In the last few decades, electronic devices have become more and more important in our lives, and semiconductors are a crucial part of this technology innovation. A semiconductor is a generic term for materials, normally solid chemical elements, that can conduct current, but only partly. A semiconductor has a conductivity which is between that of an insulator with almost no conductivity, and a conductor with almost full conductivity.
Photo: Silicon - the most common semiconductor (Source: Wikipedia Silicon)
A decade after the invention of the rectifier in 1947, the transistors were invented at Bell Laboratory in the US by John Bardeen, Walter Brattain, and William Shockley. The appearance of the transistors became a milestone in the rapid growth of the semiconductor industry. In 1959, the integrated circuit was invented by Fairchild Semiconductor and Texas Instruments, and it was a major point in the history of semiconductors. The ubiquitous usage of transistors and the integrated circuits in technology pushed the development of the semiconductors. The prices for technology decreased, making computers and other electronic devices available to most of the people.
The compound semiconductor is a semiconductor which usually consists of two or more elements. Therefore, the possible combinations are very broad, and the most common compound semiconductors are made of combinations of elements from periodic table GroupIII and GroupV (GaAs, GaP, InP, etc.). Currently, gallium arsenide (GaAs), gallium nitride (GaN), indium phosphide (InP), silicon carbide (SiC) and silicon germanium (SiGe) are the most common elements used for compound semiconductors. Compound semiconductors can be found in power amplifiers for smartphones and other wireless devices, as well as in light sources for DVDs, Blu-rays, LEDs, and solar batteries.
Example of a CPU which is made of million tiny transistors (Source: Krbo|Flickr)
The history of compound semiconductors can be only traced back to 15 years ago when gallium nitride substrate was produced for the first time after more than 30 years of research and development. Still, the high cost is one of the barriers for commercial development of compound semiconductors. Recently, the costs of manufacturing compound semiconductors have decreased, but it is still much more expensive than silicon, which is still considered to be the most common semiconductor thanks to its widespread commercial application and availability. However, silicon has a very basic and limited set of properties that restrict its application. The processing speed of compound semiconductors is much higher than that of the silicon. Other outstanding features of compound semiconductors include the ability to emit and sense light and generate microwaves while resistant to heat.
To pick a specific example, Gallium Nitride (GaN) is one of the most common compound semiconductors, which can conduct electrons 1000 times more efficiently than silicon while operating at a lower voltage. Because of its capability to assist high-speed switching, GaN and other compound semiconductors are being used in the key markets including photonics, optical and RF communication. The future development of compound semiconductors could benefit various technology areas that demand high-performance levels along with sensing and other capabilities. Therefore, people in academia and production environment are constantly researching the possibilities of (compound) semiconductors, which will help to create new technologies in a wide range of areas including safety systems, aerospace, and automotive applications.
Cathodoluminescence (CL) imaging and spectroscopy is used extensively to study GaN materials and devices. The SPARC is a high-performance cathodoluminescence detection system. With a one-of-a-kind high-precision mirror stage, the SPARC opens new avenues for researching semiconductors and compound semiconductors such as GaN-based LED materials. The energetic electron beam can efficiently excite the wide bandgap (3.4 eV) of GaN with subwavelength spatial resolution and is used to image local defects such as dislocations and probe light emission characteristics on small length scales. As LED devices become increasingly miniaturized, cathodoluminescence serves as the most important tool for examining them at the nanoscale. Furthermore, by varying the energy of the primary electrons, different depths can be probed, thus allowing one to obtain more information in the case of stratified devices. Delmic has also published an application note on Cathodoluminescence for bulk and nanostructured Gallium Nitride-based LED materials
Together with SPARC, the LAB Cube, the SPARC module for time-resolved cathodoluminescence imaging, can be used to study the time dynamics of various materials, including semiconductors. This module allows performing lifetime imaging and anti-bunching experiments, and it is a great tool for giving insights into intrinsic material properties, nanoscale quality and defects. It is highly relevant for a large range of applications including compound semiconductors for optoelectronic devices such as (In)GaN, perovskites, and GaAs. The LAB Cube module is a unique and easy-to-use system, which can be retrofitted on any Scanning Electron Microscopy and standard or new SPARC system without any extra modification.
The figure below shows an example of a g(2) measurement on an array of GaN nanorods with InGaN quantum wells inside. In this case, the electron beam is raster-scanned over the array and for each position, a g(2) curve is acquired. The bunching effect can be clearly seen in (b) where g(2)(0) is clearly larger than 1. For every scanning pixel, the bunching peak is used to extract the local lifetime τe and excitation probability γ.
Example of g(2) mapping data on InGaN/GaN nanorods shown in the SEM image in (a). (b) The g(2) data recorded at three colored squares as indicated by the arrows in panel (c) which shows SE intensity recorded together with the g(2) data set. Maps of (d) lifetime τe, (e) amplitude g(2)(0)-1, and (f) the probability of excitation γ are also shown. If the data was too noisy to extract these parameters, the pixel was left white in the map The contours of the nanorods are indicated by the black lines. Figure courtesy of Dr. Sophie Meuret (AMOLF, Amsterdam)
This information gained from imaging the semiconductor with the SPARC can be used to understand and quantify electron beam interaction with such a complex 3D semiconductor structure and to provide insight in the quality and homogeneity of the material.
If you are interested in our newest product – the LAB Cube, please download the Product sheet for more information.