Showing papers in "Semiconductor Science and Technology in 2007"

Band structure calculations of Si Ge Sn alloys: achieving direct band gap materials

Abstract: Alloys of silicon (Si), germanium (Ge) and tin (Sn) are continuously attracting research attention as possible direct band gap semiconductors with prospective applications in optoelectronics. The direct gap property may be brought about by the alloy composition alone or combined with the influence of strain, when an alloy layer is grown on a virtual substrate of different compositions. In search for direct gap materials, the electronic structure of relaxed or strained Ge1−xSnx and Si1−xSnx alloys, and of strained Ge grown on relaxed Ge1−x−ySixSny, was calculated by the self-consistent pseudo-potential plane wave method, within the mixed-atom supercell model of alloys, which was found to offer a much better accuracy than the virtual crystal approximation. Expressions are given for the direct and indirect band gaps in relaxed Ge1−xSnx, strained Ge grown on relaxed SixGe1−x−ySny and strained Ge1−xSnx grown on a relaxed Ge1−ySny substrate, and these constitute the criteria for achieving a (finite) direct band gap semiconductor. Roughly speaking, good-size (up to ~0.5 eV) direct gap materials are achievable by subjecting Ge or Ge1−xSnx alloy layers to an intermediately large tensile strain, but not excessive because this would result in a small or zero direct gap (detailed criteria are given in the text). Unstrained Ge1−xSnx bulk becomes a direct gap material for Sn content of >17%, but offers only smaller values of the direct gap, typically ≤0.2 eV. On the other hand, relaxed SnxSi1−x alloys do not show a finite direct band gap.

Enhanced device performance of AlGaN/GaN HEMTs using HfO2 high-k dielectric for surface passivation and gate oxide

Abstract: AlGaN/GaN high electron mobility transistors (HEMTs) using HfO2 as a surface passivation layer and metal–oxide–semiconductor HEMTs (MOS-HEMTs) using HfO2 as gate oxide have been investigated and compared with the regular HEMTs. In MOS-HEMTs, the HfO2 gate dielectric is also used for passivation simultaneously. Our measurements have shown that both passivated HEMTs and MOS-HEMTs outperformed the regular HEMTs in dc, high-frequency and pulsed-mode operations, with MOS-HEMTs exhibiting the best characteristics, including the highest drain current, the lowest gate leakage current, the largest gate voltage swing, the highest cut-off frequencies and the best immunity to current collapse. In addition, the decrease in transconductance of MOS-HEMTs relative to HEMTs is as low as 8.7%, most probably a consequence of the high-k value of HfO2. Our results thus indicate the great potential of HfO2/AlGaN/GaN MOS-HEMTs for high-frequency and high-power applications.

Collective coherence in planar semiconductor microcavities

Abstract: Semiconductor microcavities, in which strong coupling of excitons to confined photon modes leads to the formation of exciton?polariton modes, have increasingly become a focus for the study of spontaneous coherence, lasing and condensation in solid state systems. This review discusses the significant experimental progress to date, the phenomena associated with coherence which have been observed and also discusses in some detail the different theoretical models that have been used to study such systems. We consider both the case of non-resonant pumping, in which coherence may spontaneously arise, and the related topics of resonant pumping, and the optical parametric oscillator.

Charge trapping characteristics of atomic-layer-deposited HfO2 films with Al2O3 as a blocking oxide for high-density non-volatile memory device applications

Abstract: Charge trapping characteristics of high-relative permittivity (high-?) HfO2 films with Al2O3 as a blocking oxide in p-Si/SiO2/HfO2/Al2O3/metal memory structures have been investigated. All high-? films have been grown by atomic layer deposition. A transmission electron microscope image shows that the HfO2 film is polycrystalline, while the Al2O3 film is partially crystalline after a high temperature annealing treatment at 1000 ?C for 10 s in N2 ambient. A well-behaved counter-clockwise capacitance?voltage hysteresis has been observed for all memory capacitors. A large memory window of ~7.4 V and a high charge trapping density of ~1.1 ? 1013 cm?2 have been observed for high-? HfO2 charge trapping memory capacitors. The memory window and charge trapping density can be increased with increasing thickness of the HfO2 film. The charge loss can be decreased using a thick trapping layer or thick tunnelling oxide. A high work function metal gate electrode shows low charge loss and large memory window after 10 years of retention. High-? HfO2 memory devices with high-? Al2O3 as a blocking oxide and a high work function metal gate can be used in future high-density non-volatile memory device applications.

p-type oxide semiconductors as hole collectors in dye-sensitized solid-state solar cells

Abstract: Oxide semiconductors with p-type conductivity were found to be good candidates to be used as hole collectors in dye sensitized solid-state solar cells (DSSC). In this paper, a delofossite p-type CuAlO2 semiconductor was synthesized by a hydrothermal method and its properties were investigated as a hole collector in DSSC. The valence band (VB) and conduction band (CB) positions of CuAlO2 were appropriate to be used as the hole conductor in DSSC. Solar cells constructed with TiO2/Ru dye/CuAlO2 deliver Isc = 0.08 mA cm−2 and Voc = 525 mV. The particle size of CuAlO2 prepared by the hydrothermal method was ~300–500 nm and the large CuAlO2 particle penetrates weakly into the pores of a nanocrystalline matrix resulting in the weak interaction of the dye and hole collector, which was found to be one of the reasons for the poor solar cell performance of the solar cell constructed with CuAlO2 as the hole collector. Bigger particle size and poor p-type conductivity are found to be the major limiting factors of CuAlO2 as a hole collector. The solar cell performances of electrodes constructed with TiO2/Ru dye/NiO are also discussed.

Ferromagnetic nitride-based semiconductors doped with transition metals and rare earths

Abstract: This review summarizes the state-of-the-art in the search for room temperature ferromagnetic semiconductors based on transition-metal- and rare-earth-doped nitrides. The major methods of synthesis are reported, together with an overview of the magnetic, structural, electrical and optical characterization of the materials systems, where available. The controversial experimental results concerning the actual value of the apparent Curie temperature in magnetically doped nitrides are highlighted, the inadequacy of standard characterization methods alone and the necessity of a possibly exhaustive structural investigation of the systems are proven and underlined. Furthermore, the dependence on the fabrication parameters of the magnetic ions incorporation into the semiconductor matrix is discussed, with special attention to the fundamental concepts of solubility limit and spinodal decomposition. It is argued that high-temperature ferromagnetic features in magnetically doped nitrides result from the presence of nanoscale regions containing a high concentration of the magnetic constituents. Various functionalities of these multicomponent systems are listed. Moreover, we give an extensive overview on the properties of single magnetic-impurity states in the nitride host. The understanding of this limit is crucial when considering the most recent suggestions for the control of the magnetic ion distribution—and consequently of the magnetic response—through the Fermi level engineering as well as to indicate roads for achieving high-temperature ferromagnetism in the systems containing a uniform distribution of magnetic ions.

GaN IMPATT diode: a photo-sensitive high power terahertz source

Abstract: The prospects of wurtzite phase single-drift-region (SDR), flat and single-low-high-low (SLHL) type GaN IMPATT devices as terahertz sources are studied through a simulation experiment. The study indicates that GaN IMPATT diodes are capable of generating high RF power (at least 2.5 W) at around 1.45 THz with high efficiency (17–20%). The superior electronic properties of GaN make this a promising candidate for IMPATT operation in the THz regime, unapproachable by conventional Si, GaAs and InP based IMPATT diodes. The effect of parasitic series resistance on the THz performance of the device is further simulated. It is interesting to note that the presence of a charge bump in a flatly doped SDR structure reduces the value of parasitic series resistance by 22%. The effects of photo- illumination on the devices are also investigated using a modified double iterative simulation technique. Under photo-illumination (i) the negative conductance and (ii) the negative resistance of the devices (both flat and SLHL) decrease, while the frequency of operation and the device quality factor shift upwards. However, the upward shift in operating frequency is found to be more (~16 GHz) in the case of the SLHL SDR IMPATT device. The study indicates that GaN IMPATT is a promising opto-sensitive high power THz source.

Effects of the channel thickness on the structural and electrical characteristics of room-temperature fabricated ZnO thin-film transistors

Abstract: We report the fabrication and characteristics of ZnO thin-film transistors (TFTs) having different channel thicknesses. The ZnO films were deposited as active channel layers on SiO2/p-Si substrates by rf magnetron sputtering at room temperature. Effects of the channel thickness on the structural and electrical properties of ZnO TFTs using a bottom-gate configuration were investigated. The crystalline quality and channel conductance of the ZnO films were enhanced as the channel thickness increased. The ZnO TFT with the optimized channel thickness exhibited enhancement mode characteristics with the threshold voltage of 9.9 V, the on-to-off current ratio of ~105 and the field-effect mobility of 0.1 cm2 V−1 s−1. This research implies that ZnO TFTs produced by a simple and low-cost technique could be applicable to electronic devices.

Monolithic CMOS-compatible AlGaInP visible LED arrays on silicon on lattice-engineered substrates (SOLES)

Abstract: Monolithic CMOS compatible AlGaInP visible LED arrays have been demonstrated on a novel platform called silicon on lattice-engineered substrate (SOLES). SOLES wafers are based on Si1−xGex virtual substrate technology and are suitable for the practical fabrication of SOI CMOS circuits and III–V-based optoelectronic devices on a common silicon substrate. A combination of oxide–oxide wafer bonding and hydrogen-induced exfoliation was used to transfer a thin layer of device-quality silicon on insulator on the top of the Si1−xGex buffers graded to 100% Ge to realize SOLES. Epitaxial layers of a double heterojunction AlGaInP LED emitting near the red region of the visible spectrum (λ = 671 nm) were grown by MOCVD on SOLES wafers using a patterned oxide hard mask. CMOS compatibility was achieved by accessing the n-GaAs cathode of the LED through the underlying n-Ge layer of the Si1−xGex graded buffer rather than etching through the LED stack. The LED was capped with Si to avoid exposing CMOS tools to III–V materials during processing. The Si anode and Ge cathode of the resulting LED structure were contacted using Ti/Al CMOS compatible metallurgy. The prototype array is an important step towards the realization of monolithically integrated optical interconnects in high speed digital systems.

AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors with 4 nm thick Al2O3 gate oxide

Abstract: AlGaN/GaN metal?oxide?semiconductor heterostructure field-effect transistors (MOSHFETs) with 4 nm thick Al2O3 gate oxide were prepared and their performance was compared with that of AlGaN/GaN HFETs. The MOSHFETs yielded ~40% increase of the saturation drain current compared with the HFETs, which is larger than expected due to the gate oxide passivation. Despite a larger gate-channel separation in the MOSHFETs, a higher extrinsic transconductance than that of the HFETs was measured. The drift mobility of the MOSHFETs, evaluated on large-gate FET structures, was significantly higher than that of the HFETs. The zero-bias mobility for MOSHFETs and HFETs was 1950 cm2 V?1 s?1 and 1630 cm2 V?1 s?1, respectively. These features indicate an increase of the drift velocity and/or a decrease of the parasitic series resistance in the MOSHFETs. The current collapse, evaluated from pulsed I?V measurements, was highly suppressed in the MOSHFETs with 4 nm thick Al2O3 gate oxide. This result, together with the suppressed frequency dispersion of the capacitance, indicates that the density of traps in the Al2O3/AlGaN/GaN MOSHFETs was significantly reduced.

Ring-structured thermoelectric module

Abstract: A novel tube-shape thermoelectric module is fabricated from four ring-shaped thermoelements and its performance in electrical power generation is evaluated by measuring the power output as a function of temperature differences across the device. A theoretical model of the proposed configuration is established and employed for data analysis. The result indicates that a tube-shape thermoelectric module could achieve similar performance to that of a conventional plate-like module, and has an advantage in applications when heat flows in a radial direction.

MOCVD of HfO2 and ZrO2 high-k gate dielectrics for InAlN/AlN/GaN MOS-HEMTs

Abstract: We apply metal organic chemical vapour deposition (MOCVD) of HfO2 and of ZrO2 from β-diketonate precursors to grow high-k gate dielectrics for InAlN/AlN/GaN metal oxide semiconductor (MOS)-high electron mobility transistors (HEMTs). High-k oxides of about 12 nm–14 nm are deposited for the MOS-HEMTs incorporating Ni/Au gates, whereas as a reference, Ni-contact-based 'conventional' Schottky-barrier (SB)-HEMTs are processed. The processed dielectrics decrease the gate current leakage of the HEMTs by about four orders of magnitude if compared with the SB-gated HEMTs and show superior device characteristics in terms of IDS and breakdown.

Scattering analysis of 2DEG carrier extracted by QMSA in undoped Al0.25Ga0.75N/GaN heterostructures

Abstract: Hall effect measurements on undoped Al0.25Ga0.75N/GaN heterostructures grown by a metalorganic chemical vapour deposition (MOCVD) technique have been carried out as a function of temperature (20–350 K) and magnetic field (0–1.5 T). Magnetic field dependent Hall data were analysed using the quantitative mobility spectrum analysis (QMSA) technique. The mobility and density within the two-dimensional electron gas (2DEG) at the Al0.25Ga0.75N/GaN interface and within the underlying GaN layer were successfully separated by QMSA. Mobility analysis has been carried out using both the measured Hall data at a single field and the extracted data from QMSA. Analysis of the temperature-dependent mobility of 2DEG extracted from QMSA indicates that the interface roughness and alloy disorder scattering mechanisms are the dominant scattering mechanisms at low temperatures while at high temperatures only polar optical phonon scattering is the dominant mechanism. Al0.25Ga0.75N/GaN interface related parameters such as well width, deformation potential constant and correlation length were also accurately obtained from the fits of the simple analytical expressions of scattering mechanisms to the 2DEG mobility.

Structural and optoelectronic characterization of TiO2 films prepared using the sol–gel technique

Abstract: TiO2 is a versatile material that makes for fascinating study in any of its several physical forms: monocrystal, polycrystal, powder or thin film. Its enhanced photosensitivity to UV radiation and excellent chemical stability in acidic and aqueous media point to its excellent potential for use in a variety of applications, such as solar cells, electronic devices, chemical sensors and photocatalysts. Of late, thin films of TiO2 have permitted the study of physical and chemical properties that are almost impossible to examine in powders. Using the sol–gel technique, it was possible to prepare TiO2 films, and to specifically modify their characteristic properties by means of annealing treatments. Optical measurements carried out on sol–gel derived films produced results similar to those found in films prepared using the sputtering technique. The use of TiO2 films facilitates the study of the behaviour of crystalline structure, grain size, photoresponse, electrical conductivity in both darkness and light and energy band gap (Eg) as a function of treatment temperature. For the first time, it has been demonstrated that the photoconductivity of TiO2 becomes apparent at a treatment temperature of 350 °C, which means that below this temperature the material is not photosensitive. The photosensitivity (S) of TiO2 films prepared by the sol–gel technique reaches values between 100 and 104, surpassing by more than two orders of magnitude the photosensitivity of TiO2 in powder form. In addition, it was possible to study the surface crystalline structure, where TEM studies clearly revealed both the polycrystalline order and the atomic arrangements of the TiO2 films. Our findings will afford us an opportunity to better study the nature of TiO2 and to enhance its performance with respect to the above-mentioned applications.

The influence of the InGaP window layer on the optical and electrical performance of GaAs solar cells

Abstract: A wide band gap heterolayer is usually added to the front face of GaAs and other III–V solar cells to favour transparency and to passivate the emitter. This extra front layer influences the optical behaviour of the device and therefore must be taken into account in the optimization of the antireflection (AR) coating. A set of AR layers was optimized with respect to their thicknesses for an InGaP front layer in GaAs solar cells. Complementary, numerical simulation of the whole device was performed using the D-AMPS code. Our results confirm the importance of surface passivation and demonstrate that the thickness of 30 nm usually proposed for this window layer is not the optimal. (Some figures in this article are in colour only in the electronic version)

Influence of the surface charge on the operation of ballistic T-branch junctions: a self-consistent model for Monte Carlo simulations

Abstract: We analyse the influence of the surface charge on the operation of ballistic T-branch junctions by means of a semi-classical 2D Monte Carlo simulator. We propose a new self-consistent model in which the local value of the surface charge is dynamically adjusted depending on the surrounding carrier density. The well-known parabolic behaviour of the central branch potential VC when biasing right and left branches in a push–pull fashion is found to be much influenced by the value of the surface charge in both the horizontal and vertical branches. With the help of experimental measurements performed in real devices, the influence of the width of the central branch on the values of VC and its relation to surface charge effects are also studied.

Efficient top-emitting organic light-emitting diodes with a V2O5 modified silver anode

Abstract: We fabricated efficient top-emitting organic light-emitting diodes (OLEDs) with silver (Ag) as an anode and samarium (Sm) as a semi-transparent cathode. The hole-injection barrier at the Ag anode/hole transporter interface is reduced by inserting a buffer layer of vanadium oxide (V2O5) between them. The ultraviolet photoelectron spectroscopy analysis shows that the hole-injection barrier is reduced by 0.5 eV. Both the V2O5 thickness and the organic layer thickness are optimized. The optimized device achieves a maximum current efficiency of 5.46 cd A−1 and a power efficiency of 3.90 lm W−1, respectively.

Photoluminescence study and structural characterization of p-type ZnO doped by N and/or As acceptors

Abstract: ZnO doped with N and/or As layers was fabricated by thermal oxidation of ZnTe films grown by MBE on different substrates. Hall effect measurements demonstrated p-type conductivity with a hole concentration of ~5 × 1019 cm−3 for ZnO:As and ZnO:As:N on GaAs substrates and ~6 × 1017 cm−3 for ZnO:N on ZnTe substrates. The concentration of N and As atoms in ZnO is estimated to be ~1020 cm−3. This suggested that simple substitutional N atoms form acceptor impurities with a smaller efficiency than an As-related complex, probably AsZn–2VZn. In particular, we were able to distinguish between nitrogen and arsenic acceptor-related luminescence. Optical studies showed meaningful differences of the PL features in samples with different acceptors, grown on different substrates.

Residual thermal strain in thick GaN epifilms revealed by cross-sectional Raman scattering and cathodoluminescence spectra

Abstract: Strain can significantly alter the physical properties of a solid. We demonstrate that in a thick GaN epilayer there exists a residual thermal strain along the growth direction. This result is clearly revealed by cathodoluminescence spectra, in which the band gap of the GaN film decreases with distance away from the epifilm?substrate interface. This result is further confirmed by Raman scattering spectra in which the phonon modes show a red shift along the growth direction. Our finding is important for the understanding and application of nitride semiconductors.

Characterization of low-temperature molecular-beam-epitaxy grown GaBiAs layers

Abstract: GaBiAs layers with Bi content reaching 8.4% are grown by MBE technique at low temperatures. All layers were of p-type with carrier densities ranging from 3 × 1014 to 2 × 1015 cm−3 and resistivities exceeding 60 Ω cm. Energy bandgap of the gallium bismide alloys as determined from spectral measurements of the optical absorption, photoconductivity and photoluminescence decreases linearly with increasing Bi content. Optical pump–terahertz probe measurements made on these layers show that the carrier density dynamics is best described by a double-exponential decay. The shorter of the time constants corresponds to the electron trapping and the longer time constant corresponds to the trap emptying times. It has been found that the electron trapping cross-section is of the same order of magnitude as the corresponding parameter for As-antisite traps in LTG GaAs; therefore, it is reasonable to assume that As antisites play a significant role in carrier recombination processes in GaBiAs, too.

Fabrication of SiGe-on-insulator substrates by a condensation technique: an experimental and modelling study

Abstract: Today, to our knowledge, only two techniques are used to perform GeOI substrates: the Smart-Cut™ technique and the Ge condensation technique. The latter is very sensitive to the initial parameters but is the only one which allows Si and Ge-on-insulator co-integration. Predictions of experimental results are then necessary to associate the best processes with the defined starting structures. This paper presents for the first time studies on Ge condensation technique simulations. Enrichment kinetics occurring in the classical one-dimensional condensation process have been simulated by the TCAD Silvaco Athena tool and analytical calculations. The good correlation with experimental data permits us to confirm the process dependence to the initial parameters. The influence of the process-induced non-homogeneities for the prestructure fabrication is also highlighted by the simulation results. The analytical model is efficient to predict a lot of experimental data with short calculation times. For more complex processes involving a two- or three-dimensional oxidation, enrichment prediction accuracy obtained by the analytic model is limited. Mechanism predictions under these experimental conditions are necessary to be studied by TCAD simulations.

Deep level defects in CdTe materials studied by thermoelectric effect spectroscopy and photo-induced current transient spectroscopy

Abstract: The deep levels in semi-insulating CdTe crystals grown by vertical gradient freezing technique were studied using thermoelectric effect spectroscopy (TEES) and photo-induced current transient spectroscopy (PICTS) The measurement of TEES spectra in the temperature range 90–400 K was performed at different heating rates The positions of the levels and their capture cross-sections were obtained by using the heating rate method It was found that near midgap levels in samples doped with shallow donors (Cl, In) have a low value of capture cross section (≈10−15 − 10−17 cm2) and are hole traps Samples doped with a deep donor (Ge, Sn) have a much higher capture cross section of the midgap level (≈10−13 cm2), which acts as an electron trap Comparison of the TEES results with the PICTS method has shown that while the evaluated values of ionization energies are comparable using both methods, the PICTS technique gives systematically approximately two orders of magnitude higher capture cross sections than TEES

Twinning modulation in ZnSe nanowires

Abstract: ZnSe nanowires were grown on Si substrates by Au-catalyzed vapour phase growth at 725 °C. A detailed structural and microstructural investigation has been carried out using electron diffraction and high-resolution transmission electron microscopy (HRTEM). Modulated twins have been observed along the nanowire axial direction along the entire length of the nanowires. Faceting has been observed on the side surfaces of the wires with a larger twinning periodicity. The formation mechanism of these twinning-modulated nanowires is discussed. The optical properties are correlated with the microstructure of the nanowires. These twinning-modulated ZnSe nanowires might have great potential as building blocks for optoelectronic nanodevices.

Response of machining-damaged single-crystalline silicon wafers to nanosecond pulsed laser irradiation

Abstract: Ultraprecision diamond-machined silicon wafers have been irradiated by a nanosecond pulsed Nd:YAG laser. The results indicate that at specific laser energy intensity levels, the machining-induced subsurface damage layer of silicon has been reconstructed to a perfect single-crystalline structure identical to the bulk. Laser irradiation causes two effects on silicon: one is the epitaxial regrowth of the near-surface amorphous layer, and the other is the complete removal of the dislocations from the crystalline layer. It is the dislocation-free crystalline region that serves as the seed layer to recrystallize the amorphous layer, enabling excellent crystalline perfection. These findings may offer practical alternatives to current chemo-mechanical processing methods for silicon wafers.

High breakdown voltage C-doped GaN-on-sapphire HFETs with a low specific on-resistance

Abstract: High-quality C-doped GaN buffers grown on sapphire substrates were employed for the fabrication of high-power AlGaN/GaN heterojunction field effect transistors (HFETs). The fabricated device exhibited a very high breakdown voltage (BV) over 1350 V and low specific on-resistance (ARDS(ON)) of 3.4 mΩ cm2. This result is very close to the 4H-SiC limit and a record achievement for GaN-based HFETs realized on sapphire substrates, to the best of our knowledge.

Comparative analysis of nanoscale MOS device architectures for RF applications

Abstract: The suitability of nanoscale non-planar FinFETs and classical planar single and double gate SOI MOSFETs for rf applications is examined via extensive 3D device simulations and detailed interpretation. It is shown that although nanoscale FinFETs achieve higher values of intrinsic dc gain (nearly 20 dB higher than planar SG devices), they also present higher gate capacitance that severely undermines their rf performance. We also show that at large values of drain currents, well-designed conventional planar single and double gate SOI MOSFETs attain higher values of cut-off frequency compared to FinFETs, whereas at lower drain currents, a well-aligned planar double gate SOI MOSFET is the optimal structure. The reason for higher parasitic capacitance in FinFETs as compared to planar MOSFETs is examined in detail. An assessment of the impact of back gate misalignment on the rf performance of a 25 nm gate length planar double gate MOSFET indicates that a misalignment of 12 nm towards the source end is acceptable to give superior performance to a FinFET. The importance of source/drain extension region engineering in nanoscale FinFETs for ultra-low voltage analogue applications is also investigated. RF figures of merit for planar and vertical MOS devices are also compared based on layout-area calculations. The paper provides valuable design insights for optimizing device parameters for nanoscale planar and vertical MOSFETs.

Optimization of super-junction SOI-LDMOS with a step doping surface-implanted layer

Abstract: A super-junction (SJ) SOI-LDMOS with a step doping surface-implanted n-type layer is proposed and optimized which allows high breakdown voltage (BV) and low on-resistance (Ron). The proposed structure overcomes the field effect action in conventional SJ SOI-LDMOS devices, thus achieving the charge compensation between the n and p pillars as well as a near uniform electric field distribution in the drift region in the off-state. The surface-implanted layer also provides a low current path in the on-state. In addition, the analysis result of surface doping distribution is obtained which is a power tool for device designers to provide accurate first-order design schemes. The simulation results show that an increase in the off-state BV by 55% and a reduction of the specific on-resistance by 37.4% are obtained for the proposed device when compared with those of the conventional one.

TiO2/GeOxNy stacked gate dielectrics for Ge-MOSFETs

Abstract: In this work, we present the results of physical and electrical characterization of Ti-based high-k gate dielectrics on Ge substrates. Titanium tetrakis iso- propoxide (TTIP) was used as the organometallic source for the deposition of ultra-thin TiO2 films on p-Ge (1 0 0) at low temperature (<200 °C) by plasma enhanced chemical vapor deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of ~65 Pa. The presence of an ultra-thin lossy GeO2 interfacial layer between the deposited high-k film and the substrate, results in frequency-dependent capacitance–voltage (C–V) characteristics in strong accumulation and a high interface state density (~1013 cm−2 eV−1). To improve the electrical properties, nitrogen engineering has been employed to convert the lossy GeO2 interfacial layer to its oxynitride, thus forming TiO2/GeOxNy/Ge stacked-gate structure with improved interface/electrical properties. Different N sources, such as NO, NH3 and NO/NH3, have been used for nitrogen engineering. XPS and Raman spectroscopy analyses have been used for surface morphological study. Electrical properties, such as gate leakage current density, interface state density, charge trapping, flatband voltage shift, etc, have been studied in detail for TiO2/GeOxNy/Ge MIS capacitors using the current–voltage (I–V), capacitance–voltage (C–V), conductance–voltage (G–V) and stress (both constant voltage and current) measurements. Although a significant improvement in electrical characteristics has been observed after nitridation in general, the formation of the interfacial GeOxNy layer, obtained from NO-plasma nitridation, is found to provide the maximum improvement among all the nitridation techniques used in this study. It is shown that the insertion of an ultra-thin oxynitride (GeOxNy) interfacial layer is advantageous for producing gate-quality TiO2 high-k dielectric stacks on Ge substrates.

A room temperature, or moderately cooled, fast THz semiconductor hot electron bolometer

Abstract: A fast THz bolometer is proposed in which, unlike the conventional thermal one, electromagnetic radiation heats only electrons in a narrow gap semiconductor without its lattice inertial heating. Under determined conditions, this heating changes generation–recombination processes that cause the carrier number to decrease and the semiconductor resistance to rise. The Hg0.8Cd0.2Te detector noise equivalent power in the range of 77–300 K can reach ~10−11 W for frequencies of about 1 THz and signal gain frequency bandwidth of 1 Hz. Measurements with the device prototype confirmed the concept of the proposed bolometer creation.

The influence of post-deposition annealing on thermoelectric properties of Bi–Sb–Te films prepared by sputtering

Abstract: Bi–Sb–Te thin films were prepared for p-type thermoelectric material in micro Peltier cooling devices. Films with four different compositions were elaborated by co-sputtering of BiSb(1:3) alloy and Te targets. Rapid thermal annealing was carried out for the improvement of thermoelectric properties of the as-grown films under a reducing atmosphere. The effects of the thermal treatment on the characteristic properties of the films were investigated with measurements of the electrical transport properties and Seebeck coefficients. The variation of the charge carrier concentration of a film was found to be dependent on the film's composition and closely related to the thermoelectric properties. The decrease of carrier concentration and the increase of mobility due to the crystallization of the chalcogenide phase resulted in the enhancement of the thermoelectric properties. Maximum values of the Seebeck coefficient (~178 µV K−1) and power factor (~1.2 × 10−3 W K−2 m−1) were obtained for a film close to the stoichiometric composition (Bi0.5Sb1.5Te3).