AlAs superlattices by exciton photoluminescence

AlAs superlattices by exciton photoluminescence

Materials Science and Engineering C 19 Ž2002. 439–443 www.elsevier.comrlocatermsec Characterization of ultrashort-period GaAsrAlAs superlattices by e...

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Materials Science and Engineering C 19 Ž2002. 439–443 www.elsevier.comrlocatermsec

Characterization of ultrashort-period GaAsrAlAs superlattices by exciton photoluminescence V.G. Litovchenko ) , D.V. Korbutyak, S.G. Krylyuk, Yu.V. Kryuchenko Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Prospect Nauki 45, 03028, KieÕ, Ukraine

Abstract Ultrashort-period GaAsrAlAs superlattices of type-II were investigated by low-temperature photoluminescence spectroscopy. The photoluminescence spectra consisting of many lines were carefully analyzed to obtain energy position, full width at half maximum and relative intensity for each individual line. Influence of the energy band structure, layer thickness and interface disorder on the radiative electron-hole transitions in these structures is discussed. q 2002 Elsevier Science B.V. All rights reserved. Keywords: GaAsrAlAs superlattice; Exciton; Phonon; Indirect transitions

1. Introduction Indirect band gap of AlAs results in a staggered band alignment in short-period Žtype-II. GaAsrAlAs superlattices ŽSLs.. In such SLs the conduction band minimum is formed by the X states of AlAs and electron-hole transitions become indirect in both real and momentum spaces, see Fig. 1b,c. Because of long exciton lifetime, type-II SLs are very promising objects for studying such phenomena as biexcitons and exciton condensation phases Žsee, e.g. Ref. w1x and references therein.. A closer look at the energy band structure of the type-II SLs revealed that there is a splitting between the X z and X x, y levels that originates from the anisotropy of the effective mass tensor at the X point of AlAs and the small mismatch between the GaAs and AlAs lattice constants w2x. The lattice mismatch induces compressive in-plane strain in the barrier layers which shifts the X z level upward and the X x, y level downward. Depending on which level, X z or X x, y , has the lowest energy, the type-II SLs will be pseudodirect or indirect, respectively w2–6x. Photoluminescence ŽPL. spectroscopy was shown to be a powerful tool to distinguish between the pseudodirect and indirect samples w4x. Indirect energy gap structure for GaAsrAlAs SLs can be obtained

) Corresponding author. Tel.: q380-44-2656280; fax: q380-442656391. E-mail address: [email protected] ŽV.G. Litovchenko..

in two ways. For symmetrical SLs Ži.e. with equal well and barrier thickness., the transition from pseudodirect to indirect case occurs at a layer thickness F 0.85 nm Žthree monolayers. w4x. The energy gap for SLs with thin Ž; 2 nm. wells and rather thick Ž- 6 nm. barriers is also indirect because in this case the strain-induced shift exceeds the quantum-size effect w2,5x. In this paper, we report on low-temperature PL studies of a set of short-period SLs with different well and barrier thickness. The PL spectra consisting of many lines are analyzed with respect to energy position, full width at half maximum and relative intensity of each individual line. We discuss influence of the energy band structure, layer thickness and interface disorder on radiative transitions in these structures.

2. Experimental results ŽGaAs. nrŽAlAs. m SLs, where n and m denote the respective layer thickness expressed in monolayers ŽMLs., were grown by molecular beam epitaxy on semi-insulating Ž100.-GaAs substrates. The thickness of the GaAs wells was n s 1, 2, 3, 4, and 8 MLs and the thickness of the AlAs barriers was m s 1, 2, 3, 4, and 46 MLs Žone ML ˚ .. We will label the samples as nrm. corresponds to 2.83 A The samples were excited with the 515-nm line of an Arq-laser. The PL measurements were performed using a 0.5-m monochromator and a conventional lock-in technique for signal detection.

0928-4931r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 4 4 2 - 8

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Fig. 1. Schematic sketches illustrating direct Ža., pseudodirect Žb. and indirect Žc. transitions in ŽAlAs. nrŽGaAs. n superlattices. Thick lines represent potential energy run in the structure for G-points of conduction and valence bands, while thin solid lines represent the energy levels of the lowest electron and hole states.

Low-temperature PL spectra from the investigated SLs are shown in Fig. 2. The spectrum of the 4r4 SL is typical for pseudodirect SLs and exhibits an intensive zero-pho-

Fig. 2. PL spectra taken at 5 K of the indirect 1r1, 2r2, 3r3, and 8r46 SLs and the pseudodirect 4r4 SL. All spectra were normalized to the respective maximum.

non line and weak phonon-assisted sidebands at lower energy. Zero-phonon line originates from recombination of the excitons consisting of the X z electrons of AlAs and the G heavy holes of GaAs and will be discussed below. On the contrary, PL spectra of the 1r1, 2r2, 3r3 and 8r46 SLs are typical for indirect samples and are in agreement with earlier findings w4,5x. The shapes of the spectra are determined by superposition of several lines. To obtain the energy position, full width at half maximum and intensity of each individual line, all spectra were fitted with eight Gaussians. We will assign these lines using the PL spectrum of the 3r3 SL as an example ŽFig. 3.. First of all, it is seen that the spectra are greatly superimposed by a broad PL band Žlabeled as D. located at the low-energy side of the spectrum. Its nature is still unknown and may be presumably connected with the existence of a great density of deep states created by interface imperfections. The intensity of the line D is greatly enhanced at SL period decreasing. Another possible explanation can be the recombination of donor–acceptor pairs recently observed in pseudodirect SLs w7x. Investigations of temperature dependence of the PL intensity showed that this band completely dominates the spectrum at higher temperatures Žabove ; 20 K.. To analyze the rest PL lines, the influence of the D band must be properly taken into account. The PL line with the highest energy, I ex , originates from zero-phonon recombination of the X–G excitons. The next three lines, I 1 –I 3 , separated from the exciton line by 12, 29, and 45 meV are the phonon replicas connected, respectively, with zone-folded GaAs LA phonons and GaAs and AlAs interface phonons ŽIPs. w8x. The energy of the phonons remains constant Žwithin 1 meV accuracy. for all samples studied here. Weak features below the phonon

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Fig. 3. PL spectrum of the 3r3 superlattice at 5 K and its fit with eight Gaussians. Line labels are defined in the text.

sidebands, I 4 –I 6 , can be assigned, respectively, to the second phonon replicas of the GaAs IPs ŽI 4 , ; 59 meV., the AlAs IPs ŽI 6 , ; 92 meV. and superposition of the GaAs and AlAs IPs ŽI 5 , ; 74 meV.. It should be noted that these lines are not clearly resolved in all PL spectra but the best fits were obtained only if they were introduced.

Fig. 4. Energies of the actual electron-hole transitions in symmetric GaAsrAlAs SLs versus the well Žbarrier. thickness.

located at the X z valley of the Brillouin zone. The value of < M z < 2 becomes non-zero and grows rapidly at well and barrier thickness decrease ŽFig. 6.. Such effect takes place

3. Discussion As indicated earlier w9x, for symmetrical SLs Ž n s m. decrease of the SL period results in transition from direct energy gap configuration to pseudodirect, which occurs at n s 12 monolayers Žabout 3.5 nm.. In this case, the AlAs X z states with the wavevector parallel to the SL axis become the lowest conduction band states, while the X x, y and G states lie at higher energies due to smaller effective masses of electrons ŽFig. 4.. It seems that zero-phonon line in exciton luminescence originating from direct transitions has to disappear. For thin quantum layers, however, the wave functions of AlAs electrons and GaAs holes penetrate substantially in the neighboring layers due to the finite X and G band-offsets resulting in an increase of the electron and hole envelope wave functions overlapping as shown in Fig. 5. The probability of the direct dipole transitions from an electron state we,X z at the X z valley into a heavy hole G state characterized by the envelope wave function w h, G is proportional to the matrix element ) Ž . < M z < 2 , where M z s Hw h,G z we ,X zŽ z .expŽ ikz .d z, k is the wave vector of an electron-hole pair with an electron

Fig. 5. Overlapping of X z-electron and G-heavy-hole envelope wave functions in the case of GaAs ŽAlAs. layer thickness 0.7 nm.

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Fig. 6. Thickness dependence of the < Mz < 2 matrix element for direct dipole X z – G transition.

the same intensity as its LA-phonon satellite. In principle, appearance of the zero-phonon line becomes possible due to interface disorder Ždefects, corrugations, etc.., which provides a relaxation mechanism for momentum conservation in xy-plane. This situation is observed for the 8r46 SL. However, for n F 3 an additional effective mechanisms of the indirect zero-phonon recombination is possible. Due to smaller electron effective mass at the X x, y valley Ž0.19m 0 . as compared to the X z valley Ž1.1m 0 ., the width of the X x, y miniband is greatly increased and at n s 3 it overlaps the X z miniband. As a result, the energy of electrons in X x, y states can become equal or even somewhat lower than in X z states. We can thus assume that two pathways for exciton recombination now exist: Ži. indirect X x, y –G recombination which is responsible for the intensive phonon related lines and Žii. X x, y –G recombination due to interface defects Žas in the case of the 8r46 SL, but more intensive due to greater relative contribution of interfaces. or pseudodirect X z –G recombination, both contributing to the zero-phonon line.

4. Conclusions even in the case of single quantum well structures when no additional SL features are taken into account. It occurs simply due to the fact that electron and hole states become more spread and overlapped in the k-space, contributing thus to the direct dipole transitions. Physically, zero-phonon X z –G transition is possible due to the pulse transfer from an electron-hole pair to the quantum well structure as a whole. In SLs, the effect of minibands arises, leading to the folding of the X z valleys into the vicinity of the G point and, hence, to the equivalency of the X z and G points. In this case, the excess pulse is transferred directly to the SL itself. Moreover, due to the widening of the lowest G-miniband originating from GaAs and the X zminiband originating from the AlAs layers, they overlap at small SL periods, thus giving an additional X z –G mixing. All these effects contribute to increase of zero-phonon radiative transitions at SL period decrease. In the indirect SLs, the conduction band minimum is formed by the X x, y states of AlAs with wave vectors parallel to the basal plane of the SL. Since there is no mixing between X x, y and G electron states, the electronheavy hole transitions are strictly indirect and require momentum-conserving phonon participation. Really, the phonon related lines are the most intensive in the PL spectra. Despite this, zero-phonon line is also observable. The most prominent difference between the 8r46 SL and ultrashort-period samples Ž n F 3. is the relative intensity of the zero-phonon line. For the 8r46 SL, the zero-phonon line at 1.757 eV Žmarked by arrow in the inset in Fig. 2. is very weak and can be observed as a shoulder on the high-energy side of the spectrum. Contrary, zero-phonon line in the spectra of the ultrashort-period SLs has almost

We studied PL properties of short-period type-II GaAsrAlAs SLs, where pseudodirect and indirect electron-hole transitions can occur. It was shown that for the pseudodirect samples, penetration of electron and hole wave functions into neighboring layers results in an increased probability of direct Žwithout phonon assistance. recombination. This effect together with the X–G mixing due to the band folding leads to enhancement of the zero-phonon line. For the indirect SLs, zero-phonon recombination is observed due to transformation of the band structure and interface disorder. Another feature of the indirect samples is the broad low-energy PL band.

Acknowledgements We would like to thank Prof. H.T. Grahn and Prof. K. Ploog ŽPaul-Drude-Institut fur Berlin. ¨ Feskorperelektronik, ¨ for providing the samples. This work was supported in part by the Ministry for Education and Science of Ukraine.

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