- Email: [email protected]
Mid-infrared photoluminescence of InAsN dilute nitride alloys grown by LPE and MBE
Infrared Physics & Technology 55 (2012) 399–402
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Mid-infrared photoluminescence of InAsN dilute nitride alloys grown by LPE and MBE M. de la Mare a,⇑, Q. Zhuang a, S. Dhar b, A. Krier a a b
Physics Department, Lancaster University, Lancaster LA1 4YB, UK Department of Electronic Science, University of Calcutta 92, APC. Road, Kolkata-700 009, India
h i g h l i g h t s " Comparison between LPE and MBE growth methods. " Successful mid-infrared emission from both LPE and MBE material with nitrogen contents as high as 1%. " Narrower FWHM and XRD obtained from LPE material. " Superior activation energies from MBE increasing with N due to de-tuning of Auger recombination.
a r t i c l e
i n f o
Article history: Received 26 March 2012 Available online 29 June 2012 Keywords: Dilute nitrides Liquid phase epitaxy Molecular beam epitaxy Characterisation
a b s t r a c t Dilute nitride InAsN epitaxial layers were produced using both liquid phase and molecular beam epitaxial growth techniques. The spectral features in the photoluminescence of samples containing up to 1% N with emission energies in the mid-infrared spectral region are described and compared. The emission intensities of both LPE and MBE grown materials were found to be comparable. Increasing the N content in the InAsN alloys resulted in a significant increase in the activation energy for thermal quenching of the photoluminescence emission due to a combination of lowering of the conduction band edge and Auger de-tuning. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Narrow gap III-V dilute nitride materials such as InAsN have attracted particular interest because of their potential applications for use in optoelectronic devices operating in the mid-infrared spectral region which is rich in applications [1–4]. Small amounts of nitrogen added to III–V materials are known to result in significant band gap reduction [5], large band gap bowing [6], enhancement of the conduction band effective mass [7] and reduction in Auger recombination processes [8], all of which hold great potential for the development of improved photonic devices. These properties also make them a possible alternative to other mid infrared materials such as HgCdTe, PbSnTe and InAsSb. One potential advantage of using InAsN may be the ability to tune the band gap down to a very low value by controlling the amount of nitrogen incorporated, with previous reports of InAsN being grown onto InAs [9] GaAs [10] and InP [11] substrates displaying substantial band gap reductions. InAsN bulk layers and quantum wells have ⇑ Corresponding author. Tel.: +44 (0)1524 594750; fax: +44 (0)1524 844037. E-mail addresses: (M. de la Mare).
[email protected],
[email protected]
1350-4495/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2012.06.002
been grown by molecular beam epitaxy (MBE) [8–12] with reports of bright photoluminescence being observed [8,13,14]. Liquid phase epitaxy (LPE), which is a thermal equilibrium growth technique capable of producing epitaxial materials with excellent crystalline quality and high luminescence efficiencies, is believed to avoid some of the problems associated with ion damage from the N plasma used in MBE growth of dilute nitrides, and as such is an attractive alternative. In this paper we report on the growth of InAsN by both LPE and MBE techniques and the characterisation and comparison of the resulting epilayers. 2. Material and method LPE growth of InAsN layers was done in a conventional horizontal sliding boat reactor using a transparent gold furnace and high purity quartz tube. Prior to growth, the reactor together with the graphite boat, were baked for several hours at high temperature in Pd-diffused ultrapure hydrogen gas. Initially, 99.99999% pure In metal was loaded into the boat and baked at 780 °C for 18– 20 h under purified hydrogen flow. The required amount of 99.9999% polycrystalline InAs was then loaded to the In contained within the growth melt bin and again baked at 750 °C for a further
M. de la Mare et al. / Infrared Physics & Technology 55 (2012) 399–402 1.2
(a)
MBE InAs/InAs InAsN (0.4%) InAsN (0.65%) InAsN (0.8%)
x1/5
PL Intensity (a.u.)
18 h. Finally, a precisely weighed amount of 99.9% polycrystalline InN was added to the growth melt and baked at 700 °C for 1–2 h. Growth was typically carried out at 590 °C for 7–8 min using a melt super-saturation of 5–6 °C and a constant cooling rate of 0.3 °C/min onto InAs (1 0 0) substrates. Epitaxial layers obtained from this growth method were between 2 and 3 lm thick with smooth mirror like surface morphologies with no noticeable inclusions. For comparison, the MBE epilayers were grown onto both (1 0 0) InAs and (1 0 0) GaAs substrates. The preparation and oxide desorption in the growth chamber was carried out in the conventional manner [15]. InAsN epitaxial layers approximately 1 lm thick were grown at a growth temperature in the range of 400–440 °C using a VG-V80H MBE reactor with radio frequency nitrogen plasma source. A growth rate of 1 lm/h was employed using an As flux of 2 10 6 mbar. For the N-plasma, a power of 160 W and a flux of 5 10 6 mbar were used. The surface reconstruction was monitored by in situ reflection high energy electron diffraction (RHEED), while the substrate temperature was measured using a calibrated infrared pyrometer. After growth, the surface of the layers was inspected using a Nomarski phase contrast microscope. Structural properties were determined using high resolution X-ray diffraction. Photoluminescence (PL) was performed on all samples in the temperature range 4–300 K using a variable temperature continuous flow He cryostat. An Ar+ ion laser (514 nm, with maximum power density of 20 W cm 2 at the sample) was used for excitation. The emitted radiation was analysed using a 0.3 m Bentham M300 monochromator and detected with a cooled (77 K) InSb photodiode detector and digital lock-in amplifier.
0.8
0.4
0.0 2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
Wavelength (μm)
1.0
(b)
LPE ISN14 IABN8B IABN8A MDLM090720
InAsN(Bi) (x1/5)
0.8
Intensity (a.u.)
400
InAsN(0.5%)/InAs
0.6
InAsN(Bi)
0.4 InAsN/InAs
0.2
0.0
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
Wavelength (μm) Fig. 1. 4 K photoluminescence spectra of InAsN bulk epilayers with nitrogen contents in the range 0–1% grown by (a) MBE and (b) LPE.
3. Results and discussion Table 1 lists the various InAsN samples produced containing different amounts of N obtained from the two growth methods and summarises the corresponding properties of the PL emission spectra obtained in each case. Fig. 1 shows the 4 K photoluminescence spectra of bulk InAsN samples grown by (a) MBE and (b) LPE. While both sets of samples exhibit emission in the mid-infrared spectral range, the PL from the LPE material is centred around 3 lm. In contrast the MBE grown material displays a broad range of emission wavelengths. Smaller amounts of N are present within the LPE grown material due to N out-diffusion from the growth melt during the LPE growth process. Two of the LPE samples were grown using Bi neutral solvent
solution. The use of Bi as the solvent enables the grower to change the stoichiometry of the melt allowing one to vary the In/As ratio in the liquid phase beyond that of traditional investigations [16]. Bi is not incorporated into the solid phase and therefore does not influence the electro-physical characteristics of the material [17– 20]. The peak PL intensities of the samples from both growth methods are roughly comparable, with the MBE grown material producing slightly stronger emission. However, MBE grown homo-epitaxial InAs and LPE grown InAsN using neutral solvent solution (Bi), both exhibit 5 stronger emission intensities. We attribute this to a reduction in the number of non-radiative defect
Table 1 Summary of dilute nitride epilayers studied. Details include nitrogen content, 4 K peak PL wavelength and equivalent band gap as well as 4 K FWHM and activation energies. The LPE samples indicated with ⁄ are those grown using neutral solvent Bi, while $ indicates material grown on InAs substrates. Sample
N (%)
4 K k (lm)
4 K Eg (eV)
FWHM (meV)
DE (meV)
LPE MDLM090720 IABN8B⁄ IABN8A⁄ ISN14 SD091216
0.05 0.1 0.2 0.5 0.6
3.02 3.03 3.09 3.22 3.26
0.411 0.409 0.401 0.385 0.379
276 118 124 116 174
– 13.6 17.7 16.8 17.2
MBE A0129$ A0276 A0285 A0147$ A0300 A0282 A0213$ A0142$ A0299
0 0 0.2 0.4 0.4 0.6 0.65 0.8 1
2.99 2.97 3.04 3.2 3.14 3.27 3.4 3.54 3.66
0.415 0.417 0.408 0.388 0.395 0.379 0.365 0.35 0.339
74 97 101 167 184 190 106 261 317
17.9 6 21.2 – 20.6 25.7 – – 34.1
401
M. de la Mare et al. / Infrared Physics & Technology 55 (2012) 399–402
A0299
0.30 MDLM090720
FWHM (eV)
A0142
0.25 A0146
0.20
A0300
A0282
A0147
SD091216
0.15 ISN14
0.10
A0276
IABN8A IABN8B
A0285
A0213
A0129
0.05 0.0
0.2
0.4
0.6
0.8
1.0
N Content (%) Fig. 3. FWHM of MBE samples grown on GaAs (black squares), InAs (green circles) substrates and LPE samples (Red Stars) against nitrogen content. The dashed line is a guide to the eye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
1000000
Intensity (a.u.)
100000
(a)
MBE
10000 1000 100 10 1 0.1 -14000 -12000 -10000 -8000 -6000 -4000 -2000
0
2000
ω−2θ (arcsec) 100000
LPE
(b) 10000
1000
100
0.44 0.42
Bandgap Energy (eV)
0.35
Intensity (a.u.)
centres in both cases. All samples exhibit an intense band–band peak with a smaller low energy peak associated with deep levels in InAs, with the exception of sample ISN14 (grown using InN powder) which exhibits four peaks [21]. The MBE samples with higher N content exhibit a large low energy tail due to localised states near the conduction band edge caused by inhomogeneous N incorporation [12]. Fig. 2 shows the band gap dependence on N content for both growth methods with respect to the band anti-crossing (BAC) model. The solid line for the BAC model was determined using an isolated nitrogen level energy of EN = 1.44 eV and an interaction potential CNM = 2.5 eV. Both these values are based on previous reports [9,22] and clearly all the experimental points follow the theory very closely. The full width half maximum (FWHM) dependence on nitrogen content for both LPE and MBE samples is shown in Fig. 3. The FWHM of the LPE samples is smaller than that of their MBE grown counterparts. The FWHM of the MBE samples rises continuously with increasing N content, while the LPE samples occupy a region below that of the MBE samples. This indicates that the LPE samples have been grown with superior crystalline quality to that of the MBE samples. Although the MBE InAsN is relaxed on the GaAs substrate (as determined from 004 and 115 measurements (see Fig. 4)) it is possible that the localisation produced by the addition of N makes the material insensitive to the presence of threading dislocations and point defects, however further work will be required to determine this. The XRD spectra for both LPE and MBE grown InAsN containing 0.6% N are shown in Fig. 4. Both samples exhibit peaks from the substrate and the InAsN epilayer with no evidence of spinodal decomposition. The LPE material is of very good crystalline quality with the peak intensity and FWHM being comparable to that of the InAs substrate (112 and 94 arc s respectively). This is 2 times smaller than obtained from the MBE samples. MBE growth of InAsN results in increasing ion damage caused by the N plasma used to incorporate progressively larger amounts of nitrogen into the epilayer [24–26] Consequently, the 4 K PLspectra from these samples would be expected to exhibit larger FWHM associated with emission involving defects and tail states. This is evident in Fig. 1 which shows that the PL spectra broaden with increasing N content. Fig. 5 shows the dependence of the activation energy for thermal quenching of the PL against N content. (The corresponding activation energies are also listed in Table 1). For higher N content
EN
0.40
MBE LPE BAC Model = 1.44 eV
-4000
-2000
0
2000
4000
ω−2θ (arcsec)
CMN = 2.5 eV
0.38
Fig. 4. XRD plot of InAsN grown by (a) MBE on GaAs displaying compressive strain and (b) LPE on InAs showing tensile strain. Both samples contain 0.6% N. The high strain of InAsN/GaAs results in a weak broad peak due to large number of structural defects.
0.36 0.34 0.32 0.30 0.28 0.26 0.0
0.5
1.0
1.5
2.0
2.5
N Content (%) Fig. 2. Band Gap dependence of InAsN grown by LPE (red circles) and MBE (black squares). The blue triangles represent InAsN grown previously by MBE onto InAs substrates [19]. The solid line depicts the band gap energy of the material as predicted by the band anti crossing model [23]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the activation energy for thermal quenching of PL in the MBE samples is larger than that of the LPE samples. The activation energy is also seen to increase with increasing N content. InAs MBE samples containing no N were found to have an activation energy of 6 meV while a sample containing 0.6% N had an activation energy of 25 meV. Although LPE growth produces high quality epitaxial material, the addition of N degrades crystalline perfection. The introduction of N results in localisation in the conduction band due to compositional fluctuations while at the same time lowering the band gap
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M. de la Mare et al. / Infrared Physics & Technology 55 (2012) 399–402
30
demonstrated that high N content epilayers exhibit superior thermal quenching behaviour and such material cannot be easily achieved from LPE growth near thermodynamic equilibrium [29].
25
Acknowledgements
Activation Energy (meV)
35
20 15 10 MBE
5
The authors are grateful to the Engineering and Physical Science Research Council (EPSRC) for providing a PhD plus award for M. de la Mare and also a visiting fellowship for S. Dhar (EP/G000190/1). Work carried out at Calcutta University was funded by the Department of Information Technology, Government of India.
LPE
0.0
0.2
0.4
0.6
0.8
1.0
References
Nitrogen Content (%) Fig. 5. Activation energy of MBE (squares) and LPE (dots) samples for different N contents. Materials grown by MBE show a significant improvement with increasing N content.
enough for the N-related cluster defect levels to become incorporated into the conduction band continuum. This reduces competing non-radiative recombination and partly accounts for the increase in activation energy while also leading to a larger PL FWHM, since the incorporation of defect levels into the conduction band also increases the residual carrier concentration [27]. In addition, once the N incorporation has increased sufficiently, the resonant (CHSH) Auger process becomes detuned due to the band gap of the material becoming smaller than the spin orbit split-off band gap. The onset of this detuning has recently been determined to be near 0.5% N for InAsN [28]. One reason for the higher activation energy in MBE material may be the smaller residual carrier concentration compared with that in the LPE material; since Auger scattering Cn3, a higher residual carrier concentration would lead to a faster onset of non-radiative Auger recombination at high temperature. 4. Conclusion In summary, InAsN epitaxial layers were produced using both LPE and MBE growth techniques. The LPE samples demonstrated similar PL emission intensities and internal quantum efficiency compared with the MBE samples. However, the maximum amount of nitrogen which could be incorporated from the liquid phase (<0.5%) was substantially less, due to the evaporation and out-diffusion of N during the LPE growth process. The FWHM of the LPE samples was found to be generally narrower than that of the MBE material containing the same amount of N. Increasing the N content in the InAsN alloys resulted in a significant increase in the activation energy for thermal quenching of the PL emission. We attributed this behaviour to a combination of lowering of the conduction band edge and Auger de-tuning. This makes dilute nitrides interesting materials for development of mid-infrared devices. For low N(<0.5%) content, LPE could potentially offer a simpler, inexpensive and viable alternative to MBE for the production of good quality dilute nitride material. For contents higher than this (N > 0.5%) MBE would be the best option since it has been
[1] D. Wang, S.P. Svensson, L. Shterengas, G. Belenky, J. Cryst. Growth 312 (2010) 2705. [2] M. Debbichi, S. Ridene, H. Bouchriha, A.B. Fredj, M. Saïd, J-L. Lazzari, Y. Cuminal, P. Christol, Semicond. Sci. Technol. 24 (2009) 085010. [3] S. Kuboya, F. Nakajima, R. Katayama, K. Onabe, Phys. Status Solidi B 243 (2006) 1411. [4] P. Carrington, M. de la Mare, K.J. Cheetham, Q. Zhuang, A. Krier, Adv. Optoelectron. 2011 (2011) 145012. [5] A. Nishikawa, R. Katayama, K. Onabe, Y. Shiraki, J. Cryst. Growth 251 (2003) 427. [6] W.G. Bi, C.W. Tu, Appl. Phys. Lett. 70 (1997) 1608. [7] J. Ibáñez, R. Oliva, M. de la Mare, M. Schmidbauer, S. Herrnández, P. Pellegrino, D.J. Scurr, R. Cuscó, L. Artús, M. Shafi, R.H. Mari, M. Henini, Q. Zhuang, A. Godenir, A. Krier, J. Appl. Phys. 108 (2010) 103504. [8] B.N. Murdin, Appl. Phys. Lett. 78 (2001) 1568. [9] T.D. Veal, L.F.J. Piper, P.F. Jefferson, I. Mahboob, C.F. McConville, M. Merrick, T.J.C. Hosea, B.N. Murdin, M. Hopkinson, Appl. Phys. Lett. 87 (2005) 182114. [10] M. Kuroda, R. Katayama, S. Nishio, K. Onabe, Y. Shiraki, Phys. Status Solidi C 0 (2003) 2765. [11] D.-K. Shih, H.-H. Lin, L.-W. Sung, T.-Y. Chu, T.-R. Yang, Jpn. J. Appl. Phys. Part 1 42 (2003) 375. [12] M. Kuroda, A. Nishikawa, R. Katayama, K. Onabe, J. Cryst. Growth 278 (2005) 254. [13] M. de la Mare, Q. Zhuang, A. Krier, A. Patanè, S. Dhar, Appl. Phys. Lett. 95 (2009) 031110. [14] Q. Zhuang, A.M.R. Godenir, A. Krier, K.T. Lai ans, S.K. Haywood, J. Appl. Phys. 103 (2008) 063520. [15] S.D. Parker, R.L. Williams, R. Droopad, R.A. Stradling, K.W.J. Barnham, J.N. Holmes, J. Laverty, C.C. Phillips, E. Skuras, R. Thomas, X. Zhang, A. StatonBevon, D.W. Pashley, Semicond. Sci. Technol. 4 (1989) 663. [16] E.V. Kunitsvna, I.A. Andreev, N.A. Solov’ev, Y. P Yaltovfev, Adv. Semicond. Dev. Microsy. 55 (1998). Slovakia. [17] I.A. Andreev, E.V. Kunitsyna, Y.V. Solov’ev, Tech. Phys. Lett. 25 (1999) 792. [18] S. Tixier, M. Adamcyk, E.C. Young, J.H. Schmid, T. Tiedje, J. Cryst. Growth 251 (2003) 449. [19] H.D. Sun, A.H. Clark, S. Calvez, M.D. Dawson, D.K. Shih, H. H Lin, Indium phosphide and related materials 2005 P267 (2005). [20] P. Gladkov, Z. Sourek, A.P. Litvinchuk, M.N. Iliev, Semicond. Sci. Technol. 21 (2006) 544. [21] S. Dhar, T.D. Das, M. de la Mare, A. Krier, Appl. Phys. Lett. 93 (2008) 071905. [22] R. Kudrawiec, J. Misiewicz, Q. Zhuang, A.M.R. Godenir, A. Krier Appl, Phys. Lett. 94 (2009) 151902. [23] Q. Zhuang, A. Godenir, A. Krier, J. Phys. D. 41 (2008) 132002. [24] A. Erol, Dilute III–V Nitride Semiconductor Materials & Systems: Physics and Technology, Springer, 2008. [25] T. Kageyama, T. Miyamoto, S. Makino, F. Koyama, K. Iga, J. Cryst. Growth 209 (2000) 350. [26] M.A. Wistey, S.R. Bank, H.B. Yuen, H. Bae, J.S. Harris Jr., J. Cryst. Growth 278 (2005) 229. [27] A. Patanè, W.H. M Feu, O. Makarovsky, O. Drachenko, L. Eaves, A. Krier, Q.D. Zhuang, M. Helm, M. Goiran, G. Hill, Phys. Rev. B 80 (2009) 115207. [28] R. Kudraweic, M. Latkowska, J. Misiewicz, Q. Zhuang, A.M.R. Godenir, A. Krier, Appl. Phys. Lett. 99 (2011) 011904. [29] H. Abu-Farsakh, J. Neugebaur, Phys. Rev. B 79 (2009) 155311.
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Mid-infrared photoluminescence of InAsN dilute nitride alloys grown by LPE and MBE M. de la Mare a,⇑, Q. Zhuang a, S. Dhar b, A. Krier a a b
Physics Department, Lancaster University, Lancaster LA1 4YB, UK Department of Electronic Science, University of Calcutta 92, APC. Road, Kolkata-700 009, India
h i g h l i g h t s " Comparison between LPE and MBE growth methods. " Successful mid-infrared emission from both LPE and MBE material with nitrogen contents as high as 1%. " Narrower FWHM and XRD obtained from LPE material. " Superior activation energies from MBE increasing with N due to de-tuning of Auger recombination.
a r t i c l e
i n f o
Article history: Received 26 March 2012 Available online 29 June 2012 Keywords: Dilute nitrides Liquid phase epitaxy Molecular beam epitaxy Characterisation
a b s t r a c t Dilute nitride InAsN epitaxial layers were produced using both liquid phase and molecular beam epitaxial growth techniques. The spectral features in the photoluminescence of samples containing up to 1% N with emission energies in the mid-infrared spectral region are described and compared. The emission intensities of both LPE and MBE grown materials were found to be comparable. Increasing the N content in the InAsN alloys resulted in a significant increase in the activation energy for thermal quenching of the photoluminescence emission due to a combination of lowering of the conduction band edge and Auger de-tuning. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Narrow gap III-V dilute nitride materials such as InAsN have attracted particular interest because of their potential applications for use in optoelectronic devices operating in the mid-infrared spectral region which is rich in applications [1–4]. Small amounts of nitrogen added to III–V materials are known to result in significant band gap reduction [5], large band gap bowing [6], enhancement of the conduction band effective mass [7] and reduction in Auger recombination processes [8], all of which hold great potential for the development of improved photonic devices. These properties also make them a possible alternative to other mid infrared materials such as HgCdTe, PbSnTe and InAsSb. One potential advantage of using InAsN may be the ability to tune the band gap down to a very low value by controlling the amount of nitrogen incorporated, with previous reports of InAsN being grown onto InAs [9] GaAs [10] and InP [11] substrates displaying substantial band gap reductions. InAsN bulk layers and quantum wells have ⇑ Corresponding author. Tel.: +44 (0)1524 594750; fax: +44 (0)1524 844037. E-mail addresses: (M. de la Mare).
[email protected],
[email protected]
1350-4495/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2012.06.002
been grown by molecular beam epitaxy (MBE) [8–12] with reports of bright photoluminescence being observed [8,13,14]. Liquid phase epitaxy (LPE), which is a thermal equilibrium growth technique capable of producing epitaxial materials with excellent crystalline quality and high luminescence efficiencies, is believed to avoid some of the problems associated with ion damage from the N plasma used in MBE growth of dilute nitrides, and as such is an attractive alternative. In this paper we report on the growth of InAsN by both LPE and MBE techniques and the characterisation and comparison of the resulting epilayers. 2. Material and method LPE growth of InAsN layers was done in a conventional horizontal sliding boat reactor using a transparent gold furnace and high purity quartz tube. Prior to growth, the reactor together with the graphite boat, were baked for several hours at high temperature in Pd-diffused ultrapure hydrogen gas. Initially, 99.99999% pure In metal was loaded into the boat and baked at 780 °C for 18– 20 h under purified hydrogen flow. The required amount of 99.9999% polycrystalline InAs was then loaded to the In contained within the growth melt bin and again baked at 750 °C for a further
M. de la Mare et al. / Infrared Physics & Technology 55 (2012) 399–402 1.2
(a)
MBE InAs/InAs InAsN (0.4%) InAsN (0.65%) InAsN (0.8%)
x1/5
PL Intensity (a.u.)
18 h. Finally, a precisely weighed amount of 99.9% polycrystalline InN was added to the growth melt and baked at 700 °C for 1–2 h. Growth was typically carried out at 590 °C for 7–8 min using a melt super-saturation of 5–6 °C and a constant cooling rate of 0.3 °C/min onto InAs (1 0 0) substrates. Epitaxial layers obtained from this growth method were between 2 and 3 lm thick with smooth mirror like surface morphologies with no noticeable inclusions. For comparison, the MBE epilayers were grown onto both (1 0 0) InAs and (1 0 0) GaAs substrates. The preparation and oxide desorption in the growth chamber was carried out in the conventional manner [15]. InAsN epitaxial layers approximately 1 lm thick were grown at a growth temperature in the range of 400–440 °C using a VG-V80H MBE reactor with radio frequency nitrogen plasma source. A growth rate of 1 lm/h was employed using an As flux of 2 10 6 mbar. For the N-plasma, a power of 160 W and a flux of 5 10 6 mbar were used. The surface reconstruction was monitored by in situ reflection high energy electron diffraction (RHEED), while the substrate temperature was measured using a calibrated infrared pyrometer. After growth, the surface of the layers was inspected using a Nomarski phase contrast microscope. Structural properties were determined using high resolution X-ray diffraction. Photoluminescence (PL) was performed on all samples in the temperature range 4–300 K using a variable temperature continuous flow He cryostat. An Ar+ ion laser (514 nm, with maximum power density of 20 W cm 2 at the sample) was used for excitation. The emitted radiation was analysed using a 0.3 m Bentham M300 monochromator and detected with a cooled (77 K) InSb photodiode detector and digital lock-in amplifier.
0.8
0.4
0.0 2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
Wavelength (μm)
1.0
(b)
LPE ISN14 IABN8B IABN8A MDLM090720
InAsN(Bi) (x1/5)
0.8
Intensity (a.u.)
400
InAsN(0.5%)/InAs
0.6
InAsN(Bi)
0.4 InAsN/InAs
0.2
0.0
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
Wavelength (μm) Fig. 1. 4 K photoluminescence spectra of InAsN bulk epilayers with nitrogen contents in the range 0–1% grown by (a) MBE and (b) LPE.
3. Results and discussion Table 1 lists the various InAsN samples produced containing different amounts of N obtained from the two growth methods and summarises the corresponding properties of the PL emission spectra obtained in each case. Fig. 1 shows the 4 K photoluminescence spectra of bulk InAsN samples grown by (a) MBE and (b) LPE. While both sets of samples exhibit emission in the mid-infrared spectral range, the PL from the LPE material is centred around 3 lm. In contrast the MBE grown material displays a broad range of emission wavelengths. Smaller amounts of N are present within the LPE grown material due to N out-diffusion from the growth melt during the LPE growth process. Two of the LPE samples were grown using Bi neutral solvent
solution. The use of Bi as the solvent enables the grower to change the stoichiometry of the melt allowing one to vary the In/As ratio in the liquid phase beyond that of traditional investigations [16]. Bi is not incorporated into the solid phase and therefore does not influence the electro-physical characteristics of the material [17– 20]. The peak PL intensities of the samples from both growth methods are roughly comparable, with the MBE grown material producing slightly stronger emission. However, MBE grown homo-epitaxial InAs and LPE grown InAsN using neutral solvent solution (Bi), both exhibit 5 stronger emission intensities. We attribute this to a reduction in the number of non-radiative defect
Table 1 Summary of dilute nitride epilayers studied. Details include nitrogen content, 4 K peak PL wavelength and equivalent band gap as well as 4 K FWHM and activation energies. The LPE samples indicated with ⁄ are those grown using neutral solvent Bi, while $ indicates material grown on InAs substrates. Sample
N (%)
4 K k (lm)
4 K Eg (eV)
FWHM (meV)
DE (meV)
LPE MDLM090720 IABN8B⁄ IABN8A⁄ ISN14 SD091216
0.05 0.1 0.2 0.5 0.6
3.02 3.03 3.09 3.22 3.26
0.411 0.409 0.401 0.385 0.379
276 118 124 116 174
– 13.6 17.7 16.8 17.2
MBE A0129$ A0276 A0285 A0147$ A0300 A0282 A0213$ A0142$ A0299
0 0 0.2 0.4 0.4 0.6 0.65 0.8 1
2.99 2.97 3.04 3.2 3.14 3.27 3.4 3.54 3.66
0.415 0.417 0.408 0.388 0.395 0.379 0.365 0.35 0.339
74 97 101 167 184 190 106 261 317
17.9 6 21.2 – 20.6 25.7 – – 34.1
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A0299
0.30 MDLM090720
FWHM (eV)
A0142
0.25 A0146
0.20
A0300
A0282
A0147
SD091216
0.15 ISN14
0.10
A0276
IABN8A IABN8B
A0285
A0213
A0129
0.05 0.0
0.2
0.4
0.6
0.8
1.0
N Content (%) Fig. 3. FWHM of MBE samples grown on GaAs (black squares), InAs (green circles) substrates and LPE samples (Red Stars) against nitrogen content. The dashed line is a guide to the eye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
1000000
Intensity (a.u.)
100000
(a)
MBE
10000 1000 100 10 1 0.1 -14000 -12000 -10000 -8000 -6000 -4000 -2000
0
2000
ω−2θ (arcsec) 100000
LPE
(b) 10000
1000
100
0.44 0.42
Bandgap Energy (eV)
0.35
Intensity (a.u.)
centres in both cases. All samples exhibit an intense band–band peak with a smaller low energy peak associated with deep levels in InAs, with the exception of sample ISN14 (grown using InN powder) which exhibits four peaks [21]. The MBE samples with higher N content exhibit a large low energy tail due to localised states near the conduction band edge caused by inhomogeneous N incorporation [12]. Fig. 2 shows the band gap dependence on N content for both growth methods with respect to the band anti-crossing (BAC) model. The solid line for the BAC model was determined using an isolated nitrogen level energy of EN = 1.44 eV and an interaction potential CNM = 2.5 eV. Both these values are based on previous reports [9,22] and clearly all the experimental points follow the theory very closely. The full width half maximum (FWHM) dependence on nitrogen content for both LPE and MBE samples is shown in Fig. 3. The FWHM of the LPE samples is smaller than that of their MBE grown counterparts. The FWHM of the MBE samples rises continuously with increasing N content, while the LPE samples occupy a region below that of the MBE samples. This indicates that the LPE samples have been grown with superior crystalline quality to that of the MBE samples. Although the MBE InAsN is relaxed on the GaAs substrate (as determined from 004 and 115 measurements (see Fig. 4)) it is possible that the localisation produced by the addition of N makes the material insensitive to the presence of threading dislocations and point defects, however further work will be required to determine this. The XRD spectra for both LPE and MBE grown InAsN containing 0.6% N are shown in Fig. 4. Both samples exhibit peaks from the substrate and the InAsN epilayer with no evidence of spinodal decomposition. The LPE material is of very good crystalline quality with the peak intensity and FWHM being comparable to that of the InAs substrate (112 and 94 arc s respectively). This is 2 times smaller than obtained from the MBE samples. MBE growth of InAsN results in increasing ion damage caused by the N plasma used to incorporate progressively larger amounts of nitrogen into the epilayer [24–26] Consequently, the 4 K PLspectra from these samples would be expected to exhibit larger FWHM associated with emission involving defects and tail states. This is evident in Fig. 1 which shows that the PL spectra broaden with increasing N content. Fig. 5 shows the dependence of the activation energy for thermal quenching of the PL against N content. (The corresponding activation energies are also listed in Table 1). For higher N content
EN
0.40
MBE LPE BAC Model = 1.44 eV
-4000
-2000
0
2000
4000
ω−2θ (arcsec)
CMN = 2.5 eV
0.38
Fig. 4. XRD plot of InAsN grown by (a) MBE on GaAs displaying compressive strain and (b) LPE on InAs showing tensile strain. Both samples contain 0.6% N. The high strain of InAsN/GaAs results in a weak broad peak due to large number of structural defects.
0.36 0.34 0.32 0.30 0.28 0.26 0.0
0.5
1.0
1.5
2.0
2.5
N Content (%) Fig. 2. Band Gap dependence of InAsN grown by LPE (red circles) and MBE (black squares). The blue triangles represent InAsN grown previously by MBE onto InAs substrates [19]. The solid line depicts the band gap energy of the material as predicted by the band anti crossing model [23]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the activation energy for thermal quenching of PL in the MBE samples is larger than that of the LPE samples. The activation energy is also seen to increase with increasing N content. InAs MBE samples containing no N were found to have an activation energy of 6 meV while a sample containing 0.6% N had an activation energy of 25 meV. Although LPE growth produces high quality epitaxial material, the addition of N degrades crystalline perfection. The introduction of N results in localisation in the conduction band due to compositional fluctuations while at the same time lowering the band gap
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demonstrated that high N content epilayers exhibit superior thermal quenching behaviour and such material cannot be easily achieved from LPE growth near thermodynamic equilibrium [29].
25
Acknowledgements
Activation Energy (meV)
35
20 15 10 MBE
5
The authors are grateful to the Engineering and Physical Science Research Council (EPSRC) for providing a PhD plus award for M. de la Mare and also a visiting fellowship for S. Dhar (EP/G000190/1). Work carried out at Calcutta University was funded by the Department of Information Technology, Government of India.
LPE
0.0
0.2
0.4
0.6
0.8
1.0
References
Nitrogen Content (%) Fig. 5. Activation energy of MBE (squares) and LPE (dots) samples for different N contents. Materials grown by MBE show a significant improvement with increasing N content.
enough for the N-related cluster defect levels to become incorporated into the conduction band continuum. This reduces competing non-radiative recombination and partly accounts for the increase in activation energy while also leading to a larger PL FWHM, since the incorporation of defect levels into the conduction band also increases the residual carrier concentration [27]. In addition, once the N incorporation has increased sufficiently, the resonant (CHSH) Auger process becomes detuned due to the band gap of the material becoming smaller than the spin orbit split-off band gap. The onset of this detuning has recently been determined to be near 0.5% N for InAsN [28]. One reason for the higher activation energy in MBE material may be the smaller residual carrier concentration compared with that in the LPE material; since Auger scattering Cn3, a higher residual carrier concentration would lead to a faster onset of non-radiative Auger recombination at high temperature. 4. Conclusion In summary, InAsN epitaxial layers were produced using both LPE and MBE growth techniques. The LPE samples demonstrated similar PL emission intensities and internal quantum efficiency compared with the MBE samples. However, the maximum amount of nitrogen which could be incorporated from the liquid phase (<0.5%) was substantially less, due to the evaporation and out-diffusion of N during the LPE growth process. The FWHM of the LPE samples was found to be generally narrower than that of the MBE material containing the same amount of N. Increasing the N content in the InAsN alloys resulted in a significant increase in the activation energy for thermal quenching of the PL emission. We attributed this behaviour to a combination of lowering of the conduction band edge and Auger de-tuning. This makes dilute nitrides interesting materials for development of mid-infrared devices. For low N(<0.5%) content, LPE could potentially offer a simpler, inexpensive and viable alternative to MBE for the production of good quality dilute nitride material. For contents higher than this (N > 0.5%) MBE would be the best option since it has been
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