Low-temperature atomic assembly of stoichiometric gallium arsenide from equiatomic vapor

The low-temperature atomic assembly of homoepitaxial GaAs thin films on the (0 0 1) surface has been investigated using molecular dynamics with a Stillinger–Weber potential energy function. During equiatomic vapor deposition, crystalline growth was observed for substrate temperatures above 35% of the melting temperature. Below this temperature, the critical epitaxial thickness began to rapidly decrease as defects were increasingly incorporated and eventually nucleated an entirely amorphous structure. The atomic assembly mechanisms of arsenic dimer incorporation, as well as gallium vacancy formation, were studied just above the amorphous/crystalline growth transition temperature. The adsorption of arsenic dimers was found to show dependence upon the orientation of the deposited molecule. Atomic processes responsible for the formation of the gallium vacancy defects were observed, and the influence of growth temperature on defect formation was also identified.     

Transmission electron microscopy investigation of AlN growth on Si(111)

AlN layers with a thickness of 250 nm were grown by plasma-assisted gas source molecular-beam epitaxy on Si(111) at substrate temperatures between 600 °C and 900 °C. The surface morphology and microstructure of the AlN layers were analyzed by scanning and transmission electron microscopy. Different defect types are observed in the AlN layers and at the AlN/Si(111) interfaces as a function of the temperature: inclusions of pure Al in the Si-substrate, crystallites of the cubic AlN phase, dislocations, stacking faults and inversion domain boundaries. The formation and concentration of the defects depends strongly on the substrate temperature during the growth. X-ray diffraction rocking curves for the (0002) reflection yield minimum full width at half maximum values for the sample grown at the 900 °C under Al-rich conditions indicating optimum structural quality. However, the discussion of the entity of defects will show that a more differentiated view is required to assess the overall quality of the AlN layers.     

The Eγ′ center as a probe of structural properties of nanometer-sized silica particles

A Q-band electron spin resonance (ESR) study is reported of E′ type point defects observed in ～7 nm-sized fumed silica nanoparticles following 10-eV irradiation to photodissociate H from passivated defects. In a comparative study with bulk silica (suprasil), the E′ center is used as an atomic probe to get more in depth information on the network structure of the nm-sized particles. The nanoparticles were brought into contact with ‘bulk’ Si/SiO₂ entities at an elevated temperature in vacuum (T_an = 1105 °C), i.e., the presence of an Si/SiO₂ interface. As a result of this post manufacture treatment, the E′ density increased drastically (>order of magnitude), enabling us to resolve hyperfine (hf) structure of the E′ centers located in the core region of the nanoparticles. Two doublet structures are observed, one each assigned to O₂Si–H entities and the primary ²⁹Si hf structure of the E′ centers. Analysis of these hf spectra reveals interesting information on the network structure of the core region of the nanoparticles: (1) Fumed silica is found to contain relatively less hydrogen than suprasil. (2) An increased ²⁹Si hf splitting (439 ± 2 G) is observed compared to bulk silica (418 ± 2 G), indicating that the E′ centers located in the core of the nanoparticles exhibit on average a slightly more pyramidal defect structure, and moreover, providing evidence that the fumed silica particles are densified compared to standard bulk silica, possibly originating from the presence of more low-membered rings (n < 5) in the nm-sized silica network.     

Modeling of incorporation of oxygen and carbon impurities into multicrystalline silicon ingot during one-directional growth

We propose a simple numerical model for incorporation of oxygen and carbon impurities into multicrystalline Si during one-directional crystal growth in comparison with experimental results. The model includes parameters that are oxygen and carbon concentrations in the melt in the beginning of the growth, carbon flux form the atmosphere, oxygen fluxes from the crucible and to the atmosphere. Variation of oxygen and carbon concentrations in multicrystalline Si ingots with a diameter of 30 cm and a height of 7.5 cm solidified one-directionally was measured by infra red absorption spectroscopy at room temperature. By fitting the numerical results on the experimental results, the parameters were evaluated. In the modeling we found fruitful suggestions for suppressing and controlling the oxygen and carbon concentrations in multicrystalline Si for solar cells.     

Liquid phase epitaxy (LPE) of GaN on c- and r-faces of AlN substrates

We present the optical properties of MBE-grown GaAs–AlGaAs core–shell nanowires (NWs) grown on anodized-aluminum-oxide (AAO) patterned-Si (1 1 1) substrate using photoluminescence and Raman scattering spectroscopy. The GaAs NWs were grown via the vapor–liquid–solid method with Au-nanoparticles as catalysts. Enhancement in emission of at least an order of magnitude was observed from the GaAs–AlGaAs core–shell NWs as compared to the bare GaAs NWs grown under similar conditions, which is an indication of improved radiative efficiency. The improvement in radiative efficiency is due to the passivating effect of the AlGaAs shell. Variation in bandgap emission energy as a function of temperature was analyzed using the semi-empirical Bose–Einstein model. Results show that the free exciton energy of the GaAs core–shell agrees well with the known emission energy of zinc blende (ZB) bulk GaAs. Further analysis on the linear slope of the temperature dependence curve of photoluminescence emission energy at low temperatures shows that there is no difference between core–shell nanowires and bulk GaAs, strongly indicating that the grown NWs are indeed predominantly ZB in structure. The Raman modes show downshift and asymmetrical broadening, which are characteristic features of NWs. The downshift is attributed to lattice defects rather than the confinement or shape effect.     

Thickness-dependent structural transition in GaAs nanocrystals grown on Si (1 1 1) surface

Structural stabilities in GaAs nanocrystals grown on the Si (1 1 1) substrate have been studied by transmission electron microscopy in order to see the structure and growth mechanism. The GaAs nanocrystals grown epitaxially on the Si (1 1 1) surface kept at 573 K have thin shapes consisting of a flat surface which is parallel to the Si (1 1 1) surface. The crystalline structure of the initial growth layer approximately below 5 nm in thickness is the zincblend structure, but with increasing thickness the structure changes to the wurtzite structure by formation of orderly-arranged stacking faults. The small difference in the driving force between the wurtzite structure and the zincblende structure could bring about a situation, where the kinetic rate of nucleus formation is high for the wurtzite structure than for the zincblende structure. It would highly increase the probability that the wurtzite structure is formed as a non-equilibrium state.     

Silicon light emissions from boron implant-induced defect engineering

We studied the electroluminescence of boron-implanted p–n junction silicon light emitting-diodes (Si LEDs) engineered with the implant-induced extended defects of different types. By varying the post implant annealing conditions to tune the extended defects and by using plan-view transmission electron microscopy to identify them, we found that {1 1 3} defects along Si〈1 1 0〉 are the ones that result in strong silicon light emission of the p–n junction Si LEDs other than {1 1 1} perfect prismatic and {1 1 1} faulted Frank dislocation loops. The electroluminescence peak intensity at about 1.1 eV of {1 1 3} defect-engineered Si LEDs is about twenty-five times higher than that of dislocation defect-engineered Si LEDs.     

Spectroscopic investigation of porous GaAs prepared by laser-induced etching

We have investigated the etching mechanism in Cr–O doped GaAs wafer under super- and sub-bandgap photon illumination. A comparison of the etching rate and properties of nanostructures from two samples which are etched with argon-ion laser (2.41 eV) and Nd:YAG laser (1.16 eV), are presented here. The etching mechanism is found different for these different illuminations, which play the key role in the formation of defects. It is observed that the etching process starts vigorously under sub-bandgap photon illumination through the mediation of intermediate defect states. SEM micrograph shows the formation of distinct GaAs nanostructures in sample etched by Nd:YAG laser. Porous structure produced by Nd:YAG laser shows strong room temperature luminescence in the red region. The size and size distribution of the nanocrystals are investigated by non-destructive Raman and photoluminescence spectroscopies. The data are analyzed within the framework of quantum confinement models.     

Growth mechanisms of GaAs nanowires by gas source molecular beam epitaxy

GaAs nanowires were grown on GaAs (1 1 1)B substrates in a gas source molecular beam epitaxy system, using self-assembled Au particles with diameters between 20 and 800 nm as catalytic agents. The growth kinetics of the wires was investigated for substrate temperatures between 500 and 600 °C, and V/III flux ratios of 1.5 and 2.3. The broad distribution of Au particles enabled the first observation of two distinct growth regimes related to the size of the catalyst. The origins of this transition are discussed in terms of the various mass transport mechanisms that drive the wire growth. Diffusion of the growth species on the 2-D surface and up the wire sidewalls dominates for catalyst diameters smaller than ∼130 nm on average, while direct impingement on the catalyst followed by bulk diffusion through the Au particle appears to sustain the wire growth for larger catalyst diameters. A change in wire sidewall facets, indicating a probable transition in the crystal structure, is found to be primarily dependent on the V/III flux ratio.     

Heterophase inclusions and dissolved impurities in Ge25Sb10S65 glass

The impurity content and microhomogeneity of Ge₂₅Sb₁₀S₆₅ glass samples, prepared by direct synthesis from elements, were investigated. It was shown that the increase in temperature of synthesis of the glass-forming melt resulted in the increase of the content of impurities of H, Na, Al, Si, K, Ca and transition metals in the prepared glasses. The glasses from the melt, subjected to chemical-distillation purification, were characterized by the low content of gas-forming impurities and the increased content of Al, Si and Cl. The glasses contained heterophase impurity inclusions mainly consisting of SiO₂, and their concentration and size depended on the conditions of glass preparation. The impurity content in the purest glasses was as follows: oxygen – <0.5 ppm wt, carbon – <5 ppm wt, hydrogen – 0.1 ppm wt, Si – <1 ppm wt, transition metals – <0.25 ppm wt, heterophase impurity inclusions with sizes larger than 80 nm – <10² cm³. It was shown that heterophase impurity inclusions behaved as the centers of glass crystallization.     

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1? 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1? 1? 2 0] || Si[1 1? 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.     

Photoluminescence of Si or Ge nanocrystallites embedded in silicon oxide

This paper presents the results of photoluminescence, its temperature dependence and Raman scattering investigations on magnetron co-sputtered silicon oxide films with (or without) embedded Si (or Ge) nanocrystallites. It is shown the oxide related defect origin of the visible PL centers peaked at 1.7, 2.06 and 2.30 eV. The infrared PL band centered at 1.44–1.58 eV in Si–SiO_x, system has been analyzed within a quantum confinement PL model. Comparative PL investigation of Ge–SiO_x system has confirmed that high energy visible PL bands (1.60–1.70 and 2.30 eV) are connected with oxide related defects in SiO_x. The PL band in the spectral range of 0.75–0.85 eV in Ge–SiO_x system is attributed to exciton recombination inside of Ge NCs.     

Enhanced luminescence from Si quantum dots/SiO2 multilayers by hydrogen annealing

Si quantum dots/SiO₂ multilayers were prepared by annealing a-Si:H/SiO₂ stacked structures at 1100 °C . Photo- and electro-luminescence band around 750 nm can be observed from Si QDs/SiO₂ multilayers due to the recombination of electron-hole pairs in Si QDs/SiO₂ interfaces. The electro-luminescence intensity was obviously enhanced after post hydrogen annealing at 400 °C. Electron spin resonance measurements were used to characterize the change of the defect states after hydrogen annealing. It is found that there exists a-centers (g value = 2.006), which is related to the Si dangling bonds in Si QDs in our samples. Hydrogen annealing can significantly reduce non-luminescent a-centers and enhance the electro-luminescence intensity consequently.     

Role of ballistic transport in photoluminescence excitation of Si nanocrystals

Photoluminescence of Si NCs with the size (10–300 nm) bigger than the exciton Bohr radius in the bulk Si crystals (4.8 nm) has been considered. Photoluminescence in such NC systems is analyzed from the point of view of new concept based on the effect of hot carrier ballistic transport in excitation of suboxide defect-related photoluminescence at the Si/SiO_x interface. The dependence of the 1.70 eV PL band integrated intensity on Si NC sizes was numerically calculated on the base of the hot carrier ballistic PL model. The well correlation between calculated and experimental results has been obtained for Si NCs with the size from the 30–150 nm range.     

Radiation processes in oxygen-deficient silica glasses: Is ODC(I) a precursor of E′-center?

The accumulation of radiation-induced defects under non-destructive X-ray and destructive cathodoexcitation was studied in pure silica KS-4V glasses possessing an absorption band at 7.6 eV. The correspondence between the existence of this band and the creation of the E′-center by radiation was checked. Detection of induced defects was accomplished by measurement of the luminescence during irradiation, post irradiation afterglow or phosphorescence, induced optical absorption, and thermally stimulated luminescence. In all samples, these observed phenomena associated with charge trapping and recombination on the oxygen-deficient luminescence center. Others centers of luminescence were not significant contributors. In some samples, the intensity of the 7.6 eV absorption band was deliberately increased by treatment in hydrogen at 1200 C for 100 h. The intensity of luminescence in hydrogen-treated samples was smaller because of the known quenching effect of hydrogen on the luminescence of oxygen-deficient centers. The optical absorption method does not reveal an induced absorption band for the E′-center in the hydrogen-free samples with different levels of oxygen deficiency. Therefore, we did not detect the transformation of the defect responsible for the 7.6 eV absorption band or the ODC(I) defect into the E′-center. In the hydrogen-treated sample, the absorption of the E′-center was detected. The E′-centers creation in the hydrogen-treated sample was associated with precursors created by hydrogen treatment (≡Si–O–H and ≡Si–H) in the glass network. The destructive e-beam irradiation reveals an increase with dose of the ODC luminescence intensity in the sample exhibiting a small 7.6 eV band. That means that the corresponding luminescence centers are created. Optical absorption measurements in that case reveal the presence of E′-centers and a broad band at 7.6 eV. A compaction of the irradiated volume was detected. Therefore, we conclude that the E′-center is produced by heavy damage to the glass network or by the presence of precursors.     

Optical constants and band gap expansion of size controlled silicon nanocrystals embedded in SiO2 matrix

Silicon nanocrystals (Si-NCs) with different sizes embedded in SiO₂ matrix were synthesized by phase separation and thermal crystallization of SiO_x/SiO₂ supperlattice approach. The optical constants and band gap expansion of Si-NCs have been investigated by spectroscopic ellipsometry, based on the Maxwell–Garnett effective medium approximation and the Forouhi–Bloomer optical dispersion model. Similar spectra shapes but smaller values of Si-NCs optical constants with respect to bulk crystalline Si is observed. With the size of Si-NCs decreasing from 6 nm to 2 nm, the band gap increases from 1.64 eV to 2.56 eV. The band gap expansion, as compared to bulk crystalline Si, which agrees with the prediction of first-principles calculations based on quantum confinement effect, is presented in this paper.     

Microcrystalline silicon–germanium alloys for solar cell application: Growth and material properties

Microcrystalline silicon–germanium (μc-Si_1−xGe_x:H) alloy films have been grown by 100-MHz glow-discharge of a SiH₄/GeH₄/H₂ gas mixture. Alloys over a full range of compositions were prepared to gain a comprehensive understanding of their growth and material properties. With increasing GeH₄ concentration in the gas-phase, we observed a preferential Ge incorporation behavior in the solid. Growth rate studies revealed that the Ge incorporation efficiency from source gas to solid is about five times greater than for Si at growth temperature of 200 °C, which accounts for the variation of alloy composition. With increasing Ge incorporation in the solid, on the other hand, we find a monotonic decrease in photoconductivity, followed by an electrical transition from weak n-type to strong p-type conduction at x > 0.7. At x ≈ 0.4, however, we obtained relatively high photoconductivity gains by a factor of 20 and strong infrared response in the solar cell structure. The Ge incorporation behavior and its effect on charge carrier transport are discussed.     

Optical properties and valence state of Sm ions in aluminoborosilicate glass under β-irradiation

Sm-doped borosilicate glasses exposed to β-irradiation with doses from 8 × 10⁵ up to 4 × 10⁹ Gy have been studied by luminescence, Raman and electron paramagnetic resonance (EPR) spectroscopies. The luminescence spectra for pristine and irradiated glasses reveal that the β-irradiation process affects valence state of samarium ions. Intense emission at 684 and 727 nm excited by Ar+ laser (514.5 nm) due to the transition of Sm²⁺ ion was observed after irradiation. Relative proportion of Sm²⁺ ions estimated as a function of both Sm₂O₃ content and irradiation dose has the tendency to increase with increasing irradiation dose. In contrast, the EPR spectra of the studied samples reveal a decrease of the defect content, which are mostly hole defects, produced during irradiation, as a function of Sm₂O₃ content. Finally, the addition of Sm₂O₃ leads to a decrease of the Si–O–Si bending vibration modes shift and polymerisation changes under irradiation.     

Films thickness effect on structural and optoelectronic properties of hydrogenated amorphous germanium (a-Ge:H)

Thin films of hydrogenated amorphous germanium (a-Ge:H) deposited at high growth rate by radiofrequency (RF) glow discharge with 1 sccm GeH₄ diluted in 40 sccm H₂ have been studied. The effect of the films thicknesses on the defect density and on the structural parameters was carefully investigated by means of infrared spectroscopy, optical transmission measurements, and the photothermal deflection spectroscopy (PDS) technique. The results of this investigation show that when the films thicknesses increase, the total hydrogen content (C_H) decreases and the hydrogen-bonding configuration changes. The results of these changes appear clearly on the defects density and on the microstructure parameter of the films, while the disorder parameter E_OV and the optical gap E_T remain practically constant (E_OV ≈ 45 ± 2 meV, E_T = 1.08 ± 0.02 eV). The improvement of these parameters is mainly due to the incorporation of the hydrogen in the bulk of the material as the monohydride groups (Ge-H) rather than the polyhydride groups (Ge-H₂ and Ge-H_2n) when the films thicknesses increase.     

Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require Al_xGa_1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, Al_xGa_1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of Al_xGa_1-xN bulk crystals by any standard bulk growth techniques has not been developed so far.There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by any standard bulk growth techniques were not successful.Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA-MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100 μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite Al_xGa_1-xN wafers. Zinc-blende and wurtzite Al_xGa_1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and Al_xGa_1-xN wafers. At a thickness of ～30 µm, free-standing GaN and Al_xGa_1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and Al_xGa_1-xN wafers with thicknesses in the 30–100 μm range may be used as substrates for further growth of GaN and Al_xGa_1-xN-based structures and devices.We have compared different RF nitrogen plasma sources for the growth of thick nitride Al_xGa_1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3 µm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day.Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.