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Article

In Situ OBIC Mapping to Investigate Native Defect Dynamics in GaInN/GaN-Based Light-Emitting Diodes

by
Dong-Guang Zheng
1,
Jian-Feng Zhang
1,
Hao-Min Yu
1 and
Dong-Pyo Han
2,*
1
Department of Electronic and Communication, Information Engineering College, Hangzhou Dianzi University, Hangzhou 311305, China
2
Display & Semiconductor Engineering, School of Electrical Engineering, Pukyong National University (PKNU), Busan 48513, Republic of Korea
*
Author to whom correspondence should be addressed.
Photonics 2025, 12(9), 861; https://doi.org/10.3390/photonics12090861
Submission received: 7 July 2025 / Revised: 15 August 2025 / Accepted: 21 August 2025 / Published: 27 August 2025
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Abstract

Native defects significantly impair the electro-optical performance of GaInN/GaN-based light-emitting diodes (LEDs). Therefore, precise characterization of their properties, such as energy levels, capture kinetics, capture cross-sections, and spatial distributions, is crucial for understanding their physical origins following improvement in performance. However, modeling the impact of various defects on the electrical and optical characteristics of LEDs still remains a complex challenge. This study proposes a laser-based measurement technique for the accurate localization and screening of defects in GaInN/GaN-based LEDs by establishing a correlation model between laser excitation and defect response, which enables real-time monitoring of defect dynamics during device degradation, while simultaneously evaluating the effects of the defect state dynamics on the electro-optical characteristics of LED devices. The experimental results indicate that defects located at different spatial positions lead to distinct degradation mechanisms.

1. Introduction

As the core device of third-generation semiconductor lighting, GaInN/GaN-based light-emitting diodes (LEDs) play a critical role in solid-state lighting, advanced displays, and optical communication systems due to their high energy efficiency, long lifespan, and spectral tunability. Despite the excellent inherent thermodynamic stability of nitride material, crystallographic defects generated during heteroepitaxial growth including threading dislocations, V-pits, and point defects remain a critical challenge that limits the development of device reliability [1,2,3,4,5]. Recent research has revealed that the defect state dynamics under the accelerated aging test exacerbate the degradation processes through thermally induced self-heating, defect-assisted non-radiative recombination, imbalanced current distribution, and doping atom migration, ultimately leading to luminance decline or sudden failure of the device [6,7,8,9,10,11,12]. Therefore, accurately characterizing the spatial distribution of defects and analyzing degradation mechanisms induced by defect state dynamics remain a fundamental challenge in the development of high-reliability GaInN/GaN-based LED devices.
Conventional degradation analysis has predominantly established a macroscopic correlation between electrical properties (e.g., forward voltage drift and leakage current increase) and optical characteristics (e.g., luminous efficiency decay and wavelength shift). Various degradation models, such as self-heating effects, defect-assisted tunneling, and impurity migration, have been developed [13,14,15]. However, intrinsic limitations of conventional characterization methodologies—particularly insufficient spatial resolution and destructive measurement protocols—constrain precise spatial characterization of defects through in situ monitoring. While transmission electron microscopy (TEM) and secondary electron microscopy (SEM) provide nano-scale defect structure analysis, its inherent requirements for vacuum environment and static observation precludes real-time monitoring of defect dynamics [16,17]. Similarly, although electroluminescence (EL) imaging can map optoelectronic inhomogeneity, its spatial resolution is intrinsically limited by carrier diffusion effects [18,19]. The limitations of various methods have impeded more comprehensive understanding of the correlation between defects and device performance, thereby preventing the formulation of accurate physics models for device reliability assessment.
This paper proposes a non-contact laser-excited detection method based on the optical-beam-induced current (OBIC) technique, achieving sub-micron defect localization and time-resolved defect state tracking [20,21,22,23,24]. A focused probe-laser beam with photon energy exceeding the semiconductor bandgap energy was sequentially scanned over the LED device [25]. Through synchronized acquisition of the photo-generated current values during spatial scanning, a two-dimensional spatially resolved photo-generated current mapping was constructed to locate defects [21,26]. Photoluminescence (PL) is a highly sensitive analytical technique. Upon laser excitation, electrons in the material are promoted to higher energy levels and subsequently undergo relaxation to the ground state, emitting photons. The spontaneous radiation emission, characterized by its intensity and spectral profile, is measured to interrogate the material’s luminescent properties and electronic band structure. PL demonstrates particular sensitivity to luminescence-related defects, such as non-radiative recombination centers [27,28,29]. In the OBIC technique, targeted defect excitation was achieved through localized laser intensification, inducing the targeted defect state evolution while monitoring relaxation dynamics. The characterizations of electrical response spectra (localized photocurrent response) and optical characteristics (micro-region photoluminescence) quantitatively correlated defect state dynamics with the degradation of device performance. The critical impact of defect-related recombination on LED efficiency is well-established. Advanced models commonly adopt carrier lifetimes as material parameters directly linked to defect state evolution. In LEDs, the internal quantum efficiency (IQE) is defined as the product of the radiative recombination rate and the carrier lifetime. Given that IQE equivalently represents the ratio of external quantum efficiency (EQE) to light extraction efficiency (LEE), variations in EQE may indicate changes in carrier lifetime under constant LEE conditions [28,29,30,31,32].
The experimental results indicated that defect state dynamics revealed distinct device degradation mechanisms: (i) threading dislocations primarily contributed to quantum well carrier leakage [10,33,34], and (ii) V-pits promoted current crowding effects that accelerated metal electrode delamination [35,36]. The established correlation between defect state dynamics and device performance provided theoretical guidance for optimizing epitaxial growth processes and device structural design, thereby advancing the development of high-reliability solid-state lighting technologies.

2. Materials and Methods

Several pieces of high-power lateral-electrode-type GaInN/GaN-based LEDs from the same batch were selected in this investigation. The bare-die chips underwent identical encapsulation (lens-free configuration) to eliminate optical interference factors. The chips exhibited a square geometry (1150 × 1150 μm2). OBIC is a scanning laser microscope technique that utilizes localized laser stimulation on electrically biased devices under constant voltage. This non-destructive method generates electron–hole pairs through laser–material interactions. As a probe-laser beam (wavelength: 405 nm, spot diameter: ~3 μm) is scanned over the device, it reveals material defects by inducing measurable current fluctuations on the constant voltage applied. These fluctuations in the device are spatially plotted as a function of laser position, resulting in quantitative photo-generated current mapping (OBIC mapping). Current variations indicate electrically active sites and perhaps a defect. Figure 1 illustrates a schematic diagram of a scanning laser microscope setup specialized for OBIC mapping. EL characterization revealed a dominant emission peak at 461 nm under 10 mA injection current. Prior to stress testing, all samples underwent electrical and optical characterizations. Electrically active/inactive defects were identified through OBIC mapping and light intensity distribution (LID) measurement. Comparative analysis of electrical stress aging (−20 V) and laser-induced defect state dynamics experiments elucidated the dominant photo-generated current regime, with real-time defect state evolution monitored via synchronized electrical-optical characterization. The analysis of laser-induced defect state dynamics relies on spatially resolved measurements of defect sites and their surrounding regions. Variations in local current density at defect sites and their inherent properties generate significant spatial heterogeneity in electrical and optical properties. To eliminate potential signal contamination in the photo-generated current measurements, laser illumination is applied to only one defect site at a time, thereby ensuring that photo-generated carriers are generated exclusively at the targeted location. Furthermore, synchronous lock-in detection is employed to suppress the influence of static defects [37,38,39,40].

3. Results and Discussion

Figure 2a presents the non-biased OBIC mapping, revealing intrinsic defect distributions in the initial stage. Simultaneously, Figure 2b shows the LID mapping at an injection current of 0.5 mA. The metal wires and electrode structures exhibited pronounced chromaticity contrasts in both OBIC and LID mapping. High-resolution optical microscopy confirms that electrically inactive defects near the metal wires (indicated by black arrows) are micrometer-scale metallic particles attributed to fabrication processes. Latent defects (indicated by red arrows) were exclusively detected in OBIC mapping. The surface morphology consistency between defect location areas and defect-free regions indicates that these defects reside at buried layer interfaces within the device structure.
To determine intrinsic degradation mechanisms arising from the defect state dynamics, defect-free regions were continuously irradiated with the probe-laser beam, while synchronized electrical and optical characterizations were performed at 6 h intervals. Figure 3a demonstrates the significant leakage current increase in the electrically stressed device, confirming that the device degradation correlated with defect state dynamics [41,42]. Conversely, defect-free regions maintained stress-resistant photo-generated current characteristics in Figure 3b, indicating that the laser energy density in this experiment did not induce degradation in these regions. It was particularly noteworthy that different defect sites (P1, P2, and P3 as marked in Figure 2a) exhibited distinct dynamic responses during continuous probe-laser irradiation.
As shown in Figure 4a, the photo-generated current at the P1 site decreased in reverse bias, while no significant change was observed in forward bias. The contribution of defects to the formation of leakage current paths was proposed to originate from Mg doping in the p-type GaN layer, which induced the generation of point defects such as nitrogen vacancies, Mg interstitials, Mg-Ga-N vacancy complexes, and Mg-H defects [43,44,45,46,47,48,49,50]. Furthermore, metal diffusion in the contact metallization generated micro-pits in the p-type region, thereby altering the layer resistivity and establishing metallic leakage paths [21]. The reduction in reverse current is attributed to enhanced magnesium incorporation and significantly reduced silicon/oxygen impurity concentrations in the p-type GaN layer [51]. Figure 4b shows stress-resistant EQE after stress, implying that P1 was situated deep within the p-type layer.
Figure 5a reveals an enhanced photo-generated current at the P2 site under reverse bias conditions. The consistency of EQE in the high-current regime after stress, as shown in Figure 5b, excluded lattice damage within the active region. The observed reduction in radiative recombination rate within the low-current region (Figure 5b inset), coupled with degradation in I-V characteristics, is attributed to defect state enhancement in the cladding layer. The defect accumulation substantially altered the Schottky barrier potential, promoting localized tunneling current [10,52,53,54]. In addition, deep-level defects within the cladding layer were found to simultaneously influence carrier transport pathways and photon emission efficiency through their role as non-radiative recombination centers in the space-charge region [10].
Figure 6a reveals that the P3 site exhibits a similar enhancement trend of photo-generated current as the P2 site. Notably, Figure 6b demonstrates significant time-dependent EQE decay in the entire current region after stress, as shown in Figure 6b. The strong correlation between the degradation of electrical parameters and alterations in optical properties provided direct evidence of defect generation within the active region. These defects predominantly originated from nitrogen vacancy formation at the GaInN/GaN multiple-quantum-well (MQW) interfaces, which substantially reduced both carrier injection efficiency and radiative recombination rate [55,56,57,58].
To further investigate the defects responsible for the observed OBIC signals and variations in electrical and optical characteristics, additional measurements, including TEM, SEM, time-resolved photoluminescence (TRPL), and deep-level transient spectroscopy (DLTS), are necessary to clarify their microscopic characteristics. TEM enables atomic-level structural analysis, revealing the atomic configuration and formation mechanisms of defects. SEM facilitates the observation of surface defects, aiding in macroscopic defect localization. TRPL enables the evaluation of potential mid-gap state defect concentrations, allowing for the quantitative characterization of non-radiative recombination activity. DLTS provides quantitative analysis of deep-level defect parameters, thereby elucidating the physical nature of electrically active defects.

4. Conclusions

In this paper, the OBIC technique was used to localize defects in LED devices. Laser-induced defect state dynamics were monitored through synchronized electrical and optical characterization, establishing quantitative correlations between microscopic defect dynamics and macroscopic device degradation patterns. This approach eliminates the prerequisite for prior knowledge of specific device parameters such as barrier layer dimensions, QW configurations, or material stacking sequences. The integration of micro-defect analysis with macro-scale electro-optical characterization enables multidimensional reconstruction of defect state dynamics. The OBIC technique offers high-spatial-resolution, time-resolved, and non-destructive analytical capabilities for comprehensive LED defect characterization. This multiscale characterization framework provides critical support for the design and manufacturing of high-performance LEDs.

Author Contributions

D.-G.Z. contributed to conceptualizing the experiment, data analysis, and data collection, as well as documentation. H.-M.Y. contributed by assisting in data collection and analysis. J.-F.Z. and D.-P.H. contributed supervision of the overall experiment, conceptualization of data analysis and data collection, and documentation. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

This work was supported by the Global Joint Research Program funded by the Pukyong National University (202411810001).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental setup for OBIC mapping integrated in a scanning laser microscope system.
Figure 1. Experimental setup for OBIC mapping integrated in a scanning laser microscope system.
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Figure 2. (a) OBIC mapping under zero-bias conditions and (b) LID mapping under 0.5 mA current were performed on a 1150 × 1150 μm2 LED device. The black arrows indicate the electrically inactive defects.
Figure 2. (a) OBIC mapping under zero-bias conditions and (b) LID mapping under 0.5 mA current were performed on a 1150 × 1150 μm2 LED device. The black arrows indicate the electrically inactive defects.
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Figure 3. (a) The I-V characteristics under the electrical stress. (b) The probe-laser beam was spotted on the “defect-free” regions.
Figure 3. (a) The I-V characteristics under the electrical stress. (b) The probe-laser beam was spotted on the “defect-free” regions.
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Figure 4. (a,b) The electrical and optical characteristics around the P1 defect position.
Figure 4. (a,b) The electrical and optical characteristics around the P1 defect position.
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Figure 5. (a,b) The electrical and optical characteristics around the P2 defect position.
Figure 5. (a,b) The electrical and optical characteristics around the P2 defect position.
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Figure 6. (a,b) The electrical and optical characteristics around the P3 defect position.
Figure 6. (a,b) The electrical and optical characteristics around the P3 defect position.
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MDPI and ACS Style

Zheng, D.-G.; Zhang, J.-F.; Yu, H.-M.; Han, D.-P. In Situ OBIC Mapping to Investigate Native Defect Dynamics in GaInN/GaN-Based Light-Emitting Diodes. Photonics 2025, 12, 861. https://doi.org/10.3390/photonics12090861

AMA Style

Zheng D-G, Zhang J-F, Yu H-M, Han D-P. In Situ OBIC Mapping to Investigate Native Defect Dynamics in GaInN/GaN-Based Light-Emitting Diodes. Photonics. 2025; 12(9):861. https://doi.org/10.3390/photonics12090861

Chicago/Turabian Style

Zheng, Dong-Guang, Jian-Feng Zhang, Hao-Min Yu, and Dong-Pyo Han. 2025. "In Situ OBIC Mapping to Investigate Native Defect Dynamics in GaInN/GaN-Based Light-Emitting Diodes" Photonics 12, no. 9: 861. https://doi.org/10.3390/photonics12090861

APA Style

Zheng, D.-G., Zhang, J.-F., Yu, H.-M., & Han, D.-P. (2025). In Situ OBIC Mapping to Investigate Native Defect Dynamics in GaInN/GaN-Based Light-Emitting Diodes. Photonics, 12(9), 861. https://doi.org/10.3390/photonics12090861

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