A Scrum Master is a role that is supposed to support the Scrum Team. In general a Scrum Team is a self-organizing system. Systems can be stable or instable. I would see a Scrum Master as the one who is responsible to stabilize the system. A Scrum Team should also follow a goal, that is somehow a lower level goal of an organization, i.e. the company that the Scrum Teams works for. It's also a resposibility of the Scrum Master, but mainly of the Product Owner, that the goal of the team is in line with the company's goal.
Having said that, we cannot describe the Scrum Master as a servant of the team. He is a servant, but he is more than that. A Scrum Master must have a potetial to influence the team members. According to business sciences we have five sources of power:
- personal charisma
- expert knowledge
Which of the sources of power can be used? To create a self-organizing team, the Scrum Master cannot use punishment, because that will kill it. The Scrum Master has the legitimation to control the Scrum Rules. But the rules are made by the team and the team may choose rules that lead to the wrong direction. The Scrum Master must lead - with personality, knowledge and rewards.