- Email: [email protected]
GaAs interface
:::::::::::::::::::::::::::::::::::::::::::::::::: aOOHeo
Applied Surface Science 56-58 (1992) 691-696 North-Holland
surface s c i e n c e
Photoexcited plasmon-LO-phonon modes at the ZnSe/GaAs interface A. Krost, W. Richter and D.R.T. Zahn bzstitut fiir FestkiJrperphysik der TU Berlin. Sekr. PAl 6-1, ttardenbergstrasse 36. D.IO00 Berlin 12. Germany
Received 6 May 1991; accepted for publication 9 May 199!
ZnSe epitaxial layers grown on GaAs(10tl) substrates were studied by Raman spectroscopy. The intensity of the incident light was varied over three orders of magnitude in the range from about 1 kW/cm-' to 0.5 MW/cm-'. The spectra recorded at low laser power exhibit the pure phonon features of GaAs and ZnSe. i.e. only scattering by the longitudinal optical (LO) phonons is observed. While the lineshape of the ZnSe LO phonon peak remains constant upon increasing laser power, drastic changes can be seen in the GaAs Raman featulcs. These changes depend only on the laser power and are found to be completely reversible. The relative GaAs LO intensity decreases and simultaneously a new peak appears close to the GaAs TO phonon position. This behaviour is explained in terms of scattering by photogenerated coupled plasmon-LO-phonon (PLP) modes. Free carrier concentrations above I(1Is cm-3 are deduced from the PLP mode frequencies. At t~ : highest laser powers used the spectra are completely dominated by the Pt,P modes. The long carrier lifetime neccessary for the generation of such high concentrations suggests that the ZnSe/GaAs interface has to be of high structural quality leading to low recombination velocities. Indeed ZnSe/GaAs hetemstructures of lower quality showed the effect of photogenerated carriers only to a lesser extent. Thus Raman scattering by the photoexcited free carriers is likely to serve as an excellent new tool for interface quality assessmenl.
I. Introduction T h e g r o w t h a n d quality of Z n S e epitaxial layers o n G a A s s u b s t r a t e s using various m e t h o d s , such as m o l e c u l a r b e a m epitaxy ( M B E ) , m e t a l organic v a p o u r p h a s e epitaxy ( M O V P E ) , a n d hot wall epitaxy ( H W E ) has b e e n substantially improved in r e c e n t years [1]. W h i l e the p r o p e r t i e s of the epitaxial Z n S e film itself are well c o n t r o l l a b l e now, the interface region b e t w e e n the Z n S e epilayer and the G a A s s u b s t r a t e is not yet fully u n d e r s t o o d . Besides high resolution transmission e l e c t r o n microscopy ( H R T E M ) [2] a n d in situ grazing incidence X-ray s c a t t e r i n g [3] mainly Ram a n spectroscopy was a p p l i e d for interface chara c t e r i s a t i o n [4,5]. W h i l e the H R T E M results of t h r e e different g r o u p s [2] giv,: indirect evidence for the f o r m a t i o n of G a 2 S e 3 islands at the interface, m o r e direct p r o o f was provided recently by
R a m a n spectroscopy [6]. It was also claimed from R a m a n results that the o p t o e l e c t r o n i c p r o p e r t i e s of interfaces are affected by intrinsic instabilities which are assigned to interfacial a t o m i c processes a n d lead to a n a g i n g of the s a m p l e s [4]. lnterfacial diffusion of Z n in G a A s and G a in Z n S e has b e e n d e d u c e d from electrolyte e l e c t r o r e f l e c t a n c e m e a s u r e m e n t s [7]. In this p a p e r we r e p o r t on r e m a r k a b l e drastic c h a n g e s in the R a m a n spectra which have to be a t t r i b u t e d to the p h o t o g e n e r a t i o n of free carriers n e a r the Z n S e / G a A s interface. C a r r i e r concentrations h i g h e r t h a n 10 ~8 cm -3 were achieved in the G a A s s u b s t r a t e s at room t e m p e r a t u r e a n d C W laser powers in the o r d e r of milliwatts. Coupled p l a s m o n - l o n g i t u d i n a l o p t i c a l - p h o n o n (PLP) m o d e s associated with such high c a r r i e r densities w e r e observed. This is only possible if the interfacial defect density in comparison to the uncov-
0169-4332/92/$05.01) © 1992 - Elsevier Science Publishers B.V. All rights reserved
.4. Krost ¢t aL / Photot~Otcd pha.sm~m-LO.phonon tmnh'~ at tie" ZnSe / G(uts
LOGot,~ L O ~ ~ o~~w
cred GaAs surfitce is ~tiffi,.icntly lowered m order to lower the recombination velocity for the electron-hole pairs at the interface drastically.
2. Experimental ZnSe epitaxial layers of typical thicknes~:es around I p.m were grown on GaAs(lll(?~) substrates by MOVPE (Jenscn, MIT Boston), H W E (Hinged, Univ. Linz), and MBE (Cairns, U W C Cardiff). The M O V P E and H W E substra~es were semi-insulaUng, while the MBE substrates were highly doped In-type > lip '~ cm-3). The interface of the H W E samples grown at substratc temperatures of 4(10°C was supposed to be ot minor quality because of the highly probable formation of Ga_,Sc~ during the growth process [6]. All Raman measurements were performed at room temperature with '~he samples under the microscope stage of a Dilor triple monochromator equipped with multiehannel detection. The power of the exciting laser line (mainly A = 514.5 nm for which the ZnSc is transparent) was varied over more than two orders of magnitude, i.e. from 0.02 to 5 m W as measured at the sample position. The focusing microscope lens had a magnification of 10. The focal size was estimated slightly less than I p.m in diameter. This yields power densities in the range from 2 kW/cm-" to 1).5 M W / c m 2. In the backscattering geometry used the incident light was polarised alon~ the [0111] direction of the sample, while the scattcred light was analysed along [001].
3. Results and discussion Fig. 1 shows a typical series of R a m a n spectra for ZnSe on semi-insuh:~ting G a A s substrates taken with different laser powers. At the lowest power of 0.02 m W two distinct peaks are clearly visible. They correspond t,:) scattering by the longitudinal optical (LO) ph~mons of ZnSe at 253 c m - t and G a A s at 292 c m - ' . Consequently the symmetry selection rules for (100) surfaces are strictly obeyed.
~
osmw
t~o
2;o
3;o
~so
wavenumbers (tin -~) Fig. I. Raman spectra of ZnSe/GaAs grown by MOVPE on a semi-insulating (si) substrate. The spectra are taken with variot,s laser powers and normaliscd to the LOznsc intensitie With increasing light intensity the LOc~;,a, scattering vanishes and a new mode anpears which is attribated to a light generated coupled plasmon-LO-phonon mode ( ( l ) of the GaAs. The effect is completely reversible.
The ZnSe LO scattering intensity increases proportionally to the incident laser power. Therefore, after normalisation to the laser power the ZnSe LO peak is of equal height in all the spectra shown in fig. 1. Dramatic changes are then still be seen in the G a A s features when the laser power is raised by two orders of magnitude. The G a A s LO feature decreases in intensity and broadens. At the highest incident power there is hardly any remainder of this feature. Simultaneously a new peak of increasing intensity appears between the two LO phonon peaks at almost the position of the G a A s T O phonon. Due to the overlap of the features the exact frequency position of this peak is difficult to analyse. it should be stated that this behaviour was found to be reversible and no damage of the sample was observed even though the highest power density was close to 0.5 M W / c m z. Such sensitive dependence of the Raman spectra on the incident light power was observed in eight
A. Kro.(t et aL / Photoexcited phlsmon-l.O-phonon modes at the ZnSe / GaAs
different samples independent of the growth technique. However, we will show later that the individual growth conditions can severely affect the magnitude of this behaviour. The appearance of the scattering near the G a A s T O phonon combined with the disappearance of the GaAs LO phonon implies strongly that the creation of photoexcited carriers in the semi-insulating substrate is reponsible for the observed changes and coupled p l a s m o n - L O phonon (PLP) modes arc formed. The PLP coupling occurs through the macroscopic electric fields of the LO phonon and the collective plasmon excitations of free card, crs. This coupling results in two new modes of the L O - p h o n o n - p l a s m o n system, denoted by (! + and ~ - . Their frequencies and lineshapcs dcpcr, d er~ the plasma frequency to~,=ne'-/e0m* and plasmon damping t o , = 1/~', respectively. The f t mode with increasing carrier concentration approaches the T O phonon frequency (screened LO phonon). For wavevectors around 10 ~' c m - I and carrier concentrations above 10 TM cm -3 the frequency of this mode in G a A s practically coincides with the T O phonon frequency. The scattering intensity appearing near the G a A s T O phonon position with the higher light powers in the Ram a n spectra of fig. I is thus assigned to scattering by a photoinduced f~- mode. At such carrier concentrations and wavevectors the ft ÷ mode frequency is dominateo by the plasmon contribution and appears separated from the phonon peaks at much higher frequencies ( > 400 cm-~ for GaAs). No evidence of this mode, however, was found in the spectra of fig. 1. This is in line with previous studies [8-12] which also showed that the photoexcited l~ + :~ difficult to detect in GaAs. For some of the Z n S e / G a A s samples, however, even scattering by the fl ~ mode is recognisable in the spectra as demonstrated in fig. 2. U n d e r low power excitation (0.1 m W ) the spectrum is essentially the same as the low power ones in fig. I. The G a A s and ZnSe LO features are again dominating. When raising the incident light power to l m W the LO GaAs intensity decreases as in fig. 1 and now both PLP modes are visible: the f l - mode again close to the T O frequency, anti a new broad band centered around 500 cm - t . This new band "~ at-
w
I ~"
~mw
/\
160
?,20
t,80
6t,O
wovenumbers (cm"I ) Fig. 2. Raman spectra of ZnSe/si-GaAs. Upon higher excitation (1 m',q) both light generated coupled PLP modes lland it" are visible.
tributed to the f~+ mode. Its frequency can be utilized to evaluate the free ca, tier concentration. The R a m a n cross se~i.io,a for that purpose is described by I m [ - l / e ( t o , q)], an approximation valid for fl+>>toLO, toTO" For e(to, q) the Mermin extension of the Lindhard dielectric function evaluated for non-parabolic conduction bands was used [13]. The hole contribution to the dielectric function was neglected, since the effective mass entering the two component plasma frequency within a few percent equals the electron effective mass. In such a way a carrier concentration of 1.3 × 10 TM cm -~ was obtained from the fl + mode frequency in fig. 2. The fact. that the observation of the f~+ mode depends o n the individual sample can be explained in t~rms of quite different recombination velocities at the interface of GaAs. It has already been shown that the interface recombination velocity can be drastically reduced by three orders of magnitude when G a A s is covered with an AIGaAs epitaxial layer for which the interlace is known to be highly perfect [9]. Even in this case the photocreated EHP appeared to be highly inhomogeneous, and this affects, in particular, the 1"~+ mode which is then further broadened and hardly observable [10]. The fact that the fl + mode is fairly weak and broad ( A v = 100 cm -~) is explained either by
A. Krost et aL / Phottn,x,'ited pht~mon-LO.phonon modes at the ZnSe / (ia,4s
smw
{
.~_
,
200
300
t.~
500 600
wotvenumbers
h:rn"~)
Fig. 3. Rar0an spectra of ZoSe/GaAs grown by MBE on highly n.doped GaAs. Due to the high intrinsic carrier concentration both coupled PI.P modes are present at relatively low laser p'3~'er. Upon higher excitation the f~ mode increases whereas the t~ + m~te shifts to higher frequencies and broadens.
carrier damping or by the broadenir, g via spatial inhomogene*ty. Tills can be caused by tile Gaussfan profile of the laser beam or the exponential absorption in the GaAs. However, the measurement of a it + mode caused by a dense photoexcited EHP at the interface of z ! I - V I / l l l - V heterosystem is most remarkable, especially, considering that these results were obtained at 300 K and using continuous wave excitation. For comparison R a m a n spectra were taken of a sample based on a highly n-doped MBE grown G a A s substrate with subsequent deposition of a ZnSe layer by MBE without breaking the vacuum ensuring optimal conditions for the formation of the interface. In this case first of all we were able to observe the f l - and •÷ modes stemming from the high built-in carrier concentration as can be seen in the lower spectrum of fig. 3. The spectrum is then dominated by the strong f~peak at the G a A s T O frequency and the fl ÷ mode, now clearly visible as a broad band centered around 555 c m - ' . This corresponds to approximately n = 2 . 7 x 10 I~ cm -3 carriers introduced by n-type doping. Neve.~ Theless, also in this case of a highly n-doped (3aAs substrate it is possible to create sufficient additional photoex-
cited carriers which show up in the Raman spectrum by changing the lineshape and the frequency of the coupled modes as shown in the upper spectrum in fig 3. While the f l - mode increases in intensity the [1 + mode shifts to 610 cm - i corresponding to a carrier concentration of 3.1 × Ill 18 cm -3. Furthermore, a decrease in scattering intensity as well as an asymmetric lineshape is observed, which is attributed to spatial inhomogeneities. The spatial inhomogeneity of the free carrier plasma strongly depends on the recombination velocity at the interface, i.e. the lower the recombination velocity, the better is the homogeneity and the larger the free carrier concentration. Thus it may be concluded from our R a m a n data that the recombination velocity at the Z n S e / G a A s samples studied is extremely low. in face by comparison with a similar experiment performed on a AIGaAs c(wered G a A s sample [9] the recombination velocity at the Z n S e / G a A s interface attains obviously much smaller values. The magn;tude of the recombination velocity is determineo by the perfection of the interface wi,ieh itself dt:peltds on e.g. interfacial abruptness, defect density, interdiffusion, and surface treatment prior to ZnSe growth. All these possi',~{ifies may lead to the presence of interface stateg which act as recombination centers. T h u s the occurrence of the PLP modes reflects a high interface quality, which apparently was ensured by the MBE growth process described above. O n the other hand, if the interface is of minor quality, the effect of photoexcitation .3f carriers should be less significant. The existence of Ga2Se 3 islands at the Z n S e / G a A s interface was shown recently on samples grown by H W E at relatively high temperatures [6]. Consequently the intelfaces are expected to exhibit a high recombination velocity. Fig. 4 shows the R a m a n spectra of such a H W E sample grown on semi-insulating G a A s for a series of different incident laser powers. By comparing with fig. I it can easily be seen that although the light powers were similar the change in the spectra is much less pronounced. While a slight decrease in intensity and broadening of the G a A s LO phonon peak is visible thele is no detectable change of the scattering intensity at the G a A s T O phonon position. This behaviour
A. Krost et aL / Photo¢~tcited plasmmz -LO.plumon modes at the ZnSe / Ga~ls
LOz.~= LO~A~,
iso
2so 3~o ~so wavenurnbers (crn-I) Fig. 4. Same measurements as in fig. 1 tnr a ttWE grown sample. The non-appearance of PLP modes is attributed to a disturbed interface, probably due to Ga,Se 3 islands.
is c o n s i s t e n t with our a s s u m p t i o n that the enh a n c e d r e c o m b i n a t i o n velocity c a u s e d by the minor interface quality manifests itself in a low c o n c e n t r a t i o n and spatial i n h o m o g e n e o u s free c a r r i e r p l a s m a which Iced only to insignificant modifications of the R a m a n spectra. Only in case of high quality interfaces the PLP m o d e s show up w h e n the incident light p o w e r c r e a t e s a sufficient and spatial h o m o g e n e o u s n u m b e r of e l e c t r o n hole pairs.
4. Summary in this p a p e r we have shown that p l a s m o n L O - p h o n o n m o d e s can be g e n e r a t e d by photoexcitation in the G a A s s u b s t r a t e of Z n S e / G a A s h c t e r o s t r u c t u r e s . T h e i r p r e s e n c e was d e t e c t e d by a micro R a m a n e x p e r i m e n t at r o o m t e m p e r a t u r e and c o n t i n u o u s wave excitation. The o c c u r r e n c e and s t r e n g t h of p l a s m o n - L O - p h o n o n m o d e s upon i l l u m i n a t i o n d e p e n d s on the interface r e c o m b i n a tion velocity which in turn is g o v e r n e d by the interface defect density of the individual s a m p l e and c o n s e q u e n t l y its growth condition. T h e effect is found to be s t r o n g e r t h a n in any o t h e r photoexcitation e x p e r i m e n t so far p e r f o r m e d on G a A s
695
even w h e n the G a A s was covered with A I G a A s , which also substantially reduces the interface rec o m b i n a t i o n velocity. This u n d e r l i n e s the important role of Z n S e as a passivatio.n l.'=.ycr for G a A s [14]. T h e r e f o r e , R a m a n scattering by PLP m o d e s in such h e t e r o s t r u c t u r e s is s u g g e s t e d to be a suitable tool for interface c h a r a c t e r i s a t i o n and is possibly applicable to o t h e r heterosystems. Moreover, these R a m a n spectra r e p o r t e d here d e m o n s t r a t e that even low power optical experim e n t s at high quality interfaces have to be interp r e t e d and controlled quite carefully since the p h o t o n fl'ux present, might strongly modify the p r o p e r t i e s in the interface region.
Acknowledgement W e gratefully a c k n o w l e d g e K.F. Jensen, J. C a i r n s and K. Hingerl for providing the s a m p l e s for this study.
References [1] See publicatiop; in: Proe. 4th Int. Conf. on II-VI Compounds. Bedim 1989.J. Cryst. Growth 101 (199(1);and in: Proc. E-.~ARS Advanced Re:;eareh Workshop on Wide Gap II-VI Semiconduc~ols. Montpellier. 1991. Semicond. Sci. Technol.. to be published. [2] J. Qiu. Q.D. Qian, M. Kobayashi, R.L. Gunshor, D.R. Menke. D. Li and N. Otsuka, .I. Vac. Sci. Technol. B 8 (1991)) 7al: J.O. Williams and W. Gebhardt. private commumcations. 13] D.W. Kisker, P.H. Fuoss, S. Brennan. G. Renaud. K.L Tokuda and J.L Kahn. J. Cryst. Growth 101 (199a) 42. [4] D.J. Olego and D. Cammack. J. Cryst. Growth I01 (1990) 546. [5] O. Pages. M. Renucci, O. Briot, N. Tempier and R.L Aulombard. ]. Cryst. Growth 1(17(1991) 670. [6] A. Krost. W. Richter. D.R.T. Zahn. K. Hinged and H. Sitter, AppL Phys. Left, 57 (19911) 1981. [7] L. Kassel. J.W. Garland, P.M. Raecah, M.A. Haase and H. Cheng, in: Proc. E-MRS Advanced Research Workshop on Wide Gap II-VI Semiconductors. Montpellier. 1991. Semicond. Set. Technol., to be published. [8] T. Nakamura and T. Katoda, J. Appl. Phys. 55 (1984) 3(164. [9] J.F. Young, K. Wan, A.J. SpringThorpe and P. Mandevtile, Phys. Rev. B 36 (1987) 1316. [10] J.C. Tsang. J.A. Kash and S.S. Jha, Physica B 143 (1985) 184.
696
.4. KreJ.slel aL / Pll¢~lt~.?;cited i~lastmm-I.O-ph~m~m m¢ah,s , t the ZtlSe / GaA.~
II I] C.L. Collins and P.Y. Yu, S,~lid State C,,)mmun. 51 (1984) 123. [12] II. Nath~:r and L.G. Ouaglian..~. J. Lumin. 31) (P.~85) 51). [13[ W. Richter. U. N~,,,,,ak. il. Jhrgen~.,en and U. R6ssler, Solid State Commun. 67 (1~88) 1~)9.
[14] J. Oiu, Q.-D. Oian, M. Kobayashi, iLL. Gunsh,~)r, D.R. Menge. D. Li and N. Otsuka, J. Vac. Sci Technol. B :.l (I~'.}l)) 701.
Applied Surface Science 56-58 (1992) 691-696 North-Holland
surface s c i e n c e
Photoexcited plasmon-LO-phonon modes at the ZnSe/GaAs interface A. Krost, W. Richter and D.R.T. Zahn bzstitut fiir FestkiJrperphysik der TU Berlin. Sekr. PAl 6-1, ttardenbergstrasse 36. D.IO00 Berlin 12. Germany
Received 6 May 1991; accepted for publication 9 May 199!
ZnSe epitaxial layers grown on GaAs(10tl) substrates were studied by Raman spectroscopy. The intensity of the incident light was varied over three orders of magnitude in the range from about 1 kW/cm-' to 0.5 MW/cm-'. The spectra recorded at low laser power exhibit the pure phonon features of GaAs and ZnSe. i.e. only scattering by the longitudinal optical (LO) phonons is observed. While the lineshape of the ZnSe LO phonon peak remains constant upon increasing laser power, drastic changes can be seen in the GaAs Raman featulcs. These changes depend only on the laser power and are found to be completely reversible. The relative GaAs LO intensity decreases and simultaneously a new peak appears close to the GaAs TO phonon position. This behaviour is explained in terms of scattering by photogenerated coupled plasmon-LO-phonon (PLP) modes. Free carrier concentrations above I(1Is cm-3 are deduced from the PLP mode frequencies. At t~ : highest laser powers used the spectra are completely dominated by the Pt,P modes. The long carrier lifetime neccessary for the generation of such high concentrations suggests that the ZnSe/GaAs interface has to be of high structural quality leading to low recombination velocities. Indeed ZnSe/GaAs hetemstructures of lower quality showed the effect of photogenerated carriers only to a lesser extent. Thus Raman scattering by the photoexcited free carriers is likely to serve as an excellent new tool for interface quality assessmenl.
I. Introduction T h e g r o w t h a n d quality of Z n S e epitaxial layers o n G a A s s u b s t r a t e s using various m e t h o d s , such as m o l e c u l a r b e a m epitaxy ( M B E ) , m e t a l organic v a p o u r p h a s e epitaxy ( M O V P E ) , a n d hot wall epitaxy ( H W E ) has b e e n substantially improved in r e c e n t years [1]. W h i l e the p r o p e r t i e s of the epitaxial Z n S e film itself are well c o n t r o l l a b l e now, the interface region b e t w e e n the Z n S e epilayer and the G a A s s u b s t r a t e is not yet fully u n d e r s t o o d . Besides high resolution transmission e l e c t r o n microscopy ( H R T E M ) [2] a n d in situ grazing incidence X-ray s c a t t e r i n g [3] mainly Ram a n spectroscopy was a p p l i e d for interface chara c t e r i s a t i o n [4,5]. W h i l e the H R T E M results of t h r e e different g r o u p s [2] giv,: indirect evidence for the f o r m a t i o n of G a 2 S e 3 islands at the interface, m o r e direct p r o o f was provided recently by
R a m a n spectroscopy [6]. It was also claimed from R a m a n results that the o p t o e l e c t r o n i c p r o p e r t i e s of interfaces are affected by intrinsic instabilities which are assigned to interfacial a t o m i c processes a n d lead to a n a g i n g of the s a m p l e s [4]. lnterfacial diffusion of Z n in G a A s and G a in Z n S e has b e e n d e d u c e d from electrolyte e l e c t r o r e f l e c t a n c e m e a s u r e m e n t s [7]. In this p a p e r we r e p o r t on r e m a r k a b l e drastic c h a n g e s in the R a m a n spectra which have to be a t t r i b u t e d to the p h o t o g e n e r a t i o n of free carriers n e a r the Z n S e / G a A s interface. C a r r i e r concentrations h i g h e r t h a n 10 ~8 cm -3 were achieved in the G a A s s u b s t r a t e s at room t e m p e r a t u r e a n d C W laser powers in the o r d e r of milliwatts. Coupled p l a s m o n - l o n g i t u d i n a l o p t i c a l - p h o n o n (PLP) m o d e s associated with such high c a r r i e r densities w e r e observed. This is only possible if the interfacial defect density in comparison to the uncov-
0169-4332/92/$05.01) © 1992 - Elsevier Science Publishers B.V. All rights reserved
.4. Krost ¢t aL / Photot~Otcd pha.sm~m-LO.phonon tmnh'~ at tie" ZnSe / G(uts
LOGot,~ L O ~ ~ o~~w
cred GaAs surfitce is ~tiffi,.icntly lowered m order to lower the recombination velocity for the electron-hole pairs at the interface drastically.
2. Experimental ZnSe epitaxial layers of typical thicknes~:es around I p.m were grown on GaAs(lll(?~) substrates by MOVPE (Jenscn, MIT Boston), H W E (Hinged, Univ. Linz), and MBE (Cairns, U W C Cardiff). The M O V P E and H W E substra~es were semi-insulaUng, while the MBE substrates were highly doped In-type > lip '~ cm-3). The interface of the H W E samples grown at substratc temperatures of 4(10°C was supposed to be ot minor quality because of the highly probable formation of Ga_,Sc~ during the growth process [6]. All Raman measurements were performed at room temperature with '~he samples under the microscope stage of a Dilor triple monochromator equipped with multiehannel detection. The power of the exciting laser line (mainly A = 514.5 nm for which the ZnSc is transparent) was varied over more than two orders of magnitude, i.e. from 0.02 to 5 m W as measured at the sample position. The focusing microscope lens had a magnification of 10. The focal size was estimated slightly less than I p.m in diameter. This yields power densities in the range from 2 kW/cm-" to 1).5 M W / c m 2. In the backscattering geometry used the incident light was polarised alon~ the [0111] direction of the sample, while the scattcred light was analysed along [001].
3. Results and discussion Fig. 1 shows a typical series of R a m a n spectra for ZnSe on semi-insuh:~ting G a A s substrates taken with different laser powers. At the lowest power of 0.02 m W two distinct peaks are clearly visible. They correspond t,:) scattering by the longitudinal optical (LO) ph~mons of ZnSe at 253 c m - t and G a A s at 292 c m - ' . Consequently the symmetry selection rules for (100) surfaces are strictly obeyed.
~
osmw
t~o
2;o
3;o
~so
wavenumbers (tin -~) Fig. I. Raman spectra of ZnSe/GaAs grown by MOVPE on a semi-insulating (si) substrate. The spectra are taken with variot,s laser powers and normaliscd to the LOznsc intensitie With increasing light intensity the LOc~;,a, scattering vanishes and a new mode anpears which is attribated to a light generated coupled plasmon-LO-phonon mode ( ( l ) of the GaAs. The effect is completely reversible.
The ZnSe LO scattering intensity increases proportionally to the incident laser power. Therefore, after normalisation to the laser power the ZnSe LO peak is of equal height in all the spectra shown in fig. 1. Dramatic changes are then still be seen in the G a A s features when the laser power is raised by two orders of magnitude. The G a A s LO feature decreases in intensity and broadens. At the highest incident power there is hardly any remainder of this feature. Simultaneously a new peak of increasing intensity appears between the two LO phonon peaks at almost the position of the G a A s T O phonon. Due to the overlap of the features the exact frequency position of this peak is difficult to analyse. it should be stated that this behaviour was found to be reversible and no damage of the sample was observed even though the highest power density was close to 0.5 M W / c m z. Such sensitive dependence of the Raman spectra on the incident light power was observed in eight
A. Kro.(t et aL / Photoexcited phlsmon-l.O-phonon modes at the ZnSe / GaAs
different samples independent of the growth technique. However, we will show later that the individual growth conditions can severely affect the magnitude of this behaviour. The appearance of the scattering near the G a A s T O phonon combined with the disappearance of the GaAs LO phonon implies strongly that the creation of photoexcited carriers in the semi-insulating substrate is reponsible for the observed changes and coupled p l a s m o n - L O phonon (PLP) modes arc formed. The PLP coupling occurs through the macroscopic electric fields of the LO phonon and the collective plasmon excitations of free card, crs. This coupling results in two new modes of the L O - p h o n o n - p l a s m o n system, denoted by (! + and ~ - . Their frequencies and lineshapcs dcpcr, d er~ the plasma frequency to~,=ne'-/e0m* and plasmon damping t o , = 1/~', respectively. The f t mode with increasing carrier concentration approaches the T O phonon frequency (screened LO phonon). For wavevectors around 10 ~' c m - I and carrier concentrations above 10 TM cm -3 the frequency of this mode in G a A s practically coincides with the T O phonon frequency. The scattering intensity appearing near the G a A s T O phonon position with the higher light powers in the Ram a n spectra of fig. I is thus assigned to scattering by a photoinduced f~- mode. At such carrier concentrations and wavevectors the ft ÷ mode frequency is dominateo by the plasmon contribution and appears separated from the phonon peaks at much higher frequencies ( > 400 cm-~ for GaAs). No evidence of this mode, however, was found in the spectra of fig. 1. This is in line with previous studies [8-12] which also showed that the photoexcited l~ + :~ difficult to detect in GaAs. For some of the Z n S e / G a A s samples, however, even scattering by the fl ~ mode is recognisable in the spectra as demonstrated in fig. 2. U n d e r low power excitation (0.1 m W ) the spectrum is essentially the same as the low power ones in fig. I. The G a A s and ZnSe LO features are again dominating. When raising the incident light power to l m W the LO GaAs intensity decreases as in fig. 1 and now both PLP modes are visible: the f l - mode again close to the T O frequency, anti a new broad band centered around 500 cm - t . This new band "~ at-
w
I ~"
~mw
/\
160
?,20
t,80
6t,O
wovenumbers (cm"I ) Fig. 2. Raman spectra of ZnSe/si-GaAs. Upon higher excitation (1 m',q) both light generated coupled PLP modes lland it" are visible.
tributed to the f~+ mode. Its frequency can be utilized to evaluate the free ca, tier concentration. The R a m a n cross se~i.io,a for that purpose is described by I m [ - l / e ( t o , q)], an approximation valid for fl+>>toLO, toTO" For e(to, q) the Mermin extension of the Lindhard dielectric function evaluated for non-parabolic conduction bands was used [13]. The hole contribution to the dielectric function was neglected, since the effective mass entering the two component plasma frequency within a few percent equals the electron effective mass. In such a way a carrier concentration of 1.3 × 10 TM cm -~ was obtained from the fl + mode frequency in fig. 2. The fact. that the observation of the f~+ mode depends o n the individual sample can be explained in t~rms of quite different recombination velocities at the interface of GaAs. It has already been shown that the interface recombination velocity can be drastically reduced by three orders of magnitude when G a A s is covered with an AIGaAs epitaxial layer for which the interlace is known to be highly perfect [9]. Even in this case the photocreated EHP appeared to be highly inhomogeneous, and this affects, in particular, the 1"~+ mode which is then further broadened and hardly observable [10]. The fact that the fl + mode is fairly weak and broad ( A v = 100 cm -~) is explained either by
A. Krost et aL / Phottn,x,'ited pht~mon-LO.phonon modes at the ZnSe / (ia,4s
smw
{
.~_
,
200
300
t.~
500 600
wotvenumbers
h:rn"~)
Fig. 3. Rar0an spectra of ZoSe/GaAs grown by MBE on highly n.doped GaAs. Due to the high intrinsic carrier concentration both coupled PI.P modes are present at relatively low laser p'3~'er. Upon higher excitation the f~ mode increases whereas the t~ + m~te shifts to higher frequencies and broadens.
carrier damping or by the broadenir, g via spatial inhomogene*ty. Tills can be caused by tile Gaussfan profile of the laser beam or the exponential absorption in the GaAs. However, the measurement of a it + mode caused by a dense photoexcited EHP at the interface of z ! I - V I / l l l - V heterosystem is most remarkable, especially, considering that these results were obtained at 300 K and using continuous wave excitation. For comparison R a m a n spectra were taken of a sample based on a highly n-doped MBE grown G a A s substrate with subsequent deposition of a ZnSe layer by MBE without breaking the vacuum ensuring optimal conditions for the formation of the interface. In this case first of all we were able to observe the f l - and •÷ modes stemming from the high built-in carrier concentration as can be seen in the lower spectrum of fig. 3. The spectrum is then dominated by the strong f~peak at the G a A s T O frequency and the fl ÷ mode, now clearly visible as a broad band centered around 555 c m - ' . This corresponds to approximately n = 2 . 7 x 10 I~ cm -3 carriers introduced by n-type doping. Neve.~ Theless, also in this case of a highly n-doped (3aAs substrate it is possible to create sufficient additional photoex-
cited carriers which show up in the Raman spectrum by changing the lineshape and the frequency of the coupled modes as shown in the upper spectrum in fig 3. While the f l - mode increases in intensity the [1 + mode shifts to 610 cm - i corresponding to a carrier concentration of 3.1 × Ill 18 cm -3. Furthermore, a decrease in scattering intensity as well as an asymmetric lineshape is observed, which is attributed to spatial inhomogeneities. The spatial inhomogeneity of the free carrier plasma strongly depends on the recombination velocity at the interface, i.e. the lower the recombination velocity, the better is the homogeneity and the larger the free carrier concentration. Thus it may be concluded from our R a m a n data that the recombination velocity at the Z n S e / G a A s samples studied is extremely low. in face by comparison with a similar experiment performed on a AIGaAs c(wered G a A s sample [9] the recombination velocity at the Z n S e / G a A s interface attains obviously much smaller values. The magn;tude of the recombination velocity is determineo by the perfection of the interface wi,ieh itself dt:peltds on e.g. interfacial abruptness, defect density, interdiffusion, and surface treatment prior to ZnSe growth. All these possi',~{ifies may lead to the presence of interface stateg which act as recombination centers. T h u s the occurrence of the PLP modes reflects a high interface quality, which apparently was ensured by the MBE growth process described above. O n the other hand, if the interface is of minor quality, the effect of photoexcitation .3f carriers should be less significant. The existence of Ga2Se 3 islands at the Z n S e / G a A s interface was shown recently on samples grown by H W E at relatively high temperatures [6]. Consequently the intelfaces are expected to exhibit a high recombination velocity. Fig. 4 shows the R a m a n spectra of such a H W E sample grown on semi-insulating G a A s for a series of different incident laser powers. By comparing with fig. I it can easily be seen that although the light powers were similar the change in the spectra is much less pronounced. While a slight decrease in intensity and broadening of the G a A s LO phonon peak is visible thele is no detectable change of the scattering intensity at the G a A s T O phonon position. This behaviour
A. Krost et aL / Photo¢~tcited plasmmz -LO.plumon modes at the ZnSe / Ga~ls
LOz.~= LO~A~,
iso
2so 3~o ~so wavenurnbers (crn-I) Fig. 4. Same measurements as in fig. 1 tnr a ttWE grown sample. The non-appearance of PLP modes is attributed to a disturbed interface, probably due to Ga,Se 3 islands.
is c o n s i s t e n t with our a s s u m p t i o n that the enh a n c e d r e c o m b i n a t i o n velocity c a u s e d by the minor interface quality manifests itself in a low c o n c e n t r a t i o n and spatial i n h o m o g e n e o u s free c a r r i e r p l a s m a which Iced only to insignificant modifications of the R a m a n spectra. Only in case of high quality interfaces the PLP m o d e s show up w h e n the incident light p o w e r c r e a t e s a sufficient and spatial h o m o g e n e o u s n u m b e r of e l e c t r o n hole pairs.
4. Summary in this p a p e r we have shown that p l a s m o n L O - p h o n o n m o d e s can be g e n e r a t e d by photoexcitation in the G a A s s u b s t r a t e of Z n S e / G a A s h c t e r o s t r u c t u r e s . T h e i r p r e s e n c e was d e t e c t e d by a micro R a m a n e x p e r i m e n t at r o o m t e m p e r a t u r e and c o n t i n u o u s wave excitation. The o c c u r r e n c e and s t r e n g t h of p l a s m o n - L O - p h o n o n m o d e s upon i l l u m i n a t i o n d e p e n d s on the interface r e c o m b i n a tion velocity which in turn is g o v e r n e d by the interface defect density of the individual s a m p l e and c o n s e q u e n t l y its growth condition. T h e effect is found to be s t r o n g e r t h a n in any o t h e r photoexcitation e x p e r i m e n t so far p e r f o r m e d on G a A s
695
even w h e n the G a A s was covered with A I G a A s , which also substantially reduces the interface rec o m b i n a t i o n velocity. This u n d e r l i n e s the important role of Z n S e as a passivatio.n l.'=.ycr for G a A s [14]. T h e r e f o r e , R a m a n scattering by PLP m o d e s in such h e t e r o s t r u c t u r e s is s u g g e s t e d to be a suitable tool for interface c h a r a c t e r i s a t i o n and is possibly applicable to o t h e r heterosystems. Moreover, these R a m a n spectra r e p o r t e d here d e m o n s t r a t e that even low power optical experim e n t s at high quality interfaces have to be interp r e t e d and controlled quite carefully since the p h o t o n fl'ux present, might strongly modify the p r o p e r t i e s in the interface region.
Acknowledgement W e gratefully a c k n o w l e d g e K.F. Jensen, J. C a i r n s and K. Hingerl for providing the s a m p l e s for this study.
References [1] See publicatiop; in: Proe. 4th Int. Conf. on II-VI Compounds. Bedim 1989.J. Cryst. Growth 101 (199(1);and in: Proc. E-.~ARS Advanced Re:;eareh Workshop on Wide Gap II-VI Semiconduc~ols. Montpellier. 1991. Semicond. Sci. Technol.. to be published. [2] J. Qiu. Q.D. Qian, M. Kobayashi, R.L. Gunshor, D.R. Menke. D. Li and N. Otsuka, .I. Vac. Sci. Technol. B 8 (1991)) 7al: J.O. Williams and W. Gebhardt. private commumcations. 13] D.W. Kisker, P.H. Fuoss, S. Brennan. G. Renaud. K.L Tokuda and J.L Kahn. J. Cryst. Growth 101 (199a) 42. [4] D.J. Olego and D. Cammack. J. Cryst. Growth I01 (1990) 546. [5] O. Pages. M. Renucci, O. Briot, N. Tempier and R.L Aulombard. ]. Cryst. Growth 1(17(1991) 670. [6] A. Krost. W. Richter. D.R.T. Zahn. K. Hinged and H. Sitter, AppL Phys. Left, 57 (19911) 1981. [7] L. Kassel. J.W. Garland, P.M. Raecah, M.A. Haase and H. Cheng, in: Proc. E-MRS Advanced Research Workshop on Wide Gap II-VI Semiconductors. Montpellier. 1991. Semicond. Set. Technol., to be published. [8] T. Nakamura and T. Katoda, J. Appl. Phys. 55 (1984) 3(164. [9] J.F. Young, K. Wan, A.J. SpringThorpe and P. Mandevtile, Phys. Rev. B 36 (1987) 1316. [10] J.C. Tsang. J.A. Kash and S.S. Jha, Physica B 143 (1985) 184.
696
.4. KreJ.slel aL / Pll¢~lt~.?;cited i~lastmm-I.O-ph~m~m m¢ah,s , t the ZtlSe / GaA.~
II I] C.L. Collins and P.Y. Yu, S,~lid State C,,)mmun. 51 (1984) 123. [12] II. Nath~:r and L.G. Ouaglian..~. J. Lumin. 31) (P.~85) 51). [13[ W. Richter. U. N~,,,,,ak. il. Jhrgen~.,en and U. R6ssler, Solid State Commun. 67 (1~88) 1~)9.
[14] J. Oiu, Q.-D. Oian, M. Kobayashi, iLL. Gunsh,~)r, D.R. Menge. D. Li and N. Otsuka, J. Vac. Sci Technol. B :.l (I~'.}l)) 701.