Shawn D. Burnham

Reproducible increased Mg incorporation and large hole concentration in GaN using metal modulated epitaxy

The metal modulated epitaxy (MME) growth technique is reported as a reliable approach to obtain reproducible large hole concentrations in Mg-doped GaN grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire substrates. An extremely Ga-rich flux was used, and modulated with the Mg source according to the MME growth technique. The shutter modulation approach of the MME technique allows optimal Mg surface coverage to build between MME cycles and Mg to incorporate at efficient levels in GaN films. The maximum sustained concentration of Mg obtained in GaN films using the MME technique was above 7×1020cm−3, leading to a hole concentration as high as 4.5×1018cm−3 at room temperature, with a mobility of 1.1cm2V−1s−1 and a resistivity of 1.3Ωcm. At 580K, the corresponding values were 2.6×1019cm−3, 1.2cm2V−1s−1, and 0.21Ωcm, respectively. Even under strong white light, the sample remained p-type with little change in the electrical parameters.

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.

Reproducible reflection high energy electron diffraction signatures for improvement of AlN using in situ growth regime characterization

Recently published methods that answer the previously unresolved critical issue of in situ growth regime determination during molecular beam epitaxy of AlN are used to address issues of material quality and intergrowth nonuniformity for improved repeatability using a modulated flux technique. A shutter modulation growth technique, defined as metal modulation epitaxy (MME), using the previously published reflection high-energy electron diffraction (RHEED) signatures was developed with the goal of obtaining materials with the properties of droplet regime materials, without the adverse effect of droplets. The films grown using MME were compared to films grown with no shutter modulation, and the surface roughness determined by atomic force microscopy was improved. For an unmodulated sample without droplets, the rms surface roughness was 6.9nm, while a sample with droplets had a rms surface roughness of 1.2nm. For the same Al flux that resulted in droplets with the unmodulated sample, the MME sample had no dro...

In situ growth regime characterization of AlN using reflection high energy electron diffraction

Methods for characterizing the growth regime of AlN are presented to deal with the unresolved critical issue of in situ growth regime determination during molecular beam epitaxy. Previously, there were no known reports of in situ measurements to determine the growth regime of AlN, making reproducibility difficult. Reflection high energy electron diffraction (RHEED) intensity transitions were observed upon opening and closing Al effusion cells, and three key signatures were identified. First, the time constant of the falling RHEED specular spot intensity upon Al shutter opening was found to vary inversely with the Al flux, providing a means of in situ calibration of Al flux. Second, a RHEED intensity spike or oscillation feature was observed at the onset of Al flux. Third, the behavior of the RHEED intensity rise upon Al shutter closing can be used to identify the Al-rich intermediate and Al-rich droplet regimes. These techniques and observations provide in situ methods of determining the growth regime of AlN, improving reproducibility and control.Methods for characterizing the growth regime of AlN are presented to deal with the unresolved critical issue of in situ growth regime determination during molecular beam epitaxy. Previously, there were no known reports of in situ measurements to determine the growth regime of AlN, making reproducibility difficult. Reflection high energy electron diffraction (RHEED) intensity transitions were observed upon opening and closing Al effusion cells, and three key signatures were identified. First, the time constant of the falling RHEED specular spot intensity upon Al shutter opening was found to vary inversely with the Al flux, providing a means of in situ calibration of Al flux. Second, a RHEED intensity spike or oscillation feature was observed at the onset of Al flux. Third, the behavior of the RHEED intensity rise upon Al shutter closing can be used to identify the Al-rich intermediate and Al-rich droplet regimes. These techniques and observations provide in situ methods of determining the growth regime of ...

Mg Doped GaN Using a Valved, Thermally Energetic Source: Enhanced Incorporation, Control and Quantitative Optimization

In this study, a thermally-energetic Mg source with an independent, valved-flux control was used to study the behavior of Mg incorporation into GaN. To observe effects of the thermal energy of the Mg flux on Mg incorporation, two Mg flux temperatures were investigated: one (900°C) well above the melting point of Mg and one (625°C) slightly below the melting point of Mg. Alternating Mg-doped and undoped GaN layers were grown at steps of increasing Mg flux, retaining a constant thermal energy, from below the saturation limit, to above the saturation limit. Results were analyzed and compared using secondary ion mass spectroscopy (SIMS). For a constant measured Mg flux, the incorporated Mg increased by more than an order of magnitude when the Mg thermal source temperature was raised from 625°C to 900°C. During SIMS analysis, the energy spectra of sputtered Ga atoms were fairly constant for a Mg flux above the saturation limit, and shifts for a Mg flux slightly below the critical flux for saturation, indicating a conductivity change, and possibly providing a quantitative means of optimizing p-type conduction. Furthermore, Mg incorporation into GaN strongly depends on the III-V flux ratio. During this study it was also observed that Mg incorporation into GaN was enhanced on a rough growth-layer surface under N-rich conditions, while a smoother growth-layer surface resulted in lower Mg incorporation, even under N-rich conditions1.

Resistivity analysis of epitaxially grown, doped semiconductors using energy dependent secondary ion mass spectroscopy

A characterization technique is discussed that allows quantitative optimization of doping in epitaxially grown semiconductors. This technique uses relative changes in the host atom secondary ion (HASI) energy distribution from secondary ion mass spectroscopy (SIMS) to indicate relative changes in conductivity of the material. Since SIMS is a destructive process due to sputtering through a film, a depth profile of the energy distribution of sputtered HASIs in a matrix will contain information on the conductivity of the layers of the film as a function of depth. This process is demonstrated with Mg-doped GaN, with the Mg flux slowly increased through the film. Three distinct regions of conductivity were observed: one with Mg concentration high enough to cause compensation and thus high resistivity, a second with moderate Mg concentration and low resistivity, and a third with little to no Mg doping, causing high resistivity due to the lack of free carriers. During SIMS analysis of the first region, the energ...