Hydrogenic impurity states in wurtzite symmetric ZnO/MgZnO coupled quantum dots
Introduction
Recently, the wide-band-gap wurtzite (WZ) II–VI ZnO-based heterostructures have attracted much interest for their conspicuous optoelectronic devices applications in the visible and ultraviolet spectral regions. Attribute to its wide direct band gap of 3.37 eV [1] and high exciton binding energy 60 MeV at room temperature [2], ZnO is more suitable for technological applications, such as optically pumped nanolasers [3], nanorod heterostructure light-emitting diodes (LEDs) [4], gas nanosensors [5], nanowire field-effect transistors [6], piezoelectric nanogenerators [7]. Moreover, alloying ZnO with MgO allows the tuning of the direct band gap into deep ultraviolet (UV) regions. More recently, Ohtomo et al. [8], [9] successfully fabricated WZ MgZnO alloy and ZnO/MgZnO superlattices. In addition, WZ ZnO is a piezoelectric material. The strain originating from lattice mismatch at heterointerfaces by coherent growth creates a piezoelectric field in the quantum well (QW) layer. By this way, Park and Ahn [10] reported the spontaneous and piezoelectric polarization effects in WZ ZnO/MgZnO QW lasers. Cui [11] have found a strong built-in electric field in WZ ZnO/MgZnO superlattices. Built-in electric field effects on the exciton states and interband optical transitions in WZ ZnO/MgZnO quantum dots (QDs) have been investigated theoretically [12]. Moreover, considering the influence of polarization, the electronic structure of WZ ZnO/MgZnO superlattices has also been investigated theoretically [13]. However, to our knowledge, there is still little experimental and theoretical work focused on hydrogenic impurity states in WZ ZnO/MgZnO quantum structures to date. A deep understanding of the effects of impurities on electronic states of semiconductor nanostructures is a fundamental question in semiconductor physics because their presence can dramatically alter the performance of quantum devices. Thus, in this paper, we will investigate the hydrogenic impurity states in cylindrical WZ symmetric ZnO/MgZnO strained coupled QDs, in which both the 3D confinement of the electrons in QDs-like and the quantum-confined Stark effect (QCSE) due to the strong built-in electric fields are included.
This paper is organized as follows: in Section 2, we present a theoretical model to investigate the hydrogenic impurity states in the cylindrical WZ symmetric ZnO/MgZnO strained coupled QDs. Numerical results are discussed in Section 3. Finally, a brief conclusion is presented in Section 4.
Section snippets
Built-in electric fields in wurtzite symmetric ZnO/MgZnO strained coupled quantum wells
In order to investigate the effects of the spontaneous and piezoelectric polarization on the impurity states, let us now consider a WZ symmetric ZnO/MgZnO strained coupled QWs with corresponding layer thickness . For simplicity, we will ignore the complicated strains of the MgZnO layers due to the lattice and thermal mismatch [14].
In the following, we will calculate the built-in electric field induced by the spontaneous and piezoelectric polarizations in the WZ symmetric
Numerical results and discussion
We have calculated the ground-state donor binding energy Eb as a function of the impurity positions zi, the middle barrier width LMgZnO, the dot height LZnO, the radius R and the Mg composition x in the cylindrical WZ symmetric ZnO/MgZnO strained coupled QDs, surrounded by the WZ Mg0.02Zn0.98O material in the radial direction. The material parameters used in our calculation are as follows. The band gap energies of WZ ZnO and MgxZn1−xO are 3.37 eV [2] and 3.37 + 2.0x eV [18], respectively. The
Conclusions
In conclusion, we have calculated variationally the ground-state donor binding energy of a hydrogenic impurity in WZ symmetric ZnO/MgZnO strained coupled QDs. Numerical results show that the donor binding energy is highly dependent on the impurity positions, the coupled QDs structural parameters, such as, the middle barrier width LMgZnO, the dot height LZnO, the radius R, and the Mg composition x. It is found that the strong built-in electric field induces an asymmetric distribution of the
Acknowledgement
This work was supported by the Natural Science Foundation of the Education Bureau of Henan Provience, China under grant no. 102300410100.
References (20)
- et al.
J. Lumin.
(2008) - et al.
Physica E
(2009) - et al.
Phys. Lett. A.
(2007) - et al.
Phys. Lett. A.
(2006) - et al.
Comput. Mater. Sci.
(2007) - et al.
Appl. Phys. Lett.
(1997) - et al.
Appl. Phys. Lett.
(2001) - et al.
Science
(2001) - et al.
Adv. Mater. (Weinheim, Ger.)
(2004) - et al.
Appl. Phys. Lett.
(2004)
Cited by (10)
Phonon states of polar mixing optical modes in wurtzite ZnO-based coupling quantum dots
2014, Solid State CommunicationsCitation Excerpt :This may result in significantly different optical properties in CQDs from those in single QDs [11]. In fact, the bound electronic states, hydrogenic impurity states, and excitonic states in the Q0D ZnO-based CQDs have been investigated and reported [7,8]. However, besides a few works [3–5], the polar optical phonon states and their coupling properties with electrons in Q0D ZnO-based quantum systems have rarely been analyzed and not been fully understood by now.
Electric field induced nonlinear optical properties of a confined exciton in a ZnO/Zn<inf>1-x</inf>Mg<inf>x</inf>O strained quantum dot
2013, Physica E: Low-Dimensional Systems and NanostructuresCitation Excerpt :Alloying Mg with ZnO leads to increase the band gap whereas alloying Cd with ZnO tends to decrease the band gap. However, concentration of dopant materials should be taken into consideration for keeping these materials lattice matched with the same crystal structure to adjust the energy band structure [10]. Adding Mg with ZnO materials leads the tuning of the direct band gap into deep ultraviolet regions [11].
Optical investigation of A-plane ZnO/ZnMgO multiple quantum wells grown by pulsed laser deposition
2011, Physica E: Low-Dimensional Systems and NanostructuresCitation Excerpt :These data are extracted from PL and absorption spectra measured at 300 K. The electron and hole effective masses of ZnO are 0.24 and 0.78m0, while for ZnMgO barrier, the effective masses are 0.28 and 1.8m0, respectively, where m0 is the free electron mass [20–24]. Fig. 4 presents the transition energy of experimental data (open circle is sample B and open square is sample C) and the calculated result (solid curve) versus the well thickness changing from 1 to 2.5 nm for sample C and from 2.8 to 6 nm for sample B.
Polar optical phonon states and dispersive spectra of wurtzite ZnO nanocrystals embedded in zinc-blende MgO matrix
2011, Superlattices and MicrostructuresCitation Excerpt :Hence the investigation of various physical properties in quasi-0-dimensional (Q0D) ZnO-based quantum structures has become a hot topic during the last decade [1–3,6–15]. Among the experimental and theoretical investigations on the Q0D ZnO-based nanostructures, the ZnO nanocrystals embedded in the MgO matrix attracted substantial attention [6–12,16–18]. This is mainly due to the fact that the capping MgO shell outside of the ZnO core can not only reduce the surface-related defects and enhance the stability of nanocrystal, but also confine the charge carriers into the core region due to the wider band offset potential, which greatly improve the luminescent characteristics such as quantum efficiency and photostability [6–10].
Polar optical phonon states and their degenerative behaviors of wurtzite ZnO/MgZnO coupling quantum dots
2014, International Journal of Modern Physics BBinding Energy of an Off-Center Shallow Donor Impurity in Wedge-Shaped Quantum Dot Under Electric Field Effect
2023, Lecture Notes in Electrical Engineering