Influence of slip direction on the photoplastic effect in cadmium sulfideT. J. Garosshen, C. S. Kim, and J. M. Galligan Citation: Applied Physics Letters 56, 335 (1990); doi: 10.1063/1.102800 View online: http://dx.doi.org/10.1063/1.102800 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/56/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Thermoelectric and photothermoelectric effects in semiconductors: Cadmium sulfide films J. Appl. Phys. 45, 648 (1974); 10.1063/1.1663298 Persistent Photodielectric Lens Effect in Cadmium Sulfide J. Appl. Phys. 43, 586 (1972); 10.1063/1.1661161 Impurity Photovoltaic Effect in Cadmium Sulfide J. Appl. Phys. 37, 1660 (1966); 10.1063/1.1708581 Photoemission in the Photovoltaic Effect in Cadmium Sulfide Crystals J. Appl. Phys. 31, 968 (1960); 10.1063/1.1735786 Effect of Oxygen on Luminescence of Cadmium Sulfide J. Chem. Phys. 23, 977 (1955); 10.1063/1.1742165

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Influence of sUp direction on the photo plastic effect In cadmium sulfide T. J. Garosshen, C. S. Kim, and J. M. Galligan Department of Metallurgy and Institute of Alaterials Science, Uniuersity (~f Connecticut, Storrs. Connecticut 06269

(Received 10 July 1989; accepted for publication 16 November 1989)

Studies of the photoplastic behavior of CdS single crystals show that when slip occurs on the basal plane, a large increase in flow stress is observed in the presence of incident light. However, when the crystal is oriented for slip on the prismatic planes, a very small photoplastic effect is observed. Measurements of the current associated with dislocation motion 011 the active slip systems indicate that the photo plastic effect is proportional to the charge on the dislocations in the respective slip planes.

Compound semiconductors of the U-VI family exhibit a significant change in flow stress when irradiated with light during plastic deformation. ',2 The change in stress is related to the generation of charge carriers (electrons and holes) which interact with mobile dislocations. This effect of light on flow stress, referred to as photopiasticity, is especially pronounced in CdS. j

The mechanism by which the charge carriers interact with a dislocation is not, however, well understood. One ex­planation suggests that the absorption of electrons at the dangling bonds in the dislocation core modifies the Peierls forces.4 Another mocte1 proposes that the photoinduced electrons and holes arc caught at trapping centers and there­by modify the charge state of the traps.

The altered traps then interact in a Coulombic manner with charged dislocations. s In compounds with strong ionic character, dislocations can carry a significant electrical charge when they are comprised :;,oldy of cations or anions. Such is the case with edge dislocation on the basal planes of CdS. 6 In contrast, edge dislocations moving on prismatic planes carry very little charge because of the alternation of cations and anions along the dislocation line. This results in a net compensation of charge. A small amount of charge may, however, exist on the prismatic dislocations due to the electronic configuration of the dangling bonds at the core,7 These considerations will be exploited below to demonstrate the influence of dislocation charge on the photoplastic effect.

The experiments were performed as follows: Single­crystal specimens of high resistivity ( > 106 n. em) CdS were sectioned from a bulk crystal manufactured by the Eagle­Picher Company. A wire saw was used for sectioning. The compression surfaces of the specimens were polished to a

1 /10101

Priamalic

FIG. 1. Schematic diagram showing geometry and orientations of CdS sin­gle crystal compression specimens.

0.03 Ilm finish, and etched with a HCI-Cr03-H20 solution to remove the work-related damage at the surface. Crystallo­graphic orientation was determined by the Laue back reflec­tion technique and the orientations of specimens prepared for basal, and prismatic slip are shown schematically in Fig. 1. Final specimen dimensions were typically 3 X 3 X 3 mm. 3

Specimens were deformed in a laboratory scale com­pression rig specifically designed to detect small changes in stress with a sensitive load cell. The loaded faces of the speci­mens were lubricated with vacuum grease; deformation was initially at a strain rate of 5 X 10- 5 S -1. A light source with a 100 W halogen bulb was used to irradiate the crystal, and a container of water was used as a filter between the light and

Specimens were deformed in a laboratory scale com­pression rig specificalIy designed to detect small changes in stress with 11 sensitive load cell. The loaded faces ofthe speci­mens were lubricated with vacuum grease; deformation was initially at a strain rate of5 X 10 5 s I. A light source with a 100 W halogen bulb was used to irradiate the crystal, and a container of water was used as a filter between the light and the specimen to avoid unwanted heating. The intensity of the light reaching the specimen was on the order of 0.02 W Icm 2

,

The extent of the photoplastic effect is shown in Fig. 2. As shown, when slip occurs on the basal plane (0001), the increase in flow stress is on the order of 100% when the light

120

140~---! Prismatc

_100 t t ~ ~ 00 /oe off

~ ::!t{~-e;" 20:- t t

on 0'1 o _-.J...

o 0.02 0.04 0.06 o.ca 0.1 0.12

Strain

FIG. 2. Stres;,-strain curves of CdS single crystals deformed at room tem­perature showing effect of light on flow stress for basal and prismatic- slip.

335 Appl. Phys. lett. 56 (4), 22 January 1990 0003-6951 i90/040335-02$02.00 @ 1990 American Institute of Physics 335

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Basal Slip

on off

l

o 0.02 0.04 0.06 c.oa 0.1 0.12

Strain

FIG. 3. Stress-strain curve and associated deformation current of CdS sin­gle crystal during basal slip.

is turned on; the experimental conditions are given on the figures. Note that the change in stress during illumination does not occur while the material is deforming elastically, demonstrating that thc photo plastic effect is related to a change in the stress needed to move dislocations.

In comparison, as also shown in Fig. 2, the change in flow stress upon illumination for prismatic slip is only 1 % of that observed for basal slip (slip on both the basal and pris­matic planes occurs in the (1120) direction). Evidence of the charge associated with dislocation motion during basal slip is shown in Fig. 3. As shown, a current is produced across the specimen during basal slip, such as observed by Osipyan et af.1 (Currents were measured with a Keithley 616 digital electrometer which has a sensitivity of 10- 16 A. Ohmic specimen contacts were made with indium solder.) Figure 3 also shows that when the light is turned on, there is a drop in current due to the pinning of dislocations by the injected charge carriers. Conversely, there is a large increase in current when the light is turned off due to the unpinning of dislocations. Dislocations are then free to propagate very rapidly until the stress level returns to steady state levels. In comparison, a significantly smaller current was detected during prismatic slip as shown in Fig. 4. Prior work has not obtained quantitative measurements of current when pris­matic slip occurred. Note, further, that there is no current during elastic deformation for either slip system. It is there­fore concluded that the observed current is associated with dislocation motion.

It is interesting to note that the magnitude of current produced during slip on the two slip systems appears to be proportional to the amplitude of the photoplastic behavior on the respective slip system. During basal slip, both the magnitude of the photo plastic effect and the dislocation cur-

336 Appl. Phys. Lett., Vol. 56, No.4, 22 January 1990

18o! I' 150r Stress 1

I 0.8 ~

~120r E

~ "r r ! ~ ::rv r: ~

Curren: I (l L-L.c==~~~~----,--~ 0

o 0.02 0.04 0.06 C.OS 0.1 0.12

Strain

FIG. 4. Stress-strain curve and associated deformation current of CdS sin­gle crystal during prismatic slip. Note the ohserved current is about 1 % of that observed during basal slip.

rent are approximately 100 times greater then that observed during prismatic slip. Such a relationship supports the idea that it is the charge on the dislocation which is of primary importance to photoplasticity.

Although it might be expected that no net current should be associated with dislocation motion, other studies have indicated that the positively charged dislocations ex­hibit a higher mobility, resulting in a net current. I

In summary, the experiments described above arc con­sistent with the idea that the charge on a mobile dislocation is the most important factor governing the photoplastic ef­fect. Basal slip exhibits a much greater degree ofphotoplasti­city and deformation current due to the greater charge on the dislocations. This is also consistent with calculations of charge on various dislocations in CdS. 6 Experiments which demonstrate the dynamic change in deformation current as­sociated with the removal oflight and the related dislocation velocity and density measurements will be reported else­where.

'Y. A. Osipyan, V. F.l'etrcnko. A. V. Zaretski. ami R. W. Whitworth, Adv. Phys.35, liS (1986). 's, Takeuchi. K. Maeda, and K. Nakagawa, Defects in 5;emiconductors n, Materials Research Society Symposia Proceedings Vol. 14, edited by S. Mahajan and J. W. Corbett (North-Holland, New York, 1983), p. 461.

'V. Maver and J. M. Gal!igan, AppL Phys. Lett. 40,1020 (1982). 'Y. A. bsipyan and V. F. Petrenko, Sov, Phys. JETI' 48, 147 (1978). 'L Carlson and C. Svensson. J. Appl. Phys. 411652 (1970). "~Yo A. Osipyan and L S. Smirnova, Phy;;. Status Solidi 30, 19 (1968 l. 'R. Labusch and W. Schroter, in Disiocations in .S'oiids, edited by R. R. N. Nabarro (North-Holland, Amsterdam. The Netherlands, 19R6), pp. 127-192.

Garossllen, Kim, and Galligan 336

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