Silicon-germanium films deposited by low-frequency plasma-enhanced chemical vapor deposition: Effect of H 2 and Ar dilut
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T.E. Felter Lawrence Livermore National Laboratory, Livermore, California 94550
R. Asomoza and Y. Kudriavtsev Centro de Investigacio´n y de Estudios Avanzado del Instituto Politecnico Nacional, Mexico, D.F. 07360
R. Silva-Gonzalez Instituto de Fisica, Benemerita Universidad Autonoma de Puebla, Puebla, Pue., C.P. 72570, Mexico
E. Gomez-Barojas CIDS-IC, Benemerita Universidad Autonoma de Puebla, Puebla, Pue. C.P. 72000, Mexico
A. Ilinski Benemerita Universidad Autonoma de Puebla, Puebla, Puebla 72050, Mexico
A.S. Abramov A.F. Ioffe Phys.-Technical Institute, St.-Petersburg 194021, Russia (Received 21 December 2004; accepted 6 July 2005)
We have studied structure and electrical properties of Si1−YGeY:H films deposited by low-frequency plasma-enhanced chemical vapor deposition over the entire composition range from Y ⳱ 0 to Y ⳱ 1. The deposition rate of the films and their structural and electrical properties were measured for various ratios of the germane/silane feed gases and with and without dilution by Ar and by H2. Structure and composition was studied by Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), and Fourier transform infrared (FTIR) spectroscopy. Surface morphology was characterized by atomic force microscopy (AFM). We found that the deposition rate increased with Y, maximizing at Y ⳱ 1 without dilution. The relative rate of Ge and Si incorporation is affected by dilution. Hydrogen preferentially bonds to silicon. Hydrogen content decreases for increasing Y. In addition, optical measurements showed that as Y goes for 0 to 1, the Fermi level moves from mid gap to the conduction band edge; i.e., the films become more n-type. No correlation was found between the pre-exponential and the activation energy of conductivity. The behavior of the conductivity ␥-factor suggests a local minimum in the density of states at E ≈ 0.33 eV for the films grown with or without H-dilution and E ≈ 0.25 eV for the films with Ar dilution.
I. INTRODUCTION
Silicon-germanium alloys (Si1−YGeY:H) are used as a low-bandgap material in multi-junction amorphous silicon solar cells1–3 and are of much interest for other applications, e.g., un-cooled micro-bolometers3–4 and in un-cooled bolometers.5–7 Jordan et al.8 have very recently studied a nanocrystalline Ge p-i-n structure. Despite implementation in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0013 88
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 1, Jan 2006 Downloaded: 14 Mar 2015
device structures, Si1−YGeY:H films have been significantly less studied and their electronic properties are poorly understood in comparison to those for amorphous silicon. Reviews of earlier work on Si1−YGeY:H films can be found in Refs. 9 and 10. Although radio frequency (rf; f ⳱ 13.56 MHz) plasma-enhanced chemical vapor deposition (PECVD) remains the major technique for fabrication of Si1−YGeY:H films, alternative methods, such as, electron-cyclotron resonance (ECR) plasma deposition11 and non-plasma hot
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