SI-GaAs Substrate Research Articles

Terahertz photoconductive antenna with all-dielectric nanopillars

Photoconductive antennas (PCAs), as a popular terahertz (THz) radiation source, have been widely used in spectroscopy, material characterization, biological imaging and detection of hazardous materials. However, PCAs have a relatively low energy conversion efficiency from femtosecond laser pulses to THz radiation which often limits the signal-to-noise ratio and bandwidth of THz imaging and spectroscopy systems. To address these limitations, here we report a THz photoconductive antenna emitter with all-dielectric nanopillars integrated on top of the SI-GaAs substrate to increase the generated photocarriers, which achieves a broadband and frequency insensitive THz power enhancement factor around 1.25 at frequencies 0.05 - 1.6 THz. Our results reported here provide a new method for increasing the THz power of PCAs, which paves the way for the subsequent researches of next-generation PCAs.

Defect levels and interface space charge area responsible for negative photovoltage component in InAs/GaAs quantum dot photodetector structure

InAs/GaAs quantum dot photodetector photoresponse spectra consist of quantum dot, wetting layer and GaAs components, however, they frequently contain features attributed to defect levels. In this study we focus on the origin of an unwanted negative photovoltage component in order to find a design allowing to eliminate these undesirable effects. Photoelectric properties of vertical InAs/GaAs quantum dot structures grown on semi-insulating (si) and n+-GaAs wafers are analyzed. It is found that the photoresponse drop above 1.37 eV in the photovoltage spectrum, observed with si-GaAs substrate, is originated from shallow defect levels in the unintentional space-charge area created at the interface between the substrate and n+-GaAs buffer. The intrinsic field in that area is opposite to the top pn junction, according to modelling calculations, and it reduces the photoelectric response. Photoelectric characterization of the heterostructure grown on n+-GaAs indicates that the utilization of n+-GaAs substrate helps to avoid this band bending and to eliminate its negative effects, increasing the total photoresponse.

MOCVD grown low dislocation density GaAs-on-V-groove patterned (001) Si for 1.3 μ m quantum dot laser applications

We report the development of gallium arsenide (GaAs) films grown on V-groove patterned (001) silicon (Si) by metalorganic chemical vapor deposition. This technique can provide an advanced virtual substrate platform for photonic integrated circuits on Si. A low defect density of 9.1 × 106 cm−2 was achieved with the aspect ratio trapping capability of the V-grooved Si and dislocation filtering approaches including thermal cycle annealing and dislocation filter layers. The efficiencies of these dislocation reduction methods are quantified by statistical electron channeling contrast imaging characterization. Meanwhile, different sets of dislocation filtering layers are evaluated and optimized. To further demonstrate the suitability of GaAs on the V-grooved Si technique for Si-based photonic devices, especially for the appealing 1.3 μm quantum dot (QD) lasers, a 7-layer indium arsenide QD structure was grown on both GaAs-on-V-grooved Si and native GaAs substrates. The same photoluminescence intensity and full-width at half-maximum values were observed for both structures. The optimization methodology in this work therefore offers a feasible approach to realize high quality III–V materials on Si for large-scale integration.

Design and fabrication of terahertz quantum cascade laser with double metal waveguide based on multilayer GaAs/AlGaAs heterostructures

We have designed multilayer heterostructure (MH) based on three quantum well GaAs/Al0.15Ga0.85As active module with diagonal transitions and optimized oscillator strength – 0.425. Furthermore, we have developed a method for the fabrication of terahertz quantum cascade laser (THz QCL) with double metal waveguide via low-temperature In-Au wafer bonding followed by SI-GaAs substrate removal. ICP RIE in BCl3/Ar has been used to obtain laser ridges (widths 50-200 mkm) with vertical sidewalls. We investigate the dependence of the MH energy band structure on the electric field and thermal properties of terahertz quantum cascade lasers (THz QCL) under different operation conditions.

Direct growth of type II InAs/GaSb superlattice MWIR photodetector on GaAs substrate

We report on the direct growth and characterization of type-II InAs/GaSb superlattice (T2SL) MWIR photodetector structure grown on a GaAs substrate by molecular beam epitaxy. The designed photodetector structure contains 140 period of 8.0 ML InAs/8.3 ML GaSb p-i-n SL structure with a 50% cutoff wavelength of 3.78 μm. We achieved a peak specific detectivity (D∗) and differential resistance area product at zero bias (R0A) of 1.3 × 1012 cm Hz½ W−1 and 104 Ω cm2 at 80 K, respectively. The obtained D∗ value is the best value reported up to now for a T2SL MWIR p-i-n photodetector grown on a GaAs substrate. The crystalline quality and the uniformity of the grown structure were verified by high resolution X-ray diffraction method by measuring three different spots on grown structure on a full 4 inch SI GaAs substrate.

Material selection methodology for radio frequency (RF) microelectromechanical (MEMS) capacitive shunt switch

This paper describes the process of selecting the most optimum Radio Frequency Micro- electro- mechanical-systems (RF-MEMS) switch design using Ashby’s methodology. The switches are compared on the basis of parameters like actuation voltage, insertion loss, isolation and switching time using material selection charts. The chart shows that a low-voltage metal-to-metal contact shunt capacitive RF-MEMS having a bridge structure with Si-GaAs substrate, electroplated gold contacts and silicon nitride dielectric layer, is the most optimum of all the switches considered.

Terahertz emission characteristics of GaMnAs dilute magnetic semiconductor under 650 mT external magnetic field

We investigate the effects of an externally applied magnetic field on the terahertz (THz) emission of Gallium Manganese Arsenide (GaMnAs) films grown via molecular beam epitaxy (MBE). Results show that low Mn-doping in GaMnAs resulted to increased THz emission as compared with a SI-GaAs substrate. Further increase in Mn-doping content resulted to a comparably less THz emission, which is attributed to reduced crystallinity and higher free-carrier absorption. Under an external magnetic field, the contributions of the Bup and Bdown-related THz emission were observed to be asymmetric: possibly due to intrinsic magnetic properties of GaMnAs.

Comparative evaluation of InAs/GaSb superlattices for mid infrared detection: p–i–n versus residual doping

We report on the opto-electronic characterization of an InAs/GaSb superlattice (SL) midwave infrared p–i–n photodetector structure (pin-SL) in comparison with the same structure with no intentional doping (i-SL). Both structures were grown on an n-GaSb substrate using molecular beam epitaxy. The nominally undoped structure (i-SL) presented p–i–n like behavior and showed a photovoltaic mode photoresponse due to the residual doping and native defects in this material system. For ∼77 K operation, 0.76 and 0.11 A W–1 responsivity values were obtained at 4 μm from the pin-SL and i-SL structures, respectively. Activation energy analysis showed that the recombination current was dominant in both structures but different recombination centers were involved. The same i-SL structure was also grown on a semi-insulating (SI)-GaAs substrate to study the contribution of the substrate to the carrier density in the SL layers. Temperature dependent Hall effect measurements showed that the nominally undoped structure presented both n-type and p-type conductivities; however, the temperature at which the carrier type switched polarity was observed to be at higher values when the i-SL structure was grown on the SI-GaAs substrate. In addition, a higher carrier density was observed for i-SL on the GaSb substrate than on the GaAs substrate.

Pump-probe surface photovoltage spectroscopy measurements on semiconductor epitaxial layers.

Pump-probe Surface Photovoltage Spectroscopy (SPS) measurements are performed on semiconductor epitaxial layers. Here, an additional sub-bandgap cw pump laser beam is used in a conventional chopped light geometry SPS setup under the pump-probe configuration. The main role of pump laser beam is to saturate the sub-bandgap localized states whose contribution otherwise swamp the information related to the bandgap of material. It also affects the magnitude of Dember voltage in case of semi-insulating (SI) semiconductor substrates. Pump-probe SPS technique enables an accurate determination of the bandgap of semiconductor epitaxial layers even under the strong influence of localized sub-bandgap states. The pump beam is found to be very effective in suppressing the effect of surface/interface and bulk trap states. The overall magnitude of SPV signal is decided by the dependence of charge separation mechanisms on the intensity of the pump beam. On the contrary, an above bandgap cw pump laser can be used to distinguish the signatures of sub-bandgap states by suppressing the band edge related feature. Usefulness of the pump-probe SPS technique is established by unambiguously determining the bandgap of p-GaAs epitaxial layers grown on SI-GaAs substrates, SI-InP wafers, and p-GaN epilayers grown on Sapphire substrates.

Highly efficient and electrically robust carbon irradiated semi-insulating GaAs based photoconductive terahertz emitters

We demonstrate here an efficient photoconductive THz source with low electrical power consumption. We have increased the maximum THz radiation power emitted from SI-GaAs based photoconductive emitters (PCEs) by two orders of magnitude. By irradiating the SI-GaAs substrate with Carbon-ions up to 2 μm deep, we have created lot of defects and decreased the lifetime of photo-excited carriers inside the substrate. Depending on the irradiation dose, we find 1 to 2 orders of magnitude decrease in total current flowing in the substrate, resulting in subsequent decrease of heat dissipation in the device. This has resulted in increasing maximum cut-off of the applied voltage across PCE electrodes to operate the device without thermal breakdown from ∼35 V to >150 V for the 25 μm electrode gaps. At optimum operating conditions, carbon irradiated (1014 ions/cm2) PCEs give THz pulses with power about 100 times higher in comparison to the usual PCEs on SI-GaAs and electrical to THz power conversion efficiency has improved by a factor of ∼800.

Design of High-performance Wilkinson Power Divider on SI-GaAs Substrate using Integrated Passive Technology

An integrated passive device technology by semi-insulating (SI)-GaAs-based fabrication has been developed to meet the ever-increasing needs of size and cost reduction in wireless applications. This technology includes reliable NiCr thin film resistor, high Q spiral inductor due to thick plated Cu/Au metal process to reduce resistive loss, high breakdown voltage metal-insulator-metal capacitor due to a thinner dielectric thickness, lowest parasitic effect by multi air-bridged metal layers, air-bridges for inductor underpass and capacitor pick-up, and low chip cost using only six process layers. This paper presents the Wilkinson power divider with excellent performance for a digital cellular system application. The insertion loss of this power divider is −0.53 dB and the port isolation greater than −30 dB over the entire band. Return loss in input and output ports are −20.2 dB and −35.8 dB, respectively. The Wilkinson power divider based on SI-GaAs substrates is designed within a die size of 1.42 mm2.

A novel method for the fabrication of integrated passive devices on SI-GaAs substrate

A novel process is developed for fabricating cost-effective, high-yield, and high-quality integrated passive devices on SI-GaAs substrate. Various material and processing approaches to thin film resistors (TFRs), spiral inductors, and metal-insulator-metal (MIM) capacitors are evaluated in terms of cost, yield and device performance. For better precision in TFR resistance, we modify the bottom metal manufacturing process. For higher spiral inductor quality factor (Q-factor) and yield, a much thicker second metal and a sputter-etching process is presented. For higher MIM capacitor yield, some optimised mechanisms are used. To further decrease the cost and increase the yield, SU-8 photo resist is firstly presented as a novel material for forming the final passivation layer to replace the traditional SiNx. A wireless local area network balun, low-pass filter and digital cellular system power divider are demonstrated by using this novel manufacturing process; they show very good RF performances in spite of theirs small chip size and low cost, compared with those of the reported literature.

Residual stress stability in fiber textured stoichiometric AlN film grown using rf magnetron sputtering

The authors report on the stability of mechanical stress with aging and thermal cycling for columnar structured stoichiometric and homogeneous aluminum nitride thin films grown using radio frequency magnetron sputtering technique. The set of deposition parameters were optimized for the best possible orientation of crystallites in the c axis of compositionally stoichiometric films. The as-grown stress in the slightly nitrogen-rich film does not change when exposed to the atmosphere following deposition, while that in the nitrogen-deficient film, it changes due to oxidation. Additionally, the magnitude of as-grown stress has been found to depend on the substrate material in addition to the deposition parameters. The stress in the film grown on a Si(001) substrate was more tensile than in the film grown on a semi-insulating (si) GaAs(001) substrate for a given set of deposition parameters. Furthermore, the stress in the film grown on Si decreased with temperature, while that on si GaAs increased, indicating the thermally induced stress component to be the major component in the residual stress. Upon subsequent cooling the stress changes in both substrates followed the same path as of heating, thus exhibiting no hysteresis with thermal cycles between room temperature and 400 °C.

Elevated conductor coplanar waveguide-fed three-level proximity-coupled antenna for G-band applications

A new concept for feeding a proximity-coupled elevated patch antenna is presented. The antenna scheme maximises the antenna performance on high dielectric substrates. The patch is excited by electromagnetic coupling using an elevated loop slot. This loop slot is elevated on the substrate and located below the patch at the end of an elevated conductor coplanar waveguide (EC-CPW) feed line. The antenna is fabricated on SI-GaAs substrate based on surface micromachining technology without need of etching the substrate. The authors results show that the bandwidth is 15 GHz, determined from 10 dB return loss and the gain is about 7.18 dB at G-band frequencies (140–220 GHz) with nearly constant broadside radiation pattern over its bandwidth. The authors believe this to be the best reported performance of a micro-machined antenna operating in the G-band frequency range to date.

적외선검출소자를 위한 GaSb 결정 및 MBE로 성장한 Gasb/SI-GaAs 박막의 진성결함에 관한 연구

Sb에 기초한 응력 초격자 적외선검출소자의 구성 물질인 도핑하지 않은 기판 GaSb 결정과 GaSb/SI-GaAs 박막에 잔존하고 있는 진성결함 (intrinsic defect)을 비교 조사하였다. 상온 근처 (250 K)까지 광여기 발광 (PL)을 보이는 GaSb 결정에서의 발광 에너지의 온도의존성으로부터, 밴드갭 에너지에 관한 경험식인 Varshni 함수의 파라미터 (<TEX>$E_o$</TEX>, <TEX>$\alpha$</TEX>, <TEX>$\beta$</TEX>)를 결정하였다. GaAs 기판 위에 성장된 이종 GaSb 박막에서는 GaSb 주요 진성결함으로 알려져 있는 29 meV의 이온화 에너지를 가지는 위치반전 (antisite) Ga ([<TEX>$Ga_{Sb}$</TEX>]) 결함과 함께 위치반전 Sb ([<TEX>$Sb_{Ga}$</TEX>])와의 복합결함 ([<TEX>$Ga_{Sb}-Sb_{Ga}$</TEX>])과 관련된 것으로 분석된 732/711 meV의 한 쌍의 깊은준위 (deep level)가 관측되었다. PL의 온도 및 여기출력 의존성을 분석하여, Sb-rich상태에서 성장된 GaSb 박막에서는 잉여 Sb의 자발확산 (self-diffusion)에 의하여 치환된 위치전도 [<TEX>$Ga_{Sb}$</TEX>] 및 [<TEX>$Sb_{Ga}$</TEX>]가 결합하여 [<TEX>$Ga_{Sb}-Sb_{Ga}$</TEX>]의 깊은준위를 형성하는 것으로 해석되었다. We have investigated the intrinsic defects remaining in epitaxial GaSb layers grown on SI-GaAs substrates compared to those in bulk GaSb crystal substrate, which is a basic material of Sb-based strained-layer superlattice infrared photodetectors. From the functional dependence of the band-to-band transition energy of the photomuminescence (PL) spectra observing up to near room-temperature (250 K), the temperature parameters of [<TEX>$E_o$</TEX>, <TEX>$\alpha$</TEX>, <TEX>$\beta$</TEX>] of undoped GaSb crystal are determined by using the Varshni empirical equation describing the temperature variation of the bandgap energy. Additionally to the antisite-Ga ([<TEX>$Ga_{Sb}$</TEX>]) with an ionization energy of 29 meV that is well known to a major intrinsic defect in GaSb, epitaxial GaSb layers show a pair of deep states at the emission energy of 732/711 meV that may be related with a complex of two antisite-Ga and antisite-Sb ([<TEX>$Ga_{Sb}-Sb_{Ga}$</TEX>]). Based on the analysis of the temperature and the excitation-power dependences of PL, it suggests that excess-Sb substitutes Ga-site by self-diffusion and two anti sites of [<TEX>$Ga_{Sb}$</TEX>] and [<TEX>$Sb_{Ga}$</TEX>] could form as a complex of [<TEX>$Ga_{Sb}-Sb_{Ga}$</TEX>] in GaSb epilayers grown under Sb-rich condition.

High Performance Wilkinson Power Divider Using Integrated Passive Technology on SI-GaAs Substrate

An integrated passive device(IPD) technology by semi-insulating(SI)-GaAs-based fabrication has been developed to meet the ever increasing needs of size and cost reduction in wireless applications. This technology includes reliable NiCr thin film resistor, thick plated Cu/Au metal process to reduce resistive loss, high breakdown voltage metal-insulator-metal(MIM) capacitor due to a thinner dielectric thickness, lowest parasitic effect by multi air-bridged metal layers, air-bridges for inductor underpass and capacitor pick-up, and low chip cost by only 6 process layers. This paper presents the Wilkinson power divider with excellent performance for digital cellular system(DCS). The insertion loss of this power divider is - 0.43 dB and the port isolation greater than - 22 dB over the entire band. Return loss in input and output ports are - 23.4 dB and - 25.4 dB, respectively. The Wilkinson power divider based on SI-GaAs substrates is designed within die size of <TEX>$1.42\;mm^2$</TEX>.

Effect of the Photoquenching of EL2 in GaAs Substrate on the Piezoelectric Photothermal and Surface Photovoltage Spectra of a GaAs Single Quantum Well Confined by GaAs/AlAs Short-Period Superlattices

Two nondestructive techniques, surface photovoltage (SPV) and piezoelectric photothermal (PPT) spectroscopies, were adopted to investigate a GaAs single quantum well (SQW) confined by GaAs/AlAs short-period superlattices (SPSs) fabricated on a semi-insulating (SI) GaAs substrate, whose absorption spectra cannot be obtained easily using conventional techniques. Excitonic absorptions associated with subband transitions in a GaAs SQW and SPSs were clearly observed. We also examined how a SI-GaAs substrate affects the PPT and SPV spectra, particularly the effect of the photoquenching of the deep donor level EL2. It was found that the photoquenching of EL2 causes a significant change in the total built-in potential at the interface between the epitaxial layers and the substrate, and affected the signal intensities observed in the PPT and SPV spectra. The present experimental results have shown that a large amount of carrier leakage occurs from a GaAs SQW and SPSs to the sample surface, even in the presence of Al0.3Ga0.7As buffer layers.

Effect of Sb composition on the conduction type and photoluminescence of heavily Sn‐doped GaAs 1–x Sb x

Heavily Sn-doped GaAs1-xSbx epitaxial films were grown on SI-GaAs (001) substrates by solid source molecular beam epitaxy. A 5 nm-thick AlSb buffer layer was employed to relax the lattice mismatch between the epilayer and the substrate. X-ray diffraction (XRD), Hall effect measurements and photoluminescence measurements were performed to characterize the epitaxial films. The heavily Sn-doped GaAs1-xSbx / AlSb films with x ≤ 0.24 indicated n-type conduction while the epitaxial films with x ≥ 0.43 indicated p-type conduction. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lateral n–i–p junctions formed in an InSb quantum well by bevel etching

We have used a novel, simple technique based on bevel etching, to fabricate samples containing lateral n–i–p junctions in an InSb/InAlSb quantum well. The structure was designed by self-consistent solution of Schrödinger's and Poisson's equations, and grown by molecular beam epitaxy on a SI GaAs substrate. Current/voltage characteristics were measured as a function of temperature between 10 and 80 K, and rectifying characteristics were obtained over the whole temperature range.

Metal-organic chemical vapor deposition of indium selenide films using a single-source precursor

Indium selenide thin films have been deposited on glass, Si(1 0 0) and polycrystalline GaAs substrates from In[(SeP i Pr 2) 2N] 2Cl precursor by aerosol-assisted (AA) and low-pressure (LP) metal-organic chemical vapor deposition (MOCVD). X-ray powder diffraction (XRPD) patterns of these films indicated the growth of hexagonal γ-In 2Se 3. The morphologies of films have been studied by scanning electron microscopy (SEM) and compositions have been determined by energy dispersive analysis of X-rays (EDAX) and X-ray photoelectron spectroscopy (XPS).