References Hans Kuzmany : Solid State Spectroscopy (Springer) Chap 5 S.M. Sze

1. PHOTODETECTORS References Hans Kuzmany : Solid State Spectroscopy (Springer) Chap 5  S.M. Sze Physics of semiconductor devices (Wiley) Chap 13

2. Detection of Electromagnetic radiation • Signal and Noise • Photomultipliers • Photoelectric Detectors

3. Signal and Noise for photon counting Scattering experiment S= signal N = noise B = background D = dark level Low signal/noise ratio 23/4 = 6.5

4. Signal and Noise Origin of Noise at detector Signal = photon absorption Absorption Scattering Probability of absorption, i.e. contribution to signal= p Probability = q =1-p For coins p=q=1/2 For photons……. p <<q Low probability event

5. Signal and Noise Signal = photon absorption n incident photons Probability of 1 photon absorption = p k adsorbed photons Probability of no absorption q =1-p Or no contribution to signal Probability absorption of k photons Binomial distribution

6. Signal and Noise For photons……. p <<q np = expected value Poisson distribution n = 200 p = 0.05 k = 10 k = mean value of k

7. For large n, np  k If I have 200 photons every sec, for every second the absorbed photon number might be 7,9,10,15, depending on the probability distribution, but on average I have 10 photons absorbed per sec. So <k> is the average magnitude of the signal The noise intensity is defined as the variance  of the Poisson distribution So the signal is on average 10   By increasing the measuring time (or equivalently increasing the number of incident photons ) the magnitude increases linearly, and the noise increases as square root, so the signal to noise ration gets better as T

8. PHOTOMULTIPLIERS Elements: Photocathode Dynodes Anode e- Operation: • Photon in Photocathode • e- emission • e- on dynode • Secondary e- emission • Current on Anode Photocathode animation http://micro.magnet.fsu.edu/primer/java/digitalimaging/photomultiplier/sideonpmt/index.html

9. Photocathode Material to emit electrons by photoelectric effect Key property: low work function to allow extraction of e- The photon absorption depend on the material Hence the photocathodes are sensible to some part of the light spectrum Quantum efficiency

10. Radiation sensitivity Ic= current at photocathode P = incident light power Typical 80 mA/W

11. Dark current Due to thermal emission of electrons M = material dependent factor (0.5) T = temperature W = material work function (1.5-3 eV) J(T) increases rapidly with T, so photocathode needs to be cooled if you need to observe few e/s

12. Dynodes The dynodes work by employing secondary electron emission (SEE) SEE: When a primary beam hits a surface, then it generates electrons that are either emitted either travel into the solid and generate more electrons

13. Secondary Electron Eemission Physical principle: ionization of a solid (atom) by an electron with kinetic energy E0 E0 =[1 106 eV] Each scattering event might generate one or more e- I0 = incident beam current IS = secondary current (I emitted from surface) Secondary Electron Yield

14. Secondary Electron Eemission Contributions E0 =[1 106 eV] Ie = elastically scattered e- Ir = rediffused e- Its = true secondary e- I0 = incident beam current IS = secondary current (I emitted from surface)

15. N(E) E0 + ΔE E0 Electron Energy (eV) Collect the current by applying a voltage V so that only e- with EK  E = eV arrives at detector The signal is the sum (integral) over the electrons up to the maximum EK Usually we are interested in the value of S for a range of energy and to get N(E) we must differentiate the signal

16. For dynodes all the current originated from secondary emission is used The number of dynodes n provides the multiplication factor G (gain) of the photomultiplier Typical values  = 5, n = 10 G = 510 107 G depends on the voltage because the voltage sets the primary energy of the incident e- generated in the dynode

17. PHOTOELECTRIC DETECTORS Slab of semiconductor between two electrodes Generation of carriers: intrinsic • = mobility n,p = concentration q = charge For  < c incident radiation is adsorbed Generation of carriers: extrinsic The cutoff is determined by the energy of donor and acceptor states Performance detemined by: gain, response time, sensitivity

18. PHOTOELECTRIC DETECTORS Principle of operation n0 = density of carriers generated by a photon flux at t=0 Recombination processes n(t) = density of carriers at time t  = carrier lifetime 1/ = recombination rate Steady, uniform photon flux on A=wL P = optical power Total number of photons impinging on the surface/unit time is P/h At steady state, the carrier generation rate is equal to recombination rate Generation rate  = quantum efficiency

19. The current due to photon absorbption Defining primary photocurrent as Carrier Transit time The gain of the device depends on carrier lifetime and carrier velocity

20. PHOTODIODES Depleted semiconductor High E to separate photogenerated e--h pairs Depletion region small to reduce tr Depletion region large to increase  Reverse bias to reduce tr  depends on absorption coefficient