Metal Contacts for the Hybrid Silicon Laser Morgan Swaidan, Ventura College Major: Physics Mentor: Siddharth Jain Faculty Advisor: Dr. John Bowers Funding Agency : Intel Corp Department of Electrical and Computer Engineering
Outline • Optical communication • Hybrid Silicon Laser – Need for metal-semiconductor contacts • Measurement technique – Two and four point probe method • Anatomy of a metal contact • Measurement results 2
Optical Communication Fiber optic cables UK - For long distance communication - Niche technology USA - Expensive components What we want Trans-Atlantic cable - Adapt technology to everyday use Eg. Faster internet Why is it important - Replace existing copper cable - Fast & efficient data communication Make cheaper/better light sources Fiber optic cable array 3
Outline • Optical communication • Hybrid Silicon Laser – Need for metal-semiconductor contacts • Measurement technique – Two and four point probe method • Anatomy of a metal contact • Measurement results 4 4
Hybrid Silicon Laser Problem: “Usual” gold contacts incompatible with Si Aim: Convert electrical energy light energy What we’ll do: Study metal/semiconductor interface -Try other metals Metal -Which has lowest resistance? Indium Phosphide (light-producing) Current Light Silicon Metal Semiconductor 5
Outline • Optical communication • Hybrid Silicon Laser – Need for metal-semiconductor contacts • Measurement technique – Two and four point probe method • Anatomy of a metal contact • Measurement results 6
What Do We Want to Measure? Metal R c Semiconductor R s We want R c : Contact Resistance – R s : Inherent resistance of semiconductor What we actually record: Specific contact resistance (ρ) – takes into account area through which current flows. 7
Measuring Resistance • We want to measure R c Ω i x Rc Rc Rs • Measured Resistance = 2R c + R s 8
Finding Contact Resistance (R c ) • R s depends on spacing (x) • Vary spacing to construct graph x x x metal semiconductor R total R t = R s (x) +2R c y-int = 2R c 0 Spacing 9
Two Point Probe Method Ω i R n =needle resistance R c =contact resistance R s =semiconductor Ω resistance R n R n R c R s R c
Four Point Probe Method V A i R n =needle resistance R c =contact resistance R s =semiconductor A resistance i R n R n R c R s R c i = 0 R n R n V 11
Outline • Optical communication • Hybrid Silicon Laser – Need for metal-semiconductor contacts • Measurement technique – Two and four point probe method • Anatomy of a metal contact • Measurement results 12
Anatomy of a metal contact Thick metal layer Barrier layer Dopant layer Adhesion layer Semiconductor 13
Anatomy of a metal contact On Indium Phosphide (InP): Aluminum (Al) Tungsten (W) Germanium (Ge) Palladium (Pd) InP 14 14
Anatomy of a metal contact On Indium Gallium Arsenide (InGaAs): Aluminum (Al) Tungsten (W) X Titanium (Ti) InGaAs 15 15 15
Outline • Optical communication • Hybrid Silicon Laser – Need for metal-semiconductor contacts • Measurement technique – Two and four point probe method • Anatomy of a metal contact • Measurement results 16 16
Non-annealed samples Ti/W/Al on InP Pd/Ge/Pd/W/Al on InP 30 20 Resistance ( Ω ) Resistance ( Ω ) 20 18 10 16 0 14 0 10 20 30 0 5 10 15 20 25 Spacing ( μ m) Spacing ( μ m) Some samples not even linear [Ohmic: V = iR] non-linear = not Ohmic 17
Non-annealed samples Ti/W/Al Pd/Ge/Pd/W/Al 0.2 0.8 Current (Amp) Current (Amp) 0.1 0.4 0 0 -4 -2 0 2 4 -5 0 5 -0.1 -0.4 -0.2 -0.8 Voltage (V) Voltage (V) Ohmic : Linear relationship between voltage and current (Contacts must be Ohmic) 18
Annealed at 350°C, 30 sec Ti/W/Al Pd/Ge/Pd/W/Al 0.4 1.2 Current (Amp) Current (Amp) 0.8 0.2 0.4 0 0 -4 -2 0 2 4 -3 -2 -1 0 1 2 3 -0.4 -0.2 -0.8 -0.4 -1.2 Voltage (V) Voltage (V) After annealing, both samples are Ohmic 19
Which contact has lowest resistance? 14 Ti/W/Al 12 Resistance ( Ω ) ρ = 6.62 x 10 -6 Ω -cm 2 10 8 6 Pd/Ge/Pd/W/Al ρ= 9.50 x 10 -7 Ω -cm 2 4 2 0 -5 0 5 10 15 20 25 -2 Spacing ( μ m) 20
Looking ahead… • Two possible semiconductors: InP and InGaAs • Find contact that will work equally well on both – Simplifies manufacturing InP lots of Germanium InGaAs less Germanium Resistance InGaAs “Happy medium” InP Amount of Germanium 21
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