Impact of N2O/NH3/N2 Gas Mixture on the Interface Quality of Germanium MOS Capacitors
Authors: Dushaq Ghada , Rasras Mahmoud , Nayfeh Ammar 
Published: February 2020

The development of high quality germanium based devices has been generating a lot of interest due to the excellent optical and electrical properties of germanium [1-4]. From an optical perspective germanium has high index of refraction, low optical dispersion, and the possibility to tune the silicon physical properties and reduce the band gap by controlling Ge content which makes it very useful for photo-detectors application. Furthermore, the bulk hole and electron mobility of Ge are approximately four and two times higher than conventional Si channel, respectively, which makes it an excellent candidate for future high performance complementary metal-oxide-semiconductor (CMOS) devices. The major weaknesses in Ge technology is germanium oxide (GeO and GeO2) stability and water solubility, the poor quality of Ge/ high κ-dielectric interfaces and the difficulty in forming a low-Dit (density of traps) Ge metal-oxide-semiconductor interface. However, a superior high-k/Ge gate stack satisfying low Dit and thin EOT simultaneously is mandatory for high performance Ge-channel devices. Several techniques have been adopted for Ge interface passivation, among them the oxygen-based passivation such as GeOx which has been proved to be thermally unstable, while nitrogen-based passivation has better thermal stability. Furthermore, the energy needed to break the nitrogen-oxygen bond is smaller than that of oxygen-oxygen bond in O2 plasma which makes nitration-based passivation a low temperature technique and compatible with Ge MOSFET fabrication. In this paper a mixture of N2O, NH3 and N2passivation on Ge is performed using RF-PECVD reactor and extensively studied in terms of Ge MOS capacitor.

In order to fabricate Ge MOS capacitor a P-type (100) Ge substrate with a resistivity of (5-10) mΩ.cm is cleaned by diluted 1:10 BHF solution. After being cleaned the samples is loaded immediately to an Oxford Instruments System 100 PECVD tool at RF frequency of 13.56MHz.The plasma power was fixed at 20W and a 710 sccm flow of N2O for 2min is used in all treated samples as a pre plasma clean step. After this 380 sccm of N2 and 30 sccm of NH3 were performed at 285 ºC and pressure of 1T at different exposure time on top of the Ge surface. A sample without passivation is fabricated as a control sample. A 5nm thin film of HfO2 was deposited by plasma- Atomic Layer Deposition (ALD) at 200 ºC, followed by Al deposition using e-beam evaporator through a shadow mask to define the gates. Finally, post dielectric annealing was carried out at 400 ºC for 10min under forming gases. Capacitance-Voltage (C-V) measurements are carried out using Agilent B 1505 A curve tracer + Signatone manual 1160 prober. Fig. 1 shows a schematic of the fabricated gate stack and Fig. 2 shows the typical C–V characteristics of nMOS capacitors without (a) and (b), (c) and (d) with N2O/NH3/N2 treatment at different exposure time. As can be seen from Fig.2 the hump is invisible for the samples passivated by N2O/NH3/N2, indicating that the formed GeOxNy IL may block the impurities from diffusing from the gate dielectric into the Ge substrate. Furthermore, the hysteresis of metal–oxide–semiconductor (MOS) capacitor with HfO2 serving as gate dielectric is reduced to ~150 mV, compared with ~400mV mV of the untreated one. Moreover, this passivation technique can act as an effective method for high performance NMOSFET devices.

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Figure 1