Arthur Zhang

Ultrahigh Responsivity Visible and Infrared Detection Using Silicon Nanowire Phototransistors

Nanowire photodetectors can perform exceptionally well due to their unique properties arising from the nanowire geometry. Here we report on the phenomenal responsivity and extended spectral range of scalable, vertically etched, silicon nanowire photodetector arrays defined by nanoimprint lithography. The high internal gain in these devices allows for detection at below room temperatures of subfemtowatt per micrometer visible illumination and picowatt infrared illumination resulting from band to surface state generation.

Silicon nanowire detectors showing phototransistive gain

Nanowire photodetectors are shown to function as phototransistors with high sensitivity. Due to small lateral dimensions, a nanowire detector can have low dark current while showing large phototransistive gain. Planar and vertical silicon nanowire photodetectors fabricated in a top-down approach using an etching process show a phototransistive gain above 35 000 at low light intensities. Simulations show that incident light can be waveguided into vertical nanowires resulting in up to 40 times greater external quantum efficiency above their physical fill factor. Vertical silicon nanowire phototransistors formed by etching are attractive for low light level detection and for integration with silicon electronics.

InGaAs single photon avalanche detector with ultralow excess noise

An InGaAs single photon avalanche detector capable of sub-Geiger mode (Photomultiplier-tube-like) operation is reported. The device achieves a stable gain at around 106. The gain fluctuation is greatly suppressed through a self-quenching effect, thus an equivalent excess noise factor as low as 1.001 is achieved. In the photon counting experiment, the device is operated in the nongated mode under a dc bias. Because of its unique characteristics of self-quenching and self-recovery, no external quenching circuit is needed. The device shows a single photon response of around 30ns and a self-recovery time of about 300ns.

Fabrication of Vertical Silicon Nanowire Photodetector Arrays using Nanoimprint Lithography

Nanoimprint lithography (NIL) is an attractive method for its ability to quickly and cheaply pattern nano-scaled dimensions, and is an enabling technology for patterning large area substrates. The benefits of NIL are demonstrated through its application towards large area nanowire image arrays. In this work, we have fabricated and characterized top down silicon nanowire detector arrays by using UV curing NIL and deep Reactive Ion Etching techniques. Fabricated devices show over 106 gain value at low incident light power, which is comparable to high sensitivity of an e-beam written lithography device. This technology is suitable for fabrication of high density, addressable imager arrays.

Planar and vertical Si nanowire photodetectors

We demonstrate scalable Si nanowire photodetectors that function as phototransistors. Etched planar and vertical Si nanowire photodetectors have been fabricated and characterized, showing high (>35,000) internal gain under UV illumination.

Counting leukocytes from whole blood using a lab-on-a-chip Coulter counter

A microfluidic lab-on-a-chip Coulter counter was demonstrated to count micro particles and leukocytes from whole blood. Instead of electroplated or deposited metal electrodes, off-the-shelf gold pins were used as electrodes to simplify fabrication process, reduce cost, enhance device durability, and above all, achieve superior uniformity in E-field distribution for improved signal quality. A custom-designed, low-cost demodulation circuit was developed to detect the AC impedance signals of the particles and cells passing the detection area defined by the microfluidic channels. A mixture of polystyrene beads with three different sizes was used to characterize the device. The results showed high throughput at 2000 particles/s and clear separation among different sizes of beads with coefficients of variation (CV) of 13.53%, 10.35% and 5.67% for 7.66μm, 10.5μm and 14.7μmbeads, respectively. To demonstrate the potential for a point-of-care or self-administered device for cancer patients going through chemotherapy, we have used the lab-on-a-chip device to count leukocytes from whole blood, generating encouraging preliminary results comparable to the results from a commercial flow cytometer.

Novel HMD concepts from the DARPA SCENICC program

Access to digital information is critical to modern defense missions. Sophisticated sensor systems are capable of acquiring and analyzing significant data, but ultimately this information must be presented to the user in a clear and convenient manner. Head-Worn Displays (HWDs) offer one means of providing this digital information. Unfortunately, conventional HWDs occupy significant volume and have serious performance limitations. To truly offer a seamless man/machine interface, the display must be able to provide a wide array of information in a manner that enhances situation awareness without interfering with normal vision. Providing information anywhere in the eyes field of view at resolutions comparable to normal vision is critical to providing meaningful information and alerts. Furthermore, the HWD must not be bulky, heavy, or consume significant power. Achieving these goals of the ideal wearable display has eluded optical designers for decades. This paper discusses the novel approach being developed under DARPAs SCENICC program to create a high resolution HWD based on using advanced contact lenses. This approach exploits the radically different concept of enhancing the eyes normal focus accommodation function to enable direct viewing of high resolution, wide field of view transparent image surfaces placed directly in front of the eye. Integrating optical components into contact lenses eliminates all of the bulky imaging optics from the HWD itself creating a high performance wearable display in a standard protective eyewear form factor. The resulting quantum advance in HWD performance will enable HWDs to expand well beyond their current limited rolls.

High-sensitivity visible and IR (1550nm) Si nanowire photodetectors

Vertical silicon nanowire detectors with high phototransistive gain have been demonstrated and the principles responsible for the high gain have been reported in recent publications. The emphasis of this paper is (a) the fabrication technology of silicon nanowire array detectors that can be integrated with Si VLSI and (b) the ability of sub-bandgap detection to achieve ultrawide band (from UV to IR) responsivity. We have demonstrated responsivity of greater than 100 A/W at 1550 nm for single crystal silicon nanowires to detect picowatts of IR light, the highest record ever reported for single crystal silicon detectors.

Characterization and physics of top-down silicon nanowire phototransistors

Nanowire photodetectors of a variety of materials have been attracting increased attention due to their potential for very high sensitivity detection. Silicon photodetectors are of particular interest for detection in the visible spectrum, having many benefits including cost of substrate, ease of processing, and ability for integration with conventional fabrication techniques. Using top-down fabrication techniques results in additional benefits of precise control of number, geometry, and placement of these wires. To demonstrate the potential of these devices, top-down, vertical silicon nanowire phototransistor arrays have been fabricated using ebeam lithography and deep reactive ion and inductively coupled plasma etching. These devices show a much higher phototransistive gain over nanowire photodiodes with similar geometry under illumination from a 635nm laser. Low temperature measurements also show the dependence of dark current and sensitivity on temperature. The mechanism responsible for this gain is shown to be dominated by the large surface-to-volume ratio of nanowires where charge capture and recombination at the surface creates a radial gate bias which is modulated with light intensity. 3D numerical simulations validate this mechanism and further show the dependence of device behavior on nanowire doping, geometry, and surface state density. This will allow for the precise engineering of these devices to achieve the maximum sensitivity obtainable as we strive for the ultimate goal of single photon resolution.

High Gain ZnO Nanowire Phototransistor

We demonstrate the potential of nanowires as photo transistors with internal gain. Two- terminal single ZnO nanowire devices have been fabricated, which under UV illumination, show high photoconductive gain (approaching 1010) due to hole-trapping at surface states.