Evidence of space-charge-limited current in amorphous silicon Schottky diodes

Palladium/amorphous silicon (a-SiHx) Schottky barrier diodes have been found to exhibit superlinear dark forward current-voltage (I-V) characteristics over the temperature range 30 to 130°C. The results are consistent with expected behavior for space-charge-limited current in the presence of distributed traps. The trap parameters are deduced from I-V data.     

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     

Shot noise in silicon Schottky barrier diodes

Noise measurements have been performed on forward and reverse-biased silicon Schottky barrier diodes. Measurements were performed in the frequency range of 100 Hz to 50 kHz. Apart from excess noise observed for some diodes in a portion of this frequency range, the noise for the diodes was found to be in excellent agreement with shot-noise theory. Some refinements of the shot-noise theory have been considered, but the difference between the refined and the simple theories was not resolvable in our measurements. A useful noise-measurement technique is described.     

Theory and experiment for silicon Schottky barrier diodes at high current density

Metal-silicon Schottky barrier diodes exhibit n values which theoretically vary as a function of doping and applied voltage. The expected variation depends on which theoretical model is used to describe the current transport.Titanium n-type silicon barriers were prepared. At a doping level of 3 × 10¹⁵ cm^?3 the barrier height and n-value measured at 100 mV were 0.485±0.005 V and 1.02±0.01 whereas for a doping level of 2 × 10¹⁴ cm^?3 the corresponding values were 0.500±0.005 V and 1.18±0.05.The experimental variation of the diode n value as a function of semiconductor band bending showed good agreement with the thermionic-diffusion model of Crowell and Beguwala: n values increased rapidly as the band bending β → 2, and n values were highest at a given β for diodes with the lowest doping concentration. Similar results were obtained by measurements on magnesium and aluminium barriers on n-type silicon.An analysis of the results has shown that the variation of the diode saturation current I_s follows the predictions of the thermionic-diffusion theory, although there were some anomalies at high current densities. The anomalies did not result from variation of the width of the undepleted region of the epitaxial silicon layer or from diode self-heating effects.     

Temperature dependence of the reverse current in Schottky barrier diodes

The temperature dependences of the current I in reverse-biased Al/SiO₂/n-Si, Al/SiO₂/n-GaAs and Al/n-GaAs (with the native oxide) structures are measured. It is established that these dependences all have the property that the thermal activation energy decreases with increasing applied voltage and that at higher voltages the plots of ln I versus 1/T deviate from straight lines. The results can be explained on the basis of the fact that the current through the barrier is due to electron tunneling from surface states into the conduction band of the semiconductor. The field intensity in the Schottky barrier and the density of surface electron states in the interfacial layer of the semiconductor are estimated by comparing the experimental results with a tunneling theory that takes into account the effect of the semiconductor lattice phonons on the tunneling probability. Fiz. Tekh. Poluprovodn. 32, 882–885 (July 1998)     

Low-frequency excess noise in metal—Silicon Schottky barrier diodes

Theoretical models for the generation-recombination noise and trapping noise in metal-semiconductor Schottky barrier diodes are developed. Low-frequency excess noise in Schottky barrier diodes is found to be dominated by the modulation of the barrier height φB caused by fluctuation in the charge state of traps or generation-recombination centers. This noise mechanism does not occur in p-n junctions. The bias and the temperature dependence of the generation-recombination noise is critically compared with the experimental data for forward diode current ranges from 3 to 300 µA and operating temperatures from -25° to 100°C. Trapping noise in Schottky barrier diodes is observed at low temperatures in diodes not intentionally doped with deep level impurities. The experimental results on trapping noise can be described by assuming that the trap states have a constant capture cross section and are uniformly distributed in space, as well as in energy. The surface potential at the diode periphery also has an important effect on the Schottky barrier diode noise. The best low-frequency noise behavior is found when the surface is at the flat-band condition. An accumulated surface is always associated with a large amount of low-frequency excess noise.     

Thermal degeneration of Mo and Pt silicon Schottky diodes

In order to gain thermal stability and process compatibility, Mo and Pt silicon Schottky diodes have been annealed in an hydrogen atmosphere. A clear degeneration of the rectifying characteristics has been observed upon 450°C for Mo and 650°C for Pt.Although metal or defect diffusion could not be detected by transient capacitance measurements, the annealed devices did show anomalously high currents at low bias and at low temperatures. This excess current can be tentatively explained in terms of resonant tunnelling via deep centers near the interface metal-semiconductor induced by the annealing.The difference in the critical temperature for degeneration has its origin in the different behaviour of both barrier metals with silicon. While, in the case of platinum it seems that the silicide formation takes place at temperatures below 650°C with a subsequent diffusion of platinum or silicon vacancies into the silicon, in the case of molybdenum, the defect or metallic difusion apparently occurs before the silicide formation.     

Schottky diodes and other devices on thin silicon membranes

The fabrication of ultra-thin silicon membranes of micron or submicron sizes with thicker supporting frames makes possible improvements in several kinds of electronic devices. We show that the series resistances of some devices can be reduced significantly, thus effecting increases in the cutoff frequencies. The resistance swing of Schottky diodes can also be raised. Initial experimentation relating to some of these applications is presented.     

Behavior of various silicon Schottky barrier diodes under heat treatment

Variations of Schottky forward voltage with heat treatment have been observed for AlPd₂Si Si and Si-doped AlPd₂Si Si Schottky barrier diodes. These variations have been reduced to a minimum by the use of a barrier metal such as TiW between Al and the silicided contacts.     

Computer calculations of avalanche-induced relaxation oscillations in silicon diodes

A digital computer program has been employed to study relaxation oscillations of a silicon p-i-n junction diode reverse-biased into avalanche. The computed plots of diode current and voltage versus time and of diode current versus diode voltage are in good agreement with those obtained experimentally by Hoefflinger. The electric field and charge density distributions across the junction are given for selected times during the breakdown and current decay phases of the oscillations. The electric field is characterized by the formation of a saddle during the avalanche buildup. The variations of power output, efficiency, frequency, and average diode voltage are presented as a function of average current for one particular set of diode and circuit parameters.     

Effects of the undoped layer on characteristics of amorphous silicon Schottky diodes

The effects of undoped layer thickness on the dark and illuminated I-V characteristics of hydrogenated amorphous silicon Schottky barrier solar cells are investigated. Schottky barrier (S.B.) metals having different work function (Cr and Pd) were deposited on the 0.22 µm - 1.45 µm thick a-Si:H films. Photovoltaic performance, Jsc, Voc, FF and efficiency, are independent of thickness of the undoped layer if film thickness is larger than the depletion region width. Jscand Vocare controlled by S.B. metal and FF is independent of S.B. metal. Dark I-V characteristics depend on both S.B. metal and device thickness suggesting a barrier controlled space charge limited phenomena. Variation of turn-on (threshold) voltage with undoped layer thickness can be applied to the design of switching and memory devices.     

High-barrier Schottky diodes on p-type silicon due to dry-etching damage

It is known that the barrier height of Schottky diodes made to dry-etched silicon surfaces deviate from the barrier height values obtained for diodes fabricated on wet chemically etched or cleaved silicon. This effect, in cases where neither a substantial residue layer nor a surface film is formed, can be exploited to yield diodes on p-type Si that display barrier enhancement together with excellent diode ideality factors. It is shown that the barrier heights produced on p-type Si, by exploiting this effect of dry etching, can achieve a value of ∼0.75 eV which is ∼0.15 eV better than the best value reported for wet chemically etched or cleaved p-Si. When this barrier height value is attained, it is found to be independent of metallization. The same barrier height is achieved by two very different dry etching techniques: Ar⁺ion-beam etching (IBE) and CCl4reactive ion etching (RIE).     

Device processing and characterisation of high temperature silicon carbide Schottky diodes

High temperature silicon carbide diodes with nickel silicide Schottky contacts were fabricated by deposition of titanium-nickel metal film on 4H-SiC epitaxial wafer followed by annealing at 550 °C in vacuum. Room temperature boron implantation have been used to form single zone junction termination extension. 4H-SiC epitaxial structures designed to have theoretical parallel-plain breakdown voltages of 1900 and 3600 V have been used for this research. The diodes revealed soft recoverable avalanche breakdown at voltages of 1450 and 3400 V, respectively, which are about 80% and 95% of theoretical values. I-V characteristics of fabricated 4H-SiC Schottky diodes have been measured at temperatures from room temperature up to 400 °C. The diodes revealed unchangeable barrier heights and ideality factors as well as positive coefficients of breakdown voltage.     

New way of plotting current/voltage characteristics of Schottky diodes

It is pointed out that the empirical diode relationship I = I0 exp (qV/nkT){1 - exp (-qV/kT)} has the property that a plot of log [I/{1 - exp (-qV/kT)}] against V should be linear for all values of V, including reverse voltages. This equation has been tested by making such a plot for an Al/GaAs Schottky diode made by MBE. The plot is linear over the entire range from +0.5 V to - 1.0 V, with n = 1.01.     

The influences of traps on the generation-recombination current in silicon diodes

The recombination current in the space charge region of an abrupt Si P-N junction is calculated under the assumption that the electrochemical potentials or quasi-Fermi levels for the majority carriers are constant throughout the bulk and space-charge regions, and that the Hall-Shockley-Read centers are distributed uniformly with respect to position in the space-charge region, but can be either a single, discrete level or a continuous and uniform distribution with respect to energy. The results for the single level show that the slope can have constant value over a wide range of applied forward bias. These values are mainly dependent on the trap level position and are nearly independent of the minority carrier lifetime at a given doping concentration. They are also dependent on the lower doping concentration. Furthermore, very good linearity in the characteristic curve can be obtained if we assume that the recombination centers are uniformly distributed in energy throughout the energy gap.     

The effect of an interfacial layer on minority carrier injection in forward-biased silicon Schottky diodes

It is shown that the degree of minority carrier injection in Au-Si junctions can be substantially increased by the inclusion of a thin interfacial layer between the metal and the semiconductor. When a forward voltage is applied to the junction, a part of this voltage is developed across the interfacial layer. This favours the reduction of the barrier height to minority carriers, which tunnel from the metal into the semiconductor. The minority carrier injection current increases at the expense of the majority carrier current.For a given oxide thickness, γ(= J_minority/J_tot) increases with forward bias, approaching a saturation value for a few volts applied to the junction. For a given voltage, γ also shows a variation with interfacial layer thickness, δ, and the present results indicate that an oxide thickness can be chosen to optimise γ. In the case of a gold-silicon junction with an insulating layer of thermally-grown oxide, as δ is increased to 40 Å, the saturation value of γ increased from $\text{10}{}^{\mathrm{?4}}\text{for δ = 10}\text{A}\text{?}$ through a maximum of 2 × 10^?1 for δ ? 30 Å. For oxides prepared by r.f. sputtering, the maximum value of γ is 10^?1 and occurs for δ ? 80 Å.These results are of considerable importance in the improvement of injection luminescence in metal-semiconductor diodes.     

Tunnelling transport in Al-n-GaSb Schottky diodes

Investigations of Al Schottky contacts to n-GaSb are presented. Barrier heights of 0.60±0.02 eV are found. Forward bias ideality factors between 2 at 300 K to 60 at 10 K, are explained by electron tunneling. Ideality factors yield donor concentrations significantly greater than nominal, accentuated by annealing, suggesting modification of the reactive GaSb surface     

Low-frequency noise in Schottky barrier diodes

The low-frequency excess noise in Schottky barrier diodes has been investigated. In the ideal case where the saturation current is completely determined by thermionic emission of electrons, no 1/? noise will be produced in the barrier. The presence of trap states in the depletion region can lead to generation-recombination noise. At sufficient high forward currents 1/? noise can be generated in the series resistance of the Schottky diode. Deviations from the ideal diode, for example as a result of edge effects, produce 1/? noise and increase at the same time the ideality factor. It is empirically found that the 1/? noise level decreases very rapidly if the ideality factor tends to unity.     

Barrier-height modification in Schottky silicon diodes with highly doped 3D and 2D layers

The possibility of controlling the effective barrier height in Schottky diodes based on silicon structures grown by the molecular-beam-epitaxy method is experimentally investigated. It is shown that control of the effective barrier height is possible both when using heavily doped surface homogeneous (3D) layers (??10²⁰ cm^?3) and surface (2D) ?? layers (??10¹³ cm^?2), which provide tunnel transmission of the current through the barrier on the metal-semiconductor interface. The dependences of the effective barrier height on the parameters of the heavily doped layers are investigated. The performed simulation of electron transport in the structures makes it possible to qualitatively explain the observed experimental results.     

Correlation of the leakage current and charge pumping in silicon oninsulator gate-controlled diodes

The leakage and charge pumping currents were measured in gate-controlled MOS p-i-n diodes fabricated on thin SIMOX substrates. The efficiencies of the techniques as well as their complementary features are analyzed for various experimental conditions. The interface properties of device-grade SIMOX wafers are characterized and shown to be compatible with VLSI requirements. Special interface coupling effects, which occur only in fully depleted SOI devices and modify the conventional signature of charge pumping and leakage current, are thoroughly investigated