Showing papers in "Semiconductor Science and Technology in 2004"
TL;DR: In this paper, the authors present a review of the material properties, growth techniques, band structure and the main electronic devices of the Si/SiGe heterostructure system, in particular, the important device technologies in mainstream microelectronics.
Abstract: Silicon germanium (SiGe) has moved from being a research material to accounting for a small but significant percentage of manufactured semiconductor devices. This percentage is predicted to increase substantially as SiGe begins to be used in complementary metal oxide semiconductor (CMOS) technology in the future to substantially improve performance. It is the development of Si/SiGe heterostructures which has enabled band structure and strain engineering allowing Si/SiGe to be used in many different ways to improve conventional microelectronic device performance along with allowing new concepts to be explored. This paper presents a review of the material properties, growth techniques, band structure and the main electronic devices of the Si/SiGe heterostructure system. In particular, the important device technologies in mainstream microelectronics of the SiGe heterostructure bipolar transistor (HBT) and strained-Si CMOS will be reviewed before future device and optoelectronics concepts are explored.
536 citations
TL;DR: A review of recent results on transition metal doping of electronic oxides such as ZnO, TiO 2, SnO2, BaTiO 3, Cu2O, SrTiO3 and KTaO3 is presented in this article.
Abstract: A review of recent results on transition metal doping of electronic oxides such as ZnO, TiO2, SnO2, BaTiO3, Cu2O, SrTiO3 and KTaO3 is presented. There is interest in achieving ferromagnetism with Curie temperatures above room temperature in such materials for applications in the field of spintronic devices, in which the spin of the carriers is exploited. The incorporation of several atomic per cent of the transition metals without creation of second phases appears possible under optimized synthesis conditions, leading to ferromagnetism. Pulsed laser deposition, reactive sputtering, molecular beam epitaxy and ion implantation have all been used to produce the oxide-based dilute magnetic materials. The mechanism is still under debate, with carrier-induced, double-exchange and bound magnetic polaron formation all potentially playing a role depending on the conductivity type and level in the material.
527 citations
TL;DR: In this paper, the authors present a review of the theoretical models and related empirical expressions to evaluate parameters related to the carrier transport within Si/SiGe heterostructures, such as the band structure of strained or relaxed SiGe, the conduction and valence band offsets in the Si1−xGex/Si1−yGey heterostructure, the effective transport masses and the densities of states, have been calculated and shown to be in good agreement with existing experimental and theoretical results.
Abstract: This paper is based on a comprehensive review of the literature and our own studies. We present a summary of the theoretical models and related empirical expressions to evaluate parameters related to the carrier transport within Si/SiGe heterostructures. The models and expressions include the effects of alloy composition and mechanical strain on the band structure of Si/SiGe alloys and the corresponding interfaces. They are presented in a form suitable for implementation in various types of device simulators. Important parameters, such as the band structure of strained or relaxed SiGe, the conduction and valence band offsets in the Si1−xGex/Si1−yGey heterostructures, the effective transport masses and the densities of states, have been calculated and shown to be in good agreement with existing experimental and theoretical results. Analytical expressions of those parameters as a function of Ge composition of the SiGe alloy have been given for strained Si on relaxed Si1−yGey substrate and strained Si1−xGex on Si substrate.
155 citations
TL;DR: In this article, the authors present the results of experimental and theoretical studies concerning the temperature dependence of electron mobility in a two-dimensional electron gas (2DEG) confined at the GaN/AlGaN interface.
Abstract: We present the results of our experimental and theoretical studies concerning the temperature dependence of electron mobility in a two-dimensional electron gas (2DEG) confined at the GaN/AlGaN interface. Experimental mobility of 2912 cm2 (V s)−1 at 4.2 K, remains almost constant up to lattice temperature TL = 150 K, it then decreases rapidly down to 1067 cm2 (V s)−1 at TL = 300 K. In order to compare the experimental results with the theory we use a simple analytical formula for low-field electron mobility based on 2D degenerate statistics for a 2DEG confined in a triangular well. We consider acoustic phonon, polar-optical phonon, dislocation and interface-roughness (IFR) scattering. The polar-optical phonon scattering is the dominant mechanism at high temperatures. At low temperatures, however, both the IFR and dislocation scattering explain, equally well, the observed mobility. In reality, however, a mixture of the two mechanisms together with the deformation potential and piezoelectric scattering will determine the low temperature mobility. The experimental results are discussed in the light of the calculations.
150 citations
TL;DR: In this paper, the authors present both theoretical and experimental aspects of the spin splitting in the absence of external fields as well as its dependence on magnetic and electric fields, and the theoretical description of conduction and valence subbands is based on a multiband k p formalism.
Abstract: We review the spin splitting of subband energies caused by bulk and structure inversion asymmetries in semiconductor III–V and II–VI heterostructures. We present both theoretical and experimental aspects of the problem, and we discuss the spin splitting in the absence of external fields as well as its dependence on magnetic and electric fields. The theoretical description of conduction and valence subbands is based on a multiband k p formalism. Experimental results are summarized, as obtained by beatings of the Shubnikov–de Haas oscillations, magnetoconductance in antilocalization regime, Raman scattering, spin resonance and cyclotron resonance. This review article is motivated by recent interest in spin properties of heterostructures in view of spintronic applications.
143 citations
TL;DR: In situ Hall effect measurements have been carried out on vapour-phase-grown, n-type ZnO irradiated with 1.0 and 1.5 MeV electrons in the [000-1] direction as mentioned in this paper.
Abstract: In situ Hall-effect measurements have been carried out on vapour-phase-grown, n-type ZnO irradiated with 1.0 and 1.5 MeV electrons in the [000-1] direction. The electrical properties change very little during irradiation at temperatures as low as 130 K, the lowest temperature presently attainable under 1 MeV, 0.3 µA cm−2 irradiation. It is concluded that long-term damage in ZnO is limited by defect annihilations that are rapid on the time scale of the experiment (<1 min), even at 130 K.
125 citations
TL;DR: In this article, the electrical, optical and structural properties of Li-doped SnO2 transparent conducting films were investigated by the spray pyrolysis technique and the results of x-ray diffraction have shown that the deposited films are polycrystalline without any second phases with preferential orientations along the (110 and (211) planes and an average grain size of 28.7 nm.
Abstract: In this paper, we investigate the electrical, optical and structural properties of Li-doped SnO2 transparent conducting films deposited on glass substrates by the spray pyrolysis technique. The SnO2:Li thin films were deposited at a substrate temperature of 480 °C using an aqueous ethanol solution consisting of tin and lithium chloride with various doping levels from 0 to 25 wt% in solution. The effect of increasing Li concentration on the electrical, optical and structural properties of SnO2 films has been studied. The results of x-ray diffraction have shown that the deposited films are polycrystalline without any second phases with preferential orientations along the (110) and (211) planes and an average grain size of 28.7 nm. Also, the Hall effect and resistivity measurements of the films show that for a specific acceptor dopant concentration of ~2 wt% or [Li]/[Sn] atomic ratio equal to 37 at% in solution, the majority of carriers convert from electrons to holes and for a Li concentration of ~15 wt% in solution, p-conductivity increases sharply. The optical absorption edge for undoped SnO2 films lies at 4.11 eV, whereas for high acceptor doped films it shifts towards lower energies (longer wavelengths) in the range of 4.11 to 3.61 eV.
120 citations
TL;DR: In this article, the effect of the strained silicon thickness on the characteristics of strained silicon MOSFETs on SiGe virtual substrates was studied. And the mechanism of the leakage was examined by using photon emission microscopy.
Abstract: This paper studies the effect of the strained silicon thickness on the characteristics of strained silicon MOSFETs on SiGe virtual substrates. NMOSFETs were fabricated on strained silicon substrates with various strained silicon thicknesses, both above and below the strained silicon critical thickness. The low field electron mobility and subthreshold characteristics of the devices were measured. Low field electron mobility is increased by about 1.8 times on all wafers and is not significantly degraded on any of the samples, even for a strained silicon thickness far greater than the critical thickness. From the subthreshold characteristics, however, it is shown that the off-state leakage current is greatly increased for the devices on the wafers with a strained silicon thickness that exceeds the critical thickness. The mechanism of the leakage was examined by using photon emission microscopy. Strong evidence is shown that the leakage mechanism is source/drain electrical shorting caused by enhanced dopant diffusion near misfit dislocations.
111 citations
TL;DR: In this paper, the authors showed that the thermal excitation of holes from the quantum wells into the waveguide where they recombine, but not Auger recombination, limits the continuous-wave room-temperature output power of In(Al)GaAsSb/GaSb diode lasers.
Abstract: Measurements of gain, loss, threshold current, device efficiency and spontaneous emission of 2.5?2.82 ?m In(Al)GaAsSb/GaSb quantum-well diode lasers have been performed over a wide temperature range. The experimental results show that the thermal excitation of holes from the quantum wells into the waveguide where they recombine, but not Auger recombination, limits the continuous-wave room-temperature output power of these lasers, at least up to ? = 2.82 ?m. An approach to extend the wavelength of In(Al)GaAsSb/GaSb diode lasers beyond 3 ?m is discussed.
97 citations
TL;DR: In this article, successive ionic layer adsorption and reaction method was used to deposit CdSe thin films on glass substrates at room temperature (300 K) and optical absorption and electrical resistivity were measured.
Abstract: The successive ionic layer adsorption and reaction method was used to deposit CdSe thin films on glass substrates at room temperature (300 K). The films were characterized by x-ray diffraction, energy dispersive x-ray analysis, scanning electron microscopy, atomic force microscopy and high-resolution transmission electron microscopy. Optical absorption and electrical resistivity were measured. The CdSe layer grew with nanocrystalline cubic phase along with some amorphous phase present in CdSe film, with an optical band gap 'Eg' of 2.1 eV and room temperature electrical resistivity of the order of 106 ? cm.
94 citations
TL;DR: In this article, a complete set of material parameters, including revised values for the intrinsic concentration, the electron and hole mobilities and the absorption coefficient, is given based on extended reviews of previously published data.
Abstract: GaSb photovoltaic cells are the most common choice for receivers in thermophotovoltaic (TPV) systems. Although nowadays their manufacturing technology is well established, a theoretical simulation frame for their modelling under real TPV operating conditions is still not fully developed. This is basically due to the lack of a reliable and accurate set of GaSb material parameters as input for the semiconductor simulation tools. Thorough GaSb TPV cell models are needed to understand the electro-optical behaviour of the cells and eventually are essential in improving their design. This work will try to go beyond this key issue, carefully analysing and reviewing some of the key parameters for GaSb. A complete set of material parameters, including revised values for the intrinsic concentration, the electron and hole mobilities and the absorption coefficient, is given based on extended reviews of previously published data. For the first time, estimations for their temperature dependences are introduced. Finally, GaSb TPV cells are manufactured and characterized inside a real TPV system prototype. The comparisons between the electrical measurements and the model theoretical predictions confirm the validity of the proposed set of GaSb material parameters and their temperature dependences.
TL;DR: In this paper, the authors compared the performance of QWIPs, QRIPs, and QDIPs based on physical analysis of the factors determining their operation, and they showed that the detectivity of QRPs can be much higher than the responsivity of WIPs; however, higher responsivity is inevitably accompanied by higher dark current.
Abstract: This paper deals with the comparison of quantum well, quantum wire and quantum dot infrared photodetectors (QWIPs, QRIPs and QDIPs, respectively) based on physical analysis of the factors determining their operation. The operation of the devices under consideration is associated with the intersubband (intraband) electron transitions from the bound states in QWs, QRs and QDs into the continuum states owing to the absorption of infrared radiation. The redistribution of the electric potential across the device active region caused by the photoionization of QWs, QRs and QDs affects the electron injection from the emitting contact. The injection current provides the effect of current gain. Since the electron thermoemission and capture substantially determine the electric potential distribution and, therefore, the injection current, these processes are also crucial for the device performance. To compare the dark current, responsivity and detectivity of QWIPs, QRIPs and QDIPs we use simplified but rather general semi-phenomenological formulae which relate these device characteristics to the rates of the thermoemission and photoemission of electrons from and their capture to the QWs and the QR and QD arrays. These rates are expressed via the photoemission cross-section, capture probability and so on, and the structural parameters. Calculating the ratios of the QWIP, QRIP and QDIP characteristics using our semi-phenomenological model, we show that: the responsivity of QRIPs and QDIPs can be much higher than the responsivity of QWIPs, however, higher responsivity is inevitably accompanied by higher dark current; the detectivity of QRIPs and QDIPs with low-density arrays of relatively large QRs and QDs is lower than that of QWIPs; the detectivity of QRIPs and QDIPs based on dense arrays can significantly exceed the detectivity of QWIPs.
TL;DR: In this paper, the phase transition from FCC to hexagonal structure was suppressed by nitrogen implantation if the nitrogen implant dose is higher than 4.51 × 1016 cm−2.
Abstract: Ge2Sb2Te5 films were deposited by RF magnetron sputtering on Si(100)/SiO2 substrates. N+ ion was implanted into Ge2Sb2Te5 films. Two obvious steps were observed in the resistance–temperature curve of the Ge2Sb2Te5-N film with a minor nitrogen implant dose. The two steps may change into one step because the phase transition from FCC to hexagonal structure was suppressed by nitrogen implantation if the nitrogen implant dose is higher than 4.51 × 1016 cm−2. The favourite nitrogen implant dose is about 6.44 × 1015 to 1.92 × 1016 cm−2 in our study. This phenomenon is very important for multilevel storage. Three-level storage with Ge2Sb2Te5-N media for chalcogenide random access memory (C-RAM) can be performed easily, and hence, the capacity of C-RAM will be dramatically increased.
TL;DR: In this article, a deposition of Se thin films of 80-350 nm in thickness from a chemical bath at a deposition yield of more than 95% was reported, and these films were utilized to prepare SnSe and SnSe2 thin films either by heating a vacuum deposited Sn thin film in close contact with the Se thin film at 200 °C in a nitrogen atmosphere for 1 h or by heating Se stack in air at 400 °C for about 5 min.
Abstract: Deposition of Se thin films of 80–350 nm in thickness from a chemical bath at a deposition yield of more than 95% is reported. These films are utilized to prepare SnSe and SnSe2 thin films either by heating a vacuum deposited Sn thin film in close contact with the Se thin film at 200 °C in a nitrogen atmosphere for 1 h or by heating a Se–Sn stack in air at 400 °C for about 5 min. By controlling individual film thicknesses and the conditions of heating, SnSe2 or composite films of SnSe2 and SnSe are formed. An optical band gap of 1–1.27 eV, electrical conductivity of 0.01–0.2 Ω−1 cm−1 and the photoconductivity of these materials fulfil the basic requirements for their integration into photovoltaic structures.
TL;DR: In this article, a rate equation model for the carrier dynamics is fitted to the experimental data obtained for the integrated intensity of the photoluminescence at different temperatures, and two thermal escape mechanisms for carriers in the dots are identified that have the onsets at 110 K and 220 K, respectively.
Abstract: Photoluminescence from InAs quantum dots in a strained Ga0.85In0.15As quantum well is investigated over a temperature range from 10 to 300 K using low intensity optical excitation. A rate equation model for the carrier dynamics is fitted to the experimental data obtained for the integrated intensity of the photoluminescence at different temperatures. It is found necessary to assume a potential barrier, possibly arising from the strain, at the interface between the dots and the quantum well that makes the carrier capture in dots less effective at low temperatures. In addition, two thermal escape mechanisms for carriers in the dots are identified that have the onsets at 110 K and 220 K, respectively. At low temperatures, the ground state photoluminescence has a large full width at half maximum, while at temperatures above 220 K, the full width decreases as emission from larger dots dominate. The activation energies for different carrier thermal escape channels are estimated using the solutions of the steady-state rate equation system.
TL;DR: In this paper, a Schottky barrier diode on unintentionally doped p-type GaTe grown by the directional freezing method was obtained and characterized by the capacitance-voltage technique as a function of temperature.
Abstract: A Schottky barrier diode on unintentionally doped p-type GaTe grown by the directional freezing method was obtained and characterized by the capacitance–voltage technique as a function of temperature (100–300 K). Using vacuum evaporated Sn as the Schottky barrier contact and In for the ohmic contact, high-quality diodes were produced. The discrepancy between Schottky barrier heights (BHs) obtained from current–voltage–temperature and capacitance–voltage–temperature measurements is explained by the introduction of a spatial distribution of BHs due to barrier height inhomogeneities that prevail at the metal/GaTe interface. The deviations of apparent BHs were investigated by considering the microstructure of the metal/GaTe interface. It was found that the dispersion of this distribution across the contact area grew increasingly larger at lower temperatures and was responsible for the increasing difference between apparent BHs obtained from the two techniques.
TL;DR: In this article, the authors investigated the composition dependence of Cu-In-Se films with nominal 60 nm thickness grown on Na-free glass substrates and found that the Raman bands around 160 and 174 cm−1 observed in CuIn5Se8 are probably due to the other ordered structure phase which is very similar to chalcopyrite-related CuIn 5Se8.
Abstract: Composition dependence of Cu–In–Se films with nominal 60 nm thickness grown on Na-free glass substrates has been systematically investigated by Raman scattering and x-ray diffraction spectra. The most intense A1 mode shift from 175 cm−1 for Cu1.5InSe2 to 151 cm−1 for CuIn5Se8 indicates weakening of the bond-stretching forces with decrease in Cu content. According to the evolution of the phonon frequencies in films, we found that the Raman bands around 160 and 174 cm−1 observed in CuIn5Se8 are probably due to the other ordered structure phase which is very similar to chalcopyrite-related CuIn5Se8. Such coexistence mechanism of both structure phases should be associated with the influence of preferred (112) orientation in the films. Comparing our experimental data with the reported data of the related compounds, we explain the composition effect on phonon frequencies shift from Cu-rich to Cu-poor transition in this paper.
TL;DR: In this paper, it was shown that the crossing of ln(I) −V curves is an inherent property of any Schottky diode and that it is the presence of series resistance that keeps this intersection hidden and unobservable.
Abstract: This paper explains the phenomenon of intersection of current–voltage (I–V) curves observed recently in inhomogeneous Schottky diodes at low temperatures, generated using an analytical equation. The crossing of the ln(I)–V curves appears as an abnormality when seen with respect to the conventional behaviour of ideal Schottky diodes. Here it is shown that this crossing of ln(I)–V curves is an inherent property of any Schottky diode. For a homogeneous Schottky diode, it can be observed by plotting ln(I)–V curves with zero series resistance at various temperatures. It is the presence of series resistance that keeps this intersection hidden and unobservable in homogeneous Schottky diodes. In inhomogeneous Schottky diodes with Gaussian distribution of barrier heights, this crossing is observable in the usual range of ln(I)–V curves at low temperatures even with finite (non-zero) series resistance. The analytical model of the Gaussian distribution of barrier heights thus favours crossing. However, the calculations based on the numerical integration method do not show such intersection. The detailed aspects of this intersecting behaviour of ln(I)–V curves in inhomogeneous Schottky diodes are discussed in this paper.
TL;DR: In this article, a study of optical and magnetotransport properties at a type II arsenide-antimonide heterojunction with a broken-gap alignment is presented, which leads to unusual tunnelling assisted radiative recombination transitions and novel transport properties.
Abstract: This review deals with a study of optical and magnetotransport phenomena at a type II arsenide–antimonide heterojunction with a broken-gap alignment. A fundamental feature of this structure is partial overlapping of the InAs conduction band with the GaSb-rich solid solution valence band. In such a heterostructure, electrons and holes are spatially separated and localized in self-consistent quantum wells formed on both sides of the heterointerface. This leads to unusual tunnelling-assisted radiative recombination transitions and novel transport properties. Results of a pioneering study of interface-related luminescence in type II broken-gap GaInAsSb/InAs heterostructures, with a high quality abrupt heteroboundary, grown by the LPE method, are presented. The energy band diagram of the type II broken-gap GaInAsSb/InAs(GaSb) heterostructures and band overlapping control, depending on a doping level and epilayer composition of quaternary solid solution, are discussed. A 2D-electron channel with high Hall mobility at the p-GaInAsSb/p-InAs interface was found and examined. A great deal of attention is paid to quantum transport properties of the semimetal channel in a wide magnetic field range (up to 18 T) at low temperatures. A cyclotron resonance study was used to obtain data on the energy spectrum at the interface and to estimate effective masses for subbands in the semimetal channel. The intriguing behaviour of the 2D-electron system in the presence of localized holes in high magnetic fields and the first observation of integer quantum Hall effect plateaus on the type II single GaInAsSb/InAs heterointerface formed by LPE are demonstrated. A tunnelling-injection laser with high asymmetric band offset confinements based on the type II broken-gap heterojunction is considered. Applications of the interface-induced phenomena in luminescent and transport properties for the design of novel mid-IR optoelectronic devices and Hall sensors are briefly reviewed.
TL;DR: In this paper, a series of wet-chemical etchants for materials lattice-matched to GaSb was investigated and the selectivities for the four different etching solutions were shown.
Abstract: A series of wet-chemical etchants for materials lattice-matched to GaSb was investigated. The etch rates for GaSb, AlAsSb, InAsSb and InAs with etch solutions based on KNa–tartaric acid (C4H4KNaO6), citric acid (C6H8O7) and hydrochloric acid were determined and the selectivities for the four different etching solutions are shown. The applicability of the selectivity between GaSb and InAs (respectively InAsSb) with C4H4KNaO6:HCl: H2O2:H2O (selectivity higher than 15:1) and C6H8O7:H2O2 (selectivity around 1:100) is proved by a substrate removal experiment. Also a new etchant for AlAsSb is proposed: HCl:H2O2:H2O etches AlAsSb versus GaSb with a selectivity of 5:1 and the GaSb surface underneath is smooth and without any remaining particles of oxidized aluminium.
TL;DR: In this paper, a theoretical study of the structural, elastic, electronic and piezoelectric properties of zinc-blende AlN and GaN under the pressure effect is presented.
Abstract: In this paper we report a theoretical study of the structural, elastic, electronic and piezoelectric properties of zinc-blende AlN and GaN under the pressure effect. The study is focused on the first-principles all electron full-potential augmented plane wave plus local orbitals calculations within the density-functional theory. The results of bulk properties, including lattice constants, bulk modulus and derivatives and band structures are obtained and compared using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation functional. We find that the GGA does not give a significant improvement over LDA. We also report calculations for elastic constants as well as the internal-strained parameter and their behaviour under pressure. The electronic energy levels and ionicity factor are studied under hydrostatic pressure. We extend our investigation to the study of the stress effect on piezoelectric constants and transverse effective charges. Our results show that III–V nitrides resemble II–VI compounds in terms of the sign of the piezoelectric constants. The piezoelectric constants and transverse effective charges vary nonlinearly with pressure.
TL;DR: In this article, a modified chemical bath deposition method was developed to prepare CdSe and HgSe semiconductor thin films in cubic modification, based on the chemical reaction of complexed cadmium acetate, mercuric nitrate with sodium selenosulphate in an aqueous ammoniacal medium at 5 °C.
Abstract: A modified chemical bath deposition method has been developed to prepare CdSe and HgSe semiconductor thin films in cubic modification, based on the chemical reaction of complexed cadmium acetate, mercuric nitrate with sodium selenosulphate in an aqueous ammoniacal medium at 5 °C. The films were characterized by using x-ray diffraction, optical absorption, electrical measurements and energy dispersive x-ray analysis techniques. The films were uniform, well adherent, dark red and polycrystalline in cubic form without trace of any hexagonality. The CdSe film shows two bandgaps at 2.15 and 1.86 eV, while HgSe shows two bandgaps at 0.8 and 0.45 eV. A thermoelectric study indicated the presence of an n-type conduction mechanism.
TL;DR: In this article, the evolution of nanocrystal formation with annealing temperature, i.e., the size growth, was monitored by Raman spectrometry for several samples and the corresponding nanocrystals sizes were estimated using the phonon confinement model.
Abstract: Ge nanocrystals formed in a SiO2 matrix by ion implantation were studied by Raman spectroscopy. It is shown that Raman analysis based on the phonon confinement model yields a successful explanation of the peculiar characteristics resulting from the nanocrystals. A broadening and a shift in the Raman peak are expected to result from the reduced size of the crystals. Asymmetry in the peak is attributed to the variations in the size of the nanocrystals. These effects were observed experimentally for the Ge nanocrystals prepared by ion implantation and explained theoretically by incorporating the effect of size and size distribution into the theoretical description of the Raman shift. A comparison with the transmission electron microscopy images indicated that this analysis could be used to estimate the structural properties of nanocrystals embedded in a host matrix. The evolution of nanocrystal formation with annealing temperature, i.e. the size growth, was monitored by Raman spectrometry for several samples and the corresponding nanocrystal sizes were estimated using the phonon confinement model.
TL;DR: In this paper, the phase matching of a frequency conversion process within an isotropic semiconductor is studied, where waveguide phase matching is obtained by exploiting modal dispersion and, for energies below the reststrahlen region, the semiconductor anomalous dispersion.
Abstract: Waveguide phase matching of a frequency conversion process within an isotropic semiconductor is studied. The three-wave interaction involves pump and signal beams in the near infrared, and the idler at THz frequencies, i.e. with a wavelength greater than 50 µm in GaAs. Phase matching is obtained by exploiting modal dispersion and, for energies below the reststrahlen region, the semiconductor anomalous dispersion. This original phase matching scheme provides simultaneously a long interaction length, due to waveguide confinement, and efficient wave mixing because only the fundamental modes are involved in a modal phase matching process. Two different types of waveguides are studied: bulk semiconductor rods and plasmon waveguides.
TL;DR: In this paper, the effects of thermal annealing on the surface morphology, stoichiometric ratio, structural and optical properties of ZnO films were investigated using scanning electron microscopy, x-ray photoemission spectroscopy (XPS), xray diffraction, Raman spectra and photoluminescence spectra.
Abstract: ZnO thin films were deposited on (001) Si substrate by low-pressure metalorganic chemical vapour deposition. Thermal annealing was performed at 800 °C in air for an hour. The effects of annealing on the surface morphology, stoichiometric ratio, structural and optical properties of ZnO films were investigated using scanning electron microscopy, x-ray photoemission spectroscopy (XPS), x-ray diffraction, Raman spectra and photoluminescence spectra. The resistivity of ZnO film increased to 1.25 × 102 Ω cm after annealing. It was found that the quality of ZnO film could be improved through annealing.
TL;DR: In this paper, the energy and momentum relaxation of hot electrons in n-type epilayer InN grown on sapphire substrate using molecular beam epitaxy (MBE) was investigated.
Abstract: We report on the energy and momentum relaxation of hot electrons in n-type epilayer InN grown on sapphire substrate using molecular beam epitaxy (MBE). Hall and pulsed I–V measurements are carried out in the temperature range between 77 and 300 K. Drift velocity versus electric field characteristics show that, at 77 K, the drift velocity saturates just above vd ~ 8 × 106 cm s−1 at electric fields in excess of E ~ 12 kV cm−1. The mobility comparison method together with the power balance equation is used to obtain the electron temperature as a function of applied electric field and the electron energy loss rate as a function of electron temperature. Our results show conclusively that the effective energy relaxation time constant in InN is 200 fs. This is about six times slower than the theoretical value for the e–LO phonon scattering time of 31 fs. The effects of non-equilibrium phonon generation on slowing down of the energy relaxation and increasing the momentum relaxation processes are discussed using a theoretical model first developed for GaAs and then adapted to the III-N material systems.
TL;DR: In this paper, the authors report on the study of ZnS and X-doped X-ZnS thin films, prepared by spray pyrolysis technique using chloride precursors, which consist of mixed hexagonal (α) and cubic (β) phases with a predominance of cubic phase.
Abstract: Here we report on the study of ZnS and X-doped ZnS (with 4 at% of X = Al, Sn) thin films, prepared by spray pyrolysis technique using chloride precursors. Cathodoluminescence imaging and spectroscopy, x-ray diffraction, x-ray energy dispersive spectrometry and spectrophotometry have been used for their characterization. Deposited at their optimal substrate temperature (Ts = 773 K), these films are polycrystalline and consist of mixed hexagonal (α) and cubic (β) phases with a predominance of the cubic phase. Their growth is preferentially oriented along the (111)β direction and their optical bandgap always remains close to 3.56 eV regardless of the sample considered. The cathodoluminescence spectra of ZnS and Al–ZnS films are similar and are characterized by a blue emission peak at 407 nm (3.05 eV) and a broad blue–green one located at 524 nm (2.36 eV) due to the presence of chlorine. The insertion of Sn2+ ions in the ZnS material leads to the formation of the SnCl2 compound and to the disappearance of the blue–green emission associated with Cl ionized donors.
TL;DR: In this paper, the authors studied the strain state, film and surface morphology of SiGe virtual substrates grown by reduced pressure chemical vapour deposition (RP-CVD) and found that the misfit dislocations generated to relax the lattice mismatch between Si and SiGe are mostly confined inside the graded layer.
Abstract: We have studied the strain state, film and surface morphology of SiGe virtual substrates grown by reduced pressure chemical vapour deposition (RP-CVD). The macroscopic strain relaxation and the Ge composition of those virtual substrates have been estimated in high resolution x-ray diffraction, using Omega-2Theta scans around the (004) and (224) orders. Typically, linearly graded Si0.7Ge0.3 virtual substrates 5 ?m thick are 96% relaxed. From transmission electron microscopy, we confirm that the misfit dislocations generated to relax the lattice mismatch between Si and SiGe are mostly confined inside the graded layer. The threading dislocations density obtained for Ge concentrations of 20% and 27% is indeed typically of the order of (7.5 ? 2.5) ?105 cm?2. The surface roughness of the relaxed SiGe virtual substrates increases significantly as the Ge concentration approaches 30%. We find for the technologically important Ge concentration of 30% a surface root mean square roughness of 5 nm, with an undulation wavelength for the cross-hatch of the order of 1 ?m. We have also studied the electronic quality of our RP-CVD grown SiGe virtual substrates. We have grown a MODFET-like heterostructure for this purpose, with a buried tensile-strained Si channel 8 nm thick embedded inside SiGe 31%. We have obtained a well-behaved two-dimensional electron gas in the Si channel, with electron sheet densities and mobilities at 1.45 K of 5.7 ? 1011 cm?2 and 180?000 cm2 V?1 s?1, respectively.
TL;DR: In this paper, the authors studied the electric domain dynamics and the self-oscillation frequency dependence on the applied dc field, and constructed a typical nonlinear dynamic system with the dc bias, the ac amplitude and the ac frequency as the control parameters.
Abstract: Inflection of carriers in the ? valley, prior to the usual intervalley scattering when the electric field is approximately less than 300 kV cm?1, can cause a negative differential velocity in zinc-blende (Zb) GaN. GaN n+nn+ oscillators based on this mechanism have a self-oscillating frequency in the terahertz (THz) range. The situation is completely different from the case of traditional Gunn-effect devices. We have carefully studied the electric domain dynamics and the self-oscillation frequency dependence on the applied dc field. When the diode is driven by a dc and an ac bias, a typical nonlinear dynamic system is constructed with the dc bias, the ac amplitude and the ac frequency as the control parameters. Different mode locking and chaotic modes show up when tuning the control parameters.
TL;DR: In this article, the authors derived the optical constants of CdCr2S4 thin films from the experimental recorded transmission and reflectance spectral data over the wavelength range 300?2500 nm and found that the refractive index and extinction coefficient are almost independent of the film thickness.
Abstract: Reproducible and good quality CdCr2S4 thin films with different thicknesses were deposited on microscopic glass substrates using the chemical bath deposition process. The x-ray and electron diffraction analysis revealed that the as-deposited films were polycrystalline cubic phase. The optical constants of the deposited films were obtained from the analysis of the experimental recorded transmission and reflectance spectral data over the wavelength range 300?2500 nm. It has been found that the refractive index and extinction coefficient are almost independent of the film thickness. The complex dielectric constants of CdCr2S4 films have been calculated in the investigated wavelength range. An analysis of the optical absorption data of the as-deposited films revealed an optical direct transition with the estimation of the corresponding bandgap value. It was found that the refractive index dispersion data obeyed the single oscillator of the Wemple?DiDomenico model, from which the dispersion parameters and the high-frequency dielectric constant were determined. The electric free carrier susceptibility and the carrier concentration to the effective mass ratio were estimated according to the model of Spitzer and Fan.