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The formation of large scale, highly uniform and controllable GaN microdome arrays based on a self-assembled low cost method was investigated. The deposition of a large area, hexagonally close-packed SiO2 microsphere monolayer on top of the III-nitride semiconductor using the dip-coating method was optimized, which leads to surface coverage of 87% of SiO2 on GaN (ideal close-packed microsphere surface coverage is 90.7%). Reactive ion etching was used to simultaneously etch both SiO2 microspheres and GaN substrate to form GaN microdomes. Experiments show that GaN microdomes with controllable size, shape, and aspect ratio are achievable through controlling the plasma etching conditions.
©2013 Optical Society of America
1. Introduction
III-nitride semiconductors (Al, In, Ga, -N) with the band gap ranging from deep ultraviolet (UV) up to near infrared, have been widely applied in light-emitting diodes (LEDs) [1–3], laser diodes [4, 5], and solar cell devices [6–8]. However, III-nitride is known as a “hard” material, which has a high bonding energy (GaN: 8.7 eV / atom) and is inert to most chemicals at room temperature. Thus, patterning (i.e. etching) of III-nitride materials requires dry etching where materials are removed through chemical reactions and energetic surface bombardment.
Recently, III-nitride microdomes applied in LEDs have been numerically studied for significantly enhancing the light extraction efficiency for both InGaN quantum wells (QWs) based visible LEDs [9] and AlGaN QWs based UV /deep-UV LEDs [10]. GaN microdomes have also been applied in III-nitride based solar cell devices to harvest the light collection efficiency [11]. However, it is still challenging to achieve large scale, uniform, and controllable III-nitride microdome structures for device application.
In this work, we investigated the fabrication of GaN microdomes based on a self-assembled low cost approach, focusing on the optimization of its uniformity, scalability and structural parameter tunability. The self-assembled approach was based on the nanosphere lithography method, where dielectric colloidal particles were used as lithography mask to transform the spherical shape to the underneath substrate. Thus, a low cost, scalable and controllable process is highly demanded to fabricate uniform colloidal particle monolayers. Many processes have been developed to deposit particles such as spin-coating [12], rapid convective deposition [13–17], colloidal epitaxy [18], optical tweezers [19], electrophoretic assembly [20, 21], and vertical dip-coating deposition [22–24]. However, it is still challenging to form long-range uniform colloidal microspheres, especially on top of the hydrophobic surfaces such as III-nitride semiconductors.
2. GaN microdome fabrication process
In this work, SiO2 microspheres were used as the hard mask, which were deposited on top of the GaN substrate by using the dip-coating method, considering its simplicity and feasibility for large scale production. The GaN microdome structures were formed by reactive ion etching (RIE) of both SiO2 microspheres and GaN substrate. The experiments show that the GaN microdome array structural parameters including the size, shape, and aspect ratio can be well tuned by controlling the plasma etching conditions.
Figure 1 shows the fabrication flow chart of forming the GaN microdomes, which includes the following steps: (a) surface hydrophilic treatment for GaN; (b) SiO2 microsphere monolayer deposition via dip-coating method; (c) reactive ion etching of both SiO2 microspheres and GaN to form GaN microdomes; and (d) hydrofluoric acid wet etching of the sample to remove the residue of SiO2.
Fig. 1 The process flow chart of forming III-nitride based microdomes via reactive ion etching for both III-nitride semiconductor and self-assembled SiO2 microsphere monolayer: (a) surface treatment to convert III-nitride surface from hydrophobic to hydrophilic; (b) dip-coating of SiO2 on III-nitride semiconductor to form close-packed monolayer; (c) RIE of both III-nitride semiconductor and SiO2 microspheres to form III-nitride microdomes; (d) HF etching to remove SiO2 residue.
3. Self-assembled dip-coating of SiO2 microspheres on GaN
Here, the GaN wafers grown on sapphire substrate with thickness of 5 µm were provided by Kyma Technologies. UV ozone was used for the GaN surface treatment with the temperature of 150 °C for 120 minutes. Figures 2(a) and 2(b) show the water contact angle of the GaN surface before (Fig. 2(a)) and after (Fig. 2(b)) the hydrophilic treatment, which indicates significant reduction of the water contact angle from 41.1° to 21.7°. Our experiments have shown that the hydrophilic surface treatment is a crucial step for the SiO2 microsphere dip-coating deposition.
Fig. 2 The water contact angle of GaN substrate (a) before and (b) after surface hydrophilic treatment; (c) schematic of SiO2 microspheres dip-coating on GaN substrate; (d) SEM image of the hexagonal close-packed SiO2 microsphere monolayer deposited on GaN substrate.
The deposition of the self-assembled monolayer (SAM) of SiO2 microspheres was based on the dip-coating method, as shown in Fig. 2(c). The crystallization of the microspheres during the deposition was described by the following equation, which was proposed by Dimitrov and Nagayama [25].
where Vw represents the substrate vertical withdrawal speed that is equal to the crystallization velocity of the microspheres; x represents the number of microsphere layers; d represents the diameter of the microspheres; β is a ratio of the macroscopic mean velocity of the microspheres to that of the solvent; φ is the particle volume fraction in the suspension; je is the evaporation flux of the liquid medium; ϵ is the volume porosity of the formed array; and l is the evaporation length [23, 25]. The product βl is taken to be an experimentally determined constant.In this study, the suspensions of SiO2 microspheres were prepared with mono-dispersed silica powder from Fiber Optic Center Inc.. Two types of inorganic solvent have been utilized and optimized: 1) the mixture of de-ionized (DI) water and ethanol; and 2) pure ethanol. Due to the solvent evaporation, the capillary force between the particles leads to the self-assembly of the microspheres when the GaN substrate is withdrawn from the suspension. With optimized SiO2 microsphere suspension preparation and substrate withdrawal speed, well organized SiO2 microsphere monolayer is able to be formed. An example of the scanning electron microscope (SEM) image of the self-assembled SiO2 microsphere monolayer deposited on GaN substrate is shown in Fig. 2(d). Note that there are points and line defects exist within the SiO2 microsphere monolayer. Thus, the optimization of the deposition process to reduce the defects in large scale is crucial.
For the preparation of SiO2 microsphere suspension, the microsphere particles mixed with the solvent were placed in an ultrasonic bath for 1 hour to ensure the solution uniformity. The substrate is soaked in the well dispersed suspension for about 20 minutes before it is withdrawn from the suspension. The dip-coating apparatus was custom built with withdrawal speed ranging from 0 to 15 mm/s, as shown in Fig. 3. Our studies show that both the suspension components and the substrate withdrawal speed are key parameters for obtaining high quality microsphere monolayer deposition.
For the solution composed of silica microspheres, DI water and ethanol, the optimized weight percentage of the solution is 32%:37%:31% (silica: DI water: ethanol). With the optimized withdrawal speed of 50 µm/s, close-packed SiO2 microsphere monolayers are formed on GaN substrate. Figure 4(a) shows the microscope image of the deposited SiO2 microspheres. In order to calculate the microsphere surface coverage percentage, Fig. 4(a) is converted to Fig. 4(b), which allows the counting of the number of particles in the image. From Fig. 4(b), the microsphere surface coverage is estimated to be 65% due to the existence of high density of point and line defects.
Fig. 4 (a) The microscope image of SiO2 microsphere array by dip-coating with colloidal solution components (weight percentage) of 32% silica microspheres, 37% DI-water and 31% ethanol; and (b) the corresponding high contrast microscope image with estimated surface coverage of 65%.
In order to improve the SiO2 monolayer quality and reduce the point and line defects of the monolayer, we prepared the SiO2 microsphere suspension with only ethanol. With the weight percentage of 30%: 70% (silica: ethanol) and substrate withdrawal speed of 12.5 µm/s, a significantly improved SiO2 microsphere monolayer deposition on GaN was realized. The microscope image is shown in Fig. 5(a). The inset of Fig. 5(a) plots the ideal hexagonal close-packed microspheres, which contains the highest microsphere surface coverage of 90.7%, independent of the microsphere diameter. From Fig. 5(b), the estimated microsphere surface coverage is 87%. The enhanced microsphere surface coverage is due to the significantly reduced point and line defects during the SiO2 microsphere deposition. In addition, the SiO2 microspheres are uniformly deposited on 1/6 of 2” GaN wafer, which indicates its potential for large scale manufacturing.
Fig. 5 (a) The microscope image of SiO2 microsphere array by dip-coating with colloidal solution components (weight percentage) of 30% silica microspheres and 70% ethanol (inset: schematic of ideal close-packed hexagonal microsphere pattern with microsphere surface coverage of 90.7%); and (b) the corresponding high contrast microscope image with estimated surface coverage of 87%.
In this study, microspheres with diameters of 1 µm and 500 nm were studied. In the case of 1 μm microsphere deposition, the evaporation flux rate of the 30%: 70% (silica: ethanol) solution was calculated to be 1.01 × 10−6 m/s, the volume packing density for a hexagonal close packed monolayer is 0.605, the array height is equal to the microsphere diameter of 1 μm, and the optimized velocity is 12.5 μm/s. Under the optimized conditions for 1 μm microsphere deposition, from Eq. (1), the product of βl is estimated to be 3.66 × 10−6 m.
For the case of 500 nm diameter sphere deposition, a microsphere solution was prepared by dispersing the silica spheres in ethanol. The weight percent of the solution was 30%: 70% (silica: ethanol) and the optimized withdrawal speed is 30 μm/s. Uniform, close packed monolayers of microspheres were able to be formed on large areas of the GaN substrate as shown in Fig. 6. The estimated surface coverage of the monolayer formed by 500 nm spheres is 80%. The experimentally determined constant from Eq. (1), βl, was calculated to be 4.39 × 10−6 m in the case of 500 nm sphere deposition.
The product βl is slightly larger for the 500 nm case as compared to that of the 1 µm sphere case. Note that the evaporation rate for the dispersions does not change with different sphere diameters, so it can be assumed that the evaporation length also does not change from case to case. This indicates that the increase in βl originates from an increase in the macroscopic mean velocity of the microspheres or, in other words, better mobility of the 500 nm spheres in the suspension.
4. Formation of GaN microdomes by RIE
The RIE process was used to simultaneously etch both GaN and SiO2 microspheres so that the spherical shape of SiO2 microspheres is transferred to the underneath GaN substrate. From previous studies, chlorine based gases such as Cl2 or BCl3 are effective to etch III-nitride semiconductors [26, 27], but their etching rate on SiO2 is trivial. While the fluorine based gases such as SF6 or CF4 are effective to etch SiO2, but not to GaN [28, 29]. Thus, in this study, Cl2 and SF6 were selected to selectively etch GaN and SiO2 microspheres, respectively. We utilized the Lam Research 9400 Etcher to perform the etching process with RF power of 500 W and bias voltage of 108 V. The chamber pressure was set as 10 mTorr.
Due to the vertical bombardment of the plasma particles, the voids region between the SiO2 microspheres on GaN are mainly etched by Cl2, and meanwhile the effective etching of SiO2 from SF6 leads to the lateral shrinking of SiO2 microspheres. Thus, the mixture of Cl2 and SF6 plasma could transfer the spherical shape of SiO2 to the GaN substrate during the gradual etching process of SiO2 microspheres. The etching rates of GaN and SiO2 will determine the shape or aspect ratio of the GaN microdomes. In our studies, we found that the ratio of the Cl2/SF6 flow rate would effectively control the etching rate ratio of GaN and SiO2, which in turn determines the aspect ratio of the GaN microdomes. Table 1 lists the four conditions of RIE process that were performed with fixed Cl2 flow rate of 48 sccm and tuned SF6 flow rates of 40 (sample 1), 34 (sample 2), 30 (sample 3) and 18 (sample 4) sccm.
Table 1. Four different reactive ion etching process conditions for GaN substrates deposited with close-packed SiO2 microsphere monolayer (diameter of 1µm) to form GaN microdomes.
Figure 7 shows the 45° tilted SEM images of the GaN microdome structures of samples 1, 2, 3 and 4, respectively. By using the RIE process, the spherical shape of SiO2 microspheres is successfully transferred to the GaN substrate. With Cl2/SF6 flow rate ratio of 48/40 and 8 minutes of etching, the GaN microdome height h and diameter D are about 180 nm and 930 nm, respectively. As shown in the inset of Fig. 7(a), there exists SiO2 residues on top of the GaN microdomes, which could be etched away with longer etching time. By reducing the flow rate of SF6, the etching rate of GaN is enhanced and meanwhile the etching rate of SiO2 microspheres is reduced, which results in the increase of the aspect ratio of the GaN mirodomes. The corresponding GaN microdome height and diameter for each case are shown in Table 1. These experiments indicate that the geometrical shape of the GaN mcirodomes could be precisely controlled through controlling the plasma etching conditions.
Fig. 7 SEM images of GaN microdome structures formed via four different RIE etching conditions: (a) sample 1; (b) sample 2; (c) sample 3; and (d) sample 4.
Our recent studies for the LEDs light extraction efficiency indicated that the aspect ratio of the GaN microdomes will significantly affect the light extraction efficiency enhancement [9, 10]. The work from this letter demonstrated the feasibility to achieve large scale, low cost, uniform GaN microdomes with desired aspect ratio. In addition, the approach to form GaN microdomes can be applied for other materials.
As compared to the close-packed microsphere pattern, it is more challenging to fabricate the nonclose-packed pattern. The current approach allows to fabricating uniform nonclose-packed III-nitride microdomes. For example, with the close-packed SiO2 microspheres with diameter of D1, the selective RIE of etching SiO2 only is applied to shrink the diameter of SiO2 microspheres to D2. Then the simultaneous etching of both GaN and SiO2 is applied to form desired nonclose packed GaN microdome structure.
5. Summary
In summary, the fabrication of uniform and large-scale self-assembled close-packed SiO2 microsphere monolayer deposition on GaN was optimized by using dip-coating method. By using the RIE process to simultaneously etch both GaN and the SiO2 microsphere arrays, the GaN microdomes are formed. We demonstrated that the geometrical shape of the GaN microdomes could be precisely controlled through controlling of the RIE conditions. The microdome structures have strong application in III-nitride based LEDs for enhancing the light extraction efficiency and in solar cells for harvesting the light collection efficiency.
Acknowledgments
The authors acknowledge financial support through start-up funds from Case Western Reserve University.
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