Mj. Bevan et al., IN-SITU SENSORS FOR MONITORING AND CONTROL IN MOLECULAR-BEAM EPITAXIAL-GROWTH OF HG1-XCDXTE, Journal of electronic materials, 25(8), 1996, pp. 1371-1374
The current status of the implementation and refinement of two wafer s
tate sensors for in situ monitoring and control during molecular beam
epitaxial (MBE) growth of Hg1-xCdxTe will be reported. First a rapid s
can spectral ellipsometer has been developed and employed for precisel
y measuring compositions of Hg1-xCdxTe alloys during growth. MBE films
in the composition range x = 0.20 to 0.30 have been grown and in situ
spectra taken at the growth temperature (180 degrees C) and at room t
emperature. The MBE films were treated as single layers without the ne
ed to invoke any surface film (due to surface roughness, oxide, or of
any different composition) as required for ex situ data. The least squ
ares fit over the whole spectral range was used as a measure of the pr
ecision. The film composition was also determined ex situ by wavelengt
h dispersive analysis of x-rays and by Fourier transform infrared (FTI
R) spectrometry after verifying that there was no lateral variation. A
precision of better than +/-0.0015 has so far been demonstrated using
in situ spectral ellipsometry for Cd composition or CdTe mole fractio
n, x, measurements. This compares with +/-0.003 for single wavelength
ellipsometry. The composition of Hg1-xCdxTe films were also monitored
during growth. A spectral pyrometer based on a FTIR spectrometer has a
lso been developed for substrate temperature measurements during growt
h. The spectral pyrometer measures both the emission and reflectance t
o give the emissivity of a growing sample over a range of wavelengths
spanning the peak of the grey body emission. From the reflectivity mea
surements, the thickness (in excess of 1 mu m) of the growing film is
also determined from the interference fringes. The spectral ellipsomet
er is only capable of measuring thicknesses up to ca. 5000 Angstrom(i.
e. optically thin). Excellent agreement is obtained between the in sit
u (at growth temperature) and ex situ (at room temperature) thickness
measurements. The small discrepancy can be explained by the refractive
index of Hg1-xCdxTe being 5% higher at the growth temperature than at
room temperature. The combination of in situ sensors now provides a m
eans of continuously monitoring the composition and thickness: of the
growing Hg1-xCdxTe film.