Ph.D Thesis

Ph.D StudentRaz Tal
SubjectSelf Assembled Quantum Dots Grown on GaAs and InP Substrates
by Metal Organic Molecular Beam Epitaxy
DepartmentDepartment of Electrical and Computer Engineering
Supervisors PROF. Dan Ritter


Semiconductor quantum dots can be grown using the concept of self-assembly. Growing a highly strained thin layer of a narrow-gap semiconductor, on a wide-gap semiconductor substrate, can result in a spontaneous formation of small islands on top of a very thin wetting layer. This type of growth is termed “Stranski-Krastanow” growth mode. The formation of coherent self assembled quantum dots utilizing this method was demonstrated in various semiconductor material systems such as In(Ga)As/(Al)GaAs, InAs/InP, Ge/Si etc.

The objective of this work is studying the technology of self assembled quantum dot epitaxial growth in a metal organic molecular beam epitaxy (MOMBE) system, understanding physical processes related to charge carriers in the quantum dots, and implementing opto-electronic devices based on quantum dots.

The growth of self-assembled quantum dots on GaAs and InP substrates is studied in the first part of the research. The formation process of the quantum dots, and the properties of the quantum dots ensemble are described with respect to the growth conditions. In this part, quantum dots grown on strained layers were studied as well. Growing InAs quantum dots on strained layers of GaxIn1-xP on InP substrates results in smaller quantum dots, relative to quantum dots grown directly on InP. This modification in quantum dot size is attributed to the diffusion of Ga from the underlying layer into the quantum dots. Another unique phenomenon studied in this work is the formation of InAs quantum rings on InP substrates. The shape transformation from lens-shaped to ring-shaped structure occurs after the quantum dots are partially covered by a thin layer, thinner than the quantum dots height. The formation of quantum rings is driven by thermodynamics, where the final quantum ring is an equilibrium structure.

Charge carrier dynamics is studied by photoluminescence measurements as a function of temperature. A rate equation model, taking into account the physical processes occurring during the experiment, is presented. Processes as carrier capture by the quantum dots, and carrier thermal escape from the quantum dots back into the wetting layer are studied by comparing the model calculations with the experimental results.

First attempts at implementing quantum dot devices are reported at the last part of the work. The devices investigated here are 1.55 mm quantum dot laser, and a quantum dot infrared photo-detector. Laser structures based on quantum dots were grown and characterized. Since the optical gain in these structures is not high enough, only a spontaneous emission is observed. Quantum dot infrared detector structures were grown, with different doping levels and dot shapes. The grown structures show a photocurrent in response to normal incident infrared radiation.