The SEM image of GaN surface after EPD of Pd NPs can be seen in Figure . Rounded black spots represent Pd NPs; most of them are circular of about 10 nm in diameter and the others are their aggregations of various sizes. The image area is covered by the particles to about 10% only. The SEM image of InP surface deposited with Pd NPs from the same colloid solution following the same EPD process as in the previous case can be seen in Figure . In this case, most of the spots represent aggregated Pd NPs consisting of about ten spherical NPs of 10 nm in diameter. A similar image, but without aggregates, was obtained when a dried droplet of the colloid solution was observed on a copper grid coated with graphite. However, in such an image (not shown), there were no aggregates seen, despite that the colloid solution had been prepared several months earlier. It shows that aggregates of Pd NPs seen in Figures or did not arise in the colloid solution during storage, but they were created by the EPD process itself. A tendency to create aggregates was stronger in the case of EPD on InP than on GaN, as it can be seen by comparing Figure with Figure .
SEM image of GaN after 2.5 h of electrophoretic deposition of Pd NPs. The image was additionally processed to enhance the contrast. The scale 100 nm is shown with the bright bar at the bottom of the image.
SEM image of InP after 2 h of electrophoretic deposition of Pd NPs. The scale 100 nm is shown with the bright bar at the bottom of the image.
The SEM image of a dried droplet of colloidal graphite, forming a Schottky contact on InP, can be seen on the left side of Figure . It is seen that the graphite layer consists of irregular particles of dimensions of 1 μm order-of-magnitude. A cognate image of graphite contact on GaN can be seen in the left upper corner of Figure . Likewise in this image, graphite particles can be well seen but small Pd NPs in the lower part are less distinct due to the smaller size of single Pd NPs in GaN surface than the size of aggregated NPs in InP surface (Figure ).
SEM image of InP. After 2 h of electrophoretic deposition of Pd NPs (right side) and graphite Schottky contact (left side). The scale 1 μm is shown with the bright bar at the bottom of the image.
Figure 4 SEM image of GaN. After 2 h of electrophoretic deposition of Pd NPs (lower side) and graphite Schottky contact (left upper corner). The image was additionally processed to enhance the contrast. The scale 1 μm is shown with the bright bar at the (more ...)
Figure Shows forward and reverse current-voltage characteristics of vertical diodes with Schottky contacts made by painting colloidal graphite on Pd NPs deposited surfaces of InP (InP-Pd-C) and GaN (GaN-Pd-C) and whole area ohmic contact on the opposite surface. Besides, there are also seen characteristics of diodes with graphite Schottky contacts made on the plain InP surface (InP-C). The areas of the Schottky contacts, estimated from photographs taken on the optical microscope, were 0.0868, 0.0699, and 0.0769 mm2 for InP-Pd-C, GaN-Pd-C, and InP-C diode, respectively. It can be seen in Figure that all diodes indicate a high rectification ratio of more than seven orders-of-magnitude. Notice that plain graphite diodes give (due to smaller leakage) smaller currents at low voltages than diodes with Pd NPs. Leakage currents of GaN-based diodes are about two orders-of-magnitude smaller than leakage currents of InP-based diodes.
Forward and reverse current-voltage characteristics of Schottky diodes. Prepared by painting 0.0868, 0.0699, and 0.0769 mm2 colloidal graphite on Pd NPs deposited InP (circles) and GaN (squares) and on plain InP (triangles).
All forward current-voltage characteristics show distinct linear parts in the semi-log scale in Figure . Using these linear parts, the Schottky barrier heights and ideality factors (IF) were evaluated as described in Ref. [7
]. The height value was 0.873 eV and the ideality factor was 1.08 for InP-Pd-C diode, giving evidence that the thermionic emission primarily governed the electron transport in this case. In the case of GaN-Pd-C diode, the IF exited at 1.74 showing that a generation-recombination current (IF = 2) added to the thermionic emission current (IF = 1). When the linear part of the current-voltage curve of GaN-Pd-C diode was fitted with both currents added, the barrier height was estimated at 1.14 eV. The values of Richardson constants used in the evaluation were 9.24 and 26.4 A/(K·cm)2
for InP and GaN, respectively.
Figure shows current transient responses of the diode InP-Pd-C upon alternating exposure to the flow of various gas blends H2/N2 and air. The measurements started with the flow of air which showed virtually no change of current in comparison with that without the flow. Flows of four gas blends H2/N2 from 1,000 to 3 ppm were applied. The length of each flow was chosen to reach a stationary state when virtually no change of current was observed. It should be pointed that in the stationary state, the current did not change when the speed of flow was changed. The ratio of the current in the H2/N2 ambient to the current in the air ambient was 7 × 105 in the case of 0.1% H2/N2.
Figure 6 Current transient responses of the InP-Pd-C Schottky diode. Upon alternating exposure to the flow of gas blends H2/N2 and air. Transients upon exposure to four various gas blends are shown. The concentrations in parts per million are indicated with arrows. (more ...)
Figure shows current transient responses of the diode GaN-Pd-C upon alternating exposure to the flow of the gas blend 0.1% H2/N2 and of the air. There are two time developments in Figure : (1) measured shortly after preparing a diode and (2) measured lately, after 3 months' time. Characteristics of the two developments were the same, showing on a good time stability of the diode in this range of time. The ratio of the current in the H2/N2 ambient to the current in the air ambient was 7 × 105 in both cases. Also, the response time (change from air to H2/N2 exposure) and the recovery time (change from H2/N2 to air exposure) did not change after a 3-month history of the diode.
Figure 7 Current transient responses of the GaN-Pd-C Schottky diode. Upon alternating exposure to the flow of the gas blend 0.1% H2/N2 and of the air. Transients measured shortly after preparing the diode (1) and later after 3 months (2) are shown The diode was (more ...)
The diode InP-C, made by graphite on the plain InP, was also tested on the hydrogen sensitivity. However, there was no change of current when such voltage-biased diode was exposed to a gas containing hydrogen.
Four measured values of the current of InP-Pd-C diode were plotted in dependence on the concentration of H2/N2 in log-log scale as can be seen in Figure . The four plotted points can be well approximated with a parabolic curve. By the extension of this curve to lower concentrations, the hydrogen detection limit of the InP-Pd-C diode was estimated at 1 ppm H2/N2.
Figure 8 Dependence of the current of InP-Pd-C diode on the concentration of H2/N2. Four square points indicate values of current measured after long time exposure to the respective H2/N2 ratio (indicated in Figure 6). The full line is parabola in the log-log (more ...)