Improvement Of Crystal Quality Research Articles

High-quality InGaN∕GaN heterojunctions and their photovoltaic effects

High-quality p-GaN∕i-In0.1Ga0.9N∕n-GaN heterojunctional epilayers are grown on (0001)-oriented sapphire substrates by metal organic chemical vapor deposition. The Pendellösung fringes around the InGaN peak in high-resolution x-ray diffraction (HRXRD) confirm a sharp interface between InGaN and GaN films. The corresponding HRXRD and photoluminescence measurements demonstrate that there is no observable phase separation. The improvement in crystal quality yields high-performance photovoltaic cells with open-circuit voltage of around 2.1eV and fill factor up to 81% under standard AM 1.5 condition. The dark current-voltage measurements show very large shunt resistance, implying an insignificant leakage current in the devices and therefore achieving the high fill factor in the illuminated case.

Enhancing electron transport in Si:P delta-doped devices by rapid thermal anneal

We address the use of rapid thermal anneal (RTA) to enhance electron mobility and phase coherent transport in Si:P δ-doped devices encapsulated by low temperature Si molecular beam epitaxy while minimizing dopant diffusion. RTA temperatures of 500–700°C were applied to δ-doped layers encapsulated at 250°C. From 4.2K magnetotransport measurements, we find that the improved crystal quality after RTA increases the mobility/mean free path by ∼40% and the phase coherence length by ∼25%. Our results suggest that the initial capping layer has near optimal crystal quality and transport improvement achieved by a RTA is limited.

Integrating Selective Epitaxy in Advanced Logic & Memory Devices

In this paper, we review integration of selective epitaxy in advanced logic and memory devices. Embedded SiGe epitaxy has been adapted for logic pMOSFET fabrication from the 90 nm technology node and device performance continues to scale with higher [Ge] in the epitaxial SiGe layer. For nMOSFET performance boost, use of embedded silicon carbon (Si:C) epitaxial layer has been considered. Recent improvements in crystal quality of selective in-situ phosphorus doped Si:C epitaxy enables significant transistor drive current increase. For integration of Si:C epitaxy, effect of various thermal annealing processes on substitutional carbon incorporation and NiSi formation on Si:C epitaxy are described. Also, heavy phosphorus doping in Si and Si:C layers is discussed in terms of film resistivity and strain contribution. Selective epitaxy is being used to form elevated source/drain in DRAM memory devices where it provides immunity against short channel effects and results in improved retention time.

Metal-organic vapour-phase epitaxy of gallium nitride nanostructures for optoelectronic applications

The two main topics with respect to better gallium nitride (GaN)-based optoelectronic device performance are the improvement of crystal quality and of light extraction. Concerning the first topic, the epitaxial growth of GaN-related materials has to be improved. Since native substrates with good quality are still expensive, foreign substrates like sapphire or silicon which are much cheaper are used, but they introduce a lattice as well as a thermal mismatch leading to detrimental crystal defects. Several approaches to reduce the defect density exist, but in most cases they are coupled with a much higher technological effort.We present an alternative solution to decrease the defect density by the growth of GaN nanostructures, where the small footprint and high aspect ratio leads to a much lower influence of the substrate. Metal-organic vapour-phase epitaxy of GaN nanostructures and InGaN/GaN multi-quantum wells was carried out in a vertical reactor with close-coupled showerhead. Studies on the morphologic properties have been carried out by scanning electron microscopy and energy-dispersive X-ray spectroscopy to reveal the influence of growth parameters and material composition. Cathodoluminescence measurements show the good optical properties of the GaN as well as of the InGaN/GaN structures.

Precise control of doping profile and crystal quality improvement of SiGe HBTs using continuous epitaxial growth technology

A continuous-epitaxial-growth technique, using a cold-wall rapid thermal UHV/CVD system, for Si/SiGe multi-layers containing different doping types has been developed. Extremely high phosphorous-doping in the silicon layer grown by LPCVD with Si 2H 6 can be achieved without high-temperature activation annealing, and good crystallinity of the Si layer results in very low resistivity. High-temperature surface cleaning prior to the epitaxial growth can be omitted, since contamination occurring during wafer transfer was minimized by using a UHV transfer chamber. Good crystallinity of the HBT structure after the whole fabrication process was confirmed by reciprocal lattice space mapping of high-resolution X-ray diffraction. These results indicate that the developed continuous-epitaxial-growth technique gives a major advantage in maintaining crystal quality of Si/SiGe multi-layers for future ultra-high-speed devices.

Thermal analysis of GaN powder formation via reaction of gallium ethylenediamine tetraacetic acid complexes with ammonia

AbstractThe reaction mechanism of gallium ethylenediamine tetraacetic acid (Ga[(OOCCH2)2–N–CH2–CH2–N–(CH2COO)2]NH4) (Ga‐EDTA)with ammonia was investigated. The Ga‐EDTA·NH4 complex powders were analyzed by thermogravimetric and differential thermal analysis (TG/DTA) under flow of ammonia gas and nitrogen gas. From the TG/DTA results, it is shown that the precursors undergo a sharp mass loss at about 296 °C (step I) due to the release of organic groups, followed by further releasing of organic groups (297∼ 552 °C, step II), elimination of carbon and formation of GaN (552 ∼ 650 °C, step III), GaN crystallization and further elimination of carbon (650 ∼ 850 °C, step IV), the improvement of GaN crystal quality (600 ∼ 1100 °C, step‐IV) and the initial decomposition of GaN powders. The X‐ray diffraction analysis of the powders indicates that proper sintering temperature for GaN powders should be approximate to 1100 °C. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

New nitridation technique for mosaicity control in RF‐MBE InN growth

AbstractMosaicity control is necessary for improvement of crystal quality of III‐V nitride semiconductor materials, especially for InN. However, the mosaicity control for both of the tilt and the twist angle fluctuations has not been achieved still now in the direct growth of InN on the sapphire substrate by nitridation. Moreover, the role of nitridation process has not been so much clear from the view point of the mosaicity control. In this paper, we propose a new nitridation technique, two‐step nitridation, combined with the high‐ and the low‐ temperature nitridation to reduce the mosaicity and also to make clear the roles of the nitridation process in the InN growth. As the result of the new nitridation technique, the considerably improvement of the tilt and the twist angle fluctuations has been successfully achieved in the direct growth of InN without any buffer layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvements in a-plane GaN crystal quality by a two-step growth process

Nonpolar (112¯0) a-plane GaN films have been grown by metal-organic vapor deposition on r-plane (11¯02) sapphire. Lateral growth is favored using a low V:III ratio resulting in films with a smooth surface, while pitted films are grown at a high V:III ratio indicating preferential on-axis growth. High-resolution x-ray diffraction analysis of both film types showed a strong anisotropy in the peak width of the symmetric omega rocking curve with respect to the in-plane orientation, phi. In-plane isotropic behavior of crystallinity with overall reduced omega full width at half maximum values was achieved when the growth was initiated at a high V:III ratio before reducing the V:III ratio for film coalescence. An improvement of crystal quality through initial surface roughening was equally realized by the incorporation of partial-coverage SiNx interlayers.

Online Measurement Technique of Melt Surface Level Using Fusion Ring Image in Growth Furnace of Czochralski Method for Si Crystal

We have developed a new instrumentation technique for Si crystal growth process of Czochralski (CZ) method for the purpose of the quality improvement. The online measurement technique for the melt surface level of Si crystal in growth furnace using image of the fusion ring, which appears to surround solid liquid boundary of Si ingot, was developed. Our new technique can measure the melt surface level with high accuracy without the following influences, (i) the change of solid liquid boundary height during diameter change, and (ii) the habit lines of Si ingot. The accuracy of this method is confirmed within +/-0.6mm. This technique contributes to the improvement of CZ Si crystal quality.

Crystallogenesis Research for Biology in the Last Two Decades as Seen from the International Conferences on the Crystallization of Biological Macromolecules

The series of 11 International Conferences on the Crystallization of Biological Macromolecules (ICCBM) took place over the period 1986–2006 in the USA (four times), Germany (two times), China, France, Japan, Spain, and lastly the 11th in Canada in Quebec City. Here we review the first 10 ICCBMs. Their focus was to bring rational approaches to the field of protein crystal growth and thus overcome the rate-limiting step in macromolecular X-ray crystallography. This survey summarizes how the ICCBM series contributed to the emergence of the science of biocrystallogenesis. This was achieved through the joint efforts of scientists from the small molecule crystal growth community and from biochemists, biophysicists, and protein crystallographers. Highlights from each conference are discussed, and scientific synergies are emphasized. While the first conferences focused on fundamentals, especially from the standpoint of physics and biochemical considerations, the more recent conferences stressed applications in structural biology, to advanced methods of crystallization, and of crystal quality improvement. Particular attention will be given to themes that were recurrent through all the ICCBMs: purity and impurities, solution properties of macromolecules under precrystallization conditions, microgravity and assessment of crystal quality, as well as specific trends of practical interest to structural biology.

Epitaxial growth of the metastable phase ytterbium monoxide on gallium nitride surfaces

Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O 2 system to access the metastable phase ytterbium monoxide (YbO). Experiments exploring the growth of polycrystalline YbO x films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O 2 fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (0 0 1) GaN surface leads to the growth of epitaxial (1 1 1) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50 nm lateral dimensions. Rocking curves in ω and φ have full-width half-maximum values of 1.77° and 4.1°, respectively; further improvements in crystal quality appear to be limited by the thermal stability of the YbO phase.

Formation of Fe3Si/Ge/Fe3Si Multi-layer by Double Heteroepitaxy on High Quality Fe3Si/Ge Substrate for Spintronic Application

Double heteroepitaxial growth of Fe3Si/Ge on high quality Fe3Si/Ge substrates has been investigated by low-temperature (<300oC) molecular beam epitaxy. Reflective high-energy electron diffraction measurements suggested that Fe3Si and Ge layers were epitaxially grown on Fe3Si/Ge substrates, though the surface morphology became rough. X-ray diffraction measurements revealed that a small amount of Fe3Si(110) was included in the top Fe3Si layers. Transmission electron microscopy measurements indicated that stacking faults were formed in the intermediate Ge and the top Fe3Si layers. Improvement of crystal quality of the intermediate Ge layer is essential to realize high quality Fe3Si/Ge/Fe3Si multi-layers.

Specific features of kinetics of molecular beam epitaxy of compounds in the GaN-AlN system

The effect of growth conditions (V/III ratio, substrate temperature) on the properties of materials in AlN-GaN systems is discussed. A concept of the growth of the AlN/AlGaN/GaN multilayer heterostructure, which provides the improvement of crystal quality and surface morphology of the layers, is suggested and realized. The improvement of the properties of GaN in the AlN/AlGaN/GaN/AlGaN multilayer heterostructure is confirmed by a considerable increase in electron mobility in the two-dimensional electron gas formed at the upper heterointerface GaN/Al0.3Ga0.7N.

Prediction of improvement of protein crystal quality grown in microgravity

Prediction of improvement of protein crystal quality grown in microgravity

Relationships between material properties of piezo-electric thin films and device characteristics of film bulk acoustic resonators

The relationships between the material properties of piezo-electric thin films (such as ZnO and AlN) and the frequency response characteristics of film bulk acoustic resonator (FBAR) devices based on those films are examined. ZnO and AlN films are deposited by RF magnetron plasma reactive sputtering. Issues in film growth are focused on electric bias application, oxygen incorporation, and two-step deposition. The XRD spectra, AFM topographies, and SEM images measured from all the deposited films present the optimal condition for both ZnO and AlN films to have excellent piezo-electric properties. For ZnO films, excessive injection of oxygen deteriorates the crystal quality and increases the surface roughness. Applying negative d.c. bias during deposition contributes to the improvement of crystal quality of ZnO films as well as the enhancement of acoustic coupling capability of ZnO-based FBAR devices. By adopting the two-step method to deposit AlN films, the films reveal the highly (002)-preferred growth nature and the RMS surface roughness values are drastically reduced, which also leads to the remarkable improvement in the acoustic coupling coefficient of AlN-FBAR devices.

Copper co-crystallization and divalent metal salts cross-influence effect: A new optimization tool improving crystal morphology and diffraction quality

The effect of several metal cations (Cu 2+, Cd 2+, Co 2+, Ba 2+) was tested in attempts to improve crystallization and verify a newly discovered cross-crystallization alias cross-influence procedure (CIP) with two selected proteins; di-heme cytochrome c 4 from anaerobic purple sulphur bacterium Thiocapsa roseopersicina and sweet-tasting protein thaumatin from the African berry Thaumatococcus daniellii. Divalent cupric cations (Cu 2+) promoted the most dramatic improvement in crystal morphology, internal packing and diffraction quality. This investigation qualitatively established the influence of cupric cations on the crystal growth with use of CIP. The CIP procedure is based on using a set of additives included in additional droplets, effecting the same reservoir through slightly varying pH and vapour pressure of water over the neighbouring drop containing protein. It was found that influence of Cu 2+ ions produced evidently different outer morphology and internal packing of thaumatin crystals (hexagonal prism). Usually their shape is presented as a tetragonal bipyramids. In the case of cytochrome, the good diffractable crystals were obtained only by using CIP with metal-ion salts. Newly grown crystals (hexagonal prisms) of thaumatin and cytochrome displayed as the same tetragonal system with different unit cell parameters and diffracted up to 1.7 Å. From the analysis of copper influence and CIP effect, we propose that the copper addition with CIP provides useful technique to modify crystal morphology and improve diffraction quality.

Improvement in crystal quality of epitaxial Ag and Cu films induced by self-ion irradiation

Epitaxial Ag and Cu films ∼ 300 nm thick on Si substrates were irradiated with self-ions (Ag + and Cu +) at room temperature to improve crystal quality of the films. The minimum yield of Ag/Si(100) for [100] axial channeling, for example, was reduced from 39% of that for the un-irradiated sample to 13% after irradiation with 300 keV Ag ions to a fluence of 3 × 10 15 ions cm − 2 . The self-ion irradiation uniformly improves the crystal quality of the irradiated layer up to a depth of ∼ 200 nm, rather close to the depth corresponding to the end of range of ions, in spite of the non-uniform depth distribution of displaced Ag atoms. In channeling analysis, the energy dependences of dechanneling factors show that dominant defects present in the irradiated films are not dislocations, but twins, stacking faults and/or voids. AFM observations evidence grain growth in a lateral direction at the surface.

Key inventions in the history of nitride-based blue LED and LD

It was quite difficult to grow high quality single crystals of nitrides and impossible to control their electric conduction. These problems prevented the development of nitride-based devices for many years. In 1986, a dramatic improvement in the crystalline quality of GaN was achieved by use of low-temperature-deposited (LT) buffer layer technology in metalorganic vapor phase epitaxy. In 1989, the high quality GaN enabled us to produce p-type conduction in nitrides and to control the conductivity of n-type nitrides. These achievements led to the invention of the world's first GaN p–n junction blue/UV LED in 1989. Room temperature UV stimulated emission, which is indispensable for laser operation, was also achieved in 1990 by use of high quality GaN films grown with the LT-buffer layers. These breakthroughs inspired nitride researchers around the world to greater efforts, and eventually led to the commercialization of high-performance blue LEDs and long-life violet LDs as well as the development of nitride-based devices such as high-speed transistors. Furthermore, unique properties such as a large piezoelectric effect were also clarified due to the marked improvements in crystal quality of nitrides. In this paper, key inventions during the development of nitride-based blue LED and LD are reviewed and a recent advance in UV devices is also described.

Growth and electronic and magnetic properties of α-FeSe films on GaAs (001) substrates

FeSe films have been grown on GaAs (001) substrates by low-pressure metal organic vapor deposition at different temperatures. X-ray diffraction analysis indicated that the FeSe films grown at substrate temperatures (Ts) between 220 and 340°C were preferentially oriented with tetragonal structure. It was not possible to deposit films at Ts above 400°C. The atomic molar ratios of Se∕Fe increased with increasing the flow rate of H2Se∕Fe(CO)5 and the growth temperature. The electronic property results showed that the conductive type of FeSe could be adjusted by controlling the growth temperature. The coercivity of FeSe decreased with increasing the growth temperature from 220to300°C due to the improvement of crystal quality.

Improvement of crystal quality of thick InGaAsN layers grown on InP substrates by adding antimony

We investigated the effect of adding Sb in 1-μm-thick InGaAsN layers grown by solid source molecular beam epitaxy (MBE) method on InP substrates. Secondary ion mass spectroscopy (SIMS) analysis of the InGaAsSbN layer showed that both the nitrogen and the Sb concentration are uniform in the growth direction. The X-ray rocking curve (XRC) of InGaAsSbN was narrower than that of the InGaAsN layer, which suggests that adding Sb improved the crystallographic quality of the thick InGaAsN layer. The surface morphology of the thick InGaAsSbN layer was as smooth as that of the lattice-matched InGaAs layer, while the surface of the thick InGaAsN layer was rough. These results indicate that the crystalline quality of thick InGaAsN layer can be improved by adding Sb. The photoluminescence (PL) peak wavelength became longer by adding Sb, but the PL intensity decreased. This is supposed to be due to the sensitivity of PL intensity to the growth condition.