Understanding mesoporous nitride semiconductors using electron microscopy

Session

EMAG - Microscopy of Semiconductors

Authors

Fabien Massabuau (2, 4), Peter Griffin (2), Helen Springbett (2), Giorgio Divitini (2), Yingjun Liu (2, 3), R. Vasant Kumar (2), Maruf Sarkar (2), Francesca Adams (2), Sidra Abbas (2), Saptarsi Ghosh (2), Jordan Penn (2), Chaowang Liu (1), Hasan Hirshy (1), Menno Kappers (2), Gunnar Kusch (2), Tongtong Zhu (2, 3), Rachel Oliver (2)

Affiliations

1. IQE Ltd
2. University of Cambridge
3. Porotech
4. University of Strathclyde

Keywords

porous GaN

transmission electron microscopy

scanning electron microscopy

back scattered electrons

dislocations

distributed Bragg reflectors

Abstract text

Porosification of nitride semiconductors provides a new route to engineer previously unrealisable properties in these key optoelectronic materials. Electrochemical etching creates porosity in doped layers whilst leaving undoped layers untouched, allowing the realisation of complex three-dimensional porous nanostructures. A plethora of device sub-structures may thus be fabricated ranging from distributed Bragg reflectors (DBRs) to strain management layers used to enable long-wavelength nitride micoLEDs. In one particularly exciting variant of the prosification workflow, porous/non-porous multilayers are formed by etching whole, as-grown wafers uniformly in one simple process, without any additional processing steps. The etch penetrates from the top down, accessing doped layers through undoped layers, without substantially altering the structure of the undoped layers and leaving the top surface pristine and ready for overgrowth of further epitaxial structures.

The mechanism of this unusual process has been uncovered using high resolution scanning transmission electron microscopy (STEM), which showed that the etchant accesses the doped layers via nanometre-scale channels that form at dislocation cores and transport the etchant and etch products to and from the doped layer respectively.  Initial STEM studies provided access to a single porous layer near the surface of a DBR, but thereafter a method for sequential plan-view STEM was developed which allowed measurements to be taken at various depths through the stack. This was achieved by performing an iterative series of front-side ion milling steps followed by STEM imaging. It was thus possible to examine how a porous sample’s structure varied with depth up to several microns below the surface, with no degradation of the sample or imaging conditions throughout the experiment. The same workflow could be applied to a variety of other complex micron-scale systems which are by nature too thick for standard TEM analysis.

An alternative approach to understanding the three-dimensional structure of porous layers exploits focussed ion beam milling and serial block-face scanning electron microscopy imaging (SBI). This allows the internal pore morphology to be viewed in a reconstruction of any 3D plane, and has been found to be particularly useful in accessing the alignment of pores where this occurs on a length scales of a few to tens or hundreds of microns.   It is vital to understand alignment on these length scales since it influences the device-relevant optical properties of the porous nitride material.  However, both the STEM and SBI approaches described here are time-consuming, and would be difficult to integrate into an industrial setting where regular checking of the sub-surface porous microstructure in a non-destructive workflow may be required.  To solve this problem, we introduce high energy back-scattered electron (BSE) imaging in the scanning electron microscope.  Comparison between SBI and BSE experiments, indicates that although the interaction volume in BSE imaging extends across multiple layers in a DBR, the vast majority of the electrons involved in image formation originate in the porous layer nearest the surface.  Differences in porous structure arising from process drift in the electrochemical etching parameters can be quickly identified, and the approach is applicable to non-destructive examination of sub-surface porous layers buried beneath non-porous capping layers more than 200 nm thick.   

Overall, we have applied a wide range of electron microscopy techniques to the examination of porous multi-layers in nitride semiconductors, and have developed approaches to examine the influence of defects on the porous morphology at the atomic scale, to assess three-dimensional structure and to diagnose failures in the porosification process.  Combined, these approaches provide new insights into porous nitride structures which will facilitate their application in optoelectronic devices.