Positron annihilation is a fundamental quantum process that occurs when a positron, the antimatter counterpart of an electron, encounters an electron in matter. When these particles meet, they annihilate each other and convert their mass into energy, producing two gamma rays that travel in opposite directions. This process provides valuable information about the electronic structure and defects in materials, making it particularly useful for analyzing semiconductor materials and their dopant distributions.
Positron annihilation spectroscopy works by measuring the energy and angular correlation of gamma rays produced during annihilation. The gamma rays typically have an energy of 511 kiloelectron volts, corresponding to the rest mass energy of an electron. By analyzing the energy spectrum and timing of these gamma rays, we can determine the momentum distribution of electrons in the material. Different material environments, such as defects or impurities, create characteristic changes in the annihilation spectrum, allowing us to identify and quantify these features.
半导体材料如硅和锗具有高度有序的晶体结构，原子在周期性晶格中排列。在这些完美晶体中，电子占据明确的位置和能级。然而，真实半导体包含各种类型的缺陷，包括原子缺失的空位、间隙原子和替代杂质。这些晶格不完美性在电子环境中产生局部变化，可以使用正电子湮没谱学检测，因为正电子对电子密度的变化很敏感。
Dopant atoms such as boron, phosphorus, and arsenic are intentionally introduced into semiconductor crystals to modify their electrical properties. These impurity atoms create local changes in electron density and generate electric fields around their positions. Boron acts as a p-type dopant by accepting electrons, while phosphorus and arsenic are n-type dopants that donate electrons. Positrons are sensitive to these electronic environment changes and are attracted to or repelled by different dopant types, making positron annihilation spectroscopy an effective tool for analyzing dopant distribution and concentration in semiconductor materials.
Positron annihilation spectroscopy provides quantitative data about dopant distribution through characteristic changes in gamma ray spectra. Undoped semiconductors show sharp peaks at 511 kiloelectron volts, while doped samples exhibit peak shifts and broadening that correlate with dopant concentration. Light doping causes small peak shifts, while heavy doping results in significant spectral changes. The technique also enables depth profiling, revealing how dopant concentration varies with depth in the material. This capability makes it invaluable for analyzing semiconductor device structures and optimizing fabrication processes.