S N Holmes

Observation and control of the amphoteric behaviour of Si-doped InSb grown on GaAs by MBE

The MBE growth and doping of heteroepitaxial layers of InSb on GaAs (100) are investigated. The layers are assessed by low-field Hall and magnetoresistivity measurements and high-field Shubnikov-de Haas studies together with infrared transmission, and TEM. The mechanism for silicon incorporation is investigated as a function of growth temperature. At low growth temperatures ( approximately=340 degrees C) silicon acts only as a donor and can produce electron concentrations up to 3*1018 cm-3 with 77 K mobilities identical to those found with bulk material. Although higher concentrations than 3*1018 cm-3 can be achieved; auto-compensation appears to occur in those samples. The 77 K mobilities achieved for less heavily doped samples (>40000 cm2 V-1 s-1 for n=1.2*1017 cm-3 for samples grown at 340 degrees C) are the highest low-temperature mobilities yet reported for n-type InSb films of approximately=1 mu m thickness grown on GaAs. However, higher growth temperatures ( approximately=420 degrees C) combined with constant silicon flux are found to simultaneously decrease electron concentration and mobility measured at 77 K although the structural quality as assessed by TEM remains unchanged. Analysis of the observed behaviour in terms of the Brooks-Herring model of ionised impurity scattering, modified for nonparabolicity, suggests that silicon is acting amphoterically with compensation ratios (NA/ND) reaching 0.5 at the higher temperatures. The effect of the interface between GaAs and InSb (lattice mismatch=14%) on the electrical properties is studied by introducing doping slabs of thickness approximately=1300 AA at various distances (d) between the interface (d=0 mu m) and the surface (d approximately=1.5 mu m) of the epilayer. A series of peaks not periodic in reciprocal field (1/B) are found at low fields with B parallel to the slabs and are interpreted as arising from the diamagnetic depopulation of the large number of subbands occupied as a result of the considerable thickness of the slabs. Be doping at 2*1019 cm-3 was demonstrated and, as with silicon, the bulk mobility corresponding to this hole concentration was achieved.

Electrical and magneto-optical of MBE InAs on GaAs

The electrical quality of InAs films grown on GaAs substrates by MBE is found to be optimum for growth temperatures close to 490 degrees C. The Hall mobility for such samples is 80000 cm2 V-1 s-1 at 77 K for film thicknesses of 5 mu m but falls to about 10000 cm2V-1s-1 at a thickness of 0.05 mu m. The carrier concentration in the bulk of the films is believed to be less than 1015 cm-3. The carrier concentration rises and the mobility falls as the growth temperature is varied on either side of this optimum value, reaching 2.5 *1016 cm-3 and 15000 cm2 V-1 s-1 at 77 K respectively for a growth temperature of 350 degrees C. Extremely sharp free-carrier cyclotron resonance and shallow donor lines are observed from the bulk of the film in far-infrared magneto-optical measurements, together with a very broad but strong cyclotron resonance line from an electron accumulation layer believed to be at the surface. The width of the cyclotron resonance line is consistent with a bulk mobility of the order of 200000 cm2 V-1 s-1 and the decrease in Hall mobility, together with the apparent increase in carrier concentration with decreasing film thickness, can be explained by the parallel conductance from the two-dimensional electron gas at the surface. There is no evidence for a significant reduction in mobility from the high density of threading dislocations caused by the mismatch with the GaAs substrate. The sharpness of the cyclotron resonance allows an accurate value for the band edge effective mass to be determined of 0.0236+or-0.0003 me with a pressure coefficient of +2.0% kbar-1. The donor lines are sufficiently sharp that central cell structure due to two different donor contaminants can be detected, and these donors are thought to be sulphur and selenium originating from the As source material. Certain of the transitions detected are too energetic to be from the shallow donors and these are thought to arise from singly ionized double donors which may be arsenic antisites. Silicon is found to act as a donor dopant up to high concentrations (6*1019 cm-3 where the mobility is 2000 cm2 V-1 s-1).

Magneto-optical and transport studies of ultrahigh mobility films of InAs grown on GaAs by molecular beam epitaxy

InAs layers of very high electrical quality are grown on GaAs substrates by molecular beam epitaxy (MBE). The observation of sharp cyclotron resonance and donor lines (linewidths approximately=1 cm-1) in far-infrared magneto-optical studies suggest that the low-temperature mobilities in the bulk of the films are in the range 200 000-300 000 cm2 V-1 s-1 with an electron concentration of approximately=2*1014 cm-3. A strong but broad cyclotron resonance line and the Shubnikov-de Haas effect are observed from a two-dimensional electron gas (2DEG) at the surface or GaAs interface (nS approximately=1*1012 cm-2 and mu s approximately=20 000 cm2 V-1 s-1). As a consequence of parallel conduction from the low mobility layer the Hall mobility measured from a 5 mu m thick sample is 80 000 cm2 V-1 s-1 at 77 K and that in a 2 mu m sample is only 30 000 cm2 V-1 s-1. The width of bulk cyclotron resonance and impurity lines depend only weakly on thickness and consequently scattering from dislocations generated by the misfit at the GaAs/InAs interface is not thought to affect the bulk mobility strongly down to film thicknesses of 1 mu m. The parallel conduction from the 2DEG also produces a large magnetoresistance. Please note - the first author name has been corrected from Homes to Holmes

MBE growth and quantum transport measurements of spike-doped InSb and InAs

Molecular beam epitaxy is used to prepare high-mobility films of InSb and InAs either homoepitaxially or heteroepitaxially on GaAs substrates. Silicon donors and beryllium acceptors can be introduced at high concentrations ( approximately 1019 cm-3), although low-temperature growth (<or=300 degrees C) must be employed in the case of silicon in InSb to avoid compensating amphoteric behaviour. Atomic plane doping of these impurities is studied by quantum transport measurements. Up to five sub-bands are occupied at high doping levels. Little or no diffusion of silicon away from the doping plane is found provided that the growth temperatures are kept low.

Bulk GaAs ionization detectors for WIMP searches

Bulk GaAs ionization detectors are being developed to search for weakly interacting massive particles (WIMPs). A GaAs array of mass 1–2 kg is envisaged sensitive to ionizing events < 200 keV and incorporating event position sensitivity with < 10 mm resolution to aid photon background discrimination. Preliminary results are presented for high purity GaAs detectors operated at 4 K to produced large depletion volumes by freezing out the carriers. A variety of ohmic contacting procedures have been investigated including pure Ga contacts. Spectra have been obtained using 60Co, 137Cs, 57Co and 241Am X-ray sources with detectors of mass from 0.1–8 g including one 8 g detector array having 30 16 mm2 detector regions.

Characterization of n-channel Si/SiGe modulation doped structures grown by gas source molecular beam epitaxy

We have grown n-channel Si/SiGe modulation doped structures by gas source molecular beam epitaxy using arsine as the n-type dopant source. The structures were characterized by transmission electron microscopy, secondary ion mass spectroscopy, electrochemical capacitance voltage analysis and x-ray diffraction. Arsenic and free electron concentrations in excess of 1019 cm-3 could be obtained with substantial surface segregation. Several different structures have been grown and their transport properties investigated. Low-temperature electron mobilities of up to 60000 cm2 V-1 s-1 in the dark (75800 cm2 V-1 s-1 after illumination) were obtained with a sheet density range (4-7)*1011 cm-2. Parallel conduction is discussed in terms of the effect of illumination.

Magnetotransport measurements on InAs-GaSb quantum wells with the application of hydrostatic pressure

Abstract We present a series of magnetotransport measurements on high mobility single InAs-GaSb semimetallic quantum wells using hydrostatic pressures up to 10 kbar and temperatures down to ~100mK. Clearly defined, hole-related peaks in pϱxx close to integer electronic filling factors in the semimetallic regime, have a stronger temperature dependence than the electron Shubnikov-de Haas effect at millikelvin temperatures and with this additional structure, apparent in ϱxx the quantum Hall plateaux in ϱxy deviate from h ve 2 , where v is the filling factor. The decrease in the electron concentration, dn 0 dp is 3.0 × 1010 cm−2 kbar−1 in the dark, and 4.0 × 1010 cm−2kbar−1 in the dark following infrared illumination to reduce the Fermi energy. We relate this reduction in the electron and hole concentrations to the uncrossing of the InAs conduction band and the GaSb valence band and to the role played by a GaSb surface donor at high pressures. The fermi energy at the surface is pinned approximately 250 meV above the GaSb valence band edge.

Experimental determination of the transport properties of composite fermions with reduced Lande g-factor

We demonstrate experimentally the existence of unpolarized spin states of composite fermions in the fractional quantum Hall effect regime. The modulus of the effective Lande g-factor was reduced using hydrostatic pressure and the transport properties of the first and second hierarchy of composite fermion states were studied in an AlGaAs/GaAs heterojunction in perpendicular magnetic fields. The activation energy of states at filling factor, nu =2/3 and 2/5 show a variation with effective Lande g-factor that is consistent with a spin-unpolarized ground state at these filling factors. The rapid destruction of the energy gap in the first composite fermion hierarchy at nu =2/5 follows the pressure dependence of the effective Lande g-factor of free electrons in GaAs. We show that structure in sigma xx, initially absent at ambient pressure, develops at even filling factor fractions corresponding to metallic states at nu =3/8 and nu =5/12 under reduced Lande g-factor conditions. Consistent behaviour is observed independent of the carrier density in the dilute electron system at 4.4*1010 and 6.8*1010 cm-2.

High-resolution magneto-optical studies of the donors in InP

The magneto-optical spectrum of the shallow donors in ultra-high-purity MOCVD InP is studied. The central cell structure on the 1s-2p+ and 1s-3p+ lines shows that the dominant donor contaminant is silicon but significant amounts of sulphur, germanium and another donor (probably selenium) are also present. Strong lines from negatively charged donor ions (D- states) can be observed, indicating that the samples are relatively uncompensated. The main spectral features are very similar to those found with high-purity VPE GaAs but, because MOCVD InP is slightly purer, more lines are observed. The appearance of certain higher order lines causes the interpretation of the GaAs data to be changed in some cases. Theory can predict the observed line positions on the revised interpretation to an accuracy of about 0.3%. The 1s-2p lines are broader than expected in comparison with GaAs.

First observation of a two-dimensional electron gas at the interface of α-Sn/InSb (100) grown by molecular beam epitaxy

The authors report the first observation of a high-mobility two-dimensional electron gas (2DEG) at the interface of alpha -Sn thin films heteroepitaxially grown on InSb(100) by MBE. Two-dimensionality of the electron gas was demonstrated by the Fourier analysis of the longitudinal and transverse Shubnikov-de Haas (SDH) measurements. The total carrier concentration of the 2DEG is about 6*1012 cm-2 and at least seven subbands are occupied at 4.2 K. Possible reasons for the occurrence of a 2DEG at this novel polar/non-polar interface between narrow-gap materials of high dielectric constants are discussed in the context of band-offsets at the interface, interfacial charges and other possibilities such as cross-doping and substrate cleaning artefacts.