Non-Uniform Hole Current Induced Negative Capacitance Phenomenon Examined by Photo-Illumination in MOS(n)

In this work, the photo-induced negative capacitance (NC) phenomenon in MOS(n) capacitor with ultra-thin oxide film was observed and analyzed for the first time. The low-frequency NC have been reported in MOS tunneling diode with ultra-thin oxide film under accumulation region, which was attributed to the conductivity modulation in silicon or even measuring error in phase angle.^1,2 The recent research studied the NC phenomenon in n-type MOS with ultra-thin oxide film under depletion region, attributing it to local hole-electron recombination due to lateral non-uniformity in oxide film thickness.³ Although some circuit models of the NC have been proposed, the physical interpretation remains debatable.^4,5

    Fig. 1 shows both I-V and 1k Hz C-V characteristics in Al/SiO₂/n-Si MOS capacitor with oxide thickness of 2.4 nm. The negative capacitance occurs under inversion region when given illumination, and the maximum of NC in magnitude occurs when the gate current saturates. With increasing light intensity, both the peak value and the saturation voltage increase as well. To investigate the origin of photo-induced NC, TCAD tools were utilized. Fig. 2 shows the simulated distribution of hole current density under light illumination with an intensity of 50 mW/cm². Both recombination rate and hole current density at 0.002 μm beneath the SiO₂/Si interface are extracted and plotted in Fig. 3. The lateral non-uniformity of hole current density is clearly observed at low gate voltage bias, where the current density is highest at gate edge due to lateral diffusion of photo-generated holes. For V_G = -1V, both the recombination rate and the electron current injecting from gate are relatively low, thus the holes can recombine with the injecting electrons rapidly. For V_G = -2V, more electrons are injected into the substrate; meanwhile, the large hole current density under gate edge provides sufficient holes for higher recombination rate. However, the holes under the bulk region of gate become insufficient to maintain high recombination rate, thus the depletion width expands to supply more holes, leading to the voltage–current phase lag as well as NC. As V_G increases continuously, a large number of holes are induced under the gate to form an inversion layer, and both the recombination rate and the hole current density become more uniform as shown in Fig. 3 (a) and (b). For V_G = -3V, the holes in bulk region become sufficient for hole-electron recombination process, and the voltage–current phase lag effect is relieved gradually, which explains the presence of NC peak of C-V characteristics in Fig. 1. The concept of NC behavior described above is illustrated in Fig. 4.

    Fig. 5 (a), (b), and (c) show hole current density at 0.002 μm beneath the SiO₂/Si interface for samples under dark, 10 mW/cm² and 70 mW/cm² light illumination, respectively. According to the discussion above, the profile of hole current density can be utilized to determine the occurrence of NC phenomenon. It is clearly shown that hole current density is uniformly distributed for the sample without illumination. For the sample given 10 mW/cm² illumination, the hole current shows non-uniform distribution at V_G = -1V but uniform distribution for V_G = -2V, implying that the peak exists between gate voltage of -1V and -2V as shown in Fig 1. For the sample given 70 mW/cm² illumination, the gate voltage required for obtaining uniform hole current density is larger (V_G = -3V) due to higher lateral diffusion of photo-generated holes, and the peak shifts toward more negative region of gate voltage, which is consistent with C-V characteristics in Fig. 1.

    In this work, it is demonstrated that NC is closely related to the uniformity of hole current distribution and the hole-electron recombination rate. Besides, it is worth noting that in MOS(p) capacitor with ultra-thin oxide film, no photo-induced NC occurs since the frequency and bias dependencies of NC behavior are different from those in MOS(n) due to varied distributions of electron and hole. It is of importance to explore the NC effect in MOS structure with tunneling mechanism.

Acknowledgement

    This work was supported by the Ministry of Science and Technology, Taiwan, under Contract MOST 103-2622-E-002-031 and by the National Science Council, Taiwan, under Contract NSC 102-2221-E-002-183-MY3.

References

1. M. Mieko and H. Yutaka, Jpn. J. Appl. Phys., 39, L123 (2000).

2. H. Suto et al., in ICMTS 2004., p. 221 (2004).

3. S.-J. Chang and J.-G. Hwu, IEEE Trans. Electron Devices, 58, 684 (2011).

4. M. Ershov et al., Electron Devices, IEEE Transactions on, 45, 2196 (1998).

5. Y. Okawa et al., in ICMTS 2003. International Conference on, p. 197 (2003).