Silicon carbide LED
Silicon carbide has been widely used as material for manufacturing yellow, red, green LED and optoelecronics devices (in¬dicators, screens). The silicon carbide LED technology has been in¬vestigated for improvement of their operational characteristics. This in¬cludes the influences of the surface pr...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2002
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nasplib_isofts_kiev_ua-123456789-1211382025-02-23T18:02:52Z Silicon carbide LED Vlaskina, S.I. Silicon carbide has been widely used as material for manufacturing yellow, red, green LED and optoelecronics devices (in¬dicators, screens). The silicon carbide LED technology has been in¬vestigated for improvement of their operational characteristics. This in¬cludes the influences of the surface processing (etching, annealing), the formation method for the p-n junctions and the contacts on the LED properties. Light-emitting devices used as light sources for optical-fiber communication lines. LED fabricated by Al⁺ ion-implanted in 6H-SiC and investigated their characteristics for an effective green LED. The brightness of the ion-implanted p-n junction was found to be two orders higher than that of diffusion p-n junction, and the best value was 2000-10000 cd/m² with passing current about 0.5 mA through area 50x50 mm and applied voltage about 2.6 ± 0.2 V. The ion-implanted structures showed a high stability of light in the temperature range of 77-600 K. 2002 Article Silicon carbide LED / S.I. Vlaskina // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 1. — С. 71-75. — Бібліогр.: 2 назв. — англ. 1560-8034 PACS: 77.55.+f, 77.80. https://nasplib.isofts.kiev.ua/handle/123456789/121138 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Silicon carbide has been widely used as material for manufacturing yellow, red, green LED and optoelecronics devices (in¬dicators, screens). The silicon carbide LED technology has been in¬vestigated for improvement of their operational characteristics. This in¬cludes the influences of the surface processing (etching, annealing), the formation method for the p-n junctions and the contacts on the LED properties. Light-emitting devices used as light sources for optical-fiber communication lines. LED fabricated by Al⁺ ion-implanted in 6H-SiC and investigated their characteristics for an effective green LED. The brightness of the ion-implanted p-n junction was found to be two orders higher than that of diffusion p-n junction, and the best value was 2000-10000 cd/m² with passing current about 0.5 mA through area 50x50 mm and applied voltage about 2.6 ± 0.2 V. The ion-implanted structures showed a high stability of light in the temperature range of 77-600 K. |
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Vlaskina, S.I. Silicon carbide LED Semiconductor Physics Quantum Electronics & Optoelectronics |
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Vlaskina, S.I. |
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Vlaskina, S.I. |
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Silicon carbide LED |
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Silicon carbide LED |
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Silicon carbide LED |
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Silicon carbide LED |
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Silicon carbide LED |
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silicon carbide led |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2002 |
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Silicon carbide LED / S.I. Vlaskina // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 1. — С. 71-75. — Бібліогр.: 2 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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AT vlaskinasi siliconcarbideled |
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71© 2002, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2002. V. 5, N 1. P. 71-75.
1. Introduction
Silicon carbide LED first became a significant commer-
cial success when ÑREE (USA) got them on the market in
the early 1990�s. Today CREE�s CB series of super bright
LED are a new generation of solid-state LED emitters,
which combine highly efficient InGaN with Cree�s SiC
substrate. Cree�s main LED products now have SiC as a
substrate for gallium nitride or indium gallium nitride as
active ingredients, and emit blue as well as green and
blue�green luminescence. Yellow silicon carbide LED�s
had been made around 1970 � 1975 in the former Soviet
Union. Diffusion p-n junctions had been applied to manu-
facturing this LED devices. Single crystalline SiC with
ND - NA=(1~5)·1018 cm-3 was suitable for manufacturing
such devices. The diffusion factors depend on the con-
centrations of the impurities in the crystals. The process
of p-n junction creation is rather complicated because
the usual impurity concentrations in crystals range from
1017 to 1018 cm- 3.The optimal depth (1µm) of the p-n junc-
tion had been obtained with industrial crystals [1].
To obtain a high manufacturing yield for LED with
PACS: 77.55.+f, 77.80.
Silicon carbide LED
S.I. Vlaskina
Institute of Semiconductor Physics, NAS of Ukraine, 45 prospect Nauky, 03028 Kyiv, Ukraine
Phone: +380 (44) 269 3792; fax +380 (44) 265 8342; e-mail: businkaa@mail.ru
Dong Seoul College, 461-714, 423, Bokjung-Dong, Sungnam-city, Kyonggi-do, Korea
Phone: 82 (031) 720 2141; fax: 82 (031) 720 2261; e-mail: svitlana@haksan.dsc.ac.kr
Abstract. Silicon carbide has been widely used as material for manufacturing yellow, red,
green LED and optoelecronics devices (indicators, screens). The silicon carbide LED technol-
ogy has been investigated for improvement of their operational characteristics. This includes
the influences of the surface processing (etching, annealing), the formation method for the p-n
junctions and the contacts on the LED properties. Light-emitting devices used as light sources
for optical-fiber communication lines. LED fabricated by Al+ ion-implanted in 6H-SiC and
investigated their characteristics for an effective green LED. The brightness of the ion-im-
planted p-n junction was found to be two orders higher than that of diffusion p-n junction,
and the best value was 2000-10000 cd/m2 with passing current about 0.5 mA through area
50x50 µm and applied voltage about 2.6 ± 0.2 V. The ion-implanted structures showed a high
stability of light in the temperature range of 77-600 K.
Keywords: silicon carbide, LED, ion implantation.
Paper received 05.02.02; revised manuscript received 08.02.02; accepted for publication 05.03.02.
uniform light, the following technique had been applied.
Diffusion of Al with O was carried out for 2 hr at 1700°C.
Then, B and Al were introduced for 15 min at 1600°C.
The sources for O, Al, and B were SiO, 99.99% Al and
B2O3, respectively. After the diffusion processes, ap-
preciable changes in the surface morphology or the car-
bon traces were observed using optical microscopy. The
low-temperature (77 K) and the high-temperature (300
K) photoluminescence in the ultraviolet wavelength range
showed repeatable and clear spectra for the uniformly
doped samples. Diffusion of Al and O above 1700°C re-
sulted in a deterioration of the surface conditions, and
diffusion of B and Al at higher temperatures made the
photoluminescence spectra less clear and the surface condi-
tions even worse. The time at optimum temperature could
be adjusted to change the color of electro-luminescence from
yellow-green to red (in the wavelength range between 5600
and 6000 Å). In the case of a yellow-green color (5600 Å),
the time for the diffusion of B and Al turned out to be 20-
25 min. A further increase in the diffusion time caused
the p-n junction to be less sharp as well as the voltage drop
on the p-layer to increase, and, consequently, spreading of
72 SQO, 5(1), 2002
S. I. Vlaskina: Silicon carbide LED
the current carriers on the p-layer and less clear light draw-
ings on the displays. At the shortest diffusion time (5 min)
for B and Al, the thickness of the p-n junction was too small
to assure an effective LED. The parameters of such LED
and indicators, which had been fabricated, are listed in Ta-
ble 1. Usage of another SiC polytypes (4H, 3C, 15R) allows
to change the color of light, too.
Because of the extreme stability of silicon carbide, it is
not necessary to dope the crystal by thermal diffusion. In-
stead, dopants can be introduced by ion implantation. Once
implanted into the crystal, the dopant atoms occupy inter-
stitial positions in the lattice and must be transferred to sub-
stitution sites to become electrically active. This �activa-
tion� is accomplished by high temperature annealing in an
inert ambient such as argon.
Nitrogen and phosphorous are typical n-type dopants
in SiC.
Aluminum and boron are p-type dopants. Implanta-
tion is usually conducted with the sample at elevated tem-
peratures (600 - 800 °C) to provide some in-situ anneal-
ing of lattice damages caused by the implant. The im-
planted sample is subsequently annealed at temperatures
1000 � 1700 °C for times 5 and 90 minutes to activate the
dopants. The dynamics of activation depends both on the
dopant species (i.e. aluminum and boron, nitrogen and
phosphorus) and upon the SiC polytype (i.e., 4H or 6H).
Activation of nitrogen implants in 4H-SiC requires higher
annealing temperatures than those in 6H-SiC.
Phosphorus is an excellent n-type dopant in 4H-SiC
when implanted at high doses, such as for source and
drain regions of MOSFET. p-type dopants, aluminum
and boron, require much higher temperatures for efficient
activation and temperatures above of 1650 °C are
nescessary for that. Aluminum implants typically achieve
the same degree of activation at annealing temperatures
about 100 °C lower than boron. At any given annealing
temperature, there exists an optimum annealing time [2].
This article reports results of SiC green LED electrical char-
acterization. This LED had the same brightness as CREE�s
LED, but higher stability, more simple design (without
AlGaN or GaN layers) and can be used in various usual
applications.
2. Experiment
Green LED had been prepared by the method of ion-
implantation of impurities into 6H-SiC crystals. Al+-im-
planted for p-n junctions in 6H-SiC substrates and their
characteristics were investigated as an effective LED. The
ion-implantation was carried out on polished n-type 6H-
SiC crystals and on epitaxial layers of 6H-SiC with an im-
purity concentration of ND - NA = (2-8)·1018 cm-3. Substrate
was doped by donor nitrogen. Implantation was performed
with 80 keV Al+ by using an ion accelerator. The im-
planted depth was 0.5 µm, and the concentration of Al
was 1020 cm-3.
The optimum implantation time and the substrate
temperature were 2 min and 660°C, respectively.
Despite the low resistance of the p-layers, a thin high-
resistive layer of C was formed on the surface of the crys-
tals after implantation. This C film reduced the adhesion
of the metals deposited for the contacts, increased the
resistance of the contacts to the p-layers, and worsened
the light uniformity. The crystals were annealed at 800 -
1100 °C for 2-10 min to remove the C layer and to reduce
the quantities of radioactive defects. Before the LED fab-
rication process, the crystals were etched in an acid mix-
ture of HF and HNO3.
Contacts were prepared by deposition of Al layer on
p-SiC heated to the temperature 550 °C in vacuum (im-
planted and annealed). Al contact thickness was less or
about 4000 Å. Ni layers was deposited for protection of
Al layer from oxidation and for better contact with wire
during LED manufacturing. Thickness of Ni layer was
about 2000 Å. Contacts to the n-SiC substrate were made
by laser using Ti and Ni wire. Contact of necessary
configuration were made by photolithography meth-
ods. Photos of prepared LED are shown in Fig.1 (a, b, c).
Device Light Light Working Brightness Size of
voltage current voltage (cd/m2) light area
(V) (mA) (V) (µm2)
LED indicator
(10 elements) 2.2 5 2.4 ± 0.2 30 - 80 800 x 300
LED matrix 120 x 120
(64 elements) 2.5 ± 0.1 0.5 3.0 ± 0.2 60 step 100
LED break with 40 x 40
high resolution 2.4 ± 0.2 2.5 3.1 ± 0.1 70 step 60
(64 elements)
LED break 100 x 100
(100 elements) 2.4 ± 0.2 0.5 2.8 ± 0.1 80 step 100
LED break with 40 x 40
face conclusion step 60
of light (step 100)
Table 1. LED (diffusion p-n junction) on 6H-SiC.
S. I. Vlaskina: Silicon carbide LED
73SQO, 5(1), 2002
Fig. 1. Micro photo of ion-implanted LED:
magnitude: a - x75; b - x380; c � x730.
Just this LED was made on n-epitaxial layer grown by
Tairov�s method. The boundary between n-layer and
implanted p-layer is shown. The block structure of
epilayer and p-n junction is also shown. Single crys-
tals grown by the Lely method had been used for such
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0.0 1.0 1.8 2.4 3.4 5.0 8.0 13.0 18.0
Voltage (V)
lg
I
(A
)
Fig. 2.Voltage-current characteristic of ion implanted p-n junc-
tion in 6H-SiC.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0.0 0.8 1.8 2.3 3.2 4.5 7.0 12.0 17.0
Voltage (V)
d
lg
I/
d
V
(r
.u
)
Fig. 3. Derivation from voltage-current characteristic of ion im-
planted LED.
LEDs, too, but photos of epilayers with implanted
aluminum are more interesting than these of single crys-
tals with p-n junction.
The voltage-current characteristics of ion-implanted
p-n junctions are shown in Fig. 2.
The differential degree:
α = dlgI/dV is shown in Fig. 3.
In the low voltage area (at a voltages less then 0.08V) α
is about unity. Determined from this linear site leakage is
(1-2)·104 Ω. The reason of such resistance is carbon or sili-
con oxide on the surface of LED.
In the region of very small voltages:
I=I0 exp(V/VT).
This circumstance, as well as independence of VT on
temperature permits to think about tunnel character of
a
b
c
74 SQO, 5(1), 2002
S. I. Vlaskina: Silicon carbide LED
N about (1-4)·1020 cm-3 is indicative of a rather high dop-
ing level of the junction. The ideality factor 1.9 to 3.2
was obtained from the following exponential part of the
forward bias current-voltage characteristics. The forward
saturation current was (3-5)·10-16 to (3-5)·10-8 A/cm2 and
forward turn-on voltage of 0.8 to 2 V (at current density
about (5-8)·10-8 A/cm2. Reverse biasing produced average
leakage currents were that were of the order of 10-8
to10-3 A/cm2 (at 10 to 20V reverse bias).
The ideality factor had some trends. Namely, the
ideality factor is decreased and forward current density
is increased with increasing operation temperature in the
range of 20 °C to 400 °C. The ideality factor is of the
order of 2 for most LED suggesting that recombination/
generation current is dominant over diffusion current.
The ion-implanted p-n junctions are sharp, and the injec-
tion of carriers as well as their recombination in the p-
layer prevail. The sharpness was maintained in the tem-
perature range between 77 and 500 K. Recombination
happens in the layer of the volume charge in the p-n junc-
tion at low levels of injection, but the recombination pre-
vails in either the p- or the n-area at high levels of injec-
tion, depending on the doping degree.
In the region of voltages changing from 2.2 V to 18 V
the α degree is reduced passing the flat minimum. Then α
degree (Figs 2, 3) is again increased and can be described
by the expression:
+
+
=
∞VBI
VAI
V
4
1
0
2
1
The square-law part of the voltage-current charac-
teristics enables to calculate the major carrier lifetime near
the cathode:
s
V
L
kn
8
0
2
10)2.18.0(
2
−⋅−==
µ
τ
(µn=100 cm2/Vs).
That is in good agreement with the experimental data of
transitive characteristics, where ignition time is (2.5-
3)·10-8 s and emission time is (1-1.5)·10-8 s.
The light-brightness characteristic of the ion-implanted
p-n junction is shown in Fig. 4. The differential degree
β=dB/dlgI is shown in Fig. 5. The linear region extends
from 7·10-4 A to 10-2 A/cm2 .The high stability of the light,
even at 77-600 K, is another characteristic for the ion-im-
planted structures, which is understood mainly by the role
of the radioactive defects in the emission. The concentra-
tion of defects is constant in the temperature range of 77-
600 K.
The destructive features of the ion-implanted structures
produced narrow lines in the electro-luminescence spectra
at relatively small current (Fig. 6) A correlation between
0
1000
2000
3000
4000
5000
6000
7000
-6.1 -4.7 -4.6 -3.9 -3.2 -2.1 -1.5
lgI (A)
B
(c
d
/m
2
)
Fig. 4. Ion-implanted p-n junction light brightness characteristic
0.0
0.5
1.0
1.5
2.0
2.5
3.0
-5.80 -4.73 -4.05 -3.42 -1.94
lgI (A)
β
(r
.u
.)
Fig. 5. Derivation from brightness-current characteristic of ion-
implanted p-n junction.
the narrow lines in the spectra and the brightness of LED
had been established; most of the effective LED had narrow
lines at low temperatures. It is a very convenient way to
select crystals for green highly bright LED. When the
temperature increases, the narrow lines disappear from the
spectra. The fact that the sharp-line structures of the
electro-luminescence spectra coincide with those of the
photoluminescence spectra at low temperatures (77 K) is
an additional indication to support the suggestion that
the characteristics of the spectra are due to radioactive
defects.
The ion-implanted structures have much faster re-
sponse times than the diffused ones (Table 2). The switch-
ing time is close to 10-8 s, which provides wide op-
portunities for the applications of such LED to optical
couples and optical-fiber communication lines. The
2
2
2
32
)(1
T
n
n
V
h
m
dn
n
n
e
N
n
s
επρ == ∫
this part of the voltage-current characteristics. Concen-
tration evaluated from a volume charge is as follows:
S. I. Vlaskina: Silicon carbide LED
75SQO, 5(1), 2002
Fig.6. Ion-implanted p-n junction electroluminescence spectrum.
brightness of the diffused LED is 30-100 cd/m2 at
10 mA/mm2, while for the ion-implanted it is 500-
600 cd/m2 at 10 mA/mm2 and even (2-10)·1000 cd/m2 for
the best samples.
On the other hand, ion-implanted p-n junctions can be
applied widely in optical-fiber communication, especially,
short communication lines, because of their high speed
(10-8 s), high stability of light for 10,000 hrs, linear bright-
ness-current characteristics up to a current density of
10 A/cm2, high brightness, absence of brightness deg-
radation, and stability of characteristics over a wide in-
terval of temperatures.
Switching Switching Working Bright- Size of Switching
voltage current voltage ness light area time
(V) (mA) (V) (cd/m2) (µm2) (s)
2.6±0.2 0.5 3.2±0.1 2000-4000 50x50 0.15
80x50 0.01
Table 2. LED for the fiber communication line
3. Conclusion
The characteristics of silicon carbide LEDs were in-
vestigated. These included the influence of surface
processing, the formation method of the p-n junctions,
and the contacts on the properties of the LED. Green SiC
LEDs can be used as light sources for optical-fiber
communication lines, in the traffic lights, as indicators,
in screens and so on. p-n junctions were fabricated by Al+
ion-implantation 6H-SiC, and annealed at 800-1100°C.
Characteristics for using as effective green LED had been
investigated. The brightness of the best ion-implanted
p-n junction was found to be about 2000-10000 cd/m2.
The ion-implanted structures showed a high stability of
light in the temperature range of 77-600K.
The pulsing characteristics of these LEDs based on
ion-implanted structures had a response time close to
10-8 s.
References
1. S.I.Vlaskina, K.W.Kim, Y.S.Kim, Y.P.Lee, G.S.Svechnikov,
Optoelectronics Devices on Silicon Carbide // Journal of the
Korean Physical Society, 30(1),pp.117-121(1997).
2. M.A.Capano, R.Santhakumar, J.A.Cooper,Ir., and
M.R.Meloch, SiC Ion Implantation Research at Purdue, USA
(internet information), 2002.
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