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.     

Observation of minority-carrier traps in Schottky diodes with a high barrier and a compensated near-contact region using deep-level transient spectroscopy

Schottky diodes based on the single-crystal n-ZnSe and fabricated by nitrogen-ion implantation with subsequent postimplantation treatment employing radical-beam epitaxy in atomic oxygen were studied using deep-level transient spectroscopy. On the assumption that the Schottky barrier is high and the near-contact region is compensated, the processes resulting in the occurrence of traps of minority charge carriers under negative biases were analyzed. The procedure for determining the compensated region thickness and the concentration of minority charge carriers in this region is described. The mechanisms of defect formation in zinc selenide crystals under annealing in atomic oxygen are described on the basis of the deep-level transient spectroscopy results.     

Carrier recombination at grain boundaries and the effective recombination velocity

The recombination of excess minority carriers at grain boundaries, or other interfaces with space-charge regions, is treated theoretically for general energy distributions of interface states (recombination centers). The distinction is made between minority carrier recombination velocity at the (grain-boundary) interface itself, and the effective recombination velocity for the collection of these carriers by the adjacent space-charge region. Calculations of the effective recombination velocity are made, as a function of the excess minority-carrier concentration at the edge of the space-charge region, since this is the quantity of most convenience for device modelling.     

Influence of the doping concentration on switching processes in psn rectifiers—I Theory

The theory, formerly developed for switching power rectifiers into the reverse state, which was limited to pin rectifiers, is extended to a noticeable homogeneous doping concentration of the middle region (psn rectifier). In this case the swept-out zones originating at the boundaries of the middle region are qualitatively different. While at the side of the pn junction a Schottky space-charge zone with a quadratic voltage vs. time increase forms passing over into the stationary reverse state, at the other side of the middle region finally a zone develops that shows an approximative equilibrium concentration of the majority carriers. This zone is essentially free of space charge. its voltage drop becomes important at most in a medium period of the sweeping-out process (but only in the case of the ps_pn rectifier because of the unequal mobilities of the charge carriers), but later on it is always negligible compared with the voltage across the Schottky zone at the pn junction. The temporal development of the swept-out zones is treated more accurately than in the pin case, i.e. we consider the difference between the points of time when the building up of swept-out zones begins at the respective sides. One finds that the sweeping-out at the n side proceeds not only more slowly but also later than at the p side. Some technical conclusions for the switching behaviour of rectifiers and thyristors are drawn up. p rectifiers have a shorter reverse recovery time than n rectifiers (and n thyristors).     

Mechanism of reverse current in the Al/p-InP schottky diodes

Reverse current-voltage characteristics of the Al/p-InP Schottky diodes based on Zn-doped InP epilayers were measured in relation to bias and temperature. Temperature dependence of reverse current is characterized by the activation energies of 0.75 and 0.51 eV in the high-temperature region and at temperatures T<280 K, respectively. Results are explained by the phonon-assisted tunneling generation of charge carriers from the surface states of a semiconductor with regard to the Frenkel emission mechanism. It is found that, in the low-temperature region, tunneling occurs via the centers with energy levels of 0.51 eV. Comparing experimental results with theory, we estimated electric-field strength in the barrier at (5–13)×10⁷ V/m and the surface density of the hole charge in the boundary layer of the semiconductor.     

Concentration profiles of recombination centers in semiconductor junctions evaluated from capacitance transients

The effect of stationary charges trapped in the region near the p-n boundary and in the edge region of a semiconductor junction space-charge layer on the evaluation of the concentration profiles of the recombination centers as well as doping impurities, from high-frequency capacitance transient data, is studied. Both the theory, its simplification for junctions with low concentration of recombination centers, and two experimental examples are given to illustrate the importance of the edge effect. One of the examples is an aluminum on n-Si Schottky diode with a very low concentration of process-induced donor trap, and the other is a phosphorus and gold diffused diode with a gold concentration about 20 percent of the boron concentration.     

A mechanism of charge transport in electroluminescent structures consisting of porous silicon and single-crystal silicon

Electroluminescent structures that emit in the visible region of the spectrum and are based on porous silicon (por-Si) formed on the p-Si substrate electrolytically using an internal current source are fabricated. The photoluminescent and electroluminescent properties, as well as the current-and capacitance-voltage characteristics of the structures are studied. Electroluminescence is observed only if the forward bias voltage is applied to the structure; the electroluminescence mechanism is based on the injection and is related to the radiative recombination of electrons and holes in quantum-dimensional Si nanocrystals. The injection of holes is controlled by the condition of their accumulation in the space-charge region of p-Si and by a comparatively low concentration of electronic states at the por-Si/p-Si interface. The charge transport in por-Si is caused by the direct tunneling of charge carriers between the quantum-mechanical levels, which is ensured by an appreciable number of quantum-dimensional Si nanocrystals. The leakage currents are low as a result of a small variance in the sizes of Si nanocrystals and the absence of comparatively large nanocrystals.     

High-frequency barrier capacitance of metal-semiconductor contacts and abrupt p-n junctions

Semiconductors - High-frequency capacitance of a Schottky barrier and an abrupt p-n junction are calculated taking into account the free charge carrier concentration in the space-charge region of a...     

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.     

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.     

Dependence of GaN photoluminescence on the excitation intensity

The dependence of the edge photoluminescence (PL) intensity on the excitation intensity in (0001) HVPE-grown GaN samples has been studied. The specific behavior found is that, at a low excitation level, the dependence is markedly superlinear, namely, superquadratic, and at high excitation levels it is nearly linear. A model accounting for the observed superquadratic behavior is proposed which is based on the identity of the recombination processes in the surface space charge region (SCR) under optical excitation with those in the SCR of the Schottky barrier or a p-n junction under current flow conditions. The superquadratic dependence is obtained analytically under the assumption that the nonradiative recombination channel is associated with multiple-hopping tunneling along a dislocation, which is formed by a chain of carrier localization centers and crosses the SCR. The experimental dependence of the PL intensity on the excitation intensity is a power-law function. The distance between the neighboring localization centers, i.e., the period of the potential along the dislocation, is determined as ～4.1 nm.     

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)     

Messung der ladungsträger-konzentrationsverteilung im mittelgebiet eines legierten silizium-psn-gleichrichters bei belastung in durchlassrichtung

An experimental set-up is described that enables the measurement of the concentration distribution of charge carriers which overflow the middle region of an alloyed silicon-psn-rectifier when loaded in the forward direction. The principle of the measurement is based on an experimentally verified relationship between the charge carrier concentration and the intensity of the recombination radiation emitted from the middle region. Examples are given for the development in time of the charge carrier concentration after turn-on and turn-off of a forward current and for the stationary distribution as a function of the height of the forward current.     

Bistable amphoteric centers in semiconductors

It is shown that, at thermodynamic equilibrium, the release of charge carriers from the localized states of bistable amphoteric centers into quasi-free states depends on the degree of compensation. This brings about different functional dependences of the concentration of free charge carriers on temperature. It is found that, in uncompensated semiconductors, the concentration of free charge carriers follows the same dependence in the case of bistable amphoteric centers and bistable amphoteric U ^? centers, although the distributions of charge carriers over the charge states and configurations are different for these types of centers. The results can be used for interpreting various experimental data insufficiently explained in the context of the traditional approach.     

Experimental 4<Emphasis Type="Italic">H</Emphasis>-SiC junction-barrier Schottky (JBS) diodes

4H-SiC junction-barrier Schottky (JBS) diodes have been fabricated with local p–n junctions under the Schottky contact formed by nonequilibrium diffusion of boron. Static and dynamic characteristics of the JBS diodes are compared with those of similar 4H-SiC Schottky diodes. It is shown that, compared with ordinary Schottky diodes, the JBS diodes have leakage currents that are, on average, a factor of 200 lower at the same reverse bias. The reverse recovery charge is the same for both types of diodes and equal to the charge of majority carriers removed from the n-type base region in switching.     

The role of local potential minima on charge transport in thin organic semiconductor layers

We have performed a systematic study of dependence of time-resolved photocurrent on the point of charge excitation within the organic semiconductor channel formed by two coplanar metal electrodes. The results confirm that spatial variation of electric field between the electrodes crucially determines transport of photogenerated charge carriers through the organic layer. Time-of-flight measurements of photocurrent demonstrate that the transit time of photogenerated charge carrier packets drifting between the two electrodes decreases with increasing travelling distance. Such counterintuitive result cannot be reconciled with the spatial distribution of electric field between coplanar electrodes, alone. It is also in contrast to expected role of space-charge screening of external electric field. Supported by Monte Carlo simulations of hopping transport in disordered organic semiconductor layer, we submit that the space-charge screens the external electric field and captures slower charge carriers from the photogenerated charge carrier packet. The remaining faster carriers, exhibit velocity distribution with significantly higher mean value and shorter transit time.     

Recombination beneath ohmic contacts and adjacent oxide covered regions

By modelling a contact as a Schottky barrier space-charge region it is possible to calculate the effective surface-recombination velocity which it displays. Such a model is of use in analysing the behaviour of thin emitter regions, and of contacts, or oxide covered regions in the extrinsic base of I²L vertical transistors.It is shown necessary to take account of both the doping and electric field dependence of the carrier mobility as well as the exact form of the potential at the boundary between the space-charge region and the substrate.The values of surface recombination velocity predicted by the model are in fair agreement with available data, and a tentative model for recombination beneath an oxide is proposed.     

Recombination in the space-charge region of Schottky barrier solar cells

The expression for the recombination rate as a function of voltage, of illumination level, and of position in the space-charge region of the semiconductor is derived analytically. The recombination current density is derived by numerical integration of the above expression. The results show good agreement with experiment for the typical Au-n-type Si near-ideal Schottky barrier solar cells, and the comparison provides information on the uncovering of deep recombination centers by the hole quasi-Fermi level under increasing illumination. It is found that the principal effect of recombination under illumination is the reduction of the photocurrent. A rather surprising but gratifying result is that, once the above effect is taken into account by using short-circuit currents rather than photocurrents, the remaining (voltage dependent) effect of recombination is extremely close to the one in the dark, provided the increase in “uncovered” recombination centers with illumination is taken into account.     

On the ohmicity of Schottky contacts

The conditions under which Schottky contacts become ohmic are analyzed. Proceeding from classical concepts of the mechanisms of current flow, a generalized Schottky contact model is investigated. This model takes into account the thermionic current of majority charge carriers and the recombination current of minority carriers in Schottky contacts with a dielectric gap. Based on an analysis of the predictions of this model, ohmicity criteria are obtained for Schottky contacts and the conditions for a low injection level and the ohmicity of Si-based Schottky contacts are compared. It is shown that the conditions for Schottky-contact ohmicity do not coincide with those for p–n junctions.     

Mechanisms of charge transport in anisotype <Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-CdTe heterojunctions

Surface-barrier anisotype n-TiO₂/p-CdTe heterojunctions are fabricated by depositing thin titanium-dioxide films on a freshly cleaved surface of single-crystalline cadmium-telluride wafers by reactive magnetron sputtering. It is established that the electric current through the heterojunctions under investigation is formed by generation-recombination processes in the space-charge region via a deep energy level and tunneling through the potential barrier. The depth and nature of the impurity centers involved in the charge transport are determined.