Application of Raman spectroscopy in ZnSe heterostructure of semiconductor materials

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author: cily

2022-01-27

In the semiconductor industry, Raman spectroscopy can assist the industry to study the basic characteristics of semiconductor materials. Semiconductors containing a single element, such as silicon (Si), germanium (Ge), and semiconductors composed of multiple components, such as zinc selenium (ZnSe), gallium arsenic (GaAs) semiconductors. In the material manufacturing stage, the characteristics of these semiconductor materials will determine the quality of the subsequent wafer manufacturing and IC packaging processes. This stage is the most important step.

In the process of wafer manufacturing, using Raman spectroscopy as a tool for process analysis and quality control will help improve product quality, increase yield and increase overall output.

Common applications of Raman spectroscopy in the semiconductor industry include:

Material purity
Pollutant identification
Alloy composition
Superlattice structure
The characteristic factors of essential stress & strain
Defect analysis
Semiconductor heterostructure
Doping effect of heterojunction

Example application: Raman spectroscopy in ZnSe heterostructure testing
What is ZnSe?

ZnSe is a typical II-VI semiconductor material with a zinc blende structure and a direct band gap. The band gap at room temperature (300K) is 2.698eV, and at low temperature (<10K), its band gap is 2.821ev, corresponding to blue light with a wavelength of 459.4nm.

Application and Research of ZnSe
Thermal evaporation of ZnSe powder in high vacuum has been applied. A series of ZnSe monolayers with thicknesses between 30 nm and 1 µm were deposited on c-Si and glass substrates at room temperature. In addition, SiOx/ZnSe periodic multilayer films prepared by the same deposition technique with ZnSe layer thicknesses of 2 and 4 nm were investigated. Raman spectra were measured at 295K using the 442nm line of a He-Cd laser and different lines of an Ar+ or Ar+/Kr+ laser. The observed Raman features are related to multiple optical phonon (1LO to 4LO) light scattering and to the presence of randomly oriented crystalline ZnSe grains of ZnSe monolayers and multilayers. A relatively large linewidth (≈15 cm-1) of the 1LO band has been observed, which is related to lattice distortion in the grains and the presence of an amorphous phase in layers thinner than 100 nm.

Raman analysis
All measurements were performed in air at room temperature. All spectra plotted on the same graph are scaled the same.

Fig. 1 Raman spectra of ZnSe monolayer and SiOx(4nm)/ZnSe(4nm) multilayer (c) with thicknesses of 1 µm (a) and 30 nm (b) measured under three laser excitations
Figure 1a shows three Raman spectra of a ZnSe layer with a thickness of 1 µm deposited on a Corning7059 glass substrate. You can choose the Raman spectrometer from the domestic Raman leader ~ AOPU Tiancheng. Better resolution is obtained under excitation conditions close to those of resonant Raman scattering. The strongest enhancement of the Raman signal is observed when the excitation light is close to the optical bandgap Eg0 of the material. Since the 1LO band is the densest in the Raman spectrum excited by the 457.9nm line, the energy of this line should be close to the optical bandgap. This is consistent with previous results showing that for an optical bandgap of a 1 µm thick ZnSe layer, optical absorption follows the law of directly allowed electronic transitions in crystalline semiconductors.

Figures 1b and 1c show the Raman spectra of a ZnSe layer with a thickness of 30 nm and a SiOx(4 nm)/ZnSe(4 nm) multilayer, respectively. Both samples were deposited on c-Si substrates, and the observed strong narrow band peaked at 521 cm-1 due to the scattering from the substrate. The 1LOZnSe band intensity increases with decreasing excitation wavelength, and a series of 4 peaks can be seen only in the spectrum excited by the 457.9 nm line. Indeed, SiOx/ZnSeML (with various thicknesses between 2 and 10 nm) were characterized by Raman spectroscopy using the 442nm line, where resonant behavior was shown. It is related to the size-induced variation of the bandgap energy with layer thickness.

Figure 2 Raman spectra of ZnSe monolayers measured at four different thicknesses (as shown) using 442 nm excitation light.
Figure 2 depicts the Raman spectra of a series of ZnSe monolayers with thicknesses between 30 and 100 nm measured using a 442 nm laser line. The intensity of the 1LO band of the 30nm layer is significantly higher than that of other samples, and the Raman results show that the ZnSe layer with a thickness less than 50nm can show good chemical sensitivity due to the small grain size.
Summarize
Application of Raman spectroscopy for structural characterization of thin and ultrathin ZnSe layers deposited at room temperature using stepwise thermal vacuum evaporation. Measurements under excitation with three different Ar+ laser lines showed that the 1LO band intensity exhibited a maximum when excited with the 457.9 nm line, independent of layer thickness.

Measurements with the 442nm line show that its energy is closer to the optical bandgap of the grains in the thinnest monolayer and the ZnSe layer in the ML than the energy of the 457.9nm line. This observation indicates carrier confinement and implies that the grain size in these layers is around 10 nm.
Recommended instrument
Customized automatic focusing and automatic scanning scientific research-grade Raman microscope, with an excitation wavelength of 457nm and a wavenumber range of 200-1000cm-1.