First-principles and Quantum-transport Calculations of Metal Germanides/n-type Ge Contacts

• Main Authors: Yu, Shao-Cheng, 余紹成
• Other Authors: Lin, Chiung-Yuan
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/2b3e6g

碩士 === 國立交通大學 === 電子研究所 === 107 === In this thesis, we perform the first-principles and quantum-transport calculations to study the Pd-germanide/Ge and Pt-germanide/Ge Schottky junctions. First, we find out the compositions and crystalline planes of Pd-germanide and Pt-germanide from the GIXRD analysis, and initially narrow down the choices of atomic structures of the PdGe-&PtGe2/Ge contacts based on several crystalline planes of the stronger peak signals. After further considering the lattice mismatches and their desired computational resources, we then choose and construct PdGe(011)/Ge and PtGe2(120)/Ge as our modeled structures of the electronic-structure and transport calculations. From the calculated energetics and electronic structures, we compare the Schottky barrier height lowering effects under segregation of various dopants. Calculations reveal that they tend to segregate to the interface, and the conventional N-type dopants P, As, and Sb segregating to the semiconductor side can reduce Schottky barrier height. For transition-metal dopants, only Pt can reduce Schottky barrier height in Platinum germanide/Ge contact. However, the overall reduction of the Schottky barrier height for our calculated dopants is less than 0.1 eV, which likely results from the strong Fermi level pinning. Based on our previous quantum-transport calculations of the NiGe/Ge contacts, we speculate that the improvements of junction characteristics are mainly attributed to the high dopant concentration (within 3~5 nm) around the contact, while the reduction of Schottky barrier height only place a minor role. Finally, by employing the nonequilibrium Green's function method with the density functional theory, we calculate specific contact resistance to inspect whether the dopants at the interface atomic layer can reduce the specific contact resistance. Under quantum-size effects, the results show that dopants rarely influence the specific contact resistance of the platinum germanide/Ge contact, with its values always close to 3×10-9 Ω×cm2. We additionally find in experiments that enormous Pd atoms diffuse into the Ge semiconductor, resulting in the anomalous increase of the reverse bias current in Palladium germanide/Ge contact. Thus, by performing first-principles calculations, we first calculate the PdGe(011)/Ge(001) contact with Pd doped in Ge slab at different distances(1Å, 7Å, 12Å) away from the interface. We find that high density of the gap states appears at both ends of the energy gap no matter whether there is no interface dopoing or Pd atoms are doped near the interface. We further find that these interface states are metal bulk states, i.e., the interface states withing the energy gap are formed by the PdGe alloy wave functions as they penetrate into the Ge slab during the formation of the PdGe(011)/Ge(001) contact. On the other hand, while Pd are doped far from the interface, we find that a high density of states distributes in the midgap regime, implying that these gap states are dopant states generated by Pd dopants. To verify the above gap states are indeed dopant states, we construct a simple Ge bulk with Pd doped at an areal concentration same as that of the PdGe(011)/Ge(001) contact. Such a calculation shows that Pd dopants generate high-density gap states also in the midgap regime, consistent with the case that Pd are doped far away from the interface in the PdGe(011)/Ge(001) contact. This verifies that the high-density midgap states are dopant states generated by Pd dopants. Finally, we adopt Synopsys TCAD tools to calculate the I–V characteristic by considering of trap assisted tunneling (TAT) model as well as the dopant-generated midgap states in our first-principles calculations. In such a way, We further justify that high-density midgap states can indeed induce the increase of the reverse bias current.