Graphene-aluminum nitride NEMS resonant infrared detector

The use of micro-/nanoelectromechanical resonators for the room temperature detection of electromagnetic radiation at infrared frequencies has recently been investigated,showing thermal detection capabilities that could potentially outperform conventional microbolometers.The scaling of the device thickness in the nanometer range and the achievement of high infrared absorption in such a subwavelength thickness,without sacrificing the electromechanical performance,are the two key challenges for the implementation of fast,high-resolution micro-/nanoelectromechanical resonant infrared detectors.In this paper,we show that by using a virtually massless,high-electrical-conductivity,and transparent graphene electrode,floating at the van der Waals separation of a few angstroms from a piezoelectric aluminum nitride nanoplate,it is possible to implement ultrathin(460 nm)piezoelectric nanomechanical resonant structures with improved electromechanical performance(450% improved frequency×quality factor)and infrared detection capabilities(4100×improved infrared absorptance)compared with metal-electrode counterparts,despite their reduced volumes.The intrinsic infrared absorption capabilities of a submicron thin graphene–aluminum nitride plate backed with a metal electrode are investigated for the first time and exploited for the first experimental demonstration of a piezoelectric nanoelectromechanical resonant thermal detector with enhanced infrared absorptance in a reduced volume.Moreover,the combination of electromagnetic and piezoelectric resonances provided by the same graphene–aluminum nitride-metal stack allows the proposed device to selectively detect short-wavelength infrared radiation(by tailoring the thickness of aluminum nitride)with unprecedented electromechanical performance and thermal capabilities.These attributes potentially lead to the development of uncooled infrared detectors suitable for the implementation of high performance,miniaturized and power-efficient multispectral infrared imaging systems.

Absorption and emission modulation in a MoS2–GaN（0001） heterostructure by interface phonon–exciton coupling

Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides(TMDs)interfaced to gallium nitride(Ga N)are excellent material systems to realize broadband light absorbers and emitters due to their close proximity in the lattice constants.The surface properties of a polar semiconductor such as Ga N are dominated by interface phonons,and thus the optical properties of the vertical heterostructure are influenced by the coupling of these carriers with phonons.The activation of different Raman modes in the heterostructure caused by the coupling between interfacial phonons and optically generated carriers in a monolayer MoS_2–Ga N(0001)heterostructure is observed.Different excitonic states in MoS_2 are close to the interband energy state of intraband defect state of Ga N.Density functional theory(DFT)calculations are performed to determine the band alignment of the interface and revealed a type-I heterostructure.The close proximity of the energy levels and the excitonic states in the semiconductors and the coupling of the electronic states with phonons result in the modification of carrier relaxation rates.Modulation of the excitonic absorption states in MoS_2 is measured by transient optical pump-probe spectroscopy and the change in emission properties of both semiconductors is measured by steady-state photoluminescence(PL)emission spectroscopy.There is significant red-shift of the C excitonic band and faster dephasing of carriers in MoS_2.However,optical excitation at energy higher than the bandgap of both semiconductors slows down the dephasing of carriers and energy exchange at the interface.Enhanced and blue-shifted PL emission is observed in MoS_2.Ga N band-edge emission is reduced in intensity at room temperature due to increased phonon-induced scattering of carriers in the Ga N layer.Our results demonstrate the relevance of interface coupling between the semiconductors for the development of optical and electronic applications.