Growth And Characterization Of Nitride Semiconductors on Chemical Vapor Deposited Diamond
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Group III nitride semiconductor based devices have emerged as the best candidates for handling higher power and frequency in recent years. Performance of various devices such AlGaN/ GaN high electron mobility transistors, GaN lasers and GaN LEDs are often hindered by self-heating of these materials and poor heat removal capabilities of the substrate materials. Chemical vapor deposited (CVD) diamond has demonstrated the best heat removal capability, when employed as the substrate material for GaN based high power devices, due to its high thermal conductivity. Diamond is either grown directly on the backside or bonded with GaN using an adhesion layer to extract excessive heat from the near junction region of these devices. In both cases, thermal resistance associated with the interface of diamond and GaN limits the effectiveness of the diamond layer. In this work, single crystal GaN has been grown using metal organic chemical vapor deposition (MOCVD) directly on chemical vapor deposited diamond without any adhesion layer in a novel way which will mitigate thermal resistance between the near junction region of GaN devices and diamond substrate. The growth of GaN-on-diamond was achieved through a series of experiments and characterizations in various steps of the process.
Initial experiments were conducted to understand the effect of precursor stoichiometry of hot filament CVD (HFCVD) on diamond’s morphology, phase purity and crystal structure. By varying the methane concentration in hydrogen in the range of 1.5-4.5%, it was found that film thickness, grain size, roughness and non-diamond carbon (NDC) of polycrystalline diamond increase with increasing methane concentration. X-ray diffraction and pole figure results indicate that diamond films grown with 4.5% methane exhibit preferential orientation in the <220> direction whereas films grown with 3.0% and 1.5% methane showed preferential orientation along <111>.
A process for photolithography and reactive ion etching based selective seeding of nano-diamond and selective deposition of CVD diamond was established on Si substrate which resulted in diamond feature size as small as 1 µm. Ultraviolet (UV) micro Raman stress mapping across selectively deposited diamond stripes with various widths revealed up to 0.9 GPa compressive stress in 1.5 µm thick diamond and a moderate (~150 MPa) tensile stress within 10 nm inside Si from the diamond-Si interface. UV Raman based measurements and finite element (FE) simulations indicated that the stress is thermal in origin which resulted from the mismatch of thermal expansion coefficients (CTE) between diamond and Si. The stress on Si was relaxed away from the diamond stripe which indicate global stress relaxation occurs when diamond is grown selectively. Subsequently, the selective nano-diamond seeding and selective diamond growth process was developed for GaN substrate. Poor diamond coverage, poor selectivity and significant etching of GaN was observed when diamond was seeded on the bare GaN surface. To mitigate these problems, a thin SiNx layer was deposited on GaN before selective nano-diamond seeding which resulted in enhanced coverage, excellent selectivity and complete protection of the GaN surface. To characterize the protection of the GaN surface during CVD diamond growth, an MOCVD grown AlGaN/GaN wafer with ~25 nm AlGaN barrier layer on top of GaN was employed. The presence of the AlGaN layer after diamond deposition was confirmed form scanning electron microscopy (SEM), atomic force microscopy (AFM), Micro Raman and high-resolution x-ray diffraction (HRXRD) reciprocal space mapping (RSM) of the asymmetric (114) plane taken before and after diamond deposition.
Finally, selectively deposited diamond coated GaN wafers with various window sizes and orientations was used for epitaxial lateral overgrowth (ELO) of GaN on the windows. The effect of pressure (P), temperature (T), group V to group III molar ratio (V/III) and mask orientation on lateral growth was studied in detail to achieve complete coalescence of GaN over narrow diamond stripes. The best ELO process that resulted in complete coalescence of GaN over diamond mask was achieved using P=100 torr, T=1030 °C, V/III=7880 and GaN window on diamond mask oriented along [11 ̅00]. Cross-section SEM images indicated a continuous interface between ELO GaN and diamond. HRXRD data confirmed growth of c-axis (0002) oriented ELO GaN on overgrown region.