组分阶梯InGaN势垒对绿光激光二极管光电性能的影响

为探究不同铟(In)组分In_xGa_1-xN势垒对绿光激光二极管光电性能的影响，本文采用SiLENSe(simulator of light emitters based on nitride semiconductors)仿真软件对一系列具有不同In组分In_xGa_1-xN势垒的激光二极管进行研究，结果发现In_xGa_1-xN势垒中In组分最佳值为3%,此时结构的斜率效率最高，内部光学损耗最低，光学限制因子最大，性能最优。在具有In_0.03Ga_0.97N势垒的多量子阱结构基础上，设计了一种组分阶梯(composition step-graded, CSG)InGaN势垒多量子阱结构，提高了激光二极管的斜率效率和电光转换效率，增加了光场限制能力。仿真结果表明，当注入电流为120 mA时，具有CSG InGaN势垒的多量子阱结构，电光转换效率从17.7%提高至19.9%,斜率效率从1.09 mW/mA增加到1.14 mW...     

采用MOCVD法在p型Si衬底上生长Zn0薄膜

采用金属有机物化学气相沉积(MOCVD)法在p型Si(100)衬底上生长未掺杂的n型ZnO薄膜.在不同的生长温度下,c轴取向ZnO薄膜被生长在Si衬底上,生长所采用的锌源为二乙基锌(DEZn),氧源为氧气(O2).通过X射线衍射(XRD)、光电子能谱(XPS)和荧光光(PL)谱研究了薄膜的结构和光学特性.研究表明温度为610℃时生长的ZnO薄膜显示最好的结构和光学特性.此外,所生长n-ZnO/p-Si异质结的I-V特性曲线都表现明显的整流特性,且反向漏电流很小.在620℃生长的异质结的漏电流相对最大,大于在其它温度下生长的异质结的漏电流.     

退火温度和β-FeSi2薄膜厚度对n-β-FeSi2/p-Si异质结太阳电池的影响

本文采用室温直流磁控溅射Fe-Si组合靶的方法,并通过后续退火温度的优化得到了单一相高质量的β-FeSi2薄膜.结果表明,在本实验条件下得到的未掺杂的β-FeSi2薄膜在室温下是n型导电的,其电学特性存在一个退火温度的最优点:800 ℃.而且在这个最佳温度点上,在Si(111)衬底上外延得到的薄膜载流子迁移率比在Si(100)上高出了一倍多.在上述研究的基础上,采用p-Si(111)单晶片作为外延生长β-FeSi2薄膜的衬底,并通过退火温度和薄膜厚度的优化制备出了国内第一个n-β-FeSi2/p-Si异质结太阳电池,其Jsc=7.90 mA/cm2 ,Voc=0.21 V,FF=0.23,η=0.38;.     

立方氮化硼薄膜注入Be的Hall效应研究

本文应用RF溅射法,在n型Si(100)衬底上制备BN薄膜.首次用离子注入工艺,将铍(Be)离子注入到BN薄膜中使之成为p-型.我们用范德堡方法对该薄膜进行了室温下的霍尔效应测量,薄膜为P型导电,电阻率为10-3Ω·cm左右,迁移率14～28 cm2/V·S,载流子浓度1019～1020cm-3,霍尔系数10-1cm-3/C左右,用此法制备的P-BN/n-BN异质结,有明显的整流特性.     

p-ZnO薄膜及其异质结的光电性质

采用射频磁控溅射法,在不同的Ar∶O2条件下,以高掺磷n型Si衬底为磷掺杂源制备了p型ZnO薄膜和p-ZnO/n-Si异质结.对ZnO∶P薄膜进行了光致发光谱(PL)、霍尔参数、Ⅰ-Ⅴ特性、扫描电镜(SEM)和X射线衍射谱(XRD)等测试.结果表明,获得的ZnO∶P薄膜沿(0002)晶面高取向生长,以3.33 eV近带边紫外发光为主,伴有2.69 eV附近的深能级绿色发光峰,空穴浓度为8.982 × 1017/cm3,空穴迁移率为9.595 cm2/V·s,p-ZnO/n-Si异质结I-V整流特性明显,表明ZnO∶P薄膜具有p型导电特性.     

MoS2/SnS异质结太阳能电池的模拟研究

硫化亚锡(SnS)是一种Ⅳ-Ⅵ族层状化合物半导体材料,其禁带宽度与太阳能电池最佳带隙1.5 eV非常接近,并且在可见光范围内光的吸收系数很大(α>10⁴ cm^-1),因此SnS是一种很有应用前景的材料。本文利用太阳能电池模拟软件wxAMPS模拟了MoS₂/SnS异质结太阳能电池,主要研究SnS吸收层的厚度、掺杂浓度和缺陷态等因素对太阳能电池性能的影响。研究发现:SnS吸收层最佳厚度为2 μm,最佳掺杂浓度为1.0×10¹⁵ cm^-3;同时高斯缺陷态浓度超过1.0×10¹⁵ cm^-3时,电池各项性能参数随着浓度的增加而减小,而带尾缺陷态超过1.0×10¹⁹ cm^-3·eV^-1时,电池性能才开始下降;其中界面缺陷态对太阳能电池影响比较严重,界面缺陷态浓度超过1.0×10¹² cm^-2时,开路电压、短路电流、填充因子和转换效率迅速下降。另外,通过模拟获得的转换效率高达24.87%,开路电压为0.88 V,短路电流为33.4 mA/cm²。由此可知,MoS₂/SnS异质结太阳能电池是一种很有发展潜力的光伏器件结构。     

Nd掺杂对磁控溅射Bi4-xNdxTi3O12铁电薄膜结构和性能的影响

采用磁控溅射工艺在p-Si衬底上制备了Bi4-xNdxTi3O12铁电薄膜,研究了Nd掺杂对Bi4-xNdxTi3O12薄膜微观结构、介电和铁电性能的影响.结果表明,Nd掺杂并未改变薄膜的晶格对称性,仍然保持Bi层状钙钛矿结构,但能在一定程度上抑制晶粒的生长,使薄膜的晶粒更加细小、均匀,同时能明显改善薄膜的介电、铁电性能.Nd掺杂量x=0.30 ～0.40时,Bi4-xNdxTi3O12薄膜的综合性能较好,其介电常数εr＞250,介电损耗tanδ ＜0.1,剩余极化Pr=20.6 μC/cm2,Ec＜ 150 kV/cm.Ag/Bi4-xNdxTi3O12/p-Si异质结顺时针回滞的C-V曲线表明该异质结可实现极化存储,其记忆窗口达1.6V.但掺杂量不宜过多,当Nd掺杂量达到0.45以后,薄膜的介电、铁电性能反而有所下降.     

ε-(AlxGa1-x)2O3/ε-Ga2O3异质结电子输运性质研究

Ga₂O₃是一种新兴的宽带隙半导体,在电力和射频电子系统中具有潜在的应用前景。前期研究以β-Ga₂O₃为主,并且已经对β-(Al_xGa_1-x)₂O₃/Ga₂O₃异质结构中的二维电子气(2DEG)进行了理论计算,本文主要研究ε-(Al_xGa_1-x)₂O₃作为势垒层对ε-(Al_xGa_1-x)₂O₃/ε-Ga₂O₃异质结电子输运性质的影响,首先介绍了ε-(Al_xGa_1-x)₂O₃/ε-Ga₂O₃异质结的结构和性质,分析计算了由于ε-(Al_xGa_1-x)₂O₃/ε-Ga₂O₃异质结的自发极化和压电极化所产生的极化面电荷密度,以及极化对2DEG浓度产生的影响,接着分析了在不同Al摩尔组分下,ε-(Al_xGa_1-x)₂O₃势垒层厚度与合金无序散射、界面粗糙度散射和极性光学声子散射之间的关系。最后通过计算得出结论:界面粗糙度散射和极性光学声子散射对ε-(Al_xGa_1-x)₂O₃/ε-Ga₂O₃异质结的电子输运性质有重要影响,合金无序散射对异质结的输运性质影响较小;2DEG浓度、合金无序散射、界面粗糙度散射和极性光学声子散射的电子迁移率强弱由ε-(Al_xGa_1-x)₂O₃势垒层的厚度和Al摩尔组分共同决定。     

薄膜非晶/微晶叠层电池中NP隧穿结的影响

本文论述了薄膜非晶/微晶叠层电池中NP隧穿结对电池性能的影响.在薄膜叠层电池中非晶顶电池的N层采用微晶硅,减小了电池的内部串联电阻影响.通过调整非晶硅顶电池N层和微晶硅底电池P层的厚度,降低NP隧穿结的影响,获得薄膜叠层电池效率11.73;(Voc=1.34V,Jsc=14.53mA/cm2,FF=60.27;),电池面积为0.253cm2.     

AlxGa1-xN氮化物结构和热力学性质的第一性原理研究

为了掌握Ⅲ族氮化物微观结构对热力学性能的影响规律,进而为超高功率器件的设计提供数据支持,本文借助第一性原理计算软件CASTEP,对半导体Al_xGa_1-xN不同合金结构及其热性能进行了系统研究。结构优化和数据分析后发现,Al_xGa_1-xN的晶格常数、平均键长和晶格热容值随Al组分值x(原子数分数)增大而线性减小。热力学性质计算结果表明,在GaN中引入Al组分会在频率带隙中引入杂质模,随着Al组分浓度的增加杂质模变宽并进入低频段,在低频段顶部频率随Al组分增大而线性升高,在12.5%以上低频段顶部频率均大于 (1/2)A₁(LO)。温度从300 K到700 K变化时, 合金热容随温度变化关系结果证明,在确定温度时,Al_xGa_1-xN合金热容随Al组分增大而线性减小。本文的研究为以Al_xGa_1-xN为代表的Ⅲ族氮化物半导体高功率器件设计提供了一定的设计参考。     

Growth of bulk InGaN films and quantum wells by atmospheric pressure metalorganic chemical vapour deposition

InGaN bulk layers and single quantum wells were grown on 1.4 to 2.4 μm thick GaN on sapphire films by atmospheric pressure metalorganic chemical vapour deposition (AP-MOCVD). The morphology of the epitaxial layers was strongly affected by the V/III ratio in the gas phase. The incorporation efficiency of indium was observed to increase with higher growth rates and decreasing temperature, but was independent of the V/III ratio in the investigated parameter range. In_0.16Ga_0.84N single quantum wells showed intense quantum well related luminescence at room temperature, with a full width at half maximum of 7.9 nm at a thickness of 50 Å. Single quantum wells embedded in InGaN of graded composition showed superior properties compared to quantum wells with In_0.04Ga_0.94N barriers of constant composition.     

Anomalous behavior of excess energy curves of InxGa1−xN grown on GaN and InN

We worked out the excess energies for bulk In_xGa_1−xN and In_xGa_1−xN thin films on GaN and InN in order to investigate their thermodynamic stabilities. It has been found that the excess energy maximum shifted toward x0.80 for InGaN/GaN and x0.10 for InGaN/InN due to the lattice constraint in contrast with x0.50 for bulk. Moreover, it has been revealed that the excess energy for InGaN/GaN is larger than that for bulk at x>0.65. This suggests that In-rich films are less stable on GaN than bulk state. These results indicate that the lattice constraint has a significant influence on thermodynamic stabilities of thin films.     

Profiling band structure in GaN devices by electron holography

Electron holography in a field emission gun transmission electron microscope has been used to profile the inner potential V₀ across GaN/x nm In_0.1Ga_0.9N/GaN/(0 0 0 1) sapphire samples (x=10, 40 nm) grown by molecular beam epitaxy and viewed in cross-section. Results are presented which suggest a decrease in V₀ of 3–4 V across the InGaN layer in the [0 0 0 1] direction. It is proposed that the results can be explained by charge accumulation across the InGaN layer and that the opposing contributions due to piezoelectric and polarisation fields are effectively masked by Fermi level pinning.     

Study of group-III binary and ternary nitrides using X-ray absorption fine structure measurements

It is demonstrated that the NEXAFS spectra are a “fingerprint” of the symmetry and the composition of the binary nitrides GaN, AlN and InN, as well as of their ternary alloys In_0.16Ga_0.84N and Al_yGa_1−yN. From the angular dependence of the N-K-edge NEXAFS spectra, the hexagonal symmetry of the under study compounds is deduced and the (p_x, p_y) or p_z character of the final state is identified. The energy position of the absorption edge (E_abs) of the binary compounds GaN, AlN and InN is found to red-shift linearly with the atomic number of the cation. The E_abs of the Al_yGa_1−yN alloys takes values in between those corresponding to the parent compounds AlN and GaN. Contrary to that, the E_abs of In_0.16Ga_0.84N is red-shifted relative to that of GaN and InN, probably due to ordering and/or phase separation phenomena. The EXAFS analysis results reveal that the first nearest-neighbour shell around the N atom, which consists of Ga atoms, is distorted in both GaN and Al_xGa_1−xN for x<0.5.     

Impedance spectroscopy analysis of AlGaN/GaN HFET structures

For HFET application a series of samples with 30 nm Al_xGa_1−xN (x=0.02–0.4) layers deposited at 1040°C onto optimised 2 μm thick undoped GaN buffers were fabricated. The Al_xGa_1−xN/GaN heterostructures were grown on c-plane sapphire in an atmospheric pressure, single wafer, vertical flow MOVPE system. Electrical properties of the Al_xGa_1−xN/GaN heterostructures and thick undoped GaN layers were evaluated by impedance spectroscopy method performed in the range of 80 Hz–10 MHz with an HP 4192A impedance meter using a mercury probe. The carrier concentration distribution through the layer thickness and the sheet carrier concentration were evaluated. A non-destructive, characterisation technique for verification of device heterostucture quality from the measured C–V and G–V versus frequency characteristics of the heterostructure is proposed.     

Thermodynamic analysis of the MOVPE growth of InxGa1−xN

A chemical equilibrium model is applied to the growth of the In_xGa_1−xN alloy grown by metalorganic vapor-phase epitaxy (MOVPE). The equilibrium partial pressures and the phase diagram of deposition are calculated for the In_xGa_1−xN alloy. The vapor-solid distribution relationship is discussed in comparison with the experimental data reported in the literature. It is shown that the solid composition of the In_xGa_1−xN alloy grown by MOVPE is thermodynamically controlled and that the incorporation of group III elements into the solid phase deviates from a linear function of the input mole ratio of the group III metalorganic sources under the conditions of high mole fraction of decomposed NH₃ (high value of ), high temperature and low input V/III ratio. The origin of the deviation of the solid composition from the linear relation is also discussed.     

Indium content determination related with structural and optical properties of InGaN layers

We studied the structural and optical properties of a set of nominally undoped epitaxial single layers of In_xGa_1−xN (0<x0.2) grown by MOCVD on top of GaN/Al₂O₃ substrates. A comparison of composition values obtained for thin (tens of nanometers) and thick (≈0.5 μm) layers by different analytical methods was performed. It is shown that the indium mole fraction determined by X-ray diffraction, measuring only one lattice parameter strongly depend on the assumptions made about strain, usually full relaxation or pseudomorphic growth. The results attained under such approximations are compared with the value of indium content derived from Rutherford backscattering spectrometry (RBS). It is shown that significant inaccuracies may arise when strain in In_xGa_1−xN/GaN heterostructures is not properly taken into account. Interpretation of these findings, together with the different criteria used to define the optical bandgap of In_xGa_1−xN layers, may explain the wide dispersion of bowing parameters found in the literature. Our results indicate a linear, E_g(x)=3.42−3.86x eV (x0.2), “anomalous” dependence of the optical bandgap at room temperature with In content for In_xGa_1−xN single layers.     

MBE growth of cubic AlyGa1−yN/GaN heterostructures—structural, vibrational and optical properties

Cubic Al_yGa_1−yN/GaN heterostructures on GaAs(0 0 1) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, micro-Raman, spectroscopic ellipsometry, and cathodoluminescence measurements were used to characterize the structural, optical and vibrational properties of the Al_yGa_1−yN epilayers. The AlN mole fraction y of the alloy was varied between 0.07<y<0.20. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic Al_yGa_1−yN films. The measured Raman shift of the phonon modes of the Al_yGa_1−yN alloy was in excellent agreement with theoretical calculations. Both SE and CL of the Al_yGa_1−yN epilayer showed a linear increase of the band gap with increasing Al-content.     

AlGaN photodetectors grown on Si(1 1 1) by molecular beam epitaxy

The fabrication and characterisation of Al_xGa_1−xN (0x0.35) photodetectors grown on Si(1 1 1) by molecular beam epitaxy are described. For low Al contents (<10%), photoconductors show high responsivities (10A/W), a non-linear dependence on optical power and persistent photoconductivity (PPC). For higher Al contents the PPC decreases and the photocurrent becomes linear with optical power. Schottky photodiodes present zero-bias responsivities from 12 to 5 mA/W (x=0−0.35), a UV/visible contrast higher than 10³, and a time response of 20 ns, in the same order of magnitude as for devices on sapphire substrate. GaN-based p–n ultraviolet photodiodes on Si(1 1 1) are reported for the first time.     

Heterointerface study of perturbed LPE growth of AlGaAs/InGaP single heterostructure

This paper presents the perturbed growth of Al_0.7Ga_0.3As/In_0.5Ga_0.5P single heterostructure on a GaAs substrate by liquid-phase epitaxy. The AlGaAs-InGaP heterointerface was characterized by scanning electron microscopy, photoluminescence, Auger electron spectroscopy, and transmission electron microscopy. Evidence is provided showing that a small amount of droplets, after the slider operation of the In_0.5Ga_0.5P epitaxial growth, mixed with the Ga-rich AlGaAs melt, is sufficient to attack the In_0.5Ga_0.5P underlying layer. Even with complete melt removal, there is still a partial dissolution at the “flat” Al_0.7Ga_0.3As-In_0.5Ga_0.5P heterojunction. The Auger depth profiles reveal the composition-depth transition width at this interface to be 560Å from the 90%-10% of Al (or Ga, As, and In) Auger profile; however, the P atoms penetrate deeply into the Al_0.7Ga_0.3As layer due to the partial dissolution of In_0.5Ga_0.5P layer. By high-resolution electron-micrograph analysis, some dislocations are observed at the heterojunction leading to nonradiative recombination and to poor optical device performance, even though the heterointerface observed by scanning electron microscopy is very flat.