Metal diffusion barriers for GaAs solar cells.

In this study accelerated ageing testing (AAT), J-V characterization and TEM imaging in combination with phase diagram data from literature are used to assess the potential of Ti, Ni, Pd and Pt as diffusion barriers for Au/Cu-based metallization of III-V solar cells. Ni barriers show the largest potential as at an AAT temperature of 250 °C both cells with 10 and 100 nm thick Ni barriers show significantly better performance compared to Au/Cu cells, with the cells with 10 nm Ni barriers even showing virtually no degradation after 7.5 days at 250 °C (equivalent to 10 years at 100 °C at an Ea of 0.70 eV). Detailed investigation shows that Ni does not act as a barrier in the classical sense, i.e. preventing diffusion of Cu and Au across the barrier. Instead Ni modifies or slows down the interactions taking place during device degradation and thus effectively acts as an 'interaction' barrier. Different interactions occur at temperatures below and above 250 °C and for thin (10 nm) and thick (100 nm) barriers. The results of this study indicate that 10-100 nm thick Ni intermediate layers in the Cu/Au based metallization of III-V solar cells may be beneficial to improve the device stability upon exposure to elevated temperatures.

A multi-nuclear magnetic resonance and density functional theory investigation of epitaxially grown InGaP2.

In this paper the short and long range order in In0.483Ga0.517P thin films is investigated by solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. To this end two samples were grown on a GaAs substrate by metal-organic vapor phase epitaxy at two different growth-pressures. From band gap energy measurements, CuPt long range order parameters of SCuPt = 0.22 and 0.39 were deduced, respectively. In the (31)P spectrum five resonances are observed corresponding to the five possible P(GanIn4-n), n = 0-4, coordinations whose relative intensities correspond to the order in the material, but the intensity variations for order parameters between 0 and 0.5 are minimal. (69)Ga, (71)Ga and (115)In (MQ)MAS spectra were acquired to analyze the quadrupolar and chemical shift distributions related to the (dis)order in these materials in more detail. All these spectra clearly reflect the disorder in the sample and do not show the presence of highly ordered domains. The difference in the order parameter in the sample is not clearly reflected in the spectra. (31)P chemical shifts were calculated using Density Functional Theory. The experimentally observed shifts are well reproduced with a simple random model of the disorder, thus confirming the assignment of the resonances. The (31)P chemical shifts are very sensitive to changes in the lattice parameter and chemical surroundings. These effects nearly compensate and explain why the (31)P chemical shifts in pure InP and GaP are nearly identical whereas a large difference would be expected based on the observed shift difference for the P[In4] and P[Ga4] coordinations in In0.483Ga0.517P.

Degradation mechanism(s) of GaAs solar cells with Cu contacts.

Substrate-based GaAs solar cells having a dense Au/Cu front contact grid with 45% surface coverage were exposed to accelerated life testing at temperatures between 200 and 300 °C. TEM analysis of the front contacts was used to gain a better understanding of the degradation process. During accelerated life testing at 200 °C only intermixing of the Au and Cu in the front contact occurs, without any significant influence on the J-V curve of the cells, even after 1320 h (55 days) of accelerated life testing. At temperatures ≥250 °C a recrystallization process occurs in which the metals of the contact and the GaAs front contact layer interact. Once the grainy recrystallized layer starts to approach the window, diffusion via grain boundaries to the window and into the active region of the solar cells occurs, causing a decrease in Voc due to enhanced non-radiative recombination via Cu trap levels introduced in the active region of the solar cell. To be a valid simulation of space conditions the accelerated life testing temperature should be <250 °C in future experiments, in order to avoid recrystallization of the metals with the GaAs contact layer.

Photo-generated THz antennas.

Electromagnetic resonances in conducting structures give rise to the enhancement of local fields and extinction efficiencies. Conducting structures are conventionally fabricated with a fixed geometry that determines their resonant response. Here, we challenge this conventional approach by demonstrating the photo-generation of THz linear antennas on a flat semiconductor layer by the structured optical illumination through a spatial light modulator. Free charge carriers are photo-excited only on selected areas, which enables the realization of different conducting antennas on the same sample by simply changing the illumination pattern, thus without the need of physically structuring the sample. These results open a wide range of possibilities for the all-optical spatial control of resonances on surfaces and the concomitant control of THz extinction and local fields.