Advances in applications of positron annihilation spectroscopy to investigating semiconductor microstructures

Positron annihilation spectroscopy has unique advantage for detecting the micro-defects and microstructures in materials,especially for investigating the negatively charged defects such as cation vacancies in semiconductors.It is a powerful tool to characterize the important features for vacancy-type defects localized electron states within the forbidden energy gap and cation vacancy which provides the key information about the type and distribution of microdefects. Positron annihilation lifetime and Doppler broadening spectroscopy are the major methods of analyzing the vacancy formation,evolution and distribution mechanism.Importantly,the slow positron beam technique can provide the dependences of surface,defect and interface microstructure information on depth distribution in semiconductor thin film.Vacancy and impurity elements can change the ambient electron density in material.They also induce the middle band,which will have dramatic effects on optical and electrical performance.And the variation of electron density will exert furtherinfluences on the positron-electron annihilation mechanism and process.For the fundamental experiments in semiconductors,fabrication technology,thermal treatment,ion implantation/doping,irradiation etc, positron annihilation spectroscopy technology has been extensively applied to detecting the detailed electron density and momentum distribution,and gained the information about microstructure and defects.It can guide the fundamental researches in experiment and give optimal design of the technology and properties about semiconductors.In principle, defect concentrations can be derived and an indication can be obtained about the nature of the defect.Results are presented showing that cation vacancies can be easily detected.Also charge states and defect levels in the band gap are accessible.By combining the positron annihilation spectroscopy with optical spectroscopies or other experimental methods,it is possible to give detailed identifications of the defects and their chemical surroundings.The positron annihilation spectroscopy technology is a very special and effective nuclear spectroscopy analysis method in studying semiconductor microstructure.In this review,the research progress in applications of positron annihilation spectroscopy technology to semiconductors is reported,which focuses on the experimental results from the Positron Research Platform located in Institute of High Energy Physics,Chinese Academy of Sciences.Under different growth modes and ways of treating semiconductors,the experimental results about the internal micro-defect formation mechanism of material, evolution mechanism,and defect feature research progress are reviewed Future challenges including the analysis of electropositivity vacancy (i.e.oxygen vacancy) and of multi-ion implantation phenomena are also presented new technologies such as digitization and new theory will make the positron annihilation spectroscopy portable and reliable.