Strain tunable light emitting diodes with germanium P-I-N heterojunctions

1. A vertical p-i-n diode comprising:

a mechanically stretchable substrate;

a vertical p-i-n heterojunction structure affixed to the mechanically stretchable substrate, the p-i-n heterojunction structure comprising a layer of n-type doped Ge, a layer of p-type doped Ge, and a layer of intrinsic Ge disposed between the n-type doped and p-type doped layers of Ge, wherein the combined thickness of the layer of n-type doped Ge, the layer of intrinsic Ge, and the layer of p-type doped Ge is no greater than about 100 nm;

a first electrically conductive contact in electrical communication with the layer of n-type doped Ge;

a second electrically conductive contact in electrical communication with the layer of p-type doped Ge; and

a stretching actuator configured to stretch the mechanically stretchable substrate and the affixed vertical p-i-n heterojunction structure to induce a biaxial tensile strain in the Ge.

2. The diode of claim 1 , wherein the induced biaxial tensile strain is sufficiently large to transform the Ge into a direct-bandgap semiconductor material.

3. The diode of claim 1 , wherein the mechanically stretchable substrate comprises a polymeric elastomer.

4. The diode of claim 1 , wherein the mechanically stretchable substrate has a thickness in the range from about 20 to about 1000 μm.

5. The diode of claim 1 , wherein the stretching actuator comprises:

a cell that defines a reversibly expandable cavity; and

a fluid source in communication with the cavity and configured to introduce a fluid into the cavity under sufficient pressure to expand the cavity;

wherein the p-i-n heterojunction structure is configured such that it undergoes a biaxial stretch as the cavity expands.

6. The diode of claim 5 , wherein at least a portion of the cell that defines the cavity is comprised of the mechanically stretchable substrate.

7. The diode of claim 5 , wherein at least a portion of the cell that defines the cavity is comprised of an elastic material that is configured to stretch as the cavity expands and further wherein the mechanically stretchable substrate is disposed on the elastic material.

8. A method of inducing the emission of radiation from a vertical p-i-n diode, the diode comprising:

a mechanically stretchable substrate;

a vertical p-i-n heterojunction structure affixed to the mechanically stretchable substrate, the vertical p-i-n heterojunction structure comprising a layer of n-type doped Ge, a layer of p-type doped Ge, and a layer of intrinsic Ge disposed between the n-type doped and p-type doped layers of Ge wherein the combined thickness of the layer of n-type doped Ge, the layer of intrinsic Ge, and the layer of p-type doped Ge is no greater than about 100 nm;

a first electrically conductive contact in electrical communication with the layer of n-type doped Ge;

a second electrically conductive contact in electrical communication with the layer of p-type doped Ge; and

a stretching actuator configured to mechanically stretch the mechanically stretchable substrate and the affixed vertical p-i-n heterojunction structure, the method comprising:

stretching the mechanically stretchable substrate and the affixed vertical p-i-n heterojunction structure using the stretching actuator to induce a biaxial tensile strain in the Ge; and

applying a voltage across the tensilely strained vertical p-i-n heterojunction structure, wherein radiation is emitted via electroluminescence.

9. The method of claim 8 , wherein the induced biaxial tensile strain is sufficiently large to transform the Ge into a direct-bandgap semiconductor material.

10. The method of claim 8 , further comprising changing the wavelength range of the emitted radiation by changing the degree of biaxial tensile strain in the Ge.

11. The method of claim 8 , wherein the radiation is emitted in the wavelength range from about 2.1 to 2.5 μm.

12. The method of claim 8 , wherein the radiation is emitted in the wavelength range from about 1.5 to 2.1 μm.