Visualization of carrier depletion in semiconducting nanowires.

Semiconducting nanowires (NWs) grown by the vapour– liquid–solid (VLS) mechanism are attractive building blocks for nanoscale devices. A central question in the study of such devices is to what degree the physical concepts established to describe larger planar semiconductor devices can be extended to these new NW devices. For instance, the large surface-to-volume ratio in nanoscale devices leads to the electrostatics and transport properties becoming strongly influenced by the surrounding environment and not dependent only on the intrinsic properties of the semiconductor alone. As the most basic element of many electronic and optoelectronic devices and circuits, p-type/n-type (pn) junction diodes are a useful starting point for the physics of nanoscale devices. It has long been known that nearby dielectrics (and field-shaping electrodes) can enhance the breakdown voltage in planar junctions via a reduction of the maximum surface field. Similarly, the electric field inside SiO2-embedded NW pn junctions is smeared out and charge carriers experience a potential landscape controlled by the dielectric environment, which in turn results in even a suppression of avalanche breakdown. Thus, a technique to directly map the space–charge layer in NWs would be of general interest. Previous work used time-consuming scanning-gate and near-field scanning optical microscopy techniques to probe the concentration of charge carriers in NWs. Here, we report the visualization of the depletion width in crossed NW pn heterojunctions with a far-field optical experiment that can be readily applied to characterize chargecarrier distribution in nanoscale structures. Unlike rather laborious near-field scanning of the space–charge layer width by local illumination of a NW, global low-excitation photoluminescence (PL) of a NW and far-field optical microscopy of the PL are sufficient to probe the charge-carrier separation in NWs. Crossed-NW devices have been used to demonstrate logic operations, light-emitting devices and lab-on-chip applications. Most recently, crossed-NW devices from nCdS/p-Si were used as polarization-sensitive photodetectors at room temperature. Spatially resolved photocurrent measurements revealed the nanoscale pn-junction device as the active area of the photodiodes and by controlling the doping level of the p-Si NWs Zener and avalanche diodes were fabricated. Although the breakdown mechanism is different in both types of diode, differences in carrier concentration across the pn junction always generate a depletion layer with a strong electric field. Due to the large surfaceto-volume ratio of NWs a deviation of the nanoscale space– charge layer from bulk diodes can be expected. We have observed that the breakdown voltage of n-CdS/p-Si crossed-NW diodes cooled down to 4.2 K in an optical cryostat shifts to very high reverse biases (>60 V), which gives us access to extended space–charge layers. PL measurements of crossed p-Si-NW/n-CdS-NW diodes show that the depletion-layer thickness extends over several micrometers in CdS NWs and the width of the space–charge layer can be controlled by the illumination intensity and the reverse bias. Interestingly, these results show that the depletion width does not scale with the square root of the bias voltage but is linearly dependent on the reverse bias and thus suggests unusual carrier distributions in these crossed junctions. PL measurements of the CdS-NW in n-CdS/p-Si crossed-NW junctions and subsequent analysis of the PL was used to determine details of charge carrier recombination in the active region of the reverse biased pn junction as described below. Figure 1A shows the experimental set up. A defocused laser beam at 400 nm was used to illuminate the device area and to stimulate a uniform PL from the single n-CdS-NW (direct-bandgap material). PL spectra and images were recorded with a home-built epifluorescence microscope and a liquid nitrogen cooled CCD (Princeton Instruments Spec-10). Figure 1B and C shows a scanning electron microscopy (SEM) image of the photodiode and a CCD image of the illuminated device, respectively. Uniform PL with a near-band-edge emission of 514 nm is observed along the entire CdS NW length. We observe no feature in the PL of the junction at zero bias, which we could identify with an equilibrium depletion region. This indicates a very small value of the built-in voltage at the low temperatures of the experiment, or a spatial extent of the depletion region below the limited resolution of the microscope. No PL is observed at the position of the SiNW, which is an indirect bandgap material. The two dark areas at the ends of the CdS NW correspond to the position of optically opaque top metal contacts. With applied forward bias to the diode electroluminescence (EL) appears right at the pn junction [*] Dr. O. Hayden Current address: SIEMENS AG, Corporate Technology CT MM1 G#nther-Scharowsky-Strasse 1 91050 Erlangen (Germany) Fax: (+49)91-317-328-04 E-mail: oliver.hayden@siemens.com