Transient Grating Spectrometer
- Carrier diffusion coefficient in a matter of minutes
- Non-invasive measurement technique
- Fully automated and computer-controlled
- Sensitivity down to µJ/cm² excitation level
- Advanced measurement and analysis software
- Photoluminescence measurement option
HARPIA-TG is a transient grating spectrometer designed for measuring carrier diffusion and lifetime. Measurements are based on the laser-induced transient grating (LITG) technique, enabling the simultaneous observation of non-equilibrium carrier recombination and diffusion by all‑optical means.
HARPIA-TG allows the characterization of electrically non‑conductive or non-fluorescent samples. It is suitable for a wide range of materials, including semiconductor materials and derivatives such as silicon carbide (SiC), gallium nitride (GaN), perovskites, organic and inorganic solar cells, quantum dots, and even complex nanostructures like quantum wells.
Coupled with a CARBIDE or PHAROS femtosecond laser featuring an integrated optical parametric amplifier (I-OPA), the compact system is fully automated and computer-controlled via advanced measurement and analysis software. Thus, the user only needs to put the sample in the holder and start the measurement to obtain the diffusion coefficient within minutes.
Applications
- Carrier diffusion measurement
- Carrier lifetime measurement
- Carrier diffusion length measurement
- Single-wavelength absorption
The principle of a LITG measurement is illustrated in the figure on the right. A pair of ultrashort pulses are overlapped both spatially and temporally at the sample plane. The angular separation of the excitation beams causes them to interfere at their crossing, creating an interference pattern. The period of this pattern Λ depends on the beam intersection angle and the pump wavelength.
Excitation by this periodic pattern generates a spatially modulated distribution of excited carriers and, consequently, a periodic modulation of the refractive index. This pumping geometry forms a transient grating from which a temporally delayed probe pulse can diffract.
Over time, the laser-induced grating decays due to carrier recombination (electronic decay with the rate of τR) and carrier diffusion (spatial decay with the rate of τD ). The diffusion term depends on the transient grating period. Fine gratings (smaller Λ values) diffuse more rapidly than coarser gratings (larger Λ values). By measuring the temporal behavior of the diffracted signal across a series of different periods Λ, the carrier diffusion coefficient D (cm2/s) can be determined using the appropriate relationship:
where τG is the net decay rate of the transient grating.
The principle of a LITG measurement.
I.
Transient decay dynamics are measured across various grating periods Λ. The HARPIA-TG enables continuous tuning of the excitation grating period. Grating periods ranging from 1.15 to 15 μm can be generated at the sample plane, depending on the pump wavelength.
II.
Data obtained at each grating period Λ is fit to an exponential decay. The retrieved reciprocal decay constants are then plotted
as a function of the inverse square of the grating period (1/Λ2). The tangent of this curve provides the carrier diffusion coefficient, corresponding to the given carrier concentration and temperature. Meanwhile, the zero-intercept point Λ = ∞ μm provides the intrinsic carrier recombination rate τR.
III.
The experiment is repeated across various excitation intensities to obtain a comprehensive dependence of the diffusion coefficient on the non-equilibrium carrier concentration.
Polarization and pump wavelength control enable more advanced spin grating and thermal grating measurements, respectively.
The graphs below show the carrier diffusion coefficient, diffusion length, and lifetime of GaN at the back and front of the layer as a function of fluence. Thicker GaN layers exhibit better structural quality due to better coalescence. This is evidenced by lower diffusivity and shorter lifetimes, which indicate poor structural quality and higher defect density at the interface between the sapphire substrate and the GaN layer.
Measurements were performed using the HARPIA-TG spectrometer combined with the air-cooled CARBIDE femtosecond laser and I-OPA.
Measurement conditions: 60 kHz, 355 nm pump wavelength, and 1030 nm probe wavelength.
Diffusion coefficient of GaN as a function of fluence.
Diffusion length of GaN as a function of fluence.
Carrier lifetime of GaN as a function of fluence.
Silicon carbide (SiC) is a compound semiconductor renowned for its unique properties, including high thermal conductivity, a wide bandgap, and excellent electrical performance.
In SiC devices, where high-frequency, high-temperature, and high-voltage operation is common, managing carrier diffusion is particularly critical to ensure efficient and reliable device performance. This makes carrier diffusion a key factor in advancing SiC semiconductor technology.
Diffusion coefficient of SiC as a function of fluence.
Diffusion length of SiC as a function of fluence.
Carrier lifetime of SiC as a function of fluence.
HARPIA-TG transient grating spectrometer.
HARPIA-TG recommended layout with CARBIDE-CB5 and I-OPA.
HARPIA-TG drawing.
HARPIA-TG principal scheme.
HARPIA-TG is equipped with dedicated software that enables fully automated selection of pump and probe parameters, as well as the grating period. This simplifies the measurement process for determining diffusion coefficients and carrier lifetimes, making it as user-friendly as possible.
HARPIA-TG software, transient grating control window.
Measurements were performed with undoped GaN.
