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28 Aug. 2019

Negative capacitance field effect transistor (NCFET)

With the progress in silicon circuit miniaturization, lowering power consumption becomes the major objective. Recent developments in the creation of effective negative capacitance in ferroelectrics have further offered a solution in nano-electronics scaling such as reducing supply voltage, heat generation and leakage current, as wellie their potential application in low-power electronics, energy storage, and conversion.
 

 

 

Fig. 1. The investigated experimental configuration of the NCFET (left) and the capacitance model of the structure (right). (a). Transfer characteristics of a non-hysteretic NCFET versus the base MOSFET highlighting the gain of using the ferroelectric negative capacitance in terms of SS improvement and threshold voltage reduction (b). Figure is cited from A. Saeidi et al., 2017 47th European Solid-State Device Research (ESSDERC), European Conference.

             

 

We will take advantage of the modern x-ray techniques to explore the homogeneity of polycrystalline ferroelectric HfZrO2 with varying layer thickness and structure. Understanding the correlation between ferroelectricity and the structure will open up a new field for the study of negative capacitance field-effect transistor (NCFET) and greatly expand the functionality of low-power devices. 

Fig. 2: phase distribution (tetragonal (t), orthorhombic (o) and monoclinic (m) phase) mapped by nano x-ray beam on HZO, which takes advantage of x-ray’s element-specificity-key to solving the phase inhomogeneityissue of NCFET. HZO films were deposited on TiN and SiO2 substrates for comparison.

We used in-situ XPS to explore interface properties arising from different fabrication process. Distinct TiN/HZO interfacial states resulting from the opposite capping/annealing sequence order were clearly probed by in-situ XPS. 
 

 

Fig. 3: Core level photoelectron spectra of Hf 4f and Zr 3d recorded for TiN/HZO ((a) and (b)) and TiN/HZO-ref ((c) and (d)). The curve indicated with black symbols (experimental data) overlaps the sum curve of the spectrum (sum) and a Shirley background (BG) used in fitting XPS spectra.

We subjected the TiN/HZO sample to electrical stress by applying 3 volts for a period of 1000 seconds (stressed) and then comparing the XPS and GI-XRD results with those obtained from samples without stress (fresh). We illustrate the relationship between structural and interfacial properties and the wake-up effect in NCFET.

 

 

Fig. 4: (a) Core level photoelectron spectra of O 1s for the TiN/HZO, fresh (upper figure) and after-stress-1000s (lower figure) ; fitting heighted by green color indicates the contribution from the non-lattice oxygen (i.e., oxygen vacancy, Ox); (b) O-phase (111o) characteristic diffraction peaks and (c) P-E hysteresis loops recorded for TiN/HZO fresh (black) and post-stress-1000s (red).



Fourier-transformed EXAFS (FT-EXAFS) of TiN/HZO with/without the application of electrical stress for 1000 seconds. EXAFS is a powerful technique for investigating local atomic structure in ordered/disordered materials with the sensitivity to the coordination number and bonding status surrounding the selected atomic species. This reveals the interface bonding information while the device was subjected to applied voltage.
 

Fig. 5: The Hf L3-edge FT-EXAFS spectra (black) and their respective fitting results (red) for the TiN/HZO with (a) fresh and (b) stressed conditions. Horizontal dashed lines identify the difference in FT-EXAFS magnitude between the two samples which is associated with the level of atomic ordering around Hf. Insets show the Chi vs k spectra.

C-V responses in the devices with and without electrical stress at 1 kHz were measured. The stressed sample shows a larger capacitance value due to the enhanced FE property. The flat-band voltage shift and no apparent distortion of C-V curves in the stressed sample suggest the generated oxygen vacancy mainly occurred at near the TiN/HZO interface after the electrical stress.  
 

Fig. 6: (left) schematic of a MOS capacitor structure; (right) capacitance-voltage (C-V) performance of the TiN/HZO with fresh (black) and stressed (red) conditions.


 

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