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 τR) and carrier diffusion (spatial decay with the rate of τD ). 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 τG is the net decay rate of the transient grating.
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 τR.
- 可扩展至长波(可见光/近红外)。详情请联系 sales@lightcon.com。
- 可根据需求提供 SH(515 nm)或基于 OPA 的探测。详情请联系 sales@lightcon.com。
- 取决于激发波长。
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.
HARPIA-TG 提供了一个专用软件,可以完全自动选择泵浦和探测参数以及光栅周期,从而尽可能简化扩散系数和载流子寿命的测量。
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