Kazutoshi Nakajima

Properties and design theory of ultrafast GaAs metal-semiconductor-metal photodetector with symmetrical Schottky contacts

A GaAs metal-semiconductor-metal photodetector fabricated in a symmetrical and interdigital Schottky contact structure on a semi-insulating GaAs substrate is discussed. The device exhibits a high-speed response and a very low dark current, even with a moderate size (200 mu m/sup 2/). Ultrafast responsivity is due to the high drift velocity of the photoelectrons and low capacitance. A design theory for optimum performance which includes a compromise between the circuit time constant and the transit time is described. >

Room-Temperature 1.56 µm Electroluminescence of Highly Oriented β-FeSi2/Si Single Heterojunction Prepared by Magnetron-Sputtering Deposition

β-FeSi2/Si light-emitting devices were prepared by growing continuous and highly oriented β-FeSi2 films on Si (111) substrates using RF magnetron-sputtering deposition with a Fe target. In-plane X-ray diffraction reveals that the epitaxial relationship is such that β-FeSi2 // Si, indicating a (110) orientation in the growth direction. Post annealing was performed at 830°C and Al electrodes were deposited on both p-β-FeSi2 and n-Si sides. Room-temperature 1.56 µm electroluminescence was clearly observed from such a simple structure for the first time. Compared with the previously reported results for β-FeSi2 balls or β-FeSi2 precipitates buried in Si, the injection current density is more than one order of magnitude lower, indicating that the nonradiative recombination was effectively suppressed for the junction consisting of a continuous film.

A rapid optoelectronic half-adder logic composed of a pair of GaAs metal-semiconductor-metal photodetectors

A novel optoelectronic half-adder logic, composed of only two GaAs metal-semiconductor-metal photodetectors (MSM-PDs), is reported. The optoelectronic logic utilizes the features of the MSM-PD for both polarities of electrical bias (positive and negative). Without any other active devices such as transistors, the output delay time is short, less than 100 ps for the entire half-adder operation, which assures a very fast arithmetic operation. >

Low-Field Breakdown and Negative Differential Resistance in Semi-Insulating GaAs

In a semi-insulating GaAs (S.I. GaAs) diode with an ohmic cathode, a negative differential resistance (NDR) appears at a field as low as 2 kV/cm. In the NDR region, luminescence with a broad spectrum is observed. The luminescence area is localized just in front of the anode. These results are explained on the basis of the avalanche injection effect in S.I. GaAs material. The electron temperature estimated from the luminescent spectrum is 3200 K. The avalanche multiplication is discussed in terms of the low-field avalanche scheme.

Power-speed product of an optical flip-flop memory with optical feedback

Evaluation of optical logic gate devices in terms of a power-speed (switching time) product is discussed. An example is given for a simply fabricated optoelectronic flip-flop circuit utilizing an optical feedback. The minimum power-speed products for set and reset operations are 5 and 35 pJ, respectively. They are very small compared with previously reported optical nonlinear or bistable devices. The figure of merit is decreased by monolithic integration, by which an optical computation can be realized. >

Near-field effect in the infrared range through periodic Germanium subwavelength arrays

Using finite-difference-time-domain simulation, we have studied the near-field effect of Germanium (Ge) subwavelength arrays designed in-plane with a normal incidence. Spectra of vertical electric field component normal to the surface show pronounced resonance peaks in an infrared range, which can be applied in a quantum well infrared photodetector. Unlike the near-field optics in metallic systems that are commonly related to surface plasmons, the intense vertical field along the surface of the Ge film can be interpreted as a combination of diffraction and waveguide theory. The existence of the enhanced field is confirmed by measuring the Fourier transform infrared spectra of fabricated samples. The positions of the resonant peaks obtained in experiment are in good agreement with our simulations.

Vertical field enhanced nanostructure for quantum well infrared photodetector through Germanium subwavelength arrays

Finite-difference-time-domain (FDTD) computer simulations reveal interesting features of Germanium (Ge) subwavelength nanostructures designed in x-y plane on substrate of InP with normal incidence, which can be applied in quantum well infrared photodetector (QWIP). Unlike the mechanism of excellent near field effects through periodic metallic nanostructures, large intensity of Ez field is achieved at near-infrared range by subwavelength arrays of Ge which has no surface plasmons (SPs). The evanescent Ez field generated along the surface of Ge is interpreted due to waveguide mode interference of coupled scattering. The existence of the enhanced field is confirmed by comparing the Fourier transform infrared (FTIR) spectra of real-fabricated samples with simulation outcomes. Positions of resonant peaks obtained in experiment are in good agreement with those of simulation.

Enhanced and suppressed infrared transmission through germanium subwavelength arrays

We have studied the zero-order transmission of periodic germanium (Ge) subwavelength arrays in an infrared range by using finite-difference time-domain simulations. A special wavelength-selective peak in a triangular hole array of Ge film is observed with an enhanced transmission accompanied by a drastic suppression nearby, which cannot be found in a one-dimensional Ge subwavelength array and is different from the extraordinary transmission related to surface plasmons in a metal film. The electromagnetic field is found to be concentrated on both surfaces of the Ge film at this peak. The unique transmission is verified through measurements on fabricated samples and is interpreted using the photonic band structure.

Infrared Photoresponse in Semi-Insulating GaAs Diode

In semi-insulating GaAs (SI GaAs) photodiodes, an IR photoresponse for longer wavelength beyond the energy gap threshold is observed. The sensitivity is analyzed by classifying electrode types and configurations. The IR sensitivity originates from the optical hole injection. When a cathode has an ohmic contact, the IR sensitivity is enhanced exponentially with the bias voltage. The enhancement of the IR sensitivity corresponds to the negative differential resistance in SI GaAs. The mechanism of the IR sensitivity is discussed in terms of the avalanche injection.

Enhanced long wavelength response in a GaAs photodetector

A metal‐semiconductor‐metal photodetector (MSM‐PD) fabricated on a semi‐insulating GaAs has a long wavelength response beyond the energy gap. It is enhanced by applying a second bias voltage to the bottom electrode. When the bias is 100 V, the responsivity exceeds the unit quantum efficiency, which indicates that a photoconductive amplification function exists. Since the dark current is as small as 0.2 nano‐amperes, it may be more suitable than an InGaAs or a Ge photodiode for long wavelength detection.