Carrier Density Dependence Research Articles

The pH-dependent properties of cuprous oxide thin films prepared by electrochemical deposition for liquid petroleum gas sensing

Efficient liquid petroleum gas sensing was investigated by modulating the pH level (8, 9, 10, 11, and 12) of electrochemically deposited nanostructured cuprous oxide thin films in a lactate bath. The synthesized films were characterized by surface contact angle, XRD, SEM, UV–Vis absorption, Mott-Schottky, and AC impedance spectroscopy measurements. These displayed a unique initial increment or decrease followed by a persistent decrement or increment at pH 9–10. This turning pH point exhibited the least wetting polycrystalline Cu2O growth with the smallest crystallite size of ∼ 45 nm, the highest dislocation density of ∼50 µm−2, and a strain of 0.03 according to X-ray diffraction. SEM micrographs identified ∼ 1 µm-sized, closely structured grains with trigonal-prismatic shapes. The Mott-Schottky plots drawn from C-V measurements revealed a significant dependence of flat band potential and free carrier density on the deposition bath pH level. Cu2O films deposited using the lactate bath changed the conductivity at pH 9 from n-type to p-type when increasing the pH value. With the conductivity transition, films prepared at pH 9 displayed a relatively high direct band gap energy of 2.28 eV. Liquid petroleum gas (LPG) sensing at 70 °C demonstrated the most substantial gas response of 11.5 % at pH 10 Cu2O films. Cu2O thin films deposited at pH 9 emerged as the most stable and reproducible p-Cu2O LPG sensor, with response and recovery times ranging from 20 to 25 seconds. Remarkable changes in the film structures and morphologies were revealed with pH values above 9 after the gas exposure. The grain shape and distribution of pH 9 remained unchanged, assuring the stability of the sensing platform.

Carrier type and density dependence of impact ionization characteristics in WSe2.

The impact ionization process offers advantages in achieving low-power and high-speed switching in transistors and also provides high internal gain for photodetectors. We investigate the density dependence of both electron- and hole-initiated impact ionization in WSe2. We observe a record-low critical electric field for impact ionization and a large multiplication factor in WSe2 when the impact ionization is initiated by holes, particularly near a charge-neutral point. As the carrier density increases, the impact ionization decreases, which is attributed to the increase in Fermi energy and the weakening of the carrier-carrier interaction by increasing the screening effect. To understand the role of screening effects on the impact ionization, we examine the carrier scatterings of several scattering sources and the rate of change of the channel current with respect to the drain source voltage, ΔIDS/ΔVDS. We obtain the optimal conditions for impact ionization and apply these findings to fabricate avalanche photodetectors (APDs). This study proposes that the performance of low-power transistors or APDs, utilizing impact ionization, can be enhanced under optimized conditions by examining and controlling specific external parameters.

Carrier density dependent Auger recombination in c-plane (In,Ga)N/GaN quantum wells: insights from atomistic calculations

Understanding Auger recombination in (In,Ga)N-based quantum wells is of central importance to unravelling the experimentally observed efficiency ‘droop’ in modern (In,Ga)N light emitting diodes (LEDs). While there have been conflicting results in the literature about the importance of non-radiative Auger recombination processes for the droop phenomenon, it has been discussed that alloy fluctuations strongly enhance the Auger rate. However, these studies were often focused on bulk systems, not quantum wells, which lie at the heart of (In,Ga)N-based LEDs. In this study, we present an atomistic analysis of the carrier density dependence of the Auger recombination coefficients in (In,Ga)N/GaN quantum wells. The model accounts for random alloy fluctuations, the connected carrier localisation effects, and carrier density dependent screening of the built-in polarisation fields. Our studies reveal that at low temperatures and low carrier densities the calculated Auger coefficients are strongly dependent on the alloy microstructure. However, at elevated temperatures and carrier densities, where the localised states are starting to be saturated, the different alloy configurations studied give (very) similar Auger coefficients. We find that over the range of carrier densities investigated, the contribution of the electron-electron–hole related Auger process is of secondary importance compared to the hole-hole-electron process. Overall, for higher temperatures and carrier densities, our calculated total Auger coefficients are in excess of 10−31 cm6 s−1 and may reach 10−30 cm6 s−1, which, based on current understanding in the literature, is sufficient to result in a significant efficiency droop. Thus, our results are indicative of Auger recombination being an important contributor to the efficiency droop in (In,Ga)N-based light emitters even without defect-assisted processes.

Evaluation of radiative and nonradiative recombination lifetimes in c-plane InGaN quantum wells with emission colors ranging from blue to red

In this study, we investigate In composition and the carrier density dependences of radiative and nonradiative recombination lifetimes for a series of c-plane InGaN quantum well (QW) samples with different emission wavelengths (450 nm to 620 nm). The two lifetimes can be separately evaluated using photoluminescence (PL) decay time, obtained by time-resolved PL measurement, combined with the value of internal quantum efficiency (IQE) obtained by simultaneous photoacoustic and PL measurements. It is found that the decrease in IQE with increasing In composition is caused by the reduction in radiative recombination lifetime, not by the enhancement of nonradiative lifetime, which shows little dependence on In composition. In addition, it is found that the carrier density dependence of IQE is also mainly determined by the change in radiative recombination lifetime. These findings will lead to a comprehensive understanding of carrier dynamics in InGaN-QW optical devices.

Light-induced giant Rashba spin-orbit coupling at superconducting KTaO3 (110) heterointerfaces.

The two-dimensional electron system (2DES) at KTaO3 surface or heterointerface with 5d orbitals hosts extraordinary physical properties including the stronger Rashba spin-orbit coupling (RSOC), higher superconducting transition temperature, and potential of topological superconductivity. Herein, we report a huge enhancement of RSOC under light illumination achieved at a superconducting amorphous-Hf0.5 Zr0.5 O2 /KTaO3 (110) heterointerfaces. The superconducting transition is observed with Tc = 0.62 K and temperature dependent upper critical field reveals the interaction between spin-orbit scattering and superconductivity. A strong RSOC with Bso =1.9 T is revealed by weak antilocalization in normal state, which undergoes sevenfold enhancement under light illumination. Furthermore, RSOC strength develops a dome-shaped dependence of carrier density with the maximum of Bso =12.6 T achieved near the Lifshitz transition point nc ∼ 4.1×1013 cm-2 . The highly tunable giant RSOC at KTaO3 (110)-based superconducting interfaces show great potential for spintronics. This article is protected by copyright. All rights reserved.

High-Order Fractal Quantum Oscillations in Graphene/BN Superlattices in the Extreme Doping Limit.

Recent studies of van der Waals (vdW) heterostructures and superlattices have shown intriguing quantum phenomena, but these have been largely explored only in the moderate carrier density regime. Here, we report the probe of high-temperature fractal Brown-Zak (BZ) quantum oscillations through magnetotransport in the extreme doping regimes by applying a newly developed electron beam doping technique. This technique gives access to both ultrahigh electron and hole densities beyond the dielectric breakdown limit in graphene/BN superlattices, enabling the observation of nonmonotonic carrier-density dependence of fractal BZ states and up to fourth-order fractal BZ features despite strong electron-hole asymmetry. Theoretical tight-binding simulations qualitatively reproduce all observed fractal BZ features and attribute the nonmonotonic dependence to the weakening of superlattice effects at high carrier densities.

Impact of Carrier Transport and Capture on VCSEL Dynamics

Using a vertical-cavity surface-emitting laser (VCSEL) equivalent circuit model based on two carrier rate equations to include effects of carrier dynamics, we study the impact of carrier transport and capture on the small- and large-signal modulation response of high-speed VCSELs. The model also accounts for parasitics, current-induced self-heating, and gain compression. A variation of the effective capture time from 1 to 15 ps is found to have a large impact on the small-signal modulation response, with the 3 dB bandwidth decreasing from 40 to 15 GHz and the response transitioning from under-damped to over-damped. This is primarily due to the increasing low frequency parasitic-like roll-off with increasing effective capture time. A significant effect on the optical waveforms produced by the VCSEL under 56 Gbit/s on-off keying (OOK) non-return-to-zero (NRZ) and pulse-amplitude modulation 4 (PAM4) modulation is observed, with a short effective capture time leading to horizontal eye closure caused by timing jitter (TJ) and intersymbol interference (ISI) and a long effective capture time leading to vertical eye closure caused by long rise- and fall-times. However, for high modulation speed, a short effective capture time is needed and the photon lifetime should be set for clear eye opening. We also show the impact of the effective capture time on the output power vs current characteristics and map the dependence of internal temperature, carrier densities, carrier escape and leakage rates, and spontaneous recombination rates on current for different effective capture times.

Analysis of Recombination Dynamics in Solar Cells by Generation-Decay Time Maps

A new method of representation based on the generation-decay time (GDT) maps is presented to estimate the recombination dynamic behavior in solar cells. The method quantitively describes the open circuit voltage transients versus carrier density dependence. An accurate estimation of recombination dynamics through GDT maps enables more easily identifying the dominant recombination processes. Moreover, the method provides deeper insight on the consistent interpretation of recombination order and ideality factor.

Intralayer charge-transfer moiré excitons in van der Waals superlattices.

Moiré patterns of transition metal dichalcogenide heterobilayers have proved to be an ideal platform on which to host unusual correlated electronic phases, emerging magnetism and correlated exciton physics. Whereas the existence of new moiré excitonic states is established1-4 through optical measurements, the microscopic nature of these states is still poorly understood, often relying on empirically fit models. Here, combining large-scale first-principles GW (where G and W denote the one-particle Green's function and the screened Coulomb interaction, respectively) plus Bethe-Salpeter calculations and micro-reflection spectroscopy, we identify the nature of the exciton resonances in WSe2/WS2 moiré superlattices, discovering a rich set of moiré excitons that cannot be captured by prevailing continuum models. Our calculations show moiré excitons with distinct characters, including modulated Wannier excitons and previously unidentified intralayer charge-transfer excitons. Signatures of these distinct excitonic characters are confirmed experimentally by the unique carrier-density and magnetic-field dependences of different moiré exciton resonances. Our study highlights the highly non-trivial exciton states that can emerge in transition metal dichalcogenide moiré superlattices, and suggests new ways of tuning many-body physics in moiré systems by engineering excited-states with specific spatial characters.

Contribution to Excitonic Linewidth from Free Carrier–Exciton Scattering in Layered Materials: The Example of hBN

Scattering of excitons by free carriers is a phenomenon, which is especially important when considering moderately to heavily doped semiconductors in low-temperature experiments, where the interaction of excitons with acoustic and optical phonons is reduced. In this paper, we consider the scattering of excitons by free carriers in monolayer hexagonal boron nitride encapsulated by a dielectric medium. We describe the excitonic states by variational wave functions, modeling the electrostatic interaction via the Rytova–Keldysh potential. Making the distinction between elastic and inelastic scattering, the relevance of each transition between excitonic states is also considered. Finally, we discuss the contribution of free carrier scattering to the excitonic linewidth, analyzing both its temperature and carrier density dependence.

Temperature-linear resistivity in twisted double bilayer graphene

We report an experimental study of carrier density ($n$), displacement field, and twist angle (\ensuremath{\theta}) dependence of temperature ($T$)-linear resistivity in twisted double-bilayer graphene (TDBG). For a large twist angle ($\ensuremath{\theta}>1.{5}^{\ensuremath{\circ}}$) where correlated insulating states are absent, we observe a $T$-linear resistivity (with a slope on the order of $\ensuremath{\sim}10\phantom{\rule{0.28em}{0ex}}\mathrm{\ensuremath{\Omega}}/\mathrm{K}$) over a wide range of carrier densities, and its slope decreases with increasing $n$, which is in agreement with the acoustic phonon scattering model semiquantitatively. The slope of $T$-linear resistivity is nonmonotonically dependent on the displacement field with a single peak structure. For a device with $\ensuremath{\theta}\ensuremath{\sim}1.{23}^{\ensuremath{\circ}}$ at which correlated states emerge, the slope of $T$-linear resistivity is found to be a maximum ($\ensuremath{\sim}100\phantom{\rule{0.28em}{0ex}}\mathrm{\ensuremath{\Omega}}/\mathrm{K}$) at the boundary of the halo structure where phase transition occurs, with signatures of continuous phase transition, Planckian dissipation, and diverging effective mass. These observations are in line with quantum critical behaviors, which might be due to symmetry-breaking instability at the critical points. Our results shed new light on correlated physics in TDBG and other twisted moir\'e systems.

Carrier Density Dependence of the Mobility in Disordered Organic Semiconductors with Gaussian Disorder

In this paper, the charge transport properties and energetic disorder are studied for pristine Si-PCPDTBT, mono-PCBM, bis-PCBM, and their blends of Si-PCPDTBT:mono-PCBM, Si-PCPDTBT:bis-PCBM. We show that the current density versus voltage curves of electron-only devices for mono-PCBM, bis-PCBM, Si-PCPDTBT:mono-PCBM, Si-PCPDTBT:bis-PCBM, and hole-only devices for Si-PCPDTBT, Si-PCPDTBT:mono-PCBM, Si-PCPDTBT:bis-PCBM cannot be accurately described using a conventional mobility model within which the mobility depends only on the electric field, but that a fully consistent description of all curves can be obtained using the improved extended Gaussian disorder model (IEGDM). Within the IEGDM, the presence of energetic disorder is described by a Gaussian density of states, and the mobility depends on the electric field and carrier density. Furthermore, we found that the mobility increases as the width of the Gaussian density of states decreases for both electron-only and hole-only devices, and the mobility is closely related to the energetic disorder. We view this as an indication that the energetic disorder appears to govern the charge transport in disordered organic semiconductors.

Excitation density dependent carrier dynamics in a monolayer MoS2: Exciton dissociation, formation and bottlenecking

Research on Molybdenum Disulfide (MoS2) monolayer at high excitation densities is critical due to its rising demand in optoelectronic applications such as lasers, compact optical parametric amplifiers, and high power detectors. We show that at high excitation densities, measurements on a single monolayer MoS2 reveal a transient response rich in physical processes. Despite the fact that carriers are excited straight to the A-excitonic state, the time and fluence dependence of the observed optical response indicates exciton dissociation followed by hot-carrier generation. In the picosecond time scale, these hot-carriers form excitons, which have a longer lifetime at higher excitation densities, implying a bottlenecking process. For the use of MoS2 at high-excitation densities, the fluence dependency of the relaxation mechanisms detailed here is essential.

Photogenerated carrier density dependence of ultrafast carrier dynamics in intrinsic 6H-SiC measured by optical-pump terahertz-probe spectroscopy

Ultrafast carrier dynamics and terahertz conductivity of intrinsic 6H-SiC are investigated by optical-pump terahertz-probe spectroscopy at room temperature. The amplitude of photoinduced absorption increases gradually and followed by a biexponential recovery with photogenerated carrier densities from 7.43 × 1017 to 2.81 × 1018 cm −3 at 400 nm. With the increase of photogenerated carrier density, the fast decay time decreases may be due to higher order recombination (i.e., Auger recombination), and the slow one increases is related to the band-filling effect. The evolution of complex conductivity with photogenerated carrier density at the delay time of about 50 ps can be better fitted with Drude–Smith​ model than Drude–Lorenz model. The Drude–Smith scattering time τDS and Smith​ parameter c1 increase with photogenerated carrier density implies carriers backscattering is dominant. Our analysis provides important basic data for research and improvement ultrafast semiconductor devices of intrinsic 6H-SiC.

Nanoscale-Resolved Surface-to-Bulk Electron Transport in CsPbBr3 Perovskite.

Describing the nanoscale charge carrier transport at surfaces and interfaces is fundamental for designing high-performance optoelectronic devices. To achieve this, we employ time- and angle-resolved photoelectron spectroscopy with ultraviolet pump and extreme ultraviolet probe pulses. The resulting high surface sensitivity reveals an ultrafast carrier population decay associated with surface-to-bulk transport, which was tracked with a sub-nanometer spatial resolution normal to the surface, and on a femtosecond time scale, in the case of the inorganic CsPbBr3 lead halide perovskite. The decay time exhibits a pronounced carrier density dependence, which is attributed via modeling to enhanced diffusive transport and concurrent recombination. The transport is found to approach an ordinary diffusive regime, limited by electron–hole scattering, at the highest excitation fluences. This approach constitutes an important milestone in our capability to probe hot-carrier transport at solid interfaces with sub-nanometer resolution in a theoretically and experimentally challenging, yet technologically relevant, high-carrier-density regime.

Complex energies of the coherent longitudinal optical phonon–plasmon coupled mode according to dynamic mode decomposition analysis

In a dissipative quantum system, we report the dynamic mode decomposition (DMD) analysis of damped oscillation signals. We used a reflection-type pump-probe method to observe time-domain signals, including the coupled modes of long-lived longitudinal optical phonons and quickly damped plasmons (LOPC) at various pump powers. The Fourier transformed spectra of the observed damped oscillation signals show broad and asymmetric modes, making it difficult to evaluate their frequencies and damping rates. We then used DMD to analyze the damped oscillation signals by precisely determining their frequencies and damping rates. We successfully identified the LOPC modes. The obtained frequencies and damping rates were shown to depend on the pump power, which implies photoexcited carrier density. We compared the pump-power dependence of the frequencies and damping rates of the LOPC modes with the carrier density dependence of the complex eigen-energies of the coupled modes by using the non-Hermitian phenomenological effective Hamiltonian. Good agreement was obtained between the observed and calculated dependences, demonstrating that DMD is an effective alternative to Fourier analysis which often fails to estimate effective damping rates.

Energy diagram and parameters regarding localized states in InGaN/GaN nanocolumns

The parameters related to the localized states in green-emitting indium gallium nitride (InGaN) have been evaluated by considering the energy diagrams derived by five different methods: (1) the exponential tail of the low-energy side of photoluminescence (PL) spectra, (2) the photon energy dependence of PL decay time, (3) excitation energy dependence of the PL peak energy, (4) the PL excitation spectrum, and (5) the temperature dependence of PL peak energy. The results indicate that the energy diagram of InGaN is divided into four regions: deep localized states, migration region, transition region, and extended states. It is suggested that wider localized states and a narrower transition region are preferable in order to achieve higher PL efficiency. In addition, the dependence of carrier density on PL properties supports the fact of photo-generated carriers forming localized excitons in green-emitting InGaN, although the carriers do not form localized excitons in orange-emitting InGaN and instead exist as localized electrons and holes.

Direct Visualization of Native Defects in Graphite and Their Effect on the Electronic Properties of Bernal-Stacked Bilayer Graphene.

Graphite crystals used to prepare graphene-based heterostructures are generally assumed to be defect free. We report here scanning tunneling microscopy results that show graphite commonly used to prepare graphene devices can contain a significant amount of native defects. Extensive scanning of the surface allows us to determine the concentration of native defects to be 6.6 × 108 cm-2. We further study the effects of these native defects on the electronic properties of Bernal-stacked bilayer graphene. We observe gate-dependent intravalley scattering and successfully compare our experimental results to T-matrix-based calculations, revealing a clear carrier density dependence in the distribution of the scattering vectors. We also present a technique for evaluating the spatial distribution of short-scale scattering. Finally, we present a theoretical analysis based on the Boltzmann transport equation that predicts that the dilute native defects identified in our study are an important extrinsic source of scattering, ultimately setting the charge carrier mobility at low temperatures.

Possible dimensionality transition behavior in localized plasmon resonances of confinement-controlled graphene devices

We investigated the dimensionality transition behavior of graphene localized plasmon resonances in confinement-controlled graphene devices. We first demonstrated a possibility of dimensionality transition, based on the devices carrier-density dependence, from a two-dimensional plasmon resonance to a one-dimensional plasmon one. We fabricated optical transparent devices and electrical transport devices on the same optical transparent wafer. These devices allow detailed control and analysis between carrier density and plasmon resonance peak positions. The carrier density from (two-dimensional) to constant (one-dimensional) is consistent with the theoretical predictions based on the Dirac Fermion carriers in linear-band structure materials.

Angular dependence of magnetoresistance and planar Hall effect in semimetals in strong magnetic fields

The semiclassical transport theory is especially powerful for investigating galvanomagnetic effects. Generally, the semiclassical theory is applicable only in weak fields because it does not consider Landau quantization. Herein, we extend the conventional semiclassical theory by considering Landau quantization through the field dependence of carrier density in semimetals. The extended semiclassical theory is applicable even in strong fields, where Landau quantization is noticeable. Using this new approach, we explain the qualitative change in the angular dependence of transverse magnetoresistance (TMR), anisotropic magnetoresistance (AMR), and planar Hall effect (PHE) in bismuth with an increase in the magnetic field. This unveils the puzzle of nontrivial field-induced changes in TMR, AMR, and PHE observed recently in semimetal bismuth.