Characterization of Amorphous and Microcrystalline Silicon Using Raman Spectroscopy - - Spectroscopy
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Characterization of Amorphous and Microcrystalline Silicon Using Raman Spectroscopy


Application Notebook


Abstract

Silicon deposited on glass or silicon carbide is widely used in manufacturing photovoltaic cells. Both the proportion and distribution of amorphous and crystalline silicon are critical for performance and are therefore important to monitor. Raman spectroscopy is an ideal technique for this application, as the two forms generate readily distinguishable spectra that lend themselves to simple quantification methods using Beer's Law. Mapping generates chemical images with detailed information on the spatial distribution of the crystalline and amorphous forms. Since excess excitation laser power can be demonstrated to convert amorphous into crystalline silicon, care must be taken to limit the amount of power used. The Thermo Scientific DXR Raman microscope, which is equipped with a laser power regulator, is ideal for this application, particularly if the method has to be replicated from instrument to instrument at multiple manufacturing plants.

Introduction

One of the more widely used photovoltaic cell technologies employs silicon deposited on either glass or silicon carbide. Although panels were made initially with either crystalline or amorphous silicon, a judicious combination of the two materials takes advantage of the benefits of each while minimizing their respective disadvantages. Optimal performance depends on reliably being able to make cells with a predetermined ratio and distribution of the two forms. Monitoring ensures the manufacture of economical, highly efficient and long-lasting solar cells.

This is an application for which Raman spectroscopy is particularly well suited. Silicon-silicon bonds are symmetrical and result in strong Raman scattering. Crystalline silicon has highly uniform bond angles and bond lengths and exists in a limited number of states. This results in sharp Raman peaks with a characteristic strong band at 521 cm–1 . Amorphous silicon is less orderly in its arrangement with a wider array of bond angles, bond energies and bond lengths in addition to dangling bonds. The distribution of possible states leads to a broad Raman band centered at 480 cm–1 that is readily distinguishable from that of crystalline silicon.

Raman spectroscopy can be used to quantify the relative amounts of amorphous and crystalline silicon in thin layer deposits. By mapping an area of deposited silicon the uniformity of the distribution of the two silicon forms can be monitored. This application note shows typical results and discusses some of the practical aspects and potential pitfalls of using Raman spectroscopy for measuring amorphous and crystalline silicon.

Experimental

All spectra were collected using a Thermo Scientific DXR Raman microscope equipped with a 532 nm excitation laser, a full range grating and a motorized stage. The instrument was operated using the Thermo Scientific OMNIC 8 software suite. OMNIC™ Atlμs™ mapping software was used to collect and analyze mapping data.

Results


Figure 1
Crystalline versus amorphous silicon: Figure 1 shows typical silicon spectra from samples that range from pure crystalline to those containing significant amounts of amorphous material. The strong band at 521 cm–1 of the crystalline sample very distinct from the broad band centered at 480 cm–1 that is exhibited by the amorphous silicon. These clearly distinguishable spectra make it possible to use a simple Beer's law calculation based on the ratio of the peak heights at 521 cm–1 and 480 cm–1 to estimate the relative amounts of the two forms.


Figure 2
Distribution of amorphous and crystalline silicon: Raman mapping is an excellent way to obtain information about potential variations in crystallinity over areas of deposited silicon. By mounting the sample on a motorized stage and collecting Raman spectra at specified intervals across the sample, it is possible by analyzing the spectra to determine the distribution of the silicon forms. Figure 2 shows a 30-micron line map across a sample of deposited silicon. The strong Raman band at 521 cm–1 at about half way across the map shows that there is a small region of crystalline silicon at this location. The remainder of the mapped sample shows the broad Raman band at 480 cm–1 that is characteristic of amorphous material.


Figure 3
A two-dimensional (x, y) map collected across a boundary region between crystalline and amorphous forms is shown in Figure 3. In this case OMNIC AtlÁs software was used to display the spectral data as a chemical image of the distribution of the two forms by calculating the ratio of the peak heights at 521 cm–1 and 480 cm–1 . The map shows the gradient between the two forms and can be used as a quality check during manufacture.


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