Hydrogenic impurity states in wurtzite symmetric ZnO/MgZnO coupled quantum dots

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Abstract

Within the framework of effective-mass approximation, the ground-state donor binding energy of a hydrogenic impurity in cylindrical wurtzite (WZ) symmetric ZnO/MgZnO strained coupled quantum dots (QDs) is calculated by means of a variational method, considering the strong built-in electric field induced by the spontaneous and piezoelectric polarizations. Numerical results show that the strong built-in electric field induces an asymmetric distribution of the donor binding energy with respect to the center of the coupled QDs. When the impurity is located at the center of the dot which is opposite to the growth direction, the donor binding energy has a maximum value with increasing dot height; and it is also insensitive to the middle barrier width if the middle barrier width is large. Moreover, numerical results also indicate that the donor binding energy of impurity located at the right boundary of the dot which is opposite to the growth direction is insensitive to the Mg composition if the Mg composition is large.

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 /LZnO/LMgZnO/LZnO/. 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.

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