The analytical expression for the ambipolar in-plane diffusion constant of excess carriers in n-i-p-i doping superlattices is derived. Shockley-Haynes-type experiments quantitatively confirm the predicted huge enhancement of the diffusion constant (in the tested case by factors of about 300) and the long diffusion lengths.PACS numbers: 73.50.Pz, 73.20.Dx, 73.50.Gr n-i-p-i crystals 1,2 represent a unique system to study the dynamic behavior of strongly correlated electron and hole plasmas. Contrary to the bulk, 3 where Coulomb interaction is highly efficient in keeping the electron-hole plasma neutral and where short recombination 4 lifetimes also prohibit diffusion over large distances, the strong built-in space-charge fields in n-i-p-i systems fully separate electrons and holes from each other on the scale of the superlattice period. l ' 2 Therefore, the electrostatic repulsion between carriers of the same type is no longer compensated by the attractive interaction between carriers of opposite sign. This results in strong spacecharge-induced fields of opposite sign for both types of carriers, which enormously enhance their ambipolar diffusion. Moreover, the spatial separation suppresses electron-hole recombination to a large extent. Recent theoretical and experimental studies 5 " 7 have already indicated the existence of a new, extremely fast diffusive relaxation mechanism for locally generated charge carriers in such systems.In this paper we first present a general derivation of the analytical expression for the diffusion constant in ni-p-i systems. 5 The predicted huge value of the diffusion constant is then verified by a Shockley-Haynes-type experiment. 3 In addition, we provide the experimental proof for the dramatically increased value of the diffusion length in GaAs n-i-p-i systems. It is shown that even for diffusion over distances in the cm range recombination can be negligible.For the derivation of the diffusion constant, consider the steady-state situation of a large n-i-p-i sample which is continuously illuminated at a small spot. Recombination shall take place only via externally connected selective n-and /?-type contacts far away from the excitation point. The continuum equations for the electron and hole currents \ n and j p are j«(r)-M**V", and <*> p are the mobilities, densi-ties, and the respective quasi-Fermi-levels, and r -Gc, y,z). Note that by using the gradient of the quasi-Fermi-level as the "driving force" for the current, both diffusion and drift currents are automatically included.Since there is no external field in our case we have only a diffusion current. Its two components compensate each other such that the total current vanishes: j(r)-j"(r)+j p (r)s0.(This implies the following relation between the gradients of the electron and hole quasi-Fermi-levels:V p (r) = -Ur"(r)/a p (r)]VO"(r) .This relation is also valid in bulk material and reflects the fact that, in the ambipolar case, the diffusion of carriers with diffe...
Wavelength-shifted GaAs/AlGaAs Fabry-Pérot ridge waveguide lasers were fabricated by vacancy-enhanced quantum well disordering using dielectric cap annealing. 500 m long and 4 m wide Fabry-Pérot lasers with emission wavelengths selectively shifted by 20 nm were integrated with unshifted lasers on the same chip, characterized and further compared with lasers fabricated from as-grown material. These investigations showed that the absorption edge of a single-quantum well double heterostructure can be selectively blueshifted after epitaxial growth without compromising diode laser performance.
We report on polarization effects in surface emitting light emitting diodes based on p-i-n AlGaInP/GaInP double heterostructures grown by metal organic vapor phase epitaxy. Devices with an ordered GaInP active layer show polarized light output with a ratio of 4:3 for polarization along the [011] and [011̄] crystal direction, respectively. This polarization is nearly independent of diode current and mesa geometry, but is not observed if disordered GaInP active layers are used. The effect is considered to be an important means for polarization control in vertical cavity surface emitting lasers.
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