Radiative behavior of negatively charged excitons in CdTe-based quantum wells: A spectral and temporal analysis V. Ciulin, P. Kossacki,* S. Haacke, ² J.-D. Ganie ` re, and B. Deveaud Physics Department, Swiss Federal Institute of Technology Lausanne, CH-1015 Lausanne-EPFL, Switzerland A. Esser Institut fu ¨r Physik, Humboldt-Universita ¨t zu Berlin, AG Halbleitertheorie, Germany M. Kutrowski and T. Wojtowicz Institute of Physics, Polish Academy of Sciences, Al. Lotniko ´w 32-46, PL-02668 Warsaw, Poland Received 7 August 2000; revised manuscript received 9 October 2000 Using reflectivity and picosecond time-resolved photoluminescence, we have studied the intrinsic optical properties of negatively charged excitons in modulation doped CdTe quantum wells. In emission, we observe simultaneously a low energy exponential tail in the charged exciton spectral line and a linear increase of its radiative lifetime with temperature. In absorption, we find a consistent decrease of the charged exciton oscil- lator strength with temperature. For low electron concentrations these observations are well reproduced by a model of delocalized and thermalized three-particle complexes. The model takes into account the recoil mo- mentum of the electron during the charged exciton optical transition. It is further found to compare well with lifetime measurements up to high carrier concentrations. Small deviations from the theoretical predictions occur only below 7 K due to localization effects and for the highest carrier concentration of n e 2 10 11 cm -2 . Since their first identification, 1 negatively charged exci- tons ( X - ) have been observed as an important spectral sig- nature of quantum wells QW’s in which excess electrons are introduced by modulation doping 2–4 or optical excitation. 5 They appear a few meV below the excitonic line X and their spectral weight increases with electron concen- tration while that of X decreases. The previously reported studies of their optical properties cover a wide range of in- vestigations. They include specific polarization properties when polarizing the initial electron gas by a magnetic field, 1 the increase of their dissociation energy with the Fermi en- ergy of the initial electron gas in the QW, 6,7 their lineshape 5,8 and their lifetime and dynamics. 9–12 However, intrinsic opti- cal properties of X - still remain to be established. Several of the properties found in the above mentioned studies, which are characteristic of X - , can also be attributed to donor bound excitons ( D 0 X ). 13,14 D 0 X has its spectral line on the low energy side of X and can be seen as an X - strongly localized by the donor core. There has actually been some controversy in the above cited literature about the dis- tinction between X - and D 0 X as well as to the possible localization of X - , particularly in modulation doped QWs. Indeed, in these structures, on top of interface roughness, the presence of remote ionized donors in the barrier induces electrostatic potential fluctuations. These can localize the electrons and X - , but do not affect much the X which are neutral complexes. 2,15,16 In this context, we report here on measured photolumines- cence PL and reflectivity properties of X - in modulation doped structures which are well described in terms of delo- calized and thermalized X - . For our experimental investiga- tions, we have chosen CdTe QWs as the X - binding energy in these materials is about two times larger than in GaAs based structures. This allows us to study X - in PL over a wide range of temperatures 2–35 K before the X - disso- ciation becomes important. To obtain a comprehensive pic- ture of X - PL at low electron concentrations, we have ana- lyzed simultaneously, on the same measurement, the decay times and the lineshapes of X - following its resonant exci- tation. We find that the low electron concentration lifetimes and lineshapes are well described by a model composed of delocalized and thermalized ideal three-particle X - complexes. 8 This model is extended so that it reproduces the change of absorption of X - as a function of temperature; a property that compares very well with the inverse of the lifetime. Over a wider range of electron concentration n e 0.2-2 10 11 cm -2 , the measured and predicted X - life- time are in good agreement. The main difference between them appears below 7 K due to localization. Localization effects vanish upon raising the temperature. The sample is a one-side modulation doped CdTe/ Cd 1 -x Mg x Te heterostructure containing a single 80 Å QW. The remote donor layer of iodine is situated 100 Å from the QW. Its thickness is changed within the same sample in four steps of 0, 2, 5, and 10 monolayers labeled hereafter spot A, B, C, and D. 17 This allows us to tune the concentration of the electron gas in steps by selecting one of the four different spots on the sample. Additionally, on a given spot, the elec- tron concentration can be increased further by illuminating the sample with light of energy higher than the energy gap of the barrier. The mechanism of this effect is based on the competition between the QW and surface states which can both trap carriers and will be discussed elsewhere. 18 The maximum attainable electron concentration is n e =2 RAPID COMMUNICATIONS PHYSICAL REVIEW B 15 DECEMBER 2000-II VOLUME 62, NUMBER 24 PRB 62 0163-1829/2000/6224/163104/$15.00 R16 310 ©2000 The American Physical Society