Student Voices

Travel Report: Faezeh Mohammadbeigi, Physics, Germany

December 22, 2014

Recently, we have studied low temperature PL properties of systematically C-doped ZnO NWs and observed new PL transitions that are shallower than Ga and In donors.1 It is critical to understand the charge type of the defects responsible for these new transitions, and whether they are donor or acceptor like, as well as spatial distribution of these defects to investigate if they are related to structural defects. In order to investigate them, we initiated collaboration with Prof. Axel Hoffman’s group at the Institut für Festkörperphysik, Technische Universität Berlin in Germany. I had access to sophisticated characterization techniques such as high magnetic field PL (MPL), tip enhanced Raman spectroscopy (TERS) and time resolved photoluminescence (TRPL). They could also provide resources that make it feasible to run PL and Raman on a single nanowire which gives us valuable and unique information about spatial distribution of dopants in a single ZnO nanowire. During my stay in Berlin (May 10, 2014- August 7, 2014) I had the opportunity of being exposed to a broader range of ideas and perspectives in my research.

I had the chance to run tip enhanced Raman scattering (TERS) on our carbon doped samples which is a very good technique to observe surface related modes in ZnO. The goal was to figure out if carbon clusters are located in the surface or are in the bulk of the sample. Analysis of the data indicated that carbon related modes are coming from the bulk and the surface of the NWs and also confirmed that there is no carbon related modes in our co-doped carbon and hydrogen NWs.

We could run micro PL on a single wire to see how impurity related transitions change along the wire. According to the data we got, the carbon related impurity was uniformly distributed along the wire.

One amazing observation was to see donor-acceptor pair (DAP) transition and its related phonon replicas for the first time in our highly carbon doped sample. This transition was previously reported in nitrogen doped ZnO films. We ran temperature dependent and power excitation dependent PL measurements to confirm that it was DAP transition. This transition was only observed in our highly carbon doped sample and make this question open that if carbon related acceptor like centers are created in this sample. To confirm this we need to grow the same sample with the same condition and run micro PL to see if it is reproducible.

I also did TRPL along a single wire to see how lifetime of impurities changes spatially in the wire. We did this measurement on our Sn-doped sample as well and the results will be included in a paper which is in progress. I did the same measurement on one of highly carbon doped and carbon and hydrogen co-doped samples and comparing the results from these two illuminates the role of hydrogen in our samples better.

The most critical experiment for me was magneto photoluminescence (MPL) which tells us about the charge type of the defects responsible for newly observed PL transitions and that whether they are donor or acceptor like. Unfortunately, after working with this set up for a couple of weeks we figured out that the resolution needed for our measurements is not enough due to some misalignments in the spectrometer. Usually it takes a long time to realign the spectrometer and I couldn't use that set up during my stay in Germany anymore. One of the students from Prof. Axel Hoffman’s group is still working on it and will repeat the experiment and send us the results as soon as he fixes the issue.

In all, I got new interesting results which need to be further analyzed and discussed with our collaborators and hopefully published in near future.

Here in Simon Fraser University, our group has unrivalled capabilities to grow carefully doped ZnO NWs using metalorganic vapor phase epitaxy (MOVPE) growth method. MOVPE provides controlled intentional doping with a wide range of potentially useful dopant impurities. My thesis work focuses on the use of low temperature photoluminescence spectroscopy to identify the physical origin of these dopant impurities in ZnO NWs. Recently, our group has demonstrated that carbon can be a significant impurity in ZnO. It may act as an important compensating impurity during efforts to achieve n- or p-doping. Despite extensive evidence of carbon in ZnO the nature of C in this material is not yet understood and a systematic study on the electronic properties of C doped ZnO is not been realized.

In order to further investigate our samples, we continued our previous collaboration with the Institut für Festkörperphysik, Technische Universität Berlin in Germany. Prof. Axel Hoffman is an expert and pioneer in ZnO and it was an opportunity for me to work in his group and have access to sophisticated characterization techniques such as high magnetic field PL (MPL), tip enhanced Raman spectroscopy (TERS) and time resolved photoluminescence (TRPL). They could also provide resources that make it feasible to run PL and Raman on a single nanowire which gives us valuable and unique information about spatial distribution of dopants in a single ZnO nanowire. During my stay in Berlin I had the opportunity of being exposed to a broader range of ideas and perspectives in my research.

Most of my planned experiments worked and I got very good results which need to be further analyzed and discussed with our collaborators.

I appreciate the financial assistance I was granted through the GIRTA which allowed me to complete an important part of my research for my PhD.

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