In situ sensors for monitoring and control in molecular beam epitaxial growth of Hg_1−xCd_xTe

Abstract

The current status of the implementation and refinement of two wafer state sensors forin situ monitoring and control during molecular beam epitaxial (MBE) growth of Hg_1−xCd_xTe will be reported. First a rapid scan spectral ellipsometer has been developed and employed for precisely measuring compositions of Hg_1−xCd_xTe alloys during growth. MBE films in the composition range x = 0.20 to 0.30 have been grown andin situ spectra taken at the growth temperature (180°C) and at room temperature. The MBE films were treated as single layers without the need to invoke any surface film (due to surface roughness, oxide, or of any different composition) as required for exsitu data. The least squares fit over the whole spectral range was used as a measure of the precision. The film composition was also determinedex situ by wavelength dispersive analysis of x-rays and by Fourier transform infrared (FTIR) spectrometry after verifying that there was no lateral variation. A precision of better than ±0.0015 has so far been demonstrated usingin situ spectral ellipsometry for Cd composition or CdTe mole fraction, x, measurements. This compares with ±0.003 for single wavelength ellipsometry. The composition of Hg_1−xCd_xTe films were also monitored during growth. A spectral pyrometer based on a FTIR spectrometer has also been developed for substrate temperature measurements during growth. The spectral pyrometer measures both the emission and reflectance to give the emissivity of a growing sample over a range of wavelengths spanning the peak of the grey body emission. From the reflectivity measurements, the thickness (in excess of 1 µm) of the growing film is also determined from the interference fringes. The spectral ellipsometer is only capable of measuring thicknesses up to C.a 5000°A (i.e. optically thin). Excellent agreement is obtained between thein situ (at growth temperature) andex situ (at room temperature) thickness measurements. The small discrepancy can be explained by the refractive index of Hg_1−xCd_xTe being 5% higher at the growth temperature than at room temperature. The combination ofin situ sensors now provides a means of continuously monitoring the composition and thickness of the growing Hg_1−xCd_xTe film.