REMINDER: University Oral Examination - Vincenzo Lordi (June 8, 3:30pm)

Vincenzo Lordi vlordi at stanford.edu
Fri Jun 4 01:41:30 PDT 2004


University Oral Examination
Vincenzo Lordi
Department of Materials Science and Engineering
Stanford University

Tuesday, June 8, 2004
3:30pm (Refreshments will be served at 3:15pm)
CIS-X Auditorium

"Band Edge Optical Properties of GaInNAs(Sb) and the Relation to 
Atomic Structure"

Lasers, detectors, and electroabsorption modulators operating in the 
telecommunications wavelength range of 1300-1600 nm are important not 
only for optical fiber communications, but also for use in optical 
interconnects to replace the electrical lines limiting the future 
speed of microelectronics. The design of long wavelength optical 
interconnects allows lower voltage operation as well as seamless 
integration with optical networking. The novel dilute nitride III-V 
alloys, GaInNAs and GaInNAsSb, are promising material systems for 
realizing quantum-well (QW) optoelectronic devices on GaAs that 
operate in this wavelength range. We grow these materials using 
solid-source molecular beam epitaxy (MBE).

The luminescent quality of as-grown material is generally poor but is 
improved by ex situ rapid thermal annealing at 700-800 degC for 1-3 
min. However, the improvement in material quality is accompanied by 
an undesirable blueshift of the wavelength. Understanding the 
mechanism of this blueshift is critical for reproducible control of 
the operating wavelength of devices using these materials in the 
active region.

X-ray absorption, electroreflectance, and photoluminescence 
spectroscopies were used to study a series of transitions near the 
apparent band edge of the material that correspond to different N-In 
nearest neighbor configurations. These band-edge states are found to 
be a dominant contribution to the bandgap blueshift upon annealing. 
As-grown material contains a random distribution of bonds, which is 
dominated by N-Ga nearest neighbors, corresponding to a smaller 
bandgap. Annealing shifts the distribution of bonds toward increased 
N-In nearest neighbors and a configuration with a larger bandgap that 
is also more thermodynamically stable. The anneal-induced blueshift 
of the bandgap saturates after the material has reached equilibrium, 
as expected, although the luminescent quality of the material can 
continue to be increased.

In addition,  the electroabsorption properties of the GaInNAs(Sb) QWs 
were measured by photocurrent to determine their suitability for use 
in optical modulators. Spectra taken at room temperature demonstrate 
very nice quantum confined Stark effect (QCSE) behavior, with sharp 
exciton peaks having FWHM less than 25 meV. The peak absorption 
coefficient of fully annealed GaInNAsSb QWs was measured to be close 
to 35,000 /cm at ~1525 nm wavelength, a value higher than reported 
for competing materials, while annealed GaInNAs QWs showed peak 
absorption of ~18,000 /cm at 1250 nm. Analogous to the material's 
luminescent behavior, thermal annealing was found to increase the 
absorption coefficient of the QWs while blueshifting the bandgap, 
employing the mechanism described above. The measured 
electroabsorption characteristics indicate that optical modulators 
can be fabricated throughout the 1300-1600 nm wavelength range with 
performance comparable or superior to competing materials grown on 
InP substrates. We predict device performance for an asymmetric 
Fabry-Perot reflection modulator of up to 15-20 dB modulation ratio 
using less than 3 V swing.
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