Inductively Coupled Plasma Etching of III-Nitrides in Cl 2 /Xe, Cl 2 /Ar AND Cl 2 /He
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tudies have been focused on obtaining relatively the large etch depths (2-4ltm) typical of ridge or facet heights in LEDs or laser diodes, where the final surface morphology on the field is less important. There is increasing interest in the development of GaN-based high power/high temperature electronics for power switching and transmission applications [14-18]. In these devices, the etch depth is much shallower, but smooth morphologies and high selectivities for InN over the other nitrides are required because layers based on InN will probably be used to obtain low ohmic contact resistance. Shul et al. [1,10] first reported Inductively Coupled Plasma (ICP) etching of GaN, AIN, InN, InAIN and InGaN at low dc biases (• 750W. However, the C12/Ar discharge showed the highest etch rate of InN at 1000 W. AIN etch rate increased slightly with the source power, but resulted in low etch rates. GaN etch rates with CI2/He and CI2/Ar chemistries showed maxima as the source power increased, but relatively constant etch rates with C12/Xe. The increase in etch rate with increasing source power is due to the higher concentration of reactive species in the plasma, suggesting a reactant-limited regime, and to higher ion flux to the substrate surface. Increased numbers of ions also make the surface more active with respect to the reactive neutrals. The decrease in etch rate with further increase of the ICP power is attributed either to lower ion energies or ion-assisted desorption of the reactive species at the substrate surface prior to etch reactions. The dc bias of the sample chuck was decreased as the ICP power increased mainly due to the increased ion density. In order to reduce the currently high contact resistance in GaN-based heterostructure field transistors [23], and eventually heterojunction bipolar transistors, it is expected that InN-based contact layers will be necessary [24-26], in analogy to InGaAs on GaAs. In such a case, the ability to selectively etch InN relative to the other nitrides will be crucial. Figures 5 shows some selectivity data as functions of rf power in chlorine-noble discharges. As the rf power increased, the C12/Ar discharge showed overall the best selectivity of InN over GaN, but the C12/He chemistry yielded the lowest selectivities for InN over AIN as well as over GaN. The selectivity
data obtained in this work showed overall higher selectivity characteristics for InN over GaN and AIN in C12/He, C12/Ar and C12/Xe than that previously reported [1,12].
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Figure 4. Effect of ICP source power on etch rates of InN (top), AIN (center) and GaN (bottom) with 2C12/13He, 2C12/13Ar and 2CIll3Xe plasma chemistries (250W rf chuck power, 5mTorr).
Figure 5. Effect of rf chuck power on the selectivity for InN over GaN and AIN (750W source power, 5mTorr, 2C0 2/13
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