Epitaxial growth of AlN on silicon is highly attractive for integrated nitride-based devices but remains challenging, as conventional techniques typically require high growth temperatures or suffer from limited industrial scalability. High-power impulse magnetron sputtering (HiPIMS) has emerged as a promising alternative by enabling enhanced ionization of the sputtered species and control of ion energy through substrate biasing. However, selective acceleration of the ionized film-forming species in HiPIMS is particularly challenging for AlN, due to the low atomic mass of Al relative to Ar. In this work, we successfully demonstrate that selective Al+ ion acceleration can be achieved by tuning the HiPIMS pulse length to exploit gas rarefaction, which suppresses Ar+ ion generation while sustaining a high Al+ ion density. Time-resolved mass spectrometry, supported by process modeling, reveals that increasing the pulse length induces a pronounced temporal separation between Ar+ and Al+ ion fluxes, resulting in a metal-ion-rich time window suitable for synchronized substrate biasing. Under these conditions, epitaxial AlN growth on Si(111) is achieved at a substantially reduced substrate temperature without the use of buffer or seed layers. The resulting films exhibit a clear enhancement in crystalline quality, strain state, and surface morphology consistent with a shift from Ar-ion-dominated to metal-ion-assisted growth. These findings establish pulse-length-controlled HiPIMS as an effective strategy for metal-ion-assisted epitaxial growth of AlN on silicon at moderate temperatures.