during several years by the lack of an efficient nitrogen source. This problem being solved, MBE has recently demonstrated state-of-theart quantum well and quantum dot heterostructures, and 2D electron gas heterostructures.

Independently contacted electron–hole gas heterostructures fabricated with focused ion beam doping during molecular beam epitaxial growth

The successful combination of two mature technologies, molecular beam epitaxy (MBE) and focused ion beams into a single, versatile semiconductor growth system has recently been established. It has been shown that the focused ion MBE (FIMBE) system is able to produce spatially selected doping profiles of high quality material in both bulk-doped GaAs and modulation doped GaAs/AlGaAs heterostructures. With the versatility of a Au–Si–Be liquid metal ion source (LMIS), the range of devices that can be produced with three-dimensional topologies has been greatly increased. One such structure is the closely spaced, electron–hole gas heterostructure, which can be easily fabricated, and independently contacted using the FIMBE method. We have fabricated such structures with different barrier widths between the gases, and studied their low-temperature transport properties at various carrier densities.

Electrochemical capacitance-voltage profiling of heterostructures using small contact areas

A substantial improvement in the electrochemical capacitance-voltage (eCV) profiling technique is proposed by effectively reducing the diode area, thereby extending the range of applicability of this technique to highly spatially confined carrier systems and heterostructures. This modified technique makes use of the dielectric properties of photoresist films into which small area holes can be defined by standard photo- lithographic techniques. It is easily implemented in a commercially available system and excellent quality data across silicon (Si)/silicon-germanium (SiGe) two-dimensional electron gas (2DEG) heterostructures and delta- (-) doped gallium arsenide (GaAs) layers are obtained reproducibly. Applying this technique to modulation-doped field-effect transistors (MODFETs) it is possible to clearly identify the supply, spacer and channel layers in such structures; corresponding measured sheet densities exhibit excellent agreement with those determined by other techniques. The proposed modifications raise the eCV technique to a valuable complementary assessment tool in the determination of room temperature transport properties of heterostructures with device layer design.

Influence of electron-beam induced microporosity on masking properties of polymethylmethacrylate in wet etching of nanometer structures

The masking properties of nanometer scale polymethylmethacrylate (PMMA) features used for definition of sub-100-nm-wide bridges in InP/Ga0.25In0.75As two-dimensional electron gas heterostructures by wet etching were investigated. Bridges untreated after the development of PMMA showed very poor masking ability due to a substantial exposure by backward scattered electrons from the surrounding exposed areas. However, if the resist was post-baked after development at a temperature higher than the glass transition temperature, Tg, the masking properties were restored and wet etched nanobridges of a sufficient quality were obtained. The post-development bake temperature and time were optimized to provide enough resist “packing” and yet not to cause resist flowing resulting in unacceptable alteration of the pattern geometry.

Optoelectronic properties of n-type CdZnTe 2-DEG single-quantum-well heterostructures

Optoelectronic properties of asymmetrically strained II-VI CdZnTe single-quantum-well structures grown by molecular-beam epitaxy are reported. Indium doping CdZnTe n-type using a two-dimensional electron gas heterostructure achieved a carrier mobility of 5000 cm/sup 2//spl middot/V/sup -1/ s/sup -1/ at 40 K. A shallow donor ionization energy of 14.5 meV was determined from Hall effect measurements. Fabrication of a large-area-mesa heterostructure device allowed us to investigate exciton absorption of the mixed type-I and type-II single quantum well. Control of the electron concentration in the quantum well allows optical absorption modulation using both the quantum-confined Stark effect (QCSE) and phase-space absorption quenching. Separation of electron and hole photocurrents in different layers is demonstrated and results in photogain. A heavy-hole red-shift of 9.9 meV/V due to the reverse QCSE is reported.

Influence of Interface Roughness Scattering on Electron Mobility in GaAs-Al0.3Ga0.7 as Two Dimensional Electron Gas (2DEG) Heterostructures

Influence of Interface Roughness Scattering on Electron Mobility in GaAs-Al0.3Ga0.7 as Two Dimensional Electron Gas (2DEG) Heterostructures

Low-damage electron-beam-assisted dry etching of GaAs and AlGaAs using electron cyclotron resonance plasma electron source

Low-damage etching using an electron-beam (EB)-induced surface reaction (EB-assisted dry etching) is studied. Low-energy and high-current density EB is obtained from argon (Ar) initiated electron-cyclotron-resonance (ECR) plasma. The rate of EB-assisted dry etching, showering 120 eV EB onto a GaAs substrate in a 1×10−4 Torr chlorine gas (Cl2) atmosphere, is more than ten times larger than that for Cl2 gas etching. A 0.4 μm linewidth fine structure in GaAs was transferred. Etching of AlGaAs in Cl2 and the selective etching of GaAs from AlGaAs through the addion of SF6 gas into Ar-ECR plasma have been performed. The degree of damage induced by EB-assisted dry etching is optically and electrically characterized using a GaAs/AlGaAs quantum well (QW) structure and a two-dimensional electron gas (2DEG) heterostructure samples, respectively. The results were compared with those of samples prepared by ion-beam-assisted etching (IBAE). It is shown that EB-assisted dry etching does not degrade these properties, whereas for IBAE, reduction of both PL intensity from a QW and hall mobility in a 2DEG are observed.

Effects of electron-beam-assisted dry etching on optical and electrical properties

The degree of damage induced by electron-beam (EB)-assisted dry etching is optically and electrically characterized and compared with ion-beam-assisted etching (IBAE). The optical and electrical damage are evaluated using the photoluminescence (PL) from GaAs/AlGaAs quantum well (QW) structures and the Hall effect in two-dimensional electron gas (2DEG) heterostructures, respectively. These layered samples are etched by a newly proposed EB-assisted selective dry etching technique using Cl2 and SF6 gases. EB-assisted dry etching is proven to produce no degradation of either type of property, whereas a 54% reduction in PL intensity from a 30-nm-deep QW and 8% reduction in Hall mobility in a 51-nm-deep 2DEG are observed for IBAE.

Electrical characterization of very-low energy (0-30 eV) CI-Radical/Ion-beam-etching induced damage using two-dimensional electron gas heterostructures

Damage induced by very low energy (30 eV) Cl reactive ion beam etching (RIBE) and radical etching (RE) has been electrically characterized. GaAs/n-AlGaAs two-dimensional electron gas heterostructures were used as damage sensitive probes. Sheet carrier concentrations and Hall mobilities were measured at 77 K, under dark as well as illuminated conditions. By carefully designing the sample structure, the damaged layer thickness could be estimated by comparing dark and illuminated data. In case of RE, no degradation was detected at depths as shallow as 25 nm from the etched surface. For 30 eV-RIBE, the damage was detected but found to be reasonably small (38% decrease in electron mobility) and shallow (<50 nm). Electrical and optical damage are compared briefly.