High reactivity of H2O vapor on GaN surfaces.

Understanding the process of oxidation on the surface of GaN is important for improving metal-oxide-semiconductor (MOS) devices. Real-time X-ray photoelectron spectroscopy was used to observe the dynamic adsorption behavior of GaN surfaces upon irradiation of H2O, O2, N2O, and NO gases. It was found that H2O vapor has the highest reactivity on the surface despite its lower oxidation power. The adsorption behavior of H2O was explained by the density functional molecular dynamic calculation including the spin state of the surfaces. Two types of adsorbed H2O molecules were present on the (0001) (+c) surface: non-dissociatively adsorbed H2O (physisorption), and dissociatively adsorbed H2O (chemisorption) molecules that were dissociated with OH and H adsorbed on Ga atoms. H2O molecules attacked the back side of three-fold Ga atoms on the (0001̅) (-c) GaN surface, and the bond length between the Ga and N was broken. The chemisorption on the (101̅0) m-plane of GaN, which is the channel of a trench-type GaN MOS power transistor, was dominant, and a stable Ga-O bond was formed due to the elongated bond length of Ga on the surface. In the atomic layer deposition process of the Al2O3 layer using H2O vapor, the reactions caused at the interface were more remarkable for p-GaN. If unintentional oxidation can be resulted in the generation of the defects at the MOS interface, these results suggest that oxidant gases other than H2O and O2 should be used to avoid uncontrollable oxidation on GaN surfaces.

Effect of Deep-Defects Excitation on Mechanical Energy Dissipation of Single-Crystal Diamond.

The ultrawide band gap of diamond distinguishes it from other semiconductors, in that all known defects have deep energy levels that are less active at room temperature. Here, we present the effect of deep defects on the mechanical energy dissipation of single-crystal diamond experimentally and theoretically up to 973 K. Energy dissipation is found to increase with temperature and exhibits local maxima due to the interaction between phonons and deep defects activated at specific temperatures. A two-level model with deep energies is proposed to explain well the energy dissipation at elevated temperatures. It is evident that the removal of boron impurities can substantially increase the quality factor of room-temperature diamond mechanical resonators. The deep energy nature of the defects bestows single-crystal diamond with outstanding low intrinsic energy dissipation in mechanical resonators at room temperature or above.

Strain-enhanced high Q-factor GaN micro-electromechanical resonator.

We report on a highly sensitive gallium nitride (GaN) micro-electromechanical (MEMS) resonator with a record quality factor (Q) exceeding 105 at the high resonant frequency (f) of 911 kHz by the strain engineering for the GaN-on-Si structure. The f of the double-clamped GaN beam bridge is increased from 139 to 911 kHz when the tensile stress is increased to 640 MPa. Although it is usually regarded that the energy dissipation increases with increasing resonant frequency, an ultra-high Q-factor which is more than two orders of magnitude higher than those of the other reported GaN-based MEMS is obtained. The high Q-factor results from the large tensile stress which can be intentionally introduced and engineered in the GaN epitaxial layer by utilizing the lattice mismatch between GaN and Si, leading to the stored elastic energy and drastically decreasing the energy dissipation. The developed GaN MEMS is further demonstrated as a highly sensitive mass sensor with a resolution of 10-12 g/s through detecting the microdroplet evaporation process. This work provides an avenue to improve the f × Q product of the MEMS through an internally strained structure.

High-performance visible to near-infrared photodetectors by using (Cd,Zn)Te single crystal.

The authors report on a high-performance metal-semiconductor-metal (MSM) photodetector fabricated on the Cd0.96Zn0.04Te single crystal with the photoresponse from visible to near infrared region. Benefitting from the high-quality single crystallization, an ultra-low dark current of ~10-10 A was obtained at a high applied voltage of 10 V, leading to a photo-to-dark-current ratio of more than 103 at 700 nm light illumination. The highest responsivity is estimated to be 1.43 A/W with a specific detectivity of 3.31 × 1012 Jones at 10 V at a relatively lower injection power density. The discrimination ratio between the near infrared region of 800 nm and 900 nm is almost 102, which is high enough for the accurate spectra selectivity. The MSM photodetector also exhibits a fast response speed of ~800 µs and extremely low persistent photoconductivity (PPC), while the PPC is inhibited at high temperatures.

High detectivity solar-blind high-temperature deep-ultraviolet photodetector based on multi-layered (l00) facet-oriented ß-Ga2O3 nanobelts.

Fabrication of a high-temperature deep-ultraviolet photodetector working in the solar-blind spectrum range (190-280 nm) is a challenge due to the degradation in the dark current and photoresponse properties. Herein, ß-Ga2O3 multi-layered nanobelts with (l00) facet-oriented were synthesized, and were demonstrated for the first time to possess excellent mechanical, electrical properties and stability at a high temperature inside a TEM studies. As-fabricated DUV solar-blind photodetectors using (l00) facet-oriented ß-Ga2O3 multi-layered nanobelts demonstrated enhanced photodetective performances, that is, high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability, importantly, at a temperature as high as 433 K, which are comparable to other reported semiconducting nanomaterial photodetectors. In particular, the characteristics of the photoresponsivity of the ß-Ga2O3 nanobelt devices include a high photoexcited current (>21 nA), an ultralow dark current (below the detection limit of 10(-14) A), a fast time response (<0.3 s), a high R(λ) (≈851 A/W), and a high EQE (~4.2 × 10(3)). The present fabricated facet-oriented ß-Ga2O3 multi-layered nanobelt based devices will find practical applications in photodetectors or optical switches for high-temperature environment.

High-detectivity nanowire photodetectors governed by bulk photocurrent dynamics with thermally stable carbide contacts.

Photodetectors fabricated from one-dimensional semiconductors are always dominated by the surface states due to their large surface-to-volume ratio. Therefore, the basic 5S requirements (high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability) for practical photodetectors are difficult to satisfy. We report on high-temperature and high-detectivity solar-blind deep-ultraviolet (DUV) photodetectors based on ß-Ga2O3 nanowires, in which the photoresponse behavior is dominated by the bulk instead of the surface states. Ohmic contact to the ß-Ga2O3 nanowires was achieved by using a thermally stable tungsten carbide electrode. As a result, the DUV responsivity at 250 nm shows the highest values--4492 A W(-1) at room temperature (RT) and 3000 A W(-1) at 553 K (280 °C)--among the DUV photodetectors. The detectivity is as high as 1.26×10(16) cm Hz(1/2) W(-1) at RT, and still remains 4.1×10(14) cm Hz(1/2) W(-1) at as high a temperature as 553 K. The photocurrent dynamics from the ß-Ga2O3 nanowire is discussed in terms of the bulk dominated photoresponse behavior. Other wide bandgap DUV detectors based on nanostructures could also be developed for high-temperature applications based on this work.

A comprehensive review of semiconductor ultraviolet photodetectors: from thin film to one-dimensional nanostructures.

Ultraviolet (UV) photodetectors have drawn extensive attention owing to their applications in industrial, environmental and even biological fields. Compared to UV-enhanced Si photodetectors, a new generation of wide bandgap semiconductors, such as (Al, In) GaN, diamond, and SiC, have the advantages of high responsivity, high thermal stability, robust radiation hardness and high response speed. On the other hand, one-dimensional (1D) nanostructure semiconductors with a wide bandgap, such as ß-Ga2O3, GaN, ZnO, or other metal-oxide nanostructures, also show their potential for high-efficiency UV photodetection. In some cases such as flame detection, high-temperature thermally stable detectors with high performance are required. This article provides a comprehensive review on the state-of-the-art research activities in the UV photodetection field, including not only semiconductor thin films, but also 1D nanostructured materials, which are attracting more and more attention in the detection field. A special focus is given on the thermal stability of the developed devices, which is one of the key characteristics for the real applications.

High-pressure MOCVD growth of InGaN thick films toward the photovoltaic applications.

The highly efficient photovoltaic cells require the In-rich InGaN film with a thickness more than 300 nm to achieve the effective photo⋅electricity energy conversion. However, the InGaN thick films suffer from poor crystalline quality and phase separations by using the conventional low-pressure metal organic chemical vapor deposition (MOCVD). We report on the growth of 0.3-1 µm-thick InGaN films with a specially designed vertical-type high-pressure MOCVD at the pressure up to 2.5 atms. The In incorporation is found to be greatly enhanced at the elevated pressures although the growth temperatures are the same. The phase separations are inhibited when the growth pressure is higher than atmospheric pressure, leading to the improved crystalline quality and better surface morphologies especially for the In-rich InGaN. The In0.4Ga0.6N with the thickness of 300 nm is further demonstrated as the active region of solar cells, and the widest photoresponse range from ultraviolet to more than 750 nm is achieved.

Polarity Control of an All-Sputtered Epitaxial GaN/AlN/Al Film on a Si(111) Substrate by Intermediate Oxidization.

The ability to control the polarity of an all-sputtered epitaxial GaN/AlN/Al film on a Si(111) substrate via intermediate oxidization was investigated. A stable surface of GaN on a Si substrate is a N-terminated surface (-c surface); hence, for electric device applications, the Ga-terminated surface (+c surface) is preferable. The GaN/AlN/Al film on Si(111) showed a -c surface, as confirmed by time-of-flight low-energy atom scattering spectroscopy (TOFLAS) and X-ray photoelectron spectroscopy (XPS). The AlN layer was intentionally oxidized via air exposure during film growth. The GaN surface subjected to the oxidization process had the +c surface. Secondary-ion mass spectrometry measurements indicated a high oxygen concentration after the intentional oxidization. However, the intentional oxidization degraded the crystallinity of the GaN/AlN layer. By changing the oxidization point and repeating the GaN/AlN growth, the crystallinity of GaN was recovered. Such polarity control of GaN on Si grown by sputtering shows strong potential for the fabrication of large-diameter +c-GaN template substrates at low cost.

Stress effect on the resonance properties of single-crystal diamond cantilever resonators for microscopy applications.

Micro-cantilever beams have been widely used for surface sensing applications as well as atomic force microscope. However, surface stress appears in cantilever beams due to a variety of factors such as the absorption of molecules, temperature variations, materials imperfectness, and the fabrication process. Single-crystal diamond (SCD) has been regarded as an ideal material for cantilever sensors through the surface effect due to the outstanding mechanical rigidity and chemical inertness. In this paper, the authors report on the SCD cantilever beams fabricated by a smart-cut method with high quality factors up to 14 000 and stress characterization by surface geometry curvature observation and Raman microscopy. Although both surface geometry profile and Raman shift show the existence of surface stress in the SCD cantilever beams, the resonance properties are little influenced and maintain excellent rigidity and high quality. Therefore, the SCD-on-SCD resonator provides a promising platform for high-reliability microscopy applications.

A comparative study of interfacial thermal conductance between metal and semiconductor.

To understand and control thermal conductance of interface between metal and semiconductor has now become a crucial task for the thermal design and management of nano-electronic and micro-electronic devices. The interfacial alignments and electronic characteristics of the interfaces between metal and semiconductor are studied using a first-principles calculation based on hybrid density functional theory. The thermal conductance of interfaces between metal and semiconductor were calculated and analyzed using diffuse mismatch model, acoustic mismatch model and nonequilibrium molecular dynamics methods. Especially, according to nonequilibrium molecular dynamics, the values of thermal conductance were obtained to be 32.55 MW m-2 K-1 and 341.87 MW m-2 K-1 at C-Cu and Si-Cu interfaces, respectively. These results of theoretical simulation calculations are basically consistent with the current experimental data, which indicates that phonon-phonon interaction play a more important role than electron-phonon interaction during heat transport. It may be effective way to improve the interfacial thermal conductance through enhancing the interface coupling strength at the metal-semiconductor interface because the strong interfacial scattering plays a role in suppressing in the weaker interface coupling heterostructure, leading to the lower thermal conductance of interfaces. This could provide a beneficial reference for the design of the Schottky diode and thermal management at the interfaces between metal and semiconductor.

Electrochemical-coupling layer-by-layer (ECC-LbL) assembly.

Electrochemical-coupling layer-by-layer (ECC-LbL) assembly is introduced as a novel fabrication methodology for preparing layered thin films. This method allows us to covalently immobilize functional units (e.g., porphyrin, fullerene, and fluorene) into thin films having desired thicknesses and designable sequences for both homo- and heteroassemblies while ensuring efficient layer-to-layer electronic interactions. Films were prepared using a conventional electrochemical setup by a simple and inexpensive process from which various layering sequences can be obtained, and the photovoltaic functions of a prototype p/n heterojunction device were demonstrated.

Enhancing Delta E Effect at High Temperatures of Galfenol/Ti/Single-Crystal Diamond Resonators for Magnetic Sensing.

A conventional wisdom is that the sensing properties of magnetic sensors at high temperatures will be degraded due to the materials' deterioration. Here, the concept of high-temperature enhancing magnetic sensing is proposed based on the hybrid structure of SCD MEMS resonator functionalized with a high thermal-stable ferromagnetic galfenol (FeGa) film. The delta E effect of the magnetostrictive FeGa thin film on Ti/SCD cantilevers is investigated by varying the operating temperature from 300 to 773 K upon external magnetic fields. The multilayer structure magnetic sensor presents a high sensitivity of 71.1 Hz/mT and a low noise level of 10 nT/√Hz at 773 K for frequencies higher than 7.5 kHz. The high-temperature magnetic sensing performance exceeds those of the reported magnetic sensors. Furthermore, an anomalous behavior is observed on the delta E effect, which exhibits a positive temperature dependence with the law of Tn. Based on the resonance frequency shift of the FeGa/Ti/SCD cantilever, the strain coupling in the multilayers of the FeGa/Ti/SCD structure under a magnetic field is strengthened with increasing temperature. The delta E effect shows a strong relationship with the azimuthal angle, θ, as a sine function at 300 and 773 K. This work provides a strategy to develop magnetic sensors for high-temperature applications with performance superior to that of the present ones.

P-Channel InGaN/GaN heterostructure metal-oxide-semiconductor field effect transistor based on polarization-induced two-dimensional hole gas.

The concept of p-channel InGaN/GaN heterostructure field effect transistor (FET) using a two-dimensional hole gas (2DHG) induced by polarization effect is demonstrated. The existence of 2DHG near the lower interface of InGaN/GaN heterostructure is verified by theoretical simulation and capacitance-voltage profiling. The metal-oxide-semiconductor FET (MOSFET) with Al2O3 gate dielectric shows a drain-source current density of 0.51 mA/mm at the gate voltage of -2 V and drain bias of -15 V, an ON/OFF ratio of two orders of magnitude and effective hole mobility of 10 cm(2)/Vs at room temperature. The normal operation of MOSFET without freeze-out at 8 K further proves that the p-channel behavior is originated from the polarization-induced 2DHG.

Electrochemically Organized Isolated Fullerene-Rich Thin Films with Optical Limiting Properties.

Electrochemical assembly was applied directly to determine the aggregation of nanoclusters in isolated fullerene-rich (54-63 wt %) thin films. The electroactive reactions were achieved using electroactive carbazole and pyrene, which led to distinguishable nanoparticle-like and irregular cluster formations. These films, with amorphous and transparent states, showed good photoactivity and significant optical limiting response with an excellent threshold of 63 mJ cm(-2). This work thus paves a way to assemble highly isolated (or monodispersed) building blocks into thin films at the molecular level with control of the nanostructural formations through important molecular design.

A multilevel intermediate-band solar cell by InGaN/GaN quantum dots with a strain-modulated structure.

Multiple stacked InGaN/GaN quantum dots are embedded into an InGaN p-n junction to develop multilevel intermediateband (MIB) solar cells. An IB transition is evidenced from both experiment and theoretical calculations. The MIB solar cell shows a wide photovoltaic response from the UV to the near-IR region. This work opens up an interesting opportunity for high-efficiency IB solar cells in the photovoltaics field.

Arbitrary multicolor photodetection by hetero-integrated semiconductor nanostructures.

The typical photodetectors can only detect one specific optical spectral band, such as InGaAs and graphene-PbS quantum dots for near-infrared (NIR) light detection, CdS and Si for visible light detection, and ZnO and III-nitrides for UV light detection. So far, none of the developed photodetector can achieve the multicolor detection with arbitrary spectral selectivity, high sensitivity, high speed, high signal-to-noise ratio, high stability, and simplicity (called 6S requirements). Here, we propose a universal strategy to develop multicolor photodetectors with arbitrary spectral selectivity by integrating various semiconductor nanostructures on a wide-bandgap semiconductor or an insulator substrate. Because the photoresponse of each spectral band is determined by each semiconductor nanostructure or the semiconductor substrate, multicolor detection satisfying 6S requirements can be readily satisfied by selecting the right semiconductors.

In situ switching layer-by-layer assembly: one-pot rapid layer assembly via alternation of reductive and oxidative electropolymerization.

In situ one-pot rapid layer-by-layer assembly of polymeric films as an active layer of a photoactive device via alternation of reductive and oxidative electropolymerization has been demonstrated. This novel fabrication without moving or changing experimental gears would be a powerful strategy to develop automated layer-by-layer machines.