The diffusion of Ag in GaN-based surface plasmon light-emitting diodes under different temperature annealing

Abstract

In order to achieve effective coupling between the surface plasmon and the quantum wells (QWs) for surface plasmon LEDs, metal-embedded or thin p-GaN layer approach have been normally been used, but by taking either of these methods, the enhancement of the IQE of the fabricated SPLEDs is not remarkable, in some cases, the internal quantum efficiency even decreases. Here, this study is to clarify the origin and understand what kind of influence of temperature treatment to the LED structure. GaN based LEDs with Ag from the same epitaxial wafer were annealed under different temperatures to imitate the high temperature treatment during the SP-LED fabrication. The results show that the diffusion of Ag into the quantum wells could be one of the reasons and in order to realize SP-LEDs with high IQE, high temperature treatments need to be avoided to prevent the diffusion of metal. I. IntroductionGaN-based light-emitting diodes (LEDs) have become as a promising candidate for next generation high-brightness solidstate lighting because of many advantages, such as energy saving and environment friendly [1], and with different indium composition, it can cover the UV to longer-wavelength spectral region. However, as the emission wavelength approaches to the green light region, the internal quantum efficiency (IQE) suffers an obvious loss due to poor crystal quality [2], serious quantum confined stark effect (QCSE) [3] and efficiency droop [4]. Various methods have been proposed to address the above issues [5][6][7].Recently, surface plasmon (SP) coupling have drawn a lot of attention, since it can effectively enhance the IQE by offer an extra emission channel [8][9][10]. To achieve high performance SP-LEDs, one of the most important key points is to get close-distance coupling between surface plasmon and the QWs. In order to realize the close-distance coupling, it has been reported to use metal-embedded or thin p-GaN layer approach [11][12][13][14][15]. However, by taking either of two methods above, the enhancement of the IQE of the fabricated SP-LEDs is not remarkable. In some cases, the IQE even decreases. The reason could be related to the material degeneration, which is caused by the metal diffusion during the regrowth or the thermal treatment in the thin p-GaN layer method. Here, the aim of this study is to clarify the origin and understand what kind of influence of temperature treatment to the LED structure.Similar LED samples from the same wafer were annealed under different temperature to imitate the high temperature treatments during the SP-LED fabrication. Photoluminescence (PL) measurements were before and after the temperature treatments using 325nm He-Cd laser irradiation. Secondary ion mass spectrometry (SIMS) depth profile was performed to acquire the diffusion degree of the metal. In Sec. II of this paper, fabrication procedure and characterization results of the LED samples are presented, and discussions are provided. In Sec. III conclusions and summary are drawn. Figure 1 illustrates the procedure flow chart...

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“…(2)­It is important to note that the Ag/GaN interface is not abrupt and that the Ag metal gets diffused into the GaN semiconductor layer, as indicated from the STEM–EDX line spectrum (Figure b,c). Possible reasons involve poor adhesion between the Ag reflective (metal) and GaN (semiconductor) layers, different thermal annealing (temperature cycle), and lattice mismatch (stress) during the fabrication of GaN and Ag layers. , Furthermore, thermal annealing performed in O 2 ambient environment exhibited interdiffusion between Ag and GaN, which disrupts the Ag/GaN interface (Figure d) . In this case, interfacial Ga vacancies will be generated substantially, which can degrade the Ohmic contact (metal–semiconductor junction) or change energy levels (e.g., Schottky barrier height). , In addition, dissimilar properties between the Ag metal and the GaN semiconductor materials (Table S3) can potentially result in unwanted diffusion and intermixing.…”

“…The degradation of the crystal quality of InGaN/GaN MQWs will decrease the efficiency of the active region in generating photons by varying the energy levels (band gap). This additional reliability issue of Ag is due to its electromigration (EM) property, and such interfacial reactions between the Ag metal and GaN were also addressed by other groups using the secondary-ion mass spectrometry (SIMS) technique, , wherein Ag diffused into the light-producing active region (MQWs), thereby creating deep levels in the GaN semiconductor material and hindering the light output . Furthermore, Ag migration inside of the MQWs serves as a nonradiative recombination center that can shift the spectrum (optical wavelength) of a particular blue LED device by impairing the carrier confinement and transport mechanism at the active region of the LED.…”

“…44,45 Furthermore, thermal annealing performed in O 2 ambient environment exhibited interdiffusion between Ag and GaN, which disrupts the Ag/GaN interface (Figure 3d). 46 In this case, interfacial Ga vacancies will be generated substantially, which can degrade the Ohmic contact (metal−semiconductor junction) or change energy levels (e.g., Schottky barrier height). 13,47 In addition, dissimilar properties between the Ag metal and the GaN semiconductor materials (Table S3) can potentially result in unwanted diffusion and intermixing.…”

“…In the context of optimization for device performance enhancement in terms of LEE and output power, factors such as nanostructural composition, selection of metallic elements at reflection, barrier or adhesion, and wafer bonding layers need to be considered in correlation with electrical resistivity, coefficient of thermal expansion (CTE) or mechanical stress, fabrication methodology, and grain morphological parameters. On the basis of our analysis and results, possible optimization suggestions are given as follows: GB engineering, that is, grain structure, SB (TBs), and texture orientation need to be well-controlled as electron or photon scattering mechanism and electromigration lifetime are highly dependent on the grain size distribution. ,, Nanotwinned metals, for example, Ag, should be promoted as these structures exhibit distinctive properties (e.g., high tensile strength, good ductility, thermal stability, and electrical conductivity) as compared to the nanocrystalline or ultrafine-grained metals. , To mitigate issues of Ag diffusion into the GaN or active region and In/Ga out-diffusion into the Ag mirror layer, high thermal treatments should be avoided on Ag and p-GaN layers to prevent metal–semiconductor diffusions. , Ni/Ag/TiW metal stack is found to tolerate high-temperature annealing To reduce thermal stress between GaN and metal–alloy-based interfaces, the tunable incorporation of diamond-like carbon (DLC) layers on the reflective layer has the distinct capability to match its CTE with GaN and hence enhances the thermal diffusion. , The GaN/Ag interface needs diffusion barrier layers such as tin–zinc oxide (TZO) and nickel–titanium (NiTi)-related alloys that can effectively block Ag diffusion .…”

“…To mitigate issues of Ag diffusion into the GaN or active region and In/Ga out-diffusion into the Ag mirror layer, high thermal treatments should be avoided on Ag and p-GaN layers to prevent metal–semiconductor diffusions. , Ni/Ag/TiW metal stack is found to tolerate high-temperature annealing …”