射频等离子体分子束外延GaN的极性研究

对采用射频等离子体分子束外延（RF-plasma MBE)生长得到的GaN进行极性研究。由于镓极性（Ga-polar)比氮极性（N－polar)有更好的化学稳定性，通过比较RF－plasma MBE生长得到的不同GaN样品对光辅助湿法刻蚀的稳定性，发现缓冲层生长条件对GaN外延层的极性有着重要影响：较高缓冲层生长温度得到的GaN外延层表现为N－polar，较低缓冲层生长温度得到的GaN外延层表现为Ga-polar。     

Chloride vapor-phase epitaxy of gallium nitride on silicon: Structural and luminescent characteristics of epilayers

The structure and luminescent properties of gallium nitride (GaN) epilayers grown by hydride-chloride vapor-phase epitaxy (HVPE) in a hydrogen or argon atmosphere on 2-inch Si(111) substrates with AlN buffer layers have been studied. The replacement of hydrogen atmosphere by argon for the HVPE growth of GaN leads to a decrease in the epilayer surface roughness. The ratio of intensities of the donor-acceptor and exciton bands in the luminescence spectrum decreases with decreasing growth temperature. For the best samples of GaN epilayers, the halfwidth (FWHM) of the X-ray rocking curve for the (0002) reflection was 420 sec of arc, and the FWHM of the band of exciton emission at 77 K was 48 meV.     

Photoluminescence of CdHgTe epilayers grown on silicon substrates

We have studied the photoluminescence (PL) spectra of thin layers of mercury cadmium telluride (CdHgTe) solid solutions grown by molecular beam epitaxy on silicon substrates. It is established that a disorder in the solid solution structure in these layers does not exceed that in the layers grown by the same method on GaAs substrates. The PL spectra of CdHgTe/Si samples exhibit emission lines characteristic of the structurally perfect material, in particular, the lines due to donor-acceptor recombination and the recombination of excitons bound to impurities.     

Structural characterization of through silicon vias

Different milling strategies for the structural characterization of through silicon vias on silicon wafers and in stacked dies are examined. For investigation of the filling quality, the most appropriate analysis technique is dual beam focused ion beam and scanning electron microscopy. The characterization of thin barriers and Cu seed layers requires transmission electron microscopy. An optimized TEM specimen preparation method is discussed.     

Effect of carrier concentration on the microhardness of GaN layers

The paper presents a microhardness study of thick crack-free hydride vapor phase epitaxial GaN layers (not intentionally doped), and of thin metal-organic vapor phase epitaxial (MOVPE) GaN layers (undoped and Si-doped), grown on sapphire. A Vickers indentation method was used to determine the microhardness under applied loads up to 2 N. An increase in the microhardness was observed with decreasing carrier concentration and increasing mobility. A dip at an indentation depth of about 0.75 μm is observed in the microhardness profile in the MOVPE films, and is correlated with peculiarities in the spatially resolved cathodoluminescence spectra. The relationship between the mechanical and electrophysical parameters is discussed.     

Surface characterization of GaN and AlGaN layers grown by MOVPE

Surface properties of GaN and Al_0.17Ga_0.83N materials grown by metal organic vapor phase epitaxy (MOVPE) were systematically investigated by X-ray photoelectron spectroscopy (XPS). Air-exposed samples showed highly non-stoichiometric surfaces, which included a large amount of natural oxides. Deposition of Al on the air-exposed GaN surface caused interfacial reactions, resulting in the formation of oxide layers including Al₂O₃ and Ga oxide at the interface. A natural oxide layer of AlGaN surface possessed a complicated composition distribution in depth where the Al-oxide component was dominant on the topmost layer. Such natural oxide layers were found to be removed from GaN and AlGaN surfaces after the treatment in an NH₄OH solution at 50°C for 10 min, resulting in oxide-free and well-ordered surfaces.     

Comprehensive characterization of hydride VPE grown GaN layers and templates

GaN community has recently recognized that it is imperative that the extended, and point defects in GaN and related materials, and the mechanisms for their formation are understood. This is a first and an important step, which must be followed by defect reduction before full implementation of this material and its allied binaries/ternaries in devices. This review is based on a recent concerted effort to establish benchmarks as far as defects are concerned, and identify the basic issues involved. Samples were analyzed for extended defects by TEM and chemical etches, for polarity by electric force microscopy and convergent beam electron diffraction (CBED), for point defects by DLTS, for optical quality and deep defects by photoluminescence (PL), for vacancies by positron annihilation, for donor and acceptor like states within the gap by ODMR and EPR, and for carrier transport targeted for defects and impurities by variable temperature and magnetic field-dependent Hall measurements.Hydride VPE samples grown at Lincoln Laboratories with 1.5, 5.5 and 55 μm thicknesses were investigated for defects by TEM, and their polarity was found to be Ga-polarity, as expected, by CBDE combined with simulations. The density of misfit dislocations at the substrate/EPI interface was determined to be on the order of 10¹³ cm⁻² based on high-resolution electron microscopy images. The threading dislocation density decreased gradually with distance from the interface, reaching a value of about 10⁸ cm⁻² at the surface of a 55 μm film. A 200 μm thick laser separated and free-standing HVPE grown GaN template grown at Samsung was also characterized similarly. The free surface and substrate sides were confirmed to be Ga- and N-polarity, respectively. The density of dislocations near the N-face was determined to be, in order, (3±1)×10⁷ and (4±1)×10⁷ by cross-sectional TEM and plan-view TEM, respectively. Identical observations on the Ga-face revealed the defect concentration to be less than 1×10⁷ cm⁻² by plan-view TEM and 5×10⁵ cm⁻² by cross-sectional TEM.Defects in a 10 μm thick GaN layer grown by HVPE at Lincoln Laboratory have been investigated by photo electrochemical (PEC) etching, and by wet etching in hot H₃PO₄ acid and molten KOH. Threading vertical wires (i.e. whiskers) and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic force microscopy, one finds the density of “whisker-like” features to be 2×10⁹ cm⁻², the same value found for the etch-pit density on samples etched with both H₃PO₄ and molten KOH. Values agree well with TEM results.A free standing GaN template has been characterized for its structural and optical properties using X-ray diffraction, defect delineation etch followed by imaging with atomic force microscopy (AFM). The Ga-face and the N-face of the c-plane GaN exhibited a wide variation in terms of the defect density. The defect concentrations on Ga- and N-faces were about 5×10⁵ cm⁻² for the former and about 1×10⁷ cm⁻² for the latter, again in good agreement with TEM results mentioned above.High resolution X-ray rocking curves (omega scans) were measured. The [0 0 2] symmetric and [1 0 4] asymmetric peaks in 10 μm thick HVPE films had FWHM values between 5.8 and 7.9 arcmin, and 3.9 and 5.2 arcmin, respectively. The Samsung template investigated had wide diffraction peaks (20.6 and 24 arcmin for [0 0 2] and [1 0 4] diffractions, respectively) on the Ga-face, similar for the N-face, when a 2 mm slit size was used. When the slit size was reduced to 0.02 mm, the Ga- and N-face [0 0 2] peaks reduced to 69 and 160 arcsec. A bowing radius of 1.2 m was calculated to account for increased broadening with wider slits.In the HVPE layer studied, SIMS investigations indicate that both O and Si concentrations drop rapidly away from the surface into the sample — mainly due in part to the artifact of the technique and in part due to condensates on the surface of the sample, down to about 10¹⁷ cm⁻³ for Si and high 10¹⁶ cm⁻³ for O. The Ga-face profile in the Samsung template indicated levels below mid-10¹⁶ cm⁻³ for all three of the impurities. The picture is different for the N-side, however, as it was juxtaposed to the substrate during growth and was mechanically polished after laser separation. The impurity concentration on this face, depending on the species, is some 1–3 orders of magnitude higher than the case for the Ga-face.Transport properties as a function of the layer thickness, ranging from about 1 to 68 μm, while all the other parameters being the same, as they evolve from the sapphire/GaN interface and up were determined in epitaxial layers. A strong dependence on distance from the interface was observed with the averaged mobility figures increasing from 190 cm²/V s in the 5 μm film to 740 cm²/V s in the 68 μm film. The preliminary differential Hall measurements indicate that the mobility at the surface of the thick layer is about 1200 cm²/V s. Electron mobilities in free-standing template were 1425 cm²/V s at T=273 K and 7385 cm²/V s at T=48.2 K. By using the most recent unscreened acoustic deformation potential and allowing only the acceptor concentration to vary (2.4×10¹⁵ cm⁻³ for the best fit), one obtains an excellent fit to the measured mobility in the temperature range of 30–273 K by using an iterative BTE method. In addition, an excellent fit for the temperature-dependent electron concentration was also obtained utilizing the acceptor concentration determined from the fit to the Hall data, and the charge balance equation. This led to a donor concentration of 1.6×10¹⁶ cm⁻³, and activation energy of 26 meV, the latter being the highest reported in the literature for GaN.In the free-standing template, the Γ₅ and Γ₆ free excitons were identified from emission measurements by utilizing polarization geometries where the E field is perpendicular to the c-axis, favoring the Γ₅ exciton, and E field parallel to the c-axis (incident beam from the edge of the wafer) favoring the Γ₆ exciton. Focusing on the defect region of the PL spectrum, the N-face of the sample exhibited the usual yellow line. However, the Ga-face exhibited a broad band encompassing both yellow and green bands. The yellow luminescence in the free-standing template is weak and can be easily saturated. In contrast, the green luminescence is dominant and is attributed to the isolated defect involving gallium vacancy, whereas the yellow luminescence is related to the same defect bound to dislocation or surface-bound structural defect.Deep centers have been characterized by DLTS in HVPE-grown GaN epilayers of different thickness and dislocation densities, and templates. The main deep centers, such as A₁, B, and D, show higher concentrations in thinner samples, which suggests a correlation to the high dislocation densities. Based on the anti-correlation between A₁ and B, which is observed in thin HVPE-GaN layers, the defect B was tentatively attributed to N_Ga. Centers A₁ and E₁ found in thin HVPE-GaN are very similar to centers A₂ and E induced by electron-irradiation, indicating their point-defect nature. Centers A, C, and D are not affected by 1 MeV electron-irradiation, thus ruling out the possibility of these centers being identical to any EI-induced centers; however, their nature remains unknown. As the layer thickness decreases, an increase of deep centers, both in species and concentrations, was clearly observed, which is believed to be closely associated with the significant increase of threading dislocations as revealed by TEM. Based on a comparison with EI-induced centers and an observation of anti-correlation, A₁ is tentatively assigned to N_I, and B to N_Ga. The template exhibited a new trap B′, with parameters E_T=0.53 eV and σ_T=1.5×10⁻¹⁵ cm² on the Ga-face, in addition to the four traps commonly observed in various epitaxial GaN layers. For the N-face, an N vacancy-related trap E₁, with E_T=0.18 eV and σ_T=4×10⁻¹⁷ cm², was observed. On the other hand, the Ga-face sample contained trap C, with E_T=0.35 eV and σ_T=1.6×10⁻¹⁵ cm². This trap may be related to surface damage caused by the RIE process employed.Electron beam and optical depth-profiling of thick (5.5–68 μm) n-type HVPE-GaN samples have been carried out using electron beam-induced current (EBIC) and micro-PL to determine the minority carrier diffusion length, L, and minority carrier lifetime. The minority carrier diffusion length increased linearly from 0.25 μm, at a distance of about 5 μm from the GaN/sapphire interface, to 0.63 μm at the GaN surface for a 36 μm thick sample. The increase in L was accompanied by a corresponding increase in PL band-to-band radiative transition intensity as a function of distance from the GaN/sapphire interface. These observations in PL intensity and minority carrier diffusion length have been attributed to a reduced carrier mobility and lifetime at the interface and to scattering at threading dislocations.Positron annihilation experiments have been conducted in HVPE films with varying thicknesses from 1 to 68 μm. Mg-doped samples and bulk GaN platelets have also been investigated and the behavior of positron annihilation in Mg-doped samples established. Unlike the Mg-doped samples, the positron lifetime in the HVPE samples increased with decreasing lattice temperature. This was interpreted as acceptors in these n-type samples being due to Ga vacancies as opposed to relatively shallow acceptor impurities. The similarities in the behavior of these samples and those investigated previously where the III/V ratio was changed also lend support to the Ga vacancy argument. Previous investigations established that as the III/V is lowered by increasing the ammonia flow during the growth, the Ga vacancy concentration increased. Using Mg-doped samples as a standard, the vacancy concentration was determined to be about 10¹⁷ cm⁻³ near the surface for the layer with a thickness greater than 30 μm. Assuming that the growth parameters in the set of layers with varying thicknesses that were investigated are the same, the Ga vacancy concentration increases to mid-10¹⁹ cm⁻³ near the interface. Since the interfacial region is n-type and highly conductive, this region must also contain even larger concentrations of O and/or N vacancies which lead to n-type material. SIMS results already indicate mid-10¹⁹ cm⁻³ levels of O being present in this region. This has been attributed to O out-diffusion from sapphire as previously reported.FTIR, ODMR and EPR measurements have been performed in GaN layers and templates. In FTIR measurements, two absorption bands corresponding to binding energies of 30.9 (Si) and 33.9 meV were found. Splitting of the binding energies with magnetic field is consistent with an effective mass of 0.22m₀. Angular rotation studies were performed with the magnetic field oriented perpendicular and parallel to the c-axis to provide symmetry information. The ODMR on the 2.2 eV peak in a 5–10 μm thick GaN layer, the notorious yellow emission, showed signatures of shallow donor (effective mass like) and deep defect centers with g-values of 1.95 and 1.99, respectively. The 3.27 eV peak with resolved LO phonon replicas, which is the blue peak observed in many GaN films grown by a variety of methods, is attributed to transitions involving shallow acceptors with g2.1 and g2.0. ODMR on the 2.4 eV “green” PL band in the free-standing template also revealed evidence for shallow donors with a g-value of 1.95 and other deeper centers. The larger line width of the shallow donor signal from the template, relative to that found for the epitaxial layers, is indicative of a lower concentration of this center, which leads to an increased hyperfine interaction. EPR studies confirmed the notable difference between the epilayers and the template, particularly the larger line widths in the template due to the lower concentration of shallow donors. Specifically, the free-standing sample has about 6×10¹⁵ cm⁻³ uncompensated donors while the epilayers have a concentration about a factor of four higher.Calculations indicate that incorporation of Si has a negligible effect on the lattice constant, but O and Mg can lead to an observable expansion of the lattice. Since values of the GaN lattice constant have often been based on bulk crystals that are now known to contain large concentrations of oxygen, the “true” GaN lattice constant is actually smaller than what has been measured for such crystals. Boron is an unintentional impurity that can be introduced during MBE growth. There has been speculation about whether B might act as an acceptor in GaN; this would require it to be incorporated on the nitrogen site. Computations indicate that incorporating B on the N site is energetically unfavorable. Even if it did incorporate there, it would act as a deep, rather than a shallow acceptor. Formation energies of H in AlN and GaN have also been calculated. The behavior of H in AlN is very similar to GaN: H⁺ dominates in p-type, H⁻ in n-type. Surprisingly, H in InN behaves exclusively as a donor, i.e. it is not amphoteric as in GaN and AlN, but actually contributes to the n-type conductivity of the material.Scanning thermal microscopy (SThM) has been applied to measure the local thermal conductivity of epitaxial GaN as it is affected to a large extent by phonon scattering, and a closer to the true value of this parameter can be obtained by a local measurement in areas of lower defect concentration such as those found in the wing regions of lateral epitaxially grown GaN. The method relies on a thermo-resistive tip forming one quadrant of a Winston bridge. The bridge is balanced with the tip heated followed by bringing the tip in contact with the sample under test which cools down due to thermal dissipation. However, the feedback circuit attempts to keep the thermo-resistance and thus the tip temperature the same. The square of the feedback voltage necessary for this is proportional to the thermal conductivity. Accurate values can be obtained with calibration using known substrates such as GaSb, GaAs, InP, Si and Al metal. Using SThM, thermal conductivity, κ, values of 2.0–2.1 W/cm K in the wing regions of lateral epitaxially grown GaN, 1.70–1.75 W/cm K in HVPE grown GaN, and 3.0–3.3 W/cm K for free-standing AlN have been measured.     

Bulk GaN layers grown on oxidized silicon by vapor-phase epitaxy in a hydride-chloride system

Bulk GaN layers with a thickness of up to 1.5 mm and a lateral size of 50 mm were grown by hydride-chloride vapor-phase epitaxy (HVPE). The best samples show a half-width (FWHM) of the X-ray rocking curve of ω_θ=27′, a charge carrier density of ∼8 × 10¹⁹ cm⁻³, and a mobility of ∼50 cm²/(V s). The photoluminescence spectra of the obtained epitaxial GaN layers exhibit an edge emission band at 348 eV. The HVPE layers are characterized by a high mechanical strength: H _V = 14 GPa.     

Influence of double buffer layers on properties of Ga-polarity GaN films grown by rf-plasma assisted molecular-beam epitaxy

Ga-polarity GaN thin films were grown on sapphire (0001) substrates by rf-plasma assisted molecular beam epitaxy (MBE) using a double buffer layer, which consisted of an intermediate-temperature GaN buffer layer (ITBL) grown at 690 °C and a conventional AlN buffer layer deposited at 740 °C. Raman scattering spectra showed that the E₂ (high) mode of GaN film grown on conventional AlN buffer layer is at about 570 cm⁻¹, and shifts to 568 cm⁻¹ when an ITBL was used. This indicates that the ITBL leads to the relaxation of residual strain in GaN film caused by mismatches in the lattice constants and coefficients of thermal expansion between the GaN epilayer and the sapphire substrate. Compared to the GaN film grown on the conventional AlN buffer layer, the GaN film grown on an ITBL shows higher Hall mobility and substantial reduction in the flicker noise levels with a Hooge parameter of 3.87×10⁻⁴, which is believed to be, to date, the lowest reported for GaN material. These results imply that the quality of Ga-polarity GaN films grown by MBE can be significantly improved by using an ITBL in addition to the conventional low-temperature AlN buffer layer.     

Plasma-assisted molecular beam epitaxy of ZnO on in-situ grown GaN/4H-SiC buffer layers

Plasma-assisted molecular beam epitaxy (MBE) was used to grow ZnO(0001) layers on GaN(0001)/4H-SiC buffer layers deposited in the same growth chamber equipped with both N- and O-plasma sources. The GaN buffer layers were grown immediately before initiating the growth of ZnO. Using a substrate temperature of 440°C–445°C and an O₂ flow rate of 2.0–2.5 sccm, we obtained ZnO layers with smooth surfaces having a root-mean-square roughness of 0.3 nm and a peak-to-valley distance of 3 nm shown by AFM. The FWHM for X-ray rocking curves recorded across the ZnO(0002) and ZnO(101ˉ5) reflections were 200 and 950 arcsec, respectively. These values showed that the mosaicity (tilt and twist) of the ZnO film was comparable to corresponding values of the underlying GaN buffer. It was found that a substrate temperature >450°C and a high Zn-flux always resulted in a rough ZnO surface morphology. Reciprocal space maps showed that the in-plane relaxation of the GaN and ZnO layers was 82.3% and 73.0%, respectively and the relaxation occurred abruptly during the growth. Room-temperature Hall-effect measurements showed that the layers were intrinsically n-type with an electron concentration of 10¹⁹ cm^–3 and a Hall mobility of 50 cm²·V^–1·s^–1.     

Comparison of luminescence and physical morphologies of GaN epilayers

In this paper we examine a series of four GaN epilayers grown by MOVPE on sapphire substrates with different AlN buffer layer thicknesses. We examine the effect of the buffer layer thickness on the physical and optical properties of the samples via optical microscopy, cathodoluminescence imaging, photoluminescence, and cathodoluminescence spectroscopy. While the morphological and optical properties of all the films (excepting that with the thinnest buffer layer of 30 nm) are good, i.e. the films are smooth and the luminescence is dominated by excitonic luminescence, a number of circular island like features are observed in all the films whose density decrease with increasing buffer layer thickness. A large circular island present on the sample with the thinnest buffer layer and surrounded by cracks in the 11

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0 directions, displays some interesting acceptor related luminescence.     

图案化厚膜多孔硅制备与表征

图案化多孔硅是微电子、微机械、光电子器件的重要组成部分.实验以含Si3N4保护层的光刻单晶硅片为基底,采用电化学阳极氧化法制备图案化厚膜多孔硅,分析阳极氧化前后Si3N4保护层表面形貌变化特征和光刻尺寸对图案化多孔硅宽度、膜层厚度的影响规律,表征图案化多孔硅的结构、组成与发光性能.结果表明,氧化前Si3N4保护层局部区域出现枝晶,阳极氧化后形成不均匀孔状结构;制备的图案化多孔硅膜厚62～83μm,其横向扩展程度和膜层厚度均随光刻尺寸增大呈减小趋势;图案化多孔硅微结构含大量不规则裂纹和硅柱,新鲜制备的表面含Si-Hx键,其光致发光峰值波长650nm.     

低温缓冲层对金刚石衬底上GaN的沉积作用

采用电子回旋共振等离子体增强金属有机物化学气相沉积技术,以自支撑金刚石厚膜作为衬底,改变缓冲层参数条件,低温沉积氮化镓(Ga N)薄膜材料。实验结束之后,利用反射高能电子衍射、X射线衍射和原子力显微镜系统性对实验制备的薄膜样品进行测试分析,探究引入低温缓冲层与无缓冲层以及改变缓冲层沉积温度对Ga N薄膜质量的影响。结果表明,低温缓冲层的制备,对后续的薄膜样品沉积制备起到减小晶格失配的作用,而且低温缓冲层沉积温度在100℃时,沉积制备的薄膜样品呈高度c轴择优取向,结晶性较好;薄膜表面平整。     

Structural and magnetic characterization of Ni-filled porous silicon

During an anodization process, porous silicon (PS) consisting of pores with a diameter of about 60 nm and a depth from 10 μm to 30 μm has been produced. To achieve oriented channels in this mesoporous range, an n-type Si wafer with a doping density of 10¹⁹ cm^− 3 was electrochemically etched in an aqueous electrolyte of 10% HF. The essential parameters like the current density and bath temperature are variable only in a narrow range to influence the result with respect to sample characteristics like pore diameter, interpore spacing and degree of orientation. In a further electrochemical step, this array of quasi-regular self-assembled channels grown along the (100)-axis is filled with Ni. Pulsed electrodeposition of metallic Ni from a NiCl₂ solution offers a quite good possibility to fill the channels with Ni. The characterization of the structure was performed by various methods like AES [data presented in P. Granitzer, K. Rumpf, S. Surnev, H. Krenn. JMMM 290–291 (2005) 735], EDXS, and SEM to demonstrate the filling of the pores, to show the chemical element distribution within the channels, and to figure out the nature and regularity of the self-assembly of the pores. Additional experiments were carried out by SQUID-magnetometry to receive information about the magnetic properties. In summary, all investigations refer to a nanoscopic ferromagnetic system consisting of particles as well as wires which results in a peculiar magnetization behaviour with a second high field switching prior to saturation.     

Structural properties of relaxed Ge buffer layers on Si(0 0 1): effect of layer thickness and low temperature Si initial buffer

We have used the strain sensitive tool two-dimensional reciprocal space mapping (2D-RSM) and high resolution rocking curves (HR-RC) to assess the effect of the layer thickness and the influence of low temperature Si buffer on the properties of fully relaxed Ge on Si (0 0 1). The samples were grown by chemical vapor deposition in an ASM commercial reactor. As complementary measurements we have employed secondary ion mass spectrometry (SIMS) for chemical analysis, cross sectional transmission electron microscopy for quality assessment, and finally atomic force microscopy (AFM) for investigating the surface roughness. The investigated samples have a thickness ranging from 0.25 to 5.0 m. In addition and for a 5.0 m thick Ge layer, an initial low temperature Si (LT-Si) template was grown before the Ge epitaxy. The results indicate that high quality fully relaxed Ge layers have been achieved using the adopted growth procedure. Most of the improvement in crystalline quality was observed for Ge layers with thickness up to 1.5 m. Above this thickness the observed crystalline quality improvement was negligible. The LT-Si buffer observed to be disadvantageous for pure relaxed Ge growth.     

Structural perfection of laterally overgrown GaN layers grown in polar- and non-polar directions

The structural quality of GaN overgrown layers was evaluated using Transmission Electron Microscopy methods. Growth on polar and non-polar substrates was compared. Independent from growth polarity much better structural quality of the overgrown areas compared to the seed areas was obtained, but overgrowth on non-polar substrates is more difficult. For the latest samples, two wings on the opposite sites of the seed area grow in two different polar directions with different growth rates. Wings grown with Ga polarity are much wider than wings grown with N-polarity making coalescence of these layers difficult. Defects formed in the overgrown wings were characterized and their density was compared. It is shown that two-step growth (using two different temperatures) lead to much smaller misorientation between the wings than one step growth