An exit interview will give you a better idea of the reasons why your employees are not inclined to continue with you and prefer to opt for 'better opportunities ' ( in their view). If your exit interviews are conducted professionally without rancor or bias, chances are that the exiting employee will share a truthful feedback. Analyze these to identify the core issue and try to work around addressing those issues.
While an employment contract letter etc. gives legitimacy to the terms of employment, in practical terms, people will look out for better prospects if you do not meet their aspirational goals.
Some reasons why people will leave and you may perhaps find it difficult to counter them include:
They prefer to work with established names or larger set up
They move because they are seeking opportunities abroad
They get a much better terms of employment with better pay and perks in larger set up
Their intrinsic work goals are not being catered to by you
They take up the job just to get the training and experience.
Your focus to retain employees must not be so much driven by legal recourse as much as by meeting the aspirational needs of those you select. Hence it is also important to clearly identify personnel,who in your opinion, are less prone to move or would find it difficult to move. e.g. I am aware of a company that was a sick unit but required to recruit, to keep operations going. They shortlisted candidates who were academically sound but were not great communicators nor highly capable of cracking interviews with larger firms. Net result was that these employees not only stayed with the firm for a fairly long time, they also contributed substantially to turn around the company.