The paper presents a polycrystalline GaN thin film with a hexagonal wurtzite structure under the optimized sputtering conditions of 40 W RF power, 5 mT working pressure, using pure nitrogen gas with a substrate temperature of 700 °C. The study examines the effects of surface disorders and incorporates it in the thin films characteristics. A radio frequency (RF) Ultra High Vacuum (UHV) Magnetron Sputtering System has been used for the deposition of Gallium Nitride (GaN) on silicon, sapphire and glass substrates with different parameters. The power is varied from 40 W to 50 W, and the pressure from 4 mT to 15 mT. The effects of the RF sputtering powers and gas pressures on the structural properties are investigated experimentally. Sputtering at a lower RF power of 15 W does increase the N atomic percentage, however the deposition rate is substantially slower and the films are amorphous. GaN deposited on both silicon and sapphire wafer resulted in thin films close to stoichiometric once the N2 concentration is 60% or higher. It is also observed that the substrate cooling/heating effects improve the quality of the thin films with fewer defects present at the surface of the GaN epi-structure.
The need for concise and accurate growth of GaN thin films on a specific substrate with a specific doping is greatly emerging for the biosensor applications [
In addition, GaN HEMTs also have a high resistance to chemical corrosion by acids, have non-toxicity to living cells, allow extreme sensing environments, and are chemically inert [
Although GaN HEMTs have shown to be suitable transistors, they have not been popularized due to fabrication issues. Growing high quality GaN has been a main challenge as this material does not exist purely in nature [
The sputter grown GaN thin films are preferred because of the ease of operation, thickness control, low temperature deposition, sequential deposition of different films, and less toxicity [
Utilizing an Ultra High Vacuum (UHV) Magnetron Sputtering System, GaN films have been deposited by radio-frequency (RF, 13.5 MHz) magnetron sputtering with a GaN target. Four sets of GaN thin films are deposited on silicon, sapphire and glass substrates. The first set of sample is deposited in Ar gas; the second set of sample is deposited in mixed Ar and N2 gas (where the ratio of Ar is higher than N2); the third set of sample is deposited in mixed N2 and Ar gas (where the ratio of N2 is higher than Ar); the fourth set of sample is deposited in N2 gas. Finally, substrate heating is used to reduce incorporation of oxygen impurities during the deposition process. The base pressure is below 3 × 10−6 Torr, and the working pressure for all synthesis is above 3 × 10−3 Torr. Prior to sputtering, a pre-sputtering process is performed for more than 30 min
to eliminate any contaminants from the target. Sputtering is then conducted with varying RF power of GaN target from 15 W to 50 W. The substrate temperature is varied from 25˚C to 700˚C. Substrates are rotated at 30 - 60 RPM during deposition to enhance film uniformity. Imaging tools such as the Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDAX), X-ray Diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS) are used to characterize each sample.
GaN thin films samples are made using the magnetron sputtering system under various conditions. Sputtering with pure argon gas under 40 W - 50 W RF power and low pressure (4 mT - 5 mT) has resulted in dark shade films that are highly nitrogen (N) deficient with an average ratio of 0.16 N/Ga for all four samples.
The effects of various concentrations of sputtering gas on the deposition resulted in
Pure Argon Gas | |||||||
---|---|---|---|---|---|---|---|
Power (W) | Pressure (mT) | Thickness (nm) | Ga % | N % | O % | N/Ga Ratio | Time (hr.) |
40 | 4 | 175 | 30.11 | 4.87 | 26.22 | 0.16 | 1 |
40 | 5 | 165 | 28.25 | 5.48 | 22.04 | 0.19 | 1 |
50 | 4 | 198 | 38.63 | 6.19 | 28.45 | 0.16 | 1 |
50 | 5 | 185 | 30.97 | 4.05 | 33.09 | 0.13 | 1 |
an improvement on the N/Ga ratio as well as reduced target poisoning as the sputtering gas has a higher concentration (60% or higher) of N2.
Sputtering with 40 W RF power and 10 mT pressure, XRD spectra revealed weak crystallization at 36.7 beginning to form, however the film eventually becomes amorphous if the nitrogen concentration is lower than 60%,
Comparing the SEM surface images taken of the GaN films sputtered using 40% N2 for 1 hr vs 2 hrs, showed small grains (~15 nm) are visible initially, and over time (2 hrs) the grains become less defined,
The characteristics of the films deposited on the sapphire wafer are shown on
Nitrogen Gas (%) | Power (W) | Pressure (mT) | Thickness (nm) | Atomic percentage | N/Ga Ratio | Time (hrs.) | ||
---|---|---|---|---|---|---|---|---|
Ga (%) | N (%) | O (%) | ||||||
0 | 40 | 5 | 175.63 | 28.3 | 5.48 | 22.04 | 0.19 | 1 |
25 | 40 | 5 | 74.3 | 21.03 | 4.49 | 30.84 | 0.21 | 1 |
33 | 40 | 5 | 146.43 | 13.7 | 3.63 | 25.58 | 0.27 | 2 |
33 | 40 | 10 | 146 | 35.02 | 7.2 | 45.47 | 0.21 | 2 |
33 | 40 | 20 | 29.63 | 7.39 | 2.13 | 14.05 | 0.28 | 2 |
33 | 40 | 30 | 16.9 | 3.45 | 1.63 | 7.88 | 0.47 | 2 |
33 | 50 | 20 | 37.58 | 10.35 | 2.32 | 18.95 | 0.22 | 2 |
33 | 50 | 30 | 18.1 | 5.25 | 1.86 | 10.79 | 0.35 | 2 |
40 | 40 | 10 | 35.45 | 8.69 | 2.85 | 16.41 | 0.33 | 1 |
40 | 40 | 10 | 64.72 | 8.62 | 3.22 | 17.31 | 0.37 | 2 |
60 | 40 | 15 | 15.42 | 6.98 | 3.51 | 14.89 | 0.5 | 1 |
60 | 40 | 15 | 34.05 | 8.48 | 3.44 | 16.07 | 0.41 | 2 |
60 | 15 | 15 | 7.32 | 2.69 | 2.15 | 6.21 | 0.80 | 3 |
100 | 15 | 15 | 3.1 | 1.15 | 2.88 | 5.84 | 2.5 | 3 |
100 | 40 | 15 | 22.22 | 4.07 | 2.99 | 8.89 | 0.73 | 2 |
Nitrogen Gas (%) | Power (W) | Pressure (mT) | Thickness (nm) | Atomic percentage | N/Ga Ratio | Time (hrs.) | ||
---|---|---|---|---|---|---|---|---|
Ga (%) | N (%) | O (%) | ||||||
60 | 40 | 15 | 32 | 5.5 | 6.26 | 57.99 | 1.1 | 2 |
60 | 15 | 15 | 6 | 1.92 | 3.37 | 56.14 | 1.75 | 3 |
100 | 40 | 15 | 22 | 5.5 | 6.26 | 57.99 | 1.13 | 2 |
100 | 15 | 15 | 3 | 3.13 | 3.3 | 55.83 | 1.05 | 3 |
The 15 W RF power proved to be inefficient due to a reduction in sputtering rate, making it unpractical. In addition, although the N/Ga increased, the films are amorphous. The amorphous state of the film could be due to insufficient time given during sputtering for the film to grow and get passed the nucleation phase.
The biggest factor affecting the depositions is the atomic percentage of oxygen incorporated in the film during deposition. Even though close to stoichiometric films are produced when sputtering with 60% N2 and an RF power of 40 W, there is still a high concentration of oxygen.
The incorporation of oxygen is concluded to be from impurities already incorporated into the gas chamber. To reduce the oxygen levels in the films, the substrate temperature is utilized. The effect of the high temperature deposition results in polycrystalline films, with a significant reduction in oxygen.
Increasing the substrate temperature to 700˚C significantly reduces the oxygen and the film is mostly compost of gallium and nitrogen. XRD pattered showed polycrystalline thin film with wurtzite GaN of mixed orientations (100, 002, 101, 110, and 103) with a preferred orientation of GaN (002) on both silicon and sapphire,
One of the major challenges is the target poisoning during the sputtering. When sputtering with pure argon gas and low pressures, target poisoning is greatly observed, Once the pressure is increased to 20 mT, the effect begins to be reduced. The introducing of N2, positively affects the conditions of the target.
However, sputtering at 50 W RF power still creates the effect at low pressures. When sputtering with 40 W RF power and high pressure of 10 mT, the effect is suppressed but not completely reduced. Once the process gas is 60% or higher N2, sputtering can be done at low or high pressures. The optimized conditions are an RF power of 40 W or
Nitrogen Gas 40 W RF power and 5 mT pressure | |||||||||
---|---|---|---|---|---|---|---|---|---|
Wafer | Temperature | Thickness | Atomic percentage | N/Ga | Time | ||||
(˚C) | (nm) | Ga (%) | N (%) | O (%) | Ratio | (hours) | |||
Silicon | 400 | 40 | 9.7 | 9.28 | 8.77 | 0.96 | 1 | ||
Sapphire | 400 | 40 | 8.04 | 7.23 | 47.32 | 0.90 | 1 | ||
Silicon | 700 | 300 | 48.09 | 36.85 | 6.28 | 0.77 | 10 | ||
Sapphire | 700 | 300 | 51.35 | 37.6 | 6.48 | 0.73 | 10 | ||
Power (W) | Pressure (mT) | Thickness (nm) | Ga % | N % | O % | N/Ga Ratio | Time (hours) |
---|---|---|---|---|---|---|---|
50 | 4 | 99.53 | 33.7 | 11.08 | 34.07 | 0.33 | 1 |
40 | 4 | 78.1 | 6.42 | 1.15 | 0.18 | 1 | |
40 | 5 | 72.62 | 14.6 | 4.1 | 0.28 | 1 | |
40 | 10 | 41.2 | 11.4 | 4.62 | 20.21 | 0.40 | 1 |
40 | 10 | 43.4 | 11.9 | 5.03 | 20.41 | 0.42 | 1 |
40 | 5 | 41.19 | 25.4 | 14.41 | 26.58 | 0.56 | 2 |
below with a pressure between 10 mT and 20 mT. Sputtering at a low pressure (5 mT) is possible once these conditions are met, however some target poisoning is still observed. However, this effect is minimal and sputtering can be carried out without affecting the deposition. A similar phenomenon is observed in [
In this study, a middle-frequency magnetron-sputtering method is adopted to deposit GaN on Si (111), Sapphire and glass substrates. Experimental results revealed that polycrystalline GaN thin film with a hexagonal GaN wurtzite structure is grown on silicon and sapphire wafers. Glass substrate is not suitable for GaN. In addition, oxygen
impurities incorporated during the deposition are shown to be reduced by using temperature depositions. The samples are made with various parameters. The deposition rate of the films is observed. The study showed that the high quality crystal of the GaN thin films depend strongly on the low power (~40 W), high gas pressure (~15 mT) and annealing effects. This is comparable as in [
This project is supported in part by the National Science Foundation (NSF) under Grant No. 1229523, and by the US Army Research Office W911NF-14-1-0100.
Huq, H.F., Garza, R.Y. and Garcia-Perez, R. (2016) Characteristics of GaN Thin Films Using Magnetron Sputtering System. Journal of Modern Phy- sics, 7, 2028-2037. http://dx.doi.org/10.4236/jmp.2016.715178