Phosphor-converted light-emitting diode with dielectric spacer
A light-emitting device includes: a semiconductor diode structure, a reflector, a wavelength-converting layer, and an intermediate spacer between the diode structure and the wavelength-converting layer. The diode structure emits diode output light at a vacuum wavelength λ 0 to propagate within the diode structure. The reflector is on the back diode structure and internally reflects diode internally incident output light. The wavelength-converting layer is positioned with its back surface facing and spaced-apart from a front surface of the diode structure, and absorbs diode output light at λ 0 and emits down-converted light at a vacuum wavelength λ 1 >λ 0 , which exits the wavelength-converting layer through its front surface. The intermediate spacer includes one or more transparent dielectric layers and has an effective refractive index less than that of the wavelength-converting layer, so that the wavelength-converting layer acts as an optical waveguide supporting one or more laterally propagating optical modes at λ 0 and λ 1 .
1. A semiconductor light-emitting device comprising:
a semiconductor diode structure including first and second doped semiconductor layers and an active layer between a back surface of the first semiconductor layer and a front surface of the second semiconductor layer, the active layer being arranged for emitting diode output light, resulting from electrical current flow through the diode structure, at a nominal emission vacuum wavelength λ 0 to propagate within the diode structure;
a reflector positioned against a back surface of the second semiconductor layer that internally reflects diode output light incident thereon from within the diode structure;
a wavelength-converting layer positioned with a back surface thereof facing and spaced-apart from a front surface of the first semiconductor layer, the wavelength-converting layer absorbing diode output light at the vacuum wavelength λ 0 and in response emitting down-converted light at a vacuum wavelength λ 1 that is greater than λ 0 , at least a portion of the down-converted light exiting the wavelength-converting layer through a front surface thereof; and
an intermediate spacer positioned between the front surface of the first semiconductor layer and the back surface of the wavelength-converting layer and comprising one or more dielectric layers, each dielectric layer of the intermediate spacer being at least partly transparent at the vacuum wavelength λ 0 , the intermediate spacer having an effective refractive index n S less than an effective refractive index n C of the wavelength-converting layer so that the wavelength-converting layer acts as an optical waveguide supporting one or more laterally propagating optical modes at the vacuum wavelengths λ 0 and λ 1 .
2. The device of claim 1 wherein (i) single-pass absorption exhibited at the vacuum wavelength λ 0 by the wavelength-converting layer is less than about 50% or (ii) the wavelength-converting layer is less than about 5 μm thick.
3. The device of claim 1 , the wavelength-converting layer being arranged so as to support, at each of the vacuum wavelengths λ 0 and λ 1 , at most ten laterally propagating optical modes.
4. The device of claim 1 , the wavelength-converting layer including a multitude of quantum dots arranged in a layer or matrix structure or dispersed in a transparent solid medium, the quantum dots absorbing diode output light at the vacuum wavelength λ 0 and emitting down-converted light at the vacuum wavelength λ 1 .
5. The device of claim 1 , separation between the wavelength-converting layer and the diode structure being sufficiently large so as to prevent substantial spatial overlap of the one or more laterally propagating modes with the diode structure.
6. The device of claim 1 , the intermediate spacer including multiple layers of one or more dielectric materials arranged so as to exhibit (i) reflectance at the vacuum wavelength λ 1 greater than reflectance at the vacuum wavelength λ 0 or (ii) transmittance at the vacuum wavelength λ 0 greater than transmittance at the vacuum wavelength λ 1 .
7. The device of claim 1 , the intermediate spacer comprising only a single layer of dielectric material that is transparent at the vacuum wavelength λ 0 with a refractive index less than that of the wavelength-converting layer.
8. The device of claim 1 , further comprising a metal layer within the intermediate spacer or between the intermediate spacer and the wavelength-converting layer.
9. The device of claim 1 , further comprising a front array of micro- or nano-structured scattering elements at the front surface of the wavelength-converting layer, the scattering elements of the front array being arranged collectively so as to redirect at least a portion of the down-converted light propagating in or through the wavelength-converting layer to exit through the front surface of the wavelength-converting layer.
10. The device of claim 1 , further comprising a back array of micro- or nano-structured scattering elements at the back surface of the wavelength-converting layer, the scattering elements of the back array being arranged collectively so as to optically couple at least a portion of the diode output light propagating in or through the intermediate spacer into one or more of the laterally propagating optical modes.
11. The device of claim 1 , the reflector including a reflector array of micro- or nano-structured scattering elements arranged (i) as one or more of a MLR structure, a micro- or nano-structured diffractive MLR structure, a distributed Bragg reflector (DBR), a quasi-guided-mode (QGM) structure, or a surface-lattice-resonance (SLR) structure, or (ii) so that the reflector exhibits non-specular reflective redirection of diode output light incident thereon from within the diode structure.
12. A method for making the light-emitting device of claim 1 , the method comprising: (A) forming the first semiconductor layer, the active layer, and the second semiconductor layer; (B) forming the reflector on the back surface of the second semiconductor layer; (C) forming the intermediate spacer on the front surface of the first semiconductor layer; and (D) forming the wavelength-converting layer on the intermediate spacer.
13. The device of claim 1 , further comprising an output optical structure positioned on a front surface of the wavelength-converting layer, the output optical structure having an effective refractive index less than a refractive index of the wavelength-converting layer.
14. The device of claim 13 , the effective refractive index of the output optical structure being about equal to the effective refractive index of the intermediate spacer.
15. The device of claim 13 , the output optical structure including multiple layers of one or more dielectric materials arranged so as to exhibit (i) reflectance at the vacuum wavelength 2 greater than reflectance at the vacuum wavelength λ 1 or (ii) transmittance at the vacuum wavelength λ 1 greater than transmittance at the vacuum wavelength λ 0 .
16. The device of claim 13 , the output optical structure comprising only a single layer of dielectric material that is transparent at the vacuum wavelength λ 1 with a refractive index less than that of the wavelength-converting layer.
17. The device of claim 13 , further comprising a metal layer within the output optical structure or on a front surface of the wavelength-converting layer.
18. An array of multiple light-emitting devices of claim 1 .
19. The array of claim 18 , spacing of the light-emitting devices of the array being less than about 0.10 mm, the array exhibiting a contrast ratio for down-converted light exiting from adjacent devices that is greater than 5:1.
20. A method for operating the array of claim 18 , the method comprising supplying to one or more of the light-emitting devices of the array corresponding electrical currents so that those light-emitting devices emit down-converted light from the front surfaces of the corresponding wavelength-converting layers.