Evolution of exciton states near the percolation threshold in two-phase systems with II–VI semiconductor quantum dots

From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, “a dielectric trap” is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.     

Flicker noise in metal semiconductor Schottky barrier diodes due to multistep tunneling processes

In a practical metal semiconductor Schottky barrier diode there is a certain amount of current flow by indirect tunneling through the barrier. Although this component of current is negligibly small compared to the thermionic emission or thermionic field emission current, a large low-frequency 1/f noise is associated with this multistep tunneling process. The multistep tunneling current introduces a random fluctuation of charge density at the trap states, which trap current carriers during the indirect tunneling process, in the space-charge region of the diode. The field intensity at the metal semiconductor interface is therefore modulated, which in turn modulates the Schottky effect and produces a random fluctuation of the diode current. The spectral intensity of noise due to this mechanism is calculated. Large flicker noise is expected at low frequencies.     

Unified tunnelling-diffusion theory for Schottky and very thin MOS structures

We derive general formulae for calculating the transport of free charge carriers in a MOS structure with a thin insulating layer. In particular, we obtain relationships for boundary concentrations of free charge carriers on the insulator–semiconductor interface and for the current densities flowing through the MOS structure. Our direct tunnelling-diffusion approach makes the well known thermionic emission–diffusion theory for the Schottky interface applicable also to metal–insulator–semiconductor barriers with a very thin insulator layer. We demonstrate how direct tunnelling through the insulating layer and drift–diffusion of free charge carriers in the semiconductor affect the I–V and C–V curves and the boundary concentrations needed to numerically solve the continuity equations.     

Effect of the levels of intrinsic defects in the CdP2 band gap on electrical characteristics of corresponding structures with the Schottky barrier

The electrical characteristics of Schottky barriers formed on n-type cadmium diphosphide are studied. It is established that the space-charge region at the metal-semiconductor interface represents in fact a Schottky layer formed owing to a high concentration of deep-level centers. The charge transport in the conducting direction for these structures is related to the above-barrier emission of electrons and is consistent with the diffusion theory for one or two types of charge carriers. The high concentration of ionized centers in the space-charge region gives rise to the tunneling mechanism of breakdown in the blocking direction. The frequency dependences of the complex conductance are governed by the exchange of charge carriers between the conduction band and donors that specify the conductivity type of the material and also by the recharing of the centers with a large depth of levels. Good agreement between the reported results and the theory is obtained.     

过渡层和界面电荷对金属硅化物/硅肖特基接触特性的影响

针对金属硅化物／硅接触存在过渡层，提出了分析这种结构的肖特基接触特性的模型；讨论了过渡层厚度、界面电行及有关参数的影响，分析了不同退火条件下PtSi／Si肖特基二极管的特性。     

Accurate barrier modeling of metal and silicide contacts

This letter presents a novel technique of evaluating Si consumption, dopant segregation, and pile-up effects in metal and silicide contacts on silicide using a newly developed device model based on thermionic-field emission and tunneling phenomena. The field dependence of interface charge penetration into the semiconductor is evaluated from the measured electrical characteristics of Schottky diodes and the theoretical results obtained from the model.     

Study and modeling of the transport mechanism in a Schottky diode on the basis of a GaAs semiinsulator

The current through a metal-semiconductor junction is mainly due to the majority carriers.Three distinctly different mechanisms exist in a Schottky diode:diffusion of the semiconductor carriers in metal,thermionic emission-diffusion（TED） of carriers through a Schottky gate,and a mechanical quantum that pierces a tunnel through the gate.The system was solved by using a coupled Poisson-Boltzmann algorithm.Schottky BH is defined as the difference in energy between the Fermi level and the metal band carrier majority of the metal-semiconductor junction to the semiconductor contacts.The insulating layer converts the MS device in an MIS device and has a strong influence on its current-voltage（I-V） and the parameters of a Schottky barrier from 3.7 to 15 eV.There are several possible reasons for the error that causes a deviation of the ideal behaviour of Schottky diodes with and without an interfacial insulator layer.These include the particular distribution of interface states,the series resistance, bias voltage and temperature.The GaAs and its large concentration values of trap centers will participate in an increase in the process of thermionic electrons and holes,which will in turn act on the I-V characteristic of the diode,and an overflow maximum value[NT = 3×10²⁰]is obtained.The I-V characteristics of Schottky diodes are in the hypothesis of a parabolic summit.     

Bilayer tellurene-metal interfaces

Tellurene, an emerging two-dimensional chain-like semiconductor, stands out for its high switch ratio, carrier mobility and excellent stability in air. Directly contacting the 2D semiconductor materials with metal electrodes is a feasible doping means to inject carriers. However, Schottky barrier often arises at the metal–semiconductors interface, impeding the transport of carriers. Herein, we investigate the interfacial properties of BL tellurene by contacting with various metals including graphene by using ab initio calculations and quantum transport simulations. Vertical Schottky barriers take place in Ag, Al, Au and Cu electrodes according to the maintenance of the noncontact tellurene layer band structure. Besides, a p-type vertical Schottky contact is formed due to the van der Waals interaction for graphene electrode. As for the lateral direction, p-type Schottky contacts take shape for bulk metal electrodes(hole Schottky barrier heights(SBHs) ranging from 0.19 to 0.35 eV). Strong Fermi level pinning takes place with a pinning factor of 0.02. Notably, a desirable p-type quasi-Ohmic contact is developed for graphene electrode with a hole SBH of 0.08 eV. Our work sheds light on the interfacial properties of BL tellurene based transistors and could guide the experimental selections on electrodes.     

Study and modeling of the transport mechanism in a Schottky diode on the basis of a GaAs semiinsulator

The current through a metal-semiconductor junction is mainly due to the majority carriers. Three distinctly different mechanisms exist in a Schottky diode: diffusion of the semiconductor carriers in metal, thermionic emission-diffusion (TED) of carriers through a Schottky gate, and a mechanical quantum that pierces a tunnel through the gate. The system was solved by using a coupled Poisson-Boltzmann algorithm. Schottky BH is defined as the difference in energy between the Fermi level and the metal band carrier majority of the metal-semiconductor junction to the semiconductor contacts. The insulating layer converts the MS device in an MIS device and has a strong influence on its current-voltage (I-V) and the parameters of a Schottky barrier from 3.7 to 15 eV. There are several possible reasons for the error that causes a deviation of the ideal behaviour of Schottky diodes with and without an interfacial insulator layer. These include the particular distribution of interface states, the series resistance, bias voltage and temperature. The GaAs and its large concentration values of trap centers will participate in an increase in the process of thermionic electrons and holes, which will in turn act on the I-V characteristic of the diode, and an overflow maximum value [NT = 3 × 10²⁰] is obtained. The I-V characteristics of Schottky diodes are in the hypothesis of a parabolic summit.     

The mechanism of metal conductivity over the interface between organic insulators

The hopping mobility of charge carriers (both at the surface and in the bulk) is analyzed theoretically in the presence of electron–hole pairs. A physical model is suggested for the metal conductivity over the interface between organic materials, each being an insulator by itself. The conductivity is due to the rather high surface density of geminate pairs formed at the interface. Conditions are established wherein the transitions of a significant portion of charge carriers between molecules do not require thermal activation or tunneling. The surface conductivity and mobility of charge carriers are estimated by numerical simulation.     

Monte Carlo simulation of GaAs Schottky barrier behaviour

The transport of carriers through the space-charge region (SCR) of a GaAs Schottky barrier is studied by the Monte Carlo simulation technique. Simulation results indicate that the carrier distribution is significantly perturbed from a Max-wellian near the metal-semiconductor boundary, limiting the validity of the commonly used thermionic diffusion model. Phenomena related to the disturbed distribution include increased recombination velocity at the interface and reduced carrier concentration near the junction. The interface recombination velocity is found to be constant with applied bias and the Bethe condition is shown to be more than sufficient to ensure the validity of the thermionic emission model.     

Influence of electric field on the photoelectric effect in a Schottky barrier based on n-type cadmium diphosphide

The results of a study on photoelectric properties of a Schottky barrier based on n-type CdP₂ are considered. The effect of the barrier electric field on photocurrents caused by photoelectron emission from metal and optical generation of excess charge carriers in the semiconductor was studied. It was found that the dependence of the photocurrent on the light-modulation frequency is controlled by level recharging times at the interfaces between the space-charge and quasi-neutrality regions and between semiconductor and metal. Good agreement between the calculated and experimental results was achieved.     

UV photon assisted control of interface charge between CdTe substrates and metalorganic chemical vapor deposition CdTe epilayers

The technology to control the interface charge density between CdTe substrates and CdTe epilayers grown by metalorganic chemical vapor deposition is studied. The interface charge is determined by the modified built-in potential derived from capacitance-voltage characteristics of Schottky contacts formed on the epilayers. Novel ultraviolet (UV) photon assisted and photo thermal surface pretreatments that control the interface between p-type CdTe substrates and CdTe epilayers are reported. The substrates are exposed to UV radiation provided by a high pressure Hg lamp operating at 600 W with a wide emission spectrum between 190 and 300 nm. The UV photon assisted surface pretreatment with hydrogen is compared with additional surface pretreatments: thermal pretreatment with hydrogen (without UV photons) and UV photo thermal pretreatment with hydrogen. The UV photon assisted and the UV photo thermal surface pretreatments with hydrogen reduce the interface charge density to a practically negligible value. In addition, the p-type doping level of the substrate is reduced considerably in a layer of few microns adjacent to the interface.     

Reverse current instabilities in amorphous silicon Schottky diodes:modeling and experiments

A new model for high bias transport is reported which describes the time-dependent reverse current variations in amorphous silicon Schottky diodes. This phenomenon is of practical importance in the design and optimization of pixels for large-area optical and X-ray imaging. In the model, the main components of the reverse current, namely thermionic emission and tunneling, are both affected by the electric field at the metal/amorphous silicon interface. Time-dependent variations in this electric field arise due to the release of charges trapped in defect states in the depletion region and to charge trapping at the interface. This effect is analyzed using the approximation that the tunneling component of the current is equivalent to a lowering of the potential barrier at the interface. The calculated time-dependent reverse current is compared with the measured data     

Macroscopic ion traps at the silicon-oxide interface

The drift kinetics of the mobile charge in SiO₂ films, its capture on ion traps localized at the Si-SiO₂ interface, and ion emission from these traps are investigated by measuring the capacitance-voltage characteristics, the dynamic current-voltage characteristics, and the thermally stimulated depolarization current of the insulator. The current components (peaks) associated with the emission of particles trapped on the interface during thermofield treatment are isolated in an explicit form. The charge of the surface ions is shown to be neutralized mainly by Si electrons, and the field dependence of the ion emission currents is characterized by an anomalous Schottky effect associated with opening of the ion-trap potential by the external field. The relationship between these traps and the potential inhomogeneities, i.e., potential wells for mobile particles, on the interface under consideration is discussed. It is noted that the mobile ions in the insulator can be used for interface potential inhomogeneity diagnostics. Fiz. Tekh. Poluprovodn. 32, 1439–1444 (December 1998)     

Photoelectric spectroscopy of InAs/GaAs quantum dot heterostructures in a semiconductor/electrolyte system

The photovoltaic effect in the semiconductor/electrolyte junction is an effective method for investigation of the energy spectrum of InAs/GaAs heterostructures with self-assembled quantum dots. An important advantage of this method is its high sensitivity. This makes it possible to obtain photoelectric spectra from quantum dots with high barriers for the electron and hole emission from quantum dots into the matrix even if the surface density of the dots is low (～10⁹ cm^?2). In a strong transverse electric field, broadening of the lines of optical transitions and emission of electrons and holes from quantum dots into the matrix directly from the excited states are observed. The effect of the photovoltage sign reversal was detected for a sufficiently high positive bias across the barrier within the semiconductor. This effect is related to the formation of a positive charge at the interface between the cap layer and electrolyte and of the negative charge on impurities and defects in the quantum dot layer.     

A unified simulation of Schottky and ohmic contacts

The Schottky contact is an important consideration in the development of semiconductor devices. This paper shows that a practical Schottky contact model is available for a unified device simulation of Schottky and ohmic contacts. The present model includes the thermionic emission at the metal/semiconductor interface and the spatially distributed tunneling calculated at each semiconductor around the interface. Simulation results of rectifying characteristics of Schottky barrier diodes (SBD's) and resistances under high impurity concentration conditions are reasonable, compared with measurements. As examples of application to actual devices, the influence of the contact resistance on salicided MOSFETs with source/drain extension and the immunity of Schottky barrier tunnel transistors (SBTTs) from the short-channel effect (SCE) are demonstrated     

Residual damage effects on gate contacts formed on SiC surfaces etched by using the amorphization technique

A trench fabrication process has been proposed and experimentally demonstrated for silicon carbide using the amorphization technique. In the present work, the quality of gates [oxide for metal oxide semiconductor field-effect transistors (MOSFETs) and Schottky barrier contacts for metal semicondcutor field-effect transistors (MESFETs)] fabricated on the etched surfaces are compared with those formed on the as-grown silicon carbide surface. The resistivity and breakdown electric field of the thermal oxide grown on the etched surface was found to be comparable to that of thermal oxide grown on silicon. However, a large concentration of acceptor type interface states (0.5-1 x 10¹³ cm⁻²eV⁻¹) was observed. This results in a large negative interface charge at room temperature and a significant shift in flat band voltage as a function of temperature, which makes the process unsuitable for formation of gates in UMOSFETs. Titanium Schottky contacts formed on the etched surface showed superior reverse current-voltage characteristics and higher breakdown voltages than the Schottky diodes formed on unetched surface with similar doping concentrations. This indicates that the argon implant process for trench formation is suitable for fabrication of gate regions in high voltage vertical MESFETs (or SITs).     

Energy characteristics of excitons in structures based on InGaN alloys

Samples containing ultrathin InGaN layers that emit radiation in the spectral range from the ultraviolet to yellow region are studied. The samples are grown by metal-organic vapor-phase epitaxy. The Urbach energy, the localization energy of excitons, and the activation energy of charge carriers are determined to characterize radiative and nonradiative processes in the quantum dots and barriers of the structures. It is shown that these energy parameters are linearly dependent on the photon energy in the range from 3.05 to 2.12 eV. It is established that temperature variations in the emission intensity are due to the increase in the number of charge carriers thermally activated from the quantum wells into barriers as well as due to the enhancement of scattering of free excitons at defects.     

Power Schottky diode design and comparison with the junction diode

Because the Schottky diode is a one-carrier device, it has both advantages and disadvantages with respect to the junction diode which is a two-carrier device. The advantage is that there are practically no excess minority carriers which must be swept out before the diode blocks current in the reverse direction. The disadvantage of the Schottky diode is that for a high voltage device it is not possible to use conductivity modulation as in the pin diode; since charge carriers are of one sign, no charge cancellation can occur and current becomes space charge limited. The Schottky diode design is developed in Section 2 and the characteristics of an optimally designed silicon Schottky diode are summarized in Fig. 9. Design criteria and quantitative comparison of junction and Schottky diodes is given in Table 1 and Fig. 10. Although somewhat approximate, the treatment allows a systematic quantitative comparison of the devices for any given application.