Date of Award

Summer 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Committee Director

Lepsha Vuskovic

Committee Director

Hani Elsayed-Ali

Committee Member

Alex Godunov

Committee Member

Moskov Amaryan

Committee Member

Gon Namkoong

Abstract

Self-assembled Ge quantum dots (QD) are grown on Si(100)-(2×1) with laser excitation during growth processes 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. A Q-switched Nd:YAG laser (λ = 1064 nm, 40 ns pulse width, 5 J/cm2 fluence, and 10 Hz repetition rate) were used to ablate germanium and irradiate the silicon substrate. Ge QD formation on Si(100)-(2×1) with different substrate temperatures and excitation laser energy densities was studied. The excitation laser reduces the epitaxial growth temperature to 250 °C for a 22 ML film. In addition, applying the excitation laser to the substrate during the growth changes the QD morphology and density and improves the uniformity of quantum dots fabricated at 390 °C. At room temperature, applying the excitation laser during growth decreases the surface roughness although epitaxial growth could not be achieved.

We have also studied the surface diffusion coefficient of Ge during pulsed laser deposition of Ge on Si(100)-(2×1) with different excitation laser energy densities. Applying the excitation laser to the substrate during the growth increases the surface diffusion coefficient, changes the QD morphology and density, and improves the size uniformity of the grown quantum dots.

To study the effect of high intensity ultralast laser pulses, Ge quantum dots on Si(I00) were grown in an ultrahigh vacuum (UHV) chamber (base pressure ∼7.0x10 -10 Torr) by femtosecond pulsed laser deposition. The results show that excitation laser reduces the epitaxial growth temperature to ∼70 °C. This result could lead to nonthermal method to achieve low temperature epitaxy which limits the redistribution of impurities, reduces intermixing in heteroepitaxy, and restricts the generation of defects by thermal stress.

We have ruled out thermal effects and some of the desorption models. Although further studies are needed to elucidate the mechanism involved, a purely electronic mechanism of enhanced surface diffusion of Ge atoms is proposed.

DOI

10.25777/6jwv-nk16

ISBN

9781124973043

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