Reduction Of Carrier Density Research Articles

Studies on suppressed surface recombination of InGaN-based red light-emitting diodes with V-pits

InGaN-based red light-emitting diodes (LEDs) are a promising candidate to achieve high efficiency, which is highly desired by the display market. Understanding their size-dependent properties is crucial for further performance improvement. In this work, different sized InGaN red LEDs (from 500 × 500 to 15 × 15 µm2) based on V-pits were characterized systematically. The current–voltage characteristics show that the leakage current density, revealed either in forward or reverse bias, is independent of the chip size. Further light output power measurements and current dependent electroluminescence also reveal very weak dependence of the power density on chip size showing insensitivity the chip perimeter. The suppressed surface recombination effect may be due to effective carrier trapping effect by the V-pits, which prevents the carriers from flowing to the sidewalls. By detailed analysis of the spatially resolved electroluminescence, an anti-correlation between the emission wavelengths and the intensities was observed, with the longer wavelength emitting regions exhibit weaker intensity, which can be explained by the reduced carrier density in the V-pit-rich regions as carriers are partially captured by quantum wells associated with the V-pits and the associated dislocations. This study will pave the way for further understanding and improving the efficiency of InGaN red LEDs including micro-LEDs.

Modeling the carrier density in the exciton formation zone of organic light-emitting diode under high current injection

The carrier density in exciton formation zone is the electrical parameter most relevant to the stability of organic light-emitting diode: the decrease of carrier density improves the device stability. Here, based on the general mode of carrier device lifetimes, the carrier densities in the exciton formation zone of organic light-emitting diode have been calculated at current densities (J) ≥ 2.0 kA cm−2. With J increasing, the carrier density increases accordingly. At a given J, the carrier density decreases with the emissive layer’s thickness increasing, but increases with the emissive layer’s charge carrier mobilities. The increase of the energetic disorder of emissive layer leads to a marked decrease of carrier density. Increasing the hole (electron) mobility of hole (electron) transport layer helps reduce carrier density. The current research is believed very beneficial to the development of electrically pumped organic lasers.

15 MeV proton damage in NiO/β-Ga2O3 vertical rectifiers

15 MeV proton irradiation of vertical geometry NiO/β-Ga2O3 heterojunction rectifiers produced reductions in reverse breakdown voltage from 4.3 kV to 3.7 kV for a fluence of 1013ions·cm−2 and 1.93 kV for 1014 ions·cm−2. The forward current density was also decreased by 1–2 orders of magnitude under these conditions, with associated increase in on-state resistance R ON. These changes are due to a reduction in carrier density and mobility in the drift region. The reverse leakage current increased by a factor of ∼2 for the higher fluence. Subsequent annealing up to 400 °C further increased reverse leakage due to deterioration of the contacts, but the initial carrier density of 2.2 × 1016 cm−3 was almost fully restored by this annealing in the lower fluence samples and by more than 50% in the 1014 cm−2 irradiated devices. Carrier removal rates in the Ga2O3 were in the range 190–1200 for the fluence range employed, similar to Schottky rectifiers without the NiO.

Efficiency Boost of (Ag0.5,Cu0.5)(In1‐x,Gax)Se2 Thin Film Solar Cells by Using a Sequential Process: Effects of Ag‐Front Grading and Surface Phase Engineering

AbstractPost‐selenization‐fabricated (using elemental Se vapor) Cu(In,Ga)Se2 solar cell efficiency is limited by a low open‐circuit voltage, which is attributable to the Ga‐deficient surface and insufficient grain growth. In this study, a band‐grading structure is demonstrated by combining Ag‐front and Ga‐back grading in selenized (Ag,Cu)(In,Ga)Se2 (ACIGSe) absorbers with a properly designed precursor structure (Mo/CuGa/In/AgGa) and high Ag content ([Ag]/([Ag]+[Cu]) = 0.5). The phase evolution during post‐selenization reveals that the precursor structure suppresses Ag2Se formation and promotes the ACIGSe phase formed at a low temperature with enhanced grain growth. A widened surface bandgap by Ag‐front grading substantially increases the open‐circuit voltage. Furthermore, Ag addition promotes ordered vacancy compound (OVC) formation on the front surface to enlarge the valence band offset, which in turn reduces interface recombination. Furthermore, the OVC phase also assists interface passivation. Promoting surface OVC phase by Ag addition is also validated by first‐principles calculations. Furthermore, the K‐doped CuGa precursor is used for a ACIGSe absorber to address the significantly reduced carrier density by the Ag addition. With a band‐grading structure and surface OVC phase, the superior device achieves an efficiency of > 19%, the highest efficiency by post‐selenization with an elemental Se source.

Copper-nickel functionally magnetic gradient material fabricated via directed energy deposition

In this paper, a functional gradient material (FGM) of CuSn10 and Inconel 718 is fabricated through a laser-based directed energy deposition (DED) additive manufacturing process. Experimental studies are performed to examine the microstructures, the interface bonding behaviors, and the physical and mechanical properties of the resulting FGMs. The result shows that a sound metallurgical bonding was formed at the interface section. The electrical resistivity for CuSn10, IN718, and CuSn10 – IN718 samples are tested for a temperature range of 320 to 390 K, CuSn10 – IN718 specimen displayed its value in the middle range of its counterparts. The CuSn10 – IN718 FGM displayed the highest absolute Seebeck coefficient in the temperature range 320 K to 390 K. The change in the Seebeck coefficient of the FGM showed a 70 % increase from the single material specimens. This boost indicated a reduction in carrier density across the interface, theorized to be influenced by the interface of the FGM. During the compression strength test, a folding deformation occurred in the copper alloy region but neither buckling nor deformation was triggered at the interface section.

Reduction of carrier density and enhancement of the bulk Rashba spin-orbit coupling strength in Bi2Te3/GeTe superlattices

Featured with the large Rashba constant (αR ∼ 4.2 eVÅ) coupled with the ferroelectricity, GeTe has attracted much interest, proposing novel energy-efficient spintronic devices. However, those approaches are hampered by the high hole density of GeTe around 1020-1021 cm−3, which deteriorates both the ferroelectricity and the bulk Rashba effect of GeTe. To solve this problem, we have investigated the superlattices composed of Bi2Te3 and GeTe ([BT|GT] SL), the former of which is known as a topological insulator (TI) with typically n-type conduction and strong spin-orbit coupling. We have investigated the magnetotransport properties of 25 [BTx|GTy]z SLs with varying thicknesses (x, y: the thickness in the multiples of the unit cell of the respective layer) of each layer and the repetition number (z) systematically. We have observed a minimum carrier density of 5.7 × 1019 cm−3 in [BT6|GeTe6]6 superlattice sample smaller than that (∼4.4 ×1020 cm−3) of GeTe single film by an order. In addition, we have found that some [BT|GT] SLs with reduced carrier density have a larger Rashba constant compared to that of a single GeTe film. This is explained by the change of the spin polarization which can be modulated by the Fermi level in the Rashba band. These results provide a viable way to realize robust ferroelectricity and strong SOC in GeTe-related material systems simultaneously.

Cu2ZnSnS4 monograin layer solar cells for flexible photovoltaic applications.

Monograin powder technology is one possible path to developing sustainable, lightweight, flexible, and semi-transparent solar cells, which might be ideal for integration with various building and product elements. In recent years, the main research focus of monograin technology has centered around understanding the synthesis and optoelectronic properties of kesterite-type absorber materials. Among these, Cu2ZnSnS4 (CZTS) stands out as a promising solar cell absorber due to its favorable optical and electrical characteristics. CZTS is particularly appealing as its constituent elements are abundant and non-toxic, and it currently holds the record for highest power conversion efficiency (PCE) among emerging inorganic thin-film PV candidates. Despite its advantages, kesterite solar cells' PCE still falls significantly behind the theoretical maximum efficiency due to the large VOC deficit. This review explores various strategies aimed at improving VOC losses to enhance the overall performance of CZTS monograin layer solar cells. It was found that low-temperature post-annealing of CZTS powders reduced Cu-Zn disordering, increasing Eg by ∼100 meV and VOC values; however, achieving the optimal balance between ordered and disordered regions in kesterite materials is crucial for enhancing photovoltaic device performance due to the coexistence of ordered and disordered phases. CZTS alloying with Ag and Cd suppressed non-radiative recombination and increased short-circuit current density. Optimizing Ag content at 1% reduced CuZn antisite defects, but higher Ag levels compensated for acceptor defects, leading to reduced carrier density and decreased solar cell performance. Co-doping with Li and K resulted in an increased bandgap (1.57 eV) and improved VOC, but further optimization is required due to a relatively large difference between measured and theoretical VOC. Heterojunction modifications led to the most effective PCE improvement in CZTS-based solar cells, achieving an overall efficiency of 12.06%.

Enhancement of superconductivity upon reduction of carrier density in proximitized graphene

The superconducting transition temperature (Tc) of a single layer graphene coupled to an Indium oxide (InO) film, a low carrier-density superconductor, is found to increase with decreasing carrier density and is largest close to the average charge neutrality point in graphene. Such an effect is very surprising in conventional BCS superconductors. We study this phenomenon both experimentally and theoretically. Our analysis suggests that the InO film induces random electron and hole-doped puddles in the graphene. The Josephson effect across these regions of opposite polarity enhances the Josephson coupling between the superconducting clusters in InO, along with the overall Tc of the bilayer heterostructure. This enhancement is most effective when the chemical potential of the system is tuned between the charge neutrality points of the electron and hole-doped regions.

Structural Defects Modulate Electronic and Nanomechanical Properties of 2D Materials.

Two-dimensional materials such as graphene and molybdenum disulfide are often subject to out-of-plane deformation, but its influence on electronic and nanomechanical properties remains poorly understood. These physical distortions modulate important properties which can be studied by atomic force microscopy and Raman spectroscopic mapping. Herein, we have identified and investigated different geometries of line defects in graphene and molybdenum disulfide such as standing collapsed wrinkles, folded wrinkles, and grain boundaries that exhibit distinct strain and doping. In addition, we apply nanomechanical atomic force microscopy to determine the influence of these defects on local stiffness. For wrinkles of similar height, the stiffness of graphene was found to be higher than that of molybdenum disulfide by 10-15% due to stronger in-plane covalent bonding. Interestingly, deflated graphene nanobubbles exhibited entirely different characteristics from wrinkles and exhibit the lowest stiffness of all graphene defects. Density functional theory reveals alteration of the bandstructures of graphene and MoS2 due to the wrinkled structure; such modulation is higher in MoS2 compared to graphene. Using this approach, we can ascertain that wrinkles are subject to significant strain but minimal doping, while edges show significant doping and minimal strain. Furthermore, defects in graphene predominantly show compressive strain and increased carrier density. Defects in molybdenum disulfide predominantly show tensile strain and reduced carrier density, with increasing tensile strain minimizing doping across all defects in both materials. The present work provides critical fundamental insights into the electronic and nanomechanical influence of intrinsic structural defects at the nanoscale, which will be valuable in straintronic device engineering.

Study on device reliability for P-type FinFETs with different fin numbers

In this work, both channel coupling effect and compressive stress were found to influence the performance and reliability for p-type FinFET. With fin number increasing, the carrier density in the channel is reduced due to the coupling effect between the neighboring fins. On the other hand, with the enlarging of the area of contact etch stop layer (CESL) due to fin number increasing, hole mobility is enhanced because of active surface area (SA) induced compressive stress. As a result, multi-fin FinFET device with more fins shows lower normalized drain current due to reduced carrier density, and worse reliability could also be observed because of hotter carriers in the channel.

Toward Ultra-Low Efficiency Droop in C-Plane Polar InGaN Light-Emitting Diodes by Reducing Carrier Density with a Wide InGaN Last Quantum Well

We demonstrate an ultra-low efficiency droop in c-plane polar InGaN blue light-emitting diodes (LEDs) by reducing the carrier density using a wide InGaN last quantum well (LQW). It is found that the LEDs with a 5.2 nm thick LQW show a negligible efficiency droop, with an external quantum efficiency (EQE) reducing from a peak value of 38.8% to 36.4% at 100 A/cm2 and the onset-droop current density is raised from 3 A/cm2 to 40 A/cm2 as the LQW thickness increases from 3.0 nm to 5.2 nm. The analysis based on the ABC model indicates that small efficiency droop is caused by the reduced carrier density using a wide LQW. The peak efficiency is reduced with a wide LQW, which is caused by the reduction of the electron-hole wavefunction overlap and the deterioration of the crystal quality of the InGaN layer. This study suggests that the application of the InGaN LEDs with a wide LQW can be a promising and simple remedy for achieving high efficiency at a high current density.

Effects Of Structural Phase Transition On Thermoelectric Performance in Lithium-Intercalated Molybdenum Disulfide (Li xMoS2).

Layered transition metal dichalcogenides (TMDCs) intercalated with alkali metals exhibit mixed metallic and semiconducting phases with variable fractions. Thermoelectric properties of such mixed-phase structure are of great interest because of the potential energy filtering effect, wherein interfacial energy barriers strongly scatter cold carriers rather than hot carriers, leading to enhanced Seebeck coefficient ( S). Here, we study the thermoelectric properties of mixed-phase Li xMoS2 as a function of its phase composition tuned by in situ thermally driven deintercalation. We find that the sign of Seebeck coefficient changes from positive to negative during initial reduction of the 1T/1T' phase fraction, indicating crossover from p- to n-type carrier conduction. These anomalous changes in Seebeck coefficient, which cannot be simply explained by the effect of deintercalation-induced reduction in carrier density, can be attributed to the hybrid electronic property of the mixed-phase Li xMoS2. Our work shows that careful phase engineering is a promising route toward achieving thermoelectric performance in TMDCs.

Effects of a Reduced Effective Active Region Volume on Wavelength-Dependent Efficiency Droop of InGaN-Based Light-Emitting Diodes

In this study, wavelength-dependent efficiency droop phenomena in InGaN-based light-emitting diodes (LEDs) by a reduced effective active region volume were investigated. Different effective active region volumes can be extracted from theoretical fitting to the efficiency-versus-current curves of standard high efficiency InGaN near-ultraviolet, blue, and green LEDs. It has been found that the effective volume of the active region reduces more significantly with increasing emission wavelength, resulting in a lower onset-droop current density, as well as a more severe droop. Increasing the quantum well (QW) thickness to reduce carrier density is proposed as an effective way to alleviate the efficiency droop.

Irradiation effects on the structural and optical properties of single crystal β-Ga2O3

In the present work, we report the 25 MeV oxygen irradiation effects in n-type single crystal β-Ga2O3 at different fluences. We demonstrate that the symmetric stretching modes and bending vibrations of GaO4 and GaO6 units are impaired upon increasing O irradiation fluence. Blue and green photoluminescence (PL) emission bands are found to be mainly associated with gallium–oxygen divacancies, gallium vacancies and oxygen interstitials. The increase of optically active centers at low fluence and the PL quenching at high fluence are ascribed to the reduction of carrier density and the production of non-radiative recombination centers, respectively. The results envisage the possibility of obtaining pre-designed spectral behaviors by varying the oxygen irradiation fluence.

Quantum superconductor-insulator transition in titanium monoxide thin films with a wide range of oxygen contents

The superconductor-insulator transition (SIT), one of the most fascinating quantum phase transitions, is closely related to the competition between superconductivity and carrier localization in disordered thin films. Here, superconducting $\mathrm{Ti}{\mathrm{O}}_{x}$ films with different oxygen contents were grown on $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ substrates by a pulsed laser deposition technique. The increasing oxygen content leads to an increase of disorder, a reduction of carrier density, an enhancement of carrier localization, and therefore a decrease of superconducting transition temperature. A fascinating SIT emerges in cubic $\mathrm{Ti}{\mathrm{O}}_{x}$ films with increasing oxygen content and its critical sheet resistance is close to the quantum resistance $h/{(2e)}^{2}\ensuremath{\sim}6.45\phantom{\rule{0.16em}{0ex}}\mathrm{k}\mathrm{\ensuremath{\Omega}}$. The scaling analyses of magnetic field--tuned SITs show that the critical exponent products z\ensuremath{\nu} increase from 1.02 to 1.31 with increasing disorder. Based on the results, the SIT can be described by the ``dirty boson'' model, and a schematic phase diagram for $\mathrm{Ti}{\mathrm{O}}_{x}$ films was constructed.

Effective g Factor of Subgap States in Hybrid Nanowires.

We use the effective g factor of Andreev subgap states in an axial magnetic field to investigate how the superconducting density of states is distributed between the semiconductor core and the superconducting shell in hybrid nanowires. We find a steplike reduction of the Andreev g factor and an improved hard gap with reduced carrier density in the nanowire, controlled by gate voltage. These observations are relevant for Majorana devices, which require tunable carrier density and a g factor exceeding that of the parent superconductor.

Dynamic evolution of photogenerated carriers at complex oxide heterointerfaces

Heterointerfaces between two insulators play a central role in the study of oxide electronics owing to a spectrum of emergent properties. Manipulating transport of the interface by light can result in significant modulation of the ground state and excite localized states. However, their dynamics and mechanisms of photogenerated carries remain unclear. Here, this study presents the dynamics of carrier density and mobility under and after light illumination by Hall effect over time. It is discovered that the density and mobility after light illumination obey a stretched exponential expression, further indicating that the variation of mobility caused by the electron-electron scattering plays an important role in the recovery process in addition to the reduction of carrier density. Meanwhile, a non-linear Hall resistance at the LaAlO3/SrTiO3 interface under the illumination of a 360 nm laser at low temperature is observed. Furthermore, the gating effect can tune the recovery process after light illumination and induce a disappearance of non-linear Hall resistance. The results provide the experimental support for detailed mechanisms of the nonequilibrium process and developing of all-oxide electronic devices based on heterointerfaces.

Reducing Carrier Density in Formamidinium Tin Perovskites and Its Beneficial Effects on Stability and Efficiency of Perovskite Solar Cells

In Sn-based halide perovskite solar cells (PSCs), the oxidation of Sn2+ to Sn4+ under ambient air leads to unwanted p-type doping in the perovskite film, which is a main reason for increased background carrier density and low efficiency. Here, we find that the introduction of bromide into formamidinium tin iodide (CH(NH2)2SnI3, FASnI3) lattice significantly lowers the carrier density of perovskite absorber, which is thought to be a result of reduction of Sn vacancies. It reduces the leakage current of devices, increases recombination lifetime, and finally improves open-circuit voltage and fill factor of the resulting devices employing mesoporous TiO2 as an electron transport layer. Consequently, a high power conversion efficiency (PCE) of 5.5% is achieved with an average PCE of 5%, and after encapsulation the devices are highly stable over 1000 h under continuous one sun illumination including the ultraviolet region. This study suggests a simple approach for improving stability and efficiency in FASnI3-ba...

Ferromagnetism in vanadium-doped Bi2Se3 topological insulator films

With molecular beam epitaxy, we grew uniformly vanadium-doped Bi2Se3 films which exhibit ferromagnetism with perpendicular magnetic anisotropy. A systematic study on the magneto-transport properties of the films revealed the crucial role of topological surface states in ferromagnetic coupling. The enhanced ferromagnetism with reduced carrier density can support quantum anomalous Hall phase in the films, though the anomalous Hall resistance is far from quantization due to high carrier density. The topological surface states of films exhibit a gap of ∼180 meV which is unlikely to be magnetically induced but may significantly influence the quantum anomalous Hall effect in the system.

Analytical modeling of subthreshold characteristics of ultra-thin double gate-all-around (DGAA) MOSFETs incorporating quantum confinement effects

In this work, analytical models of subthreshold current and subthreshold swing of short channel ultra-thin double gate-all-around (DGAA) MOSFETs including quantum confinement effects have been proposed. The subthreshold current model is presented using Pao-Sah's double integration formula accounting both drift and diffusion components of the current. Quantum effects arising due to 2-D carrier confinement are incorporated in the model to consider the effective carrier density reduction in ultra-thin channel region. A detailed analysis of charge density calculation using density of states (DOS) method is also done. Further, the virtual-cathode potential has been utilised to model the subthreshold swing of the device. The effect of device design parameters like channel thickness, oxide thickness, gate length etc. on subthreshold characteristics has been extensively studied. The model results have been verified by comparing it with the numerical simulation data obtained from 3D device simulator Visual TCAD of Cogenda Int.