Dissertation Title:  Impact Excitation of Impurities Doped in Silicon for Light Emission

Date: 2024/05/17

Dissertation Title:  Impact Excitation of Impurities Doped in Silicon for Light Emission

Speaker: Xiaoming Wang

Time: 10:00, May 17, 2024(Beijing Time)

Location: 503, Longbin Building


Silicon is the primary material for complementary metal-oxide-semiconductor (CMOS) integrated circuits. Turning silicon into an efficient light emitting material is important for improving the performance of CMOS integrate circuits by taking the advantage of light for signal processing. Unfortunately, silicon is an indirect bandgap semiconductor that is extremely inefficient in light emission. Introducing impurities into silicon can significantly improve the light emission efficiency of silicon, which however is still not high enough for commercialization. In this dissertation, the author explored two strategies to improve the light emission efficiency of impurity-doped silicon. The first strategy is to manipulate the impurities and their co-dopants as well as the processing conditions. The second one is to leverage the impact excitation of impurities in a reversely biased PN junction.

For the first strategy, the author explored the light emission of erbium ions in silicon that is co-doped with fluorine (F). It was found that the co-doping of F ions significantly suppresses the thermal quenching of light emission and Auger recombination of excitons. Further investigations show that F ions can passivate the interface states between erbium nanocrystals and Si lattice, resulting in a longer decay time ~1ms of Er and a 3-order-of-magnitude increase in photoluminescence compared to Er/O-doped Si. However, theoretical analysis indicates that Er ions with the co-doping of F ions suffer from a relatively low optical activation rate due to the large electron negativity of F elements.

To improve the optical activation rate of Er ions, the author adopted O and B as the co-doping elements for Er ions in silicon. The samples were treated with the deep cooling process that was recently invented by the lab mates. The investigation shows that the deep cooling process effectively inhibits the aggregation of erbium ions, thereby enhancing optical activation, and the co-doping of boron ions reduces in the free carrier (electrons) concentration, lowering the Fermi level and emptying the defects in bandgap. As a result, the photoluminescence (PL) intensity has experienced a 16-fold enhancement in comparison with Er/F-doped Si samples. Optically and electrically pumped Er-doped waveguide amplifiers at room temperature are demonstrated. The optically pumped amplifier based on Er/O/B-doped Si exhibits an absorption loss of 15 dB/mm and a 6 dB signal enhancement at 1541.1 nm. The electrically driven amplifier based on Er/O-doped Si has an absorption loss of 92.5 dB/mm and a 25.3 dB signal enhancement at 1534 nm. Stimulated emission is observed at high pumping currents. However, it is still challenging to obtain net gain for both of them.

The author also explored the light emission from the dislocation loops by implanting boron into silicon treated by the deep cooling process. Interestingly, it was found that electrons and holes recombine through defects emitting two photons, one in near infrared (NIR, 1.3~1.6 μm) and the other in mid-infrared band (MIR, around 3.5 μm). The PL intensity at NIR increases by three folds when the temperature increases from 77 K to 300K.

For the second strategy, the author established the impact excitation theory for impurities in semiconductors and fabricated impurities doped Si light emitting diodes (LED) by deep cooling process. For Er/O/B co-doped Si LEDs and only B doped Si ones, reverse bias induces a 2-orders-of-magnitude stronger light emission than the forward bias due to impact excitation of hot electrons colliding with impurity ions. The experimental electroluminescence was well fitted with the impact excitation theory. From the fittings, it was found that the excitable Er ions in the Er/O/B co-doped Si LEDs reach a record concentration of 1.9×1019 cm-3 and up to 45% of them are in excitation state by impact excitation. The proposed model of impact excitation explains the pronounced emission under reverse bias and the distinctive spectral disparities between EL and PL.

The findings in this dissertation have important implications for developing efficient classical and quantum light sources utilizing rare earth elements and boron ions.


Xiaoming Wang received the B.S. degree in electronic science and technology from Xidian University in 2018. She is currently a Ph.D. candidate at the UM-SJTU Joint Institute, supervised by Prof. Yaping Dan. Her current research interest includes light sources based on silicon.