Gate‐controlled metal–insulator transition in the LaAlO3/SrTiO3 system with sub‐critical LaAlO3 thickness

We studied the electrical conduction in the LaAlO₃/SrTiO₃ (LAO/STO) interface electron system with a sub‐critical LAO layer thickness of ～3.5 unit cells (uc). It was found that the true dividing point between metallic and insulating behaviour without gating lies near the LAO thickness of 3.5 uc. Our marginally metallic 3.5 uc sample showed a sharp transition to insulating state at temperatures which strongly depended on the applied negative back‐gate voltage. The superior gate‐controllability of the sample was attributed to its sheet carrier density which was an order of magnitude lower than those of conducting LAO/STO samples with 4 uc or more of LAO layers. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sr掺杂对La_(1-x)Sr_xMnO_3/LaAlO_3/SrTiO_3界面电子结构的影响

采用基于密度泛函理论的第一性原理计算方法,系统地研究了La_(1-x)Sr_xMnO_3层中Sr的掺杂方式和掺杂量对4La_(1-x)Sr_xMnO_3/3LaAlO_3/4SrTiO_3(LSMO/LAO/STO)异质结构原子和电子结构的影响.结果表明:对于相同的Sr掺杂量,掺杂方式的差异对体系电子结构的影响微弱,不会导致体系发生金属-绝缘体转变;掺杂量的不同对体系电子结构有着显著的影响,当Sr的掺杂量较少时,LAO/STO界面处存在着准二维电子气,当Sr的掺杂量高于1/3时,LAO/STO界面处准二维电子气消失.我们相信,Sr的引入以及通过Sr掺杂量的改变可以对LSMO覆盖层极化进行调控,这也是导致体系LAO/STO界面处金属-绝缘体转变的可能原因,进一步为极化灾变机制导致的界面处电子重构提供了证据.     

LaAlO_3/SrTiO_3界面增强光伏效应

探索LaAlO_3/SrTiO_3(LAO/STO)界面产生的新奇物理特性对理解关联电子系统中多自由度耦合和设计功能材料器件具有重要的价值.本文通过脉冲激光沉积方法在SrTiO_3基底上制备了LAO/STO薄膜,研究了正面照射LAO/STO膜面和侧面照射LAO/STO界面时的光伏效应,探讨了LAO/STO界面对光伏效应的影响.研究结果表明,在同样光照能量下侧面照射LAO/STO界面产生的光电压远高于正面照射LAO/STO膜面产生的光电压,说明LAO/STO界面对光伏效应有明显的增强作用.通过偏压调控可以进一步增强照射LAO/STO界面产生的光电压,当偏压为60 V时, LAO/STO样品的位置探测灵敏度达到了36.8 mV/mm.这些研究结果为设计场调控位置敏感探测器等新型光电子器件提供了新的思路.     

Estimation of the concentration of charge carriers in a conductive layer between LaAlO3 and SrTiO3 dielectrics

Using the capacitor model by Goetzberger, the concentration of charge carriers on the contact layer LAO’STO is evaluated. The obtained results are shown to be comparable with those obtained within the framework of “polar catastrophe” model. Some possible interpretations of charge carriers, concentration variations, depending on the thickness of deposited layers, are presented as well, both for the “pure” (LAO) and composite (LASTO) ones.     

光电协同增强的场效应对LaAlO_3/SrTiO_3界面中持续光电导的调控

LaAlO_3/SrTiO_3异质结界面体系具有新奇的二维自由电子气现象、暂态光电导效应、持续光电导效应等丰富的光电性质,是近年来科学界研究的热点之一.本文研究了场效应对LaAlO_3/SrTiO_3界面光电导效应的调控,发现光电协同增强的场效应可以使得LaAlO_3/SrTiO_3界面产生显著的持续光电导效应,进一步研究发现:在光电协同效应的影响下,随着负的背栅门电压的增加,持续光电导的数值增大,在-70 V附近达到极值;随着负的背栅门电压处理时间的增加,持续光电导的数值单调增加.LaAlO_3/SrTiO_3异质结中这种场增强的持续光电导效应可为多参数可调的光电子记忆器件的研发提供参考依据.     

纤锌矿AlGaN/AlN/GaN异质结构中光学声子散射影响的电子迁移率

对含有AlN插入层纤锌矿Al_xGa_1-xN/AlN/GaN异质结构,考虑有限厚势垒和导带弯曲的实际 异质结势,同时计入自发极化和压电极化效应产生的内建电场作用,采用数值自洽求解薛定谔方程和泊松方程, 获得二维电子气(2DEG)中电子的本征态和本征能级.依据介电连续模型和Loudon单轴晶体模型, 用转移矩阵法分析该体系中可能存在的光学声子模及三元混晶效应.进一步, 在室温下计及各种可能存在的光学声子散射,推广雷-丁平衡方程方法,讨论2DEG分布及二维电子迁移率的 尺寸效应和三元混晶效应.结果显示: AlN插入层厚度和Al_xGa_1-xN势垒层中Al组分的增加均会 增强GaN层中的内建电场强度,致使2DEG的分布更靠近异质结界面,使界面光学声子强于其他类型的 光学声子对电子的散射作用而成为影响电子迁移率的主导因素.适当调整AlN插入层的厚度和Al组分, 可获得较高的电子迁移率.     

具有部分本征GaN帽层新型AlGaN/GaN高电子迁移率晶体管特性分析

为了优化传统Al GaN/GaN高电子迁移率晶体管(high electron mobility transistors,HEMTs)器件的表面电场,提高击穿电压,本文提出了一种具有部分本征GaN帽层的新型Al GaN/GaN HEMTs器件结构.新型结构通过在Al GaN势垒层顶部、栅电极到漏电极的漂移区之间引入部分本征GaN帽层,由于本征GaN帽层和Al GaN势垒层界面处的极化效应,降低了沟道二维电子气(two dimensional electron gas,2DEG)的浓度,形成了栅边缘低浓度2DEG区域,使得沟道2DEG浓度分区,由均匀分布变为阶梯分布.通过调制沟道2DEG的浓度分布,从而调制了Al GaN/GaN HEMTs器件的表面电场.利用电场调制效应,产生了新的电场峰,且有效降低了栅边缘的高峰电场,Al GaN/GaN HEMTs器件的表面电场分布更加均匀.利用ISE-TCAD软件仿真分析得出:通过设计一定厚度和长度的本征GaN帽层,Al GaN/GaN HEMTs器件的击穿电压从传统结构的427 V提高到新型结构的960 V.由于沟道2DEG浓度减小,沟道电阻增加,使得新型Al GaN/GaN HEMTs器件的最大输出电流减小了9.2%,截止频率几乎保持不变,而最大振荡频率提高了12%.     

Optimization of two-dimensional electron gases and – characteristics for AlGaN/GaN HEMT devices

In this paper, we take account of the spontaneous and piezoelectric polarization effect at the heterointerface in the AlGaN/GaN HEMT device, and one-dimensional Schrödinger–Poisson equations are solved self-consistently using a nonuniform mesh; using our findings, the AlGaN/GaN heterostructure conduction band and the two-dimensional electron gas (2DEG) density are investigated. The dependences of the 2DEG characteristics on the Al fraction, the thickness of each layer, the donor concentration and the gate voltage are investigated through simulation. The output characteristics are simulated using a quasi-2D model; a saturation voltage and threshold voltage are also shown. The influence of the spacer layer width on the 2DEG density is calculated for the first time. An explanation and analyses are given.     

In situ investigation of conducting interface formation in LaAlO3/SrTiO3 heterostructure

The high-mobility conducting interface (CI) between LaAlO₃ (LAO) and SrTiO₃ (STO) has revealed many fascinating phenomena, including exotic magnetism and superconductivity. But, the formation mechanism of the CI has not been conclusively explained. Here, using in situ angle-resolved photoemission spectroscopy, we elucidated the mechanisms for the CI formation. In as-grown samples, we observed a built-in potential (V_bi) proportional to the polar LAO thickness starting from the first unit cell (UC) with CI formation appearing above 3 UCs. However, we found that the V_bi is removed by synchrotron ultraviolet (UV)-irradiation; The built-in potential is recovered by oxygen gas (O₂(g))-exposure. Furthermore, after UV-irradiation, the CI appears even below 3UC of LAO. Our results demonstrate not only the V_bi-driven CI formation in as-grown LAO/STO, but also a new route to control of the interface state by UV lithographic patterning or other surface modification.     

阶梯AlGaN外延新型Al_(0.25)Ga_(0.75)N/GaNHEMTs击穿特性分析

为了优化AlGaN/GaN HEMTs器件表面电场,提高击穿电压,本文首次提出了一种新型阶梯AlGaN/GaN HEMTs结构.新结构利用AlGaN/GaN异质结形成的2DEG浓度随外延AlGaN层厚度降低而减小的规律,通过减薄靠近栅边缘外延的AlGaN层,使沟道2DEG浓度分区,形成栅边缘低浓度2DEG区,低的2DEG使阶梯AlGaN交界出现新的电场峰,新电场峰的出现有效降低了栅边缘的高峰电场,优化了AlGaN/GaN HEMTs器件的表面电场分布,使器件击穿电压从传统结构的446 V,提高到新结构的640 V.为了获得与实际测试结果一致的击穿曲线,本文在GaN缓冲层中设定了一定浓度的受主型缺陷,通过仿真分析验证了国际上外延GaN缓冲层时掺入受主型离子的原因,并通过仿真分析获得了与实际测试结果一致的击穿曲线.     

阶梯AlGaN外延新型Al0.25Ga0.75N/GaNHEMTs器件实验研究

本文报道了作者提出的阶梯AlGaN外延层新型AlGaN/GaN HEMTs结构的实验结果. 实验利用感应耦合等离子体刻蚀(ICP)刻蚀栅边缘的AlGaN外延层, 形成阶梯的AlGaN 外延层结构, 获得浓度分区的沟道2DEG, 使得阶梯AlGaN外延层边缘出现新的电场峰, 有效降低栅边缘的高峰电场, 从而优化了AlGaN/GaN HEMTs器件的表面电场分布. 实验获得了阈值电压-1.5 V的新型AlGaN/GaN HEMTs器件. 经过测试, 同样面积的器件击穿电压从传统结构的67 V提高到新结构的106 V, 提高了58%左右; 脉冲测试下电流崩塌量也比传统结构减少了30%左右, 电流崩塌效应得到了一定的缓解.     

Charge transport studies on chemically grown manganite based heterostructures

In this communication, we have successfully fabricated mixed valent La_0.7Ca_0.3MnO₃ (LCMO) manganite based (i) ZnO/LCMO/LAO and (ii) LMO/LCMO/LAO (LMO: LaMnO_3–d thin layer; LAO: LaAlO₃ substrate) thin film heterostructures using chemical solution deposition (CSD) method. 100 nm LCMO layer was initially grown on single crystalline (100) LAO substrate followed by the growth of 50 nm ZnO and LMO layers separately on the two different heterostructures. In the present study, upper layers of ZnO and LMO were intentionally prepared at 700 °C for 12 h under air environment, thereby some naturally created oxygen vacancies are expected to be present in their lattices. Presence of oxygen vacancies makes ZnO and LMO layers as n–type oxides in the heterostructures. Temperature dependent current–voltage (I–V) characteristics and interface resistivity (under different applied electric fields across interface only) were carried out to understand their charge transport behavior. A strong effect of electric field on the resistivity behavior has been observed due to a reasonable electrically polarizable (active) nature of ZnO and LMO thin layers. Zener double exchange (ZDE) polynomial law has been employed to understand various scattering processes as source of resistivity across, both, ZnO/LCMO and LMO/LCMO interfaces. Transport properties and charge conduction mechanisms have been discussed and compared for both the interfaces in the context of interface state and barrier between electrically active layer and LCMO film. Also, power consumption criteria have been discussed in detail for the presently studied heterostructures for their practical device applications such as field effect devices, memory devices, read–write head devices or any other spintronic devices.     

Effects of GaN cap layer thickness on an AlN/GaN heterostructure

In this study, we investigate the effects of GaN cap layer thickness on the two-dimensional electron gas (2DEG) electron density and 2DEG electron mobility of AlN/GaN heterostructures by using the temperature-dependent Hall measurement and theoretical fitting method. The results of our analysis clearly indicate that the GaN cap layer thickness of an AlN/GaN heterostructure has influences on the 2DEG electron density and the electron mobility. For the AlN/GaN heterostructures with a 3-nm AlN barrier layer, the optimized thickness of the GaN cap layer is around 4 nm and the strained a-axis lattice constant of the AlN barrier layer is less than that of GaN.     

AlGaN/GaN双异质结F注入增强型高电子迁移率晶体管

理论模拟了不同GaN沟道厚度的双异质结(AlGaN/GaN/AlGaN/GaN)材料对高电子迁移率晶体管(HEMT)特性的影响,并模拟了不同F注入剂量下用该材料制作的增强型器件的特性差异.采用双异质结材料,结合F注入工艺成功地研制出了较高正向阈值电压的增强型HEMT器件.实验研究了三种GaN沟道厚度制作的增强型器件直流特性的差异,与模拟结果进行了对比验证.采用降低的F注入等离子体功率,减小了等离子体处理工艺对器件沟道迁移率的损伤,研制出的器件未经高温退火即实现了较高的跨导和饱和电流特性.对14 nm GaN沟道厚度的器件进行了阈值电压温度稳定性和栅泄漏电流的比较研究,并且分析了双异质结器件的漏致势垒降低效应.     

Coexistence of superconductivity and ferromagnetism in two dimensions

Ferromagnetism is usually considered to be incompatible with conventional superconductivity, as it destroys the singlet correlations responsible for the pairing interaction. Superconductivity and ferromagnetism are known to coexist in only a few bulk rare-earth materials. Here we report evidence for their coexistence in a two-dimensional system: the interface between two bulk insulators, LaAlO(3) (LAO) and SrTiO(3) (STO), a system that has been studied intensively recently. Magnetoresistance, Hall, and electric-field dependence measurements suggest that there are two distinct bands of charge carriers that contribute to the interface conductivity. The sensitivity of properties of the interface to an electric field makes this a fascinating system for the study of the interplay between superconductivity and magnetism.     

Impact of AlxGa1-xN barrier thickness and Al composition on electrical properties of ferroelectric HfZrO/Al2O3/AlGaN/GaN MFSHEMTs

Ferroelectric (FE) HfZrO/Al$_{2}$O$_{3}$ gate stack AlGaN/GaN metal-FE-semiconductor heterostructure high-electron mobility transistors (MFSHEMTs) with varying Al$_{x}$Ga$_{1-x}$N barrier thickness and Al composition are investigated and compared by TCAD simulation with non-FE HfO$_{2}$/Al$_{2}$O$_{3}$ gate stack metal-insulator-semiconductor heterostructure high-electron mobility transistors (MISHEMTs). Results show that the decrease of the two-dimensional electron gas (2DEG) density with decreasing AlGaN barrier thickness is more effectively suppressed in MFSHEMTs than that in MISHEMTs due to the enhanced FE polarization switching efficiency. The electrical characteristics of MFSHEMTs, including transconductance, subthreshold swing, and on-state current, effectively improve with decreasing AlGaN thickness in MFSHEMTs. High Al composition in AlGaN barrier layers that are under 3-nm thickness plays a great role in enhancing the 2DEG density and FE polarization in MFSHEMTs, improving the transconductance and the on-state current. The subthreshold swing and threshold voltage can be reduced by decreasing the AlGaN thickness and Al composition in MFSHEMTs, affording favorable conditions for further enhancing the device.     

N极性GaN/AlGaN异质结二维电子气模拟

通过自洽求解薛定谔方程和泊松方程,较系统地研究了GaN沟道层、AlGaN背势垒层、Si掺杂和AlN插入层对N极性GaN/AlGaN异质结中二维电子气(2DEG)的影响,分析表明,GaN沟道层厚度、AlGaN背势垒层厚度及Al组分变大都能一定程度上提高二维电子气面密度,AlGaN背势垒层的厚度和Al组分变大也可提高二维电子气限阈性,且不同的Si掺杂形式对二维电子气的影响也有差异,而AlN插入层在提高器件二维电子气面密度、限阈性等方面表现都较为突出,在模拟中GaN沟道层厚度小于5nm时无法形成二维电子气,超过20nm后二维电子气面密度趋于饱和,而AlGaN背势垒厚度超过40nm后二维电子气也有饱和趋势,对均匀掺杂和delta掺杂而言AlGaN背势垒层Si掺杂浓度超过5×10~(19)cm~(-3)后2DEG面密度开始饱和,而厚度为2nmAlN插入层的引入会使2DEG面密度从无AlN插入层时的0.93×10~(13)cm~(-2)提高到1.17×10~(13)cm~(-2)。     

Two-dimensional electron gas densities in AlGaN/AlN/GaN heterostructures

The dependence of two-dimensional electron gas (2DEG) density and distribution in an Al_xGa_1-xN/AlN/GaN heterostructure on the thicknesses of the Al_xGa_1-xN barrier layer and the AlN interfacial layer are investigated theoretically. A competitive contribution of the AlGaN and AlN layers to the 2DEG density is revealed. For an AlN interfacial layer thinner than a critical value d_cAlN, the 2DEG density is dominated by the AlGaN barrier and the 2DEG density increases with the increase of the AlGaN barrier thickness, as in the case of a simple AlGaN/GaN heterostructure. While the AlN interfacial layer will take the dominant contribution to the 2DEG density as its thickness exceeds d_cAlN. In this case, the increase of AlGaN barrier layer thickness leads to the decrease of the 2DEG density. Detailed calculations show that the critical AlN thickness increases with the increase of Al content in the AlGaN barrier. PACS 85.30.De; 73.40.Kp; 02.60.Cb     

InAlN/GaN异质结二维电子气波函数的变分法研究

使用变分法推导了InAlN/GaN异质结二维电子气波函数和基态能级的解析表达式,并讨论了InAlN/GaN异质结结构参数对二维电子气电学特性的影响.在假设二维电子气来源于表面态的前提下,使用了一个包含两个变分参数的尝试波函数推导电子总能量期望值,并通过寻找能量期望极小值确定变分参数.计算结果显示,二维电子气面密度随InAlN厚度的增大而增大,且理论结果与实验结果一致.二维电子气面密度增大抬高了基态能级与费米能级,并保持二者之差增大以容纳更多电子.InAlN/GaN界面处的极化强度失配随着In组分增大而减弱,二维电子气面密度随之减小,并导致基态能级与费米能级减小.所建立的模型能够解释InAlN/GaN异质结二维电子气的部分电学行为,并为电子输运与光学跃迁的研究提供了解析表达式.     

Superconducting and ferromagnetic phases in SrTiO3/LaAlO3 oxide interface structures: possibility of finite momentum pairing

We introduce a model to explain the observed ferromagnetism and superconductivity in LAO/STO oxide interface structures. Because of the polar catastrophe mechanism, 1/2 charge per unit cell is transferred to the interface layer. We argue that this charge localizes and orders ferromagnetically via exchange with the conduction electrons. Ordinarily, this ferromagnetism would destroy superconductivity, but, due to strong spin-orbit coupling near the interface, the magnetism and superconductivity can coexist by forming a Fulde-Ferrell-Larkin-Ovchinikov-type condensate of Cooper pairs at finite momentum, which is surprisingly robust in the presence of strong disorder.