In the continuous scaling down of the device for logic circuits, germanium has been considered as a candidate for high performance logic device because of its high mobility. [1,2] However, one of the most difficult challenge in realizing high performance device on germanium is the reduction of source/drain (S/D) resistance because of its low dopant solubility.  The use of the shallow metal S/D is a likely way to solve this limitation. The research of germanide/germanium Schottky junction has been popular in recent years.  In this paper, we propose a simple, novel and reliable technique to modulate the barrier height of NiGe/n-Ge contact formed by microwave annealing (MWA) with capping another pure metal on top of NiGe. [5,6] The physical and electrical properties of capping metals on NiGe/n-Ge Schottky junctions will be discussed by depositing various work function metals (Al (4.26 eV), Ti (4.33 eV), Ni (5.15 eV) and Pt (5.65 eV)) [7,8] as capping metals.
After DHF cleaning on (100)-oriented n-type Ge substrate, the 10-nm-thick Ni was deposited by physical vapor deposition (PVD). Next, MWA (5.8 GHz, 600W, in N2 ambient, for 150 s) was performed for NiGe alloy formation. The X-ray diffraction (XRD) pattern shows that NiGe was the only phase formed by MWA. The transmission electron microscopy (TEM) figure shows that the 10-nm-thick Ni was consumed completely and transformed into a smooth and 20-nm-thick NiGe was formed by MWA. And the energy dispersive spectroscopy (EDS) analysis is consistent with XRD pattern on NiGe composition. Afterwards, various 100-nm-thick metals with different work function were deposited by PVD including Al, Ti, Ni and Pt.
Upon completion of the process mentioned above, we obtained NiGe Schottky junctions with various work function metals as capping metal. The first thing we want to mention is the ION/IOFF ratio. With different work function metals as capping metal including Al, Ti, Ni and Pt, the ION/IOFF ratio at +/- 1V are 2.89×104, 6.50×104, 9.12×104 and 1.31×105 respectively. Furthermore, we obtain the difference of Schottky barrier height (SBH) from 0.570 eV to 0.591 eV between Al and Pt as capping metal. These two results indicate that with different work function metals as capping layer, we can modulate the SBH of NiGe/n-Ge junction so that we can improve the ION/IOFF ratio. The last thing we want to mention is the ideality factor. The ideality factors of Al, Ti, Ni and Pt as capping metal are 1.28, 1.21, 1.13 and 1.12 respectively indicating that the higher work function of capping metal is the more approaching-to-one ideality factor is. In conclusion, by choosing higher work function metal as capping metal on NiGe, we can improve the performances of the NiGe/n-Ge Schottky diode with higher on current, ION/IOFF ratio, lower leakage current and closer to one ideality factor by SBH modulation. This is simple, novel and suitable technique for fabricating high performance Schottky Ge pMOSFET.