Bougerol Catherine, Robin Eric, Di Russo Enrico, Bellet-Amalric Edith, Grenier Vincent, Ajay Akhil, Rigutti Lorenzo, Monroy Eva
Univ. Grenoble-Alpes, Institut Néel-CNRS, 25 av. des Martyrs, 38000 Grenoble, France.
Univ. Grenoble-Alpes, CEA, IRIG, MEM, 17 av. des Martyrs, 38000 Grenoble, France.
ACS Appl Mater Interfaces. 2021 Jan 27;13(3):4165-4173. doi: 10.1021/acsami.0c19174. Epub 2021 Jan 15.
Attaining low-resistivity AlGaN layers is one keystone to improve the efficiency of light-emitting devices in the ultraviolet spectral range. Here, we present a microstructural analysis of AlGaN/Ge samples with 0 ≤ ≤ 1, and a nominal doping level in the range of 10 cm, together with the measurement of Ge concentration and its spatial distribution down to the nanometer scale. AlGaN/Ge samples with ≤ 0.2 do not present any sign of inhomogeneity. However, samples with > 0.4 display μm-size Ge crystallites at the surface. Ge segregation is not restricted to the surface: Ge-rich regions with a size of tens of nanometers are observed inside the AlGaN/Ge layers, generally associated with Ga-rich regions around structural defects. With these local exceptions, the AlGaN/Ge matrix presents a homogeneous Ge composition which can be significantly lower than the nominal doping level. Precise measurements of Ge in the matrix provide a view of the solubility diagram of Ge in AlGaN as a function of the Al mole fraction. The solubility of Ge in AlN is extremely low. Between AlN and GaN, the solubility increases linearly with the Ga mole fraction in the ternary alloy, which suggests that the Ge incorporation takes place by substitution of Ga atoms only. The maximum percentage of Ga sites occupied by Ge saturates around 1%. The solubility issues and Ge segregation phenomena at different length scales likely play a role in the efficiency of Ge as an n-type AlGaN dopant, even at Al concentrations where Ge DX centers are not expected to manifest. Therefore, this information can have direct impact on the performance of Ge-doped AlGaN light-emitting diodes, particularly in the spectral range for disinfection (≈260 nm), which requires heavily doped alloys with a high Al mole fraction.
获得低电阻率的AlGaN层是提高紫外光谱范围内发光器件效率的关键之一。在此，我们对0≤x≤1且名义掺杂水平在10¹⁷cm⁻³范围内的AlGaN/Ge样品进行了微观结构分析，并测量了Ge浓度及其在纳米尺度下的空间分布。x≤0.2的AlGaN/Ge样品未表现出任何不均匀性迹象。然而，x>0.4的样品在表面显示出微米尺寸的Ge微晶。Ge的偏析并不局限于表面：在AlGaN/Ge层内部观察到尺寸为几十纳米的富Ge区域，通常与结构缺陷周围的富Ga区域相关。除了这些局部异常情况，AlGaN/Ge基体呈现出均匀的Ge组成，其含量可能显著低于名义掺杂水平。对基体中Ge的精确测量给出了Ge在AlGaN中的溶解度图随Al摩尔分数的变化情况。Ge在AlN中的溶解度极低。在AlN和GaN之间，溶解度随三元合金中Ga摩尔分数线性增加，这表明Ge仅通过取代Ga原子而掺入。被Ge占据的Ga位点的最大百分比在1%左右饱和。不同长度尺度下的溶解度问题和Ge偏析现象可能对Ge作为n型AlGaN掺杂剂的效率产生影响，即使在预期不会出现Ge DX中心的Al浓度下也是如此。因此，这些信息可能会直接影响Ge掺杂的AlGaN发光二极管的性能，特别是在消毒光谱范围（≈260nm）内，该范围需要具有高Al摩尔分数的重掺杂合金。
