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{111} Quantum wells of dilute nitrides grown on GaAs by molecular beam epitaxy
ARTICLE IN PRESS
Physica E 23 (2004) 352–355 www.elsevier.com/locate/physe
f1 1 1g Quantum wells of dilute nitrides grown on GaAs by molecular beam epitaxy A. Arnoult*, F. Gonzalez-Posada, S. Blanc, V. Bardinal, C. Fontaine LAAS-CNRS, 7 Avenue du Colonel Roche, Toulouse 31077 Cedex 4, France
Abstract Dilute nitrides are promising alloys in view of extending potential micro- and opto-electronics applications of GaAs technology. Orientation effects on nitrogen incorporation in GaAs have been scarcely addressed. Here, GaAsN on ð1 0 0Þ and on As(B)- and Ga(A)-rich ð1 1 1Þ substrates was grown by molecular beam epitaxy at different substrate temperatures. Nitrogen content measured by secondary ion mass spectrometry as a function of the growth temperature highlights the influence of orientation on nitrogen incorporation. Furthermore, thermal annealing is shown to improve the optical quality of GaAsN quantum wells whatever their substrate orientations. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 75.55.m; 82.80.MS Keywords: Dilute nitrides; Molecular beam epitaxy; III–V semiconductors; Photoluminescence; SIMS
1. Introduction
2. Samples description
The optical emission at 1:3 mm is of major interest for fibre optics communication and can be obtained on GaAs by incorporating nitrogen in GaInAs quantum wells (QWs). Operating lasers with GaInAsN QWs have already successfully been achieved on ð1 0 0Þ oriented substrates. We have extended this study to nitrogen incorporation in GaAs on ð1 1 1Þ oriented substrates. In this paper, we discuss the link between orientation, nitrogen incorporation, and growth conditions.
The samples have been grown by molecular beam epitaxy (MBE). The RIBER-32 MBE ultrahigh vacuum system used is equipped with a HD25R Oxford Applied Research RF plasma cell in order to provide reactive N species [1]. Ultrapure N2 is obtained from 6N nitrogen flowing through a heating getter filter to remove O2 ; H2 O; CO, CO2 and other impurities. In order to relate nitrogen incorporation to the substrate orientation, differently oriented GaAs substrates were mounted using In soldering on a single silicon wafer for MBE growth. Substrates were radially soldered in order to reduce heterogeneity effects which have later been measured not to be relevant.
*Corresponding author. Tel.: +33-5-61-33-63-81; fax: +335-61-33-62-08. E-mail address: [email protected] (A. Arnoult).
1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.01.019
ARTICLE IN PRESS A. Arnoult et al. / Physica E 23 (2004) 352–355
Each Si slice supported semi-insulating GaAs pieces with the following orientations: ð1 0 0Þ nominal, ð1 1 1ÞB 2:5 off ½2 1 1 and ð1 1 1ÞA 2:5 off ½2% 1 1: Because of the ambiguity between European and American notations, it is worth to note that these misorientations lead to the f1 1 1gB vicinal surfaces consisting nominal f1 1 1gB terraces separated by f1 1 1gA type steps, and to the opposite vicinal f1 1 1gA vicinal surfaces. Two kinds of samples were grown in order to study nitrogen incorporation. The V/III ratio for all the samples was kept constant and equal to 10. First, a stack of 200 nm thick GaAsN layers separated by 50 nm thick GaAs spacer layers (sample A) was used in order to measure the nitrogen incorporation by secondary ion mass spectroscopy (SIMS). Second, 10 nm wide GaAs1x Nx =GaAs ð0pxp0:03Þ single quantum wells (QW) were grown at 520 C in order to study their optical properties and rapid thermal annealing effects by photoluminescence spectroscopy (PL). For SIMS analyses, Csþ primary ions and positive secondary ions CsMþ were used in order to provide quantitative measurements of the N content up to a few percent. A ð1 0 0Þ oriented GaAsN layer previously characterised by XRD served as a reference. PL measurements were carried out using the 514:5 nm line of an Arþ laser source. The emission light was analyzed by a Jobin–Yvon HR1000 monochromator. The scattered light was detected by means of an InGaAs photodiode, an amplifier and a lock-in detection.
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different sample orientations which were simultaneously used. The amount of nitrogen incorporated, [N], is observed to highly depend on surface orientation. It is higher for ð1 1 1ÞA than for ð1 0 0Þ; and lower for ð1 1 1ÞB as had already been reported [2]. Such a result can originate from the differences in morphology, symmetry, and reconstruction for surfaces of these three orientations. For each orientation, our results show that [N] also depends on the surface temperature. Just considering thermal effects, one would expect [N] to remain constant if N incorporation efficiency is stable or to follow an activation energy-like law, and thus to decrease as the temperature increases, if N desorption occurs from the growing surface. For the ð1 1 1ÞA case, [N] is found to be relatively constant which would indicate an efficient and stable N incorporation for the temperature range investigated. On the contrary, for ð1 1 1ÞB; [N] is observed to be divided by about three when rising temperature from 520 C to 580 C: The normalized [N] as a function of growth temperature is observed not to fit the expected Arrhenius behaviour and these N incorporation changes then have to be accounted for by another phenomenon. We believe as already proposed by
3. SIMS measurements The amount of nitrogen incorporated in GaAsN as a function of substrate temperature was measured by SIMS on sample A. The temperature was ramped by steps of 20 C from layer to layer, from 470 C up to 610 C: Finally, the sample was capped by a GaAsN layer grown at 530 C in order to check reproducibility and withdraw any suspicion of drift in the measurements. The results obtained are shown in Fig. 1, where the nitrogen content is presented as a function of the depth from the sample surface for the three
Fig. 1. Nitrogen content as measured by SIMS on three samples of different orientations simultaneously grown. The growth temperature is indicated on the graph for each GaAsN layer.
ARTICLE IN PRESS 354
A. Arnoult et al. / Physica E 23 (2004) 352–355
Zhang and Zunger [3] that the [N] incorporation is affected by a change in surface reconstruction. Indeed, the reconstruction of the ð1 1 1ÞB surfaces pffiffiffiffiffi is ffiffiffiffiffi known to evolve from a ð2 2Þ to a ð 19
p 19Þ between 520 and 570 [4]. Moreover, the ð2 p 2Þ ffiffiffiffiffi surface exhibits 2.6 times more arsenic that pffiffiffiffiffi the ð 19 19Þ surface, which is consistent with the [N] variation estimated between 520 and 580 from this SIMS analysis considering the nitrogen atoms are incorporated in the arsenic sites. It is worth recalling that the surface reconstruction for the ð1 1 1ÞA surface, on the contrary, remains a ð2 2Þ for the temperature range explored [5] which prevents any [N] desorption due to such a reconstruction change effect. The dependence of [N] on the substrate temperature for the ð1 0 0Þ case is similar to the ð1 1 1ÞB case. The ð1 0 0Þ surface is also known to exhibit a change in its static surface reconstruction from cð4 4Þ to ð2 4Þ at about 520 C when increasing the substrate temperature, while the dynamic reconstruction during the growth remains ð2 4Þ: For the present case, the [N] variation is probably more related to a change in surface morphology when passing 520 C: Indeed, the RHEED pattern of the growing GaAsN surface, which is streaky below 520 C; clearly becomes spotty above this temperature. This can reasonably be attributed to a 3D-like growth above 520 C: Moreover the RHEED of the growing GaAs layers between the GaAsN layers is streaky whatever the growth temperature, which shows that these intermediary layers allow for surface flatness recovery. On the contrary, the observed SIMS signal for nitrogen, decreases along growth of the GaAsN layers from 530 C to 590 C (see Fig. 1). This can be attributed to a better nitrogen incorporation at the beginning of each GaAsN layer where the surface is still 2D-like; then [N] continuously decreases during the growth of GaAsN as the surface progressively becomes rougher.
due to atomic gement inside the GaAs matrix. [6] we have performed thermal annealing on ð1 1 1Þ-Aand -B-oriented samples in order to check if this curing effect also occurs for these orientations. A single GaAsN/GaAs QW sample has been grown simultaneously on ð1 1 1ÞA; ð1 0 0Þ and ð1 1 1ÞB oriented substrates. The nitrogen plasma during the growth of this particular QW was operating at a power of 325 W and a N2 flow of 0:2 sccm: SIMS measurements on thick layers grown under the same conditions indicate the nitrogen concentration to be 1.9% on ð1 1 1ÞA; 1.7% on ð1 0 0Þ and 1.5% on ð1 1 1ÞB (Fig. 2). Fig. 3 shows the 12 K PL spectra for the three samples. They clearly evidence a difference in emission wavelength, and thus in nitrogen incorporation as a function of the substrate orientation too. The ð1 1 1ÞB leads to the lowest wavelength, and the ð1 1 1ÞA to the highest, as expected from their nitrogen content estimated by means of SIMS. Whereas the PL spectra of ð1 1 1ÞA and ð1 0 0Þ samples exhibit a single peak, the PL spectrum of the ð1 1 1ÞB sample is very weak and shows a peak at 1:1 mm and a broad quasicontinuum up to our detection limit ð1:6 mmÞ: These shapes have already been reported and discussed elsewhere [2]. These samples were cleaved and a series of rapid thermal annealing under different conditions, temperatures and durations, was then performed.
4. Spectroscopy Rapid thermal annealing has been shown to improve optical quality of ð1 0 0Þ GaInAsN QWs,
Fig. 2. Normalised nitrogen contents as a function of growth temperature derived from the SIMS data of Fig. 1.
ARTICLE IN PRESS A. Arnoult et al. / Physica E 23 (2004) 352–355
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broad emission at high wavelengths is less intense than the emission peak observed at low wavelengths after annealing which sustains this hypothesis. Already seen in the ð1 0 0Þ case, the thermal annealing treatment is thus observed to improve the optical quality of samples grown on ð1 1 1ÞA and ð1 1 1ÞB oriented samples too.
5. Conclusions
Fig. 3. 12 K PL spectra of three differently oriented samples before and after a thermal annealing of 750 C during 60 s:
The optimal annealing conditions obtained for these samples were found to be 750 C and 60 s durations. The PL spectra of the annealed samples are shown in Fig. 3. The PL intensity of the main peak is multiplied by an amount of two for the ð1 1 1ÞA and ð1 0 0Þ ones. A blue shift of about 20 meV is observed for ð1 1 1ÞA and ð1 0 0Þ after annealing. The full-width at half-maximum (FWHM) of the emission peak observed for the ð1 1 1ÞA sample slightly increases from 30 to 35 meV when annealed, whereas the peak FWHM of the ð1 0 0Þ QW decreases from 47 to 38 meV: The ð1 1 1ÞB sample is of poor quality compared to the two other orientations, but this quality is slightly improved by the annealing treatment: the intensity of the peak at 1:1 mm is about four times higher, and a blue shift of about 80 meV is measured. For this orientation, the blue shift is then more pronounced than for the two others. This blue shift could be due to a cure of point defects originating from nitrogen incorporation which would occur in addition to the N rearrangement generally proposed for ð1 0 0Þ: Note that the
Nitrogen incorporation as a function of crystal orientation and growth temperature has been investigated. It has been found to highly depend on surface orientation. A link between surface reconstructions and nitrogen incorporation has been proposed. Nitrogen incorporation in GaAs has been shown to be related to the atomic arrangement of the growing surface. Rapid thermal annealing of GaAsN quantum wells grown on ð1 1 1ÞA- and ð1 1 1ÞB-oriented substrates is observed to improve their optical quality and to lead to a blue shift of their main emission line. This behaviour had already been reported on samples grown on ð1 0 0Þ oriented substrates.
References [1] H. Carr"ere, A. Arnoult, A. Ricard, E. Bedel, E. BedelPereira, J. Cryst. Growth 243 (2002) 295. [2] S. Blanc, A. Arnoult, H. Carr"ere, E. Bedel, G. Lacoste, C. Fontaine, M. Cabi!e, A. Ponchet, A. Rocher, IEE Proc. Optoelectron. 150 (2003) 64. [3] S.B. Zhang, A. Zunger, Appl. Phys. Lett. 71 (1997) 677. [4] D.A. Woolf, D.I. Westwood, R.H. Williams, Semicond. Sci. Technol. 8 (1993) 1075. [5] M.R. Fahy, K. Sato, B.A. Joyce, Appl. Phys. Lett. 64 (1994) 190. [6] E.V.K. Rao, A. Ougazzaden, Y. Le Bellego, M. Juhel, Appl. Phys. Lett. 72 (1998) 1409.
Physica E 23 (2004) 352–355 www.elsevier.com/locate/physe
f1 1 1g Quantum wells of dilute nitrides grown on GaAs by molecular beam epitaxy A. Arnoult*, F. Gonzalez-Posada, S. Blanc, V. Bardinal, C. Fontaine LAAS-CNRS, 7 Avenue du Colonel Roche, Toulouse 31077 Cedex 4, France
Abstract Dilute nitrides are promising alloys in view of extending potential micro- and opto-electronics applications of GaAs technology. Orientation effects on nitrogen incorporation in GaAs have been scarcely addressed. Here, GaAsN on ð1 0 0Þ and on As(B)- and Ga(A)-rich ð1 1 1Þ substrates was grown by molecular beam epitaxy at different substrate temperatures. Nitrogen content measured by secondary ion mass spectrometry as a function of the growth temperature highlights the influence of orientation on nitrogen incorporation. Furthermore, thermal annealing is shown to improve the optical quality of GaAsN quantum wells whatever their substrate orientations. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 75.55.m; 82.80.MS Keywords: Dilute nitrides; Molecular beam epitaxy; III–V semiconductors; Photoluminescence; SIMS
1. Introduction
2. Samples description
The optical emission at 1:3 mm is of major interest for fibre optics communication and can be obtained on GaAs by incorporating nitrogen in GaInAs quantum wells (QWs). Operating lasers with GaInAsN QWs have already successfully been achieved on ð1 0 0Þ oriented substrates. We have extended this study to nitrogen incorporation in GaAs on ð1 1 1Þ oriented substrates. In this paper, we discuss the link between orientation, nitrogen incorporation, and growth conditions.
The samples have been grown by molecular beam epitaxy (MBE). The RIBER-32 MBE ultrahigh vacuum system used is equipped with a HD25R Oxford Applied Research RF plasma cell in order to provide reactive N species [1]. Ultrapure N2 is obtained from 6N nitrogen flowing through a heating getter filter to remove O2 ; H2 O; CO, CO2 and other impurities. In order to relate nitrogen incorporation to the substrate orientation, differently oriented GaAs substrates were mounted using In soldering on a single silicon wafer for MBE growth. Substrates were radially soldered in order to reduce heterogeneity effects which have later been measured not to be relevant.
*Corresponding author. Tel.: +33-5-61-33-63-81; fax: +335-61-33-62-08. E-mail address: [email protected] (A. Arnoult).
1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.01.019
ARTICLE IN PRESS A. Arnoult et al. / Physica E 23 (2004) 352–355
Each Si slice supported semi-insulating GaAs pieces with the following orientations: ð1 0 0Þ nominal, ð1 1 1ÞB 2:5 off ½2 1 1 and ð1 1 1ÞA 2:5 off ½2% 1 1: Because of the ambiguity between European and American notations, it is worth to note that these misorientations lead to the f1 1 1gB vicinal surfaces consisting nominal f1 1 1gB terraces separated by f1 1 1gA type steps, and to the opposite vicinal f1 1 1gA vicinal surfaces. Two kinds of samples were grown in order to study nitrogen incorporation. The V/III ratio for all the samples was kept constant and equal to 10. First, a stack of 200 nm thick GaAsN layers separated by 50 nm thick GaAs spacer layers (sample A) was used in order to measure the nitrogen incorporation by secondary ion mass spectroscopy (SIMS). Second, 10 nm wide GaAs1x Nx =GaAs ð0pxp0:03Þ single quantum wells (QW) were grown at 520 C in order to study their optical properties and rapid thermal annealing effects by photoluminescence spectroscopy (PL). For SIMS analyses, Csþ primary ions and positive secondary ions CsMþ were used in order to provide quantitative measurements of the N content up to a few percent. A ð1 0 0Þ oriented GaAsN layer previously characterised by XRD served as a reference. PL measurements were carried out using the 514:5 nm line of an Arþ laser source. The emission light was analyzed by a Jobin–Yvon HR1000 monochromator. The scattered light was detected by means of an InGaAs photodiode, an amplifier and a lock-in detection.
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different sample orientations which were simultaneously used. The amount of nitrogen incorporated, [N], is observed to highly depend on surface orientation. It is higher for ð1 1 1ÞA than for ð1 0 0Þ; and lower for ð1 1 1ÞB as had already been reported [2]. Such a result can originate from the differences in morphology, symmetry, and reconstruction for surfaces of these three orientations. For each orientation, our results show that [N] also depends on the surface temperature. Just considering thermal effects, one would expect [N] to remain constant if N incorporation efficiency is stable or to follow an activation energy-like law, and thus to decrease as the temperature increases, if N desorption occurs from the growing surface. For the ð1 1 1ÞA case, [N] is found to be relatively constant which would indicate an efficient and stable N incorporation for the temperature range investigated. On the contrary, for ð1 1 1ÞB; [N] is observed to be divided by about three when rising temperature from 520 C to 580 C: The normalized [N] as a function of growth temperature is observed not to fit the expected Arrhenius behaviour and these N incorporation changes then have to be accounted for by another phenomenon. We believe as already proposed by
3. SIMS measurements The amount of nitrogen incorporated in GaAsN as a function of substrate temperature was measured by SIMS on sample A. The temperature was ramped by steps of 20 C from layer to layer, from 470 C up to 610 C: Finally, the sample was capped by a GaAsN layer grown at 530 C in order to check reproducibility and withdraw any suspicion of drift in the measurements. The results obtained are shown in Fig. 1, where the nitrogen content is presented as a function of the depth from the sample surface for the three
Fig. 1. Nitrogen content as measured by SIMS on three samples of different orientations simultaneously grown. The growth temperature is indicated on the graph for each GaAsN layer.
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Zhang and Zunger [3] that the [N] incorporation is affected by a change in surface reconstruction. Indeed, the reconstruction of the ð1 1 1ÞB surfaces pffiffiffiffiffi is ffiffiffiffiffi known to evolve from a ð2 2Þ to a ð 19
p 19Þ between 520 and 570 [4]. Moreover, the ð2 p 2Þ ffiffiffiffiffi surface exhibits 2.6 times more arsenic that pffiffiffiffiffi the ð 19 19Þ surface, which is consistent with the [N] variation estimated between 520 and 580 from this SIMS analysis considering the nitrogen atoms are incorporated in the arsenic sites. It is worth recalling that the surface reconstruction for the ð1 1 1ÞA surface, on the contrary, remains a ð2 2Þ for the temperature range explored [5] which prevents any [N] desorption due to such a reconstruction change effect. The dependence of [N] on the substrate temperature for the ð1 0 0Þ case is similar to the ð1 1 1ÞB case. The ð1 0 0Þ surface is also known to exhibit a change in its static surface reconstruction from cð4 4Þ to ð2 4Þ at about 520 C when increasing the substrate temperature, while the dynamic reconstruction during the growth remains ð2 4Þ: For the present case, the [N] variation is probably more related to a change in surface morphology when passing 520 C: Indeed, the RHEED pattern of the growing GaAsN surface, which is streaky below 520 C; clearly becomes spotty above this temperature. This can reasonably be attributed to a 3D-like growth above 520 C: Moreover the RHEED of the growing GaAs layers between the GaAsN layers is streaky whatever the growth temperature, which shows that these intermediary layers allow for surface flatness recovery. On the contrary, the observed SIMS signal for nitrogen, decreases along growth of the GaAsN layers from 530 C to 590 C (see Fig. 1). This can be attributed to a better nitrogen incorporation at the beginning of each GaAsN layer where the surface is still 2D-like; then [N] continuously decreases during the growth of GaAsN as the surface progressively becomes rougher.
due to atomic gement inside the GaAs matrix. [6] we have performed thermal annealing on ð1 1 1Þ-Aand -B-oriented samples in order to check if this curing effect also occurs for these orientations. A single GaAsN/GaAs QW sample has been grown simultaneously on ð1 1 1ÞA; ð1 0 0Þ and ð1 1 1ÞB oriented substrates. The nitrogen plasma during the growth of this particular QW was operating at a power of 325 W and a N2 flow of 0:2 sccm: SIMS measurements on thick layers grown under the same conditions indicate the nitrogen concentration to be 1.9% on ð1 1 1ÞA; 1.7% on ð1 0 0Þ and 1.5% on ð1 1 1ÞB (Fig. 2). Fig. 3 shows the 12 K PL spectra for the three samples. They clearly evidence a difference in emission wavelength, and thus in nitrogen incorporation as a function of the substrate orientation too. The ð1 1 1ÞB leads to the lowest wavelength, and the ð1 1 1ÞA to the highest, as expected from their nitrogen content estimated by means of SIMS. Whereas the PL spectra of ð1 1 1ÞA and ð1 0 0Þ samples exhibit a single peak, the PL spectrum of the ð1 1 1ÞB sample is very weak and shows a peak at 1:1 mm and a broad quasicontinuum up to our detection limit ð1:6 mmÞ: These shapes have already been reported and discussed elsewhere [2]. These samples were cleaved and a series of rapid thermal annealing under different conditions, temperatures and durations, was then performed.
4. Spectroscopy Rapid thermal annealing has been shown to improve optical quality of ð1 0 0Þ GaInAsN QWs,
Fig. 2. Normalised nitrogen contents as a function of growth temperature derived from the SIMS data of Fig. 1.
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broad emission at high wavelengths is less intense than the emission peak observed at low wavelengths after annealing which sustains this hypothesis. Already seen in the ð1 0 0Þ case, the thermal annealing treatment is thus observed to improve the optical quality of samples grown on ð1 1 1ÞA and ð1 1 1ÞB oriented samples too.
5. Conclusions
Fig. 3. 12 K PL spectra of three differently oriented samples before and after a thermal annealing of 750 C during 60 s:
The optimal annealing conditions obtained for these samples were found to be 750 C and 60 s durations. The PL spectra of the annealed samples are shown in Fig. 3. The PL intensity of the main peak is multiplied by an amount of two for the ð1 1 1ÞA and ð1 0 0Þ ones. A blue shift of about 20 meV is observed for ð1 1 1ÞA and ð1 0 0Þ after annealing. The full-width at half-maximum (FWHM) of the emission peak observed for the ð1 1 1ÞA sample slightly increases from 30 to 35 meV when annealed, whereas the peak FWHM of the ð1 0 0Þ QW decreases from 47 to 38 meV: The ð1 1 1ÞB sample is of poor quality compared to the two other orientations, but this quality is slightly improved by the annealing treatment: the intensity of the peak at 1:1 mm is about four times higher, and a blue shift of about 80 meV is measured. For this orientation, the blue shift is then more pronounced than for the two others. This blue shift could be due to a cure of point defects originating from nitrogen incorporation which would occur in addition to the N rearrangement generally proposed for ð1 0 0Þ: Note that the
Nitrogen incorporation as a function of crystal orientation and growth temperature has been investigated. It has been found to highly depend on surface orientation. A link between surface reconstructions and nitrogen incorporation has been proposed. Nitrogen incorporation in GaAs has been shown to be related to the atomic arrangement of the growing surface. Rapid thermal annealing of GaAsN quantum wells grown on ð1 1 1ÞA- and ð1 1 1ÞB-oriented substrates is observed to improve their optical quality and to lead to a blue shift of their main emission line. This behaviour had already been reported on samples grown on ð1 0 0Þ oriented substrates.
References [1] H. Carr"ere, A. Arnoult, A. Ricard, E. Bedel, E. BedelPereira, J. Cryst. Growth 243 (2002) 295. [2] S. Blanc, A. Arnoult, H. Carr"ere, E. Bedel, G. Lacoste, C. Fontaine, M. Cabi!e, A. Ponchet, A. Rocher, IEE Proc. Optoelectron. 150 (2003) 64. [3] S.B. Zhang, A. Zunger, Appl. Phys. Lett. 71 (1997) 677. [4] D.A. Woolf, D.I. Westwood, R.H. Williams, Semicond. Sci. Technol. 8 (1993) 1075. [5] M.R. Fahy, K. Sato, B.A. Joyce, Appl. Phys. Lett. 64 (1994) 190. [6] E.V.K. Rao, A. Ougazzaden, Y. Le Bellego, M. Juhel, Appl. Phys. Lett. 72 (1998) 1409.