Cadmium (Zinc) Telluride

Semiconductor Material for X-ray and Gamma Ray Imaging

Cadmium (Zinc) Telluride (Cd(Zn)Te) is a semiconductor material that is used in the production of X-ray and gamma ray imaging detectors. It has a high atomic number, which makes it highly sensitive to X-rays and gamma rays. Cd(Zn)Te detectors are commonly used in medical imaging, security imaging, and industrial imaging applications.

Applications

  • Non-Destructive Testing
  • Computed Tomography
  • Quality Assurance
  • Spectral Imaging
  • Spectroscopy
  • X-Ray Diffraction

Usage of Cd(Zn)Te Sensors

Cadmium Zinc Telluride (CdZnTe) and Cadmium Telluride (CdTe) sensors are used in X-ray and gamma-ray imaging and spectroscopic detectors.

Cd(Zn)Te has a high atomic number, which makes it highly sensitive to high energy X-rays and gamma rays, and high density, which makes it sensitive to high energy electrons. Thus the Cd(Zn)Te sensors are commonly used in medical imaging, security imaging, industrial non-destructive-testing and electron microscopy applications.

Nevertheless, Cd(Zn)Te sensors have some disadvantages, such as low electron and hole mobilities, high energy X-ray fluorescence photons, large number of defects, electrical stability and fragility of the sensor material, making it challenging to grow, manufacture and package.

Here are some typical energy ranges and applications where Cd(Zn)Te sensors are employed:

  • High-Energy X-rays: Cd(Zn)Te sensors can effectively detect and measure X-rays up to 100-1000 keV depending on the sensor thickness. This includes applications such as medical computed tomography, medical X-ray radiography, non-destructive testing of heavyweight materials, X-ray and gamma-ray spectroscopy, and security applications
image-1362

X-ray absorption in Cd(Zn)Te sensor and its usable energy range. Source publication

  • High-Energy electrons: Cd(Zn)Te sensors can effectively detect and measure electrons due to its high density in the energy range from a few keV to few of MeV. This includes applications such as Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS).

Cd(Zn)Te Semiconductor Material

Cadmium (Zinc) Telluride (CdZnTe) is a compound semiconductor material composed of Cadmium (Cd), Zinc (zn) and Telluride (Te) atoms.

CdZnTe belongs to the III-VI group of semiconductors, which means it is formed by elements from Group III (Cadmium and Zinc) and Group VI (Telluride) of the periodic table. Cd(Zn)Te is a direct bandgap semiconductor, meaning it can efficiently absorb photons and convert them into electrical signals.

Cadmium (Cd): Cadmium is a transition metal element with atomic number 48. It is commonly used in semiconductors due to its unique electrical properties. In CdZnTe, cadmium provides the basis for the crystal lattice structure and contributes to the overall electronic behavior of the material.

Zinc (Zn): Zinc is another transition metal element with atomic number 30. It is often alloyed with cadmium to form CdZnTe. Zinc’s inclusion helps to improve the structural and electrical properties of the compound. The incorporation of zinc allows for the adjustment of the bandgap energy, lowering the leakage current and the enhancement of the material’s overall performance.

Tellurium (Te): Tellurium is a metalloid element with atomic number 52. It plays a crucial role in CdZnTe as it provides the majority of the material’s electrical properties. Tellurium is the main component responsible for CdZnTe’s semiconducting behavior and its ability to efficiently detect ionizing radiation.

By controlling the ratio of cadmium, zinc, and tellurium in the CdZnTe compound, the material’s properties can be tailored to suit specific applications. The elemental composition affects the bandgap energy, electrical conductivity, charge carrier mobility, and other characteristics of CdZnTe.

CdZnTe is available in small (50-100 mm) size wafers; the material is commercially available in different grades; its price is more expensive than GaAs and it is more fragile than Si or GaAs.

  • Cadmium Zinc Telluride (CdZnTe or CZT) is not a naturally occurring mineral or compound found in nature. Instead, CdZnTe is a synthetic compound semiconductor material that is created through a manufacturing process.

    Cadmium, zinc, and tellurium are individual elements that can be found in the Earth’s crust, but they do not naturally combine to form CdZnTe. The synthesis of CdZnTe involves precise control of the composition and crystal structure to achieve the desired properties.

    Cadmium (Cd) is a relatively rare element found in small quantities in various minerals, such as sphalerite (zinc sulfide) and greenockite (cadmium sulfide). Zinc (Zn) is more abundant and is commonly extracted from minerals such as sphalerite. Tellurium (Te) is a rare element often associated with copper and gold ores.

  • Semiconductor-grade Cd(Zn)Te is grown as ingots using different techniques. The growth technique has a major contribution to the number of impurities. Several methods are employed for CdZnTe crystal growth, including the following:

    1. Bridgman-Stockbarger Method: The Bridgman-Stockbarger method is a popular technique for growing large, high-quality CdZnTe crystals. In this method, a sealed ampoule containing the desired composition of cadmium, zinc, and tellurium is heated and melted. Then, a seed crystal is immersed in the melt, and the ampoule is slowly cooled, allowing the crystal to grow from the melt as the temperature gradient is maintained.
    2. Traveling Heater Method (THM): The THM is a modified version of the Bridgman-Stockbarger method. In this method, a cylindrical heater is used to move the melt zone along the ingot as it grows. This allows for better control over the crystal growth and the distribution of impurities.
    3. Vertical Gradient Freeze (VGF) Method: The Vertical Gradient Freeze method is another technique employed for growing CdZnTe crystals. In this method, a seed crystal is slowly pulled upward through a temperature gradient in a controlled atmosphere. As the crystal is pulled, the material solidifies on the seed crystal, resulting in the growth of a single crystal with the desired composition.
    4. Liquid Encapsulated Czochralski (LEC) Method: The LEC method involves growing CdZnTe crystals by dipping a seed crystal into a crucible containing the molten material. The crystal is then slowly pulled upward while being rotated, allowing the crystal to grow as the material solidifies. The use of a liquid encapsulant, such as boric oxide, helps control the growth process and reduce impurities.

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