Impact of thermal oxidation uniformity on 150 mm GaAs- and Ge-substrate VCSELs. (13th March 2023)

• Record Type: Journal Article
• Authors: Gillgrass, S J; Allford, C P; Peach, T; Baker, J; Johnson, A D; Davies, J I; Joel, A M; Shutts, S; Smowton, P M
• Is Part Of: Journal of physics. Volume 56:Number 15(2023)
• Publication Date: 2023-03-13
• Subjects: VCSEL -- thermal oxidation -- wafer bow -- Ge-substrate -- GaAs-substrate; Physics -- Periodicals; 530
• DOI: 10.1088/1361-6463/acc040
• Languages: English
• ISSNs: 0022-3727

Abstract

Abstract: Vertical cavity surface emitting laser (VCSEL) devices and arrays are increasingly important in meeting the demands of today's wireless communication and sensing systems. Understanding the origin of non-uniform wet thermal oxidation across large-area VCSEL wafers is a crucial issue to ensure highly reliable, volume-manufactured oxide-confined VCSEL devices. As VCSEL wafer diameters approach 200 mm, germanium (Ge) is emerging as an alternative substrate solution. To this end, we investigate the uniformity of 940 nm-emitting VCSEL performance across 150 mm diameter GaAs- and Ge-substrates, comparing the oxidation method in each case. Nominally identical epitaxial structures are used to evaluate the strain induced wafer bow for each substrate type with Ge exhibiting a reduction of over 100 μ m in the peak-to-valley distortion when compared with GaAs. This wafer bow is found to be the principal cause of centre-to-edge oxidation non-uniformity when utilising a conduction-heated chuck furnace, in comparison to a convection-heated tube furnace. Using on-wafer testing of threshold current, differential resistance, and emission wavelength, device performance is demonstrated for the first time across a 150 mm Ge wafer, and is shown to be comparable to performance on GaAs substrates, when the effects of oxidation uniformity are removed. These results provide evidence that there is a realistic path to manufacturing high yield VCSELs, over wafer diameters approaching those used in Si-photonics, via Ge substrates.