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.     

Synthesis and characterization of phosphorus-doped ZnO and (Zn,Mg)O thin films via pulsed laser deposition

The transport and optical properties of phosphorus-doped (Zn,Mg)O thin films grown via pulsed laser deposition (PLD) are studied. The carrier type of as-deposited (Zn,Mg)O:P films converts from n-type to p-type with increasing oxygen partial pressure. All the films exhibit good crystallinity with c-axis orientation. This result indicates the importance of oxidation conditions in realizing p-type (Zn,Mg)O:P films. The as-deposited ZnO:P film properties show a strong dependence on the deposition ambient at different growth temperatures. The resistivity of the samples deposited in O₃/O₂ mixture is two orders of magnitude higher than the films grown in oxygen and O₂/Ar/H₂ mixture. The room-temperature photoluminescence (PL) of the as-deposited films has been shown that growing in the O₂/Ar/H₂ mixture ambient significantly increases the band edge emission while inhibiting the visible emission. The enhanced ultraviolet (UV) emission in the films grown in O₂/Ar/H₂ mixture may result from hydrogen passivation of the deep level emission centers. The annealed ZnO:P films are n-type with nonlinear dependence of resistivity on annealing temperature. The resistivity increases in the films with annealing at 800°C while decreasing with further increasing annealing temperature. Strong visible light emission is observed from the ZnO:P films annealed in oxygen.     

溅射气压对氧化镓薄膜光学特性的影响

采用射频磁控溅射法在石英衬底上制备了氧化镓(Ga2O3)薄膜.利用X射线衍射仪和紫外-可见-红外分光光度计分别对Ga2O3薄膜的晶体结构和光学带隙进行了表征,并在室温下测量了 Ga2O3薄膜的光致发光(PL)谱.结果表明:制备的Ga2O3薄膜呈非晶态.吸收边随着溅射气压的增加先蓝移后红移,光学带隙值范围为5.06～5.37 eV,溅射气压为1 Pa时,制备的Ga2O3薄膜具有最大的光学带隙.在325 nm激光激发下,400 nm附近和525 nm附近处出现与缺陷能级相关的发光峰.     

Effects of annealing on the characteristics of ZnO films deposited in various O2/(O2+Ar) ratios

C-axis oriented ZnO films are deposited on polished diamond substrates in various O₂/(O₂+Ar) ratios using the radio frequency (RF) magnetron sputtering technique and are subsequently annealed in oxygen ambience under the same conditions. Structural, morphologic and electrical properties of ZnO films are characterized by X-ray diffraction (XRD), high-resistance instrument, energy dispersive X-ray spectroscopy (EDS) and scanning electronic microscopy (SEM). As the O₂/(O₂+Ar) ratio increasing from 1/12 to 5/12, the crystallinity of the as grown ZnO films becomes better and the electrical resistivity increases slowly. After annealing, the ZnO films deposited in O₂/(O₂+Ar) =1/12 and 3/12 are improved greatly in crystallinity, and their electrical resistivity is enhanced by two orders of magnitude, while those deposited in O₂/(O₂+Ar) =5/12 are scarcely changed in crystallinity, and their resistivity is only increased by one order. In addition, the ZnO films deposited in O₂/(O₂+Ar) =3/12 and annealed in oxygen are with the best crystal quality and the highest resistivity.     

Properties of Indium-Doped ZnO Films Prepared in an Oxygen-Rich Plasma

Indium-doped zinc oxide (ZnO:In) films were prepared in an Ar:O₂ plasma by reactive magnetron sputtering. The x-ray diffraction (XRD) patterns presented the crystal structures of ZnO:In films, while transmission spectra and photoluminescence (PL) spectra showed the changed band gap and the visible emission from defects, as compared to the PL spectra of undoped ZnO films. It was concluded that the increase of substrate temperature enhanced the crystal quality of ZnO:In films; the incorporation of In made the c-axis constant of the samples larger than that of undoped ZnO films; the blue emission was due to the transition from an unknown donor level by indium doping to the valance band; and the orange-green emission originated from acceptor defects (O_Zn) formed in the O-rich plasma. Meanwhile, the current- voltage characteristics and persistent photoconductivity phenomenon also could be explained by the increased acceptor defects (O_Zn) that formed when the substrate temperature was increased.     

Enhancement in optical characteristics of c-axis-oriented radio frequency–sputtered ZnO thin films through growth ambient and annealing temperature optimization

High-quality radio frequency–sputtered ZnO were grown on Si substrates at 400 °C at various partial gas pressures (Ar/Ar+O2). Subsequently, to remove as-grown defects, high temperature annealing from 700 to 900 °C on as-grown samples in constant oxygen flow for 10 s was performed. X-ray diffraction study confirmed the formation of highly crystalline films with a dominant peak at (002). The sample grown in 50% Ar and 50% O₂ ambient exhibited the lowest linewidth (2θ=~0.2728°) and highest stoichiometry. Grain size of the as grown samples decreased with increase in the partial pressure of oxygen till a certain ratio (1:1), and photoluminescence (PL) improved with increase in annealing temperature. Low-temperature (18 K) PL measurements showed a near-band-edge emission peak at 3.37 eV, and the highest peak intensity (more than six orders compared to others with narrow linewidth of ~0.01272 eV) was exhibited by the sample annealed at 900 °C and was six orders higher than that of the as-grown sample. All as-grown samples exhibited dominant visible-range peaks due to emission from defect states.     

Towards metal electrode interface scavenging of rare-earth scandates: A Sc2O3 and Gd2O3 study

Amorphous Gd₂O₃ and Sc₂O₃ thin films were deposited on Si by high-pressure sputtering (HPS). In order to reduce the uncontrolled interfacial SiO_x growth, firstly a metallic film of Gd or Sc was sputtered in pure Ar plasma. Subsequently, they were in situ plasma oxidized in an Ar/O₂ atmosphere. For post-processing interfacial SiO_x thickness reduction, three different top metal electrodes were studied: platinum, aluminum and titanium. For both dielectrics, it was found that Pt did not react with the films, while Al reacted with them forming an aluminate-like interface and, finally, Ti was effective in scavenging the SiO₂ interface thickness without severely compromising gate dielectric leakage.     

Optical characteristics of Si/SiO2 multilayers prepared by magnetron sputtering

Si/SiO₂ multilayers have been successfully prepared by magnetron sputtering and subsequently thermal annealed in an Ar atmosphere at a temperature of more than 500 °C. The surface of the as-deposited films is compact and smooth, and the distribution of grain size estimated to be 20 nm is uniform. For Si/SiO₂ multilayers annealed at 1100 °C, the Si sublayer sandwiched between potential barrier SiO₂ is crystalline structure by means of the analysis of Raman spectra and XRD data. The visible PL peak accompanying to a blue-shift with the decrease of Si sublayer thickness has been observed, and the intensity of this peak enhances with the increase of annealing temperature. The visible luminescence properties of Si/SiO₂ multilayers can be ascribed to quantum confinement of electron-hole pairs in quantum wells with grain size lower than 4.5 nm. In Si/SiO₂ multilayers, not only quantum confinement but also Si-SiO₂ interface states play an important role in the optical transition. The PL peak located at 779 nm is independent of the thickness of Si sublayer, so it may be ascribed to interface mediated transition. Typical Si dangling bonds defect could be a dominating obstacle to high luminescence efficiencies.     

Highly Conducting Transparent Indium-Doped Zinc Oxide Thin Films

Highly conducting transparent indium-doped zinc oxide (IZO) thin films have been achieved by controlling different growth parameters using radio frequency magnetron sputtering. The structural, electrical, and optical properties of the IZO thin films have been investigated for varied indium content and growth temperature (T _G) in order to find out the optimum level of doping to achieve the highest conducting transparent IZO thin films. The highest mobility and carrier concentration of 11.5 cm²/V-s and 3.26 × 10²⁰ cm^?3, respectively, have been achieved in IZO doped with 2% indium. It has been shown that as T _G of the 2% IZO thin films increase, more and more indium atoms are substituted into Zn sites leading to shift in (002) peaks towards higher angles which correspond to releasing the stress within the IZO thin film. The minimum resistivity of 5.3 × 10^?4 Ω-cm has been achieved in 2% indium-doped IZO grown at 700°C.     

溅射法制备钇掺杂铟锌氧薄膜和器件特性研究王丹丹,王卿璞*,王汉斌,张锡健,武丽伟,李福杰,袁帅中国,济南,山东大学物理学院

室温下,采用射频磁控溅射法分别在钠钙玻璃和P型硅衬底上制备了不同厚度的钇掺杂铟锌氧薄膜。研究了薄膜的结构形貌和光学特性。以P型硅为栅极制备了底栅结构的YIZO薄膜晶体管,并研究了器件的输出和转移特性。研究发现,室温下制备的所有Y掺杂IZO薄膜均为非晶结构,YIZO薄膜晶体管均为n沟道耗尽型器件。有源层厚度为20nm的器件的开关电流比超过105,亚阈值摆幅为2.20 V/decade,阈值电压为-1.0V, 饱和迁移率为0.57 cm2/ V·s。     

Photoluminescence from c-axis oriented ZnO films synthesized by sol-gel with diethanolamine as chelating agent

ZnO films were synthesized on SiO₂/Si substrates through the sol-gel technique using diethanolamine as chelating agent and annealed in Ar+O₂ atmospheres with different O₂ flow-rates in the 10–100 sccm range. Samples were studied by scanning electron microscopy and X-ray diffraction, evidencing a nanostructured morphology with a preferential orientation along the (0 0 2) direction (c-axis orientation), which is uncommon when diethanolamine is used as the chelating agent. The room temperature photoluminescence spectra show strong UV emissions at around 375 and 384 nm from near band-edge transitions and phonon replica, and a broad defect-related band extending from the visible to near infrared (∼500–800 nm). The analysis of the defect-related emission band and its various components as a function of the O₂ flow-rate is discussed in terms of contributions from specific luminescent point defect centers established during annealing.     

Deposition and doping of CdS/CdTe thin film solar cells

1% oxygen is incorporated into both CdS and CdTe layers through RF sputtering of CdS/CdTe thin film solar cells. The optical and electrical parameters of the oxygenated and O₂-free devices are compared after CdCl₂ treatment and annealing in ambient Ar and/or air. The effects of ambient annealing on the electrical and optical properties of the films are investigated using current-voltage characterization, field emission scanning electron microscopy, X-ray diffraction, and optical transmission spectroscopy. The 1% oxygen content can slightly increase the grain size while the crystallinity does not change. Annealing in ambient Ar can increase the transmission rate of the oxygenated devices.     

Hafnium oxide thin films deposited by high pressure reactive sputtering in atmosphere formed with different Ar/O2 ratios

The physical and electrical properties of hafnium oxide (HfO₂) thin films deposited by high pressure reactive sputtering (HPRS) have been studied as a function of the Ar/O₂ ratio in the sputtering gas mixture. Transmission electron microscopy shows that the HfO₂ films are polycrystalline, except the films deposited in pure Ar, which are amorphous. According to heavy ion elastic recoil detection analysis, the films deposited without using O₂ are stoichiometric, which means that the composition of the HfO₂ target is conserved in the deposition films. The use of O₂ for reactive sputtering results in slightly oxygen-rich films. Metal-Oxide-Semiconductor (MOS) devices were fabricated to determine the deposited HfO₂ dielectric constant and the trap density at the HfO₂/Si interface (D_it) using the high–low frequency capacitance method. Poor capacitance–voltage (CV) characteristics and high values of D_it are observed in the polycrystalline HfO₂ films. However, a great improvement of the electrical properties was observed in the amorphous HfO₂ films, showing dielectric constant values close to 17 and a minimum D_it of 2×10¹¹ eV⁻¹ cm⁻².     

Analysis of Nonradiative Carrier Recombination Processes in InN Films by Mid-infrared Spectroscopy

We investigate the reduction in the efficiency of band-edge radiative recombination in InN by two carrier recombination processes via mid-gap states: radiative recombination via deep states and nonradiative recombination (NR). Because of the small band-gap energy value and the existence of the surface electron accumulation layer, the carrier transition processes via deep states cannot be observed easily. We address this problem by using mid-infrared photoluminescence (PL) measurements, and observe an emission peak around 0.32 eV at room temperature, which we interpret as being caused by transition processes via deep-defect states. Since this emission is weaker than the band-edge emission, the dominant carrier recombination process is concluded to be NR by phonon emission. The NR rate is known to be determined by the NR defect density, carrier transport processes to NR defects, and thermal activation processes of carriers. Carrier transport and capture processes by NR defects are investigated using p-type samples for various carrier mobility values. It is concluded that the NR rate is highly affected by the carrier transport, and that the candidates for the NR defect species are point defects and complexes of acceptor nature. We have also observed the correlation between the thermal conductivity and the band-edge PL intensity. As a result, we have found that the NR rate is highly affected by the carrier transport and thermal activation processes in InN.     

Chemical compositions and optical properties of HfOxNy thin films at different substrate temperatures

High-k HfO_xN_y thin films have been grown by radio frequency (rf) reactive sputtering of metal Hf target in N₂/Ar/O₂ ambient at different substrate temperatures. The chemical compositions of the films have been investigated as a function of substrate temperature by X-ray photoelectron spectroscopy (XPS). XPS measurements showed that nitrogen concentration increases with an increase in substrate temperature. Room-temperature spectroscopic ellipsometry (SE) with photon energy 0.75–6.5 eV was used to investigate the optical properties of the films. SE results demonstrated that refractive index n increases with an increase in substrate temperature. Based on TL parameters which were obtained from the best fit results used in a simulation of the measured spectra, meanwhile, we conclude that the energy band gap (E_g) decreases with an increase in substrate temperature.     

Cathodoluminescence of ZnO/GaN/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> heteroepitaxial structures grown by chemical vapor deposition

The cathodoluminescent properties of ZnO films in ZnO/GaN/α-Al₂O₃ and ZnO/α-Al₂O₃ heteroepitaxial structures grown by chemical vapor deposition in a low-pressure flowing-gas reactor were studied and compared. A superlinear dependence of the excitonic-band intensity in the cathodoluminescence spectrum of the ZnO/GaN/α-Al₂O₃ structures on the electron-beam current is ascertained, which indicates that the emission is stimulated for relatively low thresholds of the excitation intensity. It is shown that the ZnO films grown on the GaN substrates exhibit a much more effective cathodoluminescence compared to the cathodoluminescence in the films grown on α-Al₂O₃. It was observed that the luminescent properties of ZnO layers in the ZnO/GaN/α-Al₂O₃ structures subjected to long-term heat treatment at 750°C in an oxygen atmosphere exhibit a high thermal stability.     

Effect of Lattice Mismatch on Luminescence of ZnO/Si Hetero-Structure

研究了ZnO薄膜中应力对发光的影响.实验样品为ZnO体单晶、在Si基片上直接生长的ZnO薄膜以及通过SiC过渡层在Si基片上生长的ZnO薄膜.测量了这三种样品的X射线衍射图形、喇曼光谱和光致发光光谱.由X射线衍射图形可以看出,由于SiC过渡层缓解了ZnO与Si之间的晶格失配,使得通过SiC过渡层在Si上生长的ZnO薄膜的结晶质量好于直接在Si上生长的ZnO薄膜的质量.进一步通过喇曼谱测量发现,与ZnO体单晶相比,直接在Si上生长的ZnO薄膜的E2(high)峰红移1.9cm-1,根据喇曼谱峰位移与应力的关系可以推出薄膜中存在0.4GPa的张应力;而通过SiC过渡层在Si上生长的ZnO薄膜的E2(high)峰红移0.9cm-1,对应着0.2GPa的张应力.对照X射线衍射图形的结果可以看出,薄膜中张应力的大小与薄膜的结晶质量密切相关,表明张应力来源于外延层和基片间的晶格失配,晶格失配越大,外延层中产生的张应力越大.有无SiC过渡层的两种薄膜样品的PL光谱中都存在紫外和绿光两种谱带,随样品热处理时氧气分压增加,两种样品都出现绿光增强的相似的变化规律,但有SiC过渡层的样品的变化幅度较小.这一结果说明,绿色发光中心与薄膜的质量,也就是与薄膜中存在的张应力大小有关.在以往研究中得出的非故意掺杂ZnO薄膜的绿色发光中心来源于氧反位缺陷(Ozn),文中研究的结果正好可以解释氧反位缺陷形成的原因.由于薄膜中存在张应力,使得样品的能量升高,其结果必然会产生缺陷来释放张应力,以便降低系统能量.而氧离子半径大于锌离子半径,氧替位锌有利于释放张应力,也就是说,在存在张应力的情况下,Ozn的形成能降低.这一结果进一步证明Si上生长的ZnO薄膜中的绿色发光中心与氧反位缺陷有关.     

Physical properties in thin films of iron oxides

We have grown hematite (α-Fe₂O₃) thin films on stainless steel substrates and magnetite (Fe₃O₄) thin films on (0 0 1)-Si single crystal substrates by a RF magnetron sputtering process. α-Fe₂O₃ thin films were grown in an Ar atmosphere at substrate temperatures around , and Fe₃O₄ thin films in an Ar/O₂ reactive atmosphere at substrate temperatures around . Conversion electron Mössbauer (CEM) spectra of α-Fe₂O₃ thin films exhibit values for hyperfine parameter characteristic of the hematite stoichiometric phase in the weak ferromagnetic state [R.E. Vandenberghe, in: Mössbauer Spectroscopy and Applications in Geology, University Gent, Belgium, 1990. [1]]. Furthermore, the relative line intensity ratio suggests that the magnetization vector of the polycrystalline film is aligned preferentially parallel to the surface. The CEM spectra of Fe₃O₄ thin films show the presence of only the stoichiometric phase, and the values for the hyperfine fields and isomer shifts of the A and B sites are consistent with bulk Fe₃O₄[1]. The X-ray diffraction (XRD) pattern of the polycrystalline thin films also corresponds to α-Fe₂O₃ and Fe₃O₄ [JCPDS, X-ray diffraction data cards, 2001. [2]]. The samples were also analyzed by atomic force microscopy (AFM) and they reveal a grain morphology common for polycrystalline films. We found an average grain size of 211 nm and surface roughness of 45 nm in α-Fe₂O₃ films and an average grain size of 148 nm and surface roughness of 1.2 nm in Fe₃O₄ films.     

The Remarkable Thermal Stability of Amorphous In‐Zn‐O Transparent Conductors

Transparent conducting oxides (TCOs) are increasingly critical components in photovoltaic cells, low‐e windows, flat panel displays, electrochromic devices, and flexible electronics. The conventional TCOs, such as Sn‐doped In₂O₃, are crystalline single phase materials. Here, we report on In‐Zn‐O (IZO), a compositionally tunable amorphous TCO with some significantly improved properties. Compositionally graded thin film samples were deposited by co‐sputtering from separate In₂O₃ and ZnO targets onto glass substrates at 100 °C. For the metals composition range of 55–84 cation% indium, the as‐deposited IZO thin films are amorphous, smooth (R_RMS < 0.4 nm), conductive (σ ∼ 3000 Ω⁻¹ · cm⁻¹), and transparent in the visible (T_Vis > 90%). Furthermore, the amorphous IZO thin films demonstrate remarkable functional and structural stability with respect to heating up to 600 °C in either air or argon. Hence, though not completely understood at present, these amorphous materials constitute a new class of fundamentally interesting and technologically important high performance transparent conductors.     

Molecular beam epitaxial growth of MgZnCdSe on (100) InP substrates

Wide-gap II-VI MgZnCdSe quaternary compounds were grown on InP substrates by molecular beam epitaxy, for the first time. Changing the Mg composition (x = 0 to 0.63), various Mg_x(Zn_yCd_{1_y})_{1_x}Se lattice-matched to InP were grown. Mirror-like surface morphologies and streaky reflection high energy electron diffraction patterns of MgZnCdSe were obtained. With increased Mg compositions, the band-edge emissions wavelength in photoluminescence spectra was shifted from 572 nm (2.17 eV) to 398 nm (3.12 eV) at 15K. Furthermore, the absolute PL peak intensity increased drastically with increased band-edge emission, being accompanied by a relative decrement in the deep level emission intensities were also observed.