I have just presented hydrogenated amorphous silicon. Let us now pass to the thin films of nanocrystalline silicon. What are the challenges of nanocrystalline silicon? The general objective is to overcome hydrogenated amorphous silicon while keeping the context of thin film depositions. The first limitation of amorphous silicon is its low mobility related to defects. One square centimeter per volt per second for electrons, 1000 times less than crystalline silicon are negligible for holes. Only end-transistors are possible with amorphous silicon, which make it impossible to manufacture logic circuits, most complimentary with crystalline silicon technology. Another difficulty of amorphous silicon is the low efficiency of solar cells. Less than 10 percent, two or three times less than crystalline silicon. This limitation is enhanced by the phenomenon of instability of the IR band gap, 1.8 eV, which does not allow the conversion of infrared photons. Let's finally mention the problems related to the doping of amorphous silicon. The low doping efficiency can easily be overcome technologically, but the increase of defects due to doping constitutes an important drawback for the device preparation. The objective of nanocrystalline silicon is to be able to decrease the consequence of these shortcomings, while keeping the main advantages of amorphous silicon. That is to say, it's low cost deposition based on plasma growth which can be extended on very large area together with a moderate thermal balance, 200 degree C. Moreover, the PECVD deposition exhibits great flexibility notably for the deposition of multilayer. Crystallization is favored thermodynamically. The crystalline phase being more stable than the amorphous one. Thus, a relatively high temperature heating of an amorphous layer induces its crystallization as illustrated here. Heated at 600 degrees for several hours induces partial crystallization. The presence of crystallites as micrometric size are clearly evidence in this electron microscopy picture. Nevertheless, crystallization by thermal annealing at high temperature, 600 degrees C, is incompatible with the cheap substrate such as glass. Moreover, it is a long process therefore, expensive and difficult to extend on large areas. It can then be shown that plasma deposition can make it possible to obtain a partially crystallized material. Examples of electron microscopy pictures, scanning or transmission of plasma deposited thin films on glass substrate are presented here. Depending on the discharge condition, it is possible to obtain a polymorphous materials on the left, presenting locally a crystalline order on the nanometric scale. By changing the plasma deposition condition, it is possible to grow nanocrystalline silicon with crystallites of the order of five nanometers or microcrystalline, if the size is micrometric, and even polycrystalline material as can be seen on the right with grains from 10 to several hundred microns. Under certain condition, it is even possible to achieve total crystallization of the thin film. The control of your discharge conditions leading to the various form of nano, micro, polycrystalline silicon is particularly complex. You will find more details in Appendix four. Let's summarize here some essential key points which favors the crystallization. I remind you that thermodynamics favor the crystalline phase, and therefore, if one uses plasma condition close to the equilibrium between growth linked to the dissociation of the saline on etching favored by atomic hydrogen or fluorine. The etching will preferably affect the amorphous phase. As a result, the crystalline volume fraction of the layer will increase during growth. The formation of crystallite seeds in the gas phase can be a favorable factor. It should be noted that the presence of hydrogen in the gas phase causes exothermic reaction favoring crystallization. The growth of nanocrystalline silicon can be induced by the gas phase as you have just seen. It can also occur in solid phase. In fact, atomic hydrogen that diffuses proton, diffuses very easily into the films and promotes exothermic reactions with induced crystallization. Let's mention the influence of the substrate. The glass substrate obviously does not favor the crystalline order. On the other hand, if a crystalline silicon is used, the propagation of the crystalline order into the film will be favored, leading to the growth of a totally crystallized materials which can be then transferred on other substrates. I gave you the main keys in order to obtain a film, partially crystallized. Animation illustrates the influence of total pressure. Dissociation of saline produces very small ions on radicals. These radicals on ions can then react with the saline forming fragments containing several silicon atoms. The increase in pressure favors the secondary reaction. It is therefore possible to manufacture agglomerates at higher pressures, some of which may be crystallized. They can be deposited directly onto the substrate. At very high pressure, these agglomerates can become microscopic. Powders are thus produced which are generally to be avoided. I have presented in these seconds, the nanocrystalline silicon thin films. Please refer to Appendix four for better understanding of these growth processes. It will be seen in the following chapter, how to prepare solar cells from the thin films semiconductors, and it will be underlined that it is possible to combine these two thin films, amorphous and microcrystalline in order to produce tandem cells. Thank you.
