High speed, low driving voltage vertical cavity germanium-silicon modulators for optical

Chapter 4 Device Fabrication
4.1 DC PIN Diode Fabrication 
Previous work demonstrated strong QCSE absorption spectra in Ge/SiGe quantum 
wells [41]. Because of the thin quantum wells and barriers, growth of these structures 
is moderately challenging, thus proving that our material exhibits significant QCSE is 
a first step before fabricating a high-speed optical modulator. 
Our DC SiGe modulator devices are SiGe p-i-n diodes on Si with Ge/SiGe 
quantum wells in the i-region. The Ge/SiGe quantum-well structures were grown by 
RPCVD. We used thin, direct deposition of SiGe buffers on Si, as described in 
Chapter 3, and 10 pairs of quantum wells on top of the buffer with a single growth 
temperature of 375°C used for all layers.
Fig. 4.1 shows the device fabrication processes. 4-inch, (001)-oriented, 
boron-doped Si wafers with resistivity 10-20 Ω-cm were used as starting substrates. 
Two boron-doped Ge-rich SiGe layers (p-type dopants with doping levels ~5x10
18
cm
-3
) were deposited on silicon sequentially and annealed. The first 250nm layer is 
annealed at 850 °C for 30-60 min, and then a second 250 nm SiGe layer is deposited at 
375 °C and annealed at 700 °C for 5 min. Undoped spacers, and Ge/SiGe quantum 
wells and barriers were then deposited and capped by arsenic-doped layers (n-type 
dopants with doping levels ~ 6x10
18 
cm
-3
).

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61 
Figure 4.1: DC p-i-n diode device process flow. 
Epi wafers were coated with 1μm thick photoresist (Shipley 3612) in the standard 
4-inch SVG coater track, using the standard recipe without edge bead removal. The 
resist was then patterned, using optical lithography in the Karl Suss MA-6 aligner and 
developed in the SVG developer. The square mesas with different sizes ranging from 
100×100 to 500×500μm were plasma dry etched to reach the bottom p-doped region 
with CF
4
etchant in the Drytek2 etcher.
Rectangular ring contact regions were patterned with 1.6μm thick photoresist 
(Shipley 3612), again using the SVG coater, Karl Suss aligner, and SVG developer. 
The size of the optical window inside ring contacts range from 80×80μm to 
450×450μm Metal layers, including 20 nm Ti and 400 nm Al, were deposited by 
electron beam evaporation. The metal was lifted-off in acetone/methanol/isopropanol 
solvents and then annealed at ~375º
C for ~30 seconds in the rapid thermal annealer 
(RTA) to form Ohmic n- and p- contacts.