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Abstract
In GaN-based vertical micro LEDs, conventional metal n-contacts on the N face n-GaN suffer from a low aperture ratio due to the high reflection of metals, resulting in low-light extraction efficiencies. Great efforts have been devoted to enhancing transparency by employing transparent conducting oxides for n-contacts, but they exhibited poor Ohmic behavior due to their large work functions. Herein, we introduce an InN/ITO n-contact to achieve both superior contact property and high transparency. At the initial stage, the ITO with thin In interlayer was utilized, and the change in contact properties was observed with different annealing temperatures in the N2 atmosphere. After annealing at 200 °C, the In/ITO n-contact exhibited Ohmic behavior with high a transparency of 74% in the blue wavelength region. The metallic In transformed into InN during the annealing process, as confirmed by transmission electron microscopy. The formation of InN caused polarization-induced band bending at the InN/GaN interface, providing evidence of enhanced Ohmic properties. In the application of vertical GaN µLED, the EQE increased from 6.59% to 11.5% while operating at 50 A/cm2 after the annealing process.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent display technologies, the development of high-resolution displays has attracted a lot of attention for realizing metaverse platforms such as virtual reality (VR), augmented reality (AR), and mixed reality (MR) [1–6]. The key technology for high-resolution displays is the implementation of a light-emitting device with superior performance. Conventional organic light emitting diodes (OLEDs), which are used in monitors, TVs, smartphones, and pads, have the potential to realize future displays due to their tremendous advantages, including flexibility, a perfect contrast ratio, and the capability of large area production. However, applying OLEDs in such technologies is challenging due to problems such as burn-in, sensitivity to moisture and oxygen, micro-scale response time, and limitations in improving resolution due to the size restriction of a fine metal mask [7–10].
Recently, gallium nitride (GaN)-based micro LEDs (µLEDs) with device size under 100${\times} $100 µm2 have emerged as promising candidates for realizing metaverse platforms due to their low cost, resistance to damage from oxygen and moisture, nano-scale response time, and low power consumption [11–16]. In particular, vertical GaN LEDs, which are fabricated through the laser-lift off (LLO) process, offer advantages over conventional lateral GaN LEDs, including excellent current spreading, low heat resistance, high extraction efficiency, and compatibility with various substrate types [17–20]. In the past, poor Ohmic properties on the n-type GaN surface (N-face polarity) posed significant challenges in achieving high-quality vertical GaN LEDs. However, these issues have been addressed by introducing metal-based electrodes with low work functions, such as Al [21], In [22], Ti/Al [23–25], Cr/Al [26], TiN/Al [27], and metal multilayered structures [28–30]. Nevertheless, when metal n-electrodes are applied to µLEDs, the ratio of metal-to-light-emitting area becomes larger with decreasing the size of µLEDs. This increased metal coverage on the light-emitting area results in a low aperture ratio for µLEDs due to high reflection of metals, leading to poor light extraction efficiencies.
To overcome these problems, transparent conductive oxides (TCOs) such as indium tin oxide (ITO), zinc oxide (ZnO), aluminum-doped ZnO (AZO), and fluorine-doped tin oxide (FTO) can be excellent candidates for achieving a high aperture ratio owing to their high transparency. However, work functions (WFs) of TCOs are relatively large (WF > 4 eV) compared to the electron affinity of N face n-GaN (χe ∼ 3.3 eV), resulting in the formation of high Schottky barrier height (SBH). Several groups have reported strategies to reduce SBH by introducing an interlayer between ITO and N face n-GaN. Hwang et al. reported that a Ti 10 nm / ITO 100 nm n-contact exhibited Ohmic behavior on Ar plasma treated n-GaN with a specific contact resistance of 3.2${\times} $10−6 Ω·cm2 [31]. Kang et al. showed that the In 20 nm / ITO 200 nm n-contact showed enhanced contact resistance of 1.8${\times} $10−3 Ω·cm2 when annealed at 300 °C [32]. Kim et al. demonstrated Cr 10 nm / ITO 40 nm n-contact with the specific contact resistance of 9.8${\times} $10−4 Ω·cm2 and a transmittance of ∼ 30% [33]. Although these metal/ITO n-contacts achieved enhanced Ohmic behavior compared to the ITO n-contact, the transmittance would be degraded due to the reflection induced by the metal layer. Furthermore, there is a lack of clear explanations with sufficient experimental results considering the enhanced Ohmic behavior compared to the ITO n-contact.
Here, we designed a highly transparent n-contact with low resistance by inserting a thin metal layer between the n-GaN and TCO, and elucidated a mechanism of enhanced contact properties. Metals with low WFs such as tin (Sn), titanium (Ti), and indium (In) were utilized as an interlayer, and the change in contact resistance and transmittance was investigated with different annealing temperature in N2 atmosphere. Among them, the ITO n-contact with a 10 nm-thick In interlayer exhibited a contact resistivity of 2.20${\times} $10−4 Ω·cm2 with an average total transmittance (%T) of ∼70% after annealing at 200 °C in N2 ambient, surpassing the ITO n-contact (contact resistivity of 3.17${\times} $102 Ω·cm2 and average %T of 70%). These enhancements attributed from the formation of InN at the interface of n-GaN/In interlayer during the annealing process. The InN induced polarization due to the lattice mismatch with the n-GaN, resulting in band bending and the formation of two-dimensional electron gas (2DEG). In the application of µLEDs with In/ITO n-contacts of size 100${\times} $100 µm2, the current density at 4 V increased from 11.6 A/cm2 to 20.0 A/cm2, and the external quantum efficiency (EQE) increased from 6.59 to 11.5% when operating at 50 A/cm2 after the annealing process.
2. Results and discussion
In GaN-based vertical LED, various types of n-contacts are employed to establish Ohmic contact with N face n-GaN (Fig. 1). Conventionally, metal n-contacts with low work function are widely employed for vertical LEDs (Fig. 1(a), e). While metal n-contacts can effectively reduce the SBH to below 0.5 eV, they suffer from the low EQE due to the intrinsic property of metals with high light reflection (∼ 100%). To address this issue, ITO, a well-known TCO with a low sheet resistance of 10 Ω/sq, can reduce the light reflection due to its high %T of ITO (over ∼ 80% in visible wavelengths) (Fig. 1(b), f). However, since the WF of ITO is approximately 4.6 eV [34], the SBH is quite large (∼1.3 eV), posing a challenging in establishing Ohmic contact with the N face n-GaN. In order to reduce the SBH while maintaining high transparency, the introduction of a thin metal layer beneath the ITO n-contact can be considered (Fig. 1(c), g). Nevertheless, partial reflection can still degrade transparency due to the presence of thin metal layer. To achieve both low contact resistance and high transparency, we propose the metal nitride/ITO n-contact produced by the annealing process of the thin metal/ITO n-contact (Fig. 1(d), h). During the annealing process, N atoms from the GaN diffuse into the thin metal layer, resulting in the transformation of metal into metal nitride. The N vacancies on the GaN surface create a donor-like state, inducing the formation of the 2DEG at the GaN surface, which can enhance the electron transfer properties. Together with the combined effect of the 2DEG and the high transparency provided by the thin metal nitride, it becomes possible to realize efficient vertical GaN µLEDs with high EQE.
Fig. 1. Schematic illustration of vertical micro LED with different n-contacts: (a) metal, (b) ITO, (c) ITO with thin metal, and (d) ITO with thin metal nitride after annealing process of ITO with thin metal. Schematic band diagram of each materials for n-contact ((e) metal, (f) ITO, (g) ITO with thin metal, and (h) ITO with thin metal nitride after annealing process of ITO with thin metal).
As a first step, we selected three metals with low WF (Sn = 4.42 eV, Ti = 4.33 eV, In = 4.12 eV) [35] and designed the optimal n-contact structure for high transparency in the blue wavelength region (λ, ranging from 400 to 500 nm), which corresponds to the emission wavelength region of GaN LEDs. The average %T in the blue wavelength region was systematically calculated using thin film design simulation tool as a function of the thickness of ITO and metal interlayers (Fig. S1). The maximum %T could be achieved with an ITO thickness of 50 nm and 150 nm thicknesses of ITO. Considering the conductance and transparency, the thickness of ITO was fixed at 150 nm. To achieve average %T above 60%, the thickness of the Sn, Ti, and In interlayer needed to be less than 2 nm, 8 nm, and 15 nm, respectively. Based on these simulation results, the In interlayer offers an advantage in achieving high %T with appropriate conductance.
To assess the effect of the metal interlayer on the contact properties, the transmission line method (TLM) was employed (Fig. S2) [36,37]. TLM structures were patterned on the N face n-GaN using photoresist (PR), followed by the sequential deposition of thin metal and ITO. After then, the PR was removed. Initially, the current-voltage (I-V) characteristics of ITO n-contact and ITO n-contact with 3 nm metal interlayers were measured between TLM pads with an inter-spacing of 5 µm (Fig. 2(a), Fig. S3, S4). The I-V curves of the ITO and Sn 3 nm/ITO contact exhibited a nonlinear shape, indicating the non-Ohmic behavior. Conversely, the Ti 3 nm/ITO and In 3 nm/ITO showed linear curves, indicative of Ohmic behavior. Furthermore, an annealing process was conducted on all samples to investigate the effect of annealing on the contact properties. When the ITO and Sn 3 nm/ITO contacts were annealed at 200 °C in a nitrogen (N2) ambient, non-Ohmic behavior persisted. In the case of Ti 3 nm/ITO, electrical degradation was observed upon annealing, while the In 3 nm/ITO exhibited enhanced Ohmic behavior. These trends remained consistent when the thickness of all metal interlayers was increased to 10 nm (Fig. 2(b), Fig. S5). Note that the I-V characteristics showed slight improvement as the thickness of the metal interlayer increased. The contact resistivity was determined by plotting the measured resistivity against the spacing between the TLM pads, using following equation [36]:
where ${\rho _c}$ is the contact resistivity, ${R_c}$ is the contact resistance, and ${R_s}$ is the sheet resistance. The contact resistivities of the contacts were plotted as a function of annealing temperature (Fig. 2(c)-(e)). The ITO contact showed very poor contact resistivities on a scale of over 102 Ω·cm2 regardless of the annealing temperature, indicating non-Ohmic contact. For the Sn/ITO contact, the ${\rho _c}$ of Sn 3 nm/ITO decreased as the annealing temperature increased, while the ${\rho _c}$ of Sn 10 nm/ITO increased with higher annealing temperatures. Note that the annealing temperature was not exceed over 300 °C due to the crack or shrinkage in Au-Sn eutectic bonding, which might diffuse to the top surface and affect the contact properties (Fig. S6) [38]. In the case of Ti/ITO contact, the ${\rho _c}$ increased as the annealing temperature increased, regardless of the thickness of Ti interlayer. Moreover, ${\rho _c}$ of the In/ITO contact decreased until annealing at 200 °C. Beyond 200 °C annealing, the ${\rho _c}$ of the In 3 nm/ITO contact increased by approximately 1 order of magnitude, while those of the In 10 nm/ITO contacts remained relatively stable. The minimum ${\rho _c}$ values for each metal interlayers are summarized in Table S1. Based on the I-V characteristics and contact properties, ITO with Ti and In interlayers could be excellent candidates for achieving Ohmic contact in vertical GaN μLEDs.Fig. 2. I-V curves of ITO contacts on N face n-GaN with (a) 3 nm and (b) 10 nm interlayer of Sn, Ti, and In before and after annealing at 200 °C in a N2 ambient. Contact resistivities of ITO with interlayer of (a) Sn, (b) Ti, and (c) In with variation of annealing temperature in a N2 ambient.
To investigate the effect of the metal interlayer on both optical and contact properties, ITO with various metal interlayers were deposited on the n-GaN, which was grown on double-sided polished (DSP) sapphire substrate (Fig. 3(a)). The %T of each sample was calculated using thin film designing simulation tool, and the actual %T was measured using a UV-vis spectrometer within the range from 400 to 500 nm wavelengths (Fig. 3(b), Fig. S7). The average %T was calculated from the average value of transmittance in the range of 400 to 500 nm wavelengths. The measured average %T of all samples was similar to the simulated average %T, indicating that all contacts were well fabrication with appropriate thickness. To determine the effect of annealing on %T, the %T of each ITO with metal interlayers was measured as a function of annealing temperature (Fig. 3(c)-(e)). The GaN without n-contact exhibited the average %T of 74%, which remained almost the same as the annealing temperature increased. When ITO was deposited on GaN, the average %T slightly reduced to 70%, and it was maintained even after annealing process. The Sn 3 nm/ITO and Sn 10 nm/ITO contacts showed average %T of 67.9% and 44.8%, respectively. After annealing at 250 °C, the average %T of both contacts increased to 74.0% and 55.3%, respectively. The average %T of Ti 3 nm/ITO and Ti 10 nm/ITO contacts was 67.8% and 51.4%, respectively, and slightly improved to 72.8% and 54.5% for each contact as the annealing temperature increased. In the case of the In interlayer, the In 3 nm/ITO and In 10 nm/ITO contacts exhibited the average %T of 69.5% and 50.3%, respectively. As the annealing temperature increased, the average %T of In 3 nm/ITO gradually increased to 74.1%, while that of In 10 nm/ITO dramatically increased to 73.3%. From the measured electrical and optical properties of ITO with thin metal layers, it can be concluded that In 10 nm/ITO with 200 °C annealing is the most suitable contact for achieving both high conductivity and transparency in vertical GaN µLEDs.
Fig. 3. (a) Schematic illustration of Macleod simulation and experimental sample structures of LED. (b) Simulated and measured average total transmittance (λ: 400 nm ∼ 500 nm) of LED with different n-contact materials. Measured average total transmittance of LED with ITO-based n-contact with the interlayer of (c) Sn, (d) Ti, and (e) In thin metal.
To quantify the performance enhancement of µLED by introducing ITO with a thin metal layer, we fabricated vertical GaN µLED with n-contacts of ITO, Ti 10 nm/ITO, and In 10 nm/ITO using PR lithography on GaN wafer (Fig. S8, S9). The size of the µLEDs was 100 µm${\times} $100 µm, and the area of the n-contact was defined as 80% of the LED’s area (Fig. S10). To facilitate probing, n- and p-pads were deposited with Cr (50 nm) / Au (200 nm). The schematic LED structure is depicted in Fig. 4(a). The current density-voltage characteristics of vertical GaN µLEDs were measured using a semiconductor analyzer (Keithley 4200-SCS) (Fig. 4(b) and (c), Fig. S11). The reverse leakage current density at -3 V was measured to be in the range of 130 to 476 nA/cm2 for all samples, and the forward voltage at 20 mA/cm2 was measured to be 2.25 V for all samples. The I-V curves of µLEDs with metal-intercalated ITO n-contact showed a similar trend, while that of µLED with ITO was forward-shifted due to the poor contact resistivity of ITO/N face n-GaN. No distinct changes were found in the reverse leakage current densities and forward voltages at 20 mA/cm2 in all samples after 200 °C annealing. After annealing at 200 °C, the I-V curve of µLED with ITO n-contact exhibited slight negative-shift, even though the contact resistivity was quite similar. This can be explained by the better current spreading of ITO after annealing process [18,39–42]. In case of Ti 10 nm/ITO n-contact, I-V curve significantly shifted in the positive direction after annealing, which is attributed to the increased contact resistivity. However, the µLED with In 10 nm/ITO n-contact showed an enhanced I-V characteristic after the annealing process owing to the reduced contact resistivity following the annealing process. The current injected optical images of µLEDs at 5 A/cm2 are displayed in Fig. 4(d). The µLED with ITO/metal interlayer n-contacts showed lower light emission in the region where n-contacts were deposited compared to the ITO n-contact. This is due to the low %T of ITO with metal interlayers. After 200 °C annealing, the μLED with ITO showed better uniform light emission due to the enhanced current spreading. Since the %T of Ti 10 nm/ITO was low even after annealing process, the Ti 10 nm/ITO still showed low light emission, whereas the In 10 nm/ITO showed uniform light emission due to the increased %T after annealing process. The external quantum efficiency (EQE), electroluminescence (EL) spectrum, and radiant flux versus current densities of µLED with different n-contacts were measured using an integrating sphere (Fig. 4(e), (f), and Fig. S12). The EQE of In 10 nm/ITO was higher than that of Ti 10 nm/ITO due to its better %T in blue wavelength region. However, the ITO exhibited the highest EQE since its %T was 1.4 times higher than those of ITO with metal interlayers. Note that the maximum EQE was measured at the current density of 5 A/cm2 due to the size effect of µLEDs [43–45]. After the annealing process, the EQEs increased for all contacts except Ti 10 nm/ITO. Although the ITO n-contact exhibited similar contact resistivity and transparency after annealing process, the EQE of µLED with ITO n-contact was enhanced, which would be induced by the better current spreading of ITO after annealing. In particular, the EQE of In 10 nm/ITO was significantly enhanced due to the increased %T after annealing. From these results, the In 10 nm/ITO n-contact is effective for applications in µLEDs with high EQE and low operating power owing to its high conductivity and transparency.
Fig. 4. (a) Schematic illustration of fabricated µLED. I-V characteristics of µLED with different n-contacts: (b) before annealing and (c) after 200 °C annealing. (d) Optical microscope images of µLEDs operating at 5 A/cm2 with different n-contacts (up: before annealing, down: after 200 °C annealing). Measured EQE of µLEDs with different n-contacts (e) before annealing and (f) after 200 °C annealing.
To investigate the origin of the enhanced contact properties of In 10 nm/ITO, cross-sectional transmission electron microscope (TEM) and an electron energy loss spectroscopy (EELS) elemental mapping were conducted (Fig. 5). The EELS elemental mapping clearly showed the presence of each element in its corresponding layer. The O signal was observed in the In metal layer, which originated from the negative Gibb’s energy of formation of In2O3 from In (-830.68 kJ/mol) [46]. After annealing at 200 °C, the O signal in In layer nearly disappeared because In2O3 is unstable over 200 °C in an N2 ambient [47]. Notably, the N signal was detected in the In layer near the GaN layer, indicating N diffusion during the annealing process. Detailed analysis is explained using high resolution cross-sectional TEM images and electron diffraction patterns (Fig. 5(c), d). In the as-deposited In 10 nm/ITO on GaN sample, each layer could be clearly distinguished based on its crystallinity: the GaN layer consisted of epitaxially grown GaN (002) plane (lattice spacing (a) = 0.260 nm) along the z-axis, the In layer consisted of mixed phase of metallic In (a = 0.246 nm, corresponding to (002) plane, and a = 0.272 nm, corresponding to (111) plane) and In2O3 (a = 0.292 nm, corresponding to (222) plane)), and the ITO layer consisted of amorphous phase. No significant changes were observed in the GaN layer after annealing at 200 °C. The ITO layer transformed into a poly-crystalline phase after annealing at 200 °C. Interestingly, InN (100) (a = 0.413 nm) and (002) (a = 0.285 nm) planes were observed after annealing at 200 °C. The presence of InN confirmed that the EQE of the µLED with In 10 nm/ITO improved after the annealing process due to the higher %T of InN compared to metallic In (Fig. S13).
Fig. 5. Electron energy loss spectroscopy (EELS) elemental maps of ITO/In 10 nm on N face n-GaN (a) before annealing and (b) after 200 °C annealing in a N2 ambient. High-resolution TEM images and electron diffraction patterns of ITO/In 10 nm on N face n-GaN (c) before annealing and (d) after 200 °C annealing in a N2 ambient.
Previously, we reported that the presence of InN between In and GaN layers could enhance the contact properties due to the polarization induced band bending [22]. Two types of polarization can occur in the InN/GaN system: spontaneous polarization (PSP) and piezoelectric polarization (PPZ) [48–50]. The PSP in GaN is induced by the dipole moment along the c-axis. The direction of the PSP is along the Ga-to-N direction and depends on the crystal’s polarity. For example, the surface of vertical GaN is terminated with N polarity, so the direction of the PSP is perpendicular to the surface. The PPZ is induced by the lattice mismatch between GaN and InN. Since the lattice constants of GaN and InN are 3.189 Å and 3.54 Å, respectively, the compressive strain occurs in InN [48]. The amount of the PPZ can be calculated using the following equation [48–50]:
Fig. 6. Schematic band diagrams of (a) ITO/N face n-GaN, (b)ITO/In/N face n-GaN, and (c) ITO/InN/N face n-GaN system.
3. Conclusion
In summary, we have successfully demonstrated Ohmic contacts to N face n-GaN with both low resistance and high transparency by introducing a thin metal layer in ITO n-contact. Three metals were investigated to achieve optimized n-contacts, and the In/ITO combination showed an improved contact resistivity of 2.20${\times} $10−4 Ω·cm2 and an average %T of ∼70% in blue wavelength regions after 200 °C annealing. The origin of the enhanced Ohmic behaviors was discussed in relation to the formation of InN during the annealing process, which was confirmed by the cross-sectional TEM and EELS analysis. The resulting InN/ITO n-contact significantly improved the I-V characteristics compared to the ITO n-contact, with a high EQE of 11.5%. Therefore, the ITO n-contact with InN interlayer holds promise as an Ohmic contact for the realization of high-resolution display technologies using vertical GaN µLEDs.
4. Experimental section/methods
Device Fabrication: A typical vertical InGaN/GaN MQW LED wafer was prepared on Si wafer by laser-lift off (LLO) process. The structure of LED wafer is consisted of Si substrate, Au-Sn bonding layer, silicon dioxide (SiO2) diffusion barrier, ITO p-contact, p-type GaN, InGaN/GaN MQW, n-type GaN, and unintentionally doped GaN (u-GaN) layers. The sample was cleaned with acetone and isopropyl alcohol in an ultrasonic bath for 5 min, washed by deionized water, and then dried by blowing with high-purity nitrogen (N2) gas. The u-GaN was removed using inductively coupled plasma (ICP) etching. The active regions were defined with ICP etching process. Subsequently, the SiO2 passivation layer was deposited using plasma-enhanced chemical vapor deposition (PECVD) system with SiH4, N2O, and He at 200 °C. And, the area of p-contact was defined with buffered oxide etcher. And then, various n-contacts were deposited; metal layers (Sn, Ti, and In) were deposited by a thermal evaporator under 1${\times} $10−6 Torr and ITO layers were deposited by a sputter system at room temperature. After then, n- and p-pad with Cr 50 nm/Au 200 nm were deposited by an e-beam evaporator at 6.5 kV under 1${\times} $10−6 Torr. Each layer was defined through conventional photolithography, and the active area was defined to be 100${\times} $100 µm2. The annealing process was conducted by rapid thermal annealing system for 3 min in N2 ambient.
Characterizations: The cross-sectional image of vertical LED on Si wafer was measured using a field-emission scanning electron microscope (XLS30s FEG, PHILIPS) with 5 kV acceleration voltage and 6 mm working distance. The device images and emission images were taken with an optical microscope (BX41, Olympus Co. Ltd). High-resolution transmission electron microscopy (HR-TEM) equipped with electron energy loss spectroscopy (EELS) was analyzed at 200 kV with Cs-correcter using JEOL JEM 2200FS.
Measurements: The total (integrating sphere) transmittance of the samples was measured at 400 $\le $ λ $\le $ 500 nm wavelength regions using a UV-vis spectrometer (Cary 4000, Agilent Technologies). I-V characteristics were measured using a precise semiconductor analyzer unit (Keithley 4200-SCS, Keithley Instruments). The contact resistivities were measured using the transmission line method (TLM) (see details in supporting information). The light output, EQE, and EL spectrum were measured using a spectrometer (SM240, Spectral Products) with an optical fiber, equipped with integrating sphere and powered with semiconductor analyzer unit (Keithley 4200-SCS, Keithley Instruments). All the LED measurements were conducted on the probe station (REL-3200, Cascade Microtech, Inc.) in an unpackaged configuration. For reliable values, each experiment was repeated at least 5 times.
Optical Simulations: The optical transmittance calculations of n-contacts on n-GaN were performed using commercial thin film designing software (The Essential Macleod, Thin Film Center, Inc.) based on the characteristic matrix method. The refractive index (n) and extinction coefficient (k) of materials were used from open library sources.
Funding
Ministry of Science and ICT, South Korea (F21YY7105002); Samsung Display Co., Ltd.
Acknowledgments
This research was financially supported by Brain Korea 21 PLUS project for Center for Creative Industrial Materials (F21YY7105002). We also acknowledge Samsung Display Co., Ltd. for financial support of this work.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained the authors upon reasonable requrest.
Supplemental document
See Supplement 1 for supporting content.
References
1. Y. Huang, E.-L. Hsiang, M.-Y. Deng, et al., “Mini-LED, Micro-LED and OLED displays: Present status and future perspectives,” Light: Sci. Appl. 9(1), 105 (2020). [CrossRef]
2. W. Meng, F. Xu, Z. Yu, et al., “Three-dimensional monolithic micro-LED display driven by atomically thin transistor matrix,” Nat. Nanotechnol. 16(11), 1231–1236 (2021). [CrossRef]
3. T. Wu, C.-W. Sher, Y. Lin, et al., “Mini-LED and micro-LED: promising candidates for the next generation display technology,” Appl. Sci. 8(9), 1557 (2018). [CrossRef]
4. V. W. Lee, N. Twu, and I. Kymissis, “Micro-LED technologies and applications,” Inf. Disp. 32, 16–23 (2016).
5. T. Zhan, K. Yin, J. Xiong, et al., “Augmented reality and virtual reality displays: perspectives and challenges,” Iscience 23(8), 101397 (2020). [CrossRef]
6. J. Xiong, E.-L. Hsiang, Z. He, et al., “Augmented reality and virtual reality displays: emerging technologies and future perspectives,” Light: Sci. Appl. 10(1), 216 (2021). [CrossRef]
7. J. Li, L. Xu, C. W. Tang, et al., “High-resolution organic light-emitting diodes patterned via contact printing,” ACS Appl. Mater. Interfaces 8(26), 16809–16815 (2016). [CrossRef]
8. T. Tsujimura, OLED display fundamentals and applications (John Wiley & Sons, Hoboken, NJ, USA, 2017).
9. Q. Lu, Z. Yang, X. Meng, et al., “A review on encapsulation technology from organic light emitting diodes to organic and perovskite solar cells,” Adv. Funct. Mater. 31(23), 2100151 (2021). [CrossRef]
10. C. Keum, C. Murawski, E. Archer, et al., “A substrateless, flexible, and water-resistant organic light-emitting diode,” Nat. Commun. 11(1), 6250 (2020). [CrossRef]
11. D.-H. Lee, J.-H. Lee, J.-S. Park, et al., “Improving the leakage characteristics and efficiency of GaN-based micro-light-emitting diode with optimized passivation,” ECS J. Solid State Sci. Technol. 9(5), 055001 (2020). [CrossRef]
12. H. Zhang and J. A. Rogers, “Recent advances in flexible inorganic light emitting diodes: From materials design to integrated optoelectronic platforms,” Adv. Opt. Mater. 7(2), 1800936 (2019). [CrossRef]
13. S. Hwangbo, L. Hu, A. T. Hoang, et al., “Wafer-scale monolithic integration of full-colour micro-LED display using MoS2 transistor,” Nat. Nanotechnol. 17(5), 500–506 (2022). [CrossRef]
14. E. L. Hsiang, Z. Yang, Q. Yang, et al., “Prospects and challenges of mini-LED, OLED, and micro-LED displays,” J. Soc. Inf. Disp. 29(6), 446–465 (2021). [CrossRef]
15. A. R. Anwar, M. T. Sajjad, M. A. Johar, et al., “Recent Progress in Micro-LED-Based Display Technologies,” Laser Photonics Rev. 16, 2100427 (2022). [CrossRef]
16. H. Jiang and J. Lin, “Nitride micro-LEDs and beyond-a decade progress review,” Opt. Express 21(S3), A475–A484 (2013). [CrossRef]
17. B. U. Ye, B. J. Kim, Y. H. Song, et al., “Enhancing Light Emission of Nanostructured Vertical Light-Emitting Diodes by Minimizing Total Internal Reflection,” Adv. Funct. Mater. 22(3), 632–639 (2012). [CrossRef]
18. J. H. Son, B. J. Kim, C. J. Ryu, et al., “Enhancement of wall-plug efficiency in vertical InGaN/GaN LEDs by improved current spreading,” Opt. Express 20(S2), A287–A292 (2012). [CrossRef]
19. M. Asad, Q. Li, M. Sachdev, et al., “Thermal and optical properties of high-density GaN micro-LED arrays on flexible substrates,” Nano Energy 73, 104724 (2020). [CrossRef]
20. P. Tian, J. J. McKendry, E. Gu, et al., “Fabrication, characterization and applications of flexible vertical InGaN micro-light emitting diode arrays,” Opt. Express 24(1), 699–707 (2016). [CrossRef]
21. H. W. Jang, S. Lee, S. W. Ryu, et al., “Interfacial Band Bendings in Al Ohmic Contacts to Laser-Irradiated Ga-Face and N-Face n-GaN,” Electrochem. Solid-State Lett. 12(11), H405–H407 (2009). [CrossRef]
22. S. Y. Moon, J. H. Son, K. J. Choi, et al., “Indium as an efficient ohmic contact to N-face n-GaN of GaN-based vertical light-emitting diodes,” Appl. Phys. Lett. 99(20), 202106 (2011). [CrossRef]
23. H. W. Jang and J.-L. Lee, “Origin of the abnormal behavior of contact resistance in ohmic contacts to laser-irradiated n-type GaN,” Appl. Phys. Lett. 94(18), 182108 (2009). [CrossRef]
24. T. Jang, S. N. Lee, O. H. Nam, et al., “Investigation of Pd∕Ti∕Al and Ti∕Al Ohmic contact materials on Ga-face and N-face surfaces of n-type GaN,” Appl. Phys. Lett. 88(19), 193505 (2006). [CrossRef]
25. H. Seo, Y. J. Cha, A. M. H. Islam, et al., “Improvement of Ti/Al ohmic contacts on N-face n-type GaN by using O-2 plasma treatment,” Appl. Surf. Sci. 510, 145180 (2020). [CrossRef]
26. J. W. Jeon, S. Y. Lee, J. O. Song, et al., “Low-resistance Cr/Al Ohmic contacts to N-polar n-type GaN for high-performance vertical light-emitting diodes,” Curr. Appl Phys. 12(1), 225–227 (2012). [CrossRef]
27. J. W. Jeon, T. Y. Seong, H. Kim, et al., “TiN/Al Ohmic contacts to N-face n-type GaN for high-performance vertical light-emitting diodes,” Appl. Phys. Lett. 94(4), 5 (2009). [CrossRef]
28. K. Rickert, A. Ellis, J. K. Kim, et al., “X-ray photoemission determination of the Schottky barrier height of metal contacts to n–GaN and p–GaN,” J. Appl. Phys. 92(11), 6671–6678 (2002). [CrossRef]
29. A. Motayed, R. Bathe, M. C. Wood, et al., “Electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au multilayer Ohmic contacts to n-type GaN,” J. Appl. Phys. 93(2), 1087–1094 (2003). [CrossRef]
30. Y. H. Song, J. H. Son, B. J. Kim, et al., “Effects of W diffusion barrier on inhibition of AlN formation in Ti/W/Al ohmic contacts on N-face n-GaN,” Appl. Phys. Lett. 99, 8 (2011).
31. J. D. Hwang, G. H. Yang, C. C. Lin, et al., “Nonalloyed Ti/indium tin oxide and Ti ohmic contacts to n-type GaN using plasma pre-treatmentnn,” Solid-State Electron. 50(2), 297–299 (2006). [CrossRef]
32. K. M. Kang, J. M. Jo, J. S. Kwak, et al., “In/ITO Ohmic Contacts to Ga-face and N-face n-GaN for InGaN-based Light-emitting Diodes,” J. Korean Phys. Soc. 55(1), 318–321 (2009). [CrossRef]
33. H. Kim, D. H. Kim, and T. Y. Seong, “Cr/ITO semi-transparent n-type electrode for high-efficiency AlGaN/InGaN-based near ultraviolet light-emitting diodes,” Superlattices Microstruct. 111, 872–877 (2017). [CrossRef]
34. K. H. Lee, H. W. Jang, K.-B. Kim, et al., “Mechanism for the increase of indium-tin-oxide work function by O 2 inductively coupled plasma treatment,” J. Appl. Phys. 95(2), 586–590 (2004). [CrossRef]
35. H. B. Michaelson, “The work function of the elements and its periodicity,” J. Appl. Phys. 48(11), 4729–4733 (1977). [CrossRef]
36. J. K. Kim, H. W. Jang, C. Jeon, et al., “Reduction of ohmic contact resistivity on p-type GaN by surface treatment,” Curr. Appl Phys. 1(4-5), 385–388 (2001). [CrossRef]
37. J. H. Son, Y. H. Song, B. J. Kim, et al., “Effect of reflective P-type ohmic contact on thermal reliability of vertical InGaN/GaN LEDs,” Electron. Mater. Lett. 10(6), 1171–1174 (2014). [CrossRef]
38. G. S. Matijasevic, C. C. Lee, and C. Y. Wang, “Au Sn alloy phase diagram and properties related to its use as a bonding medium,” Thin Solid Films 223(2), 276–287 (1993). [CrossRef]
39. V. Sheremet, M. Genç, M. Elçi, et al., “The role of ITO resistivity on current spreading and leakage in InGaN/GaN light emitting diodes,” Superlattices Microstruct. 111, 1177–1194 (2017). [CrossRef]
40. D. Kim, H. Lee, G. Yeom, et al., “A study of transparent contact to vertical GaN-based light-emitting diodes,” J. Appl. Phys. 98(5), 7 (2005). [CrossRef]
41. J. Park, C. Buurma, S. Sivananthan, et al., “The effect of post-annealing on Indium Tin Oxide thin films by magnetron sputtering method,” Appl. Surf. Sci. 307, 388–392 (2014). [CrossRef]
42. C. Guillén and J. Herrero, “Structure, optical, and electrical properties of indium tin oxide thin films prepared by sputtering at room temperature and annealed in air or nitrogen,” J. Appl. Phys. 101(7), 8 (2007). [CrossRef]
43. F. Olivier, A. Daami, C. Licitra, et al., “Shockley-Read-Hall and Auger non-radiative recombination in GaN based LEDs: A size effect study,” Appl. Phys. Lett. 111(2), 022104 (2017). [CrossRef]
44. P. Tian, J. J. McKendry, Z. Gong, et al., “Size-dependent efficiency and efficiency droop of blue InGaN micro-light emitting diodes,” Appl. Phys. Lett. 101(23), 231110 (2012). [CrossRef]
45. J. Kou, C.-C. Shen, H. Shao, et al., “Impact of the surface recombination on InGaN/GaN-based blue micro-light emitting diodes,” Opt. Express 27(12), A643–A653 (2019). [CrossRef]
46. D. D. Wagman, W. H. Evans, V. B. Parker, et al., “The NBS tables of chemical thermodynamic properties. Selected values for inorganic and C1 and C2 organic substances in SI units,” (National Standard Reference Data System, 1982).
47. H. Schoeller and J. Cho, “Oxidation and reduction behavior of pure indium,” J. Mater. Res. 24(2), 386–393 (2009). [CrossRef]
48. O. Ambacher, J. Smart, J. Shealy, et al., “Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N-and Ga-face AlGaN/GaN heterostructures,” J. Appl. Phys. 85(6), 3222–3233 (1999). [CrossRef]
49. F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B 56(16), R10024 (1997). [CrossRef]
50. J.-M. Wagner and F. Bechstedt, “Properties of strained wurtzite GaN and AlN: Ab initio studies,” Phys. Rev. B 66(11), 115202 (2002). [CrossRef]
51. W.-C. Yang, B. J. Rodriguez, M. Park, et al., “Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures,” J. Appl. Phys. 94(9), 5720–5725 (2003). [CrossRef]
52. A. G. Bhuiyan, A. Hashimoto, and A. Yamamoto, “Indium nitride (InN): A review on growth, characterization, and properties,” J. Appl. Phys. 94(5), 2779–2808 (2003). [CrossRef]
