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The authors present the results of low-temperature transport measurements on 'metallic' Si:Sb doping layers with nominal widths of 10 nm, 20 nm and 80 nm. There is clear evidence of weak localization and interaction corrections to the transport coefficients and for a 3D to 2D transition as the layer width decreases-the first observation of these effects in this system. Of particular interest is the fact that the experimental results are in good quantitative agreement with most aspects of the theory, despite the highly disordered and multi-sub-band nature of the samples.

The maximum entropy mobility spectrum analysis (ME-MSA) method has been developed on the basis of a maximum entropy (ME) principle. It allows one to calculate a smooth electrical conductivity versus mobility plot ("mobility spectrum") from the classical magnetoconductivity tensor in semiconductor multilayer structures. The advantages of the ME-MSA as compared to the MSA are demonstrated using a synthetic data set.

The effects of localized electron irradiation of the (111) surface of PbTe are described. These studies show that fast surface diffusion processes considerably extend the lateral range of electron induced effects. The modifications that occurred over a relatively large surface area were compositional, in doping levels in the underlying material and in oxygen sorption mechanisms. The irradiated surface effectively passivated the underlying material against much of its normally strong interactions with oxygen which involve conduction and valence band electrons. A model is set up to account for these observations.

Measurements of the Hall coefficients and of the resistivity of MBE-grown Si, doped with P, As, Sb, B, and Ga in the concentration range 1014 to 1020 cm−3, were carried out at 77 K and at 300 K. With the exception of Ga-doped Si, the measured mobilities were close to or higher than those of bulk materials at both temperatures. The Mott metal/non-metal transition has been observed in the present epitaxial materials and the measured values for the critical impurity concentration at which the transition occurs, agree with values reported by other workers for bulk silicon.

Describes how an argon ion gun intended for sputter cleaning, and a quadrupole residual gas analyser (QRGA) have been adapted for use in low current, low energy secondary ion mass spectroscopy (SIMS). Minor external modifications including the fitting of a new type of axial electrostatic energy filter in front of the QRGA make secondary ion detection feasible. The spectrometer may be used for surface analysis; secondary ion spectra are presented from stainless steel type 316, and from mica and sapphire.

Investigations are reported into the effect of 350 eV He+, Ar+ and Xe+ bombardment of PbTe. The studies were made on surfaces of thin films prepared by molecular beam deposition under UHV conditions. The modifications to the films were monitored by measurements of the films’ carrier transport properties. Bombardment‐induced areal densities of carriers (electrons) up to 6×1013 cm−2 with near‐bulk mobilities were observed for ion fluences up 1016 ions/cm2. The results are discussed in terms of preferential sputtering of Te in combination with beam‐induced mixing. It is concluded that the induced damage is located very close to the film surface. Whereas in the case of Ar+ and Xe+ bombardment the altered layer appears to extend over a depth commensurate with the ion range, the results for He+ bombardment suggest that only the very surface is depleted of Te. There is evidence that ion‐induced desorption of weakly bound surface species contributes significantly to the observed changes in the areal density of ca...

The effects on the electrical properties and surface composition of molecular-beam-deposited PbTe films to exposure by oxygen and monatomic hydrogen were investigated. Three stages of oxidation have been identified, depending on the pressure regime employed. Characteristic differences in the response of each of the three oxides to thermal annealing, electron irradiation during Auger analysis and to monatomic hydrogen exposure were observed. The kinetics of n-type doping by monatomic hydrogen, and its reversibility under ultra-high vacuum and oxygen-exposure conditions are presented.

By measurement of ion and sputter yields from thermally grown SiO2 and single crystal Si it has been possible to quantify 11B range profiles measured by secondary‐ion mass spectrometry from device‐type silicon on sapphire wafers implanted through a gate oxide. Contrary to theoretical predictions, no step in 11B concentration is observed at the SiO2/Si interface after calibration. Measured ion ranges in SiO2 and Si agree well with those of other works and with theoretical predictions by Johnson and Gibbons and Biersack. Ranges in SiO2 are found to be around 20% greater than those calculated by Smith. Under 4‐keV 16O2+ primary‐ion bombardment, the relative yields (SiO2/Si) are found to be the following: sputtering, 2.05; 11B+ ion yield, 2.25; degree of 11B+ ionization, 1.10. The oxygen primary beam is apparently inert in the SiO2, but reacts with the Si to form a mix SiOm where m=1.95 under steady‐state conditions.

A study of the silicon doping of MBE (molecular beam epitaxy) (100) GaAs is presented. The effects of the various growth parameters on the electrical properties of the layers have been investigated. In particular the influence of the As4:Ga flux ratio on the Si incorporation is considered. A model based on the site occupancy of the Si dopant and an As-induced defect Si-impurity complex is invoked to account for the observed electrical properties.

Potential‐enhanced doping (PED) is a method by which the incorporation efficiencies of certain low sticking coefficient, coevaporated dopants can be enhanced during Si‐MBE. PED is implemented by application of a potential to the substrate during deposition. The applied potential is used to control instantaneous dopant incorporation, thereby permitting accurate control over doping levels over a wide dynamic range, as well as offering a powerful method of profile control. PED is especially effective with Sb doping, with enhancements by up to 1000 times being realized, thus permitting doping levels up to the solubility limit to be readily achieved. PED is less efficacious in the case of As doping obtained from III–V compound sources. The electrical, optical, and crystallographic quality of PED:Sb‐doped material is comparable to that of bulk Si. Doping transitions of < 50 A/decade have been obtained for transitions over several decades.

A study of the electrical properties of InAs heteroepitaxial layers grown on (100) GaAs and their dependence upon growth parameters has been undertaken. InAs films 1 mu m thick were grown by molecular beam deposition at a substrate temperature of 370 degrees C. Layers with residual n-type carrier concentrations in the range 5*1016-2*1018 cm-3 at room temperature were obtained. These mobile carriers in arise from both bulk dopants and interface effects. It has been observed that the electrical properties of the InAs epilayers are critically dependent upon the incident indium and arsenic fluxes. High values of In and As4 fluxes produced residual bulk n-type doping levels in the 1018 cm-3 range and caused additional scattering. The lowest carrier concentrations and highest mobilities were obtained for the lowest value of indium flux used (1*1014 atoms cm-2 s-1) and with the smallest arsenic flux consistent with the deposition of stoichiometric layers. Depth profile measurements on the best films indicated that the residual carriers primarily arose from donor centres situated at the film-substrate interface. The material remote from this interface had bulk-like mobilities and near-intrinsic carrier concentrations. The use of a 0.1 mu m GaAs buffer layer isolated the films from a further source of donor centres. The presence of gaseous oxygen during film growth has been found to degrade film electrical properties. No effects were observed when hydrogen was present.

An investigation of the growth of heteroepitaxial InAs by MBE is reported. The surface morphology and electrical properties are shown to be critically dependent on growth parameters and the conditions necessary to obtain good material quality are deduced. Analysis of the thickness dependence of the electrical properties of undoped, and Si- or Te-doped InAs shows that interfacial effects contribute to the measured properties. Material remote from the interfacial region compares favourably with VPE- and bulkgrown InAs.

Investigations are reported into the effect of low-pressure oxygen exposure and thermal annealing on the carrier transport properties of native and 350 eV Ar+ bombarded PbTe films. The electrical measurements were madein situ on MBD-grown PbTe films without breaking vacuum. On native surfaces, oxidation was initially sustained by diffusion of a donor species from the film bulk to the surface, where reaction with oxygen occured. This diffusion process was apparently inhibited on ion irradiated films and direct doping of the film surface effected a gradual reduction in the ion-induced electron accumulation resident at the film surface. The native properties and behavioural characteristics of the films could be recovered by thermal annealing of the ion-irradiated and/or oxidized films at 300–350 °C.

The surface tension of liquid xenon was measured in the temperature range 189°—286°K by a capillary‐rise method. The results may be represented by γ=γ0[1 − (T/289.74) ]p, with γ0=54.6±0.1 dyn cm−1 and p=1.287±0.017.The experimental method is discussed and the results compared with existing theory and experimental data.

A method is described for determining self and impurity diffusion rates in high vapour pressure solids at cryogenic temperatures. It is based on mass spectrometer analysis of the concentration of a tracer in the vapour in equilibrium with the solid. The major source of possible error lies in determining the effective diffusion surface area of the crystal. In the case of solid argon it is estimated that the diffusion coefficient is obtained to ±3%.

Phosphorus-doped Si epilayers with bulk-like mobilities were grown by molecular beam epitaxy (Si-MBE) by coevaporation of phosphorus from a tin phosphide source. The behaviour of P doping as a function of growth parameters and of “potential enhanced doping” indicates a non-unity, almost growth-temperature independent incorporation efficiency with negligible surface segregation -a unique combination among coevaporated dopants in Si-MBE.

The feasibility of using silicon diodes as the temperature sensors for thermoelectric power measurements below room temperature has been established. The advantages and disadvantages of diodes as compared to thermocouples for this purpose are discussed.

Thin heteroepitaxial layers of InAs have been grown on (100) GaAs substrates and buffer layers by molecular beam deposition in a UHV chamber. Smooth epitaxial layers as indicated by in situ MEED patterns have been obtained at substrate temperatures in the range 330–530 °C and SEM micrographs and electron channeling patterns taken ex situ have confirmed these properties. MEED observations also indicated that epitaxial growth can be obtained at considerably lower deposition temperatures. InAs films, 0.5–2.0 μm thick, grown between 330 and 530 °C had bulklike electrical properties with residual n‐type carrier concentrations in the range 5×1016 to 1×1018 cm−3, the lowest values occurring for depositions at 530 °C.

The presence of parts per million (ppm) levels of impurities in the low-temperature arsenic charge used in molecular beam epitaxy (MBE) is shown to have a dramatic effect on the quality of GaAs grown. Source charge impurities play a vital role in determining the apparent variation in reported doping behaviour in MBE:GaAs.

Recent work has indicated that residual contamination on III‐V substrates has an adverse effect on epilayer nucleation. This letter presents preliminary results of measurements of contamination levels on methanol‐bromine polished ion‐cleaned (100) InP substrates using low [primary ion dose (

Molecular beam epitaxy has recently been developed from surface physics experiments to a device fabrication technology. The technique, its applications and potential are outlined in this article.

Semiconductor devices continue to shrink, with an immediate prospect of spatial dimensions ∼1µm laterally and

To conduct a SIMS depth profile at constant sensitivity, a fixed fraction of the secondary ion energy distribution should fall within the bandpass of the instrument. Thus, the crater surface potential, V(x,y,t), should not vary by more than a few volts. This condition is unlikely to be met when profiling resistive multilayer structures,and may sometimes be impossible to achieve using fixed instrumental conditions. Thus the use of charge compensation techniques (electron beam flooding, evaporation of conducting films) must be accompanied by sensitive procedures for monitoring the surface potential throughout the profile and for quantifying the change in instrumental sensitivity associated with any charging. It may then be possible to restore the surface potential to its required value by applying a DC offset to the sample holder, provided the charging is uniform (V→V+c). This article describes such monitoring procedures with reference to two systems, BF2 and As implants in silicon-on-sapphire (SOS) epilayers, in which small changes in surface potential can lead to profile distortions.

The SLAC Electron Trajectory Program [1] is capable of tracing ion trajectories in cylindrical or linear symmetry, with or without space charge, and in both magnetic and electrostatic fields. We have applied the program in design studies of various ion-optical components for a new Quadrupole SIMS system (EVA 3000). Amongst the topics studied have been the design of the secondary ion transfer optics, the extraction of ions from a plasma source, and the design of the primary ion column final lens. Space limitations confine us to the latter topic here. Much electrostatic lens data, obtained by other means, has been published (eg [2]). Ray tracing a specific structure gives valuable extra information, eg. non-paraxial ray behaviour, space-charge effects. We have modified the SLAG program to allow computation of cardinal points and spherical aberration. Data are reported here for the symmetric and asymmetric einzel lenses shown in Fig 1. The proportions have been chosen so that their powers are similar, and so that practical implementations will fit into the same space. The lenses are intended to focus an ion beam of perveance 3×10−6 µpery (eg. 3µa at 10keV) into a small spot at a distance of W=40mm or less (the working distance) from the reference plane (LA in the centre of the accelerating gap — dotted line in Fig. 1). Voltages V1 and V2 are measured from a point (eg. the plasma) where the ions have zero energy. Vr values > 1 (where Vr =V­2/V1) give better theoretical performance because trajectories remain closer to the axis. Often, however, practical focussing voltages are excessive (~ −30kV for l0keV +ve ions) but there may be advantages for ion energies ≤ 2keV. For Vr < 0.15, multiple crossovers occur inside the lenses as the mirror condition israpproached. This study is confined to 0.15≤Vr≤0.5.

Silicon-on-Sapphire (SOS) is the most advanced silicon-on-insulator technology for CMOS-VLSI. We have described techniques for extracting accurate boron profiles from SOS [1] and, elsewhere in these proceedings, methods for obtaining accurate depth profiles from resistive multi-layer structures like SOS [2]. The substrates used for SOS form a potential source of aluminium doping which could lead to poor mobilities and unpredictable device behaviour. Many of the processing techniques for SOS (especially self-implantation and regrowth [3]) are very likely to remove Al atoms from the substrate and redistribute them in the silicon. This SIMS study of the behaviour of Al in SOS is part of a programme to define acceptable processing schedules. Three problems may be anticipated: Firstly, we wish to obtain accurate epi-layer and interface profiles from a resistive structure on an insulator. Secondly, we require quantification from impurity (i.e. < 1%) to matrix concentrations, across two different matrices, so calibration standards must be chosen with care, and the interpretation at the interface may be semi-quantitative. Thirdly, the Al+ signal is only 1 amu away from the intense Si+ matrix emission. Thus, the dynamic range will be dependenI on the peak-shape of the mass spectrometer, and the relative ion yields of Al+ and Si+.

The problems of the thermal loading of cryopumps during bakeout of UHV systems are considered. A simple design of thermal baffle is presented which permits baking at temperatures of 200 °C without compromising the high speed of the pump during normal system operation.

A simple, low‐cost, and easily‐manufactured evaporation furnace, suitable for MBE, is described. The essential feature of the design is a composite heater‐shroud and evaporation cell, machined from high‐purity graphite. Good‐quality silicon‐doped gallium arsenide has been grown using this cell design.

A combined SIMS‐MBE UHV system has been used for growth and subsequent analysis of differently reconstructed (001)GaAs surfaces. Some small, but significant, differences are found in SIMS spectra from the different surfaces, indicating that the (2×4)As stabilized surface commonly used for MBE growth may be more ‘‘bulklike’’ than other reconstructions. Spectra obtained after oxygen exposure indicate involvement of both Ga and As at the initial stages of oxidation, and the different surface reconstructions showed very similar response to oxygen exposure. These SIMS analyses do not indicate large differences in either the surface composition or chemistry of differently reconstructed surfaces.

Photoluminescence spectra are used to investigate inadvertent impurities in Si molecular beam epitaxy (MBE) layers. The spectra show free exciton and condensed exciton (electron‐hole drop) features which are indicative of good crystalline quality. Electrically active shallow impurities are identified from near‐gap bound exciton features. In one layer radiative recombination at dislocations is dominant. For the first time using an optical technique carbon is shown to be a persistent impurity in Si MBE material. This technique provides a test for inadvertent carbon incorporation at levels which may otherwise be difficult to detect in thin epitaxial layers.

Inefficient doping and poor profile control are generally associated with the use of coevaporated dopants in Si–MBE. However, two new techniques have been developed, one applicable to p‐type and the other to n‐type doping, which alleviate these problems. Coevaporated boron ( p‐type) is shown to have a simple incorporation mechanism by which good doping and profile control can be achieved using the conventional MBE technique of changing the source cell temperature. For n‐type doping, a method of enhancing the low incorporation efficiency of Sb has been found (potential enhanced doping) which is also ideally suited to profile control. Enhancements by a factor of up to 1000 times have been achieved. Material with bulklike mobilities has been obtained over the range of 1×1015 to 8×1019 cm−3 for p‐type and up to 3×1019 cm−3 for n‐type doping. Extremely sharp doping transitions down to