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The submillimeter wave (496, 385, and 66 μm) photoresponse has been measured in an n-channel Si metal-oxide-semiconductor field-effect transistor at 4.2 K. A fast (≤10 ns) response is observed only in the low carrier density (ns) regime where the dc conductance is activated. Nonlinear photo response is found for I≥10 kW/cm2 and the signal follows a simple saturation behavior. The peak illuminated effective mobility is ∼13 000 cm2/Vs (at ns∼0.7×1011 cm-2) which is more than a factor of 2 larger than the peak 4.2-K mobility of the device (at ns∼2.4×1012 cm-2). At carrier densities greater than 5×1011 cm-2 the response is bolometric with a response time ∼1 μs. [ABSTRACT FROM AUTHOR]

Electrical transport in modulation-doped GaAs1-xSbx-AlyGa1-yAs strained-layer superlattices (SLS) grown by molecular beam epitaxy was studied, using low field Hall measurements from 4.2 to 300 K, high field quantum transport, and the far-infrared cyclotron resonance at 4.2 K. Our results show directly a degenerate two-dimensional electron gas in the SLS with an effective mass m*=0.063m0 and a mobility μ=7.7×103 cm2/Vs at a density ns =6.7×1011 cm-2, demonstrating the feasibility of high quality GaAs1-xSbx-AlyGa1-yAs SLS with good two-dimensional electronic transport properties despite the large lattice mismatch between the constituent layers. [ABSTRACT FROM AUTHOR]

We report cross-correlation measurements of the response of photoconductor pulsers and sampling gates excited by a femtosecond laser. The photoconductors were fabricated in microstrip transmission line structures on Si-on-sapphire, semi-insulating GaAs, and semi-insulating InP wafers. The photoconductor sampling gates were ion beam damaged to produce short carrier lifetimes (<3 ps in one case). Damage was introduced with 6 MeV 20Ne on the Si-on-sapphire, 2 MeV 2H on the GaAs, and 2 MeV 4He on the InP. Doses in the range 1012-1015 cm-2 were used. Our results show circuit limits to the time resolution in correlation measurements from two sources: (a) RC time constants due to photoconductor gap capacitance and transmission line characteristic impedance and (b) dispersion in microstrip transmission lines. [ABSTRACT FROM AUTHOR]

We report measurements of the energy-dependent pulse response of InP:Fe photoconductors to soft x rays in the energy range 200 eV-3 keV. The experiments were performed with x-ray pulses from the Stanford Positron Electron Asymmetric Ring storage ring at the Stanford Synchrotron Radiation Laboratory. The detector had an optical impulse response of 180 ps (full width at half-maximum) and an active area of 1×1 mm. An array of filters (carbon, magnesium, and beryllium) was used to unfold the energy response of the detector to the soft x rays. Direct measurements of the time structure of the ring bunches were limited by the bandwidth of the real time recording system (400 MHz), but inidividual ring pulses were easily recorded with a detector pulse amplitude of 65 mV for the unfiltered beam. The detector response was constant over the energy range examined at 2.7×10-3 A/W. [ABSTRACT FROM AUTHOR]

We have identified the electron paramagnetic resonance (EPR) spectrum of the As on a Ga site (AsGa) defect in bulk undoped liquid encapsulated Czochralski grown GaAs. The intensity of the EPR signal can be correlated with the concentration of compensating carbon acceptors in the GaAs crystals. These results indicate that AsGa are responsible for the compensation of the undoped semi-insulating material, and thus can be identified with the deep donor EL2 in such material. [ABSTRACT FROM AUTHOR]

We report impulse response measurements on InP:Fe photoconductors excited by laser and electron beam radiation. Measurements are reported on crystals with Fe concentrations from 2×1015 cm-3 to 4×1016 cm-3 and with excited electron-hole-pair densities of ∼1012 cm-3 and 9×1017 cm-3. Measured signal decays are purely exponential in character, and decay times are inversely related to Fe concentration. No long-lived tails are observed. Decay times show no dependence on excitation level for excited carrier concentrations that are well above and well below the Fe concentrations. The magnitude of the photoresponse indicates that electrons and not holes are the primary current carriers. The data suggest that for impulse excitation photoconductance decay in InP:Fe is due to trap-assisted recombination of electrons and holes at the Fe sites, with a rate determined by the species with the slower capture rate. [ABSTRACT FROM AUTHOR]

We demonstrate photothermally excited force modulation microscopy (PTE FMM) for mechanical property characterization across a broad frequency range with an atomic force microscope (AFM). Photothermal excitation allows for an AFM cantilever driving force that varies smoothly as a function of drive frequency, thus avoiding the problem of spurious resonant vibrations that hinder piezoelectric excitation schemes. A complication of PTE FMM is that the sub-resonance cantilever vibration shape is fundamentally different compared to piezoelectric excitation. By directly measuring the vibrational shape of the cantilever, we show that PTE FMM is an accurate nanomechanical characterization method. PTE FMM is a pathway towards the characterization of frequency sensitive specimens such as polymers and biomaterials with frequency range limited only by the resonance frequency of the cantilever and the low frequency limit of the AFM.