A study of interface characteristics in HfAlO/p-Si by deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) and high-frequency capacitance-voltage (HF-CV) measurement are used for the investigation of HfAlO/p-Si interface. The so-called “slow” interface states detected by HF-CV are obtained to be 2.68 × 10¹¹ cm⁻². Combined conventional DLTS with insufficient-filling DLTS (IF-DLTS), the true energy level position of interfacial traps is found to be 0.33 eV above the valance band maximum of silicon, and the density of such “fast” interfacial traps is 1.91 × 10¹² cm⁻² eV⁻¹. The variation of energy level position of such traps with different annealing temperatures indicates the origin of these traps may be the oxide-related traps very close to the HfAlO/Si interface. The interfacial traps’ passivation and depassivation effect of postannealing in forming gas are shown by comparing samples annealed at different temperatures.     

A Physics-Based Compact Direct-Current and Alternating-Current Model for AlGaN/GaN High Electron Mobility Transistors

A set of analytical models for the dc and small signal characteristics of AIGaN/GaN high electron mobility transis- tors （HEMTs） are presented. A modified transferred-electron mobility model is adapted and a phenomenological low-field mobility model is developed. We calculate the channel charge considering the neutralization of donors and the contribution of free electrons in the AlGaN layer. The gate-to-source and gate-to-drain capacitances are obtained analytically, and the cut-off frequency is predicted. The models are implemented into the HSPICE simulator for the dc, ac and transient simulations and verified by experimental data for the first time. A high efficiency class-E GaN HEMT power amplifier is designed and simulated by the HSPICE to verify the applicability of our models.     

High-k Al2O3 MOS structures with Si interface control layer formed on air-exposed GaAs and InGaAs wafers

This paper attempts to realize unpinned high-k insulator-semiconductor interfaces on air-exposed GaAs and In_0.53Ga_0.47As by using the Si interface control layer (Si ICL). Al₂O₃ was deposited by ex situ atomic layer deposition (ALD) as the high-k insulator. By applying an optimal chemical treatment using HF acid combined with subsequent thermal cleaning below 500 °C in UHV, interface bonding configurations similar to those by in situ UHV process were achieved both for GaAs and InGaAs after MBE growth of the Si ICL with no trace of residual native oxide components. As compared with the MIS structures without Si ICL, insertion of Si ICL improved the electrical interface quality, a great deal both for GaAs and InGaAs, reducing frequency dispersion of capacitance, hysteresis effects and interface state density (D_it). A minimum value of D_it of 2 × 10¹¹ eV⁻¹ cm⁻² was achieved both for GaAs and InGaAs. However, the range of bias-induced surface potential excursion within the band gap was different, making formation of electron layer by surface inversion possible in InGaAs, but not possible in GaAs. The difference was explained by the disorder induced gap state (DIGS) model.     

Band bending of n‐InN epilayers and exact solution of the classical Thomas–Fermi equation

The exact solution of the Thomas–Fermi equation for a planar accumulation layer of a degenerate semiconductor is presented. The obtained results are compared with theoretical literature data. The applicability of the solution is demonstrated by using results of electrochemical capacitance–voltage measurements and photoluminescence data for n‐InN epilayers. It has been found that the difference between the electron concentrations estimated from the Hall and photoluminescence measurements is a measure of the electron content in the accumulation layer with acceptable accuracy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

XPS analysis of surface chemistry of near surface region of epiready GaAs(1 0 0) surface treated with (NH4)2Sx solution

In this work we analyze the effect of (NH)₂S_x wet treatment on the GaAs(1 0 0) covered with “epiready” oxide layer without any pretreatment in order to check the removal of oxides and carbon-related contamination, and the formation of sulfur species. The sulfidation procedure consisted of epiready sample dipping (at room and 40 °C temperatures) in an ammonium polysulfide solution combined with a UHV flash annealing up to 500 °C.The inspection of the XPS As 2p_3/2 and Ga 2p_3/2 spectra taken at surface sensitive mode revealed: (i) the temperature-dependent reduction of the amount of GaAs oxides and carbon contamination after sulfidation, and almost their complete removal after subsequent annealing, (ii) the creation of sulfur bonds with both Ga and As, with more thermally stable Ga-S bonds, and (iii) the slight reduction in elemental arsenic amount.     

Effective surface passivation of crystalline silicon using ultrathin Al2O3 films and Al2O3/SiNx stacks

We measure surface recombination velocities (SRVs) below 10 cm/s on p‐type crystalline silicon wafers passivated by atomic–layer–deposited (ALD) aluminium oxide (Al₂O₃) films of thickness ≥10 nm. For films thinner than 10 nm the SRV increases with decreasing Al₂O₃ thickness. For ultrathin Al₂O₃ layers of 3.6 nm we still attain a SRV < 22 cm/s on 1.5 Ω cm p‐Si and an exceptionally low SRV of 1.8 cm/s on high‐resistivity (200 Ω cm) p‐Si. Ultrathin Al₂O₃ films are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is extremely low compared to the frequently used plasma‐enhanced chemical vapour deposition of silicon nitride (SiN_x). Our experiments on silicon wafers passivated with stacks composed of ultrathin Al₂O₃ and SiN_x show that a substantially improved thermal stability during high‐temperature firing at 830 °C is obtained for the Al₂O₃/SiN_x stacks compared to the single‐layer Al₂O₃ passivation. Al₂O₃/SiN_x stacks are hence ideally suited for the implementation into industrial‐type silicon solar cells where the metal contacts are made by screen‐printing and high‐temperature firing of metal pastes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of Oxygen Vacancy on Transport Property in Perovskite Oxide Heterostructures

Effect of oxygen vacancy on transport property of perovskite microstructures is studied theoretically. Compared with calculated and measured I-V curves, it is revealed that electron conduction plays an important role in the oxygen nonstoichiometry perovskite heterostructures even with hole-doped or un-doped material due to the oxygen vacancies. In addition, a detailed understanding of the influence of oxygen vacancy concentration and temperature on the conduction characteristics of oxide heterojunction with both forward and reverse biases is obtained by calculation.     

Deep‐level characterization of emissive interface states in Alq3‐based OLEDs

We have succesfully investigated emissive interface states in fabricated indium‐tin‐oxide (ITO)/N,N′‐di‐1‐naphthyl‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′diamine (α‐NPD)/tris(8‐hydroxyquinoline) aluminum (Alq₃)/LiF/Al organic light‐emitting diodes (OLEDs) by a modified deep‐level optical spectroscopy (DLOS) technique. In the vicinity of the α‐NPD/Alq₃ emissive interface, a discrete trap level was found to be located at ～1.77 eV below the conduction band of Alq₃, in addition to band‐to‐band transitions of carriers from α‐NPD to Alq₃. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetoquantum transport in a modulated 2D electron gas with spin-orbit interaction

We investigate the effects of spin-orbit interaction (SOI) and plane-perpendicular magnetic field on the conductivity of a two-dimensional electron system in the presence of one-dimensional electrostatic modulation. The calculations are performed when a low-intensity, low-frequency external electric field is applied. The Kubo formula for the conductivity is employed in the calculation. The single-particle eigenstates which depend on the strengths of the magnetic field, the SOI and modulation potential, are calculated and then used to determine the conductivity. We present numerical results for the conductivity along the channels as well as the tunneling conductivity perpendicular to the constrictions as functions of the modulation potential, the SOI and the magnetic field. We demonstrate that the effect of finite frequency is to related to the reduction of both the longitudinal and transverse conductivities.     

Structure and composition of chemically prepared and vacuum annealed InSb(0 0 1) surfaces

The InSb(0 0 1) surfaces chemically treated in HCl-isopropanol solution and annealed in vacuum were studied by means of X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and electron energy-loss spectroscopy (EELS). The HCl-isopropanol treatment removes indium and antimony oxides and leaves on the surface about 3 ML of physisorbed overlayer, containing indium chlorides and small amounts of antimony, which can be thermally desorbed at 230 °C. The residual carbon contaminations were around 0.2-0.4 ML and consisted of the hydrocarbon molecules. These hydrocarbon contaminations were removed from the surface together with the indium chlorides and antimony overlayer. With increased annealing temperature, a sequence of reconstructions were identified by LEED: (1 × 1), (1 × 3), (4 × 3), and (4 × 1)/c(8 × 2), in the order of decreasing Sb/In ratio. The structural properties of chemically prepared InSb(0 0 1) surface were found to be similar to those obtained by decapping of Sb-capped epitaxial layers.     

Surface and interface issues in wide band gap semiconductor electronics

Wide band gap (WGB) materials are the most promising semiconductors for future electronic devices, and are candidates to replace the conventional materials (Si, GaAs, …) that are approaching their physical limits. Among WBG materials, silicon carbide (SiC) and gallium nitride (GaN) have achieved the largest advancements with respect to their material quality and device processing. Clearly, the devices performances depend on several surface and interface properties, which in turn are often crucially determined by the quality of the available material, as well as by the device processing maturity. In this paper, some surface and interface issues related to SiC and GaN devices processing are reviewed. First, the control of metal/SiC barrier uniformity and surface preparation will be discussed with respect to the performance of Schottky-based devices. Moreover, the impact of high-temperature annealing required for high-voltage Schottky diodes and MOSFETs fabrication, on the surface morphology and device performances will also be briefly presented. In the second part, it will be shown that for GaN the material quality is still the main concern, since dislocations have a severe influence on the current transport and barrier homogeneity of metal/GaN interfaces. Other practical implications of thermal annealing and surface passivation during GaN-based devices fabrication will also be addressed.     

Oxygen plasma and high pressure H<Subscript>2</Subscript>O vapor heat treatments used to fabricate polycrystalline silicon thin film transistors

Oxygen plasma and high pressure H₂O vapor heat treatment were applied to fabrication of n-channel polycrystalline silicon thin film transistors (poly-Si TFTs). 13.56 MHz-oxygen-plasma treatment at 250 °C, 100 W for 5 min effectively reduced defect states of 25-nm-thick silicon films crystallized by 30 ns-pulsed XeCl excimer laser irradiation. 1.3×10⁶-Pa-H₂O vapor heat treatment at 260 °C for 3 h was carried out in order to improve electrical properties of SiO_x gate insulators and SiO_x/Si interfaces. A carrier mobility of 470 cm²/V s and a low threshold voltage of 1.8 V were achieved for TFTs fabricated with crystallization at 285 mJ/cm². Received: 18 November 2002 / Accepted: 25 November 2002 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +81-42/388-7109, E-mail: tsamesim@cc.tuat.ac.jp     

Escape time model for the current self-oscillation frequencies. in weakly coupled semiconductor superlattices

A semiclassical model was developed to predict the frequencies of current self-oscillations in weakly coupled semiconductor superlattices (SLs). The calculated frequency is derived from the classical round trip time in one well and the tunneling probability through the barrier, using the well and barrier width, effective masses and band offsets as well as the resulting energies of the sub- and minibands as input parameters. For all SLs, the measured frequency dependence on the sample parameters can be well described by our model. For weakly (strongly) coupled SLs, the calculated frequencies are somewhat above (below) the observed ones. The changeover from one behavior to the other occurs for SLs with miniband widths of a few meV. Received: 2 August 2000 / Accepted: 27 October 2000 / Published online: 28 February 2001     

Hysteresis in current transport in a GaAs diode containing self-assembled InAs quantum dots

We have observed hysteresis loops in current transport in a GaAs metal–semiconductor–metal diode containing InAs quantum dots. The dots in our structure are directly embedded under the GaAs–metal interface. The charging and discharging of electrons in the dots modulate the current and produce hysteresis. These processes are controlled by the applied voltages. The dots are charged by forward current flowing through the structure. The discharging of the electrons is dominated by the tunneling process under high reverse bias. The modulated currents are well fitted with an electron-trapping model considering both the ground states and the excited states of the quantum dots. Received: 5 October 2000 / Accepted: 12 December 2000 / Published online: 23 May 2001     

Defect reduction and surface passivation of SiO2/Si by heat treatment with high-pressure H2O vapor

Heat treatment with high-pressure H₂O vapor was applied to improve interface properties of SiO₂/Si and passivate the silicon surface. Heat treatment at 180–420 °C with high-pressure H₂O vapor changed SiO_x films, 150 nm thick formed at room temperature by thermal evaporation in vacuum, into SiO₂ films with a Si-O-Si bonding network similar to that of thermally grown SiO₂ films. Heat treatment at 130 °C with 2.8×10⁵ Pa H₂O for 3 h reduced the recombination velocity for the electron minority carriers from 405 cm/s (as-fabricated 150-nm-thick SiO_x/Si) to 5 cm/s. Field-effect passivation was demonstrated by an additional deposition of defective SiO_x films on the SiO₂ films formed by heat treatment at 340 °C with high-pressure H₂O vapor. The SiO_x deposition reduced the recombination velocity from 100 cm/s to 48 cm/s. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Improvement of interface properties of AlGaN/GaN heterostructures under gamma-radiation

The cathodoluminescence (CL) spectra of AlGaN/GaN heterostructures grown on sapphire substrate were studied before and after gamma irradiation treatment. The CL spectroscopy results reveal strong yellow and blue luminescence transformation under gamma radiation treatment. The changes in CL spectra are compared with changes in the electrical characteristics of two-dimensional gas in AlGaN/GaN heterostructures. The origins of the observed improvement in properties of AlGaN/GaN heterostructures after gamma radiation treatment with 1 × 10⁶ rad are discussed on the basis of compensation and structural ordering of native defects.     

Gas Source MBE-Grown Metamorphic InGaAs Photodetectors using InAlAsBuffer and Cap Layers with Cut-off Wavelength up to 2.7

We report on InP-based metamorphic InGaAs photodiodes grown by gas source molecular beam epitaxy （MBE）, in which a relatively thin compositional graded wide band-gap InxAl1-xAs buffer layer is adopted. In the photodiodes, InAiAs is also taken as cap layers, so this structure is suitable for both front and back illuminations. At room temperature the photodiodes show 50% cut-off wavelength of 2.66μm, with measured peak detectivity of 4.91×10^9 cmHz^1/2/W at 2.57μm, and the typical dark current and RoA are 7.68μA/0.94Ωcm^2 and 291 nA/24.29Ωcm^2 at 290 K and 150 K respectively for the devices in diameter 300 μm. Their performances are compared to the 2.5μm cut-off photodiodes with similar structures.     

Low-cost 3D nanocomposite solar cells obtained by electrodeposition of CuInSe2

Thin CuInSe₂ films have been prepared by electrodeposition from a single bath aqueous solution on both dense and nanoporous TiO₂. The films are deposited potentiostatically using a N₂-purged electrolyte at different potentials. Various deposition times and solution compositions have been employed. The effect of annealing in air and in argon at different temperatures and times is also investigated. Thin films and nanocomposites of TiO₂ and CuInSe₂ have been studied with electron microscopy, X-ray diffraction, Raman spectroscopy, and optical absorption spectroscopy. After a thermal anneal in argon at 350 °C for 30 min excellent CuInSe₂ is obtained. In particular the nominal crystal structure and the bandgap of 1.0 eV are found. Although pinholes are present occasionally, good samples with diode curves showing a rectification ratio of 24 at ±1 V are obtained. Upon irradiation with simulated solar light of 1000 W m⁻² a clear photoconductivity response is observed. Furthermore, also some photovoltaic energy conversion is found in TiO₂|CuInSe₂ nanocomposites.     

Schottky diode based on porous GaN for hydrogen gas sensing application

This article reports the study of Pd Schottky contact on porous n-GaN for hydrogen gas sensing. Upon exposure to 2% H₂ in N₂, porous GaN sensor exhibited significant change of current. Morphological studies revealed that Pd contact deposited on porous GaN has ridge-trench-like morphology, a dense porous network was found in between the ridges. The dramatic change of current was attributed to the unique microstructure at Pd/porous GaN interface, which allowed higher accumulation of hydrogen; this resulted in a stronger effect of H-induced dipole layer and led to a significant change in the electrical characteristics of the porous sensor.     

Surface Structure and Electronic Property of InP（001）-（2×1）S Surface： A First-Principles Study

The surface structure and electronic property of InP（001）-（2 ×1）S surface under S-rich condition are investigated based on first-principles simulations. The analyses of phase transition show that the 3B model is the most stable structure and the S-S dimer is difficult to form. The geometry of the 3B structure agrees well with the experiments. It is also found that the 3B structure has a good passivation with a band gap of about 1.24eV. The results indicate that the 3B structure is the best candidate for the sulfur-rich InP（001）（2 × 1）A phase.