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Phosphorous passivation of In0.53Ga0.47As using MOVPE and characterization of Au–Ga2O3(Gd2O3)–In0.53Ga0.47As MIS capacitor
Applied Surface Science 245 (2005) 196–201 www.elsevier.com/locate/apsusc
Phosphorous passivation of In0.53Ga0.47As using MOVPE and characterization of Au–Ga2O3 (Gd2O3)–In0.53Ga0.47As MIS capacitor S. Pala,*, S.M. Shivaprasadb, Y. Aparnab, B.R. Chakrabortyb a
Centre for Advanced Technology, Indore 452013, MP, India b National Physical Laboratory, New Delhi 110012, India Received 4 August 2004; accepted 7 October 2004 Available online 11 November 2004
Abstract A study of phosphorous passivation of the interface states of undoped In0.53Ga0.47As has been carried out. Phosphorous surface passivation has been achieved by: (1) exchange reaction of the InGaAs surface under phosphine vapor or (2) direct growth of InGaP/GaP thin epitaxial layers in a metal organic vapour phase epitaxy (MOVPE) reactor. The passivated surfaces have been characterized using X-ray photoelectron spectroscopy and capacitance–voltage measurements of the MIS devices. The minimum interface state density of 2.90 1011 eV1 cm2 was obtained for Au/Ga2O3(Gd2O3)/GaP/In0.53Ga0.47As structure. # 2004 Elsevier B.V. All rights reserved. Keywords: In0.53Ga0.47As; Surface passivation; Interface state density; XPS; C–V measurement
1. Introduction InxGa1xAs is a direct bandgap semiconductor throughout the entire composition range from x = 0 to x = 1. In0.53Ga0.47As, lattice matched to InP, with a bandgap of 0.74 eV at room-temperature, is an important material for device applications. Very high * Corresponding author. Tel.: +91 731 2488348; fax: +91 731 2488300. E-mail addresses: [email protected], [email protected] (S. Pal).
low-field electron mobility (12,000 cm2 V1 s1 at 300 K), high saturation velocity, and a large intervalley (G–L) separation in the conduction band are some of its favorable properties. Heterostructure devices using this compound, such as modulationdoped field-effect transistors, have demonstrated record dc, high frequency, and noise performance at room and cryogenic temperatures. Its bandgap also corresponds to the spectral range where optical fibers have very low loss and dispersion. Therefore, In0.53Ga0.47As is used for the development of modulators and detectors for optical communication.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.009
S. Pal et al. / Applied Surface Science 245 (2005) 196–201
Feature size-reduction for higher performance and integration density appears to be a prevailing trend in the advanced semiconductor device technology. In such microstructures, the importance of surfaces and interfaces will obviously be more pronounced than in the present-day devices. Air- or oxygen-exposed GaAs surfaces are pinned near the middle of the bandgap due to formation of As2O3, which in turn forms elemental arsenic [1]. Increasing the indium concentration of InxGa1xAs from x = 0 to 1 moves the Fermi level 0.91 eV below the conduction band minimum (CBM) for GaAs, to 0.21 eV below the CBM for In0.53Ga0.47As and thence to 0.122 eV, above the CBM for InAs [1]. Although InGaAs is characterized by low surface recombination velocity (200 cm/s), the situation is quite different for air- or oxygenexposed InxGa1xAs surfaces or for metal insulator semiconductor (MIS) structures. Problems with surface recombination have been reported in InGaAs/InP heterostructures [2–5]. It is therefore necessary to investigate further into this problem. Conventional insulators, e.g., SiO2 and Si3N4 coated In0.53Ga0.47As indicate sufficient Ga, In and As diffusion into the insulating layer. Preservation of the stoichiometry of the semiconductor surface must be considered to be a prime consideration in the choice of appropriate insulators for device purposes. Lattice matched or pseudomorphic heterojunctions with large band edge discontinuities may represent a possible solution to theimpediments associated with homomorphic or heteromorphic insulators. Phosphorus passivation has been recognized to be an attractive method for GaAs and InGaAs surface passivation. Viktorovitch et al. reported a new passivation method of the GaAs surface based on a thermal treatment under PH3 overpressure [6]. A dramatic enhancement of over 300 in the roomtemperature photoluminescence signal obtained from high-purity GaAs epilayers after a brief heat treatment in tertiarybutylphosphine vapor has also been reported [7]. These treatments result, by As/P exchange, in the formation of a thin superficial GaP layer, which prevents the formation of an arsenic oxide, as observed by XPS analysis. Sugino et al. reported the treatment of GaAs surface with a remote plasma of phosphine diluted with Ar [8]. An alternate approach that offers the possibility of better long-term stability is the use of epitaxial passivation layers such as InGaP, InP, or GaP
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surface layers prepared either by direct epitaxial growth or exchange reaction. Recently a study [9] on the effect of phosphorous passivation on the surface electric field of undoped GaAs has been presented using tertiarybutylphosphine. The first insulated gate n-channel enhancement-mode In0.53Ga0.47As MOSFET on InP semi-insulating substrate using Ga2O3(Gd2O3) as gate dielectric has been demonstrated [10]. In the present study, we have employed the technique of in situ surface passivation of In0.53Ga0.47As in a MOVPE reactor using PH3. Phosphorous surface passivation was achieved by: (1) exchange reaction of the GaAs surface under PH3 vapor or (2) direct growth of GaP or In0.5Ga0.5P thin epitaxial layers on top of In0.53Ga0.47As in a MOVPE reactor. The interface chemical states were examined by XPS while the sharpness was monitored by SIMS depth profile. The efficacy of different passivation methods was also measured by extracting the interface state density of the Au–Ga2O3(Gd2O3)–In0.53Ga0.47As capacitors from C–V and G–V measurements.
2. Experiment The growth and surface passivation of InGaAs samples were carried out in a low-pressure MOVPE system (CVD Equipment Corporation). All the growths were performed at 630 8C temperature and under a growth pressure of 100 Torr. A hydrogen carrier flow of 1.3 l/min was maintained during the growth. In this case n+-InP was used as the substrate and 0.5 mm undoped InP buffer layer was grown on it. Using 1 s pause an In0.53Ga0.47As epilayer of 1 mm thickness was grown over it (S1). Phosphorus passivation of InGaAs surface was done using two methods. In first case, in situ PH3 flow was used (exchange reaction) for 10 s (S2) after InGaAs growth. In the second case (direct growth), pseudomorphic ˚ GaP (S3) and In0.5Ga0.5P (S4) layers of 20 A thickness were grown on top of InGaAs epilayers. For the InGaAs/phosphide samples a pause of 2 s (for complete removal of AsH3) was used before the phosphide surface layer was formed on top of InGaAs. After the growth of phosphide layers the samples were cooled down to 200 8C in PH3 atmosphere and then to room-temperature in H2 flow.
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Chemical characterization of the grown oxide was performed by transferring the sample into a PerkinElmer Model 1257 X-ray Photoelectron Spectroscopy unit, operating in UHV at a base pressure of 5 1010 Torr. The sample surface is excited by a Al Ka X-ray source, and the electrons are energyresolved by a high resolution hemispherical sector analyzer. Ga2O3(Gd2O3)/In0.53Ga0.47As structures were characterized by secondary ion mass spectrometry (SIMS) to study the oxides and interfaces. In the present case, Ga+ ions were used as the primary beam of 25.0 keV energy with beam current of 4 nA and positive secondary ions were detected. The depth profiles were carried out on a surface area of 150 mm 150 mm with a 10% window gating to avoid the crator edge effect. Depth calibration was carried out using a Tencor Alpha step 500 surface profilometer of the craters after SIMS depth profiling. To study the interface properties before and after passivation, MIS structures were fabricated on the unpassivated and passivated InGaAs samples, which were exposed to air for 2 weeks after growth. The samples were cleaned using trichloroethelene, acetone and methanol, followed by a dip into 50% HCl solution before loading into the e-beam evaporation ˚ unit. An insulating layer of Ga2O3(Gd2O3) of 1000 A thickness was deposited on the surfaces of all the samples. MIS structures were fabricated by evaporating Au-gate electrodes on the top and Au–Ge as the ohmic back contact. The samples were then annealed at 350 8C for 30 s in an Ar atmosphere. The interface state densities of the Au/dielectric/In0.53Ga0.47As MOS structures were determined from the conductance–voltage (G–V) and capacitance–voltage (C–V) characteristics using a Kiethley 590 CV analyzer.
3. Results and discussions The as-grown InxGa1xAs epilayers were characterized using X-ray diffraction (XRD) and photoluminescence measurements. The measurement was carried out using a Philips X’Pert X-ray single crystal diffractometer. The lattice mismatch between the ternary InGaAs epilayer and InP substrate as well as the composition of the epilayer were evaluated by examining the diffraction from (0 0 4) planes of the
Fig. 1. Photoluminescence spectra of InGaAs lattice matched to InP substrate at 15 K.
epitaxial layer and substrate. The composition of the epilayer was found to be In0.528Ga0.472As. At 15 K, the InGaAs epilayers show (Fig. 1) two near-band-edge luminescence features (a shallow peak at 0.81 eV and a shoulder at 0.795 eV). The higher energy line is attributed to bound exciton (BE) recombinations and the low-energy band to donor–acceptor pair (DAP) recombinations [11,12]. The line width of the shallow principal peak was 5 meV. The Hall electron mobility, m, varies from 8500 cm2 V1 s1 at 300 K to 65,000 cm2 V1 s1 at 20 K. These results are indicative of the growth of a good-quality epilayers. 3.1. X-ray photoelectron spectroscopy (XPS) analysis The XPS survey scans were obtained by using a pass energy of 100 eV, while the core-level scans had 40 eV as pass energy. XPS spectra of As 3d core level for the as-grown as well as phosphorous-passivated samples are shown in Fig. 2a. In the case of as-grown sample, two peaks are observed at binding energies of 41.0 and 44. 6 eV, which are assigned to As–Ga and As2O3, respectively [13]. In the case of phosphidized samples (dotted line), the peak due to As2O3 is not observed. This indicates that As-oxides are not formed on InGaAs surface upon phosphorous passivation. In this study both the as-grown and phosphidized samples were exposed to air for several days. Fig. 2b shows the photoelectron spectrum from the P 2p core level of the PH3-treated InGaAs sample. The XPS signals observed at 128.8 eV is due to existence of
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Fig. 2. XPS spectra from, (a) As 3d core level of as grown and PH3treated In0.53Ga0.47As and (b) P 2p core level of PH3 treated In0.53Ga0.47As.
P–In bond. Existence of P–In bonds suggests substitution of phosphorous atoms for arsenic atoms at the surface. Another photoelectron peak is observed at 133.3 eV. It was reported that XPS signals of InPoxide, P-oxide and P2O5 peaked in the range of 128.7– 135.6 eV region [14,15]. So, it can be inferred that the P and InGaP related oxides are formed on the surface of all the phosphorous treated samples as these are exposed to air before the XPS measurements. XPS measurement was also done for Ga 3d and In 3d core level which also showed the formation of Ga–P and In–P bonds upon phosphidization of InGaAs surface. 3.2. Calculation of interface state density (Dit) using the MIS characteristics The efficacy of different passivation methods was measured by extracting the interface state density of the Au–Ga2O3(Gd2O3)–InGaAs capacitors from C–V and G–V measurements. The classical operational modes of ideal MIS structures with distinct inversion,
Fig. 3. (a) Capacitance–voltage plot of Au–Ga2O3(Gd2O3)-semiconductor diodes with (i) InGaAs/PH3, 10 s (S2), (ii) InGaAs/GaP (S3), and (iii) InGaAs/InGaP (S4) structures; (b) Conductance– voltage plot of Au–Ga2O3(Gd2O3)-semiconductor diodes with (i) InGaAs/PH3, 10s (S2), (ii) InGaAs/GaP (S3), and (iii) InGaAs/ InGaP (S4) structures.
depletion and accumulation are clearly revealed in the C–V characteristics (Fig. 3a). A good hysteresis behavior with hysteresis window width of 85 mV is also observed. A remarkable reduction in frequency dispersion between 10 kHz and 10 MHz is also observed upon annealing. While the C–V characteristic of the MIS device fabricated on as-grown InGaAs surface shows very little modulation, it improves significantly in case of phosphide layer capped InGaAs samples. This result is indicative of lowering of interface state densities after phosphidization of InGaAs surface. The G–V curves have been used to calculate the density of interface states using Hill’s method. Correction to the G–V characteristics is required to account for the series resistance of the passivating
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Table 1 Variation of interface state densities of Ga2O3(Gd2O3)/In0.53Ga0.47As with different phosphorous passivation conditions Flow rate of TMG (Tccm)
Flow rate of TMI (Tccm)
Flow rate of AsH3 (sccm)
Flow rate of PH3 (sccm)
Passivation time/layer thickness
Composition of the passivating layer
Sample no.
Density of interface states, Dit (eV1 cm2)
–
– – 110.0
– – –
32.0 32.0 32.0
10 s PH3 ˚ 20 A ˚ 20 A
InxGa1xAsyP1y GaP In0.5 Ga0.5P
S2 S3 S4
5.83 1011 2.90 1011 4.18 1011
81.0 101.0
layer/InGaAs/InP heterostructure used in this study. This can arise from different sources e.g., the back contact to the InP substrate, bulk resistance of the InP buffer layer and different heterointerfaces. Fig. 3b shows the dependence of Gc (corrected) on gate voltages at 1 MHz for the Au–Ga2O3(Gd2O3)– In0.53Ga0.47As capacitors under different passivating conditions. S3 shows the lowest value of Gmax giving lowest interface state density among all the samples (Table 1). The interface state densities are calculated from the Gc versus Vg plots using the relation [16]. 2 Gmax Dit ¼ qA v !1 Gmax 2 Cmax 2 þ 1 (1) vCi Ci where Dit is the interface state density, A the area of the capacitor, q the electronic charge, Gmax the peak value of the conductance in Gc versus Vg curve and Cmax the capacitance corresponding to Gmax.. The minimum interface state density for the S3 sample is calculated to be 2.90 1011 eV1 cm2. 3.3. SIMS depth profile of Ga2O3(Gd2O3)/ In0.53Ga0.47As Conventional insulators e.g., SiO2 and Si3N4coated InGaAs indicate sufficient Ga, In and As diffusion into the insulating layer. Preservation of the stoichiometry of the semiconductor surface must be considered to be a prime factor in the choice of appropriate insulators for the device purpose. The SIMS depth profile of Ga2O3(Gd2O3)/InGaAs sample (Fig. 4) does not show any inter-diffusion of In, Ga and As into the insulating layer or vice versa even after annealing at 450 8C. The profile shows a very sharp interface between the insulating layer and InGaAs.
Fig. 4. SIMS depth profile of Ga2O3(Gd2O3)/In0.53Ga0.47As interface annealed at 450 8C.
4. Conclusion The effect of phosphorous passivation on the surface states of undoped InGaAs for MOS device application has been studied in detail. For P-passivation, an in situ MOVPE technique has been used. Optimum condition for passivation was found out using different methods e.g., flowing phosphine over InGaAs surface after growth (As–P exchange reaction), growing GaP and InGaP epilayers directly on InGaAs. Phosphorous incorporation and removal of arsenic oxides upon Ptreatment of the surface was confirmed by XPS and the interface sharpness or interdiffusion of P or In inside the insulator was monitored by SIMS depth profiling.
S. Pal et al. / Applied Surface Science 245 (2005) 196–201
Interface state densities were calculated from the C–V and G–V characteristics of the MIS structures. These characteristics clearly show the significant reduction density of interface states upon P-passivation. The minimum interface state density of 2.90 1011 eV1 cm2 was obtained for Au/Ga2O3 (Gd2O3)/GaP/InGaAs structure.
[6]
[7]
[8]
Acknowledgments One of the authors (SP) is grateful to Prof. B.M. Arora, TIFR, for the growth of the InGaAs samples. Fruitful discussions with Dr. S.K. Ray and Prof. D.N. Bose are also acknowledged.
[10]
References
[11]
[1] H.H. Weider, Appl. Phys. Lett. 38 (1981) 170. [2] F. Ren, D.N. Buckley, K.M. Lee, S.J. Pearton, R.A. Bartynski, C. Constantine, W.S. Hobson, R.A. Hamm, P.C. Chao, Effect of ECR plasma on the luminescence efficiency of InGaAs and InP, Solid-State Electron. 38 (1995) 2011. [3] B. Jacobs, M. Emmerling, A. Forchel, I. Gyuro, P. Speier, E. Zielinski, Optical investigation of the sidewall recombination in wet etched InGaAs/InP-wires, Jpn. J. Appl. Phys., Part 2 32 (1993) L173. [4] H. Fukano, Y. Takahashi, M. Fujimoto, Surface currents in InP/ InGaAs heterojunction bipolar transistor produced by passivation film formation, Jpn. J. Appl. Phys., Part 2 32 (1993) L1788. [5] A. Oucha, M. Willander, B. Hammarlund, R.A. Logan, Effect of surface passivation with silicon nitride on the electrical
[9]
[12]
[13]
[14]
[15]
[16]
201
properties of InP/InGaAs heterojunction bipolar transistor, J. Appl. Phys. 74 (1993) 5602. P. Viktorovitch, M. Gendry, S.K. Krawczyk, F. Krafft, Improved electronic properties of GaAs surface stabilized with phosphorous, Appl. Phys. Lett. 58 (1991) 2387. D.A. Harrison, R. Ares, S.P. Watkins, M.L.W. Thewalt, C.R. Bolognesi, D.J.S. Beckett, A.J. Spring Thorpe, Large photoluminescence enhancements from epitaxial GaAs passivated by postgrowth phophidization, Appl. Phys. Lett. 70 (1997) 3275. T. Sugino, S. Nozu, S. Nakajima, Phosphidization of GaAs by a remote phosphine plasma process and its application to surface passivation of GaAs metal-semiconductor field-effect transistors, Appl. Phys. Lett. 74 (1999) 2999. R. Beaudry, S.P. Wattkins, X. Xu, P. Yeo, Photorefletance study of phosphorous passivation of GaAs (0 0 1), J. Appl. Phys. 87 (2000) 7838. F. Ren, J.M. Kuo, M. Hong, W.S. Hobson, J.R. Lothian, J. Lin, H.S. Tsai, J.P. Mannaerts, J. Kwo, S.N.G. Chu, Y.K. Chen, A.Y. Cho, Ga2O3(Gd2O3)/InGaAs enhancement-mode n-channel MOSFET’s, IEEE Electron. Dev. Lett. 19 (1998) 309. E.F. Schubert, W.T. Tsang, Photoluminescence line shape of excitons in alloy semiconductors, Phys. Rev. B 34 (1986) 2991. P. Bhattacharya, M. Rao, M. Tsai, Growth and photoluminescence spectra of high-purity liquid phase epitaxial In.53Ga.47As, J. Appl. Phys. 54 (1983) 5096. C.D. Chouvy, D. Ballutaud, J.C. Pesant, A. Etcheberry, X-ray photoelectron spectroscopy study of GaAs surface exposed to a r.f. hydrogen plasma, Appl. Phys. Lett. 62 (1993) 2254. T. Miyazaki, T. Sugino, K. Matsuda, J. Shirafuji, Characterization of InGaAs phosphidized by a plasma process, Jpn. J. Appl. Phys. 38 (1999) 1139. G. Hollinger, E. Berginat, J. Joseph, Y. Robach, On the nature of oxides on InP surfaces, J. Vac. Sci. Technol. A3 (1985) 2082. W.A. Hill, C.C. Coleman, A single-frequency approximation for interface-state density determination, Solid-State Electron. 23 (1980) 987.
Phosphorous passivation of In0.53Ga0.47As using MOVPE and characterization of Au–Ga2O3 (Gd2O3)–In0.53Ga0.47As MIS capacitor S. Pala,*, S.M. Shivaprasadb, Y. Aparnab, B.R. Chakrabortyb a
Centre for Advanced Technology, Indore 452013, MP, India b National Physical Laboratory, New Delhi 110012, India Received 4 August 2004; accepted 7 October 2004 Available online 11 November 2004
Abstract A study of phosphorous passivation of the interface states of undoped In0.53Ga0.47As has been carried out. Phosphorous surface passivation has been achieved by: (1) exchange reaction of the InGaAs surface under phosphine vapor or (2) direct growth of InGaP/GaP thin epitaxial layers in a metal organic vapour phase epitaxy (MOVPE) reactor. The passivated surfaces have been characterized using X-ray photoelectron spectroscopy and capacitance–voltage measurements of the MIS devices. The minimum interface state density of 2.90 1011 eV1 cm2 was obtained for Au/Ga2O3(Gd2O3)/GaP/In0.53Ga0.47As structure. # 2004 Elsevier B.V. All rights reserved. Keywords: In0.53Ga0.47As; Surface passivation; Interface state density; XPS; C–V measurement
1. Introduction InxGa1xAs is a direct bandgap semiconductor throughout the entire composition range from x = 0 to x = 1. In0.53Ga0.47As, lattice matched to InP, with a bandgap of 0.74 eV at room-temperature, is an important material for device applications. Very high * Corresponding author. Tel.: +91 731 2488348; fax: +91 731 2488300. E-mail addresses: [email protected], [email protected] (S. Pal).
low-field electron mobility (12,000 cm2 V1 s1 at 300 K), high saturation velocity, and a large intervalley (G–L) separation in the conduction band are some of its favorable properties. Heterostructure devices using this compound, such as modulationdoped field-effect transistors, have demonstrated record dc, high frequency, and noise performance at room and cryogenic temperatures. Its bandgap also corresponds to the spectral range where optical fibers have very low loss and dispersion. Therefore, In0.53Ga0.47As is used for the development of modulators and detectors for optical communication.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.009
S. Pal et al. / Applied Surface Science 245 (2005) 196–201
Feature size-reduction for higher performance and integration density appears to be a prevailing trend in the advanced semiconductor device technology. In such microstructures, the importance of surfaces and interfaces will obviously be more pronounced than in the present-day devices. Air- or oxygen-exposed GaAs surfaces are pinned near the middle of the bandgap due to formation of As2O3, which in turn forms elemental arsenic [1]. Increasing the indium concentration of InxGa1xAs from x = 0 to 1 moves the Fermi level 0.91 eV below the conduction band minimum (CBM) for GaAs, to 0.21 eV below the CBM for In0.53Ga0.47As and thence to 0.122 eV, above the CBM for InAs [1]. Although InGaAs is characterized by low surface recombination velocity (200 cm/s), the situation is quite different for air- or oxygenexposed InxGa1xAs surfaces or for metal insulator semiconductor (MIS) structures. Problems with surface recombination have been reported in InGaAs/InP heterostructures [2–5]. It is therefore necessary to investigate further into this problem. Conventional insulators, e.g., SiO2 and Si3N4 coated In0.53Ga0.47As indicate sufficient Ga, In and As diffusion into the insulating layer. Preservation of the stoichiometry of the semiconductor surface must be considered to be a prime consideration in the choice of appropriate insulators for device purposes. Lattice matched or pseudomorphic heterojunctions with large band edge discontinuities may represent a possible solution to theimpediments associated with homomorphic or heteromorphic insulators. Phosphorus passivation has been recognized to be an attractive method for GaAs and InGaAs surface passivation. Viktorovitch et al. reported a new passivation method of the GaAs surface based on a thermal treatment under PH3 overpressure [6]. A dramatic enhancement of over 300 in the roomtemperature photoluminescence signal obtained from high-purity GaAs epilayers after a brief heat treatment in tertiarybutylphosphine vapor has also been reported [7]. These treatments result, by As/P exchange, in the formation of a thin superficial GaP layer, which prevents the formation of an arsenic oxide, as observed by XPS analysis. Sugino et al. reported the treatment of GaAs surface with a remote plasma of phosphine diluted with Ar [8]. An alternate approach that offers the possibility of better long-term stability is the use of epitaxial passivation layers such as InGaP, InP, or GaP
197
surface layers prepared either by direct epitaxial growth or exchange reaction. Recently a study [9] on the effect of phosphorous passivation on the surface electric field of undoped GaAs has been presented using tertiarybutylphosphine. The first insulated gate n-channel enhancement-mode In0.53Ga0.47As MOSFET on InP semi-insulating substrate using Ga2O3(Gd2O3) as gate dielectric has been demonstrated [10]. In the present study, we have employed the technique of in situ surface passivation of In0.53Ga0.47As in a MOVPE reactor using PH3. Phosphorous surface passivation was achieved by: (1) exchange reaction of the GaAs surface under PH3 vapor or (2) direct growth of GaP or In0.5Ga0.5P thin epitaxial layers on top of In0.53Ga0.47As in a MOVPE reactor. The interface chemical states were examined by XPS while the sharpness was monitored by SIMS depth profile. The efficacy of different passivation methods was also measured by extracting the interface state density of the Au–Ga2O3(Gd2O3)–In0.53Ga0.47As capacitors from C–V and G–V measurements.
2. Experiment The growth and surface passivation of InGaAs samples were carried out in a low-pressure MOVPE system (CVD Equipment Corporation). All the growths were performed at 630 8C temperature and under a growth pressure of 100 Torr. A hydrogen carrier flow of 1.3 l/min was maintained during the growth. In this case n+-InP was used as the substrate and 0.5 mm undoped InP buffer layer was grown on it. Using 1 s pause an In0.53Ga0.47As epilayer of 1 mm thickness was grown over it (S1). Phosphorus passivation of InGaAs surface was done using two methods. In first case, in situ PH3 flow was used (exchange reaction) for 10 s (S2) after InGaAs growth. In the second case (direct growth), pseudomorphic ˚ GaP (S3) and In0.5Ga0.5P (S4) layers of 20 A thickness were grown on top of InGaAs epilayers. For the InGaAs/phosphide samples a pause of 2 s (for complete removal of AsH3) was used before the phosphide surface layer was formed on top of InGaAs. After the growth of phosphide layers the samples were cooled down to 200 8C in PH3 atmosphere and then to room-temperature in H2 flow.
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Chemical characterization of the grown oxide was performed by transferring the sample into a PerkinElmer Model 1257 X-ray Photoelectron Spectroscopy unit, operating in UHV at a base pressure of 5 1010 Torr. The sample surface is excited by a Al Ka X-ray source, and the electrons are energyresolved by a high resolution hemispherical sector analyzer. Ga2O3(Gd2O3)/In0.53Ga0.47As structures were characterized by secondary ion mass spectrometry (SIMS) to study the oxides and interfaces. In the present case, Ga+ ions were used as the primary beam of 25.0 keV energy with beam current of 4 nA and positive secondary ions were detected. The depth profiles were carried out on a surface area of 150 mm 150 mm with a 10% window gating to avoid the crator edge effect. Depth calibration was carried out using a Tencor Alpha step 500 surface profilometer of the craters after SIMS depth profiling. To study the interface properties before and after passivation, MIS structures were fabricated on the unpassivated and passivated InGaAs samples, which were exposed to air for 2 weeks after growth. The samples were cleaned using trichloroethelene, acetone and methanol, followed by a dip into 50% HCl solution before loading into the e-beam evaporation ˚ unit. An insulating layer of Ga2O3(Gd2O3) of 1000 A thickness was deposited on the surfaces of all the samples. MIS structures were fabricated by evaporating Au-gate electrodes on the top and Au–Ge as the ohmic back contact. The samples were then annealed at 350 8C for 30 s in an Ar atmosphere. The interface state densities of the Au/dielectric/In0.53Ga0.47As MOS structures were determined from the conductance–voltage (G–V) and capacitance–voltage (C–V) characteristics using a Kiethley 590 CV analyzer.
3. Results and discussions The as-grown InxGa1xAs epilayers were characterized using X-ray diffraction (XRD) and photoluminescence measurements. The measurement was carried out using a Philips X’Pert X-ray single crystal diffractometer. The lattice mismatch between the ternary InGaAs epilayer and InP substrate as well as the composition of the epilayer were evaluated by examining the diffraction from (0 0 4) planes of the
Fig. 1. Photoluminescence spectra of InGaAs lattice matched to InP substrate at 15 K.
epitaxial layer and substrate. The composition of the epilayer was found to be In0.528Ga0.472As. At 15 K, the InGaAs epilayers show (Fig. 1) two near-band-edge luminescence features (a shallow peak at 0.81 eV and a shoulder at 0.795 eV). The higher energy line is attributed to bound exciton (BE) recombinations and the low-energy band to donor–acceptor pair (DAP) recombinations [11,12]. The line width of the shallow principal peak was 5 meV. The Hall electron mobility, m, varies from 8500 cm2 V1 s1 at 300 K to 65,000 cm2 V1 s1 at 20 K. These results are indicative of the growth of a good-quality epilayers. 3.1. X-ray photoelectron spectroscopy (XPS) analysis The XPS survey scans were obtained by using a pass energy of 100 eV, while the core-level scans had 40 eV as pass energy. XPS spectra of As 3d core level for the as-grown as well as phosphorous-passivated samples are shown in Fig. 2a. In the case of as-grown sample, two peaks are observed at binding energies of 41.0 and 44. 6 eV, which are assigned to As–Ga and As2O3, respectively [13]. In the case of phosphidized samples (dotted line), the peak due to As2O3 is not observed. This indicates that As-oxides are not formed on InGaAs surface upon phosphorous passivation. In this study both the as-grown and phosphidized samples were exposed to air for several days. Fig. 2b shows the photoelectron spectrum from the P 2p core level of the PH3-treated InGaAs sample. The XPS signals observed at 128.8 eV is due to existence of
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Fig. 2. XPS spectra from, (a) As 3d core level of as grown and PH3treated In0.53Ga0.47As and (b) P 2p core level of PH3 treated In0.53Ga0.47As.
P–In bond. Existence of P–In bonds suggests substitution of phosphorous atoms for arsenic atoms at the surface. Another photoelectron peak is observed at 133.3 eV. It was reported that XPS signals of InPoxide, P-oxide and P2O5 peaked in the range of 128.7– 135.6 eV region [14,15]. So, it can be inferred that the P and InGaP related oxides are formed on the surface of all the phosphorous treated samples as these are exposed to air before the XPS measurements. XPS measurement was also done for Ga 3d and In 3d core level which also showed the formation of Ga–P and In–P bonds upon phosphidization of InGaAs surface. 3.2. Calculation of interface state density (Dit) using the MIS characteristics The efficacy of different passivation methods was measured by extracting the interface state density of the Au–Ga2O3(Gd2O3)–InGaAs capacitors from C–V and G–V measurements. The classical operational modes of ideal MIS structures with distinct inversion,
Fig. 3. (a) Capacitance–voltage plot of Au–Ga2O3(Gd2O3)-semiconductor diodes with (i) InGaAs/PH3, 10 s (S2), (ii) InGaAs/GaP (S3), and (iii) InGaAs/InGaP (S4) structures; (b) Conductance– voltage plot of Au–Ga2O3(Gd2O3)-semiconductor diodes with (i) InGaAs/PH3, 10s (S2), (ii) InGaAs/GaP (S3), and (iii) InGaAs/ InGaP (S4) structures.
depletion and accumulation are clearly revealed in the C–V characteristics (Fig. 3a). A good hysteresis behavior with hysteresis window width of 85 mV is also observed. A remarkable reduction in frequency dispersion between 10 kHz and 10 MHz is also observed upon annealing. While the C–V characteristic of the MIS device fabricated on as-grown InGaAs surface shows very little modulation, it improves significantly in case of phosphide layer capped InGaAs samples. This result is indicative of lowering of interface state densities after phosphidization of InGaAs surface. The G–V curves have been used to calculate the density of interface states using Hill’s method. Correction to the G–V characteristics is required to account for the series resistance of the passivating
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Table 1 Variation of interface state densities of Ga2O3(Gd2O3)/In0.53Ga0.47As with different phosphorous passivation conditions Flow rate of TMG (Tccm)
Flow rate of TMI (Tccm)
Flow rate of AsH3 (sccm)
Flow rate of PH3 (sccm)
Passivation time/layer thickness
Composition of the passivating layer
Sample no.
Density of interface states, Dit (eV1 cm2)
–
– – 110.0
– – –
32.0 32.0 32.0
10 s PH3 ˚ 20 A ˚ 20 A
InxGa1xAsyP1y GaP In0.5 Ga0.5P
S2 S3 S4
5.83 1011 2.90 1011 4.18 1011
81.0 101.0
layer/InGaAs/InP heterostructure used in this study. This can arise from different sources e.g., the back contact to the InP substrate, bulk resistance of the InP buffer layer and different heterointerfaces. Fig. 3b shows the dependence of Gc (corrected) on gate voltages at 1 MHz for the Au–Ga2O3(Gd2O3)– In0.53Ga0.47As capacitors under different passivating conditions. S3 shows the lowest value of Gmax giving lowest interface state density among all the samples (Table 1). The interface state densities are calculated from the Gc versus Vg plots using the relation [16]. 2 Gmax Dit ¼ qA v !1 Gmax 2 Cmax 2 þ 1 (1) vCi Ci where Dit is the interface state density, A the area of the capacitor, q the electronic charge, Gmax the peak value of the conductance in Gc versus Vg curve and Cmax the capacitance corresponding to Gmax.. The minimum interface state density for the S3 sample is calculated to be 2.90 1011 eV1 cm2. 3.3. SIMS depth profile of Ga2O3(Gd2O3)/ In0.53Ga0.47As Conventional insulators e.g., SiO2 and Si3N4coated InGaAs indicate sufficient Ga, In and As diffusion into the insulating layer. Preservation of the stoichiometry of the semiconductor surface must be considered to be a prime factor in the choice of appropriate insulators for the device purpose. The SIMS depth profile of Ga2O3(Gd2O3)/InGaAs sample (Fig. 4) does not show any inter-diffusion of In, Ga and As into the insulating layer or vice versa even after annealing at 450 8C. The profile shows a very sharp interface between the insulating layer and InGaAs.
Fig. 4. SIMS depth profile of Ga2O3(Gd2O3)/In0.53Ga0.47As interface annealed at 450 8C.
4. Conclusion The effect of phosphorous passivation on the surface states of undoped InGaAs for MOS device application has been studied in detail. For P-passivation, an in situ MOVPE technique has been used. Optimum condition for passivation was found out using different methods e.g., flowing phosphine over InGaAs surface after growth (As–P exchange reaction), growing GaP and InGaP epilayers directly on InGaAs. Phosphorous incorporation and removal of arsenic oxides upon Ptreatment of the surface was confirmed by XPS and the interface sharpness or interdiffusion of P or In inside the insulator was monitored by SIMS depth profiling.
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Interface state densities were calculated from the C–V and G–V characteristics of the MIS structures. These characteristics clearly show the significant reduction density of interface states upon P-passivation. The minimum interface state density of 2.90 1011 eV1 cm2 was obtained for Au/Ga2O3 (Gd2O3)/GaP/InGaAs structure.
[6]
[7]
[8]
Acknowledgments One of the authors (SP) is grateful to Prof. B.M. Arora, TIFR, for the growth of the InGaAs samples. Fruitful discussions with Dr. S.K. Ray and Prof. D.N. Bose are also acknowledged.
[10]
References
[11]
[1] H.H. Weider, Appl. Phys. Lett. 38 (1981) 170. [2] F. Ren, D.N. Buckley, K.M. Lee, S.J. Pearton, R.A. Bartynski, C. Constantine, W.S. Hobson, R.A. Hamm, P.C. Chao, Effect of ECR plasma on the luminescence efficiency of InGaAs and InP, Solid-State Electron. 38 (1995) 2011. [3] B. Jacobs, M. Emmerling, A. Forchel, I. Gyuro, P. Speier, E. Zielinski, Optical investigation of the sidewall recombination in wet etched InGaAs/InP-wires, Jpn. J. Appl. Phys., Part 2 32 (1993) L173. [4] H. Fukano, Y. Takahashi, M. Fujimoto, Surface currents in InP/ InGaAs heterojunction bipolar transistor produced by passivation film formation, Jpn. J. Appl. Phys., Part 2 32 (1993) L1788. [5] A. Oucha, M. Willander, B. Hammarlund, R.A. Logan, Effect of surface passivation with silicon nitride on the electrical
[9]
[12]
[13]
[14]
[15]
[16]
201
properties of InP/InGaAs heterojunction bipolar transistor, J. Appl. Phys. 74 (1993) 5602. P. Viktorovitch, M. Gendry, S.K. Krawczyk, F. Krafft, Improved electronic properties of GaAs surface stabilized with phosphorous, Appl. Phys. Lett. 58 (1991) 2387. D.A. Harrison, R. Ares, S.P. Watkins, M.L.W. Thewalt, C.R. Bolognesi, D.J.S. Beckett, A.J. Spring Thorpe, Large photoluminescence enhancements from epitaxial GaAs passivated by postgrowth phophidization, Appl. Phys. Lett. 70 (1997) 3275. T. Sugino, S. Nozu, S. Nakajima, Phosphidization of GaAs by a remote phosphine plasma process and its application to surface passivation of GaAs metal-semiconductor field-effect transistors, Appl. Phys. Lett. 74 (1999) 2999. R. Beaudry, S.P. Wattkins, X. Xu, P. Yeo, Photorefletance study of phosphorous passivation of GaAs (0 0 1), J. Appl. Phys. 87 (2000) 7838. F. Ren, J.M. Kuo, M. Hong, W.S. Hobson, J.R. Lothian, J. Lin, H.S. Tsai, J.P. Mannaerts, J. Kwo, S.N.G. Chu, Y.K. Chen, A.Y. Cho, Ga2O3(Gd2O3)/InGaAs enhancement-mode n-channel MOSFET’s, IEEE Electron. Dev. Lett. 19 (1998) 309. E.F. Schubert, W.T. Tsang, Photoluminescence line shape of excitons in alloy semiconductors, Phys. Rev. B 34 (1986) 2991. P. Bhattacharya, M. Rao, M. Tsai, Growth and photoluminescence spectra of high-purity liquid phase epitaxial In.53Ga.47As, J. Appl. Phys. 54 (1983) 5096. C.D. Chouvy, D. Ballutaud, J.C. Pesant, A. Etcheberry, X-ray photoelectron spectroscopy study of GaAs surface exposed to a r.f. hydrogen plasma, Appl. Phys. Lett. 62 (1993) 2254. T. Miyazaki, T. Sugino, K. Matsuda, J. Shirafuji, Characterization of InGaAs phosphidized by a plasma process, Jpn. J. Appl. Phys. 38 (1999) 1139. G. Hollinger, E. Berginat, J. Joseph, Y. Robach, On the nature of oxides on InP surfaces, J. Vac. Sci. Technol. A3 (1985) 2082. W.A. Hill, C.C. Coleman, A single-frequency approximation for interface-state density determination, Solid-State Electron. 23 (1980) 987.