Low-temperature Electron Transport Research Articles

Diffusive and ballistic transport in thin InSb nanowire devices using a few-layer-graphene-AlO x gate

Quantum devices based on InSb nanowires (NWs) are a prime candidate system for realizing and exploring topologically-protected quantum states and for electrically-controlled spin-based qubits. The influence of disorder on achieving reliable quantum transport regimes has been studied theoretically, highlighting the importance of optimizing both growth and nanofabrication. In this work, we consider both aspects. We developed InSb NW with thin diameters, as well as a novel gating approach, involving few-layer graphene and atomic layer deposition-grown AlO x . Low-temperature electronic transport measurements of these devices reveal conductance plateaus and Fabry–Pérot interference, evidencing phase-coherent transport in the regime of few quantum modes. The approaches developed in this work could help mitigate the role of material and fabrication-induced disorder in semiconductor-based quantum devices.

Modulation of electron transport and quantum lifetimes in symmetric and asymmetric AlGaAs/InGaAs double quantum well structures

The low-temperature electron transport (τ t ) and quantum (τ q ) lifetimes are modulated as a function of applied electric field (F) in Al0.3Ga0.7As/In0.15Ga0.85As-based symmetric and asymmetric double quantum well structures by considering ionized impurity, alloy disorder and interface roughness scatterings. The asymmetry in the structure potential is achieved through differences in the well widths, i.e. W w1 ≠ W w2. We show that in a symmetric structure (W w1 = W w2 = 120 Å) resonance of subband states (RSS) occurs at F = F rss = 0, whereas in asymmetric structures [(W w1, W w2) = (90 Å, 150 Å) and (W w1, W w2) = (150 Å, 90 Å)], the RSS is shifted and occurs at |F rss| = 16.4 kV cm−1. The symmetric and antisymmetric nature of the subband wave functions at their corresponding F rss makes τ t a minimum and this increases with change in F during multisubband occupancy. However, the absence of the factor 1 – cosθ in the quantum lifetime makes τ q one order less than the magnitude of τ t , but τ q shows a maximum at the corresponding F rss and decreases with change in F. The above changes in τ t and τ q with F make their ratio τ t /τ q maximum at the transition of subband states.

Low-temperature electron transport of rutile-type GexSn1−xO2

Rutile-type wide and ultrawide band-gap oxide semiconductors are emerging materials for high-power electronics and deep ultraviolet optoelectronics applications. A rutile-type GeO2-SnO2 alloy (r-GexSn1–xO2) recently found is one of such materials. Herein, we report low-temperature electron transport properties of r-GexSn1−xO2 thin films with x = 0.28 and 0.41. Based on resistivity and magnetoresistance measurements, along with the theory of quantum interference, it is suggested that Efros–Shklovskii variable-range hopping, i.e., hopping over the states within the Coulomb gap, is dominant at lower temperatures (T ≤ 10 and 15 K) in both r-Ge0.41Sn0.59O2 and r-Ge0.28Sn0.72O2. The negative and positive magnetoresistances observed at low temperatures are attributable to the quantum interference and field-induced spin alignment, respectively. The magnetoresistance measurements at higher temperatures suggest that both Mott variable–range hopping and thermally activated band conduction occur at T < 100 K and that almost pure thermally activated band conduction takes place at T ≥ 150 K.

Anomalous electronic transport in high-mobility Corbino rings

We report low-temperature electronic transport measurements performed in two multi-terminal Corbino samples formed in GaAs/Al-GaAs two-dimensional electron gases (2DEG) with both ultra-high electron mobility ( ≳ 20 × 106 cm2/ Vs) and with distinct electron density of 1.7 and 3.6 × 1011 cm−2. In both Corbino samples, a non-monotonic behavior is observed in the temperature dependence of the resistance below 1 K. Surprisingly, a sharp decrease in resistance is observed with increasing temperature in the sample with lower electron density, whereas an opposite behavior is observed in the sample with higher density. To investigate further, transport measurements were performed in large van der Pauw samples having identical heterostructures, and as expected they exhibit resistivity that is monotonic with temperature. Finally, we discuss the results in terms of various lengthscales leading to ballistic and hydrodynamic electronic transport, as well as a possible Gurzhi effect.

Surface conductivity in SmB6

We present a study of low temperature electron transport (resistivity, magnetoresistance and Hall coefficient) in SmB6 single crystals having different polar (100) and nonpolar (110) and (111) surfaces after mechanical polishing and chemical etching. The estimation of effective parameters for surface and bulk charge carriers allows us to conclude that surface conductivity is very sensitive to the method of surface treatment. The most pronounced change is observed for the polar (100) surface, for which the related Hall concentration of charge carriers at 2 K decreases more than by 2 orders of magnitude and the Hall mobility increases by a factor of 15 after etching these faces in diluted nitric acid. We suggest that the strong dependence of surface properties on the type of treatment may result from both topological protection, which is influenced by intrinsic defects or surface reconstruction, and band bending effects, which modulate the properties of the surface conduction layer in case of polar faces.

Low-damage electron beam lithography for nanostructures on Bi2Te3-class topological insulator thin films

Nanostructured topological insulators (TIs) have the potential to impact a wide array of condensed matter physics topics, ranging from Majorana physics to spintronics. However, the most common TI materials, the Bi2Se3 family, are easily damaged during nanofabrication of devices. In this paper, we show that electron beam lithography performed with a 30 or 50 kV accelerating voltage—common for nanopatterning in academic facilities—damages both nonmagnetic TIs and their magnetically doped counterparts at unacceptable levels. We additionally demonstrate that electron beam lithography with a 10 kV accelerating voltage produces minimal damage detectable through low-temperature electronic transport. Although reduced accelerating voltages present challenges in creating fine features, we show that with careful choice of processing parameters, particularly the resist, 100 nm features are reliably achievable.

Solution-processed pristine metal oxides as electron-transporting materials for perovskite solar cells

In recent years, perovskite material-based photovoltaic devices have attracted great attention of researchers because of an expeditious improvement in their efficiency from 3.8% to over 25%. The electron transport layer (ETL), which functions for the extraction and transportation of photogenerated electrons from active perovskite material to the electrodes, is a vital part of these perovskite solar cells (PSCs). The optoelectronic properties of these electron transport layer materials also have an impact on the performance of these perovskite solar cells, and for commercialized flexible perovskite solar cells, low-temperature and solution-processable electron transport layers having high stability and suitable optoelectronic properties are needed. In this regard, the solution-processable films of different metal oxides have been largely investigated by many research groups. So, this review summarizes the optoelectronic properties of the different metal oxide-based electron transport layers and the development in the performance of the perovskite solar cells, which have solution-processable metal oxides as electron transport layers.

Potassium hexacyanoferrate(III): A promising additive for perovskite precursors in carbon-based perovskite solar cells

Perovskite solar cells (PSCs) that use carbon electrodes and low-temperature processed electron transport layers (ETLs) show great promise in meeting global energy needs at an affordable price. Our current research is focused on the development of carbon-based perovskite solar cells (PSCs) that incorporate low-temperature titanium oxide electron transport layers (ETLs) to enhance their photovoltaic performance. In our approach, we introduce potassium hexacyanoferrate(III) material into the pre-solution of the MAPbI3 perovskite to fabricate a light-harvesting layer. This promising technique has shown great potential in improving the overall performance of PSCs. This additive reduces the formation of non-radiative recombination centers, resulting in a perovskite layer that is desirable and free from defects. Furthermore, photovoltaic devices based on potassium hexacyanoferrate(III) exhibit reduced transfer resistance, leading to faster charge transfer at the interfaces of TiO2/perovskite and perovskite/carbon electrodes. As a result, the efficiency of PSCs can be improved by up to 14.89%, which is significantly higher than the recorded efficiency of unmodified PSCs at 12.05%. In addition, PSCs based on potassium hexacyanoferrate show greater stability in ambient air compared to their unmodified counterparts.

Coherent Hole Transport in Selective Area Grown Ge Nanowire Networks.

Holes in germanium nanowires have emerged as a realistic platform for quantum computing based on spin qubit logic. On top of the large spin–orbit coupling that allows fast qubit operation, nanowire geometry and orientation can be tuned to cancel out charge noise and hyperfine interaction. Here, we demonstrate a scalable approach to synthesize and organize Ge nanowires on silicon (100)-oriented substrates. Germanium nanowire networks are obtained by selectively growing on nanopatterned slits in a metalorganic vapor phase epitaxy system. Low-temperature electronic transport measurements are performed on nanowire Hall bar devices revealing high hole doping of ∼1018 cm–3 and mean free path of ∼10 nm. Quantum diffusive transport phenomena, universal conductance fluctuations, and weak antilocalization are revealed through magneto transport measurements yielding a coherence and a spin–orbit length of the order of 100 and 10 nm, respectively.

Low‐Temperature Atomic Layer Deposited Electron Transport Layers for Co‐Evaporated Perovskite Solar Cells

Perovskite Photovoltaics In article number 2100842, Nripan Mathews, Subodh Mhaisalkar, Annalisa Bruno, and co-workers investigated low-temperature and scalable electron transport materials for rigid and flexible co-evaporated perovskite solar cells and modules. The scale up of perovskite photovoltaics is a key aspect to bring this technology to commercialization.

The synthesis and electrical transport properties of carbon/Cr2GaC MAX phase composite microwires.

While MAX phases offer an exotic combination of ceramic and metallic properties, rendering them a unique class of materials, their applications remain virtually hypothetical. To overcome this shortcoming, a sol-gel based route is introduced that allows access to microwires in the range of tens of micrometers. Thorough structural characterization through XRD, SEM, EDS, and AFM demonstrates a successful synthesis of carbonaceous Cr2GaC wires, and advanced low temperature electronic transport measurements revealed resistivity behavior dominated by amorphous carbon. The tunability of electronic behavior of the obtained microwires is shown by a halide post-synthesis treatment, allowing purposeful engineering of the microwires' electrical conductivity. Raman studies revealed the polyanionic nature of the intercalated halides and a slow decrease in halide concentration was concluded from time-dependent conductivity measurements. Based on these findings, the process is considered a viable candidate for fabricating chemiresistive halogen gas sensors.

Highly efficient and stable quantum dot light-emitting devices with a low-temperature tin oxide electron transport layer

UVO treatment is proposed to decompose the residual tin hydroxide in an SnO2 ETL film. With UVO treatment, efficient and stable QLEDs are fabricated with a low-temperature SnO2 transport layer.

Low-temperature electronic transport of manganese silicide shell-protected single crystal nanowires for nanoelectronics applications

Recently, core–shell nanowires have been proposed as potential electrical connectors for nanoelectronics components. A promising candidate is Mn5Si3 nanowires encapsulated in an oxide shell, due to their low reactivity and large flexibility. In this work, we investigate the use of the one-step metallic flux nanonucleation method to easily grow manganese silicide single crystal oxide-protected nanowires by performing their structural and electrical characterization. We find that the fabrication method yields a room-temperature hexagonal crystalline structure with the c-axis along the nanowire. Moreover, the obtained nanowires are metallic at low temperature and low sensitive to a strong external magnetic field. Finally, we observe an unknown electron scattering mechanism for small diameters. In conclusion, the one-step metallic flux nanonucleation method yields intermetallic nanowires suitable for both integration in flexible nanoelectronics as well as low-dimensionality transport experiments.

Effect of precursor solution concentrations of PCBM interlayer between low-temperature spin-coating TiO2 electron transport layer and light absorption layer on physical properties of perovskite film and photo-current hysteresis of photo-voltaic devices

we insert (6,6)-phenyl c61 butyric acid methyl ester (PC61BM) interlayer of different precursor solution concentrations between low-temperature TiO2 electron transport layer (ETL) and light absorption layer to modify charge carrier transporting, mitigate hysteresis and improve perovskite film quality. The results show that the TiO2 / perovskite interface, the perovskite film morphology and photo physical properties can be modified by optimizing the precursor solution concentrations of PC61BM interlayer, which facilitates the electron extraction from perovskite absorption layer to TiO2 ETL and reduces the hysteresis.

Kondo effect at ultimate atomic-scale two-dimensional limit: Au/Si(111) 3×3 reconstruction with embedded Cr atoms

The occurrence of the Kondo effect in the ultimate two-dimensional (2D) case, when the thickness of the conducting film is reduced down to the atomic-scale limit, was explored taking the Tl-adsorbed Au/Si(111)$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ reconstruction with embedded Cr atoms as a prototype system. The reconstruction is essentially an atomic-layer film, which resides on a semiconductor Si(111) substrate and demonstrates conductivity of a metallic type, at least down to 1.8 K. The tight binding of Cr atoms in the interstitial sites surrounded by six Si atoms prevents them from clustering. The Kondo effect in this system was investigated experimentally using low-temperature scanning tunneling spectroscopy and electronic transport measurements. Both techniques yield a Kondo temperature of $\ensuremath{\sim}70--80\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. A substantial increase of the sample resistivity caused by adding Cr atoms is believed to be a specific feature of the extremely thin metallic films, where small amounts of magnetic impurities can effectively block off the narrow conduction channel.

Pyridine-Functionalized Fullerene Electron Transport Layer for Efficient and Flexible Perovskite Solar Cells

Designing low-temperature solution-processed electron transport layer (ETL) for perovskite solar cells (PSCs) is important to realize low-cost and large-scale manufacturing of PSCs. Herein, we successfully synthesized a novel pyridine-functionalized fullerene derivative [6,6]-(4-pyridinyl)-C61-ethyl acid ethyl ester (PyCEE), which can act as an excellent ETL for n-i-p type PSCs. Due to its suitable energy levels, high electron mobility, and coordination interactions with the Pb2+ within perovskite, PyCEE-based PSCs displayed a power conversion efficiency (PCE) of 18.27% with dramatically suppressed hysteresis, superior to that of the TiO2-based control devices. Benefitting from the low-temperature and solution processability of PyCEE, it was further applied in flexible PSCs and a PCE of 15.25% was obtained, which demonstrates its applicability in flexible PSCs and provides new opportunity for large-scale production and commercialization of PSCs.

Anomalous Hall Effect in Layered Ferrimagnet MnSb2Te4**Supported by the National Natural Science Foundation of China under Grant Nos. 11961141011 and 61425015, the National Key Research and Development Program under Grant No. 2016YFA0300600, and the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No. XDB28000000.

We report on low-temperature electron transport properties of MnSb2Te4, a candidate of ferrimagnetic Weyl semimetal. Long-range magnetic order is manifested as a nearly square-shaped hysteresis loop in the anomalous Hall resistance, as well as sharp jumps in the magnetoresistance. At temperatures below 4 K, a lnT-type upturn appears in the temperature dependence of longitudinal resistance, which can be attributed to the electron-electron interaction (EEI), since the weak localization can be excluded by the temperature dependence of magnetoresistance. Although the anomalous Hall resistance exhibits a similar lnT-type upturn in the same temperature range, such correction is absent in the anomalous Hall conductivity. Our work demonstrates that MnSb2Te4 microflakes provide an ideal system to test the theory of EEI correction to the anomalous Hall effect.

Gate-tunable h/e -period magnetoresistance oscillations in Bi2O2Se nanowires

We report on the successful synthesis and low-temperature electron transport investigations of a new form of material - Bi2O2Se semiconducting nanowires. Gate-tunable 0- and $\pi$-h/e (h is the Planck constant and e the elementary charge) periodic resistance oscillations in longitudinal magnetic field were observed unexpectedly, demonstrating novel quasi-ballistic, phase-coherent surface states in Bi2O2Se nanowires. By reaching a very good agreement between the calculated density of states and the experimental data, we clarified the mechanism to be the one dimensional subbands formed along the circumference of the nanowire rather than the usually considered Aharonov-Bohm interference. A qualitative physical picture based on downward band bending associated with the complex band structure is proposed to describe the formation of the surface states.

A facile strategy for enhanced performance of inverted organic solar cells based on low-temperature solution-processed SnO2 electron transport layer

High-efficiency organic solar cells (OSCs) based on low-temperature (LT) processed SnO2 electron transport layer (ETL) are promising for their commercial use. However, high density of traps and large contact barrier for carriers at the interface between LT SnO2 and the active layer has been reported. To solve the problem, various interface modifying layer materials, such as PFN, has been introduced. Currently, the fabrication process of such interface modifying layer materials is complex and expensive. Herein, a facile strategy involved a polar solvent ethanolamine (EA) is introduced to modify LT SnO2 surface. By soaking the SnO2 film into EA solution in 2-Methoxyethanol (2-ME), EA can easily anchor into SnO2 film surface and forms a continuous monomolecular layer via dehydration reaction. The whole process is green and highly compatible with a roll-to-roll process. Further study suggests that the deep trap centers on SnO2 surface are substantially reduced and the built-in potential in OSCs is reinforced. Finally, OSCs based on EA-modified SnO2 demonstrated an enhanced power conversion efficiency from 10.71% to 12.45% which was comparable to those based on ZnO (12.26%) under the same experiment parameters. Our work boosts the development of the inverted OSCs with easy fabrication and compatibility with roll-to-roll process.

(Invited) Investigation of Interfacial Charge Transfer Doping of 2D MoS2

Layered two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) have attracted a great deal of attention because of their exciting electrical and optical properties and their potential applications in electronic and optoelectronic devices. Because of ultra low volume and chemically inert surface of 2D materials, it is challenging to achieve controlled doping of 2D materials. Recently, interfacial charge transfer induced doping of 2D materials have gained significant attention. In this talk, I will present electrical transport studies of 2D molybdenum disulfide (MoS2) field effect transistors decorated with zero dimensional (0D) gold (Au) nanoislands. I will discuss how the Au nanoisland coverage affects the interfacial charge transfer based doping and tailor the electrical properties of the host material. From the detailed low temperature electron transport measurements, I will elucidate the role of 2D-0D interface on the transport properties of MoS2 FETs.