Table of contents

The negative differential resistance properties of resonant tunnelling barrier diodes with InGaAs quantum wells developed by us for new transistor applications are reviewed. The emphasis is on the peak current density and the peak-to-valley ratio. The tunnel barriers of InAlAs, pseudomorphic AlAs, AlAsSb and GaAsSb are studied. A simple theoretical calculation which includes the mid-gap band non-parabolicity of tunnelling electrons is described. These theoretical calculations show excellent agreement with the observed peak current density at 77 K and 300 K and a reasonable agreement with the valley current density at 300 K.

The ionization coefficient, for a given semiconductor and carrier type, gives the number of ionization events directly caused by a carrier as it travels a unit length through the material. The position dependence of this impact ionization coefficient is at present poorly understood, and is usually assumed to be constant, possibly with the inclusion of a deadspace length (where the coefficient is zero). In the deadspace the carrier is being accelerated by the field but has not attained the threshold energy for ionization. In this paper an explanation of the origin of spatial oscillations in the coefficients is given, and the way their features vary with model parameters is investigated. Coefficients in both constant and piecewise linear/constant fields are modelled. The results are specifically for GaAs, but have qualitative validity for the other wide-bandgap Ill-Vs.

An S-shaped voltage-current characteristic (vcc) has been observed in the semimagnetic semiconductor p-Mn_xHg_1-xTe with a sufficiently strong compensation. It is suggested that the amplification of fluctuations of the energy of shallow impurity centres which stem from fluctuations of the composition of the semiconducting solid solution (as in the case of p-Mn_xHg_1-xTe which is considered in this paper) improves the conditions for formation of an S-shaped vcc due to development of a non-equilibrium carrier distribution in an impurity band during impact ionization.

A method for extracting the parameters of local levels from noise spectroscopic data for the case when the capture cross section of electrons sigma _n depends on temperature exponentially, sigma _n= sigma ₀ exp(-E₁/kT), has been suggested. For the limiting cases when the Fermi level E_F lies significantly below (E_F-E₀>>kT) or above (E₀-E_F>>kT) the level E₀, analytic expressions have been obtained to calculate sigma ₀, E₀, E₁ and the level concentration N_t. The results have been applied to noise-measurement data for GaAs samples with electron concentrations n₀ approximately 10¹⁵, 10¹⁶ and 10¹⁷ cm^-3. It has been shown that the technique proposed can be applied to a wide variety of experimental situations.

We study theoretically the conditions for achieving lasing action at infrared and far-infrared wavelengths based on intersubband transitions in semiconductor quantum well structures, taking into account resonant tunnelling and intersubband absorption-emission processes. Analysis of optical losses in the semiconductor quantum well structures reveals that a reasonably low threshold current density in the range of 1-5 kA cm^-2 is achievable for room temperature lasing at emission wavelength of lambda =60 mu m. A significantly higher threshold current density of 40-50 kA cm^-2 is required for lasing at lambda =10 mu m.

An investigation of strained quantum well 0.98 mu m lasers with one, two and three Ga_0.80In_0.20As wells and Ga_0.50In_0.50P cladding layers has been carried out. The threshold current density of broad-area lasers exhibited a saturation effect with increasing cavity length, and a logarithmic relationship existed between the gain per well at threshold and the threshold current density per well. This relationship was used in the determination of the variations of total gain with total injection current, which showed that a one-well structure should be best for lasers of suitable length for pumping erbium-doped fibre amplifiers. A low threshold current density of 112 A cm^-2 was obtained for a single well laser having a cavity length of 1000 mu m. The performance of broad-area lasers with structures grown at 600 degrees C on a substrate surface tilted at 3 degrees from the (001) exhibited greater thermal sensitivity than that of those grown on the (001). Thus, there seems to be no advantage in the use of off-axis substrates when the angle of tilt is small. Ridge-waveguide lasers were found to be capable of relatively high power output, but mode changes tended to occur at drive currents above 100 mA.

A centre Si delta -doped GaAs/Al_0.33Ga_0.67As double heterostructure has been studied by photoluminescence excitation spectroscopy in perpendicular magnetic fields up to 13.5 T. Landau level transitions are identified to originate from the highest occupied electron subband. With increasing field a depopulation of that electron subband is observed. The value of the field at which this depopulation occurs depends on the position of the Fermi level, which can be varied in the present sample structure by changing the intensity of the optical excitation.

Bistable operation of resonant-tunnelling phototransistors incorporating a multiple quantum well base (RTQWPT) is reported. The mechanism of operation is resonant-tunnelling hot-electron injection controlled by electron photoemission from the multiple quantum well base into the collector. The RTQWPT utilizes intersubband optical absorption in the infrared region of the spectrum. The RTQWPT can be used as a high-efficiency optoelectronic switch controlled by infrared radiation and an infrared photodetector.

Charge injection transistors have been implemented in molecular beam epitaxy grown InGaAs/InAlAs/InGaAs and InGaAs/InP/InGaAs heterostructures using a self-aligned process for the collector stripe definition. Scattering parameters have been measured in the frequency range from 100 MHz to 40 GHz. InP barrier devices show the best microwave performance ever reported for a real-space transfer transistor: at 40 GHz the short circuit current gain mod h₂₁ mod is 8 dB and the power gain is larger than unity. The slope of mod h₂₁(f) mod depends on the bias point and is generally gentler than 20 dB decade^-1. Extrapolating at the measured slope, we find mod h₂₁ mod =1 at f=115 GHz. The short circuit current gain cut-off f_T, defined by extrapolation at 20 dB decade^-1 from the point of least mean square deviation of the measured slope from 20 dB decade^-1, is f_T=73 GHz. Devices with InA/As barriers show a relatively slower performance (f_T=32 GHz). The difference is discussed in terms of the relative rates of intervalley scattering and real-space transfer in the two heterostructures.

The current flowing through a quantum point contact defined in the plane of a two-dimensional electron gas does not saturate under high source-drain bias voltages as expected from simple theoretical considerations. We propose an analytical model which explains this observation in terms of the increased current contribution from normally non-conducting subbands under high source-drain bias. Furthermore, the observed current-voltage characteristics are asymmetric with respect to changing the polarity of the bias voltage. We explain this effect by considering the influence of the bias voltage on the effective point contact geometry. A further consequence of this model is the prediction of negative differential resistance under conditions of high positive forward bias in very narrow channels.

The formation of Schottky contacts on methane/hydrogen reactive-ion-etched (RIE) n⁺In_0.52Al_0.48As surfaces has been investigated for the first time. Two metallization systems (Ti/W/Au and Pt/Au) were examined. The ion etching-induced damage in the InAlAs surface layer was evaluated by means of I-V and C-V characterization. Rapid thermal annealing (RTA) at several temperatures was applied for damage removal. Dry-etched Schottky contacts were compared with wet-etched ones. Using the optimized condition of RTA, parameters achieved on RIE-based Schottky contacts were in agreement with theoretical predictions and comparable with thermally processed wet-based ones. The role of the thermal treatment in the final determination of metal-semiconductor interface and contact properties is discussed in detail.

The effect of cap layer thickness on residual strain in partially relaxed, 250 AA thick In_0.2Ga_0.8As/GaAs single quantum well structures was examined by photoluminescence, transmission electron microscopy and high-resolution x-ray diffraction. When the cap layer thickness increased from 50 to 5000 AA, the residual strain increased from 0.88% to the misfit value of 1.4%. As a consequence the cap layer thickness should be taken as a parameter, along with misfit and growth temperature, to determine critical layer thickness in lattice-mismatched single quantum well structures.

A simple and economical electroless chemical deposition technique for deposition of several metal sulphides and selenides is presented. Aqueous solutions of metal salts and of sodium thiosulphate or sodium selenosulphate were used for the chemical bath. The technique is based on hydrolytic decomposition of the various metal-thiosulphate or metal-selenosulphate complexes in aqueous media, at a suitable temperature, concentration and pH. The technique presented in this paper has been successfully used for deposition of Cu₂S, CuS, PbS, Sb₂S₃ and Sb_2-xBi_xS₃ in acidic media, and Ag₂S, Cu₂Se and PbSe in alkaline media, on various substrates, such as glass, metal, plastics, alumina, silicon and sapphire. Depending on the deposition time and the individual system, films of thicknesses up to 0.3 mu m were obtained from a single bath. The prospects of this technique are very good for multilayer combinations, solid solutions, and possibly ternary compounds as well. This technique was found to be suitable for any size and shape of substrate. It is a non-polluting and economical technique, since the bulk precipitates are of a relatively high purity and can be used as semiconducting powders, mineral pigments etc. The optimal conditions for deposition of each kind of film are described and the corresponding X-ray diffraction patterns are presented. Optical spectra in the wavelength interval between 300 and 2500 nm for each of the films are also supplied. The sheet resistivity and type of conductivity for each kind of film have also been determined.

The structural and optical properties of RF magnetron sputtered ITO films without in situ substrate heating and post-deposition annealing are investigated. The structure and orientation of ITO films strongly depends on the energy of the sputtered particles arriving at the substrate. As the sputtering power increases from 15 to 100 W, the preferred orientation of the ITO film changes from (222) to (400). It is observed that the lattice constant of the film increases initially as sputtering power increases from 15 to 50 W and then decreases with further increase of sputtering power. The higher the sputtering power, the smaller the oxygen content of the film. The oxygen deficiency is used to explain the loss of transmittance or blackening of ITO films deposited at high sputtering power. Several material parameters, such as optical constants and energy gap, have been derived and discussed. The figure of merit of the films ranges from 81.3 to 117.6 Omega ^-1.

In multilevel interconnection processes, because thermal stress affects device characteristics and wiring reliability, low-temperature deposition has been required for interlayer dielectrics (SiO₂). This research investigated high growth rate and selective growth conditions of room-temperature interlayer dielectrics formed using the liquid-phase deposition method. The dependence of deposition rate on growth temperature and H₃BO₃ concentration is confirmed, and a concentration of 3.8 mol 1^-1 of H₂SiF₆ is used to achieve a high deposition rate of 1250 AA h^-1. The degree of supersaturation of silica of the immersing solution is found to define three ranges of growth conditions: the non-deposition range, selective deposition range, and conformal deposition range. A selective deposition model is proposed to clarify the mechanism of room-temperature selective SiO₂ growth.

The optical and electrical properties of CulnSe₂ (CIS) have shown it to be a promising photovoltaic material. The preparation of CulnSe₂, thin films by means of the stacked elemental layer (SEL) technique was developed at Newcastle Photovoltaic Centre, Northumbria University. The method is presented as a low-cost technology because of its ability to retain stoichiometry and its scalability for large-area device applications. This paper presents a modification of the technique employed to produce CulnSe₂ thin films and describes the structure, optical and electrical properties of such films. X-ray diffraction examination, energy dispersive x-ray analysis and measurements of the optical transmission and reflection and resistivity were used to determine the film properties relative to the preparation parameters and stoichiometry, The growth conditions were optimized for solar cell applications. Electrical resistivities in the range 10^-2 to 10⁵ Omega cm, energy gaps between 0.96 and 1.03 eV, and an absorption coefficient close to 10⁵ for the CIS films were obtained in this work.

The solar cell structure is based upon a heterojunction of p-type CulnSe₂ (CIS) and n-type CdS or mixed CdZnS. The use of a mixed CdZnS layer is reported to increase the open circuit voltage. The cells have been fabricated using the stacked elemental layer (SEL) technique for the fabrication of CIS thin films. The effect of layer resistivities on the performance of the device has been studied. The photoresponse was also determined and an essentially flat region between 0.6 and 1.0 mu m was found. Dark and light I-V characteristics in ambient and different solar cell temperatures have been investigated. Capacitance-voltage-frequency relationships were determined. A photovoltaic effect was observed with an ELH illumination (85 mW cm^-2). Conversion efficiencies up to 5.2% were obtained. The results are discussed and recommendations for future work are suggested.

We calculate by numerical integration the energy of a strained layer in an infinite isotropic continuous elastic medium with and without a misfit dislocation dipole of pure edge character. The equilibrium critical thickness is the thickness at which these energies are the same. Solving for a range of values of misfit strain, we compare the results with some versions of the existing approximate models, and we are thereby able to indicate the best analytic expressions to use to predict equilibrium critical thickness.

Direction-dependent transition matrix elements are calculated as a function of strain and superlattice ordering for GaInP using a 6*6 LK Hamiltonian. The predicted direction dependence is in excellent agreement with the measured polarization of laser emission from broad-area strained GaInP QW lasers. Lasers with cavities oriented along the (011) direction lase with a polarization direction tilted towards the ordering planes. Lasers with (011)-oriented stripes and disordered structures emit with the conventional TE or TM slab-waveguide polarization normally expected for strained-layer lasers.

Boron-induced strain in Si epilayers was characterized by using high-resolution x-ray reciprocal lattice mapping. Samples grown by molecular beam epitaxy with high crystalline quality and full electrical activation of dopants with concentrations from 2*10¹⁹ cm^-3 up to approximately=1*10²¹ cm^-3 were employed, with measured lattice mismatches up to approximately=0.5% (tetragonal strain approximately=1%). The measured lattice contraction coefficient, beta , was (5.6+or-0.1)*10^-24 cm³/atom, by considering the ratio of the lattice mismatch to the total dopant concentration. However, by considering only the effectively incorporated and activated dopant concentration, given by the measured carrier concentration, beta was found to be (6.3+or-0.1)*10^-24 cm³/atom. Further measurements on B-doped structures grown at high temperatures (>or=600 degrees C), which had only partial electrical activation (<20%), confirmed that the B-induced lattice contraction in Si is related not to the total dopant concentration, but rather to the carrier concentration, which in principle corresponds to the active and substitutionally incorporated dopants. The high accuracy of our strain measurements was facilitated by the reciprocal lattice mapping method. In-plane and normal components (relative to the sample surface) of the lattice parameter of the epilayers were measured independently, and minute variations in the strain/relaxation status of the structures were accounted for.

A high-performance lateral p-n photodiode in LPCVD polysilicon has been successfully fabricated and characterized. The device is shown to have a response time of 30 ps, i.e. a 3 dB cut-off frequency of 5.3 GHz, which we believe is the fastest speed reported in polysilicon. The measured device speed is analysed quantitatively on the basis of the recombination-dominated-response model. In so doing, an analytic expression for the impulse response function of such devices is given here for the first time. The general features of the device and its possible applications in optoelectronic integrated circuits are also discussed.