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a) a) b) b) Fig. 2. Reflectance and derivative of the transmittance spectra for a) N42N, b) N48N samples. The arrows point to the excitonic band gaps of the GaAs 1-x N x layers, and the dotted lines indicate GaAs band gap a) b) Fig. 3. PC spectra of GaAs 1−x N x 110 Fig. 4. Dark and illuminated I-V characteristics of the sample a) N42N, b) N48N Tabl. 2. Characteristics of the studied samples Sample J SC (mA/cm 2 ) V OC (V) P max (W) FF N42N 0.23 0.42 -1.71x10 -7 0.35 N48N 0.23 0.38 -1.41x10 -7 0.33 Our results are comparable with GaAsN homo-junction solar cells fabricated by CBE [7]. The short circuit current for the latter is bigger (8.99 mA/cm 2 ) but open circuit voltage and fill factor are 0.48 V and 0.43, respectively [7]. Conclusions In summary, we present in this study the results of studies on the layers of GaAs 1-x N x grown on n-type GaAs substrates by atmospheric pressure metal organic vapour phase epitaxy (APMOVPE). Using transmittance, reflectance, photocurrent measurements and empirical expression for E g a new nitrogen content for the studies samples was obtained. Using dark and illuminated I-V characteristics the main parameters of the Schottky contacts ( short-circuit current I sc , open circuit-voltage, V oc , and fill factor, FF) were determined. Obtained contacts are promising for solar cell application. References [1] Kondow M. et al.: Japan. J. Appl. Phys. 35, 1273 (1996). [2] Nakahara K. et al.: IEEE Photonics Technol. Lett. 10, 487 (1998). [3] Friedman D.J. et al.: J. Cryst. Growth 195, 409 (1998). [4] Kurtz S.R. et al.: 26th IEEE PVSEC (1997) 875. [5] Taliercio T., R. Intartaglia, B. Gil, P. Lefebvre, T. Bretagnon, U. Tisch, E. Finkman, J. Salzman, M.-A Pinault, M. Laugt and E. Tournie: Phys. Rev. B 69, (2004)073303. [6] Zhang Y., A. Mascarenhas, H.P. Xin, and C.W. Tu: Phys. Rev. B 63, (2001) 161303. [7] Suzuki H., K. Nishimura, T. Hashiguchi, K. Saito, B. Balasubramanian, S. Yamamoto, M. Inagaki, Y. Ohshita, N. Kojima and M. Yamaguchi: Fabrication of GaAsN homo-junction solar cells by chemical beam epitaxy, Photovoltaic Specialists Conference, 2008. PVSC ‘08. 33rd IEEE. Elektronika 6/2012
AP-MOVPE technology of AIIIBV-N heterostructures for photovoltaic applications Wojciech Dawidowski 1) , Beata Ściana 1) , Damian Pucicki 1) , Damian Radziewicz 1) , Jarosław Serafińczuk 1) , Magdalena Latkowska 2) , Marek Tłaczała 1) 1) Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology 2) Institute of Physics, Wrocław University of Technology Properties of dilute nitrides material system like large discontinuity of a conduction band (due to high electronegativity of nitrogen) [1], large band gap bowing coefficient [2], increased electron effective mass [3], large scattering rate (connected with a small radius of nitrogen atom in comparison with the others V group atoms) [4] provide a way to novel applications: telecommunications lasers [1], heterojunction bipolar transistors [5] and very efficient solar cells [6]. InGaAsN is a very promising, lattice matched to GaAs and In 0.5 Ga 0.5 P material with band gap about 1 eV. The control of indium and nitrogen amounts in the InGaAsN quaternary alloy offers the ability to tailor the value of band gap and positions of valence and conduction band edges. Main advantage of dilute nitrides is a large reduction of band gap caused by nitrogen introduction (about ~150 meV/% of N). The technology of GaAs is better developed and still much cheaper than indium phosphide and allow to fabricate much better Bragg reflectors (based on the high refractive index contrast GaAs/AlAs heterostructure) for laser applications. Moreover, this material system guarantees a good temperature performance of light emitters and a high internal quantum efficiency of solar cells. On the other hand, growth of InGaAsN quaternary alloy causes some technological problems. The most serious technological problem results from a large miscibility gap between GaAs and GaN binary alloys, which is connected with both a high electronegativity and a small radius of N atom. It complicates the growth of dilute nitrides– the incorporation of a small amount of nitrogen strongly deteriorates the optical properties of epilayers [3]. Material growth with higher nitrogen concentration would cause phase segregation (to GaN and GaAs) which could be a nonradiative recombination centres [7]. Introduction of nitrogen into GaAs or InGaAs crystalline lattice requires a low growth temperature (in comparison with GaN deposition), for MOVPE method it is below 600 ˚C. This technological limitation causes non-homogeneous distribution of the nitrogen atoms in epilayers and also favours to forming of various defects, which are the centres of non-radiative recombination and reduce a photoluminescence efficiency [8]. Split interstitials N-N and N-As, vacancies V Ga and V As , antisites As Ga are commonly occurred points defects in dilute nitrides. Another problem connected with the epitaxial growth is high concentration of impurities such as: carbon, oxygen and hydrogen. In MOVPE technique occurs unintentional doping by carbon, which is a component of all organometallic precursors. The main sources of hydrogen contamination are metaloorganic compounds and a carrier gas. Experiment The series of undoped InGaAsN/GaAs heterostructures were grown by AP-MOVPE (atmospheric pressure metal organic vapour phase epitaxy) technique with AIX 200 R&D Aixtron reactor on n-type and semi insulating (100) oriented GaAs substrates. The grown multi quantum well (MQW) structures consisted of 500 nm buffer, triple dilute nitrides quantum wells (about 15 nm) sandwiched between GaAs barriers (about 30 nm) and capped by 40…50 nm thick GaAs layer (~75 nm with last barrier). The typical MQW epitaxial structure is shown in Fig. 1. Trimethylindium (TMIn), trimethylgallium (TMGa), tertiarybutylhydrazine (TBHy) and 10% mixture of arsine (AsH 3 ) in H 2 were used as growth precursors. As a carrier gas was applied a high purity hydrogen. The growth temperature T g was changed in the range of 566 to 585˚C in order to optimize the properties of investigated epitaxial structures. For dilute nitrides the arsine and hydrogen (passed through the bubbler with nitrogen source – TBHy) flows were stable and equal 50 ml/min and 1500 ml/min, respectively. Low flow of AsH 3 ensured better incorporation of nitrogen into layer. The arsine flow during the epitaxy of GaAs barrier was equal 300 ml/min. Structural and optical properties of the obtained MQW structures were examined by high resolution X-ray diffraction (HRXRD), photoluminescence (PL) and contactless electro-reflectance spectroscopy (CER). Optical measurements were carried out at room temperature (RT). Fig. 1. Scheme of the typical MQW epitaxial structure Results The structural analysis based on XRXRD measurements of diffraction curves of the symmetric (004) reflection in ω/2θ geometry. Obtained curves for NI 43 and NI 46 samples are shown in Fig. 2. Keenness of peaks with highest intensity (these peaks represent GaAs substrate) suggests sharp interfaces between layers. The occurrence of secondary oscillations, also known as the Pendellösung oscillations allow to determine thickness of layers and their composition by comparison of the measured curves with simulations results. Complete set of parameters determined in that way is listed in Table 1. The indium content in the InGaAsN quantum wells is varied from 12.1 to 17% while the nitrogen amount does not exceed0.6%. Based on the data determined from XRXRD the temperature dependence of the efficiency of nitrogen incorporation into InGaAsN quantum wells was estimated. This relationship is shown in Fig. 3. The content of nitrogen decreases with increasing a growth temperature, as it was expected. From the measured PL and CER spectra the ground state energy (E GS ) in the investigated quantum wells was determined. The obtained values from the both mentioned methods are comparable and listed in Table 2. Elektronika 6/2012 111
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