Ultrathin Al_1−xSc_xN for Low‐Voltage‐Driven Ferroelectric‐Based Devices

Abstract

The high coercive field (E c ) and the high, stable remanent polarization separate the ferroelectric properties recently discovered in materials with wurtzite-type structure from classical ferroelectrics. [1][2][3][4] This raises hopes for particularly good scalability of wurtzite-type-based ferroelectric devices. In addition, the complementary metaloxide semiconductor compatibility and the well-established industrial deposition process make Al 1Àx Sc x N thin films highly attractive for building novel neuromorphic computing and memory devices such as ferroelectric field-effect transistors (FeFET) and ferroelectric tunnel junctions (FTJs). [5][6][7][8][9] Furthermore, it is expected that wurtzite-type ferroelectrics such as Al 1Àx Sc x N introduce ferroelectricity into III-N technology, resulting in a straightforward approach to realize, for example, GaN-embedded memory. Such ferroelectric all-epitaxial all-wurtzite type Al 1Àx Sc x N/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, Al 1Àx Sc x N offers high scalability due to its high E c , making it possible to tailor the film thickness to the ultrathin regime to achieve reasonable memory windows and low operating voltages. [12] Furthermore, in terms of device design, ultrathin ferroelectric films are a prerequisite for building FTJs. [7] Reducing the thickness to the ultrathin regime (<30 nm) is often accompanied by a material-specific diminution of remanent polarization (P r ), a drastic increase of E c , or results in a total loss of ferroelectricity. [13][14][15] To our best knowledge, up to now, no thickness scaling study on ferroelectric epitaxial Al 1Àx Sc x N heterostructures was conducted. Also for nonepitaxial heterostructures, only a small number of studies were performed. Below 20 nm film thickness, measurements that suggest ferroelectricity at elevated temperatures or indirectly through scanning nonlinear dielectric microscopy are accessible. [16,17] Very recently, partly ferroelectric switching of %12 nm-thick Al 1Àx Sc x N was reported by performing positive-up-negative-down (PUND) measurements at room temperature. [18] The availability of ferroelectric sub-20 nm films is however crucial in order to reach switching voltages in the low-single-digit volt range that is desired for advanced circuits as

In‐Grain Ferroelectric Switching in Sub‐5 nm Thin Al0.74Sc0.26N Films at 1 V

“…A slight increase in E c with decreasing film thickness down to 10 nm is consistent with the scaling properties reported so far for Al 1−x Sc x N. [11][12][13][14] Yasuoka et. al.…”

“…All films were capped in situ to prevent oxidation of the Al 0.74 Sc 0.26 N surface, which is crucial to obtain an undisturbed ferroelectric response especially of <10 nm thin films, where the thickness of the native oxide can be in the range of the total film thickness. [14][15][16] For the epitaxially grown film stacks, the interfaces are smooth with an overall low surface roughness of the respective layers, which is known to result in a reduction of the leakage currents in capacitors. [17] Structurally, the epitaxial films with a 10 nm thin Pt bottom electrode layer also have superior crystalline quality compared to non-epitaxial ones, that is, higher c-axis texture, which we investigate in detail in a separate work.…”

“…As already discussed in related work, [18,26,41] sputtered nanocrystalline films of Al 1−x Sc x N exhibit small grain diameters of 2 -6 nm featuring an in-plane tilt between the individual grains in the order of 6°which restricts the observable area to single grains with exact orientation to the incident electron beam. [14] In this respect, the ABF-STEM image contrast formation crucially depends on exact orientation conditions. [43,44] In this investigation, the directly interpretable sample area was further limited by 1 to 2 nm large Pt agglomerates present evenly spaced in the center of the Al 0.74 Sc 0.26 N layer.…”

Ferroelectric Nitride Heterostructures on CMOS Compatible Molybdenum for Synaptic Memristors

“…These results suggest that the emulation of biological STDP using a single artificial synapse based on the ferroelectric GaN/ScAlN heterostructure resistive memristor can be realized, which further demonstrates the promising applications of ferroelectric III-N architectures in neural networks and neuromorphic computing. Additionally, it is worth noting that the operating voltage can be significantly reduced by increasing the Sc content, introducing tensile strain, or using a thin ferroelectric ScAlN layer, which has been reported recently. −, …”

A Comparative Study of Pt/Al0.72Sc0.28N/Pt-Based Thin-Film Metal-Ferroelectric-Metal Capacitors on GaN and Si Substrates

“…Properties of Ultra-Thin Al 0.72 Sc 0.28 N. Following our work on describing the ferroelectric characteristics of ultra-thin 10 nm Al x Sc 1−x N MFM capacitors lately, 27 in this article, we focus on the structural characterization of such thin MFM structures. Figure 5a shows a trend of improving the texture of epitaxial Al 0.72 Sc 0.28 N with decreasing film thickness.…”