Nitridation of Sapphire as a Precursor to GaN Growth: Structure and

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The Nitridation of Sapphire as a Precursor To GaN Growth: Structure and Chemistry Krishna Yaddanapudi, Sabyasachi Saha, Srinivasan Raghavan, K Muraleedharan, and Dipankar Banerjee Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00299 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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Crystal Growth & Design

The Nitridation of Sapphire as a Precursor To GaN Growth: Structure and Chemistry Krishna Yaddanapudi1, Sabyasachi Saha3, Srinivasan Raghavan2, K. Muraleedharan4 and Dipankar Banerjee1 1

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Department of Materials Engineering, Indian Institute of Science Bangalore, India 560012 Centre for Nano Science & Engineering, Indian Institute of Science Bangalore, India 560012 3 Electron Microscopy Group, Defence Metallurgical Research Laboratory Hyderabad, India 500058 4 CSIR-Central Glass & Ceramic Research Institute, Kolkata, India 700032

Abstract Nitridation of sapphire substrates is used as precursor to the growth of GaN films to provide a wetting layer which is closer in terms of structure and chemistry to the overlayer. Nitridation has been carried out by metalorganic chemical vapour deposition at 530, 800 and 1100oC in an environment of NH3 and H2. The structure and chemistry of the nitrided layer grown at these different temperatures has been studied by x-ray photoelectron spectroscopy, electron diffraction, high resolution electron microscopy and electron energy loss spectroscopy. The low temperature nitridation process results in a nitrided layer in which oxygen has been partially replaced by nitrogen to form a cubic spinel- AlxOyNz structure. Nitridation at 8000 and 1100oC results in complete substitution of oxygen atoms by nitrogen to form a cubic rock salt AlN structure. These structures are stable on thermal anneal at 1000C prior to epitaxial GaN growth.

1. Introduction A wide and direct band gap (~ 3.4 eV), high critical electric field strength (~ 3.3 MV/cm) and high saturation velocity (~ 2E7 cm/s) enables the use of Gallium Nitride (GaN) and its alloys in various optoelectronic applications such as light emitting diodes (LEDs) [1], UV detectors [2], and high electron mobility transistors (HEMTs) for RF and power electronic applications [3, 4]. The epitaxial growth of GaN either by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) on substrates such as c-plane sapphire commonly employs a two-step growth process involving the deposition of the GaN epitaxial layer on a thin low-temperature nucleation layer of AlN or GaN [5, 6]. Prior to the two-step growth process, the sapphire substrates are thermally cleaned under the ambient of H2 at typical temperatures greater than 1000oC. The purpose of the subsequent nitridation process is to provide a wetting layer on the sapphire surface which is closer in terms of structure and chemistry to the nitrides, thereby enhancing the lateral growth of the overlying GaN layers. Importantly, nitridation of sapphire is considered as a critical step in the development of Npolar nitride layers under appropriate conditions [7, 8]. Nitridation of sapphire substrates appears to have been first introduced by Kawakami et al [9] in a study of the epitaxial growth of AlN films on a-plane sapphire substrates followed by Yamamoto et al [10] in a study of

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nitridation effects on InN growth on c-plane sapphire substrates. The importance of the nitride layer and its structure and chemistry in controlling GaN polarity has been emphasized by Rouvière et al [11] and more recently by Mohn et al [12] and Stolyarchuk et al [13]. The importance of sapphire nitridation has been also realized in the deposition of GaN layers using pulsed laser deposition [14]. In spite of the wide use of nitridation of sapphire in GaN growth, direct crystallographic structural evidence of the nature of the nitride layer is scarce in the literature and conflicting, especially under low-temperature nitridation process conditions. An amorphous AlxOyNz layer has been suggested by Uchida et al [15], while Vennéguès et al [16] proposed that the nitride phase has a hexagonal wurtzite AlN structure (wz-AlN). Rouvière et al [11] identified a cubic Al-O-N phase, while Lee et al [17] suggested a complex of wz-AlN and zincblende-AlN (zbAlN) phases in the nitride layer. More recently the wz-AlN phase has been identified in the nitrided layer by Milakhin et al [18] using reflection high energy electron diffraction technique (RHEED). In this contribution, therefore, we examine the formation of the nitride layer on c-plane sapphire substrates as a function of nitridation temperature by interrupting the two-step growth process to access the nature of intermediate layers. The structure and chemistry of nitride layers have been characterized by a variety of complementary techniques that include transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and x-ray photoelectron spectroscopy (XPS). 2. Experimental Experiments were carried out using a Aixtron 200/4 RF-S horizontal flow single wafer MOCVD reactor. The gas foil rotation of the satellite ensures the uniformity of temperature across the diameter of the substrate during growth. Trimethyl Gallium (TMG) and NH3 are used as precursors for GaN deposition with purified H2 gas as the carrier gas. c-plane sapphire wafers obtained from EPISTONE with unintentional miscut of 0 ± 0.2⁰ were used as substrates. Substrates were first cleaned ex-situ by acetone, IPA and DI water. Thermal cleaning of the substrates was carried out in-situ at 1100°C in 30 mbar of purified H2 for 10 min. The substrates were then nitrided at three different temperatures TN = 530, 800 and 1100°C for 60 sec at 200 mbar, under the environment of 1500 sccm of NH3 and 6500 sccm of H2. Some samples were cooled directly after nitridation to room temperature in the reactor, for ex-situ characterization by XPS. A thin low temperature (LT) GaN nucleation layer (NL) was deposited on the remaining nitrided samples. This was done to provide a cap on the nitride layer to facilitate


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characterization by TEM. The deposition of LT GaN was performed at 530°C, 200 mbar and at a V/III ratio of 2535. The as-nitrided samples without LT GaN capping layer were characterized by an Axis Ultra DLD XPS with monochromatic x-rays as the primary source. The x-ray gun is used in combination with a focusing monochromator due to which only the Al K-α component is diffracted from the quartz crystal. The natural line width of the component is < 0.26 eV. Charge corrections were done with respect to the C 1s (Carbon) peak present at 284.5 eV. Elemental quantification was carried out using CASA XPS software. Crosssectional TEM foils were prepared using a standard sandwich technique in which two pieces of a sample are cut and glued together keeping the film sides facing each other. The glued sample is inserted inside a slotted rod and the rod is placed and glued inside a hollow tube with a 3mm outer diameter. Thin slices from the tube were prepared using a Buehler ISOMET slow speed saw and thinned down to less than 100µm using MULTIPREPTM equipment manufactured by Allied Instruments. In the subsequent process the samples are thinned down further from both sides to electron transparency using a Gatan PIPS (Precision Ion Polishing System) equipped with a cold stage. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) studies were carried out with FEI Tecnai G2-20T 200KV, FEI F-30 and FEI TITAN 300KV TEMs. 3. Results Figure 1 shows the high resolution photoelectron N 1s peak obtained from ultra-thin nitride layers for nitridation temperatures TN = 530, 800 and 1100°C. The distinct N 1s peak indicates that there is incorporation of N into the sapphire substrate during the nitridation process. Changes in N 1s peak intensity variation, peak position shift and peak broadening are observed in the spectrum with change in nitridation temperature. The increase in peak intensity indicates the increase in incorporation of N atoms into the sapphire surface with nitridation temperature. The shift in peak position and peak broadening indicates that the chemical bond environment of N atoms in the modified sapphire surface undergoes changes with nitridation temperature. Surface modification during nitridation is expected to occur by inward diffusion of N atoms in to sapphire to replace O atoms. The intensities of N 1s and O 1s peaks are normalized to Al 2p peak intensities from the sapphire and are plotted against the nitridation temperature in Figure 2. Increase in nitridation temperature results in greater N/O ratios in the


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Figure 1: N 1s x-ray photoelectron spectrum obtained from sapphire substrates nitrided at TN = 530, 800 and 1100°C.

Figure 2: Normalized N 1s and O 1s photoelectron peak intensities obtained from sapphire o

substrates nitrided at T = 530, 800 & 1100 C. Normalization is done using Al 2p peak of N

sapphire. The horizontal line with red markers in Figure 2b indicates the O 1s intensity from the non-nitrided sapphire surface. modified surface. The normalized O 1s intensity is compared with the O 1s intensity from the non-nitrided sapphire surface as indicated by the horizontal line with red markers in Figure 2b Peak broadening and shifts in peak positions have been explored by deconvoluting the N 1s peaks as shown in Figure 3. A strong peak located at ~396.7 eV is present for TN =800


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and 1100oC (Figure 3b & 3c). The peak coincides with that of the binding energy of Al-N chemical bond in bulk AlN of 396.7 eV [19], and indicates the transformation of the sapphire surface from an environment of Al-O bonds to one with Al-N bonds by the nitridation process at TN ≥ 800°C. There are additional peaks present at ~398.5 eV and ~395.5 eV. The intensity of the former peak decreases and the latter increases with the increase in nitridation temperature from TN = 800 to 1100oC. The peak at ~398 eV has been attributed to incomplete substitution of O by N such that tetrahedral Al-N-O bonds are present [19]. The peak at ~ 395 eV binding energy corresponds to sub-stoichiometric AlNx 1000oC) prior to the growth of the high temperature GaN epitaxial layer. This thermal exposure results in a partial decomposition of the LT GaN layer and it is possible that the growth of the subsequent epitaxial layer may occur on exposed nitrided surface as a result. We have therefore characterized the nitride layers after thermal annealing of LT GaN as well. We find that thermal annealing of LT GaN NL has no effect on the nature of nitride layers for both the nitridation temperatures. An example of diffraction pattern obtained from the nitride layer after the thermal annealing treatment at 1000oC for 4 minutes under the ambient of NH3 and H2 and is shown in Figure 9. In summary, the structure and chemistry of the nitride layers are controlled by the nitridation temperature of sapphire. Low nitridation temperature (TN = 530°C) yields cubic


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spinel-AlxOyNz with its direction parallel to the c-axis of sapphire. In contrast, high temperature nitridation (TN = 1100°C) results in a rs-AlN structure, again with its direction parallel to c-axis of sapphire. The nitride layers show the presence of variants that are twin related about this direction. The annealing step of LT GaN in the two-step growth process does not affect the structure of nitride layers for either nitridation temperature.

Figure 9: Nano-diffraction pattern obtained from annealed LT-GaN NL (prior to o epitaxial layer growth) for TN=1100 C acquired near the interface, showing rs-AlN phase in [110] zone-axis orientation (green grids for twin variants) and for sapphire ;0] (blue grid). in zone-axis [10𝟏 4.

Discussion The nitridation process exposes the sapphire surface to a N precursor at a particular

temperature. During this process N atoms diffuse inward from the sapphire surface replacing O atoms, and eventually transform the surface to a structure different from that of sapphire. The relative concentration of N atoms in the nitride phase varies with nitridation temperature (Figures 2 and 8). The ratio of Al to O and N atoms will also change in the process if the nitride layer has a different structure from sapphire. Our results indicate that the structure of the nitride layer formed in the nitridation process varies with nitridation temperature. Low nitridation temperatures (TN = 530oC) results in only a partial replacement of O in sapphire to form a cubic


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spinel-AlxOyNz structure, while high nitridation temperatures (TN = 1100oC) result in the formation of rs-AlN. Direct structural evidence of the nature of the nitride layer in the MOCVD process is scarce in the literature and conflicting, especially in the low temperature nitridation condition. Thus amorphous layers, wz-AlN, zb-AlN and duplex layers with wz-AlN adjacent to the sapphire surface followed by rhombohedral AlxOyNz have all been proposed [9-16]. Uchida et al proposed an amorphous nitride phase based on cross sectional TEM, apparently directly after nitridation for 3 min at 1050oC [15]. Our results show clearly that the nitride layer is crystalline, irrespective of the nitridation temperature, in observations made after the deposition of LTGaN NL. The first suggestions of a cubic nitrided layer emerge from the work of Rouvière et al [11]. The nitridation temperature used in their study is not provided nor has the basis for the identification of a cubic structure been made clear. However Rouvière et al [11] report an interplanar spacing of 2.34 Å of the cubic {111} planes while we obtain 2.3 Å. In addition, our diffraction evidence unambiguously identifies a cubic structure for the entire range of nitridation temperatures from TN = 530 to 1100oC. Thus our results appear to be in agreement with Rouvière et al. [11], while additionally delineating the chemistry and differences in the cubic structure at low and high nitridation temperatures. Our results for the high nitridation temperature (TN = 1100oC) contradict those of Vennéguès et al [16]. This latter study concludes that the nitride layer under similar nitridation conditions to that used in our study is wz-AlN with a hexagonal structure on the basis of a high resolution micrograph (Figure 2 of [16]) that bears a remarkable similarity to our Figure 4b. Our diffraction evidence however appears quite unambiguous and in addition we detect the two twin related variants that can only form out of the observed crystallographic relationship between the cubic rs-AlN nitride layer and sapphire (our Figure 5 and 7). Finally, it is noted that Lee at al. [17] used grazing incidence x-ray diffraction to identify mixtures of wz-AlN and zb-AlN after nitridation at 1080oC for 30 min, while Mohn et al. [12] use high resolution TEM and HADDF images to propose duplex layers of wz-AlN and rhombohedral AlON structure after nitridation at 1080oC for 7 min. While these conditions are somewhat atypical of typical nitridation times, the results suggest that the structure of the nitride layer may vary with nitridation time and other nitridation process conditions, as also indicated by the results of [13,18]. The relationship between the structures of the nitride layers obtained in this study and the sapphire substrate are shown in Figure 10. The approximate thickness of the nitride layer, as experimentally observed, is indicated by the dashed lines and is a fraction of the sapphire unit cell. This thickness corresponds also to less than one unit cell of the spinel-AlxOyNz layer


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Figure 10: (a) The cross-section of projected atom positions along the [11#00] orientation of sapphire, and [110] of spinel-AlxOyNz and rs-AlN. The approximate thickness of the nitrided layer (from Figure 4 and 7) is indicated by dashed lines. The unit cell of the 3 structures in projection are shown. The average Al-Al (or O-O) interlayer spacing is indicated (b) shows the bonding between Al and)/N atoms in the 3 structures. Al atoms are blue in color while O/N atoms are red. formed at low TN = 530oC with a distortion of 6% in the growth direction. The spinel-AlxOyNz molecular structure is shown in Figure 9b. Two different types of Al-O/N bonding configurations are present, one of which closely resembles the sapphire configuration. The composition of Al in spinel varies between 40-42% and is very similar to that in sapphire. Thus spinel-AlxOyNz may form in the process of nitridation with small distortions and rearrangements of Al atoms from the sapphire lattice as shown in Figure 10 and 11, and by the substitution of O atoms by N since the arrangement of O atoms is similar in both the structures. At higher nitridation temperatures (TN = 1100oC), O atoms of sapphire are completely substituted by N resulting in the formation of rs-AlN. The thickness of the nitrided layer in the growth direction corresponds to a little more than 1 unit cell of the rs-AlN structure, again with a small distortion of 6%. We speculate that the rs-AlN structure is preferred over


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the wz-AlN structure because the ABC stacking of this structure requires smaller rearrangements of Al atoms of sapphire in the transformation to rs-AlN, as indicated in Fig 10. In addition, the distortion along the growth direction is about 14% between the Al layers in sapphire and wz-AlN. However the formation of rs-AlN requires upward diffusion of Al from sapphire to enrich the nitride layer with Al. The O/N arrangement in sapphire, spinel-AlxOyNz, and in rs-AlN is identical in all layers as shown in Figure 11.

Figure 11: (a) The plan view of projected Al atom positions in the (0001) planes of Al2O3, (111) planes of spinel-AlxOyNz and rs-AlN, corresponding to the layers A and B in the respective structures outlined by dashes lines in Figure 9 . For A layer of the spinel structure, the 3 layers of Al atoms are indicted in different colors (b) the plan view of O atoms in these planes. In conclusion, we have unambiguously determined the structure and chemistry of ultrathin nitride layers formed on c-plane sapphire wafers during the nitridation process through a combination of XPS, EELS, diffraction and high resolution TEM based techniques. The low temperature nitridation process results in transformation of sapphire surface to cubic spinelAlxOyNz. In contrast, the high temperature nitridation process transforms the sapphire surface to the rs-AlN structure. We believe that this study is an important step in understanding the evolution of structure, defects and polarity of subsequent GaN layers deposited in the conventional two-step growth process on nitrided sapphire wafers.


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Acknowledgements Funding by the Defence Research and Development Organisation, India for parts of this work is gratefully acknowledged. The Micro and Nano Characterization Facility of the Centre for Nano Science and Engineering, Indian Institute of Science, India provided access to XPS and the Advanced Facility for Microscopy and Microanalysis, Indian Institute of Science, India provided access to the TEM facility and are also acknowledged. DB, acknowledges the support of the Department of Science and Technology, Government of India through the J.C. Bose Fellowship. SS would like to thank past and present directors of Defence Metallurgical Research Laboratory for permission to carry out this work. SS would also like to thank Jawaharlal Nehru Centre for Advanced Scientific Research for allowing the use of their facilities for carrying out parts of this research work. Supporting Information All reported structures for AlN and ALON polytypes, simulated selected area diffraction patterns from spinel AlON and rs-AlN in the [110] and [112] zone axes, normalized intensity ratios of various reflections in these zone axes.

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FOR TABLE OF CONTENTS USE ONLY The Nitridation of Sapphire as a Precursor To GaN Growth: Structure and Chemistry Krishna Yaddanapudi, Sabyasachi Saha, Srinivasan Raghavan, K. Muraleedharan and Dipankar Banerjee

GaN

AlN

Sapphire

SYNOPSIS The structure and chemistry of the nitrided layer grown by MOCVD at different temperatures on c-sapphire substrates has been characterized. The low temperature nitridation process results in a nitrided layer in which oxygen has been partially replaced by nitrogen to form a cubic spinel- AlxOyNz structure. Nitridation at 8000 and 1100oC results in complete substitution of oxygen atoms by nitrogen to form a cubic rock salt AlN structure.


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Nitridation of Sapphire as a Precursor to GaN Growth: Structure and

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