Incorporation of the transition metal Hf into GaN

The perturbed angular correlation (PAC) technique was applied to study the incorporation of the transition metal Hf into GaN after implantation. To this end the PAC probe ¹⁸¹Hf(¹⁸¹Ta) was implanted into epitaxial Wurtzite GaN layers (1.3 μm on sapphire) with an energy of 160 keV and doses of 7× 10¹² at/cm². PAC spectra were recorded during an isochronal annealing programme, using rapid thermal annealing (RTA) and furnace annealing, in the 300–1000oC temperature range. After implantation the spectra show a damped oscillation corresponding to a quadrupole interaction frequency (QIF) of ν_Q= 340 MHz for 30% of the probe nuclei. Annealing up to 600oC reduces the damping of this frequency without an increase of the probe atom fraction f_s in these sites. Above 600oC f_s grows rapidly until after the 900oC RTA step more than 80% of the Hf probes experience a well defined QIF due to the incorporation of Hf on undisturbed sites of the hexagonal GaN wurtzite lattice. An interaction frequency of ν_Q= 340 MHz is derived. RTA and furnace annealing yield similar results for annealing up to 800oC, where the undisturbed fraction reaches about 60%. Then RTA at higher temperatures increases this fraction, while furnace annealing leads to a decrease down to 22% after annealing at 1000oC. To our knowledge this is the first time that a transition metal probe like Hf is incorporated to such a large extent into a semiconductor lattice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fabrication and analysis of the structural phase transition of ZrO2 nanoparticles using modified facile sol–gel route

The synthesis of zirconia nanoparticles is achieved through a modified facile sol–gel route. The as-prepared gel is analyzed thermally using TGA and DTA techniques to spot the crystallization process of zirconia nanoparticles. The prepared gel is then annealed at different temperatures and the structure was found to change between tetragonal and monoclinic crystal systems. The first stable tetragonal phase is achieved after annealing for 2?h at 400°C. The annealed powders between 600°C and 800°C demonstrate mixed tetragonal/monoclinic phases. Annealing at 1000°C and higher temperatures up to 1200°C resulted in pure monoclinic phase. Cubic phase was not detected within the annealing temperature range in this study. The elemental analysis of the annealed powder confirmed the formation of zirconia nanoparticles with the chemical formula ZrO₂. The FTIR spectra of the annealed samples introduced a variation in the vibrational bands especially around the phase transition temperature. HR-TEM images reported the formation of nano-zirconia crystals with apparently large particle sizes. The optical energy gap of zirconia nanoparticles is investigated and determined.     

增强型AlGaN/GaN高电子迁移率晶体管高温退火研究

深入研究了两种增强型AlGaN/GaN高电子迁移率晶体管(HEMT)高温退火前后的直流特性变化.槽栅增强型AlGaN/GaN HEMT在500 ℃ N₂中退火5 min后,阈值电压由0.12 V正向移动到0.57 V,器件Schottky反向栅漏电流减小一个数量级.F注入增强型AlGaN/GaN HEMT在 400 ℃ N₂中退火2 min后,器件阈值电压由0.23 V负向移动到-0.69 V,栅泄漏电流明显增大.槽栅增强型器件退火过程中Schottky有效势垒     

Optical and structural properties of Ge-ion-implanted fused silica after annealing in different ambient conditions

Ge+ ions are implanted into fused silica glass at room temperature and a fluence of 1×10 17 cm-2 . The as-implanted samples are annealed in O2, N2 and Ar atmospheres separately. Ge0 , GeO and GeO2 coexist in the as-implanted and annealed samples. Annealing in different atmospheres at 600℃ leads each composite to change its content. After annealing at 1000℃, there remains some amount of Ge 0 in the substrates. However, the content of Ge decreases due to out-diffusion. After annealing in N2 , Si–N composite is formed. The absorption peak of GeO appears at 240 nm after annealing in O2 atmosphere, and a new absorption peak occurs at 418 nm after annealing in N2 atmosphere, which is attributed to the Si–N composite. There is no absorption peak appearing after annealing in Ar atmosphere. Transmission electron microscopic images confirm the formation of Ge nanoparticles in the as-implanted sample and GeO 2 nanoparticles in the annealed sample. In the present study, the GeO content and the GeO2 content depend on annealing temperature and atmosphere. Three photoluminescence emission band peaks at 290, 385 and 415 nm appear after ion implantation and they become strong with the increase of annealing temperature below 700℃, and their photoluminescences recover to the values of as-grown samples after annealing at 700℃. Optical absorption and photoluminescence depend on the annealing temperature and atmosphere.     

Co掺杂纳米ZnO微结构的正电子湮没研究

利用正电子湮没技术研究了10 at.% Co掺杂的Co₃O₄/ZnO纳米复合物中退火对缺陷的影响. 利用X射线衍射(XRD)测量了Co₃O₄/ZnO纳米复合物的结构和晶粒尺寸. 随着退火温度升高,Co₃O₄相逐步消失,ZnO晶粒尺寸也有显著增加. 经过1000 ℃以上退火后,Co₃O₄相完全消失,并出现了CoO的岩盐结构. 正电子湮没寿命测量显示出Co₃O₄ /ZnO纳米复合物中存在大量的Zn空位和空位团. 这些空位缺陷可能存在于纳米复合物的界面区域. 当退火温度达到700 ℃后Zn空位开始恢复,空位团也开始收缩. 900 ℃以上退火后,所有空位缺陷基本消失,正电子寿命接近ZnO完整晶格中的体态寿命值. 符合多普勒展宽谱测量也显示Co₃O₄ /ZnO纳米复合物经过900 ℃以上退火后电子动量分布与单晶ZnO基本一致,表明界面缺陷经过退火后得到消除. 关键词： ZnO 界面缺陷 正电子湮没     

Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

Surface morphology of ZnO buffer layer and its effects on the growth of GaN films on Si substrates by magnetron sputtering

ZnO thin films were first prepared on Si(111) substrates using a radio frequency magnetron sputtering system. Then the as-grown ZnO films were annealed in oxygen ambient at temperatures of 700, 800, 900, and 1000°C , respectively. The morphologies of ZnO films were studied by an atom force microscope (AFM). Subsequently, GaN epilayers about 500 nm thick were deposited on the ZnO buffer layers. The GaN/ZnO films were annealed in NH₃ ambient at 900°C. The microstructure, morphology and optical properties of GaN films were studied by x-ray diffraction (XRD), AFM, scanning electron microscopy (SEM) and photoluminescence (PL). The results are shown, their properties having been investigated particularly as a function of the ZnO layers. For better growth of the GaN films, the optimal annealing temperature of the ZnO buffer layers was 900°C.     

Microscopic and spectroscopic characterization of chemo-mechanically polished, furnace-annealed, nitrogen-doped 4H:SiC

The effect of chemical–mechanical polishing and high-temperature furnace annealing at temperatures ranging from 1000 °C to 1600 °C on nitrogen-doped crystalline 4H:SiC was investigated. Techniques used to characterize the samples included environmental scanning electron microscopy (ESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy. The ESEM micrographs and EDS data indicated that there were structural defects on the unannealed sample that did not propagate into the sample or vary in composition from the bulk. The sample annealed at 1000 °C showed oxygen-rich and carbon-depleted surface defects. Annealing at temperatures above 1200 °C introduced defects that grew out of the sample surface. These were carbon and oxygen rich, but depleted in silicon. This supported the XPS data, which showed an increase in the surface C bonding with annealing temperatures above 1200 °C. The XPS data also suggested that the oxycarbide content may be increasing on annealing above 1200 °C. Raman micro-probe data from the defects on the sample annealed at 1200 °C showed the maximum shift in the transverse optical phonon mode at 776 cm^-1, indicating that the beginning of carbon out-diffusion may be accompanied with structural changes. Optimal annealing temperatures are thus below 1200 °C. PACS 78.20.-e; 81.05.-t; 81.70.Fy     

Controlling magnetic properties of iron oxide nanoparticles using post-synthesis thermal treatment

Changes in morphological and magnetic properties of Fe₃O₄ nanoparticles before and after annealing are investigated in the present work. The nanoparticles are synthesized in a standard capacitively coupled plasma enhanced chemical vapour deposition system with two electrodes using ferrocene as the source compound. Post annealing, due to the sintering process, the particles fuse along with recrystallization. This results in increased size of the nanoparticles and the interparticle interaction, which play a major role in deciding the magnetic properties. X-ray diffraction patterns of the samples before and after annealing indicate a phase change from Fe₃O₄ to Fe₂O₃. Annealing at 200 ^°C causes the apparent saturation magnetization to increase from 6 emu?g^?1 to 15 emu?g^?1. When annealed at 500 ^°C, the magnetic properties of the nanoparticles resemble those of the bulk material. The evidence for the transition from a superparamagnetic state to a collective state is also observed when annealed at 500 ^°C. Variation of the magnetic relaxation data with annealing also reflects the change in the magnetic state brought about by the annealing. The correlation between annealing temperature and the magnetic properties can be used to obtain nanocrystallites of iron oxide with different sizes and magnetic properties.     

Influence of p-GaN annealing on the optical and electrical properties of InGaN/GaN MQW LEDs

Optical and electrical properties of InGaN/GaN multiple quantum wells (MQWs) light emitting diodes (LEDs) annealed in pure O₂ ambient (500 °C) and pure N₂ ambient (800 °C) were systematically investigated. The temperature-dependent photoluminescence measurements showed that high-temperature thermal annealing in N₂ ambient can induce indium clusters in InGaN MQWs. Although the deep traps induced by indium clusters can act as localized centers for carriers, there are many more dislocations out of the trap centers due to high-temperature annealing. As a result, the radiative efficiency of the sample annealed in N₂ ambient was lower than that annealed in O₂ ambient at room temperature. Electrical measurements demonstrated that the LEDs annealed in O₂ ambient were featured by a lower forward voltage and there was an increase of ~41% in wall-plug efficiency at 20 mA in comparison with the LEDs annealed in N₂ ambient. It is thus concluded that activation of the Mg-doped p-GaN layer should be carried out at a low-temperature O₂ ambient so as to obtain LEDs with better performance.     

Annealing effect on transport properties of Nd0.67Sr0.33MnO3 thin films

Annealing dependence of the lattice parameter, resistivity, magnetoresistance and thermopower have been studied on Nd_0.7-Sr_0.33MnO₃ thin films deposited on LaAlO₃ and alumina substrates by pulsed laser ablation. Upon annealing at 800°C and 1000°C the lattice constant of the LaAlO₃ film tends toward that of the bulk target due to reduction in oxygen vacancies. This results in a metal-insulator transition at temperatures which increase with progressive annealing along with a decrease in the observed low temperature MR. Using a magnon scattering model we estimate the e _g bandwidth of the film annealed at 1000°C and show that the magnon contribution to the resistivity is suppressed in a highly oxygen deficient film and gains prominence only upon subsequent annealing. We also show that upon annealing, the polaron concentration and the spin cluster size increases in the paramagnetic phase, using an adiabatic polaron hopping model which takes into account an exchange dependent activation energy above the resistivity peak.     

Activation of Mg-doped P-GaN by using two-step annealing

One- and two-step rapid thermal annealing (RTA) for activating Mg-doped p-type GaN films had been performed to compare with conventional furnace annealing (CFA). The two-step annealing process consists of two annealing steps: the first step is performed at 750 °C for 1 min and the second step is performed at 600 °C for 5 min in pure O₂ or air ambient. It is found that the samples annealed in air ambient exhibit poor electrical properties as compared to those annealed in pure O₂. Compared to one-step RTA annealing and CFA annealing, the samples with two-step annealing exhibit higher hole concentration and lower resistivity. This means that the two-step annealing is a powerful method to enhance the electrical performance of Mg-doped p-type GaN films. Similar results were also evidenced by photoluminescence (PL) measurement. Possible mechanism was confirmed by secondary ion mass spectrometry analysis.     

Multilayer based interferential-plasmonic structure: metal cluster 3D grating combined with dielectric mirror

This paper reports strong deep-ultraviolet and visible photoluminescence (PL) of the GaN nanoparticles depending on the conversion time from Ga₂O₃ to GaN. Monoclinic β-Ga₂O₃ nanoparticles with a diameter of approximately 2.5–5.0 nm were fabricated prior to conversion to GaN. The Ga₂O₃ nanoparticles were converted to GaN in the tube furnace with NH₃ flow at 900°C for 10, 30, 60, and 120 min. Depending on the conversion time, the converted GaN nanoparticle size became bigger with the increase of the conversion time. The characteristic GaN x-ray diffraction (XRD) peaks became bigger when the conversion time increased. The PL intensity drastically increased with the increase of the conversion time. The spectra profile completely overlapped for GaN samples converted for 10, 30, and 60 min, with the maximum peak at 390 nm. However, the PL spectrum slightly narrowed and red-shifted with the maximum peak at 400 nm for the GaN nanoparticles converted for 120 min.     

掺铒GaN薄膜的背散射/沟道分析和光致发光研究

采用背散射（RBS）/沟道（channeling）分析和傅里叶变换红外光谱（FT-IR）研究了掺铒G aN薄膜的晶体结构和光致发光（PL）特性.背散射/沟道分析结果表明：随退火温度的升高， 薄膜中辐照损伤减少；但当退火温度达到1000℃，薄膜中的缺陷又明显增加.Er浓度随注入 深度呈现高斯分布.通过沿GaN的<0001>轴方向的沟道分析，对于900℃，30min退火的GaN:Er 样品，Er在晶格中的替位率约76％.光谱研究表明：随退火温度的升高，室温下样品的红外P L峰强度增加；但是当退火温度达到100 关键词： GaN Er 离子束分析 光致发光     

The electrical, optical properties of AlSb polycrystalline thin films deposited by magnetron co-sputtering without annealing

In order to fabricate AlSb polycrystalline thin films without post annealing, this paper studies a technology of magnetron co-sputtering onto intentionally heated substrate. It compares the structural characteristics and electrical properties of AlSb films which are deposited at different substrate temperatures. It finds that the films prepared at a substrate temperature of 450 oC exhibit an enhanced grain growth with an average grain size of 21 nm and the lattice constant is 0.61562 nm that goes well with unstained lattice constant (0.61355 nm). The ln(σ_dark) ～1/T curves show that the conductivity activation energy is about 0.38 eV when the film is deposited at 450 oC without an annealing. The transmittance and reflectance spectra show that the film deposited at 450 oC has an optical band gap of 1.6 eV. These results indicate that we have prepared AlSb polycrystalline films which do not need a post annealing.     

SnO$lt;sub$gt;2$lt;/sub$gt;纳米晶体的制备、结构与发光性质

使用软化学方法在碱性溶液中制备出了颗粒尺寸分布均匀的SnO₂纳米颗粒,使用透射电子显微镜（TEM）、X射线衍射（XRD）、光致发光谱（PL）和光吸收谱等方法分析与表征了SnO₂纳米颗粒的结构和光学性能.实验中通过表面活性剂的加入来控制纳米颗粒的结晶与凝聚.XRD,TEM的结果表明,原始制备出的SnO₂纳米颗粒的平均粒径小于4 nm,为完好的晶体状态.纳米颗粒经过400—1000 ℃退火后晶粒尺寸进一步增大.光吸收谱表明,相对于体材料,纳米颗粒的禁带宽度展宽并随颗粒尺寸增大而红移.光致发光谱测试表明,不同温度下退火的SnO₂纳米颗粒在350—750 nm有较强的发光,研究表明这是来源于颗粒表面的氧空位缺陷发光. 关键词： 氧化锡 表面活性剂 纳米颗粒 光致发光     

Double-electromagnetically induced transparency in a Y-type atomic system

We present secondary phase identification studies on Cr doped ZnO nanoparticles prepared by the sol-gel method. X-ray diffraction analysis confirms the formation of chromium oxides and there is found to be an increase of lattice parameter with thermal annealing. Scanning electron microscopic studies show the increase in the crystalline nature and particle size. Optical absorption measurements of the as prepared sample exhibit a strong band at 356 nm due to the free exciton absorption of the ZnO nanoparticles. An absorption band at 277 nm is due to the ³T₁ → ³T₂ transition in Cr⁴⁺ ions which appears only for the annealed samples. Photoluminescence studies show that deep level emission is completely suppressed after Cr₂O₃ formation/thermal annealing. Raman and FTIR spectra reveal formation of the Cr₂O₃ phase. Thermal annealing leads to the increase of crystalline nature which gives an enhancement to the Raman modes.     

高温退火对蓝宝石基片的表面形貌和对CeO2缓冲层以及Tl-2212超导薄膜生长的影响

研究了蓝宝石(1102)基片在不同温度和时间下退火时表面形貌和表面相结构的变化,以及它对CeO₂缓冲层和Tl-2212超导薄膜生长的影响.原子力显微镜(AFM)研究表明,在流动氧环境中1000℃温度下退火,蓝宝石(1102)的表面首先局部区域形成台阶结构,然后表面形成叠层台阶结构,随着退火时间的延长,表面发生了台阶合并现象,表面形貌最终演化为稳定的具有光滑平台的宽台阶结构.XRD测试表明,通过高温热处理可以大幅度提高蓝宝石基片表面结构的完整性.在1000℃温度下热处理20 h的蓝宝石 关键词： Tl-2212超导薄膜 蓝宝石 缓冲层     

Possible origin of room-temperature ferromagnetism in undoped GaN epilayers implanted with Mn ions

Manganese ions were implanted into unintentionally doped GaN epilayers grown by metal organic chemical vapor deposition (MOCVD). The (Ga,Mn)N and Ga_xMn_y phases were formed after Mn-implanted undoped GaN epilayers annealed at 700 and 800 °C. The samples showed ferromagnetic behavior at room temperature with the highest magnetization obtained in the sample annealed at 800 °C. Ferromagnetic signal reduces as annealing temperature increased above 900 °C. It is believed that the room-temperature ferromagnetic property of Mn-implanted undoped GaN epilayers are mainly from (Ga_,Mn)N. The Ga_xMn_y phases play a critical role in providing holes and also contribute to increasing the ferromagnetic property.     

Crystallization of hydrogenated amorphous silicon carbon films with laser and thermal annealing

The crystallization of silicon rich hydrogenated amorphous silicon carbon films prepared by Plasma Enhanced Chemical Vapor Deposition technique has been induced by excimer laser annealing as well as thermal annealing. The excimer laser energy density (E_d) and the annealing temperature were varied from 123 to 242 mJ/cm² and from 250 to 1200 °C respectively. The effects of the two crystallization processes on the structural properties and bonding configurations of the films have been studied. The main results are that for the laser annealed samples, cubic SiC crystallites are formed for E_d ≥ 188 mJ/cm², while for the thermal annealed samples, micro-crystallites SiC and polycrystalline hexagonal SiC are observed for the annealing temperature of 800 and 1200 °C respectively. The crystallinity degree has been found to improve with the increase in the laser energy density as well as with the increase in the annealing temperature.