AlisonConstantin

Abstract

In this project, we apply the theoretical learnings from class in laboratory experiments. Our goal was to identify the material of two chips that looked identical to our eyes. Using a 532~nm laser, a grating spectrometer, and a CCD, we were able to identify the materials through Raman spectroscopy. Lastly, we thought of a way how we could do the same experiment at significantly smaller financial cost.

Introduction

Material characterization is an essential part of the development of any device, from the nanoscale to macroscale and from prototyping to mass-production. Here techniques that allow to investigate the material composition and to trouble-shoot problems caused by effects such as strain. In our case, we recently found two transparent chips (Fig. \ref{chips_photo_fig}). Our lab works with two different materials that can both be clear: diamond and 4H-SiC. Thus, we set out to identify which material comprised each chip.

Photo of the two chips we are trying to identify. One is diamond and the other is 4H-SiC.

Many different methods can be used to distinguish diamond from 4H-SiC, from hardness testing to X-ray diffraction. We opted to perform Raman spectroscopy as it is a non-invasive, low-energy characterization technique. As we discuss in further detail below, diamond and 4H-SiC both have large enough Raman shifts that we expect to be able to discern the Raman scattering spectral lines from the excitation laser with the appropriate spectral filters and a commercial spectrometer.

Background

Methods

$r=\sqrt{x^2+y^2}$

• Excitation and detection

To measure the Raman spectra, we excited our sample with 532-nm laser light. We spectrally filter the excitation light with a bandpass filter centered around 532~nm with a width of 3~nm (532/3). The green excitation light is directed to and focused on the sample of interest using a confocal setup represented schematically in Fig. \ref{setup_fig}. The scattered and emitted light from the sample is collected by the objective and directed toward a collection optical fiber. Before focusing into the optical fiber, the light passes through two chromatic filters: a 532-notch and a 550 long-pass filter. The 532-notch filter transmits all light except that in a narrow band around 532~nm. The 550 long-pass filter transmits light with a wavelength $>550$~nm. This allows us to pre-select what wavelengths we study with our detector of choice. The output of the detection fiber will be sent to a spectrometer, which we shall detail in the next section.

% FIGURE: Setup \begin{figure}[h] \includegraphics[width=0.45\textwidth,]{setup_schematic.png} \caption{Schematic diagram of the setup used to guide the excitation light (in green) to the sample and collect the emitted and scattered light (in red). 532-nm excitation light passes through a 532/3 bandpass filter, is transmitted through a beamsplitter (blue square), and directed to an objective. The objective lens focuses the excitation light onto the sample. The emitted and scattered light is then collected by the objective lens, reflected by the beamsplitter, passed through a 532-notch filter and 550 long-pass filter, and collected into an optical fiber. The light guided by this collection fiber can be outcoupled to a detector of choice (\textit{e.g.}, a spectrometer).} \label{setup_fig} \end{figure}

• Spectrometer and CCD

From the fiber we send a collimated beam to the spectrometer (see Figure \ref{SpecCCD}) and focus it onto the input slit of the spectrometer using a lens. From the first mirror, the light gets reflected onto a large parabolic mirror, then onto a grating, where diffraction separates the light by wavelength. From the second parabolic mirror the light gets focused down onto the CCD. At the point when the light reaches the chip of the CCD, the light is distributed by wavelength across a line. The chip of the CCD has 2000x256 pixels and by integrating the number of detection events per pixel in one column on the CCD, we can now infer how many photons we can count per wavelength. Our CCD has a quantum efficiency of $>50\%$ and up to $>90\%$ in the wavelength range of ~500-950~nm. At low temperatures the read noise is as low as 4 electrons and the dark current as low as 0.0006 electrons per pixel and per second [\onlinecite{Andor}]. This means that even the weakest light sources, such as single-photon sources, can be detected with this CCD.

\begin{figure}[h] \includegraphics[width=0.65\textwidth,]{SpecCCD.png} \caption{Schematic illustration of a spectrometer with diffraction grating and a CCD. Figure adapted from \onlinecite{Andor}} \label{SpecCCD} \end{figure}