F. Newman

Development of GaAs space solar cells by high growth rate MOMBE/CBE

Abstract Realization of high-quality GaAs photovoltaic materials and devices by metalorganic molecular beam epitaxy (MOMBE) and chemical beam epitaxy (CBE) with growth rates in excess of 3xa0μm/h is demonstrated. Despite high growth rates, the optimization of III/V flux-ratio and growth temperatures leads to a two-dimensional layer-by-layer growth mode characterized by a (2×4) RHEED diagrams and strong intensity oscillations. The not intentionally doped layers exhibit low background impurity concentrations and good luminescence properties. Both n (Si) and p (Be) doping studies in the range of concentrations necessary for photovoltaic device generation are reported. Preliminary GaAs (p/n) solar cells fabricated at growth rates in excess of 3xa0μm/h exhibit performances comparable to state of the art and stress the potential of the high growth rate MOMBE/CBE as reduced toxicity alternatives for the production of space III–V solar cells.

Significant operating voltage reduction on high-speed GaAs-based heterojunction bipolar transistors using a low band gap InGaAsN base layer

We report the fabrication of double heterojunction bipolar transistors (DHBTs) with the use of a new quaternary InGaAsN material system that takes advantage of a low-energy band gap E/sub G/ in the base to reduce operating voltages in GaAs-based electronic devices. InGaP/In/sub 0.03/Ga/sub 0.97/As/sub 0.99/N/sub 0.01//GaAs DHBTs with improved band gap engineering at both heterojunctions exhibit a DC peak current gain over 16 with small active emitter area. The use of the lattice-matched In/sub 0.03/Ga/sub 0.97/As/sub 0.99/N/sub 0.01/ (E/sub G/=1.20 eV) base layer allows a significant reduction of the turn-on voltage by 250 mV over standard InGaP/GaAs HBTs, while attaining good high-frequency characteristics with cutoff frequency and maximum oscillation frequency as high as 40 GHz and 72 GHz, respectively. Despite inherent transport limitations at the present time, which penalize peak frequencies, this novel technology provides comparable RF performance to conventional devices with a GaAs control base layer but at much lower operating base-emitter bias conditions. This technical progress should benefit to the next generation of RF circuits using GaAs-based HBTs with lower power consumption and better handling of supply voltages in battery-operated wireless handsets.

High growth rate metal-organic molecular beam epitaxy for the fabrication of GaAs space solar cells

In this work, the epitaxial growth of GaAs photovoltaic devices using metalorganic molecular beam epitaxy (MOMBE) and chemical beam epitaxy (CBE) with growth rates in excess of 3 μm/h is undertaken. The performance of these preliminary devices offer encouraging evidence for MOMBE and CBE as possible alternatives to the more common metalorganic chemical vapor deposition (MOCVD) for the production of III-V solar cells. Copyright

Chemical beam epitaxy for high-efficiency InP solar cells

Abstract The aim of this work is to develop thin InP solar cells for space applications from more mechanically resistant, lighter, and cheaper substrates. In this paper we present the development of a p+/nn+ solar cell structure with a very thin emitter layer. A thin emitter helps to increase the collection of carriers generated by high energy incident photons from the solar spectrum. Moreover, the use of a p+/nn+ structure should improve the radiation resistance of this already radiation resistant technology. A remarkable improvement of high energy photo-response is shown for these CBE-grown very thin emitter InP solar cells. The ability to obtain a very high p-type InP doping level by using CBE avoided the sheet resistance increase that degrades the I–V characteristic even for emitters only 400xa0A thick. It is also shown that the photo-response of thin base layers is reduced, and that more than 2.4xa0μm thick base layers are necessary for a better collection of near InP band gap photons.

Radiation hardening of InP solar cells for space applications

The aim of this work is to develop a radiation resistant thin InP-based solar cells for space applications on more mechanically resistant, lighter, and cheaper substrates. In this paper, we present the development of a p+/nn+ InP-based solar cell structures with very thin emitter and base layers. A thin emitter helps to increase the collection of carriers generated by high energy incident photons from the solar spectrum. The use of a thin n base structure should improve the radiation resistance of this already radiation resistant technology. A remarkable improvement of high energy photons response is shown for InP solar cells with emitters 400 A thick.

Molecular beam epitaxy of InP single junction and InP/In0.53Ga0.47As monolithically integrated tandem solar cells using solid phosphorous source material

This work reports the first InP solar cells, InP/In0.53Ga0.47As tandem solar cells and InP tunnel junctions to be grown using a solid phosphorous source cracker cell in a molecular beam epitaxy system. High p-type doping achieved with this system allowed for the development of InP tunnel junctions. These junctions which allow for improved current matching in subsequent monolithically integrated tandem devices also do not absorb photons which can be utilized in the InGaAs structure. Photocurrent spectral responses compared favorably to devices previously grown in a chemical beam epitaxy system. High resolution x-ray scans demonstrated good lattice matching between constituent parts of the tandem cell. AM0 efficiencies of both InP and InP/InGaAs tandem cells are reported.

Improvement and optimization of InAsxP1−x/InP multi quantum well solar cells

Most of the incident photons in an InP-based multi-quantum well solar cells (MQWSC) are expected to be converted before reaching the base of the device (i.e., converted in the emitter and intrinsic region). Indeed, the external quantum efficiency of the MQWSC is found not to be very much affected by base thickness reduction. By using this property and refined growth interruption schemes, comparable efficiencies are obtained with very thin MQWSC and thick traditional InP devices. The usual MQWSC lower conversion efficiency is mostly due to interface defects, which degrades both open circuit voltage (Voc) and fill factor (FF). In this work optimization in device design and quantum well region growth have been beneficial by improving both Voc and FF of the multi quantum well solar cells.

Influence of growth transients on interface and composition uniformity of ultra thin In(As,P) and (In,Al,Ga)As epilayers grown by chemical beam epitaxy

In this work, measurements of epitaxial growth rate transients for multiple quantum wells (MQWs) in chemical beam epitaxy (CBE) have been made. Mass spectrometry measurements of typical growth conditions were made of gas source species of the InAsxP1−x/InP system, while reflection high-energy electron diffraction (RHEED) measurements were made for the GaAs/AlxGa1−xAs and InxGa1−xAs/GaAs systems. The results of these experiments went directly into predicting the transient growth rate of thin layers for multi-quantum well photovoltaic devices. The data obtained using these techniques resulted in an improved growth interruption sequence for MQW structures in the InAsxP1−x/InP system. Improvements in overall material quality have been observed by high resolution X-ray diffraction (HRXRD). HRXRD measurements of the InAsxP1−x/InP structures yield sharp satellite peaks revealing the possibility of achieving nearly perfect interfaces. From low temperature photoluminescence, narrow emission linewidths from quantum...