A.T.J. van Niftrik

Etching AlAs with HF for Epitaxial Lift-Off Applications

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 (AsH3) is formed in the etch process. Some oxygen- related arsenic compounds like AsO, AsOH, and AsO2 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 AlF3 or by solid arsenic

Strain-accelerated HF etching of AlAs for epitaxial lift-off

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.

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

The lateral etch rate of the epitaxial lift-off (ELO) process was determined as a function of the total HF concentration Cup and the O 2 partial pressure Po 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 Po 2 range varying from 0.046 to 0.98 atm and that for Po 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.

Study of wet chemical etching of AlxGa1-xInP2 films using hydrochloric acid

The etching behavior of Al x Ga 1-x InP 2 (0 ≤ x ≤ 1) 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 (ELO) 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 (AFM) 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.