Workshop 23 Molecular Beam Epitaxy of (In,Ga) 2 on Th. Hahn J. Cieslak H. Metzner J. Eberhardt M. Müller U. Kaiser U. Reislöhner W. Witthuhn J. Kräußlich Universität Jena hahn@pinet.uni-jena.de R. Goldhahn F. Hudert Technische Universität Ilmenau In recent years impressive achievements concerning thin film solar cells based on chalcopyrite compound semiconductors have been reached, resulting in the realisation of several pilot plants for commercial exploitation. However, the best reported efficiencies for (In,Ga)(e,) 2 -based devices, although remarkably high, are still considerably lower as compared to single crystalline solar cells based on or GaAs. Here, the question arises, how far the polycrystalline nature of these materials limits their efficiencies. ngle crystalline materials, either obtained by single crystal or epitaxial growth, bear the possibility to check the beneficial or disadvantageous influence of grain boundaries and interfaces in these compounds. In recent years we have developed a process for the growth of the sulphide-based chalcopyrite system (In,Ga) 2 (CIG) on substrates via molecular beam epitaxy (MBE) from elemental sources. Fig. 1 depicts the bandgap energies of varioussemiconductors as a function of their lattice constants. As can be seen, the sulphide chalcopyrite system stands very close to the lattice constant of and thus bears the possibility of a monolithic integration of a direct semiconductor into technology. Via the adjustment of the Ga-content, the bandgap energies can be tuned over a wide range and the lattice mismatch to the employed substrate can be eliminated, as illustrated in Fig. 2-4. 28
FV PV-UNI-NETZ Workshop 23 Furthermore, apart from their unique optoelectronic properties, important information about the structural variations in chalcopyrite thin films can be drawn from epitaxial layers. As an example, a metastable ordering, called Autype ordering, was predicted theoretically, but it was not until epitaxial thin layers of In 2 were available that this metastability in chalcopyrites could be verified experimentally (Fig. ). On the other hand, as depicted in Fig. 6, MBE growth of Ga 2 always leads to the highly ordered chalcopyrite structure. Thus, through variation of the deposition parameters and compositions, MBE grown CIG offers the possibility for a wide, systematic variation of electronic, optical, and structural properties of photoactive layers in thin film devices. 4 AI 2 Zn GaN Bandgap (ev) 3 2 1 Ga 2 Gae 2 Ale 2 Zne GaP Cd In 2 InP GaAs lne 2 Ge Figure 1: Bandgap energies of common semiconductors as a function of their lattice constants..2.4.6.8 6. Lattice Constant (A) 29
Workshop 23 2 In 1 In 2 1 Figure 2: Rutherford backscattering spectra (RB) of CIG thin films with varying Ga-content reveal a high homogeneity of the epitaxial layers 1 (In,Ga) 2 Ga In Normalized Yield 1 1 Ga 2 Ga. 1. 1. 2. 2. 3. 3. Energy (MeV) 21
FV PV-UNI-NETZ Workshop 23 T = 29 K Figure 3: R / R (arb. units) Ga 2 (In (1-x) Ga x ) x x =.81 x =.68 Photoreflection (PR) measurements of CIG thin films with varying Ga-content. The arrows indicate the respective bandgap-energies at room temperature which varies continuously from 1.2 ev for pure In 2 x =.42 to 2.2 ev for pure Ga 2. 1. 2. 2. 3. Photon Energy (ev) 1.6 Figure 4: d (224) [Å] 1.8 1.6 1.4 d I 222 47% Lattice constants in CIG epitaxial layers as a function of the Ga-content x. Perfect lattice match to is achieved at x = 47%. 2 4 6 8 1 x [%] 211
Workshop 23 Figure : High-resolution transmission electron microscope image of pure Au-ordering in CIG. The metastable Auordering can be identified by the alternating light and dark grey cationic planes in [1]-direction. The inset to the image shows a simulation of the Au-ordering for comparison. 1nm [1] [11] 6 Figure 6: X-ray diffraction scan of the (22)/(24) reflexions of InGa 2 taken in Bragg-Brentano geometry. The splitting of the two reflexions is due to the tetragonal nature of the highly ordered chalcopyrite structure. Intensity [a.u.] 4 3 2 1 (22) CG(22) 46. 46. 47. 48. 48. 49. 49. 2Q [Degrees] CG(24) CG(24) 212