可转移单晶AlN纳米膜的合成及特性

Single-crystalline inorganic semiconductor nanomembranes (NMs), which  have a thickness of less than a few hundred nanometers, have gained increasing  interest since 2005 due to their remarkable use for a variety of applications.  These free-standing NMs can be transferred to any arbitrary foreign substrates  by using elastomeric stamps, which was  first demonstrated by Meitl et al.Importantly, these NMs are compatible with the  traditional semiconductor manufacture  approach for fabricating various heterogeneously integrated and flexible electronic devices, including the seamless integration of semiconductor NMs with the  low-cost, complementary-metal-oxidesemiconductor-compatible silicon (Si)  substrates. Until now, several applications  of transferrable monocrystalline NMs  based on Si,germanium (Ge),gallium arsenide (GaAs)/aluminum gallium arsenide (AlGaAs),gallium nitride (GaN)/aluminum  gallium nitride (AlGaN),[27–30]  silicon carbide (SiC), gallium  oxide (Ga2O3), etc. have been reported during the last few  years. These transferrable narrow, wide, and ultrawide bandgap  NMs have enabled a variety of device applications over the past  nearly two decades.

Initially, the AlN layer was grown using MBE and further, it  was successfully transferred onto a flat sapphire (Figure S1,  Supporting Information), a flat Si (Figures 2a,b and 3a), and a  hexagon pillar-structured Si (Figure 1a–c) substrates. Figure 1a  exhibits the optical image of the transferred AlN NMs on the  pillar-structured Si substrate. The pillar-structured Si substrate  was prepared via photolithography and reactive ion etching  (RIE) and the height of the pillars is  about 4  µm. The pillars  can be seen through the AlN NM due to the high optical transparency of AlN. Figure  1b,c presents the tilt-view scanning  electron microscope (SEM) images of transferred AlN NMs on  the pillar-structured Si substrate. These SEM images clearly  confirm the uniformity of the NMs over the whole substrate  without formation of cracks. The high-resolution SEM image  (Figure 1c) confirms the smoothness of the NMs edges, which  are highly desired from the device fabrication point of view. It  is noted from Figure 1c that the AlN NM has sufficient rigidity  even with a thickness of only 30  nm (Figure 2a). The schematics of as-grown AlN and transferred AlN on flat Si substrate  have been illustrated in Figure 1d.

A few studies on transferring AlN thin films to flexible adhesive substrates have been reported recently. However, there  exist three issues: 1) the AlN crystalline quality, e.g., grain size,  crystal orientation, in the above-mentioned studies is rather low  due to the growing methods, which are limited by the growth  approaches; 2) the adhesive handling materials during the  transfer can leave residues on the AlN film; 3) only adhesive  flexible substrates have been demonstrated as the final host  substrate, which eliminate the potential of applications such as  AlN-based vertical devices.Hence, it is desirable to overcome  the above issues and achieve a high-quality transferrable singlecrystalline AlN NM that can be transferred to a broader range of  substrates.

Fig1d

It is well-known that a smooth interface is highly desirable  to fabricate the high performing electronic devices. Therefore,  the surface roughness of the AlN NMs has been measured  from both sides by AFM. Figure 1h,i depicts the morphological  images of AlN epi sample (Al-polar surface) and transferred  AlN NMs (N-polar surface), respectively. It is interesting to note  that the AlN NMs from both the polarities show a smooth surface having root mean square (RMS) of 0.585 and 0.578  nm.  After transferring the AlN NMs (N-polar surface), the RMS is  found to be comparable to that of Al-polar surface even with  plasma etching process of Al layer.

The intrinsic polarization in III-nitride semiconductors plays  an important role to fabricate the high electron mobility or high  hole mobility devices. Therefore, the piezoelectric response of  the transferred AlN (0001) NMs on the flat Si (001) substrate,  according to the conductivity requirement for the substrate,  was confirmed using PFM.