arXiv subjects

Toshihiro Ohki

Publications and source records attributed to Toshihiro Ohki.

3 recordsLinked to original sources

arXiv2025

Record negative photoconductivity in N-polar AlGaN/GaN quantum-well heterostructures

The AlGaN/GaN quantum-well heterostructures typically exhibit a positive photoconductivity (PPC) during the light illumination. Surprisingly, we found that introducing the GaN/AlN superlattice (SL) back barrier into N-polar AlGaN/GaN quantum-well heterostructures induces a transition in these heterostructures from PPC to negativie photoconductivity (NPC) as the SL period number increased at room temperature. This transition occurred under an infrared light illumination and can be well explained in terms of the excitation of hot electrons from the two-dimensional electron gas and subsequent trapping them in a SL structure. The NPC effect observed in N-polar AlGaN/GaN heterostructures with SL back barrier exhibits photoconductivity yield exceeding 85 % and thus is the largest ones reported so far for semiconductors. In addition, NPC signal remains relatively stable at high temperatures up to 400 K. The obtained results can be interesting for the development of NPC related devices such as photoelectric logic gates, photoelectronic memory and infrared photodetectors.

cond-mat.mtrl-sci↗

arXiv2025

A simple method for detection and quantitative estimation of deep levels in a barrier layer of AlGaN/GaN HEMT structures by analysis of light induced threshold voltage shift

The characterization of deep levels in AlGaN/GaN heterostructures is one of the most important problems in GaN high electron mobility transistors (HEMTs) technology. This work reports on a technique for determination of deep level concentration in AlGaN/GaN HEMT structures. The proposed method is relatively simple, and it is based on the detection of free holes created by optically induced transitions of electrons from the deep levels to the conduction band. The developed method can detect and provide quantitative estimation of deep level traps in a barrier layer of AlGaN/GaN HEMT structures. Furthermore, it provides a framework for analysis of light induced threshold voltage shift, which includes an important experimental criterion of determination whether the holes are generated or not in the AlGaN/GaN HEMT structures by sub-band gap illumination. The method was verified by applications it to a study of the deep levels in GaN HEMTs grown on various substrates, i.e. SiC and GaN.

cond-mat.mtrl-sci↗

arXiv2025

Unusually high-density 2D electron gases in N-polar AlGaN/GaN heterostructures with GaN/AlN superlattice back barriers grown on sapphire substrates

We reported on the observation of extremely high-density ($>10^{14}$cm$^{-2}$) 2D electron gas in N-polar AlGaN/GaN heterostructures grown on sapphire substrates. Due to introducing the GaN/AlN superlattice (SL) back barrier between the GaN buffer layer and AlGaN barrier layer, we observed a giant enhancement of the 2D electron gas density at the GaN/AlGaN interface from $3\times10^{13}$cm$^{-2}$ (without SL) to $1.4\times10^{14}$cm$^{-2}$ (with SL back barrier) that is only one order of magnitude below the intrinsic crystal limit of $\approx10^{15}$cm$^{-2}$. We found that the changes of 2D electron gas density with SL correlated well with the changes of the wafer warp parameter which suggests that the strains are responsible for the 2D electron gas density enhancement (reduction of the piezoelectric polarization in the GaN channel). Nevertheless, this finding is probably insufficient to fully explain the observed high 2D electron gas density. Simultaneously, the room temperature electron mobility was 169 cm$^2$/Vs, which with the electron density of $1.4\times10^{14}$cm$^{-2}$ gives a low sheet resistance of 264 $Ω$/sq (one of the lowest reported so far for the N-polar 2D electron gas channel). Finally, the possibility of application of such high-density 2D electron gas with low sheet resistance to transistors, emitters and detectors was discussed.

cond-mat.mes-hall↗