Many-body theory of pump-probe spectra for highly excited semiconductors

Kanemitsu

2002

phys. stat. sol. (c)

We have studied photoluminescence (PL) properties of GaN thin films at low temperatures for various excitation intensities. Under weak excitation intensities, the free exciton line as well as the bound exciton line appear in the PL spectrum. With an increase of the excitation intensity, the M-line and the P-line due to inelastic exciton-exciton interactions are clearly observed. Under strong excitation intensities, electron-hole plasmas (EHP) emission appears at lower energy below the M-line. The radiative recombination processes of highly excited GaN are discussed.Recently, optical properties of wide band-gap semiconductors have extensively been studied. In particular, GaN and III-nitride compounds are promising materials for blue and ultraviolet optoelectronic devices because of their large exciton binding energies (e.g., 28 meV for GaN [1]). From the viewpoint of the fundamental physics, highly photoexcited GaN provide us with an excellent stages for studying the excitonic manybody effects in semiconductors [2,3]. The M-line [4,5], P-line [6], and electron-holeplasma (EHP) emissions [2] have been reported. However, the detailed power dependence of the photoluminescence (PL) spectrum and the PL intensity are not clarified. In this work, we have studied PL spectra of highly excited GaN thin films at a wide range of the laser power (1 nJ/cm 2 to 1 mJ/cm 2 ) and discuss many-body effects of correlated electron and hole systems.All samples used here were fabricated by means of a metal organic chemical vapor deposition (MOCVD) method. 5 mm GaN epitaxial layers were grown on GaN-buffer layers (25 nm) on c-plane (0001) of sapphire substrates. Since the free exciton energy depends on the strain of GaN films in the GaN/sapphire system, the photorefectrance (PR) measurement was carried out to determine the free exciton energy in our samples. For the PR measurements, the reflectance signals were modulated under a 325 nm He-Cd laser beam chopped at 210 Hz and were detected by a Si photodiode with a lock-in amplifier. The PL spectra under weak excitation were measured under a cw He-Cd (325 nm) laser excitation. For the study of the excitation intensity dependence of the PL spectrum, 130 fs and 333 nm laser pulses from optically parametric amplifier (OPA) systems (1 kHz repetition rate) were used as an excitation source and PL signals were dispersed by a 50 cm single-grating monochromator and detected by a liquid-nitrogen cooled charge coupled device (CCD). The beam spot size was carefully determined by a knife-edge method, which allow us to precisely evaluate the energy density of the excitation light. The spatial shape of the excitation laser beam was a Gaussian shape. The incident excitation density was varied from $1 nJ/cm 2 to $1 mJ/cm 2 .

supporting

confidence: 69%

Excitons and Electron–Hole Plasmas in Highly Excited GaN

Nagai

Kanemitsu

2002

phys. stat. sol. (c)

“…27 Recently, the authors have shown that, even in very high densities, the e-h pair correlation is enhanced by a strong photoexcitation and the BCS-like gap extraordinarily grows with increasing pump-light intensity. 44 This result suggests a possibility of decisive experimental observation of e-h BCS state under strong photoexcitations. Figure 3 illustrates the e-h pair wave function k ͑Refs.…”

Section: B the E-h Pair Correlation And The Bcs-like Energy-gap Formmentioning

confidence: 92%

Many-body theory for luminescence spectra in high-density electron-hole systems

Aihara

2002

Phys. Rev. B

Self Cite

We present a unified theory for the luminescence spectra in highly excited semiconductors, which is applicable throughout the whole density regime including the electron-hole (e-h) BCS state and the excitonic Bose-Einstein condensate. The calculated spectra clearly show the crossover behavior between the e-h BCS state and the excitonic Bose-Einstein condensate. The analysis is based on the generalized random-phase approximation combined with the Bethe-Salpeter equation. This approach allows us to consider the strong and weak e-h pair correlations on the same basis. In particular, we find that the broad spectral component arising from the carrier recombination in the e-h BCS state splits into the P and P 2 lines with decreasing e-h density. This behavior can be predicted neither by the BCS-like mean-field theory nor by the interacting Boson model. The result agrees with a recent noteworthy experiment for the strongly excited ZnO, where the ultraviolet laser emission was observed at room temperature.

“…We analyze the time-independent Hamiltonian in the rotating-frame representation obtained by the Galitskii transformation 33,38,39 that is defined by the following unitary operator:…”

Section: Formulationmentioning

confidence: 99%

“…Later, the analysis has been extended to consider the relaxation processes with the nonequilibrium Green function formalism 34,35 and the interparticle Coulomb interaction. [35][36][37][38][39] However, these results are not conclusive because the BCS-like mean-field theory overestimates the BEG.…”

Section: Introductionmentioning

confidence: 99%

Infrared absorption in high-density electron-hole systems: The role of quantum fluctuations

Aihara

2002

Phys. Rev. B

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We study the infrared absorption of the macroscopic quantum state in highly excited semiconductors. The calculated spectrum clearly shows the BCS-like energy-gap ͑BEG͒ formation. We incorporate the large quantum fluctuation with the quasistatic Eliashberg equation for e-h systems that allows us to calculate the renormalized band energy, the BEG, and the wave function renormalization factor. We find that the collective phase fluctuation significantly modifies the spectra, and that the strong visible-light excitation distinctly stabilizes the e-h BCS state.