Chapter 39: Carrier mobility

　　 Sunday.
　　 Xu Qiu came to the laboratory.
　　Duan Yun is wearing headphones and concentrating on codewords, while Chen Wanqing is sorting paper documents on the table.
　　 Xu Qiu greeted Duan Yun first, then walked over to Chen Wanqing and sat down.
　　 "Senior sister, is there any news about PEN Ji?"
"Yeah, I contacted several manufacturers of optoelectronic materials, one of which has ready-made PEN substrates," Chen Wanqing said: "But they only do large-area substrates. At least 1 meter by 10 meters should be purchased. I bought it in the specifications and it is estimated to be available within a week."
　　 "Large-area substrate," Xu Qiu asked: "It refers to a PEN substrate with an area of ​​10 square meters, is it evenly plated with ITO?"
　　 "That's right." Chen Wanqing said.
　　 "But the ITO on the glass substrate we use is patterned." Xu Qiu said.
　　 "Then you need to cut a large piece of substrate into small pieces of one square inch, and then etch away the unnecessary ITO to obtain a patterned ITO substrate." Chen Wanqing said.
　　 "Well, I see," Xu Qiu said, "Does the senior sister have ready-made etching conditions?"
　　 "It doesn't seem to be there." Chen Wanqing tilted her head and thought.
　　 "It seems?" Xu Qiu muttered in his heart, and said, "Okay, I'll look for it."
　　He took out the computer and looked through the documents on the flexible substrate again.
　　 Soon, he saw the conditions for etching ITO given by them in the experimental information part of one of the documents:
　　 Add zinc powder to 2 moles per liter of dilute hydrochloric acid, and then put it on the ITO substrate, the part in contact with the solution will be etched away.
　　 However, the PEN/ITO substrate has not arrived yet, so he can only write down this method first, and then report to Chen Wanqing:
　　 "Senior sister, I found out how to etch ITO."
　　 "I found it so soon, let me see." Chen Wanqing leaned her head and looked at Xu Qiu's computer.
　　 "I remember, this is the classic wet etching method, I remember there is another dry etching method..." Seeing Xu Qiu staring at her, Chen Wanqing whispered:
　　 "If I said I didn't remember it just now, would you believe it?"
　　 "I believe it." Xu Qiu said helplessly.
　　…………
　　 There is no experiment done today. Xu Qiu walked around in the laboratory to see what other instruments he is not familiar with.
　　 As a result, he was surprised to find that in the entire laboratory, except for the electrochemical workstation, there was only one instrument he had not used yet.
　　 That instrument, he still had a vague impression, as if it appeared on Tian Qing’s graduation defense PPT.
　　 Considering that he will not come to the laboratory for nearly a month after the exam week starts next week, Xu Qiu decided to get this instrument done today.
　　 So, he found Chen Wanqing.
　　 "Sister, what is that instrument in the laboratory used for, can you teach me?"
　　 "Which one is used to test CELIV and TOF," Chen Wanqing said: "It is mainly used by Tian Qing. This is what she did for her graduation project.
　　 However, at the beginning, Mr. Wei also taught other people the test method, and I will also operate it, but the data analysis is not good enough.
　　 The two instruments are mainly used to characterize the transport of carriers in battery devices, and they are more inclined to theoretical research.
　　 By establishing models and fitting data, it is said that a lot of information on the mechanics of charge transport can be obtained.
　　 But now I will only simply calculate the carrier mobility.
　　In the semiconductor field, electrons and holes are called carriers, and their mobility can be understood as the rate at which they move in the device, and the faster the better.
　　 This instrument has never been used since I learned it. If it weren't for your reminder, I would never have thought that I could test this. "
　　 "Before the test, I will show you a baby first." Chen Wanqing said mysteriously.
　　 "Baby?" Xu Qiu suspiciously followed the senior sister, her direction of travel was a locker.
　　 Chen Wanqing took out a black square paper box from the locker and signaled Xu Qiu to open it.
　　 Xu Qiu opened it and found two black zipper boxes inside.
　　 From the outside, it’s not a glasses case, right?
　　 He unzipped the box.
　　 As expected, there are glasses inside, and the orange-red lenses seem to be made of special materials.
　　 "Sure enough, I shouldn't expect anything." Xu Qiu thought.
　　 "Is it a pleasant surprise," Chen Wanqing smiled and said: "This is goggles. Later, a high-power laser will be used. Wearing it will protect your eyes from burns."
　　 Xu Qiu forced a smile in response.
　　 After wearing the goggles, the two came to the instrument. Chen Wanqing introduced:
　　"This instrument is mainly composed of three parts, a laser light source, a signal generator, and an oscilloscope. Because there is no computer, a USB flash drive needs to be inserted in advance, and the data will be automatically stored in the USB flash drive.
　　 The test method is very simple. First, load the sample into the sample holder, then connect the sample holder to the signal generator and the oscilloscope. After setting the parameters, start the laser and signal generator, and the oscilloscope will display the waveform of the current changing with time.
　　 The first test method is the linear boosted carrier transient method, which is CELIV. UU reading www.uukａnshu.com
The principle is to give the device a short laser pulse first, the effective layer absorbs the light energy of the laser, generates carriers inside, and then uses a linearly increasing voltage to'sweep' electrons and holes to the negative and positive electrodes, respectively. An electric current is formed.
　　In this method, the current size of the waveform is not important, because we are concerned about the charge mobility, so we mainly investigate the time when the current signal generates the peak value.
　　The shorter the time, the faster the movement of carriers, the higher the mobility.
　　 In the calculation formula, the mobility is proportional to the negative power of time.
　　The mobility obtained by this method cannot distinguish between electrons and holes, but is their overall mobility.
　　The other is the time-of-flight method, TOF.
　　Using this method requires the preparation of special battery devices. First, the effective layer must be thicker, usually above 300 nanometers, to ensure that the carriers can fly in the effective layer for a long time.
　　 In addition, a barrier layer for electrons and holes needs to be added to the device structure. It can be understood from the literal meaning that a barrier layer is a material through which electrons and holes can hardly pass.
　　 Generally, an insulator can be used as a barrier layer. We generally use lithium fluoride. The thickness of 5nm vapor deposition can effectively block the passage of electrons and holes.
　　 is similar to CELIV in principle, but uses a constant voltage, and mainly examines the time when the current signal begins to decay.
　　Because of the existence of the barrier layer, it is possible to test the mobility of electrons and holes, and the respective mobility of the two types of carriers.
　　We currently don’t have any devices that can be used to test TOF, so today we only test CELIV. "
　　The parameters of the instrument are all set by default, and the operation is not difficult. After Chen Wanqing's demonstration, Xu Qiu also tested the other five systems.