Advanced transmission electron microscopy studies of III-V semiconductor nanostructures.

III -V semiconducting materials allow many novel optoelectronic devices,
such as light emitting diodes and lasers, to be developed. Furthermore, recent
development in crystal growth techniques allows the growth of low-dimensional
semiconductor heterostructures. To achieve the best performance, the crystallinity
and the growth mechanism of the devices have to be analysed. In this work, a JEOL
JEM-2010F field emission gun transmission electron microscope (TEM) is
employed to analyse the nanoscale semiconductor structures. Various techniques,
such as conventional TEM, scanning TEM, high resolution TEM and energy-filtered
TEM were employed to characterize the structural properties of III-V
semiconducting materials.
In this thesis, advanced TEM analysis on InAs/GaAs quantum dots with
InAIAs capping layer, GaInNAs/GaAs quantum wells and annealed low
temperature-grown GaAs are presented. The former investigates the impact of
varying the thicknesses of InAIAs in the combined two-level InAIAs-InGaAs
capping layer on InAs/GaAs quantum dots. Based on the energy-filtered TEM
images, the concentration of Al near the apex of the dots is significantly reduced. An
increase in the height of the quantum dots has been observed when the thickness of
InAIAs capping layer is increased. This is attributed to the suppression of indium
atom detachment rate from the InAs dots during the capping process.
Effects of growth temperature on the structural properties of 1.6 um
GaInNAs/GaAs mUltiple quantum wells were also investigated. TEM studies show
that compositional modulations and dislocations occurred in the sample grown at
400°C and possible point defect formation in the sample grown at 350 °C. The
photoluminescence intensities for samples grown at 350 and 400°C are degraded
dramatically, compared with the sample grown at 375 °C.
The effects of low temperature-growth GaAs annealed at different
temperatures were systematically investigated by TEM. Along with other
collaborative measurements, the arsenic precipitate parameters obtained from TEM
images were employed to develop a semi-quantitative model based on Ostwald
ripening to explain the precipitate formation. Furthermore, the "two-trap" model
successfully explains the anomalous features in the carrier lifetime and resistivity
trends in annealed low temperature-grown GaAs.