Application Of Raman Spectroscopy In Semiconductor Direction

       In the semiconductor industry, Raman spectrometers can assist the industry to study the basic characteristics of semiconductor materials. Semiconductor containing a single element, such as silicon (Si), germanium (Ge), and multi-component semiconductor, such as selenium zinc (ZnSe), gallium arsenic (GaAs) semiconductor. In the material manufacturing stage, the properties of these semiconductor materials will determine the quality of subsequent wafer manufacturing and IC packaging processes. This stage is the most important step.
      In the process of wafer manufacturing, the use of Raman spectrometer as a tool for process analysis and quality control will be beneficial to improve product quality, yield and overall yield.
      At present, common applications of Raman spectrometers in the semiconductor industry include:

1.X-ray purity
2.Radionuclide pollutant identification
3.Alloying composition
4.Linear superlattice structure
5.The characteristic factors of spontaneous stress-strain
6.Linear defect analysis
7.Monolayne semiconductor heterostructure
8.Doping effect of monolayne heterojunction

Example application:Raman heterostructure testing in ZnSe
      What is ZnSe?ZnSe is a typical group ii-ⅵ semiconductor material with a zinc sphalerite structure and a direct band gap. The band gap is 2.698eV at room temperature (300K) and 2.821 eV at low temperature (<10K), corresponding to the blue light with a wavelength of 459.4nm.

Application and research of ZnSe?
      Thermal evaporation of ZnSe powders in high vacuum has been applied. A series of ZnSe monolayers with thicknesses ranging from 30nm to 1µm were deposited on c-si and glass substrates at room temperature. In addition, SiOx/ZnSe periodic multilayers with thickness of 2 and 4nm were prepared by the same deposition technique. Raman spectra were measured at 295K using 442nm lines of he-Cd lasers and different lines of Ar+ or Ar+/Kr+ lasers. The observed Raman characteristics are related to the scattering of light from multiple optical phonons (1LO to 4LO) and the presence of randomly oriented crystalline ZnSe grains with single and multilayers of ZnSe. A relatively large linewidth (≈15cm-1) of the 1LO band has been observed, which is related to lattice distortion in the grains and the presence of amorphous phases in layers thinner than 100nm.

Raman Analysis
      All measurements were made in air at room temperature. All spectra plotted on the same diagram are scaled at the same scale.

FIG. 1 Raman Spectra Of ZnSe Monolayers And SiOx(4nm)/ZnSe(4nm) Multilayers (C) With Thickness Of 1µm(a) And 30nm(b) Under Three Kinds Of Laser Excitation Measurements

      Figure 1a shows three Raman spectra of ZnSe layers of 1µm thickness deposited on a Corning7059 glass substrate. Better resolution is obtained under excitation conditions close to resonant Raman scattering conditions. When the excitation light approaches the optical band gap Eg0, the strongest enhancement of Raman signal is observed. Since the 1LO band is the densest Raman spectrum excited by 457.9nm line, the energy of the line should be close to the optical band gap. This is consistent with previous results showing that for optical band gaps of 1µm thick ZnSe layers, optical absorption follows the law of directly allowable electron transitions in crystalline semiconductors.
      FIG. 1B and 1C show the Raman spectra of ZnSe layer and SiOx(4nm)/ZnSe(4nm) layers with thickness of 30nm respectively. Both samples were deposited on the C-Si substrate, and the observed strong-narrow band peaked at 521cm-1 due to the scattering of the substrate. The intensity of 1LOZnSe band increases with decreasing excitation wavelength, and a series of 4 peaks can be seen only in the spectrum of 457.9nm line excitation. In fact, characterization of SiOx/ZnSeML (with various thicknesses between 2 and 10nm) by Raman spectra using a 442nm line shows resonance behavior in Raman spectra. It is related to the change of band gap energy caused by size with layer thickness.

Figure 2. Raman Spectra Of Four ZnSe Monolayers With Different Thicknesses (As Shown) Were Measured Using 442nm Excitation Light.

      FIG. 2 depicts a series of Raman spectra of ZnSe monolayers with thicknesses between 30 and 100nm measured using a 442nm laser line. The strength of 1LO band in 30nm layer is obviously higher than that in other samples. Raman results show that ZnSe layer with thickness less than 50nm can show good chemical sensitivity due to small grain size.

Conclusion
      Raman applications for structural characterization of thin and ultrathin ZnSe layers deposited at room temperature using stepwise thermal vacuum evaporation. Measurements using three different Ar+ laser lines under excitation show that the 1LO band intensity shows a maximum when excited with 457.9nm line, independent of the layer thickness.
      Measurements with the 442nm line show that its energy is closer to the optical band gap of the grains in the thinnest monolayer and the ZnSe layer in ML than that of the 457.9nm line. This observation indicates carrier confinement and implies a grain size of about 10nm in these layers.

Recommend Instruments
      Customized autofocus autoscan scientific research grade micro Raman spectrometer, excitation wavelength 457nm, wave number range 200-1000cm-1.

Related Products:ATR8300 ATR8500,ATR8800
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