Ballistic electron emission microscopy

Ballistic electron emission microscopy sketch

The ballistic electron emission microscopy (short BEEM ) is a modification of the scanning tunneling microscopy , was developed in 1988 by L. Douglas Bell and William J. Kaiser. With the help of BEEM, the Schottky barrier at the interface between a metal and a semiconductor can be investigated with a nanometer-high lateral resolution.

Measuring principle

A Schottky diode , which consists of a metal film on a semiconductor substrate, is used as a sample . A Schottky barrier forms between the metal and the semiconductor , which electrons can not cross with an energy . Electrons are injected into the metal side of the Schottky diode through a tunnel tip . The applied tunnel voltage defines the electron energy . The electrons can be scattered in the metal film, whereby they lose at least 50% of their energy. If one chooses the electron energy of the kind that is, the Schottky barrier acts as an energy filter for the scattered electrons. Only electrons without a change in direction or energy make it through the Schottky barrier. These electrons are therefore also called ballistic because they propagate through the metal film like bullets. The ratio of the current of the BEEM electrons to the total tunnel current applies in the context of a constant current experiment ${\displaystyle \varphi _{\text{SB}}}$ ${\displaystyle \varphi _{\text{SB}}\leqq E}$ ${\displaystyle U}$ ${\displaystyle E}$ ${\displaystyle \varphi _{\text{SB}}<E<2\varphi _{\text{SB}}}$ ${\displaystyle I_{\text{BEEM}}}$ ${\displaystyle I_{t}}$

${\displaystyle {\frac {I_{\text{BEEM}}}{I_{\text{t}}}}=w{\frac {(U-\varphi _{\text{SB}})^{2}}{U}}.}$

${\displaystyle w}$describes the throughput of ballistic electrons that even reach the Schottky barrier. If you run through the tunnel voltage at a location on the sample with the tunnel tip, you get a so-called ballistic electron emission spectrum (BEES).

application

With BEES, the Schottky barrier can be measured very precisely (approx. 0.02 eV). Furthermore, one can investigate the properties and lateral variation of the electron flow through the interface between metal and semiconductor using BEEM. The mean free path in the metal can be determined from the BEEM throughput for different metal film thicknesses . In addition, the ballistic properties of adsorbates on the Schottky barrier can be investigated. ${\displaystyle w}$

See also

• Surface chemistry

literature

• LD Bell, WJ Kaiser: Ballistic-Electron-Emission Microscopy: A Nanometer-Scale Probe of Interfaces and Carrier Transport . In: Annual Review of Materials Science . tape 26 , no. 1 , 1996, p. 189–222 , doi : 10.1146 / annurev.ms.26.080196.001201 .

Individual evidence

1. ↑ LD Bell, WJ Kaiser: Observation of Interface Band Structure by Ballistic Electron Emission Microscopy . In: Physical Review Letters . tape 61 , no. 20 , November 14, 1988, pp. 2368-2371 , doi : 10.1103 / PhysRevLett.61.2368 .
2. ^ WJ Kaiser, LD Bell: Direct investigation of subsurface interface electronic structure by ballistic electron emission microscopy . In: Physical Review Letters . tape 60 , no. 14 , April 4, 1988, pp. 1406-1409 , doi : 10.1103 / PhysRevLett.60.1406 .
3. ^ RH Ritchie, JC Ashley: The interaction of hot electrons with a free electron gas . In: J. Phys. Chem. Sol. tape 26 , no. 12 , 1965, p. 1689-1694 , doi : 10.1016 / 0022-3697 (65) 90198-8 .
4. ↑ CA Bobisch: Microscopy with ballistic electrons . VDM Verlag Dr. Müller, Saarbrücken 2008, ISBN 978-3-639-05791-1 , p. 34.