Comparative study of III-phosphide- and III-nitride-based light-emitting diodes: understanding the factors limiting efficiency

Abstract

Modern light-emitting diodes (LEDs) require further improvements in the wall-plug efficiency (WPE). Thus, understanding and elucidating the physical factors limiting the WPE is of crucial importance. This study aims to understand and elucidate such factors via a comparative study of two different LED material systems, i.e. III-phosphide (AlGaInP) red and III-nitride (AlGaInN) blue LEDs. The WPE was decoupled into its component elements, which indicated that the dominant contribution to WPE reduction depends on the sample. To further investigate, thermodynamic and current component analyses were conducted. These analyses clearly demonstrated that the carrier dynamics is dependent of the material systems, i.e. internal quantum efficiency and voltage efficiency are simultaneously degraded due to the carrier accumulation in the blue sample. We found that the WPE can be improved using strategies dependent on the samples, such as improvement of the light extraction in the red sample and suppression of the carrier accumulation in the blue sample.

Pre-trimethylindium Flow Treatment of GaInN/GaN Quantum Wells to Suppress Surface Defect Incorporation and Improve Efficiency

“…5,6 Recently, there has been a rapid increase in the demand for SSL composed of three primary colors since they can generate white light with a high color-rendering index. 7,8 Despite the strong demand for such SSL, the AlGaInN-based LEDs suffer from an issue, the so-called "green gap" [a severe drop in the internal quantum efficiency (IQE) of green−yellow emitters compared to blue and near-ultraviolet ones], 9,10 which is a major obstacle that should be overcome to fabricate the SSL with high color quality.…”

“…Recent studies have revealed that the defect incorporation originated from two independent mechanisms: (i) inherent incorporation of impurities, such as carbon and oxygen, due to reduction in the growth temperature to grow the indium (In)-rich MQWs 14,15 nonradiative recombination, thereby reducing IQE. 6,8 Recently, studies on the second of the above-mentioned mechanism are being actively conducted by several research groups as it is considered a key factor in the IQE improvement. 16−21 Our recent studies showed that the SDs in an n-type GaN are most likely nitrogen vacancies (V N ), divacancies comprising Ga and N vacancies (V Ga V N ), and/or a V N impurity complex, which are intensively distributed within about 100 nm from the growth surface of n-type GaN.…”

“…A priority concern in the research of wide band gap semiconductors is the production of high-efficiency optoelectronic devices operating in visible spectral regions. Among the wide band gap material systems, III-nitride compounds attract considerable attention nowadays because they allow the direct transition of carriers in k -space and have a tunable emission wavelength. , Akasaki et al gave the first demonstration of a high-quality GaN layer on c -sapphire substrates using a low-temperature (LT) buffer layer, and their demonstration is hailed as a breakthrough in III-nitride semiconductor technology, thereby making remarkable progress in the development of AlGaInN-based light-emitting devices (LEDs). , Thanks to such a great effort, traditional white lighting can be replaced with solid-state lighting (SSL), that is, blue LEDs covered with yellow phosphors, which have tremendous benefits, such as small sizes, environmental friendly, a long lifetime span, and low energy consumption. , Recently, there has been a rapid increase in the demand for SSL composed of three primary colors since they can generate white light with a high color-rendering index. , Despite the strong demand for such SSL, the AlGaInN-based LEDs suffer from an issue, the so-called “green gap” [a severe drop in the internal quantum efficiency (IQE) of green–yellow emitters compared to blue and near-ultraviolet ones], , which is a major obstacle that should be overcome to fabricate the SSL with high color quality.…”

“…Several physical mechanisms have been suggested as the origin of the green gap, including the quantum-confined Stark effect, random alloy fluctuation, high dislocation density, and asymmetric carrier distribution. − Besides these, the high level of defect incorporation in the multiple-quantum-well (MQW) active layer has often been discussed as a potential contributor to the green gap. Recent studies have revealed that the defect incorporation originated from two independent mechanisms: (i) inherent incorporation of impurities, such as carbon and oxygen, due to reduction in the growth temperature to grow the indium (In)-rich MQWs , and (ii) incorporation of the native defects formed at an n-type GaN surface [i.e., surface defects (SDs)] into the MQWs during the LT growth. − Both mechanisms create nonradiative recombination centers (NRCs) in the MQW active layer, which act as a path for Shockley–Read–Hall (SRH) nonradiative recombination, thereby reducing IQE. , Recently, studies on the second of the above-mentioned mechanism are being actively conducted by several research groups as it is considered a key factor in the IQE improvement. − Our recent studies showed that the SDs in an n-type GaN are most likely nitrogen vacancies (V N ), divacancies comprising Ga and N vacancies (V Ga V N ), and/or a V N impurity complex, which are intensively distributed within about 100 nm from the growth surface of n-type GaN. , Note that the SDs are easily incorporated with In atoms owing to strong affinity between them; therefore, an introduction of a GaInN or AlInN layer between the n-type GaN and the MQWs [i.e., an underlying layer (UL)] is very useful to improve the IQE by trapping the SDs therein. , However, a further increase in the In content and/or thickness of the UL to enhance the SD trapping frequently has negative impacts on the device performance because it generates threading dislocations (TDs) and absorbs the photons emitted from the MQWs. , This indicates that the introduction of a UL alone is insufficient to suppress the incorporation of SDs into the MQWs. As a method to further reduce the SDs in n-type GaN other than the introduction of a UL, we have demonstrated the MQWs grown on the surface-etched n-type GaN layer, which showed an improvement in the IQE by a factor of 2.5 in green LEDs .…”

Inhomogeneous Barrier Height Characteristics of n-Type AlInP for Red AlGaInP-Based Light-Emitting Diodes

“…The fabrication of high-efficiency (Al x Ga 1−x ) y In 1-y P compound semiconductors-based light-emitting diodes (LEDs) is of considerable interest for the use in various applications including display, indicator lights, and solid-state lighting. [1][2][3][4][5][6][7] In particular, for micro-LED display application, such as virtual reality and augmented reality, 2 the enhancement of the external quantum efficiency of GaAlInP-based emitters is essential. [2][3][4][5]8,9 One of the ways of increasing the quantum efficiency is to enhance the light extraction.…”