Nitrogen Plasma-assisted Molecular Beam Epitaxy Research Articles

Ultrathin TiN Epitaxial Films as Transparent Conductive Electrodes.

Titanium nitride (TiN), a transition-metal compound with tight covalent Ti-N bonding, has a high melting temperature and superior mechanical and chemical stabilities compared to noble metals. With a reduction in thickness, the optical transmittance of TiN films can be drastically increased, and in combination with its excellent electrical conductivity, the ultrathin and continuous TiN film can be considered as an ideal alternative of the metal oxide electrodes. However, the deposition of ultrathin and continuous metallic layer with a smooth surface morphology is a major challenge for typical deposition methods such as thermal evaporation or reactive sputtering. In particular, defects mainly related with oxygen contents and surface scattering can significantly limit the performance of ultrathin TiN films. In this work, ultrathin TiN films with 2-10 nm in thickness are grown by using the nitrogen plasma-assisted molecular-beam epitaxy (MBE) method in an ultrahigh vacuum environment. Excellent surface morphology with a root-mean-square roughness of ≤0.12 nm and a high optical transparency of 75% over the whole visible regime are achieved for ultrathin TiN epitaxial films. The dielectric properties determined by the spectroscopic ellipsometry and the electrical properties measured by the terahertz spectroscopy and the Hall effect method reveal that the percolation thickness of the TiN epitaxial film is less than 2.4 nm and its electrical conductivity is higher than 1.1 × 104 Ω-1 cm-1. These features make MBE-grown ultrathin TiN epitaxial films a good candidate for robust, low cost, and large-area transparent conductive electrodes.

Method for Controlling the Polarity of Gallium Nitride Layers in Epitaxial Synthesis of GaN/AlN Heterostructures on Hybrid SiC/Si Substrates

The effect of the inversion of polarity in gallium nitride layers from the N-polar GaN layer to the Ga-polar GaN layer was discovered during the sequential growth of GaN films on hybrid SiC/Si(111) substrates by nitrogen plasma-assisted molecular beam epitaxy and chloride–hydride vapor phase epitaxy. A new method has been developed for the formation of crack-free Ga-polar GaN/AlN heterostructures on hybrid SiC/Si(111) substrates. The method includes two stages of growing gallium nitride layers. At the first stage, the transient N-polar GaN layer is grown on the SiC/Si(111) surface by nitrogen plasma-assisted molecular beam epitaxy. At the second stage, two layers are grown on the obtained N-polar GaN layer by chloride–hydride vapor phase epitaxy, namely, the AlN layer and then the GaN layer, which at this stage grows in the Ga-polar orientation. Etching in a KOH solution affects only the N-polar GaN transition layer and leads to its complete removal. This procedure separates the main Ga-polar GaN layer from the SiC/Si(111) substrate completely. The method enables one to grow crack-free and elastically unstressed thick GaN layers and transfer them to substrates of other materials.

Spontaneous Formation of Indium Clusters in InN Epilayers Grown by Molecular-Beam Epitaxy

We have studied the influence of growth conditions on the number of metallic indium clusters formed spontaneously in indium nitride (InN) layers grown by nitrogen plasma-assisted molecular-beam epitaxy (PAMBE). InN epilayers of N-and In-polarity were grown on c-sapphire substrates and GaN and AlN templates, respectively. N-polar layers were obtained in the standard PAMBE regime, while In-polar layers were grown using a three-stage regime including the stages of epitaxy with enhanced atomic migration and interruption of growth under nitrogen flow. A series of samples were prepared at various growth temperatures and relative In/N flow rates. Measurement of the magnetic-field dependences of the Hall-effect coefficient and its model approximation were used to determine the percentage content of In clusters in various InN layers and the minimum amount of such inclusions that can be achieved by varying the conditions of MBE growth.

Спонтанное формирование кластеров In в эпитаксиальных слоях InN, выращенных методом молекулярно-пучковой эпитаксии

AbstractWe have studied the influence of growth conditions on the number of metallic indium clusters formed spontaneously in indium nitride (InN) layers grown by nitrogen plasma-assisted molecular-beam epitaxy (PAMBE). InN epilayers of N-and In-polarity were grown on c-sapphire substrates and GaN and AlN templates, respectively. N-polar layers were obtained in the standard PAMBE regime, while In-polar layers were grown using a three-stage regime including the stages of epitaxy with enhanced atomic migration and interruption of growth under nitrogen flow. A series of samples were prepared at various growth temperatures and relative In/N flow rates. Measurement of the magnetic-field dependences of the Hall-effect coefficient and its model approximation were used to determine the percentage content of In clusters in various InN layers and the minimum amount of such inclusions that can be achieved by varying the conditions of MBE growth.

Deep level defects in N-rich and In-rich InxGa1−XN: in composition dependence

This study focuses on the presence and electronic properties of deep levels in In- and N-rich InxGa1−xN materials with different growth temperature, causing the different In composition, grown by nitrogen plasma-assisted molecular beam epitaxy (PAMBE). Semi-transparent Ag/Au Schottky contacts were formed on the InxGa1−xN layers and the device performances has been investigated with varying In contents. Increase in the leakage current density with the increase of In content has been observed in both In- and N-rich samples. Deep level optical spectroscopy has shown three deeper levels in both In-rich and N-rich InxGa1−xN layers. 1.4–1.6 eV level below the conduction band has been observed in all the samples, while the energy values of the other deep levels varies by sample to sample. The deepest levels are found to be the most prominent with the concentration in the range of 1016 cm−3 for high In content InxGa1−xN for both In-rich and N-rich growth conditions. Almost 4–5 times higher total trap concentrations have been observed for higher In InxGa1−xN sample.

AlxGa1−xN/GaN/AlN heterostructures grown on Si(1 1 1) substrates by MBE for MSM UV photodetector applications

AlxGa1−xN/GaN/AlN heterostructures on silicon (Si) substrate was developed by nitrogen plasma-assisted molecular beam epitaxy (MBE) and their properties were investigated by scanning electron microscopy (SEM), electron dispersive X-ray (EDX), atomic force microscopy (AFM), high resolution X-ray diffraction (XRD), Raman spectroscopy and Hall effect measurements. High purity gallium (7N) and aluminum (6N5) were used in the Knudsen cells. High purity nitrogen with 7N purity was supplied to radio frequency (RF) source to generate reactive nitrogen species. The nitrogen pressure and a discharge power were kept at 1.5×10−5Torr and 300W, respectively. From SEM measurements, the surface morphology of samples presented 2- and 3-dimensional growth modes. The EDX measurements showed that there were no foreign elements in the grown samples. The HR-XRD measurement has confirmed that the AlxGa1−xN/GaN/AlN heterostructures samples were epitaxially grown on Si substrate. All the dominant E2 phonon modes were found in Raman spectra results. Lastly, AlxGa1−xN/GaN/AlN heterostructures based metal–semiconductor–metal (MSM) UV photodetectors were fabricated and the electrical characteristics of the devices were investigated by using current–voltage (I–V) and photo-conductivity measurements. The devices presented good I–V and photoconductivity characteristics.

Temperature- and Al/N ratio-dependent AlN seed layer formation on (110) Si substrates by using plasma-assisted molecular beam epitaxy

AlN seed layers with a thickness of 50 nm were grown by using nitrogen plasma-assisted molecular beam epitaxy on (110) Si substrates with different V/III ratios in the temperature range from 850 °C to 940 °C. In varying the Al/N ratio and the growth temperature, distinct surface morphologies emerge, which are quite different from those observed in AlN growth on (111) Si substrates. Under N-rich conditions, AlN films exhibits randomly distributed islands with different sizes ranging from 10 nm to 1 m. In Al-rich conditions, two distinct surface morphologies, (1) closely-packed islands preferentially aligned along the [1120]AIN/[110]Si azimuth and (2) smooth flat surfaces, are observed at various growth temperatures. The observed morphology transition is attributed to the asymmetric strain distribution between hexagonal symmetric AlN layers and rectangular (110) Si substrates and to varying surface adatom migration rates present at different growth temperatures.

Effect of thermal annealing on ohmic contacts properties of undoped and Si-doped AlxGa1−xN on Si (1 1 1) substrate grown by PA-MBE

In this work, we study the ohmic contact properties of titanium (Ti)/aluminum (Al) bi-layer contacts on undoped and n-type doped AlxGa1−xN grown on silicon (111) substrates by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PA-MBE). The electrical stability of the contacts at various annealing temperatures of 400, 500, 600 and 700°C were investigated. Specific contact resistivity was determined using transmission line method (TLM) and current–voltage (I–V) measurements. The results reveal that the bi-layer scheme was sensitive to the change of annealing temperatures and annealing time. The optimal value of specific contact resistivities was obtained at annealing temperature of 600°C for both samples. However, the values of n-type doped sample exhibited better results compared with the undoped sample.

Fabrication and characterization of ZnO nanostructures on Si(111) substrate using a thin AlN buffer layer

In the present work, radio-frequency (RF) nitrogen plasma-assisted molecular beam epitaxy (PA-MBE) technique was used to grow AlN thin layers on Si(111) substrate. Subsequently, the thermal evaporation technique was used to deposit the zinc films on Si(111) substrates with AlN as buffer layer. ZnO nanostructures were obtained from zinc granulated (99.99%) by thermal oxidation from 400 °C to 600 °C in air for 1 hours without any catalysts. The effect of annealing temperatures were studied ranging from 400 °C to 600 °C in air for 1 hours. The AlN was introduced to accommodate the lattice mismatch and thermal expansion mismatch between ZnO layer and Si substrate. The structural and optical properties of ZnO nanostructures are studied through scanning electron microscopy (SEM), X-ray diffraction (XRD) and room temperature photoluminescence (PL) spectroscopy. The films show a polycrystalline hexagonal wurtzite structure without preferred (0002) orientation. The mean grain sizes are calculated to be about 18 nm, 22 nm and 50 nm for the ZnO films prepared at temperatures of 400 °C, 500 °C and 600 °C. The structure of the fabricated nanomaterials were characterized by scanning electron microscopy (SEM). The PL spectra of the ZnO nanostructures having a sharp excitonic ultraviolet (UV) emission and very weak defect-related deep level visible emissions. It is showed that the ZnO nanostructures thermal annealed treatment was performed at 600 °C shows the strongest UV emission intensity among the temperatures ranges studied. In addition, from the one-dimensional ZnO nanostructures thermal annealed at 600 °C, the stronger UV emission is assigned to the best crystalline quality of the ZnO film

GaN ON SILICON SUBSTRATE WITH AlN BUFFER LAYER FOR UV PHOTODIODE

Nitrogen plasma-assisted molecular beam epitaxy (PAMBE) deposited GaN thin films on (111) n-type silicon substrate with different thickness AlN buffer layers are investigated and distinguished by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman scattering. The thickness of AlN buffer layer ranged from 200 nm to 300 nm. Besides that, the electrical characteristics of the GaN thin film for ultraviolet detecting utilizations are studied by calculating the photo current/dark current ratio on a metal-semiconductor-metal (MSM) photodiode with and without the illumination of Hg-lamp source. The devices have been tested over room temperature (RT). The photocurrent analysis, together with the study of Schottky barrier height (SBH) development, ascertain that the principal mechanism of photo transport is thermionic emission. The photocurrent value is rigorously dependent on Schottky barrier height. The GaN/AlN(200 nm)/n-Si MSM photodiode produces the highest photo/dark current ratio for the lowest strain that consists of the GaN film grown on the AlN (200 nm) buffer layer.

Growth of InN layers on Si (111) using ultra thin silicon nitride buffer layer by NPA-MBE

Indium nitride (InN) epilayers have been successfully grown by nitrogen-plasma-assisted molecular beam epitaxy (NPA-MBE) on Si (111) substrates using different buffer layers. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by high resolution X-ray diffraction (HRXRD). The Raman studies show the high crystalline quality and the wurtzite lattice structure of InN films on the Si substrate using different buffer layers and the InN/β–Si 3N 4 double buffer layer achieves minimum FWHM of E 2 (high) mode. The energy gap of InN films was determined by optical absorption measurement and found to be in the range of ~ 0.73–0.78 eV with a direct band nature. It is found that a double-buffer technique (InN/β–Si 3N 4) insures improved crystallinity, smooth surface and good optical properties.

Tetrapod-Like ZnO Nanostructures Deposited on Si Substrates with AlN as Buffer Layer

In this work, radio-frequency (RF) nitrogen plasma-assisted molecular beam epitaxy (PA-MBE) technique was used to grow AlN layers on Si(111) substrate. Subsequently, the thermal evaporation technique was used to deposit the zinc films on Si(111) substrates with AlN as buffer layer. The effect of annealing temperatures ranging from 600°C to 700°C in air for 1 h on the structural and optical properties of ZnO films is evaluated using X-ray diffraction (XRD) and room temperature photoluminescence (PL) spectroscopy. Tetrapod-like ZnO nanostructures were obtained from zinc granulated (99.99%) by thermal oxidation at 600°C in air for 1 h without any catalysts. The films show a polycrystalline hexagonal (wurtzite) structure without preferred (0002) orientation. The mean grain sizes are calculated to be about 22 nm and 50 nm for the ZnO films prepared at temperatures of 600°C and 700°C. The as-synthesized ZnO nanostructures were characterized by using scanning electron microscopy (SEM). Room temperature PL spectra of the ZnO films exhibit dominant excitonic ultraviolet (UV) emission and weak deep level visible emission. It is perceived that the ZnO film annealed at 600°C shows the strongest UV emission intensity among the temperatures ranges studied. From the ZnO film annealed at 600°C, the excellent UV emission is assigned to the good crystalline quality of the ZnO film and the low rate of formation of intrinsic defects.

Tensile measurement of single crystal gallium nitride nanowires on MEMS test stages

This paper reports direct tensile tests on n-type (Si-doped) gallium nitride single crystal nanowires 1 1 Contribution of NIST, an agency of the U.S. government, not subject to copyright. that were grown by nitrogen plasma-assisted molecular beam epitaxy and which are essentially free of defects and residual strain. Nanowires were integrated with actuated, active microelectromechanical (MEMS) devices using dielectrophoresis-driven self-assembly and platinum-carbon clamps created using a gallium focused ion beam. For one nanowire, failure strain of 0.042 ± 0.011 was found. Most nanowire specimens appeared to demonstrate tensile strength in the range of 4.0 ± 1.7 GPa to 7.5 ± 3.4 GPa. Failure modes included clamp failure, transverse (nanowire c-plane) fractures, and insufficient force from the MEMS test actuator.

Ohmic contacts properties of Ni/Ag metallization scheme on p-type GaN

Abstract In the present work, we report on the characteristics of nickel(Ni)/silver(Ag) bi-layer contacts on p-type GaN. High quality doped GaN layers were grown on silicon (1 1 1) substrates using high temperature grown AlN (about 200 nm) as buffer layer by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PAMBE). Doping was done using high purity Mg as p-type dopant. In this work, a metal Ag layer was selected as the capping layer due to its reflective characteristics at visible wavelengths. The structural and electrical stability of the contacts at various annealing temperatures (400–700 °C) were investigated, as thermally stable metal semiconductor contacts are essential for high quality devices. Changes in the surface morphology of the contacts on annealing were examined using scanning electron microscopy (SEM). Specific contact resistivity was determined using transmission line method (TLM) and current–voltage ( I – V ) measurements. The low contact resistance is attributed to the reduction of the native oxide by Ni diffusion along with the formation of Ni and Ag related-gallide phases at the p-GaN surface region. We deduce that high Mg doping may lead to the creation of a large number of deep level defect in p-GaN, leading to the reduction of the depletion layer width in the p-GaN near the interface and an increase in the probability of thermionic filed emission.

Properties of InSbN grown on GaAs by radio frequency nitrogen plasma-assisted molecular beam epitaxy

We report the growth of InSbN on a lattice-mismatched GaAs substrate using radio frequency nitrogen plasma-assisted molecular beam epitaxy. The effects of a two-step thin InSb buffer layer grown at 330 and 380 °C and substrate temperature (270–380 °C) on the properties of the InSbN are studied. The crystalline quality of the InSbN is significantly improved by the two-step buffer layer due to defect suppression. The shifting in the absorption edge of the InSbN from ∼5 to 8 µm following an increase in the substrate temperature is correlated with the reduction in free carrier concentration from ∼1018 to 1016 cm−3 and increase in concentration of N substituting Sb from ∼0.2 to 1%. These results will be beneficial to those working on the pseudo-monolithic integration of InSbN detectors on a GaAs platform.

Defect characterization of InSb1−xNx grown using radio frequency nitrogen plasma-assisted molecular beam epitaxy

We report the growth of InSb1−xNx using radio frequency nitrogen plasma-assisted molecular beam epitaxy and its characterization using visible wavelength Raman scattering and x-ray diffraction. The effects of the growth temperature (330–420 °C) and the plasma power (200–500 W) on the N-induced defects were studied. Sb antisite defects from the A1gSb mode are shown to be dominant at a high growth temperature and a low plasma power. On the other hand, the high growth temperature and high plasma power induce the formation of interstitial Sb–N defects. A reduction in Sb related defects is observed at the lowest substrate temperature (330 °C) and plasma power (200 W). Based on the experimental results, a possible mechanism of defect formation is suggested.

High responsivity GaNAsSb p-i-n photodetectors at 13µm grown by radio-frequency nitrogen plasma-assisted molecular beam epitaxy

GaNAsSb/GaAs p-i-n photo notdetectors with an intrinsic GaNAsSb photoabsorption layer grown at 350 degrees C, 400 degrees C, 440 degrees C and 480 degrees C, have been prepared using radio-frequency nitrogen plasma-assisted molecular beam epitaxy in conjunction with a valved antimony cracker source. The i-GaNAsSb photoabsorption layer contains 3.3% of nitrogen and 8% of antimony, resulting in DC photo-response up to wavelengths of 1350 nm. The device with i-GaNAsSb layer grown at 350 degrees C exhibits extremely high photoresponsivity of 12A/W at 1.3 microm. These photodetectors show characteristics which strongly suggest the presence of carrier avalanche process at reverse bias less than 5V.

Growth and structural characteristics of GaN∕AlN/nanothick γ-Al2O3∕Si (111)

The authors report on the growth of GaN by nitrogen plasma-assisted molecular beam epitaxy (MBE) on a 2in. Si (111) substrates with a nanothick (∼4.8nm thick) γ-Al2O3 as a template/buffer. A thin layer of MBE-AlN ∼40nm thick was inserted prior to the growth of GaN. High-resolution transmission electron microscopy (HR-TEM) and high-resolution x-ray diffraction studies indicated that both of the nanothick γ-Al2O3 and AlN are a single crystal. Reflection high-energy electron diffraction, high-resolution x-ray scattering using synchrotron radiation, and cross-sectional HR-TEM measurements indicated an orientation relationship of GaN(0002)∥AlN(0002)∥γ-Al2O3(111)∥Si(111) and GaN[10−10]∥AlN[10−10]∥γ-Al2O3[2−1−1]∥Si[2−1−1]. A dislocation density of 5×(108–109)∕cm2 in the GaN ∼0.5μm thick was determined using cross-sectional TEM images under weak-beam dark-field conditions.

1.55 μ m GaAs∕GaNAsSb∕GaAs optical waveguides grown by radio frequency nitrogen plasma-assisted molecular beam epitaxy

We demonstrate a 1.55μm GaAs∕GaNAsSb∕GaAs optical waveguide grown by molecular beam epitaxy as an alternative to the AlGaAs∕GaAs system. The 0.4-μm-thick GaNAsSb guiding layer contains ∼3.5% of N and 9% of Sb, resulting in optical band gap of 0.88eV. The refractive index of the GaNAsSb layer was measured from 800to1700nm. The GaNAsSb layer has a refractive index value of 3.42 at 1.55μm wavelength. The propagation loss measured using the Fabry–Pérot resonance method was found to be affected by nitrogen-related defect absorption.

THE STUDY OF Pd SCHOTTKY CONTACT ON POROUS GaN FOR UV METAL–SEMICONDUCTOR–METAL (MSM) PHOTODETECTORS

High quality unintentionally doped n-type GaN layers were grown on Si (111) substrate using AlN (about 200 nm) as buffer layer by radio frequency (RF) nitrogen plasma-assisted molecular beam epitaxy (MBE). This paper presents the structural and optical studies of porous GaN sample compared to the corresponding as-grown GaN. Metal–semiconductor–metal (MSM) photodiode was fabricated on the samples. For as-grown GaN-based MSM, the detector shows a sharp cut-off wavelength at 362 nm, with a maximum responsivity of 0.254 A/W achieved at 360 nm. For porous GaN MSM detector, a sharp cut-off wavelength at 360 nm with a maximum responsivity of 0.655 A/W was achieved at 359 nm. Both the detectors show a little decrease in responsivity in the UV spectral region. The MSM photodiode based on porous GaN shows enhanced (2×) magnitude of responsivity relative to the as-grown GaN MSM photodiode. Enhancement of responsivity can be attributed to the relaxation of tensile stress and reduction of surface pit density in the porous sample.