Properties of ZnO Thin Films Codoped with Lithium and Phosphorus

The properties of ZnO thin films codoped with lithium and phosphorus have been characterized. The films were deposited from high-purity ZnO and Li₃PO₄ solid targets onto c-plane sapphire substrates by radiofrequency (RF) magnetron sputtering. A substrate temperature of 900°C was determined as optimum for depositing undoped ZnO films with background electron concentration of 9.9 × 10¹⁵ cm^?3 as the buffer layer on the sapphire substrate. Postdeposition annealing was carried out using rapid thermal processing in O₂ at temperatures ranging from 500°C to 1000°C for 3 min. Analyses performed using low-temperature photoluminescence spectroscopy measurements revealed luminescence peaks at 3.356 eV, 3.307 eV, 3.248 eV, and 3.203 eV at 12 K for the codoped samples. X-ray diffraction 2θ-scans showed a single peak at about 34.4° with full-width at half-maximum of about 0.09°. Hall-effect measurements revealed initial p-type conductivities, but these were unstable and toggled between p-type and n-type over time with Hall concentrations that varied between 2.05 × 10¹³ cm^?3 and 2.89 × 10¹⁵ cm^?3. The fluctuation in the carrier type could be due to lateral inhomogeneity in the hole concentration caused by stacking faults in the films. An additional cause could be the small Hall voltages in the measurements, which could be significantly impacted by even small spikes in signal noise inherent in the measurements.     

Fabrication of a Flexible Bismuth Telluride Power Generation Module Using Microporous Polyimide Films as Substrates

In this study, we investigated the effect of the structure of microporous p-type (Bi_0.4Te₃Sb_1.6) and n-type (Bi_2.0Te_2.7Se_0.3) BiTe-based thin films on their thermoelectric performance. High-aspect-ratio porous thin films with pore depth greater than 1 μm and pore diameter ranging from 300 nm to 500 nm were prepared by oxygen plasma etching of polyimide (PI) layers capped with a heat-resistant block copolymer, which acted as the template. The cross-plane thermal conductivities of the porous p- and n-type thin films were 0.4 W m^?1 K^?1 and 0.42 W m^?1 K^?1, respectively, and the dimensionless figures of merit, ZT, of the p- and n-type BiTe films were estimated as 1.0 and 1.0, respectively, at room temperature. A prototype thermoelectric module consisting of 20 pairs of p- and n-type strips over an area of 3 cm × 5 cm was fabricated on the porous PI substrate. This module produced an output power of 0.1 mW and an output voltage of 0.6 V for a temperature difference of 130°C. The output power of the submicrostructured module was 1.5 times greater than that of a module based on smooth BiTe-based thin films. Thus, the thermoelectric performance of the thin films was improved owing to their submicroscale structure.     

Effect of Thermal Annealing on the Characteristics of Phosphorus-Implanted ZnO Crystals

A P-doped ZnO surface layer on undoped ZnO wafers was prepared by phosphorus (P) ion implantation. Hall effect measurement revealed p-type conduction in such layers annealed at 800°C. This indicates that acceptor levels are present in P-doped ZnO, even though the ZnO is still n-type. Micro-Raman scattering in ?z(xy)z geometry was conducted on P-implanted ZnO. The E ₂ ^high mode shift observed toward the high-energy region was related to compressive stress as a result of P-ion implantation. This compressive stress led to the appearance of an A ₁(LO) peak, which is an inactive mode. This A ₁(LO) peak relaxed during thermal annealing in ambient oxygen at temperatures higher than 700°C. The P2p_3/2 peak observed at 135.6 eV by x-ray photoelectron spectroscopy is associated with chemical bond formation leading to 2(P₂O₅) molecules. This indicates that implanted P ions substituted Zn sites in the ZnO layer. In photoluminescence spectroscopy, the P-related peaks observed at energies ranging between 3.1 and 3.5 eV originated from (A⁰, X) emission, because of P_Zn-2V_Zn complexes acting as shallow acceptors. The acceptor level was observed to be 126.9 meV above the valence band edge. Observation of this P-related emission indicates that ion implantation results in acceptor levels in the P-doped ZnO layer. This suggests that the P₂O₅ bonds are responsible for the p-type activity of P-implanted ZnO.     

Structural and Electrical Properties of Ag/<Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO₂/p-Si/Al heterostructure. Thin films of pure TiO₂ were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO₂ film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO₂, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO₂/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 10⁵ Am^?2 K^?2, which is of the order of the theoretical value 3.2 × 10⁵ Am^?2 K^?2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO₂ film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO₂ films was found to be 3.60 eV.     

Thin Film Deposition of Semiconducting Ni-Co Oxide Spinel with Adequate Electrical and Optical Properties for Energy Application

Nickel-cobalt oxide with spinel structure was successfully fabricated using a wet chemical route followed by calcinations at 300 °C. In nickel-cobalt spinel oxide, Ni²⁺ ions occupy the octahedral sites and Co³⁺ ions are distributed over both octahedral and tetrahedral sites. Very interestingly, nickel-cobalt spinel oxide does not only show a p-type semi-conducting behavior material but also exhibits desired transparency in infrared wavelengths. Electrical and optical properties of the deposited films were investigated as a function of different processing conditions. The sputtering target is fabricated by homogeneously mixing oxide powders and followed by sintering at 1,500 °C. The nickel-cobalt oxide film showed a resistivity as low as 10^?2 Ω-cm by radio frequency (RF) magnetron sputtering in a pure oxygen atmosphere. The sputter-deposited nickel-cobalt oxide films also showed more than 70% transmittance in the infrared range.     

Determination of the Origin of Crystal Orientation for Nanocrystalline Bismuth Telluride-Based Thin Films Prepared by Use of the Flash Evaporation Method

We have investigated the origin of crystal orientation for nanocrystalline bismuth telluride-based thin films. Thin films of p-type bismuth telluride antimony (Bi–Te–Sb) and n-type bismuth telluride selenide (Bi–Te–Se) were fabricated by a flash evaporation method, with exactly the same deposition conditions except for the elemental composition of the starting powders. For p-type Bi–Te–Sb thin films the main x-ray diffraction (XRD) peaks were from the c-axis (Σ{00l}/Σ{hkl} = 0.88) whereas n-type Bi–Te–Se thin films were randomly oriented (Σ{00l}/Σ{hkl} = 0.40). Crystal orientation, crystallinity, and crystallite size were improved for both types of thin film by sintering. For p-type Bi–Te–Sb thin films, especially, high-quality structures were obtained compared with those of n-type Bi–Te–Se thin films. We also estimated the thermoelectric properties of the as-grown and sintered thin films. The power factor was enhanced by sintering; maximum values were 34.9 μW/cm K² for p-type Bi–Te–Sb thin films at a sintering temperature of 300°C and 23.9 μW/cm K² for n-type Bi–Te–Se thin films at a sintering temperature of 350°C. The exact mechanisms of film growth are not yet clear but we deduce the crystal orientation originates from the size of nano-clusters generated on the tungsten boat during flash evaporation.     

Effects of Thickness and Annealing on Optoelectronic Properties of Electrodeposited ZnS Thin Films for Photonic Device Applications

Thin layers of ZnS with thicknesses of 400 nm, 500 nm, and 700 nm have been electrodeposited on glass/fluorine-doped tin oxide substrates using a simple two-electrode setup under similar conditions. Structural characterization of the layers using x-ray diffraction (XRD) measurements showed that they were amorphous. The results of optical characterization carried out in the wavelength range of 315 nm to 800 nm using spectrophotometry revealed that the optical properties of the layers are strongly influenced by the film thickness as well as annealing conditions. The values of the refractive index, extinction coefficient, absorption coefficient, and dielectric constant obtained from normal-incidence transmittance spectra were generally lower after annealing, showing also the influence of postdeposition annealing on the deposited ZnS layers. Electrical characterization of the layers, using direct-current current–voltage measurement under dark conditions at room temperature, shows that the resistivity of the as-deposited and annealed layers is in the range of 1.4 × 10⁴ Ω cm to 2.5 × 10⁴ Ω cm and 2.5 × 10⁴ Ω cm to 3.1 × 10⁴ Ω cm, respectively. The results suggest that the optoelectronic properties can be tuned for particular applications by adjusting the thickness of the layers appropriately.     

Practical Contact Resistance Measurement Method for Bulk Bi2Te3-Based Thermoelectric Devices

The impact of contact resistance on thermoelectric (TE) device performance grows more significant as devices are scaled down. To improve and understand the effects of contact resistance on bulk TE device performance, a reliable experimental measurement method is needed. There are many popular methods to extract contact resistance, but they are only well suited for measuring metal contacts on thin films and do not necessarily translate to measuring contact resistance on bulk TE materials. The authors present a measurement technique that precisely measures contact resistance on bulk TE materials by making and testing stacks of bulk, metal-coated TE wafers using TE industry-standard processes. An equation that uses the Z of the stacked device to extract the contact resistance is used to reduce the sensitivity to resistivity variations of the TE material. Another advantage of this technique is that it exploits realistic TE device manufacturing techniques and results in an almost device-like structure. The lowest contact resistivity measured was 1.1 × 10^?6 Ω cm² and 1.3 × 10^?6 Ω cm² for n- and p-type materials, respectively using a newly developed process at 300 K. The uncertainty in the contact resistivity values for each sample was 10% to 20%, which is quite good for measurements in the 10^?6 Ω cm² range.     

Design of Apparatus for Ni/Mg2Si and Ni/MnSi1.75 Contact Resistance Determination for Thermoelectric Legs

In recent decades, thermoelectricity has been widely studied as a potential new source of renewable energy. One of the major challenges to improve the efficiency of thermoelectric (TE) devices is to minimize the contact resistance between the active material and the electrodes, since this represents the major loss of charge in a TE module. This article describes the fabrication of an apparatus for TE leg characterization built with commercial and custom-made parts based on the analog one-dimensional transmission-line method. This device permits contact resistance measurements of bulk TE legs. p- and n-type TE materials, Mg₂Si_0.98Bi_0.02 and MnSi_1.75Ge_0.02, respectively, were metallized with nickel foils and used as test materials for contact resistance characterization. Contact resistance values of 0.5 mΩ mm² for Ni/Mg₂Si_0.98Bi_0.02 junctions and 4 mΩ mm² for Ni/MnSi_1.75Ge_0.02 junctions have been measured. Contact resistance measurements are discussed depending on materials processing and the experimental measurement conditions.     

Preparation of CdSSe-ZnS superlattice,SrS, and CdSSe-SrS superlattice by hot-wall epitaxy,and applications to electroluminescent devices

CdSSe (manganese-doped, E_g = 1.9–2.5 eV, lattice constant a = 6.05–5.8A)-ZnS (Eg = 3.56 eV, a = 5.41A) superlattices, SrS (cerium-doped, E = 4.4 eV, a = 6.02A) layers, and CdSSe-SrS (cerium-doped) superlattice layers nave been prepared by hot-wall epitaxy, and the properties and the electroluminescent device characteristics of the active layers are reported. For the superlattices with ZnS, the maximum luminance was 800 cd/m² at an applied sinusoidal voltage (V_o-p = 200 V) with frequency 1kHz, and the wavelength of the spectral peak was 610 nm due to the large strain caused by the lattice mismatch (8–15%) between the CdSSe and ZnS layers. The maximum luminance and Comisson Internationale de Enluminure (CIE) chromaticity of CdS(Mn)-ZnS superlattices and CdSe(Mn)-ZnS superlattice devices were 557cd/m² and (x,y) = (0.58,0.41) and 982 cd/m² and (0.61, 0.38), respectively. For superlattices with SrS, the maximum luminance of the device with the SrS (cerium-doped) active layer was nearly 700 cd/m² at a voltage of 340V. Blue electroluminescent emission was observed in the photon wavelength region less than 450 nm, due to carriers dropping into the quantum wells of the device with the CdSSe-SrS superlattice active layer.     

Physical Properties of Single-Crystalline Ba8Ni3.5Ge42.1□0.4

Clathrates are candidate materials for thermoelectric applications because of a number of unique properties. The clathrate I phases in the Ba-Ni-Ge ternary system allow controlled variation of the charge carrier concentration by adjusting the Ni content. Depending on the Ni content, the physical properties vary from metal-like to insulator-like and show a transition from p-type to n-type conduction. Here we present first results on the characterization of millimeter-sized single crystals grown by the Bridgman technique. Single crystals with a composition of Ba₈Ni_3.5Ge_42.1□_0.4 show metallic behavior (dρ/dT > 0) albeit with high resistivity at room temperature [ρ (300 K) = 1 mΩ cm]. The charge carrier concentration at 300 K, as determined from Hall-effect measurements, is 2.3 e^?/unit cell. The dimensionless thermoelectric figure of merit estimated at 680 K is ZT ≈ 0.2.     

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.     

Carrier Transport Mechanism and Band Offsets at the Interface of ZnS/<Emphasis Type="Italic">n</Emphasis>-Si (111) Heterojunctions Fabricated by Vacuum Thermal Evaporation

The electrical properties and band offset of ZnS/n-Si(111) heterojunctions with and without annealing were analyzed. The result showed that the rectifying characteristics of ZnS/n-Si(111) heterojunctions became better and the leakage current increased after annealing. This phenomenon is mostly due to the volatilization of S atoms of ZnS films and leads to defect levels appearing at the interface of the ZnS/n-Si(111) hetrojunctions. The valence band offset (ΔE _V) of the ZnS/n-Si(111) heterojunctions can be calculated to be ?0.7 ± 0.15 eV by means of photoelectron spectroscopy, indicating that the band offsets of ZnS/n-Si(111) heterojunctions show a type-II band alignment.     

Depth Profiling of Electronic Transport Parameters in n-on-p Boron-Ion-Implanted Vacancy-Doped HgCdTe

We report results of a detailed study of electronic transport in n-on-p junctions formed by 150-keV boron-ion implantation in vacancy-doped p-type Hg_0.769Cd_0.231Te without postimplantation thermal annealing. A mobility spectrum analysis methodology in conjunction with a wet chemical etching-based surface removal approach has been employed to depth profile the transport characteristics of the samples. In the as-implanted samples, three distinct electron species were detected which are shown to be associated with (a) low-mobility electrons in the top 220-nm surface-damaged layer (E ₁: μ _80K = 2940 cm²/Vs), (b) the B-ion implantation region in the top 500-nm region (E ₂: μ _80K = 7490 cm²/Vs), and (c) high-mobility electrons in the n-to-p transition region at a depth of 600 nm to 700 nm (E ₃: μ _80K = 25,640 cm²/Vs). Due to the maximum magnetic field employed (2 T), hole carriers from the underlying vacancy-doped p-type region were detected only after the removal of the top 220 nm of the profiled sample (μ _80K = 126 cm²/Vs), revealing fully p-type character 800 nm below the original sample surface. A comparison of the extracted E ₂ electron concentration and calculated B-impurity profile suggests that the n-type region is due primarily to near-surface implantation-induced lattice damage.     

Carrier Lifetimes in Lightly-Doped <Emphasis Type="Italic">p</Emphasis>-Type 4H-SiC Epitaxial Layers Enhanced by Post-growth Processes and Surface Passivation

We investigated limiting factors of carrier lifetimes and their enhancement by post-growth processes in lightly-doped p-type 4H-SiC epitaxial layers (N _A ～ 2 × 10¹⁴ cm^?3). We focused on bulk recombination, surface recombination, and interface recombination at the epilayer/substrate, respectively. The carrier lifetime of 2.8 μs in an as-grown epilayer was improved to 10 μs by the combination of V_C-elimination processes and hydrogen annealing. By employing surface passivation with deposited SiO₂ followed by POCl₃ annealing, a long carrier lifetime of 16 μs was obtained in an oxidized epilayer. By investigating carrier lifetimes in a self-standing p-type epilayer, it was revealed that the interface recombination at the epilayer/substrate was smaller than the surface recombination on a bare surface. We found that the V_C-elimination process, hydrogen annealing, and surface passivation are all important for improving carrier lifetimes in lightly-doped p-type epilayers.     

Development of Polyaniline Using Electrochemical Technique for Plugging Pinholes in Cadmium Sulfide/Cadmium Telluride Solar Cells

Polyaniline (PAni) thin films were prepared by using an electrochemical polymerization technique on glass/FTO substrates by varying the deposition potential, deposition time, pH concentrations and heat treatment conditions. The structural, morphological, optical and electrical properties of electrodeposited PAni films were characterized using x-ray diffraction, scanning electron microscopy, UV–VIS spectroscopy, optical profilometry and D.C. conductivity measurements. Structural analysis shows the formation of the highest crystallinity for PAni thin film grown at V _g 1654 mV. Optical absorption measurements have demonstrated a wide variety of energy band gaps (E _g), varying from ～0.50 eV to 2.40 eV for PAni grown by tuning the pH value during the deposition. The electrical resistivity showed an increase from 0.37 × 10⁶ Ω cm to 3.91 × 10⁶ Ω cm when the pH increased from 2.00 to 6.50. The diode structures of glass/FTO/CdS/CdTe/PAni/Au were fabricated incorporating PAni as a pinhole plugging layer, and assessed for their photovoltaic activities. The results showed the enhancement of all device parameters, especially of open circuit voltage and fill factors. This improvement offers a great potential for enhancing solar cell performance and the device lifetime, and the latest results are presented in this paper.     

p+-n--n+-type power diode with crystalline/nanocrystalline Si mosaic electrodes

Using p⁺-type crystalline Si with n⁺-type nanocrystalline Si (nc-Si) and n⁺-type crystalline Si with p⁺-type nc-Si mosaic structures as electrodes, a type of power diode was prepared with epitaxial technique and plasma-enhanced chemical vapor deposition (PECVD) method. Firstly, the basic p⁺-n^--n⁺-type Si diode was fabricated by epitaxially growing p⁺- and n⁺-type layers on two sides of a lightly doped n^--type Si wafer respectively. Secondly, heavily phosphorus-doped Si film was deposited with PECVD on the lithography mask etched p⁺-type Si side of the basic device to form a component with mosaic anode. Thirdly, heavily boron-doped Si film was deposited on the etched n⁺-type Si side of the second device to form a diode with mosaic anode and mosaic cathode. The images of high resolution transmission electronic microscope and patterns of X-ray diffraction reveal nanocrystallization in the phosphorus- and boron-deposited films. Electrical measurements such as capacitance-voltage relation, current-voltage feature and reverse recovery waveform were carried out to clarify the performance of prepared devices. The important roles of (n^-)Si/(p⁺)nc-Si and (n^-)Si/(n⁺)nc-Si junctions in the static and dynamic conduction processes in operating diodes were investigated. The performance of mosaic devices was compared to that of a basic one.     

Postgrowth Annealing of CdTe Layers Grown on Si Substrates by Metalorganic Vapor-Phase Epitaxy

Annealing conditions of CdTe layers grown on Si substrates by metalorganic vapor-phase epitaxy were studied. Typically, 3-μm-thick n-type (211) CdTe layers were annealed for 60 s in flowing hydrogen at atmospheric pressure by covering their surfaces with bulk CdTe wafers. At annealing temperatures above 700°C, improvement of crystal quality was confirmed from full-width at half-maximum values of double-crystal rocking-curve measurements and x-ray diffraction measurements. Photoluminescence measurements revealed no deterioration of electrical properties in the annealed n-CdTe layers. Furthermore, annealing at 900°C improved the performance of radiation detectors with structure of p-like CdTe/n-CdTe/n ⁺-Si substrate.     

Interface Microstructure and Performance of Sb Contacts in Bismuth Telluride-Based Thermoelectric Elements

A thermoelectric joint composed of p-type Bi_0.5Sb_1.5Te₃ (BiSbTe) material and an antimony (Sb) interlayer was fabricated by spark plasma sintering. The reliability of the thermoelectric joints was investigated using electron probe microanalysis for samples with different accelerated isothermal aging time. After aging for 30 days at 300°C in vacuum, the thickness of the diffusion layer at the BiSbTe/Sb interface was about 30 μm, and Sb₂Te₃ was identified to be the major interfacial compound by element analysis. The contact resistivity was 3 × 10^?6 ohm cm² before aging and increased to 8.5 × 10^?6 ohm cm² after aging for 30 days at 300°C, an increase associated with the thickness of the interfacial compound. This contact resistivity is very small compared with that of samples with solder alloys as the interlayer. In addition, we have also investigated the interface behavior of Sb layers integrated with n-type Bi₂Se_0.3Te_2.7 (BiSeTe) material, and obtained similar results as for the p-type semiconductor. The present study suggests that Sb may be useful as a new interlayer material for bismuth telluride-based power generation devices.     

Cadmium Telluride Solar Cells on Ultrathin Glass for Space Applications

This paper details the preliminary findings of a study to achieve a durable thin-film CdTe photovoltaic (PV) device structure on ultrathin space-qualified cover glass. An aluminum-doped zinc oxide (AZO) transparent conducting oxide was deposited directly onto the cover glass using metalorganic chemical vapor deposition (MOCVD). The AZO demonstrated low sheet resistance of 10 Ω/□ and high optical transparency of 85% as well as excellent adherence and environmental stability. Preliminary deposition of PV layers onto the AZO on cover glass, by MOCVD, showed the possibility of such a structure, yielding a device conversion efficiency of 7.2%. High series resistance (10 Ω cm²) and low V _oc (586 mV) were identified as the limiting factors when compared with the authors’ platform process on indium tin oxide-coated aluminosilicate. The coverage of the Cd_1?x Zn _x S window layer along with the front contacting of the device were shown to be the major causes of the low efficiency. Further deposition of AZO/CdTe employing an oxygen plasma cleaning step to the cover glass and evaporated gold front contacts significantly improved the device performance. With a highest conversion efficiency of 10.2%, series resistance improved to 4.4 Ω cm², open-circuit voltage (V _oc) up to 667 mV, and good adhesion, this represents the first demonstration of direct deposition of CdTe solar cells onto 100-μm-thick space-qualified cover glass.