Residual Carrier Concentration Research Articles

Comparison of the Material Quality of AlxIn1-xN (x-0-0.50) Films Deposited on Si(100) and Si(111) at Low Temperature by Reactive RF Sputtering.

AlxIn1−xN ternary semiconductors have attracted much interest for application in photovoltaic devices. Here, we compare the material quality of AlxIn1−xN layers deposited on Si with different crystallographic orientations, (100) and (111), via radio-frequency (RF) sputtering. To modulate their Al content, the Al RF power was varied from 0 to 225 W, whereas the In RF power and deposition temperature were fixed at 30 W and 300 °C, respectively. X-ray diffraction measurements reveal a c-axis-oriented wurtzite structure with no phase separation regardless of the Al content (x = 0–0.50), which increases with the Al power supply. The surface morphology of the AlxIn1−xN layers improves with increasing Al content (the root-mean-square roughness decreases from ≈12 to 2.5 nm), and it is similar for samples grown on both Si substrates. The amorphous layer (~2.5 nm thick) found at the interface with the substrates explains the weak influence of their orientation on the properties of the AlxIn1−xN films. Simultaneously grown AlxIn1−xN-on-sapphire samples point to a residual n-type carrier concentration in the 1020–1021 cm−3 range. The optical band gap energy of these layers evolves from 1.75 to 2.56 eV with the increase in the Al. PL measurements of AlxIn1−xN show a blue shift in the peak emission when adding the Al, as expected. We also observe an increase in the FWHM of the main peak and a decrease in the integrated emission with the Al content in room-temperature PL measurements. In general, the material quality of the AlxIn1-xN films on Si is similar for both crystallographic orientations.

Study on the Field-effect Carrier Transport of Epitaxial Graphene on SiC

We have studied the field-effect carrier transport of graphene on 4H silicon carbide substrate. In order to extract the electrical parameters, the top-gated field effect transistor has been fabricated. By fitting the measured results with Kim’s model, the field effect carrier mobility (µ) and the metal/graphene contact resistance (Rc) and the residual carrier concentration (n0) are derived to be 3382cm2/Vs, 2250Ω▪µm and 2.18×1013cm-2, respectively. It is noted that the large contact resistance did not affect the high field effect carrier mobility of our device. The high carrier mobility suggests that the SiC epitaxial graphene may be quite suitable for the future high speed electronic applications.

High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development

New fabrication methods are strongly demanded for the development of thin-film saturable absorbers with improved optical properties (absorption band, modulation depth, nonlinear optical response). In this sense, we investigate the performance of indium nitride (InN) epitaxial layers with low residual carrier concentration (<1018 cm−3), which results in improved performance at telecom wavelengths (1560 nm). These materials have demonstrated a huge modulation depth of 23% and a saturation fluence of 830 µJ/cm2, and a large saturable absorption around −3 × 104 cm/GW has been observed, attaining an enhanced, nonlinear change in transmittance. We have studied the use of such InN layers as semiconductor saturable absorber mirrors (SESAMs) for an erbium (Er)-doped fiber laser to perform mode-locking generation at 1560 nm. We demonstrate highly stable, ultrashort (134 fs) pulses with an energy of up to 5.6 nJ.

AlxIn1-xN on Si (100) Solar Cells (x = 0-0.56) Deposited by RF Sputtering.

We investigate the photovoltaic performance of solar cells based on n-AlxIn1−xN (x = 0–0.56) on p-Si (100) hetero-junctions deposited by radio frequency sputtering. The AlxIn1−xN layers own an optical bandgap absorption edge tuneable from 1.73 eV to 2.56 eV within the Al content range. This increase of Al content results in more resistive layers (≈10−4–1 Ω·cm) while the residual carrier concentration drops from ~1021 to ~1019 cm−3. As a result, the top n-contact resistance varies from ≈10−1 to 1 MΩ for InN to Al0.56In0.44N-based devices, respectively. Best results are obtained for devices with 28% Al that exhibit a broad external quantum efficiency covering the full solar spectrum with a maximum of 80% at 750 nm, an open-circuit voltage of 0.39 V, a short-circuit current density of 17.1 mA/cm2 and a conversion efficiency of 2.12% under air mass 1.5 global (AM1.5G) illumination (1 sun), rendering them promising for novel low-cost III-nitride on Si photovoltaic devices. For Al contents above 28%, the electrical performance of the structures lessens due to the high top-contact resistivity.

Two-step epitaxial growth of NbON (100) thin films on rutile-type TiO2 (101) substrates and reduction of residual carrier concentration by RF reactive sputtering

Two-step growth makes it possible to grow NbON epitaxial films and minimize anion-related defects in the NbON films.

Low Residual Carrier Concentration and High Mobility in 2D Semiconducting Bi2O2Se.

The air-stable and high-mobility two-dimensional (2D) Bi2O2Se semiconductor has emerged as a promising alternative that is complementary to graphene, MoS2, and black phosphorus for next-generation digital applications. However, the room-temperature residual charge carrier concentration of 2D Bi2O2Se nanoplates synthesized so far is as high as about 1019-1020 cm-3, which results in a poor electrostatic gate control and unsuitable threshold voltage, detrimental to the fabrication of high-performance low-power devices. Here, we first present a facile approach for synthesizing 2D Bi2O2Se single crystals with ultralow carrier concentration of ∼1016 cm-3 and high Hall mobility up to 410 cm2 V-1 s-1 simultaneously at room temperature. With optimized conditions, these high-mobility and low-carrier-concentration 2D Bi2O2Se nanoplates with domain sizes greater than 250 μm and thicknesses down to 4 layers (∼2.5 nm) were readily grown by using Se and Bi2O3 powders as coevaporation sources in a dual heating zone chemical vapor deposition (CVD) system. High-quality 2D Bi2O2Se crystals were fabricated into high-performance and low-power transistors, showing excellent current modulation of >106, robust current saturation, and low threshold voltage of -0.4 V. All these features suggest 2D Bi2O2Se as an alternative option for high-performance low-power digital applications.

Material and device characterization of Type-II InAs/GaSb superlattice infrared detectors

This work investigates midwave infrared Type-II InAs/GaSb superlattice (SL) grown by molecular beam epitaxy on GaSb substrate. In order to compensate the natural tensile strain of the InAs layers, two different shutter sequences have been explored during the growth. The first one consists of growing an intentional InSb layer at both interfaces (namely GaSb-on-InAs and InAs-on-GaSb interfaces) by migration enhanced epitaxy while the second uses the antimony-for-arsenic exchange to promote an ‘InSb-like’ interface at the GaSb-on-InAs interface. SLs obtained via both methods are compared in terms of structural, morphological and optical properties by means of high-resolution x-ray diffraction, atomic force microscopy and photoluminescence spectroscopy. By using the second method, we obtained a nearly strain-compensated SL on GaSb with a full width at half maximum of 56 arcsec for the first-order SL satellite peak. Relatively smooth surface has been achieved with a root mean square value of about 0.19 nm on a 2 µm × 2 µm scan area. Finally, a p-i-n device structure having a cut-off wavelength of 5.15 µm at 77 K has been demonstrated with a dark-current level of 8.3 × 10−8 A/cm2 at −50 mV and a residual carrier concentration of 9.7 × 1014 cm−3, comparable to the state-of-the-art.

Charge carrier velocity in graphene field-effect transistors

To extend the frequency range of transistors into the terahertz domain, new transistor technologies, materials, and device concepts must be continuously developed. The quality of the interface between the involved materials is a highly critical factor. The presence of impurities can degrade device performance and reliability. In this paper, we present a method that allows the study of the charge carrier velocity in a field-effect transistor vs impurity levels. The charge carrier velocity is found using high-frequency scattering parameter measurements followed by delay time analysis. The limiting factors of the saturation velocity and the effect of impurities are then analysed by applying analytical models of the field-dependent and phonon-limited carrier velocity. As an example, this method is applied to a top-gated graphene field-effect transistor (GFET). We find that the extracted saturation velocity is ca. 1.4×107 cm/s and is mainly limited by silicon oxide substrate phonons. Within the considered range of residual charge carrier concentrations, charged impurities do not limit the saturation velocity directly by the phonon mechanism. Instead, the impurities act as traps that emit charge carriers at high fields, preventing the current from saturation and thus limiting power gain of the GFETs. The method described in this work helps to better understand the influence of impurities and clarifies methods of further transistor development. High quality interfaces are required to achieve current saturation via velocity saturation in GFETs.

A 400-GHz Graphene FET Detector

This letter presents a graphene field effect transistor (GFET) detector at 400 GHz, with a maximum measured optical responsivity of 74 V/W, and a minimum noise-equivalent power of 130 pW/Hz1/2. This letter shows how the detector performance degrades as a function of the residual carrier concentration in the graphene channel, which is an important material parameter that depends on the quality of the graphene sheet and contaminants introduced during the fabrication process. In this work, the exposure of the graphene channel to liquid processes is minimized resulting in a low residual carrier concentration. This is in part, an important contributing factor to achieve the record high GFET detector performance. Thus, our results show the importance to use graphene with high quality and the importance to minimize contamination during the fabrication process.

Effect of O2/Ar flow ratio on Ga and Al co-doped ZnO thin films by rf sputtering for optoelectronic device fabrication

Ga and Al co-doped ZnO thin films were prepared on glass substrates by radio frequency magnetron sputtering and the effect of O2/Ar flow ratio on their physical properties was investigated. The films exhibited a hexagonal wurtzite structure with a predominant (002) peak intensity and their crystallinity improved with O2/Ar flow ratio. Raman spectroscopy confirmed the films’ wurtzite structure and revealed a decrease in residual tensile stress and charge carrier concentration with increasing O2/Ar flow ratio. Low surface roughness resulted in high optical transmittances around 85–90% in the visible region. The Urbach energy decreased with increasing O2/Ar flow ratio indicating a reduction in structural disorders. This was consistent with Raman spectroscopy and X-ray diffraction analysis. The least electrical resistivity (2.6×101 Ωcm) and highest figure of merit (8.8×10−6Ω−1) for films prepared with oxygen admittance were obtained at 0.667 O2/Ar flow ratio, indicating their suitability in optoelectronic device fabrication.

Low temperature deposition of highly transparent and conducting Al-doped ZnO films by RF magnetron sputtering

Radio frequency (rf) magnetron sputtering process was optimized to deposit highly conducting and transparent polycrystalline Al-doped ZnO (ZnO:Al or AZO) films at low temperature. Deposition was done in pure argon atmosphere on unheated glass substrates. Various properties of AZO films, such as residual stress, crystallite size, Al content, carrier concentration and mobility, resistivity, optical transmittance, band-gap and refractive index were measured and their variation with deposition power and working pressure was analyzed. In the first step deposition power was varied from 200 W to 1200 W at a constant working pressure, and then in the next step working pressure was varied from 0.05 to 0.5 Pa while keeping deposition power constant at the optimized value. Film thickness was maintained constant at 600 ± 50 nm. Obtained results were compared with those of other researchers wherever possible. With increasing deposition power it was observed that film properties improve up to a certain power and then start to deteriorate with further increase of power. Electrical properties were found to depend more strongly on structural properties of films, compared to optical properties. At optimized deposition power film properties showed a weaker dependence on working pressure in the used range. Best quality AZO films with 6.7 × 10−4 Ω-cm electrical resistivity and 90% average visible transmittance were obtained at 600 W deposition power and 0.1 Pa working pressure.

Characterization of GaN films grown on hafnium foils by pulsed sputtering deposition

GaN films were grown on nearly lattice‐matched Hf foils by pulsed sputtering deposition (PSD) which showed highly c‐axis oriented structures. Unlike GaN films directly grown on Hf foils that showed coexisting zinc blende and wurtzite phases, as a result of interfacial reactions the insertion of HfN reaction barrier layers between GaN and Hf suppressed the inclusion of the zinc blende phase. The utilization of surface‐oxidized AlN interlayers resulted in GaN films on Hf foils with controlled polarity (i.e., Ga‐polar). The residual carrier concentration in the Ga‐polar GaN films was as low as 3.2 × 1016 cm−3. These results revealed that the proposed technique can yield high‐quality Ga‐polar GaN films on Hf foils that can be potentially used in large‐area and flexible GaN‐based optical devices.

Quasi-epitaxial Metal-Halide Perovskite Ligand Shells on PbS Nanocrystals.

Epitaxial growth techniques enable nearly defect free heterostructures with coherent interfaces, which are of utmost importance for high performance electronic devices. While high-vacuum technology-based growth techniques are state-of-the art, here we pursue a purely solution processed approach to obtain nanocrystals with eptaxially coherent and quasi-lattice matched inorganic ligand shells. Octahedral metal-halide clusters, respectively 0-dimensional perovskites, were employed as ligands to match the coordination geometry of the PbS cubic rock-salt lattice. Different clusters (CH3NH3+)(6-x)[M(x+)Hal6](6-x)- (Mx+ = Pb(II), Bi(III), Mn(II), In(III), Hal = Cl, I) were attached to the nanocrystal surfaces via a scalable phase transfer procedure. The ligand attachment and coherence of the formed PbS/ligand core/shell interface was confirmed by combining the results from transmission electron microscopy, small-angle X-ray scattering, nuclear magnetic resonance spectroscopy and powder X-ray diffraction. The lattice mismatch between ligand shell and nanocrystal core plays a key role in performance. In photoconducting devices the best performance (detectivity of 2 × 1011 cm Hz 1/2/W with > 110 kHz bandwidth) was obtained with (CH3NH3)3BiI6 ligands, providing the smallest relative lattice mismatch of ca. -1%. PbS nanocrystals with such ligands exhibited in millimeter sized bulk samples in the form of pressed pellets a relatively high carrier mobility for nanocrystal solids of ∼1.3 cm2/(V s), a carrier lifetime of ∼70 μs, and a low residual carrier concentration of 2.6 × 1013 cm-3. Thus, by selection of ligands with appropriate geometry and bond lengths optimized quasi-epitaxial ligand shells were formed on nanocrystals, which are beneficial for applications in optoelectronics.

Deep-level spectroscopy in metal–insulator–semiconductor structures

In this study we present a method for measuring bulk traps using deep-level spectroscopy techniques in metal–insulator–semiconductor (MIS) structures. We will focus on deep-level transient spectroscopy (DLTS), although this can be extended to deep-level optical spectroscopy (DLOS) and similar techniques. These methods require the modulation of a depletion region either from a Schottky junction or from a highly asymmetric p–n junction, junctions that may not be realizable in many current material systems. This is the case of wide-bandgap semiconductor families that present a doping asymmetry or have a high residual carrier concentration or surface carrier accumulation, such as InGaN or ZnO. By adding a thin insulating layer and forming an MIS structure this problem can be circumvented and DLTS/DLOS can be performed under certain conditions. A model for the measurement of bulk traps in MIS structures is thus presented, focusing on the similarities with standard DLTS, maintaining when possible links to existing knowledge on DLTS and related techniques. The model will be presented from an equivalent circuit point of view. The effect of the insulating layer on DLTS is evaluated by a combination of simulations and experiments, developing methods for the measurement of these type of devices. As a validation, highly doped ZnO:Ga MIS devices have been successfully characterized and compared with a reference undoped sample using the methods described in this work, obtaining the same intrinsic levels previously reported in the literature but in material doped as high as cm−3.

Effect of oxide traps on channel transport characteristics in graphene field effect transistors

A semiempirical model describing the influence of interface states on characteristics of gate capacitance and drain resistance versus gate voltage of top gated graphene field effect transistors is presented. By fitting our model to measurements of capacitance–voltage characteristics and relating the applied gate voltage to the Fermi level position, the interface state density is found. Knowing the interface state density allows us to fit our model to measured drain resistance–gate voltage characteristics. The extracted values of mobility and residual charge carrier concentration are compared with corresponding results from a commonly accepted model which neglects the effect of interface states. The authors show that mobility and residual charge carrier concentration differ significantly, if interface states are neglected. Furthermore, our approach allows us to investigate in detail how uncertainties in material parameters like the Fermi velocity and contact resistance influence the extracted values of interface state density, mobility, and residual charge carrier concentration.

Access to residual carrier concentration in ZnO nanowires by calibrated scanning spreading resistance microscopy

Scanning spreading resistance microscopy (SSRM) was performed on non-intentionally doped (nid) ZnO nanowires (NWs) grown by metal-organic chemical vapor deposition in order to measure their residual carrier concentration. For this purpose, an SSRM calibration profile has been developed on homoepitaxial ZnO:Ga multilayer staircase structures grown by molecular beam epitaxy. The Ga density measured by SIMS varies in the 1.7 × 1017 cm−3 to 3 × 1020 cm−3 range. From measurements on such Ga doped multi-layers, a monotonic decrease in SSRM resistance with increasing Ga density was established, indicating SSRM being a well-adapted technique for two dimensional dopant/carrier profiling on ZnO at nanoscale. Finally, relevant SSRM signal contrasts were detected on nid ZnO NWs, and the residual carrier concentration is estimated in the 1–3 × 1018 cm−3 range, in agreement with the result from four-probe measurements.

Characterization of carrier concentration in ZnO nanowires by scanning capacitance microscopy

AbstractScanning capacitance microscopy (SCM) has been investigated on Ga doped ZnO staircase multi‐layers grown by molecular beam epitaxy (MBE) and ZnO NWs grown by chemical bath deposition (CBD). It is found that SCM data amplitude experiences a monotonic decrease with increasing Ga concentration from 2×1017 cm‐3 to 3×1020 cm‐3, indicating SCM being an appropriate technique for two dimensional dopant/carrier profiling in ZnO at nanoscale. For ZnO nanowires (NWs), a planariz‐ation process was developed based on dip‐coating of silica sol‐gel and chemical‐mechanical polishing. Then ZnO NWs were well detected from the silica matrix by SCM and the residual carrier concentration inside them was estimated through calibration method to be between 5×1017 cm‐3 and 2×1018 cm‐3. Finally, the result was compared with that obtained from conventional capacitance‐voltage (C‐V) measrement and their difference was discussed. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth condition dependence of unintentional oxygen incorporation in epitaxial GaN

Growth conditions have a tremendous impact on the unintentional background impurity concentration in gallium nitride (GaN) synthesized by molecular beam epitaxy and its resulting chemical and physical properties. In particular for oxygen identified as the dominant background impurity we demonstrate that under optimized growth stoichiometry the growth temperature is the key parameter to control its incorporation and that an increase by 55 °C leads to an oxygen reduction by one order of magnitude. Quantitatively this reduction and the resulting optical and electrical properties are analyzed by secondary ion mass spectroscopy, photoluminescence, capacitance versus voltage measurements, low temperature magneto-transport and parasitic current paths in lateral transistor test structures based on two-dimensional electron gases. At a growth temperature of 665 °C the residual charge carrier concentration is decreased to below 1015 cm−3, resulting in insulating behavior and thus making the material suitable for beyond state-of-the-art device applications.

Hybrid Graphene/Fluoropolymer Field-Effect Transistors With Improved Device Performance

We demonstrate high-performance field-effect transistors consisting of hybrid graphene/fluoropolymer films by investigating the molecular interaction with strong carbon–fluorine dipoles and hydrophobic surface characteristics. A simple and reproducible solution-coating method on a chemical vapor deposition grown monolayer graphene was employed with fluoropolymer, amorphous CYTOP, and polycrystalline poly(vinylidene fluoride-co-trifluoroethylene) as an interacting layer. All of the key device metrics, including field-effect mobility, Dirac voltage, residual carrier concentration, ON–OFF current ratio, and electron–hole transport symmetry, were substantially improved. Significantly, the Dirac voltage shifted in both directions toward zero, regardless of its initial positions of the Dirac voltage after molecular encapsulation with fluoropolymers. We also demonstrate the improved stability of hybrid graphene/fluoropolymer films that possesses a hydrophobic surface through repelling hydroxyl (–OH) functional groups from the graphene surface.

Dependence of minority carrier lifetime of Be-doped InAs/InAsSb type-II infrared superlattices on temperature and doping density

We investigate the minority carrier lifetime of Be-doped InAs/InAsSb type-II superlattices as a function of doping density and temperature using time-resolved photoluminescence (TRPL) to determine if switching the superlattice type from the typical n-type residual carrier concentration to p-type may improve device performance by improving the lifetime–mobility product. The introduction of the Be dopant to the superlattice reduces the carrier lifetime, first by a factor of ∼3 for doping densities near or below the n-type residual carrier concentration, then by an order of magnitude for samples doped well above the residual carrier concentration. Further, the higher-doped p-type samples demonstrate two distinct TRPL decay regimes and two peaks in the PL spectra, suggesting the formation of an additional acceptor-related recombination pathway leading to the observed shorter carrier lifetime.