VOLUME 88, NUMBER 12 PHYSICAL REVIEW LETTERS 25 MARCH 2002 Hydrogen Vibration Modes in GaP:N: The Pivotal Role of Nitrogen in Stabilizing the H  2 Complex A. Janotti, S. B. Zhang, and Su-Huai Wei National Renewable Energy Laboratory, Golden, Colorado 80401 (Received 24 October 2001; published 11 March 2002) Atomic structures of N-related hydrogen complexes in GaP:N are calculated from first principles. As the more electronegative N bonds H stronger than P, it stabilizes the H  2 complex that is otherwise unstable against the formation of an H 2 molecule. This provides the first theoretical proof that H  2 can be stable in a III-V semiconductor. The previously proposed H-N-H dihydride model is found to be unstable against spontaneously transforming into H  2 , which involves only monohydrides, H-N and H-Ga. The calculated local vibrational frequencies and isotope shifts are in good agreement with experiment. DOI: 10.1103/PhysRevLett.88.125506 PACS numbers: 61.72.Ji, 61.72.Bb, 63.20.Pw Either intentional or unintentional, hydrogen is an important impurity in semiconductors [1]. Abundant unintentional H is often present during growth such as in metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) when hydrides are used as gas sources. Hydrogen not only interacts with host materials [2,3], but also with other impurities/defects, causing changes in the electronic properties [4]. Being a fast diffuser in semiconductors [5], atomic hydrogen may also be used to probe either electronically or structurally the various defect properties. On the other hand, nitrogen is an important first-row element known to strongly perturb and modify the electronic properties of conven- tional III-V semiconductors; e.g., isovalent doping of GaP by N leads to a quasidirect band gap with enhanced optical functionality [6]. Recently, dilute GaAsN alloys have also attracted a lot of attention due to the giant band-gap bowing effect [7] that opens up vast potentials in band-gap engineering without significantly affecting lattice matching to GaAs. Until recently, however, the interaction of hydrogen with nitrogen in III-V semiconductors has been very much unexplored, other than that hydrogen incorporation into GaP:N reduces the excitonic luminescence intensity [6]. An infrared study of this system revealed [8] at least three distinct local vibrational mode frequencies at 2885.5, 2054.1, and 1049.8 cm 21 . To explain the data, a H-N-H dihydride model of trigonal symmetry was proposed [8] where two H atoms are bonded directly to a N atom, as shown in Fig. 1(a). There are, however, several difficul- ties with this model. (1) The N is fivefold coordinated, which has never been observed in any nitride. (2) The two H stretching frequencies differ surprisingly by 830 cm 21 . In contrast, on the silicon (001) surface, the calculated difference in the stretching frequencies for Si-H 2 dihy- dride [9] is only 50 cm 21 , more than 1 order of magnitude smaller. Even for ammonia, the triplet splitting is only around 100 cm 21 . (3) The observed isotope shifts of the two high frequency modes are distinctly different: 5.4 and 1.7 cm 21 . From a more fundamental point of view, di- atomic H 2 complexes other than the H 2 molecules [10,11] have never been predicted by theory to be stable [12], nor have they been observed in any III-V semiconductors, even though in Si a H  2 complex is not only comparable in energy to the H 2 molecule [13] and has been observed [14], but it also plays an essential role in the nucleation and growth of the technologically important H platelets [3]. Using first-principles total energy calculations, we have investigated the various H structural configurations in GaP either associated with or without N. We find that nitrogen plays a pivotal role in the relative stability between the H  2 complexes and H 2 molecules. Without N, H  2 is 0.3 eV per H higher in energy than H 2 . With one N, however, H  2 is 0.26 eV per H lower than H 2 . For the typical experi- mental condition existing in Ref. [8], ´ F $ 0.5 eV, H  2 is FIG. 1. Atomic structures for the various diatomic H and N complexes in GaP:N, along with the calculated formation ener- gies (in eVH). The black atoms are P, the gray atoms are Ga, the white atoms are N, and the small gray atoms are H. 125506-1 0031-9007 02 88(12) 125506(4)$20.00 © 2002 The American Physical Society 125506-1