Optical studies of 2DEGs in ZnSe quantum wells in high magnetic fields. Page: 4 of 12
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Exciton binding energy is 30 meV and trion binding energy is 5.3 meV, i.e. the
condition EB > EF > E8 is hold.
2. Experimentals and optical spectra in the absence of magnetic field
A modulation-doped QW structure was grown by molecular-beam epitaxy on
a (100)-oriented GaAs substrate. The structure consists of a 67-A-thick ZnSe single
quantum well embedded between 1000-A-thick Zno.82Beo.osMgo.1oSe barriers. To
prevent the loss of carriers escaping into the substrate and recombining at the surface,
the structure was confined by Zno.71Beo.1iMgo.1sSe barriers. Both barrier materials are
lattice matched to the GaAs substrate. The modulation-doped layer with Iodine donors
is separated from the QW by a 100-A-thick spacer layer. Optical methods, first one
based on properties of trion absorption (reflection) [Ast02] and second one reported
here, allow us evaluation of the 2DEG density in the QW ne =5 x 10" cm-2. This
corresponds to Fermi energy of 2DEG EF = 7.7 meV with electron effective mass
mn =0.15m. For comparison trion binding energy in such a QW is E =5.3 meV
and exciton one is Ef =30 meV [Oss02, Ast02a]. Polarized magneto-luminescence
and magneto-reflectivity have been measured at T=1.6 K in long-pulse magnetic
fields up to B=47 T applied perpendicular to the QW plane (Faraday geometry).
Details of the setup are given in Ref. [Ast02a].
Fig. 1 shows PL and reflectivity spectra recorded at a zero magnetic field at
temperature T =1.6 K. The PL spectrum has been excited by a He-Cd laser with a
photon energy of 3.8 eV. It consists of a broad band with a maximum at 2.810 meV
and with a full width at a half maximum (FWHM) of 7 meV. This value is very close
to the value of the 2DEG Fermi energy 7.7 meV. In the reflectivity spectrum a strong
resonance is detected at 2.817 meV. We label it as a C-line and the origin of this
transition is a subject of discussion. The C-line is blue shifted in respect to PL
maximum at about 7 meV, the value is comparable with EF . Its position coincide
with a high energy wing of PL band, i.e. with the spectral position which is often
associated for the modulation-doped QWs with a "Fermi edge singularity (FES)". The
FES is commonly associated with a optical transition when the absorbed photon
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Ossau, Wolfgang J.; Astakhov, G. V.; Yakovlev, D. R.; Crooker, S. A. (Scott A.) & Waag, A. Optical studies of 2DEGs in ZnSe quantum wells in high magnetic fields., article, January 1, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc926265/m1/4/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.