Fisichella Gabriele, Greco Giuseppe, Roccaforte Fabrizio, Giannazzo Filippo
CNR-IMM, Strada VIII, 5, 95121 Catania, Italy.
Nanoscale. 2014 Aug 7;6(15):8671-80. doi: 10.1039/c4nr01150c.
Vertical heterostructures combining two or more graphene (Gr) layers separated by ultra-thin insulating or semiconductor barriers represent very promising systems for next generation electronics devices, due to the combination of high speed operation with wide-range current modulation by a gate bias. They are based on the specific mechanisms of current transport between two-dimensional-electron-gases (2DEGs) in close proximity. In this context, vertical devices formed by Gr and semiconductor heterostructures hosting an "ordinary" 2DEG can be also very interesting. In this work, we investigated the vertical current transport in Gr/Al(0.25)Ga(0.75)N/GaN heterostructures, where Gr is separated from a high density 2DEG by a ∼ 24 nm thick AlGaN barrier layer. The current transport from Gr to the buried 2DEG was characterized at nanoscale using conductive atomic force microscopy (CAFM) and scanning capacitance microscopy (SCM). From these analyses, performed both on Gr/AlGaN/GaN and on AlGaN/GaN reference samples using AFM tips with different metal coatings, the Gr/AlGaN Schottky barrier height ΦB and its lateral uniformity were evaluated, as well as the variation of the carrier densities of graphene (ngr) and AlGaN/GaN 2DEG (ns) as a function of the applied bias. A low Schottky barrier (∼ 0.40 eV) with excellent spatial uniformity was found at the Gr/AlGaN interface, i.e., lower compared to the measured values for metal/AlGaN contacts, which range from ∼ 0.6 to ∼ 1.1 eV depending on the metal workfunction. The electrical behavior of the Gr/AlGaN contact has been explained by Gr interaction with AlGaN donor-like surface states located in close proximity, which are also responsible of high n-type Gr doping (∼ 1.3 × 10(13) cm(-2)). An effective modulation of ns by the Gr Schottky contact was demonstrated by capacitance analysis under reverse bias. From this basic understanding of transport properties in Gr/AlGaN/GaN heterostructures, novel vertical field effect transistor concepts with high operating speed and I(on)/I(off) ratio can be envisaged.
由超薄绝缘或半导体势垒层隔开的两层或多层石墨烯（Gr）组成的垂直异质结构，由于其高速运行与通过栅极偏置进行宽范围电流调制的结合，代表了下一代电子器件非常有前景的系统。它们基于紧密相邻的二维电子气（2DEG）之间电流传输的特定机制。在这种情况下，由Gr和承载“普通”2DEG的半导体异质结构形成的垂直器件也可能非常有趣。在这项工作中，我们研究了Gr/Al(0.25)Ga(0.75)N/GaN异质结构中的垂直电流传输，其中Gr通过约24nm厚的AlGaN势垒层与高密度2DEG隔开。使用导电原子力显微镜（CAFM）和扫描电容显微镜（SCM）在纳米尺度上表征了从Gr到掩埋2DEG的电流传输。通过对使用不同金属涂层的AFM尖端在Gr/AlGaN/GaN和AlGaN/GaN参考样品上进行的这些分析，评估了Gr/AlGaN肖特基势垒高度ΦB及其横向均匀性，以及石墨烯（ngr）和AlGaN/GaN 2DEG（ns）的载流子密度随施加偏置的变化。在Gr/AlGaN界面处发现了具有优异空间均匀性的低肖特基势垒（约0.40eV），即与金属/AlGaN接触的测量值相比更低，金属/AlGaN接触的测量值根据金属功函数在约0.6至约1.1eV范围内。Gr/AlGaN接触的电学行为已通过Gr与紧邻的类施主表面态AlGaN的相互作用得到解释，这些表面态也是高n型Gr掺杂（约1.3×10(13) cm(-2)）的原因。通过反向偏置下的电容分析证明了Gr肖特基接触对ns的有效调制。基于对Gr/AlGaN/GaN异质结构传输特性的这种基本理解，可以设想具有高工作速度和I(on)/I(off)比的新型垂直场效应晶体管概念。
