NASCENT

The Effect of Surface Roughness on Chemical Vapor Deposition of Hexagonal Boron Nitride Daniel Kuddes, Ariel Ismach, Harry Chou, Richard Pinner, and Rodney S. Ruoff

Department of Mechanical Engineering and the Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States

Introduction

Interest in two-dimensional (2D) materials has surged due to their wealth of potential applications in nano and optoelectronics. 2D materials can be defined as any material in which the bond strength between atoms within a plane are much stronger than the bonds out of the plane.[1] This unique property means that single atom thick monolayers can be created. Among these materials, graphene has shown particular promise. Graphene is 95% transparent to visible light, highly thermally conductive, and has extremely high charge mobility [2]. These properties mean graphene has potential applications in high speed electronics, as a transparent electrode, in energy generation, and in energy storage.

Silicon oxide makes a poor base for graphene due to its rough surface and unsaturated bonds that act as scattering points for electrons. Hexagonal boron nitrite (h-BN), in contrast, is a favorable substrate for graphene due to its lack of interplanar covalent bonding, smooth topology, insulating properties, and a near epitaxial lattice to graphene. H-BN can also function as a support for nanotubes and other types of 2D molecules (MoS 2 , WS 2 , or MoSe 2 ). Furthermore, because of its wide band gap, h-BN has potential applications in electronics as an insulator, in transistors, and in electroluminescent devices.

1. Effect of Annealing on Polished Ni Film Procedure

Two 25μm thick Ni films were placed in the CVD quartz tube. One sample was polished to a surface roughness of 39nm root mean squared (RMS). The other was left as received from Goodfellow Corp. It had a surface roughness of 300nm RMS. A low vacuum was drawn, hydrogen flowed over the sample, and the furnace was heated to 1050 ° C for 75 minutes.

Results

a)

Polished Ni

b) c)

As Received Ni

d)

2. H-BN Growth on a Polished Ni Surface Procedure

Two 25μm thick Ni films were placed in the CVD quartz tube. One was polished to 39nm RMS and the other unpolished with 300nm RMS. The same procedure was followed as in experiment 1 with an added h-BN growth step. While hydrogen was flowing, ammonia borane gas and methane were added to the chamber for ~5 five minutes.

Polished Ni

a)

Results

b)

Grain Boundary

Figure 5- SEM images

of h-BN on Ni Taken at 1,000x and 8,000x magnification. Images a and b show polished Ni. Images c and d show unpolished Ni.

Wrinkle

50 Å x 26 Å e) f) j) k) c) d)

Adlayer As Received Ni Steps • • Figure 1- H-BN lattice. (a) A model showing the position of boron and nitrogen in a h-BN monolayer and a monovacancy where a boron atom is missing.[3] (b) A Scanning Tunneling Microscope (STM) image of h-BN which exhibits the nearest neighbor in the plane of 1.4Å. [4] (c) A model of three-layer h-BN and interlayer Van der Waals bonding.

Goals

Strategic: Find a means of large scale synthesis of h-BN with few gaps, breaks, or lattice defects.

Motivational: To better understand the role of surface roughness when using chemical vapor deposition (CVD) to grow h-BN on catalytic metal and apply that knowledge to create high quality h-BN films on Ni.

• • • • •

Methodologies

Low Pressure Chemical Vapor Deposition (LPCVD) Optical Microscopy Scanning Electron Microscope (SEM) with Everhart-Thornley detector (ETD) and Energy Dispersive X-ray Sectroscopy Detector (EDS) Mechanical Polishing using 1.0μm grit polish and a dremel Optical Profilometry Figure 2- Low Pressure Chemical Vapor Deposition (LPCVD) System Gas flows from left to right over samples placed in the quartz tube. The system base pressure is ~10mTorr.

Figure 3- SEM Images of Ni before and after a 75 Minute Anneal Images a, b, c, and d show the Ni film before annealing. e, f, g, and h show the Ni film post anneal. The surface of the polished sample seen in images a, b, e, and f remain considerably smoother than those of the unpolished sample c, d, j, and k.

75min Anneal in Hydrogen: 450 400 350 300 250 200 150 100 50 0

Before Annealing After Annealing

Unpolished Ni Polished Ni

Figure 4- Roughness After Annealing The roughness of the polished Ni sample more than doubled after annealing, but it remained at least three times smoother than the as received sample.

• • • •

Observations

Dendritic crystals, possibly nickel oxide, formed on the films after annealing. Ni grain boundaries became visible in both the polished and unpolished films after annealing.

There is less residual alumina (Al 2 O 3 ), a substance found in the polish that remains on the films despite cleaning, on the polished sample after annealing.

The tooling marks, visible before annealing, on the unpolished sample are no longer visible after annealing.

References

1. Butler, Sheneve Z., et al. "Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene." ACS nano 7.4 (2013): 2898-2926.

2.Colombo, Luigi, Robert M. Wallace, and Rodney S. Ruoff. "Graphene Growth and Device Integration." (2013): 1-21.

3. Alem, Nasim, et al. "Atomically thin hexagonal boron nitride probed by ultrahigh-resolution transmission electron microscopy." Physical Review B80.15 (2009): 155425.

4. Doktorw ¨ urde, Erlangung. "One Monolayer of Hexagonal Boron Nitride on Ni(111): An Atomically Sharp Interface." Diss. Universit ¨ at Z ¨ urich, 2003. Print.

5.Logeeswaran, V. J., et al. "Ultra-smooth metal surfaces generated by pressure-induced surface deformation of thin metal films." Applied Physics A 87.2 (2007): 187-192.

• • • • • • • • • • • • • •

Observations

The polished film seems to have more uniform coverage of h-BN.

There are fewer steps in the polished film indicating a smoother surface.

There are fewer adlayers (regions of 2 or more layers of h-BN) in the polished film.

Conclusions

Roughness of the Ni films was increased by annealing.

Polished samples remained substantially smoother than the unpolished samples after annealing.

Surface topographies role in h-BN growth on Ni can be studied in high-temperature, low pressure environments.

A flatter surface leads to more homogenous h-BN growth.

Polishing leaves residual contaminants, such as Al 2 o 3 , on the film that add extra variables to experiments involving h-BN and increases the surface roughness.

The maximum achievable smoothness is ~39nm.

Additional polishing past ~3 min/in smoothness.

2 did not contribute to significant increases in

Future Work

Use Pressure Induced Surface Deformation (PISD) in place of mechanical polishing.

Use successively smaller polishing grits to further decrease roughness.

Put samples in a chemical bath after polishing to etch Al Use lapping as apposed to mechanical polishing.

2 o 3 particles.

Figure 6- PISD PISD works by pressing a metal film against a surface that is already smooth such as a highly polished silicon wafer. With enough pressure the metal will conform to the surface becoming smoother. While the technique has been successful with Au and Ag films[5], we have yet to perfect its use with Ni. Ni presents a unique challenge due to its hardness. Vickers hardness of common metals used for PISD: Au: 216Mpa, Ag: 251Mpa, Ni: 638Mpa, and Si: 1000 Mpa.

Acknowledgements

This work is based upon work supported primarily by the National Science Foundation under Cooperative Agreement No. EEC-1160494. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

I would like to thank NASCENT and the University of Texas at Austin whose support helped foster my research.