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III-nitride materials have different crystal structures and properties than the substrates commonly used for their deposition, including silicon, silicon carbide and sapphire. These differences, such as thermal expansion coefficient and lattice constant, necessitate the use of a transition layer to accommodate the resulting stress between substrate and the epitaxially grown III-N layers. AlxGa1−xN based transition layers are one proven solution used for the growth of device quality GaN layers on Si (111) substrates. The use of such transition layers enables the deposition of state of the art AlGaN/GaN high electron mobility transistor epitaxial structures that, upon fabrication into devices, exhibit high performance and excellent reliability.
Examination of the microstructure of these AlxGa1−xN transition layers, by transmission electron microscopy (TEM) and other methods, reveals some interesting properties that can help explain how high quality III-N epitaxy can be performed in a system with significant thermal and lattice mismatch. Observations that will be reported on and discussed in this presentation are (1) the role that a thin strain absorbing amorphous SiNx layer at the Si substrate/transition layer interface plays in the reduction of the formation of misfit dislocations, (2) the low screw dislocation density (less than ȼ107/cm2) in these III-N films relative to edge and mixed dislocation densities, and (3) the role that the substrate type and quality can play on dislocation type and density.
AlGaN/GaN based high power, high frequency high electron mobility transistors (HEMTs) have been in development for over a decade. Although much progress has been made, AlGaN/GaN HEMT technology has yet to be commercialized. The choice of silicon as the substrate for the growth of GaN-based epi layers will enable commercialization of AlGaN/GaN based HEMTs, because of its maturity, scalability, reproducibility and economy. One of the epitaxial issues pertaining to the growth of AlGaN/GaN HEMTs on Si is the understanding of parasitic losses that can adversely impact the RF device performance. The effect of the III-N MOCVD process on the resistivity of the Si substrate, and correlations between the Si substrate resistivity and AlGaN/GaN HEMT RF characteristics are presented. Optimization of the MOCVD growth process led to a reduction in parasitic doping of the Si substrate. This resulted in the following improvements: (a) small signal gain increased from 17 to 21dB, (b) the cut-off frequency increased from 7 to 11GHz and (c) the maximum frequency of oscillation improved from 12 to 20GHz. This optimized process will enhance performance of AlGaN/GaN HEMTs at higher frequencies.
The emergence of III-nitride technology and fabrication of high quality GaN based devices is possible due to the advances in the heteroepitaxial growth of III-N thin-films on lattice-mismatched substrates. Typically, the substrate of choice is either SiC or sapphire. We have adopted 100mm Si as our substrate of choice; uniform substrates of high quality are inexpensive and plentiful due to decades of use in the microelectronics industry. Growth of device quality GaN on Si is challenged by the ∼17% lattice mismatch and an additional thermal expansion coefficient (TEC) mismatch of ∼56%. In order to accommodate this strain and TEC mismatch between Si and GaN, a novel transition layer was designed, grown and successfully optimized, ® obviating the need for either a PENDEO based overgrowth process or a SiC interlayer-based process. This growth technique (SIGANTIC®) does not require any wafer conditioning prior to growth and thus reduces the process complexity and maintains the cost effectiveness of the GaN on Si strategy. We will report on this manufacturable 100mm MOCVD heteroepitaxial process that consistently produces device quality AlGaN/GaN heterostructures with two dimensional electron gas (2DEG) mobilities typically around 1400 cm2/Vs at room temperature. Structural and electrical properties as determined by optical reflectance, atomic force microscopy, capacitance-voltage and van der Pauw Hall measurements, which are measured across the 100mm wafer, will be presented. Device results will be mentioned to show continuous wave (CW) RF operation at 2 GHz with competitive power output, gain and power added efficiency (PAE).
Monocrystalline GaN(0001) thin films have been grown at 950 °C on high-temperature, ≈ 100 nm thick, monocrystalline AlN(0001) buffer layers predeposited at 1100 °C on α(6H)−SiC(0001)Si substrates via OMVPE in a cold-wall, vertical, pancake-style reactor. These films were free of low-angle grain boundaries and the associated oriented domain microstructure. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound excitonic emission with a FWHM value of 4 meV. The near band-edge emission from films on the vicinal substrates was shifted slightly to a lower energy, indicative of films containing residual tensile stresses. A peak attributed to free excitonic emission was also clearly observed in the on-axis spectrum. Undoped films were too resistive for accurate Hall-effect measurements. Controlled n-type, Si-doping in GaN was achieved for net carrier concentrations ranging from approximately 1 × 1017 cm−3 to 1 × 1020 cm−3. Mg-doped, p-type GaN was achieved with nA−nD ≈ 3 × 1017 cm−3, ρ ≈ 7 Ω · cm, and μ ≈ 3 cm2/V · s. Double-crystal x-ray rocking curve measurements for simultaneously deposited 1.4 μm GaN films revealed FWHM values of 58 and 151 arcsec for deposition on on-axis and off-axis 6H−SiC(0001)Si substrates, respectively. The corresponding FWHM values for the AlN buffer layers were approximately 200 and 400 arcsec, respectively.
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