Physical and electrical characterizations of thermally oxidized/nitrided Zr thin films on Si at various temperatures in N2O environment

The use of physically thicker high dielectric constant (κ) materials is indisputably a promising yet formidable solution to replace SiO₂ as alternative gate dielectrics in Si-based metal-oxide-semiconductor (MOS) devices [1]. Of several high-κ oxides, ZrO₂ is one of the most extensively investigated insulators owing to its high enough κ value (22-25), suitably large bandgap (5.8-7.8 eV), good thermodynamic stability when in contact with Si up to ~900°C of processing temperature, minimal lattice mismatch with Si(100), and easily stabilized in the form of cubic or tetragonal polymorphs, which may further enhance its effective κ value [1-3]. Nevertheless, the quality of ZrO₂ thin film is dependent on the deposition method applied. According to the previous reports, stoichiometric ZrO₂ thin films were successfully produced by sputtering of metallic Zr followed by thermal oxidation in O₂ environment [4-6]. However, due to the diffusion and reaction between oxygen, silicon, and zirconium, an undesirable interfacial layer was unavoidably formed in between ZrO₂ and Si, which reduces the capacitance, thus degrading the performance of the metal-oxide-semiconductor capacitor. To overcome the aforementioned problem, interfacial layer thickness in between ZrO₂ and Si must be reduced. Incorporation of nitrogen in the film is a possible solution. Nitrogen-contained film can help to suppress the out-diffusion of Si by passivating the dangling bonds of Si surface and improve the hot carrier resistance at the Si-dielectric interface [7-9]. Based on the literatures, a nitrogen-incorporated ZrO₂ film has shown encouraging electrical characteristics [9-11]. In order to perform oxidation and nitridation simultaneously, NO and N₂O are the typical gases used [7]. Comparatively, N₂O is more appropriate and preferable to be used owing to its n- n-toxic property [12]. Therefore, in this work, physical and electrical properties of thermally oxidized and nitrided Zr thin films on Si using N₂O have been systematically investigated. Simultaneous oxidation and nitridation of sputtered Zr thin films on Si was performed in N₂O environment for 5 min at 500-1100°C in order to optimize the oxide properties. The atomic microscopy force results indicated that the surface root-mean-square roughness of the sample increases with the increasing oxidation and nitridation temperature (Figure 1). Figure 2 shows the normalized high-frequency capacitance-voltage curves of the oxidized/nitrided samples at different temperatures. Depletion region is generally observed in the negative bias and flatband voltage is shifted negatively for all characterized capacitors. This indicated the existence of positive effective oxide charges in the oxide [13]. Based on the capacitance-voltage curves, positive effective oxide charges, slow trap densities, and total interface-trap density are calculated (Figure 3). Interface-trap density of each investigated oxide has been calculated as well (Figure 4). Leakage current density-electric field characteristics of the investigated samples have been investigated (Figure 5). A two-step oxide breakdown labeled as E_B and E_HDB were observed in the leakage current density-electric field plot for all investigated samples, due to the presence of interfacial and ZrO₂ layers in the sample [14]. The electrical results showed that the sample oxidized and nitrided at 700°C has the highest breakdown field, owing to the lowest positive effective oxide charge, interface-trap density, and total interface-trap density.