Terahertz (THz) Spectroscopy

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Float Zone Silicon Wafes for THz Time-Domain Spectroscopy

A PhD in physics requested a quote for the following.

A peer has forwarded an email from you (see below) where you offer certain Si wafers. We would like to order the following few items but we would need a formal quote before:

2 wafers #C963 and 2 wafers #D092. We need those (100)-oriented and high-resistance wafers for our THz time-domain-spectroscopy system in order to block NIR light while the THz beam is transmitted.

Reference #114947 for specs and pricing.

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Our Best-Selling THz Substrates

High-Resistivity Silicon Wafers

Optimized for minimal free-carrier absorption in THz time-domain spectroscopy (THz-TDS). Resistivity >10,000 Ω·cm. Custom sizes available.

GaAs (Gallium Arsenide) Wafers

High mobility substrates for ultrafast carrier dynamic studies. Available in semi-insulating and doped forms for photoconductive switches.

Sapphire Wafers

Transparent in the THz range, ideal for optical access experiments and cryogenic applications.

InP (Indium Phosphide) Wafers

For advanced optoelectronic THz research applications. Available in custom thicknesses and orientations.

Why Choose UniversityWafer, Inc.?

  • Fast global shipping
  • Custom specs for academic and industrial R&D
  • Responsive technical support

Contact us today for a quote or to speak with a technical sales rep!

The perfect crystal dimensions would be: 10,0 x 10,0 x 0,2 mm;
if you do not have that, please send me a quotation for the smallest wafer you have. 
In case you do anti reflection coating, I would like one both (larger) surface sites. Otherwise it would be great if you could recommend me a company which does that. The purpose of the use is terahertz spectroscopy.
Best regards,
Christoph Testud

 

What Is Terahertz (THz) Spectroscopy?

Terahertz (THz) spectroscopy is a powerful, non-contact technique used to probe the electronic, optical, and structural properties of semiconductors using electromagnetic waves in the terahertz frequency range (0.1 to 10 THz)—which sits between the microwave and infrared regions of the spectrum.

In the context of semiconductors, THz spectroscopy is used to:

🔍 Characterize Carrier Dynamics

  • Non-contact measurement of carrier mobility, density, and lifetime.

  • THz pulses can excite and then probe the response of free carriers, revealing how they behave in the material over ultrafast timescales (sub-picoseconds to nanoseconds).

⚡ Measure Conductivity

  • THz waves are sensitive to changes in conductivity, particularly due to photoexcited carriers (like from a laser pulse).

  • This allows researchers to assess photoconductive properties and observe how semiconductors respond under real-time illumination or biasing conditions.

🔬 Investigate Band Structure and Impurities

  • THz spectroscopy can detect interband and intraband transitions, especially in narrow-bandgap materials.

  • Useful for examining dopant levels, traps, and impurity states in the band gap.

📏 Characterize Thin Films and Nanostructures

  • Determine thickness, refractive index, and absorption coefficient of semiconductor films.

  • Especially helpful for non-destructive testing of multilayer semiconductor devices.

🧪 Material Identification

  • Every semiconductor material has a unique THz spectral “fingerprint” due to lattice vibrations (phonons), making it useful for material verification and defect detection.

THz Time-Domain Spectroscopy (THz-TDS)

One of the most common types, where an ultrashort THz pulse is sent through the semiconductor, and its time delay and attenuation are measured after interacting with the sample. The Fourier transform of this signal gives the complex permittivity and conductivity spectra.

Why It's Valuable:

  • Ultrafast (picosecond or femtosecond resolution)

  • Non-contact & non-destructive

  • Works at low energies (safe for delicate or sensitive materials)

  • Can be used at various temperatures, including cryogenic

 

Here's an example using GaAs (Gallium Arsenide) and Silicon, two classic semiconductor materials, and how THz spectroscopy applies to each:

🔬 Example 1: GaAs – Ultrafast Carrier Dynamics

Material: Gallium Arsenide (GaAs)
Why THz? GaAs has high electron mobility, which makes it ideal for high-speed electronics and optoelectronics.

How THz Spectroscopy Helps:

  • In pump-probe THz time-domain spectroscopy, a femtosecond laser pulse excites carriers in the GaAs.

  • A delayed THz probe pulse detects the photoconductivity and carrier relaxation dynamics.

  • Researchers can extract carrier lifetime, mobility, and scattering times, which are critical for optimizing photodetectors, solar cells, or THz emitters.

✅ Result: Allows precise tuning of doping levels, surface passivation layers, and recombination dynamics in high-speed devices.

💡 Example 2: Silicon – Measuring Intrinsic Conductivity and Doping

Material: Intrinsic or lightly doped silicon
Why THz? Silicon is the backbone of modern electronics, and characterizing it non-invasively is very valuable.

How THz Spectroscopy Helps:

  • THz-TDS can differentiate doped vs. intrinsic Si by detecting changes in free-carrier absorption.

  • THz waves interact with free electrons and holes, and the response varies with dopant type and concentration.

  • Useful for quality control of wafers and films, especially when combined with cryogenic or elevated temperature studies.

✅ Result: Non-destructive testing of wafers, evaluation of annealing effects, or verification of SOI (Silicon-On-Insulator) layer uniformity.

🧪 Bonus Application: 2D Materials (e.g., MoS₂, Graphene)

  • THz spectroscopy is also used to study ultrafast charge transport and plasmonic behavior in 2D materials.

  • Helps in understanding how few-layer materials behave when integrated into next-gen sensors or FETs.

Let’s dive into how complex conductivity is derived from THz time-domain spectroscopy (THz-TDS)—and why it matters.

📈 What Is Complex Conductivity?

When a THz pulse interacts with a semiconductor, its amplitude and phase change. From these changes, we can extract the complex conductivity:

σ~(ω)=σ1(ω)+iσ2(ω)\tilde{\sigma}(\omega) = \sigma_1(\omega) + i\sigma_2(\omega)

Where:

  • σ1(ω)\sigma_1(\omega) is the real part (represents absorption / energy loss),

  • σ2(ω)\sigma_2(\omega) is the imaginary part (represents phase shift / energy storage),

  • ω\omega is the angular frequency.

This tells you how carriers in your sample respond dynamically to an electric field at THz frequencies.

⚙️ How It Works: Step-by-Step

1. THz-TDS Setup

  • A femtosecond laser generates an ultrafast optical pulse.

  • This pulse creates a THz pulse via photoconductive antenna or optical rectification.

  • The THz pulse passes through your sample (e.g., a GaAs wafer).

  • A delayed probe pulse measures the transmitted THz electric field in the time domain.

2. Reference vs. Sample

  • You first measure the reference signal: THz pulse through no sample (air).

  • Then, measure the sample signal: THz pulse through the semiconductor.

You now have two electric field signals:

  • Eref(t)E_{\text{ref}}(t)

  • Esam(t)E_{\text{sam}}(t)

3. Fourier Transform

Convert both time-domain signals to the frequency domain:

  • E~ref(ω)\tilde{E}_{\text{ref}}(\omega)

  • E~sam(ω)\tilde{E}_{\text{sam}}(\omega)

4. Transmission Function

Take the ratio:

T(ω)=E~sam(ω)E~ref(ω)T(\omega) = \frac{\tilde{E}_{\text{sam}}(\omega)}{\tilde{E}_{\text{ref}}(\omega)}

This gives the frequency-dependent transmission coefficient.

5. Extract Material Properties

From T(ω)T(\omega), and using knowledge of sample thickness and refractive index, you can compute:

  • Complex permittivity ε~(ω)\tilde{\varepsilon}(\omega)

  • And then the complex conductivity using:

σ~(ω)=−iωε0[ε~(ω)−1]\tilde{\sigma}(\omega) = -i\omega\varepsilon_0 [\tilde{\varepsilon}(\omega) - 1]

(Where ε0\varepsilon_0 is the permittivity of free space.)

🎯 Why It’s Powerful

  • Fast: Probes carrier behavior on sub-picosecond timescales

  • Non-contact: No electrodes needed

  • Wide spectral range: Ideal for low-energy excitations, phonons, free carriers

  • Models: You can fit σ~(ω)\tilde{\sigma}(\omega) with models like Drude or Drude-Smith to extract mobility, scattering time, and carrier concentration.

THz pulse generation

  1. THz pulse generation

  2. Sample interaction

  3. Time-to-frequency transform

  4. Extraction of σ1\sigma_1 and σ2\sigma_2