Thick adherent diamond films on AlN with low thermal barrier resistance

Growth of $>$100 $\mu$m thick diamond layer adherent on aluminium nitride is presented in this work. While thick films failed to adhere on untreated AlN films, hydrogen/nitrogen plasma treated AlN films retained the thick diamond layers. Clear differences in zeta potential measurement confirms the surface modification due to hydrogen/nitrogen plasma treatment. Areal Raman maps showed an increase in non-diamond carbon in the initial layers of diamond grown on pre-treated AlN. The presence of non-diamond carbon has minimal effect on the interface between diamond and AlN. The surfaces studied with x-ray photoelectron spectroscopy (XPS) revealed a clear distinction between pre-treated and untreated samples. The surface aluminium goes from nitrogen rich environment to an oxygen rich environment after pre-treatment. Cross section transmission electron microscopy shows a clean interface between diamond and AlN. Thermal barrier resistance between diamond and AlN was found to be in the range of 16 m$^2$K/GW which is a large improvement on the current state-of-the-art.

The ζ-potential of the substrates (AlN on Si) were measured by measuring the streaming potential of the substrates in Surpass T M 3 electrokinetic analyzer. The change in potential or current between two Ag/AgCl electrodes at the either end of streaming channel as a function of electrolyte pressure gives the streaming potential. The shearing of the counterions in the streaming channel by flowing electrolytes from the charged surface gives rise to a streaming current/potential between two electrodes positioned tangentially to the flowing electrolyte.
The flow of counterions in the channel is proportional to the electric double layer of the surfaces forming the channel. Hence, the measured streaming current/voltage is related to the ζ -potential of the surface. 1 The streaming potential technique is not only useful for well defined surfaces but also can be used for measuring ζ -potential of fibres. 2 The setup for measuring ζ -potential was suggested by Van Wagenen et al. 1 and has been used for measuring ζ -potential of variety of flat surfaces. [3][4][5][6][7][8] In our experiments, the streaming channel was formed by two plates of AlN on Si. The channel width was kept between 90-110 µm. A 10 −3 M solution of KCl in DI water was used as an electrolyte. The pressure of the electrolyte was varied between 600 and 200 mbar. For altering the pH of electrolyte, 0.1M NaOH and 0.1M HCl solution was used with the inbuilt titrator in Surpass T M 3.
The wafers were seeded with mono-dispersed diamond solution. The growth of diamond on the seeded wafers were done using a Carat Systems SDS6U microwave chemical vapour deposition systems. 15 X 15 mm 2 pieces of AlN on Si were seeded with both H -and O -terminated seeds and placed in the reactor. A gas mixture of 3% CH 4 /H 2 at 110 torr with 5kW microwave power was used for the growth. A thick diamond layer (>100µm) was grown over a time period of 48 hours. After growth the samples were cooled down in the growth mixture over a 2 hour time period. For both H-and O-terminated seeds complete diamond layers were formed after 48 hours growth. The grown samples were then laser cut to get rid of the edges sticking to the silicon substrate underneath the AlN layer. After laser cutting it was found the diamond layer on AlN immediately delaminated. This may S2 be due to excessive stress in case of H -terminated seeds and inadequate adhesion in the case of O-terminated seeds. In the literature, it has been shown 9 that as-grown AlN has large number of dangling bonds. A treatment with nitrogen plasma helps to stabilise the AlN surface and reduce the dangling bond density. In our case a 10% N 2 /H 2 gas mixture at 1.5 kW microwave plasma and 20 torr to pretreat the surface before seeding was used.
The treatment times were fixed at 5, 10 and 15mins. Since the H-terminated seeds failed to produce diamond layers which did not delaminate, only O-terminated seeds were used on the plasma treated samples. The substrates were seeded after plasma treatment and a thick diamond layer was grown. The samples were again laser cut and it was found that diamond layers stayed attached to AlN only when the pretreatment time was 10minutes or more. So, for the rest of the study, only the 10 minutes plasma treatment before seeding was used. A comparative study of the surfaces treated with H 2 plasma and O 2 plasma exposure for 10 mins was also done.

X-Ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) was used to study the surface of the as-received and plasma treated substrates. A Thermo Scientific K-Alpha + spectrometer equipped with a monochromatic Al source operating at 72W (6 mA emission current at 12kV anode potential) was used to collect the spectra. Pass energies of 150 and 40 eV were used for acquiring survey and high-resolution spectra respectively. For charge neutralisation a combination of electrons and low energy Ar ions were used. The spectra were analysed using CasaXPS software. In this study, H 2 /N 2 plasma pretreatment was used. But, for comparison pure H 2 plasma and O 2 plasma exposure for 10 mins was also used. For O 2 plasma, a Plasma Etch Inc. PE -25 operating at 30W was used. XPS spectra of surfaces treated with pure hydrogen and oxygen plasma were also acquired and the ratios of O1s and N1s peaks with respect to Al2p peak is presented in figure S1. While the peak ratios in case of H 2 /N 2 and H 2 plasma are S3 similar, the O 2 flashed surface has higher oxygen concentration than nitrogen concentration.
Even then the ζ -potential of this surface at pH 6-7 is smaller in magnitude then the H 2 /N 2 plasma treated surface. The reason for such a behaviour is not clear at the moment. 3 Raman Spectroscopy Figure S2 shows the Raman spectra of the 100µm diamond film grown on pretreated AlN.
The Raman spectroscopy was done using an inVia Renishaw confocal microscope using a 514nm laser. A clear 1332 cm −1 diamond peak is seen. There is no signature of any nondiamond carbon peak in the spectra. The small peak close to 520 cm −1 is the silicon peak.
The silicon Raman signal is from the substrate on which the AlN is grown. Areal Raman maps of 50nm diamond films grown on treated and untreated AlN films were also done. The

Atomic Force Microscopy
The substrates were seeded with monodispersed diamond/H 2 O solution in an ultrasonic bath before AFM. This type of seeding produces nucleation density in excess of 10 11 cm −2 on silicon wafers. 10 The ζ -potential of seed solution is dependent on the surface termination of the diamond particles. H -terminated diamond particles give rise to positive ζ -potential and O -terminated particles lead to negative potential. 11 Commercially available nanodiamond seeds are aggregated and oxygen terminated. 12 The procedure for creating diamond solu- tions for seeding has been described elsewhere. 11 The as-received and H 2 /N 2 plasma treated wafers were seeded with both H -and O -terminated nanodiamond solution. The seeding density was checked using a Bruker Dimension Icon Pro in PeakForce Tapping mode. Bruker SCANASYST-AIR probes with a nominal tip radius of 2 nm were employed. The analysis of the images was performed using Gwyddion 13 scanning probe microscopy analysis software.
The AFM images of seeded and unseeded films without pre-treatment is shown in figure S5.
Panel A is the as-received AlN surface. Panel B shows the surface after seeding with Hterminated diamond seeds. As expected there is high seeding density. Panel C shows the surface after seeding with O-terminated seeds showing very small number of seeds.

Scanning Electron Microscopy
In figure    columnar growth mechanism after the initial phase of growth. This leads to grain sizes on the surface proportional to the thickness of the film. In figure S6, grains as large as 100µm can be seen. This is indicative of the fact that the film is at-least 100µm thick. SEM of non-coalesced films on pre-treated AlN were taken. The images are presented in figure S7.
The samples were seeded and exposed to diamond growth conditions for 30 seconds. Panel fitted thermal properties include the thermal interfacial resistance at the Au-AlN interface (R Au-AlN ), the thermal resistance across AlN layer and AlN-diamond interface (TBR eff ) and the diamond thermal conductivity (k di ). The TBR eff is is a lumped thermal resistance, associated with the thin AlN layer, AlN/diamond boundary and diamond nucleation layer.