Pyramidal photonic crystal light emitting device
A light-emitting device (LED) is described which exhibits high extraction efficiency and an emission profile which is substantially more directional than from a Lambertian source. The device comprises a light generating layer disposed between first and second layers of semiconductor material, each having a different type of doping. An upper surface of the first layer has a tiling arrangement of pyramidal or frustro-pyramidal protrusions of semiconductor material surrounded by a material of different refractive index which together comprise a photonic band structure. The protrusions and their tiling arrangement are configured for efficient extraction of light from the device via the upper surface of the first layer and in a beam that is substantially more directional than from a Lambertian source An enhanced device employs a reflector beneath the second layer to utilize the microcavity effect. A method for fabricating the device is also described which employs anisotropic wet etching to produce the pyramidal or frustro-pyramidal protrusions
1. A method for fabricating a light-emitting device (LED) comprising the steps of:
fabricating a light-emitting device heterostructure, the device heterostructure comprising a first layer comprising a first semiconductor material having a first type of doping, a second layer comprising a second semiconductor material having a second type of doping, and a light-generating layer disposed between the first and second layers, wherein the first layer has an upper surface distal the light-generating layer and a lower surface proximate the light-generating layer and wherein light generated in the light-generating layer emerges from the LED structure through the upper surface of the first layer,
wherein the light-emitting device heterostructure is fabricated using a flip-chip process, the flip-chip process including:
forming the first layer on a substrate;
forming the subsequent layers upon the first layer; and,
removing the substrate to expose the first layer,
forming an etch mask on the exposed first layer, the mask comprising islands of mask material at locations corresponding to a predetermined tiling arrangement, wherein the step of forming the mask comprises the steps of:
depositing a layer of photoresist over the first layer;
patterning the photoresist by exposure according to the predetermined tiling arrangement; and,
removing unexposed photoresist to leave islands of photoresist at locations corresponding to the predetermined tiling arrangement;
forming pyramidal or frustro-pyramidal projections of the first semiconductor material in the first layer at locations beneath the islands of mask material by anisotropically wet etching the first semiconductor material to a predetermined depth along predetermined crystal planes; and,
removing the islands of mask material to leave behind the predetermined tiling arrangement of pyramidal or frustro-pyramidal projections, which in combination with a surrounding material of different refractive index comprise a photonic band structure.
2. A method according to claim 1 , wherein the step of forming an etch mask further comprises the steps of:
depositing a layer of hard mask material over the first layer before the step of depositing the layer of photoresist;
removing hard mask material after the step of removing photoresist to leave islands of hard mask material beneath the islands of photoresist; and,
removing the remaining islands of photoresist to leave behind the etch mask comprising the islands of hard mask material at locations corresponding to the predetermined tiling arrangement.
3. A method according to claim 1 , wherein the first layer of the light-emitting device heterostructure comprises a layer of etch stop material embedded at the predetermined depth in the first semiconductor material.
4. A method according to claim 3 , wherein the predetermined depth corresponds to a desired thickness for the light-emitting device.
5. A method according to claim 3 , wherein the predetermined depth corresponds to a desired shape of inverted frustro-pyramidal indentation.
6. A method according to claim 1 , wherein the predetermined tiling arrangement comprises a quasicrystal tiling pattern and wherein the pitch of the tiling arrangement is greater than or equal to 1.5 μm.