Crystalline Quality Of Epilayers Research Articles

Characterization of nonpolar a-plane GaN epi-layers grown on high-density patterned r-plane sapphire substrates

To reduce the number of threading dislocations (TDs) in nonpolar a-plane GaN (a-GaN) epi-layers grown on flat r-plane sapphire substrates (r-FSS), we investigated the effects on the crystalline quality of the a-GaN epi-layers of high-density patterned r-plane sapphire substrates (r-HPSS), the patterns of which were placed at intervals of several hundred nanometers. Two types of r-HPSS, the patterns of which had diameters and heights on the order of several hundred nanometers (r-NHPSS) or several micrometers (r-MHPSS), were prepared with conventional r-FSS. The effect of these r-HPSS on the a-GaN epi-layers was demonstrated by evaluating the surface morphology and the crystalline quality of the epi-layers. The surfaces of the a-GaN epi-layer grown on r-FSS and r-NHPSS were pit-free and mirror-like, whereas the surface of the a-GaN epi-layer grown on r-MHPSS was very rough due to the large, irregular GaN islands that grew on the patterns, mainly at the initial growth stage. The crystalline quality of the a-GaN epi-layer grown on r-NHPSS was better than that of the a-GaN epi-layer grown on r-FSS. We confirmed that there were fewer TDs in the a-GaN epi-layer grown on r-NHPSS than there were in the a-GaN epi-layer grown on r-FSS. The TDs propagating to the surface in a-GaN epi-layer grown on r-NHPSS were mainly generated on the flat sapphire regions between the patterns. Interestingly, it was also found that the TDs that propagated to the surface concentrated with a periodic pitch along the c-axis direction. The TD densities of a-GaN epi-layers grown on r-FSS and r-NHPSS were estimated to be approximately 5.0 × 1010 and 1.5 × 109 cm−2, respectively. This knowledge will contribute to the further development of a-GaN epi-layers for high-performance devices.

Enhanced output power of near-ultraviolet LEDs with AlGaN/GaN distributed Bragg reflectors on 6H–SiC by metal-organic chemical vapor deposition

Near-ultraviolet (UV) InGaN/AlGaN multiple quantum well (MQW) LEDs with 30 pairs AlGaN/GaN distributed Bragg reflectors (DBRs) were grown on 6H–SiC substrate by metal-organic chemical vapor deposition. A thin SiNx interlayer was introduced between the DBRs and n-GaN layer of the LED to reduce the threading dislocation density and result in enhancement the internal quantum efficiency (ηint) of the InGaN/AlGaN LED. The result indicates that the light output power for the LED with DBRs and SiNx interlayer was approximately 56% higher (at 350mA) than the LED without DBRs and SiNx interlayer on 6H–SiC substrate, and this significant improvement in performance is attributed not only to the light extraction enhancement via the DBRs but also due to improve epilayer crystalline quality.

Properties of GaN layers grown on N-face free-standing GaN substrates

GaN layers were homoepitaxially grown on N-face free-standing GaN substrates using a hot-wall metalorganic chemical vapor deposition method. By using optimized growth parameters, layers with a smooth morphology were obtained. The crystalline quality of epilayers was studied by a high resolution X-ray diffraction technique and compared to the substrates. Optical properties of the epilayers studied by low temperature time-resolved photoluminescence have shown longer recombination time for donor-bound exciton compared to the substrates.

Ge growth on porous silicon: The effect of buffer porosity on the epilayer crystalline quality

We report on the epitaxial growth of Ge virtual substrates directly on Si (001) and on different porosity porous silicon (pSi) buffers. Obtained results indicate that Ge grown on low porosity (22%) pSi buffer has a better crystalline quality compared to Ge grown on bulk Si and on higher porosity buffers. This result is attributed to the compliant nature of pSi and to its reduced Young's modulus, which leads to plastic tensile deformation of the 22% porosity buffer under the in-plane tensile stress introduced by Ge lattice. The same result is not observed for higher porosity buffers, this effect being attributed to the higher buffer fragility. A low porosity pSi layer can hence be used as buffer for the growth of Ge on Si virtual substrates with reduced dislocation content and for the growth of Ge based devices or the successive integration of III-V semiconductors on Si.

Effects of AlN nucleation layer thickness on crystal quality of AlN grown by plasma-assisted molecular beam epitaxy

In this paper, the effects of thickness of AlN nucleation layer grown at high temperature on AlN epi-layer crystalline quality are investigated. Crack-free AlN samples with various nucleation thicknesses are grown on sapphire substrates by plasma-assisted molecular beam epitaxy. The AlN crystalline quality is analysed by transmission electron microscope and x-ray diffraction (XRD) rocking curves in both (002) and (102) planes. The surface profiles of nucleation layer with different thicknesses after in-situ annealing are also analysed by atomic force microscope. A critical nucleation thickness for realising high quality AlN films is found. When the nucleation thickness is above a certain value, the (102) XRD full width at half maximum (FWHM) of AlN bulk increases with nucleation thickness increasing, whereas the (002) XRD FWHM shows an opposite trend. These phenomena can be attributed to the characteristics of nucleation islands and the evolution of crystal grains during AlN main layer growth.

Recent advances in the MOVPE growth of indium nitride

AbstractSince a few years, indium nitride promising properties for device applications have attracted much attention worldwide. Huge efforts are dedicated to optimize indium nitride growth. However, this growth is extremely challenging, in particular using the metal organic vapor phase epitaxy (MOVPE) technique which exhibits very low growth rates. This may explain why most of the samples available in the scientific community, which also present the best electrical properties, were grown by molecular beam epitaxy (MBE). However, up to date, no intrinsic indium nitride layer was obtained. InN epilayer crystalline quality suffers from the lack of lattice matched substrates, leading to the use of double‐step growth process. In this paper, we show that InN low lateral growth rate is a limiting parameter for efficient double‐step growth process. But, we also report that the use of CBrCl3 during InN growth enhances the lateral growth rate. To improve the growth rate along the c‐axis, we investigate alternative precursors for both nitrogen and indium species. We show that ammonia remains the best precursor, but combined with triethylindium, the growth rate increases with optimum crystalline quality. Finally, we compare MOVPE‐grown and MBE‐grown InN layers in order to understand the difference observed on the electrical properties. We show how thermal annealing can improve the MOVPE‐grown InN layers leading to similar electrical properties than reported in MBE‐grown samples. The role of ammonia as source of hydrogen is also discussed.

Improvement of crystalline quality of InGaN epilayers on various crystal planes of ZnO substrate by metal‐organic vapor phase epitaxy

AbstractWe demonstrated InxGa1‐xN epitaxial growth with InN mole fractions of x = 0.10 to 0.23 on an m‐plane ZnO substrate by metal‐organic vapor phase epitaxy. The crystalline quality of the epilayers was found to be much higher than that of epilayers grown on a GaN template on an m‐plane SiC substrate. The strain of the epilayers was investigated by X‐ray reciprocal space mapping. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Schottky I-V Characteristics of Au/Ni/GaN/SiNx nanonework/sapphire structures

ABSTRACTGaN layers on sapphire substrates were grown by metalorganic chemical vapor deposition using in situ porous SiNx nano-network. Crystalline quality of epilayers was characterized by X-ray rocking curve scans, and the full width at hall maximum values for (002) and (102) diffractions were improved from 252 arc sec and 405 arc sec, respectively, in control samples to 216 arc sec and 196 arc sec when SiNx was used. Ni/Au Schottky diodes (SDs) were fabricated and the SD performance was found to be critically dependent on the SiNx coverage (fewer and farther the pores the better the results) which is consistent with the trends of XRD and photoluminescence data. A 1.13eV barrier height was achieved when 5min SiNx layer was used compared with 0.78 eV without any SiNx nanonetwork. Furthermore, the breakdown voltage improved from 76 V to 250V when SiNx nanonetwork was used in otherwise identical structures.

Homoepitaxy of ZnTe on (100) oriented substrates: Technology issues and MOVPE growth aspects

AbstractThe metalorganic vapour phase epitaxy of ZnTe on single crystal (100)ZnTe:P wafers is reported. The technological steps to prepare a substrate surface suitable for the high quality homoepitaxy of ZnTe are identified and optimised in terms of structural and morphological properties of overgrown epilayers. Removal of ∼7 µm of material from the ZnTe:P wafers by chemical etching in 1% Br2‐methanol solution proved necessary to achieve a sufficiently smooth and homogeneous surface; in‐situ H2 heat treatment of the wafers at 350°C immediately before growth ensures optimal desorption of residual oxides, allowing epilayer crystalline quality comparable to the substrate. However, the structure of epilayers degrades for growth temperatures (TG) above 350 °C due to the occurrence of stacking faults (SFs) within ∼200‐300 nm from the epilayer‐substrate interface. Accordingly, the epilayer band‐edge luminescence vanishes below 350 nm, indicating a worsening of the material radiative efficiency in very thin epilayers. The epilayer surface morphology is the result of a complex interplay between SF nucleation and Te:Zn ad‐atom stoichiometry during growth. Almost featureless morphologies are obtained for growth at 350 °C, i.e. under nearly stoichiometric surface conditions. Pyramid‐like hillocks develop instead for TG ≥ 360 °C, corresponding to Te‐rich surface conditions, their density rapidly increasing up to around 9 × 106 cm–2 at TG = 400 °C. Hillocks occur in close pairs on the epilayer surface, their nucleation being strongly reduced if a thin ZnTe buffer layer is grown at low (325 °C) temperature, i.e. if SFs do not occur at the epilayer‐substrate interface. This demonstrates that hillocks form as a result of three‐dimensional growth around partial dislocations pairs bounding SFs, the phenomenon being driven by Te ad‐atoms experiencing a Schwoebel potential barrier at the surface step edges around the dislocations. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)