Shot noise in resonant tunneling through a zero-dimensional state with a complex energy spectrum

Abstract

We investigate the noise properties of a GaAs /Al x Ga 1Ϫx As resonant-tunneling structure at bias voltages where the current characteristic is determined by single electron tunneling. We discuss the suppression of the shot noise in the framework of a coupled two-state system. For large bias voltages we observe superPoissonian shot noise up to values of the Fano factor ␣Ϸ10. DOI: 10.1103/PhysRevB.69.113316 PACS number͑s͒: 73.63.Kv, 73.40.Gk, 72.70.ϩm Shot noise allows for a direct measurement of the correlation in a current of discrete charges. In the case of a totally uncorrelated current, one observes the so-called full or Poissonian shot noise. 1 The corresponding noise power S displays a linear dependence Sϭ2eI on the stationary current I as long as the applied bias voltage V SD is large compared to the thermal energy: eV SD ӷ2k B T. 2 Full current shot noise arises, e.g., in a single tunneling barrier, since the tunneling process of different electrons are independent of each other.However, if an additional source of negative correlation is introduced the noise amplitude was shown to be reduced. 3,4 For resonant tunneling through a double-barrier structure this is attributed to the dependency of successive tunneling events caused by the finite dwell time of the resonant state. 5,6 This suppression of the shot noise has also been observed for resonant tunneling through zero-dimensional systems. 7,8 For the opposite case of a positive correlation between individual tunneling events the noise power can even become super-Poissonian. 9,10 We report on noise measurements of a resonant-tunneling double-barrier structure. Under certain bias conditions the tunneling current through our sample flows through a single zero-dimensional state. 13,14 Therefore we are dealing with 3d-0d-3d tunneling in contrast to the above-mentioned experimental studies where the tunneling takes place through a two-dimensional subband (3d-2d-3d). 5,6 Our sample consists of a nearly symmetric double-barrier resonant-tunneling structure grown by molecular-beam epitaxy on a n ϩ -type GaAs substrate ͓see Fig. 1͑a͔͒. The heterostructure is formed by a 10-nm-wide GaAs quantum well sandwiched between two Al 0.3 Ga 0.7 As-tunneling barriers of 5 and 6 nm thickness. The contacts consist of 300-nm-thick GaAs layers doped with Si (4ϫ10 17 cm Ϫ3 ) and they are separated from the active region by a 7 nm thick undoped GaAs spacer layer. The geometrical diameter of the diode is 1 m.The current-voltage (I-V) characteristic is plotted in Fig. 1͑c͒. The upper inset shows the characteristic shape of the resonance due to tunneling through the two-dimensional subband at V SD ϭϮ130 mV. However, we concentrate our measurements on small bias voltages ͉V SD ͉р10 mV, where we find a pronounced current step ͑marked with ''X''͒. This feature indicates the presence of at least one single impurity in the nominally undoped GaAs, as depicted schematically in Fig. 1͑b͒. Such impurity states with an energy lower than the quantum well in between the barriers most like...

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