GaN Growth by Remote Plasma MOCVD: Chemistry and Kinetics by Real Time Ellipsometry
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While the investigation of the GaN growth mechanism is ongoing for MBE systems, where a variety of in situ diagnostic techniques can be used, such as reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), some difficulties are encountered for the MOCVD process, where high pressure and
reactive environment prevent from the use of the above techniques. In this paper, the remote plasma MOCVD (RP-MOCVD) of GaN buffer layers grown on both oc-A12 0 3 and GaAs substrates, also pre-nitrided by N 2 plasmas, is discussed. Particular emphasis is placed on the in situ real time control of the growing surface and of the substrate/GaN interface by spectroscopic ellipsometry (SE), which is a non-invasive and nonintrusive optical technique compatible with the MOCVD environment. Ellipsometry data are used to depict the chemistry and kinetics of the initial stage of the GaN growth.
EXPERIMENT The deposition system used in this study is the remote plasma MOCVD apparatus specifically designed for the growth and treatment of III-V materials [51 at reduced pressure (0.1 -
10 Torr) and reduced temperature. A unique feature of this apparatus is the presence of an in situ phase modulated ellipsometer (UVISEL-ISA Jobin-Yvon) compatible with the reactive environment of MOCVD reactors and, hence, able to monitor in real time the chemistry and
kinetics of surface modifications with a sub-monolayer resolution. The substrates used for the growth were GaAs (001) and cx-A120
3
(0001). The remote r.f. (13.56 MHz) plasma source was
G 3.12 Mat. Res. Soc. Symp. Proc. Vol. 537 0 1999 Materials Research Society
used to produce H-atoms for the substrates cleaning, and N-atoms for the nitridation process and GaN growth. The H 2 remote plasma cleaning was operated at P = I Torr, r.f. power = 60 Watt and at a surface temperature of 250'C and 350'C for GaAs (001) and (x-A120 3 (0001), respectively. The subsequent nitridation by N2-H 2 (3% in H2 ) plasmas was operated at a pressure of 0.2 Torr, a r.f. power of 200 Watt and at a temperature of 250'C and 800'C for GaAs and a-A120 3, respectively. GaN layers were grown at 600'C by trymethylgallium and N2 /H2 (=1000/3 sccm) plasma at a pressure of 1 Torr and a r.f. power of 200 W. Single wavelength ellipsometry (SWE) was used, in terms of the ellipsometric angles T and A [6], to monitor in real time the surface kinetics, and, at the growth end, ellipsometric spectra (SE) of the pseudodielectric function, = + i , were acquired in the energy range 1.5 - 5.5 eV. In order to determine the chemical and optical properties of surfaces, ellipsometric data were modeled by optical models based on the Bruggeman effective-medium approximation (BEMA) [6] and references for the used dielectric functions are given in ref. [7]. Optical emission spectroscopy (OES) was used to control the N-atom density interacting with the growth surface. Ex situ atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements were also performed to valid
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