HF Species and Dissolved Oxygen on the Epitaxial Lift-Off Process of GaAs Using AlAsP Release Layers

Abstract

The lateral etch rate of the epitaxial lift-off ͑ELO͒ process was determined as a function of the total HF concentration C HF and the O 2 partial pressure P O 2 . For this purpose samples were grown by metallorganic chemical vapor deposition and etched using a weight-induced ELO process. It was found that the etch rate increases linearly with C HF , which is in accordance with the model on the ELO process presented in a previous paper. This result and composition calculations of HF solutions show that the first step in the etch process of AlAs with an HF solution most probably takes place by chemical attack of undissociated HF on AlAs surface bonds. Furthermore, it is shown that the ELO rate increases slightly over a P O 2 range varying from 0.046 to 0.98 atm and that for P O 2 = 0.003 atm, a significantly lower etch rate is found. We suggest that the observed decrease is the result of surface passivation by elemental arsenic, which is formed by the reaction of AlAs with H+ . An oxygen-poor atmosphere may allow the build-up of elemental arsenic on the surface, thus slowing down the AlAs reaction with HF. Oxygen, by removing arsenic as As 2 O 3 , keeps the surface active. The epitaxial lift-off ͑ELO͒ technique, in which a III/V device structure is separated from its GaAs substrate by using selective wet etching of a thin Al x Ga 1−x As ͑x Ͼ 0.6͒ release layer and transferred to a foreign carrier, allows the production of single-crystalline thin films of III/V materials.1 Application of this technique is interesting for the optoelectronics industry because use of thin-film devices results potentially in a more efficient transfer of generated heat from device to carrier or heat sink and significantly reduces the amount of material needed by reuse of the substrates.2 This is of particular importance for intrinsically large-areas, thus expensive, devices like high-efficiency III/V solar cells, 2,3 and the integration of III/V-based components with, for example, silicon-based devices. 4,5 Recently, at our institute thin-film GaAs solar cells, based on the ELO technique, reached record efficiencies of 24.5%.6 This is close to the highest efficiency of 25.1% reported for regular GaAs cells on a GaAs substrate, 7 which indicates that this technique is not detrimental to the quality of the thin-film device.In the ELO process ͑see Fig. 1a͒ the AlAs release layer is etched with hydrofluoric acid ͑HF͒, which leads to the formation of gaseous arsine ͑AsH 3 ͒, solid aluminum fluoride ͑AlF 3 ·3H 2 O͒, and dissolved aluminum fluoride complexes as the major reaction products.8 This reaction is most likely described by a set of overall reactions given by AlAs + 3HF + 6H 2 O → AsH 3 + ͓AlF n ·͑H 2 O͒ 6−n ͔ ͑3−n͒+with n = 0,1,2,3. In a previous work, 9 the ELO process was described by a diffusion and reaction related model ͑DR-model͒, which is based on the notion that the overall etch rate V e is determined by both the diffusion of HF to the etch front and its subsequent reaction ͑see Fig. 1b͒. According to this model V e is given b...

Wafer reuse for repeated growth of III–V solar cells

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Epitaxial lift-off process for gallium arsenide substrate reuse and flexible electronics

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