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228
IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 3, MARCH 2008
Thin Amorphous Si/Si3N4-Based Light-Emitting
Device Prepared With Low Thermal Budget
W. K. Tan, M. B. Yu, Q. Chen, W. Y. Loh, J. D. Ye, X. H. Zhang, G. Q. Lo, and D.-L. Kwong
Abstract—This letter reports for the first time on an electrically pumped silicon light-emitting device with a thin multilayer
stacked amorphous silicon (α-Si, in thickness of 3–7 nm)/silicon
nitride (∼10 nm) structure. The observed photoluminescence (PL)
is tunable from ∼700 to ∼670 nm, and intensity increases by
decreasing the α-Si thickness. The PL intensity can be enhanced
through postdeposition annealing at relatively low temperatures
and a short annealing time (e.g., as optimized at 700 ◦ C/10 min).
Electroluminescence from devices that are built upon the proposed structure originates from electron–hole pair recombination,
and the carrier injection mechanism is through Frenkel–Poole
tunneling. Our proposed structure, being highly complimentary
metal–oxide–semiconductor compatible, benefits from a low thermal budget process coupled with an accurate layer thickness
control.
Index Terms—Electroluminescence (EL), light emitting, photoluminescence (PL), α-Si/SiN multilayer stack.
This letter reports, to the best of the authors’ knowledge,
on the first demonstration of light emission from a multilayer
stacked thin α-Si and SiN structure. The use of thin α-Si
layers in a quantum-well fashion is expected to have better
dimensional control as compared to the use of Si-nc. We choose
α-Si instead of polycrystalline-Si or Si-nc since this can reduce
the thermal budget; furthermore, it has been argued that α-Si
has higher radiative recombination efficiency, as the disorder in
the structure relaxes the selection rule [16]. We demonstrate that
the light emission most likely originates from the quantum confinement of the α-Si. The photoluminescence (PL) intensity can
be enhanced by postdeposition annealing at low thermal cycle
(e.g., 700 ◦ C/10 min) while maintaining the amorphous state
of the Si layers. From simple devices with p+-polyelectrode/
α-Si/SiN stacks/n+-Si substrate, the uniform EL that is visible
to the naked eye was observed across the whole pad.
I. I NTRODUCTION
F
OR THE light emitter in the Si photonics field, huge
efforts have been devoted to circumvent the disadvantage
of silicon to develop it as a viable optical material. Several
approaches have been reported, including Si-nanocrystals
(Si-nc) [1]–[3], Si/Ge superlattice [4], Si dislocation engineering [5], [6], and, recently, extremely thin silicon-on-insulator
transistors [7], [8]. Among these materials and device structures, structures with Si-nc embedded in SiOx have been
investigated the most since the report on its optical gain by
Pavesi et al. [9]. Such a system, while efficient under optical
excitation, is not suitable for fabricating electrically excited
devices due to the large band offset between SiO2 and Si.
Although electroluminescence (EL) has been demonstrated on
such structures, operation is limited to either pulsed operation
[10] or under high field conditions [1]. The tunability of the
emission wavelength, particularly toward the shorter wavelengths, has also been a problem because the interfaces of Si
with the surrounding SiO2 tend to form radiative sites [11].
Therefore, recent research has focused on embedding Si-nc in
the SiNx matrix [12]–[14], whereas another approach used the
field enhancement effect by forming Si nanopyramids at the
SiOx /Si interface [15].
Manuscript received October 30, 2007; revised December 3, 2007. The
review of this letter was arranged by Editor P. Yu.
W. K. Tan, M. B. Yu, Q. Chen, W. Y. Loh, J. D. Ye, G. Q. Lo, and
D.-L. Kwong are with the Institute of Microelectronics, A*STAR, Singapore
117685 (e-mail: logq@ime.a-star.edu.sg).
X. H. Zhang is with the Institute of Materials Research and Engineering,
A*STAR, Singapore 117602.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2007.915379
II. E XPERIMENTAL
For active layers, ten periods of alternating α-Si/SiN layers
are deposited through plasma-enhanced chemical vapor deposition on p-type Si substrate (100). The SiN layers were
deposited by using SiH4 /NH3 with N2 dilution, with gas flows
for SiH4 /NH3 and N2 at 110/38 and 2500 sccm, respectively,
during the deposition. The RF power was 410 W. Deposition
temperature and pressure were at 400 ◦ C and 4.2 torr, respectively. α-Si layers were deposited using SiH4 (20 sccm) with
Ar dilution (2500 sccm). The RF power was 50 W. Deposition
temperature and pressure were also at 400 ◦ C and 4.2 torr,
respectively. The α-Si layers for wafers SL5, SL6, and SL7
were ∼3, ∼5, and ∼7 nm, respectively. The SiN layers were
kept at 10 nm for all cases. Small samples were diced out of the
wafers and were subsequently subjected to annealing at various
temperatures and time in the N2 ambient.
For an electrically biased light-emitting device (LED), the
starting n-substrate was as-implanted to form the N+-bottom
layer. The same active multilayers of α-Si/SiN were deposited,
followed by poly-Si ∼100-nm deposition, which was implanted
with B11 to form the P+-poly. Circular and square structures
with size of 1–9 mm2 were patterned by lithography and
reactive ion etching through poly-Si/active layers to expose the
bottom N+-region, and were then subjected to annealing at
700 ◦ C/10 min.
III. R ESULTS AND D ISCUSSION
Fig. 1 shows the EL testing device structure. The inset in
Fig. 1 shows the TEM of the annealed samples, which did not
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TAN et al.: THIN AMORPHOUS Si/Si3 N4 -BASED LED PREPARED WITH LOW THERMAL BUDGET
Fig. 1. Schematic of the EL device. An expanded view showing the TEM
of the active region after being annealed at high temperature. The two TEM
image insets (a and b) compare the devices used for EL and PL measurements,
respectively (with identical layer structures, annealed at identical conditions).
The other inset shows a selective area electron diffraction pattern indicating the
amorphous state.
Fig. 2. Relative PL intensity measured at room temperature for samples from
SL5 annealed at various temperatures for a fixed period of 10 min. Note that
the 500 ◦ C point on the axis corresponds to the as-grown condition. The inset
shows the PL spectra of SL5, SL6, and SL7.
reveal the formation of Si-nc. The inset in the TEM is the
electron diffraction pattern of the α-Si layers, which further
confirmed the amorphous state after the annealing. Further
proofs of the amorphous state of the Si layers were collected
using micro-Raman spectroscopy.
Fig. 2 plots the PL intensity as samples being optimized
through different postdeposition annealing temperatures for a
fix annealing time of 10 min. From Fig. 2, it can be seen that the
PL intensity initially increases with increasing annealing temperature up to 700 ◦ C (enhanced ∼2.7× that of as-grown), after
which the PL intensity decreases with increasing annealing.
The effect of the annealing time was also investigated. It was
noted that for the periods investigated (10, 30, 60, and 90 min),
the annealing time has little effect on the PL intensity. The inset
in Fig. 2 shows the spectra for samples from SL5, SL6, and SL7.
It can be seen that the peak PL wavelength exhibits a blue shift
from ∼706 to ∼674 nm as the α-Si thickness is reduced from
∼7 to ∼3 nm. It is noted that the PL intensity also increases
with decreasing α-Si layer thickness. As such, we proposed that
the origin of the PL is from the quantum-confined α-Si layers
[17]. Compared to that in [18] and [19], the optimum PL was
229
Fig. 3. Plot of the current density versus the effective applied field. The
current flow shows little polarity dependence. The inset shows the Arrhenius
plot for operating temperatures between 20 ◦ C and 80 ◦ C.
also achieved at an annealing temperature of 700 ◦ C with a short
annealing time. However, the major differences between ours
and that of Dal Negro et al. [18], [19] are the amorphous nature
of Si in our structure and the fact that wavelength tunability can
be achieved.
To investigate the charge injection in the devices, the current
density–voltage (J–V ) characteristics of the devices were measured. Fig. 3 plots the typical current density versus the applied
field characteristic of a device with 9-mm2 area measured at a
chuck temperature of 30 ◦ C. There is little polarity dependence.
We note that the injected current density is low for the applied
fields even when compared to [1], where an oxide dielectric
was used. However, this could have been a result of a resistive
top electrode that resulted in a reduced applied field across
the active layers. The inset shows the Arrhenius plot for various applied fields, plotted between operating temperatures of
20 ◦ C and 80 ◦ C. The current flow through our structure is well
fitted by charge transport through Frenkel–Poole tunneling. The
barrier height of 1.024 eV is extrapolated from the data, which
is a reasonable value considering that the conduction band
offset and the valance band offset between Si and SiN are 2.4
and 1.8 eV, respectively [20].
Fig. 4 shows the EL spectra of devices fabricated from active
layers that are identical to those of SL5, SL6, and SL7. Blue
shifting of the emission wavelength is also observed for the
LEDs as the thickness of the α-Si layers decreased. There is
a blue shift for all the EL spectra compared to that of the
PL spectra of identical layers. This might have resulted from
the additional thermal processes during the fabrication of the
LEDs or a result of the absorption of the poly-Si electrodes
(we could not measure the PL from these devices due to
the huge absorption of the electrodes). EL is observed under
the forward bias (positive voltage applied to top electrode)
and reversed bias (negative voltage) conditions for currents
≥ 0.5 mA with both spectra virtually identical, whereas in
[21], EL is only observed under the forward bias condition.
The inset in Fig. 4(a) shows the integrated EL intensity of one
device for an increasing injection current. It can be seen that
the EL intensity linearly increases with an increasing current.
There is virtually no change to either the shape or the position
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IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 3, MARCH 2008
as the active region of the LED. Blue shifting of emission
wavelengths is also observed as the α-Si layers in the active
region decrease. The low external efficiency of the present
devices (as typical operating voltages are ≥ 40 V) is expected
to increase by improving the conductance of the top electrode
and with the use of even thinner dielectric layers.
ACKNOWLEDGMENT
The authors would like to thank the staff in the SPT Laboratory of the Institute of Microelectronics, Singapore, for their
assistance in sample preparations.
R EFERENCES
Fig. 4. (a) EL spectra of device active layers identical to SL5, SL6, and SL7.
The coupling has not been optimized for each measurement. This accounts
for the lower EL intensity of the device with 5-nm α-Si layer thickness. The
inset shows the integrated EL intensity with an increasing injection current.
(b) Square device lighted when biased. The inset shows the same die taken
under the lighted condition, showing the details of the die.
of the EL spectra with an increasing current. The EL most
likely originates from electron–hole pair recombination in the
quantum-confined α-Si layers [20].
Fig. 4(b) shows a picture of a die consisting of several LEDs,
with bias applied to one of the square devices (taken under very
dim light conditions). The inset shows the same die taken under
the lighted condition without any bias. The EL is visible to the
naked eye under dim light conditions. It can be clearly seen that
light is emitted only from the excited region (9-mm2 square
pad) and not from an isolated defected region that would have
otherwise resulted in a radial emission pattern. By calibrating
the measurement system to a commercial orange LED, the
measured wall plug efficiency is in the order of 10−9 . Although
this figure seems very low, we stress here that this is a gross
underestimate of the actual efficiency, as the output power is
only collected from a small area of the pad using a multimode
fiber. We believe that the actual wall plug efficiency is at least a
few orders of magnitude higher.
IV. C ONCLUSION
We have demonstrated light emission from a multilayer
α-Si/SiN stack. The structure has been successfully applied
[1] G. Franzo, A. Irrera, E. C. Moreira, M. Miritello, F. Iacona, D. Sanfilippo,
G. Di Stefano, P. G. Fallica, and F. Priolo, “Electroluminescence of silicon
nanocrystals in MOS structures,” Appl. Phys. A, Solids Surf., vol. 74,
no. 1, pp. 1–5, 2002.
[2] L. Dal Negro, M. Cazzanelli, N. Daldosso, Z. Gaburro, L. Pavesi,
F. Priolo, D. Pacifici, G. Franzo, and F. Iacona, “Stimulated emission
in plasma-enhanced chemical vapour deposited silicon nanocrystals,”
Phys. E, vol. 16, no. 3/4, pp. 297–308, Mar. 2003.
[3] D. Jurbergs, E. Rogojina, L. Mangolini, and U. Kortshagen, “Silicon
nanocrystals with ensemble quantum yields exceeding 60%,” Appl. Phys.
Lett., vol. 88, no. 23, pp. 233 116.1–233 116.3, Jun. 2006.
[4] N. D. Zakharov, V. G. Talalaev, P. Werner, A. A. Tokikh, and
G. E. Cirlin, “Room-temperature light emission from a highly strained
Si/Ge superlattice,” Appl. Phys. Lett., vol. 83, no. 15, pp. 3084–3086,
Oct. 2003.
[5] W. L. Ng, M. A. Lourenco, R. M. Gwilliam, S. Ledain, G. Shao, and
K. P. Homewood, “An efficient room-temperature silicon-based
light-emitting diode,” Nature, vol. 410, no. 6825, pp. 192–194,
Mar. 2001.
[6] T. Hoang, P. LeMinh, J. Holleman, and J. Schmitz, “The effect of dislocation loops on the light emission of silicon LEDs,” IEEE Electron Device
Lett., vol. 27, no. 2, pp. 105–107, Feb. 2006.
[7] S. Saito, D. Hisamoto, H. Shimizu, H. Hamamura, R. Tsuchiya,
Y. Matsui, T. Mine, T. Arai, N. Sugii, K. Torii, S. Kimura, and
T. Onai, “Silicon light-emitting transistor for on-chip optical interconnection,” Appl. Phys. Lett., vol. 89, no. 16, pp. 163 504.1–163 504.3,
Oct. 2006.
[8] T. Hoang, P. LeMinh, J. Holleman, and J. Schmitz, “Strong efficiency
improvement of SOI-LEDs through carrier confinement,” IEEE Electron
Device Lett., vol. 28, no. 5, pp. 383–385, May 2007.
[9] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, and F. Priolo, “Optical
gain in silicon nanocrystals,” Nature, vol. 408, no. 6811, pp. 440–444,
Nov. 2000.
[10] R. J. Walters, J. Carreras, T. Feng, L. D. Bell, and H. A. Atwater,
“Silicon nanocrystal field-effect light-emitting devices,” IEEE J. Sel.
Topics Quantum Electron., vol. 12, no. 6, pp. 1647–1656,
Nov./Dec. 2006.
[11] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and C. Delerue,
“Electronic states and luminescence in porous silicon quantum dots:
The role of oxygen,” Phys. Rev. Lett., vol. 82, no. 1, pp. 197–200,
Jan. 1999.
[12] L. Dal Negro, J. H. Yi, J. Michel, L. C. Kimerling, S. Hamel,
A. Williamson, and G. Galli, “Light-emitting silicon nanocrystals and
photonic structures in silicon nitride,” IEEE J. Sel. Topics Quantum
Electron., vol. 12, no. 6, pp. 1628–1635, Nov./Dec. 2006.
[13] K. S. Cho, N. M. Park, T. Y. Kim, K. H. Kim, G. Y. Sung, and J. H. Shin,
“High efficiency visible electroluminescence from silicon nanocrystals
embedded in silicon nitride using a transparent doping layer,” Appl. Phys.
Lett., vol. 86, no. 7, p. 071 909, Feb. 2005.
[14] L. Y. Chen, W. H. Chen, and F. C. N. Hong, “Visible electroluminescence from silicon nanocrystals embedded in amorphous silicon nitride matrix,” Appl. Phys. Lett., vol. 86, no. 19, p. 193 506,
May 2005.
[15] G. R. Lin, C. K. Lin, L. J. Chou, and Y. L. Chueh, “Synthesis of Si
nanopyramids at SiOx /Si interface for enhancing electroluminescence of
Si-rich SiOx ,” Appl. Phys. Lett., vol. 89, no. 9, pp. 093 126.1–093 126.3,
Aug. 2006.
TAN et al.: THIN AMORPHOUS Si/Si3 N4 -BASED LED PREPARED WITH LOW THERMAL BUDGET
[16] G. Allan, C. Delerue, and M. Lannoo, “Electronic structure of amorphous
silicon nanoclusters,” Phys. Rev. Lett., vol. 78, no. 16, pp. 3161–3164,
Apr. 1997.
[17] W. K. Tan, M. B. Yu, Q. Chen, J. D. Ye, G. Q. Lo, and D. L. Kwong,
“Red light emission from controlled multilayer stack comprising of thin
amorphous silicon and silicon nitride layers,” Appl. Phys. Lett., vol. 90,
no. 22, pp. 221 103.1–221 103.3, May 2007.
[18] L. Dal Negro, J. H. Yi, J. Michel, and L. C. Kimerling, “Light emission
efficiency and dynamics in silicon-rich silicon nitride films,” Appl. Phys.
Lett., vol. 88, no. 23, pp. 233 109.1–233 109.3, Jun. 2006.
231
[19] L. Dal Negro, J. H. Yi, and L. C. Kimerling, “Light emission from
silicon-rich nitride nanostructures,” Appl. Phys. Lett., vol. 88, no. 18,
pp. 183 103.1–183 103.3, May 2006.
[20] J. Robertson, “Band offsets of wide-band-gap oxides and implications for
future electronic devices,” J. Vac. Sci. Technol. B, Microelectron. Process.
Phenom., vol. 18, no. 3, pp. 1785–1791, May 2000.
[21] G. Y. Sung, N. M. Park, J. H. Shin, K. H. Kim, T. Y. Kim, K. S. Cho,
and C. Huh, “Physics and device structures of highly efficient silicon
quantum dots based silicon nitride light-emitting diodes,” IEEE J. Sel.
Topics Quantum Electron., vol. 12, no. 6, pp. 1545–1555, Nov./Dec. 2006.