I hope that the students and staff of Rooks Heath won’t mind me using the blog to do a spot of reminiscing but the good news is that it does involve physics and one of Rooks Heath ex-students occurs in it.
After graduating in 1985 I was privileged to spend three years doing research at King’s college London. The area of research was the investigation of optical properties of defects in silicon under stress.
The head of the group was Professor Edward Lightowlers (pictured on the left) and my supervisor was Dr. (now retired Professor) Gordon Davies (pictured on the right).
Sadly I don’t have pictures from my time at King’s but I was able to visit in October and take some pictures then. The experimental solid state physics group has been disbanded now and the labs will be re-developed next year so my pictures are a bit sad.
Defects in silicon can be identified by how they absorb or emit (photoluminescence) certain wavelengths of light.
There are two main methods of producing the silicon necessary for experimental purposes: Czochralski (CZ) and Float Zone.
X-ray diffraction was used to orientate a silicon crystal and allow it to be cut to the required shape (large ratio of length to width and thickness). It was then compressed along one specific axis by placing it between pistons made of a material harder than silicon (quartz or hardened steel). My experiments had to be done at very low temperatures so the stress cell (as described above) was designed to fit inside a cryostat. A cryostat (from cryo meaning cold and stat meaning stable) is a device used to maintain cold cryogenic temperatures. The low temperatures were maintained by liquid helium. It is assembled into a vessel, similar in construction to a vacuum flask. Superfluid helium was used to stop the scattering of light by bubbles which occur at 4.2 K. Reducing the pressure in the cryostat could produce temperatures of about 2K.
As liquid helium is very expensive the cryostat was pre-cooled with liquid nitrogen. The liquid nitrogen was kept at the back of King’s and I had to wheel a Dewar (a large container similar to a vacuum flask) there to obtain it.
The stress was applied to the sample via a push-rod. The force was generated by gas or oil pressure acting on a piston outside the cryostat at room temperature. The optical measurement was made through the central section of the silicon (to ensure that the stress was at a maximum and constant).
The stress cell could apply a strain of about 10^-3 without breaking the silicon. The stress caused the electronic states of the crystal to be changed (perturbed).
In solid-state physics, the electronic band structure (or simply band structure) of a solid describes those ranges of energy, called energy bands, that an electron within the solid may have (“allowed bands”), and ranges of energy called band gaps (“forbidden bands”), which it may not have. Band theory models the behaviour of electrons in solids by postulating the existence of energy bands. It successfully uses a material’s band structure to explain many physical properties of solids, such as electrical resistivity and optical absorption.
The electrons of a single isolated atom occupy atomic orbitals, which form a discrete set of energy levels (which is what A level students are taught). If several atoms are brought together into a molecule, their atomic orbitals split into separate molecular orbitals each with a different energy. This is due to the Pauli exclusion principle, which says that electrons that are close together must have different sets of quantum numbers (energy). This produces a number of molecular orbitals proportional to the number of valence electrons. When a large number of atoms (of order ×1020 or more) are brought together to form a solid, the number of orbitals becomes exceedingly large. Consequently, the difference in energy between them becomes very small. Thus, in solids the levels form continuous bands of energy rather than the discrete energy levels of the atoms in isolation. However, some intervals of energy contain no orbitals, no matter how many atoms are aggregated, forming band gaps.
Within an energy band, energy levels can be regarded as a near continuum for two reasons. First, the separation between energy levels in a solid is comparable with the energy that electrons constantly exchange with phonons (atomic vibrations). Second, this separation is comparable with the energy uncertainty due to the Heisenberg uncertainty principle, for reasonably long intervals of time. As a result, the separation between energy levels is of no consequence.
Bands have different widths, based upon the properties of the atomic orbitals from which they arise. Also, allowed bands may overlap, producing (for practical purposes) a single large band.
The stress applied to the silicon crystal (if my memory serves me right) alters these energy bands and allows them to interact.
The picture top left shows the route to the liquid nitrogen. The picture above right shows me in one of the solid-state physics labs with two cryostats visible (silver towards the back and black towards the front). The picture below left is a more modern version of a cryostat.
Defects in silicon can happen naturally and it can contain various impurities including oxygen, carbon, boron and possibly hydrogen. Depending on how the crystal was created nitrogen can be present too.
Crystals also contain structural defects derived from the grouping of intrinsic point defects as they cool from the melting temperature. The defects and impurities often show a non-uniform distribution in the form of helical swirls. Heat treatment of silicon-containing oxygen leads to the clustering of this impurity. At 450 degrees C there is formation of small complexes that act as shallow donors.
Defects can be deliberately induced in silicon by bombarding the crystals with electrons or ions or by heating.
To identify the defect by absorption I focused red laser light onto the silicon crystal (inside the cryostat) using lenses. An electronic detector was placed the other side of the crystal and the spectra was recorded on paper. The detector recorded a range of wavelengths and the absorbed wavelength was shown by a dip on the trace (of course this would be done by computer now).
The blue object in the picture above is a detector.
For my day-to-day work I didn’t use photoluminescence but I did use it when investigating a specific optical transition (The temperature dependence of the 969meV “G” optical transition in silicon). Spectroscopy was by Fourier transform and dispersive spectrometers fitted with Ge detectors. Luminescence was generated using a krypton laser, typically at 100 mW mm^-2.
Photoluminescence excitation (abbreviated PLE) is a specific type of photoluminescence and concerns the interaction between electromagnetic radiation and matter. It is used in spectroscopic measurements where the frequency of the excitation light is varied, and the luminesence is monitored at the typical emission frequency of the material being studied. Peaks in the PLE spectra often represent absorption lines of the material. PLE spectroscopy is a useful method to investigate the electronic level structure of materials with low absorption due to the superior signal-to-noise ratio of the method compared to absorption measurements.
In a quantum-mechanical description of matter, the electrons confined to a material (such as those in individual atoms, molecules or crystals) are limited to a discrete set of energy values. The ground state of such a material system is such that the most energetic electron has its minimal energy. In photoluminescence energy is transferred from light incident on the material and absorbed to electrons. The light is absorbed in minimal “quanta” or “packets” of energy of the electromagnetic radiation called photons. The amount of energy carried by a photon is proportional to its frequency. The electron is then in an excited state of higher energy. Such states are not stable and with time the material system will return to its ground state and the electron will lose its energy. Luminescence is the process whereby light is emitted when the electron drops to a lower energy level.
Often when a photon is absorbed, the system is excited in the corresponding excited state, then it relaxes in an intermediate lower energy state, with a “non-radiative relaxation” (a relaxation that doesn’t involve the emission of a photon, but e.g. involves the emission of vibrational energy) and then there is the emission of a photon with a lower energy than the absorbed one, because of the relaxation from the intermediate, lower energy state to the “ground state”. Usually the strongest luminescence of the material is from the lower levels to the ground state. This process is called fluorescence. For instance in semiconductors most of the light emitted is at the frequency corresponding to the bandgap energy, i.e. from the bottom of the conduction band to the top of the valence band. In such systems, more light absorbed by the material, results in more electrons decaying non radiatively to the lower states, and more luminescence in the emission wavelength.
After twenty four years it is great that one of my ex-students is now studying physics at King’s College.
Mohamed-Ali Al-Bhadri walking down Q corridor.
Behind the door on the left is a room where I once had a desk.
Bill and Julian. Two of the amazing technicians at King’s who are still there.
Mohamed listening to all of us reminiscing.
Inside the room where I had my desk. The blue object is a cryostat. Just to the left you can see a room that once belonged to a post-doc (somebody who has recently obtained a PhD).
Some work is being finished off. You can just make out a laser to the left. The picture on the right shows equipment that will be recycled.
Just as Mohamed and I were leaving we saw Professor Alan Collins. Alan investigated defects in diamond and was also in Professor Lightowler’s group and he was very happy to have his picture taken with Mohamed.