Composition and doping metrology

For many devices, optimized performance relies on careful control of the alloy composition of compound semiconductor crystals, epitaxial layers, or nanostructures. Whether that is producing homogeneous layers with a composition that is very close to the target value in the manufacture of GaAs solar cells, nitride-semiconductor quantum wells for light-emitting diodes, or the through-thickness graded alloy of a CIGS thin-film solar cell; cathodoluminescence (CL) proves to be a superior technique offering rapid, non-contact, analysis that also benefits from nanoscale spatial resolution.

Compositional metrology of a CdTe1-xSx  absorber layer from a thin-film solar cell (left) and InxGa1-xN quantum well from a micro-LED.  The sulfur and indium concentrations were calculated from CL spectrum images by using Gaussian fitting to the band-to-band emission corresponding to the A°X exciton peak in the CdTeS and donor-acceptor pair recombination in the InGaN quantum well. The CdTeS data provided by Dr. B. Mendis, Durham University. 

See these experiment briefs for more information:

Defect detection and characterization

CL microscopy is useful to reveal the presence and distribution of sub-surface crystal defects such as dislocations and stacking faults in semiconductor wafers and epitaxial layers. During a measurement, the CL signal shows the local recombination activity of electron-hole pairs and excitons. To add, as the recombination of electron-hole pairs is a competitive process, it reveals non-radiative recombination at electrically active defects as dark contrast in a CL map (see below).

Threading dislocations in GaN wafer revealed by unfiltered imaging (left). Misfit dislocation network observed in an InGaAs epitaxial layer by unfiltered imaging using an infrared detector (right).

Understanding the electronic structure of crystal defects can prove vital when devising methods to nullify the deleterious effect on device performance. Spectroscopic CL is one of the few methods available that can provide a detailed understanding of the distribution, concentration, and energy levels of deep-level states associated with crystal defects. At cryogenic temperatures, non-radiative recombination becomes less probable due to the reduced phonon population, making radiative transitions via the defect states possible. Therefore, spectrally resolved CL can be used to determine the mid-bandgap energy level(s) of defects from the radiative emissions, and the spatial resolution of CL enables measurement of the distribution and concentration of defects.

The distribution of three distinct stacking fault species (E, I1 and I2) revealed by spectrally resolved CL of GaN microcrystals. Measure with a sample temperature = 5 K. Image courtesy of Dr. U. Jahn, Paul Drude Inst.

Lifetime and/or minority carrier diffusion

In a semiconductor, the electron beam of the electron microscope generates free electrons and holes. Before recombination, the excess carriers are free to move within a crystal under drift and/or diffusion mechanisms. Under most experimental conditions, the concentration of these free carriers is far below the majority carrier density concentration of a doped semiconductor, so we typically only need to consider the behavior of the excess minority carriers (or excitons). CL maps may capture this effect so you can observe when the generation volume is small (e.g., at low accelerating voltages): In the CL map of a GaN wafer containing threading dislocations (above), the black dots correspond to an individual threading dislocation intersecting the surface. These defects are one-dimensional, yet the dark spot in the CL map has a diameter >50 nm. This demonstrates that the dislocation has an impact on the electrical properties of the crystal over a longer range than the dislocation’s physical extent, or a fraction of the free carriers generated within the minority carrier diffusion length diffuse to the dislocation and recombine non-radiatively. Empirical models are available to extract the minority carrier diffusion length from CL maps of crystal defects. However, recent research demonstrates that time-resolved measurements or spectrally resolved measurements at cryogenic temperatures are a superior method.