High N-type Doping Research Articles

High Efficiency n-Type Doping of Organic Semiconductors by Cation Exchange.

Achieving efficient doping in n-type conjugated polymers is crucial for their application in electronic devices. In this study, an n-type doping method is developed based on cation exchange that maintains a high doping level while ensuring a high degree of structural order, leading to significantly improved electrical conductivity. By investigating various dopants and ionic liquids, it is discovered that the choice of dopant influences doping efficiency, while the selection of ionic liquid affects cation exchange efficiency. Through careful selection of suitable dopants and ionic liquids, High doping levels are achieved remarkably in a short period, resulting in the highest conductivity (nearly 1×10- 2Scm-¹) compared to other doping methods for poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200). The findings highlight the robustness and efficiency of cation exchange doping as an effective approach for achieving high-quality n-type doping in conjugated polymers, thereby opening new avenues for the development of advanced polymer-based electronic devices.

Large-Area MoS2 Films Grown on Sapphire and GaN Substrates by Pulsed Laser Deposition.

In this paper, we present the preparation of few-layer MoS2 films on single-crystal sapphire, as well as on heteroepitaxial GaN templates on sapphire substrates, using the pulsed laser deposition (PLD) technique. Detailed structural and chemical characterization of the films were performed using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction measurements, and high-resolution transmission electron microscopy. According to X-ray diffraction studies, the films exhibit epitaxial growth, indicating a good in-plane alignment. Furthermore, the films demonstrate uniform thickness on large areas, as confirmed by Raman spectroscopy. The lateral electrical current transport of the MoS2 grown on sapphire was investigated by temperature (T)-dependent sheet resistance and Hall effect measurements, showing a high n-type doping of the semiconducting films (ns from ~1 × 1013 to ~3.4 × 1013 cm-2 from T = 300 K to 500 K), with a donor ionization energy of Ei = 93 ± 8 meV and a mobility decreasing with T. Finally, the vertical current injection across the MoS2/GaN heterojunction was investigated by means of conductive atomic force microscopy, showing the rectifying behavior of the I-V characteristics with a Schottky barrier height of ϕB ≈ 0.36 eV. The obtained results pave the way for the scalable application of PLD-grown MoS2 on GaN in electronics/optoelectronics.

Reduced Defect Density in MOCVD-Grown MoS2 by Manipulating the Precursor Phase.

Advancements in the synthesis of large-area, high-quality two-dimensional transition metal dichalcogenides such as MoS2 play a crucial role in the development of future electronic and optoelectronic devices. The presence of defects formed by sulfur vacancies in MoS2 results in low photoluminescence emission and imparts high n-type doping behavior, thus substantially affecting material quality. Herein, we report a new method in which single-phase (liquid) precursors are used for the metal-organic chemical vapor deposition (MOCVD) growth of a MoS2 film. Furthermore, we fabricated a high-performance photodetector (PD) and achieved improved photoresponsivity and faster photoresponse in the spectral range 405-637 nm compared to those of PDs fabricated by the conventional MOCVD method. In addition, the fabricated MoS2 thin film showed a threshold voltage shift in the positive gate bias direction owing to the reduced number of S vacancy defects in the MoS2 lattice. Thus, our method significantly improved the synthesis of monolayer MoS2 and can expand the application scope of high-quality, atomically thin materials in large-scale electronic and optoelectronic devices.

Phosphorus diffusion and activation in fluorine co-implanted germanium after excimer laser annealing

The diffusion and the activation of phosphorus in phosphorus and fluorine co-implanted Ge after being annealed by excimer laser are investigated. The results prove that the fluorine element plays an important role in suppressing phosphorus diffusion and enhancing phosphorus activation. Moreover, the rapid thermal annealing process is utilized to evaluate and verify the role of fluorine element. During the initial annealing of co-implanted Ge, it is easier to form high bonding energy F n V m clusters which can stabilize the excess vacancies, resulting in the reduced vacancy-assisted diffusion of phosphorus. The maximum activation concentration of about 4.4 × 1020 cm−3 with a reduced diffusion length and dopant loss is achieved in co-implanted Ge that is annealed at a tailored laser fluence of 175 mJ/cm2. The combination of excimer laser annealing and co-implantation technique provides a reference and guideline for high level n-type doping in Ge and is beneficial to its applications in the scaled Ge MOSFET technology and other devices.

Characterization of Interface and Bulk Traps in Ultrathin Atomic Layer-Deposited Oxide Semiconductor MOS Capacitors With HfO2/In2O3 Gate Stack by C-V and Conductance Method

Oxide semiconductors have attracted revived interest for complementary metal–oxide–semiconductor (CMOS) back-end-of-line (BEOL) compatible devices for monolithic 3-dimensional (3D) integration. To obtain a high-quality oxide/semiconductor interface and bulk semiconductor, it is critical to enhance the performance of oxide semiconductor transistors. Atomic layer-deposited (ALD) indium oxide (In2O3) has been reported with superior performance such as high drive current, high mobility, steep subthreshold slope, and ultrathin channel. In this work, the interface and bulk traps in the MOS gate stack of ALD In2O3 transistors are systematically studied by using the C–V and conductance method. A low EOT of 0.93 nm is achieved directly from the accumulation capacitance in C–V measurement, indicating a high-quality gate oxide and oxide/semiconductor interface. Defects in bulk In2O3 with energy levels in the subgap are confirmed to be responsible for the conductance peak in GP/ω versus ω curves by TCAD simulation of C–V and G–V characteristics. A high n-type doping of 1×1020/cm3 is extracted from C–V measurement. A high subgap density of states (DOS) of 3.3×1020 cm−3 eV−1 is achieved using the conductance method, which contributes to the high n-type doping and high electron density. The high n-type doping further confirms the capability of channel thickness scaling because the charge neutrality level aligns deeply inside the conduction band.

A highly controllable doping technique via interdiffusion between epitaxial germanium layers and GaAs

Interdiffusion between epitaxial Ge layers and GaAs substrates was used in this study to precisely control the doping concentration and doping type of the Ge layers. An extremely high n-type doping concentration (> 1020 cm−3) was achieved in a Ge layer grown at a low temperature of 300 °C; the layer exhibited a smooth surface morphology and excellent crystalline quality. Subsequent post-growth annealing processes led to a decrease of the n-type doping concentration followed by an n- to p-type transition because of the diffusivity difference between As and Ga atoms. Our experimental results indicated the preferential diffusion of As atoms and their consequent escape through the Ge top surface, resulting in the p-type transition via time-lagged Ga diffusion. We fabricated a Ge-on-insulator junctionless field-effect transistor using a heavily n-doped 10 nm-thick Ge channel, and the device demonstrated good performance.

Metal–insulator transition in single crystalline ZnO nanowires

In this work, we report on the metal–insulator transition and electronic transport properties of single crystalline ZnO nanowires synthetized by means of Chemical Vapor Deposition. After evaluating the effect of adsorbed species on transport properties, the thermally activated conduction mechanism was investigated by temperature-dependent measurements in the range 81.7–250 K revealing that the electronic transport mechanism in these nanostructures is in good agreement with the presence of two thermally activated conduction channels. More importantly, it was observed that the electrical properties of ZnO NWs can be tuned from semiconducting to metallic-like as a function of temperature with a metal-to-insulator transition (MIT) observed at a critical temperature above room temperature (T c ∼ 365 K). Charge density and mobility were investigated by means of field effect measurements in NW field-effect transistor configuration. Results evidenced that the peculiar electronic transport properties of ZnO NWs are related to the high intrinsic n-type doping of these nanostructures that is responsible, at room temperature, of a charge carrier density that lays just below the critical concentration for the MIT. This work shows that native defects, Coulomb interactions and surface states influenced by adsorbed species can significantly influence charge transport in NWs.

Key factors boosting the performance of planar ZnFe2O4 photoanodes for solar water oxidation

The interplay between high film crystallinity and n-type doping in enhancing the performance of ZnFe2O4 thin film photoanodes has been revealed. Maximum benefit was achieved for the ca. 300 nm-thick photoactive layer with superior photon harvesting.

Freestanding n-Doped Graphene via Intercalation of Calcium and Magnesium into the Buffer Layer–SiC(0001) Interface

The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied extensively, yet precisely where the Ca resides remains elusive. Furthermore, the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been reported. Here, we use low energy electron diffraction, x-ray photoelectron spectroscopy, secondary electron cut-off photoemission and scanning tunneling microscopy to elucidate the physical and electronic structure of both Ca- and Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca intercalates underneath the buffer layer and bonds to the Si-terminated SiC surface, breaking the C-Si bonds of the buffer layer i.e. 'freestanding' the buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene (Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial graphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC surface and Mg bonds to the Si-terminated SiC surface are formed, resulting in Mg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG). Ca-intercalation underneath the buffer layer has not been considered in previous studies of Ca-intercalated epitaxial graphene. Furthermore, we find no evidence that either Ca or Mg intercalates between graphene layers. However, we do find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low workfunctions of 3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to ambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG remains remarkably stable.

Sub-10 nm junctionless carbon nanotube field-effect transistors with improved performance

Carbon nanotube field-effect transistors (CNTFETs) and their growing applications are becoming part of modern nanoelectronics, which is in urgent need for high-performance ultrascaled transistors. Sub-10-nm junctionless ballistic carbon nanotube field-effect transistors (JL-CNTFET) with substantial improved performance are computationally proposed herein. The non-equilibrium Green's function (NEGF) simulation is used for the computational assessment. The proposed simple improvement technique is based on the use of high n-type doping concentration at the level of carbon nanotube underneath the gate while keeping the junctionless paradigm that facilitates tremendously the fabrication processes of ultrascaled FETs. It has been found that the proposed doping profile can significantly mitigate several ultrascaling effects while boosting the performance of sub-10-nm JL-CNTFETs. The recorded enhancements include the leakage current, current ratio, subthreshold swing, switching speed, switching energy, drain-induced barrier lowering, and threshold voltage roll-off. The significant improvements obtained in this work for sub-10-nm JL-CNTFETs, make the proposed strategy, which is simple, feasible, and efficient, as a promising technique for improving ultrascaled FETs endowed with other gate geometries and channel nanomaterials while paving the way towards high-performance sub-5-nm FETs.

Direct-Bandgap Electroluminescence From Germanium With Subband Engineering Utilizing a Metal-Oxide-Semiconductor Structure

In this article, an electrical doping method to realize the high n-type doping for a germanium (Ge) layer in a low-doped n-type Ge metal-oxide-semiconductor (MOS) structure in the accumulation condition is proposed, which maintains the initial crystal quality of Ge. The direct-bandgap electroluminescence (EL) from Ge light-emitting diode (LED) has been demonstrated with 1.5– $1.6~\mu \text{m}$ infrared emission using the metal/graphene/high- ${k}$ / interfacial layer (IL)/ ${n}$ -Ge MOS structure fabricated on bulk-Ge substrate. An onset current density of 5 A cm−2 is observed, which is, up to now, the lowest onset current density compared with the Ge LEDs fabricated on unstrained Ge substrates. These results indicate the superiority of the electrical doping method and the interface passivation technique on yielding the high-performance Ge-based LEDs and lasers.

Inadequacy of Third-Order Elastic Coefficients for Predicting Nonlinearity in Highly n-Type-Doped Silicon Resonators

Third-order nonlinear elastic coefficients of silicon have been assumed to be the dominant factor in describing the nonlinear behavior of silicon-based resonators in literature. In this article, it is postulated that in spite of the common belief, third-order elastic coefficients may not be adequate to explain the nonlinear elastic behavior of silicon micro-resonators at high n-type doping concentrations. The nonlinear behavior observed in degenerately n-type-doped bulk-extensional mode resonators aligned to $\langle {100}\rangle $ orientation is carefully studied in which a spring-hardening effect is observed at large vibration amplitudes. It is shown that the existing analytical/numerical calculations and a proposed finite element model that are based on the utilization of third-order elastic (TOE) coefficients will all predict spring-softening in such resonators, thus suggesting insufficiency of the nonlinear model.

Size effect of electronic properties in highly arsenic-doped silicon nanowires

The unique electrostatic properties of semiconductor nanowires enable the realization of novel transistor types by the possibility to use surround gate architectures resembling ideal gate electrostatic control. Nevertheless one fundamental issue of semiconducting nanowire channels is the reliable control of doping to adjust the charge carrier concentration. Indeed, as dimensions scale down the surrounding media and the interfaces become more important. In this study we experimentally investigate the role of surface depletion and dielectric mismatch on the electronic charge transport of highly arsenic doped and bottom-up grown silicon nanowires. Electrical characterization of silicon nanowires (SiNWs) synthesized by Au catalyzed vapour-liquid-solid (VLS) growth and in-situ arsine (AsH3) doping is reported for the first time. We demonstrate that high n-type doping is possible by adjusting the dopant precursor flow ratio during growth. Based on electrical measurements of individual nanowires, reproducible donor concentrations of up to 5.2 × 1019 cm−3 could be revealed. By measuring the electrical characteristics for individual nanowires in dependence of their radius, we show that the electrically active carrier density drastically reduces for small nanowires at radii much larger than those at which quantization or dopant surface segregation effects are expected to occur. Furthermore, enhancement of the contact transparency for small radii nanowires is demonstrated through dopant segregation upon metal silicidation. Size dependent measurement of electrical characteristics revealed improved contact resistivities as low as 1.4 × 10−11 Ωm2.

Ultrafast carrier recombination in highly n-doped Ge-on-Si films

We study the femtosecond carrier dynamics of n-type doped and biaxially strained Ge-on-Si films which occurs upon impulsive photoexcitation by means of broadband near-IR transient absorption spectroscopy. The modeling of the experimental data takes into account the static donor density in a modified rate equation for the description of the temporal recombination dynamics. The measurements confirm the negligible contribution at a high n-type doping concentration, in the 1019 cm−3 range, of Auger processes as compared to defect-related Shockley-Read-Hall recombination. Energy resolved dynamics reveal further insights into the doping-related band structure changes and suggest a reshaping of direct and indirect conduction band valleys to a single effective valley along with a significant spectral broadening of the optical transitions.

Effect of Ge doping on growth stress and conductivity in AlxGa1-xN

Silicon (Si) is a common n-type donor in AlxGa1-xN; however, it induces bending of edge-type threading dislocations which can generate tensile stress in the film leading to the formation of channeling cracks in thick layers. Germanium (Ge) has previously been investigated as an alternative to Si for n-type doping of GaN, but its impact on film stress in AlxGa1-xN has not been investigated in detail. In this study, we employ in situ wafer curvature measurements combined with postgrowth characterization to investigate Ge doping of AlxGa1-xN (x = 0–0.62) layers grown on 6H-SiC by metalorganic chemical vapor deposition. It was found that Ge doping (n ∼ 1.6 × 1019 cm−3) of Al0.30Ga0.70N does not induce tensile stress during growth in contrast to that observed with a similar level of Si doping. In addition, the average inclination angle of edge dislocations was similar for undoped and Ge doped films indicating that Ge does not promote surface-mediated dislocation climb. High n-type doping was achieved in Ge doped AlxGa1-xN for lower Al fraction range (x < 0.5), but resistivity increased and carrier density decreased significantly for higher Al fractions. The results demonstrate Ge doping as a viable alternative to Si doping of AlxGa1-xN (x < 0.5) for achieving thick, crack-free layers.

Enhancement-mode CdS nanobelts field effect transistors and phototransistors with HfO2 passivation

As typical direct bandgap II–VI semiconductors, quasi-one dimensional CdS nanowires, nanobelts, and nanorods have shown great potential in electronic and optoelectronic applications. However, most nano-scale CdS Field Effect Transistors (FETs) work in the depletion-mode (D-mode) due to the high unintentional n-type doping concentration, which results in high power consumption under off-state. In addition, the large dark current limits to the specific detectivity when they are fabricated into phototransistors. Here, we have synthesized single crystal CdS nanobelts (NBs) on a SiO2/Si substrate via chemical vapor deposition. The CdS NB FETs were fabricated with HfO2 as a passivation layer. It is found that the working mode of the FETs was transformed from the D-mode to the enhancement-mode with the threshold voltage changing from −22.6 to 0.7 V due to the decrease in the defect density. The HfO2 passivated CdS NB phototransistor shows a responsivity of 4.7 × 104 A/W and an ultrahigh detectivity of 9.07 × 1014 Jones at the source-drain voltage of 1 V under an illumination wavelength of 450 nm. Our work demonstrates an effective way to achieve enhancement-mode CdS FETs and high performance phototransistors.

V-groove trench gate SiC MOSFET with a double reduced surface field junction termination extensions structure

We have developed V-groove trench gate SiC MOSFETs (VMOSFETs) with a double reduced surface field junction termination extensions structure (DR-JTEs) to reduce JFET resistance by using a high n-type doping density in a surface drift layer. Simulation results showed that the DR-JTEs structure for an optimized current spreading layer doping of 5 × 1016 cm−3 has a breakdown voltage that corresponds to 94% of the ideal 1D parallel-plane value. The VMOSFETs with a DR-JTEs structure demonstrated both a breakdown voltage of 1610 V and a specific on-resistance of 2.4 mΩcm2 with a threshold voltage of 5.1 V.

High concentration phosphorus doping in Ge for CMOS-integrated laser applications

Germanium is a promising material for the laser that can be monolithically integrated on Si complementary metal-oxide-semiconductor platform and has emission wavelength of 1550 nm for optical interconnect. To obtain significant optical gain, it is necessary to achieve high n-type doping concentration level, while avoiding the damage to Ge crystalline quality. In this paper, we report an ex-situ phosphorus diffusion doping of Ge film, based on low-temperature phosphosilicate glass (PSG) pre-deposition process such as spin-on-glass and sub-atmospheric chemical vapor deposition (SACVD) methods. Closely related to optical gain properties of Ge for Ge-on-Si laser application, the photoluminescence characteristics of Ge epitaxial film after P diffusion doping were investigated. In particular, SACVD-processed PSG deposited directly on Ge film without any Si capping layer successfully led to high phosphorus doping concentration of ∼1019 cm−3 deep inside Ge and dramatically enhanced photoluminescence intensity by more than 10 times compared to intrinsic Ge film. By using the SACVD-PSG based P doping process, we developed an inverted-rib Ge waveguide structure for more effective optical gain. In the inverted-rib Ge structure, the mode will be positioned upward and stay relatively away from the Ge-Si interface where many dislocations are located and, as a result, we can expect less optical loss due to scattering and the overall higher mode gain. As a very promising preliminary result, from optical-pumping of the inverted-rib Ge, a threshold-like behavior starting at 18 kW/cm2 and amplified spontaneous emission around 1570 nm were demonstrated.

Improved performance of Al/n<sup>+</sup>Ge Ohmic contact andGe n<sup>+</sup>/p diode by two-step annealing method

Silicon based germanium devices are crucial parts of optoelectronic integration as CMOS feature size continuously decreases. Germanium has attracted increasing attention due to its higher electron and hole mobility, larger optical absorption coefficient as well as lower processing temperature than those of silicon. However, the high diffusion coefficient and low solid solubility about n-type dopant and relatively high thermal budget required for high n-type doping in Ge make it difficult to achieve high activation n-type doping and excellent n<sup>+</sup>/p shallow junction for source/drain in the nano-scaled n-MOSFET (here MOSFET stands for). The high activation concentration and shallow junction n-type doping in Ge are greatly beneficial to the scaled Ge n-MOSFET technology. In this work, the ohmic contact of Al/n<sup>+</sup>Ge and Ge n<sup>+</sup>/p junction fabricated by a combination of low temperature pre-annealing process and excimer laser annealing for phosphorus-implanted germanium are demonstrated. Prior to excimer laser annealing, the samplesare annealed at a relatively low temperature, which can heal the implantation damages preliminarily. Through the optimization of pre-annealing temperature and time, the low temperature pre-annealing step can play a critical role in annihilating the implantation damages and significantly suppressing phosphorus diffusion in the laser annealing process, resulting in a very small dopant diffusion length at a high activation level of phosphorus. Through the combination of ion implantation and two-step annealing technology, the specific contact resistivity (<i>ρ</i><sub>C</sub>) of Al/n<sup>+</sup>Ge Ohmic contact is measured by CTLM structure. The optimized annealing condition is 400 <sup>o</sup>C-10 min of low temperature annealing and 150 mJ/cm<sup>2</sup> of ELA. Under that annealing condition, the ρ<sub>C</sub> of the sample by two-step annealing is reduced to 2.61 × 10<sup>–6</sup> Ω·cm<sup>2</sup>, which is one order of magnitude lower than that by ELA alone (about 3.44 × 10<sup>–4</sup> Ω·cm<sup>2</sup>). The lower value of <i>ρ</i><sub>C</sub> for the sample with LTPA can contribute to the higher carrier concentration and better crystalline quality thanthat without LTPA, which is confirmed by SRP and TEM. Moreover, the rectification ratio of Ge n<sup>+</sup>/p junction diode is improved to 8.35 × 10<sup>6</sup> at ± 1 V, which is two orders of magnitudes higher than that by ELA alone. And a lower ideality factor of about 1.07 is also obtained than that by ELA alone, which indicates that the implantation damages can be repaired perfectly by two-step annealing method.

Silicon doping of GaP layers grown by time-modulated PECVD

We study the doping in GaP layers grown on n-type silicon wafer by time-modulated plasma-enhanced chemical vapour deposition with additional flow of silane. Classical and electrochemical (ECV) capacitance-voltage methods were performed on GaP/Si heterostructures and they demonstrate high electron concentration in the GaP layer and similar profiles. In addition, glow-discharge optical emission spectroscopy revealed high silicon content in GaP, which should be responsible for the detected high n-type doping.