High quantum efficiency of photoluminescence in GaN and ZnO
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High quantum efficiency of photoluminescence in GaN and ZnO M. A. Reshchikov,1 X. Gu,1 B. Nemeth,2 J. Nause,2 and H. Morkoç1 1 EE and Physics Dept., Virginia Commonwealth University, Richmond, VA 23284, U.S.A. 2 Cermet, Inc., Atlanta, GA 30318, U.S.A. ABSTRACT The quantum efficiency (QE) of photoluminescence (PL) has been estimated in GaN and ZnO samples. A Si-doped GaN layer grown by molecular beam epitaxy (MBE) exhibited the highest QE of about 90% at low temperatures. Recombination via the shallow donor-acceptor pair transitions dominated in this sample. In contrast, a bulk ZnO crystal with the QE of PL of about 85% contained almost no defect- or impurity-related PL signatures besides the emission attributed to free and bound excitons. The sources of radiative and nonradiative recombination in GaN and ZnO are discussed. INTRODUCTION The performance of opto-electronic devices, especially those operating in the visible and ultraviolet (UV) ranges of the optical spectrum, depends on the quantum efficiency (QE) of emission in general and photoluminescence (PL) in particular. Qualitative terms such as “very intense PL that confirms high quality of the material” are omnipresent in the literature. However, there have been very few attempts to estimate the absolute value of PL intensity or its QE. We estimated the QE of PL from GaN and ZnO crystals by direct and indirect methods [1]. METHOD The internal QE (η) of PL is a ratio between the power emitted in the form of photons and the total power of the laser light absorbed in the analyzed volume of a sample, i.e. η = IPL/ (IPL+ INR), where IPL and INR are intensities of PL and nonradiative recombination. We can also introduce QE for every recombination channel, ηi, as the ratio of the integrated PL intensity of this channel to the total laser power absorbed in the analyzed volume. η and ηi, are important characteristics of the material, which allow us to evaluate the crystal quality and analyze the complex processes taking place in a semiconductor in conditions of PL. Note, however, that the intensity of PL detected from different samples by devices is not equal and even not necessarily proportional to the internal QE. The external QE of PL can be estimated directly by measuring the power of the incident laser light and the power of PL collected from a sample by a lens with the correction for the geometry of the PL registration optics [1]. One can try to establish the internal QE from this value by accounting for the optical properties of the studied material and the geometry of the experiment [1]. In particular, the absorbed laser power is less than the incident power by the portion of the reflected light. The transmission of the 325 nm light through the 1 µm-thick GaN or ZnO layer can be ignored due to high absorption coefficient in this wavelength range. The PL is emitted equally in all directions inside the layer and refracts at the semiconductor surface according to its refraction index. Absorption of PL inside the layer can be ignored, at least for d
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