Interaction of infrared radiation with free carriers in mesoporous silicon

Features of absorption and reflection of infrared radiation in the range 500–6000 cm^?1 are investigated; these features are associated with free carriers in the layers of mesoporous Si (porosity, 60–70%) formed in single-crystal p-Si(100) wafers with a hole concentration of N _p ≈10²⁰ cm^?3. It is found that the contribution of free holes to the optical parameters of the samples decreases as the porosity of the material increases and further falls when the samples are naturally oxidized in air. The experimental results are explained in the context of a model based on the Bruggeman effective medium approximation and the Drude classical theory with a correction for additional carrier scattering in silicon residues (nanocrystals). A comparison between the calculated and experimental dependences yields a hole concentration in nanocrystals of N _p ≈10¹⁹ cm^?3 for as-prepared layers and shows a reduction of N _p when they are naturally oxidized.     

To observe bidirectional negative differential resistance at room temperature by narrowing transport channels for charge carriers in vertical organic light-emitting transistor

Bidirectional negative differential resistance (NDR) at room temperature with high peak-to-valley current ratio (PVCR) of ～10 are observed from vertical organic light-emitting transistor indium-tin oxide (ITO)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine) (α-NPD)(60 nm)/Al(30 nm)/α-NPD(60 nm)/tris-(8-hydroxyquinoline) aluminium (Alq₃)(50 nm)/Al by narrowing the transport channels for charge carriers with a thick-enough middle Al gate electrode layer to block charge carriers transporting from source electrode to drain electrode. When the transport channel for charge carriers gets large enough, the controllability of gate bias on the drain–source current gets weaker and the device almost works as an organic light-emitting diode only. Therefore, it provides a very simple way to produce NDR device with dominant bidirectional NDR and high PVCR (～10) at room temperature by narrowing transport channels for charge carriers in optoelectronics.     

Generation of minority charge carriers at the semiconductor surface during thermally activated ionic depolarization of metal-insulator-semiconductor structures

Relaxation signals represented by the temperature dependences of current J(T) and high-frequency capacitance C(T) and generated in the course of the thermally stimulated depolarization of a metal-insulator-semiconductor structure were analyzed numerically. Both the depletion of ionic traps located at the insulator-semiconductor interface and the generation of minority charge carriers from a bulk center with the activation energy _{E D} were taken into account.     

Optical generation of free charge carriers in thin films of tin oxide

The methods of infrared absorption spectroscopy and Raman spectroscopy are used to study nanocrystalline SnO _x films (1 ≤ x ≤ 2) prepared by thermal oxidation of metallic tin layers. A monotonic decrease in the transmittance of films in the infrared region has been observed as a result of exposure of the films to light with the wavelength of 380 nm at room temperature. The effect is at a maximum for the samples with x ≈ 2 and is observed for ∼10 min after switching off of illumination. The mentioned variations in optical properties, similarly to those observed in the case of heating of the samples in the dark, are accounted for by an increase in the concentration of free charge carriers (electrons) in nanocrystals of tin dioxide. The data of infrared spectroscopy and the Drude model are used to calculate the concentrations of photogenerated charge carriers (∼10¹⁹ cm⁻³); variations in these concentrations in the course of illumination and after switching off of illumination are determined. Mechanisms of observed photogeneration of charge carriers in SnO _x films and possible applications of this effect to gas sensors are discussed.     

Temperature-dependent ambipolar electrical characteristics of pentacene-based thin-film transistors: The impact of opposite-sign charge carriers

The temperature-dependent electrical and charge transport characteristics of pentacene-based ambipolar thin-film transistors (TFTs) were investigated at temperatures ranging from 77 K to 300 K. At room temperature (RT), the pentacene-based TFTs exhibit balanced and high charge mobility with electron (μ_e) and hole (μ_h) mobilities, both at about 1.6 cm²/V s. However, at lower temperatures, higher switch-on voltage of n-channel operations, almost absent n-channel characteristics, and strong temperature dependence of μ_e indicated that electrons were more difficult to release from opposite-signed carriers than that of holes. We observed that μ_e and μ_h both followed an Arrhenius-type temperature dependence and exhibited two regimes with a transition temperature at approximately 210–230 K. At high temperatures, data were explained by a model in which charge transport was limited by a dual-carrier release and recombination process, which is an electric field-assisted thermal-activated procedure. At T < 210 K, the observed activation energy is in agreement with unipolar pentacene-based TFTs, suggesting a common multiple trapping and release process-dominated mechanism. Different temperature-induced characteristics between n- and p-channel operations are outlined, thereby providing important insights into the complexity of observing efficient electron transport in comparison with the hole of ambipolar TFTs.     

Determination of the diffusion length of minority charge carriers in a semiconductor from the dynamic nonequilibrium I–V characteristics of MIS structures

A method for determination of the diffusion length L _de of minority charge carriers in a semiconductor from the dynamic nonequilibrium I–V characteristics of metal-insulator-semiconductor (MIS) structures is proposed. The method makes it possible to exclude the effect of bulk generation in a semiconductor on L _de and decrease the temperature of measurements. The possibility of investigating the effective L _de profile in the semiconductor surface layer is shown for the case of nonuniform depth distribution of electrically active defects in a material. The values of L _de in MIS structures on silicon used in integrated-circuit technology are investigated. The effect of the internal gettering of defects in silicon and the irradiation of MIS structures by low-energy electrons (10–30 keV) on the effective L _de profiles in the surface layer of a semiconductor is considered.     

Determination of the diffusion length of charge minority carriers using digital oscillography of surface photovoltage

A procedure for determining the diffusion length of charge minority carriers (CMCs), which uses digital oscillography to detect surface photovoltage (SPV), is considered. It is shown by experiments that the shape of pulses of SPV, excited by rectangular IR pulses, depends significantly on both their intensity and wavelength. It is shown that it is not valid to use a synchronous detector to measure the quasi-stationary SPV when determining the diffusion length of CMCs in semiconductors, because, in a number of cases, it results in an uncontrollable error that reaches 100% and more. A technique is developed for determining the diffusion length in silicon wafers and epitaxial structures with the specific resistance range 0.01–12 Ω cm, the diffusion length range 5–500 μm, and an error of not more than 8%.     

Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I₂ iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I₂ molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ～10¹⁸?10¹⁹ cm^?3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, P _b centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I₂ molecules.     

Measurement of the diffusion length of minority charge carriers using real Schottky barriers

The characteristic features of the field dependence of the short-circuit photocurrent of real Schottky barriers based on strongly doped semiconductors under conditions of an oscillatory dependence of the light absorption coefficient a on the field intensity in the space-charge region and the photon energy (hν>E _g ) are analyzed. An analytical expression is obtained for the dependence of the photocurrent on the thickness W of the space-charge region when the condition αW≪1 is satisfied. An improved method is proposed for determining the diffusion length of minority charge carriers by analyzing the dependence I _p (W) in the spectral region satisfying the conditions of applicability of the expressions obtained. Some improvements are also made in the method for distinguishing the capacitance of the space-charge region from the high-frequency capacitance of a real Schottky barrier. The method is tested on structures Au-GaAs with N _d =4.5×10¹⁶–1×10¹⁸ cm⁻³. The method of determining L is checked independently on the basis of a theoretical descripion of the spectral dependence of the quantum efficiency of the structure. Fiz. Tekh. Poluprovodn. 31, 781–785 (July 1997)     

Edge-photoluminescence concentration dependence in semi-insulating undoped GaAs

Dependences of the spectral peak position of edge photoluminescence, its half-width, resistivity, charge carrier mobility in crystals of semi-insulating undoped GaAs on the carbon concentration N _C at 77 K (3.0×10¹⁵ cm^?3≤N _C≤4.3×10¹⁶ cm^?3) were studied. The dependences observed are explained by the interaction of charge carriers with ionized impurity atoms and with structural defects.     

Interface charge characteristics of MOS structures with different metals on steam grown oxides

Characteristics of interface states and interface charges of MOS structures with steam grown oxides and different metal contacts were investigated. Steam oxidation was carried out in an RF furnace under the influence of a d.c. bias. Interface state density N_is as well as the interface charge density at flat band Q_ic^FB were found to be independent of the oxide thickness between 150 and 5000 Å, independent of steam oxidation temperature between 900 and 1150°C, but dependent on the contact metal. Metals with smaller work functions were found to have lower N_is and Q_ic^FB. Post oxidation annealing in nitrogen at high temperatures reduced both N_is and Q_ic^FB. The silicon type had no influence on the interface state density, but p-type silicon had a higher interface charge density at flat band than n-type silicon. Post metallization annealing in nitrogen at low temperatures completely removed a slight slow trapping instability observed before the heat treatment but increased Q_ic^FB. MOS structures with oxides grown without bias had much higher interface state density as well as Q_ic^FB, however a short postmetallization annealing was very effective in greatly reducing these. The above experimental facts suggest that the interface charges result from acceptor type interface states and fixed positive charges at or near the interface. Protons or SiH groups are very likely to be the source of the fixed positive charges while likely source for the acceptor states are the OH radicals.     

The effect of low-field injection of charge carriers on the electrical properties of the metal-oxide-semiconductor structures

The method of high-frequency capacitance-voltage characteristics was used to study the effect of low-field injection of charge carriers on the electrical properties of metal-SiO₂-Si structures with n- and p-type substrates. It is shown that in all cases of injection (irrespective of the polarity of the voltage at the gate), an effective positive charge is generated in the oxide; after completion of the injection, this charge relaxes with characteristic times that depend on the bias voltage applied to the gate and the type of the metal-oxide-semiconductor structure. In the structures with p-Si substrates, in the case of a positive voltage applied to the gate, a capacitance minimum appeared in the inversion portion of the capacitance-voltage characteristics in the course of injection; this effect became more pronounced as the gate voltage was increased. After the injection, the capacitance gradually approached the initial value (before injection).     

Investigation of the majority carriers diffusion coefficients behavioural pattern in degenerately-doped silicon

A detailed analysis of the majority carriers diffusion coefficients (D_nand D_p) behaviour in degenerately-doped n-type and p-type silicon, respectively, was performed. Simple analytical expressions showing that for electron (n) and hole (p) concentrations equal to or larger than the corresponding density of states in the conduction (N_c) and valence (N_v) bands, respectively (i.e., n > N_c and p > N/_v), the ratio of the majority carriers diffusion coefficient-to-drift mobility (D/μ) increases with the power of (2/3) of the corresponding majority carriers concentration are derived for the first time in the literature. Based upon all available experimental dependences of drift mobilities μi on impurity concentration, it is shown in this work that the majority carriers diffusion coefficient D in degenerately-doped silicon increases sharply with the dopant level in the 10¹⁹ to 5 × 10^2lcm^?3 range of the latter, regardless of the ambient temperature T. Exact values of diffusion coefficients in phosphorus-, arsenic- and boron-doped silicon are presented up to carrier densities one order of magnitude higher than the solid solubility. The main implications of the presented results for the physics of silicon-based electron devices are also outlined in this work     

Effect of electron-electron interactions on the ionization rate of charge carriers in semiconductors

The effect of the presence of a large carrier density on the ionization rate of carriers in a semiconductor is investigated. Thus e-e interactions have been considered in addition to normal scattering events, i.e. generation of optical phonons and electron-hole pair production. For a charge density n = 10²¹/m³ and above the ionization rate decreases appreciably due to e-e interactions.     

Experimental investigation of the excess charge

The open-circuit voltage of about 600 mV developed by 0.1 ohm-cm silicon solar cells under air mass zero illumination is about 100 mV less than voltages predicted from simple diffusion theory. The lower measured voltages appear to be controlled by junction current transport processes associated with the thin top diffused layer. Mechanisms such as low n⁺ layer minority carrier lifetime and bandgap narrowing due to heavy doping effects (HDE) have been suggested to explain these results. Experimental determinations of the properties of the diffused layer are required to assess which of these mechanisms predominate. While direct measurement is difficult, an indirect measurement methodology exists by which the lifetime or transit time in the diffused layer can be obtained. Nine p-type, 1×2 cm, 〈111〉 orientation silicon wafers were phosphorus diffused at 880°C for 45 minutes using P0Cl₃. Open-circuit voltages of 595-612 mV, typical of all 0.1 ohm-cm cell voltages, were obtained. From the open-circuit voltage and short-circuit current, the diffusion controlled I₀ was obtained. In addition to illuminated I-V characteristics, the time constants from the Open-Circuit Voltage Decay method, and the minority carrier diffusion lengths in the base region were measured. The base region charge was determined using the base region diffusion length measured by an X-ray method. The data from these experiments combined with simple theory can imply the minority carrier time constant and the excess charge in the diffused layer. From this, certain conclusions are drawn about the relative roles of bandgap shrinkage and recombination rates in the diffused layer.     

Metallic conductivity over an acceptor band of lightly compensated copper-doped p-Hg0.78Cd0.22Te crystals

The conductivity and Hall effect of heavily doped p-Hg_0.78Cd_0.22Te:Cu crystals were studied in the temperature range of 4.2–125 K. The conductivity over the impurity band is of a metallic type for the acceptor concentration N _A>3.8×10¹⁷ cm^?3. The conductivity and the Hall coefficient governed by the delocalized charge carriers in the impurity band are independent of temperature. The sign of the Hall effect is positive in the metallic conductivity range. Near the metal-insulator transition point, the Hall mobility increases linearly with the acceptor concentration and is independent of the acceptor concentration at N _A>1.6×10¹⁸ cm^?3. The metallic conductivity is proportional to N _A in the concentration range under study at N _A<3.1×10¹⁸ cm^?3. The Anderson transition occurs at the Cu concentration N _A=1.4×10¹⁷ cm^?3 in the A ⁺ impurity band, which is formed by positively charged acceptors. Minimum metallic conductivity corresponding to this transition equals 5.1 Ω^?1 cm^?1. It is shown that ?₂ conductivity in the subthreshold region is defined by delocalized carriers in the upper Hubbard band only for fairly heavy doping (N _A>1.4×10¹⁷ cm^?3). For N _A<1.4×10¹⁷ cm^?3, the hopping conductivity is observed.     

Measurements of bandgap narrowing in Si bipolar transistors

Theory predicts appreciable bandgap narrowing in silicon for impurity concentrations greater than about 10¹⁷ cm^?3. This effect influences strongly the electrical behaviour of silicon devices, particularly the minority carrier charge storage and the minority carrier current flow in heavily doped regions. The few experimental data known are from optical absorption measurements on uniformly doped silicon samples. New experiments in order to determine the bandgap in silicon are described here. The bipolar transistor itself is used as the vehicle for measuring the bandgap in the base. Results giving the bandgap narrowing (ΔV_g0) as a function of the impurity concentration (N) in the base (in the range of 4.10¹⁵–2.5 10¹⁹ cm^?3) are discussed. The experimental values of ΔV_g0 as a function of N can be fitted by:

$\text{δV}{}_{\mathrm{g0}}\text{= V}{}_1\text{ln}\text{N}\text{N}{}_0\text{+}\text{ln}{}^2\text{N}\text{N}{}_0\text{+C}$

where V₁, N₀ and C are constants.It is also shown how the effective intrinsic carrier concentration (n_ie) is related with the bandgap narrowing (ΔV_g0).     

Increase in the rate and discretization of the kinetics of isothermal surface generation of minority charge carriers in metal-insulator-semiconductor structures with a planar-inhomogeneous insulator

Surface generation of minority charge carriers in silicon metal-oxide-semiconductor (MOS) structures is efficient only at the initial recombinationless stage. Quasi-equilibrium between surface generation centers and the minority-carrier band is established in a time t ～ 10^?5 s. In the absence of other carrier generation channels, an equilibrium inversion state at 300 K would need t = t_∞ > 10³ years to become established. In fact, the time t ∞ is much shorter, due to excess-carrier generation via centers located at the SiO₂/Si interface over the gate periphery. This edge-related generation can easily be simulated in an MOS structure with a single gate insulated from Si by oxide layers of various thicknesses. At gate depleting voltages V _g , the role of the periphery is played by a shallow potential well under a thicker oxide, and the current-generation kinetics becomes unconventional: two discrete steps are observed in the dependences I(t), and the duration and height of these steps depend on V _g . An analysis of the I(t) curves allows determination of the electric characteristics of the Si surface in the states of initial depletion (t = 0) and equilibrium inversion (t = t_∞), as well as the parameters of surface lag centers, including their energy and spatial distributions. The functionally specialized planar inhomogeneity of a gate insulator is a promising basis for dynamic sensors with integrating and threshold properties.     

The charge accumulation in an insulator and the states at interfaces of silicon-on-insulator structures as a result of irradiation with electrons and gamma-ray photons

The accumulation of charge in an insulator and the states at interfaces in silicon-on-insulator structures irradiated with 2.5-MeV electrons and 662-keV gamma-ray photons were studied. It was found that an additional positive charge appears in the buried insulator of the structures as a result of irradiation. The concentration of hole traps generated by radiation in the oxide is higher at the boundary with substrate than at the bonding interface between a split-off silicon layer and oxide. It is shown that the presence of even a weak built-in field in the structures (F?5×10³ V/cm) gives rise to efficient separation of charge carriers. There is no generation of additional states at the Si/SiO₂ interfaces in the silicon-on-insulator structures for both irradiation types, although this generation is observed in the initial thermal oxide.     

Spectral sensitivity of p-Cu1.8S/n −-ZnS/n-(II-VI) heterostructures

Photosensitivity of multilayered p-Cu_1.8S/n ^?-(II-VI)/n-(II-VI) heterostructures beyond the fundamental-absorption edge of the wide-gap component is studied experimentally, and a simple model is suggested as an explanation of this photosensitivity. It is established that an effective method for reducing the photosensitivity of the structures beyond the ultraviolet spectral region consists in decreasing the probability of dominant tunneling processes, by increasing the thickness of the wide-gap layer, giving rise to a blocking barrier for photogenerated minority charge carriers. It is shown that the p-Cu_1.8S/n ^?-ZnS/n-CdSe heterostructures are promising for the development of efficient “solar-blind” detectors of ultraviolet radiation.