利用鎂摻雜/無摻雜氮化鋁鎵交替層結構來改善p-型氮化鋁鎵的導電特性

Abstract

我們利用分子束磊晶技術，在氮化鎵基板上成長七個鋁含量約25%不同結構的氮化鋁鎵樣品，並比較樣品之間的p-型導電特性。其中，相較於均勻鎂摻雜氮化鋁鎵樣品，其他六個奈米尺度的鎂摻雜/無摻雜氮化鋁鎵交替層結構樣品皆具有較高的導電率。由於無摻雜氮化鋁鎵層內電洞遷移率大於鎂摻雜氮化鋁鎵層，而交替層結構內鎂摻雜氮化鋁鎵層所產生的電洞可以擴散至相鄰的無摻雜氮化鋁鎵層中，並以較大的電洞遷移率傳輸，因而可以改善樣品整體的導電特性。一般而言，交替層結構樣品的導電率會隨著分層厚度減少而增加;然而，樣品內鎂摻雜和無摻雜氮化鋁鎵層的相對厚度也會影響其導電特性。我們發現鎂摻雜與無摻雜氮化鋁鎵分層厚度分別為6奈米與4奈米的交替層結構樣品的等效電阻率比均勻鎂摻雜氮化鋁鎵樣品降低了4.7-6.5倍。此外，我們也建立一套模型來釐清無摻雜氮化鋁鎵內電洞遷移率以及擴散至無摻雜氮化鋁鎵層的電洞比率兩者間的關係。

Based on molecular beam epitaxy, seven AlGaN samples of ~25 % Al content on GaN templates are prepared for comparing their p-type behaviors. In particular, six nm-scale p-AlGaN/u-AlGaN layer-structure samples are grown to show the higher sheet conductance levels, when compared with a uniform p-AlGaN thick-layer sample. The higher conductance in a layer-structure sample is caused by the diffusion of holes generated in a p-AlGaN layer into the neighboring u-AlGaN layers, in which hole mobility is significantly higher when compared with that in a p-AlGaN layer. The sheet conductance in a layer-structure sample generally increases with decreasing layer thickness. However, it is also controlled by the relative thicknesses of the p-AlGaN and u-AlGaN layers. In the layer-structure sample of 6 (4) nm for the p-AlGaN (u-AlGaN) layer thickness, the effective resistivity is reduced by a factor of 4.7-6.5, when compared to that of the uniform p-AlGaN sample. A simple model is built for understanding the relation between the hole mobility in u-AlGaN and the fraction of holes diffused into the u-AlGaN layers.