Abstract
Due to its high refractive index, reflectance is often a problem when using Germanium for optoelectronic devices integration. In this work, we propose an effective and low-cost nano-texturing method for considerably reducing the reflectance of bulk Germanium. To do so, uniform V-shape pit arrays are produced by wet electroless chemical etching in a 3:1 volume ratio of highly-concentrated hydrochloridric and nitric acids or so-called aqua regia bath using immersion times ranging from 5 to 60 min. The resulting pit morphology, the crystalline structure of the surface and the changes in surface chemistry after nano-patterning are all investigated. Finally, broadband near-infrared reflectance measurements confirm a significant reduction using this simple wet etching protocol, while maintaining a crystalline, dioxide-free, and hydrogen-passivated surface. It is important to mention that reflectance could be further reduced using deeper pits. However, most optoelectronic applications such as photodetectors and solar cells require relatively shallow patterning of the Germanium to allow formation of a pn-junction close to the surface.
Highlights
IntroductionA major inconvenience with bulk Ge is its high refractive index, which causes large reflections, preventing efficient light coupling
The instability of Germanium dioxide (GeO2 ) was a primary cause for the abandonment of Germanium (Ge) in favor of Silicon as the premier material platform for microelectronics [1,2].Ge still offers many unique properties for optoelectronic devices [3,4,5,6,7,8], especially for near-infrared applications [9].a major inconvenience with bulk Ge is its high refractive index, which causes large reflections, preventing efficient light coupling
Our results clearly show that the surface treatment using 3:1 aqua regia (HCl:HNO3 ) can allow a controllable, relatively uniform, and shallow nano-texturing of Germanium
Summary
A major inconvenience with bulk Ge is its high refractive index, which causes large reflections, preventing efficient light coupling. In the last two decades, nanoscale texturing has proved to be a successful approach to reduce the broadband reflectance of semiconductor surfaces [10,11,12,13,14]. While many low-cost nano-texturing processes were pioneered for silicon [15,16,17,18,19], researchers have been searching for an equivalent approach to achieve uniform and low-cost anti-reflection for bulk. Different types of etching agents for various concentrations and etching times were used and encouraging results were achieved using this process [20]. To allow for large-scale manufacturing at low costs, it would be better if an electroless approach could achieve similar results
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