HARPIA-TG is a transient grating spectrometer for the measurement of carrier diffusion and lifetime. Measurements are based on the laser-induced transient grating (LITG) technique. This method enables 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 semiconductors materials and derivatives, e.g., silicon carbide (SiC), gallium nitride (GaN), perovskites, organic and inorganic solar cells, quantum dots, and even complex nanostructures such as quantum wells.

Coupled with CARBIDE or PHAROS laser with 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 in a matter of minutes.

Applications

Carrier diffusion measurement
Carrier lifetime measurement
Carrier diffusion length measurement
Single-wavelength absorption

Basics of Operation

The principle of a LITG measurement is depicted in the figure on the right. A pair of ultrashort pulses are overlapped both spatially and temporally at the sample plane. Angular separation of the excitation beams causes them to interfere at their crossing. The period of the interference pattern Λ depends on the beam intersection angle and the pump wavelength.

Excitation by a periodic pattern creates a spatially-modulated excited carrier distribution and, effectively, a periodic modulation of the refractive index. Therefore, this pumping geometry produces 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 τ_R) and carrier diffusion (spatial decay with the rate of τ_D ). The diffusion term depends on the transient grating period: fine gratings (small Λ values) diffuse faster than coarser ones (large Λ values).  Accordingly, if we measure the temporal behavior of the diffracted signal over a series of different periods Λ, we can determine the carrier diffusion coefficient D[cm²/s] from the relationship,

where τ_G is the net decay rate of the transient grating.

Measurement Ranges

Parameter	Value
Diffusion coefficient	0.1 – 50 cm²/s
Carrier lifetime	1 ps – 80 ns

Measurement Process

I.

Transient decay dynamics are measured at various grating periods Λ. HARPIA-TG allows continuous tuning of the excitation grating period. Periods ranging from 1.15 to 15 μm (depending on the pump wavelength) can be formed at the sample plane.

II.

Data obtained at each grating period is fit to an exponential decay. The retrieved reciprocal decay constants are plotted
as a function of the inverse square of the grating period. Tangent of this curve provides the carrier diffusion coefficient (at the given carrier concentration and temperature), while the zero-intercept point (Λ = ∞ μm) provides the intrinsic carrier recombination rate τ_R.

III.

Experiment is repeated at various excitation intensities to obtain a comprehensive dependence of the diffusion coefficient on the non-equilibrium carrier concentration.

Polarization and pump wavelength control enables more advanced spin grating and thermal grating measurements, respectively.

Specifications

Model	HARPIA-TG
Grating recording wavelength ¹⁾	340 – 560 nm
Probe wavelength ²⁾	1030 nm
Grating period ³⁾	1.15 – 15 μm
Pulse duration	< 290 fs
Delay range	Up to 8 ns

Extendable to long-wave VIS/NIR. Contact sales@lightcon.com for details.
SH (515 nm) or OPA-based probe is available upon request. Contact sales@lightcon.com for details.
Depends on the excitation wavelength.

Measurement ranges

Model	HARPIA-TG
Diffusion coefficient	0.1 – 50 cm²/s
Carrier lifetime	1 ps – 80 ns

Dimensions

Model	HARPIA-TG
Physical dimensions (L × W × H)	730 × 420 × 188 mm

Performance

GaN

The graphs below indicate the carrier diffusion coefficient, diffusion length, and lifetime of GaN at the back and at the front of the layer as a function of fluence. The thicker the GaN, the better the quality of the grown layer due to better coalescence. It is evidenced by the lower diffusivity and shorter lifetimes that indicate poor structural quality and higher defect density at the interface between the sapphire substrate and GaN.

Measurements were performed using HARPIA-TG combined with CARBIDE-CB5 laser and I-OPA. Measurement conditions: 60 kHz, 355 nm pump wavelength, 1030 nm probe wavelength.

Diffusion coefficient of GaN as a function of fluence

Carrier lifetime of GaN as a function of fluence

Diffusion length of GaN as a function of fluence

SiC

Silicon carbide (SiC) is a compound semiconductor with unique properties, valued for its high thermal conductivity, 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, making it a key consideration in SiC semiconductor technology.

Diffusion coefficient of SiC as a function of fluence

Carrier lifetime of SiC as a function of fluence

Diffusion length of SiC as a function of fluence

Images & Drawings

Principal scheme of HARPIA-TG

Drawing of HARPIA-TG

Recommended layout with CARBIDE-CB5 and I-OPA

HARPIA-TG dedicated software

HARPIA-TG offers a dedicated software that enables fully automated selection of pump and probe parameters and grating period, thus, making the measurements of diffusion coefficient and carrier lifetime as simple as possible.