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Optical nonlinear properties of Cd1−xZnxTe epilayers grown on (100) GaAs by pulsed laser deposition
Thin Solid Films 318 Ž1998. 151–153
Optical nonlinear properties of Cd 1yx Zn xTe epilayers grown on ž100 / GaAs by pulsed laser deposition D. Ohlmann ) , J.L. Deiss, J.L. Loison, M. Robino, G. Versini Institut de Physique et Chimie des Materiaux de Strasbourg UMR 46, CNRS-ULP-EHICS, Groupe d’Optique Nonlineaire et d’Optoelectronique 23, ´ ´ ´ rue du Loess, BP 20 CR, 67037 Strasbourg Cedex, France
Abstract This paper reports induced absorption in Cd 1yx Zn xTe thin films epitaxially grown by pulsed laser deposition on Ž100.-oriented GaAs substrate. The high optical nonlinearity is adjustable in the visible over a wide spectral region as the concentration x is varied. q 1998 Elsevier Science S.A. Keywords: Deposition; Lattice; Substrate
1. Introduction Different methods are used to grow Cd 1y x Zn xTe bulk material w1,2x and thin layers w3,4x. The change of the relative concentrations of Cd and Zn atoms results in a variation of the lattice parameter and of the band gap between the values of the binary compounds CdTe and ZnTe. The change of the lattice parameter is interesting since CdZnTe is used as a substrate for IR detectors, see for example for HgCdTe w5x where the lattice parameter has to be matched with that of the sensible material. CdZnTe is also a promising material for X-ray and g-ray detectors at room temperature. The variation of the band gap is used to change the strong optical nonlinearity in a wide region of the visible spectrum w6x. This paper describes, in its first part, the epitaxial growth of Cd 1y x Zn xTe by pulsed laser deposition and, in its second one, optical properties of the layers with different Zn concentrations x. In Section 5, the nonlinear optical properties are compared to that of polycrystalline layers grown by a hot wall evaporation technique ŽHWE.. 2. Growth of the epitaxial films The films we describe in this paper are grown by pulsed laser deposition ŽPLD.. In this technique, a XeCl excimer laser emitting pulses of 20 ns duration at 10 Hz in the UV )
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 5 5 - 3
region Ž308 nm. is focalized at room temperature onto a 4 cm diameter rotating target of the ternary material CdZnTe with the desired composition. The fluence is 2 Jrcm2 and the growth rate 1 m mrh. The important fluence creates a plasma around the focus point of the beam and atoms are ejected from the target. In the vacuum of 10y6 Torr, they migrate to the substrate which is mounted in distance at 4 cm above the target and can be heated. As a substrate, we use GaAs single crystalline wafer oriented in the direction Ž100., chemically etched and pre-heated in the vacuum cell in order to eliminate any contamination of the surface. The substrate temperature during deposition is about 3008C. Two different targets were used, the first one was bulk polycrystalline material with x s 0.20, a concentration which is commercially available. For other compositions, we had to prepare the targets because there is no commercial bulk material with high Zn concentrations available. The cold-pressed targets were obtained by mixing high purity fine grained CdTe and ZnTe powders in the desired concentrations. We began our studies with a cold-pressed target having the same composition as the bulk one. The X-ray diffraction pattern exhibits peaks only in the directions Ž n00.. Strong Ž400. diffraction lines are observed which indicate a good epitaxy of the film on the Ž100. oriented GaAs substrate. Fig. 1 shows the X-ray diffraction of epilayers grown from the two targets. The peaks for the layers grown from the bulk polycrystalline target are broad compared to the peaks of binary compounds and the peaks for the layers grown with the pressed target are even broader indicating the importance of the target quality.
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D. Ohlmann et al.r Thin Solid Films 318 (1998) 151–153
Fig. 1. X-ray diffraction for the polycrystalline target and cold-pressed targets with different x values.
Ternary materials always have a distorted lattice because the lattice constants of the contributing binaries are different. In addition, the atom sizes are different and thus all the features are broaden in the diffraction pattern. AFM ŽAtomic Force Microscopy. images give a good crystalline surface quality of the film with a surface roughness of 3 nm over a 5 m m scan spot. The thickness of the samples is about 1 m m. By measuring the lattice constant, the composition of the layer is determined from Vegard’s law.
Fig. 2. Transmission of the samples.
spot of the pump beam is larger then that of the test beam in order to probe a quasi-homogeneously excited central region. The pump beam consists of quite intense Ž300 kWrcm2 . and spectrally narrow light pulses of 15 ns duration. The probe beam is a spectrally broad beam of low intensity in order to avoid induced nonlinearities in the sample. The transmission of the sample is measured with and without excitation of the pump beam. It decreases when it is excited by the intense laser beam indicating induced absorption. Without the pump beam excitation, the
3. Optical properties 3.1. Linear optical properties The GaAs substrates are not transparent in the visible part of the spectrum while we are interested in studying the vicinity of the fundamental band gap of CdZnTe Žbetween 1.54 to 2.26 eV.. Thus, we removed the substrate, by gluing the CdZnTe film on a glass plate and by polishing mechanically and finally etching chemically the GaAs. The typical transmission spectrum of such a sample is shown in Fig. 2, the interference fringes show the good quality of the film. They result from interfering multiple reflections from the two surfaces of good optical quality of the film forming a Fabry–Perot cavity. The transmission coefficient is relatively high in the transparent region Ž0.7 to 0.8.. The optical transmission coefficient and the modulation of interference fringes of these films are better than those of samples grown by HWE after annealing. 3.2. Nonlinear optical properties Nonlinear optical properties of epilayers of different composition are studied by pump and probe experiments, where two pulsed dye lasers are optically pumped with the excimer laser. We focalize the two laser beams onto the sample in spatial and nearly temporal coincidence. The
Fig. 3. Ža. Induced absorption of the epitaxial layer. Žb. Induced absorption of the polycrystalline layer.
D. Ohlmann et al.r Thin Solid Films 318 (1998) 151–153
Fig. 4. Variation of the photon energy of the maximum induced absorption as a function of the composition x.
sample absorbs the photons of the test beam to create carriers. In presence of the pump beam, such carriers are already generated when the probe pulse arrives. The occupied levels of the carriers are no longer available for the carriers excited by the probe photons which should result in a bleaching of the absorption as studied in polycrystalline films w4x. We observe, however, an important increase of the absorption Ž16%. in these epitaxial films as well as in bulk monocrystals. This induced absorption ŽFig. 3a for a sample with 16% Zn and excitation at l p s 700 nm. is attributed to defect levels near the fundamental absorption edge in the band gap. The fluctuating potential due to the substitution of Cd atoms by Zn atoms introduces states below the fundamental absorption edge. The carriers excited by the absorption of the pump beam create a bound excitonic complex in this potential. When the probe pulses arrive, the complexes are increased by binding further excitons. We have grown thin films with different compositions in Cd and Zn atoms by changing the concentration x of the ZnTe powders in the cold-pressed targets and we did the same experiment with each of the layers. Since the energy gap of CdZnTe depends on x, we had to change the emission of the dye lasers in order to study the same spectral region close to the band gap. We observe an induced absorption in all our samples indicating that the high optical nonlinearity is adjustable over a wide spectral range ŽFig. 4..
4. Comparison between layers grown by two different methods The results obtained from the epilayers deposited by PLD were then compared to those obtained on CdZnTe thin films grown by hot wall evaporation on a glass
153
substrate in our laboratory w4x. The two samples were elaborated, first with the same powders in order to avoid the influence of the origin of the material and then with powders of different origins. As shown in Fig. 3b, the films Ž x s 0.2. evaporated by HWE present an induced absorption for lower photon energies and a decrease of the absorption at higher ones. The induced absorption is found to be approximately at the same spectral position for the layers grown by the two methods and we can attribute this absorption mechanism to the defect levels as discussed above. No absorption bleaching is observed in the films grown by PLD. It appears that the main difference between the two samples is the epitaxial character of the laser deposition layers compared to the polycrystalline character of the evaporated layers. Thus, for the evaporated layers, the transmission under high excitation is the same in the spectral region studied in the previous part but at higher photon energies, additional states exist in the HWE samples which can be filled by the pump beam excitation. Therefore we observe a competition with this induced transparency also observed in an earlier paper w4x under some different experimental conditions.
5. Conclusion Pulsed laser deposition enables the growth of ternary Cd 1y x Zn xTe epilayers with different Zn concentrations. These films show an important induced absorption near the fundamental absorption edge in the band gap. This optical nonlinearity is adjustable over an important region of the visible spectrum between 1.6 and 2.1 eV by changing the composition x. This optical nonlinearity is attributed to defect levels in the band gap.
Acknowledgements This research is supported by a contract with the ‘Office ´ National d’Etudes et de Recherche Aerospatiales’. ´
References w1x R. Triboulet, G. Neu, B. Fotouhi, J. Cryst. Growth 65 Ž1983. 262. w2x J.J. Kennedy, P.M. Amirtharaj, P.R. Boyd, S.B. Qadri, R.C. Dobbyn, G.G. Long, J. Cryst. Growth 86 Ž1988. 93. w3x J.L. Reno, E.D. Jones, Phys. Rev. B 45 Ž1992. 1440. w4x V. Netiksis, B. Honerlage, R. Weil, J.L. Loison, J.B. Grun, R. Levy, ¨ J. Appl. Phys. 74 Ž1993. 5729. w5x S.L. Bell, S. Sen, J. Vac. Sci. Technol. A 3 Ž1995. 112. w6x D. Ohlmann, M. Mazilu, R. Levy, B. Honerlage, J. Appl. Phys. 82 ¨ Ž1997. to be published.
Optical nonlinear properties of Cd 1yx Zn xTe epilayers grown on ž100 / GaAs by pulsed laser deposition D. Ohlmann ) , J.L. Deiss, J.L. Loison, M. Robino, G. Versini Institut de Physique et Chimie des Materiaux de Strasbourg UMR 46, CNRS-ULP-EHICS, Groupe d’Optique Nonlineaire et d’Optoelectronique 23, ´ ´ ´ rue du Loess, BP 20 CR, 67037 Strasbourg Cedex, France
Abstract This paper reports induced absorption in Cd 1yx Zn xTe thin films epitaxially grown by pulsed laser deposition on Ž100.-oriented GaAs substrate. The high optical nonlinearity is adjustable in the visible over a wide spectral region as the concentration x is varied. q 1998 Elsevier Science S.A. Keywords: Deposition; Lattice; Substrate
1. Introduction Different methods are used to grow Cd 1y x Zn xTe bulk material w1,2x and thin layers w3,4x. The change of the relative concentrations of Cd and Zn atoms results in a variation of the lattice parameter and of the band gap between the values of the binary compounds CdTe and ZnTe. The change of the lattice parameter is interesting since CdZnTe is used as a substrate for IR detectors, see for example for HgCdTe w5x where the lattice parameter has to be matched with that of the sensible material. CdZnTe is also a promising material for X-ray and g-ray detectors at room temperature. The variation of the band gap is used to change the strong optical nonlinearity in a wide region of the visible spectrum w6x. This paper describes, in its first part, the epitaxial growth of Cd 1y x Zn xTe by pulsed laser deposition and, in its second one, optical properties of the layers with different Zn concentrations x. In Section 5, the nonlinear optical properties are compared to that of polycrystalline layers grown by a hot wall evaporation technique ŽHWE.. 2. Growth of the epitaxial films The films we describe in this paper are grown by pulsed laser deposition ŽPLD.. In this technique, a XeCl excimer laser emitting pulses of 20 ns duration at 10 Hz in the UV )
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 5 5 - 3
region Ž308 nm. is focalized at room temperature onto a 4 cm diameter rotating target of the ternary material CdZnTe with the desired composition. The fluence is 2 Jrcm2 and the growth rate 1 m mrh. The important fluence creates a plasma around the focus point of the beam and atoms are ejected from the target. In the vacuum of 10y6 Torr, they migrate to the substrate which is mounted in distance at 4 cm above the target and can be heated. As a substrate, we use GaAs single crystalline wafer oriented in the direction Ž100., chemically etched and pre-heated in the vacuum cell in order to eliminate any contamination of the surface. The substrate temperature during deposition is about 3008C. Two different targets were used, the first one was bulk polycrystalline material with x s 0.20, a concentration which is commercially available. For other compositions, we had to prepare the targets because there is no commercial bulk material with high Zn concentrations available. The cold-pressed targets were obtained by mixing high purity fine grained CdTe and ZnTe powders in the desired concentrations. We began our studies with a cold-pressed target having the same composition as the bulk one. The X-ray diffraction pattern exhibits peaks only in the directions Ž n00.. Strong Ž400. diffraction lines are observed which indicate a good epitaxy of the film on the Ž100. oriented GaAs substrate. Fig. 1 shows the X-ray diffraction of epilayers grown from the two targets. The peaks for the layers grown from the bulk polycrystalline target are broad compared to the peaks of binary compounds and the peaks for the layers grown with the pressed target are even broader indicating the importance of the target quality.
152
D. Ohlmann et al.r Thin Solid Films 318 (1998) 151–153
Fig. 1. X-ray diffraction for the polycrystalline target and cold-pressed targets with different x values.
Ternary materials always have a distorted lattice because the lattice constants of the contributing binaries are different. In addition, the atom sizes are different and thus all the features are broaden in the diffraction pattern. AFM ŽAtomic Force Microscopy. images give a good crystalline surface quality of the film with a surface roughness of 3 nm over a 5 m m scan spot. The thickness of the samples is about 1 m m. By measuring the lattice constant, the composition of the layer is determined from Vegard’s law.
Fig. 2. Transmission of the samples.
spot of the pump beam is larger then that of the test beam in order to probe a quasi-homogeneously excited central region. The pump beam consists of quite intense Ž300 kWrcm2 . and spectrally narrow light pulses of 15 ns duration. The probe beam is a spectrally broad beam of low intensity in order to avoid induced nonlinearities in the sample. The transmission of the sample is measured with and without excitation of the pump beam. It decreases when it is excited by the intense laser beam indicating induced absorption. Without the pump beam excitation, the
3. Optical properties 3.1. Linear optical properties The GaAs substrates are not transparent in the visible part of the spectrum while we are interested in studying the vicinity of the fundamental band gap of CdZnTe Žbetween 1.54 to 2.26 eV.. Thus, we removed the substrate, by gluing the CdZnTe film on a glass plate and by polishing mechanically and finally etching chemically the GaAs. The typical transmission spectrum of such a sample is shown in Fig. 2, the interference fringes show the good quality of the film. They result from interfering multiple reflections from the two surfaces of good optical quality of the film forming a Fabry–Perot cavity. The transmission coefficient is relatively high in the transparent region Ž0.7 to 0.8.. The optical transmission coefficient and the modulation of interference fringes of these films are better than those of samples grown by HWE after annealing. 3.2. Nonlinear optical properties Nonlinear optical properties of epilayers of different composition are studied by pump and probe experiments, where two pulsed dye lasers are optically pumped with the excimer laser. We focalize the two laser beams onto the sample in spatial and nearly temporal coincidence. The
Fig. 3. Ža. Induced absorption of the epitaxial layer. Žb. Induced absorption of the polycrystalline layer.
D. Ohlmann et al.r Thin Solid Films 318 (1998) 151–153
Fig. 4. Variation of the photon energy of the maximum induced absorption as a function of the composition x.
sample absorbs the photons of the test beam to create carriers. In presence of the pump beam, such carriers are already generated when the probe pulse arrives. The occupied levels of the carriers are no longer available for the carriers excited by the probe photons which should result in a bleaching of the absorption as studied in polycrystalline films w4x. We observe, however, an important increase of the absorption Ž16%. in these epitaxial films as well as in bulk monocrystals. This induced absorption ŽFig. 3a for a sample with 16% Zn and excitation at l p s 700 nm. is attributed to defect levels near the fundamental absorption edge in the band gap. The fluctuating potential due to the substitution of Cd atoms by Zn atoms introduces states below the fundamental absorption edge. The carriers excited by the absorption of the pump beam create a bound excitonic complex in this potential. When the probe pulses arrive, the complexes are increased by binding further excitons. We have grown thin films with different compositions in Cd and Zn atoms by changing the concentration x of the ZnTe powders in the cold-pressed targets and we did the same experiment with each of the layers. Since the energy gap of CdZnTe depends on x, we had to change the emission of the dye lasers in order to study the same spectral region close to the band gap. We observe an induced absorption in all our samples indicating that the high optical nonlinearity is adjustable over a wide spectral range ŽFig. 4..
4. Comparison between layers grown by two different methods The results obtained from the epilayers deposited by PLD were then compared to those obtained on CdZnTe thin films grown by hot wall evaporation on a glass
153
substrate in our laboratory w4x. The two samples were elaborated, first with the same powders in order to avoid the influence of the origin of the material and then with powders of different origins. As shown in Fig. 3b, the films Ž x s 0.2. evaporated by HWE present an induced absorption for lower photon energies and a decrease of the absorption at higher ones. The induced absorption is found to be approximately at the same spectral position for the layers grown by the two methods and we can attribute this absorption mechanism to the defect levels as discussed above. No absorption bleaching is observed in the films grown by PLD. It appears that the main difference between the two samples is the epitaxial character of the laser deposition layers compared to the polycrystalline character of the evaporated layers. Thus, for the evaporated layers, the transmission under high excitation is the same in the spectral region studied in the previous part but at higher photon energies, additional states exist in the HWE samples which can be filled by the pump beam excitation. Therefore we observe a competition with this induced transparency also observed in an earlier paper w4x under some different experimental conditions.
5. Conclusion Pulsed laser deposition enables the growth of ternary Cd 1y x Zn xTe epilayers with different Zn concentrations. These films show an important induced absorption near the fundamental absorption edge in the band gap. This optical nonlinearity is adjustable over an important region of the visible spectrum between 1.6 and 2.1 eV by changing the composition x. This optical nonlinearity is attributed to defect levels in the band gap.
Acknowledgements This research is supported by a contract with the ‘Office ´ National d’Etudes et de Recherche Aerospatiales’. ´
References w1x R. Triboulet, G. Neu, B. Fotouhi, J. Cryst. Growth 65 Ž1983. 262. w2x J.J. Kennedy, P.M. Amirtharaj, P.R. Boyd, S.B. Qadri, R.C. Dobbyn, G.G. Long, J. Cryst. Growth 86 Ž1988. 93. w3x J.L. Reno, E.D. Jones, Phys. Rev. B 45 Ž1992. 1440. w4x V. Netiksis, B. Honerlage, R. Weil, J.L. Loison, J.B. Grun, R. Levy, ¨ J. Appl. Phys. 74 Ž1993. 5729. w5x S.L. Bell, S. Sen, J. Vac. Sci. Technol. A 3 Ž1995. 112. w6x D. Ohlmann, M. Mazilu, R. Levy, B. Honerlage, J. Appl. Phys. 82 ¨ Ž1997. to be published.