[English]

Electron emission from semiconductor surfaces

(supervisor: Andrew Evans)

Nature of project: experimental, data analysis

Available to full-time physicists only.

Project description and methodology

When electromagnetic radiation is incident on a material in a vacuum, electrons are emitted when the photon energy is sufficient to enable the electron to overcome the surface potential energy (work function). For metals, the work function is an important parameter that can be measured from the emission of electrons from surfaces of known quality and such measurements must be carried out in vacuum. For semiconductors, the work function depends on the impurity doping levels and the parameter that defines electron emission in this case is the electron affinity. This parameter is particularly important in optoelectronic devices that involve interfaces between different semiconductors, for example photovoltaic cells. In some cases, for example the hydrogen-terminated diamond surface, the electron affinity is negative, effectively providing no energy barrier for the emission of electrons from the conduction band. The emitted electrons also carry information of the energy and momentum of valence and core electron states and this information can be extracted by measuring the energy and angle of emission of the electrons.

This project involves the development of a measurement and analysis system to investigate electron emission from metal and semiconductor surfaces during irradiation with visible/UV radiation sources. It will involve the calibration and control of the light source and the measurement of photocurrents within the vacuum vessel. The system will be calibrated using irradiation of standard metal foils and will progress to the measurement of electron emission from n-type and p-type semiconductors of different band gaps.

A successful project will develop beyond the above in one/some of the following directions:
The project can be extended to study interfaces of relevance in photovoltaic cells such as transparent conducting oxides and semiconductors. This will involve the use of different UV sources and electron energy analysis to measure key surface and interface parameters in order to determine the interface energy band profile. There is also scope to develop the instrumentation and software for data collection and analysis.

When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.

Additional scope or challenge if taken as a Year-4 project: As a Y4 masters project, a fixed UV or soft x-ray radiation source will be used to provide electrons with higher kinetic energy in order to determine the electronic band structure near the Fermi level for doped semiconductor surfaces. The particular focus will be on surfaces prepared using in-situ oxidation and reduction to provide a wide range of work function and electron affinity. Such measurements are important for developing new efficient cold cathode emitters and for designing more efficient materials combinations for photovoltaic energy capture.

Initial literature for students:

  1. Physics of semiconductor devices, S. M. Sze (Primo)
  2. Modern techniques of surface science, D. P. Woodruff T. A Delchar (Primo)
  3. Photoemission and the electronic properties of surfaces, B Feuerbacher; B Fitton; R. F Willis (Primo)

Novelty, degree of difficulty and amount of assistance required

The equipment needs to be put together from components and is thus a challenging task. It also requires a willingness to master the basics of vacuum technology. Some computational skills are needed for instrument control and data analysis.

Project milestones and deliverables (including timescale)

milestoneto be completed by
Identification of materials (e.g. electron emitter, PV cell)end of October
Finalise methodChristmas
First measurements on test metalsend of February
Measurements and analysis of semiconductorsEaster

Students taking this project will have to submit a full risk assessment form