Silicon Nanowires Act as Nanoscale Avalanche Photodetectors

Lauren I. Rugani

Researchers at Harvard University in Cambridge, Mass., have synthesized PIN silicon nanowires with single-crystal structures and uniform diameters that demonstrate potential for use as avalanche photodetectors in integrated photonic systems.

The scientists grew the nanowires in three sequential steps, employing complementary doping of a single wire rather than joining two individually doped ones, a technique that may lead to better-defined junctions. Gold clusters that were 20 nm in size catalyzed the chemical vapor deposition growth of the PIN sections in the presence of diborane, in the absence of chemical dopants and in the presence of phosphine, respectively.

Characterization of the nanowires’ structure by transmission electron microscopy revealed a uniform diameter over its length. The lack of radial growth eliminates the possibility of altered electronic properties caused by surface coating of dopant layers on the nanowires. The studies also revealed an average growth length of 6.5 μm, which is consistent with that expected from the growth rate, and a uniform single crystalline structure in the <112> direction along the axis.

A photocurrent map with a superimposed scanning electron microscopy image shows a 20-nm-diameter PIN nanowire. The length of the intrinsic region, where avalanche breakdown occurs, is 1.5 μm and overlaps with the maximum photocurrent intensity. Scale bar is 2 μm. Reprinted with permission of Nano Letters.

Electrostatic force microscopy and scanning gate microscopy revealed the variations in electronic properties among the three nanowire regions. A bias voltage was applied to the doped ends while a conducting tip with an applied voltage scanned the device. An electrostatic force microscopy phase-shift signal revealed a dramatic voltage drop across the 1.0-μm intrinsic middle section as well as a significant decrease in conductance over the same region in the scanning gate microscopy mode.

The researchers then measured current versus bias voltage, which showed a current flow at a forward bias >1 V and a reverse bias at 38 V. This increase in voltage, associated with the breakdown voltage, may cause avalanche breakdown in the devices. This breakdown was found to be a function of temperature, confirming that it is based on band-to-band impact ionization.

Measurements of dark current and photocurrent as a function of reverse bias in the intrinsic region of the nanowires allowed the researchers to find the avalanche gain. A scanning optical microscope combined with transport measurements revealed a large photocurrent in the intrinsic region caused by photogenerated electron-hole pairs. The photocurrent resulting from excitation of the positive region was greater than that of the negative region, indicating that the avalanche multiplication factor is greater for electrons than for holes.

These measurements, combined with the high spatial resolution and sensitivity of the nanowires, exhibit the potential for use of the devices as avalanche photodetectors in integrated nanophotonic systems.

Nano Letters, published online Nov. 16, 2006, 10.1021/nl062314b.