Dangling Bond States Research Articles

Modelling of transport and recombination of photocarriers in un-doped hydrogenated amorphous silicon (a-Si:H)

In this paper, we report on the simulation of steady state photoconductivity in un-doped a- Si:H at temperatures from 30 to 500 K. The model is based on recombination at dangling bond states and band tail states. It takes also into account the hopping transitions in the conduction hand tail states to describe the conduction in localized states at low temperatures. At high temperatures, the multiple trapping process is considered to describe the conduction in extended states. The density of states includes the exponential density of conduction band tail states and valence band tail slates and the density of dangling bond states. This later is determined by the Defect Pool Model ‘DPM’. The experimental features observed on the temperature dependence of the photoconductivity are generally the thermal quenching, the low activated region and the temperature independent photoconductivity at very low temperatures. All these observations are well reproduced by the model in un-doped a-Si:H. By the examination of the relative contributions of two processes of conduction: (i) the multiple trapping and (ii) the multiple trapping associated with the hopping, the model results show that the multiple trapping process of electrons where the conduction is assured by free carriers in the thermal quenching region above 140 K is important while the hopping process of electrons is negligible. At 140 K and below, the hopping transport of electrons in the conduction band tail states makes an important contribution in the photoconductivity. It explains successfully the low activated region and the temperature independent photoconductivity at very low temperatures.

(Invited) (Photo-)Electrochemical Reactions on Semiconductor Surfaces:Si and III-V Surfaces – Atomic and Electronic Structure

Semiconductor/electrolyte interfaces are used in many application areas where strong non linear variations in the electric performance depending on applied potential or due to illumination play a decisive role. Often a simplified explanation on charge redistribution across the semiconductor/electrolyte junctions is applied considering only the charge redistribution involving the bulk and the electrolyte following an idealized semiconductor /metal Schottky barrier model. But as will be addressed in our contribution strong deviations are usually observed in real devices which are due to Fermi level pinning effects and adsorbate related double layer potential shifts of the band edges. This is especially true for photoelectrochemical cells used in water splitting devices. For this reason we will present detailed surface science studies on the properties of Si and III-V surfaces espcially by applying photoelectron spectroscopy.In so-called model experiments the effect of surface states found on differently doped Si(111) and (100) surfaces is investigated in relation to the resulting surface electronic structure after appying different surface preparation and processing procedures. Additionally the reactivity and the changes in electronic structure following a „frozen electrolyte“ junction formation will be presented after water exposure at low sample temperature. The results are compared to photoelectrochemical cells based on Si single crystals as well as thin film multiabsorber cells used as photocathodes.For III-V semiconductors at first the surface properties of etched InP, GaP and GaInP (100) surfaces will be compared and further modified by subsequent vacuum based processing steps as heating and sputtering. The best surfaces are in their surface composition very close to samples prepared by MOCVD. Also for the III-V materials the interaction with H2O was investigated. In all cases Fermi level pinning close to the valence band maximum was observed and compared to the resulting band diagrams of photoelectrochemical devices used for HER.In combining photoelectrochemical studies with surface science investigations on the electrode properties using directly contacted but also buried junction photoelectrodes it can be shown that defect levels related to remaining dangling bond states play a dominant role for the performance of the devices. The concentration of the remaining defect states influence the obtained performance in dependence of the interfacial contact composition preferring the formation of buried junctions. In conclusion we suggest that surface science studies of photoelectrodes before and after electrochemical studies are essential and need to be further developed for in operando experiments with special emphasis also in the valence band regime to obtain improved insights into semiconductor/electrolyte junction properties.

Silicon Oxy-Nitride for the Low Refractive Index Layers in the Mirror Coatings of the Cryogenic Laser Interferometer Gravitational Waves Detector.

The low refractive index layers in the mirror coatings of the room-temperature laser interferometer gravitational waves detectors are silica deposited by the ion beam sputter method. However, the silica film suffers from the cryogenic mechanical loss peak, hindering its application for the next generation detector operated at cryogenics. New low refractive index materials need to be explored. We study amorphous silicon oxy-nitride (SiON) films deposited using the method of plasma-enhanced chemical vapor deposition. By changing the N_{2}O/SiH_{4} flow rate ratio, we can tune the refractive index of the SiON smoothly from nitridelike to silicalike of ∼1.48 at 1064nm, 1550nm, and 1950nm. Thermal anneal reduced the refractive index down to ∼1.46 and effectively reduced the absorption and cryogenic mechanical loss; the reductions correlated with the N─H bond concentration decrease. Extinction coefficients of the SiONs at the three wavelengths are reduced down to 5×10^{-6}∼3×10^{-7} by annealing. Cryogenic mechanical losses at 10K and 20K (for ET and KAGRA) of the annealed SiONs are significantly lower than the annealed ion beam sputter silica. They are comparable at 120K (for LIGO-Voyager). Absorption from the vibrational modes of the N─H terminal-hydride structures dominates over the absorption from other terminal hydrides, the Urbach tail, and the silicon dangling bond states in SiON at the three wavelengths.

Scanning tunneling microscopy study of the antiferromagnetic topological insulator MnBi2Se4

Topological insulators with intrinsic magnetic ordering such as MnBi 2 Se 4 have been predicted to host many exotic topological phenomena including dissipationless edge states and magnetoelectric effects. Here we use scanning tunneling microscopy to study the surface of MnBi 2 Se 4 epitaxial thin films . Large scale topographic images show steps between van der Waals layers and screw dislocations . While we find the Se-termination predominant on the surface, areas attributed to Bi-termination are found at the edges of small islands in as-grown films, or in larger areas upon removal of surface capping. We compare these terminations through an analysis of step heights, atomic resolution images and tunneling spectroscopy . Tunneling spectra on the Se-termination reveals semiconducting behavior, with an apparent bandgap of >0.18 eV that depends on the tip height. In contrast, tunneling spectra of the Bi-termination appears gapless, which could be an indication of dangling bond states on the surface. In both cases, a hexagonal 3.9 Å lattice is observed that is consistent with the expected crystal structure. • First STM study of MnBi 2 Se 4 surface. • Atomic resolution imaging confirms predominance of Se termination and antisite defects. • Large areas of Bi termination correlated with Se cap removal. • Tunneling spectroscopy reveals termination-dependent density of states. • Se termination exhibits ∼0.2 eV semiconducting gap.

Analysis of effects of dangling-bond defects in doped a-Si:H layers in heterojunction silicon solar cells with different electron affinities of ITO contacts

The effects of dangling-bond defects in doped hydrogenated amorphous-silicon layers (p-a-Si:H and n-a-Si:H) in heterojunction silicon (SHJ) solar cells are studied in relation to applied Indium-Tin-Oxide (ITO) contacts with different electron affinities. A state-of-the-art numerical model of the SHJ solar cell was employed, including ITO contacts as full, volumetric semiconductor layers, applying the trap-assisted, band-to-band and direct tunnelling mechanisms at heterointerfaces in the device. The levels of dangling bond defect concentrations were varied in both p-a-Si:H and n-a-Si:H layers and ITOs with two different electron affinities were considered at both sides of the device. We show that the effects of the defects on the short-circuit current density, open-circuit voltage, fill factor and conversion efficiency of the device become more pronounced if ITOs with non-optimal electron affinities are used. Possibility to reach higher doping levels of the doped a-Si:H layers would mitigate the effects of its dangling bond states, which becomes more important if ITO electron affinity is not optimized to the doped a-Si:H layers. We demonstrate that the reduced efficiency due to the increase in dangling-bond density originates from the decrease of the fill-factor and open-circuit voltage, whereas the short-circuit current density has a small effect on efficiency for the chosen variation span. The reduction of the fill-factor is further explained by a drop in maximum-power-point voltage, which is more pronounced if optimization of ITO electron affinity is not taken into account.

Electronic phase transitions in quasi-one-dimensional atomic chains: Au wires on Si(553)

The Si(553) surface, covered with half a monolayer of gold, forms a double strand of well-ordered atomic Au chains in each miniterrace. It represents one of the smallest possible realizations of quasi-one-dimensional (1D) systems. In this prototype system, by combining DC conductance and low-energy electron diffraction measurements with density functional calculations and ab initio Monte Carlo simulations, we demonstrate that nonlocal electronic correlations, together with thermal excitations, lead to peculiar phase transitions without long-range order. They are characterized by marginal (average) geometric relaxations, but with huge variations of the electronic band structure and concomitant strong temperature-dependent modifications of the density of states close to the Fermi level. Similar phenomena are expected in the large class of quasi-1D conductors and open a wide range of possibilities for their controlled manipulation. It is the increasing hybridization between spin-polarized Au and Si edge states that makes the Si dangling bond states at the step edge conducting, first as a transient between two insulating phases and finally opening a permanent new 1D conduction channel at high temperatures.

Engineering of the Topological Surface States and Topological Dangling Bond States in the (0001) Surface of Bi2Se3 via Structural Distortion

The (0001) surface band structure evolution of Bi2Se3 is studied for different broken symmetries in the top quintuple layer (QL) of 8QLs slab. Each surface electronic structure shows dangling bond states (DBS) irrespective of broken symmetries introduced by the surface termination or layer vacancy. The DBS become topological (TDBS) in the presence of spin–orbit coupling (SOC) effect, which form multiple Dirac cones (DC) at the Γ and M points in the conduction band due to Bi1 termination in top QL. For Bi1 and Se2 terminations, the contribution of p‐orbital of Bi2 atom to DBS in the gap is negligible. The DBS eliminates the topological state of Bi2Se3 for all broken symmetries except Se2 layer vacancy. The band structure with only Se2 and Se3 layer vacancies shows a narrow‐bandgap semiconducting nature with a small bandgap value of 0.18 eV and 0.08 eV, respectively. The DC feature is observed in vacant atomic layer configurations with SOC. The layer vacancies created by Bi atoms form a metallic state, whereas those produced by Se atoms form a narrow‐bandgap semiconducting state. The introduction of broken symmetries in Bi2Se3 can be utilized in the tuning of surface states and TDBS for possible technological applications.

Numerical Analysis of Selective ITO/a-Si:H Contacts in Heterojunction Silicon Solar Cells: Effect of Defect States in Doped a-Si:H Layers on Performance Parameters

The effects of defect states in doped thin-film amorphous-silicon layers (p-a-Si:H and n-a-Si:H) and at heterointerfaces of the passivated crystalline-silicon (c-Si) wafer on the performance of heterojunction silicon solar cells are investigated using optoelectrical simulations. We used a state-of-the-art numerical model of a silicon heterojunction solar cell, including indium-tin-oxide (ITO) contacts as semiconductor layers, applying the accompanying tunneling mechanisms. We show that increasing the dangling-bond density in the p-a-Si:H reduces the conversion efficiency of the device more than in the n-a-Si:H, independent of the doping type of the c-Si wafer and illumination side of the device. Simulations revealed that this is due to: a larger ITO work-function mismatch with the p-a-Si:H than the n-a-Si:H; a larger valence-band offset at i-a-Si:H/n-c-Si interface at the p-side compared to the conduction-band offset at the n-side i-a-Si:H/n-c-Si; and asymmetric distribution of defects in the doped a-Si:H layers. The effects of individual defect-state parameters in doped a-Si:H layers and c-Si heterointerfaces are shown. We demonstrate that the decrease in conversion efficiency due to increased dangling-bond states in doped layers is primarily a consequence of a reduced fill-factor and open-circuit voltage. We explain the phenomenon by charge-redistribution linked to increased dangling-bond density in p-a-Si:H, whereas the increased free-charge recombination has only little effect on the short-circuit-current density over a wide range of defect-state variation. Besides quantitative results of defect-state variation on solar cell performance, this article aims to offer a thorough understanding of the physical processes in the device, governing these effects.

Shallowing interfacial carrier trap in transition metal dichalcogenide heterostructures with interlayer hybridization

With the unique properties, layered transition metal dichalcogenide (TMD) and its heterostructures exhibit great potential for applications in electronics. The electrical performance, e.g., contact barrier and resistance to electrodes, of TMD heterostructure devices can be significantly tailored by employing the functional layers, called interlayer engineering. At the interface between different TMD layers, the dangling-bond states normally exist and act as traps against charge carrier flow. In this study, we propose a technique to suppress such carrier trap that uses enhanced interlayer hybridization to saturate dangling-bond states, as demonstrated in a strongly interlayer-coupled monolayer-bilayer PtSe2 heterostructure. The hybridization between the unsaturated states and the interlayer electronic states of PtSe2 significantly reduces the depth of carrier traps at the interface, as corroborated by our scanning tunnelling spectroscopic measurements and density functional theory calculations. The suppressed interfacial trap demonstrates that interlayer saturation may offer an efficient way to relay the charge flow at the interface of TMD heterostructures. Thus, this technique provides an effective way for optimizing the interface contact, the crucial issue exists in two-dimensional electronic community.

Superconductivity in a Hole-Doped Mott-Insulating Triangular Adatom Layer on a Silicon Surface.

Adsorption of one-third monolayer of Sn on an atomically clean Si(111) substrate produces a two-dimensional triangular adatom lattice with one unpaired electron per site. This dilute adatom reconstruction is an antiferromagnetic Mott insulator; however, the system can be modulation doped and metallized using heavily doped p-type Si(111) substrates. Here, we show that the hole-doped dilute adatom layer on a degenerately doped p-type Si(111) wafer is superconducting with a critical temperature of 4.7±0.3 K. While a phonon-mediated coupling scenario would be consistent with the observed T_{c}, Mott correlations in the Sn-derived dangling-bond surface state could suppress the s-wave pairing channel. The latter suggests that the superconductivity in this triangular adatom lattice may be unconventional.

Germanium Tin Surface Passivation and Its Effect on the Optoelectronic Performances

Germanium Tin is an emerging semiconductor, with high carrier mobility and tunable bandgap directness and energy, that has been attracting a great deal of interest for applications in silicon-compatible electronics and monolithic optoelectronics. In contrast to compound semiconductors which have in the limelight to address several challenges in these technologies, this new emerging family of silicon-compatible group IV semiconductors holds the promise to combine both cost effectiveness and performance in several devices such as tunnelling field effect transistors, infrared detectors and emitters. By tuning strain and composition of GeSn, the band structure can be modified, thus allowing to engineer a large variety of low-dimensional heterostructures relevant to these devices. In fact, even though Ge is an indirect bandgap material, the incorporation of Sn in its lattice allows a transition into a direct bandgap material. Thus, GeSn yields to higher rate of radiative transitions and band-to-band tunneling. However, there are still several outstanding challenges at the materials level that must be overcome before harnessing GeSn advantageous properties. For instance, effective processes to effectively passivate its surface at various Sn composition are yet to be established. It is known that the surface of a semiconductor contains electronically active states because of unsaturated surface bonds or dangling bonds states, which act as localized energy levels the band gap that can change the intrinsic electrical behavior of the material. Therefore, understanding and controlling the surface states are of compelling importance. Moreover, the native oxide layer that forms at the surface of GeSn contains defects resulting in trapping charge carriers thus decreasing their mobility. The poor quality of the surface can decrease the rate of radiative transitions and contribute to the dark current. An effective passivation is thus needed to enhance the optoelectronic performances of GeSn. Passivation layer allows improved charge-separation, reduces charge recombination at surface states, passivates the dangling bonds and decreases the reactivity of the surface. To address these issues, this work investigates several chemical passivation processes and evaluate their effects on the optoelectronic properties of GeSn.The GeSn samples investigated in this work were grown on silicon wafers using ~0.6-3 µm-thick Ge interlay – commonly known as virtual substrates (Ge-VS). The epitaxial growth was carried out by the chemical vapor deposition (CVD) using monogermane (GeH4) and tin-tetrachloride (SnCl4) precursors. Strain minimization and the reduced growth temperature below 350 °C are of paramount importance to enhance Sn incorporation in Ge lattice to reach compositions of 7 – 17 at.%, much larger than the equilibrium composition of 1at.%. Several passivation treatments were evaluated and their basic mechanisms were elucidated. We first evaluated the capacity to remove the native oxide, leave the surface clean and passivate the dangling bonds of GeSn. The kinetic of oxide regrowth was studied to assess the chemical stability of the passivation through X-rays photoelectron spectroscopy (XPS) analysis. For instance, for a surface treatment consisting of a dip in HF followed by a dip in (NH4)2S and then by a nitride drying. XPS measurements show that it is effective in cleaning and passivating the surface, it allows removing the major part of Ge and Sn oxides. Recorded spectra reveal that this treatment gives rise to a persistent sulfur bonds at 161.8 eV, which is the signature of sulphide compounds. Sulfur allows surface passivation and slows down the oxide regrowth. Moreover, alternative surface passivation processes will also be discussed, and their performances compared to the treatment above will be elucidated. Besides, to better understand the evolution of the surface state after the treatments, ellipsometric measurements combined with atomic force microscopy studies are conducted and will be presented. Also, complementary electrical measurements and photoluminescence emission studies will also be discussed to highlight the effectiveness of each treatment in improving the optoelectronic performances of GeSn.

Charge-transfer transition in Au-induced quantum wires on Si(553)

The Si(553)-Au system resembles a heteroatomic chain ensemble with a delicate spin-charge interplay. The ordering of the $\ifmmode\times\else\texttimes\fi{}3$ reconstruction vanishes via a phase transition taking place at ${T}_{c}=100$ K. Our directional-dependent surface transport measurements showed that this order-disorder phase transition is not driven by the formation of a charge-density wave, as previously suggested. Instead, at 65 K there is a pronounced increase of the surface-state conductivity along the wires. We attribute this to activated charge transfer between the localized Si dangling bond states and the proximate Au bands revealing a $\ifmmode\times\else\texttimes\fi{}2$ periodicity. Apparently, a quasiorthogonality between the wave functions of the two proximal reconstructions is also responsible for a missing $\ifmmode\times\else\texttimes\fi{}6$ periodicity along the wires. The electronic charge transfer is in agreement with recent band-structure calculations.

Role of Dangling Bond State Occupancy in Adsorption of Copper Phthalocyanine on Si(111)-Sn-√3 × √3

Interaction of organic molecules with solid surfaces reflects the local electronic structure of nonequivalent surface sites. The Si(111)-Sn-√3 × √3 surface with intrinsic defects offers dangling bonds with various electron occupancies. In this study, we report on the interaction of copper phthalocyanine with this surface studied with scanning tunneling microscopy and spectroscopy. Sn atoms in the adlayer contain unpaired electrons in their dangling bonds. The molecules are found to adsorb on Sb and Si substitutional defects instead of the homogenous Sn areas. The symmetry of the molecules is lowered or lost after adsorption, and we find substantial differences in the electronic structure of the molecules adsorbed on either type of defects. Furthermore, the molecules preferentially adsorb on double Si substitutional defects where they can switch between a static and a fuzzy state, for both of which a corresponding bonding model is proposed. Our study shows a pivotal role of substitutional defects in the re...

Experimental validation of the surface state distribution model in the Suzuki theory to qualify the thin film surface materials

The relevance of the theoretical Suzuki’s model of surface state density to determine the surface state distribution at the active layer/silicon dioxide interfaces is carried out in polycrystalline silicon thin film transistors (TFTs) issued from two different crystallization technologies of the active layer: furnace solid phase crystallized (FSPC) and laser solid phase crystallized (LSPC) TFTs. The experimental validation of this model is demonstrated using the field effect conductance method. Results show that distribution is higher for the FSPC TFTs in relationship with the process crystallization of the active layer. In addition, it is shown that the theoretical surface state model allows discriminating dangling bond state and tail state distributions and acts as relevant model to qualify the surface of thin film material.

Computational characterization of a-Si:H/c-Si interfaces

We use ab initio molecular dynamics to generate realistic a-Si:H/c-Si interface structures with very low defect-state density by performing a high-temperature annealing. Throughout the annealing, we monitor the evolution of the structural and electronic properties. The analysis of the bonds by means of the electron localization function reveals that dangling bonds move toward the free a-Si:H surface, leaving the interface region itself completely defect free. The hydrogen follows this movement, which indicates that in the case under consideration, hydrogen passivation does not play a significant role at the interface. A configuration with a satisfactory low density of defect states is reached after annealing at 700 K. A detailed characterization of the electronic states in this configuration in terms of their energy, localization, and location reveals that, although no dangling bond states can be found near the interface, localized interface states do exist and are attributed to a potential barrier at the interface. The quantitative description of electronic localization also allows for the determination of the a-Si:H mobility gap, which, together with the c-Si band gap, yields band offsets that are in qualitative agreement with experimental observations.

Wurtzite AlN(0001) Surface Oxidation: Hints from Ab Initio Calculations.

With superior electrical and thermal properties, aluminum nitride (AlN) exhibits wide application. However, AlN is rather oxygen-sensitive and tends to be oxidized at high temperature. The surface oxidation of AlN remains a major challenge, while the underlying physics of AlN surface oxidation is still elusive. Here, First-principles calculations were performed to study wurtzite AlN(0001) surface oxidation process. The adsorption energy of oxygen was calculated to be site-dependent on the surface with varying O coverage. Calculation indicates that oxygen atoms are preferentially adsorbed at the hollow site (H3) of the AlN(0001) surface regardless of the O coverage. N2 is determined as the dominant gas product. The procedure of N3- removal and the formation of N vacancies (VN) take place step by step. VN plays an accelerating role in the oxidation of AlN, and O2- prefers to occupy the site of VN via consuming the Al p lone-pair electrons and passivating the dangling bond states of Al. An O-Al-O layer is formed when the first Al-N bilayer is fully oxidized, which could be regarded as a precursor of γ-Al2O3. On the basis of our atomic-level simulation, a possible phase transformation mechanism from γ-Al2O3 to α-Al2O3 was further proposed.

Channelling and induced defects at ion-bombarded aligned multiwall carbon nanotubes

A detailed investigation of ion channelling and defect production for a highly-ordered array of multi-wall carbon nanotubes is presented. The effects of argon ion bombardment (0.25–5 keV) carried out either parallel (top) or perpendicular (side) to their axis, have been studied by Raman, X-Ray Photoelectron Spectroscopy and Scanning Electron Microscopy. Raman spectra provided evidence of channelling of the Ar+ ions observed for top bombardment along the whole 180 μm carbon nanotube length, while the penetration length is limited to the first 10 μm when the ions impinge from the side. The nature of defects, determined through the spectral fingerprints of the C 1s core level as a function of energy and flux, unveils a distorted sp3-like bonding increase and the π-excitation decrease till quenching. Dangling bond states due to displaced carbon atoms become significant only at beam energies higher than 0.25 keV and high flux. These results on anisotropic channelling and selective defects creation open new perspectives in the application of highly-ordered arrays of multi-wall carbon nanotubes as anisotropic detectors.

Physical criteria for the interface passivation layer in hydrogenated amorphous/crystalline silicon heterojunction solar cell

AFORS-HET (automat for simulation of heterostructures) simulation was utilized to explore the physical criteria for the passivation layer in hydrogenated amorphous/crystalline silicon heterojunction (SHJ) solar cells, by systematically investigating the solar cell current density–voltage (J–V) performance as a function of the interface defect density (Dit) at the passivation layer/c-Si hetero-interface, the thickness (t) of the passivation layer, the bandgap (Eg) of the passivation layer, and the density of dangling bond states (Ddb)/band tail states (Dbt) in the band gap of the passivation layer. The corresponding impact regulations were presented clearly. Except for Dit, the impacts of Ddb, Dbt and Eg are strongly dependent on the passivation layer thickness t. While t is smaller than 4–5 nm, the solar cell performance is less sensitive to the variation of Ddb, Dbt and Eg. Low Dit at the a-Si:H/c-Si interface and small thickness t are the critical criteria for the passivation layer in such a case. However, if t has to be relatively larger, the microstructure, i.e. the material quality, including Ddb, Dbt and Eg, of the passivation layer should be controlled carefully. The mechanisms involved were analyzed and some applicable methods to prepare the passivation layer were proposed.

Edge states in mesoscopic Bi islands on superconducting Nb(110)

Islands of Bi showing (110)- and (111)-orientated facets were grown on a Nb(110) single crystal and were studied with a scanning tunneling microscope at low temperatures. On the (111) facets, several states localized at step edges are identified from maps of the local differential conductance. We find edge states on both types of zig-zag step edges that decay exponentially over a length scale of several nanometers into the terrace. These states agree with the necessary condition for topologically nontrivial edge states. Further, we find states localized at the atomic scale at the step edge, which we interpret as dangling bond states. To investigate the nature of these states, we studied the Bi(110) surface, where the sharply localized states form stripe patterns, while the more extended edge state does not show a strong lattice-periodic pattern. Thus, our findings indicate a causal link between the strongly localized edge states on Bi(111) and the stripe pattern on Bi(110) as they are both related to the zig-zag termination of the honeycomb structure of Bi. Finally, we observed a decay of the superconducting proximity gap of the Bi islands with island thickness.

Enhanced van der Waals epitaxy via electron transfer enabled interfacial dative bond formation

Enhanced van der Waals (vdW) epitaxy of semiconductors on layered vdW substrate is identified as the formation of dative bonds. For example, despite that NbSe2 is a vdW layered material, first-principles calculations reveal that the bond strength at CdTe-NbSe2 interface is five times as large as that of vdW interaction at CdTe-graphene interface. The unconventional chemistry here is enabled by an effective net electron transfer from Cd dangling-bond states at CdTe surface to metallic non-bonding NbSe2 states, which is a necessary condition to activate the Cd for enhanced binding with Se.