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JOURNAL OF APPLIED PHYSICS 105, 083519 共2009兲

Diffusion mechanism and the thermal stability of fluorine ions in GaN after ion implantation M. J. Wang,1 L. Yuan,1 K. J. Chen,1,a兲 F. J. Xu,2 and B. Shen2 1

Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China 2 School of Physics, Peking University, Beijing 100871, China

共Received 9 November 2008; accepted 22 February 2009; published online 21 April 2009兲 The diffusion mechanisms of fluorine ions in GaN are investigated by means of time-of-flight secondary ion mass spectrometry. Instead of incorporating fluorine ions close to the sample surface by fluorine plasma treatment, fluorine ion implantation with an energy of 180 keV is utilized to implant fluorine ions deep into the GaN bulk, preventing the surface effects from affecting the data analysis. It is found that the diffusion of fluorine ions in GaN is a dynamic process featuring an initial out-diffusion followed by in- diffusion and the final stabilization. A vacancy-assisted diffusion model is proposed to account for the experimental observations, which is also consistent with results on molecular dynamic simulation. Fluorine ions tend to occupy Ga vacancies induced by ion implantation and diffuse to vacancy rich regions. The number of continuous vacancy chains can be significantly reduced by a dynamic thermal annealing process. As a result, strong local confinement and stabilization of fluorine ions can be obtained in GaN crystal, suggesting excellent thermal stability of fluorine ions for device applications. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3106561兴 I. INTRODUCTION

Recently, fluorine plasma treatment technique has been demonstrated as a robust technology to realize self-aligned enhancement-mode 共E-mode兲 or normally off AlGaN/GaN high electron mobility transistor 共HEMT兲.1 The incorporation of the fluorine ions in the AlGaN barrier layer can effectively deplete the two dimensional electron gases 共2DEGs兲 in the channel and deliver a threshold voltage shift of HEMT as large as +5 V. Significant reduction in the gate leakage can also be achieved by this technique.2,3 Several promising device applications using this technique have also been demonstrated.4–6 However, previous reliability test on GaAs-based HEMT revealed that fluorine contamination contributes to the thermal and electrical degradation of such devices and significant diffusion and instability of fluorine were observed at 400 ° C in GaAs-based materials.7,8 Naturally, there is great concern about fluorine ions’ stability in GaN and related III-nitride materials. On the other hand, promising reliability results have been reported recently in E-mode AlGaN/GaN HEMT,9 suggesting good stability of the fluorine ions in AlGaN/GaN heterostructures after thermal and electrical stresses. Thus, it is of great importance to understand the underlying physics of the fluorine ion’s diffusion process and eventually reveal the stability of fluorine ions in III-nitride semiconductor devices. Secondary ion mass spectroscopy 共SIMS兲 has been applied to study the fluorine ions’ profile with various annealing conditions in plasma treated samples.2,10 However, the surface effects 共e.g., enhanced out-diffusion on the unpassivated surface兲 can significantly affect the analysis of the measurement results and may lead to false conclusions since a兲

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high density 共1019 – 1020 cm−3兲 fluorine ions are located just a few nanometers from the surface. In this letter, we investigated the fluorine ions’ diffusion mechanism and thermal stability by implanting the fluorine ions deep into a GaN epitaxial layer so that the surface effects can be greatly mitigated. The fluorine ions distribution profiles are measured in implanted samples with various annealing times at 600 and 800 ° C for the stability study. It was found that the diffusion of fluorine in GaN is a dynamic process. A vacancy-assisted diffusion model is proposed to explain the experiment observation. The fluorine ions reached excellent stability after continuous vacancy chains induced by implantation were removed by thermal annealing.

II. EXPERIMENT

The GaN epitaxial sample was grown by means of metal-organic chemical vapor deposition on a 共0001兲 sapphire substrate. A nucleation GaN buffer layer was grown at 530 ° C, followed by a 1.7-␮m-thick unintentionally doped GaN layer deposited at 1070 ° C. The samples were degreased and sonicated in acetone and isopropyl alcohol for 5 min each and blow dried using N2 after rinsing in de-ionized water. Then 19F+ ions were implanted into the GaN sample at an energy of 180 keV with a dose of 1 ⫻ 1015 cm−2. The samples were cut into small pieces and individually annealed in N2 atmosphere at 600 ° C for a time ranging from 30 s to 72 h and at 800 ° C for time ranging from 20 min to 8 h. Time-of-flight secondary-ion mass spectrometry 共ToF-SIMS兲 was performed using a Physical Electronics PHI 7200 ToFSIMS spectrometer with depth resolution of 5 nm to determine the concentration-depth profile of fluorine. The primary ions were generated from a Cs ion source 共3 kV兲. Charge

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FIG. 1. SIMS profile and TRIM simulation of 1 ⫻ 1015 cm−2, 180 keV F implanted into the GaN layer. The figure also shows the proposed Ga vacancy profile before and after postimplantation annealing.

compensation was realized by low-energy 共0–70 eV兲 flooding electrons being pulsed out of phase with the primary ion beam. III. RESULTS AND DISCUSSION

Figure 1 shows the concentration-depth profile of fluorine after implantation measured by ToF-SIMS, along with the results from the transport-of-ions-in-matter 共TRIM兲 simulation based on the Monte Carlo method.11 In the TRIM simulation, the density of the GaN layer was set to be 6.15 g / cm3 and the displacement energies of Ga and N atoms were set to be 25 and 28 eV, respectively. A large tail is observed in the measured F profile compared to the simulated one, as a result of the strong channeling effect along the c axis of wurtzite GaN that cannot be taken into account in the TRIM simulation.12 The F distribution profiles for samples annealed at 600 ° C with various annealing times are plotted in Fig. 2共a兲. The results can be divided into three different regions: left of the peak 共I兲, right of the peak 共II兲, and the tail region 共III兲. There is rapid out-diffusion at the onset of thermal annealing from the substrate side of the project range 共R P兲 to the surface when the annealing time is shorter than 20 min as shown in Fig. 3. The quick outdiffusion is also observed in region III. Figure 4 shows the fluorine profile after the sample was annealed longer than 20 min at 600 ° C. Opposite from the quick out diffusion process, F concentration in region I decreases with annealing time, while it increases in region II, exhibiting a slower in diffusion trend. There is very little change in the fluorine profile after 24 h annealing, indicating excellent stability achieved after long time annealing. Earlier studies of ion implantation in GaN revealed that most of the dopants, such as Be, C, Mg, Si, and Zn, are thermally stable after implantation even with annealing at temperature above 1000 ° C.13,14 However, weak but detectable diffusion of fluorine ions is observed with 600 ° C thermal annealing as presented in this work. According to a recent molecular dynamics simulation study, the Ga and N vacancies outnumber the fluorine ions by two orders of magnitude and exhibit similar profiles as the fluorine ions, as shown in Fig. 1.12 The potential energy profiles of fluorine ions in GaN are also calculated.15,16 It is found that the po-

FIG. 2. SIMS profile of 1 ⫻ 1015 cm−2, 180 keV F implanted into the GaN layer before and after annealing at 共a兲 600 ° C for 30 s, 20 min, 2 h, and 72 h, and 共b兲 800 ° C for 20 min, 2 h, and 8 h, respectively.

tential energy barrier between an interstitial site 共I兲 and a Ga vacancy 共VGa兲 site is the lowest at 1.4 eV, while that between the I site and a nitrogen vacancy 共VN兲 is 2.1 eV. The potential energy barrier between two adjacent I sites is highest at 5 eV.

FIG. 3. Detailed SIMS profile of fluorine ions in GaN when the annealing time is shorter than 20 min at 600 ° C.


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FIG. 4. Detailed SIMS profile of fluorine ions in GaN after long time annealing at 600 ° C. The profile before annealing is also shown for comparison.

Thus, it is suggested that the dominant fluorine diffusion mechanism is the Ga-vacancy assisted diffusion, rather than the N-vacancy assisted diffusion or interstitial-interstitial diffusion. Such an argument is also supported experimentally by the positron annihilation spectroscopy 共PAS兲 carried out on the same samples.17 Implantation induced negatively charged Ga vacancies were found to be annihilation sites of positrons. It was observed that fluorine ions tend to form fluorine-vacancy 共F-VGa兲 complexes quickly after thermal annealing,17 providing another evidence of the proposed Ga vacancy assisted diffusion model. Besides, such a diffusion process is only detectable when plenty of Ga vacancies are available to form continuous Ga vacancy chains. Otherwise, fluorine ions will be confined within the vicinities of individual Ga vacancy sites or Ga vacancy complexes. Based on this diffusion model, the results shown in Figs. 2–4 can be explained in the following. A. Short-time annealing †shown in Fig. 5„a…‡

At the early stage of a thermal annealing process, fluorine ions tend to out diffuse toward the surface because of the concentration gradient and plenty of Ga vacancies are available along the implantation path. At the same time, the escape of fluorine ions from the GaN surface also occurred because the fluorine ions are not stable at the open surface due to the lack of atomic bonds and energy confinement. Since the escape rate is lower than the out-diffusion rate from region II to region I, the fluorine concentration increases in region I but decreases in regions II and III, as shown in Fig. 2共a兲. Similar out-diffusion trend was also observed in the samples annealed at 800 ° C, as shown in Fig. 2共b兲. B. Long-time annealing †as shown in Fig. 5„b…‡

As the annealing time gets longer, large numbers of Ga vacancies are removed. Thus, the fluorine ions tend to diffuse inward to the location of the original projection peak, where the density of Ga vacancies is the highest. The in-diffusion

FIG. 5. Illustration of the diffusion and stabilization of implanted fluorine ions in GaN after thermal annealing. The locations and numbers of Ga vacancies and fluorine ions are not one to one correspondence from 共a兲 to 共c兲.

together with the continuous escape at the open surface lead to the decreasing fluorine concentration in region I when the annealing time is longer than 20 min. It is difficult to use a concentration-dependent diffusion to explain the in-diffusion phenomenon because of the high activation energy needed for concentration-dependent diffusion of impurities in GaN layer. In addition, the in-diffusion process starts to saturate after 24 h annealing. This is not expected to happen in the concentration-dependent diffusion process. We suggest that the diffusion process with long-time annealing is also vacancy assisted. As more and more Ga vacancies induced by ion implantation are removed with the longer annealing time 共see Fig. 1兲, the out-diffusion process will be significantly weakened. The fluorine ions in region I will diffuse back to region II where more vacancies exit, as shown in Fig. 5共b兲. C. Stabilization of fluorine ions after long-time annealing †as shown in Fig. 5„c…‡

When the thermal annealing time is long enough, e.g., after 24 h annealing, the in-diffusion is eventually stopped and fluorine ions become stable in bulk GaN. The indiffusion is not so obvious in 800 ° C annealed samples, which may be due to the quicker removal of vacancies at higher annealing temperature. The vacancy concentration in region III is relatively low after implantation and the indiffusion of fluorine does not occur in region III. Typically, full recovery or complete removal of implantation induced damage 共in the form of vacancy like point defects as the crystal atoms are knocked away from their original sites by the incident fluorine ions兲 is expected to be difficult because of the high critical temperature required for atomic movement more than one lattice constant long


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共⬃1500 ° C兲.14 It is expected that the higher the annealing temperature, the more complete the damage removal. On the other hand, the higher the annealing temperature, the stronger the out-diffusion of fluorine ions, suggesting that lower annealing temperature is preferred as far as the fluorine’s stability is concerned. It should be noted that the displacement of majority of the knocked-away atoms is less than the lattice constant and is small, according to a molecular dynamics simulation. Therefore, majority of the displaced atoms can easily fall back to their original sites with sintering at relatively low temperature 共e.g., 600 ° C兲. In realistic device fabrication, the annealing process is usually carried out after the deposition of gate metal. The conventional gate metal, Ni/Au, starts to degrade as annealing temperature exceeds 500 ° C. Thus, sintering at 400 ° C for 10 min was used to remove the fluorine plasma-induced damages in the E-mode HEMT and only slight degradation 共⬃5%兲 in 2DEG mobility was observed.1 The analysis presented above suggests that fluorine ions become stable in the bulk of GaN when the number of continuous vacancy chains is significantly reduced by thermal annealing. Recently, E-mode HEMT fabricated by fluorine plasma treatment is shown to exhibit excellent thermal stability with long-term thermal stress at 350 ° C,9 providing supporting evidence to the diffusion model presented in this work. Generally speaking, fluorine ions exhibit larger diffusion coefficients in semiconductors compared to other dopants, due to fluorine’s small atomic radius, which favors the movement of fluorine in a lattice. In semiconductors 共such as GaAs, InGaAs, InAlAs, and Si兲 with larger lattice constant, the fluorine’s movement requires little activation energy and poor thermal stability has been observed in GaAs, InAlAs, Si, etc. Gallium nitride and related compound semiconductors, on the other hand, feature small lattice constants and tight lattice structures that make the movement of fluorine ions difficult. We believe that good thermal stability can be obtained for unbonded fluorine ions in III-nitride materials or other semiconductor materials that feature small lattice constants, such as SiC and AlN. IV. CONCLUSIONS

In summary, the dynamic diffusion process of fluorine ions in bulk GaN is investigated by ToF-SIMS upon postim-

plantation annealing. It is found that the diffusion of fluorine ions in GaN is assisted by implantation-induced vacancies. Fluorine ions tend to occupy open volumes, especially Ga vacancies induced by ion implantation, and diffuse to vacancy-rich regions. The fluorine ions become stable when the number of continuous vacancy chains is significantly reduced by thermal annealing. The results support the excellent thermal stabilities observed in the E-mode AlGaN/GaN HEMTs fabricated with fluorine plasma ion implantation. This work is supported by the Hong Kong Research Grant Council under the competitive earmarked research Grant No. 611706. 1

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