Zhenqiang Jack Ma

Bio: Zhenqiang Jack Ma is an academic researcher from University of Wisconsin-Madison. The author has contributed to research in topics: Quantum efficiency & Photodetector. The author has an hindex of 1, co-authored 1 publications receiving 54 citations.

Single-crystalline germanium nanomembrane photodetectors on foreign nanocavities.

Abstract: Miniaturization of optoelectronic devices offers tremendous performance gain. As the volume of photoactive material decreases, optoelectronic performance improves, including the operation speed, the signal-to-noise ratio, and the internal quantum efficiency. Over the past decades, researchers have managed to reduce the volume of photoactive materials in solar cells and photodetectors by orders of magnitude. However, two issues arise when one continues to thin down the photoactive layers to the nanometer scale (for example, <50 nm). First, light-matter interaction becomes weak, resulting in incomplete photon absorption and low quantum efficiency. Second, it is difficult to obtain ultrathin materials with single-crystalline quality. We introduce a method to overcome these two challenges simultaneously. It uses conventional bulk semiconductor wafers, such as Si, Ge, and GaAs, to realize single-crystalline films on foreign substrates that are designed for enhanced light-matter interaction. We use a high-yield and high-throughput method to demonstrate nanometer-thin photodetectors with significantly enhanced light absorption based on nanocavity interference mechanism. These single-crystalline nanomembrane photodetectors also exhibit unique optoelectronic properties, such as the strong field effect and spectral selectivity.

Mechanisms of photoconductivity in atomically thin MoS2

Abstract: Atomically thin transition metal dichalcogenides have emerged as promising candidates for sensitive photodetection. Here, we report a photoconductivity study of biased mono- and bilayer molybdenum disulfide field-effect transistors. We identify photovoltaic and photoconductive effects, which both show strong photogain. The photovoltaic effect is described as a shift in transistor threshold voltage due to charge transfer from the channel to nearby molecules, including SiO2 surface-bound water. The photoconductive effect is attributed to the trapping of carriers in band tail states in the molybdenum disulfide itself. A simple model is presented that reproduces our experimental observations, such as the dependence on incident optical power and gate voltage. Our findings offer design and engineering strategies for atomically thin molybdenum disulfide photodetectors, and we anticipate that the results are generalizable to other transition metal dichalcogenides as well.

Ultrafast All-Optical Switching of Germanium-Based Flexible Metaphotonic Devices.

Abstract: Incorporating semiconductors as active media into metamaterials offers opportunities for a wide range of dynamically switchable/tunable, technologically relevant optical functionalities enabled by strong, resonant light-matter interactions within the semiconductor. Here, a germanium-thin-film-based flexible metaphotonic device for ultrafast optical switching of terahertz radiation is experimentally demonstrated. A resonant transmission modulation depth of 90% is achieved, with an ultrafast full recovery time of 17 ps. An observed sub-picosecond decay constant of 670 fs is attributed to the presence of trap-assisted recombination sites in the thermally evaporated germanium film.

Strong Light–Matter Interaction in Lithography-Free Planar Metamaterial Perfect Absorbers

Abstract: The efficient harvesting of electromagnetic (EM) waves by subwavelength nanostructures can result in perfect light absorption in the narrow or broad frequency range. These metamaterial-based perfec...

Halide-Induced Self-Limited Growth of Ultrathin Nonlayered Ge Flakes for High-Performance Phototransistors

Abstract: 2D nonlayered materials have attracted intensive attention due to their unique surface structure and novel physical properties. However, it is still a great challenge to realize the 2D planar structures of nonlayered materials owing to the naturally intrinsic covalent bonds. Ge is one of them with cubic structure impeding its 2D anisotropic growth. Here, the ultrathin single-crystalline Ge flakes as thin as 8.5 nm were realized via halide-assisted self-limited CVD growth. The growth mechanism has been confirmed by experiments and theoretical calculations, which can be attributed to the preferential growth of the (111) plane with the lowest formation energy and the giant interface distortion effect of the Cl–Ge motif. Excitingly, a Ge flake-based phototransistor shows excellent performances such as a high hole mobility of ∼263 cm2 V–1 s–1, a high responsivity of ∼200 A/W, and fast response rates (τrise = 70 ms, τdecay = 6 ms), suggesting its great potential in the applications of electronics and optoelectr...

High-Responsivity Mid-Infrared Black Phosphorus Slow Light Waveguide Photodetector

Abstract: DOI: 10.1002/adom.202000337 transparency windows and the characteristic absorption bands of abundant biochemical molecules.[1,2] Thus, the MIR possesses enormous potential for various applications, ranging from thermal imaging for homeland security and missile guidance to label-free absorption spectroscopy for environmental monitoring, industrial process control, and medical diagnostics.[3–7] The monolithic integration of waveguides and photodetectors enables miniaturization of photonic systems and is an essential step towards the realization of on-chip sensing systems.[8,9] Moreover, the waveguide photodetector provides another advantage of decoupling the optical absorption length from the absorption material thickness, thereby offering more flexibility in the device geometry design for performance optimization.[10] However, the development of MIR waveguide photodetectors is still in infancy, which is primarily hindered by the huge lattice mismatch between silicon (Si) and typical narrowbandgap semiconductors for MIR photodetection, such as II–VI,[11] III–V,[12,13] and IV–VI[14] alloys. 2D materials, whose layered lattice structures ease their monolithic integration with Si, are regarded as promising alternatives to overcome this bottleneck. Compared with graphene that suffers from large dark current due to its zero bandgap,[15–18] the lately rediscovered black phosphorus (BP) has been attracting intense research interest for realizing high-performance MIR photodetection because of its narrow direct bandgap of around 0.3 eV in bulk form corresponding to a cut-off wavelength of 4.13 μm.[19,20] Beyond 4.13 μm, through alloying with arsenic[21,22] or exploring the Stark effect by applying a vertical electric field,[23] BP photoresponses have been extend to around 8 μm. Various MIR BP photodetectors with free-space geometry have been reported.[24–28] In addition, several BP waveguide photodetectors have been demonstrated in the near-infrared and the short-wavelength infrared.[29,30] Recently, MIR grating-couplerintegrated BP photodetectors were reported with a high responsivity of 1.333 A W−1 at 3.78 μm in a 40 nm zigzag device under 1 V bias and 1.193 μW incident power.[31] Nevertheless, the high responsivity was obtained at the expense of a long device length (i.e., channel width) of 80 μm. Further miniaturization of the photodetector is expected to improve the operation speed, the signal-to-noise ratio (SNR), and the internal quantum Black phosphorus (BP) offers unique opportunities for mid-infrared (MIR) waveguide photodetectors due to its narrow direct bandgap and layered lattice structure. Further miniaturization of the photodetector will improve operation speed, signal-to-noise ratio, and internal quantum efficiency. However, it is challenging to maintain high responsivities in miniaturized BP waveguide photodetectors because of reduced light–matter interaction lengths. To address this issue, a method utilizing the slow light effect in photonic crystal waveguides (PhCWGs) is proposed and experimentally demonstrated. A shared-BP photonic system is proposed and utilized to fairly and precisely characterize the slow light enhancement. Close to the band edge around 3.8 μm, the responsivity is enhanced by more than tenfold in the BP photodetector on a 10 μm long PhCWG as compared with the counterpart on a subwavelength grating waveguide. At a 0.5 V bias, the BP PhCWG photodetector achieves a 11.31 A W−1 responsivity and a 0.012 nW Hz−1/2 noise equivalent power. The trap-induced photoconductive gain is validated as both the dominant photoresponse mechanism and the major limiting factor of the response speed. The BP slow light waveguide photodetector is envisioned to realize miniaturized high-performance on-chip MIR systems for widespread applications including environmental monitoring, industrial process control, and medical diagnostics.