The (001) surface of molecular-beam epitaxially grown GaAs studied by scanning tunneling microscopy

In order to better understand the mechanism of molecular-beam epitaxy (MBE) growth on GaAs(OOl), itisessential to know the structure of the arsenic rich (2X 4 )/e(2X 8) surface where growth usually begins and ends. We have studied this surface with the scanning tunneling microscope (STM). The specimens were grown by MBE and arsenic capped prior to transfer to the STM. In the STM chamber the arsenic capping was removed by heating to-450 °C. The STM images show that the (2 X 4 ) unit cell consists of three arsenic dimers and a missing dimer to give the 4 X periodicity, resulting in an arsenic coverage of 0.75 monolayers. The surface consists of small domains of both (2X 4) and e(2X 8} reconstructions. The ee2 X 8) structure is made up from the basic (2 X 4 ) units. The STM images also show other features which may be important in the growth mechanism. The most striking of these are small islands one step up and small holes one step down, typically only a few unit cells in size. The step height corresponds to the spacing between arsenic planes. The raised islands are made up of complete unit cells, and can be as small as one (2 X 4) unit cell wide in the 4 X direction. This shows the three dimers ofa (2 X 4) unit cell to be a very stable structure, either in the plane when it is bordered by missing dimers, or on a raised island when bordered by a step edge.