Semiconductor Spectroscopy and Devices Group
The Semiconductor Spectroscopy and Devices Group at Strathclyde University began its activities in 1987 as a wing of Professor Brian Henderson's Solid State Materials Group. It became a separate entity in 1990 with a mission "to explore the optoelectronic device capabilities of less-well-developed semiconductors":at first, visibly emitting semiconductor compounds from the Zn(Cd)S(Se) family.
In 1991, we began studies of the novel semiconductor porous silicon. Since 1993 magneto-optic studies of strained-layer III-V heterostructures have joined the list, and more recently (since 1995) we have worked on gallium nitride, indium gallium nitride, silicon quantum dots, chalcopyrites, ZnO, cuprate superconductors, dilute nitrides, ninth century Chinese paper........in fact, just about anything that glows. In 2000 we secured funding for a state-of-the-art electron microprobe and now are the lead partner in the facility that houses this instrument.
The group's programme of research combines basic
techniques of electron beam analysis, optical spectroscopy and spectroscopic
imaging with the realisation of opto-electronic devices using 21st Century
Group home page: http://ssd.phys.strath.ac.uk/
- Electron Probe Micro-Analysis
- Copper Indium Diselenide Materials for Solar Cells
- Quantum Dots
- Atomic Force Microscopy and Luminescence Scanning Tunnelling Microscopy
- Electron Beam Studies of Solids
- Investigations of Semiconductors in the Presence of High Magnetic Fields
The Semiconductor Spectroscopy
group is responsible for the running of a state-of-the-art electron probe
micro-analyser (EPMA). This instrument is an exceptionally powerful tool
for studying the topography, composition and optical properties of a wide
range of materials on sub-micron length scales. A specially modified scanning
electron microscope lies at the heart of the EPMA, which is equipped with
multiple spectrometers to analyse characteristic X-rays emitted by the
studied sample. These tools are employed in several projects aiming to
accurately determine the micro-composition
of structures (such as InGaN-based heterostructures and rare-earth doped
semiconductors) and relate this to other material properties – most
notably their characteristics as light emitters. To achieve this we have
developed a unique cathodoluminescence spectral mapping system, which
forms the basis of a range of ground-breaking studies. Projects related
to the EPMA will thus provide experience in a powerful range of techniques,
including electron microscopy, optical spectroscopy and X-ray microanalysis.
The commercialisation of light
emitting diodes based upon InGaN has resulted in an explosion of interest
in wide bandgap III-V materials. The ability to produce light emitting
diodes spanning the UV to red spectral region makes these materials highly
attractive for the development of full colour displays. Recently Nichia
Chemicals have introduced a laser diode operating at 410 nm, generating
further interest in developing coherent light emitters for high density
optical data storage, reprographics, underwater communications and so
In spite of recent successes, much fundamental physics remains to be done on III-N materials. Working devices, surprisingly, feature huge defect densities. Unidentified deep centres contribute a strong green/yellow band in competition with band edge emission. We use atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and cathodoluminescence spectroscopy, to study both the morphological and optical properties of InGaN films and to investigate the correlation between structure and functionality. These studies use material from the III-Nitride chemical vapour deposition system at Strathclyde along with that from many collaborations round the world.
Vertical cavity surface emitting lasers (VCSELs) built from III-Nitrides promise significant advantages over the edge-emitting lasers mentioned above but prove challenging to demonstrate. We have collaborative projects underway investigating the physical performance and properties of III-Nitride surface emitters, fabricated using a variety of novel techniques.
An alternative approach to realising nitride LEDs uses the highly efficient localised emission of rare-earth (RE) ions in wide-band gap hosts. Red, green and blue emission from Eu, Er and Tm respectively practically define the primary colours of the chromaticity diagram. We will compare the efficacy of different sample preparation techniques, establish optimum conditions for RE incorporation and investigate prototype electroluminescent devices. See http://www.renibel.net.
The solar cell industry is rapidly growing in Europe and America by 20% a year. In the UK alone the solar cell market is expected to reach the level of 3.6 billion pounds in 5 years. CuInSe2 (CIS) is one of the most promising semiconductors for thin-film solar cells and is used in the absorption layer. CIS-based solar cells are becoming one of the leading technologies for solar energy generators and hold the efficiency record (which is about 19%) among thin-film devices. One of their mysterious features is super high tolerance to any radiation. Their life-time in outer space was found to be at least 50 times as long as that of amorphous silicon solar cells. In fact, irradiation with quite high doses of MeV protons and electrons improves their performance. The material seems to repair itself at room temperature. In collaboration with research groups from many countries studies our group studies the effects of energetic ions on the optical properties of CIS and other related semiconductors.
In the last 25 years, semiconductor physicists have
explored the concept of reduced dimensionality in condensed matter. Starting
with quantum wells, where the charge carrier motion is essentially two-dimensional,
semiconductor physicists have proceeded through quantum wires to the ultimate
concept in low-dimensioned semiconductors, the quantum dot (QD). Such
a dot is smaller than the electronic Bohr radius in all three spatial
co-ordinates. The quantum states of carriers confined within a dot adopt
the discrete character more usually associated with atoms. Hence the QD
is sometimes referred to as an artificial atom, although it may in fact
contain several thousand real atoms.
Assemblies of quantum dots may be prepared using a number of different fabrication techniques. To date, we have been mainly concerned at Strathclyde with QD systems that are capable of producing visible light when excited by photons (or high energy electrons) or when immersed in a semiconductor diode placed under bias. The II-VI materials such as CdSe and CdS and silicon provide such systems.
Atomic Force Microscopy of hexagonal
crystallites on a GaN surface. Atomic force microscopy involves scanning
a sharp tip attached to a cantilever spring across the surface of a solid
sample. Maintaining a constant force between tip and sample results in
the tip going up the "hills" and down into the "valleys"
of the sample as the tip is scanned. Monitoring the movements of the cantilever
leads to a high resolution (<10 nm) 3-D image of the sample’s
topography. In certain circumstances, atomic resolution may be accessible
to this technique.
At present we routinely use a Burleigh Personal atomic force microscope operating in air at room temperature to obtain high-resolution physical images of samples within a frame up to 70 µm square. To date we have investigated the surface morphology of ZnSe and GaN films grown on different substrates by different techniques, and under various conditions of growth. A new project involves the development of a luminescence scanning tunnelling microscope in which the light emission induced by a current injecting probe is detected concurrently with the height variation of the probe as it is raster scanned over the surface of a sample. This technique opens up exciting opportunities for studying comparative morphology and luminescence properties of materials down to the atomic scale, enabling, for example, the interrogation of the luminescence properties of single quantum dots and impurity ions in solids.
Using a specially built electron beam system, a variable
energy electron beam is used in non-destructive 3-D analysis of solids.
At low temperatures, the cathodoluminescence (CL) spectrum is very sensitive
to the properties of the solid, such as the crystal structure, composition,
strain and defect concentrations. If the electron beam is positioned at
different points on the sample and a CL spectrum obtained at each point,
maps may be produced which show these properties as a function of position.
Information in the 3rd dimension, i.e., depth information is obtained
by exploiting the unique property of a variable energy electron beam,
to deposit energy at a depth which depends on its kinetic energy. For
example, a 12 keV electron beam will deposit most energy at 200 nm, whilst
a 30 keV beam will deposit most energy at 1.0 µm in a typical solid.
Comparative studies of samples' topography and luminescence properties at different wavelengths are also carried out using a modified scanning electron microscope (SEM). The SEM also contains an electron backscattered diffraction (EBSD) system. From EBSD patterns crystallographic information such as crystal structure, crystal orientation and tilt can be measured. It is also possible to measure the strain in a sample. To date the properties of diamond, II-VI semiconductors, III-Nitrides and high temperature superconducting thin films have been investigated. Further research in progress/under development includes the development of novel low threshold e-beam pumped semiconductor laser structures, and time resolved CL utilising the ability to rapidly switch an electron beam.
Absorption spectra of an InGaAs/InGaAlAs MQW at
various magnetic field strengths. The application of high magnetic field
is an important investigative tool for the study of semiconductor materials.
We use a 12 Tesla superconducting magnet to study the optical and transport
properties of a range of materials. Current work includes magneto-transport
studies of the electron gases within GaN / AlGaN transistor structures
(in collaboration with Sheffield University), magneto-absorption studies
of infra-red laser structures and magneto-photoluminescence studies of
copper indium diselenide films.