6
1880
Si
1.11
1150
GaAs
1.42
870
Intrinsic Semiconductors
Charge carriers concentration in a semiconductor without impurities:
N-type Semiconductor
Some impurity atoms (donors) with more valence electrons are introduced into the crystal:
The probability that a single photon incident on the detector generates a signal
( 1 R )[ 1 e x p ( d )]
Losses: • reflection •nature of absorption • a fraction of the electron hole pairs recombine in the junction
Detectors: Quantum Efficiency
Wavelength dependence of α:
Summary: P-N photodiode
Simple and cheap solid state device No internal gain, linear response Noise (“dark” current) is at the level of
Semiconductors
Compounds
Semiconductors
electrons and “holes”: negative and positive charge carries
Energy-momentum relation of free particles, with different effective mass
several hundred electrons, and consequently the smallest detectable light needs to consist of even more photons
Avalanche photodiode
High reverse-bias voltage enhances the field in the depletion layer
Semiconductors
Thermal excitations make the electrons “jump”
to higher energy levels, according to Fermi-
Dirac distribution:
f(E )
1
E k T e x p ( E /k T )
The P-I-N junction
Larger depletion layer allows improved efficiency Smaller junction capacitance means fast response
Detectors: Quantum Efficiency
•Energy conservation •Momentum conservation •photon momentum is negligible k2≈k1 •useful to remember: E(eV ) 1240
(nm)
(T=300K) Egap(eV) λgap(nm)
Ge
P-type Semiconductor
Some impurity atoms (acceptors) with less valence electrons are introduced into the crystal:
The P-N Junction
Electrons and holes diffuse to area of lower concentration
Electrons and holes excited by the photons are accelerated in the strong field generated by the reverse bias.
Collisions causing impactionization of more electronhole pairs, thus contributing to the gain of the junction.
Single Photon Detectors
By: Kobi Cohen Quantum Optics Seminar 25/11/09
Outline
A brief review of semiconductors
P-type, N-type Excitations
Photodiode Avalanche photodiode
The P-N photodiode
The i-V curve in the reverse-biased P-N junction is changed by the photocurrent
Reverse biasing: •Electric field in the junction increases quantum efficiency •Larger depletion layer •Better signal
Electrons and holes multiply by impact ionization faster than they can be collected, resulting in an exponential growth in the current
Individual photon counting
We’ll focus on reverse biasing: 1. larger electric field in the
junction 2. extended space charge
region
The P-N photodiode
Electrons and holes generated in the depletion area due to photon absorption are drifted outwards by the electric field
Summary: Geiger mode
High detection efficiency (80%). Dark counts rate (at room temperature) below
1000/sec. Cooling reduces it exponentially. After-pulsing caused by carrier trapping and
delayed release. Correction factor for intensity (due to dead time).
Silicon Photomultipliers
SiPM is an array of microcell avalanche photodiodes (~20um) operating in Geiger mode, made on a silicon substrate, with 500-5000 pixels/mm2. Total area 1x1mm2.
Photomultiplier
Light excites the electrons in the photocathode so that photoelectrons are emitted into the vacuum
Photoelectrons are accelerated due to between the dynodes, causing secondary emission
(~20 photons) Average photocurrent is proportional to the
incident photon flux (linear mode)
Geiger mode
In the Geiger mode, the APD is biased above its breakdown voltage for operation in very high gain.
Summary: Photomultiplier
First to be invented (1936) Single photon detection Sensitive to magnetic fields Expensive and complicated
Photomultiplier
Photoelectric effect causes photoelectron emission (external photoelectric effect)
For metals the work function W ~ 2eV, useful for detection in the visible and UV. For semiconductors can be ~ 1eV, useful for IR detection
Geiger Mode
Silicon Photomultipliers (SiPM) Photomultiplier Superconducting Wire Characterization of single photon sources
HBT Experiment Second order correlation function