Chapter 6 Eliminating extra domains in m-plane ZnO by
6.4 Non-polar quantum well structures on m-sapphire
In order to realize whether the nonpolar multiple quantum wells (MQWs)
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structures could be made absence of the quantum confinement stork effect (QCSE), we fabricate 5 pairs of m-plane ZnO/MgZnO MQWs with three well widths on a m-ZnO buffer grown by two-step method including the LT and HT layers with
thicknesses of about 5 and 500 nm. The thickness of barriers is fixed to 55 nm and the well widths are 4, 8, and 16 nm, respectively. The LT-PL spectrum of the QWs with well width of 4 nm shows the barrier (MgZnO) emission at 3.599 eV and well, emissions at 3.427 and 3.395 eV in Fig. 6-14(a), respectively. The Mg content calculated according to Koike’s equation [38] from the PL spectra is close to 8.6%.
The dashed line is the near-band edge emission of bulk ZnO at ~ 3.365 eV, on the low-energy side of the dashed line is attributed to the buffer emission. The peaks at 3.428 and 3.395 eV are attributed to the confined-exciton and BSF bound exciton emissions in the QWs, respectively. The energy separations of spectral peaks on the low-energy shoulder marked with solid lines in Fig. 6-14(a) are closed to 72 meV, corresponding to the LO phonon energy in ZnO. From the phonon position, we determine the phonon replica from the BSF bound exciton emissions (3.395 eV) in the QWs.
Figure 6-14(b) show the PL spectra at 13K with various well widths. The emissions of both the confined and the BSF bound excitons in QWs blue shift with decreasing well width resulting from the quantum confinement effect. The results differ from those of the polar ZnO QW structures, which were red shift below the bulk ZnO value due to the QCSE for the well width being wider than 3 nm [39-43].
The NBE emission of our sample with well width of 16 nm is closed to the bulk one, but as the samples with well widths decrease below 8 nm, their NBE emissions show blue shift relative to the bulk ZnO. Without the internal electric field building in the nonpolar QWs, the blue shift of band edge with further decreasing well width to 4 nm is more promising. These results confirm that the nonpolar QWs structures should
resolve the QCSE to enhance optical emission efficiency.
Fig. 6-14 The PL spectra of m-plane MQWs with well widths of 4, 8 and 16 nm measured at 13K. (a) The PL spectrum of nonpolar MQWs of 4 nm well width shows the near-band edge (NBE) emissions from barriers, respectively; and (b) The PL spectra of various well widths indicated. The dashed line is the NBE emission of bulk ZnO.
6.5 Summary
We use LT-buffer layers, which show no extra (1103)ZnO domains for successfully growing m-ZnO epitaxial films without extra domains on m-sapphire.
The major crystal structure and defect properties of the two-step m-ZnO epilayers are similar to the thermal annealed m-ZnO layers in Chapter 5. After having analyzed the XRD measurement, we found absence of extra (1103)ZnO domains for the thickness of LT-buffer layers thinner than 67 nm, but presence of a few extra domains for the thickness going above 156 nm. Further raising the growth temperature to fabricate m-ZnO layers on the LT-buffers, we found the optimal thickness of LT-buffer ranging from 47 to 67 nm. For the samples grown with the thinner LT-buffers (< 47
141
nm), we could find weaker (1103)ZnO domains in the HT m-ZnO, whose content is about one order less than those direct growth m-ZnO samples without LT-buffers.
The extra domains contents decrease with increasing LT-buffer thickness and the largest extra domains content is ~ 3.6 10× −5 in m-ZnO with the 1.7 nm LT-buffer.
Determined from XRD and Raman spectra, we found that the two-step growth m-ZnO samples show the lower strain than those without LT-buffers. The Raman spectra of the mixing LO-phonon mode (A1(LO) + E1(LO)) appeared at ~ 580 cm-1 become polarization independent when the extra domain content measured by the intensity ratio of (1103)ZnO / (1100)ZnO is below ~1.8 10× −4. We also correlated the peak positions of the E2high
mode with the strain determined from the XRD results. From TEM measurement, we found a lot of BSFs density (~ 2 10× 6 cm-1) in two-step growth samples that helps to release the strains, and the D0X, BSFs and (e, A) emissions dominate the NBE emission without deep-level and surface bound excitons (~ 3.17 eV) emission from PL spectra. The more BSFs density reduces the strain effect and generates the more BSFs emission in PL spectra. The two-step growth m-ZnO possess the smoother surface attributed to the lower or without other oriented
growth in m-ZnO that will benefit for fabricating m-ZnO multilayer structures such as multiple quantum wells structures.
Finally, the m-plane nonpolar QWs show the quantum confinement effect with well widths below 8 nm, which confirm the nonpolar structure without internal field. The NBE emission from QWs is blue-shifted with respect to the bulk when the well width is narrower than 8 nm that shows the m-plane MQWs not only possess the quantum confinement effect but also prevent from the quantum confined Stark effect.
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Chapter 7 Conclusions and Prospective 7.1 Conclusions
We have investigated for two nonpolar ZnO epifilms,
( )
1120 - and( )
1100 -planeZnO epifilms on r- and m- plane sapphire substrates by the laser-molecular beam epitaxy. We observed the in-plane anisotropic strains, tensile strain perpendicular to the c-axis and compressive stain parallel to the c-axis, cause crystal symmetry breaking from wurtzite (C6V) to orthorhombic (C2V) for the nonpolar
( )
1120ZnO epilayers on r-sapphire. The as-grown a-ZnO epilayer has a small tilting angle
~ 0.3135o of ZnO c-axis with respect to the optical plane of substrate that is closely related to the r-sapphire miscut, indicating high crystal quality. The observed blue-shift
E
2(high) Raman mode contradicts with the anisotropic strain by XRDreveals violation of C6V selection rule. Determining from the polarized Raman spectra of E modes, the C2 2V configuration satisfies most of the selection rules for the Raman modes but violation of the C6V selection rule implies that the anisotropic strain on the nonpolar
( )
1120 ZnO epilayer may have changed crystal structure from C6V to C2V. The observed E1 and E2 bands in polarized optical reflection and photoluminescence spectra confirm that the anisotropic strains cause the structure change toward the orthorhombic structure for the nonpolar( )
1120 ZnO epilayer grown on r-sapphire.In the m-plane ZnO films grown on m-sapphire, we found small amount of (1103)ZnO domains provide strain relaxation of the m-ZnO matrix behaving as a low strain layer. Through carefully correlating low-temperature polarized PL spectra with the X-ray diffraction peak intensity ratio of (1103)ZnO (1100)ZnOof the samples
147
grown at different temperatures and after thermal treatment, we found that the broad-band emission around 3.17 eV may result from the interface defects trapped excitons at the boundaries between the (1103)ZnO domains and the m-ZnO matrix.
The peak positions of the free A-, B- and C-exciton emissions in the PL spectra are close to that of bulk ZnO as a result of the (1103)ZnO-oriented domains providing the
strain relaxation mechanism. The more (1103)ZnO domains in the m-ZnO layer cause the more surface boundary that makes the stronger surface-bound-exciton emission. And the a-axes of both the (1103)ZnO domains and the m-ZnO matrix are aligned with the c-axis of the sapphire (α-Al2O3) substrate. The c-axis of the
(1103)ZnO domains rotates about ±59° against the common a-axis of the m-ZnO.
We have successfully used a two-step growth method in m-ZnO on m-sapphire with LT-buffer to eliminate extra domains to reduce surface boundary trapping excitons.
The as-grown LT-buffer exist large strain to reduce the main m-ZnO by the second step growth at high temperature. The two-step grown m-ZnO showing the lower strain from the XRD and Raman spectra is due to the LT-buffer sharing a lot of stresses from lattice mismatch. The thickness of LT-buffers has an ideal window from 47 to 67 nm for growing high temperature m-ZnO; below this range there exist extra domains but whose content (≤3.6 10× −5) is one-order less than those made by direct growth m-ZnO without buffers. From AFM results, the two-step grown m-ZnO layers had smooth surface than without LT-buffer attributes the extra domains
existence to add other growth orientated and make rough surface. The TEM and PL measurements indicate that more BSFs density ~ 2 10× 6 cm-1 with smooth surfaces in m-ZnO, which is larger than that of without buffers (~ 5 10× 5 cm-1), causes the stronger BSFs emission in LT-PL spectra. There are three dominant
emission peaks of D0X, BSFs and (e, A) in LT-PL spectra of two-step m-ZnO.
Finally, we confirm successful fabrication of nonpolar m-plane multiple quantum wells structures can be achieved by the two-step growth method. The m-plane MQWs show the blue-shift NBE emission spectra for well width narrower than 8 nm, exhibiting quantum confinement effect and free of the quantum confined Stark effect (QCSE) due to nonpolar nature, which has no internal field.
We have confirmed the change of optical properties in nonpolar ZnO epilayers result from the change of crystal structure. The change in optical transitions by the anisotropic strains in a-ZnO was identified by the symmetry breaking of C6V to C2V by polarized Raman, PL and optical reflection spectroscopy prior to the similar results reported in a-GaN by using polarized Raman spectroscopy recently in 2012.
Furthermore, the first report to our knowledge in this study that the domain boundaries between the extra domains and main m-ZnO can trap the excitons to form the so-called surface-bound-excitons, proven by nicely correlating the characteristic of surface-bound-exciton emission with the extra domain content. By using the two-step growth method, we demonstrated successfully eliminating extra domains to reduce the surface-bound-exciton emission. This method provides a way of fabricating single-phase nonpolar m-ZnO structure with smooth surface and low strain state that merits for MQWs structure. The optical properties of nonpolar MQWs are consistent with other nonpolar MQWs which exhibit only quantum confinement effect with absence of the quantum confined Stark effect.
7.2 Prospective
In principle, MgxZn1-xO (MZO) could be engineered to achieve any bandgap in the range of 3.37 to 7.7 eV. However, the solubility of MgO in ZnO is limited in 0 < x
< 0.33 that restricts the band gap to the range of 3.37 eV < Eg < 3.99 eV [1]. The
149
nonpolar ZnO QW based on ZnO/MZO structures have been reported without QCSE in literatures by MBE [2, 3] and laser-MBE [4, 5]. Photonic devices with nonpolar ZnO, such as a-plane (1120) and m-plane (1100), have been proposed to improve the quantum efficiency suffered by the quantum-confinement Stark effect (QCSE). The nonpolar QWs structures not only reduce the QCSE effect but also increase the polarized emission rate. We will grow the more nonpolar a-plane (1120) and m-plane (1100) ZnO QWs on r- and m-plane sapphires with various well widths and
Mg contents. In addition, the surface roughness between the wells and barriers is also important to the QWs emission. The sharper the interface has the more efficient emission that also benefits for combining with the optical cavities. Many interesting physical properties in nonpolar QWs still need to investigate that are about excitons binding energy, exciton and phonon coupling interaction, phonon coupling with barriers, localized state, lifetime study, carriers dynamic and design barrier band engineered, etc.
If high density of excitons is excited in semiconductor materials such that average separation of excitons becomes shorter than the de Broglie wavelength of the exciton, then the excitons as quasi-Bosons would all condense to the lowest energy state. It is called the exciton Bose-Einstein condensates (BECs) [6]. The excitons BEC could generate the coherent emission as a laser does with high internal and extraction efficiencies having extreme low energy consumption. It is named the exciton polariton laser that has been recently realized in CdTe single quantum well within a microcavity at 19K under optical excitation 50 times below the lasing threshold [6].
A lot of polariton BEC effects have been observed in the semiconductor microcavity (MC) systems which often combine QWs and MCs to enhance electron and photon coupling generating polariton, including a bimodal momentum-space distribution with a narrow peak at zero momentum, long-range off-diagonal order [6,7], spatial
condensation in a macroscopic trap [7,8], spontaneous symmetry breaking [9], flow without dispersion [10], and a dramatic increase of coherence as measured in first-order and second-order correlation measurements [11-13]. Recently, L.
Sapienza, et al. [14] have realized an electroluminescent device operating in the light-matter strong-coupling regime based on a GaAs/AlGaAs quantum cascade structure embedded in a planar MC. They experimentally demonstrated that the electrons can be selectively injected into polariton states up to RT. Intrinsic decoherence mechanisms in the MC polariton condensate have been studied limited by the combination of number fluctuations and interactions. [15,16] RT low-threshold transition to a coherent polariton state has been observed as polariton lasing in bulk wide bandgap GaN MC in the strong-coupling regime. [17] The observation of polariton lasing over such a broad range of temperatures reveals a clear transition from a kinetic to a thermodynamic regime with increasing temperature.
Similar to GaN, ZnO is an environmental-friendly wide direct bandgap semiconductor.
Therefore, dominant exciton emission can be constantly observed in ZnO even at RT.
Due to its fast carrier cooling rate (< 200 fs) [18] and high Mott density (3.7 x 1019 cm-3, exciton Bohr radius ~2.34 nm) and large Rabi splitting of 120 meV [19] that have been theoretically show that as a consequence of the broadening of the upper branch by the continuum, Rabi oscillations should not be observed in ZnO MCs which nevertheless remain good candidates for polariton-based effects (polariton BEC) involving the lower polariton branch. [20]
151
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