Influences of ZnO buffer layers on the quality of ZnO films synthesized by the metal-organic chemical vapor deposition process

High-quality ZnO thin films were prepared by metal-organic chemical vapor deposition (MOCVD) on a sapphire (a-Al₂O₃) substrate. The synthesis of ZnO films was performed over a substrate temperature of 400–700°C and at chamber pressures of 0.1–10 torr. The structural and optical properties of ZnO films were investigated in terms of deposition conditions, such as substrate temperature, working pressure, and the ratio of Zn precursor (Diethylzinc (DEZn)) to oxygen. The ZnO films, preferentially oriented to 34.42° diffraction because of the (002) plane, were obtained under processing conditions of 700°C and 3 torr. This film shows a full-width at half-maximum (FWHM) of 0.4–0.6°. The results of photoluminescence (PL) spectroscopy also show a strong near band-edge emission at 3.36 eV at 10 K as well as a very weak emission at deep levels around 2.5 eV at room temperature. In addition, we are interested in the introduction of ZnO buffer-layer growth by the sputtering process to reduce lattice mismatch stress. This paper addresses how to advance the crystalline and optical properties of film. The ZnO film grown with the aid of a buffer layer shows a FWHM of 0.06–0.1° in the x-ray diffraction (XRD) pattern. This result indicates that crystalline properties were highly improved by the ZnO buffer layers. The PL spectroscopy data of ZnO film also shows a strong near band-edge emission and very weak deep-level emission similar to films synthesized without a buffer layer. Accordingly, synthesized ZnO films with buffer layers indicate fairly good optical properties and low defect density as well as excellent crystallinity.     

衬底偏压对HWCVD制备纳米晶硅薄膜结晶性的影响

通过在热丝化学气相沉积(HWCVD)制备纳米晶硅 薄膜过程中施加衬底偏压,研 究衬底偏压对HWCVD制备纳米晶硅薄膜结晶性能的影响。利用拉曼(Raman)光谱,X射线 衍射(XRD)和扫描电子显微镜(SEM)对所制备的纳米晶硅薄膜的结构性能进行分析。结果表 明,与未施加衬底偏压的薄膜相比,当衬底偏压为－300V时,薄膜 的晶化率由42.2%升高至 46.2%;当衬底偏压升高至－600V时,晶化率 又降至40.6%;未施加衬底偏压与施加－300V 偏压的纳米晶硅薄膜表面由长约200nm、宽约100nm的晶粒构成,－600V衬底偏压的薄 膜表面晶粒尺寸明显变小,并且出现大量非常细小的晶粒。分析产生上述现象的原因,主要 与 高温热丝发射电子、电子在电场作用下加速运动并与反应气体、基团碰撞发生能量传递有关 。     

A GeSi-buffer structure for growth of high-quality GaAs epitaxial layers on a Si substrate

A SiGe-buffer structure for growth of high-quality GaAs layers on a Si (100) substrate is proposed. For the growth of this SiGe-buffer structure, a 0.8-μm Si_0.1 Ge_0.9 layer was first grown. Because of the large mismatch between this layer and the Si substrate, many dislocations formed near the interface and in the low part of the Si_0.1Ge_0.9 layer. A 0.8-μm Si_0.05Ge_0.95 layer and a 1-μm top Ge layer were subsequently grown. The strained Si_0.05Ge_0.95/Si_0.1Ge_0.9 and Ge/Si_0.05Ge_0.95 interfaces formed can bend and terminate the upward-propagated dislocations very effectively. An in-situ annealing process is also performed for each individual layer. Finally, a 1–3-μm GaAs film was grown by metal-organic chemical vapor deposition (MOCVD) at 600°C. The experimental results show that the dislocation density in the top Ge and GaAs layers can be greatly reduced, and the surface was kept very smooth after growth, while the total thickness of the structure was only 5.1 μm (2.6-μm SiGe-buffer structure +2.5-μm GaAs layer).     

A new buffer layer for MOCVD growth of GaN on sapphire

High quality GaN films have been grown on sapphire substrates (C face and A face) by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) using a new buffer layer. With our reactor configuration and growth parameters, a GaN film grown on a single GaN buffer layer appears opaque with high density of hexagonal pits. Using a single A1N buffer layer results in extremely nonuniform morphology with mirror-like areas near the edge of the substrates and opaque areas in the center. The double buffer layer we report here, with GaN as the first layer and A1N as the second, each with an optimized thickness, leads to mirror-like films across the entire substrate. Scanning electron microscopy, photoluminescence, x-ray diffraction, and van der Pauw geometry Hall measurement data are presented to establish the quality of our films. The mechanism for this new buffer layer is also discussed.     

Selective silicon epitaxy by photo-chemical vapor deposition at a very low temperature of 160°C

We have grown epitaxial Si films by the photo-chemical vapor deposition (photo-CVD) technique with SiH₄ and H₂ at a very low-temperature of 160°C. Epitaxial films were grown on silicon substrates, while amorphous-like films were deposited on glass substrates. Furthermore, it was found from the atomic hydrogen etching which was produced by photo-dissociation of hydrogen that the etching rate of amorphous silicon was much higher than that of crystal silicon. By using these selectively, we have demonstrated selective epitaxial growth of silicon by the photo-CVD technique followed by the atomic hydrogen photo-etching. Furthermore, heavily phosphorus-doped silicon films (>1 × 10²¹ cm¹⁻³) were also selectively grown by this novel technique.     

Optimization of n/i and i/p buffer layers in n-i-p hydrogenated microcrystalline silicon solar cells

Hydrogenated microcrystalline silicon （μc-Si：H） intrinsic films and solar cells with n-i-p configuration were prepared by plasma enhanced chemical vapor deposition （PECVD）. The influence of n/i and i/p buffer layers on the μc-Si：H cell performance was studied in detail. The experimental results demonstrated that the efficiency is much improved when there is a higher crystallinity at n/i interface and an optimized a-Si：H buffer layer at i/p interface. By combining the above methods, the performance of μc-Si：H single-junction and a-Si：H/μc-Si：H tandem solar cells has been significantly improved.     

Optimization of n/i and i/p buffer layers in n-i-p hydrogenated microcrystalline silicon solar cells

Hydrogenated microcrystalline silicon (μc-Si:H) intrinsic films and solar cells with n-i-p configuration were prepared by plasma enhanced chemical vapor deposition (PECVD). The influence of n/i and i/p buffer layerson the μc-Si:H cell performance was studied in detail. The experimental results demonstrated that the efficiency is much improved when there is a higher crystallinity at n/i interface and an optimized a-Si:H buffer layer at i/p interface. By combining the above methods, the performance ofμc-Si:H single-junction and a-Si:H/μc-Si:H tandemsolar ceils has been significantly improved.     

Structural characteristics of as-deposited and crystallized mixed-phase silicon films

In this work, we report on the structural characteristics of as-deposited and crystallized mixed-phase silicon films prepared by thermal decomposition of silane in a low pressure chemical vapor deposition reactor. Mixed-phase films consist of crystallites embedded in an amorphous matrix. The size of these crystallites depends upon the surface diffusion length, a parameter quantitatively expressing the potential of adsorbed silicon atoms for surface diffusion. The density of the pre-existing crystallites can be related to the maximum density of critical nuclei, which develops during the deposition of the film. Both variables were quantitatively related to the deposition temperature and rate via physical models reflecting the experimental observations. Values for the parameters associated with these models were extracted by fitting the experimental data to the theoretical equations. Our theoretical analysis is the first to relate quantitatively the structural characteristics of as-deposited mixed-phase films to the prevailing deposition conditions. Mixed-phase films can crystallize in a much shorter time than as-deposited amorphous films, due to the combination of the growth of the pre-existing crystallites and the higher nucleation rate of new crystallites within the amorphous matrix of the mixed-phase film. The crystallization time and final grain size of crystallized mixed-phase films were found to decrease with increasing density of pre-existing crystallites. However, we showed that if composite films are deposited, consisting of a mixed-phase layer and an amorphous layer, the grain size after crystallization could be comparable to that of crystallized as-deposited amorphous films, with the crystallization time of such composite films about threefold shorter. The structure of both as-deposited and crystallized single and composite mixed-phase films was found to be identical for films deposited on both oxidized silicon and Corning Code 1735 glass substrates.     

Synthesis and electromagnetic transport of large-area 2D WTe2 thin film

Tungsten telluride thin films were successfully prepared on monocrystal sapphire substrates by using atomic layer deposition and chemical vapor deposition technology, and the effects of different tellurization temperatures on the properties of tungsten telluride films were investigated. The growth rate, crystal structure and composition of the film samples were characterized and analyzed by using scanning electron microscope, Raman spectroscopy and X-ray photoelectron spectroscopy. The results showed that tungsten telluride thin films with good crystal orientation in (001) were obtained at telluride temperature of 550 °C. When the telluride temperature reached 570 °C, the tungsten telluride began to decompose and unsaturated magnetoresistance was found.     

Atmospheric pressure chemical vapor deposition of blanket tungsten films on silicon substrates for integrated circuit applications

Atmospheric pressure chemical vapor deposition (APCVD) of tungsten films using WF₆/H₂ chemistry has been studied. A statistical design of experiments approach and a surface response methodology were used to determine the most important process parameters and to obtain the best quality film possible in the parameter range studied. It was found that the deposition rate depends strongly on WF₆ flow rate, temperature, and the interaction between hydrogen flow rate and temperature. The resistivity was found to have a strong dependence on WF₆ and H₂ flow rates and temperature. An activation energy of 0.4 eV was calculated for the reaction rate limited growth regime. Empirical equations for predicting the deposition rate and resistivity were obtained. The resistivity decreases with both increasing film thickness and grain size. The films grown in the studied process parameter range indicate that (110) is the preferred orientation for films deposited with low WF₆/H₂ flow rate ratios at all deposition temperatures (350–450°C), whereas, the (222) orientation dominates at high WF₆/H₂ flow ratios and high deposition temperatures. Also, the grain size is larger for (222) oriented films than for (110) oriented films. The results of this study suggest that high-quality, thin film tungsten can be deposited using APCVD.     

热丝电流对HWCVD制备α-Si∶H膜结构及钝化效果的影响

采用热丝化学气相沉积(HWCVD)方法沉积本征非晶硅薄膜,研究了热丝电流对薄膜结构及其钝化单晶硅片效果的影响.采用光谱型椭偏仪分析了非晶硅薄膜的介电常数虚部ε2和薄膜空位浓度的变化,采用傅里叶红外光谱测试仪分析了膜中Si-HX键,使用硅片的少子寿命表征钝化效果.结果表明:在热丝电流(两根直径为0.5 mm的钽丝的总电流)为20.5～23.5 A时,随着热丝电流增大薄膜中空位浓度逐渐增大,薄膜中氢总含量在热丝电流约22.0 A时出现峰值,而此时薄膜微观结构参数R*最小,钝化效果在约21.5A处出现峰值,对应的表面复合速率低至2.9 cm/s.     

Polycrystalline silicon thin film transistors fabricated in various solid phase crystallized films deposited on glass substrates

In this work, we have characterized various types of polysilicon films, crystallized upon thermal annealing from films deposited by low pressure chemical vapor deposition in the amorphous phase and a mixed phase using silane or in the amorphous phase using disilane. Polysilicon thin film transistors (TFTs) were fabricated, at low processing temperatures, in these three types of films on high strain point Corning Code 1734 and 1735 glass substrates. Double layer films, with the bottom layer deposited in a mixed phase and the top in the amorphous phase, allowed TFT fabrication at a drastically reduced thermal budget; optimum values of thicknesses and deposition rates of the layers are reported for reducing the crystallization time and improving film quality. Optimum deposition conditions for TFT fabrication were also obtained for films deposited using disilane. The grain size distribution for all types of films was shown to be wider for a larger grain size. Fabricated TFTs exhibited field effect electron mobility values in the range of 20 to 50 cm²/V·s, subthreshold swings of about 0.5–1.5 V/dec and threshold voltage values of 2–4 V.     

MPCVD中基片加热材料的温度场摄动模型研究

为改进微波等离子体化学气相沉积（MPCVD）装置中的加热系统，提出了用基片加热材料替代常规加热方式的新的技术路线，建立了基片加热材料的微波轴对称温度场模型并得到了一般解，通过对基片加热材料的微波设计，在MPCVD装置中获得大片基片台直径的均匀温度分布区。     

Pure germanium epitaxial growth on thin strained silicon-germanium graded layers on bulk silicon substrate for high-mobility channel metal-oxide-semiconductor field-effect transistors

We demonstrate epitaxially grown high-quality pure germanium (Ge) on bulk silicon (Si) substrates by ultra-high-vacuum chemical vapor deposition (UHVCVD) without involving growth of thick relaxed SiGe buffer layers. The Ge layer is grown on thin compressively strained SiGe layers with rapidly varying Ge mole fraction on Si substrates resulting in several SiGe interfaces between the Si substrate and the pure Ge layer at the surface. The presence of such interfaces between the Si substrate and the Ge layer results in blocking threading dislocation defects, leading to a defect-free pure Ge epitaxial layer on the top. Results from various material characterization techniques on these grown films are shown. In addition, capacitance-voltage (CV) measurements of metal-oxide-semiconductor (MOS) capacitors fabricated on this structure are also presented, showing that the grown structure is ideal for high-mobility metal-oxide-semiconductor field-effect transistor applications.     

采用低温缓冲层技术在Si衬底上生长高质量Ge薄膜

采用低温缓冲层技术,在Si衬底上生长了质量优良的Ge薄膜。利用原子力显微镜（AFM）、双晶X射线衍射（XRD）和拉曼散射等研究了薄膜的晶体质量。结果表明,由于无法抑制三维岛状生长,低温Ge缓冲层的表面是起伏的。然而,Ge与Si间的压应变几乎完全弛豫。当缓冲层足够厚时,后续高温Ge外延层的生长能够使粗糙的表面变得平整。在...     

Co‐optimization of SnS absorber and Zn(O,S) buffer materials for improved solar cells

Thin‐film solar cells consisting of earth‐abundant and non‐toxic materials were made from pulsed chemical vapor deposition (pulsed‐CVD) of SnS as the p‐type absorber layer and atomic layer deposition (ALD) of Zn(O,S) as the n‐type buffer layer. The effects of deposition temperature and annealing conditions of the SnS absorber layer were studied for solar cells with a structure of Mo/SnS/Zn(O,S)/ZnO/ITO. Solar cells were further optimized by varying the stoichiometry of Zn(O,S) and the annealing conditions of SnS. Post‐deposition annealing in pure hydrogen sulfide improved crystallinity and increased the carrier mobility by one order of magnitude, and a power conversion efficiency up to 2.9% was achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

MOCVD生长源流量对p型GaN薄膜特性影响的研究

利用金属有机物化学气相淀积(MOCVD)技术在蓝宝石衬底上生长p型GaN:Mg薄膜,对不同二茂镁(CP2Mg)流量和Ⅴ族和Ⅲ族摩尔(Ⅴ/Ⅲ)比生长的p型GaN:Mg薄膜特性进行研究。研究表明,增加Ⅴ/Ⅲ比,可以降低螺旋位错密度,提高p型GaN晶体质量。当Ⅴ/Ⅲ比为3 800时,Cp2Mg流量最高为170sccm,获得p型GaN(002)面峰值半高宽(FWHM)最窄为232"。同时研究发现,单纯提高Ⅴ/Ⅲ比对降低刃型位错影响较不明显。     

生长温度对Mn掺杂ZnO纳米棒铁磁性的影响

用化学气相沉积法(CVD),在生长温度为600、650和700 ℃条件下,未采用任何催化剂制备了Mn掺杂ZnO纳米棒.研究发现,随着生长温度的升高,样品中O空位的浓度逐渐增加.低浓度的O空位可以增强Mn掺杂ZnO纳米棒的铁磁性,但O空位浓度过高时,Mn掺杂ZnO纳米棒表现出超顺磁性或反铁磁性.在3个样品中,650 ℃的样品具有最好的室温铁磁性,其饱和磁化强度为0.85 μB/Mn,矫顽力为50 Oe.     

The role of the low temperature buffer layer and layer thickness in the optimization of OMVPE growth of GaN on sapphire

In agreement with previous work,¹² a thin, low temperature GaN buffer layer, that is used to initiate OMVPE growth of GaN growth on sapphire, is shown to play a critical role in determining the surface morphology of the main GaN epilayer. X-ray analysis shows that the mosaicity of the main GaN epilayer continues to improve even after several μm of epitaxy. This continuing improvement in crystal perfection correlates with an improvement in Hall mobility for thicker samples. So far, we have obtained a maximum mobility of 600 cm²/V-s in a 6 μm GaN epilayer. Atomic force microscopy (AFM) analysis of the buffer layer and x-ray analysis of the main epilayer lead us to conclude that the both of these effects reflect the degree of coherence in the main GaN epitaxial layer. These results are consistent with the growth model presented by Hiramatsu et al., however, our AFM data indicates that for GaN buffer layers partial coherence can be achieved during the low temperature growth stage.     

ZnO growth on Si with low-temperature CdO and ZnO buffer layers by molecular-beam epitaxy

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.