CdSe thin film transistor (TFT) structures which have been ion implanted with 50 keV 52Cr, 50 keV 27Al, or 15 keV 11B have a very steeply rising conductivity above some threshold dose and exhibit modulated transistor characteristics over certain ranges of implant dose, even though there is no thermal annealing during or after ion implantation. These results are interpreted using a model based on grain boundary trapping theory. The dependence of leakage current on implant dose, and of drain current (at a fixed dose) on gate voltage are described very well by this model, when the drain voltage is very small. Using this simple model, the important parameters of the polycrystalline CdSe film, namely the trap density per unit area in the grain boundary, the donor density, grain size, and electron mobility can be deduced. The effect of thermal annealing on implanted and unimplanted CdSe TFT’s has also been studied and the model appears to give a general description of the conductivity behavior in polycrystalline semiconductor TFT’s. This is illustrated by applying the model to devices fabricated by other groups from polycrystalline CdSe, poly-Si and laser-annealed poly-Si semiconductor layers.
Deposition of films by sputtering was observed first in 1852 by Grove. The technique was in general use through the 1920s for preparing reflective coatings and other thin film samples. Western Electric deposited gold on wax masters for phonograph recordings. The improvement in diffusion pump technology at that time caused thermal evaporation deposition to replace sputtering.Not till the 1950s did sputter deposition reappear… Bell Laboratories developed tantalum hybrid circuit technology using sputter deposition. Besides depositing Ta, they created a new material, Ta2N, by reactively sputtering tantalum in gas mixtures of argon and N2. Since then, these two methods, sputtering of metals and alloys and reactive sputtering of compounds, have been investigated for many applications of thin film materials.Although the general aspects of the methods have changed little in the past 30 years, the implementations have changed significantly, particularly since the introduction of magnetron systems in the 1970s. This review will concentrate mainly on these flexible, high rate magnetron deposition systems.The term sputtering actually applies to the physical processes by which atoms are removed from a material. Momentum is transferred from an incident, energetic particle, usually in the form of an ion, to atoms of the target material. A large number of these atoms are displaced from their normal sites in the crystal lattice, producing a disordered structure that also contains some of the incident particles, which are implanted. Some of the target atoms are displaced from the surface; if they have enough energy, they escape from the target as sputtered atoms.
ZnO films have been prepared by rf sputtering a Zn target in a planar magnetron system with controlled Ar/O2 gas mixtures. The films were deposited on unheated glass substrates which were either stationary in front of the target or in constant motion. Both the system pressure and plasma impedance changed when an oxide layer formed on the target surface. This occurred at an oxygen flow rate which increased almost linearly with rf power; at 500 W, the required flow rate was 9 ml/min and the pressure increased from 0.1 to 1.2 Pa due to the reduced oxygen gettering. High resistance ZnO films were deposited at oxygen flow rates above this threshold value. The target self-bias voltage increased by 30 V at this value; it is affected by both the system pressure and the power. The deposition rate increased linearly with power at approximately 0.03 (μm/min)/(W/cm2) which appears to be typical of sputtering from a ZnO layer or target. For continuous substrate motion, the average rate was approximately 7% of this value. All the films were polycrystalline ZnO with a preferred orientation, the c axis of the hexagonal structure being within a small angle of the substrate normal; this orientation was improved by motion of the substrate past the target. Films deposited at pressures of approximately 0.4 Pa had a large internal stress, as revealed both by substrate bending and x-ray measurements. Increasing the pressure to 4.7 Pa decreased the stress by an order of magnitude. SEM analysis showed that this was associated with the development of a columnar structure. The refractive indices obtained from guided wave measurements were 1.940±0.006 and 1.962±0.003, which correspond to 97% of the single crystal values. The resistivity measured normal to the film plane was greater than 107Ω cm. The changes in film stress and structure are similar to effects in metal films. The electromechanical coupling coefficients obtained from SAW measurements are approximately half the best reported value.
The scattering of sputtered atoms by the sputtering gas has been modelled to obtain values for the distances which the atoms travel normal to the sputtering target before their energies are reduced to the thermal energy of the gas. This distance increases with the mass and energy of the sputtered atom and with decreasing gas pressure; for a 5-eV atom of mass 80, it decreases from 42 cm at an argon pressure of 0.1 Pa to 0.44 cm at 10 Pa. For most diode sputtering configurations, the sputtered atoms are thermalized before reaching the substrate and the transport to the substrate is by diffusion. Relative deposition rates for substrates situated behind apertures or masks in these diffusive sputtering situations have been measured and compared with the solid angles subtended at the substrate by the effective aperture. The agreement between relative values is very good. Thus, the solid angle provides a simple measure for determining the effect of a mask on the deposition profiles. Significant changes in rates and profiles are possible. With a 5-cm-square aperture, the rate at the center decreases by 50% as the substrate moves 2 cm away from the aperture plane and there is a 30% variation in rate over the substrate. It is impossible to obtain a sharply defined edge using a mechanical mask.
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