Optical properties of Ge1-yCy alloys

The Ge_1-yC_y semiconductor alloy system offers promise as a material for use in heterostructure devices based on Si as well as other materials. We have grown Ge_1-y C_y alloys by solid source molecular beam epitaxy on Si substrates. Layer thicknesses ranged from 0.01 to 3 μm, and Auger electron spectroscopy and secondary ion mass spectrometry indicated C fractions up to 3 at. %. Optical absorption in the near-infrared region indicated a shift in the energy bandgap from that of Ge which was attributed to the effects of alloying. The dependence of the bandgap on composition was consistent with linear interpolations of the Ge and C conduction band minimums. We observed a fundamental absorption edge characteristic of an indirect bandgap material. Photoluminescence spectra at 11K of thick, relaxed layers indicated single broad peaks near the expected bandgap energy.     

Electrical properties of hydrogenated amorphous Ge0.90Si0.1:Hx films

Amorphous 1-μm thick films of the Ge_0.90Si_0.1:H_x solid solution (x=1.3, 5.1, 8.7, 14.2, and 23.7 at. %) were grown in a hydrogen atmosphere under various partial hydrogen pressures. The method of plasmachemical deposition with a rate of 0.3–0.5 ?/s was employed. Electrical conductivity of the films was measured within a temperature range of 100–420 K. The dark conductivity of the films was measured. The activation energy of hopping at T=100 K, the hop range, the mobility of electrons at the levels ɛ_F and ɛ_C, and the activation energy of conductivity were calculated. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 11, 2000, pp. 1383–1385. Original Russian Text Copyright ? 2000 by Nadzhafov.     

Nature and evolution of interfaces in Si/Si1-xGex superlattices

The interfacial properties of a series of Si/Si_1-xGe_x superlattices with the same unit period (∼10 nm Si and 5 nm Si_0.65Ge_0.35) but with different number of repetitions (5, 10, 20) or prepared at different temperatures in the range 250-750°C are studied by x-ray and Raman scattering techniques. For growth at 250°C, the interfaces are chemically abrupt, but exhibit a pronounced vertically correlated physical roughness that increases from ∼0.3 nm near the substrate to ∼2 nm at the surface. Growth at intermediate temperatures (400–550°C) resulted in structures with physically smooth interfaces, independent of the number of periods, but intermixed over at least two monolayers, and asymmetrically broadened with an alloy/Si interface width about twice (0.25 vs 0.5 nm) that of the Si/alloy interface. In that temperature range, a roughness component aligned with the substrate miscut angle (∼0.14° toward [001]) and with an inplane length scale of ∼1.2 μrn is observed. Higher growth temperatures (620–750°C) further enhanced the intermixing and caused undulation (100 nm length scale) of the interfaces.     

Long-wavelength photodiodes based on Ga1−x InxAsySb1−y wit composition near the miscibility boundary

The possibility of using liquid-phase epitaxy to obtain Ga_1−x In_xAs_ySb_1−y solid solutions isoperiodic with GaSb near the miscibility boundary is investigated. The effect of crystallographic orientation of the substrate on the composition of the solid solutions grown in this way is examined, and the indium concentration is observed to grow from 0.215 to 0.238 in the Ga_1−x In_xAs_ySb_1−y solid phase in the series of substrate orientations (100), (111)A, (111)B. A change in the composition of the solid solution leads to a shift of the long-wavelength edge of the spectral distribution of the photosensitivity. The use of a GaSb (111)B substrate made it possible, without lowering the epitaxy temperature, to increase the indium content in the solid phase to 23.8% and to create long-wavelength photodiodes with spectral photosensitivity threshold λ _th=2.55 μm. The primary characteristics of such photodiodes are described, along with aspects of their fabrication. The proposed fabrication technique shows potential for building optoelectronic devices (lasers, LED’s, photodiodes) based on Ga_1−x In_xAs_ySb_1−y solid solutions with red boundary as high as 2.7 μm. Fiz. Tekh. Poluprovodn. 33, 249–253 (February 1999)     

A GeSi-buffer structure for growth of high-quality GaAs epitaxial layers on a Si substrate

A SiGe-buffer structure for growth of high-quality GaAs layers on a Si (100) substrate is proposed. For the growth of this SiGe-buffer structure, a 0.8-μm Si_0.1 Ge_0.9 layer was first grown. Because of the large mismatch between this layer and the Si substrate, many dislocations formed near the interface and in the low part of the Si_0.1Ge_0.9 layer. A 0.8-μm Si_0.05Ge_0.95 layer and a 1-μm top Ge layer were subsequently grown. The strained Si_0.05Ge_0.95/Si_0.1Ge_0.9 and Ge/Si_0.05Ge_0.95 interfaces formed can bend and terminate the upward-propagated dislocations very effectively. An in-situ annealing process is also performed for each individual layer. Finally, a 1–3-μm GaAs film was grown by metal-organic chemical vapor deposition (MOCVD) at 600°C. The experimental results show that the dislocation density in the top Ge and GaAs layers can be greatly reduced, and the surface was kept very smooth after growth, while the total thickness of the structure was only 5.1 μm (2.6-μm SiGe-buffer structure +2.5-μm GaAs layer).     

HgCdZnTe quaternary materials for lattice-matched two-color detectors

As the number of bands and the complexity of HgCdTe multicolor structures increases, it is desirable to minimize the lattice mismatch at growth interfaces within the device structure in order to reduce or eliminate mismatch dislocations at these interfaces and potential threading dislocations that can degrade device performance. To achieve this we are investigating the use of Hg_1−x−yCd_xZn_yTe quaternary alloys which have an independently tunable lattice constant and bandgap. Lattice matching in Hg_1−x−yCd_xZn_yTe structures can be achieved using small additions of Zn (y<0.015) to HgCdTe ternary alloys. We have investigated some of the basic properties of Hg_1−x−yCd_xZn_yTe materials with x≈0.31 and 0≤y≤0.015. The quaternary layers were grown on (112)CdZnTe substrates using MBE and the amount of Zn in the layers was determined from calibrated SIMS measurements. As expected, the lattice constant decreased and the bandgap increased as Zn was added to HgCdTe to form Hg_1−x−yCd_xZn_yTe. Hall-effect results for both n-type (In) and p-type (As) Hg_1−x−yCd_xZn_yTe layers were very similar to HgCdTe control samples. We have also utilized x-ray rocking curve measurements with (246) asymmetric reflections as a novel sensitive technique to determine the correct amount of Zn needed to achieve lattice matching at an interface. MWIR/LWIR n-p-n two-color triple-layer heterojunction structures were grown to evaluate the effects of minimizing the lattice mismatch between the widest bandgap p-type collector layer, using Hg_1−x−yCd_xZn_yTe, and the HgCdTe MWIR and LWIR collector layers and compared to structures that did not incorporate the quaternary. Sequential mode two-color detectors were fabricated using a 256 × 256, 30 μm unit cell design. There were several interesting findings. Macro defects predominantly affected the LWIR band (Band 2) operability and had little effect on the MWIR band (Band 1). The incorporation of Hg_1−x−yCd_xZn_yTe p-type collector layers had little effect on MWIR detector performance, but overall the LWIR performance was generally better. These initial detector results indicate that the use of Hg_1−x−yCd_xZn_yTe alloys in multicolor detector structures are potentially promising and should be pursued further.     

Room temperature λ=1.3 µm photoluminescence from InGaAs quantum dots on (001) Si substrate

GaAs/In_xGa_1−x As quantum dot heterostructures exhibiting high-intensity λ=1.3 μm photoluminescence at room temperature have been grown on (001) Si substrate with a Si_1−x Ge_x buffer layer. The growth was done successively on two MBE machines with sample transfer via the atmosphere. The results obtained by the study of the structure growth process by means of high-energy electron diffraction are presented. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 5, 2002, pp. 565–568. Original Russian Text Copyright ? 2002 by Burbaev, Kazakov, Kurbatov, Rzaev, Tsvetkov, Tsekhosh.     

Synthesis and physical properties of Si(Ge)-Se-Te glasses

Ternary Si₁₅Ge(Ga)₅Te₈₀, Si_19.7Te_78.7Se_1.6, Si_19.2Te_76.8Se₄, and Ge₁₉Te₇₂Se₉ telluride glasses were synthesized. Electrical, acoustic, acoustooptical properties, and the dispersion of optical transmittance of these films were studied in a wide range of temperatures and frequencies. Comparative analysis of the results obtained is performed. Possible mechanisms of the observed phenomena are discussed. It is shown that Ge₁₉Te₇₂Se₉ alloy is quite competitive with Si₂₀Te₈₀ alloy for the fabrication of highly efficient acoustooptical devices with a wide range of applications in the middle IR spectral region (2–12 μm). __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 7, 2003, pp. 822–826. Original Russian Text Copyright ? 2003 by Kulakova, Melekh, Bakharev, Kudoyarova.     

A new magnetic semiconductor Cd1−x MnxGeP2

A new semiconductor material, which comprises a solid solution of a ternary diamond-like semiconductor and transition element Mn, was grown and investigated. According to X-ray diffraction data, the crystal structure of the material is similar to that of the CdGeP₂ host substance with a chalcopyrite-type crystal structure. The interplanar distances and the unit cell parameter decrease with an increase in Mn content: a=5.741 ? → 5.710 ? → 5.695 ? in the series of CdGeP₂ → Cd_1−x Mn_xGeP₂ → Cd_1−y Mn_yGeP₂ compounds (x<y). The surface composition and in-depth concentration profiles for elements of a Cd-Mn-Ge-P quaternary system were investigated using electron microscopy and energy dispersive X-ray spectroscopy. The molecular concentration ratio for Mn and Cd at a depth of 0.4 μm is Mn/Cd=0.2. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 3, 2001, pp. 305–309. Original Russian Text Copyright ? 2001 by Medvedkin, Ishibashi, Nishi, Sato.     

MBE growth and photoluminescent properties of InAsSb/AlSbAs quantum wells

Heterostructures with a single InAs_1−x Sb_x/AlSb_1−y As_y quantum well (QW) on (001) GaSb substrates have been grown by MBE and studied using X-ray diffraction, transmission electron microscopy, and photoluminescence (PL) spectroscopy. High-intensity PL was observed at a temperature of 80 K, with a peak half-width of 30–50 meV and a peak wavelength in the range from 2 to 4.5 μm, depending on the QW width, which varied between 4 and 20 nm, respectively. The fundamental absorption edge of such QWs was calculated for a wide range of alloy compositions, x and y. Good correlation between the experimental and calculated dependences of the band gap on the InAsSb/AlSbAs QW thickness was obtained. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 12, 2002, pp. 1470–1474. Original Russian Text Copyright ? 2002 by Solov’ev, Terent’ev, Toropov, Meltser, Semenov, Sitnikova, Ivanov, Meyer, Kop’ev.     

High density 2DEG in III-V semiconductor heterostructures and high-electron-mobility transistors based on them

Situation in high-electron-mobility transistor (HEMT) technology is discussed. The N-AlGaAs/InGaAs/GaAs pseudomorphic HEMT’s are now considered as most advanced for mmwave monolithic circuits, but metamorphic N-In_xAl_1−x As/In_yGa_1−y As/In_xAl_1−x As HEMT’s grown on GaAs substrates are very promising for the future high-frequency devices. High density 2DEG in HEMT’s is analyzed by means of the Hall effect and photoluminescence measurements. Processing technology of the sub-0.25-μm pseudomorphic HEMT’s, metamorphic HEMT’s and their characteristics are also described. Fiz. Tekh. Poluprovodn. 33, 1064–1065 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Photosensitivity of thin-film structures based on (CuInSe2)x(2ZnSe)1−x solid solutions

Polycrystalline (CuInSe₂)_x(2ZnSe)_1−x films (x=0.6–1.0) with p-type conductivity and a thickness of 0.5–0.9 μm were obtained by pulsed laser evaporation. It is shown that a chalcopyrite-sphalerite transition occurs in the above system for x=0.7. The obtained films were used to fabricate the photosensitive structure of the In/p-(CuInSe₂)_x(2ZnSe)_1−x and InSe(GaSe)/(CuInSe₂)_x(2ZnSe)_1−x types. Spectral dependences of photovoltaic-conversion quantum efficiency were studied, and the photosensitivity of the structures in relation to the type of energy barrier and the composition was analyzed. It is concluded that the structures under consideration can be used as broadband photovoltaic converters. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 5, 2000, pp. 576–581. Original Russian Text Copyright ? 2000 by V. Rud’, Yu. Rud’, Bekimbetov, Gremenok, Bodnar’, Rusak.     

Photoelectric phenomena in (μc xa1−x )-Si:H/c-Si heterostructures

Heterostructures (μc _xa_1−x )-Si:H/c-Si with a various volume fraction x of microcrystalline and amorphous Si phases were obtained by plasmochemical deposition. The fraction variation was achieved by changing the silane content in H₂. The steady-state current-voltage characteristics and spectral dependences of photosensitivity of the structures obtained were investigated. The latter dependences were recorded by exposing the samples to the natural and linearly polarized light in relation to the phase composition of thin (d ₁ ≅ 0.6–0.8 μm) films of microcrystalline and amorphous Si. The photovoltaic effect and induced photopleochroism of the structures obtained were detected. The prospects of using a new type of heterojunction in photoconverters of natural and linearly polarized light were assessed. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 11, 2001, pp. 1316–1319. Original Russian Text Copyright ? 2001 by Mell, Nikolaev, V. Rud’, Yu. Rud’, Terukov.     

Analysis of mechanisms for electron scattering in GaAs/AlxGa1−x As superlattices with doped quantum wells for longitudinal resonant current flow in high electric fields and at low temperatures

Formulas are derived for, and a numerical analysis made of, the dependence of the transverse phase relaxation time on electron energy for resonant current flow through GaAs/Al_xGa_1−x As superlattices with doped quantum wells. The parameters are chosen to be close to those of superlattices used for creating photodiodes for operation at λ⋍10 μm. The analysis is limited to the interactions of electrons with neutral atoms and impurity ions at low temperatures. Resonant current flow is ensured by an electric field that brings the ground state and the first excited state of the “Stark ladder” into resonance with neighboring, weakly interacting quantum wells. Fiz. Tekh. Poluprovodn. 33, 438–444 (April 1999)     

InGaAs/InP heterostructures with strained quantum wells and quantum dots (λ=1.5–1.9 µm)

Strongly strained In_xGa_1−x As/In_0.53Ga_0.47As/InP heterostructures with indium content x=0.69−1.0 in the active region were investigated experimentally and theoretically. Two types of structures were obtained by vapor-phase epitaxy from metalorganic compounds: 1) with isolated compression-strained quantum wells and 2) with self-organized nanosize InAs clusters (quantum dots). The temperature dependence of the quantum radiation efficiency of samples with quantum wells in the temperature range 77–265 K is characterized by T ₀=43 K. One reason for the low value of T ₀ is electron delocalization in the active region. The maximum radiation wavelength obtained in structures with quantum dots is 1.9 μm at 77 K. Fiz. Tekh. Poluprovodn. 33, 1105–1107 (September 1999)     

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.     

Energy spectrum of acceptors in the semimagnetic semiconductors p-Hg1−x MnxTe in the spin-glass region

The far-IR transmission and photoconductivity in the semimagnetic alloys p-Hg_1−x Mn_xTe (x=0.20–0.22) at temperatures 2–7 K were investigated at fixed frequencies of optically pumped molecular lasers in the region 49–311 μm. We report the observation of photoexcitation of acceptors from the ground into excited states under conditions when the direct interaction of the magnetic moments of the manganese ions becomes substantial and results in the formation of a spin-glass phase. It was found that the internal field produced by the spontaneous and external polarizations of the magnetic moments of the Mn²⁺ ions in the spin-glass temperature range influences the energy spectrum of acceptors in a magnetic field. Fiz. Tekh. Poluprovodn. 32, 450–452 (April 1998)     

The preparation of Gal-x Inx As by organometallic pyrolysis for homojunction LED’S

Ga_1-x}In_x}As epitaxial layers have been deposited on GaAs substrates using the technique of organometallic pyrolysis (metalorganic chemical vapour deposition). The deposition was performed in a laminar flow, resistively heated, reactor. Both n and p-type (10¹⁷}-10¹⁸} carriers/cm³}) epitaxial layers, several microns thick,were prepared, with values of x in the range 0 ≤x ≤0.3. Epitaxial layer characterisation was carried out using conventional electrical, optical and x-ray techniques. Restricted emitting area (50–75 μm diameter) zinc-diffused LED’s were prepared in ungraded epitaxial layers with emission spectral peaks in the range 0.9 —1.15 ym. External quantum efficiencies of these devices decreased rapidly with increasing x, from∼0.4% for GaAs LED’s to∼0.02% for Gao._0.75}In_0.25}As LED’s.     

Acceptor and donor doping of AlxGa1−xN thin film alloys grown on 6H-SiC(0001) substrates via metalorganic vapor phase epitaxy

Thin films of Si-doped Al_xGa_1−xN (0.03≤x≤0.58) having smooth surfaces and strong near-band edge cathodoluminescence were deposited at 0.35–0.5 μm/h on on-axis 6H-SiC(0001) substrates at 1100°C using a 0.1 μm AlN buffer layer for electrical isolation. Alloy films having the compositions of Al_0.08Ga_0.92N and Al_0.48Ga_0.52N exhibited mobilities of 110 and 14 cm²/V·s at carrier concentrations of 9.6×10¹⁸ and 5.0×10¹⁷ cm⁻³, respectively. This marked change was due primarily to charge scattering as a result of the increasing Al concentration in these random alloys. Comparably doped GaN films grown under similar conditions had mobilities between 170 and ∼350 cm²/V·s. Acceptor doping of Al_xGa_1−xN for x≤0.13 was achieved for films deposited at 1100°C. No correlation between the O concentration and p-type electrical behavior was observed.     

Pb1-xSnxTe epitaxial layers prepared by the hot-wall technique

Epitaxial layers of Pb_1-xSn_xTe were grown on polished-etched KCl substrates using the modified hot-wall technique. Four layers, each 5-30 μm thick, of area 5.5 × 5.5 mm² were simultaneously grown at growth rates of 0.3–4.8 μm/hr. A typical substrate temperature of 325°C , and baffle and source temperatures of 450-560°C were used. P-and n-type layers were obtained from stoichio-metric and metal-rich sources of (Pb_1-xSn_x)_1+δ Te_1-δ, 0.01≤δ ≤0.02, respectively. From metal-rich sources with 0<δ<0.01, the layers obtained were n-type at 300K but p-type at 77K. The layers became intrinsic between approximately 250K and 295K. Carrier concentrations of as-grown layers with 0≤x ≤0.26 were in the range of 10¹⁶-10¹⁷ /cm³ and mobilities were of the order of 10⁴ cm² /V-sec at 77K. Both n-and p-type layers were also obtained from metal-rich sources of δ = 0.01 by controlling the Te vapor pressure from a separate reservoir. With Te temperatures higher than the n-p turnover point of 240°C , p-type characteristics were obtained. Layers that were p-type at 77K grown with the Te temperatures near the n-p turnover point had the best mirror-like surfaces.