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Undergraduate Nanoelectronics Laboratory at the University at Buffalo and Demonstration

V. Mitin

Electrical Engineering Department

University at Buffalo, Buffalo, NY 14260-1920, USA

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As transistor’s size becomes much smaller than micron, Microelectronics becomes Nanoelectronics (www.itrs.net/Common/2003ITRS/ExecSum2003.pdf )

Nanoelectronics: the future of Electronics

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Challenges and Solutions

• Challenge: preparation of a new generation of workers with solid skills in Nanoelectronics and Nanotechnology, overall

• General approach to solution: acquiring the practical skills in Nanoelectronics through hands-on experience

• Our specific solution: Interdisciplinary Nanoelectronics Laboratory for the Engineering/Science Undergraduate Curriculum

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Scanning tunneling microscopy

This novel technique yields surface topographies in real space and work function profiles on an atomic scale directly in real space. We know that the removal of an electron from the conduction band of a solid, requires a certain amount of energy called the affinity. For a metal or a doped semiconductor, when the conduction band is partially filled, the energy to remove an electron is lower and it is called the work function. Let us consider two conducting solids separated by a space. In terms of classical physics, a transfer process of an electron from one solid into another can be thought of as an electron transfer over a vacuum barrier. The process requires additional energy and because of this it has a small probability. According to quantum mechanics, a particle can penetrate in classically forbidden spatial region under a potential barrier. This phenomenon was called tunnelling. Thus, electron transfer between two solids can occur as a tunnelling process through (under) the vacuum barrier.

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Different tunneling experiments have been performed, for example, by using two metal films separated by vacuum or a solid-state insulator (a sandwich structure). Each of the metal films can be considered as an electrode and when a voltage bias is applied to these electrodes a so-called tunneling electric current is produced. This current can give information on electronic properties, but obviously the information will be averaged over the area of the metal film surface. By appropriate shaping of one of the electrodes spatial resolution of far smaller scales than that of sandwich structures can be achieved. Since vacuum is conceptually a simple tunnel barrier, such experiments pertain directly to the properties of the electrodes and their bare surfaces. Clearly, vacuum tunneling offers fascinating and challenging possibilities to study surface physics and many other related areas.

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Fig. 1. The principles of operation of the Scanning Tunnelling Microscope.

δ B

CA

IT

VT

sΔs

Py

Px

PzVz

Control Unit

The tunnel current, JT, is a sensitive function of the gap between the tip and the surface, s, i.e.,

JT VT exp(-Aφ1/2 s)

where φ is the average barrier height; the numerical value of A is equal to unity if φ is measured in eV and s in Å.

The control unit, applies a DC voltage, Vz, to the piezodrive, Pz, such that JT remains constant when piezodevices Px and Py, move the tip over the surface of the sample. At constant function φ, Vz(x,y) yields the topography of the surface, that is z(x,y), directly, as illustrated at a surface step in the figure.

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Actually, the lateral resolution is given by the width of the tunnel channel, which is extremely narrow. Additionally, focusing of the tunneling current (in addition to the geometrical one) occurs due to a local lowering of the tunnel barrier height at the apex of the tip. At present, the resolution of the scanning tunneling microscopy reaches 0.05 Å vertically and well below 2 Å laterally. Scanning tunneling microscopy is subject to some restrictions in application: only conductive samples can be investigated, and measurements usually have to be performed in ultra-high vacuum.

Tunnel tips used nowadays are typically made of tungsten or molybdenum wires with the tips of overall radii of < 1 μm. However, the rough macroscopic grinding process creates many rather sharp minitips. The tunnel current is extremely sensitive to the vacuum gap, s; this is why the minitip closest to the sample defines the whole current through the tip.

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Fig. 2. Cross-sectional scanning tunneling microscopy: (a) STM image of a stack of InAs islands in GaAs; (b) comparison between a measured and simulated height profile for a similar sample; (c) lattice parameter in growth direction in an InAs island; experimental data are obtained from cross-sectional STM, solid line is obtained from a simulation assuming an In content increasing from island base to island apex. [From J. Stangl, V. Holý, et. al., Structural properties of self-organized semiconductor nanostructures, Figs. 25 and 26, Reviews of Modern Physics, v. 76, pp. 725-783 (2004).]

Cross-sectional scanning tunneling microscopy can be performed at the cleaved edge to study buried structures.

(a)(b)

(c)

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Apart from structural information, low-temperature scanning tunnelling spectroscopy has been used for wavefunction mapping of single electron states in nanostructures. Being applied to the InAs dots (islands) the STM methods directly reveal s-, p-, d-, and even f-type states as made visible by an asymmetry of the electronic structure, attributed to a shape asymmetry of the islands. Simulation of the electron ground state and first excited state of an InAs island corresponds well with the STM image, showing that the wavefunctions in such islands are indeed atom-like.

Fig. 3 Cross-sectional scanning tunneling microscopy: (d) the electronic wavefunction measured at two different tip biases, compared to simulations for the ground and the first excited states. Two measurements were performed at different voltages at the STM tip: at a low bias of 0.69 V, only s electrons contribute, and at a larger bias of 0.82 V, both s and p electrons contribute to the STM image. [From J. Stangl, V. Holý, et. al., Structural properties of self-organized semiconductor nanostructures, Figs. 25 and 26, Reviews of Modern Physics, v. 76, pp. 725-783 (2004).]

(d)

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Fig. 4. AFM in the contact mode. The size of the tip at the end is about 30-50 nm.

Atomic force microscopy An atomic force microscope measures the force between the sample surface and a very fine tip. The force is measured either by the bending of a cantilever on which the tip is mounted – the contact mode – or by measuring the change in resonance frequency due to the force – the tapping mode. A typical resolution is several nanometers laterally and several angstroms vertically.

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The top surface of PbSe/ PbEuTe multilayers is shown. Both materials are semiconductors. From Figure 5 (a), one can see that PbSe forms triangular pyramids with [001] side facets.

[101]

[110]

PbSe dots(a) (b)

nm [010]

Fig. 5. PbSe islands with [001]-type facets: (a) the AFM image of the top surface of a PbSe/PbEuTe island multilayer; (b) AFM image 3×3 μm2 of the top surface of a PbSe/PbEuTe island multilayer. Islands are arranged in a regular hexagonal array up to the sixth-nearest neighbor. [From J. Stangl, V. Holý, et. al., Structural properties of self-organized semiconductor nanostructures, Figs. 25 and 26, Reviews of Modern Physics, v. 76, pp. 725-783 (2004).]

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EE342 Lab Course ReviewEquipment: STM EasyScan-2

• Nanosurf EasyScan 2 STM• 500 nm lateral range• 200 nm Z-range, 3 pm Z-resolution• 7.6 pm lateral resolution. • Maximum 10 mm diameter sample size. • Tips are simply cut from a Pt/Ir wire without any etching in

hazardous substances • Current set point 0.1 - 100 nA in 25 pA steps• Tip voltage ± 10 V in 5mV steps• Imaging modes: Constant Current (Topography), Constant

Height (Current)

Nanosurf EasyScan2 STM unit

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Equipment: AFM EasyScan-2• Nanosurf EasyScan 2 AFM• 70 micron lateral range• 14 micron Z-range• 1.1 nm lateral resolution• 0.21 nm Z-resolution• Virtually unlimited sample size. • Sample observation optics Dual lens system  (top/side

view)• Optical magnification  Top 12 x / Side 10 x• View field Top 4 x 4 mm / Side 5 x 3 mm• Imaging modes Static Force (Contact), Const.Force

(Topography), Const.Height  (Deflection)

Nanosurf EasyScan2 AFM unit

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Lab 1: Introduction to Scanning Tunneling Microscopy

• Objectives:

– Introduce student to what STM is, how important STM is for the understanding and characterizing the nano world

– Student understand basic principles of STM and the operation of Nanosurf EasyScan2 STM

– Learn how to use the EasyScan 2 software – Obtain the atomic structure images of Highly Oriented

Pyrolytic Graphite – Be able to obtain good images at atomic scale of Highly

Oriented Pyrolytic Graphite– Analyze and present the results obtained

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Lab 1: Introduction to Scanning Tunneling Microscopy

• Some of the results obtained by our students working with Highly Oriented Pyrolytic Graphite sample

• Sample control question for write-up:

What is the distance between two bright ‘hills’ of the graphite layer?

4 nm scan8 nm scan

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Lab 4: Introduction To Atomic Force Microscopy• Objectives:

– Introduce to the basic principles of AFM and the operation of Nanosurf EasyScan2 AFM

– Understand the basic principles of the two most popular operation modes, contact mode and non-contact mode

– Understand the advantages and disadvantages of each operation mode and when to use them

• Sample control question for write-up: List up to four possible experiments when one should use

the contact and the non-contact modes

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Lab 5: AFM Images Data Acquisition• Objectives:

– Reinforce the understanding of AFM and its operation

– Practice operating the AFM with contact mode

– Obtain the images of the semiconductor microstructures

– Analyze the obtained data

• Sample control question for write-up:

Describe the microstructure from the images that you obtained (periodicity, height, width of the structure etc.)

Microstructure sample

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EE342 Lab Course Summary

• EE342 Undergraduate Nanoelectronics Lab has been established with three Nanosurf EasyScan STMs and one AFM due to NSF CCLI Program support

• The lab is targeted for Electrical Engineering sophomores and juniors

• The goal of the lab is to give students opportunity to see and analyze the nanoscale structures

• Students have two labs with the STM and two labs with the AFM at EE Department and 5 additional lab experiments on quantum phenomena at Physics Department

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Acknowledgements• NSF, for the support of this project through NSF NUE and CCLI programs • School of Applied Sciences and Engineering of UB, for providing us with various kinds of support during this project• Nanoscience Instruments Inc. for technical support