A Study on the Contact Resistance Reduction in Metal/n-type Germanium Contacts
|關鍵字:||鍺基板;接觸阻抗;蕭基位能障;介電層插入技術;蕭基位能障調變技術;Germanium;Contact resistance;Schottky barrier;Dielectric insertion process;Modified Schottky barreir process|
With the rapid progress of nano-fabrication technology, Si based MOSFETs have been successfully scaled down to 20 nm regime. However, the continued scaling will be a problem due to several physical and technical limitations, and the device performance may not be improved by further scaling down. Many methods have been purposed to solve this problem, because of the higher carrier mobilities and better process compatibility, Ge is considered a potential candidate to replace silicon as the next generation channel material. However, the contact resistance between metal and n-type Ge is very high due to the high Schottky barrier height. To implement high performance Ge NMOSFETs, reducing the contact resistance of metal/n-type Ge is critical. In this thesis, two different methods to reduce the contact resistance of metal/n-type Ge are investigated. One is inserted dielectric-inserted junction and the other is modified Schottky barrier junction. The former is to modulate the Schottky barrier height and the latter is to enhance the doping concentration at the metal/Ge interface. The dielectric insertion method has been reported in literature. Both Al2O3 and TiO2 have been used as the inserted dielectric layer. Because of the lower conduction band offset of TiO2 to Ge, it can not only reduce the Schottky barrier height but also achieve low tunneling resistance and high conduction current. However, the mechanism has not been well understood. In this thesis, we first observed that the Fermi level pinning effect is a very weak function of the TiO2 thickness so we infer that the Schottky barrier height reduction by the dielectric insertion is due to the change of the pinning position. The amount of fixed charges in the thin dielectric layer is extracted from a MIS structure and it is found that the small amount of fixed charges is not sufficient to produce such a pronounced voltage drop to modulate the Schottky barrier height. It is thus recommended that the mechanism of the dielectric insertion method mainly comes from interface dipoles. Besides, it is observed that after annealing at 300 °C for 30 minutes, the Schottky barrier height will increase. The increase of the Schottky barrier height increase with the increasing of the annealing temperature. According to the microstructural analysis, it is postulated that the crystallization of the dielectric layer after annealing is the main reason for the increase of Schottky barrier height. Due to the dielectric constant increase after crystallization, the capacitance of the dielectric capacitor increases and causes smaller voltage drop so that the Schottky barrier height increases. The thicker TiO2 dielectric layer has smaller capacitance, which might be the reason for the lower Schottky barrier height. In the modified Schottky barrier method, dopants are implanted into metal-germanide instead of Ge so that the number of defects formed in substrate could be negligible, which would mitigate the dopants diffusion caused by the interaction between defects and dopants. A thin and high doping concentration layer can be formed at the metal/semiconductor interface. Increasing the activation temperature will achieve higher doping concentration. But the allowed annealing temperature is limited by the poor thermal stability of NiGe films. To improve the thermal stability of NiGe films, a Si layer is inserted between Ni and Ge before annealing. The result shows that the thermal stability of NiGe film is improved by this Si-insertion layer from 500 °C to 550 °C. Using the Si-insertion technique, the activation temperature of MSB junctions can be raised to 550 °C so that the doping concentration is enhanced. Compared to the conventional junction with direct implantation into Ge, the junction depth is much shallower and the carrier concentration is much higher for the MSB junction which suggests that the MSB process is attractive for the short channel Ge MOSFETs. In summary, this thesis proposed the mechanism of the Schottky barrier height modulation by dielectric insertion. The thickness effect and annealing effect are also explained. A new method, modified Schottky barrier method, is propsed to reduce the contact resistance between metal and n-type Ge by forming a thin and high concentration layer at the metal/Ge interface. This method is very promising for short channel Ge NMOSFET.
|Appears in Collections:||Thesis|
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