Fabrication of Smooth GaN-Based Laser Facets
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Downloaded from https://www.cambridge.org/core. IP address: 185.101.69.109, on 17 Nov 2018 at 05:43:33, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300003446
rates and crystal planes observed for all chemicals used in this work are summarized in Table I. The etching planes listed in this table are those that appear during the etch. In all cases the etch rate in the “vertical” [0001] direction is immeasurably small, but etching occurs “horizontally,” i.e., normal to [0001]. Because the c-plane is impervious to all of the chemicals used in this study, no etch mask is required for the crystallographic etching step; the c-plane itself acts as a mask. An etch mask may be necessary, however, if long etching times are used, to prevent the development of etch pits at defect sites. For this purpose we have successfully used both titanium masks annealed at 900°C for 30 seconds in a nitrogen atmosphere and nickel masks annealed at 650°C for 2 minutes in a nitrogen atmosphere. RESULTS The { 10 1 0 } plane shown in Fig. 1 was produced by etching in 10% KOH by weight dissolved in ethylene glycol at 170°C. This plane has been examined using a high resolution field-effect scanning electron microscope (FESEM) with a resolution of 5 nm at 2.5 kV, and the surface appears perfectly smooth. This indicates that wet chemical etching may be a valuable tool for producing high-quality laser facets with reflectivities close to the ideal for a perfectly smooth surface.16 For this etching method to be useful for fabricating pn-junction laser diodes, the dependence of etch rate and surface morphology on doping must be determined. A scanning electron microscope (SEM) image of p-type GaN after anisotropic wet etching in molten KOH at a temperature of 195°C is shown in Fig. 2. Only the top portion of the epilayer is doped p-type; the lower 1 µm is undoped. The seamless morphology of the surface displayed in Fig. 2 indicates that the change in doping does not affect the etch plane or the etch rate. The quality of the crystallographically etched surfaces is generally lower in the p-type material than in Table I: Etch rates and observed etching planes for various chemicals Chemical Acetic Acid (CH3COOH) Hydrochloric Acid (HCl) Nitric Acid (HNO3) Phosphoric Acid (H3PO4) Sulphuric Acid (H2SO4) Potassium Hydroxide (KOH), molten 50% KOH in H2O 10 – 50% KOH in Ethylene Glycol (CH2OHCH2OH) 50% NaOH in H2O Tetramethylammonium Hydroxide (TMAH) Tetraethylammonium Hydroxide (TEAH) 20% NaOH in Ethylene Glycol
Temperature (°C) 30 50 81 108 – 195 93 150 – 247 83 90-182
Etch Rate (µ µm/min) < 0.001 < 0.001 < 0.001 0.013 – 3.2 < 0.001 0.003 – 2.3 < 0.001 0.0015 – 1.3
Etching Planes Observed None None None { 10 1 2 },{ 10 1 3 } None { 10 1 0 },{ 10 1 1 } None { 10 1 0 }
100 76
< 0.001 0.013
None { 10 1 2 }
91
0.007
{ 10 1 2 }
178
0.67 – 1.0
None
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