Sains Malaysiana 40(1)(2011): 59–62

Optical Constants and Electronic Transition in Hydrogenated Silicon (Si:H) Thin Films Deposited by Layer-by-Layer (LBL) Deposition Technique

(Pemalar Optik dan Peralihan Elektronik dalam Filem Nipis Silikon Terhidrogen (Si:H) disediakan melalui Teknik Mendapan Lapisan Demi Lapisan)

Goh Boon Tong*, Muhamad Rasat Muhamad & Saadah Abdul Rahman

Solid State Research Laboratory, Department of Physics

University of Malaya, 50603 Kuala Lumpur, Malaysia

Received: 7 December 2009 / Accepted: 13 July 2010

ABSTRACT

Optical constants derived from optical transmission (T) and reflectance (R) spectra in the wavelength range of 220 to 2200 nm are presented in this paper for hydrogenated silicon (Si:H) thin films deposited by plasma enhanced chemical vapor deposition (PECVD) using the layer-by-layer (LBL) deposition technique. The films were deposited on quartz substrate by decomposition of SiH₄ and H₂ gases at flow-rate of 5 sccm and 20 sccm, respectively. The substrate temperature, deposition pressure and deposition rate are 100°C, 0.8 mbar and 2.8 nm/s, respectively. The as-prepared films were annealed in nitrogen for one hour at annealing temperatures of 400°C, 600°C, 800°C and 1000°C. The as-prepared film thickness of 301 nm decreased to 260 nm when samples were annealed at 1000°C. The refractive indices (~ 3.0 to 3.4) of annealed films were determined from the interference fringes of transmission spectrum following Manifacier and Davies methods. The electronic transition from valence band to conduction band in these films are characterized from the optical energy gap; E_G (~1.64 to 2.41 eV), the dispersion energy; Ed (~26.4 to 34.0 eV) and the oscillator strength; Eo (~2.8 to 3.2 eV). It is interesting to note that EG is lowest for the films annealed at temperature of 600°C which has the lowest hydrogen content, CH in the film. Evidence of the presence of nanocrystallites formed in amorphous matrix is also observed for the films annealed at temperatures above 600°C.

Keywords: layer-by-layer; optical constants; Si:H film

ABSTRAK

Pemalar optik diterbitkan daripada spektrum pancaran (T) dan pantulan (R) optik dalam julat panjang gelombang 220 nm ke 2200 nm dibentangkan dalam kertas ini. Filem nipis silikon berhidrogen (Si:H) dimendapkan dengan sistem pemendapan wap kimia secara peningkatan plasma (PECVD) menggunakan teknik lapisan demi lapisan (LBL). Filem nipis dimendapkan di atas substrak kuartz daripada pemisahan komposisi gas SiH4 dan H2 pada kadar aliran 5 sccm dan 20 sccm masing-masing. Suhu substrak, tekanan pemendapan dan kadar pemendapan adalah 100°C, 0.8 mbar dan 2.8 nm/s masing-masing. Filem asal yang terhasil dipanaskan dalam suhu 400°C, 600°C, 800°C dan 1000°C dalam aliran nitrogen selama satu jam. Ketebalan filem asal berkurang daripada 301 nm ke 260 nm apabila filem nipis dipanaskan sehingga 1000°C. Indeks biasan (~ 3.0 to 3.4) bagi filem nipis yang telah dipanaskan ditentukan daripada pingiran interferen spektrum pancaran berdasarkan kaedah Manifacier and Davies. Transisi elektronik dari jalur valen ke jalur konduksi dalam filem nipis ini dicirikan daripada jurang tenaga optik; E_G (~1.64 to 2.41 eV), tenaga sebaran; Ed (~26.4 to 34.0 eV) and kekuatan pengayun; Eo (~2.8 to 3.2 eV). Adalah sangat penting untuk menunjukan bahawa EG paling rendah pada filem nipis yang telah dipanaskan pada suhu 600°C yang mempunyai kandungan hidrogen paling rendah. Bukti kewujudan nanohablur terbentuk di dalam amorfus matrik boleh diperhatikan dalam filem nipis yang dipanaskan pada suhu lebih 600°C.

Kata kunci: Filem nipis Si:H; lapisan-demi-lapisan; pemalar optik

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*Corresponding author; email: boontong77@yahoo.com