https://indico.ihep.ac.cn//event/9242/

Yuhang presented the JC-101 related to LGAD and the title was “Radiation hardness of gallium doped Low Gain Avalanche Detector”

He introduced that LGAD has n++-p+-p-p++ structure to achieve high enough electric field near the junction. The advantage of this kind of structure is that the timing resolution can be achieved to 26ps. But, there is problemwith this is that harsh radiation environment leads to decrease the gain. The reason behind the decrease in the gain is the acceptor removal in the multiplication layer and for this reason we can use gallium doping as gallium is more difficult to displace/dislocate from the lattice site than boron. The result of this doping was good and the removal rate of gallium was found to be 5*10^-16 cm^-2, around two times smaller than for so far studied boron LGAD detectors. This feature could lead to significantly improved performance of thin Ga-LGADs at HL-LHC.

He pointed out the Performance before irradiation: The process parameter for gallium implantation, unlike for boron, are are less studied abd known. As a result, the implantation profile of gallium differed from the planned one and this led to very high gain and consequently break down of the devices starting at around 30V with a steep rise of the leakage current. Also, there was a difference of around 25% between highest(Dose 4) and lowest(Dose 1)(see figure)

He showed the plots and made it clear that Ga-LGADs perform better in terms of charge collection when compared not only control/no-gain devices, but also better than similar B-LGADs. He also showed the gain plot of Ga-LGADs and it was clear that the stronger the irradiation intensity, the lower the gain is not necessarily.

The electric field model shows almost identical (similar positive space charge) in the detector bulk for the devices with LF doesnot equal to 0.

Questions: Amit’s Question: In point 3, Performance Before Irradiation: The process parameters or gallium implantation, unlike for boron, are less studied and known. As a result, the implantation profile of gallium differed from the planned one. why did they choose gallium if they didn’t know the process parameter of gallium? Was this a testing for gallium? Answer: The reason they choose gallium is that It was observed before that initial acceptor removal is smaller for gallium than for boron doped silicon after electron irradiation in solar cells. This was a test. Meanwhile, they don’t know the process parameter of implantation, but if the simulation is good, they can improve the process of implanation.

Yuzhen’s Question: In Section 4 “4. Charge collection and gain of LGAD devices “, can you tell us about how to test the LGAD gain? Answers:The gain can be obtained by the given formula mentioned in the talk.

Xin’s question: At the end of the paper, it says: “the concentration of gallium in the multiplication layer was too high causing an early break down of devices before irradiations.”, could you explain more? Answer: The break is when the bias voltage is greater than 30V, the leakage current is multiplied. Under normal circumstances, this value will be greater than 400V.

GuShan’s Question: What is the basis for the the multiplication layer to select dopants? Answer:1.Choose ⅢA as acceptor and form P-N junction with donor from periodic table. 2.ⅣA change the performance of semiconductors.

Suyu’s Question: Why could gallium mitigate the acceptor removal? Is there theoretical explanation? Answer: Answer: Gallium is heavier than boron and thus more difficult to displace from the lattice site and could be less susceptible to reactions with vacancies in the Si lattice (V) and interstitial silicon atoms (I).

Ryuta: Q. In page 1, the sentence, “Gallium is heavier than boron and thus more difficult to displace from the lattice site …” explains the motivation of the introduction of Gallium, as well as one of possible interpretation of the obtained results. Similarly, can we think about the indium (In) as the acceptor at the multiplication layer or not ? What would be different between Gallium/Boron and Indium ? Answer:Si-ln was used as infrared detectors. I think the ∆𝐸(Ec-Ev) is different between Si-Ga, Si-B, and Si-In. Others are not clear.

Suyu has shown us the next JC-102 Paper and the title was “High-resolution three-dimensional imaging of a depleted CMOS sensor using an edge Transient Current Technique based on the Two Photon Absorption process (TPA-eTCT)”.