Initial stages of the ion-beam assisted epitaxial GaN film growth on 6H-SiC(0001)

Thin Solid Films 520 (2012) 3936–3945

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Initial stages of the ion-beam assisted epitaxial GaN film growth on 6H-SiC(0001) L. Neumann a, J.W. Gerlach a,⁎, B. Rauschenbach a, b a b

Leibniz-Institut für Oberflächenmodifizierung (IOM), D-04318 Leipzig, Germany Universität Leipzig, Institut für Experimentalphysik II, D-04103 Leipzig, Germany

a r t i c l e

i n f o

Article history: Received 25 July 2011 Received in revised form 27 January 2012 Accepted 1 February 2012 Available online 9 February 2012 Keywords: Ion-beam assisted epitaxy Gallium nitride High-energy electron diffraction Scanning tunneling microscope Growth mode Ultra-thin films

a b s t r a c t Ultra-thin gallium nitride (GaN) films were deposited using the ion-beam assisted molecular-beam epitaxy technique. The influence of the nitrogen ion to gallium atom flux ratio (I/A ratio) during the early stages of GaN nucleation and thin film growth directly, without a buffer layer on super-polished 6H-SiC(0001) substrates was studied. The deposition process was performed at a constant substrate temperature of 700 °C by evaporation of Ga and irradiation with hyperthermal nitrogen ions from a constricted glow-discharge ion source. The hyperthermal nitrogen ion flux was kept constant and the kinetic energy of the ions did not exceed 25 eV. The selection of different I/A ratios in the range from 0.8 to 3.2 was done by varying the Ga deposition rate between 5 × 1013 and 2 × 1014 at. cm− 2 s− 1. The crystalline surface structure during the GaN growth was monitored in situ by reflection high-energy electron diffraction. The surface topography of the films as well as the morphology of separated GaN islands on the substrate surface was examined after film growth using a scanning tunneling microscope without interruption of ultra-high vacuum. The results show, that the I/A ratio has a major impact on the properties of the resulting ultra-thin GaN films. The growth mode, the surface roughness, the degree of GaN coverage of the substrate and the polytype mixture depend notably on the I/A ratio. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The investigation of the nitride semiconductor film growth, such as of gallium nitride (GaN), is of great interest for a variety of advanced applications as well as for a fundamental understanding of the occurring complex compound growth processes. Generally, first and foremost the properties of epitaxial thin films depend on their structural properties and their morphology. Those are determined to a large extent by the initial stages of growth, i.e. the growth regime that ranges from nucleation to film coalescence. Once the conditions are fixed during the initial stages, the film characteristics (e.g. presence of polytypes) cannot be easily altered, controlled, improved or even reversed by simply changing the deposition parameters. Admittedly, the crystalline quality can be improved by growing to high film thicknesses, but this is counterproductive in the case of ultra-thin films. Therefore, a thorough knowledge of the involved processes is necessary to control and determine the film growth during the initial stages in order to obtain the intended film properties. Several studies of the initial stages of wurtzitic GaN film growth by conventional molecular-beam epitaxy (MBE) exist, which demonstrate the distinct dependence of the GaN film surface topography and morphology on the atomic N flux to Ga flux ratio (see e.g. [1–3]) and/ or on the substrate temperature (e.g. [4,5]). According to them, the

⁎ Corresponding author. Tel.: + 49 341 235 3310. E-mail address: [email protected] (J.W. Gerlach). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.02.004

present growth mode can be influenced by those parameters to be three-dimensional or two-dimensional, the latter resulting in GaN films of high crystalline, optical and electronic quality when grown to larger film thicknesses [2,3]. While in some of the studies GaN templates for homoepitaxial growth experiments were used [1–3,6], thus representing the most ideal case, in other studies common substrates for heteroepitaxial GaN growth experiments were chosen like 6H-SiC(0001) with and without buffer layers [5–9]. Typical substrate temperatures were in the range from 640 to 720 °C. A frequently used way of sample characterization in the studies was the combination of in situ analysis by reflection high-energy electron diffraction (RHEED) with subsequent surface topography analysis by either atomic force microscopy or scanning tunneling microscopy (STM). In recent times, a series of publications with experimental and theoretical background appeared that revisited the N/Ga flux ratio in conventional MBE of GaN and its influence on the film growth as well as on the resulting film properties [10–13]. They confirm the importance of controlling the N/Ga flux ratio in order to obtain GaN films of high crystalline quality. The application of energetic nitrogen ions, which possess hyperthermal kinetic energies instead of the rather thermal atomic nitrogen usually used in conventional MBE of GaN, introduces additional growth parameters to this already complex heteroepitaxial growth system. Earlier studies on ion-beam assisted MBE (IBA-MBE) demonstrated the beneficial effect of the hyperthermal nitrogen ion irradiation on the growth of GaN films with several hundred nanometre thickness directly on Al2O3(0001) [14], 6H-SiC(0001) [15] and γ-LiAlO2(100) [16] substrates. In analogy to conventional MBE of GaN, the ratio of

L. Neumann et al. / Thin Solid Films 520 (2012) 3936–3945

the impinging hyperthermal nitrogen ion flux to the flux of the incoming Ga atoms (ion to atom ratio I/A) was observed to be one crucial parameter, amongst others like the kinetic energy of the ions [16], determining the growth mode and the presence of more than just the intended wurtzitic GaN polytype. Recently, the experiences gained with IBA-MBE on substrates with six fold symmetry were used to deposit zincblende GaN films on cubic 3C-SiC/Si pseudo-substrates [17]. There too, the I/A ratio was shown to strongly influence the properties of the resulting films, in particular the fraction of the in that case unintended wurtzitic polytype. The importance of the parameter kinetic ion energy for ion enhanced epitaxy was stressed already in the mentioned earlier studies. In short, there exists a window of suitable ion energies where no ion collision induced defects are created below the surface, but instead only at the surface (for details see [18]). This leads to a rearrangement of surface atoms that can be interpreted as ballistically enhanced adatom mobility. In a theoretical approach regarding the effect of hyperthermal ion irradiation in IBA-MBE on film growth compared to conventional MBE, a kinetic equations model was developed [19]. The main result of the model was that the energetic ions, detaching surface atoms from GaN islands by overcoming the Ehrlich–Schwoebel barrier, can provoke two-dimensional growth instead of three-dimensional growth, resulting in a highly reduced coalescence thickness. While until now thin GaN films prepared by IBA-MBE were several hundreds of nanometre thick and were predominantly studied ex situ, in the present paper the initial stages of nitrogen ion-beam assisted epitaxial growth of ultra-thin GaN films on super-polished 6HSiC(0001) substrates are investigated in situ by RHEED and after deposition by STM in ultra-high vacuum (UHV) at room temperature. For this purpose, growth experiments with different combinations of I/A ratio, Ga deposition rate and deposition time were performed at a constant substrate temperature of 700 °C. This is a study with the aim to determine the influence of the hyperthermal ion irradiation on the nucleation, on the formation or dissociation of clusters, as well as on the structural properties, the morphology and surface topography of the growing ultra-thin GaN films as a consequence of the near surface energy and momentum input through the impinging nitrogen ions and of the ballistic adatom rearrangement by atomic collisions at the surface.

2. Experimental details The experiments were carried out in a self-built low-energy IBA-MBE system consisting of three interconnected UHV chambers for film deposition, analysis and microscopy, respectively, which are connected via a load-lock system allowing the transfer of the samples without changing the vacuum condition (Fig. 1). Thin GaN films were grown in the preparation chamber by deposition of Ga and simultaneous irradiation of the film growing on the substrate with low-energy nitrogen ions. The residual gas pressure prior to deposition was 5×10− 7 Pa. The working pressure in the preparation chamber was 8×10− 2 Pa during ion bombardment due to a nitrogen mass flow of 12 standard cubic centimetres per minute necessary to operate the ion source. Gallium was evaporated by an effusion cell at temperatures TEC between 950 °C and 1040 °C. In this temperature range the deposition rate ФGa varied between 5 × 1013 and 2 × 1014 Ga at. cm− 2 s− 1 according to the relation ΦGa =8 × 107 × exp(TEC/71) obtained by Rutherford backscattering spectrometry (RBS) measurements of GaN films. A hollow-anode ion source, based on the principle of a constricted dc glow-discharge, was utilized to generate atomic nitrogen ions N+ and molecular ions N2+; the ratio of N +/N2+ is roughly of order 1/4. The ion energy distributions show two peaks at 4–6 eV, 15–16 eV and not exceeding 25 eV. The output of the source contains a significant but not well known fraction of dissociated and excited molecules [20,21]. The nitrogen ion flux of 1.6 × 10 14 ions cm− 2 s − 1 was kept constant.

3937

Fig. 1. Schematic of the IBA-MBE system.

Super-polished 6H-SiC(0001) substrates, sized 1 × 1 × 0.25 cm³, with a root mean square (RMS) surface roughness of 0.2 nm were ultra-sonically cleaned using ethyl alcohol in order to degrease the surface and dried before transferring into the UHV system. The substrate temperature in these experiments was kept constant at 700 °C. The N ion to Ga atom arrival ratio was changed between I/A= 0.8 and 3.2 by varying the Ga flux, i.e. the Ga effusion cell temperature was varied at otherwise constant ion source parameters. It should be noted here that GaN was deposited on bare SiC substrate surfaces without any kind of buffer or wetting layers. RHEED at an electron acceleration voltage of 30 kV (RHEED 30 S, STAIB Instruments GmbH, Germany) and an incidence angle of approximately 2° with respect to the substrate surface was performed to observe the GaN film formation in situ during the deposition process. The RHEED patterns were recorded simultaneously during film deposition with a high-resolution charge-coupled device camera. The monitoring of diffraction intensities was carried out by using a real-time image processing system. The topography of the deposited GaN islands as well as films on the SiC substrate surfaces was analyzed without interruption of the UHV by STM in the microscopy chamber (Variable temperature STM, Omicron NanoTechnology GmbH, Germany) at a base pressure of 2 × 10− 7 Pa. The STM measurements were performed at room temperature by using a W or Pt/Ir tip and were acquired at a typical sample bias voltage of −1.5 V with a set-point tunnel current of 0.5 to 2.0 nA. Typical scanned areas were 2 × 2 μm² and 200 × 200 nm². The RMS roughness of the films on the microscopic scale was determined on base of the STM measurements. The average film thickness was obtained by x-ray reflectometry (XRR), evaluating the measured film thickness oscillations. The crystalline structure of the GaN films was studied by x-ray diffraction (XRD). As RHEED patterns did not always allow for distinguishing between the thermodynamically stable hexagonal polytype (wurtzite GaN or w-GaN) and the metastable cubic polytype (zincblende GaN or z-GaN), in particular in the case
of streaky patterns, azimuthal XRD  and the z-GaN(200) reflections scans (φ scans) of the w-GaN 1011 were measured, respectively. All XRR and XRD measurements were performed ex situ in a combined high resolution reflectometer– diffractometer setup (XRD 3003 PTS-HR, Rich. Seifert GmbH, Germany) using a collimated and monochromatic Cu-Kα1 beam with a wavelength

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of 0.15406 nm. The setup was equipped with a four-circle goniometer allowing for accurate sample adjustment. X-ray photoelectron spectroscopy (XPS) with excitation by monochromatic Al-Kα radiation (E= 1486.6 eV) at a pass energy of 40 eV was conducted ex situ to estimate the degree of coverage of the substrate with GaN. The XPS system (Axis Ultra DLD, Kratos Analytical Ltd., UK) was equipped with a combination of electrostatic lens and magnetic immersion lens to enhance the photoelectron yield. The total Ga coverage, i.e. the total number of Ga atoms deposited per unit area on the substrate during the deposition process, was obtained by multiplying the Ga deposition rate ΦGa with the total deposition time t. This definition is valid, as it was found that at the selected substrate temperature of 700 °C the amount of Ga atoms in the GaN films on the substrate (as obtained by RBS measurements) did not differ from the value calculated directly from the Ga flux. Consequently, the sticking coefficient is unity here. Assuming that every single Ga atom reacts with one N atom, so that a Ga\N bond is formed, the total N coverage equals the total Ga coverage. If the deposited GaN film is compact and is thus possessing the bulk atomic density nGaN = 8.77 × 1022 atoms cm− 3, the calculated average GaN coverage θ is given in nanometres by θ ¼ ð2⋅ϕGa ⋅t Þ=nGaN :

ð1Þ

Alternatively, the GaN coverage θ can be expressed in GaN monolayers (ML) for (0001) oriented hexagonal GaN and for (111) oriented cubic GaN  c θ ¼ ð2⋅ϕGa ⋅t Þ= nGaN ⋅ and 2   a θ ¼ ð2⋅ϕGa ⋅t Þ= nGaN ⋅ pffiffiffi respectively; 3

ð2a; bÞ

with lattice constant c = 0.5185 nm or a = 0.452 nm for hexagonal or cubic GaN structure, respectively. It should be noted that the average GaN coverage θ given in ML is almost equal pffiffiffi for both polytypes in the present growth orientation, i.e. c=2≈a= 3≈0:26 nm. Thus, the calculated average GaN coverage θ is equivalent to the nominal thickness of an ideal GaN film. For real GaN films a deviation from the calculated coverage θ is usually observed that is caused e.g. by three-dimensional instead of two-dimensional growth. The typical thickness of films was in the range from 10 to 50 ML of GaN, the growth rates varied from 3 to 10 ML/min. Photoelectron spectra were taken in order to obtain a simple, macroscopic, experimentally accessible measure for the degree of coverage of the SiC substrate with GaN. For this purpose two core-level electron lines, Si 2s and Ga 3s, were selected. They are energetically well separated and simultaneously close to each other with an energetic difference of only about 10 eV. The inelastic mean free path of the measured Si 2s and Ga 3s photoelectrons is approximately 2.5 nm [22]. The degree of coverage CXPS was determined using the area SGa 3s of the GaN filmrelated Ga 3s peak at a binding energy of 161 eV and the area SSi 2s of the substrate-related Si 2s peak at 152 eV: CXPS ¼ SGa 3s =ðSSi 2s þ SGa 3s Þ:

ð3Þ

If Ga or GaN completely cover the surface of the SiC substrate, the Si 2s peak is no longer observed; consequently the degree of coverage CXPS is equal to 100%. 3. Results The substrates utilized in this study were super-polished 6HSiC(0001) pieces with Si termination [23]. STM measurements of such substrates exhibited an atomically flat surface, free of polishing scratches and without a damaged surface region, with 150 to 350 nm wide terraces of about 0.4 to 0.6 nm in height. The RHEED pattern of a

bare 6H-SiC(0001) substrate (before

deposition) in Fig. 2a with the  exhibits distinct Kikuchi lines, electron beam parallel to SiC 1120 which indicates a high surface quality of the substrate material. Although high quality GaN films can be obtained by IBA-MBE also on conventionally polished SiC substrates that exhibit a large amount of polishing scratches at the surface [15], the present study demanded a smooth substrate surface in order to avoid a falsification of the GaN growth results by substrate surface artefacts. The early steps of the low-energy nitrogen ion beam assisted GaN film formation on smooth 6H-SiC(0001) substrates were studied at constant deposition temperature of 700 °C and ion energy of maximal 25 eV. Under these conditions, the deposited GaN films grew epitaxially on 6H-SiC(0001) substrates as function of the Ga flux. The crystalline surface structure during the GaN growth was monitored in situ by RHEED to determine the present growth mode, as well as to distinguish between the formation of the thermodynamically stable hexagonal GaN polytype and the metastable cubic GaN polytype, respectively. Different exemplary RHEED patterns of GaN films on 6H-SiC(0001) are presented in Fig. 2b and c. The RHEED patterns with the electron beam parallel to

 direction before and after GaN deposition duration of the SiC 1120 150 s and 300 s are compared (for detailed deposition parameters see Table 1). The interpretation of the RHEED patterns is based on simulated patterns of two different GaN polytypes and the superimposition of these patterns. A series of simulated and corresponding experimental RHEED pattern examples obtained during GaN deposition on 6HSiC(0001) is shown in Fig. 3. Fig. 3a shows the simulated of the pattern

 azimuth. (0001) oriented hexagonal polytype of GaN in the 1120 The measured RHEED pattern in Fig. 3b is in agreement with this simulated pattern. In Fig. 3c the simulated and superimposed RHEED patterns of the oriented cubic GaN polytype and its twinned (111)

 azimuth is shown. Each of them is positioned in the phase in the 110 opposite direction by rotating 180° around the shared [111]-axis. The islands of cubic GaN are twinned in relation to the (111) plane. In the simulated pattern of Fig. 3e, the patterns of the cubic, the twinned cubic and the hexagonal polytype are superimposed, which is in good agreement with the measured RHEED pattern in Fig. 3f. 3.1. GaN deposition at constant deposition times A series of samples was prepared with different Ga deposition rate ΦGa but with a constant deposition time of 300 s (see Table 2). Shown in Fig. 4 are RHEED patterns (first row), STM images of size 2 × 2 μm2 (second row) and STM images at higher magnification with a size of 200 × 200 nm² (third row). The RHEED patterns and STM images in Fig. 4 reveal a large variation in surface topography among the different growth regimes. At the beginning of the deposition at a low Ga deposition rate of 5 × 1013 at. cm− 2 s− 1 (I/A = 3.2), the intensity of the RHEED pattern decreased rapidly due to coating the substrate with a thin layer of Ga. After about 120 s of deposition time the diffraction pattern demonstrated mixed two- and three dimensional growth modes. Finally (Fig. 4a) this surface exhibited mainly diffuse diffraction spots together with a faint streak pattern. The streaky nature in Fig. 4a indicates a rather small surface roughness. The diffuse streaks show, that the crystallites are not perfectly oriented. In Fig. 4b, a spotty final RHEED pattern for ΦGa = 9 × 1013 at. cm− 2 s− 1 (I/A = 1.8) is clearly observed, which indicates a three-dimensional island growth mode. Bright reflections appeared already in the first minute of deposition. The degrees of coverage for the two examples, as obtained by means of XPS measurements, increase from 69% to 87% with ΦGa increasing from 0.5 × 1014 to 0.9 × 1014 at. cm− 2 s− 1 (I/A decreasing from 3.2 to 1.8), respectively. The STM images in Fig. 4a and b show an island-like morphology of GaN with an average surface roughness of about 0.5 and 1.2 nm, respectively. From line scan it is found that the average diameter and height of islands are 40 nm and 1.5 nm in Fig. 4a, whereas they are 15 nm and 3 nm in Fig. 4b. Thus, although the total Ga coverage in Fig. 4a is only

L. Neumann et al. / Thin Solid Films 520 (2012) 3936–3945

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 Fig. 2. RHEED patterns obtained during one single GaN deposition on 6H-SiC(0001) with the electron beam parallel to SiC[1120].

half of that in Fig. 4b, the average lateral island size in Fig. 4a is several times bigger than the one in Fig. 4b. The lower Ga deposition rate (higher I/A ratio of 3.2) in Fig. 4a seems to reduce the height growth of islands in favour of the lateral growth. The corresponding average island height of 1.5 nm in Fig. 4a is in the range of the inelastic mean free path of the measured Si 2s photoelectrons (approximately 2.5 nm). This can lead to an underestimation of the degree of coverage CXPS in this specific case, as some Si 2s photoelectrons can still penetrate the GaN islands and get to the detector. The XRD φ scans of the film shown in Fig. 4a did not exhibit any polytype components due to the extremely small film thickness of approximately 4 nm and the mixed two- and three-dimensional growth mode observed by RHEED. In the RHEED patterns of deposition at ΦGa =1.3×1014 at.cm− 2 s− 1 (I/A=1.2) a transition was observed in which the initial spots were replaced by streaks after about 180 s of deposition time (Fig. 4c), explicable by a rapid island coalescence through favoured lateral growth. The surface topography in the corresponding STM image in Fig. 4c consists indeed of a terrace-step structure, which is evidence for the twodimensional (step flow) growth mode. The terraces are atomically smooth and the step height is about 0.2 to 0.3 nm, which agrees well with the 0.26 nm step height expected for a w-GaN(0001) monolayer (half of the c-axis lattice constant). The change in RHEED patterns from Fig. 4b to c resembles the transition from N-rich to Ga-rich growth in conventional MBE [1–3,24]. GaN films grown in the Ga-rich regime exhibit a smooth surface topography, which correlates with the streaky RHEED patterns suggesting a two-dimensional growth mode. In Fig. 4d the final RHEED pattern at ΦGa = 1.9 ×1014 at.cm− 2 s− 1 (I/A= 0.8) consists of streaks and faint double spots corresponding to (111)-oriented GaN. This type of pattern is consistent with a surface topography which is locally smooth on an atomic scale, as shown by the associated STM image (Fig. 4d). Moreover, STM investigations at a higher magnification (200 ×200 nm² scan area) reveal an interpenetration structure. The atomically flat areas are more than 80 to 100 nm in size with RMS roughness values of about 0.2 nm. The randomly distributed canyons have a depth of about 2 to 3 nm. In the case of ΦGa = 1.3 × 1014 at.cm− 2 s− 1 (I/A =1.2) (Fig. 4c), the bottom of the valley areas was covered with small islands of GaN. At ΦGa of 1.3× 1014 at. cm− 2 s− 1 and higher (I/A of 1.2 and lower) separate flat-top islands (white areas in Fig. 4c and d, second row) with an average diameter and height of about 100 nm and 15 nm, respectively, were observed on the locally smooth GaN films. The vicinity of those islands is obviously depleted of deposited material. The islands height appeared to increase slightly with the Ga deposition rate ΦGa increasing (I/A ratio decreasing). The faster-growing three-dimensional

formations of GaN are depicted only in a 2× 2 μm2 surface area (Fig. 4d and c), which exhibit a RMS roughness of over 1.5 nm in comparison to the small (flat) 200 × 200 nm2 zoom area with a RMS roughness ~0.3 nm. The degree of coverage CXPS for these two examples decreased from 89% to 84% with increasing of ΦGa from 1.3×1014 to 1.9×1014 at.cm− 2 s− 1 (I/A ratio decreasing from 1.2 to 0.8), respectively.
  For films shown in Fig. 4b–d the XRD φ scans of the w-GaN 1011 and the z-GaN(200) reflections (not shown) indicate that only the hexagonal polytype of GaN is generated. A special case is the threedimensional growth of ultra-thin (lower as 4 nm) GaN films at I/A = 3.2. The RHEED patterns exhibited diffraction spots together with faint diffuse streaks (Fig. 4a). This result could be a consequence of both polytypes, but it could not be proven by XRD due to the low signal intensity from the thin GaN film. Summarizing the results, a strong dependence of the resulting film morphology and topography as a function of the Ga deposition rate (I/A ratio) could be observed with rather two-dimensional growth favoured at higher Ga deposition rates (lower I/A ratios). But it should be considered, that the films compared here had different thicknesses. 3.2. Deposition with constant average GaN coverage For a direct and more significant comparison, a constant amount of deposited GaN at different Ga deposition rate ΦGa was achieved by adapting the deposition time (see Table 3). A total Ga coverage of approximately 4.9 × 1016 Ga at. cm− 2 was selected which corresponds to a GaN film thickness of about 11.1 ± 0.3 nm (≈42.5 ± 1.5 ML). For this total Ga coverage, two-dimensional grown and coalesced films were obtained in Section 3.1. Shown in Fig. 5 are RHEED patterns (first row), STM images of size 2 × 2 μm 2 (second row) and STM images at higher resolution with a size of 200 × 200 nm² (third row). The RHEED patterns and STM images in Fig. 5 reveal a large variation in surface topography among the different growth regimes. The typical spotty RHEED pattern after a GaN deposition time of 960 s in Fig. 5a indicates a three-dimensional, purely wurtzitic growth of GaN islands. The corresponding STM images confirm this; showing GaN island structures with an average island diameter and height of about 40 nm and 6 nm, respectively. At a Ga flux ΦGa of 1.0 × 1014 at. cm− 2 s − 1 and higher (I/A ratio of 1.6 and lower) the morphology transition from three-dimensional to predominantly two-dimensional growth of GaN was observed after about 180 s of deposition time (not demonstrated here). Consequently, streaky final RHEED patterns in Fig. 5b and c after longer deposition

Table 1 Deposition parameters and characteristic values of the GaN film deposition monitored by RHEED in Fig. 2.

(b) (c)

Effusion cell temperature [°C]

Ga deposition rate [at. cm− 2 s− 1]

I/A ratio

Deposition time [s]

Total Ga coverage [at. cm− 2]

Calculated average GaN coverage, θ [nm]

[ML]

950 950

0.5 × 1014 0.5 × 1014

3.2 3.2

150 300

0.8 × 1016 1.6 × 1016

1.8 3.6

6.9 13.9

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 azimuth (a), both twinned cubic GaN (z-GaN) polytype in 110  azimuth (c), and all superimFig. 3. Schematic reciprocal lattice projection of hexagonal GaN (w-GaN) in [1120] posed (e). Exemplary corresponding experimental RHEED patterns obtained during GaN deposition on 6H-SiC(0001) (b, d, f). Note that Fig. 3f is the same as Fig. 2b.

times (480 s and 420 s, respectively) indicate that the surface is grown smoothly. The corresponding STM images show laterally wide terracelike structures characterized by step flow growth and step bunching. The atomically flat terraces (RMS roughness ~0.4 nm) and the unfinished valley areas (Fig. 5c), i.e. still incompletely filled canyons/ gaps, are covered with small islands comparable with the GaN film in Fig. 4c. A closer examination for I/A ratios in the range from 1.1 to 1.6 of substrate regions that are not already completely covered by GaN (Table 4, Fig. 6) reveals that small isolated islands can be found in between the large two-dimensionally grown domains. The final RHEED pattern in Fig. 5d (for 300 s deposited GaN) shows a superposition of streaks and double spots, the latter corresponding to (111) oriented z-GaN. This pattern at higher ΦGa = 1.6 × 10 14 at. cm − 2 s − 1 (I/A ratio = 1.0) is consistent with a locally smooth topography, as shown by STM in Fig. 5d. Similar to the film in Fig. 4d this GaN film exhibits an interpenetration structure. It consists

of areas with an atomically flat surface of more than 90 nm in size, divided by large and irregularly shaped canyons with a maximal depth of about 7 to 8 nm. The RMS roughness of the two-dimensionally grown GaN films in Fig. 5b–d, measured in an area 2 × 2 μm 2 by STM, approached a value of 1.2 to 1.4 nm. Separate faster-growing flat-top islands of GaN with an average diameter and height of about 100 nm and 15 nm, respectively, occurred for all predominantly two-dimensionally grown GaN films (see white areas in Fig. 5b–d, second row). As already observed in Fig. 4c and d the vicinity of the islands is depleted also for the islands in Fig. 5c–d. The degrees of coverage CXPS (Fig. 7a–b) remained constant at about 93% with increasing ΦGa from 0.5 × 1014 to 1.4 × 10 14 at. cm− 2 s − 1 (I/A ratio decreasing from 3.2 to 1.1). Then it decreased significantly to 80% at ΦGa = 1.6 × 1014 at. cm− 2 s − 1 (I/A ratio = 1.0). This decrease in the degree of coverage at higher ΦGa was also obtained for GaN films at constant deposition times and high ΦGa (Section 3.1, Fig. 4d).

Table 2 Deposition parameters and characteristic values of the GaN film deposition results shown in Fig. 4 at constant deposition time. Symbols: (✓) — presence of z-GaN, (–) — absence of z-GaN, (S) — streaky RHEED pattern (distinction between w- and z-GaN not possible).

(a) (b) (c) (d)

Effusion cell temperature[°C]

Ga deposition rate [at. cm− 2 s− 1]

I/A ratio

Deposition time[s]

Total Ga coverage [at. cm− 2]

950 990 1015 1040

0.5 × 1014 0.9 × 1014 1.3 × 1014 1.9 × 1014

3.2 1.8 1.2 0.8

300 300 300 300

1.6 × 1016 2.8 × 1016 3.9 × 1016 5.6 × 1016

Calculated average GaN coverage θ [nm]

[ML]

3.6 6.3 8.9 12.7

13.8 24.2 34.5 49.1

Degree of coverage CXPS[%]

RMS roughness [nm]

69 87 89 84

0.5 1.2 0.3…2.0 0.2…1.5

z-GaN formation RHEED

XRD

S – S ✓

– – – –

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Fig. 4. Final RHEED patterns and STM images of GaN deposited on 6H-SiC(0001) substrates at different Ga deposition rate ΦGa and constant deposition time t (see Table 2). Image size: 2 × 2 μm2, smaller zoom areas image size: 200 × 200 nm2. The height scale z between lowest (dark) and highest (bright) levels is given in [nm]. The corresponding I/A ratios are 3.2 (a), 1.8 (b), 1.2 (c) and 0.8 (d).

XRD rocking curve measurements of the GaN(0002) reflections for three-dimensional and two-dimensional grown films (Fig. 7a) were performed to prove the RHEED and STM results. The rocking curve full width at half maximum (FWHM) of films consisting of threedimensional grown islands (I/A = 3.2) was below 8 arcmin, but the curves also comprised an up to 2° wide background peak correlated to a fraction of misoriented islands. The rocking curves of samples with a flat two-dimensional topography (I/A b 1.6) were similar with FWHM of 4 arcmin and did not show a distinct broad background peak, demonstrating the high quality orientation in those films.
of crystallite   φ Scans of the w-GaN 1011 and the z-GaN(200) reflections (Fig. 7c) indicate that epitaxial w-GaN(0001) was always obtained independent of the Ga deposition rate. Only at I/A = 1.6–1.3 the grown GaN films reveal the coexistence of hexagonal and cubic GaN polytypes (Fig. 7c). On the other hand, the final RHEED pattern at I/A = 1.0 (Fig. 5e) consists of faint double spots corresponding to (111)-oriented GaN. It is also clear from a number of reports that epitaxial films may contain mixtures of the two phases [25], in some cases containing small amounts of either phase in a matrix dominated by the other one, where the minority component is not detectable by XRD, anymore. At I/A=1.6–1.3 the two phases exist in comparable proportions, as they can readily be seen in the XRD patterns. At 3.2b I/Ab 1.6

the RHEED and XRD measurements agree well and reveal only the hexagonal polytype of GaN.

4. Discussion and conclusions The experimental studies have shown that ultra-thin GaN films grown directly on 6H-SiC(0001) substrates can be produced by low-energy ionbeam assisted molecular-beam epitaxy. The following results were achieved under the condition that the substrate temperature was 700 °C, the deposition time was 300 s, and the Ga deposition rate ΦGa (I/A ratio) was varied: (i) Epitaxial w-GaN(0001) films were obtained independent of the Ga deposition rate ΦGa. For the Ga-rich case (I/A ratio = 0.8) the z-GaN polytype was detected only by RHEED. For the N-rich case (I/A=3.2) no polytypes could be registered by XRD. (ii) The GaN film growth is characterized by the three-dimensional growth mode at low Ga deposition rate ΦGa ≤ 1 × 10 14 at. cm − 2 s − 1 (high I/A ratios ≥ 1.8).

Table 3 Deposition parameters and characteristic values of the GaN film deposition results shown in Fig. 5 at constant average GaN coverage θ. Symbols: (✓) — presence of z-GaN, (–) — absence of z-GaN, (S) — streaky RHEED pattern (distinction between w- and z-GaN not possible).

(a) (b) (c) (d)

Effusion cell temperature [°C]

Ga deposition rate [at. cm− 2 s− 1]

I/A ratio

Deposition time [s]

Total Ga coverage [at. cm− 2]

950 1000 1010 1030

0.5 × 1014 1.0 × 1014 1.2 × 1014 1.6 × 1014

3.2 1.6 1.3 1.0

960 480 420 300

5.0 × 1016 4.7 × 1016 5.0 × 1016 4.9 × 1016

Calculated average GaN coverage θ [nm]

[ML]

11.4 10.6 11.4 11.1

44.0 41.0 44.0 42.8

Degree of coverage CXPS [%]

RMS roughness [nm]

z-GaN formation RHEED

XRD

93 95 93 80

1.6 0.4…1.2 0.3…1.4 0.5…1.7

– S S ✓

– ✓ ✓ –

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Fig. 5. RHEED patterns and STM images of GaN deposited on 6H-SiC(0001) substrates at different Ga deposition rate ΦGa adapted for a constant GaN coverage θ by using different deposition times (see Table 3). Image size: 2 × 2 μm2, smaller zoom areas image size: 200 × 200 nm2. The height scale z between lowest (dark) and highest (bright) level is given in [nm]. The corresponding I/A ratios are 3.2 (a), 1.6 (b), 1.3 (c) and 1.0 (d).

(iii) In the three-dimensional growth regime, a decrease of the Ga deposition rate ΦGa (increase of the I/A ratio from 1.8 to 3.2) leads to stronger lateral growth at negligible height growth. (iv) For Ga deposition rate ΦGa ≥ 1 × 1014 at. cm− 2 s − 1 (I/A ratios ≤ 1.8) a transition of the three-dimensional into a twodimensional growth mode can be observed. The transition from the three-dimensional to the two-dimensional growth mode at a deposition temperature of 700 °C and a constant total Ga coverage of 4.9 × 10 16 Ga at. cm − 2, which corresponds to a GaN film thickness of about 11 nm, was investigated for different Ga deposition rates ΦGa (I/A ratios). The most important results here are: (v) Only the w-GaN polytype was formed for Ga deposition rate ΦGa b 1×1014 at.cm− 2 s− 1 (I/A ratios>1.6). (vi) For Ga deposition rate ΦGa ≥ 1 × 1014 at. cm− 2 s − 1 (I/A ratios ≤ 1.6) the coexistence of two polytypes, w-GaN and z-GaN could be proved. (vii) There is a coincidence between the formation of a second polytype, z-GaN, besides the polytype w-GaN and the transition from the three-dimensional to the two-dimensional growth mode. (viii) For Ga deposition rate ΦGa ≥ 1.2 × 10 14 at. cm − 2 s − 1 (I/A ratios ≤ 1.3) a decrease of the cubic component was obtained,

thereby it could not be detected by XRD and was indicated only by RHEED. Qualitatively, the observed variation in surface topography is similar to that achieved in initial GaN growth stages studies using metalorganic chemical vapour deposition (MOCVD) [26–29] and conventional MBE [1–3,7]. The aimed final state for all the methods is two-dimensional step-flow growth. But before step-flow growth is observed, GaN islands form on the substrate surface and grow individually until coalescence takes place. Typical published coalescence thicknesses for MOCVD GaN films on 6H-SiC(0001) vary, depending on the nucleation step, from about 50 nm to several hundreds of nanometres (see e.g. [26,27]). For conventional MBE of GaN on 6H-SiC(0001) coalescence thicknesses of as low as 8 nm for vicinal and 25 nm for flat substrate surfaces are reported [9]. For the alternative method of pulsed electron beam deposition a coalescence thickness of 50 nm was obtained [30]. In the present study, for higher Ga deposition rate (lower I/A ratios), where the final growth was two-dimensional, the average coalescence thickness, as derived from the change from spotty to streaky RHEED patterns, was only about 10 ML, i.e. 2.5 nm. In the following, as the processes in MOVPE and MBE are too different due to the large difference in substrate temperature, the results presented here are only compared to results of other research groups that used

Table 4 Deposition parameters and characteristic values of the GaN film deposition results shown in Fig. 6. Symbols: (✓) — presence of z-GaN, (–) — absence of z-GaN, (S) — streaky RHEED pattern (distinction between w- and z-GaN not possible).

(a) (b) (c) (d)

Effusion cell temperature [°C]

Ga deposition rate [at. cm− 2 s− 1]

I/A ratio

Deposition time [s]

Total Ga coverage [at. cm− 2]

1000 1010 1015 1020

1.0 × 1014 1.2 × 1014 1.3 × 1014 1.4 × 1014

1.6 1.3 1.2 1.1

480 420 300 300

4.7 × 1016 5.0 × 1016 3.9 × 1016 4.3 × 1016

Calculated average GaN coverage θ [nm]

[ML]

10.6 11.4 8.9 9.9

41.0 44.0 34.5 38.0

Degree of coverage CXPS [%]

RMS roughness [nm]

95 93 89 93

0.4…1.2 0.3…1.4 0.3…2.0 0.3…1.7

z-GaN formation RHEED

XRD

S S S ✓

✓ ✓ – –

L. Neumann et al. / Thin Solid Films 520 (2012) 3936–3945

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Fig. 6. STM images of GaN deposited on 6H-SiC(0001) substrates at different Ga deposition rate ΦGa (see Table 4). Image size: 150 × 150 nm2. The height scale z between lowest (dark) and highest (bright) level is given in [nm]. The corresponding I/A ratios are 1.6 (a), 1.3 (b), 1.2 (c) and 1.1 (d).

conventional MBE for depositing GaN films. In principle, between the N/Ga flux ratio of the conventional MBE of GaN and the I/A ratio of the IBA-MBE exist similarities, but also considerable differences. At first sight, the I/A ratio like the N/Ga flux ratio describes the incoming active N and Ga fluxes towards the sample, i.e. the supply with active N and Ga atoms to the surface of the substrate or of the growing film. Consequently, the results of a variation of both ratios may lead to similar results. But, if the much higher kinetic energy and momentum of the N+ and N2+ ions in IBA-MBE are taken into account, there should exist significant deviations of IBA-MBE growth from conventional MBE

growth. Unfortunately, the quantitative comparison of I/A ratios and N/Ga flux ratios is not possible in most of the cases due to several reasons that are related to the measurement of the reactive N atom flux in MBE and of the N ion flux in IBA-MBE. Thus, what remains possible is only a qualitative comparison (as a matter of fact, additional deviations in growth temperatures between various studies make it even more difficult to compare). A transition from three-dimensional to two-dimensional growth through the islands coalescence is an often observed phenomenon in conventional MBE growth of GaN (see e.g. [1–3,7]). It is only found

Fig. 7. XRD rocking curve FWHM of the GaN(0002) reflection and degree of coverage CXPS for different Ga deposition rates (I/A ratios) (a); selected film-related Ga 3s line and
  substrate-related Si 2s line of the core level photoelectron spectra (b); azimuthal XRD scans (φ scans) of the w-GaN 1011 and the z-GaN(200) reflections (c). The STM image insets are the same as shown in Fig. 5 in the bottom row.

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when the growth conditions are Ga-rich, i.e. when the Ga flux is slightly larger than the active N flux. In a first attempt to explain the transition, density-functional theory calculations of adatoms on GaN basal plane surfaces revealed that the lateral mobility of Ga adatoms is orders of magnitude higher than that of N adatoms, irrespective of the termination of the surface, but that the Ga adatom mobility is heavily decreased in the presence of excess N atoms [31]. By this it was explained, why N-rich growth conditions resulted in three-dimensional growth, but the observed high mobility at Ga-rich conditions could not be explained. Further studies, in experiment as well as in theory, investigated the Ga-rich growth conditions in detail, i.e. conditions that lead to larger amounts of surplus Ga on the film surface. Eventually, the formation of a laterally contracted bilayer of metallic Ga – sometimes referred to as “fluid-like” or “liquid-like” due to the low melting point of Ga – on the GaN surface was described in theory and proven by experiments, where the Ga atoms in the upmost Ga monolayer are much more mobile in comparison to isolated Ga atoms on a pure, N-saturated GaN surface [6,32–35]. In the same context, the role of Ga as an efficient surfactant in GaN growth on AlN was observed [33,34]. The effect seems also to have lead to the twodimensional growth shown in the present study with I/A ratios around or below unity, i.e. for balanced and Ga-rich growth conditions. Obviously, the boundary between N-rich and Ga-rich growth corresponds to the intermediate “crossover” regime when the I/A ratio ≈ 1. The boundary between the two Ga-stable regimes (the Ga-droplet regime and the intermediate regime) is determined by the formation of large residual Ga droplets with a diameter of several hundred nanometres at a high density [2]. In the present study, the formation of such densely arranged large Ga droplets on the GaN surface was not observed even at the lowest used I/A ratio (Figs. 4d and 5d). But related to this is the presence of faster-growing flat-top islands observed in Figs. 4c, d and 5b, c and d. Excess Ga, agglomerated early during the growth of a GaN film, can act as a sink for Ga adatoms in its vicinity within the surface diffusion length of the Ga adatoms. Therefore the surroundings of the flat-top islands are depleted of material. This is the cause for why the observed flat-top islands are higher than the surrounding GaN film, because due to volume conservation they must grow higher, when their diameter is smaller. The penetration depth of hyperthermal N ions in matter is larger than that of N atoms in conventional MBE, which possess rather thermal energies. Consequently, an influence of the kinetic energy on the nitrogen insertion probability into a Ga bilayer and thus on the GaN formation cannot be ruled out. The observed small coalescence thickness of only 2.5 nm could be a consequence of that, but this is still speculation. Nonetheless, results from density-functional theory in combination with STM of Ga or in metal film coated GaN surfaces showed, that nitrogen may be dissolved in between the metallic layers and via this additional diffusion path may have a much higher mobility in comparison to N atoms on a bare GaN surface [35]. On the other hand, the lower the I/A ratio gets (two-dimensional growth regime), the lower will be the expectable influence of the impinging ions on growth processes and the more the IBA-MBE results will resemble the conventional MBE results. For the higher I/A ratios (lower Ga deposition rate ΦGa) only threedimensional, but purely wurtzitic GaN growth was observed. The island-like morphology of GaN films grown under N-rich conditions is well known with conventional MBE (see e.g. [1,3]) and sometimes this fact is intentionally used to grow GaN nanowires in MBE setups. In the present study (Fig. 4a and b), the islands diameter to height ratio increased with the Ga flux ΦGa decreasing (I/A ratio increasing). It will be subject of further investigations to increase the I/A ratios and prove, if the indicated enhancing effect of the ion beam irradiation on the island diameter vs. height ratio is valid. But even if there was such an effect, then the lateral domain sizes in the two-dimensional growth regime might still be several times larger than those in the ion irradiation enhanced three-dimensional growth regime.

During the initial nucleation of GaN the typical RHEED pattern (see e.g. Fig. 3f) exhibited wurtzitic and zincblende polytype diffraction spots together with faint diffuse streaks. The mixed nature (polytypism) of initial grown GaN on 6H-SiC(0001) has been previously observed by metalorganic MBE [36] and was explained as a substrate effect [37]. Accordingly, terraced 6H-SiC locally exhibits both, cubic and hexagonal crystal structures on neighbouring terraces, influencing the GaN growth and as a consequence resulting in the formation of mixed polytype growth. This can be ruled out to be a dominant origin, together with surface contaminations or locally differing surface quality of the substrates as an origin of the effect in the present study. It is in contradiction to the findings here that show that purely wurtzitic GaN films can be grown on exactly the same substrate type, where with other I/A ratio a mixture of wurtzitic and zincblende GaN can be provoked. That the effect is indeed growth parameter (N/Ga ratio) related was also shown by Pavlovska et al. using transmission electron microscopy [6]. That the polytypism and the growth mode seem to be correlated is another manifestation of the important role of surplus Ga on the GaN surface during growth. This fact is still not fully understood. The appearance of faster-growing separate islands for predominantly two-dimensionally grown films at the higher Ga deposition rate in the present study might be an additional explanation for the occurring z-GaN related spots in the RHEED patterns. According to Pavlovska et al. [6], surplus Ga on GaN surfaces that amounted to Ga droplets acts as a Ga source for further GaN growth, even after the Ga supply from the effusion cell was stopped. GaN islands formed by such a process can exhibit a mixture of w-GaN and z-GaN polytypes due to the local gradients of the N/Ga ratio. Finally, a recent ab-initio based theory study also related the occurrence of mixed polytypes in GaN films with the growth conditions, in particular with the coverage of the GaN surface with excess Ga. Accordingly, growth of the metastable z-GaN polytype can be stabilized by establishing a one ML thick Ga surface layer during growth [38]. In summary, ultra-thin GaN films were obtained by ion-beam assisted molecular beam epitaxy. The initial stages of growth on bare 6HSiC(0001) were studied by a combination of RHEED and STM analysis. Depending on the Ga deposition rate (I/A ratio) at constant nitrogen ion flux and at constant substrate temperature, the growth mode changed from three-dimensional to two-dimensional growth. In the three dimensional growth mode an enhancement of the lateral growth with increasing I/A ratio was observed. Only in the two-dimensional growth mode the zincblende polytype of GaN occurred. Further experiments will be necessary to explore the GaN growth mode and polytype formation at even higher I/A ratios, as well as at lower substrate temperatures. Acknowledgements The authors would like to thank Dr. A. Anders (Lawrence Berkeley National Laboratory, Berkeley, U.S.) for kindly providing the hollow-anode source. This study was funded by the Deutsche Forschungsgemeinschaft (DFG) as part of the German Excellency Initiative in the Graduate School BuildMoNa (Universität Leipzig, Germany). References [1] E.J. Tarsa, B. Heying, X.H. Wu, P. Fini, S.P. DenBaars, J.S. Speck, J. Appl. Phys. 81 (1997) 5472. [2] B. Heying, R. Averbeck, L.F. Chen, E. Haus, H. Riechert, J.S. Speck, J. Appl. Phys. 88 (2000) 1855. [3] B. Heying, I. Smorchkova, C. Poblenz, C. Elsass, P. Fini, S. DenBaars, U. Mishra, J.S. Speck, Appl. Phys. Lett. 77 (2000) 2885. [4] V. Ramachandran, A.R. Smith, R.M. Feenstra, D.W. Greve, J. Vac. Sci. Technol., A 17 (1999) 1289. [5] P. Waltereit, S.-H. Lim, M. McLaurin, J.S. Speck, Phys. Status Solidi A 194 (2002) 524. [6] A. Pavlovska, E. Bauer, D.J. Smith, Surf. Sci. 496 (2002) 160. [7] Y. Nakada, H. Okumura, J. Cryst. Growth 189–190 (1998) 370. [8] J. Lu, L. Haworth, D.I. Westwood, J.E. MacDonald, Appl. Phys. Lett. 78 (2001) 1080. [9] S.H. Cheung, L.X. Zheng, M.A. Xie, S.Y. Tong, N. Ohtani, Phys. Rev. B 64 (2001) 033304.

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