Optical Studies of Highly-Dope Ga1-xMnxAs and Ga1-xBexAs

我們以紅外光傅立業轉換光譜儀(Fourier transform Infrared spectroscopy)研究鐵磁性半導體材料(Mn,Ga)As及重掺雜Be元素的非磁性半導體材料GaAs之光學性質。從CER(Contactless electroreflectance)的量測中，我們在GaAs band gap能量之上約40 meV處發現有一特徵訊號，意味著在(Mn,Ga)As中含有大量的電洞，將費米能階提升至價帶中，並透過計算得到自由電洞的濃度為1.4× 1019 cm-3。而對於低溫成長的(Be,Ga)As樣品，從霍爾效應的量測得知其載子濃度遠低於我們企圖掺雜的大小，而在近紅外光譜區間，從7500cm-1開始有吸收，並朝band gap (12000 cm-1)快速增加，意味著此低溫成長的樣品中有band tail state 或midgap state，其費米能階位於禁帶能隙之中。然而對於同樣是低溫成長的(Mn,Ga)As樣品，在7500cm-1到12000 cm-1之間光譜則很平坦，意味著band tail state 或midgap state被Mn所提供的電洞所填滿，而使得費米能階位於價帶之中。對於高溫成長的(Be,Ga)As樣品，從霍爾效應的量測得知其最高的自由電洞濃度是大約為1020 cm-3。另外，透過中紅外穿透光譜的量測，此高溫成長的(Be,Ga)As樣品在以600 cm-1 為中央附近有一寬廣的吸收，再此吸收之中又可發現在1600 cm-1處另有一較小的吸收，此較小的吸收我們認為此吸收是由inter-valence band transition 所貢獻。而在(Mn,Ga)As的光譜中亦可觀察到一位於1700 cm-1 為的吸收。因此，就以上的結果而言，此(Mn,Ga)As鐵磁性半導體材料其費米能階應該位於價帶之中。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.Contents Abstract (in Chinese) i Abstract (in English) ii Chapter 1. Introduction 1 1.1 Background 1 1.2 Motivation 2 1.3 Organization of the thesis 4 Reference 5 Chapter 2. Background 9 2.1 III-V Based Ferromagnetic Semiconductor 9 2.2 Magnetic Properties 13 2.3 Transport Properties 15 2.3.1 The Hall Effect 15 2.4 Temperature and Magnetic Field Dependence of Resistivity 19 2.5 Origin of Ferromagnetism 26 2.6 Metal-insulator transition in doped semiconductor 28 2.7 Results of the previous optics studies 33 Reference 39 Chapter 3. Experimental 41 3.1 Sample structures 41 3.2 Preparation of samples 43 3.2.1 Sample wedge for FIR measurement 43 3.2.2 Sample polish for MIR and NIR measurement 43 3.3 FTIR measurement 44 3.3.1 FTIR spectrometer instruments 44 3.4 CER measurement 52 3.4.1 CER basic principle 53 3.5 Hall effect measurement 57 Reference 59 Chapter 4. Result and discussion 60 4.1 Basic characterization 60 4.1.1 SQUID measurement of the GaMnAs samples 60 4.1.2 Temperature dependence of Rxx at zero magnetic field. 60 4.1.3 Hall effect results of nonmagnetic samples 66 4.2 Result and discussion of the FTIR measurement 69 4.2.1 FTIR spectra for the high temperature grown GaAs:Be sample 69 4.2.2 FTIR spectra for the low temperature grown GaAs:Be samples 76 4.2.3 FTIR spectra for the GaMnAs samples 80 4.3 Contactless electroreflectance measurement 87 Reference 94 Chapter 5 Summary and conclusion 9