Improved crystal quality of GaN film with the in-plane lattice-matched In_(0.17)Al_(0.83)N interlayer grown on sapphire substrate using pulsed metal organic chemical vapor deposition

We report on an improvement in the crystal quality of GaN film with an Ino.17Alo.83N interlayer grown by pulsed metal-organic chemical vapor deposition, which is in-plane lattice-matched to GaN films. The indium composition of about 17% and the reductions of both screw and edge threading dislocations （TDs） in GaN film with the InA1N interlayer are estimated by high resolution X-ray diffraction. Transmission electron microscopy （TEM） measurements are employed to understand the mechanism of reduction in TD density. Raman and photoluminescence measurements indicate that the InA1N interlayer can improve the crystal quality of GaN film, and verify that there is no additional residual stress induced into the GaN film with InA1N interlayer. Atomic force microscopy measurement shows that the InA1N interlayer brings in a smooth surface morphology of GaN film. All the results show that the insertion of the InA1N interlayer is a convenient method to achieve excellent crystal quality in GaN epitaxy.

Enhanced Crystal Quality of Perovskite via Protonated Graphitic Carbon Nitride Added in Carbon-Based Perovskite Solar Cells

The quality of perovskite layers has a great impact on the performance of perovskite solar cells(PSCs).However,defects and related trap sites are generated inevitably in the solutionprocessed polycrystalline perovskite films.It is meaningful to reduce and passivate the defect states by incorporating additive into the perovskite layer to improve perovskite crystallization.Here an environmental friendly 2D nanomaterial protonated graphitic carbon nitride(p-g-C_(3)N_(4))was successfully synthesized and doped into perovskite layer of carbon-based PSCs.The addition of p-g-C_(3)N_(4)into perovskite precursor solution not only adjusts nucleation and growth rate of methylammonium lead tri-iodide(MAPb I3)crystal for obtaining flat perovskite surface with larger grain size,but also reduces intrinsic defects of perovskite layer.It is found that thep-g-C_(3)N_(4) locates at the perovskite core,and the active groups-NH_(2)/NH_(3)and NH have a hydrogen bond strengthening,which effectively passivates electron traps and enhances the crystal quality of perovskite.As a result,a higher power conversion efficiency of 6.61% is achieved,compared with that doped with g-C_(3)N_(4)(5.93%)and undoped one(4.48%).This work demonstrates a simple method to modify the perovskite film by doping new modified additives and develops a low-cost preparation for carbon-based PSCs.

Crystallization tailoring of cesium/formamidinium double-cation perovskite for efficient and highly stable solar cells

Achieving high-quality perovskite crystal films is a critical prerequisite in boosting solar cell efficiency and improving the device stability,but the delicate control of nucleation and growth of the perovskite film remains limited success.Herein,a facile but effective strategy has been developed to finely tailor the crystallization of thermally stable cesium/formamidinium(Cs/FA)based perovskite via partially replacing PbI2 with PbCl2 in the precursor solution.The incorporation of chlorine into the perovskite crystal lattice derived from PbCl2 changes the crystallization process and improves the crystal quality,which further results in the formation of larger crystal grains compared to the control sample.The larger crystal grains with high crystallinity lead to reduced grain boundaries,suppressed non-radiative recombination,and enhanced photoluminescence lifetime.Under the optimized conditions,the methylammonium free perovskite solar cells(PSCs)delivers a champion power conversion efficiency(PCE)of 21.30%with an open-circuit voltage as high as 1.18 V,which is one of the highest efficiencies for Cs/FA based PSCs up to now.Importantly,the unencapsulated PSC devices retain more than 95%and 81%of their original PCEs even after long-term(over one year)storage under ambient conditions or 2000 h’s thermal aging at 850C in a nitrogen atmosphere,respectively.

Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation

We use laser-scanning nonlinear imaging microscopy in atomically thin transition metal dichalcogenides(TMDs)to reveal information on the crystalline orientation distribution,within the 2D lattice.In particular,we perform polarization-resolved second-harmonic generation(PSHG)imaging in a stationary,raster-scanned chemical vapor deposition(CVD)-grown WS2 flake,in order to obtain with high precision a spatially resolved map of the orientation of its main crystallographic axis(armchair orientation).By fitting the experimental PSHG images of sub-micron resolution into a generalized nonlinear model,we are able to determine the armchair orientation for every pixel of the image of the 2D material,with further improved resolution.This pixel-wise mapping of the armchair orientation of 2D WS2 allows us to distinguish between different domains,reveal fine structure,and estimate the crystal orientation variability,which can be used as a unique crystal quality marker over large areas.The necessity and superiority of a polarization-resolved analysis over intensity-only measurements is experimentally demonstrated,while the advantages of PSHG over other techniques are analysed and discussed.

Diamond growth in a high temperature and high pressure Fe–Ni–C–Si system:Effect of synthesis pressure

Pressure is one of the necessary conditions for diamond growth.Exploring the influence of pressure on growth changes in silicon-doped diamonds is of great value for the production of high-quality diamonds.This work reports the morphology,impurity content and crystal quality characteristics of silicon-doped diamond crystals synthesized under different pressures.Fourier transform infrared spectroscopy shows that with the increase of pressure,the nitrogen content in the C-center inside the diamond crystal decreases.X-ray photoelectron spectroscopy test results show the presence of silicon in the diamond crystals synthesized by adding silicon powder.Raman spectroscopy data shows that the increase in pressure in the Fe-Ni-C-Si system shifts the Raman peak of diamonds from 1331.18 cm^(-1)to 1331.25 cm^(-1),resulting in a decrease in internal stress in the crystal.The half-peak width decreased from 5.41 cm^(-1)to 5.26 cm^(-1),and the crystallinity of the silicon-doped diamond crystals improved,resulting in improved quality.This work provides valuable data that can provide a reference for the synthesis of high-quality silicon-doped diamonds.

Study on the relationships between Raman shifts and temperature range for α-plane GaN using temperature-dependent Raman scattering

In this paper,Raman shifts of a-plane GaN layers grown on r-plane sapphire substrates by low-pressure metal-organic chemical vapor deposition（LPMOCVD） are investigated.We compare the crystal qualities and study the relationships between Raman shift and temperature for conventional a-plane GaN epilayer and insertion AlN/AlGaN superlattice layers for a-plane GaN epilayer using temperature-dependent Raman scattering in a temperature range from 83 K to 503 K.The temperature-dependences of GaN phonon modes（A1（TO）,E2（high）,and E1（TO）） and the linewidths of E2（high） phonon peak are studied.The results indicate that there exist two mechanisms between phonon peaks in the whole temperature range,and the relationship can be fitted to the pseudo-Voigt function.From analytic results we find a critical temperature existing in the relationship,which can characterize the anharmonic effects of a-plane GaN in different temperature ranges.In the range of higher temperature,the relationship exhibits an approximately linear behavior,which is consistent with the analyzed results theoretically.

Effects of oxygen/nitrogen co-incorporation on regulation of growth and properties of boron-doped diamond films

Regulation with nitrogen and oxygen co-doping on growth and properties of boron doped diamond films is studied by using laughing gas as dopant. As the concentration of laughing gas(N2O/C) increases from 0 to 10%, the growth rate of diamond film decreases gradually, and the nitrogen-vacancy(NV) center luminescence intensity increases first and then weakens. The results show that oxygen in laughing gas has a strong inhibitory effect on formation of NV centers, and the inhibitory effect would be stronger as the concentration of laughing gas increases. As a result, the film growth rate and nitrogen-related compensation donor decrease, beneficial to increase the acceptor concentration(~3.2×10^(19)cm^(-3)) in the film. Moreover, it is found that the optimal regulation with the quality and electrical properties of boron doped diamond films could be realized by adding appropriate laughing gas, especially the hole mobility(~700cm^(2)/V·s), which is beneficial to the realization of high-quality boron doped diamond films and high-level optoelectronic device applications in the future.

Effect of oxygen on regulation of properties of moderately boron-doped diamond films

Regulation of oxygen on properties of moderately boron-doped diamond films is fully investigated.Results show that,with adding a small amount of oxygen(oxygen-to-carbon ratio<5.0%),the crystal quality of diamond is improved,and a suppression effect of residual nitrogen is observed.With increasing ratio of O/C from 2.5%to 20.0%,the hole concentration is firstly increased then reduced.This change of hole concentration is also explained.Moreover,the results of Hall effect measurement with temperatures from 300 K to 825 K show that,with adding a small amount of oxygen,boron and oxygen complex structures(especially B_(3)O and B_(4)O)are formed and exhibit as shallow donor in diamond,which results in increase of donor concentration.With further increase of ratio of O/C,the inhibitory behaviors of oxygen on boron leads to decrease of acceptor concentration(the optical emission spectroscopy has shown that it is decreased with ratio of O/C more than 10.0%).This work demonstrates that oxygen-doping induced increasement of the crystalline and surface quality could be restored by the co-doping with oxygen.The technique could achieve boron-doped diamond films with both high quality and acceptable hole concentration,which is applicable to electronic level of usage.

Low temperature photoluminescence study of Ga As defect states

Low temperature(77 K)photoluminescence measurements have been performed on different GaAs substrates to evaluate the GaAs crystal quality.Several defect-related luminescence peaks have been observed,including 1.452 eV,1.476 eV,1.326 eV peaks deriving from 78 meV GaAs antisite defects,and 1.372 eV,1.289 eV peaks resulting from As vacancy related defects.Changes in photoluminescence emission intensity and emission energy as a function of temperature and excitation power lead to the identification of the defect states.The luminescence mechanisms of the defect states were studied by photoluminescence spectroscopy and the growth quality of GaAs crystal was evaluated.

Epitaxial growth of large-area and highly crystalline anisotropic ReSe2 atomic layer

The anisotropic two-dimensional （2D） layered material rhenium disulfide （ReSe2） has attracted considerable attention because of its unusual properties and promising applications in electronic and optoelectronic devices. However, because of its low lattice symmetry and interlayer decoupling, anisotropic growth and out-of-plane growth occur easily, yielding thick flakes, dendritic structure, or flower-like structure. In this stud34 we demonstrated a bottom-up method for the controlled and scalable synthesis of ReSe2 by van der Waals epitaxy. To achieve controllable growth, a micro-reactor with a confined reaction space was constructed by stacking two mica substrates in the chemical vapor deposition system. Within the confined reaction space, the nucleation density and growth rate of ReSe2 were significantly reduced, favoring the large-area synthesis of ReSe2 with a uniform monolayer thickness. The morphological evolution of ReSe2 with growth temperature indicated that the anisotropic growth was suppressed at a low growth temperature （〈600 ℃）. Field-effect transistors employing the grown ReSe2 exhibited p-type conduction with a current ON/OFF ratio up to 10s and a hole carrier mobility of 0.98 cm^2/（V·s）. Furthermore, the ReSe2 device exhibited an outstanding photoresponse to near-infrared light, with responsivity up to 8.4 and 5.1 A/W for 850- and 940-nm light, respectively. This work not only promotes the large-scale application of ReSe2 in high-performance electronic devices but also clarifies the growth mechanism of low-lattice symmetry 2D materials.

The effect of AlN/AlGaN superlattices on crystal and optical properties of AlGaN epitaxial layers

We investigate the effect of AlN/AlGaN superlattices（SLs） on crystal and optical properties of AlGN epitaxial layers. The result indicates that the crystal quality of AlGaN layers is consistent within a wide range of SLs thicknesses, while the optical properties are opposite. With SLs thickness decreasing from 20/44 to 17/36 and 15/29 nm, the full-width at half maximum of X-ray rocking curves for （0002）- and（1012）-plane of n-AlGaN layers grown on SLs are consistent of around 250 arcsec and 700 arcsec, respectively. Meanwhile, the center of the low optical transmittance band decreases from 326 to 279 nm and less than 266 nm as the SLs thickness decreases.280 nm deep ultraviolet light-emitting diodes（DUV-LEDs） structures are further regrown on the n-AlGaN layers.The electroluminescent intensities of samples are 30% higher than that of the sample whose low optical transmittance band appears around 279 nm. Optical simulations reveal that the SLs acts as distributed Bragg reflectors, thus less photons of the corresponding wavelength escape from the sapphire backside.

Ultrahigh-resolution nonlinear optical imaging of the armchair orientation in 2D transition metal dichalcogenides

We used nonlinear laser scanning optical microscopy to study atomically thin transition metal dichalcogenides(TMDs)and revealed,with unprecedented resolution,the orientational distribution of armchair directions and their degree of organization in the two-dimensional(2D)crystal lattice.In particular,we carried out polarization-resolved second-harmonic generation(PSHG)imaging for monolayer WS2 and obtained,with high-precision,the orientation of the main crystallographic axis(armchair orientation)for each individual 120×120 nm^(2) pixel of the 2D crystal area.Such nanoscale resolution was realized by fitting the experimental PSHG images,obtained with sub-micron precision,to a new generalized theoretical model that accounts for the nonlinear optical properties of TMDs.This enabled us to distinguish between different crystallographic domains,locate boundaries and reveal fine structure.As a consequence,we can calculate the mean orientational average of armchair angle distributions in specific regions of interest and define the corresponding standard deviation as a figure-of-merit for the 2D crystal quality.

Study on growing thick AlGaN layer on c-plane sapphire substrate and free-standing GaN substrate

In this study,the thick AlGaN epilayers have been grown on the c-plane sapphire substrate and the free-standing GaN substrate using low-temperature AlN nucleation layers by low-pressure metal-organic chemical vapor deposition(LPMOCVD).High resolution X-ray diffraction(HRXRD),atom force microscopy(AFM),scanning electron microscopy(SEM),photoluminescence(PL) and Raman scattering measurements have been employed to study the crystal quality,threading dislocation density,surface morphology,optical properties and phonon properties of thick AlGaN epifilms.The results indicate that AlGaN epifilms crystal quality can be improved greatly when grown on the free-standing GaN substrate.We calculated the threading dislocation density and found that thick AlGaN epifilm grown on the free-standing GaN substrate is much lower in total threading dislocation density than that grown on the sapphire substrate,although the surface morphology is rougher than that of sapphire substrate.

MOCVD epitaxy of InAlN on different templates

InAlN epilayers were grown on high quality GaN and A1N templates with the same growth parameters. Measurement results showed that two samples had the same In content of～16%,while the crystal quality and surface topography of the InAlN epilayer grown on the AlN template,with 282.3＂（002） full width at half maximum （FWHM） of rocking curve,313.5＂（102） FWHM,surface roughness of 0.39 nm and V-pit density of 2.8×10~8 cm^（-2）,were better than that of the InAlN epilayer grown on the GaN template,309.3＂,339.1＂,0.593 nm and 4.2×10~8 cm^（-2）.A primary conclusion was proposed that both the crystal quality and the surface topography of the InAlN epilayer grown on the AlN template were better than that of the InAlN epilayer grown on the GaN template. Therefore,the AlN template was a better choice than the GaN template for getting high quality InAlN epilayers.