Intrinsic carrier multiplication in layered Bi_(2)O_(2)Se avalanche photodiodes with gain bandwidth product exceeding 1 GHz

Emerging layered semiconductors present multiple advantages for optoelectronic technologies including high carrier mobilities,strong light-matter interactions,and tunable optical absorption and emission.Here,metal-semiconductor-metal avalanche photodiodes(APDs)are fabricated from Bi2O2Se crystals,which consist of electrostatically bound[Bi2O2]2+and[Se]2−layers.The resulting APDs possess an intrinsic carrier multiplication factor up to 400 at 7 K with a responsivity gain exceeding 3,000 A/W and bandwidth of~400 kHz at a visible wavelength of 515.6 nm,ultimately resulting in a gain bandwidth product exceeding 1 GHz.Due to exceptionally low dark currents,Bi2O2Se APDs also yield high detectivities up to 4.6×1014 Jones.A systematic analysis of the photocurrent temperature and bias dependence reveals that the carrier multiplication process in Bi2O2Se APDs is consistent with a reverse biased Schottky diode model with a barrier height of~44 meV,in contrast to the charge trapping extrinsic gain mechanism that dominates most layered semiconductor phototransistors.In this manner,layered Bi2O2Se APDs provide a unique platform that can be exploited in a diverse range of high-performance photodetector applications.

Violation of the T^(−1) Relationship in the Lattice Thermal Conductivity of Mg_(3)Sb_(2) with Locally Asymmetric Vibrations

Most crystalline materials follow the guidelines of T^(-1) temperature-dependent lattice thermal conductivity(κ_(L))at elevated temperatures.Here,we observe a weak temperature dependence ofκL in Mg_(3)Sb_(2),T^(-0:48) from theory and T-0:57 from measurements,based on a comprehensive study combining ab initio molecular dynamics calculations and experimental measurements on single crystal Mg_(3)Sb_(2).These results can be understood in terms of the so-called“phonon renormalization”effects due to the strong temperature dependence of the interatomic force constants(IFCs).The increasing temperature leads to the frequency upshifting for those low-frequency phonons dominating heat transport,and more importantly,the phononphonon interactions are weakened.In-depth analysis reveals that the phenomenon is closely related to the temperature-induced asymmetric movements of Mg atoms within MgSb_(4) tetrahedron.With increasing temperature,these Mg atoms tend to locate at the areas with relatively low force in the force profile,leading to reduced effective 3^(rd)-order IFCs.The locally asymmetrical atomic movements at elevated temperatures can be further treated as an indicator of temperature-induced variations of IFCs and thus relatively strong phonon renormalization.The present work sheds light on the fundamental origins of anomalous temperature dependence of κ_(L) in thermoelectrics.