Structure and properties of GZO thin films grown on ZnO buffer layers

Thin gallium-doped zinc oxide (in GZO the Ga₂O₃ contents are approximately 3 wt%) films having different ZnO buffer layers were deposited using radio frequency (rf) magnetron sputtering. The use of a grey-based Taguchi method to determine the processing parameters of ZnO buffer layer deposition has been studied by considering multiple performance characteristics. A Taguchi method with an L₉ orthogonal array, signal-to-noise (S/N) ratio, and analysis of variance (ANOVA) is employed to investigate the performance characteristics in the deposition operations. The effect and optimization of ZnO buffer deposition parameters (rf power, sputtering pressure, thickness, and annealing) on the structure, morphology, electrical resistivity, and optical transmittance of the GZO films are studied. Annealing treatment and reduction in thickness resulted in a decrease in root-mean-square (RMS) surface roughness of the ZnO buffer layer. Using the optimal ZnO buffer layer obtained by the application of the grey-based Taguchi method, the electrical resistivity of GZO films was decreased from 2.94×10⁻³ to 9.44×10⁻⁴ Ω cm and the optical transmittance in the visible region was slightly increased from 84.81% to 85.82%.     

GaP layers grown on GaN with and without buffer layers

The growth of GaP layer on GaN with and without buffer layers by metal-organic chemical vapour deposition (MOCVD) has been studied. Results indicate that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature (HT) GaP growth. These sites can promote the two-dimensional (2D) growth of HT GaP and reduce the surface roughness. A GaP single crystal layer grown at 680°C is obtained using a 40-nm thick GaP buffer layer. The full-width at half-maximum (FWHM) of the (111) plane of GaP layer, measured by DCXRD, is 560 arcsec. The GaP layer grown on GaN without low temperature GaP buffer layer shows a rougher surface. However, the FWHM of the (111) plane is 408 arcsec, which is the indication of better crystal quality for the GaP layer grown on GaN without a low temperature buffer layer. Because it provides less nucleation sites grown at high growth temperature, the three-dimensional (3D) growth is prolonged. The crystalline quality of GaP is lightly improved when the surface of GaN substrate is pretreated by PH3 , while it turned to be polycrystalline when the substrate is pretreated by TEGa.     

Textured growth of CoFe for soft underlayers in CoCrPt:SiO2 perpendicular magnetic recording media

CoFe alloy thin films were studied with the intention of potential use as a soft underlayer (SUL) for Co-based perpendicular recording media. The effect of composition and the effects of seedlayers on the formation of crystalline phase and crystallographic texture and the magnetic properties were investigated. Films deposited on Ta/Pd seedlayer were found to have a good FCC(1 1 1) texture than those deposited on glass substrates or on Ta seedlayers. The magnetic properties were also better when deposited on Ta/Pd seedlayers. On these seedlayers, Fe concentration of 15 at% was found to be suitable for the formation of FCC phase. Disks were prepared with CoFe SULs. The noise of CoFe SUL is one of the challenges to be solved.     

Structure and properties of ZnO films grown on Si substrates with low temperature buffer layers

Zinc oxide (ZnO) thin films were grown on Si (1 0 0) substrates by pulsed laser deposition (PLD) using two-step epitaxial growth method. Low temperature buffer layer (LTBL) was initially deposited in order to obtain high quality ZnO thin film; the as-deposited films were then annealed in air at 700 °C. The effects of LTBL and annealing treatment on the structural and luminescent properties of ZnO thin film were investigated. It was found that tensile strain was remarkably relaxed by employing LTBL and the band-gap redshifted, correspondingly. The shift value was larger than that calculated from band-gap theories. After annealing treatment, it was found that the annealing temperature with 700 °C has little influence on strains of ZnO films with LTBLs other than directly deposited film in our experiments. Interestingly, the different behaviors in terms of the shift of ultraviolet (UV) emission after annealing between films with and without buffer were observed, and a tentative explanation was given in this paper.     

The influence of annealing on the structural and magnetic properties of C/CoCrPt/CrTi trilayer recording media

Post-deposition annealing was performed on trilayer films consisting of a C overcoat layer on top of CoCrPt/CrTi/glass substrate. We observed a coercivity of 3600 Oe in the films with a 4 nm C overcoat, which was about three times larger than the coercivity of similar films without a C overcoat. This is most likely due to the significant decrease in intergranular exchange coupling for the films with C overcoat, as shown by the annealing treatment. It is believed that the diffusion of C into CoCrPt grain boundaries promotes magnetic grain isolation.     

Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD

We investigate the structural and electrical properties of Al_xIn_1–xN/AlN/GaN heterostructures with AlGaN buffers grown by MOCVD, which can be used as an alternative to AlInN HEMT structures with GaN buffer. The effects of the GaN channel thickness and the addition of a content graded AlGaN layer to the structural and electrical characteristics were studied through variable temperature Hall effect measurements, high resolution XRD, and AFM measurements. Enhancement in electron mobility was observed in two of the suggested Al_xIn_1?xN/AlN/GaN/Al_0.04Ga_0.96N heterostructures when compared to the standard Al_xIn_1–xN/AlN/GaN heterostructure. This improvement was attributed to better electron confinement in the channel due to electric field arising from piezoelectric polarization charge at the Al_0.04Ga_0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same interface. If the growth conditions and design parameters of the Al_xIn_1?xN HEMT structures with AlGaN buffers can be modified further, the electron spillover from the GaN channel can be significantly limited and even higher electron mobilities, which result in lower two-dimensional sheet resistances, would be possible.     

Hard magnetic properties of rapidly annealed NdFeB thin films on Nb and V buffer layers

NdFeB thin films of the form A (20 nm)/NdFeB(d nm)/A(20 nm), where d ranges from 54 to 540 nm and the buffer layer A is Nb or V were prepared on a Si(1 0 0) substrate by magnetron sputtering. The hard Nd₂Fe₁₄B phase is formed by a 30 s rapid anneal or a 20 min anneal. Average crystallite size ranged from 20 to 35 nm with the rapidly annealed samples having the smaller crystallite size. These samples also exhibited a larger coercivity and energy product than those treated by a 20 min vacuum anneal. A maximum coercivity of 26.3 kOe at room temperature was obtained for a Nb/NdFeB (180 nm)/Nb film after a rapid anneal at 725°C. Initial magnetization curves indicate magnetization rotation rather than nucleation of reverse domains is important in the magnetization process. A Brown's equation analysis of the coercivity as a function of temperature allowed us to compare the rapidly annealed and 20 min annealed samples. This analysis suggests that rapid annealing gives higher quality crystalline grains than the 20 min annealed sample leading to the observed large coercivity in the rapidly annealed samples.     

Patterned media with composite in-plane and perpendicular anisotropy recording layers

Magnetization reversal mechanism in nanostructures composed of exchange coupled bi-layers with in-plane and perpendicular anisotropy was investigated. Micromagnetic simulation was carried out for bit-patterned media with areal density of 5 Tb/in², as example. Magnetization of thermally stable recorded bit using a single layer may not switch under write field. However, a complete and fast switching is possible with an exchange coupling to a layer with in-plane anisotropy. By adjusting the thicknesses and intrinsic properties of the two layers, the composite recording layer still can retain perpendicular anisotropy. The exchange coupled structure with dual-anisotropy can be extended to magnetic memories.     

Structural and optical properties of ZnO nanorods grown on MgxZn1−xO buffer layers

ZnO nanorod arrays were synthesized by chemical-liquid deposition techniques on Mg_xZn_1−xO (x = 0, 0.07 and 0.15) buffer layers. It is found that varying the Mg concentration could control the diameter, vertical alignment, crystallization, and density of the ZnO nanorods. The X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) data show the ZnO nanorods prefer to grow in the (0 0 2) c-axis direction better with a larger Mg concentration. The photoluminescence (PL) spectra of ZnO nanorods exhibit that the ultraviolet (UV) emission becomes stronger and the defect emission becomes weaker by increasing the Mg concentration in Mg_xZn_1−xO buffer layers.     

Analysis of waveform from perpendicular magnetic printed media

Edge printing is one of the perpendicular magnetic printing methods for writing servo signals with high speed, high accuracy and low cost. Sub-peaks of waveform from edge printed media are concerned as the sub-peaks can cause errors during the read-back process. In this study, in order to reduce sub-peaks, the influence of printing field, bit length and patterned magnetic layer thickness of master medium on sub-peaks is investigated by using a metal-evaporated (ME) tape as a slave medium. The results show that sub-peak to main peak ratio decreases with stronger printing field, smaller bit length and thicker patterned magnetic layer of master media.     

Structural and magnetic properties of FePt films grown on Cr1-xMox underlayers

FePt films were deposited on Cr_1-xMo_x underlayers by dc magnetron sputtering. The effects of the Mo content in the underlayers, underlayer thickness, substrate temperature, and FePt film thickness on the structural and magnetic properties of the FePt films were studied. Experimental results showed that the (200) textured Cr₉₀Mo₁₀ film was a promising underlayer for promoting the growth of the L1₀ FePt films with (001) preferred orientation at relatively low temperatures. With the Cr₉₀Mo₁₀ underlayers, the ordering process of the FePt films could start at 200 °C. Both the ordering degree and the out-of-plane coercivity (H_c) of the FePt films increased with an increase in substrate temperature. When the substrate temperature was ≥250 °C, the FePt films grown on the Cr₉₀Mo₁₀ underlayers could have the (001) preferred orientation. The FePt films grown on the Cr₉₀Mo₁₀ underlayers at different temperatures showed a continuous microstructure. The out-of-plane coercivities H_c decreased while the ordering degree increased with increased FePt film thickness, which could be due to the variation of the magnetic reversal mechanism from rotation predominant mode to domain wall motion predominant mode. PACS 68.55.Jk; 75.50.Ss     

热辅助磁盘探针存储的研究

描述了一种热辅助磁盘存储技术，该技术可应用于未来的高密度磁盘存储.记录介质是一种CoNi/Pt多层膜，它可用作垂直模式的磁记录介质.使用扫描隧道显微镜(STM)产生的隧道电流作为热源对磁膜进行局部加热.隧道电流随着加在STM针尖与磁膜之间的脉冲电压幅值的增大而增大.实验结果显示了圆形记录点在磁膜上生成，记录点尺寸与电压值相关，阈值电压为4V左右.当电压高于阈值时，记录点尺寸随着电压的增大而增大,平均尺寸为170nm；当电压低于阈值时，未发现记录点.一个简单的模型解释了以上实验现象. 关键词： 扫描隧道显微镜 热辅助磁盘存储技术 高密度磁盘存储     

Writability issues in high-anisotropy perpendicular magnetic recording media

Challenges and recent developments associated with writability issues in high-anisotropy perpendicular recording media are reviewed. The writing field is limited by the high coercivity caused by the high anisotropy. Some new alterna- tives are proposed to solve the writability issues, including texture-tilting-assisted, domain-wall-assisted, energy-assisted magnetic recording technologies, and so on, In addition, we propose new alternatives for the next-generation of magnetic recording media.     

Ti(Cr)缓冲层对用于垂直磁记录材料CoCrTa介质磁特性和微结构的影响

利用直流对靶磁控溅射技术在单晶Si衬底上制备了C/CoCrTa/X (X=Cr，Ti)介质材料.分别采用振动样品磁强计、X射线衍射仪、扫描探针显微镜对样品的磁性、微结构等进行了测试分析.研究发现，Ti缓冲层有利于样品中Co晶粒的易轴垂直于膜面生长.以Ti为缓冲层的样品，颗粒尺寸和表面粗糙度较小，而且磁畴明显，说明以Ti为缓冲层的薄膜样品更适宜做垂直磁记录. 关键词： CoCrTa 垂直磁记录 缓冲层 微结构     

Extracting intergranular exchange coupling in perpendicular magnetic recording media

A micromagnetic numerical technique has been used to demonstrate how intergranular exchange coupling and intrinsic anisotropy field dispersion can be extracted from measuring two types of M-H curves. A realistic grain configuration formed by planar Voronoi cells is used to simulate perpendicular magnetic media. This technique effectively separates the effects of intergranular exchange coupling and anisotropy dispersion by finding their correlation to differentiated M-H curves with different initial magnetization states, even in the presence of thermal fluctuation. The validity of this method is investigated with a series of intergranular exchange couplings and anisotropy dispersions for different media thickness. A relationship between the auto-correlation function of an ac-erased sample and dispersion of the exchange interaction is demonstrated. Utilizing magnetization auto-correlation functions, the magnetic intergranular exchange coupling statistics show a correlation with the auto-correlation function shape in terms of zero-cross and undershoot values.     

Permalloy-copper-Permalloy sandwiches with perpendicular anisotropy axes in the magnetic layers

Layerd FeNi-Cu-FeNi structures with a mutually perpendicular orientation of the easy axes in the FeNi layers are fabricated in ultrahigh vacuum, and the physical properties of these structures are investigated: ferromagnetic resonance, quasistatic magnetization reversal, and the Faraday rotation of the plane of polarization of a light wave. Zh. Tekh. Fiz. 67, 45–48 (November 1997)     

TiN underlayer and overlayer for TbFeCo perpendicular magnetic recording media

TiN thin film is prepared by DC reactive sputtering in Ar+N₂ atmosphere and its suitability as underlayer and overlayer for TbFeCo perpendicular recording media as well as its effect on the magnetic properties of the latter have been studied. Only 5 nm TiN overlayer and 20 nm under layer can successfully protect the TbFeCo film from oxidation. Initially the coercivity is increased sharply from about 2 to 6 kOe for an increase of underlayer thickness to 60 nm then the increasing rate of coercivity becomes very slow. The saturation magnetization remains almost constant with the underlayer thickness. The remanent squareness ratio remains constant at 1.0 with the underlayer thickness up to 60 nm then decreases.     

Measurement of angular-dependent hysteresis loops in perpendicular magnetic recording media

The intrinsic angular-dependent hysteresis loops in perpendicular recording media have been measured by iteratively correcting the demagnetization effect in order to maintain the internal total field at a fixed angle. Both the angle and the magnitude of the applied field are adjusted simultaneously to compensate for the demagnetization field change. Significant differences have been observed between the conventional angular-dependent hysteresis loop with a constant applied field angle and the intrinsic angular-dependent hysteresis loop with a fixed total field angle. Without demagnetization corrections, the field range in which irreversible magnetization occurs changes as a function of the field angle, whereas it remains constant if proper demagnetization field corrections are applied.     

Effect of thermal annealing in the microstructural and the optical properties of uncapped InAs quantum dots grown on GaAs buffer layers

The effect of thermal annealing on self-assembled uncapped InAs/GaAs quantum dots (QDs) has been investigated using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images showed that the lateral sizes and densities of the InAs QDs were not changed significantly up to 650 °C. When the InAs/GaAs QDs were annealed at 700 °C, while the lateral size of the InAs QDs increased, their density decreased. The InAs QDs disappeared at 800 °C. PL spectra showed that the peaks corresponding to the interband transitions of the InAs QDs shifted slightly toward the high-energy side, and the PL intensity decreased with increasing annealing temperature. These results indicate that the microstructural and the optical properties of self-assembled uncapped InAs/GaAs can be modified due to postgrowth thermal annealing.