Joshua J. Williams

In-rich InGaN thin films: Progress on growth, compositional uniformity, and doping for device applications

A number of In-rich InGaN films with In contents in the 20–40% range have been grown at moderately low temperatures on sapphire and silicon substrates at high growth rates using a versatile molecular beam epitaxy-type technology that utilizes an energetic beam of N atoms called energetic neutral atom beam lithography and epitaxy to overcome reaction barriers in the group III-nitride system. Extensive characterization results on the crystalline, optical, and electrical properties of the In-rich InGaN materials are reported. It was found that N-rich growth conditions are required to produce materials that have excellent crystallinity, uniform compositions, and bright band edge photoluminescence. For In-rich InGaN growth on sapphire, electrical transport measurements show reasonably low carrier concentrations and high mobilities. Successful p-type doping of In-rich InGaN with ∼20% and ∼40% In contents is demonstrated, and preliminary results on the formation of a p–n junction are reported. For In-rich InGaN ...

AlGaSb-Based Solar Cells Grown on GaAs: Structural Investigation and Device Performance

GaSb and alloys based on the 6.1 Å family can be grown metamorphically on substrates such as GaAs allowing for the realization of several multijunction solar cell designs. This paper investigates the molecular beam epitaxy growth, crystal quality, and device performance of Al<italic>_x</italic>Ga₁₋<italic>_x</italic>Sb-based single-junction solar cells grown on GaAs substrates. The focus is on the optimization of the growth of Al<italic>_x</italic>Ga₁₋<italic>_x</italic>Sb on GaAs (001) substrates in order to minimize the threading dislocation density resulting from the large lattice mismatch between GaSb and GaAs. Utilizing optimum growth conditions, solar cells with absorbing layers of different Al<italic>_x </italic>Ga₁<italic>_x</italic>Sb compositions are studied and compared to control cells grown on lattice-matched GaSb substrates. GaSb, Al_0.15Ga_0.85Sb, and Al_0.5Ga_0.5Sb solar cells grown on GaAs substrates show open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. Furthermore, the lattice-mismatched cells demonstrate promising carrier collection with comparable spectral response to lattice-matched control cells grown on GaSb.

High In content InxGa1−xN grown by energetic neutral atom beam lithography and epitaxy under slightly N-rich conditionsa)

Two series of In-rich InGaN films with compositions of ∼25% and ∼35% In, grown over a substrate temperature range from 490 to 620 °C, show how the film properties improve as the growth temperature is lowered below the InN decomposition temperature of ∼550 °C in vacuum. These InGaN films have been grown using a novel growth technique utilizing energetic N atoms as the active growth species. Under N-rich growth conditions, these InGaN films show how compositional uniformity, crystallinity, band edge photoluminescence, and surface morphology are improved as growth temperatures are reduced. The results emphasize the importance of energetic N atoms and lower substrate temperatures for overcoming difficulties associated with growing high-quality In-rich InxGa1−xN thin film materials. Utilizing energetic N atoms allows for the growth of high-quality, thick (>500 nm) InxGa1−xN films at temperatures below 500 °C.

Structural and optical investigations of GaN-Si interface for a heterojunction solar cell

In recent years the development of heterojunction silicon based solar cells has gained much attention, lead largely by the efforts of Panasonics HIT cell. The success of the HIT cell prompts the scientific exploration of other thin film layers, besides the industrially accepted amorphous silicon. The band gap, mobilities, and electron affinity of GaN make it an interesting candidate to solve problems of parasitic absorption while selectively extracting electrons. Using a novel MBE based growth technique, thin films of GaN have been deposited at temperature significantly lower than industry standards. Crystalline measurements and absorption data of GaN are presented. Additionally, effects of deposition on the silicon wafer lifetimes are presented.

Development of a high-band gap high temperature III-nitride solar cell for integration with concentrated solar power technology

The III-N material class of semiconductors exhibits desirable properties for construction of a cell for integration with the thermal receiver of a concentrated solar plant. We design a GaN-InGaN based solar cell for operation at 450 °C. An MQW structure for the InGaN absorber is selected to improve voltage through improved material quality. Cell performance shows a VOC of 2.4 V for room temperature and 1.7 V at operating temperature and 300x suns. EQE measurements show little cell performance decrease up to 500 °C. Repeated measurements indicate the device to be thermally robust.

“Thin silicon solar cells: A path to 35% shockley-queisser limits”, a DOE funded FPACE II project

Crystalline silicon technology is expected to remain the leading photovoltaic industry workhorse for decades. We present here the objectives and workplan of a recently launched project funded by the U.S. Department of Energy through the Foundational Program to Advance Cell Efficiency II (FPACE II), which aims at leading crystalline silicon to an efficiency breakthrough. The project will tackle fundamental approach of materials design, defect engineering, device simulations and materials growth and characterization. Among the main novelties, the implementation of carrier selective contacts made of wide bandgap material or stack of materials is investigated for improved passivation, carrier extraction and carrier transport. Based on an initial selection of candidate materials, preliminary experiments are conducted to verify the suitability of their critical parameters as well as preservation of the silicon substrate surface and bulk properties. The target materials include III-V and metal-oxide materials.

Induced junction III-nitride solar cells for wide band gap solar cells: Modeling charge transport and band bending in polarized material

III-N alloys of aluminum nitride, gallium nitride, and indium nitride are of high interest for solar cells as they span the majority of the solar spectrum from 6.2eV to 0.7eV. There are however challenges in creating conventional cells from these materials. Issues include an inability to produce high quality p-type material and polarization effects that block carrier transport in standard heterojunctions. We propose using an induced junction, a form of heterojunction, to create band bending and thus an effective p-n junction solely within n-type material. In this paper, we discuss theoretical equilibrium, transport, generation and recombination mechanisms within a III-N induced junction device. Recent experimental work with III-N material and device architectures will be added to help the models accuracy.

Inducing a junction in n-type InxGa(1−x)N

The pseudo-binary alloy of indium(x)gallium(1−x)nitride has a compositionally dependent bandgap ranging from 0.65 to 3.42 eV, making it desirable for light emitting diodes and solar cell devices. Through modeling and film growth, the authors investigate the use of InxGa1−xN as an active layer in an induced junction. In an induced junction, electrostatics are used to create strong band bending at the surface of a doped material and invert the bands. The authors report modeling results, as well as preliminary film quality experiments for an induced junction in InGaN by space charge effects of neighboring materials, piezoelectric effects, and spontaneous polarization.