Charge Carrier Concentration Measurement (THz)

THz-TDS measures how broadband terahertz pulses interact with a semiconductor. When a THz pulse strikes the surface, a part of the pulse reflects from each internal interface and is modified by the free carriers within the material.

From these reflected signals, both the amplitude and phase can be analyzed to extract electrical conductivity, plasma frequency, and ultimately charge carrier concentration (N). The underlying model follows Drude’s theory, linking the material’s dielectric response to the density and mobility of charge carriers.

By comparing the measured and simulated reflection waveforms, the charge carrier concentrations of both the epilayer and the buffer can be determined simultaneously. This unique capability enables full-stack characterization of semiconductor wafers such as 4H-SiC, even when the epilayers differ by three orders of magnitude in their doping concentration (J. Hennig et al., Opt. Express 33:22 (2025) 45828).

Unlike traditional electrical methods, THz spectroscopy works without electrodes or chemical/physical contact. The sample remains pristine, avoiding contaminations or surface damage.
Even more importantly, THz-TDS provides depth-sensitive information, being able to probe through multilayer stacks that are opaque to visible and infrared light.

Key benefits:

• Contactless measurement: No physical contact, no chemical residue, no sample preparation.
• Simultaneous layer evaluation: Access both epilayer and substrate properties in one step.
• Wide dynamic range: Detects carrier concentrations across several orders of magnitude.
• High spatial resolution: Enables full wafer mapping with sub-millimeter precision.
• Non-destructive: Suitable for production-line quality control and research metrology alike.

In industrial semiconductor environments, measurement speed and robustness are as crucial as accuracy. This is where Electronically Controlled Optical Sampling (ECOPS) has revolutionized THz metrology.

ECOPS-based THz systems such as TOPTICA’s TeraFlash smart acquire complete THz time-domain waveforms at kHz sampling speeds. This enables the rapid scanning of entire wafers.

Advantages of high-speed ECOPS for charge carrier mapping:

• Faster than contact-based methods,  up to 200× shorter measurement times than mCV.
• Insensitive to acoustic noise or mechanical vibrations. The fast sampling “freezes” disturbances, ensuring stable results in industrial environments.
• Real-time wafer mapping, with thousands of points measured in minutes.

In a collaboration between Fraunhofer Institute for Industrial Mathematics (ITWM), Fraunhofer Institute for Integrated Systems and Device Technology (IISB), Technical University Freiberg, Freiberg Instruments and TOPTICA Photonics, an entire SiC wafer was mapped in under 70 minutes with more than 17,000 data points, compared to 20 minutes for only 25 points using mCV. The results demonstrated a good agreement between THz-derived carrier densities and mCV reference measurements.

The transition to SiC and other wide-bandgap semiconductors demands new metrology tools that match the materials’ complexity and production speed. Terahertz time-domain spectroscopy fulfills this need, delivering non-contact, broadband, and high-speed measurements of charge carrier concentration and wafer uniformity.

With ECOPS-based THz systems like TOPTICA's TeraFlash smart, wafer mapping that once took hours now takes minutes. The result: precise, contamination-free, and industrially viable semiconductor inspection, paving the way towards future high-efficiency power devices.