Etching AlAs with HF for Epitaxial Lift-Off Applications

Abstract

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...

A Diffusion and Reaction Related Model of the Epitaxial Lift-Off Process

“…15 It was found that the reaction between AlAs and the HF etchant is most likely described by a set of overall reactions given by…”

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

“…The attack of the substrate by the HF and the residues formed during the sacrificial layer etching result in the increase of RMS roughness from 0.3 to 1-4 nms range. Chemical reactions between AlAs and HF have been well studied 10,15,16 6 À n ] (3 À n) þ , on the other hand, are solid and hard to dissolve into the solution. Besides these primary byproducts, solid As 2 O 3 can also be generated on the substrate depending on the oxygen concentration of the etchant 17 .…”

“…Hence, developing a new ELO process with different chemistry that enables the direct reuse of the substrate is very desirable to lower the overall cost. Although HCl-based etchant was proposed and used by several researchers to selectively etch AlGaAs 10,11 or other sacrificial layers [12][13][14] , the possibility of reusing the substrate directly with these methods has not been explored. In this paper, we demonstrate a new ELO process where aluminium-arsenide based sacrificial layer and HF-based etchant are replaced with phosphide-based materials and HCl.…”

Inverted thin film InGaP/GaAs tandem solar cells for CPV applications using epitaxial lift off

“…Since a basic wet chemical smoothing etch procedure appeared insufficient to remove all surface contamination, wafer re-preparation is done by a surface cleaning followed by a chemomechanical polishing procedure. Multiple wafer re-use without degradation in solar cell quality was shown using this procedure [4]. .…”

“…We have tested a number of different set-ups for ELO ( fig.2) [3] and investigated the mechanism of the etch process in detail [4,5].…”