Electronic properties and oxygen chemisorption at AlxGa1-xN surfaces

Materials Chemistry and Physics 239 (2020) 122106

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Electronic properties and oxygen chemisorption at AlxGa1-xN surfaces Monu Mishra a, Govind Gupta a, b, * a b

Academy of Scientific and Innovative Research, CSIR-NPL Campus, Dr. K.S. Krishnan Marg, New Delhi-110012, India Advanced Materials & Devices Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi, 110012, India

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Impact of Al molar fraction on elec­ tronic properties & chemical bonding on AlxGa1-xN/GaN heterostructures. � Nature of chemical bonding, oxygen chemisorption & VB hybridization is influenced via Al mole fraction. � Oxygen chemisorption mechanism (electron counting or oxidestoichiometry) altered due to perturbed surface energetics.

A R T I C L E I N F O

A B S T R A C T

Keywords: AlGaN XPS Electronic Properties

The presented work investigate the impact of aluminium composition on chemical states and electronic prop­ erties of AlxGa1-xN/GaN (x ¼ 0.45, 0.62 and 0.75) heterostructures using photoemission spectroscopy. Extensive core level and valence band analysis revealed significant variation in chemical coordination, electron affinity and valence band hybridization with aluminium composition. The variation in aluminium molar fraction lead to perturbed energetics and changed the oxygen chemisorption mechanism at the surface (i.e. electron-counting & oxide-stoichiometry). AlxGa1-xN surfaces with lower aluminium composition also pursued higher density of donor like surface states. However, the electron affinity of the films decreased from 2.6 to 2.1 eV with increment in aluminium composition and material bandgap.

1. Introduction

pursue two dimensional (2D) electron gas at the interface which demonstrate their potential candidature in high electron mobi­ lity/heterostructure field-effect transistors (HEMT/HEFT) [2–4]. Be­ sides the conventional HEMTs, the technological importance of AlxGa1-xN films (X > 40%) absorbing UV-B radiation lies in visible-blind

AlGaN/GaN heterostructure based devices have attracted consider­ able attention due to their high conductivity, large breakdown field and high power/frequency applications [1–4]. AlGaN/GaN heterostructures

* Corresponding author. Academy of Scientific and Innovative Research, CSIR-NPL Campus, Dr. K.S. Krishnan Marg, New Delhi, 110012, India. E-mail address: [email protected] (G. Gupta). https://doi.org/10.1016/j.matchemphys.2019.122106 Received 1 April 2019; Received in revised form 26 August 2019; Accepted 30 August 2019 Available online 5 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

M. Mishra and G. Gupta

Materials Chemistry and Physics 239 (2020) 122106

photodetectors, cold cathodes, space applications, field emission and terahertz technology [5–7]. The key parameter governing the perfor­ mance of these AlxGa1-xN devices is the modification in optical absorp­ tion and surface & electronic properties associated with aluminium molar fraction. However, AlxGa1-xN structures offering bandgap tuneablity with aluminium content is evident but its impact on surface & electronic properties has not been explored in thoroughly. Therefore, an in-depth investigation of surface states (electronic structure & chemical coordination) of AlxGa1-xN films is required for technology development and the fabrication of efficient devices. It has been reported that growth kinetics and aluminium molar fraction in AlxGa1-xN can significantly alter the nature of oxidation due to change in surface free energy, bonding mechanism, substitution of atoms etc. [5–8]. This ultimately perturb the surface energetics (states) and localized electronic structure of the grown AlN & AlGaN films [8–10]. In our previous work, we have also reported that the electronic structure (band bending) and electronic properties (electron affinity, Ionization energy) are strongly influenced by the growth parameters, inherent polarization, surface chemical states and the ambient oxidation [9–13]. It was observed that the most critical parameter (i.e. electron affinity) of AlN and AlGaN has remained controversial over years, where initial investigations reported the existence of negative electron affin­ ities for AlN and Al-rich AlGaN (Al � 0.75) surfaces [14,15]. Though, the argument were ruled out and positive electron affinities were reported by many research groups in consecutive years [16–18]. Therefore, the present study emphasizes on extensive photoemission analysis of Alx­ Ga1-xN/GaN heterostructures (x ¼ 0.45, 0.62 and 0.75) to investigate the localized band structure, chemical bonding, oxygen chemisorption mechanism and its influence on the electronic properties of the grown films.

(0.62), and 5.2 eV (0.75). These samples are abbreviated as AL-45 (Al0.45Ga0.55N), AL-62 (Al0.62Ga0.38N) and AL-75 (Al0.75Ga0.25N) in the later part of the manuscript. X-Ray/Ultraviolet Photoelectron Spectro­ scopic (XPS & UPS) experiments were carried out using mono­ chromatized AlKα (1486.7 eV) and He (I) (21.2 eV) radiation sources (Scienta Omicron, Germany). The calibration of spectrometer work function (and XPS spectra) was performed using standard gold sample practice with an uncertainty of �0.1 eV in the exploration of valence band. 3. Results and discussion In order to eliminate the possibility of morphological changes induced ambient oxidation, FESEM measurements were performed, which revealed smoother surface morphology for all the AlxGa1-xN/GaN structures (not shown here). Further, XPS core level (CL) spectra of Al (2p), Ga (3d) and O (1s) of AL-45, AL-62 and AL-75 samples were deconvoluted (using a Voigt function) to explore their chemical struc­ ture at the surface as shown in Fig. 1. The Al (2p) CL spectra (as rep­ resented in Fig. 1(a)) was deconvoluted into two identified components; major peak at binding energy (BE) of 74.0 eV corresponding to Al–N while the minor peak at ~1.0 eV higher binding energy was ascribed to Al–O species [8,9]. Similarly, Ga (3d) CL was deconvoluted into Ga–Ga, Ga–N and Ga–O components at their respective BE positions as shown in Fig. 1(b) [12–14]. It was interesting to witness that AlGaN films pur­ suing higher Al content (i.e. sample AL-75) had a small peak corre­ sponding to metallic Ga in samples which demonstrates the presence of remnant gallium. However, to perform an in-depth and comprehensive analysis of oxygen chemisorption at AlxGa1-xN surfaces, the O (1s) CL spectra (Fig. 1(c)) was deconvoluted into three components corre­ sponding to (Al/Ga)xOy, O2 and hydroxyl species [8,16] at the surface. For the ease of readability, the peak position and percentage contribu­ tion all the deconvoluted components in the Al (2p), Ga (3d) and O (1s) CLs are tabulated in Table 1. It is evident from Table 1 that Al (2p) CL of AL-75 has shifted towards lower BE and pursued least amount of native oxide in comparison to AL45 and AL-62. Also, some un-interacting metallic gallium (Ga–Ga spe­ cies) was observed in the sub-surface of AL-75 sample. Since the

2. Experimental The growth of (0001) oriented AlxGa1-xN/GaN heterostructures was carried out on c-plane sapphire (0001) substrate using plasma-assisted molecular beam epitaxy (RIBER Compact 21, France). Photo­ luminescence studies (not shown here) revealed the band gap (and aluminium composition) of the grown films as 4.4 eV (0.45) and 4.8 eV

Fig. 1. De-convoluted XPS core level spectra of a). Al (2p), b), Ga (3d) and c). O (1s) for AL-45, AL-62 and AL-75 samples. 2

M. Mishra and G. Gupta

Materials Chemistry and Physics 239 (2020) 122106

Table 1 Peak positions and the % contribution of the components in the deconvoluted Al (2p), Ga (3d) and O (1s) core levels. Al (2p) Ga (3d)

O (1s)

Position (eV) Al–N (%) Al–O (%) Position (eV) Ga–Ga (%) Ga–N (%) Ga–O (%) Position (eV) (Al/Ga)xOy (%) O2¡ (%) OH¡ (%)

AL-45

AL-62

AL-75

74.1 83 17 20.4 – 82 18 532.3 24 47 29

74.0 92 8 20.0 – 88 12 531.8 17 23 60

73.5 96 4 20.1 8 88 4 531.4 43 37 20

exposure of samples in ambient lead to surface oxidation, the deconvolution of O (1s) CL of the samples was performed to probe the different species and their chemisorption mechanism at the surface. The analysis revealed that the contribution of native oxide was dominated by O2 (i.e. Al2O3 or Ga2O3) and (Al/Ga)xOy species in AL-45 and AL-75, while it was led by hydroxyl group (i.e. OH species) in AL-62 (see Table 1). The chemisorbed oxygen on AlGaN surface act as trap centres which can be classified into two categories namely “Intrinsic” and “Extrinsic” [19]. The intrinsic traps are related to surface defects or dangling bonds while the extrinsic traps are usually associated with ambient adsorbates. Intrinsic traps refer to the surface oxide (O2 spe­ cies) caused by oxygen chemisorption on AlGaN surface due to the available dangling bonds (Al- or Ga-) interacting with the oxygen mol­ ecules present in the ambient. The formation of Al2O3 introduces N-vacancy related (substitution of Nitrogen via Oxygen) defects in the near-surface region [20] and generates virtual gate leading to current collapse [21] in devices. According to density functional theory (DFT) calculations, the stability of a surface is interpreted by the adsorbate driving mechanisms such as electron counting (EC) rule or oxide-stoichiometry (OS) matching [22]. It has been reported that ox­ ygen atoms replace nitrogen in GaN & AlN to form EC oxides and result in high density of donor states at the surface [23], while OS oxides does not require any substitution. It implies that EC oxides are more stable than OS oxides in terms of surface free energy [23]. The explanation and obtained results infers that AL-45 and AL-75 having higher amount surface oxide may possess N-vacancy related defect/trap states along with high density of donor states [23,24]. Also, as predicted by DFT calculation, the chemisorbed oxygen on these surfaces follow EC mechanism and are energetically more stable [22]. On the other hand, the extrinsic traps (i.e. hydroxyl (OH ) group) on the surface of AL-62 does not cause surface trapping and can be removed easily by vacuum annealing at 200 � C [19] or higher temperature. Therefore, it can be finally concluded that the nature of oxide in AL-45 and AL-75 are more stable in comparison to AL-62 and pursue various surface related defect/traps. The XPS valence band (VB) spectra of the AlxGa1-xN samples are shown in Fig. 2 which consist of two peaks labelled as PI and PII located at ~ 4.2 and 9.0 eV, respectively. The upper peak (PII) is occupied by the density maximum of nitrogen state of p-symmetry along with aluminium p states having the same energy whose relative intensity changes with aluminium content. Literature report that the relative intensity of these peaks (PII/PI) increases with aluminium content [24] in the AlxGa1-xN films. It was observed that the relative intensity ratio of PII/PI is higher for AL-62 and AL-75 which signifies the presence of high aluminium content. The VB maximum (VBM) was positioned at 2.1, 2.4 and 2.4 eV above the top of VB for AL-45, AL-62 and AL-75, respectively depicting the positive shift of fermi level (FL) deeper in the energy (or closer to CB) with aluminium content. Also, the surface barrier height (SBH), which is defined as the energy separation between conduction band minimum and FL was calculated to 2.3, 2.4 and 2.8 eV for AL-45, AL-62 and AL-75,

Fig. 2. High resolution XPS valence band spectra of the AlxGa1-xN samples.

respectively. Further, UPS analysis was performed to determine the electronic properties and energy band alignment of the AlxGa1-xN samples as shown in Fig. 3. UPS is a powerful technique to calculate the electronic properties (especially the electron affinity) of the semiconductor sur­ faces. The VBM values were calculated using UPS spectra was obtained to be 2.5, 2.7 and 2.9 eV for AL-45, AL-62 and AL-75, respectively. The slightly higher VBM values (compared to XPS) obtained via UPS spectra were ascribed to higher surface sensitivity of UPS technique (~2–5 nm). The electron affinity (χ) of the films was calculated using following equation [12–14]:

χ ¼ hν

W

Eg:

[1]

where, hν is the energy of the incident photons, W is the spectral width and Eg is the band gap of the material. The electron affinity which is the minimum energy required by an electron to reach the vacuum level from the conduction band minimum is a highly important property of a semiconductor. The concept & nature of metal-semiconductor (MS) contacts (i.e. schottky or ohmic) and properties like field-emission strongly depend on electron affinity of the semiconductor which is highly influenced by surface states. The electron affinities of the AlxGa1xN were calculated using equation (1) and obtained to be 2.6, 2.3 and 2.1 eV AL-45, AL-62 and AL-75, respectively (as shown in Fig. 3). The lower value of electron affinity for AL-62 and AL-75 was originated from the higher amount of aluminium content in the AlGaN film. Interest­ ingly, the photo threshold energy or the ionization energy (I) of the electrons (the energy difference between the UPS photon energy and the spectrum width or the sum of the electron affinity and band gap) was 3

M. Mishra and G. Gupta

Materials Chemistry and Physics 239 (2020) 122106

Fig. 3. (a) UPS valence band spectra of the samples displaying VBM positions and (b) schematic band-structure showing variation in electronic properties.

observed to be independent of aluminium content (i.e. nearly identical for all the samples) due to the difference in bandgap of the materials. In summary, we state that change in aluminium composition in AlxGa1-xN films can significantly alter its surface chemistry and elec­ tronic structure/properties. Ambient oxidation is dominated via chem­ isorbed oxygen on AlxGa1-xN surfaces pursing low and high Al composition (0.45 or 0.75) while it was led via physisorbed hydroxyl species at intermediate Al composition (i.e. 0.62). The Fermi Level tend to approach conduction band minimum with increment in Al content however a reverse trend was perceived for electron affinity. The ob­ tained results can contribute in optimizing the growth recipe of AlGaN films and the study of oxidation mechanism would assist in the fabri­ cation of AlGaN based efficient sensing devices [25].

[5] S. Aslam, D. Franz, C. Stahle, L. Miko, D. Pugel, J. Zhang, R. Gaska, Dual band ultraviolet AlGaN photodetectors for space applications, IEEE Electron. Lett. 43 (2007) 1382. [6] W. Zhang, J. Xu, W. Ye, Y. Li, Z. Qi, J. Dai, Z. Wu, C. Chen, J. Yin, J. Li, H. Jiang, Y. Fang, High-performance AlGaN metal–semiconductor–metal solar-blind ultraviolet photodetectors by localized surface plasmon enhancement, Appl. Phys. Lett. 106 (2015), 021112. [7] H. Durmaz, D. Nothern, G. Brummer, T.D. Moustakas, R. Paiella, Terahertz intersubband photodetectors based on semi-polar GaN/AlGaN heterostructures, Appl. Phys. Lett. 108 (2016) 201102. [8] M. Mishra, S. Krishna, N. Aggarwal, G. Gupta, Influence of metallic surface states on electron affinity of epitaxial AlN films, Appl. Surf. Sci. 405 (2017) 255. [9] S.K. Jain, M. Mishra, N. Aggarwal, S. Krishna, B. Gahtori, A. Pandey, G. Gupta, Influence of temperature and Al/N ratio on structural, chemical & electronic properties of epitaxial AlN films grown via PAMBE, Appl. Surf. Sci. 455 (2018) 919. [10] P. Tyagi, Ch. Ramesh, S.S. Kushwaha, M. Mishra, G. Gupta, B.S. Yadav, M. S. Senthil, Dependence of Al incorporation on growth temperature during laser molecular beam epitaxy of AlxGa1-xN epitaxial layers on sapphire (0001), J. Alloy. Comp. 739 (2018) 122. [11] M. Mishra, S. Krishna, N. Aggarwal, M. Kaur, S. Singh, G. Gupta, Pit assisted oxygen chemisorption on GaN surfaces, Phys. Chem. Chem. Phys. 17 (2015) 15201. [12] M. Mishra, S. Krishna, N. Aggarwal, G. Gupta, Surface chemistry and electronic structure of nonpolar and polar GaN films, Appl. Surf. Sci. 345 (2015) 440. [13] M. Mishra, S. Krishna, N. Aggarwal, S. Vihari, G. Gupta, Electronic structure analysis of GaN films grown on r- and a-plane sapphire, J. Alloy. Comp. 645 (2015) 230. [14] M.C. Benjamin, C. Wang, R.F. Davis, R.J. Nemanich, Observation of a negative electron affinity for heteroepitaxial AlN on α(6H)-SiC(0001), Appl. Phys. Lett. 64 (1994) 3288. [15] M.C. Benjamin, M.D. Bremser, T.W. Weeks Jr., S.W. King, R.F. Davis, R. J. Nemanich, UV photoemission study of heteroepitaxial AlGaN films grown on 6HSiC, Appl. Surf. Sci. 104/105 (1996) 455. [16] C.I. Wu, A. Kahn, E.S. Hellman, D.N.E. Buchanan, Electron affinity at aluminium nitride surfaces, Appl. Phys. Lett. 73 (1998) 1346. [17] W. Zhao, R.Z. Wang, X.M. Song, H. Wang, B. Wang, H. Yan, P.K. Chu, Electron field emission enhanced by geometric and quantum effects from nanostructured AlGaN/ GaN quantum wells, Appl. Phys. Lett. 98 (2011) 152110. [18] J.H. Shin, Y.J. Jo, K.C. Kim, T. Jang, K.S. Kim, Gate metal induced reduction of surface donor states of AlGaN/GaN heterostructure on Si-substrate investigated by electroreflectance spectroscopy, Appl. Phys. Lett. 100 (2012) 111908. [19] F. Gao, D. Chen, H.L. Tuller, C.V. Thompson, T. Palacios, On the redox origin of surface trapping in AlGaN/GaN high electron mobility transistors, J. Appl. Phys. 115 (2014) 124506. [20] T. Hashizume, S. Ootomo, H. Hasegawa, Suppression of current collapse in insulated AlGaN/GaN heterostructure field-effect transistor using ultrathin Al2O3 dielectric, Appl. Phys. Lett. 83 (2003) 2952. [21] M.S. Shur Koudymov, G. Simin, Compact model of current collapse in heterostructure field effect transistor, IEEE Electron. Device Lett. 28 (2007) 332. [22] M.S. Miao, J.R. Weber, C.G. Van de Walle, Oxidation and origin of the two dimensional electron gas in AlGaN/GaN heterostructure, J. Appl. Phys. 107 (2010) 123713. [23] M. Higashiwaki, S. Chowdhury, B.L. Swenson, U.K. Mishra, Effects of oxidation on surface chemical states and barrier height of AlGaN/GaN heterostructures, Appl. Phys. Lett. 97 (2010) 222104. [24] A. Rizzi, M. Kocan, J. Malindretos, A. Schildknecht, N. Teofilov K. Thonke, R. Sauer, Surface and interface properties of AlGaN (0001) epitaxial layers, Appl. Phys. Mater. Sci. Process 87 (2007) 505. [25] M. Mishra, N.K. Bhalla, A. Dash, G. Gupta, Nanostructured GaN and AlGaN/GaN heterostructure for catalyst-free low-temperature CO sensing, Appl. Surf. Sci. 481 (2019) 379.

4. Conclusion We report aluminium composition changes induced modifications in oxygen chemisorption mechanism and electronic structure/properties at AlxGa1-xN surfaces. AlxGa1-xN films grown with different aluminium composition (x ¼ 0.45, 0.62 and 0.75) revealed significant variation in the amount and nature of chemi-/physisorbed oxygen with aluminium content. It was observed that the surface oxide in AL-45 and AL-75 followed EC mechanism and was more stable compared to AL-62. The electron affinities of the surfaces varied from 2.6 to 2.1 eV for AL-45 and AL-75 while the ionization energy of the surfaces remained nearly identical. The present studies provides better understanding of oxidation mechanism and modifications in electronic properties in AlxGa1-xN surfaces which could be employed for the fabrication of efficient devices. Acknowledgement The authors acknowledge the Director, CSIR-NPL India, for his sup­ port. The work is financially supported by Department of Science and Technology (Govt. of India) under grant aid DST/TM/CERI/C245(G). MM would like to thank CSIR (Govt. of India) for financial support under CSIR-Senior Research Fellowship scheme. References [1] K.A. Jones, T.P. Chow, M. Wraback, M. Shatalov, Z. Sitar, F. Shahedipour, K. Udwary, G.S. Tompa, AlGaN devices and growth of device structures, J. Mater. Sci. 50 (2015) 3267. [2] J.H. Tsai, C.C. Chiang, F.M. Wang, High-performance AlGaN/AlN/GaN high electron mobility transistor with broad gate-to-source operation voltages, Phys. Status Solidi C 12 (2015) 596. [3] T. Palacios, A. Chakraborty, S. Rajan, C. Poblenz, S. Keller, S.P. Denbaars, J. S. Speck, U.K. Mishra, High-power AlGaN/GaN HEMTs for Ka-band applications, IEEE Electron. Device Lett. 26 (2005) 781. [4] T. Palacios, C.S. Suh, A. Chakraborty, S. Keller, S.P. Denbaars, U.K. Mishra, Highperformance E-mode AlGaN/GaN HEMTs, IEEE Electron. Device Lett. 27 (2006) 428.

4