1.Department of Physics and Electrical Engineering, Tongren University, Tongren 554300, China 2.School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11604246), the Natural Science Foundation of Guizhou Province Education Department, China (Grant No. KY[2017]053), the Natural Science Foundation of Guizhou Province Science and Technology Agency, China (Grant No. [2018]1163), and the Outstanding Young Science and Technology Talents of Guizhou Province, China.
Received Date:23 January 2019
Accepted Date:26 February 2019
Available Online:01 May 2019
Published Online:05 May 2019
Abstract:As is well known, diamond is extensively used in many fields, because of its excellent properties, such as its hardness, high thermal conductivity, high electron and hole mobility, high breakdown field strength and large band gap (5.4 eV). However, its application in semiconductor area needs to be further understood, because it is irreplaceable by conventional semiconductor materials, especially in the extreme working conditions. Furthermore, the preparation of n-type diamond semiconductors is still an unsolved problem. The reason is that an effective donor element has not yet been found. Recently, both the theoretical and experimental studies show that it is difficult to obtain n-type diamond semiconductor with excellent properties by doping single element in the synthetic system. In this paper, diamond single crystals co-doped with B and S are successfully synthesized in FeNiMnCo-C system at a pressure of 6.5 GPa and temperature ranging from 1280 ℃ to 1300 ℃, by using temperature gradient method. The impurity defects in the synthesized diamond single crystals are characterized by Fourier infrared absorption spectra and the results indicate that the corresponding characteristic absorption peaks of B and S are located at 1298 cm–1 and 847 cm–1, respectively. Furthermore, the absorption attributed to B-S group is not detected. The N concentration of the synthesized diamond crystals decreases to 195 ppm, resulting from the incorporation of B and S impurities into the diamond lattices. Additionally, the electrical properties of the typical diamond single crystals are measured in virtue of Hall effects at room temperature. The measurement results display that the electrical conductivity of the diamond doped with B is obviously enhanced, resulting from the involvement of the S when B addition amount is fixed in the synthesis system. Hall mobility of the corresponding diamond crystals increases from 12.5 cm–2·V–1·s–1 to 760.87 cm–2·V–1·s–1. And then, the relative proportion of S and B will determine the p/n properties of the obtained diamond. In order to further study the electrical properties of diamond, first-principles calculations are adopted and the theoretical calculation results show that the impurity elements involved in the obtained diamond can affect the band structures of the synthetic diamond crystals, which is consistent with the experimental result. Keywords:high pressure and high temperature/ co-doped/ diamond/ electrical properties
表2金刚石样品的电学性能参数((a)未添加硼与硫, (b)添加2.0%硫, (c)添加1.2%硼, (d)添加1.2%硼和2.0%硫, (e)添加0.8%硼和2.0 %硫) Table2.Electrical performance parameters of the diamond samples measured at room temperature ((a) without B or S additives, (b) with 2.0 wt.% S additive, (c) with 1.2% B additive, (d) with 1.2% B and 2.0% S additives, (e) with 0.8% B and 2.0% S additives).
为进一步深入解释不同掺杂对金刚石电学性质的影响, 使用第一性原理对相关掺杂进行了理论计算. 由本课题组以前的研究可知[21,22], 实验过程中由于间隙掺杂的形成能较替位掺杂更高, 本文计算过程中的硼、硫均采用替位掺杂方式. 图4(a)为无任何添加剂的金刚石能带结构. 图4(b)给出了硫掺杂比例为2%的金刚石所对应的能带图, 由该图可以看出: 费米面靠近导带底, 证明硫元素单掺杂使金刚石呈现出n型半导体性能; 然而, 由于掺杂所得金刚石禁带宽度较大(为4.439 eV), 电子从价带顶跃迁至导带底实现导电需要外界提供较大的能量, 从而表现出较高的电阻率, 与表2所述实验结果吻合较好. 当硼元素进一步协同掺杂后, 其能带图如图4(c)所示, 即: 1) B的进一步掺杂使费米面出现在价带顶部, 使金刚石呈现p型半导体特性; 2)由于硼硫共掺杂而导致禁带宽度变窄至4.013 eV左右, 较S单掺杂减小约0.4 eV, 使电子更易实现从价带顶向导带底的跃迁, 与表2中霍尔测试结果相吻合; 3)由于B掺杂比例较S少, 从而使晶体中出现富余的电子, 而在禁带中出现大量杂质能级, 进一步减小了金刚石的电阻率; 4)协同掺杂使晶体中富余的电子为未成键电子, 因此其有效质量较大, 不利于金刚石的载流子迁移率的提升; 同时杂质能级分散在费米面两侧, 易实现载流子的复合, 进一步限制了载流子浓度的提升. 当进一步减小B的含量, 其能带结构如图4(d)所示, 其能带表现出与S单掺杂类似的半导体性能, 为典型的n型半导体; 与此同时, 由于S掺杂占主导作用, 因此其杂质能级需要出现在导带底, 材料禁带宽度较S单掺杂时变化不大, 因此获得了和S单掺杂相类似的电阻率及载流子浓度(如表2所列), 与实验吻合较好. 然而, 由于B的引入, 使S杂质能级主要出现在导带底, 同时使导带底部能级出现展开的同时价带顶部能级基本不变, 利于电子的跃迁, 从而电阻率约为S单掺杂的1/3; 由于杂质能级电子未成键, 由能带曲线曲率可以看出, 其有效质量较大, 不利于迁移率的提升, 其迁移率较S单掺杂小, 与实验结果中的数值相符. 图 4 金刚石能带结构 (a)未添加硼与硫; (b)添加2.0%硫; (c)添加1.2%硼和2.0%硫; (d)添加0.8%硼和2.0%硫 Figure4. Band structures of the synthesized diamond: (a) Without B or S additive; (b) with 2.0% S additive; (c) with 1.2% B and 2.0% S additives; (d) with 0.8% B and 2.0% S additives.