利用P-MBE制备高质量Mg_xZn_(1-x)O的结构和光学特性

Structural and Optical Properties of High Quality Mg_xZn_(1-x)O Films Grown by P-MBE

利用等离子体辅助分子束外延(P-MBE)的方法,在c平面的蓝宝石衬底上制备了高质量的MgxZn1-xO合金薄膜。通过改变Mg源的温度,得到了不同Mg组份的MgxZn1-xO合金薄膜;通过引入ZnO的低温缓冲层,有效地提高了MgxZn1-xO合金薄膜的结晶质量。随着Mg组份的增加,MgxZn1-xO的X射线衍射的(002)衍射峰逐渐向大角度方向移动。对样品进行光致发光(PL)谱的测量,在室温下观察到了较强的紫外发光。随Mg浓度的增加,紫外发光峰向高能侧移动,并且发光峰逐渐展宽。通过对x=0.15的样品进行变温光谱的测量研究了紫外发光峰起因,得到了MgxZn1-xO的发光是来自于自由激子的发光。自由激子束缚能为54meV。

ZnO is a wide band-gap semiconductor with good electrical and optical properties. ZnO has higher exciton binding energy of 59 meV at room temperature, leading to a lower threshold, and is favorable for efficient operation of optical devices. Short wavelength devices based on ZnO have become even more interesting. On the other hand, band gap devices based on ZnO/ZnMgO superlattices or quantum wells can confine both excitons and photons in the low dimensions, making the stimulated exciton-related emission process more efficient. Therefore, keeping focus on the MgxZn1-xO films for purpose of exploring its potential applications in ultraviolet optoelectronics is more and more important. High quality MgxZn1-xO alloy films have been grown by plasma-assisted molecular beam epitaxy on c-sapphire （c-Al2O3 ） substrate. The growth temperature was 800 ℃ , the temperature of the zinc source is fixed at 245 ℃, and the flow rate of oxygen is 0.8 seem. The MgxZn1-xO films were obtained with different Mg contents by changing the temperature of the Mg source. The quality of the MgxZn1-xO films was improved by growing ZnO buffer layers at low temperature. Their crystal structures are characterized by X-ray diffraction spectroscopy （XRD）. The XRD patterns indicate all the MgxZn1-xO films with the （002） preference orientation of hexagonal wurtzite structure. When x value is varied from 0 to 0. 15, the （002） diffraction peak of MgxZn1-xO shifts to the large angle side with increasing Mg contents, and the full wide at half maximum （FWHM） of the diffraction peak is widen with increasing Mg contents. The lattice constant of c-axis decreases from 0. 520 5 nm to 0.518 9 nm as the Mg content increased from 0 to 0.15. The FWHM is only 0. 145° for the Mg0.15Zn0.85O film, which exhibited the high quality of the MgxZn1-xO films. The intense ultraviolet emission was shown in photoluminescence spectra at room temperature, which shifts from 3.29 eV （x = 0） to 3.54 eV（x =0.15 ） with increasing x values. The bandgaps of the films were evaluated by using the squared absorption coefficient （ α^2 ） of MgxZn1-xO films as a function of photon energy. The origin of the ultraviolet emission is studied by the PL spectra measured at the temperature from 80 K to 280 K. The emission peaks show a redshift, the FWHM of the emission peak widen and the intensities of the emission peak decreased with increasing the temperature. The temperature-dependent PL-integrated intensity of MgxZn1-xO were fitting by the equation： Ⅰ = Ⅰ0/[ 1 ＋ Aexp（ -E/kB T） J （where E is the activation energy of the thermal quenching process, kB is Boltzmann constant, Ⅰ0 is the emission intensity at 0 K, T is the thermodynamic temperature, and A is a constant）. A fit of the experimental data to the equation yields E = 54 meV, which agrees well with the exciton binding energy of 59 meV for bulk ZnO. Therefore, the ultraviolet emission peak in PL spectra of the MgxZn1-xO alloy films is attributed to the free exciton emission, indicating the high quality of MgxZn1-xO film.