IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 7, NO. 2, MARCH2017 581 Photoluminescence Spectra of Moderately Doped, Compensated Silicon Si:P,B at 79–300 K AnYao Liu, Hieu T. Nguyen, and Daniel Macdonald Abstract—Photoluminescence (PL) spectra from moderately doped, compensated silicon with boron and phosphorus concentra- tions on the order of 10 16 - 10 17 cm -3 , which is representative of the low-cost upgraded metallurgical grade silicon potentially used for photovoltaics, are presented and explained. At 79 K, the cap- tured PL spectra from the compensated silicon reveal the presence of the following radiative recombination channels in the material: band-to-band recombination, recombination through a single neu- tral dopant (phosphorus or boron), and recombination from the neutral donors (phosphorus) to the neutral acceptors (boron), i.e., the D o –A o pair recombination. The D o –A o pair luminescence peaks are found to appear as rather broad in the measured PL spectra from compensated silicon at 79 K. The relative PL intensity of the broad dopant features is shown to increase with increasing dopant concentrations. The dopant-related luminescence of the compen- sated silicon demonstrates strong excitation dependence, as a result of the D o –A o recombination channel in compensated silicon. The dopant features become increasingly suppressed at higher excita- tions due to the increasing dominance of the band-to-band recom- bination channel. With increasing temperature, the dopant-related luminescence features diminish and become undistinguishable at around 200 K, due to the increased ionization of dopants and the broadening of the band-to-band recombination peaks at higher temperatures. Index Terms—Doping, photoluminescence (PL), silicon, spec- troscopy. I. INTRODUCTION U PGRADED metallurgical-grade silicon (UMG-Si) feed- stock is a promising low-cost alternative to the electronic grade silicon feedstock used for photovoltaic applications [1]– [7]. Silicon solar cells with efficiency above 20% were recently shown to be possible using entirely UMG-Si [7]. Due to the nongaseous purification process used for UMG-Si, the material contains a larger concentration of impurities, particularly un- wanted shallow dopants of both types, resulting in a strongly compensated silicon material. UMG silicon usually contains both boron and phosphorus atoms of moderate doping concen- trations, on the order of 10 16 [5]–[7] to 10 17 cm −3 [2]. This work focuses on studying moderately doped, compensated silicon us- ing photoluminescence spectroscopy (PLS). Manuscript received October 10, 2016; revised November 28, 2016 and De- cember 12, 2016; accepted December 12, 2016. Date of publication January 16, 2017; date of current version February 16, 2017. This work was supported by the Australian Renewable Energy Agency (ARENA) through project RND009. The authors are with the Research School of Engineering, Australian National University, Canberra, A.C.T. 2601, Australia (e-mail: anyao.liu@anu.edu.au; hieu.nguyen@anu.edu.au; daniel.macdonald@anu.edu.au). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2016.2641302 PLS is a powerful characterization technique enabling de- tailed studies of the material properties, and has been extensively used in the literature for decades to extract both the fundamen- tal semiconductor properties and properties of the luminescent defects and impurities in the bandgap [8], [9]. By using PLS to study compensated silicon materials, we can examine not only the dopant properties (from the sub-bandgap defect lumines- cence) but the effect of the compensated dopants on the silicon bandgap [from the band-to-band (BB) luminescence] as well. Photoluminescence from compensated silicon codoped with boron and phosphorus was first studied by Enck and Honig [10]. Since then, however, only a few papers have been published on the PL spectra of compensated silicon [9], [11]–[17]. Most of these works were performed at the liquid helium temperature of 4.2 K, and some were focused on more heavily doped com- pensated silicon. There still lacks a comprehensive description of the luminescence spectra from moderately doped solar-grade compensated silicon, particularly over the more moderate tem- perature range of 79–300 K, which is of relevance for PL-based characterization of silicon materials for photovoltaics [9], [18]– [20]. Possible applications include the developments of nonde- structive contactless material identification and dopant quantifi- cation techniques. In this paper, we present PL spectra from moderately doped, compensated silicon, measured at a wide range of excitation power intensities, at temperatures from 79 to 300 K, and for samples of different doping concentrations in the range 4 × 10 16 − 10 17 cm −3 . Comparison was made with the PL spectra from intrinsic silicon and uncompensated silicon with similar boron or phosphorus concentrations, which allows the dominant luminescence features of the compensated silicon to be identified in terms of their physical origins. II. EXPERIMENTAL METHODS The compensated silicon wafers used in this study were from a single-crystalline Czochralski-grown silicon (Cz-Si) ingot with intentional doping of both boron and phosphorus atoms. The boron and phosphorus concentrations of some wafers along the ingot were measured by secondary ion mass spectrometry, which, together with the Scheil equation, allowed an estima- tion of the boron and phosphorus concentrations in all samples along the ingot (see [21] for details). The compensated Si sam- ples examined in this work had doping from [B]= 4.7 × 10 16 and [P]= 4.2 × 10 16 cm −3 , i.e., p-type Si at room temperature, to [B]= 7.2 × 10 16 and [P]= 1.2 × 10 17 cm −3 , i.e., n-type Si. 2156-3381 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.