Characterization and growth of high quality ZnTe epilayers by hot-wall epitaxy

j. . . . . . . . C R Y I T A L GROWTH

ELSEVIER

Journal of Crystal Growth 180 11997)47-53

Characterization and growth of high quality ZnTe epilayers by hot-wall epitaxy S u n g u n N a m a, J o n g k w a n g R h e e a, B y u n g s u n g O a'*, K i - S e o n L e e a, Y o n g D a e C h o i b, G y u n g - N a m J e o n c, C h o o n - H o L e e c aDepartment of Physics, Chungnam National University, Taejon 305-764, South Korea bDepartment of Physics, Mokwon University, Taejon 301-729, South Korea cDepartment of Physics and Semiconductor Physics Research Center. Jeonbuk National Universi~, Jeonju 561-756, South Korea

Received 12 December 1996;accepted 6 March 1997

Abstract

ZnTe epilayers of high quality have been grown by hot-wall epitaxy and their characteristics have been investigated. By Rutherford backscattering, the atomic ratio of Zn : Te in the epilayers was found to be almost constant over the wide range of the growth temperature. It was found that the quality of the films depended on the pre-heating temperature. The best value of the FWHM of the double crystal rocking curve, 93 arcsec, was obtained at the pre-heating temperature of 590-610°C. The PL spectrum with the strong free exciton peaks and no oxygen-bound exciton peaks showed the high quality of the films. From the PL spectrum and the lattice constants, it could be concluded that a tensile strain remained in the ZnTe/GaAs epilayers. PACS: 68.55.Gi; 78.55.Et; 78.30.Fs Keywords: Hot-wall epitaxy; ZnTe; Pre-heating; DCRC; Photoluminescence

I. I n t r o d u c t i o n

High quality ZnTe epilayers have been grown by many thin-film growth methods such as molecular beam epitaxy (MBE) [1, 2] metalorganic chemical vapor deposition (MOCVD) [3,4] atomic layer epitaxy (ALE) [5] and hot-wall epitaxy (HWE)

* Correspondingauthor. Fax: + 82 42 822 8011;e-mail:byung @nsphys.chungnam.ac.kr(168.188.99.66).

[6, 7]. The ZnTe epilayers on GaAs substrates have attracted interest because of the potential applications for electro-optical devices in the blue-green region of the spectrum. Furthermore, it can be used for a heterojunction with other II-VI semiconductors [8, 9]. Among the many epitaxial methods, H W E has its own advantages [10]. First of all, all the reactions such as vaporization, transportation and growth occur in the nearly-thermal equilibrium, which would give high quality films. Also, the loss

0022-0248/97/$17.00 Copyright ~, 1997 ElsevierScienceB.V. All rights reserved PII S0022-0248(97)00 1 93-0

48

Sungun Nam et al. ,/Journal o['Cr),stal Growth 180 (1997) 47 53

of the source material is small. The cost for maintenance is low. Most of the lI VI semiconductor epilayers are grown on GaAs substrates which are superior in quality, area and price. But these heterostructures give some problems such as (1) the misfit between the substrate and the film, (2) the impurities from the substrate elements and (3) the different thermal expansion coefficients. So a lot of dislocation and the remaining compressive or tensile strain in the film affect its characteristics. Usually, the strain due to the lattice mismatch which is compressive in ZnTe/GaAs films disappears gradually from the beginning of the growth. But the difference in thermal expansion coefficients and the large difference between the high growth temperature and the low measurement temperature would give a little large tensile strain, which remains in even a thick layer. In this paper, the optimum conditions have been found for the growth of the high quality ZnTe films on (1 0 0) GaAs substrates by HWE and their characteristics have been investigated. The crystal quality and the dislocation density have been examined by the double crystal rocking curves (DCRC). The obtained 93 arcsec of the full width at half maximum (FWHM) of the DCRC, which is the lowest as we know, revealed the high quality of the ZnTe epilayers. Photoluminescence (PL) and Rutherford back scattering (RBS) also have been carried out to study the characteristics and to confirm the quality of the films.

2. Experiments ZnTe epilayers were grown on Cr doped SI (1 0 0) GaAs substrates with hot-wall epitaxy system. The crucible was made of quartz and the heater was made of the tungsten wire. They were surrounded with a stainless steel tubing and all of these were inside the vacuum chamber. The source was 5N ZnTe powder. The substrates were ultrasonically cleaned with TCE, acetone and methanol, successively, and etched in 3H2SO 4 : H 2 0 2 " H 2 0 solution at 50-60°C for 1 min. After rinsing with DI water and dried with Ar gas, they were mounted on the substrate holder in the HWE system. The gap between the substrate and the top of the crucible

Substrate temp. = 390 ~C 4

/

(a)

6)

~-

2

0

c3' 0 500

=

I 510

i

I

I

520

530

,

I

i

I

540

i

550

I

560

570

S o u r c e temp.(°C)

Source temp. = 530 "C

(b) 3 °< v

~,

2

o (.9 1

280

i

i

i

i

i

300

320

340

360

380

~

•

400

I

420

i

I

440

,

460

Substrate temp. ('(3) Fig. 1. (a) Source temperature dependence and (b) substrate temperature dependence of the growth rate of ZnTe epilayers.

was about 15 mm. The vacuum was maintained at below 2 × 10 -v Tort during the growth with the liquid nitrogen trap. The pre-heating at 450-630~'C for 20 rain was done to remove the oxides and the remaining impurities on the substrate surface. Fig. 1 shows the temperature dependence of the growth rate. The growth rate was increased exponentially as the source temperature was increased. But it was decreased slowly at the elevated substrate temperature. The thickness of the films was determined with the spectrophotometer reflectance spectrum.

3. Experimental results and discussion The surface morphology was observed with the Nomarski interference microscope. The surfaces

Sungun Nam et al. ,/Journal of Cr),stal Growth 180 (1997) 47-53

Table 1 The composition of ZnTe epilayers by RBS Sample 1D ZT007-1 ZT044-2 ZT015-2 ZT021-2 ZT029-1 ZT031-1 ZT034-1 ZT038-1 ZT039-1

49

6.20

Source temp. (C)

Substrate temp.1'C)

Zn : Te ratio

480 510 540 560 530 530 530 530 530

330 330 330 330 300 320 350 380 390

51.0 : 49.0 51.3:48.7 52.5 : 47.5 51.3:48.7 51.5:48.5 51.3 : 48.7 51.3:48.7 51.3 : 48.7 51.3 : 48.7

°<

6.15

Bulk ZnTe (ao= 6.1037 ~ )

l

"E ('a

6.10 0

o o q)

Q

10

•

.2 ~m 6.05

6.00

,

0

I

i

i

2

i

4

i

l

6

8

,

I

1o

Thickness ( ~am )

were mirror-like. The surface roughness was found to increase as the growth rate was increased. To confirm the crystal orientation, X-ray diffraction was performed. Only the (2 0 0) and (4 0 0) peaks from a GaAs substrate and a ZnTe epilayer could be observed, which shows that the epilayers were grown in the same (1 00) direction as the GaAs substrate orientation. The Rutherford backscattering spectrum showed that the composition ratio of Zn : Te was independent of the growth temperature. The ratio was constant over the wide range of the substrate temperatures (300-390°C) as shown in Table 1. It was not changed much as the source temperature was changed, either. Fig. 2 shows the ZnTe lattice constant in the growth direction. It was found to be smaller than the known bulk value, 6.1037 ,~ [11]. Similar result was reported [12] for the ZnTe/GaAs epilayers grown by M O C V D . It may be because the tensile strain due to the thermal expansion coefficient difference is larger than the compressive strain due to the lattice mismatch for these films thicker than the critical thickness of 15 ,~ [13]. It would be confirmed with the PL measurement. To grow high quality ZnTe films, the effect of the thermal pre-heating has been examined. At the source temperature and the substrate temperature at 530°C and 390°C, respectively, the thermal preheating of the substrates was done for 20min. Fig. 3 shows the F W H M of the D C R C as a function of the pre-heating temperature. The films were 5-6 gm thick.

Fig. 2. The lattice constant in ZnTe epilayers.

500

400

300 O

/.

"1" LL

100

.

40

i

460

.

i

480

.

,

500

.

i

520

,

i

540

,

i

560

.

i

,

580

i

600

.

i

620

,

640

Preheating temp. (°C) Fig. 3. Pre-heating temperature dependence of the FWHM of the DCRC for ZnTe epilayers.

The best F W H M values were found at 590-610°C. As the pre-heating temperature was increased above 610°C, the F W H M was increased. It may be due to substrate surface defects generated when the As atoms were diffused out of the GaAs substrate [6]. As the temperature was decreased below 590°C, the F W H M was increased again. A sharp peak in the F W H M was found at ,-~ 550°C. N o other results below 560°C could be found. The higher values of the F W H M at the low temperature were regarded as due to the remaining oxygen in the films [6]. It may be true in the cases at the lower temperatures below 530°C.

50

Sungun Nam et al. / Journal of C~stal Growth 180 (1997) 47- 53

1200

The dislocation density affects the crystallinity and gives a change in the F W H M of the DCRC. It is given by [6]

1011

t

ZnTe

(4i0)

1000

',

~

F2 O=~-~

93 arcsec

101o

'f;L ........

800

¢n o

/

109

E v

(cm- 2),

where F is the F W H M in radians and b is the Burger vector which is given by a(1 1 0)

600

b

__ _

"O

"1"

108

400

¢/)

~ 107

200

"JI

"'I'II .......... • 0 0

I

I

I

1

!

2

4

6

8

10

2

az,Te __

,A

g o

"~- ~',

_

106 2

Thickness (p.rn) Fig. 4. The film thickness dependence of the FWHM of the DCRC and the dislocation density. The inset is the DCRC of a good quality ZnTe epilayer.

The sharp peak around 550°C could be explained as due to a high concentration of the As atoms diffused from the GaAs substrate. The photoluminescence spectrum showed a larger Asrelated peak in these films. At around 550°C, high enough temperature to remove the oxygen, the As atoms can be considered to start diffusing out of the substrate but they may remain on the substrate surface. The ZnTe epilayers grown in this condition should have a lot of As impurities inside and the quality was a little worse. Fig. 4 shows the F W H M as a function of the film thickness. For the films thinner than 4 lam, the F W H M was decreased very sharply as the thickness was increased. For the thicker films, it was decreased slowly to a saturation value of 93 arcsec, which is the best value reported ever for the epitaxial ZnTe films, shown in the inset. In the heterojunction system, the dislocation was generated mainly due to the lattice mismatch.

for the zincblende structure. Fig. 4 shows the thickness dependence of the dislocation density. As expected, it was decreased exponentially with the film thickness to ~ 1 x 107 cm -2. The photoluminescence spectrum at 11 K for a 5.5 ~tm thick epilayer is shown in the Fig. 5a. The excitation source was a 514.5 nm Ar ÷ ion laser. The near-gap excitonic emission peaks such as free exciton (FE) peaks and bound exciton (BE) peaks could be resolved clearly and the intensity of the FE peaks was larger than that of the BE peaks. The donor-acceptor pair peak around 2.3 eV [14], Y-band about 200 meV below the band-edge found in other II-VI compound semiconductors such as ZnSe and CdTe [14-1 and the oxygen bound exciton peak below 2.0 eV [-15] can be hardly observed. This is consistent with the DCRC result, which shows that the ZnTe epilayers were of high quality. Fig. 5b is the detailed spectrum near the bandedge. Their corresponding transitions to the PL peaks can be assigned by analyzing the peak positions, the line-shapes, the temperature dependence of the peak intensities and comparing the other data. The doublet at 2.3787 eV and 2.3768 eV could be assigned as a ground-state heavy hole FE recombination X l s , h h . The splitting of ~ 2.0 meV is known as due to exciton-polariton splitting [16]. The ground-state light hole FE recombination Xlsah peak appeared at 2.3728 eV. Therefore the peak at 2.3874 eV could be assigned as the first excited-state heavy hole FE recombination X2S.nh and the 2.3823 eV peak could be assigned as the first excited-state light hole FE recombination X2s,lh.

Sungun Nam et al. / Journal of Crystal Growth 180 (1997) 47-53

(a) ZnTe/GaAs

substrates [15, 17]. With this value of the splitting, the strain e could be calculated [17]

Band edge emission

11K

AEhh = ( - 2a Cll

f.¢t t-m

-

C12

1 -- C12

AEih= OBE

1.9 '

1.8

Y line

2.0 '

2.3 '

' 2.4

Photon energy (eV)

R2LO I

X sj h

ills

.

(

....

BE

) I ,,o i~X2s I

i

i

,

I

.aa i

2.34 2.36 2.38 Photon energy (eV)

2.40

1,5

Co) 1.0

J"', I

I

I

I

I

I

I

c,,

)

resulting in a tensile strain which is consistent with the lattice constant data in Fig. 2. This value of the strain was found to be very close to other published data, such as in Ref. [17]. They got 6.95 x 10 -4 and 5.54 x 10 -4 for a 4.3 gm and a 7 gm thick ZnTe/GaAs epilayer, respectively. The thermally induced strain is proportional to the temperature difference AT between the growth temperature (300°C) and the experiment temperature (10 K). gth . . . .

-°5 0.0

Cll + 2C12"~ C17 -)t~,

X s.hh

e-

2.32

+ 2C12'~ <;

1 C l l + 2 C l z ] (AEhh -- AEIh) = 6.49 x 10 -4 = 2-b

th

i

b

*

Cll

where C u are the elastic stiffness coefficients, a the hydrostatic, and b the shear deformation potential. The numerical values used here are Cll = 7.13 x 106 N cm -2, C12 = 4.07x 106 Nero -2, a = - 5.4eV and b = - 1 . 8 eV. Therefore, the strain may be calculated as

(b)

2.30

-2aC'c1

+b

DAP

2 i 1 ' 212'

¢0

51

/

I

440 460 480 500 520 540 560 580 600 620 640 Preheating temp.(oC)

Fig. 5. (a) Photoluminescencespectrum of a ZnTe epilayer,(b) the detail spectrumnear the band edgeand (c) the intensityratio of the As related BE peak (I~) to the light hole FE peak (X~sah) The splitting of the free exciton into X~s.hh and X~sah was about 5 meV and was known due to the residual strain in ZnTe films grown on GaAs

1=

K(~z,Te -- ~GaAs)AT,

where K describes the value of the lattice relaxation, ~Z,Te(~OaAs) is the thermal expansion coefficient of ZnTe (GaAs), 8.30 × 10-6/K (5.75×10-6/K). For the films thick enough, K could be set to 1. Then ~th. . . . ~could be calculated as 1.436 × 10 -3 which is tensile and larger than the strain found from the PL data. The difference may come from the assumption of K which depends on the film thickness. A binding energy of EB = 12.7 meV for the light hole exciton was obtained from the energy difference E 2 S , l h - E I s . I h = 9.5 meV. This value is very close to the theoretical one [18] and the experimental ones [19, 20]. A gap energy of 2.3855 eV at 11 K results for the light hole band.

52

Sungun Nam et al. / Journal of Crystal Growth 180 (1997) 47 53

For the heavy hole exciton, EB = 12.9meV was obtained with the average value of the doublets of the Xls.hh peaks. A energy gap is found to be 2.3906 eV for the heavy hole band. It means that the light hole band maximum is higher than the heavy hole band maximum, which is known as a result of the tensile strain. The 1~" at 2.3678 eV, 1~ at 2.3615 eV and I c at 2.3588 eV could be assigned as shallow acceptor BE transitions. The I~ peak was assigned to the transition of the BE to As atom diffused out of the substrate [-21]. It was located below the corresponding peak at 2.3750 eV in the bulk ZnTe 1-20] by 7.2 meV. It may be due to the thermal induced tensile strain [15, 21]. The IF peak could not be assigned to any known transition. It may be regarded as due to an unknown impurity from the source material. The I c peak was overlapped with the Raman peak at 2.3579eV. The similar peak at 2.3613eV in the bulk ZnTe was reported [12] and was known as the exciton transition bound to the neutral double acceptor. It was considered the Si atom in place of the Te site [22]. Or it was considered to be related with the extended defects occurring at the ZnTe/GaAs hetero-interface due to the lattice mismatch [21]. For the H W E system, it is hardly rare to get the Si atoms. Then it is more reasonable to regard it as related to the lattice defects. The three peaks at 2.3837 eV (R1LO), 2.3579 eV (R2LO) and 2.3317 eV (R3LO) could be assigned as the Raman peaks. Their differences were around 26 meV close to the LO phonon energy for ZnTe. It was confirmed with the Raman scattering. The summary of the assignment of these transitions is shown in Table 2. Fig. 5c shows the As-related peak (I~) intensity to the Xls.~h peak intensity. As the pre-heating temperature was increased, the ratio was decreased. It is thought that the As atoms were diffused out of the GaAs substrate during the pre-heating. But around 550°C the ratio was high, which means the high concentration of the As atoms in the films. Fig. 6 shows the Raman spectrum measured at 10 K. The excitation source was an Ar ÷ ion laser.

Table 2 tPL spectral peaks observed in ZnTe epilayers Peak energy (eV)

Assignments discussed

2.3874 2.3823 2.3787] 2.3768 ~ 2.3728 2.3678 2.3615 2.3588 2.3837 2.3579 2.3317

Heavy hole FE, Xzs,hh Light hole FE, Xzs.~h Heavy hole FE, doublet due to excitonpolariton splitting, X~s.hh Light hole FE, X~s,jh BE to As atom, I t Unidentified, I~ BE related to the lattice defects, I c Phonon peak, R1LO Phonon peak, R2LO Phonon peak, R3LO

R2LO (420) 2.40 RILO

o

v

5o

lO0

15o

200

Temperature(K) i-

¢R1LO I (210)

0

200

R3LO (630)

400

Raman

600

800

1000

shift(cm -1)

Fig. 6. Raman spectrum for a ZnTe epilayer. The inset is the temperature dependence of the Raman shift lines.

The Raman shifts were found at 210, 420 and 630 cm-1. The energy levels for those shifts were constant over the wide range of the temperature, from i0 to 150 K as shown in the inset.

Sungun Nam et al. / Journal of Cr3,stal Growth 180 (1997) 47 53

4. Conclusions ZnTe epilayers of high quality have been grown on the GaAs substrates by hot-wall epitaxy. Rutherford backscattering showed that the atomic ratios in the ZnTe films were almost constant regardless of the growth temperature. It was found that the quality of the films depended on the pre-heating temperature. The best value of the FWHM of the DCRC, 93 arcsec, was obtained at the pre-heating temperature of 590-610°C. Above or below this temperature the FWHM was increased. The PL spectrum showed no oxygen-bound exciton peaks. The ground and first-excited free excitonic peaks were observed. The strong and narrow free exciton peaks near the band-edge showed the high quality of the films. It could be concluded that a tensile strain remained in the ZnTe/GaAs epilayers since the PL peaks were shifted to the lower energy compared to those in the bulk and the lattice constant was found to be smaller.

Acknowledgements This work was supported by Korean Ministry of Education through Research Fund.

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