Effect of non-vacuum thermal annealing on high indium content InGaN films deposited by pulsed laser deposition

InGaN films with 33% and 60% indium contents were deposited by pulsed laser deposition (PLD) at a low growth temperature of 300 °C. The films were then annealed at 500-800 °C in the non-vacuum furnace for 15 min with an addition of N2 atmosphere. X-ray diffraction results indicate that the indium contents in these two films were raised to 41% and 63%, respectively, after annealing in furnace. In2O3 phase was formed on InGaN surface during the annealing process, which can be clearly observed by the measurements of auger electron spectroscopy, transmission electron microscopy and x-ray photoelectron spectroscopy. Due to the obstruction of indium out-diffusion by forming In2O3 on surface, it leads to the efficient increment in indium content of InGaN layer. In addition, the surface roughness was greatly improved by removing In2O3 with the etching treatment in HCl solution. Micro-photoluminescence measurement was performed to analyze the emission property of InGaN layer. For the asgrown InGaN with 33% indium content, the emission wavelength was gradually shifted from 552 to 618 nm with increasing the annealing temperature to 800 °C. It reveals the InGaN films have high potential in optoelectronic applications. ©2013 Optical Society of America OCIS codes: (160.2100) Electro-optical materials; (160.6000) Semiconductor materials. References and links 1. J. Wu, W. Walukiewicz, K. M. Yu, W. Shan, J. W. Ager, E. E. Haller, H. Lu, W. J. Schaff, W. K. Metzger, and S. Kurtz, “Superior radiation resistance of In1−xGaxN alloys: Full-solar-spectrum photovoltaic material system,” J. Appl. Phys. 94(10), 6477–6482 (2003). 2. T. Matsuoka, N. Yoshimoto, T. Sasaki, and A. Katsui, “Wide-gap semiconductor InGaN and InGaAln grown by MOVPE,” J. Electron. Mater. 21(2), 157–163 (1992). 3. S. Keller, S. F. Chichibu, M. S. Minsky, E. Hu, U. K. Mishra, and S. P. 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Introduction
InGaN alloys with a wide band gap range from 0.7 eV (InN) to 3.4 eV (GaN) are used as the active layers for light emitting diodes (LEDs) to emit blue, green and red lights [1].In order to grow the InGaN layer with high indium content by metalorganic chemical vapor deposition (MOCVD), the relatively low growth temperature from 500 to 850 °C is required.However, some drawbacks consisting of the low surface mobility of ad-atoms, indium droplets, phase separation, and composition inhomogeneity are always formed during the low-temperature growth process [2,3].Recently, the high quality of nitride films were grown by using pulsed laser deposition (PLD) technique at low temperature ranging from 25 to 380 °C [4,5].The kinetic energies of atoms were enhanced with the assistance from laser energy, leading to the surface migration of ad-atoms.On the other hand, the treatments of post-annealing at 500-1000 °C have been proposed to estimate the feasibility of InGaN films [6][7][8][9].Chalker et al. have demonstrated that the InGaN film with lower indium content was observed after annealing in vacuum due to the loss of indium from surface via inward diffusion and some surface evaporation [8].However, Seo et al. have found that there was an increase of 2% indium in the InGaN layer after 800 °C-annealing with a N 2 atmosphere in furnace due to the possible formation of InGaN with In-rich phase [9].Based on the above researches, the InGaN layer with higher indium content could be obtained by combining the PLD technique and post-annealing in the furnace.
In this study, the InGaN layers with two indium contents of 33% and 60% were prepared by PLD using a dual-composition target.After the deposition, the films were post-annealed at 500-800 °C in a non-vacuum furnace to increase the indium content.Moreover, we have investigated the influence of In 2 O 3 formation on films surface during annealing process on the characteristics of InGaN.

Experimental
InGaN films were deposited on the MOCVD-grown undoped GaN (2 μm) templates using a dual-composition target by PLD.The details of dual-composition target and growth parameter are described in our previous study [10].The as-deposited InGaN films with indium contents of 33% and 60% were denoted as samples A 0 and B 0 , respectively.The films were treated by an isochronal annealing at 500-800 °C in the non-vacuum furnace for 15 min under a N 2 atmosphere, which were denoted as A 500 -A 800 and B 500 -B 800 , respectively.The crystal orientations of InGaN films were investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM).The indium contents of InGaN films were estimated by using the Vegard's law, which calculated the InGaN (0002) diffraction peak shift relative to the GaN (0002) peak.The depth profiles of element concentration for the as-deposited and annealed samples were measured by auger electron spectroscopy (AES).The binding state and surface roughness were characterized by x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively.Room temperature micro-photoluminescence (μ-PL) measurement was analyzed to investigate the emission characterization by employing a He-Cd laser with 325 nm as an excitation source.

Result and discussion
Figures 1(a) and 1(b) show the XRD patterns of A 0 -A 800 and B 0 -B 800 , respectively.As can be seen, a weak InN (0002) peak resulted from the indium vapor directly reacted with the nitrogen plasma was formed in A 0 and B 0 .The InGaN (0002) peak position of A 0 -A 800 was shifted from 33.42° to 33.1° (Fig. 1(a)), corresponding to the increment of indium content from 33% to 41%.Meanwhile, the indium content of B 0 -B 600 was raised from 60% to 63% in Fig. 1(b).According to the past research, the InN content would be decomposed to indium and nitrogen as the annealing temperature was higher than 550 °C [11].All of the A 600 -A 800 and B 600 -B 800 samples displayed the In 2 O 3 peaks, which resulted from the reaction between the indium (produced by the decomposition of InN) and residual oxygen [12].The AES depth profiles of A 0 , B 0 , A 700 and B 700 were shown in Figs.1(c)-1(f), respectively.It can be observed that the elements of O, N, In and Ga distributions in A 0 and B 0 were clearly distinguished into the InGaN and GaN regions in Figs.1(c) and (d), respectively.Figure 1(e) demonstrated that the higher oxygen was distributed on InGaN surface.Furthermore, we also found the oxygen and indium were diffused into the regions of InGaN and GaN.Nevertheless, the serious oxidation and diffusion can be found in Fig. 1(f), resulting from the easy reaction between the higher indium content and incorporation oxygen.This is the reason why the InGaN peaks disappeared in B 700 and B 800 , as shown in Fig. 1(b).These results proved that the increase in indium content of InGaN was caused from the In 2 O 3 formation on InGaN surface, obstructing the out-diffusion of indium.Therefore, the indium can be reacted with surrounding InGaN toward the higher indium content.In our previous study [13], the InGaN films grown by PLD were then annealed at 500-800 °C in the vacuum chamber with a N 2 atmosphere.It was found that the indium contents of InGaN layers before and after annealing were almost the same, indicating a dissimilar result compared to this study.It is attributed to the In 2 O 3 formation on InGaN surface during the non-vacuum annealing process in this study.The cross-sectional TEM images of annealed InGaN (A 600 and B 600 ) were presented in Figs.2(a) and 2(b), and the thicknesses of these two samples can be determined to 121.8 and 71.9 nm, respectively.Moreover, the oxide thickness of 43.8 nm in B 600 was higher than that in A 600 (28.1 nm).It was in good agreement with the AES results, as shown in Figs.1(b) and 1(f).High resolution images of A 600 and B 600 exhibited in Figs.2(c) and 2(d) were taken at the interface between oxide and InGaN layers, marked with the red-dash line in Figs.2(a) and 2(b), respectively.From our calculation, the d-spacing of incompletely decomposed InN and crystalline In 2 O 3 were 2.85 and 2.92 Å, respectively.The InGaN layer was analyzed to a single crystal structure with (0002) plane.The d-spacing of InGaN (0002) planes in A 600 and B 600 were determined to 2.683 and 2.758 Å, which can be indexed to the indium contents of 35% and 63%, respectively.The indium contents in these samples corresponded to the XRD results in Figs.1(a) and 1(b).Figure 3 shows the XPS spectra of O1s and In3d for A 0 -A 800 at a sputtering depth of 20 nm.From the spectra of O1s binding energy (Fig. 3(a)), there was no In-O peak existed in the samples of A 0 and A 500 .However, the divided peaks corresponding to 530.5 and 530 eV formed in A 600 -A 800 were determined to In 2 O 3 phase [14], and the peak intensity was enhanced as the annealing temperature was increased.Moreover, the spectra of In3d binding energy were measured to confirm the oxidation states in the films again, as shown in Fig. 3(b).For the spectra of A 0 and A 500 , almost the same peak positions at 451 and 443.4 eV were analyzed to In3d 3/2 and In3d 5/2 , respectively, resulting from the In-N bonds [15].With increasing the annealing temperature to 600-800 °C, the peaks of In3d 3/2 and In3d 5/2 were shifted toward higher binding energy and approached to 451.8 and 444.3 eV, respectively.It can be proved the In 2 O 3 phase was appeared [16], coinciding with the above results.
As mentioned above, the In 2 O 3 was formed during the annealing process (600-800 °C), which probably leaded to the diffusion of In 2 O 3 into the InGaN layer.This would cause the existence of In 2 O 3 and small amounts of InN:O formation in the InGaN region [12], resulting in the enlargement of InGaN d-spacing and the increase in indium content.To verify if the increment of indium content in InGaN film is partially ascribed to the oxide formation, the samples of A 500 -A 800 have been etched in HCl solution.The XRD patterns of A 500 -A 800 after etching in HCl solution for 10 min are shown in Fig. 4(a).Apparently, we found that the In 2 O 3 peak was disappeared.Figures 4(b) and 4(c) show the indium contents and root-meansquared (RMS) roughnesses of A 500 -A 800 before and after the etching treatment.It can be seen the indium contents of A 500 and A 600 before and after etching were the same.Even though the annealing temperature was further increased, the indium contents of A 700 and A 800 were slightly decreased to 36.6% and 40.7%, respectively, compared with the non-etching samples (37% and 41%).In Fig. 4(c), there is an obvious decrease in roughness after etching except for the sample of A 500 .From our observation, the RMS roughnesses of 600 , A 700 and A 800 before etching were determined to be 29.5, 31.1 and 95.4 nm, respectively.Meanwhile, the RMS roughnesses of these three samples with removing surface oxide were decreased to 2.67, 5.54 and 9.34 nm, respectively.It demonstrated that the rough surface was induced by oxide formation on InGaN surface.To investigate the luminescent property of InGaN, the μ-PL measurement was performed on the A 0 -A 800 after etching in HCl solution.Figure 5 shows the μ-PL spectra of these samples measured at room temperature.The peak positions of A 0 , A 500 , A 600 , A 700 and A 800 were located at 552, 564, 571, 599 and 618 nm, respectively.It reveals that the emission of InGaN layer was shifted to long wavelength with an increase of annealing temperature.On the other hand, the indium contents of A 0 -A 800 also can be determined from the μ-PL peak positions using the following equation: where the band gaps of GaN and InN are E GaN = 3.4 eV and E InN = 0.7 eV, respectively.The x and b are represented the indium content and bowing parameter of 1.4 eV, respectively [17].The indium contents of A 0 , A 500 , A 600 , A 700 and A 800 were estimated as 32%, 33%, 34%, 37% and 39%, respectively.The indium contents determined from μ-PL measurements are very close to those from XRD results shown in Fig. 4(a).Moreover, it proves again that the indium content is increased by raising the annealing temperature.

Conclusion
InGaN films with indium contents of 33% and 60% were prepared on the MOCVD-grown undoped GaN templates by pulsed laser deposition at 300 °C.Then the films were annealed by using the non-vacuum furnace to investigate the effect of annealing temperature on crystal structure, indium content and emission characterization of InGaN.Because of the In 2 O 3 formation on surface during the non-vacuum furnace annealing, it resulted in the blocking of indium out-diffusion from InGaN.Therefore, the indium content of InGaN layer can be increased efficiently.The luminescent property of InGaN measured by microphotoluminescence indicated the emission peak was shifted to long wavelength with an increment of annealing temperature, corresponding to the trend of increased indium content.This indicates the InGaN films can be potentially useful in optoelectronic devices.

Fig. 2 .
Fig. 2. Cross-sectional TEM images of (a) A 600 and (b) B 600 .High resolution images of (c) A 600 and (d) B 600 were taken at the interface between InGaN and oxide layers.

Fig. 3 .
Fig. 3. XPS spectra of (a) O1s and (b) In3d for the samples of A 0 -A 800 at a sputtering depth of 20 nm.