Vipul Kheraj | National Institute of Technology, Surat (original) (raw)

Papers by Vipul Kheraj

Semiconductor Science and Technology, 2008

Schottky diodes have been fabricated by depositing Au on n-type CdSe thin films using the thermal... more Schottky diodes have been fabricated by depositing Au on n-type CdSe thin films using the thermal evaporation technique, and their properties have been investigated by current-voltage and capacitance-voltage measurements. At room temperature, the characteristics obey the pure thermionic emission theory and the barrier height has been found to be 0.63 eV. The barrier heights decrease, while ideality factors increase with decrease in temperature. Further, the activation energy plot does not provide the expected Richardson constant and barrier height values. The abnormal behavior of the barrier heights and the ideality factors with respect to temperature as well as the difference between the barrier heights measured from I-V and those from C-V or flat band have been explored on the basis of barrier height inhomogeneities.

Thin Solid Films, 2009

Stoichiometric compound of copper indium diselenide (CuInSe2) was synthesized by direct reaction ... more Stoichiometric compound of copper indium diselenide (CuInSe2) was synthesized by direct reaction of high-purity elemental copper, indium and selenium in an evacuated quartz ampoule. The phase structure and composition of the synthesized pulverized material analyzed by X-ray diffraction (XRD) and energy dispersive analysis of X-rays (EDAX) revealed the chalcopyrite structure and stoichiometry of elements. Thin films of CuInSe2 were deposited onto organically cleaned soda lime glass substrates held at different temperatures (i.e. 300 K to 573 K) using thermal evaporation technique. CuInSe2 thin films were then thermally annealed in a vacuum chamber at 573 K at a base pressure of 10− 2 mbar for 1 h. The effect of substrate temperature (Ts) and thermal annealing (Ta) on structural, compositional, morphological, optical and electrical properties of films were investigated using XRD, transmission electron microscopy, EDAX, atomic force microscopy (AFM), optical transmission measurements and Hall effect techniques. XRD and EDAX studies of CuInSe2 thin films revealed that the films deposited in the substrate temperature range of 423–573 K have preferred orientation of grains along the (112) plane and near stoichiometric composition. AFM analysis indicates that the grain size increases with increase of Ts and Ta. Optical and electrical characterizations of films suggest that CuInSe2 thin films have high absorption coefficient (104 cm− 1) and resistivity value in the interval 10− 2–101 Ω cm influenced by Ts and Ta.

Many applications of Laser diode require antireflection coatings either on one or both the facets... more Many applications of Laser diode require antireflection coatings either on one or both the facets of the diode. These include, for example, semiconductor optical amplifiers, optical pumping for solid state lasers and creation of broad band source for tunable external cavity. We have used single layer antireflection coating on the front facet of the laser diode using electron beam evaporation technique to enhance optical power output from the facet. To optimize the coating conditions with precise control over facet reflectance of the laser diode, we have carried out experiment for In-Situ reflectivity measurement. We have used MgF2 as a low refractive index dielectric material for antireflection coating. The actual single layer AR coating consists of λ/4 thick MgF2 film. The reflectivity of the film being deposited is measured on GaAs test substrate, kept in close vicinity of the laser diode bar, with the help of a 657 nm (red) laser diode and a photo detector. A LabVIEW programme, called Virtual Instrument (VI), has been prepared to automate the whole experiment. We have also carried out simulation of facet reflectivity subject to the film thickness being deposited.

Pramana-journal of Physics, 2009

Semiconductor Science and Technology, 2008

Synthesis and Reactivity in Inorganic Metal-organic and Nano-metal Chemistry, 2007

Solar Energy, 2009

Copper indium diselenide (CuInSe2) compound was synthesized by reacting its constituent’s element... more Copper indium diselenide (CuInSe2) compound was synthesized by reacting its constituent’s elements copper, indium and selenium in near stoichiometric proportions (i.e. 1:1:2 with 5% excess selenium) in an evacuated quartz ampoule. Synthesized pulverized compound material was used as an evaporant material to deposit thin films of CuInSe2 onto organically cleaned sodalime glass substrates, held at different temperatures (300–573 K), by means of single source thermal evaporation method. The phase structure and the composition of chemical constituents present in the synthesized compound and thin films have been investigated using X-ray diffraction and energy dispersive X-ray analysis, respectively. The investigations show that CuInSe2 thin films grown above 423 K are single phase, having preferred orientation of grains along the (112) direction, and having near stoichiometric composition of elements. The surface morphology of CuInSe2 films, deposited at different substrate temperatures, has been studied using the atomic force microscopy to estimate its surface roughness. An analysis of the transmission spectra of CuInSe2 films, recorded in the wavelength range of 500–1500 nm, revealed that the optical absorption coefficient and the energy band gap for CuInSe2 films, deposited at different substrate temperatures, are ∼104 cm−1 and 1.01–1.06 eV, respectively. The transmission spectrum was analyzed using iterative method to calculate the refractive index and the extinction coefficient of CuInSe2 thin film deposited at 523 K. The Hall effect measurements and the temperature dependence of the electrical conductivity of CuInSe2 thin films, deposited at different substrate temperatures, revealed that the films had electrical resistivity in the range of 0.15–20 ohm cm, and the activation energy 82–42 meV, both being influenced by the substrate temperature.

Pramana-journal of Physics, 2009

Optics and Laser Technology, 2007

The optical output power of a laser diode can be enhanced by anti-reflection (AR) and high-reflec... more The optical output power of a laser diode can be enhanced by anti-reflection (AR) and high-reflection (HR) facet coatings, respectively, at the front and back facet. AR and HR coatings also serve the purpose of protection and passivation of laser diode facets. In this work, we have designed and optimized a single layer λ/4 thick Al2O3 film for the AR coating and a stack of λ/4 thick Al2O3/λ/4 thick Si bi-layers for the HR coating for highly strained InGaAs quantum-well edge emitting broad area (BA) laser diodes. Effect of the front and back facet reflectivities on output power of the laser diodes has been studied. The light output versus injected current (L–I characteristics) measurements were carried out on selected devices before and after the facet coatings. We have also carried out the numerical simulation and analysis of L–I characteristics for this particular diode structure. The experimental results have been compared and verified with the numerical simulation.

Journal of Materials Science, 2009

We have investigated the electrophysical properties of metallic thin films based on Cu/Cr and Fe/... more We have investigated the electrophysical properties of metallic thin films based on Cu/Cr and Fe/Cr systems. We find that the longitudinal gauge factor of two-layer films is significantly greater as compared with one-layer films, which have the same thickness as the total thickness of a two-layer film. Interface and intensive grain–boundary electron scattering explain such an increase in the longitudinal gauge factor. We find that the longitudinal gauge factor increases in transition from elastic to plastic zone.

Journal of Materials Science, 2009

The power enhancement of laser diodes is achieved by single and multilayer facet coatings such as... more The power enhancement of laser diodes is achieved by single and multilayer facet coatings such as antireflection and high reflection respectively at the front facet and the back facet of the laser diode. In this work, we have experimented with single layer λ/4 thick Al2O3 film for the Anti Reflection (AR) coating and stack of λ/4 thick Al2O3/ λ/4 thick Si bi-layers for the High Reflection (HR) coating. The AR/HR coatings were deposited in an electron beam evaporation system. The effect of front and back facet reflectivities on the output power of diode laser has been studied. The highly strained MOVPE grown InGaAs quantum-well edge emitting broad area (BA) diode lasers have been used for this experiments. The light output versus current (L-I) measurements were made on selected devices before and after the coatings. The devices were tested under pulsed operation with a pulse width of 400 ns and a duty cycle of 1:400. We have also carried out the theoretical analysis and simulation of L-I characteristics for this particular diode structure using LabVIEW. The experimental results were compared with simulated results. The effect of facet coating on external differential efficiency of diode laser has also been studied.

Journal of Crystal Growth

The development of thin-film semiconductor compounds, such as Copper Indium Gallium Selenide (CIG... more The development of thin-film semiconductor compounds, such as Copper Indium Gallium Selenide (CIGS), has caused remarkable progress in the field of thin-film photovoltaics. However, the scarcity and the increasing prices of indium impose the hunt for alternative materials. The Copper Zinc Tin Sulphide (CZTS) is one of the promising emerging materials with Kesterite-type crystal structure and favourable material properties like high absorption co-efficient and direct band-gap. Moreover, all the constituent elements of CZTS are non-toxic and aplenty on the earth-crust, making it a potential candidate for the thin-film photovoltaics. Here we report the synthesis of CZTS powder from its constituent elements, viz. copper, zinc, tin and sulphur, in an evacuated Quartz ampoule at 1030 K temperature. The sulphur content in the raw mixture in the ampoule was varied and optimised in order to attain the desired atomic stoichiometry of the compound. The synthesised powder was characterised by X-Ray diffraction technique (XRD), Raman Scattering Spectroscopy, Energy Dispersive Analysis of X-Ray (EDAX) and UV–Visible Absorption Spectra. The XRD Patterns of the synthesised compound show the preferred orientation of (112), (220) and (312) planes, confirming the Kesterite structure of CZTS. The chemical composition of the powder was analysed by EDAX and shows good atomic stoichiometry of the constituent elements in the CZTS compound. The UV–Vis absorption spectra confirm the direct band-gap of about 1.45 eV, which is quite close to the optimum value for the semiconductor material as an absorber in solar-cells.► Synthesis and characterisation of Cu2ZnSnS4 (CZTS) from constituent elements. ► Good stoichiometry in compounds prepared with 30% and 40% excess S in the raw mixture. ► Formation of CZTS with good crystalline quality confirmed by XRD and Raman spectra. ► Band gap of about 1.45 eV, which is close to optimum value for solar PV applications.

Journal of Materials Science, 2009

Copper indium diselenide (CuInSe2) compound was synthesized by reacting its elemental components,... more Copper indium diselenide (CuInSe2) compound was synthesized by reacting its elemental components, i.e., copper, indium, and selenium, in stoichiometric proportions (i.e., 1:1:2 with 5% excess selenium) in an evacuated quartz ampoule. Structural and compositional characterization of synthesized pulverized material confirms the polycrystalline nature of tetragonal phase and stoichiometry. CuInSe2 thin films were deposited on soda lime glass substrates kept at different temperatures (300–573 K) using flash evaporation technique. The effect of substrate temperature on structural, morphological, optical, and electrical properties of CuInSe2 thin films were investigated using X-ray diffraction analysis (XRD), atomic force microscopy (AFM), optical measurements (transmission and reflection), and Hall effect characterization techniques. XRD analysis revealed that CuInSe2 thin films deposited above 473 K exhibit (112) preferred orientation of grains. Transmission and reflectance measurements analysis suggests that CuInSe2 thin films deposited at different substrate temperatures have high absorption coefficient (~104 cm−1) and optical energy band gap in the range 0.93–1.02 eV. Results of electrical characterization showed that CuInSe2 thin films deposited at different substrate temperatures have p-type conductivity and hole mobility value in the range 19–136 cm2/Vs. Variation of energy band gap and resistivity of CuInSe2 thin films deposited at 523 K with thickness was also studied. The temperature dependence of electrical conductivity measurements showed that CuInSe2 film deposited at 523 K has an activation energy of ~30 meV.

Materials Chemistry and Physics, 2009

Tungsten trioxide (WO3) thin films are of great interest due to their enormous and promising appl... more Tungsten trioxide (WO3) thin films are of great interest due to their enormous and promising applications in various opto-electronic thin-film devices. We have investigated the structural, electrical, and optical properties of the WO3 thin films grown by thermal evaporation of WO3 powder and their dependence on growth condition. The WO3 thin films were grown on glass substrates at different substrate temperature varying from room temperature to 510 °C. The structural characterization and surface morphology were carried out using X-ray diffraction and atomic force microscopy, respectively. The amorphous films were obtained at substrate temperatures below 450 °C whereas films grown above 450 °C were crystalline. The surface roughness and the grain size of the films increase on increasing the substrate temperature. The electrical characterization has been carried out using four-point-probe methods. The resistivity of the films decreases significantly while the carrier concentration and mobility increase with the substrate temperature. The transparency and optical energy band-gap, Eg, of the films are found to decrease monotonically as the substrate temperature increases.

Semiconductor Science and Technology, 2008

Schottky diodes have been fabricated by depositing Au on n-type CdSe thin films using the thermal... more Schottky diodes have been fabricated by depositing Au on n-type CdSe thin films using the thermal evaporation technique, and their properties have been investigated by current-voltage and capacitance-voltage measurements. At room temperature, the characteristics obey the pure thermionic emission theory and the barrier height has been found to be 0.63 eV. The barrier heights decrease, while ideality factors increase with decrease in temperature. Further, the activation energy plot does not provide the expected Richardson constant and barrier height values. The abnormal behavior of the barrier heights and the ideality factors with respect to temperature as well as the difference between the barrier heights measured from I-V and those from C-V or flat band have been explored on the basis of barrier height inhomogeneities.

Thin Solid Films, 2009

Stoichiometric compound of copper indium diselenide (CuInSe2) was synthesized by direct reaction ... more Stoichiometric compound of copper indium diselenide (CuInSe2) was synthesized by direct reaction of high-purity elemental copper, indium and selenium in an evacuated quartz ampoule. The phase structure and composition of the synthesized pulverized material analyzed by X-ray diffraction (XRD) and energy dispersive analysis of X-rays (EDAX) revealed the chalcopyrite structure and stoichiometry of elements. Thin films of CuInSe2 were deposited onto organically cleaned soda lime glass substrates held at different temperatures (i.e. 300 K to 573 K) using thermal evaporation technique. CuInSe2 thin films were then thermally annealed in a vacuum chamber at 573 K at a base pressure of 10− 2 mbar for 1 h. The effect of substrate temperature (Ts) and thermal annealing (Ta) on structural, compositional, morphological, optical and electrical properties of films were investigated using XRD, transmission electron microscopy, EDAX, atomic force microscopy (AFM), optical transmission measurements and Hall effect techniques. XRD and EDAX studies of CuInSe2 thin films revealed that the films deposited in the substrate temperature range of 423–573 K have preferred orientation of grains along the (112) plane and near stoichiometric composition. AFM analysis indicates that the grain size increases with increase of Ts and Ta. Optical and electrical characterizations of films suggest that CuInSe2 thin films have high absorption coefficient (104 cm− 1) and resistivity value in the interval 10− 2–101 Ω cm influenced by Ts and Ta.

Many applications of Laser diode require antireflection coatings either on one or both the facets... more Many applications of Laser diode require antireflection coatings either on one or both the facets of the diode. These include, for example, semiconductor optical amplifiers, optical pumping for solid state lasers and creation of broad band source for tunable external cavity. We have used single layer antireflection coating on the front facet of the laser diode using electron beam evaporation technique to enhance optical power output from the facet. To optimize the coating conditions with precise control over facet reflectance of the laser diode, we have carried out experiment for In-Situ reflectivity measurement. We have used MgF2 as a low refractive index dielectric material for antireflection coating. The actual single layer AR coating consists of λ/4 thick MgF2 film. The reflectivity of the film being deposited is measured on GaAs test substrate, kept in close vicinity of the laser diode bar, with the help of a 657 nm (red) laser diode and a photo detector. A LabVIEW programme, called Virtual Instrument (VI), has been prepared to automate the whole experiment. We have also carried out simulation of facet reflectivity subject to the film thickness being deposited.

Pramana-journal of Physics, 2009

Semiconductor Science and Technology, 2008

Synthesis and Reactivity in Inorganic Metal-organic and Nano-metal Chemistry, 2007

Solar Energy, 2009

Copper indium diselenide (CuInSe2) compound was synthesized by reacting its constituent’s element... more Copper indium diselenide (CuInSe2) compound was synthesized by reacting its constituent’s elements copper, indium and selenium in near stoichiometric proportions (i.e. 1:1:2 with 5% excess selenium) in an evacuated quartz ampoule. Synthesized pulverized compound material was used as an evaporant material to deposit thin films of CuInSe2 onto organically cleaned sodalime glass substrates, held at different temperatures (300–573 K), by means of single source thermal evaporation method. The phase structure and the composition of chemical constituents present in the synthesized compound and thin films have been investigated using X-ray diffraction and energy dispersive X-ray analysis, respectively. The investigations show that CuInSe2 thin films grown above 423 K are single phase, having preferred orientation of grains along the (112) direction, and having near stoichiometric composition of elements. The surface morphology of CuInSe2 films, deposited at different substrate temperatures, has been studied using the atomic force microscopy to estimate its surface roughness. An analysis of the transmission spectra of CuInSe2 films, recorded in the wavelength range of 500–1500 nm, revealed that the optical absorption coefficient and the energy band gap for CuInSe2 films, deposited at different substrate temperatures, are ∼104 cm−1 and 1.01–1.06 eV, respectively. The transmission spectrum was analyzed using iterative method to calculate the refractive index and the extinction coefficient of CuInSe2 thin film deposited at 523 K. The Hall effect measurements and the temperature dependence of the electrical conductivity of CuInSe2 thin films, deposited at different substrate temperatures, revealed that the films had electrical resistivity in the range of 0.15–20 ohm cm, and the activation energy 82–42 meV, both being influenced by the substrate temperature.

Pramana-journal of Physics, 2009

Optics and Laser Technology, 2007

The optical output power of a laser diode can be enhanced by anti-reflection (AR) and high-reflec... more The optical output power of a laser diode can be enhanced by anti-reflection (AR) and high-reflection (HR) facet coatings, respectively, at the front and back facet. AR and HR coatings also serve the purpose of protection and passivation of laser diode facets. In this work, we have designed and optimized a single layer λ/4 thick Al2O3 film for the AR coating and a stack of λ/4 thick Al2O3/λ/4 thick Si bi-layers for the HR coating for highly strained InGaAs quantum-well edge emitting broad area (BA) laser diodes. Effect of the front and back facet reflectivities on output power of the laser diodes has been studied. The light output versus injected current (L–I characteristics) measurements were carried out on selected devices before and after the facet coatings. We have also carried out the numerical simulation and analysis of L–I characteristics for this particular diode structure. The experimental results have been compared and verified with the numerical simulation.

Journal of Materials Science, 2009

We have investigated the electrophysical properties of metallic thin films based on Cu/Cr and Fe/... more We have investigated the electrophysical properties of metallic thin films based on Cu/Cr and Fe/Cr systems. We find that the longitudinal gauge factor of two-layer films is significantly greater as compared with one-layer films, which have the same thickness as the total thickness of a two-layer film. Interface and intensive grain–boundary electron scattering explain such an increase in the longitudinal gauge factor. We find that the longitudinal gauge factor increases in transition from elastic to plastic zone.

Journal of Materials Science, 2009

The power enhancement of laser diodes is achieved by single and multilayer facet coatings such as... more The power enhancement of laser diodes is achieved by single and multilayer facet coatings such as antireflection and high reflection respectively at the front facet and the back facet of the laser diode. In this work, we have experimented with single layer λ/4 thick Al2O3 film for the Anti Reflection (AR) coating and stack of λ/4 thick Al2O3/ λ/4 thick Si bi-layers for the High Reflection (HR) coating. The AR/HR coatings were deposited in an electron beam evaporation system. The effect of front and back facet reflectivities on the output power of diode laser has been studied. The highly strained MOVPE grown InGaAs quantum-well edge emitting broad area (BA) diode lasers have been used for this experiments. The light output versus current (L-I) measurements were made on selected devices before and after the coatings. The devices were tested under pulsed operation with a pulse width of 400 ns and a duty cycle of 1:400. We have also carried out the theoretical analysis and simulation of L-I characteristics for this particular diode structure using LabVIEW. The experimental results were compared with simulated results. The effect of facet coating on external differential efficiency of diode laser has also been studied.

Journal of Crystal Growth

The development of thin-film semiconductor compounds, such as Copper Indium Gallium Selenide (CIG... more The development of thin-film semiconductor compounds, such as Copper Indium Gallium Selenide (CIGS), has caused remarkable progress in the field of thin-film photovoltaics. However, the scarcity and the increasing prices of indium impose the hunt for alternative materials. The Copper Zinc Tin Sulphide (CZTS) is one of the promising emerging materials with Kesterite-type crystal structure and favourable material properties like high absorption co-efficient and direct band-gap. Moreover, all the constituent elements of CZTS are non-toxic and aplenty on the earth-crust, making it a potential candidate for the thin-film photovoltaics. Here we report the synthesis of CZTS powder from its constituent elements, viz. copper, zinc, tin and sulphur, in an evacuated Quartz ampoule at 1030 K temperature. The sulphur content in the raw mixture in the ampoule was varied and optimised in order to attain the desired atomic stoichiometry of the compound. The synthesised powder was characterised by X-Ray diffraction technique (XRD), Raman Scattering Spectroscopy, Energy Dispersive Analysis of X-Ray (EDAX) and UV–Visible Absorption Spectra. The XRD Patterns of the synthesised compound show the preferred orientation of (112), (220) and (312) planes, confirming the Kesterite structure of CZTS. The chemical composition of the powder was analysed by EDAX and shows good atomic stoichiometry of the constituent elements in the CZTS compound. The UV–Vis absorption spectra confirm the direct band-gap of about 1.45 eV, which is quite close to the optimum value for the semiconductor material as an absorber in solar-cells.► Synthesis and characterisation of Cu2ZnSnS4 (CZTS) from constituent elements. ► Good stoichiometry in compounds prepared with 30% and 40% excess S in the raw mixture. ► Formation of CZTS with good crystalline quality confirmed by XRD and Raman spectra. ► Band gap of about 1.45 eV, which is close to optimum value for solar PV applications.

Journal of Materials Science, 2009

Copper indium diselenide (CuInSe2) compound was synthesized by reacting its elemental components,... more Copper indium diselenide (CuInSe2) compound was synthesized by reacting its elemental components, i.e., copper, indium, and selenium, in stoichiometric proportions (i.e., 1:1:2 with 5% excess selenium) in an evacuated quartz ampoule. Structural and compositional characterization of synthesized pulverized material confirms the polycrystalline nature of tetragonal phase and stoichiometry. CuInSe2 thin films were deposited on soda lime glass substrates kept at different temperatures (300–573 K) using flash evaporation technique. The effect of substrate temperature on structural, morphological, optical, and electrical properties of CuInSe2 thin films were investigated using X-ray diffraction analysis (XRD), atomic force microscopy (AFM), optical measurements (transmission and reflection), and Hall effect characterization techniques. XRD analysis revealed that CuInSe2 thin films deposited above 473 K exhibit (112) preferred orientation of grains. Transmission and reflectance measurements analysis suggests that CuInSe2 thin films deposited at different substrate temperatures have high absorption coefficient (~104 cm−1) and optical energy band gap in the range 0.93–1.02 eV. Results of electrical characterization showed that CuInSe2 thin films deposited at different substrate temperatures have p-type conductivity and hole mobility value in the range 19–136 cm2/Vs. Variation of energy band gap and resistivity of CuInSe2 thin films deposited at 523 K with thickness was also studied. The temperature dependence of electrical conductivity measurements showed that CuInSe2 film deposited at 523 K has an activation energy of ~30 meV.

Materials Chemistry and Physics, 2009

Tungsten trioxide (WO3) thin films are of great interest due to their enormous and promising appl... more Tungsten trioxide (WO3) thin films are of great interest due to their enormous and promising applications in various opto-electronic thin-film devices. We have investigated the structural, electrical, and optical properties of the WO3 thin films grown by thermal evaporation of WO3 powder and their dependence on growth condition. The WO3 thin films were grown on glass substrates at different substrate temperature varying from room temperature to 510 °C. The structural characterization and surface morphology were carried out using X-ray diffraction and atomic force microscopy, respectively. The amorphous films were obtained at substrate temperatures below 450 °C whereas films grown above 450 °C were crystalline. The surface roughness and the grain size of the films increase on increasing the substrate temperature. The electrical characterization has been carried out using four-point-probe methods. The resistivity of the films decreases significantly while the carrier concentration and mobility increase with the substrate temperature. The transparency and optical energy band-gap, Eg, of the films are found to decrease monotonically as the substrate temperature increases.