Inhomogeneous Diodes Research Articles

Nonlinear Properties of an Inhomogeneous Diode Structure in a Strong Microwave Field

Results of experimental investigation of detection (rectification) of high power X-band microwave signal in diodes of various design (semiconductor p-n-junction, point-contact, Schottky, Metal-Isolator-Metal—MIM) are reported. The maximum of the detected direct voltage V vs. power P of microwave signal and subsequent polarity reversal, previously found in MIM diodes in the optical and microwave bands, have found to be characteristic of all investigated diodes as well. After the reversal of polarity, this dependence comes linear, and the sign of the voltage corresponds to thermoEMF. In some diodes, the hysteresis on V(P) was observed. All 5 types of V(P) of MIM diodes (have made from different pairs of metals), reported earlier, were reproduced on same p-n-junction diode by variable external DC bias. These results joined with abnormal frequency cutoff forced to suggest that there is an unknown mechanism for direct flow of charge carriers (and for generate direct current) in the high-frequency electrical field, which differs from the conventional rectification.

Intersection of 4H-SiC Schottky diodes I–V curves due to temperature dependent series resistance

We theoretically and experimentally analyzed the non-obvious intersections of Schottky diode I–V curves measured at different temperatures caused by increasing the series resistance of the diode with increasing temperature. We considered a homogeneous diode and an inhomogeneous diode with two ways of influencing the I–V curve by the series resistance. In each case we developed a numerical method that enabled anticipation of the I–V intersection point. We studied the Ni/Au/4H-SiC diode for which such an intersection was measured. For homogeneous diodes and temperature interval 300–400 K we found a voltage dispersion of intersection points of only ∼0.002 V, which is in accordance with experimental observations and suppositions in the literature that the curves intersect at almost the same I–V point. Even for an inhomogeneous diode with a common series resistance we obtained a dispersion of the intersection voltage of only ∼0.02 V which is hardly discernible by the common visualization of the I–V curves. The largest dispersion of intersection points was obtained for an inhomogeneous diode composed of non-interacting diode patches.

Asymmetric behavior of threshold pump power of Nd:YAG diode end-pumped laser in critical cavity configurations

The essential asymmetry in the dependences of threshold pump power on the cavity length of an Nd:YAG laser in degenerate (critical) cavity configurations under inhomogeneous diode end-pumping was investigated experimentally and theoretically. These dependences were numerically calculated using an iterative method based on the expansion of the field in Laguerre–Gaussian modes of a cold cavity. A complex gain profile, representing not only active medium gain but also phase distortions due to electronic and/or nonstationary thermal lenses, was introduced into the numerical calculations to obtain the asymmetric threshold pump power dependences on the cavity length in critical cavity configurations. In the case of the Nd:YLF crystal the threshold dependences, observed under the same experimental conditions, were symmetric and could be explained with pure real gain profile.

An Investigation of SiC Schottky Contact Barrier Inhomogeneity for Temperature Sensing Applications

This paper proposes a method of characterizing silicon carbide Schottky diodes with inhomogeneous contacts in temperature sensing applications. Using the energy activation technique, temperature intervals where the effective barrier height is constant are determined. Unlike the conventional barrier which increases with temperature for inhomogeneous diodes, the effective barrier has physical meaning and can be used for sensor performance evaluation. The utility of effective barrier analysis is confirmed on fabricated Ni/4H-SiC Schottky diodes with different annealing conditions and different degrees of barrier non-uniformity. The good agreement between calculated and experimental data proves the suitable behavior of inhomogeneous diodes as sensors for different temperature ranges.

Lasing threshold of Nd-doped crystal, ceramic, and glass lasers under spatially inhomogeneous diode pumping

The effect of transverse mode locking on the dependence of the threshold pump power on the cavity length in Nd-doped media lasers with longitudinal inhomogeneous diode pumping is experimentally and theoretically studied in the cavity stability region. An essentially nonmonotonic behavior of the dependence of the threshold pump power on the cavity length is detected. The dependences of the threshold pump power on the cavity length are calculated using the method based on the expansion of the complex field amplitude in Laguerre-Gaussian modes of a cold cavity. The best agreement with calculated results is observed for lasers with Nd:YLF active elements.

Inhomogeneous electrical characteristics in 4H-SiC Schottky diodes

Hundreds of current–voltage (I–V) measurements of Ni, Pt and Ti Schottky diodes on 4H–SiC were conducted at low applied voltages. The SiC substrates contained homoepitaxial layers grown by either chemical vapor deposition or sublimation. While near-ideal contacts were fabricated on all samples, a significant percentage of diodes (∼7%–50% depending on the epitaxial growth method and the diode size) displayed a non-ideal, or inhomogeneous, barrier height. These ‘non-ideal’ diodes occurred regardless of growth technique, pre-deposition cleaning method, or contact metal. In concurrence with our earlier reports in which the non-ideal diodes were modeled as two Schottky barriers in parallel, the lower of the two Schottky barriers, when present, was predominantly centered at one of the three values: ∼0.60, 0.85 or 1.05 eV. The sources of these non-idealities were investigated using electron-beam-induced current (EBIC) and deep-level transient spectroscopy (DLTS) to determine the nature and energy levels of the defects. DLTS revealed a defect level that corresponds with the low- (non-ideal) barrier height, at ∼0.60 eV. It was also observed that the I–V characteristics tended to degrade with increasing deep-level concentration and that inhomogeneous diodes tended to contain defect clusters. Based on the results, it is proposed that inhomogeneities, in the form of one or more low-barrier height regions within a high-barrier height diode, are caused by defect clusters that locally pin the Fermi level.

Inhomogeneities in Ni∕4H-SiC Schottky barriers: Localized Fermi-level pinning by defect states

We investigated arrays of Ni, Pt, or Ti Schottky diodes on n-type 4H-SiC epitaxial layers using current-voltage (I-V) measurements, electron beam induced current (EBIC), polarized light microscopy, x-ray topography, and depth-resolved cathodoluminescence spectroscopy. A significant percentage of diodes (∼7%–30% depending on epitaxial growth method and diode size) displayed “nonideal” or inhomogeneous barrier height characteristics. We used a thermionic emission model based on two parallel diodes to determine the barrier heights and ideality factors of high- and low-barrier regions within individual nonideal diodes. Whereas high-barrier barrier heights increased with metal work function, low-barrier barrier heights remained constant at ∼0.60, 0.85, and 1.05eV. The sources of these nonidealities were investigated with a variety of spectroscopic and imaging techniques to determine the nature and energy levels of the defects. EBIC indicated that clusters of defects occurred in all inhomogeneous diodes. Cathodoluminescence spectra revealed additional peaks in the nonideal diodes at 2.65, 2.40, and 2.20eV, which complement the low-barrier barrier heights. It is proposed that defect clusters act to locally pin the Fermi level, creating localized low-barrier patches, which account for the inhomogeneous electrical characteristics.

OCVD carrier lifetime measurements on an inhomogeneous diode structure

This paper investigates the problem of evaluating the lifetime of a carrier measured by the OCVD method on structures with a non-uniform carrier lifetime distribution. A simple model of two diodes connected in parallel (lumped charge approximation) has been used for evaluating the measured carrier lifetime. The theoretical analysis was experimentally verified.

C– V dependence of inhomogeneous Schottky diodes

We have studied the C– V dependence of inhomogeneous Schottky diodes by numerical simulation using Poisson's equation and the current continuity equations under the assumption of quasi-static approximation. Inhomogeneous barriers were modelled by a Gaussian barrier height distribution with two different space distributions of particular patches with different barrier heights. The results are compared with the simulations of homogeneous diodes. As expected, only a small difference has been found between the reverse bias C– V curves and the extracted barrier height of homogeneous diodes and inhomogeneous diodes simulated with the mean barrier height of the inhomogeneous diode equal to the barrier height of the homogeneous diode. A capacitance peak was observed for both cases in forward bias. The homogeneous and inhomogeneous diodes differ in the peak height.

Correlation of the interfacial structure and electrical properties of epitaxial silicides on Si

The Schottky barrier height (SBH) measured at epitaxial NiSi2 contacts is shown to be strongly influenced by the local interfacial structure. Diodes with guard rings, which eliminate the edge leakage current which has hampered some previous studies, were prepared by molecular-beam epitaxy of NiSi2 films on a wide range of n- and p-type doped Si substrates. Schottky diodes processed with high quality NiSi2 films on Si(111) are shown to exhibit highly ideal electrical properties. Structural inhomogeneity at the NiSi2/Si(100) interface is shown to lead to electrical properties which are consistent with the existence of spatial inhomogeneity in the SBH. Numerical simulations of the electron transport across the inhomogeneous diodes, based on a spatial variation of the interfacial SBH consistent with the observed structural inhomogeneity in the films, are shown to accurately reproduce the trends in the electrical behavior of the inhomogeneous diodes. The necessity to understand the interfacial atomic structure at metal/semiconductor contacts in order to understand the formation mechanism of the Schottky barrier is stressed by these results.