On the temperature dependence of the thermoelectric power in disordered semiconductors

In the context of the thermodynamics of irreversible processes, general expressions for the temperature dependence of the thermoelectric power in the region of hopping conductivity in disordered materials at low temperatures are derived. The effect of degeneracy of impurity levels on the thermoelectric power is taken into account. On the basis of the results, the experimental data on the thermoelectric power in amorphous and impurity-containing semiconductors are discussed.     

Processes of growth of disordered semiconductors in the context of self-organiziation theory

The possibility of applying the concepts and methods of self-organization theory to the growth of disordered semiconductors is justified. A basic method for analyzing the dynamics of complex systems is described. The invariants of random dynamics are considered in the context of the processes involved in material growth. New principles for the construction of technological systems are stated.     

Modeling anomalous charge carrier transport in disordered organic semiconductors using the fractional drift-diffusion equation

Anomalous transport is ever-present in many disordered organic semiconductor materials. The long-tail behavior observed in the transient photocurrent is a manifestation of anomalous transport. Owing to the fact that anomalous transport has dispersive and non-Gaussian transport dynamics, thus anomalous transport cannot be adequately described by the standard drift-diffusion equation which is a framework commonly used to model normal diffusive transport. In this work, we generalized the standard drift-diffusion equation to time fractional drift-diffusion equation (TFDDE) using the fractional calculus approach to model the anomalous transport in the regio-random poly(3-hexylthiophene) (RRa-P3HT) and regio-regular poly(3-hexylthiophene) (RR-P3HT). Physical elucidation of TFDDE is given by stressing how the influence of the multiple-trapping mechanisms and energy disorder lead to the long-tail behavior in the transient photocurrent curves. TFDDE is solved numerically using finite difference scheme to obtain the profiles of charge carriers density evolution and hence to reproduce the corresponding transient photocurrents of RRa-P3HT and RR-P3HT. Poisson solver is also included in the model to account for the fluctuation of localized electric field due to the evolution of charge carriers. It is found that charge carriers acquire additional energy from high electric field that helps them to escape from the trap centers more easily and then propagating at higher velocity, which yields higher transient current. Higher concentration of charge carriers can be generated at higher light intensity and they can occupy energy levels close to the mobility edge, where charge carriers will encounter smaller capturing rate and hop at a longer length in each hopping event. Thus, the transport dynamic of charge carriers at high light intensity is less dispersive than that of the low light intensity. Besides, the transport dynamic of charge carriers in RR-P3HT is relatively less dispersive and has higher mobility than that of the RRa-P3HT since RR-P3HT has lower capturing rate and is less energy disordered.     

Transmission through abrupt heterojunction potential barriers

A new expression is derived for the transmission coefficient of incident electrons scattered by a varying potential with discontinuities. The expression is applicable above, below, and near the potential peak. As an example, electron transmission across the energy barrier in the emitter of an InP/InGaAs heterojunction bipolar transistor is calculated. Excellent agreement with a numerical calculation is obtained, even for electron energies close to the potential peak, where other expressions fail. It is shown how this new approach reduces to other expressions in limiting cases, and its validity in comparison to other approximations is discussed     

Principles of flicker noise spectroscopy and its application to disordered semiconductors: IonImplanted silicon

Flicker noise spectroscopy in disordered semiconductors is gaining wide acceptance both in physical investigation and in analysis of various microelectronics processes. Its successful use as a diagnostic tool in microtechnology is a result of advances in fundamental studies of flicker noise in semiconductors with disordered structure. In this work, we systematize experimental data for flicker noise phenomena in disordered semiconductors, specifically, in ion-implanted silicon     

Ir studies of disordered zincblende semiconductors III-V-II-VI alloys and GaAs-Ga2/3Se

This paper presents the ir reflectance spectra of several III-V — II-VI alloys, GaAs-ZnSe, GaP-ZnSe and GaAs-ZnS, and the III-V — III₂ -VI₃ system, GaAs-Ga_2/3 Se. GaAs-ZnSe and GaP-ZnSe alloys each display two reflectivity peaks, where-as the GaAs-ZnS system displays a single feature. These behaviors are explained using a linear chain model which considers both mass and force constant differences and which assumes pairing of the III-V and II-VI constituents. The spectrum of Ga₂ Se₃ is dominated by disorder induced features associated with the vacancies. Alloying with GaAs reduces the number of vacancies and is accompanied by im-portant changes in both the intensity and shape of the spectral features.     

The role of the density of states on the hole mobility of disordered organic semiconductors

It was recently demonstrated that the hopping mobility in disordered organic semiconductors depends both on the charge carrier concentration and on the density of states but their relative influence is still an open question. The mobility is almost constant below a certain concentration and increases at large concentration where the density of states is conventionally assumed exponential. Hence the experimental hole mobility in polymer FETs is at least one order of magnitude larger than the hole mobility in LEDs based on the same materials. In order to investigate this issue, the mobility of disordered organic semiconductors is calculated by numerically solving the variable range hopping (VRH) equations in a wide range of carrier concentrations ranging from low concentrations, typical of hole-only diodes, to high concentrations, typical of field effect transistors. The exact mobility is numerically calculated for various density of states (DOS) and it is compared with the experimental data in order to investigate the dependence of the hole mobility on the DOS. Our calculations show that the strong dependence of the hole mobility on the charge carrier density is strictly correlated to the shape of the DOS and that a single gaussian, in general does not explain the mobility behavior in the whole range of concentrations; depending on the material, it could be accurately approximated by a single gaussian, an exponential, or by a combination of both functions.     

Formation of the nickel-platinum alloy silicide Schottky barriers

We propose a new technique for the Schottky barrier formation that involves magnetron deposition of a thin film from a multicomponent target consisting of vanadium, platinum, and nickel onto silicon and the subsequent stage thermal treatment. Using the developed technique, we fabricated device structures with the 0.69–0.71-V-high Schottky barriers. It is established that the barrier layer comprises the Ni_{1 ? x} Pt _x Si silicide phase and about 2 at % of platinum in the contact region. We show that the amount of platinum at the interface with silicon determines the barrier’s height. The highest platinum content at the interface is ensured at the two-stage thermal treatment at a first stage temperature of 240–300°C. The use of the two-stage thermal treatment in the silicide formation in the system’s silicon-composite Ni-Pt-V alloy allows obtaining a silicide layer with higher structural quality and a better silicon/silicide interface than the one-stage treatment can yield.     

Grain boundary space charge region potential barrier in polycrystalline semiconductors

The grain boundary space charge region potential barrier is derived in an analytical form under equilibrium as well as non-equilibrium conditions, using various energy distributions to represent the grain boundary surface states, and the Shockley- Read-Hall theory for the carrier generation-recombination processes. The energy value of the grain boundary states is taken as a single level in the energy band gap of the semiconductor and located at the intrinsic Fermi level or displaced from it by a given amount. The restriction of the single level is later removed by permitting a certain width to the energy distribution of these states. It is shown that the results derived for non-equilibrium case reduce to the equilibrium case results derived separately, under the appropriate limit of zero excitation potential. The numerical results obtained from this formulation for the case of polycrystalline silicon are presented graphically for different values of the bulk doping concentration, grain boundary surface state density, grain boundary state energy distributions and the excitation potential, under equilibrium and non-equilibrium conditions. It is found that the grain boundary energy states located higher than the intrinsic Fermi level give greater values of grain boundary potential barrier height for a p-type semiconductor and the reverse applies to an n-type semiconductor. Further a wider energy distribution of these states gives rise to a towering of the potential barrier height at the grain boundary. Under non-equilibrium conditions, it is seen that the grain boundary barrier height remains unaltered for values of the excitation potential below a certain threshold which in turn depends upon the bulk dopant concentration in the semiconductor and the grain boundary surface state density and its energy distribution. Above this threshold value the barrier height reduces nearly linearly with the increase of the excitation potential with a slope lying-between ?0.5 and ? 1.0. For comparison with the available experimental results of Seager and Castner for n-type polycrystalline silicon we have obtained the grain boundary surface density and energy distribution parameters that provide excellent matching with the experimentally determined variation of the grain boundary potential barrier height with the dopant concentration in the range of 10¹³ to 10¹⁷/cm³, and based upon these parameters we have calculated the non-equilibrium barrier heights at different excitation potentials for this experimental case. A comparison is also made with Seager's experimental observations of non-equilibrium grain boundary barrier height for n-type silicon bicrystals in the illumination range of 3 × 10^?6 to 3 × 10^?2 SUN. For particular values of the density of grain boundary surface states and their energy distribution parameters, we could match our theoretical results with his experimental observations of barrier height.     

Particle scattering times on one-dimensional potential barriers

The tunneling times for particles whose initial state is described by wave packets of a general form is determined on the basis of a packet analysis. It is shown that the unphysical nature of the results obtained previously by numerical modeling of the motion of wave packets in one-dimensional structures is due to the incorrect interpretation of the packet formalism. Wave-packet tunneling times observed in a numerical experiment are not at all the particletunneling times. Fiz. Tekh. Poluprovodn. 31, 427–431 (April 1997)     

Superlinear electroluminescence in GaSb-based heterostructures with high potential barriers

The electroluminescence in isotype and anisotype light-emitting diode heterostructures grown by the method of liquid-phase epitaxy with large conduction-band offset ΔE _c at the heterointerface between a narrow-band active region and a wide-band layer is studied. Two types of electroluminescence peaks are observed in the range of photon energies 0.28–0.74 eV at temperatures T = 300 and 77 K; in this case, a super-linear increase in the intensity and optical power of emission by a factor of 1.5–2 is observed in the range of pump currents 20–220 mA. This effect is attributed to the formation of additional electron-hole pairs as a result of impact ionization by hot electrons heated as a result of the band offset ΔE _c in the conduction band at the n-AlGaAsSb/n-InGaAsSb and n-GaSb/n-InGaAsSb heteroboundaries. This effect can be used to increase the quantum efficiency of semiconductor emitters (light-emitting diodes, lasers) in the mid-infrared region.     

Analysis of persistent photoconductivity due to potential barriers

Persistent photoconductivity has been seen in thin silicon resistors fabricated with SIMOX material at temperatures between 60 and 220 K. This effect has been attributed to the depletion of carriers near the interface between the top silicon layer and the buried oxide, which is due to the large number of surface traps at this interface. The depletion of carriers is accompanied by a built-in field on the order of 10,000 V/cm, which causes a potential barrier that is nearly a quarter of the energy gap of silicon. The theory of the recombination kinetics of majority carriers with minority carriers trapped at the interface on the other side of a potential barrier is studied. Both the possibilities of tunneling and thermal activation have been considered. The results show that thermal activation dominates at the temperatures of our measurements in SIMOX material, while at lower temperatures tunneling would dominate.     

Features of erbium electroluminescence observed in disordered semiconductors and related to the difference in the localized state charge

The effect of the charge of defects and donors on electroluminescence is considered. It is shown that the specific temperature and field dependences of the conductivity and electroluminescence can be explained if one takes into account the difference between the temperature and field dependences of the probability of thermally activated tunneling of electrons from D ^? defects and N ⁰ donors.     

Formation and some properties of chromium oxide nanolayers on semiconductors

Ultrathin chromium oxide layers (nanostructures) are synthesized by molecular laying (atomic layer deposition) on the (100)-oriented Si surface and the (100)- and (110)-oriented GaAs surfaces. The influence of technological conditions on the composition and basic regular trends in the formation of the layers is established. Some of the dielectric properties of the chromium oxide nanostructures and the quality of the semiconductor-insulator interface are assessed.     

Schottky barriers and ohmic contacts on n-type InP based compound semiconductors for microwave FET's

InP, and In0.73Ga0.27As0.6P0.4, and In0.53Ga0.47As lattice matched to InP are of special importance as active FET channel materials because of the high electron velocity and/or high electron mobility they offer. Using a AuGe/Ni/Au metallization system, specific contact resistances of 5 × 10^-7Ω . cm², 8 × 10^-7Ω . cm², and 5.8 × 10^-6Ω cm²were obtained for ohmic contacts on In0.53Ga0.47As, InP, and In0.89Ga0.11As0.24P0.76, respectively. Leakage currents of 10 µA at 7-V reverse bias were observed for 1 × 200-µm gates on InP. and In0.89Ga0.11As0.24P0.76FET's having a SiO2film about 50 Å thick under the gate. A thin SiO2layer underneath the gate improved the Schottky-gate I-V characteristics, but thick oxides severely degraded the microwave performance of the FET's. These excellent ohmic contacts and Schottky barriers resulted in a maximum insertion gain of 15 dB at 8 GHz and a noise figure of 2.5 dB with 8-dB gain at 7 GHz for the InP deviees. For 1.15-eV InxGa1-xAsyP1-yFET's, the resulting gain was 9 dB at 8 GHz.     

Equilibrium energy distribution of localized carriers in disordered semiconductors subjected to an external electric field at low temperature

An equilibrium energy distribution of charge carriers is possible in the 3D case in disordered semiconductors with a sufficiently rapidly decreasing density of localized states at low temperatures when the contribution of thermally activated hops of charge carriers to the hopping transport is negligible. A Boltzmann exponential with a field-dependent effective temperature describes the asymptotic behavior of this distribution.     

Deformation potential constants of deep impurity centers in semiconductors with anisotropic valence band

The effect of strain on the ground state of an impurity center with a deep Γ₈ level was considered. The model of short-range potential of the impurity center was applied to the analysis of level splitting. The impurity center deformation potential constants were obtained as functions of the band constants and deep level energies in the cases of spherical and nonspherical approximation for the valence band structure. For a deep center in GaAs and Ge, numerical calculations of the deformation potential constants were performed.     

The potential of III‐V semiconductors as terrestrial photovoltaic devices

III‐V semiconductors, GaAs and in particular InGaP, are used in many different electronic applications, such as high power and high frequency devices, laser diodes and high brightness LED. Their direct bandgap and high reliability make them ideal candidates for the realisation of high efficiency solar cells: in the past years they have been successfully used as power sources for satellites in space, where they are able to produce electricity from sunlight with an overall efficiency of around 30%. Nowadays, the use of arsenides and phosphides as photovoltaic (PV) devices is confined only to space applications since their price is much higher than conventional Si flat panel modules, the leading PV market technology. But with the introduction of multijunction solar cells capable of operating in high concentration solar light, the area and, therefore, the cost of these cells can be reduced and will eventually find an application and market also on Earth. This article will review the situation of semiconductor solar cell materials, focusing on Si, GaAs, InGaP and multijunction solar cells and will discuss future trends and possibilities of bringing III‐V technology from space to Earth. Copyright © 2006 John Wiley & Sons, Ltd.