8

State-of-the-art ALD reactors and microfabrication equipment

We are interested in exploring functional materials processing at the atomic scale. We exploit thermal and plasma-assisted processes towards self-limiting deposition/etch of nitrides, oxides, and metals with sub-monolayer precision. Our cumulative research goal is to develop highly-selective atomic layer processes which will enable self–aligned and lithograhpy–free device fabrication with atomic resolution.

Plasma-assisted ALD of III-Nitrides

Low-temperature self-limiting growth of nitride materials via ALD necessitates the usage of an external energy source, which will create nitrogen radicals to be incorporated within the film. RF plasma sources of various architectures are commonly used for this purpose. As opposed to conventional ALD materials, i.e., oxides where oxygen is a co-reactant, for nitrides it is one of the most important impurity who kills the crystal quality of nitride films. Therefore, leak-free vacuum systems with UHV-grade base vacuum levels are needed. Moreover, even the very plasma source material itself might become an oxygen contamination source.

In our lab, we have demonstrated that using a stainless-steel based capacitively-coupled hollow-cathode plasma source helps to reduce the oxygen impurity level in GaN films by orders of magnitude [JMC 2014]. By thorough growth optimization, we achieved self-limiting deposition recipes for crystalline hegzagonal GaN, AlN, and InN films and their ternary alloys [JMC 2015, AIP Adv 2016, JVSTA 2015/2016]. We successfully have shown that the entire III-nitride material family can be grown at a common substrate temperature as low as 200°C. Efforts to achieve near-single crystal quality material is ongoing.

This initial achievement might pave the way for III-nitride based flexible (opto)electronics, as it features several inherent advantages with respect to conventional oxide-based technology. In collaboration with Prof. Ali Kemal Okyay’s research group, we have demonstrated ALD-grown GaN-channel thin-film transistors and GaN-absorption layer UV photodetectors as proof-of-concept device applications [APL 2014, JVSTA 2015].

Template-assisted ALD of nanostructured semiconductors

Due to its self-limiting and conformal deposition properties, ALD provides an attractive three-dimensional templating capability with atomic-scale control and precision. We are actively collaborating with four chemistry groups to fabricate functional materials using template-assisted ALD:

1) Electrospinning + ALD: In this collaboration with Prof. Tamer Uyar’s group we have been fabricating fibrous functional materials based on electrospun polymeric nanofibers. Core-shell, hollow-core, and bi-shell nanofibers with ALD-grown oxides and nitrides are developed and used in photocatalysis, showing remarkable performance enhancement.

2) Self-assembly + ALD: Self-assembled peptide nanofiber templates synthesized by Prof. Mustafa Ozgur Guler’s group were coated with oxides using ALD to achieve high-surface area functional materials. The resulting nanomaterials were utilized for photocatalysis and dye sensitized solar cells.

3) MWCNT + ALD: Ultrathin oxide coatings were deposited via ALD on multiwall-carbon nanotubes (MWCNT), constituting a novel active cathode material for Li-O₂ batteries. We teamed up with Prof. Eda Yilmaz’s research group for the design and electro-chemical testing of these cathode materials and achieved a significant improvement in battery lifetime.

4) AAO-templated ALD: Prof. Fatih Buyukserin’s research group at TOBB University of Economics and Technology provide anodized alumina (AAO) membranes to be used as masking layers for subsequent etching and deposition steps. Using these templates, we have demonstrated the fabrication of III-nitride and metal-oxide nanopillars and hollow nanocylinders in both Si and quartz substrates with precisely controlled dimensions. These nanostructured semiconductors might find use in photocatalysis and opto-electronics, as well as serving as enhanced seed layers for subsequent epitaxial growth steps.

ALD of metallic nanoparticle coatings of nanofibrous templates for catalysis

As an extension of the collaborative work with Prof. Tamer Uyar on oxide/nitride coatings on electrospun nanofibers, in this project we aim to decorate various polymeric nanofibrous templates with ALD-grown metallic nanoparticles including Pt, Pd, and Ru. ALD growth parameters are tuned to synthesize size-controlled metal nanoparticles without harming the polymeric template material. If successful, the resulting nanomaterials will be used for catalysis reactions, in which the main goal is to achieve higher catalytic conversion efficiency using less precious material.

Selective ALD of metal-oxides using polymeric blocking layers

We are investigating a methodology to achieve area selective ALD (AS-ALD) by using inductively couple plasma (ICP) grown C₄F₈polymer film as hydrophobic inhibition layer for selective deposition of metal oxides. The robust albeit rather simple and straightforward technique overcomes various challenges associated with previous methods of AS-ALD and provides an alternative route towards nano-patterning using AS-ALD.  AS-ALD of TiO₂ demonstrated in our group offers a novel approach to fabricate close packed nanopatterns for various device architectures without any complex etching or liftoff processes.

Core-team of BIYIKLI Group

We are interested in exploring functional materials processing at the atomic scale. We exploit thermal and plasma-assisted processes towards self-limiting deposition/etch of nitrides, oxides, and metals with sub-monolayer precision. Our cumulative research goal is to develop highly-selective atomic layer processes which will enable self–aligned and lithograhpy–free device fabrication with atomic resolution.