# Presentation: 466300

## Performance Characteristics of a Spin-FET

- Above right  is a schematic diagram showing a top view of the UHV chamber for the two-tip low-temperature STM system.  This view of the chamber shows the relative position of the equipment including the STM sample and tip, the second STM tip, the wobble stick, and the load lock.

- The highlight of our first year is the completion of our Spin-FET and all the necessary peripherals.  Our Spin-FET consists of a low-temperature, ultra-high vacuum scanning tunneling microscope system with two independently controllable STM tips.  The two tunnel junctions are on perpendicular surfaces and they can be brought to within 10 nm of each other.  Above left are photographs of our Spin-FET system. (a) View of the main section of the UHV chamber showing the linear translator that holds the second STM tip (left side). (b) Close-up image of the mechanical arm holding the second STM tip.  (c) Close-up image of the electrical connector for the second STM tip.  (d) View of STM sample and the two STM tips (tip one points up while tip two points right).  (e) View of the e-beam tip cleaning facility.

**Performance Characteristics of a Spin-FET**
**Paul Thibado  ****DMR-0405036**

**Notes:**
Development results:
Interest in developing next-generation nanoscale devices is at an all time high.  New methods for fabricating nanostructures are being developed at an increasing rate.  This includes self-assemble, artificially fabricated, lithography-etching techniques, etc.  What has not kept pace is the number of new ways to characterize materials on these smaller scales.  Consequently, the majority of characterization techniques still average over a large volume or surface area containing numerous nanostructures.  This method of characterization yields a signal that is inhomogeneously broadened from the desired result.  At present, there is a critical need to add new techniques which precisely characterize individual nanostructures.
An instrument that incorporates two scanning tunneling microscope (STM) tips which can have their tunnel junctions as close together as one nanometer was designed and built.  The sample is fixed and can be imaged simultaneously with two independent STM tips.  The entire STM head is cooled with liquid helium to about four Kelvin while in an ultra-high vacuum (UHV) environment.  Macroscopic positioning of the tips is accomplished using piezoelectric “stick-slip” coarse motion stages, whereas atomic positioning is accomplished with piezoelectric tube scanners.  This instrument addresses the critical need to locally characterize individual nanostructures.
The specific design and development details have been submitted to the Review of Scientific Instruments for publication.
Significance of this work:
Currently, there is international concern that the evolution of the miniaturization of silicon technology will reach an end in less than ten years.  Consequently, there is a critical need to explore revolutionary alternative approaches.  One such approach that has gained considerable attention is electronic devices that utilize the other fundamental property of the electron, its spin (i.e., magnetic moment).  To harness this additional degree of freedom would truly be a revolutionary breakthrough in the electronic industry.  For example, to represent a letter on your keyboard requires eight normal transistors, however, see the comparison below for 8 spin-transistors:
(A)  8 normal transistors (2 levels of operation, on or off) = 28 = 256 different possible characters
  8 spin-transistors (10 levels of operation) = 108 = 100,000,000 different possible characters
By using ferromagnetic metal STM tips our system is a Spin-FET.  One tip injects a spin-polarized current, while the second tip detects it.  With this instrument we can study the current-voltage performance characteristics of the world’s first Spin-FET.

(left panel) Possible spintronic device structure.  (right panel) I-V characteristics for both a convention transistor and a spin-FET.
 The spin-polarized electron can only exit the channel if its spin is aligned with the drain's spin.The spin will precess as it moves through the channel (spin couples to magnetic field generated by the electron moving in the presence of the electric field applied to the gate).  Slight changes in the precession rate changes the total number of rotations the spin will go through and this changes the number of plateaus.  The depth of the valleys is determined by the percentage of spins polarized and the energy splitting between up and down spins in the source and drain.

## Performance Characteristics of a Spin-FET

***University of Arkansas, Fayetteville***

- Above left are STM images taken with STM tip one detecting electrons from the sample and STM tip two injecting the electrons.  These images where acquired with normal metal tungsten tips so we will not observe spin polarization effects.  These constant current STM images are of HOPG using a set point 1 nA, side tip at a bias of -3V (injecting electrons) and the underneath tip at +3 volts (detecting electrons) and sample is at ground.  (a) A 500 nm by 500 nm image with a vertical scale of 0.5 nm. (b) A 100 nm by 100 nm image with a vertical scale of 0.5 nm.  (c) A 5 nm by 5 nm image showing atomic resolution.

**Performance Characteristics of a Spin-FET**
**Paul Thibado  ****DMR-0405036**

**Notes:**
Development results:
Interest in developing next-generation nanoscale devices is at an all time high.  New methods for fabricating nanostructures are being developed at an increasing rate.  This includes self-assemble, artificially fabricated, lithography-etching techniques, etc.  What has not kept pace is the number of new ways to characterize materials on these smaller scales.  Consequently, the majority of characterization techniques still average over a large volume or surface area containing numerous nanostructures.  This method of characterization yields a signal that is inhomogeneously broadened from the desired result.  At present, there is a critical need to add new techniques which precisely characterize individual nanostructures.

An instrument that incorporates two scanning tunneling microscope (STM) tips which can have their tunnel junctions as close together as one nanometer was designed and built.  The sample is fixed and can be imaged simultaneously with two independent STM tips.  The entire STM head is cooled with liquid helium to about four Kelvin while in an ultra-high vacuum (UHV) environment.  Macroscopic positioning of the tips is accomplished using piezoelectric “stick-slip” coarse motion stages, whereas atomic positioning is accomplished with piezoelectric tube scanners.  This instrument addresses the critical need to locally characterize individual nanostructures.

The specific design and development details have been submitted to the Review of Scientific Instruments for publication.

Significance of this work:
Currently, there is international concern that the evolution of the miniaturization of silicon technology will reach an end in less than ten years.  Consequently, there is a critical need to explore revolutionary alternative approaches.  One such approach that has gained considerable attention is electronic devices that utilize the other fundamental property of the electron, its spin (i.e., magnetic moment).  To harness this additional degree of freedom would truly be a revolutionary breakthrough in the electronic industry.  For example, to represent a letter on your keyboard requires eight normal transistors, however, see the comparison below for 8 spin-transistors:
(A)  8 normal transistors (2 levels of operation, on or off) = 28 = 256 different possible characters
  8 spin-transistors (10 levels of operation) = 108 = 100,000,000 different possible characters

By using ferromagnetic metal STM tips our system is a Spin-FET.  One tip injects a spin-polarized current, while the second tip detects it.  With this instrument we can study the current-voltage performance characteristics of the world’s first Spin-FET.

## Performance Characteristics of a Spin-FET

***University of Arkansas, Fayetteville***

- The P.I. of this project periodically invites girl scouts and boy scouts to visit the lab and perform multiple hands on tasks.  This includes building atomic structural models, spot welding, removing air from a small chamber causing a marshmallow to expand, and observing several liquid nitrogen demonstrations.

***Working with Girls Scouts and Boy Scouts***

- The scouts are told about the critical need for Americans to go into physics, especially women.  Scouts are a great group of kids already organized with scout leaders and they get a science patch for their efforts.

**Performance Characteristics of a Spin-FET**
**Paul Thibado  ****DMR-0405036**