Growth and properties of Hg1-x Cdx Te epitaxial layers

The growth of epitaxial layers of mercury-cadmium-telluride (Hg_1-xCd_xTe) with relatively low x (0.2-0.3) from Te-rich solutions in an open tube sliding system is studied. The development of a semiclosed slider system with unique features permits the growth of low x material at atmospheric pressure. The quality of the films is improved by the use of Cd_1-yZ_yTe and Hg_1-xCd_xTe substrates instead of CdTe. The substrate effects and the growth procedure are discussed and a solidus line at a relatively low temperature is reported. The asgrown epitaxial layers are p-type with hole concentration of the order of 1·10¹⁷ cm⁻³, hole mobility of about 300 cm²·V⁻¹ sec⁻¹ and excess minority carrier life-time of 3 nsec, at 77 K.     

Thermoelectric quantum-dot superlattices with high ZT

Following the experimentally observed Seebeck coefficient enhancement in PbTe quantum wells in Pb_1−xEu_xTe/PbTe multiple-quantum-well structures which indicated the potential usefulness of low dimensionality, we have investigated the thermoelectric properties of PbSe_xTe_1−x/PbTe quantum-dot superlattices for possible improved thermoelectric materials. We have again found enhancements in Seebeck coefficient and thermoelectric figure of merit (ZT) relative to bulk values, which occur through the various physics and materials science phenomena associated with the quantum-dot structures. To date, we have obtained estimated ZT values approximately double the best bulk PbTe values, with estimated ZT as high as about 0.9 at 300 K.     

P-N junction observations on Pb-salt diode lasers using an ebic-mode SEM

A scanning electron microscope (SEM) operating in the elec-tron- beam induced current (EBIC) mode has been used to determine the depths and uniformities of p- n junctions in a variety of Pb- salt diode lasers. Pb- salt materials with 20K band gap values ranging from 44 meV to 450 meV have been investigated. This includes the ternary alloy systems Pb_1−xCd_xS(with 0 ≤ x ≤ 0.048), PbS_1-xSe_x (with 0 ≤ x ≤ l), Pb_1−xSn_xSe (with O ≤ x ≤ O.10),and Pb_1-xSn_xTe (with x = 0.25). With typical carrier concentrations in the 10¹⁸ – 10¹⁹ cm^{− 3} range, the junction depths were found to be independent of temperature between 77K and 300K. Small band gap devices exhibited EBIC signals strongly dependent on T, while devices with band gap values above 130 meV at 77K exhibited relatively little such temperature dependence to 300K. This non- destructive technique is capable of providing junction depth and uniformity information of all the Pb-salt materials, and is useful for estimating minority car-rier diffusion lengths at various temperatures. The method is also useful for investigating some of the more complex confinement heterostructures in these materials.     

Growth and characterization of lead-tin-ytterbium-telluride for diode lasers

PbSnYbTe is a new and potentially useful material for fabricating double hetero junction PbSnTe diode lasers by molecular b<sam epitaxy. These lasers should have improved carrier and photon confinement, resulting in improved external quantum efficiency and higher device operating temperature. It is found from room temperature optical transmission studies that the band gap of Pb_1−x Yb_x Te increases approximately as dEg/dx =; 3.3 eV for x < 0.04, and that the index of refraction decreases with increasing x. Doping studies of (Pb_1−y Sn_y )_0.97 Yb_0.03Te indicate that> it can be doped heavily p-type at low temperatures for y ∼ 0.10. The lattice constant of (Pb_0.85 Sn_0.15)_1−x Yb_x Te is independent of x up to x ∼ 0.10. these characteristics make (Pb_1−y Sn_y )_1−x Yb_x Te well-suited for the fabrication of lattice-matched double heterojunction lasers with y∼ 0.10.     

Nickel silicidation techniques for strained Si1?xGex, Si1?x?yGexCy, and Si1?yCy alloys material-device applications

A nickel silicide process for Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloy materials compatible with Si technology has been developed. Low-resistivity-phase (12–20 μΘ cm) nickel silicides have been obtained for these alloys with different low sheet-resistance temperature windows. The study shows that thin (15–18 nm) silicide layers with high crystalline quality, smooth silicide surface, and smooth interface between silicide and the underlying material are achievable. The technique could be used to combine the benefits of Ni silicide and Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloys. The technique is promising for Si or Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloy-based metal-oxide semiconductor, field-effect transistors (MOSFETs) or other device applications.     

High thermoelectric figures of merit in PbTe quantum wells

High-quality Pb_1−xEu_xTe/PbTe multiple quantum wells (MQWs) have been grown by molecular beam epitaxy. The measured 300K thermoelectric properties have been compared with that of the best bulk PbTe. This experimental investigation is the first detailed study of MQW structures designed to improve ZT of thermoelectric materials and has resulted in a breakthrough in the decades-long ZT ≅ 1 barrier for a room-temperature thermoelectric material. A value of Z_2DT >1.2 has been achieved for these PbTe quantum wells.     

Optimizing the Charge Carrier Dynamics of Thermal Evaporated TexSe1-x Films for High-Performance Short-Wavelength Infrared Photodetection

Short-wavelength infrared photo-sensing materials are dominated by germanium, indium gallium arsenide, indium antimonide, and mercury cadmium telluride. However, the complex fabrication process and high production cost hinder their widespread applications. Recently, Te_xSe_1-x has shown great potential for infrared photodetection, but Te_xSe_1-x-based devices are still suffering from extremely high dark current and poor device performance. In this work, high-quality Te_xSe_1-x films are fabricated by thermal evaporation and low-temperature annealing. The optoelectronic properties of the Te_xSe_1-x thin films are systematically investigated and optimized. The absorption spectrum is carefully tuned to cover the broad range from 300 to 1600 nm by modulating the ratio of Te and Se. Photodiodes based on the optimized Te_xSe_1-x thin films are also fabricated, and achieve high responsivity, reduced dark and low noise current density, and a fast response time of <850 ns. Then, prototypical devices based on Te_0.65Se_0.35 thin films are demonstrated for optical communications, indicating the great potential for next-generation, low-cost short-wavelength infrared photodetection.     

Studies of the Ga1-xI^As1-ySby quaternary alloy system:-II characterisation by far-infrared reflectance spectroscopy

Reflectivity spectra have been measured on LPE Ga_1-xIn_xAs_1-ySb_y, quaternary alloy layers, lattice-matched to InAs and GaSb substrates. The compositions studied were limited by a miscibility gap to the regionsx = 0,y - 1 (GaSb) tox ~ 0.2,y ~ 0.8 andx = 1,y = 0 (InAs) tox ~ 0.9,y ~ 0.1. The spectra were analysed to give values of carrier density and mobility to investigate Te doping. Fory ~ 0.8 layers werep-type as-grown,n-type conduction being obtained for Te concentrations in the melt above about 10^-6 at.%. Electron mobility for low carrier densities was found to increase sharply with decreasingy. The lattice vibrational properties of the system were also investigated. Three phonon modes were observed in the composition range studied, with GaAs, InSb and mixed InAs/GaSb characteristics. Thus the behaviour of the system may be termed “three-mode” due to the degeneracy of the GaSb and InAs modes. The oscillator strengths of the observed modes are qualitatively as expected from a comparison with appropriate binary systems.     

Liquidus temperatures of hg-rich Hg-Cd-Te alloys*

The liquidus temperatures have been measured for ten (Hg_1-xCd_{x y}) Te_1-y compositions with 0.091<x<0.401 and 0.544<y<0.952. For metal-rich melts with the same x-value, the liquidus temperature increases with y for 0.5<y<0.7. This behavior is attributed to a higher degree of association between Cd and Te than between Hg and Te in the melts. Experimentl work was supported by NASA contract NAS8-33107 and was performed while the authors were employed by the McDonnell Douglas Research Labratories, St. Louis, MO.     

Band anticrossing in highly mismatched group II-VI semiconductor alloys

We have successfully synthesized highly mismatched Cd_1−yMn_yO_xTe_1−x alloys by high-dose implantation of O ions into Cd_1−yMn_yTe crystals. In crystals with y>0.02, incorporation of O causes a large decrease in the bandgap. The bandgap reduction increases with y; the largest value observed is 190 meV in O⁺-implanted Cd_0.38Mn_0.62Te. The results are consistent with the band anticrossing (BAC) model, which predicts that a repulsive interaction between localized states of O located above the conduction-band edge and the extended states of the conduction band causes the bandgap reduction. A best fit of the measured bandgap energies of the O-ion-synthesized Cd_1−yMn_yO_xTe_1−x alloys using the BAC model for y<0.55 suggests an activation efficiency of only ∼5% for implanted O in Cd_1−yMn_yTe.     

Distinctive features of the far-infrared reflection spectra of the semimagnetic semiconductors Hg1−x MnxTe1−y Sey

Reflection spectra of single crystals of Hg_1−x Mn_xTe_1−y Se_y (0.01<x<0.14, y=0.01) in the far-infrared range (10–600 cm⁻¹) are investigated at 300 and 77 K. A series of new phonon modes is observed, in addition to the longitudinal and transverse modes corresponding to the ternary compounds. Fiz. Tekh. Poluprovodn. 32, 546–548 (May 1998)     

Effect of Ce-Doping on Thermoelectric Properties in PbTe Alloys Prepared by Spark Plasma Sintering

Ce-doped Pb_1−x Ce _x Te alloys with x = 0, 0.005, 0.01, 0.015, 0.03, and 0.05 were prepared by induction melting, ball milling, and spark plasma sintering techniques. The structure and thermoelectric properties of the samples were investigated. X-ray diffraction (XRD) analysis indicated that the samples were of single phase with NaCl-type structure for x less than 0.03. The lattice parameter a increases with increasing Ce content. The lower Ce-doped samples (x = 0.005 and 0.01) showed p-type conduction, whereas the pure PbTe and the higher doped samples (x = 0, 0.015, 0.03, and 0.05) showed n-type conduction. The lower Ce-doped samples exhibited a much higher absolute Seebeck coefficient, but the higher electrical resistivity and higher thermal conductivity compared with pure PbTe resulted in a lower figure of merit ZT. In contrast, the higher Ce-doped samples exhibited a lower electrical resistivity, together with a lower absolute Seebeck coefficient and comparable thermal conductivity, leading to ZT comparable to that of PbTe. The lowest thermal conductivity (range from 0.99 W m⁻¹ K⁻¹ at 300 K to 0.696 W m⁻¹ K⁻¹ at 473 K) was found in the alloy Pb_0.95Ce_0.05Te due to the presence of the secondary phases, leading to a ZT higher than that of pure PbTe above 500 K. The maximum figure of merit ZT, in the alloy Pb_0.95Ce_0.05Te, was 0.88 at 673 K.     

Molecular beam epitaxial growth of Cd<Subscript>1−y</Subscript>Zn<Subscript>y</Subscript>Se<Subscript>x</Subscript>Te<Subscript>1−x</Subscript> on Si(211)

We report on the first successful growth of the quaternary alloy Cd_1−yZn_ySe_xTe_1−x(211) on 3-in. Si(211) substrates using molecular beam epitaxy (MBE). The growth of CdZnSeTe was performed using a compound CdTe effusion source, a compound ZnTe source, and an elemental Se effusion source. The alloy compositions (x and y) of the Cd_1−yZn_ySe_xTe_1−x quaternary compound were controlled through the Se/CdTe and ZnTe/CdTe flux ratios, respectively. Our results indicated that the surface morphology of CdZnSeTe improves as the Zn concentration decreases, which fits well with our previous observation that the surface morphology of CdZnTe/Si is poorer than that of CdSeTe/Si. Although the x-ray full-width at half-maximums (FWHMs) of CdZnSeTe/Si with 4% of Zn + Se remain relatively constant regardless of the individual Zn and Se concentrations, etched-pit density (EPD) measurements exhibit a higher dislocation count on CdZnSeTe/Si layers with about 2% Zn and Se incorporated. The enhancement of threading dislocations in these alloys might be due to an alloy disorder effect between ZnSe and CdTe phases. Our results indicate that the CdZnSeTe/Si quaternary material with low Zn or low Se concentration (less than 1.5%) while maintaining 4% total Zn + Se concentration can be used as lattice-matching composite substrates for long-wavelength infrared (LWIR) HgCdTe as an alternative for CdZnTe/Si or CdSeTe/Si.     

Electrical properties of indium-doped LPE layers of Pb1−xSnx Te

Liquid-phase epitaxial (LPE) layers of Pb_1−xSn_xTe with an alloy composition 0≤×≤0.25 were doped n-type by adding from 0.002 to 10 at.% indium to the growth solution. Doping characteristics of indium and electrical properties of the epilayers at 77 and 4.2K were studied by Hall and resistivity measurements made directly on the grown layers. Electron concentration and mobility at 77 and 4.2K are presented as a function of indium doping for various x values. Doping coefficients of ~0.05 and ~0.03 are found for PbTe and Pb_0.8Sn_0.2Te, respectively, grown at ~450°C. For medium to high indium doping, the electron concentration saturates to a constant value independent of doping and LPE growth temperature. The saturation values decrease substantially with increasing x and increase with a decrease in sample temperature. Bulklike mobilities practically independent of doping are recorded up to an indium concentration N_ln~0.3 at.%, above which the mobility decreases with increasing indium concentration. The data shows that indium is a suitable donor in liquid-phase epitaxial layers of Pb_l-XSn_xTe.     

Highly Efficient Ge-Rich Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te Thermoelectric Alloys

The search for alternative energy sources is presently at the forefront of applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role. This paper is concerned with the development of highly efficient p-type Ge _x Pb_1−x Te alloys for thermoelectric applications, using spark plasma sintering. The carrier concentration of GeTe was varied by alloying of PbTe and/or by Bi₂Te₃ doping. Very high ZT values up to ~1.8 at 500°C were obtained by doping Pb_0.13Ge_0.87Te with 3 mol% Bi₂Te₃.     

Effect of surface encapsulation and As4 overpressure on Si diffusion and impurity-induced layer disordering in GaAs,AlxGa1-xAs,and AlxGa1-xAs-GaAs quantum well heterostructures

Data are presented demonstrating that the surface encapsulant and the As₄ overpressure strongly affect Si diffusion in GaAs and Al_xGa_1-xAs, and thus are important parameters in impurity-induced layer disordering. Increasing As₄ overpressure results in anincrease in diffusion depth in the case of GaAs, and adecrease in diffusion depth for Al_xGa_1-xAs. In addition, the band-edge exciton is observed in absorption on an Al_xGa_1-xAs-GaAs superlattice that is diffused with Si and is converted to bulk crystal Al_yGa_1-yAs via impurity-induced layer disordering. In contrast, the exciton is not observed in absorption on GaAs diffused with Si in spite of the high degree of compensation. These data indicate that the Si diffusion process, and the properties of the diffused material, are different for GaAs and for Al_xGa_1-xAs-GaAs superlattices converted into uniform Al_yGa_1-yAs (0 ≤y ≤x ≤ 1) via impurity-induced layer disordering with the amphoteric dopant Si.     

Direct synthesis of quaternary Cd(Zn,S)Se thin films: Effects of composition

In the present communication, the binary CdSe and quaternary Cd_1-xZn_xSe_1-yS_y (0 ≤ x = y ≤ 0.35) thin films were synthesized using a chemical bath deposition. Thin film deposition was carried out at the optimized conditions (pH = 10 ± 0.1, deposition temperature = 70 ± 0.1 °C, deposition time = 100 min and substrate rotation speed = 65 ± 2 rpm). X-ray diffraction studies confirmed hexagonal-wurtzite crystal structure with the formation of quaternary Cd(Zn, S)Se phase along with binary CdSe, CdS, ZnS and ZnSe, phases of the as-grown Cd_1-xZn_xSe_1-yS_y thin films. Elemental analysis showed presence of Cd²⁺, Zn²⁺, S^2- and Se^2- in the deposited films. Fourier transform infrared spectroscopy shown the bands at 911.15 cm⁻¹ – 901.62 cm⁻¹ which are assigned to the stretching frequency of Cd–Se bond. Scanning electron microscopy show transformation of the microstructure from globular crystallites to a rhomboid flake like network. The electrical conductivity was typically ≈ 10⁻⁷ Ω⁻¹ cm⁻¹. At low temperatures, the conduction was by variable range hopping, and this changed to thermally activated grain boundary dominated conduction for T > 350 K.     

Dependence of photoluminescence spectra of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 0.1) alloy layers on conditions of growth

The photoluminescence spectra of the Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.1) alloy are studied at low temperatures. Epitaxial layers were grown by molecular-beam epitaxy at different temperatures. Along with the basic line corresponding to interband radiative recombination, additional emission lines are observed in the low-energy region of the spectra. Some nonuniformities are observed at the sample surface (within an area smaller than 1% of the total surface area). The concentration and size (1–10 μm) of the nonuniformities decrease with increasing temperature of growth. The additional emission lines are attributed to local nonuniformities in the layer. The dependence of the band gap of the Pb_1−x Eu _x Te (0 ≤ x ≤ 0.1) alloy on the composition parameter x is determined at 77.4 K. The dependence is nonlinear and adequately described by the relation E _g [eV] = 0.213 + 4.8x − 18.4x ².     

The effect of trimethylaluminum concentration on the incorporation of P In AlxGa1−xPyAs1−y grown by organometallic vapor phase epitaxy

The organometallic vapor phase epitaxial (OM-VPE) growth of Al_xGa_1-xP_yAs_1-y on graded GaP_yAs_1-yGaAs in the compositional range 0 < x < 0.9 and 0 < y < 0.6 is reported. It is found that composition control can be easily achieved, and that the vapor phase ratio of trimethylaluminum to trimethylgallium strongly influences the incorporation of P in the solid. A model is developed which explains this in terms of competing reaction rates. The model gives a good fit to the experimental data.     

Realization of a New Topological Crystalline Insulator and Lifshitz Transition in PbTe

Topological materials boast exotic metallic surface states with linear dispersion and spin‐momentum locking, which makes them potential candidates for dissipationless electronic and spintronic devices. Here, it is theoretically predicted that intrinsic Te antisite defects (Te_Pb) in the narrow‐gap semiconductor PbTe induce a band inversion, turning it into a topological crystalline insulator (TCI). To experimentally verify the exotic properties, Te_Pb antisites are introduced into PbTe crystals via nonstoichiometric growth by molecular beam epitaxy. Semimetallic resistivity and distinct quantum oscillations are observed on the Te_Pb doped PbTe. Most importantly, a π Berry phase is unambiguously revealed by a Landau index analysis, demonstrating the Dirac fermion nature of the topological surface states. The discovered TCI nature in Te_Pb doped PbTe is further explored using magneto‐transport measurements under external pressure, and the theoretical calculations of band structures with applying pressure indicate a pressure‐induced Lifshitz transition. Besides, it is proposed that the contribution of bulk states to transport can be reduced by enlarging the inverted gap with strain.