APPLIED PHYSICS LETTERSVOLUME 74, NUMBER 1115 MARCH 1999

Decay dynamics of near-infrared photoluminescence from Ge nanocrystals
Shinji Takeoka
Division of Mathematical and Material Science, The Graduate School of Science and Technology, Kobe
University, Rokkodai, Nada, Kobe 657-8501, Japan

Minoru Fujii,a) Shinji Hayashi, and Keiichi Yamamoto
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,Nada, Kobe 657-8501, Japan~Received 7 December 1998; accepted for publication 14 January 1999!
Photoluminescence ~PL! decay dynamics of Ge nanocrystals (nc-Ge) 1.2–3.2 nm in average
diameter embedded in SiO2 matrices was studied. The samples showed a PL peak in thenear-infrared region with strong size dependence. A very fast component ~!1 ms! was found indecay curves for all the samples. For the samples containing relatively large nc-Ge, a slow
component of the order of microseconds was also observed. With decreasing the size, the slow
component gradually faded out, and the PL intensity increased significantly. The transition fromindirect to direct recombination of carriers with decreasing the size is considered to be responsible
for the observed PL decay dynamics. © 1999 American Institute of Physics.
@S0003-6951~99!00711-1#

Optical properties of nanometer size Si and Ge crystals
have been investigated in recent years.1 The band gap widening and the increase in the oscillator strength caused by thequantum size effects would appear in such nanocrystals. In
particular, for Si nanocrystals (nc-Si), the relationship between the size and the photoluminescence ~PL! properties
has been revealed.2–4 With decreasing the size, the PL peakshifts from the band gap of bulk Si to the visible region2–5
and the decay time becomes shorter from milliseconds tomicroseconds.5–8
In contrast to nc-Si, little is known on the PL propertiesof Ge nanocrystals (nc-Ge). Since the exciton Bohr radiusof bulk Ge crystal ~17.7 nm! is much larger than that of bulk
Si crystal ~4.9 nm!,1 quantum size effects will appear more
conspicuously for nc-Ge than nc-Si. Recently, we have succeeded in observing size dependent PL from nc-Ge in thenear-infrared ~NIR! region.9 A PL peak was observed atabout 0.88 eV for nc-Ge 5.3 nm in average diameter (d ave).
With decreasing the size, the PL peak shifted to higher energies and reached 1.54 eV for d ave50.9 nm. The PL intensity increased about two orders of magnitude as the size decreased. From these results, we concluded that the observed
PL comes from the radiative recombination of electron–holepairs confined in nc-Ge. In this work, in order to clarify therecombination process of electron–hole pairs confined innc-Ge, PL decay measurements were performed for thesame samples as those studied in our previous work.9 It will
be demonstrated that the PL decay time is much shorter thanthat reported for nc-Si. The transition of carrierrecombination process from indirect to direct one will bediscussed.
Ge nanocrystals embedded in SiO2 matrices were prepared by a radio frequency cosputtering method similar to
those used in our previous studies on nc-Ge embedded ina!

Author to whom correspondence should be addressed; electronic mail:fujii@eedept.kobe-u.ac.jp

SiO2 matrices.9–11 Small pieces of Ge chips ~23230.5 mm3, purity 99.999 9%! were placed on a SiO2 target~10 cm in diameter, purity 99.99%! and they were cosputtered in Ar gas of 2.7 Pa. Substrates were fused quartz plates.
After cosputtering, the films were thermally annealed in N2
gas ambient for 30 min at 800 °C to grow nc-Ge in SiO2matrices.
In the present preparation method, the size of nc-Ge canbe controlled by changing the volume fraction of Ge in films( f Ge). 9–11 In our previous work, we prepared nc-Ge withd ave50.9– 5.3 nm by changing f Ge from 0.2–7.2 vol %.9 The
size of nc-Ge was determined by cross-sectional highresolution transmission electron microscopic ~HRTEM! observation @JEM-2010 ~JEOL!#. For the samples with d ave<2.3 nm, nc-Ge were not observed in HRTEM images due
probably to the intense background image of SiO2 matrices.
The size of nc-Ge for these samples was estimated by therelationship between the size of nc-Ge and f Ge . 9 In thiswork, we used the same samples as those studied in ourprevious work.
The PL spectra were measured at room temperature using a HR-320 monochromator ~Jobin Yvon!, a R5509-72
photomultiplier ~Hamamatsu Photonics! and a SR830 lock-in
amplifier ~Stanford Research!. The excitation source was the457.9 nm line of an Ar-ion laser. The beam power density
was about 1 W/cm2. The spectral response of the detection
system was calibrated with the aid of a reference spectrum ofa standard tungsten lamp. For the PL decay measurements, aSR430 multichannel scaler ~Stanford Research! was used.
Excitation pulses were obtained from the 488.0 nm line of anAr-ion laser by using an acoustic-optic modulator. The pulse
width and repetition frequency were 400 ns and 11 kHz,
respectively. The time resolution of the system is about 40ns.
Figure 1 shows the PL spectra for the samples with various d ave . The PL spectra are normalized at their maximum
intensities and the scaling factors for the normalization are

0003-6951/99/74(11)/1558/3/$15.001558© 1999 American Institute of Physics
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Takeoka et al.

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FIG. 1. Dependence of PL spectra on the average diameter of nc-Ge. The
inset shows the size dependence of the PL intensity which was reported in
our previous paper ~in Ref. 9!. The arrows show the energy at which decay
dynamics was measured.

shown in Fig. 1 ~a larger factor corresponds to a smaller PL
intensity!. The PL spectra have strong size dependence. For
the sample with d ave53.2 nm, the PL peak is located atabout 0.96 eV. With decreasing d ave , the PL peak shows
blueshift and appears at 1.08, 1.21, and 1.40 eV for thesamples with d ave52.3, 1.7, and 1.2 nm, respectively. The
PL intensity increases drastically as shown in the inset ofFig. 1. The PL intensity was corrected by the amount of
nc-Ge in the films and the absorbance at the excitation
wavelength.9
The PL decay curves monitored at the PL peak energy
are shown in Fig. 2~a!. We can see that the PL decay curves
consist of fast and slow components for all the samples. The
fast component has a decay time faster than 1 ms and theslow one of the order of microseconds. The decay time offast component is shorter than the instrumental time resolution in this experiment ~'40 ns!. For the sample with d ave
53.2 nm, the contribution of the slow component to the PL
decay curve is rather large, although the fast component
dominates the PL decay curve. As the size of nc-Ge decreases, the slow component gradually fades out. The increase in the fast decay component and the drastic increase in
the PL intensity shown in Fig. 1 with decreasing the size
suggest that the oscillator strength of electron–hole recombination increases with decreasing the size.
Recently Ren calculated the electron and hole wave
function distributions in the k space of some edge states in
nc-Ge. 12 He showed that for nc-Ge 5.1 nm in diameter, theelectron wave functions are located near L point and are well
separated from the hole wave functions located near G point,
while for nc-Ge 1.5 nm in diameter, the electron and hole
wave functions spread in the k space and the overlaps of thewave functions become significant. This theoretical prediction indicates that the electron–hole pairs confined in nc-Ge
about 5 nm in diameter recombine with the aid of phonon~indirect recombination!. With decreasing the size, nophonon process ~direct recombination! begins to contributeto the recombination process, and for nc-Ge 1.5 nm in diameter, the direct recombination dominates the recombination process.

FIG. 2. PL decay curves monitored ~a! at the PL peak energies for thesamples shown in Fig. 1 and ~b! at various PL energies for the sample withd ave52.3 nm.

In general, the transition from the indirect to direct recombination process causes the increase in the oscillator
strength, resulting in the faster decay time and the enhancement of the PL intensity. As the size of nc-Ge decreases andreaches the size which begins to show the direct recombination, the fast decay component due to the no-phonon transition will be superimposed on the slow component due to thephonon-assisted transition. With decreasing the size further,
the ratio of the fast to slow components will increase. Therefore, a possible assignment of the observed fast and slow
decay components is the no-phonon and phonon-assisted recombination, respectively. Our results demonstrate that, evenfor the largest nc-Ge studied in this work ~3.2 nm in diameter!, the direct transition is the dominant carrier recombination process, and as the size decreases further, the indirecttransition becomes less probable. It is very plausible that as
the size becomes larger than 3.2 nm, the slow component
will play a significant role in the recombination process. Unfortunately, the PL signal of the samples containing nc-Gelarger than 3.2 nm was too weak to obtain PL decay curves.
Figure 2~b! shows the PL decay curves monitored atvarious PL energies for the sample with d ave52.3 nm. The
monitored PL energy is indicated by arrows in Fig. 1. The
slow component gradually fades out with increasing themonitored PL energy. Similar results were observed for othersamples. The increase in the fast decay component with increasing the monitored PL energy can be explained as follows. The PL spectra are broadened by inhomogeneous
broadening caused by the size distribution. The PL decay
curve obtained at higher energy side of the PL peak represents the signal from smaller nc-Ge in the size distribution,and at lower energy side from larger nc-Ge. Therefore, theDownloaded 10 Jun 2002 to 133.30.106.15. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

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Takeoka et al.

Appl. Phys. Lett., Vol. 74, No. 11, 15 March 1999

slow component of decay curve decreases with increasing
the monitored PL energy.
It is noted here that the observed PL decay time is much
faster than that of nc-Si previously reported.5–8 The PL decay time of red and NIR PL for nc-Si is within the range ofmicroseconds to milliseconds. No component faster than 1
ms has been reported. The slow PL decay indicates that therecombination of carriers in nc-Si is an indirect process.
Hybertsen13 and Ren14 theoretically predicted that the direct
recombination
processes
becomeimportantfor
nc-Si<1.5 nm in diameter. Recently, Kovalev et al. obtained the ratio of no-phonon transitions to phonon-assisted
ones by resonant PL measurement.15 They demonstrated thatwith increasing the confinement energy, the contribution ofno-phonon transitions increases and becomes comparablewith that of phonon-assisted transitions for confinement energy above 0.7 eV. Although the size of nc-Si has not beenreferred in their article,15 the confinement energy of 0.7 eVcorresponds to the size of about 1.5 nm from the size versus
PL peak energy relation reported so far.2–4 These theoreticaland experimental results indicate that nc-Si which show thedirect recombination of electron–hole pairs should besmaller than 1.5 nm. This may be the reason why the indirect
character is preserved for nc-Si as small as a few nanometers in diameter. On the other hand, our results demonstratethat the direct recombination is the dominant process for
nc-Ge as large as 3 nm in diameter. The large exciton Bohrradius of nc-Ge and the resulting strong confinement of carriers may be the cause of the indirect-to-direct transition at
rather large sizes. Further studies including resonant excitation of PL spectra are required to fully understand theindirect-to-direct transition in nc-Ge.
In conclusion, we have studied the PL decay dynamics
for nc-Ge embedded in SiO2 matrices as a function of thesize (d ave51.2– 3.2 nm). A very fast component faster than

1 ms was observed in decay curves for all the samples. For
the samples containing relatively large nc-Ge, a slow component of the order of microseconds was also observed. Withdecreasing the size, the slow component gradually faded out,
and the PL intensity increased drastically. These results indicate that the direct recombination process is the dominant
process for nc-Ge even for particles as large as 3 nm indiameter.
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science,Sports, and Culture, Japan.
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