Influence of initial growth conditions and Mg-surfactant on the quality of GaN film grown by MOVPE

    Corresponding author: Wei Gao, gaowei@china-led.net
  • 1. State Key Laboratory of Solid-State Lighting, Beijing Solid-State Lighting Science and Technology Promotion Center, Beijing 100083, China
  • 2. Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
  • 3. Shandong Inspur Huaguang Optoelectronics Co., LTD., Weifang 261000, China

Key words: metalorganic vapor phase epitaxygallium nitridehigh resolution X-ray diffraction

Abstract: The initial growth conditions of a 100 nm thick GaN layer and Mg-surfactant on the quality of the GaN epilayer grown on a 6H-SiC substrate by metal-organic vapor phase epitaxy have been investigated in this research. Experimental results have shown that a high V/III ratio and the initially low growth rate of the GaN layer are favorable for two-dimension growth and surface morphology of GaN and the formation of a smoother growth surface. Mg-surfactant occurring during GaN growth can reduce the dislocations density of the GaN epilayer but increase the surface RMS, which are attributed to the change of growth mode.

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1.   Introduction
  • III-nitrides have been considered to be the most favorable materials in the field of white light illumination due to their direct band gaps[1, 2, 3]. This is because they can form a continuous range of solid solution and also, they allow a wide wavelength distribution from the visible to the deep ultraviolet region[4]. Beside the lighting and display applications, III-nitride (GaN) material can also be used in the field of microwave devices[5, 6, 7]. However, the principal difficulty for GaN application is the absence of a suitable homogeneous substrate. The most popular substrates for the GaN epitaxial are sapphire and SiC[8, 9]. However, the mismatches of the crystal lattice and the thermal expansion coefficient between GaN and these heterogeneous substrates will lead to a large density of defects in the GaN epilayer[10, 11]. To reduce the density of defects, first a buffer layer has to be inserted and annealed at a high temperature before growing the GaN layer[12, 13]. In addition, the growth parameters for GaN needed to be optimized to achieve an improvement of the GaN epilayer's crystal quality. This is especially important in the early stage of GaN layer growth. In the past several years, a lot of work on the growth of high quality GaN epilayers has been done. Li et al.[14] investigated the influence of the initial growth rate of the GaN layer at high temperature on the quality of GaN. Their studies found that the quality of GaN can be improved by decreasing the growth rate of the initial stage. However, if the growth rate was decreased to some extent, the quality of the GaN layer started to deteriorated again. Yang et al.[15] studied the impact of the V/III ratio on the GaN layer quality during the initial stage of GaN growth. They declared that a low V/III ratio could reduce the nuclei density of GaN, and hence, an improvement in the GaN's quality. All researchers have come to a similar conclusion, that the control of the growth mode at the initial stage of GaN growth plays a significant role in improving GaN quality. Furthermore, a smooth surface morphology could also enhance the quality of GaN grown afterwards[16, 17].

    Pits are the most widely observed defects on the surface of GaN film. They are caused by edge-type dislocations[18], which will lower the reliability of the GaN device and shorten the device lifetime. So, controlling the density of dislocations is significant in improving the performance of GaN devices.

    In this paper, the influence of the growth rate and V/III ratio, at the high temperature (HT) initial stage of the growth of GaN, on the quality of GaN film and the effect of Mg-surfactant on the density of dislocation was studied. The results indicate that a high V/III ratio and a low growth rate at the initial stage of HT GaN growth could improve the surface morphology of GaN. In addition, the Mg-surfactant could reduce the density of edge dislocation significantly.

2.   Experimental procedure
  • The GaN films were grown on (0001) 6H-SiC substrates by the metal organic vapor phase epitaxy (MOVPE) technique. Trimethylaluminum (TMAl), trimethylgallium (TMGa) and ammonia (NH$_3$) were used as precursors, whereas bis-magnesium (Cp2Mg) was used as the Mg-surfactant source. Prior to the growth, SiC substrates were heated to 1050 C for 10 min to remove the contaminants before a 10 nm thick AlN buffer was deposited at 1100 C[19]. After the growth of the AlN buffer, HT GaN films were grown at different conditions, as depicted below. The growth temperature of HT GaN was 1070 C. The growth of HT GaN consists of three stages. The initial stage is the growth of the first 100 nm GaN film. The second stage is the growth of the 1.2 $\mu $m thick main GaN film. The last stage is the deposition of a 100 nm thick Mg-surfactant GaN on the stratosphere at a Mg concentration of about 5 $\times$ 10$^{17}$. Seven samples were prepared in this work. Samples A, B, E and F correspond to the first stage growth. Second stage growth are completed for samples C and D. All three growth stages are completed for sample G. The basic and the first stage growth parameters for the seven samples are listed in Table 1.

    The surface morphology of the initial 100 nm GaN film is affected by the growth rate and V/III ratio. This conclusion can be drawn by comparing the surface morphologies for samples A, B, E and F. The influence of the initial 100 nm GaN film growth rate on the quality of the main GaN was investigated by comparing the surface morphologies for samples C and D. The effects of the Mg-surfactant on the quality of the GaN film can be obtained by comparing the surface morphologies for samples G and D.

    The surface morphologies of the samples were observed by AFM. The structural quality of the main GaN is evaluated by high-resolution X-ray diffraction (HR-XRD).

3.   Results and discussion

    3.1.   The influence of the growth rate and V/III ratio at the initial stage on the quality of GaN film

  • Figure 1 shows the AFM images of samples A and B, which have a thickness of 100 nm. To investigate the effect of the GaN growth rate during the early stage on the quality of GaN film, two samples were grown at the rates of 1.6 and 0.8~$\mu $m/h, respectively. The surface RMSs of the two samples with a 10 $\times $10 $\mu $m$^{2}$ area are 14.75 nm and 8.34 nm respectively. It can be seen that sample A with a high GaN growth rate (1.6~$\mu $m/h) has a higher density of holes on the surface, but sample B with a low GaN growth rate (0.8 $\mu $m/h) forms a continuous film. Thus, it can be seen that lowering the GaN growth rate will enhance the 2D-growth and promote the formation of a smooth surface. It can be explained as follows. The lower GaN growth rate could allow atoms to have enough time to migrate on the surface, which leads to the lateral growth. Then the coalescence of the GaN film is accelerated. Therefore, the growth rate at the early stage of GaN should be kept slow to obtain a smoother surface.

    After growing 1.2 $\mu $m of GaN with a growth rate of 1.6~$\mu $m/h and a V/III ratio of 1150 for samples A and B, we obtained samples C and D. The surface RMSs of the two GaN films in the 2 $\times$ 2 $\mu $m$^{2}$ dimension are 0.25 and 0.17 nm respectively, as illustrated in Figure 2. In contrast to sample C, sample D has a much flatter surface. It can be attributed to the lower GaN growth rate in the initial stage of sample D. Many pits can be observed on the surfaces of the two samples. These pits actually correspond to the presence of dislocations[18]. The depths of the pits for the two samples are 2.0 and 1.6 nm, respectively. A low pit depth means an improvement of the GaN crystal quality. This can be supported by the following analysis: symmetric 0002 reflection rocking curves are taken by HR-XRD as the XRD's FWHM will reflect (indirectly) the density of dislocation; a narrower FWHM reveals a lower dislocation density. The X-ray full widths at half maximum (FWHM) of samples C and D are 377 and 297~arcsec, respectively. It means the quality of sample D is better than sample C, indicating the quality of the main GaN grown afterwards will benefit from the low growth rate in the initial growth stage.

    Figure 3 is the surface morphologies of samples E, B and F whose V/III ratios are 1800, 1150 and 600, respectively. The thickness and the growth rate of the three samples are 100 nm and 0.8 $\mu $m/h. The surface RMSs of the samples E, B and F, with an area of 10 $\times$ 10 $\mu $m$^{2}$, are 1.29, 8.34 and 45.14 nm respectively, as shown in Figure 4. The V/III ratio of GaN has a significant influence on the surface morphology. This result shows that the RMS of the GaN surface is decreased with the increase of the V/III ratio. When the V/III ratio is dropped to 600, the GaN epilayer did not coalesce, as shown in Figure 3. It means a high V/III ratio will enhance the lateral overgrowth of GaN and induces the formation of a smooth 2D-growth surface, which would be beneficial to the growth of high quality main GaN.

  • 3.2.   The influence of Mg-surfactant on the density of dislocations

  • Figure 5 shows the AFM images of samples D and G. Sample G is a main GaN sample with a 100 nm Mg-surfactant layer deposited on the top. It can be seen from Figure 5 that the surface of the main GaN (sample D) has a high density of pits, of about 8.5 $\times$ 10$^{8}$~cm$^{-2}$. After growing a 100 nm Mg-surfactant layer on the top, the density of pits is reduced to 5.5 $\times $ 10$^{8}$~cm$^{-2}$, which means the Mg-surfactant layer could reduce the density of pits, i.e. the density of dislocations. In early literature[20], Mg was reported to enhance the lateral overgrowth of GaN, which was generally used in the ELOG technique. However, after the deposition of the Mg-surfactant layer, the surface RMS of GaN film increased from 0.17 to 0.29 nm, as shown in Figure 5. It implies that Mg promotes the 3D growth of GaN film rather than enhancing the lateral overgrowth of GaN film. However, a comparison of both experiments may be inappropriate since the growth condition in our experiment is different from the abovementioned literature. From our experiment, we can draw a conclusion that the Mg-surfactant GaN layer can reduce the density of dislocation.

4.   Summary
  • The effect of the growth condition and the Mg-surfactant on the quality of GaN grown by MOVPE has been investigated. In this study, the RMS of the 100 nm thick GaN surface is reduced to 1.29 nm in an area of 10 $\times$ 10 $\mu $m$^{2}$ by lowering the growth rate to 0.8 $\mu $m/h while increasing the V/III ratio to 1800. The pits depth of the main GaN is also reduced from 2 to 1.6 nm. The result from X-ray FWHM shows a decrease from 377 to 297 arcsec when the growth rate of the first 100~nm GaN is decreased from 1.6 to 0.8 $\mu $m/h. Hence, this shows that the Mg-surfactant in GaN can lower the dislocation density of GaN. The dislocation density of the main GaN dropped from 8.5 $\times$ 10$^{8}$ to 5.5 $\times$ 10$^{8}$ cm$^{-2}$ after the deposition of a 100~nm Mg-surfactant layer. The change of the growth mode is the main cause of the reduction of dislocation density.

Figure (5)  Table (1) Reference (20) Relative (20)

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