Download Characterization of ultrashort-period GaAsrAlAs superlattices by exciton photoluminescence V.G. Litovchenko

Materials Science and Engineering C 19 Ž2002. 439–443
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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
440
V.G. LitoÕchenko et al.r Materials Science and Engineering C 19 (2002) 439–443
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
V.G. LitoÕchenko et al.r Materials Science and Engineering C 19 (2002) 439–443
441
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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V.G. LitoÕchenko et al.r Materials Science and Engineering C 19 (2002) 439–443
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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