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SiO2 superlattices
JOURNAL OF
Journal of Luminescence 52(1992) 335—343 North-Holland
Radiative recombination in short-period a-Si/Si02 superlattices A.V. Zayats, Yu.A. Repeyev, D.N. Nikogosyan, E.A. Vinogradov Institute of Spectroscopy, Troitzk, Moscow region, 142092, Russia Received 20 March 1991 Revised 26 November 1991 Accepted 23 December 1991
Photoluminescence and photoluminescence excitation spectra of short-period ( ~ 20 A) a-Si/Si02 superlattices have been investigated under Continuous and picosecond laser excitation. Three types of radiative transitions (intersubband luminescence in a-Si layers, cross-luminescence between states of well and barrier layers and impurity luminescence in a-SiO, layers) were observed in ps excited SL, as contrasted4 to times) continuous in the impurity excitationluminescence when only impurity intensityrecombination in a-Si0 in a-Si02 layers took place. A significant increase (by about i0 2 was observed in superlattices compared to that in isolated a-Si02 layers. It is caused by a transfer of excited carriers between SL layers. Two types of hot luminescence were observed at transitions between the higher subbands of conduction and valence bands of a-Si layers and between subbands of a-Si and a-Si02 impurity states in SLs. Relaxation of excited carriers between the subbands within the same band is low. No evidence was found of the influence of quantum size effects on deep impurity states in a-Si02. Fast nonradiative recombination in a-Si layers of SLs results in the absence of luminescence in these layers under continuous excitation.
1. Introduction The optical and electric properties of semiconductor superlattices (SLs) are of great interest [1—41.Quantum size effects which depend on the SL period give rise to appreciable changes in the spectra of electron states in such structures [4,51. The photoluminescence (FL) spectrum, luminescence intensity and the lifetime of excited carriers are determined by the quantum confinement as well as by the interaction between the electron states of the adjacent layers which form the SL [4]. Observation of luminescence caused by new electron states allows determination of their energy positions without additional assumptions which are required to obtain the same information from absorption spectra [6]. At the same
time the influence of quantum size effects on the energy state of defects in SL layers and radiative recombination with their participation have not been well studied. The reason is that in the majority of SLs (e.g. those based on 111—V cornpounds or a-Si H/SiN H, etc.) the difference between the band gaps of layers, which form the potential well and the potential barrier, is comparable with the values of these band gaps. So, impurity states in the layers, whose density is low compared to the density of conduction or valence band states, play an insignificant role in the determination of SL optical properties under these conditions. The situation differs drastically for a-Si/Si02 superlattices. The depth of the one-dimensional potential created in such superlattice by alternation of layers, i~E Eg(a-Si02) Eg(a-Si) 7 eV greatly =
Correspondence to: Dr. A.V. Zayats, Institute of Spectroscopy, Moscow Region, Troitzk, 142092, Russia. 0022-23l3/92/$05.00 © 1992
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—
exceeds the band gap of a-Si (‘~ 0.9 eV) which forms the potential well in the SL. Therefore, for
Elsevier Science Publishers B.V. All rights reserved
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/ Radiatii’e recombination
such SLs, impurity states whose density is low but whose energy position is close to the a-Si band states become of primary importance. Due to their low density the impurity states can be invisible against the background of a-Si interband transitions in absorption and in reflection spectra, but they can appear in luminescence, The magnitude of the potential modulation, ~E, in amorphous Si/Si02 superlattices exceeds by several times the value of defect formation energy in crystalline silicon and quartz. Therefore, under the crystallization of a-Si/Si02 SL with ultrathin layers, a new substance must appear whose crystalline structure will be determined by the SL one-dimensional potential [7,8]. As a result, investigations of the electron structure and the properties of short-period (~20 A) a-Si/Si02 superlattices attracts particular attention. The influence of quantum size effects in such SLs leads to the appearance of subbands (m 0, 1, 2...) of the conduction and valence bands of a-Si so that interband absorption transitions between the states of the same (~m 0) subbands of c- and v-bands occur [10]. This results in a pronounced “stair”-like structure observed in differential absorption spectra of the SLs investigated [8,9]. The mth step of the “stair” corresponds to the transition between the mth subbands. This observed structure confirms the presence of size quantization in a-Si/5i02 superlattices and gives the possibility of determining the energy positions of the m = 0, m = 1 and m = 2 subband edges, coincident with the energy positions of the “steps” that are visible in the spectra [8,9]. The present work investigates recombination in amorphous Si/Si02 superlattices in which the thickness of the a-Si02 layers is 4 monolayers and the thickness of a-Si varies from 2 to 8 monolayers. Investigation of FL spectra under continuous and pulsed excitation, and of photoluminescence excitation (PLE) spectra, yields information on the band and impurity electron structure of SLs and on the relaxation channels of the excited carries. Comparison of PLE spectra of different bands and SL absorption gives information on the interaction between SL layers. =
=
in short-period a-Si / SiO, superlattices
2. Experimental technique The SLs to be investigated (table 1) were prepared by RF magnetron sputtering of, in turn, amorphous Si02 and Si in spectrally pure argon onto silicon single crystal substrates with (1 0 0) or (1 1 1) orientation [7]. The thickness of the SL layers, and their periodicity and quality were checked by profile Auger spectroscopy and electron microscopy [7,8]. A a-Si02 film with 1000 A thickness was studied for comparison. These films were prepared by the same technique on the same substrates as SLs. Photoluminescence spectra of SLs were studied under continuous and pulse excitation. For continuous excitation an Ar~—Kr~ (the excitation wavelengths are 647, 514.5 and 488 nm) and a He—Cd (441 nm) laser were used. The measurements were made at room or liquid helium ternperatures. The FL and PLE spectra with picosecond laser excitation were recorded as follows. Second harmonic pulses (532 nm, 10 mJ, 37 ±2 ps) of a passively mode-locked Nd : YAG laser were directed into a generator of parametric superluminescence (PSL) consisting of two DKDP crystals cut in the direction Q = 45°, J = 0°. For PSL generation three-wave interaction of the e—oe type was used [11]. With synchronous rotation of both DKDP crystals (the range of rotation angles was = 26°) the wavelength of the generated PSL with ordinary polarization fell within the limits 740—1200 nm and that with extraordinary polarization within the limits 960—1890 nm. The pulse energy of the ordinary wave over the range of 800—1000 nm was about I mJ. For second harmonic generation of the PSL radiation a set of
Table 1 Parameters of amorphous Si/Si02 superlattices No a-Si layer a-Si02 layer Total a-SL thickness thickness thickness
2 3 4
2.7 A 5.5 A 8.2A 11
A
10 A 10 A IOA
1143 A 775 A 910A
10 A
1050 A
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/
Radiatii’e recombination in short-period a-Si / Si0
7 super/a ttices
Li103 crystals cut at angles ranging from 20°to 45° was used. Thus, picosecond (~20 ps) light pulses were generated whose wavelength could vary continuously from 370 to 1890 nm. The energy of the pulses for the wavelengths most often used (450 and 580 nm) was 0.1 mJ. For spectral analysis a MDR-3 diffraction monochromator (LOMO) was used. For observation of PL over various spectral ranges different photomultipliers were used. Under pulse excitation the PL signal was averaged over 20 pulses whose intensity lay within the preset range (only those pulses were taken into account for which the pumping energy of the parametric generator was 9—il rnJ). The FL excitation spectra were normalized with allowance for the spectral dependence of the energy of the exciting light pulses.
3. Experimental results 3.1. Luminescence under continuous excitation Photoluminescence spectra of SLs under continuous excitation are shown in fig. 1. In other spectral ranges up to 1.1 eV no luminescence was observed under continuous excitation. At both room and liquid helium temperatures broad structureless FL bands appear. The luminescence intensity depends weakly on the temperature of
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3
to
a
________________________
0
1
.~
1 • 6 1 •8 2•0 Photon energy (eV)
Fig. 2. Photoluminescence spectra of SLs under picosecond excitation. SL-l at ~ = 580 nm (1), SL-2 at = 600 nm (2), SL-3 at Aex = 580 nm (3). T 300 K. The arrows show the position of the subband m = 0 in SLs
191.
the samples: on cooling down from 300 to 4.2 K it increases 3—4 times. Redistribution of the shape of a PL band in this case is negligible though the relative intensity of the long- and short-wave shoulders decreases. The FL spectrum depends on the wavelength of the exciting light. Under 514.5 nm light excitation FL intensity of all SLs decreases 2—3 times compared to 488 nm excitation. Under excitation by light with shorter (330 nm) and longer (647 nm) wavelengths no luminescence was observed. As the absorption coefficients at these wavelengths differ only slightly [9], the PL intensity dependence on Aex cannot be attributed to variation in absorption. PL spectra of superlattices are analogous to PL spectra of fused quartz in this spectral region (fig. 1) but the luminescence intensity in SLs is about i0~times higher. 3.2. Luminescence under picosecond excitation
w
~ ~ 1
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I
9
2. 1 2. Photon energy (eV) Fig. 1. Photoluminescence spectra of SLs under continuous excitation A~ 488 nm. SL-1 at T = 4.2 K (dot-and-dash line); SL-4 at T = 300 K (solid line 1) and T = 4.2 K (solid line 2); a-SiO, at T = 300 K (dash line, intensity >< i0~).
In contrast to continuous excitation, the PL spectra of SLs under picosecond excitation are of considerable interest (figs. 2 and 3). It should be noted that the possibility of observing FL bands strongly depends on the energy of exciting quanta. Under long-wave (580 nm) excitation a number of PL bands are observed (fig. 2) whose spectral positions correspond well to the energy distance between the nearest subbands (m = 0) arising
338
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/ Radiatice recombination in short-period a-Si / Si02
cence and for SL-3 a broad new line near 2.9 eV is observed whose nature will be clarified below.
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super/a ttices
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3.3. Excitation of photoluminescence
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lines can be observed only for certain energies of
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1
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Fig. 3. Photoluminescence spectra of SLs under picosecond excitation A0.’ = 450 nm (solid line) and 370 nm (dashed line) for SL-1 (1), SL-2 (2), SL-3 (3) and a-Si02 (4, intensity X 10k). T = 300 K, The arrows show the position of the subband m = 1 in SLs [9].
due to spatial quantization in the conduction and valence band of a-Si layers [9]. As the SL period decreases, these PL lines move in the long-wave direction with respect to the position of the subband edge. PL bands at 1.74 and 1.96 eV (fig. 2) are also observed in the energy range of the first subband for SL-2 and SL-3 where no peculiarities in the absorption spectrum [8,9] were detected. A change of excitation wavelength to 450 nm (near ,k~.’ of the continuous excitation 480) nm leads to the disappearance of all the previously observed bands (fig. 3, solid). The FL spectrum in this case becomes similar to the luminescence spectrum obtained under continuous excitation but in contrast to it, additional FL lines are observed besides the luminescence bands of fused quartz. For the SL-2 sample FL bands were observed near the edge of and inside the second (m = 1) subband (analogous to the first subband in fig. 2). For SL-3 the FL band near the edge of the second subband is considerably shifted to longer wavelength relative to the PL due to the first subband. In the FL spectrum of SLs 1 and 3 the a-quartz band 2.1 eV has a short-wave branch. At still shorter-wave (370 nm) excitation (fig. 3, dashed) in the spectrum of SL-1 there is only one broad FL band near the edge of the second subband of a-Si, the sample SL-2 has no lumines-
showninterband in section 3.2, in some luminescence theAs tors excitation under light excitation bulk allinvestigate PL semiconduclines are seen). Therefore it is(whereas interesting to the FL excitation spectra. It turns out that those luminescence lines, whose spectral positions correlate with those of the edges of the nearest a-Si subbands, are cxcited by the light absorbed at the electronic transitions between these subbands (fig. 4; PLE spectra for SL-2 are not shown as their behavior is absolutely analogous to that described above). As the excitation energy increases, the absorption due to transitions between the next subbands (m = 1) begins and luminescence associated with transitions between m = 0 subbands disappears, but, simultaneously, other FL bands appear whose positions are determined by the edge of subbands m = 1. A further decrease in the excitation wavelength leads to repetition of the situation when the edge of the next subband m = 2 is reached.
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Fig. 4. Photoluminescence excitation spectra: (a) for SL-1 at 1.83 eV (1) and at 2.78 eV (2); (b) for SL-3 at 1.42 eV (1), at 1.96 eV (2) and at 1.74 eV (3). T=300 K. The arrows show the positions of the subbands in SLs [9].
/ Radiatite
A. V Zayats et a!.
recombination in short-period a-Si / 5i02 super/a ttices
;i~I
339
relation between the PLE intensities corresponds to that observed for the FL intensity of SLs and of a-SiO2.
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Photon energy (eV) Fig. 5. Photoluminescence excitation spectra at 2.1 eV for SL-1 (solid line), SL-3 (dash line) and a-Si02 (dot-and-dash line, intensity X I0~).T = 300 K.
Investigation of the excitation (fig. 4) of the FL lines at 1.96 eV in SL-2 and 1.74 eV in SL-3, whose spectral positions are in the energy range of the m = 0 SL subbands and which do not correlate with the subband edges (fig. 2), shows that these PL lines are excited both in the Stokes and in anti-Stokes range, and that the long- and short-wavelength boundaries of the PLE spectra are determined by the edges of the m = 0 subbands. The PL lines within the energy range of the second subband (m = 1) do not show antiStokes excitation and the short-wave edge of cxcitation spectra coincides with the edge of the m = 1 SL subband. Figure 5 shows the PLE spectra of the band at 2.1 eV observed both in SLs and in isolated a-Si02 layers under picosecond and continuous excitation. In fused quartz this line has its excitation peak far away from the luminescent transitions and it is not resonantly excited. The shape of the excitation spectrum of this PL in SL-3 looks similar to that of the previous one in the short-wave spectral region but excitation near the PL line is also observed. For SL-l the excitation spectrum is analogous rather to PLE of the bands considered above; it has an anti-Stokes component and is determined by absorptive transitions between the states of the m = 0 subband. The
4. Discussion Investigation of luminescence spectra and PL excitation in amorphous Si/SiO2 superlattices and in isolated layers of fused quartz, and their comparison with the spectrum of SL electron states makes it possible to divide all the observed FL bands into three groups: FL lines observed both in SLs and in isolated a-SiO layers, FL .
lines whose spectral positions correspond to the edges of subbands in a-Si layers, and PL lines whose energy positions are inside these subbands. After excitation of an electron the following processes of energy relaxation are possible in a SL: thermalization by emission of phonons in the subband in which the electron is excited [12]; thermalization between subbands due to the emission of phonons as well as of light [13]; a transition of excited carriers into the adjacent SL layer and further relaxation in this layer [4]; and, finally, radiative and nonradiative recombination. If no transitions between neighbouring layers occurs, the luminescence excitation spectrum reflects the absorption spectrum of a SL. Thus, the observed PL and PLE spectra are determined by the time required for leaving the layer or nonradiative capture of the carriers in the SL layer relative to the time necessary for relaxation (intra- and intersubband) to the energy at which luminescence occurs.
4.1. Subband-to-subband recombination PL lines whose spectral positions correspond to the energy distances between the edges of a-Si subbands are observed not only for the nearest (m = 0) but also for the higher (m = I and 2) subbands in the SLs investigated. Usually, due to thermalization of excited carriers, only the lower states of the electronic spectrum show up in luminescence. As follows from fig. 4, the lumines-
340
A. V Zayats et a!.
/
Radiatite recombination in short-period a-Si / SiO, super/a itices
cence at the mth subband edge occurs only if carriers are excited directly to the same mth subbands (between which the FL transition occurs) of the conduction and valence bands. When carriers are excited to other, nth, n ~ m subbands the luminescence at the mth subband edge is not observed. These facts are evidence for slow relaxation between different subbands. Such low intersubband relaxation is probably caused by the large energy difference between adjacent Am = 1 subbands of the conduction and valence bands in the SLs investigated. If this difference increases, the relaxation rate due to LO-phonon assisted processes decreases [17]. PL spectral positions (near the edges of subbands), and their PLE dependences (excitation within its own subband), which are analogous for all these FL bands in all SLs investigated, gives us the opportunity to attribute these PL lines to subband-to-subband recombination. It is the low intersubband relaxation of excited carriers in aSi/Si02 SLs that results in observation of hot luminescence due to higher subbands of well layers. Both hot electrons and hot holes simultaneously take part in the recombination. Apparently, the short-wave tail in the PLE spectrum of the line at 1.96 eV caused by the m = 1 subbands in SL-3 is determined, analogous to the excitation spectra of subband-to-subband transitions m = 0 and 1 in SL-l, 2, and 3, by the edge of the next subband m = 2 in SL-3. This correlates with the appearance of a 2.9 eV line in the PL spectrum of SL-3. This line appears under excitation above this edge (fig. 3). Thus, this PL line can be attributed to transitions from the m = 2 a-Si subband in SL-3. The position of this subband has not been determined before in absorption studies as it lies outside their spectral range [9]. Excitation spectra of the FL lines in the region where electron processes related to impurities in a-Si02 layers are important (see figs. 3, 4 and 5), are influenced by transitions of electrons between band states of the a-Si layers and impurity states of the a-Si02 layers, resulting in a PLE spectrum which deviate from the SL absorption spectrum. At the same time, in the spectral range where this transition does not occur (<2.0 eV) the PLE
spectrum reflects the absorption spectrum of transitions between the a-Si subbands. The FL bands discussed have long-wave tails and are of greater width (—~0.1 eV) that the thermal width expected for “ideal” subband-tosubband transitions. This can be due to the participation of localized states in the subband tail in recombination. As the SL period decreases, the long-wave tails of these FL bands increase, apparently due to increasing density of localized states in the tails of permitted bands when silicon layers are disordered in short-period SLs [8,14]. 4.2. Impurity luminescence in a-Si07 in SLs The PL line at 2.1 eV which appears under continuous and pulse excitation in isolated a-SiO2 layers is considered to be caused by background Ge impurities in fused quartz [15]. Excitation of this luminescence in isolated a-Si02 layers occurs with low efficiency as the energy of excitation quanta is less than one third the a-Si02 band gap and only transitions between impurity states can be excited. The fact that this FL is not resonantly excited in a-Si02 and the FLE peak is shifted by about 0.7 eV to the short-wave side speaks in favor of a three-level scheme of transitions: only the lowest of the three levels is filled in the absence of excitation. Explanation of other specific features of the FL and FLE spectra of SLs (e.g. the presence of an anti-Stokes shift in FLE of some bands) also requires a three-level scheme. As pointed out above, the deviation of the PLE spectrum of subband-to-subband luminescence in a-Si layers from the absorption spectrum is caused by electron transfer between a-Si and a-Si02 layers. The PL intensity (the 2.1 eV FL intensity is iO~ times greater in SLs than in isolated a-Si02 layers, showing the increase of excitation efficiency in SLs) and the PLE spectral dependence (the fact that the short-wavelength boundaries of the PLE spectra of impurity luminescence of a-Si02 in SLs is determined by the subband edges of a-Si, the deviation of the PLE spectra of subband-to-subband FL from the absorption spectra, and the differences between the PLE spectra in SLs and in isolated a-SiO., layers) show that the excitation light is absorbed in a-Si
A. V Zayats ci a!.
/ Radiative recombination
~j/Si02
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2 superlattices
341
available. Due to the absence of relaxation of excited carriers between the various a-Si subbands, the conditions for transfer are no longer fulfilled when absorption with the participation
~ I
in short-period a-Si / Si0
b
c
Fig. 6. Scheme of electron states and radiative transitions in SL-1 (a), SL-2 (b), and SL-3 (c). Discontinuity of c- and v-bands of a-Si and a-Si02 are assumed to be independent of the SL period. The position of the edge of subbands of a-Si layers is taken from ref. 191, the edge of the subband m = 2 for SL-3 (dash line) is determined in the present paper. Horizontal arrows show transfer of electrons between SL layers.
layers and excited carriers then transfer to impurity states in the adjacent a-Si02 layers, recombining (the inverse effect to resonant transitions of carriers between the band states of different layers of the GaAs/AlGaAs superlattices [4,16]). The probability of such a transition between the states of adjacent layers in a superlattice depends on their mutual energy position and, therefore, it varies with varying thicknesses of layers due to size quantization. As the excitation wavelength varies, the initial energy of carriers in the subbands changes, affecting the resonance transition condition [4], so that the FL intensity changes as well, In SL-1 the impurity PL is excited in the entire first subband of a-Si band states (fig. 5). The impurity level of a-Si02 is close to the bottom of the c-band subband in a-Si (fig. 6). Consequently, excitation of an electron into this subband, and its subsequent thermalization, give it the correct energy for resonant transfer to an impurity state in the adjacent layer of a-Si. The radiative transition then takes place to a vacant impurity state. The presence of an anti-Stokes component in the PLE spectrum confirms that these states are
of the next subband begins, and the impurity FL does not manifest itself in the spectra. The presence of resonant PLE and of the specific feature near 2.4 eV in the excitation spectrum is due to the fact that at these photon energies an increase in the depopulation rate of the lower impurity states after recombination occurs due to resonant transfer of holes from the neighbouring a-Si layers and due to absorptive transitions from this level to c-subbands m = 0 and m = 1. Excitation due to transitions connected with the impurity absorption in a-SiO2 does not show up in SL-i, since absorption in a-Si layers is many orders of magnitude stronger, and the m = 1 subband-tosubband PL is resonantly excited; this is the rcason for the step near 2.6 eV in the PLE spectrum .
(see figs. 3 and 5). In contrast to this, in the spectrum of the a-Si states in SL-3 no specific features are observed near the energy of impurity absorption in a-5i02 layers, and they are seen in the PLE spectrum owing to their role in deactivation of the lower recombination level. In SL-3 the upper impurity level in a-SiO2 is populated via the states of the m = 1 subband of the a-Si conduction band (fig. 6) and this determines the shape of the FLE spectrum. As seen from the comparison of FL and PLE spectra of fused quartz and SLs, positions of the impurity transitions are independent of the material in which they are observed. Consequently, impurity centers in a-SiO2 layers are not influenced by the SL potential even in such shortperiod SLs. This is, apparently, due to the fact that energy states of deep defects are mainly determined by the short-range potential and, owing to considerable localization of the electron wave function, the states of such centres are not affected by the potential profile. 4.3. Subband-to-impurity recombination The PLE spectra of the third type of luminescence bands, situated between the bands of subband-to-subband recombination, are similar to
342
A.V Zayats eta!.
/ Radiative recombination in short-period a-Si/SiO,
those of both impurity and subband-to-subband recombination. The presence of an anti-Stokes component in the excitation spectrum, as for the a-Si02 impurity luminescence, allowing to relate these radiative transitions to impurity bands of a-Si02 which are not filled in equilibrium. On the other hand, such PL is observed for a number of a-Si subbands and the character of the FL excitation is analogous to the subband-to-subband luminescence (see section 4.1). Excitation anywhere within the whole mth subband absorption range leads to this type of luminescence within the mth subband. This fact confirms the participation of a-Si subband states (m = 0, 1 ...) in this luminescence. The above scheme of levels (fig. 6) allows one to explain this PL by transitions of electrons from different conduction band subbands of a-Si to impurity states in the a-Si02 layers. The validity of the model proposed is confirmed by the fact that the lines of such cross-luminescence, which at first sight are absent from the PL spectra for the 1st and 2nd subbands of SL-I and for the second subband of SL-3, according to the scheme in fig. 6, must lie near the line of the impurity PL. In the FL spectrum of SL-1 a peculiarity at 2.15 eV is clearly seen, which may correspond to this cross-recomiination. Broad PL bands at 2.1 eV in SL-3 and at 2.8 eV in SL-1 are, apparently, due to superposition of crossluminescence lines from the second SL subband and impurity FL in SL-3 and subband-to-subband PL SL-1, respectively. This type of hot luminescence differs from the subband-to-subband hot FL because only hot electrons participate in cross-luminescence, while both hot electrons and hot holes are needed for subband-to-subband hot PL. As a result of rapid nonradiative recombination in a-Si layers caused, apparently, by the states arising from disorder in the layers [8], only luminescence associated with the states of a-Si05 in the SL shows up under continuous excitation. The smaller influence of nonradiative relaxation of carriers in a-Si layers under picosecond excitation makes it possible to observe subband-to-subband and cross-luminescence associated with the states of subbands of a-Si layers. The proposed electron state scheme in SLs explains the observed shifts of lines caused by
superlattices
subband-to-subband transitions in a-Si layers with respect to the position of the edge of these subbands (figs. 2 and 3). Apparently, this is caused by interaction of the states of carriers in different layers of the SLs. The a-Si subbands m = 0 in SL-3 and m = 1 in SL-1 and SL-2 are far in energy from the impurity states of a-Si02 layers, and the shift of FL lines associated with these subbands is negligible. As the energy positions of the edges of subbands of the c- or v-band and of the impurity come closer, the interaction between the states of both layers leads to a shift of the subband edge (without a shift of the impurity level, as shown above) to lower energies. This shift is larger due to interaction with the states of the valence band than with those of the conduction band. The shifts observed confirm that the two upper impurity states in a-Si02 are empty in equilibrium. In the absorption spectra the position subband edge is different from that in luminescence whenever the presence of nonequilibrium carriers causes a shift in the subband edge due to interaction with an impurity level near the edge.
5. Conclusions Investigation of luminescence and its excitation in short-period (<20 A) amorphous Si/Si02 superlattices shows the presence of three types of radiative transitions: those between the subbands of the conduction and valence bands which arise due to quantum size effect in the a-Si layers, those between impurity states in a-Si02 layers, and those between subbands of the a-Si conduction band and impurity levels in a-Si07 (crossluminescence). This directly confirms the presence of subbands of spatial quantization in a-Si layers in SLs which are due to the two-dimensional character of electron states in these layers. For one of the superlattices (SL-3) the energy position of the m = 2 subband, E2 = 2.9 eV, has been found, which could not be obtained when absorption was investigated. It turned out that, in contrast to the well-investigated SLs based on Ill—V and other cornpounds, in a-Si/Si02 SLs relaxation of excited
A. V. Zayats ci a!.
/ Radiative recombination
carriers between various subbands of c- and vbands is slow. This allows observation of two types of hot PL caused by higher subbands in a-Si layers: subband-to-subband hot luminescence with participation of both hot electrons and hot holes, and hot cross-luminescence in which only hot electrons take part. In the SLs investigated the electrons excited in a-Si layers can transfer to adjacent a-Si02 layers, either with photon emission under recombination at impurity levels of fused quartz, or by resonant transfer of electrons to neighbouring impurity states of a-SiO-, with subsequent recombination in these layers. Excitation of impurity transitions in a-Si02 layers via the states of the subbands of a-Si layers in SLs leads to an increase in intensity of this impurity luminescence by about iO~times compared to isolated a-Si02 layers. Comparison of FL and PLE spectra of a-Si/5i02 superlattices and thick a-5i02 layers show that quantum size effects do not influence the energy states of deep defects in a-Si02 layers even for such shortperiod SLs. Rapid nonradiative recombination of excited carriers in a-Si layers even at low temperatures (down to 4.2 K) leads to the absence of luminescence in these layers under continuous excitation when radiative transitions occur only in a-Si02 layers in SLs. The decreasing influence of nonradiative transitions under picosecond excitation permits observation of FL related with the states of size quantization of a-Si layers in SLs: subband-to-subband PL and cross-luminescence. On the basis of PL, PLE and absorption investigations a scheme of electronic states of aSi/SiO2 superlattices is proposed which allows a satisfactory explanation from the unified standpoint of the specific observed features of the spectra.
in short-period a-Si / SiO, super/a itices
343
References [11M.
Jaros, Physics and applications of semiconductor microstructures (Oxford Univ. Press, 1989). [21M. Hirose and S. Miyazaki, J. Non-Cryst. Solids 66(1988)
327. 131 L.J. Sham and Y.-T. Lu, J. Lumin. 44 (1989) 207. [41Y. Murayama, Phys. Rev. B 36 (1986) 2500. [51B. Abeles and T. Tiedje, Phys. Rev. Lett. 51(1983) 2003.
[6] To obtain information about band states from absorption spectra the absorption should be treated within the model assumptions such as type of electron transition, conservation or nonconservation of wave vector, constant momentum or dipole matrix element. The choice between the absorption models is ambiguous in many cases and resuIts in the different energy position of band states (A. Frova, in: Tetrahedrally-Bonded Amorphous Semiconductors, ed. D. Adler (Plenum, New York, 1985), and ref.
1101). [7] A.F.
Plotnikov, F.A. Pudonin and yB. Stopachinsky, Soy. Phys. JETP Lett. 46 (1987) 443. [8] E.A. Vinogradov, V.N. Denisov, A.V. Zayats, B.N. Mavrin, G.I. Makarov, SR. Oktyabrsky and F.A. Pudonin, in: Optoelectronics and Spectroscopy in Semiconductors and Related Materials, eds. S.C. Shen et al. (World Scientific, Singapore, 1990) p. 121.
[91E.A.
Vinogradov, A.V. Zayats and F.A. Pudonin, Soy. Phys. Solids 33 (1991) 197. [101 K. Hattori, T. Mori, H. Okamoto and Y. 1-lamakawa, Phys. Rev. Lett. 60 (1988) 825. [11] PG. Krukov, Yu.A. Matveyets, D.N. Nikogosyan and A.V. Sharkov, Soy. J. Quantum Electronics 8(1978)1319. [12] J. Shah, A. Pinczuk, AC. Gossard and W. Wiegmann, Phys. Rev. Lett. 54 (1985) 2045. [13] E.A. Imhoff, M.I. Bell and R.A. Horman, Solid State Commun 54 (1985) 845. [14] E.A. Vinogradov, V.N. Denisov, B.N. Mavrin and F.A. Pudonin, Soy. Phys. Solids 32 (1990) 2174. [15] A.A. Gordeev and A.P. Gorchakov, Soy. Phys. Opt. 66 (1989) 662. [161 AR. Bonnefoi, T.C. McGill and RD. Burnham, Phys. Rev. B 37 (1988) 8754. [17] J.A. Levenson, G. Doligue, J.A. Oudar and 1. Abram, Phys. Rev. B 41(1990) 3688.
Journal of Luminescence 52(1992) 335—343 North-Holland
Radiative recombination in short-period a-Si/Si02 superlattices A.V. Zayats, Yu.A. Repeyev, D.N. Nikogosyan, E.A. Vinogradov Institute of Spectroscopy, Troitzk, Moscow region, 142092, Russia Received 20 March 1991 Revised 26 November 1991 Accepted 23 December 1991
Photoluminescence and photoluminescence excitation spectra of short-period ( ~ 20 A) a-Si/Si02 superlattices have been investigated under Continuous and picosecond laser excitation. Three types of radiative transitions (intersubband luminescence in a-Si layers, cross-luminescence between states of well and barrier layers and impurity luminescence in a-SiO, layers) were observed in ps excited SL, as contrasted4 to times) continuous in the impurity excitationluminescence when only impurity intensityrecombination in a-Si0 in a-Si02 layers took place. A significant increase (by about i0 2 was observed in superlattices compared to that in isolated a-Si02 layers. It is caused by a transfer of excited carriers between SL layers. Two types of hot luminescence were observed at transitions between the higher subbands of conduction and valence bands of a-Si layers and between subbands of a-Si and a-Si02 impurity states in SLs. Relaxation of excited carriers between the subbands within the same band is low. No evidence was found of the influence of quantum size effects on deep impurity states in a-Si02. Fast nonradiative recombination in a-Si layers of SLs results in the absence of luminescence in these layers under continuous excitation.
1. Introduction The optical and electric properties of semiconductor superlattices (SLs) are of great interest [1—41.Quantum size effects which depend on the SL period give rise to appreciable changes in the spectra of electron states in such structures [4,51. The photoluminescence (FL) spectrum, luminescence intensity and the lifetime of excited carriers are determined by the quantum confinement as well as by the interaction between the electron states of the adjacent layers which form the SL [4]. Observation of luminescence caused by new electron states allows determination of their energy positions without additional assumptions which are required to obtain the same information from absorption spectra [6]. At the same
time the influence of quantum size effects on the energy state of defects in SL layers and radiative recombination with their participation have not been well studied. The reason is that in the majority of SLs (e.g. those based on 111—V cornpounds or a-Si H/SiN H, etc.) the difference between the band gaps of layers, which form the potential well and the potential barrier, is comparable with the values of these band gaps. So, impurity states in the layers, whose density is low compared to the density of conduction or valence band states, play an insignificant role in the determination of SL optical properties under these conditions. The situation differs drastically for a-Si/Si02 superlattices. The depth of the one-dimensional potential created in such superlattice by alternation of layers, i~E Eg(a-Si02) Eg(a-Si) 7 eV greatly =
Correspondence to: Dr. A.V. Zayats, Institute of Spectroscopy, Moscow Region, Troitzk, 142092, Russia. 0022-23l3/92/$05.00 © 1992
—
—
exceeds the band gap of a-Si (‘~ 0.9 eV) which forms the potential well in the SL. Therefore, for
Elsevier Science Publishers B.V. All rights reserved
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/ Radiatii’e recombination
such SLs, impurity states whose density is low but whose energy position is close to the a-Si band states become of primary importance. Due to their low density the impurity states can be invisible against the background of a-Si interband transitions in absorption and in reflection spectra, but they can appear in luminescence, The magnitude of the potential modulation, ~E, in amorphous Si/Si02 superlattices exceeds by several times the value of defect formation energy in crystalline silicon and quartz. Therefore, under the crystallization of a-Si/Si02 SL with ultrathin layers, a new substance must appear whose crystalline structure will be determined by the SL one-dimensional potential [7,8]. As a result, investigations of the electron structure and the properties of short-period (~20 A) a-Si/Si02 superlattices attracts particular attention. The influence of quantum size effects in such SLs leads to the appearance of subbands (m 0, 1, 2...) of the conduction and valence bands of a-Si so that interband absorption transitions between the states of the same (~m 0) subbands of c- and v-bands occur [10]. This results in a pronounced “stair”-like structure observed in differential absorption spectra of the SLs investigated [8,9]. The mth step of the “stair” corresponds to the transition between the mth subbands. This observed structure confirms the presence of size quantization in a-Si/5i02 superlattices and gives the possibility of determining the energy positions of the m = 0, m = 1 and m = 2 subband edges, coincident with the energy positions of the “steps” that are visible in the spectra [8,9]. The present work investigates recombination in amorphous Si/Si02 superlattices in which the thickness of the a-Si02 layers is 4 monolayers and the thickness of a-Si varies from 2 to 8 monolayers. Investigation of FL spectra under continuous and pulsed excitation, and of photoluminescence excitation (PLE) spectra, yields information on the band and impurity electron structure of SLs and on the relaxation channels of the excited carries. Comparison of PLE spectra of different bands and SL absorption gives information on the interaction between SL layers. =
=
in short-period a-Si / SiO, superlattices
2. Experimental technique The SLs to be investigated (table 1) were prepared by RF magnetron sputtering of, in turn, amorphous Si02 and Si in spectrally pure argon onto silicon single crystal substrates with (1 0 0) or (1 1 1) orientation [7]. The thickness of the SL layers, and their periodicity and quality were checked by profile Auger spectroscopy and electron microscopy [7,8]. A a-Si02 film with 1000 A thickness was studied for comparison. These films were prepared by the same technique on the same substrates as SLs. Photoluminescence spectra of SLs were studied under continuous and pulse excitation. For continuous excitation an Ar~—Kr~ (the excitation wavelengths are 647, 514.5 and 488 nm) and a He—Cd (441 nm) laser were used. The measurements were made at room or liquid helium ternperatures. The FL and PLE spectra with picosecond laser excitation were recorded as follows. Second harmonic pulses (532 nm, 10 mJ, 37 ±2 ps) of a passively mode-locked Nd : YAG laser were directed into a generator of parametric superluminescence (PSL) consisting of two DKDP crystals cut in the direction Q = 45°, J = 0°. For PSL generation three-wave interaction of the e—oe type was used [11]. With synchronous rotation of both DKDP crystals (the range of rotation angles was = 26°) the wavelength of the generated PSL with ordinary polarization fell within the limits 740—1200 nm and that with extraordinary polarization within the limits 960—1890 nm. The pulse energy of the ordinary wave over the range of 800—1000 nm was about I mJ. For second harmonic generation of the PSL radiation a set of
Table 1 Parameters of amorphous Si/Si02 superlattices No a-Si layer a-Si02 layer Total a-SL thickness thickness thickness
2 3 4
2.7 A 5.5 A 8.2A 11
A
10 A 10 A IOA
1143 A 775 A 910A
10 A
1050 A
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Radiatii’e recombination in short-period a-Si / Si0
7 super/a ttices
Li103 crystals cut at angles ranging from 20°to 45° was used. Thus, picosecond (~20 ps) light pulses were generated whose wavelength could vary continuously from 370 to 1890 nm. The energy of the pulses for the wavelengths most often used (450 and 580 nm) was 0.1 mJ. For spectral analysis a MDR-3 diffraction monochromator (LOMO) was used. For observation of PL over various spectral ranges different photomultipliers were used. Under pulse excitation the PL signal was averaged over 20 pulses whose intensity lay within the preset range (only those pulses were taken into account for which the pumping energy of the parametric generator was 9—il rnJ). The FL excitation spectra were normalized with allowance for the spectral dependence of the energy of the exciting light pulses.
3. Experimental results 3.1. Luminescence under continuous excitation Photoluminescence spectra of SLs under continuous excitation are shown in fig. 1. In other spectral ranges up to 1.1 eV no luminescence was observed under continuous excitation. At both room and liquid helium temperatures broad structureless FL bands appear. The luminescence intensity depends weakly on the temperature of
(0 .,-1
O
i”~~ I
~ ——-..
337
3
to
a
________________________
0
1
.~
1 • 6 1 •8 2•0 Photon energy (eV)
Fig. 2. Photoluminescence spectra of SLs under picosecond excitation. SL-l at ~ = 580 nm (1), SL-2 at = 600 nm (2), SL-3 at Aex = 580 nm (3). T 300 K. The arrows show the position of the subband m = 0 in SLs
191.
the samples: on cooling down from 300 to 4.2 K it increases 3—4 times. Redistribution of the shape of a PL band in this case is negligible though the relative intensity of the long- and short-wave shoulders decreases. The FL spectrum depends on the wavelength of the exciting light. Under 514.5 nm light excitation FL intensity of all SLs decreases 2—3 times compared to 488 nm excitation. Under excitation by light with shorter (330 nm) and longer (647 nm) wavelengths no luminescence was observed. As the absorption coefficients at these wavelengths differ only slightly [9], the PL intensity dependence on Aex cannot be attributed to variation in absorption. PL spectra of superlattices are analogous to PL spectra of fused quartz in this spectral region (fig. 1) but the luminescence intensity in SLs is about i0~times higher. 3.2. Luminescence under picosecond excitation
w
~ ~ 1
I .
I
9
2. 1 2. Photon energy (eV) Fig. 1. Photoluminescence spectra of SLs under continuous excitation A~ 488 nm. SL-1 at T = 4.2 K (dot-and-dash line); SL-4 at T = 300 K (solid line 1) and T = 4.2 K (solid line 2); a-SiO, at T = 300 K (dash line, intensity >< i0~).
In contrast to continuous excitation, the PL spectra of SLs under picosecond excitation are of considerable interest (figs. 2 and 3). It should be noted that the possibility of observing FL bands strongly depends on the energy of exciting quanta. Under long-wave (580 nm) excitation a number of PL bands are observed (fig. 2) whose spectral positions correspond well to the energy distance between the nearest subbands (m = 0) arising
338
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/ Radiatice recombination in short-period a-Si / Si02
cence and for SL-3 a broad new line near 2.9 eV is observed whose nature will be clarified below.
I
Co
I
“-4
super/a ttices
0:3 _
3.3. Excitation of photoluminescence
a p.’ 4.’
lines can be observed only for certain energies of
o
(0 O C)
•I” I
1
.9
I
2.3 2.7 Photon energy (eV)
Fig. 3. Photoluminescence spectra of SLs under picosecond excitation A0.’ = 450 nm (solid line) and 370 nm (dashed line) for SL-1 (1), SL-2 (2), SL-3 (3) and a-Si02 (4, intensity X 10k). T = 300 K, The arrows show the position of the subband m = 1 in SLs [9].
due to spatial quantization in the conduction and valence band of a-Si layers [9]. As the SL period decreases, these PL lines move in the long-wave direction with respect to the position of the subband edge. PL bands at 1.74 and 1.96 eV (fig. 2) are also observed in the energy range of the first subband for SL-2 and SL-3 where no peculiarities in the absorption spectrum [8,9] were detected. A change of excitation wavelength to 450 nm (near ,k~.’ of the continuous excitation 480) nm leads to the disappearance of all the previously observed bands (fig. 3, solid). The FL spectrum in this case becomes similar to the luminescence spectrum obtained under continuous excitation but in contrast to it, additional FL lines are observed besides the luminescence bands of fused quartz. For the SL-2 sample FL bands were observed near the edge of and inside the second (m = 1) subband (analogous to the first subband in fig. 2). For SL-3 the FL band near the edge of the second subband is considerably shifted to longer wavelength relative to the PL due to the first subband. In the FL spectrum of SLs 1 and 3 the a-quartz band 2.1 eV has a short-wave branch. At still shorter-wave (370 nm) excitation (fig. 3, dashed) in the spectrum of SL-1 there is only one broad FL band near the edge of the second subband of a-Si, the sample SL-2 has no lumines-
showninterband in section 3.2, in some luminescence theAs tors excitation under light excitation bulk allinvestigate PL semiconduclines are seen). Therefore it is(whereas interesting to the FL excitation spectra. It turns out that those luminescence lines, whose spectral positions correlate with those of the edges of the nearest a-Si subbands, are cxcited by the light absorbed at the electronic transitions between these subbands (fig. 4; PLE spectra for SL-2 are not shown as their behavior is absolutely analogous to that described above). As the excitation energy increases, the absorption due to transitions between the next subbands (m = 1) begins and luminescence associated with transitions between m = 0 subbands disappears, but, simultaneously, other FL bands appear whose positions are determined by the edge of subbands m = 1. A further decrease in the excitation wavelength leads to repetition of the situation when the edge of the next subband m = 2 is reached.
__________________________ k0 C))
b I~
k1
.4-’
0 :3
-‘-4
I1
Fl to p.’ “.4 to
_____________________________
0 0)
0
~1Ia 1.6Photon 2.0
2.4 2.8 energy (eV)
Fig. 4. Photoluminescence excitation spectra: (a) for SL-1 at 1.83 eV (1) and at 2.78 eV (2); (b) for SL-3 at 1.42 eV (1), at 1.96 eV (2) and at 1.74 eV (3). T=300 K. The arrows show the positions of the subbands in SLs [9].
/ Radiatite
A. V Zayats et a!.
recombination in short-period a-Si / 5i02 super/a ttices
;i~I
339
relation between the PLE intensities corresponds to that observed for the FL intensity of SLs and of a-SiO2.
‘,-l
0•
i
I
I
‘jIl
I..’ :•‘ ~
• to .~
•
~9
I
‘. —.
2 0
,1
2 4
•
2 8
Photon energy (eV) Fig. 5. Photoluminescence excitation spectra at 2.1 eV for SL-1 (solid line), SL-3 (dash line) and a-Si02 (dot-and-dash line, intensity X I0~).T = 300 K.
Investigation of the excitation (fig. 4) of the FL lines at 1.96 eV in SL-2 and 1.74 eV in SL-3, whose spectral positions are in the energy range of the m = 0 SL subbands and which do not correlate with the subband edges (fig. 2), shows that these PL lines are excited both in the Stokes and in anti-Stokes range, and that the long- and short-wavelength boundaries of the PLE spectra are determined by the edges of the m = 0 subbands. The PL lines within the energy range of the second subband (m = 1) do not show antiStokes excitation and the short-wave edge of cxcitation spectra coincides with the edge of the m = 1 SL subband. Figure 5 shows the PLE spectra of the band at 2.1 eV observed both in SLs and in isolated a-Si02 layers under picosecond and continuous excitation. In fused quartz this line has its excitation peak far away from the luminescent transitions and it is not resonantly excited. The shape of the excitation spectrum of this PL in SL-3 looks similar to that of the previous one in the short-wave spectral region but excitation near the PL line is also observed. For SL-l the excitation spectrum is analogous rather to PLE of the bands considered above; it has an anti-Stokes component and is determined by absorptive transitions between the states of the m = 0 subband. The
4. Discussion Investigation of luminescence spectra and PL excitation in amorphous Si/SiO2 superlattices and in isolated layers of fused quartz, and their comparison with the spectrum of SL electron states makes it possible to divide all the observed FL bands into three groups: FL lines observed both in SLs and in isolated a-SiO layers, FL .
lines whose spectral positions correspond to the edges of subbands in a-Si layers, and PL lines whose energy positions are inside these subbands. After excitation of an electron the following processes of energy relaxation are possible in a SL: thermalization by emission of phonons in the subband in which the electron is excited [12]; thermalization between subbands due to the emission of phonons as well as of light [13]; a transition of excited carriers into the adjacent SL layer and further relaxation in this layer [4]; and, finally, radiative and nonradiative recombination. If no transitions between neighbouring layers occurs, the luminescence excitation spectrum reflects the absorption spectrum of a SL. Thus, the observed PL and PLE spectra are determined by the time required for leaving the layer or nonradiative capture of the carriers in the SL layer relative to the time necessary for relaxation (intra- and intersubband) to the energy at which luminescence occurs.
4.1. Subband-to-subband recombination PL lines whose spectral positions correspond to the energy distances between the edges of a-Si subbands are observed not only for the nearest (m = 0) but also for the higher (m = I and 2) subbands in the SLs investigated. Usually, due to thermalization of excited carriers, only the lower states of the electronic spectrum show up in luminescence. As follows from fig. 4, the lumines-
340
A. V Zayats et a!.
/
Radiatite recombination in short-period a-Si / SiO, super/a itices
cence at the mth subband edge occurs only if carriers are excited directly to the same mth subbands (between which the FL transition occurs) of the conduction and valence bands. When carriers are excited to other, nth, n ~ m subbands the luminescence at the mth subband edge is not observed. These facts are evidence for slow relaxation between different subbands. Such low intersubband relaxation is probably caused by the large energy difference between adjacent Am = 1 subbands of the conduction and valence bands in the SLs investigated. If this difference increases, the relaxation rate due to LO-phonon assisted processes decreases [17]. PL spectral positions (near the edges of subbands), and their PLE dependences (excitation within its own subband), which are analogous for all these FL bands in all SLs investigated, gives us the opportunity to attribute these PL lines to subband-to-subband recombination. It is the low intersubband relaxation of excited carriers in aSi/Si02 SLs that results in observation of hot luminescence due to higher subbands of well layers. Both hot electrons and hot holes simultaneously take part in the recombination. Apparently, the short-wave tail in the PLE spectrum of the line at 1.96 eV caused by the m = 1 subbands in SL-3 is determined, analogous to the excitation spectra of subband-to-subband transitions m = 0 and 1 in SL-l, 2, and 3, by the edge of the next subband m = 2 in SL-3. This correlates with the appearance of a 2.9 eV line in the PL spectrum of SL-3. This line appears under excitation above this edge (fig. 3). Thus, this PL line can be attributed to transitions from the m = 2 a-Si subband in SL-3. The position of this subband has not been determined before in absorption studies as it lies outside their spectral range [9]. Excitation spectra of the FL lines in the region where electron processes related to impurities in a-Si02 layers are important (see figs. 3, 4 and 5), are influenced by transitions of electrons between band states of the a-Si layers and impurity states of the a-Si02 layers, resulting in a PLE spectrum which deviate from the SL absorption spectrum. At the same time, in the spectral range where this transition does not occur (<2.0 eV) the PLE
spectrum reflects the absorption spectrum of transitions between the a-Si subbands. The FL bands discussed have long-wave tails and are of greater width (—~0.1 eV) that the thermal width expected for “ideal” subband-tosubband transitions. This can be due to the participation of localized states in the subband tail in recombination. As the SL period decreases, the long-wave tails of these FL bands increase, apparently due to increasing density of localized states in the tails of permitted bands when silicon layers are disordered in short-period SLs [8,14]. 4.2. Impurity luminescence in a-Si07 in SLs The PL line at 2.1 eV which appears under continuous and pulse excitation in isolated a-SiO2 layers is considered to be caused by background Ge impurities in fused quartz [15]. Excitation of this luminescence in isolated a-Si02 layers occurs with low efficiency as the energy of excitation quanta is less than one third the a-Si02 band gap and only transitions between impurity states can be excited. The fact that this FL is not resonantly excited in a-Si02 and the FLE peak is shifted by about 0.7 eV to the short-wave side speaks in favor of a three-level scheme of transitions: only the lowest of the three levels is filled in the absence of excitation. Explanation of other specific features of the FL and FLE spectra of SLs (e.g. the presence of an anti-Stokes shift in FLE of some bands) also requires a three-level scheme. As pointed out above, the deviation of the PLE spectrum of subband-to-subband luminescence in a-Si layers from the absorption spectrum is caused by electron transfer between a-Si and a-Si02 layers. The PL intensity (the 2.1 eV FL intensity is iO~ times greater in SLs than in isolated a-Si02 layers, showing the increase of excitation efficiency in SLs) and the PLE spectral dependence (the fact that the short-wavelength boundaries of the PLE spectra of impurity luminescence of a-Si02 in SLs is determined by the subband edges of a-Si, the deviation of the PLE spectra of subband-to-subband FL from the absorption spectra, and the differences between the PLE spectra in SLs and in isolated a-SiO., layers) show that the excitation light is absorbed in a-Si
A. V Zayats ci a!.
/ Radiative recombination
~j/Si02
____
I I
\\
•
•
I
I
•
~•
~
I
~
I
III
~l
~ ~I
II
~
-~-T---- ~ —
—
a
2 superlattices
341
available. Due to the absence of relaxation of excited carriers between the various a-Si subbands, the conditions for transfer are no longer fulfilled when absorption with the participation
~ I
in short-period a-Si / Si0
b
c
Fig. 6. Scheme of electron states and radiative transitions in SL-1 (a), SL-2 (b), and SL-3 (c). Discontinuity of c- and v-bands of a-Si and a-Si02 are assumed to be independent of the SL period. The position of the edge of subbands of a-Si layers is taken from ref. 191, the edge of the subband m = 2 for SL-3 (dash line) is determined in the present paper. Horizontal arrows show transfer of electrons between SL layers.
layers and excited carriers then transfer to impurity states in the adjacent a-Si02 layers, recombining (the inverse effect to resonant transitions of carriers between the band states of different layers of the GaAs/AlGaAs superlattices [4,16]). The probability of such a transition between the states of adjacent layers in a superlattice depends on their mutual energy position and, therefore, it varies with varying thicknesses of layers due to size quantization. As the excitation wavelength varies, the initial energy of carriers in the subbands changes, affecting the resonance transition condition [4], so that the FL intensity changes as well, In SL-1 the impurity PL is excited in the entire first subband of a-Si band states (fig. 5). The impurity level of a-Si02 is close to the bottom of the c-band subband in a-Si (fig. 6). Consequently, excitation of an electron into this subband, and its subsequent thermalization, give it the correct energy for resonant transfer to an impurity state in the adjacent layer of a-Si. The radiative transition then takes place to a vacant impurity state. The presence of an anti-Stokes component in the PLE spectrum confirms that these states are
of the next subband begins, and the impurity FL does not manifest itself in the spectra. The presence of resonant PLE and of the specific feature near 2.4 eV in the excitation spectrum is due to the fact that at these photon energies an increase in the depopulation rate of the lower impurity states after recombination occurs due to resonant transfer of holes from the neighbouring a-Si layers and due to absorptive transitions from this level to c-subbands m = 0 and m = 1. Excitation due to transitions connected with the impurity absorption in a-SiO2 does not show up in SL-i, since absorption in a-Si layers is many orders of magnitude stronger, and the m = 1 subband-tosubband PL is resonantly excited; this is the rcason for the step near 2.6 eV in the PLE spectrum .
(see figs. 3 and 5). In contrast to this, in the spectrum of the a-Si states in SL-3 no specific features are observed near the energy of impurity absorption in a-5i02 layers, and they are seen in the PLE spectrum owing to their role in deactivation of the lower recombination level. In SL-3 the upper impurity level in a-SiO2 is populated via the states of the m = 1 subband of the a-Si conduction band (fig. 6) and this determines the shape of the FLE spectrum. As seen from the comparison of FL and PLE spectra of fused quartz and SLs, positions of the impurity transitions are independent of the material in which they are observed. Consequently, impurity centers in a-SiO2 layers are not influenced by the SL potential even in such shortperiod SLs. This is, apparently, due to the fact that energy states of deep defects are mainly determined by the short-range potential and, owing to considerable localization of the electron wave function, the states of such centres are not affected by the potential profile. 4.3. Subband-to-impurity recombination The PLE spectra of the third type of luminescence bands, situated between the bands of subband-to-subband recombination, are similar to
342
A.V Zayats eta!.
/ Radiative recombination in short-period a-Si/SiO,
those of both impurity and subband-to-subband recombination. The presence of an anti-Stokes component in the excitation spectrum, as for the a-Si02 impurity luminescence, allowing to relate these radiative transitions to impurity bands of a-Si02 which are not filled in equilibrium. On the other hand, such PL is observed for a number of a-Si subbands and the character of the FL excitation is analogous to the subband-to-subband luminescence (see section 4.1). Excitation anywhere within the whole mth subband absorption range leads to this type of luminescence within the mth subband. This fact confirms the participation of a-Si subband states (m = 0, 1 ...) in this luminescence. The above scheme of levels (fig. 6) allows one to explain this PL by transitions of electrons from different conduction band subbands of a-Si to impurity states in the a-Si02 layers. The validity of the model proposed is confirmed by the fact that the lines of such cross-luminescence, which at first sight are absent from the PL spectra for the 1st and 2nd subbands of SL-I and for the second subband of SL-3, according to the scheme in fig. 6, must lie near the line of the impurity PL. In the FL spectrum of SL-1 a peculiarity at 2.15 eV is clearly seen, which may correspond to this cross-recomiination. Broad PL bands at 2.1 eV in SL-3 and at 2.8 eV in SL-1 are, apparently, due to superposition of crossluminescence lines from the second SL subband and impurity FL in SL-3 and subband-to-subband PL SL-1, respectively. This type of hot luminescence differs from the subband-to-subband hot FL because only hot electrons participate in cross-luminescence, while both hot electrons and hot holes are needed for subband-to-subband hot PL. As a result of rapid nonradiative recombination in a-Si layers caused, apparently, by the states arising from disorder in the layers [8], only luminescence associated with the states of a-Si05 in the SL shows up under continuous excitation. The smaller influence of nonradiative relaxation of carriers in a-Si layers under picosecond excitation makes it possible to observe subband-to-subband and cross-luminescence associated with the states of subbands of a-Si layers. The proposed electron state scheme in SLs explains the observed shifts of lines caused by
superlattices
subband-to-subband transitions in a-Si layers with respect to the position of the edge of these subbands (figs. 2 and 3). Apparently, this is caused by interaction of the states of carriers in different layers of the SLs. The a-Si subbands m = 0 in SL-3 and m = 1 in SL-1 and SL-2 are far in energy from the impurity states of a-Si02 layers, and the shift of FL lines associated with these subbands is negligible. As the energy positions of the edges of subbands of the c- or v-band and of the impurity come closer, the interaction between the states of both layers leads to a shift of the subband edge (without a shift of the impurity level, as shown above) to lower energies. This shift is larger due to interaction with the states of the valence band than with those of the conduction band. The shifts observed confirm that the two upper impurity states in a-Si02 are empty in equilibrium. In the absorption spectra the position subband edge is different from that in luminescence whenever the presence of nonequilibrium carriers causes a shift in the subband edge due to interaction with an impurity level near the edge.
5. Conclusions Investigation of luminescence and its excitation in short-period (<20 A) amorphous Si/Si02 superlattices shows the presence of three types of radiative transitions: those between the subbands of the conduction and valence bands which arise due to quantum size effect in the a-Si layers, those between impurity states in a-Si02 layers, and those between subbands of the a-Si conduction band and impurity levels in a-Si07 (crossluminescence). This directly confirms the presence of subbands of spatial quantization in a-Si layers in SLs which are due to the two-dimensional character of electron states in these layers. For one of the superlattices (SL-3) the energy position of the m = 2 subband, E2 = 2.9 eV, has been found, which could not be obtained when absorption was investigated. It turned out that, in contrast to the well-investigated SLs based on Ill—V and other cornpounds, in a-Si/Si02 SLs relaxation of excited
A. V. Zayats ci a!.
/ Radiative recombination
carriers between various subbands of c- and vbands is slow. This allows observation of two types of hot PL caused by higher subbands in a-Si layers: subband-to-subband hot luminescence with participation of both hot electrons and hot holes, and hot cross-luminescence in which only hot electrons take part. In the SLs investigated the electrons excited in a-Si layers can transfer to adjacent a-Si02 layers, either with photon emission under recombination at impurity levels of fused quartz, or by resonant transfer of electrons to neighbouring impurity states of a-SiO-, with subsequent recombination in these layers. Excitation of impurity transitions in a-Si02 layers via the states of the subbands of a-Si layers in SLs leads to an increase in intensity of this impurity luminescence by about iO~times compared to isolated a-Si02 layers. Comparison of FL and PLE spectra of a-Si/5i02 superlattices and thick a-5i02 layers show that quantum size effects do not influence the energy states of deep defects in a-Si02 layers even for such shortperiod SLs. Rapid nonradiative recombination of excited carriers in a-Si layers even at low temperatures (down to 4.2 K) leads to the absence of luminescence in these layers under continuous excitation when radiative transitions occur only in a-Si02 layers in SLs. The decreasing influence of nonradiative transitions under picosecond excitation permits observation of FL related with the states of size quantization of a-Si layers in SLs: subband-to-subband PL and cross-luminescence. On the basis of PL, PLE and absorption investigations a scheme of electronic states of aSi/SiO2 superlattices is proposed which allows a satisfactory explanation from the unified standpoint of the specific observed features of the spectra.
in short-period a-Si / SiO, super/a itices
343
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