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Monoclinic gallium oxide (β−Ga2O3) has found great research interest in solar blind photodetector (SBP) applications due to its’ bandgap ∼4.85 eV and availability of high quality native crystal growth. Applications includ- ing missile guidance, flame detection, underwater/intersatellite communication and water purification systems require SBPs. β−Ga2O3 SBPs with high responsivity values have been published indicating internal gain in these devices. The gain has been attributed to accumulation of self-trapped hole (STH) below Schottky metal which the lowers Schot- tky barrier in these devices based on some approximations rather than a proper device simulation. In this paper, technology computer-aided design (TCAD) simulation of β−Ga2O3 SBPs are performed to numerically investigate the effect of low hole mobility STHs on Schottky barrier lowering (SBL). The simulations revealed that based on the theoretical hole mobility of 1 × 10−6 cm2V−1s−1 , photoconductive gain in β−Ga2O3 based photodetectors cannot be attributed to STH related hole accumulation near Schottky contact. It is found that hole mobility in the range of 1 × 10−10 cm2 V−1 s−1 − 1 × 10−12 cm2 V−1 s−1 is required to induce ∼ 0.3 eV of SBL potential. Unless such low hole mobility is reported either experimentally or theoretically, it is not reasonable to attribute gain to STH formation in these devices.

Objective The 2014 report by European Medicines Agency (EMA) restricted the use of thiocolchicoside for all reproductive-age women. In this study, we aim to expand the systematically-collected human data and discuss it within the frame provided by this report. Methods We identified and evaluated the outcomes of 48 prospectively recorded pregnancies referred to Terafar (Teratology Information Service, Izmir, Turkey). Results Of 42 pregnancies with first-trimester exposure and known outcomes, 31 resulted in live births, four in miscarriage and seven ended with elective terminations. There were 26 normal outcomes, two major and three minor congenital malformations among the live births. Conclusions Despite a number of limitations, our results and previous case series collectively strengthen the view that thiocolchicoside is unlikely to be a major teratogen. EMA’s 2014 report should be revised to reflect this finding, while current restrictions on use should continue until more detailed safety information is available.

We report on ultra-wide bandgap (UWBG) Al0.7Ga0.3N channel metal–oxide–semiconductor field-effect transistors (MOSFETs) grown by metal-organic chemical vapor deposition. Employing reverse Al composition graded ohmic contact layers and 20 nm Al2O3 gate-dielectric, 250 nm thick Al0.7Ga0.3N:Si channel MOSFETs resulted in the maximum current density of 0.5 A/mm, which is the highest value reported for AlGaN channels with Al composition >0.25. Transistors with a gate-drain spacing ( $L_{\mathrm{ GD}})$ of $1.7~\mu \text{m}$ demonstrated a breakdown voltage ( $V_{\mathrm{ DG}})$ of ~620 V, translating in an average lateral breakdown field of ~3.6 MV/cm. This work establishes UWBG AlGaN as a promising candidate for advanced RF applications.

We report on the small-signal high frequency characteristics of highly scaled graded AlGaN channel polarization-doped field-effect transistors (PolFETs) that show constant current gain cutoff frequency ( ${f}_{T}$ ) and maximum oscillation frequency ( ${f}_{\textsf {max}}$ ) profiles as a function of current density or gate bias. The device design includes upward and downward Al composition grading to induce a distributed 3-D charge profile, and eliminate abrupt heterojunction band offsets to achieve nonalloyed ohmic contacts with low resistance. The highest extrinsic ${f}_{T}$ of 52 GHz and ${f}_{\textsf {max}}$ of 67 GHz were measured at ${V}_{\textsf {GS}} = -1.5$ V and ${V}_{\textsf {DS}} = 9$ V, and constant ${f}_{T}$ and ${f}_{\textsf {max}}$ over wide input voltage ( ${V}_{\textsf {GS}}$ ) and output current range ( ${I}_{\textsf {DS}}$ ) were achieved.

We report on the design, demonstration and current status of tunnel-injected ultra-violet light emitting diodes (UV LEDs). III-Nitride ultraviolet light emitting diodes (UV LEDs) are promising in various applications including sterilization, water purification and medical sensing. However, both the light extraction efficiency and electrical efficiency face fundamental challenges for the conventional UV LED structures. This stems from the poor p-type conductivity and high p-type contact resistance. Hole injection using interband tunneling provides an elegant solution to the fundamental issues of UV LEDs, and can resolve both the hole injection and light extraction issues that have been the primary problems for UV LEDs. In this talk, we will discuss in detail the heterostructure design and demonstration through polarization engineering to realize efficient interband tunneling in ultra-wide band gap AlGaN material. We will then outline some of the growth and fabrication challenges, and discuss our approaches to overcome these. Finally, we will present our results on tunnel-injected UV LEDs that have enabled us to achieve efficient UV light emission in the UVA and UVB wavelength ranges with on-wafer efficiencies comparable to state-of-the-art values [1,2,3]. References: 1. Yuewei Zhang, et al. ''Interband tunneling for hole injection in III-nitride ultraviolet emitters", Appl. Phys. Lett. 106, 141103 (2015); 2. Yuewei Zhang, et al. "Design of p-type cladding layers for tunnel-injected UV-A light emitting diodes", Appl. Phys. Lett. 109, 191105 (2016); 3. Yuewei Zhang, et al. “Tunnel-injected sub-260 nm ultraviolet light emitting diodes”, Appl. Phys. Lett. 110, 201102 (2017).

We present small-signal measurements on graded AlGaN channel polarization-doped field-effect transistors (PolFETs) that show constant current gain cutoff frequency, f T , and power gain cutoff frequency, f max , profiles as a function of current density or gate bias. AlGaN/GaN high electron mobility transistors (HEMTs) are suitable candidates for mm-wave and THz amplifiers. However their transconductance, current gain and power gain profiles reach a peak value at low current density, and then drop off significantly as the current density is increased. Reduction in power gain with gate voltage is detrimental for linearity under large signal operation, and translates into intermodulation distortion.

In this work, we investigate the use of nanoscale polarization engineering to achieve efficient hole injection from metals to ultra-wide band gap AlGaN, and we show that UV-reflective aluminum (Al) layers can be used for hole injection into p-AlGaN. The dependence of tunneling on the work function of the metal was investigated, and it was found that highly reflective Al metal layers can enable efficient hole injection into p-AlGaN, despite the relatively low work function of Al. Efficient tunneling hole injection was confirmed by light emission at 326 nm with on-wafer peak external quantum efficiency and wall-plug efficiency of 2.65% and 1.55%, respectively. A high power density of 83.7 W/cm2 was measured at 1200 kA/cm2. The metal/semiconductor tunnel junction structure demonstrated here could provide significant advantages for efficient and manufacturable device topologies for high power UV emitters.

With a wide bandgap of ∼4.85 eV, high chemical and thermal stability, and melt growth availability, β-Ga2O3 has been found in a large number of solar blind photodetector (SBP) applications including missile guidance, flame detection, water purification, and intersatellite communication. The modelling of a Schottky diode (SD) based SBPs is crucial in order to reach high external quantum efficiency (EQE), especially for self-powered applications and also to extract hole mobility in these devices. The EQE performance of β-Ga2O3 vertical SD SBPs with various Schottky contact finger spacings is obtained using highly controversial hole mobility values reported in the literature. By modelling experimentally demonstrated EQE values of the existing β-Ga2O3 vertical SD SBPs, average nonequilibrium hole mobility value of ∼20 cm2 V−1 s−1 is extracted, which is quite higher than the claimed theoretical value of 1 × 10−6 cm2 V−1 s−1 and motivates for the efforts of technologically important p-type β-Ga2O3. By modelling the efficiency of full Schottky metal covered vertical SD SBPs by using hole mobility value of 20 cm2 V−1 s−1, internal quantum efficiency of 92% is obtained at an optimum n-type doping concentration of 1 × 1016 cm−3.

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919., The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade.

We report on ultra-wide bandgap (UWBG) Al0.75Ga0.25N channel metal-insulator-semiconductor field-effect transistors (MISFETs) with heterostructure engineered low-resistance ohmic contacts. The low intrinsic electron affinity of AlN (0.6 eV) leads to large Schottky barriers at the metal-AlGaN interface, resulting in highly resistive ohmic contacts. In this work, we use a reverse compositional graded n++ AlGaN contact layer to achieve upward electron affinity grading, leading to a low specific contact resistance (ρsp) of 1.9 × 10−6 Ω cm2 to n-Al0.75Ga0.25N channels (bandgap ∼5.3 eV) with non-alloyed contacts. We also demonstrate UWBG Al0.75Ga0.25N channel MISFET device operation employing the compositional graded n++ ohmic contact layer and 20 nm atomic layer deposited Al2O3 as the gate-dielectric.

We report GaN n++/p++ interband tunnel junctions with repeatable negative differential resistance and low resistance. Reverse and forward tunneling current densities were observed to increase as Si and Mg doping concentrations were increased. Hysteresis-free, bidirectional negative differential resistance was observed at room temperature from these junctions at a forward voltage ∼1.6 V. Thermionic PN junctions with GaN homojunction tunnel contact to the p-layer exhibited forward current density of 150 kA/cm2 at 7.6 V, with a low series device resistance of 1 × 10−5 Ω cm2.

Ultra violet light emitting diodes (UV LEDs) face critical limitations in both the injection efficiency and light extraction efficiency due to the resistive and absorbing p-type contact layers. In this work, we investigate the design and application of polarization engineered tunnel junctions for ultra-wide bandgap AlGaN (Al mole fraction higher than 50%) materials towards highly efficient UV LEDs. We demonstrate that polarization-induced 3D charge is beneficial in reducing tunneling barriers especially for high composition AlGaN tunnel junctions. The design of graded tunnel junction structures could lead to low tunneling resistance below 10-3 Ohm cm2 and low voltage consumption below 1 V (at 1 kA/cm2) for high composition AlGaN tunnel junctions. Experimental demonstration of 292 nm emission was achieved through non-equilibrium hole injection into wide bandgap materials with bandgap energy larger than 4.7 eV, and detailed modeling of tunnel junctions shows that they can be engineered to have low resistance, and can enable efficient emitters in the UV-C wavelength range., Comment: 7 pages, 7 figures

We discuss the engineering of p-AlGaN cladding layers for achieving efficient tunnel-injected III-Nitride ultraviolet light emitting diodes (UV LEDs) in the UV-A spectral range. We show that capacitance-voltage measurements can be used to estimate the compensation and doping in p-AlGaN layers located between the multi-quantum well region and the tunnel junction layer. By increasing the p-type doping concentration to overcome the background compensation, on-wafer external quantum efficiency and wall-plug efficiency of 3.37% and 1.62% were achieved for tunnel-injected UV LEDs emitting at 325 nm. We also show that interband tunneling hole injection can be used to realize UV LEDs without any acceptor doping. The work discussed here provides new understanding of hole doping and transport in AlGaN-based UV LEDs, and demonstrates the excellent performance of tunnel-injected LEDs for the UV-A wavelength range., Comment: 9 pages, 5 figures

We investigate Al2O3/AlGaN interface in GaN MOSHEMTs to engineer channel mobility and threshold voltage suitable for power switching applications. Using oxygen-plasma and annealing treatments, we find the optimal window for high mobility and threshold voltage. Next, we discuss the power switching figure of merit of high composition AlGaN based HEMTs and their potential to achieve large threshold voltages. Finally, we characterize the electrical properties of the interface between Al2O3/high composition Al0.7Ga0.3N, and measure the conduction band offset of approximately 1 eV with a low positive interface fixed charge density of +2.5 × 1012 cm−2.

We report on the first demonstration of tunnel-injection-based III-Nitride ultraviolet light emitting diodes (LEDs) with emission wavelength lower than 300 nm. The III-Nitride system could enable a new class of solid state ultraviolet sources that could surpass the efficiency and cost of traditional gas-based ultraviolet bulbs. However, III-Nitride based ultraviolet emitters have significantly lower efficiency (10–100x lower) than visible wavelength emitters, even though the internal quantum efficiency (IQE) of AlGaN emitters is high. The main reason for the lower efficiency in these devices is the high acceptor activation energy, poor hole availability, and high p-contact resistance in p-type AlGaN. Interband tunnel junctions (TJ) can overcome these fundamental limitations by non-equilbrium injection of holes into the LED active region, offering an approach to increase both the electrical injection as well as light extraction efficiency. However, interband PN tunnel junctions become increasingly more resistive as the band gap is increased. We show here that polarization engineering can enable non-equilibrium hole injection into wide bandgap AlGaN with bandgap larger than 4.7 eV (Al composition > 50%), enabling UV LEDs at a wavelength of 295 nm, with a record low differential resistance of 1.6 × 10−3 Ωcm2 at a current density of 1 kA/cm2.

We report on the high efficiency tunnel-injected ultraviolet light emitting diodes (UV LEDs) emitting at 287 nm. Deep UV LED performance has been limited by the severe internal light absorption in the p-type contact layers and low electrical injection efficiency due to poor p-type conduction. In this work, a polarization engineered Al0.65Ga0.35N/In0.2Ga0.8N tunnel junction layer is adopted for non-equilibrium hole injection to replace the conventionally used direct p-type contact. A reverse-graded AlGaN contact layer is further introduced to realize a low resistance contact to the top n-AlGaN layer. This led to the demonstration of a low tunnel junction resistance of 1.9 × 10−3 Ω cm2 obtained at 1 kA/cm2. Light emission at 287 nm with an on-wafer peak external quantum efficiency of 2.8% and a wall-plug efficiency of 1.1% was achieved. The measured power density at 1 kA/cm2 was 54.4 W/cm2, confirming the efficient hole injection through interband tunneling. With the benefits of the minimized internal absor...

Ultra-violet emitters have several applications in the areas of sensing, water purification, and data storage. While the III-Nitride semiconductor system has the band gap region necessary for ultraviolet emission, achieving efficient ultraviolet solid state emitters remains a challenge due to the low p-type conductivity and high contact resistance in wide band gap AlGaN-based ultra-violet light emitters. In this work, we show that efficient interband tunneling can be used for non-equilibrium injection of holes into ultraviolet emitters. Polarization-engineered tunnel junctions were used to enhance tunneling probability by several orders of magnitude over a PN homojunction, leading to highly efficient tunnel injection of holes to ultraviolet light emitters. This demonstration of efficient interband tunneling introduces a new paradigm for design of ultra-violet light emitting diodes and diode lasers, and enables higher efficiency and lower cost ultra-violet emitters., Comment: 13 pages, 7 figures, Submitted

We report a combination of highly doped layers and polarization engineering that achieves highly efficient blue-transparent GaN/InGaN/GaN tunnel junctions (In content = 12%). NPN diode structures with a low voltage drop of 4.04 V at 5 kA/cm2 and a differential resistance of 6.51 × 10−5 Ωcm2 at 3 kA/cm2 were obtained. The tunnel junction design with n++-GaN (Si: 5 × 1020 cm−3)/3 nm p++-In0.12Ga0.88N (Mg: 1.5 × 1020 cm−3)/p++-GaN (Mg: 5 × 1020 cm−3) showed the best device performance. Device simulations agree well with the experimentally determined optimal design. The combination of low In composition and high doping can facilitate lower tunneling resistance for blue-transparent light-emitting diodes.

We report on tunnel-injected deep ultraviolet light emitting diodes (UV LEDs) configured with a polarization engineered Al0.75Ga0.25N/ In0.2Ga0.8N tunnel junction structure. Tunnel-injected UV LED structure enables n-type contacts for both bottom and top contact layers. However, achieving Ohmic contact to wide bandgap n-AlGaN layers is challenging and typically requires high temperature contact metal annealing. In this work, we adopted a compositionally graded top contact layer for non-alloyed metal contact, and obtained a low contact resistance of Rc=4.8x10-5 Ohm cm2 on n-Al0.75Ga0.25N. We also observed a significant reduction in the forward operation voltage from 30.9 V to 19.2 V at 1 kA/cm2 by increasing the Mg doping concentration from 6.2x1018 cm-3 to 1.5x1019 cm-3. Non-equilibrium hole injection into wide bandgap Al0.75Ga0.25N with Eg>5.2 eV was confirmed by light emission at 257 nm. This work demonstrates the feasibility of tunneling hole injection into deep UV LEDs, and provides a novel structural design towards high power deep-UV emitters.

We discuss the design and demonstration ultra-low resistance III-nitride tunnel junctions, and how tunnel junctions could solve the long-standing problem of efficiency droop in solid state lighting. We have used nanoscale band engineering based on polarization and mid-gap states to reduce tunneling resistance by four orders of magnitude. We will discuss experimental demonstration of highly efficient tunnel junctions (resistivity ~ 0.1 mOhm-cm 2 ) in PN junctions, p-contact free LEDs, and multiple junction structures. Finally we will show how tunneling based carrier regeneration in multiple active region cascade LEDs could help to enable low current, high voltage operation to overcome the efficiency droop problem.

InGaN/GaN tunnel junction contacts were grown using plasma assisted molecular beam epitaxy (MBE) on top of a metal-organic chemical vapor deposition (MOCVD)-grown InGaN/GaN blue (450 nm) light emitting diode. A voltage drop of 5.3 V at 100 mA, forward resistance of 2 × 10−2 Ω cm2, and a higher light output power compared to the reference light emitting diodes (LED) with semi-transparent p-contacts were measured in the tunnel junction LED (TJLED). A forward resistance of 5 × 10−4 Ω cm2 was measured in a GaN PN junction with the identical tunnel junction contact as the TJLED, grown completely by MBE. The depletion region due to the impurities at the regrowth interface between the MBE tunnel junction and the MOCVD-grown LED was hence found to limit the forward resistance measured in the TJLED.

We report on the design, fabrication, and characterization of record high efficiency inter-band tunnel junctions in GaN. We have achieved tunnel junction specific resistivity values as low as 3×10−4 Ωcm2 by using polarization engineered GaN/InGaN/GaN tunnel junctions. An alternate approach of using rare earth nitride (GdN) nano-islands embedded in degenerately doped GaN p n junction has resulted in tunnel junction specific resistivity of 2.7×10−3 Ωcm2. This is the first report of mid gap states assisted tunneling in GaN.

Enhanced interband tunnel injection of holes into a PN junction is demonstrated using P-GaN/InGaN/N-GaN tunnel junctions with a specific resistivity of 1.2 X 10-4 {\Omega} cm2. The design methodology and low-temperature characteristic of these tunnel junctions is discussed, and insertion into a PN junction device is described. Applications of tunnel junctions in III-nitride optoelectronics devices are explained using energy band diagrams. The lower band gap and polarization fields reduce tunneling barrier, eliminating the need for ohmic contacts to p-type GaN. This demonstration of efficient tunnel injection of carriers in III-Nitrides can lead to a replacement of existing resistive p-type contact material in light emitters with tunneling contact layers, requiring very little metal footprint on the surface, resulting in enhanced light extraction from top emitting emitters., Comment: 5 pages, 4 figures

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors. Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 x 10^7 cm/s at a low sheet charge density of 7.8 x 10^11 cm-2. A new optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated LO phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.

We report on the cascading of blue light-emitting diodes (LEDs) up to three junctions using low-resistance InGaN tunnel junctions (TJs). At a forward current density of 10 A/cm2, triple- and dual-junction LEDs operated at 9.09 and 6.07 V with total differential resistances of 6.31 × 10−2 and 4.16 × 10−2 Ω cm2, respectively. A significant increase in output power was observed from the triple-junction LED compared to the dual one, showing that all LED layers contribute to the luminescence output. Enabling high brightness at low current, cascaded LEDs can circumvent efficiency droop mechanism.

We report on the potential of high electron mobility transistors (HEMTs) consisting of high composition AlGaN channel and barrier layers for power switching applications. Detailed two-dimensional (2D) simulations show that threshold voltages in excess of 3 V can be achieved through the use of AlGaN channel layers. We also calculate the 2D electron gas mobility in AlGaN channel HEMTs and evaluate their power figures of merit as a function of device operating temperature and Al mole fraction in the channel. Our models show that power switching transistors with AlGaN channels would have comparable on-resistance to GaN-channel based transistors for the same operation voltage. The modeling in this paper shows the potential of high composition AlGaN as a channel material for future high threshold enhancement mode transistors.

In this work, we demonstrate visible wavelength transparent GaN/GdN/GaN tunnel junction interconnects to cascade multiple p-n junctions with low resistance (5.7 × 10−4 Ω × cm2). We model a device structure using cascaded light emitting diodes (LEDs) with tunnel junction-based carrier regeneration to create high-power LEDs operating at low current density. Experimental LED characteristics are used to model the use of these low resistance tunnel junctions in cascaded multiple active region LEDs that can effectively provide high power by operating under low current and high forward voltage. The adoption of cascaded LED structures can enable high power LEDs while maintaining high efficiency.

In this letter, we experimentally demonstrate direct correlation between efficiency droop and carrier overflow in InGaN/GaN green light emitting diodes (LEDs). Further, we demonstrate flat external quantum efficiency curve up to 400 A/cm2 in a plasma assisted molecular beam epitaxy grown N-polar double quantum well LED without electron blocking layers. This is achieved by exploring the superior properties of reverse polarization field of N-face polarity, such as effective carrier injection and higher potential barriers against carrier overflow mechanism. The LEDs were found to operate with a low (∼2.3 V) turn-on voltage.

We report the demonstration of a N-polar InGaN based green light emitting diode (LED) grown by N2 plasma-assisted molecular beam epitaxy (PAMBE). High quality multiple quantum well LEDs with In0.29Ga0.71N quantum wells were grown at a temperature of 600 °C by applying a new growth model. LED structures exhibited green emission, and electroluminescence measurements on the test structure showed peak emission wavelengths varying from 564.5 to 540 nm. The full width at half-maximum reduced from 74 to 63 nm as the drive current was increased to 180 A/cm2. This work is the first demonstration of an N-polar LED with emission in the green wavelength range.