A new approach to realize ultraviolet (UV) light emitting diodes (LEDs) is using AlN/GaN or AlxGa1-xN/GaN SL structure as active layers. Effect of a uniaxial strain on the degree of polarization (DOP) of Al0.26Ga0.74N/GaN superlattices (SLs) grown on c-plane sapphire substrates has been investigated. Compared with AlN/AlxGa1-xN quantum wells, the DOP of the light emission from Al0.26Ga0.74N/GaN SLs shows an opposite variation tendency with in-plane strain and quantum confinement. The results would be helpful to the structural design of c-plane deep-UV and UVA LEDs to enhance surface emission.
© 2014 Optical Society of America
As a direct band-gap material, ternary nitride alloy AlxGa1-xN has a tunable band-gap energy from 3.4 eV to 6.2 eV when Al composition x varies from 0 to 1, corresponding to wavelength from 365 nm to 200 nm. It is a promising candidate for ultraviolet light emitting diodes (UV-LEDs) and laser diodes (LDs), which have potential applications in bioagent detection, sterilization, water purification, UV curing, spectroscopy, photolithography, and optical data storing . A direct approach to realize deep-ultraviolet (DUV) emission is to adjust Al composition x of AlxGa1-xN active layer to higher composition. However, for the DUV-LEDs grown on c-plane sapphire with high Al content, it is hard to emit from the surface of DUV-LED structure [2, 3]. The weak c-plane surface emission originates from anisotropy of light emission of AlGaN alloy active layer.
The anisotropy of emission light from high Al composition AlGaN originates from remarkable difference of the valence band structure between AlN and GaN. In wurtzite nitride semiconductors, owing to both crystal-field splitting and spin-orbit splitting, the degeneracy of the p-like states at the Γ point is lifted, resulting in three valence sub-bands at the Brillouin zone center. The crystal-field splitting energy is −217 meV in AlN [4–7], whereas it is 11 meV in GaN . The difference in crystal-field splitting energy leads to valence-band arrangements in the order of Γ7, Γ9, and Γ7 for AlN, and Γ9, Γ7, and Γ7 for GaN, respectively. The topmost Γ7 in AlN is the crystal-field split off hole (CH) band governed by pz-like state, and Γ9 in GaN is the heavy hole (HH) band governed by px-like and py-like states. Therefore, the edge emission of c-AlGaN is expected to be switched from E⊥c-polarized to E∥c-polarized with increasing Al content. In fact, such polarization state switching from E⊥c to E∥c has been reported for AlxGa1-xN thick layers . In order to study the anisotropy of light emission, research about optical polarization properties of AlGaN and AlN (or AlxGa1-xN)/GaN quantum wells (QWs) were carried out widely in both theory and experiment, especially via polarization dependent photoluminescence (PL) spectra [10–15].
Another approach to realize UV and DUV emission is using AlN/GaN or AlxGa1-xN/GaN SL structure as active layers. GaN well layer in AlxGa1-xN/GaN SLs will be beneficial to c-plane surface emission in DUV-LEDs because the emission from GaN is governed by px-like and py-like states of the HH band. In order to avoid unfavorable anisotropic emission pattern from high Al composition AlGaN, recently, AlN/GaN SLs were designed to be active layers for DUV-LEDs with high emission efficiency [16, 17]. Although the strain effect on polarization for AlGaN and AlN/AlGaN superlattice has been studied by many groups [3, 10–13], the direct measurement of degree of polarization (DOP) of light emission from GaN/AlxGa1-xN SLs as a function of the strain has not been reported yet.
In this letter, we experimentally studied the optical polarization properties of c-plane AlxGa1-xN/GaN SLs. This SL structure is a good candidate for the active layer of UV-LEDs in UVA region. The structural parameters are accurately determined by high resolution x-ray diffraction (HR-XRD). The strain effect on the DOP of AlxGa1-xN/GaN SLs was studied experimentally. Compared with AlN/AlxGa1-xN QWs, the Al0.26Ga0.74N/GaN SLs show an opposite variation tendency of polarization with strain and quantum confinement. The results should be helpful to the structural design of c-plane-based DUV or UVA-LEDs with high light extraction efficiency.
The c-plane AlxGa1-xN/GaN SLs were grown on c-plane GaN/sapphire template by metal organic chemical vapor deposition (MOCVD). Trimethylaluminum, trimethylgallium, and ammonia were used as Al, Ga, and N sources, respectively. Hydrogen was used as carrier gas. A 6-μm-thick GaN/sapphire was grown under the same condition as a contrasting sample. Structural characteristics of samples have been measured by HR-XRD including ω-2θ scan and reciprocal space mapping (RSM) measurements to evaluate the composition, lattice tilt, strain state of SLs.
Polarization dependent PL system includes a 325-nm He-Cd laser, a Glan-Taylor prism, and a spectrometer. A uniaxial strain device consists of a micrometric system which is able to bend the wafer to achieve both tensile and compressive strain. In order to bend the sample easily, the wafer was grinded to 100-μm thick. Then, it was scribed into rectangle (length-width ratio greater than 3) by excimer laser scribing and cleavage to get an intact cleaved m-plane. The edge emitting light of the sample was collected through an aperture and a Glan-Taylor prism. The light was then focused with lenses into the spectrometer. To minimize the error from misalignment of optical components, the integrated intensities of spectra, plotted against prism angle, were fitted with cosine functions to obtain intensities for the light with electric field vector parallel to the c direction IE∥c (TM mode) and perpendicular to the c direction IE⊥c (TE mode). The DOP was defined by P = (IE⊥c - IE∥c)/(IE⊥c + IE∥c). Rectangle sample’s coordinate diagram is as follows: X-axis and Y-axis are in the c-plane, with Z-axis perpendicular to c-plane. Fluorescence are collected by optical fiber along the Y-axis(perpendicular to m-plane) in Fig. 1.
3. Results and discussion
Figure 2(a) shows the HR-XRD ω-2θ scan results of the 30-period AlxGa1-xN/GaN SLs sample. The zero-order and higher-order satellite diffraction peaks are clearly distinguishable, indicating the good integrity and periodicity of SLs. The strain of the AlxGa1-xN/GaN SLs structure is determined by the RSM of diffraction from an asymmetric (105)-plane, as shown in Fig. 2(b). The observed GaN-template diffraction peaks, zero-order and higher-order satellite diffraction peaks share the same value for the parallel component Qx, demonstrating that all layers have the same in-plane lattice constant and the AlxGa1-xN/GaN SL structure is fully strained. The extracted composition and thickness of the constituents of the samples are Al0.26Ga0.74N(5.5 nm)/GaN(2.5 nm). Because the lattice parameter of GaN in well layer of SL is the same with that of GaN template, the GaN well layer in Al0.26Ga0.74N/GaN SLs is in unstrained state, while the barrier Al0.26Ga0.74N is in a tensile strain state. Therefore, in the measurement of DOP dependence on strain, only the external strain caused by bending needs to be considered.
The PL results of GaN and SLs samples are shown in Fig. 3. At room temperature, the Al0.26Ga0.74N/GaN SLs and GaN exhibit light emission at a wavelength of 347 nm and 365 nm, respectively. The integrated intensity distribution of edge emission from SLs and GaN against prism angle is also depicted in the inset of Fig. 3. The similar intensity distribution of SLs and GaN against prism angle indicates a close DOP and the same dominant TE mode emission. If the emission at 347 nm is realized by adjusting Al composition of AlxGa1-xN, the desirable Al composition is about x = 0.1. As shown later, the DOP of edge emission from SLs is slightly smaller than that from GaN and much bigger than that of Al0.1Ga0.9N (P ≈0.15) reported in Ref. 9. Obviously, as an active layer, c-plane AlxGa1-xN/GaN SLs structure will show higher DOP of light emission than that of AlxGa1-xN resulting in enhancement of surface emission.
The remarkable difference of DOP between our Al0.26Ga0.74N/GaN SLs and reported AlxGa1-xN at the same emission wavelength originates from the modification of valence band structure. However, The apparent advantage of using GaN as well layer is that wave functions in SL structure are much more localized in the GaN layer due to quantum confinement effect. Thus the optical transition originates from conduction band and the quantized HH level of GaN which is corresponding to a dominant TE emission. The first-principle calculations indicate that even the 224 nm emission wavelength can be realized by adjusting the GaN well layer thickness in AlN/GaN SLs to 1 monolayer . Even to the DUV region, the DOP of light emission from AlN/GaN SLs does not decrease obviously and the E⊥c emission is still dominant .
In fact, for a practical structure in the UV range, the active layer of AlxGa1-xN/GaN SLs is a better choice. Both the width of GaN well layer and the Al content of AlGaN barrier can be adjusted to actualize a certain wavelength. Compared with GaN/AlN superlattices, AlxGa1-xN/GaN SLs are able to reduce the series resistance in the banausic devices. In this letter, the Al0.26Ga0.74N is used as the barrier layer of active region of SLs to realize the UVA light emission. The shorter wavelength emission can be obtained by increasing the barrier height and decreasing the well width of SLs.
The DOP results of SLs and GaN as a function of the uniaxial strain εxx are shown in Fig. 4(a). The external strain can be achieved by a uniaxial strain device illustrated in Fig. 4(b). And the strain at center of the sample will be 
where L is the length of the sample, h is the thickness of the wafer, and J0 is the flexivity of the stripe. For the SLs sample, the tensile strain is favorable to increase DOP. In contrast, the compressive strain decreases DOP. Meanwhile, for the GaN sample, there is no obvious monotonical change dependent on the strain. The experimental DOP data can be linear fitted against the strain εxx. The best fit gives
It is obvious that the DOP of light emission from SLs is more sensitive to the strain than that from the bulk GaN.
There are two main factors which affect the DOP of SLs or QWs. One is the quantum confinement originating from a quantum structure of active layer, and the other is the strain in active region which affects valence bands arrangement of well layer material . For the AlN/AlxGa1-xN QWs, where the Al composition x is so high that the DOP of light emission from the QWs switches to negative value, the quantum confinement effect on DOP has been studied theoretically and experimentally [3, 14, 15]. As shown in region (1) of Fig. 5, for the AlN/AlxGa1-xN QWs, the favorable polarization for light extraction from DUV-LEDs can be realized by making use of quantum confinement by decreasing QW width and/or introducing in-plane compressive strain.
For the Al0.26Ga0.74N/GaN SLs in this work, our results show an opposite tendency of polarization dependence on quantum confinement and strain in the well layer. As shown in region (2) of Fig. 5, quantum confinement and the in-plane compressive strain will reduce the DOP of light emission. In other words, the tensile strain will benefit the c-plane surface emission.
In the Al0.26Ga0.74N/GaN SLs, due to the piezoelectric polarization effect, the energy band structure of GaN layers is triangular QWs as demonstrated by the PL measurement. A visible red shift of PL peak due to the quantum confinement Stark effect (QCSE) has been observed in our Al0.26Ga0.74N/GaN SLs sample when the strain switched from tensile strain to compressive strain, which means that the effective well width becomes thinner, leading to stronger quantum confinement. That is why the DOP of light emission from SLs is more sensitive to the strain than that of bulk GaN. So the strain in AlxGa1-xN/GaN SLs can affect the DOP of light emission in two ways: (1) the strain adjusts the valence bands arrangement; (2) the strain induces a piezoelectric polarization effect resulting in a thinner effective width of QW layer (stronger quantum confinement). These dual impacts lead to a sensitive change of DOP in the SLs under uniaxial strain.
As an active layer in GaN-based QW structure for LEDs, compressive strain is unavoidable. When SLs structure UV-LED was grown on GaN templates, thick GaN is favorable to reduce compressive strain in the well layers. Therefore, AlxGa1-xN/GaN SLs on GaN templates are promising candidates for UVA region LEDs. However, the absorption from GaN template must be considered. In contrast, for DUV-LED structure on AlN templates using AlN (or AlxGa1-xN)/GaN as active layers, AlN template will introduce an in-plane compressive strain, because aSL>aAlN. Since the DOP of AlN (or AlxGa1-xN)/GaN is positive even for one-monolayer-thick GaN well in SLs, a strain relief technique is necessary to avoid the reduction of DOP caused by compressive strain.
In summary, the optical polarization properties of c-plane Al0.26Ga0.74N/GaN SLs were experimentally studied under uniaxial strain. The c-plane Al0.26Ga0.74N/GaN SLs show E⊥c-polarized light emission similar to the c-plane GaN. The Al0.26Ga0.74N/GaN SLs show an opposite dependence on polarization with increasing strain and the quantum confinement, compared with the AlN/AlxGa1-xN QWs with high Al content. It is found that within the measured strain range, the relationship between the polarization degree P(SLs) of light emission from Al0.26Ga0.74N/GaN SLs and the in-plane strain εxx is in linearity, fitted as P(SLs) = 0.70 + 46.39εxx. The results should be helpful to the structural design of c-plane-based DUV or UVA-LEDs with high light extraction efficiency. Thus, the AlxGa1-xN/GaN short-period SLs is a promising emission layer for high-efficiency UV-LEDs.
This work was supported by National Basic Research Program of China(Nos: 2012CB619301 and 2012CB619306) and National High Technology Research and Development Program of China (2011AA03A111).
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