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

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

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

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

5. Signal and Noise For photons……. p <<q np = expected value Poisson distribution n = 200 p = 0.05  k  = 10  k  = mean value of k, i.e. number of photons absorbed 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.

6. For large n, np   k  So is the average magnitude of the signal 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 ratio gets better as  t Definition: The noise intensity is the variance  of the Poisson distribution So the signal is on average 10  

7. Slab of semiconductor between two electrodes PHOTOELECTRIC DETECTORS Generation of carriers: intrinsic 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

8. Principle of operation steady, uniform photon flux on A = wL at t = 0 P = optical power  = carrier lifetime P/h  = Total number of photons impinging on the surface/unit time Generation rate  = quantum efficiency (number of carriers generated/photon) PHOTOELECTRIC DETECTORS total steady-state carrier density D >> 1/  light penetration depth (all radiation absorbed) n = Excess carrier density Recombination rate

9. Principle of operation Recombination processes n(t) = density of carriers at time t PHOTOELECTRIC DETECTORS n 0 = density of carriers generated by steady, uniform photon flux at t = 0 if the light is taken off, the concentration decay with time Apply field E between electrodes Intrinsic semiconductor photocurrent

10. Primary photocurrent Carrier transit time For high gain,  long, L should be short and  high The response time of a photoconductor is also determined by the lifetime. trade-off between gain and speed

11. PHOTODIODES Depleted semiconductor High E to separate photogenerated e - -h pairs Depletion region small to reduce t r Depletion region large to increase  Reverse bias to reduce t r trade-off between speed of response and quantum efficiency In reverse bias the maximum photocurrent is set by diode J S so maximum gain is 1

12. PHOTODIODES Reverse bias to reduce t r Special case of the p-n junction photodiodes Most-common photodetector Depletion region = intrinsic layer thickness can be tailored to optimize the quantum efficiency and frequency response. The p-i-n photodiode - Light absorption produces e-h pairs In the depletion region e-h will be separated by E --> current flow in the external circuit as carriers drift across the depletion layer Outside carriers has to diffuse to depletion region first

13. Absorption coefficient  It determines whether light can be absorbed for photoexcitation Indicates where light is absorbed high  : light is absorbed near the entry surface low  : light can penetrate deeper into the semiconductor Very low  : material can be transparent for long wavelengths (no photoexcitation)

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

15. Material to emit electrons by photoelectric effect Wavelength to transition energy  E relation in the device operation Key property: low work function to allow high extraction rate of e- Is the minimum wavelength limit for detection transition energy  E can be: energy gap of the semiconductor barrier height in a metal-semiconductor photodiode impurity level - band edge transition energy in an extrinsic photoconductor Photocathode

16. The photon absorption depend on the material Hence the photocathodes are sensible to some part only of the light spectrum Quantum efficiency number of carriers produced per photon photocurrent flux Photocathode

17. Radiation sensitivity Typical 80 mA/W I ph = current at photocathode P = incident light power Photocathode

18. 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 Photocathode Behavior vs work function

19. 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

20. Secondary Electron Eemission Physical principle: ionization of a solid (atom) by an electron with kinetic energy E 0 Each scattering event might generate one or more e - Secondary Electron Yield I 0 = incident beam current I S = secondary current (I emitted from surface) E 0 =  [1  10 6 eV]

21. Secondary Electron Eemission Contributions I e = elastically scattered e - I 0 = incident beam current I S = secondary current (I emitted from surface) E 0 =  [1  10 6 eV] I r = rediffused e - I ts = true secondary e -

22. Collect the current by applying a voltage V so that only e - with E K  E = eV arrives at detector The signal is the sum (integral) over the electrons up to the maximum E K Usually we are interested in the value of S for a range of energy and to get N(E) we must differentiate the signal Electron Energy (eV) N(E) E0E0 E 0 + ΔE

23. 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 = 5 10  10 7 G depends on the voltage because the voltage sets the primary energy of the incident e - generated in the dynode