The nonpolar m-plane ͑1100͒ thin film GaN and InGaN / GaN light-emitting diodes ͑LEDs͒ grown by metal-organic chemical vapor deposition on LiAlO 2 ͑100͒ substrates are reported. The LEDs emit green light with output power of 80 W under a direct current of 20 mA for a 400 ϫ 400 m 2 device. The current versus voltage ͑I-V͒ characteristic of the diode shows soft rectifying properties caused by defects and impurities in the p-n junction. The electroluminescence peak wavelength dependence on injection current, for currents in excess of 20 mA, saturates at 515-516 nm. This proves the absence of polarization fields in the active region present in c-plane structures. The light output intensity versus current ͑L-I͒ characteristic of the diode exhibits a superlinear relation at low injection current caused by nonradiative centers providing a shunt path and a linear light emission zone at high current level when these centers are saturated. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2825419͔ III-nitride materials provide great practical benefits in producing short wavelength light-emitting diodes ͑LEDs͒. These devices are conventionally grown on c-plane sapphire or SiC substrates, which suffer from the quantum-confined Stark effect. 1 The quantum efficiency decreases due to the spatial separation of the electron and hole wave functions within the quantum wells, thereby reducing recombination probability of electron-hole pairs, especially when the emission wavelength exceeds 500 nm. This is caused by the electrostatic fields along the ͓0001͔ axis of hexagonal III nitrides due to the spontaneous and piezoelectric polarizations. In contrast, the growth direction of optoelectronic devices normal to the direction of the polarization ͑e.g., growth along the a and m directions͒ is one promising way to avoid these polarization effects. Recently, significant progress has been made in the fabrication of nonpolar LEDs on r-plane sapphire 2 or on m-plane and a-plane freestanding GaN substrates. 3,4 The ␥-LiAlO 2 ͑100͒ substrate is another candidate substrate for growing m-plane GaN films. 5,6 The m-plane InGaN / GaN multiple quantum wells ͑MQWs͒ have been synthesizing substrates by molecular-beam epitaxy, 7 and nonpolar LED structures have been grown by metalorganic chemical vapor deposition ͑MOCVD͒ on LiAlO 2 ͑100͒. 8,9 However, the device performance of m-plane LEDs has still not been reported yet. In this letter, we report on the MOCVD growth of m-plane GaN thin film and nonpolar m-plane InGaN / GaN LEDs on LiAlO 2 ͑100͒ substrates. The electrical and optical properties of these structures are mentioned.The In x Ga 1−x N / GaN LED structures, shown in Fig. 1, were grown on LiAlO 2 ͑100͒ using a Thomas Swan MOCVD system which has a close-coupled-showerhead reactor. Epigrade trimethylgallium, Trimethylindium, and high-purity ammonia were used as the source precursors for In, Ga, and N, respectively. Silane ͑SiH 4 ͒ and bis͑cyclopentadienyl͒mag-nesium were used for n-and p-type dopings. Only nitrogen ͑N 2 ͒ was used as car...

An integrated process to fabricate controllable arrays of semiconductor nanorings and nanodots on patterned surfaces is presented. This approach is based on pattern transfer of nanopores to a SiO2 layer, followed by selective epitaxial growth of InGaN onto an underlying GaN substrate using metalorganic chemical vapor deposition. Using this approach, crystalline InGaN nanorings and nanodots ∼80nm in diameter have been grown on GaN surfaces. The formation mechanism of the nanorings and nanodots is described based on the initial stage of selective growth and restricted atom migration in a confined hole. Strong photoluminescence obtained at room temperature from the noncapped nanorings indicates strong confinement of the excitons in the nanostructures. This approach enables fabrication of dense, uniform arrays of epitaxial nanostructures and is potentially applicable to a variety of materials systems.

The m-plane GaN films grown on LiAlO 2 ͑100͒ by metal-organic chemical vapor deposition exhibit anisotropic crystallographic properties. The Williamson-Hall plots point out they are due to the different tilts and lateral correlation lengths of mosaic blocks parallel and perpendicular to GaN͓0001͔ in the growth plane. The symmetric and asymmetric reciprocal space maps reveal the strain of m-plane GaN to be biaxial in-plane compress xx = −0.79% and zz = −0.14% with an out-of-plane dilatation yy = 0.38%. This anisotropic strain further separates the energy levels of top valence band at ⌫ point. The energy splitting as 37 meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2951618͔The wurtzite structure of III-nitrides leads to electrostatic fields along the ͓0001͔ direction due to spontaneous and piezoelectric polarization, when the film is grown on c-oriented substrates. 1 These built-in electric fields along the ͓0001͔ direction separate the electron and hole wave functions in a quantum well ͑QW͒ thereby reducing the recombination probability of electron-hole pairs. Consequently, it results in reduction of quantum efficiency of light-emitting diodes. 2 A way to overcome this deficiency is to grow GaNbased QWs along nonpolar orientations, for example, m plane 3,4 or a plane. 5,6 It has been shown that the electric field can be avoided in such nitride QWs. When ͓1100͔ or ͓1120͔ becomes the growth direction, the c axis of GaN lattice lies down in the growth plane. As a consequence, the C 6v hexagonal symmetry of the c growth plane is reduced to C 2v symmetry of the m growth plane. Firstly, it would show the anisotropic crystallographic characteristics. 7,8 Secondly, the nonpolar GaN films experience strong anisotropic deformation due to different in-plane lattice mismatches and thermalexpansion coefficients between GaN and underlying substrates. In the case of c-plane GaN films, isotropic strain in the c-plane preserves C 6v symmetry in the x-y plane so that no significant in-plane physical anisotropy occurs. The situation is quite different for nonpolar GaN films. Anisotropic in-plane strain components further lift the symmetry in the growth plane, which significantly modify the top three valence band ͑VB͒ states at ⌫ point. 9,10 Both the energy splitting and polarization selection of the transitions between conduction band ͑CB͒ and VB have been observed by absorption, reflectance, and photoreflectance spectroscopy. 11,12 Recently, transmission anisotropy spectroscopy has been utilized to obtain the energy splitting and polarization information of m-plane GaN films. 13 Although the in-plane strain components are important for the modifications in band structure of nonpolar GaN, it is still difficult to experimentally determine the full state of strain. In the case of m plane, GaN films are grown along GaN ͓1100͔ direction. The growth plane is the x-z plane, and the growth direc...

No abstract

Alumina nanotube (ANT) and nanowire (ANW) arrays were produced from highly ordered porous anodic alumina (PAA) by etching in phosphoric acid and subsequent thermal treatment. The domain structures that have small sizes provide more boundaries and break the hexagonal symmetry of the PAA template, thereby inducing star-like splitting modes. Using very thin PAA films, ANTs and their arrays are fabricated, whereas ANWs and their arrays are produced when using thick films. This is due to the thin wall between nanopores. Our results provide some mechanistic and fundamental information pertaining to the fabrication of nanostructures via a PAA template.