APPLIED PHYSICS LETTERS 87, 211919 共2005兲

Photoluminescence from impurity codoped and compensatedSi nanocrystals
Minoru Fujii,a兲 Yasuhiro Yamaguchi, Yuji Takase, Keiichi Ninomiya, and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering,
Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

共Received 7 September 2005; accepted 12 October 2005; published online 18 November 2005兲
Photoluminescence 共PL兲 properties of B and P codoped and compensated Si nanocrystals werestudied. The compensation of carriers in nanocrystals was confirmed by the annihilation ofconfined-carrier optical absorption in the infrared region. In the PL spectra obtained under theresonant excitation condition, the codoped samples did not exhibit structures related tomomentum-conserving phonons, which were clearly observed for pure Si nanocrystals. The result
strongly suggests that in impurity codoped Si nanocrystals, nonphonon quasidirect optical transitionis the dominant recombination path for electron-hole pairs, and thus impurity codoping is a possible
approach to further improving PL efficiency of Si nanocrystals. © 2005 American Institute ofPhysics. 关DOI: 10.1063/1.2135214兴
It has been recently demonstrated that photoluminescence 共PL兲 properties of Si nanocrystals 共Si-nc’s兲 can be
drastically modified by impurity doping.1–3 The doping ofeither n- or p-type impurities results in the quenching of thePL because of the efficient Auger interaction between photoexcited electron-hole pairs and carriers supplied by thedoping.3–7 The PL quenching by impurity doping is effectively suppressed when n- and p-type impurities are doped
simultaneously2,3 due probably to the compensation of carriers in nanocrystals. The codoped Si-nc’s exhibit roomtemperature PL at the energy lower than that of the bulk Si
band gap 共⬃0.9 eV兲.2 Therefore, by properly controlling impurities in Si-nc’s together with the size, the tunable range of
the PL energy of Si-nc’s can be largely extended without
losing much of the intensity.
Another advantage of impurity doping is that it localizes
carriers further and increases the probability of nonphonon
quasidirect recombination of electron-hole pairs, and thus theradiative recombination rate. Therefore, impurity doping isuseful even in the energy range where pure Si-nc’s can cover.
In fact, in the previous work on Si-nc’s doped into SiO2 orborophosphosilicate glasses 共BPSG兲, we observed the enhancement of PL intensity by P and B codoping compared tothat of pure Si-nc’s emitting at almost the same energy.3
If PL from the samples containing impurity codoped Sinc’s appears below the bulk Si band gap, it is clear that
impurity states are involved in the luminescence process. On
the other hand, if the PL appears above the bulk band gap, itcannot easily be distinguished whether the PL arises fromimpurity doped Si-nc’s or those that remain undoped becauseof the statistical fluctuation of impurity concentration.
The purpose of this paper is to study PL properties of P
and B codoped Si-nc’s assemblies having the PL maximum
above the band-gap energy of bulk Si crystals and demonstrate that the mechanism of the PL is different from that ofpure Si-nc’s. We will show that under resonant PL excitation
condition, impurity codoped samples exhibit quite different
PL spectra from those of pure Si-nc’s. The result suggestsa兲Author to whom correspondence should be addressed; electronic mail:fujii@eedept.kobe-u.ac.jp

that quasidirect nonphonon optical transition is dominant in
impurity-doped samples.
The samples studied were Si-nc’s embedded in glass matrices prepared by a cosputtering method. Details of thepreparation procedure are given in our previous paper.3 PL
spectra were measured using a single monochromatorequipped with a liquid N2 cooled InGaAs near-infrared diodearray. The spectral response of the detection system was calibrated with the aid of a reference spectrum of a standard
tungsten lamp. The wavelength of excitation light for conventional PL measurements was 532 nm 关frequency-doubled
cw Nd:yttrium vanadium oxygen 共YVO兲 laser兴 and for resonant PL measurements it was changed from 700 to 1000 nm
共cw Ti:Sapphire laser兲. The excitation light was loosely focused on the samples 共spot size ⬃2 mm兲 to avoid heating.
For the resonant PL measurements, to suppress the laser
light, the asynchronous mechanical chopping technique wasapplied.8 The samples were mounted in a continuous-flow
He cryostat and kept at 5 K.
First we will show the evidence that impurities aredoped into substitutional sites of Si-nc’s and are electricallyactive. Figure 1共a兲 shows the infrared absorption spectra of P
and B doped Si-nc’s. For the P-doped sample 共without B
doping兲, we can see the rising of the absorption coefficient toboth the shorter and longer wavelengths. The absorption at
the short wavelength side is due to the valence to conduction
band transition of electrons, while that at the long wavelength side is due to the intra-conduction-band transitions ofelectrons supplied by P doping, i.e., free 共or in this case“confined”兲 carrier absorption.9 By keeping the P concentration the same and increasing the B concentration, the absorption becomes weaker, indicating that carriers are compensated in nanocrystals by simultaneously doping P and B. By
further increasing the B concentration, the absorption
reaches minimum and again increases. This time the absorption is due to the intra-valence-band transitions of holes supplied by excess B. In Fig. 1共b兲, the absorption coefficient at2.5 ␮m and the PL intensity of the same samples are plotted
as a function of B concentration. We can clearly see that thePL intensity is the largest when the confined carrier absorption is the weakest. The result demonstrates that the exact

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Fujii et al.

Appl. Phys. Lett. 87, 211919 共2005兲

FIG. 1. 共a兲 Infrared absorption spectra of P and B codoped samples. The P
concentration is fixed to be 1.25 mol %, while the B concentration ischanged from 0 to 1.68 mol %. 共b兲 The absorption coefficient at 2.5 ␮m共left axis兲 and the PL intensity 共right axis兲 are plotted as a function of B
concentration. P concentration is fixed to be 1.25 mol %.

compensation of carriers is crucial to achieve maximum PL
intensity.3
Figure 2 shows the PL spectra at 5 K of pure 关Fig. 2共a兲兴,B-doped 关Fig. 2共b兲兴, P-doped 关Fig. 2共c兲兴, and B and P
codoped 关Fig. 2共d兲兴 samples. The excitation power ischanged from 0.7 mW to 1.43 W. The spectra are normalized at the maximum of the higher energy band to compare
the shape. It is worth noting that PL intensities of either B- orP-doped samples are much smaller than those of pure andcodoped samples because of the Auger effect. In the case ofpure Si-nc’s, with increasing the excitation power, the defectrelated PL at around 0.9 eV grows significantly. The different excitation power dependence of the two bands arises
from the difference in the lifetime; the lifetime of the exciton
PL at low temperatures is very long 共⬃5 ms兲 and it saturates
easily at relatively low excitation power, while that of thedefect-related PL is of the order of microseconds or less, andthus it grows in proportion to the excitation power. The lifetime of the exciton PL also depends on the size and is longer
for larger Si-nc’s, resulting in easier saturation of the intensity at the low-energy side of the exciton PL band. This size

FIG. 2. Excitation power dependence of the PL spectra at 5 K for 共a兲 pure,共b兲 B-doped, 共c兲 P-doped, and 共d兲 B and P codoped Si nanocrystals. The
excitation power is changed from 0.7 mW to 1.43 W.

dependence of the lifetime results in the high-energy shift ofthe exciton PL with increasing the excitation power. In thepresent case, it shifts about 100 meV by changing the excitation power from 0.7 mW to 1.43 W.
Compared to pure Si-nc’s, impurity-doped Si-nc’s areless sensitive to the excitation power. In B-doped samples,

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Appl. Phys. Lett. 87, 211919 共2005兲

Fujii et al.

FIG. 3. Normalized resonant PL spectra of 共a兲 pure and 共b兲 B and P codoped
Si-nc’s at 5 K. Excitation energies are indicated by arrows.

the low-energy band is broader and weaker than that of pureSi-nc’s, and also the excitation power dependence of the highenergy band is smaller. In the P-doped sample, the lowenergy band almost disappears.1 The excitation power dependence of the spectral shape is the smallest for the codoped
sample and is almost unaffected by the excitation power.
Figure 2 strongly suggests that in impurity-codoped samples,
the PL arises not from Si-nc’s remaining undoped but fromthose really doped with B and P, and the mechanism of thePL is different from that of pure Si-nc’s.
To demonstrate more clearly that the mechanism of PL isdifferent between pure and codoped Si-nc’s, we measured PL
under resonant excitation condition. The essence of this technique is the size selectivity of the optical excitation.8 In pureSi-nc’s, the low-energy part of the PL band is governed by
the emission coming from larger nanocrystals. When the energy of the exciting laser light is chosen to fall inside the low
energy part of the PL band, only a very small subset ofnanocrystals having band gaps below the laser energy isprobed. Figure 3共a兲 shows the normalized resonant PL spectra of pure Si-nc’s at different excitation energies. The energy
positions of the exciting light are shown by arrows. Whenthe excitation energy is moved down and into the band of theconventional PL spectrum, distinct features appear in thespectra. This PL feature becomes more pronounced when theexcitation energy is at the low-energy tail of the PL band.

From the energy separation of these PL features they can beassigned to the optical transitions involving 1TA, 1TO, TO+ TA, and 2TO momentum-conserving phonons in theabsorption/emission cycle. The observation of these features
is the direct evidence that pure Si-nc’s strongly inherit theindirect band-gap nature of bulk Si crystals.8
Figure 3共b兲 shows the resonant PL spectra of B- andP-codoped Si-nc’s. In contrast to pure Si-nc’s, the spectra arefeatureless even when the excitation energy is below themaximum of the band of the conventional PL spectrum 共solidcurve兲, although the spectra become narrower with a decrease in the excitation energy. The absence of the phonon
structures is the strong indication that in codoped Si-nc’s
electrons and holes recombine directly without the participation of momentum conserving phonons.
In Fig. 3, the maxima of the PL bands when excited
nonresonantly 共solid curves兲 are different between the pureand codoped samples. The PL maximum of the pure sample
can be brought to lower energy by higher temperatureannealing.10 The higher temperature annealing usually resultsin more distinct phonon features under the resonant excitation condition. Therefore, to make the comparison fair, inFig. 3 we fixed the annealing temperature to be 1100 ° C. If
we chose a set of samples having the PL maximum at almost
an identical energy prepared by applying different annealing
temperatures between pure and codoped samples, the difference in the spectral shape under resonant excitation will be
more pronounced.
In conclusion, we demonstrated that by simultaneously
doping n- and p-type impurities in Si-nc’s, we can change
the indirect band-gap nature of pure Si-nc’s to direct ones
without losing the intensity. This approach may further improve the luminescence efficiency of Si-nc’s as well as extend the tunable range of the luminescence energy. However,
to make the benefit of this effect maximum, carriers in all
nanocrystals in a sample should be compensated. Technological development to realize this condition is stronglydemanded.
This work was supported by a Grant-in-Aid-Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
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