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Replacing each C atom of the diamond structure alternately with Ga and As so that each Ga is bonded to four As and each As is bonded to four Ga leads to the zincblende or sphalerite structure (actually zincblende is the cubic form of ZnS and the mineral sphalerite is cubic ZnS with some Fe)• As at corners: (0,0,0)• As at face centers: (a/2,a/2,0),
(a/2,0,a/2), (0,a/2,a/2)• Ga 4 internal sites: (a/4,a/4,a/4),
Bonding in GaAsMaking a covalent bond between to each atoms, one might have expected tetrahedral As to make 3 bonds with a left over lone pair pointing away from the 3 bonds, while Ga might be expected to make 3 covalent bonds, with an empty sp3 orbital point away from the 3 bonds, as indicated here, where the 3 covalent bonds are shown with lines, and the donor acceptor (DA) or Lewis acid-Lewis base bond as an As lone pair coordinated with and empty orbital on Ga
Of course the four bonds to each atom will adjust to be equivalent, but we can still think of the bond as an average of ¾ covalent and ¼ DA
Similar zincblende or sphalerite compounds can be formed with Ga replaced by B, Al,In and /or As replaced by N, P, Sb, or Bi. They are call III-V compounds from the older names of the columns of the periodic table (new UIPAC name 13-15 compounds).
In addition a hexagonal crystal, called Wurtzite, also with tetrahedral bonding (but with some eclipsed bonds) is exhibited by most of these compounds.
In addition there are a variety of similar II-VI systems, ZnS, ZnSe, CdTe, HgTe, etc
The surface unit cell, P(1x1) is ½ the cross-section for the (110) plane outlined in the unit cell cube at the right. Note that top surface has equal number of Ga and As
Reconstruction of (110) surface, side view along [-1,1,0]
[001]
[110]
54.7º54.7º
54.7º
Si (110)
Ga As
GaAs (110)
Si has dangling
bond electron at
each surface atom
Surface As has 3 covalent bonds to Ga, with 2 e in 3s lone pair, relaxes upward until average bond angle is 95º Surface Ga has 3 covalent bonds leaving 0 e in 4th orbital, relaxes downward until average bond angle is 119º. GaAs angle 0º 26º
We consider that bulk GaAs has an average of 3 covalent bonds and one donor acceptor (DA) bond. But at the surface can only make 3 bonds so the weaker DA bond is the one broken to form the surface.
The result is that GaAs cleaves very easily compared to Si. No covalent bonds to break.
As has 3 covalent bonds, leaving 2 electrons in 3s lone pair. AsH3 has average bond angle of 92º. At the GaAs surface As relaxes upward until has average bond angle of 95º Ga has 3 covalent bonds leaving 0 eletrons in 4th orbital. GaH3 has average bond angle of 120º. At the GaAs surface Ga relaxes downward until has average bond angle of 119º.This changes the surface Ga-As bond from 0º (parallel to surface to 26º. Observed in LEED experiments and QM calculations
Analysis of chargesBulk structure: each As has 3 covalent bonds and one Donor-accepter bond(Lewis base – Lewis acid). This requires 3+2=5 electrons from As and 3+0=3 electrons from Ga.We consider that each bulk GaAs bond has 5/4 e from As and ¾ e form Ga. Each surface As has 5/4+1+1+2 = 5.25e for a net charge of -0.25 each surface Ga has ¾+1+1+0= 2.75 e for a net charge of +0.25 Thus considering both surface Ga and As, the (110) is neutral
1st Layer RED2nd Layer GREEN3rd Layer ORANGE4th Layer WHITE
Every red surface atom is As bonded to two green 2nd layer Ga atoms, but the other two bonds were to two Ga that are now removed. This leaves three non bonding electrons to distribute among the two dangling bond orbitals sticking out of plane (like AsH2)
For the perfect surface, As in top layer, Ga in 2nd layer, As in 3rd layer, Ga in 4th layer etc. For the unreconstructed surface each As has two bonds and hence three electrons in two nonbonding orbitals. Expect As atoms to dimerize to form a 3rd bond leaving 2 electrons in nonbonding orbitals. Surface As-As bonds
Terminating the bulk charges onto the surface layer and considering the lone pairs and broken bonds on the surface should lead to:• the atomic valence configuration on each surface atom.
For example As with 3 covalent bonds and a lone pair and Ga with 3 covalent bonds and an empty fourth orbital
• A neutral surface
This leads to the permissible surface reconstructions
Trends: overlaps between bonded atoms decrease from 2p to 3p to 4p etcThus bonds are weaker, but antibonds are not as bandThus cohesive energy and band gaps decrease as go down the periodic table
Consider the case in which one Si atom of Si crystal is replace by a P atom (substitutional impurity)Main effect is that P has one more electron than Si
The substituted P can make covalent bonds to 3 of Si neighbors but the extra electron is in the way of making the 4th bond. Thus it is very easy to ionize this extra electron (IP=4.05 eV) donating it to the conduction band (EA=4.0 eV) leaving behind a P making covalent bonds to all four Si neighbors. The net excitation energy is just 4.05-4.00=0.05 eV. Thus as room temperature lots of electrons in conduction band. Get n type semiconductor and P is called an n-type dopant
Consider the case in which one Si atom of Si crystal is replace by a Al atom (substitutional impurity)Main effect is that Al has one less electron than Si
The substituted Al can make covalent bonds to 3 of the Si neighbors but it lacks the electron to make a 4th bond 2-e bondThus the EA of add an electron to make the 2 electron bond is EA=5.033 eV, which is nearly as great as the IP=5.1 eV. Thus removing an electron from the valence band and adding it to the Al-Si bond costs only 5.1-5.033=0.067eV. leaving behind an Al making covalent bonds to all four Si neighbors.
Since the net excitation energy 0.067 eV there are lots of holes in the valence band at room temperature. Get p type semiconductor and Al is called a p-type or acceptor dopant