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Abstract
High Al-content AlGaN epilayers were grown on AlN template by using indium (In) surfactant with plasma-assisted molecular beam epitaxy (PA-MBE), and deep ultraviolet emission at 235 nm was obtained at room temperature. The effects and mechanisms of In-surfactant on the crystalline quality and optical properties of AlGaN were investigated. It was found that In-surfactant could facilitate two-dimensional AlGaN growth by reducing activation barrier for Al/Ga atoms to cross steps and effectively increasing the migration rate on the growth surface, and thus improve surface morphology and decrease defect density. The photoluminescence measurements revealed that the optical properties were remarkably improved by adopting In as surfactant, and phase separation was also effectively eliminated. Furthermore, the concentration of impurities including oxygen and silicon was decreased, which is attributed to higher defects formation energy for these impurities with In-surfactant assisted epitaxy growth.
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1. Introduction
In the past decade, AlGaN alloys have emerged as one of the most potential candidates for high power and high frequency electronic devices [1–3], as well as optoelectronic devices including ultraviolet (UV) light emitting diodes (LEDs), lasers diodes (LDs) [4–6], and solar-blind avalanche photodiodes (APD). Recently, it has been reported that the peak external quantum efficiencies (EQEs) of UV-LEDs at 275 nm reached 20% [7]. Nevertheless, the EQEs of UV-LED are still less than 10% and drop sharply at wavelength shorter than 250 nm, which mainly attributed to high threading dislocation density (TDD) and other factors including the poor electrical conductivities of Si-doped and Mg-doped AlGaN layers, especially at high Al mole fractions. AlGaN-based LEDs operating in the deep ultraviolet (DUV) spectral range are usually grown on AlN template deposited on c-plane sapphire substrate. However, the AlN layers grown on sapphire substrate usually exhibit a large number of threading dislocations (TDs) [8], which extend into the AlGaN epi-layer deposited on the top. In order to overcome this difficult, many approaches have been explored for the growth of AlGaN [9,10].
Due to the strong Al-N bond, the mobility of Al adatom on the growth surface is very low, so that Al adatoms are unable to move sufficiently from the point of impact to the most energetically favorable lattice sites. It thus promotes the three-dimensional growth mode and the formation of coalescence-related defects [11]. Therefore, it is very difficult to grow AlGaN with high Al content by conventional growth technologies. So far, several growth methods including epitaxial lateral overgrowth, metal modulated epitaxy, pulsed and migration enhanced epitaxy were proposed to achieve AlGaN layers with medium Al content of over 40%. Besides, it has been reported that indium (In) can act as an effective surfactant to promote the mobilities of Gallium (Ga) and aluminum (Al), and then improve the surface morphology significantly and optical properties of GaN or AlGaN [12–16]. In addition, In-surfactant-assisted technology is particularly well adapted for the growth of AlGaN by plasma-assisted molecular beam epitaxy (PA-MBE), because of unique advantages of MBE technique including but not limited to high-purity simple substance material and low growth temperature. However, there are few detailed and in-depth studies on the effects and mechanisms of In surfactant on the structural and optical properties of high-Al-content AlGaN materials grown by PA-MBE.
In this paper, the effects of In surfactant on the characteristics of high Al-content AlGaN epitaxial layers grown by PA-MBE have been investigated. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD), Photoluminescence (PL), high-resolution X-ray photoelectron spectroscopic (HR-XPS) and Secondary ion mass spectrometry (SIMS) were used to characterize the crystal quality, optical properties and impurity defect of AlGaN epitaxial layers. The results revealed that surface morphology and optical properties of AlGaN were improved with the introduction of In-surfactant, and the enhancement in crystal quality of AlGaN was attributed to regulating surface kinetics and promoting metal atoms diffusion in the growth process for AlGaN.
Materials and methods
All AlGaN epitaxial samples were grown by Veeco GEN20A PA-MBE on 3 µm-thick AlN template on c-plane sapphire substrates. The AlN template wafers were solvent cleaned and loaded into the Loadlock chamber. Prior to initiating the growth, two out-gassing bakes were performed for thermal cleaning of the substrate surface at 200 °C for 2 h and followed by 900 °C for 0.5 h to remove surface contamination and oxide film. Subsequently, a 50 nm-thick high temperature grown AlN buffer was deposited at 900 °C. Then, 200 nm-thick high-Al-content AlGaN films were grown under different growth conditions. The nitrogen flow rate and the RF plasma power were fixed at 1.0 sccm and 370 W, respectively. The group-III element fluxes in beam equivalent pressure (BEP) were regulated by controlling the temperatures of Al, Ga and In effusion sources. To study the effects of In-surfactant on the characteristics of AlGaN epilayers, the substrate temperature was fixed at 850 °C. Four AlGaN samples named as from S1 to S4 were grown under identical conditions except for increasing In flux. The growth conditions are tabulated in Table 1. Besides, another sample named S5 was prepared at excessive Ga flux.
Table 1. The main growth parameters of AlGaN samples grown by MBE
The surface morphology and structural properties of these as-grown epitaxial films were characterized with AFM and HR-XRD, respectively. PL spectroscopy measurements were performed for S1 and S2 at different temperature of 10∼300 K using a 213 nm excitation source. HR-XPS were carried out in a PHI5000 system equipped with a monochromatic Al Kα (1486.7 eV) radiation and an Ar+ plasma gun. SIMS was performed to evaluate the impurity atom concentration in AlGaN films.
Results and discussion
The “stability windows” of In coverage thickness become narrower for higher Al contents [17], so different In fluxes will lead to different thicknesses of In coverage during the high Al content AlGaN growth. The effects of In fluxes on the surface morphology for AlGaN films were studied in terms of AFM images, as shown in Fig. 1. These pictures actually demonstrate AFM images of AlN template for a scanning area of 5 µm × 5 µm and AlGaN samples grown with different In fluxes for a scanning area of 3 µm × 3 µm. It can be observed that the surface height of AlN template fluctuates greatly, which has a great influence on the epitaxial layer of AlGaN. The measured root-mean-square (RMS) roughness values of S1, S2 and S3 over the 3 µm × 3 µm scans are 6.44 nm, 5.80 nm and 3.21 nm, respectively. In other words, the surface morphology was enhanced with increasing In flux during the epitaxial growth process. In addition, atomic steps could be observed in the range of 300 nm × 300 nm for S1. The spotty and streaky RHEED diffraction patterns appeared at the same time, implying that there were both three-dimensional island growth and two-dimensional layered growth on the growth surface. This phenomenon indicate that the surfactant capabilities of Ga are insufficient to overcome the decreased adatom kinetic energy for S1 growth process [18]. Hence, In atoms were introduced as surfactant to improve the growth of AlGaN films. It can be found that the fluctuation and roughness of the surface of AlGaN film decreased with the increase of In flux. This is because In atoms as a surfactant can improve the mobilities of atoms on the growth surface, allowing Al and Ga adatoms to reach the kink of the step, and thus inhibit three-dimensional island growth.
Because In-N bond energy is weaker than Al-N bond and Ga-N bond, the effective bond strengths satisfy the inequalities ${{V_{Al - N}} > \; {V_{In - N}} \gg \; {V_{In - In}}}$ for Al adatoms on the growth surfaces. The surface activity of In adatom are mainly attributed to two factors. Firstly, In surfactant atoms attached to a step edge can lower the Ehrlich-Schwoebel (ES) barrier for Al and Ga atoms to descend steps as shown in Fig. 2(a) [19], and add an additional barrier at ascending steps [20], which are instrumental in two-dimensional AlGaN growth. Secondly, Fig. 2(b) illustrates the Al (Ga) and In adatoms exchange mechanism. Al and Ga atoms on the In atomic layer need to be exchanged with In atoms to incorporate into the crystal lattice. In other words, In surfactant increase Al and Ga atoms mean free path by passivating the growth surface. Consequently, the atoms can reach the most energetically favorable lattice sites and incorporated into the crystal lattice, eventually resulting in flat surface morphology. However, too large flux of In lead to the appearance of Al metal droplets on the surface of AlGaN film, which may due to the excessive In make the nitrogen free bonds more saturated and preventing Al atoms from participating in growth.
Fig. 2. Schematic picture of (a) showing In surfactant lower the barrier for Al/Ga atoms to step down and (b) Al/Ga atoms lifts an In surfactant atom by an exchange process.
To assess crystal quality of AlGaN samples, the rocking curves of symmetric HR-XRD were conducted, as presented in Fig. 3. The densities of screw dislocation and edge dislocation in the material are related to the full width at half maximum (FHWM) of the (002) and (102) rocking curves. It was found that the FHWM of (102) rocking curves for AlGaN epitaxial layer was almost equal to that for AlN substrate. The result shows that the edge dislocation in AlGaN epitaxial layer is closely related to the edge dislocation in AlN substrate. Figure 3 shows the (002) rocking curves of AlGaN epitaxial layers grown under different In fluxes. Their diffraction intensities were normalized and their diffraction peak positions were shifted to zero. The FHWM of S1, S2, S3 and S4 were 782, 630, 616 and 494 arcsec, respectively. The reduction in FHWM signified that screw dislocation in AlGaN layer could be suppressed effectively by introducing In surfactant.
Figure 4 shows the room temperature photoluminescence spectrum of AlGaN samples. The PL emission peaks of S1 ∼ S4 were 297 nm, 235 nm, 265 nm and 240 nm respectively. Thus, the Al content of AlGaN dominating PL emission at room temperature were estimated to be 35%, 74%, 54% and 70% corresponding to S1 ∼ S4. The In coverages resulted in blue shift of the PL spectrum with different degrees. It implies that the In surfactant improved the homogeneity during the growth of high Al content AlGaN. In the absence of In surfactant, the S1 had the longest wavelength of PL emission peak. In addition, there was another “shoulder” appearing on the high energy side of the major peak observed. The component fluctuates were greatly due to the low surface mobility of atoms in the process of AlGaN growth without surfactant. Obviously, three AlGaN samples with In-surfactant had shorter wavelength of PL emission peak and narrower peak width. For S2, deep ultraviolet emission at 235 nm had been obtained at room temperature. Although further studies are required to reveal the inherent links between the quantity of In-surfactant atoms and the homogeneity of AlGaN, it is reasonable to conclude that In-surfactant is effective to improve the homogeneity of AlGaN material.
Further, the temperature-dependent PL measurements were performed on S1 and S2, as shown in Fig. 5. The PL emission of S1 separated two peaks corresponding to two different Al-content of AlGaN at low temperature, while single emission peak was observed for S2 without phase separation. In addition, Fig. 5(b) and (d) shows the variation curves between PL emission peak energy of S1 and S2 with different temperature, respectively. It was found that the luminescence peak positions of the two samples showed red-shift with the test temperature increasing generally. The variation curve of P1 (S1) conforms to the Varshni formula [21], which typically describes semiconductor bandgap narrowing with the increase of temperature.
Where Eg (T) and Eg (0) are bandgap at a temperature T (K) and T = 0 K, α and β are Varshni thermal coefficients. But the variation curve of P2 (S1) showed a significant “S-shaped” emission shift behavior with increasing temperature, which is the feature of localized-state emission due to component disorder [22]. Localized excitons can't overcome the potential barriers and freeze out, which dominates PL emission energy at low temperature. The phase separation tends to occur in the AlGaN films grown on the AlN substrate with step-bunched morphology [23,24]. In terms of S2 grown by In-surfactant assisted growth, the variation of peak positions was also consistent with the Varshini equation at temperature above 70 K, which had a little deviation caused by the localized state at low temperature. What’s more, assuming that the nonradiative recombination in the material is completely frozen at 10 K, the IQE of S1 and S2 can be estimated to be 21% and 32% at room temperature respectively. The activation energies Ea1 and Ea2 of the nonradiative recombination center were obtained by Arrhenius fitting of dependence between the PL intensity and temperature, which correspond to the low temperature and high temperature stages respectively [25], as the following formula:Fig. 5. PL spectra of AlGaN film in the temperature range 10K∼300K and Variation curve of PL emission peak position of S1 and S2 with different temperature.
Considering Ga atoms are also able to act as surfactant in AlGaN growth, S5 was grown under Ga-rich conditions with a certain amount of In atoms. AlGaN thin film with a luminescence peak at 235 nm at room temperature was obtained, indicating that S5 and S2 have the same Al component (74%). Because Al atoms easily bond with oxygen (O) atoms, higher Al content AlGaN tend to have higher concentration of oxygen impurities, and so as C impurities [26]. The existence of C, O and other impurity atoms will cause a strong self-compensation effect and worsen the crystal structure. The Ar+ ion beam bombardment can clean the C and O contaminants at the AlGaN surface, which lead to the monotonical increase in the (Al + Ga) / N ratio due to preferential sputtering of N [27]. After in situ Ar+ plasma treatment, XPS core level (CL) spectra of Al (2p) and Ga (3d) of S2 and S5 were deconvoluted to identify their chemical states near the surface of AlGaN films as shown in Fig. 7. The Al (2p) CL spectra was deconvoluted into three components assigned to Al-Al metal, Al-N and Al-O bonding states [28]. And the Ga (3d) CL was deconvoluted into Ga-Ga metal, Ga–N and Ga-O, and N2s components at their respective binding energy positions [29,30]. the percentage contribution of the deconvoluted components in the Al (2p) and Ga (3d) CLs are tabulated in Table 2. It is evident from Table 2 that In-rich surfactant method is more propitious to decrease native oxide content than Ga-rich condition, where S2 has a 75% reduction in the percentage contribution of Al-O bonding states compared to S5. In addition, The Ga-Ga bonding percentage of S5 was larger than that of S2, which may imply more VN defects in S5 due to excessive Ga adatoms during the growth of S5.
Table 2. Percentage contribution of various components in deconvoluted Al (2p) and Ga (3d) core level.
Figure 8(a) compares the SIMS depth profiles of impurities concentration along the growth direction in the two samples. It can be clearly observed that the concentrations of C, O, Si and other impurity atoms in AlGaN film grown only with In as surfactant were decreased compared with the film grown with In as surfactant at Ga-rich condition. Due to the long exposure time of S2 to the atmosphere, the O intensity curve has a tail. Energy band of AlGaN near the growth surface is bent when Ga / In metal layer covers the growth surface. Since the work function (4.12 eV) of In is smaller than that of Ga (4.2 eV), the coverage of the In atom layer can raise the Fermi level higher near the growth surface compared with the Ga atoms, as shown in Fig. 8(b). Because the formation energy of some impurities is related to the distance between Fermi level and the valence band maximum, the increasing of the distance caused by In surfactant enable an increase in the formation energy of ON and SiAl defects [31], which imply that the concentration of impurity defects decrease in the crystal. Therefore, using In alone as surfactant can reduce the concentration of impurity such as O and Si during AlGaN crystal growth.
2. Conclusion
In summary, High quality AlGaN films with deep ultraviolet emission at 235 nm were grown by PA-MBE and the effects of In-surfactant on the growth and characteristic of high Al-content AlGaN were investigated. The AFM and XRD results revealed that the surface roughness and screw defect density are remarkably decreased, which is attribute to increased migration rate of Al/Ga atoms on the growth surface with In as surfactant. PL measurements indicated that In-surfactant can not only suppress phase separation, but also effectively enhance luminescent properties of AlGaN films by decreasing the nonradiative recombination. Moreover, according to the results of SIMS and HR-XPS, AlGaN films grown with In surfactant alone have lower impurity concentration than Ga-rich conditions. Therefore, In-surfactant assisted growth method is a promising candidate for the development of AlGaN-based deep ultraviolet devices.
Funding
Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics (B2006, Y8AAQ21001); Guangxi Key Laboratory of Precision Navigation Technology and Application, Guilin University of Electronic Technology (DH202011); the Key R&D Program of Jiangsu Province (BE2018005); National Natural Science Foundation of China (61827823, 61875224).
Acknowledgments
The authors are grateful for the technical support from Platform for Characterization & Test, Vacuum Interconnected Nanotech Workstation of SINANO, Chinese Academy of Sciences.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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