Research Plan

CONTENT

1.Introduction 2.Literature survey 3.Objective 4.Work done so far 5.Future plan 6.Details of course work 7.References

Introduction:-

ZnO is a II-VI semiconductor of wurtzite structure and has a direct band gap 3.34 eV with large exciton bound energy of 60 meV which paves the way for an intense near-band edge exciton emission at room and high temperature. It has also attracted immense interest as a material for electronic and optoelectronic devices such as blue/UV LEDs and laser diodes. For full use of ZnO in technological and commercial purpose, high quality of n-type as well as p-type ZnO are needed. But in ZnO it is difficult to dope p-type due to native defects such as Zn interstitial oxygen vacancy and hydrogen impurity from growth environment. [1] ZnO crystallizes in hexagonal crystal structure with lattice parameters a=3.249A and c=5.206A, where each Zn ion is tetrahedrally coordinated by four oxygen ions and vice-versa.

Fig-1:

Unit cell of the

ZnO Dar k blue color - O ion Yell ow color - Zn ion

p-type doping:Group I doping:- Li, Na, and K, substituting on the Zn site, are predicted to have acceptor energy levels of 0.09, 0.17, and 0.32 eV, respectively [2]. These levels are shallower than those predicted for NO, PO, and AsO. However, it is well known that Li doping actually produces semi-insulating (SI) ZnO [3], and the reason likely involves formation of the Li interstitial LI, which has a donor nature [2]. That is, as the Fermi level EF drops due to the formation LiZn acceptor centers, it will become more and more favourable for LiI donors to form, and eventually EF will settle at an energy between the donor and acceptor levels, i.e., near mid gap. The same mechanism should hold for Na doping. For K doping, on the other hand, it will probably be the formation of O-vacancy donors that will keep the sample from transforming to p-type [2]. Group –V doping:- N, P, As, Sb , substitute O site in ZnO. Out of these N is considered best dopant because of its ionic radius matches with that of O ion. However these dopants introduce deep acceptor levels with energies 0.4eV, 0.93eV and 1.25eV respectively.[2]

Other Codopants:- Cu, Ag, Au these codopants with valance state +1 substitute Zn to produce a hole in the ZnO.

Codoping in ZnO:The codoping method using acceptors (A) and donors (D) as the reactive codopant, in a ratio of A/D 2:1, contributes (i) to the enhancement of the incorporation of the acceptors because the strong attractive interactions between the acceptor and donor dopants dominate the repulsive interactions between the acceptors, where the driving force is the electrostatic energy gain associated with partial compensation, and (ii) to lowering of the energy levels of the acceptors and raising those of the donors in the band gap due to the strong attractive interactions between the acceptor and donor as reactive codopants, as shown in Fig. 2.[4] Basic codopant pairs are: N) (Ga, P) etc

(B, N) (Ga, N) (In, P) (Al,

Fig-2 Schematic energy band diagram of ptype codoped semiconductor. Literature Survey:ZnO is very potential material in electronic and optoelectronic devices such as UV LEDs and LDs. But for the full realization of ZnO, both high qualities of n-type as well as p-type are needed. The n-type ZnO:( In and Al ) with high quality are available but p-type ZnO with high mobility with low resistivity is still to optimize. Since p-type ZnO is difficult to dope due deep acceptor levels of dopants. T.Yamamoto et al proposed the theory of Codoping, in which codopants having Donor (B, Ga, In, Al..) with acceptor (N, P, As,…) are used. This codoping theory enables, how codoping increases the solubility of dopant which are acceptor like and also how their ionization energy can be decreased. Y.R Sui et al in 2009 reported a p-type B–N codoped ZnO film was grown on quartz by magnetron sputtering and postannealing techniques. It has room temperature resistivity of 2.3ohm_cm , Hall mobility of 11 cm2/

V−1 s−1 and carrier concentration of 1.2 × 1017 cm−3, better than the electrical properties of the Ndoped p-type ZnO.[5] The ZnO homojunction fabricated by deposition of an undoped n-type ZnO layer on the B–N codoped p-type ZnO layer showed clear p–n diode characteristics. In 2006 L.L. Chen suggested the fabrication p-type ZnO by In-N codoping. The carrier type in the In–N codoped ZnO can be controlled by adjusting the growth conditions and good p-type conductivity is obtained at temperatures between 490 and 580 °C. The p-type behavior is improved when a buffer layer is used. The lowest reliable room temperature resistivity is found to be 7.85 ohm-cm in the presence of a buffer layer.[6] Apart from doping Bin Wang et al (2009) realized p-type conductivity in undoped film of ZnO by ultrasonic spray pyrolysis .[7] In which they realized the fact that in oxygen rich condition experimentally, that is enthalpy formation for oxygen vacancy and zinc interstitial is high while for oxygen interstitial and zinc vacancy have low formation enthalpy. The undoped p-type ZnO thin films have also been realized by K.H. Nam et al (2008) by plasma assisted chemical vapor technique. in which they claimed that films with n type conductivity change into p-type ZnO on increasing the oxygen flow rate .[8] Further moving to monodoped p-type ZnO thin films by Vth group element N, P, As, Sb,theoretically it has been realized that only N substitute the O in ZnO. while P, As, Sb ,due to large size mismatch they do not substitute O but they substitute Zn by creating two vacancies with the mechanism- AsZn-2VZn which creates shallow acceptor levels because two VZn connected by the AsZn antisite

donor through a cation sublattice.[9] Still much efforts are needed to improve mobility with higher carrier concentration because high mobility is required for the better performance of p-n homojunction.

Objective:• To fabricate reproducible and stable p-type ZnO thin films. • To optimize low resistive with high mobility ptype ZnO thin films. •

To fabricate p-n homojunction of ZnO .

Work done so far:

Operation of measurements like resistance Vs temperature (R-T) measurement, Hall Effect measurement, Telestep profilometer and MOKE.

 Operation of XRD.  Vacuum system- Leybold, Varian and Excel have also been learnt.  Deposition of ZnO thin films.  Completion of course work with (7.8) CGPA.

Details of course work:Registration date -26-07-08 In 1st semester

S.No

Course title

credit

Grade

1

Numerical & computational method PHL-800

3

B-

2

Vacuum science & cryogenics PHL-723

3

B

3

Material technology

3

B-

4

Material characterization PHL707

3

B-

second semester:-

S.No

Course title

credit

grade

1

Science & technology of thin films PHL702

3

A

2

Nanostructuredmarerials PHL726

3

B

3

Communication Skill HUL-810

audit

NP

• Total CGPA obtained is 7.8

References:1-

Z.B.Whang, S.H.Wei, and Alex Zunger Physical Review B,

2-

Volume 63,075205 (2001).

C. H. Park, S. B. Zhang, and S. H. Wei, Phys. Rev. B 66, 073202 (2002).

3-

D. C. Look, D. C. Reynolds, C. W. Litton, R. L. Jones, D. B. Eason, and G. Cantwell, Appl. Phys. Lett. 81, 1830 (2002).

4- T. Yamamoto Thin Solid Films 420 –421 (2002) 100– 106. 5-

Y.R.Sui, B. Yao, Z. Hua,G. Z. Zing, J. Phys. D: Appl. Phys. 42 (2009) 065101.

6-

L.L. Chen, Z.Z.Ye, J.G. Lu, Paul K. Chu, APPLIED PHYSICS LETTERS 89, 252113 _2006.

7-

Bin Wang, Jiahua Min, Yue Zhao, Wenbin Sang, and Changjun Wang ,APPLIED PHYSICS LETTERS 94, 192101 _2009.

8-

K.H. Nam, H. Kim, H.Y. Lee, D.H. Han, J.J. Lee K.H. Nam⁎, H. Kim, H.Y. Lee, D.H. Han, J.J. Lee ,Surface & Coatings Technology 202 (2008).

9-

Sukit Limpijumnong, S. B. Zhang, Su-Huai Wei, and C. H. Park, Phy Rev. Lett ,VOLUME 92, NUMBER 15 (2004).