Showing papers in "Semiconductor Science and Technology in 2013"

Gallium nitride devices for power electronic applications

Abstract: Recent success with the fabrication of high-performance GaN-on-Si high-voltage HFETs has made this technology a contender for power electronic applications. This paper discusses the properties of GaN that make it an attractive alternative to established silicon and emerging SiC power devices. Progress in development of vertical power devices from bulk GaN is reviewed followed by analysis of the prospects for GaN-on-Si HFET structures. Challenges and innovative solutions to creating enhancement-mode power switches are reviewed.

N-polar GaN epitaxy and high electron mobility transistors

Abstract: This paper reviews the progress of N-polar () GaN high frequency electronics that aims at addressing the device scaling challenges faced by GaN high electron mobility transistors (HEMTs) for radio-frequency and mixed-signal applications. Device quality (Al, In, Ga)N materials for N-polar heterostructures are developed using molecular beam epitaxy and metalorganic chemical vapor deposition. The principles of polarization engineering for designing N-polar HEMT structures will be outlined. The performance, scaling behavior and challenges of microwave power devices as well as highly-scaled depletion- and enhancement-mode devices employing advanced technologies including self-aligned processes, n+ (In,Ga)N ohmic contact regrowth and high aspect ratio T-gates will be discussed. Recent research results on integrating N-polar GaN with Si for prospective novel applications will also be summarized.

III-nitride semiconductors for intersubband optoelectronics: a review

Abstract: III-nitride nanostructures have recently emerged as promising materials for new intersubband (ISB) devices in a wide variety of applications. These ISB technologies rely on infrared optical transitions between quantum-confined electronic states in the conduction band of GaN/Al(Ga)N nanostructures, namely quantum wells or quantum dots. The large conduction band offset (about 1.8 eV for GaN/AlN) and sub-picosecond ISB relaxation of III-nitrides render them appealing materials for ultrafast photonic devices in near-infrared telecommunication networks. Furthermore, the large energy of GaN longitudinal-optical phonons (92 meV) opens prospects for high-temperature THz quantum cascade lasers and ISB devices covering the 5?10 THz band, inaccessible to As-based technologies due to phonon absorption. In this paper, we describe the basic features of ISB transitions in III-nitride quantum wells and quantum dots, in terms of theoretical calculations, material growth, spectroscopy, resonant transport phenomena, and device implementation. The latest results in the fabrication of control-by-design devices such as all-optical switches, electro-optical modulators, photodetectors, and lasers are also presented.

Prospects for the application of GaN power devices in hybrid electric vehicle drive systems

Abstract: GaN, a wide bandgap semiconductor successfully implemented in optical and high-speed electronic devices, has gained momentum in recent years for power electronics applications. Along with rapid progress in material and device processing technologies, high-voltage transistors over 600?V have been reported by a number of teams worldwide. These advances make GaN highly attractive for the growing market of electrified vehicles, which currently employ bipolar silicon devices in the 600?1200?V class for the traction inverter. However, to capture this billion-dollar power market, GaN has to compete with existing IGBT products and deliver higher performance at comparable or lower cost. This paper reviews key achievements made by the GaN semiconductor industry, requirements of the automotive electric drive system and remaining challenges for GaN power devices to fit in the inverter application of hybrid vehicles.

Current status and scope of gallium nitride-based vertical transistors for high-power electronics application

Abstract: Gallium nitride (GaN) is becoming the material of choice for power electronics to enable the roadmap of increasing power density by simultaneously enabling high-power conversion efficiency and reduced form factor. This is because the low switching losses of GaN enable high-frequency operation which reduces bulky passive components with negligible change in efficiency. Commercialization of GaN-on-Si materials for power electronics has led to the entry of GaN devices into the medium-power market since the performance-over-cost of even first-generation products looks very attractive compared to today's mature Si-based solutions. On the other hand, the high-power market still remains unaddressed by lateral GaN devices. The current and voltage demand for high-power conversion application makes the chip area in a lateral topology so large that it becomes difficult to manufacture. Vertical GaN devices would play a big role alongside silicon carbide (SiC) to address the high-power conversion needs. In this paper vertical GaN devices are discussed with emphasis on current aperture vertical electron transistors (CAVETs) which have shown promising performance. The fabrication-related challenges and the future possibilities enabled by the availability of good-quality, cost-competitive bulk GaN material are also evaluated for CAVETs.

III-nitrides for energy production: photovoltaic and thermoelectric applications

Abstract: In this review article, we discuss the recent advances in the III-nitrides, in particular GaN and its ternary alloys, for photovoltaic and thermoelectric devices. The advantages of using the III-nitrides for electronic and optoelectronic applications are well understood and III-nitride devices are already seen in many consumer based products. Recently, an emerging potential of III-nitrides for energy harvesting has been investigated due to its unique materials properties that include a tunable direct band gap, mechanical and thermal stability at high temperatures and excellent electronic transport properties. This review will show that the potential of using the III-nitrides for photovoltaic and thermoelectric applications will be realized as the impact of intrinsic and extrinsic defects on the photovoltaic and thermoelectric properties are better understood.

Trapping phenomena in AlGaN/GaN HEMTs: a study based on pulsed and transient measurements

Abstract: Slow trapping phenomenon in AlGaN/GaN HEMTs has been extensively analyzed and described in this paper. Thanks to a detailed investigation, based on a combined pulsed and transient investigation of the current/voltage characteristics (carried out over on an 8-decade time scale), we report a detailed description of the properties of trap levels located in the gate–drain surface, and in the region under the gate of AlGaN/GaN HEMTs. More specifically, the following, relevant results have been identified: (i) the presence of surface trap states may determine a significant current collapse, and reduction of the peak transconductance. During a current transient measurement, the emission of electrons trapped at surface states proceeds through hopping, as demonstrated by means of temperature-dependent measurements. The activation energy of the de-trapping process is equal to 99 meV. (ii) The presence of a high density of defects under the gate may induce a significant shift in the threshold voltage, when devices are submitted to pulsed transconductance measurements. The traps responsible for this process have an activation energy of 0.63 eV, and are detected only on samples with high gate leakage, since gate current allows for a more effective charging/de-charging of the defects.

Derivation of 12- and 14-band k ? p Hamiltonians for dilute bismide and bismide-nitride semiconductors

Abstract: Using an sp3s* tight-binding (TB) model we demonstrate how the observed strong bowing of the band gap and spin-orbit-splitting with increasing Bi composition in the dilute bismide alloy GaBixAs1 ? x can be described in terms of a band-anticrossing interaction between the extended states of the GaAs valence band edge (VBE) and highly localized Bi-related resonant states lying below the GaAs VBE. We derive a 12-band k ? p Hamiltonian to describe the band structure of GaBixAs1 ? x and show that this model is in excellent agreement with full TB calculations of the band structure in the vicinity of the band edges, as well as with experimental measurements of the band gap and spin-orbit-splitting across a large composition range. Based on a TB model of GaBixNyAs1 ? x ? y we show that to a good approximation N and Bi act independently of one another in disordered GaBixNyAs1 ? x ? y alloys, indicating that a simple description of the band structure is possible. We present a 14-band k ? p Hamiltonian for ordered GaBixNyAs1 ? x ? y crystals which reproduces accurately the essential features of full TB calculations of the band structure in the vicinity of the band edges. The k ? p models we present here are therefore ideally suited to the simulation of the optoelectronic properties of these novel III?V semiconductor alloys.

Growth and characterization of electrodeposited Cu2O thin films

Abstract: This work demonstrates the electrodeposition of cuprous oxide (Cu2O) thin films onto a fluorine-doped tin oxide (FTO)-coated conducting glass substrates from Cu(II) sulfate solution with C6H8O7 chelating agent. During cyclic voltammetry experiences, the potential interval where the electrodeposition of Cu2O is carried out was established. The thin films were obtained potentiostatically and were characterized through different techniques. From the Mott–Schottky measurements, the flat-band potential and the acceptor density for the Cu2O thin films are determined. All the films showed a p-type semiconductor character with a carrier density varying between 2.41 × 1018 cm−3 and 5.38 × 1018 cm−3. This little difference is attributed to the increase of the stoichiometric defects in the films with the deposition potential. Atomic force microscopy analysis showed that the Cu2O thin films obtained at high potential are more homogenous in appearance and present lower crystallites size. X-ray diffraction measurements indicate a cubic structure with good crystallization state and the deposition potential was found to have an influence on the size of the crystallites. The optical measurements show a direct band gap between 2.07–2.49 eV depending on the applied potential.

Thermoelectric transport and Hall measurements of low defect Sb2Te3 thin films grown by atomic layer deposition

Abstract: Sb2Te3 has recently been an object of intensive research since its promising applicability in thermoelectric, in phase-change memory devices and as a topological insulator. In this work, we report highly textured Sb2Te3 thin films, grown by atomic layer deposition on Si/SiO2 wafers based on the reaction of SbCl3 and (Et3Si)2Te. The low deposition temperature at 80 °C allows the pre-patterning of the Sb2Te3 by standard lithography processes. A platform to characterize the Seebeck coefficient S, the electrical conductivity σ as well as the Hall coefficient RH on the same film has been developed. Comparing all temperature-dependent transport properties, three different conductive regions in the temperature range of 50–400 K are found. Room temperature values of S = 146 × 10–6 VK−1, σ = 104 Sm−1 and mobility µ = 270.5 × 10−4 m2 V−1 s−1 are determined. The low carrier concentration in the range of n = 2.4 × 1018 cm−3 at 300 K quantifies the low defect content of the Sb2Te3 thin films.

Epitaxial growth of sexi-thiophene and para-hexaphenyl and its implications for the fabrication of self-assembled lasing nano-fibres

Abstract: Over the last few years, epitaxially grown self-assembled organic nano-structures became of increasing interest due to their high potential for implementation within opto-electronic devices. Exemplarily, the epitaxial growth of the rod-like molecules para-hexaphenyl (p-6P) and α-sexi-thiophene (6T) is discussed within this review. Both molecules tend to crystallize in highly asymmetric elongated entities which are also called nano-fibres. It is demonstrated that the obtained needle orientations and morphologies result from a complex interplay between various parameters e.g. substrate surface symmetry, molecular adsorption, crystal structure and contact plane. The interplay and its implications on the fabrication of self-assembled waveguiding nano-fibres and optical resonator structures are discussed and substantiated by a comparison with the reported literature. In further consequence, it is demonstrated that a precise control on the molecular adsorption geometry and the crystal contact plane represents a fundamental key parameter for the fabrication of self-assembled nano-fibres. As both parameters are basically determined by the chosen molecule–substrate material couple, the possible spectrum of molecular building blocks for the fabrication of waveguiding and lasing nano-structures can be predicted by the discussed growth model. A possible expansion of this common valid concept is presented by the utilization of organic–organic heteroepitaxy. Based on the reported p-6P/6T heterostructures which have been fabricated on various substrate surfaces, it is substantiated that the fabrication of organic–organic interfaces can be effectively used to gain control on the molecular adsorption geometry. As the proposed strategy still lacks a precise control of the obtained crystal contact plane, further strategies are discussed which potentially lead to a controlled fabrication of opto-electronic devices based on self-assembled organic nano-structures.

Molecular beam epitaxy for high-performance Ga-face GaN electron devices

Abstract: Molecular beam epitaxy (MBE) has emerged as a powerful technique for growing GaN-based high electron mobility transistor (HEMT) epistructures. Over the past decade, HEMT performance steadily improved, mainly through the optimization of device fabrication processes. Soon, HEMT performance will be limited by the crystalline quality of the epistructure. MBE offers heterostructure growth with highly abrupt interfaces, low point defect concentrations, and very low carbon and hydrogen impurity concentrations. Minimizing parasitic leakage pathways and resistances is essential in the growth of HEMTs for high-frequency and high-power applications. Through growth on native substrates with very low threading dislocation density, low-leakage HEMTs with very low on-resistance can be realized. Ga-rich plasma-assisted MBE (PAMBE) has been studied extensively, and it is clear that this technique has inherent limitations, including a high density of leakage pathways and a very small growth parameter space. Relatively new MBE growth techniques—high-temperature N-rich PAMBE and ammonia-based MBE—are being developed to circumvent the shortcomings of Ga-rich PAMBE.

Merits of gallium nitride based power conversion

Abstract: Gallium nitride (GaN) based power electronics devices are actively being evaluated to determine if their theoretical advantages over silicon (Si) based switches can translate into improved performance of existing hardware as well as open the doors to new types of applications, such as high temperature implementations, or very high frequency power conversion. The following paper presents an overview of this activity. A brief summary about power electronics and the requirements of semiconductor devices used in this field is provided. Detailed analysis of the advantages and the challenges of using GaN devices is included along with a survey of demonstrations. This work also presents the test results from the evaluation of GaN devices from Efficient Power Conversion (EPC) and Transphorm. Included is a demonstration of EPC's devices in a high frequency, high efficiency, switched-capacitor voltage doubler. This circuit achieves an output of 480 W at a switching frequency of 893 kHz.

Fabrication challenges of electrical injection metallic cavity semiconductor nanolasers

Abstract: The recently emerged metallic-cavity nanolasers have opened a new phase of miniaturization of semiconductor lasers down to sub-wavelength scale. This new type of semiconductor lasers is suitable for many low-power applications due to its small size, tight optical confinement and good heat dissipation. However, there are major technical challenges in the fabrication of such nanolasers that must be overcome to make high-quality devices with high yield needed for practical applications. Here we will discuss several fabrication issues that are critical to the device performance. These issues, including device patterning, pillar etching, surface passivation and metal deposition, will determine both optical and electrical properties, especially the lifetime, threshold, and efficiency of a nanolaser.

Gallium nitride electronics

Abstract: In the past two decades, there has been increasing research and industrial activity in the area of gallium nitride (GaN) electronics, stimulated first by the successful demonstration of GaN LEDs While the promise of wide band gap semiconductors for power electronics was recognized many years before this by one of the contributors to this issue (J Baliga), the success in the area of LEDs acted as a catalyst It set the field of GaN electronics in motion, and today the technology is improving the performance of several applications including RF cell phone base stations and military radar GaN could also play a very important role in reducing worldwide energy consumption by enabling high efficiency compact power converters operating at high voltages and lower frequencies While GaN electronics is a rapidly evolving area with active research worldwide, this special issue provides an opportunity to capture some of the great advances that have been made in the last 15 years The issue begins with a section on epitaxy and processing, followed by an overview of high-frequency HEMTs, which have been the most commercially successful application of III-nitride electronics to date This is followed by review and research articles on power-switching transistors, which are currently of great interest to the III-nitride community A section of this issue is devoted to the reliability of III-nitride devices, an area that is of increasing significance as the research focus has moved from not just high performance but also production-worthiness and long-term usage of these devices Finally, a group of papers on new and relatively less studied ideas for III-nitride electronics, such as interband tunneling, heterojunction bipolar transistors, and high-temperature electronics is included These areas point to new areas of research and technological innovation going beyond the state of the art into the future We hope that the breadth and quality of articles in this issue will make it an excellent reference for newcomers and experienced researchers in this field for several years We thank Alice Malhador at IOP Publishing for her constant encouragement and guidance in putting together this special issue on GaN electronics

Simulation of thermal management in AlGaN/GaN HEMTs with integrated diamond heat spreaders

Abstract: We investigated the impact of diamond heat spreading layers on the performance of AlGaN/GaN high-electron-mobility-transistors (HEMTs). A finite element method was used to simulate the thermal and electrical characteristics of the devices under dc and pulsed operation conditions. The results show that the device performance can be improved significantly by optimized heat spreading, an effect strongly dependent on the lateral thermal conductivity of the initial several micrometers of diamond deposition. Of crucial importance is the proximity of the diamond layer to the heat source, which makes this method advantageous over other thermal management procedures, especially for the device in pulsed operation. In this case, the self-heating effect can be suppressed, and it is not affected by either the substrate or its thermal boundary resistance at the GaN/substrate at wider pulses. The device with a 5 µm diamond layer can present 10.5% improvement of drain current, and the self-heating effect can be neglected for a 100 ns pulse width at 1 V gate and 20 V drain voltage.

Study of band-structure, optical properties and native defects in AIBIIIO2 (AI = Cu or Ag, BIII = Al, Ga or In) delafossites

Abstract: In this work, employing a first-principles approach, the structural, electronic, optical, as well as the defect physics of AIBIIIO2 (AI = Cu or Ag, BIII = Al, Ga or In) compounds are discussed. We show that all these delafossite compounds have indirect band gaps with gap energy in the range = 1.6–3.6 eV. We also estimate the lowest direct band gap energies to be in the range = 2.6–4.0 eV. Optical characteristics reveal that AIBIIIO2 compounds exhibit a significant anisotropy for both the real and imaginary parts of the dielectric function. Furthermore, we find that absorption onset for these compounds is energetically well above (>1.5 eV) the fundamental band gaps. Moreover, we demonstrate that the copper delafossites have larger absorption coefficients compared to the corresponding BIII cation silver delafossites in the visible range. Defect calculations reveal that Cu or Ag vacancy has the lowest formation energy followed by the O vacancy while the BIII cation vacancy has the highest formation energy.

Non-local exchange correlation functionals impact on the structural, electronic and optical properties of III-V arsenides

Abstract: Exchange correlation (XC) energy functionals play a vital role in the efficiency of density functional theory (DFT) calculations, more soundly in the calculation of fundamental electronic energy bandgap. In the present DFT study of III-arsenides, we investigate the implications of XC-energy functional and corresponding potential on the structural, electronic and optical properties of XAs (X = B, Al, Ga, In). Firstly we report and discuss the optimized structural lattice parameters and the band gap calculations performed within different non-local XC functionals as implemented in the DFT-packages: WIEN2k, CASTEP and SIESTA. These packages are representative of the available code in ab initio studies. We employed the LDA, GGA-PBE, GGA-WC and mBJ-LDA using WIEN2k. In CASTEP, we employed the hybrid functional, sX-LDA. Furthermore LDA, GGA-PBE and meta-GGA were employed using SIESTA code. Our results point to GGA-WC as a more appropriate approximation for the calculations of structural parameters. However our electronic bandstructure calculations at the level of mBJ-LDA potential show considerable improvements over the other XC functionals, even the sX-LDA hybrid functional. We report also the optical properties within mBJ potential, which show a nice agreement with the experimental measurements in addition to other theoretical results.

Backside-configured surface plasmonic structure with over 40 times photocurrent enhancement

Abstract: In this paper, we report a quantum dot infrared photodetector (QDIP) enhanced by a backside-configured surface plasmonic structure with an over 40 times peak photocurrent enhancement. The QDIP enhancement by the backside-configured plasmonic structure is compared with that by the top-configured plasmonic structure. The backside-configured plasmonic structure shows much higher photocurrent and photodetectivity D* enhancement. We analyze the excitation of the surface plasmonic waves by the backside-configured and top-configured plasmonic structures. The higher enhancement is attributed to the more efficient surface plasmonic excitation by the backside-configured plasmonic structure.

Reliability studies of AlGaN/GaN high electron mobility transistors

Abstract: AlGaN/GaN high electron mobility transistors are gaining commercial acceptance for use in high power and high frequency applications, but the degradation mechanisms that drive failure in the field are not completely understood. Since some of these mechanisms are current or field driven, reliability studies must go beyond the typical Arrhenius-accelerated life tests. In this paper, we summarize recent work on electric field or current driven degradation in devices with different gate metallization, device dimensions, electric field mitigation techniques (such as source field plates) and the effect of device fabrication processes for both dc and RF stress conditions.

Reduction of defect density by rapid thermal annealing in GaAsBi studied by time-resolved photoluminescence

Abstract: Time-resolved photoluminescence was performed on as-grown and annealed bulk GaAsBi samples. Rapid thermal annealing was carried out at a temperature of 750 °C. With annealing, we observed a significant change in the photoluminescence decay time at low temperature and low excitation power, which is likely due to a reduction of localized states. Although the time-integrated photoluminescence intensity did not show a large variation, this enhancement was confirmed by the observed removal after annealing of the S-shape behaviour present in the as-grown sample.

Formation of III–V-on-insulator structures on Si by direct wafer bonding

Abstract: We have studied the formation of III?V-compound-semiconductors-on-insulator (III?V-OI) structures with thin buried oxide (BOX) layers on Si wafers by using developed direct wafer bonding (DWB). In order to realize III?V-OI MOSFETs with ultrathin body and extremely thin body (ETB) InGaAs-OI channel layers and ultrathin BOX layers, we have developed an electron-cyclotron resonance (ECR) O2?plasma-assisted DWB process with ECR sputtered SiO2?BOX layers and a DWB process based on atomic-layer-deposition Al2O3?(ALD-Al2O3) BOX layers. It is essential to suppress micro-void generation during wafer bonding process to achieve excellent wafer bonding. We have found that major causes of micro-void generation in DWB processes with ECR-SiO2?and ALD-Al2O3?BOX layers are desorption of Ar and H2O gas, respectively. In order to suppress micro-void generation in the ECR-SiO2?BOX layers, it is effective to introduce the outgas process before bonding wafers. On the other hand, it is a possible solution for suppressing micro-void generation in the ALD-Al2O3?BOX layers to increase the deposition temperature of the ALD-Al2O3?BOX layers. It is also another possible solution to deposit ALD-Al2O3?BOX layers on thermally oxidized SiO2?layers, which can absorb the desorption gas from ALD-Al2O3?BOX layers.

Opportunities and pitfalls in patterned self-catalyzed GaAs nanowire growth on silicon

Abstract: Periodic arrays of self-catalyzed GaAs nanowires (NWs) were grown on Si substrates by gas source molecular beam epitaxy (GS-MBE) using patterned oxide templates. The various challenges of the patterning process that result in undesired outcomes are described, such as pattern transfer by wet/dry etching, oxide thickness variations, and native oxide re-growth. Transmission electron microscopy (TEM) results are used to illustrate each case. In particular, we show that a linearly increasing length–radius distribution, analogous to that observed for unpatterned self-catalyzed growth on substrates with thin oxides, may be obtained even when using patterned oxide masks due to an unintended residual layer of oxide, as confirmed by TEM analysis. We explain how a linear length–radius dependence can result from the individual NWs beginning their growth at different times, accompanied by significant radial growth. The spread in obtained NW dimensions was decreased by improving the patterning method.

ZnO nanorod ultraviolet photodetector on porous silicon substrate

Abstract: Vertically high-density ZnO nanorods were successfully synthesized on a porous silicon (PS) substrate by chemical bath deposition method. The structural and optical investigations revealed that the ZnO nanorods grown on the PS substrate had high structural and optical quality. The photoelectric properties of the fabricated photodetector were investigated with 325?nm UV light illumination under 1?V bias voltage. Based on the current?voltage curve, the responsivity of the ZnO nanorod photodetector was 1.738 A W?1?at 1?V bias voltage. Under a bias voltage of 1?V, the sensitivity of the ZnO nanorod device was 20. The response and recovery time of the ZnO nanorod photodetector under these conditions were 0.032 and 0.041?s, respectively.

Electrical properties and microstructure of vanadium-based contacts on ICP plasma etched n-type AlGaN:Si and GaN:Si surfaces

Abstract: Light-emitting diodes emitting in the UV-B spectral range usually contain an n-type AlxGa1?xN contact layer with x of about 0.4 and require a low specific contact resistivity in the range of 10?6?? cm2?to operate at their optimal capacities. We compare the Ti/Al/Mo/Au metal system commonly used on GaN with the V/Al//V/Au metal system on n-Al0.4Ga0.6N:Si and GaN:Si surfaces. On Al0.4Ga0.6N, the lowest specific contact resistivity, (2.3 ? 0.2)??10?6?? cm2, was reached with V/Al/V/Au whereas for Ti/Al/Mo/Au the lowest resistivity was (1.7 ? 0.7)??10?3?? cm2. Independently of the metal system, lower contact resistivities are obtained on GaN where Ti/Al/Mo/Au is still performing at least one order of magnitude better than V/Al/V/Au. The contact resistivity of V/Al/V/Au on Al0.4Ga0.6N was found to be sensitive to surface treatments (wet chemical etching or sputtering) applied prior to metal deposition, to the thickness of the first vanadium layer and to that of the aluminum layer above. Structural and compositional investigations showed that a low contact resistivity is accompanied by the formation of an interfacial layer between AlGaN and V/Al/V/Au on the nm-scale, which contains aluminum, in the form of aluminum nitride, and separately vanadium as metal or metal nitride.

Quantum-cascade lasers as local oscillators for heterodyne spectrometers in the spectral range around 4.745 THz

Abstract: Local oscillators in terahertz heterodyne spectrometers have to be operated in continuous-wave mode at precisely defined target frequencies. In particular, for advanced airborne instruments, several specifications such as operating temperature and cooling requirements are necessary to be considered. We have developed a quantum-cascade laser (QCL) applicable as a local oscillator for heterodyne spectroscopy of the OI line at 4.745 THz, which is of particular interest for astronomy. We demonstrate a distributed-feedback QCL operating in continuous-wave mode up to about 60 K, which can be tuned precisely to the target frequency when operated in a mechanical cooler.

Dual-dielectric-constant spacer hetero-gate-dielectric tunneling field-effect transistors

Abstract: Hetero-gate-dielectric tunneling field-effect transistors (HG TFETs) with dual-dielectric-constant (k) spacers have been fabricated and characterized. They have a heterogeneous gate dielectric layer which consists of high-k material (HfO2) and SiO2 at the source and drain side, respectively. The dual-k spacer has an inner high-k and outer SiO2 spacer. The fabricated dual-k-spacer HG TFETs show higher on/off current ratio, on-current and better subthreshold slope than control TFETs which use only SiO2 for a gate dielectric layer and spacer. Moreover, the impact of the length of high-k material inserted under the gate (Lhigh-k) has been investigated. It turns out that the optimization of Lhigh-k can significantly improve performance and power efficiency.

Effects of vacuum annealing on the optical and electrical properties of p-type copper-oxide thin-film transistors

Abstract: We have investigated the effects of vacuum annealing on the optical and electrical properties of the p-type copper-oxide thin-film transistors (TFTs). The vacuum annealing of the copper-oxide thin-film was performed using the RF magnetron sputter at various temperatures. From the x-ray diffraction and UV-vis spectroscopy, it is demonstrated that the high-temperature vacuum annealing reduces the copper-oxide phase from CuO to Cu2O, and increases the optical transmittance in the visible part of the spectrum. The fabricated copper-oxide TFT does not exhibit the switching behavior under low-temperature vacuum annealing conditions. However, as the annealing temperature increases, the drain current begins to be modulated by a gate voltage, and the TFT exhibits a high current on?off ratio over 104?as the vacuum annealing temperature increases over 450??C. These results show that the vacuum annealing process can be an effective method of simultaneously improving the optical and electrical performances in p-type copper-oxide TFTs.

Effect of oxygen partial pressure on the behavior of dual ion beam sputtered ZnO thin films

Abstract: Undoped ZnO thin films were grown on p-type Si (1 0 0) substrates at different oxygen partial pressure by dual ion beam sputtering deposition system at a constant growth temperature of 400 °C. The crystallinity, surface morphology, optical, elemental and electrical properties of these ZnO thin films was studied. The minimum value of full-width at half-maximum of the θ-rocking curve obtained from x-ray diffraction of the ZnO (0 0 2) plane, was reported to be 0.1865° from ZnO film grown at 50% of (O2/(O2 + Ar))%. Crystalline property of ZnO films was observed to degrade with the increase in oxygen partial pressure. Photoluminescence measurements demonstrated sharp near-band-edge emission at ∼381 nm at room temperature. X-ray photoelectron spectroscopy study revealed presence of oxygen interstitials and vacancies as point defects in ZnO films. Electrical resistivity of ZnO was found to increase with the increase in oxygen partial pressure.

Simultaneous two-color lasing in a single CdSSe heterostructure nanosheet

Abstract: The ability of a single monolithic semiconductor structure to emit or lase in a broad spectrum range is of great importance for many applications such as solid-state lighting and multi-spectrum detection. But spectral range of a laser or light-emitting diode made of a given semiconductor is typically limited by its emission or gain bandwidth. Due to lattice mismatch, it is typically difficult to grow thin film or bulk materials with very different bandgaps in a monolithic fashion. But nanomaterials such as nanowires, nanobelts, nanosheets provide a unique opportunity. Here we report our experimental results demonstrating simultaneous lasing in two visible colors at 526 and 623 nm from a single CdSSe heterostructure nanosheet at room temperature. The 97 nm wavelength separation of the two colors is significantly larger than the gain bandwidth of a typical single II‐VI semiconductor material. Such lasing and light emission in a wide spectrum range from a single monolithic structure will have important applications mentioned above. (Some figures may appear in colour only in the online journal)