V. Rajagopal Reddy

Tetragonal site of transition metal ions doped sodium phosphate glasses

Abstract Vanadium and copper doped sodium phosphate glasses are prepared. EPR and optical absorption investigations are carried out. The results and analysis of the copper doped glass indicate that the copper ions enter the glass matrix as Cu 2+ ions. The Cu 2+ ion spectra are characteristic of tetragonally elongated octahedral site. The spectra of vanadium doped glass are characteristic of VO 2+ and because of the short VO band the distortion appears to be a tetragonally compressed one. Crystal field and spin-Hamiltonian parameters are evaluated. The bonding parameters indicate that bonding between the metal ions and ligands is partially covalent.

Analysis of the current-voltage characteristics of the Pd/Au Schottky structure on n-type GaN in a wide temperature range

We report on the temperature-dependent electrical characteristics of the Au/Pd/n-GaN Schottky diode in the temperature range of 90–410 K. The barrier heights and ideality factors of Schottky diodes were found in the range 0.23 eV and 3.5 at 90 K to 0.97 eV and 1.9 at 410 K, respectively. It was observed that the zero bias barrier height Φbo decreases and the ideality factor n increases with a decrease in temperature. Such behavior is attributed to barrier inhomogeneities by assuming a Gaussian distribution of barrier heights at the interface. The estimated values of series resistance (RS) are in the range of 636 Ω at 90 K to 220 Ω at 410 K using Cheungs method. Based on the above observations, the Φbo, n and RS values are seen to be strongly temperature dependent. The flat-band barrier height Φbf(T = 0 K) and temperature coefficient α were found to be 0.67 eV and 2.81 × 10−3 eV K−1, respectively. Further, the homogeneous barrier height is estimated from the linear relationship between temperature-dependent experimental effective barrier heights and ideality factors and the value is approximately 1.31 eV. The effective Richardson constant is determined to be 20.43 A cm−2 K−2 and is in good agreement with the theoretical value. It is concluded that the temperature-dependent I–V characteristics of the Au/Pd/n-GaN Schottky diode can be successfully explained on the basis of thermionic emission (TE) mechanism with the Gaussian distribution of the barrier heights.

Schottky Barrier Parameters and Interfacial Reactions of Rapidly Annealed Au/Cu Bilayer Metal Scheme on N-type InP

The influence of rapid thermal annealing effect on the electrical and structural properties of Au/Cu Schottky contacts on n-InP has been investigated by the current-voltage (I-V), capacitance-voltage (C-V), auger electron spectroscopy (AES) and the X-ray diffraction (XRD) techniques. As-deposited sample has a barrier height of 0.64 eV (I- V), 0.76 eV (C-V) which increases to 0.82 eV (I-V), 1.04 eV (C-V) after annealing at 400°C for 1min in nitrogen ambient. However, the barrier height decreases to 0.75 eV (I-V), 0.88 eV (C-V) after annealing at 600°C for 1min. Norde method is also employed to calculate the barrier heights of Au/Cu Schottky rectifiers and the values are 0.66 eV for as-deposited, 0.83 eV for 400°C and 0.76 eV for 600°C annealed contacts. These values are in good agreement with the values obtained from I-V method. From the above observations, it is clear that the Schottky barrier height increases with annealing temperatures up to 400°C. Thus, the optimum annealing temperature for the Au/Cu Schottky contact is 400 o C. Based on the AES and XRD results, the formation of indium phases at the Au/Cu/n-InP interface may be responsible for the increase in the barrier height for the contact annealed at 400°C and a corresponding decrease in leakage current. The decrease in the barrier height after annealing at 500 o C may be due to the formation of Cu-P and Au-P interfacial compounds at the interface. The AFM results showed that the surface morphology of Au/Cu Schottky contact is fairly smooth even after annealing at 500°C.

Electrical properties of Au/Bi0.5Na0.5TiO3-BaTiO3/n-GaN metal?insulator?semiconductor (MIS) structure

We investigated the electrical properties of solution processed high-k Bi0.5Na0.5TiO3(BNT)-BaTiO3(BT) on n-GaN with Au electrode. Higher barrier height is obtained for Au/BNT-BT/n-GaN structure compared to Au/n-GaN structure. Thin interfacial layer is formed in between BNT-BT and n-GaN confirmed by TEM results. The interface state density of Au/BNT-BT/n-GaN structure is lower than that of Au/n-GaN structure due to the existence of interfacial layer (Ga-O) at the interface. It is observed that the frequency dispersion is decreased in the Au/BNT-BT/n-GaN structure. Poole?Frenkel mechanism is found to dominate the reverse leakage current in both Au/n-GaN and Au/BNT-BT/n-GaN structures.

Microstructural, electrical and carrier transport properties of Au/NiO/n-GaN heterojunction with a nickel oxide interlayer

Nickel oxide (NiO) films are prepared on n-type GaN by an e-beam evaporation technique and its structural and chemical characteristics analysed by XRD, TEM and XPS measurements first at room temperature. XRD and TEM results reveal that the NiO films are oriented and that the NiO/n-GaN interface has a good quality. XPS analysis demonstrated that the NiO films clearly showed Ni 2p3/2 and 2p1/2 peaks at 854 eV and 872 eV along with the O 1s peak at ∼529.1 eV. Then, we fabricated an Au/NiO/n-GaN heterojunction Schottky diode with a NiO insulating layer and compared its electrical properties with the Au/n-GaN Schottky junction. The Au/NiO/n-GaN heterojunction presents excellent rectifying behaviour with a low reverse-leakage current compared to the Au/n-GaN Schottky junction. Calculation revealed that a higher barrier height is achieved for the Au/NiO/n-GaN heterojunction than for the Au/n-GaN Schottky junction, implying the barrier height was modified by the NiO insulating layer. Using Cheungs and Norde functions and an ΨS–V plot, the barrier heights are estimated and we found that the values are comparable with one another. The results suggest that the interface state density (NSS) of the Au/NiO/n-GaN heterojunction decreases compared to the Au/n-GaN Schottky junction, which indicates the NiO insulating layer plays a significant role in the reduced NSS. The results demonstrate that Poole–Frenkel emission governs the reverse leakage current in both junctions, which could be associated with structural defects and trap levels in the insulating layer.

The effect of annealing temperature on electrical and structural properties of Rh/Au Schottky contacts to n-type GaN

The effect of thermal annealing temperature on electrical and structural properties of Rh/Au Schottky contacts to n-type GaN (~4 × 1017 cm−3) has been investigated by current–voltage (I–V), capacitance–voltage (C–V), x-ray diffraction (XRD) and Auger electron microscopy (AES). Calculations showed that the Schottky barrier height of the as-deposited Rh/Au contact was 0.57 eV (I–V) and 0.62 eV (C–V), respectively. However, the Schottky barrier height increased with annealing temperature up to 500 °C, reaching maximum values of 0.84 eV (I–V) and 1.05 eV (C–V). Based on the Auger electron microscopy and x-ray diffraction results, the formation of gallide phases at the Rh/Au/n-GaN interface could be the reason for the increase of Schottky barrier heights after annealing at temperatures 400 °C and 500 °C.

Microstructural, electrical and frequency-dependent properties of Au/p-Cu2ZnSnS4/n-GaN heterojunction

An Au/Cu2ZnSnS4 (CZTS)/n-GaN heterojunction (HJ) is fabricated with a CZTS interlayer and probed its chemical states, structural, electrical and frequency-dependent characteristics by XPS, TEM, I-V and C-V measurements. XPS and TEM results confirmed that the CZTS films are formed on the n-GaN surface. The band gap of deposited CZTS film is found to be 1.55eV. The electrical properties of HJ correlated with the Au/n-GaN Schottky junction (SJ). The Au/CZTS/n-GaN HJ reveals a good rectification nature with high barrier height (0.82eV) compared to the Au/n-GaN SJ (0.69eV), which suggests the barrier height is influenced by the CZTS interlayer. The barrier height values assessed by I-V, Cheungs and Norde functions are closely matched with one other, thus the methods used here are reliable and valid. The extracted interface state density (NSS) of Au/CZTS/n-GaN HJ is lower compared to the Au/n-GaN SJ that suggests the CZTS interlayer plays an important role in the reduction of NSS. Moreover, the capacitance-frequency (C-f) and conductance-frequency (G-f) characteristics of SJ and HJ are measured in the range of 1kHz-1MHz, and found that the capacitance and conductance strappingly dependent on frequency. It is found that the NSS estimated from C-f and G-f characteristics is lower compared to those estimated from I-V characteristics. Analysis confirmed that Poole-Frenkel emission dominates the reverse leakage current in both SJ and HJ, probably related to the structural defects and trap levels in the CZTS interlayer.

Electrical Properties of Rapidly Annealed Ir and Ir/Au Schottky Contacts on n-Type InGaN

The effect of annealing temperature on electrical characteristics of iridium (Ir) and iridium/gold (Ir/Au) Schottky contacts to n-type InGaN have been investigated by means of current-voltage (I-V) and capacitance-voltage (C-V) techniques. It is observed that the barrier height of Ir/n-InGaN and Au/Ir/n-InGaN Schottky diodes increases after annealing at 300∘C for 1 min in N2 ambient compared to the as-deposited. However, the barrier heights are found to be decreased somewhat after annealing at 500∘C for the both Ir and Ir/Au Schottky contacts. From the above observations, it is clear that the optimum annealing temperature for both Ir and Ir/Au Schottky contacts is 300∘C. Moreover, the barrier height (𝜙𝑏), ideality factor (n) and series resistance (𝑅𝑆) are determined using Cheung’s and Norde methods. Besides, the energy distribution of interface state densities are determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height. Based on the above results, it is clear that both Ir and Ir/Au Schottky contacts exhibit a kind of thermal stability during annealing.

Electrical and structural properties of Ti/W/Au ohmic contacts on n-type GaN

The electrical and structural properties of Ti/W/Au ohmic contacts to moderately doped n-GaN (4.07 × 1018 cm−3) have been investigated. The as-deposited and annealing contacts at temperatures below 750 °C exhibit non-linear behaviour. However, the contact showed ohmic behaviour after annealing at a temperature in excess of 850 °C. Specific contact resistance as low as 8.4 × 10−6 Ω cm2 is obtained from the Ti(12 nm)/W(20 nm)/Au(50 nm) contact annealed at 900 °C for 1 min in N2 ambient. It is observed that annealing results in a large reduction (by ~160 meV) in the Schottky barrier height of the contacts, compared to the as-deposited one. The atomic force microscopy results showed that the surface morphology of the contact annealed at 900 °C is fairly smooth with a RMS roughness of 3.8 nm. Based on the Auger electron spectroscopy and glancing angle x-ray diffraction results, possible explanations are given to describe the annealing temperature dependence of the specific contact resistance of the contacts.

Modified electrical properties and transport mechanism of Ti/p-InP Schottky structure with a polyvinylpyrrolidone (PVP) polymer interlayer

This paper reviews the electrical and transport properties of Ti/polyvinylpyrrolidone (PVP)/p-InP metal/interlayer/semiconductor (MIS) Schottky diode and compared its results with the Ti/p-InP metal/semiconductor (MS) diode. Analysis results showed that the barrier height (BH) and ideality factor of the MIS diode are found to be improved compared to the MS diode. This indicates that the effective BH is modified by the PVP interlayer since it creates physical barrier between the Ti metal and the p-InP substrate. It is noted that the evaluated BH by Cheung’s and ψS-V plots is in good concurrence with one another. Also, the series resistance of the MS and MIS diodes is estimated by Cheung’s functions. The insertion of the PVP interlayer led to a decrease of the interface state density in the Ti/p-InP MS diode. In addition, the relevant junction mechanisms are explained by feasible energy level band diagrams. The Poole–Frenkel emission is the governing conduction mechanism in the Ti/p-InP MS diode. However, at lower voltage region the Poole–Frenkel is conquered, whereas at higher voltage region the Schottky emission is occupied for the Ti/PVP/p-InP MIS diode.