A.K. Kulkarni

Dependence of the sheet resistance of indium-tin-oxide thin films on grain size and grain orientation determined from X-ray diffraction techniques

ITO thin films (100‐200 nm) are deposited on glass and plastic (PET and polycarbonate) substrates by r.f. sputtering. Process parameters such as oxygen partial pressure, r.f. power, and post deposition annealing parameters are varied to determine the dependence of the sheet resistance on process parameters. The microstructure of these thin films is determined using an X-ray diffractometer (XRD) and a transmission electron microscope (TEM). The experimentally observed dependence of the sheet resistance on the grain size and grain orientation of these films is correlated to the dependence of the electron mobility on grain boundary scattering. Larger grain sizes (<25 nm) in ITO films result in lower sheet resistance (250 V/A). This type of large grain size microstructure is produced with moderate r.f. power (<100 W) and low oxygen partial pressure (<10%). There is a unique correspondence between grain size and grain orientation. ITO films with a strong peak intensity ratio of (400) orientation to all other orientations (<0.35) have the largest grain size (<25 nm) resulting in the lowest sheet resistance (250 V/A) and high transmission ( < 86.7%) at la 550 nm. q 1999 Published by Elsevier Science Ltd. All rights reserved.

Electrical, optical and structural characteristics of indium-tin-oxide thin films deposited on glass and polymer substrates

Abstract The sheet resistance, optical transmittance and microstructure of tin-doped indium oxide (ITO) thin films (50–100-nm thick) rf sputter deposited on polymer substrates are investigated using a four-point probe, spectrophotometer, X-ray diffractometer and a transmission electron microscope (TEM). Sheet resistances vary from 250 Ω/sq. to 170 kΩ/sq. Sheet resistances for the ITO films on polycarbonate substrates are at least an order of magnitude higher than those ITO films deposited on glass substrates at the same time. Annealing ITO films on polycarbonate substrates at 100°C in air for 1 h decreased the sheet resistances significantly (almost by 50%). The X-ray diffraction data indicate polycrystalline films with grain orientations predominantly along (222) and (400) directions. TEM photographs show two distinct regions of growth: a dense growth close to the substrate and a sparse growth away from the substrate. The vertical growth is columnar and rod shaped. Changes in the ITO film sheet resistance either due to the types of substrate used or due to annealing can be correlated to the grain size and grain orientation.

Electrical and structural characteristics of chromium thin films deposited on glass and alumina substrates

Abstract The electrical resistivities of electron beam deposited and sputter deposited chromium thin films (25–500 nm) on glass and alumina substrates are correlated with the structural characteristics of the films determined by X-ray diffraction, scanning electron microscope and Auger electron spectrometer. The resistivities of as-deposited chromium films on glass substrates range from 58 to 107 μΩ cm, whereas, those deposited on alumina substrates range from 155 to 573 μΩ cm. The surface roughness appears to be the most important parameter in determining the resistivity on different substrates. The electron-beam deposited films have lower resistivities compared to the sputter deposited films because there is less oxygen and carbon contamination in the e-beam deposited films. The average grain sizes are determined from the X-ray diffraction data and are compared to the resistivity of Cr films.

Electrical, optical, and structural properties of indium-tin-oxide thin films deposited on polyethylene terephthalate substrates by rf sputtering

Indium-tin-oxide (ITO) is a transparent conducting material which is deposited as a thin film on glass substrates for use in opto-electronic devices. However, there are several applications for which ITO films on polymeric substrates are desirable. The sheet resistance, optical transmittance, and microstructure of as-deposited ITO thin films on unheated polyethylene terephthalate substrates were studied using rf sputter deposition. During separate deposition runs, the partial pressure of oxygen was varied from 5% to 20% and the deposition time was varied from 15 to 120 min. No significant variations are observed in the sheet resistance with respect to oxygen partial pressure; however, changes in sheet resistance were observed in ITO films deposited on different substrates for short deposition times (15 min). Additionally, the thickness of the film (assumed to be proportional to the deposition time) is shown to have a considerable impact on the sheet resistance and the optical transmittance. The x-ray diff...

Electrical, optical and chemical properties of indium-tin oxidized films grown by sequential electron beam deposition of indium and tin

Abstract A novel technique of layer-by-layer deposition of indium and tin by electron beam deposition and subsequent annealing in oxygen atmosphere to form indium tin oxide was attempted here. The tin content in the sample was varied by changing the relative thickness of the indium and tin layers. The as-deposited samples did not result in the expected layered structures as evidenced by the Auger depth profile plots. Instead, an In-Sn alloy of almost uniform composition was achieved. Resistivity measurements on as-deposited samples by the four-point probe technique yielded a graph typical of binary eutectic alloys. These samples were also highly reflective and exhibited no optical transmission. Annealing 50% Sn samples at 500°C for 60 min in flowing oxygen gas resulted in 50% transmission in the range from 330 nm to 820 nm with a resistivity of 4.56 × 10 -4 Ω cm . Many samples were tin deficient after annealing, resulting in poor optical transmission data. Possible reasons for the absence of tin after annealing are discussed.

Electrical properties of diamond thin films grown by chemical vapor deposition technique

Diamond thin films grown on high resistivity, 〈100〉-oriented silicon substrates by the hot filament chemical vapor deposition method have been characterized by four-point probe and Hall effect measurements. The resistivities of both as-grown and chemically etched samples were lower than expected. The Raman spectra showed dramatic changes from two broad bands (one starting at 1250 cm−1 and peaking at 1350 cm−1 and the other starting at 1500 cm−1 and peaking at 1580 cm−1) for the as-grown samples to a sharp peak centered at 1332 cm−1 with a full width at half-maximum of 10.7 cm−1 for the chemically treated sample. Hall measurements yielded carrier concentrations in the temperature range 180–300 K. From a plot of carrier concentration vs. inverse temperature, activation energies of 0.36 and 0.20 eV are obtained for the two chemically treated samples. The resistivity values at room temperature of 100 Ω cm are significantly lower than the values recently observed for other undoped samples (ϱ=103−104 Ω cm). The low resistivity values observed in this work are attributed to the disordered graphitic regions between the diamond crystalline grains.

A circuit model for a thin film ferroelectric memory device

Thin film ferroelectric memory devices have displayed ideal memory characteristics such as nonvolatility, radiation hardness and fast switching times. The simulation of the digital circuits using thin film ferroelectric memory devices necessitates an equivalent circuit model for these thin film device structures. The basic switching characteristics of a device is nonlinear in nature and requires a generating source in the equivalent circuit model. This current source is related to the polarization current as a function of time.The capacitance-voltage and hysteresis data on capacitor type device structures suggest the existance of an interfacial layer at the metal-ferroelectric interface. The interfacial layers are characterized by appropriate RC networks. The bulk ferroelectric material is modeled as a linear capacitance (dielectric) in parallel with a bulk resistance in parallel with the current generating source. This equivalent circuit model is simulated on the IBM 4381 mainframe computer using SPICE. ...

Electrical characterization of CVD diamond thin films grown on silicon substrates

Abstract Diamond thin films grown on high resistivity, 〈100〉 oriented silicon substrates by the hot filament chemical vapor deposition (HFCVD) method have been characterized by four-point probe and current-voltage (through film) techniques. The resistivities of the as-grown, chemically etched and annealed samples lie in the range of 102 Ω cm to 108 Ω cm. The Raman measurements on these samples indicate sp3 bonding with a sharp peak at 1332 cm−1. The surface morphology as determined by scanning electron microscope shows polycrystalline films with (100) or (111) faceted structures with average grain size of ≈2.5 μm. The through film current-voltage characteristics obtained via indium contacts on these diamond films showed either rectifying or ohmic behavior. The difference in Schottky and ohmic behavior is explained on the basis of the high or low sheet resistivities measured by four-point probe technique. 5% methane to hydrogen concentration during film growth resulted in poor surface morphology, absence of sp3 bonds, and low resistivity.

Electrical properties of ferroelectric thin film KNO3 memory devices

Abstract Thin film non-volatile memory devices are fabricated on glass substrates by evaporating KNO3 in an ultra high vacuum system. The top and bottom gold electrodes [25 μm × 25 μm] are deposited by shadow masking in the same vacuum system. The KNO3 is protected from moisture by depositing a passivation layer of SiO. Pulse switching characteristics of these thin film memory devices show excellent signal-to-noise ratios for nonlinear and linear behavior. The integration of these transient current vs. time curves yields large values of spontaneous polarization (Ps ≈ 103 μC cm−2). Capacitance-voltage measurements made at high frequency (1 MHz) with slow ramp rates (200 mV s−1−2 V s−1) indicated well-defined thershold voltages and the presence of mobile carriers.

Degradations in the electrical and structural characteristics of Schottky diodes and ohmic contacts to GaAs due to thermal aging

Abstract The reliability of GaAs microwave devices is directly related to the integrity of Schottky and ohmic contacts for Schottky barrier devices and metal/semiconductor field effect transistor devices. The interface analysis of these device structures using surface analysis techniques has become extremely important in the study of the degradation of these devices. The research reported here focuses on three different metallic systems, namely Au/In, Au-12 wt.% Ge and Ni/AuGe, for both Schottky and ohmic contacts. The three metallic systems were evaporated onto 〈100;〉- oriented GaAs substrates ( N D =3×10 17 cm -3 ) in an ultrahigh vacuum system. These samples were thermally aged by keeping them at 150°C for 500h. Current-voltage and capacitance-voltage measurements were made on as-deposited and thermally aged samples. The ideality factor decreased in all the samples. There was an apparent large increase in barrier height in AuGe/GaAs and Ni/AuGe/GaAs Schottky diodes. There was an insignificant change in the contact resistivity of ohmic contacts after thermal aging. The changes in the electrical characteristics of these device structures are explained on the basis of the formation of an oxide layer after thermal aging. A comparison of the Auger depth profiles of the as-deposited and the thermally aged samples substantiates the electrical observations. However, Au/In/GaAs Schottky diodes do not show the existence of an oxide layer at the interface. The out-diffusion of indium to the surface might have removed the oxygen from the interface to result in an Au-GaAs interface in the thermally aged sample. A slight increase in the barrier height of this sample is due to the Au-GaAs interface rather than the In-GaAs interface.