Suppressing spontaneous polarization of p-gan by graphene oxide passivation: augmented light output of gan uv-led

ABSTRACT GaN-based ultraviolet (UV) LEDs are widely used in numerous applications, including white light pump sources and high-density optical data storage. However, one notorious issue is

low hole injection rate in p-type transport layer due to poorly activated holes and spontaneous polarization, giving rise to insufficient light emission efficiency. Therefore, improving hole

injection rate is a key step towards high performance UV-LEDs. Here, we report a new method of suppressing spontaneous polarization in p-type region to augment light output of UV-LEDs. This

was achieved by simply passivating graphene oxide (GO) on top of the fully fabricated LED. The dipole layer formed by the passivated GO enhanced hole injection rate by suppressing

spontaneous polarization in p-type region. The homogeneity of electroluminescence intensity in active layers was improved due to band filling effect. As a consequence, the light output was

enhanced by 60% in linear current region. Our simple approach of suppressing spontaneous polarization of p-GaN using GO passivation disrupts the current state of the art technology and will

be useful for high-efficiency UV-LED technology. SIMILAR CONTENT BEING VIEWED BY OTHERS DUAL POLARIZATION FOR EFFICIENT III-NITRIDE-BASED DEEP ULTRAVIOLET MICRO-LEDS Article Open access 02

August 2024 OPTIMIZING CHARGE TRANSPORT IN HYBRID GAN-PEDOT:PSS/PMMADEVICE FOR ADVANCED APPLICATION Article Open access 04 June 2024 ACHIEVING 9.6% EFFICIENCY IN 304 NM P-ALGAN UVB LED VIA

INCREASING THE HOLES INJECTION AND LIGHT REFLECTANCE Article Open access 16 February 2022 INTRODUCTION GaN-based LED consists of active quantum wells, electron and hole transport layers and

contact metals1,2. One technological bottleneck in GaN LED is strong spontaneous electric polarization formed in p-GaN (hole transport layer), leading to poor hole injection rate, although

heavily p-doped GaN has been successfully grown on c-plane sapphire substrates3,4. Owing to this polarization, hole carriers in p-GaN are locally bound by potential gradient, preventing them

from contributing to the radiative recombination in multiple quantum wells (MQWs)5. To further improve light output power, increase of hole injection rate by suppression of spontaneous

electric polarization is mandatory6. Graphene oxide (GO) nanosheets possess numerous oxygen functional groups and therefore may form strong dipole layers to modify such electric

polarization7,8,9,10. In this study, we propose a simple and effective method to suppress spontaneous polarization of p-GaN transport layer by simply spin-casting GO layers on the

conventionally fabricated UV-LED to eventually enhance the light output power. RESULTS AND DISCUSSION A GaN-based UV-LED was grown by metal-organic chemical vapor deposition. We used a

commercially optimized UV-LED structure with an AlGaN carrier blocking layer11 and Mg-doped graded p++ layers to achieve an improved hole injection efficiency, as illustrated in Fig. 1a. Mg

content in _p_-GaN reached as high as ~1020 cm−3 (see Supplementary Information, Figure S1). Next, the prepared GO nanosheet solution was spin-casted onto the UV-LED12. The GO nanosheets

possess hydroxyl, phenol and epoxy groups in the basal plane and carbonyl and carboxyl groups on the edge sites. These oxygen functional groups possess negative charges on the GO nanosheets,

which induce strong dipole field13,14. The final structure including the GO nanosheets is shown in Fig. 1a. Using the conventional LED fabrication technique, we obtained a full chip with

GO/ITO as a transparent conducting layer and Cr/Au as a p–n-type electrode, as shown in Fig. 1b. The GO nanosheets of an area of approximately 1 μm2 and a thickness of 1 nm, were distributed

at relatively low density (referred to as _l_-GO) owing to hydrophobic nature of the pristine ITO layer, as shown in Fig. 1c. The inset in Fig. 1c is the atomic force microscopy image of

graphene oxide sheets on the ITO surface. In addition, these were revealed as dark regions compared to the bare ITO layer. To increase the density of GO nanosheets and to achieve uniform

distribution on the substrate, the ITO surface was treated with O2 plasma prior to GO spin-casting, which increases hydroxyl groups on the surface15. In Fig. 1d, high density GO nanosheet

was obtained with improved uniformity (see Supplementary Infromation, Figure S2), hence referred to as _h_-GO. We conducted ultraviolet photoelectron spectroscopy to observe the charge

distribution near the surface of ITO passivated with _h_-GO. As depicted in Fig. 1e, the work function of ITO increased considerably from 4.3 to 4.55 eV by passivation of the _h_-GO. The

value of work function was estimated from the secondary electron cutoff using the relation Φ = hν-(EF-Ecutoff), where hν, EF and Ecutoff are the photon energy of the excitation light (21.22 

eV), the Fermi level edge and the measured secondary electron cutoff, respectively16. This results in charge transfer from ITO to GO. O1s spectra from X-ray photoelectron spectroscopy

revealed three oxygen-related peaks (Fig. 1f). The O1 and O2 peaks in the inset originate from bulk In2O3 near ~530.0 eV and O-deficient “sub-oxide” sites associated with O vacancies near

~531.3 eV. The O3 peak near 532.4 eV is related to hydroxyl groups17. This peak position was unchanged with GO passivation but the intensity was increased by approximately 5 times (see

Supplementary Information, Figure S3). This indicates hydroxyl groups strongly adhere to ITO. The consequence of charge transfer and adhesion is schematically drawn in Fig. 1g. Large D-band

near 1,350 cm−1 and G-band near 1,620 cm−1 did not appreciably change after deposition18 (see Supplementary Information, Figure S4). The light output powers of conventional UV-LED,

UV-LED/_l_-GO and UV-LED/_h_-GO were measured as a function of injection current using a probe station and photodiode detector to investigate the effect of GO nanosheets (Fig. 2a). The light

output power of the UV-LED increased linearly at low current and saturated and reduced at high current, called as efficiency droop, due to the insufficiently activated hole concentration or

electron over flow, which is a typical light output power behavior19,20. On the other hand, the light output power of UV-LED/_l_-GO was enhanced by approximately 40% for a typical operating

current of 20 mA and almost twice at the saturated current region compared with that of conventional UV-LED without GO nanosheets. This positive contribution was more significant for

UV-LED/_h_-GO, which exhibited a 60% increase in light output power at 20 mA. In both cases, the saturated current was prolonged to large values. This can be attributed to the improved hole

injection rate in _p_-GaN region by the strong dipole field induced from the GO layers. Images of the electroluminescence emission of UV-LED/_h_-GO at 1 and 5 mA are shown in Fig. 2b and 2c,

respectively. The electroluminescence emission is uniform and sufficiently bright. Enhanced light output power in GaN-based UV-LEDs is generally attributed to improvement in electrical

properties, enhancement in light extraction efficiency, increase in the hole injection rate between ITO and _p_-GaN and an increase in the hole concentrations inside active layer of

LED21,22,23. In our samples, the current-voltage (I-V) characteristics and light output power were measured simultaneously, as plotted in Fig. 2d. The I-V curves for the all three samples

were nearly identical. Although the work function of ITO increased by 0.25 eV, this only allows to reduce Schottky barrier height for hole injection. Since the I-V of the device is dominated

by electron current, this contribution is negligible. This confirms that passivation of the GO nanosheet did not affect the series resistance and current level of the device and that the

improvement in light output power is not attributed to the change in electrical properties. To examine the effect of GO nanosheet passivation on the light extraction efficiency of the LEDs,

transmission spectra were compared (see Supplementary Information, Figure S5). The transmittance of the ITO/_h_-GO was quite similar to that of the ITO without GO nanosheets. This

observation confirms that the improvement in the light output power of GO-passivated UV-LEDs is not due to an enhancement in light extraction efficiency. To investigate the origin of

enhanced light output in LED with GO, source-drain hole currents in bare _p_-GaN were measured for both perpendicular and parallel directions to _p_-GaN c-axis. For the measurement, we

prepared four pieces of _p_-GaN which is grown by same conditions with _p_-GaN of UV-LED. Before coating GO, two pieces of p-GaN was etched by inductively coupled plasma to measure vertical

I-V curves. GO was coated on the center of both samples with same area. Distance of cathode and anode tips were kept for comparing all I-V curves. The vertical source-drain hole current was

enhanced with GO passivation (Fig. 2e). This is a direct evidence of the reduced spontaneous polarization, in other words, the negative charges at the surface of _p_-GaN are compensated by

the induced positive charges (Figs. 2g and 2h and supplementary Information Fig. S6). The effect of such charge compensation was also visible in the in-plane source-drain current (Fig. 2f).

In this case, due to the gating effect of the reduced negative charges at the _p_-GaN surface, the in-plane hole current was reduced. As a consequence, the hole injection rate is enhanced to

supply more hole carriers to the active quantum well region in the real device, giving rise to enhanced light output power in Fig. 2a. Since this is the minority hole current contribution

(hole mobility is about 100 times lower than electron mobility in the device), I-V is still dominated by electron current and not modified appreciably, as shown in Fig. 2d24. It is also

intriguing to see the reduced current fluctuation with GO passivation in both cases. This is ascribed to the enhanced field uniformity in _p_-GaN region due to the random location of GO

flakes. The Ga-faced c-plane GaN has, in general, spontaneous polarization from top to bottom inherently due to dipole-like N-Ga arrays that is typical for GaN crystal grown on _c_-plane

sapphire substrate4. Local charge distributions in each layer near Ga-faced _p_-GaN are depicted in the inset of Fig. 2g. In this arrangement, the hole carriers in the valence band of

_p_-GaN are bound by potential valleys at the ITO/_p_-GaN interface that are induced by a tilted energy band, thus suppressing hole injection rate to the active layer. In this study, GO

passivation creates dipole field outside ITO/_p_-GaN layer and induces positive charges (indicated by red color) at the top surface of _p_-GaN region. This dipole field (PGO) is opposite of

the spontaneous electric polarization (Fig. 2h). Because of the suppressed internal polarization field in _p_-GaN region with GO passivation, the hole injection rate to active layer is

improved. Hole carriers are therefore injected more efficiently into active layer so as to increase electron-hole radiative recombination, contributing enhanced light output power, as

observed in Fig. 2a. This concept is schematically demonstrated in Fig. 2i. To examine enhancement of the hole concentration in the active layer, electroluminescence spectrum was obtained at

room temperature by collecting all the lights with lens from the whole device (Fig. 3a). The intensity of UV-LED/_h_-GO was increased by about 60% compared to that without GO by the

increased hole carrier concentration in the active layer as a consequence of enhanced hole injection rate in the _p_-GaN transport layer. The peak position shift in the EL spectra of the

conventional UV-LED was about 0.8 nm (68 meV) at 100 mA, known as the conduction band filling effect25. This should be distinguished from die-to-die variation in the same wafer, since the EL

was measured by varying injection current in the same device, although the fluctuation is in the same order of magnitude. This shift in UV-LED/_h_-GO was amplified to 1.6 nm (137 meV) at

100 mA26. This additional blueshift is ascribed to the valance band filling effect by the enhanced hole carrier concentration in the MQWs27. The enhanced hole carrier injection rate in

_p_-GaN supplies more hole carriers to lead to valence band filling in the active layer (Fig. 3c). Because the number of hole carriers injected into the MQWs is small in the conventional

UV-LED, radiative recombination primarily occurs in In-rich region, where the band gap (Eg1) is minimum in the QW28. On the other hand, in the UV-LED/_h_-GO, more hole carriers can

contribute to radiative recombination throughout all the regions of the device, including both In-rich and In-poor regions (Eg2), resulting in a blue-shift in the EL spectrum and emission

uniformity improvement. To confirm emission uniformity of the device, we performed two-dimensional confocal scanning electroluminescence microscopy29. An injection current of 5 mA was fixed

during emission measurements for the UV-LEDs with and without _h_-GO nanosheets (Figs. 4a and 4b). For the UV-LED with _h_-GO, not only the emission uniformity was improved but also absolute

emission intensity was augmented compared to the UV-LED without _h_-GO, which is confirmed by the broad width in the low intensity region in the conventional UV-LED and narrow width in the

high intensity region in the UV-LED/_h_-GO (Fig. 4c). The average EL spectra collected over the entire scanning area presented in Figs. 4a and 4b are shown in Fig. 4d, again revealing the

increased areal intensity in the UV-LED/_h_-GO. Overall EL spectra still revealed clearly the blue shift. The similar conduction band-filling phenomenon beyond the localized states inside

the InGaN/GaN MQWs by increasing the injection current have been discussed previously30,31. This improvement in the intensity and uniformity becomes more severe by valence band filling

effect at higher injection current limit as shown in Fig. 3b. Towards the application of white LED, we carried out a fluorescence test of the GO-passivated UV-LED with yellow phosphor.

Figure 4e is an optical image of fully fabricated UV-LED with yellow phosphor pads without applied current in UV-LED. In case of GO-passivated UV-LED at 20 mA, Yellow phosphor pads emitted

bright fluorescence at 20 mA due to UV light irradiation (Fig. 4f). This fluorescence is significantly brighter than that of the conventional UV-LED at the same current (Fig. 4g). This

demonstrates the white LED performance with improved light output power in the GO-passivated UV-LED, which can be further applied for commercialized white LEDs as well as solar cells and

laser diode. In such applications, a degradation of GO by UV irradiation might be a problem. Reminding that such degradation take place only when the GO sheets are exposed in air or

chemicals, protecting GO by conventional encapsulation should be enough, which is discussed in detail in Supplementary information (see Supplementary Information, Figure S7). In addition,

THz transmission were measured to determine change of hole concentration in the _p_-GaN (see Supplementary Information, Figure S8). The transmission amplitude of ITO/_p_-GaN/GO decreased by

6.5% in average compared to ITO/_p_-GaN. This increased absorption means conductivity increase, which implies that the carrier density is increased. From these results, we observed that

improved hole injection rate induces an increase of free carrier concentration in the _p_-GaN. CONCLUSION We have shown that the spontaneous polarization was suppressed by simply passivating

GO on top of the fully fabricated UV-LED chips under ambient conditions. As a consequence, the hole injection rate in p-GaN was increased. It leads to augmented electron-hole radiative

recombination in active layer so as to enhance the light output power of GaN-LED device up to 60%. No further treatment is required. GO solution is easily obtained with cheap cost. We

believe that our simple method of passivation with GO is a breakthrough for further augmenting the emission efficiency in UV-LEDs. METHODS UV-LED FABRICATION Trimethylgallium,

trimethylindium and ammonia were employed as precursors for Ga, In and N, respectively. H2 was used as the carrier gas and N2 gas was used as the carrier gas for the InGaN, MQWs and GaN

barrier layers. A 20 nm thick GaN nucleation layer, a 2 µm thick undoped GaN layer, a 2-µm-thick n-type GaN layer, 5 pairs of InGaN/GaN MQW active layers, p-type AlGaN carrier blocking

layer, a 150 nm thick p-GaN layer and graded p++GaN layers were subsequently grown on a c-plane sapphire substrate (see Supplementary Information, Figure S1). Following epitaxial growth,

hole carriers of p-GaN were activated by thermal annealing process at 940°C for 40 s. Then, the surface of the LED epitaxial layers was partially etched down to the n-type GaN layer using

inductively coupled plasma etching with Cl2/BCl3/Ar plasma. A 200 nm thick ITO layer was deposited as a transparent conductive layer. Using a conventional LED fabrication technique, we

produced a full chip with ITO as the transparent top conductive layer and Cr/Au as the p–n-type electrode, as shown in the top of Fig. 1b. PREPARATION AND PASSIVATION OF THE THE GO

NANOSHEETS TO UV-LED GO nanosheets were prepared by using a modified Hummers method. Natural graphite was purchased from Alfa Aesar, 99.999% purity, 200 mesh. Natural graphite of 5 g was

dissolved in 350 ml of 10 M H2SO4. KMnO4 of 15 g was slowly added in the solution over approximately an hour. Stirring was continued for two hours in an ice-water bath. To obtain highly

oxidized graphite, the mixture was stirred vigorously for three days at room temperature. De-ionized water was subsequently added, followed by the stirring for 10 min. Aqueous solution of

H2O2 (30 wt%) was then added and the mixture was stirred for 2 h at room temperature. Aqueous solution of HCl (35 wt%) was then added and stirred for 30 min at room temperature. After the

supernatant solution was decanted, deionized water was slowly added and stirred for 30 min. The graphite oxide solution of 1 g/l in water was sonicated for an hour to exfoliate the GO

nanosheets. To obtain highly dispersed GO, centrifugation at 10,000 rpm was performed for an hour and the supernatant solution was decanted. The prepared GO solution was spin-casted onto the

ITO surface at 4,000 rpm for 10 s. To achieve a high density and uniform distribution of GO nanosheets on the ITO surface, the ITO surface was treated with O2 plasma for 5 min. The

hydrophilicity of ITO was enhanced, as indicated by the contact angle measurement (Surface Electro Optics/Phoenix 300, Fig. S2). As a result, the _h_-GO nanosheet was uniformly distributed

on the ITO surface without a concomitant loss of transmittance relative to the pristine ITO and _l_-GO/ITO, as indicated in Fig. 2e. CHARACTERIZATION OF THE GO/GAN-BASED LED Raman scattering

measurements were performed using a micro-Raman system with He-Ne laser (633 nm). I-V curves were measured using a conventional probe station system equipped with a Keithley source meter

2400. Transmittance spectra were measured on a JASCO model V-560 at room temperature. ultraviolet photoelectron spectroscopy data (Kratos model AXIS-NOVA) were obtained with a helium

discharge lamp at 7.9 × 10−9 Torr. To confirm the change in the carbon to oxygen atomic ratio in the _h_-GO, X-ray photoelectron spectroscopy analysis was performed using a Multilab2000

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Google Scholar  Download references ACKNOWLEDGEMENTS This work was supported by IBS-R011-D1, the KERI Primary research program of MSIP/ISTK (No. 14-12-N0101-14) and a grant from the Center

for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (2014M3A6A5060953), Korea. AUTHOR INFORMATION Author notes * Jeong Hyun,

Jeong Seung Yol and Park Doo Jae contributed equally to this work. AUTHORS AND AFFILIATIONS * Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS),

Sungkyunkwan University, Suwon, 440-746, Republic of Korea Hyun Jeong, Doo Jae Park, Hyeon Jun Jeong, Young Hee Lee & Mun Seok Jeong * Graphene Hybrid World Class Laboratory, Nano Carbon

Materials Research Group, Korea Electrotechnology Research Institute, Changwon, 641-120, Republic of Korea Seung Yol Jeong, Sooyeon Jeong, Joong Tark Han, Hee Jin Jeong, Sunhye Yang, Ho

Young Kim, Kang-Jun Baeg & Geon-Woong Lee * Department of Physics and Division of Energy Systems Research, Ajou University, Suwon, 443-749, Republic of Korea Sae June Park & Yeong

Hwan Ahn * School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju, 561-756, Republic of Korea Eun-Kyung Suh *

Laboratoire de Nanotechnologie et d'Instrumentation Optique, Institut Charles Delaunay, CNRS-UMR 6279, Université de Technologie de Troyes, BP 2060, Troyes, 10010, France Hyun Jeong *

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View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS H.J., S.Y.J. and D.J.P. contributed to this work in experiment planning, experiment

measurements, data analysis and manuscript preparation. The correlation between theoretical and experimental results was discussed with J.T.H. and H.J.J. The fabrication of the LED device

was carried out by H.J.J. and E.-K.S. The THz transmission measurements were performed by S.J.P. and Y.H.A. Graphene oxide nanosheets were synthesized by S.J. Scanning electron microscopy

and atomic force microscopy were performed by S.Y. and H.Y.K. The measurement and analysis of X-ray and ultraviolet photoelectron spectroscopies were carried out by K.J.B. and G.-W.L.,

Y.H.L. and M.S.J. contributed to experiment planning, data analysis and manuscript preparation. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests.

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p-GaN by graphene oxide passivation: Augmented light output of GaN UV-LED. _Sci Rep_ 5, 7778 (2015). https://doi.org/10.1038/srep07778 Download citation * Received: 26 September 2014 *

Accepted: 10 December 2014 * Published: 14 January 2015 * DOI: https://doi.org/10.1038/srep07778 SHARE THIS ARTICLE Anyone you share the following link with will be able to read this

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