Wrong Bonds Research Articles

Reproducibility of the Optical Absorption Edge in Amorphous GeS2

Herein, poor reproducibility of optical absorption edges in GeS2 glasses and films is seen. Reported spectral positions of the absorption edge in melt‐quenched glasses spread over ≈0.2 eV at ħω ≈ 3 eV. In deposited films, the edge red‐shifts to ħω ≈ 2.5 eV showing wider variations of ≈1 eV. This work considers plausible reasons of such low, spectral reproducibility, with the aid of ab initio molecular orbital analyses of Ge–S clusters and known insights on optical gaps, electron‐spin‐resonance signals, and structural data. The variation in the glass is likely to be governed by several factors including compositional fluctuation, edge/corner‐shared configurations, wrong bonds, and intimate valence‐alternation pairs. The conspicuous red‐shift in the films seems to be affected also by neutral dangling bonds.

Defects in Se, As2S3, and GeS2 glasses: Ab initio analyses of related clusters

Dangling bonds in Se, As2S3, and GeS2 glasses and wrong bonds in the compounds have been comparatively studied using an ab initio molecular-orbital software. Defect energies and the densities in the chalcogenide glasses are estimated from the total energy Et of small clusters such as H-nSe-H, As3S4H, and Ge4S4H. Properties of corner- and edge-shared connections are also explored for the As-S and Ge-S systems. The three kinds of clusters show for a meta-stable energy ΔEt, relative to the total energy Et of normal bonding structures, a magnitude order of ΔEt(IVAP) < ΔEt(2D0) < ΔEt(D+/D−). Based on this result, we discuss why only GeS2 exhibits unpaired electron spins. The analysis also manifests that ΔEts of clusters containing wrong bonds (As-As, Ge-Ge, and S-S) are comparable with those of ΔEt(IVAP). Regarding electronic levels, the three glasses seem to possess marked gap states having different origins.

Theoretical predictions of the structural stability and property contrast for Sb-rich Ge3Sb6Te5 phase-change materials

Improving the structural stability and property contrast of phase-change materials is important to make phase-change random access memories work in prolonged service stably. Based on the density functional theory and ab initio molecular dynamics simulations, we analyze the structural, electronic, and optical properties of Sb-rich amorphous Ge3Sb6Te5, in comparison with the traditional amorphous Ge2Sb2Te5. The results show that excess Sb concentration can promote the formation of wrong bonds, tetrahedrons, and fivefold rings, which are beneficial for the structural stability of amorphous phases. In addition, Ge3Sb6Te5 has a bigger difference in the bandgap and dielectric function between the crystalline and amorphous phases, indicating that excess Sb concentration can improve the electrical and optical property contrast between the two phases. Our calculation will provide a theoretical basis for applying Sb-rich Ge3Sb6Te5 to retain data stably in prolonged service.

Optical spectroscopy study of damage in ion-irradiated 3C-SiC epilayers on a silicon substrate

Epitaxial cubic (100) 3C-SiC films on a (100) silicon wafer were irradiated at room temperature with 2.3-MeV Si+ or 3.0-MeV Kr+ ions up to a fluence of 1 × 1016 cm−2. The evolutions of the epilayer and the substrate were followed as a function of ion fluence by using micro-Raman spectroscopy, optical absorption, and diffuse reflectance spectroscopy in the UV-visible and near infrared range. Raman spectra evidence the amorphization of SiC films at an estimated dose of about 0.1 displacement per atom (dpa) for both ion irradiations. The narrow peaks of the Raman-allowed TO and LO modes of SiC and Si are recorded in the virgin sample, together with few peaks assigned to zone-edge modes of SiC arising from the intrinsic disorder in the strained films. Those crystal phonon peaks broaden or disappear with increasing fluence. The spectra finally exhibit broad extra peaks assigned to the formation of Si–Si and C–C wrong homonuclear bonds in the local order of the amorphous phase. The optical transmission and diffuse reflectance spectra feature interference fringe patterns in the SiC film that are smoothened out with irradiation due to the matching of refractive indices of the amorphous SiC film and Si substrate. The evolution of the refractive index of SiC and optical gap of Si are deduced from those spectra. The respective roles of ballistic effects and electronic excitations in the radiation damage of both SiC and Si are discussed for those two ions with about the same electronic stopping power and about one order-of-magnitude difference in nuclear stopping power. The damage is dominated by the nuclear collision processes and rather well correlated with the estimated irradiation dose in dpa. Optical spectra show that electronic excitations induce damage recovery of the amorphized substrate below the SiC/Si interface. Raman spectra and optical absorption/reflection spectra yield complementary pictures of the radiation damage.

Photoexcitation-Induced Nonthermal Ultrafast Loss of Long-Range Order in GeTe.

Nonadiabatic quantum molecular dynamics is used to investigate the evolution of GeTe photoexcited states. Results reveal a photoexcitation-induced picosecond nonthermal path for the loss of long-range order. A valence electron excitation threshold of 4% is found to trigger local disorder by switching Ge atoms from octahedral to tetrahedral sites and promoting Ge-Ge bonding. The resulting loss of long-range order for a higher valence electron excitation fraction is achieved without fulfilling the Lindemann criterion for melting, therefore utilizing a nonthermal path. The photoexcitation-induced structural disorder is accompanied by charge transfer from Te to Ge, Ge-Te bonding-to-antibonding, and Ge-Ge antibonding-to-bonding change, triggering Ge-Te bond breaking and promoting the formation of Ge-Ge wrong bonds. These results provide an electronic-structure basis to understand the photoexcitation-induced ultrafast changes in the structure and properties of GeTe and other phase-change materials.

Cr-doped Sb2Te materials promising for high performance phase-change random access memory

Transition metal doping is an effective strategy to improve the properties of phase-change random access memory (PCRAM), while the underlying mechanism remains to be sufficiently investigated. Herein, a Cr-doped Sb2Te film is fabricated by co-sputtering and the enhancement mechanism of Cr-doping is deeply investigated by experiments and first-principles calculations. The CrxSb2Te film exhibits excellent performances involving high crystallization temperature (238.9 °C) and good data retention (10 years @161.7 °C) for Cr0.56Sb2Te, low density change rate (3.8%) and high reversible switching speed (5 ns) for Cr0.29Sb2Te, providing a variety of options for different applications. The calculated results show that Cr atoms preferentially substitute the Sb1 site and the CrTe bonds display a stronger ionic bond character in contrast to the covalent bonded SbTe, thus improving the crystalline phase stability and crystallization temperature. Moreover, ab initio molecular dynamics simulations demonstrate that in the amorphous phase Cr atoms locate in defective octahedral coordination. Meanwhile, the homopolar SbSb wrong bond increased with Cr doping and the Cr atoms tend to form tight cluster in the amorphous phase which could benefit for the fast phase-change speed. Our present findings of Cr0.29Sb2Te PCRAM device clearly reveal the enhancement mechanism by Cr doping which provides guidance for developing high-performance PCRAM devices.

From surface data to bulk properties: a case study for antiphase boundaries in GaP on Si(001)

The orientation, atomic structure, and charge state of defects in crystals have a large influence on the functionality of resultant semiconductor devices. Two-dimensional defects are especially challenging to analyze, as they are embedded within the bulk. We developed a data analysis procedure using information from atomically resolved cross-sectional surface data from two non-parallel surfaces, enabling us to draw conclusions about two-dimensional defects within the bulk. Using antiphase boundaries (APBs) in GaP on Si as a model system, we show that this approach can be applied to any material and microscopy method. For the GaP/Si system and for two-side cross-sectional scanning tunneling microscopy as the experimental tool, the procedure shows that the commonly discussed and APBs do not arise in the investigated sample, whereas the data strongly indicates that previously not recognized APBs are present. Furthermore, a statistical analysis allows a calculation of the net excess charge carrier density introduced into the crystal by the APBs. Its value is negligible compared to the density of wrong bonds that characterize the APBs, since negative and positive excess charges compensate each other almost completely. Moreover, we suggest that controlled enhancement of the formation of APBs may also lead to applications in thermoelectric devices.

Progress on defects and interfacial passivation layers in hybrid perovskite solar cells

<p indent=0mm>Perovskite, as a class of semiconductor materials with unique optoelectronic properties, such as low exciton binding energy, long carrier diffusion length and tunable bandgap, is widely studied in many research fields. Particularly, organic–inorganic hybrid perovskite solar cells (PSCs) have attracted wide attention due to their low cost and high power conversion efficiencies. Currently, inherent defects in perovskites are restricting their further development. As is known to all, both deep level defects and shallow level defects play key roles in the dynamic process of carriers and the stability of perovskites. Recombination loss, unfavorable band bending, unwanted surface reaction and ionic migration caused by these defects, can lead to serious damages to the device lifetime and performance. In this review article, the formation processes and physical properties of the defects, including point defects, surface defects and defects around perovskite grain boundaries, as well as their effects on the PSC performances, are carefully summarized and analyzed. According to the previous studies, conduction band and valence band of perovskites and defect energy states in perovskites are all related to the anti-bonding coupling of Pb-s and I-p atom orbitals. Certain anti-site and interstitial defects caused by the wrong ionic bonds as well as the surface states can create deep level defects. Moreover, shallow level defects produced by the point defects and grain boundaries will aggravate the unfavorable ion migration and surface reaction, which will dramatically deteriorate the device lifetime and performance of the PSCs. Therefore, defect passivation is a crucial and powerful route to optimizing the power conversion efficiency and long-term stability of the PSCs. Moreover, recent advances in interfacial layer used for the defect passivation, categorized as Lewis acid, Lewis base, alkylammonium, polymer, metal oxide, 3D perovskite, graphene and inorganic salt, are described in detail. We comprehensively present the passivation mechanisms and significant effects of these materials in this paper. Note that the main passivation mechanisms of these materials have been summarized as following: (1) Reducing the density of surface defects via the interaction between functional groups of the passivation molecules and the defects; (2) forming passivation layer on the surface through <italic>in situ</italic> reaction between the passivation molecules and the perovskite during post-treatment; (3) optimizing the energy level arrangement of the PSCs by molecular dipole to promote the transport and extraction of photo carriers; (4) adding an insulating layer between the active layer and the charge transport layer to improve the stability of the perovskite and the PSCs; (5) controlling the crystallization and wettability of the perovskite film and each function layer through surface interaction to obtain flat and high-quality films. Lastly, some potential design strategies of passivation materials are proposed, such as intentionally integrating multifunction groups, polymerization of small organic molecules and exploring novel inorganic materials. The theoretical understanding of the defects properties and the mechanisms for defect passivation summarized by this review should be very helpful in further developing the PSCs.

Photoexcitation Induced Ultrafast Nonthermal Amorphization in Sb2Te3

Phase-change materials are of great interest for low-power high-throughput storage devices in next-generation neuromorphic computing technologies. Their operation is based on the contrasting properties of their amorphous and crystalline phases, which can be switched on the nanosecond time scale. Among the archetypal phase change materials based on Ge-Sb-Te alloys, Sb2Te3 displays a fast and energy-efficient crystallization-amorphization cycle due to its growth-dominated crystallization and low melting point. This growth-dominated crystallization contrasts with the nucleation-dominated crystallization of Ge2Sb2Te5. Here, we show that the energy required for and the time associated with the amorphization process can be further reduced by using a photoexcitation-based nonthermal path. We employ nonadiabatic quantum molecular dynamics simulations to investigate the time evolution of Sb2Te3 with 2.6, 5.2, 7.5, 10.3, and 12.5% photoexcited valence electron-hole carriers. Results reveal that the degree of amorphization increases with excitation, saturating at 10.3% excitation. The rapid amorphization originates from an instantaneous charge transfer from Te-p orbitals to Sb-p orbitals upon photoexcitation. Subsequent evolution of the excited state, within the picosecond time scale, indicates an Sb-Te bonding to antibonding transition. Concurrently, Sb-Sb and Te-Te antibonding decreases, leading to formation of wrong bonds. For photoexcitation of 7.5% valence electrons or larger, the electronic changes destabilize the crystal structure, leading to large atomic diffusion and irreversible loss of long-range order. These results highlight an ultrafast energy-efficient amorphization pathway that could be used to enhance the performance of phase change material-based optoelectronic devices.

Effect of Nitrogen on the Amorphous Structure and Subthreshold Electrical Conduction of GeSeSb‐Based Ovonic Threshold Switching Thin Films

Herein, the amorphous structure of Ge–Se–Sb–N chalcogenide thin films is investigated through Raman, infrared, and X‐ray absorption spectroscopies in the light of the electrical performances of such materials once integrated in ovonic threshold switching (OTS) selector devices. In particular, it is shown that the presence of homopolar and wrong bonds in the amorphous structure has a detrimental impact on the subthreshold leakage current of the OTS devices. Although the presence of Sb–Sb and Ge–Sb bonds tends to increase the leakage current in pristine devices, the presence of Se–Se bonds is correlated to a significant device‐to‐device dispersion of subthreshold characteristics after the device initialization. Finally, the incorporation of a proper N concentration in Ge–Se–Sb glass permits to suppress the homopolar bonds, leading to a very low leakage current and a low device‐to‐device dispersion.

Detailed Identification of the Progression of Antiphase Boundaries in GaP/Si(001)

We present a detailed analysis of two-dimensional defects arising at the interface of the nonpolar semiconductor Si to the polar III/V-semiconductor GaP. These defects consist of so called wrong bonds, i. e. two of the same atoms bound to one another. On the two different sides of the defect, the polarity of the GaP crystal is flipped. Accordingly, the defects are called antiphase boundaries. They occur regularly during growth of polar semiconductors onto nonpolar substrates [1] and form three-dimensional antiphase domains investigated in this work.

The [10-10] edge dislocation in the wurtzite structure: A high-resolution transmission electron microscopy investigation of [0001] tilt grain boundaries in GaN and ZnO

A detailed topological analysis and atomic structure investigation of [0001] tilt grain boundaries (GBs) in GaN and ZnO has been carried out using atomistic simulation and high-resolution transmission electron microscopy (TEM). The analysis of Σ7 asymmetric GBs in GaN, shows that they are only made of well-separated a=13<112¯0> edge dislocations based on the three basic structural units: 4-, 8- and 57-atom rings. In this compound, the Burgers vectors adapt their orientations in order to accommodate the rotation angle of the grain boundary. However, in ZnO, the topology of such GBs is shown to comprise two types of dislocation contents: one of them follows the behavior of GaN in which individual a edge dislocations are dominant; the second is the [101¯0] edge dislocation which comes out as a distinct structure with a large core as pointed out in the Σ13 (27.8°/32.2°) GBs. The determined low-energy configuration of this [101¯0] dislocation is made of connected 4-8-6-atom rings in agreement with an early proposal of its possible geometrical model. The electronic structure of these GB dislocations shows the evidence of a deep state induced above the valence band edge in all the structural units comprising the 57-atom ring. In contrast to previous reports which solely ascribed such states to dangling bonds, this one is ascribed to the cooperative effect of Zn–Zn wrong bonds and broken O–O bonds inside the 57-atomic ring.

Interface of GaP/Si(001) and antiphase boundary facet-type determination

GaP/Si(001) virtual substrates are highly interesting for solar cells and optoelectronic device applications. While antiphase disorder at the resulting surface of the virtual substrate—after a few tens of nm GaP—can be suppressed, the structural and electronic properties of the actual GaP-to-Si interface and of the antiphase domains within the GaP are still of high importance. Here, we compare scanning tunneling microscopy data of the prepared Si(001) 2° off-oriented substrate with cross-sectional scanning tunneling microscopy data (XSTM) taken after GaP0.98N0.02/GaP growth. Besides regions where an intermixing of Si with GaP cannot be excluded, we also observe sections with a quite abrupt appearance. In addition, basic knowledge for use of contrast mechanisms occurring in XSTM experiments at antiphase boundaries will be established in order to understand their atomic structure. Thereby, we present a structural model for an entire antiphase domain cross section, including antiphase boundary facet-type determination. Furthermore, we find indications that the majority of the antiphase boundaries within this sample exhibit an equal number of so-called wrong bonds and that XSTM will allow to directly determine the electronic impact of the antiphase boundaries on its surroundings in the future.

First-principles calculations on effects of Al and Ga dopants on atomic and electronic structures of amorphous Ge2Sb2Te5

Effects of post-transition metal dopants M (M = Al, Zn, and Ga) on structural and electronic properties of amorphous Ge2Sb2Te5 (a-GST) are investigated through first-principles calculations based on the density functional theory. The doped a-GST is generated through the melt-quench procedure using molecular dynamics simulations. It is found that the three dopants behave similarly in a-GST, and they are mostly coordinated by Te atoms in tetrahedral geometry, which is similar to those in crystalline MxTey. This is in contrast with crystalline GST wherein the most stable position of dopant M is the octahedral vacancy site. The number of wrong bonds such as Ge–Ge, Ge–Sb, or Sb–Sb increases as dopant atoms predominantly bond with Te atoms. The number of 4-fold ring structures, especially ABAB-type, decreases significantly, explaining the enhanced thermal stability of doped a-GST in the experiment. The bandgap estimated from density of states and the optical gap obtained from Tauc plot increase upon doping, which is also in good agreement with the experiment. By successfully relating the experimental doping effects and changes in the atomic structure, we believe that the present work can serve as a key to offer better retention and lower power consumption in phase-change memory.

Thermodynamics of inversion-domain boundaries in aluminum nitride: Interplay between interface energy and electric dipole potential energy

Aluminum nitride (AlN) has a polar crystal structure that is susceptible to electric dipolar interactions. The inversion domains in AlN, similar to those in GaN and other wurtzite-structure materials, decrease the energy associated with the electric dipolar interactions at the expense of inversion-domain boundaries, whose interface energy has not been quantified. We study the atomic structures of six different inversion-domain boundaries in AlN, and compare their interface energies from density functional theory calculations. The low-energy interfaces have atomic structures with similar bonding geometry as those in the bulk phase, while the high-energy interfaces contain N-N wrong bonds. We calculate the formation energy of an inversion domain using the interface energy and dipoles' electric-field energy, and find that the distribution of the inversion domains is an important parameter for the microstructures of AlN films. Using this thermodynamic model, it is possible to control the polarity and microstructure of AlN films by tuning the distribution of an inversion-domain nucleus and by selecting the low-energy synthesis methods.

Mechanical properties of amorphous Ge2Sb2Te5 thin layers

The working principle of a Phase Change Memory (PCM) cell exploits the repeated reversible transition between a crystalline and an amorphous phase of chalcogenide alloys typically Ge2Sb2Te5, that are characterized respectively by a high (SET) and a low (RESET) conductive state. The change in density between the two phases (6%) induces a very high compressive stress to the active amorphous region by the surrounding crystalline materials. Moreover, the physical iterative transformation between crystalline and amorphous phase transformation introduces a swelling and deswelling effect. This is one of the key failure mechanisms that are limiting the reliability of the final integration of the PCM system. Knowledge of the mechanical properties of the amorphous phase is then an important factor. Amorphous structure, i. e. its short-range order, depends on the adopted formation procedure. In this paper we analyze the mechanical characteristics of sputtered amorphous Ge2Sb2Te5 thin layers and the modification introduced by ion irradiation, a procedure adopted to simulate the amorphous state produced by melt quenching. Measurements of Young's Modulus and Hardness were performed using Ultra High-Nano Indentation on plane samples. The values of both quantities increase of about 10–20% in the 30 keV Ge+ irradiated samples. This trend is due to the reduction of homopolar wrong bonds (GeGe and TeTe) present in the as deposited film. Thermal spikes associated to the impinging ion cause a local atomic rearrangement that results in a structure similar to that of the crystalline phase. The investigation was extended to cantilevers of length in the range 10–200 μm, with a layer of 100 nm Ge2Sb2Te5 deposited on 280 nm thick SiN. The cantilever modal analysis and the out of plane deflection measurements were correlated using a Finite Element modeling, that makes use of the mechanical values measured by Ultra high Nano Indentation. After deposition the amorphous Ge2Sb2Te5 layer is subject to a compressive mechanical shrinkage, this internal stress is released after ion implantation.

Atomic and electronic structure of perfect dislocations in the solar absorber materials CuInSe2 and CuGaSe2 studied by first-principles calculations

Structural and electronic properties of screw and ${60}^{\ensuremath{\circ}}$-mixed glide and shuffle dislocations in the solar absorber materials ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ are investigated by means of electronic structure calculations within density functional theory (DFT). Screw dislocations present distorted bonds but remain fully coordinated after structural relaxation. Relaxed ${60}^{\ensuremath{\circ}}$-mixed dislocations, in contrast, exhibit dangling and ``wrong,'' cation-cation or anion-anion bonds, which induce deep charge transition levels and are electrically active. Analysis of Bader charges and local density of states (LDOS) reveals that acceptor and donor levels are induced by $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ cores, respectively. Moreover, there is local charge accumulation in the surrounding of those cores which contain dangling or ``wrong'' bonds. Thus the apparently harmless nature of dislocations is not because they are electrically inactive, but can only be a result of passivation by segregating defects.

Corrigendum: Natural product-derived quaternary ammonium compounds with potent antimicrobial activity.

Correction to: The Journal of Antibiotics (2016) 69, 344–347; doi:10.1038/ja.2015.107; published online 18 November 2015 The authors of the above article noted an error in publication of this paper in Figures 1 and 2, whereby the wrong bond was used to show the configuration of C9 in (-)-quinine andderivatives thereof.

Effect of intermixing at CdS/CdTe interface on defect properties

We investigated the stability and electronic properties of defects in CdTe1−xSx that can be formed at the CdS/CdTe interface. As the anions mix at the interface, the defect properties are significantly affected, especially those defects centered at cation sites like Cd vacancy, VCd, and Te on Cd antisite, TeCd, because the environment surrounding the defect sites can have different configurations. We show that at a given composition, the transition energy levels of VCd and TeCd become close to the valence band maximum when the defect has more S atoms in their local environment, thus improving the device performance. Such beneficial role is also found at the grain boundaries when the Te atom is replaced by S in the Te-Te wrong bonds, reducing the energy of the grain boundary level. On the other hand, the transition levels with respect to the valence band edge of CdTe1−xSx increases with the S concentration as the valence band edge decreases with the S concentration, resulting in the reduced p-type doping efficiency.

Structural analysis of RF sputtered Ge-Sb-Se thin films by Raman and X-ray photoelectron spectroscopies

Chalcogenide thin films (GeSe2)100−x(Sb2Se3)x (with x=10 and 50) were deposited by Radio-frequency (RF) magnetron sputtering. In order to study the impact of Ar pressure on the structure and the composition of selenide thin films structural properties of thin films and targets were investigated by means of Raman scattering spectroscopy and X-ray photoelectron spectroscopy (XPS). Under low pressure (5·10−3mbar), the increase of wrong bonds like Ge(Sb)-Ge(Sb) was confirmed by Raman and also XPS for both composition. The observed structural changes with Ar pressure are linked with modification of the composition of the selenide films analyzed by EDS and XPS. Furthermore for higher Ar pressure (5·10−2mbar), RF sputtered thin film and target structure present a great similarity. These differences driven by Ar pressure modification are probably related to distinctive sputtering rate and mean free path of the particles ejected from target for the different Ar pressures.