Table of contents

The microwave noise technique is used to estimate the hot-electron energy relaxation time in an AlGaN/GaN heterostructure containing a two-dimensional electron gas subjected to a strong electric field applied in the plane of electron confinement. Room-temperature data show that the energy relaxation time decreases monotonously from about 1 ps at 2 kV cm⁻¹ to 0.4 ps at 10 kV cm⁻¹ electric field. The estimated low-field value is 1.4 ps.

Grooved gate metal–oxide-semiconductor field-effect transistors (MOSFETs) are known to alleviate many of the short channel and hot carrier effects that arise when MOSFET devices are scaled down to very short channel lengths. However, they exhibit much higher parasitic capacitance with stronger bias dependence when compared to conventional planar devices. In this paper, we present a model for gate-to-drain and gate-to-source capacitance characteristics of a deep submicrometre grooved gate MOSFET. Both the intrinsic and extrinsic parts of the capacitance are modelled separately. In particular, the model presents a novel but simple way to account for the accumulation layer formation in the source/drain region of MOSFETs due to the application of the gate voltage. The results are compared with those obtained from a two-dimensional device simulator. The close match between the modelled and simulated data establishes the validity of the model. The model is then used to account for the superiority of capacitance characteristics of planar device structures and to arrive at optimization guidelines for grooved gate devices to match these characteristics.

We systematically studied the effect of thermal annealing on the optical and electrical properties of amorphous semiconducting thin films in the system Ge₂₀Te_80−xBi_x (x = 0, 0.19, 2.93, 7.35) prepared by flash evaporation in a vacuum of 1 × 10⁻⁶ Torr. The films are characterized by x-ray diffraction (XRD) and electron probe micro analysis. The annealing temperature is kept at 150 °C, 180 °C and 220 °C. No crystallization of the thin films is achieved on annealing up to the temperature of 150 °C. At a higher temperature of annealing, microcrystals of Te, Bi₂Te₃, Ge–Te, etc, are observed along with an amorphous phase as indicated by XRD analysis. The fundamental optical absorption edge and reflection spectra of as-prepared and annealed films are determined. Optical interband transitions are observed for various films (as-prepared and annealed). The presence of crystalline Bi₂Te₃ in films annealed at 220 °C is also supported by the reflection spectrum. The optical energy gap (E_g), the slope parameter (Δ) of the absorption edge and the tailing parameter (B⁻¹) of the energy band tails are computed from the optical data. The dc electrical conductivity (σ_dc) of various films is studied in the temperature range of 150–450 K. We observe that two types of conduction take place: conduction through extended states in the higher temperature region, and conduction through localized states in the band tails and at the Fermi level by the hopping process assisted by phonons at lower temperatures. The data at higher temperatures have been fitted with the expression σ_dc = σ₀exp(−ΔE/kT) and the electrical parameters, ΔE and σ₀, are also determined. It is observed that the bismuth concentration and annealing temperature dependences of the optical and electrical parameters are different in the two regions of compositions, x ≤ 2.93 and x > 2.93, indicating structural differences in the two sets of compositions. It is pointed out that the bulk form of these amorphous semiconductors exhibits a carrier sign reversal at a bismuth concentration of about 3.5 at%. However, the thin-film form of these amorphous semiconductors does not show such a carrier sign reversal in the electrical transport.

We have performed large and small angle x-ray scattering measurements on Si/Si_0.7Ge_0.3 superlattices grown by gas-source molecular beam epitaxy. The thickness of the Si and SiGe layers as well as their Ge content were extracted through a correlation between large-angle x-ray diffraction and photoluminescence measurements. A SiGe growth rate enhancement was observed during the first stages of the SiGe layer deposition, which might be due to a slow build-up of the Ge surface population. The Fresnel optical method was used to analyse the small-angle specular x-ray reflectivity data. The interface root mean square roughness, 7 ± 1 Å for the samples grown at 520 and 580 °C, and 5 ± 1 Å for the sample grown at 550 °C, are quite comparable to those found in Si/SiGe superlattices grown by solid source MBE. The crystallographic quality of those stackings is very good; no measurable period dispersion has been obtained in x-ray diffraction.

A borderless contact (BLC) technique and double-implant structure (DIS) have been developed successfully to fabricate high-performance Ti-salicide sub-quarter-micron CMOS devices. A SiO_xN_y film grown by low-temperature chemical vapour deposition is used to act as the selective etch-stop layer. The n⁺ and p⁺ DIS can reduce the junction leakage current which is usually enhanced by BLC etching near the edge of shallow trench isolation. Based on the use of the BLC process, the process window can be enlarged. In addition, the employed low-thermal oxynitride and high deposition rate can improve the salicide thermal stability and avoid the salicide agglomeration. Experimentally, by combining the BLC and DIS techniques, low leakage and low sheet resistance CMOS devices and low standby current and high yield 1 Mb SRAMs are fabricated successfully.

The resistivity ρ(P), Hall coefficient R(P) and electron Hall mobility μ_H(P) in Ge with doubly charged level of gold are presented in relation to hydrostatic pressure in the range up to P = 7 GPa at T = 295 K. The effect of interband electron scattering, associated with electron exchange between the (111) and (100) minima of the conduction band, on the electron mobility and kinetic interband electron scattering coefficients is discussed. The parameters S = 0.57 ± 0.10 and S' = 0.29 ± 0.12 are calculated, characterizing the intensity of interband electron scattering in comparison with intraband scattering of electrons from the L₁ and Δ₁ valleys of the conduction band in the vicinity of the intervalley transition.

Extended balance equations accounting for the conduction–valence interband impact ionization (II) process in semiconductor heterostructures are presented. The II effect and terahertz (THz) field influence on electron transport in InAs/AlSb heterostructures is studied. It is shown that the II process usually results in a decrease in electron velocity and temperature when compared to the case without the II process. Qualitative agreement is obtained between the calculated electron–hole generation rates and available experimental data. Comparison with published experimental data on THz-driven heterostructures is also made.

L'industrie des composants VLSI exige des investissements financiers de plus en plus lourds pour mesurer la sophistication grandissante des produits fabriqués ainsi que pour les équipements nécessaires à leur élaboration. De ce fait, la modélisation électrique des composants électriques constitue actuellement un axe de recherche très convoité à travers le monde. Pour suivre cette évolution, les modèles existants doivent être améliorés et de nouveaux modèles doivent être développés. C'est ainsi que nous assistons régulièrement à des améliorations des logiciels de simulation. Dans ce travail, nous avons mis en oeuvre un modèle de la technique de pompage de charge pour l'étude de la dégradation des TMOS. Ce modèle, implanté dans le simulateur SPICE3F4, prend en considération la majorité des effets physiques décrivant le comportement réel du dispositif. La validation de notre modèle nous a fourni des résultats concernant le courant pompé I_cp (en négligeant l'effet de la température sur les paramètres du TMOS), en fonction des niveaux haut et bas de l'impulsion de grille (V_gh et V_gl) et en fonction de la température. Nos résultats sont comparés à des résultats expérimentaux, analysés et commentés de manière à pouvoir en tirer des conclusions pratiques qui sont de nature à intéresser tous ceux qui sont appelés à réaliser des circuits de technologie VLSI.

The VLSI-component industry requires ever increasing financial investment in order to measure the growing sophistication of the manufactured products and for the equipment necessary to their development. So the modelling of electric components constitutes a research field that is currently very important throughout the world. To follow this evolution, the existing models must be improved and new models must be developed. So we regularly see improvements of simulation software. In this work, we develop a model of the charge pumping technique to study the degradation of a MOSFETs. This model, implemented in the simulator SPICE3F4, takes into account the majority of the physical effects describing the real behaviour of the device. The validation of our model provided us with results concerning the charge pumping current I_cp (while disregarding the effect of the temperature on parameters of the TMOS), versus high and low levels of the gate impulse (V_gh and V_gl) and versus the temperature. Our results are compared with experimental results, analysed and discussed in order to be able to extricate some practical convenient conclusions that are of such a nature as to interest all those who have to realize circuits involving VLSI technology.

An interacting electron–hole pair in a two-dimensional quantum dot is studied within the framework of the effective-mass approximation. It is shown that non-local separable potentials may be used to obtain the ground state of confined excitons. To this end, we replace the actual Coulomb potential by a projective operator to determine in a closed form the exciton energy as a function of the quantum dot size. Several potential functions are considered and results are compared to well-established approaches. Improvements to the present method are also discussed.

The etching characteristics of InP by reactive ion etching (RIE) using Cl₂ and CH₄ gases were investigated systematically as functions of various etching parameters. The etching parameters, such as Cl₂ flow rate, CH₄ flow rate, rf power and process pressure, were varied. The effects of H₂ or Ar addition to the Cl₂ and the CH₄ discharges were compared. The etch rates of SiO₂ mask during Cl₂ RIE process were also measured. The etched profiles, sidewall roughness and surface morphology were observed by scanning electron microscopy and by atomic force microscopy. Auger electron spectroscopy was used to examine the elemental composition of etched surfaces and to evaluate contamination. The possible mechanism for these etching phenomena is discussed. The results suggest that a CH₄/H₂ gas mixture is the most promising for achieving ideal etching profiles (i.e., anisotropy, good surface morphology, smooth sidewall roughness) for optoelectronic device applications.

We present results of numerical calculations of low-lying energy states of the negatively charged exciton in the rectangular CdTe_1−xMg_xTe/CdTe/Cd_1−xMg_xTe quantum well in the presence of a perpendicular magnetic field. Specific results for the trion binding energy in 40 Å, 80 Å and 160 Å wide quantum wells are discussed as a function of the magnetic field strength (up to 40 T). These three widths of the quantum wells correspond to three different relationships between the magnetic length, Bohr radius and quantum well width. The 80 Å quantum well is examined in greater detail. In particular, we show the behaviour of the total energy of various trion states, and the behaviour of the allowed optical transitions with an external magnetic field. We discuss also the role of the Zeeman Hamiltonian on the energy spectra of the trion.

In this paper we present a GaAs surface passivation technique using (NH₄)₂S_x treatment and plasma-enhanced chemical vapour deposition silicon nitride. The passivation effects are demonstrated by the improvement in the capacitance–voltage (C–V) and the photoluminescence (PL) data. Post-metallization annealing (PMA) at 450 °C is required to obtain a good 1 MHz C–V curve and high band-edge PL intensities. Even though higher-temperature PMA shows better values of C–V and lower interface state density (D_it), it reduces the band-edge PL intensities. This is attributed to the hydrogen incorporation into GaAs during higher-temperature annealing cycles.

The microstructural features of the high-temperature-stable ohmic contact system Ti/Al/Ti/Au/WSiN on AlGaN/GaN were investigated using analytical transmission electron microscopy and thin film x-ray diffraction. For two typical rapid thermal annealing steps at 750 °C (non-ohmic behaviour) and 850 °C (ohmic behaviour) the intermetallic phases at the metal-semiconductor interface are presented. Increased annealing leads to the transformation of an Al₂Au-AlAuTi phase mixture to a mixture of Al₂Au-Al₃Au₈ phases and the formation of Ti-Al-nitride layers at the interfaces. In light of these results the electrical contact properties are discussed.

A computer simulation based on an advanced self-consistent thermal–electrical finite-element approach has been carried out to analyse electrical and thermal phenomena in possible electrically-biased axially-symmetric annular-contacted nitride VCSELs (vertical-cavity surface-emitting lasers). Joule heating within the upper p-type GaN layer has been determined to be the distinctly dominant heat source in this device. Our analysis revealed that during a continuous-wave (CW) room-temperature (RT) device operation, the highest-temperature area constitutes a ring within the above-mentioned layer close to the active-region perimeter. It dramatically enhances radial current flow from the annular p-side contact towards the centre of the structure before crossing the p–n junction. Therefore current injection into a central active region is considerably improved (by almost 170%!) in RT CW-operating nitride devices as compared with pulse-operating ones. Hence temperature-enhanced radial current spreading within the upper part of the annular-contacted CW nitride VCSELs may enable one to obtain their first RT operation.

We study the structural, electrical and optical properties of silver selenide thin films prepared by reactive evaporation. From the Hall effect study, at room temperature we find a mobility of 2000 cm² V⁻¹ s⁻¹ and a carrier concentration of 10¹⁸ cm⁻³. The thermoelectric power is measured from room temperature up to 200 °C. X-ray diffraction indicates that the as-prepared films are polycrystalline in nature. The composition and morphology are determined using energy dispersive x-ray analysis and scanning electron microscopy (SEM). The optical bandgap, which is direct-allowed, is 1.58 eV.

CdTe films have been deposited onto HgCdTe by potentiostatic electrodeposition at deposition potentials of −0.4 V and −0.5 V with respect to a saturated calomel reference electrode in ethylene glycol base electrolyte. Films deposited with and without nitrogen bubbling to reduce dissolved oxygen in electrolytes were investigated by atomic force microscopy (AFM) and x-ray diffraction (XRD). Significant reduction in roughness (>1 orders) was observed by bubbling the electrolyte with 6 N nitrogen gas and the resulting films were highly oriented. The reduction in roughness is believed to be the result of the suppression of the reduction reaction whereby the dissolved molecular oxygen is converted to hydrogen peroxide and water that disturbs the ordered CdTe electrodeposition onto HgCdTe wafers.

Amorphous selenium films were prepared at room temperature using the simple and safe method of chemical bath deposition. This was achieved with an acidified solution of Na₂SeSO₃ at a pH value of 4.5. In order to confirm the formation of amorphous selenium (a-Se), samples were characterized using x-ray diffraction, x-ray photoelectron spectroscopy, secondary ion mass spectroscopy and optical absorption studies. The optical bandgap of the material was found to be 2.09 eV. Indium selenide could be prepared by annealing this a-Se in a multilayer structure glass/SnO₂/a-Se/In.