In infrared optical systems, coated germaniumwindow paneDue to its excellent wave transmission performance and mechanical stability, it is widely used in fields such as aerospace, precision instruments, and high-power laser transmission. As the core substrate material, the selection of single crystal germanium directly affects the optical efficiency and reliability of the device. This article explores in depth the key value of single crystal germanium as a substrate for coated germanium window plates from three aspects: material properties, preparation processes, and application advantages.
1、 Core physical and chemical properties of single crystal germanium
1. Wide spectral transmittance
Single crystal germanium has a transmittance of over 95% in the mid infrared range of 2-14 μ m, and the refractive index uniformity error is less than ± 0.0005, which can effectively reduce light scattering losses. Its bandgap width (0.67eV) makes it one of the few semiconductor materials in this band that can operate without cooling.
2. Thermal stability
The melting point reaches 938.25 ℃, and the coefficient of thermal expansion is 6.1 × 10 ⁻⁶/℃ (25 ℃). It can still maintain dimensional stability within the special temperature range of -200~+150 ℃. The low thermal conductivity (68W/(m · K)) characteristic helps to reduce the thermal lens effect under high-power laser action.
3. Mechanical processing performance
The Mohs hardness is 7.5, close to quartz glass, but with higher fracture toughness. Through diamond cutting and polishing processes, ultra smooth surfaces with a surface roughness Ra<0.5nm can be obtained, meeting the adhesion requirements of anti reflective films such as ZnS/Y ₂ O3 multilayer films.
2、 Crystal Growth and Defect Control Technology
1. Purification by Czochralski method
Using high-purity polycrystalline germanium raw material (≥ 99.999%), single crystal rods with a diameter of 50~100mm are grown by direct drawing method. The doping concentration is precisely controlled at the ppb level to ensure that the carrier concentration is less than 1 × 10 ¹³ cm ⁻ ³, avoiding infrared attenuation caused by free carrier absorption.
2. Optimization of dislocation density
The introduction of crucible accelerated rotation (ACCR) technology reduces the radial temperature gradient of the crystal to<0.5 ℃/cm, and lowers the dislocation density from the traditional process of 10 ⁴/cm ² to<10 ²/cm ². According to X-ray morphology analysis, the lattice integrity can reach 99.9%.
3. Innovation in surface treatment
Chemical mechanical polishing (CMP) combined with atomic layer deposition (ALD) passivation layer can control the depth of subsurface damage layer within 50nm. Subsequent plasma cleaning removes residual organic matter and enhances the bonding strength of the film layer to>50MPa.
3、 Comparative analysis of typical application scenarios
| Application field | Technical Requirements | Advantages of Single Crystal Germanium Reflected |
| Spacecraft infrared window | Radiation resistance, low thermal distortion | Anti gamma radiation dose up to 1Mrad, surface accuracy PV<λ/10 |
| CO ₂ laser output window | High damage threshold (>10J/cm ²) | No grain boundary scattering, continuous working temperature rise<2 ℃ |
| Terahertz filter | Broadband transmission (0.1~3THz) | The real part of the dielectric constant ε=16.3, the imaginary part<0.001 |
4、 Future Development Trends
With the increasing demand for deep ultraviolet transparent materials in extreme ultraviolet lithography technology, fluoride modified single crystal germanium (Ge-F co doped) is expected to break through the wavelength limit of 193nm. Meanwhile, the strain superlattice structure based on molecular beam epitaxy (MBE) technology is driving the development of single crystal germanium towards multifunctional integration.