Micro-transfer printing of micro-structured, ultra-thin light-emitting devices
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Date
2023-03
Authors
Shaban, Zeinab
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Publisher
University College Cork
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Abstract
3D integration of optoelectronic devices is a crucial future technology, which can be applied in the areas of photonic integrated circuits, flexible displays, communication and more. Among the various technologies, micro-transfer printing has emerged as a precise and cost-effective way to assemble devices for 3D integration. To enable this technology, devices must be released from their native substrates, which open up a lot of possibilities. It can achieve integration with flexible or heat-conductive backplanes, as well as heterogeneous integration of multiple materials on a common platform, resulting in miniaturised chips. Also one can benefit from reclaiming and reusing the original substrates to reduce the production cost significantly. On the other hand, GaN devices exhibit unique optical properties in optoelectronics compared to other semiconductors, and GaN-based LEDs have established themselves in a variety of applications, due to their low power consumption, long lifetime, short response time, and high brightness. This thesis has focused on releasing high performance GaN LEDs and addressing
their associated issue for micro-transfer printing.
The first part of this work is focused on releasing and transfer-printing of GaN LEDs grown
on Si substrate. There are several factors that limit the performance and manufacturing of
GaN LEDs on Si. One issue is related to the deformation of the released coupons due to their high inbuilt strain, which could result in transfer-printing failures as well as challenges during the post-print integration process. To address this issue, COMSOL software was used to study the stress effect on the devices. Experimentally, the intrinsic deformation of the released LEDs was compensated by using compressed SiNx layers that resulted in flat devices after release. Another issue is related to the low light extraction for GaN LEDs on Si. To solve this problem, the underside of the released LEDs is roughened during the coupon preparation process prior to transfer printing. Furthermore, using the unique properties of transfer printing, the roughened LEDs are printed inside a fabricated reflective trench with 10 μm depth to direct the light to the surface normal. Results showed that roughening along with the reflective trench increased the collected power by a factor of ∼ 7 compared with LEDs on the original substrate.
A second part of this study examines the release of GaN-based structures from substrates (i.e. sapphire or bulk GaN) by photoelectrochemical (PEC) etching when pure chemical etching is not possible. A sacrificial layer which can obtain smooth etch surfaces and uniform etching with high selectivity is needed. Also, from the perspective of transfer printing, thick rather than thin sacrificial layers are preferred to facilitate the releasing and picking process. In this work, 300 nm-thick releasing layers comprising of InGaN/AlInN stacks are proposed for PEC etching. The presence of two-dimensional hole gas at the interface of InGaN/AlInN due to the strong polarization field are indicated by modelling and capacitance-voltage measurement. This resulted in a smoother surface with a three times higher etch rate compared to the conventional InGaN/GaN superlattice structures used for PEC etching. Moreover, various electrolytes and post-PEC treatment were studied to improve the surface smoothness.
Further work should be done to determine the impact of the adhesion layer in transfer printing on heat generation and device performance. Using the optimized sacrificial layer to release other structures like lasers should also be investigated.
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Keywords
Micro-transfer printing , GaN LEDs , Photoelectrochemical etching , Wet etch
Citation
Shaban, Z. 2023. Micro-transfer printing of micro-structured, ultra-thin light-emitting devices. PhD Thesis, University College Cork.