Impurity survey analysis of CdxHgi−xTe by laser scan mass spectrometry

A laser scan mass spectrometry (LSMS) technique has been applied to the analysis of epitaxial layers and bulk CdxHg1−xTe (CMT) materials. The samples are raster scanned under a focused Q-switched Nd–YAG laser beam which ionises all elements with approximately unit sensitivity. The ions are extracted into a high-resolution mass spectrometer and interpretation of the mass spectra gives a complete impurity survey of the material with detection limits down to 1 part in 109 (3 × 1013 cm−1) in bulk materials and 5 parts in 109 in epitaxial layers. Surface impurities are effectively removed in the first scan and subsequent scans over the same area give a true measurement of the impurities present in the material. Successive erosion of the sample surface gives impurity depth profiles with the thickness of material eroded in each scan variable between 1 and 4 μm. Results are given for impurities found in bulk CMT grown by standard Bridgman and ACRT (accelerated crucible rotation technique) processes and in epitaxial CMT produced by liquid phase epitaxy (LPE), metal organic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE). Bulk material is shown to be purer in general than epitaxial material. Semiquentitative depth profiles of dopants in CMT, such as iodine, arsenic and phosphorus, can also be obtained and comparative LSMS and secondary ion mass spectrometry (SIMS) depth profiles showing good agreement are given for arsenic in an MOVPE layer.