Al2O3 Buffer Layer Separates Strain from Interface Effects in Crystallized Ge Films

Germanium thin films made by solid-phase crystallization are attractive for electronics built on low-temperature budgets, but engineers have struggled to tell apart two influences that shape their quality: the mechanical stress a substrate imposes through thermal expansion mismatch, and the local chemical and structural environment right at the film’s lower boundary. A new study by Jae Hyeok Choi and Woong Choi, published in the Journal of Materials Science, reports a way to pull these effects apart using a thin aluminum oxide interlayer.

The researchers worked with 200-nanometer-thick germanium films crystallized on three very different substrates: sapphire, silicon, and fused quartz. On each, they compared films grown directly against the substrate with films separated from it by a 20-nanometer aluminum oxide buffer layer deposited by atomic layer deposition. Because the buffer inserts a chemically distinct, uniform interface while the underlying substrate still sets the thermal mismatch, any change in behavior could be traced to one influence or the other.

Raman spectroscopy and X-ray diffraction showed that the macroscopic residual strain baseline in the films is dictated by thermal expansion mismatch between germanium and the substrate. In other words, the large-scale mechanical state of the crystallized film follows the substrate’s thermal behavior, just as conventional models of stress in thin films would predict.

The interfacial picture was different. Electron backscatter diffraction revealed that the aluminum oxide buffer changed how grains nucleate at the interface during crystallization. On sapphire and quartz, the buffer promoted grain growth, while on silicon it suppressed substrate-templated crystallization, in which the underlying crystal would otherwise influence the orientation and development of the growing germanium grains.

Despite these substrate-dependent differences in microstructure, Hall effect measurements told a consistent story: inserting the aluminum oxide buffer layer enhanced hole mobility and reduced hole concentration on all three substrates. That uniformity across sapphire, silicon, and quartz indicates that the local interfacial environment exerts a governing influence on carrier transport that goes beyond the strain baseline and beyond the specific grain structures seen on each substrate.

The findings offer practical guidance for interface engineering of high-mobility polycrystalline semiconductor thin films. By decoupling substrate-induced strain from interfacial effects, the study suggests that designers of low-thermal-budget germanium devices can tune carrier transport through the choice of interlayer, independently of the mechanical constraints imposed by the substrate beneath it.

Subject of Research: Disentangling strain and interfacial effects in solid-phase-crystallized germanium thin films using an Al2O3 buffer layer

Article Title: Disentangling substrate-induced strain and interfacial environment in solid-phase-crystallized Ge thin films using an Al2O3 buffer layer

Article References: Choi, J. H., & Choi, W. (2026). Disentangling substrate-induced strain and interfacial environment in solid-phase-crystallized Ge thin films using an Al2O3 buffer layer. Journal of Materials Science. https://doi.org/10.1007/s10853-026-14017-8

Image Credits: AI Generated

DOI: 10.1007/s10853-026-14017-8

Keywords: germanium thin films, solid-phase crystallization, Al2O3 buffer layer, atomic layer deposition, residual strain, thermal expansion mismatch, electron backscatter diffraction, Raman spectroscopy, X-ray diffraction, Hall effect, hole mobility, interface engineering