奈米尺寸層狀p-型氮化鎵的高導電率與其等效導電率量測方法

Abstract

我們使用分子束磊晶法成長p摻雜與未摻雜氮化鎵多層奈米結構，達到超低電阻率 0.038 ohm-m，低電阻率之原理是靠著p型氮化鎵層的電洞擴散至兩側未摻雜氮化鎵奈米層，藉由未摻雜層的高電洞遷移率特性，使用未摻雜氮化鎵層為通道，大幅提高整體導電率。我們也進行模擬研發基於於Brooks-Herring理論建立受離子化後雜子散射的電洞遷移率分布和隨深度變化的電洞濃度分布，藉由此分布我們也可以瞭解電流流經的區域，進而計算與實驗結果吻合的等效電洞濃度、等效遷移率與等效電阻率。 為瞭解在量測電流與電壓關係時，電流在多層p摻雜與未摻雜氮化鎵之交替層結構中的分布，我們建立了一個模型。在這模型中，於較低等效導電率的多層結構內，我們假設側向電流隨深度指數下降，而在高導電率多層結構內的側向電流隨深度均勻分布。藉由改變上部分低導電結構的厚度和下部分高導電結構的厚度比例，我們可以量測多組樣品的片導電率，然後用我們建立的模型對此實驗量測數據以獲得出最佳擬合曲線，由此可得到上部分低導電結構的導電率與電流穿透深度。我們發現降低電流穿透深度的重要因素是低導電結構中未摻雜氮化鎵層的厚度太大，此厚度太大使得附近p摻雜氮化鎵層的電洞無法擴散至未摻雜層的中間，以致於在未摻雜層中間形成一高電阻的薄層。

p-GaN/u-GaN alternating-layer nanostructures are grown with molecular beam epitaxy to show a low p-type resistivity level of 0.038 Ohm-cm. The obtained low resistivity is due to the high hole mobility in the u-GaN layers, which serve as effective transport channels of holes diffused from the neighboring p-GaN layers. The Mg doping in a thin p-GaN layer can lead to a high Mg-doping concentration for supplying holes to the neighboring u-GaN layers. Simulations based on a one-dimensional drift diffusion charge control model and the Brooks-Herring theory of ionized impurity scattering are undertaken to first obtain the depth-dependent distributions of hole concentration, mobility, and hence resistivity. Then, weighted averaging processes are used for evaluating the effective hole concentration, mobility, and resistivity of a p-GaN/u-GaN alternating-layer nanostructure to give consistent results with the measured data. A model for estimating the current penetration behavior in a p-GaN/u-GaN alternating-layer structure when its I-V characteristics is to be measured is proposed. In this model, an exponential decay with a characteristic penetration depth is assumed for a layered structure of low effective conductivity. In a high-conductivity structure, this penetration depth is regarded as infinity such that the depth-dependent current distribution is uniform. By growing p-type structures with an upper portion of a layered structure of unknown effective conductivity and a lower portion of high conductivity, which can be a layered or a uniform structure, of different individual thicknesses, we can have a sequence of sheet conductance data for best-fitting with the proposed model to simultaneously obtain the characteristic penetration depth and effective conductivity of the upper layered structure. Simulation studies are performed to provide the results supporting the proposed model. From the simulation results, it is found that the key factor hindering the current penetration is the low conductivity and finite thickness of a sub-layer around the middle of a u-GaN layer, which is not covered by the hole diffusion range from the neighboring p-GaN layers.