The physics of terahertz negative photoconductivity in low-dimensional materials

Negative Photoconductivity: Bizarre Physics in Semiconductors

“…To date, almost all commercialized photodetector devices are based on positive photoconductivity. The opposite NPC phenomenon has been widely demonstrated in inorganic semiconductors. − Various types of mechanisms/hypotheses have been proposed behind the NPC generation in these semiconductors. In the following few subsections, we will discuss the NPC phenomenon in inorganic semiconductors and the possible mechanism behind this feature.…”

“…As we discussed in the above sections that NPC was widely observed in various types of materials. ,,,,− Different origin has been proposed for NPC generation, which can be suitable for various kind of applications. The NPC effect has been used for the fabrication of humidity and light sensors, highly sensitive photodetectors, and nonvolatile memories. ,− Qin et al demonstrated that the NPC in nanodiamonds can be applied for humidity sensors with the sensitivity of 1.26 × 10 6 %, which is the highest in carbon-based humidity sensors .…”

Carrier Dynamics in Solution-Processed CuI as a P-Type Semiconductor: The Origin of Negative Photoconductivity

“…Thus, photoinduced increments in n c will translate in a positive ΔA (positive photoconductivity response (PPC)), while increments in m* and τ c will yield negative ΔA (negative photoconductivity response (NPC)). 30,31 The kinetic trace extracted at 4550 nm was successfully fitted with three exponential decays, two of them with a positive amplitude (τ 1 ≈ 0.4 ps (98%) and τ 2 ≈ 60 ps (2%)) and one with a negative amplitude (τ 3 ≈ 2 ps). A schematic representation of the signal deconvolution is presented in the Figure 2b inset.…”

“…In the Drude formalism, by considering that for a probe in the 4000–5000 nm range ω 2 τ c 2 ≫ 1, (ω is the angular frequency and τ c the carrier mean free path), σ(ω) can be expressed as double-struckR { σ false( ω false) } ≈ σ 0 ω 2 τ c 2 = n c q 2 m * ω 2 τ c where σ 0 corresponds to the DC conductivity, q corresponds to the carrier charge, n c is the carrier population density, and m * is the carrier effective mass. From eq and eq , a last relation can be established: normalΔ A ( ω ) ∝ normalΔ σ ( ω ) ∝ normalΔ ( n c m * τ c ) Thus, photoinduced increments in n c will translate in a positive Δ A (positive photoconductivity response (PPC)), while increments in m * and τ c will yield negative Δ A (negative photoconductivity response (NPC)). , …”

“…No matter the origin, negative Δ A are rare in 3D semiconductors. Generally, photoexcitation leads to the generation of free carriers in great numbers, outweighing any influence that a change in m * or τ c could have, i.e., PPC signal. , NPC responses are more common in metals and semimetals commonly detected in the THz range. , …”