JOURNAL OF APPLIED PHYSICSVOLUME 83, NUMBER 12

15 JUNE 1998

Photoluminescence from B-doped Si nanocrystals
Minoru Fujii,a) Shinji Hayashi, and Keiichi Yamamoto
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657, Japan~Received 30 December 1997; accepted for publication 17 March 1998!
Boron-doped Si nanocrystals as small as 3.5 nm were prepared and their photoluminescence ~PL!
properties were studied. The PL properties were found to be very sensitive to the B concentration.
For the sample without B doping the temperature-dependent shift of the PL peak was almost thesame as that of the bulk band gap. As the B concentration increased, the temperature dependence
deviated from that of the bulk band gap, and the peak exhibited a low-energy shift as thetemperature decreased. The anomalous temperature dependence is considered to be due to thecontribution of the PL from excitons bound to the neutral B states. © 1998 American Institute ofPhysics. @S0021-8979~98!06012-5#

pellets ~10 mm in diameter! were placed on a SiO2 sputteringtarget ~10 cm in diameter! and they were cosputtered in Ar
gas of 2.7 Pa. Films about 2 mm in thickness were deposited
on fused quartz plates and Si wafers. The films were thenannealed under N2 gas flow at 1100 °C for 30 min to grownc-Si in the SiO2 matrices. During the growth of nc-Si, B
atoms are thought to be incorporated into nc-Si.
In this method, the size of the nc-Si can be controlled bychanging the number of Si chips during the cosputtering andby changing the annealing temperature. The B concentration
can also be controlled by changing the number of B2O3 pellets during the cosputtering. In the annealed samples, Bdoped nc-Si are embedded in the matrices that consist ofB2O3 and SiO2 ~borosilicate glass!. Although the B concentration in each nanocrystal is impossible to determine, theconcentration of B2O3 in the matrix region @C B ~mol %!# canbe determined from the intensity ratio of the B-O(;1400 cm21) and Si-O (;1080 cm21) vibration peaks in
the infrared ~IR! absorption spectra.8 Since C B is considered
to be roughly proportional to the B concentration in a film asa whole, in the following we will use C B to distinguish thesamples. The maximum C B studied was about 1.3 mol %.
The size of nc-Si was determined by cross-sectional highresolution transmission electron microscopic ~HRTEM! observations @using JEM-2010 ~JEOL!#. The samples for the
HRTEM observations were prepared by the standard procedures including mechanical and ion thinning methods. Photoluminescence spectra were measured using a HR-320~Jobin-Yvon! monochromator and a EO-817L ~North Coast!
Ge detector. The spectral responses of the detection systems
were corrected by reference spectra of a standard tungsten
lamp. The excitation source was the 457.9 nm line of an
Ar-ion laser with a power density of less than 1 W/cm2. The
measurements were made in the temperature range between
5 and 300 K in a CF1204 continuous-flow He cryostat ~Oxford Instruments!.

I. INTRODUCTION

During the past decade, optical properties of Si nanostructures have been the subject of intensive investigations
because the quantum confinement effects of electrons andholes lead to optical properties much different from those ofbulk Si crystals.1–7 Photoluminescence ~PL! properties inparticular have widely been studied and the relationship between the size of nanostructures and the PL peak energy has
been revealed experimentally for at least red and infrared
PL.4,7 However, there remain large differences between the
PL peak energies observed and the calculated band gap,4,7
and thus the mechanism of the PL is not yet fully understood. Furthermore, the role of impurity atoms on photoluminescence properties has not been clarified and little is
known about the electronic states associated with impurity
atoms. To fully understand the optical and electrical properties of Si nanostructures and to achieve various device applications, systematic studies of impurity-doped Si nanostructures are indispensable.
In our previous work,5,6 we succeeded in preparing Bdoped Si nanocrystals ~nc-Si! by a cosputtering method. We
have confirmed the doping of B atoms into Si nanocrystals
by Raman spectroscopy. The transverse optical ~TO! peak of
B-doped nc-Si observed under excitation of a 676.5 nm lineof a Kr-ion laser was asymmetric with a tail at the highenergy side of the peak due to the discrete-continuum Fanotype interference between the phonon scattering and the scattering of free carriers generated by the B doping. In thiswork, we have studied PL properties of B-doped nc-Si as
small as 3.5 nm in diameter prepared by the same method.
We will demonstrate that the PL properties of Si nanocrystals, in particular the temperature dependence, are very sensitive to the B concentration.
II. EXPERIMENTThe B-doped nc-Si were prepared by the cosputtering
method.5,6 Small pieces of Si chips (5315 mm2) and B2O3

III. RESULTS

Figure 1 shows a typical HRTEM image of a sample.
We can clearly see lattice fringes corresponding to the $111%a!

Electronic mail: fujii@eedept.kobe-u.ac.jp

0021-8979/98/83(12)/7953/5/$15.00

7953© 1998 American Institute of Physics

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J. Appl. Phys., Vol. 83, No. 12, 15 June 1998

FIG. 1. Typical cross-sectional HRTEM image of a SiO2 film containing Si
nanocrystals ~without B doping!. Lattice fringes corresponding to the $111%
planes of Si nanocrystals with the diamond structure can clearly be seen.

planes of nc-Si with the diamond structure. The Si nanocrystals are well dispersed in a SiO2 matrix. The average size
obtained from the HRTEM images was about 3.5 nm. It
should be noted that the size of nc-Si was almost independent of the B concentration.
In our previous work, we have studied B-doped nc-Si as
small as 8 nm in diameter by Raman spectroscopy.5,6 We
demonstrated that, under blue excitation, the nanocrystals
show a broad Raman peak with a tail towards the low-energy
side of the peak due to phonon confinement effects. As theexcitation wavelength was increased, the spectral shapechanged gradually and under the excitation of 676.5 nm
light, we observed an asymmetric peak with a tail towards
the high-energy side of the peak. The asymmetric spectral
shape and its excitation wavelength dependence can be explained well by the discrete-continuum Fano-type interference between the phonon scattering and the scattering of free
carriers generated by the B doping. The observation of the
Fano-type spectral shape indicates that the Fermi level of thenanocrystals is located in the valence band due to the very
high doping (.1019/cm3).
Similar in the present nc-Si, B atoms are also considered
to be heavily doped, because the B concentration of thepresent samples is nearly the same as that of the previous
ones (C B '1 mol %). In the case of a nc-Si 3.5 nm in diameter, one B atom corresponds to the very high concentrationof about 831019/cm3, provided that the crystal structure andthe lattice constant of the nanocrystals are the same as thoseof the bulk Si crystal. Since the solid solubility of B atoms inbulk Si crystal at room temperature is smaller than 431020/cm3 ~solid solubility at 1000 °C!,9 the maximum
number of B atoms doped into each nanocrystal is expectedFujii, Hayashi, and Yamamoto

FIG. 2. PL spectra of B-doped Si nanocrystals. C B is the concentration~mol %! of B2O3 in the matrix region estimated from IR absorption spectra.

to be very small. Therefore, it should be noted here that, if
the B concentration in a film is low, both doped and undoped
nanocrystals are thought to coexist and, as the B concentration increases, the ratio of the doped nanocrystals increases.
Figure 2 shows the room temperature PL spectra of Bdoped nc-Si with three different C B and that of an undopednc-Si. All the spectra are normalized at their maximum intensities. The scaling factors are shown ~a larger factor corresponds to a larger PL intensity!. For the sample without B
doping, we can see a peak at about 1.4 eV. In our previous
work,7 we demonstrated that the PL peak energy is very
sensitive to the size of the nc-Si and the peak exhibits ahigh-energy shift as the size decreases. From the observed
size dependence, we attribute the peak to the recombinationof electrons and holes confined in the nc-Si. As the B concentration increases ~i.e., as the number of doped nanocrystals increases!, the peak becomes weak and is broadened,
while the peak energy is almost independent of the B concentration. The decrease in the PL efficiency may be ascribedto the increase in the nonradiative Auger recombination process in a nanocrystal, because in B-doped nanocrystals therecombination energy of an exciton can be transferred to ahole in a neutral acceptor by exciting the hole deep into thevalence band. As the B concentration increases, the numberof doped nanocrystals increases. This results in an increase in
the number of ‘‘dark’’ nanocrystals and thus the PL is
quenched.
Figure 3 shows the temperature dependence of the PL
spectra for the samples without doping @Fig. 3~a!# and with
C B '1.3 mol % @Fig. 3~b!#. In both the samples, as the temperature decreases, another peak appears at about 0.9 eV.
This peak becomes intense with decreasing temperature.
Similar PL spectra have often been observed for porous Si
and the origin of the low-energy peak is considered to be due

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J. Appl. Phys., Vol. 83, No. 12, 15 June 1998

Fujii, Hayashi, and Yamamoto

7955

FIG. 4. Temperature dependence of the PL peak energy for the samples
with and without B doping. The curves are drawn as a guide to the eye.

shown as a solid curve. Figure 4 demonstrates that the temperature dependence of the peak energy is very sensitive tothe B concentration. For the sample without doping, the temperature dependence is almost the same as that of the bulk
band gap. As C B increases, the temperature dependence deviates from that of the bulk band gap. For the sample with
C B >0.8 mol %, the peak shifts in the direction opposite tothat of the bulk band gap.
IV. DISCUSSION

FIG. 3. Temperature dependence of the PL spectra for the samples ~a!
without B doping and ~b! with C B '1.3 mol %.

to the recombination of carriers trapped at P b centers at thesurface of Si nanostructures.10,11 We have studied in detailthe size and temperature dependence of the low-energy peak.
The results will be published elsewhere. In this article wewill restrict our attention to only the high-energy peak.
In Fig. 3~a!, as the temperature decreases, the highenergy peak at about 1.4 eV shifts monotonously to higher
energies. On the other hand, in Fig. 3~b!, the peak shifts tolower energies. Figure 4 shows the peak energies of the highenergy peak as a function of temperature. The ordinate in
Fig. 4 represents the peak energy shift relative to the peakenergy at 290 K. The band-gap shift of bulk Si crystal is also

In the B-doped nc-Si at low temperatures, excitons are
considered to be bound to the neutral B states and the boundexcitons may recombine radiatively. The PL spectra will thus
consist of the main band originating from the recombinationof unbound electron-hole pairs and the additional band related to the bound excitons ~bound-exciton band!. The
bound-exciton band will appear at the low-energy side of themain band. The energy difference between the two bands
corresponds to the binding energy of the bound exciton. If
the widths of the bands are much larger than the binding
energy, the two bands will not appear as two isolated peaks;
instead they will overlap resulting in a broad peak. By assuming that the PL peak observed for the B-doped nc-Si~Figs. 2 and 3! consists of the two bands, we can explain theobserved anomalous temperature dependence of the PL spectra as follows.
The relative intensity of the main band and the boundexciton band will depend on the temperature. At room temperature, excitons are considered to be thermally released
from the neutral B states. The PL peak energy at room temperature is thus not affected by the B doping. At low temperatures, a part of the excitons is bound to the neutral B
states and emits light at the low-energy side of the main
band. This causes the low-energy shift of the observed peak

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J. Appl. Phys., Vol. 83, No. 12, 15 June 1998

from that of the undoped nc-Si. As the temperature decreases
further, the relative intensity of the bound-exciton band increases and the deviation of the peak energy from that of theundoped nc-Si becomes large. If the binding energy of boundexcitons is larger than the widening of the band gap causedby the temperature variation from 300 to 5 K ~about 45
meV!, the widening is canceled out and the PL peak of the
doped nc-Si will shift to lower energies as the temperature
decreases.
In actual B-doped samples, both the doped and undopednc-Si coexist. The PL spectra will thus be a mixture of doped
and undoped nanocrystals, and the observed PL peak energy
versus temperature curve is expected to be located between
the curves of the undoped and doped nanocrystals. In Fig. 4,
the temperature dependence of the undoped sample is almost
the same as that of the bulk band gap. As C B increases, thetemperature dependence deviates from that of the undoped
sample. The deviation becomes large as C B increases. This
may result from the increase in the ratio of the doped nc-Si
in the samples. If we assume that all the nanocrystals in thesample with C B '1.3 mol % are doped with B atoms andthat only the bound-exciton band appears at 5 K, a roughestimate of the binding energy of the bound excitons can beobtained from Fig. 4 by the deviation of the peak energy
from that of the undoped sample at 5 K. The estimated value
is about 80 meV.
The energy levels of hydrogenic impurities in nc-Si havebeen studied on the basis of the tight binding theory byDelerue et al.12 In their calculations, the binding energy of ahole on a bare acceptor located at the center of a nanocrystal
as small as 3.5 nm in diameter is about 550 meV. In the bulk
Si crystal, the binding energy of a bound exciton on a neutral
donor ~acceptor! is about 0.1 of the binding energy of anelectron ~hole! on a bare donor ~acceptor!.13 If we assume
that this empirical rule can be applied to the nc-Si, the binding energy of bound excitons in the present samples becomes
about 55 meV. Although this value is derived under the assumptions stated and in spite of the fact that size distributions exist in actual samples, the agreement between the estimated binding energy and the experimentally obtained one~80 meV! is rather good.
In the above discussion, we assumed that the widths ofthe main and bound-exciton bands are much larger than thebinding energy of excitons. The width of the main band isthought to be nearly the same as the PL peak width of theundoped sample ~full width at half maximum is about 300
meV!. This value is much larger than the experimentallyobtained binding energy. The large width is considered to bemainly due to the inhomogeneous broadening caused by thesize distribution ~standard deviation is about 1 nm!. The size
distribution also causes the broadening of the bound-exciton
band, and thus the width of the band is also considered to bemuch larger than the binding energy.
The above discussion is based on the assumption that theobserved PL peak consists of the bound and unbound exciton
bands. Although the model could well explain the experimental results, the assumption is not fully justified. Study of
the PL decay dynamics may provide useful information for amore definite determination of the origin of the bands.

Fujii, Hayashi, and Yamamoto

In previous PL studies of porous Si, both positive andnegative temperature dependences of the PL peak have been
observed.14–19 However, no uniform explanation of the observed temperature dependence has been given so far. Porous
Si is prepared by the electrochemical anodization of p-type
Si wafers. The resistivities of the Si wafers used in the previous studies were distributed over a wide range ~0.001–50
V cm!.14–19 This suggests that the impurity concentration of
the nanostructures in the porous Si prepared from these wafers was also distributed over a wide range. The results of thepresent work strongly suggest that the variety in the temperature dependences previously reported for porous Si wascaused by the variety of impurity concentrations in the nanostructures resulting from the variety of resistivities of the Si
wafers used to prepare porous Si.
V. CONCLUSION

We have studied photoluminescence properties of Bdoped nc-Si as small as 3.5 nm. We demonstrated that thetemperature dependence of the PL peak energy strongly depends on the B concentration. For the sample without B doping, the temperature dependent shift of the PL peak was almost the same as that of the bulk band gap. As the B
concentration increased, the temperature dependence deviated from that of the undoped sample, and the peak exhibiteda low-energy shift as the temperature decreased. The anomalous temperature dependence is considered to be due to thecontribution of the PL from excitons bound to the neutral B
state.
In previous PL studies of porous Si, both the positive
and negative temperature dependences of the PL peak energy
were reported. The present results strongly suggest that thedifferent impurity concentrations in the nanostructures in porous Si result in the different temperature dependences reported.
ACKNOWLEDGMENTS

The authors are grateful to Atsushi Mimura for his valuable assistance in this work. This work was supported by aGrant-in-Aid for Scientific Research from the Ministry of
Education, Science, Sports and Culture, Japan, and a grantfrom the Hyogo Science and Technology Association.
L. T. Canham, Appl. Phys. Lett. 57, 1046 ~1990!.
H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki, and T. Nakagiri, Appl.
Phys. Lett. 56, 2379 ~1990!.
3
K. A. Littau, P. J. Szajowski, A. J. Muller, A. R. Kortan, and L. E. Brus,
J. Phys. Chem. 97, 1224 ~1993!.
4
S. Schuppler, S. L. Friedman, M. A. Marcus, D. L. Adler, Y. H. Xie, F. M.
Ross, Y. J. Chabal, T. D. Harris, L. E. Brus, W. L. Brown, E. E. Chaban,
P. F. Szajowski, S. B. Christman, and P. H. Citrin, Phys. Rev. B 52, 4910
~1995!.
5
Y. Kanzawa, M. Fujii, S. Hayashi, and K. Yamamoto, Solid State Commun. 100, 227 ~1996!.
6
Y. Kanzawa, M. Fujii, S. Hayashi, and K. Yamamoto, Mater. Sci. Eng., A
217/218, 155 ~1996!.
7
Y. Kanzawa, T. Kageyama, S. Takeoka, M. Fujii, S. Hayashi, and K.
Yamamoto, Solid State Commun. 102, 533 ~1997!.
8
A. S. Tenney, J. Electrochem. Soc. 118, 1658 ~1971!.
9
S. M. Sze, Physics of Semiconductor Devices ~Wiley, New York, 1981!.
10
B. K. Meyer, D. M. Hofman, W. Stadler, V. Petrova-Koch, F. Koch, P.
Emanuelsson, and P. Omling, J. Lumin. 57, 137 ~1993!.
1
2

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J. Appl. Phys., Vol. 83, No. 12, 15 June 1998
11

B. K. Meyer, D. M. Hofman, W. Stadler, V. Petrova-Koch, F. Koch, P.
Omling, and P. Emanuelsson, Appl. Phys. Lett. 63, 2120 ~1993!.
12
C. Delerue, M. Lannoo, G. Allan, and E. Martin, Thin Solid Films 255, 27~1995!.
13
J. R. Haynes, Phys. Rev. Lett. 4, 361 ~1960!.
14
X. L. Zheng, W. Wang, and H. C. Chen, Appl. Phys. Lett. 60, 986 ~1992!.
15
Z. Y. Xu, M. Gal, and M. Gross, Appl. Phys. Lett. 60, 1375 ~1992!.

Fujii, Hayashi, and Yamamoto

7957

16

K. L. Narasimhan, S. Banerjee, A. K. Srivastava, and A. Sardesai, Appl.
Phys. Lett. 62, 331 ~1993!.
17
S. M. Prokes and O. J. Glembocki, Phys. Rev. B 49, 2238 ~1994!.
18
G. G. Qin, H. Z. Song, B. R. Zhang, J. Lin, J. Q. Duan, and G. Q. Yao,
Phys. Rev. B 54, 2548 ~1996!.
19
A. Shimizu, Y. Yamada, G. Izutsu, K. Yano, and M. Kasuga, Jpn. J. Appl.
Phys., Part 2 35, L276 ~1996!.

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