James S. Harris

Dilute nitride GaInNAs and GaInNAsSb solar cells by molecular beam epitaxy

Dilute nitride films with a roughly 1 eV band gap can be lattice-matched to gallium arsenide and germanium, and therefore could become a critical component in next-generation multijunction solar cells. To date most dilute nitride solar cells have been plagued with poor efficiency, due in large part to short diffusion lengths. This study focuses on two techniques aimed at improving the quality of dilute nitride films grown by molecular beam epitaxy: the utilization of biased deflection plates installed in front of the nitrogen plasma source, and the introduction of antimony during growth. Results from GaInNAs cells grown with and without deflection plates, and GaInNAsSb solar cells are reported. The use of biased deflection plates during GaInNAs growth improved every aspect of solar cell performance. For the GaInNAs devices grown with deflection plates, the dark current density, open-circuit voltage, and fill factor were the best of the devices studied. The GaInNAsSb cells had the highest quantum efficienc...

Atom-resolved scanning tunneling microscopy of vertically ordered InAs quantum dots

We present cross-sectional scanning tunneling microscopy (STM) images of strain-induced, self-organized InAs quantum dots grown on GaAs. Samples containing 5 and 10 sequentially grown dot layers are investigated, and dots from different layers are seen to align in vertical columns. Our STM images are in general agreement with previous structural imaging, such as cross-sectional transmission electron microscopy, except that dot crowns appear more truncated. Although the size of the dots in most columns is roughly constant, monotonic changes in diameter are observed in some cases. STM analysis also reveals many new atom-resolved details of electronic structure, including dissolution of the InAs wetting layer and the presence of indium between the dot columns, which we attribute to segregation and diffusion of indium out of the wetting layer during overgrowth.

Reactive ion etching of GaN using CHF3/Ar and C2ClF5/Ar plasmas

III–V nitride semiconductors have great potential for optoelectronic and electronic devices due to their wide direct band gaps. Because GaN is chemically very stable, dry etching techniques must be established in order to fabricate devices. In this work, we report the reactive ion etching (RIE) of GaN using CHF3/Ar and C2ClF5/Ar plasmas. GaN films on (001) GaAs were grown by electron cyclotron resonance (ECR) plasma associated molecular beam epitaxy (MBE) and the films showed (0001)hex orientation along the surface normal. We used a Drytek DRIE‐184 rf plasma discharge RIE system with no ECR discharge or high energy Ar ion beam. The effects of rf plasma power, pressure, and gas flow rates on the etch rate were investigated and the surface contamination due to RIE was examined by XPS. The etch rate varied between 60 and 470 A/min. The conditions for etching were 100–500 W of rf plasma power, 60–300 mTorr of pressure, a CHF3 or C2ClF5 flow rate of 5–50 sccm and an Ar flow rate of 0–50 sccm. The highest etch ...

Integrated bio-fluorescence sensor.

Due to the recent explosion in optoelectronics for telecommunication applications, novel optoelectronic sensing structures can now be realized. In this work, we explore the integration of optoelectronic components towards miniature and portable fluorescence sensors. The integration of these micro-fabricated sensors with microfluidics and capillary networks may reduce the cost and complexity of current research instruments and open up a world of new applications in portable biological analysis systems. A novel optoelectronic design that capitalizes on current vertical-cavity surface-emitting laser (VCSEL) technology is explored. Specifically, VCSELs, optical emission filters and PIN photodetectors are fabricated as part of a monolithically integrated near-infrared fluorescence detection system. High-performance lasers and photodetectors have been characterized and integrated to form a complete sensor. Experimental results show that sensor sensitivity is limited by laser background. The laser background is caused by spontaneous emission emitted from the side of the VCSEL excitation source. Laser background will limit sensitivity in most integrated sensing designs due to locating excitation sources and photodetectors in such close proximity, and methods are proposed to reduce the laser background in such designs so that practical fluorescent detection limits can be achieved.

The role of antimony on properties of widely varying GaInNAsSb compositions

Antimony has been used as a surfactant to improve the quality of GaInNAs∕GaAs quantum wells for long-wavelength optoelectronics. We demonstrate the importance of antimony as a reactive surfactant and the proper usage of it with dilute nitrides in order to tailor the properties of solar cell and laser devices. The effects of the addition of antimony to low indium concentration (∼8%) and low strain GaInNAs material (for 1.0eV solar cell applications) were investigated. It was assumed previously that adding antimony helped all GaInNAs alloys, but the validity of this was not previously tested. The addition of antimony to high indium concentration (∼32%) and high strain GaInNAs samples led to a dramatic improvement in optical quality and a widening of the growth window, while it led to a degradation in the low indium (low strain) composition samples. The addition of indium under constant antimony flux also improved the optical quality of the GaInNAs material. Variations in the indium and antimony compositions...

Growth of epitaxial AlxGa1−xN films by pulsed laser deposition

Epitaxial AlxGa1−xN films have been grown on c-cut sapphire substrates at 800 °C and 10−2 Torr N2 by pulsed laser deposition (PLD) using a KrF laser. Throughout the composition range from x=0 to 0.6, the films show epitaxial patterns in reflection high-energy electron diffraction, in agreement with the results from x-ray diffraction. The lattice constants of the films vary linearly with x. The composition dependence of the band gaps of the films deviates from linearity and bows downward. This letter reports the application of PLD to controlling the lattice constant and band gap by varying the proportion of AlN and GaN in the target mixture.

Basic mechanisms of an atomic force microscope tip-induced nano-oxidation process of GaAs

An atomic force microscope (AFM) tip-induced direct nano-oxidation process of GaAs(100) substrates has been investigated, and is viewed as a promising method for the fabrication of nanometer-scale electronic devices such as single electron tunneling transistors. The effects of the AFM drawing parameters such as tip bias voltage and writing speed as well as the ambient humidity on the oxide line height and width were explored. The rate of reaction and its dependence on electric field strength and oxide thickness were examined to understand the basic mechanisms involved in the tip-induced oxidation of GaAs. The rate of oxidation/anodization was found to decrease rapidly with oxide film growth, which was explained at the simplest level in terms of a self-limiting influence of decreasing electric field strength.

Comparison of GaNAsSb and GaNAs as quantum-well barriers for GaInNAsSb optoelectronic devices operating at 1.3–1.55μm

GaNAsSb∕GaAs quantum wells were grown by solid-source molecular-beam epitaxy utilizing a radio-frequency nitrogen plasma source. The GaNAsSb layers, originally the quantum well barrier materials for GaInNAs(Sb) devices, were studied for their general growth characteristics as well as their structural and optical properties, which give an indication of its quality as a quantum well barrier material. Reflection high-energy electron diffraction, high-resolution x-ray diffraction, secondary-ion mass spectroscopy, and photoluminescence (PL) measurements were used to study those properties. The growth parameters including arsenic overpressure and substrate temperature were changed systematically to determine the properties during deposition and to optimize these conditions. It was found that the addition of antimony to GaNAs did not improve the material as it did for GaInNAs. PL measurements indicated a decreasing optical quality with an increasing substrate temperature and no change with the arsenic overpressu...

Theoretical Analysis of GeSn Alloys as a Gain Medium for a Si-Compatible Laser

In this paper, a theoretical analysis of unstrained GeSn alloys as a laser gain medium was performed. Using the empirical pseudopotential method, the band structure of GeSn alloys was simulated and verified against experimental data. This model shows that GeSn becomes direct bandgap with 6.55% Sn concentration. The optical gain of GeSn alloys with 0-10% Sn concentration was calculated with different n-type doping concentrations and injection levels. It is shown theoretically that adding Sn greatly increases the differential gain owing to the reduction of energy between the direct and indirect conduction bands. For a double-heterostructure laser, the model shows that at a cavity loss of 50 cm-1, the minimum threshold current density drops 60 times from Ge to Ge0.9Sn0.1, and the corresponding optimum n-doping concentration of the active layer drops by almost two orders of magnitude. These results indicate that GeSn alloys are good candidates for a Si-compatible laser.

Raman scattering study of GaN films

Raman spectra of GaN films grown by molecular‐beam epitaxy and hydride vapor‐phase epitaxy on GaAs and Al2O3 substrates have been studied. It was found that longitudinal phonon modes disappear from the spectra of n+ films due to screening by free carriers, but coupled plasmon phonon modes of the higher‐energy branch are not observed because of strong damping of plasmons. Precise values for phonon frequencies and linewidths are presented. No differences in phonon frequencies for the films of different thicknesses grown on different substrates have been found which indicates that the strain due to lattice and thermal‐expansion mismatch is relaxed by the formation of the dislocations very close to the substrate–film interface.