Stacking atoms

A sidebar to the Bell Labs special report, Reviving an icon

When Al Cho arrived at Bell Labs in 1968, he was given a desk and a pad of paper, and was then told to figure out what he wanted to research. It didn’t take him to long to decide. While many of his colleagues were tackling the hot new field of optical lasers, Cho decided focus on epitaxy, the science of growing the crystalline structures of semiconductors. As its name implies, a semiconductor is both an insulator and a conductor, meaning it can both allow the flow of electricity or resist it depending on its state. To bring out the unique electrical properties of semiconducting materials like silicon, however, the material needs be modified by adding elemental impurities in a process called “doping.”

At the time, semiconductor doping was done through a process called Liquid Phase Epitaxy, which layered impurities into a semiconductor crystal as it wasn’t being grown. The process wasn’t very precise, and Cho thought he could do better. He proposed superheating the semiconductor materials and the impurities and firing them separately as beams onto a wafer in a high-vacuum chamber. There the different elements would condense into crystals one atom layer at time. The method, called Molecular Beam Epitaxy, created the most pure semiconductor crystals ever seen.

Cho moved from Murray Hill to a Western Electric facility in Reading, Penn., where he saw his new MBE machine through to commercial production. MBE machines are now used to construct the advanced microelectronics in some of our most common devices, such as the RF amplifiers in cell phones, but the technique has also been used to advance multiple fields of science. MBE has been used to build the component materials for quantum computing and the burgeoning area of nanotechnology.

Ironically, Cho’s MBE machine made significant contributions to the field of optical asers he eschewed back in 1968. MBE processes now produce the semiconductor lasers used in CD players, and in 1994 Cho and fellow Bell Labs scientist Federico Capasso used MBE to create the precise semiconductors powering the quantum cascade laser. The QCL’s high optical can be used to detect pollutant levels in the atmosphere, conduct advanced medical diagnostics by analyzing the gases in a patient’s breath and in other fields of advanced spectroscopy.

In 2005, Cho was awarded the National Medal of Technology for his work.