- Email: [email protected]
ZnSe quantum wires
PERGAMON
Solid State Communications 114 (2000) 435–440 www.elsevier.com/locate/ssc
Length dependence of the longitudinal optical phonon properties in CdZnSe/ZnSe quantum wires B. Schreder a, A. Materny a, W. Kiefer a,*, T. Ku¨mmell b, G. Bacher b, A. Forchel b, G. Landwehr b a
Institut fu¨r Physikalische Chemie der Universita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany b Physikalisches Institut der Universita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany
Received 7 January 2000; received in revised form 7 February 2000; accepted 17 February 2000 by J. Kuhl
Abstract This paper presents the results of Raman spectroscopic investigations on etched CdxZn1⫺xSe/ZnSe quantum wires with different wire lengths and wire widths. The recorded spectra show intensive signals in the region of the longitudinal optical phonons (LO) of the ZnSe barrier and the CdZnSe wire as well as several overtones. A strong outgoing resonance can be observed when the overtone position matches the luminescence peak position. The asymmetric LO signal can be separated into three distinct peaks: two higher wavenumber peaks, which are observed especially for the long and thick wires for short excitation wavelengths, are attributed to the LO phonon modes of the ZnSe barrier material and the strained CdZnSe wire; a shoulder at the low wavenumber region, which appears in the spectra of the short and thin wires, is attributed to an LO phonon excited in the strain relaxed wire edges. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; D. Phonons; E. Inelastic light scattering
Low dimensional II–VI semiconductors like quantum wires and quantum dots have attracted much attention over the last several years. This is due to the lateral confinement of charge carriers and phonons in the semiconductor structure. This confinement effect causes the appearance of new physical phenomena, which strongly depend on the size of the wires and dots, for example a blue-shift of the bandgap [1,2]. One of the most interesting systems for the investigation of basic physical properties within the group of the II–VIsemiconductor nanostructures are the CdZnSe/ZnSe quantum wires and dots. For these systems strain effects have to be taken into consideration because of the lattice mismatch between the ZnSe barrier and the CdZnSe well [3]. This biaxial strain causes a blueshift of the wire longitudinal optical phonon (LO) frequency, whereas the LO phonon of the ZnSe barrier shows no shift because of the high fraction of unstrained material [4]. The etching process leads to * Corresponding author. Tel.: ⫹49-931-888-6330; fax: ⫹49-931888-6332. E-mail address: [email protected] (W. Kiefer).
a strain relaxation on the edges of the wires. Lermann et al. [5–7] have shown, that the combined signal of the strained wire centre and the relaxed edges of the wires show a redshift with decreasing wire width due to the increase of the relaxed fraction to the strained fraction. For wires having widths of 30 nm and less no further shift could be observed. Hence, they concluded that these wires should be nearly strain relaxed [4]. In this paper, we present a Raman spectroscopic investigation of CdZnSe quantum wires with Cd contents of 20% (A) and 35% (B) and with different wire lengths (L) and widths (W). The Raman spectra of CdxZn1⫺xSe show one LO phonon mode for the ZnSe-like phonon, whose wavenumber shifts depending on the Cd content from 256 x 0 to 210 cm ⫺1 x 1: The expected LO phonon position for the unstrained wires is about 251 cm ⫺1 for 20% Cd content and 247 cm ⫺1 for 35% Cd content [8]. As a consequence of the small cadmium concentration of the CdZnSe layer of sample A, no separation between the LO phonon of the ZnSe barrier layer and the strained phonon of the quantum well layer can be observed. For the strain relaxed LO phonon of the short and narrow wires, the separation is more clearly at an excitation wavelength of 458 nm,
0038-1098/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00077-6
436
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
Fig. 1. (a) Emission spectra (Raman and luminescence) of two CdZnSe wires, (18/1D) (I), (27/50) (II), recorded at excitation wavelengths 465 (I) and 458 nm (II) at a temperature of 10 K. (b) An enlargement of the spectra given in (a) by a factor of 16 is shown. The dashed lines mark the positions of the fundamental, the first, second, and third overtone.
whereas for longer excitation wavelengths no signal from the ZnSe barrier was detected due to the poor resonance enhancement and the decreased scattering volume of the barrier material. Because of their relatively large sizes the wires are not expected to show phonon confinement; composition dependent shifts could be also excluded. Therefore, the observed shifts of the LO phonon position must be due to strain effects. ZnSe/CdxZn1⫺xSe quantum well layers of 5 nm (A) and 3 nm (B) thickness were grown by a molecular beam epitaxy Riber system. To obtain wire widths and lengths down to 18 nm, the quantum wells were patterned by electron beam lithography and wet chemical etching as described elsewhere [1,2]. The samples used in our experiments consist of several patterned areas of 100 × 100 mm2 each containing wires with well-defined widths and lengths. These areas are separated by several 100 mm from each other. Resonance Raman spectroscopy was performed on these CdxZn1⫺xSe quantum wires at 10 K by means of a micro Raman setup. The latter enabled us to perform measurements on particular areas having well-defined wire widths and lengths. For details of the experimental set-up we refer to Ref. [9]. In short, the Raman spectra were recorded using the different wavelengths of an argon ion laser (Spectra Physics
Model 166). The sample was placed in a closed cycle Hecryostat (CTI Cryogenics), which enabled us to achieve temperatures of about 10 K. The best signals were obtained for excitation wavelengths of 465 and 458 nm (A) and 476 nm (B). In this contribution, we only present spectra for these three wavelengths. The scattered light was focused onto the entrance slit of a Spex 1404 double monochromator equipped with a charge coupled device (CCD) camera system (Photometrics Model 9000). The spectra were recorded applying the scanning multichannel technique [10]. The presented spectra concentrate on wires with two fixed widths W 105 A=W 150 nm B and W 18 A= W 35 nm B: The wires with widths above 100 nm show only the beginning strain relaxation, whereas the wires with widths less than about 30 nm should be nearly relaxed according to the investigations of Lermann et al. [3– 7]. For both wire widths the length of the wire decreases from L 100 mm to L ⬇ W: The latter structures are too large to show any phonon confinement effects but too small to show the vibrational properties of a two-dimensional quantum well. Therefore, we decided to call these structures also short wires and not dots or wells. For more clearness, the sample fields are classified in the text by labels (W/L),
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
437
Table 1 Ratios of the integrated intensities of the LO phonon peak and its overtones as well as the electron phonon coupling parameter D (calculated from the ratio 2LO/1LO; for details of the calculation see Refs. [11–13]) Wire width/wire length (nm)
1LO:2LO
1LO:3LO
1LO:4LO
D (2LO/1LO)
18/1D lex 458 18/1D lex 465 18/24 lex 458 27/50 lex 458
1:0.28 1:15.5 1:1.29 1:1.25
1:0.91 1:40 1:0.3 1:7.5
– – – 1:50
1.05 7.85 2.3 2.25
where W is the width and L the length in nm. “1D” refers to a wire length of 100 mm. Fig. 1 shows the emission spectra (Raman and luminescence) of two wire fields with the sizes (18/1D) (I) and (27/ 50) (II). One can observe up to three intensive overtones of the LO phonon band when the overtone positions are matching the luminescence region. This strong outgoing resonance leads to very high ratios of the integrated intensities of the respective overtone to the fundamental (Table 1), which characterizes the electron phonon coupling in this system. From the data in the table one can see that the intensity ratio of the overtone to the fundamental 2LO/ 1LO and the electron phonon coupling parameter calculated from this ratio depend on the sample size. But there is also a strong dependence of this ratio on the excitation wavelength due to the resonance enhancement of the signals. The excitation wavelength dependence varies for each wire size because of the shift of the bandgap energy. Therefore, the resonance profile is a function of the wire size. According to Shiang et al. [11], it is possible to calculate the electron– phonon coupling parameter from the integrated excitation profile of the fundamental and the first overtone of the samples and by these gains the excitation wavelength
independent value of the coupling parameter. But to obtain this wavelength independent coupling parameter, it would be necessary to separate the part of the outgoing resonance from that of the incoming resonance. Measurements of the excitation profile of the two wire fields are currently in progress. With these excitation profiles it should be possible to obtain the wavelength dependent overtone to fundamental ratio for incoming and outgoing resonance separately. A factor, which complicates these investigations, is the dependence of the overtone fundamental ratio on the aging of the sample. With increasing sample age both luminescence and the relative intensities of the overtones to the fundamental band decrease drastically. Detailed results of these investigations will be published in a forthcoming paper. The Raman spectra of quantum wires of sample A with wire widths of W 105 nm and different wire lengths (from L 100 mm to L 114 nm recorded at excitation wavelengths of 465 (a) and 458 nm (b) are shown in Fig. 2. All spectra exhibit a well-resolved vibrational mode at about 254 cm ⫺1, which is close to the 1LO phonon of the ZnSe bulk material (256 cm ⫺1). The observed signal is strongly asymmetric or has a shoulder on the lower energy side. Therefore, we conclude that the observed Raman signal is
Fig. 2. Raman spectra of the CdZnSe wires with a wire width of W 105 nm and different wire lengths, recorded at excitation wavelengths (a) 465 and (b) 458 nm at a temperature of 10 K. The dashed lines mark the LO phonon positions of the unstrained CdZnSe wire material (251 cm ⫺1 for 20% Cd content) and the ZnSe barrier material (256 cm ⫺1).
438
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
Fig. 3. Raman spectra of the CdZnSe wires with a wire width of W 18 nm and different wire lengths, recorded at excitation wavelengths (a) 465 and (b) 458 nm at a temperature of 10 K. The dashed lines mark the LO phonon positions of the unstrained CdZnSe wire material (251 cm ⫺1 for 20% Cd content) and the ZnSe barrier material (256 cm ⫺1).
due to an overlapped signal of the ZnSe lattice vibration of the barrier material and the ZnSe-like LO-phonon of the wire. When varying the excitation wavelength l ex from 488 to 458 nm, the peak maximum of the broader wires shifts to higher energies (Fig. 2(a) and (b)). This is due to the different resonance conditions of the wire and the barrier material, which lead to an enhancement of the ZnSe part of the signal when approaching the ZnSe gap. For lex 458 nm; the main part of the signal is due to the ZnSe barrier material because of the large scattering volume of ZnSe and the weak resonance enhancement of the ZnSe-like signal, whereas lex 488 nm is more resonant with the bandgap of the CdZnSe wire and therefore the wire signal is dominating. This excitation wavelength dependent shift was not observed for the wires with smaller wire widths and the short wires as can be seen in Fig. 3(a) and (b), where Raman spectra of wires having widths of W 18 nm and different lengths from L 100 mm to 24 nm are shown. Sample A consists of deep etched wires; thus, the scattering volume of the ZnSe barrier decreases strongly with the wire size. The decrease of the scattering volume of the wire is compensated by an increasing resonance enhancement due to the electronic confinement shift of the absorption gap [1,2]. Therefore, the detected signal from the thin and short wires at all excitation wavelengths is mostly due to the CdZnSe wire with nearly no contribution of the ZnSe barrier, whereas for the thick and long wires and for short excitation wavelengths the signal is dominated by the ZnSe signal. The plotted spectra also show a redshift of the wavenumber position of the peak maxima with decreasing wire length. The detected signals were fitted by a sum of Gaussians to obtain the positions of the single peaks. Because of the low Cd content, there was no clear separation between the LO
phonon signals of the barrier and the well material of the strained wires. This fact complicated the fits and made it impossible to determine the wavenumber position of all peak maxima with high accuracy. The fits resulted in three different kinds of LO phonon peaks. The ZnSe LO phonon mode at about 256 cm ⫺1 appears only in the spectra of the thick and longest wires and for short excitation wavelengths with high intensity, whereas for the thin and short wires only a weak or no ZnSe signal could be resolved. The intensity of the second peak, which is due to the ZnSe-like LO phonon of the strained wire centre, increases in intensity relative to the ZnSe LO phonon with increasing excitation wavelength and decreasing wire size. The wavenumber of this signal shows a small redshift depending on the wire length, which is more pronounced for the wires with widths smaller than 30 nm (Fig. 3), despite the fact that these wires should be nearly completely relaxed according to the results of Lermann et al. [5–7]. The fit of the Raman bands also showed the appearance of a third line in the Raman spectra. This contribution is on the low energy side of the ZnSe-like peak and increases in significance with decreasing wire length and width. The wavenumber position of this peak is near the position of the wavenumber for the LO phonon of the unstrained wire material at 251 cm ⫺1 and does not change significantly with the decreasing wire length or with the excitation wavelength. Therefore we assign this peak to a LO phonon of the CdZnSe, which belongs to the strain relaxed sides of the wire. The thin wires also show an additional peak at about 245 cm ⫺1, which was not yet identified but also increases significantly with the decreasing wire length. So this peak is also affected by a size effect. The existence of at least three distinct peaks in the LO phonon region has been proven by Raman experiments on CdZnSe wires and short wires with higher Cd content
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
Fig. 4. Raman spectra of Cd0.35Zn0.65Se wires with widths W 150 (a) and 35 nm (b) and decreasing length, recorded at an excitation wavelength of 476 nm at a temperature of 10 K. The dashed lines mark the LO phonon positions of the unstrained CdZnSe wire material (247 cm ⫺1 for 35% Cd content) and the ZnSe barrier material (256 cm ⫺1).
(35%). In this sample, the wires were not deeply etched, thus the scattering volume of the barrier does not decrease by the same amount as the CdZnSe scattering volume and a well resolved signal of the ZnSe barrier can be observed for nearly all excitation wavelengths and wire sizes. The wavenumber of the unstrained bulk-like CdZnSe LO phonon of this sample is about 247 cm ⫺1. Thus, the separation between the ZnSe-like signal of the strained wire centre and the signal of the strain relaxed edge should be more clearly. Fig. 4(a) and (b) show the Raman spectra of wires with widths of W 150 and 35 nm and the decreasing wire length for 35% Cd content. The spectra clearly show three peak contributions in the region of the ZnSe LO phonon
439
with the same characteristics and length dependence as the LO signals for the sample with 20% Cd content. The ZnSelike LO contribution of the wire centre at about 254 cm ⫺1 shows a decrease in intensity with decreasing wire width and especially wire length, but with nearly no shift towards the contribution of the strain relaxed edges. This can be explained by the rather large sizes of the investigated wires. From the calculations of Lermann et al. [6] and Ku¨mmell et al. [3] the width of the relaxation zone is about 10 nm, thus an observable relaxation of the wire centre should occur only for wires with widths less than 30 nm. The peak contribution of the strain relaxed wire edges also shows no shift but increasing intensity with decreasing wire size. Fits of Raman signals also reveal the existence of another, broad contribution with relative low intensity (see exemplary fits in Fig. 5). Due to the position of this contribution, this Raman signal is assigned to the relaxation zone between the strained centre and the relaxed edges. The large width of this contribution reflects the inhomogeneities of the lattice constant in this zone. In summary, we observe a strong outgoing resonance of the LO phonon overtones of the samples. The observed LO fundamental splits into three modes: one is due to the barrier material, whereas the signal of the wire material is split into two modes. The lower energy peak of the wire material occupies the wavenumber position of the unstrained wire material and shows no shift with decreasing wire length. Because of this, we assign it to the totally relaxed edges of the wire. The higher energy peak is assigned to the wire centre and shows a small redshift with decreasing wire size towards the other peak, which is due to a further strain relaxation of the wire centre with decreasing wire length and width. A weak additional peak at about 245 cm ⫺1, which can only be observed in short wires and quantum dots, could not be assigned yet, but might be due to a relaxation process in a second dimension. Raman experiments on another CdZnSe sample with 35% Cd content confirm our results. Due to the higher Cd content, a better separation of the two dominating peak contributions of the ZnSe-like LO phonon was achieved. Further, by fitting the Raman spectra, a broad contribution of the relaxation zone between the strained wire centre and relaxed wire edges could be detected.
Acknowledgements The authors gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 410, Teilprojekte A1, C1 and C3). Three of us (B.S., A.M. and W.K.) also acknowledge the financial support from the Fonds der Chemischen Industrie.
440
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
Fig. 5. (a) Exemplary fits of two Raman spectra of Cd0.35Zn0.65Se wires with W 106 nm and L 100 mm; and (b) of Cd0.2Zn0.8Se wires with W 18 nm and L 24 nm: The fits show the peak contributions of the ZnSe barrier (LO(B)), the wire centre (LO(Wc)) and edges (LO(We)) as well as the “intermediate zone” between relaxed edges and strained centre (LO(Wiz)). It can be clearly seen, that for the wires with W 18 nm the centre is also partially relaxed, whereas for the W 106 nm wires the phonon position of the strained centre is the same as for the totally strained two-dimensional quantum wells.
References [1] M. Illing, G. Bacher, T. Ku¨mmell, A. Forchel, T.G. Anderson, D. Hommel, B. Jobst, G. Landwehr, Appl. Phys. Lett. 67 (1995) 124. [2] M. Illing, G. Bacher, T. Ku¨mmell, A. Forchel, D. Hommel, B. Jobst, G. Landwehr, J. Vac. Sci. Technol. B 13 (1995) 2792. [3] T. Ku¨mmell, G. Bacher, A. Forchel, G. Lermann, W. Kiefer, B. Jobst, D. Hommel, G. Landwehr, Phys. Rev. B 57 (1998) 15439. [4] S. Nakashima, A. Fujii, K. Mizoguchi, A. Mitsuishi, K. Yoneda, Jpn. J. Appl. Phys. 27 (1988) 1327. [5] G. Lermann, T. Bischof, A. Materny, W. Kiefer, T. Ku¨mmell, G. Bacher, A. Forchel, G. Landwehr, J. Appl. Phys. 81 (1997) 1446. [6] G. Lermann, T. Bischof, A. Materny, W. Kiefer, T. Ku¨mmell, G. Bacher, A. Forchel, G. Landwehr, Phys. Rev. B 56 (1997) 7469.
[7] G. Lermann, T. Bischof, B. Schreder, A. Materny, W. Kiefer, T. Ku¨mmell, G. Bacher, A. Forchel, G. Landwehr, Ber. Bunsenges. Phys. Chem. 101 (1997) 1665. [8] Theoretical calculations based on the MREI model by: L. Genzel, T.P. Martin, C.H. Perry, Phys. Status Solidi B 62 (1974) 83. Raman experimental data by: R.G. Alonso, E.-K. Suh, A.K. Ramdas, N. Samarth, H. Luo, J.K. Furdyna, Phys. Rev. B 40 (1989) 3720. [9] M. Lankers, D. Go¨ttges, A. Materny, K. Schaschek, W. Kiefer, Appl. Spectrosc. 46 (1992) 1331. [10] V. Deckert, W. Kiefer, Appl. Spectrosc. 46 (1992) 322. [11] J.J. Shiang, S.H. Risbud, A.P. Alivisatos, J. Chem. Phys. 98 (1993) 8433. [12] A.P. Alivisatos, T.D. Harris, P.J. Carroll, M.L. Steigerwald, L.E. Brus, J. Chem. Phys. 90 (1989) 3463. [13] M.C. Klein, F. Hache, D. Richard, C. Flytzanis, Phys. Rev. B 42 (1990) 11123.
Solid State Communications 114 (2000) 435–440 www.elsevier.com/locate/ssc
Length dependence of the longitudinal optical phonon properties in CdZnSe/ZnSe quantum wires B. Schreder a, A. Materny a, W. Kiefer a,*, T. Ku¨mmell b, G. Bacher b, A. Forchel b, G. Landwehr b a
Institut fu¨r Physikalische Chemie der Universita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany b Physikalisches Institut der Universita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany
Received 7 January 2000; received in revised form 7 February 2000; accepted 17 February 2000 by J. Kuhl
Abstract This paper presents the results of Raman spectroscopic investigations on etched CdxZn1⫺xSe/ZnSe quantum wires with different wire lengths and wire widths. The recorded spectra show intensive signals in the region of the longitudinal optical phonons (LO) of the ZnSe barrier and the CdZnSe wire as well as several overtones. A strong outgoing resonance can be observed when the overtone position matches the luminescence peak position. The asymmetric LO signal can be separated into three distinct peaks: two higher wavenumber peaks, which are observed especially for the long and thick wires for short excitation wavelengths, are attributed to the LO phonon modes of the ZnSe barrier material and the strained CdZnSe wire; a shoulder at the low wavenumber region, which appears in the spectra of the short and thin wires, is attributed to an LO phonon excited in the strain relaxed wire edges. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; D. Phonons; E. Inelastic light scattering
Low dimensional II–VI semiconductors like quantum wires and quantum dots have attracted much attention over the last several years. This is due to the lateral confinement of charge carriers and phonons in the semiconductor structure. This confinement effect causes the appearance of new physical phenomena, which strongly depend on the size of the wires and dots, for example a blue-shift of the bandgap [1,2]. One of the most interesting systems for the investigation of basic physical properties within the group of the II–VIsemiconductor nanostructures are the CdZnSe/ZnSe quantum wires and dots. For these systems strain effects have to be taken into consideration because of the lattice mismatch between the ZnSe barrier and the CdZnSe well [3]. This biaxial strain causes a blueshift of the wire longitudinal optical phonon (LO) frequency, whereas the LO phonon of the ZnSe barrier shows no shift because of the high fraction of unstrained material [4]. The etching process leads to * Corresponding author. Tel.: ⫹49-931-888-6330; fax: ⫹49-931888-6332. E-mail address: [email protected] (W. Kiefer).
a strain relaxation on the edges of the wires. Lermann et al. [5–7] have shown, that the combined signal of the strained wire centre and the relaxed edges of the wires show a redshift with decreasing wire width due to the increase of the relaxed fraction to the strained fraction. For wires having widths of 30 nm and less no further shift could be observed. Hence, they concluded that these wires should be nearly strain relaxed [4]. In this paper, we present a Raman spectroscopic investigation of CdZnSe quantum wires with Cd contents of 20% (A) and 35% (B) and with different wire lengths (L) and widths (W). The Raman spectra of CdxZn1⫺xSe show one LO phonon mode for the ZnSe-like phonon, whose wavenumber shifts depending on the Cd content from 256 x 0 to 210 cm ⫺1 x 1: The expected LO phonon position for the unstrained wires is about 251 cm ⫺1 for 20% Cd content and 247 cm ⫺1 for 35% Cd content [8]. As a consequence of the small cadmium concentration of the CdZnSe layer of sample A, no separation between the LO phonon of the ZnSe barrier layer and the strained phonon of the quantum well layer can be observed. For the strain relaxed LO phonon of the short and narrow wires, the separation is more clearly at an excitation wavelength of 458 nm,
0038-1098/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00077-6
436
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
Fig. 1. (a) Emission spectra (Raman and luminescence) of two CdZnSe wires, (18/1D) (I), (27/50) (II), recorded at excitation wavelengths 465 (I) and 458 nm (II) at a temperature of 10 K. (b) An enlargement of the spectra given in (a) by a factor of 16 is shown. The dashed lines mark the positions of the fundamental, the first, second, and third overtone.
whereas for longer excitation wavelengths no signal from the ZnSe barrier was detected due to the poor resonance enhancement and the decreased scattering volume of the barrier material. Because of their relatively large sizes the wires are not expected to show phonon confinement; composition dependent shifts could be also excluded. Therefore, the observed shifts of the LO phonon position must be due to strain effects. ZnSe/CdxZn1⫺xSe quantum well layers of 5 nm (A) and 3 nm (B) thickness were grown by a molecular beam epitaxy Riber system. To obtain wire widths and lengths down to 18 nm, the quantum wells were patterned by electron beam lithography and wet chemical etching as described elsewhere [1,2]. The samples used in our experiments consist of several patterned areas of 100 × 100 mm2 each containing wires with well-defined widths and lengths. These areas are separated by several 100 mm from each other. Resonance Raman spectroscopy was performed on these CdxZn1⫺xSe quantum wires at 10 K by means of a micro Raman setup. The latter enabled us to perform measurements on particular areas having well-defined wire widths and lengths. For details of the experimental set-up we refer to Ref. [9]. In short, the Raman spectra were recorded using the different wavelengths of an argon ion laser (Spectra Physics
Model 166). The sample was placed in a closed cycle Hecryostat (CTI Cryogenics), which enabled us to achieve temperatures of about 10 K. The best signals were obtained for excitation wavelengths of 465 and 458 nm (A) and 476 nm (B). In this contribution, we only present spectra for these three wavelengths. The scattered light was focused onto the entrance slit of a Spex 1404 double monochromator equipped with a charge coupled device (CCD) camera system (Photometrics Model 9000). The spectra were recorded applying the scanning multichannel technique [10]. The presented spectra concentrate on wires with two fixed widths W 105 A=W 150 nm B and W 18 A= W 35 nm B: The wires with widths above 100 nm show only the beginning strain relaxation, whereas the wires with widths less than about 30 nm should be nearly relaxed according to the investigations of Lermann et al. [3– 7]. For both wire widths the length of the wire decreases from L 100 mm to L ⬇ W: The latter structures are too large to show any phonon confinement effects but too small to show the vibrational properties of a two-dimensional quantum well. Therefore, we decided to call these structures also short wires and not dots or wells. For more clearness, the sample fields are classified in the text by labels (W/L),
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
437
Table 1 Ratios of the integrated intensities of the LO phonon peak and its overtones as well as the electron phonon coupling parameter D (calculated from the ratio 2LO/1LO; for details of the calculation see Refs. [11–13]) Wire width/wire length (nm)
1LO:2LO
1LO:3LO
1LO:4LO
D (2LO/1LO)
18/1D lex 458 18/1D lex 465 18/24 lex 458 27/50 lex 458
1:0.28 1:15.5 1:1.29 1:1.25
1:0.91 1:40 1:0.3 1:7.5
– – – 1:50
1.05 7.85 2.3 2.25
where W is the width and L the length in nm. “1D” refers to a wire length of 100 mm. Fig. 1 shows the emission spectra (Raman and luminescence) of two wire fields with the sizes (18/1D) (I) and (27/ 50) (II). One can observe up to three intensive overtones of the LO phonon band when the overtone positions are matching the luminescence region. This strong outgoing resonance leads to very high ratios of the integrated intensities of the respective overtone to the fundamental (Table 1), which characterizes the electron phonon coupling in this system. From the data in the table one can see that the intensity ratio of the overtone to the fundamental 2LO/ 1LO and the electron phonon coupling parameter calculated from this ratio depend on the sample size. But there is also a strong dependence of this ratio on the excitation wavelength due to the resonance enhancement of the signals. The excitation wavelength dependence varies for each wire size because of the shift of the bandgap energy. Therefore, the resonance profile is a function of the wire size. According to Shiang et al. [11], it is possible to calculate the electron– phonon coupling parameter from the integrated excitation profile of the fundamental and the first overtone of the samples and by these gains the excitation wavelength
independent value of the coupling parameter. But to obtain this wavelength independent coupling parameter, it would be necessary to separate the part of the outgoing resonance from that of the incoming resonance. Measurements of the excitation profile of the two wire fields are currently in progress. With these excitation profiles it should be possible to obtain the wavelength dependent overtone to fundamental ratio for incoming and outgoing resonance separately. A factor, which complicates these investigations, is the dependence of the overtone fundamental ratio on the aging of the sample. With increasing sample age both luminescence and the relative intensities of the overtones to the fundamental band decrease drastically. Detailed results of these investigations will be published in a forthcoming paper. The Raman spectra of quantum wires of sample A with wire widths of W 105 nm and different wire lengths (from L 100 mm to L 114 nm recorded at excitation wavelengths of 465 (a) and 458 nm (b) are shown in Fig. 2. All spectra exhibit a well-resolved vibrational mode at about 254 cm ⫺1, which is close to the 1LO phonon of the ZnSe bulk material (256 cm ⫺1). The observed signal is strongly asymmetric or has a shoulder on the lower energy side. Therefore, we conclude that the observed Raman signal is
Fig. 2. Raman spectra of the CdZnSe wires with a wire width of W 105 nm and different wire lengths, recorded at excitation wavelengths (a) 465 and (b) 458 nm at a temperature of 10 K. The dashed lines mark the LO phonon positions of the unstrained CdZnSe wire material (251 cm ⫺1 for 20% Cd content) and the ZnSe barrier material (256 cm ⫺1).
438
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
Fig. 3. Raman spectra of the CdZnSe wires with a wire width of W 18 nm and different wire lengths, recorded at excitation wavelengths (a) 465 and (b) 458 nm at a temperature of 10 K. The dashed lines mark the LO phonon positions of the unstrained CdZnSe wire material (251 cm ⫺1 for 20% Cd content) and the ZnSe barrier material (256 cm ⫺1).
due to an overlapped signal of the ZnSe lattice vibration of the barrier material and the ZnSe-like LO-phonon of the wire. When varying the excitation wavelength l ex from 488 to 458 nm, the peak maximum of the broader wires shifts to higher energies (Fig. 2(a) and (b)). This is due to the different resonance conditions of the wire and the barrier material, which lead to an enhancement of the ZnSe part of the signal when approaching the ZnSe gap. For lex 458 nm; the main part of the signal is due to the ZnSe barrier material because of the large scattering volume of ZnSe and the weak resonance enhancement of the ZnSe-like signal, whereas lex 488 nm is more resonant with the bandgap of the CdZnSe wire and therefore the wire signal is dominating. This excitation wavelength dependent shift was not observed for the wires with smaller wire widths and the short wires as can be seen in Fig. 3(a) and (b), where Raman spectra of wires having widths of W 18 nm and different lengths from L 100 mm to 24 nm are shown. Sample A consists of deep etched wires; thus, the scattering volume of the ZnSe barrier decreases strongly with the wire size. The decrease of the scattering volume of the wire is compensated by an increasing resonance enhancement due to the electronic confinement shift of the absorption gap [1,2]. Therefore, the detected signal from the thin and short wires at all excitation wavelengths is mostly due to the CdZnSe wire with nearly no contribution of the ZnSe barrier, whereas for the thick and long wires and for short excitation wavelengths the signal is dominated by the ZnSe signal. The plotted spectra also show a redshift of the wavenumber position of the peak maxima with decreasing wire length. The detected signals were fitted by a sum of Gaussians to obtain the positions of the single peaks. Because of the low Cd content, there was no clear separation between the LO
phonon signals of the barrier and the well material of the strained wires. This fact complicated the fits and made it impossible to determine the wavenumber position of all peak maxima with high accuracy. The fits resulted in three different kinds of LO phonon peaks. The ZnSe LO phonon mode at about 256 cm ⫺1 appears only in the spectra of the thick and longest wires and for short excitation wavelengths with high intensity, whereas for the thin and short wires only a weak or no ZnSe signal could be resolved. The intensity of the second peak, which is due to the ZnSe-like LO phonon of the strained wire centre, increases in intensity relative to the ZnSe LO phonon with increasing excitation wavelength and decreasing wire size. The wavenumber of this signal shows a small redshift depending on the wire length, which is more pronounced for the wires with widths smaller than 30 nm (Fig. 3), despite the fact that these wires should be nearly completely relaxed according to the results of Lermann et al. [5–7]. The fit of the Raman bands also showed the appearance of a third line in the Raman spectra. This contribution is on the low energy side of the ZnSe-like peak and increases in significance with decreasing wire length and width. The wavenumber position of this peak is near the position of the wavenumber for the LO phonon of the unstrained wire material at 251 cm ⫺1 and does not change significantly with the decreasing wire length or with the excitation wavelength. Therefore we assign this peak to a LO phonon of the CdZnSe, which belongs to the strain relaxed sides of the wire. The thin wires also show an additional peak at about 245 cm ⫺1, which was not yet identified but also increases significantly with the decreasing wire length. So this peak is also affected by a size effect. The existence of at least three distinct peaks in the LO phonon region has been proven by Raman experiments on CdZnSe wires and short wires with higher Cd content
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
Fig. 4. Raman spectra of Cd0.35Zn0.65Se wires with widths W 150 (a) and 35 nm (b) and decreasing length, recorded at an excitation wavelength of 476 nm at a temperature of 10 K. The dashed lines mark the LO phonon positions of the unstrained CdZnSe wire material (247 cm ⫺1 for 35% Cd content) and the ZnSe barrier material (256 cm ⫺1).
(35%). In this sample, the wires were not deeply etched, thus the scattering volume of the barrier does not decrease by the same amount as the CdZnSe scattering volume and a well resolved signal of the ZnSe barrier can be observed for nearly all excitation wavelengths and wire sizes. The wavenumber of the unstrained bulk-like CdZnSe LO phonon of this sample is about 247 cm ⫺1. Thus, the separation between the ZnSe-like signal of the strained wire centre and the signal of the strain relaxed edge should be more clearly. Fig. 4(a) and (b) show the Raman spectra of wires with widths of W 150 and 35 nm and the decreasing wire length for 35% Cd content. The spectra clearly show three peak contributions in the region of the ZnSe LO phonon
439
with the same characteristics and length dependence as the LO signals for the sample with 20% Cd content. The ZnSelike LO contribution of the wire centre at about 254 cm ⫺1 shows a decrease in intensity with decreasing wire width and especially wire length, but with nearly no shift towards the contribution of the strain relaxed edges. This can be explained by the rather large sizes of the investigated wires. From the calculations of Lermann et al. [6] and Ku¨mmell et al. [3] the width of the relaxation zone is about 10 nm, thus an observable relaxation of the wire centre should occur only for wires with widths less than 30 nm. The peak contribution of the strain relaxed wire edges also shows no shift but increasing intensity with decreasing wire size. Fits of Raman signals also reveal the existence of another, broad contribution with relative low intensity (see exemplary fits in Fig. 5). Due to the position of this contribution, this Raman signal is assigned to the relaxation zone between the strained centre and the relaxed edges. The large width of this contribution reflects the inhomogeneities of the lattice constant in this zone. In summary, we observe a strong outgoing resonance of the LO phonon overtones of the samples. The observed LO fundamental splits into three modes: one is due to the barrier material, whereas the signal of the wire material is split into two modes. The lower energy peak of the wire material occupies the wavenumber position of the unstrained wire material and shows no shift with decreasing wire length. Because of this, we assign it to the totally relaxed edges of the wire. The higher energy peak is assigned to the wire centre and shows a small redshift with decreasing wire size towards the other peak, which is due to a further strain relaxation of the wire centre with decreasing wire length and width. A weak additional peak at about 245 cm ⫺1, which can only be observed in short wires and quantum dots, could not be assigned yet, but might be due to a relaxation process in a second dimension. Raman experiments on another CdZnSe sample with 35% Cd content confirm our results. Due to the higher Cd content, a better separation of the two dominating peak contributions of the ZnSe-like LO phonon was achieved. Further, by fitting the Raman spectra, a broad contribution of the relaxation zone between the strained wire centre and relaxed wire edges could be detected.
Acknowledgements The authors gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 410, Teilprojekte A1, C1 and C3). Three of us (B.S., A.M. and W.K.) also acknowledge the financial support from the Fonds der Chemischen Industrie.
440
B. Schreder et al. / Solid State Communications 114 (2000) 435–440
Fig. 5. (a) Exemplary fits of two Raman spectra of Cd0.35Zn0.65Se wires with W 106 nm and L 100 mm; and (b) of Cd0.2Zn0.8Se wires with W 18 nm and L 24 nm: The fits show the peak contributions of the ZnSe barrier (LO(B)), the wire centre (LO(Wc)) and edges (LO(We)) as well as the “intermediate zone” between relaxed edges and strained centre (LO(Wiz)). It can be clearly seen, that for the wires with W 18 nm the centre is also partially relaxed, whereas for the W 106 nm wires the phonon position of the strained centre is the same as for the totally strained two-dimensional quantum wells.
References [1] M. Illing, G. Bacher, T. Ku¨mmell, A. Forchel, T.G. Anderson, D. Hommel, B. Jobst, G. Landwehr, Appl. Phys. Lett. 67 (1995) 124. [2] M. Illing, G. Bacher, T. Ku¨mmell, A. Forchel, D. Hommel, B. Jobst, G. Landwehr, J. Vac. Sci. Technol. B 13 (1995) 2792. [3] T. Ku¨mmell, G. Bacher, A. Forchel, G. Lermann, W. Kiefer, B. Jobst, D. Hommel, G. Landwehr, Phys. Rev. B 57 (1998) 15439. [4] S. Nakashima, A. Fujii, K. Mizoguchi, A. Mitsuishi, K. Yoneda, Jpn. J. Appl. Phys. 27 (1988) 1327. [5] G. Lermann, T. Bischof, A. Materny, W. Kiefer, T. Ku¨mmell, G. Bacher, A. Forchel, G. Landwehr, J. Appl. Phys. 81 (1997) 1446. [6] G. Lermann, T. Bischof, A. Materny, W. Kiefer, T. Ku¨mmell, G. Bacher, A. Forchel, G. Landwehr, Phys. Rev. B 56 (1997) 7469.
[7] G. Lermann, T. Bischof, B. Schreder, A. Materny, W. Kiefer, T. Ku¨mmell, G. Bacher, A. Forchel, G. Landwehr, Ber. Bunsenges. Phys. Chem. 101 (1997) 1665. [8] Theoretical calculations based on the MREI model by: L. Genzel, T.P. Martin, C.H. Perry, Phys. Status Solidi B 62 (1974) 83. Raman experimental data by: R.G. Alonso, E.-K. Suh, A.K. Ramdas, N. Samarth, H. Luo, J.K. Furdyna, Phys. Rev. B 40 (1989) 3720. [9] M. Lankers, D. Go¨ttges, A. Materny, K. Schaschek, W. Kiefer, Appl. Spectrosc. 46 (1992) 1331. [10] V. Deckert, W. Kiefer, Appl. Spectrosc. 46 (1992) 322. [11] J.J. Shiang, S.H. Risbud, A.P. Alivisatos, J. Chem. Phys. 98 (1993) 8433. [12] A.P. Alivisatos, T.D. Harris, P.J. Carroll, M.L. Steigerwald, L.E. Brus, J. Chem. Phys. 90 (1989) 3463. [13] M.C. Klein, F. Hache, D. Richard, C. Flytzanis, Phys. Rev. B 42 (1990) 11123.