Journal of Alloys and Compounds 739 (2018) 122e128 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom Dependence of Al incorporation on growth temperature during laser molecular beam epitaxy of AlxGa1 xN epitaxial layers on sapphire (0001) Prashant Tyagi a, b, Ch Ramesh a, b, S.S. Kushvaha a, Monu Mishra a, b, Govind Gupta a, B.S. Yadav c, M. Senthil Kumar a, b, * a b c CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, Dr. K.S. Krishnan Marg, New Delhi 110012, India Solid State Physics Laboratory, Timarpur, Lucknow Road, New Delhi 110054, India a r t i c l e i n f o a b s t r a c t Article history: Received 15 September 2017 Received in revised form 20 December 2017 Accepted 21 December 2017 Available online 22 December 2017 We report the successful growth of AlxGa1 xN (0  x  0.25) epitaxial films on c-plane sapphire substrate by using laser molecular beam epitaxy technique. The role of growth temperature on the Al incorporation and the structural, electronic and optical properties of the AlxGa1 xN epitaxial layers grown in the temperature range 500e700  C have been systematically studied. The atomic force microscopy analysis shows that the grain size of AlxGa1 xN increases with increase in growth temperature and flat surface epilayers are obtained at  600  C. The Al incorporation is confirmed with high resolution x-ray diffraction, x-ray photo electron microscopy and photoluminescence studies. It is observed that the growth temperature plays a critical role in determining the Al composition, which increases with increasing growth temperature. AlxGa1 xN layer with about 23% of Al composition is obtained on sapphire (0001) substrate at a growth temperature of 700  C, which is about 100e150  C lower than the conventional molecular beam epitaxy growth. © 2017 Elsevier B.V. All rights reserved. Keywords: Group III-Nitrides Hetero-epitaxy Atomic force microscopy High resolution X-ray diffraction X-ray photoelectron spectroscopy Photoluminescence spectroscopy 1. Introduction In the past two decades, AlGaN epitaxial layers have gained a great amount of research interest due to their large, direct and tunable band gaps ranging from 3.4 to 6.2 eV. This ternary alloy of group III-nitrides are being successfully employed for fabricating high electron mobility transistor (HEMT) and solid state ultra violet (UV) radiation sources and detectors in the wavelength range 210e365 nm [1e7]. AlGaN/GaN based HEMTs are known for their high switching speed and high power density and require less thermal management as compared to conventional silicon solid state devices. AlGaN based UV light-emitting diodes (LEDs) and laser diodes (LDs) could be used in lithography, bacteria disinfection and water purification. Moreover, they have a potential to replace hazardous mercury based conventional UV sources used for domestic as well as industrial applications. The crucial step in the * Corresponding author. CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India. E-mail address: senthilmk@nplindia.org (M. Senthil Kumar). https://doi.org/10.1016/j.jallcom.2017.12.220 0925-8388/© 2017 Elsevier B.V. All rights reserved. fabrication of III-nitride based UV devices is the growth of good crystalline quality AlGaN epilayer that acts as active emission and absorption medium. But, the growth of high quality ternary AlGaN alloy, especially with high Al content, is very challenging due to increased alloy scattering, phase segregation and the large lattice and thermal mismatch with substrates. Mainly, Al adatoms are much less mobile on the growth front as compared to Ga adatoms due to higher sticking co-efficient and fail to incorporate at more energetically favorable lattice sites, which results in higher defect densities [8]. The straight forward approach to reduce the defect density is to develop a low-temperature growth process for AlGaN so as to employ closely lattice-matched uncommon substrates like ZnO, Spinel, LiGa2O3, etc. that are vulnerable to high temperature process [9e12]. Laser molecular beam epitaxy (LMBE) technique [also referred as ultra high vacuum pulsed laser deposition or UHV-PLD] is being developed as a low temperature growth technique for III-nitrides since the laser power assists the kinetic energy of the growth precursors for surface mobility and compound formation [13,14]. LMBE technique is relatively naive compared to conventional P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128 techniques for III-nitrides such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) and the influence of experimental parameters on LMBE growth of AlGaN is not well understood yet. Huang et al. first grew AlGaN on c-plane sapphire employing PLD using GaN and AlN sintered targets and nitrogen gas as precursors [15]. But, the layers grown using sintered target are characterized with a high level of impurities that trapped air in the sintered targets. Masuyama et al. could solve the impurity issue of AlGaN growth on sapphire substrates by employing high purity metal based targets and nitrogen gas as precursor sources but the crystalline quality of the grown layers remained an issue due to lack of availability of high density nitrogen radicals [16]. These problems were addressed by Kobayashi et al. and Ueno et al. by growing AlGaN layers by laser ablation of high pure metal targets in the presence of r.f. nitrogen plasma on lattice-matched ZnO substrate [10,17]. Considering the commercial viability of sapphire substrate for III-nitride device fabrication, it is important to develop LMBE growth process for high quality AlGaN on sapphire. In this paper, we report, for the first time to the best of our knowledge, on the growth of AlxGa1 xN (0.0 < x < 0.25) epitaxial layers on the cplane sapphire substrate by using AlxGa1 x alloy target and r.f. nitrogen plasma as the precursors. Specifically, we discuss the effect of growth temperature on the Al mole fraction and the quality of LMBE grown AlGaN layers possess the epitaxial quality comparable to that grown on lattice-matched ZnO substrate [10]. 123 laser power and frequency were used in the range 3e5 J/cm2 and 45 Hz. The growth sequence followed for the AlGaN growth is presented in Fig. 1(a) [18]. The growth rates are observed to be 0.22 and 0.1 mm/h for the growth temperatures of 500 and 700  C, respectively. The grown AlxGa1 xN layers have been characterized for their structural, morphological, compositional, electronic and optical properties using high resolution x-ray diffraction (HR-XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements. A PANalytical X'Pert PRO MRD HR-XRD system, with CuKa1 source, was employed to characterize the crystalline quality of AlGaN films grown on sapphire substrate. A Multimode-V Veeco AFM was employed in tapping mode using silicon tips of curvature radii less than 10 nm for the surface morphology studies of AlxGa1 xN films. The surface morphology of grown layers was also characterized using field emission scanning electron microscopy (FE-SEM) (FIB, ZEISS, Germany) at an operating voltage of 5 KV. The PL measurements were done using an excitation laser of wavelength 266 nm at room temperature. The XPS measurements were performed in an UHV based Omicron Multiprobe Surface Analysis System operating 2. Experimental The growth of epitaxial AlxGa1 xN on sapphire (0001) substrates was carried out in an LMBE system equipped with reflection high energy electron diffraction (RHEED), laser ablation target and a radio-frequency (RF) nitrogen plasma source to supply nitrogen radicals to the growth surface. The base pressure of the main chamber was maintained at 2  10 10 Torr. The backside of sapphire wafer was coated with a layer of molybdenum of thickness about 1 mm in order to increase the absorption of heat radiation and uniform heating all around the substrate. The substrates were cleaned with standard organic solvents and de-ionized water before loading into the system. The AlxGa1 x alloy target was prepared using high purity Al and Ga elemental metals. Al and Ga metals (20:80 at.%) were put together in a quartz ampoule and sealed under a high vacuum of order 10 6 Torr. The ampoule was then annealed in a hot furnace of temperature at 700  C for 60 h. The substrates and targets were loaded inside the load-lock chamber and degassed to remove any surface contaminants due to their exposure to the atmosphere. The degassing was done under a high vacuum of 10 7 Torr. The temperature was raised gradually to 250  C and maintained for 2 h. After degassing, the target and substrate were transferred into the growth chamber. Thermal cleaning of sapphire substrate was carried out under UHV condition by gradually increasing the temperature to 850  C by radiative heating using infra-red (IR) heater for 10 min. Thermal cleaning improves the surface quality with terraces suitable for epitaxial growth. Afterwards, the substrate temperature was brought down to 700  C for sapphire nitriadation that was carried out for 35 min under RF nitrogen plasma ambient. The RF plasma power and N2 gas flow rate were kept constant at 400 W and 1.2 sccm with a nitrogen partial pressure of 6.7  10 5 Torr. During the nitridation process, a few monolayers of the sapphire substrate are converted into AlN, which has a close lattice match to AlGaN and acts as a buffer for AlGaN growth. We grew the AlGaN layer for 2 h on nitridated sapphire substrate in the temperature range 500e700  C under the nitrogen plasma power of 250 W and the nitrogen partial pressure of 2.2  10 5 Torr. A KrF laser of 248 nm wavelength and 25 ns pulse width was used for the ablation of the alloy target. The Fig. 1. (a). The growth sequence followed during growth of AlxGa1 xN layers by LMBE technique (b). In-situ RHEED observations along [11e20] and [10e10] incident azimuths at various stages of AlxGa1 xN growth process: Sapphire cleaning, nitridation, and growth under metal-rich and N-rich conditions. 124 P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128 at a base pressure of 5  10 11 Torr. For XPS analysis, MgKa radiation source (1253.6 eV) was employed. An OMICRON EA125 hemispherical analyzer (operating at pass energy of 20 eV) equipped with a seven channeltron parallel detection unit was used to collect the XPS spectra. The calibration of binding energy in photoemission spectra was done referring to Au 4f7/2 emission line [19]. 3. Results and discussion The entire growth sequence of AlGaN layer was monitored using in-situ RHEED observations as shown in Fig. 1(b). A short streaky RHEED pattern with kikuchi lines was observed for the bare sapphire at the end of UHV cleaning. The intensity of the kikuchi lines became poor after sapphire nitridation and additional streaky patterns belonging to AlN appeared along with sapphire features as indicated in the respective images. It revealed the formation of AlN on the sapphire surface, which is well known to improve the crystalline quality of the epitaxial III-nitride layers [20]. The epitaxial growth of AlGaN layers was examined under different III/ N flux conditions. A well defined streaky (2-dimensional growth) or a spotty (3-dimensional growth) RHEED pattern was obtained during AlGaN growth under metal-rich or N-rich condition, respectively, as shown in Fig. 1(b). This observation is similar to the conventional MBE growth process of III-nitrides. The surface mobility of adatoms is reported to be relatively higher under Garich flux condition that leads to 2D growth of III-nitride epilayers [21]. For the current study, all AlGaN samples were grown under slightly metal-rich condition in the temperature range 500e700  C. From HRXRD 2q-u scan measurements, a shift of XRD peak positions towards higher angle was noticed for the AlGaN layers as compared to GaN. The XRD peak shift increased with growth temperature and was minimal for the AlGaN layers grown up to 600  C due to the close lattice constants of GaN and AlN (lattice mismatch~2.4%) [22]. An apparent peak shift in the XRD position has been obtained for the layer grown at 700  C as presented in Fig. 2(a). It implies that the Al incorporation in the AlGaN layer increases with the increase of growth temperature from 500 to 700  C. The XRD peaks corresponding to only (0001) crystalline planes are obtained exhibiting that the AlGaN layer is grown along c-axis with a wurtzite hexagonal crystalline structure. Moreover, the shift in XRD peak position towards higher angle when compared to GaN peaks indicates a decrease in lattice parameter due to the incorporation of Al in the grown layer. Using Vegard's Law [23], we could further calculate the Al composition from the peak shift of (0004) plane to be ~24% for the layer grown at 700  C, which is close to the composition of Al20Ga80 target alloy [24]. In order to examine the tilt and twist mosaic structures, the xray rocking curve measurements were employed along symmetric (0002) and asymmetric (10e12) reflections of AlGaN layers. It is found that the full width at half maximum (FWHM) of both curves decreases with increasing growth temperature as given in Fig. 2(b). The inset shows the (0002) rocking curve of AlGaN layer grown at various temperatures. The rocking curves are symmetric that indicates a minimal of any Al inhomogenity in the grown layer. A rocking curve FWHM of 0.4 and 0.95 is obtained along the (0002) and (10e12) planes of AlGaN layer, respectively, grown at 500  C. However, the FWHM values of (0002) and (10e12) plane rocking curves decreased to 0.27 and 0.68 when the AlGaN growth temperature is increased to 700  C. These are the lowest values reported for AlGaN layer grown on sapphire by employing LMBE technique with an Al composition >20% and are comparable with that grown on closely lattice-matched ZnO substrate [10]. Here, it should be noted that the crystalline quality of the AlGaN layers was retained even with increased Al mole fraction with increasing Fig. 2. (a) HRXRD 2q-u scan of GaN and AlxGa1 xN layer grown on sapphire (0001) by LMBE at 500 and 700  C, respectively. (b) X-ray rocking curve FWHM values of symmetric and asymmetric plane reflections of AlxGa1 xN layer as a function of growth temperature. The inset shows the normalized (0002) plane x-ray rocking curves of AlxGa1 xN layers grown at various temperatures. growth temperature. In case of MOCVD process, Kucukgok et al. obtained a (0002) x-ray rocking FWHM of 0.146 for 600 nm thick of Al0.23Ga0.77N layer grown on c-sapphire using AlN buffer [25]. Monroy et al. reported ~0.2 of x-ray rocking curve FWHM from the (0002) reflection of AlGaN layer (0.5e0.7 mm thick) with Al content of 23% grown by plasma-assisted MBE on GaN/sapphire template [26]. The surface morphology of LMBE grown AlGaN layers at various temperatures as imaged by SEM and AFM measurements are presented in Fig. 3. As seen in Fig. 3(a), at 500  C, the AlGaN layer was grown as a high density of nano-sized islands and localized agglomeration of islands are also found across the surface as bright spots. With increase of growth temperature, the grain size increased and a flat surface growth is observed for the layers grown at and above 600  C without any particle-like agglomerations. For the layer grown at 600  C, deep valley-like features are observed on the surface due to incompletion of coalescence process of AlGaN layer growth. AlGaN nucleates and grows as islands at the P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128 125 Fig. 3. SEM and AFM surface morphologies of AlxGa1 xN layer grown on sapphire (0001) at (a) 500, (b) 600 and (c) 700  C. beginning and with the progress of growth the islands coalescence due to enhanced lateral growth and form a smooth layer surface. The rms surface roughness values are noted to be less than 2 nm for these layers. It should be noted that several micron-sized Aldroplets are found on the surface of AlGaN layers grown at  600  C. For the AlGaN layer grown at 700  C, smooth surface with a high density of small-sized surface pits is observed in the AFM morphology. A surface roughness (rms) value of ~1.2 nm is obtained for 2 mm  2 mm scan-area. The surface pits are inverted hexagonal in feature and normally occur as a result of strain relaxation. In many cases, the pits are associated with threading dislocations arise from the interface due to large lattice and thermal mismatch between AlGaN and sapphire substrate [27]. The density of surface pits generally increases as the Al composition increases in the layer due to increased strain [28]. We studied the compositional and electronic properties of LMBE grown AlGaN layers using XPS measurements. Two AlGaN samples grown at 500 and 700  C were taken for the XPS analysis. The core level (CL) spectra have been plotted after necessary carbon correction and the position of valence band maxima (VBM) was calculated by extrapolating a linear fit to the leading edge of the valence band photoemission to the baseline [29,30]. Fig. 4(a) shows the survey scans of both the AlGaN samples displaying the presence of aluminum, gallium and nitrogen with small amount of oxygen and carbon contaminations at their respective binding energy positions [31]. The angle dependent XPS analysis revealed that oxygen and carbon contaminations are present only on the layer surface where the intensity of these peaks increased with variation in TakeOff Angle (TOA) from 0 to 60 . The Al (2p) CL spectra are shown in Fig. 4(b) where the intensity of AlGaN layer grown at 700  C was observed to be significantly higher than that grown at 500  C. The Al (2p) CL peak is observed at a binding energy of 73.8 and 74.0 eV for the AlGaN layers grown at 500 and 700  C, respectively, attributed to the Al-N bonding [32,33]. The stoichiometry of both the AlGaN samples were calculated from survey scan using respective sensitivity factors [34] and are obtained to be about 3 and 23% for the layers grown at 500 and 700  C, respectively. The survey scan as well as the CL spectra confirms that the Al incorporation into the layer increases at higher growth temperature. Fig. 4(c) shows the valence band (VB) spectra of the AlGaN layers grown at 500 and 700  C. The VBM was calculated to be positioned at 2.9 ± 0.1 and 3.6 ± 0.1 eV below the 126 P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128 Fig. 4. XPS survey scans of LMBE grown AlxGa1 xN layers at 500 and 700  C. (a) Core level survey scans, (b) Al (2p) spectra and (c) Valence Band Spectra. Fermi Level for the AlGaN layers grown at 500 and 700  C, respectively. It indicates that the as-grown layers have n-type conductivity. It is also noted that the VBM shift towards the higher binding energy with increase of Al content and is attributed to the increase of band gap of AlxGa1 xN. The bandgap of GaN is 3.4 eV while that of AlN is 6.2 eV. The bandgap of AlxGa1 xN ternary compound increases with increment of Al content (x). Hence, the position of Fermi level shifts towards higher binding energy with increase in Al content or Bandgap or simply the growth temperature in the present scenario. Fig. 5 represents the room temperature PL spectra recorded for Fig. 5. Room temperature PL Spectra of AlxGa1 xN layers grown on sapphire (0001) by LMBE at various growth temperatures along with GaN PL spectrum. the LMBE AlGaN layers grown on sapphire (0001) at various temperatures. The PL spectra were acquired with the same incident beam aperture size and excitation for all the samples. A strong UV emission is seen for all layers with a weak deep band yellow luminescence (YL) emission under the measurement conditions adopted. The UV peak corresponds to the near band edge (NBE) emission due to band-to-band transition. The NBE peak position blue shifts with increasing growth temperature implying that the Al composition increases in the AlGaN layer as a function of growth temperature. It is also noticed that the PL emission peak broadens with increase of Al composition, which is normally attributed to the band tail states, which arise due to the shallow energy states of point defects present in AlGaN ternary alloy [35e38]. Moreover, the increase of Al mole fraction in the AlGaN layer can also cause further broadening of the PL emission peak because of the alloy broadening effect [39]. The alloy fluctuations occur due to inhomogeneities present in the grown ternary compound at nanoscale level. The PL spectra of 700  C grown sample exhibits an additional low intensity peak at 3.27 eV, which is generally accepted to the transitions associated with point defects such as VIII and its complexes [40]. The strong NBE emission with less intensity defect peaks indicates the good optical quality of the AlGaN epitaxial layers grown by LMBE technique. Fig. 6 shows the graph of band gap obtained by room temperature PL versus Al composition in the AlxGa1 xN layer estimated by using XPS and EDX analyses. Theoretical band gap values for AlxGa1 xN are calculated by using Vegard's Law that is assumed to be linear for semiconductor alloy system [41]. Eg ðxÞ ¼ xEg ðAlNÞ þ ð1 xÞEg ðGaNÞ (1) where Eg(GaN) ¼ 3.4 eV and Eg(AlN) ¼ 6.2 eV [42]. The experimental values are near to the theoretical calculation of band gap based on equation (1) indicating the absence of any bowing for the Al incorporation up to 23% in the AlGaN layers grown by LMBE. This observation is similar to the one reported by Ochalski et al. on the MOVPE and MBE grown AlGaN layers [43]. In general, GaN requires a lower growth temperature while AlN demands a high temperature due to the strong Al-N bond-strength compared to Ga-N [44]. Hence, AlGaN layers are grown at an optimal temperature between GaN and AlN. Using LMBE, we have shown the growth of good quality GaN epitaxial layers at temperatures as low as 500  C and the growth rate decreased drastically Fig. 6. Theoretical and experimental Band gap energy E(g) versus Aluminum composition incorporated into AlxGa1 xN thin films. P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128 when the growth temperature is raised above 600  C due to increased Ga desorption from the growth surface [45,46]. In case of AlGaN growth by LMBE, the Al incorporation becomes difficult due to relatively less surface mobility of Al atoms and poor formation of Al-N bond at low growth temperatures of 600  C irrespective of the high Al content in the target material. The excess Al remains in the form of droplets on the layer surface at the end. Increase of growth temperature promotes more Al incorporation in the growing layer due to enhanced Al-N bond formation. On the other hand, Ga surface desorption becomes very significant at higher temperatures above 600  C. In addition, when the layer is grown under metal-rich condition, the lifetime of the physisorbed groupIII adatoms is increased leading to longer diffusion length and thus enhanced Ga desorption rate at high temperature [47]. As a result, the Al to Ga flux ratio is increased on the growth surface at higher temperatures favouring more Al incorporation in the AlGaN layer. In LMBE process, the growth at 700  C has led to a strong increase of Al composition in the AlxGa1 xN layer even higher than the composition of the alloy target. These observations reveal that, in LMBE AlGaN growth, the designated Al content cannot simply be achieved by appropriately choosing the target composition but the growth temperature should also be taken into account as it plays a very critical role in determining the degree of Al incorporation. 4. Conclusion We have successfully grown epitaxial AlGaN layers on prenitridated sapphire (0001) using LMBE technique. The effect of growth temperature on the Al incorporation and physical properties of grown AlGaN layers have been studied in detail. It is found that the Al mole fraction increases with increase of growth temperature and a maximum Al content of about 23% is achieved at a growth temperature of 700  C. The growth of good quality AlGaN layers has been achieved in the temperature range of 600e700  C in LMBE technique, which is 100e150  C lower than the typical AlGaN growth temperature of conventional MBE growth. Acknowledgements The authors thank Dr.K.K. Maurya for the help in XRD measurements. The authors would like to acknowledge the financial support from Council of Scientific and Industrial Research (CSIR) through network project (PSC-0109). Prashant Tyagi and Ch. Ramesh are grateful to CSIR for the award of senior research fellowship. References [1] D. Morita, M. Yamamoto, K. Akaishi, K. Matoba, K. Yasutomo, Y. Kasai, M. Sano, S. Nagahama, T. Mukai, Watt-class high-output-power 365 nm ultraviolet light-emitting diodes, Jpn. J. Appl. Phys. 43 (2004) 5945e5950. [2] Y. Taniyasu, M. Kasu, T. 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