3.1 Absorption and Transparency

Transcription

1 3.1 Absorption and Transparency Optical Devices (definitions) Photon and Semiconductor Interactions Photon Intensity Absorption

2 3.1 Absorption and Transparency Objective 1: Recall that optical devices are designed to detect or generate optical signals. Objective 2: Recall that photon energy is absorbed by a semiconductor material when the photon energy is greater than the band gap of the material. Objective 3: Recall that the absorption coefficient is material specific, and that it increases with increase in photon energy or decrease in photon wavelength. Objective 4: Calculate photon intensity as a function of wavelength or distance.

3 3.1.1 Optical Devices (definitions) Optical Devices designed to detect or generate optical signals. Optical Power Electrical Power Solar Cells Photodetectors Optical energy is absorbed and EXCESS electron-hole pairs are generated producing photocurrents. Electrical Power Optical Power LEDs Lasers EXCESS carriers are generated in these devices, and when they recombine they emit photons for forward bias PN junctions - ELECTROLUMINESCENCE

4 3.1.2 Photon and Semiconductor Interactions The energy of a photon is equal to E = hf = hc/λ Three types of photon semiconductor interactions 1. Photon lattice: photon energy converted to heat 2. Photon impurity (donors/acceptors): photon energy converted to heat 3. Photon valence electron: electron elevated to conduction band, and electron hole pair is generated.

5 3.1.2 Photon and Semiconductor Interactions Photon Valence Electron Interactions Photons have energy equal to E = hf = hc/. One photon will interact with 1 valence electron 3 Cases Case 1: E < E g Case 2: E E g Case 3: E>> E g

6 3.1.2 Photon and Semiconductor Interactions Case 1: E < E g The energy of the photon is not high enough to break the Sielectron bond, and the light is transmitted through the semiconductor. In this case, the semiconductor is TRANSPARENT to the light. Ex. CdTe/CdS Solar Cell CdS is transparent layer. E g (CdS) >> E g (CdTe) Source: Figure from

7 3.1.2 Photon and Semiconductor Interactions Case 2: E E g The energy of the photon is greater than the band gap and Si-electron bonds are broken resulting in a valence electron having higher energy or conduction band energy; electron-hole pairs are formed. * This is a function of the number of electrons in the valence band and the number of empty states in the conduction band!!! Source: Figure from

8 3.1.2 Photon and Semiconductor Interactions Case 3: E >> E g Similar to Case II, bonds are broken and electron-hole pairs are formed, and excess energy (energy above E g ) is converted to heat that is dissipated in the semiconductor.

9 3.1.3 Photon Intensity Intensity of photon flux as a function of distance I ( x ) I o e x The intensity of the photon flux decreases exponentially as a function of distance from the semiconductor surface. I o is the initial intensity of the photon flux (J/cm 2 s or W/cm 2 ) I(x) is the intensity of photon flux at a distance x in the semiconductor α is the absorption coefficient and is dependent on the photon wavelength. For a given material, α increases with increase in photon energy or decrease in photon wavelength. x is the distance from the surface of the semiconductor

10 3.1.3 Photon Intensity Absorption coefficient, α Source: MICON/ABSCOEF.HTM α is the absorption coefficient and is dependent on the photon wavelength. For a given material, α increases with increase in photon energy or decrease in photon wavelength.

11 (W/cm 2 ) Absorption For high α materials, photons do not penetrate large distances and I decreases rapidly with x and absorption is high. For low α materials, photons penetrate large distances and I decreases at lower rate with x and absorption is low. If light is transmitted through a material, the material has not absorbed the light ( is very low or E < E g ). I large α small α x

12 3.1.4 Absorption Example 1 A photon flux with an intensity of I o = 0.15 W/cm 2 illuminates the surface of Si. If the wavelength of the light is μm, what is the photon flux intensity at (a) 1 μm, (b) 2 μm, (c) 3 μm, (d) 4 μm, (e) 5 μm, (f) 10 μm, and (g) 20 μm? The absorption coefficient for μm photon wavelength on Si is 100/cm. I ( x ) I o e x I( x) W 0.15 cm exp cm 5 m 1cm 4 10 m I( x) 0.143W cm 2

13 3.1.3 Photon Intensity Absorption coefficient, α Source: MICON/ABSCOEF.HTM α is the absorption coefficient and is dependent on the photon wavelength. For a given material, α increases with increase in photon energy or decrease in photon wavelength.

14 3.1.4 Absorption Example 1 A photon flux with an intensity of I o = 0.15 W/cm 2 illuminates the surface of a Si semiconductor. If the wavelength of the light is 0.9μm, what is the photon flux intensity at (a) 1 μm, (b) 2 μm, (c) 3 μm, (d) 4 μm, (e) 5 μm, (f) 10 μm, and (g) 20 μm? The absorption coefficient for 0.9 μm photon wavelength on Si is 100/cm. Io, W/cm 2 x, um I, W/cm The intensity of the light decreases exponentially as a function of x

15 3.1.5 Summary Optical Devices are designed to detect or generate optical signals. In solar cells, optical energy is absorbed and excess electron-hole pairs are generated producing photocurrents. In LEDs and Lasers, when excess carriers recombine, they emit photons. When E < E g, the semiconductor will be transparent to the radiation; material is transparent to photon energy. When E > E g, the photon energy will produce electron hole pairs; photon energy is absorbed by the material.

16 3.1.5 Summary Intensity of photon flux decreases exponentially as a function of distance. I x I e x ( ) o For a given material, α increases with increase in photon energy or decrease in photon wavelength. For high α materials, photons do not penetrate large distances and I decreases rapidly with x and absorption is high. For low α materials, photons penetrate large distances and I decreases at a lower rate with x and absorption is low.

17 References D. Neamen, An Introduction to Semiconductor Devices, McGraw-Hill, New York, Photon absorption image from Absorption Coefficient graph from