Table of contents

An alternative doping process to ion implantation for the production of very shallow p⁺-n junctions has been investigated. n-type silicon wafers covered with a boron-doped silicon oxide were submitted to thermal cycles with incoherent light radiation at temperatures between 900 and 1250 degrees C for times ranging from 20 to 120 s, in order to promote boron in-diffusion. The samples were characterised by sheet resistance measurements combined with sequential layer removal to obtain the carrier concentration-depth profiles. The results show the formation of a boron-rich surface layer above the normal boron solubility levels and a linear dependence of the sheet conductance on diffusion time, which suggests a chemical reaction at the silicon surface is the controlling factor for the boron in-diffusion. Moreover it was demonstrated that defect-free p⁺-n junctions with a junction depth as shallow as 50 nm are readily obtained.

A band of disorder is observed following the annealing of surface amorphous layers produced by ion bombardment. The depth of the band's centre is just below the original amorphous/crystalline interface. The ion bombardment simulation computer program TRIM.CASCADE has been used in conjunction with ion channelling measurements to predict the areal density of atoms available for the formation of this band of disorder. The variation in the amounts of disorder expected due to changing the implantation parameters: ion dose, ion energy and implantation temperature are reported. The predictions are compared with the published transmission electron microscopy results and electrical measurements of other workers. The implications of this work for the technique of pre-amorphisation are also discussed.

The statistics of random fluctuations in the concentrations of donors and acceptors is applied to determine the mean position of the Fermi level and its variance via the concept of a fundamental volume whose magnitude is determined by the screening length. It is shown that the fluctuations are strongest at a carrier concentration corresponding to the onset of degeneracy. For a given carrier concentration it is shown that the electrical effect of fluctuations increases with compensation ratio and effectively converts the material into a two-component system, one component being normally doped, the other being intrinsic. When this happens the RMS Fermi level fluctuation rises and it is pointed out that this provides a measure of the effective barrier height associated with photovoltaic effects. The theory is illustrated for the case of GaAs at 300 K.

The influence of dislocations on the electrical properties of 1" partially dislocation-free LEC GaAs has been studied as a function of the fraction of melt solidified (g). Microscopic electrical characterisation was performed using spreading resistance and anodisation imaging techniques, and Hall measurements at 294-400 K were employed for macroscopic characterisation. The zero-dislocation-density seed end regions are microscopically uniform with consistently high mobility, resistivity and thermal activation energy. However, dislocated regions are microscopically non-uniform with lower mobilities and activation energies. At g>0.4, carrier concentrations increase due to the presence of a shallow donor, possibly at E_c-0.42 eV, at the dislocations, and this causes large microscopic fluctuations in resistivity. Radial dependences of the electrical parameters vary with dislocation distribution and sub-structure along the crystals, with the mobility having an 'M' shaped radial variation at g=0.5 where dislocations lie in slip bands and a 'W' shape at g approximately 0.97 where the resistivity is low and dislocations form cellular networks. Good correlation of electrical inhomogeneity with slip bands, cell structure and lineage on a microscopic scale is obtained.

The authors have obtained evidence from galvanomagnetic and PL measurements that the electron mobility compensation ratio relationship of Taguchi and Yamada (1987) provides more realistic values of N_A/N_D than Walukiewicz and co-workers earlier relationship (1980). They also point out the possible use of FIR photosignal decay measurements as a means of determining N_A and N_D, particularly in the difficult case of low-compensation materials.

Examines the Hall angle ( theta ) dependence of geometrical magnetoresistance, with emphasis on the theta to 90 degrees limit. A comparison of the four cases mentioned in the title benefits from the use of alternative notations for the two kinds of limits, and from the basic idea that the Corbino disc (CD) and the Hall bar (HB) have two and one theta -dependent effective dimensions, respectively. One of these dependences is cancelled under QHE conditions, relative to the behaviour in the traditional high-magnetic-field limit. As theta to 90 degrees , this produces no qualitative changes in the CD, but a fundamental difference, with useful experimental consequences, in the HB. The alternative notation also provides a clear explanation for some unusual characteristics of the CD under QHE conditions, namely a finite current density with zero current, and a zero resistivity with infinite resistance. The paper also includes some clarifying remarks on the relationships between the conductivity and resistivity tensors in the two kinds of limits, and on the classical and quantum-mechanical origins of a number of phenomena which ultimately make it possible to achieve highly accurate QHE measurements.

High-quality homo- and hetero-epitaxial GaSb has been grown from TMGa and TMSb using atmospheric pressure metal-organic vapour-phase epitaxy (MOVPE). Unintentionally doped material was p-type. At 295 K it had a carrier concentration of around 3.0*10¹⁶ cm^-3 and a corresponding Hall mobility in the range of 670 to 1000 cm² V^-1 s^-1. Growth parameters were carefully correlated with surface morphology, electrical quality and photoluminescence data to establish the growth window for this material.

The effects of interface states upon the electrical properties of metal-semiconductor diodes are investigated with the purpose of casting some light on current interpretations of capacitance and conductance measurements. The predominance of majority-carrier mechanisms in the ideal diode is quantitatively reassessed. Several kinds of interface state are considered, accounting as well for states in equilibrium with the semiconductor as for states in equilibrium with the metal. Electrostatic and kinetic effects lead to three types of admittance that are evaluated and compared under a small signal voltage superimposed on a forward bias. The major contribution arises from the reaction of interface charge variations with the diode conductance and depletion width. Corresponding conductances and capacitances are analysed as a function of the energy distribution of states, bias voltage, frequency and temperature. Densities about 10¹³ eV^-1 cm^-2 or higher and the existence of several types of states lead to non-linear effects as a function of the density of states and even to negative differential capacitance when recombining centres are concerned. Some fundamental and technological implications are discussed.

The operation of a special GaAs-MESFET (metal-semiconductor field-effect transistor), originally proposed by M. Pepper (1977) for studies of low dimensional transport at low temperatures, is analysed in terms of a modified oscillator model designed for the quantum limit (narrow channels, low temperatures). The relation between the electron concentration and the gate voltage is calculated in the quantum limit and compared with the results of the bare oscillator model and the abrupt depletion model. The sublevels and the depopulation of sub-bands in the case of an applied magnetic field are calculated and compared with experiments. The major advantage of this model is the simple analytical structure of the measurable quantities.

The authors calculated self-consistently the electronic properties of Si-NIPI doping superlattices with general designed parameters, using the method of anisotropic effective masses and expansion of plane waves, in the frame-work of the local density functional approximation. It is shown that the overlap between the functions of electrons and holes is very small, which means that the recombination lifetime between electrons and holes is very long. In their calculations, they found that the self-consistent results are almost the same as the approximative results of Wang and Huang (1988) when d=2d_n, but for d>>d_n self-consistent results are 10 meV higher than previous approximative results. So they conclude that the zigzag-shaped potential approximation is applicable only when d>>d_n.

In GaAs/Al_xGa_1-xAs superlattices, GaAs and AlGaAs epitaxial layers, the results of photoluminescence excitation spectroscopy are shown to depend strongly on carrier mobilities when, at a given distance from the surface, an escape mechanism exists, i.e. a possibility for carriers to escape from the active layer after moving through it. When there is no escape mechanism close enough to the surface, excitation spectra are roughly flat apart from the excitonic resonances. When an escape mechanism is present at a distance of around 1 mu m from the surface, if the mobility is high enough as in GaAs and superlattices with periods smaller than about 70 AA (for x approximately 0.3 and nearly equal well and barrier widths), the excitation spectra exhibit a decreasing high-energy tail; on the contrary in AlGaAs and superlattices with greater periods the excitation spectra have the same shape as in similar samples without an escape mechanism. Superlattices with small periods constitute better structures than equivalent AlGaAs layers for device applications.

Infrared absorption measurements have been made of the localised vibrational modes (LVM) of ⁹Be_Ga acceptors in GaAs grown by MBE for as-grown, electron-irradiated and hydrogen-passivated samples. The dipole moment has been determined for the Be⁰ line at 482.1 cm^-1. The strength of this line decreased during electron irradiation and it is inferred that Be impurities form complexes with intrinsic defects. In passivated samples H-Be complexes form and stretching modes from H and D are found at 2037 and 1471 cm^-1 respectively. Surprisingly the width of the former line is some three times greater than that of the latter. Only one mode of the complexes Be has been found at 555.7 cm^-1. The hydrogen is believed to occupy a bond centre position between the Be atom and a nearest neighbour As atom. A comparison is made with H-Zn complexes.

A photoluminescence study at low temperatures of Si-doped Al_xGa_1-xAs (0<x<0.6) reveals the existence of four donor levels D₁, D₂, D₃, D₄, in order of increasing ionisation depth. The number of donor levels and the dependence of their ionisation energies on the composition parameter x accords qualitatively with theoretical expectations. D₁ can be identified with the common 'shallow donor'. D₂ and D₃ have been reported in an earlier PL study by Dingle and co-workers (1977). The position of the D₄ level suggests an identification with the DX centre. Its luminescence properties have been the subject of a previous publication.

Photoluminescence studies of Ga_1-xIn_xAs/Al_1-yIn_yAs quantum wells nominally lattice matched to InP are reported. The photoluminescence from two samples each having six quantum wells of thicknesses ranging from about 6 to 122 AA is studied in detail. The main emission peak associated with each quantum well is attributed to excitonic recombination. The emission from the 6 AA quantum well at 897.3 nm is found to have a peak width of only 18 meV at 4.2 K. Temperature dependence studies show that at 4.2 K the excitons are localised at potential fluctuations in the quantum wells. Impurity-related emission is also observed from some of the quantum wells.

An improved Czochralski method with the alternate rotation of a seed crystal was developed in order to homogenise impurities doped in InSb single crystals. When the crystal was pulled under one-directional rotation, the fluctuation width of the growth rate increased as the crystal periphery was approached from the centre, and rotational striations were formed. Under alternative rotation of the angle range of 18 degrees and 90 degrees , the fluctuation width of the growth rate was decreased to 15 and 70% in comparison with that of the one-directional rotation, respectively. The fluctuation of the growth rate then became small as the angle of alternate rotation became narrow. Thus, in a crystal pulled under 18 degrees alternate rotation, rotational striations were excluded and moreover the fluctuation of spreading resistance was also decreased from 22 to 15 Omega .

Photodiodes have been formed by ion implantation into epitaxial mercury cadmium telluride grown by MOCVD on 2 degrees (100) CdTe substrates. The diodes exhibit R₀A products up to 7*10⁴ Omega cm² with 5.3 mu m cut-off at 77 K, and quantum efficiencies of 88% when anti-reflection coated. This is believed to be the first report of photovoltaic device performance in this waveband resulting from ion implantation of MOCVD-grown mercury cadmium telluride, and shows promise for the exploitation of this growth technique in the production of infrared image sensors.

A thin layer of dye sandwiched between p-CuCNS deposited on a copper substrate and n-SnO₂ coated transparent glass was found to yield photocurrents resulting from light absorption in the dye. The mechanism of photocurrent generation is discussed.

A new technique, photonuclear transmutation doping (PTD), which can be used to produce p-type silicon by nuclear transmutation, is presented. Aluminium acceptors are produced in silicon by photoproton and photoneutron nuclear reactions on the silicon isotope ²⁸Si. The technique can be used to produce easily controllable uniform dopant densities. The PTD technique can also be used to dope other semiconductor materials. In an experiment, wafers of silicon were irradiated using 25 MeV bremsstrahlung on the Harwell electron linear accelerator to induce ²⁸Si( gamma , p)²⁷Al and ²⁸Si( gamma , n)²⁷Si-^{beta +} to ²⁷ Al transmutations. Analysis, including photoluminescence measurements, showed that electrically active aluminium acceptors were produced in the concentrations predicted.

The authors consider electronic transport in a multiple-quantum-well structure (MQW). The transport is described in terms of a superposition of various transport paths each associated with a given length (coherence length as determined by energy relaxation). Their starting point is the simple double-barrier situation, and the sequential tunnelling approximation. They estimate the elastic and inelastic transfer rates including electron-phonon, electron-electron, and impurity-assisted processes. They find that in most cases one should expect defects in the barrier and impurities to be the reason for the absence of marked non-linearities in the I-V characteristics.

Negative results from two kinds of experiments were obtained: the lack of any change in the DLTS peak of EL2 after annealing a GaAs Schottky diode at 180 degrees C under reverse bias conditions, and the lack of any deformation-induced persistent dichroism in the quenching rate of photocapacitance of the Schottky diode. These results are in conflict with earlier findings reported by two research groups and they question the belief that the highly mobile As_i interstitial is one of the two components of EL2.

In electron inversion layers on (100) Si MOS structures with laterally periodic oxide modulation plasmons are investigated with far-infrared transmission experiments. In such structures an applied gate voltage causes a static inversion charge density modulation which in turn gives rise to mini-gaps in the plasmon dispersion relation. The authors investigate the lowest mini-gap at plasmon wavevectors near the zone boundary of the first mini-zone induced by the periodic charge density modulation. The experimental results are compared with perturbation theory.