A. Sužiedėlis

Hot carrier photocapacitive effect

Semiconductor p-n and metal-insulator-semiconductor structures containing a capacitive element were investigated under pulsed ir laser irradiation. It is shown that the carrier heating in the structures is responsible for the rise of the photosignal. A direct correlation between the photosignal and the capacitance of the structures is confirmed. Possible applications of the effect are discussed.

Microwave Response of La2/3Ca1/3MnO3 Thin Films

The investigations of microwave (MW) response of both epitaxial and polycrystalline La0.67Ca0.33MnO3 (LCMO) thin films, magnetron sputtered onto lattice matched NdGaO 3(100) and MgO(100) substrates, respectively, are presented. Both the MW radiat ion nduced anomalous resistance increase and fast resistance relaxation observed in this work for epitaxial LCMO film just below the paramagnetic (PM) to ferromagnetic (FM) transition t emperature Tc demonstrate a clear evidence of dynamical phase coexistence in the material. Similar investigations of LCMO/MgO films exhibited the importance of intergrain boundaries on the high fre quency conductivity of polycrystalline material. The MW response of the films, both below and above Tc, has been explained assuming influence of strong microwave radiation on the hopping c onductance of high resistance intergrain material.

Peculiarities of Photovoltage Formation across p-n Junction under Illumination of Laser Radiation

Photovoltage formation across Si and GaAs p-n junctions exposed to laser radiation is experimentally investigated. When the photon energy is lower than semiconductor forbidden energy gap, the photovoltage is found to consist of two components, U=Uf+ Uph. The first one Ufis fast having polarity of thermoelectromotive force of hot carriers. The second one Uphis slow component of opposite polarity, and it is caused by electron-hole pair generation due to two-photon absorption. Uph was shown to decrease with the rise of radiation wavelength due to diminution of two-photon absorption coefficient with wavelength. Predominance of each separate component in the formation of the net photovoltage depends on both laser wavelength and intensity.

Detecting microwave radiation on metal–porous silicon contacts

We have studied the rise of electromotive force (emf) across ohmic and nonohmic metal–porous silicon contacts under the action of 10-GHz microwave radiation. It is established that emfs of opposite polarities with different growth (and decay) rates are generated across the contacts of different types. The emf growth across the potential barriers of contacts is explained using a model of the microwave-induced heating of charge carriers. It is shown that the use of a porous semiconductor layer is a promising way to increase the voltage–power responsivity of microwave detectors and eliminate the “point contact” effects.

Detection of microwave radiation on porous silicon nanostructures

We report on possibility to detect pulsed microwave radiation across the metal/oxide/porous silicon structures and analyse possible physical reasons causing the rise of the emf voltage signal. The n-type porous layers were fabricated according to conventional electrochemical etching procedure, and were exposed to pulsed 10 GHz microwave radiation. The results of investigation show that the porous Si samples have higher by at least one order voltage-to-power sensitivity than the samples without the porous layer, and are considered to have high potential to increase it further. Free carrier heating phenomenon is considered to be responsible for the signal formation.

Dynamical study of thermal conductivity of nanostructured layers by use of the photoinduced transient thermoelectric effect

We propose a new fast technique to determine thermal conductivity of a nanostructured material and demonstrate it for porous silicon. Transient thermoelectric voltage is measured after a pulsed laser irradiation, and analysis of the voltage decay time constant and porosity of the structure gives the value of the thermal conductivity. For n-type Si of 70% porosity we obtain the value of 35 W m-1 K-1 what is in good agreement with the results of other investigations The method can be easily applied for any other porous or otherwise structured low-dimensional materials.

New Planar Heterojunction Diode for Microwave and Infrared Applications

Planar design of AlxGa1−xAs/GaAs heterojunction diode is proposed for the detection of microwaves as well as infrared radiation. Suitable choice of the AlAs mole fraction (x = 0.3) and doping level of the ternary AlxGa1−xAs semiconductor (Nd = 1016 cm−3) allowed us to fabricate a semiconductor structure consisting of two diodes connected in opposite direction to each another: a Schottky diode with lowered energetic barrier and a heterojunction diode. Microwave and infrared experiments were performed at room temperature. High voltage sensitivity of the planar diodes (∼1 kV/W) was inherent in microwave Ka frequency range, while at higher frequencies substantial drop of the sensitivity was observed (∼10 V/W in D frequency range). Rapid decrease of the voltage sensitivity with frequency is determined by the reduction of microwave current rectification on the Schottky and heterojunction barriers. Still, under the action of pulsed CO2 laser radiation the planar heterojunction diodes revealed themselves as fast ...

GaAs/AlGaAs heterojunction as a fast detector of infrared laser pulses

We present experimental study of photoresponse in small area GaAs/AlGaAs heterojunction planar detector induced by nanosecond CO2 laser pulses. This device revealed itself as a fast IR sensor operating at room temperature. Hot carrier effects are proposed to be responsible for the photoresponse formation.

GaAs/AlGaAs structures with δ-doped layer for microwave detection

Planar microwave detectors on the base of modulation doped AlGaAs/GaAs structures with &dgr;-doped layer were investigated in (26÷120) GHz frequency range. Comparison of the features of the microwave diodes on the base of modulation doped structures with &dgr;- and smoothly-distributed doping impurities in the AlGaAs barrier is presented. Influence of the layers composing the modulation doped structure onto detective properties of the microwave diodes is ascertained both theoretically and experimentally. In the case of the structure with &dgr;-doping this influence was less, especially, in the case of symmetrically shaped structure with n-n+ and homogeneous asymmetrically shaped modulation doped structure.

THz/subTHz Detection by Asymmetrically‐Shaped Bow‐Tie Diodes Containing 2DEG Layer

We present asymmetrically‐shaped bow‐tie diodes based on a modulation‐doped GaAs/AlGaAs structure. One of the bow‐tie leaves is metallized in order to concentrate the incident radiation into the apex of the other half which contains the 2DEG layer: Here the electrons are heated non‐uniformly by incident radiation inducing a voltage signal over the ends of the device. The diode sensitivity at room temperature within 10 GHz – 0.8 THz is close to 0.3 V/W, while with an increase of frequency up to 2.52 THz it decreases due to weaker coupling. We consider options to improve the operation of the device.