High Hole Density Research Articles

Weak Localization in p-Type Heterostructures in the Presence of Parallel Magnetic Field

Theory of weak localization is developed for two-dimensional holes in the presence of in-plane magnetic field. The Zeeman splitting even in the hole momentum results in the spin-dependent phase changing the quantum interference. The negative correction to the conductivity is shown to decrease by a factor of two by the in-plane magnetic field. The positive magnetoconductivity in a classically weak perpendicular field caused by the weak localization is calculated for both quadratic and quartic in momentum Zeeman hole splittings. Calculations show that the conductivity corrections are very close to each other in these two cases of low and high hole density.

Excimer Laser Patterned Holey Graphene Oxide Films for Nonenzymatic Electrochemical Sensing.

The existence of point defects, holes, and corrugations (macroscopic defects) induces high catalytic potential in graphene and its derivatives. We report a systematic approach for microscopic and macroscopic defect density optimization in excimer laser-induced reduced graphene oxide by varying the laser energy density and pulse number to achieve a record detection limit of 7.15 nM for peroxide sensing. A quantitative estimation of point defect densities was obtained using Raman spectroscopy and confirmed with electrochemical sensing measurements. Laser annealing (LA) at 0.6 J cm-2 led to the formation of highly reduced graphene oxide (GO) by liquid-phase regrowth of molten carbon with the presence of dangling bonds, making it catalytically active. Hall-effect measurements yielded a mobility of ∼200 cm2 V-1 s-1. An additional increase in the number of pulses at 0.6 J cm-2 resulted in deoxygenation through the solid-state route, leading to the formation of holey graphene structure. The average hole size showed a hierarchical increase, with the number of pulses characterized with multiple microscopy techniques, including scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The exposure of edge sites due to high hole density after 10 pulses supported the formation of proximal diffusion layers, which led to facile mass transfer and improvement in the detection limit from 25.4 mM to 7.15 nM for peroxide sensing. However, LA at 1 J cm-2 with 1 pulse resulted in a high melt lifetime of molten carbon and the formation of GO characterized by a high resistivity of 3 × 10-2 Ω-cm, which was not ideal for sensing applications. The rapid thermal annealing technique using a batch furnace to generate holey graphene results in structure with uneven hole sizes. However, holey graphene formation using the LA technique is scalable with better control over hole size and density. This study will pave the path for cost-efficient and high-performance holey graphene sensors for advanced sensing applications.

Novel Backside Structure for Reverse Conducting Insulated-Gate Bipolar Transistor With Two Different Collector Trench

A novel reverse-conducting insulated gate bipolar transistor (RC-IGBT) with two different collector trench (DCT) is proposed. One of the collector trenches is filled with heavily doped N-type polysilicon (N-poly) and the other is filled with heavily doped N- and P-poly. An electron accumulation layer is formed along the sidewall of trench owing to built-in potential difference between the N-poly and the N-drift region. The electron accumulation layer and collector trenches block the electric field, which behaves just like the N-buffer layer of the conventional RC-IGBT (Con. RC-IGBT). Similarly, due to potential difference between the P-poly and N-drift region, a high-density hole inversion layer is formed to narrow the electron current path as a high resistance. The DCT RC-IGBT achieves snapback-free in a small cell pitch without additional control. Moreover, the DCT RC-IGBT shows better tradeoff between turn-off loss and ON-state voltage drop than that of con. RC-IGBT. For the same forward voltage drop, the turn-off loss is reduced by 26%.

Gas permeation through graphdiyne-based nanoporous membranes

Nanoporous membranes based on two dimensional materials are predicted to provide highly selective gas transport in combination with extreme permeance. Here we investigate membranes made from multilayer graphdiyne, a graphene-like crystal with a larger unit cell. Despite being nearly a hundred of nanometers thick, the membranes allow fast, Knudsen-type permeation of light gases such as helium and hydrogen whereas heavy noble gases like xenon exhibit strongly suppressed flows. Using isotope and cryogenic temperature measurements, the seemingly conflicting characteristics are explained by a high density of straight-through holes (direct porosity of ∼0.1%), in which heavy atoms are adsorbed on the walls, partially blocking Knudsen flows. Our work offers important insights into intricate transport mechanisms playing a role at nanoscale.

Bipolar Junction Transistor Exhibiting Excellent Output Characteristics with a Prompt Response against the Selective Protein

AbstractBipolar junction transistors (BJTs), the basic building blocks of integrated circuits, are deployed to control switching applications and logic operations. However, as the thickness of a conventional BJT device approaches a few atoms, its performance decreases substantially. The stacking of atomically thin 2D semiconductor materials is advantageous for manufacturing atomically thin BJT devices owing to the high carrier density of electrons and holes. Here, an atomically thin n–p–n BJT device composed of heavily doped molybdenum ditelluride (n‐MoTe2) and germanium selenide (p‐GeSe) sheets stacked over each other by van der Waals interactions is reported. In a common‐emitter configuration, MoTe2/GeSe/MoTe2 BJT devices exhibit a considerably high current gain (β = Ic /Ib = 29.3) at Vbe = 2.5 V. The MoTe2/GeSe/MoTe2 BJT device is employed to detect streptavidin biomolecules as analytes within <10 s. The real‐time response of the functionalized BJT device is examined at various concentrations of streptavidin biomolecules ranging from 250 to 5 pm. Such vdW BJT devices can trigger the development of state‐of‐the‐art electronic devices that can be used as biosensors to detect the various kinds of target DNA and proteins like spike protein of Covid‐19.

Hole‐Selective Ultrathin Al‐Doped SiOxPassivation Layer Formed by Immersing in Aluminum Nitrate Aqueous Solution

Herein, ultrathin Al‐doped SiOxlayer formed by immersing in Al(NO3)3aqueous solution is introduced. This layer enables high‐level surface passivation with a maximum effective surface recombination velocity (Seff,max) < 16 cm s−1at an excess carrier density (Δn) of 1 × 1015 cm−3for 2.5 Ω cm n‐type c‐Si without any other processes such as film deposition in a vacuum chamber, high‐temperature annealing, and hydrogenation. This passivation effect presumably comes from negative fixed charges localized in Al‐doped SiOxlayers, forming inversion layers due to upward Si band bending at the SiOx/Si interface. In addition, the Al‐doped SiOxlayers facilitate accumulated holes at the interface to tunnel through the layers even though the applied bias is extremely low. The high tunnel current density of holes accumulated in the inversion layers implies that these ultrathin Al‐doped SiOxlayers are expected to be favorably implemented for high‐efficiency passivating‐contact c‐Si solar cells.

Efficient p‐Type Doping of Tin Halide Perovskite via Sequential Diffusion for Thermoelectrics

Metal halide perovskites (MHPs) hold great potential in thermoelectric (TE) applications, thanks to their regular and soft lattice in nature. However, the poor electrical conductivity caused by low charge carrier density (<1014 cm−3 for lead‐based MHPs) strongly impedes its TE development. In this scenario, tin halide perovskites (THPs) emerge as promising TE candidates owing to their high background hole densities (>1019 cm−3). However, further electrical doping remains challenging, originating from the limited capability of accommodating heterogeneous dopants and the heavy compensation in THPs. Herein, a novel diffusion‐mediated doping approach is demonstrated to prominently increase the p‐type doping level of THPs by a sequence of air exposure and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) surface treatments. In paradigm photovoltaic THP materials—CH(NH2)2SnI3 (namely FASnI3), the electrical conductivity is dramatically increased by 300× from 0.06 to 18 S cm−1 in thin films, leading to a remarkable enhancement of power factor by 25× up to 53 μW m−1 K−2. In contrast, only a slight variation of thermal conductivity is observed after F4TCNQ deposition, which is in accordance with the increase in electrical conductivity, indicating that the lattice structures of FASnI3 remain intact after doping. This study paves an illuminating way to ameliorate TE properties in halide perovskites.

Unconventional Doping Effect Leads to Ultrahigh Average Thermoelectric Power Factor in Cu3 SbSe4 -Based Composites.

Thermoelectric materials are typically highly degenerate semiconductors, which require high carrier concentration. However, the efficiency of conventional doping by replacing host atoms with alien ones is restricted by solubility limit, and, more unfavorably, such a doping method is likely to cause strong charge-carrier scattering at ambient temperature, leading to deteriorated electrical performance. Here, an unconventional doping strategy is proposed, where a small trace of alien atoms is used to stabilize cation vacancies in Cu3 SbSe4 by compositing with CuAlSe2 , in which the cation vacancies rather than the alien atoms provide a high density of holes. Consequently, the hole concentration enlarges by sixtimes but the carrier mobility is well maintained. As a result, a record-high average power factor of 19µWcm-1 K-2 in the temperature range of 300-723K is attained. Finally, with further reduced lattice thermal conductivity, a peak zT value of 1.4and a record-high average zT value of 0.72are achieved within the diamond-like compounds. This new doping strategy not only can be applied for boosting the average power factor for thermoelectrics, but more generally can be used to maintain carrier mobility for a variety of semiconductors that need high carrier concentration.

High-densities of free holes in homoepitaxial n-GaN induced by fluorine-plasma ion implantation

In this work, fluorine plasma ion implantation is used to induce high densities of free holes in homoepitaxial n-GaN for the first time, as evidenced by electroluminescence and p-n junction rectification. F+ ions implanted into the near-surface layer capture electrons from shallow bulk donors, bending the band upward until a high density of free holes is induced at the valence band maximum, and surface donors, depinning the Fermi level to form p-type ohmic contact. Free holes tunnel from the metal into states at the valence band maximum, where they form a conductive p-type channel, benefitting the development of GaN-based junction devices.

High-Performance P-Channel Tin Halide Perovskite Thin Film Transistor Utilizing a 2D-3D Core-Shell Structure.

Metal halide perovskites (MHPs) are plausible candidates for practical p‐type semiconductors. However, in thin film transistor (TFT) applications, both 2D PEA2SnI4 and 3D FASnI3 MHPs have different drawbacks. In 2D MHP, the TFT mobility is seriously reduced by grain‐boundary issues, whereas 3D MHP has an uncontrollably high hole density, which results in quite a large threshold voltage (V th). To overcome these problems, a new concept based on a 2D–3D core–shell structure is herein proposed. In the proposed structure, a 3D MHP core is fully isolated by a 2D MHP, providing two desirable effects as follows. (i) V th can be independently controlled by the 2D component, and (ii) the grain‐boundary resistance is significantly improved by the 2D/3D interface. Moreover, SnF2 additives are used, and they facilitate the formation of the 2D/3D core–shell structure. Consequently, a high‐performance p‐type Sn‐based MHP TFT with a field‐effect mobility of ≈25 cm2 V−1 s−1 is obtained. The voltage gain of a complementary metal oxide semiconductor (CMOS) inverter comprising an n‐channel InGaZnO x TFT and a p‐channel Sn‐MHP TFT is ≈200 V/V at V DD = 20 V. Overall, the proposed 2D/3D core–shell structure is expected to provide a new route for obtaining high‐performance MHP TFTs.

Influence of minority charge carrier lifetime and concentration on crystalline silicon solar cells based on double antireflection coating: A simulation study

In this work, the optoelectrical properties of silver doped zinc oxide (Ag–ZnO)/ZnO double antireflection coating (ARC) layer were investigated by a simulation approach in terms of their minority charge carriers and surface charge carrier concentrations. From the experimental results, the Ag–ZnO/ZnO double ARC layer on Si solar cells presented the reduced average reflectance by 7.58% which was lowered to ZnO and Ag–ZnO single ARC layer. The simulation study demonstrated the reasonable electron and hole densities of 1.75 × 10 16 cm −3 and 1.51 × 10 16 cm −3 on c-Si solar cells with Ag–ZnO/ZnO double ARC layer. The simulated I–V characteristics evidenced that the performances of Ag–ZnO/ZnO double ARC layer-based Si solar cells gradually increased with the increase of the minority carrier lifetimes. As compared to a single ARC layer, the Ag–ZnO/ZnO double ARC layer-based Si solar cell achieved the highest conversion efficiency 14.32% with a fill factor of 81.35% at a minority carrier lifetime of 10 μs and carrier concentration of 1 × 10 17 cm −3 . The enhanced photovoltaic performance in Ag–ZnO/ZnO double ARC layer-based Si solar cells might be attributed to the high generation of electron-hole pairs, improved minority lifetime, and excellent carrier concentrations. • A simulation study was performed for optoelectrical properties of Ag–ZnO/ZnO double antireflection coating layer > Photovoltaic properties with respect to minority charge carriers/charge carrier concentrations studied > High electron and hole densities of 1.75 × 10 16 cm −3 and 1.51 × 10 16 cm −3 were observed > Ag–ZnO/ZnO double ARC layer based Si solar cell achieved the highest PCE 14.32%.

Two-dimensional hole gas in organic semiconductors.

A highly conductive metallic gas that is quantum mechanically confined at a solid-state interface is an ideal platform to explore non-trivial electronic states that are otherwise inaccessible in bulk materials. Although two-dimensional electron gases have been realized in conventional semiconductor interfaces, examples of two-dimensional hole gases, the counterpart to the two-dimensional electron gas, are still limited. Here we report the observation of a two-dimensional hole gas in solution-processed organic semiconductors in conjunction with an electric double layer using ionic liquids. A molecularly flat single crystal of high-mobility organic semiconductors serves as a defect-free interface that facilitates two-dimensional confinement of high-density holes. A remarkably low sheet resistance of 6 kΩ and high hole-gas density of 1014 cm-2 result in a metal-insulator transition at ambient pressure. The measured degenerate holes in the organic semiconductors provide an opportunity to tailor low-dimensional electronic states using molecularly engineered heterointerfaces.

Flexible Chemiresistive Cyclohexanone Sensors Based on Single-Walled Carbon Nanotube-Polymer Composites.

We report a chemiresistive cyclohexanone sensor on a flexible substrate based on single-walled carbon nanotubes (SWCNTs) functionalized with thiourea (TU) derivatives. A wrapper polymer containing both 4-vinylpyridine (4VP) groups and azide groups (P(4VP-VBAz)) was employed to obtain a homogeneous SWCNT dispersion via noncovalent functionalization of SWCNTs. The P(4VP-VBAz)-SWCNT composite dispersion was then spray-coated onto an organosilanized flexible poly(ethylene terephthalate) (PET) film to achieve immobilizing quaternization between the pyridyl groups from the polymer and the functional PET substrate, thereby surface anchoring SWCNTs. Subsequent surface functionalization was performed to incorporate a TU selector into the composites, resulting in P(Q4VP-VBTU)-SWCNT, for the detection of cyclohexanone via hydrogen bonding interactions. An increase in conductance was observed as a result of the hydrogen-bonded complex with cyclohexanone resulting in a higher hole density and/or mobility in SWCNTs. As a result, a sensor device fabricated with P(Q4VP-VBTU)-SWCNT composites exhibited chemiresistive responses (ΔG/G0) of 7.9 ± 0.6% in N2 (RH 0.1%) and 4.7 ± 0.4% in air (RH 5%), respectively, upon exposure to 200 ppm cyclohexanone. Selective cyclohexanone detection was achieved with minor responses (ΔG/G0 < 1.4% at 500 ppm) toward interfering volatile organic compounds (VOC). analytes. We demonstrate a robust sensing platform using the polymer-SWCNT composites on a flexible PET substrate for potential application in wearable sensors.

P-NiO junction termination extensions for GaN power devices

We report the demonstration of p-NiO layers as junction termination extensions (JTEs) for realizing high-performance GaN power devices. The p-NiO was deposited by RF sputtering, a flexible process which enables to achieve high hole densities. In this work, we investigated the material and transport properties of p-NiO on GaN, and demonstrated its application as JTEs for GaN-on-Si Schottky barrier diodes (SBDs). p-NiO JTEs resulted in a similar ON-state behavior compared to a control SBD without any termination, while providing 1.9×-higher breakdown voltage of 443 V, the highest reported for SBDs on GaN-on-Si substrates.

Optoelectronic Properties of Tin-Lead Halide Perovskites.

Mixed tin–lead halide perovskites have recently emerged as highly promising materials for efficient single- and multi-junction photovoltaic devices. This Focus Review discusses the optoelectronic properties that underpin this performance, clearly differentiating between intrinsic and defect-mediated mechanisms. We show that from a fundamental perspective, increasing tin fraction may cause increases in attainable charge-carrier mobilities, decreases in exciton binding energies, and potentially a slowing of charge-carrier cooling, all beneficial for photovoltaic applications. We discuss the mechanisms leading to significant bandgap bowing along the tin–lead series, which enables attractive near-infrared bandgaps at intermediate tin content. However, tin-rich stoichiometries still suffer from tin oxidation and vacancy formation which often obscures the fundamentally achievable performance, causing high background hole densities, accelerating charge-carrier recombination, lowering charge-carrier mobilities, and blue-shifting absorption onsets through the Burstein–Moss effect. We evaluate impacts on photovoltaic device performance, and conclude with an outlook on remaining challenges and promising future directions in this area.

Optimization of Doping CdTe with Group-V Elements: A First-Principles Study

Group-V substitution at the $\mathrm{Te}$ site, ${X}_{\mathrm{Te}}$ (X = $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Sb}$), under $\mathrm{Cd}$-rich conditions is an effective way to enhance the hole density and, in the meantime, suppresses the dominant nonradiative carrier recombination center in $\mathrm{Cd}\mathrm{Te}$, thus, improving the performance of $\mathrm{Cd}\mathrm{Te}$ thin-film solar cells. However, it is not clear which group-V dopant, X, is the most effective dopant, because it is expected that ${\mathrm{P}}_{\mathrm{Te}}$ will have the shallowest acceptor level due to its high electronegativity, whereas ${\mathrm{Sb}}_{\mathrm{Te}}$ will have the smallest formation energy due to its small size mismatch with $\mathrm{Te}$. Our systematic first-principles study shows that the hole concentration contributed by the acceptor ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ is limited by the related compensating $A{X}^{+}$ center that increases simultaneously with ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ as the chemical potential of dopant X increases. However, the ratio of ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ acceptors to the $A{X}^{+}$ donors can be significantly increased if the sample is grown at high temperature and then annealed to room temperature, achieving a high hole density and low Fermi level $({E}_{F})$. We find that all group-V ($\mathrm{P}$, $\mathrm{As}$, and $\mathrm{Sb}$) dopings can achieve maximum hole densities of about ${10}^{17}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, which are consistent with previous experimental results. Despite the relatively deep acceptor level of 150 meV, $\mathrm{Sb}$ doping can achieve a considerable hole density due to the low formation energy of substituting $\mathrm{Sb}$ for $\mathrm{Te}$ with similar atomic radii. $\mathrm{P}$ doping can achieve a higher hole density than that of $\mathrm{Sb}$ due to its shallow transition energy at \ensuremath{\epsilon}(0/\ensuremath{-}) = 70 meV. However, the highest hole density is achieved through $\mathrm{As}$ doping, which is attributed to its balanced defect level at \ensuremath{\epsilon}(0/\ensuremath{-}) = 80 meV and relatively small formation energy.

Valley relaxation of resident electrons and holes in a monolayer semiconductor: Dependence on carrier density and the role of substrate-induced disorder

Using time-resolved optical Kerr rotation, we measure the low temperature valley dynamics of resident electrons and holes in exfoliated WSe$_2$ monolayers as a systematic function of carrier density. In an effort to reconcile the many disparate timescales of carrier valley dynamics in monolayer semiconductors reported to date, we directly compare the doping-dependent valley relaxation in two electrostatically-gated WSe$_2$ monolayers having different dielectric environments. In a fully-encapsulated structure (hBN/WSe$_2$/hBN, where hBN is hexagonal boron nitride), valley relaxation is found to be monoexponential. The valley relaxation time $\tau_v$ is quite long ($\sim$10~$\mu$s) at low carrier densities, but decreases rapidly to less than 100~ns at high electron or hole densities $\gtrsim$2 $\times 10^{12}$~cm$^{-2}$. In contrast, in a partially-encapsulated WSe$_2$ monolayer placed directly on silicon dioxide (hBN/WSe$_2$/SiO$_2$), carrier valley relaxation is multi-exponential at low carrier densities. The difference is attributed to environmental disorder from the SiO$_2$ substrate. Unexpectedly, very small out-of-plane magnetic fields can increase $\tau_v$, especially in the hBN/WSe$_2$/SiO$_2$ structure, suggesting that localized states induced by disorder can play an important role in depolarizing spins and mediating the valley relaxation of resident carriers in monolayer transition metal-dichalcogenide semiconductors.

Efficiency enhancement of CZTS solar cells using structural engineering

We numerically performs structural engineering for lowering interfacial recombination, and enhancing efficiency of CZTS solar cells. Structural modification of controlling carrier density profile at CZTS/CdS interfaces, CZTS absorber surface region carrier density, CZTS surface region bandgap modification, Fermi level pining and tunneling at the CZTS/CdS junction are studied for their impact device performance. We observed that the lowering hole concentration and widening bandgap of CZTS in vicinity of CZTS/CdS junction, lower interface recombination and improve performance. Fermi level pinning near to conduction band edge at CZTS/CdS interface showed lower recombination as compared to the case where Fermi level is pinned at midgap at CZTS/CdS interface. The donor defects from buffer layer pins the Fermi level near conduction band and acceptor defect from absorber CZTS layer pins Fermi level near mid gap at CZTS/CdS interface. Intra band tunneling through high conduction band offset barrier maintain performance for up to 0.5 eV spike barrier. High hole density profile at CZTS/metal lower back contact interfacial recombination. These structural engineering numerical performed here, present bright aspect for improving efficiency of CZTS solar cells. • Device engineering is performed for controlling interfacial recombination in CZTS solar cells. • Absorber surface layer parameters such as carrier density profile and bangap influence interfacial recombination. • Fermi level pinning near conduction band edge is advantageous than Fermi level pinned at midgap at CZTS/CdS interface. • High majority carrier concentration near back contact in CZTS minimize the effect of high Schottky barrier and high SRV.

Parametric Analysis of Laser Beam Percussion Drilling for Thin Titanium Alloy Sheet Using Yb: Yag Fiber Laser

Laser beam percussion drilling (LBPD) can create high density holes in aerospace materials with the repeated application of laser pulses at a single spot. In this study, one-parameter-at-a-time approach has been used to investigate the individual effect of peak power, pulse width and pulse frequency on geometrical accuracy and metallurgical distortion during LBPD of 0.85[Formula: see text]mm thick Ti–6Al–4V sheet using 200[Formula: see text]W Yb:YAG fiber laser. It has been found that the output parameters behave differently at the higher and lower values of a particular input process. The increase of pulse width from 1 to 1.50[Formula: see text]ms increases hole taper by 20% whereas the same corresponding change from 1.50 to 2.00[Formula: see text]ms reduces the taper by 20%. The increase of pulse frequency from 10 to 50[Formula: see text]Hz reduces hole circularity by 40% but the same proportionate change from 50 to 90[Formula: see text]Hz reduces circularity by 79%. Increase of peak power from 1.70 to 2.0[Formula: see text]kW increases hole taper by 8% but the corresponding increase from 2 to 2.30[Formula: see text]kW is 143%.

H-diamond MOS interface properties and FET characteristics with high-temperature ALD-grown HfO2 dielectric

The performance of hydrogen-terminated diamond MOSFETs with a large gate–drain interspace (LGD) was analyzed on a polycrystalline diamond sample. A 300 °C atomic layer deposition grown (ALD-grown) HfO2 dielectric (28 nm) was used the gate dielectric and device passivation layer. The HfO2/H-diamond interfacial valence band offset of 1.98 eV was determined by x-ray photoelectron spectroscopy, indicating that the high temperature grown HfO2 dielectric has great potential for accommodating a high density of holes on the H-diamond surface, and the high dielectric constant of HfO2 is also useful for improving the control capability of the gate on the device channel. A breakdown voltage of 657 V was achieved on the device. Baliga’s figure of merit of the device was calculated to be ∼2.0 MW/cm2, which is comparable to that of the H-diamond FETs with Al2O3 dielectrics that are more than 200 nm thick. The HfO2 dielectric shows great potential for use in H-diamond power devices.