Plasma Potential Measurements and Strain Effects in Epitaxial GAN Grown on AlN Buffered Si(111) by Radio Frequency React
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potentials of rf Ar/N 2 glow discharges used for growth of GaN thin films. Using ultra high vacuum (UHV) rf reactive sputtering, we have grown epitaxial wurtzitic GaN films on AIN buffered Si(111) substrates[8]. Growth of epitaxial and oriented wurtzitic AIN thin films on Si substrates by reactive sputtering has been reported previously[9]. Characterization of epitaxial GaN films have been carried out by combining the techniques of x-ray diffraction, reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM), and Raman scattering. Here we report on measurements of plasma potentials and strain in epitaxial GaN films. EXPERIMENTAL An UHV sputter deposition system with a base pressure - 2 x 10"1 Torr was used for growth of the AIN buffer layers on Si(111) and for subsequent growth of GaN films. The planar magnetron sputter sources used for this work were designed for both dc and
rf operation. Metallic Ga (99.999+%) contained in a Mo cup and metallic Al (99.999+%) were used as sources for GaN and AIN growth. Thin films of AIN and GaN were grown in the dc and rf mode, respectively. Power input into the plasma was regulated in the constant current and constant power mode for dc and rf operation, respectively. Both Al and Ga can be sputtered in pure Ar (99.999+%) or an Ar/N 2 (99.999+%) mixture. For growth of both AIN and GaN thin films, the Ar/N 2 input flow ratio and the total pressure were kept constant at 1:9.9 and 1.6 mTorr, respectively. All AiN buffer layers were 450A in thickness and grown at 710 ' C. GaN films were - lym in thickness. Heatable Langmuir probes were constructed using W-3%Re wires, the portion of the probe exposed to the plasma consisted of a 50 1 um diameter wire. The electrical circuit used for measuring (I) - V as well as (dI/dV) - V spectra followed closely those reported in the literature[6]. X-ray diffraction measurements were carried out at room temperature on a Siemanns D500 9 - 9 diffractometer with Cu koz radiation. Raman scattering measurements were made at room temperature using an Ar ion laser (488 nm) as the excitation source, further details were reported elsewhere[9,10]. RESULTS and DISCUSSION Fig. 1(a) shows a 9 - 20 x-ray diffraction pattern of a GaN film grown at 560 'C. In addition to Si reflections, only A1N(000/) and GaN(000I) reflections are present, showing the film orientation GaN[0001j//AIN[0001]//Si[111]. Fig. 1(b) shows superimposed Si(111), A1N(0002), and GaN(0002) rocking curves of the same film. The Si(111) rocking curve width of 0.09 deg is indicative of the diffractometer resolution. The AIN(0002) and GaN(0002) rocking curve widths are 0.85 and 0.61 deg, after the instrumental correction. The in-plane orientation was previously shown by cross-sectional TEM to be GaN[ll20]//A1N[1120]//Si[1iO][8]. RHEED patterns from final GaN surfaces showed spotty transmission features indicative of atomic scale roughness[8]. Fig. 1(c) shows an optical interference pattern taken during growth of a GaN film on A1N/Si(111) at 550 'C, good agreement is
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