M. Fukuda

Probing the nature of carrier localization in GaInNAs epilayers by optical methods

Photoluminescence (PL), optical pumping, and reflectance studies of nominally undoped and p-type GaInNAs epilayers are presented. The PL peak energy of the nominally undoped sample exhibits an S-shaped dependence on temperature for T < 50 K. This is attributed to recombination of bound excitons localized on traps. The energy of the PL circular-polarization maximum coincides with the energy of the free-exciton related reflectance feature at all temperatures. In heavily p-type samples the S-shaped temperature-dependence of the PL energy disappears, and the PL peak and circular polarization maximum coincide with the reflectance feature at all temperatures, indicating that the PL is free-exciton-like.

Improved performance in GaInNAs solar cells by hydrogen passivation

The effect of UV-activated hydrogenation on the performance of GaInNAs solar cells is presented. A proof-of-principle investigation was performed on non-optimum GaInNAs cells, which allowed a clearer investigation of the role of passivation on the intrinsic nitrogen-related defects in these materials. Upon optimized hydrogenation of GaInNAs, a significant reduction in the presence of defect and impurity based luminescence is observed as compared to that of unpassivated reference material. This improvement in the optical properties is directly transferred to an improved performance in solar cell operation, with a more than two-fold improvement in the external quantum efficiency and short circuit current density upon hydrogenation. Temperature dependent photovoltaic measurements indicate a strong contribution of carrier localization and detrapping processes, with non-radiative processes dominating in the reference materials, and evidence for additional strong radiative losses in the hydrogenated solar cells.

Photoluminescence study of Be-acceptors in GaInNAs epilayers

We have studied Be-acceptors in a p-type GaInNAs epilayer using magneto-luminescence spectroscopy. The band edge photoluminescence (PL) spectra at T = 7 K contain two features: the first is associated with the free exciton while the second with the conduction band to acceptor (CB  →  A) transition. The intensity of the latter decreases with increasing temperature while the excitonic feature survives up to T = 250 K. From the energies of the two PL features, as well as the exciton binding energy in GaInNAs, we determined the Be-acceptor binding energy to be equal to 42 meV. The energy of the CB  →  A feature varies linearly with magnetic field B and has a slope of 5.5×10−4 eV/T.

The role of N-H complexes in the control of localized center recombination in hydrogenated GaInNAs (Conference Presentation)

A significant improvement in the quality of dilute nitrides has recently led to the ability to reveal depletion widths in excess of 1 μm at 1 eV [1]. The real viability of dilute nitrides for PV has been recently demonstrated with the reporting of a record efficiency of 43.5% from a 4J MJSC including GaInNAs(Sb) [2]. Despite the progress made, these materials remain poorly understood and work continues to improve their lifetime and reproducibility. We have investigated the possibility of improving the functionality of GaInNAs using hydrogenation to selectively passivate mid-gap defects, while preserving the substitutional nitrogen. Temperature dependent photoluminescence measurements of the intrinsic region of a GaInNAs p-i-n solar cell show a classic “s-shape” associated with localization prior to hydrogenation. No sign of this “s-shape” is evident after hydrogenation, despite the retention of substitutional nitrogen as evidenced by the band absorption of 1 eV. The absence of an “s-shape” at low-temperature in hydrogenated GaInNAs is rather curious since, even in high quality nitrides this behavior is due to the emission of isoelectronic centers created via N-As substitution [3]. The potential origins of this behavior will be discussed. The promise of this process for GaInNAs solar cells was demonstrated by a three-fold improvement in the performance of a hydrogenated device with respect to an as-grown reference [4]. [1] “Wide-depletion width GaInNAs solar cells by thermal annealing,” I. R. Sellers, W-S. Tan, K. Smith, S. Hooper, S. Day and M. Kauer, Applied Physics Letters 99, 151111 (2011) [2] “43.5% efficient lattice matched solar cells,” M. Wiemer, V. Sabnis, and H. Yuen, Proc. SPIE 8108, 810804 (2011) [3]“Probing the nature of carrier localization in GaInNAs, epilayers using optical methods,” T. Ysai, B. Barman, T. Scarce, G. Lindberg, M. Fukuda, V. R. Whiteside, J. C. Keay, M. B. Johnson, I. R. Sellers, M. Al Khalfioui, M. Leroux, B. A. Weinstein and A. Petrou. Applied Physics Letters 103, 012104 (2013) [4] “Improved performance in GaInNAs solar cells by hydrogen passivation by hydrogen passivation,” M. Fukuda, V. R. Whiteside, J. C. Keay, A. Meleco, I. R. Sellers, K. Hossain, T. D. Golding, M. Leroux, and M. Al Khalfioui, Applied Physics Letters 106, 141904 (2015)

Improved performance of GaInNAs solar cell after UV-activated hydrogenation

While GaInNaAs has been earmarked as a possible candidate [1] for a 1 eV solar cell material in a multi-junction solar it has not made a lot of traction. This is due primarily to serious material limitations that arise from the non-ideal growth conditions that are necessary to incorporate nitrogen while preventing phase segregation. There has been some success, most notably Solar Junctions world record power conversion efficiency of a solar cell with a GaInNAsSb cell [2]. Yet, there remains pertinent questions concerning lifetime, reproducibility, and yield in order for this material to become widely used as part of a commercial solar cell. Previously, we reported on the improved photoluminescence of GaInNAs materials which had been achieved through the selective passivation of defect centers. Now, we present results that show that GaInNAs solar cells can be improved using this UV-activated hydrogenation process. Hydrogenated solar cells show a greater than two times improvement with respect to reference cells in terms of short circuit current and external quantum efficiency.