Influences of initial nitridation process on the optical and structural characterization of GaN layer grown on sapphire (0001) by metalorganic chemical vapor deposition

In this paper. we investigate the influences of the initial nitridation of sapphire substrates on the optical and structural characterizations in GaN films. Two GaN samples with and without 3 min nitridation process were investigated by photoluminescence (PL) spectroscopy in the temperature range of 12-300 K and double-crystal X-ray diffraction (XRD). In the 12 K PL spectra of the GaN sample without nitridation, four dominant peaks at 3.476, 3.409 3.362 and 3.308 eV were observed, which were assigned to donor bound exciton, excitons bound to stacking faults and extended structural defects. In the sample with nitridation, three peaks at 3.453, 3.365. and 3.308 eV were observed at 12 K, no peak related to stacking faults. XRD results at different reflections showed that there are more stacking faults in the samples without nitridation.

Defect states in cubic GaN epilayer grown on GaAs by metalorganic vapor phase epitaxy

Defect states in cubic GaN epilayers grown on GaAs were investigated with the photoluminescence technique. One shallow donor and two acceptors were identified to be involved in relevant optical transitions. The binding energies of the free excitons, the bound excitons. the donor and the acceptors were determined. These values are in good agreement with recent theoretical results.

长白山阔叶红松林CO2收支特征

植被-大气间CO2交换研究对准确评价陆地生态系统碳收支有重要意义。本研究以我国北方典型的植被类型—长白山阔叶红松林为研究对象，以国际公认的涡度相关技术为主要手段，结合微气象观测和生物学实验，探讨了森林碳交换过程与机制，建立了CO2通量观测数据的质量评价与控制体系。主要结论： （1）长白山阔叶红松林湍流通量贡献区主要集中在0.01~2Hz频率范围内，在弱湍流条件下存在通量低估现象。经过质量控制，CO2通量数据能够反映阔叶红松林与大气间CO2交换的真实过程； （2）森林生态系统CO2通量变化范围为-1.5~1.0 mg CO2•m-2•s-1，白天主要受光合有效辐射PAR的影响，但高的空气饱和水汽压差VPD对植被光合有一定的抑制作用，夜间CO2通量主要受温度的控制； （3）5cm温度是控制土壤碳排放的主导因素，由于雨热同季，因此干旱胁迫现象并不明显。土壤呼吸Q10变化范围为3.88~4.67，其值大小与温度区间有关； （4）2003~2005年森林GPP分别为1441、1312和1502gC•m-2，NEE分别为-188、-157和-186gC•m-2，森林碳收支的年际差异是由水热条件引起。叶片和群落高的弱光利用率和较强的光合吸收能力是其作为有200多年林龄的温带原始林，仍保持较高固碳能力的重要原因之一； （5）2003~2005年生态系统碳排放量分别为1252、1155和1326gC•m-2，土壤呼吸约占生态系统呼吸排放的80%； （6）长白山阔叶红松林具有较高的碳代谢能力，但由于光合吸收和呼吸释放都很活跃，森林总体表现为一稳定、适度的碳汇。这与其温带成熟原始林在碳循环领域的生态功能相一致。