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

 
 
 
xii 
Figure 3.4: (a) Ge profile on SiGe sandwich structure grown by MBE. (b) Ge profile of 
SiGe heterostructure grown by CVD [78-79]............................................................... 39
 
Figure 3.5: Applied Materials Centura Epi RPCVD reactor used in this work ........... 40
 
Figure 3.6 Fig 3.6: Applied Materials Centura Epi RPCVD schematic ....................... 41
 
Figure 3.7: Schematic diagram of the Centura gas flow control system ...................... 42
 
Figure 3.8: SIMS measurement of SiGe layers grown on Si using RPCVD ............... 43
 
Figure 3.9: (a) Temperature dependence of Si and Ge growth. (b) Temperature 
dependence of Ge composition in SiGe layers ............................................................. 44
 
Figure 3.10: (a) SiGe concentration ratio vs Si/Ge flux ratio. (b) Growth rate vs.Ge 
composition. ................................................................................................................. 45
 
Figure 3.11: (a) Most likely heterogeneous reactions in Si
1−x
Ge
x
 CVD from GeH
4
 and 
SiH
4
. (b) H desorption from surface sites and H diffusion between Ge and Si sites. .. 47
 
Figure 3.12: Doping profile: SIMS measurement of a SiGe p-i-n test diode. Red line is 
the Ge composition, green line is the Si composition. Blue line is the As doping level 
and yellow line is the B doping level. .......................................................................... 49
 
Figure 3.13: Buffer growth methods: (a) graded buffer; (b) direct buffer with two cycles 
of low T growth and high T anneal; (c) direct buffer with two growth-temperatures. 50
 
Figure 3.14: [12] AFM image of as-grown surface. (a) MBE-grown Ge-on-Si with 
2-growth-temperatures. (b) RPCVD-grown SiGe-on-Si at a single growth temperature. 
Both samples were annealed......................................................................................... 53
 
Figure 3.15: Strained Ge/Si
1-x
Ge
x
 quantum well structure on relaxed Si
1-z
Ge
z
 buffer and 
its strain balance. .......................................................................................................... 54
 
Figure 3.16: Cross-sectional TEM image of 10-pair MQW grown on SiGe on Si. ..... 55
 
Figure 3.17: 2-D XRD reciprocal-space map of quantum well sample ....................... 56
 
Figure 3.18: (a) Schematic of selective growth. (b) SEM of actual growth ................. 57
 
Figure 3.19: Temperature dependence of the selective growth .................................... 58
 
Figure 4.1: DC p-i-n diode device process flow. .......................................................... 61
 
Figure 4.2: Schematic of the side view of PIN diode for DC measurements ............... 62
 
Figure 4.3: Optical absorption current measurement (DC) device size: 100×100μm .. 63
 
Figure 4.4: Optical absorption current measurement after the material and processing 
improvements ............................................................................................................... 64
 
Figure 4.5: High-speed modulator device process flow (a) epi wafer; (b) two mesa etch; 
(c) oxide passivation deposition; (d) double contact for p and n regions; (e) metal 
deposition for contacts .................................................................................................. 65
 
Figure 4.6: (a) Schematic side view of the modulator; (b) schematic top view of the 
modulator; (c) SEM top view of the modulator ........................................................... 67
 
Figure 5.1: High speed measurement setup .................................................................. 69
 
Figure 5.2: Optical eye diagram of the 40

m device at 3.125GHz (Measured in HP Labs, 
Palo Alto, CA) .............................................................................................................. 70
 
Figure 5.4: Two-port system model: variable definitions ............................................ 71
 
Figure 5.5: High frequency two-port system model ..................................................... 72
 
Figure 5.6: Small signal measurement for 6μm×6μm device (Experimental data taken 
by Lars Thylen) ............................................................................................................ 73
 
Figure 5.7: Equivalent circuit with comparison to modulator layout ........................... 76
 
Figure 5.9: Pump probe measurement of reflectivity change ....................................... 77
 

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Figure 5.10: Pump probe measurement setup .............................................................. 79
 
Figure 5.11: Relative transmission variation of the modulator, immediately after 
perturbation from the pump pulse, vs the bias voltage for various pump wavelengths.
 ...................................................................................................................................... 81
 
Figure 5.12: (a) Differential transmission spectroscopy at 1.43μm (b) Fitting for lifetime 
at 6V and 6.5V .............................................................................................................. 82
 
Figure 6.1: Ge quantum well waveguide modulator with SOI bus waveguide ............ 86
 

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