(Invited) Electrical Properties of Group 4 Metal-Nitride/Ge Contacts and the Application to Ge Optoelectronic Devices

1) Introduction

Ge is of great interest as a candidate for the channel material in CMOS devices. In order to integrate Ge-CMOS with current Si platforms, metal source/drain (S/D) MOSFETs are promising candidate because of simple structures and low-temperature processing. To realize high-performance, metal/Ge contacts with low electron barrier height (Φ_BN) and low hole barrier height (Φ_BP) are needed for n- and p-MOSFETs, respectively. However, one difficulty is the Fermi-level pinning (FLP) where the Fermi level is pinned near the valence band edge at the metal/Ge interface. Thus, the formation of a p-MOSFET is straight forward, but an n-MOSFET is difficult. We found that TiN/Ge contacts can alleviate the FLP position toward the conduction band edge [1-3]. By using this contact technique, optical devices with function of light emission and detection are possible [4], which are useful for optical communication. This is because the direct band gap of Ge is 0.80 eV, corresponding to a wavelength of 1.55 mm.

In this presentation, from investigations on electrical properties and interfacial structures of group-4 metal-nitride/Ge contacts such as TiN/Ge, ZrN/Ge, and HfN/Ge, we show that an amorphous interlayer (a-IL) with N plays an important role in the FLP alleviation. We also present direct-bandgap room temperature electroluminescence as well as light detection using bulk-Ge diodes with a fin type lateral PtGe/Ge/TiN structure.

2) Electrical and structural properties of group-4 metal-nitride/Ge contacts

Metal nitrides were directly deposited on (100) Ge by rf magnetron sputtering using a nitride target. Then, a 50-nm-thick Al film was deposited by evaporation, and the contact pattern was formed by liftoff technique. Finally, PMA was carried out at temperatures below 600°C. HAADF-STEM was used to clarify the interfacial structures.

A TiN/Ge contact with 350°C-PMA showeda high Φ_BP of 0.53 eV. In the HAADF-STEM images for samples with no-PMA and 350°C-PMA, an IL was present at the TiN/Ge interface. The IL thicknesses were 1.3 and 1.9 nm for the no-PMA and 350°C-PMA samples, respectively. The IL structure was determined to be amorphous (a). EDX for the no-PMA and 350°C-PMA samples revealed that the ILs consisted of Ge (90 at%)/Ti (10 at%) and Ge (76 at%)/Ti (24 at%), respectively. The N atoms in the a-IL could be also detected by the EDX. The a-IL changed to a crystalline phase after 600°C-PMA, where the TiN was in direct contact with the Ge crystal, resulting in pinned contact property.

An a-IL with a thickness of 1.9 nm exists at an interface in a ZrN/Ge contact with 450°C-PMA, leading to strong FLP alleviation (Φ_BP=0.56 eV). An a-IL in a HfN/Ge contact is thin (0.9 nm) even for 350°C-PMA samples, leading to small FLP alleviation (Φ_BP=0.39 V). Thus, it was found that FLP alleviation for metal nitride/Ge contacts is strongly associated with existence of an a-IL with N. The extrinsic states associated with bonding defects at an a-IL/Ge interface cause the creation of an interfacial dipole. Considering the upward shift of the FLP position, the dipole should be positively charged at the Ge side of the interface. The dipole is likely caused by N-related defects near the a-IL/Ge interface.

3) Light emission and detection Ge devices

We fabricated fin type lateral PtGe/Ge/TiN device using above contact techniques. The contacts were passivated by SiO₂/GeO₂ bilayer. The light emission with a wavelength of 1.55 mm could be observed when forward currents were flowed in the diode. The wavelength corresponded to direct energy gap of Ge (0.80 eV). This is because electron and hole were effectively injected from both contacts. At the wavelength of 1.55 μm, a linear dependence of photo current intensity on laser power was observed with a responsivity of 0.44 A/W at a reverse bias voltage of -1 V.

Acknowledgement

This work was supported by (JSPS) KAKENHI (grant No. 25249035 & 26289090) and was partially supported by JSPS Core-to-Core Program, A. Advanced Research Networks.

References

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[4] D. Wang, T. Maekura, S. Kamezawa, K. Yamamoto, and H. Nakashima, Appl. Phys. Lett. 106, 071102(2015).