Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications

Axial p-n and p-i-n junctions in GaAs0.7P0.3 nanowires are demonstrated and analyzed using electron beam induced current microscopy. Organized self-catalyzed nanowire arrays are grown by molecular beam epitaxy on nanopatterned Si substrates. The nanowires are doped using Be and Si impurities to obtain p- and n-type conductivity, respectively. A method to determine the doping type by analyzing the induced current in the vicinity of a Schottky contact is proposed. It is demonstrated that for the applied growth conditions using Ga as a catalyst, Si doping induces an n-type conductivity contrary to the GaAs self-catalyzed nanowire case, where Si was reported to yield a p-type doping. Active axial nanowire p-n junctions having a homogeneous composition along the axis are synthesized and the carrier concentration and minority carrier diffusion lengths are measured. To the best of our knowledge, this is the first report of axial p-n junctions in self-catalyzed GaAsP nanowires

Подтверждение методом картирования тока, наведенного электронным пучком, самопроизвольного легирования GaN нитевидных нанокристаллов из вицинальной подложки SiC/Si

Данная работа посвящена подтверждению спонтанного легирования GaN нитевидных нанокристаллов, выращенных на вицинальных гибридных подложках SiC/Si, методом картирования тока, наведенного электронным пучком. Нитевидные нанокристаллы (ННК) GaN выращивались на сингулярных и вицинальных подложках SiC/Si методом молекулярно-пучковой эпитаксии с плазменной активацией азота. Морфологические свойства нитевидных нанокристаллов исследуются методами растровой электронной микроскопии. Электрофизические свойства выращенных наноструктур исследуются методом картирования тока, наведенного электронным пучком. Методом картирования тока, наведенного электронным пучком, нами было подтверждено спонтанное легирование GaN ННК, выращенных на вицинальных пластинах SiC/Si. В свою очередь, было показано, что выращенные на сингулярных подложках SiC/Si GaN ННК не демонстрирует сигнала наведённого тока, что указывает на отсутствие легирования в таком НН

Caractérisation électrique de nanofils de semi-conducteurs III-V pour des applications photovoltaïques

Despite the potential of semiconductor nanowires (NWs) for photovoltaic applications, the performance of NW solar cells remains far behind the present record efficiency for planar devices. To enhance their photovoltaic conversion efficiency, in-depth analyses of their properties down to the nanoscale are needed to understand the origin of losses and solve the issues. Today with the nanotechnology revolution it became possible to perform characterizations on single NWs with nanoscale resolution. In this PhD work we focus on the analysis of III/V semiconductor nanowires using EBIC microscopy. Two types of materials have been characterized in this thesis. First, GaAsP NWs weredeveloped and characterized by EBIC. The EBIC studies first allowed to determine the doping type in the NWs as well as to quantify the carrier concentration. Secondly, the diffusion of Be atoms and the existence of an unintentional shell around the NW core was demonstrated. The second part of the work was dedicated to the study of nitride NWs. GaN and InGaN NWs were studied by EBIC, photo- and cathode-luminescence. The correlation of the results obtained by these three measurements made it possible to extract the Mg and Si doping concentrations in the NWs. The analysis of InGaN/GaN NWs showed that this type of nanostructures is promising for solar cells.Malgré le potentiel des nanofils (NFs) semi-conducteurs pour des applications photovoltaïques, la performance des cellules solaires à NFs reste toujours en deçà de celle des dispositifs à base de couches bidimensionnelles. Pour augmenter l’efficacité de conversion, l’analyse de leurs propriétés jusqu’à l’échelle nanométrique est nécessaire afin de comprendre l’origine des pertes de conversion et trouver des solutions adéquates pour les éliminer. Aujourd’hui avec l’arrivée massive des nanotechnologies il est devenu possible de caractériser des nanofils uniques avec une résolution nanométrique. Dans ce travail de thèse nous analysons des nanofils de semiconducteur III/V avec la microscopie EBIC dans la perspective d’extraire puis d’optimiser leurs paramètres électriques et ainsi améliorer le rendement photovoltaïque. Tout d’abord, des NFs de GaAsP élaborés par épitaxie par jets moléculaires (EJM) ont été caractérisés par des mesures EBIC.Sur des nanofils individuels, mais aussi sur des ensembles de NFs. Les études EBIC nous ont permis tout d’abord de déterminer le type de dopage dans les NFs ainsi que de quantifier la concentration des électrons et des trous. En second lieu, la diffusion des atomes de Be et l’existence d’une coquille parasite autour du coeur des NFs ont été mis en évidence. La seconde partie du travail a été consacré à l’étude de NFs de nitrures crus par EJM assisté par plasma. Les NFs de GaN et d’InGaN ont été étudiés par EBIC, photo- et cathodo-luminescence. La corrélation entre les résultats de ces trois mesures a permis d’extraire la concentration du dopage Mg et Si dans les NFs. L’analyse des NFs de GaN/InGaN a montré que ces nanostructures sont prometteuses pour des applications photovoltaïques

Electrical characterization of III-V semiconductor nanowires for photovoltaic applications

Malgré le potentiel des nanofils (NFs) semi-conducteurs pour des applications photovoltaïques, la performance des cellules solaires à NFs reste toujours en deçà de celle des dispositifs à base de couches bidimensionnelles. Pour augmenter l’efficacité de conversion, l’analyse de leurs propriétés jusqu’à l’échelle nanométrique est nécessaire afin de comprendre l’origine des pertes de conversion et trouver des solutions adéquates pour les éliminer. Aujourd’hui avec l’arrivée massive des nanotechnologies il est devenu possible de caractériser des nanofils uniques avec une résolution nanométrique. Dans ce travail de thèse nous analysons des nanofils de semiconducteur III/V avec la microscopie EBIC dans la perspective d’extraire puis d’optimiser leurs paramètres électriques et ainsi améliorer le rendement photovoltaïque. Tout d’abord, des NFs de GaAsP élaborés par épitaxie par jets moléculaires (EJM) ont été caractérisés par des mesures EBIC. Sur des nanofils individuels, mais aussi sur des ensembles de NFs. Les études EBIC nous ont permis tout d’abord de déterminer le type de dopage dans les NFs ainsi que de quantifier la concentration des électrons et des trous. En second lieu, la diffusion des atomes de Be et l’existence d’une coquille parasite autour du coeur des NFs ont été mis en évidence. La seconde partie du travail a été consacré à l’étude de NFs de nitrures crus par EJM assisté par plasma. Les NFs de GaN et d’InGaN ont été étudiés par EBIC, photo- et cathodo-luminescence. La corrélation entre les résultats de ces trois mesures a permis d’extraire la concentration du dopage Mg et Si dans les NFs. L’analyse des NFs de GaN/InGaN a montré que ces nanostructures sont prometteuses pour des applications photovoltaïques.Despite the potential of semiconductor nanowires (NWs) for photovoltaic applications, the performance of NW solar cells remains far behind the present record efficiency for planar devices. To enhance their photovoltaic conversion efficiency, in-depth analyses of their properties down to the nanoscale are needed to understand the origin of losses and solve the issues. Today with the nanotechnology revolution it became possible to perform characterizations on single NWs with nanoscale resolution. In this PhD work we focus on the analysis of III/V semiconductor nanowires using EBIC microscopy. Two types of materials have been characterized in this thesis. First, GaAsP NWs were developed and characterized by EBIC. The EBIC studies first allowed to determine the doping type in the NWs as well as to quantify the carrier concentration. Secondly, the diffusion of Be atoms and the existence of an unintentional shell around the NW core was demonstrated. The second part of the work was dedicated to the study of nitride NWs. GaN and InGaN NWs were studied by EBIC, photo- and cathode-luminescence. The correlation of the results obtained by these three measurements made it possible to extract the Mg and Si doping concentrations in the NWs. The analysis of InGaN/GaN NWs showed that this type of nanostructures is promising for solar cells

Electron beam induced current investigation of Ga(In)N nanowires (Conference Presentation)

International audienc

Investigation of the effect of the doping order in GaN nanowire p–n junctions grown by molecular-beam epitaxy

International audienceAbstract We analyse the electrical and optical properties of single GaN nanowire p–n junctions grown by plasma‐assisted molecular-beam epitaxy using magnesium and silicon as doping sources. Different junction architectures having either a n-base or a p-base structure are compared using optical and electrical analyses. Electron-beam induced current (EBIC) microscopy of the nanowires shows that in the case of a n-base p–n junction the parasitic radial growth enhanced by the magnesium (Mg) doping leads to a mixed axial-radial behaviour with strong wire-to-wire fluctuations of the junction position and shape. By reverting the doping order p-base p–n junctions with a purely axial well-defined structure and a low wire-to-wire dispersion are achieved. The good optical quality of the top n nanowire segment grown on a p-doped stem is preserved. A hole concentration in the p-doped segment exceeding 10 18 cm −3 was extracted from EBIC mapping and photoluminescence analyses. This high concentration is reached without degrading the nanowire morphology

Health-status outcomes with invasive or conservative care in coronary disease

BACKGROUND In the ISCHEMIA trial, an invasive strategy with angiographic assessment and revascularization did not reduce clinical events among patients with stable ischemic heart disease and moderate or severe ischemia. A secondary objective of the trial was to assess angina-related health status among these patients. METHODS We assessed angina-related symptoms, function, and quality of life with the Seattle Angina Questionnaire (SAQ) at randomization, at months 1.5, 3, and 6, and every 6 months thereafter in participants who had been randomly assigned to an invasive treatment strategy (2295 participants) or a conservative strategy (2322). Mixed-effects cumulative probability models within a Bayesian framework were used to estimate differences between the treatment groups. The primary outcome of this health-status analysis was the SAQ summary score (scores range from 0 to 100, with higher scores indicating better health status). All analyses were performed in the overall population and according to baseline angina frequency. RESULTS At baseline, 35% of patients reported having no angina in the previous month. SAQ summary scores increased in both treatment groups, with increases at 3, 12, and 36 months that were 4.1 points (95% credible interval, 3.2 to 5.0), 4.2 points (95% credible interval, 3.3 to 5.1), and 2.9 points (95% credible interval, 2.2 to 3.7) higher with the invasive strategy than with the conservative strategy. Differences were larger among participants who had more frequent angina at baseline (8.5 vs. 0.1 points at 3 months and 5.3 vs. 1.2 points at 36 months among participants with daily or weekly angina as compared with no angina). CONCLUSIONS In the overall trial population with moderate or severe ischemia, which included 35% of participants without angina at baseline, patients randomly assigned to the invasive strategy had greater improvement in angina-related health status than those assigned to the conservative strategy. The modest mean differences favoring the invasive strategy in the overall group reflected minimal differences among asymptomatic patients and larger differences among patients who had had angina at baseline

Initial invasive or conservative strategy for stable coronary disease

BACKGROUND Among patients with stable coronary disease and moderate or severe ischemia, whether clinical outcomes are better in those who receive an invasive intervention plus medical therapy than in those who receive medical therapy alone is uncertain. METHODS We randomly assigned 5179 patients with moderate or severe ischemia to an initial invasive strategy (angiography and revascularization when feasible) and medical therapy or to an initial conservative strategy of medical therapy alone and angiography if medical therapy failed. The primary outcome was a composite of death from cardiovascular causes, myocardial infarction, or hospitalization for unstable angina, heart failure, or resuscitated cardiac arrest. A key secondary outcome was death from cardiovascular causes or myocardial infarction. RESULTS Over a median of 3.2 years, 318 primary outcome events occurred in the invasive-strategy group and 352 occurred in the conservative-strategy group. At 6 months, the cumulative event rate was 5.3% in the invasive-strategy group and 3.4% in the conservative-strategy group (difference, 1.9 percentage points; 95% confidence interval [CI], 0.8 to 3.0); at 5 years, the cumulative event rate was 16.4% and 18.2%, respectively (difference, 121.8 percentage points; 95% CI, 124.7 to 1.0). Results were similar with respect to the key secondary outcome. The incidence of the primary outcome was sensitive to the definition of myocardial infarction; a secondary analysis yielded more procedural myocardial infarctions of uncertain clinical importance. There were 145 deaths in the invasive-strategy group and 144 deaths in the conservative-strategy group (hazard ratio, 1.05; 95% CI, 0.83 to 1.32). CONCLUSIONS Among patients with stable coronary disease and moderate or severe ischemia, we did not find evidence that an initial invasive strategy, as compared with an initial conservative strategy, reduced the risk of ischemic cardiovascular events or death from any cause over a median of 3.2 years. The trial findings were sensitive to the definition of myocardial infarction that was used