Method for etching multi-layer epitaxial material
A single-step wet etch process is provided to isolate multijunction solar cells on semiconductor substrates, wherein the wet etch chemistry removes semiconductor materials nonselectively without a major difference in etch rate between different heteroepitaxial layers. The solar cells thus formed comprise multiple heterogeneous semiconductor layers epitaxially grown on the semiconductor substrate.
1. A method comprising the steps of:
providing a wafer comprising a substrate and three or more subcells of a multijunction solar cell overlying the substrate, wherein,
the substrate comprises a gallium arsenide-containing material or a germanium-containing material;
at least one of the three or more subcells comprises GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, or GaNAsSbBi;
each of the three or more subcell comprises a back surface field, a base, a depletion region, an emitter and a front surface field; and
an interconnection region overlying each of the three or more subcells;
patterning the wafer with a mesa etch pattern using photolithography; and
etching in exposed areas the three or more subcells and part or all of the substrate according to the mesa etch pattern using a single nonselective etchant mixture comprising hydrochloric acid, iodic acid, and water, for yielding mesa isolation of individual multijunction solar cells characterized by a macroscopically smooth substantially inwardly curved sidewall profile,
wherein the etchant mixture comprises a molar ratio of 0.95 moles to 1.05 moles iodic acid:59 moles to 65 moles hydrochloric acid:760 moles water.
2. The method of claim 1 ,
wherein the etchant mixture has a temperature 10° C. to 140° C.
3. The method of claim 1 , wherein the etchant mixture has a temperature of 30° C. to 45° C.
4. The method of claim 1 , wherein patterning comprises using a photoresist, using a dielectric hard mask, or using both a photoresist and a dielectric hard mask.
5. The method of claim 1 , wherein
patterning comprises using a photoresist to pattern a dielectric hard mask, and
the photoresist is retained during removal of the heteroepitaxial layers during mesa etching.
6. The method of claim 1 , wherein etching comprises agitating the wafer.
7. The method of claim 1 , wherein the three or more subcells comprises:
a multijunction photovoltaic cell comprising a first subcell overlying the substrate; and
a second subcell overlying the first subcell;
wherein the first subcell comprises GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, or GaNAsSbBi.
8. The method of claim 1 , wherein each of the three or more subcells is lattice matched to the substrate.
9. The method of claim 1 , wherein,
the substrate comprises Si, Ge, SiGe, or GaAs; and
each of the three or more subcells is lattice matched to the substrate.
10. A multijunction solar cell fabricated using the method of claim 1 .