Melt Flow and Species Transport in μg‐Gradient‐Freeze Growth of Germanium

The result of a μg‐experiment on the Gradient‐Freeze growth of Ge:Zn with doping from the vapour phase shows a homogeneous distribution of the zinc in the melt, indicating the dominating role of a gravity‐independent transport mechanism. This effect is investigated numerically on the basis of a global model of the growth setup. The numerical simulation includes the melt flow and the transport of the dopant taking into account buoyant and thermocapillary forces. The results confirm the minor influence of gravity on the species transport. The complete mixing of the melt can be explained by thermocapillary (Marangoni) convection only.     

Radial segregation due to weak convection in a floating zone

The influence of weak convection, caused by surface tension forces, on radial dopant segregation occurring in crystals grown under microgravity conditions is studied numerically. The geometry considered corresponds to a floating-zone configuration with partially coated melt surfaces consisting of small evenly distributed spots of free surfaces. In order to distinguish dopant distribution due to weak convection clearly from distribution due to diffusion the spots only cover one quarter of the periphery. Thus, surface tension-driven convection is allowed only over one quarter of the floating-zone configuration resulting in an asymmetric dopant distribution. The percentage of free surfaces present is varied in order to alter the Marangoni flow rates. The maximum dopant concentration due to radial segregation is plotted as a function of a certain convection level. The results of the present numerical study are supposed to be used to design corresponding space experiments launched at the end of the year 2000.     

Growth,structure and optical characterization of Al‐doped ZnO nanoparticle thin films

Al‐doped ZnO nanoparticle thin films were prepared on glass substrate at the optimum temperature of (410±10) °C by spray pyrolysis technique using zinc nitrate as a precursor solution and aluminium chloride as a dopant. The dopant concentration (Al/Zn at%) was varied from 0 to 2 at%. Structural analysis of the films shows that all the films are of polycrystalline zinc oxide in nature, possessing hexagonal wurtzite structure. The films exhibit variation in peak intensities corresponding to (100), (002) and (101) reflection planes on Al‐doping. The crystallite size calculated by Scherrer formula has been found to be in the range of 35‐65 nm. The optical absorption study shows that the optical band gap in the Al‐doped films varies in the range of 3.11 – 3.22 eV. The width of localized states in the band gap estimated by the Urbach tail analysis has been found to be minimum in case of the 1 at% Al‐doped zinc oxide thin film. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of CeO2-doping on surface and catalytic properties of CuO-ZnO system

The effect of doping CuO-ZnO system with CeO₂ on its surface and catalytic properties was investigated using nitrogen adsorption at −196 °C, EDX technique and catalysis of CO oxidation by O₂ at 100-200 °C. Pure mixed solids were prepared by thermal decomposition of copper/zinc mixed hydroxides at 400 °C. The doped solids were obtained by impregnating a known mass of mixed hydroxides with calculated amount of cerium ammonium nitrate followed by drying then calcination at 400 °C. The dopant concentration was 1.5, 3.0 and 4.5 mol% CeO₂. The results revealed that CeO₂-doping modified the surface atomic Cu/Zn ratio of the system investigated and changed the crystallite size of both CuO and ZnO phases. The increase of the amount of dopant added changed the major phase present. This treatment decreased the specific surface area of doped solids. The doping process modified also the catalytic activity in a manner dependent on both mode of preparation and dopant concentration. However, CeO₂-doping did not modify the mechanism of the catalytic reaction but changed the concentration of catalytically active sites involved in the catalyzed reactions.     

Characterization of large‐sized Nd:YAG single crystals grown by horizontal directional solidification

Nd³⁺‐doped Y₃Al₅O₁₂ single crystals have been grown by the horizontal directional solidification (HDS) method in different thermal zone. The Grashof (G_r), Prandtl (P_r), Marangoni (M_a) and Rayleigh (R_a) numbers of melt in HDS system have been discussed for our experimental system to understand the mechanism of melt flow patterns and concentration gradient of dopant. The concentration gradient of Nd³⁺ ions was explained with melt flow processes during crystal growth in different thermal zone, and results indicated that high growth temperature will be helpful for uniformity of dopant in HDS‐grown single crystal. The main microscopic growth defects such as bubbles and irregular inclusions in HDS‐grown Nd:YAG crystals were observed, and the causes were discussed as well. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Axial macrosegregation in Ga‐doped germanium grown by the vertical gradient freeze technique with a rotating magnetic field

The impact of a rotating magnetic field (RMF) on the axial segregation in Vertical Gradient Freeze (VGF) grown, Ga doped germanium is investigated. Growth experiments were performed using the VGF‐RMF as well as the conventional VGF technique. Carrier concentration profiles characterising the Ga segregation were measured by the Spreading Resistance method and calibrated using Hall values of carrier concentration and mobility. The Ga concentration rises more gradually under RMF action, i.e., the dopant segregation is significantly reduced by the rotating field. This effect is attributed to a better mixing of the melt. Numerical results on the flow velocity confirm this explanation. The RMF induced flow is much more intense than the natural buoyant convection due to the radial temperature gradient and leads to a pronounced decrease of the effective partition coefficient k_eff. In the early stages of growth a k_eff value close to k₀ was obtained, i.e., the gallium was almost homogeneously distributed within the melt. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of large doped Bi12SiO20 crystals

The conditions for growing Cr-, Mn-, Fe-, Co-, Ni-, and Cr + Al-doped BSO crystals from a stoichiometric melt by the Czochralski method have been the subject of research. A decrease of the absorption coefficient along the growth axis was observed in undoped crystals of 50 mm diameter and 150 mm long, which is indicative of melt stoichiometry change in the course of the growth run. The transmission and reflection spectra of different crystals were measured and the absorption coefficients was determined in the range 0.38–0.70 μm. When Fe dopant concentration exceeds a limit value, the absorption spectrum is blue-shifted relative to the undoped case and the absorption coefficient decreases. Co, Cr, Mn, and Cr + Al dopants produce a red shift and increase the absorption coefficient. Combined Cr + Al doping shift the absorption spectrum to the blue region as compared to Cr-only doping with the same Cr concentration.     

Photoluminescence from screen printed ZnO based nanocrystalline films

Photoluminescence from ZnO based films deposited by screen printing is studied. PL spectra of undoped films show strong broad green emission around 500 nm and a weak (nearly one fourth of visible) UV emission around 398 nm. Further enhancement of 30% in the green emission is observed as 5 at.% Ca is doped in ZnO which for higher dopant concentration falls monotonously. The films are polycrystalline and the particle size lies between 33 to 47 nm as determined by Debye‐Scherrer method. Film with 5 at.% doping is under tensile strain and the others are under compressive strain. SEM shows microclusters, consisting of nanoparticles, scattered throughout the substrate which club together with increase in dopant amount. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Convection in melts and impurity distribution in semiconductor crystals

Impurity distributions in semiconductor melts and crystals grown from these melts are experimentally and numerically studied on an example of Ga-doped Ge crystals. It is shown that inhomogeneous dopant distribution is observed in the form of striations and is caused by the convective flows in the melt and their nonstationary rearrangement in the vicinity of the crystallization front. The character of heat and mass transfer under the microgravity conditions is predicted. The necessity of precision experiments under terrestrial and, especially, space conditions is emphasized.     

Investigation of the effect of aluminum doped zinc oxide (ZnO:Al) thin films by spray pyrolysis

ABSTRACT

In this study, aluminum doped zinc oxide thin films were deposited on ordinary glass substrates at the temperature of 425°C by spray pyrolysis technique for various doping concentrations of aluminum ranging from 1 to 5 at.%. The effect of dopant concentration on structural, morphological and optical properties of ZnO:Al thin films was studied. Optical band gap of the films increased with doping percentage and started to decrease from 2 at.% of dopant. The average transmittance for 2at.% ZnO:Al film is significantly increased over 90% in the visible region at 450 nm which is crucial for optoelectronic applications.     

Structural and optical response of 2‐Mercaptoethonal capped CdS nanocrystals to Fe3+ ions

Cadmium sulfide (CdS) semiconductor nanocrystals (NCs) doped with Fe³⁺ have been synthesized via a solution‐based method utilizing dopant concentrations of (0–5%) and employing 2‐mercaptoehonal as a capping agent. X‐ray diffraction (XRD) results showed that the undoped CdS NCs are in mixed phase of cubic and hexagonal, where as the doped CdS NCs are in hexagonal phase. The crystallite size was increased from ∼1.2 nm to ∼2 nm. Diffuse reflectance spectroscopy studies (DRS) reveals that the band gap energy was decreased with Fe doping and it lies in the range of 2.58 ‐ 2.88 eV. Photoluminescence (PL) spectra of undoped CdS NCs show a strong green emission peak centered at 530 nm and a weak red emission shoulder positioned at 580 nm. After doping all the luminescence intensity was highly quenched and the green emission peak was shifted to orange region (580 nm), but the position of weak red emission shoulder was unaltered with doping. FTIR studies revealed that the NCs were sterically stabilized by 2‐mercaptoethanol. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of Zn‐doped talc in hydrothermal atmosphere

Zn‐doped talc were synthesized under hydrothermal conditions at constant reaction time and pressure of 160 hours and 2 kbar respectively, at three different temperatures (300, 500 and 650 °C) with pH‐values of 5 and 7. The starting materials and run products were characterized by X‐ray powder diffraction (XRPD), scanning electron microscopy with annexed energy‐dispersive spectrometry (SEM‐EDS), differential scanning calorimetry thermogravimetric analysis (DSC‐TG) and Fourier transform infrared (FT‐IR). The results showed that the temperature, pH‐value of the reaction mixture and amount of zinc in the starting mixture affected the growth of the Zn‐doped talc. When synthesized at low temperature talc presents low crystallinity, flawed morphology but higher content in zinc in its lattice. A thermal treatment at, or above 500 °C allowed a significant flaw reduction in talc morphology, a higher crystallinity and a lower content in dopant. When large amounts of Zn were present in the starting mixtures, Zn‐doped talc grew small in size and poorly shaped. The effect of Zn doping on some chemical/physical characteristics of the synthesized talc was also discussed.     

Results of crystal growth of bismuth-antimony alloys (Bi100–xSbx) in a microgravity environment

Results of two experiments are presented for growth of crystals from (Bi_100–xSb_x) alloys in a microgravity environment. In the growth experiments different variants of the Bridgman technique were used. It was shown that in crystal growth from the melt in closed ampoules under microgravity conditions convection can be prevented completely. Therefore it is possible to grow crystals from melts of some components under diffusion controlled conditions of mass transfer. In microgravity a reduced interaction between the melt and the confining walls was observed even if they have large contact with each other. The investigation of surface morphology corroborated the importance of surface effects for crystal growth from the melt under microgravity conditions. Measurements of electronic properties of crystals grown in microgravity showed a good quality in comparison to earth grown crystals. Because under microgravity conditions in closed ampoules the diffusion controlled mass transfer can be realized and the interaction between the melt and confining wall is reduced, homogeneous crystals with high perfection can be grown melts of some components.     

The current understanding of epitaxial CVD silicon layer doping in the light of modeling and theory development (VII)

Extension and maximum concentration of autodoping profile are discussed for both lateral and vertical autodoping phenomena and, additionally, bearing in mind the typical course of autodoping profile as obtainable by spreading resistance technique, on the background of previously published theoretical concepts of dopant incorporation. It is shown that the “improved” (three-step mechanism) as well as the “consequent” dopant incorporation concepts (two-step mechanism) are suited for theoretically explaining autodoping phenomena. If no additional supposition will be stated, however, it is a consequence of the former that the concentration maximum in the profile of vertical autodoping equals the buried-layer surface dopant concentration in agreement with non-steady state layer doping behaviour after dopant source flow has been immediately interrupted. In the contrary the latter conception simply identifies lateral and vertical autodoping effect since autodoping above as well as beyond buried layer is controlled in the same way by parasitie dopant partial pressure in the gas.     

In-process monitoring and control of doping gas concentration during vapor phase silicon epitaxial growth by using a flame photometric detector

An in-process monitoring and control method of the doping gas concentration during epitaxial growth of Si was developed. A flame photometric detector (FPD) can be used as a monitor for the PH₃ and B₂H₆ dopant concentrations in the injected doping gases. A combination of this dopant monitor with an automatic control system of the silicon source (SiHCl₃) gas concentration using an infrared spectrophotometer as a monitor, makes possible an automatic in-process control of the concentrations of dopant and of silicon source gas supplied to the reactor. The present system provides an accurate and reproducible control of impurity concentrations in Si epitaxial layers. Good correlation between the monitored signal (or the doping gas concentration) and the impurity concentration incorporated into the growth layers was confirmed for PH₃ (n-type) and B₂H₆ (p-type) doping. For the B₂H₆ doping, a divergence from the linear relationship between the doping gas concentration and the impurity concentration in the layers was observed in the level of acceptor concentration below about 10¹⁵ atoms/cm³. The transient response of the present system was measured by growing epitaxial layers with increasing and decreasing step-changes in the dopant gas flow during continuous deposition of the layers. Some interesting, but complicated, transient responses of impurity concentration in the growth layer were observed. The responses are different between the PH₃ doping and the B₂H₆ doping, and also different between increasing and decreasing steps especially for the B₂H₆ doping.     

Oxygen and hydrogen profiles and electrical properties of unintentionally doped gallium nitride grown by hydride vapor phase epitaxy

Commercially available hydride vapor phase epitaxy gallium nitride (GaN) is characterized with the aim to correlate the oxygen and hydrogen secondary ion mass spectrometry profiles of a GaN wafer with the electrical properties of the sample. A GaN layer model, including doping profile and mobility, is derived, utilizing electrical (capacitance–voltage, Hall), structural (high resolution X‐ray diffraction) and optical (polarized infrared spectroscopy) methods. Oxygen and hydrogen are easily incorporated during hydride vapor phase epitaxy growth of GaN. Oxygen is an n‐type dopant in GaN, whereas hydrogen may passivate some of the donors. Electrical and optical properties correlate with a low defect concentration top GaN layer and a high defect concentration GaN interlayer.     

FTIR,powder XRD,TEM and dielectric studies of pure and zinc doped nano‐hydroxyapatite

Hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ or HAP, is an important bio‐material, which is having application in bone implants and dentistry. In the present study, zinc doped nano‐hydroxyapatite (Zn‐HAP) was synthesized via chemical precipitation route using surfactant mediated approach. The doping of zinc was confirmed by EDAX. The powder X‐ray diffraction (XRD) pattern revealed the typical hydroxyapatite pattern with broadening and extra peaks were observed for higher concentration. The average crystallite size was calculated by applying the Scherrer's formula to powder XRD pattern and was found in the range of 16 to 33 nm. The morphology of synthesized nano‐particles was also confirmed using TEM. FTIR spectroscopy was used to confirm the presence of various bonds. The dielectric study was carried out at room temperature within the frequency range from 10² Hz to 10⁷ Hz and the variations of dielectric constant with frequency of applied field as well as with the concentration of zinc were studied. It was found that as the concentration of zinc increased the dielectric constant increased. The variations of dielectric loss and a.c. conductivity with frequency of applied field were studied. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simplified Treatment of Transient Doping Behaviour in CVD(I)

Physiochemical reasons are discussed for the delayed response of doping concentrations within epitaxial semiconductors (Si and III-V compounds) to changes of the input partial pressure of dopant species for CVD processes. A simplified circuit representation for doping is used that takes care of five serial process steps, the first three of which are the topic of this work (Part I). Among the corresponding storage phenomena in which the gas phase is involved, those with adsorptional influences are to be expected as most important for transient behaviour. Storage of dopant species at the semiconductor surface is included into the circuit representation, as it has been done by Reif et al. for silicon doping. Moreover, adsorption at the reactor wall is represented approximately, assuming equilibrium with the gas phase.     

Thermosolutal Convection during Vertical BRIDGMAN Growth of Semiconductor Melts

An attempt is made to investigate the influence of the solutal effect on convection in BiSbTe₃ melts as a representative for semiconductor melts of low PRANDTL number. Calculations have been performed for a 3D BRIDGMAN configuration applying an experimentally measured temperature profile at the outer surface of the ampoule. The FIDAP^TM FEM code was used to solve the transient hydrodynamic moving-boundary problem. Results are presented for the excessive tellurium as a melt component as well as for an additive lead doping. It is shown that, according to the growth conditions, the melt components differently contribute to the thermosolutal convection. For Bridgman growth experiments in space, arbitrary orientations of the ampoule axis with respect to the residual gravity vector can occur. Calculations have been performed for certain orientations (angles) at a constant microgravity level. They show an influence of the solutal effect only for large deviations of the ampoule axis from the direction of the residual gravity vector.     

Habit modification and improvement in properties of potassium hydrogen phthalate (KAP) crystals doped with metal ions

Potassium hydrogen phthalate (KAP) single crystals were grown by slow evaporation and slow cooling techniques. The growth procedure like temperature cooling rate, evaporation rate, solution pH, concentration of the solute, supersaturation ratio etc., has been varied to have optically transparent crystals. Efforts were made to dope the KAP crystals with rubidium, sodium and lithium ions. The dopant concentration has been varied from 0.01 to 10 mole percent. Good quality single crystals were grown with different concentrations of dopants in the mother phase. Depending on the concentration of the dopants and the solution pH value, there is modification of habit. Rubidium ions very much improve the growth on the prismatic faces. The transparency of the crystals is improved with rubidium and sodium doping. The role of the dopants on the non‐linear optical performance of KAP indicates better efficiency for doped crystals. The grown crystals were characterized with XRD, FT‐IR, chemical etching, Vickers microhardness and SHG measurements. The influence of the dopants on the optical, chemical, structural, mechanical and other properties of the KAP crystals was analysed. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim