# Presentation: 894121

## Imaging quantum and other small-size effects at nanostructure contacts 

**     ****Jon Pelz and Steve Ringel,  ****DMR-**0076362, DMR-0505165

- 	There is great interest in making new kinds of electronic devices using *semiconductor nanostructures*.  Nanometer-sized metal contacts are necessary for most nanostructure devices to function, but little work has been done to measure how *small size effects at contacts* (such as quantum confinement) affect device behavior.
- 	 We are developing Ballistic Electron Emission Microscopy (BEEM) to directly image and quantify small-size effects at nanocontacts.  This was tested on a *cleaved quantum well* (QW) *heterostructure* (see Fig. (**a)**) which had gold/QW contacts as narrow as 1 nm.  Figure (**b**) shows the measured top surface of a thin gold film (the bumps are small gold grains) while Fig. (**c**) shows the *simultaneously measured* BEEM image, revealing the buried gold/QW contacts. BEEM can measure (with nm-scale resolution) the local Schottky energy barrier at the gold/QW contact, which  controls how electrons move from the gold film into the QW nanostructure.  Fig. (d) shows that this energy barrier increases strongly as the QW width is made narrower.   We can understand this increase as a combination of quantum-confinement effects (green line) modified by a small reduction due to nm-scale “pinning” effects (red line).  We will next study how the transmission efficiency is affected by small-size effects, and then apply this tool to other nanostructure systems.

- (a) Schematic of gold/quantum-well nanocontacts.  (b) Image of top gold (Au) surface, showing small gold grains.  (c) BEEM image, revealing the buried quantum well contacts.  (d) Data points: measured Schottky Barrier Height (SBH) vs. quantum-well width, showing strong size effects.  Lines are model predictions.

**(a)**

**(b)**

**(c)**

**(d)**

**Probe Tip**

- Phys. Rev. Lett **94**, 206803 (2005).

**Notes:**
Who: Profs. Jon Pelz and Steve Ringel, with graduate student Cristian Tivarus and Post-doc Mantu Hudait.

Where: Ohio State University

What/So-what: 
     There has been a lot of interest in making semiconducting nanostructures (such as carbon nanotubes or semiconductor nanowires) into electronic, opto-electronic, or spintronic devices.  But so far most work has focused on making the nanostructure, making prototype devices, and measuring device behavior (i.e., current-voltage curves or basic FET response).  But much less work has been done to determine whether this behavior comes from the properties of the nanostructure itself, or from the CONTACTS.  In fact, IBM has argued that the the behavior of simple carbon nanotube transistors may in fact be dominated by the contacts.  To resolve issues like this, and ultimately to improve device performance, it is important to be able to directly measure the electronic properties of the contacts themselves. 
     We have applied the technique of Ballistic Electron Emission Microscopy (BEEM), which is based on scanning tunneling microscopy (STM), to directly image and quantify small-size effects at nanocontacts.  BEEM can measure the metal/semiconductor energy barrier (called a Schottky barrier) that controls carrier transport across an abrupt metal/semiconductor interface.  BEEM can measure this energy barrier with very high spatial resolution (~5 -10 nm) and high energy resolution (<20 meV).  We tested this on a metal/nanostructure system in which metal contacts were made to cleaved GaAs quantum wells (QW), where we could vary the width of the QW (and hence the metal/QW contact dimension) down to 1 nm (see Fig. (a)).  BEEM can simultaneously measure both the shape of the top metal surface (Fig. (b)) and also the electronic properties of the buried metal/QW interface (Fig. (c)).  The probe tip can then be located directly over a particular QW, and the Schottky barrier at just this part of the contact can be accurately measured (Fig. (d)).  We observe that the size of the nanostructure (and the contact) have a strong influence on the energy barrier.  We were able to model this using finite element modeling, so we now understand what part of this barrier increase comes from quantum confinement (the “particle in a box” increase to the energy as the box becomes smaller), and what part is due to non-uniform local electric fields around the QW contact.  
     We will next study how the transmission efficiency is affected by small-size effects, i.e., whether small size effects cause poor coupling between the metal contact and the nanostructure.  Again, understanding this is very important for making devices out of nanostructures.  We will  then apply this technique to other nanostructure systems, such as GaAs nanowires or carbon nanotubes.

## Imaging quantum and other small-size effects at nanostructure contacts 

- Three undergraduate REU students (Adam Champion, Sylvia Schaepe, and Corey Campbel) and two graduate students of Jon Pelz (Kibog Park, and Cristian Tivarus) contributed to this work. Sylvia and Corey have both gone on to graduate school, while Adam continues his undergraduate studies.
**Outreach:**
- In addition to giving lab tours to high school students and other visitors, the PI presents yearly Physics demonstrations in elementary school classrooms (1st – 6th Grade), and has worked with elementary school teachers to help prepare them (and their students) for the Ohio State Proficiency Test.
- See for example: _**http://www.physics.ohio-state.edu/ outreach/gallery/Cassingham2-04/index.htm**_

- From visit to a 2nd grade classroom. The air in the balloon will soon “shrink” in liquid nitrogen.

***Imaging quantum and other small-size effects at nanostructure contacts ***
**     ****Jon Pelz and Steve Ringel,  ****DMR-**0076362, DMR-0505165

**Notes:**
Both the 4th grade and 6th grade Ohio proficiency tests have (or had) questions about Newton’s laws.  So in discussions with the elementary school teachers, we decided some demonstrations and explanations would be useful.