Vegard’s law and superstructural phases in Al<Subscript>x</Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs(100) epitaxial heterostructures

The lattice constants of Al_xGa_1?x As epitaxial alloys with various AlAs (x) contents are determined for AlxGa_1?xAs/GaAs(100) heterostructures grown by MOC-hydride epitaxy using X-ray diffractometry and an X-ray back-reflection method. An ordered AlGaAs₂ (superstructural) phase is found in epitaxial heterostructures with x ≈ 0.50. The lattice constant of this phase is smaller than the lattice constants of an Al_0.50Ga_0.50As alloy and GaAs single-crystal substrate.     

Experimental studies of the effects of atomic ordering in epitaxial Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>P alloys on their optical properties

The properties of epitaxial Ga _x In_{1 – x} P alloys with an ordered arrangement of atoms in the crystal lattice are studied by a number of spectroscopic methods. The alloys are grown by metal-organic chemical vapor deposition onto single-crystal GaAs(100) substrates. It is shown that, under conditions of the coherent growth of an ordered Ga _x In_{1 – x} P alloy on a GaAs(100) substrate, atomic ordering results in radical modifications of the optical properties of the semiconductor compared to the properties of disordered alloys. Among these modifications are a decrease in the band gap and an increase in the luminescence intensity. From the data of dispersion analysis of the infrared dispersion spectra and from ultraviolet spectroscopy data obtained in the transmittance–reflection mode of measurements, the basic optical characteristics, specifically, the dispersion of the refractive index and the high-frequency permittivity of Ga _x In_{1 – x} P alloys with ordering are determined. All of the experimental data are in good agreement with the developed theoretical concepts.     

Thermodynamic stability of bulk and epitaxial CdHgTe,ZnHgTe, and MnHgTe alloys

The thermodynamic stability of Cd_1?xHg_xTe, Mn_xHg_1?xTe, and Zn_xHg_1?xTe alloys is studied. Calculations performed in the context of the δ lattice-parameter model indicate that CdHgTe and ZnHgTe alloys are stable over the entire range of compositions at typical growth temperatures. At the same time, a miscibility gap is found in Mn_xHg_1?xTe at 0.33 < x < 1 at T = 950 K, which is consistent with the known experimental data. It is shown that the biaxial strains observed in Mn_xHg_1?xTe/CdTe and Mn_xHg_1?xTe/Cd_0.96Zn_0.04Te thin epitaxial films lead to a narrowing of the miscibility gap and to insignificant lowering of critical temperatures.     

Resonant Raman scattering and atomic force microscopy of InGaAs/GaAs multilayer nanostructures with quantum dots

The transition from two-dimensional (2D) pseudomorphic growth to the three-dimensional (3D) (nanoisland) growth in In_xGa_1?xAs/GaAs multilayer structures grown by molecular-beam epitaxy was investigated by atomic force microscopy, photoluminescence, and Raman scattering. The nominal In content x in In_xGa_1?xAs was varied from 0.20 to 0.50. The thicknesses of the deposited In_xGa_1?xAs and GaAs layers were 14 and 70 monolayers, respectively. It is shown that, at these thicknesses, the 2D-3D transition occurs at x ≥ 0.27. It is ascertained that the formation of quantum dots (nanoislands) does not follow the classical Stranski-Krastanov mechanism but is significantly modified by the processes of vertical segregation of In atoms and interdiffusion of Ga atoms. As a result, the In_xGa_1?xAs layer can be modeled by a 2D layer with a low In content (x < 0.20), which undergoes a transition into a thin layer containing nanoislands enriched with In (x > 0.60). For multilayer In_xGa_1?xAs structures, lateral alignment of quantum dots into chains oriented along the \([\overline 1 10]\) direction can be implemented and the homogeneity of the sizes of quantum dots can be improved.     

High-efficiency dual-junction GaInP/GaAs tandem solar cells obtained by the method of MOCVD

Monolithic dual-junction GaInP/GaAs solar cells grown by the MOCVD method were studied. The conditions of the growth of ternary Ga _x In_1?x P and Al _x In_1?x P alloys lattice-matched to GaAs are optimized. Technology for fabrication of a tunneling diode with a high peak current density of 207 A/cm² on the basis of heavily doped n ⁺⁺-GaAs:Si and p ⁺⁺-AlGaAs:C layers is developed. Cascade GaInP/GaAs solar cells obtained as a result of relevant studies featuring a good efficiency of the solar-energy conversion both for space and terrestrial applications. The maximum value of the GaInP/GaAs solar-cell efficiency was 30.03% (at AM1.5D, 40 suns).     

Chemical bonding and elastic constants of certain ternary III–V solid solutions

The chemical bonding of GaP, InP, InAs, InSb, and InBi binary compounds was investigated as well as the modification of the bonding in Ga_xIn_1?xP, InAs_xSb_1?x, and InSb_1?xBi_x semiconductor alloys; this modification occurs as a result of variation in the composition characterized by the parameter x. An approach based on the consideration of the total valence charge density, the chemical-bonding polarity, and the transverse effective charge was used. The elastic constants of the aforementioned ternary solid solutions were calculated, and the influence of chemical-bonding modification on these constants was analyzed. Local strains and composition disordering in the alloys under consideration drastically affect the dependences of the quantities under investigation on solid-solution composition. Thus, the above effects cannot be neglected in investigating substitutional solid solutions.     

Specific features of electrical properties of the In<Subscript>x</Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>N alloy

Temperature dependences of the Hall coefficient and resistivity of the In_xGa_{1 ? x} N alloys (0 ≤ x ≤ 1) are investigated. It is found that, at x ≤ 0.4, the temperature dependences of the Hall coefficient and resistivity have the activation-related portion. The activation energy depends linearly on the In content in the alloy. At x ≈0.5, the activation portion vanishes. The main scattering mechanism depends on the temperature, on the defect density in the film (this density is largely determined by the used intermediate GaN layers), and on the alloy composition x.     

Study of optical characteristics of structures with strongly strained In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As quantum wells

Results of photoluminescence (PL) studies of heterostructures with strongly strained InxGa_{1 ? x} As quantum wells (QWs) are presented. It is shown that the dependence of the PL intensity on the QW thickness has a maximum whose position depends on the composition of the In _x Ga_{1 ? x} As solid solution. The PL wavelength at the maximum intensity is 1.13 µm at a QW thickness of 60 µm at a QW thickness of 50 Å for x = 0.39 and 0.42, respectively.     

Transition from the type-II broken-gap heterojunction to the staggered one in the GaInAsSb/InAs(GaSb) system

Conditions for the transition from the staggered heterojunction to the type-II broken-gap one were considered for isolated Ga_1?x In_xAs_ySb_1?y /InAs(GaSb) heterostructures in relation to the quaternary alloy composition. Energy-band diagrams of such heterojunctions were estimated and energy band offsets Δ at the heterointerface were determined. It was experimentally found that the type-II broken-gap heterojunction in the Ga_1?x In_xAs_ySb_1?y /p-InAs structure is observed in the entire range of composition parameters under study, 0.03 < x < 0.23, and becomes staggered in the range 0.3 < x < 1. In p-Ga_1?x In_xAs_ySb_1?y /p-GaSb heterostructures with the indium content 0.85 < x < 0.92 in the solid phase, the p-type conductivity is observed, which is indicative of the staggered heterojunction. At x > 0.92, the contribution of electrons of the semimetal channel at the heterointerface to the total conductivity was observed, as well as the transition from the staggered heterojunction to the type-II broken-gap one.     

Spin effects in magnetoresistance induced in an <Emphasis Type="Italic">n</Emphasis>-In<Subscript>x</Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs double quantum well by a parallel magnetic field

Magnetoresistance in n-In_xGa_1?xAs/GaAs (x ≈ 0.18) heterostructures with double quantum wells (DQWs) was studied in the magnetic field parallel to the DQW layer. Specific features of the magnetoresistance, related to the passing of the tunnel gap edges across the Fermi level, are revealed and studied. Agreement between the calculated and experimental positions of the observed features is obtained when the spin splitting of the energy spectrum is taken into account. Earlier, similar features were observed in the magnetoresistance of n-GaAs/Al_xGa_1?xAs DQW heterostructures, but the spin effects did not manifest themselves.     

Theoretical Study and Simulations of an InGaN Dual-Junction Solar Cell

This study aims to determine the optimal configuration of the dual-junction InGaN solar cell. Several parameters of the dual-InGaN-junction solar cell have been investigated as the band gap combination and the thicknesses of the layers. Physical models and the optical properties of the In _x Ga_1?x N according to the indium content have been used. The dual-junction solar cell has been designed and simulated for each chosen band gap combination. The current densities drawn from the sub-cells were matched by adjusting their emitter layers thicknesses. The best conversion efficiency obtained for the optimized dual-junction In_0.49Ga_0.51N/In_0.74Ga_0.26N solar cell, under standard conditions, was 34.93% which corresponds to the band gap combination of 1.73 eV/1.13 eV. The short-circuit current density and the open circuit voltage obtained from the tandem cell In_0.49Ga_0.51N/In_0.74Ga_0.26N are respectively, 21.3941 mA/cm² and 1.9144 V. The current mismatch was 0.057%. The effects of the front and back layers thicknesses of the top and bottom cells on the efficiency were also studied. Furthermore, the electrical characteristics of the dual-junction solar cell and its sub-cells were also discussed.     

Study of certain properties of Si-Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript> (0 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 1) structures grown from a restricted tin-based solution-melt by liquid-phase epitaxy

The lattice parameters and band gap are experimentally determined for Si-Si_1?xGe_x structures in relation to the component ratiox (0 ≤ x ≤ 1). The distribution of components over the hickness of the Si_1?xGe_x alloys and certain photoelectric properties are studied. The experimental data indicate that the structures obtained are of high quality. Graded-gap Si_1?xGe_x alloys (0 ≤ x ≤ 1) can be used for the fabrication of photoelectric devices sensitive in the visible and near-IR regions. They can also be used as substrates for GaAs and GaAs-based layers.     

Tuning the energy spectrum of InAs/GaAs quantum dots by varying the thickness and composition of the thin double GaAs/InGaAs cladding layer

It is shown that the ground state transition energy in quantum dots in heterostructures grown by atmospheric-pressure MOCVD can be tuned in the range covering both transparence windows of the optical fiber at wavelengths of 1.3 and 1.55 μm by varying the thickness and composition of the thin GaAs/In_xGa_1−x As double cladding layer. These structures also exhibit a red shift of the ground state transition energy of the In_xGa_1−x As quantum well (QW) as a result of the formation of a hybrid QW In_xGa_1−x As/InAs (wetting layer) between the quantum dots (QDs). The Schottky diodes based on these structures are characterized by an increased reverse current, which is attributed to thermally activated tunneling of electrons from the metal contact to QD levels. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 4, 2004, pp. 448–454. Original Russian Text Copyright ? 2004 by Karpovich, Zvonkov, Levichev, Baidus, Tikhov, Filatov, Gorshkov, Ermakov.     

Population inversion between Γ subbands in quantum wells under the conditions of Γ-<Emphasis Type="Italic">L</Emphasis> intervalley transfer

Monte Carlo simulations of electron transport in Al_xGa_1−x As/GaAs/In_yGa_1−y As double-quantum-well heterostructures in high lateral electric fields are carried out. It is shown that, under the conditions of intervalley Γ-L electron transfer, there exists a population inversion between the first and the second quantum-confinement subbands in the Γ valley. The population inversion appears in the fields exceeding 4 and 5.5 kV/cm at 77 and 300 K, respectively. The gain in a superlattice composed of such quantum wells is estimated to be on the order of 100 cm⁻¹ for radiation with a wavelength of 12.6 μm. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 224–229. Original Russian Text Copyright ? 2003 by Aleshkin, Andronov, Dubinov.     

On the preparation and photoelectric properties of Tl1–x In1–x Sn x Se2 (x = 0.1–0.25) alloys

The technological conditions for growing single crystals of Tl_1–x In_1–x Sn _x Se₂ (x = 0.1–0.25) alloys are developed. The spectral distribution of the photoconductivity of the grown crystals at T = 300 K and thermally stimulated conductivity are studied. The effect of In³⁺cation substitution with Sn⁴⁺ in Tl_1–x In_1–x Sn _x Se₂ (x = 0.1–0.25) alloys on their photoelectric properties is shown.     

Dielectric waveguide for middle and far infrared radiation

The possibility of using the normal skin effect in dielectric waveguides for long-wavelength radiation is analyzed. A design of a waveguide integrated with a heterolaser is suggested, in which an undoped layer of GaAs is clad between heavily-doped n- and p-Al _x Ga_{1 ? x} As alloy layers, reflecting radiation because of the normal skin effect. It is shown that an efficient waveguide can be formed using n-Al _x Ga_{1 ? x} As layers with x < 0.45 and the electron concentration N > 5 × 10¹⁸ cm^?3 and p-Al _x Ga_{1 ? x} As layers of any composition with the hole concentration P ≥ 3 × 1019 cm^?3.     

1 550 nm long-wavelength vertical-cavity surface emitting lasers

A 1 550 nm long-wavelength vertical cavity surface emitting laser (VCSEL) on InP substrate is designed and fabricated. The transfer matrix is used to compute reflectivity spectrum of the designed epitaxial layers. The epitaxial layers mainly consist of 40 pairs of n-AlxGayIn(1-x-y)As/InP, and 6 strain compensated AlxGayIn(1-x-y)As/InP quantum wells on n-InP substrate, respectively. The top distributed Bragg reflection (DBR) mirror system has been formed by fabricating 4.5 pairs of SiO2/Si. The designed cavity mode is around 1 536 nm. The dip of the fabricated cavity mode is around 1 530 nm. The threshold current is 30 mA and the maximum output power is around 270 μW under CW operation at room temperature.     

Phase separation, effects of biaxial strain, and ordered phase formations in cubic nitride alloys

The thermodynamics as well as the energetics and the structural properties of cubic group-III nitrides alloys have been investigated by combining first-principles total energy calculations and cluster expansion methods. In particular results are shown for the ternary In_xGa_1−xN and the quaternary Al_xGa_yIn_1−x−yN alloys. Phase separation is predicted to occur at growth temperatures, for both fully relaxed alloys. A remarkable influence of an external biaxial strain on the phase separation, with the formation of ordered phase structures has been found for the InGaN alloy. These findings are used to clarify the origin of the light emission process in InGaN-based optoelectronic devices. Results are shown for the composition dependence of the lattice constant and of the energy gap in quaternary Al_xGa_yIn_1−x−yN alloys.     

Optical properties of hybrid quantum-well–dots nanostructures grown by MOCVD

The deposition of In _x Ga_1–x As with an indium content of 0.3–0.5 and an average thickness of 3–27 single layers on a GaAs wafer by metalorganic chemical vapor deposition (MOCVD) at low temperatures results in the appearance of thickness and composition modulations in the layers being formed. Such structures can be considered to be intermediate nanostructures between ideal quantum wells and quantum dots. Depending on the average thickness and composition of the layers, the wavelength of the photoluminescence peak for the hybrid InGaAs quantum well–dots nanostructures varies from 950 to 1100 nm. The optimal average In _x Ga_1–x As thicknesses and compositions at which the emission wavelength is the longest with a high quantum efficiency retained are determined.