'Doors' Founder Ray Manzarek Traveled to Germany for Special Cancer Treatment

"The Doors" co-founder Ray Manzarek traveled across the world to seek out special world class treatment for his rare form of cancer ... which is why he was in Germany when he died, TMZ has learned.

Sources close to Ray's most recent band -- The Manzarek-Rogers Band -- tell us, Ray did tons of research on the best places to get treatment for his bile duct cancer ... and his search led him to the RoMed Clinic in Rosenheim, Germany.

As we reported, Ray eventually passed away at RoMed yesterday.

According to sources, Ray broke news of his cancer to the band in March ... and traveled to Germany shortly after.

Up until Ray told his band about the cancer, we're told they had no idea he was sick. Ray had repeatedly just told them he was "tired," so the news was a shock.

Manzarek's next appearance was scheduled for August at the Orange County Fair ... but that's obviously been canceled.