Arrangement of nitrogen atoms in GaAsN alloys determined by scanning tunneling microscopy
Arrangement of nitrogen atoms in GaAsN alloys determined by scanning tunneling microscopy
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扫描隧道显微镜测定GaAsN合金中氮原子的排列
DOI：
10.1063/1.1337625
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发表时间：
2001
影响因子：
4
通讯作者：
U. W. Pohl
中科院分区：
文献类型：
--
作者：
H. McKay;R. Feenstra;T. Schmidtling;U. W. Pohl
用扫描隧道显微镜测量了GaAs0.983N0.017中氮原子对的分布函数。在相对于解理(10)面的第一和第三平面上的氮原子被成像。虽然在较大的间隔下，N对间隔的分布是随机的，但发现最近邻对的数量略有增加，特别是在[001]取向上。近年来，人们对低氮含量的GaAsN和InGaAsN合金有了很大的兴趣，通常只有几个百分点。在这个高度失配的阴离子体系中预测的大带隙弯曲导致在适度N含量的情况下相当大的带隙减小的可能性[1，2]。重要的应用包括波长在1.31.55μm范围内的激光器，以及带隙在1.0eV左右的太阳能电池[3]。一般来说，GaAsN和InGaAsN合金都表现出了不均匀性，如宽的光致发光线宽度、可变的光致发光衰减时间和较短的少数载流子扩散长度[4-7]。这种观察通常被认为是材料成分波动的指标，尽管缺乏对这种波动的直接结构表征。在这项工作中，我们使用横截面扫描隧道显微镜(STM)直接探测了Ga As 0.983 N 0.017合金中N原子的排列。两个不同对比度的氮原子被成像，我们将其分配给相对于(10)表面的第一和第三表面的占据。通过对连续带状合金材料中约1000N原子位置的精确测定，我们计算了原子对分离的分布函数。结果表明，N原子的排列与随机占据的结果非常一致，只是发现最近邻N对的出现增加了。采用金属有机气相外延(MOVPE)技术，在530~570℃温度范围内，用TMGa、TbAs或AsH3和叔丁基肼(TBHy)在氢气载气下在GaAs001衬底上生长了GaAsN合金。有关材料生长和表征的更多详细信息，请参阅参考文献。[8]。所研究的特殊薄膜由GaAs缓冲层、0.983 N 0.017层、52 nm厚的GaAs隔离层、0.972 N 0.028层和370 nm厚的GaAs帽层组成。高分辨X射线衍射仪测得GaAsN层的厚度约为18 nm；STM测量其厚度为14-19 nm，具体取决于晶片中的位置。用高分辨X射线衍射仪测定了上述样品中的氮含量；对这些量的扫描隧道显微镜测量也得到了类似的结果。在砷化镓衬底、缓冲层和帽层中一次掺入硅
The pair distribution function of nitrogen atoms in GaAs 0.983N0.017 has been determined by scanning tunneling microscopy. Nitrogen atoms in the first and third planes relative to the cleaved (1 0) surface are imaged. A modest enhancement in the number of nearest-neighbor pairs particularly with [001] orientation is found, although at larger separations the distribution of N pair separations is found to be random. Considerable interest has developed in recent years concerning GaAsN and InGaAsN alloys with low N content, typically a few %. The large predicted band gap bowing in this system of highly mismatched anions leads to the possibility of considerable band gap reduction with modest N content [1,2]. Important applications include lasers with wavelength in the 1.3‐1.55 μm range, as well as solar cells with band gap around 1.0 eV [3]. Generally speaking the GaAsN and InGaAsN alloys have displayed evidence of inhomogeneities, such as broad photoluminescence (PL) line widths, variable PL decay times, and short minority carrier diffusion lengths [4-7]. Such observations are often taken as an indicator of compositional fluctuations in the materials, although direct structural characterization of such fluctuations is lacking. In this work we use cross-sectional scanning tunneling microscopy (STM) to directly probe the arrangement of N atoms in GaAs 0.983 N 0.017 alloys. Nitrogen atoms of two distinct contrast levels are imaged, which we assign to occupation in the first and third surface planes relative to the (1 0) surface. From an accurate determination of the position of about 1000 N atoms in a continuous strip of alloy material, we compute the distribution function of pair separations. The arrangement of N atoms is found to be quite consistent with that expected from random occupation, with the exception that an enhanced occurrence of nearest-neighbor N pairs is found. The GaAsN alloys studied here were grown on GaAs(001) substrates by metal organic vapor phase epitaxy (MOVPE) at temperatures between 530 and 570 C using TMGa, TBAs or AsH3, and tertiarybutylhydrazine (TBHy) under hydrogen carrier gas. Additional details of the growth and characterization of the material can be found in Ref. [8]. The particular film studied here consists of a GaAs buffer layer followed by a GaAs 0.983 N 0.017 layer, a 52 nm thick GaAs spacer layer, a GaAs 0.972 N 0.028 layer, and a 370 nm thick GaAs cap layer. The thickness of the GaAsN layers was determined by high-resolution x-ray diffraction (HRXRD) to be about 18 nm; STM measurements of their thickness gave results of 14‐19 nm depending on location in the wafer. The N contents quoted above were also determined by HRXRD; STM measurements for those quantities gave similar results. The GaAs substrate, buffer layer, and cap layer were doped with Si at a con1