Dislocations in HgCdTe-CdTe and HgCdTe-CdZnTe heterojunctions

Dislocations in HgCdTe-CdTe and HgCdTe-CdZnTe heterojunctions

446 Journal of C r~sEal Gross th ~6 (I 9~) 446 4S I North Holland. \msterdani DISLOCATIONS IN HgCdTe-CdTe AND HgCdTe-CdZnTe HETEROJUNCTIONS H. TAKIG...

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446

Journal of C r~sEal Gross th ~6 (I 9~) 446 4S I North Holland. \msterdani

DISLOCATIONS IN HgCdTe-CdTe AND HgCdTe-CdZnTe HETEROJUNCTIONS H. TAKIGAWA. M. YOSHIKAWA and 1. MAEKAWA infrared Dei ir ci I.aboratori. Fujitsu Laboratories Lid. 10 1 Marina saw ~4al. omit a, .4% s ugi 24 01, Japan

Mechanisms that gosern the generation and movement of dislocations in (Ill) heter~unctzons of HgCdTe Cd Fe and UgCdTe CdZnTe grown h’s liquid phase epitaxy have been insestigated ssith etch pit studies The density and distribution of the misfit dislocations in these heterojunctions have been measured quantitatisel’ 5’ and anakzed to clarif’s the mechanisms that generated them. I he origins of the dislocations threading through the epilaser are also reported

1. Introduction

2. Experimental

HgCdTe is a direct band-gap semiconductor that has many advantages for use in infrared detectors. A high quality HgCdTe crystal is re quired for the next generation of two-dimensional infrared detector arrays. To produce HgCdTe crystals with good crystallinity, liquid phase epitaxy is thought to have the greatest promise among the growth techniques recently developed. CdTe and Cd1 Zn 5Te are conventionally used for the substrate. The lattice constants of Hg1 5Cd~Te and CdTe differ by 0.2~ for ~ 0.3. A Cd1 ZnTe substrate holds promise for lattice matching with the Hg1 5Cd 5Te epilayer. The lattice mismatch between HgCdTe epilayers and these substrates may introduce strain-relieving misfit dislocations. Furthermore, there are many dislocations in these substrates. The dislocation density of most substrates is more than 5 >< iO~ cm 2 These dislocations in the substrates may cause the dislocations threading through the epilayers. Dislocations in HgCdTe crystals have been reported to degrade the performance of detectors made from the material [1.2]. The purpose of the present paper is to Investtgate the mechanisms which govern the generation and movement of dislocations in HgCdTe CdTe and l-lgCdTe CdZnTe hetero~uncttons, and to clarify the origins of the dislocations. The growth conditions that eliminate dislocations in the epilayers are presented in the conclusion.

We grew HgCdTe epilayers in Te-rich growth solutions on substrates using the tipping liquid phase epitaxy technique. The substrates were (111)B CdTe and (l1l)B Cd1 ~Zn~Te wafers cut from ingots grown by the Bridgman method. The size of the substrate was 22 x 15 mm. The substrates were etched with a 5% by volume solution of bromine in methanol to remove the polishing damage. Most epilayers were grown at about 480°C with a cooling rate of 0.05°C mm for about 200 mm. Typically the thickness of the epilayer was about 30 /.tm. The exact growth conditions were reported previously [3]. The alloy composition of the epilayer was measured by infrared transmission spectroscopy and by determining the photoconductive cut-off wavelength. The Zn content of the Cd1 Zn Te substrate was measured by atomic absorption spec troscopy. The compositional profile across the heterojunction was examined on the cleaved (110) plane of the epilayer by X-ray microprobe analysis (XMA). Dislocations in the epilayer were investigated quantitatively by an etch pit study on a cleaved (~10) plane perpendicular to the epilayer sub strate interface, and on a (111 )A or (111 )B plane parallel to the interface. The dislocations of CdTe and CdZnTe substrate were estimated on the (ll1)A plane using the Nakagawa etch [4].

0022-0248/88 $03.50 Elsevier Science Publishers (North-Holland Physics Publishing Division)

By.

H. Takzgasva et a!.

447

Dislocations In HgCdTe CdTe and HgCdTe CdZnTe heterojunctions

3. Results and Discussions

10

i I

8

Epilayer

3.]. Misfit di.slrssution.s

Fig. 1 shows the cleaved, etched cross-sectional (110) plane of an Hg07Cd03Te epilayer grown on a CdTe substrate. Many dislocation etch pits spreading into the epilayer can be clearly seen. Etch pit density varies along the growth direction of the epilayer, and is highest around the interface, decreasing gradually in the direction of the growth. The compositional profile of the heterojiinction is shown in fig. 2a. The gradual compositional change around the heterojunction is due to interdiffusion of Hg and Cd during epilayer growth. The dislocation etch pits were found to appear entirely within the compositional transient region on the (110) plane. It has been shown that the misfit dislocation lines for (111) heterojunctions lie in the <011), <101) and <110) directions [5], and are of the 60° type with the Burgers vector lying in the misfit plane [6]. If the lattice mismatch at abrupt Hg1 X(1)CdX(1)Te Hg~ r(i)Cdx(2)Te heterojunction is entirely accommodated by these misfit dislocations, the mean spacing between dislocation lines is given by

_____________________

~

~

Hg

07Cd0~Te epilayer

4

4 _____

~

2

“N~

~

a

• ~••

‘-~-....,.,,

—2

‘N I

0

.

.

I

.

I

02 0.4 0.6 08 x (a)

1

c

. ~.



Substrate L —

(b)

.

Fig. 2. Schematic diagram of the distribution of dislocation etch pits on a (110) plane of a HgCdTe CdTe heterostructure: (a) compositional profile of the heterostructure; (b) distribution of the dislocation etch pits.

h



3/~a~(l)aX(2)

2I~(a2ti1



a2~2>)

>

a50~ a5~2~,

where a5(1) and aX(2) are the lattice constants for x(1) and x(2) respectively. This spacing is v~/2 of the value calculated on the assumption that the dominant misfit dislocations are pure edge dislocations [7]. The lattice constants of CdTe and HgTe are 6.481 x 10 8 cm and 6.461 X 10 ~cm at room temperature [8]. The lattice constant of Hg~ 5Cd5Te varies approximately linearly with the x value across the entire composition range. The linear thermal expansion coefficients CdTe 6/°Cand 40 x 10for 6/°C and Hgle are 49 >< 10 [8] Therefore the mean spacing h at the growth temperature is given approximately by 5/ax (2) h(cm) 1 7 X 10 The =relationship between the distnbution of dislocation etch pits on the (110) plane of a heterojunction and the compositional profile of the heterostructure is shown schematically in fig. 2. Dislocation etch pits on the cleaved (110) plane were produced only by dislocation lines aligned in the <110) direction. Dislocation lines aligned in the <011> and <101> directions may form only —

~ CdTe substrate 10 ijm Fig. 1. Photoniicrograph of a cleaved, etched cross-sectional (110) plane of a Hg 07Cd03Te epilayer grown on a CdTe substrate at a growth temperature of 480°C and with growth time of 200 mm.



invisible shallow etch pits. Therefore, if the lattice mismatch originating from the compositional dif-

445

ii

Takigawa CI a!.

Drslocazionv in iigC die

C’dle and Hg( die

(‘dLn Fe hetero;unctio,is

ference of ix is entirely accommodated by dislo-

cations, the number of dislocation etch pits i~ in a narrow area with a width of ~t and length L is given by N

4i~L.

L’h

5.9x10

2)

N/(~tL)

/

E

5.9XlO4dx/dt.

-~

a

(4)

different conditions, experimental data for other epilayers [9] is added in fig. 3. This fact indicates that many dislocations generated in the original abrupt heterojunction at the beginning of growth move in the (Ill) direction with the interdiffu-

sion during epilayer growth in order to accommodate lattice mismatch entirely,

~

L

A A

~

~7

/

°

~Calculated ___

iO~~ ~ II 10 0 Compositional gradient (cm

The relationship between the experimental data

and the curve calculated from the model mentioned above is shown in fig. 3. To confirm that the previous model fits epilayers grown under

7’

480°C, 200 mm A510°C 400 mmnAYO A / 0

(3)

Thus, the dislocation etch pit density n on the (110) plane is given by ,i(cm

I

.480°C 90 mtn

1)

Fig. 3. 1-speriniental and calculated dislocation etch pit densit’s on (110) planes as a function of the compositional gradient in this area for three epilayers grown under different conditions

dislocation lines lying in the (111) plane parallel to the interface. They were observed only on the cleaved (110) plane, and were not seen for the dislocation-etched (111) surface of the epilayer.

To investigate the movement of misfit dislocations in another direction, a thin epilayer was grown for 3 mm by the supercooling technique. Figs. 4a and 4b are the surface and the cleaved, etched cross-sectional (110) plane of the thin HgCdTe epilayer on a CdTe substrate. As shown in fig. 4b, the dislocation etch pits are distributed unevenly and the epilayer where many dislocation etch pits exist is thinner than the other portions of

The dents on the surface of the thin epilayer were

the epilayer. These etch pits result from misfit

along the direction parallel to the interface. The

~

~

~

~ -

thought to be generated by the following mechanism. Islands of HgCdTe grow at first on the substrate, and the lattice mismatch accumulated

on the boundaries of the islands prevents the combination of the islands with each other and decreases the epitaxial growth velocity at the

boundaries. From the results shown in fig. 4h, the misfit dislocations were confirmed to move also

____________

~—Hg

07Cd03Te epilayer

CdTe substrate 100 ~im

10pm

(~) Fig. 4 Photomicrographs of a thin HgCdTe epilayer grown with growth time of 3 mm on a (‘die substrate. (a) a (II l)A surface morphology of the epilayer: (b) a cleaved, etched cross-sectional (110) plane of the epilayer

/

H. Takigawa eta!.

449

Dislocations In HgCdTe CdTe and HgCdTe CdZnTe helerojunctions

4

2x10

I

I

Li

I

.

x~0.2

~

x

~ Hg

07Cd03Te

epitayer

-~

1x10 ,,.“O

It)



C

~~LUJr~ $

0.3

4 ,‘ -.

N

Cd

0 95Zn004Te substrate

10 pm Fig. 5. Photomierograph of a cleaved, etched cross-sectional (110) plane of a Hg07Cd03Te epilayer grown on a Cd5596Zn51~Tesubstrate at a growth temperature of 480°C and with growth time of 200 mm.

high mobility of the dislocations is one of the most distinctive features of the HgCdTe-CdTe heterojunction. The distribution of misfit dislocations in a HgCdTe CdZnTe junction differs remarkably from that of a HgCdTe CdTe one._Fig. 5 shows the cleaved, etched cross-sectional (110) plane of an Hg0 7Cd0 3Te epilayer grown on a CdZnTe substrate with a ZnTe mole fraction of 4%. The region where the etch pits clustered on the cleaved plane is narrower than that in HgCdTe CdTe heterojunctions grown under the same conditions. The dislocation etch pits were confirmed to be located only at the original surface of the CdZnTe substrate [10]. These results indicate that all misfit dislocations are generated at the initial stage of the epitaxial growth and do not move during the growth into the transient region of the lattice constant. The lattice constant profile was estimated from the measured compositional profile. The Zn atoms of the substrate were found to diffuse into the epilayer by the same measurement. Zn has been reported to stabilize HgTe bonds and increase dislocation energy [11]. Therefore, Zn diffused into the epilayer is thought to pin the misfit dislocations and to prevent the movement of those dislocations,

-

2

3

4 ZnTe mole fraction (°/~)

5

Fig. 6. Experimental linear etch pit density as a function of the ZnTe mole fraction of the CdZnTe substrate for Hg0 5Cd55 2Te and Hg07Cd03Teepilayers.

The linear dislocation-pit density, which is defined as the number of pits per unit length along the (112) direction on a (110) cleaved plane, was investigated. The relationship between the linear etch pit density and the ZnTe mole fraction of the CdZnTe substrates for Hg08Cd02Te and Hg07Cd03Te epilayers is shown in fig. 6. The linear densities change proportionately with the Zn content in the substrates. The Zn mole fraction where the lowest density of misfit dislocations will be obtained were found to differ slightly from the values reported previously as the lattice-matching compositions [12]. This difference seems to be caused by diffusion of Hg vaporized from the growth solution into the surface of the substrate before epitaxial growth. The linear etch pit density is thought to be fixed with the lattice mismatch between the composition of the epilayer and the surface composition of the substrate at the moment when the epitaxial growth begins. Compared with the HgCdTe CdTe heterojunction, the surface of the thin HgCdTe epilayer grown on the lattice-matched CdZnTe substrate was smooth and the thickness of the epilayer was uniform. Epitaxial growth on the substrate with a lattice mismatch of less than 0.1% produced an approximately 30 ~.iin thick HgCdTe epilayer with good surface morphology (density of surface pits <5 cm 2), even when it was grown by the supercooling technique.

450

H. Tahigawa et al.

Dolocationi in i-IgC‘dFe (‘die and HgCdTe

3.2. Threading dislocations

(d7n Fe heterojunctioni

O 1 ~7 5)

The origin of the dislocatinn’~threading through the epilayers has been investigated for HgCdTe CdTe and HgCdTe CdZnTe heterojunctions [13]. A variation in etch pit density

~

0 •

a) >~

~

CdTC dZnTe

a a)

along the growth direction was observed on (111) planes and is shown in fig. 7. The lattice mismatch

between the

a)

Hg0 7Cd0 ~Te epilayer

S

and the

bjj~

,°6~/

CdZnTe the epilayer substrate and theis CdTe about substrate. 1/15 of that The between etch pit density near the interface regions is higher than that of other regions. and is decreased by lattice matching. The region where the high etch pit density was observed on (ill) planes coincides with the high etch pit region on the (110) plane. Fig. 7 also shows that the density of the etch pits away from the interface is almost the same as the dislocation etch pit density of the substrate, and independent of the amount of lattice mismatch. The relationship between the etch pit density near the surface of the Hg0 7Cd~Teepilayer and that of substrate is shown in fig. 8. Each value of the density was measured as dislocation an average etch over pit an 2. The area of about 4 mm density of the epilayer was confirmed to coincide with the that of the substrate except for the interfacial region. Therefore, we have concluded that the origin of threading dislocations through the

0

1o~

a) ‘10

10

°

0~

Et h pit dr’rs:ty 0f ‘ubstr0te ~TY 2) Fig 5. Relationship het~een the etch pit densit~ near the surface of a Hg07Cd0 je epilayer and that of the (‘die and CdJnTe substrates

epilayer is not the misfit dislocations in the interface, but the dislocations in the substrate.

~

4. Conclusions It was found that misfit dislocations in HgCdTe CdTe heterojunctions moved during epitaxial growth in order to accommodate lattice mismatch entirely originating from the gradual compositional change around the heterojunction, and the density of the misfit dislocation etch pits was proportional to the compositional gradient. In contrast, it was found that misfit dislocations in H 8CdTe CdZnTe heterojunction were localized at the original surface of the substrate because the



CdZnTe

epilayer. We confirmed that an epilayer with low

o

Surface Cd~

misfit density andZngood surface morphologydislocation dislocations can were obtained pinned by by using the diffused lattice into matchthe ing CdZnTe substrate. However, the threading

__________

.~-

a

decreased by lattice matching, and coincides with dislocation density in the HgCdTe epilayer is not

~ ~

r5) I

10

0

10

20

30

40

Distance from interface (tim) Fig. 7 Etch pit densities along the growth direction observed on (1)1) planes of HgCdTe CdTe and HgCdTe CdZnTe bet erojunctions

the dislocation density of the substrate. These threading dislocations were found to originate from the dislocations in the substrate. Thus we concluded that, in order to improve the crystallinity of the HgCdTe epilayer, a substrate with not only a exact lattice-matching composition but also a low dislocation density should be used.

H Takigawa eta!

/ Dislocalions in

HgCdTe CdTe and HgCdTe CdZnTe heterojunctions

References

451

181 Landolt-Bornstein, New Series, Vol. 17/b Semiconductors: Physics of 11 VI and I VII Compounds, Ed. 0.

[11 Y. Miyamotn, H. Sakai and K. Tanikawa, Proc. SPIE ~72 (1985) 115. [2] T. Yamamoto, Y. Mmyamoto and K. Tanikawa, J. Crystal Growth 72 (1985) 270. 13] M. Yoshikawa, S. ijeda, K. Maruyama and H. Takigawa, J. Vacuum Set. l’echnol. A3 (1985) 153. 14] K. Nakagawa, K. Maeda and S. Takeuchi, App]. Phys. Letters 34 (1979) 574. 15] W.G. Oldham and AG. Mimes, Solid-State Electron. 7 (1964) 153. [6) G.R. Woolhouse, T.J. Magee, HA. Kawayoshi, C.S.H. Leung and RD. Ormond, J. Vacuum Sci. Technol. A3 (1985) 83. [7] R.B. Schoolar, Proc. SPIE 409 (1983) 32.

Madelung (Springer, Berlin. 1982).

[91 H. Takigawa, M. Yoshikawa, M. Ito and K. Maruyama, in: Mater. Res. Soc. Symp. Proc. Vol. 37 (Mater, Res. Soc., Pittsburgh, PA, 1985) p. 97. 110] T. Maekawa, T. Saito, M. Yoshikawa and H. Takigawa, in: Mater. Res. Soc. Symp. Proc. Vol. 56 (Mater. Res. Soc., Pittsburgh, PA, 1986) p. 109. [111 A. Sher, A.B. (‘hen, W.E. Spicer and C.K. Shih, J. Vacuum Sci. Technol. A3 (1985) 105. [12[ J.H. Basson and H. Booyens. Phys. Status Solidi (a) 80 (1983) 663. [131 M. Yoshikawa, K. Maruyama, I. Saito, T. Maekawa and H. Takigawa. J. Vacuum Sci. Technol. A, to be published.