Low Defect Density Research Articles

Study on the vertical Bridgman method of melt-grown CsPbBr3 single crystals for nuclear radiation detection.

As an excellent representative of all-inorganic perovskite materials, CsPbBr3 has been widely used in high-energy rays or high-energy particles detection for its outstanding high carrier mobility and long diffusion length. The great challenges and opportunities in these fields are crystal growth technology, especially the high-quality and large-sized CsPbBr3 single crystals. In this work, the influences of growth parameters (temperature gradient, growth rate, cooling rate) and thermal stress by the vertical Bridgman method on the quality and performance of CsPbBr3 crystals are systematically studied. The final results show that 10°C cm-1 is the optimized temperature gradient and 0.5 mm h-1 is the suitable growth rate for CsPbBr3 crystal growth. The study also shows that a cooling rate of 10°C h-1 for the general temperature interval and 1°C h-1 for the phase transition temperature interval is helpful to balance crystal growth efficiency as well as crystal quality. Crystal cracks caused by thermal stress as well as crystal adhesion on the ampoule can be effectively solved by depositing a uniform carbon film on the ampoule in advance. The optical, electrical and detection performance are also investigated. The optical characterization in the wavelength region ranging from ultraviolet to infrared indicates the crystal has a low density of deep-level defects and good crystal quality. The resistivity over 109 Ω cm and μτ of electrons over 10-2 cm-2 V-1 proves that the electrical performance of the crystal has met the basic requirement for nuclear radiation detection. The metal-semiconductor-metal structure Ti/Ni/CsPbBr3/Ni/Ti detector fabricated from the optimized CsPbBr3 single crystal has an energy resolution of 12.85% (137Cs, 662 keV). The purpose of this work is to provide a useful guide and reference for the future exploration of repeatable and improvable CsPbBr3 crystal growth technology.

Control of growth kinetics during remote epitaxy of complex oxides on graphene by pulsed laser deposition

Remote epitaxy through 2D materials opens new opportunities for research and application, overcoming some limitations of classical epitaxy and allowing the creation of freestanding layers. However, using graphene as a 2D interlayer for remote epitaxy of metal oxides is challenging, particularly when carried out by pulsed laser deposition (PLD). The graphene layer can be easily oxidized under the typically applied high oxygen pressures, and the impact of highly kinetic particles of the plasma plume can lead to severe damages. In this study, both aspects are addressed: Argon is introduced as an inert background gas in order to avoid oxidation and to reduce the kinetic impact of the plasma species on graphene. The laser spot size is minimized to control the plasma plume and particle flux. As a model system, strontium titanate (STO) is quasi-homoepitaxially grown on graphene buffered STO single crystals. Raman spectroscopy is performed to evaluate the 2D, G, and D band fingerprints of the graphene layer and to assess the defect structure of the interlayer after the deposition. Our results prove that control of the growth kinetics by reducing the laser spot size and by using high argon pressures provides a key strategy to conserve graphene with a low defect density during PLD while allowing a layer-by-layer growth of structurally coherent oxide layers. This strategy may be generalized for the PLD remote epitaxy of many complex oxides, opening the way for integrating 2D materials with complex oxides using widely accessible PLD processes.

Revealing the influence of aerosol particle state on the high-quality α-Ga2O3 single crystal thin film growth by Mist CVD epitaxy

There is little research on α-Ga2O3 single crystal film because it is a metastable phase of Ga2O3 crystal and difficult for controllable fabrication. In this paper, highly crystallized Ga2O3 thin films of poly-crystalline, α-Ga2O3 and β-Ga2O3 were prepared by modulating the growth temperature from 400 °C to 700 °C by mist-assisted CVD (Mist CVD) method. The results show that films grown at 525 °C owes the best crystalline quality of α-Ga2O3 film. To further improve the crystallinity of α-Ga2O3 thin film, a systematic simulation revealing the distribution changes of aerosol particle size, flow field, and temperature gradient in the reaction chamber were explored. The optimal growth region C with uniform particle size, well-distribute temperature gradient and stable flow field is determined. Meanwhile, the experimental results of α-Ga2O3 thin films grown at different positions in the reaction chamber was proved to consistent with the simulation results. Furthermore, the optimal single crystallized α-Ga2O3 thin films with smooth surface morphology and optical property were obtained. The PL spectrum operated at room temperature show the α-Ga2O3 film with low defect density and high crystalline quality. It shows that the single crystallized α-Ga2O3 is orientated grown on sapphire substrate with epitaxial relationship of α-Ga2O3 (0006)//c-Al2O3 (0006). Although, slight lattice mismatch is observed on the selected area electron diffraction pattern of the interface between sapphire and α-Ga2O3 film, it is ideal for further application in devices. This work reveals the key point for α-Ga2O3 single crystal film growth by mist CVD is to construct a steady and orderly environment through controlling aerosol particle state.

Investigations of MoS2-Based Self-Powered M–S–M Photodetectors with Low Defect Density and Fast Response and Low-Temperature Characteristics

Investigations of MoS₂-Based Self-Powered M–S–M Photodetectors with Low Defect Density and Fast Response and Low-Temperature Characteristics

Heterogeneous integration of high-k complex-oxide gate dielectrics on wide band-gap high-electron-mobility transistors

Heterogeneous integration of dissimilar crystalline materials has recently attracted considerable attention due to its potential for high-performance multifunctional electronic and photonic devices. The conventional method for fabricating heterostructures is by heteroepitaxy, in which epitaxy is performed on crystallographically different materials. However, epitaxial limitations in monolithic growth of dissimilar materials prevent implementation of high quality heterostructures, such as complex-oxides on conventional semiconductor platforms (Si, III-V and III-N). In this work, we demonstrate gallium nitride (GaN) high-electron-mobility transistors with crystalline complex-oxide material enabled by heterogeneous integration through epitaxial lift-off and direct stacking. We successfully integrate high-κ complex-oxide SrTiO3 in freestanding membrane form with GaN heterostructure via a simple transfer process as the gate oxide. The fabricated device shows steep subthreshold swing close to the Boltzmann limit, along with negligible hysteresis and low dynamic on-resistance, indicating very low defect density between the SrTiO3 gate oxide and GaN heterostructure. Our results show that heterogeneous integration through direct material stacking is a promising route towards fabricating functional heterostructures not possible by conventional epitaxy.

Sulfur‐Supplemented Vapor Transport Deposition of Sb2S3 and Sb2(S,Se)3 for High‐Performance Hydrogen Evolution Photocathodes

Chalcogen vacancy has long been recognized as a detrimental deep‐level defect that can induce severe charge recombination and diminish the quasi‐Fermi‐level splitting in Sb2S3 and Sb2Se3, especially when they are prepared by physical vapor deposition in vacuum. In order to counter this limitation, a sulfur‐supplemented vapor transport deposition technique is proposed, which intentionally introduces low pressure sulfur vapor and can thus afford low‐defect‐density antimony chalcogenide films. The resultant photocathodes modified with conformal TiO2 protection layer and Pt cocatalyst demonstrate a photocurrent as high as 20 mA cm−2 along with a much improved fill factor and onset potential, achieving an applied bias photoconversion efficiency above 1%. This work highlights the importance of the vacancy defect passivation and the preferential crystalline orientation in the film in promoting the photocathode performance.

Quantifying defects in graphene oxide structures

Oxidation of graphite and subsequent exfoliation leads to single layer graphene oxide (GO). One of the key structural features of GO is the presence of different kinds of defects that dictates their various physical and chemical properties. Molecular dynamics simulations with ReaxFF force fields have been widely used to generate realistic models of GO. In these simulations, the extent and distribution of the defects are varied by changing the initial oxygen (O)/carbon (C) ratio while the defect density is often measured by the total number of non-graphitic carbon (non-gC) atoms. Our calculations suggest that this parameter overestimates the defect densities at low O/C ratio. Herein, we employ the relative area of the defects as an alternative metric to gauge the defect density. Being exclusive to the defects and sensitive to the structural irregularities, this metric works well at both low and high defect densities. In another example, we consider the reduction of GO to reduced graphene oxide (rGO) for different O/C ratios, where the decrease in the number of non-gC atoms is associated with the formation of comparatively larger defects. Hence, it is unclear whether the extent of reduction in the defect density (if at all) should vary monotonically with the O/C ratio. The defect area, unlike the count of the non-gC atoms, mirrors this ambiguity and the change with respect to O/C ratio is not strictly monotonic. Additionally, we also investigate the dependence of the defect distribution and defect area on the size of the simulation cell.

Nitrogen-Doped Graphene Materials with High Electrical Conductivity Produced by Electrochemical Exfoliation of Graphite Foil.

Nitrogen-doped graphene-based materials are of utmost importance in sensing and energy conversion devices due to their unique physicochemical properties. However, the presence of defects such as pyrrolic nitrogen and oxygenated functional groups reduces their electrical conductivity. Herein, a two-step approach based on the electrochemical exfoliation of graphite foils in aqueous mixed electrolytes followed by thermal reduction at 900 °C is used to prepare high-quality few layers of N-doped graphene-based materials. The exfoliations were conducted in 0.1 M (NH4)2SO4 or H2SO4 and HNO3 (5 mM or 0.1 M) electrolytes mixtures and the HNO3 vol% varied. Chemical analysis demonstrated that the as-prepared graphene oxides contain nitro and amine groups. Thermal reduction is needed for substitutional N-doping. Nitrogen and oxygen surface concentrations vary between 0.23-0.96% and 3-8%, respectively. Exfoliation in (NH4)2SO4 and/or 5 mM HNO3 favors the formation of pyridinic-N (10-40% of the total N), whereas 1 M HNO3 favors the formation of graphitic-N (≈60%). The electrical conductivity ranges between 166-2705 Scm-1. Raman spectroscopy revealed a low density of defects (ID/IG ratio between 0.1 and 0.7) and that most samples are composed of mono-to-bilayer graphene-based materials (IG/I2D integrated intensities ratio). Structural and compositional stability of selected samples after storage in air for three months is demonstrated. These results confirm the high quality of the synthesized undoped and N-doped graphene-type materials.

Pioneering non-thermal plasma as a defect passivator: a new Frontier in ambient metal halide perovskite synthesis.

Growing energy demands make cost-effective, high-performance perovskite solar cells (PSCs) desirable. However, their commercial applications are limited due to defect formation and instability. Passivation technologies help enhance their favorable traits. Herein, we propose a pioneering technique utilizing non-thermal plasma (NTP) synthesis for passivating inherent defects and optimizing the energy levels of perovskites. AC-NTP utilizes ionic charges and uniform electric fields to effectively neutralize defect-induced charge traps, acting as a field-effect passivator. This approach not only mitigates energetic defects, but also facilitates the transformation of NH4PbI3 into a CH3NH3PbI3 perovskite through a self-degassing mechanism. The perovskites synthesized using this method demonstrate notable advancements in their properties, as evidenced by X-ray diffraction, UV-vis spectroscopy, and scanning electron microscopy. These improvements include enhanced crystalline quality, superior optical characteristics, and precise nanoparticle size control, with an average size of 54 nm. In situ Rietveld refinement analysis reveals minimal PbI2 formation, resulting in fewer lead iodide inversion defects. Accordingly, the PSC fabricated by AC-NTP shows a PCE of 15.25%, significantly higher than that fabricated by the DC one (13.29%), which demonstrates improved stability under ambient conditions for over 160 hours. Hysteresis assessment, SCLC analysis, and Shockley diode modeling show our PSCs' low defect densities and high interface quality. Moreover, DFT was applied to indirectly analyze the effects of NTP on the perovskites, focusing on quantum confinement effects and lattice arrangement's influence on the optoelectronic characteristics of MAPbI3 nanoparticles. The findings confirm that NTP synthesis leads to more optimal PSCs, showing notable improvement in photovoltaics.

GeSn-on-GaAs with photoconductive carrier lifetime >400 ns: role of substrate orientation and atomistic simulation.

Group IV GeSn quantum material finds application in electronics and silicon-compatible photonics. Synthesizing these materials with low defect density and high carrier lifetime is a potential challenge due to lattice mismatch induced defects and tin segregation at higher growth temperature. Recent advancements in the growth of these GeSn materials on Si, Ge, GaAs, and with substrate orientations, demonstrated different properties using epitaxial and chemical deposition methods. This article addresses the effect of GaAs substrate orientation and misorientation on the materials' properties and carrier lifetimes in epitaxial Ge0.94Sn0.06 layers. With starting GaAs substrates of (100)/2°, (100)/6°, (110) and (111)A orientations, Ge0.94Sn0.06 epitaxial layers were grown with an intermediate Ge buffer layer by molecular beam epitaxy and analyzed by several analytical tools. X-ray analysis displayed good crystalline quality, and Raman spectroscopy measurements showed blue shifts in phonon wavenumber due to biaxial compressive strain in Ge0.94Sn0.06 epilayers. Cross-sectional transmission electron microscopy analysis confirmed the defect-free heterointerface of Ge0.94Sn0.06/Ge/GaAs heterostructure. Minority carrier lifetimes of the unintentionally doped n-type Ge0.94Sn0.06 epilayers displayed photoconductive carrier lifetimes of >400 ns on (100)/6°, 319 ns on (100)/2°, and 434 ns on (110) GaAs substrate at 1500 nm excitation wavelength. On the other hand, Ge0.94Sn0.06 layer showed poor carrier lifetime on (111)A GaAs substrate. The observed differences in carrier lifetimes were correlated with the formation energy of the Ge on (100)/6° and (100)/2° GaAs heterointerface using Stillinger-Weber interatomic potential model-based atomistic simulation with different heterointerfacial bonding by Synopsys QuantumATK tool. Total energy computation of 6280-atom Ge/GaAs supercell on (100)/6° leads to lower formation energy than (100)/2°, making it more thermodynamically stable. Hence, the growth of the GeSn/III-V material system using misoriented (100) substrates that are more thermodynamically stable will enhance the performances of optoelectronic devices.

Green anti-solvent engineering for high-efficiency and environmentally friendly perovskite solar cells.

Flat and dense perovskite films with low defect density are essential for high-performance perovskite solar cells (PSCs). Anti-solvent-assisted crystallization (ASAC) is one of the effective ways to obtain high-quality perovskite films with low cost and simple operation. However, most of the traditional anti-solvents such as chlorobenzene, toluene, and diethyl ether have strong toxicity, which would be harmful to people's physical and mental health. It can be anticipated that when these toxic anti-solvents are widely applied in the industry, they will have destructive effects on humans and the environment, which is contrary to the current promotion concepts of green environmental protection. In September 2015, the United Nations Development Program regulated the Sustainable Development Goals (SDGs) for Mankind, which clearly emphasized the use of economically viable clean energy that was compatible with the goals for climate action, good health and well-being. So the development of non-hazardous green anti-solvents is an important direction in the research field of PSCs. In this review paper, the outstanding research achievements on green anti-solvents in recent years are summarized, including different types of perovskite films using different green anti-solvents with/without additives, the physical and chemical properties of different green anti-solvents, and their effects on the performance of perovskite films and PSCs. Moreover, five types of non-anti-solvent green preparation methods regulated by physical processes are also assessed. It provides references for the manufacturing of efficient, stable, low-cost and environmentally friendly perovskite devices.

Post-hot-cast annealing deposition of perovskite films with infused multifunctional organic molecules to enhance the performance of large-area light-emitting devices.

All-inorganic perovskites show great promise as an emission layer in perovskite light-emitting diodes (PeLEDs) owing to their easy solution processing, low manufacturing cost, and excellent optoelectronic properties. However, there is still an immense performance gap from small-area devices to large-area PeLED devices. The inhomogeneity of large-area high-quality perovskite films inevitably leads to vast defects and electroluminescence performance losses. Herein, a post-hot-cast annealing deposition scheme and the introduction of the multifunctional molecule 2-amino-1,3-propanediol (APDO) were proposed to regulate the crystallization of the perovskite film. As a result, uniform APDO:CsPbBr2.5Cl0.5 perovskite films with high crystallinity and lower defect density were deposited by post-hot-cast annealing. A decent maximum brightness of 2659 cd m-2 was achieved for the large-area cyan PeLEDs with an emitting area of 400 mm2.

The impact of different flexible substrates on the photothermal reduction quality of graphene oxide.

In this work, we demonstrate the impact of the photothermal reduction quality of graphene oxide (GO), which is affected by the material composition, roughness, and thermal properties of the membrane substrates. We show high efficiency reduced graphene oxide (rGO) conversion by applying a 405 nm pulsed laser in ambient conditions onto different flexible substrates. Three filter membranes, such as nylon, cellulose acetate, and nitrocellulose, are used as rGO thin film substrates, achieving sheet resistance of 51 ± 2, 58 ± 3, and 620 ± 40 Ω sq-1, respectively, which has been the lowest resistance reported in ambient conditions. Finally, we demonstrate that such flexible materials can be applied as temperature sensors ranging from 35 °C to 100 °C. The best sensitivity is achieved using nylon membranes, showing a smoother rGO surface and lower defect density.

Boosting the performance of Quasi-2D perovskite Light-Emitting diodes by tailoring molecular configurations of Carboxyl-Based additives

Quasi two-dimensional (quasi-2D) perovskites have shown huge potential for perovskite light-emitting diodes (PeLEDs). Nevertheless, the serious non-radiative recombination induced by numerous defects in polycrystalline quasi-2D perovskite films hinders their further development. Herein, three carboxyl-based additives with different molecular configurations, i.e. butyric acid (BAC), glutaric acid (GAC) and tricarballylic acid (TAC), are adopted as passivation agents for quasi-2D PeLEDs. Benefiting from the strong coordination interaction between –COOH and perovskites, the inclusion of carboxyl-based additives can effectively optimize film morphologies, regulate the distribution of small-n phases, passivate defects and enhance film stability. With the increase of the carboxyl group number and molecular spatial dimension of carboxyl-based additives, more compact and smoother quasi-2D perovskite films, together with narrower phase distribution, lower defect density and better stability, can be obtained. Consequently, the unpacked quasi-2D PeLED treated with TAC, which is tested in the ambient air with the relative humidity of about 50 %, delivers an external quantum efficiency (EQE) of 14.21 % and prolonged operational lifetime (T50) of 2160 s, while the pristine device only gives an EQE of 7.63 % and T50 of 460 s. Our work provides guidance for boosting the performance of quasi-2D PeLEDs by rationally tailoring molecular configurations of carboxyl-based additives.

Component control and additive engineering of all-inorganic perovskite films and carbon-based solar cells under ambient air environment

The new all-inorganic CsPb<i>X</i><sub>3</sub> perovskite material is expected to be used as an absorbing layer to prepare solar cells for efficient and stable commercial devices. However, the problems of high cost and poor stability, caused by precious metal electrodes and hole transport materials, urgently need solving. Therefore, carbon-based perovskite solar cells (C-PSCs) based on the HTL-free all-inorganic system have attracted widespread attention. This work adopts a strategy of finely regulating the ratio of I to Br in X-site of perovskite. Using the one-step anti-solvent method, CsPbI<sub><i>x</i></sub>Br<sub>3–<i>x</i></sub> films and HTL-free C-PSCs are prepared under ambient air condition. By comparing their light absorption characteristics, carrier transport, and corresponding optoelectronic properties, a balance point between efficiency and stability is found. Finally, HTL-free C-PSCs achieve an optimal efficiency of 10.10% and can be stably prepared under ambient air conditions. In order to further improve the performance of the corresponding devices, phenylethylammonium bromide (PEABr) is introduced into the perovskite, and the crystallinity, carrier transport, defect situation, and corresponding optoelectronic properties of perovskite films and devices are compared under different conditions. Ultimately, the perovskite film treated with PEABr reaches better crystallinity and lower defect density, while generating a small amount of two-dimensional perovskite which can passivate the perovskite film and suppress non-radiative recombination of charge carriers. After appropriate PEABr treatment, the photoelectric conversion efficiency (PCE) of the device is significantly enhanced, increasing from 10.18% of the optimal device in the control group to 12.61%. Thus, this method provides an optimal approach for preparing efficient and low-cost HTL-free C-PSCs under ambient air environments.

Effect of the rigidity of an organic interlayer on the nonradiative recombination and exciton dissociation in hybrid Dion–Jacobson 2D lead iodide perovskites

3AMP-based 2D perovskites produce higher rigidity octahedral structures through more symmetric hydrogen bonding, ultimately resulting in weaker electron–phonon coupling and lower defect density.

Novel solution-processed 2D organic semiconductor crystals for high-performance OFETs

2D organic semiconductor crystals have the advantage of ultrathin thickness, long-range ordered molecular structures, the absence of grain boundaries, and low defect and impurity densities. They are of great significance for preparing high-performance OFET devices.

Benzoic acid inhibits intrinsic ion migration for efficient and stable perovskite solar cells.

An efficient perovskite solar cell (PSC) has the following characteristics: (1) large perovskite grain size; (2) small ion migration; (3) low defect density states. Here, benzoic acid was employed as an additive to a perovskite solution to improve the thin film quality. Surprisingly, 1.0%-BA can implement all of these features. Therefore, the power conversion efficiency (PCE) of the champion PSC is 18.05%, which is superior to that of the control device (15.42%). In addition, BA-doped PSC kept 86% of its primary PCE after 30 days (RH: 35%), but the control device only retained 75% under the same conditions. The improvement of its stability is because of the inhibition of the cation migration of perovskite by the addition of BA and the passivation of perovskite defects. The results can acquire a better understanding of the potential applications of small organic molecules in improving the PCE and stability of PSC devices.

“Freezing” intermediate phases for efficient and stable FAPbI3 perovskite solar cells

Preparation of low defect density FAPbI3 perovskite thin films by adjusting the process temperature to −20 °C for high-performance perovskite solar cells.

Growth and surface structrue of hydrogen terminal diamond thin films

The conductivity of hydrogen-terminated diamond is a limiting factor in its application in field-effect transistor devices. The traditional preparation process hinders the improvement of the electrical properties of hydrogen-terminated diamond due to impurity elements in the diamond bulk and surface damage caused by processing near the diamond surface. To overcome this, researchers have explored the epitaxial growth of a high-purity and flat-surfaced diamond thin film on a diamond substrate. However, this approach still faces challenges in film characterization and achieving high surface smoothness. In this study, microwave plasma chemical vapor deposition technology is used to epitaxially grow a sub-micron thick diamond film on a nitrogen-doping chemical vapor deposition diamond substrate of 10 mm × 10 mm × 0.5 mm in size. The influence of methane concentration on the growth and conductivity of diamond film is investigated. The test results reveal that the growth thickness of the diamond film ranges from 230 to 810 nm, and the nitrogen concentration in the epitaxial layer is lower than 1×10<sup>16</sup> atom/cm<sup>3</sup>. Three growth modes are observed for the homoepitaxial growth of the diamond thin film under different methane concentrations. A methane concentration of 4% enables two-dimensional planar growth of diamond, resulting in a smooth and flat surface with a roughness of 0.225 nm (10 μm×10 μm). The formation of different surface morphologies is attributed to the growing process and etching process of diamond. Surface low-energy electron diffraction testing indicates that the surface of the diamond film undergoes a structural transition from oxygen terminal (1×1: O) to hydrogen terminal (2×1: H) when grown for a short period of time. X-ray photoelectron spectroscopy analysis reveals an extremely low ratio of oxygen element to nitrogen element, giving the grown diamond film P-type conductivity characteristics. The Hall test results demonstrate that the hydrogen-terminated diamond film grown with a methane concentration of 4% exhibits the highest conductivity, with a square resistance of 4981 Ω/square and a hole mobility of 207 cm<sup>2</sup>/(V·s). This enhanced conductivity can be attributed to the lower defect density observed under these specific conditions. The findings of this study effectively improve the electrical properties of hydrogen-terminated diamond, and contribute to the development and practical application of high-power diamond devices.