SQL Query how to combine two fields Year and Month to one field Period

Hello,

I have the following Query:

SELECT t0.[Docnum], t0.[docdate],  (Datepart(Year,T0.[DocDate])) , (Datepart(Month,T0.[DocDate])), t0.[doctotal] FROM OINV T0 where t0.[docdate] >= '20140101'

This results in a list of AR/Invoices with Docnum, Docdate, Year, Month, Doctotal in a separate columns.

I would like to combine the two columns "Year" and "Month" in one column "Period" which is written as follows:

2014-01

2014-02

.....

2014-09

2014-10

2014-11

2014-12

2015-01

The zero at the months lower than 10 is essential because of sorting issues.

what statement do i have to use to realise this ?

Who can help me with this ? - Thanks in advance