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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.

A photodetector having a sensitivity only in a narrow-wavelength region has been fabricated by utilizing a thick semi-insulating GaAs (SI GaAs). Because the photon energy at the peak response is nearly equal to the bandgap energy of GaAs, this detector is called a band-edge-emphasizing photodetector. A theory is proposed to explain this phenomenon. Since GaAs is characterized by a short carrier lifetime and a steep change of the absorption coefficient, the active layer is assumed to be composed of two different parts connected in series, which are specified by a function delta . Good agreement has been shown between the experimental and the theoretical results on the sensitivity characteristics. >

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. >

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.

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.

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.

A peculiar photoconductive property with a positive temperature coefficient in a semi-insulating GaAs photodiode is found in the room temperature range. Also, the low-light-level photoconductivity is higher than usual. We suggest that an ionised-impurity scattering mechanism is dominant. Theoretical responsivity is compared with the measured value

A new behavior of highly accelerated hot electrons in a magnetic field is observed in semi-insulating GaAs (S.I.GaAs). In an electric field at a few kV/cm, the conduction current in S.I.GaAs increases steeply. The abrupt current increase has been interpreted by the trap-filling model of Lampert. The abrupt current increase shows interesting behavior in a magnetic field. By magnetic field application (at ∼0.3 T), the current increment is cut off. This cutoff behavior requires a new concept of the conduction mechanism in S.I.GaAs. This report suggests that cyclotron cutoff occurs. It implies that the drifting electrons suffer no scattering, just as in vacuum. The scattering relaxation time of an electron is estimated to be extremely long, such as 50 ps.