From Fully Strained to Relaxed: Epitaxial Ferroelectric Al_1‐xSc_xN for III‐N Technology

Abstract

The recent emergence of wurtzite-type nitride ferroelectrics such as Al 1-x Sc x N has paved the way for the introduction of all-epitaxial, all-wurtzite-type ferroelectric III-N semiconductor heterostructures. This paper presents the first in-depth structural and electrical characterization of such an epitaxial heterostructure by investigating sputter deposited Al 1-x Sc x N solid solutions with x between 0.19 and 0.28 grown over doped n-GaN. The results of detailed structural investigations on the strain state and the initial unit-cell polarity with the peculiarities observed in the ferroelectric response are correlated. Among these, a Sc-content dependent splitting of the ferroelectric displacement current into separate peaks, which can be correlated with the presence of multiple strain states in the Al 1-x Sc x N films is discussed. Unlike in previously reported studies on ferroelectric Al 1-x Sc x N, all films thicker than 30 nm grown on the metal (M)polar GaN template feature an initial multidomain state. The results support that regions with opposed polarities in as-grown films do not result as a direct consequence of the in-plane strain distribution, but are rather mediated by the competition between M-polar epitaxial growth on an M-polar template and a deposition process that favors nitrogen (N)-polar growth.

Ultrathin Al1−xScxN for Low‐Voltage‐Driven Ferroelectric‐Based Devices

“…[ 18 ] As a consequence, the internal parameter u decreased, which is known to result in an increase of the coercive field. [ 11 ] However, XRD measurements (not shown here) of our films reveal no scaling of u . Very recently the low scaling of nonepitaxial down to 20 nm film thickness was related to charge‐injection effects through a very thin interfacial dielectric layer.…”

“…This epitaxial growth enables significantly better crystal properties of compared with previous studies on materials with cylindrical symmetry. [ 11,16,17,19 ] In addition, as reported in several studies, epitaxial growth of induced an improved coupling coefficient and quality factor in resonators as well as a reduction of the coercive field due to strain effects. [ 11,20–22 ] In our study, the low root mean square (RMS) roughness (see Figure S1, Supporting Information) and the sharp interfaces combined with an in situ Pt capping result in an improved ferroelectric response.…”

“…[ 11,16,17,19 ] In addition, as reported in several studies, epitaxial growth of induced an improved coupling coefficient and quality factor in resonators as well as a reduction of the coercive field due to strain effects. [ 11,20–22 ] In our study, the low root mean square (RMS) roughness (see Figure S1, Supporting Information) and the sharp interfaces combined with an in situ Pt capping result in an improved ferroelectric response. Consequently, we were able to record typical butterfly‐shaped capacitance–voltage ( C – U ) loops as well as current density over the electric field ( J – E ) loops at room temperature, demonstrating clearly distinguishable ferroelectric switching down to 10 nm film thickness.…”

“…Such ferroelectric all‐epitaxial all‐wurtzite type /GaN heterostructures were demonstrated recently. [ 10,11 ] However, a very low film thickness of the ferroelectric layer is needed for following the general trend of miniaturization and increasing storage density in all of the aforementioned devices. In this context, offers high scalability due to its high , making it possible to tailor the film thickness to the ultrathin regime to achieve reasonable memory windows and low operating voltages.…”

Ferroelectric Nitride Heterostructures on CMOS Compatible Molybdenum for Synaptic Memristors

“…Furthermore, the negligible capacitance fluctuation in unipolar and bipolar sweeps suggests a wake-up-free nature for this ScAlN. For comparison, it has remained challenging to achieve such wake-up-free ferroelectric heterostructures by the conventional sputter deposition. ,, The realization of wake-up-free ferroelectric nitride semiconductors is beneficial for a wide range of applications, including ferroelectric memory, self-powered photodetectors, acoustic filters and resonators, and piezoelectric energy harvesters, to name just a few. , …”