Electrical Properties and Carrier Transport Mechanism of Au/n-GaN Schottky Contact Modified Using a Copper Pthalocyanine (CuPc) Interlayer

Janardhanam, V.,Jyothi, I.,Lee, J.-H.,Kim, J.-Y.,Reddy, V.R.,Choi, C.-J. JAPAN INSTITUTE OF METALS 2014 MATERIALS TRANSACTIONS Vol.55 No.5

We investigated the electrical characteristics of Au/n-GaN Schottky rectifier incorporating a copper pthalocyanine (CuPc) interlayer using current voltage (I-V), capacitance voltage (C-V) and conductance voltage (G-V) measurements. The barrier height of the Au/CuPc/n-GaN Schottky contact was higher than that of the Au/n-GaN Schottky diode, indicating that the CuPc interlayer influenced the space charge region of the n-GaN layer. The series resistance of Au/CuPc/n-GaN Schottky contact extracted from the C-V and G-V methods was dependent on the frequency. In addition, the series resistance obtained from C V and G V characteristics was comparable to that from Cheung's method at sufficiently high frequencies and in strong accumulation regions. The forward log / log V plot of Au/CuPc/n-GaN Schottky contact exhibited four distinct regions having different slopes, indicating different conduction mechanisms in each region. In particular, at higher forward bias, the trap-filled space-charge-limited current was the dominant conduction mechanism of Au/CuPc/n-GaN Schottky contact, implying that the most of traps were occupied by injected carriers at high injection level.

Influence of rapid thermal annealing on electrical and structural properties of double metal structure Au/Ni/n-InP (1 1 1) diodes

M. Bhaskar Reddy,V. Janardhanam,A. Ashok Kumar,V. Rajagopal Reddy,P. Narasimha Reddy 한국물리학회 2010 Current Applied Physics Vol.10 No.2

The effect of rapid thermal annealing on the electrical and structural properties of Ni/Au Schottky contacts on n-InP have been investigated by current–voltage (I–V), capacitance–voltage (C–V), auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The Au/Ni/n-InP Schottky contacts are rapid thermally annealed in the temperature range of 200–500 ℃ for a duration of 1 min. The Schottky barrier height of as-deposited Ni/Au Schottky contact has been found to be 0.50 eV (I–V) and 0.86 eV (C–V),respectively. It has been found that the Schottky barrier height decreased with increasing annealing temperature as compared to as-deposited sample. The barrier height values obtained are 0.43 eV (I–V),0.72 eV (C–V) for the samples annealed at 200 ℃, 0.45 eV (I–V) and 0.73 eV (C–V) for those at 400 ℃. Further increase in annealing temperature to 500 ℃ the barrier height slightly increased to 0.46 eV (I–V) and 0.78 eV (C–V) compared to the values obtained for the samples annealed at 200 ℃ and 400 ℃. AES and XRD studies showed the formation of indium phases at the Ni/Au and InP interface and may be the reason for the increase in barrier height. The AFM results showed that there is no significant degradation in the surface morphology (rms roughness of 1.56 nm) of the contact even after annealing at 500 ℃.

Rapid Thermal Annealing Effects on the Electrical, Structural and Morphological Properties of Yb/p-type InP Schottky Structure

V. Rajagopal Reddy,D. Sri Silpa,V. Janardhanam,윤형중,최철종 대한금속·재료학회 2015 ELECTRONIC MATERIALS LETTERS Vol.11 No.1

The electrical, structural and surface morphological properties of Yb/p-InPSchottky barrier diode (SBD) have been investigated at different annealingtemperatures. The determined Schottky barrier height (SBH) and idealityfactor n of the as-deposited Yb/p-InP SBD are 0.68 eV (I-V)/0.81 eV(C-V)and 1.44 respectively. After annealing at 300°C, the SBH of Yb/p-InP SBDincreases to 0.72 eV (I-V)/0.88 eV (C-V). When the contact is annealed at400°C, the SBH slightly decreases to 0.67 eV (I-V)/0.80 eV (C-V). Theseresults reveal that the optimum annealing temperature for Yb/p-InP SBD is300°C. Cheung’s functions are also employed to determine the seriesresistance of the Yb/p-InP SBD. Using Terman’s method, the interface statedensity is estimated for Yb/p-InP SBD at different annealing temperatures. The XPS results reveal that the existence of phosphorous-rich surface afterthe annealing. The AES and XRD results showed that the formation ofphosphide phases at the Yb/p-InP interface may be the reason for theincrease of SBH after annealing at 300°C. The decrease in the BH afterannealing at 400°C may be due to the formation of indium phases at theinterface. The overall surface morphology of the Yb Schottky contact isfairly smooth at elevated temperatures.

Electrical and frequency-dependent properties of Au/Sm2O3/n-GaN MIS junction with a high-k rare-earth Sm2O3 as interlayer

V. Manjunath,V. Rajagopal Reddy,P.R. Sekhar Reddy,V. Janardhanam,최철종 한국물리학회 2017 Current Applied Physics Vol.17 No.7

High-k rare-earth samarium oxide (Sm2O3) films are formed on n-GaN surface and analyzed its compositional properties by XPS measurements. XPS results specify that the Sm2O3 films are formed at the interface. Then, the Au/Sm2O3/n-GaN MIS junction is prepared with a Sm2O3 as insulating layer and correlated its electrical properties with the Au/n-GaN MS junction. The MIS junction shows highest barrier height ((0.81 eV (I-V)/1.0 eV (C-V)) for MIS junction than the MS junction (0.68 eV (I-V)/0.90 eV (C-V)). Excellent rectifying property is observed with lowest reverse leakage current and higher barrier height for the MIS junction than the MS junction, implying that the Sm2O3 insulating layer effectively modified the barrier height. The barrier heights determined from I-V, Cheung's, Norde and JS eV plot closely matched with each other, suggesting that these techniques are reliable and valid. The estimated interface state density of the MIS junction (1.990 1011 cm2eV1 (EC-0.82 eV)) is lower than the MS junction (9.204 1012 cm2eV1 (EC-0.70 eV)), which demonstrates that the Sm2O3 insulating layer performs an important role in lowering the interface state density. The frequency-dependent characteristics of the MS and MIS junctions are discussed in the frequency range of 10 kHz to 1 MHz and found that the determined capacitance values decrease with increasing frequency. The forward I-V characteristic of the MS and MIS junctions reveals the ohmic behavior at low voltage regions and space-chargelimited conduction at higher voltage regions. Results reveal that the reverse leakage current in the studied MS and MIS junctions is controlled by a Poole-Frenkel emission.

Double Gaussian barrier distribution of permalloy (Ni_0.8Fe_0.2) Schottky contacts to n-type GaN

Janardhanam, V.,Jyothi, I.,Sekhar Reddy, P.R.,Cho, Jaehee,Cho, Jeong-Mook,Choi, Chel-Jong,Lee, Sung-Nam,Rajagopal Reddy, V. Elsevier 2018 Superlattices and microstructures Vol.120 No.-

<P><B>Abstract</B></P> <P>The temperature-dependent current-voltage (<I>I-V</I>) characteristics of permalloy (Ni<SUB>0.8</SUB>Fe<SUB>0.2</SUB>) Schottky contacts to n-type GaN have been investigated. Magnetization measurements revealed the ferromagnetic behavior of Ni<SUB>0.8</SUB>Fe<SUB>0.2</SUB> film on n-type GaN. The Schottky barrier parameters, such as the barrier height and ideality factor, determined by thermionic emission depended on the measurement temperature, suggesting the presence of lateral inhomogeneity in the Schottky barrier. The experimental data modified by the thermionic emission model along with a Gaussian distribution of the barrier heights indicated the presence of a double Gaussian barrier distribution in the Ni<SUB>0.8</SUB>Fe<SUB>0.2</SUB>/n-type GaN Schottky contact. The mean barrier heights and standard deviations for each Gaussian distribution were 0.84 & 1.32 eV and 0.10 & 0.17 eV over temperature range of 125–200 K and 225–400 K, respectively. The noise spectral density of the current fluctuations measured as a function of frequency (<I>f</I>) at room temperature followed a 1/<I>f</I> <SUP> <I>γ</I> </SUP> dependence with a <I>γ</I> value close to unity, irrespective of the applied forward bias. The 1/<I>f-</I>type noise was attributed to the barrier inhomogeneity existing at the Ni<SUB>0.8</SUB>Fe<SUB>0.2</SUB>/n-type GaN Schottky interface as revealed from the temperature-dependent <I>I</I>–<I>V</I> characteristics.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The Ni<SUB>0.8</SUB>Fe<SUB>0.2</SUB>/n-type GaN structure showed ferromagnetic behavior. </LI> <LI> <I>I</I>–<I>V</I>–<I>T</I> characteristics of permalloy (Ni<SUB>0.8</SUB>Fe<SUB>0.2</SUB>)/n-GaN Schottky diodes were studied. </LI> <LI> Thermionic emission model with a Gaussian distribution of barriers indicates double barrier distribution. </LI> <LI> At 300 K, current noise spectral density against frequency (<I>f</I>) showed a 1/<I>f</I> dependence. </LI> <LI> 1/<I>f-</I>type noise was attributed to barrier inhomogeneity at the Ni<SUB>0.8</SUB>Fe<SUB>0.2</SUB>/n-type GaN Schottky interface. </LI> </UL> </P>

Current–voltage–temperature (I–V–T) characteristics of Pd/Au Schottky contacts on n-InP (111)

M. Bhaskar Reddy,A. Ashok Kumar,V. Janardhanam,V. Rajagopal Reddy,P. Narasimha Reddy 한국물리학회 2009 Current Applied Physics Vol.9 No.5

We have investigated the current–voltage–temperature (I–V–T) characteristics of Pd/Au/InP Schottky barrier diodes in the temperature range of 220–400 K. The I–V analysis based on thermionic emission (TE) theory shows an abnormal decrease of apparent barrier height and increase of ideality factor at low temperatures. The conventional Richardson plot exhibits nonlinearity with activation energy of 0.17 eV and the Richardson constant value of 5.63 × 10-6 A cm-2 K-2. The nonlinearity in the Richardson plot and strong dependence of Schottky barrier parameters on temperature may be attributed to the spatial inhomogeneity in the interface. Further, the homogeneous barrier height has been obtained from the linear relationship between experimentally obtained effective barrier heights and ideality factors. Φb versus (2kT)-1 plot has been drawn to obtain the mean barrier height [Φbo(T = 0 K)] and the standard deviation (σs) at zero-bias which are found to be 0.84 eV, 138 meV, respectively. The series resistance is also estimated from the forward current–voltage characteristics of Pd/Au/InP Schottky contacts using Cheung’s method and found that it is strongly dependent on temperature and also decreases with increase in temperature. We have investigated the current–voltage–temperature (I–V–T) characteristics of Pd/Au/InP Schottky barrier diodes in the temperature range of 220–400 K. The I–V analysis based on thermionic emission (TE) theory shows an abnormal decrease of apparent barrier height and increase of ideality factor at low temperatures. The conventional Richardson plot exhibits nonlinearity with activation energy of 0.17 eV and the Richardson constant value of 5.63 × 10-6 A cm-2 K-2. The nonlinearity in the Richardson plot and strong dependence of Schottky barrier parameters on temperature may be attributed to the spatial inhomogeneity in the interface. Further, the homogeneous barrier height has been obtained from the linear relationship between experimentally obtained effective barrier heights and ideality factors. Φb versus (2kT)-1 plot has been drawn to obtain the mean barrier height [Φbo(T = 0 K)] and the standard deviation (σs) at zero-bias which are found to be 0.84 eV, 138 meV, respectively. The series resistance is also estimated from the forward current–voltage characteristics of Pd/Au/InP Schottky contacts using Cheung’s method and found that it is strongly dependent on temperature and also decreases with increase in temperature.

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

Rajagopal Reddy, V.,Janardhanam, V.,Won, Jonghan,Choi, Chel-Jong Elsevier 2017 JOURNAL OF COLLOID AND INTERFACE SCIENCE - Vol.499 No.-

<P><B>Abstract</B></P> <P>An Au/Cu<SUB>2</SUB>ZnSnS<SUB>4</SUB> (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>I</I>-<I>V</I> and <I>C</I>-<I>V</I> 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>I</I>-<I>V</I>, Cheung’s and Norde functions are closely matched with one other, thus the methods used here are reliable and valid. The extracted interface state density (<I>N<SUB>SS</SUB> </I>) 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 <I>N<SUB>SS</SUB> </I>. Moreover, the capacitance-frequency (<I>C</I>-<I>f</I>) and conductance-frequency (<I>G</I>-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 <I>N<SUB>SS</SUB> </I> estimated from <I>C</I>-<I>f</I> and <I>G</I>-<I>f</I> characteristics is lower compared to those estimated from <I>I</I>-<I>V</I> 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.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Electrical properties and the double Gaussian distribution of inhomogeneous barrier heights in Se/n-GaN Schottky barrier diode

Rajagopal Reddy, V.,Janardhanam, V.,Leem, C.H.,Choi, C.J. Academic Press 2014 Superlattices and microstructures Vol.67 No.-

The temperature dependent electrical characteristics of Se/n-GaN Schottky barrier diode have been investigated in the temperature range of 130-400K in the steps of 30K. The estimated barrier height (φ<SUB>bo</SUB>) and ideality factor n are found to be 0.46eV and 3.83 at 130K, 0.92eV and 1.29 at 400K. The φ<SUB>bo</SUB> and n are found to be strongly temperature dependent and while the φ<SUB>bo</SUB> decreases and the n increase with decreasing temperature. Such behavior of φ<SUB>bo</SUB> and n is attributed to Schottky barrier inhomogeneities, explained by the assumption of Gaussian distribution of barrier heights at the metal/semiconductor interface. Experimental results revealed the existence of a double Gaussian distribution with mean barrier height values of 1.33 and 0.90eV and standard deviations (σ<SUB>o</SUB>) of 0.0289 and 0.010V, respectively. The modified ln(I<SUB>o</SUB>/T<SUP>2</SUP>)-(q<SUP>2</SUP>σ<SUB>o</SUB><SUP>2</SUP>/2k<SUP>2</SUP>T<SUP>2</SUP>) versus 10<SUP>3</SUP>/T plot gives φ<SUB>bo</SUB> and Richardson constant (A<SUP>*</SUP>) values as 1.30 and 0.88eV, 23.6 and 19.2 A/cm<SUP>2</SUP> K<SUP>2</SUP> at 400 and 130K, respectively without using the temperature coefficient of the barrier height. Further, the barrier height obtained from C-V method decreases with an increase in temperature. It is also noted that the barrier height value estimated from the C-V method is higher than that estimated from the I-V method at various temperatures. Possible explanations for this discrepancy are presented. The interface state density (N<SUB>ss</SUB>) is found to be decreased with an increasing temperature. The reverse-bias leakage current mechanism of Se/n-GaN Schottky diode is investigated. Both Poole-Frenkel and Schottky emissions are described and discussed.

Microstructural, chemical states and electrical properties of Au/CuO/n-InP heterojunction with a cupric oxide interlayer

Balaram, N.,Rajagopal Reddy, V.,Sekhar Reddy, P.R.,Janardhanam, V.,Choi, Chel-Jong Elsevier 2018 Vacuum Vol.152 No.-

<P><B>Abstract</B></P> <P>Cupric oxide (CuO) is synthesized by simple chemical bath deposition method and deposited on n-type InP substrate using e-beam evaporation technique. First, the structural and chemical compositional analysis of CuO/n-InP are analysed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD and XPS results confirmed that the formation of CuO on n-type InP substrate. Then, Au/CuO/n-InP heterojunction is fabricated with a CuO interlayer and correlated its results with the Au/n-InP Schottky junction (SJ). The barrier height (Φ<SUB>b</SUB>) and ideality factor (n) are extracted through I-V and C-V methods and the respective values are 0.66 eV (I-V)/0.80 eV (C-V) and 1.24, and 0.78 eV (I-V)/0.94 eV (C-V) and 1.62 for the SJ and HJ diodes, respectively. By applying Cheung's and Norde functions, the Φ<SUB>b</SUB>, ideality factor and series resistance (R<SUB>S</SUB>) are derived for the SJ and HJ diodes. The derived interface state density (N<SUB>SS</SUB>) of HJ is lower than the SJ; results demonstrated that the CuO interlayer plays an important role in the decreased N<SUB>SS</SUB>. The Poole-Frenkel emission is the dominant current conduction mechanism in reverse bias of both SJ and HJ diodes.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Structural and chemical properties of CuO films are analysed by XRD and XPS. </LI> <LI> Barrier height of Au/n-InP SJ was modified by CuO interlayer. </LI> <LI> Heterojunction has a good rectification ratio compared to the Schottky junction. </LI> <LI> The interface state density of HJ is lower as compared to the SJ. </LI> <LI> Poole-Frenkel mechanism is found to dominate in both SJ and HJs. </LI> </UL> </P>

Conduction Mechanism of Se Schottky Contact to n-Type Ge

Janardhanam, V.,Yang-Kyu Park,Hyung-Joong Yun,Kwang-Soon Ahn,Chel-Jong Choi IEEE 2012 IEEE electron device letters Vol.33 No.7

<P>The conduction mechanism of Se/n-type-Ge Schottky diodes is investigated using temperature-dependent current-voltage (<I>I</I>- <I>V</I>) characteristics. The presence of microscopic inhomogeneity at the Se/Ge interface could be the primary cause of the differences between the barrier heights measured from the <I>I</I>-<I>V</I> and capacitance-voltage (<I>C</I>-<I>V</I> ) characteristics. The position of the quasi-Fermi level suggested the dominance of thermionic emission in the forward bias region. The electric field dependence of the reverse current revealed that Schottky emission, along with the generation mechanism, has dominance over the current conduction in the reverse bias region.</P>