Rachel S. Goldman

Professor

rsgold@umich.edu

2094 H.H. Dow Building

T: (734) 647-6821

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Fundamental Phenomena in Semiconductors: Strain Relaxation

Sponsor: National Science Foundation DMR-9773707, DMR-0210714; National Renewable Energy Laboratory ACQ-1-31619
During heteroepitaxial growth of films and heterostructures, misfit strain may be relaxed elastically and/or plastically. Although the mechanisms of these strain relaxation processes have been studied for decades, the interplay between elastic and plastic relaxation, as well as the effects of diffusion and segregation on both processes, are not well understood. For example, in highly-mismatched epitaxial systems, stress relaxation is generally driven by elastic relaxation of strain via island nucleation, a so-called "Stranski-Krastanow"(SK) growth mode transition, resulting in the formation of self-assembled quantum dots. We have been examining this transition in real-time, using simultaneous in-situ wafer curvature and reflection high-energy electron diffraction measurements. Our studies of InAs/GaAs reveal an onset of stress relaxation following the SK growth mode transition, suggesting that the islands formed during the SK growth mode transition do not relax strain immediately. In addition, we find that In surface segregation plays a substantial role in the stress relaxation of capped and stacked quantum dot structures, often influencing the subsequent island nucleation. Finally, we find evidence for diffusion-enhanced suppression of plastic relaxation in stacked quantum dot structures. We are currently in the process of investigating the transition from elastic to plastic relaxation using real-time measurements of the stress evolution during growth. We are also in the process of developing a generic model for these effects, through investigations in a variety of semiconductor systems.
Highlights (Click an image for more information)
  • In Situ Stress Measurements via Wafer Curvature Detection Technology

    In a multi-beam optical stress sensor (MOSS) system mounted on MBE, the laser diode produces a laser beam as incident beam to the wafer in MBE chamber. An etalon, with a highly reflective dielectric coatings on each side, is placed at an angle to a laser beam. The incident angle of the laser beam leads to multiple internal reflection within the etalon, which generates a linear array of parallel beams. These beams then pass through a second etalon to produce a 2-dimensional array of beams. The number and spacing of these beams can be controlled by the rotation of each etalon. The array of parallel laser beams is then reflected from the sample surface and directly imaged with a CCD camera. The relative changes in spacing of all spots can be simultaneously measured by the CCD camera. These changes of spot spacing can be directly related to stress changes in the film vis Stoney equation. Thus, stress-thickness product can be recorded as a function of time. The figure shows a MOSS measurement of InAs/GaAs growth, with simultaneous RHEED patterns. The Stranski-Krastanov transition occurs at 20s: steak-spotty transition of RHEED patterns, while significant stress relaxation commences at 24s: slope decrease in the plot. This reveals the S-K growth mode transition occurs prior to stress relaxation.