The Effect of Growth Method on GaN Films and Their Interfaces with CdTe and CdS
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This work has analyzed the complex interfaces of GaN and InGaN grown by sputter deposition and GaN grown by metal-organic chemical vapor deposition (MOCVD) with CdTe and CdS. First, the GaN and InGaN films were characterized by AFM and XRD, and it has been shown that the MOCVD samples have a very smooth surface and are single crystalline with growth in the (002) direction. On the other hand, the sputter deposited samples have rougher surfaces and are polycrystalline. Furthermore, complete interface studies have been performed using in-situ XPS and deposition between GaN grown by sputter deposition and MOCVD and CdTe and CdS to determine the band alignments, conduction and valence band offsets, and Fermi level positions. These interface studies will help determine basic properties to see if these GaN films can be incorporated in a CdTe solar cell to improve its efficiency. It was determined that all the interfaces between the sputtered GaN/InGaN and CdTe/CdS have small conduction band offsets of less than 0.1 eV that do not significantly prevent electron flow across the interface. However, the valence band offsets were much more significant, as they ranged from 0.43 eV to over 1.8 eV. For purposes of the desired positions of the GaN in the CdTe solar cell, the conduction band offsets are much more crucial, and very small conduction band offsets are desired. An interesting effect was that the interfaces between InGaN/CdTe and InGaN/CdS showed In migration into the CdTe and CdS, causing a rise in the Fermi level for the CdTe and CdS, which has been known to worsen the performance of the CdTe solar cell. The MOCVD GaN/CdTe and CdS interfaces showed a slightly higher conduction band offset of about 0.15 eV, but this barrier still should not significantly prevent current flow.
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