Analysis of the Temperature Dependence of Trap-Assisted Tunneling in Ge pFET Junctions

Abstract

In this work, the temperature behavior of trap-assisted tunneling (TAT) in Ge pFET junctions selectively grown in STI substrates is evaluated, whereby the impact of the electric field and the threading dislocation density (TDD) is studied for temperatures ranging from 233 to 418 K. The experimental results are compared with the TAT model for Si proposed by [G. A. M. Hurkx, D. B. M. Klaassen, and M. P. G. Knuvers, IEEE Trans. Electron Devices, 39, 331 (1992)], where several parameters (such as the effective mass and trap level values) are adapted in order to obtain the best agreement between the model and the experimental results. In the frame of channel engineering, Ge pMOS devices are currently being investigated as a potential candidate to extend the technology roadmap below the 22 nm node. This is motivated as follows. For the most recent technology, the implementation of high-k gate dielectrics, which reduces the low-field mobility due to remote phonon and Coulomb scattering, leads to the necessity of searching for new channel materials to achieve a significant carrier mobility enhancement and higher drive current. This offers an excellent opportunity for other materials than Si. For example, Ge is considered for pFETs and Ge or III-V materials for nFETs. Germanium has a 4.4 times higher bulk hole mobility than silicon. This can enhance the circuit speed, through an increase of the transistor drive current or I ON . However, in order to integrate germanium using silicon process tools, Ge transistors need to be fabricated in thin Ge layers processed on a Si handle or carrier wafer, which raises new concerns. One such issue is the presence of extended defects at the Ge/ Si hetero-interface, owing to the 4% lattice mismatch between Si and Ge, which can significantly degrade the mobility enhancement, thus reducing the on-state current.Furthermore, the presence of defects can increase/aggravate the leakage current in the sub-threshold region or I OFF state leakage, through the drain-to-substrate junction leakage.2 Moreover, one of the key concerns for germanium is the narrow bandgap (0.66 eV) compared to unstrained Si (1.1 eV). For advanced CMOS nodes, where higher halo implantations are used and high electric fields are present, the smaller bandgap can considerably increase the drainsubstrate junction leakage for the shorter channel MOSFETs, through field assisted mechanisms like Trap-Assisted Tunneling (TAT) and Band-To-Band Tunneling (BTBT). The high drain-substrate leakage will increase the off-state current and the power consumption. Therefore, it is important to keep a good defect control with an acceptable current level. For this purpose, a better understanding of the field-assisted leakage current mechanisms occurring in the Ge drain-substrate junction is required. ExperimentalProcessing Details.-The Ge pFET junctions were fabricated on 200 mm diameter (100) n-type Czochralski silicon wafers. Active diode regions were defined by shallow trench isolation (STI) followed by P implantations and an n-well anne...

Current leakage mechanisms related to threading dislocations in Ge-rich SiGe heterostructures grown on Si(001)

“…In particular it can be responsible of an increase of the leakage current contribute related to the BTB tunneling with the TDD even if the BTB tunneling does not depend explicitly on defect levels [35]. Field-enhanced tunneling mechanisms such as TAT and BTB tunneling have been observed previously in Ge p-i-n photodetectors [36], p + -n Ge junctions for MOSFETs [37], strained SiGe source/drain junctions [38] and Ge pFET junctions [39]. In order to assess the dominant mechanism of transport along TDs in i-Si 0.06 Ge 0.94 /Ge layers, the J-V diode characteristics were measured at temperatures ranging from 210K to 475K.…”

Review—Device Assessment of Electrically Active Defects in High-Mobility Materials

“…A further reduction of the depletion width W yields an increase of the maximum electric field at the junction (∼1/W), which promotes field-assisted generation mechanisms like Trap-Assisted Tunneling (TAT) and, finally, Band-To-Band Tunneling (BTBT). [103][104][105] This explains the strong, nearly exponential increase of the leakage current density for doping densities above a few 10 18 cm −3 .…”

Dark current analysis of germanium-on-insulator vertical p-i-n photodetectors with varying threading dislocation density

“…It has been also reported that the BTBT leakage is dominant in a strong electric field of >5 × 10 8 V/m in Ge junctions. 9 Therefore, BTBT leakage current contribution is negligible in this analysis.…”

“…The leakage current has been widely associated with the amount of threading dislocation density (TDD) for Ge photodetectors and for shallow Ge pn junctions. [6][7][8][9][10] Tremendous efforts have been made to decrease dark currents of Ge p-i-n diodes and p + /n junctions by reducing TDDs of Ge layers. The dark current density for Ge p + /n junctions with the TDD of 4.2 × 10 8 cm −2 was reduced by roughly 10× compared to the one with the TDD of 1.6 × 10 10 cm −2 .…”