Crystal Gallium Research Articles

A distinctive material removal mechanism in the diamond grinding of (0001)-oriented single crystal gallium nitride and its implications in substrate manufacturing of brittle materials

Single crystal gallium nitride (GaN) substrates are highly demanded for fabricating advanced optoelectronic devices. It is thus essential to develop high efficiency machining technologies for this difficult-to-machine material, which in turn necessitates a thorough understanding of its deformation mechanism. In this study, the deformation and removal characteristics of (0001)-oriented single crystal GaN involved in diamond grinding were systematically investigated. The material removal exhibited a brittle mode when using relatively coarse diamond abrasives of 2000 in mesh size, while ductile removal was achieved when diamond abrasives of 6000 in mesh size were utilized. A novel peeling phenomenon was observed along (0001) lattice plane (c-plane) in the coarse grinding, as the crystal has a hexagonal crystal structure with c-planes serving as the preferable slip/cracking planes. Peeling observed in material removal agrees well with the findings that lateral planar defects were prone to initiate in nanoscratching in comparison to nanoindentation in the ductile regime, indicating that the effect of tangential grinding force is significant. The application of Molecular dynamics (MD) simulations, employing smaller indentation/scratching models, provided additional confirmation of the crucial role played by lateral force in initiating planar defects on c-planes. Furthermore, larger-scale MD scratching models substantiated the occurrence of peeling in the deformation process on c-plane, a finding corroborated by scratching experiments conducted in the brittle regime. Conversely, such peeling is absent on m- and a-planes. Complementary to the simulations, specifically designed grinding experiments were conducted to empirically demonstrate that peeling phenomena were intensified with elevated rotational wheel speeds. This enhancement was attributed to the increased tangential grinding force associated with higher speeds. These findings contribute to a comprehensive understanding of the intricate relationship between rotational wheel speed, tangential grinding force, and the observed peeling mechanisms in the context of single crystal GaN machining.

Unveiling the temperature-dependent deformation mechanisms of single crystal gallium nitride in nanoscratching

The surface grinding process of single crystal gallium nitride (GaN) is intricate, and localized high temperature generated during interactions between the workpiece and the abrasives on grinding wheel is inevitable. Elevated temperatures significantly affect the deformation mechanisms of GaN, which remains inadequately elucidated. In this study, nanoscratching experiments were systematically conducted on the (0001) plane of GaN single crystal across a temperature range up to 500 ℃, and the resultant deformation patterns were thoroughly characterized. The results indicate that temperature elevation delays the brittle-to-ductile transitions in GaN and enhance its anisotropy. Additionally, GaN exhibits increased plasticity as temperature rises, leading to distinct scratch-induced subsurface deformation patterns at varying temperatures: at room temperature, the thickness of the subsurface damaged layer is primarily influenced by the penetration depth of cross slips, whereas at 500 ℃, basal slipping predominantly defines the thickness of the subsurface damaged layer. The plastic deformation patterns at both room temperature and 500 ℃ mainly involve phase transition, polycrystal formation, slips, stacking faults, dislocations and lattice distortions. The discrepancies in deformations at varying temperatures were thoroughly investigated by transmission electron microscopy and molecular dynamics simulations, and the underlying mechanisms were elucidated.

Anisotropic fracture energy and toughness of single crystal gallium arsenide by microindentation

Although single crystal gallium arsenide (GaAs) is a technologically important semiconductor material, its mechanical behavior is poorly understood. In this study, the Vickers microindentation technique was leveraged with a quantitative fractographic approach to determine the fracture energy and toughness corresponding to various crystal planes. Cracks preferentially propagated along the {110} planes associated with the primary crack system {110} / 〈110〉. The normalized fracture toughness was used to calculate the directional fracture energy on the {001} plane and was consistent with a theoretical model based on calculating the number of broken bonds.

Acoustic Studies of the Melting and Crystallization of Eutectic Gallium–Silver Alloys in Porous Glasses

Acoustic Studies of the Melting and Crystallization of Eutectic Gallium–Silver Alloys in Porous Glasses

Redefined interface error, 2D verification and validation for pure solid-gallium phase change modeling by enthalpy-porosity methodology

For simulating phase change process of pure solid gallium, previously published literature contains some key problems, including the inappropriate definition & computation for interface errors, and the unsatisfactory numerical verifications & validations. Herein, the 2D verifications and 2D validations for the enthalpy-porosity modeling of pure gallium melting starting from a vertical end wall have been reported comprehensively. The well-known and classic experimental data presented in literature are used to verify and validate the model. The groundbreaking contributions in this work are that: i) The definition and computational method of the interface-position errors between the numerical and experimental results are revisited and proven carefully; ii) The numerical schemes as well as major parameters of 2D modeling gallium melting are selected with overall interface error <9%. However, these schemes and parameters are not appropriate for the solidification modeling of gallium due to the irregular and non-reproducible interface shapes caused by highly anisotropic properties of solid crystal gallium as experimentally reported in literature; iii) The 2D validations of enthalpy-porosity modeling of gallium melting are investigated in detail, and the maximum interface error is within the limits of ±12%. Moreover, the simulated liquid volume fractions and dimensionless heat transfer coefficients are well correlated in terms of related criteria parameters, and they are compared with correlations available in literature. It is concluded that to solidly validate the modeling of pure metal melting, both the global parameters (e.g., volume-averaged liquid fraction) and the local parameters (e.g., interface position) need to be considered together.

Acoustic Studies of the Melting and Crystallization of Eutectic Gallium–Silver Alloys in Porous Glasses

The paper presents the results of acoustic studies of the melting and crystallization of Ag–Ga alloys with a silver content of 1.5 and 3 at % embedded into porous glasses with an average pore size of 13 nm. The temperature dependences of the velocity of longitudinal ultrasonic waves are measured by a modified pulse-phase method at a frequency of 7 MHz in the 200–325 K range for complete and partial cooling–heating cycles. The temperature dependences of the ultrasonic velocity showed regions corresponding to phase transitions. Significant changes in the phase diagram of the bulk alloy due to nanostructuring have been revealed. It is shown that segregates with different crystal structures are formed in the pores for alloy of different compositions.

Thermal conductivity of wurtzite gallium nitride

This paper reviews the theoretical and experimental works concerning one of the most important parameters of wurtzite gallium nitride – thermal conductivity. Since the heat in gallium nitride is transported almost exclusively by phonons, its thermal conductivity has a temperature behavior typical of most nonmetallic crystals: the thermal conductivity increases proportionally to the third power of temperature at lower temperatures, reaches its maximum at approximately 1/20 of the Debye temperature and decreases proportionally to temperature at higher temperatures. It is shown that the thermal conductivity of gallium nitride (depending on fabrication process, crystallographic direction, concentration of impurity and other defects, isotopical purity) varies significantly, emphasizing the importance of determining this parameter for the samples that closely resemble those being used in specific applications. For isotopically pure undoped wurtzite gallium nitride, the thermal conductivity at room temperature has been estimated as high as 5.4 W/(cm·K). The maximum room temperature value measured for bulkshaped samples of single crystal gallium nitride has been 2.79 W/(cm·K).

Analysis of crack initiation load and stress field in double scratching of single crystal gallium nitride

Precision processing such as grinding of single crystal gallium nitride is processed through complex interactions between many diamond particles. However, single-grain scratching can only study the basic performance of abrasive grain processing, and cannot study the mutual influence of multi-abrasive grain processing. Therefore, this paper studies the situation of double scratching. The influence of the distance between two scratches on the crack initial load has been studied by the analysis model during simultaneous double scratching and continuous double scratching. During continuous double scratching, when the separation distance L < 1.8b, lateral cracks on adjacent side of the second scratch nucleate and expand under a smaller initial load than single-indenter scratching, and the smaller the L is, the smaller the corresponding initial load of lateral cracks will be. When the separation distance is 0.4b and 0.8b, the median crack nucleates at the middle of two scratches. During continuous double scratching, the median cracks always nucleate and expand at the bottom of the second scratch.

Deformation and removal mechanism of single crystal gallium nitride in nanoscratching

Understanding the deformation mechanism of a brittle material under mechanical loading is of value for unveiling the material removal in an abrasive machining process. In this work, instrumented nanoscratching was performed on the (0001) plane of single crystal gallium nitride (GaN) using a conical diamond indenter and the scratch-induced surface/subsurface deformation patterns were characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A finite element model of scratching was established to relate stress distribution and deformation patterns. The stress threshold for the elastic-plastic deformation transition of the crystal was found being relatively high in comparison to single crystal silicon and gallium oxide. The plastic deformation in the GaN sublayer was dominated by slip, dislocation and lattice distortion under relatively low normal loads, but those defects were accompanied with grain rotation when the load was sufficiently high. The thickness of the damage layer was directly related to the ratio of lateral force to normal force during scratching. Such a force ratio can be used as an indicator for controlling the damage layer thickness during abrasive machining, thus providing a valuable guidance for developing the optimal parameters of an ultraprecision abrasive machining process.

Friction and wear mechanisms for single crystal GaN based on an electro-Fenton enhanced chemical reaction

Electro-Fenton polishing is a chemical mechanical polishing (CMP) technology with enhanced chemical reaction, which hydroxyl radicals (•OH) can be efficiently and controllably generated by using an electric Fenton method. The oxidation and corrosion of single crystal gallium nitride (GaN), and the synergistic mechanism of chemical and mechanical effects in the electro-Fenton polishing process are key to the electro-Fenton polishing of GaN. This article evaluated the effects of different media (electrolyte, H2O2, Fenton, and electro-Fenton) and different conditions (H2O2 mass fraction, means of H2O2 addition, cathode potential, Fe–C catalyst mass fraction, and pH) through ball-disk friction and wear tests. The results showed that electro-Fenton media demonstrated the best oxidation and corrosion effects on the GaN surface among the four media. In the electro-Fenton slurries, the oxidation was very sensitive to the H2O2 mass fraction which showed that 5 wt % H2O2 was the optimal mass fraction; the oxidation efficiency was greatly improved by regularly supplementing H2O2. The friction and wear behavior of GaN exhibited a good response to the cathode potential with the optimal cathode potential being −0.5 V. Meanwhile, the best Fe–C catalyst mass fraction was 3 wt % and the optimal pH was 2. In addition, the friction and wear behavior were investigated, and the energy spectrum tests was carried out to verify the effect of the chemical reaction. The synergistic mechanism of chemical action and mechanical removal in the friction and wear process was revealed. This research provides theoretical support for on-line electro-Fenton polishing technology.

Study on the Size Effect of Single Crystal Gallium Oxide During the Nanoindentation Process

In order to accurately define the nanomechanical performance parameters of single crystal gallium oxide, the microscopic deformation mechanism during processing is analyzed. On the G200 nanoindenter, the Berkovich indenter made of diamond was used to perform nanoindentation experiments on single crystal gallium oxide (100) and (010) crystal planes by using a quasi-static method to set the load. According to the power law relationship and Nix-Gao model proposed by Manika, the relationship between the hardness of the two crystal faces of single crystal gallium oxide and the depth of indentation is fitted, and the single crystal gallium oxide (100) and (010) crystal faces without considering the size influence factor. The hardness is 8.238[Formula: see text]GPa and 9.824[Formula: see text]GPa, the micro feature length is 101.196[Formula: see text]nm and 88.033[Formula: see text]nm, and the size influence factor is 0.1725 and 0.1706, respectively. The size influencing factors of the two crystal planes are between common metal materials and semiconductor materials. Compared with the (010) crystal plane, the (100) plane exhibits a larger elastic modulus and lower hardness. (100) The indentation size effect of the crystal plane is more obvious.

Research on material removal mechanism and radial cracks during scribing single crystal gallium nitride

A diamond conical indenter with the cone angle of 60° and a tip arc radius of 5 μm was used to perform the gradual loading scribing experiments on gallium nitride (0001) crystal plane along the (Li et al., 2019; Wang et al., 2018; Yao et al., 2020; Fang and Zhang, 2013; Feng et al., 2009; Zhang et al., 2007; Jing et al., 2007; Yonenaga et al., 2018; Nowak et al., 1999; Shimada et al., 1998) [11-20] crystal orientation, and the material removal mechanism and machining properties of gallium nitride are studied. Single crystal gallium nitride will have the surface and sub-surface damage during machining, and the study of the stress field during scribing can effectively analyze and control the surface and sub-surface damage. Therefore, the stress field for scribing gallium nitride is acquired by superposing the boussinesq field, cerruti field and sliding blister field which have been led-in anisotropy parameters. Then on the basis of the distribution of the maximum tensile stress on the scribing surface, the nucleation and initial spread angle of radial cracks under our experimental conditions are analyzed, and for gallium nitride, the more severe the brittle spalling accompanying radial crack spread is, the greater the initial spread angle of radial cracks is.

An atomistic investigation on the wear of diamond during atomic force microscope tip-based nanomachining of gallium arsenide

This paper investigated the wear mechanism of diamond during the atomic force microscope (AFM) tip-based nanomachining of Gallium Arsenide (GaAs) using molecular dynamics (MD) simulations. The elastic–plastic deformation at the apex of the diamond tip was observed during the simulations. Meanwhile, a transition of the diamond tip from its initial cubic diamond lattice structure sp3 hybridization to graphite lattice structure sp2 hybridization was revealed. Graphitization was, therefore, found to be the dominant wear mechanism of the diamond tip during the nanometric cutting of single crystal gallium arsenide for the first time. The various stress states, such as hydrostatic stress, shear stress, and von Mises stress within the diamond tip and the temperature distribution of the diamond tip were also estimated to find out the underlying mechanism of graphitization. The results showed that the cutting heat during nanomachining of GaAs would mainly lead to the graphitization of the diamond tip instead of the high shear stress-induced transformation of the diamond to graphite. The paper also proposed a new approach to quantify the graphitization conversion rate of the diamond tip.

An analytical model for estimation of the stress field and cracks caused by scratching anisotropic single crystal gallium nitride

Single crystal gallium nitride will produce subsurface damage during processing, and studying the total stress field during scratching gallium nitride is advantageous for predicting and controlling subsurface damage. The total stress field during scratching gallium nitride (0001) crystal plane is obtained by superimposing the elastic and residual stress field. Thus the distribution of the stress field under the indenter can be acquired. It was found that the median crack nucleated at the elastoplastic interface of the front of the indenter, and the extension angle of the median crack is linearly related to half-angle of the conical indenter. Next the anisotropic model in this paper and the method simplified to isotropic materials are used for calculating the total stress field, and the consequences of the two are obviously different. The ratio between the stresses obtained by the two methods under different indenter half-angle has been obtained. Then by introducing the corresponding ratio into the traditional equation of the median crack depth and the lateral crack depth, the results considering the anisotropy can be obtained by the isotropic method.

MHz-repetition-rate, sub-mW, multi-octave THz wave generation in HMQ-TMS

We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10-4 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm-2 at 10 MHz with a 1/e2 pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm-1 and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications.

Luminescence and Nonlinear Optical Properties of Borates LnGa3(BO3)4 (Ln = Nd, Sm, Tb, Er, Dy, or Ho)

Luminescence spectra of single crystals of rare-earth gallium borates LnGa3(BO3)4 (Ln = Nd, Sm, Tb, Er, Dy, or Ho) at room (300 K) and cryogenic (10 K) temperatures are presented for the first time. Photoluminescence has been recorded in the wavelength range of 470–5000 nm (2000–21 300 cm–1) with a high spectral resolution (down to 0.1 cm–1) upon excitation by different diode lasers. The spectra obtained cannot be unambiguously interpreted within one luminescent center, which can be due to the presence of defects and/or inclusions of other crystalline phases. The optical nonlinearity of rare-earth–gallium borates has been estimated using the Kurtz–Perry powder technique. The typical intensities of the second-harmonic generation in gallium borate powders are 30–40 (with respect to quartz), and the optical nonlinearity is as good as the nonlinearity of the efficient rare-earth aluminum borate YAl3(BO3)4.

Structure dependence of GaN-based nanopillar-shaped crystals grown on a quartz glass substrate on their growth conditions

Nanopillar-shaped crystals of gallium–nitride-based semiconductors were grown on a quartz glass substrate using molecular beam epitaxy apparatus. Two sets of radio-frequency plasma-assisted active nitrogen sources were simultaneously operated and almost same amounts of indium and gallium were supplied during the growths. The effects of their growth conditions, i.e., the amount of the indium supply, flow rate of nitrogen gas, and growth temperature, on the structures of the nanopillar-shaped crystals were investigated.

Люминесцентные и нелинейно-оптические свойства боратов LnGa-=SUB=-3-=/SUB=-(BO-=SUB=-3-=/SUB=-)-=SUB=-4-=/SUB=- (Ln = Nd, Sm, Tb, Er, Dy, Ho)

AbstractLuminescence spectra of single crystals of rare-earth gallium borates LnGa_3(BO_3)_4 (Ln = Nd, Sm, Tb, Er, Dy, or Ho) at room (300 K) and cryogenic (10 K) temperatures are presented for the first time. Photoluminescence has been recorded in the wavelength range of 470–5000 nm (2000–21 300 cm^–1) with a high spectral resolution (down to 0.1 cm^–1) upon excitation by different diode lasers. The spectra obtained cannot be unambiguously interpreted within one luminescent center, which can be due to the presence of defects and/or inclusions of other crystalline phases. The optical nonlinearity of rare-earth–gallium borates has been estimated using the Kurtz–Perry powder technique. The typical intensities of the second-harmonic generation in gallium borate powders are 30–40 (with respect to quartz), and the optical nonlinearity is as good as the nonlinearity of the efficient rare-earth aluminum borate YAl_3(BO_3)_4.

Structural connection between gallium crystals and near-Tm liquids under ambient pressure

We study the short-range structural order in liquid gallium prepared by ab initio molecular dynamics simulations. We found that at 400K, which is close to the melting point (Tm) of Ga, the dominant motif in the liquid phase is identical to that in the stable solid phase Ga-I. Strong directional bonding in this motif prevents it from nucleation in the liquid phase. Meanwhile, a newly identified motif, topologically distinct from yet related to those in β-Ga and Ga-III phases, is also abundant in Ga liquid. This new motif could serve as precursors for β-Ga or Ga-III during crystallization.

Fabrication of Single Crystal Gallium Phosphide Thin Films on Glass

Due to its high refractive index and low absorption coefficient, gallium phosphide is an ideal material for photonic structures targeted at the visible wavelengths. However, these properties are only realized with high quality epitaxial growth, which limits substrate choice and thus possible photonic applications. In this work, we report the fabrication of single crystal gallium phosphide thin films on transparent glass substrates via transfer bonding. GaP thin films on Si (001) and (112) grown by MOCVD are bonded to glass, and then the growth substrate is removed with a XeF2 vapor etch. The resulting GaP films have surface roughnesses below 1 nm RMS and exhibit room temperature band edge photoluminescence. Magnesium doping yielded p-type films with a carrier density of 1.6 × 1017 cm−3 that exhibited mobilities as high as 16 cm2V−1s−1. Due to their unique optical properties, these films hold much promise for use in advanced optical devices.