Abstract

The optical nonlinear effects of bandfilling and bandgap renormalisation were studied by means of photoluminescence (PL) of highly excited CdTe/CdMnTe quantum wells. We find that excitons survive with little change in energy to carrier densities > 10¹² cm⁻². This is in contrast to earlier experiments carried out on GaAs/AlGaAs¹ and CdTe/CdMnTe² quantum wells which arc interpreted in terms of free electron-hole recombination. Figure 1 shows the PL from a typical sample using continuous wave excitation, and 10 ns pulses of 58 kW cm⁻² and 15 MW cm⁻² from a dye laser. Mesas of 100 μm diameter were etched into the material to ensure uniform excitation. When cw excitation is used (Fig 1a), the PL originates primarily from a donor bound exciton with a less intense peak from the free electron 1-hole 1 (e1-h1) exciton transition. When pulsed excitation is used, bandfilling results in broadened spectra as observed in Figures 1b and 1c. In Figure 1c the peaks at 748.2 nm and 767.7 nm result from the el-h3 and the e1-h1 exciton transitions respectively and the low energy broad peak at 777.3 nm originates from the CdTe buffer layer. Many-body theory predicts a Mott transition with a disappearance of the exciton resonances because of Coulomb and exchange screening at carrier densities above 5 × 10¹¹ cm⁻². Experimentally, the exciton transitions are retained and do not shift significantly in energy with carrier density. We believe that the reduced screening results mainly from biexciton formation. When the sample is highly excited.(fig 1c), the integrated intensity ratio of the forbidden e1-h3 and allowed e1-h1 exciton resonances is 1.4:1 This compares with relative strengths of 1:18 observed in photoluminescence excitation at low excitation densities and in agreement with calculations. Bandfilling and exciton-carrier scattering reduce the oscillator strength of the e1-h1 transition. Density dependent changes in the oscillator strength of the e1-h3 transition will be discussed and experimental evidence for biexciton formation presented.

© 1996 IEEE