Date of Award
Doctor of Philosophy (PhD)
Self-assembled Ge quantum dots (QD) are grown on Si(100)-(2×1) by pulsed laser deposition (PLD). In situ reflection-high energy electron diffraction (RHEED) and post-deposition atomic force microscopy (AFM) are used to study the growth dynamics and morphology of the QDs. Several films of different thicknesses were grown at a substrate temperature of 400°C using a Q-switched Nd:YAG laser (λ = 1064 mu, 40 ns pulse width, 23 J/cm2 fluence, and 10 Hz repetition rate). At low film thicknesses, but clusters that are faceted by different planes, depending on their height, are observed after the completion of the wetting layer. With increasing film thickness, the size of the clusters grows, and they gradually lose their facetation and become more rounded. With further thickness increase, the shape of these clusters becomes dome-like with some pyramids observed among the majority of domes. The effect of the laser fluence on the morphology of the grown clusters was studied. The cluster density was found to increase dramatically while the average cluster size decreased with the increase in the laser fluence. For a laser fluence of 70 J/cm2, dome-shaped clusters that are smaller than the large huts formed at 23 J/cm2 were observed. At a substrate temperature of 150°C, misoriented three-dimensional (3D) clusters formed producing only a RHEED background. At 400 and 500°C, huts and a lower density of domes formed, respectively. Above 600°C, 3D clusters formed on top of a discontinuous textured layer.
As an application, pulsed laser deposition is used to fabricate multilayered Ge quantum-dot photodetector on Si(100). Forty successive Ge quantum dot layers, each covered with a thin Si layer, were deposited. Deposition and growth are monitored by in situ reflection-high energy electron diffraction and the morphology is further studied by ex situ atomic force microscopy. The difference in the current values in dark and illumination conditions was used to measure the device sensitivity to radiation. Spectral responsivity measurements reveal a peak around 2 μm, with responsivity that increases three orders of magnitude as bias increases from 0.5 to 3.5 V.
The effects of laser-induced electronic excitations on the self-assembly of Ge quantum dots on Si(100)-2×1 grown by pulsed laser deposition are also studied. Electronic excitations, due to laser irradiation of the Si substrate and the Ge film during growth, are shown to decrease the roughness of films grown at a substrate temperature of ∼120°C. At this temperature, the grown films are nonepitaxial. However, electronic excitation results in the formation of an epitaxial wetting layer and crystalline Ge quantum dots at ∼260°C, a temperature at which no crystalline quantum dots form without excitation under the same deposition conditions.
Finally, the very early stages of formation of Ge but clusters on Si(100) has been studied by UHV STM. Growth starts by the formation of a very low density of asymmetric huts with high aspect ratios. Further deposition results in a higher density of clusters characterized by their narrow size and height distributions. These clusters are almost of the same lateral size as those deposited at lower thicknesses.
Hegazy, Mohammed S..
"Self-Assembly and Characterization of Germanium Quantum Dots on Silicon by Pulsed Laser Deposition"
(2007). Doctor of Philosophy (PhD), Dissertation, Electrical/Computer Engineering, Old Dominion University, DOI: 10.25777/6sr8-7f34