| Gallium nitride(GaN)has been widely used in optoelectronics,high-frequency power electronic devices and microwave devices due to its excellent properties such as wide band gap(～3.4 eV),high field breakdown,high saturation electron drift rate and so on.These mature applications mainly use metal-organic chemical vapor deposition(MOCVD),molecular beam epitaxy(MBE)and hydride vapor phase epitaxy(HVPE)to grow GaN.The growth technologies require a high growth temperature(>700℃).At present,the development of GaN has reached a new stage.We should not only promote the application of GaN,but also explore new applications and solve new problems.In recent years,great progress has been made in the fabrication of GaN by plasma enhanced atomic layer deposition(PE-ALD)at low temperature(≤350℃),which provides a new opportunity for the low-temperature fabrication of GaN and GaN-based devices.Graphene is a kind of two-dimensional material with excellent physical and chemical properties.It is transferred to various substrates as a template to play a supporting role which has drawn great attention.At home and abroad,many studies have reported on the high-temperature growth of high-quality single-crystalline GaN on graphene.However,there are few researches on low-temperature growth of GaN on graphene and related physical mechanism.This paper systematically studies the possible role of graphene in the growth of high-quality GaN via PE-ALD.In addition,the possible related physical mechanism was furthur studied.Major progress has been made in finding the over-functionalization of graphene in the depsition of GaN,demonstrating how to obtain nearly single-crystalline GaN and revealing the physical mechanism of using graphene to improve the quality of GaN grown on flexible substrates.The detailed results are shown as follows:(1)It is found that the graphene defects are overactivated after the Ar/N2/H2 mixed plasma treatment.The growth of three cycles aluminium oxide(Al2O3)via thermal atomic layer deposition(T-ALD)is used to suppress the problem,and then GaN films with preferred orientation are prepared.Firstly,hexagon polycrystalline GaN was directly deposited on graphene/SiO2 substrate by PE-ALD,which proved the feasibility of GaN growth on graphene by PE-ALD.Subsequently,the optimal growth parameters of PE-ALD GaN deposited on graphene were explored,and it was found that both growth temperature and plasma interaction time can affect the crystallization quality of GaN.Because PE-ALD is a low-temperature growth technology,we focused on the effect of Ar/N2/H2 mixed plasma treatment on graphene.And PE-ALD GaN with high crystalline quality was grown on functionalized graphene.However,the plasma treatment will lead to excessive activation of graphene defects.To solve the problem,three cycles T-ALD Al2O3 was used to repair the surface defects of graphene.This allows the growth of GaN with extremely preferred orientation while avoiding excessive activation of graphene defects.(2)It is found that the substrate under graphene and crystallization quality of graphene will both affect the quality of PE-ALD GaN.Based on the experimental conclusion,nearly single-crystalline GaN is obtained.Firstly,PE-ALD GaN thin films were grown on graphene/sapphire and graphene/SiO2 substrates under the same growth conditions for comparation.The crystalline quality of PE-ALD GaN grown on graphene/sapphire substrate was better.Based on the experimental results,the crystalline quality of GaN films grown on sapphire,graphene/sapphire and large domian graphene/sapphire under the same growth conditions was compared.The results show that graphene insertion layer can affect the crystal orientation of PE-ALD GaN films.And the influence is more obvious when using high crystal quality graphene insertion layer.Accoding to the finding,the direct growth of c-axis-oriented nearly single-crystalline GaN was performed at 300℃on large-domain graphene.The X-ray rocking curve taken on the(002)diffraction plane of the 27.3 nm thick GaN displayed a FWHM of 466 arcsec.(3)Polycrystalline GaN films with low impurity content were prepared using graphene/Al2O3/Kapton structure.It is found that plasma can cause surface damage to plastic substrates.And the diffusion behavior of GaN in Kapton was found when the growth temperature exceeds 250℃.The GaN diffusion problem was solved through the insertion of Al2O3.However,due to the lack of epitaxial interaction between the amorphous Al2O3 and GaN to be deposited,the GaN deposited on Al2O3 is amorphous.Therefore,graphene was introduced as a growth buffer layer to improve the crystallization quality of GaN which leads to the obtaining of polycrystalline GaN.However,the improvement of the crystal quality of GaN is limited by the high oxygen content during the early growth stage.The problem was solved by adding a "TMA clean up" process which results in the fixing of most of the oxygen in the chamber onto the surface of functionalized graphene and plastic substrate.And the growth of flexible polycrystalline GaN with(002)preferred orientation and low impurity content on plastic substrate was achieved.(4)On the premise of avoiding graphene activation,we tried to activate ultra-thin Al2O3 deposited by T-ALD.We found that high quality GaN was also obtained when using the composite substrate and the method.And the related physical mechanism was analyzed.Graphene is an ideal material to act as electrode or heat dissipation layer.Therefore,how to avoid the damage of graphene during the growth of PE-ALD GaN was studied.It was found that the use of ultra-thin Al2O3 layer and vacuum annealing can avoid the degradation of graphene properties which was caused by the growth of PE-ALD GaN.The related physical mechanism is also studied.Based on the results,the surface of T-ALD Al2O3 was modified by in-situ plasma treatment which leads to the increase of active sites on the surface of T-ALD Al2O3.The preparation of high-quality GaN on ultra-thin Al2O3/graphene structure was achieved. |
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