N-type Contact Research Articles

Van der Waals interfacial reconstruction in monolayer transition-metal dichalcogenides and gold heterojunctions

The structures and properties of van der Waals (vdW) heterojunctions between semiconducting two-dimensional transition-metal dichalcogenides (2D TMDs) and conductive metals, such as gold, significantly influence the performances of 2D-TMD based electronic devices. Chemical vapor deposition is one of the most promising approaches for large-scale synthesis and fabrication of 2D TMD electronics with naturally formed TMD/metal vdW interfaces. However, the structure and chemistry of the vdW interfaces are less known. Here we report the interfacial reconstruction between TMD monolayers and gold substrates. The participation of sulfur leads to the reconstruction of Au {001} surface with the formation of a metastable Au4S4 interfacial phase which is stabilized by the top MoS2 and WS2 monolayers. Moreover, the enhanced vdW interaction between the reconstructed Au4S4 interfacial phase and TMD monolayers results in the transition from n-type TMD-Au Schottky contact to p-type one with reduced energy barrier height.

Pressure and electric field tuning of Schottky contacts in PdSe2/ZT-MoSe2 van der Waals heterostructure

A two-dimensional van der Waals (vdW) heterostructure (PdSe2/ZT-MoSe2) has been investigated through vdW corrected density functional theory. ZT-MoSe2 acts as a Dirac material with an anisotropic Dirac cone and variable Fermi velocity (0.52–1.91 × 105 ms−1). The intrinsic Schottky barrier height can be effectively tuned by applying external pressure and an electric field to the heterostructure. The p-type Schottky barrier transforms into a p-type ohmic contact at pressure P ≈ 16 GPa. A positive electric field induces p-type ohmic contact while a negative electric field results in the transition from p-type Schottky contact to n-type Schottky contact, and finally to n-type ohmic contact at the higher values of the field. Moreover, the external positive (negative) electric field induces n-type (p-type) doping of ZT-MoSe2 in the heterostructure and remarkably controls the charge carrier concentration. Our results demonstrate that controlling the external pressure and electric field in a PdSe2/ZT-MoSe2 heterostructure can result in an unprecedented opportunity for the design of high-performance nanodevices.

Tuning Schottky barrier in graphene/InSe van der Waals heterostructures by electric field

The contacts between semiconductor and metal are vital in the fabrication of nano electronic and optoelectronic devices. The contact type has a great influence on the function realization and performance of the device. In order to prepare multifunctional devices with high performance, it is necessary to modulate the barrier height and contact type at the interface. First-principles calculations based on the density functional theory (DFT) are implemented in the VASP package. The generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) with van der Waals (vdW) correction proposed by Grimme (DFT-D3) is chosen due to its good description of long-range vdW interactions. It is demonstrated that weak vdW interactions dominate between graphene and InSe with their intrinsic electronic properties preserved. We find that the n-type ohmic contact is formed at the graphene/InSe interface with the Fermi level through the conduction band of InSe (<i>Φ</i><sub>Bn</sub> < 0). The Fermi level of graphene/InSe heterostructure moves down to below the Dirac point of graphene layer, which results in p-type (hole) doping in graphene. Moreover, the external electric field is effective to tune the Schottky barrier, which can control not only the Schottky barrier height but also the type of contact. With the negative external electric field varying from 0 to –1 V/nm, the conduction band minimum of InSe below the Fermi level declines gradually but the n-type ohmic contact is still preserved. Nevertheless, with the positive external electric field varying from 0 to 0.8 V/nm, the conduction band minimum of InSe shifts upward and across the Fermi level, the conduction band minimum of InSe is closer to the Fermi level than the valence band maximum, which indicates that the n-type Schottky contact is formed. The Fermi level moves from the the conduction band minimum to the valence band maximum of InSe when the positive external electric field increases from 0.8 V/nm to 2 V/nm. The n-type Schottky barrier height exceeds the p-type Schottky barrier height gradually, which demonstrates that the positive external electric field transforms the n-type Schottky contact into the p-type Schottky contact at the graphene/InSe interface. When the positive external electric field exceeds 2 V/nm, the valence band of InSe moves upward and cross the Fermi level (<i>Φ</i><sub>Bp</sub> < 0), the ohmic contact is obtained again. Meanwhile, p-type (hole) doping in graphene is enhanced under negative external electric field and a large positive external electric field is required to achieve n-type (electron) doping in graphene. The external electric field can control not only the amount of charge transfer but also the direction of charge transfer at the graphene/InSe interface.

Effects of different surface functionalization on the electronic properties and contact types of graphene/functionalized-GeC van der Waals heterostructures.

Constructing vertical heterostructures by placing graphene (Gr) on two-dimensional materials has recently emerged as an effective way to enhance the performance of nanoelectronic and optoelectronic devices. In this work, first principles calculations are employed to explore the structural and electronic properties of Gr/GeC and Gr/functionalized-GeC by H/F/Cl surface functionalization. Our results imply that the electronic properties of the Gr, GeC and all functionalized-GeC monolayers are well preserved in Gr/GeC and Gr/functionalized-GeC heterostructures, and the Gr/GeC heterostructure forms a p-type Schottky contact. Interestingly, we find that the p-type Schottky contact in Gr/GeC can be converted into the n-type one and into an n-type ohmic contact by H/F/Cl surface functionalization to form Gr/functionalized-GeC heterostructures. Furthermore, we find that electric fields and strain engineering can change both the Schottky barrier heights and the contact types of the Gr/functionalized-GeC vdWHs. These findings suggest that Gr/functionalized-GeC heterostructures can be considered as a promising candidate for designing high-performance optoelectronic and nanoelectronic devices.

Ohmic contact in graphene/SnSe2 Van Der Waals heterostructures and its device performance from ab initio simulation

Van der Waals (vdW) type metallic/semiconducting heterostructures have attracted much attention for applications like nanoelectronics. The electronic properties of graphene/SnSe2 vdW heterostructure are investigated by the first-principles calculation. The band dispersions of both the graphene and SnSe2 layers are well preserved in the graphene/SnSe2 vdW heterostructure. Notably, n-type Ohmic contact is found at the graphene/SnSe2 vdW interface so that graphene is a fantastic electrode for the SnSe2 Schottky barrier field-effect transistor (SBFET). The on-state current of the 10-nm-gate-long ML SnSe2 SBFET with graphene electrode is 1535 µA/µm for high-performance (HP) application, which is twice that of the ML MoS2 SBFET with bulk Ti electrode and exceeds the requirement of the International Technology Roadmap for Semiconductors for HP devices.

Passivated Metal Oxide n-Type Contacts for Efficient and Stable Organic Solar Cells

Suppressing trap states and localized electronic states in the forbidden gap of semiconductors as either active layers or contacts is critical to the enhancement of optoelectronic device performanc...

Broadband high quantum efficiency InGaAs/InP focal plane arrays via high precision plasma thinning.

We report on the fabrication of a 160×120 visible (Vis)-extended InGaAs/InP focal plane array (FPA) by means of the inductively coupled plasma etching. Compared to the conventional Vis-InGaAs FPAs, higher quantum efficiency in the Vis spectrum has been achieved. High precision thinning of the n-type InP contact layer down to 10 nm has led to quantum efficiencies higher than 60% over a broad wavelength range of 500-1700 nm. Benefiting from the textured surface after plasma etching, 17% lower reflectance over the entire response range was also found. Enhanced Vis/near-infrared (NIR) laboratory imaging capability has also been demonstrated, which has proved the feasibility of such processes for fabrication of future higher definition Vis/NIR InGaAs imagers.

Janus PtSSe and graphene heterostructure with tunable Schottky barrier

Janus transition metal dichalcogenides with a built-in structural cross-plane asymmetry have recently emerged as a new class of two-dimensional materials with a large cross-plane dipole. By using the density functional theory calculation, we report the formation of different Schottky barriers for Janus PtSSe and graphene based van der Waals heterostructures, where the Schottky barrier height (SBH) and type of contact can be controlled by adjusting the interlayer distance, by applying an external electric field, and by having multiple layers of Janus PtSSe. It is found that the effects of tuning are more prominent for SPtSe/graphene as compared to SePtS/graphene. Besides, a transition from n-type Schottky contact to p-type Schottky contact and to Ohmic contact is also observed in the SPtSe/Gr heterostructure for different SPtSe stackings from 1 layer, to 2- and 3-layers, respectively. Our findings indicate that the SPtSe/graphene heterostructure is a suitable candidate for applications that require a tunable SBH.

High-Performance Schottky-Barrier Field-Effect Transistors Based on Monolayer SiC Contacting Different Metals

Schottky-barrier field-effect transistors (SBFETs) based on monolayer SiC contacting four metals (Ag, Au, Al, and Pd) have been proposed using density functional theory and ab initio simulations. The n-type Schottky contacts could be formed between monolayer SiC and Ag, Au electrodes with electron Schottky barrier heights (SBHs) of 0.7 and 1.1 eV, respectively, whereas the p-type Schottky contacts were formed between monolayer SiC and Al, Pd electrodes with hole SBHs of 1.0 and 0.1 eV, respectively. When the gate length was equal to 5.1 nm, SiC-SBFETs with four metals could all overcome the short-channel effect due to the wide bandgap of SiC. In addition, the SiC-SBFET with Pd electrode provided the best performance and could satisfy the performance requirement of high-performance transistor with gate length of 5.1 nm outlined by the International Technology Roadmap for Semiconductors (ITRS). Our study gives an insight into the monolayer SiC–metal interfaces and, thus could be developed into a viable 5-nm “More than Moore” device in the future.

Tunable Electronic Properties of Graphene/g-AlN Heterostructure: The Effect of Vacancy and Strain Engineering.

The structural and electronic properties of graphene/graphene-like Aluminum Nitrides monolayer (Gr/g-AlN) heterojunction with and without vacancies are systematically investigated by first-principles calculation. The results prove that Gr/g-AlN with nitrogen-vacancy (Gr/g-AlN-VN) is energy favorable with the smallest sublayer distance and binding energy. Gr/g-AlN-VN is nonmagnetic, like that in the pristine Gr/g-AlN structure, but it is different from the situation of g-AlN-VN, where a magnetic moment of 1 μB is observed. The metallic graphene acts as an electron acceptor in the Gr/g-AlN-VN and donor in Gr/g-AlN and Gr/g-AlN-VAl contacts. Schottky barrier height by traditional (hybrid) functional of Gr/g-AlN, Gr/g-AlN-VAl, and Gr/g-AlN-VN are calculated as 2.35 (3.69), 2.77 (3.23), and 1.10 (0.98) eV, respectively, showing that vacancies can effectively modulate the Schottky barrier height. Additionally, the biaxial strain engineering is conducted to modulate the heterojunction contact properties. The pristine Gr/g-AlN, which is a p-type Schottky contact under strain-free condition, would transform to an n-type contact when 10% compressive strain is applied. Ohmic contact is formed under a larger tensile strain. Furthermore, 7.5% tensile strain would tune the Gr/g-AlN-VN from n-type to p-type contact. These plentiful tunable natures would provide valuable guidance in fabricating nanoelectronics devices based on Gr/g-AlN heterojunctions.

High-Voltage AlInGaN LED Chips

A high-voltage light-emitting diode (LED) flip chip based on an AlInGaN heterostructure is developed and fabricated. The LED flip chip consists of 16 elements connected in series, each of which is a convential LED. The chip with a total area of 1.25 × 1.25 mm is intended for a working current of 20 mA and a forward voltage of 48 V. To improve the current-distribution uniformity over the active region of the chip elements and to minimize the losses of the element area occupied by the n-type contact, the n-type contact pads in them are arranged inside the p-type contact region due to the two-level metallization layout with an intermediate insulating layer of dielectric. The arrangement topology of the contact pads is developed using numerical simulation. An increase in the quantum efficiency of the chip is provided by the application of combinations of metals with a high reflectance at the LED emission wavelength, which are used when fabricating n- and p-type contacts as well as current-carrying strips.

An Investigation of Internal Quantum Efficiency of Bifacial Interdigitated Back Contact (IBC) Crystalline Silicon Solar Cell

In this article, we investigated the internal quantum efficiency (IQE) properties of n-type bifacial interdigitated back contact crystalline silicon solar cells using an IQE mapping system. In the cell structure, high and low IQE values were observed above the emitter and back surface field (BSF) regions. The IQE values above the BSF busbar were drastically reduced due to electrical shading loss. Line scan profiles at different wavelengths showed the detailed distribution of IQE values. The IQE values varied greatly depending not only on the difference between emitter and BSF regions but also on the rear side structure such as the electrode width and the distance between the emitter and BSF regions. On the other hand, the IQE spectra at over 950 nm improved by increasing the light absorbance ratio from the rear side. After module formation, the IQE spectra at short wavelengths were significantly reduced. The IQE properties were obtained from the front and rear sides. The difference in the short-circuit current between front side illumination and rear side illumination was mainly due to optical shading loss and carrier recombination loss at the BSF region. For a high cell efficiency, it is necessary to improve the passivation properties of the BSF region and optimize the electrode design.

Impact of phosphorus diffusion on n-type poly-Si based passivated contact silicon solar cells

This paper describes the impact of phosphorus dopant concentrations in n-type passivated contact structures. Free carrier absorption (FCA) of the phosphorus doped poly-Si layers was studied by using different phosphorus dopant concentrations, with a range of from 9 × 1019 to 2.3 × 1020 cm−3, and different poly-Si layer thicknesses, with a range of from 30 to 200 nm. In the worst case, the effect of FCA reduce the short circuit current density by 0.89 mA/cm2. The impacts of tunnel SiO2 thicknesses, poly-Si thicknesses and diffusion temperatures on the performances of the poly-Si passivated contacts were studied. Phosphorus concentrations induced silicon surface field passivation play an important role in passivated contact structure. On the planar n-type mono crystalline-silicon substrates, excellent J0 value of 6.5 fA/cm2 was achieved by optimizing phosphorus doping in poly-Si films and silicon substrate. Different poly-Si thickness and diffusion temperature were studied simultaneously to optimize the phosphorous dopants in the poly-Si passivated contacts for the specified poly-Si thickness. N-type silicon solar cell have been fabricated by using the optimized poly-Si passivated contacts, as addressed above. Average efficiency of 22.52%, and best cell efficiency of 23.04% were demonstrated.

High-performance flat-type InGaN-based light-emitting diodes with local breakdown conductive channel

Flat-type InGaN-based light-emitting diodes (LEDs) without an n-type contact electrode were developed by using a local breakdown conductive channel (LBCC), and the effect of the In content of the InGaN quantum wells (QWs) on the local breakdown phenomenon was investigated. Electroluminescence and X-ray analyses demonstrated that the homogeneity and crystallinity of the InGaN QWs deteriorated as the In content of the InGaN QWs increased, thereby increasing the reverse leakage current and decreasing the breakdown voltage. After reverse breakdown with a reverse current of several mA, an LBCC was formed on the GaN-based LEDs. The surface size and anisotropic shape of the LBCC increased as the indium content of the InGaN QWs in the LEDs increased. Moreover, a flat-type InGaN LED without an n-type electrode was developed by using the LBCC. Notably, the resistance of the LBCC decreased with increasing indium content in the InGaN QWs, leading to lower resistance and higher light emission of the flat-type InGaN-based LEDs without an n-type contact electrode.

Controlling a crystalline seed layer for mirocrystalline silicon oxide window layer in rear emitter silicon heterojunction cells

Remarkable progress has been made in the improvement of rear-emitter silicon heterojunction (RE-SHJ) solar cells with the use of very thin n-type front contact layers. However, further reducing the thickness of the front window layers while maintaining high conductivity for mitigating the parasitic absorption and carrier collection loss has proven challenging. In this study, we implement controlling a seed layer for achieving ultra-thin, high crystalline and conductivity of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiOx:H) front window layer in RE-SHJ solar cells. By using a seed layer, the crystallinity confirmed by Raman and TEM measurements, and the conductivity of the n-µc-SiOx:H front layers are significantly enhanced compared with that without using the seed layer. This leads to a remarkable increase in the open-circuit voltage (Voc) by 6 mV and fill factor (FF) by 4.11% while maintaining a high short-circuit current density (Jsc) in range of 38 mA/cm2. A high cell performance of 21.1% is obtained with the use of an optimised seed layer.

Theoretical Insights into Two-Dimensional IV-V Compounds: Photocatalysts for the Overall Water Splitting and Nanoelectronic Applications.

Two-dimensional (2D) materials have attracted enormous attention in many fields because of their appealing performances. In this contribution, we perform first-principles calculations on the photocatalytic properties of IV-V compounds, along with the design of a functional Schottky device based on a graphene/SiAs van der Waals heterostructure (vdWH). Our results indicate that eight IV-V compound materials are all excellent photocatalysts for water-splitting reactions with high efficiency of visible light, with the conduction band minimum (CBM) and valence band maximum (VBM) both involving the corresponding band-gap region. It is examined whether a weak acid environment is beneficial for the hydrogen production process. Monolayer GeAs is characterized by an excellent absorption coefficient of up to 105-2 × 105 cm-1 in the visible region. The other nanostructures also have a considerable optical absorption as high as approximately half of 105 cm-1. These illustrate fascinating application prospectives for IV-V compounds in photocatalysis for water splitting under the irradiation of visible light, predicting tremendous significance in the fields of energy conversion and hydrogen production. The graphene/SiAs vdWH nanocomposite at the equilibrium state is featured for an n-type Schottky contact. External strain and electric-field applications are employed to practically present the transition for interface contact between the n- and p-type Schottky contacts or between the Schottky and ohmic contacts, which suggests appealing applications for the graphene/SiAs vdWH as a competitive candidate for functional Schottky devices and nanoelectronic materials.

Graphene/GeTe van der Waals heterostructure: Functional Schottky device with modulated Schottky barriers via external strain and electric field

Vertical heterogeneous combinations of two-dimensional (2D) materials have been evident as an effective strategy to design novel photovoltaic and optoelectronic devices. Herein, we perform the first-principles calculations about electronic and optical properties of graphene/GeTe van der Waals heterostructures (vdWHs), accompanied by the efficient modulations about Schottky barriers via an external strain and electric field. A slightly opened band gap of 2.0 meV is obtained for graphene/GeTe vdWH in the equilibrium configuration, and the Dirac point is well preserved at K point. The built-in electric field caused by the charge transfer is devoted to its visible contribution to the efficient hindrance for photoexcited electron-hole pair recombination, consequently increasing the number of carriers and extending their lifetimes. The graphene/GeTe vdWH possesses enhanced optical absorption in the visible region in contrast to the separate GeTe monolayer, indicating its significant potential applications in optoelectronic devices. Besides, the graphene/GeTe vdWH at the equilibrium state exhibits an n-type Schottky contact with n-type Schottky barrier height (SBH) of 0.684 eV. By applying suitable external vertical strains or electric fields, the SBHs can be efficiently tuned, making it of significant possibility to realize the transportations between p-type Schottky contact and n-type Schottky contact, and between Schottky contact and Ohmic contact at the graphene/GeTe interface. These findings together predict the graphene/GeTe vdWH nanocomposite as a competitive candidate in next-generation optoelectronics and Schottky devices.

Computational understanding of electronic properties of graphene/$${\mathrm{{PtS}}_2}$$ heterostructure under electric field

Graphene-based two-dimensional van der Waals heterostructures (vdWH) have recently shown a great potential for high-performance nanodevices. Here, we design a novel vdWH, consisting of the graphene and a noble transition metal dichalcogenide $${\mathrm{{PtS}}_2}$$ monolayer and investigate its electronic properties as well as the effect of electric field. We find that the key intrinsic characteristics of both the graphene and $${\mathrm{{PtS}}_2}$$ monolayers are well preserved due to the weak vdW interactions. The graphene is found to form an n-type Schottky contact with the $${\mathrm{{PtS}}_2}$$ monolayer with a small barrier height of 0.11 eV. Moreover, this small barrier height of the $$\mathrm{{G}}/{\mathrm{{PtS}}_2}$$ vdWH is known to be very sensitive to the external electric field. It can be controlled and turned to the Ohmic contact under electric field. These findings could provide significant knowledge for designing novel electronic and optoelectronic devices of such heterostructure.

Intriguing electronic and optical properties of M2CX2 (M = Mo, W; X = O, F) MXenes and their van der Waals heterostructures

M2CX2 (M = Mo,W; X = O,F) MXenes and their van der Waals heterostructures are investigated by first-principle calculations. Phonon spectrum has shown that Mo2CO2/W2CO2 and Mo2CF2/W2CO2 heterostructures and corresponding monolayers are dynamically stable. W2CO2 exhibits the properties of topological insulator and Mo2CO2 is narrow band gap semiconductor, while Mo(W)2CF2 are metals. Mo2CO2/W2CO2 heterostructure indicates a possible topological feature similar to corresponding monolayers and reveals type-II band alignment. Mo2CF2/W2CO2 heterostructure generating n-type Schottky contact with 0.139 eV barrier height. Large absorption in the visible region is observed for Mo2CO2 and blue shift is noticed for W2CO2 and W2CO2/Mo2CO2heterostructure, especially the part contributed by Mo2CO2.

Schottky barrier modulation of a GaTe/graphene heterostructure by interlayer distance and perpendicular electric field

Two-dimensional materials have recently been the focus of extensive research. Graphene-based vertical van der Waals heterostructures are expected to design and fabricate novel electronic and optoelectronic devices. Monolayer gallium telluride is a graphene-like nanosheet synthesized in experiment. Here, the electronic properties of GaTe/graphene heterostructures are investigated under the interlayer coupling and the applied perpendicular electric field. The results show that the electronic properties of GaTe and graphene are preserved, and the energy bandgap of graphene is opened to 13.5 meV in the GaTe/graphene heterostructure. It is found that the n-type Schottky contact is formed in the GaTe/graphene heterostructure, which can be tuned by the interlayer coupling, and the applied electric field. Moreover, a transformation from n-type to p-type Schottky contact is observed when the interlayer distance is smaller than 3.15 Å or the applied electric field is larger than 0.05 V Å−1. These properties are fundamental to the design of new Schottky nanodevices based on the GaTe/graphene heterostructure.