very good questions indeed. I will try answer your questions the best I can. Below are just some quickies for now. Will get back to you for more details later.. 4.: No band gap is b/c no ions. Eg concept is from the existence of ions where electrons cannot penetrate. This is shown in detail by my video below: th-cam.com/video/ClRbvmhFxZA/w-d-xo.html 1&5: As I mentioned in the lecturer, CNL is a materials surface intrinsic property. It has no connection with the Fermi level. Fermi is not an intrinsic property since it is altered by doping. The detail calculation of CNL is very complex. Please check out the ref. mentioned in the talk. The polarity of charge at surface, however, depends upon the position of EF w.r.t. CNL. It's negative when EF is above CNL charges and positive when below. CNL and EF forms a relationship from which the resulting band bending (QSC), Qit and Qm are balanced in a way to preserve neutrality. A perfect alignment between CNL and EF is an ideal case in which no band bending.. 2. No dangling bonds for 2D, but metal can react with 2D (hydridation) to form MIGS. 2D crystal in not perfect as well (subsurface impurity still can cause havoc.). BTW, you don't need much to pin. The Dit of Si-CMOS is in the range of E10/cm2. The surface atomic density of any material is around E14-E15. Any impurity on the surface, not observed by any physical apparatus (their sensitivity is only 0.1% at best, i.e., E13) , can still pin the Fermi level (electrical signal is a lot sensitive). 3. I would think so.
@@Scola_Hao contact me thru my gmail in my LinkedIn . We can the connect thru LINE for easier discussion www.linkedin.com/in/wei-e-wang-4911533/ You need to keep in mind (as I mentioned earlier). All physical measurements are quite limited in understanding defects. This is because (a) your STS measurement area is so small and (b) the sensitivity of STS is inadequate to see the electrical response (down to E12/cm2 or below). This is just statistics. The sampling area is too small. You can use a TEM to see no defect and yet there could be tons of defect in reality, e..g, to see defects density of E8/cm2 need to sample 1umx1um. TEM area is a few nm or a bit more. There is no way for them to see defect density < E8/cm2. BTW, the surface has no band gap is b/c electron can go anyway since they do not see ions blocking their paths... 2D materials is a bit more complex. Please check the most recent papers on this subject (VIGS?) pubs.aip.org/aip/jap/article/135/10/100901/3269952/Native-point-defects-in-2D-transition-metal pubs.aip.org/aip/sci/article/2020/30/301104/364696/Extending-the-metal-induced-gap-state-model-to
相當感謝您製作這部影片
我感到受益良多
根據老師的整理與介紹，我理解到的是
半導體大多會有Fermi-level pinning的原因是因為半導體表面有缺陷或因邊界條件不同而產生surface states
這些surface states 是否帶電荷會由有一個電中性的能量位置(CNL)所決定
而半導體內部的電荷就先會與surface的電荷先做一次電中性平衡
當金屬與半導體接觸時，金屬的電荷將與surface的電荷做平衡而不是半導體內部電荷
因此才導致了fermi level pinning
以下有些問題非常想了解
1. 半導體內部的電荷要與surface 的電荷做平衡，是因為半導體表面態的CNL與半導體內部的fermi level不相同嗎?
在電荷平衡後，為甚麼CNL 與 fermi level 不是在同一個位置?
2. 根據老師給的文章Dangling bonds, the charge neutrality level, and band alignment in semiconductors
半導體的CNL會與材料缺陷(dangling bonds)的種類高度相關，但在二維材料上，他們是沒有dangling bond的
但仍然會有fermi level pinning的現象，他們的CNL 是由甚麼決定的呢?
3. surface states (defect induced)與interface states(Metal induced gap states)他們是共用同一個CNL嗎?
4. 1:59:38處，在半導體的表面，是沒有能隙的
我能把它想像成在從半導體內部過渡到表面的時候，能隙逐漸變小，然後消失嗎?
5. 他們是如何量測出CNL與fermi level 之間的差異呀？
望老師能解惑
very good questions indeed. I will try answer your questions the best I can. Below are just some quickies for now. Will get back to you for more details later..
4.: No band gap is b/c no ions. Eg concept is from the existence of ions where electrons cannot penetrate. This is shown in detail by my video below:
th-cam.com/video/ClRbvmhFxZA/w-d-xo.html
1&5: As I mentioned in the lecturer, CNL is a materials surface intrinsic property. It has no connection with the Fermi level. Fermi is not an intrinsic property since it is altered by doping. The detail calculation of CNL is very complex. Please check out the ref. mentioned in the talk.
The polarity of charge at surface, however, depends upon the position of EF w.r.t. CNL. It's negative when EF is above CNL charges and positive when below.
CNL and EF forms a relationship from which the resulting band bending (QSC), Qit and Qm are balanced in a way to preserve neutrality. A perfect alignment between CNL and EF is an ideal case in which no band bending..
2. No dangling bonds for 2D, but metal can react with 2D (hydridation) to form MIGS. 2D crystal in not perfect as well (subsurface impurity still can cause havoc.). BTW, you don't need much to pin. The Dit of Si-CMOS is in the range of E10/cm2. The surface atomic density of any material is around E14-E15. Any impurity on the surface, not observed by any physical apparatus (their sensitivity is only 0.1% at best, i.e., E13) , can still pin the Fermi level (electrical signal is a lot sensitive).
3. I would think so.
老師好
感謝老師的回覆與分享了MIGS這部影片的連結(th-cam.com/video/ClRbvmhFxZA/w-d-xo.html)
我也順道把在另一部影片談論了surface states看完了(th-cam.com/video/cxJPkBx-Jhw/w-d-xo.html)
還有後續幾個影片也一併看了
前陣子真的尋找好多相關文獻，在試圖了解這個Fermi level pinning的這個問題
根據老師的影片我終於理解到的是就是surface states與 metal induced gap states是可以類比的東西，一個是要描述在半導體表面因邊界條件改變而形成的能態，另一個則是因為半導體表面接了金屬而變成介面態，而他們都共用一個圖像就是會形成了一堆連續能量的states在原始半導體的帶隙中，這些states的數量會成指數衰減到到CNL的位置
而表面缺陷或雜質則會在材料中創造一個在特定能量位置的states
先前我也一直覺得沒辦法很好的把這些states的區別給搞清楚，但現在有比較理解了
但我還是有好多問題在我仔細思考過一樣存在
1. 是關於CNL的問題
在上一個問答的內容裡面，我想我忽略的這個CNL與Fermi level之間的關係是討論在電中性條件之前還是之後了
想跟老師確認的事情是在電中性平衡之後，CNL與Fermi level 應該會在同一個位置對嗎?
否則能帶應該要繼續彎曲直到達成電中性平衡對嗎?
2. 關於這些連續能量的states
在老師的影片與一些發表的文章都可以看到這些states的數量是指數形式的衰減到CNL的位置
想知道為甚麼這些states的數量會是以這個形式指數衰減到CNL的位置？
而CNL為甚麼會是states數量最少的能量位置？
3. 關於這些連續能量states的存在區域
在老師的影片中描述了這些states的存在是以指數形式從介面/表面衰減到半導體內部
如果是這樣的話，假如觀察半導體從內部到表面的能隙大小，是否也是會呈現一個指數變化，從內部有能隙，能隙大小指數衰減到0而進入表面
我只是在想這個半導體的內部與表面應該會有個過渡區域，那能隙的大小好像就會像是被這些states給填上而縮小的感覺
不知道這樣的觀念是不是有誤
4. 關於使用絕緣體來減少MIGS
依照絕緣體的圖像，絕緣體可以是能隙比較大的半導體
金屬與絕緣體接觸不會產生MIGS而造成Pinning嗎？
---------------------------------------------------
近年對一些單層的過度金屬二硫族化物感興趣(TMD)
有看到一些文章講二維半導體生長在金屬上，如單層的MoS2生長在Au(111)，即便表面沒有看到任何缺陷，其實從能譜上也無法辨識能隙的準確位置
這些能譜除了特定能量位置有峰值以外，其他能量位置都還是有DOS可以量測到
在稍微了解了MIGS，之後，我頓時感受到很神奇，儘管沒有任何晶體缺陷產生被觀察到，但這些交互作用確實創造了states，並填滿了整個能隙
相比MoS2生長在HOPG這種半金屬基板上，能隙就顯得乾淨的多
為了去了解MIGS，我深感半導體與金屬的接觸真的很複雜，儘管很多文章發表，好像仍然有很多規則還沒有被確立
好比說CNL應該存在的位置，從老師的影片中就有看到有的人說跟H在晶體內的能量位置有關，有的說在dangling bond的能量位置有關
有的又會說跟缺陷的states在哪裡有關
但我會感覺CNL存在的位置應該是最容易產生缺陷的那個能量位置，就會是CNL的位置(雖然我也不知道對不對，也沒有證據，只是瞎猜)
但這個想法是源自於MIGS在TMD材料生長在金屬上的文章，由於大部分金屬與這些TMD都是pinning在N-type 的區域(即便材料上沒有觀察到任何缺陷)
而這些TMD材料，最容易生成的缺陷大多數都是硫、硒空缺，這些空缺都是被預期會在CBM下方產生一個in-gap states
恰好與pinning在N-type 區域好像有連結性
總之還是相當感謝老師辛苦製作影片，我真的收穫良多
也盼望能與老師持續有交流
Bests
@@Scola_Hao have fun!
@@Scola_Hao contact me thru my gmail in my LinkedIn . We can the connect thru LINE for easier discussion
www.linkedin.com/in/wei-e-wang-4911533/
You need to keep in mind (as I mentioned earlier). All physical measurements are quite limited in understanding defects. This is because (a) your STS measurement area is so small and (b) the sensitivity of STS is inadequate to see the electrical response (down to E12/cm2 or below). This is just statistics. The sampling area is too small.
You can use a TEM to see no defect and yet there could be tons of defect in reality, e..g, to see defects density of E8/cm2 need to sample 1umx1um. TEM area is a few nm or a bit more. There is no way for them to see defect density < E8/cm2.
BTW, the surface has no band gap is b/c electron can go anyway since they do not see ions blocking their paths...
2D materials is a bit more complex. Please check the most recent papers on this subject (VIGS?)
pubs.aip.org/aip/jap/article/135/10/100901/3269952/Native-point-defects-in-2D-transition-metal
pubs.aip.org/aip/sci/article/2020/30/301104/364696/Extending-the-metal-induced-gap-state-model-to