Polla
Rouf
*a,
Nathan J.
O’Brien
a,
Sydney C.
Buttera
b,
Ivan
Martinovic
a,
Babak
Bakhit
a,
Erik
Martinsson
a,
Justinas
Palisaitis
a,
Chih-Wei
Hsu
a and
Henrik
Pedersen
a
aDepartment of Physics, Chemistry and Biology, Linköping University, SE-58183 Linköping, Sweden. E-mail: polla.rouf@liu.se
bDepartment of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada
First published on 25th May 2020
Low temperature deposition of high-quality epitaxial GaN is crucial for its integration in electronic applications. Chemical vapor deposition at approximately 800 °C using SiC with an AlN buffer layer or nitridized sapphire as substrate is used to facilitate the GaN growth. Here, we present a low temperature atomic layer deposition (ALD) process using tris(dimethylamido)gallium(III) with NH3 plasma. The ALD process shows self-limiting behaviour between 130–250 °C with a growth rate of 1.4 Å per cycle. The GaN films produced were crystalline on Si (100) at all deposition temperatures with a near stochiometric Ga/N ratio with low carbon and oxygen impurities. When GaN was deposited on 4H-SiC, the films grew epitaxially without the need for an AlN buffer layer, which has never been reported before. The bandgap of the GaN films was measured to be ∼3.42 eV and the Fermi level showed that the GaN was unintentionally n-type doped. This study shows the potential of ALD for GaN-based electronic devices.
The disadvantage of TMG and TEG for ALD of GaN is their strong M–C bonds, which makes it difficult to remove all of their ligands from the deposited precursor at low temperatures.18 Amides (M–NR2) have more reactive and desirable M–N bonds, which would improve the surface chemistry during the NH3 precursor pulse. Herein, we report low temperature deposition of GaN using tris(dimethylamido)gallium(III), Ga(N(CH3)2)31, with NH3 plasma by ALD. Precursor 1 has been previously used for ALD of Ga2O319,20 and Ga2S321 and CVD of GaN,22 but not for ALD of GaN. Notably, all previous reports using 1 has resulted in amorphous films at all deposition temperatures.19,21,22 We show that 1 displayed self-limiting behaviour in a temperature range from 130–250 °C on Si (100). Deposition of GaN on 4H-SiC(0001) rendered epitaxial films with near stochiometric composition and very low impurity levels of carbon and oxygen. To the best of our knowledge, this is the first report of GaN grown epitaxially directly on SiC substrate.
1: Colourless solid, m.p. 101–104 °C. 1H NMR (300 MHz, C6D6) δ 2.34 (s, 12H, μ-NMe2), 2.68 (24H, μ-NMe2).
Elemental composition of the films was obtained using Rutherford backscattering spectrometry (RBS) and time-of-flight elastic recoil detection analyses (ToF-ERDA). The measurements were carried out in a 5-MV NEC-5SDH-2 pelletron tandem accelerator. 2 MeV 4He+ ions were employed for RBS and detected in a scattering angle of 170°. Two different geometries, azimuth angle 5° + tilt angle 2° and azimuth angle 40° + tilt angle 2°, were chosen in order to minimize channelling effects. In addition, more suppression of the probable channelling effects was undertaken by multiple small random-angular movements around the equilibrium angles within a range of 2°. RBS spectra were fitted by SIMNRA 7.02 code26 with an ∼1% statistic uncertainty to determine elemental compositions. Recoils, in ToF-ERDA, were detected at 45° angle between the primary beam and a ToF-E detector telescope in a gas ionization chamber (GIC) using a 36 MeV 127I8+ beam incident at 67.5° with respect to the sample surface normal. The ToF-E detector telescope consisted of two circular carbon foils with 8 and 5 μg cm−2 thicknesses, 6 mm radius, a 0.05 msr solid angle (ΔΩ), and a flight distance of 425 mm between the foils. Utilizing a ToF-GIC setup provides the system with a good energy resolution and enhanced ion species separation in terms of mass and energy.27 Average elemental compositions were also obtained from ToF-ERDA time-energy coincidence spectra using two different software packages, CONTES28 and Potku.29 Systematic uncertainties of the experiment, discussed in more detail elsewhere30 in particular for light elements, were estimated to be a maximum of 5–10%, whereas statistic uncertainties arisen from the number of experimental counts were <2.3%. However, the relative elemental concentrations were obtained with higher accuracy.31,32 The stopping power data required for both RBS and ERDA simulations was retrieved from SRIM2013 code.33
Cross-sectional transmission electron microscope (TEM) samples were prepared by the traditional sandwich approach, which includes sample cutting, gluing, polishing and ion milling. A Gatan Precision Ion Polishing System Model 691 operated at 5 kV and 40 mA, with an Ar ion source, was used to make the samples electron transparent. Scanning transmission electron microscopy (STEM) and selective area electron diffraction (SAED) characterization were performed using the Linköping double Cs corrected FEI Titan3 60–300 operated at 300 kV.
To study if 1 could be used in self-limiting deposition of GaN, ALD experiments on Si (100) substrates were undertaken using varied pulse times for 1 and NH3 plasma whilst the temperature was maintained at 200 °C. The GaN growth per cycle (GPC) saturates at 1.4 Å per cycle when the pulse time of 1 is 4 s or longer (Fig. 3a). The GPC saturates at the same value for NH3 plasma when pulse times of 6 s or longer were used. This is indicative of a self-limiting growth and surface chemistry between 1 and NH3 plasma. The growth per cycle was constant between 130–250 °C when using a 4 s pulse time for 1 and 9 s NH3 plasma pulse time (Fig. 3b). As the bubbler temperature was set to 120 °C, lower deposition temperature was not conducted due to the risk of condensation of 1 on the substrate. Reactor temperatures of ≥250 °C resulted in a decreased GPC, indicating desorption of the precursor from the substrate.
This GPC is roughly 3–5 times that of previous reports of GaN plasma processes with TMG/TEG.11,12,14 It can be noted here that the wurtzite GaN lattice has a c lattice constant of 5.182 Å. To obtain such a unit cell, three monolayers are required (Ga source, N source followed by Ga source) which corresponds to 1.5 ALD cycles and therefore the theoretical maximum growth per cycle is approximately 3.45 Å per cycle. The growth of 1.4 Å per cycle obtained with the M–N bonded precursor 1 is lower than the theoretical growth making it reasonable despite its higher growth rate compared to other GaN processes.
The films deposited in the temperature window with constant GPC rendered crystalline GaN with a preferred (002) orientation on Si (100) (Fig. 4a). The intensity of the XRD peaks, which is especially clear from the GIXRD measurements (Fig. 4b), increases with higher deposition temperature suggesting that film crystallinity increases with temperature. The (002) plane is present in both the θ–2θ XRD and the GIXRD indicating that grains tilted with respect to the substrate normal are present in the film. The stress and strain in the 70 nm GaN films were estimated from the GIXRD measurement by a ε–sin2ψ plot (Fig. 4c). The positive slope of the fitted line to the plotted data is indicative of tensile strain of the film. The strain was obtained from the slope (7475.61 ppm) and gives a value of 0.00747561. This corresponds to roughly 0.75% of tensile strain. To calculate the stress in the film, Young's modulus (E) and Poisson's ratio (v) for GaN was used (eqn (1)).
(1) |
The Youngs's modulus and Poisson's ratio of GaN have been determined from previous studies to be 325.3 GPa and 0.25, respectively.34 This gives a tensile stress of about 1.95 GPa for the GaN thin film on a Si substrate. Tensile strain and stress in the GaN film is expected due to the large difference in lattice constant and crystal structure of cubic Si to hexagonal GaN.
Top-view SEM analysis showed that the films deposited at 250 °C is composed of many small crystalline grains with little variation in grain size (Fig. 4d). The little variation in grain size indicates that the film grows in a layer-by-layer fashion without any observable secondary nucleation.
The chemical bonding environment and atomic composition in the GaN films deposited at 250 °C were analysed by XPS (Fig. 5). High resolution XPS spectra were used to analyse the chemical environment of the Ga 2p3/2 and N 1s regions. Two peaks were used to obtain a good fit for the Ga 2p3/2 at 1117.5 eV and 1119.4 eV, which were attributed to Ga–N and Ga–O bonds, respectively (Fig. 5a). Three peaks were used to fit the N 1s at 397.1 eV, 394.8 eV and 392.7 eV, which were assigned to N–Ga and two Ga Auger peaks, respectively (Fig. 5b). This is in accordance with previous XPS measurements on GaN thin films.11,13,35–37 After sputter cleaning the film surface, XPS measurements gave an initial overall composition of 25.0 at% Ga, 74.5 at% N, 0.5 at% O with no detectable C. It should be noted that the N 1s peak overlaps with Ga Auger peaks, which results in a broad N 1s peak and an overestimate of the N content. To obtain the Ga/N ratio in the film without Ga Auger interference, the Auger peaks were subtracted from the contribution to the N content. The N–Ga peak contribution is 33% of the overall N content, which gives 24.6 at% of N that is attributed to the actual N content of the film. This gives a corrected Ga/N ratio of 1.02, which is close to stochiometric GaN. The high resolution XPS peak of Ga 2p3/2 can be fitted with contributions from Ga–N and Ga–O bonds while the high resolution XPS N 1s peak cannot be fitted with a N–O peak, suggesting that all oxygen in the film is bonded to Ga. If oxygen is subtracted from Ga 2p3/2 spectra and only the Ga–N and N–Ga bonds are considered, a near stoichiometric Ga/N ratio of 0.99 is obtained. To investigate if the subtraction of the Ga Auger peaks from the N 1s XPS region is a sound strategy, RBS combined with ERDA was conducted on the GaN film. The RBS/ERDA measurement showed that the GaN film contained 45.7 at% Ga, 47.2 at% N, 2.8 at% C, 3.1 at% O and 1.2 at% H giving a Ga/N ratio of 0.97, which indicates a slightly under-stochiometric film composition. This shows that even when subtracting the Ga Auger peak from the N 1s XPS region, the film composition and Ga/N ratio is not reliable using XPS and other techniques are required to determine an accurate atomic content. In addition, C impurities were not detected by XPS after clean sputtering whilst RBS/ERDA showed C present in the film, showing the latter is preferred for detection of impurities. In addition, sputtering was used in the XPS measurement, which could cause preferential sputtering and change the composition and impurity levels in the film. Nonetheless, XPS is highly important for accessing the chemical environment of each element.
Fig. 5 HR-XPS of Ga 2p (a) and N 1s (b) with 4 s of 1 and 9 s NH3 plasma pulse deposited at 250 °C with a thickness of 70 nm. |
An absorption measurement was conducted on the GaN film and the data was used to construct a Tauc plot to calculate the bandgap. The optical bandgap for a direct bandgap material is expressed by eqn (2). Extrapolating the linear part in the Tauc plot and setting (αhv)2 to 0 gives the band gap.
(2) |
The Tauc plot for GaN films deposited at 250 °C gave a band gap value of approximately 3.42 eV (Fig. 6a). This is close to the previously reported single crystalline GaN value of 3.40 eV.5 The valence band (VB) spectrum obtained from XPS has two distinct features, labelled A and B in Fig. 6b, which correspond to Ga 4p–N 2p and Ga 4p–N 4s hybridized orbitals, respectively. These features are in line with previous studies on GaN thin films.38,39 The VB near the Fermi level (EF) was obtained by XPS (Fig. 6c). By extrapolating the linear part of the leading edge and the baseline, the valence band maxima (VBM) could be obtained. It was found that the VBM lies approximately 2.20 eV below the surface Fermi level and is in line with a previous GaN study deposited with by MBE.38 In combination with the estimated band gap of 3.42 eV, this result indicates that the Fermi level is not positioned in the middle of the bandgap, but closer to the conduction band (CB) instead of the VB (Fig. 6d). This shift of the Fermi level indicates that the deposited GaN is n-type doped. This is expected as GaN normally shows n-type behaviour due to impurities and defects in the film such as oxygen and nitrogen vacancies.40–42
By replacing the Si (100) substrate with 4H-SiC (0001), which is a more suitable substrate for GaN with respect to the lattice mismatch and crystal structure, XRD in θ–2θ geometry showed only the GaN (0002) and (0004) peaks together with the substrate peaks. This indicates that growth occurred only in the c-direction (Fig. 7a). The GaN film was deposited at 250 °C using a 4 s pulse of 1 and 9 s NH3 plasma pulse. GIXRD of the GaN film grown on SiC showed no peaks, indicating no tilted grains present in the film (Fig. 7b). Pole figures were constructed to investigate the epitaxial relationship between the film and the substrate (Fig. 7c and d). The GaN (103) plane was used due to large difference in the psi angle between GaN and SiC, making it possible to distinguish between the GaN and SiC poles. The pole figure showed six poles, representing the hexagonal GaN lattice with its six-fold symmetry (Fig. 7c). The SiC (102) plane show six poles, representing its hexagonal six-fold symmetry (Fig. 7d). These pole figures show that the hexagonal GaN crystals have grown exactly on top of the SiC crystals, confirming that epitaxial GaN has been grown directly on SiC substrate. The epitaxial relationship between the film and the substrate is GaN (0002)‖SiC (0004) and GaN (103)‖SiC (102). It is worth noting that no AlN interlayer was used to aid the growth of GaN on SiC, which is required for CVD of GaN on SiC.7,43 This opens up the possibility for deposition of GaN on transistor structures without the need of a buffer layer between SiC and GaN.
Top-view SEM of the GaN film on SiC deposited at 250 °C was composed of larger grains compared to GaN on Si substrate (Fig. S3, ESI†).
Further structural characterization was undertaken using scanning transmission electron microscopy (STEM) with high angle annular dark field (HAADF) imaging and SAED). An overview cross-sectional STEM-HAADF image, along [110], together with corresponding SAED patterns (shown in the insets) acquired from the film and substrate are shown in Fig. 8a. According to the STEM-HAADF images recorded under strong elemental contrast (Z-contrast) conditions, two distinct regions in the film were observed. The film appears uniform starting from film/substrate interface for the first ∼6 nm. Afterward, the growing film starts to develop columnar microstructure, leading to a pronounced surface roughening. The darker STEM-HAADF contrast dotted pattern in the columns and columnar boundaries can be attributed to the presents of point defects, e.g. vacancies and impurities, in the film.
The epitaxial grown GaN film on 4H-SiC can be identified by SAED pattern shown as insets in Fig. 8a. The GaN is oriented with the c-axis perpendicular to the 4H-SiC substrate surface, GaN [0002]‖SiC [0002] and the in-plane relation GaN [010]‖SiC [010]. According to our HRSTEM-HAADF imaging and the Fast Fourier Transforms (FFTs), the stacking of the GaN lattice planes is more ordered close to the GaN–SiC interface compared to the upper part of the film (Fig. 8b). The arc-shaped diffraction spots associated with GaN (inset in Fig. 8a), indicating part of GaN crystal is deviated from the ideal c-axis epitaxy, are ascribed to the curved stacking of (0002) planes for the upper part of the film (Fig. 8c). The lattice curvature together with various types of structural imperfections becomes more and more pronounced for the top part of the film. The GaN film can accommodate the stress in the beginning of the growth, the first ∼6 nm as evidenced from Fig. 8b. We believe that when the GaN grows over this critical thickness the increased stress (due to compressive lattice mismatch of 3% with the substrate) forces the GaN planes to curve. The curvature of the planes increases along the c-direction, further away from film/substrate interface. X-ray Rocking Curve (XRC) was employed on the epitaxial GaN, showing that the full width at half maximum (FWHM) for the (0002) plane was 523 arcsec (0.1452°), Fig. S2 (ESI†). This is on par with previous high temperature CVD study of GaN films were an AlN seed layer was deposited on the SiC substrate.8 These findings could revolutionize the transistor industry, especially for the production of high electron mobility transistors (HEMTs). Being able to deposit very thin GaN directly on SiC without the need for an AlN seed layer allows for increased design freedom when producing HEMTs, which could in turn improve their performance.
Finally, a thermal ALD route to GaN was investigated using 1 and NH3 without plasma discharge. The same pulse parameters were used as the plasma process, but a 16 s of NH3 pulse was used in a temperature range of 150–400 °C. GaN films could also be obtained via this thermal process, however, the films were X-ray (θ–2θ and GIXRD) amorphous. The XPS analysis showed the films contained high amounts of C and O impurities, 4 at% and 15 at%, respectively (Fig. S4, ESI†). The Ga/N ratio could not be accurately be measured due to the Ga Auger overlap with N 1s. The high level of impurities could explain the amorphous nature of these films, which are likely to distort the long-range order of the atoms in the lattice. This also highlights the efficiency of the NH3 plasma to remove carbon and oxygen impurities from the growing film surface.
These findings could open up the doors for new and smaller HEMTs based on group 13 nitrides. By being able to deposit high quality GaN films at these low temperatures also present the opportunity to deposit on temperature sensitive materials which has previously been hindered due to the high deposition temperatures. The ability to deposit GaN directly on SiC also reduces the number of material interfaces which could potentially improve the transistor performance.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc02085k |
This journal is © The Royal Society of Chemistry 2020 |