Mechanisms of recombination of nonequilibrium charge carriers in epitaxial Cd<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te (<Emphasis Type="Italic">x</Emphasis> = 0.20–0.23) layers

The experimental temperature dependences of the photosensitivity and the data on the lifetime of nonequilibrium charge carriers in epitaxial Cd_xHg_1?x Te layers with x = 0.20–0.23 were used to show that, in the region of intrinsic and extrinsic conductivity in n-type films grown by molecular beam epitaxy, CHCC Auger recombination is the prevailing recombination mechanism. At the same time, in p-type films grown by liquid-or vapor-phase epitaxy, it is observed that, in the region of extrinsic conductivity, CHLH Auger recombination competes with Shockley-Read recombination. The n-type films grown by molecular beam epitaxy contain a much lower concentration of recombination centers than the p-type films grown by liquid-or gasphase epitaxy.     

Synthesis of thin p-type rutile films

The structure and electrical and optical properties of heterostructures formed on the surface of single-crystal silicon wafers as a result of the heat treatment and pulsed photon treatment of Ti films in oxygen, air, and nitrogen are investigated. It is shown that a TiO₂/Ti₅Si₃/p-Si heterostructure is formed upon heat treatment in air, whereas a TiO₂/TiSi₂/p-Si heterostructure is formed upon photon treatment. It is established that rutile films with pronounced n-type conductivity are formed as a result of the heat treatment of Ni-doped Ti films in oxygen. Rutile films with p-type conductivity are formed upon the thermal annealing of Ti films in air with subsequent photon treatment in nitrogen.     

Synthesis and Characterization of Magnesium-Alloyed Hematite Thin Films

We have synthesized pure and Mg-alloyed hematite thin films on F-doped, SnO₂-coated glass substrates by radiofrequency magnetron cosputtering of iron oxide with and without MgO sources in mixed Ar/O₂ and mixed N₂/O₂ ambient. We found that hematite films deposited in N₂/O₂ ambient exhibited much poorer crystallinity than those deposited in Ar/O₂ ambient. We determined that Mg alloying led to increased crystallinity and bandgap. Furthermore, we found that Mg alloying inverted the type of conductivity of the thin films: pure hematite thin films exhibited n-type conductivity, whereas Mg-alloyed hematite thin films exhibited p-type conductivity.     

Charge-carrier lifetime in Hg1−x CdxTe (x=0.22) structures grown by molecular-beam epitaxy

Measurements of the charge carrier lifetime in epitaxial structures based on narrow-gap Hg_1−x Cd_xTe (x=0.22), grown by molecular-beam epitaxy with pulsed excitation using radiation at different wavelengths, are reported. It is shown that in p-type epitaxial films the lifetime is determined by the Auger recombination mechanism at temperatures corresponding to the impurity conductivity, and for n-type epitaxial films recombination via local centers is characteristic. Fiz. Tekh. Poluprovodn. 31, 774–776 (July 1997)     

Photoluminescence Studies of Si-Doped Epitaxial GaAs Films Grown on (100)- and (111)A-Oriented GaAs Substrates at Lowered Temperatures

The electrical properties and photoluminescence features of uniformly Si-doped GaAs layers grown on GaAs substrates with the (100) and (111)A crystallographic orientations of the surface are studied. The samples are grown at the same As₄ pressure in the growth temperature range from 350 to 510°C. The samples grown on GaAs(100) substrates possess n-type conductivity in the entire growth temperature range, and the samples grown on GaAs(111)A substrates possess p-type conductivity in the growth temperature range from 430 to 510°C. The photoluminescence spectra of the samples exhibit an edge band and an impurity band. The edge photoluminescence band corresponds to the photoluminescence of degenerate GaAs with n- and p-type conductivity. The impurity photoluminescence band for samples on GaAs(100) substrates in the range 1.30–1.45 eV is attributed to V_As defects and Si_As–V_As defect complexes, whose concentration varies with sample growth temperature. Transformation of the impurity photoluminescence spectra of the samples on GaAs(111)A substrates is interpreted as being a result of changes in the V_As and V_Ga defect concentrations under variations in the growth temperature of the samples.     

Fabrication by Coaxial-Type Vacuum Arc Evaporation Method and Characterization of Bismuth Telluride Thin Films

We prepared both n- and p-type bismuth telluride thin films by using a coaxial-type vacuum arc evaporation method. The atomic compositions of the as-grown thin films and several annealed thin films were comparable to that of bulk bismuth telluride. Their thermoelectric properties were measured and found to be comparable to those of bulk materials. The Seebeck coefficient and electrical conductivity of the as-grown thin films were improved by the annealing process. The measured figures of merit (ZT) of the films were 0.86 for the n-type and 0.41 for the p-type at 300 K for annealing temperatures of 573 K and 523 K, respectively.     

Modeling of Thermoelectric Properties of Magnesium Silicide (Mg2Si)

Thermoelectric (TE) materials based on alloys of magnesium (Mg) and silicon (Si) possess favorable properties such as high electrical conductivity and low thermal conductivity. Additionally, their abundance in nature and lack of toxicity make them even more attractive. To better understand the electronic transport and thermal characteristics of bulk magnesium silicide (Mg₂Si), we solve the multiband Boltzmann transport equation within the relaxation-time approximation to calculate the TE properties of n-type and p-type Mg₂Si. The dominant scattering mechanisms due to acoustic phonons and ionized impurities were accounted for in the calculations. The Debye model was used to calculate the lattice thermal conductivity. A unique set of semiempirical material parameters was obtained for both n-type and p-type materials through simulation testing. The model was optimized to fit different sets of experimental data from recently reported literature. The model shows consistent agreement with experimental characteristics for both n-type and p-type Mg₂Si versus temperature and doping concentration. A systematic study of the effect of dopant concentration on the electrical and thermal conductivity of Mg₂Si was also performed. The model predicts a maximum dimensionless figure of merit of about 0.8 when the doping concentration is increased to approximately 10²⁰?cm^?C3 for both n-type and p-type devices.     

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.     

Characterization of n-Type and p-Type ZnS Thin Layers Grown by an Electrochemical Method

Electrodeposition of n-type and p-type thin-film layers of ZnS was carried out using a simple two-electrode system and aqueous solutions of ZnCl₂ and (NH₄)₂S₂O₃ with different Zn²⁺ concentrations. X-ray diffraction measurements show that the ZnS layers deposited from both solutions are amorphous. Optical absorption measurements show low absorbance of the layers with energy bandgap in the range of 3.68 eV to 3.78 eV after postdeposition annealing. Photoelectrochemical cell measurements show that both n-type and p-type ZnS thin layers can be electrodeposited by simply changing the concentrations of the deposition solutions. With higher Zn²⁺ concentration in the bath, n-type ZnS films were deposited, while p-type ZnS films were deposited with lower Zn²⁺ concentration. The estimated resistivity of layers from both solutions using I–V measurements were 3.0 × 10⁴ Ω cm and 2.0 × 10⁴ Ω cm, respectively, for n-ZnS and p-ZnS. Scanning electron microscopy shows that the deposited films consist of particles with good surface coverage of the glass/fluorine-doped tin oxide substrate.     

Electrical characteristics of r.f.-sputtered CdTe thin-films for photovoltaic applications

A method of preparing self-doped p- and n-type and In-doped n-type CdTe thin-films for photovoltaic applications has been developed using r.f. sputtering. Ohmic contacts to n-type films with contact resistivity less than 10^?2 Ω — cm² have been obtained. Schottky barrier diode test devices, formed by evaporation of various metals including Au on n-CdTe films, have been examined for electrical and photovoltaic evaluation of the sputtered films. Although S.B. diodes based on In doped films, prepared under Cd overpressure, show promising electrical and photovoltaic performance (V_oc ～ 315 mV, I_sc ～ 4.6mA/cm²), much improvement remains to be made by further control of dopant concentration and structural details of films.     

Effect of bismuth impurity on carrier density in PbSe:Bi:Se epitaxial layers

The dependence of the Hall carrier density on bismuth concentration, n, p=f(N _Bi), in PbSe:Bi:Se/BaF₂ films has been studied. The films were grown by vacuum condensation from two independent molecular beams (PbSe:Bi and Se₂) mixed directly at the surface of a (111)BaF₂ substrate heated to 350°C. The bismuth concentration in the stock was 0–0.3 at. %. Two specific portions can be distinguished in the experimental n, p=f(N _Bi) dependence. At N _Bi>0.0375 at. %, the electron density is close to N _Bi; at low bismuth concentrations, N _Bi<0.0375 at. %, the linear run of the n=f(N _Bi) dependence is violated, and the conduction changes to p-type. All the doped films under study are saturated with selenium. This is a necessary condition for obtaining the highest electron densities in the films at N _Bi corresponding to the linear portion of the n=f(N _Bi) dependence. The results are discussed in terms of a thermodynamic model of the impurity interaction with intrinsic defects in PbSe, taking into account the amphoteric behavior of bismuth atoms in lead selenide.     

Determining the position of antimony impurity atoms in PbS by 119Sb(119m Sn) emission Mössbauer spectroscopy

Emission Mössbauer spectroscopy based on the isotope ¹¹⁹Sb(^119m Sn) is used to show that the location of antimony impurity atoms in the PbS lattice depends on the conductivity type of the material: in n-type samples the antimony is localized primarily on the anion sublattice, while in p-type material it is primarily on the cation sublattice. It is noteworthy that when the ^119m Sn center appears in the anion PbS sublattice (i.e., as an antisite defect) after radioactive conversion of ¹¹⁹Sb, its charge state does not depend on the position of the Fermi level. When the ¹¹⁹Sb-center is in the cation sublattice of PbS, it acts like an electrically active substitutional impurity: in n-type samples the spectrum corresponds to the neutral state of a donor center (^119m Sn²⁺), while in p-type material it corresponds to the doubly ionized state of this center (^119m Sn⁴⁺).     

Effect of Ti Substitution on Thermoelectric Properties of W-Doped Heusler Fe2VAl Alloy

Effects of element substitutions on thermoelectric properties of Heusler Fe₂VAl alloys were evaluated. By W substitution at the V site, the thermal conductivity is reduced effectively because of the enhancement of phonon scattering resulting from the introduction of W atoms, which have much greater atomic mass and volume than the constituent elements of Fe₂VAl alloy. W substitution is also effective to obtain a large negative Seebeck coefficient and high electrical conductivity through an electron injection effect. To change the conduction type from n-type to p-type, additional Ti substitution at the V site, which reduces the valence electron density, was examined. A positive Seebeck coefficient as high as that of conventional p-type Fe₂VAl alloy was obtained using a sufficient amount of Ti substitution. Electrical resistivity was reduced by the hole doping effect of the Ti substitution while maintaining low thermal conductivity. Compared with the conventional solo-Ti-substituted p-type Fe₂VAl alloy, the ZT value was improved, reaching 0.13 at 450 K.     

Hall mobility in dielectrically isolated single-crystal silicon films defined by electrochemical etching

The majority-carrier Hall mobility has been measured in thin, single-crystal silicon films defined by electrochemical etching. Both n-type and p-type films with dopant concentrations of about 10¹⁵ cm^?3 were studied. The mobilities observed in p-type thin films and in epitaxial control samples were almost identical while the mobilities measured in n-type films were markedly less than those in epitaxial control samples. This apparent anomaly is attributed to the presence of an n-type surface-charge layer with lower carrier mobility near the bottom of the thin films, although it may possibly be related to voids formed in the n-type films. Measurements on very thin samples indicated that an t-type surface layer is left on the top surface of p-type films immediately after electrochemical etching.     

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.     

Conduction type conversion in ion etching of Au- and Ag-doped narrow-gap HgCdTe single crystal

Fundamental aspects of the p-to-n conductivity type conversion induced by ion etching in HgCdTe single crystals diffusion-doped with Au and Ag have been studied. A conversion mechanism is suggested, according to which interstitial mercury atoms rapidly diffuse from the surface source with high concentration and “kick out” impurity atoms from the cation sublattice into interstitial positions and thereby convert impurity atoms from the acceptor state to the donor state. It is shown that the structure of defects in the converted layer is unstable and the electrical parameters of this layer vary when stored at room temperature. The most probable mechanism of this process at room temperature is decomposition of an oversaturated solution of the impurity. A re-conversion to the p-type, observed in isochronous annealing of samples, is due to impurity diffusion into the converted layer from the unconverted bulk of the sample or from microscopic impurity inclusions.     

The nature of thermoacceptors in electron-irradiated high-resistance silicon

We have carried out an analysis of the possibility for the deep acceptor centers in silicon to participate in the formation of the thermoacceptor effect observed experimentally in a number of works, which consists in a change of conductivity from the n- to the p-type by annealing after irradiation of high-resistance silicon with electrons or neutrons. Based on the solution of the electroneutrality equation in a compensated monocrystalline silicon, we have estimated the concentration of the deep acceptor centers which are needed for obtaining p-type conductivity depending on the acceptor ionization energy and concentration of a shallow donor impurity. It is shown that deep acceptor centers (with ionization energy of up to 0.4 eV) can substantially contribute to the thermoacceptor effect in high-resistance n-type silicon prepared by floating zone melting. The concentrations of deep acceptors needed to overcompensate a sample with a low initial donor concentration (10¹²–10¹³ cm^–3) are on the order of 10¹²–10¹⁴ cm^–3 and seem to be quite achievable. Such centers can be divacancy–impurity (Fe, P) complexes with the ionization energy of up to 0.34 eV. In this case, the thermal activation of interstitial boron is also not excluded.     

Effect of non-uniformly doped surface layer on the barrier height of a Schottky contact

The presence of a highly doped p-type surface layer at the interface of a metal n-type semiconductor Schottky contact increases the barrier height. An analysis of such an increase in barrier height is given for different impurity distributions in the surface layer. It has been shown that of the three distributions considered, namely exponential, Gaussian and erfc, the increase of barrier height (Δφ_B) is a maximum for the Gaussian and minimum for the exponential distribution. It is found that the increase of barrier height results in degradation of the diode ideality factor n, which can be minimised when Δφ_B is obtained by changing the doping concentration of the surface layer and keeping the thickness to a minimum.     

Power Factor Anisotropy of <Emphasis Type="Italic">p</Emphasis>-Type and <Emphasis Type="Italic">n</Emphasis>-Type Conductive Thermoelectric Bi-Sb-Te Thin Films

The best films for thermoelectric applications near room temperature are based on the compounds Bi₂Te₃, Sb₂Te₃, and Bi₂Se₃, which as single crystals have distinct anisotropy in their electrical conductivity σ regarding the trigonal c-axis, whereas the Seebeck coefficient S is nearly isotropic. For p- and n-type alloys, P _⊥c > P _||c, and the power factors P _⊥c of single crystals are always higher compared with polycrystalline films, where the power factor is defined as P = S ² σ, ⊥c and ||c are the direction perpendicular and parallel to the c-axis, respectively. For the first time in sputter-deposited p-type (Bi_0.15Sb_0.85)₂Te₃ and n-type Bi₂(Te_0.9Se_0.1)₃ thin films, the anisotropy of the electrical conductivity has been measured directly as it depends on the angle φ between the electrical current and the preferential orientation of the polycrystals (texture) using a standard four-probe method. The graphs of σ(φ) show the expected behavior, which can be described by a weighted mixture of σ _⊥c and σ _||c contributions. Because (σ _⊥c/σ _||c) _p  < (σ _⊥c/σ _||c) _n , the n-type films have stronger anisotropy than the p-type films. For this reason, the angular weighted contributions of P _||c lead to a larger drop in the power factor of polycrystalline n-type films compared with p-type films.     

Thin-Film Thermoelectric Modules for Power Generation Using Focused Solar Light

We demonstrated the fabrication of thin-film thermoelectric generators and evaluated their generation properties using solar light as a thermal source. Thin-film elements of Bi_0.5Sb_1.5Te₃ (p-type) and Bi₂Te_2.7Se_0.3 (n-type), which were patterned using the lift-off technique, were deposited on glass substrates using radiofrequency magnetron sputtering. After annealing at 300°C, the average Seebeck coefficients of p- and n-type films were 150???V/K and ?104???V/K, respectively, at 50°C to 75°C. A cylindrical lens was used to focus solar light to a line shape onto the hot side of the thin-film thermoelectric module with 15 p?Cn junctions. The minimum width of line-shaped solar light was 0.8?mm with solar concentration of 12.5 suns. We studied the properties of thermoelectric modules with different-sized p?Cn junctions on the hot side, and obtained maximum open voltage and power values of 140?mV and 0.7???W, respectively, for a module with 0.5-mm p?Cn junctions. The conversion efficiency was 8.75?×?10^?4%, which was approximately equal to the value estimated by the finite-element method.