Nucleation and Growth of GaAs on a Carbon Release Layer by Halide Vapor Phase Epitaxy

We couple halide vapor phase epitaxy (HVPE) growth of III-V As-P materials with liftoff from an ultrathin carbon release layer to address two significant cost components in III-V devices – epitaxial growth and substrate cost. HVPE is a potentially lower-cost high-throughput manufacturing technique of III-V devices, and the use of carbon layer has been shown to enable substrate reusability to mitigate the large cost of the single crystal wafers. We investigate nucleation and growth of GaAs layers by HVPE on a thin, amorphous carbon layers that can be mechanically exfoliated, enabling recovery of the substrate for re-use. The carbon layers were grown in an organo-metallic vapor phase epitaxy (OMVPE) reactor with a capability to grow III-V epitaxial layers <i>in situ</i>. This setup allows for the growth of both the carbon layers and GaAs buffers in the same reactor to avoid surface oxidation before deposition of the carbon layer on the GaAs substrates. Samples were then transferred to an HVPE reactor, through air, to demonstrate compatibility with a potentially low-cost and high throughput deposition method of III-V material. Degradation of the carbon release layer was observed with prolonged air exposure during sample transfer between OMVPE and HVPE reactors. The nature of the degradation still requires further study, but is reproducibly demonstrated by increased roughness of GaAs films grown on carbon release layers with more exposure to air before HVPE growth. As a result, air exposure was kept to a minimum during transfer before conducting any HVPE growth studies. Growth was performed under a nitrogen environment to eliminate any degradation of the carbon layer via reaction with high temperature hydrogen present in typical HVPE growth. We studied HVPE nucleation as a function of carbon layer thickness and growth rate and found island-like nucleation during the initial stages of HVPE growth. The island nucleation density during HVPE growth decreases with increasing carbon layer thickness. We then studied various GaAs growth conditions including V/III ratio, growth temperature, and growth rate to minimize film roughness. Lower III/V ratios and thicker films during HVPE growth led to drastically smoother surfaces with reduced threading dislocation density. Finally, we grew an initial GaAs photovoltaic device on a carbon release layer that has an efficiency of 7.2%. A GaAs cap layer was added on top of the carbon layer before removing it from the OMVPE reactor to avoid air exposure of the carbon layer, since air exposure could still be affecting material quality despite efforts to minimize it. This resulted in improved solar cell efficiency up to 12.3%. Device efficiency remains limited by the large dislocation density of the initial GaAs buffer layer due to island growth on top of the carbon release layer.