Electrical and optical properties of AgInS2

Solid-Slate

Electronics.

1976, Vol. 19, pp. 31-35.

Pergamon Press.

Printed in Great Britain

ELECTRICAL AND OPTICAL PROPERTIES OF AgInS, K. OKAMOTO and K. KINOSH~TA Universityof Electra-Communications, Tokyo, Japan (Received 15 November 1974; in reuised form 8 April 1975) Abstract-Crystal sizes of AgInS2 grown by a directional freezing depend on sulphur pressures at the preparation. The conductivity is only n-type and nominally undoped AgInS, has the resistivity of 25 &cm and the Hall mobility of 64 cm2/V sec. Sulphur vacancies of AgIn& become electron-trapping levels in the forbidden band. It is obtained from the measurements of thermally stimulated current that the levels lie at ET1 = 0.19? 0.01 eV and ET2 = 0.40 t 0.01 eV, and the concentrakions depend on sulphur pressures at the crystal preparations. Au-AgInS, contacts operate as a Schottky barrier diode and the barrier he@ht is 0.97 eV. AgInS, has a dichroism because of its uniaxial lattice structure. The transition is direct for Elc and indirect for E//c, and the values for the energy gap are E,’ = 1.88 r 0.01 eV and E,” = 1.77 + 0.01 eV, respectively.

INTRODUCTION AgIn& is a ternary compound semiconductor which has a wide band gap lying in the visible region of the spectrum which crystalizes in the chalcopyrite structure, and which is uniaxial lattice structure[l]. So, AgInS, is a candidate as a material for optoelectronic devices. Here it is the purpose to discuss its fundamental properties from a viewpoint of optoelectronic materials. In this paper, we report the sulphur-pressure dependence of AgIn& crystal growth, electrical and optical properties of bulk and Au-AgIn& contacts and, especially, optical anisotropy. 1.

pressure increased, the ingots became harder and crystal sizes became larger. In case of the highest sulphurpressure (8 at. %), a boule of single crystal was obtained, whose diameter was about lOmm, and with residual sulphur on the top surface of the ingot. The optical transmission of AgIn& also depended on the sulphurpressure and the transmittance at 1OOOnmare shown in Fig. 2 as a function of excess sulphur contents. Tem~ (‘Cl

2. CRYSTAL GROWTH OF AgInS, The constituent elements, weighed in a stoichiometric ratio, silver (99.999%), indium (99.999%) and sulphur (99.99%), were sealed in a quartz ampoule at pressures less than 10-5Torr. At first, one end of the ampoule was heated by a gas burner while the other end was being cooled by water. Then, the elements were reacted with each other and, the whole ampoule was heated up to 1000°C without ampoule explosion. After the ampoule was heated at 1ooo”Cfor 10hr in a horizontal furnace, it was suspended in a vertical furnace and lowered at a rate of O?mm/hr for 40 hr. After the freezing process, the furnace was cooled down slowly to prevent the ampoule cracking. The temperature distribution of the vertical furnace and the initial setting position of the ampoule are shown in Fig. I. The ingots obtained usually consisted of three parts: (1) Apexes of the ingots were metallic and opaque. (2) The central parts contained several single crystals, which were dark red and transparent. (3) The upper parts were dark grey and porous. The sulphur-pressure dependence of crystalization was examined by adding an excess of sulphur during the ampoule preparation and the maximum contents of excess sulphur were limited under lOat. %, above which ampoules tended to explode. The resulting ingots and crystals showed obvious sulphur-pressure dependence and their outside appearances are as follows. In case of low sulphur-pressure, the ingots were porous and fragile, and contained small lumps of single crystals. As sulphur-

Fig. I. Temperature distribution of vertical furnace setting position of ampoule.

60

and initial

-

X- IOOOnm

I

1 -3

I

1

t

0

3

6

content of excess s (at.W

Fig. 2. Optical transmittance of AgIn& at 1000 nm as a function of excess-sulphur contents. 31

K.

32

OKAMOTO

and K. KINOSHITA

Crystalization of AgInS: occurred at 890°C. The conductivity of AgInS is only n-type and has not been converted to p-type either by doping impurities (Zn, Sn, P and I) or thermal annealing. Nominally undoped AgInS has a resistivity of 25 n-cm, electron concentration of 4, lOI cm-’ and electron mobility of 64 cm*/V sec. An activation energy of 0.0013 eV was obtained from the temperature dependence of conductivity. AgInS? was easy to cleave in parallel with the (110) plane and acute-angled triangle patterns were observed on the etched surfaces as shown in Fig. 3. A (221)-network pattern of electron diffraction was observed at the tilting angle of 21”, so that the surface of triangle-etch pattern corresponds to a (221) plane and crosses the cleaved (1 IO)-plane at an angle of 21”. The vertical angle of the triangle-etch pattern is given as 39” by projecting the vertical angle of 43” measured from the photograph to (221) plane. Then, the unit cell of AgIr&, which has the chalcopyrite structure, is able to be constructed as shown in Fig. 4. The lattice constants were determined to be a = 5.46 A and c = IO.07 A, and the built-in compressive distortion was given as c/2a = 0.92 by electron diffraction.

3. ELECTRICAL

PROPERTIES

OF Au-A@&

DIODES

Gold electrodes of 1 mm diameter were evaporated on one side of AgInS substrate polished in parallel with a cleaved surface and indium was deposited on the other side as an ohmic contact. The Au-AgInS diode operated as a Schottky barrier diode and typical V-1 characteristics are shown in Fig. 5. After the diode was kept sufficiently long time in the dark, the forward current initially increased with time and then arrived at a steady state as shown in Fig. 5 by arrow heads. On the contrary. the reverse current showed an initial decrease but the amount of current drift was usually very small. The initial drift of the forward current depended strongly on sulphur pressure during the crystal preparation and the amounts of the drift were larger for the samples prepared at low sulphurpressure than those prepared at high sulphur-pressure. When the polarity of applied voltage was reversed for a time interval at the steady state, the forward current restarted again from a decreased current level and, then, reached the original steady state with a time constant. Figure 6 shows the normalized levels of initial forwardcurrent as a function of the reverse-biasing time. From the above results, it is suggested that the initial drifts of the

400 a 2

E

300

:

:

*

200

c=1.840

! (I-

Fig. 5. V-I

Fig. 4. Unit cell of AgInS,.

0

20

40

60

characteristics of Au-A&& indicate current drift.

120

180

Timdsec)

Fig. 6. Normalized

initial-forward-current

as a function of reverwbiasing

time.

diode.

Arrow

heads

33

Properties of AgInS, forward current are due to the electron-charging effects of traps, that is, the positively charged traps lying in the space charge region are neutralized by the forward biasing and the narrowing of the space charge region brings about the increase of forward current. On the contrary, the effects of the bias-reversing can be explained by the electron-discharging of the traps. From the correspon-

dence between the amounts of the current drift and the sulphur pressure at the crystal preparation, it is thought that the traps originate from sulphur vacancies in AgIt&. In order to investigate the trap levels, the thermally stimulated current (TSC) was measured for AgInS. The dimensions of the samples were 3 x2x0.5mm3 and indium was deposited on both ends as ohmic contacts. After being cooled down to the liquid nitrogen temperature, the samples were illuminated and, then, warmed up at the rate of 25”C/min in the dark. The samples were biased at 10V and the current picked up from a small series resistance was recorded by an X-Y recorder as a function of temperature. Figure 7 shows the TSC of -3% excesssulphur sample and 8% excess-sulphur sample, which were obtained by subtracting the dark current from the above results. The both TSC of -3% excess-sulphur sample and 8% excess-sulphur sample show a peak at -170”K, but the value of the former is about thirteen times as high as that of the latter. Therefore, it is inferred from the sulphur-pressure dependence of the TSC that the levels are ascribed to sulphur vacancies. The TSC of 8% excess-sulphur sample shows another peak at -120”K, which, in case of -3% excess-sulphur sample, is buried in the skirt of the former peak. The energy levels of ET1= 0.19 * 0.01 eV and ET>= 0.40 + 0.01 eV were obtained from the slope of the TSC-vs-l/kT plots. The temperature dependence of forward current was measured for Au-AgInS diode and shown in Fig. 8. The diode current is given as,

I

2.6

I

I

3.0

3.2

I

I

3.6

3.6

I

I/T

34

I

4.0

(x103KJ)

Fig. 8. Temperature dependence of forward current of Au-AgInS, diode.

and the voltage across the junction is, V = V, - IRS

where & is the barrier height, V,, is the applied voltage and RS is the series resistance. The barrier height of r#~= 0.97 2 0.05 eV was obtained from the slope of Fig. 8. The energy band diagram of an Au-AgInS contact is shown in Fig. 9. 4. OPTICAL

PROPERTIES

OF AgIn&

AgInS has a dichroism because of its uniaxial lattice structure. Fig. 10 shows the spectrum dependence of the optical transmittance for polarized incident light, and the dichroism is observed in the wavelength region between 630 and 665 nm. The absorption coefficients were measured for polarized incident light and shown in Fig. 11 as a function of photon energies. In case that the electric AU

AglnSp

.40eV

I

I

00

100

I

120

I

140

Temperature

I

160

,

160

1

20(

(K)

Fig. 7. Thermally stimulated current of AgInS,. Solid line corresponds to sample of -3% excess-sulphur content, and dashed line to sample of 8% excess-sulphur content.

i

Fig. 9. Energy band of Au-AgInS, contact.

34

K.

620

640

660

660 Wavelength

720

700

K.

OK~MOTO and

(nm)

Fig. 10. Spectrum dependence of optical transmittance for polarized incident light

KINOSHITA

vector of light was perpendicular to the optic axis (I?_Lc). the absorption coefficients showed the square-root dependence of photon energies, so that the transition was direct and the band gap energy was obtained as E,’ = 1.88 r 0.01 eV. In case of J$, the absorption coefficients showed the quadratic dependence of photon energies, so that the transition was indirect and the band gap energy was obtained as E,” = I ,77 t 0.01 eV. The polarization dependence of optical transmittance was measured at 6324 nm of a He-Ne laser as shown in Fig. 12, and it was confirmed that AgIn$ was able to polarize with the electric vector perpendicular to the optic axis. The spectrum dependence of photovoltaic responses for Au-AgIn& diode are shown in Fig. 13. The photovoltaic responses are able to be divided into two regions: first, a large response corresponding to the across-the-gap pair generation (A-region); and, second, a tail extending to

1200

6

I

I

I

I

I

/

1000

5

I

1 -

B

4

800

3

600

2

400

I

200

N c(

0 I.9

Photon Fig. Il.

1

0 I.9

500

I

I

I

700

000

900

energy (eV)

Wavelength

Photon energy dependence of absorption coefficients for ,??lc and hilt.

Fig.

13. Spectrum

I

0

90’

12. Polarization

dependence

nm

I

I

450 Angle

Fig.

I

X-632.8

-

135’

of polarizer of optical

1

transmittance

at 632%nm.

I

1000 1100 I<

(nm)

dependence of photovoltaic Au-AgInS, diode.

I

I

20

I

600

responses

of

35

Properties of AgIn&

I

I

1

I

:

8 15

6

5%

k

IO 4

I.0 5 2

0 I.9

I.6

Photon energy

0

(eV)

Fig. 14. Photon energy dependence of ph_otovoltaic responses of Au-AgInS, diode for E IC and E 11~.(A region).

lower energies due to the migration of hot electrons from the gold over the barrier height into AgInS (B-region)[21. In the A-region, the photovoltaic responses depended on the polarization of light, and showed the square-root dependence of photon energies for Elc and the quadratic dependence of photon energies for E\lc as shown in Fig. 14. The results coincided well with those obtained from the absorption coefficient measurements, and, independently, E,- = 1.90 eV and Ed’= 1.76eV were obtained. In the B-region, the photovoltaic responses did not show polarization dependence and the photovoltage-vs-photonenergy plots of Fig. 15 seem to be consisted of three straight lines, which give energies, El = 0.93 eV, Ez = 1.13 eV and E1 = 1.33 eV. E, = 0.9 is the activation from the conduction band over the Schottky barrier; E2 i= 1.2 is the activation from the 0.2 eV trap over the same barrier and El- 1.3 is the activation from the 0.4eV trap over the same barrier. 5CONCLUSlONS

AgInS crystalized at 890°C and dark red and transparent crystals were obtained by directional freezing. The crystalization depended strongly on the sulphur pressure at the crystal preparation. In order to get large and high-quality crystals, it is necessary to grow the crystals under high sulphur pressure. The conductivity of AgInS was only n-type and has not been converted to p-type either by doping impurities or by thermal annealing. Therefore, it is thought to be difficult to prepare a homo-junction of AgIt&

1.4 1.2 Photon energy

1.6 (eV)

Fig. 15. Photon energy dependence of photovoltaic Au-AgInS diode. (B region).

I.6

responses of

The sulphur vacancies of AgInS become the electrontrapping levels in the forbidden band, and affect the optical transmittance and the current drift. From the TSC measurements, the energy levels of 0.19 +O*OleV and 0.40 + 0.01 eV were obtained and the concentrations of the levels depended on the sulphur pressure at the crystal preparation. Au-AgIt& contact operated as a Schottky barrier diode and the barrier height was 0.97 eV. The initial increase of the forward current was due to the electroncharging effects of the traps in the space charge region, and the amounts of the current drift depended on the concentrations of the sulphur vacancies. Although such levels are usually undesirable for carrier transport, it is expected to apply them to a memory device [3,4]. AgInS crystallizes in the chalcopyrite structure, which is an uniaxially ordered superstructure of zincblende, so that it shows a dichroism for the optical transmittance and the photovoltaic responses in the wavelength region between 630 nm and 665 nm. For Elc, the transition was direct and the energy band gap was 1.88 t 0.01 eV and, for Ellc, the transition was indirect and the energy band gap was 1.77t 0.01 eV. Because of the wide energy band gap lying in the visible region of the spectrum, the optical anisotropy and the semiconducting properties, AgInS is expected, in the future, to be applied to optoelectronic devices, and it is the matter in hand to grow high-quality crystals. Acknowledgement-The his encouragement.

authors wish to thank Prof. K. Takei for

REFERENCES I. J. L. Shay, B. Tell, L. M. Schiavone, H. M. Kasper and F. Thiel, Phys. Rev. B9, 1719 (1974). 2. J. L. Pankove, Optical Processes in Semiconductors. PrenticeHall, Englewood Cliffs, New Jersey (1971). 3. H. J. Hovel and .I. J. Urgell, J. Appl. Phys. 42, 5076 (1971). 4. N. Romeo and A. Dallaturca, Appf. Phys. Letts 22,21 (1973).