Chemical Vapor Deposition of Diamond Films on Patterned GaN Substrates via a Thin Silicon Nitride Protective Layer Y. S. Zou,† Y. Yang,† Y. M. Chong,† Q. Ye,† B. He,† Z. Q. Yao,† W. J. Zhang,*,† S. T. Lee,† Y. Cai,‡ and H. S. Chu‡
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1770–1773
Center of Super-Diamond and AdVanced Films (COSDAF), and Department of Physics and Materials Sciences, City UniVersity of Hong Kong, Hong Kong SAR, China, and Hong Kong Applied Science and Technology Research Institute Company Limited, Hong Kong SAR, China ReceiVed March 19, 2007; ReVised Manuscript ReceiVed January 9, 2008
ABSTRACT: Integrating diamond films with GaN-based devices may enhance heat dissipation and thus improve device performance for high power loading. Direct deposition of diamond films on GaN layers has been hampered by GaN degradation in the chemical vapor deposition environment for diamond growth. In this work, three approaches were introduced to grow high-quality diamond films on patterned GaN substrates via a thin silicon nitride protective layer, that is, (i) a two-step process involving an initial rapid growth step, (ii) addition of nitrogen to hydrogen-based plasma to suppress reactions between GaN and hydrogen, and (iii) deposition in argon-based plasma. Continuous, adherent, and high-quality micro- and nanocrystalline diamond films were successfully deposited. All three approaches were effective in reducing plasma-induced GaN decomposition and etching, and in eliminating film cracks and delamination.
1. Introduction GaN is widely used to fabricate optoelectronic devices such as light-emitting diodes (LEDs), photodetectors, and laser diodes working in the blue and violet light regions.1–3 GaN-based devices are usually constructed on sapphire substrates. However, excessive heating due to high thermal resistance of sapphire and high operation current densities may reduce device performance and lifetime. Therefore, improvement in thermal management of GaN-based devices is desired. One approach to reduce device temperature during operation is to spread the heat over a large area with the assistance of high heat dissipation materials. Diamond, with its outstanding physical and chemical properties, is a promising material for mechanical and electronic applications.4–11 Because of its highly insulating nature and highest thermal conductivity (20 W cm-1 K-1 at room temperature) diamond is arguably the best candidate for heat dissipation applications, in particular in high-power electronic devices. Thick hexagonal GaN layers have been grown on (110) single-crystal diamond wafers via an AlN transition layer by metal organic chemical vapor deposition (MOCVD).12 However, the delicate GaN-based devices cannot endure the harsh environment during chemical vapor deposition (CVD) of diamond, where the substrates are exposed to hydrogen plasmas and high temperature. Although the melting point of GaN is around 2500 °C, GaN reacts with hydrogen around 800 °C.13 In addition, GaN is decomposed and etched under the conditions for CVD diamond growth. The poor chemical stability of GaN under typical diamond CVD conditions has hampered the application of diamond films for GaN-based devices. There are relatively few reports on the deposition of continuous and high-quality diamond films on GaN. By combining carburization with biasenhanced nucleation and growth, oriented diamond was deposited on hexagonal GaN layers via conventional microwave * Corresponding author. E-mail:
[email protected]. † City University of Hong Kong. ‡ Hong Kong Applied Science and Technology Research Institute Company Limited.
plasma CVD.14,15 However, the nucleation density of diamond was very low, and only isolated diamond crystals were obtained on GaN surface. Continuous diamond films have been grown on epitaxial GaN films via hot filament CVD where the reactivity of hydrogen is much lower than in microwave plasma.16 However, the diamond films deposited were porous with low crystal quality and poor adhesion. Recently, deposition of continuous boron-doped diamond films on p-GaN substrate by hot-filament CVD was reported.17 In this work, we report three approaches to grow high-quality diamond films on patterned GaN layers in a microwave plasma CVD system. All three approaches are effective in restraining the decomposition and etching of GaN substrates in the plasma. Continuous, adherent, and high-quality micro- and nanocrystalline diamond films were grown on GaN via a thin silicon nitride transition layer.
2. Experimental Procedures GaN epitaxial layers used in the present study were grown on c-face (0001) sapphire substrates by MOCVD. Trimethylgallium and ammonia were used as the Ga and N sources, respectively. The thickness of GaN epitaxial layer was about 5 µm. To improve heat dissipation, patterns with different size and geometry including circles, grooves, and crosses were formed on GaN surfaces by lithography-assisted chemical etching. To protect GaN surfaces against abrasion and plasma CVD environment, a thin SiNx layer of about 120 nm in thickness was precoated on GaN by using rf-assisted CVD. Diamond films were then deposited on SiNx-coated GaN layers using a commercial 1.5 kW ASTeX microwave plasma CVD system.18 The substrates were ultrasonically cleaned with acetone and ethanol for 15 min successively, then rinsed with deionized water. To achieve a high nucleation density of diamond, the substrates were ultrasonically abraded for 60 min in a suspension of nanodiamond powder (grain size ∼ 5 nm) in ethanol. After abrasion, the diamond seeds were actually embedded in the SiNx layer. Observations via cross-sectional scanning electron microscopy (SEM, Philips 30 XL FEG) revealed that the SiNx layer remained intact after abrasion and diamond growth (as shown below), indicating that the underlayer GaN should remain nondamaged during the diamond abrasion process. In the reactor, the substrates were mounted on a Mo substrate holder located on an inductively heated plate. The substrate temperature was directly measured by an optical pyrometer through a sapphire window. Growth of microcrystalline and nanocrystalline diamond films was conducted in two gas systems, that
10.1021/cg070267a CCC: $40.75 2008 American Chemical Society Published on Web 04/18/2008
CVD of Diamond Films on Patterned GaN Substrates
Figure 1. The SEM images of the nanodiamond films deposited on patterned GaN substrates. (a, b) Surface image, without initial rapid growth stage. (c, d, e) Surface image, two-step process with an initial fast growth stage. (f) Cross-section image, two-step process with an initial fast growth stage.
is, hydrogen-based (1–20%CH4/H2 or 10%CH4/45%H2/45%N2) and argon-based (1%CH4/3%H2/96%Ar). For the hydrogen-based plasma, the microwave power was maintained at 1000 W and the total pressure at 20 torr with a total gas flow rate of 200 sccm. For diamond deposition in argon-based CVD environment, the microwave power was 700 W, the total pressure was 100 Torr, and the total gas flow rate was 200 sccm. The substrate temperature was varied from 600 to 700 °C. The growth duration for all diamond samples was 6 h. The morphology, structure, and phase composition of the deposited diamond films were characterized by SEM, visible (Renishaw 2000) and UV Raman spectroscopy (Renishaw inVia Raman microscope).
3. Results and Discussion Deposition of diamond films was first attempted on GaN substrates directly, using different deposition parameters typically for the growth of nano- and microcrystalline diamond. However, the diamond film deposited showed very poor adhesion to the substrate; the film cracked and even delaminated from the GaN layer upon removal from the CVD chamber. Therefore, a protective SiNx film with a thickness of about 120 nm was predeposited on the GaN layer before diamond deposition. Under typical deposition conditions for growth of nano- and microcrystalline films, decomposition and etching of GaN still persisted during diamond growth, and the diamond films deposited were not continuous, as shown in Figure 1a. For nanodiamond films deposited on SiNx protective layer under the following conditions: microwave power was 1000 W; CH4 content of 10% in CH4/H2; total pressure of 20 torr and substrate temperature of 700 °C, many cracks were observed, and the crack width was as large as several tens of microns, as shown in Figure 1b. This result implies that the SiNx layer cannot provide effective protection for the underlying GaN layer against decomposition and etching at high temperature and in the hydrogen-based plasma. As reported in the previous work, hydrogen atoms may diffuse through the SiNx layer and react with GaN.19 In this case, the following approaches are designed
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Figure 2. The visible (a) and UV (b) Raman spectra of the diamond films deposited on patterned GaN substrates. Spectrum I and II for the nanodiamond and microcrystalline diamond films deposited using twostep process with an initial fast growth stage, respectively. Spectrum III and IV for the nanodiamond films deposited with addition of nitrogen in hydrogen-based plasma and in Ar-based plasma, respectively.
to deposit continuous and adherent diamond films on top of the SiNx protective layer. 3.1. Two-Step Process with an Initial Rapid Growth Stage. A two-step deposition process involving an initial rapid growth and subsequent high-quality growth was designed. In the first stage, diamond films were deposited with a high methane concentration (20%CH4/H2) and at a low temperature of 600 °C for 1 h. A high growth rate of diamond was achieved, and the SiNx/GaN layer was quickly covered by a continuous nanodiamond layer. Then the methane concentration was reduced to 10%, and the substrate temperature increased to 700 °C to improve the phase purity of nanodiamond films. The growth duration for the second stage was 5 h. Figure 1c shows the surface morphology of the nanodiamond films deposited on a patterned SiNx/GaN layer. It can be seen that the diamond film was deposited uniformly over a large area, with no observable cracks and delamination. Moreover, continuous nanodiamond films were deposited on both the flat surface and the normal wall of the patterns, as shown in Figure 1d,e. The cross-section SEM image in Figure 1f shows that the thickness of the nanodiamond film is about 1.55 µm, revealing a growth rate of 0.26 µm/h. The nanodiamond film deposited is very dense, and the underlying SiNx protective layer can be clearly distinguished. No gaps were observed between the nanodiamond film and substrate, indicating very good adhesion of the diamond film. The phase composition of the nanodiamond film was studied by utilizing visible and UV Raman spectroscopy (spot size: 1 µm, wavelength: 514.5 and 226 nm, respectively). In the visible Raman spectrum (spectrum I in Figure 2a), besides the peaks at about 1350 and 1560 cm-1 for the amorphous (D peak) and graphitic carbon (G peak), respectively, the two characteristic peaks at 1140 and 1470 cm-1 indicate the formation of nanodiamond. The peaks at 1332 and 1580 cm-1 in the UV Raman spectrum (spectrum I in Figure 2b) are characteristic of diamond and graphitic carbon, respectively, further verifying the deposition of nanodiamond film. Moreover, using the two-step process, continuous microcrystalline diamond films were also deposited on the SiNx/GaN layer. In this case, the methane content was reduced to 1%, and the substrate temperature increased to 700 °C for the second
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Figure 3. The SEM images of microcrystalline diamond films deposited on patterned GaN substrates using two-step deposition process. (a, b, and c) Surface image; (d) cross-section image.
Figure 4. The SEM images of the nanodiamond films deposited on patterned GaN substrates with addition of nitrogen in hydrogen-based plasma. (a, b, and c) Surface image. (d) Cross-section image.
stage. The growth duration was still 5 h. Again, as shown in Figure 3, continuous microcrystalline diamond films were deposited uniformly over the whole substrate. The grain size was about 500–800 nm. Similar to the nanodiamond film in Figure 1c-f, no cracks and delamination were observed, even when deposition was conducted in plasma with a very high hydrogen concentration. As depicted in Figure 3b, the walls of the patterned circles were also covered with uniform microcrystalline diamond. The thickness of the diamond film was about 1.4 µm (Figure 3d), and the growth rate was calculated to be 0.23 µm/h. The visible and UV Raman spectra of the sample (spectra II in Figure 2a,b) show only a sharp peak at 1332 cm-1, which is characteristic of microcrystalline diamond, demonstrating the deposition of high-quality diamond on the GaN layer. Contrary to the previous works on diamond deposition on GaN substrates,14–16 the two-step process reported here enables the deposition of adherent, continuous nano- and microcrystalline diamond films uniformly on GaN substrates. The results indicate that by quickly forming a protective nanodiamond layer on GaN, the substrate surface could remain stable enough in the harsh environment for diamond growth. 3.2. Addition of Nitrogen in Hydrogen-Based Plasma. During exposure to hydrogen plasma environment, GaN would react with hydrogen (atoms or ions) at a temperature of around 800 °C and decompose to volatile species as follows:16,20
nation were observed. The results imply that GaN decomposition was significantly suppressed due to increased nitrogen partial pressure in the plasma. As shown in Figure 4d, the nanodiamond film is dense and adheres well to the substrate. The film thickness is about 1 µm after the deposition for 6 h. The influence of nitrogen addition in the range of 0.5–5% in the gas mixture 1%CH4/H2 was investigated for diamond deposition on GaN layers by hot filament CVD.16 It showed that for nitrogen content of less than 5% only porous diamond films with poor adhesion were obtained. Comparison to our work shows that a high partial pressure of nitrogen in the gas mixture is required to suppress effectively GaN decomposition during diamond growth. The visible and UV Raman spectra (spectra III in Figure 2a,b) also confirm the nanodiamond nature of the film. Similar to spectra I, the additional small peak centered at 1190 cm-1 was attributed to C-N bonds arising from nitrogen impurities.21,22 3.3. Deposition in Ar-Based Plasma. Diamond with nanosized grains can also be grown in a CVD environment with a little hydrogen or no hydrogen.23,24 Ultrananocrystalline diamond films were deposited by partially or totally substituting hydrogen with argon.25 We used the similar approach to deposit diamond on GaN, as we suppose that hydrogen depletion in argon-based plasma (1%CH4 /3%H2 /96%Ar) would improve suppression of GaN decomposition and etching by hydrogen. The pressure was 100 Torr and the substrate temperature was 700 °C for diamond growth. After 6 h growth, a thick (2.63 µm) and continuous nanodiamond film was formed on patterned SiNx/GaN substrate including the surface and pattern walls, as shown in Figure 5. The film shows good adhesion to the substrate and no damage to the SiNx protective layer. Visible and UV Raman spectra (spectra IV in Figure 2a,b) are characteristic of typical nanodiamond films with a relatively low sp2 carbon phase. The results indicate that decomposition and etching of GaN surface was indeed suppressed in the hydrogendepleted plasma. Visible Raman spectroscopy was carried out to monitor the crystal quality of the GaN layer before and after diamond deposition, as shown in Figure 6. For the substrate, a Raman peak located at about 570 cm-1 was observed, which is assigned to E2 (high) mode of GaN.26,27 The full-width at half-maximum (fwhm) of the peak is about 4.2 cm-1. After diamond deposition via different approaches, that is, nanodiamond (spectrum I) and microcrystalline diamond (spectrum II) film deposition using two-step process with an initial rapid growth stage, and the
N(surface) + xH(g) f NHx(g)
(1)
Ga(surface) + H(g) f GaH(g)
(2)
GaN(s) f Ga(l) + N(g)
(3)
The decomposition rate of GaN depends on the partial pressure of hydrogen, and increases with increasing hydrogen partial pressure. Therefore, hydrogen plays an important role in the decomposition of GaN. If hydrogen partial pressure is reduced or nitrogen partial pressure in the CVD plasma environment is increased, then the stability of GaN is expected to be enhanced. In this work, 45% of N2 was added to the plasma, and diamond deposition was performed in a gas mixture of CH4(10%)/H2(45%)/N2(45%), with microwave power at 1000 W, pressure at 20 Torr, and substrate temperature at 700 °C. Figure 4 shows the SEM images of the nanodiamond films deposited for 6 h. It can be seen that, similar to the two-step process, addition of nitrogen to the plasma leads to the deposition of a continuous nanodiamond film over a large area including the walls of patterns. Again, no cracks and delami-
CVD of Diamond Films on Patterned GaN Substrates
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eliminating film cracks and delamination. The successful integration of diamond with GaN would increase heat dissipation and improve the performance and lifetime of GaN-based devices for high power loading. Acknowledgment. This work was financially supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 122805 and CityU 123806). Support from the Croucher Foundation is also gratefully acknowledged.
References
Figure 5. The SEM images of the nanodiamond films deposited on patterned GaN substrates in Ar-based plasma. (a, b, and c) Surface image. (d) Cross-section image.
Figure 6. The visible Raman spectra of the underlying GaN substrates before and after diamond deposition. Spectrum I and II for the nanodiamond and microcrystalline diamond films deposited using twostep process with an initial fast growth stage, respectively. Spectrum III and IV for the nanodiamond films deposited with addition of nitrogen in hydrogen-based plasma and in Ar-based plasma, respectively.
nanodiamond film deposition with addition of nitrogen in hydrogen-based plasma (spectrum III) and in Ar-based plasma (spectrum IV), the peak location and fwhm are almost identical to that before diamond deposition, indicating that the GaN underlayers are maintained intact after diamond deposition with our approaches.
4. Conclusions We introduced three approaches via MWCVD and a SiNx protective layer which enabled the deposition of continuous and adherent diamond films (including micro- and nanocrystalline diamond) uniformly on patterned GaN substrates. They are (i) a two-step process involving an initial rapid growth step, (ii) addition of nitrogen into the hydrogen-based plasma to suppress reactions between GaN and hydrogen, and (iii) deposition in argon-based plasma. All three approaches are effective in restraining GaN decomposition and etching in the plasma, and
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