Simultaneous improvement on light extraction and directional
radiation of LEDs by nanocones
Jialun He, Version 7, 08/06/2015
Abstract
we fabricated cone- and rod-like ZnMgO nanostructures (NCs and NRs) on
InGaN/GaN light emitting diodes(LEDs) by a co-evaporated chemical vapor
deposition method. Without sacrificing the electrical performances of
LEDs ,cone-like NRs can enhance the light extraction efficiency by 10.5% at 20
mA and shape the radiation profile collimated from 136°to 121°. From optical
experiments and simulation, we find the waveguiding effect of NCs and
accommodation of abrupt index change by the tapered diameter of cone-like
ZnMgO nanostructures .
Introduction
Nitride based LEDs act as promising candidates in displays, solid-state lighting,
signaling.The performance is unsatisfactory due to low light extraction efficiency
and poor directional emission [1].
The low light extraction efficiency is mainly due to the considerable Fresnel loss
and total internal reflection loss caused by the large difference in refractive index (n)
at air/GaN(n~2.43) or the air/ITO(n~2.06) interfaces.[2] It is unavoidable to
experience a severe Fresnel reflection loss at the surface of the semiconductor,
considering the abrupt interface between the two media. As for the total internal
reflection, phonons emitted from the MQWs would be reflected from the interface,
reabsorbed, and internally confined if the escaped angle is larger than the critical
angle. Based on Snell’s law, the critical angles in LEDs are 54.83°and 29.04°at
the p-GaN/ITO and ITO/air interface, respectively[3][4]. A small view angle is
desired for many LED applications, such as pocket lamps, smart phone cameras,
and vehicle head lamps[5]. Briefly speaking, it is important to develop high
light-extraction efficiency LEDs with directional radiation profiles for
next-generation solid-lighting devices[6].
In this work, ZnMgO nanocones(NCs) and nanorods(NRs) were synthesized on
nitride-based LEDs by a low pressure chemical phase deposition(LPCVD) method.
The LEDs with ZnMgO NCs(NCLED) exhibit an increased extraction efficiency by
20% at 15 mA with the view angle improved from 150°to 120°， as compared to
the bare LEDs(BLED), and LEDs with nanorods (NRLED). We experimentally and
theoretically demonstrated that better light extraction efficiency and a more
collimated radiation pattern are attributed to the waveguiding effect of ZnMgO
nanocones and the mitigation of abrupt index change by the tapered finish of
cone-like ZnMgO nanostructure. Based on the optical experiments and theoretical
simulations, our findings provide prominent design criteria for the morphology of
nanostructures for high-efficiency optoelectronic devices.
Experimental
The epitaxial layer of the LEDs followed the conventional growth steps.
Nitride-based epitaxial layers were grown on sapphire substrates with c-face
orientation by the MOCVD technique. The device structure of the InGaN/GaN
MQW
LEDs
were
formed
by
sandwiching
nine
periods
of
In0.25Ga0.75N(5nm)/GaN(15nm) MQWs between n-type (2μm) and p-type(0.3μm)
GaN layer. ITO was deposited by electron beam evaporation on p-GaN to form
transparent ohmic contacts, followed by the deposition of metal electrodes of
Ti/Al/Ni/Au on the top surface of ITO and n-GaN. The experimental setup for
ZnMgO nanocones growth is depicted schematically in Figure 1a. The precursors
used in the CVD synthesis of ZnMgO nanocones were a stoichiometric mixture of
Zn and Mg powders. In short, the raw material powders were ground thoroughly
and uniformly put in a long fused silica boat. Then the boat was placed into the hot
center of the furnace. The LED devices were then placed downstream from the tube
reactor separated from the starting materials by 6 in. The distance is vital to the
shape of ZnMgO and growth rate. The quartz tube was purged using N2 to drive out
air before heating. Then the quartz tube was heated to 500 ℃ with heat rate of
10 ℃ min-1. During this heating period, N2 was introduced with a flow rate of 10
sccm (standard cubic centimeter per minute).Once the temperature reached
350 ℃, O2 was introduced with a flow rate of 2 sccm. After 30 min of reaction, the
O2 supply was cut off and the furnace began to cool down.
After the reaction, a layer of white product was found deposited on the LED
surface. The microstructures of the structures were observed using field emission
scanning electron microscope (FE-SEM, Hitachi S-4800, 10 kv) with INCA x-act
energy dispersive Spectrometer(EDS). Transmission electron microscopy (TEM)
AND high resolution TEM (HRTEM) images were obtained on a JEOL JEM-2100F
microscope at 200 kv. The crystal structure of the samples was investigated using
an X-ray diffractometer (D/MAX-2500) equipped with a nickel filtered Cu Kα
radiation (1.54056Å) at 40 kv and 140 mA. XRD spectra were collected in a 2Ө
range of 30-80°at a scanning rate of 0.02°per step and 0.15s per step.
Photoluminescence (PL) spectra were detected by a PMT928 PL spectrometer
with excitation wavelength of 325nm.
Fig. 1 Schematic diagram of the experimental apparatus for growth of ZnMgO
nanostructures.
Results and discussion
Fig. 2 SEM images of ZnMgO nanocones. The inset in (a) highlights the tapered
ending on the top of the nanostructures. (b) Schematic of a typical cone-like
nanstructure.
Fig.2 shows the morphology of the as-grown ZnMgO cone-like nanostructures.
The cross-sectional scanning electron microscopy (SEM) presented in Fig.1(a) and
(b) is the schematic of an individual cone-like nanostructure. One can see that the
cone-like nanostructures consists of two fragments: the rod-like body and the
tapered finish at the top. From the high-magnification SEM image shown in the
inset of Fig.1(a), the gradual decrease in radius of this tapered part can be perceived.
The dimensions of the nanostructures are highlighted in Fig.1(b): the average
diameter and height of the rod-like bodies are ~120nm and ~1.75μm， respectively,
and the length of the tapered part is 230±200nm.
Fig.3 Typical I-V and L-I characteristics of the bare LEDs and LEDs with ZnMgO
nanocones and nanorods. The inset depicts the EL spectra of the LEDs under 20 mA
injection current.
The forward current-voltage(I-V) characteristics of the LEDs are presented in
Fig.3. The forward voltage (Vr ) under 20mA injection current are 2.88 and 2.89 V
for the LEDs with and without cone-like ZnMgO nanostructures, respectively. The
I-V curves are almost identical, indicating that the CVD method for ZnMgO
cone-like nanostructures does not degrade the electrical properties of the LEDs.
Fig.3 also demonstrates the relative light output intensity as a function of injection
current, and the inset in Fig.3 reveals the electroluminescence (EL) spectra of two
kinds of LEDs measured at 20mA injection current. After introducing cone-like
ZnO nanostructures, the peak intensity of the LED is increased by 10.5% and
16.8% at injection currents of 20 and 100mA, respectively, indicating excellent
light extraction characteristics of cone-like ZnMgO nanostructures. Moreover, the
external quantum efficiency (EQE) is an important characteristic for revealing the
LED performance. The EQE values were obtained by varying the input current and
measuring the output light power of the LEDs in an integrating sphere. The EQE of
the bare device is 13% at 20mA. By applying cone-like ZnMgO nanostructures, the
EQE achieves 14.4%. The enhanced light extraction can be attributed to the
waveguiding and the tapered tip morphology of the cone-like ZnMgO
nanostructures, which will be discussed with experimental and simulation results in
the following paragraphs.
Fig.4 OM images of the ZnMgO nanocone before and after laser illumination. The
532nm laser was injected into the nanocone at the bottom end.
In order to prove the waveguiding effect of the ZnMgO nanocones, several
nanocones were dispersed on a SiO2/Si substrate. Fig. 4(a) shows the optical
microscope image of a single ZnMgO nanocone. As shown in Fig. 4(b), When a
532nm laser light was injected into the nanocone at the bottom by the objective lens,
it was guided out at the top end, clearly evidencing the waveguiding characteristics
of the nanocones.
Fig.5 Time-averaged and normalized TE field distribution near the surface of the
LEDs with different surface morphologies: (a) bare, (b) flat-end ZnMgO nanorods,
and (c) ZnMgO nanocones.
To further reveal the light propagation through the cone-like ZnMgO
nanostructures, a simulation based on the finite-difference time-domain(FDTD)
method is carried out[7]. For simplicity, the nanocones are set to be perpendicular to
the ITO surface and equally spaced. The diameter and the height of the nanocone
bodies are 130nm and 1.5μm, respectively, and the the length of the tapered part is
300nm. Three different LEDs: the bare LED, the LED with flat-end nanorods and
nanocones , are compared in the simulation. To match with peak emission
wavelength in the EL spectra, the monochromatic excitation source is chosen to be
at 528.5nm. A detector is set up at a distance far enough to detect the time-averaged
power emitted from the LEDs.
Fig. 5(c)-(e) displays the transverse electric (TE) field intensity distributions
inside the three different surface structures. From the coloring at the top of Fig.
5(a)-(c), one can note that the LEDs with nanorods or nanocones exhibit stronger
field intensity. The employment of the nanostructures effectively reduces the back
reflection and ensures better propagation of light by matching the optical
impedance at the interface[8]. Furthermore, the cone-like nanostructure show better
light extraction capability than the flat-end nanorods. The electric fields inside the
nanostructures reveals the impact of the tapered diameter of the cone-like
nanostructures. Due to the mismatch in optical impedance of ZnO and air, some of
the light is trapped in the flat-end nanorods. On the other hand, the nanocone
ensures a good coupling of the traveling light into the air. The light extraction
enhancement via the nanocones can be explained by two steps. Firstly, a major
portion of the photons emitted from the MQWs are easily captured by the ZnMgO
nanostructure due to the well matched refractive index between ZnMgO and ITO[9].
The captured photons are then guided within the ZnMgO nanocones. Secondly, the
traveling light inside the ZnMgO can escape into air because of the improved
impedance match between air and and nanocones[10]. However, the light would be
trapped in the nanorods owing to the large optical impedance difference between air
and the ZnMgO.
Conclusions
In this paper, we reported for the first time the application of cone-like ZnMgO
nanostructures grown on nitride-based LEDs, using a simple, low temperature and
etching-free CVD method, for highly enhancing the light extraction by 10.5% at
20mA and shaping the radiation pattern from 136°to 121°without complexly
tailoring the configuration of MQW regions. This is attributed to the waveguiding
effect and the creation of gradual refractive index profile at the interface between
air and NC layers by the tapered diameter of the cone-like ZnMgO nanostructures,
confirmed by the experiments and optical simulations. The cone-like ZnMgO
nanostructures using scalable, cost-effective fabrication technique demonstrates a
new approach to achieve better light extraction and more collimated radiation
pattern in the existing solid-state lighting technology.
Acknowledgements
The authors would like to thank the financial support from the ,,,
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