Planar Doping Research Articles

Breaking the Symmetry of Nitrogen‐Coordinated Single‐Atom Catalysts for Advanced Peroxymonosulfate Oxidation

AbstractTuning the electronic distribution of the single‐atom sites in single‐atom catalysts (SACs) is crucial for unlocking their catalytic potential. The four‐nitrogen‐coordinated transitional metal (M−N4) configuration has been widely investigated in peroxymonosulfate (PMS)‐based advanced oxidation processes (PMS‐AOPs), but restricted by the sluggish electron transfer from PMS to generate the high‐valent metal‐oxo (HVMO), reactive species with high redox potentials and long half‐lives, for the degradation of organic pollutants in water due to its symmetric structure. Recently, the SACs with asymmetric coordination configurations are found to break the symmetric electronic distribution of M−N4, which facilitates the O−H and O−O bond breaking in PMS, thus promoting HVMO formation. Asymmetric coordination has emerged as a novel and effective strategy for single‐atom coordination modulation. In this paper, two strategies for breaking the symmetry of nitrogen‐coordinated SACs by planar heteroatom doping and axial optimization engineering are outlined, and the reaction mechanisms on the formation of HVMO species over asymmetrically coordinated SACs via PMS activation are highlighted. Finally, we prospect the development of asymmetrically coordinated SACs in water purification.

Theoretical analysis of intersubband optical absorption in asymmetric AlInN/GaN/AlInN quantum well heterostructures

The intersubband optical absorption coefficients were theoretically investigated in asymmetric AlInN/GaN/AlInN quantum wells. The electron band parameters are calculated by solving, self-consistently, the one-dimensional Poisson and Schrödinger equations under Hartree and effective mass approximations. The intersubband optical absorption between the two lowest levels was calculated based on the density matrix formalism. The effects of polarization, indium composition and delta-doping on the optical absorption coefficients in AlInN/GaN double heterostructures were studied. Results have revealed: i) an enhancement of the total (linear plus nonlinear) absorption coefficient peak and a reduction of wavelength due to the polarization effects ii) that the peak of the total optical absorption coefficient increased and showed a red shift upon increasing indium composition iii) that the peak of the total absorption coefficient increased and red-shifted or blue-shifted through optimizing the concentration of the planar doping in the AlInN barriers. Furthermore, the use of a back doping with a delta-shaped profile was identified as a consistent approach to carry out the transition 1.55 µm. The results in this study could be useful for the design and applications of optoelectronic devices operating at 1.55 µm.

(Invited) Ion Implantation and Activation of n- and p-Type Dopants in GaN

GaN-based power switching devices are of significant interest for high efficiency power conversion circuits in medium voltage applications. However, there are still significant challenges preventing mass production and widespread adoption. Recently, GaN-based vertical and lateral power devices have attracted significant interest due to promising device results coupled with progress in native substrate, epitaxial growth, and processing technology developments. However, there are two significant limiting factors for vertical GaN power devices at present. The first issue is substrate and epitaxial layer uniformity and reliability, and the second is the lack of a planar selective-area doping process. While p-type epitaxial growth capability and the ability to stack layers are quite mature from the LED industry, power devices require planar selective-area doping, which is a routine step by ion implantation in the Si and SiC device industry. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge as implant activation typically requires annealing at temperatures ~ 2/3 of the melting point, which is a region where the GaN crystal is unstable and readily decomposes to Ga + N2 at atmospheric pressure. The NRL-developed symmetric multicycle rapid thermal annealing (SMTRA) technique has been the most successful, demonstrating electrical activation of up to ~10% of the implanted Mg dopant atoms using a combination of a temporary, thermally stable capping layer, annealing in a moderate nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. Advances in the SMRTA process development, including optimization of capping processes, implantation profile, and processing steps such as metallization and surface treatments will be reviewed here. In addition, the evaluation of components of the SMRTA process, such as the cap and overpressure anneal, have been applied to Si implantation for n-type doping and will be presented here. This is a critical step to reduce contact resistance in lateral GaN structures as well as form the source regions of vertical devices that is typically accomplished by selective area regrowth. The use of ion implantation and activation utilizes fewer process steps and avoids impurity incorporation at sidewalls compared to the selective area regrowth process, and has been shown to yield lower contact resistance as well. This presentation will review advances in both Mg and Si ion implantation and assess the viability of this process for practical device fabrication.

Nip the Sodium Dendrites in the Bud on Planar Doped Graphene in Liquid/Gel Electrolytes

AbstractSodium (Na) metal is the most promising alternative anode to metallic lithium for high‐energy batteries due to the low cost and high abundance of Na resources, but it suffers from severe dendritic/mossy growth at high current densities. Understanding Na nucleation/growth mechanism in different electrolyte systems is the key to tackling this issue but is complicated by the structural complexities of existing substrates for Na plating/stripping. Herein, well‐defined planar doped graphene substrates are synthesized as model plating platforms to unravel a binding energy dominant Na nucleation‐growth mode. The dopants (e.g., boron) in doped graphene and the regions close to the dopants possess high binding energies with Na atoms, providing abundant preferential nucleation sites and contributing to uniform Na plating/stripping. Accordingly, the boron‐doped graphene regulated Na anode exhibits long‐term stability at high current densities in both liquid and polymer electrolytes. The results enhance the understanding of Na nucleation/growth for stabilizing Na metal batteries.

Enhancement of Ge-based p-channel vertical FET performance by channel engineering using planar doping and a Ge/SixGe1–x–ySny heterostructure model for low power FET applications

High mobility materials are being studied to replace Si with the aim of enhancing the performance of nanoelectronic devices. Ge and III–V channels have recently received a lot of attention, where the combination of III–V channels in n-MOSFETs and Ge channels in p-MOSFETs integrated on Si substrates is regarded as a promising CMOS design. Ge integrated on Si is a very promising choice due to its superior transport properties and compatibility to CMOS technology. The main challenges faced by Ge-based FETs are the channel/gate interface quality, crystal defects due to integration on Si and the smaller bandgap compared to Si, which leads to elevated band-to-band-tunnelling leakage currents, setting limitations on the achievable off state current (IOFF). In this work, we present results on the fabrication and characterization of vertical Ge-based p-channel planar doped barrier FET together with a simulation model based on extracted material data from our experimental work and literature. Based on the model, a design of a modified device using both planar doping and a heterostructure in the channel is presented. The channel engineered design uses a Ge/SixGe1–x–ySny heterostructure, which is lattice matched to Ge, within the channel at different positions. The results show improved performance; the larger bandgap of the ternary alloy SixGe1–x–ySny compared to Ge leads to a suppression of the IOFF as well as a reduced subthreshold swing, making the heterostructure device promising for energy efficient FET applications.

GaAs/AlGaAs potential well barrier diodes: Novel diode for detector and mixer applications

Heterostructure potential well barrier (PWB) diodes in GaAs/AlGaAs system operating in a similar way to Planar Doped Barrier (PDB) diodes, though exploiting a potential well to trap charge, rather than a fixed doping spike have recently been realized and reported in the literature. This paper analyses the complex operation of these devices and determines the sensitivity of the current–voltage (I–V) characteristics to various design parameters. The PWB diode displays a significant temperature sensitivity, opposite in nature to PDB temperature dependence, and this suggests the possibility of temperature stabilised hybrid designs. The active bias dependence nature of charge within the well is found to have a significant effect on the device and impacts the ideality factor, which is more bias dependent than other comparable devices such as PDBs. However, the same mechanism offers the prospect of improvement in the current asymmetry and this effect can be greatly improved upon by changing the design and shape of the potential well.

Structural, electronic, and magnetic properties of non-planar doping of BeO in graphene: a DFT study

The non-planar molecular doping of BeO is more efficient in inducing a band gap relative to its planar doping with no magnetic effect realization.

High-density Two-Dimensional Small Polaron Gas in a Delta-Doped Mott Insulator

Heterointerfaces in complex oxide systems open new arenas in which to test models of strongly correlated material, explore the role of dimensionality in metal-insulator-transitions (MITs) and small polaron formation. Close to the quantum critical point Mott MITs depend on band filling controlled by random disordered substitutional doping. Delta-doped Mott insulators are potentially free of random disorder and introduce a new arena in which to explore the effect of electron correlations and dimensionality. Epitaxial films of the prototypical Mott insulator GdTiO3 are delta-doped by substituting a single (GdO)+1 plane with a monolayer of charge neutral SrO to produce a two-dimensional system with high planar doping density. Unlike metallic SrTiO3 quantum wells in GdTiO3 the single SrO delta-doped layer exhibits thermally activated DC and optical conductivity that agree in a quantitative manner with predictions of small polaron transport but with an extremely high two-dimensional density of polarons, ~7 × 1014 cm−2.

Perturbation of Au-assisted planar GaAs nanowire growth by p-type dopant impurities

III-V compound semiconductor nanowires (NWs), with their direct bandgaps and high mobilities, have been shown to be promising materials for many applications including solar cells, light emitting diodes, transistors, and lasers. Self-aligned, twin-plane-defect free, planar GaAs NWs can be grown by metalorganic chemical vapor deposition (MOCVD) through the Au-assisted vapor-liquid-solid mechanism. In this report, planar GaAs NW growth on GaAs (100) substrates is perturbed by introducing common p-type dopant impurities, zinc (Zn) or carbon (C), and characterized structurally and electrically. The implications of the results on planar NW growth and doping mechanism are discussed.

Reduction of Low-Temperature Nonlinearities in Pseudomorphic AlGaAs/InGaAs HEMTs Due to Si-Related DX Centers

The linearity of conventional pseudomorphic AlGaAs/InGaAs/AlGaAs high-electron mobility transistors with planar doping in the AlGaAs layers is shown to degrade at low temperatures down to -40°C, as measured by the adjacent-channel power ratio under wideband code-division multiple-access modulation. A modified structure, in which the planar Si doping layers are placed within thin single GaAs quantum wells inside the AlGaAs barrier layers, eliminates this degradation. Deep-level transient spectroscopy and persistent photocapacitance measurements show that trapping on DX centers is effectively eliminated. The linearity improvements are therefore attributed to the elimination of this trapping. Self-consistent solutions of the Schro?dinger and Poisson equations show that the transfer of the donor electrons into the channel is essentially the same in the modified and conventional structures.

Optimization and Realization of Planar Isolated GaAs Zero-Biased Planar Doped Barrier Diodes for Microwave/Millimeter-Wave Power Detectors/Sensors

A high tangential signal sensitivity (TSS) zero-bias GaAs planar doped barrier (PDB) diode for microwave and millimeter-wave power detection applications is presented. The fabricated PDB diodes have shown 4 dB better TSS at 35 GHz than that of reported devices, considerably increasing the minimum detectable power and widening the dynamic range. The high TSS was obtained by optimizing the PDB layer structures, namely, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$delta$</tex> -doped <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$hboxp^++$</tex> layer and the two intrinsic layers, and by employing ion bombardment to better define the device and reduce parasitic effects. The isolation properties of ion bombarded epitaxial layers on GaAs substrates were examined and optimized to have a sheet resistivity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$hbox10^8~Omega/hboxsq$</tex> . The temperature dependence of the barrier height of the PDB diode has been investigated experimentally, showing positive temperature coefficient and, hence, better thermal stability. We have also defined the critical barrier height and derived its analytical expression, which gives the theoretically lowest possible barrier height of a PDB diode.

Nominal doping and partition of doped holes between planar and apical orbitals inLa2−xSrxCuO4

By considering both the average structural parameters obtained from Rietveld refinement of neutron powder diffraction data, and the local structural parameters obtained from the atomic pair distribution function, we have tested the recent hypothesis of Perry et al. [Phys. Rev. B 65, 144501 (2002)] that doping in a ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ system occurs as localized defects of predominantly $\mathrm{Cu}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}\text{\ensuremath{-}}\mathrm{O}{p}_{z}$ character associated with the Sr dopants accompanied by a local destruction of the Jahn-Teller distortion. While the structural parameters behave qualitatively according to the prediction of this model, a quantitative analysis indicates that doped holes predominantly appear in the planar $\mathrm{Cu}{d}_{{x}^{2}\ensuremath{-}{y}^{2}}\text{\ensuremath{-}}\mathrm{O}{p}_{x,y}$ band as is normally assumed. However, a small amount of the doped charge does enter the $\mathrm{Cu}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}\text{\ensuremath{-}}\mathrm{O}{p}_{z}$ orbitals and this should be taken into account when theoretical phase diagrams are compared to experiment. We present a calibration curve, $p=x[1.00(1)\ensuremath{-}0.45(7)x]$, for the planar charge doping, $p$, vs strontium content, $x$, for the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ system.

Doping engineering of p-type ZnO

The formation energies of defects in ZnO taken from recently published first-principles calculations are used to simulate the incorporation and compensation of group-V impurities. A detailed analysis is carried out for the specific case of nitrogen-doped ZnO as a function of doping temperature, O/Zn stoichiometric ratio, and dopant activity. This study shows that net p-type doping of ZnO:N in thermodynamic equilibrium conditions is rather limited (∼10 16 cm −3) and involves a delicate balance between the achievement of high dopant solubility (on the Zn-rich side) and low compensation by donor defects (on the O-rich side). Efficient doping appears possible in various alternative ways: simultaneous incorporation of hydrogen as an interim compensating donor to be subsequently eliminated; use of a high activity dopant in the external phase (atomic N, NO) while preventing the formation of (N 2) O compensating donors; planar doping for separate optimization of nitrogen incorporation and a defect-free crystal. The other group-V impurities (P, As) introduce deep acceptor levels when substituted for O, but have been predicted to form a single acceptor complex in the antisite position, like [ As Zn–2V Zn], with both low formation and ionization energies. This leads to a moderate p-type behaviour. Comparison of the present theoretical modelling with experiments shows that accounting for the wide range of reported hole concentrations (10 16–10 19 cm −3) necessarily implies some departure from equilibrium.

Design and characteristics of strained InAs/InAlAs composite-channel heterostructure field-effect transistors

We report composite-channel heterostructure field-effect transistors (HFETs) with an InAs channel and an In0.9Al0.1As subchannel. The HFETs are grown on antimonide buffer layers. Two composite-channel structures with different planar Te doping schemes are designed, fabricated, and characterized. High radio-frequency transconductances of above 0.9 S/mm and ∼55GHz current gain cutoff frequencies are achieved in devices with 500 nm gates. Planar Te doping in the buffer layers reduces the high kink-effect currents otherwise found in InAs/AlSb HFETs, an effect which can be attributed to either increased breakdown field in the In0.9Al0.1As subchannel or to suppression of hole blocking in the buffer. The present limitations to device performance and suggested approaches for their elimination are discussed.

DX− center formation in planar-doped GaAs:Si in strong electric fields

The kinetics of current decay and partial restoration in planar doped GaAs:Si due to the formation of DX− centers in strong electric fields has been experimentally studied. The existence of thresholds with respect to the field strength and donor concentration is explained. A model of the DX− center formation is proposed, which is based on the notions about variation of the depth and width of a potential well created by planar doping, caused by the redistribution of hot electrons between quantum confinement subbands. As a result, the energy level of DX− centers, which is situated above the potential well depth in the absence of strong field, decreases and falls within the potential well. This makes possible the DX− center formation, provided that hot electrons, occupying the resonance electron levels in the conduction band, simultaneously excite local vibrational modes.

Interface roughness and planar doping in superlattices: weak localization effects

We examine the effects of interface roughness and/or planar impurity doping in a superlattice, in the frame of a weak disorder description. We find that these two types of disorder are equivalent, and that they can be viewed as effective ‘bulk’ disorder, with anisotropic diffusion coefficients. Our results offer quantitative insight to transport properties of multilayers and devices, which contain inadvertently structural disorder at the interfaces.

Magnetoresistance decay and switching-field change in SiO2–Ni dusted Co/Cu/Co structures

The effect of interfacial planar doping with an insulating granular layer in a sandwiched structure was studied. By inserting a thin SiO2–Ni layer at either the Co/Cu (top) or Cu/Co (bottom) interface in sandwiched Co/Cu/Co structures, the magnetoresistance curve as a function of the magnetic field changed significantly. This change was due to a reduction of interlayer coupling and to a change in the switching mechanism of the magnetizations caused by modification of the interface. In addition, the dusted Co/Cu/Co structures showed flat peaks and small switching fields that would be very useful for practical applications.

Influence of carrier heating on potential well depth and screening in highly planar-doped GaAs:Si

In highly planar-doped GaAs:Si structures the application of strong electric fields leads to the formation of DX −-centers as deduced from current transients at low temperatures. The effect can be observed above a lower limit of electrically active donors and above a critical field strength depending on the donor concentration. Those thresholds can be explained by the strong influence of carrier heating on the depth of the potential wells created by the planar doping. Furthermore, the observed square-root dependence of the current change on time hints at the collective character of the effect connected with diminished screening of the interaction between the dopants promoting the shift of Si-atoms from the substitutional to an interstitial site.

Effect of planar doping in Co/SiO 2–Ni/Cu/Co structures

The effect of interfacial planar doping with insulating granular layer in a sandwiched structure was studied. By inserting a thin SiO 2–Ni layer into the sandwiched Co/Cu/Co structures, the magnetoresistance curve as a function of the magnetic field changed significantly because of reduction of interlayer coupling and the change of its switching mechanism of the magnetizations by the interface modification. The switching fields are 10 and 60 Oe for the magnetization reversals in two magnetic layers. The MR peaks are square-shaped with the width of about 30 Oe and the peak MR ratio of 3.3%.

The effect of band nonparabolicity on the range function of RKKY interaction

Using an analytical method, we provide the exact derivation of the spatial dependence of the RKKY exchange integral in a system with nonparabolic dispersion. We prove that the RKKY exchange integral shows a sign reversal, being a superposition of oscillations with two different periods. We show that our results describe the magnetic behaviour of semimagnetic semiconductors with planar doping ( δ-doping).