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Quantitative analysis of light emission from SiGe quantum wells
,. . . . . . . .
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 157 (1995) 1-10
Quantitative analysis of light emission from SiGe quantum wells S. Fukatsu
a,*,
H. Akiyama
b, y .
Shiraki b, H. Sakaki b
Department of Pure and Applied Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153, Japan b Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan
Abstract
Spontaneous emission from strained Si s _xGex/Si quantum wells (QW) and allied structures is explored with primary focus on optical characterization in the time domain. Time-correlated single photon counting method is used with complementary steady-state analysis to solicit a wealth of otherwise evasive information on the basis of steady-state measurement alone. Two-dimensional nature of quantum-confined free excitons in strained Si]_xGeJSi QWs, carrier dynamics associated with exciton localization in superlattices, and surface recombination kinetics in near surface quantum confined geometry are described to demonstrate the potential of combined use of time-resolved and steady-state luminescence analysis for characterization of indirect gap strained Si s _ xGeJSi QWs.
1. I n t r o d u c t i o n
Rapid advancement in recent crystal growth technology has stirred considerable interest in Si-based quantum confined structures as represented by strained Si s _ x G e J S i quantum wells (QWs). Extensive studies have emerged in growing numbers, which seem to be more or less pursuing Si-based optoelectronic applications [1-10] in spite of major obstacles arising from the indirect nature of interband transition and built-in strain due to the lattice mismatch between SiGe and Si. Recently, significant improvement in terms of optical quality has been achieved by employing high temperature (T > 700°C) and/or clean vacuum/materials in the growth environment both with solid source molecular beam epitaxy (MBE) [3,4,6,10-12] and gas source epitaxy [1,2,5,8,9]. Ac-
* Corresponding author. Fax: +81 3 5454 4311; E-mail: [email protected],ac.jp.
cordingly, there have been an ever increasing number of reports on the observation of clear band-edge luminescence in strained Si s - x G e J S i QWs and their allied structures so far. Fundamental properties of spontaneous emission based on stead-state (CW) measurement are already well documented in the literature [1-12]. However, little has been explored in the field of luminescence characterization in the time domain [9,13,14]. The significance of detailed knowledge in the time domain will be recognized when we remind the reader of apparently enhanced no-phonon (NP) luminescence in SimGe . short-period superlattices, where a quasi-direct transition due to conduction valley switch of A to F is expected [15-17]. The enhanced NP line observed in CW measurement has been plausibly attributed to an increase in the oscillator strength as an immediate consequence of the zonefolding [18]. However, it is not justified that the expected effect is in fact achieved unless the anticipated " r a p i d " radiative decay lifetime, i.e., the hallmark of direct transition, is observed. This is plausi-
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00365-7
2
S. Fukatsu et al./Journal of Crystal Growth 157 (1995) 1-10
ble since an analogous enhanced NP luminescence is obtained when there are excitons localized at minima of spatially fluctuating potential a n d / o r when excitons sense fluctuation of the in-plane QW interface potential, which provides appropriate Fourier components admixing A - F conduction band wavefunctions [13,14,16]. In such cases, strong enhancement of the NP transition is likely to be manifest again but indirect nature is still retained thereby giving "slower" radiative decay lifetimes. Unfortunately, it is not straightforward to discriminate these conflicting cases and identify the exact character of the observed transition by using CW luminescence measurement alone. Apart from this, the lifetime evaluation is of profound importance when one considers the luminescence efficiency, r/, taking account of nonradiative pathways since nonradiative channels control r/ through ZNR as " r / = ( 1 / T R ) / ( 1 / r R q1/~'NR). All these arguments obviously warrant the necessity of luminescence characterization in the time domain. In this paper, analysis of luminescence from SiGe-based indirect gap QWs is described in a fairly quantitative manner, addressing the great potential of time-resolved analysis in the field of optical characterization and interpretation of optical aspects of SiGe-based indirect gap QWs and allied structures.
2. Experimental procedure Samples were grown in a purpose-built gas source Si molecular bea epitaxy (MBE) system (Daido Hoxan VCE-S2020) using hydride sources, Si2H 6 and GeH 4 at 700°C [19]. The substrates are nominally p-type on-axis Si(100) with resistivity in the range of 5-10 f~-cm. Following standard cleaning, wafers were briefly dipped in 2.5 wt% diluted hydrofluoric acid to establish a hydrogen-passivated surface. Extended thermal cleaning was adopted at 800°C in ultra-high vacuum. Nominally undoped QWs were grown. The Ge composition (x) and layer thickness were obtained separately using double crystal X-ray diffraction with thick alloy layers and fully strained multiple QWs (MQWs). The details are found in Ref. [19]. Steady-state (or CW) photoluminescence (PL) was taken in standard lock-in configuration featuring an
75.4MHz1053nm50psec SHG~
]~
File(R70) r
ModeIA ,mator Cavitydu OCMdye 754kHz5 ps.
StandardTAC ] systemwithPHA Fig. 1. Schematic diagram of the setup for time-correlated single photon counting luminecence measurement. Note that the stop trigger is provided electronically delayed pulses from the oscillator (OSC) of the cavity dumped dye laser, and stop and start pulses are fed via constant fraction discriminator (CFD) to standard pulse height analyzing (PHA) system featuring time-amplitude converter (TAC).
Ar + ion laser (488 and 514.5 nm) with a typical power density of 0.1-1 W / c m -2 on the sample, a l m grating monochromator, and a liquid-nitrogencooled Ge detector. The samples were mounted on a closed cycle refrigerator with thermal anchor established by copper heat sink (T > 16 K) [9]. Time-correlated single photon counting (SPC) was performed using a standard TAC setup (Fig. 1) [20] with a cavity-dumped DCM-dye laser (631 nm, 754 kHz) synchronously pumped by a frequency-doubled (526.5 nm) mode-locked YLF ion laser (1053 nm) operating at 75.4 MHz [9,13]. For detectors, either an ultra-low dark-current S-1 photomultiplier (PMT) (Hamamatsu R3236) or an infrared sensitive PMT with InGaAs photocathode (Hamamatsu R5509-71) were used wherever appropriate [9,13]. The nominal temporal resolution of the two PMTs are < 1 ns and ~ 3 ns for R3236 and R5509-71, respectively. The two PMTs cover the whole spectrum range relevant to SiGe luminescence; R3236 for h < 1.2 /zm, and R5509-71 to cover h < 1.7/zm. The start trigger due to the PMT and the electronically delayed stop signal were fed to the TAC system through a 4 ch. constant fraction discriminator. The temporal window was
S. Fukatsu et al. /Journal of Crystal Growth 157 (1995) 1-10
fixed at 1.3/zs with a time resolution of 2.59 ns over 512 ch. The time-averaged optical power density was adjusted so that there was essentially no carrier heating and extrinsic lifetime reduction due to high fluence. The sample temperature was controlled by a varitemp cryostat ( > 7.6 K).
3. Results and discussion To demonstrate the great potential of luminescence characterization by SPC in the time domain, a selected choice of prominent examples are shown. First, the two-dimensional character of strained Sil_xGex/Si quantum wells is described on the basis of dependence of luminescence decay lifetimes on well width and temperature. 3.1. Characterization o f indirect excitons
3
width (Lz). Note that the decay profiles are drawn on a linear scale. The free exciton lifetimes are seen to take on values of the order of 100 n s - 1 /zs. Apparently, the lifetime ~- scales with the well width. The monotonic increase for L z > 50 ,~ is indicative of two-dimensional (2D) confinement and in essence runs parallel with previous results on GaAs/AIGaAs QWs [20,21]. On the other hand, it is noted that there is a sudden jump of the lifetime as one approaches vanishing L z. Hence, the lifetime is likely to reach the maximum for L: = 35-45 ,~. The unexpectedly reduced lifetime for smaller Lz'S may be related with nonradiative channels in the Si barrier with a typical lifetime, rNR < 200 ns [13]. This is plausibly understood by considering the wavefunction penetration into Si with decreasing L z. In general, the lifetime of free excitons should be inversely proportional to the exciton binding energy [22], as schematically shown in the inset • ~ (1/eb) A(T)-',
3.1.1. Well width dependence
As noted earlier, being an indirect gap material, luminescence decay lifetimes of strained Sil_xGex/Si quantum wells are typically extended as long as 1 /.is [9,13,14]. Fig. 2 is a synopsis of free exciton luminescence decay profiles by SPC of strained S i l _ x G e x / S i single QWs (SQWs) ( x = 0.15) with a Si cap of ~ 3000 A and the associated exponential decay lifetimes (~-) as a function of well
(1)
where E b is the exciton binding energy and A(T) represents the thermalization factor [20,21]. The inset shows schematically the inverse of E b. Therefore, the almost 3-fold decrease in the lifetime is translated in terms of the same order of increase in the oscillator strength which is well accounted for by an increase in the exciton wavefunction overlap, l&(0)[2, and hence the exciton binding energy, E b [22]. Inter-
,~o~ '°° 1
L,
7.6 K
l Z ax, l 0
~ 100 150 Well width (A)
200
000
200-~ 100-
~~ 0 Delay aX~O 8O0 (nsec)
8(10
t
• ttt
6O0
400
1 0
I 50
I 100
I 150
200
Well width (A)
Fig. 2. (Left) Synopsisof temporal decay transients of luminescencefrom strained Si I_ ~Ge,/Si SQWs with x = 0.15; (Right) The decay lifetime as a functionof well width. The inset schematicallyshows the well width dependenceof the exciton binding energy.
4
S. Fukatsu et al./Journal of Crystal Growth 157 (1995) 1-10
typ¢-I
type-II (staggered)
__1
CB [ AEc,>O
that the reduced dimensionality is dictated by hole confinement with the deeper potential height, e.g., ~ 160 meV for x = 0.2 [24], and is further strengthened by exciton formation.
I AEc
3.1.2. Temperature dependence
Si
SiGe Si
Si
SiGe
Si
Fig. 3. Schematic band lineup of strained Si t _,Ge,/Si QW with type-I and type II (staggered) conduction band offset.
An alternative way to assess the dimensionality rests with checking the thermal population of free excitons, which is dependent on the density of states. The thermalization factor in Eq. (1) is specifically written depending on the dimensionality of the QW system as [21]
estingly, the factor of 3 is fairly close to the predicted 4 times enhancement for idealistic 2D excitoffs, Eb ( 2 D ) = 4Eb(3D) [22]. The results unambiguously demonstrate that a two-dimensional system is realized in strained Si]_ x G e J S i QWs although there has been dispute over the conduction band offset as to whether it is of type-I or type-II (staggered) (Fig. 3) [23]. Importantly, even for type-I band lineup, the confinement potential height for electrons are extremely small, not larger than 10 meV, and hence electrons are extended over a fairly large distance and most likely to retain a 3 D character. Nevertheless, the lifetime variation w i t h L z is indicative of ever increasing 2D character with decreasing L z. Therefore, we infer
(2)
A(T) OC( Aol/T) m,
where the power index m characterizes the dimension as m = 3 / 2 (3D), 1 (2D), and 1 / 2 (1D) [20,21]. A 0 sets the upper bound of the exciton momentum allowed in the interband transition. Note that Eq. (2) is a good approximation in the limit Ao << kT. Then the decay lifetime r of 2D excitons scales with T. This is readily visible in the PL decay transients taken with SPC and the decay lifetimes in the left and right panels of Fig. 4, respectively. The sample is a strained S i I _ , G e , / S i SQW ( L z = 68.4,) cladded by symmetric strained-layer Sil_xGe ~ superlattice (SLS) barriers ( S i G e ' S i = 34 A : 5 7 A, 99 period) with x = 0.18. The linear part is seen to persist up to
I i
1400-
I
24 K .~. soo-
,'"'""
ooo
0 ,
,
,
110 2i0
3t0
4r0
510 610
Temperature (K)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 Delay(~s)
Fig. 4. (Left) Synopsis of luminescence decay transients of the centered strained Si~_~Ge,/Si SQW (L: = 68 ,g,) between strained Si I_,Ge~ superlattice barriers (SiGe: Si = 34:57 ,g,, 99 period) with x = 0.18. (Right) The decay lifetimes as a function of temperature.
S. Fukatsu et al. / Journal of Crystal Growth 157 (1995) 1-10
from the outer SLS barriers, which will be given in more detail in the next section. Interestingly, no discernible change was observed in the NP luminescence linewidth with temperature due to lifetime extension for higher temperatures. Hence the inhomogeneous contribution is likely to dominate the free exciton NP linewidth of strained Sil_xGex/Si QWs, typically 4 - 8 meV and rather broad. Rugged interface morphology and alloy broadening may account for this.
~_~ centered QW i SLS barrier Total i)-..--o
5
~
3.2. Carrier dynamics (exciton delocalization)
3
4
i 1040
•
i
f
i
1050
1060
~070
Energy (meV)
Fig. 5. Temperature dependence of CW PL intensity of the SQW-centered-SLS used in Fig. 2. The inset shows a magnification of the SLS NP peaks.
The well-established aspect of time-resolved measurement is the capability of closely tracking down very fast phenomena such as carrier dynamics. To demonstrate an example of carrier dynamics in the time domain, the exciton (de)localization will be explored using the same SQW-centered-SLS sample.
3.2.1. CW luminescence 60 K. Such a linearity of decay lifetimes was commonly observed over the whole series of QWs. With reference to the above results, it is straightforward to identify that the observed lifetimes are indeed determined by free exciton recombination. As a matter of fact, temperature variation of CW luminescence and exciton ionization experiments are in favor of the spirit of free exciton picture. By contrast, localized excitons behave quite differently, as demonstrated in the next section. The persistent linearity of free exciton decay lifetimes with temperature is indicative of a high optical grade of QWs. This is plausibly attributed to negligible contribution of nonradiative channels. In fact, there is no marked reduction in the measured lifetimes which would be modulated by nonradiative lifetime, ~-~, as 1/•-= 1/~-~ + 1/rNR. Alternative justification comes from appropriate reference to CW luminescence as shown in Fig. 5. The CW PL intensity shown by the solid circles was less dependent on the temperature when T < 60 K and no marked thermal roll-off via nonradiative channels is recognized. Such a well-behaved immunity against thermal quenching is fairly standard for strained Sil xGex/Si QWs [1-12]. Admittedly, there is a slight increase of the QW luminescence intensity but this is due to thermal influx of delocalized carriers
As already pointed out, exciton localization sets in the SLSs of extended period, n > 10, at cryogenic temperatures [9]. The excitons that have been trapped at local potential minima in the SLS are thermally released, i.e. delocalized, as the temperature is increased [9]. Hence, CW PL intensity of the centered QW is plausibly increased due to increased influx of delocalized carriers from the SLS. In Fig. 5, a clear turnover is noticed of spectral dominance between the SLS and centered SQW due to the scenario as outlined above. The SLS luminescence is dominant at lower temperatures due to exciton localization, whereas the SQW takes over as the temperature is increased. Note that the integrated intensity is almost independent of temperature, which shows clearly that the nonradiative contribution is negligible in the whole structure. The roll-off of SLS luminescence with temperature is well characterized by the activation energy ~ 4.6 meV plausibly attributed to the localization potential in analogy to thermal roll-off with QW detrapping [9]. This roughly corresponds to the SLS peak shift as shown in the inset.
3.2.2. Temporal profile of centered QW Exciton delocalization dynamics can be traced by time-resolved measurement paying closer attention to the rise time of the temporal evolution of decay
6
S. Fukatsu et al./Journal of Crystal Growth 157 (1995) 1-10
profiles when the whole system is viewed as an analogue with standard 3 level system as shown in Fig. 6a [25]. Then it is seen that the relaxation time (sometimes referred to as rise time), r~, is a good indicator representing the exciton delocalization dynamics although it involves an ambipolar process of carrier relaxation to the SQW. In qualitative terms, the longer the r,, the less the delocalized excitons. Hence, a reduction of r r is a clear indication of "enhanced delocalization". As a matter of fact, close inspection of Fig. 3 reveals a significant delay of decay transients, which amounts to ~ 100 ns at 7.6 K. Shown in Fig. 6b is the relaxation times obtained from the curve fit to the temporal evolution of the " Q W luminescence" based on the 3 level system [25], I ( t ) at
e x p ( t / r d ) -- exp(//0"r) 1 -
,
(3)
r./r,
where r d is the decay time. An almost perfect fit is obtained with Eq. (3) as shown in the inset. The relaxation time decreases from 40 ns at 7.6 K with
(a) ~
no ~ Tr
~
40-
~" 30-
20-
10-
¢0
I ,,Rw i
I
i
i
0
200
400
600
i
i
i
800
1000
1200
Time (ns) Fig. 7. Synopsis of decay transients of SLS luminescence at different temperatures. The inset illustrates appropriate radiative and relaxation pathways which modulate the lifetimes of SLS luminescence.
temperature down to ~ 10 ns at 35 K beyond which the CW luminescence intensity is totally leveled out. Thus, clear correlation between the time-resolved analysis and CW measurement is apparent. However, although the relaxation time is expected to vary as (4)
a satisfactory fit is not obtained with the experimental results. This may be related with the complicated nature of •, which plausibly involves relaxation dynamics to the centered SQW from the SLS barrier.
50
0
2°~2K
r r¢x e x p ( E a / k T ) ,
nl
"ra
i
8K,
;o
3'0
,'0
o'0
°'0
Teml~rature(K) Fig. 6. (a) Schematic diagram of the 3 level relaxation model showing correspondence scheme of exciton delocalization in SLS and subsequent relaxation to the centered SQW. (b) The relaxation time as a function of temperature. The inset shows a decay transient at 8 K and its best fit using Eq. (3).
3.2.3. Temporal profile o f SLS barriers The exciton delocalization can be alternatively traced by monitoring the decay transients of "SLS luminescence" instead of the QW luminescence. This is readily understood when one imagines that the exciton delocalization deprives SLS of contributing excitons thereby giving a faster radiative recombination decay lifetime as schematically illustrated in the inset of Fig. 7. Again, the decay lifetime is a well-described simple relationship, an analogue of nonradiative- or tunneling-controlled lifetime [9]. The only difference is substitution of the extrinsic exciton outflow, r r, for the nonradiative term,
1/%xp = l / r , + 1/rSL s.
(5)
S. Fukatsu et al. / Journal of Crystal Growth 157 (1995) 1-10
The decay transients of the NP luminescence of SLS barriers with different temperatures are shown in Fig. 7. Obviously, the overall behavior with temperature is at variance with what was observed in Fig. 4. The decay lifetimes, ~'ses, first decrease from ~ 130 ns at 7.6 K and are almost leveled between 10-30 K at 60-80 ns and then it turns up to restore a linearity again which is characteristic of 2D free excitons for T > 30 K. The reduction of decay lifetimes corresponds to delocalization of excitons. However, the decay lifetimes of the control SLS for T = 10-30 K ranges from 400 to 700 ns and almost an order of magnitude longer. Importantly, the extrapolation to 0 K of the decay lifetimes of these two SLS structures merge at ~ 300 ns. So, a simple but universal picture of exciton (de)localization with temperature seems to hold. At very low temperatures, exciton localization hampers their transport to the centered SQW thereby giving comparable decay lifetimes with reference to the SLS without SQW. As the temperature increases, exciton delocalization sets in, which becomes increasingly pronounced with temperature (up to 30 K). Exciton transport then results in apparent reduction in the SLS decay lifetimes, but this is not confused with nonradiative contribution by complementary monitor of clear correspondence with QW luminescence decay transients. Usual linear temperature dependence is observed in the SLS luminescence when excitons are fully delocalized. For increased temperatures, extended QW luminescence lifetimes on the order of 1 p~s seem to suspend the ambipolar relaxation process to the QW from the SLS due to filling effects. However, the exciton transport toward the centered QW still provides a shorter lifetime for the SLS luminescence as compared to the SLS control without SQW.
3.3. Relevant issues (surface recombination) The potential of time-resolved measurement can be more clearly demonstrated in the context of nanofabrication. Futuristic nanodevices are expected to more or less depend on quantum confined systems located in the vicinity of the surface. However, a near-surface geometry is adversely influenced by surface recombination, which more than often turns out to be nonradiative and poses severe limitation on
CTO
7
CNp
~__
Lc=60A _=
NS~To
NSNpSiTO
1, , ,8, K , •~/~,/%, Lc=3600A 960 1000 1040 1080 Energy (meV)
Fig. 8. CW luminescence profiles of near surface QWs. Note the dominant peaks (labeled C) are due to the control SQWs buried 1 p.m from the surface. Surface recombination influences the luminescence of near surface QW (labeled NS) with thin Si caps. The inset shows possible carrier outflow towards the surface. the resultant device function. As a matter of fact, CW PL intensity of near surface SQWs are dramatically reduced in comparison with the first grown QW control at 1 /xm in depth, as shown in Fig. 8 [26]. Note that the Si barrier is excited under illumination. The observed PL quenching is plausibly attributed to surface recombination and clearly scales with Si cap thickness, L c, and the overall variation is an analogy of asymmetrically coupled QW (ACQW) system with the wide well replaced with the surface in the near surface QW. It is, however, premature to assert at this stage that either electron or hole controis the PL quenching upon decreasing L c In the ACQWs, on the other hand, clear correspondence was observed between the dominance switch of CW luminescence and the decay lifetime reduction of the narrow well, which evidences that the PL of the narrow well is controlled by hole tunneling [9]. Hence, PL quenching in near surface QWs seems to support the hole tunneling toward hypothetical surface states. The inset schematically depicts the relevant particle tunneling process. The decay transient measurement is shown to be successful in sorting out the two possible cases. Temporal evolution of decay profiles for 3 samples with different L c are shown in Fig. 9. It is clearly seen that the decay lifetimes are not much different, ~ 600 ns. This plausibly rules out the possibility that hole tunneling is the limiting process. Rather, the data suggest that there is essentially no exciton and
8
S. Fukatsu et al./Journal o f Crystal Growth 157 (1995) 1-10
18K m,
8K 1
!
4-
110°~ 0t2_"-2"6:= 4-
0
TO
400
600
800
~000
/'~i
SurfaceSQW
laser Qwl i
1
Si ( ] . ErW
,
t~=6oA 12o A 3600 A
I I I I I I
200
(a)
I
800
I
I
900
1000
[ 1100
Energy (meV)
laO0
Time (ns) Fig. 9. Temporal luminescence decay profiles of near surface SQWs with x = 0.15 and L. = 52 ~.. The associated decay lifetimes remain almost unchanged even for the QWs with thin Si caps, clearly showing that hole tunneling is not the controlling process of surface recombination. The inset shows schematically the hypothetical hole tunneling towards the surface.
ambipolar carrier transfer toward the surface. In addition, rapid rise times at the onset o f temporal evolution demonstrates that there is no time delay in carrier relaxation and subsequent exciton formation. The results clearly indicate that it is the electron overflow out of the Si barrier band edge that controls
Pig. II. 18 K PL spectra of a surface S Q W (b) with x = 0.64 and L_ = 32 ,~.
(a) and control S Q W
the PL quenching. Complementary biased luminescence analysis is summarized in Fig. 10. Luminescence of the near surface Q W is dramatically enhanced when the surface is positively biased, i.e., when the electrons are collected towards the surface. Hence, it is seen that electrons are the minority carriers in the fiat band condition, and that the surface recombination is of saturable character [26]. The biased PL results are apparently in agreement with our recent C W luminescence observation of the
Negativebias
Flatband
20 K
~o
9~o ~' -~
1000 1050 Photon energy (mcV)
Positivebias
11100
Fig. 10. 20 K CW PL profiles of near surface SQW (hatched) under different biases. The inset figures show the corresponding carrier distribution. The bias voltage is specified with respect to the substrate.
S. Fukatsu et al. /Journal of Crystal Growth 157 (1995) 1-10
saturable trend of surface recombination with increased excitation density, > 20 m W over 2 m m in diameter. The saturation power is translated in terms of surface recombination flux density of ~ 3 × 1012 c m - 2 / s or 0.01 M L / s . Taking advantage of such a saturable character of surface recombination, it is feasible to obtain luminescence from a genuine surface QW, i.e. without Si cap. Fig. 11 shows the PL spectra of a strained Si~_ xGex/Si "surface S Q W " and a control S Q W of x = 0 . 6 4 and L z = 3 2 ,~ with a ~ 3 0 0 0 A Si cap under excitation of typical saturable power. Successful demonstration of surface QWs is good news and would open up a way toward realistic optoelectronic studies using near surface quantum confined geometry.
4. Conclusions Light emission from strained Si l _ x G e , / S i quantum wells and allied structures was studied using time-correlated single photon counting in a complementary fashion with steady-state luminescence measurement. The potential of time-resolved SPC measurement was demonstrated by clear observation of well width and temperature dependence which support the general recognition of free exciton picture and two dimensionality associated with strained S i l _ x G e x / S i indirect gap quantum wells. Exciton delocalization in superlattices and surface recombination kinetics relevant to near surface quantum confined geometry were further demonstrated to justify that appropriate combination of time-resolved and steady-state luminescence measurement is a powerful tool in analyzing the otherwise evasive optical aspects of strained Si l _ x G e x / S i QWs with indirect gap nature.
Acknowledgements The authors would like to acknowledge N. Usami, H. Sunamura, Y. Kishimoto, Y. Akai and S. Ohtake for their expert technical assistance. S.F. would like to acknowledge the support from the Toray Science Foundation, Asahi Glass Foundation and Murata Science Foundation. The work was in part supported by
9
a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan.
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S. Fukatsu et al. / Journal of Crystal Growth 157 (1995) 1-10
[22] C. Weisbuch and B. Vinter, Quantum Semiconductor Structures (Academic Press, New York, 1991). [23] T. Baier, U. Mantz, K. Thonke, R. Sauer, F. Sch~iffler and H.-J. Herzog, Phys. Rev. B 50 (1994) 15191; S. Fukatsu and Y. Shiraki, Appl. Phys. Lett. 63 (1993) 2378.
[24] C.G. Van de Walle and R.M. Martin, Phys. Rev. B 34 (1986) 5621. [25] A. Fujiwara, Y. Takahashi, S. Fukatsu and Y. Shiraki, Phys. Rev. B 51 (1995) 2291. [26] S. Fukatsu, J. Mater. Sci., to be published.
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 157 (1995) 1-10
Quantitative analysis of light emission from SiGe quantum wells S. Fukatsu
a,*,
H. Akiyama
b, y .
Shiraki b, H. Sakaki b
Department of Pure and Applied Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153, Japan b Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan
Abstract
Spontaneous emission from strained Si s _xGex/Si quantum wells (QW) and allied structures is explored with primary focus on optical characterization in the time domain. Time-correlated single photon counting method is used with complementary steady-state analysis to solicit a wealth of otherwise evasive information on the basis of steady-state measurement alone. Two-dimensional nature of quantum-confined free excitons in strained Si]_xGeJSi QWs, carrier dynamics associated with exciton localization in superlattices, and surface recombination kinetics in near surface quantum confined geometry are described to demonstrate the potential of combined use of time-resolved and steady-state luminescence analysis for characterization of indirect gap strained Si s _ xGeJSi QWs.
1. I n t r o d u c t i o n
Rapid advancement in recent crystal growth technology has stirred considerable interest in Si-based quantum confined structures as represented by strained Si s _ x G e J S i quantum wells (QWs). Extensive studies have emerged in growing numbers, which seem to be more or less pursuing Si-based optoelectronic applications [1-10] in spite of major obstacles arising from the indirect nature of interband transition and built-in strain due to the lattice mismatch between SiGe and Si. Recently, significant improvement in terms of optical quality has been achieved by employing high temperature (T > 700°C) and/or clean vacuum/materials in the growth environment both with solid source molecular beam epitaxy (MBE) [3,4,6,10-12] and gas source epitaxy [1,2,5,8,9]. Ac-
* Corresponding author. Fax: +81 3 5454 4311; E-mail: [email protected],ac.jp.
cordingly, there have been an ever increasing number of reports on the observation of clear band-edge luminescence in strained Si s - x G e J S i QWs and their allied structures so far. Fundamental properties of spontaneous emission based on stead-state (CW) measurement are already well documented in the literature [1-12]. However, little has been explored in the field of luminescence characterization in the time domain [9,13,14]. The significance of detailed knowledge in the time domain will be recognized when we remind the reader of apparently enhanced no-phonon (NP) luminescence in SimGe . short-period superlattices, where a quasi-direct transition due to conduction valley switch of A to F is expected [15-17]. The enhanced NP line observed in CW measurement has been plausibly attributed to an increase in the oscillator strength as an immediate consequence of the zonefolding [18]. However, it is not justified that the expected effect is in fact achieved unless the anticipated " r a p i d " radiative decay lifetime, i.e., the hallmark of direct transition, is observed. This is plausi-
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00365-7
2
S. Fukatsu et al./Journal of Crystal Growth 157 (1995) 1-10
ble since an analogous enhanced NP luminescence is obtained when there are excitons localized at minima of spatially fluctuating potential a n d / o r when excitons sense fluctuation of the in-plane QW interface potential, which provides appropriate Fourier components admixing A - F conduction band wavefunctions [13,14,16]. In such cases, strong enhancement of the NP transition is likely to be manifest again but indirect nature is still retained thereby giving "slower" radiative decay lifetimes. Unfortunately, it is not straightforward to discriminate these conflicting cases and identify the exact character of the observed transition by using CW luminescence measurement alone. Apart from this, the lifetime evaluation is of profound importance when one considers the luminescence efficiency, r/, taking account of nonradiative pathways since nonradiative channels control r/ through ZNR as " r / = ( 1 / T R ) / ( 1 / r R q1/~'NR). All these arguments obviously warrant the necessity of luminescence characterization in the time domain. In this paper, analysis of luminescence from SiGe-based indirect gap QWs is described in a fairly quantitative manner, addressing the great potential of time-resolved analysis in the field of optical characterization and interpretation of optical aspects of SiGe-based indirect gap QWs and allied structures.
2. Experimental procedure Samples were grown in a purpose-built gas source Si molecular bea epitaxy (MBE) system (Daido Hoxan VCE-S2020) using hydride sources, Si2H 6 and GeH 4 at 700°C [19]. The substrates are nominally p-type on-axis Si(100) with resistivity in the range of 5-10 f~-cm. Following standard cleaning, wafers were briefly dipped in 2.5 wt% diluted hydrofluoric acid to establish a hydrogen-passivated surface. Extended thermal cleaning was adopted at 800°C in ultra-high vacuum. Nominally undoped QWs were grown. The Ge composition (x) and layer thickness were obtained separately using double crystal X-ray diffraction with thick alloy layers and fully strained multiple QWs (MQWs). The details are found in Ref. [19]. Steady-state (or CW) photoluminescence (PL) was taken in standard lock-in configuration featuring an
75.4MHz1053nm50psec SHG~
]~
File(R70) r
ModeIA ,mator Cavitydu OCMdye 754kHz5 ps.
StandardTAC ] systemwithPHA Fig. 1. Schematic diagram of the setup for time-correlated single photon counting luminecence measurement. Note that the stop trigger is provided electronically delayed pulses from the oscillator (OSC) of the cavity dumped dye laser, and stop and start pulses are fed via constant fraction discriminator (CFD) to standard pulse height analyzing (PHA) system featuring time-amplitude converter (TAC).
Ar + ion laser (488 and 514.5 nm) with a typical power density of 0.1-1 W / c m -2 on the sample, a l m grating monochromator, and a liquid-nitrogencooled Ge detector. The samples were mounted on a closed cycle refrigerator with thermal anchor established by copper heat sink (T > 16 K) [9]. Time-correlated single photon counting (SPC) was performed using a standard TAC setup (Fig. 1) [20] with a cavity-dumped DCM-dye laser (631 nm, 754 kHz) synchronously pumped by a frequency-doubled (526.5 nm) mode-locked YLF ion laser (1053 nm) operating at 75.4 MHz [9,13]. For detectors, either an ultra-low dark-current S-1 photomultiplier (PMT) (Hamamatsu R3236) or an infrared sensitive PMT with InGaAs photocathode (Hamamatsu R5509-71) were used wherever appropriate [9,13]. The nominal temporal resolution of the two PMTs are < 1 ns and ~ 3 ns for R3236 and R5509-71, respectively. The two PMTs cover the whole spectrum range relevant to SiGe luminescence; R3236 for h < 1.2 /zm, and R5509-71 to cover h < 1.7/zm. The start trigger due to the PMT and the electronically delayed stop signal were fed to the TAC system through a 4 ch. constant fraction discriminator. The temporal window was
S. Fukatsu et al. /Journal of Crystal Growth 157 (1995) 1-10
fixed at 1.3/zs with a time resolution of 2.59 ns over 512 ch. The time-averaged optical power density was adjusted so that there was essentially no carrier heating and extrinsic lifetime reduction due to high fluence. The sample temperature was controlled by a varitemp cryostat ( > 7.6 K).
3. Results and discussion To demonstrate the great potential of luminescence characterization by SPC in the time domain, a selected choice of prominent examples are shown. First, the two-dimensional character of strained Sil_xGex/Si quantum wells is described on the basis of dependence of luminescence decay lifetimes on well width and temperature. 3.1. Characterization o f indirect excitons
3
width (Lz). Note that the decay profiles are drawn on a linear scale. The free exciton lifetimes are seen to take on values of the order of 100 n s - 1 /zs. Apparently, the lifetime ~- scales with the well width. The monotonic increase for L z > 50 ,~ is indicative of two-dimensional (2D) confinement and in essence runs parallel with previous results on GaAs/AIGaAs QWs [20,21]. On the other hand, it is noted that there is a sudden jump of the lifetime as one approaches vanishing L z. Hence, the lifetime is likely to reach the maximum for L: = 35-45 ,~. The unexpectedly reduced lifetime for smaller Lz'S may be related with nonradiative channels in the Si barrier with a typical lifetime, rNR < 200 ns [13]. This is plausibly understood by considering the wavefunction penetration into Si with decreasing L z. In general, the lifetime of free excitons should be inversely proportional to the exciton binding energy [22], as schematically shown in the inset • ~ (1/eb) A(T)-',
3.1.1. Well width dependence
As noted earlier, being an indirect gap material, luminescence decay lifetimes of strained Sil_xGex/Si quantum wells are typically extended as long as 1 /.is [9,13,14]. Fig. 2 is a synopsis of free exciton luminescence decay profiles by SPC of strained S i l _ x G e x / S i single QWs (SQWs) ( x = 0.15) with a Si cap of ~ 3000 A and the associated exponential decay lifetimes (~-) as a function of well
(1)
where E b is the exciton binding energy and A(T) represents the thermalization factor [20,21]. The inset shows schematically the inverse of E b. Therefore, the almost 3-fold decrease in the lifetime is translated in terms of the same order of increase in the oscillator strength which is well accounted for by an increase in the exciton wavefunction overlap, l&(0)[2, and hence the exciton binding energy, E b [22]. Inter-
,~o~ '°° 1
L,
7.6 K
l Z ax, l 0
~ 100 150 Well width (A)
200
000
200-~ 100-
~~ 0 Delay aX~O 8O0 (nsec)
8(10
t
• ttt
6O0
400
1 0
I 50
I 100
I 150
200
Well width (A)
Fig. 2. (Left) Synopsisof temporal decay transients of luminescencefrom strained Si I_ ~Ge,/Si SQWs with x = 0.15; (Right) The decay lifetime as a functionof well width. The inset schematicallyshows the well width dependenceof the exciton binding energy.
4
S. Fukatsu et al./Journal of Crystal Growth 157 (1995) 1-10
typ¢-I
type-II (staggered)
__1
CB [ AEc,>O
that the reduced dimensionality is dictated by hole confinement with the deeper potential height, e.g., ~ 160 meV for x = 0.2 [24], and is further strengthened by exciton formation.
I AEc
3.1.2. Temperature dependence
Si
SiGe Si
Si
SiGe
Si
Fig. 3. Schematic band lineup of strained Si t _,Ge,/Si QW with type-I and type II (staggered) conduction band offset.
An alternative way to assess the dimensionality rests with checking the thermal population of free excitons, which is dependent on the density of states. The thermalization factor in Eq. (1) is specifically written depending on the dimensionality of the QW system as [21]
estingly, the factor of 3 is fairly close to the predicted 4 times enhancement for idealistic 2D excitoffs, Eb ( 2 D ) = 4Eb(3D) [22]. The results unambiguously demonstrate that a two-dimensional system is realized in strained Si]_ x G e J S i QWs although there has been dispute over the conduction band offset as to whether it is of type-I or type-II (staggered) (Fig. 3) [23]. Importantly, even for type-I band lineup, the confinement potential height for electrons are extremely small, not larger than 10 meV, and hence electrons are extended over a fairly large distance and most likely to retain a 3 D character. Nevertheless, the lifetime variation w i t h L z is indicative of ever increasing 2D character with decreasing L z. Therefore, we infer
(2)
A(T) OC( Aol/T) m,
where the power index m characterizes the dimension as m = 3 / 2 (3D), 1 (2D), and 1 / 2 (1D) [20,21]. A 0 sets the upper bound of the exciton momentum allowed in the interband transition. Note that Eq. (2) is a good approximation in the limit Ao << kT. Then the decay lifetime r of 2D excitons scales with T. This is readily visible in the PL decay transients taken with SPC and the decay lifetimes in the left and right panels of Fig. 4, respectively. The sample is a strained S i I _ , G e , / S i SQW ( L z = 68.4,) cladded by symmetric strained-layer Sil_xGe ~ superlattice (SLS) barriers ( S i G e ' S i = 34 A : 5 7 A, 99 period) with x = 0.18. The linear part is seen to persist up to
I i
1400-
I
24 K .~. soo-
,'"'""
ooo
0 ,
,
,
110 2i0
3t0
4r0
510 610
Temperature (K)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 Delay(~s)
Fig. 4. (Left) Synopsis of luminescence decay transients of the centered strained Si~_~Ge,/Si SQW (L: = 68 ,g,) between strained Si I_,Ge~ superlattice barriers (SiGe: Si = 34:57 ,g,, 99 period) with x = 0.18. (Right) The decay lifetimes as a function of temperature.
S. Fukatsu et al. / Journal of Crystal Growth 157 (1995) 1-10
from the outer SLS barriers, which will be given in more detail in the next section. Interestingly, no discernible change was observed in the NP luminescence linewidth with temperature due to lifetime extension for higher temperatures. Hence the inhomogeneous contribution is likely to dominate the free exciton NP linewidth of strained Sil_xGex/Si QWs, typically 4 - 8 meV and rather broad. Rugged interface morphology and alloy broadening may account for this.
~_~ centered QW i SLS barrier Total i)-..--o
5
~
3.2. Carrier dynamics (exciton delocalization)
3
4
i 1040
•
i
f
i
1050
1060
~070
Energy (meV)
Fig. 5. Temperature dependence of CW PL intensity of the SQW-centered-SLS used in Fig. 2. The inset shows a magnification of the SLS NP peaks.
The well-established aspect of time-resolved measurement is the capability of closely tracking down very fast phenomena such as carrier dynamics. To demonstrate an example of carrier dynamics in the time domain, the exciton (de)localization will be explored using the same SQW-centered-SLS sample.
3.2.1. CW luminescence 60 K. Such a linearity of decay lifetimes was commonly observed over the whole series of QWs. With reference to the above results, it is straightforward to identify that the observed lifetimes are indeed determined by free exciton recombination. As a matter of fact, temperature variation of CW luminescence and exciton ionization experiments are in favor of the spirit of free exciton picture. By contrast, localized excitons behave quite differently, as demonstrated in the next section. The persistent linearity of free exciton decay lifetimes with temperature is indicative of a high optical grade of QWs. This is plausibly attributed to negligible contribution of nonradiative channels. In fact, there is no marked reduction in the measured lifetimes which would be modulated by nonradiative lifetime, ~-~, as 1/•-= 1/~-~ + 1/rNR. Alternative justification comes from appropriate reference to CW luminescence as shown in Fig. 5. The CW PL intensity shown by the solid circles was less dependent on the temperature when T < 60 K and no marked thermal roll-off via nonradiative channels is recognized. Such a well-behaved immunity against thermal quenching is fairly standard for strained Sil xGex/Si QWs [1-12]. Admittedly, there is a slight increase of the QW luminescence intensity but this is due to thermal influx of delocalized carriers
As already pointed out, exciton localization sets in the SLSs of extended period, n > 10, at cryogenic temperatures [9]. The excitons that have been trapped at local potential minima in the SLS are thermally released, i.e. delocalized, as the temperature is increased [9]. Hence, CW PL intensity of the centered QW is plausibly increased due to increased influx of delocalized carriers from the SLS. In Fig. 5, a clear turnover is noticed of spectral dominance between the SLS and centered SQW due to the scenario as outlined above. The SLS luminescence is dominant at lower temperatures due to exciton localization, whereas the SQW takes over as the temperature is increased. Note that the integrated intensity is almost independent of temperature, which shows clearly that the nonradiative contribution is negligible in the whole structure. The roll-off of SLS luminescence with temperature is well characterized by the activation energy ~ 4.6 meV plausibly attributed to the localization potential in analogy to thermal roll-off with QW detrapping [9]. This roughly corresponds to the SLS peak shift as shown in the inset.
3.2.2. Temporal profile of centered QW Exciton delocalization dynamics can be traced by time-resolved measurement paying closer attention to the rise time of the temporal evolution of decay
6
S. Fukatsu et al./Journal of Crystal Growth 157 (1995) 1-10
profiles when the whole system is viewed as an analogue with standard 3 level system as shown in Fig. 6a [25]. Then it is seen that the relaxation time (sometimes referred to as rise time), r~, is a good indicator representing the exciton delocalization dynamics although it involves an ambipolar process of carrier relaxation to the SQW. In qualitative terms, the longer the r,, the less the delocalized excitons. Hence, a reduction of r r is a clear indication of "enhanced delocalization". As a matter of fact, close inspection of Fig. 3 reveals a significant delay of decay transients, which amounts to ~ 100 ns at 7.6 K. Shown in Fig. 6b is the relaxation times obtained from the curve fit to the temporal evolution of the " Q W luminescence" based on the 3 level system [25], I ( t ) at
e x p ( t / r d ) -- exp(//0"r) 1 -
,
(3)
r./r,
where r d is the decay time. An almost perfect fit is obtained with Eq. (3) as shown in the inset. The relaxation time decreases from 40 ns at 7.6 K with
(a) ~
no ~ Tr
~
40-
~" 30-
20-
10-
¢0
I ,,Rw i
I
i
i
0
200
400
600
i
i
i
800
1000
1200
Time (ns) Fig. 7. Synopsis of decay transients of SLS luminescence at different temperatures. The inset illustrates appropriate radiative and relaxation pathways which modulate the lifetimes of SLS luminescence.
temperature down to ~ 10 ns at 35 K beyond which the CW luminescence intensity is totally leveled out. Thus, clear correlation between the time-resolved analysis and CW measurement is apparent. However, although the relaxation time is expected to vary as (4)
a satisfactory fit is not obtained with the experimental results. This may be related with the complicated nature of •, which plausibly involves relaxation dynamics to the centered SQW from the SLS barrier.
50
0
2°~2K
r r¢x e x p ( E a / k T ) ,
nl
"ra
i
8K,
;o
3'0
,'0
o'0
°'0
Teml~rature(K) Fig. 6. (a) Schematic diagram of the 3 level relaxation model showing correspondence scheme of exciton delocalization in SLS and subsequent relaxation to the centered SQW. (b) The relaxation time as a function of temperature. The inset shows a decay transient at 8 K and its best fit using Eq. (3).
3.2.3. Temporal profile o f SLS barriers The exciton delocalization can be alternatively traced by monitoring the decay transients of "SLS luminescence" instead of the QW luminescence. This is readily understood when one imagines that the exciton delocalization deprives SLS of contributing excitons thereby giving a faster radiative recombination decay lifetime as schematically illustrated in the inset of Fig. 7. Again, the decay lifetime is a well-described simple relationship, an analogue of nonradiative- or tunneling-controlled lifetime [9]. The only difference is substitution of the extrinsic exciton outflow, r r, for the nonradiative term,
1/%xp = l / r , + 1/rSL s.
(5)
S. Fukatsu et al. / Journal of Crystal Growth 157 (1995) 1-10
The decay transients of the NP luminescence of SLS barriers with different temperatures are shown in Fig. 7. Obviously, the overall behavior with temperature is at variance with what was observed in Fig. 4. The decay lifetimes, ~'ses, first decrease from ~ 130 ns at 7.6 K and are almost leveled between 10-30 K at 60-80 ns and then it turns up to restore a linearity again which is characteristic of 2D free excitons for T > 30 K. The reduction of decay lifetimes corresponds to delocalization of excitons. However, the decay lifetimes of the control SLS for T = 10-30 K ranges from 400 to 700 ns and almost an order of magnitude longer. Importantly, the extrapolation to 0 K of the decay lifetimes of these two SLS structures merge at ~ 300 ns. So, a simple but universal picture of exciton (de)localization with temperature seems to hold. At very low temperatures, exciton localization hampers their transport to the centered SQW thereby giving comparable decay lifetimes with reference to the SLS without SQW. As the temperature increases, exciton delocalization sets in, which becomes increasingly pronounced with temperature (up to 30 K). Exciton transport then results in apparent reduction in the SLS decay lifetimes, but this is not confused with nonradiative contribution by complementary monitor of clear correspondence with QW luminescence decay transients. Usual linear temperature dependence is observed in the SLS luminescence when excitons are fully delocalized. For increased temperatures, extended QW luminescence lifetimes on the order of 1 p~s seem to suspend the ambipolar relaxation process to the QW from the SLS due to filling effects. However, the exciton transport toward the centered QW still provides a shorter lifetime for the SLS luminescence as compared to the SLS control without SQW.
3.3. Relevant issues (surface recombination) The potential of time-resolved measurement can be more clearly demonstrated in the context of nanofabrication. Futuristic nanodevices are expected to more or less depend on quantum confined systems located in the vicinity of the surface. However, a near-surface geometry is adversely influenced by surface recombination, which more than often turns out to be nonradiative and poses severe limitation on
CTO
7
CNp
~__
Lc=60A _=
NS~To
NSNpSiTO
1, , ,8, K , •~/~,/%, Lc=3600A 960 1000 1040 1080 Energy (meV)
Fig. 8. CW luminescence profiles of near surface QWs. Note the dominant peaks (labeled C) are due to the control SQWs buried 1 p.m from the surface. Surface recombination influences the luminescence of near surface QW (labeled NS) with thin Si caps. The inset shows possible carrier outflow towards the surface. the resultant device function. As a matter of fact, CW PL intensity of near surface SQWs are dramatically reduced in comparison with the first grown QW control at 1 /xm in depth, as shown in Fig. 8 [26]. Note that the Si barrier is excited under illumination. The observed PL quenching is plausibly attributed to surface recombination and clearly scales with Si cap thickness, L c, and the overall variation is an analogy of asymmetrically coupled QW (ACQW) system with the wide well replaced with the surface in the near surface QW. It is, however, premature to assert at this stage that either electron or hole controis the PL quenching upon decreasing L c In the ACQWs, on the other hand, clear correspondence was observed between the dominance switch of CW luminescence and the decay lifetime reduction of the narrow well, which evidences that the PL of the narrow well is controlled by hole tunneling [9]. Hence, PL quenching in near surface QWs seems to support the hole tunneling toward hypothetical surface states. The inset schematically depicts the relevant particle tunneling process. The decay transient measurement is shown to be successful in sorting out the two possible cases. Temporal evolution of decay profiles for 3 samples with different L c are shown in Fig. 9. It is clearly seen that the decay lifetimes are not much different, ~ 600 ns. This plausibly rules out the possibility that hole tunneling is the limiting process. Rather, the data suggest that there is essentially no exciton and
8
S. Fukatsu et al./Journal o f Crystal Growth 157 (1995) 1-10
18K m,
8K 1
!
4-
110°~ 0t2_"-2"6:= 4-
0
TO
400
600
800
~000
/'~i
SurfaceSQW
laser Qwl i
1
Si ( ] . ErW
,
t~=6oA 12o A 3600 A
I I I I I I
200
(a)
I
800
I
I
900
1000
[ 1100
Energy (meV)
laO0
Time (ns) Fig. 9. Temporal luminescence decay profiles of near surface SQWs with x = 0.15 and L. = 52 ~.. The associated decay lifetimes remain almost unchanged even for the QWs with thin Si caps, clearly showing that hole tunneling is not the controlling process of surface recombination. The inset shows schematically the hypothetical hole tunneling towards the surface.
ambipolar carrier transfer toward the surface. In addition, rapid rise times at the onset o f temporal evolution demonstrates that there is no time delay in carrier relaxation and subsequent exciton formation. The results clearly indicate that it is the electron overflow out of the Si barrier band edge that controls
Pig. II. 18 K PL spectra of a surface S Q W (b) with x = 0.64 and L_ = 32 ,~.
(a) and control S Q W
the PL quenching. Complementary biased luminescence analysis is summarized in Fig. 10. Luminescence of the near surface Q W is dramatically enhanced when the surface is positively biased, i.e., when the electrons are collected towards the surface. Hence, it is seen that electrons are the minority carriers in the fiat band condition, and that the surface recombination is of saturable character [26]. The biased PL results are apparently in agreement with our recent C W luminescence observation of the
Negativebias
Flatband
20 K
~o
9~o ~' -~
1000 1050 Photon energy (mcV)
Positivebias
11100
Fig. 10. 20 K CW PL profiles of near surface SQW (hatched) under different biases. The inset figures show the corresponding carrier distribution. The bias voltage is specified with respect to the substrate.
S. Fukatsu et al. /Journal of Crystal Growth 157 (1995) 1-10
saturable trend of surface recombination with increased excitation density, > 20 m W over 2 m m in diameter. The saturation power is translated in terms of surface recombination flux density of ~ 3 × 1012 c m - 2 / s or 0.01 M L / s . Taking advantage of such a saturable character of surface recombination, it is feasible to obtain luminescence from a genuine surface QW, i.e. without Si cap. Fig. 11 shows the PL spectra of a strained Si~_ xGex/Si "surface S Q W " and a control S Q W of x = 0 . 6 4 and L z = 3 2 ,~ with a ~ 3 0 0 0 A Si cap under excitation of typical saturable power. Successful demonstration of surface QWs is good news and would open up a way toward realistic optoelectronic studies using near surface quantum confined geometry.
4. Conclusions Light emission from strained Si l _ x G e , / S i quantum wells and allied structures was studied using time-correlated single photon counting in a complementary fashion with steady-state luminescence measurement. The potential of time-resolved SPC measurement was demonstrated by clear observation of well width and temperature dependence which support the general recognition of free exciton picture and two dimensionality associated with strained S i l _ x G e x / S i indirect gap quantum wells. Exciton delocalization in superlattices and surface recombination kinetics relevant to near surface quantum confined geometry were further demonstrated to justify that appropriate combination of time-resolved and steady-state luminescence measurement is a powerful tool in analyzing the otherwise evasive optical aspects of strained Si l _ x G e x / S i QWs with indirect gap nature.
Acknowledgements The authors would like to acknowledge N. Usami, H. Sunamura, Y. Kishimoto, Y. Akai and S. Ohtake for their expert technical assistance. S.F. would like to acknowledge the support from the Toray Science Foundation, Asahi Glass Foundation and Murata Science Foundation. The work was in part supported by
9
a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan.
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