Coupled luminescence centres in erbium-doped silicon rich silicon oxide thin films
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abstract
Silicon has been the mainstay of the microelectroncs industry for over four
decades. There is no material which can match the balance it affords between
cost-benefit, mass consumability, process versatility, and nano-scale electron
device performance. It is, therefore, the logical (and perhaps inevitable) platform
for the development of integrated opto-electronics - a technology that is being
aggressively developed to meet the next generation of bandwidth demands that
are already beginning to strain interconnect architectures all the way down to the
intra-chip level. While silicon-based materials already provide a variety of
passive optical functionalities, the success of a genuine silicon-based optoelectronics
will depend upon the ability of engineers to overcome those
limitations in the optical properties of bulk silicon that occur at critical junctions
in device requirements (eg. modulator and laser). Such solutions must not render
the device processing incompatible with CMOS, for then the "silicon advantage"
is lost. Achieving reliable and efficient electroluminescence in silicon remains
the most intractable of these problems to date.
Reliability problems in recently developed light emitting devices operating
near a wavelength of 1.54 f..Lm, based on the thermally induced formation of
silicon nano-clusters in erbium-doped silicon rich silicon oxide thin films, has reinforced
the need for a further understanding of the luminescence mechanisms in
this material. Indeed, the efficient and stable sensitized photoluminescence from
Er3+ ions (near the telecom wavelength), embedded in an oxide matrix, based on a
quasi-resonant energy transfer from nanostructured silicon, has the potential to
make possible compact waveguide amplifiers and thin film electroluminescence.
This thesis represents a study into the luminescence mechanisms in
erbium-doped silicon oxide (SiOx, x~2) thin films grown by electron cyclotron
resonance plasma enhanced chemical vapour deposition. Importantly, the film
growth relies on in-situ erbium doping through the cracking of a volatile organalanthanide
Er(tmhd)3 source. Rutherford backscattering spectroscopy has been
used to map the film composition space generated from an ECR-PECVD
parameter subspace consisting of precursor gas flow rates and the erbium
precursor temperature. The response of the film photoluminescence spectra in
both visible and infrared bands consistenly reveals three classes of luminescence
centres, whose relative ability to emit light is shown in this study to exhibit a
considerable degree of variability through the control of the film composition,
subsequent thermal anneal temperature, duration, and process ambient. These
three classes consist of optically active Er3
+ ions, silicon nano-clusters phase
separated during thermal annealing, and oxide-based defects (which may
additionally include organic chromophores). The latter two of these species show
the ability to sensitize the Er3 + luminescence. In fact, sensitization by intrinsically
luminescent defects is a rarely studied phenomenon, which seems to be an
important phenomenon in the present films owing to a potentially unique Er
incorporation complex. To further investigate the ability of the oxide defects in
this regard, an optimally luminescent film has been subject to a damaging ion
irradiation to induce a photoluminescence quenching. The subsequent recovery
of this luminescence with stepwise isochronous annealing has been correlated
with Doppler broadening positron annihilation spectroscopy measurements made
with a slow positron beam. Irradiation to a sufficiently high fluence has
demonstrated a unique ability to de-couple luminescent sensitizers and Er3+ ions,
producing enhanced blue and violet emissions.
