Correlations between spatially resolved Raman shifts and dislocation density in GaN films

Spatially resolved Raman spectra were measured on thick GaN samples with known dislocation density grown by hydride vapor phase epitaxy. The frequencies of the E-2 (high) and E-1 (transverse optical) phonons shift to lower wave number over a distance of 30 mum from the sapphire substrate/GaN interface. The shifts are linearly correlated with the dislocation density suggesting that the strain due to the lattice mismatch at the interface determines both quantities

Electron beam and optical depth profiling of quasibulk GaN

Electron beam and optical depth profiling of thick (5.5-64 mu m) quasibulk n-type GaN samples, grown by hydride vapor-phase epitaxy, were carried out using electron beam induced current (EBIC), microphotoluminescence (PL), and transmission electron microscopy (TEM). The minority carrier diffusion length, L, was found to increase linearly from 0.25 mu m, at a distance of about 5 mu m from the GaN/sapphire interface, to 0.63 mu m at the GaN surface, for a 36-mu m-thick sample. The increase in L was accompanied by a corresponding increase in PL band-to-band radiative transition intensity as a function of distance from the GaN/sapphire interface. We attribute the latter changes in PL intensity and minority carrier diffusion length to a reduced carrier mobility and lifetime at the interface, due to scattering at threading dislocations. The results of EBIC and PL measurements are in good agreement with the values for dislocation density obtained using TEM

Two-photon Absorption In Direct Bandgap Semiconductors Quantum Dots

We present degenerate and nondegenerate two-photon absorption spectra in a series of CdSe and CdTe quantum dots. The measurements show that the two-photon absorption (2PA) spectrum is strongly dependent on the quantum dot size and that the 2PA coefficient decreases as the quantum dot size decreases, and it is larger for the frequency nondegenerate process. Previously we had shown a theoretical analysis of these results based on a simple model using the effective mass approximation. Although this model works well for larger quantum dots, it fails for the smaller ones. Here we use the more realistic k→ p→ model for the band structure and consider the hole band mixing in quantum dots to describe our data. This theory better describes the spectral structures for smaller quantum dots and also predicts the decrease of the 2PA coefficient with the decrease of quantum dot size. This is due to the reduction of the number of possible transitions and the blue shift of the optical bandgap from quantum confinement. This theory predicts the reduction of the 2PA coefficient with size, although our experimental results show an even stronger reduction.6327Larson, D.R., Zipfel, W.R., Willians, R.M., Clark, S.W., Bruchez, M.P., Wise, F.W., Webb, W.W., (2003) Science, 300, pp. 1434-1436Sargent, E.H., (2005) Adv. Matt., 17, pp. 515-522Padilha, L.A., Neves, A.A.R., Rodriguez, E., Cesar, C.L., Barobosa, L.C., Cruz, C.H.B., (2005) Appl. Phys. Lett., 86, pp. 1611111-1611113Uskov, A.V., O'Reilly, E.P., Manning, R.J., Webb, R.P., Cotter, D., Laemmlin, M., Ledentsov, N.N., Bimberg, D., (2004) IEEE Phot. Tech. Lett, 16, pp. 1265-1267Cerletti, V., Coish, W.A., Gywat, O., Loss, D., (2005) Nanotech., 16, pp. R27-R49Sercel, P.C., Vahala, K.J., (1990) Phys. Rev. B, 42, pp. 3690-3710Cotter, D., Burt, M.G., Manning, R.J., (1992) Phys. Rev. Lett., 68, pp. 1200-1203Seo, J.T., Yang, Q., Creekmore, S., Temple, D., Qu, L., Yu, W., Wang, A., Kim, J.H., (2003) Phys. E, 17, pp. 101-103Banfi, G.P., Degiorgio, V., Ricard, D., (1998) Adv. Phys., 47, pp. 447-510Padilha, L.A., Fu, J., Hagan, D.J., Van Stryland, E.W., Cesar, C.L., Barbosa, L.C., Cruz, C.H.B., (2005) Opt. Exp., 13, pp. 6460-6467Padilha, L.A., Fu, J., Hagan, D.J., Van Stryland, E.W., Cesar, C.L., Barbosa, L.C., Cruz, C.H.B., (2005) Proc. SPIE, 5931, pp. 226-235Fedorov, A.V., Baranv, A.V., Inoue, K., (1996) Phys. Rev. B, 54, pp. 8627-8632Sheik-Bahae, M., Said, A.A., Wei, T.H., Hagan, D.J., Van Stryland, E.W., (1990) IEEE J. of Quantum Electron., 26, pp. 760-769Negres, R.A., Hales, J.M., Kobyakov, A., Hagan, D.J., Van Stryland, E.W., (2002) IEEE J. Quantum Electron., 38, pp. 1205-1216Hales, J.M., Hagan, D.J., Van Stryland, E.W., Schafer, K.J., Morales, A.R., Belfield, K.D., Pacher, P., Bredas, J.L., (2004) J. Chem. Phys., 121, pp. 3152-3160Barbosa, L.C., Reynoso, V.C.S., De Paula, A.M., De Oliveira, C.R.M., Alves, O.L., Craievich, A.F., Marotti, R.E., Cesar, C.L., (1997) J. Non-cryst. Solids, 219, pp. 205-211Yu, W.W., Qu, L.H., Guo, W.Z., Peng, X.G., (2003) Chem. Mater., 15, p. 2854Bunge, S.D., Krueger, K.M., Boyle, T.J., Rodriguez, M.A., Headley, T.J., Colvin, V.L., (2003) J. Mater. Chem., 13, p. 1705Qu, L.H., Peng, X.G., (2002) J. Am. Chem. Soc., 124, p. 2049L.A. Padilha, J. Fu, D.J. Hagan, E.W. Van Stryland, C.L. Cesar, L.C. Barbosa, C.H.B. Cruz, D. Buso, and A. Martucci, to be published (2006)Kane, E.O., Semiconductors & Semimetals, 1. , Cap. 3Ekimov, A.I., Hache, F., Schanne-Klein, M.C., Richard, D., Flytzanis, C., Kudryavtsev, I.A., Yazeva, T.V., Efros, Al.L., (1993) J. Opt. Soc. Am. B, 10, pp. 100-10

Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots

We examine the multiple exciton population dynamics in PbS quantum dots by ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We simultaneously probe the first three excitonic transitions over a broad spectral range. Transient spectra show the presence of first order bleach of absorption for the 1S_h-1S_e transition and second order bleach along with photoinduced absorption band for 1P_h-1P_e transition. We also report evidence of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures of carrier multiplication (multiexcitons for the single absorbed photon) from analysis of the first and second order bleaches, in the limit of low absorbed photon numbers (~ 10^-2), at pump energies from two to four times the semiconductor band gap. The multiexciton generation efficiency is discussed both in terms of a broadband global fit and the ratio between early- to long-time transient absorption signals.. Analysis of population dynamics shows that the bleach peak due to the biexciton population is red-shifted respect the single exciton one, indicating a positive binding energy.Comment: 16 pages, 5 figure

Enhancement Of Minority Carrier Transport In Forward Biased Gan P-N Junction

The electron beam induced current (EIBC) effects in GaN p-n junctions were demonstrated. The possible impact of EIBC on heterojunction bipolar transistors was also discussed. The results showed an enhanced minority electron diffusion in the forward biased junctions. Analyzation revealed that electron minority diffusion length increases due to electron beam irradiation

Two-Photon Absorption In Core-Shell And Core-Only Semiconductor Quantum-Dots

We report experimental studies of two-photon absorption spectra of CdSe and CdSe/ZnS core/shell quantum-dots. The influence of the ZnS shell on the two-photon absorption is observed and discussed. © 2007 Optical Society of America

Nonlinear Optical Characterization Of Organic Molecules Using A White-Light Continuum Z-Scan

We report measurements of nonlinear refraction and nonlinear absorption spectra for organics molecules, in a range from 450nm to 800nm using a white-light-continuum Z-scan. The results show the nonlinear refraction changes sign. © 2007 Optical Society of America