The cluster study on doping energy of a-Si : H

Solid State Communications, Vol. 103, No. 11, pp. 615-618, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All tights reserved 0038-1098/97 $17.00+,00

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THE CLUSTER STUDY ON DOPING ENERGY OF a-Si : H Yufang Zhou, a'* Jiajian Wang a and Chengbu Liu b aDepartment of Physics, Shandong University, Jinan 250100, P.R. China blnstitute of Theoretical Chemistry, Shandong University, Jinan 250100, P.R. China

(Received 13 May 1997; accepted 29 May 1997 by A. Okiji) Calculations of substitutional electronic energies, doping energies and energy levels for structural clusters simulating phosphorus and boron doped hydrogenated amorphous silicon have been performed with Si17-nXnH36 (n = 1, 2; X ----P, B) atomic clusters using ab initio molecular orbital theory. The results show that doping energies vary with substitutional sites. Doping can change the electron cloud distribution and energy levels of the cluster. Doping sites can influence the doping states in the gap as well as interaction between doping states. These are factors which decide the electronic energy and the doping energy of a cluster. © 1997 Elsevier Science Ltd. Keywords: A. semiconductors.

1. INTRODUCTION As a low-priced semiconductor material, a - S i : H has been widely applied to make solar cell and other photoelectrical devices [1-4]. This is due to a-Si : H showing a efficient doping property. Substitutional impurities are an important factor for photoelectrical properties of the material [5]. Because of different applying purposes, various doped structures are required. To understand the doping mechanism can help people control doping conditions to obtain a desired material. This is the reason why so many people work at it [6-8]. Most of mechanism studies were examined by experiments, less of them were done by theoretical calculations. There may be much less experimental data to be contrast with theoretical results. But theoretical study can predict some useful information that is helpful for experimental experts to design their experiments. So, it is of great significance to make a detailed theoretical study of the doping mechanism in a-Si : H alloys. In this communication we report on doping energies of phosphorus and boron doped hydrogenated silicon using ab initio molecular orbital theory at HF/STO-3G

* Corresponding author.

level. The results will reveal some regularity in doping mechanism. 2. MODEL AND THEORETICAL METHOD The character of amorphous material is order in short distance and disorder in long distance. In the very small range the structure of amorphous is the same as crystalline lattice. So the Si17H36 cluster model and its substitutional doped cluster models should be simulated by local structures of a - S i : H alloys, and lattice defects were neglected. The Si17H36 cluster model (see Fig. 1) was built in the following manner: first, one silicon atom is laid on center (numbered 1), then, four first nearest neighbour silicon atoms were combined with it according to the tetrahedral coordination (numbered 2 to 5). Twelve silicon atoms as the first neighbours were combined with above atoms in the same manner (numbered 6 to 17). Finally, thirty-six hydrogen atoms were used to saturate the dangling bonds. The cluster possesses T d symmetry. The distance of Si-Si is equal to 2.35 A and Si-H is 1.48 ,~. Phosphorus and boron atoms were substituted into Si 17H36 cluster taking up one or two silicon atom sites, respectively, thus SilT-nXnH36 ( n = 1, 2; X --- P, B ) clusters were constructed. Ab initio molecular orbital theory, which has successfully been applied to some studies of amorphous

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Fig. 1. Structure diagram of 8i17H36 cluster model. semiconductors [8-12], was adopted to investigate energies of clusters and derive doping energies of doped clusters. Because of larger clusters and because we only concern relative energies, ab initio HF/STO-3G level was used in calculations. 3. RESULTS AND DISCUSSION Electronic energies and doping energies were calculated for undoped Si17H36 cluster and Si 16XIH36 (X = P, B) clusters (see Table 1). It shows that electronic energies and doping energies vary with X taking up different sites. For X is phosphorus atom, the doping energy (-1391.6280eV) is lowest when P atom taking up the center site (number 1), the doping energy (-1390.7003eV) is highest when P atom taking up number 17 site (near edge of the cluster). It needs 0.9277 eV energy more than the former. To form the (Si 16P IH36)5 cluster (P atom taking up middle layer site), the doping energy is median. While, for X is boron atom is not the same as phosphorus atom. B atom prefers to take up the number 17 site, the doping energy (7114.1943 eV) is the lowest. And it is harder to take up number 5 site (needs 0.2780 eV energy more) than

taking up number 1 site. Table 1 also shows changes • (AAE0 for various doping energies. When two impurity atoms were implanted into cluster we construct SilsX2H36 clusters (X = P, B). Electronic energies and doping energies are shown in Table 2. The easiest doping sites of two impurities in the same time are number 1 and 5 for phosphorus doped cluster (doping energy -2783.3285 eV) and number 5 and 15 for boron doped cluster (doping energy 14228.2351 eV). When two phosphorus atoms taking up (outer layer) number 13, 17 sites, the doping energy ( - 2781.1379 eV) is highest and when two boron atoms taking up (inner layer) number 4, 5 sites, the doping energy (14231.5991eV) is highest. There need 2.1406 eV and 3.3640eV, respectively, energies more to form laters than formers. When impurities take up middle layer sites, doping energies are median. Changes of doping energies (AAE2) are also shown in Table 2. The results mentioned above indicate that doping site is the origin to cause disparities of doping energies• But the real reason can be explained as follows• First, the situation is not the same for phosphorus and boron doped clusters. The undoped Si17H36 cluster possesses Td symmetry. When silicon atoms were substituted by phosphorus or boron atoms, the electronic cloud distribution deviates from regular tetrahedral symmetry. And because of the difference coordination numbers, phosphorus and boron atoms would make different deviations due to their bonding with silicons around it. As a result electronic energy levels are changed or energy bands are moved (see Fig. 2). Second, various doping states make deviations of cloud as well as changes of energy levels in different extent. Third, doping states and interaction between them are also important factors for electronic energies and doping energies• The two doping states separate from each other due to interaction between them, which affect the energy of the cluster. We can see from Fig. 2, for (Si16P1H36)l and (SiI6BIH36)I 7 clusters, doped atoms cause valence

Table 1. Electronic energies and doping energies of Si 16XIH36 clusters Cluster

E (hartree)

Si 17H36 (SiI6P1H36) I (Si 16PIH36)5 (Si 16PIH 36) 17 (Si 16B1H36) 17 (Si 16BIH36) 1 (Si t6B 1H36)5

-4876.1715 -4927•3155 -4927.3068 -4927.2814 -4614.7164 -4614.6885 -4614.6847

AEI (eV) a - 1391.6280 - 1391.3921 - 1390.7003 7114.1943 7114.9527 7115.0563

a AE I = E (SiI6XIH36) - E (Si17H36) b AAE1 ----EI(SiI6XIH36)i _ EI(Si16X1H36)j forX=P,i=5,17.j=1;forX=B,i= 1,5.j=17.

AAEI (eV) b

0.2359 0.9277 0.7584 1.0364

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Table 2. Electronic energies and doping energies of Si 15X2H36clusters Cluster

E (hartree)

(Si 15P2H36)1,5 (Si 15P2H36)5,15

-4978.4603 -4978.4554 -4978.4407 -4978.3817 -4353.2669 -4353.2465 -4353.1904 -4353.1432

(Si 15P2H36)4,5 (SiI5P2H36) 13.17 (Si 15B2H36)5,15 (Si 15B2H36)1,5 (Si15B2H36) 13.17 (SilsB2H36)4,5

AE2 (eV) a

AAE2 (eV) ~

-2783.2785 -2783.1435 -2782.7452 -2781.1379 14228.2351 14228.7899 14230.3199 14231.5991

0.1350 0.5333 2.1406 0.5548 2.0849 3.3640

aAE2 = E (SilsX2H36) - E (Si17H36); X = P, B. bAAE2 = E2 (SilsX2H36)i.j -E2 (SiI5X2H36)k,/ for X = P, (i,j) = (5, 15), (13, 17), (4, 5) and (k,l) = (1,5); f o r X = B, (i,j) = (5, 15), (13, 17), (4, 5) and (k,l) = (5, 15). states [see Fig. 2(h, 1)], So, they are in low energies. The doping states interaction in (Si16PlH36) 13,17 and (SiI6BIH36)4.5 clusters are weak and valence and conduction bands are high, those result in clusters in high energies [see Fig. 2(k, o)], therefore it is harder to dope impurities. In order to investigate more practical clusters in a-Si : H, we also calculated some unsaturated, dangling bonded clusters. The calculated results gave the same

bands to be moved downward besides that doping states were introduced in the gap [see Fig. 2(b, e)]. This is the reason they are in low energies and easy to be doped. (SiI6PIH36)l7 and (SiI6B IH36)5 clusters have high energies because they make both valence and conduction bands moved upward [see Fig. 2(d, g)]. For (SilsP2H36)1,5 and (SilsB2H36)s,j5 clusters, they have not only lower valence and conduction bands but also stronger interaction of doping states, which result in lower doping

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Fig. 2. Energy level diagrams: a. Si17H36; b. (Si16PiH36)l; c. (SiI6PIH36)5; d. (SiI6PIH36)I7; e. (Si16BIH36)I7; f. (SiI6BIH36)I; g. (SiI6BIH36)5; h. (SiIsP2H36)1,5; i. (SilsP2H36)5,15; j- (SilsP2H36)4,5; k. (SiI5P2H36)I3,17; 1. (SilsB2H36)5,15; m. (Si15B2H36)I,5; n. (SilsB2H36)13,17; o. (SilsB2H36)4,5. C, conduction band; V, valence band; D, doping state.

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DOPING ENERGY OF a-Si : H

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conclusion as above except that some boundary states 2. Carlson, D.E., U.S. Patent No, 4,064, 521, 1977. were introduced into the gap. For simplification, calcu- " 3. Hamakawa, Y., Okamoto, H. and Nitta, Y., Appl. Phys. Lett., 35, 1979, 187. lated results are not given here. 4. Okamoto, H., Nitta, Y. and Hamakawa, Y., Japan J. Appl. Phys., 19, 1980, 545. 4. CONCLUSIONS 5. Hamasaki, T., Kuato, H., Hirose, M. and Osaka, Y., J. Appl. Phys., 2, 1981, 84. The calculating results in this paper indicate that 6. Adler, D., Phys. Rev. Lett., 41, 1978, 1755. when impurities were implanted into a-Si:H, silicon 7. Street, R.A., Phys. Rev. Lett., 49, 1982, 1187. atoms in different sites have different opportunities to be 8. Chang, R.Q., Da, G.C., Cai, Z.T. and Guan, D.R., substituted. To form various doped structures need difSolid State Commun., 65, 1988, 1625. ferent energies. By choosing specified condition one can 9. Chang, R.Q., Solid State Commun., 69, 1989, 681. 10. Redondo, A., Goddard, W.A., Swarts, C.A. and obtain required structural material. McGill, T.C., J. Vac. Sci. Technol., 19, 1981, 498. 11. Goddard, W.A., Redondo, A. and McGiI1, T.C., REFERENCES Solid State Commun., 18, 1976, 981. . Carlson, D.E. and Wronski, C.R., Appl. Phys. Left., 12. Batra, Inder, P., Bagus, P.S. and Hermarm, K., 28~ 1976, 671. Phys. Rev. Lett., 52, 1984, 384.