夏建白院士：超宽禁带半导体高效p型掺杂

超宽禁带半导体高效P型掺杂

超宽带半导体（UWBG）具有直接带隙可调、击穿电压高、化学稳定性好和热稳定性好等优点。然而大多数UWBG半导体都存在严重的掺杂不对称问题，即它们可以很容易地掺杂p型或n型，但不能同时实现两种掺杂。影响掺杂极限的主要因素有三个：（1）高生成焓导致掺杂剂的溶解度有限；(2）掺杂能级深导致的高激活能；以及（3）缺陷或复合物的存在产生严重的自补偿。随着材料生长技术的迅速发展，低溶解度和自补偿问题得到了很大的改善。然而，高激活能问题是由宿主材料和掺杂剂的自身物理性质决定的，如何降低激活能仍然是重大挑战。

中国科学院半导体所夏建白院士在最新出版的《半导体学报》2021年第6期上发表题为《Efficient p-type doping in ultra-wide band-gap nitrides using non-equilibrium doping method》的评论短文。中国科学院长春光学精密机械与物理研究所黎大兵研究员课题组和中国科学院半导体所邓惠雄研究员课题组合作，提出了一种基于量子工程的非平衡掺杂方法，并成功应用于高Al组分AlxGa1-xN的p型掺杂。首先，通过第一性原理分析，揭示了该方法的物理过机制，然后对这一掺杂方案进行了系统实验研究和器件验证。他们通过非平衡掺杂技术将GaN量子点（QDs）嵌入Mg掺杂的AlxGa1-xN合金中，引入了局域高能级VBM。由于GaN量子点与AlxGa1-xN之间存在VBM带阶，激发电子从GaN的VBM跃迁到受主能级，激活能明显降低。本研究不仅可以提高超宽带氮化物光电器件的量子效率，而且为实现超宽带材料的高效掺杂提供了新的解决方案。

专刊

#Full Text

Al-rich nitride, as one of the most important ultra-wide band-gap (UWBG) semiconductors, currently plays the key role of deep ultraviolet (DUV) optoelectronics and potentially possesses the advantages of the huge global investment in the manufacturing infrastructure associated with InGaN material that has become the second most important semiconductor material after Si in the late 2010s[1, 2]. However, the p-doping of Al-rich nitrides has long been blocking the improvement of quantum efficiency of DUV optoelectronics. The activation energy (Ea) of the most-frequently used acceptor dopant Mg increases from 200 meV in GaN to as high as 630 meV in AlN[3-5]. Once the p-doping problem of Al-rich nitrides is solved, the DUV or even the high-frequency and high-power industries probably usher in an era of rapid development based on the existing manufacturing infrastructure.

Actually, most wide band-gap (WBG) semiconductors usually experience an asymmetry doping problem, i.e., they can only be easily doped n-type or p-type, while not both, which is because that they either have a low valance band maximum (VBM) or a high conduction band minimum (CBM), resulting extremely high acceptor or donor Ea[6, 7]. And for UWBG semiconductors, the problem gets even worse. For example, the Ea of N-doped or P-doped n-type diamond is generally higher than 0.5 eV and that of Mg-doped or N-doped p-type β-Ga2O3 is even higher than 1 eV[8-10]. These doping asymmetry problems have seriously hindered the potential applications of many WBG materials.

In the past decades, great efforts have been devoted to theoretically overcome the high Ea problem in WBG semiconductors. In these investigations, researchers tried their best to develop novel approaches to tune the dopant level. For n-type doping, to lift the impurity level up close to the CBM of the host, while for p-type doping, to lower the impurity level down close to the VBM of host. The generally used dopant delta-doping and co-doping in WBG semiconductors are all based on the principle[11, 12]. Later, Yanfa Yan et al. proposed an approach to solve the asymmetry doping problem by introducing impurity band below the CBM or above the VBM via passive donor–acceptor complexes or isovalent impurities and effectively doping the passivated impurity band, which essentially changed the band edge[13]. Clas Persson et al. found the S dopants in ZnO would form local ZnS like bonds in the ZnO host and could result in a strong VB offset bowing, making the p-doping of ZnO enhanced[14]. From this point of view, it is possible to reduce the Ea by tuning the band edge. However, it is difficult and uncontrollable to form such impurity band or local bonds as the authors declared.

Recently, Prof. Dabing Li’s group in Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences (CAS), cooperating with Prof. Hui-Xiong Deng in Institute of Semiconductors, CAS, reported an interesting work in p-doping of Al-rich nitrides[15]. They proposed a non-equilibrium doping model to achieve low acceptor Ea in Al-rich nitrides, in which GaN quantum dots (QDs) are buried in AlN host to lift the VBM up, the Mg dopants are doped at the AlN host, and the Mg dopants are concentrated near the interface between GaN QDs and AlN host, as shown in Figs. 1(a)–1(c). In their model, Mg acceptor Ea below 0.1 eV is achieved according to the first principles calculations. Based on the model, p-type Al-rich AlGaN materials with Al contents of 50%–70% are experimentally realized. The hole concentrations reach the magnitude of 1018 cm–3 at room temperature and the measured acceptor Ea is several tens of meV as expected. The turn-on voltage of the DUV light-emission diode (LED) based on the non-equilibrium doping method is reduced compared to that based on the uniform doping method, as shown in Fig. 1(d). It is an exciting result for Al-rich nitrides.

Figure 1. (Color online) Non-equilibrium doping method to lower the acceptor Ea in UWBG nitride semiconductors and its application in DUV-LED. Acceptors are randomly doped in (a) AlN and (c) GaN. Both have high Ea in this condition. (b) GaN-QDs are embedded in AlN host and acceptors are doped in AlN host and concentrate near the interface. (d) Current–voltage curves of the devices. Device A uses the non-equilibrium doping method and Device B uses the uniform doping method. The insets are the device structure diagram and the cross-sectional scanning transmission electronic microscopy for the active region of the devices. Cited from Ref. [15].

This work is an important progress in WBG semiconductor doping. It has strongly developed the non-equilibrium doping process that to lower the dopant Ea by tuning the band edge of the host. Besides, it has found a good method to bury narrow band-gap QDs in their wide band-gap congener host, which will not significantly affect the optical properties of the host and is feasible in many element and compound semiconductors. It seems more controllable and designable compared to the formation of impurity band or local bonds. Moreover, it also demonstrates that not only the dopants formed based on non-equilibrium techniques like the dopant delta-doping, but also controlled growth of host materials based on non-equilibrium technology can power up the doping efficiency of WBG semiconductors. Therefore, this work has developed the doping conception that to lower the Ea of WBG semiconductors by tuning the band edge using non-equilibrium doping method. The follow-up studies should be carried out soon, breaking new frontiers in the doping of WBG and UWBG semiconductors.

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[15] Jiang K, Sun X J, Shi Z M, et al. Quantum engineering of non-equilibrium efficient p-doping in ultra-wide band-gap nitrides. Light: Sci Appl, 2021, 10, 1

夏建白院士文章：

Efficient p-type doping in ultra-wide band-gap nitrides using non-equilibrium doping method

Jianbai Xia

J. Semicond. 2021, 42(6): 060402

doi: 10.1088/1674-4926/42/6/060402