Ab initio studies of the band parameters of III–V and II–VI zinc-blende semiconductors

Electronic band-structure calculations are performed for zinc-blende III–V (AlP, AlAs, AlSb, GaP, GaAs, GaP, InP, InAs, and InSb) and II–VI (ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe) semiconductors using an ab initio pseudopotential method within a local-density approximation (LDA). Lattice parameters, band gaps, Luttinger parameters, momentum matrix elements and effective masses are studied in detail. It is shown that LDA calculations cannot systematically give accurate band parameters. It is found that LDA band parameters calculated using experimentally determined lattice constants are more accurate than those using LDA lattice constants. We found that inclusion of the d electrons of Group-II atoms in the core gives more accurate band parameters.     

Molecular beam epitaxy of II–VI wide bandgap semiconductors

The paper presents the issues and challenges for molecular beam epitaxy (MBE) of the II–VI wide-bandgap semiconductors used for blue/green lasers. Use of reflection high-energy electron diffraction (RHEED) addresses many of these challenges, permitting characterization and control of various aspects of wide-bandgap II–VI MBE growth. The paper describes their use to control composition of Zn_{1 − x}Mg_xSe and ZnS_ySe_{1 − y}, layers, and to measure and control the growth rates of ZnSe, ZnTe and CdSe during migration-enhanced epitaxy (MEE) growth. RHEED oscillations reveal additional information about growth processes during II–VI MBE. The Mg sticking coefficient is found to be independent of substrate temperature, flux ratios, and electron beam excitation. Re-evaporation of Se, but not of Zn, is found to occur during pauses in growth. The effects of an electron beam on growth may be quantitatively determined.     

Mössbauer studies of two-electron centers with negative correlation energy in crystalline and amorphous semiconductors

The results of the study of donor U ^?-centers of tin and germanium in lead chalcogenides by Mössbauer emission spectroscopy are discussed. The published data regarding the identification of amphoteric U ^?-centers of tin in glassy binary arsenic and germanium chalcogenides using Mössbauer emission spectroscopy, and in multicomponent chalcogenide glasses using Mössbauer absorption spectroscopy are considered. Published data concerning the identification of two-atom U ^?-centers of copper in lattices of semimetal copper oxides by Mössbauer emission spectroscopy are analyzed. The published data on the detection of spatial inhomogeneity of the Bose-Einstein condensate in superconducting semiconductors and semimetal compounds, and on the existence of the correlation between the electron density in lattice sites and the superconducting transition temperature are presented. The principal possibility of using Mössbauer U ^?-centers as a tool for studying the Bose-Einstein condensation of electron pairs during the superconducting phase transition in semiconductors and semimetals is considered.     

Boron-doped Ⅲ–Ⅴ semiconductors for Si-based optoelectronic devices

Optoelectronic devices on silicon substrates are essential not only to the optoelectronic integrated circuit but also to low-cost lasers,large-area detectors,and so forth.Although heterogeneous integration of III-V semiconductors on Si has been welldeveloped,the thermal dissipation issue and the complicated fabrication process still hinders the development of these devices.The monolithic growth of III-V materials on Si has also been demonstrated by applying complicated buffer layers or interlayers.On the other hand,the growth of lattice-matched B-doped group-III-V materials is an attractive area of research.However,due to the difficulty in growth,the development is still relatively slow.Herein,we present a comprehensive review of the recent achievements in this field.We summarize and discuss the conditions and mechanisms involved in growing B-doped group-III-V materials.The unique surface morphology,crystallinity,and optical properties of the epitaxy correlating with their growth conditions are discussed,along with their respective optoelectronic applications.Finally,we detail the obstacles and challenges to exploit the potential for such practical applications fully.     

Transport Properties of Doped,Nanostructured IV–VI Epitaxial Films Grown by MBE

Here we report on the first results of PbTe epitaxial films with high Bi content above doping concentrations grown on BaF₂ in order to obtain nanoscale precipitates. The crystal structure is investigated by x-ray diffraction (XRD) measurements and no other phases than well-oriented PbTe could be found. These layers have been investigated by Hall-effect and Seebeck-effect measurements. The dependence of the Seebeck coefficient on the carrier concentration cannot be explained by simple Boltzmann statistics. Fourier-transform infrared (FTIR) transmission spectra also show some irregularities. These phenomena are interpreted as hints to nanoscale inclusions. The thermal conductivity is measured with the time-domain thermal reflectance method, developed by D. Cahill, to complete the thermoelectric characterizations.     

Boron-doped III–V semiconductors for Si-based optoelectronic devices

Optoelectronic devices on silicon substrates are essential not only to the optoelectronic integrated circuit but also to low-cost lasers, large-area detectors, and so forth. Although heterogeneous integration of III–V semiconductors on Si has been well-developed, the thermal dissipation issue and the complicated fabrication process still hinders the development of these devices. The monolithic growth of III–V materials on Si has also been demonstrated by applying complicated buffer layers or interlayers. On the other hand, the growth of lattice-matched B-doped group-III–V materials is an attractive area of research. However, due to the difficulty in growth, the development is still relatively slow. Herein, we present a comprehensive review of the recent achievements in this field. We summarize and discuss the conditions and mechanisms involved in growing B-doped group-III–V materials. The unique surface morphology, crystallinity, and optical properties of the epitaxy correlating with their growth conditions are discussed, along with their respective optoelectronic applications. Finally, we detail the obstacles and challenges to exploit the potential for such practical applications fully.     

Bromine ion-beam-assisted etching of III–V semiconductors

Bromineion-beam-assisted etching produces smooth vertical sidewalls in GaAs, GaP, InP, AlSb, and GaSb as well as in the usual alloys formed from these materials. Care must be taken, however, during etching to match the specific material system with an appropriate substrate etch temperature. For example, vertical walls were obtained using substrate temperatures in the range of 150 to 200°C with InP, 80 to 140°C with GaAs and GaP, and below 30°C with AlSb and GaSb. GaN has also been etched with the technique. Our etching experience and the vapor pressure data for bromine with group III and group V elements lead us to believe that all of the various technologically important III-V binaries, ternaries, and quaternaries can be etched. Etch rates of most of the materials can be varied from several nm/min to 0.16 μm/min through the bromine flow rate, Ar⁺ ion beam density and energy, and the substrate temperature. Bromine ion-beam-assisted etching also appears to have an advantage over chlorine ion-beam-assisted etching in many situations, in that substrate temperature ranges can be found for which vertical sidewalls are maintained while etching through layered structures composed of various alloys of the materials. Here we present results obtained from etching a number of III-V binaries, alloys, and heterostructures.     

IV–VI Compound heterostructures grown by molecular beam epitaxy

Structural and optical characterization of some IV–VI superlattices (SL) and multi-quantum wells (MQW) grown by molecular beam epitaxy (MBE) on BaF₂ (111) substrates are shown. Three different types of systems were investigated, namely, PbTe/PbSnTe, PbTe/SnTe and PbTe/PbEuTe. High-resolution X-ray diffraction analysis was performed to determine the strain in the structures. The analysis revealed sharp interfaces and good thickness control. The transition energies between the confined levels in the wells were obtained from the absorption steps observed in infrared transmission measurements. Preliminary results on PbTe/Si heterojunction grown by MBE are also presented.     

Dynamic nuclear self-polarization of III–V semiconductors

III–V semiconductors exhibit dynamic nuclear self-polarization (DYNASP) owing to the contact hyperfine interaction (HFI) between optically excited conduction electrons and lattice nuclei. In the self-polarization process at a low temperature, electron spin state and the nuclear polarization (magnetization) exchange a positive feedback, increasing energy splitting of the conduction electron states, thereby a large nuclear polarization. This phenomenon was theoretically predicted previously for conduction electrons excited linearly and elliptically polarized light. The polarization of the conduction electrons was represented by a parameterα in a formula for nuclear polarization (Eq. (9) in Ref. [1]); however, the effect of external magnetic fields on the nuclear polarization was not considered. Therefore, this study introduces this effect by further extending the previous studies. Herein, α′ represents the combination of the effects of elliptically polarized electrons and an external magnetic field, which is used in the equations presented in previous studies. When α′ = 0, a large nuclear polarization is obtained below critical temperature T_c, but no polarization occurs above T_c. When α′ > 0, the nuclear polarization is enhanced above T_c. Below T_c, the nuclear polarization follows a hysteresis curve when α′ is partially manipulated by adjusting the degree of the polarization of the exciting laser.     

Resonance states, heavy quasiparticles, and the thermoelectric figure of merit of IV–VI materials

A high thermoelectric figure of merit ZT, with a maximum value ZT = 1.5 at 670?C800 K for the composition Ge_0.9Pb_0.05Bi_0.05Te, has been obtained for GeTe solid solutions with Bi and Pb impurities. This is conducive to a decrease in the hole concentration, an increase in the Seebeck coefficient, and a decrease in the lattice thermal conductivity. The main attention in interpretation of the experimental data is given to specific features of the energy spectrum of holes in the initial GeTe compound. The model of resonance states, formed with involvement of Ge atoms and metal vacancies, has been further developed. The types of defects and their transformation depending on temperature and the concentration of superstoichiometric Te have been considered. The experimental results give reason to believe that the interaction of localized and free charge carriers at elevated temperatures leads to a pronounced hybridization of their states and the formation of heavy quasiparticles, a situation in many ways similar to that observed in materials with heavy fermions.     

Seebeck Effect in IV–VI Semiconductor Films and Quantum Wells

Theoretical calculations of the Seebeck coefficients of IV–VI semiconductors and quantum wells were performed, taking into account temperature-dependent band gaps, nonparabolicity, and anisotropy of effective masses in the framework of the Boltzmann equation. The Seebeck coefficient depends on both the density of states and a kinetic term determined from the balance between drift and diffusion currents. The theoretical Seebeck coefficients of PbTe and PbSe films were compared with experimental values, and excellent agreement was obtained. In lead-salt IV–VI materials with a positive temperature dependence of the band gap, the kinetic part becomes small or has a negative effect on the Seebeck coefficient. Strong enhancement of the Seebeck coefficient is expected theoretically in n-type PbTe/PbSe superlattices, resulting from the increase of the kinetic part due to the effective-mass difference between PbTe and PbSe.     

Interface studies of ALD-grown metal oxide insulators on Ge and III–V semiconductors (Invited Paper)

Atomic layer deposited (ALD) HfO₂/GeO_xN_y/Ge(1 0 0) and Al₂O₃/In_0.53Ga_0.47As(1 0 0) ? 4 × 2 gate stacks were analyzed both by MOS capacitor electrical characterization and by advanced physical characterization to correlate the presence of electrically-active defects with chemical bonding across the insulator/channel interface. By controlled in situ plasma nitridation of Ge and post-ALD annealing, the capacitance-derived equivalent oxide thickness was reduced to 1.3 nm for 5 nm HfO₂ layers, and mid-gap density of interface states, D_it = 3 × 10¹¹ cm^?2 eV^?1, was obtained. In contrast to the Ge case, where an engineered interface layer greatly improves electrical characteristics, we show that ALD-Al₂O₃ deposited on the In_0.53Ga_0.47As (1 0 0) ? 4 × 2 surface after in situ thermal desorption in the ALD chamber of a protective As cap results in an atomically-abrupt and unpinned interface. By avoiding subcutaneous oxidation of the InGaAs channel during Al₂O₃ deposition, a relatively passive gate oxide/III–V interface is formed.     

Nanostructured Thermoelectric Materials for Temperatures of 200–1200 K Obtained by Spark Plasma Sintering

Semiconductors - The broad application of thermoelectricity is constrained by the low efficiency of thermoelectric elements, which is mainly determined by the thermoelectric figure of merit of...     

Optimization of II–VI based heterojunctions

The performance of heterojunction devices can be optimized by proper choice of characteristics of the two semiconductors forming the junction and by proper design. One very important element in design is the termination of the active volume of the device with surfaces of low, preferably zero surface recombination velocity. This paper discusses the rules governing the choice of ratios of energy gaps, electron affinities, doping levels, minority carrier lifetimes, lattice constants, etc. for the semiconductors comprising the pair. It also discusses the use of heterostructures to produce the low surface recombination velocity surfaces bounding the active volume of the device. The paper reviews principles of materials science which govern the choice of semiconductors for both the active regions of the device and the heterostructure “encapsulants”. The few attempts to realize electronic structures having the properties required by optimization theory will be described.     

Structural Characterization of Integrated II–VI and III–V Heterostructures for Solar Cell Applications

We report on the microstructural characterization of monolithically integrated II–VI and III–V heterostructures that are being developed for possible solar cell applications. Observations by transmission electron microscopy have shown that the growth of lattice-matched ZnTe layers on GaSb(100) substrates resulted in very high quality interfaces and very few structural defects. We then investigated short-period superlattices (SSLs) of CdSe and CdTe digital alloys grown using ZnTe buffer layers on GaSb substrates. The effect of rapid thermal annealing was also studied. High-resolution electron micrographs showed that the CdSe-CdTe SSL had very high quality for approximately 25–50 periods closest to the substrate but that considerable stacking faults and microtwins were visible in layers near the top surface of the sample. For comparison purposes, CdSe layers grown on InAs(100) substrates, again using ZnTe as a buffer layer, were also characterized. The quality of the CdSe layer was not as good as that observed in the SSL sample, possibly because of the larger lattice mismatch between the materials.     

Electronic properties and pinning of the Fermi level in irradiated II–IV–V<Subscript>2</Subscript> semiconductors

Experimental data on the electronic properties of II–IV–V₂ semiconductors irradiated with high-energy particles (electrons, protons, and neutrons) are reported. The limiting electrical characteristics of irradiated ternary compounds are evaluated and compared with the corresponding data for the binary III–V analogues of the compounds. Particular emphasis is placed on the determination of the limiting position of the Fermi level, F _lim, in irradiated II–IV–V₂ compounds. The results of calculations of the energy position of the intrinsic charge-neutrality level are presented.     

Dislocation Reduction by Glide in Epitaxial IV–VI Layers on Si Substrates

It is explained with a simple model why the reduction of threading dislocation (TD) densities in epitaxial lattice and thermal expansion mismatched IV?CVI layers such as PbSe(111) on Si(111) substrates follows a 1/h ² dependence where h is the thickness of the layer. This is in contrast to the 1/h dependence for III?CV and II?CVI layers grown on mismatched substrates. The 1/h ² dependence results since the thermal mismatch strain is mainly reduced by glide and reactions of the TD in their main {100}-type glide system of the NaCl-type IV?CVI semiconductors. In addition, multiple thermal cycles lead to further reduction of the TD densities by glide and fusion since fusion does not cause dislocation blocking.     

Cost–Performance Analysis and Optimization of Fuel-Burning Thermoelectric Power Generators

Energy cost analysis and optimization of thermoelectric (TE) power generators burning fossil fuel show a lower initial cost compared with commercialized micro gas turbines but higher operating cost per energy due to moderate efficiency. The quantitative benefit of the thermoelectric system on a price-per-energy ($/J) basis lies in its scalability, especially at a smaller scale (<10 kW), where mechanical thermodynamic systems are inefficient. This study is based on propane as a chemical energy source for combustion. The produced heat generates electric power. Unlike waste heat recovery systems, the maximum power output from the TE generator is not necessarily equal to the economic optimum (lowest $/kWh). The lowest cost is achieved when the TE module is optimized between the maximum power output and the maximum efficiency, dependent on the fuel price and operation time duration. The initial investment ($/W) for TE systems is much lower than for micro gas turbines when considering a low fractional area for the TE elements, e.g., 5% to 10% inside the module. Although the initial cost of the TE system is much less, the micro gas turbine has a lower energy price for longer-term operation due to its higher efficiency. For very long-term operation, operating cost dominates, thus efficiency and material ZT become the key cost factors.