Heterogeneous integration of dissimilar semiconductor materials via epitaxial lift-off and membrane transfer proved to be an effective technique to integrate structurally mismatched materials with minimal alteration of their structural and electrical properties. Based on this recent progress, we integrate photovoltaic devices fabricated on single-crystalline GaSb membranes with Si substrates. The potential applications of this material system are multi-fold ranging from IR detectors to solar cells. In the field of photovoltaics, integrating a narrow band-gap semiconductor cell with a Si sub-cell will broaden the absorption from the visible to the infrared regions of the solar spectrum. The epi lift-off of GaSb membranes from GaSb substrate is achieved by a combination of selective etching of sacrificial layer and directional etch of the substrate followed by a stamp assisted transfer onto the Si substrate
A grand challenge in incorporating epitaxial lift-off technique to III-Sb systems relates to the poor selectivity of available etching solutions between the sacrificial layer and the GaSb membrane itself. Hence this process is challenging in terms of successfully achieving epi lift-off without affecting the active layer. Al0.4Ga0.6Sb (~40nm) is chosen as the optimum sacrificial layer preventing the occurrence of threading dislocations into the membrane as well as exhibiting some degree of selectivity with respect to the active layer. An etch stop layer is also incorporated on top of the sacrificial layer (N periods of InAs/GaSb) to prevent the etching of GaSb membrane. After growth, the GaSb membrane was patterned in a 5x5 mm2 array of pixels with each pixel area of ~30 µm2. Upon capping the membrane to protect it from the etching solution, a combination of the sacrificial layer etch and a directional etch of the substrate in the wet etchant (diluted HF) facilitate the pixels to be weakly bonded to the growth substrate. A stamp (e.g., water-soluble tape) is then used to transfer the pixelated GaSb membranes onto Si.
A comparative analysis of the structural and functional characteristics of III-V devices bonded to the native growth substrates and to Si establishes the effect of the process and the interface with the new host on device performance.
ACKNOWLEDGEMENT. This work was primarily supported by the National Science Foundation (NSF) and the Department of Energy (DOE) under NSF CA No. EEC-1041895.