Showing papers in "Semiconductor Science and Technology in 2018"
TL;DR: In this paper, a review of β-Ga2O3 at the research level that spans from the material preparation through characterization to final devices is presented, including material preparation (bulk crystals, epi-layers, surfaces), an exploration of optical, electrical, thermal and mechanical properties, as well as device design / fabrication with resulted functionality suitable for different fields of applications.
Abstract: β-Ga2O3 is an emerging, ultra-wide bandgap (energy gap of 4.85 eV) transparent semiconducting oxide (TSO), which attracted recently much scientific and technological attention. Unique properties of that compound combined with its advanced development in growth and characterization place β-Ga2O3 in the frontline of future applications in electronics (Schottky barrier diodes, field-effect transistors), optoelectronics (solar- and visible-blind photodetectors, flame detectors, light emitting diodes), and sensing systems (gas sensors, nuclear radiation detectors). A capability of growing large bulk single crystals directly from the melt and epi-layers by a diversity of epitaxial techniques, as well as explored material properties and underlying physics, define a solid background for a device fabrication, which, indeed, has been boosted in recent years. This required, however, enormous efforts in different areas of science and technology that constitutes a chain linking together engineering, metrology and theory. The present review includes material preparation (bulk crystals, epi-layers, surfaces), an exploration of optical, electrical, thermal and mechanical properties, as well as device design / fabrication with resulted functionality suitable for different fields of applications. The review summarizes all of these aspects of β-Ga2O3 at the research level that spans from the material preparation through characterization to final devices.
242 citations
Abstract: Chalcogenide Phase-Change Materials (PCMs), such as Ge-Sb-Te alloys, are showing outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, recently, in non-volatile resistive memories. The latter, known as Phase-Change Material memories or Phase-Change Random Access Memories (PCRAMs), are the most promising candidate among emerging Non-Volatile Memory (NVM) technologies to replace the current FLASH memories at CMOS technology nodes under 28 nm. Chalcogenide PCMs exhibit fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties leading to a unique set of features for PCRAMs, such as fast programming, good cyclability, high scalability, multi-level storage capability and good data retention. Nevertheless, PCM memory technology has to overcome several challenges to definitively invade the NVM market. In this review paper we examine the main technological challenges that PCM memory technology must face and we illustrate how new memory architecture, innovative deposition methods and PCM composition optimization can contribute to further improvements of this technology. In particular, we examine how to lower the programming currents and increase data retention. Scaling down PCM memories for large scale integration means incorporation of the phase-change material into more and more confined structures and raises material science issues to understand interface and size effects on crystallization. Other material science issues are related to the stability and ageing of the amorphous state of phase-change materials. The stability of the amorphous phase, which determines data retention in memory devices, can be increased by doping the phase-change material. Ageing of the amorphous phase leads to a large increase of the resistivity with time (resistance drift), which has hindered up-to-now the development of ultra-high multilevel storage devices. A review of the current understanding of all these issues is provided from a material science point of view.
180 citations
TL;DR: In this paper, the properties of group 2, group 5 and group 12 acceptor impurities in β-Ga2O3 were explored using hybrid density functional calculations, and all impurities exhibited acceptor transition levels above 1.3 eV. After examining formation energies as a function of chemical potential, Mg was determined to be the most stable acceptor species.
Abstract: With a wide band gap, high critical breakdown voltage and commercially available substrates, Ga2O3 is a promising material for next-generation power electronics. Like most wide-band-gap semiconductors, obtaining better control over its electrical conductivity is critically important, but has proven difficult to achieve. Although efficient p-type doping in Ga2O3 is not expected, since theory and experiment indicate the self-trapping of holes, the full development of this material will require a better understanding of acceptor dopants. Here the properties of group 2, group 5 and group 12 acceptor impurities in β-Ga2O3 are explored using hybrid density functional calculations. All impurities are found to exhibit acceptor transition levels above 1.3 eV. After examining formation energies as a function of chemical potential, Mg (followed closely by Be) is determined to be the most stable acceptor species.
132 citations
TL;DR: In this article, a comprehensive device simulation is used to study various possible parameters that can influence the performance of perovskite solar cell with CuI as HTM, including the effect of doping density, defect density and thickness of absorber layer, along with the influence of diffusion length of carriers as well as electron affinity of electron transport layer (ETM) and HTM on the performance.
Abstract: Hole transport material (HTM) plays an important role in the efficiency and stability of perovskite solar cells (PSCs). Spiro-MeOTAD, the commonly used HTM, is costly and can be easily degraded by heat and moisture, thus offering hindrance to commercialize PSCs. There is dire need to find an alternate inorganic and stable HTM to exploit PSCs with their maximum capability. In this paper, a comprehensive device simulation is used to study various possible parameters that can influence the performance of perovskite solar cell with CuI as HTM. These include the effect of doping density, defect density and thickness of absorber layer, along with the influence of diffusion length of carriers as well as electron affinity of electron transport layer (ETM) and HTM on the performance of PSCs. In addition, hole mobility and doping density of HTM is also investigated. CuI is a p-type inorganic material with low cost and relatively high stability. It is found that concentration of dopant in absorber layer and HTM, the electron affinity of HTM and ETM affect the performance of solar cell minutely, while cell performance improves greatly with the reduction of defect density. Upon optimization of parameters, power conversion efficiency for this device is found to be 21.32%. The result shows that lead-based PSC with CuI as HTM is an efficient system. Enhancing the stability and reduction of defect density are critical factors for future research. These factors can be improved by better fabrication process and proper encapsulation of solar cell.
120 citations
112 citations
TL;DR: In this paper, the authors discuss the development and exploitation of THz radiation detectors in human activity applications and discuss the operation conditions of the THz detectors and their upper limit performance.
Abstract: The phenomena and recent progress in the evolution of the basic classes of the THz detectors that exist today are considered. Issues associated with the development and exploitation of THz radiation detectors in human activity applications are shortly addressed. The operation conditions of the THz detectors and their upper limit performance are discussed. Because of the lack of space not all the important Refs. will be mentioned. Where it was reasonable mainly publications after 2011 were addressed.
109 citations
TL;DR: In this paper, layer dependent structural, electronic and vibrational properties of the 1T phase of two dimensional (2D) platinum diselenide are investigated by means of state-of-the-art first-principles calculations.
Abstract: The layer dependent structural, electronic and vibrational properties of the 1T phase of two dimensional (2D) platinum diselenide are investigated by means of state-of-the-art first-principles calculations. The main findings of the study are: (i) monolayer platinum diselenide has a dynamically stable 2D octahedral structure with 1.66 eV indirect band gap, (ii) the semiconducting nature of 1T-PtSe2 monolayers remains unaffected even at high biaxial strains, (iii) top-to-top (AA) arrangement is found to be energetically the most favorable stacking of 1T-PtSe2 layers, (iv) the lattice constant (layer-layer distance) increases (decreases) with increasing number of layers, (v) while monolayer and bilayer 1T-PtSe2 are indirect semiconductors, bulk and few-layered 1T-PtSe2 are metals, (vi) Raman intensity and peak positions of the A1g and E g modes are found to be highly dependent on the layer thickness of the material, hence; the number of layers of the material can be determined via Raman measurements.
78 citations
67 citations
TL;DR: In this article, the authors mainly focus on recent advances in strain-tunable properties of self-assembled InAs quantum dots embedded in semiconductor nanomembranes and photonic structures.
Abstract: The tailoring of the physical properties of semiconductor nanomaterials by strain has been gaining increasing attention over the last years for a wide range of applications such as electronics, optoelectronics and photonics. The ability to introduce deliberate strain fields with controlled magnitude and in a reversible manner is essential for fundamental studies of novel materials and may lead to the realization of advanced multi-functional devices. A prominent approach consists in the integration of active nanomaterials, in thin epitaxial films or embedded within carrier nanomembranes, onto Pb(Mg1/3Nb2/3)O3-PbTiO3-based piezoelectric actuators, which convert electrical signals into mechanical deformation (strain). In this review, we mainly focus on recent advances in strain-tunable properties of self-assembled InAs quantum dots embedded in semiconductor nanomembranes and photonic structures. Additionally, recent works on other nanomaterials like rare-earth and metal-ion doped thin films, graphene and MoS2 or WSe2 semiconductor two-dimensional materials are also reviewed. For the sake of completeness, a comprehensive comparison between different procedures employed throughout the literature to fabricate such hybrid piezoelectric-semiconductor devices is presented. Very recently, a novel class of micro-machined piezoelectric actuators have been demonstrated for a full control of in-plane stress fields in nanomembranes, which enables producing energy-tunable sources of polarization-entangled photons in arbitrary quantum dots. Future research directions and prospects are discussed.
62 citations
60 citations
TL;DR: In this article, a tradeoff between higher Ge doping concentration and high quality surface morphology on (001) β-Ga2O3 films grown by plasma-assisted molecular beam epitaxy was revealed.
Abstract: Ge and Sn as n-type dopants in (001) β-Ga2O3 films were investigated using plasma-assisted molecular beam epitaxy. The Ge concentration showed a strong dependence on the growth temperature, whereas the Sn concentration remains independent of the growth temperature. The maximum growth temperature at which a wide range of Ge concentrations (from 1017 to 1020 cm−3) could be achieved was 675 °C while the same range of Sn concentration could be achieved at growth temperature of 750 °C. Atomic force microscopy results revealed that higher growth temperature shows better surface morphology. Therefore, our study reveals a tradeoff between higher Ge doping concentration and high quality surface morphology on (001) β-Ga2O3 films grown by plasma-assisted molecular beam epitaxy. The Ge doped films had an electron mobility of 26.3 cm2 V−1 s−1 at the electron concentration of 6.7 × 1017 cm−3 whereas the Sn doped films had an electron mobility of 25.3 cm2 V−1 s−1 at the electron concentration of 1.1 × 1018 cm−3.
TL;DR: In this paper, the authors investigated the transport and optical properties of monolayer semiconducting transition metal dichalcogenides (STMDs) in the absence and presence of NH3, NO, NO2, and O2 gas molecules to assess their potentials as gas sensors.
Abstract: In this paper, we investigate the transport and optical properties of the monolayer semiconducting transition metal dichalcogenides (STMDs) in the absence and presence of the NH3, NO, NO2, and O2 gas molecules to assess their potentials as gas sensors. The first-principles calculations based on density functional theory indicate that absorption of the O2, NO2, NO gas molecules on the surface of these materials leads to significant changes in their transmission spectrum. Our calculations predict a charge transfer between the adsorbent gas and any of these STMDs. However, the presence of NH3 molecule has little effect on the transport properties of these materials. The results show that when the STMDs are exposed to NO, NO2, and O2 molecules, the dielectric function changes. Therefore, these materials can be employed as the sensing element in an optical gas sensor.
TL;DR: In this paper, the authors used super-hydrophilic copper sulfide (CuS) films with double-layer structures as light absorbers for solar steam generation, where a porous mixed cellulose ester (MCE) membrane is used as a supporting layer.
Abstract: Solar steam technology is one of the simplest, most direct and effective ways to harness solar energy through water evaporation. Here, we report the development using super-hydrophilic copper sulfide (CuS) films with double-layer structures as light absorbers for solar steam generation. In the double-layer structure system, a porous mixed cellulose ester (MCE) membrane is used as a supporting layer, which enables water to get into the CuS light absorbers through a capillary action to provide continuous water during solar steam generation. The super-hydrophilic property of the double-layer system (CuS/MCE) leads to a thinner water film close to the air-water interface where the surface temperature is sufficiently high, leading to more efficient evaporation (~80 ± 2.5%) under one sun illumination. Furthermore, the evaporation efficiencies still keep a steady value after 15 cycles of testing. The super-hydrophilic CuS film is promising for practical application in water purification and evaporation as a light absorption material.
TL;DR: In this paper, a review of O-band III-V QD lasers for Si photonic optical interconnects, including Fabry-Perot lasers, distributed-feedback laser array, and micro-lasers, is presented.
Abstract: The monolithic growth of III–V semiconductor lasers on Si remains the 'holy grail' for full-scale deployment of Si photonics with reduced cost and added flexibility. Semiconductor lasers with active regions made from quantum dots (QDs) have shown superior device performance over conventional quantum well (QW) counterparts and offer new functionalities. Furthermore, there are other advantages of QDs for monolithic III–V-on-Si integration over QWs, such as QD devices being less sensitive to defects. It is, therefore, not surprising that the past decade has seen rapid progress in research on monolithic III–V QD lasers on Si, with a view to leveraging the benefits of QD gain region technology while benefitting from the economics of scale enabled by monolithic growth. This review has a special focus on O-band III–V QD lasers monolithically grown on Si for Si photonic optical interconnects, including Fabry–Perot lasers, distributed-feedback laser array, and micro-lasers. The successes and challenges for developing monolithic III–V lasers on Si are discussed.
TL;DR: In this paper, the growth rate of β-Ga2O3 increased with increasing Ga-flux, reaching a clear plateau of 56 nm h−1, and then decreased at higher Gaflux.
Abstract: We investigated the homoepitaxial growth and etching characteristics of (001) β-Ga2O3 by plasma-assisted molecular beam epitaxy. The growth rate of β-Ga2O3 increased with increasing Ga-flux, reaching a clear plateau of 56 nm h−1, and then decreased at higher Ga-flux. The growth rate decreased from 56 to 42 nm h−1 when the substrate temperature was increased from 750 °C to 800 °C. The growth rate was negative (net etching) when only Ga-flux was supplied. The etching rate proportionally increased with increasing the Ga-flux, reaching 84 nm h−1. The etching was enhanced at higher temperatures. It was found that Ga-etching of (001) β-Ga2O3 substrates prior to the homoepitaxial growth markedly improved the surface roughness of the film.
TL;DR: In this paper, the authors review recent progress in the exploration of interlayer coupling in 2D semiconducting vdWHSs for potential applications in electronics and optoelectronics.
Abstract: Two-dimensional (2D) graphene-like layered semiconductors provide a new platform for materials research because of their unique mechanical, electronic and optical attributes. Their in-plane covalent bonding and dangling-bond-free surface allow them to assemble various van der Waals heterostructures (vdWHSs) with sharply atomic interfaces that are not limited by lattice matching and material compatibility. Interlayer coupling, as a ubiquitous phenomenon residing among 2D materials (2DMs) systems, controls a thin layer exfoliation process and the assembly of vdWHSs and behaves with a unique degree of freedom for engineering the properties of 2DMs. Interlayer coupling provides an opportunity to observe new physics and provides a novel strategy to modulate the electronic and optoelectronic properties of materials for practical device applications. We herein review recent progress in the exploration of interlayer coupling in 2D semiconducting vdWHSs for potential applications in electronics and optoelectronics.
TL;DR: In this paper, the fabrication aspects of SiGe heterojunction bipolar transistors which record high-speed performance are discussed. But the impact of critical process steps on radio frequency performance is discussed.
Abstract: This paper addresses fabrication aspects of SiGe heterojunction bipolar transistors which record high-speed performance. We previously reported f T values of 505 GHz, f MAX values of 720 GHz, and ring oscillator gate delays of 1.34 ps for these transistors. The impact of critical process steps on radio frequency performance is discussed. This includes millisecond annealing for enhanced dopant activation and optimization of the epitaxial growth process of the base layer. It is demonstrated that the use of a disilane precursor instead of silane can result in reduced base resistance and favorable device scalability.
TL;DR: In this article, a temperature-dependent chemical vapor transport (CVT) strategy was used to synthesize few-layer InSe and In2Se3 single crystals at 400 °C and 450 °C on mica substrate, respectively.
Abstract: Controllably selective growth of two-dimensional (2D) layered single crystals plays an important role for their applications in electronic and optoelectronic devices. Among these 2D materials, indium selenide (InxSey), especially InSe and In2Se3, are typical III-VI compound semiconductors which have attracted particular attention due to their excellent electronic transport properties, photoresponse and nonlinear effect. However, controllably selective growth of 2D InSe and In2Se3 by a common method is still a great challenge due to the existence of various stoichiometric substances and crystalline phases of each substance in the indium selenide system. Here, we demonstrate a temperature-dependent chemical vapor transport (CVT) strategy to synthesize few-layer β-InSe and α-In2Se3 single crystals at 400 °C and 450 °C on mica substrate, respectively. A few-layer β-InSe photodetector was fabricated, which displayed high sensitivity to a broad photoresponse range from visible light (500 nm) to near infrared light (850 nm) with a high responsivity of 2103 A W−1 and detectivity of 4.25 × 1012 Jones illuminated by 500 nm. Moreover, we first demonstrate the piezo-phototronic effect of 2D α-In2Se3. The photodetection performance of α-In2Se3 photodetector was increased by 35% with a uniaxial tensile strain of 0.44% under 700 nm light. Our work demonstrates that few-layer β-InSe and α-In2Se3 single crystals can not only be synthesized selectively, but also exhibit excellent optoelectronic properties, which paves a way for their application in optoelectronic and flexible devices.
TL;DR: In this paper, the authors showed that polymethyl methacrylate passivation is more effective to reduce the shallow trap states, and Al2O3 was more effective in reducing the deep traps in the SnO channel.
Abstract: Bottom gated thin-film transistors (TFTs) with various sputtered SnO active layer thicknesses ranging from 10 to 30 nm and different passivation layers have been investigated. The device with 20 nm SnO showed the highest on/off ratio of 1.7 × 104 and the smallest subthreshold swing of 8.43 V dec−1, and the mobility (0.76 cm2 V−1 s−1) was only slightly lower than in TFTs with a thicker SnO layer. However, both the mobility and the on/off ratio of the 15 nm SnO TFT dropped significantly by one order of magnitude. This indicated a strong influence of the top surface on the carrier transport, and we thus applied an organic or an inorganic encapsulation material to passivate the top surface. In the 20 nm TFT, the on/off ratio was doubled after passivation. The performance of the 15 nm TFT was improved even more dramatically with the on/off ratio increased by one order of magnitude and the mobility increased also significantly. Our experiment shows that polymethyl methacrylate passivation is more effective to reduce the shallow trap states, and Al2O3 is more effective in reducing the deep traps in the SnO channel.
TL;DR: In this paper, the thermal characteristics of GaN-based VCSELs with three typical structures were investigated both theoretically and experimentally, and the simulation results based on a steady state quasi-three-dimensional cylindrical model showed that the thermal resistance (R th) is affected by cavity length, mesa size, as well as the bottom distributed Bragg reflector (DBR) size.
Abstract: Thermal characteristics of GaN-based vertical cavity surface emitting lasers (VCSELs) with three typical structures were investigated both theoretically and experimentally. The simulation results based on a steady state quasi three-dimensional cylindrical model show that the thermal resistance (R th) is affected by cavity length, mesa size, as well as the bottom distributed Bragg reflector (DBR) size, and the detail further depends on different structures. Among different devices, GaN VCSEL with a hybrid cavity formed by one nitride bottom DBR and another dielectric top DBR is featured with lower R th, which is meanwhile affected strongly by the materials of the epitaxial bottom DBR. The main issues affecting the thermal dissipation in VCSELs with double dielectric DBRs are the bottom dielectric DBR and the dielectric current-confinement layer. To validate the simulation results, GaN-based VCSEL bonded on a copper plate was fabricated. R th of this device was measured and the results agreed well with the simulation. This work provides a better understanding of the thermal characteristics of GaN-based VCSELs and is useful in optimizing the structure design and improving the device performance.
TL;DR: In this article, a mild oxygen plasma treatment was used to fabricate air-stable few-layer Ultrathin indium selenide (InSe) nanosheets.
Abstract: Ultrathin indium selenide (InSe), as a newly emerging two-dimensional material with high carrier mobility and broadband optical absorption, is considered as a promising candidate for electronic and optoelectronic devices. It has been a challenge to produce air-stable mono- or few-layer InSe nanosheets because they degrade rapidly in an ambient environment. Here, we demonstrate a facile mild oxygen plasma treatment route to fabricate air-stable few-layer InSe samples. Photoluminescence (PL) and x-ray photoelectron spectroscopy (XPS) analysis indicate that the plasma treatment can introduce an oxidation process that forms a dense InSe1−xOx capping layer on the surface, and thus prevent the sample from degradation. Furthermore, the oxide layer would introduce a PL quenching of the pristine InSe, while a new and strong PL peak belonging to the emission from InSe1−xOx appears. This work offers a practical and efficient route to improve the performance of InSe based electrical and optoelectronic devices.
TL;DR: In this article, the 25 MeV oxygen irradiation effects in n-type single crystal βGa2O3 at different fluences were investigated and it was shown that the symmetric stretching modes and bending vibrations of GaO4 and GaO6 units are impaired upon increasing O irradiation fluence.
Abstract: In the present work, we report the 25 MeV oxygen irradiation effects in n-type single crystal βGa2O3 at different fluences. We demonstrate that the symmetric stretching modes and bending vibrations of GaO4 and GaO6 units are impaired upon increasing O irradiation fluence. Blue and green photoluminescence emission bands are found to be mainly associated with galliumoxygen divacancies, gallium vacancies and oxygen interstitials. The increase of optically active centers at low fluence and the photoluminescence quenching at high fluence are ascribed to the reduction of carrier density and the production of non-radiative recombination centers, respectively. The results envisage the possibility of obtaining pre-designed spectral behaviors by varying the oxygen irradiation fluence.
TL;DR: In this article, a vertical gaussian-doped tunnel-FET on SOI (VG-SOI-TFET) employing electrostatically doped (ED) p-type source is proposed and investigated.
Abstract: In this paper, a vertical gaussian-doped tunnel-FET on SOI (VG-SOI-TFET) employing electrostatically doped (ED) p-type source is proposed and investigated. A thin silicon film with n-type Gaussian doping in the vertical direction is used initially. By using the ED concept, a p+ source region is realized with Platinum having suitable work function ( m = 5.93 eV). Various dc and analog/RF parameters are studied as a function of peak donor density (N p) and doping gradient (g) of the gaussian distribution function. A 2D calibrated simulation study of the proposed VG-SOI-TFET has revealed that the I ON is increased by ~7 times while the ambipolar current (I AMB) is reduced by ~5 orders of magnitude at an optimum N p of 5 × 1018 cm−3 and g value of 2 nm/decade, compared to the uniformly-doped SOI TFET (UD-SOI-TFET). Furthermore, a superior analog/RF figures of merit are achieved such as transconductance, capacitances, cutoff-frequency, and gain bandwidth product. An improvement of ~5 times in peak g m and ~5 times in peak f T are also observed at an optimum N p of 5 × 1018 cm−3 and g value of 2 nm/decade.
TL;DR: In this paper, a first principle calculation was applied to separately obtain the scattering rates due to polar and non-polar phonon-electron interactions, and then spherically averaged coupling coefficients for each polar optical mode were calculated.
Abstract: The momentum scattering time for electron–phonon interaction in β-Ga2O3 was derived within the relaxation time approximation considering all infra-red active optical modes. A first principle calculation was applied to separately obtain the scattering rates due to polar and non-polar phonon–electron interactions, and then spherically averaged coupling coefficients for each polar optical mode were calculated. The method was tested to analyze, in the framework of the relaxation time approximation, transport data in semiconductors having different optical phonons. This approach can be reliably applied if the band may be considered as isotropic. Hall density and mobility curves were fitted simultaneously with the same parameters, after Hall-to-drift data conversion through a Hall scattering factor calculated self-consistently within the routine. In the theoretical mobility calculations, both polar and non-polar phonon interactions were considered besides impurity scattering. The Farvacque correction was included in the momentum scattering rate for electron interaction with the optical phonons, and its effect on mobility calculation is critically discussed. Hall transport data of β-Ga2O3 taken from the literature were fitted to test the approach, and good agreement between the experimental and calculated mobilities was obtained.
TL;DR: In this paper, the state-of-the-art regarding the production of silicon optical fibres in amorphous and crystalline form and then looks at the post-processing techniques and the improved material quality and new functionality that they afford.
Abstract: Semiconductor optical fibre technologies have grown rapidly in the last decade and there are now a range of production and post-processing techniques that allow for a vast degree of control over the core material's optoelectronic properties. These methodologies and the unique optical fibre geometry provide an exciting platform for materials engineering and fibres can now be produced with single crystal cores, low optical losses, tunable strain, and inscribable phase composition. This review discusses the state-of-the-art regarding the production of silicon optical fibres in amorphous and crystalline form and then looks at the post-processing techniques and the improved material quality and new functionality that they afford.
TL;DR: In this paper, an ultrathin AlN layer with surface hole gas was demonstrated to provide sufficient hole injection to a 280 nm light-emitting diode, demonstrating a record electrical-optical conversion efficiency exceeding 8.5% at 20 mA (55 A cm).
Abstract: The inherent deep-level nature of acceptors in wide-band-gap semiconductors makes p-ohmic contact formation and hole supply difficult, impeding progress for short-wavelength optoelectronics and high-power high-temperature bipolar electronics. We provide a general solution by demonstrating an ultrathin rather than a bulk wide-band-gap semiconductor to be a successful hole supplier and ohmic contact layer. Free holes in this ultrathin semiconductor are assisted to activate from deep acceptors and swept to surface to form hole gases by a large electric field, which can be provided by engineered spontaneous and piezoelectric polarizations. Experimentally, a 6 nm thick AlN layer with surface hole gas had formed p-ohmic contact to metals and provided sufficient hole injection to a 280 nm light-emitting diode, demonstrating a record electrical-optical conversion efficiency exceeding 8.5% at 20 mA (55 A cm?2). Our approach of forming p-type wide-band-gap semiconductor ohmic contact is critical to realizing high-efficiency ultraviolet optoelectronic devices.