Growth of In-Rich In x Al1−x N Films on (0001) Sapphire by RF-MBE and their Properties

In-rich In _x Al_1−x N films (0.47 ≤ x ≤ 1) were grown directly on nitridated (0001) sapphire substrates without employing a buffer layer by radiofrequency molecular-beam epitaxy. Both photoluminescence peak energy and optical absorption-edge energy of the In _x Al_1−x N films decreased monotonically with increasing In composition, which was consistent with the recently reported InN bandgap energies of ∼0.7 eV. The bowing parameter of this alloy was estimated to lie between 2.9 eV and 6.2 eV. Substrate nitridation with lower temperature and longer period conditions not only reduced misoriented In _x Al_1−x N phases remarkably but also produced narrower tilt distribution in the c-axis-oriented In _x Al_1−x N matrix.     

Characterization of Zn1?xMgxO Films Prepared by the Sol–Gel Process and Their Application for Thin-Film Transistors

Transparent semiconductor thin films of Zn_1−x Mg _x O (0 ≤ x ≤ 0.36) were prepared using a sol–gel process; the crystallinity levels, microstructures, and optical properties affected by Mg content were studied. The experimental results showed that addition of Mg species in ZnO films markedly decreased the surface roughness and improved transparency in the visible range. A Zn_1−x Mg _x O film with an x-value of 0.2 exhibited the best average transmittance, namely 93.7%, and a root-mean-square (RMS) roughness of 1.63 nm. Therefore, thin-film transistors (TFTs) with a Zn_0.8Mg_0.2O active channel layer were fabricated and found to have n-type enhancement mode. The Zn_0.8Mg_0.2O TFT had a field-effect mobility of 0.1 cm²/V s, threshold voltage of 6.0 V, and drain current on/off ratio of more than 10⁷.     

X-ray Diffraction Imaging of Improved Bulk-Grown CdZnTe(211) and Its Comparison with Epitaxially Grown CdTe Buffer Layers on Si and Ge Substrates

Large-area high-quality Hg_1–x Cd _x Te sensing layers for infrared imaging in the 8 μm to 12 μm spectral region are typically grown on bulk Cd_1–x Zn _x Te substrates. Alternatively, epitaxial CdTe grown on Si or Ge has been used as a buffer layer for high-quality epitaxial HgCdTe growth. In this paper, x-ray topographs and rocking-curve full-width at half-maximum (FWHM) data will be presented for recent high-quality bulk CdZnTe grown by the vertical gradient freeze (VGF) method, previous bulk CdZnTe grown by the vertical Bridgman technique, epitaxial CdTe buffer layers on Si and Ge, and a HgCdTe layer epitaxially grown on bulk VGF CdZnTe.     

Electric and photoelectric properties of n-GaxIn1−x N/p-Si anisotypic heterojunctions

We have developed a technology for producing n-type Ga_xIn_1−x N/p-Si heterostructures by combined pyrolysis of indium and gallium monoammoniate chlorides, making it possible to obtain heterolayers with composition varying over wide limits (from GaN up to InN). The composition and basic electric and optical characteristics of nitride films were determined. The electric and photoelectric properties of the heterostructures with Ga_xIn_1−x N films of different composition were investigated. It was shown that the anisotypic heterojunction n-Ga_xIn_1−x N/p-Si is a promising photosensitive element for detecting visible-range radiation. The maximum values of the specific detectivity were D*=1.2×10¹¹ Hz^1/2·W⁻¹ at 290 K. A band diagram of the heterojunction was constructed. Fiz. Tekh. Poluprovodn. 32, 461–465 (April 1998)     

Growth of HgTe Quantum Wells for IR to THz Detectors

We zone-engineered HgCdTe/HgTe/HgCdTe quantum wells (QWs) using the molecular-beam epitaxy (MBE) method with in situ high-precision ellipsometric control of composition and thickness. The variations of ellipsometric parameters in the ψ–Δ plane were represented by smooth broken curves during HgTe QW growth with abrupt composition changes. The form of the spiral fragments and their extensions from fracture to fracture revealed the growing layer composition and its thickness. Single and multiple (up to 30) Cd _x Hg_1−x Te/HgTe/Cd _x Hg_1−x Te QWs with abrupt changes of composition were grown reproducibly on (013) GaAs substrates. HgTe thickness was in the range of 16 nm to 22 nm, with the central portion of Cd _x Hg_1−x Te spacers doped by In to a concentration of 10¹⁴ cm⁻³ to 10¹⁷ cm⁻³. Based on this research, high-quality (013)-grown HgTe QW structures can be used for all-electric detection of radiation ellipticity in a wide spectral range, from far-infrared (terahertz radiation) to mid-infrared wavelengths. Detection was demonstrated for various low-power continuous-wave (CW) lasers and high-power THz pulsed laser systems.     

Thermoelectric Properties of SnO<Subscript>2</Subscript> Ceramics Doped with Sb and Zn

Polycrystalline SnO₂-based samples (Sn_0.97−x Sb_0.03Zn _x O₂, x = 0, 0.01, 0.03) were prepared by solid-state reactions. The thermoelectric properties of SnO₂ doped with Sb and Zn were investigated from 300 K to 1100 K. X-ray diffraction (XRD) analysis revealed all XRD peaks of all the samples as identical to the rutile structure, except for the x = 0.03 sample, which had a small amount of Zn₂SbO₄ as a secondary phase. We found that the power factor of the x = 0.03 sample was significantly improved due to the simultaneous increase in the electrical conductivity and the Seebeck coefficient. A power factor value of ∼2 × 10⁻⁴ W m⁻¹ K⁻² was obtained for the x = 0.03 sample at 1060 K, 126% higher than that for the undoped sample.     

Thermal Stability Study of Cu(MoN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>) Seed Layer on Barrierless Si

This study reports the good thermal stability of a sputtered Cu(MoN _x ) seed layer on a barrierless Si substrate. A Cu film with a small amount of MoN _x was deposited by reactive co-sputtering of Cu and Mo in an Ar/N₂ gas mixture. After annealing at 560°C for 1 h, no copper silicide formation was observed at the interface of Cu and Si. Leakage current and resistivity evaluations reveal the good thermal reliability of Cu with a dilute amount of MoN _x at temperatures up to 560°C, suggesting its potential application in advanced barrierless metallization. The thermal performance of Cu(MoN _x ) as a seed layer was evaluated when pure Cu is deposited on top. X-ray diffraction, focused ion beam microscopy, and transmission electron microscopy results confirm the presence of an ∼10-nm-thick reaction layer formed at the seed layer/Si interface after annealing at 630°C for 1 h. Although the exact composition and structure of this reaction layer could not be unambiguously identified due to trace amounts of Mo and N, this reaction layer protects Cu from a detrimental reaction with Si. The Cu(MoN _x ) seed layer is thus considered to act as a diffusion buffer with stability up to 630°C for the barrierless Si scheme. An electrical resistivity of 2.5 μΩ cm was obtained for the Cu/Cu(MoN _x ) scheme after annealing at 630°C.     

Gradient Conduction Band Energy Engineering Driven High-Efficiency Solution-Processed Cu2ZnSn(S,Se)4/ZnxCd1–x S Solar Cells

The photovoltaic performance of the environmentally friendly Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is lower than its predecessor Cu(In,Ga)Se₂ solar cells. Severe carrier recombination at the CZTSSe/CdS interface is one major reason that results in a large open-circuit voltage loss. Doping zinc into CdS is a feasible strategy to modifying the CdS buffer layer film, but the present methods are not satisfactory. In this study, novel zinc incorporation strategy is developed to deposit a gradient composition ternary Zn_xCd_1–xS buffer layer for optimizing the heterojunction interface. The application of gradient composition Zn_xCd_1–xS buffer layer constructs a gradient conduction band energy configuration in the CZTSSe/buffer layer interface, which highly reduces the interface recombination. The suppressed interface recombination contributes to the enhanced open circuit voltage and device performance. Consequently, the CZTSSe solar cell based on gradient composition Zn_xCd_1–xS buffer layers achieves champion efficiency of 12.35% with V_OC of 504.81 mV, J_SC of 36.90 mA cm⁻², and FF of 66.28%. It is worth noting that flammable and the toxic hydrazine solvent are replaced by the safe and low-toxic 2-methoxyethanol, making it more promising for the future commercialization of CZTSSe solar cells.     

Asymmetric Dislocation Densities in Forward-Graded ZnS<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>/GaAs (001) Heterostructures

We report an experimental and modeling study of ZnS _y Se_1−y /GaAs (001) structures, all of which comprised a uniform top layer of ZnS_0.014Se_0.986 grown on a compositionally graded buffer layer or directly on the GaAs substrate. High-resolution x-ray diffraction was used to estimate dislocation densities on type A slip systems, with misfit dislocation (MD) line segments oriented along the [1[`1]0] [1\bar{1}0] direction, and type B slip systems, with MD line segments oriented along a [110] direction. A control sample having no graded buffer exhibits equal dislocation densities on the two types of slip systems (D _A ≈ D _B ≈ 1.5 × 10⁸ cm⁻²), but a forward-graded (FG) structure (grading coefficient of 27 cm⁻¹) exhibits 20% more dislocations on the type B slip systems (D _A ≈ 1.6 × 10⁸ cm⁻² and D _B ≈ 1.9 × 10⁸ cm⁻²) and a steep forward-graded structure (grading coefficient of 54 cm⁻¹) exhibits 50% more type B dislocations (D _A ≈ 2 × 10⁸ cm⁻² and D _B ≈ 3 × 10⁸ cm⁻²). The insertion of an overshoot interface reduced the dislocation densities in the uniform top layer by promoting annihilation and coalescence reactions, but type B dislocations were removed more effectively. Based on equilibrium calculations the overshoot graded layer in the steep graded overshoot structure is expected to exhibit large compressive and tensile strains, with a reversal in the sign of the strain near its middle, which may promote annihilation and coalescence reactions between threading dislocations.     

Evolution of Threading Dislocation Density and Stress in GaN Films Grown on (111) Si Substrates by Metalorganic Chemical Vapor Deposition

We have studied the evolution of threading dislocations (TDs), stress, and cracking of GaN films grown on (111) Si substrates using a variety of buffer layers including thin AlN, compositionally graded Al _x Ga_1-x N (0 ≤ x ≤ 1), and AlN/Al _y Ga_1-y N/Al _x Ga_1-x N (0 ≤ x ≤ 1, y = 0 and 0.25) multilayer buffers. We find a reduction in TD density in GaN films grown on graded Al _x Ga_1-x N buffer layers, in comparison with those grown directly on a thin AlN buffer layer. Threading dislocation bending and annihilation occurs in the region in the graded Al _x Ga_1-x N grown under a compressive stress, which leads to a decrease of TD density in the overgrown GaN films. In addition, growing a thin AlN/Al _y Ga_1-y N bilayer prior to growing the compositionally graded Al _x Ga_1-x N buffer layer significantly reduces the initial TD density in the Al _x Ga_1-x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film. In-situ stress measurements reveal a delayed compressive-to-tensile stress transition for GaN films grown on graded Al _x Ga_1-x N buffer layers or multilayer buffers, in comparison to the film grown on a thin AlN buffer layer, which subsequently reduces the crack densities in the films.     

Preparation and Thermoelectric Properties of LaGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> and SmGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript>

Thermoelectric materials are attractive since they can recover waste heat directly in the form of electricity. In this study, the thermoelectric properties of ternary rare-earth sulfides LaGd_1+x S₃ (x = 0.00 to 0.03) and SmGd_1+x S₃ (x = 0.00 to 0.06) were investigated over the temperature range of 300 K to 953 K. These sulfides were prepared by CS₂ sulfurization, and samples were consolidated by pressure-assisted sintering to obtain dense compacts. The sintered compacts of LaGd_1+x S₃ were n-type metal-like conductors with a thermal conductivity of less than 1.7 W K⁻¹ m⁻¹. Their thermoelectric figure of merit ZT was improved by tuning the chemical composition (self-doping). The optimized ZT value of 0.4 was obtained in LaGd_1.02S₃ at 953 K. The sintered compacts of SmGd_1+x S₃ were n-type hopping conductors with a thermal conductivity of less than 0.8 W K⁻¹ m⁻¹. Their ZT value increased significantly with temperature. In SmGd_1+x S₃, the ZT value of 0.3 was attained at 953 K.     

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.     

Analysis of Dislocation Density in Pb<Subscript>(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se Grown on ZnTe/Si by MBE

The growth of (211) Pb_(1−x)Sn _x Se on Si is achieved with a thick ZnTe buffer layer. The obtained films are specular, but contain widely dispersed void defects. Because the lattice misfit between Pb_(1−x)Sn _x Se and ZnTe is small, dislocation density values on the order of 10⁶/cm² in the Pb_(1−x)Sn _x Se are obtained. The variation of the dislocation density as a function of Pb_(1−x)Sn _x Se thickness, h, is analyzed in terms of dislocation annihilation. The analysis predicts an inverse quadratic dependence of dislocation density on h, and quantitative agreement with experimental measurements of the dislocation density is obtained.     

High-Quality (211)B CdTe on (211)Si Substrates Using Metalorganic Vapor-Phase Epitaxy

High-quality (211)B CdTe buffer layers are required during Hg_1−x Cd _x Te heteroepitaxy on Si substrates. In this study, direct metalorganic vapor-phase epitaxy (MOVPE) of (211)B CdTe on Si, as well as CdTe on Si using intermediate Ge and ZnTe layers, has been achieved. Tertiary butyl arsine was used as a precursor to enable As surfactant action during CdTe MOVPE on Si. The grown CdTe/Si films display a best x-ray diffraction rocking-curve full-width at half-maximum of 64 arc-s and a best Everson etch pit density of 3 × 10⁵ cm⁻². These values are the best reported for MOVPE-grown (211)B CdTe/Si and match state-of-the-art material grown using molecular-beam epitaxy.     

Preparation and Thermoelectric Properties of La-Doped SrTiO<Subscript>3</Subscript> Ceramics

Single-phase polycrystalline La _x Sr_1−x TiO₃ (x = 0, 0.04, 0.06, 0.08, and 0.12) ceramics were prepared by the conventional solid-state reaction method using high-activity hydroxides as the raw materials. The electrical conductivity of all the samples increased with increasing x value and decreased with measurement temperature, while the thermal conductivity decreased with increasing x value and measurement temperature. The La_0.12Sr_0.88TiO₃ sample showed the lowest thermal conductivity of 2.45 W m⁻¹ K⁻¹ at 873 K and the largest ZT of 0.28 at 773 K. The present work revealed that hydroxides with high activity as raw materials are beneficial to improve the thermoelectric properties, especially to decrease the thermal conductivity.     

Controlled p-type Sb doping in LPE-grown Hg1−x Cdx Te epilayers

A Te-rich liquid-phase-epitaxial growth process is reported whereby reproducible Sb-doped layers are prepared with hole concentrations and hole mobilities ranging from 1.8×10¹⁶ to 1.3×10¹⁹ cm⁻³ and 280 to 29 cm²/V s, respectively, at 77K for x-values ranging from 0.23 to 0.29. An effective electronic distribution coefficient for Sb of 0.01 is calculated from the hole concentration at 77K and the concentration of Sb in the growth solution. The process for group Va doping of low-x Hg_1−x Cd_x Te from Te-rich solutions and the procedure for the growth of a CdZnTe buffer layer on a CdTeSe substrate are described. For Te-rich Cd−Zn−Te growth solutions the distribution coefficient of Zn was found to be 18. The growth of a structure consisting of an Sb-doped HgCdTe epilayer on a CdZnTe buffer layer lattice matched (Δa/a<10⁻⁴) to a CdSeTe substrate has been demonstrated.     

Correlation Between Threading Dislocations and Nonradiative Recombination Centers in InN Observed by IR Cathodoluminescence

The correlation between threading dislocations (TDs) and nonradiative recombination centers in InN films was investigated by infrared cathodoluminescence (CL). Samples were grown on nitridated (0001) sapphire substrates with a low-temperature-grown InN buffer layer by radio frequency molecular-beam epitaxy (RF-MBE). Panchromatic CL images of the InN films showed a high density of dark spots in a range of 10⁸ cm⁻² to 10⁹ cm⁻². The sample with a higher density of TDs had a higher density of CL dark spots. A depth-dependent CL measurement confirmed that CL dark spots aligned almost vertically in the film like TDs. Reasonable correlation between TDs and the nonradiative regions was also observed by a cross-sectional CL image of the InN film regrown on a microfaceted InN template, in which the TD density was dramatically reduced in part. These results suggest that threading dislocations act as nonradiative recombination centers in InN.     

Effects of Growth Temperature on Indium Incorporation in InAlN Alloys Grown by GSMBE on Si(111)

In _x Al_1−x N alloys with low indium content (0.025 < x < 0.080) were grown on Si(111) substrates, with an AlN buffer layer, using gas source molecular beam epitaxy with ammonia under nitrogen-rich conditions. Composition was varied by changing the growth temperature from 580°C to 660°C. Growth temperature in excess of 580°C was found to be necessary to obtain compositional uniformity. As temperature was varied from 590°C to 660°C, both the growth rate and indium incorporation decreased substantially. Rising In content observed near the surface of each sample was attributed to native indium oxide formation.     

Preparation of Well-Tiled <Emphasis Type="Italic">γ</Emphasis>-Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> by a Novel and Simple Sodium Alginate Gel Method and Its Electrical Properties

In this paper, a novel and simple sodium alginate (SA) gel method was developed to prepare γ-Na _x Co₂O₄. This method involved the chemical gelling of SA in the presence of Co²⁺ ions by cross-linking. After calcining at 700°C to 800°C, single-phase γ-Na _x Co₂O₄ crystals were obtained. The arrangement of about 1 μm to 4 μm flaky particles exhibited a well-tiled structure along the plane direction of the flaky particles. SA not only acted as the control agent for crystal growth, but also provided a Na source for the γ-Na _x Co₂O₄ crystals. The electrical properties of γ-Na _x Co₂O₄ ceramics prepared via ordinary sintering after cold isostatic pressing were investigated. The Seebeck coefficient and power factor of the bulk material were 177 μV K⁻¹ and 4.3 × 10⁻⁴ W m⁻¹ K⁻² at 850 K, respectively.     

Enhanced Thermoelectric Properties Obtained by Compositional Optimization in p-Type BixSb2?xTe3 Fabricated by Mechanical Alloying and Spark Plasma Sintering

Bi _x Sb_2−x Te₃ bulk alloys are known as the best p-type thermoelectric materials near room temperature. In this work, single-phase Bi _x Sb_2−x Te₃ (x = 0.2, 0.25, 0.3, 0.34, 0.38, 0.42, 0.46, and 0.5) alloys were prepared by spark plasma sintering (SPS) using mechanical alloying (MA)-derived powders. A small amount (0.1 vol.%) of SiC nanoparticles was added to improve the mechanical properties and to reduce the thermal conductivity of the alloys. The electrical resistivity decreases significantly with increasing ratio of Sb to Bi in spite of the weaker decreasing trend in Seebeck coefficient, whereby the power factor at 323 K reaches 3.14 × 10⁻³ W/mK² for a sample with x = 0.3, obviously higher than that at x = 0.5 (2.27 × 10⁻³ W/mK²), a composition commonly used for ingots. Higher thermal conductivities at low temperatures are obtained at the compositions with lower x values, but they tend to decrease with temperature. As a result, higher ZT values are obtained for Bi_0.3Sb_1.7Te₃, with a maximum ZT value of 1.23 at 423 K, about twice the ZT value (about 0.6) of Bi_0.5Sb_1.5Te₃ at the same temperature.