P. Frajtag

Electric field control of room temperature ferromagnetism in III-N dilute magnetic semiconductor films

We report on the electrical field control of ferromagnetism (FM) at room temperature in III-N dilute magnetic semiconductor (DMS) films. A GaMnN layer was grown on top of an n-GaN substrate and found to be almost always paramagnetic. However, when grown on a p-type GaN layer, a strong saturation magnetization (Ms) was observed. This FM in GaMnN can be controlled by depletion of the holes in the GaMnN/p-GaN/n-GaN multilayer structures. We have demonstrated the dependence of the FM on the thickness of the p-GaN in this heterostructure and on the applied bias to the GaN p-n junction. The Ms was measured by an alternating gradient magnetometer (AGM) and a strong correlation between the hole concentration near the GaMnN/p-GaN interface and the magnetic properties of the DMS was observed. At room temperature an anomalous Hall effect was measured for zero bias and an ordinary Hall effect for reverse bias in a fully depleted p-GaN layer. This is in close agreement with the AGM measurement results.

Embedded voids approach for low defect density in epitaxial GaN films

We have developed a technique for defect reduction in GaN epitaxial films grown on sapphire substrates. This technique relies on the generation of high densities of embedded microvoids (∼108/cm2), a few microns long and less than a micron in diameter. These voids are located near the sapphire substrate, where high densities of dislocations are present. Network of embedded voids offer free surfaces that act as dislocation sinks or termination sites for the dislocations generated at the GaN/sapphire interface. Both transmission electron and atomic force microscopy results confirm the uniform reduction of the dislocation density by two orders of magnitude.

Improved light-emitting diode performance by conformal overgrowth of multiple quantum wells and fully coalesced p-type GaN on GaN nanowires

We demonstrate a light-emitting diode (LED) structure with multiple quantum wells (MQWs) conformally grown on semipolar and nonpolar plane facets of n-GaN nanowires (NWs), followed by deposition of fully coalesced p-GaN on these nanowires. Overgrowth on the nanowires’ tips results in inclusion of high density voids, about one micron in height, in the GaN film. The light output intensity of NWs LEDs is more than three times higher than corresponding c-plane LEDs grown simultaneously. We believe this results from a reduced defect density, increased effective area of conformally grown MQWs, absence of polar plane orientation, and improved light extraction.

Growing thin films that contain embedded voids

Over the last decade gallium nitride (GaN) has become the semiconductor material of choice in several optical and electronic devices. In particular, GaN can help realize the potential of solid-state lighting (SSL), a multi-billion dollar emerging technology using LEDs that promises to fundamentally alter lighting and contribute to energy savings. Unfortunately, GaN and other III-nitride materials—such as aluminum and indium nitrides or their alloys—suffer from a high density of dislocations and other defects ( 108–1010cm 2) because of the lack of lattice-matched substrates. In general, these defects act as nonradiative recombination and scattering centers that impact the diffusion length1 and minority carrier lifetime, reduce thermal conductivity,2 and form easy pathways for impurity diffusion. Thus, they limit the performance, reliability, breakdown voltage, and lifetime3 of both optoelectronic and power devices. Here, we discuss our approach to reduce GaN defects by controlling the void density. Dislocations generated at the GaN/sapphire interface run through the crystalline structure of the films and terminate at free surfaces, affecting the function of active, multi-quantum well (MQW) layers. To combat this, our embedded voids approach (EVA)4 intentionally introduces a high density of micro-voids—a few micrometers in length and less than a micrometer in diameter—into the GaN layer near its interface with the substrate. By doing so, we create an efficient ‘trapping zone’ near the GaN/sapphire interface, where the voids act as both sinks for defects as well as expansion joints for lattice mismatches. The active layers of III-nitride epitaxial films are then grown on the void-embedded layer, free from dislocation disturbance. EVA is a three-step process (see Figure 1). First, we grow bulk GaN by metal organic chemical vapor deposition on the sapphire substrate. Then, we form GaN nanowires (NWs) from the Figure 1. 3D schematics of (a) bulk-grown gallium nitride (GaN) film on a sapphire substrate, (b) GaN nanowires (NWs) formed using the maskless inductively coupled plasma-reactive ion etching (ICP-RIE), and (c) NW overgrowth and void formation.5