AlAs single quantum wells

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Superlattices and Microstructures, Vol. 21, No. 2, 1997

Effect of well width variation on type-I/type-II photoluminescence in GaAs/AlAs single quantum wells L. J. B Duke University, Department of Physics, Box 90305, Durham, NC 27708-0305, U.S.A.

T. D-R Duke University, Department of Electrical Engineering, Box 90291, Durham, NC 27708-0291, U.S.A.

C. N. Y, L. E. MN University of North Carolina at Chapel Hill, Department of Physics and Astronomy, Chapel Hill, NC 27599-3255, U.S.A. (Received 18 December 1995) ˚ GaAs/AlAs We have investigated type-I/type-II transitions in MBE-grown 20, 35, and 50 A ˚ wells single quantum wells using photoluminescence (PL) spectroscopy. The 20 and 50 A ˚ well has PL peaks corresponding to both type-I and show a type-I peak, while the 35 A ˚ -well sample is based upon excitation intentype-II transitions. Peak assignment for the 35 A sity dependent measurements, and the strength of the type-II transition in this sample is attributed to C-X mixing. Superperiodicity has been put forth as a possible explanation for C-X mixing effects in superlattice structures, but the observation of this phenomenon in a single quantum well structure suggests that superperiodicity is not required for C-X mixing to occur. ( 1997 Academic Press Limited

1. Introduction The role of bandgap engineering, as it affects charge distribution and confinement, electronic wavefunction mixing, and carrier transport behavior in semiconductor heterostructures, is critically important to the realization of next-generation device technology [1–3]. By varying the thickness of a heterostructure layer, it is possible to vary the bandgap and to thereby vary the minima of electron energy levels in the conduction band. The position of the conduction band minima in both real and reciprocal space affects whether electron-hole transitions are direct or indirect. Studies have been undertaken to delineate direct versus indirect transitions in GaAs/AlAs and GaAs/Al Ga As hetx 1~x erostructures [4–6]. These studies have been carried out extensively for superlattices and multiple quantum wells. We present results for narrow single quantum well heterostructures. The C-band energy level of a GaAs quantum well lies below the X-band energy level of the ˚ . As the GaAs well is AlAs barriers for GaAs well thicknesses greater than approximately 35 A narrowed, the C state is raised in energy, due to quantum confinement, with respect to the original ˚ , the AlAs X-band level will be lower in conduction band edge. For GaAs well widths below 35 A 0749–6036/97/020187]07 $25.00/0

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( 1997 Academic Press Limited

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Type I AlAs

B

AlAs

AlGaAs

AlGaAs

AlGaAs – GaAs

Γ

Type II AlAs

AlAs

AlGaAs

– GaAs

Γ confined X X confined +

+

Fig. 1. A, Type-I and B, Type-II band alignment for an AlAs/GaAs/AlAs single quantum well clad in Al Ga As. For the type I case, the lowest energy level in the conduction band is the C state in the GaAs 0.45while 0.55 well, the lowest energy level in the conduction band for the type II case is the X state in the AlAs barrier.

energy than the GaAs C-band level, and electrons will prefer to sit in the AlAs X band. Quantum confinement energies are much smaller for X-point electrons in AlAs of comparable layer thicknesses, primarily due to the much larger effective mass. Electronic transitions occurring betwen the conduction and valence bands of the GaAs/AlAs heterojunction are labeled as either type I or type II. Type-I direct transitions correspond to the recombination of electrons in the GaAs C conduction band of the quantum well with holes in the GaAs C valence band (see Fig. 1A). Type-II indirect transitions describe the recombination of conduction band electrons in the AlAs X level with holes in the C valence band of the GaAs layer (see Fig. 1B). These transitions can be observed using photoluminescence (PL) spectroscopy, and the two types can be distinguished. In this paper, we describe the study of three representative GaAs/AlAs single quantum well structures designed with type-I, crossover, and type-II band alignment. Unlike previous studies, which primarily examined electronic wavefunction characteristics in short-period superlattices [4,5] and coupled quantum well structures [6–8], our work focuses on undoped variable-thickness GaAs/ AlAs single quantum well heterostructures.

2. Experiment The samples were grown by molecular beam epitaxy (MBE) on (100) semi-insulating GaAs substrates in a RIBER-32 R&D machine. The epitaxial structure is shown in Fig. 2. Each structure ˚ undoped AlAs barriers, 50 A ˚ consists of an undoped GaAs quantum well surrounded by 50 A ˚ Al Al Ga As undoped spacer layers, and 5000 A Ga As Si-doped (1]1016 cm~3) layers. 0.45 ˚ 0.55 0.45 0.55 ˚ for the three A 100 A GaAs layer caps the structure. Well widths are nominally 20, 35, and 50 A samples discussed here. Layer thicknesses are based on RHEED oscillation calibration of growth rates prior to the growth. The PL measurements utilized the 488 nm or 514.5 nm line of an Ar` laser. These two strongest lines have sufficient excitation energy to probe the quantum well region of the structures under investigation. The PL intensity was measured with a SPEX-1403 double monochromator equipped with a HAMAMATSU R-928 photomultiplier with photon-counting electronics.

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100 Å GaAs cap

5000 Å Al0.45Ga0.55As Si-doped 1 × 1016 cm–3 50 Å Al0.45Ga0.55As spacer 50 Å AlAs barrier 20, 35, or 50 Å GaAs well 50 Å AlAs barrier 50 Å Al0.45Ga0.55 As spacer

5000 Å Al0.45Ga0.55As Si-doped 1 × 1016 cm–3

(100) semi-insulating GaAs substrate

Fig. 2. Epitaxial structure.

3. Results and discussion ˚ well sample is shown in Fig. 3. This spectrum was measured The PL spectrum for the 50 A at 27 K and was excited by the 514.5 nm line. The peak at 1.512 eV corresponds to the bandgap in bulk GaAs at 27 K less the exciton binding energy in bulk GaAs (4.2 meV). There is a shoulder in this peak at 1.516 eV which corresponds to bulk GaAs without the exciton. The peak at 1.494 eV is associated with an impurity acceptor level. Of greatest interest is the peak at 1.639 eV from recombination in the GaAs well which is separated from the bulk GaAs by 123 meV. This energy separ˚ corrected by the binding ation is consistent with confinement energy in a finite square well of 49 A energy of an exciton confined to the well (quasi-2D) [9]. ˚ well sample were made at 26 K with the 488 nm line. The actual Measurements of the 35 A ˚ from an iterative well width of this sample—based on the PL measurement—was found to be 36.2 A calculation used to measure valence band offset [10]. The PL spectrum for this sample is shown in ˚ sample, but there are two Fig. 4. The bulk GaAs and acceptor peaks are the same as for the 50 A peaks from the quantum well region of the sample. The higher intensity (lower energy) peak is assigned to type-II recombination, whereas the lower intensity (higher energy) peak corresponds to type-I recombination. This assignment is consistent with the excitation intensity dependent measurements shown in Fig. 5. The dependence of peak energy and intensity as a function of incident laser intensity was also investigated for laser intensities ranging from 3.8]102 W cm~2 to 1.3]104 W cm~2. The increase in the peak energy of the type-II transition with increasing excitation intensity is due to spatial separation of electrons in the AlAs from holes in the GaAs [10,11]. Confining electrons

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PL Intensity (a.u.)

dw = 50 Å T = 27 K

1.40

1.50

1.60

1.70

Energy (eV)

Fig. 3.

˚ well. Photoluminescence spectrum for the 50 A

PL Intensity (a.u.)

dw = 35 Å T = 26 K

1.40

Fig. 4.

1.50

1.60 Energy (eV)

1.70

1.80

˚ well. Photoluminescence spectrum for the 35 A

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T = 26 K

PL Intensity (a.u.)

4I 3I I

1.65

1.70 Energy (eV)

1.75

Fig. 5. Photoluminescence spectra for 4I (solid line), 3I (dashed line), and I (long-dashed line) incident laser intensity, where I\3.2]103 W cm~2.

to the AlAs and holes to the GaAs results in an internal electric field. As the excitation intensity is increased, more electron-hole pairs are created. In turn, as the number of electron-hole pairs increases, this internal electric field increases and raises the potential energy in the AlAs. We therefore expect to observe an increase in the energy of the type-II peak with increasing excitation energy. Applying the argument of Chen et al. [11] for a GaAs/InGaP heterostructure to our GaAs/AlAs system, we estimate the energy shift to be e2 DED 4

A

d d 1` 2 M M 1 2

B

Igs 2. Ahm

(1)

This predicts an energy shift, DED4 meV, consistent with the observed shift as shown in Fig. 5, where ˚ (d \50 A ˚ ) and M \13 (M \10) are the layer thickness and e is the electronic charge, d \35 A 1 2 1 2 dielectric constant of GaAs (AlAs), I is the shift in the incident laser power (see Fig. 5), gD10~4 is the quantum efficiency, s D10~6 s [6] is the lifetime of the type-II transition, AD8]10~5 cm2 is the 2 area of the beam, and hm\2.54 eV is the excitation energy. ˚ well sample was measured at 27 K and excited by the 514.5 nm The PL spectrum for the 20 A line. In addition to the usual bulk and impurity peaks, we have a single peak due to recombination in the quantum well region (see Fig. 6). Though this well is thin enough that the type-II recombination energy is clearly lower than the type-I recombination energy, the peak is assigned to type-I recombination. The increase in background intensity with increasing energy corresponds to the tail of the peak from recombination in the Al Ga As. To the degree allowed by this structure and 0.45 0.55 the capabilities of our optical characterization apparatus, we have concluded that there is no observ-

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PL Intensity (a.u.)

dw = 20 Å T = 27 K

1.40

Fig. 6.

1.50

1.60 1.70 Energy (eV)

1.80

1.90

˚ well. Photoluminescence spectrum for the 20 A

˚ GaAs able type-II recombination in this sample. This is consistent with the PL spectrum for a 20 A well as shown by Shieh and Lee [7]. PL spectra were measured at temperatures ranging from 25 K–120 K. The energies and intensities of the quantum well peaks drop as the temperature of the samples is increased to 120 K. At 120 K, the intensity of the quantum well peaks is indistinguishable from the background intensity for ˚ well samples, and the intensity drops off at 45 K for the 20 A ˚ well sample. For the the 35 and 50 A ˚ well sample, the type-I and type-II peaks remain distinguishable up to 80 K. In the range 35 A 25 K–80 K, there is little variation in energy or intensity of the type-II peak with temperature. These results should be discussed with the understanding that momentum must be conserved when the electrons and holes recombine. This poses no problem in the type-I case where electrons and holes are both sitting in the C band in the GaAs well, but we need to more carefully consider the momentum-conserving recombination mechanism in the type-II transition where the X-band electrons in the AlAs barriers recombine with C-band holes in the GaAs well. One type-II momentum-conserving recombination process is phonon-assisted recombination. In this second-order process, the electron would absorb a phonon that would give it sufficient momentum to move to the C band in the GaAs well before recombining with the hole in the GaAs valence band. The second-order nature of such a transition, coupled with the small phonon population at the temperatures at which the PL spectra were measured (D10~7 at 27 K), indicates why we only observe a type-I peak in the ˚ structure which has clear type-II band alignment. 20 A Given the small phonon population at these temperatures, it is interesting that we see such a ˚ sample. The enhancement of this peak in the 35 A ˚ well case can be strong type-II peak in the 35 A understood by considering what occurs when two energy levels are brought close together, as occurs ˚ well. here in the conduction band for the confined X-levels in AlAs and C-level in GaAs for a 35 A In this case, the type-II transitions can be observed even in the absence of a phonon due to the mixing

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˚ well sample, the AlAs X-band level is suffiof the C- and X-electron wavefunctions. For the 20 A ciently separated from the GaAs C-band level that mixing is negligible, which is consistent with our observation of only one peak corresponding to a type-I direct transition. Since the luminescence comes from a single well and the associated barriers in our structures, we expect the luminescence intensity to be less than it would be in multilayer structures such as multiple quantum wells or superlattices. Nakayama et al. [5] and Meynadier et al. [4] have investigated C-X mixing in superlattices, and both suggest the superperiodicity may contribute to the mixing observed. Effects of periodicity, such as zone folding and minibands, are not present in single quantum well structures, so other explanations must be put forth to better understand the mixing in our structure. It would be possible to further investigate the mixing of C and X states by applying an electric field to a sample with well width in the crossover region [4] and observing the resulting changes in the PL line shape and peak positions. The results of such an investigation will be reported in a future publication.

4. Conclusions ˚ GaAs wells. The quantum well In summary, we looked at the PL spectra of 50, 35, and 20 A ˚ sample was assigned to a type-I transition, consistent with the band alignment for peak in the 50 A ˚ sample displayed two peaks, a lower energy type-II such a well width. The spectrum of the 35 A peak and a higher energy type-I peak. This peak assignment was verified by excitation intensity dependent measurements. The strength of the type-II transition is understood to be the result of C-X ˚ sample, only one peak was observed, and it was assigned to a type-I transition. mixing. In the 20 A Though such a well is clearly in the type-II regime, the absence of a type-II peak is consistent with the low phonon population for the temperatures at which the spectra were measured and with the negligible mixing due to the separation of C and X levels. Acknowledgements—The authors wish to thank Stephen Teitsworth for helpful discussions and Richard Kendall for his assistance with the sample growths. This work was supported by NSF grant #DMR-92-08381.

References [1] J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson and A. Y. Cho, Science 264, 553 (1994). [2] T. Daniels-Race and S. Yu, Solid-State Electronics 38, 1347 (1995). [3] J. Feldmann, R. Sattmann, E. O. Go¨bel, J. Kuhl, J. Hebling, K. Ploog, R. Muralidharan, P. Dawson and C. T. Foxon, Physical Review Letters 62, 1892 (1989). [4] M.-H. Meynadier, R. E. Nahory, J. M. Worlock, M. C. Tamargo, J. L. de Miguel and M. D. Sturge, Physical Review Letters 60, 1338 (1988). [5] M. Nakayama, K. Imazawa, I. Tanaka and H. Nishimura, Solid State Communications 88, 1 (1993). [6] J. Feldmann, J. Nunnenkamp, G. Peter, E. Go¨bel, J. Kuhl, K. Ploog, P. Dawson and C. T. Foxon, Physical Review B 42, 5809 (1990). [7] T. H. Shieh and S. C. Lee, Applied Physics Letters 63, 1219 (1993). [8] T. H. Shieh and S. C. Lee, Applied Physics Letters 63, 3350 (1993). [9] R. Zimmerman and D. Bimberg, Physical Review B 47, 15 789 (1993). [10] C. N. Yeh, L. E. McNeil, L. J. Blue and T. Daniels-Race, Journal of Applied Physics 77, 4541 (1995). [11] J. Chen, D. Patel, J. R. Sites, I. L. Spain, M. J. Hafich and G. Y. Robinson, Solid State Communications 75, 693 (1990).