Controlled Phase Stabilization Enabled Tunable Optical Properties of Nanocrystalline GeO₂ Films

Abstract

Germanium oxide (GeO2) exhibits two polymorphs, namely, α-quartz and rutile, which belongs to the ultrawide band gap (UWBG) semiconductor group. With exceptional thermal conductivity and ambipolar capability by doping, GeO2 has tremendous prospects for next-generation semiconductor electronics and other multifunctional device applications. However, the primary bottleneck for GeO2 utilization in advanced applications is the complexity in selectively synthesizing the desired polymorph with a controlled phase and properties. Free formation energies of these polymorphs are relatively close to each other and prone to stabilize in various metastable states or a complex mixture of respective phases. In this context, we present an approach to successfully demonstrate nanotextured hexagonal (h) and tetragonal (t) phases of GeO2 on Si and quartz substrates through a hybrid synthesis route, which involves film deposition by magnetron sputtering followed by postdeposition thermal oxidation. The resulting GeO2 thin films exhibit exceptional optical behavior with a high band gap at 6.21 eV for the α-quartz phase and 5.29 eV for the rutile phase. Precise control over the deposition and annealing conditions under oxygen atmosphere results in the formation of amorphous (a) and crystalline phases of GeO2 films. The evolution of h-GeO2 and t-GeO2 phases is further analyzed using a variety of analytical methods selectively determining the crystal structure, surface morphology, and optical properties. The findings of the current study can be further endorsed while fabricating phase-pure bulk and nanostructured GeO2 on numerous platforms as a potential candidate for UWBG semiconductors for advanced technological applications.

Effects of Heat Treatment on the Microstructure and Optical Properties of Sputtered GeO2 Thin Films

“…[ 29,30 ] The observed temperature required for the crystallization ( T ≈ 800 °C) is consistent with previous reports. [ 31–33 ] We are not able to completely rule out the possible presence of the amorphous phase and partial crystallization in this temperature range. [ 34 ] Note that XRD peaks due to the tetragonal r‐GeO 2 phase are not observed in this set of experiments.…”

“…[18,21,22] Several routes have also been suggested for fabricating r-GeO 2 thin films. [31,35,36] Changes in the surface morphologies of the GeO 2 thin films after the postdepositional heat treatments under a dehydrated atmosphere can be seen in the SEM and AFM surface images (Figure 2), and additional SEM images are provided in Figure S4, Supporting Information. Granular surfaces are observed for the as-grown thin film (Figure S4, Supporting Information) and for the A300 film (Figure 2a) without significant change between these, whereas the A600 (Figure S4, Supporting Information) and A700 (Figure 2b) films exhibit smooth and homogenous surfaces.…”

“…35-0729 (r-GeO 2 ) and 36-1463 (q-GeO 2 ). -Density [g cm À3 ] 6.3 [22] 4.3 [18] <4 [59,62] Bandgap [eV] 4.6 [23,63] 5.2-5.7 [31] 5.7 [64,65] 6.1-6.2 [31] 5.2-5.5 [31,66] Melting point [°C]…”

“…[ 18,21,22 ] Several routes have also been suggested for fabricating r‐GeO 2 thin films. [ 31,35,36 ]…”

Rationally Engineered Vertically Aligned β‐Ga2−xWxO3 Nanocomposites for Self‐Biased Solar‐Blind Ultraviolet Photodetectors with Ultrafast Response

“…Recently, the scientific, engineering, and technology development community explored various wide and ultrawide bandgap material systems, such as gallium nitride, aluminum gallium nitride, zinc oxide, magnesium zinc oxide, titanium oxide, germanium oxide, nickel oxide, tin oxide, and so on, where the focus was 3-S criteria. [3,[5][6][7][8][9][10] Undoubtedly, recent efforts have demonstrated various material synthesis and fabrication processes in addition to novel device architectures. However, there exist many fundamental, technical drawbacks in those aforementioned specific approaches, which are not ideal in the context of multi-functionality and the success and longevity of emerging technologies.…”