Laser-Controlled Etching of (Al, Ga)As Epitaxial Layers
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Power (W) Figure 1: Etch rate of p-Al0.3Gao. 7As (ND=1.4x 10s cm- 3) as a function of power of laser beam unchopped and chopped at 5% duty cycle. Scan speeds are 4.5pim/s and 3pm/s respectively. immersed in a solution containing aqueous KOH (0.25 M). This was prepared from carefully weighed anhydrous pellets of KOH, and diluted appropriately with deionised water. After the etching experiment, the samples were cleaned and cleaved across the etched lines. Both scanning electron microscope (SEM) and surface profilometer were used for the observation and measurement of the etch depths and etch profiles. RESULTS Effects of Laser Parameters Effects of laser parameters, namely, laser power, beam scanning speed, number of scans and duty cycle of beam, have been systematically investigated for different AlGal_.,As samples. For easy discussion, we will present the results for p-Al0. 3 Gao.7 As, but similar results are generally observed for other AlzGal-XAs samples. Figure 1 shows the etch rate for p-Al0. 3 Gao.7As as a function of the laser beam power for two different duty cycles. The etch rate exhibits typical exponential increase with power when the beam is unchopped and linear increase when the beam is chopped. This result is expected as both the local temperature rise and the effective photon flux reaching the sample surface are reduced for chopped laser beam. In such a case, photochemical process becomes the main etching mechanism and hence the etch rate is proportional to the laser power [3]. The etch depth as a function of number of scans with chopped and unchopped beam at a fixed laser power and scan speed is shown in Figure 2. It can be seen that the etching proceeds slower when the laser beam is chopped. This implies that chopping the laser beam will allow etch depths to be better controlled to a finer resolution. The effect of beam scanning speed on the etch rate is shown in Figure 3. For this case, the etch rate 492
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Figure 2: Variation of etch depth of p-Alo.aGao. 7As (ND=1.4x 1018cm-3 ) due to repeated scanning of chopped and unchopped laser beam. The laser power is 0.01W and the scan speed is 4.5yim/s.
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Scan speed (ftm/s) Figure 3: Etch rate of p-A1O. 3 Ga0 .7 As (ND=1.4x10 18cm- 3 ) as a function of laser scanning speed for CW laser beam. The laser power is 0.01W.
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Power (W) Figure 4: Comparison of etch rates of n-Al.GalpAs as a function of laser power for x values of 0.3, 0.4, and 0.5. The laser is pulsed at 5% duty cycle. decreases linearly, which is typical of p-type samples. This is pr
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