A comparative study of selective dry and wet etching of germanium–tin (Ge_1−xSn_x) on germanium

Abstract

A comparative study of selective dry and wet etching methods for germanium-tin (Ge 1−x Sn x ) alloys (3.5% < x < 7.7%) and germanium (Ge) is carried out. Both etching methods are optimized from the perspectives of selectivity and morphology, and then compared. A special behavior of the selective dry etching process is discovered and explained, whereby the selectivity has a dramatic increase to as high as 336 when the Sn concentration is above 6%. Different morphologies of suspended microstructures fabricated by different etching methods are investigated. Comparative study shows that the selective dry etching is a better choice for high Sn concentration GeSn (above 7%) against Ge to have better morphology, selectivity and verticality. While for low Sn concentration GeSn (below 6%), wet etching is a better way to fabricate a suspended GeSn microstructure on Ge. This work provides a comparative understanding of both methods of selective etching for GeSn. This comparative understanding is expected to be applied in the processing of next generation electronic and photonic devices.

Fabrication of High-Quality and Strain-Relaxed GeSn Microdisks by Integrating Selective Epitaxial Growth and Selective Wet Etching Methods

“…The similar line tendency indicates that the etch rate of Ge is roughly equal for Ge 0.902 Sn 0.098 disks with different diameters of 3 μm and 5 μm, and is in agreement with the value of the etch rate from Ref. [30]. Moreover, the minimum diameter of Ge pedestal for standing GeSn microdisk is about 300 nm.…”

“…It will reduce the wet etching rate of GeSn and lead to a higher etch selectivity of Ge over GeSn. The previous study has reported that the H 2 O 2 based wet etch (H 2 O 2 :NH 4 OH: H 2 O = 2:0.5:80), the same with this work, achieves an etch selectivity of Ge over Ge 0.928 Sn 0.072 of 9:1 [30]. Figure 3a shows the cross-sectional TEM micrograph of the 5 μm diameter GeSn circular mesa without the top polysilicon layer.…”

“…c, microdisk structures were formed successfully for Ge 0.902 Sn 0.098 alloy after 510 s wet etching. Previously, Han et al [30] reported that they fabricated partly suspended Ge 0.928 Sn 0.072 microdisks (5 μm in dimension) with about 1.2 μm undercut using the H 2 O 2 based wet etching with a selectivity of 7:1. However, after the selective wet etching, the edges of their disk become bent and fractured because of strong attractive capillary forces developed between the suspended structure and the substrate as the etchant liquid is removed, which pulls the released microdisk structure into contact with the substrate [24].…”

“…1 The cross-sectional schematics illustration for the fabrication of the suspended GeSn microdisks Fig. 2 Top-view SEM image of isolated 5 μm diameter GeSn circular mesas after removal of the shadow mask NH 4 OH (28 wt%), and DI-H 2 O with a volume ratio of 2: 0.5:80 [30]. Finally, the top protective polysilicon layer was removed by KOH.…”

“…The structure can not only relax the compressive strain in GeSn layer through elastic deformation at free surfaces but also confine optical models near the edge of the microdisk due to the strong refractive index contrast between the GeSn and the surrounding medium (air), such as whispering-gallery-mode [16,25]. Up to now, only one method to prepare GeSn microdisk through the post-growth optical lithography and top-down etching of the two-dimensional GeSn film is reported [16,24,29,30]. However, the process may suffer thermal mismatch effects during the post-growth etching process, which will lead to a degraded material quality of GeSn microdisks.…”