Hot Carrier-Assisted Intrinsic Photoresponse in Graphene

Hot Carrier-Assisted Intrinsic Photoresponse in Graphene

| Nathaniel M. Gabor, Justin C. W. Song, Qiong Ma, Nityan L. Nair, Thiti Taychatanapat, Kenji Watanabe, Takashi Taniguchi, Leonid S. Levitov, Pablo Jarillo-Herrero
Graphene, a promising material for optoelectronics, photonics, and energy-harvesting, has an intrinsic optoelectronic response that remains unclear. This study reports on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation at the p-n interface leads to striking six-fold photovoltage patterns as a function of gate voltages. These patterns, along with the measured spatial and density dependence of the photoresponse, provide strong evidence that non-local hot-carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This novel regime, which features a long-lived and spatially distributed hot carrier population, may open the doorway for optoelectronic technologies exploiting efficient energy transport at the nanoscale. The photoresponse of semiconductors is governed by the energy relaxation pathways of photoexcited electron-hole pairs. In graphene, energy relaxation pathways are strongly altered by the vanishing electronic density of states. After initial relaxation, electron-lattice energy relaxation can be quenched, creating a bottleneck that limits further energy redistribution into the lattice. With electron-to-lattice energy relaxation quenched, a novel transport regime is reached in which thermal energy is redistributed solely among electronic charge carriers. The photo-generated carrier population remains hot while the lattice stays cool. In graphene, hot carriers should play a key role in optoelectronic response, yet no measurements have clearly determined the photocurrent generation mechanism. Recent studies suggest that photothermoelectric effects may play an important role. This work reports optoelectronic transport measurements of gate voltage-controlled graphene p-n junction devices in the presence of local laser excitation that determine the intrinsic photoresponse. Our measurements unambiguously indicate that hot electronic carriers dominate graphene’s intrinsic optoelectronic response, at temperatures ranging from room temperature down to 10 K and in the linear optical power regime. The hot carrier regime manifests as a strong photothermoelectric effect that results in a striking six-fold photovoltage pattern as a function of gate voltages. Additionally, the spatial and charge density dependence of the optoelectronic response establishes a strong connection between thermal energy transport and electronic charge carriers. The study shows that the six-fold photovoltage pattern indicates the presence of a strong photothermoelectric effect. The photothermoelectric effect is attributed to the difference in temperature between the excited region and its surroundings, resulting in a thermal current accompanied by a charge current. The sign and magnitude of the PTE voltage depend on the Seebeck coefficient in each region. The non-monotonic dependence of the Seebeck coefficient results in multiple sign reversals for the quantity (S1 - S2), which occur along three nodal lines, giving rise to the six-fold pattern. The six-fold pattern is also observed in the bilayer graphene photoresponse. The study confirmsGraphene, a promising material for optoelectronics, photonics, and energy-harvesting, has an intrinsic optoelectronic response that remains unclear. This study reports on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation at the p-n interface leads to striking six-fold photovoltage patterns as a function of gate voltages. These patterns, along with the measured spatial and density dependence of the photoresponse, provide strong evidence that non-local hot-carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This novel regime, which features a long-lived and spatially distributed hot carrier population, may open the doorway for optoelectronic technologies exploiting efficient energy transport at the nanoscale. The photoresponse of semiconductors is governed by the energy relaxation pathways of photoexcited electron-hole pairs. In graphene, energy relaxation pathways are strongly altered by the vanishing electronic density of states. After initial relaxation, electron-lattice energy relaxation can be quenched, creating a bottleneck that limits further energy redistribution into the lattice. With electron-to-lattice energy relaxation quenched, a novel transport regime is reached in which thermal energy is redistributed solely among electronic charge carriers. The photo-generated carrier population remains hot while the lattice stays cool. In graphene, hot carriers should play a key role in optoelectronic response, yet no measurements have clearly determined the photocurrent generation mechanism. Recent studies suggest that photothermoelectric effects may play an important role. This work reports optoelectronic transport measurements of gate voltage-controlled graphene p-n junction devices in the presence of local laser excitation that determine the intrinsic photoresponse. Our measurements unambiguously indicate that hot electronic carriers dominate graphene’s intrinsic optoelectronic response, at temperatures ranging from room temperature down to 10 K and in the linear optical power regime. The hot carrier regime manifests as a strong photothermoelectric effect that results in a striking six-fold photovoltage pattern as a function of gate voltages. Additionally, the spatial and charge density dependence of the optoelectronic response establishes a strong connection between thermal energy transport and electronic charge carriers. The study shows that the six-fold photovoltage pattern indicates the presence of a strong photothermoelectric effect. The photothermoelectric effect is attributed to the difference in temperature between the excited region and its surroundings, resulting in a thermal current accompanied by a charge current. The sign and magnitude of the PTE voltage depend on the Seebeck coefficient in each region. The non-monotonic dependence of the Seebeck coefficient results in multiple sign reversals for the quantity (S1 - S2), which occur along three nodal lines, giving rise to the six-fold pattern. The six-fold pattern is also observed in the bilayer graphene photoresponse. The study confirms
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Understanding Hot Carrier%E2%80%93Assisted Intrinsic Photoresponse in Graphene