(Invited) The Assessment of Border Traps in High-Mobility Channel Materials

While the issues of Fermi level pinning by interface states at the dielectric/semiconductor interface has first attracted the concern of device engineers working on high-mobility channel devices, more and more attention is going nowadays to the so-called border traps (BTs) at some distance from the interface in the gate dielectric. According to their definition [1] they are able to communicate with the inversion layer in a MOS devices with time constants on the order of 1 ms and higher. As such, they will contribute to the frequency dispersion and hysteresis in capacitance-voltage and transconductance characteristics [2] and generally dominate the low-frequency noise spectrum [3]. Historically speaking, the 1/f noise in MOSFETs has always been interpreted in terms of trapping/detrapping towards BT (Fig. 1), whereby in many cases, pure elastic tunneling was assumed, yielding a simple relationship between the trap depth z and the noise frequency f, which is based on the tunneling parameter a_t. The latter mainly depends on the tunneling barier Φ_it at the interface and the tunneling effective mass m_ox [3]. Based on that, several methods to derive a BT density profile from a noise spectrum have been developed; an example is given in Fig. 2 for the spectrum obtained on a Si-passivated Ge pMOSFET.

            It is the intension of this work to illustrate the 1/f noise method for oxide trap profiling in high-mobility channel devices with a high-k gate stack. As shown in Fig. 3 for InGaAs/InP/Al₂O₃ nMOSFETs, quite uniform trap density profiles have been obtained, irrespective of the architecture, i.e., buried channel with InP cap or surface channel (InP etched) or the use of surface passivation with S [4]. On the other hand, the absolute value of the trap density strongly depends on pre- and post-high-k deposition treatments. While there is quite some literature on the LF noise of Ge pMOSFETs with silicon passivation, little results have been reported on alternative gate stacks or n-channel Ge MOSFETs. As shown in Fig. 4, the noise spectrum for a Ge nMOSFET with GeO_x/Al₂O₃/HfO₂ gate stack is close to 1/f, yielding a uniform profile, according to the constant fxS_I function. It corresponds with a high N_BT in the range of 6×10¹⁹ cm^-3 eV^-1, indicating a rather poor gate stack quality [5]. Finally, the impact of inelastic tunneling in the modeling of BT profiles from 1/f noise spectra will be discussed and a possible internal depth versus frequency calibration will be outlined [6].

References

[1] D.M. Fleetwood, IEEE Trans. Nucl. Sci., 39, pp. 269-271 (1992).

[2] D. Lin et al., in IEDM Tech. Dig., The IEEE (New York), pp. 645-648 (2012).

[3] E. Simoen, H.-C. Lin, A. Alian, G. Brammertz, C. Merckling, J. Mitard and C. Claeys, IEEE Trans. Device and Mater. Reliability, 13, pp. 444-455 (2013).

[4] M. Scarpino, S. Gupta, D. Lin, A. Alian, F. Crupi, and E. Simoen, IEEE Electron Device Lett., 35, pp. 720-722 (2014).

[5] W. Fang, E. Simoen, H. Arimura, J. Mitard, S. Sioncke, H. Mertens, A. Mocuta, N. Collaert, J. Luo, C. Zhao, A. Thean, and C. Claeys, submitted for publication in IEEE Trans. Electron Devices.

[6] E. Simoen, J.W. Lee and C. Claeys, IEEE Trans. Electron Devices, 61, pp. 634-637 (2014).