P-type Contact Research Articles

Optimization of p-type contacts to InGaN-based laser diodes and light emitting diodes grown by plasma assisted molecular beam epitaxy

We investigated the influence of the In0.17Ga0.83N:Mg contact layer grown by plasma assisted molecular beam epitaxy on the resistivity of p-type Ni/Au contacts. We demonstrate that the Schottky barrier width for p-type contact is less than 5 nm. We compare circular transmission line measurements with a p-n diode current-voltage characteristics and show that discrepancies between these two methods can occur if surface quality is deteriorated. It is found that the most efficient contacts to p-type material consist of In0.17Ga0.83N:Mg contact layer with Mg doping level as high as 2 × 1020 cm–3.

Enhanced wall plug efficiency of AlGaN-based deep-UV LEDs using Mo/Al as p-contact

P-type contacts with a high reflectivity in the ultraviolet spectral region made of molybdenum/aluminum (Mo/Al) on AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) emitting at 265 nm have been investigated. Optimized Mo/Al contacts are shown to have a high optical reflectivity above 75% at 265 nm. DUV LEDs with an absorbing p-AlGaN heterostructure operated at 20 mA show a 15% higher light output power and a 1 V lower voltage when Mo/Al instead of Pt is used as p-contact. The effect on the voltage of DUV LEDs with a UV-transparent p-side heterostructure is similar. Moreover, DUV LEDs with a Mo/Al contact show a lower operation voltage compared to LEDs with an indium tin oxide/aluminum (ITO/Al) p-contact where the ITO is intended to form a semitransparent low-resistance contact and the Al serves as a reflector.

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.

Initial results from the Majorana Demonstrator

The Majorana Collaboration has assembled an array of high purity Ge detectors to search for neutrinoless double-beta decay in 76Ge with the goal of establishing the required background and scalability of a Ge-based next-generation ton-scale experiment. The Majorana Demonstrator consists of 44 kg of high-purity Ge (HPGe) detectors (30 kg enriched in 76Ge) with a low-noise p-type point contact (PPC) geometry. The detectors are split between two modules which are contained in a single lead and high-purity copper shield at the Sanford Underground Research Facility in Lead, South Dakota. Following a commissioning run that started in June 2015, the full detector array has been acquiring data since August 2016. We will discuss the status of the Majorana Demonstrator and initial results from the first physics run; including current background estimates, exotic low-energy physics searches, projections on the physics reach of the Demonstrator, and implications for a ton-scale Ge-based neutrinoless double-beta decay search.

Design improvements to cables and connectors in the Majorana Demonstrator

The MAJORANA DEMONSTRATOR is an experiment constructed to search for neutrinoless double-beta decays in germanium-76 and to demonstrate the feasibility to deploy a ton-scale experiment in a phased and modular fashion. It consists of two modular arrays of natural and 76Ge-enriched germanium p-type point contact detectors totaling 44.1 kg, located at the 4850’ level of the Sanford Underground Research Facility in Lead, South Dakota, USA. The DEMONSTRATOR uses custom high voltage cables to bias the detectors, as well as custom signal cables and connectors to read out the charge deposited at each detectors point contact. These low-mass cables and connectors must meet stringent radiopurity requirements while being subjected to thermal and mechanical stress. A number of issues have been identified with the currently installed cables and connectors. An improved set of cables and connectors for the MAJORANA DEMONSTRATOR are being developed with the aim of increasing their overall reliability and connectivity. We will discuss some of the issues encountered with the current cables and connectors as well as our improved designs and their initial performance.

Progress Toward A 2νββ Measurement For The Majorana Demonstrator

The MAJORANA DEMONSTRATOR is a 76Ge-based neutrinoless double-beta decay (0νββ) experiment. Staged at the 4850 ft level of the Sanford Underground Research Facility, the DEMONSTRATOR operates an array of high-purity p-type point contact Ge detectors deployed within a graded passive shield and an active muon veto system. The present work concerns the two-neutrino double-beta decay mode (2νββ) of 76Ge. For Ge detectors, having superior energy resolution (0.1%), this mode poses negligible background to the 0νββ mode, even for a ton-scale experiment. However, the measurement of the 2νββ mode allows for careful systematics checks of active detector mass, enrichment fraction, and pulse shape discrimination cuts related to both the 0νββ and 2νββ decay modes. A precision measurement of the 2νββ shape also allows searches for spectral distortions, possibly indicative of new physics, including 0νββχ. Work is underway to construct a full experimental background model enabling a Bayesian fit to the measured energy spectrum and extraction of a precise 2νββ spectrum and half-life.

CDEX Dark Matter Experiment: Status and Prospects

The China Dark Matter Experiment (CDEX) aims at direct searches of light Weakly Interacting Massive Particles (WIMPs) at the China Jinping Underground Laboratory (CJPL) with an overburden of about 2400m rock. Results from a prototype CDEX-1 994 g p-type Point Contact Germanium(pPCGe) detector are reported. Research programs are pursued to further reduce the physics threshold by improving hardware and data analysis. The CDEX-10 experiment with a pPCGe array of 10 kg target mass range is being tested. The evolution of CDEX program into “CDEX-1T Experiment” with ton-scale germanium detector arrays will also be introduced in this study.

Self-assembled Zn phthalocyanine as a robust p-type selective contact in perovskite solar cells.

The use of self-assembled monolayers (SAMs) as selective charge extracting layers in perovskite solar cells is a great approach to replace the commonly used charge selective contacts, as they can easily modify the interface to enhance the final solar cell performance. Here, we report a novel synthetic approach of the commonly known zinc phtalocyanine (ZnPc) molecule TT1, widely employed in dye-sensitized solar cells and previously used in perovskite solar cells. TT1 is used as a p-type selective contact, and it demonstrates its ability to form SAM on top of the indium tin oxide (ITO) transparent electrode, obtaining higher efficiencies compared to Pedot:PSS based perovskite solar cells. The differences observed, with an enhanced open-circuit voltage and overall efficiency in TT1 devices are correlated with differences in energetics rather than recombination kinetics.

Semiconducting few-layer PdSe2 and Pd2Se3: native point defects and contacts with native metallic Pd17Se15.

PdSe2 is a unique layered two-dimensional (2D) material with pentagonal structural motif and anisotropic properties. In addition, its strong interlayer interaction leads to new 2D form of the exfoliated monolayer, that is, Pd2Se3. Despite the increasing interest in these emerging 2D materials, the landscape of the native point defects, as a fundamental materials property, has not been revealed. In this work, we systematically investigate different types of defects in mono- and bi-layer PdSe2 and monolayer Pd2Se3. In contrast to the common expectation, Se vacancy is not the readily formed defect. Instead, Se-excess defects, such as SePd antisite and Se interstitial, are more likely to form over a majority of the allowed range of the atomic chemical potentials. Se-deficiency defect, Pd interstitial, is able to form under the Se-poor condition in bilayer PdSe2. The defect-mediated interlayer fusion model in the formation of monolayer Pd2Se3 from bilayer PdSe2 is reformulated. These dominant defects are found to stay in the neutral charge state, partly explaining the ambipolar behavior of the PdSe2 transistors. Finally, the stacked and lateral contacts between these few-layer semiconductors and the native Pd17Se15 metal are also studied. All these interfaces show p-type contact properties. This work reveals the important materials properties of few-layer PdSe2 and Pd2Se3 for the better development of new functional devices.

Graphene/WSeTe van der Waals heterostructure: Controllable electronic properties and Schottky barrier via interlayer coupling and electric field

The formation of the graphene-based van der Waals (vdW) heterostructures has shown great potential for designing novel electronic and optoelectronic nanodevices. Here, we construct the Graphene/WSeTe heterostructure and investigate its structural, electronic, optical and transport properties through first-principles calculations. We find that the electronic properties of both graphene and Janus WSeTe are well preserved in Graphene/WSeTe heterostructure because of the weak vdW interactions. The optical absorption of the Graphene/WSeTe heterostructure is enhanced in both regions of the visible and UV lights in comparison with that of the graphene and Janus WSeTe monolayers. The absorption coefficient of the Graphene/WSeTe heterostructure for the visible light can reach 5 × 104 cm−1, which is as twice as that of Janus WSeTe monolayer. Whereas, for the UV light, the absorption coefficient of such heterostructure can reach up to 105 cm−1. Moreover, the Graphene/WSeTe heterostructure tends to own a high carrier mobility for both electrons and holes as compared with single-layered Graphene. Especially, a band gap of about 10 meV at the Dirac cone of graphene in such heterostructures can be opened. Depending on the stacking configurations, the Graphene/WSeTe heterostructure can form the p-type Ohmic contact or p-type Schottky contact with a small Schottky barrier of 0.35 eV. Furthermore, our results demonstrate that the electric fields and vertical strains can be effectively used to tune both the contact types and the Schottky barrier height of Graphene/WSeTe heterostructure from the p-type Schottky contact to n-type one or to p-type Ohmic contact. Our results could provide a significant guidance for understanding the physical properties of the Graphene/WSeTe heterostructure towards nanoelectronic and optoelectronic devices.

Anti-LID Process with a Remote Direct Heating Method Using a Half-Bridge Resonance Circuit for a PERC Solar Cell Module

As the importance of the levelized cost of electricity (LCOE) increases in the solar cell industry, the demand for high-efficiency solar cells is rapidly increasing. Typically, p-type passivated emitter rear contact (PERC) solar cells are the most commonly used in the industry, and their efficiency is approximately 22–23%. P-type solar cells are reported to exhibit a light-induced degradation (LID) phenomenon, in which their output constantly decreases during power generation under solar radiation, and the output significantly reduces as their reference efficiency increases. Ultra-high-efficiency solar cells, which are on high demand, have a considerable output reduction due to the LID phenomenon; hence, technologies to prevent the LID phenomenon are required. However, research on this phenomenon has not been conducted because there is no method to transfer heat to solar cells inside the encapsulant when the modules are produced. In this study, a regeneration state was formed by remotely heating solar cells without damaging the encapsulant of the solar cell module. This was accomplished by using a heating method based on an induction magnetic field. A half-bridge resonance circuit was used to apply the induction magnetic field, and the temperature of the solar cell was controlled by adjusting the magnitude of the current flowing through the coil. To determine whether only the solar cell was heated, the temperature distribution inside the module was analyzed using an IR camera. The minority carrier lifetime was examined by real-time observation of the open-circuit voltage pattern of the solar cell. Finally, the observed real-time open-circuit voltage data were used, and dynamic simulation of the regeneration process was applied to analyze the LID activation energy generated in the regeneration process of the solar cell module. In conclusion, research was conducted on applying the regeneration state to prevent the LID phenomenon in the solar-cell-module stage, and the LID activation energy of the solar cell module was extracted. Based on this, a nondestructive degradation prevention technology for the solar cell module was developed.

Fabrication and characteristics of back-gate black phosphorus effect field transistors based on PET flexible substrate

Black phosphorus (BP) is a layered material that is stacked together between independent atomic layers by internal van der Waals forces. In this study, we qualitatively found that the peak value of BP film with different thickness under the AFM characterization would increase with the increase of thickness, and the friction ring had a tendency of expansion. The characteristic peak within a certain range under Raman characterization will redshift with the increase of thickness, and the redshift will weaken with the continuous increase of thickness. These efforts provide the basis for selecting a suitable BP film material as the semiconductor channel for the transistor. The output characteristics and transfer characteristics of the back-gate BP field effect transistor (FET) based on PET flexible substrate are also investigated in detail. A back-gate BP FET based on a PET flexible substrate exhibits a transistor characteristic of a p-type doped ohmic contact in an output characteristic. In the transfer characteristics, the hole mobility of the back-gate BP FET device based on the PET flexible substrate was 266.6 cm2v−1 s−1 and electron mobility was 3.5 cm2v−1 s−1, and the hole mobility was 75 times that of the electron. It has a switch ratio exceeding 8.6 × 103 at the hole end. Experimental results show that the electrical properties of the device show good Schottky diode characteristics. The back-gate BP FET based on PET flexible substrate has good performance and low cost, which adds to the research and application of nano-transistor, and also contributes to the development of flexible electronic equipment.

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.

Strain tunable Schottky barriers and tunneling characteristics of borophene/MX2 van der Waals heterostructures

Based on first-principle calculations, we report the strain induced changes in electronic properties and their influence on current-voltage ( I − V ) characteristics of the borophene (β 12 )/MX 2 (M = Mo, W and X = S, Se) vdW heterostructures. The results reveal that the intrinsic electronic nature of borophene and MX 2 is retained because of weak van der Waals interactions. However, p-type Schottky contacts are formed at the interface of the heterostructures. Application of the in-plane tensile and compression strains is effective in tuning the Schottky contacts and controlling the SBHs. Also, at the vertical pressure values of 5.46 and 5.25 GPa for β 12 /MoS 2 and β 12 /WS 2 respectively, Schottky contact changes from p-type to n-type. The I − V characteristics exhibit an ohmic behavior at low bias ± 0.1 V and noticeable NDR on changing positive (negative) biases. Such strain tunable Schottky barriers may be influential in β 12 /MX 2 based high-performance nano- and optoelectronic devices.

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.

Pulsed laser annealing for advanced technology nodes: Modeling and calibration

Pulsed laser annealing is one of the promising low thermal budget approaches to overcome process limitations and develop alternative schemes to achieve better device performance and enable 3D architectures. Its applications range from the Front End Of the Line (doping, contacts, strain engineering) to Back End Of the Line (Cu grain engineering) in logic and memory devices. One key enabler for integrating this disruptive technology in the coming highly challenging technology nodes is an accurate time-resolved modeling of laser matter interaction, thermal diffusion, phase change and species diffusion at the nanosecond timescale, all to be solved self-consistently. In this paper, we will present the TCAD simulation package of the laser annealing process (LASSE Innovation Application Booster or LIAB), with a specific focus on the phase field model and calibration of relevant materials. The coupled partial differential equation system is described and a methodology for materials calibration, especially challenging in the melting regime, is detailed with results shown for Ge and SiGe, with a application on a typical p-type finFET contact region anneal 2D use case.

Influence of a thin amorphous surface layer on de-channeling during aluminum implantation at different temperatures into 4H-SiC

Ion implantation is an important technique in semiconductor processing and has become a key technology for 4H-SiC devices. Today, aluminum (Al) implantations are routinely used for p-type contacts, p+-emitters, terminations and many other applications. However, in all crystalline materials, quite a few ions find a path along a crystal channel, so-called channeling, and these ions travel deep into the crystal. This paper reports on the channeling phenomenon during Al implantation into 4H-SiC, and in particular, the influence of a thin native oxide will be discussed in detail. The effects of thermal lattice vibrations for implantations performed at elevated temperatures will also be elucidated. 100 keV Al ions have been implanted along the [000-1] direction employing samples with 4° miscut. Before implantation, the samples have been aligned using the blocking pattern of backscattered protons. Secondary ion mass spectrometry has been used to record the Al depth distribution. To predict implantation profiles and improve understanding of the role of crystal structure, simulations were performed using the Monte-Carlo binary collision approximation code SIIMPL. Our results show that a thin surface layer of native oxide, less than 1 nm, has a decisive role for de-channeling of aligned implantations. Further, as expected, for implantations at elevated temperatures, a larger degree of de-channeling from major axes is present due to increased thermal vibrations and the penetration depth of channeled aluminum ions is reduced. The values for the mean-square atomic displacements at elevated temperatures have been extracted from experimental depth profiles in combination with simulations.

A critical review of theory and progress in Ohmic contacts to p-type SiC

Abstract Silicon carbide (SiC) is a promising candidate in high-temperature, high-frequency and high-power applications due to its outstanding properties such as wide band gap, high critical electric field and high thermal conductivity. However, its applications are still limited by the formation of Ohmic contacts to SiC, especially for p-type materials. To date, the specific contact resistance of metal contacts to very highly doped p-type SiC could reach the order of 10 - 5 - 10 - 6 Ω · c m 2 after an extremely high temperature annealing process. However, the obtained Ohmic contacts exhibit poor reproducibility and stability. In addition, the formation mechanism of p-type SiC Ohmic contact is still unclear. In this paper, based on the formation principle and barrier theory, we present a review of recent progress in Ohmic contacts to p-type SiC reported in literature. Emphasis is placed on the formation mechanism of p-type SiC Ohmic contact along with future perspectives for research approaches.

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.

Dependence of channel thickness on MoTe2 transistor performance with Pt contact on a HfO2 dielectric

We investigated the dependence of MoTe2 thickness on it is electrical properties by fabricating several transistors with channel thickness between ∼6 layers and ∼46 layers. The transistor with ∼10-layer channel has the highest hole current, whereas the electron current shows monotonically increase with channel thickness. The measurements of Ids–Vds and temperature dependence show ohmic contact of p-type conduction and Schottky contact of n-type conduction. Thus, the highest hole current is observed in ∼10-layer MoTe2 due to the highest mobility. However, the bandgap of MoTe2 decreases with it is thickness, leading to the reduce of Schottky barrier and the increase of electrons current.