(Invited) Sputtered Metal Oxide Broken Gap Junctions

Both photovoltaic and light emitting devices require the unimpeded flow of electrons, holes, and photons. Often this represents a problem in high performance devices. The problem is sometimes resolved through the use of a tunnel junction formed by epitaxially growing a stack consisting of degenerately doped p- and n-type layers. Because the Fermi level resides in the valence and conduction band, respectively, the conduction band minimum is at a lower energy than the valence band maximum (Fig 1a). Such heavily doped layers are not easily formed in amorphous or polycrystalline semiconductors however, especially for p-type films. Alternatively, one can use a broken gap junction (BGJ) which relies on the difference in affinity to achieve the same result as the tunnel junction (Fig. 1b). The n-type side must have an electron affinity equal to or greater than the sum of the affinity and band gap of the p-type side. Numerical simulations of a metal-oxide broken-gap heterojunction between p-Cu₂O and n-In₂O₃suggest that low-resistance ohmic behavior can be achieved for even for only moderate doping concentrations.

N-type zinc-stannate (ZnSnO₃) was deposited by RF magnetron sputtering using an AJA ATC 2000 sputtering system in a reactive oxygen environment using a ceramic target made of 50% ZnO and 50% SnO₂ by atomic percent. The target-substrate distance, oxygen partial pressure, total pressure, rf power, substrate temperature, and post deposition anneal were optimized to get the desired optical and electronic properties. P-type Cu₂O was chosen due to the band alignment with ZnSnO₃. Films were sputtered with a variety of techniques. RF sputtering of a ceramic target gave the best results – the films were stable and film stacks using this method showed very little interdiffusion, even for a 500 ^oC anneal. Both films were characterized by UV-vis spectroscopy, 4 point measurements, Hall measurements, x-ray diffraction, AFM, EDS, and Auger depth profiling. The latter proved to be problematic for Cu₂O due to knock-on effects.

The best films were then sputtered sequentially to make a BGJ. Contacts were applied and the devices were characterized with a three terminal measurement. As expected, ohmic behavior was observed with a resistance below 1 Ohm-cm². The results were found to be temperature independent between 38 K and 300 K. When the stable Cu₂O films were used, the performance of the BGJ did not change upon annealing up to 500 ^oC. This suggests that hopping mechanisms are not involved, and that a true broken gap alignment was achieved. Freeze out is not observed since the band alignment autodopes both materials. We will also discuss applications of the BGJ to photovoltaics and quantum dot light emitters.