JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 2, JANUARY 15, 2010 223 Conversion of Direct to Indirect Bandgap and Optical Response of B Substituted InN for Novel Optical Devices Applications Bin Amin, Iftikhar Ahmad, and Muhammad Maqbool Abstract—Optical properties of B In N are calculated as a function of the varying concentration of Boron and Indium. Indium is gradually replaced by Boron and optical properties of the resulting materials are studied. The fractional concentration of Boron is increased gradually from to in steps of 0.25. The bandgap increases with the increasing Boron concentration, from 0.95 eV for pure InN to 5.6 eV for BN. A unique behavior of BN in zinc-blend phase is observed, that is, it shifts from indirect to direct bandgap semiconductor by the substitution of In on B sites. This behavior can be used to make novel and advanced optical devices. Frequency dependent reflectivity, absorption coefficient, and optical conductivity of B In N are calculated and found to be the constituent’s concentration dependent. The region of reflectivity, absorption coefficient and optical conductivity shifts from lower frequency into the higher frequency as the material goes from pure InN to pure BN. Index Terms—Boron, indium, optical materials, optical proper- ties. I. INTRODUCTION N ITRIDE semiconductor thin filmshave received consider- able attention primarily for their use in semiconductor and photonic devices. Recent progress toward nitride-based light- emitting diode and electroluminescent devices (ELDs) has been made using crystalline and amorphous gallium nitride (GaN), aluminum nitride (AlN) and boron nitride (BN) doped with a variety of rare-earth elements [1]–[12]. In order to meet the de- mands arising from the rapidly growing field of information pro- cessing it is very important to understand the fundamental prop- erties of these materials over a wide wavelength range. Accurate knowledge of the reflectivity, absorption coefficients and optical conductivity of nitride semiconductors is indispensable for the design and analysis of various optoelectronic devices. These op- tical properties reflect essentially the density of states and thus their analysis is one of the most effective tools for understanding the electronic structure of these solids [13]–[17]. Recently, there has been a discussion of the optical bandgap of crystalline indium nitride (InN). Our finding of the photolu- minescence (PL) measurement show that the bandgap energy of crystalline InN is between 0.65 and 0.90 eV [4], [5], which Manuscript received July 16, 2009; revised September 23, 2009. First pub- lished October 13, 2009; current version published January 15, 2010. B. Amin and I. Ahmed are with the Department of Physics, Hazara University, Mansehra NWFP, Pakistan. M. Maqbool is with the Department of Physics and Astronomy, Ball State University, Muncie, IN 47306 USA (e-mail: mmaqbool@bsu.edu). Digital Object Identifier 10.1109/JLT.2009.2034027 is much smaller than the previously accepted value 1.9 eV [6]. Butcher suggested that the origin of the different measured bandgap values in crystalline InN is due to Mott—Burstein effect [16]. States in the conduction band are filled with free carriers when the free carrier concentration is high, so that the first empty state available for an optical transition is larger than the intrinsic bandgap value. So a bandgap larger than approximately 0.9 eV is an indication of a large free carrier concentration. The reported wide bandgap of BN ( eV), is of particular interest in studying the ultraviolet (UV) applications of phos- phors [18]. Thus, codoping In and B in a single nitride film, can reveal interesting results about the bandgap and optical proper- ties of such materials. In the present work, the bandgap and the optical properties of B In N and the effect of variation in the concentration of In and B on these properties are studied. The varying concentra- tions of wide bandgap BN and narrow bandgap InN play signif- icant role in bandgap engineering and can be exploited to make various optical and photonic devices. The entire work is per- formed to obtain the values of respective parameters at lowest energy levels. II. METHOD AND CALCULATIONS One of the most accurate scheme for solving Kohan-Sham equations is the full potential linearized augmented plane wave (FP-LAPW) suggested by Andersen [19]. The method is based on the first-principle density-functional theory with the generalized gradient approximation (GGA) [20]. The details of FP-LAPW calculations, formulas and the wien2k code used in the present investigations are reported by Schwarz and Blaha et al. [21], [22]. In the present calculations, a muffin-tin (MTA) is used. The potential inside muffin-tin is spherically symmetric while outside is constant. The core electrons are treated rela- tivistically and the valence ones are semi-relativistically [23]. Inside the sphere of muffin-tin wave function is expanded in the basis of spherical harmonics , while in the interstitial region in the plane wave basis. On the basis of convergence tests on our response functions with a varying number of k points we are confident that 3500 k points and basis functions up to ensures an accurate and well converged result [24]. The muffin-tin radii, for B, In and N are 1.47, 1.41 and 1.41 a.u respectively. We used a mesh of 3500 K points and in the first brillouin zone integration in the corresponding irreducible wedge. 0733-8724/$26.00 © 2010 IEEE