1. Introduction

Self-assembled GaN quantum dot (QDs) have been the recent focus owing to the promising applications for different devices such as ultraviolet light-emitting-diodes (LEDs) [1, 2], single-photon sources [3], single-electron transistors [4], memory devices [5], and infrared photodetectors [6]. The fabrication of self-assembly GaN QDs has been extensively investigated, and the most frequently used approach is self-assembly process in Stranski-Krastanov (SK) growth modes which requires sufficient lattice mismatch and usually results in a 2D wetting layer [7, 8]. An alternative self-assembly growth approach is Ga droplet epitaxy which is based on vapor-liquid-solid (VLS) mechanism [9]. This technique offers distinct advantages in size and density manipulation which can be finely tailored by tuning the amount of deposited Ga and substrate temperature [10]. Although some researchers have reported the successful fabrication of GaN QDs using Ga droplet epitaxy, only few of them studied the optical performances of the obtained GaN QDs at room temperature because this technique brings serious birth defects due to the incomplete nitridation of Ga droplet at low temperature, leading to the poor optical performances of the obtained GaN QDs [11, 12]. Therefore, it is important to investigate the post-growth annealing (PGA) treatments to enhance the nitridation of Ga droplet. However, most PGA studies concerning GaN QDs are about the SK grown QDs [13–15]. In this letter, we have investigated the influence of high-temperature PGA under NH₃/H₂, NH₃/N₂ ambience on the morphologies and nitridation of GaN QDs grown via Ga droplet epitaxy. Strong PL emissions of GaN QDs after PGA are observed at room temperature.

2. Experiment

The GaN QDs were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD) under the reactor pressure of 40 torr. Triethylgallium (TEG), trimethylaluminum (TMAl) and ammonia (NH₃) were used as precursors for Ga, Al and N sources with H₂ as carrier gas. Prior to GaN deposition, a 300 nm thick AlN consisting of ~20 nm thick AlN nucleation layer grown at 670 °C and ~280 nm thick AlN epilayer grown at 1050 °C was first deposited, the details of which can be found elsewhere [16]. After that, NH₃ were purged for 2 minutes. Then Ga droplets were formed on the AlN templates by introducing the TEG into the reactor for 12 seconds in the absence of NH₃ flow at 600 °C, and the flow of TEG is 7.5 μmol/min. Subsequently, the initial nitridation of Ga droplet was carried out by importing the NH₃ flow of 2000 sccm (standard cubic centimeters per minute) for 12 seconds at the same temperature. Afterwards, 12-second PGA at different elevated temperatures was performed in two different ambient, NH₃/H₂ and NH₃/N₂, in which the NH₃ flow rate was kept constant of 2000 sccm. For comparison, the ramp time for these samples are all 3 min and 2000 sccm NH₃ is supplied during this time. What’s more, 2000 sccm NH₃ is supplied during cooling down to prevent the GaN QDs from decomposing.

3. Results and discussion

Figure 1(a) exhibits the morphologies of as-grown GaN dots or Ga metallic dots, which was directly cooled down after 12 seconds introduction of NH₃. The size of the dots is quite uniform. Most dots have an average diameter of 40 nm and height of 3.5 nm. The total density of dots is 8.0 × 10¹⁰ cm⁻². Figure 2 shows the morphologies of GaN QDs annealed at elevated temperatures from 700 °C to 900 °C under NH₃/H₂ [(a)-(d)] and NH₃/N₂ [(e)-(h)] ambience and the quantitative statistics of morphologies are listed in Table 1 . For the samples annealed under NH₃/H₂ ambience, the following behaviors are observed. After the QDs were annealed at 700 °C, some larger QDs with the average diameter of 60 nm and height of 4 nm are observed as shown in Fig. 2(a), and the density is reduced to 2.2 × 10¹⁰ cm⁻². With the increase of the annealing temperature to 750 °C, the QDs density is further reduced (4.0 × 10⁹ cm⁻²) and even larger QDs present with the average diameter of 80 nm and height of 5 nm. This is the typical Oswald ripening process [17], during which larger islands grow at the expense of the smaller ones. However, as the temperature increased to 800 °C and 850 °C, GaN QDs disappeared, leaving a flat surface. This is expected due to the decomposition of GaN at high temperature with the presence of H₂ [18, 19].

 

Fig. 1 AFM images of GaN dots or Ga metallic dots without annealing in scan area of 2 × 2 um², the height scale is 20 nm.

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Fig. 2 AFM images of GaN QDs annealed under NH₃/H₂ ambience at 700 °C (a); 750 °C (b); 800 °C (c); 850 °C (d), and GaN QDs annealed under NH₃/N₂ ambience at 750 °C (e); 800 °C (f); 850 °C (g); 900 °C (h) in scan area of 2 × 2 um², the height scale for all the images is 20 nm.

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Table 1. The quantitative statistics of morphologies

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For the samples annealed under NH₃/N₂ ambience, some different behaviors are observed. After the QDs were annealed at 750 °C, the average diameter of the QDs (38 nm) is getting smaller compared with the as-grown ones (40 nm) [Fig. 2(e)], the corresponding height is getting larger (4 nm). And the density is reduced to 3.6 × 10¹⁰ cm⁻². Unlike the samples annealed under NH₃/H₂ ambience, the QDs still exist after annealing at 800 °C and 850 °C [Fig. 2(f)-2(g)]. With the increase of annealing temperature to 800 °C, the uniformity is destroyed with the presence of larger GaN QDs (the diameter varies from 40 to 85 nm and the height varies from 1.7 nm to 6 nm) and the density is decreased to 1.3 × 10¹⁰ cm⁻² accordingly. After the annealing temperature was increased to 850 °C, the patches of GaN “wetting layer” emerge on the surface as what reported by M. Gherasimova et al. [12]. The thickness of these patches is not uniform which varies from 0.55 nm to 0.98 nm. Most of QDs have diameter of 45-68 nm, height of 4.5-5.8 nm and the density is 6.1 × 10⁹ cm⁻². The QDs finally disappear with the annealing temperature increased to 900 °C, which is 100°C higher than that under NH₃/H₂ ambience. These phenomena suggest that NH₃/N₂ ambience can prevent GaN QDs from decomposition at high temperature, which agrees well with the results of other studies [12].

Indeed, the PGA is a temperature-dependent dynamic balance of several competing mechanisms. During high temperature thermal annealing, some unstable GaN decomposes and Ga droplets escape from their nucleation sites, resulting in the formation of free Ga atoms ($GaN\to Ga+N$). These free Ga atoms may migrate in the growth front to form larger QDs at stable GaN QDs ($Ga+N\to GaN$) or evaporate out [12]. Also, the migration of Ga atoms can be suppressed under sufficient NH₃ vapor. Therefore, NH₃/N₂ ambience is better condition than NH₃/H₂ in terms of maintaining the QDs morphology by suppressing the migration and evaporation of Ga atoms and preventing the GaN from decomposition. When QDs annealed at lower temperature, the migration and nucleation mechanisms are dominant, leading to the ripening of GaN QDs. As the temperature further increased, the QD structures become more unstable, thus GaN decomposition and Ga evaporation dominate, which cause the disappearance of QDs.

However, GaN material still exists on the surface. As shown in Fig. 3 , X-ray photoelectron spectroscopy (XPS) measurement for samples annealing at 900 °C and 950 °C is carried out. For both samples, the Ga 3d and Ga 2p photoelectron lines were detected. GaN probably change from QDs into a two-dimensional layer at such a high temperature. This indicates that QDs disappear along with the formation of a more stable films on the surface when annealed at relatively high temperature.

 

Fig. 3 XPS survey spectra of samples annealed at 900 °C and 950 °C under H₂/NH₃ ambient.

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To get better understanding of PGA processes, the chemical bondings of the samples were characterized by XPS. Figure 4 shows the XPS spectra of Ga 3d core level for samples annealed at different conditions. Each peak is further fitted by three sub-peaks, Ga metal from residual metallic Ga, Ga-N and Ga-O with the corresponding binding energy of 18.2 eV [20, 21], 19.9-20.3 eV and 20.7-21 eV [22]. The proportion of Ga metal, Ga-N and Ga-O components in the configuration of Ga 3d core level are listed in Table 2 . These results reveal that the nitridation is enhanced effectively as the annealing temperature rising from 750 °C to 850 °C (800 °C) in NH₃/N₂ (NH₃/H₂) ambience, then it fall dramatically at exceedingly high temperature. This can be attributed to the fact that the evaporation of Ga is enhanced and Ga droplet constantly transforms into GaN as the anneal temperature increasing. And at higher temperature, the QDs structure on the growth surface become unstable to decompose. Additionally, the proportion of Ga-N component in the Ga 3d core level of the samples annealed under NH₃/H₂ ambience are all weaker than those of samples annealed under NH₃/N₂ ambience at same annealing temperature, which suggests that the NH₃/N₂ ambience is more helpful to enhance the nitridation. However, for all samples there is still a XPS signal of the Ga-metal which can be seen in Fig. 4. Therefore, we plan to study the in situ N₂ plasma treatment to completely transform the Ga droplet into GaN in the further.

 

Fig. 4 XPS spectra from Ga 3d core levels of the GaN QDs annealed at different temperature (a) in H₂ ambient, (b) in N₂ ambient. Dashed lines correspond to fitted curves for each bond.

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Table 2. The proportion of Ga-O, Ga-N and Ga metal

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To study the influence of PGA on the optical properties of GaN QDs, the room-temperature photoluminescence (PL) excited by 224 nm excitation laser was intensively carried out. Figure 5 displays the PL spectra of surface GaN/AlN QDs and AlN template. It is worth noting that a peak near 295 nm (4.2 eV) is present in AlN template and all the GaN/AlN QDs samples. This peak is origin from the deep level impurity related transition in AlN, as well as the peak near 388 nm (3.2 eV) [23]. When the annealing temperature is not exceeding 750 °C under NH₃/N₂ ambience, the samples just exhibit the luminescence related to AlN. As the samples are annealed under NH₃/N₂ at 750 °C and 800 °C, the emission of GaN QDs emerges at 332 nm. With the annealing temperature increased to 850 °C, the luminescence intensity of GaN QDs is further enhanced and the peak shifts to 334 nm, which can be ascribed to the increase of QDs height owing to the ripening process. Because the uncapped GaN QDs is lack of the strain from upper caplayer, the quantum confined stark effect depending on piezoelectric polarizations is weak, which is responsible for the small energy-shift. Thus it can be conclude that the PGA under NH₃/N₂ at 850 °C can sufficiently improve the optical properties of GaN QDs grown by Ga droplet, which suggests the enhancement of GaN crystal quality.

 

Fig. 5 Room temperature PL spectra of surface GaN QDs.

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However, there is even a problem that the luminescence intensity related to defect of AlN is much larger than that of naked GaN QDs. In order to explain this fact, a model is proposed as shown in Fig. 6 . Indeed, the PL emission at 3.2 eV is a donor-acceptor-pair type transition involving a shallow donor and a V^3-/2- deep acceptor [24]. And the 4.2 eV emission is also assigned to the recombination of a shallowly trapped electron with a deeply trapped hole in a (V_Al-O_N)^2-/1- center, which is observed as a ~3.9 eV line due to some coulomb energy on the order of ~0.3 eV [25]. Room temperature band gaps of GaN QDs and AlN template were taken as 3.72 eV and 6.05 eV, respectively. The valence band offset at the GaN/AlN hetero-interface is 0.67 eV (30%). The top of valence band is a reference zero level. The ionization energies (E_D) of the shallow donors in AlN are about 86 meV [26]. So, the acceptor level of (V_Al-O_N)^2-/1- is about 1.76 eV. For uncapped GaN QDs, the surface states located within band-gap can be occupied by majority electrons, leading to an upward band bending. Based on the measurements of band bending at GaN (0001) surface reported by Dhesi et al. and Valla et al., this value is 2.1 ± 0.1 eV [27] or about 1.5 eV [28]. The band bending results in a reduced overlap of the wave-functions of photo-generated electrons and holes in GaN QDs. Thus the near band emission of naked GaN QDs becomes weak. However, the level of surface states filling holes is close to that of (V_Al-O_N)^2-/1- center and the reduced probability of forming an exciton in GaN QDs will extend the lifetime of holes at the surface to increase the probability that a hole trapped at the surface tunnels into the acceptor level of (V_Al-O_N)^2-/1- in AlN, which participate in the PL emission through the recombination with the electron in shallow donor. There the emission peak near 4.2 eV can be greatly enhanced.

 

Fig. 6 A schematic energy diagram about the band alignment of surface GaN/AlN QDs

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4. Conclusion

In summary, the influence of high-temperature PGA under NH₃/H₂ and NH₃/N₂ ambience on the surface morphology and nitridation degree of GaN QDs grown via Ga droplet has been analyzed. The PGA is a temperature-dependent dynamic balance of GaN decomposition, Ga atoms migration, nucleation and evaporation out. As the temperature increased within a certain scope of temperature, the degree of nitridation of the GaN QDs is constantly enhanced regardless of the annealing ambience, which is mainly the result of the residual Ga atoms migration and nucleation. When the annealing temperature is too high, the GaN QDs will transform into GaN film due to the coaction of the decomposition of QDs, migration and nucleation of Ga atoms. Moreover, the PGA ambiences have also affected the dot formation on the surface significantly, and a NH₃/N₂ ambience is demonstrated as a necessary condition for maintaining optimal QDs morphology by suppressing the migration and evaporation of Ga atoms and preventing the GaN decomposition.