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GaAs multi-quantum wells: effects of compensation
Superlattices
and Microstructures,
47
Vol. 8, No. 7, 7990
0PTIcALlLYPUMPED -BAND MULl’TQUANTUM
WEUS:
SPECRO!XOPY IN AlGaAs/GaAs EE’PECTS OF COMPENSATION
W.J. Li and B.D. McCombe State University of New York at Buffalo, Buffalo NY 14260 F.A. Chambers and G.P. Devane Amoco Corporation, Naperville, IL 60540 J. Ralston and G. Wicks Cornell University, Ithaca, NY 14850 (Received 1 August, 1990)
Measurements of intersubband absorption and cyclotron resonance on lightlydoped GaAs/AlGaAs quantum-well structures with a sensitive optical pumping technique show that the observed intersubband transitions are due to free electrons and that the large photo-generated excess free electron density is due to compensation by acceptors.
Optical pump and probe measurements constitute a sensitive class of techniques which have recently been applied to the study of weak intersubband resonances in nominally undoped and lightly doped GaAs/AlGaAs multiple-quantum-well (MQW) structures.‘~2 Results for nominally undoped samples were attributed to heavy hole excitonic intersubband transitions, and the oscillator strength associated with such transitions was found to be 25 times that of free electrons based on an assumed excitonic lifetime, T, of - 5 nanoseconds.’ We have carried out detailed experimental studies of intersubband optical transitions and the photo-generated cyclotron resonances (CR) in a series Metallic of GaAs/Al,,,Ga&s MQW structures. grating couplers were deposited on the sample surfaces in order to couple the in-plane electric field of the incident infrared radiation into the direction of confinement, the correct polarization to induce intersubband transitions. The grating arrangement allows us to measure intersubband absorption under simultaneous optical pumping with photon energies greater than the gap of the wider gap semiconductor, so that excess carriers could be generated in the wells. We have numerically calculated the coupling efficiency as a function of p/1 for the particular grating and sample geometry 3 (here p is the grating period and I is the wavelength at resonance). The coupling efficiency, the photo-generated carrier density, and the intersubband absorption coefficient are needed to calculate the oscillator strength, f, of the intersubband transitions. Carrier densities were measured directly 0749-6036/90/050047+02S02.00/0
via cyclotron resonance (CR) absorption under identical optical pumping conditions as those fhr the For two lightlydoped intersubband absorption. samples (-1 x 10n’/cm3) with different well widths (240A and 320A), f is found to be 1.020.3 and 1.1 eO.3, respectively. These values are very different from those attributed to excitonic intersubband transitions, f>20-100 (for 7=1-5 nsec). We have found that as many as 109/cm2 free electrons per well can be generated with a pump intensity as low as 100~W/cm2 of red light. This pump intensity would only yield - 2x 10s/cm2 electron-hole pairs per well with a quantum efhciency of 1, and ~=l nsec. We suggest that compensating acceptors could be responsible for this large excess carrier density. These acceptors are unintentionally incorporated into the sample during growth. To test this, we carried out detailed CR measurements on two separate samples grown under similar conditions to the previous two. These two samples have identical nominal well width of ZIOA withSi dopant of 1 x 10r6/cm3 over the central l/3 of each well. They are known to have very different compensation from photoluminescence experiments. A 35mW He-Ne laser was used as a pump source, and the light was directed by an optical fiber onto the sample in the cryostat. Saturation at different values of laser intensity and at different values of CR absorption (number of free electrons) for the two samples is evident from a, plot of CR absorption strength, I,, vs laser intensity, I,, incident on the sample surface. Least squares fits of 0 1990 Academic Press Limiter
48
Superlattices
the expression log(~,)=ylog(td+constant to the data at low pump intensities yield r values of 0.50+0.04 and 0.56*0.05 for these two samples. The square root dependence of absorption strength on laser power (at low power levels) indicates that the recombination in both samples is dominated by bimolecular-type process in which carriers are liberated from trapping centers under light illumination.’ The combination of saturation at high intensity at different values for the two samples consistent with the known differences in compensation, and the square root dependence at low intensity confirms the impurity-related carrier generation. The above evidences suggest that large values of oscillator strength obtained in Ref.1 may result from an underestimate of the optically induced carrier density. To investigate this point further, we compare the intersubband transition of an exciton to that of a free electron. The strength of a transition from subband state 1*I> to subband state 1qz> is determined in general by the dipole matrix element in the z-direction:
&(p) are identical plane wave states; for free excitons, 4,(p) and&(p) are very similar.’ Since Fi(z) is essentially the same for both exciton and free electron, Eq. (1) implies that the strength of excitonic intersubband transitions is comparable to that of free electrons, and there is no substantial enhancement of oscillator strength in such transitions.
where &(p) (i=1,2) is the two-dimensional hydrogen-like excitonic envelope function associated with subband i, and Fi(z) is the corresponding quantum well envelope function. For free electrons, 4,(p) and
and Microstructures,
Vol. 8, No. 7, 1990
Acknowledgement - This work was supported by ONR/SDIO under the MFEL program and by ONR.
References 1. M. Olszakier, E. Ehrenfreund, E. Cohen, J. Bajaj and G.J. Sullivan, Physcal Review Letters a2997 (1989) 2. W.J. Li and B.D. McCombe, F.A. Chambers, G.P. Devane, J. Ralston and G. Wicks, 4th International Conference on Modulated Semiconductor Structures, 1989, Surface Science 22& 164 (1990). 3. W.J. Li and B.D. McCombe, to be published. 4. D.A. Anderson and W.E. Spear, Philosophical Magazine & 695 (1977). 5. The behaviour of 4i(p) for donor impurities in the confined systems has been discussed by Cheng and McCombe, to be published in Physical Review B.
and Microstructures,
47
Vol. 8, No. 7, 7990
0PTIcALlLYPUMPED -BAND MULl’TQUANTUM
WEUS:
SPECRO!XOPY IN AlGaAs/GaAs EE’PECTS OF COMPENSATION
W.J. Li and B.D. McCombe State University of New York at Buffalo, Buffalo NY 14260 F.A. Chambers and G.P. Devane Amoco Corporation, Naperville, IL 60540 J. Ralston and G. Wicks Cornell University, Ithaca, NY 14850 (Received 1 August, 1990)
Measurements of intersubband absorption and cyclotron resonance on lightlydoped GaAs/AlGaAs quantum-well structures with a sensitive optical pumping technique show that the observed intersubband transitions are due to free electrons and that the large photo-generated excess free electron density is due to compensation by acceptors.
Optical pump and probe measurements constitute a sensitive class of techniques which have recently been applied to the study of weak intersubband resonances in nominally undoped and lightly doped GaAs/AlGaAs multiple-quantum-well (MQW) structures.‘~2 Results for nominally undoped samples were attributed to heavy hole excitonic intersubband transitions, and the oscillator strength associated with such transitions was found to be 25 times that of free electrons based on an assumed excitonic lifetime, T, of - 5 nanoseconds.’ We have carried out detailed experimental studies of intersubband optical transitions and the photo-generated cyclotron resonances (CR) in a series Metallic of GaAs/Al,,,Ga&s MQW structures. grating couplers were deposited on the sample surfaces in order to couple the in-plane electric field of the incident infrared radiation into the direction of confinement, the correct polarization to induce intersubband transitions. The grating arrangement allows us to measure intersubband absorption under simultaneous optical pumping with photon energies greater than the gap of the wider gap semiconductor, so that excess carriers could be generated in the wells. We have numerically calculated the coupling efficiency as a function of p/1 for the particular grating and sample geometry 3 (here p is the grating period and I is the wavelength at resonance). The coupling efficiency, the photo-generated carrier density, and the intersubband absorption coefficient are needed to calculate the oscillator strength, f, of the intersubband transitions. Carrier densities were measured directly 0749-6036/90/050047+02S02.00/0
via cyclotron resonance (CR) absorption under identical optical pumping conditions as those fhr the For two lightlydoped intersubband absorption. samples (-1 x 10n’/cm3) with different well widths (240A and 320A), f is found to be 1.020.3 and 1.1 eO.3, respectively. These values are very different from those attributed to excitonic intersubband transitions, f>20-100 (for 7=1-5 nsec). We have found that as many as 109/cm2 free electrons per well can be generated with a pump intensity as low as 100~W/cm2 of red light. This pump intensity would only yield - 2x 10s/cm2 electron-hole pairs per well with a quantum efhciency of 1, and ~=l nsec. We suggest that compensating acceptors could be responsible for this large excess carrier density. These acceptors are unintentionally incorporated into the sample during growth. To test this, we carried out detailed CR measurements on two separate samples grown under similar conditions to the previous two. These two samples have identical nominal well width of ZIOA withSi dopant of 1 x 10r6/cm3 over the central l/3 of each well. They are known to have very different compensation from photoluminescence experiments. A 35mW He-Ne laser was used as a pump source, and the light was directed by an optical fiber onto the sample in the cryostat. Saturation at different values of laser intensity and at different values of CR absorption (number of free electrons) for the two samples is evident from a, plot of CR absorption strength, I,, vs laser intensity, I,, incident on the sample surface. Least squares fits of 0 1990 Academic Press Limiter
48
Superlattices
the expression log(~,)=ylog(td+constant to the data at low pump intensities yield r values of 0.50+0.04 and 0.56*0.05 for these two samples. The square root dependence of absorption strength on laser power (at low power levels) indicates that the recombination in both samples is dominated by bimolecular-type process in which carriers are liberated from trapping centers under light illumination.’ The combination of saturation at high intensity at different values for the two samples consistent with the known differences in compensation, and the square root dependence at low intensity confirms the impurity-related carrier generation. The above evidences suggest that large values of oscillator strength obtained in Ref.1 may result from an underestimate of the optically induced carrier density. To investigate this point further, we compare the intersubband transition of an exciton to that of a free electron. The strength of a transition from subband state 1*I> to subband state 1qz> is determined in general by the dipole matrix element in the z-direction:
&(p) are identical plane wave states; for free excitons, 4,(p) and&(p) are very similar.’ Since Fi(z) is essentially the same for both exciton and free electron, Eq. (1) implies that the strength of excitonic intersubband transitions is comparable to that of free electrons, and there is no substantial enhancement of oscillator strength in such transitions.
where &(p) (i=1,2) is the two-dimensional hydrogen-like excitonic envelope function associated with subband i, and Fi(z) is the corresponding quantum well envelope function. For free electrons, 4,(p) and
and Microstructures,
Vol. 8, No. 7, 1990
Acknowledgement - This work was supported by ONR/SDIO under the MFEL program and by ONR.
References 1. M. Olszakier, E. Ehrenfreund, E. Cohen, J. Bajaj and G.J. Sullivan, Physcal Review Letters a2997 (1989) 2. W.J. Li and B.D. McCombe, F.A. Chambers, G.P. Devane, J. Ralston and G. Wicks, 4th International Conference on Modulated Semiconductor Structures, 1989, Surface Science 22& 164 (1990). 3. W.J. Li and B.D. McCombe, to be published. 4. D.A. Anderson and W.E. Spear, Philosophical Magazine & 695 (1977). 5. The behaviour of 4i(p) for donor impurities in the confined systems has been discussed by Cheng and McCombe, to be published in Physical Review B.