Formation of Porous Layers by Electrochemical Etching of Germanium and Gallium Arsenide for Cleave Engineered Layer Transfer (CELT) Application in High Efficiency Multi-Junction Solar Cells
- Author(s): Fong, David Michael
- Advisor(s): Goorsky, Mark S
- et al.
This thesis investigates a method to reduce substrate costs of multi-junction solar cells by recycling handle wafers through Cleaved Engineered Layer Transfer (CELT) using porous Ge and GaAs. Multi-junction solar cells based on GaAs and Ge substrates are the most efficient photovoltaic devices with a world record efficiency of 43.5%, more than double that of common terrestrial solar cells. In order for multi-junction solar to expand to terrestrial power generation, significant reduction in device cost must be achieved; substrate costs represent up to 50% of the cell costs making substrate recycling an opportunity for significant cost reductions.
Ge and GaAs wafers are subjected to electrochemical etching with the goal of creating a porous layer that is suitable for epitaxial growth and Cleave Engineered Layer Transfer (CELT) for high efficiency III-V devices. Both n and p type Ge and GaAs were investigated with the doping concentration ranging from 1017-1019 cm-3. Electrolytes at various concentrations were tested including HCl, HF, and H2SO4. In addition, current profile, density, and etch time were changed to optimize porous layer formation. After electrochemical etching, samples were analyzed with a scanning electron microscope (SEM) to determine pore size, morphology, layer thickness, and substrate dissolution rate. Energy dispersive x-ray spectroscopy (EDS) was also used to find the chemical composition of the porous layer, and transmission electron microscopy (TEM) was used to determine crystallography of the porous layer. Finally, the effect of annealing on porous layer surface morphology and composition were investigated.
The results of this work showed that both n-Ge and p-GaAs yield inhomogeneous low-density pores that are not suitable for CELT. Heavily doped p-Ge (1018 cm-3 or greater) yields a uniform pore structure on the nm scale that potentially could be used in a layer transfer process. However, this porous layer includes a significant portion of germanium oxide (< 10 atomic % oxygen). Subsequent annealing in an atmospheric envi4ronment did not remove this oxide, though annealing in a reducing atmosphere such as hydrogen has not yet been investigated.
Etching of p-type GaAs in concentrated HF yielded nm sized porous layer that was completely devoid of any gallium content. This contradicts the claims made by Rojas et al.. who show a similar porous layer using the same etching parameters, but stated it was composed of stoichiometric GaAs without presenting any chemical composition data to support this assertion.
The most promising results were realized with n-GaAs doped between 1-4x1018 cm-3. Uniform pore nucleation is achieved using 5% H2SO4 with a 300 mA/cm2 current pulse for 1.4 s followed by 20 mA/cm2 for 1 hour. This profile creates primarily (111) oriented pores on the order of hundreds of nm in size. In addition, the electrochemical etching leaves a high density of pyramid shaped arsenic oxide structures on the surface, which are then removed by a 20 min dip in 17.5% HCl. After the HCl treatment, EDS showed the porous layer to be comprised of 3% O, 48% Ga, and 49% As by atomic percent, which could be suitable for high quality epitaxial growth to be carried out on the porous surface. Annealing of the GaAs porous layer at 450 °C for over 80 hours covered by a GaAs proximity cap showed minimal surface reorganization as well as a decrease in the layer's As content, which is due to the high vapor pressure of As as compared to Ga.
These results demonstrate that the electrochemical etching of n-GaAs can create a porous layer that could reduce the cost of high efficiency multi-junction solar cells through a CELT process. This was achieved with n+-GaAs etched in sulfuric acid with a short duration high current pulse followed by a long duration low current pulse. Additionally a dip in hydrochloric acid is needed to remove arsenic oxide that forms on the surface. Furthermore this work shows that pores made in p-type GaAs with HF primarily consist of arsenic oxide and are not useable for a layer transfer application.