Local Carrier Density Research Articles

Improved high voltage operation of amorphous In–Ga–Zn–O thin film transistor by carrier density enhancement

We report on improved high voltage operation of amorphous-In–Ga–Zn–O (a-IGZO) thin film transistors (TFTs) by increasing carrier density and distributing the high bias field over the length of the device which utilizes an off-set drain structure. By decreasing the O2 partial pressure during sputter deposition of IGZO, the channel carrier density of the high voltage a-IGZO TFT (HiVIT) was increased to ∼1018 cm−3. Which reduced channel resistance and therefore the voltage drop in the ungated offset region during the on-state. To further decrease the electric field in the offset region, we applied Ta capping and subsequent oxidation to locally increase the oxygen vacancy levels in the offset region thereby increasing local carrier density. The reduction of the drain field in the offset region from 1.90 V μm−1 to 1.46 V μm−1 at 200 V drain voltage, significantly improved the operational stability of the device by reducing high field degradation. At an extreme drain voltage of 500 V, the device showed an off-state current of ∼10−11 A and on-state current of ∼1.59 mA demonstrating that with further enhancements the HiVIT may be applicable to thin-film form, low leakage, high voltage control applications.

Electrical resistivity mapping of potassium-doped few-layer CVD graphene by EBAC measurements

The electron beam absorbed current (EBAC) method identifies the open and/or short points in various semiconductor devices, which can be applied to characterize the current path and local resistance in graphene. In this study, potassium (K)-doped few-layer graphene with inhomogeneous K atoms seemed to be one of the appropriate materials to characterize the current path by EBAC. Nonuniform contrast in the EBAC image due to inhomogeneous local resistances is observed, which is explained by the variation of the Fermi level in the graphene channel from the G-band peak shifts from Raman spectroscopy. The changes in the contrast of the EBAC images are obtained by applying a gate voltage. These changes are attributed to the modulation of the local carrier densities by applying the gate voltage. For comparison, uniform contrast in EBAC images and uniform G-band peak positions of undoped few-layer graphene field effect transistors are confirmed. The obtained results suggest that homogeneous Fermi level leads to a uniform current path. EBAC enables us to evaluate the uniformity of local resistance and current through a pass in the graphene channel, which can be applied to other two-dimensional materials, such as transition metal dichalcogenides, graphene oxide, and hexagonal boron nitride.

Linearity improvement in graded channel AlGaN/GaN HEMTs for high-speed applications

AlGaN HEMTs are popular devices for high-frequency applications. At higher frequencies, nonlinearity effects are major concerns and need serious attention. In this work, we have attempted to improve the linearity performance of graded channel Al x Ga1−x N/GaN HEMTs in comparison with abrupt channel GaN HEMTs through Technology Computer Aided (TCAD) simulation. In graded channel HEMTs, linearly grading of Al composition in the AlGaN layer, reduces the local carrier densities and provides improvement in the carrier saturation velocity. This provides an avenue to improve the overall linearity performance. A complete set of figures of merits (FOMs) such as and for both devices are presented. The simulation results have been calibrated with reported experimental results available in the literature. The graded devices are observed to have better transconductance (g m ) and drain current for gate voltages greater than −2 V. The drop in g m is reduced by nearly 50% at the higher gate-to-source voltages. Moreover, the RF figure of merits such as the current gain cutoff frequency (f T) and a maximum frequency of oscillation (f max) have been calculated for both devices. Due to the graded channel f T increases by 20% and f max becomes twice which makes it a better choice for high-voltage and high-power applications. The impact of interface trap and leakage current performance on the graded channel device are also reported. The cutoff frequency reduces by 25% with an increase in trap charge concentration from 9e17 cm−2 to 1e19 cm−2. Also, it has been found that the graded channel device shows better linearity and lower intermodulation distortion in comparison to the abrupt device which has been calculated from the linearity parameters. The high gate leakage current in the case of the graded channel device can be reduced by adding a thin GaN cap layer on the AlGaN channel which can be a future prospect of improving the performance of the graded GaN device.

Experimental and theoretical studies into longitudinal spatial hole burning as a power limit in high-power diode lasers at 975 nm

Spatial-hole-burning as a limit to the continuous-wave (CW) output power of GaAs-based diode lasers is experimentally studied. For 90 μm stripe lasers with 6 mm resonator length and 0.8% front facet reflectivity, spontaneous emission (SE) intensity data show that the carrier density in the device center rises rapidly at the rear facet with bias and falls at the front, consistent with simulation. At the front, the carrier density at the edge of the laser stripe also rises rapidly with bias (lateral carrier accumulation, LCA), consistent with previous observations of increased local current flow. Devices with 20% front facet reflectivity for a flat longitudinal optical field profile show smaller variation in the local carrier density. Weak variation is seen in the carrier density outside the stripe; hence, current spreading is not a power limit. SE wavelength data show higher temperatures at the front with a twofold higher increase in temperature for 0.8% than for 20% front facet. The increased front temperature likely triggers lateral spatial-hole-burning and LCA in this region, limiting power. Finally, pulsed threshold current is more strongly temperature dependent for devices with 0.8% than 20% front facets, attributed to the higher rear facet carrier density. The temperature dependence of slope in pulsed is comparable for both devices at low bias but is more rapid for 0.8% at 20 A, likely due to non-clamping at the back. The temperature dependence of slope for CW is strong with 0.8% facets, likely due to the high temperature and LCA at the front but reduced for 20% facets.

Real-space observation of a two-dimensional electron gas at semiconductor heterointerfaces.

Mobile charge carriers are essential components in high-performance, nano-engineered semiconductor devices. Employing charge carriers confined to heterointerfaces, the so-called two-dimensional electron gas, is essential for improving device performance. The real-space visualization of a two-dimensional electron gas at the nanometre scale is desirable. However, it is challenging to accomplish by means of electron microscopy due to an unavoidable strong diffraction contrast formation at the heterointerfaces. We performed direct, nanoscale electric field imaging across a GaN-based semiconductor heterointerface using differential phase contrast scanning transmission electron microscopy by suppressing diffraction contrasts. For both nearly the lattice-matched GaN/Al0.81In0.19N interface and pseudomorphic GaN/Al0.88In0.12N interface, the extracted quantitative electric field profiles show excellent agreement with profiles predicted using Poisson simulation. Furthermore, we used the electric field profiles to quantify the density and distribution of the two-dimensional electron gas across the heterointerfaces with nanometre precision. This study is expected to guide the real-space characterization of local charge carrier density and distribution in semiconductor devices.

Superior External Quantum Efficiency of LEDs via Quasi-2D Perovskite Crystals Implanted with Phenethylammonium Acetate.

The multiple quantum well structure of a quasi-two-dimensional (quasi-2D) perovskite leads to nonradiative Auger recombination (AR). This is due to high local carrier density in recombination centers, although the radiative recombination is improved by efficient energy transfer. In this study, we suppress the AR by introducing phenethylammonium acetate (PEAAc) into the quasi-2D PEA2Csn-1PbnBr3n+1 perovskite. The recombination centers of n ≥ 4 phases can be promoted because the COO- preferentially coordinates with Pb2+, inhibiting the fast formation of n = 1, 2, 3 phases with phenethylammonium anion (PEA+). Thus, the AR is suppressed due to the lower density of local charge carriers. To balance the AR suppression and decreasing binding energy in promoting the n ≥ 4 phases, the PEAAc:PEABr molar ratios are adjusted. At the optimal molar ratio, perovskite light-emitting diodes (PeLEDs) with a maximum luminescence of ∼29942 cd m-2 and a maximum external quantum efficiency of ∼20.2% are achieved. These results confirm the most efficient PeLEDs based on PEA2Csn-1PbnBr3n+1 without passivation. Moreover, the efficiency roll off is significantly mitigated with a high threshold of over 3.51 mA/cm2. This study develops high-efficiency PeLEDs with a low efficiency rolloff.

Symmetric domain segmentation in WS2 flakes: correlating spatially resolved photoluminescence, conductance with valley polarization

The incidence of intra-flake heterogeneity of spectroscopic and electrical properties in chemical vapour deposited (CVD) WS2 flakes is explored in a multi-physics investigation via spatially resolved spectroscopic maps correlated with electrical, electronic and mechanical properties. The investigation demonstrates that the three-fold symmetric segregation of spectroscopic response, in topographically uniform WS2 flakes are accompanied by commensurate segmentation of electronic properties e.g. local carrier density and the differences in the mechanics of tip-sample interactions, evidenced via scanning probe microscopy phase maps. Overall, the differences are understood to originate from point defects, namely sulfur vacancies within the flake along with a dominant role played by the substrate. While evolution of the multi-physics maps upon sulfur annealing elucidates the role played by sulfur vacancy, substrate-induced effects are investigated by contrasting data from WS2 flake on Si and Au surfaces. Local charge depletion induced by the nature of the sample-substrate junction in case of WS2 on Au is seen to invert the electrical response with comprehensible effects on their spectroscopic properties. Finally, the role of these optoelectronic properties in preserving valley polarization that affects valleytronic applications in WS2 flakes, is investigated via circular polarization discriminated photoluminescence experiments. The study provides a thorough understanding of spatial heterogeneity in optoelectronic properties of WS2 and other transition metal chalcogenides, which are critical for device fabrication and potential applications.

Robust estimation of charge carrier diffusivity using transient photoluminescence microscopy.

Transient microscopy has emerged as a powerful tool for imaging the diffusion of excitons and free charge carriers in optoelectronic materials. In many excitonic materials, extraction of diffusion coefficients can be simplified because of the linear relationship between signal intensity and local excited state population. However, in materials where transport is dominated by free charge carriers, extracting diffusivities accurately from multidimensional data is complicated by the nonlinear dependence of the measured signal on the local charge carrier density. To obtain accurate estimates of charge carrier diffusivity from transient microscopy data, statistically robust fitting algorithms coupled to efficient 3D numerical solvers that faithfully relate local carrier dynamics to raw experimental measurables are sometimes needed. Here, we provide a detailed numerical framework for modeling the spatiotemporal dynamics of free charge carriers in bulk semiconductors with significant solving speed reduction and for simulating the corresponding transient photoluminescence microscopy data. To demonstrate the utility of this approach, we apply a fitting algorithm using a Markov chain Monte Carlo sampler to experimental data on bulk CdS and methylammonium lead bromide (MAPbBr3) crystals. Parameter analyses reveal that transient photoluminescence microscopy can be used to obtain robust estimates of charge carrier diffusivities in optoelectronic materials of interest, but that other experimental approaches should be used for obtaining carrier recombination constants. Additionally, simplifications can be made to the fitting model depending on the experimental conditions and material systems studied. Our open-source simulation code and fitting algorithm are made freely available to the scientific community.

Electrical Scanning Probe Microscopy Investigation of Schottky and Metal-Oxide Junctions on Hetero-Epitaxial 3C-SiС on Silicon

This paper presents a macro-and nanoscale electrical investigation of Schottky and metal-oxide junctions with hetero-epitaxial 3C-SiC layers grown on Si. Statistical current-density-voltage (J-V) characterization of Pt/3C-SiC Schottky diodes showed an increase of the reverse leakage current with increasing the devices diameters. Furthermore, C-V and J-V analyses of SiO2/3C-SiC capacitors revealed non-idealities of the thermal oxide, such as a high trapped positive charge (3×1012 cm−2) and a reduced breakdown field (EBD=6.5 MV/cm) compared to ideal SiO2. Nanoscale electrical characterizations by conductive atomic force microscopy (CAFM) and scanning capacitance microscopy (SCM) allowed to shed light on the origin of non-ideal behavior of Schottky and thermal oxide junctions, by correlating the morphological features associated to 3C-SiC crystalline defects with local current transport and carrier density.

Nanoscale imaging of dopant incorporation in n-type and p-type GaN nanowires by scanning spreading resistance microscopy

The realization of practical semiconductor nanowire optoelectronic devices requires controlling their electrical transport properties, which are affected by their large surface/volume ratio value and potentially inhomogeneous electrical dopant distribution. In this article, the local carrier density in Si-doped and Mg-doped GaN nanowires grown catalyst-free by molecular beam epitaxy was quantitatively measured using scanning spreading resistance microscopy. A conductive shell surrounding a more resistive core was observed in Mg-doped, p-type GaN nanowires, balancing the formation of a depleted layer associated with sidewall surface states. The formation of this conductive layer is assigned to the peripheral accumulation of Mg dopants up to values in the 1020 /cm3 range, as determined by quantitative energy dispersive x ray spectroscopy measurements. By contrast, Si-doped n-type GaN nanowires exhibit a resistive shell, consistent with the formation of a depleted layer, and a conductive core exhibiting a decreasing resistivity for increasing Si doping level.

Low-Threshold Blue Quasi-2D Perovskite Laser through Domain Distribution Control.

Quasi-2D perovskites, composed of self-organized quantum well structures, are emerging as gain materials for laser applications. Here we investigate the influence of domain distribution on the laser emission of CsPbCl1.5Br1.5-based quasi-2D perovskites. The use of 2,2-diphenylethylammonium bromide (DPEABr) as a ligand enables the formation of quasi-2D film with a large-n-dominated narrow domain distribution. Due to the reduced content of small-n domains, the incomplete energy transfer from small-n to large-n domains can be greatly addressed. Moreover, the photoinduced carriers can be concentrated on most of the large-n domains to reduce the local carrier density, thereby suppressing the Auger recombination. By controlling the domain distribution, we achieve blue amplified spontaneous emission and single-mode vertical-cavity surface-emitting lasing with low thresholds of 6.5 and 9.2 μJ cm-2, respectively. This work provides a guideline to design the domain distribution to realize low-threshold multicolor perovskite lasers.

Emergence of Quantum Tunneling in Ambipolar Black Phosphorus Multilayers without Heterojunctions

AbstractNegative differential resistance (NDR) is an exotic quantum tunneling phenomenon that is exhibited in narrow p‐n junctions with heavy doping concentrations. However, the presence of multiple heterojunctions in a conventional tunneling device often hampers the observance of NDR and a deep understanding of its origin, particularly in 2D van der Waals heterojunctions. Herein, the emergence of quantum tunneling at the charge neutrality point (VCNP) in ambipolar multilayered black phosphorus (BP) transistors without heterojunctions is reported. The nearly identical electron and hole carrier densities at VCNP in the presence of a drain bias (VD) result in a lateral p‐i‐n configuration inside the BP multilayers similar to that in a tunneling field‐effect transistor. The variation of the local carrier density profile and tunneling barrier with VD at VCNP drives a sharp enhancement of the activation energy and local resistance, which consequently allows to observe band‐to‐band tunneling at up to 340 K. The enhancement of the local doping profile along the BP channel and the NDR behavior in the fabricated reconfigurable top‐gate BP device with an h‐BN top‐dielectric provide further evidence for the origin of NDR in 2D ambipolar materials.

Conductivity-Type Conversion in Self-Assembled GeSn Stripes on Ge/Si(100) under Electric Field

We report on the electronic properties of self-assembled Ge99Sn1 stripes surrounded by inhomogeneous Ge96.6Sn3.4 area resulting from the segregation and migration of Sn droplets on Ge89.5Sn10.5/Ge/Si(100). The sample with microscale patterns was characterized at the nanoscale by a combination of techniques including X-ray diffraction, Raman spectroscopy, and scanning probe microscopy. Kelvin probe force microscopy (KPFM) nanoscale maps of the local work function show a spatial inhomogeneity up to 200 meV, which is explained by the band bending induced by holes trapped at the surface states of the Sn droplets. The scanning capacitance microscopy (SCM) of the local charge carrier density shows the dominant p-type conductivity with lateral variation in the hole-density by 1 order of magnitude. The microscale patterns are depleted by holes, and we experimentally revealed an electric-field-induced conductivity-type conversion from p to n in a self-assembled GeSn stripe. Finally, we presented a model and calculations that support the described above experimentally observed phenomena. This work explicitly provides an understanding of the electronic properties of the GeSn micrometer-scale stripes that is important for the implementation of the GeSn in photonic, optoelectronic, electronic, and energy storage devices.

Three-dimensional luminescence microscopy for quantitative plasma characterization in bulk semiconductors

Important challenges remain in the development of ultrafast laser writing inside semiconductor materials because the properties of narrow gap materials cause strong propagation distortions to intense infrared light. Here, we introduce a simple and robust imaging method for high-dynamic-range investigations of the laser–matter interactions in bulk semiconductors. Supported by measurements in gallium arsenide and silicon, we show how z-scan imaging of the band-to-band radiative recombination enables quantitative reconstruction of the three-dimensional distributions of free-carriers generated by nonlinear ionization with ultrashort pulses. The validity is confirmed by comparisons with ultrafast transmission microscopy (shadowgraphy) images. The superior sensitivity of the zero-background luminescence method allows the measurement of local carrier densities as low as ≈1016 cm−3 inside GaAs that is inaccessible by shadowgraphy. It provides the first direct evidence of the low density plasma generated far prior to the focus that causes the previously reported intensity clamping phenomenon. The potential of this non-coherent 3D imaging method to assess complex beam distortion features is also exemplified by real-time pre-compensation of aberration for an intense interacting beam.

Phononics of Graphene Interfaced with Flowing Ionic Fluid: An Avenue for High Spatial Resolution Flow Sensor Applications.

While ionic flow over graphenic structures creates electromotive potential, there is a need to understand the local carrier density induced in graphene without any electrode-induced Fermi-level pinning. Here, we show the electrolyte-flow induced localized doping in graphene via inspecting its Raman phononic energy. Graphene's Fermi energy level has a logarithmic dependence to the flow velocity over 2 orders of magnitude of velocity (∼100 μm s-1 to 10 mm s-1). A theoretical model of the electric double layer (EDL) during ionic transport is used to correlate the Fermi level of graphene with the flow rate and the electronic structure (HOMO-LUMO levels) of the ionic species. This correlation can allow us to use graphene as a reliable, non-invasive, optical flow-sensor, where the flow rates can be measured at high spatial resolution for several lab-on-a-chip applications.

WSe2 Homogeneous in-Plane Pin Junction Diode with Electric Double Layers

Transition metal dichalcogenide (TMDC) materials are in the spotlight as next-generation semiconductor materials because of their advantages over other semiconductor materials. Research on WSe2 among the materials is underway, and the feature of WSe2 has an ambipolar characteristic, and the fermi level change and the doping effect are different according to the electrode type.In the present study, we have fabricated a homogeneous WSe2 PIN junction diode. Our in-plane multilayer WSe2 p-i-n homogeneous diode exhibited photovoltaic properties by asymmetric physical contact doping effect with electric double layer(EDL). Owing to the EDL placed between the cathode and anode, local charge carrier type and density profile are modulated in the same layers.Compared to general diode mode, PIN mode enhance the photo response of the device in our visible measurement range. The diode exhibits excellent rectifying behavior with a current rectification ratio of 104 or higher. Photovoltaic performance of short circuit current and open circuit voltage achieved 3.2 nA, 0.55 V, respectively under the 550 nm green light. And spectrum responsivity is about ~100 mA/W (max point) at 3.2 eV to 1.6 eV. Figure 1

Gate‐Controlled Polarity‐Reversible Photodiodes with Ambipolar 2D Semiconductors

AbstractA photosensor with an amplitude‐tunable and polarity‐reversible response under gate modulation has potential as a computational photosensor, which can provide more recognition degree of data to enhance signal processing efficiency. Although, the ambipolar 2D semiconductors possess unique gate‐tunable properties, the question of how to utilize this property to design polarity‐reversible photodiodes for intelligent applications remains unanswered. Here, gate‐controllable polarity‐reversible photodiodes based on ambipolar 2D semiconductors with an asymmetrically metal‐contacted architecture are proposed. By controlling the gate‐field, the local carrier type and density profile can be manipulated in the channel due to the partial shielding feature of the asymmetrically metal‐contacted architecture, resulting in a polarity‐reversible photodiode. The reported WSe2‐based photodiode possesses excellent rectifying behavior with a rectification ratio over 105, photovoltaic performance with 90% external quantum efficiency, and 2.3% power conversion efficiency under gate regulation. Meanwhile, the device exhibits reversible polarity of photovoltage from a negative to positive state under gate control. By utilizing the reversible photovoltage of the WSe2 photodiode, an optoelectronic switch with a photovoltage polarity signal is demonstrated without a bias voltage. This photovoltage‐reversible homodiode paves the way to develop 2D devices with multiple operation modes for potential applications in high‐efficiency photovoltaics, intelligent vision sensors, and logic optoelectronics.

Robust quantum point contact via trench gate modulation

Quantum point contacts (QPC) are a primary component in mesoscopic physics and have come to serve various purposes in modern quantum devices. However, fabricating a QPC that operates robustly under extreme conditions, such as high bias or magnetic fields, still remains an important challenge. As a solution, we have analyzed the trench-gated QPC (t-QPC) that has a central gate in addition to the split-gate structure used in conventional QPCs (c-QPC). From simulation and modelling, we predicted that the t-QPC has larger and more even subband spacings over a wider range of transmission when compared to the c-QPC. After an experimental verification, the two QPCs were investigated in the quantum Hall regimes as well. At high fields, the maximally available conductance was achievable in the t-QPC due to the local carrier density modulation by the trench gate. Furthermore, the t-QPC presented less anomalies in its DC bias dependence, indicating a possible suppression of impurity effects.

Investigating the Efficiency Droop of Nitride-Based Blue LEDs with Different Quantum Barrier Growth Rates

In this study, GaN-based blue InGaN/GaN light-emitting diodes (LEDs) with different growth rates of the quantum barriers were fabricated and investigated. The LEDs with quantum barriers grown with a higher growth rate exhibit a lower leakage current and less non-radiative recombination centers in the multiple quantum wells (MQWs). Therefore, the LED with a higher barrier growth rate achieves a better light output power by 18.4% at 120 mA, which is attributed to weaker indium fluctuation effect in the QWs. On the other hand, the localized states created by indium fluctuation lead to a higher local carrier density, and Auger recombination in the QWs. Thus, the efficiency droop ratio of the LEDs with a higher barrier growth rate was only 28.6%, which was superior to that with a lower barrier growth rate (39.3%).

Understanding Interlayer Contact Conductance in Twisted Bilayer Graphene.

Bilayer or few-layer 2D materials showing novel electrical properties in electronic device applications have aroused increasing interest in recent years. Obtaining a comprehensive understanding of interlayer contact conductance still remains a challenge, but is significant for improving the performance of bilayer or few-layer 2D electronic devices. Here, conductive atomic force microscope (C-AFM) experiments are reported to explore the interlayer contact conductance between bilayer graphene (BLG) with various twisted stacking structures fabricated by the chemical vapor deposition (CVD) method. The current maps show that the interlayer contact conductance between BLG strongly depends on the twist angle. The interlayer contact conductance of 0° AB-stacking bilayer graphene (AB-BLG) is ≈4 times as large as that of 30° twisted bilayer graphene (t-BLG), which indicates that the twist angle-dependent interlayer contact conductance originates from the coupling-decoupling transitions. Moreover, the moiré superlattice-level current images of t-BLG show modulations of local interlayer contact conductance. Density functional theory calculations together with a theoretical model reproduce the C-AFM current map and show that the modulation is mainly attributed to the overall contribution of local interfacial carrier density and tunneling barrier.