Research

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961 KiB

Open AccessArticle

Room Temperature Ferromagnetic, Anisotropic, Germanium Rich FeGe(001) Alloys

by George A. Lungu, Nicoleta G. Apostol, Laura E. Stoflea, Ruxandra M. Costescu, Dana G. Popescu and Cristian M. Teodorescu

Materials 2013, 6(2), 612-625; https://doi.org/10.3390/ma6020612 - 21 Feb 2013

Cited by 14 | Viewed by 5592

Abstract

Ferromagnetic Fe_xGe_1−x with x = 2%–9% are obtained by Fe deposition onto Ge(001) at high temperatures (500 °C). Low energy electron diffraction (LEED) investigation evidenced the preservation of the (1 × 1) surface structure of Ge(001) with Fe deposition. [...] Read more.

Ferromagnetic Fe_xGe_1−x with x = 2%–9% are obtained by Fe deposition onto Ge(001) at high temperatures (500 °C). Low energy electron diffraction (LEED) investigation evidenced the preservation of the (1 × 1) surface structure of Ge(001) with Fe deposition. X-ray photoelectron spectroscopy (XPS) at Ge 3d and Fe 2p core levels evidenced strong Fe diffusion into the Ge substrate and formation of Ge-rich compounds, from FeGe₃ to approximately FeGe₂, depending on the amount of Fe deposited. Room temperature magneto-optical Kerr effect (MOKE) evidenced ferromagnetic ordering at room temperature, with about 0.1 Bohr magnetons per Fe atom, and also a clear uniaxial magnetic anisotropy with the in-plane easy magnetization axis. This compound is a good candidate for promising applications in the field of semiconductor spintronics. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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231 KiB

Open AccessArticle

“Property Phase Diagrams” for Compound Semiconductors through Data Mining

by Srikant Srinivasan and Krishna Rajan

Materials 2013, 6(1), 279-290; https://doi.org/10.3390/ma6010279 - 21 Jan 2013

Cited by 30 | Viewed by 7101

Abstract

This paper highlights the capability of materials informatics to recreate “property phase diagrams” from an elemental level using electronic and crystal structure properties. A judicious selection of existing data mining techniques, such as Principal Component Analysis, Partial Least Squares Regression, and Correlated Function [...] Read more.

This paper highlights the capability of materials informatics to recreate “property phase diagrams” from an elemental level using electronic and crystal structure properties. A judicious selection of existing data mining techniques, such as Principal Component Analysis, Partial Least Squares Regression, and Correlated Function Expansion, are linked synergistically to predict bandgap and lattice parameters for different stoichiometries of Ga_xIn_1−xAs_ySb_1−y, starting from fundamental elemental descriptors. In particular, five such elemental descriptors, extracted from within a database of highly correlated descriptors, are shown to collectively capture the widely studied “bowing” of energy bandgaps seen in compound semiconductors. This is the first such demonstration, to our knowledge, of establishing relationship between discrete elemental descriptors and bandgap bowing, whose underpinning lies in the fundamentals of solid solution thermodyanamics. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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1563 KiB

Open AccessArticle

Growth of High-Density Zinc Oxide Nanorods on Porous Silicon by Thermal Evaporation

by Nurul Izni Rusli, Masahiro Tanikawa, Mohamad Rusop Mahmood, Kanji Yasui and Abdul Manaf Hashim

Materials 2012, 5(12), 2817-2832; https://doi.org/10.3390/ma5122817 - 13 Dec 2012

Cited by 62 | Viewed by 10349

Abstract

The formation of high-density zinc oxide (ZnO) nanorods on porous silicon (PS) substrates at growth temperatures of 600–1000 °C by a simple thermal evaporation of zinc (Zn) powder in the presence of oxygen (O₂) gas was systematically investigated. The high-density growth [...] Read more.

The formation of high-density zinc oxide (ZnO) nanorods on porous silicon (PS) substrates at growth temperatures of 600–1000 °C by a simple thermal evaporation of zinc (Zn) powder in the presence of oxygen (O₂) gas was systematically investigated. The high-density growth of ZnO nanorods with (0002) orientation over a large area was attributed to the rough surface of PS, which provides appropriate planes to promote deposition of Zn or ZnO_x seeds as nucleation sites for the subsequent growth of ZnO nanorods. The geometrical morphologies of ZnO nanorods are determined by the ZnO_x seed structures, i.e., cluster or layer structures. The flower-like hexagonal-faceted ZnO nanorods grown at 600 °C seem to be generated from the sparsely distributed ZnO_x nanoclusters. Vertically aligned hexagonal-faceted ZnO nanorods grown at 800 °C may be inferred from the formation of dense arrays of ZnO_x clusters. The formation of disordered ZnO nanorods formed at 1000 °C may due to the formation of a ZnO_x seed layer. The growth mechanism involved has been described by a combination of self-catalyzed vapor-liquid-solid (VLS) and vapor-solid (VS) mechanism. The results suggest that for a more precise study on the growth of ZnO nanostructures involving the introduction of seeds, the initial seed structures must be taken into account given their significant effects. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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1632 KiB

Open AccessArticle

Spectroscopy of Deep Traps in Cu₂S-CdS Junction Structures

by Eugenijus Gaubas, Ievgen Brytavskyi, Tomas Ceponis, Vidmantas Kalendra and Audrius Tekorius

Materials 2012, 5(12), 2597-2608; https://doi.org/10.3390/ma5122597 - 04 Dec 2012

Cited by 5 | Viewed by 5376

Abstract

Cu₂S-CdS junctions of the polycrystalline material layers have been examined by combining the capacitance deep level transient spectroscopy technique together with white LED light additional illumination (C-DLTS-WL) and the photo-ionization spectroscopy (PIS) implemented by the photocurrent probing. Three types of junction [...] Read more.

Cu₂S-CdS junctions of the polycrystalline material layers have been examined by combining the capacitance deep level transient spectroscopy technique together with white LED light additional illumination (C-DLTS-WL) and the photo-ionization spectroscopy (PIS) implemented by the photocurrent probing. Three types of junction structures, separated by using the barrier capacitance characteristics of the junctions and correlated with XRD distinguished precipitates of the polycrystalline layers, exhibit different deep trap spectra within CdS substrates. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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441 KiB

Open AccessArticle

First Principles Study on Electronic Structure and Optical Properties of Ternary GaAs:Bi Alloy

by Lifei Yu, Dechun Li, Shengzhi Zhao, Guiqiu Li and Kejian Yang

Materials 2012, 5(12), 2486-2497; https://doi.org/10.3390/ma5122486 - 26 Nov 2012

Cited by 42 | Viewed by 8305

Abstract

The electronic structure and optical properties of ternary GaAs:Bi alloy are investigated by first principles calculations. It is found that the band gap of GaAs_1-xBi_xdecreases monotonously with the increasing of Bi concentration, resulting in the fundamental absorption edge and [...] Read more.

The electronic structure and optical properties of ternary GaAs:Bi alloy are investigated by first principles calculations. It is found that the band gap of GaAs_1-xBi_xdecreases monotonously with the increasing of Bi concentration, resulting in the fundamental absorption edge and main absorption peaks of GaAs_1-xBi_x shifting toward lower energy with the increase of the Bi content. The optical constants of GaAs_1-xBi_x, such as the optical absorption coefficient, refractive index, extinction coefficient and optical conductivity, are greater than those of pure GaAs when x > 3.1%, but less than those of pure GaAs when x < 3.1%, which is primarily decided by the intraband level repulsions between Bi-induced states and host states on the valence bands; the contribution of Bi-6s, Bi-6p orbitals and Ga-4p, Ga-4s orbitals on conduction bands is also crucial. Bi doping plays an important role in the modulation of the static dielectric constant and the static refractive index. These results suggest a promising application of GaAs_1-xBi_x alloy as a semiconductor saturable absorber. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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523 KiB

Open AccessArticle

Graphene as a Buffer Layer for Silicon Carbide-on-Insulator Structures

by Budi Astuti, Masahiro Tanikawa, Shaharin Fadzli Abd Rahman, Kanji Yasui and Abdul Manaf Hashim

Materials 2012, 5(11), 2270-2279; https://doi.org/10.3390/ma5112270 - 09 Nov 2012

Cited by 10 | Viewed by 7068

Abstract

We report an innovative technique for growing the silicon carbide-on-insulator (SiCOI) structure by utilizing polycrystalline single layer graphene (SLG) as a buffer layer. The epitaxial growth was carried out using a hot-mesh chemical vapor deposition (HM-CVD) technique. Cubic SiC (3C-SiC) thin film in [...] Read more.

We report an innovative technique for growing the silicon carbide-on-insulator (SiCOI) structure by utilizing polycrystalline single layer graphene (SLG) as a buffer layer. The epitaxial growth was carried out using a hot-mesh chemical vapor deposition (HM-CVD) technique. Cubic SiC (3C-SiC) thin film in (111) domain was realized at relatively low substrate temperature of 750 °C. 3C-SiC energy bandgap of 2.2 eV was confirmed. The Si-O absorption band observed in the grown film can be caused by the out-diffusion of the oxygen atom from SiO₂ substrate or oxygen doping during the cleaning process. Further experimental works by optimizing the cleaning process, growth parameters of the present growth method, or by using other growth methods, as well, are expected to realize a high quality SiCOI structure, thereby opening up the way for a breakthrough in the development of advanced ULSIs with multifunctionalities. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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748 KiB

Open AccessArticle

Electronic and Optical Properties of Substitutional and Interstitial Si-Doped ZnO

by Hsuan-Chung Wu, Yen-Chun Peng and Tsu-Ping Shen

Materials 2012, 5(11), 2088-2100; https://doi.org/10.3390/ma5112088 - 29 Oct 2012

Cited by 65 | Viewed by 7804

Abstract

This study investigates the formation energies, electronic structures, and optical properties of pure and Si-doped ZnO using density functional theory and the Hubbard U (DFT + U_d + U_p) method. The difference in lattice constants between calculated results and experimental [...] Read more.

This study investigates the formation energies, electronic structures, and optical properties of pure and Si-doped ZnO using density functional theory and the Hubbard U (DFT + U_d + U_p) method. The difference in lattice constants between calculated results and experimental measurements is within 1%, and the calculated band gap of pure ZnO is in excellent agreement with experimental values. This study considers three possible Si-doped ZnO structures including the substitution of Si for Zn (Si_s(Zn)), interstitial Si in an octahedron (Si_i(oct)), and interstitial Si in a tetrahedron (Si_i(tet)). Results show that the formation energy of Si_s(Zn) defects is the lowest, indicating that Si_s(Zn) defects are formed more easily than Si_i(oct) and Si_i(tet). All three of the Si defect models exhibited n-type conductive characteristics, and except for the Si_i(oct) mode the optical band gap expanded beyond that of pure ZnO. In both the Si_i(oct) and Si_i(tet) models, a heavier effective mass decreased carrier mobility, and deeper donor states significantly decreased transmittance. Therefore, the existence of interestitial Si atoms was bad for the electric and optical properties of ZnO. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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Review

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3553 KiB

Open AccessReview

Surface Stability and Growth Kinetics of Compound Semiconductors: An Ab Initio-Based Approach

by Yoshihiro Kangawa, Toru Akiyama, Tomonori Ito, Kenji Shiraishi and Takashi Nakayama

Materials 2013, 6(8), 3309-3360; https://doi.org/10.3390/ma6083309 - 06 Aug 2013

Cited by 82 | Viewed by 8850

Abstract

We review the surface stability and growth kinetics of III-V and III-nitride semiconductors. The theoretical approach used in these studies is based on ab initio calculations and includes gas-phase free energy. With this method, we can investigate the influence of growth conditions, such [...] Read more.

We review the surface stability and growth kinetics of III-V and III-nitride semiconductors. The theoretical approach used in these studies is based on ab initio calculations and includes gas-phase free energy. With this method, we can investigate the influence of growth conditions, such as partial pressure and temperature, on the surface stability and growth kinetics. First, we examine the feasibility of this approach by comparing calculated surface phase diagrams of GaAs(001) with experimental results. In addition, the Ga diffusion length on GaAs(001) during molecular beam epitaxy is discussed. Next, this approach is systematically applied to the reconstruction, adsorption and incorporation on various nitride semiconductor surfaces. The calculated results for nitride semiconductor surface reconstructions with polar, nonpolar, and semipolar orientations suggest that adlayer reconstructions generally appear on the polar and the semipolar surfaces. However, the stable ideal surface without adsorption is found on the nonpolar surfaces because the ideal surface satisfies the electron counting rule. Finally, the stability of hydrogen and the incorporation mechanisms of Mg and C during metalorganic vapor phase epitaxy are discussed. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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806 KiB

Open AccessReview

Degradation Mechanisms for GaN and GaAs High Speed Transistors

by David J. Cheney, Erica A. Douglas, Lu Liu, Chien-Fong Lo, Brent P. Gila, Fan Ren and Stephen J. Pearton

Materials 2012, 5(12), 2498-2520; https://doi.org/10.3390/ma5122498 - 27 Nov 2012

Cited by 35 | Viewed by 11347

Abstract

We present a review of reliability issues in AlGaN/GaN and AlGaAs/GaAs high electron mobility transistors (HEMTs) as well as Heterojunction Bipolar Transistors (HBTs) in the AlGaAs/GaAs materials systems. Because of the complex nature and multi-faceted operation modes of these devices, reliability studies must [...] Read more.

We present a review of reliability issues in AlGaN/GaN and AlGaAs/GaAs high electron mobility transistors (HEMTs) as well as Heterojunction Bipolar Transistors (HBTs) in the AlGaAs/GaAs materials systems. Because of the complex nature and multi-faceted operation modes of these devices, reliability studies must go beyond the typical Arrhenius accelerated life tests. We review the electric field driven degradation in devices with different gate metallization, device dimensions, electric field mitigation techniques (such as source field plate), and the effect of device fabrication processes for both DC and RF stress conditions. We summarize the degradation mechanisms that limit the lifetime of these devices. A variety of contact and surface degradation mechanisms have been reported, but differ in the two device technologies: For HEMTs, the layers are thin and relatively lightly doped compared to HBT structures and there is a metal Schottky gate that is directly on the semiconductor. By contrast, the HBT relies on pn junctions for current modulation and has only Ohmic contacts. This leads to different degradation mechanisms for the two types of devices. Full article

(This article belongs to the Special Issue Compound Semiconductor Materials)

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