Epitaxial growth of GaN films on nearly lattice-matched hafnium substrates using a low-temperature growth technique Open Access

Gallium nitride (GaN) is an attractive semiconductor material for various optical device applications, such as light-emitting diodes (LEDs).^1–3 To date, most presently available commercial GaN-based LEDs have been fabricated by metal-organic chemical vapor deposition (MOCVD) on single-crystalline sapphire substrates to take advantage of their chemical and thermal stability. However, the large mismatches in lattice constants (14%) and thermal expansion coefficients (34%) between GaN and sapphire cause high crystalline defect densities in the GaN films, which leads to degradation of device performance.^4,5 To address these issues, numerous attempts have been made to replace the sapphire substrates for GaN heteroepitaxy with various materials having smaller lattice mismatches.⁶ 

Metallic hafnium (Hf) is one of the most promising materials for this purpose, because Hf has a hexagonal closed packed (hcp) structure with an a-axis lattice constant and thermal expansion coefficient close to those GaN having mismatches of 0.3% and 5.3%, respectively. An additional advantage of the metal substrates for GaN growth lies in the possibility that they are used as lower electrodes of vertical devices.

Despite these advantages, Hf substrates have not been a practical candidate for GaN films because of the difficulty in GaN epitaxial growth, which is associated with significant interfacial reactions during high-temperature growth above 1000 °C in conventional MOCVD.⁷ 

We have recently developed a growth technique called pulsed excitation deposition (PXD), where a pulsed excitation source such as a laser, electron beam, or plasma is used to ablate the target material.^8–10 This technique allows us to grow epitaxial films of group III nitride compounds even at room temperature, thanks to the high kinetic energy and pulsed supply of group III atoms, which enhance the surface migration of film precursors on substrate surfaces.^8,10–13 Such low-temperature growth enables epitaxial growth of GaN and AlN films even on chemically vulnerable substrates such as metal because the interfacial reactions between nitride films and substrates are suppressed.^14–17 

In this study, we have investigated the feasibility of epitaxial growth of GaN films on matched Hf (0001) substrates with small lattice mismatches using the low-temperature (LT) growth technique involving PXD.

All GaN films were grown on Hf (0001) substrates using PXD. We used commercially available single crystalline Hf substrates with a size of 1 cmϕ. A typical full width at half maximum (FWHM) value of the x-ray rocking curve (XRC) for 0002 diffraction of the Hf substrates was 0.15°. Before film growth, surface contaminants and native oxide layers were removed from the Hf substrates by vacuum annealing. After cleaning, GaN films were grown either at 700 °C or at LT below 200 °C in a PXD chamber. The detailed growth conditions are described elsewhere.^13,16,18 The film surfaces were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), and Al K α X-ray photoelectron spectroscopy (XPS). The sharpness of the heterointerfaces between GaN and Hf was investigated using X-ray reflectivity (XRR). The structural properties of the GaN films were characterized by X-ray diffraction (XRD) using a Bruker D8 diffractometer and by electron backscatter diffraction (EBSD) using an INCA Crystal EBSD system (Oxford Instruments).

To investigate the removal of contaminants and oxide layers during the annealing process, an XPS apparatus equipped with a high-temperature sample heater was utilized. Oxygen and carbon were detected as contaminants on Hf (0001) surfaces before annealing. Figures 1(a)–1(c) show in situ XPS spectra for peaks of O 1s, C 1s, and Hf 4f, respectively, at various annealing temperatures between RT and 1000 °C. One can clearly see that the intensities of the O 1s and C 1s peaks diminish as the annealing temperature increases. The surface concentrations of oxygen and carbon were reduced to less than 5% of their initial concentrations after annealing at 1000 °C. In the Hf 4f spectra prior to annealing, the peaks from oxide layers were clearly detected at binding energies around 16 and 14 eV for Hf 4f_5/2 and 4f_7/2, respectively. These peaks nearly disappear after annealing at over 800 °C, which is consistent with the change in the O 1s and C 1s spectra. We also monitored the Hf surfaces by RHEED before and after annealing. A halo pattern was observed prior to annealing, which probably originates from amorphous Hf-O₂ layers on the initial surfaces. The RHEED pattern changed to sharp streaks after annealing at 1000 °C (inset of Fig. 1(d)), indicating that clean and smooth Hf surfaces were obtained with this procedure. Figure 1(d) shows an AFM image of a Hf surface after annealing. AFM characterization found that the root-mean-square (rms) value of the surface roughness was as low as 1.8 nm, which is consistent with the sharp, streaky RHEED patterns. The combined results above led us to conclude that epitaxy-ready surfaces can be prepared on Hf substrates by annealing.

We attempted to grow GaN films at 700 °C on the annealed Hf substrates. Figures 2(a)–2(d) show RHEED images for the resulting GaN films of various thicknesses. The RHEED image changed to a ring pattern just after initiation of GaN growth, which indicates growth of polycrystalline GaN films at this substrate temperature. Poorly crystalline GaN film formation is probably caused by interfacial reactions between GaN and Hf. Beresford et al. reported that high-temperature growth results in the formation of a Ga-Hf intermetallic compound layer during GaN growth on Hf,⁷ and XRR analysis on this sample did indeed reveal the presence of a 4-nm-thick interfacial layer at the GaN/Hf heterointerface.

To suppress the interfacial reaction between GaN and Hf, we performed LT growth of GaN below 200 °C. Figures 3(a)–3(c) show RHEED images of LT-GaN films with thicknesses of 8, 20, and 30 nm, respectively. The sharp streaks in the RHEED patterns indicate that the LT-GaN films grew epitaxially on the Hf substrates. Careful interpretation of the RHEED patterns led us to conclude that the epitaxial relationship is GaN[11$2 ̄$0]//Hf[11$2 ̄$0] and GaN[0001]//Hf[0001]. The distinct oscillations of the specular-spot intensity were observed at an early stage during LT growth of GaN as shown in Fig. 3(d), corresponding to two-dimensional nucleation and layer-by-layer growth of the GaN film. This layer-by-layer growth is probably attributed to the small lattice mismatch between GaN (0001) and Hf (0001). The interfacial layer thickness was estimated to be as small as 1 nm based on XRR analysis, indicating that the interfacial reaction was effectively suppressed by the reduced growth temperature.

Figure 4(a) shows the XPS spectra for GaN films prepared at 700 °C and at LT. In this case, the Hf 4d_5/2 and 4d_3/2 peaks were clearly detected on GaN surfaces, indicating that diffusion of Hf atoms onto the GaN surfaces took place due to interfacial reactions. In contrast, the Hf signal on the LT-grown GaN surfaces was below the detection limit, which is consistent with the results of RHEED observations and XRR measurements. The crystal orientations of the GaN films were investigated by EBSD measurements. Figures 4(b) and 4(c) show EBSD pole figures of a 20 × 20 μm² area of the GaN films grown at 700 °C and LT, respectively. The 700 °C-grown GaN film exhibits randomly oriented patterns in the {0001} and {11$2 ̄$4} pole figures, indicating the polycrystalline character of the films. In contrast, the {0001} spot for the LT-grown GaN film is sharp, and a clear six-fold rotational symmetry appears in the {11$2 ̄$4} pole figure. These results suggest that a reduction in growth temperature leads to drastic improvement in the crystalline quality of GaN films on Hf substrates.

It is important to investigate the thermal stability of the GaN/Hf heterostructures prepared at LT because fabrication of GaN-based LEDs requires high process temperatures; p-type GaN must be grown above 480 °C.¹³ Figure 5(a) shows the post-annealing temperature dependence on the interfacial layer thickness between LT-grown GaN films and Hf substrates. One can see that the interfacial layer thickness at the heterointerfaces remains unchanged even after post-annealing for 60 min up to approximately 700 °C. The inset in Figure 5(a) shows an XPS spectrum for a GaN surface after post-annealing. The absence of Hf 4d peaks in this spectrum indicates that no significant diffusion of Hf atoms into the GaN surface occurs during this post-annealing. We also confirmed that the post-annealed GaN surface was not degraded and remained smooth with stepped and terraced structures as shown in Fig. 5(b). These results suggest that the GaN/Hf structure prepared at LT is highly stable once it is formed, and that LT-grown epitaxial GaN films on Hf can serve as buffer layers for the growth of high quality GaN films at higher temperatures.

After LT-growth of an epitaxial GaN buffer layer, we grew GaN films at 700 °C on this initial LT-grown layer. The thickness and the growth time for the 700 °C-grown GaN films were 1.2 μm and 60 min, respectively. As shown in the EBSD pole figures of Fig. 6(a), the GaN film exhibited a distinct sharp spot in the {0001} pole figure and six distinct peaks in the {11$2 ̄$4} pole figure, indicating epitaxial growth of GaN without the presence of 30° rotational domains. Figure 6(b) shows the out-of-plane 0002 XRC for 700 °C-grown GaN films with and without LT-grown GaN buffer layers. The FWHM values decreased dramatically from 2.42° to 0.49° by inserting LT-grown GaN layers. This result indicates that the LT-grown epitaxial GaN buffer layer plays an important role in improving the crystalline quality of GaN films grown on Hf substrates.

In summary, we demonstrated epitaxial growth of GaN films on nearly lattice-matched Hf (0001) substrates by using a LT epitaxial growth technique. Growth of GaN at high temperatures results in the formation of polycrystalline films due to significant reaction at GaN/Hf heterointerfaces, while growth of GaN at LT allows us to suppress the interfacial reactions and to obtain high quality GaN films on Hf with an epitaxial relationship of GaN[11$2 ̄$0]//Hf[11$2 ̄$0] and GaN[0001]//Hf[0001]. The LT-grown GaN layers can act as buffer layers for GaN growth at high temperature. The interfacial layer thickness at the LT-GaN/Hf heterointerface was as small as 1 nm and the sharpness of the contact remained unchanged even after annealing up to approximately 700 °C, which likely accounts for the dramatic improvement in GaN crystalline quality on Hf substrates with the use of a LT-GaN buffer layer.