Servicios de apoyo a las actividades de diagnóstico y tratamiento Generalidades.

Finished devices were wirebonded to a printed circuit board (PCB) chip and tested in a complementary electrical socket that was built into the cryostat holder. Figure 3.4a shows a finished device under a 10x objective. Multiple electrodes were patterned on top of the same NB to increase the chances of a successful device. This device showed very strong electroluminescence (EL) as seen in figure 3.4b. Additionally this device also showed very strong waveguiding, even though it is on a highly absorbing Si substrate. The white arrow in figure 3.4b indicates EL that is waveguided over 30 μm on p- Si. When a 60x objective was used, the EL appears to be very localized, showing EL at discrete points around the contact region (fig. 3.4d).

Figure 3.3 A) SEM image of Ti/Au electrode patterned on crosslinked PMMA, attempting to contact a CdS NW. The small diameter makes fabrication very difficult. B) SEM image of electrode patterned on top of CdS NB after PMMA crosslinking. The dotted region shows where a window was opened on the NB to make electrical contact.

64

This likely demonstrates the asymmetry in the CdS NB – Si contact. It is possible that the NB is not uniformly in contact with the p-Si substrate. As the applied voltage is increased from 4 to 5 V, EL seems to be generated at new points on the device (fig 3.5a-f). The initial locations of EL all appear on the side of the device where the electrical contact is being made. This side of the device is probably in more intimate contact with the surface, therefore it makes sense that there is more EL from this side of the device. This illustrates the importance of good physical contact between the NB and the Si surface is in generating efficient EL.

Figure 3.4 A) Optical image at 10X magnification of CdS NB LED. B) Optical image of (A) in dark when CdS LED is emitting EL. The white arrow points to waveguided light over 30 μm away. C) Optical image at 60X magnification of CdS NB LED. D) Optical image of (C) in dark when CdS LED is emitting EL. The EL emission appears from discrete points around the contact region.

65

The current-voltage (IV) behavior shows a rectifying shape, which is expected for a p – n junction (black curve fig. 3.6a). Although, when trying to fit the curve to an ideal diode the ideality factor n is close to 10 where as it should be between one and two. This indicates some source of non-ideality that is not being considered in this model. This model does not consider, for example, recombination of electron-hole pairs in the depletion layer. This model also doesn’t account for tunneling, Auger processes, trap, and surface state recombination which may all play a role to various degrees. It is likely that there is an oxide on the surface of the p-Si. This oxide may act as a barrier for carriers similar to a tunnel diode. As seen in figures 3.4 and 3.5, this device shows strong EL, although the rectifying characteristics seem to change with the numbers of times the EL is turned on (fig. 3.6b).

Figure 3.5 A-F) Optical images at 60X magnification of CdS NB LED under different applied biases from 4 to 5 V. EL is generated at discrete points that change with bias voltage. It appears that the EL location may be dependent on the contact with the Si surface.

66

Figure 3.6b shows the IV characteristics as the device is being swept between different voltages. Initially at 4 V the device only shows a current value of around 100 μA, but after a few cycles the current is close to 1.2 mA at 4 V. Fitting the ideality factor to the ideal diode equation only shows a change from ~10 to ~13 during these measurements. Concurrently during IV measurements, the EL was collected from this device. Compared to the PL, the EL slightly redshifted, probably due to heating (fig. 3.7a). As the applied voltage on the device increases from 4 to 6.5V the intensity initially rises and then falls again (fig. 3.7b). A clear redshiting of the EL is also noticeable as the voltage increases from 4 to 6.5V. The redshifting is a strong indicator of heating taking place inside the device, which is reasonable considering the high current density (~4 x 104 A/cm-2). The decreasing intensity also points to the fact that the high current density is causing some damage in the structure. Joule heating and electromigration in the device may be causing damage by increasing the defect density at the CdS-Si interface through dislocation pile-up and increasing non-radiative pathways22.

Figure 3.6 A) IV behavior of NB LED (black curve) and exponential fit using the ideal diode equation (blue curve). The high ideality factor indicates other processes taking place besides recombination B) Log-linear plot of IV curves after applying different biases. The current increases irreversibly over time.

67

Photoluminescence (PL) was taken on this device before and after device operation at high currents (fig. 3.7c). A noticeable change in the PL intensity is observed by over 60%, which indicates that, in fact the high currents have damaged the structure. Since we observe that the EL intensity actually goes down at higher applied biases, this points to more trap assisted non-radiative recombination taking place and less band to band recombination. It appears that it is important to reduce the carrier density to prevent damage to the device.