Recent research has focused on ferroelectric (FE) thin films due to their scalability and compatibility with modern field-effect transistors, as well as their potential to enhance energy-efficient electronic devices [1]. Hafnium zirconium oxide thin film (HfxZr1-xO2, HZO) is a promising FE material for silicon-based applications, particularly because it exhibits good ferroelectricity on Si [2]. However, the performance of HZO in devices is influenced by the quality of the interface between the thin film and silicon, which can be affected by impurities, defects, or vacancies introduced during fabrication.
In this study, we use terahertz emission spectroscopy (TES) to non-destructively assess the properties of HZO/Si interface [3]. TES measures the electric potential change at the interface. Our setup involves exciting photocarriers in a p-type silicon sample covered with a 30~50 nm thick HZO layer using a femtosecond laser, as shown Fig. 1(a). The photocarriers are accelerated by the electric field due to band bending at the interface, generating a transient current that produces a THz wave. This THz wave is then detected in the time domain.
Our results, shown in Figure 1(b), indicate that the THz emission amplitude is highest for the sample with an Hf content of x = 0.05, followed by the sample with x = 0.25. Samples with higher Hf contents of x = 0.5 and 0.75 exhibit lower THz amplitudes, similar to the emission from p-Si without HZO. These findings suggest that an Hf content of x < 0.5 leads to stronger silicon conduction band bending and higher THz emission. This stronger band bending may be due to interface states caused by vacancies or impurities. We will also present the waveforms of THz emissions from ITO/HZO/Si heterojunctions under the various bias voltages.
This work was supported by JST, CREST Grant Number JPMJCR22O2, Japan.
References
[1] S. S. Cheema et al., Nature 604, 65–71 (2022).
[2] J. Muller et al., Nano Lett., 12, 4318-4323 (2012)
[3] D. Yang, A. Mannan, F. Murakami, and M. Tonouchi, Light. Sci. Appl. 11, 334(2022).
