Can you Greenflame Blade / Booming Blade with a Shield? [duplicate]

Both these cantrips want a weapon as the material Component, and then later want a weapon attack. However if you are an Arcana Cleric (learns a wizard cantrip), and as a cleric you can use your shield as the spellcasting focus (ignoring the 1st requirement).

But can the shield be considered an improvised weapon for the 2nd requirement?