Abstract
GaN-on-Si has become a useful fabrication route for many GaN devices and applications, but the mechanical stress incorporated throughout the material stack can impact the viability of this approach. The transfer printing of GaN membrane devices, a promising emerging technology, is most effective with flat membranes, but in practice many GaN structures released from their Si substrate are highly bowed due to the strain in the epitaxial nitride stack. Our approach uses the optical profiles of epitaxial wafers and membranes as inputs for inferring the mechanical strain state of the material by multi-variable numerical model fitting using COMSOL Multiphysics. This versatile, adaptable and scalable method was tested on samples from two GaN-on-Si wafers, revealing the relationship between built-in strain and material bow in principal-component fashion, returning 3–4×10−4 strain estimates for the AlGaN (compressive) and GaN (tensile) layers, and suggesting the occurrence of plastic deformation during transfer printing.
Highlights
GaN devices have shown superior and unique performance in optoelectronic and high-power devices compared with alternative semiconductor technologies, but GaN bulk substrates are still prohibitively expensive for the majority of applications and hetero-epitaxial growth of GaN on dissimilar substrates is commonly employed [5,6,7].The choice of Si as a substrate for GaN epitaxy is advantageous compared with alternatives, such as sapphire or SiC, as Si wafers benefit from wide availability, low cost, compatibility with existing processing lines [8] and relatively high thermal conductivity [9]
The use of GaN-on-Si material for transfer printing increases the importance of understanding and controlling the mechanical state of the material beyond the usual wafer-flatness criterion for optical lithography
The combined experimental and numerical technique described here infers the signs and levels of mechanical strain existing in such epitaxial structures by observing the wafer and membrane profiles and fitting these observations to a high-fidelity finite-element analysis (FEA) model
Summary
GaN devices have shown superior and unique performance in optoelectronic and high-power devices compared with alternative semiconductor technologies, but GaN bulk substrates are still prohibitively expensive for the majority of applications and hetero-epitaxial growth of GaN on dissimilar substrates is commonly employed [5,6,7].The choice of Si as a substrate for GaN epitaxy is advantageous compared with alternatives, such as sapphire or SiC, as Si wafers benefit from wide availability, low cost, compatibility with existing processing lines [8] and relatively high thermal conductivity [9]. GaN-on-Si technology permits the use of Si micro-machining techniques for selectively removing substrate regions to create suspended nitride structures, including cantilevers [10], anchored chiplets [11], and membranes [12]. Such suspended GaN structures show interesting mechanical and thermal characteristics while inheriting many strengths of free-standing GaN, e.g. high break-down voltage [13]. The manufacturing and processing of GaN-on-Si wafers are, limited by several mechanical aspects These are predominantly related to the significant mismatch between the crystal lattice parameters and thermal expansion coefficients of GaN and Si, resulting in large levels of tensile stress in the epitaxial material. The effects of the built-in mechanical stress are detrimental both at the material level (promoting the development and propagation of crystal defects) and from a processing perspective (bowed wafers cannot be accurately processed using standard optical lithography processes)
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