低温缓冲层对氧化锌薄膜质量的影响

利用金属有机化学气相沉积(MOCVD)法,在Si衬底上外延生长ZnO薄膜。为了改善氧化锌薄膜的质量,首先在Si衬底上生长低温ZnO缓冲层,然后再生长高质量的ZnO薄膜。通过XRD、SEM、光致发光(PL)光谱的实验研究,发现低温ZnO缓冲层可有效降低ZnO薄膜和Si衬底之间的晶格失配以及因热膨胀系数不同引起的晶格畸变。利用低温缓冲层生长的ZnO薄膜的(002)面衍射峰的强度要比直接在Si上生长的ZnO薄膜样品的高,并且衍射峰的半高宽也由0.21°减小到0.18°,同时有低温缓冲层的样品室温下的光致发光峰也有了明显的增高。这说明利用低温缓冲层生长的ZnO薄膜的结晶质量和光学性质都得到了明显改善。     

Strain relaxation and surface morphology of high indium content InAlAs metamorphic buffers with reverse step

Low-temperature step-graded high indium content InAlAs (In% = 0.75) metamorphic buffer layers with reverse step layer grown on GaAs substrate by molecular beam epitaxy are investigated in this paper. The composition and the strain relaxation of the top InAlAs layer are determined by high-resolution triple-axis X-ray diffraction measurements, which show that the top InAlAs layer is nearly fully relaxed and the growth parameters for these samples have little influence on the strain relaxation ratio. Surface morphology is observed by reflection high-energy electron diffraction pattern and atomic force microscopy. The surface morphology is found to depend strongly on both the growth temperature and the As flux. Compared with other samples, the sample growth under the optimized conditions has the smallest value of root mean square surface roughness. Furthermore, the ω − 2θ and ω scans of the triple-axis X-ray diffraction and transmission electron microscopy result also show the sample grown under the optimized conditions has good crystalline quality.     

GaN基异质结缓冲层漏电分析

通过对GaN基异质结材料C-V特性中耗尽电容的比较,得出AlGaN/GaN异质结缓冲层漏电与成核层的关系.实验结果表明,基于蓝宝石衬底低温GaN成核层和SiC衬底高温AlN成核层的异质结材料比基于蓝宝石衬底低温AlN成核层异质结材料漏电小、背景载流子浓度低.深入分析发现,基于薄成核层的异质结材料在近衬底的GaN缓冲层中具有高浓度的n型GaN导电层,而基于厚成核层的异质结材料的GaN缓冲层则呈高阻特性.GaN缓冲层中的n型导电层是导致器件漏电主要因素之一,适当提高成核层的质量和厚度可有效降低GaN缓冲层的背景载流子浓度,提高GaN缓冲层的高阻特性,抑制缓冲层漏电. 关键词： AlGaN/GaN异质结 GaN缓冲层 漏电 成核层     

Layer by layer growth of ZnO on (0001) sapphire substrates by radical-source molecular beam epitaxy

Layer by layer growth of ZnO epilayers on (0001) Al₂O₃ substrates is achieved by radical-source molecular beam epitaxy. A thin MgO buffer, followed by a low-temperature ZnO buffer was used in order to accommodate the lattice mismatch between ZnO and sapphire. Reflection high-energy electron diffraction intensity was employed for the optimization of the ZnO growth. The surface morphology of the samples was studied with atomic force microscopy. Investigation of the nature of the influence of the MgO buffer layer on the formation of ZnO on sapphire substrate was carried out using Transmission Electron Microscopy. For the first time it was shown that a thin spinel (magnesium aluminum oxide) layer is formed on the interface of the sapphire substrate and MgO buffer layer leading to the crystalline quality improvement of the ZnO main layer. X-Ray diffractometry measurements of the obtained ZnO layers show excellent quality of the single crystalline ZnO heteroepitaxially grown on sapphire. The crystalline quality of the ZnO layers is even better than that of our previously reported layers grown employing hydrogen peroxide as an oxidant. The full width at half maximum of the XRD (0002) rocking curve is as low as 25 arc s. The influence of growth parameters (Zn/O flux ratio, temperature, etc.) on the structural properties as well as on the surface morphology of the zinc oxide layers on sapphire is investigated and discussed.     

Characteristics of GaN epilayer grown on Al2O3 with AlN buffer layer by molecular beam epitaxy

Double crystal rocking curve (DCRC), atomic force microscopy (AFM), scanning electron microscopy (SEM), and photoluminescence (PL) measurements on GaN epilayers grown with various thicknesses (3.4, 8.5, 17, 34, and 51 nm) of the AlN buffer layer were carried out to investigate the surface, the structural, and the optical properties of the GaN epilayers. The results of the DCRC, AFM, SEM, and PL measurements showed that the GaN epilayer grown on a 3.4 nm AlN buffer layer had the best quality. These results indicate that GaN active layers grown on 3.4 nm AlN buffer layers hold promise for potential applications in optoelectronic devices.     

0.6-eV bandgap In0.69Ga0.31As thermophotovoltaic devices with compositionally undulating step-graded InAsyP1-y buffers

Single-junction,lattice-mismatched In0.69Ga0.31As thermophotovoltaic(TPV) devices each with a bandgap of 0.6 eV are grown on InP substrate by metal-organic chemical vapour deposition(MOCVD).Compositionally undulating stepgraded InAsyP1-y buffer layers with a lattice mismatch of ～1.2% are used to mitigate the effect of lattice mismatch between the device layers and the InP substrate.With an optimized buffer thickness,the In0.69Ga0.31As active layers grown on the buffer display a high crystal quality with no measurable tetragonal distortion.High-performance single-junction devices are demonstrated,with an open-circuit voltage of 0.215 V and a photovoltaic conversion efficiency of 6.9% at a short-circuit current density of 47.6 mA/cm2,which are measured under the standard solar simulator of air mass 1.5-global(AM 1.5 G).     

缓冲层对InSb/GaAs薄膜质量的影响

InSb是制作3~5μm红外探测器的重要材料。在GaAs衬底上外延生长InSb,存在的主要问题在于两种材料间14.6%的晶格失配度,会引入较大的表面粗糙度以及位错密度,使外延材料的结构和电学性能均会受到不同程度的影响。通过系列实验,研究了在生长过程中缓冲层对薄膜质量的影响。利用高能电子衍射仪(RHHEED)得到了合适的生长速率和Ⅴ/Ⅲ比,研究了异质外延InSb薄膜生长中低温InSb缓冲层对材料生长质量以及不同外延厚度对材料电学性质的影响。采用原子力显微镜(AFM)、透射电子显微镜(TEM)、X射线双晶衍射(DCXRD)等方法研究了InSb/GaAs薄膜的表面形貌、界面特性以及结晶质量。通过生长合适厚度的缓冲层,获得了室温下DCXRD半高峰宽为172″,77 K下迁移率为64300 cm²·V^-1·s^-1的InSb外延层。     

Effect of diamine treatment on the conversion efficiency of PbSe colloidal quantum dot solar cells

Polymer-free photovoltaic devices were fabricated via a solution process using PbSe colloidal quantum dots (QDs) and an organic semiconductor (tetrabenzoporphyrin, BP) that can be prepared in situ from a soluble precursor. The device structure was ITO/PEDOT/BP:QD/QD/Al, where the BP:QD and QD layers correspond to a bulk heterojunction and a buffer layer, respectively. The buffer layer was treated with ethylenediamine (EDA) to crosslink the QDs. As a result, the energy conversion efficiency of the EDA-treated device was superior to that of an untreated device, mainly due to an increase in the short-circuit current density.     

Characterization of high-quality MBE-grown GaN films on intermediate-temperature buffer layers

High-quality GaN thin films are grown by rf-plasma assisted molecular beam epitaxy. The quality of the GaN epitaxial layer is significantly improved by using an intermediate-temperature GaN buffer layer (ITBL) in addition to a conventional 20-nm-thick low-temperature buffer layer. The GaN epitaxial layers demonstrate systematic improvements in the electron mobility increasing from 82 cm² V^-1 s^-1, for films grown with just the low-temperature buffer layer, to about 380 cm² V^-1 s^-1 for films grown with an ITBL of thickness 800 nm. The photoluminescence also indicates systematic improvements in the intensity and the full-width-half-maximum with the use of ITBL. Photoreflectance spectra are measured from the GaN films. Detailed analyses of the excitonic transition energy demonstrate that the residual strain relaxes rapidly with the use of ITBL, which is attributed to the observed improvements in the mobility and the PL spectra. Received: 30 November 2000 / Accepted: 4 December 2000 / Published online: 9 February 2001     

Effect of double AlN buffer layer on the qualities of GaN films grown by radio-frequency molecular beam epitaxy

This paper reports that the GaN thin films with Ga-polarity and high quality were grown by radio-frequency molecular beam epitaxy on sapphire （0001） substrate with a double A1N buffer layer. The buffer layer consists of a high-temperature （HT） A1N layer and a low-temperature （LT） A1N layer grown at 800℃ and 600℃, respectively. It is demonstrated that the HT-A1N layer can result in the growth of GaN epilayer in Ga-polarity and the LT-A1N layer is helpful for the improvement of the epilayer quality. It is observed that the carrier mobility of the GaN epilayer increases from 458 to 858cm^2/V.s at room temperature when the thickness of LT-A1N layer varies from 0 to 20nm. The full width at half maximum of x-ray rocking curves also demonstrates a substantial improvement in the quality of GaN epilavers by the utilization of LT-A1N layer.     

Performance improvement of MEH-PPV:PCBM solar cells using bathocuproine and bathophenanthroline as the buffer layers

In this work, bathocuproine (BCP) and bathophenanthroline (Bphen), commonly used in small-molecule organic solar cells (OSCs), are adopted as the buffer layers to improve the performance of the polymer solar cells (PSCs) based on poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction. By inserting BCP or Bphen between the active layer and the top cathode, all the performance parameters are dramatically improved. The power conversion efficiency is increased by about 70% and 120% with 5-nm BCP and 12-nm Bphen layers, respectively, when compared with that of the devices without any buffer layer. The performance enhancement is attributed to BCP or Bphen (i) increasing the optical field, and hence the absorption in the active layer, (ii) effectively blocking the excitons generated in MEH-PPV from quenching at organic/aluminum (Al) interface due to the large band-gap of BCP or Bphen, which results in a significant reduction in series resistance (Rs), and (iii) preventing damage to the active layer during the metal deposition. Compared with the traditional device using LiF as the buffer layer, the BCP-based devices show a comparable efficiency, while the Bphen-based devices show a much larger efficiency. This is due to the higher electron mobility in Bphen than that in BCP, which facilitates the electron transport and extraction through the buffer layer to the cathode.     

Effects of sic buffer layer on the optical properties of ZnO films grown on Si (1 1 1) substrates

ZnO films have been grown by a sol-gel process on Si (1 1 1) substrates with and without SiC buffer layers. The influence of SiC buffer layer on the optical properties of ZnO films grown on Si (1 1 1) substrates was investigated. The intensity of the E₂ (high) phonon peak in the micro-Raman spectrum of ZnO film with the SiC buffer layer is stronger than that of the sample without the SiC buffer layer, and the breadth of E₂ (high) phonon peak of ZnO film with the SiC buffer layer is narrower than that of the sample without the SiC buffer layer. These results indicated that the crystalline quality of the sample with the SiC buffer layer is better than that of the sample without the SiC buffer layer. In photoluminescence spectra, the intensity of free exciton emission from ZnO films with the SiC buffer was much stronger than that from ZnO film without the SiC buffer layer, while the intensity of deep level emission from sample with the SiC buffer layer was about half of that of sample without the SiC buffer layer. The results indicate the SiC buffer layer improves optical qualities of ZnO films on Si (1 1 1) substrates.     

Dependences of the surface and the optical properties on the O2/O2 + Ar flow-rate ratios for ZnO thin films grown on ZnO buffer layers

ZnO active layers on ZnO buffer layers were grown at various O₂/O₂ + Ar flow-rate ratios by using radio-frequency magnetron sputtering. Atomic force microscopy images showed that the surface roughnesses of the ZnO active layers grown on ZnO buffer layers decreased with decreasing O₂ atmosphere, indicative of an improvement in the ZnO surfaces. The type of the ZnO active layer was n-type, and the resistivity of the layer increased with increasing O₂ atmosphere. Photoluminescence spectra from the ZnO active layers grown on the ZnO buffer layers showed dominant peaks corresponding to local levels in the ZnO energy gap resulting from oxygen vacancies or interstitial zinc vacancies, and the peak positions changed significantly with the O₂/O₂ + Ar flow rate. These results can help improve understanding of the dependences of the surface and the optical properties on the O₂/O₂ + Ar ratio for ZnO thin films grown on ZnO buffer layers.     

Influence of double A1N buffer layers on the qualities of GaN films prepared by metal-organic chemical vapour deposition

<正>In this paper we report that the GaN thin film is grown by metal-organic chemical vapour deposition on a sapphire (0001) substrate with double AlN buffer layers.The buffer layer consists of a low-temperature(LT) AlN layer and a high-temperature(HT) AlN layer that are grown at 600℃and 1000℃,respectively.It is observed that the thickness of the LT-AlN layer drastically influences the quality of GaN thin film,and that the optimized 4.25-min-LT-AlN layer minimizes the dislocation density of GaN thin film.The reason for the improved properties is discussed in this paper.     

Residual strain and alloying effects on the vibrational properties of step-graded InxAl1−xAs layers grown on GaAs

Lattice-relaxed In_xAl_1−xAs-graded buffer layers grown by MBE on GaAs substrate have been studied by micro-Raman scattering and photoluminescence (PL). In these heterostructures, the indium composition was gradually increased in six or four intermediate layers each of 100 nm thickness. The alloying effect in the In_xAl_1−xAs layers has been interpreted using the modified random element isodisplacement (MREI) formalism. The dependence in the MREI model of the longitudinal optical (LO) phonon energy and the In composition in the InAlAs alloy with PL measurements and Raman analysis allow the evaluation of disorder degree and give the In composition in the InAlAs active layers. The obtained InAs- and AlAs-like phonon frequencies from the fitting of Raman spectra are in reasonable agreement with those calculated according to the MREI model. Raman spectra show that InAs-like phonon frequencies are not strongly dependent either on the buffer structure or on the residual strain in the active layers. Using the AlAs-like LO phonon frequency shifts, we have calculated the residual strain in the In_xAl_1−xAs active layer. Raman results show that the slope of the grade is an important parameter that allows the growth of samples with good quality. The lower residual strain value was obtained by thick buffer with a smaller grading rate. PL measurements show that In compositions of active layers of the different studied samples are slightly higher than those measured during growth.     

Demonstration of nanophotonic NOT gate using near-field optically coupled quantum dots

We propose and demonstrate the operation of a nanometric optical NOT gate using CuCl quantum dots coupled via an optical near-field interaction. The device was smaller than 20 nm and its repeated operation was verified. The operating energy of this device was much lower than that of a conventional photonic device. We also introduce all-optical NAND and NOR gates using coupled quantum dots. Toward an actual nanophotonic device, we discuss the possibility of coupled InAlAs quantum dots. A double layer of InAlAs quantum dots for nanophotonic device operation was prepared using molecular beam epitaxial growth. We obtained a near-field spectroscopy signal, indicating that the InAlAs quantum dots coupled with the optical near field acted as a NOT gate. The experimental results show that the sample has great potential as an actual nanophotonic device. PACS 78.67.Hc; 07.79.Fc; 42.79.Ta     

High-resolution X-ray diffractometry and transmission electron microscopy as applied to the structural study of InAlAs/InGaAs/InAlAs multilayer transistor nanoheterostructures

InAlAs/InGaAs/InAlAs nanoheterostructures with different structures of metamorphic buffer layer and quantum well, which were grown by means of molecular-beam epitaxy on GaAs and InP substrates, are investigated. The laboratory technology of the growth of the given nanoheterostructures with predicted properties is perfected. The potential of an approach based on the comprehensive analysis of experimental data obtained via different techniques, namely, X-ray diffractometry, electron diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and atomic-force microscopy is studied. The metamorphic buffer layer design is improved on the basis of the results of the performed investigations. A method whereby balanced-mismatched superlattices are introduced directly inside the metamorphic buffer layer is proposed. It is established that the technological parameters of the growth of nanoheterostructures affect their structural perfection and electrophysical properties.     

Growth and optical properties of Ge/Si quantum dots formed on patterned SiO2/Si(0 0 1) substrates

Single and stacked layers of Ge/Si quantum dots were grown in SiO₂ windows patterned by electron-beam lithography on oxidized Si (0 0 1) substrates. The growth of a silicon buffer layer prior to Ge deposition is found to be an additional parameter for adjusting the Ge-dot nucleation process. We show that the silicon buffer layer evolves towards [1 1 3]-faceted pyramids, which reduces the area of the topmost (0 0 1) surface available for Ge nucleation. By controlling the top facet area of the Si buffer layers, only one dot per circular window and a high cooperative arrangement of dots on a striped window can be achieved. In stacked layers, the dot homogeneity can be improved through the adjustment of the Ge deposited amount in the upper layers. The optical properties of these structures measured by photoluminescence spectroscopy are also reported. In comparison with self-assembled quantum dots, we observed, both in single and stacked layers, the absence of the wetting-layer component and an energy blue shift, confirming therefore the dot formation by selective growth.     

Stranski–Krastanow growth of multilayer In(Ga)As/GaAs QDs on Germanium substrate

We have described Stranski–Krastanow growth of multilayer In(Ga)As/GaAs QDs on Ge substrate by MBE. The growth technique includes deposition of a thin germanium buffer layer followed by migration-enhanced epitaxy (MEE) grown GaAs layer at 350°C. The MEE layer was overgrown by a thin low-temperature (475°C) grown GaAs layer with a subsequent deposition of a thick GaAs layer grown at 590°C. The sample was characterized by AFM, cross-sectional TEM and temperature-dependent PL measurements. The AFM shows dense formation of QDs with no undulation in the wetting layer. The XTEM image confirms that the sample is free from structural defects. The 8 K PL emission exhibits a 1051 nm peak, which is similar to the control sample consisting of In(Ga)As/GaAs QDs grown on GaAs substrate, but the observed emission intensity is lower. The similar slopes of Arrhenius plot of the integrated PL intensity for the as-grown QD sample grown on Ge substrate as well as for a reference QD sample grown on GaAs substrate are found to be identical, indicating a similar carrier emission process for both the samples. This in turn indicates coherent formation of QDs on Ge substrate. We presume due to the accumulated strain associated with the self-assembled growth of nanostructures on Ge that nonradiative recombination centers are introduced in the GaAs barrier in between the QD layers, which in turn degrades the overall optical quality of the sample.     

X-ray diffractometric study of the influence of a buffer layer on the microstructure of molecular-beam epitaxial InN layers of different thicknesses

Thin layers of InN are grown by molecular beam epitaxy on (0001) sapphire substrates. The influence of thin (15 nm) InN buffer layers and their temperature treatment on the structural quality of the grown layers is investigated by double-crystal and triple-crystal x-ray diffractometry. It is revealed that the preliminary high-temperature (900°C) annealing of the buffer layer leads to a notable improvement in the quality of the layers grown on this buffer. The densities of vertical screw and vertical edge dislocations decrease (to 1.9×10⁸ cm⁻² and 1.3×10¹¹ cm⁻², respectively) with an increase in the distance from the interface (by ∼1 μm). __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 5, 2001, pp. 913–917. Original Russian Text Copyright ? 2001 by Ratnikov, Mamutin, Vekshin, Ivanov.