Measurements of non-Gaussian noise in quantum wells A. Ben Simon, 1,2 Y. Paltiel, 1 G. Jung, 2 V. Berger, 3 and H. Schneider 4 1 Solid State Physics Group, Electro-Optics Division, Soreq NRC, Yavne 81800, Israel 2 Department of Physics, Ben Gurion University of the Negev, Beer Sheva 84105, Israel 3 Matériaux et Phénomènes Quantiques, Université Paris 7, Case 7021, 2 Place Jussieu, 75251 Paris, France 4 Institute of Ion-Beam Physics and Materials Research, Forschungszentrum Dresden Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany Received 30 May 2007; revised manuscript received 5 August 2007; published 11 December 2007 Gaussian generation-recombination is accepted to be a dominant mechanism of current noise source in quantum well systems biased by electric field normal to the layers. We have found pronouncedly non-Gaussian excess current noise in n-type and p-type multiple quantum wells. The non-Gaussian noise has been attributed to metastable spatial configurations of electric field. The metastability likely originates from negative differ- ential conductance caused by intervalley scattering in n-type wells and heavy and light holes tunneling in p-type wells. At a constant bias, the quantum well system randomly switches between a high resistivity state with low current flow and low resistive state with high current flow. The non-Gaussianity of the noise is more pronounced in p-type wells where the time traces of current fluctuations resemble closely a two-level random telegraph signal, which has not been straightforwardly observed in n-type wells. The non-Gaussian character of the noise in n-type systems has been revealed by measurements of nonzero skewness of the amplitude distri- butions. The difference between noise properties of n- and p-type systems has been attributed to small capture probability of electrons in n-type wells, as opposed to very high capture probability of holes in p-type wells. As a consequence, the noise of any p-type multiwell system is dominated by fluctuations of a single well, while in the n-type the noise appears as a superposition of many fluctuators associated with individual wells. DOI: 10.1103/PhysRevB.76.235308 PACS numbers: 73.21.Fg, 72.20.Jv, 72.80.Ey I. INTRODUCTION Noise is frequently regarded only as an annoying nuisance disturbing the experiment. In reality, noise measurements can provide a unique insight into the dynamics of the investi- gated physical system. 1 Noise in quantum wells QWs and, in particular, in quantum well infrared photodetectors QWIPs was extensively studied in the last years. 2–5 Al- though the noise measurements were primarily aimed at a practical goal of optimization of the signal-to-noise ratio in QWIP devices, they have also significantly contributed to our understanding of transport processes in QW systems. It is now generally accepted that generation-recombination GR noise constitutes the dominant source of current fluctuations in QWs. 6,7 In practice, QWs are always biased with a voltage source. At very low voltages, the current noise in QWs arises from trapping and detrapping of charge carriers from bound states. 8 At moderate and low bias voltages, the bias depen- dent GR noise prevails. 9–11 At very high bias voltages, 1 / f noise, associated with the action of electronic defects in QW systems, appears and becomes most pronounced at low fre- quencies. A simple relation based on considerations of statistical fluctuations of the number of charge carriers due to their emission and capture by the wells connects the dark current and the power spectral density PSD of the GR noise, 4 S i V =4qI ¯ g 1- P c 2  , 1 where g is the gain, defined as a probability that a charge carrier reaches the collector, q is the electron charge, I ¯ is the dc current in the system, and P c is the probability of captur- ing a carrier from the continuum to a QW. In deriving Eq. 1, each well was treated as a discrete independent source of GR noise. Relation 1 was the subject of controversial dis- cussions over the years. 2–5 The power spectral density of GR noise has a Lorentzian form. PSD is frequency independent at low frequencies, up to a cutoff frequency located in the GHz range. Above the cutoff, the PSD decays as 1 / f 2 . 12 Recently, fast and slow noise components of the current fluctuations, manifesting themselves as two plateaus in the PSD of the current noise, were found in quantum wells. 13,14 The plateau at higher frequencies originates from a conven- tional GR mechanism, while the low frequency plateau is considered to be a signature of an excess noise mechanism. 14 Alternatively, the time constant related to the recharging pro- cess of depleted QWIP wells has been claimed to be respon- sible for additional low frequency cutoff in the GR noise spectra. 13 The time constant of the recharging process is con- trolled by the QWIP resistance R and capacitance C. In a typical QWIP, the time constant is of the order of 10 -4 s. Since the low frequency plateau in the noise PSD appears at frequencies coinciding with the typical operating frequency range of practical QWIPs, the excess noise may significantly deteriorate the performance of QWIP devices. Therefore, un- derstanding of origins and mechanisms of excess noise be- comes an important issue also from a practical point of view. Important information about the physical nature of the excess noise was obtained in our first experiments when we have determined that in p-type quantum wells the excess noise has a non-Gaussian character. 14 Consistent with the central limit theorem, the classical generation-recombination noise originating from an action of many elementary fluctua- tors should be characterized by a Gaussian amplitude PHYSICAL REVIEW B 76, 235308 2007 1098-0121/2007/7623/2353089 ©2007 The American Physical Society 235308-1