Improved light extraction efficiency of AlGaN deep-ultraviolet light emitting diodes combining Ag-nanodots/Al reflective electrode with highly transparent p-type layer

In recent years, AlGaN-based solid-state deep ultraviolet (DUV) light emitting diodes (LEDs) have attracted much attention because of their extensive application in air and water purification, sterilization, DUV curing, and so on [1,2]. Compared with traditional gas DUV light sources, such as mercury lamps, AlGaN-based DUV-LEDs are considered to have many advantages such as small size, high power, low energy consumption, and environment friendly, etc., thus being a kind of promising DUV light source. Especially, the recent outbreak of the COVID-19 around the world further evokes the passion to apply DUV light irradiation to kill the virus [3], hence the realization of high-performance AlGaN-based DUV-LEDs becomes more urgent and essential. To date, a great progress has been achieved for AlGaN-based DUV-LEDs, and the external quantum efficiency (EQE) as high as 20.3% has been realized, whereas the performance of AlGaN-based DUV-LEDs still remains at a relatively low level compared to commercial GaN-based visible LEDs [4–6]. As is well known, the EQE is determined by the internal quantum efficiency (IQE), carrier injection efficiency (CIE), and light extraction efficiency (LEE). Up to now, high IQE and CIE have been realized [7,8], however the effective ways to increase LEE are very limited [9], which thus becomes the biggest obstacle hindering the improvement of the EQE [10,11]. Therefore, it is very important to develop effective approaches to promote the light extraction effectively.

For AlGaN-based DUV-LEDs, the main factors that hinder the improvement of LEE are the optical polarization characteristics [12,13], total reflection at the interface [14], and strong absorption of DUV light by the p-GaN layer, which are all serious challenges at present [15,16]. In order to improve the LEE, many researchers have made great efforts. For example, Liu et al. used AlN-delta-GaN quantum well to achieve a large transverse electric (TE)-polarized emission, thus improving the light polarization [17]. Wang et al. weakened the total reflection at the interface of sapphire and air and enhanced the light extraction angle by using a moth-eye microstructure [18]. And enhanced light extraction was also achieved by high-reflectance metals [19,20], reflective photonic crystals (PCs) [9], distributed Bragg reflectors [21] or omnidirectional reflectors [22]. Especially interestingly, Maeda et al. adopted a transparent p-AlGaN contact layer and highly reflective Ni/Mg, Rh, Ni/Al as the p-electrodes to obtain higher LEE, showing a great potential for high performance DUV-LED devices [23,24]. However, due to the difficulties to obtain p-AlGaN with high hole concentration, poor Ohmic contact with the p-AlGaN layer remains a big obstacle, as it leads to an increasing working voltage and decreased device stability as well as device life. Therefore, it is of great significance to seek for an optimized structure which can balance the light extraction and electrical properties.

In this study, a novel approach that can effectively enhance the light extraction and meanwhile maintain the acceptable electrical properties of AlGaN-based DUV-LEDs has been achieved by combining an Ag nanodots/Al reflective electrode with a highly transparent complex p-type layer. The p-type layer is composed of a p-AlGaN layer and a p-GaN layer with a thickness of several nm, which can greatly reduce the DUV light absorption. The Ag nanodots can ensure good Ohmic contacts with p-GaN, and the area without Ag nanodots can improve the reflectivity of DUV light through Al film. Based on such a complex p-type layer, a highly reflective p-electrode comprising Ag nanodots and Al film is adopted to maintain the electrical properties and enhance the light extraction.

Two schematic DUV-LED device structures used in this study are shown in Fig. 1 for comparison, including one with the proposed structure (defined as structure A) and another one with commonly used Ni/Au electrodes structure (defined as structure B). The DUV-LED wafers were firstly grown on (0001) sapphire substrates by a 3×2′′ close-coupled-showerhead (CCS) Aixtron metal-organic chemical vapor deposition (MOCVD) system. At beginning, a 1.5-μm-thick AlN buffer layer was grown and followed by a 0.5-μm-thick AlN/AlGaN stress modulation multilayer and a 1.5-μm-thick n-Al_0.55Ga_0.45N layer (n∼2.5×10¹⁸ cm⁻³) in sequence. Then, the active region with five pairs of nominally Al_0.37Ga_0.63N quantum well layer and Si-doped Al_0.5Ga_0.5N barrier layer (n∼1.0×10¹⁸ cm⁻³) was grown. Subsequently, a 10-nm-thick Mg-doped AlGaN electron blocking layer and a 96-nm-thick Mg-doped (∼1.5×10¹⁹ cm⁻³) p-Al_0.4Ga_0.6N/Al_0.67Ga_0.33N superlattice were grown. Finally, a thin Mg-doped p-GaN (p∼1.0×10¹⁸ cm⁻³) layer was grown as the Ohmic contact layer.

 

Fig. 1. Schematic structures of DUV-LEDs with (a) Ag nanodots/Al electrodes and (b) Ni/Au electrodes.

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After the growth, device fabrication for structures A and B was conducted respectively, while the process was consistent except for the p-type electrodes. Firstly, the wafers were fabricated by inductively coupled plasma (ICP) etching to expose the n-AlGaN region, and then Ti/Al/Ni/Au metals were evaporated. Subsequently, the samples were annealed in a N₂ atmosphere at 850 ℃ for 35 s to form Ohmic contacts. After that, 4-nm-thick Ag was deposited on the p-GaN layer for structure A, followed by annealing in a N₂ atmosphere to form Ag nanodots. Then, 150-nm-thick Al film was evaporated on the Ag nanodots region to complete the complex Ag nanodots/Al p-type electrodes. For comparison, Ni/Au metals were evaporated as the p-type electrodes for structure B. Finally, a flip-chip technology was applied to obtain the DUV-LED devices with the size of 889×889 μm² without adopting any special light extraction and packaging. The device performance was measured by an integrating sphere from EVERFINE Corporation. Besides, to evaluate the electrical properties of the p-electrodes, the transmission line method (TLM) patterns with 10 ∼ 40 μm gaps were defined using photolithography.

Morphology evolution of Ag film is firstly studied under different annealing conditions on the p-GaN layer, before depositing the Al layer. Figure 2(a) shows the surface scanning electron microscopy (SEM) image after annealing at 300 ℃ for 3 minutes. It can be seen that Ag tends to form small particles with a diameter varying from 20 to 100 nm. With further increasing the annealing temperature to 400 ℃ for 3 minutes, the Ag nanodots become larger and the diameter varies from 50 to 200 nm as shown in Fig. 2(b). Figure 2(c) further shows the surface morphology evolution with temperature increasing to 500 ℃ just for 1 minute. The reason for choosing 1 minute lies in the fact that Ag cannot withstand such high temperature for a long time. It can be noted that the particles shown in Fig. 2(c) become much larger than those in Figs. 2(a) and 2(b), clearly presenting strong size dependence on the annealing conditions. To check the transmittance of the Ag nanodots for DUV light, Ag film was deposited on a double-polished sapphire and then annealed under the conditions as mentioned above for evaluation. The Ag nanodots also form on sapphire, which are similar to the features as shown in Fig. 2(a)–2(c) under different annealing temperatures (not shown here). Figure 2(d) shows the transmittance spectra of Ag nanodots formed on double-polished sapphire under different annealing conditions. It is found that a high transmittance above 80% around the wavelength of 280 nm can be obtained with the annealing temperature varying between 300 and 400 ℃. However, obvious transmittance degradation occurs when the temperature is further increased to 500 ℃.

 

Fig. 2. Morphology evolution of Ag under different annealing conditions: (a) 300 ℃ for 3 minutes, (b) 400 ℃ for 3 minutes, and (c) 500 ℃ for 1 minute. (d) Transmittance spectra of Ag nanodots on sapphire under different annealing conditions.

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We further explore the comprehensive optical characteristics of the complex p-type layer and the p-electrodes for structure A. Figure 3(a) presents the transmission electron microscopy (TEM) image of the p-type layer structure. The thickness of p-GaN is only 7.7 nm. If only one reflection is considered, the transmittance of 7.7-nm thick p-GaN layer is approximately 88% at 280 nm according to the absorption coefficient for GaN [25–27]. Figure 3(b) shows atomic force microscopy (AFM) image of this thin p-GaN. It features a smooth surface with clear and straight atomic steps, whose root-mean square (RMS) roughness value is 0.137 nm (3×3 μm²). Moreover, the transmittance of p-AlGaN superlattice structure with hole concentration of 3.7×10¹⁸ cm⁻³ at room temperature (see Supplement 1) used in the DUV-LEDs is also measured. By directly growing the same superlattice structure on an AlN template grown on double-polished sapphire, the transmittance of the p-AlGaN superlattice layer can reach above 92% at 280 nm as shown in Fig. 3(c), presenting a high DUV transparency. Besides, the reflectance of Ag nanodots/Al complex electrodes on double-polished sapphire is also measured, as shown in Fig. 3(d). As can be seen that the reflectivity of this complex electrodes reaches 69% at the wavelength of 280 nm, which is more than twice of that of Ni/Au electrodes at 280 nm (∼30%) [9]. It can thus be expected that the combination of the Ag-nanodots/Al reflective electrodes and highly transparent complex p-type layer will greatly reflect DUV light to the sapphire side.

 

Fig. 3. (a) Cross-sectional TEM image, (b) AFM image of the LED with thin p-GaN. (c) Transmittance of the p-AlGaN superlattice. (d) Reflectivity of the Ag nanodots/Al complex electrodes.

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The electrical characteristics of structures A and B are also compared as shown in Fig. 4(a). It is found that Ohmic contact characteristic occurs in both of these two structures, and the current of sample B with Ni/Au electrodes is higher than that with Ag nanodots/Al electrodes at the same voltage, revealing a certain degree of degradation when adopting Ag nanodots to achieve Ohmic contacts [28]. Figure 4(b) further exhibits the typical I-V characteristics of structures A and B. The forward voltage at the injection current of 20 mA for structure A reaches 9.2 V, which is obviously greater than the value of 7.6 V in structure B. The increased voltage difference of 1.6 V comes from the increased contact resistance between the p-GaN and Ag nanodots/Al electrodes, which possibly originates from the significantly decreased contact area of Ag nanodots on the p-GaN. Compared with the efforts to use reflective electrodes directly on p-AlGaN, such as Rh metal [5], the increased voltage cost of the proposed scheme is significantly reduced, which indicates that the proposed device structure has great potential in maintaining acceptable electrical properties and enhanced light extraction.

 

Fig. 4. (a) Comparison of lateral I-V characteristics of Ni/Au contact and Ag nanodots/Al complex electrodes on p-GaN. (b) The I-V characteristics of DUV-LED structures A and B.

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The device performance adopting these two different electrode structures are further compared. The electroluminescence (EL) spectra at 100 mA for structures A and B present single peak emission with wavelength of 282.6 nm and 280.3 nm, respectively, as shown in Fig. 5(a). Figure 5(b) shows the variation of the full width at half maximum (FWHM) and wavelength peak with the current for structures A and B. The FWHM of DUV-LED structures A and B increases gradually with injection current, and the FWHM of the EL spectra are severally 9.4 nm and 9.5 nm at 100 mA, presenting little difference. Similarly, the wavelength peak also increases gradually with current, from 281.7 nm and 279.6 nm at 5 mA to 283.7 nm and 281 nm at 200 mA for structures A and B, respectively. We also perform the EQE and light output power (LOP) measurement with different injection current, as shown in Fig. 5(c). The EQE increases with the injection current, and reaches the maximum at 20 mA. After that, it decreases. The LOP over the whole current range (0-200 mA) increases with the current, but the increasing speed becomes slower at high current. The maximum LOP and EQE over the whole current range (0-200 mA) for the proposed structure A are severally 11.1 mW and 1.76%, which are significantly increased by 52% and 58% respectively compared to structure B with the commonly used Ni/Au electrodes. These results prove that the proposed approach adopting Ag-nanodots/Al reflective electrodes based on a highly transparent complex p-type layer can significantly improve the LEE and thus device performance of DUV-LEDs as a whole.

 

Fig. 5. (a) The EL spectra of structures A and B at 100 mA. (b) The FWHM and wavelength, and (c) The EQE and LOP of structures A and B under different injection current.

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In summary, enhanced LEE for AlGaN-based DUV-LEDs has been achieved based on a proposed approach that can enhance light extraction and maintain acceptable electrical properties. By thinning the p-GaN layer to several nm, p-type AlGaN/GaN complex layer becomes highly DUV transparent, which makes it possible for the application of reflective electrodes composed of Ag-nanodots and Al film, and thus allow more light emission upward to be reflected to the sapphire side. Applying this approach, the maximum LOP and the EQE of DUV-LEDs with optimized Ag nanodots/Al electrodes are severally increased by 52% and 58% respectively compared to that adopting traditional Ni/Au electrodes.