【作者】 唐鑫；

【导师】 张庆瑜；

【作者基本信息】 大连理工大学 ， 材料物理与化学， 2008， 博士

【摘要】 ZnO属于Ⅱ-Ⅵ族宽带半导体，其禁带宽度为3．37eV，激子束缚能为60meV，在短波光电器件上具有很大的潜力，使得ZnO成为当今半导体材料研究领域关注的热点。在ZnO光电特性研究中，制备结型器件是ZnO薄膜实用化的关键。实现ZnO基结型器件需要满足两个基本的条件：ZnO可靠p型掺杂的实现以及通过掺杂有效地调节ZnO的能带。本文采用基于密度泛函的第一性原理方法，讨论了涉及能带工程的Cd、Be和Mg掺杂；以及涉及p型导电的Cu、Ag以及Li-N和Be-N共掺问题。通过计算这几种常见掺杂体系的晶体结构和电子结构，得到的主要研究结果如下：1．通过对掺杂前后电子能带结构、态密度以及分态密度的计算和比较，发现Cd_xZn_1-xO价带顶端（VBM，conduction band minimum）始终由O-2p占据；而导带顶端（CBM，conduction band minimum）则由Cd-5s与Zn-4s杂化轨道控制。随着掺杂浓度的增加，带隙宽度的变窄由CBM位置下降和VBM位置上升共同决定，是产生荧光光谱红移的主要原因。此外，我们发现，Cd掺杂引起晶胞发生膨胀也是导致Cd_xZn_1-xO禁带宽度变小的原因之一，晶格膨胀对带隙减小的贡献约为20％-30％。通过研究Cd原子对ZnO中本征缺陷形成能及离化能的影响，我们发现，当Cd原子与V_O（O空位）靠近时，可以显著降低V_O的形成能，导致Cd_Zn-V_O复合体(其中Cd_Zn为Cd原子替位原子)的形成，但对其它本征缺陷的形成能影响不大。然而，计算结果显示Cd_Zn-V_O复合体的离化能较高，与V_O接近，不太可能是n型载流子的起源。因此，我们进一步研究了更为复杂的复合体结果，如Zn_i-Cd_Zn-V_O（其中Zn_i为Zn填隙），并由此推论n型载流子的起源可能与Zn_i-Cd_Zn-V_O类似的复合体结构有关，如Zn_i-2Cd_Zn-2V_O和Zn_i-3Cd_Zn-3V_O等。2．利用第一性原理计算，对Be掺杂及Mg掺杂ZnO的带结构，DOS和轨道能级进行了系统研究。结果显示，由于Be和Mg没有d电子，使得掺杂后合金的p-d排斥减弱，从而使得VBM下降。同时由于Be和Mg的最外层s点的轨道能量较高，有效地提升了合金中的CBM。VBM与CBM的共同作用使得掺杂后的ZnO带隙展宽。此外，我们研究了Be掺杂导致的合金压应变对带隙展宽的贡献，发现晶格缩小对带隙展宽的影响约为20％-30％。掺杂合金形成焓的结果计算结果显示，显示由于Mg与Zn的离子半径差别较小，Mg掺杂ZnO比Be掺杂更稳定。3．对纤锌矿结构Cu_xZn_1-xO和Ag_xZn_1-xO的晶体结构和电子结构进行了第一性原理计算。与具有相似离子半径的Mg和Cd掺杂相比，Cu和Ag掺杂的形成焓高于Mg和Cd掺杂，合金稳定性下降。电子结构的计算结果显示，Cu和Ag掺杂导致ZnO带隙的减小。通过分析Cu和Ag掺杂ZnO的电子结构，我们发现Cu和Ag掺杂属于受主掺杂，其杂质能级位于VBM附近，重掺杂使得Cu-3d和Ag-4d电子的杂质能级展宽，并与O-2p电子占据的VBM相连接是导致ZnO带隙减小的主要贡献。因此，Cu，Ag掺杂引起的ZnO带隙减小主要发生在掺杂浓度足以使得杂质能带与VBM相连之后；一旦杂质能带与VBM相连，掺杂浓度增加对带隙的影响不大，此时，Cu-4s和Ag-5s电子对CBM的影响是带隙变化的重要原因。4．通过计算Li，N共掺和Be，N共掺ZnO中缺陷的形成能、离化能以及结合能，发现对于Li，N共掺，在O充足的情况下，Li_Zn（Li替位Zn）是产生p型的原因，而在Zn充足的情况下，Li_i-Li_Zn-N_O复合体（其中Li_i为Li填隙，N_O为N替位O）有着相对较低的形成能和离化能，以及最低的结合能，因此可能成为这种情况下p型的起源。对于Be，N共掺ZnO，讨论了nBe_Zn-N_O及Be_i-nN_O两种类型的缺陷(其中Be_Zn为Be替位Zn，Be_i为Be填隙)。结果显示，在O充足的情况下，4Be_Zn-N_O是p型导电性的起源，而在Zn充足的情况下，除了4Be_Zn-N_O之外，Be_i-3N_O也可能是对p型导电性产生贡献。5．通过第一性原理赝势计算对HX（Hexagonal）以及WZ（Wurtzite）ZnO的比较研究，讨论了两相在临界相变压时电子结构以及光学属性包括介电函数，折射率和吸收系数。结果显示WZ转变为HX时，能带从直接带隙变为了间接带隙，而且带隙宽度也相应的减小。HX的介电函数与WZ存在一定的不同，特别是在E／／c方向。相似的情况也出现在折射率以及吸收系数上。这些差异可以归结为HX结构在c轴上比WZ多一个键。此外对HX的电子结构以及光学属性也进行了讨论。更多还原

【Abstract】 ZnO is a direct wide-band gap （3.37 eV） II-IV compound semiconductor with large exciton binding energy （60 meV） at room temperature. More attention has been paid to ZnO-based films due to their potential applications in optoelectronic devices in blue and ultraviolet spectra of light. A crucial step in designing modern optoelectronic device is the realization of band-gap engineering to create barrier layers and quantum wells in device heterostructures. In order to realize such optoelectronic devices, two important requirements should be satisfied: one is p-type doping of ZnO and the other is modulation of the band gap. This work is focus on the stability and electronic structure of the ZnO-based materials. Using first principle calculation, Cd, Be and Mg-doped ZnO are studied for band gap engineering. For p type doping, Cu, Ag-doped and Li-N, Be-N co-doped ZnO are studied. The results are summarized as follow:1. Analysis of the band structures, density of states （DOS） and partial density of states （PDOS） of Cd_xZn_1-xO shows that the valence band maximum （VBM） is determined by O-2p states and the conduction band minimum （CBM） is occupied by the hybrid Cd-5s and Zn-4s orbital. With increasing Cd-doping concentrations, the energies of CBM decrease and the energies of VBM increase, hence leading to narrowing of the band gap. Furthermore, we find that the reduction of band gap due to Cd-doping can be attributed to the tensile strain, which is about 20-30% in the reduction of the band gap. We also study the interaction between Cd and native defects. It is found that the Cd incorporation can lower the formation energy of V_o defect and results in the production of Cd_Zn-V_o complex when the Cd dopant is close to V_o. The high transition level, however, suggests that the Cd_Zn-V_o complex cannot be the source resulting in the increase of the concentration of n-type carriers due to Cd doping. We study a more complicated structure Zn_i-Cd_Zn-V_o and suggest that the increase of n-type carriers caused by Cd doping might result from a structure similar to Zn_i-Cd_Zn-V_o, such as Zn_i-2Cd_Zn-2V_o and Zn_i-3Cd_Zn-3V_o.2. The band structure, DOS, and orbital energy level of Be-doped and Mg-doped ZnO are investigated. The results show that the absence of d electrons in Be and Mg can weaken p-d repulsion, hence resulting in the decrease of VBM in the doped ZnO. The energy levels of CBM are found increasing in the doped ZnO. The increase can be attributed to the higher orbital energy levels of s electrons in Be and Mg atoms. In addition, Be-doping can cause compressive strain, causing the increase of band gap about 20%-30%. The formation enthalpy of alloy is also discussed. Mg-doped ZnO is more stable than Be-doped ZnO due to the little difference in radii between Mg and Zn ions.3. The electronic structures of the Cu_xZn_1-xO and Ag_xZn_1-xO wurtize alloys are calculated. Although Cu and Ag have the similar ion radius with Mg and Cd, respectively, the formation enthalpy of Cu-doped and Ag-doped ZnO is found to be higher than Mg-doped and Cd-doped ZnO. The results of electronic structures show that Cu-doped and Ag-doped ZnO have narrower band gap than ZnO, which can be attributed to the extension of impurity levels induced by Cu-3d and Ag-4d. Because the impurity levels is close to the VBM of ZnO, heavy doping results in the impurity levels extending to the VBM and merging into one band. Therefore, the reduction of band gap appears sharply at a concentration of Cu or Ag dopant. Thereafter, the band gap varies slowly, which can be attributed to the influence of Cu-4s or Ag-5s electrons on the CBM energy.4. Formation energy, transition energy and binding energy of defects in Li, N and Be, N co-doped p-type ZnO are calculated. For Li, N co-doped ZnO, it is found that Li_Zn is the cause of p-type conductivity in O-rich condition. The p-type conductivity is dominated by the Li_i-Li_Zn-No in Zn-rich condition because it has relatively small formation energy and transition energy and lowest binding energy. For Be, N co-doped ZnO, nBe_Zn-N_o and Be_i-nN_o are discussed. In O-rich condition, acceptor 4Be_Zn-N_o may be the source of p-type conductivity. In Zn-rich condition, both of 4Be_Zn-N_o and Be_i-5N_o are found to be the most possible candidates to provide the p-type conductivity.5. First-principles ultrasoft pseudopotential method is applied to study HX ZnO, which has a novel graphitelike hexagonal structure transformed from wurtzite phase under tensilestress along [01 （?） 0] direction or compressive stress along [0001] direction. The electronic structure and optical properties, including dielectric function, reflectivity and absorption coefficient, of HX ZnO are calculated and compared with those of WZ ZnO under the given uniaxial stress. It is found that HX ZnO is an indirect semiconductor, being different from WZ ZnO. HX ZnO has a dielectric response different from WZ ZnO at ambient conditions or under the given uniaxial stress, especially in the case of E//c. Similar variation is also observed in the reflectivity and absorption coefficient. The variation in the optical properties is attributed to the additional Zn-0 bond along c-axis HX ZnO. In addition, the electronic structure and optical properties of LY ZnO are reported.更多还原