[Show abstract][Hide abstract] ABSTRACT: In a two-dimensional electron gas at ν=1 (where the filling factor ν=neh/eB measures how many Landau levels (LL) are filled) the ground state should be regarded as a ferromagnet since all the spin down states in the lowest LL are occupied while all the spin up states are empty. In GaAs the single particle (SP) Zeeman energy (ZE) gμBB is very small 0.3 K/T, and Coulomb interactions are very significant. This has led several authors to suggest that novel charged excitations with number of reversed spin R > 1, and with non-trivial spin order, known as charged spin-texture excitations or Skyrmions, may occur (1,2). This type of excitation is based on a spin texture that consists of a central reversed spin surrounded by rings of spin that gradually cant in a radial direction until at the edge they are aligned with the external magnetic field. The balance between the SPZE and the Coulomb energy is determined by the parameter η= gμBB /Ec which determines whether Skyrmions with R>1 (small η) or spin waves (large η) will be the lowest lying excitations. To date two transport measurements have inferred the existence of Skyrmions. Increasing η by tilting the magnetic field suggested a 7 spin excitation for η~0.01 (3). In a narrow (6.8 nm) GaAs quantum well where g is already reduced by quantum confinement energy and penetration of the wavefunction into the AlGaAs barriers, it was decreased further by hydrostatic pressure becoming zero at ~4.8kbar where the energy gap at ν=1 showed a minimum (4). This indicated a much larger Skyrmionic excitation consistent with R=33 when |η|
[Show abstract][Hide abstract] ABSTRACT: A high mobility single subband occupancy InSb/InAlSb quantum well was grown by molecular beam epitaxy. The low-temperature,
high-field magnetotransport properties are measured as a function of gate bias. Spin-resolved Shubnikov-de Haas oscillations
are observed. A preliminary analysis of the Shubnikov-de Haas oscillations indicates a strong gate bias dependence of the
Rashba spin-orbit term.
[Show abstract][Hide abstract] ABSTRACT: We report on investigation of the spin dynamics in InAs and InSb films grown on GaAs at a temperature range from 77K to 290K.
For both materials, the large lattice mismatch with the GaAs substrate results in the formation of an interface accumulation
layer with a large defect concentration, which strongly affects the spin relaxation in these areas. Moreover, the native surface
defect in the InAs films resulted in an additional charge accumulation layer with high conductivity, but very short spin lifetime.
In contrast, in InSb layers, the surface states introduce a depletion region. We have correlated the spin relaxation with
a multi-layer analysis of the transport properties, and find that in a 1μm thick InAs film, approximately 70% of the total
current flows through the interface and surface accumulation layers, which have sub-picosecond lifetimes, whereas in InSb
films of the same thickness, the semiconducting layer carries more than 90% of the total current, and the spin lifetime in
the accumulation layer is only slightly less than that of the central semiconducting layer. We suggest that InSb could be
a more attractive candidate for spintronic applications than InAs.
Journal of Superconductivity and Novel Magnetism 07/2007; 20(6):461-465. DOI:10.1007/s10948-007-0237-4 · 0.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe measurements of electron diffraction by a magnetic-field-induced Wigner crystal formed at a GaAs heterojunction. The crystal is formed by dopant electrons in the lowest subband of the confining potential at the interface and is pinned by defects: the probe electrons are photoexcited into higher subbands. The signature of the electron crystal is the break-up of the density of states of the probe electron into Hofstadter bands which is detected directly in the luminescence spectrum. The measured value of the principal gap agrees well with theoretical estimates.
[Show abstract][Hide abstract] ABSTRACT: The spin relaxation in undoped InSb films grown on GaAs has been investigated in the temperature range from 77 to 290 K. Two distinct lifetime values have been extracted, 1 and 2.5 ps, dependent on film thickness. Comparison of this data with a multilayer transport analysis of the films suggests that the longer time (similar to 2.5 ps at 290 K) is associated with the central intrinsic region of the film, while the shorter time (similar to 1 ps) is related to the highly dislocated accumulation region at the film-substrate interface. Whereas previous work on InAs films grown on GaAs showed that the native surface defect resulted in an additional charge accumulation layer with high conductivity but very short spin lifetime, in InSb layers the surface states introduce a depletion region. We infer that InSb could be a more attractive candidate for spintronic applications than InAs.
[Show abstract][Hide abstract] ABSTRACT: The electrical properties of relaxed InSb
grown on GaAs substrates depend primarily on an inherent magnetoresistance in the layers and not on surface morphology. The magnetoresistance of these films can be described well using a simple model that incorporates the gradation of properties away from the InSb/GaAs interface and the interplay between conduction and impurity bands. By engineering the layers, inherent MR can be tuned accordingly.
[Show abstract][Hide abstract] ABSTRACT: We report on the use of a commercially available band-edge detection system for substrate temperature monitoring of gallium arsenide substrates. The extension of the technique to the cases where strong absorption by either the substrate or substrate holder might normally preclude the use of such systems due to poor signal levels is discussed. For indium-mounted wafers, a background subtraction/removal is applied which allows unambiguous determination of the band edge across the full temperature range. An alternative method of operation of the instrument as a highly configurable pyrometer allows measurements to be made on highly conducting p-type substrates where free carrier absorption swamps the band edge.
[Show abstract][Hide abstract] ABSTRACT: Optical measurements of substrate temperature during the growth by plasma-assisted molecular beam epitaxy of GaN on sapphire and GaAs (1 1 1)B were obtained using two commercial BandiT systems for visible and infrared wavelengths. With the visible system, an absorption edge at ∼400 nm was seen after 20–50 nm of GaN growth, and although this became clearer and more stable for the film on sapphire, strong interference oscillations precluded further measurements on the GaAs substrate. Using the infrared version, we observed a real initial increase in temperature for GaN films grown on GaAs substrates followed by a subsequent decrease to a steady-state value.
[Show abstract][Hide abstract] ABSTRACT: The recent development of a commercial band-gap thermometry system for wide band-gap materials such as GaN (the k-Space Associates’ “BandiTTM”) has allowed a systematic study of the relationship between pyrometric or thermocouple temperature-monitoring methods and the directly determined layer temperature. The wide band gap of GaN gives a weak and sample-dependent coupling to radiant heat sources, and it is common in molecular beam epitaxy (MBE) to use a sputtered Mo coating on the rear of the sapphire substrate to improve the efficiency and consistency of heat transfer. We have investigated the role of this backing layer and the use of a PBN diffuser in an MBE chamber, but similar results are expected to be obtained from band-gap thermometry in a metal-organic vapour phase epitaxy (MOVPE) system. The wide range spectrometer used for the band-edge determination can also be employed in a pyrometric mode, at wavelengths both within the band gap of GaN and above it. The latter is insensitive to thickness oscillations, and is less affected by the presence or absence of a Mo backing layer. Results are presented comparing all these measurement techniques, and analysed to show that the Mo backing does not significantly increase the power coupling into the epitaxial layer, although it does improve the accuracy of pyrometric measurements.
[Show abstract][Hide abstract] ABSTRACT: A description is given of the occurrence of fractional quantum Hall effects, and how these are influenced by the presence of disorder. It is shown that when electrons are photoexcited into a GaAs-GaAlAs heterojunction, the fractional state existing at a Landau level occupancy of 7/5 can dominate over that at 4/3. Activation energy measurements of the resistivity show how the energy gap of the fractional states is reduced by the presence of disorder. A new method of analysis of the resistivity minima is presented which is used to give information on the correlation length of the many body ground state.
Physica Scripta 02/2007; 1987(T19A):72. DOI:10.1088/0031-8949/1987/T19A/012 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a review of magneto-optical experiments and theoretical calculations which have been used to examine the influences of dimensionality and many electron effects on electron-optic-phonon coupling in low dimensional structures of GaAs-(Ga,Al)As.
Physica Scripta 01/2007; 1991(T39):308. DOI:10.1088/0031-8949/1991/T39/047 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Photoluminescence measurements of two-dimensional electrons in GaAs/GaAlAs heterojunctions in the integer and fractional quantum Hall regimes are reviewed. Intensity and energy anomalies observed experimentally are discussed in terms of many-body effects in the recombination process. Optical evidence for the formation of an electron solid in the extreme quantum limit is presented.
Physica Scripta 01/2007; 1991(T39):223. DOI:10.1088/0031-8949/1991/T39/034 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The interaction between 2D electrons and photoexcited holes is of fundamental importance in determining the photoluminescence spectrum in the fractional quantum Hall regime. We present new measurements on a low-density electron system which reveal a distinct Fermi edge singularity at zero and low magnetic fields, indicating dynamical response of the Fermi sea to the presence of the hole. The photoluminescence intensity from this system is very sensitive to the effects of electron correlation, and hierarchies of FQH states are clearly identified. The photoluminescence energy, however, is quite insensitive to electron ground state in the FQH regime.
Physica Scripta 01/2007; 1992(T45):164. DOI:10.1088/0031-8949/1992/T45/034 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The low-lying bulk excitations of two-dimensional electron systems subject to a strong perpendicular magnetic field have been studied using phonon absorption. Results reported earlier lead to the conclusion that the magnetoroton minimum energy in the fractional quantum Hall effect can be measured by phonon absorption. Recent time-resolved measurements have demonstrated fundamental differences in phonon absorption between fractional quantum Hall states, non-rational filling factors and filling factors close to 1/2. At non-rational filling factors the response is bolometric, whereas at filling factors corresponding to FQHE states the response also depends on the spectrum of the heater pulse. Around ν = 1/2 the response is largely independent of magnetic field with a weak maximum at ν = 1/2.
Physica Scripta 12/2006; 1996(T66):163. DOI:10.1088/0031-8949/1996/T66/029 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Shubnikov-de Haas and persistent photoconductivity measurements are used to study the mobility, free electron density (n) and the occupancy of the DX centre in heavily doped n-GaAs as a function of doping level and hydrostatic pressure. The results show that the DX centre produces a resonant donor level between the Γ - and L-conduction band minima at a concentration comparable with the doping level. For the Si-doped samples, comparison with local vibration mode measurements indicates that the DX level can be identified with SiGa. The level acts to pin the Fermi energy at electron concentrations around 1.8 × 1019 cm−3. Analysis of the results indicates that macroscopic charge separation is not responsible for persistent photoconductivity in these samples.
Physica Scripta 11/2006; 38(4):605. DOI:10.1088/0031-8949/38/4/018 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The depth-dependent electrical properties of InSb thin films grown on GaAs substrates result in an inherent magnetoresistance in the layers. For certain applications it is important to be able to manipulate this effect controllably. This paper demonstrates both experimentally and theoretically that the magnitude of the magnetoresistance can be dramatically changed by epilayer design. We show that the inclusion of a doped region in part of the layer structure allows the inherent magnetoresistance to be changed by a factor of 40.
[Show abstract][Hide abstract] ABSTRACT: We report investigation of the spin relaxation in InAs films grown on GaAs at a temperature range from 77 K to 290 K. InAs is known to have a surface accumulation layer and the depth profile of the concentration and mobility is strongly nonuniform. We have correlated the spin relaxation with a multilayer analysis of the transport properties and find that the surface and the interface with the GaAs substrate both have subpicosecond lifetimes (due to the high carrier concentration), whereas the central semiconducting layer has a lifetime of an order of 10 ps. Even for the thickest film studied (1 mu m), the semiconducting layer only carried 30% of the total current (with 10% through the interface layer and 60% through the surface accumulation layer). Designs for spintronic devices that utilize InAs, which is attractive due to its narrow gap and strong Rashba effect, will need to include strategies for minimizing the effects of the surface.
Physical Review B 08/2006; DOI:10.1103/PhysRevB.74.075331 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the 300 K inherent magnetoresistance of undoped InSb epilayers grown on GaAs(001) by molecular-beam epitaxy. The magnetoresistance of these films can be described well using a simplified model that incorporates gradation of properties away from the InSb/GaAs interface and the interplay between conduction and impurity bands. Although there is no significant intrinsic contribution in InSb bulk crystalline (001) materials due to its isotropic Fermi surface and mobility tensor, the linear and quadratic terms in the magnetoresistance as well as the overall magnitude can be tuned by varying the film thickness from 100 to 2000 nm.
[Show abstract][Hide abstract] ABSTRACT: There is great advantage to controlling the magnetoresistance (MR) in high mobility semiconductors for a number of applications which require thin active surface layers. Previously we have produced n type thin epilayers of InSb with the highest reported mobility and we have used these epilayers to explore novel geometries that enhance the high field MR. Here we show that by virtue of the inherent inhomogeneity in the growth direction, thin InSb epilayers can be designed to have significant MR without external geometric manipulation. The observations can be explained using a transport model that describes the electrical properties of the layers including contributions from conduction and impurity bands. We will explore using the model, the possibility of maximizing or minimizing the inherent MR in these layers and we show experimentally how to create thin high mobility layers where the inherent MR is significantly reduced or enhanced without compromising the layer mobility. T. Zhang et al. Appl. Phys Lett. 84, 4463 (2004). W.R. Branford et al., Appl. Phys Lett. 86, 202116 (2005). J.J. Harris et al., Semicond. Sci. Tech. 19, 1406 (2004). T. Zhang et al., Semicond. Sci. Tech., in press.