Electrical Characterization On SOI Nanomembranes After The Surface/Interface Hydrogen Passivation

Silicon-on-insulator (SOI) nanomembrane, that is the very thin top silicon template of SOI, for its large surface-to-volume ratio, it has special properties which can not be observed for bulk silicon. For example, SOI nanomembrane is very sensitive to the surface/interface modifications, which may be shown by the significant change in electrical conductivity. Such sensitivity leads to diverse applications such as biological and chemical sensors based on SOI nanomembrane. Therefore, it is very meaningful to explore the influence on electrical properties of SOI nanomembranes after the surface/interface modifications, which is also helpful for leading us a step closer to an atomistic understanding of the complicated problem of the influence of surfaces on charge transport in semiconductor nanosystems.There are many ways to modify the surface/interface of SOI nanomembranes. Hydrogen passivation is one of simplest ways. Hydrogen-terminated surface can be got by hydrofluoric acid (HF) immersion and vacuum-hydrogenation (VH) treatment; hydrogen-passivated interface can be achieved through forming gas anneal. Hydrogen passivation to the surface/interface plays a very important role in the production and improving the performance of semiconductor devices, which has been widely studied. However, people seldom report hydrogen passivation via all the three methods together. In this thesis, SOI nanomembranes with native oxide surface were treated by HF immersion and VH treatment to get hydrogen-terminated surface, and annealed by forming gas (5%H2in N2) to gain hydrogen-passivated interface, respectively. The sheet resistances were measured by van der Pauw method to reveal the influence on the electrical conductivities of SOI nanomembranes after those modifications to the surface/interface. X-ray photoelectron spectroscopy (XPS) measurements were also performed to confirm the electrical measurements. The main results are as follows:1. HF treatment was performed on220nm thick silicon template of SOI (220nmSOI) with native oxide surface. It is found the sheet resistance drops from107Ω/□to105Ω/□, then increases because of the surface re-oxidation, and finally reaches the sheet resistance of samples with native oxide surface. If the sheet resistances of samples were measured after HF treatment without DI water rinse, it is found the sheet resistance of the sample treated by HF with higher concentration is smaller than that of the sample treated by HF with lower concentration, which indicates that fluorine brought by HF treatment on the surface plays an important role in the conductivities of SOI nanomembranes. It is also observed that the sheet resistance of sample rinsed by de-ionized (DI) water after HF treatment is a little larger than that of sample without DI water rinse, and increases with the rinse time, that is because the number of (OH)-1group is increasing on the surface during the DI-water rinse, and (OH)-1plays a very important role in re-oxidation of the surface. Different surface oxidation rates are also observed from the slope of the sheet resistance curves:the surface of the sample rinsed with longer time or treated by HF with lower concentration has higher oxidation rate. All these results are consistent with the results from XPS measurements.2. VH treatment can produce pure hydrogen-terminated surface of SOI nanomembranes. VH treatment and HF immersion were performed on220nm and28nm SOI nanomembranes with native oxide surface. It is observed that for SOI nanomembrane with the same thickness, HF immersion can cause one order of magnitude larger drop in sheet resistance than that caused by VH treatment at the beginning, which implies that the difference in surface components caused by the two treatments can be revealed by the different change in conductivities of SOI nanomembranes. It is found that thinner SOI nanomembranes are more sensitive to the either surface treatment, shown by larger drop in sheet resistance. Compare the slopes of sheet resistances curves of28nmSOI nanomembranes treated by the two surface modifications, it is found that the surface oxidation rate of sample treated by HF is slower initially and becomes faster later than that of sample treated by VH. All these results can be confirmed by the results from XPS measurements. It is found that the surface of28nmSOI nanomembrane treated by VH returns to native oxide level much slower than that of the sample treated by HF, indicating VH treatment is better than HF immersion to get hydrogen passivated surface which can survive longer time. Besides, it is also found that F does not ultimately play the decisive role the re-oxidation kinetics on HF-treated surface.3.77nm Unibond SOI and89nm SIMOX SOI were annealed by forming gas. It is found that the sheet resistances of the two kinds of SOI nanomembranes dropped. However, the sheet resistance of89nm SIMOX SOI nanomembrane drops larger than that of77nm Unibond SOI nanomembrane treated by the same procedure of forming gas anneal. For89nm SIMOX SOI nanomembranes, vary the temperature or the period of forming gas anneal, and it is found that before the dangling silicon bonds on the Si/SiO2interfaces of SIMOX SOI are all saturated, the improvement of the conductivity has much to do with the condition of anneal procedure. That is, at the same anneal temperature (period), the conductivity is improved better for longer anneal period (higher anneal temperature).