Amphoteric Phosphorus Doping for Stable p‐Type ZnO
Amphoteric Phosphorus Doping for Stable p‐Type ZnO
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DOI：
10.1002/adma.200700083
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发表时间：
2007-10
影响因子：
29.4
通讯作者：
A. Allenic;Weiqing Guo;Yanbin Chen;M. Katz;Guangyuan Zhao;Y. Che;Zhendong Hu;Bin Liu;
中科院分区：
文献类型：
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作者：
A. Allenic;Weiqing Guo;Yanbin Chen;M. Katz;Guangyuan Zhao;Y. Che;Zhendong Hu;Bin Liu;
氧化锌是一种在光电子器件中具有潜在应用的宽带隙半导体材料。然而，这些应用的最大挑战仍然是制造可靠和稳定的p型ZnO薄膜。本文报道了用脉冲激光烧蚀法在（0001）蓝宝石衬底上生长的稳定的磷掺杂p型ZnO薄膜。在600 °C下生长的薄膜在氧气气氛中退火后变为p型，电阻率为4.9 × 10 - 3cm-1，霍尔迁移率为1 cm-2 Vs，空穴浓度为1.3 × 10 - 3cm-1。这种p型薄膜在环境条件下稳定16个月，迄今没有明显的降解。透射电子显微镜显示，p型薄膜由高密度的位错组成，这增强了磷的溶解度和Zn空位的形成，以促进导电性的n-到-p转换。这些研究提供了ZnO中磷掺杂剂的两性性质的微观证据。最近，由于ZnO的宽带隙（3.37 eV），ZnO在光电子学中的应用越来越受到关注，例如发光二极管、紫外（UV）激光器和UV光探测器。与GaN相比，ZnO由于单晶衬底的可用性、相对低的生长温度（TG）和大的激子结合能（~ 60 meV）而在光电应用方面具有一些明显的优势。报道了室温下ZnO薄膜的光泵激子激光。在ZnO纳米线阵列中的激光效应已被证明，并在室温下在薄膜ZnO同质结中观察到电致发光（EL）。虽然有几个小组报道了p型ZnO薄膜，但它们表现出高电阻率和/或差的稳定性和再现性。因此，ZnO光电子学最大的挑战是可重复制造稳定的p型ZnO薄膜。像许多其他II-VI族半导体一样，ZnO具有不对称的掺杂限制：它可以很容易地掺杂为n型，但仍然强烈抵抗p型掺杂。虽然氮在理论上是ZnO最有希望的受体，但其低溶解度和由诸如氢和Zn金属的供体补偿是主要障碍。作为N的替代物，更大尺寸的V族元素如P、As、Sb和Bi已被广泛研究。在这种材料中的p型导电性的令人困惑的观察刺激了对ZnO中由P、As或Sb引起的缺陷的电子结构的理论研究。Limpijumnong等人预测，在富氧生长条件下，包含V族反位和两个锌空位（VZn）的复合物将具有低形成能，并表现为电离能为150-160 meV的浅受主。Lee等人使用相同的概念来研究ZnO中的磷络合物。从这些研究中得出的最重要的结论之一是，这样的V族掺杂剂是双电性的，作为一个孤立的反位杂质的施主，但作为一个受体时，形成一个复杂的两个VZn。虽然已经报道了p型导电性，但是关于磷及其相关的缺陷复合物的微观信息很少，这些缺陷复合物是p型导电性的原因。在这项工作中，我们报告了稳定的p型ZnO薄膜的脉冲激光沉积的制造和表征。我们研究了由磷掺杂的ZnO引起的微观缺陷，并确定了p型ZnO薄膜可以重复制造的生长和退火温度。确定了p型ZnO的最佳实验条件为TG = 600 °C，然后在TA= 600 °C下在O2气体中退火。良好的p型导电性的开始总是伴随着位错密度的显著增加，如透射电子显微镜（TEM）研究所揭示的。同时，二次离子质谱（西姆斯）揭示了磷的溶解度显着增加。这些结果表明，TG和TA控制位错的密度，这反过来又控制磷的溶解度。此外，位错作为锌杂质的吸杂中心，也促进了PZn-2 VZn受主的锌空位的形成。我们的物理见解进一步允许通过在加工过程中仅调整TG和TA来制造具有令人印象深刻的整流特性的P掺杂ZnO同质结。不同ZnO膜的室温电性质总结于表1中。沉积态的P掺杂ZnO（PZO）薄膜具有n型导电性，
Zinc oxide is a wide bandgap semiconductor with potential applications in optoelectronic devices. The greatest challenge for these applications, however, remains the fabrication of reliable and stable p-type ZnO thin films. Here we report stable phosphorus-doped p-type ZnO thin films grown on (0001) sapphire substrates by pulsed laser ablation. While as-deposited films all show n-type conductivity, films grown at 600 °C become p-type after annealing in oxygen atmosphere with a resistivity of 4.9 × 10 X cm, a Hall mobility of 1 cm V s, and a hole concentration of 1.3 × 10 cm. Such p-type films have been stable under ambient conditions for 16 months so far without apparent degradation. Transmission electron microscopy reveals that the p-type films consist of a high density of dislocations, which enhance both the solubility of phosphorus and the formation of Zn vacancies to facilitate the n-to-p conversion of electrical conductivity. These studies provide microscopic evidence of the amphoteric nature of the phosphorus dopant in ZnO. There has recently been an increasing interest in ZnO for applications in optoelectronics such as light emitting diodes, ultraviolet (UV) lasers, and UV light detectors because of its wide bandgap (3.37 eV). In comparison with GaN, ZnO has some obvious advantages for optoelectronic applications due to the availability of single crystal substrates, relatively low growth temperatures (TG), and a large exciton binding energy (∼ 60 meV). Optically pumped excitonic lasing of ZnO thin films at room temperature (RT) has been reported. Lasing effects in ZnO nanowire arrays have been demonstrated, and electroluminescence (EL) has been observed at room temperature in thin-film ZnO homojunctions. Although p-type ZnO thin films were reported by several groups, they showed high resistivity and/or poor stability and reproducibility. Thus, the greatest remaining challenge for ZnO optoelectronics is the reproducible fabrication of stable p-type ZnO thin films. Like many other II-VI semiconductors, ZnO has asymmetric doping limits: it can be easily doped n-type, but remains strongly resistant to p-type doping. Though nitrogen is theoretically the most promising acceptor for ZnO, its low solubility and compensation by donors such as hydrogen and Zn interstitials are major obstacles. As alternatives to N, larger-size group V elements such as P, As, Sb and Bi have been widely studied. Puzzling observations of p-type conductivity in such materials have stimulated theoretical investigations into the electronic structure of the defects induced by P, As or Sb in ZnO. Limpijumnong et al. predicted that under oxygen-rich growth conditions, a complex involving a group V antisite and two zinc vacancies (VZn) would have a low formation energy, and behave as a shallow acceptor with an ionization energy of 150–160 meV. Lee et al. used the same concept to study phosphorus complexes in ZnO. One of the most important conclusions from these studies is that such group V dopants are amphoteric—acting as a donor as an isolated antisite impurity, but as an acceptor when forming a complex with two VZn. Although p-type conductivity has been reported, little microscopic information on phosphorus and its related defect complex responsible for the p-type conductivity, has emerged. In this work, we report the fabrication and characterization of stable p-type ZnO films by pulsed laser deposition. We studied microscopic defects induced by phosphorus doping of ZnO and determined the growth and annealing temperatures at which p-type ZnO films can be reproducibly fabricated. The best experimental condition for p-type ZnO is determined to be TG = 600 °C, followed by annealing at TA= 600 °C in O2 gas. The onset of good p-type conductivity is always accompanied by a considerable increase in the density of dislocations, as revealed by transmission electron microscopy (TEM) studies. Meanwhile, secondary ion mass spectroscopy (SIMS) reveals a significant increase in the solubility of phosphorus. These results suggest that TG and TA control the density of dislocations, which in turn controls the solubility of phosphorus. Furthermore, as gettering centers for zinc interstitials, the dislocations also facilitate the formation of zinc vacancies for PZn–2VZn acceptors. Our physical insights further allow for the fabrication of P-doped ZnO homojunctions with impressive rectifying characteristics by adjusting only TG and TA during processing. The room temperature electrical properties of different ZnO films are summarized in Table 1. As-deposited P-doped ZnO (PZO) films show n-type conductivity and are more conC O M M U N IC A IO N