Francisco Castro

Current transport in as‐grown and annealed intermediate temperature molecular beam epitaxy grown GaAs

Molecular beam epitaxy (MBE) GaAs grown in the intermediate temperature range of about 400 °C may provide combination of low lifetime, high resistivity, and high mobility. We compare current conduction in unannealed and annealed material grown at 400 °C by fabricating photodetectors on substrates grown between temperature ranges of 250–500 °C. The unannealed version of the device grown at 400 °C shows substantial difference of conduction properties in dark and under light. It is shown that while at low biases the unannealed material may be semi‐insulating, at high biases more current is conducted than in annealed material. We attribute this to the effect of intergap states on current conduction and suggest that defect state assisted tunneling is the dominant current transport mechanism in these ranges. Quenching of response by light suggests that occupancy of traps can eliminate their role in current conduction.

Numerical computation of the complex dielectric permittivity using Hilbert transform and FFT techniques

Abstract A procedure is described to efficiently obtain the complex dielectric permittivity of semiconductor materials with numerical routines available in current software applications. This procedure calculates the real component of the dielectric permittivity from its imaginary component by making use of the Hilbert transform properties of the Kramers–Kronig relations. We show the reliability of this approach by reconstructing the real component from the experimentally known dispersion relations of GaAs and InAs. In addition, we use this procedure for the case when the imaginary component is obtained from analytical expressions as is the case for the Franz–Keldysh effect in the absorption of photons in GaAs. We show how the spectral photoreflection of structures where this effect is present can be easily modeled and simulated by this method.

Intermediate temperature grown GaAs/AlGaAs photodetector with low dark current and high sensitivity

A photodetector in which Schottky metal laterally contacts the molecular beam epitaxy grown heterointerface of intermediate-temperature GaAs and Al0.24Ga0.76As is reported. The device processed on 400 °C shows very low dark current, less than 25 fA/μm2 (0.5 pA/μm), with a high dc responsivity of about 10 A/W at low optical power levels. The device is process compatible with high electron mobility transistor technology.

The design, processing, evaluation and characterization of pyroelectric PVDF copolymer/silicon MOSFET detector arrays

We have developed a 64 element linear array of pyroelectric elements fully integrated on silicon wafers with MOS readout devices. The ferroelectric polymer film sensor deposited and polarized on the extended gate of the MOSFET results in a hybrid circuit, the pyroelectric-oxide-semiconductor field effect transistor (POSFET). The fabrication of the wafers included the design of the various masks required to produce the layers which made up the transistor array: stopper layer, active layer, poly-silicon layer, contacts layer, and bottom electrode layer. A thin film of the ferroelectric copolymer P(VDF/TrFE) was spin coated onto the wafer. Patterned gold electrodes were sputtered as the top electrode layer. The ferroelectric copolymer was hysteresis poled in situ. Tests performed included the arrays response to a CO/sub 2/ laser operating in the CW and single pulse modes at 10.6 /spl mu/m. We present details of the design, processing, and testing of the fabricated devices.

Design of a dual-effect lens on lanthanum-modified lead zirconate titanate for continuous variation of focal length

The design of a Fresnel lens with continuous focal length is proposed for use in optical processing. A convex lens is induced in lanthanum-modified lead zirconate titanate through the application of an electric-field profile supplied by the indium tin oxide electrodes that make up the zones of a Fresnel lens. The use of a numerical method based on fast Fourier transform algorithms was required to analyze accurately the induced field inside a Fresnel lens with an initial focal length of 0.4 m (at 470 nm) and 20 indium tin oxide electrodes. The effective focal location obtained by the combined mechanisms is derived. This design is expected to produce continuous variations of ~16% in focal length; the ability of previous designs to achieve focal length switching is maintained.

Closed-form electric-field profile model for AlGaAs/GaAs heterostructures

We propose a closed-form analytical model to describe the electric-field profile inside the GaAs layer of a modulation-doped AlGaAs/GaAs heterostructure. Derived from an accurate two-dimensional electron gas density expression, this model serves as a tool in the simulation of field effects on the spectral response of AlGaAs/GaAs metal–semiconductor–metal photodetectors. It is also suitable for analyzing surface field effects on lateral confinement mechanisms in mobility calculations for enhancement-mode heterostructure-based transistors. Results from this analytical expression are shown to agree well with numerical simulations performed on a physical device simulator and a modified self-consistent solution of Schrodinger and Poisson equations.

High-speed high-sensitivity low power photodetector with electron and hole charge plasma

High speed photodetectors’ intrinsic response is limited by the transit time of the carriers that light generates to the contacts that collect these carriers generating an electric response in the external circuitry. By contrast, charge plasma confined in a semiconductor can transfer energy, hence respond much faster, than the field-induced carrier drift current. The analogy is to a drop exciting a wave in a reservoir, which is detected more rapidly than the drop’s transport by current flow. Here we construct a photodetector device in which charge reservoirs of confined two dimensional electron and hole gasses (2DEG, 2DHG) mediate the photodetector response circumventing charge transport limitations in both expended energy and required velocity. In response to short optical pulses, this device produces electrical pulses which are almost two orders-of-magnitude shorter than the same device without the charge reservoirs. In addition to speed, the sensitivity of this process allows us to measure, at room temperature, as low as 11,000 photons. The device is shown to operate without applied bias, with high responsivity, at hundreds of gigahertz. These micro plasma devices can have a range of applications such as optical communication with fraction of a microwatt power compared to the present tens of milliwatts, ultrasensitive detection of light without need for cryogenic cooling, photovoltaic devices that are capable of harvesting dim light, detectors of THz radiation, and in detection of charged particles.

Gain enhancement of low-temperature GaAs heterojunction MSM photodetectors

Summary form only given. Molecular beam epitaxy (MBE) growth of the GaAs at temperature ranges of 200/spl deg/C and 600 /spl deg/C substantially affects its optical and electronic properties. Low-temperature (LT), around 200/spl deg/C, and intermediate temperature (IT), around 400/spl deg/C, growth by MBE has been employed in a variety of device applications including transistors, mixers, and photodetectors. In the device proposed here, a heterojunction of AlGaAs is grown on top of an LT/IT active layer. Trenches are formed in the AlGaGs and Schottky contacts of Ti:Au are deposited in the trenches, laterally contacting the heterojunction interface.