We report the effect of the built-in electric field emerging in elastically strained multiple InGaAs/GaAs quantum wells on the THz generation efficiency upon irradiation of the surface of these heterostructures, as well as photoconductive antennas based on them, with femtosecond optical laser pulses. The built-in field arises as a result of the piezoelectric effect in heterostructures with multiple {InGaAs/GaAs} x 10 quantum wells grown on GaAs substrates with (110) and (111)A crystallographic orientations. Terahertz radiation produced under the same excitation conditions from films with the same composition, but grown on substrates with different orientations, is compared. The most intense THz radiation is obtained from the surface of the {In0.2Ga0.8As/GaAs} x 10 heterostructure on a GaAs (110) substrate. Among photoconductive antennas, the highest efficiency of THz generation is demonstrated by antennas made on (110) and (100)-oriented {In0.2Ga0.8As/GaAs} x 10 heterostructures. In this case, the influence of the substrate orientation, which is clearly manifested upon generation of THz radiation directly by the surface of the films, is much weaker for photoconductive antennas on the same films.

In this study we investigate a relatively new material for terahertz (THz) photoconductive antennas (PCAs): GaAs epitaxial films grown on (111)A-oriented semi-insulated GaAs substrates and doped with Si. GaAs:Si (111)A films with the required high resistance without postgrowth annealing have been grown by molecular-beam epitaxy. The Si doping of (111)A-oriented GaAs was expected to lead to the formation of acceptors that facilitate the activation of As[Formula: see text] traps. The relaxation times of nonequilibrium charge carriers in the films were measured in pump-probe experiment, and, finally, the electron mobility was estimated from the carrier lifetime and current–voltage characteristics measured under pulsed pumping by Ti:sapphire femtosecond laser. Dipole and bow-tie PCAs were fabricated, and the THz emission spectra and the total THz emission power were measured with varying applied voltage and optical excitation power. The properties of GaAs:Si (111)A films and the PCAs based on them are compared with LTG-GaAs (100) and (111)A-oriented films.

The production of superlattices with pseudomorphically strained quantum wells (QWs) {In x Ga 1– x As/GaAs} grown by molecular beam epitaxy on GaAs substrates with (100), (110), and (111)A crystallographic surface orientations is reported. The quality of the crystal structure of epitaxial samples is assessed using the atomic force microscopy of their surface. The manifestation of a piezoelectric field in the photoluminescence spectra is reported.

The production of superlattices with pseudomorphically strained quantum wells (QWs) {InxGa1–xAs/GaAs} grown by molecular beam epitaxy on GaAs substrates with (100), (110), and (111)A crys-tallographic surface orientations is reported. The quality of the crystal structure of epitaxial samples is assessed using the atomic force microscopy of their surface. The manifestation of a piezoelectric field in the photoluminescence spectra is reported.

Utilizing the amphoteric property of silicon impurity in GaAs (111)A for acceptor doping of low‐temperature‐grown (LTG‐) GaAs films and LTG‐GaAs‐based heterostructures for THz photoconductive applications is proposed and realized. The electronic properties of superlattice structures {LTG‐GaAs/GaAs:Si} grown by molecular beam epitaxy on (100) and (111)A GaAs substrates are experimentally investigated, where GaAs:Si are silicon‐doped GaAs layers grown at standard conditions. The use of GaAs substrates with surface orientation (111)A results in spatially indirect p‐type doping of LTG‐GaAs layers. In addition, the charge redistribution at the LTG‐GaAs/GaAs:Si interfaces, due to the presence of silicon atoms in GaAs:Si layers and antisite point defects in adjacent LTG‐GaAs layers, is also analyzed by means of band structure modeling.

Molecular-beam epitaxial growth of Si-doped GaAs single-crystal layers on (110)-oriented GaAs substrates has been studied. The surface morphology of grown films was analyzed by scanning electron microscopy and atomic force microscopy, and the crystal structure of grown films was estimated by X-ray grazing incidence diffraction, in-plane pole figures, reciprocal space mapping, and photoluminescence spectroscopy. The type, concentration, and mobility of charge carriers in films were measured by the four-probe method in van der Pauw geometry at temperatures of 300 and 77 K. The possible existence of two areas in growth conditions, where increased concentration and mobility of electrons are achieved, was shown: the first, main area with the highest concentration and mobility values is Tg = 450–500 °C and V/III ratio γ = 20–40, the second, minor one is Tg = 600–680 °C and γ = 40–70. The hole conductivity was obtained at a growth temperature of 580 °C and a low γ value of 16. It was also shown that the defect-free crystal structure of the films grown at high temperatures is not necessarily accompanied by a smooth surface.

The manifestation of the piezoelectric effect in the photoluminescence spectra of superlattices with elastically strained {InxGa1 – xAs/GaAs} quantum wells grown by molecular-beam epitaxy on GaAs substrates with the crystallographic surface orientations (100), (110), and (111)A is reported.

A new structure for photoconductive antennas is proposed, which represents a multilayer epitaxial film grown on a GaAs(111)A substrate and consisting of alternating low-temperature grown undoped GaAs (LTG-GaAs) layers and GaAs layers formed in the standard high-temperature mode and doped with silicon (GaAs:Si). The As4 and Ga flow ratio γ is chosen such that the GaAs:Si layers have p-type conductivity. The LTG-GaAs layers are grown at a high γ value. A similar structure is grown on a (100) substrate as a reference. The structural properties of the samples are investigated: the phase composition is explored by high-resolution X-ray diffractometry; the surface relief, by atomic-force microscopy, and the Si-impurity thickness distribution, by secondary-ion mass spectrometry.

The production of superlattices with pseudomorphically strained quantum wells (QWs) {InxGa1–xAs/GaAs} grown by molecular beam epitaxy on GaAs substrates with (100), (110), and (111)A crys-tallographic surface orientations is reported. The quality of the crystal structure of epitaxial samples is assessed using the atomic force microscopy of their surface. The manifestation of a piezoelectric field in the photoluminescence spectra is reported.

10-period superlattices with pseudomorphic quantum wells {InxGa1-xAs/GaAs} with indium molar fractions x = 0.1 and 0.2 were grown by molecular beam epitaxy, each of them on GaAs substrates with crystallographic surface orientations (100), (110), and (111)A. A built-in piezoelectric field in the quantum wells of superlattices on GaAs (111)A substrates is formed in the direction perpendicular to the plane of the layers, and it causes a red shift of the peak in the photoluminescence spectrum compared to superlattices on conventional GaAs (100) substrates due to the quantum-confined Stark effect. This red shift increases with reducing photoexcitation power, as the photoexcited carriers screen the piezoelectric field. On the other hand, the peak in the photoluminescence spectra of superlattices on GaAs (110) substrates is not redshifted and does not change its energy position with a reducing in the photoexcitation power, since the piezoelectric field, even if it was formed, is directed laterally in the plane of the layers and does not cause quantum-confined Stark effect.

Epitaxial In0.53Ga0.47As films, grown on GaAs substrates with the (100) and (111)А crystallographic orientations in the standard high-temperature and low-temperature modes, have been studied using X-ray diffraction (XRD). The parameters of GaAs substrates and In0.5Ga0.5As films were matched using step metamorphic buffers. A technique for determining the strain parameters of the crystal structure of elastically strained In0.53Ga0.47As layers with the (111) crystallographic orientation using high-resolution XRD data has been developed. The strain parameters of the crystal structure of the samples under study are determined. A correlation between the tilt angle of the In0.53Ga0.47As layers with respect to the GaAs substrate and degree of their relaxation are calculated; layers with weak relaxation are found to have larger tilt angles with respect to the substrate.

Results on the creation and properties of transistor-type MIS structures (MIST) with an Al2O3 thin-film gate dielectric based on PbSnTe:In films obtained by molecular beam epitaxy are presented. The source-drain current-voltage characteristics (CVC) and gate characteristics of the MIST at Т = 4.2 К have been investigated. It is shown that in MIST based on PbSnTe:In films with n ~ 1017 cm-3 the modulation of the channel current reaches 7 – 8 % in the range of gate voltages – 10 V < Ugate < + 10 V. The features of the source-drain CVC and the gate characteristics for a pulsed and sawtooth variation of Ugate are considered.

Results on the creation and properties of transistor-type MIS structures (MIST) with an Al2O3 thin-film gate dielectric based on PbSnTe : In films obtained by molecular beam epitaxy are presented. The source-drain current-voltage characteristics (CVC) and gate-controlled characteristics of the MIST at [BT=4.2 K have been investigated. It is shown that in MIST based on PbSnTe : In films with n~1017 cm-3 the modulation of the channel current reaches 7-8% in the range of gate voltages -10 Vgate<+10 V. The features of the source-drain CVC and the gate-controlled characteristics for a pulsed and sawtooth variation of Ugate are considered. Keywords: solid solution PbSnTe : In, field effect, MIS structure, Al2O3.

The surface topology of epitaxial films of lead tin telluride solid solution (including with In additive, Pb1 – xSnxTe:In) has been examined using atomic force microscopy. The films have been grown on BaF2(111) single-crystal substrates and on a CaF2/BaF2 buffer layer covering a Si(111) wafer. It has been shown that the relief statistical parameters depend on film growth conditions and the incorporation mechanism of indium, an excess amount of which has been detected on the surface by ex situ XPS.

In present work, the effect of Pb1-xSnxTe In In (1 1 1) surface in HCl-isopropanol (HCliPA) solution led to removal of native oxides and surface enrichment by elemental tellurium of several nm thickness. Subsequent anneals in vacuum led to desorption of elemental tellurium and revealing (1x1) surface structure. We present angle resolved photoemission spectroscopy (ARPES) measurements of the surface states on chemically prepared (1 1 1) oriented MBE-grown films of Pb1-xSnxTe, a three-dimensional topological crystalline insulator (TCI). The surface states with Dirac-like dispersion at Gamma in the surface Brillouin zone were detected. The dark current and photocurrent were found in strong dependence on the surface chemical procedure and composition. The relative simplicity of the preparation technique is encouraging, and suggests a clear path for future investigations of TCI states on alternative surface orientations in (Pb,Sn)Te solid solutions and applied aspects of TCI research.

•The chemical treatment of Pb1-xSn xTe (111) films with HCl-iPA solution removed native oxides.•Further annealing in vacuum led to the formation of structurally ordered Pb1-xSn xTe (111)-(1x1) surface.•ARPES demonstrated the surface topological states of a crystalline insulator with Dirac dispersion.•A significant effect of the HCl-iPA treatment on electrophysical properties was demonstrated.

In article 2100041, Arseniy Buryakov and co-workers report on a new approach to significantly improve the performance (efficiency and spectrum width) of already well-developed GaAs THz antennas. The cover shows three components of the proposed approach: the choice of the direction of the crystallographic cut of the GaAs substrate, a new design of the photoconductive material, and the use of a nanoplasmonic grid.

An alternative approach is proposed to improve the conventional (based on the low‐temperature grown GaAs and Si‐doped GaAs superlattice) photoconductive antenna (PCA) performance by modification of the planar electrodes design and crystallographic orientations of the GaAs substrate ((100) and (111)‐A). The electrode scheme design includes a combination of logarithmic spiral, bow‐tie, and plasmonic antennas and results in appearance of sharp resonant peaks, high spectral bandwidth and high signal‐to‐noise ratio, and significant enhancement of the output terahertz (THz) power. The material design leads to significant increase in the THz output power (by 6.4 in GaAs (100), by 5.6 in GaAs (111)‐A PCAs) regarding to a conventional antenna. The substrate crystallographic cut direction influences the relaxation time constant of photoexcited charge carriers being an order of magnitude smaller in the sample grown on the GaAs (111)‐A than in the one on the GaAs (100). The simulation model supports experimental results demonstrating that the optimal period of the plasmonic antenna grid providing the highest efficiency of THz radiation generation, is about 200 nm. Comparison of the THz spectra in manufactured antennas against the conventional stripline PCA shows a broadening band toward the low‐frequency region down to 0.1 THz with a resonance at 0.2 THz.

The topology of the surface of epitaxial films of lead tin telluride solid solution, including those with the addition of indium (Pb1-xSnxTe:In), grown on single-crystal BaF2 (111) substrates and a CaF2/BaF2 buffer layer on Si (111) was studied by atomic force microscopy. It is shown that the characteristic statistical indicators of the relief are due to the peculiarities of film growth and the mechanism of incorporation of Indians, the excess content of which was registered on the surface ex situ by X-ray photoelectron spectroscopy.