arXiv subjects

Manoranjan Ghosh

Publications and source records attributed to Manoranjan Ghosh.

17 recordsLinked to original sources

arXiv2024

Fabrication of n+ contact on p-type high pure Ge by cathodic electrodeposition of Li and impedance analysis of n+/p diode at low temperatures

Fabrication of diode by forming n-type electrical contact on germanium (Ge) and its AC impedance analysis is important for radiation detection in the form of pulses. In this work lithium (Li) metal has been electro-deposited on p-type Ge single crystal from molten lithium nitrate at 260°C. The depth of Li diffusion in Ge was successfully varied by changing the electroplating time as determined by sheet resistance (SR) measurement after successive lapping of Ge surface. Li is found to diffuse up to 500 micron inside Ge by heat treatment of as deposited Li/Ge at 350°C for 1 hour. A stable n-type electrical contact on Ge with SR ~1 ohm/square and impurity concentration ~3.7x10^15/cm^3 is developed by Li incorporation in p-type Ge crystal showing net carrier concentration ~3.4x10^10/cm^3 and SR ~100 Kohm/square. Acceptor concentration determined from the 1/C^2 vs V plot shows similar temperature dependence as found by Hall measurement. The fabricated n+/p junction exhibit ideal diode characteristics with gradual increase in cut off voltage at low temperatures. Under forward bias, junction capacitance mainly comprises of diffusion capacitance (~10 micro.F) showing strong frequency dependence and the impedance is partly resistive resulting in semicircular Cole-Cole plot. Imaginary impedance spectra reveal that the relaxation time for the diffusion of majority carriers decreases at higher temperatures and increased forward voltages. The diode is purely capacitive under reverse bias showing a line parallel to the y-axis in the Cole-Cole plot with frequency independent (100Hz-100MHz) depletion capacitance ~10pF.

physics.app-ph↗

arXiv2020

Characteristics of Al/Ge Schottky and Ohmic contacts at low temperatures

Schottky barrier contact has been fabricated by thermal deposition of Al on (100) Ge (impurity concentration~1010/cm3 at 80K) that shows extrinsic p-type to intrinsic n-type transition near 180K. Both p and n-type Ge exhibits ideal Schottky behaviour with low reverse current and near unity ideality factors obtained from the linear form of temperature dependent current-voltage (I-V) characteristics. The diode current at various temperatures change its direction at non-zero applied bias that reflects a shift in position of charge neutrality level (CNL) from the Fermi level of Ge. With the rise in temperature, Schottky barrier height (SBH) steadily increases for p-Ge that can be understood on the basis of observed variation in CNL. Values of SBH determined from the zero bias Richardson plot agrees well with that estimated from the Schottky-Mott rule for strongly pinned interface. Activation energies are determined from the Richardson plot at various forward voltages and found to decrease with applied bias for n-Ge but reduces to zero for p-Ge that shows work function similar to Al. Annealing of Al/Ge induces regrowth of p-type Al doped Ge layer that exhibits gradual reduction of Al concentration towards p-Ge crystal. Al doped Ge(P+)/Ge (P) junction thus fabricated shows linear current-voltage (I-V) characteristics in the extrinsic region (below 180K). In the intrinsic region (above 180K), rectification is observed in the I-V curve due to temperature dependent change in conductivity of both Al doped Ge layer and Ge crystal.

cond-mat.mtrl-sci↗

arXiv2019

Fabrication of p+ contact by thermally induced solid state regrowth of Al on p-type Ge crystal

Formation of p+ contact on Germanium is important for applications in diode detector and other electronic devices. In this work, thermally deposited Al on Ge crystal is annealed at 350oC followed by slow cooling for solid-state regrowth of Al-Ge p+ contact on Ge. Depth profile analysis by secondary ion mass spectrometry (SIMS) is carried out to investigate the occurrence of Al and Ge along the depth of the regrown Al-Ge layer. Evidence of regrowth is observed due to inter-diffusion of both Ge and Al across the layers although Ge diffusion in Al layer is found to be more prevalent. Thickness of the evaporated Al layer is varied to understand the diffusion profile of Al, Ge and estimate the depth of Al incorporation in Ge crystal underneath. Hall measurement at different depth of Al-Ge regrown layer reveals that Al impurity induces p+ doping in p-type Ge and its concentration gradually reduces towards the Ge crystal. Top surface of the Al-Ge layer exhibits lowest sheet resistance that varies with the thickness of the as deposited Al layer.

cond-mat.mtrl-sci↗

arXiv2019

Impurity concentration dependent electrical conduction in germanium crystal at low temperatures

Germanium single crystal having 45 mm diameter and 100 mm length of 7N+ purity has been grown by Czochralski method. Structural quality of the crystal has been characterized by Laue diffraction. Electrical conduction and Hall measurements are carried out on samples retrieved from different parts of the crystal along the growth axis. Top part of the crystal exhibits lowest impurity concentration (~10^12/cm3) that gradually increases towards the bottom (10^13/cm3). The crystal is n-type at room temperature and the resistivity shows non-monotonic temperature dependence. There is a transition from n-type to p-type conductivity below room temperature at which bulk resistivity shows maximum and dip in carrier mobility. This intrinsic to extrinsic transition regions shift towards room temperature as the impurity concentration increases and reflects the purity level of the crystal. Similar trend is observed in boron implanted high purity germanium (HPGe) crystal at different doping level. The phenomena can be understood as a result of interplay between temperature dependent conduction mechanism driven by impurity band and intrinsic carrier in Ge crystals having fairly low acceptor concentrations (<10^12/cm3).

cond-mat.mtrl-sci↗

arXiv2013

Effect of size and aspect ratio on structural parameters and evidence of shape transition in zinc oxide nanostructures

Dependence of structural parameters on the size of nanoparticles is a topic of general interest where the effect of shape is often neglected. We report a comprehensive study on size dependent structural parameters of ZnO nanostructures (NS) having a wide range of aspect ratios (length/diameter). It reveals that with increase in size, ZnO NS undergo a shape transition from spherical to rod like morphology that induces sudden change in internal parameter (u) which represents relative position of two hexagonal close-packed sublattices. The change in u introduces concomitant changes in anion-cation (Zn-O) bond lengths as well as bond angles and thereby bears a linear dependence with the aspect ratio (AR). Further, the unit cell volume and microstrain decrease with increase in particle size and show a drastic reduction when flat crystal faces begin to appear at the spherical surface (AR~1.3). The significant change in structural parameters associated with the shape transition arises due to surface dipole induced electrostatic relaxation that may be further influenced by interaction with the ambient gases as evidenced from the Extended X-ray Absorption Fine Structure (EXAFS) measurement. The present study addresses the underlying reasons of shape induced change in structural and electronic properties of ZnO NS.

cond-mat.mtrl-sci↗

arXiv2011

Probing defects in chemically synthesized ZnO nanostrucures by Positron Annihilation and Photoluminescence Spectroscopy

The present article describes the size induced changes in the structural arrangement of intrinsic defects present in chemically synthesized ZnO nanoparticles of various sizes. Routine X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) have been performed to determine the shapes and sizes of the nanocrystalline ZnO samples. Detailed studies using positron annihilation spectroscopy reveals the presence of zinc vacancy. Whereas analysis of photoluminescence results predict the signature of charged oxygen vacancies. The size induced changes in positron parameters as well as the photoluminescence properties, has shown contrasting or non-monotonous trends as size varies from 4 nm to 85 nm. Small spherical particles below a critical size (~ 23 nm) receive more positive surface charge due to the higher occupancy of the doubly charge oxygen vacancy as compared to the bigger nanostructures where singly charged oxygen vacancy predominates. This electronic alteration has been seen to trigger yet another interesting phenomenon, described as positron confinement inside nanoparticles. Finally, based on all the results, a model of the structural arrangement of the intrinsic defects in the present samples has been reconciled.

cond-mat.mtrl-sci↗

arXiv2011

Electric field induced reversible control of visible photoluminescence from ZnO nanoparticles

Reversible control of the photoluminescence of ZnO occurring in the visible range, has been achieved by application of a few volts (< 5V) to a device consisting of nanostructured ZnO film sandwiched between Indium Tin Oxide electrode and polyethylene oxide-lithium perchlorate, a solid polymer electrolyte. The photoluminescence intensity shows nearly 100% modulation with a response time less than 30 seconds, when the bias is applied at the electrolyte-electrode. A model is proposed for the observed effect that is based on defect states of ZnO and the band bending at the ZnO-electrolyte interface that can be changed by the applied bias.

cond-mat.mtrl-sci↗

arXiv2011

Structural and Optical properties of Zn(1-x)MgxO nanocrystals obtained by low temperature method

In this paper we report structural and optical properties of Magnesium substituted Zinc Oxide (Zn1-xMgxO) nanocrystals (~10-12nm) synthesized by low temperature route. In the low temperature synthesis route it was possible to reach x = 0.17 without segregation of Mg rich phase. The exact chemical composition has been established by quantitative analysis. Rietveld analysis of the XRD data confirms the Wurzite structure and a continuous compaction of the lattice (in particular the c-axis parameter) as x increases. There is an enhancement of the strain in the lattice as the Mg is substituted. The bandgap also gets enhanced as x is increased and reaches a value of 4eV for x = 0.17. From the TEM and the XRD data it has been concluded that when there is a phase segregation for x > 0.17, there is a shell of Mg(OH)2 on the ZnO. The absorption also shows persistence of the excitoinc absorption on Mg substitution. The nanocrystals show near band edge photo luminescence (PL) at room temperature which shows blue shift on Mg incorporation. In addition to the near band edge emission the ZnO and Zn1-xMg xO alloy nanocrystals show considerable emission in the blue-green region at wavelength of ~550 nm. We find that the relative intensity of the green emission increases with the Mg concentration for very low x (upto x = 0.05) and on further increase of the Mg concentration there is a sharp decrease of relative intensity of the green emission eventually leading to a complete quenching of blue emission. It is concluded that due to phase segregation (for x \geq 0.20), the formation of the shell of Mg(OH)2 on the ZnO leads to quenching of the green emission .However, this shell formation does not have much effect on the near band edge PL.

cond-mat.mtrl-sci↗

arXiv2011

Phonon dynamics of Zn(Mg,Cd)O alloy nanostructures and their phase segregation

In this paper we report phonon dynamics in chemically synthesized Zn1- xMgxO (0\leqx\leq0.07) and Zn1-yCdyO (0\leqy\leq0.03) alloy nanostructures of sizes ~10 nm using non-resonant Raman and Fourier Transformed Infrared Spectroscopy (FTIR). Substitution by Mg makes the unit cell compact while Cd substitution leads to unit cell expansion. On alloying, both A1(LO) and E1(LO) mode of wurtzite ZnO show blue shift for Zn1-xMgxO and red shift for Zn1-yCdyO alloy nanostructures due to mass defect and volume change induced by the impurity atoms. Significant shift has been observed in E1(LO) mode for Zn1-xMgxO (73 cm-1 for x = 0.07) and Zn1-yCdyO (17 cm-1 for y = 0.03) nanostructures. The variation in Zn(Mg,Cd)-O bond length determined from the blue (red) shift of IR bands on alloying with Mg (Cd) is consistent with their respective ionic sizes and the structural changes predicted by X-ray diffraction study. However, on progressive alloying one can detect phase segregation (due to presence of interstitial Mgand Cd ions) in the alloy nanostructures for relatively higher Mg and Cd concentrations. This is confirmed by the gradual absence of the characteristic IR and Raman bands of wurtzite ZnO near 400-600 cm-1 as well as by X-Ray and TEM studies.

cond-mat.mtrl-sci↗

arXiv2011

Role of ambient air on photoluminescence and electrical conductivity of assembly of ZnO Nanoparticles

Effect of ambient gases on photoluminescence (PL) and electrical conductivity of films prepared using ZnO nanoparticles (NPs) have been investigated. It is observed that NPs of size below 20 nm kept inside a chamber exhibit complete reduction in their visible PL when oxygen partial pressure of the surrounding gases is decreased by evacuation. However the visible PL from ZnO NPs is insensitive to other major gases present in the ambient air. The rate of change of PL intensity with pressure is inversely proportional to the ambient air pressure and increases when particle size decreases due to the enhanced surface to volume ratio. On the other hand an assembly of ZnO NPs behaves as a complete insulator in the presence of dry air and its major components like N2, O2 and CO2. Electrical conduction having resistivity ~102 - 103 Ωm is observed in the presence of humid air. The depletion layer formed at the NP surface after acquiring donor electrons of ZnO by the adsorbed oxygen, has been found to control the visible PL and increases the contact potential barrier between the NPs which in turn enhances the resistance of the film.

cond-mat.mtrl-sci↗

arXiv2011

Structure and optical properties of Cd substituted ZnO (Zn1-xCdxO) nanostructures synthesized by high pressure solution route

We report synthesis of Cd substituted ZnO nanostructures (Zn1-xCdxO with x upto \approx .09) by high pressure solution growth method. The synthesized nanostructures comprise of nanocrystals that are both particles (~ 10-15 nm) and rods which grow along (002) direction as established by Transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. Rietveld analysis of the XRD data shows monotonous increase of the unit cell volume with the increase of Cd concentration. The optical absorption as well as the photoluminescence (PL) shows red shift on Cd substitution. The line width of the PL spectrum is related to the strain inhomogenity and it peaks in the region where the CdO phase separates from the Zn1-xCdxO nanostructures. The time resolved photoemission showed a long lived (~10ns) component. We propose that the PL behavior of the Zn1-xCdxO is dominated by strain in the sample with the redshift of the PL linked to the expansion of the unit cell volume on Cd substitution.

cond-mat.mtrl-sci↗

arXiv2011

Role of Site-selective Doping on Melting Point of CuTi Alloys: A Classical Molecular Dynamics Simulation Study

Effect of site-selective substitution of Ti in Cu on the thermal stability of CuTi alloy is investigated using classical molecular dynamics simulations with Embedded Atom Method potentials. It has been observed experimentally that melting point of all the naturally occurring stable phases of CuTi alloys do not show a definite trend with gradual increase in Ti concentration. To understand the phenomenon, super cells of CuTi alloy are constructed where Cu atom is substituted by Ti randomly and at selective sites. For random substitution, the melting point decreases linearly with increase in Ti concentration. A non-monotonous dependence is seen when Cu atoms at selective sites are replaced by Ti. For a particular doping concentration, the melting point shows a wide range of variation depending on the order of atomic arrangement, and can be fine tuned by selecting the sites for substitution. The variations in melting points in different cases are explained in terms of the peak height, width and position of the corresponding radial distribution functions. Finally, it is verified that initial structures of the naturally occurring CuTi alloys are responsible for the non-definite trend in their melting points.

cond-mat.mtrl-sci↗

arXiv2010

Optical Emission Based Oxygen Sensing by ZnO Nanoparticles

ZnO nanoparticles (NP) of size less than around 20 nm inside a chamber exhibits complete reduction in their visible photoluminescence (PL) intensity when partial pressure of oxygen in the surrounding gaseous environment is decreased by evacuation. However the visible PL from ZnO nanoparticles are insensitive to the gases like N2, He and Ar. The rate of change of PL intensity with pressure (or the sensitivity) is inversely proportional to the ambient air pressure. Sensitivity increases when surface to volume ratio increases for lower particle size. The charged oxygen vacancies are attributed as the responsible candidates for surface related visible PL from ZnO NP.

cond-mat.mtrl-sci↗

arXiv2010

Photovoltaic Properties of ZnO Nanoparticle Based Solid Polymeric Photoelectrochemical Cells

In this work, we report optical and photovoltaic properties of ZnO based solid polymeric photoelectrochemical (PEC) cell. In this device construction, film of ZnO nanoparticle (NP) has been sandwiched between ITO and LiClO4-PEO solid electrolyte. ZnO NP has been used as photo-anode and the electrolyte LiClO4 has been dissolved in a solid solution polyethylene oxide that makes the device lightweight and portable. The charge transfer is performed by Li+ and ClO4- ions which serve the purpose of a redox couple in a conventional dye sensitize solar cell (DSSC). Short Circuit Current (JSC) shows maximum when the ZnO/ITO interface has been illuminated by a UV light (350 +/- 15 nm) passing through the transparent ITO glass substrate. JSC and Open Circuit Voltage (VOC) of the best performing device are found to be 7 microA/cm2 and 0.24 V respectively under white light of intensity 15 mW/cm2.

cond-mat.mtrl-sci↗

arXiv2009

Field induced reversible control of visible luminescence in ZnO nanostructures

In this work, a reversible control over the visible luminescence of phosphor ZnO is achieved by the application of a few volts (<5V) to nanostructured ZnO film sandwiched between ITO and LiClO4/PEO solid electrolyte. Since ZnO is a good absorbing material but ITO-Glass substrate is transparent in the range of 320-375 nm, the ZnO-ITO interface has been illuminated by a 345 nm light passing through the ITO glass substrate and the emitted light near 545 nm has been collected in reflection mode. When the solid polymer electrolyte is negatively biased, up to 107 percent enhancement in the visible luminescence has been observed, whereas a complete quenching (87 percent reduction) of the visible emission has been seen when it is positively biased. The sharp near band edge (NBE) emission in the ultraviolet region however has no dependence within the voltage range we are working. The luminescence output follows variations in the applied voltage up to maximum frequency limited by response time of the device which lies within the range of 20-30 sec. The upward band bending created by application of negative bias to the conductive polymer populates increasing number of defect levels above the Fermi level which enhances the visible photoluminescence and vice versa.

cond-mat.mtrl-sci↗

arXiv2008

Ionic environment control of visible photo-luminescence from ZnO nanoparticles

We report a novel effect that the visible photoluminescence (in the blue-green band) from ZnO anoparticles can be controlled by changing the ionic or polar nature of the medium in which the nanoparticles are dispersed. We find that the presence of sufficient amount of electrolytes can even quench the emission. We propose an explanation based on surface charge of the ZnO nanoparticles which control the band bending in the depletion layer at the surface of the anoparticles. The band bending in turn, decides the predominant nature of the visible emission. The explanation is validated by establishing a direct correlation between the visible emission and the zeta potential.

cond-mat.mtrl-sci↗

arXiv2008

Shape transition in ZnO nanostructures and its effect on blue-green photoluminescence

We report that ZnO nanostructures synthesized by chemical route undergo a shape transition at ~ 20 nm from spherical to hexagonal morphology thereby changing the spectral components of the blue-green emission. Spherically shaped nanocrystals (size range 11 -18 nm) show emission in the range of 555-564 nm and the emission shifts to the longer wavelength as the size increases. On the other hand, rods and hexagonal platelets (size range 20-85 nm), which is the equilibrium morphology after the shape transition, show emission near 465-500 nm and it shifts to shorter wavelength as the size increases. The shape transition also leads to relaxation of microstrain in the system. Our analysis shows that the visible emission originates from a defect layer on the nanostructure surface which is affected by the shape transition. The change in the spectral component of the blue green emission on change of shape has been explained as arising from band bending due to depletion layer in smaller spherical particles which is absent in the larger particles with flat faces.

cond-mat.mtrl-sci↗