Growth of AlN single crystalline boules

We have obtained high-quality, crack-free AlN wafers using a convex thermal field inside the growth chamber. Free-standing AlN boules of 15 mm in height and 15 mm in diameter were grown. The carbon concentration was found to be similar in all parts of the boule (∼8×10¹⁸ cm⁻³) while the initial O concentration was higher (∼1×10¹⁹ cm⁻³) and slightly decreased during growth. It was found that O incorporated differently on different crystallographic faces. High resolution XRD showed a continuous improvement in crystal quality as a function of boule length. The full width at half maximum (FWHM) of the double crystal rocking curves decreased from 78 in at the beginning of growth to 13 in at the growth end. To the best of our knowledge, this is the first report on impurity incorporation on different crystallographic facets obtained from the same boule.     

Submicron rectangular hollow tube crystals of rutile-type GeO2

Single crystals of rutile-type GeO₂ having a structure equivalent to that of TiO₂, a well-known photocatalyst, have been grown for the first time in supercritical oxygen at approximately 5 GPa and 3000 K. The obtained crystals exhibit a rectangular hollow tube structure with submicron size (cross section with sides of ∼500 nm, wall thickness of ∼20 nm, and longitudinal length of ∼5 μm). These single crystals were grown within 1 s and along the c-axis surrounded by the (1 1 0) faces. The crystal growth mechanism strongly depends on the growth mechanism of rutile-type oxides, and the extremely short growing time is an important factor in the formation of hollow tube crystals.     

Czochralski-growth of germanium crystals containing high concentrations of oxygen impurities

Oxygen-containing germanium (Ge) single crystals with low density of grown-in dislocations were grown by the Czochralski (CZ) technique from a Ge melt, both with and without a covering by boron oxide (B₂O₃) liquid. Interstitially dissolved oxygen concentrations in the crystals were determined by the absorption peak at 855 cm⁻¹ in the infrared absorption spectra at room temperature. It was found that oxygen concentration in a Ge crystal grown from melt partially or fully covered with B₂O₃ liquid was about 10¹⁶ cm⁻³ and was almost the same as that in a Ge crystal grown without B₂O₃. Oxygen concentration in a Ge crystal was enhanced to be greater than 10¹⁷ cm⁻³ by growing a crystal from a melt fully covered with B₂O₃; with the addition of germanium oxide powder, the maximum oxygen concentration achieved was 5.5×10¹⁷ cm⁻³. The effective segregation coefficients of oxygen in the present Ge crystal growth were roughly estimated to be between 1.0 and 1.4.     

Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H₂ and NH₃, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10⁸ cm⁻².     

Synthesis,growth and characterization of organic nonlinear optical bis-glycine maleate (BGM) single crystals

A new organic compound of bis-glycine maleate was synthesized in the alkaline medium of 10% ammonium hydroxide solution. The bulk single crystals of Bis-Glycine Maleate (BGM) have been grown by slow cooling method. The grown crystals were characterized by employing single crystal and powder X-ray diffraction, Fourier transform infrared, optical absorption spectral studies and thermo gravimetric analysis. The microhardness studies confirmed that the BGM has a fairly high Vicker’s hardness number value (41 kg mm⁻²) in comparison to other organic NLO crystals. Second harmonic generation efficiency of the crystal measured by Kurtz–Perry powder method using Nd:YAG laser is found to be comparable to that of potassium dihydrogen phosphate (KDP). Frequency dependent dielectric studies were carried out along the major growth axis.     

Growth and Raman spectroscopic characterization of As4S4 (II) single crystals

As described by Kutoglu (1976 [16]), single crystals of As₄S₄ (II) phase have been grown using a new two-step synthesis that drastically increases the reproducibility that is attainable in synthetic experiments. First, through photo-induced phase transformation, pararealgar powder is prepared as a precursor instead of AsS melt. Then it is dissolved and recrystallized from CS₂ solvent. Results show that single crystals of the As₄S₄ (II) phase were obtained reproducibly through the dissolution–recrystallization process. Single crystals of As₄S₄ (II) obtained using this method were translucent and showed a uniform yellow-orange color. The crystal exhibits a platelet-like shape as a thin film with well-developed faces (0 1 0) and (0 1¯ 0). The grown crystals are as large as 0.50×0.50×0.01 mm. They were characterized using powder and single crystal X-ray diffraction techniques to confirm the phase identification and the lattice parameters. The As₄S₄ (II) phase crystallizes in monoclinic system with cell parameters a=11.202(4) Å, b=9.954(4) Å, c=7.142(4) Å, β=92.81(4)°, V=795.4(6) Å³, which shows good agreement with the former value. Raman spectroscopic studies elucidated the behavior of the substance and the relation among phases of tetra-arsenic tetrasulfide.     

Improvement of crystal quality in ammonothermal growth of bulk GaN

Several key improvements in crystal quality of bulk GaN grown by the ammonothermal method are presented. Full width at half maximum of (0 0 2) X-ray rocking curve was reduced to 53 and 62 arcsec for Ga-side and N-side, respectively. Transparent bulk GaN crystal was also demonstrated. Oxygen and sodium concentrations were reduced to mid-10¹⁸ and mid-10¹⁵ cm⁻³, respectively. We are currently searching for a growth condition that produces transparent bulk GaN with high structural quality and low impurities. Small-sized, semi-transparent GaN wafers were fabricated by slicing the grown bulk GaN crystals, which demonstrate the high feasibility of ammonothermal growth for production of GaN wafers.     

Large-size and high-quality Zn2TiO4 single crystal grown by the optical floating zone method

Crack-free and transparent Zn₂TiO₄ single crystals of 4–6 mm in diameter and 30 mm in length have been grown by the optical floating zone method. The powder X-ray diffraction (XRD) results show that the as-grown crystals have the spinel-type Zn₂TiO₄ structure. XRD² measurements on Zn₂TiO₄ wafers cut perpendicular to the growth direction display only one peak at 42.7°, which indicates that the Zn₂TiO₄ single crystals grow along the 〈4 0 0〉 direction (a-axis). The formation of bubble inclusions was effectively suppressed by lowering rotation rate. Transmission polarized-light microscopy results showed that as-grown crystals were free of low angle grain boundaries.     

Unidirectional growth of l-asparagine monohydrate single crystal: First time observation of NLO nature and other studies of crystalline perfection,optical, mechanical and dielectric properties

L-asparagine monohydrate (LAM), a new amino acid single crystal, was grown by slow evaporation solution technique (SEST) as well as by recently invented Sankaranarayanan–Ramasamy (SR) method in aqueous medium. Using SR method, LAM single crystal of diameter ∼18 mm and length ∼52 mm was grown for the first time. The growth conditions were optimized and the maximum growth rate of 1.0 mm per day was observed for the SR crystal. The crystal structure was confirmed by powder XRD. The crystalline perfection was assessed by high resolution XRD and etching studies and found that the quality of the SR crystal is better than the SEST crystal. The UV–vis–NIR spectroscopic study revealed that the SR crystal has good optical transparency than that of SEST crystal. The relative second harmonic generation efficiency was measured and found to be ∼0.35 times to that of KDP. The laser damage threshold (LDT) was measured and found that the SR crystal has higher LDT value (5.76 GW cm⁻²) than SEST crystal (4.75 GW cm⁻²). The Vickers’s microhardness and dielectric studies were also carried out and discussed.     

Growth of ZnO crystal by self-flux method using Zn solvent

Zinc oxide (ZnO) single crystal was grown, for the first time, by the self-flux method using a high-purity zinc solvent saturated with oxygen. ZnO crystals were successfully grown in the solvent by moving a growing crystal slowly, at about 17 mm/day along a temperature gradient (1.5 °C/mm) in a furnace, from 1050 to 500 °C. Hexagonal single crystals 1–3 mm in length and 0.5–1 mm in thickness were obtained. Energy-dispersive X-ray diffraction, secondary ion mass spectrometry, and photoluminescence confirmed that the purity and crystallinity of the ZnO crystals grown in this work were high.     

Hydrothermal crystal growth of fresnoite

The growth of fresnoite, Ba₂TiSi₂O₈, by hydrothermal synthesis has led to spontaneous generation of large, (4-5 mm) optically clear crystals from 6 M KF mineralizer solutions. Growth was achieved at relatively low synthesis temperatures (575 °C) comparative to fresnoite synthesis by Czochralski or flux methods. Bulk crystal growth possibilities were explored by transport reactions performed in both fluoride and hydroxide mineralizers with 25-45 °C temperature gradients. Growth rates of 0.14×0.19×0.22 mm³/week were established in 6 M KOH, which is significantly slower than standard hydrothermal rates of 1 mm/week. Although relatively slow, the hydrothermal method has been demonstrated as a synthesis route to high quality single crystals of fresnoite.     

The influence of the Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate

The influence of Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition (MOCVD) has been systematically studied. Compared with the sample without Al pre-deposition, optimum Al pre-deposition time could improve the AlN buffer layer crystal quality and reduce the root mean square (RMS) roughness. Whereas, overlong Al-deposition time deteriorated the AlN crystal quality and Al-deposition patterns could be found. Cracks and melt-back etching patterns appeared in the GaN layer grown without Al pre-deposition. With suitable Al-deposition time, crack-free 2.0 μm GaN was obtained and the full-width at half-maximum (FWHM) of (0 0 2) plane measured by double crystal X-ray diffraction (DCXRD) was as low as 482 arcsec. However, overlong Al-deposition time would result in a great deal of cracks, and the crystal quality of GaN layer deteriorated. The surface of GaN layer became rough in the region where the Al-deposition patterns were formed due to overlong Al-deposition time.     

Chemically assisted vapour transport for bulk ZnO crystal growth

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10⁻³ to 1 atm) and temperatures (800–1200 °C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182 cm²/(V s) and n=7 10¹⁵ cm⁻³) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10⁺¹⁵ cm⁻³). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.     

Flux growth and characterizations of NdPO4 single crystals

Neodymium phosphate single crystals, NdPO₄, have been grown by a flux growth method using Li₂CO₃-2MoO₃ as a flux. The as-grown crystals were characterized by X-ray powder diffraction(XRPD), differential thermal analysis (DTA) and thermogravimetric analysis (TG) techniques. The results show that the as-grown crystals were well crystallized. The crystal was stable over the temperature range from 26 to 1200 °C in N₂. The specific heat of NdPO₄ crystal at room temperature was 0.41 J/g °C. The absorption and the fluorescence spectra of NdPO₄ crystal were also measured at room temperature.     

Bulk GaN crystals grown by HVPE

We succeeded in preparing very thick c-plane bulk gallium nitride (GaN) crystals grown by hydride vapor phase epitaxy. Growth of the bulk GaN crystals was performed on templates with 3 μm GaN layer grown by metal organic chemical vapor deposition on (0 0 0 1) sapphire substrates. Colorless freestanding bulk GaN crystals were obtained through self-separation processes. The crystal's diameter and thickness were about 52 and 5.8 mm, respectively. No surface pits were observed within an area of 46 mm diameter of the bulk GaN crystal. The dislocation density decreased with growth direction (from N-face side to Ga-face side) and ranged from 5.1×10⁶ cm⁻² near the N-face surface to 1.2×10⁶ cm⁻² near the Ga-face. A major impurity was Si, and other impurities (O, C, Cl, H, Fe, Ni and Cr) were near or below the detection limits by SIMS measurements.     

Recent achievements in AMMONO-bulk method

In this paper we present progress made recently in the development of the growth of truly bulk GaN crystals by the ammonothermal method in basic environment. High quality 2-in c-plane GaN seeds are shown. Non-polar wafers can also be cut out from thick GaN crystals grown by ammonothermal method. Perfect crystallinity manifests in very narrow peaks in X-ray rocking curves (the full width at half maximum equals about 15 arcsec). GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines, which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving the truly non-polar character of such AMMONO-GaN substrates. Other challenges like homogenous insulating properties or high p-type conductivity have been also accomplished by means of ammonothermal method. Semi-insulating crystals of resistivity up to 10¹¹ Ω cm and p-type conductivity within hole concentration up to 10¹⁸ cm⁻³ are already available in diameters up to 1.5-in.     

Growth and property of Zn-doped near-stoichiometric LiTaO3 crystal

Near-stoichiometric LiTaO₃ (SLT) and Zn-doped near-stoichiometric LiTaO₃ (Zn:SLT) crystals with 10–15 mm in diameter and 10 mm in length were grown by using TSSG technique with K₂O as the flux. The effect of adding amount of K₂O was discussed in the growing process. The crystals were characterized by inductively coupled plasma-optical emission (ICP-OES), X-ray diffraction (XRD) and differential thermal analysis (DTA). The lattice constants of Zn:SLT were smaller than those of SLT and Curie temperature was higher than that of SLT. It was found that Zn doping is an efficient way to improve the optical damage resistance ability of SLT crystal. Compared with SLT crystal, Zn:SLT exhibited a much higher optical damage threshold, more than 500 MW/cm², which was attributed to Zn self-compensated effect that formed the charge compensated complexes, (Zn_Ta)³⁻–3(Zn_Li)⁺ in SLT crystal.     

Improvements in a-plane GaN crystal quality by AlN/AlGaN superlattices layers

Non-polar (1 1 2¯ 0) a-plane GaN films have been grown by low-pressure metal-organic vapor deposition on r-plane (1 1¯ 0 2) sapphire substrate. We report on an approach of using AlN/AlGaN superlattices (SLs) for crystal quality improvement of a-plane GaN on r-plane sapphire. Using X-ray diffraction and atomic force microscopy measurements, we show that the insertion of AlN/AlGaN SLs improves crystal quality, reduces surface roughness effectively and eliminates triangular pits on the surface completely.     

Solution growth of single crystals of 4-hydroxycyanobenzene (4HCB) suitable for electronic applications

Squared, platelet-like single crystals of 4-hydroxycyanobenzene (4HCB) have been grown from solutions based on ethylic ether and petroleum ether. Properly modifying the growth conditions, in terms of both solvent used for the growth and concentration of 4HCB in the starting solution, allowed one to tune the planar dimensions of the platelets in the range 2–6 mm, and their thickness in the range 150–600 μm. In this way samples well suited for desired practical manipulation and electronic measurements may be obtained. Moreover, lowering the growth temperature resulted in larger but still thin 4HCB crystals. The ability to tailor crystal thickness has allowed one to study their Space-Charge Limited Current (SCLC) behaviour along that dimension, showing that the so-contacted samples exhibit intrinsic-like bulk conduction behaviour, and are hence well suitable for electronic studies and applications.     

Characterization of bulk GaN crystals grown from solution at near atmospheric pressure

The properties of GaN crystals grown from solution at temperatures ranging from 780 to 810 °C and near atmospheric pressure ∼0.14 MPa, have been investigated using low temperature X-band (∼9.5 GHz) electron paramagnetic resonance spectroscopy, micro-Raman spectroscopy, photoluminescense spectroscopy, and photoluminescence imaging. Our samples are spontaneously nucleated thin platelets of approximate dimensions of 2×2×0.025 mm³, or samples grown on both polycrystalline and single crystal HVPE large-area (∼3×8×0.5 mm³) seeds. Electron paramagnetic resonance spectra consists of a single Lorentzian line with axial symmetry about the c-axis, with approximate g-values, g_∥=1.951 and g_⊥=1.948 and a peak-to-peak linewidth of∼4.0 G. This resonance has been previously assigned to shallow impurity donors/conduction electrons in GaN and attributed to Si- and/or O impurities. Room temperature photoluminescence and photoluminescence imaging data from both Ga- and N-faces show different dominant emission bands, suggesting different incorporation of impurities and/or native defects. Raman scattering and X-ray diffraction show moderate to good crystalline quality.