Epitaxial Growth and Layer Transfer of InP through Electrochemically Etched and Annealed Porous Buried Layers

Heterogeneous material integration, especially for III-V semiconductors, has enabled more possibilities in designing optoelectronic and electronic devices, and the ability to exfoliate the bonded devices layers and reuse the expensive substrates makes it economically advantageous.   In our case, we propose a method using easily fractured porous layer to transfer an InP thin film from an InP substrate to an alternate substrate; e.g., glass or a flexible structure such as PDMS.  Here, we demonstrate the formation of such a porous InP layer which exhibits a surface that is nearly fully dense as well as the ability to grow InP overlayers with low (< 10⁶ cm^-2) threading dislocation density. Also, we were able to transfer large area (> 1 cm²) InP layers onto PDMS substrates and glass slides.

In our studies on (001) InP, we have demonstrated that the pore formation can be controlled as a function of depth during electrochemical etching. For as-etched porous layers, AFM scans on a 40 X 40 μm² area show the lowest surface r.m.s. less than 40 Å despite of the pore features. In order to achieve both high-quality epitaxial growth and facile layer fracture, a dual porous layer structure with a more porous (mechanically weaker) under-layer was designed and fabricated by either adjusting the etching current density or changing the switching electrolyte concentration. 

The structures were annealed to understand the morphological evolution of the pore structure and to test the structural stability under simulated epitaxial growth condition. Under N₂-purged environments, we observed that the surface remains stable (no change in surface pore morphology) with even slight improvement on roughness up to 550 °C and the layer maintained its porous structure at 650 °C.  Large voids were observed after annealing at the interface between the high porosity buried layer and substrate, which is consistent with the ease by which it is to remove top layers from the substrate after annealing. Epitaxial layers were grown about 2 μm thick on porous surfaces with different porosities and both a cross-section TEM sample and a plan-view sample (> 35 μm²) were used to characterize the epitaxial growth. Since no observable threading dislocations were found in the plan-view sample, it sets a limit on the threading dislocation density in the mid-10⁶ cm^-2 range.

Samples with dual porous layers were bonded to flexible PDMS substrates and easily peeled off due to the fracture at the high porosity layer. Another set of samples were bonded to glass slides with epoxy glue and the fractured surface was characterized under SEM.