Xun Yuan

Comparative study of structural and electronic properties of Cu-based multinary semiconductors

We present a systematic and comparative study of the structural and electronic properties of Cu-based ternary and quaternary semiconductors using first-principles electronic structure approaches. The important role that Cu d electrons play in determining their properties is illustrated by comparing results calculated with different exchange‐correlation energy functionals. We show that systematic improvement of the calculated anion displacement can be achieved by using the Heyd-Scuseria-Ernzerhof (HSE06) functional compared with the Perdew-Burke-Ernzerhof (PBE) functional. Quasiparticle band structures are then calculated within the G 0 W 0 approximation using the crystal structures optimized within the HSE06 functional and starting from the PBE+U mean-field solution. Both the calculated quasiparticle band gaps and their systematic variation with chemical constituents agree well with experiments. We also predict that the quasiparticle band gap of the prototypical semiconductor Cu2ZnSnS4 in the kesterite phase is 1.65 eV and that the gap of the stannite phase is 1.40 eV. These results are consistent with available experimental values, which vary from 1.45 to 1.6 eV.

Structural properties and quasiparticle band structures of Cu-based quaternary semiconductors for photovoltaic applications

Cu-based quaternary chalcogenide semiconductors (Cu2-II-IV-VI4) are a large group of materials that hold great promises for a variety of applications, especially as thin-film solar cell absorbers. However, despite intensive research activities, a systematic understanding of the evolution of the electronic and structural properties with chemical compositions of these materials is still lacking. In this paper, we present first-principles calculations of the structural and electronic properties of eight such semiconductors (Cu2-II-IV-VI4, with II = Zn and Cd; IV = Ge and Sn; VI = S and Se). The variation of the structural parameters with chemical compositions, investigated using the HSE06 hybrid functional, follows a few interesting trends. The quasiparticle bandgap, calculated using the state-of-the-art GW approximation, also varies systematically with chemical compositions. Effects of cation disordering on the band gaps are also investigated. This systematic understanding of the structural parameters and q...

An unexpected softening from WB3 to WB4

We report a drastic reduction of hardness of about 61% from WB3 to WB4. The three-dimensional covalent network consisting of boron honeycomb planes interconnected with strong zigzag W-B bonds underlies the high hardness of WB3. Despite the strong intralayer and interstitial B-B bonds, the interlayer B-B nonbonding and the considerably weak zigzag W-B bonding allow the layers of WB4 to cleave readily, which results in the anomalous softening of WB4. The results provide robust evidence that the highest boride of tungsten, which is characterized experimentally by an inexpensive superhard material, should be stoichiometric WB3, not WB4.

Unexpectedly hard and highly stable WB3 with a noncompact structure

A highly stable tungsten triboride with the Pearson symbol hP24 (hP24-WB3) is identified by using density functional theory calculations. This new structure can be derived from the well-known hP3 configuration by removing one third of tungsten atoms systematically so that the remaining tungsten atoms form three cycled layers of open hexagons with each a layer displaced by one atom. Such a porous and metallic system has an unexpectedly high Vickers hardness of 38.3 GPa. It is revealed that a three-dimensional covalent framework composed of hexagonal boron planes interconnected with strong zigzag W-B bonds is responsible for its unusual high hardness.

Carrier concentration and mobility in GaN epilayers on sapphire substrate studied by infrared reflection spectroscopy

We report the measurement of carrier concentration and mobility of metalorganic chemical vapor deposited GaN thin films on the sapphire substrate by an infrared reflection technique. By fitting with the experimental data we obtain all the parameters of the lattice vibration oscillators and of the plasmon. From the plasmon frequency and the damping constant we have derived the carrier concentration and the electron mobility. The concentration agrees with the Hall data very well while the mobility values are smaller than that of the Hall measurement by a factor of about 0.5. We attribute such mobility lowering to the increase of scattering for the electrons coupling with the incident photons.

An unusual variation of stability and hardness in molybdenum borides

Molybdenum borides are currently raising great expectations for superhard materials, but their crystal structures and mechanical behaviors are still under discussion. Here, we report an unexpected reduction of stability and hardness from porous hP16-MoB3 and hR18-MoB2 to dense hP20-MoB4 and hR21-Mo2B5, respectively. Furthermore, we demonstrate that this anomalous variation has its electronic origin. These findings not only manifest that the long-recognized hP20-MoB4 (hP3-MoB2) and hR21-Mo2B5 should be hP16-MoB3 and hR18-MoB2, respectively, but also challenge the general design principle for ultrahard materials only pursuing the dense transition-metal borides with high boron content.

Ultrastiffness and metallicity of rhenium nitrides

The equation of states, mechanical properties and electronic structures of the recently synthesized rhenium nitrides (Re3N and Re2N) and the pure metal Re have been investigated by the density functional theory calculations considering the effect of spin-orbit coupling. Our results not only indicate Re3N and Re3N to be ultrastiff and hard materials but also reveal that they exhibit mechanical stability and metallic character. Furthermore, the mechanical behaviors for Re, Re3N, and Re3N can be qualitatively clarified from their crystal and electronic structures. The metallic, ultrastiff, and hard Re3N and Re2N may find their promising applications as cutting tools and hard conductors at the extreme conditions.

Fine structures of photoresponse spectra in quantum well infrared photodetector

We study the fine structures in the photoresponse spectra of GaAs/AlGaAs quantum well infrared photodetectors and the influences from γ irradiation (1-16 Mrad), rapid time annealing (800, 850 and 900 °C for 30 s) and ion implantation (450 keV 0.7×, 1.0× and 5.0×1015 cm−2). With the theoretical analysis a ±-monolayer change has been concluded in the well width of the as-grown GaAs quantum wells in our photodetector. The γ irradiation decreases the carrier lifetime (the relaxation energy Γ increases with the irradiation dose α in the manner of Γ∝α1.5), while the rapid thermal annealing and ion-implantation processes enhance the Al diffusion across the GaAs/AlGaAs heterointerfaces the relaxation energy increases at the same time.

Discovery of elusive structures of multifunctional transition-metal borides

A definitive determination of crystal structures is an important prerequisite for designing and exploiting new functional materials. Even though tungsten and molybdenum borides (TMBx) are the prototype for transition-metal light-element compounds with multiple functionalities, their elusive crystal structures have puzzled scientists for decades. Here, we discover that the long-assumed TMB2 phases with the simple hP3 structure (hP3-TMB2) are in fact a family of complex TMB3 polytypes with a nanoscale ordering along the axial direction. Compared with the energetically unfavorable and dynamically unstable hP3-TMB2 phase, the energetically more favorable and dynamically stable TMB3 polytypes explain the experimental structural parameters, mechanical properties, and X-ray diffraction (XRD) patterns better. We demonstrate that such a structural and compositional modification from the hP3-TMB2 phases to the TMB3 polytypes originates from the relief of the strong antibonding interaction between d electrons by removing one third of metal atoms systematically. These results resolve the longstanding structural mystery of this class of metal borides and uncover a hidden family of polytypic structures. Moreover, these polytypic structures provide an additional hardening mechanism by forming nanoscale interlocks that may strongly hinder the interlayer sliding movements, which promises to open a new avenue towards designing novel superhard nanocomposite materials by exploiting the coexistence of various polytypes.