Study of GaN Thick Films Grown on Different Nitridated Ga₂O₃ Films

Abstract

In this paper, various Ga₂O₃ films, including amorphous Ga₂O₃ films, β-Ga₂O₃, and α-Ga₂O₃ epitaxial films, have been nitridated and converted to single-crystalline GaN layers on the surface. Although the original Ga₂O₃ films are different, all the converted GaN layers exhibit the (002) preferred orientation and the porous morphologies. The ~200 µm GaN thick films have been grown on the nitridated Ga₂O₃ films using the halide vapor phase epitaxy (HVPE) method. Raman analysis indicates that all the HVPE-GaN films grown on nitridated Ga₂O₃ films are almost stress-free. An obvious GaN porous layer/Ga₂O₃ structure has been observed in the interface between GaN thick films and sapphire substrates. The porous GaN layers can be used as promising templates for the preparation of free-standing GaN substrates.

1. Introduction

Gallium nitride (GaN), as one of the III-nitride semiconductor materials, has been widely studied for the preparation of high-performance optoelectronic and high-power electronic devices due to its wide bandgap, high electron mobility, and excellent physical properties [1,2,3]. GaN-based light-emitting and photodetection devices cover the entire wavelength range from ultraviolet to visible, which plays an important role in the transmitting and receiving ends of optical wireless communication systems [4,5]. Recently, GaN substrates with low threading dislocation density (less than 10⁴–10⁵ cm⁻²) and low residual strain have shown potential for use in the homoepitaxial growth of GaN-based devices and show promise to further enhance GaN-based device performance. However, the high price, small size, and inconsistent quality of free-standing GaN (FS-GaN) has limited its applications [6,7,8]. Generally speaking, halide vapor phase epitaxy (HVPE) is the most commonly used technique to obtain FS-GaN substrates, due to its high growth rate and low-cost. FS-GaN substrates are usually prepared by first growing the HVPE-GaN thick film on MOCVD-GaN/sapphire templates and then removing the sapphire substrates by laser-induced lift-off [9,10,11,12]. It was found that the residual strain and lattice distortion caused by hetero-epitaxy could not be completely suppressed and will also affect the quality of the subsequent epitaxial film and the performance of devices. Even now, reducing the stress and obtaining stress-free GaN substrates remains a challenge. Some elements of GaN porous templates and nanorod arrays have been optimized in many ways [13,14,15,16]. However, these methods are not cost effective and require special process.

Recently, β-Ga₂O₃ has been used as a substrate for the epitaxial growth of GaN epilayers after the surface reconstruction by the nitridation under NH₃ atmosphere [17,18,19] due to the minimum lattice mismatch between Ga₂O₃ and GaN (~2.6%) [20,21]. In our previous reports [22,23], (-201)-oriented β-Ga₂O₃ epitaxial films and (200)-oriented bulk β-Ga₂O₃ have been converted to (002)-oriented GaN porous layers by nitridation, and the porous GaN layers have been used as templates for further quasi-homoepitaxy to obtain low-stress free-standing GaN films through a self-separation or chemical lift-off (CLO) process due to the selective etching of β-Ga₂O₃ [22,23,24]. However, the preparation of large-area single-crystal β-Ga₂O₃ films and bulk β-Ga₂O₃ is complex, high cost, and has low reproducibility. In this work, the nitridation of Ga₂O₃ films prepared by various methods and GaN re-epitaxy on the nitridated Ga₂O₃ films has been studied and compared. The aim was to find a simple and low-cost method to prepare Ga₂O₃ films and porous templates for obtaining stress-free GaN substrates that are low in cost and high quality using nitridation and re-epitaxy.

2. Materials and Methods

Here, the different Ga₂O₃ films, including β-Ga₂O₃ (S1), α-Ga₂O₃ (S2), and amorphous Ga₂O₃ (S3) films, have been nitridated in the quartz tube under ammonia atmosphere. β-Ga₂O₃ epitaxial films were grown on c-plane sapphire substrates by HVPE at 1000 °C [25]. The metastable α-Ga₂O₃ single-crystalline films were obtained using the Mist-CVD method [26], while the amorphous Ga₂O₃ films were prepared by sputtering deposition. Samples were first cleaned using acetone, ethanol, and de-ionized water, followed by nitrogen drying, and were then put into the center of a quartz tube. Nitrogen gas was introduced into the quartz tube while the reactor temperature was increased. At 1050 °C, the N₂ flow was shut off and then NH₃ gas flowed into the quartz tube with a flow rate of 200 sccm. The nitridation time was set as 30 min. The flow of NH₃ gas was only maintained during the reaction and was replaced by N₂ gas when the nitridation process was completed. Afterwards, the quartz tube was cooled down to room temperature with N₂ gas and the samples turned yellow on the surface (named NS1–NS3). After the nitridation, GaN thick films (~200 µm) were grown on these nitridated films using the HVPE method. The growth temperature was 1050 °C and the growth time was 30 min.

The surface morphologies of these samples were characterized using scanning electron microscopy (SEM) at an operating voltage of 5 kV. The crystalline nature was analyzed using X-ray diffraction (XRD). Raman analyses were performed by using a LabRam HR evolution instrument from Horiba, with lasers emitting at 514 nm as excitation sources. PL measurements were carried out using a He-Cd laser operating at 325 nm as an excitation source at room temperature.

3. Results

Figure 1a shows the XRD patterns of different Ga₂O₃ films. In addition to the diffraction peak of sapphire (0006), the peaks located at 38.3° for S1 and 40.2° for S2 indicate that β-Ga₂O₃ and α-Ga₂O₃ epitaxial films are (-201)-orientated and (004)-orientated, respectively. For amorphous Ga₂O₃ film, there is only a sapphire-related peak, appearing in the XRD pattern. The XRD patterns of the nitridated Ga₂O₃ films are displayed in Figure 1b. The peaks located at 34.6° appear in all the patterns, which corresponds to wurtzite GaN (0002) and demonstrates the conversion of Ga₂O₃ to GaN. The amorphous Ga₂O₃ film (S3) has been completely converted to GaN film after nitridation because the amorphous film is thinner than other films in this study. Apart from this, some peaks related to β-Ga₂O₃, with slight intensity, have been observed in the patterns of NS1 and NS2 owing to the incomplete nitridation and conversion. The effects of the nitridation time have been investigated [18,19,20,21,22], and for thinner Ga₂O₃ films, it is easier to obtain the totally nitridated porous GaN layers in the same nitridation time. The metastable α-Ga₂O₃ will be converted to β-Ga₂O₃ above 800 °C, leading to the appearance of β-Ga₂O₃-related peaks in the pattern of the nitridated α-Ga₂O₃ (NS2) [27]. The conversion process from α-Ga₂O₃ to β-Ga₂O₃ and then to GaN during nitridation will be discussed in a future report. The XRD results indicate that the conversion from Ga₂O₃ to GaN also follows the epitaxial relationship, as displayed by GaN (002)||β-Ga₂O₃ (-201).

In order to investigate the crystalline quality of GaN films converted from different Ga₂O₃ films, X-ray rocking curves (XRCs) have been measured for the symmetric (0002) and asymmetric (10–12) planes of GaN and the full width at half maximum (FWHM) values are described in Figure 1c. It is obvious that the GaN layer converted from the amorphous Ga₂O₃ exhibits relatively better crystalline quality with (0002) and (10–12) FWHMs of 1.07° and 1.13°, larger than that of the GaN converted from the bulk β-Ga₂O₃ [22]. Therefore, the high-quality β-Ga₂O₃ would produce less defects in GaN formed by the nitridation process.

The Raman spectra of the nitridated Ga₂O₃ films are shown in Figure 1d. The peaks at 568 cm⁻¹ and 733 cm⁻¹ correspond to E₂(high) and A₁(LO) of the wurtzite GaN phonon modes, respectively. The strong Raman E₂(high) phonon mode reflects the characteristics of wurtzite GaN in the nitridated films. Ga₂O₃-related peaks have also been seen in the spectra (as marked in *) of nitridated Ga₂O₃ films (NS1, NS2) due to incomplete nitridation and phase-transformation, which is in agreement with XRD analysis. The peak at 420 cm⁻¹ is related to octahedral GaN (GaN₆) structures [28]. In the nitridation process, the oxygen atoms in Ga₂O₃ were partially substituted by nitrogen atoms. And it has been reported that the oxygen atom of the octahedron site was preferably substituted by the nitrogen atom rather than that of the tetrahedron site in the nitridation process [23,29]. The E₂(high) mode has usually been thought to be sensitive to biaxial strain in GaN epilayers, and the frequency shift (Δω) linearly depends on the residual stress (σ), according to the following relationship [30]:

⟨Δω⟩ = K⟨σ⟩

where K represents the stress coefficient, equal to 4.3 cm⁻¹/GPa for GaN. The frequency shift, Δω, is defined as Δω = ω − ω₀, where ω is the peak frequency center and ω₀ accounts for the corresponding stress-free E₂(high) peak frequency center, fixed at 567.8 cm⁻¹ [31]. Therefore, the stress can be calculated as 0.069 Gpa.

The surface morphologies of the original Ga₂O₃ films and as-nitridated films are illustrated in Figure 2. After the nitridation, the surfaces become rough with a considerable amount of voids. The nitridated amorphous Ga₂O₃ films (NS3) show porous island-like network surfaces; the size of the islands is about 200–800 nm. The formation of the voids was caused by the etching of H₂, NH, or NH₂, decomposed from NH₃ under high temperature [18,22,23].

From the above results, GaN layers with (002)-preferred orientation have been obtained after the nitridation of different Ga₂O₃ films, and a lot of voids have been formed in the nitridated surface. The porous GaN layers could be used as templates for the subsequent homoepitaxy of GaN thick films without obvious stress [18,19]. Therefore, the growth of thick GaN films grown on these templates has been carried out using the HVPE method.

Figure 3a displays the XRD patterns of as-grown HVPE-GaN films. The XRD patterns of all the HVPE-GaN thick films grown on these templates show a sharp GaN (002) diffraction peak, which indicates the preferred orientation and high quality of HVPE-GaN. XRCs have also been used to characterize the crystal quality (not shown here) and found that the quality of as-grown GaN films is higher than that of the converted GaN layers, and can also be better than that of GaN on bulk β-Ga₂O₃ crystal with only slight nitridation treatment. Oxygen in porous templates can be removed by hydrofluoric acid (HF) etching and re-nitridation [32], so as to improve the crystal quality of porous template and then improve the quality of re-epitaxial GaN. The further optimization of the GaN growth conditions on the converted porous GaN templates is also in progress. Raman spectra of as-grown HVPE-GaN thick films have been shown in Figure 3b. It is clear that the characteristic E₂(high) mode of GaN is in good agreement with that of bulk GaN [31], which reveals that as-grown HVPE-GaN thick films are free of stress. The optical properties of the as-grown GaN films were investigated using room temperature photoluminescence measurements. The grown HVPE-GaN films possess sharp and narrow near-band-edge (NBE) emissions, located at 3.40 eV, which appear due to the radiative recombination of excitons bound to neutral donors [33]. Note that the value of the NBE peak for unstressed bulk GaN film was taken to be 3.40 eV [34]; therefore, the GaN thick films are free of stress.

Figure 4 shows the top-view and cross-sectional SEM images of as-grown HVPE-GaN thick films on nitridated Ga₂O₃ films. The surface of GaN films grown on NS1–NS3 are flat (Figure 4a), with high-density nanoscale pits (observed at high magnification). A clear dividing line can be observed between the as-grown GaN thick films and sapphire substrate from the cross-sectional SEM images in Figure 4b. The interface layer between the GaN epilayers and sapphire is thought to consist of a porous GaN layer and un-nitridated Ga₂O₃. The porous GaN layer converted from Ga₂O₃ would decrease the stress of the subsequent GaN epitaxy and would also realize the low-stress free-standing GaN substrates by a self-separation or chemical lift-off (CLO) process [35,36,37].

Based on the above results, it is concluded that Ga₂O₃ films with smooth surface morphologies, whether amorphous or single crystalline films, can be converted by the nitridation to the porous GaN layers. In our past reports [18,19,27], the use of nitridated β-Ga₂O₃ layers as a porous template for epitaxy and self-separation has been well studied. In this work, amorphous or other polymorph Ga₂O₃ films were converted to (-201) β-Ga₂O₃ at high temperatures, above 800 °C, and then nitridated to (002) GaN layers. The porous GaN layers are promising templates for the re-epitaxy of low-stress GaN to prepare a free-standing GaN substrate. It is noted that the amorphous or metastable polymorph Ga₂O₃ films in this work would be more suitable for the preparation and large-scale applications of porous GaN templates, as their production requires only simple equipment and mild conditions, as well being low-cost and having higher repeatability.

4. Conclusions

In summary, the growth of stress-free GaN thick films on nitridated Ga₂O₃ films has been successfully performed using the HVPE method. Single crystalline GaN layers have been formed by the nitridation of the Ga₂O₃ films prepared by various methods under an NH₃ atmosphere. XRD and SEM results reveal that all the converted GaN layers exhibit an (002)-preferred orientation with porous morphologies. The correlation between Raman frequency shifts in the E₂ mode of the converted GaN layer was accurately quantified, revealing a stress of approximately 0.069 GPa. As-grown HVPE-GaN films are almost stress-free, with stronger photoluminescence (PL) intensity. These results suggest that Ga₂O₃ films for the preparation of porous GaN templates can be obtained by a simpler method with lower cost and better repeatability, which will be beneficial for the preparation of high-quality strain-released free-standing GaN substrates.