以分子束磊晶技術生長摻鎂p-型氮化鋁鎵

Abstract

我們利用分子束磊晶技術，在氮化鎵基板上成功的以低溫345oC的鎂蒸發源，生長出p-型氮化鋁鎵，量測到的鎂摻雜濃度為8 x 1018 cm-3，鋁含量約為25%，電洞濃度為2.8 x 1017 cm-3，電洞遷移率為15.2 cm2/V-s，而電阻率為1.5 Ω-cm，以上霍爾量測結果是在頂部沒有成長p+薄層的樣品上量測。雖然在較高的鎂蒸發源溫度下(鎂蒸發源溫度365 oC)，也可以得到p-型效果，但在鎂相對低溫的條件下成長較穩定。我們控制鎂蒸發源溫度由低而高，逐步增大鎂的摻雜量，成長出一系列樣品，並探討其表面變化及霍爾量測結果。我們發現，隨著鎂摻雜量增加，樣品表面V型小凹洞的密度隨之增加，而這些凹洞所顯示之材料缺陷會補償掉摻雜的鎂所提供的電洞，鎂摻雜的結果為n-型。我們進一步進行穿透式電子顯微鏡術的探討，從拍攝的照片中看到，有些V型小凹洞是由基板的差排一路向上延伸，另外有一些則是在成長氮化鋁鎵的過程中才生成。透過調整磊晶的相關條件，改善了表面平整度及晶體品質，我們成功透過摻雜較少量的鎂達到p-型的效果。

A p-AlGaN layer on a GaN template is successfully grown with molecular beam epitaxy. By using a low Mg effusion cell temperature of 345 oC, we achieve an Mg doping concentration of 8 x 1018 cm-3, a hole concentration of 2.8 x 1017 cm-3, hole mobility of 15.2 cm2/V-s, and resistivity of 1.5 -cm. The Hall measurement results are obtained under the condition without a p+ thin layer at the top. The Al content of the AlGaN layer is about 25 %. Although a similar p-AlGaN behavior can be achieved with a higher Mg effusion cell temperature, the aforementioned lower temperature growth condition is more stable. In searching for the stable growth condition, by increasing the Mg effusion cell temperature for increasing Mg doping, surface V-pit density is increased, leading to the compensation of generated holes such that Mg doping results in n-type behaviors. Some V-pits are formed with threading dislocations extended from the GaN layer into the AlGaN layer. Others are formed in the AlGaN layer. After the crystal quality is improved and the V-pit density is reduced by properly adjusting the growth conditions, p-type behaviors can be observed with Mg doping.