Beyond graphene, transition metal dichalcogenides (TMDCs) and black phosphorus (BP), group-IV monochalcogenides are emerging as a unique class of layered materials. We report in this paper experimental and theoretical investigations of germanium selenide (GeSe) and its heterostructures. We find that GeSe has a prominent anisotropic electronic transport with maximum conductance along the armchair direction. Density functional theory (DFT) calculations reveal that the effective mass along the zigzag direction is 2.7 times larger than that in the armchair direction, and the anisotropic effective mass explains the observed anisotropic conductance. The crystallographic direction of the GeSe is confirmed by angle-resolved polarized Raman measurement, which is further supported by calculated Raman tensors for the orthorhombic structure. Novel GeSe/MoS2 pn heterojunctions are created, taking advantage of the natural p-type doping in GeSe and n-type doping in MoS2. The temperature-dependence of the junction current measurement reveals that GeSe and MoS2 form a type II band alignment with a conduction band offset of ~ 0.234 eV. The anisotropic conductance in GeSe may enable a new series of electronic and optoelectronic devices such as plasmonic devices with resonance frequency continuously tunable with light polarization direction and high-efficiency thermoelectric devices. The unique GeSe/MoS2 pn junctions with type II alignment will be an essential building block for vertical tunneling field-effect transistors for low power applications. This new p-type layered material GeSe can also be combined with ntype TMDCs to form heterogeneous complementary metal oxide semiconductor (CMOS) circuits.Keywords: monochalcogenides, germanium selenide, anisotropic conductance, polarized Raman, pn heterojunction. ------------------------------------------------------------* These authors contributed equally to this paper. # Corresponding author's email: wjzhu@illinois.edu 2 Layered group-IV monochalcogenides are a newly emerging materials platform. As compared to graphene, transition metal dichalcogenides (TMDCs) and black phosphorus (BP), layered monochalcogenides such as SnS, SnSe, GeS and GeSe have many unique electrical, thermal, and optical properties that could prove useful for diverse applications. In particular, layered monochalcogenides have an orthorhombic (distorted rock-salt) structure and feature anomalously high Grüneisen parameters, which lead to ultralow thermal conductivity and an exceptionally high thermoelectric figure of merit, 1, 2 which makes them promising for thermoelectric applications. At high temperatures, layered monochalcogenides undergo a displacive phase transition from a lower symmetry Pnma space group to a higher symmetry Cmcm space group, 1, 3 which makes them an interesting candidate for phase change memory. The bandgap of layered group-IV monochalcogenides is in the range of 0.5 to 1.5eV, 4 lining up fairly well with the solar spectrum, which makes them attractive for solar cells and photodete...
We systematically investigate the spatial/temporal photocurrent in photodetectors and electronic transport in transistors/Hall-bar devices based on monolayer MoS2 grown by chemical vapor deposition (CVD). We found that the maximum photocurrent occurs when the laser spot is close to the metal/MoS2 contact and is tunable by the applied drain voltage, which can be explained by the modulation of the local electric field at the Schottky barrier, consistent with predictions from our quantum transport simulation. We observed that the maximum photocurrent at drain contact is much larger than the one at the source contact, and the DC currents show rectifying behavior. These phenomena can be explained by the different Schottky barrier heights at the two contacts. By measuring Hall-bar structure at various temperatures from 100 K to 400 K, we extracted the barrier heights at the source and drain contacts, separately. We found that the barrier height at drain contact is about 50 mV larger than the one at the source contact, consistent with the photocurrent and DC current observations. We measured the photocurrent at various powers, and a photoresponsivity of 3.07 mA/W was extracted at low powers. When the power increases above 20 μW, the photocurrent starts to saturate. Temporal response of the photocurrent is also dependent on the laser power. At high laser powers, photocurrent overshoot was observed. The photocurrent saturation at high powers and the overshoot in temporal photocurrent are likely due to the same mechanism: an accumulation of electrons in the channel, flattening out the band structure, since the laser spot is located near the drain contact in these measurements. These studies of photocurrents and electronic transport in CVD MoS2 highlight the importance of the contacts in the electronic/optoelectronic devices and reveal the physical mechanism of the photocurrent/electronic transport in these devices.
CdS/CdTe thin film photovoltaics were produced with transition metal nitrides (TMNs) as back contacts. The devices show photovoltaic activity but a significant Schottky barrier was found at the back contact. Solar cells perform better as the ZrN films are thicker due to improved microstructure. The good reflectance of ZrN makes it acts as a reflective layer to reduce optical losses and lower the thickness of CdTe. Pure N2 plasma treatment of the back surface of the CdTe and annealing did not improve the performance. A thin layer of Cu was introduced to dope the CdTe and completed with a ZrN film but more amount of Cu deteriorates the performance. Although the nitrides are highly stable and have good electrical properties, wehave not been able to obtain devices to date that are not limited by the back contacts.
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