Ultra-wide Bandgap Semiconductor Research Articles

(Invited) Sub-Bandgap Thermoreflectance Imaging of Ultra-Wide Bandgap Semiconductors

Over the past few decades, wide bandgap semiconductors (WBG) have been used to fabricate high power and high frequency devices. Device structures, such as Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs), have begun to be implemented in commercial products such as electrical vehicles and laptop power chargers. With the continuous increase in energy demand, however, extremely high voltage (>10 kV) applications are needed to increase the grid resiliency, storage, and transmission efficiency. Ultra-wide bandgap (UWBG) Semiconductors, with bandgaps >4 eV, could potentially be used to overcome this barrier. Pushing the maximum voltage limit of these devices, however, can lead to significant power dissipation and excessive joule heating.Unfortunately, the thermal conductivity of some UWBG semiconductors, such as Gallium Oxide, is inherently low and thus requires both device and package level thermal management. Innovative solutions, such as high thermal conductivity substrates, are required to reduce the active channel temperature and extend the device lifetime and reliability. In order to successfully implement these technologies, accurate measurement of the peak temperature and thermal distribution (both vertical and lateral) is necessary. Depth-averaged vertical temperature gradients have been demonstrated via Raman thermometry but this approach can become significantly time consuming if attempting to perform transient or lateral maps. On the other hand, Transient Thermoreflectance Imaging (TTI) has shown the potential to provide high throughput thermal images with high spatial resolution (100 nm) and temporal resolution (50 ns).Traditionally, TTI has been used to probe the temperature of metals due to their inherent high reflectivity. Nevertheless, recent studies have demonstrated the ability to probe the surface temperature of the semiconductor when using near-bandgap illumination sources. Due to the high bandgap of UWBG semiconductors, the maturity of high transmission UV sources, optics and detectors has not yet been fully developed. This study investigates the applicability of sub-bandgap wavelengths to probe the active channel temperature. The thermal properties of both Gallium Oxide transistors and Transfer Length Method (TLM) structures are investigated using TTI. The accuracy of the channel temperature is assessed by direct comparison to the adjacent metal contact temperature. A finite element model (FEM) is developed to understand the thermal transport in low thermally conductive semiconductors. Finally, a 3D analytical model is used to extract thermal properties such as the thermal conductivity of the thin film layers.Figure a) Optical CCD Image of Gallium Oxide Transfer Length Method (TLM) Structure b) Transient Thermoreflectance Image (TTI) of Gallium Oxide TLM using sub-bandgap illumination wavelength. Figure 1

Doping the Undopable: Hybrid Molecular Beam Epitaxy Growth, n-Type Doping, and Field-Effect Transistor Using CaSnO3.

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all of the perovskite oxides. CaSnO3 has the highest measured band gap in this family and is thus a particularly promising ultrawide band gap semiconductor. However, discouraging results from previous theoretical studies and failed doping attempts had described this material as "undopable". Here we redeem CaSnO3 using hybrid molecular beam epitaxy, which provides an adsorption-controlled growth for the phase-pure, epitaxial, and stoichiometric CaSnO3 films. By introducing lanthanum (La) as an n-type dopant, we demonstrate the robust and predictable doping of CaSnO3 with free electron concentrations, n3D, from 3.3 × 1019 cm-3 to 1.6 × 1020 cm-3. The films exhibit a maximum room-temperature mobility of 42 cm2 V-1 s-1 at n3D = 3.3 × 1019 cm-3. Despite having a comparable radius as the host ion, La expands the lattice parameter. Using density functional calculations, this effect is attributed to the energy gain by lowering the conduction band upon volume expansion. Finally, we exploit robust doping by fabricating CaSnO3-based field-effect transistors. The transistors show promise for CaSnO3's high-voltage capabilities by exhibiting low off-state leakage below 2 × 10-5 mA/mm at a drain-source voltage of 100 V and on-off ratios exceeding 106. This work serves as a starting point for future studies on the semiconducting properties of CaSnO3 and many devices that could benefit from CaSnO3's exceptionally wide band gap.

Analysis and Classification of Charge Trapping States in β-Ga2O3 Mosfets Using I-V and C-V in Dark and Illumination Conditions

Beta-gallium oxide (β-Ga2O3) is a promising ultra-wide bandgap semiconductor for high power applications. Its breakdown electric field of 8 MV cm-1, a Baliga figure of merit 3-10 times larger than that of SiC and GaN, and the ability to be grown from the melt make it highly attractive for the semiconductor industry. Significant improvements have been made in β-Ga2O3 device performance for both high power and high frequency operation. Currently, aluminum oxide (Al2O3) is the preferred gate dielectric for β-Ga2O3 transistors due to its electron and hole barriers, and similar atomic structure. Process optimizations to improve the interface quality has been explored by chemical treatments1 to the β-Ga2O3 surface before dielectric deposition, during the deposition of Al2O3, and with post-deposition annealing. Therefore, there is a desire to understand and classify defects at and near the Al2O3/β-Ga2O3 interface. Here, we report on a study of the Al2O3/β-Ga2O3 interface and border traps in planar depletion-mode β-Ga2O3 MOSFETs using DC I-V and C-V measurements in dark and under illumination with wavelengths from 730 nm (1.7 eV) to 230 nm (5.4 eV). The MOSFETs are fabricated on a 50 nm Si-doped epitaxial layer with a target doping concentration of 2.4 x 1018 cm-3 grown on a (010) semi-insulating β-Ga2O3 substrate. A Ti/Al/Ni/Au metal stack is deposited and annealed to form the Ohmic contacts. Plasma-assisted atomic layer deposition (PE-ALD) is used to deposit the 20 nm Al2O3 gate dielectric. The fabricated transistor structures have a LG of 1.94 µm, LS of 0.5 µm, and LD varying from 0.5 µm, 5.5 µm, and 10.5 µm. Measurements were performed at room temperature. Most FETs exhibited a threshold voltage of approximately -4 V, high linearity, and ION/IOFF ratios between 107 – 109. Using the conductance method, we extract an interface trap state density, Dit, of 3 x 1011 eV-1 cm-2 up to 0.44 eV below the conduction band. Illumination is used to obtain deep trap concentration and behavior. We observe little to no change in IV and CV curves for wavelengths above 365 nm but see a significant voltage shift in the CV curves for illumination wavelengths between 365 nm (3.4 eV) to 230 nm (5.4 eV), indicating significant activated traps from 3.4 eV below the conduction band. We will present the analysis of photo-assisted CV and stress-CV curves at various wavelengths to obtain densities of fixed, fast, and slow trap states and discuss their impact on device performance.

2D Amorphous GaOX Gate Dielectric for β-Ga2O3 Metal-Insulator-Semiconductor Field-Effect Transistors

Beta gallium oxide (β-Ga2O3) is an n-type ultra-wide bandgap semiconductor material with a reported bandgap from 4.5 eV to 4.9 eV. β-Ga2O3 is a frontrunner in the research for next-generation power devices owing to its numerous merits including the wide range of donor doping concentrations, large bandgap, high theroretical value of critical electric field (~8 MV/cm) leading to substantial Baliga’s figure of merit of ~1900, and the low production cost for crystalline wafers. The last is enabled by the commercial development of several bulk growth techniques, namely, melt growth methods including Czochralski and edge-defined film-fed growth, and epitaxial growth. The metal-insulator-semiconductor field-effect transistor (MISFETs) is the most widely used device structure since it often requires less complicated fabrication process and offers better device scaling and faster switching speed than their counterparts such as junction field-effect transistors or bipolar junction transistors. Various dielectric materials, such as silicon dioxide, hafnium oxide, and aluminium oxide, have been explored for incorporation into β-Ga2O3 MISFETs. However, many of the dielectrics require high-temperature and/or high-vacuum processes during fabrication. Recent studies have reported on the self-limiting formation of native oxide film on the surface of liquid gallium at room temperature and the application of the oxide film as a passivation or a dielectric layer in a nano-scale device. In this study, two-dimensional (2D) amorphous gallium oxide (a-Ga2O3) film is fabricated through the liquid gallium squeezing technique and is integrated with a β-Ga2O3 field-effect transistor (FET) to demonstrate an a-Ga2O3/β-Ga2O3 MISFET.In this work, β-Ga2O3 nano-flakes were obtained through mechanical exfoliation and were dry-transferred onto Si/SiO2 substrates. Electron-beam lithography and electron-beam evaporation were performed to define and deposit Ti/Au electrodes, respectively, where rapid thermal annealing was performed afterward to form Ohmic source and drain contacts. Polypropylene carbonate (PPC) solution was spin-coated and baked on a polydimethylsiloxane (PDMS) film, placed on a glass slide for mechanical support, to form a uniform PPC layer on the PDMS film. A liquid gallium droplet was placed on the PPC/PDMS film and was squeezed with a cleaned glass slide to form a uniform 2D a-Ga2O3 layer. The fabricated a-Ga2O3 layer was dry-transferred onto a β-Ga2O3 FET during which temperature of 90 °C was applied to the sample chuck to completely release the PPC layer with the a-Ga2O3 layer from the PDMS layer. The resulting a-Ga2O3/β-Ga2O3 FET was submerged in acetone to remove the PPC layer. The above dry-transfer process of the a-Ga2O3 layer was conducted twice to form double-layered a-Ga2O3 on the β-Ga2O3 FET, before performing electron-beam lithography and electron-beam evaporation to respectively define and deposit the Ni/Au top-gate electrode.The electrical properties of the fabricated device is characterized as a Ni/a-Ga2O3/β-Ga2O3 heterojunction and an a-Ga2O3/β-Ga2O3 MISFET at temperatures of 25 °C, 150 °C, and 250 °C. The Ni/a-Ga2O3/β-Ga2O3 heterojunction of the device shows a clear rectification characteristic with a wider operating voltage compared to those of standard Ni/β-Ga2O3 devices in literatures. The double-layered a-Ga2O3 The a-Ga2O3/β-Ga2O3 MISFET exhibits normally-on n-type conductivity with stable operation at temperatures from 25 °C to 250 °C, where the obtained on/off current ratio and mobility range from 5.3×108 to 4.8×107 and 4.7 cm2/V·s to 6.9 cm2/V·s, respectively. The subthreshold swing and the threshold voltage of the a-Ga2O3/β-Ga2O3 MISFET showed increases and negative shifts at higher temperatures, respectively. This work demonstrates the facile fabrication process of the a-Ga2O3/β-Ga2O3 MISFET and its high-temperature stability through the integration of the 2D amorphous Ga2O3 gate dielectrics with a β-Ga2O3 semiconductor.

Photo-Enhanced Inverse Metal-Assisted Chemical Etching of α-Ga2O3 grown on Al2O3

Si-based semiconductor devices are not suitable for stable operations in the aerospace environments. Among the ultra-wide bandgap semiconductor materials, gallium oxide (Ga2O3) is attracting attention as the next-generation material for high-power semiconductor devices in extreme condition beyond Si, owing to its large band gap of 4.5-5.3 eV, high breakdown electric fields of 7-10 MV/cm, excellent chemical and thermal stability and radiation hardness. Alpha gallium oxide (α-Ga2O3) has the largest bandgap (4.8-5.3 eV) and the highest breakdown electric fields (~10 MV/cm) among the five Ga2O3 polymorphs, which facilitates the application of α-Ga2O3 as a high power device. However, the research on etching technology for α-Ga2O3 is rare. Etching technology is a crucial step of device fabrication. Chemical etching is free from plasma-induced damage which leads to superior device performance compared with dry etching, and high throughput with low cost is beneficial to industrial implementations. Chemical stability of Ga2O3 hinders the effective reaction with most chemical etchants. However, the photo-enhanced inverse metal assisted chemical (I-MAC) etching, where a noble metal with a high work function is utilized as a catalyst under UV irradiation, has emerged as a candidate for the chemical etching of Ga2O3. Deep-UV irradiation generates electron-hole pairs (EHPs) in the uncovered region of Ga2O3, and the metal electrode withdraws carriers from the photo-generated EHPs. The holes that are accumulated in the uncovered region of Ga2O3 react with gallium ions which produce gallium fluoride (GaF3) in a reaction with Hydrofluoric acid (HF). The etching process continues as the oxidant re-oxidizes the metal. The etch rate can be controlled by various parameter, including concentration and temperature of the etchant. In this work, for the I-MAC etch of α-Ga2O3 on sapphire substrate grown by halide vapor phase epitaxy, Pt was deposited on α-Ga2O3. Etch rate and surface roughness were characterized by atomic force microscopy after each step of the I-MAC etch. The I-MAC etch using HF and potassium persulfate (K2S2O8) solution with 185-nm UV irradiation was performed at constant durations under different etchant temperature conditions. The etch rates obtained at each temperature condition were fitted with the Arrhenius plot to estimate the activation energy of 0.898 eV. The etch depth showed a linear increase with time and the etch rate exhibits a direct dependency on the temperature of the etchant, indicating that the I-MAC etching of α-Ga2O3 is an activation-controlled reaction. As the I-MAC etching proceeded, surface roughness of α-Ga2O3 showed a tendency to increase. The rate at which the surface roughness increased increased with raising the etchant temperature. In the case of Pt there was no significant difference in the surface roughness after etching. After the completion of the I-MAC etch, the remaining Pt can be removed using aqua regia. The I-MAC etch process of α-Ga2O3 can allow us to fabricate α-Ga2O3 without plasma-damage. Figure 1

(Invited) Gallium Oxides Devices for Grid-Scale Power Electronics and High Power Switched Mode RF Amplifiers

Within a decade of the first demonstration of a MESFET device, monoclinic β-Ga2O3 (Ga2O3) devices have made incredible progress in breakdown voltages, power device figure of merit and high-speed performance. It has emerged as a promising ultra-widebandgap semiconductor for next generation power, GHz switching and RF applications. In this talk, we will present an overview of our studies on fundamental transport properties, kilovolt class power MOSFETs and aggressively scaled RF devices. The large bandgap of Ga2O3 leads to a high critical field strength. This high field strength in combination with demonstrated room temperature mobility and calculated electron velocity leads to a higher Baliga’s Figure of Merit (BFoM) and Johnston Figure of Merit (JFoM) than current commercially available widebandgap technologies. Additionally, the large bandgap also enables high temperature operation and radiation hardness making it attractive for space applications such as Mars and Venus missions.In the first part, we will discuss the deep insights gained from the first-principles based transport calculations in Ga2O3. We will discuss the limits to low field mobility, saturation velocities and impact ionization coefficients. The impact of the plasmon-phonon coupling on the phonon limited transport will be discussed. We will present the experimental signatures of plasmon-phonon coupling and anti-screening behavior in ionic gated accumulation layers.Next, I will describe our work on the lateral MOSFETs with improved field plate design and beyond-kV breakdown. Temperature dependent analysis and device simulation suggest an extrinsic breakdown mechanism outside the channel. A simple and yet effective SU-8 polymer passivation technology was developed that results in significant improvement in breakdown voltages. The higher field strength of the SU-8 polymer enables a significant increase in breakdown voltage to 8 kVs in lateral MOSFETs. However, these devices show a high Ron, which is due to the depletion caused by RIE of the channel. We will present the use of ultra-high vacuum annealing techniques to improve the on-resistance of the devices still maintaining the multi-kilo-volt rating of the devices.Due to its high JFoM, Ga2O3 is a potential candidate for high power density RF amplifiers. We will also present the performance advancement that can be obtained in RF power performance using Ga2O3 technology. We will present the challenges in the technology and potential solutions. We will present the experimental DC and pulsed measurements on laterally scaled MOSFETs with 100 nm gate length. Severe DC-RF dispersion was observed which is attributed to the RIE damage and surface states on the oxide. Passivation of the devices shows reduced current collapse. Using aggressive gate lengths and gate-source spacing scaling, (AlxGa1-x)2O3/Ga2O3 heterostructure FETs were fabricated that show record high RF performance. The device transconductance was limited by the regrowth interface resistance. Ex-situ silicon nitride passivation was shown to be effective to reduce DC-RF dispersion. Recently, thin channel MOSFETs with deep-sub-micron gate lengths and source/drain regrowth have demonstrated devices with simultaneously low on resistance, high frequency, high voltages.

Theoretical study on structural and optical properties of β-Ga2O3 with O vacancies via shell DFT-1/2 method

As one of the most common and important defects, O vacancy has a great influence on β-Ga2O3, which has not been studied systematically with appropriate functional approximation. Herein, the effects of O vacancies on the structural, electronic, and optical properties of bulk β-Ga2O3 were systemically investigated by using the shell DFT-1/2 method. For intrinsic β-Ga2O3, the calculated bandgap is 4.77 eV, which corresponds well to the experimental value. Band structures and density of states (DOS) prove that O vacancy will introduce a deeper donor level in the bandgap. Charge density distribution demonstrates the covalent properties of the Ga–O bond and reveals the charge transfer near the O vacancy. Phonon dispersion spectra prove the dynamical instability of the O vacancy systems. The formation energy curves show that the +2 charge state is energetically favorable at low Fermi energy, while the neutral defect is the most stable one at high Fermi energy. The transition energy of ɛ(+2/0) also proves that the O vacancy acts as a deep donor. The introduction of O defects introduces extra peaks in the lower energy regions of the absorption and electron energy loss spectra, and the peak positions are in good agreement with the previous experimental results. This study gives a point of view on the effects of O vacancies on β-Ga2O3 and proves the advantages of shell DFT-1/2 calculation in ultrawide bandgap semiconductors.

Atomic understanding of elastic-plastic deformation and crack evolution for single crystal AlN during nanoscratch

Single crystal aluminum nitride (AlN) as a new generation of ultra-wide band gap semiconductor materials is the preferred substrate material for high power, high RF devices, but single crystal AlN is hard and difficult to process. To obtain high-quality substrate requires comprehensive understanding of the material deformation and removal mechanism. In this paper, the molecular dynamics simulation method is used to scratch the AlN on the (0001) plane. Set up multiple sets of scratch depths to study the elastic-plastic deformation, material removal mechanism and crack evolution of single crystal AlN. The simulation results show that with the increase of scratch depth, the single crystal AlN workpiece undergoes elastic deformation to continuous plastic deformation and then to brittle fracture. The deformation mode in the elastic stage is phase-change, from wurtzite-structure to graphite-like (GL) phase. The plastic deformation of single crystal AlN is achieved by dislocations moving on the basal and pyramidal planes, and horizontally extended stacking faults. There is almost no subsurface damage below the stacking faults. When the scratch depth exceeds 6 nm, the material will undergo brittle fracture, and the fracture surface is the cleavage surface {10-10} of single crystal AlN.

Growth-favored nonpolar BAlN digital alloy with cation-order based tunable electronic structure

BAlN, an ultra-wide bandgap semiconductor, has attracted the interest of many due to its potential in high-power electronics and high-efficiency optoelectronics. However, the solid solution alloy (SSA) grown along [0001] direction experiences phase separation, which poses a challenge for device applications. To overcome the issues, we proposed BAlN-based digital alloy (DA) arranged along [10–10] and [11–20] directions. Among the whole range of B composition, the mixing enthalpy of DA is at most 16 % and 13 % of the SSA for [10–10] and [11–20] directions, suggesting an improved growth feasibility of DA. The characteristics of DA, including phonon dispersions, bandgap value, dielectric constant, and potential piezo-response along [0001] direction, were thoroughly investigated. Interestingly, cation orders at the same composition offer a wide range of electric, optical, and mechanical properties, some of which are even better than the conventional SSA. It was possible to achieve a maximum tunability of 25 % for the bandgap, while still maintaining direct bandgap properties for up to 67 % boron component. The DA exhibits excellent thermodynamic stability and its characteristics can be easily adjusted, making it a suitable material for high-quality ultra-wide bandgap BAlN alloy growth and applications.

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Luminescence Properties of the Hexagonal Boron Nitride Epilayer

AbstractAs an emerging 2D ultrawide bandgap semiconductor, hexagonal boron nitride (h‐BN) is gaining significant attention for its superior properties and wide applications. Although optical properties of h‐BN have been partially revealed by using h‐BN bulk single crystal, luminescence properties of the h‐BN few‐layer are rare due to its poor crystallinity. In this work, the h‐BN epilayers have been synthesized on sapphire substrates by the submicron‐spacing vapor deposition method. The crystalline quality of the h‐BN epilayer is improved with the increase of the growth temperature, and the full width at half‐maximum of the Raman peak is only ≈13 cm−1. The low temperature cathodoluminescence spectra of h‐BN epilayers exhibit two strong deep ultraviolet (DUV) luminescence peaks at 5.49 and 5.35 eV, as well as a weaker and broader defect‐related emission band at ≈4.0 eV. The DUV emission band is ascribed to the recombination of bound excitons and its phonon replica. Based on the theoretically predicted energy levels, the 4.0 eV emission band is tentatively attributed to the radiative transition from C impurities occupying the B sites to the B vacancies. Moreover, the h‐BN‐based DUV photodetectors exhibit an outstanding performance, making it a promising prospect for optoelectronic applications.

An avalanche-and-surge robust ultrawide-bandgap heterojunction for power electronics

Avalanche and surge robustness involve fundamental carrier dynamics under high electric field and current density. They are also prerequisites of any power device to survive common overvoltage and overcurrent stresses in power electronics applications such as electric vehicles, electricity grids, and renewable energy processing. Despite tremendous efforts to develop the next-generation power devices using emerging ultra-wide bandgap semiconductors, the lack of effective bipolar doping has been a daunting obstacle for achieving the necessary robustness in these devices. Here we report avalanche and surge robustness in a heterojunction formed between the ultra-wide bandgap n-type gallium oxide and the wide-bandgap p-type nickel oxide. Under 1500 V reverse bias, impact ionization initiates in gallium oxide, and the staggered band alignment favors efficient hole removal, enabling a high avalanche current over 50 A. Under forward bias, bipolar conductivity modulation enables the junction to survive over 50 A surge current. Moreover, the asymmetric carrier lifetime makes the high-level carrier injection dominant in nickel oxide, enabling a fast reverse recovery within 15 ns. This heterojunction breaks the fundamental trade-off between robustness and switching speed in conventional homojunctions and removes a key hurdle to advance ultra-wide bandgap semiconductor devices for power industrial applications.

Classes of O–D centers in unintentionally and Fe-doped β-Ga2O3 annealed in a D2 ambient

β-Ga2O3 has attracted much recent attention as a promising ultrawide bandgap semiconductor. Hydrogen can affect the conductivity of β-Ga2O3 through the introduction of shallow donors and the passivation of deep acceptors. The introduction of H or D into β-Ga2O3 by annealing in an H2 or D2 ambient at elevated temperature produces different classes of O–H or O–D centers. This work is a study of the interaction of D with VGa1 and VGa2 deep acceptors as well as other impurities and native defects in Ga2O3 by infrared spectroscopy and the complementary theory. (We focus primarily on the deuterium isotope of hydrogen because the vibrational modes of O–D centers can be detected with a higher signal-to-noise ratio than those of O–H.) O–D centers in β-Ga2O3 evolve upon annealing in an inert ambient and are transformed from one type of O–D center into another. These reactions affect the compensation of unintentional shallow donors by deep acceptors that are passivated by D. Defects involving additional impurities in β-Ga2O3 compete with VGa deep acceptors for D and modify the deuterium-related reactions that occur. The defect reactions that occur when D is introduced by annealing in a D2 ambient appear to be simpler than those observed for other introduction methods and provide a foundation for understanding the D-related reactions that can occur in more complicated situations.

Temperature-dependent thermal conductivity of MBE-grown epitaxial SrSnO3 films

As an ultrawide bandgap (∼4.1 eV) semiconductor, single crystalline SrSnO3 (SSO) has promising electrical properties for applications in power electronics and transparent conductors. The device performance can be limited by heat dissipation issues. However, a systematic study detailing its thermal transport properties remains elusive. This work studies the temperature-dependent thermal properties of a single crystalline SSO thin film prepared with hybrid molecular beam epitaxy. By combining time-domain thermoreflectance and Debye–Callaway modeling, physical insight into thermal transport mechanisms is provided. At room temperature, the 350-nm SSO film has a thermal conductivity of 4.4 W m−1 K−1, ∼60% lower than those of other perovskite oxides (SrTiO3, BaSnO3) with the same ABO3 structural formula. This difference is attributed to the low zone-boundary frequency of SSO, resulting from its distorted orthorhombic structure with tilted octahedra. At high temperatures, the thermal conductivity of SSO decreases with temperature following a ∼T−0.54 dependence, weaker than the typical T−1 trend dominated by the Umklapp scattering. This work not only reveals the fundamental mechanisms of thermal transport in single crystalline SSO but also sheds light on the thermal design and optimization of SSO-based electronic applications.

Solar-blind UV photodetector array spotlights efficacy of gallium oxide

Ultrawide bandgap semiconductor shows promise for Industry 4.0 optoelectronics.

Combinatorial tuning of structure and optoelectronic properties of Zn-Ga-O thin films for deep ultraviolet photodetection

Ternary complex oxides exhibit strong compositional dependence on their structure and functionalities. While the profound impact of the cation stoichiometry on the physical properties of ZnGa2O4 is widely recognized, how it affects the ultraviolet photodetection behavior remains not clear. Here we apply a combinatorial pulsed laser deposition technique to establish graded composition in ZnxGa1-xO (0 ≤ x ≤ 1) across a 2-inch sapphire substrate. In combination with structural and optical characterizations, a systematic investigation of the compositional effect on the phase, crystallinity and bandgap for the ZnxGa1-xO system is performed, and their further impact on the photodetection performance is compared by fabricating multiple metal–semiconductor-metal (MSM) photodetectors based on the graded ZnxGa1-xO films. Decent photodetection behavior is seen for the photodetectors with x ≤ 0.33, above which the performance is largely limited by the conductive phase formation. An optimized compositional range of x = 0.18–0.27 is identified to deliver the best device performance, which includes a responsivity of 12.88 A/W and a photo-to-dark current ratio of 2.8 × 108. This slight zinc deficiency is considered beneficial for the formation of moderate level of oxygen vacancies or antisite defects that boosts carrier transport. This study sheds light on the composition-dependent evolution of structure and properties in ternary ultrawide-bandgap semiconductors for high-performance ultraviolet photodetection.

Impact of surface preparation on the epitaxial growth of SrTiO3 on ScAlN/GaN heterostructures

Heterogeneous integration of functional oxides with ultra-wide bandgap (UWBG) semiconductors is desired for the realization of novel hybrid systems applicable to a wide array of commercial electronics and defense applications. In this work, we demonstrate the growth of crystalline SrTiO3 (STO) thin films on high-electron-mobility transistor (HEMT) heterostructures based on an emergent UWBG semiconductor ScAlN, used as the barrier layer on a GaN channel, and determine the effects of the pre-growth chemical treatments of the ScAlN surface on resultant heterostructure properties. We investigate wet chemical cleans of ScAlN with solvents, piranha solution, UV ozone and hydrofluoric acid, and a sulfuric-phosphoric acid mix prior to STO growth, and show that the commonly used piranha solution degrades the ScAlN surface, thereby reducing the crystal quality of the deposited STO layers and lowering the channel mobility. We determine that among the treatments studied, the solvent and sulfuric-phosphoric acid cleans were the least disruptive to the electrical properties of the GaN channel as evidenced from Hall effect measurements, but the sulfuric-phosphoric acid clean results in best oxide crystallinity, as determined from structural characterizations. We perform transmission electron microscopy imaging on the piranha-treated and sulfuric-phosphoric-treated samples to compare the microstructure and find that while intermixing occurs at the oxide-nitride interfaces for both samples, the interface roughness is lower and the STO grain size is larger in the sample with sulfuric-phosphoric acid treatment. This work demonstrates the first epitaxial growth of STO on an UWBG semiconductor and motivates STO/ScAlN/GaN as material platforms for high-frequency, high-power-density HEMTs.

Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications

Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"