Temperature induced resonant Raman scattering of MOVPE grown ZnSxSe1−xGaAs(100) heterostructures

........

ELSEVIER

CRYSTAL GROWTH

Journal of Crystal Growth 170 (1997) 767-771

Temperature induced resonant Raman scattering of MOVPE grown ZnS xSel _JGaAs(100) heterostructures A. Schneider a,*, D. Drews a, j. SiSllner b, M. Heuken b, D.R.T. Zahn a ~' P r @ s s u r fi~r Halbleiterphysik. Institut fiir Physik, TU Chemnit=-Zwickau. D-09107 Chemnit=, Germany b hzstitutfiir Halbleitertechnik. RWTH Aachen. Templer,~raben 55, D-52056 Aachen. Germany

Abstract

ZnS,Se I , layers with various nominal compositions between x - 0 . 0 5 and 0.31 grown by low-pressure MOVPE on GaAs(100) were investigated by resonant Raman scattering in the temperature range from room temperature (RT) up to approximately 400°C. After growth the samples were transferred into an ultra-high vacuum chamber attached to a Raman set-up with multi-channel detection, that allows Raman spectra to be measured continuously while the sample temperature is increased. Owing to the temperature dependence of the bandgap, strong resonance enhancement of the phonon scattering efficiency is observed at certain temperatures when the energy of the scattered light equals the gap energy. Under these conditions first order scattering as well as higher order processes are observed. From the frequency difference of the ZnSeand ZnS-like longitudinal optical (LO) phonons, which is derived consistently from 1st and 2rid order structures, the sulphur content x can be determined with high accuracy. From the resonance maxima for different excitation energies the bandgap energy at RT as a function of the composition x is extrapolated. The strong signals at elevated temperatures obtained by adjusting the resonance conditions clearly demonstrate the feasibility of applying Raman spectroscopy as an in situ and on line growth monitor in the MOVPE process.

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

Since the last two decades metal organic vapor phase epitaxy (MOVPE) has increasingly been applied for the growth of I I - V I semiconductors on GaAs in particular in the view of their relevance for opto-electronic devices in the blue-green spectral range (e.g. laser diodes). The first success in producing such devices was achieved by molecular beam epitaxy (MBE) [1] but tbr technical and industrial applications the M O V P E technique is usually preferred as a more economical method. Consequently it is of great interest to understand growth mechanisms

* Corresponding author. Fax: +49 371 531 3060.

and material properties of the devices as well as of the basic materials. Various optical methods are often employed ex situ for the characterization of these semiconductors after the growth process. Raman spectroscopy is one of these methods having a sensitivity for the detection of ultra-thin I I - V I films [2]. This can be achieved by resonance enhancement of the scattering efficiency when excitation energies in the vicinity of the E 0 gap of the I I - V I material are used. Resonance enhancement also allows spectra to be detected with a sufficient signal to noise ratio even at elevated temperatures in the range of typical growth temperatures. Recently the application of Raman spectroscopy for in situ and on line monitoring the growth of ZnS.,Se~ ~ in MBE has been reported [3]. Substantial information can be obtained

0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 6 0 5 - 7

768

A. Schneider et al. / Journal of Co'stal Growth 170 (1997) 76 7- 771

in situ from the Raman spectra using this novel monitoring technique. Because of its chemical sensitivity it is e.g. possible to determine the composition of ternary alloys, here the sulphur content of ZnSxS% x, while the full width at half maximum (FWHM) provides a measure for the crystalline quality. Furthermore, the growth rate and the actual temperature of the sample can also be evaluated. The advantage of this optical method in comparison to other in situ characterization methods like reflection high energy electron diffraction (RHEED) is that its application is not restricted to ultra-high vacuum (UHV) conditions and its use for monitoring MOVPE growth processes is thus feasible. In this paper the conditions for achieving resonance enhancement at high temperatures are investigated for ZnS,Se I J G a A s ( 1 0 0 ) MOVPE grown heterostructures with 0.05 < x < 0.31. The knowledge of these conditions is the essential prerequisite for the successful application of Raman spectroscopy as a monitoring technique of MOVPE growth processes. Furthermore, the use of these measurements for investigating the dependence of the E 0 gap on sulphur content x and temperature is demonstrated.

2. Experimental procedure The ZnS, Se i - J G a A s ( 1 0 0 ) heterostructures used here for the resonant Raman experiments were grown in an AIX 2 0 0 / 4 low pressure (20 hPa) MOVPE system. They have nominal sulphur contents of x = 0.05, 0.07, 0.16, and 0.31, respectively, which was determined from the excitonic transition observed in photoluminescence (PL) spectra at 12 K [4]. The layer thickness of the samples is in the range from 150 to 610 nm. In all cases the growth temperature was TG = 500°C. Dimethylzinc triethylamine (DMZn(TEN)), diisopropylselenide (DiPSe), diethylsulphide (DES) and H 2 S w e r e employed as precursors. The variation of the sulphur content was achieved by changing the Z n / S e / S molar flux ratio of the MO Precursors and the hydride from 1 / 1 / 0 . 0 9 2 to 1 / 1 / 4 . The Raman spectra were taken using a triple Dilor XY monochromator equipped with a CCD camera for multi-channel detection in a backscattering configuration. The monochromator is attached to an

UHV chamber where the sample is mounted on a holder with resistive heating. The UHV environment is beneficial since it protects the sample against oxidation while increasing the temperature. The same chamber which is similar to that described in detail in Ref. [5] is also used for recording Raman spectra during ZnSxSe I x MBE growth. For the optical excitation of the samples, different laser lines of Ar +, Kr +, and HeCd lasers were used with a laser power of about 15 mW. The scattering configuration was chosen so that both deformation potential as well as Fr/Shlich scattering contribute to the spectra. All spectra were taken on line while heating the sample from room temperature (RT) up to approximately 400°C.

3. Results and discussion Fig. 1 shows as an example a series of Raman spectra recorded during the increase of temperature. The investigated sample in this particular case has a nominal sulphur content of x = 0.05 (E 0 > 2.7 eV). The excitation energy is 2.66 eV which is below the bandgap energy of the sample at RT. As a result the scattering intensity of the typical phonon modes, namely ZnSe-like and ZnS-like LO phonons observable at about 250 and 300 cm -1, respectively, are extremely weak in the front spectrum because this is an off-resonance condition. The sample temperature of the following spectra is directly obtained from the frequency shift of the observed phonon structures Intensity

Eeounts/mWs] E~x = 2 . 6 6 eV

0.4 0.2 0.0 )

~ture

Ramen

Shift

w_

700'

[vC]

[cm -13 Fig. 1. Raman spectra of ZnS00sSe09 s on GaAs(100) monitored on line during the increase of the temperature (E~x = 2.66 eV).

A. Schneider et al. / Journal of Cry'stal Growth 170 (1997) 767-771

relative to their peak positions at RT. The temperature dependent frequency shifts of the LO phonons in pure ZnSe (2.4 cm-~/100°C) and ZnS (1.5 c m - 1 / 1 0 0 ° C ) were obtained from previous calibration measurements. Investigation of a ternary ZnS0.31Se0.69 sample revealed the same slopes for the ZnSe-like and the ZnS-like LO, respectively. This is in agreement with the parallel slopes of the compositional dependence of these modes when measured at different temperatures (100 and 300 K [6]), indicating that the slopes do not vary with the sulphur content. The accuracy in determining the peak position of the LO modes by a curve fitting routine is in the range of 0.1 c m - ~ so that a temperature resolution of -+ 5°C is typically achievable. While the sample temperature is increased the bandgap energy decreases, thereby approaching the excitation energy. As a result the scattering intensity of the phonon modes is enhanced until maximum resonance is achieved at about 130°C for the 1st order modes in this case. Additionally the 2rid order modes involving two phonons in the scattering process are visible (two ZnSe-like LO phonons at 500 c m -1 and a mixed mode of ZnSe-like + ZnS-like LO phonons at 550 cm l). The frequency separation of ZnSe- and ZnS-like phonon structures can now be used to determine the sulphur content of the sample e.g. by comparison with the theoretical values of the modified random element isodisplacement (MREI) model [7]. Using relative frequency shifts this determination is to the first order independent from shifts induced by temperature or stress which may be present in the ZnSxSe L ~ layer due to the lattice mismatch to the GaAs substrate. Under resonant Raman scattering conditions this energy difference can be obtained from both the 1st and 2nd order structures as shown in Fig. 2 providing two independent measures for the sulphur content and thus increasing the accuracy. It is also evident from Fig. 2 that the 2nd order features reveal improved resolution. For this particular sample a composition of x = 0.03 _+ 0.005 is obtained which is close to the nominal value of 0.05. The uncertainty in determining x is always < 0.01. Considering Fig. 1 again it is apparent that the resonance maximum of the 2nd order modes appears at even higher temperature than the I st order modes. This indicates that the resonance mechanism is corre-

769

460 480 500 520 540 560 580 I

'

i

'

i

,

i

'

I

'

I

'

I

2LO ~ LOI: ZnSe-like ]'/ LO2: ZnS-like 'I[ LO2-LOI .~'~

0.05

i

I

,

I

,

I

*

I

,

i

,

I

.

I

200 220 240 260 280 300 320 340

Raman Shift [cm~] Fig. 2. Comparison of the 1st and 2nd order Raman scattering of the same sample as shown in Fig. 1 at resonance condition. From the peak separation LO2-LO I the sulphur content is deduced. Obviously the 2nd order spectrum (top) exhibits better resolution.

lated with the scattered light (outgoing resonance) as has been observed in other I I - V I compounds as well [8]. The outgoing character is further confirmed by the fact that maximum scattering intensity of a phonon structure is observed when the phonon peak coincides with the maximum of the bandedge luminescence observable as a background in the spectra in Fig. 1. This luminescence shifts with temperature according to the variation of the bandgap. Fig. 3 now shows the temperature dependence of the scattering intensity of the ZnSe-like LO phonon structure for the sample with XnominaI 0.05 for various excitation energies. All scattering intensities are corrected for temperature effects introduced by the Bose-Einstein statistic. It can be seen that the resonance maximum is shifted to higher temperatures when lower excitation energies are used. The same behaviour is observed in all other samples. Depending on the composition x, however, different excitation energies have to be used in order to achieve maximum scattering efficiency at a given temperature. The insert in Fig. 3 reveals that the resonance maxima in Fig. 3 obey a linear dependence upon temperature. Due to the outgoing resonance this dependence reflects the variation of the E 0 gap of =

A. Schneider et al. / Journal q/"Co'stal Growth 170 (1997) 767-771

770

i i . i , i . i . i , i .

1.0 :'~ b~ .: ~

"v

2.72

2.60

(a r~

E (RT) =

>

> 2.68 2.61

©

Table 1 Sulphur content x, E0(RT) and temperature dependence of E0

510 L

i

,

I

,

I

evaluated from r e s o n a n c e R a m a n m e a s u r e m e n t s

,

100 150 200 25(1

._2,0,5

d Eo/dT

-r,~,,mi,,al

XR ........

Eo(RT) (eV)

0.05

0.03

2.698

6.4

0.07 0.16 0.31

0.05 0.14 0.32

2.698 2.713 2.791

- 5.9 -5.6 -4.3

(10 - 4 eV K - i)

Temperature ["CI

! 2.71 eV

nominal

ei

e"

Zn S0.0sSe0.9_~

6

2.66 eV '

,'

~'.o oo~'¢'~.~

,, 0

50

2.62 eV 2.60eV

c]". -, . ,..,~__:L)~=~O.~,

100

150

200

250

300

350

Temperature [°C] Fig. 3. Temperature dependence of the scattering intensity' of the ZnSe-like LO phonon structurefor the sample with XnominaI = 0.05 for various excitation energies. The insert shows that the resonance maxima decrease linearly upon temperature. 2.95

the ternary compound with temperature according to E 0 = E~x - h ~Qco. The LO phonon energy h~QLO 31 meV can be obtained directly from the Raman shift. Consequently the gap energy at RT can be extrapolated from this data. The error bar for the RT value of E 0 is estimated to be around + 10 meV, but could significantly be improved by taking into account a larger number of resonance curves. The E o values for all samples investigated are plotted in Fig. 4 as a function of x. The solid line illustrates the dependence of the E o gap on the sulphur content as determined by Newbury et al. from the bandedge PL at RT [4]. As can be seen the current results reveal a somewhat modified dependence of E 0 on the sulphur content which can be approximated by Eo(x)

"~90 ......... best

>"~ 2,80

2,75

2,70

i

0,0

0,1

i 0,2

~ZnS

o

x+E

ZnSc ( 1 - x ) - b ( 1 - x ) x .

o

Here F -0zns and Fz'lse 40 are the energy gaps of ZnS (3.700 eV [9]) and ZnSe (2.694 eV [10]) at 300 K. The value of the so-called bowing parameter b has been calculated to be 1.03 _+ 0.01 eV. The deviation from the empirical data of Ref. [4] is not yet fully understood and necessitates further confirmation by further measurements and thus expanding the data set. Finally the results for all samples obtained from the resonance spectra, i.e. measured sulphur content XR~m,~, E0(RT), and d E o / d T are summarized in the Table 1.

quadratic Z / / / .

2,85

2,65

=L

i 0,3

4. Conclusion

Sulphur Content Xmeasured

Fig. 4. D e p e n d e n c e

of

E 0 on the sulphur

content

c o m p a r e d to the literature [4]. E 0 ( R T ) w a s evaluated

x

at R T

from the

linear r e g r e s s i o n o f the r e s o n a n c e m a x i m a (see e.g. insert o f Fig. 3) minus the e n e r g y o f the L O p h o n o n . A quadratic fit to the E o ( R T ) values taking E z ° s and E(z'~s~ into a c c o u n t is included ( d a s h - d o t t e d line).

In this paper it is demonstrated that the resonant Raman experiments allow the evaluation of the sample temperature, the sulphur content in the temary ZnS,S%_~ alloy as well as the bandgap energy and its temperature dependence. This has been achieved

A. Schneider et al. / Journal of Crystal Growth 170 (1997) 767- 771

independent of any other experimental technique in a straight forward manner revealing the potential of temperature induced resonant Raman spectroscopy for the characterization of ternary ZnSxSe ~_x compounds. Considering the sulphur content improved accuracy can be achieved by considering 2nd order scattering processes. In addition, the results illustrate that the application of Raman spectroscopy for in situ and on line growth monitoring in the MOVPE process is feasible.

Acknowledgements Financial support of this work by Deutsche Forschungsgemeinschaft (Za 146/2-3) is gratefully

acknowledged.

771

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