S. A. Dvoretsky

Molecular beam epitaxy of high quality Hg1 − xCdxTe films with control of the composition distribution

Abstract An investigation of Hg 1 − x Cd x The heterostructure (MCT HS) growth processes using in situ ellipsometry by molecular beam epitaxy (MBE) was carried out. High quality MCT HS with a low density of V-shaped defects (near 10 2 cm −2 ) were grown on (103) GaAs substrates. Electron probe micro analysis (EPMA) revealed the presence of elemental tellurium inside the V-shaped defects. The appearance of V-shaped defects was connected with processes in which tellurium molecules take part on the growing surface. The MCT HS with wide band gap layers at the interface with the substrate and at the surface had the best electrical characteristics. In as-grown n-type material the electron concentrations, mobilities and life time of the minority carriers were 2 × 10 14 –5 × 10 15 cm −3 , 50000–350000 cm 2 /V · s, and up to 10 −6 s, respectively. In as-grown p-type material the hole concentrations, mobilities and life time of the minority carriers were 5 × 10 15 –2 × 10 16 cm −3 , over 500 cm 2 /V · s, and over 10 −7 s, respectively. Photoconductors and photodiodes were fabricated on the base of MCT HS with good photoelectrical parameters.

Peculiarities of the MBE growth physics and technology of narrow-gap II–VI compounds

Copyright (c) 1997 Elsevier Science S.A. All rights reserved. Experimental research and a crystallochemical consideration of the chemical interaction of the film and substrate components in the A 2 B 6 /GaAs heterostructure were carried out. It was found that surface faceting, twinning and breaking of stoichiometry of the growing structure can be explained by the excess of valent electrons at the A 2 B 6 /GaAs interface. The films were grown in a multichamber mercury cadmium telluride (MCT) MBE system with different compositions of the residual gas phase in different chambers. The MCT growth chamber is equipped with a system of molecular sources of original design and a built-in automatic ellipsometer. The used system of sources allows for growing MCT films without sample rotation, uniform in composition over the substrate area of 51 mm in diameter (the composition gradient does not exceed 0.002 cm −1 ). Because there is no substrate rotation, continuous measurements of film composition can be performed during growth, and structures with specified composition over thickness can be grown. The introduction of layers with a varying energy gap into the heteroepitaxial structure increases the carriers lifetime up to 2 μs for x=0.22 (77 K). A model of defects, determining the electric parameters of MBE grown films in the presence of gallium sources, is suggested. Linear photoconductor arrays for the 8-12 μm band with background limited parameters were fabricated from the grown MCT films.

Defect formation during growth of CdTe(111) and HgCdTe films by molecular beam epitaxy

(111)B CdTe films and (111)B and (112)B HgCdTe films were grown by MBE. The films were examined by optical and transmission electron microscopy. Formation of the microtwins and dislocations was observed in the films. The HgCdTe films contained some complicated defects also. The interaction of partial dislocations in the twinning plane results in the creation of threading dislocations. The increase of twin boundary density leads to an increase of the threading dislocation density in CdTe (111)B films. The existence of the elemental tellurium phase is possible during Hg0.8Cd0.2Te film growth by MBE. This circumstance in combination with the twinning process leads to the avalanche-type multiplication of the structure defects.

The controlled growth of high-quality mercury cadmium telluride

The investigations of HgCdTe heterostructures (MCT HS) and molecular beam epitaxy (MBE) growth processes using in-situ ellipsometry were carried out. The high-quality MCT HS with low density of V-shape defects (10 2 cm -2 ) were grown on (103) CdTe and GaAs substrates. The MCT HS with wide band gap layers at the interface and at the surface exhibited the best electrical characteristics. The electron concentrations, mobilities and life-time of minority carriers were 2 X 10 14 -5 X 10 15 cm -3 , 50000-350000 cm 2 V -1 s -1 , 10 -6 s, respectively. The hole concentrations, mobilities and life-time of minority carriers were 5 X 10 15 -2 X 10 16 cm -3 , above 500 cm 2 V -1 s -1 , 10 -7 s, respectively. On the base of the grown MCT HS, photoconductors and photodiodes were fabricated with good photoelectrical parameters. Electron probe microanalysis studies revealed the presence of elemental tellurium inside V-shape defects. Thermodynamic calculations suggested the possibility of elemental tellurium crystallization during the MCT HS growth by MBE.

Cyclotron-resonance-assisted photcurrents in surface states of a three-dimensional topological insulator based on a strained high-mobility HgTe film

We report on the observation of cyclotron-resonance-induced photocurrents, excited by continuous wave terahertz radiation, in a three-dimensional topological insulator (TI) based on an 80-nm strained HgTe film. The analysis of the photocurrent formation is supported by complementary measurements of magnetotransport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample, we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages, we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR-assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the cyclotron masses and the mobility of two-dimensional Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band.

Giant photocurrents in a Dirac fermion system at cyclotron resonance

We report on the observation of the giant photocurrents in HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum emerges. At an exciting QW of 6.6 nm width by terahertz (THz) radiation and sweeping magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative (−0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical doping. The photocurrent data, accompanied by measurements of radiation transmission as well as Shubnikov–de Haas and quantum Hall effects, prove that the photocurrent is caused by cyclotron resonance in a Dirac fermion system, which allows us to obtain the effective electron velocity v≈7.2×105 m/s. We develop a microscopic theory of the effect and show that the inherent spin-dependent asymmetry of light-matter coupling in the system of Dirac fermions causes the electric current to flow.

Defect study in molecular beam epitaxy-grown HgCdTe films with activated and unactivated arsenic

A defect study was performed on molecular beam epitaxy-grown HgCdTe films in situ doped with arsenic. Doping was performed from either effusion cell or cracker cell, and studied were both as-grown samples and samples subjected to arsenic activation annealing. Electrical properties of the films were investigated with the use of ion milling as a means of “stirring” defects in the material. As a result of the study, it was confirmed that the most efficient incorporation of electrically active arsenic occurs at the cracking zone temperature of 700u2009°C. Interaction between arsenic and tellurium during the growth was observed and is discussed in the paper.

Ellipsometry as a powerful tool for the control of epitaxial semiconductor structures in-situ and ex-situ

Abstract The results of the consistent use of in-situ and ex-situ ellipsometry as a control of MCT/CdTe/GaAs heterostructures growth by molecular beam epitaxy (MBE) are presented. It is shown that ellipsometry is able to control the quality of a GaAs substrate preparation prior to epitaxy, to measure the buffer layers and MCT films growth rate, to monitor in-situ composition and surface morphology during deposition process. Temperature measurements of CdTe dielectric functions were made which show the possibility of in-situ determination of surface temperature.

Probing Quantum Capacitance in a 3D Topological Insulator

We measure the quantum capacitance and probe thus directly the electronic density of states of the high mobility, Dirac type two-dimensional electron system, which forms on the surface of strained HgTe. Here we show that observed magnetocapacitance oscillations probe-in contrast to magnetotransport-primarily the top surface. Capacitance measurements constitute thus a powerful tool to probe only one topological surface and to reconstruct its Landau level spectrum for different positions of the Fermi energy.

Photogalvanic probing of helical edge channels in two-dimensional HgTe topological insulators

We report on the observation of a circular photogalvanic current excited by terahertz (THz) laser radiation in helical edge channels of HgTe-based 2D topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from right-handed to left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a 7