以雙溫區固態再結晶法生長碲化汞鎘單晶的成長機構及其性質研究

Abstract

應用雙溫區固態再結晶法可以直接生長出 n-type 的 (HgCd)Te 單晶, 本 實驗主要是探討最適宜的雙溫區製程及單晶成長機構, 並對於所生成之單 晶進行微觀結構, 成份分佈以及電性特質之測試分析. 實驗結果顯示, 為 確保單晶之形成, 配料中之excxss Te 應大於 1E20cm-3 . MCT 由複晶轉 變成單晶的機構與傳統的再結晶理論不同, 其與析出物, 汞壓, 多邊形化 及成份, 溫度梯度有關. 雙溫區固態再結晶法不僅可直接生長 n-type 的 MCT 單晶, 亦可生長 p-type 的MCT 單晶. 以雙溫區法生長之單晶成 份分佈極佳, 且差排密度極低(<100cm-2), 唯覺遺憾的是單晶中易產生孔 洞及退火雙晶, 需進一步改善. Using a two zone process, it is possible to grow directly n- type (HgCd)Te crystal by solid state recrystallization method. The goal of this investigation is to develop the optimal two zone process and to study the mechanism of single crystal growth, microstructure, compositional unifomity, and electrical property of the grown crystals. It was found that it is imperative to keep the excess Te content of the ingot above 1.0E20 cm-3 to assure single crystal growth. The temperature of the two zone are 665 and 525 C, respectively. The annealing time should be longer than 10 days. The mechanism of converting a polycrystal MCT ingot to single crystal differ from the conventional recrystallization theory but is related to second phase precipitates, Hg pressure, polygonization, coalesence and chemical potential gradient and temperrature gradient zone melting process. The cooling rate after annealing affects the electrical properties siginificantly and it should be kept below 3 C/hr. In addition to p-type materials, the two-zone process can also be used to grow high quality p-type materials. The best p-type material grown so far has a carrier concentration of 1.4E15 cm-3, and mobility of 786 cm2V-1s-1. The best as-grown n-type material has a carrier concentration of 1.2E15 cm-3 and mobility of 6755 cm2V-1s-1. The properties of the n-type material can be further improved by slicing the ingots first followed by annealing the slices in saturated Hg pressure at low temperature. The compositional unifomity of the two-zone annealed materials is very good. The dislocation content is extremly low (etch pit density is less than 100 cm-2). The only major problems are the presence of pores and annealing twins in the ingot, the origin of these twins are suspected to be caused by the instability of the annealing furnace.