Photoconductivity spectra of CdSnP2

J. Phys.Chem.Solids, 1972,Vol.33, pp. 1565-1569. PergamonPress. Printedin Great Britain

PHOTOCONDUCTIVITY SPECTRA OF CdSnP2 I. P. AKIMCHENKO P.N. Lebedev Physical Institute, Academy of Sciences of the USSR, Moscow and

E. I. LEONOV, V. M. ORLOV and V. I. SOKOLOVA A.F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Leningrad (Received 19 July 1971 ; in revised form 24 November 197 I)

Abstract--Spectra of photoconductivity and photocurrent optical quenching were investigated in monocrysta'ls CdSnP2 in the impurity-range of spectrum. The influence of different doping impurities on the photoconductivity spectra was established. A multi-charge acceptor (compensating) centre increasing the photoconductivity was detected in crystals doped with copper. 1. INTRODUCTION

THE TERNARY semiconducting compounds of the A 2 B 4 C 2 5 type, which include CdSnP2, are the nearest isoelectrical analogs of the compounds A3BS. Theoretical and experimental studies of the band structure and electronic properties of the individual representatives of this class revealed that in AzB4C,: a complication of the band energy spectrum (as compared to A3B 5) takes place, which is due to the decrease of the crystal symmetry in transition from sphalerite to chalcopyrite [I,2]. The direct-band compounds A2B4C25 are characterized by the small effective mass of carriers and high mobility [1], maintenance of direct transitions, determining the width of forbidden band at least up to 1.6eV (CdSiAs2) nonlinear optical properties[I]. The CdSnP2 is the isoelectronic analog of InP (at 300°K, E g = 1.16eV) with small effective mass of carriers and multivalley structure of the conduction band. In CdSnP2 were observed: a stimulated radiation, when excitation was provided by an electron beam [3] and by H e - N e laser[4], generation of the second harmonic of the ruby laser radiation [5], two-photon photoconductivity (PhC)[6], L.F. and H.F. current oscillations[7]. In papers [4, 8, 9] some photoelectric properties

of CdSnP2 and of heterojunctions n CdSnPsp Cu2S were described. In this work the spectra of photoconductivity (PhC) of monocrystals CdSnP2 were observed in the extrinsic (impurity) range of the spectrum at the temperatures of 10°, 80 ° and 300°K, both for the usual and combined excitation. In addition to that, the influence of different dopings on the PhC spectra was investigated to elucidate the nature of some sensitivity centres. 2. PREPARATION OF SPECIMENS AND EXPERIMENTAL PROCEDURE

Specimens of CdSnP2 single crystals 4 × 15 x 0.3 mm in size were obtained by the free crystallization method from solution in the tin melt in evacuated quartz containers. The concentration of the ternary compound in the solution was 20 wt. %, the maximum temperature of the process was 700°C at the rate of cooling - 5 ° C / h r . The separation of single crystals from the solution was carded out by pickling in nitric acid ( 1 : 1). Crystals were doped during their growth by means of adding doping components in the starting mixture. Purposefully non-doped CdSnP2 crystals featured n-type of conductivity, resistivity of 10-~l~cm, the con-

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I . P . A K I M C H E N K O et aL

centration and mobility of electrons at 300°K within the ranges o f ( 5 . 1016-1.10 lr) cm -3 and (1500-1300) cm2/v.sec respectively. On introducing Li and Cu in the solution-melt in amounts of (0.07-0.1)wt. %, the resistivity increased, ranging (10L 105) lacm at 300°K and (10s-1011) lqcm at 77°K. The concentration of current carriers and their mobility in the specimens with resistivity of (102-103)f~cm were (10'3-7.10 ~2) cm -s and (1000-800)cm2/ v.sec respectively at 300°K. For more high resistivity specimens we failed to perform reliable measurements of Hall S constant. It should be noted that resistivity of crystals grows in proportion to an increase in the amount of Li and Cu dopants introduced in the solution tin melt. The photoconductivity spectra were taken by the spectrometer I/IKC-I2 in the range of the wavelengths 0.8-12/z, both for the directand alternation current, on a 20 Hz modulation frequency of light flux. The CI~-IO tubes or 'globar' served as a source of radiation. The signal from the sample was amplified by a narrow-band amplifier, tuned to the modulation frequency, and was delivered to the recorder. The installation sensitivity amounted t o - 10 -s V. When the. optical quenching was investigated, the crystal CdSnP2 was illuminated by two light fluxes simultaneously: the modulated light from the intrinsic absorption range and permanent brightening from the infrared spectrum. In some cases, in order to exclude the diffused light, a Si-filter was installed in front of the input window of the cryostat. 3. EXPERIMENTAL RESULTS

Figure 1 presents the photoconductivity spectra of undoped CdSnP2 (curve 1) and of CdSnP.., doped with Li (curve 2). Both spectra were taken at 10°K. It is apparent from this figure that the photosensitivity of the doped sample in the range, which is near to the intrinsic one, is greater than that of the

undoped sample by 6 orders. Besides, curve 2 is of a structure whose analysis enabled to establish the transitions at the quantum energies of 1.0 and 1.19 eV*. The increase of the photoconductivity signal in CdSnP2 (Li) in the range of 0-95-0.9/, is due to the bandto-band transitions which distinctly manifest themselves only at the resolution of 10 -s eV.

109

IJ9eV

I0 8

107

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b I ;=-,

io 5

<11 b

,o4 1.0 eV io 3

[0 I

L

i

t

t

t

I

,

I

,

,

i

I,,5

,~,

ht

Fig. 1. Photoconductivity spectra of n-type CdSnP2, undoped (curve 1) and doped with Li (curve 2).

We did not succeed in detecting the increase of the photoconductivity signal, which results from the band-to-band transitions, in the undoped crystal. Figure 2 presents the photoconductivity spectra of the CdSnP2 crystal, doped with Cu so that the distance, at which the Fermi level *The energy of the impurity level was calculated using the experimental data on the quantum energies corresponding to the half-height of the step on the spectral response curve of PhC, the estimated accuracy being +0.02 eV.

P H O T O C O N D U C T I V I T Y S P E C T R A O F CdSnP2

1567

,5 4

,°,I

~

. 17eV

~ 0 . 1 3 e ~

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,

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1,5

-

[ 2

I 2,5

I 3

"

I 3,5

"

1

5

6

I

I

7

I

8

9 X,

Fig. 2. Photoconductivity spectrum of the CdSnP2 (Cu) crystal (Ez-E~--0.35 eV) ; T = 80°K.

at 80°K was spaced from the bottom of the conduction band, did not exceed 0.35 eV*. As shown by this figure, the crystal is photosensitive up to 4/x. Curve PhC indicates the existence of transitions for the following quantum energies: 0.42 eV, 0.88, 1-07 and 1.19eV. In the range of wavelengths from 2-5 to l-5/z a structure was noticed, that we did not analyse. In the crystals, whose Fermi level was at a distance of 0.1 eV from the bottom of the conduction band, a signal of photoconductivity was observed up to 11/x. Figure 3 presents the PhC spectrum of this kind of crystal, taken at 80°K. As follows from the picture, transitions take place at the energies of 0.13 and 0-16 eV (Ec-- 0-13 and Ec-- 0.17 eV). In paper[8], where the crystals CdSnP2 (Cuma×) were described, negative photoconductivity was observed, which indicated the presence of a multi-charge centre located higher than the Fermi level. To determine the energy position of the multi-charge centre acceptor level, investigations of the photocurrent optical quenching were conducted. *These crystals are obtained from the solution-tin melt containing (0.07-0.1) wt. % of copper. Such specimens are subsequently referred to as CdSnP2 (Cum,D.

I

I0

I

II

/J.

Fig. 3. Photoconductivity spectrum of the CdSnP2 (Cu) crystal ( E r E c - - 0 " l eV); T = 80°K.

Figure

4

presents the quench spectra where lph and 1~ are the photocurrents produced by the light from the region of intrinsic absorption and from the infrared range, obtained, respectively, on the undoped sample (curve 4, T = 10°K) and on the sample doped with Li (curve 3, T = 10°K), Cu (curve 1, T-----80°K and curve 2, T = 300°K). As shown by Fig. 4, quenching in CdSnPz (Cu)* begins at a photon energy of

[(Iph--IIg)/Iph]%

,oo

5o o~ o

I

I

I

I

r,5

2

X.

tz

Fig. 4, Quench spectra: 1 and 2 - C d S n P 2 (Cu) 3 CdSnP.,(Li)4-CdSnP~. 1; 3; 4 - T = 10°K; 2 - T = 80°K. *These crystals are obtained from the solution-tin melt containing less than 0-07 wt. % of copper.

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0 . 9 e V and only the value of quenching depends on the temperature. In the undoped crystals and in crystals doped with Li, the high energy limit of quenching is located at the quantum energy o f - 1.16 eV. Table 1 summarizes the values of quantum energy at which transitions of carders in the forbidden band of n-type CdSnP2 are observed.

In this work, the acceptor level of the multicharge centre in the crystals under study was filled up. This is indicated by the presence of a wide photosensitivity band, extending up to IIp.. The quench spectra of these crystals confirm our assumption that the level (E~+ 0.92) eV belongs to the centre, which in the filled-up state has a charge of (-2). The

Table 1. Doping impurity Undoped Cu ..... Cu Li

Extrinsic transition energy (eV) 1.16" 1"19 (PhC) 1-19 (PhC) 1.16"

1.07 (PhC) 1.07 (PhC) 1.0 (PhC)

0"92 (PhC) 0.88 (PhC)

0.60 (SPh) --

0"40 (SPh) 0.42 (PhC)

-0.13 (PhC)

*Quenching of Photoconductivity. SPh - Stimulated extrinsic photoconductivity. P h C - Photoconductivity. 4. DISCUSSION OF EXPERIMENTAL RESULTS

As mentioned above, the undoped CdSnPz crystals displays the electronic type of conductivity, low specific resistance and insignificant photosensitivity. The electronic type of conductivity of the undoped crystals of CdSnP2, apparently, is due to the defects, whose concentration is so great that in the vicinity of the band edges appear the density tails of states, so that the values Eo prove to be underestimated (Fig. 1, curve 1). The Li or Cu doping involves the compensation of a considerable number of shallow donor centres, the increase of resistance and photosensitivity of samples. Formerly[10], while we investigated the extrinsic and induced extrinsic photoconductivity in CdSnP2 ( C u m a x ) , and revealed the levels (E~-- 1-07) eV and (E~+ 0-92) eV as well as the transitions at the quantum energies of 0.05 and 0.62 eV. In the same samples we observed negative photoconductivity in the spectral range of (0.5-0.90) eV. Accordingly, it was suggested that the levels (E~ + 0-47) eV and ( E v + 0 - 9 2 ) e V belong to the multi-charge centre, which is in a state once and twice negatively charged, respectively.

analysis of Table 1 data enables us to assume that this level is due either to a copper atom or to a complicated defect, whose component is a copper atom. The level (Ev+0"42) eV, as indicated by all these data, belongs to the same defect as the level (E~,+0-92)eV, but in another charging-state. Besides the levels (Ev+ 0-42) eV and (Ev+ 0.92) eV, transitions (Ec--1.07) eV are observed in the spectra of the crystals CdSnP2 photoconductivity. The investigation of cathodoluminiscence demonstrates the presence of a wide extrinsic band of 1.07 eV. Therefore it is possible that this level also is due to a complicated interaction of the copper atom and the lattice defect. In the crystals doped with Li or Cu, in the vicinity of band-to-band transitions, appears a signal resulting from the level (E~ + 1.19) eV. The optical quenching, observed in the undoped crystals CdSnP2 and crystals CdSnP2 (Li) at the energy o f - 1.16 eV attests that the transition at this energy takes place to the level of the multi-charge centre (acceptor centre), which is, apparently, an inherent compensating defect. The availability of the levels ( E c - - 0 - 1 3 ) e V and (Ec -- 0-17) eV is confirmed by the studies of photoconductivity

PHOTOCONDUCTIVITY

spectra of the samples with a low degree of compensation. The absence of quenching at the quantum energies of 1.16eV in the crystals CdSnP2 (Cu) is explained by the availability of a more intense band of quenching in the centre due to copper. Thus, there exist in the crystals CdSnP2 inherent multi-charge centres (probably, due to the lattice defects), whose concentration is not sufficient for the compensation of inherent defects of donor type. The copper doping results in the appearance of a new multi-charge centre, which increases the degree of compensation and the photosensitivity of the crystals CdSnP2. REFERENCES

1. G O R Y U N O V A N. A., Proceedings of the IX Conference on the Physics of the Semiconductors. p. 1267, Moscow (1969); Hayka Leningrad (1969). 2. G O R Y U N O V A N. A., P O P L A V N O I A. S., P O L Y G A L O V L. T. and C H A L D Y S H E V V. A., Phys. Status Solidi 39, 9 (1970).

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3. B E R K O V S K I I F. M., G O R Y U N O V A N. A., O R L O V V. M., R I V K I N S. M., S O K O L O V A V. I., T S V E T K O V A E. V. and S H P E N K O V G. P., • Til 2, 1218 (1968). 4. S H A Y I. L., L E H E N Y R. F., B U C H L E R E. and W E R N I K l.,Appl. Phys. Lett. 16, 357 (1970). 5. G O R Y U N O V A N. A.. G R I N B E R G A. A., R I V K I N S. M., F I S H M A N 1. M., S H P E N K O V G. M. and Y A R O S H E Z K I I I. D., d~Tll 2, 1525 (1968). 6. G O R Y U N O V A N. A., K O V A L S K A Y A V. A., L E O N O V E. I., O R L O V V. M., P U S H K I N S. L., R A D A N T S A N S. I., S O K O L O V A V. I. and F E R D M A N N. A., Phys. Status Solidi (a) I, K 161 (1970). 7. G O R Y U N O V A N. A., L E O N O V E. I., O R L O V V. M., S O N D A E V S K I I V. P. and A K I M C H E N K O I. O., Proceedings of the X International Conference of the Physics of the Semiconductors, U.S.A. p. 402 (1970). 8. A K I M C H E N K O I. P., G O R Y U N O V A N. A., L E O N O V E. I. and O R L O V V. M., Phys. Status Solidi (a) 3, K 51 (1970). 9. G O R Y U N O V A N. A., A N S H O N A. V., KARPOVICH I. A., L E O N O V E. I. and O R L O V V. M., Phys. Status Solidi (a) 2, K 117 (1970). 10. A K I M C H E N K O I. P., G O R Y U N O V A N. A., L E O N O V E. I. and O R L O V V. M., Phys. Status Solidi (a) 3, K 149 (1970).