The main focus of the work is on a detailed understanding of the pyroelectric properties (PE) in thin doped HfO2 layers. Pyroelectric properties were characterized for various doped HfO2 and mixed HfxZr1-xO2 films.
The ferroelectric and pyroelectric properties of 10 nm thick Si-doped HfO2 capacitors in a Si-doping range of 1.6 – 3.8 at. % were investigated. The pyroelectric coefficient and figures of merit were used to assess Si-doped HfO2 for infrared sensing and energy harvesting applications. The properties of Si-doped HfO2 were observed to be paraelectric from 0 – 1.6 at. % Si, ferroelectric between 1.6 – 2.5 at.% Si, antiferroelectric at 2.5 – 3.5 at. % Si, and paraelectric again for higher Si-doping concentrations (Fig. 1). Such transformations in the electrical behavior of Si-doped HfO2 with increasing doping were observed to arise from the stabilization of the nonpolar monoclinic to polar orthorhombic and then to the nonpolar tetragonal crystal phases with increasing Si-doping. A maximum in the pyroelectric coefficient, 46.2 μC K-1 m-2, coincided with a maximum in the remanent polarization at 2.0 at.% Si. The pyroelectric coefficients in all of the Si-doped HfO2 films were stable over a 0 – 170°C temperature range which is very promising for sensing applications using the pyroelectric effect.
A survey of the pyroelectric behavior was undertaken for a wide variety of dopants incorporated into HfO2 including Al, Gd, Sr, and La as well as the Hf0.5Zr0.5O2 composition. Hf0.5Zr0.5O2 exhibited the largest remanent polarization and the largest pyroelectric coefficient of 70 μC K-1 m-2. Through the Landau-Devonshire Gibbs energy equation for a ferroelectric material, a relationship between the dielectric, ferroelectric, and pyroelectric properties was established by the Curie constant (Fig. 2). It was discovered that the Curie constant could predict the relationship between the remanent polarization, dielectric constant, and the pyroelectric coefficient independent of dopant type and doping concentration (Fig. 3). Pyroelectric performance can thus be predicted by the dielectric constant, remanent polarization, and Curie constant which is convenient for circuit designers and device integration. High aspect ratio and CMOS-compatible fabrication processes give HfO2-based pyroelectric devices an advantage over other high-performance ferroelectric materials.
Upcoming studies will investigate the pyroelectric effect with new structures and operating conditions for sensing and energy harvesting applications.
Dr. Uwe Schroeder
RWTH Aachen (Germany), Hochschule München (Germany), Oak Ridge National Labs (USA), TU Bergakademie Freiberg (Germany)