Advances in cathodoluminescence characterisation of compound semiconductors with spectrum imaging

Abstract

New advances in cathodoluminescence (CL) instrumentation allow more advanced characterisation of compound semiconductor materials and devices. CL offers the advantage of combined high spatial and spectral information in one experiment. However, until now, CL results have typically been two dimensional, in the form of images at discrete wavelengths, or spectra chosen from specific points on the specimen. Spectrum Imaging allows large data sets to be collected with relative ease so that the full spectroscopic information can be recorded at every pixel position in an image. Gaussian fitting and advanced subtraction techniques are shown to be valuable in mapping spectral shifts, peak widths, and in the extraction of hidden luminescence features. This helps characterise important luminescence features such as impurities, doping, stress, extended defects, and alloy and quantum confinement inhomogeneities.Introduction Cathodoluminescence (CL) is the emission of photons from a specimen stimulated using an electron beam. The CL emitted is governed by competition between non-radiative and radiative electron-hole pair recombination events. In the field of compound semiconductors, where the ability to emit and detect photons efficiently and of a specific wavelength is of paramount importance in their application, CL is increasingly used as a research and evaluation tool. The growth in the application ranges from materials and growth characterisation through to device design and failure analysis.The technique has the advantage of giving relatively high spatial resolution results using a flexible injection source. In a scanning electron microscope (SEM or STEM) the electron probe is easy to control in terms of position, focusing, energy and flux. In addition it has a high depth of field so can cope with complex structures, and takes place in a vacuum which is ideal for cryogenic temperatures. The spatial resolution which is achievable is dependent on the specimen, microscope and CL equipment. Recent results have demonstrated resolution in the range of 20 nm [1]. CL performed in a transmission electron microscope also offers the promise of high spatial resolution due to the reduced dimensions of the generation volume for a thin TEM sample.Growth in CL as a technique has matched advances in the importance of compound semiconductors and has been associated with advances in electron microscope and cathodoluminescence technology. Early CL studies were primarily limited to panchromatic imaging. This simple method of collecting and detecting luminescence without recourse to dispersion or filtering remains valuable, especially in terms of mapping non radiative recombination sites.Spectral CL has gained in popularity over the last decade. The most favoured and successful approach involves direct optical coupling between a specimen, a monochromator, and finally a photo-multiplier

Cathodoluminescence microcharacterization of point defects in α-quartz

“…Normalized energy transfer function for quartz (r = 2.7 g.cm À3 ) for a range of incident beam energies. The inserted plot shows the maximum penetration depth in quartz as a function of incident electron beam energy (Goldberg et al, 1998).…”

“…In Fig. 2, the energy transfer function, dE/dz is plotted as a function of penetration depth for quartz (Goldberg et al, 1998). Approximately 70% of the CL is generated between 20% and 50% of the maximum electron penetration depth.…”

“…For example, optical cathodoluminescence microscopy (OCLM) systems, combining CL imaging with light microscopy, allow mineral phases to be distinguished (Marshall, 1988;Götze, 2000). Cathodoluminescence systems for scanning and transmission electron microscopes (SEM, TEM) and electron microprobes (MacRae et al, 2005) have also been developed with capabilities ranging across 'imaging only' through 'imaging and spectroscopy' to 'spectrum imaging' techniques (Galloway et al, 2003) as well as time, polarization, radiation dose, depth and temperature resolved experiments. Cathodoluminescence microanalysis in an electron microscope allows the defect microstructure to be investigated with high spatial resolution and sensitivity (Yacobi and Holt, 1990).…”

Subsurface analysis of semiconductor structures with helium ion microscopy

“…Channeling of the incident particles can be used to map crystalline structures [11] or detect thin films through dechanneling contrast [12]. Second, a materialand defect dependent photon yield can be measured with a luminescence detector and used to establish the presence of defects or dopants [13,14]. The third particle that is used for imaging is backscattered helium, sometimes referred to as the Rutherford backscattered ion (RBI) mode of imaging [3].…”

Improved Microanalysis Using Cathodoluminescence Spectrum Imaging with Higher Spatial Sampling

“…Traditionally, CL detectors captured spectrum images by collecting wavelength-resolved spectra on a point-by-point basis using a pixelated detector such as a CCD camera [3]. This approach captures spectrum images with high spectral resolution (0.1 -1 nm typically) however, due to the low signal levels, slow frame rates and (read out) noise of the detector, it was often necessary to compromise spatial sampling in order to capture data in a reasonable timeframe.…”