Characterization of InGaGdN layers prepared by molecular beam epitaxy

Gd‐doped InGaN layers were prepared by plasma‐assisted molecular‐beam epitaxy in search of new functional diluted magnetic semiconductors for their potential use in spintronics. The local structure around the Gd atoms was examined by the Gd L_III‐edge of X‐ray absorption fine structure. It was found that the majority of Gd atoms substitutionally occupied the cation sites in the InGaGdN layers. Clear hysteresis and saturation magnetization were observed from the magnetization versus field curves examined by means of a superconducting quantum interference device magnetometer at low and room temperatures. In addition, the incorporation of extra shallow donors by co‐doping InGaN with both Gd and Si showed higher magnetization than the undoped InGaGdN. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman scattering studies on manganese ion-implanted GaN

This paper reports that the Raman spectra have been recorded on the metal-organic chemical vapour deposition epitaxially grown GaN before and after the Mn ions implanted. Several Raman defect modes have emerged from the implanted samples. The structures around 182 cm^-1 modes are attributed to the disorder-activated Raman scattering, whereas the 361 cm^-1 and 660 cm^-1 peaks are assigned to nitrogen vacancy-related defect scattering. One additional peak at 280 cm^-1 is attributed to the vibrational mode of gallium vacancy-related defects and/or to disorder activated Raman scattering. A Raman-scattering study of lattice recovery is also presented by rapid thermal annealing at different temperatures between 700 °C and 1050 °C on Mn implanted GaN epilayers. The behaviour of peak-shape change and full width at half maximum (FWHM) of the A₁(LO) (733 cm^-1) and E^H₂ (566 cm^-1) Raman modes are explained on the basis of implantation-induced lattice damage in GaN epilayers.     

分子束外延PbTe单晶薄膜的反常拉曼光谱研究

采用分子束外延(MBE)方法在BaF₂(111)衬底上生长了高质量的PbTe单晶薄膜， 拉曼光谱测量观察到了表面氧化物的振动模、布里渊区中心(q≈0)纵光学(LO)声子振动模以 及声子-等离子激元耦合模振动.随着显微拉曼光谱仪激光光斑聚焦深度的改变，各拉曼散射 峰的峰位、积分强度、半高宽等都表现出不同的变化趋势. 随着激光光斑聚焦位置从样品表 面上方3μm处变化到表面下方3μm处，PbTe外延薄膜的LO声子频率从119cm^-1移 动到124cm^{-1关键词： PbTe外延薄膜 拉曼散射 纵光学声子}     

Temperature dependences of Raman scattering in different types of GaN epilayers

First-order Raman scatterings of hexagonal GaN layers deposited by the hydride vapour phase epitaxy and by metal-organic chemical vapour deposition on SiC and sapphire substrates are studied in a temperature range between 303 K and 503 K. The temperature dependences of two GaN Raman modes (A₁ (LO) and E₂ (high)) are obtained. We focus our attention on the temperature dependence of E₂ (high) mode and find that for different types of GaN epilayers their temperature dependences are somewhat different. We compare their differences and give them an explanation. The simplified formulas we obtained are in good accordance with experiment data. The results can be used to determine the temperature of a GaN sample.     

Properties of GaN on different polarity buffer layers by hydride vapour phase epitaxy

This paper reports on N-, mixed-, and Ga-polarity buffer layers are grown by molecular beam epitaxy (MBE) on sapphire (0001) substrates, with the GaN thicker films grown on the buffer layer with different polarity by hydride vapour epitaxy technique (HVPE). The surface morphology, structural and optical properties of these HVPE-GaN epilayers are characterized by wet chemical etching, scanning electron microscope, x-ray diffraction, and photoluminescence spectrum respectively. It finds that the N-polarity film is unstable against the higher growth temperature and wet chemical etching, while that of GaN polarity one is stable. The results indicate that the crystalline quality of HVPE-GaN epilayers depends on the polarity of buffer layers.     

GaP layers grown on GaN with and without buffer layers

The growth of GaP layer on GaN with and without buffer layers by metal-organic chemical vapour deposition (MOCVD) has been studied. Results indicate that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature (HT) GaP growth. These sites can promote the two-dimensional (2D) growth of HT GaP and reduce the surface roughness. A GaP single crystal layer grown at 680°C is obtained using a 40-nm thick GaP buffer layer. The full-width at half-maximum (FWHM) of the (111) plane of GaP layer, measured by DCXRD, is 560 arcsec. The GaP layer grown on GaN without low temperature GaP buffer layer shows a rougher surface. However, the FWHM of the (111) plane is 408 arcsec, which is the indication of better crystal quality for the GaP layer grown on GaN without a low temperature buffer layer. Because it provides less nucleation sites grown at high growth temperature, the three-dimensional (3D) growth is prolonged. The crystalline quality of GaP is lightly improved when the surface of GaN substrate is pretreated by PH3 , while it turned to be polycrystalline when the substrate is pretreated by TEGa.     

Raman scattering by folded acoustic phonons in InGaN/GaN superlattices

We use Raman scattering to investigate the folded longitudinal acoustic (LA) phonons in a series of In_xGa_1−xN/GaN superlattices (SLs) grown by molecular beam epitaxy with different compositions (15% < x < 38%) and SL periods (from 8 to 20 nm). A novel, ultralow wavenumber filtering module, which provides access to ultralow wavenumber Raman modes on single‐grating spectrometers, has been used to perform the Raman measurements. Zone‐folding effects are observed, and the wavenumber behavior of the folded LA modes is well reproduced with a linear dispersion for the folded LA modes as predicted by elastic continuum theory. We employ the wavenumber of the doublets to evaluate the period of the SLs. Copyright © 2011 John Wiley & Sons, Ltd.     

GaN薄膜拉曼散射光谱的研究

利用拉曼散射光谱对在SiC衬底上采用MOCVD异质外延的未故意掺杂GaN薄膜特性进行研究发现E2模式向频率低的方向漂移表明在GaN薄膜中存在张力,由于SiC衬底不平整度增加引起更多位错的出现,从而引起拉曼谱中E2模式的加宽,因此通过选择平整度较好的衬底可以减小缺陷密度,提高薄膜的质量,此外A1(LO)模式的出现与强度可以用来表征未掺杂GaN的薄膜质量。     

利用聚碳酸酯模板制备的金纳米棒的表面增强Raman散射效应研究

采用聚碳酸酯模板和电化学沉积法制备基于金纳米棒的Raman场增强衬底, 制备的金纳米棒直径大约36 nm, 长约1 μm, 测试结果显示其共振吸收峰的位置约为540 nm. 比较了谐振和非谐振条件下的场增强情况, 并确定了场增益系数, 结果显示谐振激光激发下的增益比非谐振情况下提高了7.36倍. 本研究相对于前人的工作取得了如下进展: 一是讨论了谐振模式与非谐振模式下的金纳米棒的场增益系数, 利用谐振波长的激光激发金纳米棒, 进一步提高了场增益; 二是消除了聚碳酸酯模板分子的荧光背底, 使其在表面增强 Raman 散射方面的应用进一步变得可行. 关键词： 金纳米棒 表面增强Raman散射 聚碳酸酯模板     

GaN quantum dots by molecular beam epitaxy

The conditions to grow GaN quantum dots (QDs) by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value, the growth mode of GaN deposited on AlN can be either of the Stranski–Krastanow (SK) or of the Frank–Van der Merwe type. Accordingly, quantum wells or QDs can be grown, depending on the desired application. In the particular case of modified SK growth mode, it will be shown that both plastic and elastic strain relaxation can coexist. Growth of GaN QDs with N-polarity will also be discussed and compared to their counterpart with Ga polarity.     

First order Raman scattering analysis of transition metal ions implanted GaN

Transition Metal (TM) ions V, Cr, Mn and Co were implanted into GaN/sapphire films at fluences 5×10¹⁴, 5×10¹⁵ and 5×10¹⁶ cm⁻². First order Raman Scattering (RS) measurements were carried out to study the effects of ion implantation on the microstructure of the materials, which revealed the appearance of disorder and new phonon modes in the lattice. The variations in characteristic modes 1GaN i.e. E₂(high) and A₁(LO), observed for different implanted samples is discussed in detail. The intensity of nitrogen vacancy related vibrational modes appearing at 363 and 665 cm⁻¹ was observed for samples having different fluences. A gallium vacancy related mode observed at 277/281 cm⁻¹ for TM ions implanted at 5×10¹⁴ cm⁻² disappeared for all samples implanted with rest of fluences. The fluence dependent production of implantation induced disorder and substitution of TM ions on cationic sites is discussed, which is expected to provide necessary information for the potential use of these materials as diluted magnetic semiconductors in future spintronic devices.     

First-order Raman scattering in a free-standing GaN nanowire with ring geometry

We have investigated one phonon resonant Raman scattering in GaN nanowires (NWs) with ring geometry. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. The selection rules are studied. For the GaN NWs with small radius, results reveal that the main contribution to the differential cross-section (DCS) stems from the surface optical (SO) phonons especially from the high-frequency of SO phonons, with a minor contribution from the longitudinal optical (LO) phonons. Meanwhile, dispersions of the two branches of the SO phonon modes are obvious when the wire is thin. Moreover, compared to GaAs NWs, the GaN NWs make more contribution to the DCS in the small quantum size.     

MOCVD生长的GaN和GaN:Mg薄膜的拉曼散射

通过显微拉曼散射对用金属有机化学气相沉积(MOCVD)法在Al2O3衬底上生长的六方相CaN和掺Mg的P型GaN薄膜进行了研究。在两个样品的拉曼散射谱中同时观察到位于640,660cm^-1附近的两个峰。640cm^-1的峰归因于布里渊区边界(L点)最高声学声子的二倍频,而660cm^-1的峰为布里渊区边界的光学声子支或缺陷诱导的局域振动模。掺Mg的GaN在该处的峰型变宽是Mg诱导的缺陷引起的加宽或Mg的局域模与上述两峰叠加的结果。在掺Mg的样品中还观察到276,376cm^-1几个局域模并给予了解释。同时掺Mg的GaN中出现了应力弛豫的现象,掺Mg引起的失配位错和电子-声子相互作用都有可能对E2模的频率产生影响。     

发散光长程空气传输受激转动拉曼散射效应

研究强激光长程空气传输受激转动拉曼散射(SRRS)效应,寻求抑制措施是高功率固体激光驱动器建造中需要解决的关键问题之一。采用数值模拟的方法研究了发散光长程空气传输SRRS效应的演化规律,通过几何近似理论推导了该效应有效建立时的强度距离乘积阈值和抑制判据。结果表明:发散光长程空气传输SRRS效应的建立速度小于平行光,二者阈值有理论关系;能够有效抑制SRRS效应的发散角设计判据由光束口径、激光强度及安全传输距离决定。     

UV-Raman scattering study of lattice recovery by thermal annealing of Eu -implanted GaN layers

Lattice recovery of Eu-implanted GaN has been studied by means of Raman scattering under UV excitation. GaN epilayers implanted at 300 keV with doses ranging from 2×10¹⁴ to 4×10¹⁵ cm⁻² and subsequently annealed at 1000 C for 20 min show an increasing degree of disorder as the implantation dose increases. Higher temperature annealings up to 1300 C were also investigated in samples having an AlN capping layer. Disorder related modes, observed in samples annealed at 1000 C, disappear at higher annealing temperatures, indicating an incomplete lattice recovery at 1000 C. The Raman scattering spectra show resonant A₁(LO) multiphonon scattering up to sixth order, whose relative intensities depend on the implantation dose. The intensity ratios between multiphonon peaks observed for the highest implantation doses suggest a spread of the resonance, which could be related to a heterogeneous strain distribution, also indicative of incomplete lattice recovery.     

受激旋转拉曼散射效应对基频光光束质量的影响

 采用瞬态受激旋转拉曼散射(SRRS)和空间相位扰动模型,定量分析了泵浦光强度和脉冲宽度对基频光在空气长程传输过程中产生的SRRS效应的阈值距离和转换效率的影响,讨论了泵浦光空间相位畸变对长程传输后的基频光光束质量的影响。研究结果表明：SRRS效应的阈值距离随着泵浦光强度和脉宽的增大而变短;空间相位畸变对斯托克斯光和泵浦光的光强分布存在一定影响,对斯托克斯光相位分布的影响比对泵浦光相位的影响大;SRRS效应的存在将导致基频光光束质量明显变差,对谐波转换效率产生明显影响。     

Stimulated Raman scattering and spontaneous Raman solitons in ammonia

In this study we report the first observation of spontaneous Raman solitons in stimulated Raman scattering (SRS) by the gas NH₃. The scattered radiation is called Stokes radiation. Raman solitons are of considerable interest, because their existence can be explained by quantum-mechanical fluctuations of the electromagnetic field in vacuum. We have observed spontaneous Raman solitons in a forward SRS configuration for two different molecular transitions of NH₃, the laser emissions at 58 μm and 72.6 μm wavelength. These are optically pumped by 10 μm CO₂-laser pulses with a duration of 100 ns and an energy of 150 mJ. Spontaneous Raman solitons are short spikes in the pump pulse which occur during its depletion. Their origin is the rapid π phase change of the Stokes seed. In contrast to other laboratories we have used single-pass cells. Thus, we have succeeded in observing multiple spontaneous Raman solitons during one pump pulse. Previous experiments with multi-pass cells never showed multiple solitons. Since multiple spontaneous Raman solitons have already been reported in an earlier experiment with a single-pass cell filled with hydrogen at high pressure, we conclude that such multiple Raman solitons can be observed mainly in this type of gas cell. Subsequently, we have performed statistical measurements on the delay time and the height of the spontaneous Raman solitons in the depleted pump pulse for the 58 μm-NH₃ emission. We have compared these statistics with theory and equivalent experimental results of other laboratories. They are in good agreement with the assumption that quantum-mechanical fluctuations are the origin of spontaneous Raman solitons. The most recent theories postulate that the origin of the formation of spontaneous Raman solitons can be explained by the rapid π phase change of the Stokes seed as well as that of the laser or polarization wave. Therefore, we have determined the phase of the spontaneous Raman solitons relative to the depleted pump pulse. Although, such changes of sign of the relative phase have already been observed in an earlier SRS experiment with hydrogen at high pressure, we did not detect any in our experiment. Therefore, we conclude that in this experiment the π phase change occurs in the Stokes or polarization wave.     

热丝CVD生长金刚石薄膜喇曼散射研究

本文报道热丝化学气相沉积法（ＨＦＣＶＤ）生长金钢石薄膜的喇曼散射结果。选取多种峰型，对金钢石薄膜喇曼谱（１１０－１８００ｃｍ－１）采用最小二乘法进行非线性拟合，得到最佳拟合模型，其计算得到的拟合曲线怀实验谱图符合得较好。该模型揭示，石墨Ｄ峰（１３５５ｃｍ－１）是金刚石薄膜喇曼谱中不可缺少的一个组份，并且结合石墨Ｄ峰和金刚石喇曼的空间相关线型，可以解释金刚石喇曼区特殊峰形的物理机制，拟合参量的进一步     

发散光长程空气传输受激转动拉曼散射效应

研究强激光长程空气传输受激转动拉曼散射(SRRS)效应,寻求抑制措施是高功率固体激光驱动器建造中需要解决的关键问题之一。采用数值模拟的方法研究了发散光长程空气传输SRRS效应的演化规律,通过几何近似理论推导了该效应有效建立时的强度距离乘积阈值和抑制判据。结果表明：发散光长程空气传输SRRS效应的建立速度小于平行光,二者阈值有理论关系;能够有效抑制SRRS效应的发散角设计判据由光束口径、激光强度及安全传输距离决定。