Without any assumptions regarding residual impurity species in an undoped semiconductor, it is experimentally demonstrated that the densities and energy levels of impurities can be precisely determined by the graphical peak analysis method based on Hall-effect measurements, referred to as free carrier concentration spectroscopy (FCCS). Using p-type undoped GaSb epilayers grown by molecular beam epitaxy (MBE), the densities and energy levels of several acceptor species are accurately determined. Five acceptor species are detected in the undoped GaSb epilayers grown by MBE, while two are also found in p-type undoped GaSb wafers. A 21–41 meV acceptor and a 75–99 meV acceptor exist both in the epilayers and in the wafer. On the other hand, a 164–181 meV acceptor is detected in epilayers grown at an Sb4/Ga flux beam equivalent pressure ratio of 8 or 10, while a 259 meV acceptor is found in the epilayer grown at Sb4/Ga = 6. In addition, a very shallow acceptor, which is completely ionized at 80 K, is found in the epilayers. The densities of the very shallow acceptor and the 21–41 meV acceptor are minimum at Sb4/Ga = 8, which makes the hole concentration lowest in the temperature range of the measurement.
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