Evan Pickett

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.

Temperature dependence of diffusion length, lifetime and minority electron mobility in GaInP

The mobility of electrons in double heterostructures of p-type Ga0.50In0.50P has been determined by measuring minority carrier diffusion length and lifetime. The minority electron mobility increases monotonically from 300 K to 5 K, limited primarily by optical phonon and alloy scattering. Comparison to majority electron mobility over the same temperature range in comparably doped samples shows a significant reduction in ionized impurity scattering at lower temperatures, due to differences in interaction of repulsive versus attractive carriers with ionized dopant sites. These results should be useful in modeling and optimization for multi-junction solar cells and other optoelectronic devices.

Formation of an oxide-free Ge∕TiO2 interface by atomic layer deposition on brominated Ge

Atomic layer deposition (ALD) of titanium dioxide (TiO2) high-κ dielectric films on brominated Ge substrates using titanium tetrachloride and water has been studied. A strong temperature dependence was observed for the TiO2 deposition rate. An accelerated growth rate was measured for the first 15 ALD cycles at 300°C; this effect is attributed to bromine desorption and resultant deposition on halide-free Ge. Results suggest that TiO2 films were deposited with no interfacial oxide layer at 300°C. The films were in a crystalline anatase phase at 300°C, and were amorphous when deposited at 100°C.Atomic layer deposition (ALD) of titanium dioxide (TiO2) high-κ dielectric films on brominated Ge substrates using titanium tetrachloride and water has been studied. A strong temperature dependence was observed for the TiO2 deposition rate. An accelerated growth rate was measured for the first 15 ALD cycles at 300°C; this effect is attributed to bromine desorption and resultant deposition on halide-free Ge. Results suggest that TiO2 films were deposited with no interfacial oxide layer at 300°C. The films were in a crystalline anatase phase at 300°C, and were amorphous when deposited at 100°C.

Temperature dependencies of annealing behaviors of GaInNAsSb∕GaNAs quantum wells for long wavelength dilute-nitride lasers

The photoluminescence (PL) spectra of GaInNAs(Sb)∕GaNAs quantum well samples emitting around 1.5μm, annealed at different temperatures and for different durations, were compared. Two distinct processes with widely different temperature dependencies are identified: PL intensity improvement at the beginning of annealing and PL intensity degradation when overannealed. The degradation process has a much steeper temperature dependence than the improvement process, so lower-temperature, longer-duration annealings result in both a higher photoluminescence intensity and a broader process window than higher-temperature, shorter-duration annealings. The lowest threshold of 1.55 GaInNAs(Sb) lasers up to now was obtained exclusively with short, hot annealings, this finding offers another method of further improving dilute-nitride laser performance. Similar trends are found for different compositions and thicknesses of GaInNAs(Sb).

Doping dependence and anisotropy of minority electron mobility in molecular beam epitaxy-grown p type GaInP

Direct imaging of minority electron transport via the spatially resolved recombination luminescence signature has been used to determine carrier diffusion lengths in GaInP as a function of doping. Minority electron mobility values are determined by performing time resolved photoluminescence measurements of carrier lifetime on the same samples. Values at 300 K vary from ∼2000 to 400 cm2/V s and decrease with increasing doping. Anisotropic diffusion lengths and strongly polarized photoluminescence are observed, resulting from lateral composition modulation along the [110] direction. We report anisotropic mobility values associated with carrier transport parallel and perpendicular to the modulation direction.

Faceting and disorder in nanowire solar cell arrays

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.

Monolithic GaInNAsSb vertical cavity surface emitting lasers at 1534nm

Electrically pulsed GaInNAsSb-based, vertical cavity surface emitting lasers (VCSELs) at 1534 nm are reported. The lasers operated below -25C due to a gain-cavity misalignment. These are the first monolithic, C-band VCSELs on GaAs.