Growth and characterization of GaN and AlN films on (1 1 1) and (0 0 1) Si substrates

GaN and AlN films were grown on (1 1 1) and (0 0 1) Si substrates by separate admittances of trimethylgallium (or trimethylaluminum) and ammonia (NH₃) at 1000°C. A high temperature (HT) or low temperature (LT) grown AlN thin layer was employed as the buffer layer between HT GaN (or HT AlN) film and Si substrate. Experimental results show that HT AlN and HT GaN films grown on the HT AlN-coated Si substrates exhibit better crystalline quality than those deposited on the LT AlN-coated Si substrates. Transmission electron microscopy (TEM) of the HT GaN/HT AlN buffer layer/(1 1 1)Si samples shows a particular orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. High quality HT GaN films were achieved on (1 1 1) Si substrates using a 200 Å thick HT AlN buffer layer. Room temperature photoluminescence spectra of the high quality HT GaN films show strong near band edge luminescence at 3.41 eV with an emission linewidth of ∼110 meV and weak yellow luminescence.     

Growth of ZnTe layers on (111) GaAs substrates by metalorganic vapor phase epitaxy

ZnTe layers were grown on (111) GaAs substrates by metalorganic vapor phase epitaxy using dimethylzinc and diethyltelluride as the source materials. X-ray diffraction analysis revealed that epitaxial ZnTe layers can be obtained on (111) GaAs substrates. X-ray rocking curves, Raman spectroscopy, and photoluminescence measurements showed that the crystal quality of ZnTe layers depends on the substrate temperature during the growth. A high-crystalline quality (111) ZnTe heteroepitaxial layer with strong near-band-edge emission at 550 nm was obtained at a substrate temperature of 440 °C.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

High-quality AlN layers grown by hot-wall MOCVD at reduced temperatures

We report on a growth of AlN at reduced temperatures of 1100 °C and 1200 °C in a horizontal-tube hot-wall metalorganic chemical vapor deposition reactor configured for operation at temperatures of up to 1500–1600 °C and using a joint delivery of precursors. We present a simple route—as viewed in the context of the elaborate multilayer growth approaches with pulsed ammonia supply—for the AlN growth process on SiC substrates at the reduced temperature of 1200 °C. The established growth conditions in conjunction with the particular in-situ intervening SiC substrate treatment are considered pertinent to the accomplishment of crystalline, relatively thin, ～700 nm, single AlN layers of high-quality. The feedback is obtained from surface morphology, cathodoluminescence and secondary ion mass spectrometry characterization.     

Structural,electrical and optical properties of in-situ phosphorous-doped Ge layers

We have studied the impact of temperature and pressure on the structural and electronic properties of Ge:P layers grown with GeH₄+PH₃ on thick Ge buffers, themselves on Si(0 0 1). The maximum phosphorous atomic concentration [P] exponentially decreased as the growth temperature increased, irrespective of pressure (20 Torr, 100 Torr or 250 Torr). The highest values were however achieved at 100 Torr (3.6×10²⁰ cm^?3 at 400 °C, 2.5×10¹⁹ cm^?3 at 600 °C and 10¹⁹ cm^?3 at 750 °C). P atomic depth profiles, “box-like” at 400 °C, became trapezoidal at 600 °C and 750 °C, most likely because of surface segregation. The increase at 100 Torr of [P] with the PH₃ mass-flow, almost linear at 400 °C, saturated quite rapidly at much lower values at 600 °C and 750 °C. Adding PH₃ had however almost no impact on the Ge growth rate (be it at 400 °C or 750 °C). A growth temperature of 400 °C yielded Ge:P layers tensily-strained on the Ge buffers underneath, with a very high concentration of substitutional P atoms (5.4×10²⁰ cm^?3). Such layers were however rough and of rather low crystalline quality in X-ray Diffraction. Ge:P layers grown at 600 °C and 750 °C had the same lattice parameter and smooth surface morphology as the Ge:B buffers underneath, most likely because of lower P atomic concentrations (2.5×10¹⁹ cm^?3 and 10¹⁹ cm^?3, respectively). Four point probe measurements showed that almost all P atoms were electrically active at 600 °C and 750 °C (1/4th at 400 °C). Finally, room temperature photoluminescence measurements confirmed that high temperature Ge:P layers were of high optical quality, with a direct bandgap peak either slightly less intense (750 °C) or more intense (600 °C) than similar thickness intrinsic Ge layers. In contrast, highly phosphorous-doped Ge layers grown at 400 °C were of poor optical quality, in line with structural and electrical results.     

Low temperature molecular beam epitaxy growth of cubic GaCrN

Cubic GaCrN layers are grown on MgO(0 0 1) substrates at 350–700 °C by plasma-assisted molecular beam epitaxy. Substrate temperature dependences of their structural and magnetic properties were systematically studied. It is found that the solubility limit of Cr atoms in cubic GaCrN is dramatically improved by the low temperature (350 °C) growth, though crystalline quality becomes poorer. It is also observed that the magnetic ordering increases with Cr content in the low Cr content region, but after showing highest ordering it decreases in the high Cr content region. The Cr content range showing ferromagnetic behavior increases with lowering substrate temperature. However, the magnetization vs. magnetic field curve shows “emaciated” hysteresis for the low temperature grown samples.     

Crystalline growth of germanium thin films on single crystal silicon substrates by solid phase crystallization

We have studied the epitaxial-like growth of germanium (Ge), due to solid phase crystallization (SPC) from amorphous Ge (a-Ge) deposited on single crystal silicon (Si) substrate. The crystalline growth of Ge following the orientation of Si substrates was successfully obtained by the SPC at 400 °C or higher. The preferential growth on Si (111) substrates continues up to 10,000 Å. Different orientations from the substrate orientation in XRD patterns are slightly observed in the growth on Si (100) substrates at 450 °C, but the preferential growth of (100) orientation continued in the whole film thickness in TEM images. The epitaxial-like growth of Ge may be more preferable on the Si (111) substrate than the (100) one.     

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm⁻¹ was found in Raman scattering spectra for all films with buffers, which is corresponding to the E₂ mode.     

Low-temperature growth of high-quality ZnO layers by surfactant-mediated molecular-beam epitaxy

High-quality ZnO layers are grown on Zn-polar ZnO substrates by surfactant-mediated plasma-assisted molecular-beam epitaxy (P-MBE) using atomic hydrogen as a surfactant. Careful investigation with atomic force microscopy (AFM) and reflection high-energy electron diffraction (RHEED) reveals that two-dimensional growth is preserved down to 400 °C by irradiating atomic hydrogen during growth, while the low-temperature limit of two-dimensional growth is 600 °C without atomic hydrogen irradiation. The crystal quality of ZnO layers grown at 400 °C by surfactant-mediated MBE is evaluated to be the same as those grown at 600 °C by conventional MBE in terms of X-ray diffraction and photoluminescence properties.     

Intersubband transition at 1.55 μm in AlN/GaN multiple quantum wells by metal organic vapor phase epitaxy using the pulse injection method at 770 °C

Intersubband transition (ISBT) at 1.55 μm in AlN/GaN multi quantum wells (MQWs) was realized by metal organic vapor phase epitaxy (MOVPE) using the pulse injection (PI) method to grow GaN well layers at 770 °C. It was shown that a main factor for shifting ISBT wavelength to shorter region to cover 1.55 μm and improving ISBT properties of MQWs is the growth temperature of MQWs. Best structural and ISBT properties are observed at low growth temperature of 770 °C in this study. Carbon incorporation level in GaN layer grown by the PI method (PI-GaN) showed one order smaller value compared with that by the conventional continuous method. Moreover, further decrease in growth temperature to 770 °C did not show significant increase in carbon incorporation in PI-GaN layer. It clearly indicates that the PI method is very effective in reducing carbon concentration in GaN layer, especially at low temperature region. The low carbon concentration of 4×10¹⁸ cm^?3 released by the PI method was indispensable for realizing enough carrier concentration of 1.6×10¹⁹ cm^?3 to achieve strong ISBT at 1.55 μm.     

Correlation between the photoluminescence and different types of Si nano-clusters in amorphous silicon

This paper presents the results of PL spectrum study for Si nano-clusters in amorphous silicon matrix. The hydrogenated amorphous Si layers were prepared by the hot-wire CVD method on glass substrates. The layers were deposited at different wafer temperatures 280, 360, 420 and 460 °C and at one filament temperature of 1650 °C. The joint analysis of PL and X-ray diffraction spectra in dependence on the technological conditions and on different sizes of Si nano-clusters has been done. The mechanisms of PL are discussed as well.     

The study of removing hydroxyl from silica glass

The removal of hydroxyl from silica glass produced by melting quartz powder under an atmosphere containing hydrogen was investigated. After heat-treatment at the temperature range (700–1200 °C) in nitrogen atmosphere, the effective hydrogen diffusion coefficients were evaluated based on the law of nonsteady-state diffusion. The activation energy obtained is 254 kJ mol⁻¹ for the dehydroxylation process in the heat-treatment temperature range of 700–900 °C, and a different activation energy calculated is 32 kJ mol⁻¹ in the temperature range of 900–1200 °C. The activation energies for the dehydroxylation process at the temperature (700–900 °C) and the higher temperature (900–1200 °C) correspond to the binding energy of SiO–H bond and the activation energy for the diffusion of hydrogen in silica glass respectively, which indicate there is a change of mechanism for dehydroxylation with heat-treatment temperature.     

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100 nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577 °C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10 nm has been observed.     

Polarities of AlN films and underlying 3C-SiC intermediate layers grown on (1 1 1) Si substrates

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.     

Pyrolytic conversion studies of blended polycarbosilane and polyaluminasilazane

Multinuclear solid-state NMR spectroscopy, FTIR and Raman experiments are employed to investigate the pyrolytic conversion of blended polycarbosilane and polyaluminasilazane (denoted CA) up to 800 °C, with the aim of studying structural evolutions and interactions between polycarbosilane and polyaluminasilazane during the pyrolysis process. Vinyl and SiCH₃ units can react with Si–H, SiCH₃ and Si–CH₂–Si groups below 400 °C. These crosslinking reactions can increase the ceramic yield of the blended precursors. At 500 °C aromatic carbon is formed, and N–H and Si–H groups vanish at 600 °C and 700 °C, respectively. At 600 °C, SiCH₃ and Si–H units can further react with SiCN₃, SiC₂N₂, N–H and C–H units. An amount of amorphous carbon and CSi₄ and CSi₃H groups are detectable at 800 °C. Even at this temperature there are still many aromatic protons. In addition, there are also SiC₄, SiC₃N, SiCN₃ and SiN₄ units. Silicon forms SiN₄ more readily than SiC₄. Many AlN₅ groups transform into AlN₆ groups. The D and G bands of graphite are observed in CA pyrolyzed at 1400 °C. According to the XRD patterns, the reflection of crystalline β-Si₃N₄ vanishes at 1700 °C, and the residue pyrolyzed at 1800 °C mainly contains a large number of 2H-SiC/AlN solid solution crystals and a few β-SiC crystals.     

Growth kinetics and boron doping of very high Ge content SiGe for source/drain engineering

We have studied the in-situ boron doping of high Ge content Si_1?xGe_x layers (x=0.3, 0.4 and 0.5). These layers have been grown at low pressure (20 Torr) and low temperature (600–650 °C) with a heavily chlorinated chemistry on blanket Si(0 0 1) substrates. Such a chemistry yields a full selectivity versus SiO₂ (isolation) and Si₃N₄ (sidewall spacers) on patterned wafers with gate stacks. We have quantified the impact of the diborane flow on the SiGe layer crystalline quality, its resistivity, the SiGe:B growth rate and the apparent Ge concentration. Resistivity values lower than 1 mΩ cm are easily achieved, all the more so for high Ge content layers. The SiGe growth rate increases and the apparent Ge concentration (from X-ray diffraction) decreases as the diborane flow increases. B atoms (much smaller than Si or Ge) indeed partially compensate the compressive strain in the SiGe:B layers. We have also probed the in-situ boron and phosphorus doping of Si at 750 °C, 20 Torr with a heavily chlorinated chemistry. The B ions concentration increases linearly with the diborane flow, then saturates at a value close to 4×10¹⁹ cm^?3. By contrast, the P ions concentration increases sub-linearly with the phosphine flow, with a maximum value close to 9×10¹⁸ cm^?3. Adding diborane (phosphine) to the gaseous mixture leads to a sharp increase (decrease) of the Si:B (the Si:P) growth rates, which has to be taken into account in device layers. All the know-how acquired will be most handy for the formation of in-situ doped recessed or raised sources and drains in metal-oxide semiconductor devices.     

Structural and optical properties of TiO2 thin films derived by sol–gel dip coating process

Nanocrystalline thin films of titanium dioxide have been fabricated on glass and silica substrates from partially hydrolyzed precursor solution. These films were subjected to heat treatment for 1 h at temperatures 100, 200, 300, 400, 500, 600, 700, 800 and 900 °C and characterized by XRD, SEM, XPS and optical techniques. As deposited films are found to be amorphous and also contain hydroxyl and organic functional groups. Films heat treated above 100 °C do not contain hydroxyl and organic functional groups. Microcrystalline behavior is observed in the films heat treated above 300 °C. Crystallite size increases from ∼5 to 50 nm as sintering temperature is increased from 300 to 700 °C. Formation of anatase phase with c-axis length 7.03 Å is observed in the films annealed up to 700 °C. These films peel off from the substrate beyond 700 °C annealing temperature. Density as well as refractive index of the films increases with increase in annealing temperature up to 700 °C. Refractive index is found to show Cauchys behavior. Transmission better than 70% is observed in the visible range. There is a strong absorption around 370 nm, which is attributed to band gap absorption of the material.     

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO₂ nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiO_x (a-SiO_x) films with 0 < x < 1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 °C. The series resistance through Ag/[Si-nanodots in SiO₂]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiO_x films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.     

Growth of YFeO3 crystal by edge-defined film-fed growth method

A sizeable single crystal of YFeO₃ (YIP) with the dimensions of 19×15×15 mm³ has been successfully grown by the edge-defined film-fed growth method. Thermal magnetic analysis shows that Curie temperature of as-grown YIP crystal is about 363.5 °C. The hardness of YIP crystal was measured as 900 VDH, equivalent to about 7.1 moh. Moreover, the optical transmittance of as-grown YIP crystal can be significantly enhanced if this crystal was annealed at 700 °C in oxygen atmosphere.     

High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer

In this work we report on the growth and characterization of high quality MOCVD GaN film grown on Al₂O₃ substrates by using a HT (>1150 °C)-AlN buffer layer. We have investigated the most favorable growth conditions in terms of temperature, thickness and growth rate of AlN buffer layer in order to optimize the high temperature GaN layer. The improved morphological and structural properties of GaN layer were verified by AFM and XRD measurements. The optimized GaN layer presents a smooth surface with a rms value of 1.4 Å. The full width at half maximum (FWHM) for 800 nm thick GaN films is 144″. Furthermore PL measurements and C–V analysis confirm that in GaN layer grown on HT-AlN buffer layer defect density is drastically reduced.