1.5 μm luminescence of silicon nanowires fabricated by thermal evaporation of SiO

Article abstract of DOI:10.1002/pssa.200622143

Silicon nanowires (NWs) fabricated by thermal evaporation of SiO were studied by cathodoluminescence. A band around 1550 nm (0.8 eV) was observed. It appears above 225 K and its intensity increases with increasing temperature. The broad band consists of the defect-related D1 and D2 lines and is supposed to be formed by extended defects within the NWs that are decorated with oxygen. Moreover, luminescence bands are found that are related to Si oxide and/or the interface between Si and Si oxide. In addition, the Si band-to-band line and the G center are observed.

Full text of DOI:10.1002/pssa.200622143

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phys. stat. sol. (a) 203, No. 8, R55–R57 (2006) / DOI 10.1002/pssa.200622143
1.5 µm luminescence
www.pss-rapid.com
of silicon nanowires fabricated
by thermal evaporation of SiO
G. Jia1, 2, M. Kittler*, 1, 2, Z. Su3, D. Yang4, and J. Sha3
1
IHP microelectronics, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany
2
IHP/BTU Joint Lab, Konrad-Wachsmann-Allee 1, 03046 Cottbus, Germany
3
Department of Physics, Zhejiang University, Hangzhou 310027, P.R. China
4
State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, P.R. China
Received 20 March 2006, revised 10 April 2006, accepted 26 April 2006
Published online 3 May 2006
PACS 61.72.–y, 78.55.Ap, 78.60.Hk, 78.67.Lt, 81.07.Vb
*
Corresponding author: e-mail kittler@ihp-microelectronics.com
Silicon nanowires (NWs) fabricated by thermal evaporation supposed to be formed by extended defects within the NWs
of SiO were studied by cathodoluminescence. A band around that are decorated with oxygen. Moreover, luminescence
1550 nm (0.8 eV) was observed. It appears above 225 K and bands are found that are related to Si oxide and/or the inter-
its intensity increases with increasing temperature. The broad face between Si and Si oxide. In addition, the Si band-to-band
band consists of the defect-related D1 and D2 lines and is line and the G center are observed.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Optical interconnection is needed for future integrated
Si-NWs were fabricated on top of p-type (111) Si sub-
circuits. Many key photonic components compatible with strate with 0.001 Ω cm resistivity by thermal evaporation
CMOS technology have already been demonstrated, as for of Si monoxide [6, 7]. Transmission electron microscopy
example a fast Si based electro-optical modulator, allowing (TEM) reveals that the NWs have a diameter of about
the use of continuous-wave light sources. However, a sili- 20–30 nm, a length of tens of micrometers and exhibit a
con based light emitter which can be integrated in the chip crystalline structure. Oxygen was detected at the NWs
is still lacking. The desired light emitter should exhibit a by energy-dispersive X-ray spectroscopy (EDX) because
high luminescence efficiency at room temperature (RT) the Si core of the NWs is surrounded with a Si oxide
and emit at about 1.55 µm or 1.3 µm [1, 2].
Semiconductor nanowires exhibit new properties. Re-
shell.
The NWs were investigated by cathodoluminescence
cently, it was shown that the light extraction from an (CL) using a Zeiss EVO 40 scanning electron microscope
InGaN/GaN LED was markedly enhanced by using nano- equipped with a cold stage, a Gatan MonoCL system and a
rod/nanowire arrays instead of the entire area of the LED Hamamatsu detector. The measurements were executed
[3]. This approach could also be used for silicon if Si na- with an acceleration voltage of 7 kV and a beam current of
nowires (Si-NWs) with appropriate luminescence behav- about 50 nA. The temperature of the sample was varied be-
iour are available. Si is believed to develop a direct band tween 78 K and RT. The sample was scanned usually over
2
gap in nanometer-sized structures. Calculations showed an area of about thousand µm with normal incidence of
that [110] oriented Si-NWs have a direct bandgap. How- the electron beam to the wafer surface.
ever, the density of states was found to be very small at the
Figure 1 shows a characteristic spectrum of the
conduction band edge [4]. Er surface-enriched Si-NWs we- Si-NWs measured at 78 K. Three main bands can be clearly
re reported to cause the desired light emission at about seen. The bands 1, 2 and 3 appear at about 660 nm, 920 nm
1.55 µm [5]. Here we show that Si-NWs are capable to and 1280 nm, respectively. Upon increasing temperature
yield efficient RT emission around 1.55 µm without an ad- their intensities are found to decrease but a new band
ditional Er doping.
around 1550 nm appears, see Fig. 2. Details about the
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G. Jia et al.: 1.5 µm luminescence of silicon nanowires fabricated by thermal evaporation of SiO
Table 1 Features deduced from the luminescence spectra.
peak/band wavelength (nm) cause
0
~460
660
Si oxide and
1
interface Si/Si oxide
nd
2 order of peak 0 (?)
2
920
2*
3
1080
1280
1320
1550
1420
BB line of Si
G center
nd
2 order of peak 1
3*
4
D1 line
D2 line
Peaks 0, 1, 2 and 3* are related to Si oxide and/or the
interface between Si and Si oxide. Recent observation of
the peaks 0, 1 and 2 in CL spectra, emitted from multiple
Figure 1 CL spectrum of Si nanowires measured at 78 K. The
bands 1, 2 and 3 can clearly be seen. Additional lines 0, 2* and
3* can be found by deconvolution; for lines 2* and 3* compare
insets.
quantum wells of alternating Si and SiO layers on quartz
2
substrate, confirms this view [8].
Features related to band 0 were reported in the litera-
ture. A dominating band at 460 nm, arising from the inter-
face between the SiO layer and Si substrate, was observed
2
spectra, which will be discussed below, are summarized in
Table 1.
by CL [9]. A similar band was reported for Si-NWs fabri-
cated by evaporation of SiO [10]. It was attributed to the
oxygen vacancy in Si oxide. Peak 2 (920 nm) might be in-
The spectral region around band 2 exhibits two fea-
tures, namely peak 2 at 920 nm and an additional peak 2*
at 1080 nm, see left inset in Fig. 1. Detailed analysis of the
spectral region around band 3 is represented in the right in-
set of Fig. 1. It shows a sharp peak 3 at 1280 nm and a
broad peak 3* with a maximum at about 1320 nm. On the
left shoulder of peak 1 a band in the range 450–550 nm,
denoted here band 0, was found by deconvolution. Due to
decreasing sensitivity of the CL system on the low wave-
length side the measured intensity for band 0 is strongly
reduced. We believe that its real position is closer to
450 nm than to 550 nm.
nd
terpreted as 2 order of peak 0 (460 nm). However, the
fact that the intensity of peak 0 is much smaller than that of
peak 2 is in opposition to this view.
Features related to peak 1 are reported by photolumi-
nescence (PL) analysis of Si-NWs surrounded with a Si
oxide shell [10, 11]. They are attributed to Si oxide and the
nd
Si/Si oxide interface. Peak 3* at about 1320 nm is the 2
order of peak 1 (660 nm).
Peak 2* at 1080 nm reflects clearly the band-to-band
(BB) transition in crystalline Si, see e.g. [12]. It can be
generated by radiative recombination within the Si-NWs
and/or in the Si substrate, but Auger recombination is be-
lieved to suppress its formation in the highly doped sub-
strate. The sharp peak 3 at 1280 nm (0.97 eV) can be as-
cribed to the G center. The G center was observed by CL
to be connected with grain boundaries/twins in Si [13].
Consequently, its appearance indicates the existence of
twin boundaries within the Si-NWs. Indeed, twins and
other extended defects have been directly observed by
TEM within Si-NWs, e.g. [14–16].
The G center is known to exist in carbon-rich Si, i.e. its
appearance points to carbon (C) contamination of the NWs.
A C-induced twinning process has been reported for Si as a
rather general process where a single layer of C atoms is
incorporated in the twin planes [17]. For example, twin
boundaries in EFG and block-cast Si were found to ac-
commodate C atoms, e.g. [17]. We suppose that also twins
in the Si-NWs accommodate a large portion of C. Thus, we
believe that according to the above discussions the forma-
tion of peaks 0, 1, 2, 2* and 3* is not related to C, except
for the G center (peak 3).
Figure 2 CL spectra of Si nanowires measured at room tempera-
ture exhibiting a new intensive band 4 around 1550 nm. The up-
per spectrum (P) was observed with planar incidence of the elec-
tron beam and the lower spectrum (X) was measured on the
cross-section after cleaving the sample.
Figure 2 shows CL spectra measured at RT with planar
geometry (P) and, after cleaving the sample, with the beam
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Acknowledgment We would like to thank T. Arguirov for
PL measurements.
scanning the cross-section in the region of the NWs (X).
The bands that are found for low temperature (see e.g.
Fig. 1) exhibit at RT a smaller intensity or disappear. How-
ever, an intensive new band 4 is observed around 1550 nm
(about 0.8 eV). In spectrum X only this band can be seen.
Band 4 becomes visible at about 225 K and its intensity in-
creases with increasing temperature. A similar band around
0.8 eV (1550 nm) was reported for nanocrystalline Si,
namely for CVD-grown nanocrystalline Si films [18] and for
Si nanocrystals fabricated by mechanical milling of Si [19].
Within the nanocrystals a high density of extended defects
was found by TEM in both types of samples.
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Si-NWs fabricated by evaporation of SiO is supposed to be
caused by extended defects within the NWs that are con-
taminated with oxygen. We also assume that the interface
between the Si oxide shell and the Si core of the NWs acts
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