High Room-temperature Mobility Research Articles

Non-volatile rippled-assisted optoelectronic array for all-day motion detection and recognition

In-sensor processing has the potential to reduce the energy consumption and hardware complexity of motion detection and recognition. However, the state-of-the-art all-in-one array integration technologies with simultaneous broadband spectrum image capture (sensory), image memory (storage) and image processing (computation) functions are still insufficient. Here, macroscale (2 × 2 mm2) integration of a rippled-assisted optoelectronic array (18 × 18 pixels) for all-day motion detection and recognition. The rippled-assisted optoelectronic array exhibits remarkable uniformity in the memory window, optically stimulated non-volatile positive and negative photoconductance. Importantly, the array achieves an extensive optical storage dynamic range exceeding 106, and exceptionally high room-temperature mobility up to 406.7 cm2 V−1 s−1, four times higher than the International Roadmap for Device and Systems 2028 target. Additionally, the spectral range of each rippled-assisted optoelectronic processor covers visible to near-infrared (405 nm–940 nm), achieving function of motion detection and recognition.

Record high electron mobilities in high-purity GaN by eliminating C-induced mobility collapse

Impact of carbon impurities on the electrical properties of lightly doped n-type GaN [electron concentration ∼(1–2)×1015 cm−3 at room temperature] was investigated using temperature-dependent Hall effect measurements. GaN crystals with a threading dislocation density of (1–3)×106 cm−2 were grown by our originally developed quartz-free hydride vapor phase epitaxy method, which enabled the background Si, O, and C concentrations to be suppressed to below the mid-1014 cm−3 range. We prepared three samples with different C concentrations ([C]) by intentional C doping. The C incorporation induced severe mobility collapse at temperatures greater than 60 K, where the measured mobility decreased and deviated from the theoretical value as [C] increased. The mobility collapse was eliminated for the purest GaN crystal with [C] ∼1.4×1014 cm−3, exhibiting a record high room-temperature mobility of 1480 cm2/(V·s), as well as a record high maximum mobility of 14 300 cm2/(V·s) at 62 K. The latter was almost double the previous record. We found that the overall mobility behavior can be well reproduced by adding an empirical [C]-dependent mobility component expressed as μUNK=K/TnUNK with 1≤nUNK≤2 and K∝C−1 to the conventional mobility theory (phonon and impurity scattering). Although the mechanism of the component remains uncertain, our findings provide insight into the unsolved issue of mobility collapse.

The Discovery of a High-Mobility Two-Dimensional Bismuth Oxyselenide Semiconductor and Its Application in Nonvolatile Neuromorphic Devices.

The development of two-dimensional (2D) electronics is always accompanied by the discovery of 2D semiconductors with high mobility and specific crystal structures, which may bring revolutionary breakthrough on proof-of-concept devices and physics. Here, Bi3O2.5Se2, a 2D bismuth oxyselenide semiconductor with non-neutral layered crystal structure is discovered. Ultrathin Bi3O2.5Se2 films are readily synthesized by chemical vapor deposition, displaying tunable band gaps and high room-temperature field-effect mobility of >220 cm2 V-1 s-1. Moreover, the as-synthesized Bi3O2.5Se2 nanoplates were fabricated into top-gated transistors with a simple device configuration, whose carrier density can be reversibly regulated in the range of 1014 cm-2 just by a facile method of electrostatic doping at room temperature. These features enable it to be functionalized into nonvolatile synaptic transistors with ultralow operating energy consumption (∼0.5 fJ), high repeatability, low operating voltage (0.1 V), and long retention time. Our work extends the family of bismuth oxyselenide 2D semicondutors.

Symmetry-Guaranteed High Carrier Mobility in Quasi-2D Thermoelectric Semiconductors.

Quasi-2D semiconductors have garnered immense research interest for next-generation electronics and thermoelectrics due to their unique structural, mechanical, and transport properties. However, most quasi-2D semiconductors experimentally synthesized so far have relatively low carrier mobility, preventing the achievement of exceptional power output. To break through this obstacle, a route is proposed based on the crystal symmetry arguments to facilitate the charge transport of quasi-2D semiconductors, in which the horizontal mirror symmetry is found to vanish the electron-phonon coupling strength mediated by phonons with purely out-of-plane vibrational vectors. This is demonstrated in ZrBeSi-type quasi-2D systems, where the representative sample Ba1.01 AgSb shows a high room-temperature hole mobility of 344 cm2 V-1 S-1 , a record value among quasi-2D polycrystalline thermoelectrics. Accompanied by intrinsically low thermal conductivity, an excellent p-type zT of ≈1.3 is reached at 1012 K, which is the highest value in ZrBeSi-type compounds. This work uncovers the relation between electron-phonon coupling and crystal symmetry in quasi-2D systems, which broadens the horizon to develop high mobility semiconductors for electronic and energy conversion applications.

Resonant x-ray scattering method for measuring cation stoichiometry in BaSnO3 thin films

We develop a resonant scattering technique to measure cation stoichiometry of lanthanum-doped BaSnO3 (BSO) thin films on a DyScO3 substrate. Samples are grown by a hybrid molecular beam epitaxy method and display high room-temperature carrier mobilities. The measured thin films are grown with widely differing cation arrival rates, with Ba being evaporated from an elemental source and Sn from a SnO2 source. Differences in mobilities in these films may arise from differences in Ba/Sn cation stoichiometry. Owing to the similar scattering strength of the Ba and Sn cations, odd-order Bragg peaks of BSO are particularly sensitive to the material's cation stoichiometry, i.e., the Ba/Sn ratio. Sensitivity to cation stoichiometry is further enhanced using the technique of resonant x-ray scattering, which changes the scattering strength of a single element across the Ba L absorption edges. We determine that the Ba/Sn cation stoichiometry varies from unity by less than 1% for films of mobility ranging from 84.8 to 144 cm2/(V s) and conclude that the mobility of these films scales with film thickness and growth rate rather than stoichiometry.

Mixed-dimensional Te/CdS van der Waals heterojunction for self-powered broadband photodetector

The 2D layered crystals can physically integrate with other non-2D components through van der Waals (vdW) interaction, forming mixed-dimensional heterostructures. As a new elemental 2D material, tellurium (Te) has attracted intense recent interest for high room-temperature mobility, excellent air-stability, and the easiness of scalable synthesis. To date, the Te is still in its research infancy, and optoelectronics with low-power consumption are less reported. Motivated by this, we report the fabrication of a mixed-dimensional vdW photodiode using 2D Te and 1D CdS nanobelt in this study. The heterojunction exhibits excellent self-powered photosensing performance and a broad response spectrum up to short-wave infrared. Under 520 nm wavelength, a high responsivity of 98 mA W−1 is obtained at zero bias with an external quantum efficiency of 23%. Accordingly, the photo-to-dark current ratio and specific detectivity reach 9.2 × 103 and 1.9 × 1011 Jones due to the suppressed dark current. This study demonstrates the promising applications of Te/CdS vdW heterostructure in high-performance photodetectors. Besides, such a mixed-dimensional integration strategy paves a new way for device design, thus expanding the research scope for 2D Te-based optoelectronics.

Damage-Free Atomic Layer Etch of WSe2: A Platform for Fabricating Clean Two-Dimensional Devices

The development of a controllable, selective, and repeatable etch process is crucial for controlling the layer thickness and patterning of two-dimensional (2D) materials. However, the atomically thin dimensions and high structural similarity of different 2D materials make it difficult to adapt conventional thin-film etch processes. In this work, we propose a selective, damage-free atomic layer etch (ALE) that enables layer-by-layer removal of monolayer WSe2 without altering the physical, optical, and electronic properties of the underlying layers. The etch uses a top-down approach where the topmost layer is oxidized in a self-limited manner and then removed using a selective etch. Using a comprehensive set of material, optical, and electrical characterization, we show that the quality of our ALE processed layers is comparable to that of pristine layers of similar thickness. The ALE processed WSe2 layers preserve their bright photoluminescence characteristics and possess high room-temperature hole mobilities of 515 cm2/V·s, essential for fabricating high-performance 2D devices. Further, using graphene as a testbed, we demonstrate the fabrication of ultra-clean 2D devices using a sacrificial monolayer WSe2 layer to protect the channel during processing, which is etched in the final process step in a technique we call sacrificial WSe2 with ALE processing (SWAP). The graphene transistors made using the SWAP technique demonstrate high room-temperature field-effect mobilities, up to 200,000 cm2/V·s, better than previously reported unencapsulated graphene devices.

A two-dimensional electron gas based on a 5s oxide with high room-temperature mobility and strain sensitivity

The coupling of optical and electronic degrees of freedom together with quantum confinement in low-dimensional electron systems is particularly interesting for achieving exotic functionalities in strongly correlated oxide electronics. Recently, high room-temperature mobility has been achieved for a large bandgap transparent oxide – BaSnO3 upon extrinsic La or Sb doping, which has excited significant research attention. In this work, we report the realization of a two-dimensional electron gas (2DEG) on the surface of transparent BaSnO3 via oxygen vacancy creation, which exhibits a high carrier density of ~7.72 × 1014 cm−2 and a high room-temperature mobility of ~18 cm2·V−1·s−1. Such a 2DEG is rather sensitive to strain and a less than 0.1% in-plane biaxial compressive strain leads to a giant resistance enhancement of ~350% (more than 540 kΩ/□) at room temperature. Thus, this work creates a new path to exploring the physics of low-dimensional oxide electronics and devices applicable at room temperature.

Electronic properties of p-type BaSnO3 thin films

Cubic perovskite oxides are attracting wide attention for applications in electronics and optoelectronics due to their high electronic mobility, however, realization of p-type conductivity in these oxides is still a matter. Barium stannate (BaSnO3) is one of such materials, wherein low-amount of lanthanum-doping can induce an extremely high room-temperature electron mobility. In this work, anion-doping strategy was explored to realize the p-type conductivity in BaSnO3, and the underlying physics of transport was investigated. It is found that nitrogen substitution for oxygen can induce the p-type conductivity in BaSnO3 thin films. By analyzing the temperature-dependent resistance and negative magnetoresistance data, Efros-Shklovskii variable-range hopping conduction is identified as the dominant transport mechanism in p-type BaSnO3 thin films. Additionally, nitrogen-doped BaSnO3 homojunctions exhibit good rectifying and photo-response properties. This study provides an alternative strategy for realizing the p-type conductivity in stannate-based perovskite oxides, and is helpful for designing the properties of stannate-based devices.

High mobility and high thermoelectric power factor in epitaxial ScN thin films deposited with plasma-assisted molecular beam epitaxy

Scandium nitride (ScN) is an emerging rock salt III-nitride semiconductor and has attracted significant interest in recent years for its potential thermoelectric applications as a substrate for high-quality epitaxial GaN growth and as a semiconducting component for epitaxial single-crystalline metal/semiconductor superlattices for thermionic energy conversion. Solid-solution alloys of ScN with traditional III-nitrides such as AlxSc1−xN have demonstrated piezoelectric and ferroelectric properties and are actively researched for device applications. While most of these exciting developments in ScN research have employed films deposited using low-vacuum methods such as magnetron sputtering and physical and chemical vapor depositions for thermoelectric applications and Schottky barrier-based thermionic energy conversion, it is necessary and important to avoid impurities, tune the carrier concentrations, and achieve high-mobility in epitaxial films. Here, we report the high-mobility and high-thermoelectric power factor in epitaxial ScN thin films deposited on MgO substrates by plasma-assisted molecular beam epitaxy. Microstructural characterization shows epitaxial 002 oriented ScN film growth on MgO (001) substrates. Electrical measurements demonstrated a high room-temperature mobility of 127 cm2/V s and temperature-dependent mobility in the temperature range of 50–400 K that is dominated by dislocation and grain boundary scattering. High mobility in ScN films leads to large Seebeck coefficients (−175 μV/K at 950 K) and, along with a moderately high electrical conductivity, a large thermoelectric power factor (2.3 × 10−3 W/m-K2 at 500 K) was achieved, which makes ScN a promising candidate for thermoelectric applications. The thermal conductivity of the films, however, was found to be a bit large, which resulted in a maximum figure-of-merit of 0.17 at 500 K.

Temperature-dependent transport properties of two-dimensional hole gas in Ge channel modulation-doped square quantum wells

We present a theoretical study of the transport properties of two-dimensional hole gas (2DHG) in Ge channel modulation-doped square quantum wells, with very high room-temperature drift mobilities. Within the variational approach, we obtain analytic expressions for the carrier distribution, and autocorrelation functions for various scattering mechanisms. The results were used to determine the hole mobility where only the relevant scattering processes, namely acoustic phonons (Deformation potential and Piezoelectric coupling), ionized impurity, surface roughness were considered. The partials and total mobility dependences on temperature were found. It is shown that acoustic phonon and surface roughness scattering are the dominant mechanism. The interpretation of these results is carried out and the scattering mechanisms limiting the 2DHG mobility in Ge quantum wells of SiGe heterostructure are discussed.

Band structure, effective mass, and carrier mobility of few-layer h-AlN under layer and strain engineering

Wide bandgap two-dimensional semiconductors are of paramount importance for developing van der Waals heterostructure electronics. This work reports the use of layer and strain engineering to introduce the feasibility of two-dimensional hexagonal (h)-AlN to fill the scientific and application gap. We show that such one- to five-layer h-AlN has an indirect bandgap, tunable from 2.9 eV for a monolayer to ∼3.5 eV for multilayer structures, along with isotropic effective masses and carrier mobilities between zigzag and armchair directions. With an increase in the layer number to bulk AlN, the bandgap will experience a transition from an indirect gap to direct gap. Surprisingly, high room-temperature mobilities of electrons and holes (of the order of 1000 cm2 V−1 s−1) in a relaxed monolayer h-AlN system and widely adjustable effective masses and carrier mobilities in a different layer h-AlN are observed. In the presence of strain engineering, the bandgap decreases obviously with an increase in tensile strain; meanwhile, the isotropy and value of effective mass or carrier mobility in monolayer h-AlN can also be modulated effectively; the hole mobilities in the armchair direction, especially, will be enhanced dramatically. With a tunable bandgap, high carrier mobilities, and modifiable isotropy, our results indicate that few-layer h-AlN has potential applications in future mechano-electronic devices.

Prediction of staggered stacking 2D BeP semiconductor with unique anisotropic electronic properties

By comprehensive structure design and first-principles calculations, we report a novel two-dimensional (2D) BeP nanomaterial with exotic structural and properties. This BeP 2D material is formed by a couple honeycomb sheets by slab staggered stacking and strong interlayer bondings. It behaves as a natural 2D semiconductor with several notable properties: a modest bandgap (~1.34 eV), high room-temperature electron mobility (~104 cm2 V−1 s−1) and high visible-light absorption coefficient (~105 cm−1); Moreover, due to the unique stacking topology, BeP may display distinctive direction-dependent electric transport by the anisotropic polarity of electron and hole mobilities, that is, it exhibits n-type (electron mobility > hole mobility) along the armchair direction while acts as p -type (hole mobility > electron mobility) in the zigzag direction, thus promising for applications in nanoelectronics. The BeP has good dynamic and thermal stabilities and is also the lowest-energy structure of 2D space indicated by particle swarm search, implying the high feasibility of experimental synthesis.

Separating Electrons and Donors in BaSnO3 via Band Engineering.

Separating electrons from their source atoms in La-doped BaSnO3, the first perovskite oxide semiconductor to be discovered with high room-temperature electron mobility, remains a subject of great interest for achieving high-mobility electron gas in two dimensions. So far, the vast majority of work in perovskite oxides has focused on heterostructures involving SrTiO3 as an active layer. Here we report the demonstration of modulation doping in BaSnO3 as the high room-temperature mobility host without the use of SrTiO3. Significantly, we show the use of angle-resolved hard X-ray photoelectron spectroscopy (HAXPES) as a nondestructive approach to not only determine the location of electrons at the buried interface but also to quantify the width of electron distribution in BaSnO3. The transport results are in good agreement with the results of self-consistent solution to one-dimensional Poisson and Schrödinger equations. Finally, we discuss viable routes to engineer two-dimensional electron gas density through band-offset engineering.

First principles calculations of intrinsic mobilities in tin-based oxide semiconductors SnO, SnO2, and Ta2SnO6

The development of high-performance p-type oxides with high hole mobility and a wide bandgap is critical for the applications of metal oxide semiconductors in vertically integrated CMOS devices [Salahuddin et al., Nat. Electron. 1, 442 (2018)]. Sn2+-based oxides such as SnO and K2Sn2O3 have recently been proposed as high-mobility p-type oxides due to their relatively low effective hole masses, which result from delocalized Sn s-orbital character at the valence band edge. Here, we introduce a promising ternary Sn-O-X compound, Ta2SnO6, which exhibits strong valence band dispersion and a large bandgap. In order to evaluate the performance of this oxide as a p-type semiconductor, we perform first-principles calculations of the phonon-limited room-temperature carrier mobilities in SnO, SnO2, and Ta2SnO6. Electron relaxation time is evaluated, accounting for the scatterings from acoustic deformation potentials and polar optical phonons (POP), within the isotropic and dispersionless approximation. At room temperature, the electron/hole mobilities in a given material (SnO, SnO2, and Ta2SnO6) are found to be limited by POP scattering. SnO2 shows high room-temperature electron mobility of 192 cm2/(V s), while SnO and Ta2SnO6 exhibit impressive hole mobilities, with the upper limit at 60 and 33 cm2/(V s), respectively. We find that carrier effective mass largely accounts for the differences in mobility between these oxides with correspondingly different POP scattering rates. The theoretically predicted intrinsic mobilities of each material will provide the upper limit to the real mobilities for their device applications. Our findings also suggest a necessity of further investigation to identify even higher mobility p-type oxides with smaller hole effective masses.

Reliable Nonvolatile Memory Black Phosphorus Ferroelectric Field-Effect Transistors with van der Waals Buffer.

Two-dimensional material-based ferroelectric field-effect transistors (2D-FeFETs) hold great promise in information storage and processing. However, an often-observed and hard-to-control anti-hysteresis response of 2D-FeFETs, for example, hysteretic switching of the resistance states of the devices opposite to that of the actual polarization of the ferroelectric dielectric, represents a major issue in the industrial applications of such devices. Here, we demonstrate a van der Waals buffer technique that eliminates anti-hysteresis in black phosphorus (BP) 2D-FeFETs and restores their intrinsic hysteretic behavior. Our modified BP 2D-FeFETs showed outstanding performance including high room-temperature carrier mobility, robust bistable states with fast response to a gate, a large on/off ratio at zero gate voltage, a large and considerably more stable memory window, and a long retention time. During repeated gate operation, the memory window of the buffered device is ∼7000 times more stable than the unbuffered device. Such a method could be crucial in future information technological applications that utilize the intrinsic properties of 2D-FeFETs.

High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition.

We report high room-temperature mobility in single-layer graphene grown by chemical vapor deposition (CVD) after wet transfer on SiO2 and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm2 V-1 s-1 at room temperature and ∼120 000 cm2 V-1 s-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepared by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycrystalline graphene, achieving room-temperature mobilities of ∼30 000 cm2 V-1 s-1. These results show that, with appropriate encapsulation and cleaning, room-temperature mobilities well above 10 000 cm2 V-1 s-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.

Data-driven and probabilistic learning of the process-structure-property relationship in solution-grown tellurene for optimized nanomanufacturing of high-performance nanoelectronics

Two-dimensional (2-D) semiconductors have been intensely explored as alternative channel materials for future generation ultra-scaled transistor technology [1–8]. However, significant roadblocks (e.g., poor carrier mobilities [9–11], instability [4,5,10], and vague potential in scaling-up [10,12–15]) exist that prevent the realization of the current state-of-the-art 2-D materials’ potential for energy-efficient electronics. The emergent solution-grown tellurene exhibits attractive attributes, e.g., high room-temperature mobility, large on-state current density, air-stability, and tunable material properties through a low-cost, scalable process, to tackle these challenges [16]. Nevertheless, the fundamental manufacturing science of the hydrothermal processing for tellurene remains elusive. Here, we report on the first systematic, data-driven learning of the process-structure-property relationship in solution-grown tellurene, revealing the process factors’ effects on tellurene’s production yield, dimensions, and transistor-relevant properties, through a holistic approach integrating both the experimental explorations and data analytics. We further demonstrate the application of such fundamental knowledge for developing tellurene transistors with optimized and reliable performance, which can enable the cost-effective realization of high-speed, energy-efficient electronics.

Charge Mobility and Recombination Mechanisms in Tellurium van der Waals Solid.

Trigonal tellurium is a small band gap elemental semiconductor consisting of van der Waals bound one-dimensional helical chains of tellurium atoms. We study the temperature dependence of the charge carrier mobility and recombination pathways in bulk tellurium. Electrons and holes are generated by irradiation of the sample with 3 MeV electrons and detected by time-resolved microwave conductivity measurements. A theoretical model is used to explain the experimental observations for different charge densities and temperatures. Our analysis reveals a high room temperature mobility of 190 ± 20 cm2 V–1 s–1. The mobility is thermally deactivated, suggesting a band-like transport mechanism. According to our analysis, the charges predominantly recombine via radiative recombination with a radiative yield close to 98%, even at room temperature. The remaining charges recombine by either trap-assisted (Shockley–Read–Hall) recombination or undergo trapping to deep traps. The high mobility, near-unity radiative yield, and possibility of large-scale production of atomic wires by liquid exfoliation make Te of high potential for next-generation nanoelectronic and optoelectronic applications, including far-infrared detectors and lasers.

Depletion Mode MOSFET Using La-Doped BaSnO3 as a Channel Material.

The high room-temperature mobility that can be achieved in BaSnO3 has created significant excitement for its use as channel material in all-perovskite-based transistor devices such as ferroelectric field effect transistor (FET). Here, we report on the first demonstration of n-type depletion-mode FET using hybrid molecular beam epitaxy grown La-doped BaSnO3 as a channel material. The devices utilize a heterostructure metal-oxide semiconductor FET (MOSFET) design that includes an epitaxial SrTiO3 barrier layer capped with a thin layer of HfO2 used as a gate dielectric. A field-effect mobility of ∼70 cm2 V-1 s-1, a record high transconductance value of >2mS/mm at room temperature, and the on/off ratio exceeding 107 at 77 K were obtained. Using temperature- and frequency-dependent transport measurements, we quantify the impact of the conduction band offset at the BaSnO3/SrTiO3 interface as well as bulk and interface traps on device characteristics.