Effects of polarization field on formation of two-dimensional electron gas in (0001) and (11(2)over-bar0) plane AlGaN/GaN heterostructures

Photoluminescence (PL) and temperature-dependent Hall effect measurements were carried out in (0001) and (11 (2) over bar0) AlGaN/GaN heterostructures grown on sapphire substrates by metalorganic chemical vapor deposition. There are strong spontaneous and piezoelectric electric fields (SPF) along the growth orientation of the (0001) AlGaN/GaN heterostructures. At the same time there are no corresponding SPF along that of the (1120) AlGaN/GaN. A strong PL peak related to the recombination between two-dimensional electron gas (2DEG) and photoexcited holes was observed at 3.258 eV at room temperature in (0001) AlGaN/GaN heterointerfaces while no corresponding PL peak was observed in (11 (2) over bar0). The existence of a 2DEG was observed in (0001) AlGaN/GaN multi-layers with a mobility saturated at 6000 cm(2)/V s below 80 K, whereas a much lower mobility was measured in (11 (2) over bar0). These results indicated that the SPF was the main element to cause the high mobility and high sheet-electron-density 2DEG in AlGaN/GaN heterostructures. (C) 2004 Elsevier B.V. All rights reserved.

Effects of In surfactant on the crystalline and photoluminescence properties of GaN nanowiires

GaN nanowires have been grown with and without In as an additional source. The effects of In surfactant on the crystal quality and photoluminescence property of GaN nanowires are reported for the first time. X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy, and photoluminescence measurements are employed to analyse the products. The results show that introducing a certain amount of In surfactant during the growth process can improve the crystal quality of the GaN nanowires, and enhance the photolurainescence of them. In addition, the as-prepared GaN nanowires have the advantage of being easy to be separated, which will benefit the subsequent nanodevice fabrication.

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photo luminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system. (c) 2004 Elsevier Ltd. All rights reserved.

Effects of growth temperature of AlGaN buffer layer on the properties of A1(0.58)Ga(0.42)N epilayer by NH3-MBE

Al0.58Ga0.42N epilayers are grown by ammonia gas source molecular beam epitaxy (NH3-MBE) on (0001) sapphire substrate using AlGaN buffer layer. The effects of the buffer layer growth temperature on the properties of Al0.58Ga0.42N epilayer are especially investigated. In-situ high-energy electron diffraction (RHEED), double-crystal X-ray diffraction (DCXRD), atomic force microscopy (AFM), photoconductivity measurement and cathodoluminescence (CL) are used to characterize the samples. It is found that high growth temperature of AlGaN buffer layer would improve the crystalline quality, surface smoothness, optical quality and uniformity of the Al0.58Ga0.42N epilayer. The likely reason for such improvements is also suggested. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Effects of doping on the crystalline quality and composition distribution in InGaN/GaN structure grown by MOCVD

The effects of Si and Mg doping on the crystalline quality and In distribution in the InGaN films were studied by atomic force microscope (AFM), triple crystal X-ray diffraction (TCXRD) and Rutherford backscattering spectrometry (RBS). The undoped, Si-doped and Mg-doped InGaN films were grown by metalorganic chemical vapor deposition (MOCVD) on (0 0 0 1) sapphire substrates. The electronic concentration in the Si-doped InGaN is about 2 x 10(19) cm(-3). It is found that the crystalline quality and In distribution in InGaN is slightly affected by the Si doping. In the Mg doped-case, the hole concentration is about 4 x 10(18) cm(-3) after annealing treatment. The surface morphology and crystalline quality of the Mg-doped InGaN are deteriorated significantly compared with the undoped InGaN. The growth rate of Mg-doped InGaN is higher than the undoped InGaN. Mg doping enhances the In incorporation in the InGaN alloy. The increase in In composition in the growth direction is more severe than the undoped InGaN. (c) 2006 Elsevier B.V. All rights reserved.

Effects of seed dot layer and thin GaAs spacer layer on the structure and optical properties of upper In(Ga)As quantum dots

The structure and optical properties of In(Ga)As with the introduction of InGaAlAs or InAlAs seed dot layers are investigated. The area density and size homogeneity of the upper InGaAs dots are efficiently improved by the introduction of a buried layer of high-density dots. Our explanation for the realization of high density and size homogeneity dots is presented. When the GaAs spacer layer is too thin to cover the seed dots, the upper dots exhibit some optical properties like those of a quantum well. By analyzing the growth dynamics, we refer to this kind of dot as an empty-core dot. (C) 2003 American Institute of Physics.

Influence of strain on annealing effects of In(Ga)As quantum dots

Post-growth rapid thermal annealing has been performed with In(Ga)As quantum dots (QDs) at different strain statuses. It is confirmed that the strain-enhanced interdiffusion decreases the inhomogeneous size distribution. The preferential lateral interdiffusion of QDs during annealing was observed. we attribute it to the naturally anisotropic strain distribution in/around the dots and the saturation of strain difference between the base boundary and the top of the dots. There exist strain-enhanced mechanism and vacancy diffusion enhanced mechanism during the annealing. As to which one dominates the QD interdiffusion depends on the thickness of capping layer and the annealing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

The effects of carbonized buffer layer on the growth of SiC on Si

Carbonized buffer layers were formed with C2H4 on Si(100) and Si(111) substrates using different methods and SIC epilayers were grown on each buffer layer at 1050 degrees C with simultaneous supply of C2H4 and Si2H6. The structure of carbonized and epitaxy layers was analyzed with in situ RHEED. The buffer layers formed at 800 degrees C were polycrystalline on both Si(100) and Si(111) substrates whereas they were single crystals, with twins on Si(100) and without tu ins on Si(111)substrates. when formed with a gradual rise in substrate temperature from 300 degrees C to growth temperature. Raising the substrate temperature slowly results in the formation of more twins. Epilayers grown on carbonized polycrystalline lavers are polycrystalline. Single crystal epilayers without twins grow on single crystalline buffer layers without twins or with a few twins. (C) 1999 Elsevier Science B.V. All rights reserved.

Effects of growth interruption on self-assembled InAs/GaAs islands

The effect of growth interruption on the InAs deposition and its subsequent growth as self-assembled island structures, in particular the material transport process of the InAs layers has been investigated by photoluminescence and transmission electron microscopy measurements. InAs material in structures with only coherent islands transfers from the wetting layer to the formed islands and the growth interruption causes a red shift of PL peak energy. On the other hand, the PL peak shifts to higher energy in structures containing simultaneously coherent and noncoherent islands with dislocations. In this case, the noncoherent islands capture InAs material from the surrounding wetting layer as well as coherent islands, which casues a reduction in the size of these islands. The variations in the PL intensity and line width are also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.

Quantum-confined Stark effects of exciton states in V-shaped GaAs/AlxGa1-xAs quantum wires

Quantum-confined Stark effects are investigated theoretically in GaAs/AlxGa1-xAs quantum wires formed in V-grooved structures. The electronic structures of the V-shaped quantum wires are calculated within the effective mass envelope function theory in the presence of electric field. The binding energies of excitons are also studied by two-dimensional Fourier transformation and variational method. The blue Stark shifts are found when the electric field is applied in the growth direction. A possible mechanism in which the blueshifts of photoluminescence peaks are attributed to two factors, one factor comes from the asymmetric structure of quantum wire along the electric field and another factor arises from the electric-field-induced change of the Coulomb interaction. The numerical results are compared with the recent experiment measurement.

Effects of growth temperature on highly mismatched InAs grown on GaAs substrates by MBE

InAs layers were grown on GaAs by molecular beam epitaxy (MBE) at substrate temperature 450 and 480 degrees C, and the surface morphology was studied with scanning electron microscopy (SEM). We have observed a high density of hexagonal deep pits for samples grown at 450 degrees C, however, the samples grown at 480 degrees C have smooth surface. The difference of morphology can be explained by different migration of cations which is temperature dependent. Cross-sectional transmission electron microscopy (XTEM) studies showed that the growth temperature also affect the distributions of threading dislocations in InAs layers because the motion of dislocations is kinetically limited at lower temperature. (C) 1998 Elsevier Science B.V. All rights reserved.

Effects of annealing treatment on the formation of CO2 in ZnO thin films grown by metal-organic chemical vapor deposition

Post-growth annealing was carried out on ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD). The grain size of ZnO thin film increases monotonically with annealing temperature. The ZnO thin films were preferential to c-axis oriented after annealing as confirmed by Xray diffraction (XRD) measurements. Fourier transformation infrared transmission measurements showed that ZnO films grown at low temperature contains CO2 molecules after post-growth annealing. A two-step reaction process has been proposed to explain the formation mechanism of CO2, which indicates the possible chemical reaction processes during the metal-organic chemical vapor deposition of ZnO films.

Effects of polarization field on formation of two-dimensional electron gas in (0001) and (11(2)over-bar0) plane AlGaN/GaN heterostructures

Photoluminescence (PL) and temperature-dependent Hall effect measurements were carried out in (0001) and (11 (2) over bar0) AlGaN/GaN heterostructures grown on sapphire substrates by metalorganic chemical vapor deposition. There are strong spontaneous and piezoelectric electric fields (SPF) along the growth orientation of the (0001) AlGaN/GaN heterostructures. At the same time there are no corresponding SPF along that of the (1120) AlGaN/GaN. A strong PL peak related to the recombination between two-dimensional electron gas (2DEG) and photoexcited holes was observed at 3.258 eV at room temperature in (0001) AlGaN/GaN heterointerfaces while no corresponding PL peak was observed in (11 (2) over bar0). The existence of a 2DEG was observed in (0001) AlGaN/GaN multi-layers with a mobility saturated at 6000 cm(2)/V s below 80 K, whereas a much lower mobility was measured in (11 (2) over bar0). These results indicated that the SPF was the main element to cause the high mobility and high sheet-electron-density 2DEG in AlGaN/GaN heterostructures. (C) 2004 Elsevier B.V. All rights reserved.

The effects of carbonized buffer layer on the growth of SiC on Si

Carbonized buffer layers were formed with C2H4 on Si(100) and Si(111) substrates using different methods and SIC epilayers were grown on each buffer layer at 1050 degrees C with simultaneous supply of C2H4 and Si2H6. The structure of carbonized and epitaxy layers was analyzed with in situ RHEED. The buffer layers formed at 800 degrees C were polycrystalline on both Si(100) and Si(111) substrates whereas they were single crystals, with twins on Si(100) and without tu ins on Si(111)substrates. when formed with a gradual rise in substrate temperature from 300 degrees C to growth temperature. Raising the substrate temperature slowly results in the formation of more twins. Epilayers grown on carbonized polycrystalline lavers are polycrystalline. Single crystal epilayers without twins grow on single crystalline buffer layers without twins or with a few twins. (C) 1999 Elsevier Science B.V. All rights reserved.

Effects of forced solution flow on lysozyme crystal growth

In order to study quantitatively the effects of forced solution on crystal growth, we designed a new set of experimental equipment, in particular, a microchannel mixer was used as crystallization container so that the consumption of protein samples was much reduced and thus an exact syringe pump could be used for precise control of the flow rates. Since the mixer’s section was designed to be rectangular, the solution velocity in its center was steady and constant, and thus repeatable experiments were facilitated. Experimental results showed that the effects of forced solution on protein crystal growth were different under different levels of supersaturation, and new results were obtained for cases of high supersaturation. When the supersaturation is σ = 2.3, with increasing flow rates the growth rates of the lysozyme crystal’s (110) face hardly change when the flow rates are lower than 1300 μm/s, and decrease quickly afterwards. When the flow rate reaches 2000 μm/s, the crystal nearly ceases to grow. When the supersaturation is σ = 2.7, with increasing flow rates the (110) face growth rates increase at the beginning then reach the maximum values at 1700 μm/s – 1900 μm/s and decrease afterwards, approaching zero or so when the flow rate reaches 12000 μm/s. The higher the supersaturation, the larger the flow rate at which the crystal ceases to grow. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim