Renying Cheng a b, Gangqiang Zha c, Kun Cao b, Hao Zhang d, Xin Wan b, Heming Wei b, Ran Jiang b, Jiahu Liu b, Yu Liu b, Xue Tian b, Tingting Tan b, Donglei Chen a
a State Key laboratory of Nuclear Power Safety Monitoring Technology and Equipment, China Nuclear Power Engineering Co., Ltd., Shenzhen, Guangdong 518172, China
b Key Laboratory of Radiation Detection Materials and Devices, Ministry of Industry and Information Technology, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China
c Research and Development Institute of Northwestern Polytechnical University in Shenzhen, Shenzhen, Guangdong 518057, China
d School of Civil Aviation, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China
## Abstract
The preparation of high quality II-VI semiconductor single crystal still is a challenging task. The II-VI ternary compound Cd1-xZnxTe is an outstanding candidate for room temperature nuclear radiation detectors. Therefore, the epitaxial mechanism of its film is profound to clarify for accurate growth regulation and quality improvement. However, determining the essence of epitaxial growth remains challenge only using experiments to explore the effects of growth conditions on film epitaxy. In this work, the growth pits filling mechanism of CdZnTe epitaxial film has been elucidated by combining the experimental results with the first-principles calculation. Based on the Atomic Force Microscope data, the structural characteristics of growth pits were analyzed, and the layered growth model and the lateral step growth model were proposed. According to Gibbs-Wulff crystal growth law, the stability of two growth pits filling models was compared through the first-principles simulations. By comparing surface energies, we identified that the filling process of growth pits relies on the movement of steps along the interface. These results well describe the deposition mechanism of CdZnTe film prepared by close-spaced sublimation and provide inspirations to accurately regulate the II-VI film growth and obtain high quality films.
## Introduction
From a practical application point of view, group II-VI and III-V semiconductors have a high degree of photostability and a high quantum efficiency of luminescence which make them promising semiconductor materials[1], [2], [3]. Meanwhile, compared with isovalent semiconductors (e.g., II-VI/II-VI and III-V/III-V), the combination of II-VI and III-V compound greatly improves the tunability of material properties and diversity of semiconductor device design[4], [5]. Several II-VI and III-V materials have similar zinc blende structure and appropriate lattice matching, and the dislocation and structure distortion can be minimized at the time when forming heterojunction[6]. Heterovalent II-VI/III-V structures have been widely applied in the field of light-emitting diodes[7], high-efficiency solar cells[8] and photoelectric devices[9] However, II-VI semiconductors have a longer distance between the two elements in the periodic table, a larger electronegativity difference, and a larger ionic bond component than III-V compound semiconductors, so they have high vapor pressure and extremely high melting points, which leads to the difficulty in the single crystal preparation of II-VI compound[10].
The II-VI ternary semiconductor compound Cd1-xZnxTe presents excellent properties with tunable band gap, low dark current, high resistivity, high energy resolution and high density for the application of room temperature nuclear radiation detectors[11], [12], [13]. Therefore, CdZnTe film is an outstanding candidate for the fabrication of solar cells[14], nuclear radiation detectors[15], [16], photoelectronic devices[17], etc. Hot wall epitaxy (HWE)[18], molecular beam epitaxy (MBE)[19], magnetron sputtering (MS)[20], metal organic chemical vapor deposition (MOCVD)[21] and close-spaced sublimation (CSS)[22] have been studied to grow CdZnTe films. Among these methods, CSS has advantages of fast deposition rate (1 μm/min < v < 20 μm/min), low cost, simple equipment and has gradually become a prospective method for CdZnTe film preparation[23], [24], [25]. Nevertheless, obtaining high quality CdZnTe film using CSS method is limited due to lattice mismatch between substrate and film, low stacking fault energy and high defect density[22], [26], [27].
Wu et al. successfully obtained CdZnTe epitaxial films with a thickness of 1110 μm on GaAs(001) substrates by CSS method[28]. The full width at half maxima (FWHM) of (004) plane from double-crystal X-ray rocking curve (DCXRC) data is 166 arcsec showing the good crystalline quality of the films. The average dislocation density of the CdZnTe film surface is calculated to be 1.05×105 cm-2 by dislocation revealing techniques and scanning electron microscope (SEM) observation. An excellent energy resolution of 31% was first observed from the 241Am @59.5 KeV gamma-ray pulse height spectrum at 20V bias, which demonstrates the greater thickness and better crystal quality of the CdZnTe epitaxial film. These results are prominent in the development of CdZnTe epitaxial films prepared by CSS[29], [30], [31], [32]. However, compared with CdZnTe crystals, these spectral performances are insufficient[33], [34] in the energy spectra detection application which is partly caused by the mismatch strain between substrate and epitaxial film[29], [35]. Many previous studies have shown that the formation and migration of dislocations at interface can contribute to the stress relief[36], [37]. Cao et al. investigated the generation of threading dislocations in the interface of CdZnTe/GaAs[38]. It was found that the interactions of dislocation-APB and dislocation-dislocation are revealed by the movement of growth pits. Hence the lateral migration of dislocations is hindered and the annihilation and deflection effect are limited by growth pits in CdZnTe epitaxial films.
Similar growth pits appear in the epitaxial growth of GaN[39], SiGe[40], etc[41]. In epitaxial growth of GaN, the presence of V-pits is associated with threading dislocations[42], and the morphology of the pits will propagate with the defect motion. This propagation leads to degradation of the crystal quality and stress accumulation, increasing the roughness of the film and even changing the film growth morphology[43]. The triggers of non-radiative recombination center[44] and deep trap[45] are verified to be V-pits which have a positive effect on the electrical properties of the films. B. Heying[46] and Romain[43] explained the development mechanism of pits on GaN epitaxial films from the perspective of dislocation and surface energies respectively, and provided guidance on the process regulation of V-pits growth. Therefore, the quality of crystalline, on the other hand, is determined by the filling process of growth pits.
Controlling the growth interface is the basis of pit filling, which requires an adequate understanding of the changes in the morphology of the interface during the growth process. In general, the heteroepitaxial growth pattern of films is three-dimensional island growth[47]. Most reports on the interface evolution mechanism of film epitaxial growth focus on nucleation[48], grain growth[49], dislocation migration[50], etc., while the reports on the filling of growth pits need to be enriched. Before our work, Cao et al. have also analyzed the nucleation and islands growth stages of CdZnTe film epitaxy, respectively, to make preparation for high crystalline CdZnTe film growth[51]. It was found that the internal strain, dislocation density and average domain size in films are heavily dependent on the substrate temperature and chamber pressure. The CdZnTe islands grow along a certain direction till to a large size then the anisotropic growth proceeds. In a high substrate temperature and a high chamber pressure, the island growth process will be controlled by stress and defect sites on the substrate surface will become the main sites of nucleation. This work provides an excellent guidance for the parameter adjustment in the process of CdZnTe film preparation. However, the filling process of growth pits, which plays a decisive role in film quality, has not been researched because of the complex exploration and conception of it. Meanwhile, the lack of theoretical calculation research also greatly slackens the corresponding research process.
Hence, focusing on the filling process of growth pits, we fundamentally elucidated the epitaxial mechanism of CdZnTe films prepared by close-spaced sublimation based on first-principles calculation combining AFM, TEM and other experimental characterization results. It has sufficient value for the preparation of CdZnTe films by the CSS method, and also for the growth of II-VI semiconductor materials.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/the-growth-pits-filling-mechanism-of-cdznte-epitaxial-film-prepared-by-close-spaced-sublimation-based-on-the-first-principles-calculation.html