Fabrication of Ag nanoparticle/ZnO thin films using dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with photoreduction method and its application

We report a novel method to grow silver nanoparticle/zinc oxide (Ag NP/ZnO) thin films using a dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with a photoreduction method. The crystalline quality, optical properties, and electrical characteristics of Ag NP/ZnO thin films depend on the AgNO₃ concentration or Ag content and annealing temperature. Optimal Ag NP/ZnO thin films have been grown with a AgNO₃ concentration of 0.12 M or 2.54 at%- Ag content and 500 °C- rapid thermal annealing (RTA); these films show orientation peaks of hexagonal-wurtzite-structured ZnO (002) and face-center-cubic-crystalline Ag (111), respectively. The transmittance and resistivity for optimal Ag NP/ZnO thin films are 85% and 6.9×10⁻⁴ Ω cm. Some Ag NP/ZnO transparent conducting oxide (TCO) films were applied to InGaN/GaN LEDs as transparent conductive layers. The InGaN/GaN LEDs with optimal Ag NP/ZnO TCO films showed electric and optical performance levels similar to those of devices fabricated with indium tin oxide.     

Epitaxial growth of nonpolar GaN films on r-plane sapphire substrates by pulsed laser deposition

Single-crystalline nonpolar GaN epitaxial films have been successfully grown on r-plane sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) with an in-plane epitaxial relationship of GaN[1-100]//Al₂O₃[11-20]. The properties of the ~500 nm-thick nonpolar GaN epitaxial films grown at temperatures ranging from 450 to 880 °C are studied in detail. It is revealed that the surface morphology, the crystalline quality, and the interfacial property of as-grown ~500 nm-thick nonpolar GaN epitaxial films are firstly improved and then decreased with the growth temperature changing from 450 to 880 °C. It shows an optimized result at the growth temperature of 850 °C, and the ~500 nm-thick nonpolar GaN epitaxial films grown at 850 °C show very smooth surface with a root-mean-square surface roughness of 5.5 nm and the best crystalline quality with the full-width at half-maximum values of X-ray rocking curves for GaN(11-20) and GaN(10-11) of 0.8° and 0.9°, respectively. Additionally, there is a 1.7 nm-thick interfacial layer existing between GaN epitaxial films and r-plane sapphire substrates. This work offers an effective approach for achieving single-crystalline nonpolar GaN epitaxial films for the fabrication of nonpolar GaN-based devices.     

Effect of AlN buffer layer thickness on the properties of GaN films grown by pulsed laser deposition

The GaN films are grown by pulsed laser deposition (PLD) on sapphire, AlN(30 nm)/Al₂O₃ and AlN(150 nm)/Al₂O₃, respectively. The effect of AlN buffer layer thickness on the properties of GaN films grown by PLD is investigated systematically. The characterizations reveal that as AlN buffer layer thickness increases, the surface root-mean-square (RMS) roughness of GaN film decreases from 11.5 nm to 2.3 nm, while the FWHM value of GaN film rises up from 20.28 arcmin to 84.6 arcmin and then drops to 31.8 arcmin. These results are different from the GaN films deposited by metal organic chemical vapor deposition (MOCVD) with AlN buffer layers, which shows the improvement of crystalline qualities and surface morphologies with the thickening of AlN buffer layer. The mechanism of the effect of AlN buffer layer on the growth of GaN films by PLD is hence proposed.     

The effect of temperature on the growth and properties of green light-emitting In0.5Ga0.5N films prepared by reactive sputtering with single cermet target

We have grown In_0.5Ga_0.5N films on SiO₂/Si (100) substrate at 100–400 °C for 90 min by rf reactive sputtering with single cermet target. The target was made by hot pressing the mixture of metallic indium, gallium and ceramic gallium nitride powder. X-ray diffraction (XRD) measurements indicated that In_0.5Ga_0.5N films had wurtzite structure and showed the preferential (1 0 -1 0) diffraction. Both SEM and AFM showed that In_0.5Ga_0.5N films were smooth and had small roughness of 0.6 nm. Optical properties were measured by photoluminescence (PL) spectra from room temperature to low temperature of 20 K. The 2.28 eV green emission was achieved at room temperature for all our InGaN films. The electrical properties of In_0.5Ga_0.5N films on a SiO₂/Si (100) substrate were measured by the Hall measurement at room temperature. InGaN films showed the electron concentration of 1.51×10²⁰–1.90×10²⁰ cm⁻³ and mobility of 5.94–10.5 cm² V⁻¹ s⁻¹. Alloying of InN and GaN was confirmed for the sputtered InGaN.     

N2O treatment enhancement-mode InAlN/GaN HEMTs with HfZrO2 High-k insulator

A normally-off InAlN/GaN MIS-HEMT with HfZrO₂ gate insulator was realized and investigated. By using N₂O plasma treatment beneath the gate region, 13 nm InAlN Schottky layer was oxidized to AlON_x + 4 nm InAlN Schottky layer. The strong polarization induced carriers in traditional InAlN/GaN 2 DEG quantum well was reduced for enhancement-mode operation. High-k thin film HfZrO₂ was used for gate insulator of E-mode device to further suppress gate leakage current and enhance device gate operation range. The maximum drain current of E-mode InAlN/GaN MIS-HEMT was 498 mA/mm and this value was higher than previous published InAlN/GaN E-mode devices. The measurement results of low-frequency noise also concluded that the low frequency noise is attributed to the mobility fluctuation of the channel and N₂O plasma treatment did not increase fluctuation center of gate electrode.     

The excellent spontaneous ultraviolet emission of GaN nanostructures grown on silicon substrates by thermal vapor deposition

In this study, GaN nanostructures were grown on p-Si (111) substrate by thermal chemical vapor deposition (TCVD). Ga vapor directly reacted with NH₃ solution in N₂ carrier gas flow of 2 L/min at different temperatures (950–1050 °C). The influence of NH₃ solution and growth temperature on the morphology, structure, optical and photoresponse properties of GaN nanostructures was investigated. Scanning electron microscopy images showed that the densities of the NWs varied with increasing temperature. The use of NH₃ solution and increased growth temperature improved the crystalline quality of GaN nanostructures. The photoluminescence (PL) spectra of nanostructures displayed a near band-edge (NBE) emission at around 363–367 nm. Higher growth temperature (1050 °C) resulted in a strong NBE emission with no yellow emission peak. With +5 V applied bias, the NWs metal–semiconductor–metal UV photodetector exhibited a high photocurrent of 1.6×10⁻³ A. The photocurrent to dark current contrast ratio was 120.     

Direct MBE growth of GaN on GaAs substrates for integrated short wavelength emitters

The growth and material properties of GaN heteroepitaxial layers on vicinal (1 0 0) and exact (1 1 1)B substrates have been investigated, using molecular beam epitaxy (MBE) with N₂ RF-plasma source. We examined the approach to grow GaN directly on the oxide desorbed GaAs, without the incidence of an As beam during oxide desorption or the following stages of growth. Perfect smooth surfaces were obtained on (1 1 1)B GaAs but excellent luminescence properties were observed on vicinal (1 0 0) GaAs. Four growth temperatures (T_G) were compared for the (1 0 0) orientation and a monotonic increase of photoluminescence intensity with increasing T_G, in the range of 570–680°C, was observed. The best surface morphology of less than 10 nm rms roughness was also determined, by atomic force microscopy, for the maximum (680°C) temperature. The layers exhibited up to 10¹⁷ cm⁻³ electron concentration and it could be compensated by Mg impurities. Metallizations of Pt and Pd gave ohmic contacts on GaN/GaAs (1 0 0) but a Schottky diode contact was achieved by Ir metallization. The obtained material properties are probably sufficient for realizing efficient GaN light emitters on (1 0 0) GaAs substrates.     

Synthesis of GaN nanowires by ammoniation of Ga2O3 films on Nb layer deposited on Si(1 1 1) substrates

Single-crystalline GaN nanowires have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniation of Ga₂O₃/Nb films at 900 °C in a quartz tube. The as-synthesized GaN nanowires are confirmed to be single-crystalline GaN with wurtzite structure by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and field-emission transmission electron microscopy (FETEM); scanning electron microscopy (SEM) shows that the GaN nanowires are smooth, with diameters of about 50 nm and lengths typically up to several microns, which could provide an attractive potential for incorporation in future GaN electronic devices into Si-based large-scale integrated circuits. Finally, the growth mechanism of GaN nanowires is also briefly discussed.     

Structural properties of β-Ga2O3 formed by dry thermal oxidization process on GaN

Dry thermal oxidation of GaN thin films grown on Al₂O₃ (0001) has been performed at different temperatures. The oxidized samples were investigated through X-ray diffraction (XRD) and atomic force microscope (AFM). For samples oxidized at temperatures from 800 °C to 950 °C, XRD peaks from the (−201), (−402) and (−603) planes of β-Ga₂O₃ were observed, indicating that a β-Ga₂O₃ layer was formed on GaN epitaxially. The epitaxial relationships were determined to be β-Ga₂O₃(−201)||GaN(002) and an in-plane orientation of β-Ga₂O₃[010]||GaN[110]. When the oxidation temperature is increased further to 1000 °C, in addition to the peaks from the (−201), (−402) and (−603) planes, extra peaks corresponding to other planes appeared, indicating that the oxidized layer had deteriorated to polycrystalline Ga₂O_3.     

Ultra-low-energy ion-beam-synthesis of Ge nanocrystals in thin ALD Al2O3 layers for memory applications

Structural and electrical properties of ALD-grown 5 and 7 nm-thick Al₂O₃ layers before and after implantation of Ge ions (1 keV, 0.5–1 × 10¹⁶ cm^?2) and thermal annealing at temperatures in the 700–1050 °C range are reported. Transmission Electron Microscopy reveals the development of a 1 nm-thick SiO₂-rich layer at the Al₂O₃/Si substrate interface as well as the formation of Ge nanocrystals with a mean diameter of ～5 nm inside the implanted Al₂O₃ layers after annealing at 800 °C for 20 min. Electrical measurements performed on metal–insulator–semiconductor capacitors using Ge-implanted and annealed Al₂O₃ layers reveal charge storage at low-electric fields mainly due to location of the Ge nanocrystals at a tunnelling distance from the substrate and their spatial dispersion inside the Al₂O₃ layers.     

Study on mobility enhancement in MOVPE-grown AlGaN/AlN/GaN HEMT structures using a thin AlN interfacial layer

Al_0.26Ga_0.74N/AlN/GaN high-electron-mobility transistor (HEMT) structures with AlN interfacial layers of various thicknesses were grown on 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy, and their structural and electrical properties were characterized. A sample with an optimum AlN layer thickness of 1.0 nm showed a highly enhanced Hall mobility (μ_Hall) of 1770 cm²/Vs with a low sheet resistance (ρ_s) of 365 Ω/sq. (2DEG density n_s = 1.0 × 10¹³/cm²) at room temperature compared with those of a sample without the AlN interfacial layer (μ_Hall = 1287 cm²/Vs, ρ_s = 539 Ω/sq., and n_s = 0.9 × 10¹³/cm²). Electron transport properties in AlGaN/AlN/GaN structures were theoretically studied, and the calculated results indicated that the insertion of an AlN layer into the AlGaN/GaN heterointerface can significantly enhance the 2DEG mobility due to the reduction of alloy disorder scattering. HEMTs were successfully fabricated and characterized. It was confirmed that AlGaN/AlN/GaN HEMTs with the optimum AlN layer thickness show superior DC properties compared with conventional AlGaN/GaN HEMTs.     

Effects of thermal annealing on In-induced metastable defects in InGaN films

We investigated the effects of thermal annealing on the properties of InGaN layers. From secondary ion mass spectroscopy results, it was found that severe In desorption occurred after annealing. Photoluminescence and X-ray diffraction results indicate that significant amounts of In vacancy-related defects exist in the annealing samples. It was also found that persistent photoconductivity decay time constants were 211, 893 and 1040 s, while the decay exponents were 0.153, 0.120 and 0.213 for the as-grown, 800 °C-annealed and 1000 °C-annealed InGaN epitaxial layers, respectively.     

Role of pre-annealing treatment in improving the porosity of gallium nitride on cubic silicon (100) substrate

This report describes the improved properties of porous gallium nitride (GaN) via pre-annealing treatment using a conventional furnace system. Prior to this work, non-porous GaN samples were annealed at the temperature of 600–1000 °C in order to rise the quality of the samples. From the microscopic, structural and optical measurements, the optimum annealing temperature was found to be 800 °C. Next, the sample that was annealed at the optimum temperature was fabricated into a porous structure by using an electrochemical etching technique. The characteristic of the porous GaN was then investigated by observing its morphology and crystallography properties. For a comparative analysis, a porous GaN sample without the annealing treatment and a porous GaN sample that was then annealed at 800 °C (post-annealing treatment) were also prepared. It was found that the pre-annealing treatment promotes a better quality in porosity of the GaN than other counterparts.     

High thermal annealing effect on structural and optical properties of ZnO–SiO2 nanocomposite

The effect of annealing temperature on photoluminescence (PL) of ZnO–SiO₂ nanocomposite was investigated. The ZnO–SiO₂ nanocomposite was annealed at different temperatures from 600 °C to 1000 °C with a step of 100 °C. High Resolution Transmission Electron Microscope (HR-TEM) pictures showed ZnO nanoparticles of 5 nm are capped with amorphous SiO₂ matrix. Field Emission Scanning Electron Microscope (FE-SEM) pictures showed that samples exhibit spherical morphology up to 800 °C and dumbbell morphology above 800 °C. The absorption spectrum of ZnO–SiO₂ nanocomposite suffers a blue-shift from 369 nm to 365 nm with increase of temperature from 800 °C to 1000 °C. The PL spectrum of ZnO–SiO₂ nanocomposite exhibited an UV emission positioned at 396 nm. The UV emission intensity increased as the temperature increased from 600 °C to 700 °C and then decreased for samples annealed at and above 800^°C. The XRD results showed that formation of willemite phase starts at 800 °C and pure willemite phase formed at 1000 °C. The decrease of the intensity of 396 nm emission peak at 900 °C and 1000 °C is due to the collapse of the ZnO hexagonal structure. This is due to the dominant diffusion of Zn into SiO₂ at these temperatures. At 1000 °C, an emission peak at 388 nm is observed in addition to UV emission of ZnO at 396 nm and is believed to be originated from the willemite.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

Influence of processing and annealing steps on electrical properties of InAlN/GaN high electron mobility transistor with Al2O3 gate insulation and passivation

We report on preparation and electrical characterization of InAlN/AlN/GaN metal–oxide–semiconductor high electron mobility transistors (MOS HEMTs) with Al₂O₃ gate insulation and surface passivation. About 12 nm thin high-κ dielectric film was deposited by MOCVD. Before and after the dielectric deposition, the samples were treated by different processing steps. We monitored and analyzed the steps by sequential device testing. It was found that both intentional (ex situ) and unintentional (in situ before Al₂O₃ growth) InAlN surface oxidation increases the channel sheet resistance and causes a current collapse. Post deposition annealing decreases the sheet resistance of the MOS HEMT devices and effectively suppresses the current collapse. Transistors dimensions were source-to-drain distance 8 μm and gate width 2 μm. A maximum transconductance of 110 mS/mm, a drain current of ～0.6 A/mm (V_GS = 1 V) and a gate leakage current reduction from 4 to 6 orders of magnitude compared to Schottky barrier (SB) HEMTs was achieved for MOS HEMT with 1 h annealing at 700 °C in forming gas ambient. Moreover, InAlN/GaN MOS HEMTs with deposited Al₂O₃ dielectric film were found highly thermally stable by resisting 5 h 700 °C annealing.     

Structural,magnetic and electrical properties of Fe/Si system

Interface studies in metal/semiconductor systems are important due to their potential technological application in microelectronics. A total of 80 nm Fe film was deposited on Si(1 1 1) substrate using electron beam evaporation technique at a vacuum of 2×10⁻⁷ Torr. The samples were annealed at temperatures 500 and 600 °C for 1 h in 3×10⁻⁵ Torr for the formation of silicide phases. GIXRD results show a stable disilicides FeSi₂ formation at the interface at annealing temperature 600 °C. The coercivity determined from MOKE hysteresis curves for as-deposited and annealed samples are 14.91, 29.82 and 31.01 Oe. The Schottky barrier height, as estimated by the current–voltage measurement is 0.59, 0.54 and 0.49 eV for pristine and annealed samples at 500 and 600 °C, respectively, and concludes that the barrier height values as a function of the heat of formation of the silicides.     

Influence of annealing temperature on the structural and optical properties of nanocrystalline zirconium oxide

Nanocrystalline zirconium oxide powder was prepared by sol-gel method using zirconyl chloride octahydrate (ZrOCl₂·8H₂O) and ethylenediaminetetraacitic acid (EDTA) in ammonium hydroxide (NH₄OH) solution. The as-synthesized complex product was annealed at 650 °C, 750 °C and 850 °C for 2 h to get fine ZrO₂ powder. These samples were further analyzed by Scanning electron microscopy (SEM), X- ray diffraction (XRD), Energy-dispersive X- ray spectroscopy (EDX), UV-vis analysis, Fourier transform infrared (FT-IR) spectroscopy, Photoluminescence spectroscopy (PL) and Raman Spectroscopy to study their structural and optical properties. The structural studies revealed that nanocrystalline ZrO₂ powder exhibits monoclinic phase with variation in crystallite size with annealing temperature. The UV–vis absorption band edge of ZrO₂ decreases from 514 nm to 451 nm as annealing temperature rises from 650 °C to 750 °C. It seems that the drastic reduction in band gap energy may be one of the novel unexpected characteristics of ZrO₂. The FTIR analyses further confirmed the formation of nanocrystalline monoclinic ZrO₂. PL analysis revealed the novel emission peaks at 305 and 565 nm. The Raman spectroscopy confirmed the transformation of amorphous zirconium hydroxide to m-ZrO₂ with increase in temperature from 650 °C to 850 °C.     

Growth temperature dependence of epitaxial Gd2O3 films on Si(1 1 1)

This article reports on the epitaxy of crystalline high κ oxide Gd₂O₃ layers on Si(1 1 1) for CMOS gate application. Epitaxial Gd₂O₃ thin films have been grown by Molecular Beam Epitaxy (MBE) on Si(1 1 1) substrates between 650 and 750 °C. The structural and electrical properties were investigated depending on the growth temperature. The C–V measurements reveal that equivalent oxide thickness (EOT) equals 0.7 nm for the sample deposited at the optimal temperature of 700 °C with a relatively low leakage current of 3.6 × 10^?2 A/cm² at |V_g ? V_FB| = 1 V.     

Influence of crystallinity of as-deposited Ge film on formation of quantum dot in carbon-mediated solid-phase epitaxy

The influence of crystallinity of as-deposited Ge films on Ge quantum dot (QD) formation via carbon (C)-mediated solid-phase epitaxy (SPE) was investigated. The samples were fabricated by solid-source molecular beam epitaxy (MBE). Ge/C/Si structure was formed by sequential deposition of C and Ge at deposition temperature (T_D) of 150–400 °C, and it was heat-treated in the MBE chamber at 650 °C. In the case of amorphous or a mixture of amorphous and nano-crystalline Ge film grown for T_D ≤250 °C, density of QDs increased with increasing T_D due to the increase of C-Ge bonds in Ge layer. Ge QDs with diameter of 9.2±2.1 nm were formed in the highest density of 8.3×10¹¹ cm⁻² for T_D =250 °C. On the contrary, in the case of polycrystalline Ge film for T_D ≥300 °C, density of QDs decreased slightly. This is because C incorporation into Ge layer during SPE was suppressed due to the as-crystallized columnar grains. These results suggest that as-deposited Ge film in a mixture of amorphous and nano-crystalline state is suitable to form small and dense Ge QDs via C-mediated SPE.