Paper: Transparent yttrium hydride films prepared by reactive sputtering

Last week the first scientific paper where I am the first author became available on the internet. That is of course a grand step for me on the way to obtaining my PhD degree.

It was a bit more than one year ago that I had just started working with a new material in my sputtering machine. I had an idea that I could make some nanometer-thick layers of a semiconducting material called yttrium hydride by putting layer by layer of yttrium atoms on a surface, while the whole process was going on under the presence of hydrogen gas. Yttrium is a somewhat rare metal (not so rare as to be extremely expensive), and by reacting with hydrogen it transforms into yttrium hydride. This yttrium hydride can appear metallic, black or yellow-transparent, depending on the amount of hydrogen absorbed. The transparent state was what interested me as a potential material for solar cells.

Thin layers of yttrium hydride had already been made by others, and was first reported in 1996 by a group in the Netherlands. However, as it is difficult to make yttrium take up hydrogen directly, the films were made by putting a thin layer of palladium on top of the yttrium film, and then exposing them to hydrogen gas. The palladium is helping to take up the hydrogen, but there are some problems. Firstly, it is an extremely expensive material, which has a similar price to gold. Secondly, in addition to help to take up hydrogen, it also helps the hydrogen to escape, so if you take the yttrium-palladium sample out of the hydrogen gas, it does not remain in the same state. These were my reasons to try this new method reactive sputtering, which had not been utilized before for this exact material.

Since this was a new method for this material, my expectations were not so high. First, I started just laying layers of yttrium atoms on some glass, and got some metallic-looking films that had the expected properties for yttrium metal films. Then I put some hydrogen on, and I was content to observe that I obtained films that were darker, similar to the black state of yttrium hydride. Then, increasing the hydrogen pressure a little bit more, I got a film with the transparent state of yttrium hydride! I was very happy to have been able to make this kind of material by a completely new method, and surprised by the facility with which I had done it.

Figure 1 - I was very happy to see how easy it was to form transparent, black and metallic (left to right on the photo), just by adjusting the hydrogen pressure in my process.

The last year, I have spent working on these samples. Unfortunately I have not been able to make any solar cells of this material, but I have discovered a lot of other interesting things. Some of them, like the finding that these films have a cubic crystal structure as opposed to hexagonal in other findings for the transparent yttrium hydride material, can be read about in the present paper. And for the people that are not metal hydride geeks, I can already tell you that some much more interesting reports will come out soon, subject of one or more papers I hope to wright in the near future.

Published in Journal of Alloys and Compounds, DOI: 10.1016/j.jallcom.2010.12.032

Get the paper (open access PDF).