Epitaxial lift off in inverted metamorphic multijunction solar cells

1. A method for manufacturing a solar cell using a process for selectively freeing an epitaxial layer from a single crystal substrate upon which it was grown, comprising:

providing a first substrate of gallium arsenide;

depositing a separation layer on said first substrate;

depositing on the separation layer a sequence of epitaxial layers of semiconductor material forming a solar cell including: forming over the separation layer an upper first solar subcell having a first band gap and including a semiconductor contact layer; forming over said first solar subcell a middle second solar subcell having a second band gap smaller than said first band gap; forming a graded interlayer over said second solar subcell; and forming over said graded interlayer a lower third solar subcell having a fourth band gap smaller than said second band gap such that said third subcell is lattice mismatched with respect to said second subcell;

mounting the first substrate and the sequence of epitaxial layers of semiconductor material on a flexible film;

mechanically holding said first substrate against a bottom support of an etching apparatus;

attaching a connecting link element that connects at least two opposed points on the periphery of the flexible film to an upper portion of the etching apparatus;

introducing said solar cell having said flexible film bonded thereon into an etchant environment in said etching apparatus;

etching said separation layer while applying mechanical tension to said connecting link element so as to remove said flexible film with said epitaxial layers from said first substrate;

attaching the flexible film directly to a surrogate substrate by an electrostatic technique;

depositing a contact metal layer over the semiconductor contact layer;

lithographically processing the contact metal layer to form grid lines;

attaching a cover glass over the grid lines; and

removing the surrogate substrate.

2. A method as defined in claim 1 , wherein the etchant environment is a liquid solution.

3. A method as defined in claim 1 , wherein the etchant environment is an atmosphere containing an etchant aerosol.

4. A method as defined in claim 1 , wherein the surrogate substrate is composed of GaAs, Ge, or Si.

5. A method as defined in claim 1 , wherein the etchant is hydrofluoric acid.

6. A method as defined in claim 1 , wherein the separation layer has a thickness of approximately 50 Angstroms.

7. A method as defined in claim 1 , wherein the electrostatic technique utilizes an electrostatic chuck.

8. A method as defined in claim 1 , wherein the mechanical tension applied to said connecting link element allows said epitaxial layers and the surrogate substrate to curl upward, permitting the outdiffusion of reaction products of the etching process.

9. A method as defined in claim 1 , wherein the upper subcell is composed of InGa(Al)P.

10. A method as defined in claim 1 , wherein the middle subcell is composed of an GaAs, GaInP, GaInAs, GaAsSb, or GaInAsN emitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN base region.

11. A method as defined in claim 1 , wherein the lower solar subcell is composed of an InGaAs base and emitter layer, or a InGaAs base layer and a InGaP emitter layer.

12. A method as defined in claim 1 , wherein the graded interlayer is compositionally graded to lattice match the middle subcell on one side and the lower subcell on the other side, and is composed of (In x Ga 1-x ) y Al 1-y As with x and y selected such that the band gap of the interlayer remains constant throughout its thickness and greater than said second band gap.

13. A method as defined in claim 12 , wherein the graded interlayer has approximately a 1.5 eV band gap throughout its thickness.

14. A method as defined in claim 1 , wherein the graded interlayer is composed of any of the As, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater or equal to that of the second solar cell and less than or equal to that of the third solar cell, and having a band gap energy greater than that of the second solar cell.

15. A method for manufacturing a solar cell using a process for selectively freeing an epitaxial layer from a single crystal substrate upon which it was grown, comprising:

providing a first substrate of gallium arsenide;

depositing a separation layer having a thickness of approximately 50 Angstroms on said first substrate;

depositing on the separation layer a sequence of epitaxial layers of semiconductor material forming a solar cell including:

forming over the separation layer an upper first solar subcell composed of InGa(Al)P and having a first band gap and including a semiconductor contact layer;

forming over said first solar subcell a middle second solar subcell composed of an GaAs, GaInP, GaInAs, GaAsSb, or GaInAsN emitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN base region and having a second band gap smaller than said first band gap;

forming a graded interlayer over said second solar subcell, wherein the graded interlayer is compositionally graded to lattice match the middle subcell on one side and the lower subcell on the other side, and is composed of (In x Ga 1-x ) y Al 1-y As with x and y selected such that the band gap of the interlayer remains constant at approximately 1.5 eV throughout its thickness and greater than said second band gap; and

forming over said graded interlayer a lower third solar subcell composed of an InGaAs base and emitter layer, or a InGaAs base layer and a InGaP emitter layer, and having a fourth band gap smaller than said second band gap such that said third subcell is lattice mismatched with respect to said second subcell;

mounting the first substrate and the sequence of epitaxial layers of semiconductor material on a flexible film;

mechanically holding said first substrate against a bottom support of an etching apparatus;

attaching a connecting link element that connects at least two opposed points on the periphery of the flexible film to an upper portion of the etching apparatus;

introducing said solar cell having said flexible film bonded thereon into an etchant environment with a liquid solution of hydrofluoric acid in said etching apparatus;

etching said separation layer while applying mechanical tension to said connecting link element so as to remove said flexible film with said epitaxial layers from said first substrate, wherein the mechanical tension applied to said connecting link element allows said epitaxial layers and the surrogate substrate to curl upward, permitting the outdiffusion of reaction products of the etching process;

attaching the flexible film directly to a surrogate substrate composed of GaAs, Ge, or Si by an electrostatic technique utilizing an electrostatic chuck;

depositing a contact metal layer over the semiconductor contact layer;

lithographically processing the contact metal layer to form grid lines;

attaching a cover glass over the grid lines; and

removing the surrogate substrate.

16. A method as defined in claim 15 , wherein the etchant environment contains an etchant aerosol of the liquid hydrofluoric acid solution.

17. A method for manufacturing a solar cell using a process for selectively freeing an epitaxial layer from a semiconductor substrate upon which it was grown, comprising:

providing a first semiconductor substrate;

depositing a separation layer on said first substrate;

depositing a sequence of epitaxial layers of semiconductor material on the separation layer to form a multi junction solar cell;

mounting the first substrate and the sequence of epitaxial layers of semiconductor material on a flexible film;

mechanically holding said first substrate against a bottom support of an etching apparatus;

attaching a connecting link element that connects at least two opposed points on the periphery of the flexible film to an upper portion of the etching apparatus;

introducing said solar cell having said flexible film bonded thereon into an etchant environment in said etching apparatus;

etching said separation layer while applying mechanical tension to said connecting link element so as to remove said flexible film with said epitaxial layers from said first substrate;

attaching the flexible film directly to a surrogate substrate by an electrostatic technique; and

removing the surrogate substrate.

18. A method as defined in claim 17 , wherein the etchant is hydrofluoric acid.

19. A method as defined in claim 17 , wherein the electrostatic technique utilizes an electrostatic chuck.

20. A method as defined in claim 17 , wherein the mechanical tension applied to said connecting link element allows said epitaxial layers and the surrogate substrate to curl upward, permitting the outdiffusion of reaction products of the etching process.