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Abstract
Low light extraction efficiency (LEE), high forward voltage and severe self-heating effect greatly affect the performance for AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). In this work, surface-textured Ga-face n-AlGaN is fabricated low-costly using self-assembled SiO2 nanosphere as hard mask. The experimental results manifest that when compared with conventional DUV LEDs, the optical power, the forward voltage and the thermal characteristics for the DUV LEDs with surface-textured Ga-face n-AlGaN are improved obviously. It is because the surface-textured Ga-face n-AlGaN between mesa and the n-electrode can be used as the scattering center for trapped light, and this leads to the enhanced LEE. Furthermore, thanks to the surface-textured n-AlGaN under the n-electrode, the n-type ohmic contact area can be increased effectively. Therefore, the n-type ohmic contact resistance can be reduced and the better heat dissipation can be attained for the proposed flip-chip DUV LED.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) have attracted tremendous attention due to their rapidly increased applications in water disinfection, sterilization, curing and light communication [1–3]. Such DUV LEDs are expected to replace the conventional mercury lamps because of many potential advantages such as low power consumption, long lifetime and environment-friendly [4–6]. However, the performance of conventional DUV LEDs is severely limited by the extremely low external quantum efficiency (EQE) due to the very poor light extraction efficiency (LEE) [7], which is directly related to absorptive p-GaN contact layer [8]. On the other hand, the large differences in refractive indices of AlGaN (n ∼2.46), sapphire (n ∼1.78), and air (n = 1) also make much of the light with large incidence angles behave as guided modes. Especially, transverse-magnetic (TM) polarized light with the electric field component parallel to the C-axis increases rapidly with the increasing Al-content for AlGaN quantum wells (QWs) [9]. Hence, the high fraction of the TM polarized light for DUV LED further increases the number of guided modes. If these guided modes cannot be scattered, the DUV photons shall be absorbed by p-GaN layer and active layer. Moreover, both the p-type and the n-type electrodes will also absorb DUV photons [10]. For suppressing the guided modes, various scattering structures have been developed such as utilizing nanopatterned sapphire substrate [11–17], roughened outlight surface [18,19], inclined sidewall structure [20,21]. Generally, the patterned p-GaN surface shows the best effect in improving LEE for flip-chip DUV LEDs due to reduced p-GaN area and the improved scattering efficiency. However, the reduced p-type ohmic contact area shall result in the increased forward voltage [22]. In addition, it also has been reported that roughened outlight surface maybe harmful to the LEE of TE-polarized light because some light with small incident angle could be scattered back towards p-GaN and then be absorbed [23]. Therefore, it is important to explore another effective scatter way to avoid the scattered light back towards p-GaN and not affect the forward voltage.
In this work, the exposed Ga-face n-AlGaN is patterned using SiO2 nanosphere array as mask to improve the performance of flip-chip DUV LED. The experimental results show that when compared with the conventional flip-chip DUV LED, the DUV LED with patterned Ga-face n-AlGaN surface possesses the enhanced optical power, decreased forward voltage and reduced self-heating effect. It is because the patterned n-AlGaN can increase the n-type ohmic contact area and the escape probability of light by scattering those light. The detailed physics analysis is conducted through various characterization tools and finite-difference time-domain (FDTD) simulation.
2. Experiment
AlGaN-based DUV LED wafer is grown on flat c-plane sapphire substrate by using metal organic chemical vapor deposition (MOCVD) system. A 2 µm-thick AlN buffer layer, a 1.0 µm-thick Si-doped n-Al0.60Ga0.40N electron injection layer, five periods of 3-nm-thick Al0.45Ga0.55N/12-nm-thick Al0.56Ga0.44N multiple quantum wells (MQWs), a 20 nm-thick Mg-doped p-Al0.60Ga0.40N electron blocking layer (p-EBL), a 50 nm-thick p-Al0.40Ga0.60N layer and a 200 nm-thick p-GaN layer are epitaxially deposited in sequence. Then, the DUV LED wafer is processed into chips as shown in the flow charts in Fig. 1. After growing the DUV LED epitaxial wafer, the mesa is dry etched to expose the n-AlGaN by using inductively coupled plasma (ICP) etching system and photoresist as mask. The etching gas mixture are chlorine (Cl2), boron trichloride (BCl3) and argon (Ar) and etching gas flow rate for Cl2 BCl3 and Ar are 150 sccm, 5sccm and 5sccm, respectively. The chamber pressure is 3 mTorr. The up RF power is 400 W and the lower RF power is 150 W. The etching depth of mesa is about 900 nm. Next, the SiO2 nanosphere suspension is dropped onto the interface of air and water to form a close-packed monolayer SiO2 nanospheres film. After that, the close-packed monolayer SiO2 nanospheres film is transferred to the surface of the fabricated DUV LED wafer [24]. There are no special requirements for the substrate during nanosphere monolayer transferring process [25]. Therefore, the close-packed monolayer SiO2 nanospheres can be well self-assembled among mesas. Then, ICP etching is applied again and the SiO2 nanospheres are used as hard mask to pattern n-AlGaN surface. Note, the p-GaN layer can be protected well by the residual photoresist mask during the dry etching process. Next, the residual photoresist and the SiO2 nanosphere array are removed by using stripping liquid and buffer oxide etchant (BOE) solution. After that, by using electron-beam deposition, the Ti/Al/Ti/Au (20/30/60/100 nm) multi-layers are deposited onto the exposed n-AlGaN as the n-electrode which is annealed in the N2 ambient for 1 min at the temperature of 650 °C. The Ni/Au (10/10 nm) as p-electrode is fabricated onto the p-GaN layer which is annealed in the O2 ambient for 3 min at the temperature of 450 °C. Subsequently, a layer of 100 nm thick SiO2 is deposited as the passivation layer at the growth temperature of 80 °C. The deposited SiO2 layer is patterned and etched so that n-electrode and p-electrode are exposed. Then, a 800/20/100 nm thick Al/Ti/Au electrode is deposited on both n-electrode and p-electrode. At last, the sapphire substrate is thinned to 250 µm, which is beneficial for dicing. The chips are flip-chip bonded on ceramic substrate. Firstly, we put Scaling Powder onto the function area of package substrate. Then the chip is put on the die-bond area. After that we put the joint-union into the vacuum furnace at a temperature of 300 °C for 45 s to achieve eutectic bonding. The DUV LED with textured n-AlGaN is named as Device 2. For comparison, a conventional DUV LED with flat n-AlGaN is also fabricated and is named as Device 1.
3. Results and discussion
Figure 2(a) shows the surface morphology of the Ga-face n-AlGaN nanostructure arrays measured by Atomic Force Microscopy (AFM) system. The three-dimensional (3D) AFM image is shown in the inset of Fig. 2(a). It can be seen that the nanostructure close-packed array with truncated cones has been fabricated on the exposed n-AlGaN. To probe the size for the nanostructure, the line profile along the line of two adjacent nanopattern centers in Fig. 2(a) is shown in Fig. 2(b). It can be obtained that the top diameter, the bottom diameter, the height and the period for nanostructure array are about 450 nm, 700 nm, 200 nm and 700 nm, respectively.
Fig. 2. (a) Morphology of the surface textured n-AlGaN in top-view AFM image, inset: 3D AFM image; (b) AFM line profiles.
Figure 3(a) shows the current-voltage (I-V) curves for all studied devices. The forward voltages at the current of 30 mA for Device 1 and Device 2 are 12.5 V and 11.1 V, respectively. The forward voltage for Device 2 is significantly lower than that for Device 1. Normally, the forward voltage for LED is determined by the voltage drop at the p-n junction, n- and p-type semiconductor and ohmic contact resistances [23]. Obviously, the main difference between Device 1 and Device 2 lies in the n-type ohmic contact resistance. It is well known that the Si-doping efficiency is very low for Al-rich n-AlGaN layer [26], which causes the large n-type specific contact resistivity. Moreover, n-type ohmic contact resistance also depends on the ohmic contact area. A large n-type ohmic contact area can effectively reduce the n-type ohmic contact resistance which helps to reduce the forward voltage for the DUV LED [7]. Here, the size of n-electrode for Device 1 and Device 2 are same. However, the n-electrode is deposited on the textured n-AlGaN for Device 2. Thus, when compared with Device 1, the effective contact area for the n-type ohmic contact of Device 2 is larger because the nanopatterns increase the surface area of n-AlGaN. As a result, the n-type ohmic contact resistance for Device 2 is smaller than that for Device 1.
Fig. 3. (a) Current-voltage characteristics for Devices 1 and 2. (b) Optical power and peak wavelength in terms of the injection current for Devices 1 and 2.
Figure 3(b) shows the optical power and peak wavelength in terms of the injection current for Devices 1 and 2. It can be observed that at the current of 30 mA, the optical power for Device 2 is enhanced by 52% when compared with Device 1. It is because the textured n-AlGaN for Device 2 serves as the scattering center and makes more light escape out from the chip, which reduces the optical absorption by the n-electrode. Furthermore, we can also observe from Fig. 3(b) that as the current increases, the optical power for both Device 1 and Device 2 reaches a saturation level and then drops and this power rollover is due to the self-heating effect. However, the current at which the power rollover occurs for Device 2 is larger than that for Device 1. There are two reasons for the reduced self-heating effect for Device 2. On one hand, the LEE for Device 2 is higher than that for Device 1, which suppresses the re-absorption to photons and generates less heat in Device 2. On the other hand, for flip-chip DUV LED, the heat diffuses mainly through the n- and p-electrode. Undoubtedly, the larger contact area between n-electrode and n-AlGaN for Device 2 is beneficial for heat diffusion through the n-electrode. To further confirm the better heat diffusion for the Device 2, Fig. 3(b) also shows the peak emission wavelengths for Device 1 and Device 2 in terms of the injection current. It can be found that the red shift of the peak emission wavelength for Device 2 is ∼1.6 nm in the current range from 10 mA to 100 mA, which number is 2.3 nm for Device 1. This further illustrates that the surface textured n-AlGaN helps to reduce the Joule heating effect. In addition, the wavelength for Device 2 is shorter than that for Device 1 at various injection currents, which can be well attributed to the better heat diffusion and thus the reduced junction temperature for Device 2.
To investigate the far-field radiation pattern, we further carry out the measurement of the angular distribution of the EL spectra for Devices 1 and 2. The tested results are shown in Fig. 4(a). It can be observed that the emission patterns for the two devices exhibit a typical heart-like shape, which originates from the anisotropic emission pattern caused by the unique photon polarization property of c-plane Al-rich AlGaN MQWs [27]. In addition, Device 2 exhibits much stronger far-field radiation intensity in each angular direction than Device 1. It is because the photons in Device 2 can be scattered randomly into the escaped cones due to the textured n-AGaN. To verify it, the optical emission photographs for Device 1 and 2 are taken at 10 mA by microscope. The optical emission photographs for Devices 1 and 2 are shown in Figs. 4(b) and 4(c), respectively. The optical intensity for Device 2 is stronger than that for Device 1 thanks to the enhanced LEE. Moreover, if we further carefully compare the light intensity in the dotted boxes in Figs. 4(b) and 4(c), it can be observed that the emitted light between n-electrode and mesa of Device 2 is stronger than that for Device 1, but there is little light for Device 1. It manifests that for Device 1, the light reaching the region between mesa and n-electrode cannot escape. However, for Device 2, the light reaching the region between mesa and n-electrode can be scattered by the nanopatterns on n-AlGaN layer and then escape into external space. Therefore, the enhanced LEE for Device 2 is mainly attributed to the scattering effect of nanopatterns between mesa and n-electrode.
Fig. 4. (a) Far-field radiation patterns for Devices 1 and 2. Light emission photograph for (b) Device 1 and (c) Device 2 under the microscope.
In order to further reveal the underlying mechanism, we utilize two-dimensional finite-difference time-domain (2D FDTD) simulation to analyze the LEEs for the two devices. The inset in Fig. 5(a) illustrates the simulation model for Device 2. Due to the limitation of computer memory, the thickness of sapphire substrate is reduced to 1 µm, and the lateral size for simulation structure is set to 50 µm. A single dipole source with a peak emission wavelength 280 nm is positioned in the middle of MQW layer [28]. The dipole source will emit TM-mode light or TE-mode light when its polarized direction is parallel to the Y-axis or the X-axis, respectively. The perfectly matched layer (PML) is applied for all simulation boundary so that the electromagnetic energy incident upon the PML can be absorbed [29]. The LEE can be derived by the ratio between the total extracted power collected from the power monitor and the total light power emitted from dipole source [30].
Fig. 5. (a) TE- and TM- polarized LEE for Devices 1 and 2. Inset: Schematic diagram of 2D-FDTD simulation models for Device 2. (b) Angular distribution patterns of TE-polarized light for Devices 1 and 2. Electric field distributions in the XY cross section of TE-polarized for (c) Device 1 and (d) Device 2.
Figure 5(a) shows the LEEs for TM- and TE-polarized light for Devices 1 and 2. The LEEs for TM- and TE-polarized light of Device 1 are ∼1.6% and 9.7%, respectively. For Device 2 with textured n-AlGaN, the LEEs for TM- and TE-polarized light are 3% and 11.1%, respectively. For both Device 1 and Device 2, the LEEs for the TM- polarized light are far lower than that for the TE- polarized light as a result of the lateral propagation for the TM-polarized light. Compared with Device 1, the LEEs of TE- and TM-polarized light for Device 2 are enhanced by 15% and 87%, respectively. The enhancement of LEE for TM-polarized is significantly higher than that for TE-polarized light. This is mainly caused by the fact that more TM-polarized light is restricted in Device 1 because of the lateral propagation properties. The light can be partly scattered into external space by surface textured n-AlGaN in Device 2. To further analyze the scattering properties, Figs. 5(c) and 5(d) show electric field distributions in the XY cross section of TE-polarized light for Devices 1 and 2. It can be seen that the central electric field distributions for the two devices are similar. Most light escapes from the central region due to vertical propagation feature for TE-polarized light. It also can be clearly observed that when the position is far away from x = 0 µm, the electric field intensity in the air for Device 2 is stronger than that for Device 1. To explain further it, we mark the propagation paths of the TE-polarized light for Device 1 and Device 2 in Figs. 5(c) and 5(d), respectively. Beam ① is emitted from the dipole source and directly propagates into the escape cone. According to Snell’s Law, the escape cone depends on the critical angle of θc = arcsin(nair /nAlGaN) ≈ 22.6°, where nair and nAlGaN are the refractive indices for the air and the AlGaN material, respectively. When the light propagation angle is larger than 22.62°, the total internal reflection will take place. As shown in Fig. 5(c), for beam ② in Device 1, it cannot enter the escape cones and is constantly reflected back, then absorbed by various materials and eventually converted into heat. However, as we can see in Fig. 5(d), for beam ② in Device 2, the total reflected light can be scattered into escape cones by the n-AlGaN nanostructure arrays and is extracted into air. Therefore, the electric field intensity in the air for Device 2 is stronger. Figure 5(b) shows the angular distribution patterns of TE-polarized light for Devices 1 and 2. The angle distribution pattern in the external space can be obtained by the near-to-far field transformation (NTFF) method. It can be observed that the light intensity for Device 2 is stronger than that of Device 1 due to the higher LEE. Moreover, the light intensity at various emission angles for Device 2 is stronger than that for Device 1 because the n-AlGaN nanostructure arrays can randomly scatter light into various angles as beam ② shows in Fig. 5(d)
4. Conclusions
In summary, well-ordered n-AlGaN nanostructure arrays have been fabricated on the Ga-face n-AlGaN surface for the DUV LED through low-cost nanosphere template process. The experimental results show that the optical, electrical and thermal characteristics for the DUV LED with Ga-face n-AlGaN nanostructure arrays all are improved significantly. It is because that the n-electrode is deposited on the nanostructure arrays and this design helps to increase the effective contact area. Then, ohmic contact resistance is decreased and the heat dissipation capability is accordingly improved. In addition, the LEE for DUV LED with Ga-face n-AlGaN nanostructure arrays is enhanced due to the scattering effect of the nanostructure arrays between mesa and n-electrode. Therefore, the Ga-face n-AlGaN nanostructure arrays is of great significance to improve various characteristics for AlGaN-based DUV LEDs.
Funding
National Natural Science Foundation of China (61904176, 61975051, 62022080, 62074050, 62135013); Natural Science Foundation of Hebei Province (F2018202080, F2020202030); Research fund by State Key Laboratory of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology (No. EERI_PI2020008); Jiangsu Provincial Key Laboratory of Photonic and Electronic Materials Sciences and Technology, Nanjing University (njuzds2021-005); National Key Research and Development Program of China (2016YFB0400800); Shanxi Key RD Program (20201102013).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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