Paper EM-ThM6
Impact of Threading Dislocation Density and Dielectric Layer on I-V Characteristics of Schottky Diodes Fabricated from Ti and Epitaxially Grown p-Type Ge on Si

Thursday, November 1, 2012, 9:40 am, Room 009

Epitaxially grown Ge and III-V on Si have emerged as a promising candidate for the next generation high performance devices, including high-mobility complementary metal-oxide-semiconductor field-effect transistors. For high-mobility transistors integrated on Si substrates, in particular, managing dislocations and metal-semiconductor contacts has become an important engineering challenge. Herein, we have investigated the impact of threading dislocations and metal-semiconductor interfacial states on Schottky diode characteristics made of Ti and wafer-scale Ge grown on Si. For the purpose of comparison, we have grown epitaxial Ge on Si with two threading dislocation densities: 2x10⁸ and 5x10⁷ cm^-2. The p-type carrier density in the Ge layer is approximately 5x10¹⁶ cm^-3. To prevent Fermi-level pinning, we have also deposited a thin layer of SiO₂ and Al₂O₃ between Ti and Ge with varying thickness, ranging from 5 to 30 nm. With a thin dielectric layer (5 nm), Schottky diodes on two Ge epilayers resulted in an on/off current ratio of approximately 1. This result indicates that there is a significant amount of leakage current. When the dielectric thickness is optimized to 30 nm, we observe that the on/off ratio improves by a factor of 40 and 2000 for SiO₂ and Al₂O₃, respectively. In the case of Al₂O₃, we were able to achieve an ideality factor of 1.67 at 300 K and the reverse leakage current density of ~4.3x10^-10 A/μm² at 300 K. The ideality factor increases to 2.44 at 77 K. This result suggests that the thermionic emission might be the dominant current transport mechanism for Schottky diodes fabricated from Ti and epitaxially grown p-type Ge on Si. However, the slight increase in ideality factor at low temperatures implies a change in the dominant current transport mechanism. In summary, the use of 30-nm-thick Al₂O₃ between Ti and Ge provides improved I-V characteristics for the Schottky diodes. In this presentation, we will further discuss our latest approaches[1] to reduce the dislocations in the Ge epilayer to low 10⁶ cm^-2 level and device characteristics of Schottky diodes fabricated on these low-dislocation-density Ge on Si substrates.

[1] Darin Leonhardt and Sang M. Han, Appl. Phys. Lett. 99, 111911 (2011).