R. A. Khabibullin

Investigation and Fabrication of the Semiconductor Devices Based on Metamorphic InAlAs/InGaAs/InAlAs Nanoheterostructures for THz Applications

A metamorphic high electron mobility transistor (MHEMT) with “zig–zag” –like gate of a length of 46 nm and cut-off frequencies for the current and power gain fr=0.13 THz and fmax=0.63 THz respectively was fabricated on the base of InAlAs/InGaAs/InAlAs nanoheterostructure.

Terahertz radiation in In0.38Ga0.62As grown on a GaAs wafer with a metamorphic buffer layer under femtosecond laser excitation

The results of time-domain spectroscopy of the terahertz (THz) generation in a structure with an In0.38Ga0.62As photoconductive layer are presented. This structure grown by molecular-beam epitaxy on a GaAs substrate using a metamorphic buffer layer allows THz generation with a wide frequency spectrum (to 6 THz). This is due to the additional contribution of the photo-Dember effect to THz generation. The measured optical-to-terahertz conversion efficiency in this structure is 10–5 at a rather low optical fluence of ~40 μJ/cm2, which is higher than that in low-temperature grown GaAs by almost two orders of magnitude.

Fabrication of a terahertz quantum-cascade laser with a double metal waveguide based on multilayer GaAs/AlGaAs heterostructures

The Postgrowth processing of GaAs/AlGaAs multilayer heterostructures for terahertz quantumcascade lasers (QCLs) are studied. This procedure includes the thermocompression bonding of In–Au multilayer heterostructures with a doped n+-GaAs substrate, mechanical grinding, and selective wet etching of the substrate, and dry etching of QCL ridge mesastripes through a Ti/Au metallization mask 50 and 100 μm wide. Reactive-ion-etching modes with an inductively coupled plasma source in a BCl3/Ar gas mixture are selected to obtain vertical walls of the QCL ridge mesastripes with minimum Ti/Au mask sputtering.

Investigation of the fabrication processes of AlGaN/AlN/GaN НЕМТs with in situ Si3N4 passivation

The optimum mode of the in situ plasma-chemical etching of a Si3N4 passivating layer in C3F8/O2 medium is chosen for the case of fabricating AlGaN/AlN/GaN НЕМТs. It is found that a bias of 40–50 V at a high-frequency electrode provides anisotropic etching of the insulator through a resist mask and introduces no appreciable radiation-induced defects upon overetching of the insulator films in the region of gate-metallization formation. To estimate the effect of in situ Si3N4 growth together with the heterostructure in one process on the AlGaN/AlN/GaN НЕМТ characteristics, transistors with gates without the insulator and with gates through Si3N4 slits are fabricated. The highest drain current of the AlGaN/AlN/GaN НЕМТ at 0 V at the gate is shown to be 1.5 times higher in the presence of Si3N4 than without it.

Terahertz radiation generation in multilayer quantum-cascade heterostructures

I–V and radiative characteristics of multilayer quantum-cascade GaAs/AlGaAs heterostructures with a double metal waveguide are investigated. The I–V characteristics typical of the quantum-cascade lasers are seen. The observed threshold-intensity growth and narrow radiation spectrum are characteristic of laser generation. The measured radiation frequency was found to be about 3 THz, which coincides with the calculated value. Thus, fully domestic terahertz quantum-cascade lasers are demonstrated for the first time.

The influence of gate length on the electron injection of velocity in an AlGaN/AlN/GaN НЕМТ channel

Field-effect high-electron-mobility transistors (HEMTs) based on AlGaN/AlN/GaN heterostructures with various gate lengths Lg have been studied. The maximum values of current and power gaincutoff frequencies (fT and fmax, respectively) amounted to 88 and 155 GHz for HEMTs with Lg = 125 nm, while those for the transistors with Lg = 360 nm were 26 and 82 GHz, respectively. Based on the measured S-parameters, the values of elements in small-signal equivalent schemes of AlGaN/AlN/GaN HEMTs were extracted and the dependence of electron-injection velocity vinj on the gate–drain voltage was determined. The influence of Lg and the drain–source voltage on vinj has been studied.

Electrical and thermal properties of photoconductive antennas based on In x Ga1 – x As (x > 0.3) with a metamorphic buffer layer for the generation of terahertz radiation

The results of studies of the electrical and thermal properties of photoconductive antennas for terahertz-radiation generation are reported; these antennas are fabricated on the basis of low-temperaturegrown GaAs (LT-GaAs) and InxGa1 – xAs with an increased content of indium (x > 0.3). It is shown that the power of Joule heating PH due to the effect of “dark” current in InxGa1 – xAs exceeds the same quantity in LT-GaAs by three–five times. This is due to the high intrinsic conductivity of InxGa1 – xAs at x > 0.38. Heatremoval equipment for the photoconductive antenna has been developed and fabricated. The results of numerical simulation show that the use of a heat sink makes it possible to reduce the operating temperature of the antenna based on LT-GaAs by 16%, of the antenna based on In0.38Ga0.62As by 40%, and for antennas based on In0.53Ga0.47As by 64%.

Energy spectrum and thermal properties of a terahertz quantum-cascade laser based on the resonant-phonon depopulation scheme

The dependences of the electronic-level positions and transition oscillator strengths on an applied electric field are studied for a terahertz quantum-cascade laser (THz QCL) with the resonant-phonon depopulation scheme, based on a cascade consisting of three quantum wells. The electric-field strengths for two characteristic states of the THz QCL under study are calculated: (i) “parasitic” current flow in the structure when the lasing threshold has not yet been reached; (ii) the lasing threshold is reached. Heat-transfer processes in the THz QCL under study are simulated to determine the optimum supply and cooling conditions. The conditions of thermocompression bonding of the laser ridge stripe with an n+-GaAs conductive substrate based on Au–Au are selected to produce a mechanically stronger contact with a higher thermal conductivity.

Intensive Terahertz Radiation from InxGa1-xAs due to Photo-Dember Effect

We have proposed and investigated InyGa1-yAs photoconductor grown by molecular-beam epitaxy on low-temperature step-graded metamorphic buffer. It exhibits superior bandwidth up to 6 THz and provides optical-to-terahertz conversion efficiency up to ~ 10−5 for rather low optical fluence ~ 40 μJ/cm2. The intensity of THz generation for the given structure is two orders higher than for lowtemperature grown GaAs due substantial contribution of photo-Dember effect.

Electron transport and optical properties of structures with atomic tin nanowires on vicinal GaAs substrates

Electron transport and optical properties are studied for structures with atomic tin nanowires (Sn-NWs) on vicinal GaAs substrates with misorientation angles of 0.3 and 3° with respect to the exact (100) orientation. Saturation-current anisotropy is revealed in the current–voltage characteristics of the samples for current flows along (‖ orientation) and across (⊥ orientation) the Sn-NWs: the current ratios I‖/I⊥ are ∼1.2 and ∼2.5 for homostructures and pseudomorphic high electron mobility transistor (PHEMT) structures, respectively. The effect of the pulling voltage and illumination on current oscillations is studied in real time in the case of current flows perpendicular to the Sn-NWs. Clear anisotropy of the PHEMT frequency characteristics is shown.