effect of material structure on pl spectra from silicon nc
TRANSCRIPT
7/28/2019 Effect of Material Structure on PL Spectra From Silicon NC
http://slidepdf.com/reader/full/effect-of-material-structure-on-pl-spectra-from-silicon-nc 1/4
Effect of material structure on photoluminescence spectra from siliconnanocrystalsS. M. Orbons, M. G. Spooner , and R. G. Elliman Citation: J. Appl. Phys. 96, 4650 (2004); doi: 10.1063/1.1790058 View online: http://dx.doi.org/10.1063/1.1790058 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v96/i8 Published by the American Institute of Physics. Related Articles
Upper limit of two-dimensional hole gas mobility in strained Ge/SiGe heterostructures Appl. Phys. Lett. 100, 222102 (2012) Shape evolution in glancing angle deposition of arranged Germanium nanocolumns J. Appl. Phys. 111, 104309 (2012) Dopant effects on the photoluminescence of interstitial-related centers in ion implanted silicon J. Appl. Phys. 111, 094910 (2012) Nano-electron beam induced current and hole charge dynamics through uncapped Ge nanocrystals
Appl. Phys. Lett. 100, 163117 (2012) Experimental and theoretical analysis of the temperature dependence of the two-dimensional electron mobility ina strained Si quantum well J. Appl. Phys. 111, 073715 (2012) Additional information on J. Appl. Phys.
Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
Downloaded 07 Jun 2012 to 59.162.23.73. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
7/28/2019 Effect of Material Structure on PL Spectra From Silicon NC
http://slidepdf.com/reader/full/effect-of-material-structure-on-pl-spectra-from-silicon-nc 2/4
Effect of material structure on photoluminescence spectra from siliconnanocrystals
S. M. Orbons, M. G. Spooner, and R. G. Elliman a)
Electronic Materials Engineering Department, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia
(Received 30 March 2004; accepted 16 July 2004)
The thickness of a silicon dioxide layer is shown to have a significant effect on the measured
photoluminescence spectrum from silicon nanocrystals embedded in the layer. This can range from
significant but subtle spectral distortions that are difficult to detect, including changes in intensity,
peak position and peak width, to gross distortions that are readily apparent as a modulation of the
measured spectrum. These distortions are shown to be a simple consequence of the wavelength
dependent reflectivity of the sample structure but to have important implications for determining
nanocrystal properties from such data. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1790058]
Bulk silicon generally exhibits weak luminescence at
room temperature due to its indirect band gap and the domi-
nance of nonradiative recombination processes.1
However, it
has been shown
2,3
that silicon based nanostructures can ex-hibit efficient photoluminescence at room temperature. This
discovery has sparked considerable debate regarding the
mechanism for improved emission efficiency and an intrinsic
element of this debate has been the comparison of experi-
mentally measured photoluminescence (PL) spectra with the-
oretical predictions based on various models.4–6
Such analy-
sis is predicated on the assumption that measured PL spectra
reflect the properties of the silicon nanostructures. However,
the broad spectral emission that is typically observed from Si
nanocrystals can be distorted by the spectral response of the
measurement system as well as by the optical properties of
the sample.7–9
A broad range of material structures have been employedby researchers studying Si nanocrystals, each with a charac-
teristic, wavelength-dependent reflectivity.7–9
Indeed, the op-
tical properties of such structures are often employed to
modify the nanocrystal emission, as exemplified in micro-
cavity structures employing distributed Bragg mirrors.10–13
However, in other cases, the impact of the sample structure
on the measured nanocrystal emission is unintentional and
often misleading. The spectral distortion due to simple ma-
terial structures is a much more subtle and often misinter-
preted variance in the observed nanocrystal emission, and
one that is of critical importance when comparing PL spectra
from different structures, even when they differ only slightly.In this study, commercially prepared͑ 100͒ oriented sili-
con wafers were oxidized to produce SiO2 layers of 5 m,
970 nm, 650 nm, and 103 nm. These, together with a 1 mm
thick fused silica plate were implanted at room temperature
with 30 keV Si− ions to a fluence of 2.5ϫ1016 ions/cm2.
Figure 1 shows the Si depth distribution resulting from the
implant. Each sample was subsequently annealed at 1050°C
for 1 h in a forming gas͑ 95%N2 , 5 % H2͒ ambient. By keep-
ing both the implantation and annealing conditions constant
for each sample, it is expected that they will have the same
Si concentration-depth profile and the same nanocrystal sizedistribution. The only significant parameter that differs from
sample to sample is the thickness of the oxide layer as illus-
trated schematically in Fig. 2. PL spectra were collected at
room temperature using a 532 nm Rumzing diode pumped
solid state laser as the pump source (incident angle 14°) and
a TRIAX 320 spectrometer with a liquid nitrogen cooled
SpectrumOne charge-coupled device as the detection system.
The PL emission was collected with f4 optics consisting of
two matched plano-convex lens of 50 mm diameter and
200 mm focal length, giving a collection angle of up to ±7°.
Reflectivity measurements were performed at an incident
angle of 5° using a Shimadzu UV-3101PC spectrophotom-
eter with a specular reflectance attachment (P/N 206-14046).The bandwidth for the illumination system was set at 5 nm.
Figures 3(a)–3(d) show both the normalized PL intensity
and the measured reflectance for each sample as a function of
wavelength. It is immediately apparent that the measured PL
spectra vary significantly as a function of oxide thickness,
with variations in peak position, width and structure clearly
evident. Inspection of panel 3(a) indicates that the fused
silica sample yields an approximately constant reflectance
over the entire wavelength range of interest. The PL spec-
trum is therefore not expected to be distorted significantly by
a)Author to whom correspondence should be addressed; electronic mail:
FIG. 1. Depth distribution of implanted ions as calculated using transport of
ions in matter TRIM software.20
JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 8 15 OCTOBER 2004
0021-8979/2004/96(8) /4650/3/$22.00 © 2004 American Institute of Physics4650
Downloaded 07 Jun 2012 to 59.162.23.73. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
7/28/2019 Effect of Material Structure on PL Spectra From Silicon NC
http://slidepdf.com/reader/full/effect-of-material-structure-on-pl-spectra-from-silicon-nc 3/4
the material structure, even though the PL intensity will still
be affected. The strong correlation between the reflectivity of
the material structure and the measured PL spectra is most
readily apparent for the 5 m thick SiO2 layer, shown in Fig.
3(d). In this case the PL spectrum is clearly modulated by the
reflectivity of the structure resulting in an obvious distortion
of the emission spectrum.
Figures 3(b) and 3(c) show much more subtle distor-
tions. For example, when compared to the spectrum from thesilica sample, the PL spectrum from the 103 nm SiO 2 layer,
Fig. 3(b), shows higher peak intensity but a similar full width
at half maximum (FWHM) (143 nm, compared to 147 nm
observed for silica). On the other hand the spectrum from the
970 nm SiO2 layer, Fig. 3(c), exhibits two obvious peaks as
a direct consequence of the spectral distortion caused by the
surrounding material structure, with the main peak being
centered at a wavelength of 780 nm compared to 740 nm
recorded for the silica sample. Such a peak shift can readily
lead to misinterpretation of the mean nanocrystal size.8,14,15
For example, using a simple expression for the relationship
between the PL emission energy and nanocrystal diameter16
suggests that PL peaks at 780 nm and 740 nm correspond to
nanocrystals with mean diameters of Ϸ4.8 or 4.1 nm, respec-
tively.
The often subtle distortion of spectra is further high-
lighted in Fig. 4 which compares PL spectra from the
103 nm and 650 nm thick SiO2 layers. The PL spectrum
from the latter shows an increase in peak intensity as well as
a considerable reduction in FWHM from 143 nm to 103 nm
for the thicker layer. Assuming inhomogeneous broadeningof the emission this corresponds to a change in the standard
deviation of the nanocrystal size distribution from 0.3 nm for
the narrower peak to 0.6 nm for the wider peak.5
It is inter-
esting to note that this difference occurs despite the fact that
the nanocrystal preparation conditions are identical, the only
difference between the two samples being an additional
547 nm of oxide. Clearly, the assumption that the width of
PL spectra from Si nanocrystals results primarily from inho-
mogeneous broadening due to the size distribution of nanoc-
rystals assumes that due care has been taken to account for
effects such as those illustrated in Figs. 3 and 4.
The data in Fig. 3 and 4 highlight the relationship be-
tween the observed PL and sample reflectance, demonstrat-ing that this is the dominant effect leading to spectral distor-
tion. Assuming that the structure is nonabsorbing, the
transmission and reflection characteristics of a given struc-
ture are inversely related (i.e., R + T =1, where R is the re-
flectivity and T the transmissivity of the structure). Hence, a
local maximum in reflectivity corresponds to a local mini-
mum in transmissivity and therefore to a minimum in the
measured PL emission. A first-order estimate of the position
of reflectivity maxima can be made from a simple construc-
tive interference model, with max = 2nt / ͑ m + 1 / 2͒ , where
max is the wavelength for maximum reflectivity, n is the
refractive index of the SiO2 layer͑ nϳ1.46͒ , t is the film
FIG. 2. Schematic representation of typical sample structures under inves-
tigation. The implantation and annealing condition are the same for all
samples. Only the thickness of the oxide layer is different for each sample.
FIG. 3. Photoluminescence (bold line) and reflectivity (thin line) spectra
from different sample structures: (a) fused silica sample, (b) 103 nm oxide
layer, (c) 970 nm oxide layer, (d) 5 m oxide layer.
FIG. 4. Photoluminescence (bold line) and reflectivity (thin line) spectra
from different sample structures: (a) 103 nm oxide layer, (b) 650 nm oxide
layer.
J. Appl. Phys., Vol. 96, No. 8, 15 October 2004 Orbons, Spooner, and Elliman 4651
Downloaded 07 Jun 2012 to 59.162.23.73. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
7/28/2019 Effect of Material Structure on PL Spectra From Silicon NC
http://slidepdf.com/reader/full/effect-of-material-structure-on-pl-spectra-from-silicon-nc 4/4
thickness, and m is the interference order. (Comparison of
the PL and reflectivity spectra reveals a small phase shift
between the reflectance and PL spectra, most obvious in Fig.
3(d), an effect that is attributed to the different illumination
conditions employed for each measurement.)
It should be noted that it is not always trivial to predict
the reflectivity of the material structure as this depends not
only on the initial material structure but also on the distribu-
tion and concentration of implanted silicon. By using thecalculated implant profile, together with Maxwell-Garnet ef-
fective medium theory17
and the Fresnel equations,18
a rea-
sonable estimate of the reflectivity can be achieved.19
How-
ever, such analysis confirms the significant role of the
implanted silicon distribution in determining interference ef-
fects and highlights its influence on measured reflection and
absorption characteristics.19
(In previous measurements19
we
have shown that this can lead to misinterpretation of optical
absorption data.) Also implicit in the above discussion is the
fact that the PL intensity and distribution will depend on the
angle of detection, and on the wavelength and angle of inci-
dence of the probe beam.
In conclusion, it has been shown that the PL emissionfrom Si nanocrystals can be distorted by the optical proper-
ties of the sample structure, even in cases where that struc-
ture is relatively simple. These distortions affect the inten-
sity, peak position, and width of the PL spectra and it is vital
that such distortion be taken into account both when compar-
ing PL spectra from different structures and when extracting
nanocrystal parameters from observed PL spectra.
1D. Kovalev and H. Heckler, Phys. Status Solidi B 215, 871 (1999).
2H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki, and T. Nakagiri, Appl.
Phys. Lett. 56, 2379 (1990).3L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990).
4A. Kux and M. B. Chorin, Phys. Rev. B 51, 17535 (1995).
5V. Ranjan, V. A. Singh, and G. C. John, Phys. Rev. B 58, 1158 (1998).
6G. Ledoux, O. Guillois, D. Porterat, C. Reynaud, F. Huisken, B. Kohn, and
Paillard, Phys. Rev. B 62, 15942 (2000).7F. Iacona, G. Franzo, E. Moreira, D. Pacifici, A. Irrera, and F. Priolo,
Mater. Sci. Eng., C 19, 377 (2002).8D. Lockwood and Z. Lu, Phys. Rev. Lett. 76, 539 (1996).
9D. Lockwood and B. Sullivan, J. Lumin. 80, 75 (1999).
10J. Bellessa, S. Rabaste, J. Plenet, J. Dumas, J. Mugnier, and O. Marty,
Appl. Phys. Lett. 79, 2142 (2001).11
L. Pavesi, G. Panzarini, and L. Andreani, Phys. Rev. B 58, 15794 (1998).12
V. Pellegrini, A. Tredicucci, C. Mazzoleni, and L. Pavesi, Phys. Rev. B
52, 14328 (1995).13
E. F. Schubert, Appl. Phys. Lett. 61, 1381 (1992).14
M. Brongersma, A. Polman, K. Min, E. Boer, T. Tambo, and H. Atwater,
Appl. Phys. Lett. 72, 2577 (1998).15
R. Soni, L. Fonseca, O. Resto, M. Buzaianu, and S. Weisz, J. Lumin. 83,
187 (1999).16
G. Ledoux, O. Guillois, D. Porterat, C. Reynaud, F. Huisken, B. Kohn, and
V. Paillard, Phys. Rev. B 62, 15942 (2000).17
C. Bohren and D. Huffman, Absorption and Scattering of Light by Small
Particles (Wiley, New York, 1983).18
M. Born and E. Wolf, Principles of Optics (Pergamon, New York, 1959).19
R. Elliman, M. Lederer, and B. Luther-Davies, Appl. Phys. Lett. 80, 1325
(2002).20
J. F. Ziegler, J. P. Biersack, and U. Littmark, The Stopping and Range of
Ions in Solids (Pergamon, New York, 1985).
4652 J. Appl. Phys., Vol. 96, No. 8, 15 October 2004 Orbons, Spooner, and Elliman
D l d d 07 J 2012 t 59 162 23 73 R di t ib ti bj t t AIP li i ht htt //j i / b t/ i ht d i i