Polycrystalline Source Research Articles

Growth kinetics of basic ammonothermal gallium nitride crystals

The ammonothermal method has been extensively studied for the last two decades because of its potential to produce high-quality bulk gallium nitride (GaN) boules at low cost. Currently, the ammonothermal literature lacks in situ, internal fluid temperature measurements during the crystal growth process. This study reports on internal fluid temperature measured simultaneously in the dissolution and growth zones for twelve basic ammonothermal GaN growth experiments (pressure range = 180–261 MPa; molar NH3:Na fill ratio = 20:1) on {0 0 0 1} and {1 0 1¯ 0}-orientation GaN seeds. Knowledge of the fluid temperature combined with mass-change measurements of the polycrystalline source and seed crystals has enabled the distinction of three growth regimes: a mass transport-limited regime at internal fluid density differences below 1.2 mol/L between dissolution and growth zones; a surface reaction-limited regime above the aforementioned critical fluid density difference and below growth zone fluid temperatures of ∼570 °C; and an apparent loss of solvent-limited regime above growth zone fluid temperatures of ∼570 °C. Analysis of growth in [0 0 0 1], [0 0 0 1¯], and [1 0 1¯ 0] has enabled the determination of activation energies of each respective GaN crystallographic orientation in the surface reaction-limited regime.

Growth of bulk ZnO crystals by self-selecting CVT method

Bulk ZnO crystals were grown by self-selecting CVT method using carbon as the transport agent. The crystal growth process took place on the top of the polycrystalline source material, and deep-red colored ZnO crystals of several millimeters were obtained. The as-grown crystals were characterized by X-ray diffraction (XRD), Energy Dispersive Spectrometer (EDS), Raman scattering (RS) spectroscopy, visible–near infrared (VIS–NIR) spectrophotometer and room temperature photoluminescence (PL) spectroscopy. XRD results indicate good crystallinity of the ZnO crystal. The EDS analysis shows that the crystal has a stoichiometry ratio Zn: O = 52: 48. The results suggest the existence of native defects of oxygen vacancies (OV) in the as-grown ZnO samples, which is caused by the stoichiometry shift to Zn-rich.

Scanning proximal microscopy study of the thin layers of silicon carbide-aluminum nitride solid solution manufactured by fast sublimation epitaxy

The objective of the study is a growth of SiC/(SiC) 1− x (AlN) x structures by fast sublimation epitaxy of the polycrystalline source of (SiC) 1− x (AlN) x and their characterisation by proximal scanning electron microscopy and atomic force microscopy. For that purpose optimal conditions of sublimation process have been defined. Manufactured structures could be used as substrates for wide-band-gap semiconductor devices on the basis of nitrides, including gallium nitride, aluminum nitride and their alloys, as well as for the production of transistors with high mobility of electrons and also for creation of blue and ultraviolet light emitters (light-emitted diodes and laser diodes). The result of analysis shows that increasing of the growth temperature up to 2300 K allows carry out sublimation epitaxy of thin layers of aluminum nitride and its solid solution.

Thermoelectric Properties of Sb-doped Sintered Mg2Si Fabricated using Commercial Polycrystalline Sources

AbstractThe thermoelectric (TE) properties, such as the Seebeck coefficient, the electrical and thermal conductivities, and the output power, of Sb-doped n-type Mg2Si were studied. A commercial polycrystalline source was used for the source material for the Mg2Si. TE elements with Ni electrodes were fabricated by using a monobloc plasma-activated sintering (PAS) technique. Compared with undoped samples, the ZT values of the Sb-doped samples were higher over the whole temperature range in which measurements were made; the maximum value for the Sb doped Mg2Si was 0.72 at 864 K. The TE characteristics of Sb-doped samples were found to be comparable to those of Bi-doped ones, and no significant difference in ZT value was observed between them. Provisional results showed that the maximum value of the output power was 6.75 mW for the undoped sample, 4.55 mW for a 0.5 at% Sb doped sample, and 5.25 mW for a 1 at% Sb doped sample with ΔT = 500 K (between 873 K and 373 K).

Composition dependent thermoelectric properties of sintered Mg 2Si 1− xGe x ( x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg 2Si 1− x Ge x ( x = 0–1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg 2Si 1− x Ge x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg 2Si 1− x Ge x with higher Ge content ( x = 0.6–1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10 − 5 Wcm − 1 K − 2 was observed at 420 K for Mg 2Si and Mg 2Si 0.6Ge 0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg 2Si 1− x Ge x . The values close to 0.02 Wcm − 1 K − 1 were obtained for Mg 2Si 1− x Ge x ( x = 0.4–0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm − 1 K − 1 at 773 K for Mg 2Si 0.4Ge 0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg 2Si 0.6Ge 0.4.

Boron oxide encapsulated Bridgman growth of high-purity high-resistivity cadmium telluride crystals

High-purity semi-insulating CdTe crystals have been successfully grown by encapsulated (B 2O 3) Bridgman technique. The procedure strongly limits component losses allowing the achievement of stoichiometry control material and keeps a low level of impurity contamination as shown by mass spectroscopy analysis data. When strictly stoichiometry-controlled and high-purity polycrystalline source material has been used, high-resistivity crystals have been obtained without any intentional doping. EPD values in the range of 1–3×10 4 cm −2 have been observed in a wide region of the crystals. Luminescence spectroscopy confirms the purity and good structural quality of the material. The proposed method avoids the technical problems posed by the High Pressure Bridgman technique and fits the requirements for CdTe/CdZnTe crystals large-scale production.

A Study of HTCVD Renewing of the SiC Polycrystalline Source during the PVT Process

A Study of HTCVD Renewing of the SiC Polycrystalline Source during the PVT Process

Continuous Feed Physical Vapor Transport : Toward High Purity and Long Boule Growth of SiC

A new reactor concept for the growth of silicon carbide bulk crystals and/or thick epitaxial layers is presented. A coupled approach involving process modeling and numerical simulation and experimental results and characterization was used. This new process combines both high-temperature chemical vapor deposition (HTCVD) for continuous feeding of the polycrystalline source and physical vapor transport (PVT) for single-crystal growth. A special crucible design was built to perform both steps simultaneously. For the feeding step (HTCVD), tetramethylsilane diluted in argon was used. The typical growth rate obtained by the continuous feed PVT process is 100 μm/h at 1900°C. The growth of thick epitaxial layers is demonstrated with a pure two-dimensional growth regime. © 2003 The Electrochemical Society. All rights reserved.

Vapor growth of Hg 1− xCd xI 2 on glass using CdTe buffer

Vapor phase epitaxy (VPE) of Hg 1− x Cd x I 2 layers on glass substrates covered by a CdTe buffer layer has been studied. The buffer layers of 2–4 μm thickness were formed by VPE using polycrystalline CdTe and Cd metal sources. The Hg 1− x Cd x I 2 layers were grown using a (Hg 1− y Cd y ) 1− z (I 2) z polycrystalline source, with a composition in the range of y=0.1–0.5 and z=0.5–0.8. Scanning electron microscopy and X-ray diffraction studies have shown that the composition and structure of Hg 1− x Cd x I 2 layers depend strongly on the VPE conditions. Varying the growth time and source composition, it has been possible to obtain Hg 1− x Cd x I 2 layers with the composition x in the range from approximately 0 (HgI 2 tetragonal structure) to 1 (CdI 2 hexagonal structure).

Vapor phase epitaxy of Hg 1− xCd xI 2 on sapphire

We demonstrate the possibility of growing Hg 1− x Cd x I 2 layers on sapphire substrates by vapor-phase epitaxy (VPE). The successful growth has been carried out using an α-HgI 2 polycrystalline source and a CdTe buffer layer grown on sapphire by metalorganic vapor phase epitaxy (MOVPE) before the Hg 1− x Cd x I 2 VPE growth. The Hg 1− x Cd x I 2/sapphire 20–40 μm thick layers with a uniform composition in the range of x=0.2–0.6 were grown at 220–250°C for 70–300 h. The layers were studied by scanning electron microscopy, energy disperse X-ray analysis and X-ray diffractometry. Results on the layer characterization are reported and the effect of VPE conditions on the layer properties is considered.

Hg 1−x Cd x I 2 /CdTe heterostructures for nuclear radiation detectors: Effect of epitaxial growth on substrate properties

We demonstrate the possibility to fabricate nuclear radiation detectors operating at room temperature from CdTe substrates affected by the vapor phase epitaxy (VPE) growth of Hg1−xCdxI2 layers. The VPE layers with the thickness 10–30 μm were grown using an α-HgI2 polycrystalline source at 220 °C and time in the range of 30–100 h. The as-grown heterostructures were chemically etched to remove the epilayers, and Au–CdTe–Au detectors were made. The substrates were characterized by synchrotron x-ray topography before and after the VPE growth, and the current–voltage (I–V) and spectroscopic measurements of the detectors were carried out. The effect of the VPE growth on the substrates and detectors has been studied and on the basis of this it has been possible to fabricate γ-ray detectors with Ohmic I–V characteristic and good spectral response.

Structural defects in Hg1−xCdxI2 layers grown on CdTe substrates by vapor phase epitaxy

Hg 1−x Cd x I 2 20–25-μm-thick layers with a uniform composition in the range of x=0.1–0.2 were grown on CdTe substrates by vapor phase epitaxy (VPE). The growth was carried out using an α-HgI2 polycrystalline source at 200 °C and in the time range of 30–100 h. The layers were studied by scanning electron microscopy (SEM) and high resolution synchrotron x-ray topography (SXRT). The SEM and SXRT images of Hg1−xCdxI2 VPE layers allow one to identify the defects affecting the layer structure. The two main types of structural defects in the layers are subgrain boundaries and densely spaced striations similar to those referred generally to as vapor grown HgI2 bulk crystals. The effect of the growth time on these defects has been analyzed and on the basis of this it has been possible to grow Hg1−xCdxI2 layers with low defect density.

Vapor phase epitaxy of Hg 1− xCd xI 2 layers on CdTe substrates

Vapor phase epitaxy (VPE) has been studied to grow Hg 1 − x Cd x I 2 epitaxial layers on CdTe bulk substrates. The effect of the VPE growth conditions on the morphology, composition and crystalline quality of Hg 1 − x Cd x I 2 CdTe heterostructures has been investigated. It has been shown that 10–30 μm thick Hg 1 − x Cd x I 2 layers can be successfully grown using an α-HgI 2 polycrystalline source under isothermal conditions at a temperature in the range 170–240°C for the time period 20–50 h. Interestingly, the VPE growth was found to consist of two successiv stages with different kinetics as follows: (1) a fast growth of an HgI 2 platelet layer on the CdTe substrate surface and (2) a slow growth of an Hg 1 − x Cd x I 2 layer on the surface of the platelet layer. The Hg 1 − x Cd x I 2 layers exhibited single-crystalline structure when the layer composition has been varied in the wide range x = 0.24−0.67. The particular role of the HgI 2 platelet layer morphology in controlling the composition uniformity throughout the thickness of the Hg 1 − x Cd x I 2 layer has been demonstrated.

Sensitivity of spinI=5/2 perturbed-angular-correlation measurements to combined magnetic-dipole and electric-quadrupole hyperfine interactions

To design, analyze, and interpret perturbed-angular-correlation (PAC) experiments, the theory of the combined nuclear-magnetic-dipole and nuclear-electric-quadrupole interaction is applied. The applications involve polycrystalline sources that contain spinI=5/2 intermediate-quantum-level PAC probes. Frequencieswkk′, amplitudesakk′, and perturbation functionsG22(t) are calculated for various values of the combined-interaction parametersη, β, andα, wheny=wL/WQ is fixed at 30. For relatively large values ofy, the weak shift in them-state energies caused by the electric-quadrupole interaction generally produces a large change in the resulting perturbation function, and the effect is sensitive to the specific values of the parametersη, β, andα. This sensitivity arises because the weak shift inm-state energies produces heterodyning. Moreover, this analysis indicates that PAC measurements of combined interactions in highly-distorted crystals (WQ≠0 andη≠0) can provide potentially more information than previously thought. This analysis strongly suggests that more experimental investigations should be performed on highly-distorted crystals.

Growth of CdTe single crystals by vapour condensation on the surface of polycrystalline source material

A method of crystal growth from the vapour phase, which had been worked out for A IVB VI compounds, has been applied to CdTe. Single crystals shaped with (110) and (111) planes of approximately 10 mm size have been obtained. A high level of structure perfection has been confirmed by means of X-ray rocking curve measurements resulting in half-width values down to approximately 15″. The suitability for production of homogeneous CdTe-based A IVB VI solid solution crystals is discussed.

Perturbation functions: PAC probe nuclei, [formula omitted

The program, ACOEFF, calculates several theoretical quantities that are useful for analyzing Perturbed Angular Correlation (PAC) experiments. In particular, ACOEFF calculates perturbation functions, G k k ( t), a( k) n, n'-coefficients, and interaction frequencies for nuclear electric quadrupole interactions of PAC probe nuclei with non-axially symmetric electric field gradients. The program calculates these quantities for the specific cases of spin, I = 2, 5 2 , and 3, for polycrystalline sources. The program is written in FORTRAN IV and should run on any mainframe or minicomputer. It was tested using the WATFIV and VS compilers. The input consists of one data card, and the output is straightforward to understand.

Electric quadrupole interactions in non-axially symmetric electric field gradients of spinI=2 and 3 nuclei in polycrystalline sources

Calculations and formulae are presented for the purpose of understanding and analyzing Perturbed Angular Correlation (PAC) experiments that use spin-two and spin-three probe nuclei. For electric quadrupole interactions of probe nuclei in a polycrystalline source with non-axially symmetric electric field gradients, the interaction frequencies and perturbation functions for spin-two and spin-three nuclei differ qualitatively and quantitatively from those for spin-five-halves nuclei. These differences result primarily because them-states of integral-spin nuclei are nondegenerate in an asymmetric electric field gradient. To help the experimenter deal with the added complexity of the integral-spin casesI=2 and 3, closed-form expressions are provided for the energy eigenvalues and the eigenvectors as a function of the asymmetry parameter η. To deal with the problem of calculating the perturbation functionsGkk(t) for η>0, the summation overm-state quantum numbers is formulated in terms ofan′,n(k)-coefficients. These coefficients are analogous to theSkn-coefficients used in the case for η=0. To illustrate the differences between the half-integral-spin caseI=5/2 and the integral-spin casesI=2 and 3, energy difference diagrams andan′,n′(2) diagrams are presented and discussed.

Plasma nitrided oxide films as a thin gate dielectric

In integrated circuit technology, the strong need for thin dielectrics that exhibit greater resistance to impurity diffusion and high electrical properties makes SiO 2 nitridation very attractive. In this work, 10 and 15 nm thermally grown silicon dioxide films were nitrided in NH 3 at 950°C for various times between 1 and 6 h. Treatments were performed in a thermally assisted RF plasma equipment. Auger electron spectroscopy analysis and step-by-step chemical etch profiling measurements provide us with the as-obtained film composition emphasizing a preferential nitridation of the surface. Elastic recoil detection analysis shows that less than 3 at% of hydrogen was incorporated after a 6 h nitridation. To investigate the nitrided oxide electrical properties, capacitors were fabricated. The plasma nitridation step results in interfacial characteristics very similar to those of SiO 2 films. Finally, the ability of the nitrided films to prevent boron diffusion from an ion implanted polycrystalline source was studied using spreading resistance measurements.

Effects of interfacial Oxide layer on short-channel polycrystalline source and drain MOSFET's

Using a novel self-alignment approach, the characteristics of polycrystalline source and drain MOSFET's with and without a deliberately grown oxide under the polycrystalline regions are compared. The interfacial oxide is shown to suppress short-channel effects in the shortest channel devices studied, but this improvement is at the expense of increased source-to-drain contact resistance in the present devices. The devices without the interfacial oxide are also expected to have superior hot-carrier performance.

Mass spectrometric study of the vapour composition in a hot-wall-reaction tube for CdTe

During the past few years epitaxial layers of II–VI compound semiconductors were successfully grown by Hot-Wall Epitaxy (HWE). However, detailed knowledge about the processes in the reactor is rather poor. In order to investigate the gas in the reactor during growth, a Quadrupole-Mass Spectrometer was connected for the first time to the HWE system. We evaporated CdTe from a polycrystalline source and identified the different vapour species by testing the isotope distribution. Cd +, Te 2 + and Te + ions were observed. The Cd- and Te 2-ion currents were monitored as a function of the CdTe-source temperature. From this measurements the equilibrium enthalpy of evaporation was obtained in agreement with literature data. Intensities of Cd + and Te 2 + signals were studied as a function of different temperature profiles in the system. For all thermodynamic conditions investigated we found that the partial pressures are governed by the mass action law. This gives rise to the assumption that it is possible to grow CdTe layers with desired composition.