<sec>Density functional theory is used to calculate the adsorption and diffusion behavior of Al atoms on clean, H-terminate, Cl-terminate Si(100) and Si(111) surfaces. The most stable position of Al atom adsorption and the diffusion path are different on Si(100) surface terminated by different methods. On the surface of clean Si(100), the <i>Tr</i> site is the most stable site for Al atom with an adsorption energy of 4.01 eV, and the <i>H</i> and <i>M</i> sites are the sub-stable stable sites with the adsorption energies of 3.51 eV, and 3.63 eV, respectively. When the Al atom is adsorbed at the <i>Tr</i> site on the clean Si(100) surface, it bonds with the Si atom to destroy the Si—Si bond in the dimer. Therefore Al is easily adsorbed at the <i>Tr</i> site of the trench and diffuses in a zigzag pattern along the trench. On the H-terminate and Cl-terminate Si(100) surface, Si—Si bonds in the dimer column are changed from cross to parallel. Al is easily adsorbed at the <i>H</i> position at the top of the dimer column, and diffuses along the line at the top of the dimer. The differential charge density shows that the Al atom transfers electrons to the Si atoms on the surface, and the surface H-terminate and Cl-terminate weaken the interaction between Al atoms and Si, and reduces the diffusion energy barrier of Al atoms. </sec><sec>The Si(111) surface terminated by different methods has the same stable position (<i>T</i><sub>4</sub> site) for the adsorption of Al atoms. When Al atom adsorbs at the <i>T</i><sub>4</sub> site on the clean Si(100) surface, it bonds to Si atom, which located at the three <i>T</i><sub>1</sub> site, then Al atom is firmly fixed by the three Al—Si bonds with a bond length of 2.55 Å. Thus Al atom can has the largest adsorption energy and form the most stable state at the <i>T</i><sub>1</sub> site. With the diffusion and migration of Al atom, the bond between Al atom and the <i>T</i><sub>1</sub> site in the opposite direction appears to be broken. When Al atom migrating to the saddle point position is the most unstable. Here Al atom bonds to the Si atoms of the two adjacent <i>T</i><sub>1</sub> sites to form a bond with a length of 2.49 Å, which is 0.06 Å shorter than the initial Al—Si bond (2.55 Å). What’s more, the diffusion energy barrier of Al atom at this position is 0.65 eV, which impede Al atom to diffuse and migrate. When Al atom migrates to the <i>H</i> site, it rebonds to the three Si atoms on the adjacent surface and forms a bond with a length of 2.52 Å, which is 0.03 Å shorter than the Al—Si bond (2.55 Å) at the initial position. On the H-terminate and Cl-terminate Si(111) surface, Al atom doesn’t bond with Si atom for the H or Cl saturates the dangling bonds on the Si surface. The Si(111) surface terminated by different methods has the same stable position for adsorption of Al atoms. The diffusion paths of Al atoms are similar, and they are easy to be adsorbed to the top position (<i>T</i><sub>4</sub> site) of the second Si atom, and the path along <i>T</i><sub>4</sub> to <i>H</i><sub>3</sub> is diffused. Similarly, the H-terminate or Cl-terminate of Si(111) surface weakens the electron transfer between Al and Si atoms and reduces the diffusion energy barrier of Al atoms. Regardless of the Si(100) or Si(111) surface, the H-terminate and Cl-terminate Si surfaces are effective in reducing the diffusion barrier of Al atoms.</sec>
Two-dimensional (2D) materials have shown great potential for electronic and optoelectronic applications. Among the 2D materials, molybdenum disulfide (MoS2) has received great attention in the transition metal dichalcogenides family. Unlike graphene, 2D MoS2 can exhibit semiconducting properties and its band gap is tunable with thickness. A demonstration of a single-layer MoS2 based field-effect transistor (FET) with a high on/off current ratio (about 108) has aroused the considerable interest. Although 2D MoS2 exhibits fascinating intrinsic properties for electronics, the contact may limit the device performance severely. In a real device such as FET, semiconducting 2D MoS2 needs contact with a metal electrode, and a Schottky barrier is always formed at the semiconductor-metal interface. The formation of low-resistance contact is a challenge, which is important for achieving high on current, large photoresponse and high-frequency operation. Therefore, understanding and tuning the interfaces formed between metals and 2D MoS2 is critical to controlling the contact resistance. In this work, some efforts have been made to investigate the 2D MoS2-metal interface in order to reduce the Schottky barrier height. By using the first-principles calculations based on density function theory, we investigate the effects of halogen doping-on metal-MoS2 interface, including the formation energy of defect, electronic structure, charge difference, and population. All calculations are performed using the ultrasoft pseudopotential plane wave method implemented in the CASTEP code. We use the generalized gradient approximation for the exchange and correlation potential as proposed by Perdew-Burke-Ernzerhof. Firstly, we calculate the formation energy to find the thermodynamically stable positions for the halogen elements located in 2D MoS2. It is shown that the halogen elements tend to occupy the S site of a MoS2 monolayer. Meanwhile, for the MoS2 monolayer, the halogen doping may introduce the defect level into the forbidden gap and make the Fermi level shift. For the metal-MoS2 interface, halogen doping can modulate its Schottky barrier height effectively in terms of Schottky-Mott model. This is because the Schottky barrier height at the metal-semiconductor interface depends on the difference between the Fermi level and the band edge position of the semiconductor. At the metal-MoS2 interface, the Fermi level is partially pinned as a result of the interface dipole formation and the production of the gap states. Therefore, using different metals with different work functions cannot modify the Schottky barrier height effectively. Here we demonstrate that F and Cl doping can reduce the Schottky barrier height, while Br and I doping can increase it. According to the results of the differential charge density analysis, we can ascribe the tuning of Schottky barrier height to the influence of the dipole caused by the charge transfer among the interfaces. This study can explain the relevant experimental results very well and provide a potential route to achieving low-resistance contact in the future applications of 2D materials.
Blocked impurity band (BIB) detectors, developed from extrinsic detectors, have long been employed for ground-based and airborne astronomical imaging and photon detections. They are the state-of-the-art choice for highly sensitive detection from mid-infrared to far-infrared radiation. In this work, we demonstrate the existence of an interfacial barrier in blocked impurity band structures by evidence of temperature-dependent dark currents, bias-dependent photocurrent spectra and corresponding theoretical calculations. The origin of the build-in field is studied. The temperature-dependent characteristics of space charge effects are also investigated in detail. It is found that at higher temperature (T 14 K), the space charge influence is negligible, and the interfacial barrier is mainly caused by bandgap narrowing effects. Based on interfacial barrier effects, a dual-excitation model is proposed to clarify the band structure of BIB detectors. The photocurrent spectra related to the two excitation processes, i.e., the direct excitation over the interfacial barrier and excitation to the band edge with subquent tunneling into blocking layer, are successfully extracted and agree reasonably well with the calculated band structure results. The effects of interfacial barrier on the photocurrent spectrum, peak responsivity and internal quantum efficiency of the devices are investigated. With the consideration of interfacial barrier effects, the calculated peak responsivity shows good agreement with the experimental result. It is suggested that interfacial barrier effects should be considered for successfully designing the BIB detectors. Additionally, the build-in field is found to equivalently lower the critical field for impact ionization. This study provides a better understanding of the working mechanism in BIB detectors and also a better device optimization.
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