High-k LaxCeyOz for Passivation of Si Substrate

High dielectric constant rare earth lanthanum cerium oxide (LaxCeyOz) films have been studied as the passivation layers for silicon substrate. Effects of post-deposition annealing time (15, 30, and 45 min) at 700°C towards capacitance-voltage characteristics of the films were investigated. As the annealing time was increased from 15 to 45 min, negative flatband voltage shift was observed, signifying the presence of positive charges in the samples. Interface trap density value calculated for the samples has shown to be the lowest for the film annealed for 45 min, owing to the presence of silicate interfacial layer to improve the overall interfacial quality.


Introduction
Low dielectric constant (k) silicon dioxide (SiO2) has long been recognized as the passivation layer-ofchoice for silicon (Si)-based metal-oxide-semiconductor (MOS) devices. Nevertheless, as the device dimension has been designed smaller to fit for different applications, SiO2 thickness has encountered its limit to be further scaled down. Alternatively, gate oxide materials with higher k value than SiO2 (k = 3.9), such as Y2O3 [1], Al2O3 [2], CeO2 [3], and La2O3 [4] have been proposed as the replacement oxide for MOS devices. Amongst these materials, particular interest has been placed on CeO2 due to the presence of oxygen vacancies in the CeO2 itself, which made it differentiated from other high k oxides. The origin of having oxygen vacancy formation was due to the reducibility of CeO2 [3].
Literatures have reported about the uniqueness of oxygen vacancies in CeO2 in term of assisting in improving the current-voltage characteristics of the MOS device through an annihilation of positive charges (oxygen vacancies) by the negatively charged electrons injected from the MOS device during forward bias. On the other hands, the capability of CeO2 to accept electrons and neutralize the charges was affected if CeO2 was treated at a temperature higher than 800ºC [5] while onset temperature for the CeO2 reducibility was 600ºC [6]. Previous studies reported about the introduction of lanthanum (La) as a foreign cation in the CeO2 lattice to yield more oxygen vacancy formation, whereby the presence of La 3+ could substitute Ce 4+ in CeO2, causing the neighbouring Ce 4+ cations to be reduced to Ce 3+ , accompanied with oxygen vacancy formation [7][8]. With these, CeO2 reducibility would not be affected at a higher temperature than 800ºC and temperature lower than 600ºC. Nonetheless, no attempt was performed at 700ºC to investigate the performance of CeO2 in terms of capacitancevoltage characteristics after incorporation of La 3+ .

Experimental Process
Chemical solution precursors of LaxCeyOz were prepared using metal-organic decomposition (MOD) method [3] before spin-coated on Radio Corporation of America (RCA)-cleaned n-type Si(100) substrates at spin rate of 4000 rpm for 30s. The samples were subsequently put in a horizontal tube furnace for post-deposition annealing at 700°C in argon ambient for a certain duration (15, 30, and 45min), followed by slow cooling. Thermal evaporator (AUTO 306) was used to evaporate an array of Al gate electrodes (area = 0.0025 cm 2 ) on the samples. Crystalline phases of the samples were determined using X-ray diffraction (XRD, P8 Advan-Bruker) while capacitance-voltage (C-Vg) characteristics of the Al/LaxCeyOz/Al MOS capacitors were measured using inductance-capacitanceresistance (LCR) meter (Agilent 4248A).

Results and discussion
Diffraction patterns of LaxCeyOz films for all of the investigated samples are presented in Figure 1. It could be observed from the figure that a single diffraction peak oriented in (200) was present in the sample annealed for 15 min. As the annealing time was increased from 15 to 30 min, the (200)oriented diffraction peak became sharper. In the sample annealed for 45 min, additional peaks allied with LaxCeyOz oriented in (220) and (222) planes were detected. This observation suggested that a prolonged annealing time during post-deposition annealing process has encouraged the formation of crystalline phases of LaxCeyOz in the samples.  where Cox is the maximum accumulation capacitance, q is the electronic charge, and A is the capacitor area.  Effective oxide charges (Qeff) values lying between 10 11 -10 12 cm -2 for the investigated samples are depicted in Figure 3. In comparison, the largest Qeff value was perceived by the sample annealed for 30 min, followed by the sample annealed for 15 min while the lowest one was obtained at 45 min. The acquired Qeff trend as a function of annealing time could be associated with the fluctuation in the positive charges present in the LaxCeyOz films. As the annealing time was increased from 15 to 30 min, the increase in the positive charges was because of the increase in the formation of LaxCeyOz Further investigation was carried out by calculating interface trap density (Dit) present in the samples using Terman's method, as conveniently shown in equation (2) where ∆Vg = Vg-Vg(ideal) is the voltage shift of the experimental curve from the ideal curve, Vg is the experimental gate voltage, and s is the surface potential of Si at a specific gate voltage. Figure 4 presents the calculated Dit values as a function of energy trap level (Ec-Et) for the investigated samples. The lowest Dit value was obtained by the sample annealed for 45 min, followed by 15 and 30 min. This observation was in agreement with the aforementioned trend obtained for Qeff. The acquisition of the lowest Dit for 45 min-annealed sample was an indication to show an improvement in the interfacial quality for the sample, contributed by the existence of SiOx or LaSiOx interfacial layer.

Conclusion
Effects of post-deposition annealing time (15, 30, and 45 min) for LaxCeyOz films at 700ºC were investigated in terms of capacitance-voltage characteristics. The lowest Qeff, and Dit values have been perceived by the film annealed for the longest duration (45 min). The improvement was related to the formation of silicate layer at the interface between LaxCeyOz and underlying Si substrate.