Deep ultraviolet (UV) light-emitting diodes (LEDs) at a wavelength of 226 nm based on AlGaN/ AlN multiple quantum wells using p-type Si as both the hole supplier and the reflective layer are demonstrated. In addition to the description of the hole transport mechanism that allows hole injection from p-type Si into the wide bandgap device, the details of the LED structure which take advantage of the p-type Si layer as a reflective layer to enhance light extraction efficiency (LEE) are elaborated. Fabricated LEDs were characterized both electrically and optically. Owing to the efficient hole injection and enhanced LEE using the p-type Si nanomembranes (NMs), an optical output power of 225 lW was observed at 20 mA continuous current operation (equivalent current density of 15 A/cm 2) without external thermal management. The corresponding external quantum efficiency is 0.2%, higher than any UV LEDs with emission wavelength below 230 nm in the continuous current drive mode. The study demonstrates that adopting p-type Si NMs as both the hole injector and the reflective mirror can enable high-performance UV LEDs with emission wavelengths, output power levels, and efficiencies that were previously inaccessible using conventional p-in structures.

Ultraviolet (UV) light emission at 229 nm wavelength from diode structures based on AlN/Al0.77Ga0.23N quantum wells and using p-type Si to significantly increase hole injection was reported. Both electrical and optical characteristics were measured. Owing to the large concentration of holes from p-Si and efficient hole injection, no efficiency droop was observed up to a current density of 76 A/cm 2 under continuous wave operation and without external thermal management. An optical output power of 160 µW was obtained with corresponding external quantum efficiency of 0.027%. This study demonstrates that by adopting p-type Si nanomembrane contacts as hole injector, practical levels of hole injection can be realized in UV light-emitting diodes with very high Al composition AlGaN quantum wells, enabling emission wavelengths and power levels that were previously inaccessible using traditional p-i-n structures with poor hole injection efficiency.Demand for ultraviolet (UV) light emitting diodes (LEDs) is increasing due to broad applications in biological and chemical detections, decontamination, medical treatment, high density optical recording, and lithography 1-6 . The group III-nitride materials system is the most attractive candidate for UV LEDs spanning the UVA, UVB, and UVC 7-15 owing to its wide bandgap range (GaN: 3.3eV -AlN: ~6.2eV). However, as the wavelength gets shorter, the external quantum efficiency (EQE) becomes significantly degraded. Along with challenges in growth of high Al composition AlxGa1-xN material with low defect densities, the doping concentration limitations and high ionization energy of acceptors for wide gap AlGaN render the p-side of the diode structure quite resistive and the resulting hole injection efficiency is poor. In addition, achieving an Ohmic metal contact to typical p-layers with low contact resistance remains a critical limitation to obtaining an electrically efficient DUV LED. The approach used in this work overcomes both limitations.A variety of approaches have been resorted to circumventing the fundamental p-type doping challenges, such as polarization doping 1,5,16,17 and tunnel junctions [18][19][20] . Both methods require careful control of precursor fluxes for grading Al composition over the growth process, which complicates the epitaxy technique. We have reported a 237 nm UV LED using silicon as an efficient hole injector and postulated that shorter wavelength emission would be obtainable

2000759 (6 of 38) www.advmattechnol.de Figure 4. Flexible microwave transistors. a) Schematic illustration of fabrication process flow of flexible microwave TFTs based on Si nanomembrane. Left, two-step transfer-printing of Si nanomembrane on flexible substrate with assist of PDMS stamp. Right, direct flip-printing Si nanomembrane on flexible substrate. Reproduced with permission. [72] Copyright 2010, Wiley-VCH. b) Flexible microwave Si TFT using direct flip-printing approach in (a). Reproduced under the terms of the Creative Commons Attribution 4.0 International License. [75] Copyright 2016, Springer Nature. c) Flexible microwave Si TFT using two-step approach in (a). Reproduced with permission. [72] Copyright 2010, Wiley-VCH. d) Cross-section SEM image of flexible Si MOSFET made by thinning a 65 nm node SOI wafer. Reproduced with permission. [45] Copyright 2013, American Institute of Physics. e) Optical images of flexible GaAs HBT on cellulose nanofibril (CNF) substrate. Reproduced under the terms of the Creative Commons Attribution 4.0 International License. [90] Copyright 2015, Springer Nature. f) Schematic illustration and normalized on-state conductance (G/G 0), maximum transconductance (g m /g m0) of flexible InAs MOSFET with self-aligned gate. Reproduced with permission. [53] Copyright 2012, American Chemical Society. g) Cross-section schematic illustration and gain curves of flexible InGaAs/InAlAs HEMT as function of frequency under various bending conditions. Reproduced with permission. [91] Copyright 2013, American Institute of Physics. h) Output power density (P OUT), power gain (G P) and power-added efficiency (PAE) of the flexible GaN HEMT on thermal conductive tape with thermal conductivity of 1.6 W m −1 K −1. Reproduced with permission. [92] Copyright 2017, Wiley-VCH. i) Photography of AlGaN/GaN film grown on BN coated sapphire substrate and printed on flexible tape. Reproduced with permission. [19] Copyright 2017, Wiley-VCH. j) Cross-section schematic illustration of bottom gate flexible graphene FET (GFET). Reproduced with permission. [93] Copyright 2013, American Chemical Society. k) Cross-section schematic illustration of top gate flexible GFET with self-aligned gate configuration. Reproduced with permission. [94] Copyright 2014, American Chemical Society. l) High-frequency characteristics of flexible GFET with gate length of 50 nm. Reproduced with permission. [95] Copyright 2018, Wiley-VCH. m) Schematic illustration and gain curves of flexible microwave transistor based on MoS 2 as function of frequency. Reproduced with permission. [96] Copyright 2014, Springer Nature. n) High-frequency characteristics of flexible microwave transistor based on black phosphorus (BP) as function of frequency. Reproduced with permission.

Understanding the band bending at the interface of GaN/dielectric under different surface treatment conditions is critically important for device design, device performance, and device reliability. The effects of ultraviolet/ozone (UV/O) treatment of the GaN surface on the energy band bending of atomic-layer-deposition (ALD) AlO coated Ga-polar GaN were studied. The UV/O treatment and post-ALD anneal can be used to effectively vary the band bending, the valence band offset, conduction band offset, and the interface dipole at the AlO/GaN interfaces. The UV/O treatment increases the surface energy of the Ga-polar GaN, improves the uniformity of AlO deposition, and changes the amount of trapped charges in the ALD layer. The positively charged surface states formed by the UV/O treatment-induced surface factors externally screen the effect of polarization charges in the GaN, in effect, determining the eventual energy band bending at the AlO/GaN interfaces. An optimal UV/O treatment condition also exists for realizing the "best" interface conditions. The study of UV/O treatment effect on the band alignments at the dielectric/III-nitride interfaces will be valuable for applications of transistors, light-emitting diodes, and photovoltaics.

Low-cost flexible microwave circuits with compact size and light weight are highly desirable for flexible wireless communication and other miniaturized microwave systems. However, the prevalent studies on flexible microwave electronics have only focused on individual flexible microwave elements such as transistors, inductors, capacitors, and transmission lines. Thinning down supporting substrate of rigid chip-based monolithic microwave integrated circuits has been the only approach toward flexible microwave integrated circuits. Here, we report a flexible microwave integrated circuit strategy integrating membrane AlGaN/GaN high electron mobility transistor with passive impedance matching networks on cellulose nanofibril paper. The strategy enables a heterogeneously integrated and, to our knowledge, the first flexible microwave amplifier that can output 10 mW power beyond 5 GHz and can also be easily disposed of due to the use of cellulose nanofibril paper as the circuit substrate. The demonstration represents a critical step forward in realizing flexible wireless communication devices.