Using gallium nitride nanowires as STM probes

Abstract (Swedish)

Gallium nitride (GaN) nanowires grown using catalyst-free metal organic vapor phase epitaxy were used as scanning tunneling microscope (STM) probes. The probes were prepared by placing a GaN nanowire on a tungsten STM probe using a nanomanipulator in a scanning electron microscope (SEM) and welding them together using an electron beam induced platinum deposition. STM imaging was performed on indium arsenide (InAs) (111)B samples and atomic steps were observed. Furthermore, (I-V) scanning tunneling spectroscopy was performed which consistently showed combined band gaps of both GaN and InAs semiconductors.

Popular Abstract

A microscopy field known as Scanning Probe Microscopy (SPM), uses various methods to get a picture of a sample surface down to such a small scale that individual atoms can be observed. One particular method to perform such microscopy uses the phenomena of quantum tunneling rather than using visible light. This method is called Scanning Tunneling Microscopy (STM) and the way it works is extremely delicate. First, a probe, which is essentially a needle with its tip etched down to a radius of a nanometers scale, is moved so close to the sample that electrical conduction becomes possible under low current, yet the probe does not physically touch the sample. Then, in the selected area on the sample, the computer checks at which vertical position the exact same current was detected. This produces an image which shows exactly how the surface of the sample looks like.

There is however a big drawback to the STM method. Most of the common probes are made of tungsten, which is a dense metal with good electric conductivity. Although it is dense, even a slightest physical contact between a tip and a sample will deform the probe. Considering the fact that a sharper probe tip produces a sharper scan image, any deformation to the tip will affect the resolution of an image. This is where the first feature of using the semiconductors come in. For example gallium nitride (GaN) semiconductor can be grown into crystal structures known as nanowires. The nanowires can be made to be utterly small and extremely sharp. This alone already fulfills the main requirement for a probe to be useful. However in addition to that, the nanowires are also crystals which makes them tough enough to withstand most nudges against the sample which cannot be said about the metallic probes.

What has been done in this thesis, is that GaN nanowires were used for STM imaging which was performed on indium arsenide sample. The obtained STM images showed atomically flat terraces. This result shows that GaN can be used as STM probes. Unfortunately atomic resolution was not achieved during the length of the thesis, however this step was very close and can be achieved. Further research towards this projects is likely to succeed in achieving atomic resolution.
An interesting feature of a semiconductor device is that they have what is called a band gap. A band gap is a range of energy levels within a material which cannot be occupied and it lies between a conduction band and a valance band. If some material has no band gap, then it is a conductor, if material has a large band gap, then it is an insulator. However, if a band gap is small enough, then the room temperature will be enough to move electrons between the bands, meaning that the material is neither a conductor nor an insulator. Hence the idea of a semiconductor was born. But how can the existence of a band gap benefit in this research? Well, gallium nitride has a reasonably big band gap for a semiconductor, which means that the movement of the electrons is not that easy within the material. This implies that such material can be see-through to lasers up to a certain wavelength. This can be used in another SPM method called Near-field Scanning Optical Microscopy (SNOM) as a light guide for a laser.

This is just one attempt at applying the semiconducting nanowires in the field of microscopy. There is still a lot that can be achieved with this research going further, for example nanowires have promising opportunities in the Atomic Force Microscopy (AFM) area due to their rigidity. Although this is more of a speculation, such nanowires could help creating multi-purpose devices which can perform an array of different microscopy methods with a single probe. This is a fresh field of research which is definitely promising to have great value to the modern scientific research.

• author: Sulinskas, Karolis
• supervisor: Rainer Timm; Sofie Yngman
• organization: Synchrotron Radiation Research; Department of Physics
• course: FYSK02 20181
• year: 2018
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: STM, STS, Nanowire, GaN, InAs, Semiconductors
• language: English
• id: 8954629
• date added to LUP: 2018-07-10 18:18:28
• date last changed: 2018-07-10 18:18:28