Showing papers by "Stephen J. Pearton published in 2021"

GaN and Related Materials

Abstract: 1. Plate Type Exchangers 2. Dynamic Systems 3. A Historical Survey of Research on Gallium Nitride 4. Growth of Group III Nitrides from Molecular Beams 5. Ternary Alloys 6. Optical Characterization of GaN and Related Materials 7. Theoretical Studies in GaN 8. GaAsN Alloys and GaN/GaAs Thin Layer Structures 9. The Contribution of Defects to the Electrical and Optical Properties of GaN 10. Growth of GaN Single Crystals Under High Nitrogen Pressure 11.Ion Implantation Doping and Isolation of III-Nitride Materials 12. High-Density ECR Etching of Group-III Nitrides 13. Contacts on III-Nitrides 14. III-V Nitride Based LEDs 15. III-V Nitride Electronic Devices 16. Physical Properties of the Bulk GaN Crystals Grown by the High-Pressure, High Temperature Method 17. Microstructure of Epitaxial III-V Nitride Thin Films 18. The Role of Hydrogen in GaN and Related Compounds

Artificial Neuron and Synapse Devices Based on 2D Materials

Abstract: Neuromorphic systems, which emulate neural functionalities of a human brain, are considered to be an attractive next-generation computing approach, with advantages of high energy efficiency and fast computing speed After these neuromorphic systems are proposed, it is demonstrated that artificial synapses and neurons can mimic neural functions of biological synapses and neurons However, since the neuromorphic functionalities are highly related to the surface properties of materials, bulk material-based neuromorphic devices suffer from uncontrollable defects at surfaces and strong scattering caused by dangling bonds Therefore, 2D materials which have dangling-bond-free surfaces and excellent crystallinity have emerged as promising candidates for neuromorphic computing hardware First, the fundamental synaptic behavior is reviewed, such as synaptic plasticity and learning rule, and requirements of artificial synapses to emulate biological synapses In addition, an overview of recent advances on 2D materials-based synaptic devices is summarized by categorizing these into various working principles of artificial synapses Second, the compulsory behavior and requirements of artificial neurons such as the all-or-nothing law and refractory periods to simulate a spike neural network are described, and the implementation of 2D materials-based artificial neurons to date is reviewed Finally, future challenges and outlooks of 2D materials-based neuromorphic devices are discussed

Effect of probe geometry during measurement of >100 A Ga2O3 vertical rectifiers

Abstract: The high breakdown voltage and low on-state resistance of Schottky rectifiers fabricated on β-Ga2O3 leads to low switching losses, making them attractive for power inverters. One of the main goals is to achieve high forward currents, requiring the fabrication of large area (>1 cm2) devices in order to keep the current density below the threshold for thermally driven failure. A problem encountered during the measurement of these larger area devices is the dependence of current spreading on the probe size, resistance, number, and geometry, which leads to lower currents than expected. We demonstrate how a multiprobe array (6 × 8 mm2) provides a means of mitigating this effect and measure a single sweep forward current up to 135 A on a 1.15 cm2 rectifier fabricated on a vertical Ga2O3 structure. Technology computer-aided design simulations using the floods code, a self-consistent partial differential equation solver, provide a systematic insight into the role of probe placement, size (40–4120 μm), number (1–5), and the sheet resistance of the metal contact on the resultant current-voltage characteristics of the rectifiers.

Experimental estimation of electron–hole pair creation energy in β-Ga2O3

Abstract: The applicability of using Electron Beam Induced Current (EBIC) measurements on Schottky barriers to obtain the mean electron–hole pair creation energy in β-Ga2O3 is reported. It is shown that, when combined with Monte Carlo simulation, this approach yields for Si, GaN, and 4H–SiC a data set consistent with empirical expressions proposed earlier in the literature for many different semiconductors. The method is then applied to β-Ga2O3, where complications related to hole trapping in the material give rise to a strong gain in EBIC and have to be carefully treated and taken into account. When this is done, the mean electron–hole pair energy formation is found to be 15.6 eV, in reasonable agreement with the values predicted by empirical expressions.

Electrical properties and deep trap spectra in Ga2O3 films grown by halide vapor phase epitaxy on p-type diamond substrates

Abstract: Films of Ga2O3 were grown by Halide Vapor Phase Epitaxy (HVPE) on bulk heavily B-doped (001)-oriented diamond substrates using thin interlayers of Al2O3 deposited by HVPE or AlN/AlGaN deposited by metalorganic chemical vapor deposition. The growth with AlN/AlGaN was dominated by the formation of a highly conducting ɛ-phase with poor crystalline quality. For these samples, excessive leakage of Schottky diodes and of the Ga2O3/diamond heterojunction prevented meaningful electrical characterization. The film grown with the Al2O3 interlayer was mainly composed of (−201) β-Ga2O3 with an admixture of the ɛ-phase. The film had a low density of residual shallow donors, 5 × 1015 cm−3, with deep electron traps spectra consisting of the well documented centers for β-Ga2O3 near Ec 0.27, Ec 0.7, and Ec 1 eV, all of which are often ascribed to native defects or their complexes. The electrical properties of heterojunctions were mostly determined by the properties of the Ga2O3 films. Both Schottky diodes and heterojunctions showed measurable photosensitivity for 259 nm wavelength excitation, but very low photocurrent for near-UV (365 nm wavelength excitation).

Fast SARS-CoV-2 virus detection using disposable cartridge strips and a semiconductor-based biosensor platform

Abstract: Detection of the SARS-CoV-2 spike protein and inactivated virus was achieved using disposable and biofunctionalized functional strips, which can be connected externally to a reusable printed circuit board for signal amplification with an embedded metal-oxide-semiconductor field-effect transistor (MOSFET). A series of chemical reactions was performed to immobilize both a monoclonal antibody and a polyclonal antibody onto the Au-plated electrode used as the sensing surface. An important step in the biofunctionalization, namely, the formation of Au-plated clusters on the sensor strips, was verified by scanning electron microscopy, as well as electrical measurements, to confirm successful binding of thiol groups on this Au surface. The functionalized sensor was externally connected to the gate electrode of the MOSFET, and synchronous pulses were applied to both the sensing strip and the drain contact of the MOSFET. The resulting changes in the dynamics of drain waveforms were converted into analog voltages and digital readouts, which correlate with the concentration of proteins and virus present in the tested solution. A broad range of protein concentrations from 1 fg/ml to 10 μg/ml and virus concentrations from 100 to 2500 PFU/ml were detectable for the sensor functionalized with both antibodies. The results show the potential of this approach for the development of a portable, low-cost, and disposable cartridge sensor system for point-of-care detection of viral diseases.

Magneto-optical properties of Cr3+ in β-Ga2O3

Abstract: β-Ga2O3 is a wide bandgap semiconductor that is attractive for various applications, including power electronics and transparent conductive electrodes. Its properties can be strongly affected by transition metal impurities commonly present during the growth such as Cr. In this Letter, we determine the electronic structure of Cr3+ by performing a correlative study of magneto-photoluminescence (magneto-PL) and electron paramagnetic resonance. We unambiguously prove that the so-called R1 and R2 PL lines at around 1.79 eV originate from an internal transition between the first excited state (2E) and the 4A2 ground state of Cr3+. The center is concluded to have monoclinic local symmetry and exhibits a large zero-field splitting (∼147 μeV) of the ground state, which can be directly measured from the fine structure of the R1 transition. Furthermore, g-values of the first excited state are accurately determined as ga = 1.7, gb = 1.5, and gc* = 2.1. Our results advance our understanding of the electronic structure of Cr in β-Ga2O3 and provide a spectroscopic signature of this common residual impurity.

OH-Si complex in hydrogenated n-type β-Ga2O3:Si

Abstract: Si is an n-type dopant in Ga2O3 that can be intentionally or unintentionally introduced. The results of Secondary Ion Mass Spectrometry, Hall effect, and infrared absorption experiments show that the hydrogen plasma exposure of Si-doped Ga2O3 leads to the formation of complexes containing Si and H and the passivation of n-type conductivity. The Si-H (D) complex gives rise to an O-H (D) vibrational line at 3477.6 (2577.8) cm−1 and is shown to contain a single H (or D) atom. The direction of the transition moment of this defect has been investigated to provide structure-sensitive information. Theory suggests possible structures for an OH-Si complex that is consistent with its observed vibrational properties.

Temperature dependent performance of ITO Schottky contacts on β-Ga2O3

Abstract: Sputtered indium tin oxide (ITO) was used as a rectifying contact on lightly n-type (n ∼ 1016 cm−3) β-Ga2O3 and found to exhibit excellent Schottky characteristics up to 500 K, with no thermally driven degradation to this temperature. The barrier height extracted from current–voltage characteristics was 1.15 ± 0.04 eV at 300 K and 0.78 ± 0.03 eV at 500 K, with thermionic behavior of charge carriers over the image force lowered Schottky barriers dominating the carrier transport at low temperatures. The breakdown voltages were 246, 185, and 144 V at 300, 400 and 500 K, respectively. At 600 K, the diodes suffered irreversible thermal damage. The diode on/off ratio was >105 for reverse biases up to 100 V. At higher reverse voltage, the current shows an I ∝ Vn relationship with voltage, indicating a trap-assisted space-charge-limited conduction (SCLC) mechanism. We observed this SCLC relation when the reverse voltage was larger than 100 V for 300 and 400 K and at <100 V at 500 K. The ITO can also be used to make Ohmic contacts on heavily doped Ga2O3 suggesting the possibility of completely optically transparent power devices.

Design of Ga2O3 modulation doped field effect transistors

Abstract: The design of β-Ga2O3-based modulation-doped field effect transistors is discussed with a focus on the role of self-heating and resultant modification of the electron mobility profile. Temperature- and doping-dependent model of the electron mobility as well as temperature- and orientation-dependent approximations of the thermal conductivity of β-Ga2O3 are presented. A decrease in drain current was attributed to a position-dependent mobility reduction caused by a coupled self-heating mechanism and a high electric-field mobility reduction mechanism. A simple thermal management solution is presented where heat is extracted through the source contact metal. Additionally, it is shown that an undesired secondary channel can form at the modulation-doped layer that is distinguished by an inflection in the transconductance curve.

Crystal orientation dependence of deep level spectra in proton irradiated bulk β-Ga2O3

Abstract: The effects of 20 MeV proton irradiation with fluences of 5 × 1014 and 1015 p/cm2 on electrical properties of lightly Sn doped n-type (net donor concentration 3 × 1017 cm−3) bulk β-Ga2O3 samples with (010) and (−201) orientation were studied. Proton irradiation decreases the net donor density with a removal rate close to 200 cm−1 for both orientations and similar to the electron removal rates in lightly Si doped β-Ga2O3 epilayers. The main deep electron traps introduced in the β-Ga2O3 crystals of both orientations are near Ec−0.45 eV, while in Si doped films, the dominant centers were the so-called E2* (Ec−0.75 eV) and E3 (Ec−0.1 eV) traps. Deep acceptor spectra in our bulk –Ga2O3(Sn) crystals were dominated by the well-known centers with an optical ionization energy of near 2.3 eV, often attributed to split Ga vacancies. These deep acceptors are present in a higher concentration and are introduced by protons at a higher rate for the (010) orientation. Another important difference between the two orientations is the introduction in the surface region (∼0.1 μm from the surface) of the (010) of a very high density of deep acceptors with a level near Ec−0.27 eV, not observed in high densities in the (−201) orientation or in Si doped epitaxial layers. The presence of these traps gives rise to a very pronounced hysteresis in the low temperature forward current–voltage characteristics of the (010) samples. These results are yet another indication of a significant impact of the orientation of the β-Ga2O3 crystals on their properties, in this case, after proton irradiation.

Electron beam probing of non-equilibrium carrier dynamics in 18 MeV alpha particle- and 10 MeV proton-irradiated Si-doped β-Ga2O3 Schottky rectifiers

Abstract: Minority hole diffusion length and lifetime were measured in independent experiments by electron beam-induced current and time-resolved cathodoluminescence in Si-doped β-Ga2O3 Schottky rectifiers irradiated with 18 MeV alpha particles and 10 MeV protons. Both diffusion length and lifetime exhibited a decrease with increasing temperature. The non-equilibrium minority hole mobility was calculated from the independently measured diffusion length and lifetime, indicating that the so-called hole self-trapping is most likely irrelevant in the 77–295 K temperature range.

Assessment of the (010) $\beta$-Ga$_2$O$_3$ Surface and Substrate Specification

Abstract: Recent breakthroughs in bulk crystal growth of the thermodynamically stable beta phase of gallium oxide ($\beta$-Ga$_2$O$_3$) have led to the commercialization of large-area beta-Ga$_2$O$_3$ substrates with subsequent epitaxy on (010) substrates producing high-quality films. Still, metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and processing of the (010) $\beta$-Ga$_2$O$_3$ surface are known to form sub-nanometer scale facets along the [001] direction as well as larger ridges with features perpendicular to the [001] direction. A density function theory calculation of the (010) surface shows an ordering of the surface as a sub-nanometer-scale feature along the [001] direction. Additionally, the general crystal structure of $\beta$-Ga$_2$O$_3$ is presented and recommendations are presented for standardizing (010) substrates to account for and control the larger-scale ridge formation.

Assessment of the (010) β-Ga2O3 surface and substrate specification

Abstract: Recent breakthroughs in bulk crystal growth of the thermodynamically stable beta phase of gallium oxide (β-Ga2O3) have led to the commercialization of large-area β-Ga2O3 substrates with subsequent epitaxy on (010) substrates producing high-quality films. Still, metalorganic chemical vapor deposition, molecular beam epitaxy, and processing of the (010) β-Ga2O3 surface are known to form subnanometer-scale facets along the [001] direction as well as larger ridges with features perpendicular to the [001] direction. A density function theory calculation of the (010) surface shows an ordering of the surface as a subnanometer-scale feature along the [001] direction. Additionally, the general crystal structure of β-Ga2O3 is presented, and recommendations are presented for standardizing (010) substrates to account for and control the larger-scale ridge formation.

Parasitic p–n junctions formed at V-pit defects in p-GaN

Abstract: The luminescent and recombination properties of V-pit defects in p-GaN(Mg) grown by metalorganic chemical vapor deposition (MOCVD) were studied by scanning electron microscopy (SEM) in the secondary electron, cathodoluminescence (CL), and electron beam induced current (EBIC) modes, combined with CL spectra measurements and EBIC collection efficiency measurements. Similar studies were performed on low-dislocation-density freestanding n-GaN crystals. For MOCVD p-GaN films, the SEM investigations were supplemented by capacitance–voltage, current–voltage, deep level transient spectroscopy analysis with Ni Schottky diode, and Ohmic contacts. These experiments show that V-pits in p-GaN increase the leakage current of Schottky diodes, as in n-GaN films and crystals. EBIC imaging and EBIC collection efficiency results suggest that in the region of V-pits, a parasitic p–n junction is formed. We also observe that, in V-pits, the CL spectra the contribution of the 3.2 eV defect band is strongly enhanced compared to the 3 eV blue CL band that dominates the spectra.