Semiconducting Light-Semiconducting Light-Emitting DevicesEmitting Devices

James A. JohnsonJames A. Johnson

16 December 200616 December 2006

Semiconducting MaterialsSemiconducting MaterialsDirect SemiconductorsDirect Semiconductors Indirect SemiconductorsIndirect Semiconductors

Conduction band exists directly Conduction band exists directly “above” valence band.“above” valence band.

Electrons and holes may recombine Electrons and holes may recombine without diverting energy to maintaining without diverting energy to maintaining conservation of momentum.conservation of momentum.

Excess energy resulting from Excess energy resulting from recombination is converted to photons.recombination is converted to photons.

Prime example of direct semiconductor Prime example of direct semiconductor material is Gallium Arsenide (GaAs)material is Gallium Arsenide (GaAs)

Conduction band and valence bands are Conduction band and valence bands are offset by some vector k.offset by some vector k.

In order for electrons and holes to In order for electrons and holes to recombine, they must travel across the k-recombine, they must travel across the k-space.space.

Traveling across k-space requires Traveling across k-space requires conservation of energy and momentum.conservation of energy and momentum.

Energy that would have become photons in Energy that would have become photons in direct semiconductor is used for travel.direct semiconductor is used for travel.

Conservation of momentum is attained by Conservation of momentum is attained by production of phonons.production of phonons.

Prime example of indirect semiconductor Prime example of indirect semiconductor material is Silicon (Si)material is Silicon (Si)

Light-Emitting Device StructureLight-Emitting Device Structure

Four layers in the basic Four layers in the basic Light-emitting device Light-emitting device structure: Substrate, n-structure: Substrate, n-type material, active type material, active region, p-type material.region, p-type material.Substrate is typically Substrate is typically constructed from n-type constructed from n-type Si or Sapphire.Si or Sapphire.n-type layer is typically n-type layer is typically a GaAs or GaN based a GaAs or GaN based alloy.alloy.Active region is typically Active region is typically p-type GaAs or GaN. p-type GaAs or GaN.p-type layer is typically p-type layer is typically a pa p++-type GaAs or GaN -type GaAs or GaN based alloy.based alloy.

Common gallium based alloys include:Common gallium based alloys include: Indium Gallium Arsenide - InGaAsIndium Gallium Arsenide - InGaAs Aluminum Gallium Arsenide - AlGaAsAluminum Gallium Arsenide - AlGaAs Indium Gallium Nitride - InGaNIndium Gallium Nitride - InGaN Aluminum Gallium Nitride - AlGaNAluminum Gallium Nitride - AlGaN

Basic Light-Emitting Device TypesBasic Light-Emitting Device Types

Edge Light-Emitting Edge Light-Emitting DeviceDevice

Edge emitting device structure lends Edge emitting device structure lends itself to use current fabrication itself to use current fabrication technologies with little modification.technologies with little modification.

Double heterojunction structure Double heterojunction structure provides greatest efficiency by creating provides greatest efficiency by creating waveguides for new photons.waveguides for new photons.

Laser diodes have highly polished Laser diodes have highly polished surfaces for focused light emission and surfaces for focused light emission and roughened surfaces to minimize roughened surfaces to minimize reflection.reflection.

Basic Light-Emitting Device TypesBasic Light-Emitting Device TypesImproving Efficiency in the Edge Light-Improving Efficiency in the Edge Light-

Emitting DeviceEmitting Device

Homojunction – Low internal reflection. Homojunction – Low internal reflection. Photon energy is absorbed and lost in Photon energy is absorbed and lost in the surrounding semiconductors.the surrounding semiconductors.

Single Heterojunction – Improves Single Heterojunction – Improves efficiency of light emission. Energy is efficiency of light emission. Energy is still lost in the homogenous still lost in the homogenous semiconductor layer.semiconductor layer.

Double Heterojunction – Greatest Double Heterojunction – Greatest efficiency. Both semiconductor layers efficiency. Both semiconductor layers reflect photons and guide them to the reflect photons and guide them to the edge of the structure.edge of the structure.

Basic Light-Emitting Device TypesBasic Light-Emitting Device Types

Surface Light-Emitting Surface Light-Emitting DeviceDevice

Surface emitting device is fabricated using Surface emitting device is fabricated using current technologies, but process is more current technologies, but process is more complicated as all layers do not have complicated as all layers do not have uniform areas.uniform areas.

Multiple wavelengths may be constructed in Multiple wavelengths may be constructed in the same device.the same device.

Light with longer wavelengths are Light with longer wavelengths are constructed deeper in the device.constructed deeper in the device.

All electrical contacts are placed on the All electrical contacts are placed on the same side; rather than “sandwiching” the same side; rather than “sandwiching” the device as observed in the edge emitter. device as observed in the edge emitter.

Allows better control of directionality of Allows better control of directionality of emitted light.emitted light.

Challenges with Light-Emitting Challenges with Light-Emitting DevicesDevices

Edge Emitting Edge Emitting DevicesDevices

It is difficult to construct It is difficult to construct 2-dimensional arrays of 2-dimensional arrays of edge-emitting devices.edge-emitting devices.

Packaging must be used Packaging must be used to improve directionality to improve directionality of light emission.of light emission.

Surface Emitting Surface Emitting DevicesDevices

Fabrication requires Fabrication requires etching of portions of etching of portions of layers to place electrical layers to place electrical contacts.contacts.

Due to placement of the Due to placement of the electrical contacts, it is electrical contacts, it is difficult to guarantee difficult to guarantee uniform current in each uniform current in each layer of the device.layer of the device.

Light-Emitting Device Applications Light-Emitting Device Applications and Packagingand Packaging

Packaging depends greatly upon Packaging depends greatly upon the application of the device.the application of the device.

Traditional through-hole and Traditional through-hole and SMD components use reflective SMD components use reflective cups to deflect emitted light.cups to deflect emitted light.

Devices are being redesigned to Devices are being redesigned to accommodate more efficient accommodate more efficient packaging such as epitaxial lift-packaging such as epitaxial lift-off.off.

Surface emitting devices are Surface emitting devices are being designed to have lenses being designed to have lenses fabricated on the emitter area to fabricated on the emitter area to focus or disperse light and to focus or disperse light and to interface with fiber optic cables.interface with fiber optic cables.

A Sample of ApplicationsA Sample of Applications PCB ComponentsPCB Components Traffic LightsTraffic Lights Motor Vehicle LightsMotor Vehicle Lights Railroad Crossing BarsRailroad Crossing Bars FlashlightsFlashlights Remote ControlsRemote Controls Christmas DecorationsChristmas Decorations Fiber Optic TransmittersFiber Optic Transmitters Laser PointersLaser Pointers

Emerging Light-Emitting DevicesEmerging Light-Emitting DevicesLED’s using quantum dots.LED’s using quantum dots.

This LED combines the edge emitting This LED combines the edge emitting structure with the efficiency of surface structure with the efficiency of surface emitting devices.emitting devices.

A layer of CdSe/ZnS core-shell A layer of CdSe/ZnS core-shell nanocrystal quantum dots (NQD’s) is nanocrystal quantum dots (NQD’s) is placed on top of the light-emitting placed on top of the light-emitting device structure.device structure.

NQD’s have exceptional luminescent NQD’s have exceptional luminescent properties, but are difficult to energize.properties, but are difficult to energize.

Wavelength may be tuned at time of Wavelength may be tuned at time of fabrication based on the size of the fabrication based on the size of the NQD’s.NQD’s.

NQD’s are energized when NQD’s are energized when recombination energy is transferred to recombination energy is transferred to the quantum dots through a process the quantum dots through a process called Förster Energy Transfer.called Förster Energy Transfer.

This offers a practical solution to the This offers a practical solution to the problem of general lighting sources problem of general lighting sources suitable to illuminate objects from large suitable to illuminate objects from large cities to optical microscopes due to cities to optical microscopes due to efficiency, stability, expected longevity, efficiency, stability, expected longevity, and potential ease of fabrication.and potential ease of fabrication.