MonoCL4 detector – The right tool, whatever the application

The MonoCL4™ is the leading spectroscopic cathodoluminescence (CL) detector providing unique insights into a sample’s composition and optical and electronic properties. A key factor in the MonoCL4 detector’s favored position is the outstanding sensitivity and reproducibility of its optical system as demonstrated in a publication record that spans many years. This application note provides insight into how leading publications are enabled by the MonoCL4 detector’s exclusive design, demonstrating unrivaled mapping speeds at better than 50 nm spatial resolution and analysis of the smallest metallic nanoparticles to date.

Inspirational results – sensitivity is the key

A key metric on which to judge a CL detector by is the system’s sensitivity at the wavelength of interest.  For many applications (lack of) sensitivity fundamentally limits the results that can be attained in a particular application.  Depending on the experiment of interest, that may be: i) setting the detection limit of trace elements, ii) the best spatial resolution at which a measurement can be performed or iii) the smallest nanoparticle that can be investigated. [1] The CL technique can be considered light-starved, necessitating the most sensitive collection and detection technologies together with the accurate and robust alignment of the optical system. The MonoCL4 detector utilizes a high numerical aperture mirror to collect 50x more light than some systems and due to its unique achromatic architecture, demonstrates unrivaled throughput in the wavelength range of 200 – 2500 nm.

With the growth of nanotechnology and nanophotonics, sensitivity will remain the key requirement of a spectroscopic CL systems in the coming years.

Analyzing the smallest nanophotonic structures

The optical properties of metallic nanostructures have received a great deal of attention in recent years due to a wide variety of sensing and telecommunications applications and their potential use in on-chip optical communication. In this field, the researcher has a broad choice of devices due to one's ability to tailor the optical properties by varying the material selection, the shape of a structure and its size. By confining the dimensions of a particle to sizes smaller than the classical diffraction limit of light, control has been demonstrated over the polarization, and wavelength range a device may operate over with demonstrations from the deep ultraviolet through the visible wavelengths to the near infrared.[2], [3], [4]

A good theoretical understanding is available for many nanophotonic systems. However, the computational time can be considerable, and experimental verification can often be faster. Moreover, manufacturing processes that create ‘perfect’ nano-sized objects are not available yet. Thus experimental methods that enable characterization of real nanoparticles are required. Two techniques that have emerged as key tools are the related electron microscopy techniques of CL and electron energy loss spectroscopy (EELS); both techniques enable optical properties to be correlated directly with morphology (size, shape, and smoothness). The EELS technique reveals the energy absorption processes that occur at optical frequencies when a fast electron interacts with a specimen. The EELS technique has an inherent advantage in detection efficiency as all interactions of interest contribute to the signal being measured. However, unlike the CL technique, the EELS technique is unable to distinguish between modes which interact with light (bright mode) and those that do not (dark mode) and the effects of polarization. Furthermore, the capital investment for a microscope and electron spectrometer suitable for optical characterization is much higher than a microscope and CL system. Therefore, CL would seem an ideal technique; however, few photons are emitted from nanoparticles. The number of photons emitted depends on the material and the particle’s volume; light output decreases with a particle’s size, d, as the cubed root,    making small nanoparticles significantly more challenging than larger particles. Researchers recently demonstrated the MonoCL4 system's excellent sensitivity by analyzing the smallest particle to date using CL – a 50 x 40 nm aluminum nanorod. Even more impressively, aluminum is a poor plasmonic material compared to noble metals and emits at deep UV wavelengths making the task doubly challenging.

Spectral mapping of compound semiconductors in just a few minutes

CL microscopy has found widespread use in the analysis of compound semiconductor devices and materials. It helps reveal the presence of, and properties of, point and extended defects,[5] mapping compositional variation,[6] and quantifying stress or strain.[7] Due to a perceived restriction in the spatial resolution achievable, inflicted largely by an inability to acquire data sets with sufficient pixels in a practicable time, spectrum-imaging (hyperspectral imaging) techniques[8] have been somewhat less widely adopted. Here again, the sensitivity of the CL system employed limits the quality of work done, as the spectral rate is restricted by signal-to-noise considerations. However, the MonoCL4 system breaks this widely held perception as the experiment below demonstrates.

Gallium Nitride (GaN) is widely used in optoelectronic, and power electronic applications and as such, is investigated intensively in an ongoing effort to expand the market by reducing costs and improving device efficiency. One approach that many researchers are pursuing is the development of GaN grown on lower cost silicon substrates. However, GaN grown on silicon is of poorer quality with more crystallographic defects such as dislocations and stacking faults than GaN grown on sapphire substrates. CL in the scanning electron microscope (SEM) is an ideal tool to determine the quality of GaN, as defects are revealed readily over wide areas with a spatial resolution of a few tens of nanometres and with little or no sample preparation.[9]

Here we examine ~2 µm sided GaN micro-pyramids grown by the metal organic chemical vapor deposition (MOCVD) method using a MonoCL4 Elite system installed on a field emission gun SEM in just 120 s. To achieve the highest spatial resolution, the SEM was operated with an accelerating voltage of only 2 keV, thus limiting the lateral extent of the interaction volume to ~20 – 30 nm. Under these conditions, the number of photons emitted from a sample is low and previously thought to be beyond the limit of practical spectral mapping. Nevertheless, the outstanding optical throughput of the MonoCL4 system (which is maintained throughout the lifetime of the instrument) and utilization of optimized detectors with the lowest noise level enables full spectral mapping of many micro-pyramids with a spatial resolution better than 50 nm in just a few minutes; a feat many thought impossible until now.

Conclusion

The MonoCL4 detector's supreme sensitivity, based on collection efficiency up to 50x more than other designs, combined with its permanently aligned optical system, achromatic optics, and ultra-efficient detector designed specifically for cathodoluminescence microscopy, ensures that the MonoCL4 system maintains its position as the system of choice for researchers from all applications.

 

[1]Ideal operating conditions vary per the specimen under investigation with insulating and semiconducting specimens benefitting from low accelerating voltages and beam currents. On the other hand, metallic samples are measured optimally at high accelerating voltages and beam currents.

[2] Knight et al., Nano Letts., 2012

[3] Xinli Zhu et al., Advanced Materials, 2010

[4] Hoffman et al., Nano Letts., 2007

[5] L J Brillson, J. Appl. Phys. D, 2012

[6] J Haarstrich et al., Thin Solid Films, 2012

[7] Xu-Wen Fu et al., Front. Physics, 2013

[8] S A Galloway et al., PSS C, 2003

[9] C M Parish and P E Russell, Adv. In Imaging and Electron Phys., 2007