Light Extraction Efficiency Enhancement of
GaN Blue LED by Liquid-Phase-Deposited
ZnO Rods copy
老師：管 鴻
學生：黃順源
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教授
outline
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Abstract
INTODUCTION
EXPERIMENT
RESULTS AND DISCUSSION
CONCLUSION
REFERENCES
Abstract
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We investigate the light extraction efficiency of a GaN lightemitting diode (LED) by using liquid-phase-deposited ZnO rods
at near-room temperature.
Zinc nitrate and hexamethylenetetramine were used as the
deposition precursors. Compared with the conventional GaN
LED, the optical power output of the GaN LED with crystalline
ZnO rods on its surface has about 1.6 times enhancement
INTODUCTION
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The internal quantum efficiency of GaN blue LEDs is close to 100%,
however, most light is lost in the materials.
From the refractive indexes of GaN ( n=2.5 at 440 nm) and the air,
the critical angle is 23.6 . Only about 4% emitted light radiates
through the top and bottom [1].
ZnO film is a promising material for the optoelectronic devices as its
wide bandgap (3.37 eV), large excitonic binding energy (60 meV)
and high optical gain (300 cm^-1 ).
In this letter, a simple and nonexpensive liquid-phase-deposition
(LPD) method was used to grow high crystalline and uniform size
ZnO rods on GaN blue LED at near-room temperature to enhance
the light extraction efficiency.
EXPERIMENT
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The GaN LED grown by metal–organic chemical vapor deposition
consists of buffer and Si-doped layers, InGaN–GaN quantum wells
(QWs), followed by Mg-doped GaN layers. The active region for blue
LED was five-period In0.18Ga0.82N–GaN QWs(at 440 nm) and the
chip area is 300×300 μm^2 . Ti–Au and Ni–Al metals were used as
ohmic contacts on p- and n-GaN, respectively.
An HP4145B semiconductor-parameter analyzer was used for
current–voltage characterization. The electroluminescence (EL) was
measured at the forward current from 20 to 100 mA
The aqueous solutions of zinc nitrate (Zn (NO3)2 , 0.1 M, 20 ml)
and hexamethylenetetramine (C6H12N4 , (HMT), 0.1 M, 20 ml)
were used to deposit ZnO rods [11]. The LED chip was immersed
in the solution and the temperature is kept at 50 C.
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RESULTS AND DISCUSSION
(FE-SEM)
(a)2.6×10^9㎝^-2
(b)8.0×10^9㎝^-2
(c)1.9×10^9㎝^-2
(e)Height 240 nm
and diameter 200
nm with ±10%
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HR-TEM
The lattice spacing is
approximately 2.6 A
between two (002)
crystal planes
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The refractive indexes of ZnO and GaN are 2.0 and 2.5 at the
wavelength of 440 nm. The critical angle related to the air, ZnO and
GaN is calculated by Snell’s law 【θC＝sin^-1（n2/n1）】. From the
aspect ratio (240 nm/200 nm) and rod density of 8.0×10^9cm^-2 in
the above
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The incident angle of the light emission from the GaN
active layer has an increase of 40% 【（53.2°-43.9°）
/23.63°×100％】through the side wall of each ZnO rod
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RESULTS AND DISCUSSION
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By increasing the aspect ratio with a fixed density, the light
escaping though the top surface decreases due to the increase of
ZnO absorption loss from the additional total reflection and
transmission length
By decreasing the aspect ratio, part of the original light escaping
though the side wall will be totally reflected and then absorbed
in GaN.
By increasing the rod density with a fixed diameter, the light
coupling between rods increases and hence the absorption
increases
By decreasing the rod density, the light enhancement decreases
due to the decrease of ZnO rod coverage
Therefore, the light emission intensity depends on the rod shape
EL spectra and I-V characteristics
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Optical power output
30° enhanced to1.65 times
60° enhanced to1.73 times
90° enhanced to3.25 times
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CONCLUSION
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ZnO rods have been directly grown on GaN LED by LPD. We have
achieved in the LED an enhancement of optical power output to 1.6
times with the ZnO rod density of 8.0×10^9cm^-2 . It also shows a
stronger and wider angular optical power distribution. The
dependence of light emission efficiency on the rod density and size
will be further studied.
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Thank you for your attention
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