Fabrication of low-density GaN/AlN quantum dots via GaN thermal decomposition in MOCVD

With an appropriate high anneal temperature under H2 atmosphere, GaN quantum dots (QDs) have been fabricated via GaN thermal decomposition in metal organic chemical vapor deposition (MOCVD). Based on the characterization of atomic force microscopy (AFM), the obtained GaN QDs show good size distribution and have a low density of 2.4 × 108 cm-2. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the GaN QDs were formed without Ga droplets by thermal decomposition of GaN.


Background
Low-dimensional III-nitrides materials have gained much research attention because of their strong carrier confinement which may lead to the realization of next-generation electronic and optoelectronic applications [1][2][3][4][5]. Among these low-dimensional III-nitride materials, the study of single GaN quantum dot has become the recent focus due to its promising applications in the solid-state quantum computation, single-photon sources, and single-photon detectors, in which the density of quantum dots is required to be as low as approximately 10 8 cm −2 [6][7][8][9]. However, challenges remains in fabrication of low-density GaN quantum dots (QDs) with high quality. On the one hand, the most frequently used fabrication approach is self-assembly process via Stranski-Krastanov (SK) growth mode which requires sufficient lattice mismatch, but it is harder to acquire low-density GaN QDs and usually results in randomly distributed QDs with different sizes [10,11]. On the other hand, although some low-density GaN nanodots can be obtained by the droplet epitaxy technique based on a vapor-liquid-solid process which offers distinct advantages in size and density manipulation of QDs, the droplet epitaxy technique usually results in QDs with the incomplete transition from Ga droplet to crystal GaN. What is more, there is almost no report about fabrication of low-density GaN QDs via the droplet epitaxy technique [12,13]. Motivated by the above issues, recently, we have demonstrated the fabrication of GaN nanodots on AlN templates via GaN thermal decomposition in H 2 atmosphere, which does not involve the induction of strain or the crystallization of the Ga droplets [14]. In addition, the recent studies and applications of GaN-based materials growth have been demonstrated [15][16][17][18][19][20]. In this letter, the thermal decomposition conditions are further optimized and low-density GaN/AlN QDs with high quality are achieved. This study provides an alternative approach to fabricating low-density GaN QDs for single-photon devices.

Methods
GaN QDs were formed on AlN/sapphire templates by metal organic chemical vapor deposition (MOCVD). Triethylgallium (TEGa), trimethylaluminum (TMAl), and ammonia were used as precursors for Ga, Al, and N sources with H 2 as carrier gas. The total pressure was maintained at 40 Torr. The sapphire substrates were introduced into the MOCVD reactor and 800-nm-thick AlN buffer layers were deposited. Then, 800-nm-thick GaN epilayers were grown on the AlN templates at 940°C. Subsequently, the samples were annealed in H 2 atmosphere at different conditions: sample A, at 1,050°C for 5 min; sample B, at 1,100°C for 5 min and sample C, at 1,100°C for 8 min, as shown in Figure 1. After that, these samples were cooled down to room temperature at the presence of NH 3 + H 2 . Besides, two controlled experiments were also conducted. One was the growth of 800-nm-thick GaN on 800-nm-thick AlN/sapphire without decomposition in H 2 (sample D), and another one is an 800-nm-thick AlN buffer template on sapphire without decomposition in H 2 (sample E). The surface morphologies of all samples were characterized by atomic force microscopy (AFM) measurements. The surface chemistries of obtained GaN QDs and some control samples were investigated using X-ray photoelectron spectroscopy (XPS) measurements with monochromatic Mg Kα X-ray source (hν = 1,253.6 eV).

Results and discussion
The surface morphologies of all samples were studied by atomic force microscopy (AFM), and the results are shown in Figure 2. Compared with the surface morphology of controlled sample D (Figure 2d), it is obvious that GaN decomposition occurs for Sample A (Figure 1a). Figure 1f is the corresponding three-dimensional (3D) AFM image of Figure 1a, in which distributed dots are on terraces and abrupt peaks are to be buried in the side wall, indicating the decomposition process for the formation of GaN dots. As the decomposition occurred toward the inner of the side wall, the abrupt peaks are then exposed to H 2 flow and decomposed. Since the heights of peaks decrease faster than the diameters of peaks, the side wall is etched away and the peaks are etched to small dots with a longer etching time, which is consistent with our previous observation [14]. With increasing of the annealing temperature from 1,050°C to 1,100°C, the decomposition of GaN has an interesting phenomenon that the steps disappear and wellshaped dots are just left on a flat surface, as shown in   To further investigate the size distribution of the obtained GaN QDs, the AFM images of sample B with scan area 10 × 10 μm 2 is shown in Figure 3a. The QDs have a low density of approximately 2.4 × 10 8 cm −2 and no obvious big dots are observed, showing the good uniformity. Figure 3b illustrates the diameter histograms obtained by the AFM analysis of Figure 3a. It is clear that the most probable diameter is in the range from 70 to 80 nm. The inset of Figure 3b shows a detailed 3D AFM image of the QDs in 1 × 1 μm 2 , indicating the similar well-formed dot structure. According to the results above, the obtained GaN QDs have a good size distribution. To the best of our knowledge, this is the first report of low-density GaN QDs fabricated via GaN thermal decomposition in MOCVD.
As is shown in Figure 4, since XPS analysis was performed for samples A, B, and C, Ga2p and N1s core level spectra were measured. For both of the XPS spectra, the C1s peak at approximately 285.0 eV was used as  binding-energy reference. Baselines were fixed using a Shirley background subtraction model and all peaks were fitted using a linear combination of 80% Gaussian and 20% Lorentzian line shapes. On the one hand, the Ga2p spectra are analyzed in Figure 4a. Both samples A and B have a Ga2p peak which can be fitted as only one subpeak located at 1,117.1 eV, which is assigned to Ga-N bond [22][23][24]. So there are no Ga droplets but GaN on the surface of samples A and B, indicating that the Ga desorption rate exceed the GaN decomposition rate. On the contrary, if the Ga desorption rate is less than the GaN decomposition rate, Ga droplets will generate in a chemical manner and Ga-Ga bond will be observed. No Ga2p peaks were observed in sample C, confirming that sample C is just the AlN buffer after H 2 decomposition. On the other hand, the N1s spectra are analyzed in Figure 4b. For sample A, the N1s spectra can be decomposed into a total of four fitted subpeaks at 397.0, 398.7, and 400.3 eV, which were assigned to N-Ga bond, N-H 2 bond and N-H 3 bond [25,26], respectively. Only GaN existed on the surface of sample A. For sample C, the N1s spectra can be decomposed into one subpeaks at 398.7 eV, which is assigned to N-Al bond [27]. Only AlN existed on the surface of sample C. For sample B, the N1s spectra were decomposed into a total of four fitted subpeaks at 396.2, 397.0, 398.7, and 400.3 eV, which can be assigned to N-Al bond, N-Ga bond, N-H 2 bond, and N-H 3 bond, respectively. These fitted subpeaks coincide with the fitted subpeaks of samples A and C, providing a chemical evidence for the existence of GaN QDs formed on the AlN buffer. In addition, the N-H 2 bond and N-H 3 bond were obtained in samples A and B but did not exist in sample C, indicating that the appearance of N-H 2 bond and N-H 3 bond were caused by the interaction of decomposed GaN and hydrogen at high temperature.

Conclusions
In summary, GaN QDs were fabricated on flat AlN buffer via H 2 decomposition of GaN film at high temperature. The AFM measurements show that the obtained GaN QDs have good size uniformity and a low dot density about 2.4 × 10 8 cm −2 . The XPS spectra analysis actually demonstrated that the GaN QDs do not contain Ga droplets. The results provide an alternative approach to fabricate low-density GaN QDs for applications in singlephoton devices.