Covalent Semiconductors Research Articles

Controlled asymmetric aggregation advances n → π* electronic transition and charge separation for enhanced photocatalytic hydrogen synthesis

Photocatalytic water splitting into hydrogen is considered as a promising approach for solar energy storage. The existing organic semiconductor photocatalysts are mainly dominated by π-conjugated covalent structures featuring π → π* electronic transition. Recent studies found that activating n → π* electronic transition is an effective strategy for improving photocatalytic activity of organic semiconductors. Nevertheless, n → π* electronic transition is generally forbidden in perfect symmetric structures. Herein, we have successfully synthesized asymmetric trithiocyanuric acid (TA) aggregates by a solvent chemistry strategy. The asymmetric degrees of TA aggregates can be greatly tuned by changing the pulling strength of solvent molecules. The asymmetric characteristic of TA aggregates increases the orbital overlap between the lone pair electrons on sulfur atoms and the adjacent π* orbitals of triazine, thus allowing n → π* electronic transition. Simultaneously, the asymmetric aggregation induces a large dipole moment, significantly improving the separation and transfer of charge carriers. After an assay of a hydrogen evolution reaction, a quantum yield of 42.9 % at 400 nm was recorded for the asymmetric TA aggregates, which is much higher than those of most existing covalent semiconductors. This study unlocks a fresh realm of artificial photosynthesis, which uses asymmetric aggregates featuring n → π* electronic transition for efficient photocatalytic hydrogen production.

Machine learning accelerated design of 2D covalent organic frame materials for thermoelectrics

Both thermodynamic stability and high thermoelectric figure of merit are required for organic thermoelectric materials for practical application. However, it is extremely time-consuming to design advanced organic thermoelectric materials via conventional theoretical calculation. In this work, combining first-principles calculations, machine learning fitted potential and solving phonon Boltzmann transport equation, we propose a new group of two-dimensional covalent organic frame semiconductors 2AL-PR-X (X = 2H, Ni, Pt, Zn) as promising organic thermoelectric materials. We found that the embedding metal atoms in the center can enhance remarkably the stability of porphyrin ring, demonstrated by evaluating the system total energy and phonon dispersion relationship. Moreover, the interaction between the metal atoms and the porphyrin ring will affect the phonon modes in the porphyrin ring, result in reduction in thermal conductivity. We found that 2AL-PR-Pt is the best thermoelectric candidate material among the studied 2AL-PR-X, which has a peak ZT of 0.32 at room temperature and can reach around 0.9 at 800 K. Our work provides strategy for the thermal transport regulation of two-dimensional covalent organic frame materials and demonstrates the large potential of 2AL-PR-X materials for thermoelectric applications.

High hole mobility and non-localized states in amorphous germanium

Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and germanium (a-Ge), are commonly believed to have localized electronic states at the top of the valence band and the bottom of the conduction band. Electrical conductivity is thought to occur through the hopping mechanism via these localized states. The carrier mobility of these materials is usually very low, in the order of ∼10−3–10−2 cm2/Vs at room temperature. In this study, we show that pure high-density amorphous Ge has exceptionally high carrier mobility, in the order of ∼100 cm2/Vs, and a high hole concentration of ∼1018 cm−3. The temperature-dependent conductivity of the material is also very-well defined with two distinctive regions, extrinsic and intrinsic conductivity, as in crystalline Ge. These results provide direct evidence for a largely preserved band structure and non-localized states within the valence band in high-density amorphous Ge, as previously suggested by Tauc et al. from optical characterization alone.

Direct evidence of high Tc superconductivity in glossy carbon films

Recent achievements and predictions in the field of intrinsic superconductivity in column IV-based covalent semiconductors and the nature of coupling, which determines the prospects for higher transition temperatures as well as the influence of disorder, are debated issues that can determine the future of the field. With this context, here we report the experimental findings of intrinsic superconductivity in glossy carbon (GC) films. These films are one of the forms of carbon materials obtained by pyrolysis of Pongamia glabra oil under high pressure and temperature. A zero-resistance state for films was observed at a critical Zero Resistance temperature of 134 K. Such relatively high superconducting critical temperature of films puts it among the superconductors with the strongest pairing strength between electrons and therefore an ideal material for investigations of high critical temperature superconductors.

Accurate thermochemistry of covalent and ionic solids from spin-component-scaled MP2.

We study the performance of spin-component-scaled second-order Møller-Plesset perturbation theory (SCS-MP2) for the prediction of the lattice constant, bulk modulus, and cohesive energy of 12 simple, three-dimensional covalent and ionic semiconductors and insulators. We find that SCS-MP2 and the simpler scaled opposite-spin MP2 (SOS-MP2) yield predictions that are significantly improved over the already good performance of MP2. Specifically, when compared to experimental values with zero-point vibrational corrections, SCS-MP2 (SOS-MP2) yields mean absolute errors of 0.015 (0.017) Å for the lattice constant, 3.8 (3.7) GPa for the bulk modulus, and 0.06 (0.08) eV for the cohesive energy, which are smaller than those of leading density functionals by about a factor of two or more. We consider a reparameterization of the spin-scaling parameters and find that the optimal parameters for these solids are very similar to those already in common use in molecular quantum chemistry, suggesting good transferability and reliable future applications to surface chemistry on insulators.

'Molecular Beam Epitaxy' on Organic Semiconductor Single Crystals: Characterization of Well-Defined Molecular Interfaces by Synchrotron Radiation X-ray Diffraction Techniques.

Epitaxial growth, often termed “epitaxy”, is one of the most essential techniques underpinning semiconductor electronics, because crystallinities of the materials seriously dominate operation efficiencies of the electronic devices such as power gain/consumption, response speed, heat loss, and so on. In contrast to already well-established epitaxial growth methodologies for inorganic (covalent or ionic) semiconductors, studies on inter-molecular (van der Waals) epitaxy for organic semiconductors is still in the initial stage. In the present review paper, we briefly summarize recent works on the epitaxial inter-molecular junctions built on organic semiconductor single-crystal surfaces, particularly on single crystals of pentacene and rubrene. Experimental methodologies applicable for the determination of crystal structures of such organic single-crystal-based molecular junctions are also illustrated.

Defect-modulated thermal transport behavior of BAs under high pressure

Boron arsenide (BAs) is a covalent semiconductor with a theoretical intrinsic thermal conductivity approaching 1300 W/m K. The existence of defects not only limits the thermal conductivity of BAs significantly but also changes its pressure-dependent thermal transport behavior. Using both picosecond transient thermoreflectance and femtosecond time-domain thermoreflectance techniques, we observed a non-monotonic dependence of thermal conductivity on pressure. This trend is not caused by the pressure-modulated phonon–phonon scattering, which was predicted to only change the thermal conductivity by 10%–20%, but a result of several competing effects, including defect–phonon scattering and modification of structural defects under high pressure. Our findings reveal the complexity of the defect-modulated thermal behavior under pressure.

Enabling Ab Initio Material Design of InAs/GaSb Superlattices for Infrared Detection

Type-II superlattices (T2SLs) exemplified by $\mathrm{In}\mathrm{As}/\mathrm{Ga}\mathrm{Sb}$ are promising for infrared detection as the cutoff wavelength can simply be tuned by the layer thickness. Computational material design is thus highly desirable, but traditional methods such as tight binding and k\ifmmode\cdot\else\textperiodcentered\fi{}p depend on the choice of parameters. Ab initio density-functional-theory (DFT) calculations, on the other hand, suffer from a dilemma because band-gap accuracy and computational efficiency cannot be reached simultaneously due to band-gap underestimation. Here we show the great potential of the shell DFT-1/2 method in the material design of T2SLs. With a single parameter input, the lattice constant, or even without parameter, the method yields satisfactory electronic structures for a series of $\mathrm{In}\mathrm{As}/\mathrm{Ga}\mathrm{Sb}$ T2SLs considering spin-orbit coupling, with the band-gap mismatch limited to 5% compared with our industry-class samples. Detailed miniband analysis is presented based on the ab initio results. For covalent semiconductors without strong correlation, shell DFT-1/2 proves to be an excellent candidate for accurate band-structure calculation with local-density-approximation-level efficiency. It enables the material design of III-V superlattice materials at the engineering level without introducing empirical parameters.

Shell DFT-1/2 method towards engineering accuracy for semiconductors: GGA versus LDA

The Kohn-Sham gaps of density functional theory (DFT) obtained in terms of local density approximation (LDA) or generalized gradient approximation (GGA) cannot be directly linked to the fundamental gaps of semiconductors, but in engineering there is a strong demand to match them through certain rectification methods. Shell DFT-1/2 (shDFT-1/2), as a variant of DFT-1/2, is a potential candidate to yield much improved band gaps for covalent semiconductors, but its accuracy depends on the LDA/GGA ground state, including optimized lattice parameters, basic Kohn-Sham gap before self-energy correction and the amount of self-energy correction that is specific to the exchange-correlation (XC) functional. In this work, the LDA/GGA as well as shDFT-1/2 results of six technically important covalent semiconductors Si, Ge, GaN, GaP, GaAs and GaSb were tested, with an additional ionic insulator LiF for comparison. The impact of XC flavor (LDA, PBEsol, PBE and RPBE), either directly on the gap value, or indirectly through the optimized lattice constant, is examined comprehensively. Moreover, the impact of XC flavor on LDA/GGA and shDFT-1/2 gaps was investigated under the condition of fixed experimental lattice constants. In-depth analysis reveals the rule of reaching the best accuracy in calculating the electronic band structures of typical covalent semiconductors. Relevant parameters like lattice constant, self-consistency in shDFT-1/2 runs, as well as the exchange enhancement factor of GGA, are discussed in details.

Integration atoms in octa- and tetrahedrical internodes of BCC crystals with a free surface

The physical properties of crystals are determined by their chemical nature, the structure of both the entire volume and the surface layer. The surface of a metal, ionic or covalent crystal and semiconductor can be considered as a special state of matter with its own chemistry and physics. The interest in the problem of solid state surfaces, in the science of surface, in the physics of thin films is stimulated by advances in solid state physics, technical innovations, opportunities to create new film materials with unique properties, the challenges of modern technology, the requirements of technology to create devices for computing technology and microelectronics. In this work the processes of redistribution of infiltration atoms along the surface and bulk inter-nodes are considered, the equilibrium distribution of infiltrated atoms on the surface and in the crystal volume, as well as the effect of external pressure on the surface and bulk distribution of infiltration atoms, are studied. Knowledge of the processes of redistribution of infiltration atoms along the inter-nodules of a crystal and of the reasons causing them may allow the purposeful formation of these processes for the purpose of obtaining certain physical characteristics of materials. The kinetics of inter-nodal rearrangement of infiltration atoms has been studied for metals and ordering alloys of various structures.

Electrodeposition of Crystalline Germanium on Glass

The research objective of this work is to test the hypothesis that crystalline germanium can be deposited on insulating substrates through electrochemical-liquid phase epitaxy (ec-LPE). In doing so, this work establishes a new method of semiconductor-on-insulator fabrication. Semiconductor-on-insulator device constructs are at the heart of all modern electronics. Methods that simplify fabrication by lowering the thermal budget and eliminating hazardous precursors are highly desirable. Recently, a new method for the direct preparation of covalent semiconductor crystals at ambient conditions, ec-LPE, was demonstrated by our group. This method utilizes thin liquid metal films simultaneously to perform electrochemical reduction of precursors and to direct heterogeneous nucleation at an underlying substrate of interest. In this work, the fabrication of germanium-on-glass using ec-LPE is demonstrated for the first time. Amperometry, microscopy, and Raman spectroscopy will be presented to demonstrate the proof of principle, i.e. the growth of Ge crystals electrochemically on insulating glass. The surface chemistry of glass substrates will be discussed, including glass functionalized with (3-Aminopropyl)triethoxysilane (APTES), and N-(Trimethoxysilylpropyl)ethylene-diamine triacetic acid (EDTA–TMS). Surfaces with a higher chelating group density improved the efficacy of Ge ec-LPE. These results set the groundwork for semiconductor-on-insulator process development based on ec-LPE.

Ligand-Free Processable Perovskite Semiconductor Ink.

Traditional covalent semiconductors require complex processing methods for device fabrication due to their high cohesive energies. Here, we develop a stable, ligand-free perovskite semiconductor ink that can be used to make patterned semiconductor-based optoelectronics in one step. The perovskite ink is formed via the dissolution of crystals of vacancy-ordered double perovskite Cs2TeX6 (X = Cl-, Br-, I-) in polar aprotic solvents, leading to the stabilization of isolated [TeX6]2- octahedral anions and free Cs+ cations without the presence of ligands. The stabilization of the fundamental perovskite ionic octahedral building blocks in solution creates multifunctional inks with the ability to reversibly transform between the liquid ink and the solid-state perovskite crystalline system in air within minutes. These easily processable inks can be patterned onto various materials via dropcasting, spraying or painting, and stamping, highlighting the crucial role of solvated octahedral complexes toward the rapid formation of phase-pure perovskite structures in ambient conditions.

Atomic-scale understanding on the physics and control of intrinsic point defects in lead halide perovskites

Lead halide perovskites (LHPs) have attracted considerable attention as promising materials for photovoltaic and optoelectronic applications. Intrinsic point defects play an important role in determining the performance of semiconductor devices. LHPs exhibit strong ionic character and unique electronic structure; thus, their defect properties are quite different from conventional covalent bond semiconductors. Understanding the defect science is crucial to the performance optimization of LHP-based devices. State-of-the-art first-principles calculation methods enable one to explore atomistic mechanisms of various defect-related processes, and tremendous efforts from theoretical simulations have provided invaluable insights to the defect physics and defect control of LHPs. In this review, we summarize recent progress, made with the help of theoretical modeling, on atomic-scale understanding about intrinsic point defects and related processes in LHPs. The fundamental properties of intrinsic point defects in LHPs are first introduced, including defect formation energy, charge transition level, and defect tolerance and its origin. A particular emphasis is given to the effects of band edge position on calculated defect properties. The impact of these defects on structural properties, carrier dynamics, and photoluminescence of LHPs is then presented. Advanced strategies to engineer the defects in LHPs are also reviewed, such as growth condition, defect passivation, and doping. Finally, we discuss open issues and outline directions toward a better understanding of defects of LHPs from a theoretical perspective. The goal of the review is to provide a comprehensive summary of atomic-scale understanding of intrinsic point defects in LHPs and to help further related research in the perovskite community.

A Practical Model of Hole Effective Mass of Si on Si1−x Ge x Modulated by External Electric Field and External Strain

This paper reconsiders the mathematical formulation of the conventional nonparabolic band dispersion model of holes of Si on SiGe materials, and proposes a practical model of hole effective mass of covalent semiconductors modulated by the external electric field and external strain. This paper also discusses how the nonparabolicity of the valence band impacts the effective masses of holes that are confined within the barriers. Since the conventional simplified model for band nonparabolicity does not include the external potential effect as a perturbation, it is examined whether this perturbation can be implemented into the conventional model to enhance its usefulness. This consideration is also applied to the impact of strain on the effective mass of holes. The conventional dispersion model for the nonparabolic hole band is examined on the basis of the Hamiltonian operator representation because the insertion of perturbation energy must be validated mathematically. When the perturbation energy is smaller than the unperturbed energy, the insertion of the perturbation energy term into the conventional expression for the nonparabolic band dispersion model is reasonably valid. For example, it is anticipated that this approximation is accurate for a sub-10 nm thick Si physically confined on the SiGe layer, so this study contributes to the analysis of nano-scale devices.

Superior Mechanical Properties of GaAs Driven by Lattice NanotwinningSupported by the National Key Research and Development Program of China (Grant No. 2018YFA0703400), the National Natural Science Foundation of China (Grant Nos. 11704044 and 11974134), the Jilin Province Outstanding Young Talents Project (Grant No. 20190103040JH), and the China Postdoctoral Science Foundation (Grant No. 2018M631870).

Gallium arsenide (GaAs), a typical covalent semiconductor, is widely used in the electronic industry, owing to its superior electron transport properties. However, its brittle nature is a drawback that has so far significantly limited its application. An exploration of the structural deformation modes of GaAs under large strain at the atomic level, and the formulation of strategies to enhance its mechanical properties is highly desirable. The stress-strain relations and deformation modes of single-crystal and nanotwinned GaAs under various loading conditions are systematically investigated, using first-principles calculations. Our results show that the ideal strengths of nanotwinned GaAs are 14% and 15% higher than that of single-crystal GaAs under pure and indentation shear strains, respectively, without producing a significantly negative effect in terms of its electronic performance. The enhancement in strength stems from the rearrangement of directional covalent bonds at the twin boundary. Our results offer a fundamental understanding of the mechanical properties of single crystal GaAs, and provide insights into the strengthening mechanism of nanotwinned GaAs, which could prove highly beneficial in terms of developing reliable electronic devices.

Curie–Weiss behavior of the low-temperature paramagnetic susceptibility of semiconductors doped and compensated with hydrogen-like impurities

For the first time, a quantitative model of the Curie–Weiss behavior of a low-temperature paramagnetic susceptibility of electrically neutral donors in n-type diamagnetic covalent semiconductors is proposed. The exchange interaction between nearest two neutral donors was calculated with the use of the Heitler–London model. In this model, we take into account the change in the thermal ionization energy of donors due to the shift of the bottom of the conduction band to the bandgap with doping and compensation. The energy of the exchange spin–spin interaction between electrons localized on donors is calculated as a function of the donor concentration and the degree of their compensation by acceptors. The broadening of the donor band due to the Coulomb interaction of the nearest impurity ions was taken into account. We considered crystals of n-type germanium doped with arsenic up to the concentration close to the insulator–metal phase transition (Mott transition) and compensated with gallium. The compensation ratio K is the ratio of the concentration of compensating acceptors KN to the concentration of doping donors N. The model predicts a change in the sign of the Curie–Weiss temperature from minus to plus (a transition from the antiferromagnetic to ferromagnetic local ordering of electron spins on donors) for K ≈ 0.15–0.3, reaching its maximum positive values of ≈1.3 K for K ≈ 0.5 with the following decrease (a transition to paramagnetism) for K > 0.85. The calculated behavior of the paramagnetic susceptibility of donors is consistent with the experimental data for compensated n-Ge:As,Ga samples close to the Mott transition.

Calculation and interpretation of classical turning surfaces in solids

Classical turning surfaces of Kohn–Sham potentials separate classically allowed regions (CARs) from classically forbidden regions (CFRs). They are useful for understanding many chemical properties of molecules but need not exist in solids, where the density never decays to zero. At equilibrium geometries, we find that CFRs are absent in perfect metals, rare in covalent semiconductors at equilibrium, but common in ionic and molecular crystals. In all materials, CFRs appear or grow as the internuclear distances are uniformly expanded. They can also appear at a monovacancy in a metal. Calculations with several approximate density functionals and codes confirm these behaviors. A classical picture of conduction suggests that CARs should be connected in metals, and disconnected in wide-gap insulators, and is confirmed in the limits of extreme compression and expansion. Surprisingly, many semiconductors have no CFR at equilibrium, a key finding for density functional construction. Nonetheless, a strong correlation with insulating behavior can still be inferred. Moreover, equilibrium bond lengths for all cases can be estimated from the bond type and the sum of the classical turning radii of the free atoms or ions.

Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.

Temperature-dependent band gaps in several semiconductors: from the role of electron–phonon renormalization

Temperature dependence of band gap is one of the most fundamental properties for semiconductors, and has strong influences on many applications. The renormalization of the band gap at finite temperatures is due to the lattice expansion and the phonon-induced atomic vibrations. In this work, we apply the recently-developed electron–phonon renormalization (EPR) method to study the temperature-dependent band gap in some classical covalent (diamond, Si, and SiC) and ionic semiconductors (MgO and NaCl). The contributions from both the lattice expansion and the phonon-induced atomic vibrations at finite temperatures are considered. The results show that the band gaps Eg all decrease as temperature T increases, consistent with the experiments and other theoretical studies (e.g., from 0 K to 1500 K, the reductions are ∼0.451 eV for diamond and ∼1.148 eV for MgO, respectively). The covalent compounds investigated show weaker temperature dependences of Egs than the ionic compounds, due to the much weaker lattice expansions and therefore low contributions from these. The zero-point motion effect has greater influence on the band gap in semiconductors with light atoms, such as diamond (reduction ∼0.437 eV), due to larger atomic displacements. By decomposing the EPR effect into respective phonon modes, it is found that the high-frequency optical phonon vibrations dominate the temperature-dependent band gap in both covalent and ionic compounds. Our work provides the fundamental understandings on the temperature-dependent band gaps caused by lattice dynamics.

Defect physics and doping engineering in semiconductor optoelectronic materials

Semiconductor materials play a central role in the integrated circuits, photovoltaics, information and communication, microelectronic devices, lighting, and so on. However, the performance of semiconductor optoelectronic devices depends critically on the dopability and defect engineering of semiconductor materials. In this paper, firstly, we introduce the doping properties in semiconductors and the theoretical progress of charged defect calculations over the past decades, including the traditional jellium model. To overcome the disadvantages and limitations of the traditional jellium model, recently, we propose a straightforward and universal theory, i.e., transfer real state model (TRSM), which can calculate directly the charged defect properties in both bulk and low-dimensional semiconductors. In the jellium model for a finite supercell size calculation, it suffers a serious problem to determine the defect properties in low-dimensional semiconductors due to charge distribution in the vacuum region. However, our TRSM method by putting the ionized electrons or holes on a real host band edge states naturally keeps the supercell neutral and provides clear physical meaning. For three-dimensional bulk materials, the defect formation energy and transition energy level calculated by our TRSM method are almost the same as the results obtained by using the traditional jellium model. For low dimensional semiconductors, however, the TRSM method cures the divergence issue that occurred in the jellium model due to long-range electrostatic Coulomb interactions. Secondly, we discuss the wide band gap semiconductors. By analyzing the doping limit law, we elucidate the effective methods for defect engineering in oxide wide band gap semiconductors, and then how their p-type conductivity can be improved. Those methods are based on two rules: (1) Reducing the ionization energy of acceptors; (2) suppressing the formation of compensating donors. Further, the physical mechanism of the difference in conductivity between ionic and covalent compounds in the amorphous wide band gap semiconductors is introduced. A band coupling model is employed to clarify the difference between pseudo-hydrogen passivated and real-hydrogen passivated low dimensional wide band gap semiconductors. Thirdly, we discuss the semiconductor alloys with high doping concentration. In the nonisovalent semiconductor alloys, due to strong wave function localization of the band edge states, the physical properties of nonisovalent alloys do not meet the statistical average law, distinct from the isovalent alloys. The band crossing phenomenon in large mismatched isovalent semiconductor alloys is often misinterpreted by a phenomenological two-level band anti-crossing model. Fortunately, this phenomenon has now been properly explained by the band broadening picture. When doping small concentration of Bi or N impurities into GaAs, the defect levels are localized due to the weak interactions between impurities. The band edges of GaAs1− x Bi x and GaAs1− x N x consist of the host atoms. The impurity level gradually broadens out with the increase of the impurity concentration, and thus the band edges of defective GaAs are dominated by impurities. Finally, we focus on the diffusion of metal impurities in semiconductors. The fundamental reason for the differences in the diffusion of Ag and Cu atoms between ionic and covalent semiconductors is clarified. The s-d coupling between the d orbitals of the diffusors and the s orbitals of the host materials are responsible for the behavior of diffusive atoms. The deep understanding of metal impurities in semiconductor materials offers effective theoretical guidance for controlling the diffusion properties of impurities in different types of semiconductor materials.