P. Mulder

High rate epitaxial lift-off of InGaP films from GaAs substrates

Centimeter sized, crack-free single crystal InGaP films of 1 μm thickness were released from GaAs substrates by a weight-induced epitaxial lift-off process. At room temperature, the lateral etch rate of the process as a function of the applied Al0.85Ga0.15As release layer thickness was found to have a maximum of 3 mm/h at 3 nm. Using 5-nm-thick AlAs release layers, the etch rate increased exponentially with temperature up to 11.2 mm/h at 80 °C. Correlation of the experimental data with the established theoretical description of the process indicate that the model is qualitatively correct but fails to predict the etch rates quantitatively by orders of magnitude.

Influence of radius of curvature on the lateral etch rate of the weight induced epitaxial lift-off process

The ‘Weight Induced Epitaxial Lift-Off’ (WI-ELO) process is used to free single crystalline films from the GaAs substrates on which they have been deposited by etching a sacrificial AlAs release layer. The lateral etch rate Ve of this process is influenced by the weight induced radius of curvature R of the film. Bulk-etch experiments of AlxGa1−x As layers were conducted to compare an unhampered etching process with WI-ELO. It is found that standard WI-ELO etching demonstrates etch rates exceeding the bulk etch rate. Further experiments have shown that the WI-ELO etch rate is not constant in time, but consists of a slow initial etch rate Ve,i, followed by the faster nominal etch rate Ve,n. The latter part is influenced by the applied radius of curvature R via Ve,n=3.1+293R−1.2 with R in mm and Ve,n in mm h−1. This result implies an etch rate consisting of a constant plus a radius-induced part, resulting in both a qualitative and quantitative discrepancy with established theory. The explanations could be the different reaction kinetics and the occurrence of stress or strain in the film.

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

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.

Enhancement of current collection in epitaxial lift-off InAs/GaAs quantum dot thin film solar cell and concentrated photovoltaic study

We report the fabrication of a thin film InAs/GaAs quantum dot solar cell (QD cell) by applying epitaxial lift-off (ELO) approach to the GaAs substrate. We confirmed significant current collection enhancement (∼0.91 mA/cm2) in the ELO-InAs QD cell within the wavelength range of 700 nm–900 nm when compared to the ELO-GaAs control cell. This is almost six times of the sub-GaAs bandgap current collection (∼0.16 mA/cm2) from the wavelength range of 900 nm and beyond, we also confirmed the ELO induced resonance cavity effect was able to increase the solar cell efficiency by increasing both the short circuit current and open voltage. The electric field intensity of the resonance cavity formed in the ELO film between the Au back reflector and the GaAs front contact layer was analyzed in detail by finite-differential time-domain (FDTD) simulation. We found that the calculated current collection enhancement within the wavelength range of 700 nm–900 nm was strongly influenced by the size and shape of InAs QD. In addition, we performed concentrated light photovoltaic study and analyzed the effect of intermediate states on the open voltage under varied concentrated light intensity for the ELO-InAs QD cell.

Deep junction III-V solar cells with enhanced performance

The influence of junction depth in III–V solar cell structures was investigated for GaAs and InGaP cells. Typical III–V solar cells employ a shallow junction design. We have shown that for both investigated cell types, a deep junction close to the back of the cell structure performs better than shallow junction cells. At the maximum power point the deep junction cells operate mainly in the radiative recombination regime, while in the shallow junction cells non-radiative recombination is dominant. The steeper slope of the IV curve boosts the fill-factor by 3–4%, which is thereby the most improved cell parameter. In order to minimize collection losses in the upper part of the solar cell, the optimal thickness of the GaAs deep junction cell is only two-thirds of a shallow junction cell. The associated lower cell current is more than compensated by the higher fill-factor and open circuit voltage. The best deep junction GaAs cell shows a record efficiency of 26.5% for a GaAs cell on substrate. In the thinner InGaP deep junction cell the absence of current loss, leads to 1.6% higher efficiency than for the shallow junction cell.

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

The epitaxial lift-off technique has been applied to dual-junction III–V solar cells grown in inverted order (subcell with highest band gap is grown first). It is shown that growing in inverse order is not trivial since both the tunnel junction and the InGaP subcell perform differently.

Steps to minimize the characterization errors in steady state IV curve measurements taken with C.O.T.S. equipment

The continuing search for clean energy solutions is driving a huge increase in the number of laboratories, non-profit and commercial, working on solar cell development. These laboratories need to characterize their solar cells in order to steer their development process. The most popular method is the measurement of the current voltage curve, better known as the IV curve. IV curves can be measured by Commercial Off The Shelf (COTS) equipment which typically consists of a steady state solar simulator, a probe station, an electronic load, a reference cell, and software driven data-acquisition and analysis means.

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.

Thin-Film III–V Solar Cells Using Epitaxial Lift-Off

Epitaxial lift-off is used to create thin-film III–V solar cells without sacrificing the GaAs wafer. It is based on selective etching of an AlAs release layer between the wafer and the cell structure using an HF solution. The wafer can be reused for subsequent deposition runs thereby reducing the cost of the cells. The thin-film cell can be transferred to any new carrier, e.g. glass, plastic, silicon, or metal foil. Although epitaxial lift-off was first demonstrated in 1978, it took until the 1990s to make significant progress in understanding the process and devising new ways to increase the etch rate. The first single-junction epitaxial lift-off cells were made in 1996. Thin-film cells offer new cell applications based on their flexibility, low weight, and possibility to deposit the cell structure in reverse order. Today the world record for single-junction cells is held by a thin-film GaAs cell, who’s performance is partly based on the increased photonrecycling factor in cells with a back contact acting as a mirror. Also state of the art tandem and inverted metamorphic thin-film cells have been demonstrated.