N-i-p-i and hetero n-i-p-i doping superlattices physics and device applications

Abstract

Artificial semiconductors compared of a periodic sequence of thin n- and p-doped layers, possibly with undoped (i-) layers in between, exhibit a number of unique features by which they differ from uniform bulk crystals as well as from semiconductors with a compositional superlattice. The properties which make them particularly appealing as a new kind of semiconductor are the tunability of carrier density, bandgap, two-dimensional subband structure, and recombination lifetime in combination with an enormous flexibility in tailoring. This tailoring includes the possibility of an additional periodic modulation of composition ('hetero n-i-p-i'). This can be used to reduce the disorder induced potential fluctuations by separating the free carriers from their parent impurity atoms, similar to the well-known 'modulation doping.' The possibility of varying conductivity, absorption coefficient, optical gain, and luminescence spectra by light or external electrical potentials implies new concepts for photodetectors, tunable light sources, and modulators. The long recombination lifetimes result in large, low-power nonlinearities of the optical absorption coefficient and the refractive index, including optical bistability. After a short review of the basic theory and fundamental properties of n-i-p-i structures we will demonstrate a number of properties which have turned out to be of particular interest for applications such as spectrally tunable electroluminescence, huge photoconductivity, strong electro-absorption and -refraction, and large optical non- linearities on homo- and hetero n-i-p-i systems. We also present results on a newly discovered peculiarity. N-i-p-i structures exhibit a giant ambipolar diffusion coefficient, which can exceed typical bulk values by more than two orders of magnitude and which allows for a control of the 'adjustable recombination lifetimes' down into the sub-ns regime. Finally we will present a new n-i-p-i-based concept of 'smart pixels' as fast optical logic gates at extremely low optical and electrical power dissipation.