FAR INFRARED MAGNETO-ABSORPTION STUDY OF BARRIER IMPURITIES AND SCREENING IN GaAs/AlGaAs MULTIPLE QUANTUM WELLS

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FAR INFRARED MAGNETO-ABSORPTION STUDY OF BARRIER IMPURITIES AND SCREENING IN

GaAs/AlGaAs MULTIPLE QUANTUM WELLS

E. Glaser, B. Shanabrook, R. Wagner, R. Hawkins, W. Moore, D. Musser

To cite this version:

E. Glaser, B. Shanabrook, R. Wagner, R. Hawkins, W. Moore, et al.. FAR INFRARED MAGNETO- ABSORPTION STUDY OF BARRIER IMPURITIES AND SCREENING IN GaAs/AlGaAs MUL- TIPLE QUANTUM WELLS. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-239-C5-242.

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J O U R N A L D E PHYSIQUE

Colloque C5, supplement au noll, Tome 48, novembre 1987

FAR INFRARED MAGNETO-ABSORPTION STUDY OF BARRIER IMPURITIES AND SCREENING I N GaAs/AlGaAs MULTIPLE QUANTUM WELLS")

E. GLASER( 2 ) , B .V

.

.

.

.

W.J. M O O R E and D. MUSSER"

Naval Research Laboratory, Washington, D C 20375, U.S.A.

a art in

Far infrared magneto-absorption experiments have been performed at 4.2K on MBE-grown GaAs/AlGaAs MQW heterostructures that were selectively doped with Si donors in the centers of the quantum wells and barrier layers. Data were obtained for several values of dopant concentration in the barriers in order to study in detail the binding of electrons in the quantum wells to posi- tively charged ions in the barriers and the effects of the screening of the Coulomb interaction by free carriers. The results are compared with recent calculations.

Recently, the nature of isolated hydrogenic impurities in GaAs/AlGaAs multiple quantum well (MQW) heterostructures has received much theoretical [I] and experimen- tal [2] attention. Experimental reportings L3.41 of the intra-impurity transition energies (1s-2p') associated with confined neutral donors in the GaAs wells as functions of the well width, the position of the impurity centers within the wells, and the strength of an applied magnetic field are in very good agreement with recent calculations [I]. In the current study, we report the results of far infrared magneto-absorption experiments performed at 4.2K on GaAs/Alo,sGao.iAs MQW (-50 layers) grown by MBE that were selectively doped with Si donors over the central third of the GaAs quantum wells (L*=200A) AlGaAs barriers (Lb=200A). The electron concentration (n) in the quantum wells was adjusted by varying the dopant concentration in the barriers. This variation in n allowed several interesting phe- nomena to be studied. Thase include (a) the binding of electrons in the wells to positively charged impurities in the barriers and (b) the effects of the screening of the Coulomb interaction by free carriers. An additional motivation for these studies was to investigate previously unidentified absorption features that were observed in recent investigations [3,4] of shallow donors in similar MQW structures with nominally undoped barrier layers.

The far infrared measurements were made in a light plpe system with a Fourier- transform spectrometer in conjunction with a photoconductive detector and a 13T sup- erconducting magnet. Data were obtained with the sample growth axis parallel to the applied magnetic field and incident radiation. The density of free electrons in the QW's was determined in-situ from analyses of the cyclotron resonance absorption pro- files while the binding energy of the donors in the quantum wells as a function of n was monitored by the Is-2pt transitions.

Representative transmission spectra obtained at 4.2K for several values of dopant concentration in the AlGaAs barrier layers ^{( N b )} are shown in Fig. 1. All of the peaks in Fig. 1, labeled by 1, 2 and 3, occur at energies bigher than cyclotron resonance (CR). The origin of peak 3, which occurs at -178 cm- , is well understood

his ^work i s supported i n part by t h e U.S. O f f i c e o f Naval Research '2'NR~-NR~ Research A s s o c i a t e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987550

JOURNAL DE PHYSIQUE

L

80 120 160 200 240

F R E Q U E N C Y ( c m - I )

Fig. 1. Transmission spectra obtained for several values of dopant concentra- tion (Nb) in the barrier layers. Peaks 1 and 3 are assigned to intra-impurity transitions of electrons bound to donors in the centers of the barriers and wells, respectively.

Fig. 2. Compilation of the IS-2pt transition energies at 4.2K for samples with minimal free electron density (n) in the wells (closed symbols) and with n = 1 . 5 ~ 1 0 1 ~ (open symbols). The 1s-2p+ transition energies calculated [5] for donors located at the centers of the wells and barriers are shown by the solid and dashed lines, respectfully.

and arises from Is-2pt transitions associated with the neutral donors whose ions are located in the center of the GaAs wells. Two additional peaks, labeled 1 and 2, are observsd at lower frequencies. The strength of these absorption features increases significantly with increasing Nb. Additional measurements were performed on a sample selectively doped with Si atoms in the wells and barriers with well widths similar to the samples discussed above but with slightly narrower barrier widths (Lb=150h). In this sample, peak 1 shifts to higher energy (by 2-3 cm-'1, while the positions of peaks 2 and 3 are unchanged. This observation and the corre- lation between the intensity of peak 1 and the doping concentration in the barriers suggests that this feature arises from an intra-impurity transition of an electron in the quantum well bound to a positively charged ion in the barrier. This transi- tion occurs at a higher energy for samples with a smaller barrier width because the mean separation between the positively charged ion in the barrier and the weakly bound electron in the quantum well becomes smaller.

A compilation of the transition energies as a function of magnetic field for the samples with barrier widths of 200A is shown in Fig. 2 with the closed symbols.

Recent calculations [5] have been performed for the binding energies of the ground and first few excited states for electrons in the wells in the presence of a uniform distribution of donors and a magnetic field applied perpendicular to the layers.

The results for the 1s-2pt transition energies for donors located at the centers of the wells (Lw=200A) and barriers (Lb=200A) are shown in Fig. 2 by the solid and dashed lines, respectfully. In addition, the oscillator strength for electronic transitions associated with donors located at the center of the barrier layers is predicted to be significantly greater than that for donors located at the center of the quantum wells. The present observations of the substantial difference in line intensities between the high and low energy peaks, the dramatic increase of the integrated intensity of peak 1 with dopant density in the barrier, and the excellent agreement between theory and experiment for the 1s-2pt transition energies are consistent with our assignment of peak 1.

The energy of peak 2 lies approximately at the 1s-2p+ transition energy of bulk donors in GaAs. However, because the energy of peak 2 is sensitive to the angle of magnetic field with respect to the growth direction of the MQW, the association of peak 2 with bulk donors in the substrate or buffer layers can be excluded. At the current time, the origin of peak 2 is not understood and is undergoing further examination.

Additional measurements were made after illumination with a red light-emitting- diode of the sample nominally undoped in the barrier layers. After this procedure, features were observed at the resonance energies of peaks 1 and 2. However, the line strengths were much weaker than those for the corresponding peaks found in the spectra of the samples intentionally doped with Si atoms in the barriers. These absorption features were not observed initially because of compensation. However, after exposure to light, these absorption features are observed due to trapping of optically created electrons in the quantum wells. These electrons could arise from ionization of deep levels in the AlGaAs or from persistent photoconductivity effects [6]

.

infrared studies provide a valuable probe

for estimating the concentration of resid-

-

u

& positively charged defects in the B112T Nb=81 1, II L; L,,=ZOOA

nominally undoped barrier layers. Previ- ous infrared measurements [3,4] on MQW's doped with donors in the center of the wells have observed features similar to that of peak 1. The current investiga- tions suggest that these features arise from residual positively charged defects

2

in the AlGaAs barriers. V)

Representative transmission spectra for the sample investigated at 4.2K with

5

the largest dopant concentration in the f barrier layers (Nb=8x10i6 ~ m - ~ ) are shown

2

in Fig. 3. In contrast to the spectra discussed above, the dominant peak corre- 2 sponds to cyclotron resonance absorption by free carriers in the quantum wells.

2

Fits to the transmission spectrum of Fig. c 3 reveal a cyclotron mass of 0.069mo and an electron concentration per well of 1.5x101 ~ m - ~ . Because free carriers exist in the quantum wells, it is expected that the effects of screening of the Cou- lomb interaction of donor ions placed in

the center of the quantum wells should be 8 0 120 160 200 240 observable. Two additional weak absorp- FREQUENCY (cm-l)

tion features are observed at frequencies well above the cyclotron resonance -energy.

The approximate resonance energies of the Fig. 3. Transmission spectra obtained feature observed in the high-energy tails with a large

of the cyclotron resonance lineshapes concentration (n=1.5x1011 cm-2 ) in the (marked by the arrows in Fig. 3) are shown wells at 4.2K. The arrows indicate the in Fig. with the open symbols. This approximate peak energies (open symbols feature is suggested to arise from in Fig. 2) of the weak absorption fea- transitions associated with the neutral tures observed in the high-energy tails donor- in the center of the wells shifted of the cyclotron resonance lineshapes- to lower energies due to the screening of The top three spectra were obtained with t h e x l o m b interaction by the free elec- the sample growth axis tilted at trons. In agreement with this assignment, with respect the magnetic recent calculations predict a decrease in

the binding energy of a hydrogenic impu- rity as a function of free electron con- centration. However, if we treat the

difference between the 1s-2p+ transition and cyclotron resonance energies as a lower limit for the binding energy, then this difference is three to four times larger than the 1s binding energy (-14 cm-l) predicted [7] for the measured value of n.

More sophisticated calculations that include effects of phase space filling [a], the exclusion principle 181, depolarization shifts [9] and screening in strong magnetic fields may be necessary to adequately model the present experimental results.

The absorption feature at approximately 235 cm-I was observed & with the sample growth axis tilted at an angle with respect to the magnetic field (top three

C5-242 JOURNAL DE PHYSIQUE

traces of Fig. 3). This feature, which is also observed in the charge density polarization of resonant electronic Raman scattering experiments, arises from tran- sitions of electrons between the lowest and first excited subband levels. The dependence of the transition energy and line strength on the magnitude and orienta- tion of the magnetic field is presently undergoing further investigation.

In summary, the far infrared magneto-absorption experiments reveal an absorption that is characteristic of very weakly bound electrons in the GaAs quantum wells.

Because this absorption feature (peak 1 in Fig. 1) exhibits a strength that is pro- portional to the dopant concentration in the barriers and a transition energy that is sensitive to the barrier width, it appears to arise from intra-impurity transi- tions of electrons in the GaAs quantum wells bound to the positively charged Coulomb centers in the AlGaAs layers. This assignment is consistent with results of recent calculations [5]. In addition, the effects of carriers in screening the Coulomb interaction in this quasi two-dimensional system have been examined. These prelimi- nary results indicate a reduction of the Is-2pt transition energies as a function of the applied magnetic field with free carrier densities of - 1 . 5 ~ 1 0 ~ ~ cm-=. However, the magnitude of the observed shift is less than that inferred from results of recent theory [7] for the measured value of free electron concentration in the wells.

The authors are grateful to R.L. Greene, J.M. Mercy, and B.D. McCombe for ongoing interactions and helpful discussions and to D. Gammon for resonant Raman scattering studies of these samples.

REFERENCES

Greene, R.L. and Bajaj, K.K., Phys. Rev. 8% (1985) 913 and references therein.

Shanabrook, B.V., Surf. Sci.

170

Jarosik, N.C., McCombe, B.D., Shanabrook, B.V., Comas, J., Ralston, J., and Wicks, G., Phys. Rev. Lett.

54

Wagner, R.J., Shanabrook, B.V., Furneaux, J.E., Comas, J., Jarosik, N.C., and McCombe, B.D., in: Proc. 11th Int'l. Symp. on GaAs and Related Com- pounds, Biarritz (1984) Inst. Phys. Conf. Ser. 74 (Inst. Phys., London- Bristol, 1985) 315.

Lane, Pat and Greene, R.L., Phys. Rev. B z (1986) 5871 and private communi- cation.

See, e.g., Stormer, H.L., Dingle, R., Gossard, A.C., Wiegmann, W., and Sturge, M.D., Solid State Commun. 2 (1979) 705.

Brum, J.A., Bastard, G., and Guillemot, C., Phys. Rev. B a (1984) 905.

See, e.g., Kleinman, D.A., Phys. Rev. 8 2 (1985) 3766 and Schmitt-Rink, S.

and Ell, C., J. Lumin.

30

Ando, T., 2. Phys. B g (1977) 263 and references therein.