The interaction between the itinerant carriers, lattice dynamics, and defects is a problem of long-standing fundamental interest for developing quantum theory of transport. Here, we study this interaction in the compositionally and strain-graded AlGaN heterostructures grown on AlN substrates. The results provide direct evidence that a dimensional crossover (2D–3D) occurs with increasing temperature as the dephasing scattering events reduce the coherence length. These heterostructures show a robust polarization-induced 3D electron gas and a metallic-like behavior down to liquid helium temperature. Using magnetoresistance measurements, we analyze the evolution of the interaction between charge carriers, lattice dynamics, and defects as a function of temperature. A negative longitudinal magnetoresistance emerges at low temperatures, in line with the theory of weak localization. A weak localization fit to near zero-field magneto-conductance indicates a coherence length that is larger than the elastic mean free path and film thickness ($l_{\phi }>t>l_{el}$), suggesting a 2D weak localization in a three-dimensional electron gas. Our observations allow for a clear and detailed picture of two distinct localization mechanisms that affect carrier transport at low temperature.

中文翻译：

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成分分级的AlxGa1-xN中的弱定位和尺寸交叉

流动载流子，晶格动力学和缺陷之间的相互作用是发展运输量子论的长期存在的基本问题。在这里，我们研究了在AlN衬底上生长的成分梯度梯度AlGaN异质结构中的这种相互作用。结果提供了直接的证据，随着相移散射事件减小了相干长度，随着温度的升高，发生了尺寸交叉（2D–3D）。这些异质结构显示出强大的极化诱导3D电子气和低至液氦温度的类金属行为。使用磁阻测量，我们分析了载流子，晶格动力学和缺陷之间的相互作用随温度的变化。在低温下会产生负的纵向磁阻，符合弱本地化的理论。接近零磁场磁导的弱局部拟合表明相干长度大于弹性平均自由程和膜厚度（${升}_{\phi }>Ť>{升}_{Ë升}$），表明在三维电子气中存在二维弱定位。我们的观察结果可以清楚而详细地了解影响低温下载流子运输的两种不同的定位机制。