PHYSICAL REVIEW B VOLUME 44, NUMBER 11 15 SEPTEMBER 1991-I Rapid Communications Rapid Communications are intended for the accelerated publication of important new results and are therefore given priority treatment both in tlte editorial once and in production. A Rapid Communication in Physical Review B should be no longer than 4 printed pages and must be accompanied by an abstract Pag. e proofs are sent to authors Structure of hydrogenated amorphous silicon from ab inrtio molecular dynamics F. Buda Department of Physics, The Ohio State University, 174 yt'est 18th Avenue, Columbus, Ohio 43210 Guido L. Chiarotti International School for Advanced Studies, Strada Costiera II, 1-34014 Trieste, Italy and Iaboratorio Tecnologie Avanzate Superftci e Catalisi del Consorzio Interuniversitario Nazionale di Fisica della Materia, Padriciano 99, I-34012 Trieste, Italy R. Car International School for Advanced Studies, Strada Costiera I I, 1-34014 Trieste, Italy, Institut Romard de Recherche Numerique en Physique des Materiaux, CH-1015 Lausanne, Switzerland, and Department of Condensed Matter Physics, University of Geneva, CH-1211 GenevaSwit, zerland M. Parrinello MM Research, Division, Zurich Research Laboratory, CH-8803 Ruschlikon, Switzerland (Received 29 May i99i) We have generated a model of hydrogenated amorphous silicon by first-principles molecular dynam- ics. Our results are in good agreement with the available experimental data and provide insight into the microscopic structure of this material. The calculation lends support to models in which monohy- dride complexes are prevalent, and indicates a strong tendency of hydrogen to form small clusters. Hydrogenated amorphous silicon (a-Si:H) is an impor- tant material for electronic applications, yet its properties to a large extent are not fully understood. Much effort has been devoted to determining its structural properties, which is a prerequisite for further progress. The short- range order of this material has been studied using several experimental techniques including neutron scattering, ' extended x-ray-absorption fine structure, and infrared absorption. In particular, infrared spectroscopy iden- tifies hydrogen in a-Si:H as bonded in diff'erent complexes. Monohydride (Si-H), dihydride (Si-H2), trihydride (Si- H3), and polymeric (Si-H2)„configurations have been detected. Unfortunately, quantitative predictions on the relative concentration of different Si-H„groups are difficult to extract from vibrational spectroscopy and strongly depend on the hydrogen concentration and preparation conditions. Furthermore, data obtained by different probes can provide inconsistent pictures, leaving room for controversies. Other issues concern the distribu- tion of hydrogen within the sample and the characteriza- tion of structural heterogeneity in a-Si:H. These ques- tions have been studied mainly by multiple quantum nu- clear magnetic resonance (MQ-NMR), and small-angle x-ray scattering. This experimental eH'ort has not been accompanied by a comparable theoretical understanding, thus our picture of the a-Si:H structure is still somewhat controversial. This state of affairs is due, in part, to the lack of a reliable theoretical model of the a-Si:H network, since it is difficult to achieve an accurate description of the inter- atomic interactions and of the delicate chemical competi- tion between the two species. The recent development of ab initio molecular-dynamics techniques circumvents these difficulties. One can now calculate interatomic forces from stat-of-the-art quantum-mechanical calcula- tions in a practical manner. This scheme is parameter free and suited to describing systems, such as covalent semiconductors, where chemical bonds may break and form as a consequence of the atomic motion. In this paper we report the analysis of an a-Si:H net- work generated by means of the computer-simulation ap- proach. We have considered an atomic H concentration of — 11%, typical of a device quality material. The aver- age structural properties of our model are in good agree- ment with the results of several experimental probes of the short-range order. We also resolve some of the outstand- ing issues by finding that (1) the most stable H con- figuration consists of monohydride (Si-H) complexes, in which the H atom is bonded to an otherwise threefold coordinated Si atom. (2) In agreement with the sophisti- cated MQ-NMR data reported by Baum et al. , we find that small clusters of H atoms form. (3) The complex 5908 O1991 The American Physical Society