Ideal Semiconductor Research Articles

A self-consistent algorithm to extract interface trap states of MOS devices on alternative high-mobility substrates

A new self-consistent technique is proposed to simultaneously extract the density of interface traps (Dit) and flat-band voltages of MOS structures fabricated on technologically relevant high-mobility semiconductors with arbitrary combination of gate stacks. The technique is based on novel analysis of the low-frequency C–V measurement. The two major problems associated with the existing low-frequency C–V technique for arbitrary substrate/oxide combinations are resolved by (i) accurate calculation of the ideal semiconductor capacitance using a self-consistent, quantum–mechanical model including wave function penetration effect, and (ii) accurate determination of the flat-band voltage utilizing an iterative scheme. The proposed technique has been applied to extract Dit profiles of a number of MOS structures fabricated on III–V semiconductors like InGaAs (with ALD grown Al2O3 gate dielectric) and elemental semiconductors like Ge (with GeON gate dielectric). The advantages of the proposed technique have been demonstrated by comparing with Dit profiles extracted from other capacitor-based extraction methods.

The detailed balance limit of photochemical energy conversion

Limits and optimization of a solar energy conversion system consisting of a photochemical charge separating unit coupled to an energy storage state are explored by multi-objective genetic algorithms. Pareto fronts were evaluated to obtain information about the ideal parameter combinations, guaranteeing highest efficiency. The light absorbing and charge separating unit is described by a chain of chromophores and electron carriers, connected by Marcus type electron transfer processes. It is coupled to the thermal equilibrium of charge conduction and transport in an energy storage system according to the principle of detailed balance. In addition to our previous findings for an optimal charge separation unit, consisting of a minimum number of charge carriers with adapted recombination and reaction rates, the complete photochemical unit must fulfil further requirements. Low reorganization energies are found to be essential for the initial charge separation steps and can be realized by a low dielectric constant in the local environment. The identified optimal operation rates can be realized by antenna systems adapted to the illumination conditions. For standard solar illumination and a realistic parameter setting energy conversion efficiencies up to 26.8% are predicted, comparable to the limit (31.8%) of ideal single junction semiconductor solar cells.

Preparation and characterization of heterojunction semiconductor YFeO3/TiO2 with an enhanced photocatalytic activity

YFeO3/TiO2 heterojunction semiconductors were prepared by the milling–annealing method. Effects of structure and surface phase by annealing temperature and TiO2 weight content were investigated. The physical and photophysical properties of heterojunction semiconductors were characterized by x-ray diffraction (XRD), UV-visible diffraction (UV-vis/DRS), and x-ray photoelectron spectroscopy (XPS), N2 adsorption. YFeO3 is presented as p-type semiconductor and disperses on the surface of n-type TiO2 to constitute a heterojunction composite. Results show that the presence of p–n junction not only has visible light harvesting but potential force for hole–electron pair separation. A preliminary investigation of photodegradation effect on Orange II showed that YFeO3/TiO2 heterojunction semiconductors exhibited better photocatalytic properties than the single phase of YFeO3 or TiO2. The ideal heterojunction semiconductor composition was ω (TiO2) = 0.9 sample annealed at 600 °C. The mechanism of the electric-field-driven electron–hole separation initiated by the chemically bonded p–n heterojunction and enhanced photocatalytic activity were discussed.

Kinetics of Charge Transport in Wide-Band Semiconductors at the Detection of X-Ray Radiation

As a result of absorption of X-ray quantum in a semiconductor, the generation of electron-hole pairs takes place in a small volume (diameter < 0.5 μm). Their surplus energy is lost due to the scattering on phonons of the crystal lattice. Spatial distribution of the charge carriers makes the form of current pulse on electrodes of the crystal complicated when an external electric field is applied. We present a logical chart of construction of basic kinetic model of X-ray conductivity (XRC) in semiconductors that uses the successive in time calculation of the spatial distribution of free charge carriers and the diffusive-drift model of motion of free carriers in a solid. The basic form of current pulse in an external circle was obtained in the analytical kind for the case of an ideal semiconductor, e.g. that does not contain deep traps and recombination centers, as well as for the case of a crystal with dominant shallow or deep traps of electrons and holes.

Compact models considering incomplete voltage swing in complementary metal oxide semiconductor circuits at ultralow voltages: A circuit perspective on limits of switching energy

This paper presents a circuit perspective on complementary metal oxide semiconductor (CMOS) limits by analyzing a standard CMOS inverter driving a capacitive load at ultralow supply voltages. While the behavior of an inverter is well understood and modeled in the superthreshold region and for sufficiently high supply voltages in the subthreshold region, the analysis at ultralow supply voltages is different. In this region of operation, the voltage levels corresponding to logic “1” and logic “0” are not Vdd and Gnd, respectively. We model the incomplete voltage swing and show that the minimum supply voltage for a CMOS inverter (with pull-up and pull-down devices as ideal conventional metal oxide semiconductor field effect transistors) is, indeed, 2kT/q ln 2. The novelty of this approach lies in the fact that it gives an explicit mathematical relationship between the supply voltage and a measure of distinguishability of the binary states defined in terms of the expected voltage swing. The analysis shows that the minimum supply voltage corresponds to the state when the two states become completely indistinguishable. Further, we obtain a relationship between the switching energy and robustness of CMOS inverters and interconnects, valid for the entire range of supply voltages. This analysis also shows that a minimum supply voltage of 2kT/q ln 2 may not correspond to the fundamental energy dissipation of kT ln 2.

Spin-Polarized Electron Transport in Silicon

Spin transport in semiconductors holds promise for beyond-CMOS logic processing because the orientation of spin (coupled to the electron's intrinsic magnetic moment) can encode a binary state. However, the experimental difficulties of achieving coherent spin-polarized electron transport in the most prevalent semiconductor, Silicon, were overcome for the first time only recently. I will present our solution to this long-standing problem, including observations of exceptionally long spin lifetimes and transit lengths, which confirms Silicon's status as the ideal semiconductor for spintronics.

Attractive Tetraceno[2,3-b]thiophene Derivatives for Organic Transistor Applications: A Theoretical Analysis

We propose new semiconductor candidates based on recently synthesized tetraceno[2,3-b]thiophene for p-type organic transistor applications. The proposed model semiconductor organics are 5,12-bis(trimethylsilylethynyl)-tetraceno[2,3-b]thiophene and 5,12-bis(triethylsilylethynyl)-tetraceno-[2,3-b]thiophene. The suitability of these proposed model organics as p-type semiconductor materials for the organic thin-film transistor applications were tested theoretically by studying their electronic parameters that are essential for ideal semiconductor candidates. For this purpose, density functional theory calculations were made to calculate their band gaps, excitation energies, and hole-reorganization energies. Eleven other p-type semiconductor organics including the well-known, and popular alkynyl pentacenes and anthradithiophenes were also studied. The results derived in the present study clearly indicate that these proposed organics show better oxidative stability and have band gaps lower than those of many of...

A review of nucleation, growth and low temperature synthesis of diamond thin films

Diamond thin films have outstanding optical, electrical, mechanical and thermal properties, which make these attractive for applications in a variety of current and future systems. In particular, the wide band gap, optical transparency and unusually high thermal conductivity of diamond thin films make them an ideal semiconductor for applications in current and future electronics. However, synthesis of diamond thin films with adequate quality remains a challenging task. Synthesis of diamond at low temperatures is even more challenging because of the difficulties in the nucleation and growth steps involved in diamond thin film deposition on a variety of substrates. Among the several deposition techniques for synthesising diamond films, plasma enhanced chemical vapour deposition is the most promising technique because of its potential for low temperature synthesis. Consequently, the first part of this paper reviews the current state of the nucleation and growth of diamond during chemical vapour deposition. A uniform and high nucleation density is a prerequisite for getting a good quality diamond film with significant growth rate. Also, since the process conditions for nucleation and growth steps are different, attempts are made in this review to identify the important parameters responsible for enhancing the nucleation and growth rates of diamond film. Describing the indispensable need for low temperature growth of diamond film, the second part of the paper reviews the research on the growth of diamond thin films at low temperatures. In spite of the slower kinetics at lower deposition temperatures, an attempt is made to identify the key processing conditions, which enhance the low temperature growth process. In addition, the mechanisms of diamond nucleation and growth are discussed based on the observations from in situ characterisation techniques such as Fourier transform infrared reflection absorption spectroscopy, real time spectroscopic ellipsometry and molecular beam mass spectroscopy.

Carrier-controlled ferromagnetism in transparent oxide semiconductors

The search for an ideal magnetic semiconductor with tunable ferromagnetic behaviour over a wide range of doping or by electrical gating is being actively pursued as a major step towards realizing spin electronics. A magnetic semiconductor having a high Curie temperature, capable of independently controlled carrier density and magnetic doping, is crucial for developing spin-based multifunctional devices. Cr-doped In(2)O(3) is such a unique system, where the electrical and magnetic behaviour-from ferromagnetic metal-like to ferromagnetic semiconducting to paramagnetic insulator-can be controllably tuned by the defect concentration. An explicit dependence of magnetic interaction leading to ferromagnetism on the carrier density is shown. A carrier-density-dependent high Curie temperature of 850-930 K has been measured, in addition to the observation of clear magnetic domain structures in these films. Being optically transparent with the above optimal properties, Cr-doped In(2)O(3) emerges as a viable candidate for the development of spin electronics.

The Chemical Potential

The definition of the fundamental quantity, the chemical potential, is badly confused in the literature: there are at least three distinct definitions in various books and papers. While they all give the same result in the thermodynamic limit, major differences between them can occur for finite systems, in anomalous cases even for finite systems as large as a cm3. We resolve the situation by arguing that the chemical potential defined as the symbol μ conventionally appearing in the grand canonical density operator is the uniquely correct definition valid for all finite systems, the grand canonical ensemble being the only one of the various ensembles usually discussed (microcanonical, canonical, Gibbs, grand canonical) that is appropriate for statistical thermodynamics, whenever the chemical potential is physically relevant. The zero–temperature limit of this μ was derived by Perdew et al. for finite systems involving electrons, generally allowing for electron–electron interactions; we extend this derivation and, for semiconductors, we also consider the zero–T limit taken after the thermodynamic limit. The enormous finite size corrections (in macroscopic samples, e.g. 1 cm3) for one rather common definition of the c.p., found recently by Shegelski within the standard effective mass model of an ideal intrinsic semiconductor, are discussed. Also, two very–small–system examples are given, including a quantum dot.

Macroscopic coherence of a single exciton state in an organic quantum wire

Macroscopic quantum coherence has been observed in some many-body systems including superconductors, quantum liquids1 and cold atom condensates2, but never for a single quasi-particle state. In an ideal semiconductor, excitons (electron–hole pairs bound by the Coulomb interaction) can, in principle, exist as delocalized plane waves extending over the entire volume. However, any kind of disorder prevents long-range spatial coherence from emerging. There has been evidence for the formation of macroscopic coherent states only in condensate phases such as in the case of microcavity polaritons condensation3,4 or in a dense quasi-two-dimensional exciton gas5. It is unclear however, whether in this latter case the observations are really related to macroscopic coherence6. Here, we show that a single exciton state in an individual ordered conjugated polymer chain7,8, shows macroscopic quantum spatial coherence reaching tens of micrometres, limited by the chain length. The spatial coherence of the k=0 exciton state is demonstrated by selecting two spatially separated emitting regions of the chain and observing their interference.

Silicon radiation detectors

The rapid progress of the past 20 years in silicon radiation detectors is reviewed. The availability of silicon as an almost ideal semiconductor material is one of the main reasons for this progress. The well-defined properties of the silicon-silicon dioxide interface allowed the development of detector structures beyond the structure of a classical diode detector, which was practically the only silicon detector structure 20 years ago. All new structures make use of an electric field parallel to the large surface of the detector. Improvements in the production of silicon detectors has lead to a substantial decrease of the detector leakage current. These detectors can achieve their best performance very close to or at room temperature.

The chemical potential of an ideal intrinsic semiconductor

The standard result for the chemical potential μ of an ideal intrinsic semiconductor as calculated in the canonical ensemble indicates that μ goes to the middle of the band gap as the temperature T goes to zero. However, the definition of μ implies that μ goes to the bottom of the conduction band as T→0. The solution to this puzzle is that the Fermi–Dirac distribution function ceases to be accurate for the thermal occupation probabilities when the temperature is so low that the number of electrons in the conduction band is of order unity. The use of the correct occupation numbers results in μ going to the bottom of the conduction band as T→0.

The synthesis of MWNTs/SWNTs multiple phase nanowire arrays in porous anodic aluminum oxide templates

In order to utilize the unique properties of carbon nanotubes (CNTs) in microelectronic devices, the special hetero-junction or multiple phase structure of multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) array has been produced by chemical vapor deposition (CVD) method in porous anodic aluminum oxide (AAO) templates. The approach involves CVD process, fabrication of catalytic nanoparticle, and nanotube interconnection for orientation. The synthesized MWNT/SWNT hetero-junctions or multiple phase nanowire arrays were fully characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectrum. The multiple phase structures have the one-dimensional features of CNTs and build an ideal semiconductor–metal or metal–metal hetero-junction nanowire in the axial direction. The structural character and growing mechanism of MWNT/SWNT hetero-junctions or multiple phase nanowire arrays were also analyzed and discussed.

Quantum-statistical equilibrium and the “law” of constant Fermi potential

We apply the general quantum-statistical density-matrix formalism to an independent-electron gas within a space-dependent external electric potential, under equilibrium conditions. This problem is analogous to an ideal semiconductor homojunction diode. We solve the resulting equilibrium density-matrix equation using a perturbation theory. Next, we derive a first-order quantum correction to the classical Maxwell-Boltzmann density-potential formula. The correction appears as an added curvature term in external potential. It represents expected quantum-mechanical scattering against a spatially varying potential. Our results indicate that the commonly encountered thermodynamic or statistical-mechanical “law” of constant, equilibrium Fermi potential—with Fermi potential a parameter in the Maxwell-Boltzmann density-potential formula—is not fundamentally exact. In a general space-dependent potential, this “law,” we prove, is simply a classical approximation.

Thermal Modelling and Characterisation of Semiconductor Bolometers

A revised version of the Griffin & Holland ideal semiconductor bolometer model is presented and its use in determining bolometer properties and parameters from experimental load curve measurements is discussed. We show that degeneracy between some bolometer parameters can only be broken by model fitting a family of load curves over a range of bath temperatures, and that measurements with the bolometer blanked (zero absorbed radiant power) are essential for unambiguous determination of the main parameters. The influence of measurement errors on parameter recovery is analysed using synthetic noisy data sets.

Microscopic nature of the Er3+ emission in mixed amorphous-nanocrystalline Si:H films

ErO 6 complexes, where every Er3+ ion is surrounded by six oxygen atoms forming an octahedron with C3v point symmetry, are found to best describe the strong Stark splitting of the characteristic Er3+ emission in the 1460–1610 nm range. An a-Si:H matrix serves as an ideal semiconductor host to permit codoped O atoms to form an optimal octahedral ligand field around the Er3+ ions. The observation of sharp intense Stark peaks for temperatures between 153 and 300 °C and of strong enhancement of the Er3+ emission after 350 °C annealing can be understood first by diffusion and then outdiffusion of weakly bound hydrogen atoms in the a-Si matrix.

Current mechanisms in VLWIR Hg1−xCdxTe photodiodes

VLWIR (c∼15 m to 17 m at 78 K) detectors have been characterized as a function of temperature to determine the dominant current mechanisms impacting detector performance. Id−Vd curves indicate that VLWIR detectors are diffusion limited in reverse and near zero bias voltages down to temperatures in the 40 K range. At 30 K the detectors are limited by tunneling currents in reverse bias. Since the detectors are diffusion limited near zero bias down to 40 K, the R0Aimp versus temperature data represents the diffusion current performance of the detector as a function of temperature. The detector spectral response measurement and active layer thickness are utilized to calculate the HgCdTe layer x value and the optical activation energy Ea optical. The activation energy, Ea electrical, obtained from the measured diffusion limited R0Aimp versus temperature data is not equal to the activation energy, Ea optical, obtained from the spectral response measurement for all x values measured. Ea electrical=*Ea optical, where ranges between 0.64 and 1.0 For cutoff wavelengths in the 9 m at 78 K, Ea electrical=Ea optical. Ea electrical=0.65* Ea optical have been measured forc=17 m at 78 K detectors. As the band gap energy decreases to values in the range of 70 meV and lower, it is reasonable to expect a more dominant role of band tailing effects on the transport properties of the material system. In such a picture, one would expect the optical band gap to be unmodified, whereas the intrinsic concentration could be enhanced from its value for the ideal semiconductor. Such a picture could explain the observed behavior. Further probing experiments and modeling efforts will help clarify the physics of this behavior.

Microscopic origin of Er 3+ emission in mixed amorphous-nanocrystalline Si:H films

ErO 6 complexes, where every Er 3+ ion is surrounded by six oxygen atoms forming an octahedron with C 3v point symmetry are found to explain the strong Stark splitting of the characteristic Er 3+ emission observed in the 1460–1610 nm range. An a-Si:H matrix serves as an ideal semiconductor host to permit co-doped O atoms to form an optimal ligand field around Er 3+ ions.

Characterization of silicon wafer bonding by observation in transmitted infrared radiation from an extended source

The bonding quality of silicon wafers used in SOI technology, [1] was estimated by the application of infrared transmission measurements. The investigated wafers were directly welded at high or low temperature by using an intermediary layer. Uniform and extended radiation source was applied. The intensity of the radiation transmitted through the wafer was recorded by the thermovision camera scanning the sample. The proposed way of the experiment, enabled very fast transmission visualization of the entire wafer surface and rough estimation of bonding quality. The more exact analysis of the geometric parameters of the measurement set-up and transmissions of ideal and real semiconductor wafers shown below, enabled in some cases the determination of the sort and size of defect occurring between the bonding wafers.