Properties of Mott contacts with an ultralow metal-semiconductor barrier

The current-voltage and capacitance characteristics of Mott contacts with an ultralow metal-semiconductor barrier are investigated. The analysis is based on the analytical solution of the Poisson equation for the space charge of carriers in the “metal-i-layer-n ⁺-substrate” structure without regard for bulk doping of the i layer. For contacts with ultralow metal-semiconductor barriers (comparable in magnitude to the thermal energy of charge carries), it is demonstrated that the reverse current becomes greater than the forward current, the sign of rectification is reversed, and the capacitance of the contact acquires a strong dependence on the voltage. This means that the mechanism of nonlinearity of the structure changes and the nonlinearity governed by charge carriers injected into the i layer becomes dominant. In a specific range of bias voltages close to zero, the differential resistance and the capacitance of the structure exponentially increase with increasing voltage. The observed behavior is not typical of conventional metal-semiconductor contacts. The obtained dependences of the electric current and capacitance on the voltage determine the characteristics of new advanced instruments, in particular, highly sensitive microwave detectors operating without a bias voltage.     

Alternating space-charge-limited currents in hydrogenated amorphous silicon

Space-charge-limited currents in n⁺-i-n⁺ sandwich samples of hydrogenated amorphous silicon have been investigated under alternating current (AC) conditions. When the voltage sweeping rate exceeds the release rate of carriers from deep traps, the AC current-voltage characteristics differ markedly from the direct current (DC) characteristics. This is attributed to the remaining space charge in the samples which acts as a potential barrier and reduces the current. A simple model is proposed to describe the time-dependence of this injection-induced barrier.     

Estimation of the parameters of charge carriers in polymers in the vicinity of the threshold of thermally stimulated switching

The temperature dependences of the parameters of charge carriers in thin films of an electroactive poly(diphenylenephthalide) polymer are investigated in the range preceding the thermally stimulated electronic switching (110–400 K). The thermally stimulated current spectra and current-voltage characteristics are measured in the same temperature range. The parameters of charge carriers are estimated in the framework of the model of injection currents limited by the space charge. It is revealed that the charge carrier mobility decreases with an increase in the temperature in the range from 110 to 400 K. A correlation between the temperature behavior of the current-voltage characteristics and the thermally stimulated current spectra is established. The possible contribution of the Pool-Frenkel effect is considered, and the inference is made that the electric field plays an important role in the thermally stimulated electronic switching.     

Mechanism of charge transfer in <Emphasis Type="Italic">n</Emphasis>-CdS/<Emphasis Type="Italic">p</Emphasis>-CdTe heterojunctions

The capacitance-voltage and current-voltage characteristics of the n-CdS/p-CdTe heterosystem are investigated. Analysis of these characteristics demonstrates that the CdTe_1?x S _x solid solution formed at the n-CdS/p-CdTe heterointerface is inhomogeneous in both the conductivity and composition. The thickness of solid solutions is estimated from the capacitance-voltage characteristics. It is shown that, for the n-CdS/p-CdTe heterosystem, the current-voltage characteristic in the current density range 10^?8-10^?5 A cm^?2 is governed by the thermal electron emission, whereas the current in the heterostructure at current densities in the range 10^?4-10^?2 A cm^?2 is limited by recombination of charge carriers in the electroneutral region of the CdTe_1?x S _x solid solution. The lifetime and the diffusion length of minority charge carriers in the CdTe_1?x S _x solid solution and the surface recombination rate at the interface between the CdS layer and the CdTe_1?x S _x solid solution are determined. It is demonstrated that the n-CdS/p-CdTe heterostructure operates as a p-i-n structure in which CdTe is a p layer, CdTe_1?x S _x is an i layer, and CdS is an n layer.     

Relaxation phenomena in TlGa<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript>Se<Subscript>2</Subscript> single crystals

The relaxation electronic phenomena occurring in TlGa_0.99Fe_0.01Se₂ single crystals in an external dc electric field are investigated. It is established that these phenomena are caused by electric charges accumulated in the single crystals. The charge relaxation at different electric field strengths and temperatures, the hysteresis of the current-voltage characteristic, and the electric charge accumulated in the TlGa_0.99Fe_0.01Se₂ single crystals are consistent with the relay-race mechanism of transfer of a charge generated at deep-lying energy levels in the band gap due to the injection of charge carriers from the electric contact into the crystal. The parameters characterizing the electronic phenomena observed in the TlGa_0.99Fe_0.01Se₂ single crystals are determined to be as follows: the effective mobility of charge carriers transferred by deep-lying centers μ_f=5.6×10^?2 cm²/(V s) at 300 K and the activation energy of charge transfer ΔE=0.54 eV, the contact capacitance of the sample C _c =5×10^?8 F, the localization length of charge carriers in the crystal d _c =1.17×10^?6 cm, the electric charge time constant of the contact τ=15 s, the time a charge carrier takes to travel through the sample t _t =1.8×10^?3 s, and the activation energy of traps responsible for charge relaxation ΔE _σ = ΔE _Q = 0.58 eV.     

Charge carrier injection and transport in polymer blend films

The steady current-voltage characteristics of single layer organic devices based on MEH-PPV and N,N′-diphenyl-N,N′-bis(4′-[N,N-bis(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) blend with different TPTE concentrations was investigated. The thickness dependence of the current-voltage relationship clearly demonstrates that the current at low voltage and at high voltage are all space charge limited. The current density-electric field characteristic proves the blend polymer LEDs to operate in the tunneling-controlled model. The effective hole mobility is directly determined by space charge limited current at high voltage and increases with increasing TPTE content in the blend. The EL efficiency shows concentration dependence, which is attributed to the change of the transport of holes in the blend film.     

Characteristics of a photovoltaic X-ray detector based on a GaAs epitaxial structure

The characteristics of a photovoltaic X-ray detector based on the GaAs p ⁺-n-n′-n ⁺ epitaxial structure grown using gas-phase epitaxy are studied. Typical current-voltage and capacitance-voltage characteristics of the epitaxial structures are analyzed together with the built-in electric field profile in the n-GaAs depleted region. The efficiency of charge accumulation in the photovoltaic detector is measured for zero bias and for a bias voltage of 17 V. It is shown that the GaAs-based photovoltaic X-ray detector can operate with zero bias voltage at room temperature. The sensitivity of the detector is measured as a function of the effective energy of X-rays and the angle of incidence of X-ray photons.     

Photovoltaic properties of a heterojunction based on copper phthalocyanine films on the surface of polycrystalline cadmium sulfide

The photovoltaic effect has been detected and studied in thin-film structures based on thermally deposited 200-nm-thick copper phthalocyanine (CuPc) films on the surface of polycrystalline CdS. The structures under study demonstrate the linear current-voltage characteristics at external electric fields to 3.5 × 10⁴ V/cm. Two components of the photovoltage of different signs have been revealed when the sample is illuminated in the wavelength range from 350 to 700 nm. The first component has the positive sign on the CuPc film side and is observed when using the radiation with a wavelength lesser than 500 nm, i.e., in conditions of predominant absorption of the radiation in the CdS layer. The second component has the negative sign on the CuPc film side and is observed when using the radiation with a wavelength in the range from 500 to 570 nm, corresponding to the spectral region of the absorption edge of the CuPc films. The dependences of the photovoltage on the radiation intensity studied in the range from 5 × 10¹² to 10¹⁴ photons cm^?2 s^?1 are different in the cases of the two detected components. Mechanisms of generation of the photovoltage components associated with a change in the band bending during photogeneration of charge carriers in the region of space charge in CdS and a change in conditions of the charge transfer in the interfacial CuPc/CdS region during the radiation absorption in the CuPc film have been proposed.     

Analysis of the charge transfer mechanisms responsible for the current-voltage characteristics of Ca<Subscript>4</Subscript>Ga<Subscript>2</Subscript>S<Subscript>7</Subscript>: Eu<Superscript>3+</Superscript> single crystals

The current-voltage characteristics of Ca₄Ga₂S₇: Eu³⁺ single crystals are measured for the first time, and the processes affecting these characteristics are analyzed theoretically. It is demonstrated that Ca₄Ga₂S₇: Eu³⁺ single crystals are high-resistance semiconductors with a resistivity of ～10⁹ Ω cm and a relative permittivity of 10.55. The electrical properties of the studied materials are governed by traps with activation energies of 0.13 and 0.19 eV and a density ranging from 9.5×10¹⁴ to 2.7×10¹⁵ cm^?3. The one-carrier injection is observed in weak electric fields. In electric fields with a strength of more than 4×10³ V/cm, traps undergo thermal field ionization according to the Pool-Frenkel mechanism. At low temperatures and strong fields (160 K and 5×10⁴ V/cm), the electric current is most likely due to hopping conduction by charge carriers over local levels in the band gap in the vicinity of the Fermi level.     

Electric field distribution in the base of semiconductor <Emphasis Type="Italic">p-n</Emphasis> junction diode

An expression for distribution of the electric field in the base of a semiconductor p-n junction diode is obtained valid for arbitrary injection levels in the approximation of exponential distribution of non-equilibrium carriers in the diode base. It is shown that at a certain current the field is constant across the base which allows one to determine the mobility of majority carriers (whose concentration is determined from the barrier capacity measurements). For commercial D223B and D219A diodes the current-voltage characteristics and the differential resistance are measured and the mobility of electrons is calculated. The electric field distribution in the D223B diode is derived at three injection levels.     

The physics of space charge instabilities and temporal chaos in extrinsic photoconductors

This paper reviews experimental and theoretical work carried out on space charge instabilities and temporal chaotic behavior in cooled extrinsic p-type Germanium photoconductors. Measured dc current-voltage (I–V) characteristics of these devices are strongly nonlinear for moderate electric fields 0.1 V/cm due to field dependence of the rates of free hole capture and impurity impact ionization. Below the threshold field for impurity breakdown, Ge samples behave like damped nonlinear oscillators, exhibiting characteristic chaotic response when driven by a time-periodic voltage. Above impurity breakdown, we observe voltage-controlled negative differential resistance (NDR) in the I–V curves accompanied by spontaneous current oscillations due to moving space charge domains with velocities 10³ to 10⁴ cm/s. Measurements are well explained by a simple rate equation model in which negative differential behavior in the impact ionization rate plays a crucial role. Related work on semiconductor chaos and possible future directions for research are also mentioned.     

Deformation of a charge density wave in the vicinity of a point contact with a normal metal

The current-voltage characteristics of Cu-K_0.3MoO₃ point contacts between a metal and a semiconductor with a charge density wave (CDW) are studied for various diameters of the contacts in a wide range of temperatures T and voltages V. In the interval 80 K ? T ? 150 K, the current-voltage characteristics are correctly described in the framework of a semiconductor model: screening of an external electric field causes CDW deformation, shifts the chemical potential of quasiparticles, and changes the point contact resistance. It is shown that the chemical potential is above the middle of the Peierls gap in equilibrium and approaches the middle upon an increase in temperature. The current-voltage characteristics of point contacts with a diameter d ? 100 Å exhibit a sharp decrease in resistance for |V| > V _t , which is associated with the beginning of local CDW sliding within the contact region. The V _t (d, T) dependence can be explained by the size effect in the CDW phase slip.     

Current transfer through dielectric in irradiated mis-structures

The current-voltage and current-temperature characteristics of MIS-structures under the different electric fields in dielectric (10⁴÷5·10⁶ V.cm^?1) within the temperature interval 77–470K are investigated before and after γ-irradiation. The mechanisms of the current transfer are considered within the limits of accepted models. There are some discrepancies between the present data and known mechanisms of current transfer in such structures. The possible reasons for these discrepancies are analysed. The effects of γ-irradiation on the I-V and I-T-characteristics are discussed on the bases of information about the nature of different components responsible for the current formation.     

CdTe/CdS太阳电池I-V，C-V特性研究

测量了CdTe太阳电池器件从50kHz至1MHz频率范围的电容-电压特性，计算了吸收层的载流子浓度和空间电荷区的位置，电容-电压特性测试结果出现两个峰，峰特征与测试频率有关，用多结模型进行模拟分析，解释了实验结果.测量了电池从220K至300K的变温暗电流-电压特性，得出电池的反向暗饱和电流密度J₀和二级管理想因子A，分析了J₀，A随测量温度的变化,并讨论了电池器件的电流特性. 关键词： CdTe太阳电池 电流-电压特性 电容-电压特性     

Stripe like inhomogeneities,spectroscopies, pairing and coherence in the high Tc cuprates

It is found that the carriers of the high-T_c cuprates are polaron-like ‘stripons’ carrying charge and located in stripe-like inhomogeneities, ‘quasi-electrons’ carrying charge and spin, and ‘svivons’ carrying spin and lattice distortion. This is shown to result in the observed anomalous spectroscopic properties of the cuprates. The AF/stripe-like inhomogeneities result from the Bose condensation of the svivon field, and the speed of their dynamics is determined by the width of the double-svivon neutron-resonance peak. Pairing results from transitions between pair states of stripons and quasi-electrons through the exchange of svivons. The obtained pairing symmetry is of the d_x²−y² type; however, sign reversal through the charged stripes results in features not characteristic of this symmetry. The phase diagram is determined by a pairing and a coherence line, associated with a Mott transition, and the pseudogap state corresponds to incoherent pairing.     

Electrical and galvanomagnetic effects in new sodium-doped fullerene composites

The Hall effect, magnetoresistance, the temperature dependence of the electrical resistance, and the current-voltage characteristics of new C₆₀-based carbon-carbon composites were studied in magnetic fields of up to 28 kOe at temperatures T = 1.8–336 K. Sodium-doped samples were used. It is shown that there is weak localization of charge carriers and strong electron-electron interactions. Superconductivity occurs in the range T ≤ 15 K. A mixed current may exist at higher temperatures. Cooper pairs are assumed to form due to the interaction of the conduction electrons with the π-electron system of a C₆₀ molecule.     

The symmetry,inferable from Bogoliubov transformation,between processes induced by a mirror in two-dimensional and a charge in four-dimensional space-time

We consider the symmetry between creation of pairs of massless bosons or fermions by an accelerated mirror in (1+1)-dimensional space and emission of single photons or scalar quanta by an electric or scalar charge in (3+1)-dimensional space. The relation of Bogoliubov coefficients describing the processes generated by a mirror to Fourier components of the current or charge density implies that the spin of any disturbances bilinear in the scalar or spinor field coincides with the spin of quanta emitted by the electric or scalar charge. The mass and invariant momentum transfer of these disturbances are essential for the relation of Bogoliubov coefficients to invariant singular solutions and the Green functions of wave equations for both (1+1)-and (3+1)-dimensional spaces, and especially for the integral relations between these solutions. One of these relations leads to the coincidence of the self-action changes and vacuum-vacuum amplitudes for an accelerated mirror in two-dimensional space-time and a charge in four-dimensional space-time. Both invariants of the Lorentz group, spin and mass, play an essential role in the established symmetry. The symmetry embraces not only the processes of real quanta radiation, but also the processes of the mirror and charge interactions with fields carrying spacelike momenta. These fields accompany their sources and determine the Bogoliubov matrix coefficients α _ω′ω ^{B, F} . It is shown that the Lorentz-invariant traces ±trα^B,F describe the vector and scalar interactions of the accelerated mirror with a uniformly moving detector. This interpretation rests essentially on the relation between propagators of the waves with spacelike momenta in two-and four-dimensional spaces. The traces ±trα^{B, F} coincide with the products of the mass shift Δm_{1, 0} of the accelerated electric or scalar charge and the proper time of the shift formation. The symmetry fixes the value of the bare fine structure constant α₀=1/4π.     

Multiple roles of bathocuproine employed as a buffer-layer in organic light-emitting diodes

Efficiency and brightness and carriers injection have been obviously improved by using bathocuproine (BCP) as a buffer-layer in organic light-emitting diodes. Compared with the bufferless device, the quantum efficiency of device ITO/NPB (10 nm)/Alq₃ (10 nm)/BCP (2.4 nm)/Al has increased four times at the same current density (32 mA/cm²). Moreover, the buffer layer has changed the current-voltage properties and the turn-on voltage has obviously decreased. Considering BCP and Al³⁺ can react conveniently under room temperature, we suggest that a complex cathode structure of BCP/[(Al)_x(BCP)_y]^3x+/Al has formed under electric field and the new cation [(Al)_x(BCP)_y]^3x+ at the BCP/Al interface has improved the internal electric field and then enhanced the electrons injection. we conclude that: for a very thin (<1 nm) BCP buffer layer, improving electron injection will principally responsible to the improvement of the performance of the OLEDs; for a thicker BCP layer, there will be a synthetic function of BCP: improving electron injection, hole-blocking and electron-transporting.     

Electrical and photoresponse characteristics of 8-(1H-indol-3-ylazo)-naphthalene-2-sulfonic acid/n-Si photodiode

In this paper, of primary interest is to synthesis 8-(1H-indol-3-ylazo)-naphthalene-2-sulfonic acid (INSA) and to evaluate the main parameters of Au/INSA/n-Si/Al diode in dark and under illumination. Different techniques are used for interpreting the proposed INSA chemical structure. The dark current-voltage measurements were achieved in the temperature range of 293?413 K. It is noticed that INSA films modify the interfacial barrier height of classical Au/n-Si junction. At low applied voltages, the I–V relation shows exponential behavior. The values ideality factor, n, and the barrier height, φ, are improved by heating. The abnormal trend of n and φ is discussed, and a homogenous barrier height of 1.45 eV is evaluated. The series resistance is also calculated using Norde's function and it changes inversely with temperature. The space charge limited current ruled with exponential trap distribution dominates at relatively high potentials, trap concentration and carriers mobility are extracted. The reverse current of the diode has illumination intensity dependence with a good photosensitivity indicating that the device is promising for photodiode applications.