Table of contents

An investigation of dynamic-threshold (DT) performance was carried out on a 0.5 µm gate length p-type Si:SiGe heterostructure field effect transistor, for temperatures ranging from T = 300 K to T = 10 K. The maximum low-field transconductance of DT-mode operation was found to be 30% higher than in normal mode, through a better control of carriers in the channel. The subthreshold slope of the device, when operated in DT mode, also improved, decreasing by 28%. The sensitivity of the threshold voltage to substrate bias was extracted from experimental data over the whole temperature range, and was found to be higher in the SiGe device than in a corresponding Si control device. The substrate sensitivity was used together with equations derived from classical Si MOSFET theory to successfully predict the performance improvement due to DT-mode operation, at all temperatures.

In the last few years, the understanding of information delivered by reflectance difference/anisotropy spectroscopy (RAS) has grown considerably. However, a full understanding of this optical technique is not yet achieved because surface, interface and bulk effects are present particularly where heteroepitaxial systems are concerned. This is especially true for the case of resonances at the bulk critical points of the dielectric function, which either resemble the dielectric function or its derivative. Previous RAS experiments on zincblende and diamond structure semiconductors found optical anisotropies in the vicinity of the E₀, E₁ and E₁ + Δ₁ critical points. In this review, the origin of these structures is discussed and it is shown that anisotropic in-plane strain in the epilayer or bulk induces resonances at these critical points. This in-plane strain is either caused by the boundaries of the epilayer system (i.e. the surface or the interface) or by symmetry breaking via a surface electric field or a preferred orientation of dislocations. These findings are best supported by applying additionally photoreflectance difference spectroscopy (PRD), where the difference between a spectrum taken with linearly polarized light and with unpolarized light is measured. In contrast to RAS, PRD spectroscopy is specific to the symmetry breakdown occurring due to band bending via the surface or interface electric field and stress.

Engineers usually adopt multilayered design for semiconductor and electron devices, and an accurate electrostatic analysis is indispensable in the design stage. For variable design of electron devices, the BEM has become a better method than the domain-type FEM because BEM can provide a complete solution in terms of boundary values only, with substantial saving in modelling effort. Since dual BEM still has some advantages over conventional BEM for singularity arising from a degenerate boundary, the dual BEM accompanied by subregion technology, instead of tedious calculation of Fourier–Bessel transforms for the spatial Green's functions, was used to efficiently simulate the electric effect of diverse ratios of permittivity between arbitrarily multilayered domain and the fringing field around the edge of conductors. Results show that different ratios of permittivity will affect the electric field seriously, and the values of surface charge density on the edge of conductors are much higher than those on the middle part because of fringing effect. In addition, if using the DBEM to model the fringing field around the edge of conductors, the minimum allowable data of dielectric strength for keeping off dielectric breakdown can be obtained very efficiently.

Transient charging in response to pulsed bias of copper phthalocyanine (CuPc) organic thin films on indium–tin–oxide (ITO) was investigated by charge transient spectroscopy (QTS) in situ under ultra-high vacuum conditions. The charge relaxation phenomena in response to a bias pulse are modelled in terms of dipolar effects, considering alternatively possible orientation of the dipoles (polarization) under an applied external electric field and capture of charge carriers in dipole-induced electron states. Assuming a Gaussian energetic distribution of the density of states (DOS) as a consequence of disorder, a simple formalism is derived to determine the width (variance) σ_d of the DOS from QTS data. The isothermal QTS response of single-layer Ag/CuPc/ITO samples provides evidence for dipolar effects in CuPc, an example of using the new formalism in practice. The dipole moment of the CuPc molecule proved to be too small for the assignment of the amount of transient charges to the reorientation of dipoles. The model of trapping of charge carriers at a Gaussian DOS induced by polar molecules in a non-polar solid has mediated an estimate of the fraction of polar molecules from the experimentally assessed variance σ_d of the Gaussian DOS. The resulting dependence of σ_d on the dipole moment is in striking coincidence with literature data related to other types of polar molecules in molecular glasses.

From a comparison between the degradation rates, induced by proton and electron irradiations, of the performances of GaAs quantum well based vertical cavity surface emitting laser, we deduce a coefficient of equivalence between the two types of irradiations. This coefficient allows us to calculate the fluence of protons of a given energy which produces the same degradation as a fluence of electrons of a standard energy. We apply the result of this analysis to the degradation of GaAs solar cells, demonstrating that this coefficient allows a quantitative prediction of the degradation induced by a proton irradiation, when the degradation induced by electrons at one energy is known.

We present an investigation of thermal annealing effects on spectral photoconductivity and in-plane photovoltage, at temperatures between 30 K and 300 K, in sequentially grown GaInNAs/GaAs and GaInAs/GaAs quantum well structures. Our results indicate that thermal annealing not only improves the sample quality but also causes the blueshift as commonly observed by other groups in optical studies. We show that the observed anneal-induced blueshift behaviour can be explained in terms of two competing mechanisms: the redistribution of nearest neighbour configuration and the change of quantum well profile. We also show that thermal annealing increases the intensity of photoconductivity signal but reduces the in-plane photovoltage signal drastically.

The current–voltage (I–V) characteristics of Ag Schottky contacts on a Bridgman–Stockbarger grown p-type SnSe layered semiconducting material have been measured over the temperature range of 80–350 K. Their analysis based on the thermionic emission (TE) theory has revealed an abnormal decrease of zero-bias barrier height and increase of ideality factor at lower temperatures. This behaviour has been interpreted on the basis of the assumption of a Gaussian distribution of barrier heights due to barrier height inhomogeneities that prevail at the interface. The inhomogeneities are considered to have Gaussian distribution with a mean barrier height of and standard deviation of σ_s0 = 0.075 V at zero-bias. Furthermore, the mean barrier height and the Richardson constant values were obtained by means of the modified Richardson plot, ln(I₀/T²) − (q²σ²_s0/2k²T²)versus 1000/T, as 0.603 eV and 7.72 A K⁻² cm⁻² respectively, of which latter is close to its theoretical value of 18 A K⁻² cm⁻² used for the determination of the zero-bias barrier height. Hence, it has been concluded that the temperature dependence of the I–V characteristics of the Schottky barrier on p-type SnSe can be successfully explained on the basis of TE mechanism with Gaussian distribution of the barrier heights.

The current–voltage (I–V) characteristics of Ag/p-SnS Schottky barrier diodes were measured in the temperature range of 100–300 K and have been interpreted on the basis of the assumption of a Gaussian distribution of barrier heights (BHs) due to barrier height inhomogeneities that prevail at the interface. It is shown that the occurrence of a Gaussian distribution of the BHs is responsible for the decrease of the apparent barrier height Φ_B0, increase of the ideality factor n and nonlinearity in the activation energy plot at low temperatures. The inhomogeneities are considered to have a Gaussian distribution with a mean barrier height of and a standard deviation of σ_s0 = 0.093 V at zero bias. Furthermore, the mean barrier height and the Richardson constant values were obtained as 0.642 eV and 12.89 A K⁻² cm⁻², respectively, by means of the modified Richardson plot, ln(I₀/T²) − (q²σ²_s0/2k²T²) versus 10³/T. Hence, it has been concluded that the temperature dependence of the I–V characteristics of the Schottky barrier on p-type SnS can be successfully explained on the basis of TE mechanism with a Gaussian distribution of the barrier heights. Furthermore, the value of the Richardson constant found is much closer than that obtained without considering the inhomogeneous barrier heights.

A study of the cavity polariton scattering processes in a InGaAs/GaAs system under increasing resonant excitation is made at both positive and negative detunings. Collisional broadening is experimentally investigated by resonantly pumping both polariton branches, the observed behaviour is shown to be similar to that of the bare quantum well excitons. However, it is shown that in contrast to this when resonantly exciting just the lower polariton branch the collisional broadening is comparatively suppressed. These results are explained by comparing the modified dispersion curve of the polariton to that of the quantum well exciton. Finally, an increase in coherence, the inverse of collisional broadening, of the lower polariton branch is seen with an increase in excitation under certain experimental conditions. We suggest that this result provides evidence for a condensate in the polariton system. No similar effect occurs in the bare quantum well excitons.

We have identically prepared as many as 60 Ti/p-Si (100) Schottky barrier diodes (SBDs) with a doping density of about 10¹⁵ cm⁻³. The Si (100)-H surfaces were obtained by wet chemical etching in diluted hydrofloric acid. We have made a statistical study related to the experimental barrier heights (BHs) and ideality factors of the diodes, and we have looked at linear relationship between BHs and ideality factors. The BHs obtained from the current–voltage (I–V) characteristics varied from 0.556 to 0.617 eV, and the ideality factor varied from 1.019 to 1.196. The experimental BH and ideality factor distributions obtained from the I–V characteristics were fitted by a Gaussian function, and their mean values were found to be 0.577 ± 0.013 eV and 1.098 ± 0.044, respectively. Furthermore, the homogeneous BH value of approximately 0.602 eV for the device was obtained from the linear relationship between experimental effective BHs and ideality factors.

A numerical model for current conduction in single layer OLEDs including both injection and bulk effects is proposed. Based upon this model, the dependences of current on the barrier height, device thickness and operation voltage, as well as the field distribution, are numerically investigated. It is found that the barrier height at the injection electrode limits the current conduction much more than the thickness of the organic layer, and, except near the electrodes, the electric field is nearly linearly distributed versus the distance from the injection electrode in the organic bulk, and the slope of the distribution decreases as the barrier height increases.

Requirements of a higher output and the single-fundamental-mode operation are usually contradictory to each other. In the present paper, an essential modification of a design of the GaAs-based oxide-confined vertical-cavity surface-emitting lasers (VCSELs) is proposed. Functions of both the oxide apertures are separated: while the larger upper one is designed to create the radial waveguiding effect, the smaller bottom one is only funnelling the current injection into the active region. Hence both may be optimized separately. As a result, single-fundamental-mode operation is confirmed by our simulation to be preserved at much higher output and/or at much higher temperatures than in standard VCSELs.

We have performed magneto-photoluminescence (PL) experiments on a self-assembled coupled double quantum dot (QD) system consisting of adjacent CdSe and CdZnSe QD layers. We have observed well-separated two PL peaks in such a double layer QD system, corresponding to the CdSe and the CdZnSe QDs. The PL signal was studied using circularly polarized detection. The intensities of the two PL peaks exhibited significant differences for the two circular polarizations when a magnetic field applied, in contrast to the behaviour shown by single layer CdSe or CdZnSe QD systems, which show nearly identical polarization dependence on the field. The observed behaviour can be interpreted in terms of interaction between carriers localized in pairs of QDs that are electronically coupled.

The design of SiGe heterojunction bipolar transistors for high frequency (10 GHz), high power applications has been investigated by numerical device modelling using a commercial simulator. For the SiGe base layer, boron outdiffusion can give rise to displacement of the base-collector p–n junction from the collector heterojunction, which contributes to the development of a parasitic barrier in the conduction band. In this paper, using a Gaussian distribution for the base's boron profile, we examine the effects of the device's epitaxial layer design on the device's unilateral power gain at 10 GHz and its relation to this barrier. Degradation in the device's microwave power gain was found to correlate with formation of the collector junction parasitic barrier, where the barrier height was found to depend upon the extent of the p–n junction displacement from the heterojunction, the Ge concentration in the base and the dc bias point. Trade-offs in device design to reduce its sensitivity to outdiffusion were examined including the use of undoped SiGe spacer layers, increased collector doping and the insertion of a thin, n+ layer at the collector junction.

We have fabricated a very bright top-emitting organic light-emitting device directly based on silicon substrate utilizing the surface-modified Ag as anode. By inducing a thin silver oxide at the surface of Ag, the hole injection from Ag anode into organic light-emitting device is largely enhanced yet with rather high reflectivity retained. It is known to us that high-reflectivity bottom is essential for high-performance top-emitting organic light-emitting devices and Ag has the highest reflectivity for visible light among metals. The brightness of the top-emitting device in silicon using such surface-modified Ag anode reaches 14 090 cd m⁻² at 13 V. To our knowledge, it is rather high in top-emitting devices based on silicon.

HfO₂ thin films were atomic-layer deposited using different-precursor partial pressures and at different growth temperatures on n- and p-type silicon substrates. The effect of processing parameters and film thickness on the electrical quality of the oxide–semiconductor interface was studied. Deep-level-transient spectroscopy and conductance-transient techniques revealed 3–10 × 10¹¹ cm⁻² eV⁻¹ interface trap densities, somewhat dependent on the processing conditions. Charge trapping took place mainly between the semiconductor and defects located at energies close to the majority-carrier-semiconductor-band edge.

This paper presents a self-consistent method to directly determine the effective refractive-index spectrum of a semiconductor quantum-well (QW) laser diode from the measured modal gain spectrum for a given current. The dispersion spectra of the optical waveguide confinement factor and the strongly carrier-density-dependent refractive index of the QW active layer of the test laser are also accurately obtained. The experimental result from a single QW GaInP/AlGaInP laser diode, which has 6 nm thick compressively strained Ga_0.4InP active layer sandwiched by two 80 nm thick Al_0.33GaInP, is presented.

The lateral photovoltage (LP) generated in a two-dimensional electron gas (2DEG) by a focussed laser spot has been shown to yield the potential distribution of a current carrying Hall-bar. The width of this distribution, which exhibits an abrupt change at even integer filling factors, is determined by the equilibration between the bulk and the edge states of the 2DEG. The effects of an anisotropic surface morphology and sample dimensions are reported.

Spin aligners and spin filters are necessary ingredients for spintronic devices. While the principle of operation is now well understood for a variety of spin filters, we point out that the efficiency of a given type of injector as to spin selectivity depends drastically on the actual device or circuit in which it is used. We present an approach based on loadlines that can be used to determine the spin injection efficiency of spin aligners and spin filters in combination with complex loads.

The near-surface deformation behaviour of ground silicon wafers was determined by using microindentation technique. Surface softening phenomenon was observed during the indentation, which was due to the formation and propagation of subsurface damage. The residual indentation depth increases with the subsurface damage introduced in the lapping process, while the reduced contact modulus decreased with increasing the subsurface damage. An empirical relation was used to correlate the reduced contact modulus to the subsurface damage in the ground silicon wafers. This could provide a simple approach to characterize the subsurface damage by using the microindentation technique.