Daichi Imai

Growth kinetics and structural perfection of (InN)1/(GaN)1–20 short-period superlattices on +c-GaN template in dynamic atomic layer epitaxy

The growth kinetics and structural perfection of (InN)1/(GaN)1–20 short-period superlattices (SPSs) were investigated with their application to ordered alloys in mind. The SPSs were grown on +c-GaN template at 650 °C by dynamic atomic layer epitaxy in conventional plasma-assisted molecular beam epitaxy. It was found that coherent structured InN/GaN SPSs could be fabricated when the thickness of the GaN barrier was 4 ML or above. Below 3 ML, the formation of SPSs was quite difficult owing to the increased strain in the SPS structure caused by the use of GaN as a template. The effective or average In composition of the (InN)1/(GaN)4 SPSs was around 10%, and the corresponding InN coverage in the ∼1 ML-thick InN wells was 50%. It was found that the effective InN coverage in ∼1 ML-thick InN wells could be varied with the growth conditions. In fact, the effective In composition could be increased up to 13.5%, i.e., the corresponding effective InN coverage was about 68%, by improving the capping/freezing speed b...

Systematic study on dynamic atomic layer epitaxy of InN on/in +c-GaN matrix and fabrication of fine-structure InN/GaN quantum wells: Role of high growth temperature

The growth kinetics and properties of nominally 1-ML (monolayer)-thick InN wells on/in +c-GaN matrix fabricated using dynamic atomic layer epitaxy (D-ALEp) by plasma-assisted molecular beam epitaxy were systematically studied, with particular attention given to the effects of growth temperature. Attention was also given to how and where the ∼1-ML-thick InN layers were frozen or embedded on/in the +c-GaN matrix. The D-ALEp of InN on GaN was a two-stage process; in the 1st stage, an “In+N” bilayer/monolayer was formed on the GaN surface, while in the 2nd, this was capped by a GaN barrier layer. Each process was monitored in-situ using spectroscopic ellipsometry. The target growth temperature was above 620 °C and much higher than the upper critical epitaxy temperature of InN (∼500 °C). The “In+N” bilayer/monolayer tended to be an incommensurate phase, and the growth of InN layers was possible only when they were capped with a GaN layer. The InN layers could be coherently inserted into the GaN matrix under se...

Carrier recombination processes in Mg-doped N-polar InN films

We investigate the nonradiative carrier recombination (NR) process in Mg-doped p-InN films having lower photoluminescence (PL) intensity than n-InN films. The NR activation energy in the p-type films is found to be in a range of 9–15 meV, which is smaller than that in n-InN films (40–65 meV). We also investigate the effect of the greater mean free path of minority carriers in p-InN. At room temperature the collision rate of minority carriers with NR centers within the radiative lifetime in p-InN is found to be three orders of magnitude greater than that in n-InN.

Proposal of leak path passivation for InGaN solar cells to reduce the leakage current

We propose some general ways to passivate the leak paths in InGaN solar cells and report some experimental evidences of its effectiveness. By adopting an AlOx passivation process, the photovoltaic performances of GaN pn-junctions and InGaN solar cells, grown by molecular beam epitaxy, have been significantly improved. The open circuit voltage under 1 sun illumination increases from 1.46 to 2.26 V for a GaN pn junction, and from 0.95 to 1.27 V for an InGaN solar cell, demonstrating evidence of leak path passivation (LPP) by AlOx. The proposed LPP is expected to be a realistic way to exploit the potential of thick and relaxed but defective InGaN for solar cell applications.

Systematic study on dynamic atomic layer epitaxy of InN on/in +c-GaN matrix and fabrication of fine-structure InN/GaN quantum wells: Impact of excess In-atoms at high growth temperature

The growth kinetics of nominally one-monolayer (∼1-ML)-thick InN wells on/in the +c-GaN matrix fabricated using dynamic atomic layer epitaxy (D-ALEp) by plasma-assisted molecular beam epitaxy were systematically studied, with particular attention given to the impacts of excess In atoms and/or In droplets at a high growth temperature of 650 °C. Even at a constant growth temperature of 650 °C, the thickness of the sheet-island-like InN-well layers could be controlled/varied from 1-ML to 2-ML owing to the effect of excess In atoms and/or In droplets accumulated during growth. The possible growth mechanism is discussed based on the ring-shaped bright cathodoluminescence emissions introduced along the circumference of the In droplets during growth. The effective thermal stability of N atoms below the bilayer adsorbed In atoms was increased by the presence of In droplets, resulting in the freezing of 2-ML-thick InN wells into the GaN matrix. It therefore became possible to study the difference between the emiss...

Leak path passivation by in situ Al-N for InGaN solar cells operating at wavelengths up to 570 nm

A leak path passivation (LPP) technology for InGaN solar cells with photo-response up to 570 nm was developed by inserting in situ monolayers of Al-N into active layers. The InGaN layer in the passivated sample is partially relaxed and incorporates more than 23.5% In. By adopting in situ Al-N LPP, the open circuit voltage increases from 0.96 V to 1.35 V under one sun illumination (1.45–1.68 V under 72 suns), and the dark shunt resistance increases from 3.6 kΩ cm2 to 12.6 kΩ cm2, leading to an increase in power conversion efficiency by a factor of 2.0–2.26 (1–72 suns). This in situ Al-N LPP approach paves a way to exploit the full potential of InGaN for high efficiency solar cell application, accepting the reality of defective high-In-content thick and relaxed InGaN.

Erratum: “Carrier recombination processes in Mg-doped N-polar InN films” [Appl. Phys. Lett. 98, 181908 (2011)]

Daichi Imai, Yoshihiro Ishitani, Masayuki Fujiwara, Kazuhide Kusakabe, Xinqian Wang et al. Citation: Appl. Phys. Lett. 99, 089901 (2011); doi: 10.1063/1.3628458 View online: http://dx.doi.org/10.1063/1.3628458 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i8 Published by the AIP Publishing LLC.