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Polarization Dependence of the Optical Modal Gain in ZnO
Bumjin Kim,김광석,조채용 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.56 No.1
The modal gain spectrum due to the electron-hole plasma in ZnO was measured using a variablestripe-length method. A modal gain contour map for energy and stripe length allowed the modal gain saturation to be analyzed in terms of the spectral and the stripe length dependences. The modal gain was observed to be more significant in the TE mode than in the TM mode due to the polarization asymmetry of the wurtzite structure. We also found that the degree of polarization are enhanced as the stripe length increased, despite of modal gain saturation.
Generating Multi-agent Patrol Areas by Reinforcement Learning
Bumjin Park,Cheongwoong Kang,Jaesik Choi 제어로봇시스템학회 2021 제어로봇시스템학회 국제학술대회 논문집 Vol.2021 No.10
In this paper, we designed reinforcement learning environment for distributed patrolling agents. In the partially observable environment, the agents take actions for each one’s interest and the non-stationary problem in multi-agent setting encourages the agents not to invade other agent’s region. In our environment, the patrolling routes for the agents are generated implicitly. We suggested different types of the environments and evaluated with different initial positions of the agents. We also show how the reinforcement learning algorithm changes the distribution of agents as training time goes.
Recent Progress in Inorganic Hole Transport Materials for Efcient and Stable Perovskite Solar Cells
Bumjin Gil,Alan Jiwan Yun,Younghyun Lee,Jinhyun Kim,Byungho Lee,Byungwoo Park 대한금속·재료학회 2019 ELECTRONIC MATERIALS LETTERS Vol.15 No.5
Organic–inorganic hybrid perovskite solar cells (PSCs) are considered as one of the most promising emerging photovoltaicswith outstanding performance. However, the commonly used organic hole transport materials (HTMs) suffer fromheat-, light-, and bias-induced degradation along with defect diffusion and hygroscopic properties. To resolve these issuesin conventional HTMs, inorganic materials with superior chemical stability, high carrier mobility, and low cost have beendeveloped, demonstrating improved stability under rigorous conditions such as high temperature and long-term illumination. Understanding the properties of alternative inorganic HTMs is of prominent importance to realize more stable and efficientPSCs. This review summarizes the recent progresses in inorganic HTMs adopted in various device architectures, with theirremarkable achievements in efficiency and long-term stability.
A 62.6-pJ/Conversion Temperature Sensor with a Capacitor Voltage Division
Bumjin Park,Youngwoo Ji,Jae-Yoon Sim 대한전자공학회 2021 Journal of semiconductor technology and science Vol.21 No.1
This paper presents an ultra-low-power temperature sensor that detects the temperature with a leakage-based bandgap circuit and converts it with an asynchronous SAR ADC. The implemented temperature sensor consumes 487 ㎺ at 20℃ with an energy efficiency of 62.6 pJ/conversion. A 1-point and 2-point calibrations show peak-to-peak inaccuracies of -3.43/+2.77 ℃ and -1.63/+1.63℃, respectively, in a temperature range of -30-to-100 ℃. The supply sensitivity is 0.61 ℃/V at 20 ℃.
Optical Modal Gain Saturation of Exciton-Exciton Scattering and Electron-Hole Plasma in ZnO
Bumjin Kim,김광석 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.5
The optical modal gain spectrum for a ZnO thin film was measured by using a variable stripe length method. Using the differential equation for amplified spontaneous emission in a stripe geometry, we obtained stripe-length-dependent gain spectra, which enabled us to investigate the gain saturation in long stripe lengths. When the stripe was shorter than the saturation length, our analysis was in a good agreement with the gain obtained by commonly-used analysis\cite{1}. For intermediate density ($<$$n_{\rm{Mott}}\sim 3.7~\times~10^{19}~$cm$^{-3}$), the gain was attributed to inelastic exciton-exciton scattering while both exciton-exciton scattering and the electron-hole plasma contribute beyond the Mott densities ($>$$n_{\rm{Mott}}$). Both gains saturate with increasing stripe length, but the electron-hole plasma becomes relatively dominant for long stripe lengths ($\geq$$1000~\rm\mu m$) while the exciton-exciton scattering is relatively suppressed. This result suggests that exciton-exciton scattering gain saturation is more vulnerable to increase in the stripe length increase since the excitonic stimulated emission occurs on a coherence scale of the localized area.