High-Performance and Temperature-Stable InGaN Single-Quantum-Well Red Light-Emitting Diodes via Selective Hydrogen Passivation

Herein, a selective passivation of p-GaN via hydrogen plasma treatment for InGaN single-quantum-well (SQW) red light-emitting diodes (LEDs) is reported. Insulating regions are formed on the p-GaN top surface via hydrogen plasma treatment, suppressing current injection beneath the p-pad and along the mesa perimeter to increase light output and mitigate non-radiative recombination. The fabricated LEDs demonstrate a high on-wafer light output power density of > 88 mW cm (cid:1) 2 , a peak on-wafer external quantum ef ﬁ ciency of 0.65%, and on-wafer wall-plug ef ﬁ ciency of 0.41% with a 645 nm peak emission wavelength at 10 mA (7.2 A cm (cid:1) 2 ) current injection. Further, the temperature dependence of InGaN SQW red LEDs is compared with their AlGaInP counterparts. InGaN SQW red LEDs exhibit a high characteristic temperature of 208 K and a small redshift coef ﬁ cient of 0.072 nm K (cid:1) 1 at 72 A cm (cid:1) 2 current injection, which are almost 3 and 2 times better than the characteristics of AlGaInP red LEDs, respectively.


Introduction
InGaN-based light-emitting diodes (LEDs) are ubiquitous in our daily life as efficient light sources; [1,2] they show excellent performances in the blue [3] and green emission ranges. [4][7] This challenge hinders the integration of III-nitride LEDs for full-color micro-LED displays. [8]Currently, most blue and green LEDs are made of InGaN materials, but AlGaInP is used to obtain the red LEDs. [9]The broad-area AlGaInP red LEDs exhibit high performances approaching InGaN blue-green LEDs. [10]However, AlGaInP has high surface recombination velocity and long carrier diffusion lengths, which induce dramatic drops in efficiency when the device dimensions are reduced to the size of micro-LEDs. [7,11]Also, AlGaInP red LEDs exhibit a strong temperature dependence. [12]The high operating temperature causes severe carrier leakage through AlGaInP red LEDs.Therefore, AlGaInP must be replaced by InGaN red LEDs for future technological applications.
In addition to overcoming inherent material-and growth-related challenges, improvements in chip technology must be introduced to increase the performance of InGaN red LEDs.[15][16] P-type electrodes are formed on top of the device stacks for most InGaN-based LEDs.In such a configuration, a current crowding effect around the p-pad often exists due to the poor conductivity of the p-type GaN layer. [16]This may further reduce the quantum efficiency and device reliability.Moreover, severe sidewall damage occurs during the dry etching process used for the mesa (chip) formation, [17,18] which generates current leakage paths through the mesa sidewalls, deteriorating the performance of LEDs.This issue might be alleviated by introducing insulating regions along the perimeter of mesa edges.
Hydrogen plasma treatment on the p-GaN is a promising way to form insulating regions beneath the p-pad and along the mesa perimeter.Hydrogen plasma treatment can form Mg-H complexes in Mg-doped GaN, limiting the electrical activity of Mg.[21] We have recently utilized hydrogen plasma treatment in InGaN green broad area [22] and micro-LEDs, [23] and improved the on-wafer external quantum efficiency (EQE) of these devices by 23% and 40% compared to non-treated LEDs, respectively.
[26] Li et al. reported a 6% EQE by decreasing the thickness of SQW in red micro-LEDs. [25]In addition, Hsiao et al. compared red micro-LEDs based on SQW and multiple quantum wells (MQWs) and revealed that the former LEDs (6.95% of EQE) exhibit nearly 20% increase in EQE along DOI: 10.1002/pssa.202400048Herein, a selective passivation of p-GaN via hydrogen plasma treatment for InGaN single-quantum-well (SQW) red light-emitting diodes (LEDs) is reported.Insulating regions are formed on the p-GaN top surface via hydrogen plasma treatment, suppressing current injection beneath the p-pad and along the mesa perimeter to increase light output and mitigate non-radiative recombination.The fabricated LEDs demonstrate a high on-wafer light output power density of >88 mW cm À2 , a peak on-wafer external quantum efficiency of 0.65%, and onwafer wall-plug efficiency of 0.41% with a 645 nm peak emission wavelength at 10 mA (7.2A cm À2 ) current injection.Further, the temperature dependence of InGaN SQW red LEDs is compared with their AlGaInP counterparts.InGaN SQW red LEDs exhibit a high characteristic temperature of 208 K and a small redshift coefficient of 0.072 nm K À1 at 72 A cm À2 current injection, which are almost 3 and 2 times better than the characteristics of AlGaInP red LEDs, respectively.with improvements in emission and charge-transport characteristics. [26]Therefore, future research trends might evolve to fabricate InGaN SQW red LEDs.However, no study has been published so far investigating the temperature characteristics of InGaN SQW red LEDs thoroughly.InGaN MQWs red LEDs demonstrated characteristic temperatures, a parameter to evaluate the temperature stability of LEDs, close to 400 K [27] and better than AlGaInP red LEDs. [28]However, MQWs InGaN LEDs might display better temperature characteristics than the SQW ones due to the relatively greater carrier confinement of the high number of quantum wells. [29]Therefore, the temperature characteristics of InGaN SQW red LEDs should be examined in detail for future display applications.
Here, we report on investigating the fabrication and characterization of InGaN SQW red LEDs with an insulating region beneath the p-pad electrode and along the mesa perimeter, realized via hydrogen plasma treatment of p-GaN.We aimed to prevent the current injection right underneath the p-pad and mesa sidewalls, the first suppressing the current crowding effect, the latter mitigating the current leakage through the mesa etchinginduced sidewall defects.Moreover, we characterized InGaN SQW red LEDs' temperature dependence, compared it with their AlGaInP counterparts, and revealed superior temperature tolerance of InGaN SQW red LEDs.

Results and Discussion
Figure 1 shows the schematic structure of fabricated devices and their digital photos without and with current injection under an optical microscope.The hydrogen-treated perimeter of LEDs can be seen in Figure 1b.Although it cannot be recognized in the images, the p-GaN region beneath the p-pad was treated as being in the perimeter of devices.Hereafter, all devices refer to InGaN SQW red LEDs with hydrogen treatment along the perimeter and underneath the p-pad unless otherwise stated.Figure 1c shows the EL image of the device under the current injection of 1 mA (7.2A cm À2 ).A homogeneous and intense light emission and good current spreading can be observed from the figure even at current injections as low as 1 mA.Also, LED chips did not exhibit any dark spots or current crowding at high current injections (not shown here), which indicates the good crystal quality of InGaN quantum wells.
We studied the electrical properties of LEDs. Figure 2 shows typical current-voltage curves of a red LED at room temperature (RT).The curve at the forward bias displayed a sharp onset voltage and exponentially increased with the injection current, exhibiting the standard behavior of a p-n junction diode.The devices exhibit a forward voltage of ≈3.3 V at 20 mA in the forward bias region and a leakage current of ≈10 À2 A cm À2 at -5 V in the reverse bias region.Both results are lower than those of non-hydrogen plasma-treated InGaN red LEDs (≥3.45 V of forward voltage at 20 mA, 10 À1 A cm À2 of leakage current density at À5 V) reported in our previous publication, [27] which shows the positive effects of hydrogen plasma treatment on red LEDs' electrical properties.
The peak emission wavelength of the red LEDs displayed a blueshift from 670 to 613 nm with injection currents ranging from 1 to 100 mA, as shown in Figure 3a.This blueshift is typical in InGaN red LEDs and is caused by screening the internal electric field as the current injection increases. [24]In addition, hydrogen-passivated InGaN SQW red LEDs displayed emission wavelengths of 645 and 634 nm at 10 mA (7.2A cm À2 ) and 20 mA (14.4A cm À2 ), respectively, which are among the longest emission wavelengths for InGaN red LEDs grown on a c-plane sapphire substrate.Our group reported InGaN red LEDs with  621, [24] 633, [6] and 634 nm [30] wavelength emission at 20 mA.Vadivelu et al. demonstrated InGaN red LEDs with 628 nm wavelength emission at 20 mA (633 nm at 5 mA). [31]More results can be found in the ref.[5].In contrast, the full width at half maximums (FWHMs) decreased to their minima of 56.5 nm (634 nm emission wavelength) at 20 mA (14.4A cm À2 ) before increasing slightly to 58 nm at 100 mA (72 A cm À2 ).The Coulomb screening of the polarization field reduces the FWHM at the low current injection regime. [32]At the high current injection regime, the heat generation related to Shockley-Read-Hall recombination becomes dominant and induces the broadening in FWHM. [33]Also, the FWHM (56.5 nm) reported here is on par with c-plane InGaN LEDs with wavelength emission ≥630 nm, ranging from 52 [34] to 69 nm. [5,26,35]The Commission Internationale de l'Eclairage (CIE) 1931 chromaticity diagram displays the color coordinates of red LEDs at injection currents ranging from 1 to 100 mA, as shown in Figure 3b.The LEDs exhibited the CIE 1931 color coordinates of (x, y) = (0.702, 0.296) at 1 mA operation, which is one of the closest coordinates to Rec. 2020 standards (x, y = 0.708, 0.292), a metric for ultrahigh-definition displays, reported to date for red emission.With the current injection increases, the coordinate position of the LEDs slightly shifted away from the Rec.2020.Although the developed InGaN SQW red LEDs exhibit changes in color coordinates, all these points were located at the edge of the chromaticity diagram, implying the good color purity of LEDs.We believe that the shifts in the peak wavelengths and color coordinates of InGaN red LEDs can be mitigated by using novel epitaxial growth techniques and strain control methods.
Figure 4a shows the forward voltage and the on-wafer light output power (LOP) density of the InGaN SQW red LEDs at different injection currents.The LOP density of the InGaN red LEDs was >88 mW cm À2 at 10 mA (7.2A cm À2 ) current injection.In addition, we calculated the on-wafer EQEs and WPEs of the LEDs, as shown in Figure 4b.The on-wafer EQE and WPE were 0.65% and 0.41% at 10 mA (7.2A cm À2 ) with a peak emission wavelength of 645 nm, respectively.Our group previously demonstrated InGaN red LEDs with a peak EQE of 1.6% (633 nm) at 20 mA (14.4A cm À2 ), [6] and peak on-wafer EQE of 0.19% (665 nm), and a peak on-wafer WPE of 0.14% at 20 mA (14.4A cm À2 ). [36]Chan et al. and Hu et al. reported InGaN red LEDs with a peak on-wafer EQE of 0.05% (633 nm) at 200 A cm À2 and with a peak EQE of 1.3% (634 nm) at 10 A cm À2 , respectively. [35,37][26] These results show that hydrogen passivation is promising to lower the driving current injection (as low as 7.2 A cm À2 ) for peak EQE and WPE.Note that packaging and measuring inside the integrating sphere improves the performance of LEDs by more than double. [35]herefore, it is expected that our hydrogen-passivated LEDs' performance (LOP, EQE, and WPE) will approach state-of-theart devices after packaging and operating inside the integrated sphere.
To further evaluate the EL property of the hydrogen-passivated InGaN SQW red LEDs, we investigated the temperature dependence of the EL and compared it with AlGaInP red LEDs. [28]he EL intensities dropped with the temperatures for both LEDs, usually called the thermal droop. [38]The thermal droop can be characterized using the following equation [39,40] where I is the EL intensity at the corresponding temperature, I T = 293 K is the EL intensity at the ambient temperature of 20 °C, T is the stage temperature, and T 0 is the characteristic temperature, which is usually considered as a quantitative parameter to evaluate the temperature stability of LEDs. [10]A larger T 0 implies weaker temperature dependence.The T 0 can be deduced by fitting (1) to the experimental data with the T 0 as a fitting parameter (Figure 5a).The temperature characteristics of LEDs are listed in Table 1.The T 0 value of hydrogen-passivated InGaN SQW red LEDs was estimated to be 208 K (at 100 mA, 72 A cm À2 ), almost three times higher than that of AlGaInP red LEDs (76 K). [28] Moreover, we analyzed the temperature dependence of emission peak wavelength at a constant current of 100 mA for InGaN red LEDs.The redshift coefficient was calculated as 0.072 nm K À1 by linear fitting (Figure 5b).In the case of AlGaInP, the redshift coefficient was reported as 0.142 nm K À (Figure 5b). [28]These results show that InGaN SQW red LEDs are inferior to their MQWs counterparts (400 K, 0.066 nm K À1 ). [27]owever, InGaN SQW red LEDs perform better than AlGaInP LEDs in terms of temperature dependency.
The temperature behavior of LEDs can be understood by examining carrier overflow, which depends on temperature, carrier density, and barrier height. [41]As the temperature increases, charge carriers in QWs get energetic, enabling them to overcome barriers, thereby reducing radiative recombination and EL in LEDs.The high carrier density and the small barrier height might induce more reduction when a device is operated at high temperatures.Due to their higher carrier density, SQW structures are more prone to carrier overflow than MQWs, where injected carriers are distributed across multiple wells.Moreover, escaping carriers from one quantum well can be captured in others, decreasing carrier leakage in MQWs structures.Therefore, SQW LEDs exhibit lower temperature tolerance than their MQW counterparts.In contrast, InGaN LEDs, with deeper quantum wells and higher barrier heights, experience better carrier confinement and lower carrier escape rates and exhibit better temperature behavior compared to AlGaInP-based LEDs. [12]

Conclusion
We investigated the electrical, optical, and thermal characteristics of InGaN red LEDs fabricated using patterned p-GaN passivation via hydrogen plasma.The on-wafer LOP density, EQE, and WPE at 10 mA (7.2A cm À2 ) were >88 mW cm À2 , 0.65%, and 0.41% with a peak emission wavelength of 645 nm, respectively.In addition, a characteristic temperature of 208 K and a redshift coefficient of 0.072 nm K À1 were obtained, which means a better temperature performance than AlGaInP red LEDs.This indicates that InGaN red LEDs can replace AlGaInP in future temperature-tolerant applications.

Experimental Section
InGaN red LED epitaxial wafers were grown on conventional c-planepatterned sapphire substrates using a metal-organic vapor phase epitaxy in a single-wafer horizontal reactor. [42,43]Note that the active region was  3.5 nm thick SQW red InGaN.More details on the epitaxial growth of layers can be found in our previous report. [24]Hydrogen passivation patterns were developed on wafers using standard photolithography and liftoff techniques before the mesa etching step.After that, the wafers were exposed to hydrogen plasma via a plasma-enhanced chemical vapor deposition chamber.The plasma exposure was conducted for 4 min with a radio frequency power of 250 W and at a substrate temperature of 300 °C.This step is the most critical: p-GaN along the expected mesa perimeter and underneath the metal p-pad was deactivated using hydrogen plasma.After this step, device fabrication was completed using standard photolithography and liftoff processes, realizing mesa etching and metal contact deposition. [22]The mesa width and length are 280 and 800 μm, respectively.After excluding hydrogen-passivated regions, the active area of LED chips was calculated as ≈0.138 mm 2 .Electrical and optical characterization of LEDs was carried out in a probe station equipped with an integrating sphere and spectrometer.The electroluminescence (EL) characteristics were measured at temperatures ranging from 293 K (RT) to 373 K.The temperatures were measured from the sample-holding stage of the probe station calibrated by the thermocouple.

Figure 1 .
Figure 1.a) Schematic device structure and digital images of InGaN red LEDs b) without and c) with a current injection.

Figure 3 .
Figure 3. a) Current dependences of the EL peak wavelength and FWHM of the LEDs.b) CIE 1931 color coordinates of the EL spectra at different injection currents.

Figure 4 .
Figure 4. a) Light-output power density and forward voltage of the LEDs at different current injections.b) On-wafer EQE and WPE at different injection currents.

Figure 5 .
Figure 5. a) The normalized EL intensities, and b) the peak wavelengths of red LEDs with a function of stage temperature.Note: normalized EL intensity is on a logarithmic scale, and AlGaInP red LED data were reproduced from the ref.[28].Solid lines are the fitting lines of InGaN red LEDs, and dashed lines are guides for the eye of AlGaInP LEDs.

Table 1 .
Temperature characteristics of LEDs.