Low Carrier Density Research Articles

Density-Dependent Carrier Recombination in MoS2 Quantum Dots and Its Implications for Luminescence Sensing of Ammonium Hydroxide

Zero-dimensional molybdenum disulfide (MoS2) quantum dots (QDs) have attracted remarkable interest due to their peculiar properties such as the quantum-confinement effect and high surface area. Exploring recombination dynamics in MoS2 QDs is not only expected to gain a deeper insight into their fundamental physics, it is also important for potential applications in optoelectronics and energy-conversion technology. This study synthesized p-type MoS2 QDs doped with diethylenetriamine (DETA) using pulsed laser ablation method. A hole concentration as high as 2.08 × 1012 cm–2 has been demonstrated by gate-dependent conductance measurements. A 110-fold enhancement of photoluminescence in the p-type MoS2 QDs has been found after the introduction of DETA, and the dependence of the radiative and nonradiative recombination of MoS2 QDs on carrier densities were studied. As the carrier density was increased, a decrease of the radiative lifetime was found, which is similar to the behavior of the radiative lifetime in monolayer MoS2. The Shockley-Read-Hall (SRH) and Auger recombination dominates the nonradiative recombination at low and high carrier densities, respectively. The SRH lifetime of MoS2 QDs increases with the increased carrier density, suggesting that the recombination mechanism at the low carrier density is dominated by the SRH recombination. This study found that as the carrier densities exceeded 0.53 × 1012 cm–2, the Auger recombination was responsible for the reduction of PL. Furthermore, MoS2 QDs was used as a fluorescent sensor for the detection of ammonium hydroxide (NH4OH). The PL intensity of MoS2 QDs demonstrates a gradual decrease with increasing NH4OH concentration. By investigating the time-resolved PL (TRPL), the mechanism that leads to the decrease of PL in MoS2 QDs is addressed. This investigation is expected to demonstrate a promising development of an effective and low-cost MoS2 QDs-based fluorescent sensor with superior sensitivity for the rapid detection of ammonia in aqueous media.

Two-Dimensional-Dirac Surface States and Bulk Gap Probed via Quantum Capacitance in a Three-Dimensional Topological Insulator.

BiSbTeSe2 is a 3D topological insulator (3D-TI) with Dirac type surface states and low bulk carrier density, as donors and acceptors compensate each other. Dominating low-temperature surface transport in this material is heralded by Shubnikov-de Haas oscillations and the quantum Hall effect. Here, we experimentally probe and model the electronic density of states (DOS) in thin layers of BiSbTeSe2 by capacitance experiments both without and in quantizing magnetic fields. By probing the lowest Landau levels, we show that a large fraction of the electrons filled via field effect into the system ends up in (localized) bulk states and appears as a background DOS. The surprisingly strong temperature dependence of such background DOS can be traced back to Coulomb interactions. Our results point at the coexistence and intimate coupling of Dirac surface states with a bulk many-body phase (a Coulomb glass) in 3D-TIs.

Large-Scale Thin CsPbBr3 Single-Crystal Film Grown on Sapphire via Chemical Vapor Deposition: Toward Laser Array Application.

Single-crystal perovskites with excellent photophysical properties are considered to be ideal materials for optoelectronic devices, such as lasers, light-emitting diodes and photodetectors. However, the growth of large-scale perovskite single-crystal films (SCFs) with high optical gain by vapor-phase epitaxy remains challenging. Herein, we demonstrated a facile method to fabricate large-scale thin CsPbBr3 SCFs (∼300 nm) on the c-plane sapphire substrate. High temperature is found to be the key parameter to control low reactant concentration and sufficient surface diffusion length for the growth of continuous CsPbBr3 SCFs. Through the comprehensive study of the carrier dynamics, we clarify that the trapped-related exciton recombination has the main effect under low carrier density, while the recombination of excitons and free carriers coexist until free carriers plays the dominate role with increasing carrier density. Furthermore, an extremely low-threshold (∼8 μJ cm-2) amplified spontaneous emission was achieved at room temperature due to the high optical gain up to 1255 cm-1 at a pump power of 20 times threshold (∼20 Pth). A microdisk array was prepared using a focused ion beam etching method, and a single-mode laser was achieved on a 3 μm diameter disk with the threshold of 1.6 μJ cm-2. Our experimental results not only present a versatile method to fabricate large-scale SCFs of CsPbBr3 but also supply an arena to boost the optoelectronic applications of CsPbBr3 with high performance.

PH-sensitive GaInAsP photonic crystal fractal band-edge laser.

The light emission characteristics of GaInAsP semiconductors in aqueous solutions are modified by the surface charge, depending on the balance between radiative and nonradiative recombination. This Letter demonstrates the application of a GaInAsP photonic crystal band-edge laser in sensing a solution pH, which reflects the surface charge. The sensitivity is enhanced by a hybrid of fractal honeycomb and close-packed structures with a large surface-to-volume ratio and a high quality factor (Q), which allows a low threshold carrier density. We experimentally obtained a maximum pH sensitivity of 1.9 dB/pH near the threshold and a signal-to-noise ratio of 52/pH (pH resolution of 0.019) at a pump power 2.4 times the threshold where the intensity noise diminished.

Gate-Modulated Quantum Interference Oscillations in Sb-Doped Bi2Se3 Topological Insulator Nanoribbon

Topological insulator nanoribbons (TI NRs) provide a useful platform to explore the phase-coherent quantum electronic transport of topological surface states, which is crucial for the development of topological quantum devices. When applied with an axial magnetic field, the TI NR exhibits magnetoconductance (MC) oscillations with a flux period of h/e, i.e., Aharonov-Bohm (AB) oscillations, and h/2e, i.e., Altshuler-Aronov-Spivak (AAS) oscillations. Herein, we present an extensive study of the AB and AAS oscillations in Sb doped Bi$_2$Se$_3$ TI NR as a function of the gate voltage, revealing phase-alternating topological AB oscillations. Moreover, the ensemble-averaged fast Fourier transform analysis on the Vg dependent MC curves indicates the suppression of the quantum interference oscillation amplitudes near the Dirac point, which is attributed to the suppression of the phase coherence length within the low carrier density region. The weak antilocalization analysis on the perpendicular MC curves confirms the idea of the suppressed coherence length near the Dirac point in the TI NR.

Artificial graphene: Unconventional superconductivity in a honeycomb superlattice

Artificial lattices have served as a platform to study the physics of unconventional superconductivity. We study semiconductor artificial graphene -- a honeycomb superlattice imposed on a semiconductor heterostructure -- which hosts the Dirac physics of graphene but with a tunable periodic potential strength and lattice spacing, allowing control of the strength of the electron-electron interactions. We demonstrate a new mechanism for superconductivity due to repulsive interactions which requires a strong lattice potential and a minimum doping away from the Dirac points. The mechanism relies on the Berry phase of the emergent Dirac fermions, which causes oppositely moving electron pairs near the Dirac points to interfere destructively, reducing the Coulomb repulsion and thereby giving rise to an effective attraction. The attractive component of the interaction is enhanced by a novel antiscreening effect which, in turn, increases with doping; as a result there is a minimum doping beyond which superconducting order generically ensues. The dominant superconducting state exhibits a spatially modulated gap with chiral $p$-wave symmetry. Microscopic calculations suggest that the possible critical temperatures are large relative to the low carrier densities, for a range of experimentally realistic parameters.

High-performance graphene/InSb heterojunction photodetectors for high-resolution mid-infrared image sensors

Graphene/semiconductor heterojunction photodetectors have the potential to outperform conventional infrared (IR) sensors. A high-performance graphene/indium antimonide (InSb) heterojunction photodetector for high-resolution mid-IR image sensors was developed using an undoped InSb substrate with low carrier density. The width of the depletion layer at the graphene/InSb interface was increased because of the low carrier density of undoped InSb, which resulted in a low dark current for the proposed device. In addition, the low carrier density resulted in an increased capacitance change due to photocarrier generation for the depletion layer upon light irradiation. As a result, the carrier density modulation effect of graphene was also amplified owing to the photogating effect. Consequently, low dark currents on the order of nanoamperes and high responsivities of over 2 A/W were achieved over a wide voltage range of −0.05 to −0.5 V using our proposed photodetector. The best performance achieved for the developed photodetector corresponded to a specific detectivity (D*) of 2.28 × 1010 cm Hz1/2/W and a noise-equivalent-power of 0.13 pW/Hz1/2, which makes these detectors suitable for use in mid-IR image sensors, realizing a low dark current of just −7.5 nA over the wide wavelength range of 3–5 μm. These results demonstrate that an IR image sensor with detection performance better than those developed using existing graphene-based photodetectors can be obtained by adopting a simple graphene/InSb heterojunction with undoped InSb, owing to the photogating effect.

Comparison of growth interruption and temperature variation impact on emission efficiency in blue InGaN/GaN MQWs

Pulsed growth, i.e. the MOCVD growth method, when the metal precursors are delivered with interruptions, allows increasing the internal quantum efficiency (IQE) of InGaN multiple quantum well (MQW) structures. On the other hand, similar results are achieved by increasing the growth temperature. In this work, we analyze the physical mechanisms behind the IQE improvement in differently grown QWs at low and high carrier densities. At low excitations, the In0.1Ga0.9N/GaN MQW structures grown at 800 and 815 °C exhibited the PL efficiency of 4% and 6%, respectively. At high excitations, the benefit of higher growth temperature disappeared: peak IQE at the droop onset was ~34% in all structures. Moreover, the structure grown at 815 °C had lower than intended indium content (9% instead of 10%) and QW width (3.5 nm instead of 4.0 nm). The pulse-grown structures, while very sensitive to interruption duration, provided peak IQE up to 40% without decline in structural quality. From the provided photoluminescence, carrier lifetime and diffusion coefficient data we speculate that higher growth temperature leads to lower point defect density, which increases IQE at low excitations, but becomes saturated at high excitations. Meanwhile, the pulsed growth mainly affects the localizing potential, which governs carrier recombination pathways.

Spin transport in multilayer graphene away from the charge neutrality point

Graphene is considered as a promising material in spintronics due to its long spin relaxation time and long spin relaxation length. However, its spin transport properties have been studied at low carrier density only, beyond which much is still unknown. In this study, we explore the spin transport and spin precession properties in multilayer graphene at high carrier density using ionic liquid gating. We find that the spin relaxation time is directly proportional to the momentum relaxation time, indicating that the Elliott-Yafet mechanism still dominates the spin relaxation in multilayer graphene away from the charge neutrality point.

High magnetic field spin-valley-split Shubnikov–de Haas oscillations in a WSe2 monolayer

We study Shubnikov-de Haas oscillations in a p-type WSe$_2$ monolayer under very high magnetic field. The oscillation pattern is complex due to a large spin and valley splitting, in the non-fully-resolved Landau level regime. Our experimental data can be reproduced with a model in which the main parameter is the ratio between the Zeeman energy and the cyclotron energy. The model takes into account the Landau levels from both valleys with the same Gaussian broadening, which allows to predict the relative amplitude of the resistance oscillation originating from each valley. The Zeeman energy is found to be several times larger than the cyclotron energy. It translates into a large and increasing effective Land\'e factor as the hole density decreases, in the continuity of the values reported in the literature at lower carrier density.

Amplified Methanol Sensitivity in Reduced Graphene Oxide FET Using Appropriate Gate Electrostatic

The current study concerns a technique to amplify the gas sensitivity of reduced graphene oxide (rGO)-field-effect transistor (FET) sensor for the detection of very low concentration gaseous species. rGO channel exhibits tunable carrier density with ambipolar transport in FET structure by the change of electrostatic potential through the gate terminal. At low carrier density of channel, the exchange of carriers due to surface adsorption and desorption is significant that eventually enhances the sensitivity of the rGO-FET sensor at a typical gate voltage ( ${V}_{GS}$ ). The back-gated rGO-FET sensor was fabricated on ${p}$ -Si substrate with 90-nm-thick SiO2 layer. The defects, disorders, and morphology of single-layer (major) rGO were characterized with the spectroscopic and microscopic study. The modulation of ambient sensitivity of rGO film was tested in the exposure of different ambient like dry air, argon, oxygen, and a test vapor methanol. 1.95% methanol (at 150 ppm) sensitivity at ${V}_{GS}= 0$ V was amplified up to 127.93% at ${V}_{GS}= 5$ V and 82.38% at ${V}_{GS}= 13$ V where the temperature and drain-to-source voltage ( ${V}_{DS}$ ) were 100 °C and 1.4 mV, respectively.

Ultrahigh Doping of Graphene Using Flame-Deposited MoO3

The expected high performance of graphene-based electronics is often hindered by lack of adequate doping, which causes low carrier density and large sheet resistance. Many reported graphene doping schemes also suffer from instability or incompatibility with existing semiconductor processing. Here we report ultrahigh and stable p-type doping up to ~7x10^13 1/cm^2 (~2x10^21 1/cm^3}) of monolayer graphene grown by chemical vapor deposition. This is achieved by direct polycrystalline MoO3 growth on graphene using a rapid flame synthesis technique. With this approach, the metal-graphene contact resistance for holes is reduced to ~200 Ohm-um. We also demonstrate that flame-deposited MoO3 provides over 5x higher doping of graphene, as well as superior thermal and long-term stability, compared to electron-beam deposited MoO3.

Extremely large magnetoresistance, anisotropic Hall effect, and Fermi surface topology in single-crystalline WSi2

We report on the observation of a nonsaturating, extremely large magnetoresistance (XMR) and the Fermi surface topology of a high quality $\mathrm{W}{\mathrm{Si}}_{2}$ single crystal grown by the Czochralski method. The magnetoresistance at $T=2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ reaches a value $\ensuremath{\approx}{10}^{5}%$ in 14 T magnetic field with no sign of saturation. The Hall resistivity data of $\mathrm{W}{\mathrm{Si}}_{2}$ was found to be highly anisotropic. The analysis of magnetoconductivity data of $\mathrm{W}{\mathrm{Si}}_{2}$ revealed a near compensation of charge carrier with relatively low carrier density as compared to that of a normal metal. The observed anisotropic Hall resistivity in $\mathrm{W}{\mathrm{Si}}_{2}$ is due to the presence of multiple bands and Fermi pockets responsible for the transport phenomena in it. The extremely large carrier mobility and near compensation of charge carriers are responsible for the nonsaturating XMR behavior in $\mathrm{W}{\mathrm{Si}}_{2}$ crystal. The band structure calculation and de Haas-van Alphen effect measurement depict a cylindrical Fermi surface from which the associated quantum parameters have been obtained. The magnetotransport data of $\mathrm{W}{\mathrm{Si}}_{2}$ along both the crystallographic directions follows the universal temperature-field triangular phase diagram as observed in other materials exhibiting XMR behavior.

Superconductivity mediated by polar modes in ferroelectric metals

The occurrence of superconductivity in doped SrTiO3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We report experimental results showing the pressure dependence of the superconducting transition temperature, Tc, near to optimal doping that sheds light on the nature of this interaction. We find that Tc increases dramatically when the energy gap of the ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached in a way reminiscent to behaviour observed in magnetic counterparts. However, in contrast to the latter, the coupling of the carriers to the critical modes in ferroelectrics is predicted to be small. We present a quantitative model involving the dynamical screening of the Coulomb interaction and show that an enhancement of Tc near to a ferroelectric quantum critical point can arise due to the virtual exchange of longitudinal hybrid-polar-modes, even in the absence of a strong coupling to the transverse critical modes.

Highly tunable topological system based on PbTe-SnTe binary alloy

Despite lots of research activities on topological materials, a highly tunable system is yet to be realized as a material platform for exploration of versatile magneto-/opto-/thermoelectronic properties. The authors followed an important scheme for topological phase transitions proposed by S. Murakami et al. and could find a highly tunable topological system based on PbTe-SnTe alloy doped with In. By varying parameters, Pb/Sn ratio and In doping, they can tune the system from trivial to topological-crystalline insulators across a finite range of materials, where the system shows polar distortion and topological semi-metal behaviors with low-carrier density, high mobility and anomalous (in-plane as well as out-of-plane field) Hall effects.

Carrier Density Modulation in PbSe Quantum Dot Films via In-Solution Ligand Exchange

In-solution ligand exchange of PbSe QDs is used to examine the effect of capping ligand on the carrier density of PbSe QD films. Results show that carrier density can be modulated by a factor of 5 by choice of ligand without any additional post deposition treatments. Proper fabrication and measurement conditions for calculating carrier densities from C-V measurements using a sandwich structure on P-doped Si/SiO2/Al2O3/QD/Au structure capacitance devices are outlined. Combining carrier density results with field-effect-transistor measurements, promising ligands which display lower carrier densities without having a significant drop off in carrier mobility are identified as candidates for photodetection purposes.

Complete removal of phenolic contaminants from bismuth-modified TiO2 single-crystal photocatalysts

Exploring low-cost and highly active photocatalysts is very urgent to accomplish complete removal of phenolic contaminants and overcome the limitations of the existing photocatalysts. In this study, we designed and synthesized noble metal-free TiO2 photocatalysts by introducing bismuth nanoparticles as modifiers of a TiO2 single crystal (Bi-SCTiO2). The Bi-SCTiO2 can make full use of the synergistic effect of a small band overlap and low charge carrier density (Bi) with a high conductivity (single crystal), significantly boosting the separation and migration of the photogenerated charge pairs. Therefore, the Bi-SCTiO2 photocatalyst exhibits a significantly enhanced degradation rate (12 times faster) of 4-nitrophenol than a TiO2 single crystal under simulated sunlight irradiation. Notably, the complete removal of phenolic contaminants is achieved in various water matrices, which not only successfully overcomes the incomplete degradation in many reported photocatalytic systems, but also manifests a significant practical potential for sewage disposal. Therefore, this work presents a new insight in designing and constructing noble metal-free decorated semiconductor single-crystal photocatalysts with excellent activity and cyclability.

A core analysis of oxygen role in physical properties of alkaline ferrite AFe2O4 (with A = Mg, Ca, Sr and Ba)

It is predicted that the oxygen behaviour in A-site may be different from that of B-site. The magnetic behaviour of oxygen in alkaline ferrites having the composition AFe2O4 (A = Mg, Ca and Sr) has been discussed using electronic structure calculations. The calculations have shown that the physical properties of the studied compounds are exclusively sensitive to the magnetic moment carried by the 2p-electrons of the oxygen anions. The compounds, with A = Ca, Sr and Ba, have a closely related electronic structures different from MgFe2O4. Fe with oxygen is more active in Ca, Sr and Ba than in the Mg compound this means that Oxygen atom is inactive in alkaline ferrites when carrying no moment. The calculations have shown that the alkaline impurities, with relatively large covalent radius, can form an excitonic state near the band edge of the ferrite. The electron pockets formed at the high symmetry point N in up and down channel for MgFe2O4, but these pockets not appear for the other studied compounds. MgFe2O4 has shown a lower current carrier density than the others compounds. The spin relaxation is expected to be suppressed as moving from Mg to Ba. It is speculated that as u increases, the ferrite becomes more efficient for the electronics that depends mainly on the electron spin.

Effect of Ge doping on the electrical properties of amorphous Zn–Sn–O thin films

We report the effect of germanium doping on the active layer of amorphous Zinc–Tin-Oxide (a-ZTO) thin film transistor (TFT). Amorphous thin film samples were prepared by RF magnetron sputtering using single targets composed of Zn2Ge0.05Sn0.95O4 and Zn2SnO4 with variable oxygen contents in the sputtering gases. In comparison with undoped, Ge-doped a-ZTO films exhibited five order of magnitude lower carrier density with a significantly higher Hall-mobility, which might be due to suppressed oxygen vacancies in the a-ZTO lattice since the Ge substituent for the Sn site has relatively higher oxygen affinity. Thus, the bulk and interface trap densities of Ge-doped a-ZTO film were decreased one order of magnitude to 7.047 × 1018 eV−1cm−3 and 3.52 × 1011 eV−1cm−2, respectively. A bottom-gate TFT with the Ge-doped a-ZTO active layer showed considerably improved performance with a reduced SS, positively shifted Vth, and two orders of magnitude increased Ion/Ioff ratio, attributable to the doped Ge ions.

Slow carrier cooling in methylammonium lead bromide quantum dots: Evidence of hot phonon bottleneck

Photoinduced charge carrier dynamics studies in lead halide perovskites helps in explaining key processes like carrier trapping, diffusion and multiexciton dynamics etc. which are of importance to understand the fate of excitons. In the present study, we investigate the dynamics in methylammonium lead bromide perovskite nanocrystals under low and high pump intensity to study hot carrier cooling mechanisms. The single exciton studies conducted under low pump fluences (<N> = 0.3) revealed slow carrier decay with a significant fraction of >1 ns decay times as expected due to long radiative carrier recombination times. This also indicated that fast carrier trapping does not occur as evidenced by absence of any fast decay components. The hot carrier (HC) cooling mechanisms were verified by studying carrier density dependence of carrier temperature decays as extracted from the higher energy part of exciton bleach. These studies revealed a very fast cooling of ∼150 fs under low carrier density 1.5 × 1017/ cm3. However additional experiments under higher carrier densities of ∼1018-1019/cm3 showed a very slow carrier temperature decay with decay time ∼100 ps for the highest intensity investigated. The analysis of hot carrier lifetimes revealed Auger heating and acoustic phonon up-conversion as reason for hot carrier relaxations at higher intensities. Additionally, we carried out experiments under hole quencher, we found lower initial carrier temperatures and faster decay indicating highly efficient hole transfer to the quencher in a time scale faster than cooling of carrier at higher intensities. These results hint at possibility of molecular design for enabling efficient hot carrier extraction from these Quantum Dots (QDs).