摘要
半导体固态照明有取代传统白炽灯的趋势,代表着人类未来光源的发展方向。1960年以来,科学家陆续开发出红光,绿光发光二极管。而正是科研工作者开发出了高质量p型GaN薄膜的生长技术后,高效率蓝光LED走向了商业化。目前为止,全世界很多科研工作者仍然为不断的提高GaN LED的发光效率而努力。相比较GaN, ZnO同样作为一种宽禁带半导体,有着在发光效率,材料制备,环境友好方面的优势：1)室温下激子结合能高达60 meV,是GaN的两倍多,这使得室温受激辐射能在较低阈值下出现,2)两英寸的ZnO单晶衬底已经被制备出来而且ZnO材料可以在较低温度下生长,Zn源在自然界中储量很丰富而Ga源较稀缺,3)良好的生物相容性和比GaN更强的抗辐射性能。这些方面的优势使得ZnO成为热点研究材料。ZnO的禁带宽度为3.37 eV,而宽禁带半导体的掺杂普遍存在非对称性的困难。具体对于ZnO而言,低电阻率、高空穴迁移率、高晶体质量、稳定的p型ZnO很难重复获得。本文正是在这一难点上研究了ZnO中的施主、受主掺杂行为。具体内容有以下几部分：
1)由于纳米线优越的光学性能,我们研究了ZnO纳米线中的Al、In族元素掺杂,得出Al、In的施主能级分别为75 meV和102 meV。同时,我们发现MOCVD方法生长的纳米线表面晶体质量较完美,比文献报导中气相输运法制备的纳米线表面质量要好。
2)较为系统实验研究和理论计算发现Na是ZnO的一种有效受主掺杂源。脉冲激光沉积技术可以实现Na掺杂p型ZnO薄膜,在此基础上制备的ZnO pn结能实现低温电致发光。NaZn受主能级距离价带顶距离为～164 meV。研究表明H对于稳定NaZn受主起着重要的作用。在理解Na掺杂含量、生长温度、H、氧气压对Na掺杂ZnO薄膜电学性能的影响方面后,我们提出了Na掺杂ZnO的p型导电机理模型。模型中总结了三点重要影响因素：1)H在掺杂过程中钝化和激活Nazn受主的作用,2)Nai和NaZn受主之间的竞争需要合适的Na掺杂含量,3)足够的动力学能量和非平衡态条件。
3)更进一步,研究了Mg含量对Na掺杂p型Zn1-xMgxO(0≤x≤0.25)电学性能的有较为显著的影响,10 at.%-15 at.%的Mg含量的ZnMgO薄膜的p型导电性能比ZnO:Na薄膜得到了提高。另外,在室温电注入条件下,以ZnO/ZnMgO多量子阱做为有源层的p-ZnO:Na/n-ZnO pn结能发蓝紫外光而缺陷发光非常微弱。
4)考虑到ZnO多晶体薄膜实际上由许多纳米线组合而成,晶界处的负电荷中心(可能是氧吸附)应该降低p型ZnO的迁移率。通过紫外灯光照射削弱晶界势垒后,ZnO：Na薄膜的电学性能可以得到增强,在光照后ZnO:Na薄膜甚至可以发生从n型薄膜到p型的转变。对于光照前在比较优化条件下生长的ZnO:Na薄膜：电阻率为13.8-19Ωcm、霍尔迁移率为0.12-1.42 cm2/V s、空穴浓度为4.66×1018-4.78×1018 cm-3,紫外灯光照后它的电阻率可以达到3.8Ωcm、空穴迁移率为7.91 cm2/V s、空穴浓度为2.09×1017cm-3。其中空穴迁移率能在光照后提高一个数量级,这可能是削弱晶界影响后p型ZnO:Na薄膜的导电信息。
5)文章后面的重点放在提高p型ZnO的晶体质量上,我们使用脉冲激光沉积技术,实现了P掺杂的单晶体ZnO纳米线阵列的生长,并发现单根ZnO纳米线中P掺杂可能存在不均匀现象。
6)最后,考虑到纳米技术和薄膜技术各自的缺陷和优点,我们结合纳米科技和薄膜技术,实现了单晶体p型ZnO和ZnMgO材料的制备,得到的单晶体ZnO和ZnMgO材料的p型性能在11个星期后依然稳定。
Semiconductor solid-state-lighting, which represents the future of human light source, has the tendency to replace the traditional bulb. Since 1960s, scientists have developed red, green light-emitting diodes (LEDs) based on solid semiconductors. After successfully fabrication of high-quality p-type GaN film, high-efficiency blue LED is commercialized. Up to now, researchers around the world have been still working hard to further improve the luminescence-efficiency of GaN based LED. Compared with GaN, ZnO has unique advantages such as:1) exciton energy is as large as 60 meV, which is two times of that of GaN and allows the realization of stable room-temperature stimulated laser; 2) ZnO single-crystal substrate as large as two inches is available and ZnO is amenable to low-temperature growth with a vast zinc resources in nature; 3) ZnO is bio-compatible and high-resist to the radiation. All these advantages promote ZnO as one of the hottest semiconductor in the world. As the problem of asymmetry doping widely exists in the area of wide-bandgap semiconductor, ZnO is suffered from p-type doping. Low resistivity, high hole mobility, high crystal quality and stable ZnO has been rarely reproducibly fabricated. This dissertation focuses on the donor and acceptor behavior in the ZnO film and nanowires. The detailed investigations can be divided into several parts:
1) Because of the excellent crystal-quality of ZnO, we have investigated the donor behavior of Al, In in ZnO nanorods. The donor levels of the Al and In are 75 meV and 102 meV, respectively, which add difficulty to the p-type doping in ZnO. On the other hand, we found that the surface crystal quality of ZnO nanowires grown by MOCVD method is superior to that of nanowires grown by VPT method.
2) Through theory and systematical experiments, we deduce that Na is a good acceptor dopant for ZnO. Utilizing PLD technique and Na doping, relative stable, low-resistivity p-type ZnO has been achieved and p-n homojunction LED has been realized. We pointed out that H is critical for promoting formation of Nazn acceptor, which has an acceptor level of-164 meV.
3) Furthermore, through Na doping, we have realized p-type Zn1-xMgxO (0≤x≤0.25) films and found that an appropriate content of Mg (0.11≤x≤0.15) may enhance the p-type behavior. Alternatively, we have observed UV-blue emission from a p-ZnO:Na/n-ZnO junction containing a Zn0.9Mg0.1O/ZnO multi-quantum wells at room temperature.
4) Considering ZnO films are composed of numerous nanorods, we propose that negative charged adsorptions existed in the crystal grain boundaries degrade the hole mobility. After reducing the barrier height in the grain boundaries by UV illumination, we observed an enhanced p-type behavior or even a transition from n-type to p-type conductivity for ZnO:Na films. As for the p-type ZnO:Na film with a resisitivity of 13.8-19Ωcm, a hole concentration of 4.78×1018-4.66×1018cm-3 and a hole mobility of 0.12-1.42 cm2/V s, UV illumination can improve its p-type behavior, and a best p-type film with a resisitivity of 3.8 Qcm, a hole concentration of 2.09×1017 cm-3 and a hole mobility of 7.91 cm2/V s can be reproducibly obtained.
5) We put emphasis on the growth of single-crystalline acceptor doped ZnO related materials. We have developed a PLD technique to grow highly-oriented phosphorus doped ZnO nanowires. However, we found that the phosphorous doping concentration in a single ZnO nanowire may be not uniform, as indicated by the varying defect density along the nanowire.
6) Finally, regarding the disadvantages and advantages of a single nanowire or film growth technique, we demonstrated a new strategy to grow single-crystalline Na doped p-type ZnO or ZnMgO materials in combination of nano technique and film technique. The p-type behavior of the obtained single-crystalline p-type ZnO materials is stable over 11 weeks.
引文
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