摘要
通过磁控贱射在玻璃基底上制备出了的厚度约为630 nm氧化锌(ZnO)薄膜,在室温下用能量为140 keV,剂量为2×1016 cm-2的氮(N)离子对该膜进行注入。注入后的样品在200到600℃范围内真空退火1小时。分别采用了X射线衍射(XRD)、扫描电子显微镜(SEM)、电子能量色谱仪(EDX)、紫外-可见分光光度计(UV-Vis)、荧光分光光度计(PL)和物理性能测试系统(PPMS)等分析手段对各种处理条件下的ZnO薄膜进行了分析,详细的研究了它们的结构特性、表面形貌、化学成分、光学性质和电学特性。
X射线衍射(XRD)结果表明,在未注入的ZnO薄膜中,(002)峰和(004)的强度相对较高,表明ZnO薄膜具有较好的结构特性。在高剂量N离子注入以后导致了(002)峰和(004)的强度急剧减小。这表明N离子注入已经导致了薄膜表面结构的严重受损。随后的退火会导致(002)峰和(004)的强度逐渐增加,这反映了ZnO的晶格结构的恢复。同时,在300℃退火时,(002)的峰位向小角度发生了偏移,这可能是跟ZnO薄膜的氮氧化物形成有关;而在600℃时,(002)峰和(004)又恢复原位,这说明在较低温度下形成的氮氧化物在高温下已经分解为气体。而在扫描电子显微镜(SEM)观测到了400℃下退火的ZnO薄膜,发现晶粒大小为30-50 nm。当退火温度达到600℃的时候,在薄膜表面发现了大小约为50 nm左右的孔洞,这说明高温退火导致了薄膜表面大量氮氧化物的分解。
紫外可见吸收和透射光谱(UV-Vis)显示未注入的ZnO薄膜的光学性质非常好,其平均透射率高达85%。然而,N离子注入以后光学透射率急剧降低到50%左右,并且透射截至波长从370 nm蓝移到330 nm。这说明N离子注入已经影响了ZnO的光学性质。随后的退火导致了ZnO薄膜光学性质的恢复。随着退火温度的逐渐增加,光学透射率也从50%增加到了90%左右。此外,研究中使用了光学带隙(Eg)和带尾参数(E0)来描述ZnO薄膜的带边和近带边特性。结果发现,离子注入导致了Eg和E0的增加。较高的Eg表明了离子注入导致了B-M漂移,而较高的E0说明薄膜内部有很高的杂质浓度。随着退火温度的增加,光学带隙增加,带尾参数减小,这表明ZnO薄膜内的杂质得到了纯化和充分扩散,光学性质得到了恢复。
光致发光(PL)测试结果表明,在未注入的ZnO薄膜中观测到三个发光带,分别是2.65,2.8和3.0 eV。2.65 eV的发光带归因于氧空位(VO),而2.8和3.0 eV发光带归因于锌的间隙子(Zni)。N离子注入导致了光致发光的严重恶化,三个发光带几乎完全消失。随后的退火使得ZnO薄膜发光强度的得到了部分恢复,这种恢复与ZnO内部的各种缺陷演变有关。当退火温度到达300℃时激活了锌的间隙子,所以2.8和3.0 eV的发光带开始增强。当退火到600℃时,氧的空位成为主要ZnO薄膜中的主要缺陷形式,而锌间隙子被部分抑制,所以2.65 eV的发光带逐渐增加,同时2.8和3.0 eV的发光带发生了红移。
霍尔效应(Hall)测量结果表明N离子注入的薄膜在退火到600℃的时候仍然是n型,载流子浓度高达6.67×1022 cm-3。强烈的自补偿效应、过高的掺杂浓度和严重的晶格失配导致了制作p型ZnO薄膜的失败。使用较低剂量的N离子注入(~1×1015 cm-2)生长在GaN基底上的ZnO薄膜可能是制作p型ZnO薄膜的有效途径。通过理论研究认为N的受主能级过高,并不是制作高效、可重复的p型ZnO薄膜的最理想的元素。通过Mg、N共掺不仅可以减小受主能级,而且能避免N和Mg的p-d轨道杂化。此外,在掺杂后,其电介质函数虚部的变化也比较小。因此, N和Mg共掺是制作高效的、稳定的、可重复性的p型ZnO的有效途径,这部分工作有待于以后在实验上进行验证。
The ZnO thin films deposited on glass substrate were implanted at room temperature with 140 keV N ions at a dose of 2×1016/cm2. Techniques, such as x-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible spectrophotometer (UV-Vis), fluorescence spectrophotometer, physical properties measurement system (PPMS), have been used to study film structures, optical, electrical properties and their thermal evolutions.
The results from XRD measurements show that high dose N ion implantation can induce deterioration of the crystallinity and sharp decrease of (002)-peak. The damaged structures have been found to be partially recovered upon annealing, which leads to growth of grain size. Images from the SEM reveal that the grain size is about 30~ 50 nm and the annealing at 600℃induce lots of pores with size of 50 nm,which may be suggest that the nitrogen oxygen compound has been evaporated at the 600℃.
The optical transmission and absorption in N-implanted ZnO films and their thermal evolutions have been investigated. Optical band gap (Eg) and the parameters of band tail (Eo) have been used to analyze the optical absorption edge and near-absorption edge characteristics. Our results show that optical band edge has shifted to higher energy and effect of band tail can be clearly seen after N ion implantation. It indicates that the Burstein-Moss shift has occurred. Subsequently, with the increase of annealing temperature, optical band gap is widened while effect of band tail is weakened. Electrostatic potentials in the grain boundary have been discussed to explain the band-gap widening. Moreover, impurity and structural disorder of the film are responsible for band tail.
The three luminescence band at 2.65, 2.8 and 3.0 eV has been observed by photoluminescence spectra. The luminescence band at 2.65 eV is related to the oxygen vacancies, and the 2.8 and 3.0 eV band is attributed to the zinc interstitial. N ion implantation induces the decease of luminescence bands. However, subsequent annealing gives rise to the increase of luminescence. At 300℃, the luminescence band at 2.8 and 3.0 eV can be clearly observed, which can be ascribed to activation of zinc interstitial. Owing to the lots of oxygen vacancies at 600℃, the band at 2.65 eV is notable. Therefore, the results suggest that the green luminescence band can be attributed to the oxygen vacancies.
N-implanted ZnO films annealed at 600℃are still n-type. Strain mismatch induced by thermal expansion and the high doping levels are responsible for the difficulty in fabrication of p-type ZnO thin film by N ion implantation. Meanwhile, our results show that ZnO thin film deposited on GaN substrate with low dose N ion implantation may be promising for development of p-type ZnO. Howerer, the theoretical calculations conceive that the acceptor level of nitrogen doping is relatively high and it is not suitable to develop reproductive p-type ZnO. The calculated results suggest that the N and Mg co-doping may be effective methods to fabricate the p-type ZnO. The further experimental results are eager to be made to confirm our conclusions.
引文
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