RESONANT-CAVITY-ENHANCED

SCHOTTKY PHOTO DETECTOR

By : Rajan Singh Prashant Singh M.E (EXTC) SPIT SEM 1 (2015-2016)

Introduction• Resonant-cavity-enhanced photo

detectors (or, RCE photo detectors) enable improved performance over their predecessors by placing the active device structure inside a Fabry–Pérot resonant cavity

• The effect of the optical cavity, which allows wavelength selectivity and an enhancement of the optical field due to resonance, allows the photo detectors to be made thinner and therefore faster, and improving quantum efficiency.

Theory• The RCE photo detectors can provide: 1.Higher quantum efficiency.

2.Higher detection speed,

3.Wavelength selective detection • All the above points are advantages over

conventional photodetectors.

Fabry–Perot Resonant Cavity• Fabry–Pérot

interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly reflecting mirrors.

• Its transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon.

Explanations• Light amplification in

the photo detector when a photon collides within an atom causing avalanche multiplication

• The optical cavity formed is more of oscillator as it provides positive feedback by reflections.

Equations for wavelength selectivity

Graph for wavelength

Schottky Diode Walter H. Schottky

• Very fast switching action. • Silicon diode : 0.6–0.7 V Schottky diode : 0.15–0.45 V.• Molybdenum, Platinum, Chromium or Tungsten,

and certain  silicides  (e.g., palladium silicide and platinum silicide)

RCE Schottky Photodiodes• GaAs–AlAs–InGaAs RCE Schottky PD’s

designed for 900-nm wavelength.• Presence of the micro cavity causes

wavelength selectivity• Semitransparent Schottky contact can also

serve as the top reflector of the resonant cavity ultrafast high-efficiency

Construction

At each reflection, the amplitude is reduced by   , while at each transmission

through an interface the amplitude is reduced by  . Assuming no absorption, conservation of energy requires T + R = 1.

Formulation of Quantum Efficiency

The values of the refractive indices used for Si N , InP, AlGaAs, AlAsSb and the In Ga As are 2.0, 3.15, 3.54, 3.0 and 3.71, respectively

Dual-wavelength RCE Schottky photodetector.

Variation of top and bottom mirrors reflectivities R1 and R2 with wavelength λ .

The revised Quantum efficiency

• Multiplying the internal power increment factor by results in increase in quantum efficiencies by 0.95.

• The typical value of conventional photodetectors of 0.72 is improved to 0.945 to 0.976 by usage of active devices in resonant cavity.

Applications

• A systematic optimization procedure of RCE photodetector is used in high-speed operations of 1.30µm and 1.55µm and also the increased bandwidth of 40 GHz to 270 GHz.

• The useful applications include communications, military applications including RADAR and SONAR.

Materials Used• The estimated superior performance of the RCE

devices critically depends on the realization of very low loss active region.

• The Materials used are:• 1.GaAs(M,C) / AlGaAs(M) / InGaAs(A) near 830-

920nm.• 2.InP(C) / In0.53Ga0.47As(M) / In0.52Al0.48As(M) / In0.53–

0.7GaAs(A) near 1550nm.• 3.GaAs(M,C) / AlAs(M) / Ge(A) near 830-920nm.

ADVANTAGES• A RCE schottky detector improves the bandwidth

significantly. The constructive interference of a Fabry–Perot cavity enhances the optical field inside the photodetector at the resonance wavelengths to achieve a quantum efficiency of close to unity.

• The RCE photodetectors have both wavelength selectivity and high speed response making them ideal for wavelength division multiplexing applications. Optical modulators situated in an optical cavity require fewer quantum wells to absorb the same fraction of the incident light, and can therefore operate at lower voltages.

Disadvantage• The increased components over conventional

photodetectors increases not only the silicon area but also the cost of fabrications

• The plane mirrors used in Fabry- perot resonant cavity are not purely reflective and thus the optical signal might get lost.

• The optical modulators used are generally compatible to conventional photodetectors than to RCE ones

References