▁Si - Si ▁dim ers ▁are ▁combined ▁to ▁form ▁a ▁solid , ▁defines ▁the ▁condu ction ▁and ▁val ence ▁bands . ▁If ▁a ▁vac ancy ▁were ▁to ▁exist , ▁such ▁as ▁those ▁on ▁each ▁atom ▁at ▁the ▁solid / v acu um ▁interface , ▁it ▁would ▁result ▁in ▁at ▁least ▁one ▁broken ▁sp 3 ▁bond ▁which ▁has ▁an ▁energy ▁equal ▁to ▁that ▁of ▁single ▁self ▁hy brid ized ▁Si ▁atoms ▁as ▁shown ▁in ▁Figure ▁ 1 . ▁This ▁energy ▁corresponds ▁to ▁roughly ▁the ▁middle ▁of ▁the ▁band g ap ▁of ▁Si , ▁~ 0 . 5 5 e V ▁above ▁the ▁val ence ▁band . ▁C ertain ly ▁this ▁is ▁the ▁most ▁ideal ▁case ▁whereas ▁the ▁situation ▁would ▁be ▁different ▁if ▁bond ▁pass iv ation ▁( see ▁below ) ▁and ▁surface ▁reconst ruction , ▁for ▁example , ▁were ▁to ▁occur . ▁Ex periment ally , ▁the ▁energ ies ▁of ▁these ▁states ▁can ▁be ▁determined ▁using ▁absor ption ▁spect ro sc opy ▁or ▁X - ray ▁photo elect ron ▁spect ro sc opy , ▁for ▁example , ▁if ▁instrument ▁sens itivity ▁and / or ▁defect ▁density ▁are ▁high ▁enough . ▁▁ ▁Comp ound ▁sem icon duct ors , ▁such ▁as ▁Ga As , ▁have ▁d ang ling ▁bond ▁states ▁that ▁are ▁near er ▁to ▁the ▁band ▁edges ▁( see ▁Figure ▁ 2 ). ▁As ▁bond ing ▁becomes ▁increasing ly ▁more ▁ion ic , ▁these ▁states ▁can ▁even ▁act ▁as ▁dop ants . ▁This ▁is ▁the ▁cause ▁of ▁the ▁well ▁known ▁difficulty ▁of ▁Ga N