Photoluminescence and Bandgap Narrowing in Heavily Doped n -GaAs
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PHOTOLUMEXIESCENCE AND BANDGAP iN HEAVILY DOPED n-GaAs NARROWENG H.D. Yao* Center for Microelectronic and Optical Materials Research, and Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511 A. Compaan Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606
ABSTRACT Extremely heavily doped n-GaAs was produced by pulsed-laser annealing of Si implanted GaAs, achieving carrier concentrations exceeding 3.2x1019/cm3. Our photoluminescence (PL) spectra indicate a bandgap narrowing due to heavy ntype doping with a functional form of A•g(eV)=- 6.3 x 10"8In(era-3)]1/3. At the highest carrier concentration, the bandgap shrinkage reaches -200 meV and the electron Fermi energy is -410 meV. These large values indicate that there exists a considerable conduction band "stretch" between the F and L-valley of GaAs for very high n-type concentrations.
LNTRODUCTION In heavily doped n-type GaAs the valence and conduction bands are shifted in energy due to the presence of the ionized dopant ions and free carriers. Quantitative analysis of the bandgap shifts is an arei• of active investigation theoretically [1,2] and experimentally [3,4]. We report here the results of our photoluminescence (PL) measurements of bandgap narrowing due to heavy ntype doping. In this work we have taken advantage of the technique of ion implantation followed by pulsed laser annealing to extend the range of n-type dopant densities from the conventional limit of -7 x 1018 cm3 to beyond 3.2 x 1019 cm3. Pulsed-YAG-dye-laser annealing (X=728 nm) of Si-implanted GaAs was used for sample preparation. The carrier concentrations were estimated from the positions of the Raman-plasmon mode (L+) with inclusion of the effective mass changes due to the nonparabolic effects of the conduction band. The pulsed laser annealing procedure and the results of Raman scattering are reported separately [5]. i
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* This work was performed while this author was at the University of Toledo, Toledo, Ohio 43606.
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EXPERIMENTAL PL spectra were obtained by a 514.5 nm probe beam with a power of -200 mW. The samples were mounted in a cryogenic dewar and cooled by liquid N 2. Fig.1 shows a composite plot of the PL spectra from a series of GaAs samples with electron concentrations ranging from undoped to -3.2x101 9 /cm 3 . The top three spectra were prepared by pulsed dye-laser annealing of Si-implanted GaAs. These three upper spectra were obtained with a photomultiplier tube (PMT) of S1 response (Hamamatsu R632). The lower two spectra are from an undoped GaAs sample and from a sample conventionally doped with 5x10 17 /cm 3 Si and were obtained with a GaAs PMT (Hamamatsu R943-02). Each spectrum has been corrected to remove the spectrometer-PMT efficiency effects. The width of the PL band increases monotonically as the carrier concentration increases. This is a consequence of the increasing Fermi energy as electron concentration rises. In addition, the lower edge of the PL peak shifts towards lower energy as electron concentra
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