The challenge: The wide variety of properties observed in the oxide systems provides many opportunities for exploring new physics or creating novel electrical components, but to date this is hindered as the origin of these properties is not yet fully understood. In the heart of the understanding is a sharp insulator-to-metal transition: When depositing an ultrathin oxide film of less than 1.2nm on top of the oxide SrTiO3 ('the silicon of oxides') the entire structure remains insulating. However, increasing the film thickness to 1.6nm by adding only a single atomic layer gives a highly conducting system. This insulator-to-metal transition is remarkable with an increase in the conductance exceeding 6 orders of magnitude by adding a single atomic layer, but yet the reason is not fully understood.
Figure 1: Preparation of the oxide heterostructure in 3 steps: 1) Making atomically sharp SrTiO3. 2) Grow an oxide thin film on top with atomic resolution. 3) Connect the interface with wires and mount a back-gate underneath the sample for application of volt that will change the resistance of the interface drastically. Example of studying current paths with thermography (right).
The solution: Our group at DTU Energy are able to grow films with thicknesses controlled below a single atomic layer, and measuring the insulator-to-metal transition directly when growing the film. Thus, we can terminate the deposition at any desired point between the perfectly insulating and conducting state, and study how the conductivity arises and whether it is homogeneous. For instance, applying a current to the sample creates heat and infrared light where the current flows. Using an infrared camera (thermography), this heat can be used to make maps of how the current flows from one contact to another. Therefore, each of these thermographs is an elegant snap shot of the transition from an insulator to a conductor, which eventually can be stitched together to a small movie.
Figure 2: Example of a thermography showing a simple current path from an anode to a cathode.
Your task: Closely supervised by a PhD student and our section leader, you will:
1. Learn how create atomically smooth surfaces using SrTiO3 purchased online (step 1 in Fig. 1).
2. Learn how to grow an ultra-thin film layer-by-layer on top of SrTiO3 using a high-quality deposition technique (step 2 in Fig. 1). During the growth, you will directly measure how the conductance abruptly changes by orders of magnitude when adding only a single nanometer to the thickness of the thin film.
3. Make wires to the interface (step 3 in Fig. 1), and use an infrared camera to visualize the current paths in the sample with varying film thicknesses (Fig. 2).
4. If time allows, try to change the current paths (without further film deposition) while filming with the infrared camera. This can be done by the so-called resistive switching, which is used in possible next-generation solid-state memory devices (ReRAM) for computer/smartphones. Here, you apply volt to the structure using e.g. a back-gate, which is some metal placed on the backside of the sample.
Step 1 and 2 plus making resistive switching is well established in the lab and rather straightforward thus leaving good time for working on thermography.
If you find this interesting, please contact Dennis Christensen (dechr@dtu.dk, 20961946) or Nini Pryds (nipr@dtu.dk) tel: 4677 5752. The project description can easily be modified to fit your interests.