The observation of a many-body, Fermi-energy edge singularity in the low-temperature photoluminescence spectra of InGaAs-InP quantum wells is reported. Strong enhancement of the photoluminescence intensity towards the electron Fermi energy {E%) is observed, due to multiple electron-hole scattering processes to states above E%. Recombination of electrons in states up to E% is allowed by hole localization. The many-body processes are analogous to the core-hole phenomena in the soft-x-ray emission spectra of metals.

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A detailed study of the optical properties of InGaAs-lnP single quantum wells (aws) grown by atmospheric-pressure metal-organic chemical vapour deposition is described. Photoluminescence (PL), photoluminescence excitation (PLE), photoconductivity (PC) and electroreflectance (ER) are employed to study both undoped and modulation-doped quantum wells. The role of extrinsic processes in determining the low-temperature PL spectra is demonstrated from the variation of peak position and linewidth with temperature. The best PL linewidth obtained for a 1 5 0 A well is 5.3 meV, fairly close to the limit imposed by alloy fluctuations in the InGaAs. The role of free carriers in the undoped aws in determining the energy threshold for optical absorption is demonstrated from a comparison of PLE and PC spectra. Lineshape fitting of the PL spectra is described, and it is deduced that at 160 K recombination processes in both doped and undoped aws proceed with wavevector conservation, although at lower temperatures highly anomalous lineshapes are found in modulation-doped samples. The observation of a threshold in PC spectra under forward bias is interpreted as a transition from the valence-band well to the top of the conduction well. The ratio of the conduction-to valence-band discontinuities is deduced to be approximately 0.4 :0.6.

The observation of many body shakeup in the photoluminescence spectra of InGaAs-InP quantum wells, in the quantum Hall regime, is reported. The occurrence of this many body effect is demonstrated from the observation of low energy satellites, corresponding to inter-Landau-level excitations of the degenerate electron gas. Comparison of the zero and finite magnetic field spectra shows that the low energy tail at 5=0 also arises from Fermi sea shakeup. The strength of the shakeup is controlled by the localization of the recombining hole, in agreement with recent theoretical predictions.PACS numbers: 73.20. Mf, 78.20.Ls, 78.55.Cr Many body processes can have a dramatic effect on the optical spectra of a degenerate electron gas (DEG) [1]. It has been known for more than twenty years that the spectra of x-ray transitions between the core states and the conduction band of a metal are strongly influenced by many body processes [2][3][4]. The most notable effects are the Fermi-energy edge singularity [2], and shakeup, the latter giving rise to the Anderson orthogonality catastrophe [3]. Recent advances in the growth of modulation-doped semiconductor heterostructures have made it feasible to investigate the similarities and differences between x-ray transitions of bulk three-dimensional metals and conduction-to-valence-band transitions of the quasitwo-dimensional (Q2D) DEG in semiconductors. In addition to the Fermi-energy edge singularity which was observed in 1987 [5], theory suggests that shakeup processes should occur in Q2D semiconductor systems [6-8]. We report an unequivocal experimental observation of shakeup in the photoluminescence (PL) of such a system. We also show that shakeup is strongly dependent on the localization of the valence-band hole, in agreement with theoretical predictions [7,8].Shakeup is a fundamental many body effect that occurs in optical transitions in the presence of a DEG, when the transition creates excitations of the DEG. These excitations are additional to the (electron or hole) quasiparticle that is added to the DEG in a one-electron picture of the transition. By energy conservation, the photon energy for shakeup processes in optical emission is lower than in the absence of shakeup, the difference corresponding to the energy of the excitations created. Pair excitations (composed of an electron and a hole quasiparticle on either side of the Fermi level) have a continuous range of energies, leading to a low-energy tail in the emission spectrum. In semiconductors there are many other sources of low-energy tails, with the result that the attribution of a low-energy tail to shakeup is usually based upon detailed analysis of the PL line shape. For example, low-energy tails in the recombination spectra of high density electron-hole plasmas in semiconductors have been attributed to many body plasmon emission processes, on the basis of a curve-fitting analysis of the PL line shape [9].

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