Optical Studies of Highly-Dope Ga1-xMnxAs and Ga1-xBexAs
Date Issued
2007
Date
2007
Author(s)
Lee, Chih-Shan
DOI
en-US
Abstract
Near, middle and far infrared Fourier transform transmission spectroscopy was used to study the optical response of ferromagnetic Ga1-xMnxAs and heavily Be-doped nonmagnetic GaAs samples. In the CER experiment, in addition to the features commonly observed for GaAs, above band gap feature could be observed at low temperature for some of the samples. The appearance of the above band gap feature is interpreted as due to the band filling effect of the Ga1-xMnxAs layer. The large amount of free holes raises the Fermi level of the Ga1-xMnxAs above the band edge of the valence band and increases the effective bandgap observed in the CER measurement. The Fermi level obtained from the measurement is 40meV which correspond to a free hole concentration of around 1.4× 1019 cm-3. For the Low T grown (Ga,Be)As samples, from the Hall effect measurement, it was found that the free carrier concentration is many orders of magnitude smaller than the intended doping concentration. In the near-IR regime we found that, for these samples, the absorption starts to appear at 7500 cm-1, and increase quite rapidly toward the band gap (11200 cm-1) of GaAs, indicating there are a lot of band tail states or midgap states for the samples grown at low T and the Fermi-level of the system is pinned inside the forbidden gap. However, although the (Ga,Mn) As samples were also grown at LT, the optical response is flat from 7500 cm-1 to 11200 cm-1, indicating that the midgap or bandtail states are filled up by free holes provided by Mn, and the Fermi level of the system is in the valence band. For the high T grown samples, from the Hall effect it is found that the highest free hole concentration could be obtained is around 1020 cm-3. The mid-IR response of these samples is dominated by a broad absorption dip centered around 600 cm-1, and inside the broad transmission dip there is a shoulder at around 1600 cm-1.and this absorption could be attributed to the inter-valence band transition. Since (Ga,Mn)As has a typical free carrier concentration of around 0.14×1020 cm-3. The Fermi energy is higher, and the separation of heavy hole band and light hole band energy is larger. The absorption dips centered around 1700 cm-1 observed in the (Ga,Mn)As is thus identified as due to inter-valence band transition. In the far-IR response, non-Drude like responses were observed for low T grown (Ga,Be)As and (Ga,Mn)As, but Drude-like responses were observed for high T grown (Ga,Be)As samples. The results indicate that the Fermi level of metallic Ga1-xMnxAs resides in the valence band and metallic Ga1-xMnxAs is an ordinary metal.
Subjects
砷化鎵錳
砷化鎵鈹
傅利葉紅外光譜
電場調制反射光譜
霍爾效應
鐵磁性半導體
價帶間傳輸
(Ga,Mn)As
(Ga,Be)As
FTIR
CER
Hall effect
ferromagnetic semiconductor
inter-valence band transition
Type
thesis
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