Optimization of p-type contact for electrical injection and light extraction for 365 nm UV-A LEDs

This paper demonstrates low-resistance and high-transparency p-type contact materials for ultraviolet (UV) micro-light-emitting diodes (LEDs) at 365 nm. As a commonly used p-type LED contact, indium tin oxide (ITO) and nickel/ITO (Ni/ITO) contacts were studied before and after rapid thermal annealing (RTA) treatments. The transmittance at 365 nm wavelength of 200 nm thick ITO films increased from approximately 57%–90% after RTA at a temperature exceeding 400 °C, while the Ni/ITO film had a transmittance of about 73% after annealing. Micron-sized UV-LEDs with Ni/ITO p-contact were fabricated. Electrical characterization shows that Ni/ITO films annealed at 600 °C demonstrated good ohmic contact behavior and the highest on-wafer external quantum efficiency, despite slightly lower transmittance. This paper shows the potential of annealed Ni/ITO films as promising p-contact materials for high-performance 365 nm UV-LEDs.

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. and 0.7 eV respectively [1,2].Wavelengths can be adjusted from UV to the entire visible spectrum by changing the aluminum (Al), gallium (Ga), and indium (In) composition ratios [1,2,7].Among those wavelength spectra, UV-A LEDs cover wavelengths 315-400 nm, which can be used in various applications such as sterilization, biotechnology, and medical treatment [8][9][10].
Developing low-resistance and high-transparency ohmic contacts is essential to improve UV-A LED's electrical and optical performance.Currently, n-type LED metal contact (n-GaN or n-Al x Ga 1−x N, x Al < 0.05) is usually performed using metals with work functions smaller than n-GaN.These metals include titanium (Ti), aluminum (Al), and tantalum (Ta) [11].However, p-GaN ohmic contact requires a metal or transparent conducting oxide with a work function larger than p-GaN which are difficult to find [11,12].Thin Ni/Au or indium tin oxide (ITO) contacts have been investigated as commonly used semitransparent or transparent p-type ohmic electrodes for blue LEDs (450 nm) due to their large work function [13][14][15].The transmittance of semi-transparent thin Ni/Au contacts is typically under 40% at blue and near UV wavelength (400-450 nm), resulting in the absorption of a significant amount of emitted light, which decreases the external quantum efficiency (EQE) [16][17][18].ITO as a p-contact material has up to 90% transparency in the visible range; however, ITO does not make as good of an electrical contact as Ni/Au on p-GaN due to its higher contact resistance [17].
Researchers have explored various methods to improve the transmittance and contact properties of ITO contact for LEDs [19,20].One approach is using thermal treatment, demonstrated by Kim et al, who annealed ITO at 600 • C, which resulted in transmittance of over 90% for the ITO film at a wavelength of 420 nm [17,21].Another method involves the use of metal interlayers, such as Ni/ITO.Lin et al reported that a Ni (3.5 nm)/ITO (60 nm) formed an excellent ohmic contact on p-GaN, achieving high transparency of more than 87% at 470 nm without the need for post-deposition thermal annealing [7,16].Horng et al show Ni/ITO (10 nm/250 nm) layers annealed at 600 • C for 2 min under vacuum and air conditions, with 74% and 88% transmittance [13].Most of these studies are focused on blue LEDs (>420 nm); however, there is no study mainly focusing on optimizing p-type electrodes for both electrical injection and optical transmission for the UV-A LED range.
This paper addresses the need for low-resistance and hightransparency ohmic contact for UV-A micro-LEDs.Low resistance ohmic contacts will lead to lower applied bias, while transparent contacts will aid in enhanced light extraction efficiency [15,22,23].Together, these will result in high wall plug efficiency (WPE).Here, we present an optimized pcontact for UV-micro-LEDs at 365 nm by introducing Ni interlayers between p-GaN and ITO.After annealing, the Ni/ITO contact demonstrates a high transmittance of over 72%, and the electrical injection performance surpassed both ITO and unannealed Ni contacts when serving as p-contact material for LED.This paper also presents the experimental results of ITO and Ni/ITO transmittance before and after thermal treatment.To investigate electrical behavior, J-V characteristics of 100 µm × 100 µm LEDs with Ni, ITO, and Ni/ITO contacts were measured using an HP 4155B semiconductor parameter analyzer.Additionally, the on-wafer EQE of the UV micro-LEDs with Ni/ITO contact has been measured.
The 365 nm UV LED epitaxial structure was grown on sapphire substrate using metal organic chemical vapor deposition technique.From the bottommost layer up, the epitaxial structure consisted of a 25 nm AlN buffer, a 4.7 µm unintentionally doped Al 0.06 Ga 0.94 N layer, a 1.6 µm Sidoped Al 0.06 Ga 0.94 N layer with 5 × 10 18 cm −3 doping concentration, ten periods of In 0.01 Ga 0.99 N/Al 0.05 Ga 0.95 N multiquantum wells (MQWs) (3 nm/10 nm), followed by a 130 nm Mg-doped p-GaN/AlGaN superlattice electron blocking layer (EBL) layer with 5 × 10 19 cm −3 doping concentration, along with a 85 nm Mg-doped Al 0.06 GaN 0.94 layer with 5 × 10 19 cm −3 doping concentration and finally a 35 nm Mgdoped GaN layer with 5 × 10 20 cm −3 doping concentration for p-type ohmic contact.
In this paper, we have fabricated two sets of samples for the purpose of transmittance analysis and electrical measurements.The first set is designed explicitly for transmittance analysis.ITO (200 nm) and Ni (5 nm)/ITO (200 nm) films were deposited on double-side polished sapphire substrates using an electron beam evaporator (Ni) and RF magnetron sputtering (ITO).The RF sputtering of the ITO layer was performed using a Denton Discovery 24 sputtering system with a 3 inch ITO target (10 wt% SnO 2 ) used for the deposition.The sputtering chamber pressure during the deposition was 10 mT.ITO sputtering was performed in an argon gas (20 sccm) environment with an RF power of 140 W.The samples underwent rapid thermal annealing (RTA) in nitrogen ambient at various temperatures: 400 • C, 450 • C, 500 • C, and 550 • C for ITO films, and 400 • C, 500 • C, and 600 • C for Ni/ITO films.The experiment was repeated twice for validation.The optical transmittances of the films were measured using a UV VIS NIR Spectrometer Perkin-Elmer Lambda 19.
The other set included the fabrication of 100 µm × 100 µm micro-LEDs for electrical characterization.5 nm of Ni was deposited, followed by a 200 nm deposition of ITO on the LED wafer.The samples were annealed by RTA at the same temperature as the first set of Ni/ITO samples.After photoresist patterning, the n-type mesa was defined using a dry etch process, where ITO plasma etching was performed using argon/methane/hydrogen (Ar/CH 4 /H 2 ) gas chemistry and III-nitride layers were etched using chlorine/argon (Cl 2 /Ar) gas chemistry.The devices were then passivated with 30 nm of Al 2 O 3 using atomic layer deposition followed by 270 nm of SiNx using plasma-enhanced chemical vapor deposition.The via etch for n and p contact regions was performed to remove the Al 2 O 3 and SiN x films.Finally, 50 nm/250 nm Ti/Ni was deposited for n-contact and probe pads; the cross-sectional schematic is shown in figure 1(b).
In this study, electroluminescence (EL) measurements were performed from the back side of the samples.The LED devices were probed from the top, and the light transmitted through the substrate was collected from the backside using a cosine corrector (Ocean Insight CC-3-UV-S).The collected light was then directed through an optical fiber.In order to ensure compatibility with the f -number of the spectrometer, the output light from the optical fiber was collimated and refocused using two plano-convex lenses before being introduced into the spectrometer (Horiba iHR320).EL spectra were obtained using a thermoelectrically cooled charge coupled device (CCD) detector (Horiba Synapse CCD) with the iHR320 spectrometer.The optical system was calibrated using a radiometric-calibrated light source (Ocean Insight DH-3P-CAL) to ensure accurate measurements.
A p-contact material with high transmittance is essential to improve light extraction efficiency.Hu et al have demonstrated that annealing ITO films at temperatures of 400 • C and above significantly improves optical transmittance due to the  improvement of ITO crystallinity [21].The increased crystallite size after RTA reduces light scattering and improves the optical and electrical properties [21,24,25].In order to determine the optimal annealing condition, the optical transmittance of the ITO (200 nm) and Ni/ITO (5 nm/200 nm) before and after RTA was measured (figure 2).The result shows no significant differences in the transmittance of the ITO and Ni/ITO film when annealed at temperatures higher than 400 • C. The transmittance of as-deposited 200 nm ITO film at 365 nm was only 56.5%.However, after RTA at various temperatures (400 • C, 450 • C, 500 • C, and 550 • C) for 1 min, the transmittances improved to 80.9%, 87.9%, 83.0%, and 82.3%, respectively.The as-deposited Ni/ITO film showed a transmittance of 43.27%.After post-annealing process at temperature ranging from 400 • C to 600 • C, the transmittance of Ni/ITO film greatly improved and showed similar value of around 72% at 365 nm.This transmittance was only slightly decreased compared to the annealed ITO film.The transmittances across the wavelength ranging from 300 to 1500 nm of ITO and Ni/ITO with different annealing conditions are shown in figures 3(a) and (b), respectively.
In addition to the optical transmittance, the electrical properties of the p-contact materials are crucial for improving LEDs' EQE.Therefore, J-V measurements are performed to evaluate the electrical properties of different contacts, as shown in figures 4(a) and (b).The LEDs with as-deposited ITO contact demonstrated high leakage current under both forward and reverse bias, which might be attributed to suboptimal amorphous ITO.On the other hand, as-deposited Ni/ITO and Ni-/ITO annealed at 400 • C and 600 • C showed the lowest leakage current among all the samples.Contrary to the annealing results at 400 • C and 600 • C, the 500 • C annealed sample demonstrated increased leakage current in both forward and reverse bias.This high reverse leakage in 500 • C annealed sample is difficult to explain and will require further investigation.
The 600 • C annealed and the unannealed Ni/ITO sample demonstrated the lowest on-resistance in among all the samples.An increase in series resistance for the 400 • C annealed sample was observed with increasing annealing temperature.This increase in resistance may be attributed to the crystallization of the ITO film.The authors hypothesize that as the ITO film crystallizes from the amorphous layer, the film roughness gradually increases, [26,27] which may result in the formation of micro-voids at the interface between the ITO layer and Ni layer, [28] hence can potentially impact the transport properties and an increased series-resistance.Furthermore, due to the relatively short annealing time (2 min), this short duration at relatively low temperature may not be sufficient to form a suitable NiO to help improve the p-type contact resistance [29].Further investigation would be needed to thoroughly understand this behavior.With a subsequent rise in the annealing temperature to 500 • C, the ITO film may experience deterioration, leading to the enlargement and worsening of defects, such as voids formed during the ITO crystallization process, [30][31][32] hence leading to higher leakage and resistance.However, when the annealing temperature was raised to 600 • C, which is close to the optimum annealing temperature for Ni, the Ni formed an improved contact with the p-GaN.During the annealing of the Ni/ITO contact, ITO provides the oxide source for NiO formation [11].NiO demonstrated a good band line-up with p-GaN, resulting in a thin Schottky barrier and smaller barrier height [33,34].A high  temperature annealing of Ni may also result in an improved intermixing or contact for Ni and ITO.Therefore, both the leakage and the resistance decrease greatly and this can overcome the impact caused by the crystalized ITO film.Although as-deposited Ni/ITO contact and 600 • C annealed Ni/ITO contact showed similar electrical properties, the transmittance of 600 • C annealed contact is much higher than the as-deposited Ni/ITO, which will show an advantage in EQE.
The efficiency of the LED devices with Ni/ITO contact was evaluated by measuring the EQE.In figure 5, the relative EQE data for LEDs with mesa sizes of 100 µm × 100 µm are presented at current densities ranging from 10 A cm −2 to 200 A cm −2 .The relatively low EQE values are due to onwafer measurement and does not include any light extraction techniques such as flip-chip, substrate removal, and packaging [35][36][37][38].Furthermore, the presented EQE values may be conservative due to the unaccounted light loss from the wafer to the cosine corrector.With suitable and state-of-the-art light extraction techniques, we can expect 2-5× improvement in the EQE of these UV-LEDs [39].However, this study was focused on obtaining relative trends in EQE of the UV LEDs with different p-type contact, therefore the EQE values are valid for comparison to find optimal annealing temperature.The EQE results align with the findings from the J-V measurements that the LED with Ni/ITO contact annealed at 600 • C shows the best J-V characteristics and highest EQE among all the devices tested.The LED annealed at 400 • C demonstrates the second-best EQE performance, while the LED annealed at 500 • C shows the lowest EQE along with the non-annealed sample.Among all the samples, 600 • C sample showed highest WPE of around 4.15% at 100 A cm −2 , while the 500 • C sample showed the lowest WPE, which is around 2.57% at 100 A cm −2 .The WPE result showed similar trend as the EQE plot for most of the samples except the not-annealed Ni/ITO sample.From J-V plot (figure 4(b)), not-annealed ITO showed low resistance compared to samples annealed at lower temperatures, therefore, led an improvement in WPE%.Owing to the low resistance, not-annealed and 600 • C annealed sample will experience efficiency droop at higher current density.On the contrary, due to the large resistance and accumulated heat at high current density, samples annealed at 400 • C and 500 • C showed a reduced WPE with increasing current density.The reason for poor J-V and EQE behaviors for annealing at 500 • C is not apparent to the authors, and further investigations will be needed to understand the trend completely.The findings from the EQE measurements further confirm the promising potential of the Ni/ITO film annealed at 600 • C as an excellent p-contact material for UV-LEDs.
In conclusion, this paper demonstrates low-resistance and high-transparency Ni/ITO films as p-contact materials for micron-sized UV-LEDs.We have explored thermal treatment to enhance the optical transmittance of ITO films.The J-V experiment shows that Ni/ITO, as a p-contact material, even with slightly lower transmittance compared to annealed ITO films, exhibits improved electrical properties when annealed at 600 • C. The J-V measurements demonstrated the advantages of Ni/ITO, and the EQE results confirmed its efficacy in enhancing LED performance.These results pave the way for developing more efficient, high-performance UV light sources.

Figure 4 .
Figure 4. J-V characteristics of LED with Ni/ITO contact and ITO contacts on p-GaN (a) from −10 V to 5 V in log scale, and (b) 2-5 V in linear scale.