Abstract
Porosification of nitride semiconductors provides a new paradigm for advanced engineering of the properties of optoelectronic materials. Electrochemical etching creates porosity in doped layers while leaving undoped layers undamaged, allowing the realization of complex three-dimensional porous nanostructures, potentially offering a wide range of functionalities, such as in-distributed Bragg reflectors. Porous/non-porous multilayers can be formed by etching the whole, as-grown wafers uniformly in one simple process, without any additional processing steps. The etch penetrates from the top down through the undoped layers, leaving them almost untouched. Here, atomic-resolution electron microscopy is used to show that the etchant accesses the doped layers via nanometer-scale channels that form at dislocation cores and transport the etchant and etch products to and from the doped layer, respectively. Results on AlGaN and non-polar GaN multilayers indicate that the same mechanism is operating, suggesting that this approach may be applicable in a range of materials.
Highlights
An alternative method of engineering the materials properties of nitride semiconductors has emerged: by introducing porosity into monolithic nitride layers, it is possible to achieve very significant changes in a wide range of properties including electrical6 and thermal7 conductivity, refractive index,8 birefringence,9 mechanical properties,10 strain relief,11 chemical activity,12 and piezo-electricity.13 Porous GaN layers are already enabling enhanced device performance in LEDs14,15 and laser diodes (LDs)16 and the development of novel composites of GaN with other materials.17 The porosification of other nitride alloys has allowed the formation of novel quantum structures.18 Introducing porosity to nitride materials provides a new degree of freedom, allowing hitherto unanticipated physical phenomena and device concepts to be explored
For porous GaN to be integrated into existing device processing paradigms, it would be preferable to develop a simple process that allows uniform porosification of sub-surface layers across whole industrial-size wafers, without the requirement for surface coating and trench etching
We showed that etching can occur through an undoped surface GaN layer and through the multiple undoped layers in the distributed Bragg reflector (DBR) stack, porosifying the doped layers and leaving the undoped GaN almost unaltered
Summary
An alternative method of engineering the materials properties of nitride semiconductors has emerged: by introducing porosity into monolithic nitride layers, it is possible to achieve very significant changes in a wide range of properties including electrical6 and thermal7 conductivity, refractive index,8 birefringence,9 mechanical properties,10 strain relief,11 chemical activity,12 and piezo-electricity.13 Porous GaN layers are already enabling enhanced device performance in LEDs14,15 and laser diodes (LDs)16 and the development of novel composites of GaN with other materials.17 The porosification of other nitride alloys has allowed the formation of novel quantum structures.18 Introducing porosity to nitride materials provides a new degree of freedom, allowing hitherto unanticipated physical phenomena and device concepts to be explored. Such a structure can be formed by growing a simple (nonporous) GaN structure, consisting of alternating layers of undoped GaN (which will remain non-porous) and highly n-doped GaN (which will be porosified).
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