Elaissa Trybus

Metal modulation epitaxy growth for extremely high hole concentrations above 1019cm−3 in GaN

The free hole carriers in GaN have been limited to concentrations in the low 1018cm−3 range due to the deep activation energy, lower solubility, and compensation from defects, therefore, limiting doping efficiency to about 1%. Herein, we report an enhanced doping efficiency up to ∼10% in GaN by a periodic doping, metal modulation epitaxy growth technique. The hole concentrations grown by periodically modulating Ga atoms and Mg dopants were over ∼1.5×1019cm−3.

Transient atomic behavior and surface kinetics of GaN

An in-depth model for the transient behavior of metal atoms adsorbed on the surface of GaN is developed. This model is developed by qualitatively analyzing transient reflection high energy electron diffraction (RHEED) signals, which were recorded for a variety of growth conditions of GaN grown by molecular-beam epitaxy (MBE) using metal-modulated epitaxy (MME). Details such as the initial desorption of a nitrogen adlayer and the formation of the Ga monolayer, bilayer, and droplets are monitored using RHEED and related to Ga flux and shutter cycles. The suggested model increases the understanding of the surface kinetics of GaN, provides an indirect method of monitoring the kinetic evolution of these surfaces, and introduces a novel method of in situ growth rate determination.

III-nitrides on oxygen- and zinc-face ZnO substrates

The characteristics of III-nitrides grown on zinc- and oxygen-face ZnO by plasma-assisted molecular beam epitaxy were investigated. The reflection high-energy electron diffraction pattern indicates formation of a cubic phase at the interface between III-nitride and both Zn- and O-face ZnO. The polarity indicates that Zn-face ZnO leads to a single polarity, while O-face ZnO forms mixed polarity of III-nitrides. Furthermore, by using a vicinal ZnO substrate, the terrace-step growth of GaN was realized with a reduction by two orders of magnitude in the dislocation-related etch pit density to ∼108cm−2, while a dislocation density of ∼1010cm−2 was obtained on the on-axis ZnO substrates.

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

Dual-color emission in hybrid III-nitride/ZnO light emitting diodes

We report dual-color production of the blue and green regions using hybrid nitride/ZnO light emitting diode (LED) structures grown on ZnO substrates. The blue emission is ascribed to the near-band edge transition in InGaN while green emission is related to Zn-related defect levels formed by the unintentional interdiffusion of Zn into the InGaN active layer from the ZnO substrates.

Deeply degenerate p-type GaN grown by metal modulated epitaxy

Metal Modulation Epitaxy (MME) [1] was introduced as a new growth technique wherein only the metal fluxes (Al, Ga, In, Si, and Mg) are modulated in a short periodic fashion in a plasma-assisted MBE system, while maintaining a continuous nitrogen plasma flux. This initially lead to dramatic improvements in grain size and demonstrated hole concentrations in excess of 4.5×10¹⁸ cm³ [1, 2, 3] for Mg-doped GaN grown on c-plane sapphire, eventually leading to p-type GaN with hole concentrations of ~1.5×10¹⁹ cm⁻³[4] and >4e19 cm³ [5]. Herein, we report the MME growth and characterization of deeply degenerately doped p-GaN with hole concentrations of ~7.9× 10¹⁹ cm³.