Ultra-Thin, High Quality HfO2 on Strained-Ge MOS Capacitors with Low Leakage Current

Strained-Ge is a promising channel material for future CMOS nodes due to its very high hole mobility [1] and compatibility with existing processes. In comparison to Si, high quality dielectric interfaces for Ge are challenging due to the lack of an ideal native oxide. In this work, for the first time, we show an extremely-scaled (EOT < 5 Å), high quality dielectric on s-Ge. CV measurements of a MOS capacitor structure show areal capacitance scaling, little hysteresis, and low gate leakage current (< 0.2 A/cm²).

The epitaxial structure (shown in Fig. 1(a)) was grown by low-pressure CVD, and the s-Ge layer is compressively strained (2.3% biaxial) due to the lattice mismatch with the relaxed Si_0.55Ge_0.45 virtual substrate. The epitaxial wafer was cleaved into pieces, and a 5-minute 1:10 (HCl:H₂O) clean was performed prior to atomic layer deposition (ALD) of 1 minute O₃/~3 nm HfO₂ (both at 250 °C), followed by ~10 nm of TiN at 300 °C. After ALD, Al contacts were sputtered and patterned followed by a 30-minute forming gas anneal at 450 °C. The final MOS capacitor structure (Fig. 1(a)) reflects the loss in the s-Ge due to the HCl clean and O₃ oxidation.

CV measurements of a 100×100 µm MOS capacitor show a CET of 8.6 Å (Fig. 2). Large frequency dispersion exists for frequencies > 50 kHz due to large series resistance likely caused by a 500 meV valence band offset [2] between s-Ge and Si_0.55Ge_0.45 (Fig. 1(b)). The IV measurements (Fig. 3) show small gate leakage current (< 0.2 A/cm²), which translates to I_G < 20 pA/µm for L_G= 10 nm, well below ITRS requirements [3].

Quasistatic one-dimensional self-consistent coupled Poisson-Schrödinger electrostatic simulations were performed in order to extract EOT. A strain-dependent 6×6 k∙p Hamiltonian (with deformation potentials from [2]) was used to calculate hole quantization in the s-Ge layer. In accordance with [4], a 2.1 eV valence band offset between Ge and GeO_x was used in the simulations (Fig. 1(b)). The 500 meV valence band offset between between s-Ge and Si_0.55Ge_0.45creates a deep quantum well confining most of the holes in the s-Ge layer.

Fig. 5 shows the 10 kHz experimental CV in addition to simulated quasistatic CV curves for structures with varying EOT. The mismatch of the simulated and experimental curves in strong hole accumulation is due to both a large series resistance (R_s ~ 0.1 Ω∙cm²) and gate leakage, which causes a sizable error when transforming the measured impedance to a two-element parallel capacitance C_P, parallel conductance G_P circuit model [5]. A circuit model (inset Fig. 6) incorporating parasitic gate tunneling conductance G_t and series resistance R_sis applied to an ideal simulated CV in order to recover the measured CV result, yielding an extracted EOT of 4.9 Å (Fig. 6). This is the smallest known EOT for a high quality dielectric on s-Ge published to date. Other recent results include a 7-Å EOT on s-Ge [6], and an 8-Å CET on unstrained Ge [7].

References

[1] W. Chern et al., IEEE EDL, vol. 35, no. 3, p. 309, 2014.

[2] J. T. Teherani et al., PRB, vol. 85, no. 20, p. 205308, 2012.

[3] ITRS Front End Processes, 2012 update.

[4] V. V. Afanas’ev et al., APL, vol. 92, no. 2, p. 022109, 2008.

[5] E. M. Vogel et al., IEEE TED, vol. 47, no. 3, p. 601, 2000.

[6] R. Zhang et al., IEEE IEDM, p. 26.1.1, 2013.

[7] B. Duriez et al., IEEE IEDM, p. 20.1.1, 2013.