Band structure and properties of Zn3P2— promising new infrared material∗

BAND STRUCTURE AND PROPERTIES OF Zn,P2-PROMISING NEW INFRARED MATERIAL* JANUSZ M. PAWLIKOWSKI Institute of Physics, Technical University of Wroclaw. St Wyspiariskiego 27, 50-370 Wroctaw. Poland (Received 6

December

1980)

Abstract-A short review of the current status of investigations of zinc phosphide is given. Special attention is paid to the examination of energy-band structure and photoelectric properties. Finally, possible applications are discussed.

INTRODUCTION

Zinc phosphide Zn,P, was, until quite recently, a relatively poorly known semiconducting compound of the A”B” type. Not long ago, a Zn,P,-based infrared photoconvertor was designed which enhanced our knowledge of its band structure parameters and optical and transport properties. The basic semiconducting properties of Zn,P, were outlined in Ref. (1); some of them were recently reviewed in Ref. (2). This paper is based on results of new investigations and focussed on optical and photoelectric data. Special emphasis is placed on the energyband structure near the r point and application to solar-cell technology. CRYSTAL

STRUCTURE AND GALVANOMAGNETIC PROPERTIES

The ZnJP, lattice has (up to ca 1053K) tetragonal symmetry belonging to the D:z space group. The dimensions of the unit cell are a = b = 0.8097 nm and c = 1.145 nm, and this cell contains 40 atoms. At cu. 1053K, the tetragonal a-phase transforms to the cubic B-phase with lattice constant 0.582 nm, and this cell contains 10 atoms.“’ Some information on thermodynamic properties of zinc phosphide has also been given by Shevchenko et ~1.‘~’ The electrical transport properties of Zn,P, have been studied by a number of workers over a wide temperature range (from 80 to 7OOK, approximately). Only p-type conductivity has been observed, at 3OOK, and hole mobilities of the order of 10-l m’/vsec have been found. The temperature dependence of a conductivity and carrier concentration indicate that the dominant acceptor levels are fully ionized near room temperature. A great effort to understand p-type conductivity was recently made by Catalan0 et ~1.‘~~”A study of the dependence of electrical resistivity and hole concentrations on temperature, vapour composition and pressure were performed in Ref. (4). Zn,P, specimens were annealed at various fixed partial pressures of zinc and phosphorus. Figure 1 shows the hole concentration vs the equilibrium partial pressure of P for crystals annealed at 573K. A least-squares fit yields N, = 1.32 x 1016p(P4)o.‘3.

(1)

A simple theory was also given in Ref. (4) assuming that phosphorus anion interstitials and zinc cation vacancies are the only native point defects which may act as acceptors. The dependences of hole concentrations (equal to fully ionized acceptor concentrations) is given by lg Nh x Qlg PV',) W * This work is carried out under contract IM-132. 181

182

J. M.

PAWLIKOWSKI

id4 Id'

(Torrid-

p Fig. 1.

for

Acceptor concentration

N, vs phosphorus partial pressure for Zn3P2 samples annealed at 574K after Ref. [4].

interstitial phosphorus acting as acceptors, by lg Nh

for

singly

x

T%g

P(p4)

ionized zinc vacancies acting as acceptors, k Nh K +&

(2b)

and by

P(p,)

(W

for double ionized zinc vacancies. Comparison with experimental data showed good agreement for the case given by equation (2a)--see Fig. 1. The possibility of change of the conductivity type from p to n and experimental evidence of p--n junction formation in Zn,P, were discussed in Ref. (5). The n-type or inversion layer near the surface was made by heating a p-type sample covered by magnesium contact, at lOO”C,in air. It should be noted that this is the first announcement about changing the conductivity type of a II-V compound. OPTICAL

PROPERTIES

A short review of prior literature data on optical properties was recently given in Ref. (2). Now, there are four fundamental works concerning this matter: Refs (2), (6), (7) and (8). In Ref. (6) the absorption coefficient and reflectivity spectrum at 300 and 80K are reported for bulk crystals. The analysis of absorption data for both bulk crystals and thin films is given in Refs (2) and (7), but at room temperature only. Measurements of single crystals over a wide temperature range 5-300K are performed in Ref. (@-see Fig. 2. In Refs (2) and (6) the smallest energy gap is ascribed to direct transitions, whereas in Refs (7) and (8) to indirect ones. All suggested values of the Zn3Pz energy gap are listed in Table 1. The fundamental question is whether the smallest gap is indirect or direct. In our paper t8) the absorption measurements were also performed on relatively thick specimens for the purpose of a more precise analysis of the low-absorption region, in fact from CI= 1 cm- ‘, approximately. A reasonable fit was obtained with a square-root law: c&w = &

(ho + kO - 17;)’ + 1 _z;_,,TJ

(hw - k@ - EfJ2

(3)

where Ei is an indirect gap and k@ is the energy of the phonon which is either necessary to the phonon-absorption process or produced in the phonon-emission process. This fit indicates that these transitions are indirect and that the smallest energy gap in Zn$, corresponds to band extrema at different points in the Brillouin zone. In very recent experimental work on the anisotropy of Zn,P, optical properties in the region of the fundamental absorption edge, (lo) the values of the indirect gap were confirmed and the smallest gap was ascribed to two different transitions at and near the r point.

Band structure and properties of ZnJPa

183

Fig. 2. Absorption plots of single ZnsPz crystal in the middle- and high-absorption regions, after Ref. 18-J.Inset shows the absorption at the lowest absorption level.

We have well fitted a fundamental part of the absorption edge to direct interband transitions in the simple parabolic two-band model

Table 1. Optical-i~du~d

transition energies (in eVf for Z&P,

Reference

(6)

3OOK

P

1.30 1.44

P P P,R

1.61 1.84

(2)

3 1.6

(7) 1.35 1.42 1.52 1.66 1.82

1.32 SOK

P

1.46

P

1.72 2.0

P,R

(8)

SK Note: P and R denote the photoresponse

1.315 1.46 1.505 R 1.84

1.335 1.50 1.645

(15) P

Comments to Refs (8) and (IS)

1.32

Indirect gap Direct gap

P

1.49

Pz Ps

1.6

P

1.34

Indirect gap

P

1.63

Direct gap

1.8

P z 2.0 1.335 1.51 1.685

indirect gap Direct gap

and reflectivity measurements, respectively.

184

J.

M. PAWLIKOWSKI

where H,,,(O) is the optical matrix element at k = 0 and Ei is a direct energy gap. In our opinion, in spite of the simplicity of the model and ignoring the electron-hole interactions, the fundamental absorption edge is connected with valence-to-conduction band transitions at the r point of the ZnJP, band structure. The values of the direct gap are also listed in Table 1. An analysis of absorption data in the middle-absorption region requires additional measurements. Some suggestions on this matter are discussed in Ref. (8). Real and imaginary parts of dielectric constant for Zn,Pz were computed by KramersKronig analysis method on the basis of reflectivity measurements and are discussed in Ref. (11). Several transition energies taken from this analysis are also given in Table 1. Lattice modes of Zn3P, were studied by infrared absorption in Ref. (12). The principal absorption bands lie in 20-25 pm waverange and numerous smaller bands are seen, mainly attributed to multiphonon processes. PHOTOELECTRIC

PROPERTIES

Semiconductors for potential application in photovoltaic solar-cell systems have to satisfy three basic criteria: (i) appropriate location and shape of fundamental absorption edge; (ii) sufficient diffusion length of photo-carriers and (iii) easy adaptability to device fabrication. From this point of view Zn,P, is one of the most promising semiconductors for such applications. There are several papers in which the photoelectric properties of Zn3P, are studied.(6~g~13-17’In our recent papers (i5*i6) the effect of different lighting configurations on spectral plots of photoresponses was researched (see Fig. 3) and two main response peaks were ascertained. These peaks are ascribed to indirect and direct band-to-band transitions (see above), their threshold energies are listed in Table 1. The effect of additional monochromatic irradiance on the spectral plots of photoresponses was also investigated. The lack of measurable differences between all particular plots makes it possible to state a lack of recombination centers within the ZnJP, energy gap or, at least, their insignificant influences on measured results. Similar results were obtained in Ref. (18). In Ref. (10) the anisotropy of photoresponses was also found, and a low-energy response, near 1.1 eV, obtained at 300K. In a former paper Cl‘) this low-energy response was observed at 80K, as well. Its energy position is well fitted to the values l&l.1 eV, found in Ref. (19) from photo-luminescence experiments. A simple model of photo-voltage effects for the metal-Zn,P, Schottky-barrier solar cell was presented in Ref. (20). This model took into account the optical absorption of semiconductor and metal contact as well as carrier recombinations in the volume and on surfaces. Carrier-distribution equations were solved and hence the photo-responses computed for experimental data for metal-Zn,Pz contacts. A comparison of results computed and experimental photo-voltage plots of the Au-Zn3P, structure yielded sufficiently good agreement,“‘) see also Fig. 3. Extensive investigations on metal-thin Zn3P, film solar cell technology and properties were recently made by Catalan0 et al. (g,14.17) These Zn,P, films were grown by the close-space vapour transport method. Highly-oriented polycrystalline films of large grain size were obtained. An effective minority-carrier diffusion length of the order of several pm was found. Metal-Zn,P* grid devices fabricated on thin Zn,P, films exhibited a total-area conversion efficiency of 2: 1.5% and an active-area of 4%. The spectral response of these devices was relatively flat in the short wavelength range (0.45-0.8 pm) indicating negligible losses due to surface recombination.” ‘) Figure 4 shows a comparison between typical photoresponse of a metal/Zn3Pz Schottky-diode convertor and the irradiance plot under AM1 (air-mass unity) conditions. The optimal fit of both location and shape of the photoresponse plot is easy to observe. This good fit as well as the relatively large minority-carrier diffusion length indicate that there are promising applications of Zn3Pz as a photovoltaic convertor in the nearinfrared region.

Band structure

and properties

of ZXI~P,

, t

L 0.:5

06

Of

06

0.9 1.0 I.1 I.2 1.3 1.4 X (pm)-

Fig. 3. Recorded spectral plots of photoresponse of single Zn,PI crystal, at 300K. for different lighting configurations, after Ref. [lS]. Contact is marked by the open rectangle and the lighting spot by the blacked one. Theoretical plots after Ref. [20] are also marked, for two configurations (broken lines).

185

J. M. PAWLIKOWSKI

186

r

Fig.

4. Photoresponse

of metal-Zn

a P 2 structure on schematically.

the

solar

spectrum

ground

marked

CONCLUSIONS

Only one band-structure calculations, by the pseudopotential method(22’ have been made for Zn3P2 so far. A pseudocubic structure was assumed, and missing atoms were represented by a matched pseudopotential. In these calculations spin-orbit and crystalfield interactions were neglected. These reasons strongly restrict the usefulness of computation results. A spin-orbit splitting of about 0.11 eV and a crystal-field splitting of about 0.03 eV were computed in Ref. (23) however, on the lines of Lin-Chung’s approach. In conclusion, there are no energy-structure calculations for real (tetragonal) Zn,P, crystal structure, so far. Absorption data, both accumulated in the literature and obtained by ourselves, do not enable us to decide definitely whether the smallest gap of Zn,P, is direct or indirect. However, the indirect gap is most probable in our opinion.@’ The results obtained on the photoelectricity of Zn,P,, both for bulk crystals and thin films, indicate the great potential possibilities of the use of Zn,P, as a high-sensitivity material for solar-cell applications.

Acknowledgements-The author is indebted to all his co-workers, and to Dr J. Misiewiez especially, for helpful comments. Kind exchange of information with Dr A. W. Catalan0 is also cordially acknowledged. This work is based on the review paper presented at The 1st international Symposium on the Physics and Chemistry of II-V Compounds, Mogilany (1980).

Band structure

and properties

of ZnSP,

187

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14.

15. 16. 17. 18. 19. 20. 21. 22. 23.

~DANOWICZW. & L. ~DANOWICZ,Ann. Rec. mater. Sci 5, 301 (1975). FAGEN E. A., J. appl. PhJls. 50, 6505 (1979). GREENBERGJ. H., V. B. LAZAREV,S. E. KOZLOV & V. J. SHEVCHENKO,J. them. Therm. 6, 1005 (1974). CATALANOA. W. & R. B. HALL, J. PhJjs. Chem. Solids 41, 635 (1980). CATALANOA. W. & M. BHUSHAN,J. appl. Phys. (in press). SOBOLEVV. V. & N.. N. SYRBU, Physica status solidi b64, 432 (1974). ZDANOWICZL., W. ZDANOWICZ,D. PETELENZ& K. KLOC, Acta phvs. pal. A57, 159 (1980). PAWLIKOWSKIJ. M., J. MISIEWICZ & N. MIROWSKA, J. Phys. C&m: S&ids 40, 1027 (1979). CATALANOA. W.. V. DALAL. E. A. FAGEN. R. B. HALL. J. V. MASI. J. D. MEAKIN. G. WARFIELD & A. M. BARNET, Proc. Int. Conf: an Photovaltaic’Solar Energy, Luxembuig, 1977, p. 644. D. Reidel, Dordrecht (1978). MISIEWICZ J. & J. A. GAJ, Proc. Int. Symp. on Physics and Chemistry of II-V Compounds, Mogilany, 1980, p. 163. M. J. Gelten & L. Zdanowicz. Eindhoven (1981). JEZIERSKI K.. J. MISIEWICZ & J. M. PAWLIKOWSKI.in Ref. (101. D. 193. RADAUTSANS. I.. N. N. SYRBU, 1. I. NEBOLA& V. I. VOLODINA,Sot. Solid-St. Phys. 19, 1290 (1977). PAWLIKOWSKIJ. M., N. MIROWSKA & F. KROLICKI, Infrared Phys. 18, 343 (1978). CATALANOA. W., V. DALAL, W. E. DEVANEY, E. A. FAGEN, R. B. HALL, J. V. MASI, J. D. MEAKIN. G. WARFIELD. N. CONVERS WYETH & A. M. BARNET. Proc. 13th IEEE Photovoltaic Specialists Conf Washington D.C., p. 288 (1978). PAWLIKOWSKIJ. M., N. MIROWSKA. P. BECLA & F. KROLICKI, Solid-st. Electron. 23, 755 (1980). KROLICKI F.. N. MIROWSKA. K. NAUKA & J. M. PAWLIKOWSKI,in [lo]. p. 199. CATALANOA. W.. M. BHUSHAN& N. CONVERSWYETH, submitted for publication. VOLODINAV. I., I. RADAUTZAN,V. I. KADYGROB & N. N. SYRBU, Fir. Tekh. Poluproc. 11, 609 (1977). HULDT L., N. G. NILSSON. B. 0. SUNDSTRBM& W. ~DANOWICZ,Physica Status Solidi a53. K15 (1979). NAUKA K. & J. M. PAWLIKOWSKI.Acta phw. pal. A59, 351 (1981). MIROWSKA N., J. MISIEWICZ. K. NAUKA & J. k. PAWLIKOWSKI,Acta phqs. PO/. A60, 179 (1981). LIN-CHUNG P. J., Physica Status Solidi b47, 33 (1971). DOWDIALLO-PLENKIEWICZ B. & P. PLENKIEWICZ.PhJsica Status Solidi b87, 309 (1978).