Optically detected magnetic resonance of excess electrons in type-I ...

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PHYSICAL REVIEW B

VOLUME 58, NUMBER 4

15 JULY 1998-II

Optically detected magnetic resonance of excess electrons in type-I quantum wells with a low-density electron gas C. Y. Hu,* W. Ossau, D. R. Yakovlev,† and G. Landwehr Physikalisches Institut der Universita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany

T. Wojtowicz, G. Karczewski, and J. Kossut Institute of Physics, Polish Academy of Sciences, Al. Lotniko´w 32/46, 02-668 Warsaw, Poland ~Received 22 April 1998! Optically detected magnetic resonance is applied to study the fast optical transition processes on a ns time scale with the microwave-induced magnetic transition rate much lower than the optical transition rate. The spin resonance of excess electrons ~EESR! is observed with the detection on either the direct exciton or the negatively charged exciton X 2 emission in type-I CdTe/Cd12x Mgx Te quantum wells with excess electrons of low density. It is found that the electron-spin-dependent and electron-spin-conserved formation and recombination of X 2 make the optical detection of EESR feasible. @S0163-1829~98!51628-1#

Optically detected magnetic resonance ~ODMR! has been proved to be a useful tool for studying recombination processes in bulk semiconductors.1 The magnetic resonance is detected by the monitoring of microwave-induced changes in the luminescence intensity between magnetic sublevels. It is generally acknowledged that a prerequisite for ODMR is that the microwave-induced transition ~which is a magnetic dipole transition! rate should be higher than or comparable to the optical transition ~which is an electric dipole transition! rate. To satisfy this condition, the optical lifetime should be on a ms time scale considering the usual microwave powers available for the ODMR measurements. This is why, up to now, ODMR has been limited to studying only the slow optical transition on a ms time scale, such as the indirect excitonic transition in bulk semiconductors1 or quantum wells ~QW’s!.2–4 In this paper we demonstrate that ODMR can also be applied to studying the fast optical transition on a ns time scale, such as the direct excitonic transition in type-I QW’s with a low-density electron gas. In our case the ODMR of excess electrons detected on the neutral and negatively charged exciton emission occurs with the microwaveinduced magnetic transition rate much lower than the optical transition rate. In the presence of excess electrons of low density (;1010 cm22), besides the neutral exciton (X) a new resonance identified as negatively charged exciton X 2 , i.e., two electrons bound to a hole, has been recently observed in CdTe/Cd1-x Znx Te QW’s ~Ref. 5! and GaAs/Alx Ga12x As QW’s.6,7 A X 2 can be formed by an exciton trapping an electron that has antiparallel spin with the electron in exciton, thus X 2 shares the luminescence intensity with the exciton. If the electron population between two spin states ( u 6 21 & ) is varied by the spin resonance of excess electrons ~EESR!, the formation probability of X 2 will be influenced. It is therefore expected that EESR ~or ODMR of excess electrons! can be detected by the monitoring of microwaveinduced changes in X 2 or X emission intensity. To realize the idea mentioned above we choose the type-I undoped CdTe/Cd0.7Mg0.3Te multiple QW’s grown by mo0163-1829/98/58~4!/1766~4!/$15.00

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lecular beam epitaxy on a GaAs substrate. The sample used in this work had six QW units, each of which consisted of an 80-Å-thick CdTe QW separated by 200-Å-thick Cd0.7Mg0.3Te barriers from 500-Å-thick CdTe/Cd0.7Mg0.3Te superlattices ~20 Å/20 Å! @see the inset of Fig. 1~a!#. This

FIG. 1. ~a! Photoluminescence spectrum of the 80-Å CdTe 200-Å Cd0.7Mg0.3Te QW’s at T51.7 K and B50 T. Inset: Energy band structure of the QW unit. ~b!, ~c! Microwave-induced luminescence changes at B50 T and B54 T, respectively. R1766

© 1998 The American Physical Society

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OPTICALLY DETECTED MAGNETIC RESONANCE OF . . .

structure is similar to that in Ref. 8. Under excitation above the superlattice miniband gap the different tunneling probabilities for electrons and holes from the superlattice miniband into the QW through the 200-Å-thick barriers provide the low-density excess electrons in the QW’s, necessary for the formation of X 2 . The back wave oscillator ~BWO! was used as the microwave source. Its frequency can be tuned from 55 GHz to 80 GHz by applying a different dc voltage. The power output of the BWO is in the range of 100 to 250 mW. The sample was placed at the exit of the rectangular waveguide in an optical cryostat with a split coil magnet system with a magnetic field perpendicular to the QW plane. Microwaves were irradiated onto the sample from its backside with the spreading direction parallel to the magnetic field. The linearly polarized light ~514-nm line! from an argon-ion laser was focused on to the sample from its front side to generate the excitation above the superlattice miniband gap (E g 51.8 eV) and the luminescence was observed in the direction of the magnetic field. This setup is shown in detail in Ref. 9. For ODMR experiments microwaves were chopped at 45 Hz and the synchronous changes in the luminescence intensity were recorded by a two-channel photoncounter. The low-temperature (T51.7 K) photoluminescence spectrum taken under an excitation intensity of 0.1 W/cm2 is shown in Fig. 1~a!. There are two lines observed: one line is identified as heavy-hole exciton X and another line, 4.1 meV lower in energy, is identified as X 2 . 8 Microwave-induced luminescence changes are shown in Fig. 1~b! at B50 T and Fig. 1~c! at B54 T. At B50 T microwaves decrease X 2 and increase X emission, whereas at B54 T microwaves increase X 2 and decrease X emission in s 2 polarization and have no obvious influence on X 2 and X emission in s 1 polarization. The ODMR spectra detected on X and X 2 emission are shown in Fig. 2~a! for s 2 and Fig. 2~b! for s 1 polarization with microwaves at 70 GHz. The positive and negative signs of DI mean the increase and decrease of luminescence intensity, respectively. The maximal change reaches about 8% of the total intensities of X and X 2 . With increasing the magnetic field, the s 2 ODMR signals decrease fast to zero ~for B