W. S. Truscott

Insitu, near‐ideal epitaxial Al/AlxGa1−xAs Schottky barriers formed by molecular beam epitaxy

The Schottky barrier height of in situ epitaxial aluminum on AlxGa1−xAs was measured as a function of aluminum mole fraction from x=0 to x=1, using I/V, C/V and activation energy plots of current‐voltage dependence on temperature. The excellent electrical properties of the molecular beam epitaxy grown AlGaAs layers, with residual deep levels concentrations of less than 1014 cm−3 combined with the in situ deposition of single‐crystal epitaxial aluminum resulted in extremely high quality Schottky diodes from x=0 (GaAs) to x=1 (AlAs) with accurately exponential current‐voltage characteristics over up to 10 decades and with ideality factors less than 1.03. Both the C−2−V and activation energy plots were linear and yielded barrier heights in very good agreement with the I/V ones. The near‐ideal characteristics of these diodes were compared with several models of Schottky barrier formation and the dependence of the Schottky barrier height on the aluminum mole fraction was found to agree with the anion vacancy m...

MBE growth of BaF2/(Ga, In)(As, Sb) structures

Abstract We have made a wide-ranging study of the growth of structures combining BaF2 (a− 0.62001 nm) and nearly lattice matched (GaSb, InAs) or lattice mismatched (GaAs) semiconductors by MBE. BaF2 grown by MBE on (100) GaAs shows a variety of crystal forms and orientation depending on growth temperature. An attempt to grow single crystal GaAs on one of these orientations was not successful. BaF2 grows epitaxially on GaSb and InAs; conversely we have grown GaSb and InAs on BaF2. BaF2 grown on these substrates at high temperatures is conducting, but lower growth temperatures give insulating films which have allowed us to make C-V measurements on MIS structures. Overgrowth of GaSb gives material with a photoluminescence spectrum showing excitonic lines. The temperature required for this overgrowth results in a conducting BaF2 layer; these structures cannot therefore be used as isolated samples for Hall measurements. We believe that this conductivity may result from the diffusion of ions into the BaF2 lattice.

A detailed Hall‐effect analysis of sulfur‐doped gallium antimonide grown by molecular‐beam epitaxy

Experimental data are presented for the Hall coefficient and the apparent Hall mobility over the temperature range 160–500 K for five samples of molecular‐beam epitaxially grown sulfur‐doped GaSb. The donor concentration of the different samples varied between 3.3×1017 and 7.5×1016 cm−3, and the native acceptor concentration between 8×1016 and 1.2×1016 cm−3. The samples show a large spread in the apparent carrier activation energy. A two valley compensated conduction model is presented that shows that the variation in apparent carrier activation energy results from different compensation ratios in the samples. This model also shows that the constant value of the Hall coefficient observed at high temperatures is not due to donor exhaustion but carrier promotion to the lower mobility L1 band. Using constraints provided by secondary ion mass spectrometry and capacitance‐voltage measurements on the samples, as well as growth data, it is shown that a narrow spread of values for the donor binding energy around ...

Charge accumulation in GaAs/AlGaAs triple barrier resonant tunneling structures

In this article we present photoluminescence and photoluminescence excitation spectroscopy data from three triple barrier resonant tunneling structures. The spectroscopic techniques are used to estimate the charge accumulation in both tunneling quantum wells of the devices as a function of bias. The charging behavior is extremely asymmetrical, with significant charge accumulation only in the quantum well adjacent to the emitter region of the device and not in the quantum well adjacent to the collector region, irrespective of the direction of bias. This asymmetry in the charging behavior is analogous to highly asymmetrical double barrier resonant tunneling structures. However, due to the two quantum wells present in the triple barrier design it provides a more flexible system to study charge density dependent effects. We also present evidence for negatively charged exciton formation in the first quantum well for both directions of applied bias.

Comprehensive analysis of epitaxial Al/AlxGa1−xAs Schottky barriers made by MBE: barrier heights and band edge discontinuities

Abstract Extremely high quality epitaxial aluminium on Al x Ga 1−x As Schottky diodes have been prepared by MBE. The excellent electrical properties of the MBE grown AlGaAs layers, with residual deep-level concentrations of less than 10 14 cm −3 , combined with the in-situ deposition of single crystal epitaxial aluminium resulted in Schottky diodes with accurately exponential current-voltage characteristics over up to 6 decades of current and with ideality factors, for all but one of the diodes, less than 1.04 for x from 0 (GaAs) to 1 (AlAs). The dependence of the Schottky barrier heights on the aluminium mole fraction was determined using I V and C/V measurements. A comparison of the data from n- and p-type diodes shows the pinning level responsible for the barrier height to be the same in both cases; the sum of the barrier heights giving a bandgap dependence on composition consistent with other data. The compositional trends of the barrier heights are shown to be in close agreement with accepted GaAs/AlGaAs conduction and valence band offsets, supporting the idea of a relationship between Schottky barriers and heterojunction band offsets.

Comparison and origin of electronic states of the free GaAs (100) surface and the GaAs/CaF2 interface

An in situ assessment technique has been used on forming a metal‐insulator‐semiconductor structure in which the vacuum of the molecular‐beam epitaxy system is used as the insulating film. The electrical characteristics of the metal‐vacuum‐semiconductor structure were measured in situ at room temperature using the capacitance‐voltage (C‐V) technique. Investigations were also carried out on the effect of the arsenic volatility on such structures. These results were compared to those obtained on a true MIS capacitor where CaF2 was used as insulator. The C‐V hysteresis phenomena recorded on most of the Al/CaF2/GaAs diodes were investigated using a computer program. The results suggest that slow surface states rather than fast ones were responsible for the hysteresis phenomena and an insulator deposited under carefully controlled conditions might reduce defects at the GaAs surface, presumably through structural reordering.

MBE growth of pseudomorphic GaAsySb1−y in a GaAs host lattice

Abstract We present results of Nomarski microscopy and X-ray diffraction studies of a series of GaAs/GaAsySb1-y structures grown by MBE, incorporating either single alloy regions or superlattice structures, in order to determine the limits for pseudomorphic growth for alloys with antimony concentrations between 3% and 40%. The results arecompared with theoretical models and those obtained on other semiconductor alloys systems.

Full wafer optical characterisation of resonant tunnelling structures using photoluminescence excitation spectroscopy

We report on a novel optical technique for accurately characterising unprocessed View the MathML source tunnelling structures having two or more thin barriers and thick, heavily doped, contact layers. The results from photoluminescence excitation spectroscopy measurements are presented in which the intensity of the photoluminescence from the heavily doped contact layers rather than photoluminescence from the quantum wells is monitored. This method gives essential information for growth and quality control of “as-grown” layers prior to device fabrication. The usefulness and precision of this technique is illustrated by the measurement of a 0.3% variation in layer thickness from the centre to the edge of an as-grown wafer, and a 1% wafer to wafer variation in layer thickness in a series of triple barrier resonant tunnelling structures that have been designed for use at THz frequencies.

An electrical and optical study of electrons in triple barrier structures

Current–voltage curves measured over the temperature range 13–300 K show three shoulders and/or peaks common to most of a series of MBE-grown GaAs/AlxGa1−xAs triple-barrier resonant tunnelling structures. The widths of the barriers (x=∼0.33) were ∼49, ∼58 and ∼49 A; one well width was fixed between 72 and 68 A and the other varied between 68 and 45 A. Photoluminescence (PL) and PL excitation spectroscopy and a self-consistent Poisson–Schrodinger model suggest that the features are associated with (i) the maximum in the transparency of the barrier region when the two electron levels in the wells are aligned, (ii) a cut-off in phonon emission assisted tunnelling between the wells when the separation of these levels exceeds the optical phonon energy of 36 meV, and (iii) cut-off of coupling between electrons in the negative contact and the states in the adjacent well.

The use of Ga2Se3 and Ga2S3 as donor doping sources for MBE-grown AlxGa1-xSb and AlxGa1-xAs

The first results obtained with a use of Ga2S3 and Ga2Se3 compounds as sources of donor elements for the molecular beam epitaxy of AI,Ga1-xSb (0<or=x<or=1) on GaSb and GaAs substrates and AlxGa1-xAs (0<or=x<or=0.4) are reported. In GaAs, free electron concentrations obtained when incorporating the donors from these sources can be easily controlled up to a maximum of 5*1018 cm-3. For AlxGa1-xSb it was possible to compensate the high concentration of native acceptors and obtain n-type conductivity over the full composition range of the alloy. For GaSb maximum free electron concentrations were 6*1017 cm-3 and 5*1016 cm-3 for selenium and sulphur elements respectively.