GaAlAs superlattices

applied surface science ELSEVIER

Applied Surface Science 123/124 (1998) 391-394

Optical anisotropy of (11 N) GaAs/ GaA1As superlattices Z. Yang *, Y.H. Chen l, y.Q. Wang Department of Physics, Hong Kong Unit,ersity of Science and Technology, Clearwater Bay, Kowloon, Hong Kong

Abstract

The in-plane anisotropy of a series of (113), (115), (001) vicinal, and singular (001) oriented GaAs/Ga0.7A10.3As superlattices (SL's) has been studied by reflectance difference spectroscopy (RDS) in the photon energy range covering both the E o and the E l energies of GaAs, and from 80 to 300 K. The polarity and the energy position of the observed RDS resonances near the E 0 energy confirm that these resonances are indeed originated from the heavy hole (HH), the light hole (LH), and the F 7 LH subbands in the GaAs wells. The transition strength anisotropy is in agreement with the multiband k * p model calculations. Sizable RD resonances have been observed in the (100) singular SL's at low temperatures, which are believed to be due to the HH and the LH exciton bound to anisotropic interface structural defects. The optical anisotropy of the SL's near the E~ critical energy of GaAs shows complicated resonance patterns. Some of the resonances occur below the Ej energy, which the simple effective-mass theory for the L-point states cannot explain. © 1998 Elsevier Science B.V.

1. Introduction

Molecular beam epitaxy (MBE) of GaAs on (11N) surface orientations has attracted much interest, as it opens up new possibilities of growing spontaneously ordered corrugation microstructures such as quantum wires [1,2]. Further studies showed that under certain growth conditions smooth (311) GaAs/A1As interfaces [3] or interfaces with much reduced microsteps [4] could also be formed. The reduced symmetry of (11N)-oriented superlattices (SL's) as compared to the (001)-oriented ones leads to the intrinsic in-plane optical anisotropy other than that is related to the interface corrugation microstructures [5-9]. In most cases, the contribution due to the intrinsic anisotropy

Corresponding author. Tel.: +852-2358-7485; fax: +8522358-1652; e-mail: [email protected]. On leave from the Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Box 912, Beijing 100083, China.

dominated the observed optical anisotropy. Broad resonance features were also reported in the optical anisotropy spectra near the E 1 energy [ 10-13], which are attributed to the splitting of the L-states predicted by simple effective-mass [11] and tight binding theories [14]. We report here our RDS study of a series of G a A s / G a 0 y A 1 0 3 A s SL's grown by MBE in the photon energy range covering both the E 0 and the E~ energies of GaAs. The orientation of the SL's are (113), (115), (001) vicinal, and nominally singular (001), respectively, grown side by side by MBE under the As-rich conditions that were known to produce smooth interfaces [3]. Each SL contains 10 pairs of GaAs(25 A)/Gao.vA10.3As(50 A) well/barrier layers.

2. A n i s o t r o p y near E o and E o + A o energies

Fig. 1 shows the imaginary RD spectra, Im(Ap), where A p =- 2 * ( r t - r 2 ) / ( r I + r2), of several SL's

0169-4332/98/$t9.00 © 1998 Elsevier Science B.V. All rights reserved. PH S01 6 9 - 4 3 3 2 ( 9 7 ) 0 0 4 9 1 - 1

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Yang et al./Applied Surfiwe Science 123/124 (1998) 391 394

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1.6 1.8 2.0 2.2 Photon Energy(eV)

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bottom, are the simulation the (115) spectrum

SL, the RD

spectra

(solid

of several SL's.

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and

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DR

T h e c u r v e s , f r o m t o p to

spectra, the RDS and the DR spectra of

spectrum

of the (113)

of the (001) vicinal SL. The spectra

SL,

and

have been

the RD shifted

vertically for clarity.

at 80 K (solid curves) together with the differential reflection (DR) spectrum don R ) / d E (dashed curve). Here r~ and r 2 represent the complex reflectance of light polarized along the two in-plane principal axes, the [1,1,0] and the [N, N,2] crystal directions, respectively. As will be shown below, I r a ( p ) manifests the imaginary part of the dielectric anisotropy Ira(A e), and directly reflects the absorption anisotropy of the SL's, because the dielectric function is mostly real in the energy range of interests. Two resonances, one positive and the other negative, are clearly seen near 1.7 eV for the RD spectrum. The amplitude of the resonances for the (115) and the (113) SL's are about the same, while that of the (100) vicinal SL are much smaller. This is expected because the in-plane anisotropy of the (100) vicinal SL's is roughly proportional to the miscut angle, and at 4 ° the anisotropy is several times smaller than the (113) and the (115) ones. The polarity of the RD resonances suggest that the positive resonance is due to the HH

subband to the conduction subband transition, and the negative one is due to the LH subband to the conduction subband transition. The DR spectrum shows two corresponding lines at the same energies. This confirms that the RDS resonances are indeed due to the HH and the LH subband states localized in the wells, rather than possible interface states at the G a A s / G a A I A s interfaces. The RD resonances are wider than the DR lines because the RD ones also contain contributions from the continuous states of subbands while the DR lines are from the discrete excitons only. A third resonance labeled 'S.O.' is seen near 2.1 eV which is due to the LH subband of the F 7 states to the conduction band transition. Its polarity is the same as the LH resonance, which is consistent with the theoretical predictions [6,13]. The intensity of the three RD resonances decrease slightly at higher temperatures, and at 300 K the intensities are about 80% of that at 80 K, while the line widths remain unchanged. The temperature dependence of the three resonance energies follow that of the GaAs E 0 energy, further confirming that these resonances are due to the subband states. A model dielectric function (MDF) for the SL is used to mimic the dielectric anisotropy of the RD spectra. The MDF of the SL's consists of the usual terms of bulk GaAs coming from the E l and higher critical points [15] plus three Lorentzian oscillator terms in the form of Aj

Ei- Ei_E_iT

(1)

to describe the HH, the LH, and the S.O. excitons. The average amplitude Aj of the oscillators are adjusted to fit the DR HH and the LH lines (see Fig. l). An additional AAi which describes the difference in oscillation amplitude between the two inplane principal axes is introduced for each RDS resonance. Typical RDS and DR simulation spectra are shown in Fig. 1. The parameters Aj and A A i / A j for the three transitions are 0.01 eV and 2% for the HH resonance, 0.01 eV and - 4 % for the LH resonance, and 0.005 eV and - 4 % for the S.O. resonance. The exciton oscillation strength anisotropy, A A / A , is therefore a few percent, which agrees with the multiband k * p calculations for the shallow quantum wells investigated here. This is consistent

393

Z. Yang et a l . / Applied Surface Science 123/124 (1998) 391 394

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defects only at low temperatures, the RD resonances disappear quickly as the temperature is raised. The anisotropy interface defects may also exist in the ( l l N ) SL's, which may explain why some S L ' s have stronger HH and the LH exciton resonances than the others even when the SL compositions are the same. Further study is being conducted to confirm the existence of the proposed interface defects and reveal their formation mechanisms.

4. Anisotropy near E t and E l + A t energies In the energy range near the E~ energy of GaAs the dielectric anisotropy can be obtained directly from the RD spectra by using the expression A~

with the TEM observations which show the absence of regular interface corrugation in the SL's.

3. Interface anisotropy The singular (100) S L ' s are expected to be isotropic within the interface plane. However, sharp RD resonances are observed at low temperatures. Shown in Fig. 2 are the imaginary RD spectra (solid curves) of an (100) SL at several temperatures together with its DR spectrum (dashed curve) at 80 K. Two resonances similar to the HH and the LH resonances of the (11 N ) S L ' s are clearly seen at 80 K. The width and the energy of the resonances are about the same as the DR lines, indicating that the RD resonances are related to the subband states. However, unlike the cases of the ( l l N ) S L ' s the RDS resonance amplitudes quickly decrease as the temperature is raised, and the resonances disappear completely at above 170 K. The DR lines, like the case of (11 N ) samples, are present even at 300 K. The origin of these RD resonances are not known at present, but their temperature dependence clearly indicates that they are different from the (11 N ) ones. One possibility is that the resonances are due to the HH and the LH excitons bound to anisotropic interface defects similar to the ones reported in Ref. [16], and the structural anisotropy of the defects give raise to the observed RD resonances through the HH and the LH excitons. Since the excitons bound to the

~-1 f~

Ap.

(2)

The M D F of bulk GaAs [15] is used for the e of S L ' s in Eq. (2) because the reflection spectra of the S L ' s show no difference from that of bulk GaAs. Fig. 3 shows the A e 2 - - - I m ( A e ) o f the ( 1 1 5 ) o r i ented SL at several temperatures. The (113) SL has very similar RD spectra. Notice that there are four positive resonances, A1 through A4, and two negative resonances, B I and B2. The resonances gain in intensity as the temperature is lowered. The intensities at 80 K are about 4 times of those at 293 K. The line widths, however, show no noticeable change

(511) SL - 62, 25 ,~

01 1 0.0 2.0

2.5

3.0 3.5 4.0 P h o t o n E n e r g y (eV)

4.5

Fig. 3. The imaginary dielectric anisotropy, Ae 2, of the (115) SL at several temperatures. Notice that two of the resonances are below the E I critical energy.

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Z Yang et al. /Applied Surface Science 123 / 124 (1998) 391-394

over the entire temperature range. The energy of the six resonances all have the same temperature dependence as the E 1 energy. This indicates that these resonances are closely related to the E 1 and the E 1 + A 1 critical points which are resulted from the electronic transitions of the L-point states. The single band effective-mass theory which is often used in describing the energy splitting and anisotropy of the L-point subbands of ( l l N ) SL's [11-13] fails to account for two major features of the experimental results here. First, there are two strong resonances which are below the E 1 energy of bulk GaAs, i.e., below the bottom of the quantum well. This is only possible if the effective mass at L-point is negative. Second, the theory predicts that the valence band at the L-point will split into three branches, and give rise to two positive RD resonances and one negative resonance. The energy of the negative resonance should be in between the two positive ones. The observed spectra show a different pattern, with four positive resonances A1 through A4 on the lower energy side and two negative resonances B1 and B2 on the higher energy side. All these point out that the L-point states are much more complicated than what the simple picture of effective mass model shows. The results presented here shed important light into the true nature of these states which calls for more realistic theoretical models which may include interface defects bands.

5. Summary We have shown by reflectance difference spectroscopy the polarity of the heavy hole, the light hole, and the F v state light hole electronic transition anisotropy in the (113) and the (115) G a A s / G a A 1 A s superlattices, and prove that the observed RD resonances are indeed from these SL subband states. The transition strength anisotropy, A A/A, is of the order of a few percent, in agreement with the multiband k * p model calculations. Sizable RD resonances have been observed in the (100) singular SL's at low

temperatures. The resonances are probably due to the HH and the LH excitons bound to elongated interface structural defects. The optical anisotropy of the SL's near the E l critical energy of GaAs shows complicated resonance patterns in terms of polarity and energy positions, and some of the resonances occur below the E l energy. The temperature dependence of the resonance energies indicates that they are originated from the L-point quantum well states. The results of the simple effective-mass theory for the L-point states fit the experimental ones poorly. More realistic models are therefore required to describe the L-point states.

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