University Oral Examination - Kai Ma (July 9, 1:00pm)

Kai Ma kaima at stanford.edu
Fri Jul 2 11:17:32 PDT 2004


University Ph.D. Oral Examination

"Fast" Photoconductive Materials Used in a Wideband A/D Conversion System
and Monolithic Integration of GaAs Devices with a Finished CMOS Chip

Kai Ma
Department of Materials Science and Engineering
Stanford University
Friday, July 9, 2004, 1:00pm, CIS-X Auditorium
(Refreshments served at 12:45pm)

ABSTRACT

Modern communications and high-speed instrumentations require high speed
analog-to-digital converters (ADCs) with bandwidths up to several tens of
GHz. The performance of state-of-the-art electronic ADCs is limited by low
input bandwidth and aperture uncertainty. Photonic devices have wide
bandwidth and superior timing accuracy. To combine the advantages of both
technologies, we investigated a photoconductive-sampling-based photonic
A/D conversion system utilizing low-temperature-grown GaAs (LT-GaAs)
metal-semiconductor-metal (MSM) photoconductive switches flip-chip bonded
with silicon-CMOS ADCs and demonstrated a two-channel prototype system
with ~3.5 effective-number-of-bits of resolution over an input bandwidth
up to 40 GHz and an estimated sampling jitter less than 80 fs.

This talk focuses on three materials used for the ultrahigh-speed
photoconductive switches. LT-GaAs is first chosen due to its
subpico-second carrier lifetime that allows for ultra-fast switching and
reasonably high mobility to provide good responsivity. Using an
electro-optic sampling technique, high-speed characterization of the MSM
switches exhibits ~2 ps full-width at half-maximum switch window. By
optimizing the growth conditions of LT-GaAs according to design of
experiments principle, we significantly improved the switch responsivity
and the optical pulse energy needed to trigger the switches was reduced to
~50pJ per pulse for the prototype system.

One of the key innovations of this work is the monolithic integration of
GaAs switches with Si CMOS circuits to minimize the input parasitics. A
common approach for monolithic integration is to finish the metallization
of the Si-ICs after growth of the GaAs devices, which creates significant
fabrication perturbation. We investigated two alternative options,
attempting to monolithically integrate the GaAs-base devices with
completely finished Si-ICs. One is to grow LT-GaAs on Si substrates and
the other is to directly grow GaAs on dielectric-coated Si substrates
which simulate the actual surface of a finish chip with passivation. All
the growths are done at temperatures safe for a finished chip. Switches
made from both materials show comparable or even better responsivity and
speed than their counterpart on a GaAs substrate.

As a final proof-of-principle demonstration, we directly grew GaAs on top
of a completely finished CMOS amplifier chip previously designed for
flip-chip bonding purpose and achieved a properly functioning optical
receiver without modifying the performance of the Si circuit. In addition
to the multiple advantages brought by monolithic integration, the beauty
of our approach is its simplicity, minimum fabrication perturbation and
greater applicability to CMOS optoelectronic interconnects and other
areas. To our knowledge, this is the first time that GaAs devices are
monolithically integrated to a completely fabricated Si chip.

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