1. Studies of Minority Carrier Recombination Mechanisms in Beryllium Doped GaAs for Optimal High Speed LED Performance An Phuoc Doan Department of Electrical Engineering Senior Project Presentation April 30, 2002

2. Introduction We are interested in LEDs as high speed emitters for optical communications. Desirable to have bright, fast, and inexpensive devices. LED’s high speed performance relies on recombination mechanisms.

3. Minority Carrier Lifetime and Recombination Minority carrier lifetime  is the average time an excess minority carrier will exist before recombining with majority carriers.  is dependent on doping concentration. –Higher doping concentration, lower . Lower  means faster LEDs. Higher doping results in faster LEDs.

4. Motivation Previous studies show that as the doping concentration increases, the internal quantum efficiency also decreases as the minority carrier lifetime decreases. Degradation of performance due to nonradiative recombination mechanisms (i.e. Auger, impurity trappings, surface recombination, etc.), self- absorption effects. Optical characterization techniques designed to probe possible mechanisms of intensity degradation.

5. GaAs Sample Purpose of structure: To confine carriers to region of interest and reduce surface effects and maximizes pumping efficiency. Sample doped p-type because electron injection is generally more efficient than hole injection. Ga 0.6 Al 0.4 As: 0.2  m; p=5*10 18 GaAs: 1  m S1: p=2.0x10 18 to p=6.0x10 19 Ga 0.6 Al 0.4 As: 0.2  m; p=5*10 18 Grading: 500Å GaAs: Substrate Ga 0.6 Al 0.4 As: 0.2  m; p=5*10 18 GaAs: 1  m S1: p=2.0x10 18 to p=6.0x10 19 Ga 0.6 Al 0.4 As: 0.2  m; p=5*10 18 Grading: 500Å GaAs: Substrate Ec Ev

6. Doping Profile with Electrochemical Capacitance Voltage (ECV) Profiler

7. Minority Carrier Lifetimes

8. Photoluminescence

9. Experimental Details Measure Photoluminescence Intensity as Function of Pump Intensity: To quantify the effect of minority carrier concentration on the recombination mechanisms. Self-Absorption: Varying the pumping depth within the sample by changing the pump energy.

10. Pump Intensity v. PL Intensity

11. Pump Intensity v. PL Power Intensity

12. Self – Absorption Analysis

13. Conclusion We do not know why our samples have longer lifetimes, yet not very bright. The two experiments presented here eliminated two possible failure mechanisms. Much work needs to be done in order to have fast and bright light emitters.

14. Double Heterostructure LEDs DH-LED as application of radiative recombination in direct bandgap semiconductors. –Recombination of electron from conduction band and hole from valence band result in photons. n – type emitter p – type barrier p – type active region

15. Auger Recombination

16. Time Resolved Photoluminescence (TRPL) TRPL measures photoluminescence decay by photon counting over many excitation cycles.