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Heterojunction Assisted Impact Ionization at the ZnS/Si Interface and Cu10Te4S13 Photodiodes

Heterojunction Assisted Impact Ionization at the ZnS/Si Interface and Cu10Te4S13 Photodiodes

Friday, June 29, 2018 at 11:30 am
328 Weniger (new location)
Christopher Reidy

Several topics are presented in this dissertation, each of which has applications to solar cells and photodetectors. First, we discuss the growth of Cu10Te4S13, copper tellurium tetrahedrite. This material has interesting optical properties; it has a large joint density of states at the conduction band maximum and valence band minimum, which gives high absorption right at the band gap energy while remaining transparent for sub gap photons. As the photon energy increases, this material has a very large absorption coefficient (α > 105 cm-1) allowing for very thin solar cells. We were able to grow this material by PLD and anneal it in situ, creating phase pure films. We were additionally able to construct a functioning solar cell with this material.

We also grew single crystal epitaxial sphalerite Zinc Sulfide (ZnS). This ZnS was grown by pulsed laser deposition (PLD) for the first time without the use of a process gas. Using <111> silicon as a growth substrate, we were able to form epitaxial ZnS with a growth temperature between 400 C and 500 C. This was done under ultra-high vacuum (<5x10-9 Torr) and after surface processing of the silicon wafer, removing its native oxide layer. This work was then extended to the fabrication of Si/ZnS/Si heterostructures.

The ZnS/Si material system was chosen specifically because it may show evidence of heterojunction assisted impaction ionization (HAII). In a HAII based photovoltaic device, energy normally lost to hot carrier relaxation is instead efficiently captured and converted into additional extractable current. With an internal quantum efficiency (IQE) greater than 1, such a device could exceed the Shockley-Quiesser limit for single junction solar cells. We show evidence that not only does HAII take place at our epitaxial ZnS/Si interfaces, the IQE improvement vs bare Si is in practice > 10% for photons with energy larger than the harvester band gap.