Semiconductor thin films, preparation methods

After the precursor stock solution is prepared, various techniques can be used to coat the substrate, depending on the solution viscosity, required film thickness and coverage. The most common methods in the semiconductor industry are spin- and dip-coating. Other processes that are used for deposition include spray coating and stamping. A summary of the uses, limitations, and advantages of the various thin film deposition methods is reported in Table 2.2. 

Ceramic and semiconductor thin films have been prepared by a number of methods including chemical vapor deposition (CVD), spray-coating, and sol-gel techniques. In the present work, the sol-gel method was chosen to prepare uniform, thin films of titanium oxides on palladium Titanium oxide was chosen because of its versatility as a support material and also because the sol-gel synthesis of titania films has been clearly described by Takahashi and co-workers (22). The procedure utilized herein follows the work of Takahashi, but is modified to take advantage of the hydrogen permeability of the palladium substrate. Our objective was to develop a reliable procedure for the fabrication of thin titania films on palladium, and then to evaluate the performance of the resulting metalloceramic membranes for hydrogen transport and ethylene hydrogenation for comparison to the pure palladium membrane results. 

As well as optical properties, the method of thin film preparation also influences the electrical properties. As-deposited WO3 films are n-type semiconductors. The room temperature conductivity usually lies in the range (Q.crn) , as shown in Table 38.1. Moreover, it... 

Specific physicochemical properties of the supercritical fluids offer flexible alternatives to established processes like chemical vapor deposition (CVD), which is used in the preparation of high-quality metal and semiconductor thin films on solid surfaces. Watkins et al. [43] reported a method named chemical fluid deposition (CFD) for the deposition of CVD-quality platinum metal films on silicon wafers and polymer substrates. The process proceeds through hydrogenolysis of dimethyl-(cyclooctadiene)platinum(ll) at 353 K and 155 bar. 

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

The redox behavior of the SeSO -Zn-EDTA system has been discussed on the basis of Pourbaix and solubility diagrams [11], Different complexes and substrates have been employed in order to optimize the electrodeposited thin films. By the selenosulfate method it is generally possible to grow ZnSe with an almost stoichiometric composition however, issues of low faradaic efficiency as well as crystallinity and compactiveness of the product, remain to be solved. Interestingly, in most reports of photoelectrochemically characterized ZnSe electrodeposits, the semiconductor film was found to be p-type under all preparation conditions (ZnSe is normally n-type unless deliberately doped p-type). 

It may finally be mentioned that molecular beam methods have been used for the preparation of semiconductors as GaP and of thin films of higher polyphosphides. 

ZnS-CdS (bandgap = 2.3-2.4 eV) composite semiconductor photoelectrodes show a broad spectral response and n-type behavior, with saturation of the anodic photocurrent upon increasing anodic potential making the system suitable for use as a photoelectrochemical cell photoanode [72], Nanostructured ZnS-CdS thin film electrodes show that anodic photocurrent saturation can be attained with the application of a small, 0.1 V, bias [73], while hydrogen evolution is observed at the Pt cathode. The performance of the ZnS-CdS photoanodes appear strongly dependent upon the method of film preparation [72,73], with Zn rich films demonstrating superior photocurrent generation, and stability, in comparison to Cd rich films. 

Armelao et al. (2005) fabricated LaCoOs thin films by the combination of chemical vapor deposition (CVD) and sol-gel methods. Two sequences were adopted to prepare the target film (i) sol-gel of Co-O on CVD La-O (ii) CVD of Co-O on sol-gel La-O. Losurdo et al. (2005) further investigated the spectroscopic properties of these films by ellipsometry in the near-IR and UV range. The former film has a larger crystallite size, a lower refractive index, and a higher extinction coefficient. It also presents a semiconductor-to-metal transition at a temperature of 530 K. Contrarily, the latter film has a smaller crystallite size, a higher refractive index, a lower extinction coefficient and a semiconductor behavior. 

Molecular beam methods are now widely used for the preparation of common semiconductors such as GaP and their intergrowth with other compounds. Thin films of iron phosphides of 0.5 to 25 J,m thickness can be electrodeposited from sulfate solutions. Depending on the deposition conditions (time, pH, temperature), the phosphides FeP, Fe2P, and/or FesP occur. The preparation of higher polyphosphides, for example, KP15, as thin films demonstrates the range of still yet unexplored preparative methods. This is also valid for the electrolysis of phosphates. The latter techniques is mostly suited for metal-rich phosphides. ... 

PLD of crystalline and stoichiometric ferroelectric materials has been successful for a number of materials. For example, crystaUine PbTiOs thin films have been deposited at only 350 °C using a KrF excimer laser (A. = 248 mn). The low deposition temperature makes this a potentially attractive method for directly fabricating ferroelectric/semiconductor hybrid devices. A related solid that is more difficult to prepare in thin film form is PbZri- ThOs. To maintain the Pb stoichiometry and ensure random mixing of Ti and Zr requires careful control of film synthesis conditions. ... 

As a probe of lattice vibrations, Raman spectroscopy is very sensitive to intrinsic crystal properties and extrinsic stimuli, especially in semiconductors. It may be employed to study crystal structure and quality, crystal orientation, optical interactions, chemical composition, phases, dopant concentration, surface and interface chemistry, and local temperatme or strain. As an optical technique, important sample information may be obtained rapidly and nondestructively with minimal sample preparation. Submicron lateral resolution is possible with the use of confo-cal lenses. These features have made it a vital tool for research labs studying semiconductor-based technologies. They also are increasingly important for the study of semiconductor NWs fabricated by both top-down and bottom-up approaches since many of the common characterization methods used with bulk crystals or thin films cannot be applied to NWs in a direct manner. 

For metal, dielectric and semiconductor films fabrication, optical and silica glass are popular substrate materials because of their availability, cost-effectiveness, and inert character, i.e., they are stable in the required temperature range for common photonic, optoelectronic and photovoltaic applications, they do not chemically react with the prepared films, and the hard plane surface makes the formation of optically smooth thin films fairly easy. Generally, it is preferable to form films by a simple, low-temperature, inexpensive and environment friendly method. Sol-gel technique and thermal evaporation is found suitable for the preparation of film parts of efficient solar cells [1], emitters, transformers [2], detectors and modulators of light [3], as well as optically stimulated luminescence dosimeters [4]. Here, we present the experimental data on the resistance to high-power optical and ionizing irradiations of the undoped components of film compositions with nanociystais. 

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