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.     

Molecular beam epitaxy passivation studies of Ge and III–V semiconductors for advanced CMOS

Future CMOS technologies will require the use of substrate material with a very high mobility in order to fulfil the performance requirements. Therefore, combination of Ge p-MOS with n-MOS devices made out of high mobility III/V compounds, such as GaAs, has recently received some attention for its possible use in advanced CMOS applications. In this work, the physical, chemical and electrical properties of Al₂O₃ high-κ oxide deposited on Ge and GaAs, using Molecular Beam Deposition (MBD) technique, have been investigated.     

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.     

The role of underbarrier transitions in processes of annihilation of excess charge carriers in II–VI semiconductors

The loss kinetics of charge carriers generated by the emission pulses of nitrogen laser in II–VI semiconductor compounds, namely, n- and p-CdTe and CdS, is studied by the method of microwave photoconductivity. The decays of photoresponse after switching off the light consisted of fast (manifesting itself at times <30 ns) and slow (at times >50 ns) components. The shape of decay of a slow component was virtually independent of temperature, and at long times, the decay law corresponded to the linear dependence of the photoresponse on the logarithm of time. The assumption follows from the results that the slow component of the photoresponse decay reflects the loss process of charges captured in the traps due to the tunnel recombination.     

Towards the Use of Cu–S Based Synthetic Minerals for Thermoelectric Applications

Semiconductors - Most of the energy produced is lost, mainly as waste heat. Thermoelectricity, which can directly convert heat into electricity, is seen with huge potential to recover part of such...     

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.     

Structure of the Energy Spectrum of Holes in IV–VI Materials from a Different Viewpoint

Semiconductors - A twofold decrease in the hole concentration for all PbTe〈NaTe〉 samples at 77–450 K and the same effect attained at 77 K and upon heavy doping due only to the...     

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.     

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.     

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.     

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.     

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.     

Design and analysis of a proposed transformerless/non-isolated high-‎gain DC-DC converter for renewable energy applications

ABSTRACT

This paper proposes a transformerless/non-isolated, high-gain DC-DC converter required to increase low voltages in renewable energy applications. The converter topology uses only a single-switch with minimum voltage stress over the semiconductors. The design is obtained by integrating both the boost and the quadratic boost converter topology. Operating principles of the converter circuit topology are detailed, and the steady-state performance is analysed. The design results in lower switching voltage that significantly improves the efficiency of the controlled switches. The results obtained by computer simulation demonstrate that the high voltage gain is obtained at lower values of duty ratio with an efficiency of more than 94%. The merits of the proposed converter are given in comparison with other high-gain DC-DC converters. Finally, a laboratory model is built using the digital signal processor [dSPACE (DS-1104)] to verify the converter theory. The theoretical, simulation and experimental results that indicate the claimed converter performance and capability are given.     

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...