Anjia Gu

Design and growth of III–V nanowire solar cell arrays on low cost substrates

State-of-the-art III–V multijunction cells have achieved a record efficiency of 42.8%, which has fueled great interest in the utility sector for large-scale deployment. However, III–V solar cells have thus far proven too expensive for widespread terrestrial applications due to the combined cost of substrates, growth processes, and materials. Here, we propose a novel III–V solar cell based on the epitaxial growth of AlGaAs/GaAs on Ge nanowires, pre-patterned on low cost substrates to achieve cost-effective, large-scale deployment. This approach is based on our recent discovery that the surface kinetics and epitaxial growth by MBE and MOCVD are dramatically altered when growing on nanostructures instead of planar surfaces. These growth kinetics enable uniform, single crystal growth of low-defect, lattice mismatched materials on nanostructures with high aspect ratios. We present the device design, TCAD simulation results, and experimental growth results for GaAs/Ge core-shell nanowires on silicon substrates. Finite-difference time-domain (FDTD) simulation results show that this GaAs/Ge nanowire array has reduced reflection and wider incident angle acceptance than its planar counterpart, and outperforms planar anti-reflective coatings under some conditions. GaAs is epitaxially grown on Ge nanowires via MBE and MOCVD. TEM measurements on the wires confirm that the GaAs/Ge core-shell structure is single crystal. Based on these results, we are in the process of fabricating GaAs/Ge nanowire solar cell arrays. We will present further characterization of these core-shell arrays as well as electrical measurements of solar cell devices.

Design and fabrication of nano-pyramid GaAs solar cell

We demonstrate a genetic method to fabricate large-area nano-structure III-V solar cells with conformal epitaxial growth on pre-patterned substrate. The design, simulation, fabrication, and characterization of a nano-structure gallium arsenide (GaAs) solar cell device are presented. The optical simulation illustrates that the nano-pyramid array is able to suppress the reflection and enhance the absorption in a wide spectrum range. The IV characterization shows that the short circuit current of the nano-pyramid GaAs solar cell with 200 nm thick junction is as high as 18.5 mA/cm2, which is more than triple of the planar cell. Our results suggest this nano-structure thin film absorber could significantly reduce epitaxial growth cost and increase yield, thus provides a pathway towards high-efficiency and low-cost solar cells.

GaAs thin film nanostructure arrays for III-V solar cell applications

State of art III-V multi-junction solar cells have demonstrated a record high efficiency of 43.5%. However, these cells are only applicable to high concentration systems due to their high cost of substrates and epitaxial growth. We demonstrate thin film flexible nanostructure arrays for III-V solar cell applications. Such nanostructure arrays allow substrate recycling and much thinner epitaxial layer thus could significantly reduce the cost of traditional III-V solar cells. We fabricate the GaAs thin film nanostructure arrays by conformally growing GaAs thin film on nanostructured template followed by epitaxial lift-off. We demonstrate broadband optical absorption enhancement of a film of GaAs nanostructure arrays over a planar thin film with equal thickness. The absorption enhancement is about 300% at long wavelengths due to significant light trapping effect and about 30% at short wavelengths due to antireflection effect from tapered geometry. Optical simulation shows the physical mechanisms of the absorption enhancement. Using thin film nanostructure arrays, the III-V solar system cost could be greatly reduced, leading to low

Faceting and disorder in nanowire solar cell arrays

/W and high kW/kg flexible solar systems.

New THz sources for bio-medical imaging

Arrays of semiconductor nanowires have been discussed as a method of fabricating lower-cost, higher-efficiency solar cells [1]. This is accomplished by shortening the minority carrier path to the contacts and by nanoscale light trapping effects [1, 2]. Numerical simulations have played a large role in the development of these cells [1, 3–5]. However, the approximation of the nanowire array as a group of uniformly spaced cylinders has limitations, as disorder is often present in fabricated devices. Here, we show that introducing disorder into simulated arrays of semiconductor nanowires enhances the calculated absorption. Additionally, facets and other surface features serve to reduce reflection and enhance light trapping over the model of the nanowire as a cylinder. An optimal disorder between 10–20% from uniform is predicted for both cylindrical and hexagonally arranged wires. This effect holds for various semiconductor materials. Preliminary electrical simulations are also presented for Si, GaAs, and Ge nanowires.

Comparative Analysis of Bio-Medical Imaging at 3.7 Terahertz with a High Power Quantum Cascade Laser

We present an evaluation of new terahertz sources for biomedical imaging based upon quantum cascade lasers (QCL) and orientation patterned gallium arsenide (OP-GaAs) optical parametric oscillator (OPO). The recently developed terahertz quantum cascade laser emits a peak output power of up to 40mW at 3.7 THz (&lgr;=81&mgr;m). Utilizing coherent terahertz radiation greatly improves the signal to noise ratio of the detection, where it provides a relatively large dynamic range and high spatial resolution. We demonstrated biomedical imaging of malignant tissue contrast in an image of a mouse liver with developed tumors with a THz imaging system based on a QCL. In addition, images of various tissues, such as lung, liver, and brain sections from the laboratory mouse were also obtained. We also explored distinct images from fat, muscle and tendon and measured the absorption coefficient and compared this with FTIR measurements. Another recent technological advance in THz source is based on cascaded optical down-conversion in an OP-GaAs OPO, which provides a tunable THz source over a broad wavelength range with an average power of 1mW at room temperature (RT). The tunability of the OPO source provides additional imaging modes through the ability to excite molecular vibrations and obtain biochemical and structural information in addition to normal absorption or reflectivity contrast.

Optical Absorption Enhancement in Freestanding GaAs Thin Film Nanopyramid Arrays

The comparative analysis of two major factors, i.e., contrast and input terahertz beam power in constructing bio-medical images using a terahertz quantum cascade lasers at 3.7 THz is investigated here. Image contrast based on water/fat content ratios in different tissues, is studied and were able to demonstrate distinguished contrast based on fat/water contents from the freshly cut pieces from various different parts (i.e. liver, fat, muscle and tendon) of untreated tissue. Liver contains more fat cells than muscle or tendon tissues, therefore liver and fat exhibit similar transmission in this measurement. Muscle and tendon contain higher water content and very little fat, therefore, exhibits rather small transmission and in turn produces a very dark image. The absorption coefficients of four different tissues were obtained at frequency 3.7 THz from the intensity measurement of the transmitted power of THz radiation