1.
Introduction

The growth of GaN epitaxy on Si has become very popular for its potential for uses in light emitting diodes, high-frequency electronic devices, ultraviolet detectors, and related technologies^{[1, 2]}. Although some GaN based devices are already commercialized, it is still a challenge to produce high quality material, due to the lack of GaN homo-epitaxial substrates. The GaN epitaxial layer is usually grown on hetero-epitaxial substrates (Sapphire, SiC, Si). Sapphire is one of the favorable substrates for it is available in a large diameter dimension; however, its low thermal and electrical conductivity limits its application, and it shows a high hardness hampering wafer dicing. Compared to SiC, silicon substrate is a more cost effective choice for GaN growth^[3–5]. As the most favored substrate orientation for wurtzite nitrides, Si (111) substrate has a lattice mismatch of 17% with GaN and 19% with AlN. It is easy for GaN epitaxy to generate high dislocation densities. Meanwhile, the large thermal mismatch of 56% also leads to large tensile strain during cooling down time^[6]. Many methods have been used to reduce the tensile and improve the crystalline quality of GaN^[7–10].



This experiment introduced two methods to improve the quality and strain condition of GaN. By comparing the XRD, Raman spectrum and PL spectrum, this article illustrates the two different growth mechanisms of the two methods.

2.
Experiment

The experiment is completed by metal-organic chemical vapor deposition (MOCVD). TMGa, TMAl, and NH₃ are used as Ga, Al, and N precursors, respectively. H₂ is the carrier gases. The structures of samples with different growth methods are shown in Fig. 1. In sample A, as a reference, only AlN buffer is adopted, followed by GaN epitaxial layer. In sample B, the GaN transition layer follows the AlN buffer. In sample C, the AlGaN buffer is introduced. The AlN buffers are 100 nm for all the samples. For sample B, GaN transition layer is grown with the V/III ratio of 590 for 100 s, while all the GaN epitaxial layers are grown with the V/III ratio of 2000. For sample C, the AlGaN buffer is 300 nm.






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Figure1.
The schematic diagrams of structures for (a) sample A, (b) sample B, and (c) sample C.




In this study, the crystal quality of GaN epitaxial layer is investigated with X-ray diffraction (XRD). It is also very important to understand the knowledge of the stress condition or strain for improving the properties of nitride-based devices. The Raman spectra have been recorded at room temperature and the He–Ne laser (632.8 nm) is used for the excitation. The photoluminescence spectrum (PL) is used to analyse the optical properties.

3.
Results and discussion

The full widths at X-ray diffraction half maximum (FWHMs) of GaN on the (0002) plane of samples A, B and C are 0.686°, 0.223° and 0.089° respectively, as shown in Fig. 2. It shows that a GaN epitaxial layer grown directly on the AlN buffer turns out to have a poor quality. With the introduced GaN transition layer or the AlGaN buffer, the crystalline quality of GaN epitaxy is greatly improved, and the latter method is better.






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Figure2.
The FWHMs of (0002) XRD of different samples: (a) sample A, (b) sample B, and (c) sample C.




Raman spectra of samples B and C were measured at room temperature to analyse the stress condition of the samples. As shown in Fig. 3, there are two peaks in both samples in the range from 480 to 600 cm^?1. The peak located at 520 cm^?1 is the optical phonon peak of Si. The nominal E₂phonon peaks of samples B and C are located at 564 and 565.8 cm^?1 respectively. The nominal E₂ phonon peak frequency of the relaxed GaN bulk material^[11] is 567.6 cm^?1. As it is reported, the move of the E₂ peak frequency is attributed to different stress conditions (red shift corresponds to tensile stress and blue shift corresponds to compressive stress)^[12]. Obviously, both samples are under tensile stress. In the linear approximation, the biaxial stress σ_xx can be calculated by the following equation with the deviation in E₂ peak frequency if the linear stress coefficient K_γ is known:






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Figure3.
(Color online) Raman spectra of GaN films with different structures: sample B and sample C.

$$Delta {omega _gamma } = {K_gamma }{sigma _{xx}}.$$

(1)

According to the stress coefficient of 4.3 cm^?1GPa^?1 for GaN^[13], the calculated residual stress in the GaN epitaxial layer is approximately 0.72 GPa for sample B and 0.42 GPa for sample C. It is obvious that, compared with sample B, which introduces the GaN transition layer, sample C with AlGaN buffer expresses much less residual tensile stress.



This result is probably attributed to the different growth mechanism of the two samples. For sample B, by introducing the GaN transition layer, GaN islands with proper density and size can be formed on the AlN buffer layer. By controlling the growth and merge of the islands, comparative improvement of the crystal quality of the GaN epitaxial layer is achieved (Details are shown in Ref. [14]). For sample C, more compressive stress is built up in the subsequent GaN due to the introduction of AlGaN buffer^[15]. The compressive stress leads to larger dislocation inclination and consequently to reduction of the density in a-type dislocations^[16]. What is more, the higher compressive stress leads to less residual tensile stress to GaN epitaxy.



The scanning curves of room temperature PL of samples B and C are shown in Fig. 4. The peaks of the band gap edge of hexagonal GaN for samples B and C are located at 3.38 and 3.41 eV separately. The difference of the peak location is mainly attributed to the different residual stress of GaN epitaxial layers^[17]. Besides, there is an additional peak located at 3.36 eV for sample B. The mechanism of this peak is not fully explained. It may be attributed to the low crystalline quality or stacking faults (SFs)^[17]. There are also some additional peaks located lower than the band gap energy for sample C, which may also be attributed to stacking faults (SFs). As shown in Fig. 4, the PL intensity of sample C is higher than sample B. Furthermore, the PL intensity of the band gap edge is much higher than that of peaks located lower for sample C. For sample B, the PL intensity of the band gap edge is lower than that of peaks located at 3.36 eV. It indicates that the crystal quality of sample C is higher than sample B, which is consistent with the XRD result.






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Figure4.
(Color online) Scanned spectra of GaN films with different structure by room temperature PL: sample B and sample C.

4.
Conclusion

In this study, two additional methods for improving the property of GaN films on Si (111) substrates are researched. The methods are proposed to improve the crystalline quality and reduce the residual tensile stress of the GaN epitaxial layer. The first method is to introduce a GaN transition layer. For the second method, AlGaN buffer is grown on AlN buffer. Both methods result in a significant improvement of the quality of GaN epitaxial layer. What is more, with the second method, the residual tensile thermal stress decreases. This is because both methods lead to a larger dislocation inclination and consequently to a reduction of the a-type dislocation density. Besides, more compressive stress is built up in the subsequent GaN due to the introduction of AlGaN buffer and the higher compressive stress leads to less residual tensile stress to GaN epitaxy.