Current path in light emitting diodes based on nanowire ensembles

With the ability to synthesize semiconductors in high crystal quality on foreign substrates, nanowires (NWs) offer new and promising possibilities for the future miniaturization of optoelectronic devices and their integration with Si technology. The bottom-up fabrication of NW-based light emitting diodes (LEDs) on Si represents an example of this integration.

Figure 1 EL of the NW-LED under 10 V forward bias imaged through a microscope at (a) 200x and (b) 500x magnification. The inset shows an enlarged part of the image.

Figure 2 (a) Room temperature CL image recorded around 560 nm superimposed on a SEM side-view image and (b) corresponding EBIC map with color-coded intensity (increasing from black via blue, green, red, and yellow to white). The measurements were performed under a reverse bias of -3.5 V. The dashed horizontal lines indicate the substrate-NW (white) and NW-top-contact (red) interface, respectively. The yellow arrows highlight broken NWs that do not contribute to the EBIC signal.

We have synthesized a p-i-n (In, Ga)N/GaN NW structure on a Si substrate using molecular-beam epitaxy. The NW ensemble was subsequently processed into a working LED emitting in the green spectral range as shown in Fig. 1(a). The microscopic analysis of the electroluminescence (EL) [Fig. 1(b)] reveals that the EL pattern is spatially inhomogeneous and apparently consists of isolated emission spots with uniform size. The density of these spots is two orders of magnitude lower than the NW density, i.e., only 1% of the NWs are contributing to the EL. This observation has also been reported by other research groups. 


We consider three possible reasons of this low number of luminescent NWs.  First, the internal quantum efficiency of most NWs may be too low for a significant emission intensity, second, many NWs may not have been contacted successfully, and third, the current may flow only through those wires with minimum series resistance. To identify the culprit, we simultaneously measure cathodoluminescence (CL) and the electron-beam induced current (EBIC) in a scanning electron microscope (SEM). In Fig. 2(a), a CL image at the emission wavelength of the (In, Ga)N quantum wells is superimposed onto a scanning electron micrograph of the NWs in cross-section. Most of the NWs are seen to exhibit strong CL. An EBIC signal is detected for essentially all NWs [Fig. 2 (b)], evidencing that at least the majority of them is contacted. These two experiments eliminate the first two of the above potential reasons for the low number of NWs contributing to EL. Hence, the only remaining explanation for this effect is a pronounced inhomogeneity in their series resistance which results in a preferential current injection into a few NWs defined by the path of least resistance. The homogeneity of such nanostructures is therefore a critical issue, and has to be improved in the future to obtain viable LEDs based on NW ensembles.

1 Author F. Limbach , C. Hauswald , J. Lähnemann , M. Wölz , O. Brandt , A. Trampert , M. Hanke , U. Jahn , R. Calarco , L. Geelhaar , H. Riechert

Current path in light emitting diodes based on nanowire ensembles

Source Nanotechnology , 23 , 465301 ( 2012 )
DOI : 10.1088/0957-4484/23/46/465301 | Download arXiv: 1210.7144 | 2339 Cite : Bibtex RIS
F. Limbach, C. Hauswald, J. Lähnemann, M. Wölz, O. Brandt, A. Trampert, M. Hanke, U. Jahn, R. Calarco, L. Geelhaar, and H. Riechert