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 for making a solar cell device comprising the steps of:
providing a wafer comprising multijunction photovoltaic cells, wherein the multijunction photovoltaic cells comprise:
a substrate;
a first subcell overlying the substrate; and
a second subcell overlying the first subcell;
patterning the wafer with a mesa etch pattern using photolithography; and
etching in exposed areas the multijunction photovoltaic cells according to the mesa etch pattern using a single etchant mixture comprising hydrochloric acid, iodic acid, and water, for yielding mesa isolation of individual multijunction photovoltaic cells comprising mesa sidewalls, wherein,
the mesa sidewalls comprise the substrate, the first subcell, and the second subcell;
the mesa sidewalls are characterized by a macroscopically smooth surface without significant undercutting that substantially continuously widens toward and along the substrate; and
the sidewalls comprise traces of iodine.
2. The method of claim 1 , wherein the etchant mixture comprises:
a volumetric ratio of hydrochloric acid of 10% to 50%; and
a volumetric ratio of iodic acid of 10% to 50%;
wherein the mixture has a temperature 10° C. to 140° C.
3. The method of claim 1 , wherein the etchant mixture comprises:
a volumetric ratio of hydrochloric acid of 30% to 35%; and
a volumetric ratio of iodic acid of 14% to 19%;
wherein the 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. A solar cell device comprising:
a multijunction photovoltaic cell comprising a first subcell overlying a substrate; and a second subcell overlying the first subcell; and
mesa sidewalls comprising sidewalls of the substrate, the first subcell, and the second subcell, wherein:
the mesa sidewalls are characterized by a macroscopically smooth surface without significant undercutting that substantially continuously widens toward and along the substrate; and
wherein the sidewalls comprise traces of iodine.
8. The solar cell device of claim 7 , wherein
at least one of the first subcell and the second subcell comprises a base layer comprising an alloy comprising one or more elements from group III of the periodic table, nitrogen, arsenic, and an element selected from Sb, Bi, and a combination thereof; and
the first subcell and the second subcell are lattice matched.
9. The solar cell device of claim 7 , wherein the first subcell and the second subcell are lattice matched to a material selected from Si, Ge, SiGe, GaAs, and InP.
10. The solar cell device of claim 7 , wherein the first subcell comprises a base layer selected from GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, and GaNAsSbBi.
11. The solar cell device of claim 7 , comprising a gallium arsenide-containing material underlying the first subcell.
12. The solar cell device of claim 7 , comprising a germanium-containing material underlying the first subcell.
13. The solar cell device of claim 7 , wherein the first subcell comprises a germanium-containing material.
14. The method of claim 1 , wherein at least one of the first subcell and the second subcell comprises a base layer comprising an alloy comprising one or more elements from group III of the periodic table, nitrogen, arsenic, and an element selected from Sb, Bi, and a combination thereof; and
the first subcell and the second subcell are lattice matched.
15. The method of claim 1 , wherein the first subcell and the second subcell are lattice matched to a material selected from Si, Ge, SiGe, GaAs, and InP.
16. The method of claim 1 , wherein the first subcell comprises a base layer selected from GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, and GaNAsSbBi.
17. The method of claim 1 , comprising a gallium arsenide-containing material underlying the first subcell.
18. The method of claim 1 , comprising a germanium-containing material underlying the first subcell.
19. The method of claim 1 , wherein the first subcell comprises a germanium-containing material.
20. The solar cell device of claim 7 , wherein the first subcell is characterized by a first bandgap and a second subcell characterized by a second bandgap, wherein the first bandgap is less than the second bandgap and the first bandgap and the second bandgap are selected from a bandgap within a range of 0.67 eV to 2.25 eV.