14-1-1Optoelectronics 光电子课件.ppt

1、Chapter1 Optical Properties of Semiconductors 半导体光学特性,1 Introduction 引言 2 Maxwell Equations and Vector Potential Maxwell方程组和矢量势 3 Electrons in an Electromagnetic Field 电磁场中的电子 4 Interband Transitions 带间跃迁 5 Indirect Interband Transitions 间接带间跃迁 6 Intraband Transitions 带内跃迁,7 Charge Injection and Rad

2、iative Recombination 电荷注入及辐射复合 8 Charge Injection ：Non-Radiative Effects 电荷注入及非辐射效应 9 Non-Radiative Recombination：Auger Process 非辐射复合：俄歇复合 10 The Continuity Equation:Diffusion Length 连续性方程：扩散长度 11 Charge Injection and Bandgap Renormalization 电荷注入及带隙重整 12 Summary 小结,1.1 Introduction,Optoelectronic de

3、vices depend upon the interactions of photons or electromagnetic fields with semiconductors.,Interaction: photon semiconductor Electromagnetic Field semiconductor,Scattering theory,Electron (hole),Initial state,Final state,This interaction is described by the scattering theory in which the electrons

4、 scatter from one state to another by the photon field.,We will first discuss a macroscopic（宏观） theory for the optical effects in materials represented by macroscopic quantities such as conductivity（电导率）, dielectric constant（介电常数）, etc.,1.2 Maxwell Equations and Vector Potential,Maxwells Equations:,

5、Wave vector（波矢） k Refractive index（折射率） nr Relative Dielectric constant （相对介电常数） Absorption coefficient （吸收系数） Reflection coefficient（反射系数） R,Its amplitude is damped exponentially .,The damping of the wave is associated with the absorption of the electromagnetic energy.,The absorption coefficient ca

6、n be measured for any material system and it provides information on nr”,Another quantity which can be easily measured is the reflection coefficient R.,re: reflection in: incident,1.3 Electrons in an Electromagnetic Field,Absorption,electron,ki,+nph photons,Initial state,electron,kf,+(nph-1) photons

7、,Final state,“Destroy” A photon,A schematic of an absorption process where a photon is absorbed (destroyed) and the energy and momentum of the electron is altered.,Emission,electron,ki,+nph photons,Initial state,electron,kf,+(nph+1) photons,Final state,“Create” A photon,The stimulated emission（受激辐射）

8、 is due to the initial photons present in the system and the emitted photons maintain phase coherence with the initial photons. The spontaneous emission （自发辐射） comes from the perturbations and the emitted photons are incoherent with no phase relationship.,The difference between these two kinds of pr

9、ocesses is the key to understanding the differences between a light emitting diode (to be discussed in Chapter 3) and the laser diode (Chapter 4 and 5).,stimulated emission,spontaneous emission,Key Topics About Last,II. Main macroscopic quantities,Interactions between photons and semiconductors disc

10、ussed by scattering theory,III. The relationship between k, nr and ,Refractive index（折射率） Absorption coefficient （吸收系数）,IV.,V. Schematic of absorption and emission,1.4 Direct Interband Transitions,Conduction band,Photon,Valence band,A schematic of direct interband transitions,Vertical in k-space,The

11、 position of the electron and hole energies at vertical k-values. The electron and hole energies are determined by photon energy and the carrier masses.,Direct Interband Transitions,Reduced eh mass ( 折合质量 ),For GaAs:,Direct Interband Transitions,State density Ncv (),Absorption coefficient ,Ncv (),Ex

12、ample 1.2 (Homework),Condition: GaAs,Calculation:,the energy of E and H,A 1.6 eV photon is absorbed by a valence band electron in GaAs. If the bandgap of GaAs is 1.41 eV, calculate the energy of the electron and heavy hole produced by the photon absorption.,Example 1.2,Solution:,1.5 Indirect Interba

13、nd Transitions,Indirect bandgap,Phonon,Photon,Phonon assisted Transitions (Second order process),Indirect transitions: transitions between electron states that are not vertical in k-space and they can be mediated by a phonon interaction.,(The phonons are required to ensure momentum conservation in t

14、he process.),Phonon(声子):晶格振动的能量量子。其行为像一个粒子，所以是一种准粒子。,The process is second order,The electron is first scattered by a photon to the direct band conserving momentum and then scattering to the indirect band by a phonon.,Indirect transitions:,Not vertical in k-space,Mediated by a phonon interaction or

15、other scattering process (impurity),Second order process,In a real situation, the effects of the e-h interaction cause the absorption coefficient to be modified at the bandedge .,Absorption coefficient for direct bandgap semiconductors,Absorption coefficient for indirect bandgap semiconductors,For t

16、he direct gap material, the absorption coefficient is very strong once the photon energy exceeds the bandgap.,Compare:,For indirect materials, the absorption coefficient rises much more gradually.,Note that: amorphous silicon has an absorption coefficient almost like a direct gap semiconductor, since k-selection is not applicable.,Example 1.4 (Homework),Condition: Si,GaAs,Calculation:,the thickness of sample for 90% incident light be absorbed,and,The absorption coefficient near the bandedges of GaAs and Si are and respectively.,What is the minimum thickness of a samp