V. N. Sheremet

Mechanism of dislocation-governed charge transport in schottky diodes based on gallium nitride

A mechanism of charge transport in Au-TiBx-n-GaN Schottky diodes with a space charge region considerably exceeding the de Broglie wavelength in GaN is studied. Analysis of temperature dependences of current-voltage (I–V) characteristics of forward-biased Schottky barriers showed that, in the temperature range 80–380 K, the charge transport is performed by tunneling along dislocations intersecting the space charge region. Estimation of dislocation density ρ by the I–V characteristics, in accordance with a model of tunneling along the dislocation line, gives the value ρ ≈ 1.7 × 107 cm−2, which is close in magnitude to the dislocation density measured by X-ray diffractometry.

Mechanism of contact resistance formation in ohmic contacts with high dislocation density

A new mechanism of contact resistance formation in ohmic contacts with high dislocation density is proposed. Its specific feature is the appearance of a characteristic region where the contact resistance increases with temperature. According to the mechanism revealed, the current flowing through the metal shunts associated with dislocations is determined by electron diffusion. It is shown that current flows through the semiconductor near-surface regions where electrons accumulate. A feature of the mechanism is the realization of ohmic contact irrespective of the relation between the contact and bulk resistances. The theory is proved for contacts formed to III-V semiconductor materials as well as silicon-based materials. A reasonable agreement between theory and experimental results is obtained.

The mechanism of contact-resistance formation on lapped n-Si surfaces

Anomalous temperature dependences of the specific contact resistance ρc(T) of Pd2Si-Ti-Au ohmic contacts to lapped n-Si with dopant concentrations of 5 × 1016, 3 × 1017, and 8 × 1017 cm−3 have been obtained. The anomalous dependences of ρc(T) have been accounted for under the assumption that the current flows along nanodimensional metallic shunts, which are combined with dislocations with a diffusionrelated limit in the supply of charge carriers taken into account. The densities of conducting and scattering dislocations in the surface region of the semiconductor are determined.

Temperature dependence of contact resistance for Au-Ti-Pd2Si-n+-Si ohmic contacts subjected to microwave irradiation

Based on a theoretical analysis of the temperature dependence of the contact resistance Rc for an Au-Ti-Pd2Si-n+-Si ohmic contact, a current-transfer mechanism explaining the experimentally observed increase in Rc in the temperature range 100–380 K is proposed. It is shown that microwave treatment of such contacts results in a decrease in the spread of Rc over the wafer and a decrease in the value of Rc whilst retaining an increase in Rc in the temperatures range 100–380 K.

Temperature Dependence of the Contact Resistance of Ohmic Contacts to III-V Compounds with a High Dislocation Density

A new mechanism describing the rise in the contact resistance ρc of ohmic contacts to n-n+-n++-GaAs(GaP, GaN, InP) structures with increasing measurement temperature T, experimentally observed in the temperature range 100–400 K, is suggested on the basis of a theoretical analysis of the temperature dependence of ρc. Good agreement between the experimental and theoretical ρc(T) dependences is obtained and explained for a case where there is a high density of dislocations (on which metallic shunts are localized) in the near-contact region of the semiconductor.

Features of temperature dependence of contact resistivity in ohmic contacts on lapped n-Si

The temperature dependence of contact resistivity ρc in lapped silicon specimens with donor concentrations of 5 × 1016, 3 × 1017, and 8 × 1017 cm−3 was studied experimentally. We found that, after decreasing part of the ρc(T) curve in the low temperature range, an increasing part is registered with increasing temperature T. It is demonstrated that the formation of contact to a lapped Si wafer results in the generation of high dislocation density in the near-surface region of the semiconductor and also in ohmic contact behavior. In this case, current flows through the metal shunts associated with dislocations. The theory developed is in good agreement with experimental results.

Radiation effects and interphase interactions in ohmic and barrier contacts to indium phosphide as induced by rapid thermal annealing and irradiation with γ-ray 60Co photons

The radiation resistance of Au-Pd-Ti-Pd-n++-InP ohmic contacts and Au-TiBx-n-n+-n++-InP barrier contacts—both initial and subjected to a rapid thermal annealing and irradiated with 60Co γ-ray photons with doses as high as 109 R—has been studied. Before and after external effects, the electrical characteristics of the barrier and ohmic contacts, distribution profiles for components, and phase composition in the metallization layers have been measured. In ohmic Pd-Ti-Pd-Au contacts subjected to rapid thermal annealing and irradiation, a significant distortion of the layered structure of metallization occurs; this distortion is caused by the thermal and irradiation-stimulated transport of Pd over the grain boundaries in polycrystalline Ti and Au films. However, the specific contact resistance ρc does not change appreciably, which is related to a comparatively unvaried composition of the contact-forming layer at the Pd-n+-InP interface. In the initial sample and the sample subjected to the rapid thermal annealing at T = 400°C with the Au-TiBx-n-n+-n++-InP barrier contacts and irradiated with the dose as high as 2 × 108 R, a layered structure of metallization is retained. After irradiation with the dose as high as 109 R, in the samples subjected to a rapid thermal annealing at T = 400°C, the layered structure of metallization becomes completely distorted; however, this structure is retained in the initial sample. The electrical properties of the contact structure appreciably degrade only after irradiation of the sample preliminarily subjected to a rapid thermal annealing at T = 400°C.

Mechanism of current flow in a Au-Ti-Al-Ti-n+-GaN ohmic contact in the temperature range of 4.2–300 K

The temperature dependence of the contact resistivity ρc(T) of Au-Ti-Al-Ti-n+-GaN ohmic contacts is studied experimentally and substantiated theoretically in the temperature range T = 4.2–300 K. It is shown that the saturation portion of ρc(T) is observed in the low-temperature measurement region (4.2–50 K). As the temperature increases, ρc decreases by the exponential law. The experimental and calculated dependences ρc(T) are in agreement. The obtained results make it possible to conclude the field nature of the current transfer for the saturation region of ρc(T) and the thermal-field one, for the exponential region.

Radiation damage of contact structures with diffusion barriers exposed to irradiation with 60Coγ-ray photons

The effect of ionizing radiation of 60Co γ-ray photons in the dose range 104–2 × 109 rad on metal-semiconductor Au-ZrBx-AlGaN/GaN and Au-TiBx-Al-Ti-n-GaN contacts and Au-ZrBx-n-GaN Schottky diodes is examined. The contacts with the TiBx and ZrBx diffusion barriers do not degrade under the effect of ionizing radiation if the dose does not exceed 108 rad. The Au-ZrBx-n-GaN Schottky diodes remain stable in the dose range 104–106 rad. As the radiation dose is increased to ≳108 rad, the damage to the contact metallization increases and is accompanied by formation of through pores, which is conducive to accumulation of oxygen at the Au-ZrBx(TiBx) interfaces and to an increase in mass transport of atoms in contact-forming layers. In this case, irradiation-caused degradation of the Schottky diodes is observed. Possible mechanisms of radiation damage of contact structures with diffusion barriers are analyzed.

Formation mechanism of contact resistance to III–N heterostructures with a high dislocation density

The temperature dependences of the contact resistance ρc(T) of ohmic Pd-Ti-Pd-Au contacts to n-GaN and n-AlN wide-gap semiconductors with a high dislocation density are studied. The dependences ρc(T) for both contacts contain portions of exponential decrease ρc(T) and very weak dependence ρc(T) at higher temperatures. Furthermore, a plateau portion ρc(T) is observed in the low-temperature region for the Au-Pd-Ti-Pd-n-GaN contact. This portion appears only after rapid thermal annealing (RTA). In principle, the appearance of the plateau portion can be associated with preliminary heavy doping of the near-contact region with a shallow donor impurity and with doping during contact fabrication as a result of RTA, if the contact-forming layer contains a material that is a shallow donor in III–N. The dependences obtained are not explained by existing charge-transport mechanisms. Probable mechanisms explaining the experimental dependences ρc(T) for ohmic contacts to n-GaN and n-AlN are proposed.