Is a surface electromyography (semg scan) done by a chiropractor?

Not usually. Apparently a few chiros have acquired this type of equipment but they have inadequate training and background to properly interpret the results. This type of testing should be done by a real doctor such as a neurologist.
Yes, limited use. Surface EMG does not provide much useful information in the diagnosis of nerve and muscle disorders. It may be performed in the office of a chiropractor or therapist to assist with biofeedback as part of an overall relaxation training program to help patients learn how to control muscle spasm. Surface EMG may also be used to analyze gait in athletes or amputees.