GaAs Counterparts Research Articles

Monolithically integrated 940 nm VCSELs on bulk Ge substrates.

This research successfully developed an independent Ge-based VCSEL epitaxy and fabrication technology route, which set the stage for integrating AlGaAs-based semiconductor devices on bulk Ge substrates. This is the second successful Ge-based VCSEL technology reported worldwide and the first Ge-based VCSEL technology with key details disclosed, including Ge substrate specification, transition layer structure and composition, and fabrication process. Compared with the GaAs counterparts, after epitaxy optimization, the Ge-based VCSEL wafer has a 40% lower surface root-mean-square roughness and 72% lower average bow-warp. After device fabrication, the Ge-based VCSEL has a 10% lower threshold current density and 19% higher maximum optical differential efficiency than the GaAs-based VCSEL.

Photovoltaic performance improvement of GaAs1-xBix nanowire solar cells in terms of light trapping capability and efficiency

This paper investigates optoelectronic performance of nanowire (NW) solar cells using GaAs and its alloy GaAs1-xBix for three different mole fractions of Bi e.g., x = 0.01, 0.043, 0.078 using extensive numerical simulation in order to evaluate the potential of the alloy as a solar cell material. Finite-difference time-domain (FDTD) method is used to obtain the optimum geometric dimensions of the nanowire in order to achieve maximum photocurrent. GaAs0.922Bi0.078 NW solar cell of length 2-µm and optimized diameter D = 140 nm with filling ratio (FR) of 0.5, produces a maximum Jsc of 49.31 mA/cm2 which is 23% higher than its GaAs counterpart. Additionally, GaAsBi NW exhibits much higher absorption, and lower reflection as well as transmission coefficients for the incident optical irradiation, due largely to its bandgap bowing property owing to incorporation of Bi, as compared to GaAs NW. Furthermore, we present electric field distribution and optical generation rate profiles under the incident irradiation in NWs to obtain a better insight into the device physics. Various material and transport parameters of the novel alloy material required to carry out electrical simulation using Lumerical 3D charge transport simulator, are presented in detail, and the effect of SRH recombination lifetime on the electrical performance i. e., open circuit voltage Voc and power conversion efficiency (PCE) of the NW solar cells is studied. Our findings reveal that GaAs0.99Bi0.01 NW exhibits the maximum PCE of 34.34% found to be 6.425% larger than that of GaAs NW under AM 1.5G illumination.

High-Efficiency Millimeter-Wave CMOS Switching Rectifiers: Theory and Implementation

This article proposes the concept of millimeter-wave (mm-wave) switching rectifiers originated from drain-pumped mixers. The topology and circuit design methodology are distinctly different from other CMOS mm-wave rectifiers which adopt voltage multiplier techniques. Operation principle and RF-to-dc power conversion efficiency (PCE) of the proposed switching rectifiers are analyzed to overcome the fundamental limits on achievable performance of switching rectifiers at mm wave. In order to demonstrate the utility of the proposed concept, two prototypes in 65-nm bulk CMOS technology have been implemented: two fully integrated switching rectifiers operating at Ka-band and W-band. A peak PCE of 36.5% at 35 GHz with a load resistance of $50~\Omega $ at about 15-dBm input power is measured for the Ka-band rectifier. Measurement results for the W-band rectifier also indicate that the PCE peaks at 27% at 91 GHz for a load resistance of $43~\Omega $ at about 16-dBm input power. To the best of our knowledge, the two fully integrated rectifiers which occupy the smallest chip area exhibit a competitive PCE reported among Ka-band and W-band CMOS rectifiers, competing well with GaAs counterparts where the Schottky diode is available.

Magneto-transport in inverted HgTe quantum wells

HgTe quantum wells (QWs) are two-dimensional semiconductor systems that change their properties at the critical thickness dc, corresponding to the band inversion and topological phase transition. The motivation of this work was to study magnetotransport properties of HgTe QWs with thickness approaching dc, and examine them as potential candidates for quantum Hall effect (QHE) resistance standards. We show that in the case of d > dc (inverted QWs), the quantization is influenced by coexistence of topological helical edge states and QHE chiral states. However, at d ≈ dc, where QW states exhibit a graphene-like band structure, an accurate Hall resistance quantization in low magnetic fields (B ≤ 1.4 T) and at relatively high temperatures (T ≥ 1.3 K) may be achieved. We observe wider and more robust quantized QHE plateaus for holes, which suggests—in accordance with the “charge reservoir” model—a pinning of the Fermi level in the valence band region. Our analysis exhibits advantages and drawbacks of HgTe QWs for quantum metrology applications, as compared to graphene and GaAs counterparts.

Phonon-assisted relaxation and decoherence of singlet-triplet qubits in Si/SiGe quantum dots

We study theoretically the phonon-induced relaxation and decoherence of spin states of two electrons in a lateral double quantum dot in a SiGe/Si/SiGe heterostructure. We consider two types of singlet-triplet spin qubits and calculate their relaxation and decoherence times, in particular as a function of level hybridization, temperature, magnetic field, spin orbit interaction, and detuning between the quantum dots, using Bloch-Redfield theory. We show that the magnetic field gradient, which is usually applied to operate the spin qubit, may reduce the relaxation time by more than an order of magnitude. Using this insight, we identify an optimal regime where the magnetic field gradient does not affect the relaxation time significantly, and we propose regimes of longest decay times. We take into account the effects of one-phonon and two-phonon processes and suggest how our theory can be tested experimentally. The spin lifetimes we find here for Si-based quantum dots are significantly longer than the ones reported for their GaAs counterparts.

Review—Ionizing Radiation Damage Effects on GaN Devices

Gallium Nitride based high electron mobility transistors (HEMTs) are attractive for use in high power and high frequency applications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaN-based blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20–30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN–based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. In InGaN/GaN blue LEDs irradiated with protons at fluences near 1014 cm−2 or electrons at fluences near 1016 cm−2, both current-voltage and light output-current characteristics are degraded with increasing proton dose. The optical performance of the LEDs is more sensitive to the proton or electron irradiation than that of the corresponding electrical performances.

(Invited) Radiation Effects in AlGaN/GaN and InAlN/GaN High Electron Mobility Transistors

GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. In this paper we review data on the radiation resistance of GaN-based transistors such as AlGaN/GaN and InAlN/GaN High Electron Mobility Transistors (HEMTs) to different types of ionizing radiation. In this paper, the primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects are discussed. The carrier removal rates are a function of initial carrier concentration, dose and dose rate but not of hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. Neutron irradiation created more extended damage regions and at high doses leads to Fermi level pinning while 60Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation.

Quantum Hall resistance standards from graphene grown by chemical vapour deposition on silicon carbide.

Replacing GaAs by graphene to realize more practical quantum Hall resistance standards (QHRS), accurate to within 10−9 in relative value, but operating at lower magnetic fields than 10 T, is an ongoing goal in metrology. To date, the required accuracy has been reported, only few times, in graphene grown on SiC by Si sublimation, under higher magnetic fields. Here, we report on a graphene device grown by chemical vapour deposition on SiC, which demonstrates such accuracies of the Hall resistance from 10 T up to 19 T at 1.4 K. This is explained by a quantum Hall effect with low dissipation, resulting from strongly localized bulk states at the magnetic length scale, over a wide magnetic field range. Our results show that graphene-based QHRS can replace their GaAs counterparts by operating in as-convenient cryomagnetic conditions, but over an extended magnetic field range. They rely on a promising hybrid and scalable growth method and a fabrication process achieving low-electron-density devices.

(Invited) Radiation Effects in AlGaN/GaN and InAlN/GaN High Electron Mobility Transistors

GaN HEMTs are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. We review data on the radiation resistance of GaN-based transistors such as AlGaN/GaN and InAlN/GaN High Electron Mobility Transistors (HEMTs) to different types of ionizin gradiation. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects are discussed. The carrier removal rates are a function of initial carrier concentration, dose and dose rate but not of hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. Neutron irradiation created more extended damage regions and at high doses leads to Fermi level pinning while 60Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation.

Radiation Effects in GaN-Based High Electron Mobility Transistors

GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. In this paper, we review data on the radiation resistance of GaN-based transistors such as AlGaN/GaN and InAlN/GaN high electron mobility transistors (HEMTs) to different types of ionizing radiation. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects are discussed. The carrier removal rates are a function of initial carrier concentration, dose and dose rate but not of hydrogen concentration in the nitride material grown by metal organic chemical vapor deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two-dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. Neutron irradiation created more extended damage regions and at high doses leads to Fermi level pinning, while 60Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation.

Type II GaSb quantum ring solar cells under concentrated sunlight.

A type II GaSb quantum ring solar cell is fabricated and measured under the concentrated sunlight. The external quantum efficiency confirms the extended absorption from the quantum rings at long wavelength coinciding with the photoluminescence results. The short-circuit current of the quantum ring devices is 5.1% to 9.9% more than the GaAs reference's under various concentrations. While the quantum ring solar cell does not exceed its GaAs counterpart in efficiency under one-sun, the recovery of the open-circuit voltages at higher concentration helps to reverse the situation. A slightly higher efficiency (10.31% vs. 10.29%) is reported for the quantum ring device against the GaAs one.

STRAIN INDUCED ACTIVE LAYER DESIGN OF GaN-THz QUANTUM CASCADE LASERS

GaN -based pseudomorphic heterostructures with their demonstrated superior thermal performance suggest an alternative to the standard GaAs -based technology to realize high power lasing at THz frequencies. A larger electron effective mass in GaN based system results in the energy levels lying deeper within the quantum well compared to its GaAs counterpart resulting in longer carrier lifetimes assisting transitions required for THz radiation while reducing tunneling current. However, the presence of spontaneous and piezoelectric polarization and dependence of bandgap and band offsets on structural and bias induced strain reduces carrier lifetime.

A 25–75 GHz Miniature Double Balanced Frequency Doubler in 0.18-μm CMOS Technology

A 25–75 GHz compact double balanced frequency doubler fabricated in standard 0.18-μm CMOS process is demonstrated. The resistive doubler is composed of two identical asymmetric broadside-coupled baluns, and a quad GS-connected diode. The fabricated doubler achieves a radio frequency bandwidth from 25 to 75 GHz with a maximum output power better than ┼3 dBm; the fundamental signal rejection is ranging from 32 to 59 dB, and only occupies a chip size of 0.24 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . To the knowledge of the authors, this double balanced frequency doubler is the first demonstration with an operating frequency up to 75 GHz in 0.18-μm CMOS technology and shows this silicon-based frequency doubler can compare with its GaAs counterpart.

RF MEMS, BST, and GaAs varactor system-level response in complex modulation systems

This article presents the response of RF microelectromechanical systems (RF MEMS), barium strontium titanate (BST), and gallium arsenide (GaAs)-based tunable filters and reconfigurable matching networks to a wideband code-division-multiple-access signal cen- tered at 1.95 GHz. The RF MEMS tunable filter and impedance tuner result in very low inter- modulation distortion and spectral regrowth compared to their BST and GaAs counterparts. The linearity of the BST and GaAs tunable networks improves considerably by using a series combination of BST and GaAs varactors, but the RF MEMS-based networks still show the best linearity of all three technologies. Also, it is shown that the reconfigurable networks, tuned with capacitive RF MEMS can handle up to 1 W of RF power with no self-actuation. V C 2007

Growth of p-type GaAs∕AlGaAs(111) quantum well infrared photodetector using solid source molecular-beam epitaxy

A p-type GaAs∕AlGaAs multi-quantum-well infrared photodetector (QWIP) was fabricated on a GaAs (111)A substrate by molecular-beam epitaxy using silicon as dopant. The same structure was also grown on a GaAs (100) wafer simultaneously to compare the material and structural properties. It was found that Si acts as a p-type dopant in the GaAs (111)A sample while it is n-type in the GaAs (100) counterpart. The growth rate was found to be appreciably enhanced for GaAs (111)A compared with that of GaAs (100) orientation, while the Al composition in the barriers was found to be 20% smaller for a (111) orientation which results in a smaller barrier height. A peak responsivity of 1mA∕W with a relatively wide wavelength response (Δλ∕λp∼53%) was observed for the GaAs (111)A QWIP, mainly due to the location of the excited state far above the barrier. The photoresponse also showed a relatively strong normal incident absorption probably originating from the mixing of the conduction and valence Bloch states. The optimization of the quantum well parameters should further enhance the responsivity of this p-type QWIP with Si as dopant species.

Transmission electron microscopy studies of the bonded SiC-SiC interface

SiC is a promising material for high-power and hightemperature electronics due to its chemical inertness, high thermal conductivity (∼5 W cm−1 K−1), and wide band gap (2.4–3.3 eV, depending on the polytype). SiC diodes, field effect transistors, as well as bipolar homojunction transistors fabricated to date, showed that their performance exceeds even theoretical limits of Si and GaAs counterparts [1, 2]. Heterojunction devices can offer improved efficiency compared to the homojunction devices [3]. Koitzsch et al. suggested a waferbonding technique [4] to fabricate higher efficiency heterojunction devices using different SiC polytypes [5]. The interface obtained during bonding will determine the device operation. This work focuses on transmission electron microscopy (TEM) studies of the bonded SiC/SiC interfaces. In all bonding experiments, on-axis Si-face (0001) SiC wafers (either 6H or 4H) obtained from Cree, Inc., were used. The root-mean-square (RMS) surface roughness of as-received SiC wafers was measured by atomic force microscopy (AFM) using a Digital Instruments D3000 microscope. These measurements revealed an RMS surface roughness of ∼2 nm, which is higher than the 0.5 nm limit required for room temperature bonding [4]. Fig. 1 shows an AFM image of a typical SiC surface. Dark lines in the image correspond to the polishing scratches. The macroscopic surface roughness (waviness) of SiC wafers was studied on a Zygo GPI XP Laser Interferometer. Although the observed surface profile (Fig. 2) varied from wafer-to-wafer, the value for the peak-to-valley waviness was consistently 5–7 μm for our 12 × 22 mm2 samples. This is similar to the waviness of similarly sized Si samples, which was measured to be 3–6 μm. This level of macroscopic flatness was found suitable for the well-established Si/Si (or Si/SiO2) wafer bonding and thus was not expected to create any major obstacles in bonding of SiC wafers. To prepare particle-free samples for our bonding experiments, all wet cleaning procedures were performed in a class 100 clean-room environment. SiC samples were ultrasonically cleaned in BakerClean©R solution for 10 min, in 10% hydrofluoric acid for 10 min, in BakerClean©R solution for another 10 min, in deionized water for 1–3 min, and finally blow-dried with nitrogen. The final cleaning procedure was performed in a

Characterization of one-dimensional quantum channels in InAs/AlSb

We report the magnetoresistance characteristics of one-dimensional electrons confined in a single InAs quantum well sandwiched between AlSb barriers. As a result of a novel nanofabrication scheme that utilizes a 3nm-shallow wet chemical etching to define the electrostatic lateral confinement, the system is found to possess three important properties: specular boundary scattering, a strong lateral confinement potential, and a conducting channel width that is approximately the lithography width. Ballistic transport phenomena, including the quenching of the Hall resistance, the last Hall plateau, and a strong negative bend resistance, are observed at 4K in cross junctions with sharp corners. In a ring geometry, we have observed Aharonov-Bohm interference that exhibits characteristics different from those of the GaAs counterpart due to the ballistic nature of electron transport and the narrowness of the conducting channel width.

Optical modes within III-nitride multiple quantum well microdisk cavities

Optical resonance modes have been observed in optically pumped microdisk cavities fabricated from 50 Å/50 Å GaN/AlxGa1−xN(x∼0.07) and 45 Å/45 Å InxGa1−xN/GaN(x∼0.15) multiple quantum well structures. Microdisks, approximately 9 μm in diameter and regularly spaced every 50 μm, were formed by an ion beam etch process. Individual disks were pumped at 300 and 10 K with 290 nm laser pulses focused to a spot size much smaller than the disk diameter. Optical modes corresponding to (i) the radial mode type with a spacing of 49–51 meV (both TE and TM) and (ii) the Whispering Gallery mode with a spacing of 15–16 meV were observed in the GaN microdisk cavities. The spacings of these modes are consistent with those expected for modes within a resonant cavity of cylindrical symmetry, refractive index, and dimensions of the microdisks under investigation. The GaN-based microdisk cavity is compared with its GaAs counterpart and implications regarding future GaN-based microdisk lasers are discussed.

Bias dependent microwave performance of AlGaN/GaN MODFET's up to 100 V

1 /spl mu/m gate-length AlGaN/GaN modulation doped field effect transistors (MODFET's) have been fabricated on an insulating GaN buffer layer for better carrier confinement. These devices demonstrate simultaneously high current levels (>500 mA/mm), excellent pinch-off and high gate-drain breakdown voltages (220 V for 3 /spl mu/m gate-drain spacing). In contrast to their GaAs counterparts, the current-gain cutoff frequency of the AlGaN/GaN devices shows little degradation at high drain voltage biases. A power-gain cutoff frequency of 19 GHz is obtained at 100 V. ACW power density of 1.57 W/mm at 4 Ghz is also achieved when biased at 28 V and 205 mA/mm.

The Effect of Strain on the Formation of Dislocations at the SiGe/Si Interface

Recently much interest has been devoted to Si-based heteroepitaxy, and in particular, to the SiGe/Si system. This is mostly for economical reasons: Si-based technology is much more advanced, is widely available, and is cheaper than GaAs-based technology. SiGe opens the door to the exciting (and lucrative) area of Si-based high-performance devices, although optical applications are still limited to GaAs-based technology. Strained SiGe layers form the base of heterojunction bipolar transistors (HBTs), which are currently used in commercial high-speed analogue applications. They promise to be low-cost compared to their GaAs counterparts and give comparable performance in the 2-20-GHz regime. More recently we have started to investigate the use of relaxed SiGe layers, which opens the door to a wider range of application and to the use of SiGe in complementary metal oxide semiconductor (CMOS) devices, which comprise strained Si and SiGe layers. Some recent successes include record-breaking low-temperature electron mobility in modulation-doped layers where the mobility was found to be up to 50 times better than standard Si-based metal-oxide-semiconductor field-effect transistors (MOSFETs). Even more recently, SiGe-basedp-type MOSFETS were built with oscillation frequency of up to 50 GHz, which is a new record, in anyp-type material for the same design rule.