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The epitaxial lift-off process allows the separation of a thin layer of III/V material from the substrate by selective etching of an intermediate AlAs layer with HF. In a theory proposed for this process, it was assumed that for every mole of AlAs dissolved three moles of H 2 gas are formed. In order to verify this assumption the reaction mechanism and stoichiometry were investigated in the present work. The solid, solution and gaseous reaction products of the etch process have been examined by a number of techniques. It was found that aluminum fluoride is formed, both in the solid form as well as in solution. Furthermore, instead of H 2 arsine (AsH 3 ) is formed in the etch process. Some oxygen-related arsenic compounds like AsO, AsOH, and AsO 2 have also been detected with gas chromatography/mass spectroscopy. The presence of oxygen in the etching environment accelerates the etching process, while a total absence of oxygen resulted in the process coming to a premature halt. It is argued that, in the absence of oxygen, the etching surface is stabilized, possibly by the sparingly soluble AlF 3 or by solid arsenic. The epitaxial lift-off ͑ELO͒ process allows the production of single-crystalline thin films of III/V materials. The technique is interesting for the optoelectronics industry, because the use of thin film devices results 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. Furthermore, ELO allows the integration of III/V-based components with, e.g., silicon-based devices.In 1978, Konagai et al. 1 first reported on peeled-film technology ͑PFT͒; they separated a Ϯ5 m thick GaAs epilayer from the GaAs substrate by etching a thin intermediate AlGaAs release layer with aqueous HF solution. It was found that this process stopped at certain depths, because etchant and reaction products could not be exchanged sufficiently fast through the narrow etch slit.2 In 1987, Yablonovitch et al. 3 reported that for thinner epilayers with a thickness in the order of 1 m this problem could be overcome by placing a droplet of black wax on top of the GaAs layer. The GaAs epilayers experience some stress due to the wax and curl up, thereby forcing open the small crevice between substrate and epilayer. As a result, the etch process, now referred to as ELO, no longer stopped at a certain depth. In a model to describe this process, Yablonovitch et al.3 assumed that in etching AlAs release layers with HF solution in water each mole of AlAs forms three moles of H 2 gas and that the out-diffusion of this H 2 gas through the etch crevice is the limiting factor for the lateral etch rate. By assuming the rate of diffusion of H 2 out of the etch slit to be equal to the rate of production at the etch front, the maximum attainable etch rate was found to bewhere N and n are the molar concentrations of AlAs and dissolved H 2 , respectively, D the diffusion constant of H 2 in the solution, R the radius of curvature of the fi...

In the present work a so-called diffusion and reaction related model ͑DR model͒ is derived based on the notion that the overall etch rate in the epitaxial lift-off ͑ELO͒ process is determined both by the diffusion rate of hydrofluoric acid to the etch front and its subsequent reaction rate in the process. In contrast to the model that was previously described in the literature, the DR model yields etch rates which are in quantitative agreement with those obtained experimentally. In order to verify the DR model, the ELO etch rate of AlAs 1−y P y release layers is determined as a function of the phosphorus percentage, the release layer thickness and the temperature. In accordance with the DR model, it is shown that the etch rate is reaction rate related by the dependence on the phosphorus percentage in the release layer, and that the etch rate is diffusion rate related by the dependence on the release layer thickness. From the temperature dependence, an activation energy of 0.31 eV could be deduced for the ELO process under the present conditions. © 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2779968͔ All rights reserved. The "epitaxial lift-off" ͑ELO͒ technique ͑see Fig. 1a͒, 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 singlecrystalline thin films of III/V materials.1 Application of this technique is interesting for the optoelectronics industry, because use of thin-film devices potentially results 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. This is of particular importance for an intrinsically large area, thus expensive devices like high efficiency III/V solar cells, 2,3 and the integration of III/V based components with, e.g., silicon-based devices. 4,5 Recently, at our institute thin-film GaAs solar cells were made based on the ELO technique, which 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 the ELO process is not detrimental to the quality of the thin-film device.In 1978 Konagai et al. 8 described the separation of devices from a GaAs substrate using the extreme selectivity of hydrofluoric acid ͑HF͒ for Al x Ga 1−x As with a high Al fraction. A wax layer was applied to support the 30 µm thick fragile films during the process. Yablonovitch et al. 9 noted that the tension induced by the wax caused the thinner films, of micrometer thickness, to curl up with a radius of curvature r as they became undercut. This was concluded to be beneficial for removal of the etch products, leading to an increased lateral etch rate V e of the AlAs release layer. By assuming that three moles of hydrogen ͑H 2 ͒ gas are produced for each mole of AlAs etched and that the ability of dissolved H 2 , which has a low solubil...

Epitaxial lift-off (ELO) is a process which allows for the separation of a single crystalline III/V thin film or device from the substrate it was deposited on. This process is based on the selective etching of an intermediate AlAs release layer in an aqueous HF solution. The lateral etch rate of the AlAs release layer through a narrow crevice in the weight-induced epitaxial lift-off (WI-ELO) process is much larger than observed for unobstructed planar AlAs layers. It is possible that this increase in etch rate is caused by the tensile strain induced upon the AlAs layer in the WI-ELO setup. In order to verify this assumption, planar AlAs layers, subjected to a controlled curvature, were etched in HF solutions and their etch duration was measured. The applied curvature reduced the already present compressive strain due to lattice mismatch. For large applied bending radii no change in etch rate was observed, because the induced bending is smaller than the already present bending due to the lattice mismatch. Further bending induces a total compressive strain from −0.126% to −0.11%, resulting in an etch rate variation from 0.054 up to 0.066 mm h−1. Measurements on AlAs layers experiencing a tensile strain of +0.286% showed much higher etch rates of 0.134 mm h−1. The present results obtained on etching experiments in the lateral plane are extrapolated to the perpendicular direction so that a combination with the data from previous work becomes feasible. This results in a better microscopic picture of the etch front in the WI-ELO process. It is found that the force exerted by the weight can be projected on an area, limited by the sample width and a depth of approximately 6 µm.

In this paper, the influence of intrinsic strain on the epitaxial lift-off (ELO) process induced by local lattice mismatch is determined as a function of the composition variation of two In x Ga1–x As or two GaAs1–y P y layers surrounding the AlAs etch layer. For this purpose, samples were grown by metal organic chemical vapor deposition, etched using a weight-induced ELO process, and analyzed by differential interference contrast microscopy and atomic force microscopy. It is shown that the etch rate decreases significantly for increasing In and P fractions in the surrounding layers. Morphology examinations after the ELO process revealed a relatively smooth interface roughness for the GaAs and GaAs1–y P y samples plus the In x Ga1–x As samples over a composition range from x = 0 to x = 0.075, indicating that the growth mechanism is controlled by a two-dimensional (2D) layer-by-layer mode. For In0.10Ga0.90As samples, however, a transition from a 2D layer-by-layer to a three-dimensional (3D) islanding growth mode was obtained.

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...

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The etching behavior of Al x Ga 1−x InP 2 ͑0 ഛ x ഛ 1͒ in aqueous HCl was investigated for layers on their native GaAs substrates as well as for layers after releasing from their substrate and transferring to a foreign plastic carrier utilizing the epitaxial lift-off ͑ELO͒ technique. For InGaP 2 layers on their native substrates the activation energy of the etching rate was determined to be 22 kcal/mol for HCl concentrations of both 6 and 12 M. The surface roughness of the partially etched Al x Ga 1−x InP 2 layers as determined with atomic force microscopy ͑AFM͒ was found to decrease with increasing aluminum fraction and to be smaller for 6 M than for 12 M HCl. Al x Ga 1−x InP 2 layers on foreign plastic carriers were often found to be not etched in HCl, in contrast to layers on substrates. This could not be attributed to a single cause and it is suggested that the nonetching behavior is related to a combination of factors, like exposure of the layers to the ELO process and strain induced by the foreign carrier. AFM studies showed an increased density of irregularities at the surfaces of the Al x Ga 1−x InP 2 samples that later showed nonetching behavior.