The brighter intensity and narrower line width of NBE and the low (IDLE/INBE) intensity ratio are considered as signatures of better optical properties. Comparing the PL spectra of ZnO grown at 200 and 300oC, we found that the optical performance of ZnO grown at 300oC was better in the NBE but worse in the DLE region. We also observed that tilt/twist angle of sample grown at 300oC is smaller/larger than that grown at 200oC. Previous studies pointed out that tilt angle alone, which is coupled
105
with the screw type TDs, is not sufficient to describe the crystalline quality of the ZnO films [15, 20]. Twist angle, which is a measure of edge type TD density, should be taken into consideration to account for the optical properties. The different trend of increase/decrease of the tilt and twist angles with growth conditions observed in this case allows us to independently examine the influence of edge and screw TDs.
The (IDLE/INBE) ratio vs. Δφ(1011), the FWHM of the azimuthal scan across ZnO
off-normal (1011) reflection, and the FWHM of NBE vs. Δθ(0002),the FWHM of the rocking curve of (0002) specular reflection, are illustrated in Fig. 5-6 (a) and (b), respectively. Here Δφ(1011)/Δθ(002) bears the same physical meaning as the twist/tilt
angle and can be considered as a measure of the density of edge/screw TDs. In both figures, (IDLE/INBE) ratio and FWHM of NBE exhibit monotonic increases with
(1011)
φ
Δ and Δθ(0002), respectively. In contrast, the distribution of (IDLE/INBE) ratio
scatters randomly with Δθ(0002) andthe line width of NBE didn't show clear correlation withΔφ(1011), either. Evidently, the NBE emission is predominantly governed by the
screw component of the TDs and the edge TDs play the key role in affecting the DLE intensity, as shown in Fig 5-7 (a) and (b). After exchanging abscissa of Fig. 5-7 (a) with (b), no clear correlation is observed, as plotted in Figure 5-8 (a) and (b). Our observations support the arguments that screw TDs can act as nonradiative centers in reducing the NBE emission intensity and the existence of edge TDs leads to
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aggregation of point defects due to stress field near the dislocation core resulting in the enhancement of DLE intensity [9, 21].
The electric properties of the ZnO/γ-Al2O3/Si samples were also investigated by Hall effect measurements (Ecopia Corporation HMS-3000 with 1 T magnet). The major carriers in ZnO layers are verified to be electrons, revealing the n-type nature of the ZnO layers, same as the usual undoped ZnO. The dependence of net carrier concentration and mobility on Δφ(1011) and Δθ(0002) were examined. The obtained
mobility is in the order of a few tens cm2/Vs and no obvious correlation between the carrier mobility and twist/tile angle was found. We also observed that the carrier concentration decreased with the increase of Δφ(1011)but distributed randomly with
Δθ(0002), as plotted in Fig. 5-6 (a) and (b). This implies that the decrease of net carrier concentration is dominantly affected by the increasing density of edge TDs but is not remarkably influenced by the screw TDs, as shown in Fig. 5-7 (a) and (b).
Similar phenomenon was also reported by D.G. Zhao et al. on GaN grown on c-plane sapphire by MOCVD, where the carrier concentration decreased with the increase of the FWHM of XRD (1012) reflection [22, 23]. It is known that there exist many dangling bonds along the edge dislocation lines, and they can induce deep acceptor-like trap states which may capture the electrons from conduction band in n-type semiconductors [24, 25]. In the case of n-type GaN, the edge dislocation
107
108
cores are normally negatively charged and the decrease of free electron concentration can be attributed to the compensation effect from the increasing acceptor levels introduced by the edge dislocations. In our case, SCM and CAFM measurement results presented in chapter 4 indicate the lower local carrier concentration and higher Dit in the boundary regions with high density of edge dislocations in ZnO grown in c-plane sapphire by the same method. Consequently, we ascribe the reduction of carrier concentration with increasing edge dislocation density to the compensation effect of the edge dislocation induced acceptor levels, similar to the case of n-type GaN. Edge type TDs seem to be more harmful to the optical and electric properties of ZnO as compared to screw type TDs. How to optimize growth parameters and/or develop new growth method to effectively cut down the density of TDs, especially the edge type ones, in ZnO epitaxial films is still an important issue in the prospective applications to photonic devices.
0.35 0.40 0.45 0.50 0.55 0.60 0.65
Fig. 5-6 The ratio (IDLE/INBE) and net carrier concentration dependence of Δφ of ZnO (1011)diffracted peak (a). The NBE width and net carrier concentration dependence of Δθ of ZnO (0002) diffracted peak (b). The dash curves were plotted to guide the eyes.
0 4 8 12 16 20
Fig. 5-7 The ratio (IDLE/INBE) and net carrier concentration dependence of edge TDs density for pile-up model calculation (a). The NBE width and net carrier concentration dependence of screw TDs density (b). The dash curves were plotted to guide the eyes. [unpublished]
2 3 4 5 6
Fig. 5-8 After exchanging abscissa of Fig. 5-7 (a) with (b), the ratio (IDLE/INBE) dependence of screw TDs density (a), The NBE width dependence of edge TDs density (b). [unpublished]
5.6 Summary
In this work, we have successfully grown high-quality ZnO epitaxial films by PLD on Si(111) substrate with a nano-thick γ-Al2O3 buffer layer. There exist two (111)-oriented γ-Al2O3 domains rotated 60° from each other relative to the surface normal. The in-plane epitaxial relationship between the wurtzite ZnO, cubic γ-Al2O3
and cubic Si follows
2 3 2 3
{1010}ZnO || {224}γ−Al O or {422}γ−Al O || {224}Si as determined by XRD and TEM. The connection between the defect characteristics and optical properties of ZnO layer was established by correlating XRD and LT-PL results. Our results indicate that the (IDLE/INBE) ratio and carrier concentration are dominantly affected by edge TDs and the line width of NBE emission is mainly related to screw TDs.
110
111
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