Alan Doolittle

Characterization and analysis of InGaN photovoltaic devices

The InGaN material system is investigated to achieve high efficiency solar cells, using tandem and quantum-well structures to implement high efficiency concepts. Here InGaN p-i-n and quantum-well solar cells are designed, grown by MOCVD and fabricated into mesa devices. They are electrically characterized by I-V response under dark, white light and UV illumination and internal quantum efficiency (IQE). Material characterization is done by X-ray diffraction, photoluminescence and photoemission. InGaN solar cells with high In compositions are grown in two configurations, one incorporating it into the i-region of a p-i-n solar cells, and the other incorporating as the well-region of a quantum-well device. A QE of 8% was measured from these quantum-wells. Solar cells with In lean In/sub 0.07/Ga/sub 0.93/N p-i-n device structures show an IQE of 19% as well as photoemission at 500 nm, confirming the suitability of the material for photovoltaic applications.

Design, Growth, Fabrication and Characterization of High-Band Gap InGaN/GaN Solar Cells

One of the key requirements to achieve solar conversion efficiencies greater than 50% is a photovoltaic device with a band gap of 2.4 eV or greater. lnxGa1-xN is one of a few alloys that can meet this key requirement. InGaN with indium compositions varying from 0 to 40% is grown by both metal-organic, chemical-vapor deposition (MOCVD) and molecular beam epitaxy (MBE), and studied for suitability in photovoltaic applications. Structural characterization is done using X-ray diffraction, while optical properties are measured using photoluminescence and absorption-transmission measurements. These material properties are used to design various configurations of solar cells in PC1D. Solar cells are grown and fabricated using methods derived from the III-N LED and photodetector technologies. The fabricated solar cells have open-circuit voltages around 2.4 V and internal quantum efficiencies as high as 60%. Major loss mechanisms in these devices are identified and methods to further improve efficiencies are discussed

Doping and oxygen dependence of efficiency of EFG silicon solar cells

The doping dependence of polycrystalline EFG silicon cell efficiency differs markedly from single-crystal float zone (FZ) cells. The optimum resistivity for EFG cells is decisively higher than 0.2-0.3 Omega -cm, the absolute efficiency is lower than the counterpart single-crystal cells, and a broad maximum in efficiency is observed in the range of 1 to 5 Omega -cm, as opposed to a sharp maximum at 0.2-0.3 Omega -cm for single-crystal FZ cells. It is shown by model calculations that all three effects can be explained on the basis of lifetime-limiting discrete trap levels whose activity increases with doping concentration. The addition of a small amount of oxygen (<or=5*10/sup 17/ cm/sup -3/) is found to improve the EFG cell performance and change the doping dependence such that the optimum resistivity shifts to a slightly higher value and the efficiency becomes nearly flat in the 1-5 Omega -cm range. J-V-T measurements revealed that oxygen passivates a 0.35 eV trap and exposes a 0.54 eV trap.<<ETX>>

Double heterojunction bipolar transistors with InP epitaxial layers grown by solid-source MBE

Epitaxial layers containing phosphorus (P) find numerous applications in electronic and photonic devices such as GaInP/GaAs HBTs, InAlAs/InGaAs/InP double HBTs, GaP LEDs and InP/InGaAsP MQW lasers operating at 1.3-1.55 /spl mu/m. These structures have been realized traditionally either by MOCVD, GSMBE or CBE relying on highly toxic hydrides as the gas sources for phosphorus and arsenic. Solid-source MBE provides the simplest and most elegant solution for growing complex epitaxial structures; however, the unique physical properties of solid phosphorus make it difficult to grow P-containing layers. By using a recently developed phosphorus valved cracker, excellent InP heterostructures have been achieved. We have explored the use of InP active layers in InP-based HBT grown entirely by solid-source MBE and compared them to InAlAs/InGaAs HBTs.

Growth dynamics of InGaAsGaAs by MBE

The growth dynamics of the InGaAs/GaAs system have been investigated by desorption mass spectrometry (DMS). Indium desorption spectra indicate the presence of one or two desorption mechanisms depending on the V/III beam equivalent pressure ratio. The activation energy associated with one of the desorption processes is found to be 1.3 eV and independent of V/III ratio and arsenic species. Analysis of the decay curve allows the calculation of the indium surface population during growth. This population is compared for the different growth conditions investigated. Indium incorporation coefficient curves as a function of substrate temperature are presented. Indium incorporation is found to be enhanced using high V/III ratio and the arsenic dimer, As2.

Heterogeneous integration: from substrate technology to active packaging

Heterogeneous integration of dissimilar materials and devices is necessary for the continued advancement of electronic and optoelectronic systems. A range of processes has been developed in recent years that will enable system integration and advanced packaging. Herein, we outline our approaches towards heterogeneous integration.

Phosphor-Free Solid State Light Sources

The objective of this work was to demonstrate a light emitting diode that emitted white light without the aid of a phosphor. The device was based on the combination of a nitride LED and a fluorescing ZnO substrate. The early portion of the work focused on the growth of ZnO in undoped and doped form. The doped ZnO was successfully engineered to emit light at specific wavelengths by incorporating various dopants into the crystalline lattice. Thereafter, the focus of the work shifted to the epitaxial growth of nitride structures on ZnO. Initially, the epitaxy was accomplished with molecular beam epitaxy (MBE). Later in the program, metallorganic chemical vapor deposition (MOCVD) was successfully used to grow nitrides on ZnO. By combining the characteristics of the doped ZnO substrate with epitaxially grown nitride LED structures, a phosphor-free white light emitting diode was successfully demonstrated and characterized.