Photocurrent Of Hydrogenated Nanocrystalline Silicon Thin Films

By the experiments of Fourier transform infrared spectrum, We have investigated the optoelectronic properties of the hydrogenated nanocrystalline silicon (nc-Si:H) thin film, especially for the impact of its internal microscope structure on the photo-carriers'generation and collection efficiency. As we all know, the efficiency of photo-carriers generation in the traditional bulk crystalline silicon material is much lower than that in the direct bandgap semiconductors, since the virtue of indirect bandgap in the crystalline silicon requires the attendance of phonons to satisfy the moment conversation during optical interband transitions. The indirect band and the phonon's assistance to the optical interband transition in crystalline silicon are mainly determined by the micro structure in atomic distribution level. Currently, the breakdown of long-range order in crystalline silicon has been found to be one of the effective ways to improve the optical absorption efficiency. Nowadays, some silicon materials have been developed by chemical etching, sputtering, or plasma-enhanced chemical vapor deposition methods with low-temperature growth technique, such as porous silicon, amorphous silicon (a-Si), and poly-silicon. The new foundings will not only improve the optoelectronic performance tremendously, but also reduce the cost of optoelectronic devices.Recently, the a-Si and porous-Si thin films have been widely used to fabricate many kinds of optoelectronic devices such as light emitting diodes, the thin film transistors and solar cells. In these silicon thin film devices, the moment conversation had been violated, so that optical absorption and emission efficiency can be upgraded greatly. However, the high density of disordered structure and traps are induced, which will significantly frusturate the photocarriers transport and collection, leading to the decrease of photo-electrical energy transformation efficiency. In addition, the photoconduction will also become unstable and degraded.According to the current status in the silicon optoelectronic devices described above, we have proposed a new idea of nc-Si:H thin film to improve the photoconductivity performance, which can be well achieved by enchancing the photo carriers'generation and collection simultaneously. The nc-Si:H thin film is firstly prepared on glass substrate, and it consists of high density nanometer silicon crystals seperated by the boundaries with amorphour silicon or viods, which is also in nanometer size. This kind of microstructure can effectively break up the long-range order of crystalline silicon, thus improve the photo-carriers generation rate as that in amourphours silicon. Furthermore, the carriers transport conductance in the nc-Si:H thin film is much higher than that in a-Si, since the bondary size of nanostructure is basically in the magnitude of the electron wave length at room temperature. The studies in this thesis demonstrate that the optical interband transitions in the nc-Si:H thin film are quite different from the indirect transition in c-Si, the transition probability and optical absorption in nc-Si:H are much higher than those in c-Si. In the glass based nc-Si:H thin film material, we have sucessfully observed photo current signals with intensity close or even higher than that in c-Si. With such high photo conductance and photo current, nc-Si:H thin film is possible to be a good candidant in the application of infrared photo detection and image devices.Based on the experimental photo conduction results, the traditional diffusion-recombination theory and model have been extended to describe the photo conductance in the nc-Si:H thin film. We have found that the photo carriers are generated mostly from the interband transitions, different to the indirect transition mechanism in c-Si. The experimental results of the crystalline fraction depending on the photo conductance further demonstrate that the photo carriers are originated from the inter band transitions within the nanometer grains.In addition, we also have investigate the photocurrent response of nc-Si:H/c-Si heterostructure. As we know, the inner depletion electric field in heterosture is a good condition for the collection of photo carriers. Moreover, the lattice match between the c-Si substrate and nc-Si:H thin film ensures the highly quality of nc-Si:H thin film with more ordered structure and thinner a-Si boundary (only several atomic layers). Such a thin bondary may cause the overlapping of the electron waves easily between the neighboring nano structures, then form a mini band, which is helpful for the carriers to transport in the nc-Si:H thin film. From our experiment results, we have not only observed high photocurrent in the nc-Si:H/c-Si heterostucture but also found the evidence of the existence of mini band in the nc-Si:H thin film. In this dissertation, the photocurrent response property has been studied in detail, including the temperature and external bias voltage dependency. These investigations open a new way to develop high performance optoelectronic devices by silicon or nc-Si:H/c-Si heterosturcture.