The unique structural and physical properties of two-dimensional (2D) atomic layer semiconductors render them promising candidates for electronic or optoelectronic devices. However, the lack of efficient and stable approaches to synthesize large-area thin films with excellent uniformity hinders their realistic applications. In this work, we reported a method involving atomic layer deposition and a chemical vapor deposition chamber to produce few-layer 2H-MoSe₂ thin films with wafer-level uniformity. The reduction of MoO₃ was found indispensable for the successful synthesis of MoSe₂ films due to the low vaporization temperature. Moreover, a metal-semiconductor-metal photodetector (PD) was fabricated and investigated systematically. We extracted an ultrahigh photoresponsivity approaching 101 A/W with concomitantly high external quantum efficiency up to 19,668% due to the produced gain arising from the holes trapped at the metal/MoSe₂ interface, the band tail state contribution, and the photogating effect. A fast response time of 22 ms was observed and attributed to effective nonequilibrium carrier recombination. Additionally, the ultrahigh photoresponsivity and low dark current that originated from Schottky barrier resulted in a record-high specific detectivity of up to 2×10¹³ Jones for 2D MoSe₂/MoS₂ PDs. Our findings revealed a pathway for the development of high-performance PDs based on 2D MoSe₂ that are inexpensive, large area, and suitable for mass production and contribute to a deep understanding of the photoconductivity mechanisms in atomically thin MoSe₂. We anticipate that these results are generalizable to other layer semiconductors as well.