1. Introduction

As direct wide-band-gap semiconductor, zinc oxide (ZnO) has become a material with great potential for ultraviolet (UV) light emitting diode (LED), modulators and transistors [1–4]. However, spontaneous and piezoelectric polarizations which usually exist in conventionally grown c-oriented ZnO-based devices due to its wurtzite crystal structure [5], result in the strong built-in electrostatic fields and the overall reduction in radiative recombination efficiency, leading to poor performance [6]. Therefore, the growth of ZnO-based devices along nonpolar directions (e.g. a- and m- directions) have attracted attentions because they can effectively circumvent the adverse effects of undesirable spontaneous and piezoelectric polarizations [7]. Ascribed to the intrinsic asymmetry of wurtzite lattice structure [8], the a-plane ZnO films present an in-plane anisotropy of optical characteristics [9, 10], and have been used in the potential applications such as UV modulators and novel polarization sensitive devices. Currently, it’s still difficult to achieve homogeneous ZnO-based devices, since controllable and reliable p-type doped ZnO has not been realized yet. Therefore, alternative materials [11, 12], especially GaN [12] have been employed as high efficient hole injection layer to fabricate heterojunctions with n-type ZnO. And the foreign substrates usually applied strong anisotropic in-plane strain on a-ZnO films by introducing anisotropic lattice constant and thermal mismatching [13], modifying the energy structure and influence the optical properties. Presently, AlGaN with low-Al concentration is also adopted as the p-type layer considering the higher valence-band offset at AlGaN/ZnO interface compared with that at GaN/ZnO interface, which can prevent electrons injecting from n-type ZnO layer into p-type layer and realize pure emission from ZnO layer [14]. Although Al_xGa_1-xN ($0\le \text{x}\le 1$) layers have been considered as p-type epitaxial template, little work concentrated on the influence of anisotropic in-plane strain applied by Al_xGa_1-xN template substrates on the optical and crystal properties of a-ZnO layers which decide the performance of ZnO based LEDs.

In this paper, the anisotropic optical properties of three heteroepitaxial a-ZnO layers grown on a-AlGaN, a-GaN and r-sapphire respectively are studied. The DOP of PL, in-plane anisotropic crystalline quality and surface morphology of these ZnO films is discussed. Meanwhile, the in-plane strains of ZnO layers was estimated by Raman spectrum measurement of ZnO layers to claim the relationship between DOP and in-plane strains.

2. Experiment details

The heteroepitaxial ZnO films were grown on r-Sapphire substrate, a-GaN template and a-Al_0.08GaN template respectively. The schematic structures of them are shown in Fig. 1(a). The a-GaN and a-AlGaN substrates were prepared using metal organic chemical vapor deposition (MOCVD) system. Trimethylaluminium (TMA) was used as Al source to grow AlGaN template. The three substrates were then put into the reaction chamber of pulsed laser deposition (PLD) system for ZnO deposition after ultrasonic cleaning by acetone, alcohol and deionized water for 10 minutes, respectively. The crystalline quality and growth orientation were investigated by high resolution X-Ray diffraction (HRXRD, PANalytical X’pertPRO MRD, Holland) using Cu Kα1 (λ = 1.54056 Å) radiation. The thickness of Al_xGa_1-xN and ZnO layers is 3 µm and 0.3 µm, respectively. And their surface morphologies were examined by atomic force microscopy (AFM) in contact mode (Veeco NanoScope MultiMode).

 

Fig. 1 (a) Schematic structure of heteroepitaxial a-ZnO layers grown on r-Sapphire (left), a-GaN (middle) and a-AlGaN (right). (b) Schematic diagrams of polarization-dependent PL measurement setup in this study. (c) Microscope image of a-plane ZnO sample grown on a-GaN template and crystallographic directions.

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Room temperature polarization photoluminescence (PL) measurements were performed by using the 4th harmonic of a Q-switched Cr:YAG laser with the wavelength of 266 nm as an excitation laser, and Glan-Taylor prism as polarizer to distinguish the light polarized in a specific direction shown in Fig. 1(b). In Fig. 1(c), Microscope images show obvious periodic fluctuations of a-ZnO film surface along crystallographic c-axis due to anisotropic surface diffusion and strain-relaxation. The fluctuations in the surface shape of the partially relaxed thin film have been correlated to the presence of interface misfit dislocations which are produced as a consequence of strain relaxation in lattice mismatched thin films [13].

When in-plane anisotropic strains are introduced into a-plane ZnO, the crystal symmetry changes from C_6v to C_2v, the valence band structures are modified from the unstrained $|X\pm iY$ heavy hole and light hole states into $|X$-like and $|Y$-like states, leading to anisotropic optical properties [10]. This modified band structure is expected to be detectable in the polarization-dependent PL measurement shown in Fig. 2(a). Through rotating the Clan-Taylor Prism, PL emission along different polarization direction can be collected by the spectrometer. The polarization angle θ is defined as angle between the polarization direction of the Clan-Taylor Prism and the direction of surface fluctuations as shown in Fig. 2 (b). Based on the results of polarized PL spectra of the three samples shown in Fig. 2, the differences of the intensity and peak position between TE and TM modes can be observed. It indicates that the a-ZnO layers possess obvious anisotropic optical property. To discuss the optical polarization property accurately, DOP is defined as below [15]:

 

Fig. 2 Room temperature PL spectra of a-ZnO layers grown on (a) r-Sapphire, (b) a-GaN and (c) a-AlGaN at different polarization angle. (d) Polarization-dependent PL results with integrated PL intensity against rotation angle of the Glan-Taylor prism, and the data was fitted with sine function.

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3. Results and discussion

Figure 2(d) shows the normalized PL intensity of the edge emitting light against rotation angles of the Glen-Taylor prism. Solid lines refer to experimental data fitting with sine function. All the three samples show the strongest PL intensity under 90° polarization angle in the angle dependent PL measurement, indicating that the emission from the conduction band to heavy-hole band is the predominant. The strain modification of the band structure of the three strained ZnO layers were investigated by the PL normalized intensity and peak shifts of TE modes and TM modes in each sample, as depicted in Fig. 3. The TE and TM modes origin from the transitions between the conduction band and the heavy-hole band, as well as the crystal field splitting band, respectively. Therefore, it can be fully proved that the topmost valence band is not switched to crystal field splitting band, as the peak wavelength of TE modes are consistently larger than those of TM modes, shown in Fig. 3(a). The polarize transmission spectrum showed the same trend as shown in Fig. 3(b) . The a-ZnO layer grown on a-GaN template has the largest DOP 0.8907, greater than that of a-ZnO grown on r-Sapphire and a-AlGaN. It is worth noting that a-ZnO grown a-AlGaN has smaller DOP than a-ZnO grown a-GaN which mainly caused by different effect of in-plane stress on the modification of valence band structure. of the ZnO layer largely depends on. The lattice mismatch and thermal mismatching between it and the substrate are the major factor that the anisotropic stress depends on.

 

Fig. 3 (a) The peak wavelength of PL spectrum at different polarization angles. (b) Absorption edge of polarization transmission spectra against rotation angle of the Glan-Taylor prism.

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Figure 4(a) shows the Raman scattering spectra. The two dashed lines (379.0 and 439.0 cm⁻¹) [17] show the phonon frequencies A₁(TO) and E₂(high), respectively, for strain-free ZnO. Although the intensity is weak, the peak of E₂(high) mode can be seen clearly at partial enlarged Raman scattering spectra shown in Fig. 4(b). In-plane strains ${\text{ε}}_{yy}$ and ${\text{ε}}_{zz}$ were estimated by using frequency shifts $\Delta \text{ω}=(\text{ω}-{\omega }_0)$ of A₁(TO) and E₂(high) modes [18]. Summary of in-plane strains and DOP of three samples are listed in Table 1.

 

Fig. 4 (a) Raman scattering spectra for three nonpolar a-ZnO layers. $\Delta \text{ω}\left({\text{A}}_1\left(\text{TO}\right)\right)$ and $\Delta \text{ω}\left({\text{E}}_2\left(\text{high}\right)\right)$ for each sample are labeled. (b) Partial enlarged Raman scattering spectra of E₂(high).

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Table 1. Summary of in-plane strains and DOP of each sample

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As shown in Table 1, unlike the large lattice and thermal mismatch between r-Sapphire and a-ZnO, a-ZnO and a-GaN have similar lattice constants and a small difference between in-place linear thermal expansion coefficient [19]. Therefore, the in-plane strains of a-ZnO grown on a-GaN and a-AlGaN are much small than that of a-ZnO grown on r-Sapphire. As the Al component x in a-Al_xG_1-xaN template increases from 0 to 0.08, both the in-plane tensile strain ${\text{ε}}_{yy}$ and in-plane compressive strain ${\text{ε}}_{zz}$ increase. This should be attributed to the smaller lattice constant of AlN than GaN and ZnO. We can speculate that the ${\text{ε}}_{yy}$ and ${\text{ε}}_{zz}$ will continue to increase with the increasing of x, leading to smaller DOP. With the increasing of in-plane strains, the DOP exhibited an opposite tendency. This indicates that DOP of a-ZnO grown on a-Al_xG_1-xaN template can be reduced by increasing the Al component x. From this experimental outcome, it shows that strain status is a major factor that the DOP depends on.

The 2θ/omega-scan spectra of the three samples are presented in Fig. 5(a), showing the standard diffraction peaks of sapphire ($20\overline{2}4$) plane diffraction at 52.6°. The diffraction peak of ZnO ($1\overline{1}20$) show that ZnO layers of three samples are all grown along a-axis orientation. Since materials grown along nonpolar direction typically exhibit strong anisotropic behaviors of crystallinities with respect to in-plane orientations, the XRCs were measured as a function of azimuthal angle φ, shown in Fig. 5(b) and (c). Interestingly, the anisotropy of crystal quality is related to in-plane strains, as well as DOP of PL. Therefore, the anisotropy of crystal quality is introduced to represent the difference of the in-plane stresses along two orthogonal axes.

 

Fig. 5 (a) HRXRD 2theta/omega scan curve of a-ZnO layers grown on r-Sapphire, a-GaN and a-AlGaN. (b) The FWHMs of symmetric XRCs of ZnO layers grown on (a) r-Sapphire, (b) a-GaN and (c) a-AlGaN as a function of azimuthal angle φ.

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Figures 5(b) and (c) shows the anisotropic crystal qualities of a-Al_xGa_1-xN template and a-ZnO layers of the samples in the in-plane direction. The rocking curves for the full width at half maximum (FWHM) exhibit the parabolic shape as the azimuth angle changes, which is similar to the reported M shape [19]. As shown in Fig. 5(b), a-GaN template showed larger anisotropy of crystalline quality than a-AlGaN template. The a-ZnO grown on a-GaN template also showed larger anisotropy of crystalline quality than a-ZnO grown on a-AlGaN template. This consistency indicates that the crystal anisotropy of the template plays a major role in the crystal anisotropy of the heteroepitaxial a-ZnO layer. The in-plane crystalline quality anisotropy is manifested through surface fluctuations which are produced as a consequence of strain relaxation in lattice mismatched thin films. For the three samples in which the a-plane ZnO are grown on r-Sapphire, nonpolar a-plane GaN and AlGaN substrates, respectively, the anisotropic in-plane stresses of ZnO layers are also different, due to the three substrates with difference of lattice mismatch, crystal quality and surface fluctuations. In general, the greater the anisotropic crystal quality is, the more obvious the fluctuations of a-plane ZnO crystal is. Moreover, the anisotropy of crystal quality can be defined as:

The ${\epsilon }_{FWHM}$ of a-ZnO layers grown on r-Sapphire, a-GaN and a-AlGaN is 0.1285, 0.4718 and 0.2973, positive related to in-plane strains of a-ZnO layers. According to AFM results shown in Fig. 6, a-GaN has more obvious undulation than a-AlGaN corresponding to anisotropy of crystalline quality. Furthermore, most obvious undulation of ZnO layer is indeed observed on the substrate with largest anisotropy of crystal quality (a-GaN).

 

Fig. 6 AFM images of (a) r-Sapphire, (b) a-GaN, (c) a-AlGaN and ZnO layers grown on (d) r-Sapphire, (e) a-GaN, (f) a-AlGaN at a scanning area of 2 × 2 μm²

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4. Summary

In summary, the angle dependent PL of three a-plane ZnO layers heteroepitaxied respectively on r-sapphire and p-doped-available template a-GaN and a-Al_0.08GaN were investigated in detail. a-ZnO grown on a-GaN has the greatest DOP, ascribed to smallest in-plane strains estimated by Raman scattering spectrum. The anistropy of crystalline quality and surface morphology were found to be also depend on the in-plane strains. We speculate that with the increasing of Al component x of a-Al_xGa_1-xN, the DOP of PL of a-ZnO layer grown on a-Al_xGa_1-xN template will be slightly reduced.