p-GaN Contact Research Articles

Study on the Degradation Performance of AlGaN-Based Deep Ultraviolet LEDs under Thermal and Electrical Stress

AlGaN-based deep-ultraviolet (DUV) LEDs could realize higher optical power output when adopting a p-AlGaN contact layer instead of a p-GaN contact layer. However, this new type DUV LEDs exhibit poor reliability. Thus, this study thoroughly investigates the degradation behaviors of AlGaN-based DUV LEDs with a p-AlGaN contact layer through different aging tests, including single thermal stress, single electrical stress with air-cooling, single electrical stress, and thermoelectric complex stress. It can be found that both high temperature and large working current play crucial roles in accelerating the degradation of optoelectronic properties of the DUV LEDs, and the single high thermal stress without electrical stress can also bring obvious performance degradation to the DUV LEDs, which is a significantly different finding from previous studies. This is because thermal stress on DUV LED could bring some metal electrode elements entering the p-AlGaN layer. Thus, the degradation of optical and electrical properties under the thermal and electrical stress could be not only attributed to the degradation of the device’s ohmic contacts, but also due to the metal electrode elements entering the p-AlGaN layer through thermal diffusion, leading to the generation of tunneling current and the generation of defects within or around the active region. Despite that the peak wavelengths of the DUV LEDs remained stable, the turn-on voltage and series resistance increased. Particularly worth mentioning is that the value of the optical power degradation under thermoelectric conditions is larger than the sum of the single thermal and single electrical optical power degradation, which is a result of the mutual reinforcement of thermal and electrical stresses to exacerbate the defect generation and ohmic contact degradation. Based on the study above, preparing p-AlGaN layers with hyperfine gradient aluminum fractions and reducing the junction temperature may help to improve the reliability of AlGaN-based DUV LEDs with the p-AlGaN contact layer.

Improvement of p-type ohmic contact of GaN laser diodes by using delta-doped p-GaN contact layer

The p-type ohmic contact of GaN laser diodes has been optimized in several ways. The heavily doped p-GaN contact layer obtained by using delta-doping method, which effectively suppressed the self compensation effect of Mg ions during the heavy doping. By optimizing the delta-doping period, temperature, and thickness of the p-GaN contact layer, the activation energy of Mg atoms is reduced, and the hole concentration in the surface contact layer is further increased. Finally, the specific contact resistivity of the p-type ohmic contact of the GaN laser, measured more accurately by using the transmission line module, was successfully reduced to the order of 1E-3 Ω cm2.

High-Efficiency Vertical-Chip Micro-Light-Emitting Diodes via p-GaN Optimization and Surface Passivation

Micro-LEDs have found widespread applications in modular large-screen TVs, automotive displays, and high-resolution-density augmented reality glasses. However, these micron-sized LEDs experience a significant efficiency reduction due to the defects originating from the dry etching process. By controlling the current distribution via engineering the electrode size, electrons will be less concentrated in the defect region. In this work, we propose a blue InGaN/GaN compound parabolic concentrator micro-LED with a metallic sidewall to boost efficiency by combining both an optical dipole cloud model and electrical TCAD (Technology Computer-Aided Design) model. By merely modifying the p-GaN contact size, the external quantum efficiency (EQE) can be improved by 15.6%. By further optimizing the passivation layer thickness, the EQE can be boosted by 52.1%, which helps enhance the display brightness or lower power consumption.

(Invited) Characterization of Metal/GaN Schottky Contacts - Review from the Early Days

GaN-based materials have been intensively developed for blue light-emitting-diodes, blue laser diodes, power amplifiers in wireless base-stations, and high-power switching devices. Schottky contacts are one of the most important components in electron device fabrication. The electrical characteristics of GaN Schottky barrier contacts have been continuously studied for more than twenty years as improvement of GaN crystal quality and understanding physics of metal/semiconductor interfaces.In the early days, for the achievement of the high-power and high-temperature operation, thermally stable contacts were explored. Interfacial reactions and electrical characteristics were intensively examined by annealing studies for various kinds of metal contacts such as Ni, Pd, Pt, and Re. As a result of those, Ni has been widely used as a stable Schottky contact, because less thermal reaction with N atoms, high Schottky barrier height (qφ B), and good adhesion.As for the correlation with GaN crystal quality, in the early days, because a GaN epitaxial layer contained large-structure killer defects, rectifying characteristics were hardly observed in the Schottky contacts. Since after the n-GaN crystal quality improved with electron mobility over 100 cm2V−1 s−1, rectifying characteristics have been reproducibly obtained.[1] Even though the dislocation density was as high as 109–1010 cm−2 in epitaxial GaN layers, good current–voltage (I–V) characteristics and qφ B values below 1.1 eV were reported in Ni/n-GaN contacts. Moreover, in order to clarify the effect of dislocations on I–V characteristics, a 0.5-µm-diameter Schottky-dot array was formed on n-GaN grown by MOCVD, and I –V measurements were conducted using AFM with a conductive probe.[2] Neither mixed nor pure edge dislocations affected the I –V characteristics, but a large structural defect with a diameter of a few hundred nanometers shorted the contact.In addition, further improvement of the GaN layer grown on low dislocation (∼106 cm−2) free-standing GaN substrates provided ideal Schottky contacts with high qφ B values around 1.1 eV. It has been reported that the forward and reverse I–V characteristics of Ni contacts formed on hydride-vapor-phase-epitaxy grown n-GaN obeyed the thermionic emission and the thermionic field emission (TFE) models, respectively.[3] The same trend was also reported in nine different metal contacts on both c- and cleaved m-plane n-GaN crystals.[4] We also reported a post-metallization annealing at 400°C for 10 min was effective to reduce the reverse biased current significantly to the prediction by the TFE model.[5]Basic understanding of p-GaN Schottky contacts was slower than that of n-GaN, because p-GaN contacts were intensively studied for ohmic contacts of p-n-junction-base optical devices. In the early days, very leaky characteristics were reported and a reasonable qφ B value was not obtained. We improved leaky characteristics of Ni/p-GaN Schottky contacts by low Mg doping. The qφ B as high as 2.4 eV and the n-values of 1.8 were obtained from I –V.[6] We found a memory effect that carrier capture and emission from acceptor-like deep level defects caused depletion layer width to vary significantly.[7]Finally, as our original technique, scanning internal photoemission spectroscopy was developed to map the electrical characteristics of metal/semiconductor interfaces nondestructively.[8] We have demonstrated the mapping of characteristics for interfacial reactions of Ni/n-GaN at 400°C; degradation under applied voltage stress up to 45 V for vertical n-GaN contacts; and process induced surface damages by ion-implantation, dry etching, and photo-electrochemical etching.[9] This technique was confirmed to be useful for the development of the wide-bandgap-semiconductor high-power devices. K. Shiojima, J. Woodall, P. Grudowski, C. Eiting and R. Dupuis, J. Vac. Sci. Tech., B17(5), p. 2030 (1999).K. Shiojima, and T. Suemitsu, J. Vac. Scie. Tech., B21(2) p. 698(2003).J. Suda, K. Yamaji, Y. Hayashi, T. Kimoto, K. Shimoyama, H. Namita, and S. Nagao, Appl. Phys. Express, 3, p. 101003 (2010).H. Imadate, T. Mishima, and K. Shiojima, Jpn. J. Appl. Phys., 57, p. 04FG13 (2018).K. Shiojima, R. Tanaka, S. Takashima, K. Ueno, and M. Edo, Jpn. J. Appl. Phys., 60, 056503 (2021).K. Shiojima, T. Sugahara and S. Sakai, Appl. Phys. Lett., 74, p. 1936 (1999).K. Shiojima, T. Sugahara and S. Sakai, Appl. Phys. Lett. 77, p. 4353 (2000).T. Okumura and K. Shiojima, Jpn. J. Appl. Phys., vol. 28, p. L1108 (1989).K. Shiojima, ECS Transactions, 104, p. 69 (2021).

Key role of initial interface on contact characteristics of Pd/p-GaN

Sequential three-layers of Pd/Pt/Au were deposited on p-GaN by magnetron sputtering under high vacuum (HV, ∼1.33 × 10−4 Pa) and ultra-high vacuum (UHV, ∼1.33 × 10−7 Pa) background conditions, respectively, for investigating the electrical contact properties. Linear I–V curves are observed in the samples deposited under the UHV conditions, whereas nonlinear I–V characteristics are obtained in the samples deposited under the HV vacuum. The depth profiles of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) of the as-deposited samples were probed in detail. It is found that the amounts of O and OH as well as Pd oxide of the sample deposited under the HV background condition are larger than those of the sample deposited under the UHV conditions due to the higher residual gases such as water in the HV chamber. The oxide layer leads to an extra barrier, influencing the electrical characteristics of Pd/Pt/Au/p-GaN contact. This study demonstrates that metal deposition under the UHV environmental conditions can reduce and even prevent formation of oxide on p-GaN surface, and hence favor making a good ohmic contact on p-GaN.

Formation and Optical Characteristics of GaN:Eu/GaN Nanowires for Applications in Light-Emitting Diodes

This paper reviews our recent research about the formation and optical characteristics of GaN:Eu/GaN nanowires (NWs) by metalorganic vapor phase epitaxy for application in GaN-based red light-emitting diodes (LEDs). Two types of GaN:Eu/GaN NWs with different configurations are introduced, core–shell and axial geometries. The configuration of GaN:Eu layers on GaN core NWs can be controlled by changing the growth conditions, and affects the properties of Eu luminescence in the GaN NWs. Next, the optimization of the p-GaN growth conditions is performed to allow to form the p-GaN shell layers on the NWs with the pedestal of the NWs free from radial overgrowth, resulting in efficient electrical isolation between top and bottom part of the NWs. Then, the fabrication process of the NW LEDs towards future possible realization of flexible devices is established, including an etch-back process of the PDMS membranes to expose the top p-GaN contact layers. Finally, a proto-type of p-GaN/GaN:Eu/n-GaN NW LEDs on sapphire substrates is fabricated to characterize the device properties. Sharp red luminescence at room temperature from Eu3+ ions is observed under current injection. These results would pave the way towards the realization of flexible light-emitting devices utilizing NW structures based on compound semiconductors.

InGaN/GaN superlattice underlayer for fabricating of red nanocolumn μ-LEDs with (10-11) plane InGaN/AlGaN MQWs

In this study, the growth behavior of Indium gallium nitride (InGaN)-based nanocolumn arrays was investigated, and red emission nanocolumn micro-light emitting diodes (μ-LEDs) were fabricated. The internal structure of the InGaN/GaN superlattice (SL) layer under the multiple-quantum-well (MQW) active layers was evaluated using scanning transmission electron microscopy (STEM) analysis. It was revealed that the InGaN crystal plane at the top of the nanocolumn changed from the c-plane, (1-102) plane, to the (10-11) plane as the number of SL pairs increased. A semipolar (10-11) plane was completely formed on top of the nanocolumn by growing InGaN/GaN SLs over 15–20 pairs, where the InGaN/GaN SL layers were uniformly piled up, maintaining the (10-11) plane. Therefore, when InGaN/AlGaN MQWs were grown on the (10-11) plane InGaN/GaN SL layer, the growth of the (10-11) plane semipolar InGaN active layers was observed in the high-angle annular dark field (HAADF)-STEM image. Moreover, the acute nanocolumn top of the (10-11) plane of the InGaN/GaN SL underlayer did not contribute to the formation of the c-plane InGaN core region. Red nanocolumn μ-LEDs with an φ12 μm emission window were fabricated using the (10-11) plane MQWs to obtain the external quantum efficiency of 1.01% at 51 A cm−2. The process of nanocolumn μ-LEDs suitable for the smaller emission windows was provided, where the flat p-GaN contact layer contributed to forming a fine emission window of φ5 μm.

Ag nanodot/Mg/Al reflective Ohmic contacts simultaneously suitable for n-type and p-type GaN

This work reports on high-reflectivity Ag nanodot (AgND)/Mg/Al Ohmic contacts suitable for both p-GaN and n-GaN. The lowest specific contact resistances are found to be 2.25 × 10−2 Ω·cm2 on p-GaN and 2.56 × 10−5 Ω·cm2 on n-GaN. Ag was deposited and converted into AgNDs by annealing, and Mg/Al was then deposited. A second annealing process at different temperatures was performed to check the thermal stability of the contacts. Both the p-GaN and n-GaN contacts were Ohmic after annealing at 300 °C or below. The AgND/Mg/Al contacts annealed at 250 °C or less showed a reflectivity of over 91% for wavelengths from 400 to 550 nm. X-ray photoelectron spectroscopy and x-ray diffraction measurements were performed to investigate the contact mechanisms. We propose that in AgND/Mg/Al on p-GaN, the effective barrier is lowered due to the presence of an Ag2O intermedia layer and the tunneling effect enables Ohmic contact. When the annealing temperature is 350 °C or higher, the Ag2O changes to β-AgGaO2 or is decomposed and the height and width of the barrier for holes increase, which causes the Ohmic contact to deteriorate. The Ohmic behavior of AgND/Mg/Al contacts on n-GaN is assumed to be mainly due to the high direct coverage ratio of Mg and the good Ohmic contact behavior of Mg/n-GaN. These results show that AgND/Mg/Al Ohmic contacts can be fabricated simultaneously on both p-GaN and n-GaN, which is a possible solution for improving the density of both GaN-based ICs and micro-light emitting diodes (LEDs). This contact scheme can also improve the light output efficiency of GaN-based LEDs.

Transparent p-type layer with highly reflective Rh/Al p-type electrodes for improving the performance of AlGaN-based deep-ultraviolet light-emitting diodes

We attempt to improve the light extraction of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) by introducing a highly transparent p-type layer together with reflective Rh/Al p-type electrodes. The p-GaN contact layer is thinned to balance the Ohmic contact and DUV light transmittance, which helps the Rh/Al p-type electrodes realize high reflection as well as good electrical performance. After optimization, the Rh/Al reflective p-type electrodes present reflectance of greater than 70% and specific contact resistivity of 3.75 × 10−4 Ω·cm2. Due to the improvement in the light extraction efficiency, the highest wall-plug efficiency of 278 nm DUV-LEDs is improved by 57% compared to the conventional configuration with Ni/Au as the p-type electrodes.

Enhancement of the light output efficiency and thermal stability of AlGaN-based deep-ultraviolet light-emitting diodes with Ag-nanodot-based p-contacts and an 8-nm p-GaN cap layer.

The efficiency of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is limited by the high absorption issue of the p-GaN contact layer or poor contact properties of the transparent p-AlGaN contact layer. Enhancement of the light output efficiency and thermal stability of DUV LEDs with an emission wavelength of 272 nm was investigated in this work. Ag nanodots on an 8-nm p-GaN cap layer were used to form ohmic contact, and Al and Mg reflective mirrors were employed to enhance the light output power (LOP) of DUV LEDs. However, serious deterioration of LOP occurred after the high-temperature process for the LEDs with Al and Mg reflective mirrors, which can be attributed to the damage to the ohmic contact properties. A Ti barrier layer was inserted between the Ag/p-GaN and Al layers to prevent the degeneration of ohmic contact. The wall-plug efficiency (WPE) of DUV LEDs fabricated by the Ag-nanodot/Ti/Al electrode is 1.38 times that of LEDs fabricated by adopting a thick Ag layer/Ti/Al at 10 mA after a high-temperature process. The Ag-nanodot/Ti/Al electrode on thin p-GaN is a reliable technology to improve the WPE of DUV LEDs. The experimental and simulated results show that the ohmic contact is important for the hole-injection efficiency of the DUV LEDs when p-GaN is thin, and a slight increase in the contact barrier height will decrease the WPE drastically. The results highlighted the importance of thermally stable ohmic contacts to achieve high-efficiency DUV LEDs and demonstrated a feasible route for improving the LOP of DUV LEDs with a thin p-GaN cap layer and stable reflective electrodes.

P-GaN Contact Study by Means of Electrochemical Short Loop

Surface states and contamination affects the efficiency of Gallium Nitride (GaN) based devices such as HEMTs, LEDs, photodetectors etc. In this work, several wet chemical treatment has been performed for the surface treatment of p-GaN, in order to achieve low-resistance ohmic contacts. The effect of surface treatment is observed by studying the flatband potential across the p-GaN/electrolyte interface via, electrochemical characterization. The electrochemical method of “Mott-Schottky characterization” gives simple and quick way of analysing the effect of different treatments, without the need of device fabrication. The effect of surface treatment are further investigated via, X-ray Photoelectron Spectroscopy and Atomic Force Microscopy. The potential surface treatments from experimental findings are integrated into device processing steps in order to realize effect of surface treatment on the p-GaN/metal contacts. The electrochemical characterization shows that HCl results in least flatband potential. Further, The electrical test shows that HCl treatment results in significant orders of magnitude improvement in contact resistance and electrical performance of the devices. This shows that short loop electrochemical characterization can be used for predicting surface treatment in order to improve electrical characteristics of the device.

Performance evaluation of tunnel junction-based N-polar AlGaN deep-ultraviolet light-emitting diodes

In this study, an N-polar AlGaN-based deep-ultraviolet (DUV) light-emitting diode (LED) incorporating a tunnel junction (TJ) as the p-side contact layer, named the N-TJ-LED, was proposed. Compared with regular N-polar LEDs (N-LEDs) with a p-GaN contact layer, the N-TJ-LEDs exhibited 50% enhanced internal quantum efficiency, 2.7 times higher light output power at an injection current of 40 mA, and dramatically reduced turn-on voltage. In addition, it was found that the N-TJ-LED can still maintain outstanding device performance at a low p-type doping level in the electron blocking layer and p-AlGaN current injection layer, significantly outperforming the regular N-LED. All these performance enhancements are derived from the higher electron and hole concentration in the active region of the N-TJ-LED, thanks to the TJ-facilitated efficient hole injection and effective electron blocking in the device. The results demonstrated in this work provide an effective strategy for the future experimental optimization of N-polar AlGaN-based DUV LEDs.

Physical and technological analysis of the AlGaN-based UVC-LED: an extended discussion focused on cubic phase as an alternative for surface disinfection.

Crisis in coronavirus times requires understanding the effects on society and establishing efficient mechanisms to prevent infections. The disinfection of personal protection equipment by UVC light remains a key opportunity area. Therefore, this letter presents the main drawbacks and challenges on the fabrication of deep ultraviolet LEDs based on III-nitrides, such as the substrate selection, dislocation reduction, the increase of external quantum efficiency, enhancement of the radiative recombination in the active region, the complications to reach high Al content in AlGaN-based UVC LED avoiding the reduction of the p-doping, replacing the p-GaN contact layer by p-AlGaN without hindering the deposition of ohmic contacts. Furthermore, the cubic phase is suggested as a promising candidate for AlGaN UVC-LEDs applications as is discussed in this work.

Improving light output power of AlGaN-based deep-ultraviolet light-emitting diodes by optimizing the optical thickness of p-layers

In this study, we investigated the relationship of light output power with the optical thickness of the p-layers in AlGaN-based deep ultraviolet light-emitting diodes with a transparent high-Al-composition p-AlGaN clad layer, a thin p-GaN contact layer, and a reflective p-type electrode. By adjusting the thickness of the transparent high-Al-composition p-AlGaN clad layer, we observed a marked change in light output power. A maximum light output power of 385 mW at 1500 mA, a maximum external quantum efficiency of 15.7% at 10 mA, and a maximum wall-plug efficiency of 15.3% at 10 mA were obtained at an emission wavelength of 275 nm.

Degradation analysis with characteristics and simulations of 265 nm UV-C LED

We report the degradation study on AlGaN-based 265 nm ultraviolet light-emitting diodes (UV-LEDs) under a series of constant current stress. The failure mechanisms were investigated systematically by measuring the optical and electrical characteristics of the LEDs before and after aging. The variation of carrier concentration in the active region was analyzed by capacitance–voltage. Combining the extracted apparent charge distribution profiles with the simulation results of the devices before and after the stress, we found that the change of carrier concentration in the multiple quantum wells was related to the donor diffusion on the n-side. On the p-side, both the acceptor concentration of electron blocking layer (EBL) and the defects in p-GaN contact layer were also found to be under constant change. The reduction of the EBL doping concentration has contributed to an increase of the diode depletion width during the stress. The changes in the LEDs before and after stressing indicate a compensating effect occurred in the p-type EBL close to the quantum wells, which leads to the degradation of the optical power of the 265 nm UV-LEDs.

Enhanced microLED efficiency via strategic pGaN contact geometries.

Micro light-emitting diode (microLED) structures were modeled and validated with fabricated devices to investigate p-type GaN (pGaN) contact size dependence on power output efficiency. Two schemes were investigated: a constant 10 μm diameter pGaN contact and varying microLED sizes and a constant 10 μm diameter microLED with varying contact sizes. Modeled devices show a 17% improvement in output power by increasing the microLED die size. Fabricated devices followed the same trend with a 70% improvement in power output. Modeled microLED devices of a constant size and varying inner contact sizes show optimized power output at different current densities for various contact sizes. In particular, lower current densities show optimized output for smaller pGaN contacts and trend towards larger contacts for higher current densities in a balance between undesirable efficiency losses at high-current injection and preventing surface recombination losses. We show that for all device geometries, it is preferential to shrink the pGaN contact to maximize efficiency by suppressing surface recombination losses and further improvements should be carefully considered to optimize efficiency for a desired operational brightness.

Analysis of the Light-Extraction Efficiency of Flip-Chip Deep-Ultraviolet Light-Emitting Diodes with Embedded Photonic Crystals

In the present study, embedded photonic crystals were introduced into the p-GaN contact layer of light-emitting diodes to increase the light-extraction efficiency of flip-chip deep-ultraviolet light-emitting diodes. As demonstrated by performing three-dimensional finite-difference time domain simulation, the embedded photonic crystals could be designed to increase the total reflectivity on the p-GaN contact layer, thereby increasing the light-extraction efficiency. In addition, the effects of the embedded photonic crystals configurations on the light-extraction efficiency of deep-ultraviolet light-emitting diodes were examined. Light-extraction efficiency over 21% for transverse magnetic polarized emission could be generally expected by rigorously optimizing the active layer position, as well as the depth, period, and filling factor of photonic crystals. Moreover, an investigation was conducted on the light-extraction efficiency over the whole ultraviolet spectrum, and more light-extraction efficiency harvest on a broadband ultraviolet spectrum has been achieved as compared with flip-chip planar deep-ultraviolet light-emitting diodes. The optimized structure will be promising for increasing the light-extraction efficiency of AlGaN-based flip-chip deep-ultraviolet light-emitting diodes.

Efficiency improvement analysis of nano-patterned sapphire substrates and semi-transparent superlattice contact layer in UVC light-emitting diodes

UVC light-emitting diodes (LEDs, λ = 275 nm) with different types of contact layers and sapphire substrates were demonstrated on high-quality AlN templates. For LEDs on flat sapphire substrates (FSSs), replacing the absorbing p-GaN contact with p-AlGaN short-period superlattices (p-SPSLs) strongly enhanced the emission along the substrate normal. The integrated external quantum efficiency (EQE) increased from 2.4% to 3.9% under I = 350 mA. For LEDs with a p-SPSL contact, replacing the FSS with nano-patterned sapphire substrates slightly deteriorated the quality of epitaxy, but the overall EQE is still enhanced to 4.4% under I > 350 mA without lens encapsulation. According to the far-field intensity measurement, the light extraction is better improved along the high emission angle to the substrate normal. The interplay among substrates, dipole polarization, and EQE enhancement factors was further analyzed and discussed in the context.

Ultraviolet Light-Emitting Diodes Using Aluminium Indium Nitride Nanowire Structures

Ultraviolet (UV) light-emitting diodes (LEDs) have various applications in the field of biochemistry, medicine, water/air purification, etc. To achieve efficient UV LEDs, researchers have essentially focused on the use of AlGaN alloys in the active region. However, the practical applications of AlGaN based UV LEDs are severely limited due to poor external quantum efficiency (EQE). Some reasons behind the low EQE in AlGaN based UV LEDs are efficiency droop, mostly due to electron leakage into the p-region, poor light extraction efficiency (LEE) which is mainly due to the total internal reflection and the absorption of UV light in the p-GaN contact layer. Identifying and developing the potential of alternative UV materials will be critical to make further progress in the development of deep UV emitters. In this regard, AlxIn1-xN alloy has not been widely studied even though it holds great potential applications in UV and visible light-emitting devices [1]. In this study, we have performed the epitaxial growth, electrical and optical properties of spontaneously grown AlInN nanowires on Si (111) substrate by molecular beam epitaxy (MBE) and demonstrated a high crystalline quality of AlInN nanowire LED structures with stable and strong EL emission at ~299 nm peak wavelength.The AlInN nanowire LED structure, shown in Fig. 1(a), was spontaneously grown on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II MBE system equipped with a RF plasma-assisted nitrogen source. The growth conditions for GaN nanowire segments include a growth temperature of ~ 770 °C, nitrogen flow rate of 1 sccm, and forward plasma power of ~ 400 W. The device active region consists of ~ 100 nm n-Al0.78In0.22N, 40 nm undoped Al0.75In0.25N well and 100 nm p-Al0.78In0.22N layers. Growth temperature of the active region was at ~ 670 ̶ 700 °C to increase the In incorporation in AlInN segments and the nitrogen flow rate was kept at 2.5 sccm, while the plasma power was fixed at 400 W. After being grown by MBE, the nanowire LED samples were fabricated using standard lithography method as reported elsewhere [2]. The device characterization was performed on LED with an area size of 500×500 µm2. Photoluminescence (PL) spectrum of the grown nanowire LEDs exhibits strong emission in the UV wavelength range as shown in Fig. 1(b). The peak emission at ~ 299 nm and ~ 280 nm is responsible for the emission from the AlInN quantum well and barriers, respectively. The AlInN nanowire UV LEDs have strong EL intensity with the peak wavelength at ~ 299 nm, as shown in Fig. 1(c). This work is being developed toward the achievement of high power AlInN nanowire UV LEDs, and the result will be presented. Reference R. T. Velpula, B. Jain, M. R. Philip et al., Sci. Rep., 10(1) 1-10 (2020).R. T. Velpula, B. Jain, H. Q. T. Bui et al., Optical Materials Express, 10(2) 472-483 (2020) Figure 1

Improvement of p-electrode structures for 280 nm AlGaN LED applications

An improvement of Ni/Au/p+-GaN p-electrode for AlGaN deep-ultraviolet light-emitting diodes (DUV LEDs) with the emission wavelength of 280 nm is proposed for both p-side-up and flip-chip structures. An interdigitated multi-finger Ni/Au was employed in p-side-up DUV LED, where the p-GaN contact layer was partially removed to improve the light extraction efficiency without a serious current-crowding effect. The 9- and 12-finger LEDs were determined to have higher thermal dissipation and lower surface temperatures and correlated well with the theoretical simulation. For the comparison of p-side-up emission LEDs, the output power of 9-finger LED is 172% higher than that of conventional LED at the current injection of 350 mA. The optimum p-electrode pattern was further applied to the flip-chip LED structure. It is determined that the output power of 9-finger flip-chip LED at 350 mA is still 14.6% higher than that of a conventional flip-chip LED. The higher output power of 9-finger flip-chip LED with a wall-plug efficiency of 1.05% is attributed to the combination of the improved current-spreading path and the higher reflection through the moderate removal of partial p+-GaN absorbing layer.