Table of contents

Neutron depth profiling (NDP) is a non-destructive analytical technique for the quantitative determination of a few nuclides of particular importance to the semiconductor industry, The first few micrometres of a material can be probed irrespective of the matrix and chemical composition, including across interfacial structures. Recently, it has been used to certify a boron implant in silicon standard reference materials for calibration of secondary ion mass spectrometry (SIMS) instruments. Detection limits and examples of its use for the analysis of boron-doped silicon wafers and diamond films are provided.

In the past five years, a sufficient understanding of hot-carrier degradation has been obtained to slow reliability predictions of digital and analogue CMOS circuits. The proven quasi-static behaviour and a universal time dependence of the degradation, which is independent of the specific stress condition, prove helpful in developing simple formulae for predicting n-channel lifetimes in digital circuits. In p-channel devices, in contrast, a detailed quantitative understanding is obtained which leads to a precise description of the degradation in this type of device. Reliability predictions of digital CMOS circuits are performed on this basis. Analogue circuits operate with long-channel devices and in regimes differing from those for digital applications, thus requiring additional considerations. Channel-length dependences of the hot-carrier degradation have been found for a variety of electrical parameters relevant to the analogue operation of n- and p-MOSFETs. In particular, a new degradation effect independent of channel length that is important for modelling analogue circuits has been discovered in p-MOSFETs. The reliability of devices in analogue circuits is described and tested on this basis.

The effect of gamma ray irradiation was investigated in HEMTs. Devices with different layer structures were employed for the better understanding of failure mechanism sources. The introduction of a charge control model allowed the determination of buffer layer degradation.

The structural and electronic properties of the zinc-blende, beta -Sn and rock-salt phases of BP are calculated within the local-density approximation. Two competing methods-LMTO and the pseudopotential plane-wave method-have been used. The agreement between the two electronic structure methods is excellent. We obtain the pressure versus volume equation of state and the pressure dependence of the structural and electronic parameters in good agreement with experiment and some previous calculations. Our calculations confirm that for BP crystal in the zinc-blende phase there is an indirect gap (of approximately 1.5 eV) which decreases quadratically with pressure and that the phase transition from zinc-blende to rock-salt phase occurs at extremely high pressure (approximately 150 GPa). We observe that BP crystal in the beta -Sn phase is stable only at very high pressure.

Theoretical studies so far on piezoelectric scattering have resulted in a mobility behaviour of mu approximately T^-1/2 in which mu continues to fall indefinitely with increasing temperature T. Such a behaviour cannot tell us where this scattering will become unimportant, as might be expected on physical grounds. The correct behaviour should be a falling curve first, followed by a gradually increasing one when the scattering ceases to be effective. In this work, by using the force-force correlation function technique within the linear response formalism, we obtained a mu versus T curve that shows just this behaviour.

Electric current due to hopping between localized states in a one-dimensional (1D) chain of quantum boxes is characterized by a set of zero minima (antiresonances) due to the momentum selection rule for the interaction of acoustic phonons with Wannier-Stark localized electrons. These antiresonances result from vanishing electron transitions between quantum wells at certain values of the phonon longitudinal momentum due to Bragg reflection. We suggest the possibility for experimental observation of this effect by applying a longitudinal magnetic field to existing superlattices. Antiresonances also arise due to the folded phonon gaps. Elastic scattering and optical phonons result in resonant maxima in the current-voltage characteristics. Owing to Wannier-Stark localization, the longitudinal magnetoresistance is expected to be negative over a substantial range of magnetic fields. We also discuss the general procedure of current calculation and clarify the role of boundary conditions in the hopping regime.

Localized vibrational modes (LVMs) of Si DX centres in GaAs are worked out for different pressures using the Green function technique, introducing lattice relaxation. This yields a very good agreement with experimental results for LVMs of DX centres.

The spin-flip Raman scattering of electrons bound to donors in CdTe/Cd_1-xMn_xTe multiple quantum well structures has been studied. The spin-flip energies are influenced strongly by the competing potentials of the quantum wells and of the donor ion. As a result, the spin-flip energy is a function of the donor position relative to the well centre. Because of resonance effects, the Raman spectra are sensitive to the laser photon energy and the results demonstrate a correlation between the binding energy of the electron to the donor (which determines the Raman shift) and the localization energy of an exciton at the neutral donor (which determines the energy of the Raman resonance). Studies of a structure containing two coupled quantum wells reveal similar effects and lend further support to the model.

We report on nonlinear optical absorption at the band edge of tensile strained In_1-xGa_xAs/InP MQW structures. While at low strain the well known exciton bleaching is observed, a new nonlinearity dominates at high tensile strain, where the conduction band discontinuity becomes very small. Calculations of the spatial band diagram with and without optical pumping show that charge-carrier-induced band bending and localization of the previously poorly confined n=1 electron wavefunction leads to enhanced absorption at the band edge. We find a good agreement between experimental results and theory.

Morimoto and Chiba (1991 J. Phys. Soc. Japan. 60, 2446) have observed a threshold behaviour of current-induced interband light emission from n-InSb samples in a magnetic field, attributing this to the onset of stimulated emission. We show that their experimental results can be explained quite satisfactorily by the combined effect of sample heating by current and surface accumulation of minority carriers under the action of the Lorentz force. We also derive for intrinsic narrow-gap semiconductors the condition for achieving population inversion within a narrow accumulation layer by passing current in a transverse magnetic field.

We present a new approach for defect engineering by ultrasound treatment (UST) in solar-grade polycrystalline silicon wafers, leading to significant enhancement of minority carrier diffusion length (L) in wafer regions with short L. The maximum value of the UST effect is a 2.7 times increase of the diffusion length in regions of the sample with L=10 to 25 mu m. Using the surface photovoltage (SPV) method for non-contact mapping of the diffusion length, we have found both a positive and negative variation of L after UST in different wafer regions. The UST effect depends upon temperature, showing an activation energy of about 0.17 eV. A relaxation study of the UST effect shows two different behaviours: (i) stable result versus post-UST holding time, and (ii) partial or complete recovery of the diffusion length. Process (ii) is linked to the ultrasound-enhanced dissociation of FeB pairs followed by pairing kinetics.

In order to confine quasi two-dimensional electron systems (2DES) on Hg_1-xCd_xTe laterally, we prepared metal-oxide-semiconductor (MOS) structures with accessory comb-shaped electrodes buried in the insulator. In these dual-stacked gate devices we can propel the electron gas from a 2DES, over a density-modulated system, to a 1DES. Accordingly, the far-infrared spectra exhibit subband cyclotron resonances, magnetoplasmons and 1D intersubband shifted cyclotron resonances in the three different regimes. Lateral quantization energies of the 1DES of up to 9 meV are observed. We find that fixed, chargeable states at the Hg_1-xCd_xTe/insulator interface play a significant role in the lateral confinement of the electron gas.

An energy-resolved DLTS technique was used to determine the distributions of both interface trap density and thermal capture cross section along the bandgap of an MOS structure. The resulting experimental values are criticized and tested in comparison with those obtained from a conventional a.c. conductance technique applied on the same diode. In this respect, incomplete filling of traps has an important effect on the interface trap density distribution. Furthermore, experimental measurements indicate that the capture cross section strongly depends on temperature, which might be interpreted as being due to a multiphonon emission process.

n-type conductivity was observed in thin films of BN deposited by microwave plasma-assisted chemical vapour deposition using a single-source organometallic precursor. The depositions were carried out on various substrates using N₂ as the carrier gas. These films were characterized by FTIR, XRD and SEM which revealed the presence of crystallites of boron nitride. The deposition conditions were optimized to obtain the maximum concentration of c-BN in the thin films of boron nitride. Seebeck studies indicate that electrons are the majority charge carriers. I-V studies of the p-n junction between p-Si and n-BN indicate typical behaviour of diode characteristics.

The coupling efficiencies of sputtered, c-axis-orientated zinc oxide (ZnO) films in guided wave resonance optical modulators have been measured. The ZnO planar waveguide is sandwiched between two 3 nm thick chromium layers on top of a SiO₂/Si(100) substrate. This novel design facilitates application of a modulation voltage directly across the dielectric film in a prism coupler set-up and thus avoids voltage losses across the silicon dioxide (SiO₂) optical isolation layer. We present a comparison of four differently prepared ZnO waveguides/modulators to investigate the influence of film crystallinity on the coupling efficiency and electro-optical parameters. The coupling efficiency of a ZnO waveguide is found to be dependent on both film thickness and average grain diameter measured in the plane of the film. A high efficiency (0.76) can be achieved for film thicknesses below 300 nm when the average grain diameter is 26 nm. This value is comparable to the maximum value of typically around 0.81 expected from model calculations for uniform couplers.

Room-temperature time-resolved photocurrent measurements have been performed on novel InGaAs/GaAs multiple-quantum-well (MQW) P-I-N diodes on (111)B GaAs substrates, having an average electric field of opposite sign to the built-in field. Direct evidence for dipole formation, displacement photocurrents and out-of-well screening by photocarriers is found. In these piezoelectric structures the sign of the internal field can be controlled by an external modulating voltage and the optically pumped charges stored at the extremes of the diode MQW active region can be extracted by appropriate pulsed biasing. The generated dipole moment and its time evolution have been determined by an optical pump and electrical probe technique: the spatial separation of electrons and holes accounts for the observation of slow recombination processes. The piezoelectric constant is evaluated for In_xGa_1-xAs (x=17% In) from the zero-average-field condition.

A fabrication process of 0.5 mu m gate GaAs-MESFETs combined with high-dielectric-constant thin-film capacitors was investigated. A buried p layer (BP) is employed to suppress short-channel effects by using the carbon ion implantation technique. These BP-MESFETs have a self-aligned n⁺ asymmetric lightly doped drain (LDD) structure to realize low source resistance and high drain breakdown voltage. The planarization technique was adopted to deposit an Au overlayer on top of the 0.5 mu m WSi gate to improve the high-frequency performance by reducing the gate resistance. These BP-MESFETs have a high K value, over 230 mA V^-2 mm^-1, and a maximum frequency of oscillation f_max of 60 GHz with an n⁺ asymmetric structure. We also have developed high-dielectric-constant SrTiO₃ thin-film capacitors by using the low-temperature RF sputtering method. These SrTiO₃ films have high epsilon _rs over 100, and low leakage current density, under 1*10^-6 A cm^-2 at 1 MV cm^-1. By using this SrTiO₃ capacitor for the bypass capacitors of the MMIC, we can reduce the pin counts of the MMIC and will be able to realize a shrinkage of the total size of the RF unit of a cellular telephone. We have fabricated the front-end IC of a cellular telephone using this newly developed process technology, and can achieve low power consumption.

By using an analytically derived dynamical density response function of a unidirectional periodically modulated two-dimensional electron gas under a perpendicular magnetic field, we study the magnetoplasmon modes in this system within the random-phase approximation. It is found that the modulation potential does not introduce an additional low frequency mode nor cause high frequency magnetic oscillations in the principal magnetoplasmon mode, as has been previously reported. However, there exists a shift in the principal mode due to the modulation, and this shift is a slow oscillatory function of the inverse magnetic field.