Vuv reflection spectra and fundamental optical constants of CdIn2S4, CdInGaS4, Cd3InGaS6 single crystals

Solid State Communications, Vol 67, No 7, pp 739-743, 1988 Printed in Great Britain

0038-1098/88 $3 00 + 00 Pergamon Press plc

VUV REFLECTION SPECTRA AND FUNDAMENTAL OPTICAL CONSTANTS OF CdIn2S4, CdlnGaS4, Cd3InGaS6 SINGLE CRYSTALS Takeo Taklzawa, Hlroshl Ohwada, Hideo Kobayashl and Kohj1 Kanbara Department of Physics, College of Humanities and Sciences, Nlhon University, Setagayaku, Tokyo 156, Japan Talzo Irle, Saburo Endo and Hlsayukl Nakamshl Department of Electrical Engineering, Faculty of Engineering, Science Umverslty of Tokyo, Shmjuku-ku Tokyo 162, Japan H~roo Kato Photon Factory, National Laboratory for High Energy Physics, Oho-machl, Tsukuba-gun, Ibarakl 305, Japan and Hlrohlto Fukutanl Institute of Physics, Umverslty of Tsukuba, Ibarakl 305, Japan

(Recetved 4 March 1988, zn revtsedform 3 Aprd 1988 by H Kamtmura) Reflection spectra of CdIn2S4, CdlnGaSa4 and Cd3InGaS6 were measured in the photon energy from 2 0 to 35 eV Optical dielectric constants, the effective number of electrons contributing to optical transitions and the energy loss function were also obtained in terms of the Kramers-Kromg transformation It was shown that electromc structures for these crystals are very s~mllar to each other m sp~te of the different crystal structures between them 1 INTRODUCTION VARIOUS compounds can be made from the mixing of II-VI, III-V elements and their adjacent neighbors In the Periodic Table In recent years much attention has been paid to the materlal research on such ternary and multmary systems m expectations of findmg new functional materials available for the photo-electronic devices So far, we have studied the electrical and optical properties of single crystals of Cdln2S4, CdlnGaS4, Cd2InGaS5 and Cd3InGaS6, which we calll as C-0, C-I, C-2 and C-3, respectwely The single crystal of C-0 has been well studied m the last two decades, and many reports concerning the electrical, optical and electro-optical properties have been presented [1-14] The crystal of C-I can be made by exchanging one of the In atoms in Cdln2S4 with Ga belonging to the same family as In It has been known as a photosensitive semiconductor having a wide band gap, showing the red luminescence [15-22] However, it was also observed that some of C-1 crystals showed weak green-luminescence at 77 K [20] By mixing CdS and 2CdS with CdInGaS4, we grew single crystals of C-2

and C-3, respectively, having layered structures slmdar to CdlnGaS4, and confirmed that the green luminescence m some C-1 samples ~s due to these crystals To understand electronic structures of C-0, C-I, C-2 and C-3, we measured the reflection spectra in the energy range from 2 0 to 35 eV and obtained fundamental optical constants of the crystals except for C-2, data on C-2 were not obtained because good reflection surfaces were not available In this paper, we report the results and discuss the electronic band structures m terms of the effective electron numbers and the optical energy loss spectra 2 EXPERIMENTAL All the single crystals used were grown from melt of the sto~chiometnc composition of more than 5N purity elements by the horizontal Bndgman method The crystallographic structure of C-0 crystals was the spinel type and that of the other crystals was the hexagonally layered one Their lattice constants were determined by the X-ray diffraction as shown m Table 1

739

740

VUV R E F L E C T I O N S P E C T R A

Table 1 Lattwe Constants of Crystals for Reflection Measurements m Umt of Angstrom

Cdln2 $4 CdlnGaS4 Cd3InGaS6

a

c

10 853 3 858 3 88

37 0 37 65

cubic hexagonal hexagonal

The measurement of reflectivlty m the photon energy from 2 0 to 6 0 eV was carried out using a usual double-beam spectroscopic system equipped with Halogen and D 2 lamps The reflectivity from 5 0 to 35 0 eV was obtained using the synchrotron radiation as a hght source together w~th a Seya-Namioka type 1 m grating m o n o c h r o m a t o r attached to the storage ring of the Photon Factory in the Natmnal Laboratory for High Energy Physics of Japan Reflection measurement were done on cleaved surfaces for C-0 crystals, and on smooth natural ones for layered crystals Since our layered samples were all in thin flakes, we measured the spectra only with the electric vector parallel to the layer 3 RESULTS AND DISCUSSION Figure 1 shows the reflection spectra of C-0, C-1 and C-3 crystals m the photon energy from 2 0 to 30 eV There observed 9 distinct peaks shown as R0 to Rs m the reflection spectra The reflection spectra of C-0 crystals were In good agreement w~th data reported by Grllh et al In the photon energy from 2 to 10 eV [4] except that we could not observe any distinct structure between R0 and R~, where they had found

Cd~lnG

01

0 0I

. . . . . . . . . . 2

4

6

8 10

'~. 20

.I 30

Photon Energy (eV) Fig I Reflection spectra of Cdln2S4, CdlnGaS4 and Cd3InGaS 6 at room temperature

Vol 67, N o 7

two additional ones, they stated In their paper [4] that one of them was not fully reproduoble Since their samples were grown by the chemical transport method, we suspect that their samples might suffer from the effect of environmental substances such as carrier gases, which might cause some defects m samples, giving additional structures The energies of structures m reflection spectra were listed m Table 2 together with those in the imaginary part of the dielectric functions mentioned later A good correspondence between the observed structures is shown by dashed lines in Fig 1 The shapes of reflection spectra near absorption edges (R0 and Rl in Fig 1) are similar between three crystals as if the edges might be just shifted in parallel manner according to the change of the constituent elements Thus it is suggested that the absorption-band-edges in these materials mainly consist of bonds due to the same pmrs of atoms, which, we suppose, are those of S-Cd and S-In A discussion on this point will be given later again Using the K r a m e r s - K r o n i g analysis, we obtained the optical dielectric constants of C-0, C-1 and C-3 crystals as shown in Fig 2 The spectra were extrapolated out of the experimental region using conventional arithmetic functions for the computation of the K - K Integral The functions were

R(E) =

R0exp[qE],

(l)

on the low-energy side and

R(E) = Rs(Es/E)p,

(2)

on the high-energy one, where E is the photon energy, R0 the reflectlvlty at 0eV, and q the number determined so that the value of R(E) coincides with the reflectlvity RA at E = EA, the lowest photon energy of the measurement The quanUty RB is the reflectlvlty at E = EB, the highest energy limit Two unknown parameters, R0 and p are so chosen that the absorpUon coefficients agree with the observed ones at any two points near the absorption edge Using the relation that e2 IS proportional to - E0 at the absorption edge in Fig 2, we obtained energies of the direct band gaps as listed In Table 2 The energies of indirect edges which have been obtained by absorption measurements are also shown, only the C-3 crystals have no Indirect edges Figure 3 represents the number of valence electrons per formula unit, neff, of each crystal contributing to the optical transmons over the energy interval from 0 to a particular photon energy It is clearly seen with the aid of the energy-derivative spectra of neff shown as broken lines in Fig 3 that there are four slope-changes m n~er spectra at 6 l, 8 5, 12 3 and

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741

V U V R E F L E C T I O N SPECTRA

Table 2 Energies of Structures m Reflecnvmes and F,2 Spectra m Unit of eV Cdln2 S4

E, Ed R0 R, R2 R3

R4 R5 R6 R7

R8

CdlnGaS4

R

e2

2 89 4 32 5 16 5 58 6 57 7 12 990 13 6 169 21 9

2 28* 2 67 2 98 4 50 5 18 5 50 6 52 7 10 986 13 5 168 22 0

band assign

Cd 3InGaS6

R

e2

R

e2

3 17 4 44 5 43 6 10 7 50 8 22 108 13 2 167 21 3

2 52** 3 10 3 37 4 52 5 46 6 02 7 35 8 05 109 13 5 167 21 3

3 09 4 48 5 27 5 68 6 97 8 48 119 14 3 170 21 5

3 04 3 44 4 65 5 50 5 70 7 36 8 05 100 13 8 167 21 2

valence bands after XPS***

A

- 1 5 (Sap)

B

- 3 7 (Cdss + Insp)

C O

- 5 9 (Ins,) - 9 6 (Cd4d) - 11 5 ($3~)

G

- 17 0 (In~)

E, the energies of the indirect edge Ed the energies of the direct edge * after ref 10 ** after [22] *** after [6] and [7] 5O

CdalnGaSs

10

40

8 6

30

4

2O

2

10.

Cd31nGaS6

, ,,

,/

0

5g

CdInGaS,

10

4O

8 6

30

4

2O

2

CdnGaS,~ I

10

0 c

10

13 5O

8

4O

6

313

4

2(]

2

113

0

13 2

4

Photon

6

8 10

Energy

20

30

(eV)

Fig 2 Dielectric constants ofCdln2S4, CdlnGaS4 and Cd3 InGaS6 20 0eV for C-0 crystals The dips in the derivative spectra show falls o f the rate of increasing In nar, indicating a kind of boundaries in band structures, so that five different optical contributions are expected to exist in e2 of C-0 crystals According to the band schemes reported in hteratures [4, 23-27], we specify those as A (2 6-6 0 eV), B (6 0-8 5eV), C (8 5-12eV), D (12-15 eV) and G (20-

2

Photon 4

6

8 10

2O

30

Energy (eV)

Fig 3 Effective valence electron numbers per formula unit as a functmn of photon energy for Cdln2S4, CdlnGaS4 and Cd3 InGaS6 Broken lines represent the derivative spectra with respect to the photon energy Vertical lines show the points where the increasing rate of neffIS reduced, by which the bands designated as A, B, C, D and G In Table 2 were determined 25 eV) (see Fig 1 and vertical lines In Figs 3 and 4) XPS [6] and X-ray sulfur K- and L-spectra [7] of C-0 crystals have revealed structures below the top o f the valence band, which are shown at the nght-end co-

742

VUV REFLECTION SPECTRA 06. 05. 04. 03. 02. 01 E I

°!l O0

0

0

0 0

4

G

8 10

20

30

Phof.on Energy (eV) Fig 4 Dielectric loss functions for CdIn2S4, CdInGaS4 and Cd 3InGaS6 Vertical lines show dips in the spectra, which correspond to boundaries of the bands designated as A, B, C, D and G in Table 2 lumn in Table 2, where the symbols in the parentheses indicate the principal atomic levels contributing to bands Since the optical transitions are determined by the joint density of states between the conduction and the valence band, it is not easy to correlate the optical structures with those of the valence band The theoretical considerations ever made [23, 26, 27] show that owing to the large number of atoms per unit cell of this crystal, m a n y electronic levels having different atomic characters contribute to form the electronic bands Thus, we can assume that optically allowed levels are always existing near the lowest conduction band for the optical transition between any state in the valence bands and the states in the conduction bands In other words, the final states of the optical transitions can be located within a few eV above the conduction bottom The energy of the band gap is about 2 7 eV Thus, the structure centered at 1 5eV below the valence band top (see Table 2), is considered to give the maxim u m in the joint density of states around 4 2 ( = 1 5 + 2 7) eV, giving rise to the A band (2 7-6 0 eV In Fig l) In the same way, the structure, 3 7eV below the valence band generates the B band Structures above 10eV in Fig 1 (R6, R7 and Rs) do not depend on the kind of samples Accordingly, they are considered as transitions associated with the c o m m o n atoms of constituent elements, that is, Cd and In F r o m XPS spectra [6, 7], valence bands due to Inss, Cd4d and In4d (see the last column in Table 2) are

Vol 67, No 7

located at ca 5 9, 9 6 and 17 0 eV from the top of the valence band, respectively Since the lowest conduction band consists of s-states [27], the transition from s- or d-levels in the valence band to the lowest conduction band must be almost forbidden Thus, the energies of allowed transitions between the conduction band and the states due to Inss, Cd4d and In4d are expected to be somewhat larger than the energy of direct gap plus binding energies of electrons in those states, giving rise to the structures R6(10eV), R7(13 5eV) and R8(21 eV) Bands due to S3s is flat [24, 25] at anergy of 11 5 eV below the top of the valence band [6] Thus, the small shoulder at about 17 eV In Fig l may be due to Sas levels The number neff up to 35 eV can be calculated as 62, 62 and 98 by taking d-electrons of Cd, In and G a into account for respective crystals, where the numbers of the formula unit contained in the unit cells are 8 and 9 for C-0 and C-1 The crystal structure of C-3 has not yet been determined, so that we assumed one for this crystal similar to that of C-l with extra layers of Cd and S atoms periodically inserted between hexagonal atomic planes of C-1 Experimentally values of neffat 30 eV in Fig 3 are somewhat smaller for C-0, C-l and much smaller for C-3 The contribution from d-electrons of In and G a may exist in a higher energy than 30 eV, so that the value neff obtained experimentally is expected to be small compared with the theoretical one The experimental value for C-3, however, is too small to be explained with the lack of d-electrons Since the effective number of electrons is proportional to the volume of the unit cell of the crystal and is inversely proportional to the number of formula unit contained, it suggests that there are some problems in the assumed lattice-structure of C-3 Laue photographs of C-3 showed various diffuse patterns, indicating a periodicity strongly disturbed along the crystal axis, which makes one consider that these crystals may have stacking faults between layers On the other hand, the X-ray powder diffraction shows that C-3 crystals are not simply the mlxIngs of CdInGaS4 and 2CdS but new materials having layer structures Thus, we now suspect that C-3 are the modified crystals of C-l, having CdS layers in a rand o m manner along the crystals axis, while keeping almost the same periodicity of C-1 in that direction Figure 4 shows the spectra of - I m ( l / e ) , corresponding to the electron energy loss spectra for each crystal It is worth noting that peaks correspond well to the dips in energy-derivative spectra of neer, and vice versa Since dn¢er/dco shows the rate of the increase in electrons optically absorbed, while the loss spectra

VUV REFLECTION SPECTRA

Vol 67, No 7

represent the longitudinal energy loss of electrons m crystals, it seems natural that dne~/doJ becomes small when the loss is large The total number of electrons m s and p orbitals of the constituent elements for C-0 and C-1 crystals which amounts to 32, gives the energy of the plasma resonance as 16 7 eV The electron energy loss spectra using 100 keV electron beam of the electron microscope (JEM-2000EX) showed the plasma loss peak at 15 0 _ 0 5eV for C-0, C-I, C-2 and C-3 crystals Thus, the peaks around 15 eV m Fig 4 are due to the plasma resonances assocmted with the collective oscillations of the electrons m the valence bands There are stdl many problems concernmg optical and electrical properties, especially on C-3 crystals, and further studies are now in progress

7 8 9 10 11 12 13 14 15 16

Acknowledgements - - The authors thank Dr K Fukushsma for help with the electron energy loss spectra by JEM-2000EX This work has been conducted under the approval of the Photon Factory Program Advisory Committee (Proposal No 87-110)

17

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20

1 2 3 4 5 6

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