Short NoteOptical characterizations and reverse-bias electroluminescence observation for reliability investigations of the InGaN light emitting diode
Graphical abstract
Forward-bias emission of a GaN LED with and without a 550 nm high pass filter. Reverse-bias emission of a GaN LED with and without a 550 nm high pass filter.
Highlights
► The reverse-bias luminescence of the InGaN LED reveals device-reliability problem. ► Surface temperature measurements and two-dimensional XRF have been investigated. ► Hot electron-induced emissions may be a mechanism of the reverse-bias emission. ► Reverse-bias stress is performed to investigate reliability issues. ► These techniques promise a screening tool.
Introduction
Researchers prefer using the InGaN/GaN light-emitting diode in the forward-bias condition, functioning as a solid-state light source. Previous research discussed the reliability issues of the LED device under reverse bias. Meneghini et al. [1] reported reverse-bias electroluminescence (EL) due to a band-to-band recombination of the stressed devices caused by leakage current flowing through preferential paths. Chen et al. investigated damage of the device induced through high reverse-bias stress by observing the reverse-bias EL [2]. In contrast to other groups, this study observes the reverse-bias electroluminescence, which was almost undetectable from the fresh device, since the luminescence behavior of the device in the reverse-bias condition could also shed light on fresh device-reliability problems. During the reverse-bias operations, field-dependent tunneling current at low voltage and local impact ionization in high-electric-field regions dominate the leakage current, which affects the device’s electrical properties [3]. The goal of this study is to use optical characterization techniques, including 2D electroluminescence observation, 2D surface temperature measurements, 2D X-ray fluorescence (XRF) element analysis, and electrical measurements to explore potential reliability problems with the InGaN LED device performance. This paper examines the EL light-emission behavior under forward-bias operations and reverse-bias operations. The reverse-bias leakage and the forward-bias subthreshold current measurements provide additional information to explore any potential reliability problems. In addition, reverse-bias stress has been performed on the LED devices. The degradation and failure caused by the reverse-bias stress further proved the possible reliability concerns with reverse-bias operations. Under a combinational study of material analysis and electrical performance, the reverse-bias electroluminescence behavior, which was a result of the hot electron-induced emission, proved relevant to the device performance. Building a nondestructive screening method could detect the current path’s leakage around the metal contact and evaluate the LED device quality from the fresh device. Though the first GaN LED was demonstrated in the early 90s, the reliability problem may cloud the promise of its future development [4], [5]. Therefore, numerous of studies have been conducted until now to unveil the GaN LED reliability issue. In LED industry, stress test around 6000 h is required [6], [7]. This study provides fast screening techniques and locating the weakness promising for future LED industrial applications. In contrast to current industrial reliability test techniques, reverse-bias electrical analysis and reverse-bias luminescence evaluation have been conducted to exam the device reliability [8]. Understanding the mechanism and locating the weak or defect area can help engineers can improve the fabrication process and increase the yield rate [9].
Section snippets
Device fabrication
This experiment uses metal–organic chemical–vapor deposition (MOCVD) on a c-face 2 inch sapphire (0 0 0 1) substrate to grow the InGaN/GaN multiple quantum well (MQW) LED. The device structure consists of a 30 nm GaN nucleation layer, a 2-m-thick undoped GaN, 2-m-thick Si-doped n-type GaN, 100-nm-thick active layers, 50-nm-thick Mg-doped AlGaN electron-blocking layer, and a 15-nm-thick Mg-doped p-GaN layer. The InGaN/GaN MQW active region consists of 10 pairs of 3-nm-thick InGaN well layers and
Experiments and discussion
To examine the device operations, this study investigates the electroluminescence behavior in both the forward bias and the reverse bias condition. It is common knowledge that band-to-band recombination is responsible for the forward-bias EL during the LED device operations, while band-to-band recombination [1], hot-electron emission [10], [11], or yellow luminescence defect [12] and its influence on device reliability [13] may generate the reverse-bias electron emission. Not only is the
Conclusion
This study investigates the mechanisms of the fresh InGaN LED device and the relationship between its performance and the reverse-bias EL with optical characterization techniques. This paper attributes the origin of the reverse-bias luminescence from the fresh device to hot electron-induced emission. As for reliability investigations of the fresh LED, we prove that the emission under reverse bias is relevant to large leakage current in the forward-bias condition, the reverse-bias condition, and
Acknowledgments
This work was supported by the National Science Council, Taiwan, Republic of China, under Contract No. NSC-98-2221-E-260-006.
References (21)
- et al.
J. Cryst. Growth
(2009) Microelectron. Reliab.
(2000)- et al.
Microelectron. Reliab.
(1999) - et al.
Solid-State Electron.
(2003) - et al.
J. Cryst. Growth
(2004) - et al.
Appl. Phys. Lett.
(2009) - et al.
Semiconductors
(1998) - et al.
J. Disp. Technol.
(2007) - et al.
A non-contact method for determining junction temperature of phosphor-converted white LEDs
SPIE
(2004) - et al.
Electron. Lett.
(2000)