1. A.Carbone, R.Introzzi and H.C. Liu Physics Department and INFM, Politecnico di Torino C.so Duca degli Abruzzi, 24 – 10129 – Torino, (Italy) Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, K1A 0R6 (Canada) Photocurrent Noise in QWIPs: Theory and Experiment

2. V b L b L b L w V e V c I e E E F qw 1 E F e 0 V w Emitter Collector

3. Redistribution of potential in the presence of radiation  M.Ershov, H.C.Liu, M.Buchanan, Z.R.Wasileski, V. Ryzhii, Appl.Phys.Lett., 70, 414 (1997).  V. Ryzhii, J.Appl. Phys. 86, 6586, (1997)  V.Letov, M.Ershov, S.G.Matsik, A.G.U.Perera, H.C.Liu, Z.R.Wasilewski and M.Buchanan, Appl.Phys. Lett. 79, 2094, (2001)

4. Redistribution of potential at different applied bias

5. Noise gain S i /4eI

7. Samples AlGaAs/GaAs QWIPs with the same design except the number of wells N Well number: N = 4, 8,16, 32 Barrier width: L b =241Å Well width: L w =62Å Area=240x240  m 2

8. Dark-current noise

9. Photo-current noise A.Carbone, R.Introzzi and H.C.Liu, Appl.Phys.Lett., 82, (2003)

10. Dark-current noise

11. Photo-current noise

12. Power spectra over 4eI/N f=20Hz

13. Contents  Noise measurements (in the dark and in the presence of IR radiation) in QWIPs with different number of wells.  Limits of the noise models, based on the continuity equation valid for standard semiconductors (models proposed so far).  Necessity of a noise model based on the QWIP continuity equation (proposed in this work).

14. To calculate the spectral intensity of the fluctuations in homogeneous semiconductors, a white noise excitation source  (x,t) is applied to the right side of the first-order continuity equation : Under the assumption that the transit time is much longer than the carrier recombination time the well-known expression:

15. If the carriers are photogenerated, the above relationship still holds provided that the thermal generation rate is replaced by the photo- generation rate. The responsivity is obtained by the same differential equation:

16. Since each photogenerated carrier induces a current pulse, the current noise power spectral density and the responsivity are obtained:

17. If, taking into account the expression of the photoconductivity gain g p for QWIPs Since for QWIPs having identical growth sequence but different N, the quantum efficiency:

18. The transport and the generation-recombination of carriers in QWIPs are described by the equation: In order to calculate the spectral intensity of the fluctuations, a white noise excitation source  k (x,t) is applied to the right side of the previous relationship

21. J-E zoom

23. Conclusion The photocurrent noise, measured in QWIPs having the same growth sequence but different number of wells, shows features related to the discrete structure of the device. A noise model based on the continuity equation developed for QWIPs instead of that valid for homogeneous semiconductors, seems to be able to account for the experimental results.