Semi-insulating Substrates Research Articles

AlGaN/GaN High Electron Mobility Transistor Grown and Fabricated on Lattice Matched Metallic Layers

AlGaN/GaN high electron mobility transistors (HEMTs) showed its promising features in high power and high frequency applications such as inverter units in hybrid electric vehicles, advanced radar systems, and satellite-based communication networks with its high density of sheet carrier concentration, high electron mobility, and radiation hardness. In order to lower the cost of HEMTs, larger diameters substrates are needed. Currently, the HEMT structures are grown on non-native sapphire, SiC and Si substrates or on native substrate of vapor phase epitaxy (VPE) GaN wafers. HEMTs grown on sapphire substrate suffer from poor heat dissipation and higher defect density due to the relative larger lattice mismatch to GaN. SiC has a very good thermal conductivity and is less lattice mismatch to GaN, but high quality semi-insulating SiC substrates are quite expensive. Although, Si substrates are fair low cost and available for larger diameter substrates, due to larger lattice mismatch to GaN, it requires to grow thick complex AlGaN buffers prior to the growth of GaN layer to reduce the threading dislocation density. These thick AlGaN layers are very defective and thermal resistive, which will hinder device heat dissipation. VPE grown GaN wafers are limited to 2-3” diameter and very expensive. Recently, it was reported that high quality GaN layers were grown by RF plasma-assisted molecular beam epitaxy (PA-MBE) or metal organic chemical vapor deposition (MOCVD) on high-quality, single crystal ZrTi refractory metal alloys deposited by plasma sputtering on c-plane sapphire and Si substrates. These ZrTi refractory metal alloys are not only lattice match to GaN, but have a similar coefficient of thermal expansion (CTE) as the CTE of GaN [1]. By employing these ZrTi alloys, there is no need to grow thick AlGaN buffer, both photonic and electronic GaN based device structure can be grown on large area Si substrates to achieve low cost GaN based device fabrication. In this work, we have demonstrated AlGaN/GaN HEMTs grown on ZrTi refractory metal alloys, which were deposited on both Si and sapphire substrates. HEMTs with a gate dimension of 1 µm × 100 um were fabricated and characterized. Both dc and rf performance of AlGaN/GaN HEMTs fabricated on the ZrTi metal alloys will be presented. 1. “Epitaxial Growth of High Quality GaN Films on Lattice Matched Metallic Layers”, A.M. Dabiran, F. Machuca, I. De, and R. Weiss, ECS Trans. 66, 113-117 (2015).

(Invited) Manufacturing Microwave AlGaN/GaN High Electron Mobility Transistors (HEMTs) on Truly Bulk Semi-Insulating GaN Substrates

The substrate of choice for high power microwave GaN-based devices is silicon carbide. However recently novel semi-insulating truly bulk GaN substrates with excellent crystalline and electrical parameters have been developed by Ammono S..A. These allow to elaborate AlGaN/GaN heterostructures with high electron mobility values and density of two-dimensional electron gas. Developed processing steps, especially planar isolation by ion implantation and formation of low resistivity regrown ohmic contacts enabled fabrication of high quality devices. An 1000 mA/mm on-state current density along with low 4.4 Ω/mm on-state resistance were achieved. For the devices with rectangular, 0.8 μm gate length the fMAG and fs were 31 GHz and 22 GHz. The maximum output power density was more than 4.15 W/mm in S-band.

Highly repeatable room temperature negative differential resistance in AlN/GaN resonant tunneling diodes grown by molecular beam epitaxy

AlN/GaN resonant tunneling diodes grown on low dislocation density semi-insulating bulk GaN substrates via plasma-assisted molecular-beam epitaxy are reported. The devices were fabricated using a six mask level, fully isolated process. Stable room temperature negative differential resistance (NDR) was observed across the entire sample. The NDR exhibited no hysteresis, background light sensitivity, or degradation of any kind after more than 1000 continuous up-and-down voltage sweeps. The sample exhibited a ∼90% yield of operational devices which routinely displayed an average peak current density of 2.7 kA/cm2 and a peak-to-valley current ratio of ≈1.15 across different sizes.

Structural, optical and electrical characterization of InAs0.83Sb0.17 p-π-n photodetector grown on GaAs substrate

High quality InAs0.83Sb0.17 mid-wavelength infrared p-π-n photodetector structure was grown with the aid of a GaSb transition layer on semi-insulating GaAs substrate by molecular beam epitaxy. The lattice mismatch and the crystal quality of the structure were investigated by high resolution X-ray diffraction rocking curve measurement. The full width at half maximum values for InAs0.83Sb0.17 and GaSb crystals were determined as 215 and 238arcsec, respectively. The lattice mismatch between InAs0.83Sb0.17/GaSb structure and GaAs substrate was identified as 8.44%. Activation energies for two different temperature regimes were extracted from Arrhenius plot which was derived from temperature dependent dark current measurements. The dominant dark current mechanisms were determined as generation-recombination limited and surface leakage based for higher- and lower-temperature regimes, respectively. The cut off wavelength and the peak quantum efficiency of the photodetector were obtained from the spectral photoresponse as 4.23μm and 24.3% at 80K, respectively.

Microstrip antenna–generator based on gallium arsenide

A design of the microstrip antenna with an epitaxial structure used as a dielectric substrate is developed. The epitaxial structure contains a thin layer of n-type gallium arsenide of on a GaAs semi-insulating substrate. The layer thickness is selected with consideration for the carrier depletion region that appears at the interface between the metal layer, which forms the antenna, and the doped semiconductor layer. We demonstrated experimentally the possibility of application of this design as a FET antenna–generator in a frequency range of 10–15 GHz.

Fabrication of Ohmic contact on semi-insulating 4H-SiC substrate by laser thermal annealing

The Ni contact layer was deposited on semi-insulating 4H-SiC substrate by magnetron sputtering. The as-deposited samples were treated by rapid thermal annealing (RTA) and KrF excimer laser thermal annealing (LTA), respectively. The RTA annealed sample is rectifying while the LTA sample is Ohmic. The specific contact resistance (ρc) is 1.97 × 10−3 Ω·cm2, which was determined by the circular transmission line model. High resolution transmission electron microscopy morphologies and selected area electron diffraction patterns demonstrate that the 3C-SiC transition zone is formed in the near-interface region of the SiC after the as-deposited sample is treated by LTA, which is responsible for the Ohmic contact formation in the semi-insulating 4H-SiC.

Low-noise epitaxial graphene on SiC Hall effect element for commercial applications

In this report, we demonstrate a complete Hall effect element that is based on quasi-free-standing monolayer graphene synthesized on a semi-insulating on-axis Si-terminated 6H-SiC substrate in an epitaxial Chemical Vapor Deposition process. The device offers the current-mode sensitivity of 87 V/AT and low excess noise (Hooge's parameter αH < 2 × 10−3) enabling room-temperature magnetic resolution of 650 nT/Hz0.5 at 10 Hz, 95 nT/Hz0.5 at 1 kHz, and 14 nT/Hz0.5 at 100 kHz at the total active area of 0.1275 mm2. The element is passivated with a silicone encapsulant to ensure its electrical stability and environmental resistance. Its processing cycle is suitable for large-scale commercial production and it is available in large quantities through a single growth run on an up to 4-in SiC wafer.

Thermal Stability of Deep-Level Defects in High-Purity Semi-Insulating 4H-SiC Substrate Studied by Admittance Spectroscopy

Effects of high temperature treatments on the deep-level defects in high-purity semi-insulating substrates are studied by employing electrical and chemical analyses. Thermal admittance spectroscopy reveals the presence of a deep-level defect, labeled XMID, with a state near the middle of the bandgap, playing a decisive role to realize the semi-insulating characteristics. The concentration of the XMID level decreases with increasing treatment temperature from 1400 to 1700 °C, and consequently, the substrate becomes more conductive toward p-type due to residual boron impurities. The identity of XMID is yet to be known but the carbon vacancy is briefly discussed as a possible candidate.

Wavelength selective UV/visible metal-semiconductor-metal photodetectors

We report on growth, fabrication and characterization of metal-semiconductor-metal (MSM) photodetectors based on periodic heterostructures with ZnCdS quantum wells separated by ZnMgS and ZnS barrier layers. Heterostructures were grown on semi-insulating GaP substrates by MOVPE. Detecting properties of the MSM-heterophotodiodes have been investigated. We observe electrically tunable spectral response of these detectors. At low bias detectors provide narrowband response at the wavelength 350 nm with FWHM = 18 nm. Increasing bias shifts maximum detector sensitivity at the wavelength 450 nm while narrowband response at 350 nm remains. Thus, a two-color detection of light emission is provided.

Ferroelectric films of barium strontium titanate on semi-insulating silicon carbide substrates

Thin ferroelectric BaxSr1–xTiO3 (BST) layers have been grown for the first time on semi-insulating silicon carbide substrates by RF magnetron sputtering of a ceramic target without using buffer sublayers. Results of investigation of the structure of obtained BST films and the electrical properties of related planar capacitors are presented. The obtained structures are characterized by high nonlinearity and low dielectric losses at microwave frequencies.

Supersensitive graphene-based gas sensor

Epitaxial graphene layers are produced with the aid of thermal destruction of the surface of a semi-insulating SiC substrate. Raman spectroscopy and atomic-force microscopy are employed in the study of the film homogeneity. A prototype of the gas sensor based on the films is fabricated. The device is sensitive to the NO2 molecules at a level of 5 ppb (five particles per billion). A possibility of the industrial application of the sensor is discussed.

Small signal model parameters analysis of GaN and GaAs based HEMTs over temperature for microwave applications

Thermal and small-signal model parameters analysis have been carried out on 0.5μm×(2×100μm) AlGaAs/GaAs HEMT grown on semi-insulating GaAs substrate and 0.25μm×(2×100μm) AlGaN/GaN HEMT grown on SiC substrate. Two different technologies are investigated in order to establish a detailed understanding of their capabilities in terms of frequency and temperature using on-wafer S-parameter measurement over the temperature range from −40 to 150°C up to 50GHz. The equivalent circuit parameters as well as their temperature-dependent behavior of the two technologies were analyzed and discussed for the first time. The principle elevation or degradation of transistor parameters with temperature demonstrates the great potential of GaN device for high frequency and high temperature applications. The result provides some valuable insights for future design optimizations of advanced GaN and a comparison of this with the GaAs technology.

Deep Traps in AlGaN/GaN High Electron Mobility Transistors on SiC

Capacitance-voltage (C-V) and current-voltage (I-V) characteristics and deep trap spectra were measured for high-power HEMT structures grown by metallorganic chemical vapor deposition on semi-insulating 4H-SiC substrates and comprised of an AlGaN barrier and GaN(Fe) semi-insulating buffer with the upper (∼100 nm) portion unintentionally doped and lightly n-type. The C-V measurements were performed in the 85–400 K range in the dark and showed a decrease in the threshold voltage for temperatures higher than 200 K. The change was most prominent for temperatures from 290 K to ∼330 K and occured due to deep traps capturing electrons in the AlGaN barrier and near the AlGaN/GaN interface. C-V measurements at low temperature showed that the traps in the AlGaN barrier are located near Ec−2 eV and have a density of ∼1012 cm−2, while the traps at the interface have an optical ionization threshold near 1.7 eV and the density of ∼4 × 1012 cm−2. Capacitance deep level transient spectroscopy (DLTS) and current DLTS (CDLTS) revealed electron traps with levels near Ec−0.56 eV and hole traps with levels near Ev+0.9 eV and Ev+0.5 eV. The Ec−0.56 eV traps are known to be located near the AlGaN/GaN interface in the GaN buffer. The Ev+0.9 eV hole traps are located in the buffer.

Low-temperature growth of GaSb epilayers on GaAs (001) by molecular beam epitaxy

Non-intentionally doped GaSb epilayers were grown by molecular beam epitaxy (MBE) on highly mismatched semi-insulating GaAs substrate (001) with 2 offcut towards [110]. The effects of substrate temperature and the Sb/Ga flux ratio on the crystalline quality, surface morphology and electrical properties were investigated by Nomarski optical microscopy, X-ray diffraction (XRD) and Hall measurements, respectively. Besides, differential Hall was used to investigate the hole concentration behaviour along the GaSb epilayer. It is found that the crystal quality, electrical properties and surface morphology are markedly dependent on the growth temperature and the group V/III flux ratio. Under the optimized parameters, we demonstrate a low hole concentration at very low growth temperature. Unfortunately, the layers grown at low temperature are characterized by wide FWHM and low Hall mobility.

Extraction of second harmonic from an InP based planar Gunn diode using diamond resonator for milli-metric wave frequencies

Planar Indium Gallium Arsenide (In0.57Ga0.47As) Gunn diode was fabricated on a semi-insulating indium phosphide substrate with on chip matching circuit to enable the extraction of second harmonic in millimeter and terahertz frequencies. The In0.57Ga0.47As planar Gunn diodes were designed with an active length of 4μm, channel width of 120μm and integrated with a novel diamond resonator to suppress the fundamental and extract the second harmonic. The experimental results gave good fundamental suppression and extraction of second harmonic (121.68GHz) with an RF output power of −14.1dBm. This is highest recorded power at the second harmonic from a planar Gunn diode.

1.3-μm-Wavelength AlGaInAs Multiple-Quantum-Well Semi-Insulating Buried-Heterostructure Distributed-Reflector Laser Arrays on Semi-Insulating InP Substrate

1.3-μm-wavelength AlGaInAs distributed-reflector laser arrays on semi-insulating InP substrate for high-speed direct modulation are investigated to realize compact and low-power-consumptive optical modules. Combination of AlGaInAs quantum wells with large differential gain and semi-insulating buried-heterostructure for reduction of the volume in active region achieved high-speed direct modulation. The fabricated lasers in the array lase with different wavelengths with stable single-longitudinal mode. Clear eye-opening and large mask margin were obtained in push-pull driving at 25.8 Gb/s direct modulation with coplanar electrode and semi-insulating InP substrate. Comparison of modulation waveform under simultaneous operation of neighbor lasers shows that crosstalk between lasers in the array is small. Clear eye-openings, large mask margins, and 10-km fiber transmission of four different wavelength lasers on LAN-WDM grid are demonstrated under 28-Gb/s operation at 50 °C. For further high-speed operation, 43-Gb/s direct modulation were performed. These results indicate that the 1.3-μm AlGaInAs DR laser arrays are promising as light sources for 100-Gb/s Ethernet and higher speed transmission.

Selective UV radiation detection on the basis of low-dimensional ZnCdS/ZnMgS/GaP and ZnCdS/ZnS/GaP heterostructures

The detecting properties of periodic heterostructures with ZnCdS quantum wells separated by ZnMgS or ZnS barrier layers are studied. Heterostructures are grown on semi-insulating GaP substrates by metal organic vapor-phase epitaxy (MOVPE). On their basis, metal–semiconductor–metal (MSM) diodes with interdigital Schottky contacts 3 μm, distances between them of 3 μm, and a total detector area of 100 × 100 μm are fabricated. The detectors have low dark currents (10–12 A); at low bias voltages, they provide a narrow- band response (full-width at half-maximum of FWHM = 18 nm at a wavelength of 350 nm) which is controlled by the ZnCdS quantum-well composition. As bias is increased to 70 V, the maximum detector sensitivity shifts by a wavelength of 450 nm, which is caused by penetration of the external-bias electric field into the semi-insulating GaP substrate. In this case, the narrow-band response of the detector at a wavelength of 350 nm is retained, i.e., the two-color detection of light is provided.

Surface photovoltage spectroscopy study of InAs quantum dot in quantum well multilayer structures for infrared photodetectors

Inter-band optical transitions in InAs submonolayer and Stranski-Krastanov quantum dot (QD) in quantum well (QW) nanostructures are studied by means of room temperature surface photovoltage (SPV) spectroscopy taking advantage of its high sensitivity and contactless nature. The QD optical transitions are identified by the combined analysis of SPV amplitude and phase spectra and are in agreement with photoluminescence results. The SPV spectra have further revealed the optical transitions in all other relevant layers in the structures - wetting layer, QWs, and AlGaAs barriers. The analysis of the SPV phase spectra has revealed that the carrier separation and transport in the QD structure is determined by the energy band bending, resulting from the slight residual p-type doping. The complicated interaction between the SPV signals from the nanostructure and the semi-insulating GaAs substrate is discussed and clarified. The advantages of the SPV spectroscopy for characterizing complicated nanostructures at room temperature are highlighted.

Nonpolar m-plane GaN/AlGaN heterostructures with intersubband transitions in the 5–10 THz band

This paper assesses intersubband (ISB) transitions in the 1–10 THz frequency range in nonpolar m-plane GaN/AlGaN multi-quantum-wells deposited on free-standing semi-insulating GaN substrates. The quantum wells (QWs) were designed to contain two confined electronic levels, decoupled from the neighboring wells. Structural analysis reveals flat and regular QWs in the two perpendicular in-plane directions, with high-angle annular dark-field scanning transmission electron microscopy images showing inhomogeneities of the Al composition in the barriers along the growth axis. We do not observe extended structural defects (stacking faults or dislocations) introduced by the epitaxial process. Low-temperature ISB absorption from 1.5 to 9 THz (6.3–37.4 meV) is demonstrated, covering most of the 7–10 THz band forbidden to GaAs-based technologies.

Growth and Characterization of InAs1–xSbx with Different Sb Compositions on GaAs Substrates**Supported by the Aeronautical Science Foundation of China under Grant No 20132435, the National High-Technology Research and Development Program of China under Grant No 2013AA031903, the National Natural Science Foundation of China under Grant Nos 61106013 and 61275107, and the China Postdoctoral Science Foundation under Grant No 2014M560936.

InAs1–xSbx with different compositions is grown by molecular beam epitaxy on (100)-oriented semi-insulating GaAs substrates. The increase of Sb content in the epilayer results in the deterioration of crystal quality and surface morphology. Hall measurements show that the carrier concentration increases with the composition of Sb. The electron mobility decreases initially, when Sb composition exceeds a certain value, and the mobility increases slightly. In this work, we emphasize the comparison of crystal quality, surface morphology and electrical properties of epilayers with different Sb compositions.