以TiO_2为缓冲层在GaN上外延生长PZT铁电薄膜

采用脉冲激光沉积技术在(0001)取向的GaN基片上以TiO2为缓冲层外延生长了PZT(111)单晶薄膜。X射线衍射分析表明PZT(111)衍射峰的摇摆曲线半高宽为0.4°,说明薄膜结晶性能良好。PZT薄膜疲劳特性测试结果表明,在经过107次翻转后PZT薄膜的剩余极化强度开始出现下降。P-E电滞回线和I-V测试表明PZT薄膜矫顽场(2Ec)为350 kV/cm,剩余极化(2Pr)约为96μC/cm2,在1 V电压下薄膜的漏电流密度为1.5×10-7A/cm2。以上性能测试结果表明,在半导体GaN上外延生长的PZT铁电薄膜性能基本满足铁电随机存储器的需要。     

LaNiO3缓冲层对Pb(Zr,Ti)O3铁电薄膜的影响

采用化学溶液法在Pt／Ti／SiO2／Si衬底上制备了PbZr0．4Ti0.6O3／LaNiO3(PZT／LNO)多层薄膜。X射线衍射测量表明LNO缓冲层的引入使PZT薄膜(111)择优取向度减小,(100)取向增加。原子力显微镜测量表明引入LNO缓冲层使得PZT薄膜表面更加平整、致密。在LNO缓冲层上制备的PZT薄膜具有优良的铁电特性和介电特性：LNO缓冲层厚度为40nm时,500kV／cm的外加电．场下。剩余极化(Pr)为37．6μC／cm^2,矫顽电场(Ec)为65kV／cm;100kHz时,介电常数达到822,并且发现LNO缓冲层的厚度为40nm,PZT的铁电、介电特性改进最为显著。     

锆钛酸铅铁电薄膜电容的研究现状

介绍了Pb(Zr,Ti)O3,(PZT)铁电薄膜电容的研究现状,列举了不同电极和缓冲层所制备出的PZT铁电薄膜电容的结构,并对不同结构进行了分析比较,结果表明由于氧化物电极材料的各种优越性,已被证明可用于替代现有的金属电极材料,从而有效解决了PZT薄膜铁电性能退化的问题,是未来铁电薄膜电容的发展方向.     

脉冲激光沉积制备Al-LiNbO_3-GaN结构研究

采用脉冲激光沉积设备(PLD)在GaN(0001)晶向上成功沉积了铁电多晶膜LiNbO_3,然后采用热蒸发法在薄膜表面镀铝电极。并对制备的LiNbO_3薄膜进行XRD表征以及对金属-铁电体-半导体(MFS)结构进行了C-V表征,XRD结果表明,沉积温度对薄膜的晶化有很大的影响;C-V测量结果表明,MFS的GaN激活层在5V下就能得到反转,这是一般半导体基集成电路的要求电压。GaN基的MFS结构在GaN基场效应晶体管的实际应用中是非常有前景的。     

xBiInO_3-(1-x)PbTiO_3薄膜的制备与铁电、介电特性研究

利用射频磁控溅射方法,在单晶MgO(100)及缓冲层异质结构基底上沉积La掺杂的xBiInO_3-(1-x)PbTiO_3薄膜。通过调控靶材配比和溅射参数,研究了溅射中由于基底加热造成的溅射过程中的薄膜失铅、失铋现象,同时对磁控溅射参数进行了调控和研究,得到了优化的溅射制备条件。利用XRD、SEM、EDS、铁电分析仪等仪器,并对不同BiInO_3组分对薄膜性能的影响进行研究,包括对薄膜的生长取向、形貌、铁电性和居里温度等进行研究,获得综合性能良好的高铁电性、高居里温度的铁电薄膜。     

MgO缓冲层上钡铁氧体(BaFe_(12)O_(19))薄膜的制备与性能研究

采用脉冲激光沉积(PLD)技术在(0006)取向的蓝宝石基片上,通过MgO缓冲层诱导生长了BaFe12O19(BaM)薄膜,研究了沉积温度对BaM薄膜的晶体结构和磁性能的影响规律。X射线衍射(XRD)分析结果表明,在激光脉冲频率6Hz、激光能量180mJ、MgO缓冲层的厚度50nm和沉积温度为750℃的条件下,制得的BaM薄膜c轴取向最好,其(0008)面的ω摇摆曲线半高宽(FWHM)仅为0.289°。扫描电子显微镜(SEM)结果显示此时薄膜的晶粒已有部分呈六角片状。振动样品磁强计(VSM)测试表明,在750℃时沉积的BaM薄膜面外饱和磁化强度为190A/m,剩磁比0.82,薄膜磁性能良好。     

PLD工艺制备高质量ZnO/Si异质外延薄膜

采用脉冲激光沉积工艺在不同条件下以Si(111)为衬底制备了Zno薄膜.通过对不同氧压下(0～50Pa)沉积的样品的室温PL谱测试表明,氧气氛显著地提高了薄膜的发光质量,在50Pa氧气中沉积的ZnO薄膜具有最强的近带边UV发射.XRD测试说明在氧气氛中得到的薄膜结晶质量较差,没有单一的(002)取向.利用-低温(500℃)沉积的ZnO薄膜作缓冲层,得到了高质量的ZnO外延膜.与直接沉积的ZnO膜相比,生长在缓冲层上的ZnO膜展现出规则的斑点状衍射花样,而且拥有更强的UV发射和更窄的UV峰半高宽(98meV).对不同温度下沉积的缓冲层进行了RHEED表征,结果表明,在600～650℃之间生长缓冲层,有望进一步改善ZnO外延膜的质量.     

铁电相BiFeO3对多铁复合薄膜CoFe2O4-BiFeO3铁磁性能的影响

为了研究铁电相BiFeO₃对复合薄膜磁性能的影响,在LaNiO₃ (LNO)缓冲层的Si (100)衬底上旋涂制备了含有0、6、9、10层等不同厚度BiFeO₃的层状CoFe₂O₄-BiFeO₃ (CFO-BFO) 多铁复合薄膜。采用XRD、SEM以及TEM对其结构和形貌进行了表征,采用振动样品磁强计测量磁性,研究了不同厚度BFO对复合薄膜磁性的影响。结果表明: CFO和BFO在异质结构薄膜中共存。缓冲层LNO和铁磁相CFO薄膜具有精细微观结构及明显界面。铁电相BFO的厚度对CFO-BFO复合薄膜的磁性能产生了很大影响。在含有不同层数铁电相BFO的复合薄膜中,含有9层BFO复合薄膜的饱和磁化强度最大,达到了230 emu·cm^-3,相比无铁电相BFO的薄膜,饱和磁化强度提高了18.6%。初步讨论认为: 随着铁电相BFO厚度的增加,CFO与BFO之间的应力传导引起了复合薄膜饱和磁化强度的提高。     

Al_2O_3衬底上Pb(Zr,Ti)O_3铁电薄膜的外延生长与性能

以SrRuO3(SRO)作为缓冲层,利用脉冲激光沉积的方法,首次在蓝宝石(Al2O3)衬底上制备了PbZr0.52-Ti0.48O3(PZT)外延薄膜。利用X射线衍射分析得到了PZT薄膜与Al2O3衬底的外延关系,即(111)[110]PZT//(0001)[1010]Al2O3。所制备的PZT薄膜具有较强的铁电极化性能,饱和极化值达到138.3μC/cm2。同时,利用对电流-电压性质的测量研究了铁电极化的退反转现象。     

单晶PMnN-PZT铁电薄膜制备与表征

利用磁控溅射方法,在MgO基底上沉积三元系铁电薄膜6%PMnN-94%PZT(6%Pb(Mn1/3,Nb2/3)O3-94%Pb(Zr0.45,Ti0.55)O3),采用淬火方法对薄膜进行后期热处理。分别运用面内和面外X射线衍射(XRD)技术分析薄膜长相及晶体结构,运用高分辨率扫描电镜(SEM)观察薄膜的晶粒表面及截面结构。实验结果表明,所沉积薄膜为单晶钙钛矿结构薄膜,薄膜结构优良。     

Study of the integrated growth of dielectric films on GaN semiconductor substrates

Typical perovskite oxides SrTiO? (STO) and PbZr?.??Ti?.??O? (PZT) were fabricated on GaN semiconductor substrates by pulsed laser deposition. STO and PZT films were deposited on bare GaN, TiO?, and MgO-buffered GaN. The effects of TiO? and MgO buffer-layers on the orientations and electric properties of the perovskite films were systematically studied. The crystalline properties were in situ monitored by reflective high energy electron diffraction and ex situ characterized by X-ray diffraction. It was found that the epitaxial temperature of STO and PZT was reduced by inserting a buffer layer. (111)-oriented films were obtained on bare and TiO?-buffered GaN. However, the orientations of the perovskite films were changed to be (110) when deposited on MgO buffer layer. Furthermore, PZT films deposited on MgO- and TiO?-buffered GaN show better electric performance compared with these films directly deposited on GaN. These results show that perovskite oxide could be epitaxially grown on GaN semiconductor substrates by inserting a proper buffer layer.     

Effect of high temperature AlGaN buffer thickness on GaN Epilayer grown on Si(111) substrates

Crack-free GaN epitaxial layer was obtained through inserting 80 nm graded AlGaN buffer layer between GaN epilayer and high temperature AlN buffer on 2-in Si(111) substrates by metal organic chemical vapor deposition. This paper investigated the influence of AlGaN buffer thickness on the structural properties of the GaN epilayer. It was confirmed from the optical microscopy and scanning electronic microscopy that the graded AlGaN buffer with optimized thickness had a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and atomic force microscopy analysis showed that AlGaN buffer with proper thickness could improve the crystal quality and surface morphology of the GaN film. Transmission electron microscopy analysis revealed that a significant reduction in threading dislocations was achieved in GaN epilayer by the insertion of graded AlGaN buffer.     

Normally-off characteristics of LiNbO3/AlGaN/GaN ferroelectric field-effect transistor

With the help of MgO mask layer, LiNbO₃ (LN) ferroelectric films were etched effectively using wet etching method and LN/AlGaN/GaN ferroelectric field-effect transistors (FFETs) were fabricated. The electrical properties of the FFETs were studied. Due to the ferroelectric polarization nature of LN films, normally-off characteristics with a turn-on voltage of about + 1.0 V were exhibited in the device. The operation mechanisms of the LN/AlGaN/GaN FFET devices were proposed by the numerical calculations of the electronic band structure and charge distribution.     

AlGaN/GaN Schottky Barrier Photodetector With Multi-${rm Mg}_{rm x} {rm N} _{rm y} $/GaN Buffer

AlGaN/GaN heterostructure Schottky barrier photodetector (PD) with multi-Mg_xN_y/GaN buffer was proposed and fabricated. Compared with AlGaN/GaN heterostructure PD prepared on conventional low-temperature GaN buffer, it was found that we can reduce dark leakage current by more than three orders of magnitude. It was also found that we can use the multi-Mg_xN_y/GaN buffer to suppress photoconductance gain, enhance UV-to-visible rejection ratio, reduce noise level and enhance the detectivity.     

The thickness effect of Bi3.25La0.75Ti3O12 buffer layer in PbZr0.58Ti0.42O3/Bi3.25La0.75Ti3O12 (PZT/BLT) multilayered ferroelectric thin films

A series of PbZr_0.58Ti_0.42O₃ (PZT) thin films with various Bi_3.25La_0.75Ti₃O₁₂ (BLT) buffer layer thicknesses were deposited on Pt/TiO₂/SiO₂/p-Si(100) substrates by RF magnetron sputtering. The X-ray diffraction measurements of PZT film and PZT/BLT multilayered films illustrate that the pure PZT film shows (111) preferential orientation, and the PZT/BLT films show (110) preferential orientation with increasing thickness of the BLT layer. There are no obvious diffraction peaks for the BLT buffer layer in the multilayered films, for interaction effect between the bottom BLT and top PZT films during annealing at the same time. From the surface images of field-emission scanning electron microscope, there are the maximum number of largest-size grains in PZT/BLT(30 nm) film among all the samples. The growth direction and grain size have significant effects on ferroelectric properties of the multilayered films. The fatigue characteristics of PZT and PZT/BLT films suggest that 30-nm-thick BLT is just an effective buffer layer enough to alleviate the accumulation of oxygen vacancies near the PZT/BLT interface. The comparison of these results with that of PZT/Pt/TiO₂/SiO₂/p-Si(100) basic structured film suggests that the buffer layer with an appropriate thickness can improve the ferroelectric properties of multilayered films greatly.     

GaN基底上集成介电薄膜材料的生长方法研究

将以极化为特征、具有丰富功能特性的介电氧化物材料通过外延薄膜的方式,在半导体GaN上制备介电氧化物/GaN集成薄膜,其多功能一体化与界面耦合效应可推动电子系统单片集成化的进一步发展。然而,由于2类材料物理、化学性质的巨大差异,在GaN上生长介电薄膜会出现严重的相容性生长问题。采用激光分子束外延技术(LMBE),通过弹性应变的TiO2的缓冲层来减小晶格失配度,降低介电薄膜生长温度,控制界面应变释放而产生的失配位错,提高了介电薄膜外延质量;通过低温外延生长MgO阻挡层,形成稳定的氧化物/GaN界面,阻挡后续高温生长产生的扩散反应;最终采用TiO2/MgO组合缓冲层控制介电/GaN集成薄膜生长取向、界面扩散,降低集成薄膜的界面态密度,保护GaN半导体材料的性能。所建立的界面可控的相容性生长方法,为相关集成器件的研发提供了一条可行的新途径。     

Theoretical investigation of the SAW properties of ferroelectric film composite structures

The characteristics of surface acoustic waves (SAW) propagating on a three-layered structure consisting of a perovskite-type ferroelectric film, a buffer layer and a semiconductor substrate have been studied theoretically. Large coupling coefficients (K(2)) can be obtained when the interdigital transducer (IDT) is on top of the perovskite-type ferroelectric film, with (type 4) and without (type 3) the floating-plane electrode at the perovskite-type ferroelectric film-buffer layer interface. In the above cases, the peak values of K (2) Of the Pb(Zr,Ti)O(3) (PZT) films (3.2%-3.8%) are higher than those of the BaTiO(3) (BT) and PbTiO(3) (PT) films. In the IDT configuration of type 4, there exists a minor peak of the coupling coefficients for the PZT and BT films, but not for the PT films when the normalized thickness (hK) of the perovskite-type ferroelectric film is about 0.3. The minor peak values of the coupling coefficients (0.62%-0.93%) for different layered structures (PZT/STO/Si, PZT/MgO/Si, and PZT/MgO/GaAs) all decrease when we increase hK value from 0 to 0.25. The results could be useful in the integration of ferroelectric devices, semiconductor devices, and SAW devices on the same substrate.     

Screening effect in contactless electroreflectance spectroscopy observed for AlGaN/GaN heterostructures with two dimensional electron gas

AlGaN/GaN heterostructures with a two dimensional electron gas (2DEG) at the interface have been investigated by contactless electroreflectance (CER) and photoreflectance (PR) spectroscopies. It has been shown that the 2DEG effectively screens the GaN layer and hence no signal related to a bandgap transition in the GaN layer is observed in CER spectra whereas the CER signal related to a bandgap transition in the AlGaN layer is very strong. The screening phenomenon is unimportant for PR spectroscopy due to different mechanism of the electromodulation. As a result both GaN and AlGaN related transitions are clearly observed in PR spectra. It has been proposed that the screening phenomena observed in CER can find application in contactless detection of the 2DEG in AlGaN/GaN heterostructures.     

Preparation of Ferroelectric NaNbO/sub 3/Thin Films on MgO Substrate by Pulsed Laser Deposition

We successfully fabricated good quality NaNbO₃ (NN) films on MgO substrate by pulsed laser deposition using a K-Ta-O (KTO) buffer layer. An SrRuO₃ (SRO) lower electrode layer was deposited on a KTO buffer layer/(100)MgO substrate and then the NN film was deposited on top. X-ray diffraction showed that the SRO and NN films were epitaxially grown on (100)MgO substrate. Transmission electron microscopy showed a crystal- lographic relationship of [001]_NN//[001]_MgO between NN and MgO. The relative dielectric constant, epsiv_r, and dielectric loss, tan delta, of the film were 350 and 0.05 at 1 kHz, respectively. The polarization vs. electric field (P-E) hysteresis loop of the NN film was characteristic of ferroelectric behavior.     

The Effect of the Method by Which a High-Resistivity GaN Buffer Layer Is Formed on Properties of InAlN/GaN and AlGaN/GaN Heterostructures with 2D Electron Gas

AlGaN/AlN/GaN and InAlN/AlN/GaN structures with 2D electron gas have been grown on sapphire substrates by metal-organic vapor-phase epitaxy. The suppression of the parasitic conductivity of buffer GaN layers was provided either by intentionally raising the density of edge dislocations or by doping with iron (GaN:Fe). It was shown that using GaN buffer layers with a better crystal perfection and more planar surface results in the electron mobility in the 2D channel for carriers becoming 1.2–1.5 times higher.