Surface and bulk absorption in germanium at 10.6 microm.

Area of Science:

• Materials Science
• Optical Engineering
• Solid State Physics

Background:

• Germanium (Ge) is a critical semiconductor material with applications in optics and electronics.
• Understanding optical absorption is crucial for optimizing Ge-based device performance.
• Surface conditions significantly impact the optical properties of semiconductor materials.

Purpose of the Study:

• To quantify the surface and bulk optical absorption of germanium at 10.6 μm.
• To investigate the influence of resistivity and surface preparation on optical absorption.
• To determine the optimal resistivity range for minimizing bulk absorption in germanium.

Main Methods:

• Adiabatic laser calorimetry was employed to measure optical absorption.
• Experiments were conducted on germanium samples with varying resistivity and surface treatments.
• Surface preparation methods included conventional optical polishing (Linde, Al2O3), colloidal silica (Syton) polishing, and single-point diamond machining.

Main Results:

• Conventional polishing resulted in surface absorption contributing up to 50% of total absorption.
• Colloidal silica polishing reduced surface absorption to less than 10%.
• The optimal resistivity range for low bulk optical absorption in germanium at room temperature is 10-40 Ω-cm.
• Single-point diamond machining sometimes yielded lower surface absorption than colloidal silica polishing.

Conclusions:

• Surface preparation is a critical factor in minimizing optical absorption in germanium.
• Colloidal silica polishing offers a significant improvement over conventional methods for reducing surface absorption.
• Resistivity plays a key role in bulk optical absorption, with an optimal range identified for germanium.