Abstract
We introduce excitonic polarized photoluminescence (PL) of nonpolar ZnO layers and related quantum well (QW) structures in terms of crystal symmetries and lattice distortions. Polarized PL characters are attributed to in-plane anisotropic strains in the host, which are fully demonstrated on A-plane ZnO. Theoretical evaluations propose that in-plane compressive strains induced in ZnO layers play an important role in obtaining highly polarized optical properties. We experimentally achieve polarized PL responses in strain-controlled A-plane ZnO layers. Furthermore, we find interesting relationship between polarization degree of PL and in-plane anisotropic strains. Finally, highly polarized PL at room temperature is obtained by controlling well width in Cd0.06ZnO0.94O/ZnO QWs as a consequence of change in crystal symmetry from C6v to C2v at interfaces between Cd0.06Zn0.94O well and ZnO barrier layers in the QW samples.
Keywords
- ZnO
- nonpolar
- luminescence
- anisotropy
- crystal symmetry
1. Introduction
Zinc oxide (ZnO) has been one of the candidates of important materials towards the fabrications of optoelectronic platforms such as transistors, light-emitting diodes, transparent electrodes, and magnetism. ZnO has large exciton energy of 64 meV [1], which has received much attention for the possibility of utilizing excitonic-based optical applications at room temperature. In addition, MgO–ZnO and CdO–ZnO alloys are attracting great deal of interests since these alloys possess higher and lower band gaps than that of ZnO [2–6], which have been applied to MgxZn1–xO/ZnO and CdxZn1–xO/ZnO multiple and single quantum wells (QWs) [7–10]. These low-dimensional heterostructures can produce interesting quantum phenomena such as an increased excitonic binding energy and two-dimensional (2D) electron transport. These physical properties have been received much attention by many researchers for science and practical applications.
ZnO has a non-centrosymmetric structure with no center of inversion. This crystal symmetry builds spontaneous and piezoelectric polarizations in the host, which provides interesting excitonic luminescence. When the growth direction is chosen along the polar [0001] axis, these polarizations result in potentially detrimental effects such as the lowering of the probability of radiative recombination of active layers in QWs due to spatial separation of electron and hole carriers because of the quantum-confinement Stark effect (QCSE) [11, 12]. To suppress the QCSE, it is needed to deposit ZnO films and quantum structures on substrates other than
To date, some studies have reported on nonpolar ZnO-based QWs in terms of preventing the QCSE caused by
This chapter is organized as follows. Theoretical evaluations of optical anisotropies are described in Section 2 in order to investigate polarized PL in nonpolar
2. Optical polarization and electronic band structure
2.1. Electronic band calculations
Polarization control is theoretically demonstrated on the basis of excitonic selection rules at the band edge in ZnO, relating to the electronic band structures (EBSs) of the CB and VB. The transition energies of anisotropic strained
where
In Figure 1, the energy of the
The polarization anisotropy is dependent on the three polarization components of the oscillation strength for the transitions
where the coefficients of
2.2. In-plane strains and crystal symmetries
We investigated crystallographic polarizations of the layers using micro-Raman scattering. The polarized Raman scattering was measured in two distinct backscattering geometries under a 514.5 nm laser at 300 K. We select the
where the coefficients of
The lattice parameters of the layers were determined by high-resolution x-ray diffraction (HR-XRD). ZnO layers deposited at different
2.3. In-plane strains and optical absorptions
Polarized absorption spectra were measured for the
where the values of
In this section, we theoretically investigated relationship between energy separations and in-plane anisotropic strains on
3. In-plane lattice strains and polarized luminescence
3.1. A -plane ZnO homoepitaxial layer growth
We report homoepitaxial growth for strain-free and strained
The hydrothermally synthesized substrates were supplied by Crystec GmbH (Germany) and Goodwill (Russia). Both substrates were annealed at 1100°C for 1 h prior to PLD growth.
Growth processes of the homoepitaxial layers on Crystec ZnO substrates were monitored using reflection high electron energy diffraction (RHEED) with the [0001] azimuth. Atomic force microscopy (AFM) was used to observe surface morphologies. At the beginning of layer growth, up to 10 nm in thickness, the V-groove was seen with a RHEED pattern of three-dimensional (3D) spot (Figure 5a and d). However, continued layer growth of ZnO up to 28 nm, changed to a slightly smooth surface by the filling of the V-groove structure (Figure 5b). As a consequence, the 3D spot of the RHEED pattern were weakened (Figure 5c). Finally, the layer morphology at a thickness of 140 nm showed a very flat surface. The RHEED pattern showed a sharp blight stripe with a high contrast to the background, which resulted from a clean and smooth surface (Figure 5c and f).
A cross-sectional transmittance electron microscopy (X-TEM) image with the [0001] zone axis showed that layer surface had nano-facet structures (Figure 6a and b). The nano-facets were consisted of the
Strained
3.2. Polarized PL from anisotropic strained ZnO layers
The in-plane lattice strains were highlighted to polarized PL. An oxygen pressure [
The strain-free ZnO layer showed polarization to
where
In this section, the in-plane anisotropic strains were introduced in
4. Control of polarized luminescence by quantum well geometries
4.1. CdxZn1–xO/ZnO quantum wells
The anisotropic lattice distortion causes strain-induced modification of the EBS in ZnO, leading to polarization modulation of PL. That is, polarization ratio of PL is strongly highlighted by the anisotropic lattice distortion introduced in nonpolar ZnO. In this section, we report on quantum size effects of polarized PL on
Cd0.06Zn0.94O/ZnO QWs with different
4.2. Polarized PL properties in A -plane Cd0.06Zn0.94O/ZnO QWs
Figure 10a and b shows polarized PL spectra of a QW with
The polarization ratio (
The calculated Δ
4.3. Control of polarized PL in the QWs
Figure 11a shows the PL peak energies of QWs at 300 K as a function of well width. The PL peak energies systematically shifted to the higher energy with decreasing
TEM images of QWs with
Makino et al. reported lattice parameters of relaxed CdxZn1–xO (up to
In this section, the dependence of polarized PL on well width was studied in
5. Conclusion remarks
This chapter was reported polarized PL of nonpolar ZnO layers and their QW structures in terms to the crystal symmetry and the in-plane lattice strain during PLD growth. Anisotropic optical properties were closely related to in-plane anisotropic strains introduced into the layers, which were demonstrated on
Acknowledgments
This work was supported in part by a grant-in-aid from the JSPS Core-to-Core Program, A. Advanced Research Network, a grand from the Japan Science and Technology Agency (JST: A-Step) and a grant-in-aid from Exploratory Research and Scientific Research (B).
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