Experimental observation of spontaneous chaotic current oscillations in GaAs/Al0.45Ga0.55As superlattices at room temperature

Abstract

Spontaneous self-sustained chaotic current oscillations are observed experimentally in lightly-doped weakly-coupled GaAs/ Al 0.45 Ga 0.55 As superlattices at room temperature for the first time. The mole fraction of Aluminum in the barrier is chosen to be 0.45 to suppress the thermal carrier leakage through the X-band valley. The effective nonlinearity induced by the sequential well-to-well resonant tunneling can still be strong enough to induce spontaneous chaotic current oscillations even at room temperature. The frequency spectrum of the chaotic current oscillations is ranged from DC to 4 GHz, which can be used as ultra-wide-band noise sources with a bandwidth of several Giga Hertz. The study of semiconductor superlattices (SLs), one of the typical low dimensional structures, has formed a major branch of solid-state physics [1]. In 1970, Esaki and Tsu proposed the concept of superlattices that the bulky Brillouin zone can be folded by several tens of time through superlattice effects [2]. Electrons in the center of the shrunk Brillouin zone could be easily drifted into the boundary that negative differential conductance (NDC) could be resulted from, which was expected to used as microwave oscillation resources with a frequency much larger than that of Gunn diodes. However, there are almost no practical applications yet even though the periodic self-sustained current oscillations were be realized in SLs [3,4], since the oscillation frequency cannot achieved to be large as expected and the overall performance was not comparable with GaAs-or InP-based HEMT devices. Another kind of NDC effect was observed experimentally in weakly-coupled SLs induced by well-to-well sequential resonant tunneling [5]. A lightly-doped weak-coupled SL represent as an ideal one-dimensional nonlinear dynamical system with many degrees of freedom, and the effective nonlinearity is originated from NDC. Indeed, there were a diversify of spatio-temporal patterns observed in DC biased SLs, such as static high-field domains, self-sustained periodic current oscillations, quasi-periodic current oscillations and spontaneous chaotic current oscillations, etc. These spatio-temporal patterns can be observed only below liquid-nitrogen temperature range except that the periodic current oscillations with a frequency range of several GHz were realized at room temperature [6,7].The occurrence of chaos in a dynamical system relies on much more nonlinearity than that of periodicity. The increase of temperature would reduce the nonlinearity in weakly-coupled SLs. The nonlinearity in SLs is characterized by NDC resulted from the well-to-well sequential resonant tunneling. There are two mechanisms that are responsible for the degradation of the nonlinearity induced by the increasing temperature. As the temperature is increased, the resonant tunneling process would be disturbed by the en-

Noise-enhanced spontaneous chaos in semiconductor superlattices at room temperature

“…. , N (N = 50 is the number of SL periods [15]), and J i→i+1 , ξ i (t), J(t) are the tunneling current density from well i to well i + 1, the corresponding fluctuating current density, and the total current density, respectively. The tunneling current density in Eq.…”

“…(2) from well i to i + 1 depends on the electric field F i in well i and the two-dimensional electron (2D) densities in the corresponding wells, n i and n i+1 . The forward velocity v (f ) (F i ) is a function given in [22,26] with peaks corresponding to three energy levels at 53, 207, and 440 meV calculated by solving a Kronig-Penney model for the SL configuration of References [5,15,27]. The level broadenings due to scattering are 2.5, 8 and 24 meV for the three energy levels [26].…”

“…The mechanisms that produce spontaneous chaos in SLs need to be understood to improve design of superlattice-based fast RNGs. Spontaneous chaos at room temperature appears in SLs whose configuration and aluminum content in the barriers are tuned to suppress leakage current through the X valley of the barrier [5,15]. Below liquid nitrogen temperatures, spontaneous chaos was observed in SLs with AlAs barriers in 1996 [16], but leakage currents through their X valley suppressed all oscillations (chaotic or not) for higher temperatures [17,18].…”

Fast Detection of a Weak Signal by a Stochastic Resonance Induced by a Coherence Resonance in an Excitable GaAs/Al0.45Ga0.55As Superlattice

“…A few years ago, spontaneous chaotic [17,18] as well as periodic and quasiperiodic current self-oscillations [19] were observed in GaAs=Al x Ga 1−x As SLs at room temperature, when the Al mole fraction x in the barrier layers was chosen to be 0.45 to suppress the thermal leakage current through the X valley [20]. In this way, the effective nonlinearity is sufficiently strong so that a dc-biased GaAs=Al 0.45 Ga 0.55 As SL can become an excitable nonlinear dynamical system even at room temperature.…”

Enhancing chaotic behavior at room temperature in GaAs/(Al,Ga)As superlattices

“…Table I gives the numerical values of the parameters used in the simulations. The GaAs/Al 0.45 Ga 0.55 As configuration corresponds with the configuration used in experiments [10,18,23]. The rest of the parameters are as follows: l b and l w , with l = l b + l w , are the barrier and well lengths, respectively, and A = s 2 is the transversal area of the superlattice.…”