Is It Viable to Improve Light Output Efficiency by Nano-Light-Emitting Diodes?
Chao-Hung Wang, 1 Yu-Wen Huang, 1 Shang-En Wu,2 and Chuan-Pu Liu1, 3, 4, a)
1
Department of Materials Science and Engineering, National Cheng Kung University, Tainan
70101, Taiwan
2
Genesis Photonics Incorporation, Tainan 70101, Taiwan
3
Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan
70101, Taiwan
4
Research Center for Energy Technology and Strategy, National Cheng Kung University,
Tainan 70101, Taiwan
Supplementary Materials
Sample structure, experimental procedure, strain, ideality factor and EL peak shift
issues on Nano-LEDs
Sample Structure:
The epitaxial film used in this study was an LED structure grown on a c-plane sapphire
(α-Al2O3) substrate by MOCVD. The structure comprised a 30-nm-thick GaN buffer layer, a
4-µm-thick Si-doped n-GaN layer, 14-pair InxGa1-xN/GaN MQWs, and a 600-nm-thick
Mg-doped p-GaN layer.
Experimental Procedure:
The FIB instrument used for patterning was an SMI3050SE FIB-SEM (scanning
electron microscopy) hybrid system from SII NanoTechnology Company. Wet chemical
etching by 10 wt% KOH was used to remove the surface damage layer with an etching time
of less than 3 minutes.1 For characterization, the morphology, size, and density of the
nanopillars upon removal of the surface damage layer were examined by SEM (Hitachi
SU-8000) and the crystallinity of multiple quantum disks (MQDs) after chemical etching was
studied
by
transmission
electron
microscopy
(TEM;
JEOL
JEM-2100F).
Temperature-dependent micro-photoluminescence (μ-PL) spectra were obtained with a liquid
nitrogen cryostat to calculate IQE. For the device structure shown in Figure 1 of the
manuscript, spin-on-glass (SOG) was used as a transparent insulator. Indium-tin-oxide (ITO)
transparent conductive films were deposited as transparent electrodes and Cr/Au
double-layered metal thin films were deposited as n- and p- electrodes. Finally,
current-voltage (IV) characteristics, electroluminescence (EL), output power, external
quantum efficiency (EQE), and wall-plug efficiency were determined for every LED.
Strain in Nanopillars:
In Figure 2(d), this TEM dark-field image was taken under two-beam condition using
g=0002. One can judge whether or not strain relaxation occurs across the interface of an
epitaxial material system simply from the contrast of the image under such imaging
conditions. For an epitaxial heterostructure without any strain relaxation across the interface,
only long range contrast variation can be seen as thickness fringes due to the variation of
local foil thickness and bending contour due to local tilting of the foil. The contrast in Figure
2(d) exhibits only thickness fringes, manifestation of no strain relaxation. For clear
demonstration of contrast variation, we compare TEM dark-field images of multiple quantum
wells using the same g=0002 between a small diameter nanopillar and a thin film case, shown
in Figure S1. Apparently, contrast is drastically different across each interface between these
two images. While the foil thickness variation is represented as continuous long range dark
and bright band as explained before for the nanopillar, the contrast in the thin film case
clearly reveals local black-and-white lobe-like contrast across the interfaces as illustrated by
red arrows. This contrast is arisen from the local variation of lattice constant, typical
representation of strain relaxation. Thus, we believed that there is almost free of strain in the
small diameter nanopillar, relaxed through small volume confinement.
Figure S1: TEM images of (a) quantum disks in a small diameter nanopillar and (b) quantum
wells in a thin film case taken under two-beam condition using g= 0002.
Ideality Factor:
The ideality factor can be extracted from I-V characteristics using the Shockley equation
by taking parasitic (Rp) and series (Rs) resistances into account, as given below:
𝑞(𝑉−𝐼𝑅𝑠 )
(𝑉 − 𝐼𝑅𝑠 )
(
)
𝐼−
= 𝐼𝑠 [𝑒 𝑛𝑖𝑑𝑒𝑎𝑙 𝑘𝑇 − 1]
𝑅𝑝
where I is the diode current, Is being the reversed saturation current, q being the electron
charge, V being the diode voltage, Rp and Rs being the parasitic and series resistance , k being
the Boltzmann constant, T being temperature and nideal is the ideality factor.
In the I-V characteristics of our LEDs shown in Figure S2, both I-V characteristics can
be divided into three regions and each region is dominated by different factors. In region I at
the low operating voltage near origin, the parasitic resistance strongly dominates the I-V
behavior. The current of the nanopillar LED is much higher than the conventional LED in
this region, implying that the formation of nanopillars would reduce the parasitic resistance.
Hence, the leakage current of the nanopillar LED is much increased due to more side surfaces
created for the nanopillar array. As for region III at high operating voltage, the series
resistance plays an important role and the joule heating effect becomes inevitable. To avoid
involving the parasitic and series resistances in calculating ideality factor, region II near
diode turn-on is the most appropriate since the space charge region dominates the device I-V
characteristics in this region. The ideality factors exceeding 2.0 have been ascribed to
trap-assisted tunneling, carrier leakage and poor metal-semiconductor contact in III-nitride
semiconductor diodes.2-4 For 1-D nanostructures, the ideality factor may be reduced because
of the high crystal quality of nanostructures,5 or may be raised by a large tunneling effect.6 In
our case, the nanopillar LED has abnormally higher ideality factor than conventional LED
although the crystal quality is better and the diffusion-recombination process is reduced since
the parasitic resistance effect is enhanced. The most probable reason for such high ideality
factor of the nanopillar LED is the influence of metal-nanopillar nanocontact. This
nanocontact would increase the tunneling current at the metal- semiconductor junction and
consequently induce the current crowding effect7, which would raise the ideality factor
because high current density may contribute electron overflowing inside the quantum-disks.
Figure S2: I-V characteristics of nanopillar and conventional LED plotted in semi-log scale
with extracted ideality factor and series resistance.
EL Peak Shift in Nanopillar LED:
The reason why the nanopillar LED exhibits a little red-shift at low operating current (<20
mA) is that it is contributed from the device-to-device variation, not from the strain-relaxation
issue in the nanopillars. Xia et al.8 showed that the emission in the EL spectrum may red-shift
by almost about 50 nm (from 458 nm to 509 nm) within three devices on the same wafer, which
was attributed to indium inhomogeneity effect. According to this phenomenon, the emission
wavelength varying from device to device is possible because of indium inhomogeneity.
We have examined the emission properties of different nanopillar sizes compared with a
planar LED, as shown in Figure S3. In Figure S3(a) and (b), the emission wavelength of the
quantum-wells exhibits monotonically blue-shift from the planar LED to the decrease of the
nanopillar size in the nanopillar LEDs under the same pumping condition. The fact that a large
blue-shift accompanied by the nanopillars with the size ~330 nm compared with the film case
implies large strain relaxation in the quantum wells when nanopillars form. As the nanopillar
size is shrunk down even to ~100 nm, the emission wavelength only undergoes a slight further
blue-shift, which seems to be saturated, proving that the strain in the quantum disks is almost
relaxed.
As for the discrepancy in the EL spectrum at low current regime, however, the pattern area
is too small to dominate the whole emission spectrum. Hence, it is hard to characterize the
strain-relaxation behavior through EL spectrum since EL spectrum is still dominated by the
whole device behavior. To explain this problem, we have checked different devices to see
whether it is the variation from device to device. From Figure S4, two conventional LEDs are
shown together with the nanopillar LED, which does exhibit a variation in wavelength from
device to device. Obviously, the nanopillar LED with a slight red-shift at low injection currents
is originated from the device-to-device variation. However, this would not be a problem for
explaining the thermal effect of the nano-LEDs for high injection currents because the
emission wavelength shows red-shift earlier and much severe than both conventional LEDs,
proving that the thermal effect is the dominant effect for the nano-LEDs operating at high
injection currents.
Figure S3: (a) PL spectra of the InGaN multiple-quantum-wells in a conventional LED and the
nanopillar LEDs with two different nanopillar sizes, and (b) the extracted PL peak position as a
function of nanopllar size.
Figure S4: EL peak wavelength versus injection current for the nanopillar LED with the size
of 100 nm and two conventional LEDs on the same LED wafer.
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