[QE-users] Regarding bands

[Giuseppe Mattioli]

Dear Satya

Like Giovanni, I also don't know why you don't obtain what you expect.  
But I want to offer my own penny to the discussion.

1) Due to the well known effect of delocalization errors in GGA-DFT,  
the band gap of Si is generally reduced to around 0.6 eV. The  
reduction involves both the lowering of the conduction band potential  
and the raising of the valence band potential. In the case of ZnO you  
have a strongly curved conduction band around the minimum at Gamma,  
which is not affected by delocalization errors, and a very problematic  
valence band, in which the interaction between the narrow O 2p and Zn  
3d contributions may raise the valence band maximum by more than 2 eV,  
shrinking the band gap to less than 1 eV in bulk ZnO.

2) First of all, therefore, I would check *in the same supercell* if  
isolated Si and isolated ZnO slabs envisage a metallic character of  
the junction due to the unsymmetrical band gap underestimation of the  
two parts.

3) The most stable ZnO surface is (10-10), not 0001 (or its 000-1  
counterpart having reverse polarity). The polar surfaces of ZnO are  
tricky to simulate as slabs due to the strong polarity of the Zn-O  
bond. In the past, I obtained some weird layered hexagonal structure  
similar to BN! I suppose that the choice of (0001) is purposeful. If  
it is not, try to reconsider your choice.

4) Such a strongly polar surface in contact with a non-polar Si  
surface may induce a huge interfacial charge transfer, even enhanced  
by delocalization errors mentioned above, or can lead to significant  
shifts of band edges, yielding a metallic character of the  
semiconductor junction.

HTH
Giuseppe

Quoting Satyasiban Dash ph19d005 <ph19d005 at smail.iitm.ac.in>:

> Yes . Thank you for the clarification. I am trying to match (0001) surface.
> But I have observed upon relaxing ZnO loosing its symmetry and this may be
> due to the polar surfaces but experimentally we do observe a band gap. I
> have also tried dipole correction but results are same. But if I remove Si
> its giving a gap. Is there anyway to reproduce it with Si (001).
>
> On Thu, 28 Apr, 2022, 4:55 pm Giovanni Cantele, <
> giovanni.cantele at spin.cnr.it> wrote:
>
>> Dear Satya,
>>
>> it is not that easy (at least for me) to provide an answer to your
>> question using the data you have provided. Maybe the output files (of scf
>> and bands) and a plot of your band structure might help to infer if there
>> is any issue in your calculation.
>>
>> However, before wondering whether you get a metal whereas experiments
>> measure a gap, you should also check that you are simulating a system that
>> is as close as possible to the experimental one (in terms of thickness
>> and/or surface orientation of both ZnO and Si, and so on). As far as the
>> atomic positions are concerned, they look reasonable. It seems (if I’m not
>> wrong) that you are trying to match a Si(100) surface with a ZnO(0001)
>> surface, aren’t you? Is this what you expect?
>>
>> Giovanni
>>
>> --
>>
>> Giovanni Cantele, PhD
>> CNR-SPIN
>> c/o Dipartimento di Fisica
>> Universita' di Napoli "Federico II"
>> Complesso Universitario M. S. Angelo - Ed. 6
>> Via Cintia, I-80126, Napoli, Italy
>> e-mail: giovanni.cantele at spin.cnr.it <giovanni.cantele at spin.cnr.it>
>> Phone: +39 081 676910
>> Skype contact: giocan74
>>
>> ResearcherID: http://www.researcherid.com/rid/A-1951-2009
>> Web page: https://sites.google.com/view/giovanni-cantele/home
>>
>>
>> On 28 Apr 2022, at 07:09, Satyasiban Dash ph19d005 <
>> ph19d005 at smail.iitm.ac.in> wrote:
>>
>> Dear Users
>>  I have been trying to calculate a heterostructure with a slab model. I
>> have followed most of the examples given in the forum and tried it still I
>> am getting a metallic band structure. I have used the K-path generation
>> tool provided in "material project" website  to generate k-path. I have
>> given the scf and bands.in file as attached files. Please have a look.
>> Experimentally this system has a wide band gap.
>>
>>
>> Thanks.
>> Satya
>> Research Scholar
>> IIT Madras.
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GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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