n型磷化銦鎵之歐姆接觸研究

Abstract

我們選擇了以AuGe、PdGe及NiGe化合物為基本的三種常被用做n型砷 化鎵(n-type GaAs)的歐姆接觸的金屬材料系統，來做為n型磷化銦鎵( n-type InGaP)的歐姆接觸。並 且以低壓金屬有機化學氣相沉積(LP- MOCVD)系統來成長濃度為2x10^18 cm^-3之Si參雜的 InGaP磊晶層，利用 電子束蒸鍍系統將金屬鍍在InGaP上，再以Transfer Length Method (TLM)的方法來求出rc值。在AuGe系統方面，接面阻值約為5x10^-4 ohm- cm^2左右，且在 300℃退火以後表面變得粗糙。以400℃做10小時的退火 不會造成接面阻值有明顯的改 變。PdGe系統在375℃的退火可得到最 低的rc值為3x10^-5 ohm-cm^2，但是在400℃退 火之後表面已不平整 ，並且這個金屬系統在400℃長時間的退火過程中熱穩定性並不 好。 對於NiGe系統來說，由於NiGe及Ni3In等具有高熔點的合金形成，所以有 較平整的表 面和較佳的熱穩定性，且接面阻值約為10^-4 ohm-cm^2。少 量的Au在NiGe系統中會降低 接面阻值，但會破壞表面的平整性及其熱穩 定性。此外，Ni的厚度亦會影響其歐姆的特 性。當Ni的厚度為60 nm時 可得到低於10^-4 ohm-cm^2等較低的rc值。有Au加入且Ni的 厚度為60 nm之NiGe系統和AuGe及PdGe系統比較，具有較佳的歐姆特性。因此，有Au 加 入且Ni的厚度為60 nm之NiGe系統是一較好的n型InGaP歐姆接觸系統 。 在InGaP的表面附近形成一層薄的Ge參雜層是形 成InGaP歐姆接觸的主要機制。在加 熱退火的過程中，Ga和In會擴散出 去而Ge會擴散進來且會佔據Ga及In的空缺變成施體而 造成一層薄的高參 雜層。不過，P在較高的退火溫度會擴散至表面並破壞表面的平整。Ge 會 佔據P的位置而變成受體造成補償，降低Ge參雜層的參雜濃度。此外，P嚴 重的向外擴 散會造成靠近接觸接面的InGaP的分解，因此而使其歐姆特 性變差。 Three commonly used metallization systems-AuGe-based, PdGe- based, and NiGe-based ohmic contacts to n-type GaAs are chosen for making ohmic contacts to n-type InGaP. The n-type InGaP epitaxial layer is grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) with Si-doping concentration of 2x10^18 cm^-3. The contact metals are deposited onto InGaP by electron-beam evaperation system. The specific contact resistance, rc, is calculated by transfer length method (TLM). For AuGe-based system, rc is around 5x10^-4 ohm- cm^2. The surface morphology becomes rough after annealed at 300℃. rc not change significantly after annealed for 10 hr at 400℃. The minimum rc of 3x10^-5 ohm-cm^2 is obtained by PdGe-based system annealed at 375℃. But the contact surface is not smooth after annealed at 400℃, and this metallization system is thermally unstable during long time annealing at 400℃. The NiGe-based system has a smoother surface morphology and better thermal stablility due to the formation of high melting point alloys, NiGe and Ni3In. rc is about 10^-4 ohm-cm^2. A small amount of Au in NiGe-based system will reduce rc. But the additional Au will deteriorate the surface morphology and the thermal stability. In addition, the thickness of Ni layer determines ohmic characteristics. Lower rc, below 10^-4 ohm-cm^2, can be achieved when 60 nm Ni is deposited. The NiGe-based system with additional Au and 60 nm Ni layer has better features of ohmic contacts than AuGe-based and PdGe-based systems. Therefore, the NiGe-based system with additional Au and 60 nm Ni layer is a better ohmic contact metallization system for n-InGaP. The formation of a thin heavily Ge-doped layer near the InGaP surface is the main mechanism of making ohmic contacts to InGaP. During annealing, Ga and In will diffuse out and Ge will diffuse in and occupy Ga and In vacancies as donors to form a thin heavily doped layer. However, at high temperatures, P will diffuse out to the surface and degrade the contact surface. Ge will occupy the P sites as acceptors and will compensate the doping concentration of the Ge-doped layer. Morevoer, the serious outdiffusion of P will cause the decomposition of InGaP at contact interface. Therefore, the ohmic characterictics is degraded.