It is theoretically accepted that the temperature (T) dependence of the exciton radiative lifetime is proportional to 3/2 power of the temperature. However, the various experimental temperature dependence of the exciton lifetime has been reported for samples with different crystal qualities, and it is not clear which factor of crystal quality determines the temperature dependence of the exciton radiative lifetime. We calculated the exciton radiative lifetime using the phononic-excitonic-radiative model, which is an analytical model that integrates excitons of various principal quantum numbers, free carriers, and phonons. For the components determining the crystal quality, the effects of nonradiative recombination, carrier trapping, bound excitons, exciton-polaritons, and energy-level broadening on the exciton radiative lifetime are considered. Our calculation model fits the experimental radiative lifetime roughly proportional to T3/2 in a wide range from cryogenic to room temperature and also minor deviation from the T3/2 dependence. We show that the exciton radiative lifetime reflects the effects of crystal quality, whereas conventional theories suggest that it is due to the momentum distribution of excitons.

In ultraviolet light emission devices, excitons are a high-efficiency emission source. However, the mechanism of experimentally observed dependence of excitonic radiative lifetimes on temperature (T) has not been discovered. We present a numerical simulation based on the phonon-exciton-radiation system, which reveals that the dependence of the radiative lifetime of GaN excitons on T is dominated by the population distribution among discrete and continuum energy states. This finding is in contrast to an existing model considering the existence of 1S exciton only. This population distribution is determined by the temperature-dependent integrated effect of background carrier density and exciton energy broadening, which induces a combination of high-order excitonic states and the continuum. Various experimental results on the dependence on temperature, including the functions of T3/2 and higher or lower power of T, are interpreted by a model integrating the interactions with the electron and phononic fields. The proposed model elucidates the corresponding effects of electronic and phononic processes in this complex system and provides a platform for the discussion of Wannier exciton dynamics under various thermal conditions, including nonequilibrium cases beyond the Saha-Boltzmann relation in population distribution.

Longitudinal optical (LO) phonon has strong electric interaction with particles and fields. Particularly, the interaction in III-nitrides is more significant than that in conventional III-V materials. We show phonon-exciton interaction properties in experimental PL spectrum analysis and theoretical calculation of population transfers of excitonic levels. Thermally nonequilibrium occupations of LO phonons and other modes generated by the LO-phonon decomposition are thought to shift the population distribution in principal quantum number states and kinetic energy to the higher energy side. The radiative exciton recombination lifetime is determined by the population distribution in the excitonic states, which is determined by the balance of the electronic and phononic elementary processes. The interaction of excitons and phonons releases the excess energy to the thermal bath of the lattice system, which sometimes yields negligible lattice temperature increase in the excited region or the nonequilibrium state between electron and phonon systems. A Raman scattering imaging measure is introduced to exhibit spatial transport of phonons generated by the energy relaxation and nonradiative recombination of the excited electrons and holes, where pump-probe measurements are enabled by the simultaneous irradiation of two laser beams. It is found that the phonon transport is blocked by the misfit dislocations located on a Ga0.84In0.16N/GaN heterointerface.

The 1S-exciton density NX(1) in GaN, AlN, and ZnO is calculated for thermal-nonequilibrium states where the temperatures of an electron–hole system and phonons are different in the range of 10–400 K. The ratio of NX(1) in AlN to that in ZnO reaches 21 despite the similar exciton-binding energies of AlN and ZnO, which is due to the higher rate of excitation by LO-phonon absorption in ZnO. This result reveals that thermal-nonequilibrium states significantly affect the validity of evaluation methods for physical parameters such as internal quantum efficiency of radiation. The ratio of NX(1) in AlN to that in ZnO is enhanced from 2.2 to 18 by the occupation of states of the principal quantum number n from 2 to 5. This result demonstrates the importance of the discussion on the n≥3 states which have not been taken into account in other analyses. The main reason for the decrease in NX(1) is found to be the increase in the temperature of LO phonons rather than LA phonons, which indicates the importance of LO-phonon control in light-emitting devices. The results for general thermal-nonequilibrium states are nontrivial because the mechanisms of the population balance are complicated owing to the several-n occupation and the transition rates determined by various factors. Our analyses and discussions quantitatively unveil the LO- and LA-phonon effects on the thermal-nonequilibrium excitation and deexcitation dynamics of excitons and provide the basis for design of highly efficient light-emitting devices particularly in the ultraviolet region.

Since heat generation in electronic devices induces the degradation of performance, the understanding of their thermal properties is required. InGaN and AlInN alloys are possibly the key materials for optical devices in green gap and high power transistors. Characteristic properties of local strain, fluctuation of energy gap, carrier dynamics, and so forth due to spatial nonuniformity of alloy composition are now under investigation, while nonuniform thermal conduction remains unclear. In this report we show nonuniform heat or phonon conductance in the nonuniform field of InGaN alloy composition. An In 0.16 Ga 0.84 N (110nm) film grown on GaN grown on a sapphire substrate was characterized by micro Raman spectroscopy with controlling spot positions of two color lasers: 325nm for electronic excitation and phonon generation and 532nm for Raman probe. Thermal properties were analyzed by two-dimensional mapping of Raman signal by 532 nm line and its energy shift by introducing the 325nm line. It is found that the phonon transport mainly takes place along the region with low GaN mole fraction or low phonon energy, while some of phonons are transported through narrow regions with higher GaN mole fraction. This pump and probe Raman method gives us phonon transport properties in semiconductors.

Excitation and deexcitation dynamics of excitons in GaN are analyzed by theoretical simulation using a set of rate equations based on a phononic-excitonic-radiative (PXR) model, which is applied to the analysis of experimentally-observed photoluminescence (PL) properties using a short pulse excitation. In phononic processes, deformation and piezoelectric interactions of the LA phonon and Frohlich interaction of the LO phonon are taken into account. This model is successfully applied to the analysis of experimentally-observed emission line intensity ratios for excitons. This analysis reveals that the strong population exchange between the state of the principal quantum number n = 2 and the continuum takes place due to the increase in temperature. Further, the long experimental radiative lifetime component in the temporal PL decay curve up to 100 ns at room temperature (RT) is attributed to the shift of the population distribution to higher n states, which work as population reservoirs of the n = 1 state. Theoretical calculation using this model suggests that the dominant phonon mode in the excitation transfers from the n = 1 and 2 states shifts from the LA phonon to the LO phonon due to the increase in temperature from 130 K to 240 K. The PXR simulation model is feasible for the analysis of exciton-carrier dynamics and radiation efficiency analyses. (C) 2019 The Japan Society of Applied Physics

GaN、AlN、ZnO 等では励起子の束縛エネルギ ーが室温と同程度以上に高く、励起子が材料の光 学特性に大きな影響を与える。GaN のレーザデ バイスでは 150K以上で励起子による発振がなく なることが観測されている[1]が、その詳細なメ カニズムは明らかになっていない。種々の条件下 における光学的デバイスの解析、開発のためには、 励起子や自由キャリア等の各状態のダイナミク スを記述する理論体系が必要である。各状態の密 度の定常解や時間発展を記述するレート方程式 のモデルが広く使われているが、各状態間の遷移 確率を表すレート係数の値の詳細な計算はこれ までほとんど行われてこなかった。 本研究では、各密度のダイナミクスの理論体系 の構築を目指し、バルクの GaN における、主量 子数 nが 5までの励起子と自由キャリアのレート 係数を理論的に詳細に計算した。輻射・電子衝突 遷移のレート係数の計算には水素プラズマモデ ルを利用し、フォノン吸放出による遷移のレート 係数はフェルミの黄金律から厳密に計算した。 Fig. 1 はレート方程式の定常解である各ポピュレ ーション密度の温度依存性である。温度が上がる 程、1s励起子の密度 NX(1)に対する上準位の密度 (nが 2以上の励起子密度NX(n)と自由電子密度Ne) の比 ru が上がる。ru が上がる程、1s 励起子の定 常状態からの減衰時間d が再結合時間rec(単位時 間当たりの再結合確率の逆数)に比べて上がって いき、d/recが 10以上にもなる(Fig. 2)。これらの d とrec の値を同じとみなしている実験研究が多 いが、特に室温付近の場合等ではそれらは一致し ないと考えられる。 [1] S. Bidnyk, et. al, Appl. Phys. Lett. 74, 1 (1999).

Population distributions and transition fluxes of the A exciton in bulk GaN are theoretically analyzed using rate equations of states of the principal quantum number n up to 5 and the continuum. These rate equations consist of the terms of radiative, electron-collisional, and phononic processes. The dependence of the rate coefficients on temperature is revealed on the basis of the collisional-radiative model of hydrogen plasma for the electron-collisional processes and theoretical formulation using Fermi's "golden rule" for the phononic processes. The respective effects of the variations in electron, exciton, and lattice temperatures are exhibited. This analysis is a base of the discussion on nonthermal equilibrium states of carrier-exciton-phonon dynamics. It is found that the exciton dissociation is enhanced even below 150 K mainly by the increase in the lattice temperature. When the thermal-equilibrium temperature increases, the population fluxes between the states of n > 1 and the continuum become more dominant. Below 20 K, the severe deviation from the Saha-Boltzmann distribution occurs owing to the interband excitation flux being higher than the excitation flux from the 1S state. The population decay time of the 1S state at 300K is more than ten times longer than the recombination lifetime of excitons with kinetic energy but without the upper levels (n > 1 and the continuum). This phenomenon is caused by a shift of population distribution to the upper levels. This phonon-exciton-radiation model gives insights into the limitations of conventional analyses such as the ABC model, the Arrhenius plot, the two-level model (n = 1 and the continuum), and the neglect of the upper levels.

In the tandem mirror GAMMA 10/PDX, molecular activated recombination (MAR) leading to plasma detachment has been observed by additional hydrogen gas injection to the divertor simulation plasma (i.e. end loss plasma) which is exposed to the V-shaped target in the divertor simulation experimental module (D-module). The temperature near the corner of the V-shaped target decreased from ∼23eV to ∼2eV as the neutral pressure in the D-module increased. A clear density rollover was observed at ∼2Pa. A position of the density maximum moves to upstream of the plasma with increase in the neutral pressure and the density near the corner of the target decreases to detach the plasma from the target. After the occurrence of the density rollover, the Balmer β intensity decreases as with the density but the Balmer α intensity continues to increase, indicating the dissociative attachment process in MAR is more dominant than the ion conversion process although the rate coefficient of the former process is lower than that of the latter one, which is calculated by using a collisional radiative model. This would be caused by the MAR process related to triatomic hydrogen molecules which significantly contributed to the detachment process.

励起子及び励起子分子は高い輻射再結合確率を持つため、窒化物半導体などワイドギャップ半 導体において注目されている。GaN のフォトルミネッセンス測定では室温での励起子発光が確認 されているものの 、レーザ素子の閾値直下の動作条件では 150 K以上において励起が解離してい るとの報告がある 。またパルス励起により生成された励起子では非熱平衡状態が数百 ps 続くこ とが分かっているが、励起子分子の生成・消滅のダイナミクスは明らかになっていない。我々は、 励起子-励起子分子系の非熱平衡状態におけるダイナミクスの理論計算を行うことにより励起子 分子の安定性評価を進めている。本研究では、バルクGaNの励起子及び励起子分子における励起、 脱励起のダイナミクスのシミュレーションを電子衝突、フォノンの吸放出、光学遷移を考慮して 行った。電子衝突過程では水素プラズマ系での状態遷移確率を基に GaN の物性値を取り入れ、フ ォノン過程ではフレーリッヒ、変形ポテンシャル及びピエゾ電気の各相互作用を考慮してダイナ ミクス計算を行った。励起子主量子数は 5 つ取り入れ、これらの励起状態、自由電子及び励起子 分子の基底準位と励起準位のそれぞれのポピュレーションに関するレート方程式をたて、定常解 を求めた。電子温度 Te、励起子温度 Tx及び格子温度 Tlをそれぞれ独立に設定して非熱平衡状態の 考察を行った。状態遷移概要図を Fig.1に示す。 計算の結果、励起子及び励起子分子の脱励起には電子温度や励起子温度に比べて格子温度が大 きく影響することが分かった。主量子数 p=1における励起子のポピュレーションは格子温度が 100 K程度から大きく減少してゆくことが分かった。これは主量子数 p=1 から p=2 へ励起し、それ以 上の準位で連続的なはしご状励起になっていることが原因であると考えられる。この励起子の脱 励起は励起子分子の減少にも影響している。Fig2に示すように、Tl=10,100,300[K]と変化させたと きの励起子分子のポピュレーションは格子温度の上昇に対して減少している。Tl が大きくなるに つれ励起子分子解離確率が大きくなると同時に、Tl=100K以上では 1s状態の励起子のポピュレー ションが減少するため 3Tl=300Kでは励起子分子のポピュレーションが減少していると考えられる。

In GAMMA 10/PDX, a temperature-controlled V-shaped target has been utilized to study an effect of target temperature on hydrogen recycling. The V-shaped tungsten target in the divertor simulation experimental module (D-module) is heated up to 573 K and it is exposed to the end loss plasma. It is found that the H-alpha intensity and the electron density of the plasma in front of the target are positively correlated with the target temperature, indicating that the recycling is enhanced due to increase in the target temperature. Moreover, additional hydrogen gas is injected into the D-module with the temperature-controlled target. As the amount of hydrogen gas injection increased, enhancement of hydrogen recycling by high temperature target becomes smaller.

Excitation and deexcitation dynamics of excitons and biexcitons are calculated on the basis of the collisions of excitons, electrons, and phonons. Fröhlich, deformation, and piezo-electric interactions are taken into account for the phonon collision system. A set of rate equations on populations of exciton states of several principal quantum numbers (p), free electrons, biexciton states of hydrogen like X1Σg+ and several excited states is solved under steady state condition. The temperatures of electron, phonon, and excion are varied independently to represent the non-thermal equilibrium states. As a result, the dissociation of excitons and biexcitons are found to be dominated by phonon temperature rather than other temperatures of electron and exciton. The exciton states p=1 decreases drastically at higher temperatures than approximately 150 K, which is caused by the excitation from p=1 to p=2 and consecutive ladder like excitations to upper states. This dissociation causes the decrease in biexciton population.

In GAMMA 10/PDX, divertor simulation studies have been started as a new research plan by using end-loss plasma flux at the end-mirror region. A divertor simulation experimental module (D-module) was installed in the west end-cell. Here we introduce a novel microwave interferometer system with a phase imaging method, and all the crucial components are designed and tested. Finally, we tested the interferometer system using a Teflon plate in substitution for plasma. In order to improve the system accuracy, the phase detection circuit was modified and the phase difference of each channel was verified.