Properties and use of cycled grown OMVPE GaAs:Zn,GaAs:Se,and GaAs:Si layers for high-conductance GaAs tunnel junctions

Heavily doped GaAs layers for high conductance GaAs tunnel junctions have been grown by atmospheric pressure organometallic vapor phase epitaxy (OMVPE) using Zn as the dopant for thep ⁺ regions and either Se or Si as the dopant for then ⁺ regions. At a growth temperature of 700° C using a “cycled” growth technique for the Zn-dopedp ⁺⁺-GaAs layer, both the conductance and the peak current density of the tunnel diode has been increased by a factor of ∼65 compared to a tunnel junction with a continuously grown Zn-doped p⁺-GaAs. The conductance of the tunnel junction, which is maximized at a growth temperature of 650° C using cycled growth, is comparable to the best reported values for tunnel junctions grown by molecular beam epitaxy. Cycled growths forn ⁺ Se-doped regions are found to reduce the conductance of a tunnel junction by more than two orders of magnitude. However, cycled growth for the n⁺-GaAs regions with Si doping show no conductance degradation. A model based on incorporation sites of these dopants during OMVPE growth of GaAs is presented to account for the experimental observations.     

Performance comparison of AlGaAs,GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10⁻⁴ Ω cm² followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10⁻⁴ Ω cm² within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Thickness determination of ultrathin oxide films and its application in magnetic tunnel junctions

In this study we propose a method for utilizing x-ray photoelectron spectroscopy (XPS), a surface sensitive technique, coupled with a wedge-shaped sample to determine the thickness of an ultrathin aluminum oxide tunnel barrier layer (∼2 nm) in a magnetic tunnel junction (MTJ). The uncertainty of the measured thickness is analyzed and the factors affecting the accuracy of this measurement are discussed as well as the advantages over the use of high-resolution transmission electron microscopy. Using this approach, we were able to quickly optimize the thickness of an aluminum oxide layer in a fabricated MTJ, yielding a high magnetoresistance ratio. In addition to XPS, one can also use Auger electron spectroscopy to determine the thickness of the oxidized tunnel barrier layer. This method can also be applied to other tunnel barrier materials such as the nitrides.     

栅控电势磁性隧道结的隧穿磁阻研究

基于Slonczewski理论模型和矩阵方法研究了由栅控制中间层电势高度的磁性隧道结的隧穿磁阻效应。数值计算了中间层势垒为0~3 V以及中间层势阱为0~3 V的磁性隧道结的隧穿磁阻随着中间层厚度改变的变化曲线。计算结果表明,当中间层为势垒时,隧穿磁阻随着中间层厚度单调下降;当中间层为势阱时,隧穿磁阻随着中间层厚度振荡,并且相比于势垒情况时明显提高。这说明栅控中间层磁性隧道结相比于传统磁性隧道结具有更好的可控性和提高隧穿磁阻效应的潜力。     

Electrical properties and structural optimization of GaN/InGaN/GaN tunnel junctions grown by molecular beam epitaxy

The InGaN films and GaN/InGaN/GaN tunnel junctions (TJs) were grown on GaN templates with plasma-assisted molecular beam epitaxy. As the In content increases, the quality of InGaN films grown on GaN templates decreases and the surface roughness of the samples increases. V-pits and trench defects were not found in the AFM images. p⁺⁺-GaN/InGaN/n⁺⁺-GaN TJs were investigated for various In content, InGaN thicknesses and doping concentration in the InGaN insert layer. The InGaN insert layer can promote good interband tunneling in GaN/InGaN/GaN TJ and significantly reduce operating voltage when doping is sufficiently high. The current density increases with increasing In content for the 3 nm InGaN insert layer, which is achieved by reducing the depletion zone width and the height of the potential barrier. At a forward current density of 500 A/cm², the measured voltage was 4.31 V and the differential resistance was measured to be 3.75 × 10⁻³ Ω·cm² for the device with a 3 nm p⁺⁺-In_0.35Ga_0.65N insert layer. When the thickness of the In_0.35Ga_0.65N layer is closer to the “balanced” thickness, the TJ current density is higher. If the thickness is too high or too low, the width of the depletion zone will increase and the current density will decrease. The undoped InGaN layer has a better performance than n-type doping in the TJ. Polarization-engineered tunnel junctions can enhance the functionality and performance of electronic and optoelectronic devices.     

Pushing the limits of concentrated photovoltaic solar cell tunnel junctions in novel high‐efficiency GaAs phototransducers based on a vertical epitaxial heterostructure architecture

A monolithic compound semiconductor phototransducer optimized for narrow‐band light sources was designed for achieving conversion efficiencies exceeding 50%. The III‐V heterostructure was grown by metal‐organic chemical vapor deposition, based on the vertical stacking of 5 partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p‐type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered near 830 nm. Devices with active areas of ~3.4 mm² were fabricated and tested with different emitter gridline spacings. The open circuit voltage (Voc) of the electrical output is five times or more than that of a single GaAs n/p junction under similar illumination. The device architecture allows for improved Voc generation in the individual base segments because of efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no needs for complicated fabrication processes or reflecting layers. With illumination powers in the range of a few 100 mW, the measured fill factor (FF) varied between 88 and 89%, and the Voc reached over 5.75 V. The data also demonstrated that a proper combination of highly doped emitter and window layers without gridlines is adequate for sustaining such FF values for optical input powers of several hundred milliwatts. As the optical input power is further increased and approaches 2 W (intensities ~58 W/cm²), the multiple tunnel junctions sequentially exceed their peak current densities in the case for which typical (n++)GaInP/ (p++)AlGaAs concentrated photovoltaic tunnel junctions are used. Lower bandgap tunnel junctions designed with improved peak current densities result in phototransducer devices having high FF and conversion efficiencies for up to 5 W optical input powers (intensities ~144 W/cm²). Measurements at different temperatures revealed a Voc reduction of −6 mV/°C at ~59 W/cm². Copyright © 2015 John Wiley & Sons, Ltd.     

老炼过程对表面放电型ACPDP显示屏内MgO薄膜二次电子发射系数的影响

在Paschen定律基础上,使用计算出的表面放电型ACPDP的击穿电压数据,拟合实验测得的ACPDP显示屏内Ne Xe混合气体的击穿电压特性曲线(Ub～pd),从而确定了击穿电压(Ub)和MgO介质保护膜有效二次电子发射系数(γeff)之间函数关系式中的相关参数,并由此关系研究了老炼过程对表面放电型ACPDP显示屏内MgO介质保护膜γeff的影响.结果表明,老炼开始时,击穿电压随着老炼时间的延长迅速降低,2 h后击穿电压逐渐趋于稳定.与此对应,MgO介质保护膜的γeff会随着老炼时间的延长迅速增大并在2 h后趋于稳定.当Xe的含量从0.5%升高到4%时,击穿电压会随着Xe的含量升高而升高,而γeff会随着Xe的含量升高而降低.本文使用的计算γeff的方法可以用于计算ACPDP屏内介质保护膜的γeff.     

质子交换MgO：Linbo_3波导及其镁含量对波导性能影响的研究

报道了不同镁含量质子交换ＭｇＯ：ＬｉＮｂＯ３波导的制备与表征。采用多个测量波长，测量并计算了不同镁含量的Ｚ切质子交换ＭｇＯ：ＬｉＮｂＯ３波导的阶跃式折射率分布，结果表明多波长测量结果一致。给出了波导层折射率增量的波长色散，发现了波导层折射率增量和同一温度下波导的扩散系数随晶体中镁含量的增加而减小，最后讨论了镁含量对波导性能影响的机制。     

磁场退火对FePt/Ag薄膜磁性能的影响

采用磁控溅射法室温沉积获得FePt/Ag薄膜,然后在500℃下,于真空磁退火炉中对薄膜进行退火处理。利用XRD和振动样品磁强计(VSM),研究了磁场退火对薄膜结构和磁性能的影响。结果表明,500℃零磁场退火获得了矫顽力为0.763 4 MA.m–1、平均晶粒尺寸21 nm的L10-FePt薄膜。磁场提供了FePt成核生长的驱动力,0.8 MA.m–1磁场退火后FePt的平均晶粒尺寸为26 nm,矫顽力增大至0.804 3 MA.m–1。非磁性Ag的掺杂可有效抑制磁性FePt晶粒的团聚生长。     

MgO缓冲层对Si衬底上制备Fe3Si薄膜性能的影响

采用磁控溅射方法和热加工工艺在n型Si衬底上溅射不同厚度的MgO层并制备Fe-Si薄膜层,退火后形成Fe3Si/MgO/Si多层膜结构.利用MgO缓冲层对退火时Si衬底扩散原子进行屏蔽,并分析MgO层对Fe3Si薄膜结构和电学性质的影响.通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)和四探针测试仪对Fe3Si薄膜的晶体结构、表面形貌、断面形貌和电阻率进行表征与分析.研究结果表明:当MgO层厚度为20 nm时生成Fe0.9Si0.1薄膜,当厚度为50,100,150和200 nm时都生成了Fe3Si薄膜,生成的Fe3Si和Fe0.9Si0.1薄膜以(110)和(211)取向为主.随MgO缓冲层厚度增加,Si衬底扩散原子对Fe3Si薄膜的影响减小,Fe3 Si薄膜的晶格常数逐渐减小,晶粒大小趋向均匀,平均电阻率呈现先增大后减小趋势.研究结果为后续基于Fe3 Si薄膜的器件设计与制备提供了参考.     

Dependence of critical current density on oxygen exposure in Nb-AlOx-Nb tunnel junctions

We demonstrate that a large fraction of the available data relating the critical current density J_c of superconducting Nb-AlO_x-Nb tunnel junctions to oxidation parameters can be accounted for by a single, nearly universal dependence. For fixed oxidation temperature, J_c does not depend independently on oxygen partial pressure and oxidation time, but only on their product. There are two distinct regimes in this dependence, corresponding to high and low J_c     

Study on the mechanism of perpendicular magnetic anisotropy in Ta/CoFeB/MgO system

The mechanism of perpendicular magnetic anisotropy (PMA) in a MgO-based magnetic tunnel junction (MTJ) has been studied in this article. By comparing the magnetic properties and elementary composition analysis for different CoFeB-based structures, such as Ta/CoFeB/MgO, Ta/CoFeB/Ta and Ru/CoFeB/MgO structures, it is found that a certain amount of Fe-oxide existing at the interface of CoFeB/MgO is helpful to enhance the PMA and the PMA is originated from the interface of CoFeB/MgO. In addition, Ta film plays an important role to enhance the PMA in Ta/CoFeB/MgO structure.     

Analysis of dielectric breakdown in CoFeB/MgO/CoFeB magnetic tunnel junction

The time-dependent dielectric breakdown has been investigated in a series of nominally identical Co–Fe–B/MgO/Co–Fe–B junctions by voltage ramp experiments. The results divulge that the breakdown voltage strongly depends on the polarity of the applied voltage, junction area, ramp speed and the annealing temperature. Magnetic tunnel junctions (MTJs) with positive bias on the top electrode show higher breakdown voltage than MTJs with negative bias. We found that there is a significant decrease in the breakdown voltage when the annealing temperature is increased above 350 °C. The experimental data can be described by different specific forms of breakdown probability functions which lead to different extrapolation of life time of junctions.     

Influence of oxygen on the ion-beam synthesis of silicon carbide buried layers in silicon

The properties of silicon structures with silicon carbide (SiC) buried layers produced by high-dose carbon implantation followed by a high-temperature anneal are investigated by Raman and infrared spectroscopy. The influence of the coimplantation of oxygen on the features of SiC buried layer formation is also studied. It is shown that in identical implantation and post-implantation annealing regimes a SiC buried layer forms more efficiently in CZ Si wafers or in Si (CZ or FZ) subjected to the coimplantation of oxygen. Thus, oxygen promotes SiC layer formation as a result of the formation of SiO_x precipitates and accommodation of the volume change in the region where the SiC phase forms. Carbon segregation and the formation of an amorphous carbon film on the SiC grain boundaries are also discovered. Fiz. Tekh. Poluprovodn. 32, 1414–1419 (December 1998)     

Influence of substoichiometry,hydrogen content and crystallinity on the optical and electrical properties of HxWOy thin films

We have prepared H_xWO_y amorphous thin films both by evaporation of tungsten trioxide powder and by cathodic sputtering of a tungsten target in an argon/oxygen/hydrogen reactive gas mixture. The evaporated layers have the composition H_xWO_2.7 (0.2 < × < 0.5). Their oxygen content seems rather insensitive to the evaporation parameters. We do not observe any correlation between x and these parameters. Evaporated virgin layers are nearly transparent. Annealing,under vacuum leaves y unchanged, under oxygen increases y to 3. Annealing of the virgin layer under vacuum induces the growth of the 1.38 eV absorption band (giving blue coloration) and a decrease of the activation energy for conduction. Annealing the blue layers in oxygen destroys the 1.38 eV band and increases the activation energy for conduction. In both cases annealing at high temperature induces a microcrystalline phase with an absorption band centered about 0.72 eV (giving also a blue coloration) and a jump in electronic conductivity. As in the case of the 1.38 eV band, an increase of the intensity of the 0.72 eV band induces a decrease of the activation energy for conduction. The two bands are interpreted as polaronic like. They can be induced in transparent layers without any change in global composition by excitation of the hydrogen atoms from a “ passive” state to an “ active” state. In addition to the hydrogen content, the existence of the 1.38 eV band requires some substoichiometry. The study of the optical and electrical properties of amorphous H_xWO_y sputtered layers, supports our previous conclusions about the composition range (C) for the coloration capability of transparent thin films. In addition there is a composition range (B) where the virgin layers are blue, and a composition range (M) where they have a metallic like behavior. On the other side of (C), there is a range (C’) where uv illumination only induces a decrease in the activation energy for conduction, then a range (T) where the layers are completely nonresponsive. One can pass from (T) to (B) through (C') and (C) either at constant hydrogen content by increasing the departure from stochiometry, or at constant substoichiometry by increasing the hydrogen content. A part of this work was presented at EMC Cornell, New York July 1, 1977.     

MgO对镍基碳化钨金属陶瓷激光熔覆层组织和耐磨性的影响

采用G112+镍包碳化钨复合合金粉末在45＃钢基材表面进行激光熔覆。对比研究了添加不同含量MgO在不同激光功率条件下对激光熔覆层的宏观形貌、显微组织、硬度分布及摩擦学性能的影响。实验表明:适量MgO的加入能使镍基碳化钨金属陶瓷熔覆层中的裂纹大为减少甚至消失,熔覆层组织得到细化。在同样的摩擦条件下,掺入0.15％MgO的激光熔覆层的摩擦系数与未掺入MgO的摩擦系数相比,随时间的变化量很小。MgO添加剂具有减摩抗磨作用。     

TbFex薄膜的成分控制及其对磁结构的影响

利用直流磁控溅射工艺制备了TBFex薄膜，通过改变溅射气压调节薄膜的成分，利用磁力显微镜研究了薄膜成分对畴结构的影响，最后从理论上讨论Ar气压对磁致伸缩薄膜TBFex成分的影响，并建立了理论模型。结果表明：Ar气压在相当大的范围内影响着TBFex膜的成分，当气压从0．2Pa增加到0．6Pa时，Tb原子数百分比从30％增加到45％，磁力显微镜（MFM）观察到当TB含量减少时，在零场下的磁化强度由与膜面垂直方向变为平行于薄膜平面方向。     

热处理时间对铁磁性微晶玻璃磁性能的影响

以Fe2O3-CaO-SiO2-B2O3-P2O5系统为基础,采用基础玻璃析晶法制备铁磁性微晶玻璃热种子材料。通过XRD,确定了热处理样品中的主晶相为磁铁矿、硅灰石和赤铁矿;采用振动样品磁强计(VSM),测试样品室温下的磁性能;通过SEM,观察样品中晶体的形貌。研究了热处理时间对其磁性能的影响。结果表明:合理的热处理时间为2h,制得的铁磁性微晶玻璃的比饱和磁矩为26.2A·m2·kg–1。     

TiO2陶瓷在磁控溅射中电导率改变机理研究

在Ar气氛中对TiO2陶瓷进行射频磁控溅射时，TiO2陶瓷电导经发生显著变化。该文对这一现象进行了研究。采用多种分析手段对实验结果进行了分析。分析表明，辉光放电电离Ar气所产生的Ar离子冲击TiO2陶瓷表面的热量和动量被O^2-吸收，引进O^2-溢出TiO2陶瓷，导致TiO2化学配比偏离正常，从而提高了TiO2陶瓷的电导率。     

玻璃粉含量对钡铁氧体厚膜磁性能的影响

采用丝网印刷法制备了钡铁氧体厚膜,系统研究了玻璃粉含量对钡铁氧体厚膜磁性能的影响。结果表明,随着玻璃粉含量的增大,样品的致密度逐渐提高;饱和磁化强度、剩磁和矫顽力均呈现出先增大后减小的趋势;而剩磁比(Mr/Ms)单调上升。当玻璃粉含量增大到质量分数7%时,所制得样品的饱和磁化强度、剩磁和矫顽力均达到最大值,分别为151,76.6,310 kA/m。当玻璃粉含量为质量分数13%时,剩磁比达到0.615。