znse dislocations

8/2/2019 Znse Dislocations

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Study of dislocations in ZnSe and ZnS by scanning force microscopyo. Nickolayev and V. F. PetrenkoThayer School of Engineering. Dartmouth Col/ege. Hanover. New Hampshire 03755(Received 29 December 1993; accepted 24 May 1994)Images of grown-in and deformation-introduced dislocations penetrating cleaved {IIO} surfaces ofZnSe and ZnS were obtained by scanning force microscopy. No electrical effects associated withdislocations were found.

I. INTRODUCTIONDislocations playa very significant role in the electrical

properties of semiconductors. Electronic states in the bandgap introduced by dislocations result in a high density ofelectric charge at the dislocation cores which in turn resultsin lower carrier mobility and lifetime, hence impairing thedevice performance. A decade ago, significant progress in thegrowth of defect-free crystals generated hope that the problem of defects was solved completely. Unfortunately, itturned out to be otherwise. First, dislocations are produced inthe fabrication of integrated circuits due to the enormouselastic stresses that build up (for example, during formationof shallow isolation trenches I). Second. in such semiconductors as II - VI compounds dislocations are so mobile at roomtemperature that they can easily multiply during the operation of optoelectronic devices. The multiplication is drivenby temperature gradients or electric fields. 2 These circumstances bring about a new interest in the study of the effect ofdislocations on the physical properties of semiconductors.

A very attractive idea is to use scanning tunneling microscopy (STM) or scanning force microscopy (SFM) for theinvestigation of the electronic structure of dislocations. sincethese methods combine atomic resolution with high sensitivity to electric fields. Dislocations intersecting the surfacehave been studied by STM.'-9 In this article we report theobservation of dislocations penetrating cleaved {IIO} surfaces of ZnSe and ZnS by SFM. Unlike STM, SFM can beused for the imaging of nonconducting samples and usuallydoes not require ultrahigh vacuum (UHV) conditions.

II. EXPERIMENTAL PROCEDUREThe experiments were performed using a NanoScope III

(Digital Instruments Inc.) operating in the contact and noncontact modes under ambient conditions. During electricalmeasurements the sample was isolated from the ambientlight to prevent charge carrier generation (due to photoconductivity), which may lead to the screening of the dislocationcharge. as d i s c u ~ s e d in Sec. IV. The maximum scanning fieldwas 120X 120 /1-m2. The radius of the tip which limits thelateral resolution was 10 nm. Scan velocities were typically /0 f.1,m/s. All images were digitally processed to remove high-frequency noise. In addition, to eliminate largeheight variations (arising due to tilt of the sample surface) abest-fit line was subtracted from each of the scan lines.

Samples were cut from a monocrystal in such a way thatone of the surfaces was the plane of easy cleavage {II O} andone of the surfaces was close to {OO I}; see Fig. I. Mono-

crystals were grown from the melt under pressure. The conductivity was in the range 1O-6_1O-lofrlm-1 for ZnSe and10-8_10- 12 0- 1 m- 1 for ZnS. The density of grown-in dislocations, revealed by chemical etching of fresh cleavages,was between 106 and 108 m- 2. Microindentation was used tointroduce new dislocations and to determine the exact orientation of the specimen from the geometry of slip bands.

ZnSe and ZnS have the sphalerite structure with perfectBurgers vectors of the type a/2< 110>.10 The primary slipsystem is well established to be of the type < 11O>{ III}; seeFig. 1. ZnS contains 10% of wurtzite which results in onlyone of the primary slip systems being active.2 Directions[110]. [112], and [112] on the surface of observation (011)correspond to the < 111 >{ Ill} primary slip system; directions [221] and [221] correspond to the < II O>{ 112} system(this will be used for data interpretation in Sec. III).

According to transmission electron microscopy (TEM)studies, perfect dislocations in the primary slip system aredissociated into Shockley partials with Burgers vectors of thetype aI6.2.11.12 The average distance between thepartials (stacking fault) in ZnSe is about 15 nm; 11 partials inZnS can move an arbitrarily large distance apart. 12 Since thelateral resolution was 10 nm. individual partials in ZnSecould not be resolved.

Samples were cleaved using razor edges in air. Some regions of the resulting surface were covered with a dirt layer(most probably of organic origin) with a typical thickness of20 nm and some regions were atomically clean. A boundarybetween atomically clean regions (on the left) and contaminated regions (on the right) is shown in Fig. 2(a). We foundthat this dirt layer can be removed from the surface directlyby the cantilever tip in the contact mode. Figure 2(b) showsa square (500x500 nm2) cleaned by the tip. The organicmaterial is collected on the perimeter of the square. No damage to the tip could be seen after such an operation. Most ofthe tests, however, were performed in the noncontact modeusing surface areas that were originally clean to avoid potential problems associated with contamination of the tip.

The amplitude of tip vibrations (noncontact mode) is afunction of the derivative of the force acting on the tip.Changing the force derivative changes the effective springconstant and hence the amplitude of the lever (tip) vibration.Methods of noncontact imaging based on the monitoring ofvery weak forces (electrostatic. magnetic) acting between thesample and the tip have found wide use (a detailed treatmentis given in Ref. 13). These techniques are collectively termedelectric force microscopy (EFM). In our case the force is dueto the interaction between the tip and the charged dislocation.

2443 J. Vac. Sci. Techno\. B 12(4), Jul/Aug 1994 0734-211 X/94/12(4)12443/8/$1.00 @1994 American Vacuum Society 2443


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2444 O. Nickolayev and V. F. Petrenko: Study of dislocations in ZnSe and ZnS by SFM 2444

[1

FI(i. I. Sample orientation.

surface ofobservation

[110]

EFM allows m e a ~ u r e m e n t of extremely small force derivatives. The minimum force derivative typically detected is ofthe order of 10-4 N/m. 14. 15 The minimum surface charge thathas been detected is 3e. with lateral resolution 0.2 tlm.15 Theminimum voltage induced in the tip that has been detected isI mV, with a lateral resolution of 0.05 tlm.1 > The sensitivityto electric fields achieved in this study is discussed in detailin Sec. IV.

III. EXPERIMENTAL RESULTSAll images presented in this section were acquired in the

noncontact mode. Contact atomic-scale images had significantly poorer quality. Figures 3(a) and 4(a) show typical(80X80 tlm") images of diamond-shape indentations and slipbands emanating from them for ZnSe and ZnS, respectively.These images w e n ~ acquired in the deflection mode when theerror signal (variation in the amplitude of cantilever oscillations) is captured instead of the voltage applied to the piezo.This mode can reveal the orientation of small features (which

3 pm a

TABl.E I. Dislocations in ZnSe.Figure 3 Image size

ic l 410 nmId) R40 nmIe) 560 nm(f ) 410 nm

Direction of the step[l12J[221J[221] and [112][221J

Height of the step2.1 :'::0.2 A2.2:'::0.2 A4.2:'::0.2 A3.8:'::0.2 A

are not seen in the height mode) over scan areas with largeheight variations. In the case of ZnSe at least three primaryslip systems are active; in the case of ZnS only one slipsystem is active.

A (3X3 tlm2) image of a slip band parallel to the [112]direction is shown in Fig. 3(b). The height of the step associated with this slip band decreases from 10 nm. in the bottom part of the picture, to zero. Unfortunately, no clear images of individual dislocations composing this slip bandwere obtained. Note the presence of steps running in the[112] direction emanating from the slip band. These stepscould be produced by dislocations which cross slipped fromthe initial slip band. The height of these steps is in the range2-5 A and was difficult to determine precisely because ofthe presence of high topological features.

Figures 3(c)-3(f) display images of grown-in dislocationsin ZnSe. The heights and directions of surface steps originating from dislocations are summarized in Table 1. The valuesof the heights were determined by taking the average of 20sections across the step. Note that the step in Fig. 3(e)changes its direction from [221] in the bottom part of thepicture to the direction close to n 2]. Note also that steps inFigs. 3(d). 3(e). and 3(f) do not lie in the primary slip system{lll}< 112>.

The height of a surface step produced by a dislocationintersecting the surface is equal to the component of the Burgers vector normal to the surface (see Sec. V). In sphaleritethe Burgers vectors of perfect dislocations are either perpendicular (in case of (1/2[110)) or inclined at 30 0 to the surface(in the case of a/2 [101] and a/2[011]); in the latter case, the

1.5 pm bFIG. 2. (a) A boundary between atomically clean and contaminated regions. (b) Atomically clean square cleaned by the cantilever tip in the contact mode.

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80J..lma

410 nmc

560 nme

3000 nmb

840 nmd

410 nmfFIG. 3. Surface steps produced by dislocations emerging on 111O} surface of ZnSe.

component normal to the surface is half the perfect Burgersvector. Thus, theoretically we can expect values of the stepsto be 4.0 and 2.0 A, respectively (for ZnSe, a=5.67 A) inreasonable agreement with the observed vaiues, see Table I.

Figure 4(b) shows a(IOX 10 ,um 2) image revealing thestructure of grown-in dislocations in ZnS (deflection mode).The direction of the steps is close to the primary slip system.The places where surface steps terminate correspond to dislocations emerging at the surface. The dislocation density issignificantly higher than that determined by chemical etchingtechniques.JVST B - Microelectronics and Nanometer Structures

Figure 4(c) shows an image of a dislocation in ZnS (introduced by indentation) on which a slip band emanatingfrom the indentation pit terminates. Figure 4(d) shows agrown-in dislocation in ZnS. There is no apparent differencebetween the images of grown-in and plastically introduceddislocations. The parameters of the steps associated withthese dislocations are summarized in Table II.

Again, the heights of the steps are close to the expectedvalues of 1.95 A (the lattice parameter. a = 5.5 A). Bothsteps lie in the primary slip system. These dislocations couldbe either perfect 60 dislocations or isolated 30 partials with


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2446 O. Nickolayev and V. F. Petrenko: Study of dislocations in ZnSe and ZnS by SFM

80 11ma

'900 nmc

e

'Ol1mb

540 nmd

PH;. 4. Surface steps produc ed hy dislocations emerging on {IIO} surface of ZnS.

J. Vac. Sci. Technol. B, Vol. 12, No.4, JuUAug 1994

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x

y O.S

FiG. 5. Spatial distrihution of the electric potential produced hy a chargeddislocation.

a Burgers vector of the type a/6[1l2]. Since perfect dislocations were never observed in ZnS, we believe that these areisolated partials.2 We did not observe dislocations producing3.9 A steps (perfect screw dislocations), conforming to theresults of TEM studies.2 Figure 4(e) shows a 3D image of thedislocation from Fig. 4(d). Note the surface roughness whichis most probably due to thermal noise (discussed in Sec. IV).

Measurements of the dislocation charge were performedin the lift mode. In this mode. first a normal scan is performed with the feedback on. Then the feedback is switchedoff. A second scan follows the topography from the preceding scan with a constant tip-sample separation II (typicallyh= 10 nm). The variation in the amplitude of tip oscillation(which in our case is due to electric forces) is captured. Thetip is oscillated at the steepest slope of its resonant curve. Toincrease sensitivity a dc bias was applied to the tip. Themaximum voltage that could be applied without deteriorationof the image quality was about 3 Y.

Both regular and metal coated cantilevers were tried. Inaddition, to further increase sensitivity, an alternating voltageV(!l) was appJied to the tip. The force derivative Fl (andconsequently the amplitude of tip vibrations) in this case ismodulated with the same frequency n. This signal is amplified using a lock-in amplifier.

No electrical effects associated with dislocations in eithermethod were observed. To make sure that our system is sensitive to real surface charges we tried to deposit an electriccharge on the surface by applying short-voltage (40 V)pulses to the tip. As a result, a significant amount of dirt wasaccumulated in the spot where the voltage pulse was applied,but again no evidence of electric charges was found. Thisnegative result is discussed in the following section.

IV. INTERACTION BETWEEN THE TIP AND THECHARGED DISLOCATION

A charged dislocation emerging at the surface produces anelectric field which interacts with the cantilever tip. ConsiderJVST B Microelectronics and Nanometer Structures

TARLE II. Dislocations in ZnS.Figure J Image sife Direction of the step Height of the step

k l 1900 nm r1121 n:'::O.2 AIll) 500 nm [112J 2.2:'::0.2 A

an electric charge Q located in the dielectric near the surface.It can be shown that the potential produced by this charge invacuum is given by the formulaI 2 Qc p ( r l = - - - - ~ 4m:() 1+ JrJ . (I)where 11 and e are the dielectric permittivity of vacuum andsemiconductor. respectively. and r is a vector between thecharge and the point of observation. The potential cp(x.r)produced by a charged dislocation can be calculated by integration of Eq. (I ) over the dislocation line and the screeningcylinder. The resulting potential (in the case of a dislocationline normal to the surface) is plotted in cylindrical coordinates (x.y) in Fig. 5. The integration can be performed analytically for the cases x = 0 (perpendicular to the surface) andy=O (along the surface):

q 2 { \ / ~ + x c p ( x . O ) = - - - - In ")47Teo I + e _x+ ~ . fl ~ ( . ~ . ) 2 + I _ ~ . ] -O.5}.A, ,A , A, (2)

CP(0.YI=-Q__ _rl2 I n ( ~ ) - I + ( ~ . )2].47Tt:o I + ' Y , . A, (3 )where q is the linear charge density at the dislocation coreand A is the radius of the screening cylinder which is relatedto q by the following formula:

(4)where e is the electron charge and Nt! is the concentration ofshallow defects." Equations (2) and (3) are valid down todistances of the order of a few atomic spacings. These functions are plotted in Fig. 5 in thick lines. In fact. the linearcharge density q is not uniform along the dislocation line andincreases close to the surface. But this variation is not large(about 20%) and can be neglected. t7 In the case of SFM, thedistance between the tip and the surface is typically muchless than the screening radius ( X ~ A , ) . In this case Eq. (2)reduces to

q 2cp(x,0)=-4 -1- ln(A/2x).7T(J + (5 )The potential at the tip (with respect to Fermi level) is equalto the sum of cp(x.y) and the external bias V. The tip isusually modeled as a conducting sphere (with a radius R). Infact, the actual geometry corresponds to the case of a sphereover a plane, but since on average the distance from thesphere to the plane is much greater then R. the change of thecapacitance due to the presence of the plane can be ne-


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2448 O. Nlckolayev and V. F. Petrenko: Study of dislocations in ZnSe and ZnS by SFM 2448glected. The electrostatic energy of a spherical capacitor isgiven by a well-known formula from electrostatics:

V(x) = 2 7TeoR[ c,o(x) + vf, (6)where x corresponds to the position of the center of thesphere (tip). The force on the sphere (tip) is equal to thederivative of its energy. Substituting c,o(x,O) from Eq. (5) weobtain

dV 2 IF ( x ) = ~ d =RVq - l - -.x +e xUsually the tip is vibrated on the steepest slope of the

resonant curve. In this case the change in amplitude of the tipvibration 8A resulting from the change in the force derivativeF I is

(8)where Ao is the amplitude of bimorph vibration, Q is thequality factor, and k is the spring constant. 13 The amplitudeof the tip vibrations in this case is

(9)The average value of the force derivative F I along the

path of tip oscillations can be found asF(xmin) - F(xmax)F I = - - - - ~ - - - - - - Xmax - Xmin (10)

where Xmin = h +R and x max = h +R+ 2A are the maximumand the minimum distances between the center of the sphere(tip) and the surface, h is the tip-sample separation in the liftmode.Substituting the following typical values (A 0 =0.18 nm,A=30 nm, Q=300, k=30 N/m, R= 10 nm, V=3 V) andfor ZnSe q=0.6X 10- 10 C/m,2 we obtain F I=2X 10- 4N/m and 8A = 0.5 A. The lateral size of the region of electricpotential is of the order of As as follows from Eq. (3) whichis much larger than the lateral resolution. A 8A of this magnitude should have been detected. The thermal noise thatlimits the sensitivity is

(II)where k8 is the Boltzman constant and T is the absolutetemperature. In our case 8A T=0.1 A.The absence of any observed electrical effects is mostprobably due to the laser-induced impurity photoconductivityof the samples. Though the helium-neon laser used had awavelength A=638 nm, which exceeds the wavelength offundamental absorption (466 nm for ZnSe and 344 nm forZnS), the laser light does generate a significant impurityphotoconductivity.ls The diameter of the laser beam shiningon the cantilever is larger than the lateral cantilever size (40Mm) and an area outside the perimeter of the cantilever isilluminated. Nonequilibrium charge carriers generated by thelaser can diffuse into the area shaded by the cantilever andscreen any electric charges. The diffusion length isJ. Vac. Sci. Technol. S, Vol. 12, No.4, Jul/Aug 1994

(12)where D is diffusion coefficient and T is the lifetime of car-riers. Substituting numerical values (for ZnSeD "5 ' -I 10 - s ) ' b "=L. cm" S,7= . s," we 0 tam tor Ad=20 Mmwhich is comparable to the size of the shaded area under thecantilever. The magnitude of the screening potential can beestimated as

( 13)where 0"1 and 0"0 are the conductivity under illumination andin the dark, respectively. The conductivity under the illumination by the laser was measured to be approximately0"1 = 10-4 n - I m-I . Assuming the typical value of the conductivity in the dark to be 0"0= IO-R n- I m-I , we obtain forthe magnitude of the screening potential cP, = 0.24 Y. Thispotential is less than the potential associated with the dislocation: E F -E d=O.5 V (for ZnSe\ where Er and Ed arcthe positions of the Fermi and the dislocation levels, respectively. This implies that the screening can account for only apartial reduction of the apparent dislocation charge. What ismore essential is that a dislocation is an extremely efficienttrap and recombination center for the minority charge carriers (in our case, ho\es).2 Recombination of the minoritylight-induced charge caITiers at the dislocation core may result in a dramatic reduction of the dislocation charge." Inaddition, a scattered light with ~ = 6 3 8 nm causes opticaltransitions of electrons from a dislocation to the conductionband, decreasing the dislocation charge. IS In summary, laserinduced photoconductivity may account for the failure to observe any electrical effects associated with the dislocationcharge.

Another possible reason for the failure to observe electrical phenomena is the presence of various impurities and ionsthat can get stuck to the spot where a dislocation emerges atthe surface. However, such large objects should have beenclearly seen in our images and were not.

To confirm the validity of our model. we have measuredexperimentally and calculated theoretically the change in theamplitude of tip vibrations when a dc voltage was applied tothe tip. In this case the imaginary charge induced in thesample plays the role of the surface charge. Experimentaland theoretical data were in agreement.

v. SURFACE MORPHOLOGY ASSOCIATED WITHDISLOCATIONS

In this section the expected surface morphology in thevicinity of a dislocation emerging at the surface is discussed.The following simple estimation shows that in ZnSe and ZnSdislocations come out perpendicular to the surface, i.e., arather long segment of the dislocation line adjacent to thesurface is always oriented normal to the surface. This occursbecause a dislocation is attracted to the surface and tends toassume an equilibrium orientation with the lowest elastic energy: i.e., with the dislocation line normal to the surface. Inthe region where these forces exceed the yield stress, the


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2449 O. Nickolayev and V. F. Petrenko: Study of dIslocatIons in ZnSe and ZnS by SFM 2449

a

bFIG. 6. Surface morphology associa ted with a screw (a) and an edge (b)dislocations.

dislocation line is oriented normally to the surface. The forceper unit length with which dislocation is attracted to a freesurface is roughly

Eb 2F=- (14)x 'where x is the distance to the surface, E is Young's modulus,and b is the magnitude of Burgers vector. The stress can beestimated as F divided by b. Equating this stress to the yieldstress CTy, we obtain for the characteristic distance Xo overwhich the dislocation line is normal to the surface:

Exo=b- .CTy (15)Substituting numbers E= 100 OPa and CTy=lO MPa,2 weobtain x0 =5 ,um. This distance is very large, and for thepurpose of calculating the electric potential (Sec. IV) andsurface displacement, the dislocation line can be consideredto be normal to the surface. This simple calculation is supported by observations of lOO-,um-long straight dislocationsnormal to the surface in CdS. 19

As was shown by Honda2o for an isotropic case, the surface displacement Uscrew and Uedge produced by screw andJVST B - Microelectronics and Nanometer Structures

edge dislocations, respectively, in the case of a dislocationline normal to the surface are given by the following formulas:

vbu e d g e = - sin cp,1T

(16)

(I7)where cp is the polar angle and v is the Poisson's ratio. Thesurfaces corresponding to Eqs. (16) and (17) are plotted inFigs. 6(a) and 6(b) [the scale in Fig. 6(b) is by a factor of 5greater than in Fig. 6(a)]. The magnitude of surface displacement associated with an edge dislocation is smaller than thatof a screw dislocation roughly by a factor of 5 (2Iv). Thismakes it possible but quite difficult to resolve a surface distortion associated with the edge components of dislocations.

VI. CONCLUSIONSSFM has been demonstrated to have an ability to identify

the type of dislocations in insulators and poor conductors.Also SFM has an advantage over STM in the sense that itcan operate in air without UHV conditions. A sufficientlyhigh sensitivity of SFM to dislocation electric fields could beachieved if the problem of the screening of the dislocationcharge by laser-induced charge carriers can be solved.

ACKNOWLEDGMENTSThe authors are very grateful to Digital Instruments, Inc.

who provided the NanoScope III used in this project and toDr. R. Whitworth for useful comments.

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2450 O. Nickolayev and V. F. Petrenko: Study of dislocations in ZnSe and ZnS by SFM 245011'1'. Martin. C. C 'V,illiaill'. alld H. K. \\ilckrclIlla,inghc. J. Apr!. Phy'. 61.. n 2 3 1 1 9 ~ 7 1 .

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znse dislocations

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