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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

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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:

rob.elliman@anu.edu.au

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

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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

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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.

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4652 J. Appl. Phys., Vol. 96, No. 8, 15 October 2004 Orbons, Spooner, and Elliman

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