Professor Roger A. Lewis

Terahertz Emission from Mercury Cadmium Telluride

M. L. Smith — Mon, 02 Feb 2009 14:36:57 PST

THz-frequency electromagnetic radiation may be generated by illuminating a suitable target with short pulses of near-infrared radiation. We present results of experiments employing pulses of < 12 fs irradiating a HgCdTe thin film. The resulting THz emission was detected using a liquid-He-cooled bolometer. Strong azimuthal angle dependence on the generated THz power is observed. The THz power varies quadratically with the pump power.

Quantum point contact in a magnetic field: Far-infrared resonant heating observed in photoconductivity

R. J. Heron — Mon, 02 Feb 2009 14:36:57 PST

We report on the far-infrared photoresponse of a quantum point contact device fabricated on a top-gated GaAs/AlGaAs heterostructure. The top-gated architecture avoids the disorder built into conventional modulation-doped structures. We observe a distinctive far-infrared magneto-photoresponse. This depends on the wavelength of the radiation and on the carrier density, which is controlled by the gate voltage. We conclude by comparison with transport data that the oscillations observed in photoconductivity and which are centred around the cyclotron energy arise from the resonant heating of electrons by the far-infrared radiation.

Electrical and Optical Properties of La0.7Ca0.3MnO3 Polycrystalline and Thin Film

F. Gao — Mon, 02 Feb 2009 14:36:56 PST

The La0.7Ca0.3MnO3 thin film shows a metal-insulator transition at temperature around 150 K, about 100 K lower than that of the polycrystalline sample. It is found that the activation barrier in the thin film is lower than that in the bulk sample. The FIR reflectance of the thin film exhibits a new mode which is not present in the bulk sample. We propose that the new mode is the corresponding surface bending mode.

Central-cell corrections for Si and S in GaAs in a strong magnetic field

R. J. Heron — Mon, 02 Feb 2009 14:36:56 PST

The central-cell correction has been determined experimentally for the two donor impurities S and Si in GaAs. Data have been obtained for magnetic fields to 39 T, corresponding to γ~6. The observed behavior is in good agreement with theory. The analysis permits accurate evaluation of zero-field central-cell corrections, yielding 0.110 and 0.059 meV for S and Si, respectively.

Electroresistance of Perovskite Manganites

D. C. Pond — Mon, 02 Feb 2009 14:36:55 PST

A comprehensive delineation of current-voltage-temperature space is given for La0.66Sr0.33MnO3 and La0.8Li0.2MnO3. The first shows little electroresistance; the second exhibits marked electroresistance over a wide temperature range.

Selective laser excitation spectroscopy of gallium and phosphorous in germanium

C. A. Freeth — Thu, 29 May 2008 15:03:02 PDT

Shallow impurity states of donors and acceptors in Ge have been investigated by far-infrared laser magnetospectroscopy. The Zeeman splitting of the impurity states produces resonant absorption of the laser radiation at certain magnetic fields. The data are compared to measurements made using a conventional spectrometer and good agreement is found.

Thermal and electrical characteristics of a multilayer thermionic device

B. C. Lough — Thu, 29 May 2008 15:02:59 PDT

We report our recent experimental and numerical investigation into the thermal and electrical transport in GaAs-AlGaAs semiconductor multilayer structures. Electrical and thermal conduction measurements were performed on multilayer structures to determine the temperature gradient across the sample. AuGe was used for top contact metallisation, and an InGa eutectic for bottom substrate contact. Metallisation contacts were also grown directly onto the substrate in order to compare results with and without the device included. By using a variable load resistor connected in series with the device, we can accurately determine the current-voltage characteristics of the device. Thus the power input can be obtained. The temperature distribution on the top and bottom substrate was measured with micro thermocouples. Since the cooling device is grown on an n-type semiconductor substrate the effects of joule heating in the substrate had to be considered. Treating the substrate as bulk material and calculating joule heating showed that this effect is negligible. Comparing experimental measurements of the device and of the substrate alone support this. The experimental I-V characteristics of the device differ significantly in shape from theoretical I-V characteristics. This may be due to that fact that space-charge effects are not included in the currently accepted model (Richardson's equation). Due to the small size of the devices and therefore very large electric fields, this effect may be important. Work is currently being carried out to modify the model. The devices studied so far have been made from undoped GaAs-AI/sub 0.07/Ga/sub 0.03/As heterostructures. For large cooling power it is a requirement that the conduction band of the layers be close to the Fermi level.

Piezospectroscopy of the p3/2 and Fano series of singly ionized zinc in germanium

G. Piao — Thu, 29 May 2008 15:02:57 PDT

The absorption spectrum of singly ionized zinc in Ge has been studied using Fourier spectroscopy. Improved experimental conditions give more details for the p3/2 series than previously. Quantitative piezospectroscopy has been performed with compression along <111> and <100>. It is deduced that the final states of the compound C line include 1Gamma₇^-, 3Gamma₈^-, 3Gamma₈⁺, and 4Gamma₈⁺. More reliable deformation potential constants of some energy states have been obtained. Fano resonances associated with bound-hole states have been studied with and without applied uniaxial compression. The piezospectroscopic behavior of these is compared with that of the parent p3/2 series.

Magnetospectroscopy of Be in GaAs

R. A. Lewis — Thu, 29 May 2008 15:02:54 PDT

Far-infrared absorption measurements have been made of the Lyman series of the acceptor Be in GaAs up to magnetic fields of 30 T, supplemented by photothermal ionization spectroscopy measurements to 6 T. The results confirm and greatly extend previous work and reveal several phenomena. The strongest component of the G line moves to higher energies and increases in intensity with field. Around 20 T the field dependence of the transition displays a marked kink. At higher fields the dependence of energy on field is linear. This behavior is attributed to unresolved components that change in relative intensity with field and leads to a reappraisal of the identity of the low-field transition. The D line is followed to high fields allowing an accurate determination of the splitting of the field-induced components of the second excited state. Photothermal ionization spectra for the C line permit the determination of the splitting of the field-induced components of the ground state and of the third excited state. At high magnetic field an absorption appears which increases in energy with field at a much greater rate than any of the lines mentioned above. It is thought to originate in the valence-band Landau levels.

Characterisation of indium phosphide using terahertz radiation

R. L. Causley — Thu, 29 May 2008 15:02:52 PDT

Specimens of undoped, n-type and p-type bulk InP have been investigated using THz radiation. Reflection spectroscopy permits the determination of the TO and LO phonon energies as well as the carrier concentration. Absorption spectroscopy reveals transitions between impurity states. We report the first absorption spectrum of an acceptor in InP. The transition energies agree well with electronic Raman scattering and photoluminescence data.