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The crystallization enthalpy of pure amorphous silicon (a-Si) and hydrogenated a-Si was measured by differential scanning calorimetry (DSC) for a large set of materials deposited from the vapour phase by different techniques. Although the values cover a wide range (200 -480 J/g), the minimum value is common to all the deposition techniques used and close to the predicted minimum strain energy of relaxed aSi (240 ± 25 J/g). This result gives a reliable value for the configurational energy gap between a-Si and crystalline silicon. An excess of enthalpy above this minimum value can be ascribed to coordination defects.In contrast to glasses, whose lower energy states can be accessed by either cooling the liquid at lower rates or by thermal annealing [1], the energy of amorphous tetrahedral semiconductors must be lowered by thermal annealing [2]. Among them, amorphous silicon (a-Si) has been extensively studied due to its technological relevance and because it is usually taken as a model material for covalent amorphous networks.We have recently shown [3] that after thermal annealing, the rms deviation from the tetrahedral angle, Δθ, is approximately 9° for a broad range of pure and hydrogenated a-Si materials. This "relaxed state" of pure a-Si (no H-atoms remain after annealing) gives support to the theoretical prediction [4] that it is not possible to build stable amorphous models below minimum bond-angle dispersion. In other words, there is a 'configurational gap' between a-Si and c-Si that ensures higher entropy in the amorphous state. This discontinuity makes it impossible for the material to evolve from one state to the other by continuously varying its short-range order.The bond-angle dispersion entails a built-in strain energy that is released during crystallization [5] and currently detected by differential scanning calorimetry (DSC) as an exothermic peak [6]. By measuring the crystallization enthalpy, ΔH cryst , we can, thus, determine the value of the 'configurational energy gap' between a-Si and c-Si. To date, the only systematic measurements of ΔH cryst have been taken from a-Si obtained by ion implantation of c-Si [6] (measurements on hydrogenated thin films being very scarce [7]). Since the experimental values felt within a narrow range (see the right side of Fig. 1), it is generally accepted that they correspond to the minimum energy of a-Si.However, since this method of synthesis is very far from equilibrium, there is good reason to suspect that, even after thermal annealing, the material has not reached its lowest energy level. In fact, we suggested [8] that one half of ΔH cryst of this material was due to high concentration of coordination defects and, consequently, predicted that the crystallization enthalpy could be reduced by this factor in defect-free materials.Recent studies of organic glasses [9] have shown that, thanks to a higher molecular mobility at the film free surface [10], deposition from the vapour phase is a way to achieve low-energy amorphous states. This behaviour could also apply ...