S. Yu. Shapoval

High-responsivity terahertz detection by on-chip InGaAs/GaAs field-effect-transistor array

Terahertz detection by a one-dimensional dense array of field-effect transistors (FETs) is studied experimentally. Such terahertz detector demonstrates greatly enhanced responsivity without using supplementary antenna elements because a short-period grating formed by metal contact fingers of densely ordered transistors in the array serves as an effective antenna coupling incident terahertz radiation to the transistor channels. Asymmetrical position of the gate contact in each FET in the array enables strong photovoltaic response.

ELECTRON TRANSPORT PROPERTIES OF DOUBLE DELTA-DOPED GAAS STRUCTURES GROWN BY LOW-PRESSURE METALORGANIC CHEMICAL VAPOR DEPOSITION

Electron transport properties of double delta-doped GaAs structures based on high electron mobility single delta-doped layers with a sheet concentration of 3×1012 cm−2 have been investigated for spacer thicknesses up to 1100 A at 77 and 300 K. At an optimized spacer thickness of about 200 A, a maximum in conductivity is observed, which exceeds the conductivity of a single delta-doped layer with the same total concentration by 30% and 20% at 300 and 77 K. The mobility and concentration as a function of spacer thickness are also presented and analyzed.

Wide-aperture detector of terahertz radiation based on GaAs/InGaAs transistor structure with large-area slit grating gate

Terahertz photoresponse of a GaAs/InGaAs transistor structure with large-area slit grating gate has been measured. Peaks in the photoresponse curve are assigned to plasmon resonances excited in the structure. More effective excitation of plasmon resonances is achieved in a grating gate structure with narrow slits, which increase the photoresponse amplitude by an order of magnitude.

Detection of terahertz radiation by tightly concatenated InGaAs field-effect transistors integrated on a single chip

A tightly concatenated chain of InGaAs field-effect transistors with an asymmetric T-gate in each transistor demonstrates strong terahertz photovoltaic response without using supplementary antenna elements. We obtain the responsivity above 1000 V/W and up to 2000 V/W for unbiased and drain-biased transistors in the chain, respectively, with the noise equivalent power below 10−11 W/Hz0.5 in the unbiased mode of the detector operation.

Tungsten thin-film deposition on a silicon wafer: The formation of silicides at W-Si interface

The interphase boundary formed in the process of tungsten thin-film deposition on a silicon wafer is investigated. These films are produced via (1) a CVD technique relying on hydrogen reduction of tungsten hexafluoride, (2) the same technique supplemented with plasmochemical action, and (3) magnetron deposition used for comparison purposes. It is shown that a nanometer tungsten silicide W5Si3 layer is formed at the tungsten-silicon interface only under gas-phase deposition. The effect of annealing on the specimen composition and surface resistance is investigated. It is shown that the formation and growth of a silicide WSi2 layer commences at 700°C for CVD films and at above 750°C for films obtained with plasmochemical deposition; this results in a drastic increase in their electrical resistance. Under optimal conditions, tungsten films of 8 × 10 −6 Ω cm resistivity are produced.

A study of the effect of the structure of plasma-chemical silicon nitride on its masking properties

Dependence of the chemical composition of a film of plasma-chemical silicon nitride on the technological parameters of the deposition process is studied. The stages in which the process parameters are optimized in order to improve the masking properties of the film are described. A systematic study of the Fourier-transform infra-red (IR) spectra of plasma-chemical silicon-nitride films is carried out. It is found that the chemical resistance of a silicon-nitride film depends on the configuration in which hydrogen is incorporated into chemical bonds.

Detection of Terahertz Radiation by Dense Arrays of InGaAs Transistors

Detection of terahertz radiation by GaAs transistor structures has been studied experimentally. The two types of samples under study included dense arrays of HEMTs and large-apertures detectors. Arrays consisted of parallel and series chains with asymmetric gate transistors for enhanced photoresponse on terahertz radiation. We investigated two types of wide-aperture detectors: grating gate detector, and single gate detector with bow-tie antenna. Wide-aperture detectors were symmetrical. Studies of transistor chains have shown that two essential features for this type of detector are the presence of asymmetry in the gate, and the type of connection between individual transistors themselves. Wide-aperture detectors have also been tested by narrow beams of terahertz radiation, which allows analyzing the role influence of individual parts of the detector for total sensitivity to terahertz excitation. The sensitivity and noise equivalent power of the detectors were evaluated.

Conductivity and persistent photoconductivity in GaAs epitaxial films containing single and double delta-doped layers

Electrophysical parameters of single and double delta-doped layers in GaAs epitaxial films grown by the metal-organic chemical vapor deposition have been systematically investigated in the temperature range of 4.2 to 300 K. The 2D electron gas density distribution is affected by the overlap of wave functions in neighboring quantum wells, as a result of which the peak on the curve of the Hall mobility in the 2D electron gas versus the separation between the quantum wells shifts. The persistent photoconductivity in delta-doped layers is due to the change in the surface potential caused by the neutralization of the negative charge of surface states by photoexcited holes. A method for comparing delta-doped layers grown under different conditions at different depths from the sample surface has been suggested.

On the electron mobility of δ layers in the presence of diamagnetic “ejection” of size-quantization levels

The Hall mobility of electrons is investigated as a function of the population of size-quantization subbands in the two-dimensional electron gas of a δ-doped layer in GaAs with constant total electron density Ns=3.2×1012 cm−2 (three initially filled subbands) at T=4.2 K. The population of the subbands is varied by diamagnetic “ejection” of size-quantization levels (i.e., pushing them over the Fermi level) by a magnetic field oriented parallel to the plane of the δ-doped layer. The measurements are made in magnetic fields making small angles (5°) with the plane of the doping. The magnetic field component normal to the plane was used to measure the Hall mobility and density. It is found that the measured Hall mobility as a function of the ejecting magnetic field has a distinct maximum. This maximum is due to an increase in the electron mobility in the first subband (the ground subband is assigned the index 0) and electron redistribution between subbands with in increasing ejecting magnetic field parallel to the plane of the δ layer.