Surface Carbonization of GaN and the Related Structure Evolution during the Annealing Process

Jin-Long Liu(刘金龙)$^{1}$, Liang-Xian Chen(陈良贤)$^{1}$, Jun-Jun Wei(魏俊俊)$^{1}$, Li-Fu Hei(黑立富)$^{1}$,\\ Xu Zhang(张旭)$^{2}$, Cheng-Ming Li(李成明)$^{1\hyperlink{s}{**}}$

doi:10.1088/0256-307X/35/1/016101

⇦Chinese Physics Letters, 2018, Vol. 35, No. 1, Article code 016101⇨ Surface Carbonization of GaN and the Related Structure Evolution during the Annealing Process * Jin-Long Liu(刘金龙)1, Liang-Xian Chen(陈良贤)1, Jun-Jun Wei(魏俊俊)1, Li-Fu Hei(黑立富)1, Xu Zhang(张旭)2, Cheng-Ming Li(李成明)1** Affiliations 1Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083 2Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing Radiation Center, Beijing 100875 Received 21 September 2017 ^*Supported by the National Natural Science Foundation of China under Grant No 51402013, the National Key Research and Development Program of China under Grant No 2016YFE0133200, and the European Union's Horizon 2020 Research and Innovation Staff Exchange Scheme under Grant No 734578.
^**Corresponding author. Email: chengmli@mater.ustb.edu.cn Citation Text: Liu J L, Chen L X, Wei J J, Hei L F and Zhang X et al 2018 Chin. Phys. Lett. 35 016101 Abstract To explain the stabilization mechanism of the carbon-ion-implanted GaN under the diamond growth environment, the luminescence characteristics and structure evolution correlative with sites' carbon atoms located for high-fluence carbon-ion-implanted GaN are discussed. GaN is implanted with carbon ion using fluence of $2\times10^{17}$ cm$^{-2 }$ and energy of 45 keV. Then the implanted samples are annealed at 800$^{\circ}\!$C for 20 min and 1 h under the N$_{2}$ atmosphere. The luminescence characteristics of carbon-ion-implanted GaN are evaluated by photoluminescence spectrum at wavelength 325 nm. The lattice damage of GaN is characterized by Raman spectrum and the corresponding vacancy-defect evolution before and after annealing is measured by slow positron annihilation. The results show that most of the carbon atoms will be located at the interstitial sites after carbon ion implantation due to the weak mobility. As the implanted samples are annealed, strong yellow luminescence is emitted and the vacancies for Ga (V$_{\rm Ga})$ are reduced resulting from the migration of interstitial carbon (C$_{\rm i})$ and formation of complexes (C$_{\rm Ga}$ and/or C$_{\rm Ga}$-C$_{\rm i}$) between them. As the annealing time is prolonged, the carbon ions accommodated by the vacancies are saturated, vacancy clusters with carbon atoms appear and the concentration of C$_{\rm Ga}$ diminishes, which will have an adverse effect on the diamond film nucleation and growth. DOI:10.1088/0256-307X/35/1/016101 PACS:61.66.Fn, 61.72.jd, 61.72.uj, 61.80.Jh © 2018 Chinese Physics Society Article Text GaN is considered as one of the most promising broadband gap semiconductor materials. Since the 1990s, with the development of the buffer layer process and breakthrough of p-type doping technology, GaN has been widely used as blue and green light-emitting devices, as well as ultraviolet detectors and high-power devices.[1-7] Recently the development of GaN-based devices has been restricted by the heat dissipation problem due to its low thermal conductivity. It was reported that GaN combined with diamond film could be used as the heat sink.[8,9] This composite could enhance the performance of GaN-based power devices dramatically. Due to the large mismatch stress between GaN and diamond and the instability of GaN in the diamond growth environment, generally it is hard to combine both of them directly. Some approaches have been explored to integrate diamond with GaN through some interlayers such as AlN and Si$_{3}$N$_{4}$ dielectric adhesive layers.[10,11] However, the thick interlayer may become a thermal barrier to impede the heat dissipation. It has been found that the diamond film could be deposited on the high-concentration carbon-ion-implanted GaN by dc arc jet CVD due to the C element transition between GaN and diamond film with no obvious interface and weak mismatch stress between GaN and diamond films.[12] Similar design has also been carried out for Al$_{2}$O$_{3}$ substrates.[13] This means that the carbon-implanted GaN could be more stable in the diamond growth environment and would be preferential nucleation and growth sites for diamond films. Carbon is a common impurity in GaN and it is usually used as the dopant in the electronic[14,15] and optical applications.[16,17] Although the fluorescence emission of the carbon-doped GaN was studied previously, the structure evolution has not been clarified, especially for the high-concentration carbon atoms implantation. In this work, the role of carbon ion implantation on the stability of GaN under the diamond deposition environment were studied by investigating the lattice structure evolution of carbon-ion-implanted GaN. The GaN/sapphire composite with GaN thickness of about 25 μm was prepared by metal organic chemical vapor deposition (MOCVD). Then the samples were implanted with carbon ion with fluence of 2$\times$10$^{17}$ cm$^{-2}$ and energy of 45 keV using the metal vapor vacuum arc (MEVVA) ion implantation technique. After that, the sample was annealed at 800$^{\circ}\!$C for 20 min and 1 h under the N$_{2}$ atmosphere. The luminescence characteristics of carbon-ion-implanted GaN was evaluated by photoluminescence (PL) spectrum with laser wavelength of 325 nm. The Raman spectrometer with laser wavelength of 532 nm was used to test the micro-zone structure of GaN samples after implantation. Defect evolution of carbon-ion-implanted GaN layers after annealing was investigated using slow positron annihilation spectroscopy, which is considered one of the most well-established techniques to characterize the vacancy defects.[18,19] This study was carried out on the slow positron beam research platform in Beijing, with 22 Na source as a positron source, and the slow positron energy incident on the sample was continuously adjustable in the range of 0.18–20 keV. Doppler broadening spectra were obtained by detecting the gamma photons produced by the positron annihilation, using the high-purity germanium detectors, and the low and high annihilation momentum parameters $S$ and $W$ were used to characterize the annihilation behavior. The total power range of gamma energy spectrum peak was from 504.2 keV to 517.8 keV. Parameter $S$ was defined as the ratio between the counts of energy ranging from 510.24 keV to 511.76 keV and the sum counts of energy within 504.2 and 517.8 keV. Parameter $W$ was defined as the ratio between the sum counts ranging from 513.6 keV to 517.8 keV and from 504.2 keV to 508.4 keV, and the counts within the total energy range of 504.2–517.8 keV.

Fig. 1. Typical Raman spectra of the sample in the as-implanted and annealed stages.

The Raman spectra of the sample in different stages are shown in Fig. 1. For the as-implanted GaN on the sapphire, in addition to the peak at 568 cm$^{-1}$ corresponding to the E$_{2}$ mode, the weak peak at 737 cm$^{-1}$ for the A$_{1}$(Lo) mode of GaN and sapphire characteristic peak at 414 cm$^{-1}$, no other obvious peaks could be found. After annealing at 800$^{\circ}\!$C for 20 min, the E$_{2}$ mode and A$_{1}$(Lo) mode become stronger. It could also be observed that a wide band in the region 1360–1620 cm$^{-1}$ appears after annealing for 20 min especially for the peak of 1616 cm$^{-1}$, which could be attributed to the in-plane vibration of $sp^2$ carbon. As the annealing time increases to 1 h, there is no apparent change for the Raman spectra, while the intensities of E$_{2}$ mode and A$_{1}$(Lo) mode of GaN and the intensity of carbon-related peaks decrease. The typical PL spectra of the sample in different stages are shown in Fig. 2. As the GaN is implanted by carbon ions, it could be seen that in addition to the intrinsic near-band edge emission at 361 nm and 2nd-order over-tones of near-band edge emission at 723 nm, no other obvious peaks and photoluminescence can be observed. After annealing at 800$^{\circ}\!$C for 20 min, a wide band corresponding to the yellow light emission appears. Meanwhile, the intensity of intrinsic emission and 2nd-order overtones of near-band edge emission of GaN are reduced dramatically. As the annealing time is prolonged to 1 h, the similar luminescence characteristic can be observed and the luminescence intensity shows a slight decrease.

Fig. 2. PL spectra of the GaN layer in the as-implanted and annealed stages.

Fig. 3. The $S$–$E$ curves of the as-implanted and annealed samples.

The measured $S$–$E$ curves of the sample in the as-implanted and annealed stages are plotted in Fig. 3. The implanted results in larger $S$ parameter on the surface indicate the generation of a large number of vacancies. After annealing at 800$^{\circ}\!$C for 20 min, it could be seen that $S$ parameter on the surface decreases slightly and increases in the tail region. With longer annealing time, the $S$ parameter on the surface increases, while for the carbon ions in the region around the projected range, it maintains relatively smaller values in the tail region showing similar characteristic with the as-implanted sample. Figure 4 shows the $W$–$E$ curves of the sample in the as-implanted and annealed stages. A decrease in the $W$ parameter corresponding to an increase in the $S$ parameter is caused by a large number of Ga vacancies. After annealing for 1 h, the $W$ parameter decreases on the surface. Then it follows the same trend of the as-implanted sample in the bulk.

Fig. 4. The $W$–$E$ curves of the as-implanted and annealed samples.

Fig. 5. The $W$–$S$ curves of the as-implanted and annealed samples.

The $W$–$S$ characteristics measured in the implanted and annealed samples are plotted in Fig. 5. It is shown that the characteristic points of the surface and bulk follow the same straight line in the as-implanted sample. After annealing for 20 min, it also shows the similar line on the surface. In the bulk, slight deviation from the straight line could be observed. As the annealing time increases to 1 h, it could be found that the characteristic points significantly deviated from the straight line on the near surface, while it shows similar characteristics on the surface and in the bulk. The straight line of the $W$–$S$ plot in Fig. 5 indicates that the surface state and Ga vacancies (mainly single vacancies) are two dominant annihilation sites in the as-implanted sample.[20] Meanwhile, the structure of GaN crystalline is damaged by high-energy carbon ions accompanied with numerous vacancies and carbon interstitial atoms, which can also be speculated from the weak intrinsic peak of GaN appearing in Raman spectrum of Fig. 1. In this structure, no luminescence could be found because there are no deep-level centers that could provide sites to jump for the electrons.[21] After annealing for 20 min, the luminescence intensity of the sample is stronger than that of 2nd-order overtone of near-band edge emission of GaN, which indicates the structure evolution of the GaN crystalline lattice. We could see that the $S$ parameter in Fig. 3 shows slight decrease on the surface, while it increases in the bulk, which means that the amount of vacancies on the surface reduces and that in the bulk increases, and the vacancy distribution changes. It has been reported that the yellow light emission of carbon ion implanted GaN results from the formation of C related complexes.[21] In fact, the carbon impurity in GaN can be a shallow donor or deep acceptor in GaN as the C$_{\rm Ga}$ (carbon in the Ga site) or C$_{\rm N}$ (carbon in the N site). After carbon atom implantation, the carbon atoms will locate the interstitial sites (C$_{\rm i}$), which will occupy the center level in the band gap. It can be a deep acceptor or a deep donor site. When it recombines with V$_{\rm Ga}$ (Ga vacancy) or V$_{\rm N}$ (N vacancy), it will become the shallow donor (C$_{\rm Ga}$) or deep acceptor (C$_{\rm N}$), which contributes to the yellow luminance. Although previous reports suggested that it was hard to remove defects induced by ion implantation fully in GaN because of the high critical annealing temperature,[22] it is believed that the defects in the GaN layer will recombine after annealing at 800$^{\circ}\!$C for 20 min. In fact, based on the MD calculation, generally the diffusion potential energy barrier of atoms from an interstitial site (C$_{\rm i}$) and a Ga vacancy (V$_{\rm Ga}$) in GaN crystalline is the lowest.[23] Thus most of the carbon ions are located at the interstitial site of GaN crystal lattice after implantation and this leads to the lattice distortion of GaN. When it is annealed at high temperature, the mobility of C$_{\rm i}$ increases, it diffuses into the V$_{\rm Ga}$ site and combines with the Ga vacancy and the GaN lattice distortion recovers. The formed complexes such as C$_{\rm Ga}$(V$_{\rm Ga}$-C$_{\rm i}$) and/or C$_{\rm Ga}$-C$_{\rm i}$ are responsible for the yellow light emission. The Raman spectrum provides the direct evidence of the formation of the C$_{\rm Ga}$ in Fig. 1. Generally, the GaN is hexagonal wurtzite structure and Ga atoms locate the vertices of the hexagon. The carbon ion implantation leads to a large amount of Ga vacancies. When the V$_{\rm Ga}$ site and C$_{\rm i}$ site combines after annealing at 800$^{\circ}\!$C for 20 min, the carbon atoms constitute the hexagon structure, which shows the typical disordered vibration (D peak at 1385 cm$^{-1}$) and in-plane vibration (G peak at 1616 cm$^{-1}$) of $sp2$ carbon. As the annealing time is prolonged to 1 h, the similar yellow light emission could be observed, which shows that the formed complexes after annealing 20 min still exist. However, the $S$–$E$ and $W$–$E$ curves show different tendencies after the sample is annealed for 1 h. The $S$ parameter increases and $W$ parameter decreases correspondingly on the surface, which means that the vacancies increase again. It could be found that the characteristic points for the sample annealed for 1 h show three distinctive annihilation processes in Fig. 5, including on the surface, the zone of deviated straight line in the subsurface and the bulk state. Compared with that annealed for 20 min, the $W$–$S$ curve for the sample annealed for 1 h shows obvious deviation from the implanted sample near the surface, which means that new types of vacancies form. This type vacancy will destroy the hexagon structure consisting of carbon atoms, making the D and G peaks weaker. Also, it reduces the concentration of C$_{\rm Ga}$ and/or C$_{\rm Ga}$-C$_{\rm i}$ and diminishes the luminescence intensity. It should be the vacancy clusters with carbon atoms. As the sites of carbon ions accommodated by the vacancies are saturated, the vacancy clusters will expand. Correspondingly, the fraction of annihilation with low-momentum electrons goes up and the fraction of annihilation with high-momentum electrons goes down, which shows similar characteristics to the F-ion-implanted GaN.[20] Taking the direct growth of diamond film on carbon-ion-implanted GaN for consideration, it is speculated that it will be helpful when the C-related complexes form, saturate and expand to the surface, because preferred nucleation and growth can be conducted on the high concentration C$_{\rm Ga}$ complexes with hexagon structure. Also it is speculated that the nitrile-like carbon-nitride bonds will form after carbon atoms move to the Ga vacancy sites. This structure could enhance the stability of GaN avoiding the GaN decomposition.[24] The implanted GaN sample without annealing will not generate the C$_{\rm Ga}$ complexes, while the long-time annealing will lead to the decreased concentration of C$_{\rm Ga}$ complexes. The most stable GaN could be obtained by maximizing the concentration of C$_{\rm Ga}$ complexes. In summary, the surface state and Ga vacancies are two dominant annihilation sites in the as-implanted GaN. As the carbon ions are implanted in the GaN, the structure of GaN crystalline is damaged by high-energy carbon ions accompanied by a large number of vacancies, and most of the carbon atoms are located at the interstitial sites. As the implanted samples are annealed, strong yellow luminescence will emit and the vacancies for Ga (V$_{\rm Ga}$) are reduced, resulting from the migration of interstitial carbon (C$_{\rm i}$) and formation of complexes (C$_{\rm Ga}$ and/or C$_{\rm Ga}$-C$_{\rm i}$) between them. With the increasing annealing time, the carbon ions accommodated by the vacancies are saturated and vacancy clusters with carbon atoms appear, which will diminish the concentration of C$_{\rm Ga}$ and have an adverse effect on the diamond film nucleation and growth. 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