Strain Effects in GaN on Sapphire: Towards a Quantitative Comprehension
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O.BRIOT, J.P. ALEXIS, B.GIL and R.L.AULOMBARD CINRS-GES 5 -France
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Universitd de Montpellier II, C.C. 074 Place E.Bataillon, 34095 Montpellier Cedex:
ABSTRACT We present here a detailed investigation of the optical properties of GaN epilayers grown by low pressure MOVPE on sapphire substrates, using 2K photoluminescence as well as 2K reflectance spectroscopy. A large series of samples has been grown under eclectic conditions (V/IlI ratio, growth temperature, nitridation steps,....) which allows us to propose the first semi-quantitative investigation of the sample-dependent band gap energies. This dependence of the bandgap is analyzed in terms of i) residual stress, ii) exciton parameters and iii) deformation potential characteristics of the A, B, C excitons. the residual stress cannot be simply explained in terms of the differences between the thermal expansion coefficients of the various compounds, but are strongly correlated to the V/III growth ratio imposed during the growth, and subsequently to the influence of this parameter at the scale of electronic and structural characteristics of the deposited layers.
CRYSTAL GROWTH AND EXPERIMENTS Spectroscopic data and their analysis were taken from GaN epilayers grown by MOVPE on sapphire. A thin GaN buffer layer was grown at low temperature (.-6000 C) on the nitridated substrate and recrystallized before to proceed to the high temperature ( between 980°C and 1010°C) growth of the GaN epilayers having device-type electronic and structural qualities [1]. The precursors we used are NH3 and TEGa. Nomarski microscopy technique was used to assess the surface morphology, and electrical characterization was made via room temperature Hall measurements. Details about this has been given in proceedings of preceding conferences [2,3] and in the present volume [4]. Photoluminescence spectra result of sample excitation by the 325 nm of a HeCd laser while reflectivity features were taken using a 100W tungsten wire lamp. All optical information was analyzed behind a TI-R Jobin Yvon 1.5m focal length equipped with a 1200 grooves/mm grating blazed at 250 nm. RESULTS AND DISCUSSION A typical photoluminescence spectrum is given on figure 1 together with three reflectivity features. We note a sample-dependent energy position of both photoluminescence (PL) and reflectance (R) structures. The stokes-shift between PL and R is deduced from a lineshape analysis of the R spectra, made following the method given in [2], and is found to be 5.8 meV within a scattering of 15% in the data. This moderate value is experimental evidence of the dominating extrinsic recombination processes in these samples. A weak residual intrinsic PL is also observed lying on the high energy wing of the PL peak. Selection rules investigations [5] have shown that we deal with a donor bound exciton where according to Pauli's uncertainty principle, the two electrons of the complex form a F1 state and combine with a F'9 hole [6]. 411 Mat. Res. Soc. Symp. Proc. Vol. 395 ©01996 Materials Research Society
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