Band alignment and enhanced breakdown field of simultaneously oxidized and nitrided Zr film on Si
© Wong and Cheong; licensee Springer. 2011
Received: 11 April 2011
Accepted: 10 August 2011
Published: 10 August 2011
The band alignment of ZrO2/interfacial layer/Si structure fabricated by simultaneous oxidation and nitridation of sputtered Zr on Si in N2O at 700°C for different durations has been established by using X-ray photoelectron spectroscopy. Valence band offset of ZrO2/Si was found to be 4.75 eV, while the highest corresponding conduction offset of ZrO2/interfacial layer was found to be 3.40 eV; owing to the combination of relatively larger bandgaps, it enhanced electrical breakdown field to 13.6 MV/cm at 10-6 A/cm2.
Comparison of the obtained values of E g(ZrO2), E g(IL), ΔE v, and ΔE c
Atomic layer chemical vapor deposition
Electron beam deposition of Zr + oxidation in O2
3.30 to 3.50
Sputtering of Zr + oxidation and nitridation in N2O
6.20 to 6.50
8.20 to 8.80
Results and discussion
where, E g(ZrO2) and E g(IL) are the bandgaps of ZrO2 and IL, respectively. ΔE v(ZrO2/Si) and ΔE v(IL/Si) are the valence band offsets of ZrO2 and IL, respectively, with respect to Si substrate. The calculated values of E g(ZrO2), E g(IL), ΔE c(ZrO2/Si), ΔE c(IL/Si), and ΔE c(ZrO2/IL) are presented in Figure 3. The highest value of ΔE c(ZrO2/IL), i.e., 3.40 eV, was attained by sample oxidized/nitrided for 15 min (Figure 3) when compared to other samples. A schematic of the band alignment of the ZrO2/IL/Si system is illustrated in Figure 1. The E g value of Si substrate is obtained from literature [3, 18]. It is found that values of E g(ZrO2), E g(IL), ΔE c, and ΔE v obtained in this study are higher than the values reported in literatures (Table 1).
A two-step oxide breakdown (BK-1 and BK-2) is being recorded in the J-E plot for all investigated samples (inset of Figure 4). The existence of interfacial and ZrO2 layers in the sample is the main cause of this two-step breakdown . The breakdowns can be explained as follows. One of the layers may experience an electrical breakdown at a lower field, which is labeled as BK-1. Subsequently, another layer would block the carriers. Due to the increment of the electric field, the concentration of the carrier increases until the layer is electrically broken down at a higher electric field at BK-2. The instantaneous increment of leakage current density at BK-1 is relatively small when compared with others, and it is defined as soft breakdown. The magnitude of BK-1 increases gradually as the oxidation time is increased (inset of Figure 4). In contrast, the instantaneous increment of current density at BK-2 is large, and this is considered as hard breakdown. The highest dielectric breakdown field, which is referred to as hard breakdown, is attained by sample oxidized/nitride for 15 min (13.6 MV/cm at 10-6 A/cm2). The lowest one is recorded by sample oxidized/nitride for 10 min (4.8 MV/cm at 10-6 A/cm2). In comparison, dielectric breakdown field recorded in this work is higher than the previous reported works [4, 5, 14].
In summary, the band alignment of ZrO2/IL/Si structure produced by simultaneous oxidation and nitridation of sputtered Zr thin film on Si in N2O has been established. Via this method, higher ΔE c and ΔE v values have been attained. Hence, a higher electrical breakdown field at low leakage current density has been achieved.
The n-type Si(100) substrate with a resistivity of 1 to 10 Ω cm was used in this study. After undergoing a standard wafers cleaning process, a 5-nm thick Zr film was sputtered on the cleaned Si substrates by an RF sputtering system. Following that, samples were loaded into a horizontal tube furnace and were heated up from room temperature to 700°C in an Ar flow ambient, and the heating rate was fixed at 10°C/min. Once the set temperature was achieved, N2O gas was introduced with a flow rate of 150 mL/min for a set of durations (5, 10, 15, and 20 min). After the furnace was cooled down to room temperature in an Ar ambient, the samples were withdrawn from the furnace. To experimentally determine band alignment of the dielectric/semiconductor structure, XPS measurements were conducted using Kratos Axis Ultra DLD (Kratos Analytical, Chestnut Ridge, NY, USA). with a monochromatic Al-K α X-ray source (hν = 1,486.69 eV) performed at the Research Center for Surface and Materials Science, The Auckland University, New Zealand. The spectra of survey or wide scan (binding energy of -5 to 25 eV) were collected at a take off angle of 0° with respect to surface normal, with low pass energy of 20 eV and small step size of 0.1 eV. Due to the onset of single particle excitation and band-to-band transition, the energy loss spectrum of O 1s photoelectron provides further insight on the bandgaps of ZrO2 and IL . Subsequently, a detail scan of O 1s was carried out using the same pass energy and step size of 1.0 eV. Ar ion gun (5 keV) was employed to etch the sample in order to perform chemical depth profiling (results are not shown here), in order for the boundary of ZrO2 and IL to be identified. A Shirley background function, which is proportional to the integrated photoelectron peak area, was subtracted from all of the XPS spectra to correct for the inelastic photoelectron scattering effect . Band alignment extraction was based on Kraut method [15, 16]. As to characterize the leakage characteristic and electrical breakdown field of the film, MOS capacitor test structure was formed by thermally evaporated a 100-nm thick aluminum (Al) film, acting as a gate electrode, on top of the films. The area of a capacitor was photolithographically defined at 9 × 10-4 cm2. In order to obtain an Ohmic back contact, a 100-nm thick Al film was thermally evaporated on the backside of the Si substrate after removal of native oxide. I-V measurements were performed by a computer-controlled Agilent HP4155-6C semiconductor parameter analyzer (Agilent Technologies, Santa Clara, CA, USA).
The authors would like to acknowledge the support provided by USM fellowship, USM-RU-PRGS (8032051), and The Academy of Sciences for the Developing World (TWAS) through the TWAS-COMSTECH research grant (09-105 RG/ENG/AS_C) during the study.
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