Dr Benedict N. Murdin

Research interests:

Study of electronic and optical properties of semiconductors and semiconductor nanostructures using high-pressures,  magnetic-fields, and linear, nonlinear and time resolved infrared spectroscopy.

At the moment my main research is devoted to the study of optical transitions between quantum confined states in semiconductor quantum wells and dots (intersubband transitions). This includes applications to strained-layer mid-infrared laser diodes based on narrow gap interband structures and Quantum Cascade devices. I am a regular user of the free-electron lasers FELIX and CLIO (in Holland and Paris).
This is a picture of FELIX:

This is where FELIX is (and this map shows how to get there from Utrecht)

Doing experiments:

with a forest of mirrors:

 

Near-IR/Far-IR Double resonance spectroscopy of semiconductor quantum dots

Using a novel technique of far-infrared modulated photoluminescence (PL) using the free-electron laser FELIX, we have performed FIR spectroscopy of self-assembled InAs/GaAs quantum dots. The far-infrared resonance observed is unambiguously associated with a bound-bound intraband transition within the dots. The results also show that higher PL lines that appear under high excitation, are from conduction band levels with successively increasing in-plane quantum number, and that the primary cause of inhomogeneous broadening of the PL is neither size fluctuation nor well depth fluctuation. A candidate mechanism consistent with the results is the Coulomb interaction.  The technique is being extended to time-resolve the scattering of electrons between such states in dots, wires and wells.

Electron subband dynamics of low dimensional systems

There is an important class of unipolar semiconductor optoelectronic devices based on transitions between the confined states (subbands) created in quantum wells. These include Quantum Well Infrared Photodetectors (QWIPs) and QC lasers. We have studied the scattering of electrons by phonons, which is a limiting factor in such devices. The use of the picosecond pulsed, far-infrared Free Electron Laser (FELIX) in The Netherlands allowed us to perform the first far-infrared pump-probe determinations of electron lifetimes in the picosecond regime. The relaxation lifetimes and subband dynamics associated with intersubband absorption were measured in GaAs/AlGaAs n-type quantum wells, p-type GaAs/AlGaAs, and also Si/SiGe n-type wells. All samples had a subband separation smaller than the optical phonon energy which suppresses the phonon emission at low temperatures. However, even at around 35K the relaxation lifetime was of order 10ps. These timescales illustrate the very fast nature of the non-radiative competition to be overcome in QC devices these lifetimes should be compared with many nanoseconds for non-radiative recombination in interband lasers.

Scattering processes between Landau quantised dot levels

A natural question to arise from the above study was the possibility of suppressing the LO phonon emission, and this is most easily addressed through the use of quantising magnetic fields. When electrons are confined in the growth direction, x, by a quantum well, and are then confined additionally to cyclotron orbits in the y-z plane by the B-field, they can be said to behave as if in a quasi-dot, at least in so far as their density of states is concerned. The lateral size of the quasi-dot is tunable with magnetic field. The electrons show an oscillatory scattering rate with B-field, with strongly enhanced cooling when the LO phonon energy is equal to a multiple of the cyclotron energy. Looked at another way, the cooling is strongly suppressed away from these resonant energies. Landau level lifetimes have been determined in the narrow gap quantum well systems InAs/AlSb, InAs/GaSb, and PbTe/PbEuTe. Strong suppression of the phonon scattering occurs away from resonance. The lifetime varies from 0.5ps in the phonon scattering regime to 60ps in the phonon bottleneck regime.
We have recently shown using magnetic fields to give a peaked DOS that this phonon suppression can have a detrimental effect on interband devices as described in the next section.

Characterisation of interband InSb laser devices

MIR laser diodes are desirable for many environmental, medical and military applications. We are working in collaboration with DERA Malvern to develop room temperature operating MIR laser diodes based on interband transitions in InSb.
Measurements have been made of the light-current characteristics of bulk InSb-based laser devices. These materials have long been known to suffer from Auger recombination and we have been able to show, using device characteristics only, that this is indeed the limiting process in our devices. The programme uses the technique of interband electro-luminescence spectroscopy of devices under high applied pressure and magnetic field to study the possibility of inhibition of the Auger in lower dimensional structures with built-in strain. Additionally, the B-field allows quantification of the effects of the bottleneck mentioned above in real devices. Striking results were obtained when the magnetic field was swept at constant bias. Peaks in the light output were seen at exactly the field positions when an inter-Landau-level splitting equals the LO phonon energy, i.e. giving enhanced electron cooling. Our observation provides unambiguous evidence for the phonon bottleneck effect independently of arguments concerning different growth techniques and the quality of different sample structures.


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This page last updated July 2000.