Optical properties of wurtzite GaN epilayers grown on A -plane sapphire

We investigated the polarization and temperature dependence of photoluminescence ͑PL͒ of m-plane GaN grown on ␥-LiAlO 2 ͑100͒ substrate. The calculated electronic band structure with k • p Hamiltonian points out the energy splitting as well as polarization selection originate from the m-plane GaN epilayer under anisotropic strain. The polarization-angle dependence PL spectra are found to be selected from in-plane x-and z-polarized emission, corresponding to T 1 and T 2 transition. And the intensity distribution of the fitting peaks satisfies the Malus' law. An S-shape energy evolution of near band edge peak on temperatures is observed, which originates from the transition between the localized holes and electrons in triangular potentials induced by basal stacking faults. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3204453͔ Spontaneous and piezoelectric polarization along the ͓0001͔ axis of wurtzite GaN and its alloys greatly affect the efficiency of luminescence in GaN-based optoelectronic devices. 1 In order to eliminate these polarization effects, one way is to deposit an m-or a-plane GaN film along unconventional non-polar directions, such as ͓1100͔ ͑Ref. 2-4͒ or ͓1120͔. 5-7 ␥-LiAlO 2 ͑LAO͒ has a tetragonal structure and its ͑100͒ plane exhibits a comparatively small lattice mismatch to GaN ͑1100͒. Since the m-plane GaN was grown by molecular-beam epitaxy on this substrate, 2 the development of nonpolar GaN based optoelectronic devices has attracted much attention. Unlike c-plane GaN, nonpolar GaN usually suffers from anisotropic in-plane strain, which leads to a splitting in the electronic band structure ͑EBS͒. Therefore, understanding the EBS of nonpolar GaN is significant for the application of optoelectronic devices. Previously, optical measurements, such as reflectance, transmission, and photoreflectance, were used to determine the EBS near the ⌫ point and verify the theoretical calculation of the energy splitting of the m-plane GaN. [8][9][10][11] Luminescence properties of a-plane GaN, including temperature and polarization dependent, spatially resolved photoluminescence ͑PL͒ and cathodoluminescence, were also particularly studied. [12][13][14] Recently, the authors reported the splitting and anisotropy of emission light polarization of the m-plane GaN on LAO͑100͒ by using PL measurements. 15 In this letter, we investigate the polarization-angle and temperature dependence of the polarized PL transitions of the m-plane GaN film on LAO͑100͒ in order to explore the evolution of splitting valence band under anisotropic strain conditions.The studied sample of a typical 0.92 m thick m-plane GaN with background electron concentration ϳ2 ϫ 10 18 cm −3 ͑named as sample I͒ was grown on LAO͑100͒ by metalorganic chemical vapor deposition. 4 High-resolution x-ray diffraction technique revealed the m-plane GaN epilayer under biaxial compressive strain of xx = −0.79% and zz = −0.14% with an out-of-plane dilatation yy = 0.38%. 15 An unintentionally doped n-type c-plane GaN with thickness of 2.5 m on ␣-Al 2 O...

supporting

confidence: 72%

mentioning

confidence: 99%

Polarization and temperature dependence of photoluminescence of m-plane GaN grown on γ-LiAlO2 (100) substrate

Liu

Kong

Zhang

et al. 2009

“…Under such circumstances, original ͉X ± iY͘ VB states are broken into ͉X͘-like and ͉Y͘-like ones. [19][20][21] Accordingly, VBs are reconstituted to ͉X͘-like, ͉Z͘-like, and ͉Y͘-like ones in order of decreasing electron energy. Therefore, the transition lowest in energy is allowed for E Ќ c, and the transition involving the ͉Z͘-like VB must occur at the higher energy under E ʈ c. Although the emissions from the MQW were polarized to E Ќ c, observable peak shift could not be found, as shown in Fig.…”

mentioning

confidence: 99%

Prospective emission efficiency and in-plane light polarization of nonpolar m-plane InxGa1−xN∕GaN blue light emitting diodes fabricated on freestanding GaN substrates

Koyama

Onuma

Masui

et al. 2006

Prospective equivalent internal quantum efficiency ͑ int ͒ of approximately 34% at 300 K was demonstrated for the blue emission peak of nonpolar m-plane ͑1100͒ In x Ga 1−x N / GaN multiple quantum well light emitting diodes ͑LEDs͒ fabricated on freestanding m-plane GaN substrates. Although the int value is yet lower than that of conventional c-plane blue LEDs ͑Ͼ70% ͒, the results encourage one to realize high performance green, amber, and red LEDs by reducing the concentration of nonradiative defects, according to the absence of the quantum-confined Stark effects due to the polarization fields parallel to the quantum well normal. The electric field component of the blue surface emission was polarized perpendicular to the c axis with the in-plane polarization ratio of 0.58 at 300 K.

“…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.…”

mentioning

confidence: 99%

Anisotropic crystallographic properties, strain, and their effects on band structure of m-plane GaN on LiAlO2(100)

Liu

Zhang

Xie

et al. 2008

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...