Structural, optical and annealing studies of nitrogen

Physica B: Condensed Matter 544 (2018) 47–51

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Structural, optical and annealing studies of nitrogen implanted GaAs M.S. Saleem a b c

a,b

a,∗

a,b

, W.A.A. Syed , N. Rafiq

T

, S. Ahmed , M.S.A. Khan , J. –Ur–Rehman c

a

a

Department of Physics, International Islamic University, Islamabad, 45320, Pakistan Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Centre for Advanced Electronics & Photovoltaic Engineering, International Islamic University, Islamabad, 45320, Pakistan

A R T I C LE I N FO

A B S T R A C T

Keywords: Nitrogen implantation Annealing effects Photoluminescence intensity Defects in GaAsN GaN

Nitrogen implanted III-V compounds semiconductors have attracted a great deal of attention of the research community due to their interesting fundamental physical properties and potential applications in optoelectronic devices. A small amount of nitrogen incorporation in gallium arsenide (GaAs) causes significant reduction in the band gap energy of fabricated alloys. In this study the GaAsN layer was prepared by implantation of nitrogen ion on thick GaAs wafer and the samples were thermally annealed for studying annealing effects. The structural, compositional and optical properties have been investigated by X-Ray Diffraction (XRD), Rutherford Backscattering Spectroscopy (RBS), Scanning Electron Microscopy (SEM), Photoluminescence (PL) and Photothermal Deflection Spectroscopy (PDS). The results reveal that the samples prepared with higher implantation dose and annealed at higher temperature exhibit strong optical signal of GaN, while the samples annealed at lower temperature show weak optical signal. Furthermore in PDS spectra, the band gap reduction was observed indicating the diluted nitrogen content in GaAs. We have demonstrated that annealing conditions and implanted nitrogen doses could be helpful to achieve the desired variation in band gap and low physical damages on surface.

1. Introduction GaAs is one of the most versatile semiconducting materials for various optoelectronic devices. Implantation of nitrogen into GaAs has been extensively studied for preparation of GaNxAs1−x compounds [1–4]. The bombardment of energetic ions during the implantation process causes distortion in the crystal structure of host material and non-radiative defects [5,6]. Post-growth thermal annealing is mostly used to remove the influence of non-radiative defects [7,8], which activates the immersed nitrogen to make bonds with GaAs and provides better insight in the host material. The band gap energy of GaAs1-xNx varies from 0.8 to 3.4 eV, with increasing the nitrogen content (x) from 0 to 1 [9,10]. However, to achieve the desired band gap variation through nitrogen implantation is extremely challenging. The band gap depends on crystal structure distortion and the amount of nitrogen activation in host GaAs, therefore in device fabrication, less physical damages and activation of nitrogen is a need for a desired band gap tuning. Here, we report the optimization of physical damages and band gap of GaAsN formed by nitriding the gallium arsenide wafer with a variety of doses of nitrogen ion implantation. We have compared the samples obtained at different annealing temperature by studying surface

∗

Corresponding author. E-mail address: [email protected] (W.A.A. Syed).

https://doi.org/10.1016/j.physb.2018.05.017 Received 24 February 2018; Received in revised form 9 May 2018; Accepted 10 May 2018 Available online 11 May 2018 0921-4526/ © 2018 Elsevier B.V. All rights reserved.

morphology and photoluminescence. In addition a brief description on ions modification, damages including blistering exfoliation as a result of ion irradiation and effect of annealing temperature is presented. 2. Experiment Nitrogen ions (N+) were implanted on 0.5 mm thick semi-insulating GaAs (100) wafer. The implantation was carried out on four different wafers with doses of 1 × 1016, 5 × 1016, 1 × 1017 and 5 × 1017atoms/ cm2. Nitrogen ions were generated by a source of 40 keV from m/s CORE, Sunnyvale California. The ions beam angle was fixed at 7° and substrate temperature was maintained at 300 °C. After the implantation the wafers were cleaved into 1 cm2 pieces. Pieces were classified in two sets; each has 4 pieces of different nitrogen implantation doses as mentioned above. Both the sets were thermally annealed in quartz tube furnace for 10 min, the first one at 650 °C and second one at 800 °C in the N2 gas environment. The residual gases were flashed out with nitrogen gas from the tube, while during annealing 20 sc-cm nitrogen gas flow was maintained. In order to prevent the impurity adsorption, the samples were placed face to face in the tube furnace. The depth dependent elemental composition was determined by Rutherford back scattering (RBS). 2 MeV of helium ion (He++) beam


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Fig. 1. RBS spectra of nitrogen implanted samples at different doses.

with a beam current of ∼100 nA/cm−2 was used and the backscattering angle was fixed at 170°. The obtained RBS data was fitted by ‘RUMP software’, which was in order to determine depth-wise elemental composition of the samples. The structural properties were studied by x-ray diffraction (XRD); Joel x-ray diffractometer with CuKα source of wavelength 1.54 Å. The scanning speed was 3° per minutes with step of 0.05 per degree. The surface morphology and optical properties were studied by SEM and room temperature photoluminescence; NeCu laser with 240 nm wavelength was used for sample excitation in photoluminescence.

3. Results and discussion The compositional depth profile was determined by the Rutherford Back Scattering (RBS) technique. The RBS measurements of implanted wafers with different nitrogen doses are given in Fig. 1. The normalized yield verses the channeling graph was obtained and the data points were fitted by RUMP software. Fitted plot has been used to determine depth-wise composition of the samples as given in Table 1. The scale is defined as 50 channels in number equal to 150 nm thickness of the sample. In sample of higher implantation doses, a dip was appeared in RBS graphs, indicating the upper layer of GaAs as amorphous with high number lattices disorder due to energetic bombardment of nitrogen ions. The observed decrease in the normalized yield is the evidence of the addition of nitrogen at the surface of GaAs wafer. The XRD patterns of unimplanted GaAs and implanted wafers are shown in Fig. 2. A strong peak at 66° confirms the single crystalline structure of unimplanted wafer (Fig. 2(a)). For the implanted samples, the peak at 66° gradually decreases with increasing nitrogen dose, indicating the lattice disorder in GaAs. The lattice disorder were homogeneously increased in GaAs with increased flounce of nitrogen and finally a complete amorphous layer at the surface was formed. The

Fig. 2. The XRD graphs of un-implanted and implanted samples, (a) before annealing (b) Nitrogen implanted samples annealed at 800 °C.

strained layer at the surface is due to displacement of gallium and arsenic atoms from their lattice positions as a result of nitrogen ions collision. The recrystallization took place after annealing at 800 °C for 10 min. The annealed samples have become polycrystalline making it difficult to analyze the observed peaks (Fig. 2(b)). The main peak at 30.05° of XRD pattern represents gallium element and some peaks due to oxides formation may be seen at similar site. We cannot conclude that the peak was entirely due to gallium itself; there is another possibility of the presence of GaAs, which may be interesting for our future study. The 2 theta values at 45.5° and 51.45° correspond to (220) and (022) planes of GaAs respectively, whereas the peaks at 44° and 32.25° correspond to (200) and (100) planes of GaN respectively. Since the addition of nitrogen causes broadening of peak, therefore we may say that the peak at 66° corresponds to GaAsN. The peak at 43.5° is due to Ga2O3, the oxides appeared due to the residual oxygen remained in the tube furnace during annealing. All the peaks were matched with references [11,12]. The surface morphology has been investigated by means of scanning electron microscope (SEM). The SEM images of as-implanted samples are shown in Fig. 3. The surface of samples implanted with low dose seems very smooth without any sort of mechanical damages, however the surface received with higher implantation dose seems a little different as shown in Fig. 3 (c,d). This is because, the larger amount of nitrogen implantation creates mechanical deformation in crystal structure of GaAs, and micro/nano cracks inside the host material. These cracks act as trapping center for implanted nitrogen. Further trapping of nitrogen creates internal pressure with the temperature

Table 1 Depth-wise elemental compositions of implanted samples determined by RBS spectra, 5% errors are included. Implantation dose (atoms/ cm2)

Sub-layer thickness (nm)

Sub-layer atomic composition (%)

1 × 1016 5 × 1016

300 300 300 170 150 160

Ga = 47, Ga = 46, Ga = 48, Ga = 45, Ga = 48, Ga = 42,

1 × 1017 5 × 1017

As = 43, As = 44, As = 44, As = 35, As = 44, As = 28,

N = 10 N = 10 N = 08 N = 20 N = 08 N = 30

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Fig. 3. (a–d) SEM images of as-implanted samples, blistering areas are marked with arrows. The atomic composition at surface of sample (c) obtained with EDX is as, gallium: 44.23%, arsenic: 40.73%, nitrogen: 12.74 and oxygen: 2.30%.

maintained during implantation, and finally reaches to a point where it causes the surface blistering phenomena. The images of set 1 and set 2 samples which were annealed at 650 and 800 °C respectively are given in Fig. 4. The samples annealed at 650 °C were not much affected and look similar to as implanted samples. The sample prepared at lower doses kept their smooth surface and remained without blistering. Some cracks and broken blisters were appeared on the sample with higher implantation doses are marked with the arrows (Fig. 4(d)). The appeared cracks are due to the difference of the lattice constant, stresses in crystalline material and difference in the coefficient of thermal expansion in the newly formed nitride film and GaAs substrate [13]. On the other hand both blistered and exfoliated regions were observed in the samples annealed at 800 °C shown in Fig. 4(g and h). The samples annealed at 800 °C show higher number of blistering areas as compared to the samples implanted at 650 °C. Comparing both sets of samples we can conclude that the surface blistering increases at higher implantation dose and annealing temperature. Fig. 5(a) and (b) shows the room temperature photoluminescence spectra for the set 1 and 2 respectively. In both Figures, we drew a GaN range corresponds to the 3.0–3.5 eV of x-axis energy values (represented by range 1). Range 1, belonging to both cubic (optical energy value ∼3.1–3.25 eV) and hexagonal (∼3.25–3.5 eV) structural phase of GaN [14–16]. For the set 1 samples, which were annealed at 650 °C, the maximum peak intensity of PL signals was appeared in range of 2.5–2.9 eV (corresponding to range 2). No peak was observed in highlighted GaN range. A significant shift in PL spectra towards higher energy value with increasing implantation dose was observed. For the set 2 samples, which were annealed at 800 °C, the PL signal was observed in highlighted GaN range with a significant shift towards higher energy value accompanied by increasing implantation dose. The signal also appeared in range 2 indicating the presence of As in GaN phase [9,10]. The PL signal in range 2 decreases with increasing implantation

does and shifted toward energy range of GaN phase. The spectra clearly show a blue shift indicating more dominant hexagonal phases in the samples with higher implantation doses. In our opinion, the observed signal of GaN is due to arsenic evaporation during annealing at 800 °C and formation of bonds between gallium and nitrogen. The bond between gallium and arsenic was broken due to high temperature and energetic ion implantation. In this process the arsenic atoms are replaced with nitrogen atoms as suggested in Ref. [14]. It may be noted that the whole surface was not converted into GaN layer only due to the formation of nano or micro precipitates [15,17]. In set 1, temperature of 650 °C was not sufficient enough to break GaAs bonding, however a small amount of arsenic could evaporate during the process and the surface remained partly as GaAsN, showing very low intensity peaks of GaN appeared in PL spectra. Comparing both the plots (a) and (b) we understand that, for the high annealing temperature the PL signals were shifted toward the higher energy value, therefore the set 1 samples shows less energy spectra as compared to the that of set 2 samples. The origin of such energy shift corresponds to the temperature and could be helpful in deciding the annealing conditions for desired optical properties. Another significant difference was appeared in signal intensity. In Fig. 5 (c) the plot shows maximum peak intensities as function of implanted doses for both set of samples. The results illustrate that the peak intensity depends on the annealing temperature and implantation dose. The samples annealed at higher temperature show higher intensity GaN peaks, which were due to the improved crystallinity, significant reduction in non-radiative recombination centers and stresses in GaN material. The peak intensity was observed 15 times higher for the samples of set 2 as compared to that of set 1. Moreover, if we look towards the implanted doses, the intensity decreases gradually for the higher value, causing damage in crystal structure. PL intensities degradation with increasing dose is probably related to an increased density of non-radiative centers [8,18,19]. For set 2, the sample 49


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Fig. 4. (a–d) SEM images of set 1 samples annealed at 650 °C, (e–h) images of set 2 samples annealed at 800 °C.

implanted at 1 × 1017 shows less PL intensity signal as compared to 5 × 1017sample. The higher dose is suitable for hexagonal GaN phase, and signal appeared near to ∼3.4 eV (see Fig. 5(b)). While the sample 1 × 1017 the signal appeared at ∼3.4 eV and also in range 2 indicates the presence of As in GaN phase. As a result maximum PL intensity signal for hexagonal GaN phase is less in sample 1 × 1017 as compared to 5 × 1017sample. For 1 × 1016 and 5 × 1016 samples the maximum peak signal shows the presence of cubic GaN phase, which is different from higher dose samples. The energy band gap of the GaAs lies near 1.42 eV. A dilute addition of nitrogen into GaAs causes reduction in the band gap of GaAs [20–22]. To find the low energy absorption edge we used PhotoThermal Deflection Spectroscopy (PDS). The PDS is a high sensitivity powerful technique for the investigation of low energy absorptions edge in thin films. The experimental detail about the setup is given in Ref. [23]. The obtained PDS spectra from selected samples are given in Fig. 6. In addition, the PDS spectrum of a semi-insulating GaAs (without nitrogen implantation) wafer is also shown for comparison. The absorption in semi-insulating GaAs appears at 1.42 eV and no signal was observed below this energy. Whereas, the nitrogen implanted samples shows the additional absorption signal below 1.42 eV indicated by arrow. The absorption signal in range of 1.2–1.35 eV are due to dilute addition of nitrogen in GaAs, which creates extra energy states close to

Fig. 5. (a,b) PL spectra of set 1 and 2 samples annealed at 650 and 800 °C respectively. (c) Comparison of PL peak intensities with implantation doses.

the edge of conduction band minima [23,24]. The signal from the samples annealed at low temperature is quite weak as compared to that of the sample annealed at higher temperatures. This is because of annealing at high temperature improved the crystallization in GaAsN and reduced the nonradioactive defects. The concept of band gap and its engineering revolves around the three materials: GaAs, GaAsN and GaN illustrated in model Fig. 7. We have found both the band gap reduction as well as band gap increment in same samples after the nitrogen implantation. We expect that the 50


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4. Conclusion Structural, optical and compositional analyses are carried out to investigate the band gap variation in nitrogen implanted GaAs wafer. We have shown that the surface blistering and exfoliation was induced as a result of nitrogen implantation that further increases with the increasing implantation dose and annealing temperature. Significant blue shift was observed in RTPL peak position from 3.0 to 3.4 eV with increasing implantation doses and with increasing annealing temperatures (650–800 °C) under similar fluence. The PDS spectrum shows a long absorption tail indicating the reduction in band gap of GaAs due to dilute addition of nitrogen. Detailed optical characterization of these samples provided an evidence of possible dilution of GaAs in forms of GaAsN and GaN. We expect that this work may provide a better understanding of nitriding process of the GaAs semiconductors for versatile potential applications. Acknowledgment Fig. 6. Photo-thermal deflection spectroscopy spectra of two samples with common implantation dose of 5 × 1016 atoms/cm2 and annealed at 650 and 800 °C. The spectra of GaAs represent the pure semi-insulating GaAs sample without nitrogen implantation.

This work was supported by the Higher Education Commission of Pakistan, Islamabad (project No. 995, under National Research Program for Universities). Waqar Syed would like to thanks to M. Beaudoin of Advanced Materials and Process Engineering Laboratory (AMPEL), University of British Columbia, Vancouver, Canada. References [1] W. Shan, K.M. Yu, W. Walukiewicz, J.W. Ager, E.E. Haller, M.C. Ridgway, Appl. Phys. Lett. 75 (1999) 1410. [2] X. Weng, W. Ye, S.J. Clarke, R.S. Goldman, V. Rotberg, A. Daniel, R. Clarke, J. Appl. Phys. 97 (2005) 064301. [3] H. Ch Alt, Y.V. Gomeniuk, G. Lenk, B. Wiedemann, Physica B340–342 (2003) 394. [4] P. Jayavel, K. Santhakumar, S. Rajagopalan, V.S. Sastry, K. Balamurugan, K.G.M. Nair, Mater. Sci. Eng. B94 (2002) 66. [5] D. Pons, J.C. Bourgoin, J. Phys. C Solid State Phys. 18 (1985) 3839. [6] R. Sreekumar, A. Mandal, S. Chakrabarti, S.K. Gupta, J. Phys. D Appl. Phys. 43 (2010) 505302. [7] E.V.K. Rao, A. Ougazzaden, Y. Le Bellego, M. Juhel, Appl. Phys. Lett. 72 (1998) 1409. [8] F. Bousbih, S.B. Bouzid, A. Hamdouni, R. Chtourou, J.C. Harmand, Mater. Sci. Eng. B123 (2005) 211. [9] K.M. Yu, S.V. Novikov, R. Broesler, I.N. Demchenko, J.D. Denlinger, Z. LilientalWeber, F. Luckert, R.W. Martin, W. Walukiewicz, C.T. Foxon, J. Appl. Phys. 106 (2009) 103709. [10] N. Bouarissa, S.A. Siddiqui, M. Boucenna, M.A. Khan, Optik 131 (2017) 317. [11] JCPDS Card Nos. 00-044-1013 and 00-029-0615, for GaAs, 00-050-0792 and 01070-2562 for GaN, 00-005-0601 for Ga, 01-074-1776 and 01-074-1610 for Ga2O3. [12] K.C. Lo, H.P. Ho, K.Y. Fu, P.K. Chu, K.F. Li, K.W. Cheah, J. Appl. Phys. 95 (2004) 8178. [13] R. Singh, S.H. Christiansen, O. Moutanabbir, U. Gösele, J. Electron. Mater. 39 (2010) 2177. [14] S. Dhara, P. Magudapathy, R. Kesavamoorthy, S. Kalavathi, K.G.M. Nair, G.M. Hsu, L.C. Chen, K.H. Chen, K. Santhakumar, T. Soga, Appl. Phys. Lett. 87 (2005) 261915. [15] A. Yu Bumai, D.S. Bobuchenko, A.N. Akimov, L.A. Vlasukova, A.R. Filipp, Vacuum 78 (2005) 119. [16] S. Amine, G. Ben Assayag, C. Bonafos, B. de Mauduit, H. Hidriss, A. Claverie, Mater. Sci. Eng. B93 (2002) 10. [17] A.W. Wood, R.R. Collino, B.L. Cardozo, F. Naab, Y.Q. Wang, R.S. Goldman, J. Appl. Phys. 110 (2011) 124307. [18] N. Kovac, C. Künneth, H.C. Alt, J. Appl. Phys. 123 (2018) 161583. [19] J. Li, X. Han, C. Dong, C. Fan, Y. Ohshita, M. Yamaguchi, J. Alloy Compd. 687 (2016) 42. [20] F. Bousbih, S. Ben Bouzid, R. Chtourou, F.F. Charfi, J.C. Harmand, G. Ungaro, Mater. Sci. Eng. C 21 (2002) 251. [21] K. Uesugi, I. Suemune, T. Hasegawa, T. Akutagawa, T. Nakamura, Appl. Phys. Lett. 76 (2000) 1285. [22] M. Biswas, A. Balgarkashi, R.L. Makkar, A. Bhatnagar, S. Chakrabarti, J. Lumin. 194 (2018) 341. [23] M. Beaudoin, I.C.W. Chan, D. Beaton, M. Elouneg-Jamroz, T. Tiedje, M. Whitwick, E.C. Young, J.F. Young, N. Zangenberg, J. Cryst. Growth 311 (2009) 1662. [24] M. Weyers, M. Sato, H. Ando, Jpn. J. Appl. Phys. 31 (1992) 853.

Fig. 7. Energy band gap representation: (a) GaAs, (b) GaAsN, and (c) GaN.

band gap variation accompanied by ‘x’ value of content from 0 to 1 in GaAs1-xNx. The dilute addition of nitrogen into GaAs creates extra energy states close to the edge of conduction band as represented in GaAsN band model. These extra energy levels cause the reduction in GaAs band gap that is why we observed a long absorption tail in PDS spectra. The large content of nitrogen introduction and post annealing replace the arsenic atoms with nitrogen, whereas the GaAs band gap start increasing due to disappearance of energy levels at valence band maxima consequently the crystal of host material behaves as a GaN as observed in PL spectra. In some of the previous reports, researchers highlighted the appearance of GaN phase in nitrogen implanted GaAs, while the physical damages are due to dilute addition of nitrogen at higher nitrogen dose in host material. Significant decrease of band gap and annealing condition were completely overlooked, therefore, not only the implantation dose but also annealing conditions play a crucial role to obtain the desired variation in band gap and recovery of defects.

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