Download A Study of the Relaxation Dynamics of Local Vibrational Modes... with Hydrogen in Diamond

A Study of the Relaxation Dynamics of Local Vibrational Modes Associated
with Hydrogen in Diamond
Hydrogen is abundant in the source gases for the growth of diamond by chemical vapour
deposition (CVD). Hydrogen is of considerable experimental and theoretical interest
because of its ability to interact with virtually any lattice defect, including impurities,
intrinsic defects, surfaces, and interfaces, thus possibly changing both the electronic and
optical properties of the material.
Until now, all spectroscopic studies of hydrogen-related in diamond have been carried out in
the frequency domain, which probes the time-averaged optical response of the modes.
Consequently, very little is known about the dynamics of the modes, i.e., the time scales and
mechanisms for population and relaxation upon excitation. Such information is crucial since
excited vibrational states may be involved in the dissociation of the bond between hydrogen
and the lattice and in optical absorption at frequencies well above that of the fundamental
vibration.
In silicon, the degradation of some electronic devices is believed to be caused by the
dissociation of vibrationally excited Si-H bonds. To understand such processes in diamond, it
is necessary to know the time scale on which excited vibrational states decay. Such
measurements are now possible with highspeed pump-probe laser systems. In silicon the SiH vibration lifetimes are found to be extremely dependent on the defect structure, varying
by more than two orders of magnitude, and the same is expected for diamond.
Knowledge of excited state lifetimes would be terrifically useful. For example we are
currently unable to relate the strength of a C-H local vibrational mode to the concentration
of the defect in diamond, for all except one defect where charge transfer has allowed
calibration. The radiative lifetime relates the strength of the absorption to the concentration
of the defect. Thus the lifetime information is vitally important for determining the amount
of hydrogen incorporated in different defects in diamond.
Furthermore, we know little about the mechanism(s) for the diffusion of hydrogen in
diamond. Control of impurity diffusion is an important element in materials processing and
device fabrication technologies. Thermal annealing is unselective and unwanted diffusion of
a specific species may be stimulated while endeavouring to activate diffusion of another. It
would be highly desirable to diffuse a specific species only (selective diffusion). It may be
possible by irradiation with the resonant frequency could cause excitation of a C-H local
vibrational mode to selectively enhance the diffusion of hydrogen in diamond. Since the
optical excitation time should be short compared to the relaxation time knowledge of later
would be terrifically useful.
This PhD project combines time resolved pump-probe techniques, to measure the excited
state lifetimes of a number of local vibrational modes, with a range of other spectroscopic
measurements to investigate some of the outstanding hydrogen related defect
identification, quantification and interaction issues in diamond. The project will involve
working with a number of collaborators, both academic and industrial, and making use of
strong existing links with theoretical researchers investing defect dynamics.
For further details please contact Prof. Mark Newton:
[email protected]