Success stories
Joe Trodahl and Ben Ruck: Structure and Optoelectronic Properties of Amorphous GaN films
Joe, Ben, Joe, Ben at the elipsometer.
Setting the scene
Everywhere we look, millions of photons rush at the speed of light into our eyes. The brain, our portable super-computer transforms these photons into information that is interpreted as the visible world we live in. This age is exploding with information, visual and electrical that needs converting from one form to the other. You can see the importance of the opto-electronic devices used to make this conversion.
Direct band gap Group 3 nitride semiconductors have attracted tremendous scientific attention because of their applications in this area. Electrons in these materials can move between the valence band and the conduction band with the exchange of a photon only (no momentum change is required) so they are ideal for making opto-electronic devices. Currently semiconductors used in devices such as lasers are only sensitive to red light, which puts a limit on the size of the features that they are sensitive to. Although a-GaN couldn't be used to make lasers or LED's (they require a crystalline structure) it is much cheaper and more versatile than its crystalline counterpart and has some very important applications. Ben Ruck, Joe Trodahl and their team are depositing thin films of a-GaN and investigating their properties and possible applications.
The Ion deposition technique
There are three possible types of GaN structure: amorphous (a) with order up to a 0.5nm, nano-crystalline (nx) ordered up to 3nm and crystalline. Unlike the expensive and painstaking process required to make crystalline GaN, the ion deposition technique that Joe and Ben use to make the other two structures is relatively cheap and easy. Within a high vacuum chamber of the ion deposition unit gallium is heated to form a vapour and nitrogen ions are shot at high speeds out of an ion source to form a thin GaN film on the surface of a substrate.
The ion Deposition System
The structure and composition of the film can be adjusted by changing the energy and flux of nitrogen ions, the deposition time, the temperature, pressure, by letting impurities or extra species into the chamber. The list goes on. Two years into the research program the team is still perfecting these ion deposition techniques. New ultra high vacuum chambers at Victoria University reach an incredibly low pressure, normally used at 10 -9 mbar, they can reach down to10 11 mbar. This facility is allowing research to reach a level of accuracy and penetration previously unattainable. The challenge now is to find out how and why these films form as they do and to begin to explore the myriad of possible applications for the materials.
High Resolution TEM of a GaN film. The scale in the bottom right corner reads 6nm. The image shows regularity on a nanometer scale, one of the attractive features of the films.
Photo-Response
The difference between the photo-responce of GaN with and without Oxygen has been the focus of much recent study for Joe Trodahl, Ben Ruck and their team. When light is shone on the films, photons push electrons up to the conduction band and for a time the material conducts. This is called photo-conductivity. They have recently discovered that the films are only amorphous when oxygen (and perhaps hydrogen) is present in the chamber. As yet there is no explanation for these unusual observations but the photoconductivity appears to be affected by the change. It was expected that the nx-GaN, having fewer defects in the structure to disrupt conduction, would have stronger photoconductivity than a-GaN. Nature had a different idea. The opposite turned out to be true. The a-GaN (with O in it) had a large short sharp photo-responce and was transparent in the visible (a blue-green response was expected) and the nx-GaN, had a small photo-responce that lasted for a long time. Why would this be? A proposed reason is that the Oxygen in a-GaN fills up an extra energy level that is left free in nx-GaN. Instead of falling straight back down to the valence band (as in a-GaN) the electron gets stuck in this trapped state where conduction still occurs but not to the same extent. You can imagine the valence band as being like a carpark for electrons, the conduction band like an open highway and the trapped states in between like gravel roads where travel is slow. After a certain time (dictated by recombination kinematics) the electrons park back in the valence band and conductivity ceases.
Time Decay of Photo-conductivity in a GaN film.
The Future
The research is still in its fledgling stage preparing to spread its wings and fly. A cheap portable UV photo-detector is on the way. Some exciting new work is taking place introducing the magnetic Mn 2+ ion. The result is a magnetic semiconducting material that could be used in spintronics (controlling and utilizing the spin properties of electrons). There is also the possibility of using a-GaN to make batteries. Batteries rely on the circulation of lithium in and out of electrodes. Currently electrodes are made out of graphite but a-GaN or nx-GaN may prove to store Li better than graphite, which would mean more efficient batteries. By creating nano-layers of GaN they will be able to look at improving superlattices, quantum wells and diode structures. So the future is full of promise for Joe, Ben and the team.
