Semi-insulating Research Articles

Comprehensive study of the structural, optical and electrical properties of InAlAs: Mg films lattice matched to InP grown by MOVPE

An extensive photoreflectance (PR) and photoluminescence (PL) study is done on an undoped n-type and p-type InAlAs metalorganic vapor phase epitaxy (MOVPE) grown layers on Fe-doped semi-insulating (SI)-InP substrates. A p-type doping with doping concentration of (1.57 × 1018 cm−3) at a growth temperature of 560 °C and V/III ratio of 50 is achieved. Photoluminescence (PL) spectra show a significant improvement in peak intensity of the type II transition for the Mg-doped InAlAs sample proving a better quality of the interface. Moreover, the abundant narrow satellite peaks of X-ray diffraction and the reciprocal space mapping recorded in the vicinity of (004) using high resolution X-ray diffraction (HRXRD) demonstrate a good quality of interfaces. Besides, photoreflectance spectroscopy (PR) at room temperature is useful to experimentally establish the valence-band splitting, a band-gap energy of InP substrate and InAlAs layers. A good correlation between experimental results was obtained.

Terahertz-Wave Characterization of LTG-GaAs Thin-Film Photoconductive Antenna

Low temperature grown (LTG) Gallium Arsenide (GaAs) thin-film based photoconductive antennas (PCAs) are fabricated and their terahertz (THz) performance is compared with conventional bulk LTG-GaAs PCAs. To fabricate the LTG-GaAs thin-film based PCAs, LTG-GaAs thin-films are separated from their semi-insulating (SI)-GaAs substrate and integrated onto bow–tie PCA electrode tips patterned on a silicon dioxide (SiO2) layer thermally grown on a silicon (Si) substrate. Conventional bulk LTG-GaAs PCAs are fabricated by direct photolithography and metallization on the surface of a conventional bulk LTG-GaAs material. THz waves radiated from each of the excited PCA emitters are characterized by a variation of applied bias voltages maintaining the incident optical pump power constant at 10 mW. It was observed that the peak THz radiation signal from the LTG-GaAs thin-film based PCA is higher than the one obtained from the conventional bulk LTG-GaAs PCA structure with the same bias voltage and optical pump laser beam conditions. The performance difference was analyzed using numerical simulations. Based on the unique integration structure, the LTG-GaAs thin-film based PCAs have a potential to improve the THz radiation and enable heterogeneously integrated THz systems.

Effect of dopant density on contact potential difference across n-type GaAs homojunctions using Kelvin Probe Force Microscopy

In this study, cross-sectional surface potential imaging of n+/semi-insulating GaAs junctions is investigated by using amplitude mode kelvin probe force microscopy. The measurements have shown two different potential profiles, related to the difference in surface potential between the semi-insulating (SI) substrate and the epilayers. It is shown that the contact potential difference (CPD) between the tip and the sample is higher on the semi-insulating substrate side than on the n-type epilayer side. This change in CPD across the interface has been explained by means of energy band diagrams indicating the relative Fermi level positions. In addition, it has also been found that the CPD values across the interface are much smaller than the calculated values (on average about 25% of the theoretical values) and increase with the electron density. Therefore, the results presented in study are only in qualitative agreement with the theory.

Current status of modelling the semi-insulating 4H–SiC transient photoconductivity for application to photoconductive switches

In this paper we present the current status of modelling the time evolution of the transient conductivity of photoexcited semi-insulating (SI) 4H–SiC taking into account the properties of defect centres. A comprehensive model that includes the presence of six, the most significant, point defects occurring in SI 4H–SiC crystals is presented. The defect centres are attributed to the two kinds of nitrogen-related shallow donors, a boron-related shallow acceptor, deep electron and hole traps, and the Z 1/2 recombination centre. We present the results of the state-of-the-art numerical simulations showing how the photoconductivity transients change in time and how these changes are affected by the properties of defect centres. The properties of defect centres assumed for modelling are compared with the results of experimental studies of deep-level defects in high purity (HP) SI 4H–SiC wafers performed by the high-resolution photoinduced transient spectroscopy (HRPITS). The simulated photoconductivity transients are also compared with the experimental photocurrent transients for the HP SI 4H–SiC wafers with different deep-level defects. It is shown that a high-temperature annealing producing the C-rich material enables the fast photocurrent transients to be achieved. The presented analysis can be useful for technology of SI 4H–SiC high-power photoconductive switches with suitable characteristics.

Carbon doped GaN layers grown by Pseudo‐Halide Vapour Phase Epitaxy

Pseudo‐Halide Vapour Phase Epitaxy (PHVPE) was utilized to grow thick carbon doped GaN layers to study their suitability as semi‐insulating (SI) substrates. With in‐situ waste gas analysis by FTIR spectroscopy, the carbon doping from the HCN precursor gas was indirectly controlled in a wide range up to 5 × 1019 cm–3. The grown layers were characterized by Scanning Electron Microscopy (SEM), Secondary Ion Mass Spectroscopy (SIMS), Photoluminescence (PL), Micro‐Raman‐Spectroscopy, Hall effect measurement, and High Resolution X‐Ray Diffractometry (HRXRD). Similar to the impurity incorporation (O, Si, Mg and Zn), carbon distribution proved to be inhomogeneous, due to its increased incorporation on facets of so‐called V‐pits.

Characterization of high electron mobility transistor fabricated on hydride vapor phase epitaxy‐grown GaN templates containing various Fe concentrations

An AlGaN/GaN high electron mobility transistor (HEMT) structure was grown on a semi‐insulating (SI) GaN layer or substrate to suppress a leakage current and achieve a high breakdown voltage. Fe can be used as doping material to obtain SI‐GaN. However, the relationship between the Fe concentration in the GaN layers and its influence on HEMT characteristics has not yet been investigated. In this study, we therefore investigated the influence of Fe concentration in the GaN layers on HEMT characteristics. The GaN layers containing several Fe concentrations were grown by hydride vapor phase epitaxy (HVPE). The Fe concentration was varied from 3 × 1017 to 3 × 1020 atoms cm−3. It was determined that the source‐drain current (IDS) decreased as the Fe concentration increased. In the case of a low Fe concentration of 3 × 1017 atoms cm−3, IDS was almost the same as that of HEMT on the non‐Fe‐doped GaN template. Thus, HEMT characteristics deteriorated as the Fe concentration increased. The channel layer played a role in blocking the adverse effects of Fe doping on the two‐dimensional electron gas concentration in AlGaN/GaN. Moreover, it was shown that the GaN channel layer must become thicker as the Fe concentration of the GaN layers increases.

Optimization of UV-assisted wet oxidation of GaAs

Ultraviolet (UV) light-assisted wet oxidation in deionized water (photowashing) was done on commercial semi-insulating (SI) and N-type (Nd ∼ 7.5 × 1017 cm−3) (100) GaAs substrates. Oxidation rate, depth elemental composition profiling, and structural quality of prepared oxide layers and oxide/GaAs interfaces were evaluated for various oxidation times and for four selected intensities of UV light (320–480 nm) irradiation by means of x-ray reflectivity, x-ray photoelectron spectroscopy, and transmission electron microscopy (TEM), respectively. N-type GaAs exhibited ∼30% higher oxidation rates as compared to SI GaAs. Oxidized layers were found to consist predominantly of Ga2O3. Peak gallium concentrations in oxidized layers related to Ga2O3 achieved ∼45 at. %, while concentrations of arsenic related to unfavorable As2O3 were found to be as low as 4–5 and 4–6 at. % for SI GaAs and N-type GaAs, respectively. As2O3 occurrence was not spatially confined and occurred throughout the oxidized layer. Arsenic depth concentration profiling showed no notable humps suggesting arsenic pile-up at the oxide/GaAs interface. TEM revealed strong correlation between UV light intensity and oxide/GaAs interface roughness. Inhomogeneous oxide layers, rough oxide/GaAs interface, and dislocation formation under excessive UV light intensity were observed. Optimized conditions were found to produce homogeneous oxide layers with smooth oxide/GaAs interface.

Low-Loss MMICs Viable Transmission Media for GaN-on-Low Resistivity Silicon Technology

In this work a novel ultra-low loss transmission media for RF GaN-on-low-resistivity silicon (LR-Si) substrates (σ <; 40 Ω.cm) has been successfully demonstrated. The developed shielded-microstrip lines achieve comparable performance to those on semi-insulating (SI) GaAs substrates with transmission loss of 0.9 dB/mm for frequencies up to 67 GHz. Line performance was further enhanced by additional elevation of the shielded-microstrip lines using air-bridge technology above a 5 μm layer of benzocyclobutene (BCB) on shielded metalized ground planes. Transmission loss of 0.6 dB/mm for frequencies up to 67 GHz was obtained as a result of the extra elevation. Structure parameters were designed and optimized based on EM simulation for best performance. The work shows that the RF energy coupled into the substrate was eliminated, indicating the suitability of III-V-on-LR Si technology for millimeter-wave applications.

Roles of voltage in semi-insulating GaAs photoconductive semiconductor switch**Project supported by the National Natural Science Foundation of China (Grant No. 31470822) and the Advanced Research Foundation of China (Grant Nos. 9140A05030114DZ02068, 9140A07030514DZ02101, and 9140A07010715DZ02001).

An experimental study of leakage current is presented in a semi-insulating (SI) GaAs photoconductive semiconductor switch (PCSS) with voltages up to 5.8 kV (average field is 19.3 kV/cm). The leakage current increases nonlinearly with the bias voltage increasing from 1.2×10 A to 3.6×10 A. Furthermore, the dark resistance, which is characterized as a function of electric field, does not monotonically decrease with the field but displays several distinct regimes. By eliminating the field-dependent drift velocity, the free-electron density n is extracted from the current, and then the critical field for each region of n(E) characteristic of PCSS is obtained. It must be the electric field that provides the free electron with sufficient energy to activate the carrier in the trapped state via multiple physical mechanisms, such as impurity ionization, field-dependent EL2 capture, and impact ionization of donor centers EL10 and EL2. The critical fields calculated from the activation energy of these physical processes accord well with the experimental results. Moreover, agreement between the fitting curve and experimental data of J(E), further confirms that the dark-state characteristics are related to these field-dependent processes. The effects of voltage on SI-GaAs PCSS may give us an insight into its physical mechanism.

Sublinear Current–Voltage Characteristics of Linear Photoconductive Semiconductor Switch

This letter presents the measurement of photoconductive semiconductor switch (PCSS) based on semi-insulating (SI) GaAs in its linear mode, through which output voltage saturation and sublinear current–voltage characteristics were obtained. The measurements of its dark-state characteristics were also taken to explore the underlying physical mechanism. Both the resistance and free-carrier concentration of ON-state and dark-state PCSS were extracted and analyzed. Remarkable contrasts between the resistance characteristics and obvious parallels for the features of free-carrier density indicate that the sublinear current–voltage characteristics of PCSS are related to the negative differential mobility and the native defect EL2 levels of SI GaAs. The sublinear current–voltage characteristics and the saturation of output voltage exhibited in terahertz pulse generator based on GaAs PCSS are ascribed to the intrinsic characteristics of PCSS.

Semi-insulating GaAs based detector of fast neutrons produced by D–T nuclear reaction

We have examined semi-insulating (SI) GaAs detectors with high density polyethylene (HDPE) conversion layer by a mono-energetic neutrons with kinetic energy of 16.755 MeV generated by a deuterium—tritium nuclear reaction. First, the influence of HDPE layer thickness on the relative detection efficiency of fast neutrons was studied. The MCNPX (Monte Carlo N-particle eXtended) code has been used to support the analysis of the experiment. The theoretical optimum thickness of the conversion layer was determined to 1.9 mm using the MCNPX code. The HDPE conversion layers of various thicknesses, in the range from 50 μ m to 3200 μ m, were glued on the top Schottky contact of SI GaAs detector in the experiment. The neutron detection efficiency was evaluated from measured spectra and compared to results from simulations. The experimental data showed very good agreement with simulation results. Then the effect of active detector thickness modified by detector reverse bias on neutron detection efficiency was studied. Finally, the effect of the angle of irradiation on neutron detection efficiency was evaluated exhibiting decreasing tendency with increasing deviation from perpendicular direction of impinging neutrons.

Spectrometric properties of semi-insulating GaAs detectors irradiated by 5 MeV electrons at different dose rates

The radiation hardness of Semi-Insulating (SI) GaAs detectors against 5 MeV electrons is investigated in this paper. The influence of two parameters, the accumulative absorbed dose (from 1 to 120 kGy) and the applied dose rate (20, 40 or 80 kGy/h), on detector spectrometric properties was studied. The electron irradiation has negatively affected the detector CCE (Charge Collection Efficiency). Un-irradiated detectors exhibited the CCE of 79% at maximum operating reverse voltage of 300 V and reached the maximum CCE of 51% at 200 V after irradiation by a dose of 120 kGy. Relative energy resolution was also affected by electron irradiation. Its global degradation was observed in the range of doses from 24 up to 120 kGy, where an increase from 19% up to 39% at 200 V reverse voltage was noticed. On the other hand, a global increase of detection efficiency with dose, by about 30% at 120 kGy, was observed with all samples. We did not observe any significant influence of chosen dose rates applied during irradiation on investigated spectrometric properties of detectors.

Photocurrent spectra of semi-insulating GaAs M–S–M diodes: Role of the contacts

Current–voltage (I–V) characteristics and photocurrent (PC) spectra (600–1000nm) of the metal–semiconductor–metal (M–S–M) structures based on high-quality undoped semi-insulating (SI) GaAs with AuGeNi backside contact and different semitransparent top contacts (AuGeNi, Pt, Gd and Nd) are reported, and analysed with the help of a simple physical model. It is shown that the dominant peak in the PC spectra and the change of photocurrent sign can be explained by a presence of two Schottky-like barriers at the top and bottom surfaces. In addition, I–V and PC results show dependence on the bias and its polarity, and on the contact metal used. The possible origins of these effects are discussed.

Semi-insulating GaAs detectors with HDPE layer for detection of fast neutrons from D–T nuclear reaction

Bulk semi-insulating (SI) GaAs detectors optimized for fast-neutron detection were examined using mono-energetic neutrons. The detectors have an active area of 7.36 mm2 defined by a multi-pixel structure of a AuZn Schottky contact allowing a relatively high breakdown voltage (300 V) sufficient for full depletion of the detector structure. The Schottky contact is covered by a HDPE (high density polyethylene) conversion layer, where neutrons transfer their kinetic energy to hydrogen atoms through elastic nuclear collisions. The detectors were exposed to mono-energetic neutrons generated by a deuterium (D)–tritium (T) nuclear reaction at a Van de Graaff accelerator. Neutrons reached a kinetic energy of 16.8 MeV when deuterons were accelerated by 1 MV potential. The influence of the HDPE layer thickness on the detection efficiency of the fast neutrons was studied. The thickness of the conversion layer varied from 50 [Formula: see text]m to 1300 [Formula: see text]m. The increase of the HDPE layer thickness led to a higher detection efficiency due to higher conversion efficiency of the HDPE layer. The effect of the active detector thickness modified by the detector reverse bias voltage on the detection efficiency was also evaluated. By increasing the detector reverse voltage, the detector active volume expands to the depth and also to the sides, slightly increasing the neutron detection efficiency.

Carbon doped GaN buffer layer using propane for high electron mobility transistor applications: Growth and device results

The creation of a semi insulating (SI) buffer layer in AlGaN/GaN High Electron Mobility Transistor (HEMT) devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). In this investigation, we evaluate the use of a gaseous carbon gas precursor, propane, for creating a SI GaN buffer layer in a HEMT structure. The carbon doped profile, using propane gas, is a two stepped profile with a high carbon doping (1.5 × 1018 cm−3) epitaxial layer closest to the substrate and a lower doped layer (3 × 1016 cm−3) closest to the 2DEG channel. Secondary Ion Mass Spectrometry measurement shows a uniform incorporation versus depth, and no memory effect from carbon doping can be seen. The high carbon doping (1.5 × 1018 cm−3) does not influence the surface morphology, and a roughness root-mean-square value of 0.43 nm is obtained from Atomic Force Microscopy. High resolution X-ray diffraction measurements show very sharp peaks and no structural degradation can be seen related to the heavy carbon doped layer. HEMTs are fabricated and show an extremely low drain induced barrier lowering value of 0.1 mV/V, demonstrating an excellent buffer isolation. The carbon doped GaN buffer layer using propane gas is compared to samples using carbon from the trimethylgallium molecule, showing equally low leakage currents, demonstrating the capability of growing highly resistive buffer layers using a gaseous carbon source.

Continuous wave terahertz radiation from antennas fabricated on C¹²-irradiated semi-insulating GaAs.

We demonstrate continuous wave (CW) terahertz generation from antennas fabricated on C12-irradiated semi-insulating (SI) GaAs substrates. The dark current drawn by the antennas fabricated on irradiated substrates is ∼3 to 4 orders of magnitude lower compared to antennas fabricated on un-irradiated substrates, while the photocurrents decrease by only ∼1.5 orders of magnitude. This can be attributed to the strong reduction of the carrier lifetime that is 2.5 orders of magnitude, with values around τ(rec)=0.2 ps. Reduced thermal heating allows for higher bias voltages to the irradiated antenna devices resulting in higher CW terahertz power, just slightly lower than that of low-temperature grown GaAs (LT GaAs)at similar excitation conditions.

Infrared Transmission and Reflectivity Measurements of 4H- and 6H-SiC Single Crystals

Room temperature infrared transmittance and reflectance spectra of 4H and 6H-SiC single crystals were measured by a NEXUS 670 Fourier Transform Infrared-Raman spectrometer. The transmittance and reflectance of non-doped, V-doped semi-insulating (SI), high purity semi-insulating, n-type and p-type SiC wafers have been compared and assessed. The effect of nitrogen and boron concentration on the transmittance is discussed. In addition, the carrier concentrations in 4H-SiC wafers were measured by Raman spectroscopy at room temperature. The influence of nitrogen concentration on the transmittance is also discussed.

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.

Effects of recess process and surface treatment on the threshold voltage of GaN MOSFETs fabricated on a AlGaN/GaN heterostructure

GaN MOSFETs on a AlGaN/GaN heterostructure with a recess gate were fabricated. The charges near the SiO2/GaN interface of the GaN MOSFETs with different etching conditions were evaluated. It was found that stronger bombardment damage in the dry process will bring more charges near the interface and finally make the threshold voltage of the device negative. Nitrogen plasma treatment and ammonia water (NH4OH) treatment were investigated to recover or remove the damaged layer in order to achieve an enhancement-mode (E-mode) device with positive threshold voltage on the dry-recessed semi-insulating (SI) GaN surface. The influence of these treatments on the interface state density was also characterized using the Terman method by using the GaN MOS capacitor. An E-mode GaN MOSFET with a maximum field-effect mobility of 148.1 cm2 V−1 s−1 and a MOS capacitor with an interface state density of 3 × 1011 cm−2 eV−1 were realized by the NH4OH treatment.

Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection.

We report here a photoconductive material for THz detection with sub-picosecond carrier lifetime made by C(12) (Carbon) irradiation on commercially available semi-insulating (SI) GaAs. We are able to reduce the carrier lifetime of SI-GaAs down to sub-picosecond by irradiating it with various irradiation dosages of Carbon (C(12)) ions. With an increase of the irradiation dose from ~10(12) /cm(2) to ~10(15) /cm(2) the carrier lifetime of SI-GaAs monotonously decreases to 0.55 picosecond, whereas that of usual non-irradiated SI-GaAs is ~70 picosecond. This decreased carrier lifetime has resulted in a strong improvement in THz pulse detection compared with normal SI-GaAs. Improvement in signal to noise ratio as well as in detection bandwidth is observed. Carbon irradiated SI-GaAs appears to be an economical alternative to low temperature grown GaAs for fabrication of THz devices.