Low-temperature and global epitaxy of GaN on amorphous glass substrates by molecular beam epitaxy via a compound buffer layer
Introduction
III-group nitride semiconductor epilayers, e.g. GaN and InGaN, have been widely applied to light emitting diodes (LED) [[1], [2], [3], [4], [5]]. However, most of GaN-based LEDs are grown on single-crystal substrates, such as c-plane sapphire, Si (111) and 6H-SiC [[6], [7], [8], [9], [10], [11], [12]]. It is difficult to realize directly flat panel light sources on these single-crystal substrates as they have relatively high costs and small sizes [13]. The amorphous substrates, such as glass, have large sizes but lack of epitaxial template which provides two-dimensional periodic lattices to match GaN-based epitaxial layers [14]. Hence, a buffer layer, which has the similar epitaxial relationship with III-group nitrides, is necessary for the epitaxy of III-group nitrides on amorphous substrates [15]. Ti film is an appropriate candidate of the buffer layer as hexagonal c-oriented (002) Ti has the same lattice structure as the wurtzite GaN and the theoretical lattice mismatch between them is 7.4%. J.H. Choi et al. fabricated nearly single-crystalline GaN LEDs on glass substrates deposited with a c-oriented Ti film successfully [[16], [17], [18]]. However, the complicated selective area epitaxy including the pattern of micro-holes and the formation of lateral pyramids array was involved as the deposited Ti film had randomly-oriented a-axe in the in-plane direction. Furthermore, the high temperature up to 1040 °C was needed during the epitaxy. Therefore, it is impossible to apply those processes to the float glass substrates as their melting point is generally no more than 600 °C.
J.W. Shon et al. fabricated full-color InGaN-based LED on amorphous substrates by pulsed sputtering via multilayer graphene buffer layer. However, the multilayer graphene layers were grown by chemical vapor deposition on Ni foil and had to be transferred onto amorphous glass substrates, and the growth temperature is up to 760 °C, which is also higher than the melting point of the float glass [19]. In our previous study, c-oriented (002) Ti film was obtained on glass substrates by electron beam (EB) evaporation as a pre-orienting layer (OL) and GaN globally grown on Ti/glass template by plasma-assisted molecular beam epitaxy (PA-MBE) at 530 °C exhibits the same epitaxial relationship to Ti OL [14]. But the resulting GaN is an assembly of randomly a-axes oriented grains, i.e., poly-crystalline film, as the a-axes orientation of Ti film is random. An interlayer which enables the global epitaxy of III-nitrides is necessary to further improve the performance of III-nitrides grown on amorphous glass substrates no more than 600 °C. In this paper, the growth of GaN on amorphous glass substrates via an AlN/Ti compound buffer layer (BL) by PA-MBE at the temperatures of 530 °C is studied, since there are many kinds of cheap non-single-crystalline substrates can bear this temperature. The crystal quality will deteriorate or the non-single-crystalline substrates is at risk of softening (or melting) when the growth temperature is too low or too high. The growth of AlN is optimized to form a single crystalline epitaxial template on a c-oriented (002) Ti film. Then the bulk GaN layer is grown on AlN template and its crystallinity is studied.
Section snippets
Experimental details
All the samples were deposited and grown on amorphous glass substrates by an EB and a PA-MBE system. Firstly, a 300-nm-thick Ti film was deposited on 2-in. diameter glass substrates by EB evaporation. Ti film is (002) c-oriented and its surface has a root mean square (RMS) roughness of ~ 1.5 nm in a scanning area of 10 × 10 μm2 [14]. AlN film was used to act as a NL to form a compound BL, because AlN can suppress the formation of cubic phase at low temperature [20, 21] and the lattice mismatch
Results and discussion
The AlN NL was characterized only by in-situ RHEED installed our PA-MBE as AlN and GaN epilayers were grown in succession during one epitaxy. Fig. 2(a) shows the RHEED patterns of AlN grown at different N2 flux. The RHEED patterns for AlN grown at N2 flux of 1.0 and 1.5 sccm are spotty rings, which indicates the polycrystalline AlN films. With the increasing of N2 flux (such as 2.0 sccm), there are more and stronger diffraction spots, which means more highly-preferred orientation and better
Conclusion
In conclusion, GaN epilayer was grown on amorphous glass substrates via the compound buffer structure consisting of Ti OL and AlN NL by PA-MBE at the temperatures lower than the softening point of the float glass. The growth of AlN NL by PA-MBE has been studied to obtain the smooth and single-crystalline GaN epilayers. It is found that the ratio of V/III source play a key role in the crystallinity and morphology of AlN NL as a trade-off of the formation of Al cluster and the mobility of Al
Acknowledgements
This work is supported by the National Key Research and Development Program (Grant No. 2017YFA0205800, 2016YFB0401803), National Basic Research Program of China (Grant No. 2015CB351900, 2013CB632804), the National Natural Science Foundation of China (Grant Nos. 61574082, 51561165012, 51561145005, 61210014, 61321004, and 61307024), the High Technology Research and Development Program of China (Grant No. 2015AA017101), the Science and Technology Planning Project of Guangdong Province (Grant No.
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