Wide Bandgap Compound Semiconductors Research Articles

An Aerosol-Assisted Chemical Vapor Deposition Route to Tin-Doped Gallium Oxide Thin Films with Optoelectronic Properties.

Gallium oxide is a wide-bandgap compound semiconductor material renowned for its diverse applications spanning gas sensors, liquid crystal displays, transparent electrodes, and ultraviolet detectors. This paper details the aerosol assisted chemical vapor deposition synthesis of tin doped gallium oxide thin films using gallium acetylacetonate and monobutyltin trichloride dissolved in methanol. It was observed that Sn doping resulted in a reduction in the transmittance of Ga2O3 films within the visible spectrum, while preserving the wide bandgap characteristics of 4.8 eV. Furthermore, Hall effect testing revealed a substantial decrease in the resistivity of Sn-doped Ga2O3 films, reducing it from 4.2 × 106 Ω cm to 2 × 105 Ω cm for the 2.5 at. % Sn:Ga2O3 compared to the nominally undoped Ga2O3.

Controlling Surface Wettability and Plasmonic Resonance of Au/ZnO Heterostructured Films

This work investigated the (0002) textured ZnO films without and with the addition of an Au continuous top layer and its effects on their surface wettability and plasmonic resonance characteristics. The ZnO films were directly fabricated onto glass substrates at the synthesized temperature of 300 °C via a plasma-enhanced chemical vapor deposition (PECVD) system, and the as-synthesized ZnO film exhibited an average optical transmittance value of 85%. The ultraviolet (UV) light irradiation can be applied to enhance the hydrophilicity, changing it from a hydrophobic status to hydrophilic status due to the existing and adjustable characteristics of the photocatalytic activity. On the other hand, the surface wetting/contact angle (CA) value of the ZnO film with a controllable surface wettability switched from 94° (hydrophobicity) to 44° (hydrophilicity), after it was exposed to UV light irradiation for 5 min, and stably reversed back to hydrophobicity (92°) via a post-annealed treatment using rapid thermal annealing (RTA) at 350 °C for 5 min in air. A fast, simple, and reversible method for switching between hydrophilic and hydrophobic status is claimed in this present work. The improved surface plasmonic resonance is owning to the coupled electron and photon oscillations that can be obtained and produced at the interface between the flat Au layer and ZnO (metal/metallic oxide) heterostructured films for future applications of various wide-bandgap compound semiconductors.

Giant and Controllable Photoplasticity and Photoelasticity in Compound Semiconductors.

We show that the wide-band gap compound semiconductors ZnO, ZnS, and CdS feature large photoplastic and photoelastic effects that are mediated by point defects. We measure the mechanical properties of ceramics and single crystals using nanoindentation, and we find that elasticity and plasticity vary strongly with moderate illumination. For instance, the elastic stiffness of ZnO can increase by greater than 40% due to blue illumination of intensity 1.4 mW/cm^{2}. Above-band-gap illumination (e.g., uv light) has the strongest effect, and the relative effect of subband gap illumination varies between samples-a clear sign of defect-mediated processes. We show giant optomechanical effects can be tuned by materials processing, and that processing dependence can be understood within a framework of point defect equilibrium. The photoplastic effect can be understood by a long-established theory of charged dislocation motion. The photoelastic effect requires a new theoretical framework which we present using density functional theory to study the effect of point defect ionization on local lattice structure and elastic tensors. Our results update the longstanding but lesser-studied field of semiconductor optomechanics, and suggest interesting applications.

Enhanced light collection from a gallium nitride color center using a near index-matched solid immersion lens

Among wide-bandgap compound semiconductors, gallium nitride is the most widely available material due to its prevalence in the solid state lighting and high-speed/high-power electronics industries. It is now known that GaN is one of only a handful of materials to host color centers that emit quantum light at room temperature. In this paper, we report on a bright color center in a semi-polar gallium nitride substrate emitting at room temperature in the near-infrared. We show that a hemispherical solid immersion lens, near index matched to the semiconductor, can be used to enhance the photon collection efficiency by a factor of 4.3 ± 0.1 while improving the lateral resolution by a factor equal to the refractive index of the lens.

Hybrid optical measurement technique for detection of defects in epitaxially grown 4H-SiC layers

Recent developments in power electronics require the use of new wide bandgap compound semiconductor. We demonstrate the use of the ellipsometry and white light interference microscopy to detect defects in epitaxially grown SiC layers on SiC substrates. Such hybrid optical metrology methods can be used to better understand the mechanism of the development of the defects as well as their effects on the material´s optoelectronic properties.

Vertical β-Ga₂O₃ Power Transistors: A Review

With projected performance advantages over silicon and incumbent wide-bandgap compound semiconductors, gallium oxide (Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) has garnered worldwide attention as an ultrawide-bandgap semiconductor material suitable for high-voltage, high-temperature, and radiation-hard electronics. Thanks to recent breakthroughs in crystal growth and device processing technologies, the research and development of vertically oriented Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> power transistors has made rapid strides. In this article, we review the important engineering achievements and performance milestones of the two major types of vertical Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> transistors-current aperture vertical metal-oxide-semiconductor field-effect transistors (MOSFETs) and vertical fin-channel MOSFETs. Challenges underlying the unique processing approaches to these devices and their implications on device reliability are also discussed.

Electron paramagnetic resonance signatures of Co2+ and Cu2+ in β -Ga2O3

Gallium oxide (β-Ga2O3) is a wide-bandgap compound semiconductor with a bandgap of ∼4.9 eV that is currently considered promising for a wide range of applications ranging from transparent conducting electrodes to UV optoelectronic devices and power electronics. However, all of these applications require a reliable and precise control of electrical and optical properties of the material, which can be largely affected by impurities, such as transition metals commonly present during the growth. In this work, we employ electron paramagnetic resonance (EPR) spectroscopy to obtain EPR signatures of the 3d-transition metals Co2+ and Cu2+ in β-Ga2O3 bulk crystals and powders that were unknown so far. Furthermore, we show that both Co2+ and Cu2+ preferentially reside on the octahedral gallium lattice site.

Silicon Carbide Microstrip Radiation Detectors

Compared with the most commonly used silicon and germanium, which need to work at cryogenic or low temperatures to decrease their noise levels, wide-bandgap compound semiconductors such as silicon carbide allow the operation of radiation detectors at room temperature, with high performance, and without the use of any bulky and expensive cooling equipment. In this work, we investigated the electrical and spectroscopic performance of an innovative position-sensitive semiconductor radiation detector in epitaxial 4H-SiC. The full depletion of the epitaxial layer (124 µm, 5.2 × 1013 cm−3) was reached by biasing the detector up to 600 V. For comparison, two different microstrip detectors were fully characterized from −20 °C to +107 °C. The obtained results show that our prototype detector is suitable for high resolution X-ray spectroscopy with imaging capability in a wide range of operating temperatures.

(Invited) Coupled Experimental and Numerical Investigation of High-Voltage GaN Hemts

The wide band gap compound semiconductor Gallium Nitride and its ternary alloys have found commercial acceptance in the fabrication of high electron-mobility transistor (HEMT) rf devices. In addition, high power applications have been very attractive due to their high breakdown voltage and thus high output power. Higher power limit improvements can be reached by several means, including the integration of field-plates above the conduction channel, which tend to improve the uniformity of the electric-field distribution and reduce its peaks. These field-plates can be connected to the source or to the gate. The reduction of electrical and thermal “hotspots,” where severe degradation is thought to initiate, increases the allowable power levels for HEMT devices by as many as five folds. In this study, GaN HEMT devices with source-connected field-plates are characterized thermally to quantify their performance. Deep submicron resolution thermoreflectance (TR) temperature imaging is used to measure the temperature profile of the exposed surface features of the device, which are mainly the source and drain metal contacts as well as the GaN channel region. The TR thermography method is based on detecting changes in surface reflectivity (ΔR) that result from changes in local surface temperature (ΔT). The relationship between the changes in reflectivity and temperature is linear to first-order effects, and is typically referred to as the coefficient of thermoreflectance (CTR). As expected, this coefficient depends on the material and the wavelength of light, but also on any transparent layers (e.g., passivation) in the optical path between the first reflective surface and the sensor where the light is collected. Therefore, the success of a TR measurement requires that the surface of interest exhibit both a high reflective index and be thermally-reflective, i.e., have a good value of CTR. By using a near ultra-violet source to illuminate the device under test (DUT), the GaN is rendered opaque, thus ensuring that the obtained temperature fields are those of the GaN surface and not an integrated average over multiple semi-transparent layers. However, the presence of the field-plate obstructs imaging access to the gate where the peak temperatures are expected. Therefore, this investigation couples the measurements to 3D computations in order to produce temperature fields that would otherwise be inaccessible to direct measurement. Both quasi-steady-state and transient temperature fields of the GaN HEMTs are obtained at different activation power levels and an experimental assessment is made with regard to the electrical activation limits and the thermal self-heating. In addition to the measurements, thermal computations are performed using a self-adaptive, multi-scale, numerical simulation engine. The 3D transient thermal model is then driven by the experimentally-obtained temperature maps to provide a more detailed and device-specific interpretation of the coupled electro-thermal effect of the field-plates on reducing the temperature and increasing the break-down voltage. Moreover, the full 3D thermal model also helps provide physically-accurate temperature fields of regions of critical channel regions, where the heat sources are located, and where localized hotspots that are hidden underneath field-plates are inaccessible to optical measurement approaches. The attached figure contains two temperature fields, numerical on the left and experimental on the right. These results are representative of the type of useful data that can be obtained from the computational and measurement techniques. The presentation will additionally include the results of the coupling of the two approaches and the insights that can be gained, especially for those features that are optically inaccessible. Figure 1

Deposition and Optical Characterization of ZnO Thin films on Glass Substrate

ZnO is a wide band gap (3.37 eV) compound semiconductor which can be used in short wavelength optoelectronic devices.Here in this work we chose spin coating method as growth mechanism as it is simple, less expensive and in addition film uniformity is maintained in this method.In the growth process zinc acetate dihydrate, iso-propanol and monoethanolamine were used for producing Sol-gel Solution.The prepared solution was kept for 3 days (44 h) and 7 days (188h) for aging. After deposition, optical characterization has been performed by UV-VIS-Spectrometry The Transmittance values of the films deposited at the speeds of 2500 rpm (with 44 h aged solution) and 3500 rpm (with 188h aged solution) are at the range of 70-73% and the Energy band gaps of the films are 3.1 eV and 3.25eV, respectively. Whereas films deposited at the speed of 4000 rpm shows 86%transmittance and band gap energy of 3.3 eV which is much closer to the optimum value of 3.37 eV. Lower absorption values have been shown for higher coating speed.So with the increase in coating speed,the transmittance and the band gap both are increased. Also we can see that there is very negligible effect of aging time on optical properties of deposited films.

Oxygen out-diffusion and compositional changes in zinc oxide during ytterbium ions bombardment

Oxygen release and out-diffusion in zinc oxide crystals during heavy ions bombardment has been suggested by many experimental techniques. In this work we have employed secondary ion mass spectrometry to study ZnO implanted with ytterbium ions. Our measurements confirm formation of an oxygen-depleted layer and oxygen out-diffusion and agglomeration at the surface. Moreover, an average compositional change in a heavily damaged near-surface region can also be monitored. This reproducible measurement procedure with subnanometer depth resolution allows to localize precisely these altered layers at the depth of 14–28 nm (oxygen-depleted layer) and 9 nm (maximum of the amorphized region). Such precise measurements may prove to be valuable for further characterization of ion beam induced defects in wide bandgap compound semiconductors and optimization of optoelectronic devices based on these materials.

Self-rectifying resistive switching behavior observed in Al2O3-based resistive switching memory devices with p-AlGaN semiconductor bottom electrode

In this work, self-rectifying unipolar resistive switching (RS) behavior is demonstrated in Al2O3-based resistive random access memory (RRAM) devices by employing wide bandgap compound semiconductors, such as p-GaN and p-AlGaN, as a bottom electrode (BE). Compared to an RS characteristic of the Al2O3 RRAM with p-GaN BE, the memory cell with p-AlGaN BE shows a lager current ratio while a current-limited region is also more clearly obtained in a low voltage region, which can result in one diode type RRAM with self-selecting properties. Consequently, these results infer that the proposed Al2O3-based RRAM cells with a wide bandgap BE warrant the realization of selector-free RRAM cell without any additional peripheral elements to suppress a disturbance in reading operation.

Mechanism of damage buildup in ion bombarded ZnO

Ion beam induced defect processes in wide bandgap compound semiconductors are rather complex because of the formation of different types of defects and their transformations. In this paper, we report results of the study on the mechanism of defect formation, migration and agglomeration in 300 keV Ar-ion bombarded ZnO single crystals. RBS/c analysis performed with the use of the unique McChasy code allowed determination of depth distributions for different defect types separately. Complementary HRXRD analysis was applied to study lattice deformation due to ion bombardment. It has been observed that migration of simple defects and their agglomeration lead to the formation of two types of dislocation loops: basal loops located at the depth corresponding to the range of incident Ar ions and prismatic loops located beyond the ion range. The stress induced by defects in the bombarded layer has been identified as the driving force for defect migration and loop formation. As soon as the critical stress is attained bombarded layer undergoes plastic deformation resulting in the formation of a dense dislocation tangle.

Preparation and study of doped ZnS thin films

Zinc sulfide (ZnS) is a wide-bandgap compound semiconductor that is broadly used for various micro- and opto-electronic applications. ZnS is also applied as an emitter layer in thin-film photovoltaic devices. The preparation of thin-film photovoltaic devices requires suitable methods for doping semiconductor layers up to the desired level. For this purpose, in the present work, an ion assisted deposition system was developed and evaluated. We used a combination of simple thermal vacuum evaporation coupled with plasma discharge in the reactive gas. The results of this study show that the developed deposition system allows for in-situ doping control of ZnS thin films.

Assessing the Composition of Wide Bandgap Compound Semiconductors by Atom Probe Tomography: A Metrological Problem

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Low Temperature Metalorganic Chemical Vapor Deposition of Semiconductor Thin Films for Surface Passivation of Photovoltaic Devices

ABSTRACT Three II-VI wide bandgap compound semiconductors have been investigated for surface passivation of various photovoltaic devices. First part of this work focuses on the surface passivation of HgCdTe IR detectors using CdTe. A new metalorganic chemical vapor deposition (MOCVD) process has been developed that involves depositing CdTe films at much lower temperature (&amp;lt; 175°C) than the conventional processes used till now. Deposition rate as high as 420nm/h was obtained using this novel experimental setup. Favorable conformal coverage on high aspect ratio HgCdTe devices along with a significant minority carrier lifetime improvement was obtained. Another II-VI semiconductor, namely, CdS was investigated as a surface passivant for HgCdTe IR detectors. It was deposited by MOCVD as well as atomic layer deposition (ALD) and was studied for optimal conformal coverage on high aspect ratio structures. Surface passivation of p-type Si wafer has also been demonstrated using p-ZnTe grown by MOCVD, for possible application in solar cells. Preliminary work showed a remarkable improvement in the minority carrier lifetime of Si light absorbing layer after passivation with a thin layer of ZnTe.

Bare and boron-doped cubic silicon carbide nanowires for electrochemical detection of nitrite sensitively.

Fabrication of eletrochemical sensors based on wide bandgap compound semiconductors has attracted increasing interest in recent years. Here we report for the first time electrochemical nitrite sensors based on cubic silicon carbide (SiC) nanowires (NWs) with smooth surface and boron-doped cubic SiC NWs with fin-like structure. Multiple techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) were used to characterize SiC and boron-doped SiC NWs. As for the electrochemical behavior of both SiC NWs electrode, the cyclic voltammetric results show that both SiC electrodes exhibit wide potential window and excellent electrocatalytic activity toward nitrite oxidation. Differential pulse voltammetry (DPV) determination reveals that there exists a good linear relationship between the oxidation peak current and the concentration in the range of 50–15000 μmoL L−1 (cubic SiC NWs) and 5–8000 μmoL L−1 (B-doped cubic SiC NWs) with the detection limitation of 5 and 0.5 μmoL L−1 respectively. Compared with previously reported results, both as-prepared nitrite sensors exhibit wider linear response range with comparable high sensitivity, high stability and reproducibility.

Enhanced Electrical and Nuclear Radiation Detection Performance in BiI3 Wide Bandgap Semiconductor Detectors

BiI3 is a wide band-gap compound semiconductor with a high effective atomic number that is anticipated to exhibit higher detection efficiency than other compound semiconductors such as HgI2, PbI2, and CdZnTe. This makes BiI3 of particular interest for moderate and high energy gamma-ray detection applications. However, the low resistivity of BiI3 results in high leakage currents and degrades the electrical properties and detecting performance of the detectors. Here we show that the main reason for the low resistivity in BiI3 is due to the high volatility of iodine and the high concentration of intrinsic Schottky defects. Furthermore, we will discuss novel defect engineering strategies that successfully mitigate the obstacles associated with iodine vacancies in the material. Density functional theory (DFT) calculations, as well as experimentally measured electrical properties, and radiation response will be presented for undoped and Sb-doped BiI3 (SBI) single crystals grown via the vertical Bridgman growth technique. Most importantly, we will demonstrate the first ever recorded gamma-ray spectrum using BiI3-based detectors.

Application of Electron Backscatter Diffraction (EBSD) for Crystallographic Characterization of Aluminum-Doped Zinc Oxide Sputtered Films

Zinc Oxide is a wide band-gap compound semiconductor that has been used in optoelectronic and photovoltaic applications due to its good electrical and optical properties. Aluminium has been an efficient n-type dopant for ZnO to produce low resistivity films and high transparency to visible light. In addition, the improvement of these properties also depends on the morphology, crystalline structure and deposition parameters. In this work, ZnO:Al films were produced by d.c. pulsed magnetron sputtering deposition from a ZnO ceramic target (2.0 wt% Al2O3) on glass substrates, at a temperature of 250 ºC.The crystallographic orientation of aluminum doped zinc oxide (ZnO:Al) thin films has been studied by Electron Backscatter Diffraction (EBSD) technique. EBSD coupled with Scanning Electron Microscopy (SEM) is a powerful tool for the microstructural and crystallographic characterization of a wide range of materials.The investigation by EBSD technique of such films presents some challenges since this analysis requires a flat and smooth surface. This is a necessary condition to avoid any shadow effects during the experiments performed with high tilting conditions (70º). This is also essential to ensure a good control of the three dimensional projection of the crystalline axes on the geometrical references related to the sample.Crystalline texture is described by the inverse pole figure (IPF) maps (Figure 1). Through EBSD analysis it was observed that the external surface of the film presents a strong texture on the basal plane orientation (grains highlighted in red colour). Furthermore it was possible to verify that the grain size strongly depends on the deposition time (Figure 1 (a) and (b)). The electrical and optical film properties improve with increasing of the grain size, which can be mainly, attributed to the decrease in scattering grain boundaries which leads to an increasing in carrier mobility (Figure 2).The authors kindly acknowledge the financial support from the Portuguese Foundation for Science and Technology (FCT) scientific program for the National Network of Electron Microscopy (RNME) EDE/1511/RME/2005.

Temperature dependence of the average electron-hole pair creation energy in Al0.8Ga0.2As

The temperature dependence of the average energy consumed in the creation of an electron-hole pair in the wide bandgap compound semiconductor Al0.8Ga0.2As is reported following X-ray measurements made using an Al0.8Ga0.2As photodiode diode coupled to a low-noise charge-sensitive preamplifier operating in spectroscopic photon counting mode. The temperature dependence is reported over the range of 261 K–342 K and is found to be best represented by the equation εAlGaAs = 7.327–0.0077 T, where εAlGaAs is the average electron-hole pair creation energy in eV and T is the temperature in K.