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GaN heterostructures with an in situ Si3N4 passivation layer
Solid-State Electronics 118 (2016) 12–17
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Scattering analysis of 2DEG mobility in undoped and doped AlGaN/AlN/GaN heterostructures with an in situ Si3N4 passivation layer G. Atmaca a,⇑, S. Ardali b, E. Tiras b, T. Malin c, V.G. Mansurov c, K.S. Zhuravlev c,d, S.B. Lisesivdin a a
Gazi University, Faculty of Science, Department of Physics, 06500 Teknikokullar, Ankara, Turkey Department of Physics, Faculty of Science, Anadolu University, Yunus Emre Campus, Eskisehir 26470, Turkey c Institute of Semiconductor Physics of Siberian Branch of Russian Academy of Sciences, Acad. Lavrentiev Ave 13, Novosibirsk 630090, Russia d Novosibirsk State University, 2, Pirogov Street, Novosibirsk 630090, Russia b
a r t i c l e
i n f o
Article history: Received 29 November 2015 Received in revised form 4 January 2016 Accepted 11 January 2016
Keywords: AlGaN GaN 2DEG SiN passivation Scattering analysis Dislocation density
a b s t r a c t The scattering mechanisms limiting mobility for low-dimensional charge carriers in a two-dimensional electron gas (2DEG) in undoped and doped AlGaN/AlN/GaN heterostructures with and without Si3N4 passivation are investigated. Hall effect measurements were carried out at temperatures from 1.8 K to 262 K and at a fixed magnetic field of 1 T. A good consistency was found between the calculated and the experimental results. The effects of in situ Si3N4 passivation on the 2DEG mobility are also discussed with majority scattering mechanisms. Interface-related parameters including quantum well width, deformation potential constant and correlation length of interface roughness were obtained from the fits of the analytical expressions of scattering mechanisms and compared for each heterostructure. After in situ Si3N4 passivation, we found that the effect of the interface roughness scattering, which was the dominant scattering mechanism at low temperatures, on the 2DEG mobility was more effective in undoped and doped AlGaN/GaN heterostructures. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction AlGaN/GaN heterostructures have attracted enormous attention for high-power and high-frequency device applications because of their polarization properties, high two-dimensional electron gas (2DEG) densities, wide band-gap and large breakdown electric field [1–5]. The high 2DEG carrier density in order of 1013 cm2 formed at the AlGaN/GaN interface is induced by spontaneous and piezoelectric polarization [6,7]. On the other hand, among the various assumptions, the origin of the 2DEG carriers is mostly assumed to be donor-like surface states on the heterostructure surface [8,9]. These surface states lead to reduced transistor output characteristics according to several studies [10,11]. The reduced output characteristics can be improved by applying various passivation materials such as SiO2, HfO2 and Si3N4 [11–13]. Several studies have revealed that Si3N4 material is one of most promising passivation materials in AlGaN/GaN heterostructures [13,14]. Also, Tadjer et al. reported that in situ Si3N4 passivation increases heterostructure crystal quality and reduces leakage of current related to surface states [15]. They therefore suggested that
⇑ Corresponding author. E-mail address: [email protected] (G. Atmaca). http://dx.doi.org/10.1016/j.sse.2016.01.006 0038-1101/Ó 2016 Elsevier Ltd. All rights reserved.
in situ Si3N4 passivation is suitable for high power applications [15]. In recent years, in situ Si3N4 passivation has been used in several studies [15–17]. With Si3N4 surface passivation, when the number of surface states is decreased, 2DEG carrier density seems to increase [10]. A decrease in 2DEG carrier mobility is another effect of Si3N4 surface passivation [18]. This decrease has been attributed to the increase in 2DEG carrier density by various studies [18–20]. Moreover, although this behavior observed in 2DEG carrier density and mobility after in situ or ex situ Si3N4 passivation for varying Al mole fractions, AlGaN barrier layer thicknesses, doping levels in barrier layer, the source of decrement in 2DEG mobility wasn’t examined in terms of scattering analysis [18–22]. The reasons for this decrement can be explained by investigating the low-field transport properties. Investigation of the low-field transport properties is important in describing transport mechanisms. To improve heterostructure crystal quality and device performance, an understanding of the low-field transport properties of carriers in these heterostructures is required. The low-field transport properties can be determined from knowledge of the 2DEG carrier density and mobility and the related scattering mechanisms. This is because the performance and output characteristics of the device can be improved by understanding and managing the effects of scattering mechanisms on 2DEG carrier mobility.
G. Atmaca et al. / Solid-State Electronics 118 (2016) 12–17
In this study, single-field Hall effect measurements depending on temperature were carried out for undoped and doped Al0.3Ga0.7N/AlN/GaN heterostructures with and without in situ Si3N4 passivation grown on sapphire substrate by molecular-beam epitaxy (MBE). We performed analytical scattering analysis relying on the 2DEG carrier density and mobility obtained from single magnetic field Hall effect measurements. We also determined interface-related parameters such as effective quantum well width (Z0), deformation potential (N) and correlation length of interface roughness (K) using scattering mechanism analyses, and we discussed the effects of in situ surface passivation on the 2DEG mobility using these parameters. 2. Experiment Fig. 1 shows the cross-sectional structure of unpassivated samples for undoped and doped Al0.3Ga0.7N/GaN heterostructures. In passivated samples, the in situ Si3N4 passivation layer was deposited on the GaN cap layer. AlGaN/AlN/GaN heterostructures were grown by MBE on (0 0 1)oriented 400-lm-thick sapphire substrates in a Riber 32 machine. We fabricated four samples. 951N and 951Y are undoped AlGaN/ AlN/GaN heterostructures without and with Si3N4 passivation, respectively. Also, 952 and 953 are doped AlGaN/AlN/GaN heterostructures without and with Si3N4 passivation, respectively. These heterostructures were grown following conditions: Highpurity ammonia served as the source of active nitrogen and standard effusion cells were used as the sources of group-III metals. In order to minimize contamination, substrates were annealed at 900 °C for several hours in the loading dock before being introduced, on a sample holder, into the growth chamber. Prior to the growth of the AlGaN/AlN/GaN heterostructures, the substrates were first annealed in the growth chamber at a temperature of 1000 °C and were then subjected to twenty minutes of nitridation at 950 °C in a flow of ammonia at an equivalent pressure of 3.5 105 Torr. The next step, at which the desired polarity of the films was defined, was the deposition of a low-temperature (600 °C) AlN nucleation layer. The deposition of such layers in an Al-rich ambient imparts
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GaN and AlGaN layers with Ga – polarity. Further buffer growth steps included a low-temperature (875 °C) deposition of a 30-nm thick AlN layer with subsequent annealing of the samples at 1000 °C. The growth of the AlN buffer layer was then continued at 980 °C until the thickness of the layer had reached a value of 0.25 lm. After that, a ten-layer-period AlN/Al0.3Ga0.7N superlattice with a total thickness of 130 nm followed by a 1.1–1.5-lm-thick GaN buffer layer were grown at 840 °C. A 1-nm-thick AlN spacer layer and a 25-nm-thick undoped Al0.3Ga0.7N capped by 2-nmthick GaN layer barrier were grown on top of the obtained complex buffer. Then a Si3N4 dielectric film was deposited at 850 °C immediately following the AlGaN/AlN/GaN heterostructure growth in the same MBE chamber, using silane and ammonia as precursors. 951N of sample was wet-etched to remove the Si3N4-passivation layer. For doped heterostructures, a 10-nm-thick undoped Al0.3Ga0.7N layer followed by a 15-nm-thick Si-doped Al0.3Ga0.7N barrier layer capped by 2-nm-thick GaN layer were grown on a 1-nmthick AlN spacer layer as it is shown in Fig. 1(b). Si concentration in doped AlGaN barrier layer is 1 1018 cm3. In passivated samples, Si3N4 passivation layer thickness was estimated to be about 1 nm using XPS technique. The passivation layer is stable and very useful. In our previous study, we demonstrated that no obvious strain relaxation of these AlGaN/AlN/GaN heterostructures with Si3N4 passivation layer was found using the XRD measurements [23]. We also demonstrated the utility of this layer by fabrication and study of HEMT devices on the base of Si3N4-passivated AlN/ GaN structures [24]. Fig. 2 shows the conduction band profiles of the investigated samples. As it can be seen in Fig. 2, bending in the doped AlGaN barrier layer in doped heterostructures is due to doping in the barrier layer. The Hall measurements were taken at temperatures ranging from 1.8 to 262 K in a cryogen-free superconducting magnet system (Cryogenics Ltd.) using an orthodox dc technique. Also, magnetic field was fixed at 1 T. On the other hand, atomic force microscopy (AFM) measurements were made for determination of surface roughness in undoped and doped heterostructures with and without in situ Si3N4 passivation.
Fig. 1. The cross-sectional structures of (a) undoped and (b) doped samples for unpassivated Al0.3Ga0.7N/AlN/GaN heterostructures.
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G. Atmaca et al. / Solid-State Electronics 118 (2016) 12–17
Fig. 2. The conduction band profiles of the investigated samples. 951N and 952 are unpassivated undoped and doped AlGaN/AlN/GaN heterostructures, respectively. 951Y and 953 are passivated undoped and doped AlGaN/AlNGaN heterostructures, respectively.
3. Results and discussion Fig. 3(a) shows 5 lm 5 lm AFM image sample of with in situ Si3N4 surface passivation layer in undoped heterostructures. The surface roughness of the sample showed a root-mean-square (RMS) value of around 1.38 nm. Fig. 3(b) shows 5 lm 5 lm
AFM image of same sample after wet-etched removal of the in situ Si3N4 surface passivation layer. In this case of after wetetched, RMS value obtained as around 1.52 nm. Fig. 3(c) and (d) shows 5 lm 5 lm AFM images of passivated and unpassivated samples of doped heterostructures. For doped heterostructures, RMS value was also changed from 1.74 to 1.64 nm after in situ Si3N4 passivation. Reduction of RMS value with Si3N4 passivation layer has been reported by various studies [18,25]. In addition to this, we calculated dislocation densities using AFM images and the dislocation densities were found to change from 1.60 109 to 1.88 109 cm2 after removing in situ Si3N4 layer for undoped heterostructures. In doped heterostructures, it was decreased from 1.56 109 to 6.16 108 cm2 after passivation. Therefore, we observed that dislocation densities in surface slightly reduced with application of in situ Si3N4 passivation layer. Actually, since dislocations act as trapping centers for both bulk and surface traps, reduction of dislocations in surface could lead to decrease the number of surface traps [26–29]. As can be seen in Table 1, the sheet carrier density was almost independent of temperature. It increased with increasing temperature due to thermally-generated bulk-related carriers toward higher temperatures. The increase of the carrier density after passivation is observed. While passivation-induced carrier density at 1.8 K was 1 1012 cm2 in undoped heterostructures, it was 2.7 1012 cm2 in doped heterostructures. On the other hand, 2DEG mobility at 1.8 K in undoped and doped heterostructures was decreased after passivation from 2544 to 2348 cm2/V s and from 2342 to 1450 cm2/V s, respectively. Therefore after passivation, changes in 2DEG mobility in these heterostructures can be
Fig. 3. 5 lm 5 lm AFM images of (a) passivated (951Y) and (b) unpassivated (951N) samples layer in undoped heterostructures. Also, 5 lm 5 lm AFM images of (c) passivated (953) and (d) unpassivated (952) samples in doped heterostructures.
G. Atmaca et al. / Solid-State Electronics 118 (2016) 12–17
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Table 1 Sheet carrier density and 2DEG mobility at different temperatures for each sample group. Si3N4 passivation
951N 951Y 952 953
None Exist None Exist
Sheet carrier density (cm2)
Mobility (cm2/V s)
1.8 K
262 K
1.8 K
262 K
1.20 1013 1.30 1013 1.17 1013 1.44 1013
1.25 1013 1.34 1013 1.20 1013 1.45 1013
2544 2348 2342 1450
1021 982 1015 789
result from changes in sheet carrier densities. Investigation of dominant scattering mechanisms limiting 2DEG mobility at especially low temperatures can be helpful to understanding these changes. Low-field transport of electrons in the 2DEG quantum well was determined by scattering mechanisms. To determine the effect of in situ Si3N4 passivation on low-field electron transport, it was necessary to investigate the scattering mechanisms limiting 2DEG mobility. In our calculations, polar optical phonon scattering (lPO), acoustic phonon scattering (lAC), which included deformation potential scattering (lDP) and piezoelectric scattering (lPZ), background impurity scattering (lBI), and interface roughness scattering (lIFR) were considered. Mobility components limited by all the individual scattering mechanisms were calculated from the expressions given in Refs. [30–32]. lTOT is the calculated total mobility as the combination of individual mobilities using Mattheisen’s rule and lH represents mobility measured by singlefield Hall effect measurements. The material parameters listed in Table 2 were used for mobility calculations [30,32]. Dislocation scattering mechanism was not considered among these scattering mechanisms. Because, the effect of dislocation scattering mechanism on 2DEG mobility is quite weak in higher carrier densities as high as 1 1013 cm2 [26,33]. Alloy scattering mechanism was not also included in the calculations due to the use of an AlN interlayer among the scattering mechanisms limiting 2DEG mobility [34,35]. Fig. 4(a) and (b) shows the scattering mechanisms limiting the 2DEG mobility of unpassivated and passivated samples for undoped heterostructure, respectively. At high temperatures, polar optical phonon scattering was the dominant scattering mechanism limiting 2DEG mobility. After passivation, the mobility component of polar optical phonon scattering had a slight change. At low temperatures, interface roughness scattering was the dominant scattering mechanism. After passivation, the effect of interface roughness scattering on 2DEG mobility increased due to the increase in sheet carrier density. In samples with a Si-doped AlGaN barrier, we also considered remote impurity scattering (lRI) as the major scattering mechanism due to remote donors in the Si-doped AlGaN barrier layer [31]. As can be seen in Fig. 5, polar optical phonon scattering was the dominant scattering mechanism for both the unpassivated
Table 2 Material parameters used in scattering analysis of total electron mobility [30,32]. Parameters
Value
High frequency dielectric constant Static dielectric constant LO-phonon energy LA-phonon velocity Density of crystal Electron wave vector The electromechanical coupling coefficient LA elastic constant TA elastic constant
e1 = 5.35 es = 8.9
⁄x = 0.092 eV ul = 6.56 103 m s1 q = 6.15 103 kg m3 k = 7.3 108 m1 K2 = 0.039 cLA = 2.650 1011 N m1 cTA = 0.442 1011 N m1
(a)
(b)
Fig. 4. The scattering mechanisms limiting 2DEG mobility of (a) 951N and (b) 951Y samples.
and the passivated samples at high temperatures. At low temperatures, interface roughness scattering mechanism was also dominant scattering mechanism. In Fig. 5(b), after passivation, it seems to be a significant increase in effect of the interface roughness scattering mechanism on 2DEG mobility. Since interface roughness component of 2DEG mobility is decreased with lIFR n2D2 according to expressions in Refs. [30,32], this increase can explain by variations in sheet carrier density. In undoped heterostructures, while passivation-induced carrier density is 1 1012 cm2, net change in low temperature 2DEG mobility is 196 cm2/V s according to Table 1. In doped heterostructures, while passivation-induced carrier density is 2.7 1012 cm2, net change in low temperature 2DEG mobility is 896 cm2 V s. While passivation-induced carrier density is change more than twice, change in the 2DEG mobility is more than four times. Accordingly, in doped heterostructures, increment in sheet carrier density seems responsible from increase in effect of interface roughness scattering mechanism on the 2DEG mobility. Table 3 presents adjustable-fit parameters including effective quantum well width, deformation potential and correlation length of interface roughness. These values were found by assuming a lateral size of D = 2.58 1010 m (1ML) and background impurity of NBI = 1023 m3 [31,36]. According to Table 3, it seems that the effective width of pseudotriangular quantum well is slightly decreased by in situ Si3N4 passivation. With increasing sheet carrier
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G. Atmaca et al. / Solid-State Electronics 118 (2016) 12–17
GaN layer is increase after passivation [39]. They also suggested source of increment in sheet carrier density is increase in tensile strain and decrease in surface states. Hence, we calculated tensile strain of each sample using expressions in Refs. [39,40]. We also used sheet carrier density values measured from Hall-effect measurements. Consequently, tensile strain in undoped heterostructures was increased from 0.0079 to 0.0105 after passivation, while it was increased from 0.0064 to 0.0136 in doped heterostructures. This additional strain on the interface may lead to an increase in interface roughness and in narrowing quantum well width [41]. Increase in interface roughness scattering also can occur due to wave function of 2DEG carriers is pushed closer to interface by increased the tensile strain. The correlation length of the interface roughness parameters obtained from the scattering analysis by using the single-field Hall measurements is comparable with previous D–K estimation calculations [42,43]. Also the quantum well width parameter values are acceptable when compared with the Fermi wavelength well-width approximation [43]. Consequently, in especially doped heterostructures, we evaluated several parameters such as n2D, K and D to explain decrement in the low temperature 2DEG mobility. We concluded that decrement in 2DEG mobility after passivation arises from interface roughness scattering mechanism because of increase in sheet carrier density.
(a)
4. Conclusion
(b)
Fig. 5. The scattering mechanisms limiting 2DEG mobility of (a) 952 and (b) 953 samples.
density, the increased electrical field at the interface can result in the narrowing of the effective quantum well width. As quantum well width is narrow and sheet carrier density is increase after passivation, effect of background impurity scattering mechanism on 2DEG mobility was reduced in passivated samples. On the other hand, it well known that effect of interface roughness on the 2DEG mobility is more important in narrower quantum wells [37]. In doped heterostructures, after passivation correlation length of interface roughness was decreased from 3.22 to 2.96 nm. This decrement is an acceptable variation due to interface roughness related to geometrical irregularities at the interface of samples [38]. This reduction trend in the correlation length may be also associated with increase in tensile strain after passivation. Dinara et al., reported tensile strain of Al0.3Ga0.7N barrier layer on
Table 3 The effect of in situ Si3N4 surface passivation on 2DEG interface-related parameters for the investigated samples. 951N and 952 are unpassivated undoped and doped AlGaN/AlN/GaN heterostructures, respectively. 951Y and 953 are passivated undoped and doped AlGaN/AlNGaN heterostructures, respectively. Interface related parameters
951N
951Y
952
953
Z0 (nm) N (eV) K (nm)
4.81 5.00 3.39
4.79 5.00 3.35
4.28 5.60 3.22
4.24 5.60 2.96
In this study, we have investigated the effect of Si3N4 surface passivation on 2DEG well parameters for undoped and doped Al0.3Ga0.7N/AlN/GaN heterostructures. The increment in 2DEG carriers due to in situ Si3N4 passivation arises from reduce in dislocations and increase in tensile strain of Al0.3Ga0.7N barrier layer on GaN layer after passivation. This increment in sheet carrier density also leading to interface roughness scattering was more effective. Consequently, the large reduction in 2DEG mobility especially at low temperatures arose from the interface roughness scattering mechanism after passivation. A direct correlation between Si3N4 surface passivation and interface roughness scattering depending on the sheet carrier density was also observed. We also suggested that quantum well width was slightly narrow due to increase in sheet carrier density and tensile strain with in situ Si3N4 surface passivation. Acknowledgements This work is supported by TUBITAK under Project No. 113F364 and RFBR (under Grants No. 13-02-00985 and 14-02-91371). References [1] Khan MA, Shur MS, Kuznia JN, Chen Q, Burn J, Schaff W. Temperature activated conductance in GaN/AlGaN heterostructure field effect transistors operating at temperatures up to 300 °C. Appl Phys Lett 1995;66:1083. [2] Khan MA, Chen Q, Yang JW, Shur MS, Dermott BT, Higgins JA. Microwave operation of GaN/AlGaN-doped channel heterostructure field effect transistors. IEEE Electron Dev Lett 1996;17(7):325. [3] Kim H, Thompson RM, Tilak V, Prunty TR, Shealy JR, Eastman LF. Effects of SiN passivation and high-electric field on AlGaN–GaN HFET degradation. IEEE Electron Dev Lett 2003;24(7):421–3. [4] Sheppard ST, Doverspike K, Pribble WL, Allen ST, Palmour JW, Kehias LT, et al. High-power microwave GaN/AlGaN HEMTs on semi-insulating silicon carbide substrates. IEEE Trans Electron Dev Lett 1999;20(4):161. [5] Morkoc H, Strite S, Gao GB, Lin ME, Sverdlov B, Burns M. Large-band-gap SiC, III–V nitride, and II–VI ZnSe-based semiconductor device technologies. J Appl Phys 1994;76:1363. [6] Ambacher O, Foutz B, Smart J, Shealy JR, Weimann NG, Chu K, et al. Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures. J Appl Phys 2000;87:334. [7] Asgari A, Kalafi M, Faraone L. Effects of partially occupied sub-bands on twodimensional electron mobility in AlxGa1xN/GaN heterostructures. J Appl Phys 2004;95:1185.
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Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Scattering analysis of 2DEG mobility in undoped and doped AlGaN/AlN/GaN heterostructures with an in situ Si3N4 passivation layer G. Atmaca a,⇑, S. Ardali b, E. Tiras b, T. Malin c, V.G. Mansurov c, K.S. Zhuravlev c,d, S.B. Lisesivdin a a
Gazi University, Faculty of Science, Department of Physics, 06500 Teknikokullar, Ankara, Turkey Department of Physics, Faculty of Science, Anadolu University, Yunus Emre Campus, Eskisehir 26470, Turkey c Institute of Semiconductor Physics of Siberian Branch of Russian Academy of Sciences, Acad. Lavrentiev Ave 13, Novosibirsk 630090, Russia d Novosibirsk State University, 2, Pirogov Street, Novosibirsk 630090, Russia b
a r t i c l e
i n f o
Article history: Received 29 November 2015 Received in revised form 4 January 2016 Accepted 11 January 2016
Keywords: AlGaN GaN 2DEG SiN passivation Scattering analysis Dislocation density
a b s t r a c t The scattering mechanisms limiting mobility for low-dimensional charge carriers in a two-dimensional electron gas (2DEG) in undoped and doped AlGaN/AlN/GaN heterostructures with and without Si3N4 passivation are investigated. Hall effect measurements were carried out at temperatures from 1.8 K to 262 K and at a fixed magnetic field of 1 T. A good consistency was found between the calculated and the experimental results. The effects of in situ Si3N4 passivation on the 2DEG mobility are also discussed with majority scattering mechanisms. Interface-related parameters including quantum well width, deformation potential constant and correlation length of interface roughness were obtained from the fits of the analytical expressions of scattering mechanisms and compared for each heterostructure. After in situ Si3N4 passivation, we found that the effect of the interface roughness scattering, which was the dominant scattering mechanism at low temperatures, on the 2DEG mobility was more effective in undoped and doped AlGaN/GaN heterostructures. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction AlGaN/GaN heterostructures have attracted enormous attention for high-power and high-frequency device applications because of their polarization properties, high two-dimensional electron gas (2DEG) densities, wide band-gap and large breakdown electric field [1–5]. The high 2DEG carrier density in order of 1013 cm2 formed at the AlGaN/GaN interface is induced by spontaneous and piezoelectric polarization [6,7]. On the other hand, among the various assumptions, the origin of the 2DEG carriers is mostly assumed to be donor-like surface states on the heterostructure surface [8,9]. These surface states lead to reduced transistor output characteristics according to several studies [10,11]. The reduced output characteristics can be improved by applying various passivation materials such as SiO2, HfO2 and Si3N4 [11–13]. Several studies have revealed that Si3N4 material is one of most promising passivation materials in AlGaN/GaN heterostructures [13,14]. Also, Tadjer et al. reported that in situ Si3N4 passivation increases heterostructure crystal quality and reduces leakage of current related to surface states [15]. They therefore suggested that
⇑ Corresponding author. E-mail address: [email protected] (G. Atmaca). http://dx.doi.org/10.1016/j.sse.2016.01.006 0038-1101/Ó 2016 Elsevier Ltd. All rights reserved.
in situ Si3N4 passivation is suitable for high power applications [15]. In recent years, in situ Si3N4 passivation has been used in several studies [15–17]. With Si3N4 surface passivation, when the number of surface states is decreased, 2DEG carrier density seems to increase [10]. A decrease in 2DEG carrier mobility is another effect of Si3N4 surface passivation [18]. This decrease has been attributed to the increase in 2DEG carrier density by various studies [18–20]. Moreover, although this behavior observed in 2DEG carrier density and mobility after in situ or ex situ Si3N4 passivation for varying Al mole fractions, AlGaN barrier layer thicknesses, doping levels in barrier layer, the source of decrement in 2DEG mobility wasn’t examined in terms of scattering analysis [18–22]. The reasons for this decrement can be explained by investigating the low-field transport properties. Investigation of the low-field transport properties is important in describing transport mechanisms. To improve heterostructure crystal quality and device performance, an understanding of the low-field transport properties of carriers in these heterostructures is required. The low-field transport properties can be determined from knowledge of the 2DEG carrier density and mobility and the related scattering mechanisms. This is because the performance and output characteristics of the device can be improved by understanding and managing the effects of scattering mechanisms on 2DEG carrier mobility.
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In this study, single-field Hall effect measurements depending on temperature were carried out for undoped and doped Al0.3Ga0.7N/AlN/GaN heterostructures with and without in situ Si3N4 passivation grown on sapphire substrate by molecular-beam epitaxy (MBE). We performed analytical scattering analysis relying on the 2DEG carrier density and mobility obtained from single magnetic field Hall effect measurements. We also determined interface-related parameters such as effective quantum well width (Z0), deformation potential (N) and correlation length of interface roughness (K) using scattering mechanism analyses, and we discussed the effects of in situ surface passivation on the 2DEG mobility using these parameters. 2. Experiment Fig. 1 shows the cross-sectional structure of unpassivated samples for undoped and doped Al0.3Ga0.7N/GaN heterostructures. In passivated samples, the in situ Si3N4 passivation layer was deposited on the GaN cap layer. AlGaN/AlN/GaN heterostructures were grown by MBE on (0 0 1)oriented 400-lm-thick sapphire substrates in a Riber 32 machine. We fabricated four samples. 951N and 951Y are undoped AlGaN/ AlN/GaN heterostructures without and with Si3N4 passivation, respectively. Also, 952 and 953 are doped AlGaN/AlN/GaN heterostructures without and with Si3N4 passivation, respectively. These heterostructures were grown following conditions: Highpurity ammonia served as the source of active nitrogen and standard effusion cells were used as the sources of group-III metals. In order to minimize contamination, substrates were annealed at 900 °C for several hours in the loading dock before being introduced, on a sample holder, into the growth chamber. Prior to the growth of the AlGaN/AlN/GaN heterostructures, the substrates were first annealed in the growth chamber at a temperature of 1000 °C and were then subjected to twenty minutes of nitridation at 950 °C in a flow of ammonia at an equivalent pressure of 3.5 105 Torr. The next step, at which the desired polarity of the films was defined, was the deposition of a low-temperature (600 °C) AlN nucleation layer. The deposition of such layers in an Al-rich ambient imparts
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GaN and AlGaN layers with Ga – polarity. Further buffer growth steps included a low-temperature (875 °C) deposition of a 30-nm thick AlN layer with subsequent annealing of the samples at 1000 °C. The growth of the AlN buffer layer was then continued at 980 °C until the thickness of the layer had reached a value of 0.25 lm. After that, a ten-layer-period AlN/Al0.3Ga0.7N superlattice with a total thickness of 130 nm followed by a 1.1–1.5-lm-thick GaN buffer layer were grown at 840 °C. A 1-nm-thick AlN spacer layer and a 25-nm-thick undoped Al0.3Ga0.7N capped by 2-nmthick GaN layer barrier were grown on top of the obtained complex buffer. Then a Si3N4 dielectric film was deposited at 850 °C immediately following the AlGaN/AlN/GaN heterostructure growth in the same MBE chamber, using silane and ammonia as precursors. 951N of sample was wet-etched to remove the Si3N4-passivation layer. For doped heterostructures, a 10-nm-thick undoped Al0.3Ga0.7N layer followed by a 15-nm-thick Si-doped Al0.3Ga0.7N barrier layer capped by 2-nm-thick GaN layer were grown on a 1-nmthick AlN spacer layer as it is shown in Fig. 1(b). Si concentration in doped AlGaN barrier layer is 1 1018 cm3. In passivated samples, Si3N4 passivation layer thickness was estimated to be about 1 nm using XPS technique. The passivation layer is stable and very useful. In our previous study, we demonstrated that no obvious strain relaxation of these AlGaN/AlN/GaN heterostructures with Si3N4 passivation layer was found using the XRD measurements [23]. We also demonstrated the utility of this layer by fabrication and study of HEMT devices on the base of Si3N4-passivated AlN/ GaN structures [24]. Fig. 2 shows the conduction band profiles of the investigated samples. As it can be seen in Fig. 2, bending in the doped AlGaN barrier layer in doped heterostructures is due to doping in the barrier layer. The Hall measurements were taken at temperatures ranging from 1.8 to 262 K in a cryogen-free superconducting magnet system (Cryogenics Ltd.) using an orthodox dc technique. Also, magnetic field was fixed at 1 T. On the other hand, atomic force microscopy (AFM) measurements were made for determination of surface roughness in undoped and doped heterostructures with and without in situ Si3N4 passivation.
Fig. 1. The cross-sectional structures of (a) undoped and (b) doped samples for unpassivated Al0.3Ga0.7N/AlN/GaN heterostructures.
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Fig. 2. The conduction band profiles of the investigated samples. 951N and 952 are unpassivated undoped and doped AlGaN/AlN/GaN heterostructures, respectively. 951Y and 953 are passivated undoped and doped AlGaN/AlNGaN heterostructures, respectively.
3. Results and discussion Fig. 3(a) shows 5 lm 5 lm AFM image sample of with in situ Si3N4 surface passivation layer in undoped heterostructures. The surface roughness of the sample showed a root-mean-square (RMS) value of around 1.38 nm. Fig. 3(b) shows 5 lm 5 lm
AFM image of same sample after wet-etched removal of the in situ Si3N4 surface passivation layer. In this case of after wetetched, RMS value obtained as around 1.52 nm. Fig. 3(c) and (d) shows 5 lm 5 lm AFM images of passivated and unpassivated samples of doped heterostructures. For doped heterostructures, RMS value was also changed from 1.74 to 1.64 nm after in situ Si3N4 passivation. Reduction of RMS value with Si3N4 passivation layer has been reported by various studies [18,25]. In addition to this, we calculated dislocation densities using AFM images and the dislocation densities were found to change from 1.60 109 to 1.88 109 cm2 after removing in situ Si3N4 layer for undoped heterostructures. In doped heterostructures, it was decreased from 1.56 109 to 6.16 108 cm2 after passivation. Therefore, we observed that dislocation densities in surface slightly reduced with application of in situ Si3N4 passivation layer. Actually, since dislocations act as trapping centers for both bulk and surface traps, reduction of dislocations in surface could lead to decrease the number of surface traps [26–29]. As can be seen in Table 1, the sheet carrier density was almost independent of temperature. It increased with increasing temperature due to thermally-generated bulk-related carriers toward higher temperatures. The increase of the carrier density after passivation is observed. While passivation-induced carrier density at 1.8 K was 1 1012 cm2 in undoped heterostructures, it was 2.7 1012 cm2 in doped heterostructures. On the other hand, 2DEG mobility at 1.8 K in undoped and doped heterostructures was decreased after passivation from 2544 to 2348 cm2/V s and from 2342 to 1450 cm2/V s, respectively. Therefore after passivation, changes in 2DEG mobility in these heterostructures can be
Fig. 3. 5 lm 5 lm AFM images of (a) passivated (951Y) and (b) unpassivated (951N) samples layer in undoped heterostructures. Also, 5 lm 5 lm AFM images of (c) passivated (953) and (d) unpassivated (952) samples in doped heterostructures.
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Table 1 Sheet carrier density and 2DEG mobility at different temperatures for each sample group. Si3N4 passivation
951N 951Y 952 953
None Exist None Exist
Sheet carrier density (cm2)
Mobility (cm2/V s)
1.8 K
262 K
1.8 K
262 K
1.20 1013 1.30 1013 1.17 1013 1.44 1013
1.25 1013 1.34 1013 1.20 1013 1.45 1013
2544 2348 2342 1450
1021 982 1015 789
result from changes in sheet carrier densities. Investigation of dominant scattering mechanisms limiting 2DEG mobility at especially low temperatures can be helpful to understanding these changes. Low-field transport of electrons in the 2DEG quantum well was determined by scattering mechanisms. To determine the effect of in situ Si3N4 passivation on low-field electron transport, it was necessary to investigate the scattering mechanisms limiting 2DEG mobility. In our calculations, polar optical phonon scattering (lPO), acoustic phonon scattering (lAC), which included deformation potential scattering (lDP) and piezoelectric scattering (lPZ), background impurity scattering (lBI), and interface roughness scattering (lIFR) were considered. Mobility components limited by all the individual scattering mechanisms were calculated from the expressions given in Refs. [30–32]. lTOT is the calculated total mobility as the combination of individual mobilities using Mattheisen’s rule and lH represents mobility measured by singlefield Hall effect measurements. The material parameters listed in Table 2 were used for mobility calculations [30,32]. Dislocation scattering mechanism was not considered among these scattering mechanisms. Because, the effect of dislocation scattering mechanism on 2DEG mobility is quite weak in higher carrier densities as high as 1 1013 cm2 [26,33]. Alloy scattering mechanism was not also included in the calculations due to the use of an AlN interlayer among the scattering mechanisms limiting 2DEG mobility [34,35]. Fig. 4(a) and (b) shows the scattering mechanisms limiting the 2DEG mobility of unpassivated and passivated samples for undoped heterostructure, respectively. At high temperatures, polar optical phonon scattering was the dominant scattering mechanism limiting 2DEG mobility. After passivation, the mobility component of polar optical phonon scattering had a slight change. At low temperatures, interface roughness scattering was the dominant scattering mechanism. After passivation, the effect of interface roughness scattering on 2DEG mobility increased due to the increase in sheet carrier density. In samples with a Si-doped AlGaN barrier, we also considered remote impurity scattering (lRI) as the major scattering mechanism due to remote donors in the Si-doped AlGaN barrier layer [31]. As can be seen in Fig. 5, polar optical phonon scattering was the dominant scattering mechanism for both the unpassivated
Table 2 Material parameters used in scattering analysis of total electron mobility [30,32]. Parameters
Value
High frequency dielectric constant Static dielectric constant LO-phonon energy LA-phonon velocity Density of crystal Electron wave vector The electromechanical coupling coefficient LA elastic constant TA elastic constant
e1 = 5.35 es = 8.9
⁄x = 0.092 eV ul = 6.56 103 m s1 q = 6.15 103 kg m3 k = 7.3 108 m1 K2 = 0.039 cLA = 2.650 1011 N m1 cTA = 0.442 1011 N m1
(a)
(b)
Fig. 4. The scattering mechanisms limiting 2DEG mobility of (a) 951N and (b) 951Y samples.
and the passivated samples at high temperatures. At low temperatures, interface roughness scattering mechanism was also dominant scattering mechanism. In Fig. 5(b), after passivation, it seems to be a significant increase in effect of the interface roughness scattering mechanism on 2DEG mobility. Since interface roughness component of 2DEG mobility is decreased with lIFR n2D2 according to expressions in Refs. [30,32], this increase can explain by variations in sheet carrier density. In undoped heterostructures, while passivation-induced carrier density is 1 1012 cm2, net change in low temperature 2DEG mobility is 196 cm2/V s according to Table 1. In doped heterostructures, while passivation-induced carrier density is 2.7 1012 cm2, net change in low temperature 2DEG mobility is 896 cm2 V s. While passivation-induced carrier density is change more than twice, change in the 2DEG mobility is more than four times. Accordingly, in doped heterostructures, increment in sheet carrier density seems responsible from increase in effect of interface roughness scattering mechanism on the 2DEG mobility. Table 3 presents adjustable-fit parameters including effective quantum well width, deformation potential and correlation length of interface roughness. These values were found by assuming a lateral size of D = 2.58 1010 m (1ML) and background impurity of NBI = 1023 m3 [31,36]. According to Table 3, it seems that the effective width of pseudotriangular quantum well is slightly decreased by in situ Si3N4 passivation. With increasing sheet carrier
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GaN layer is increase after passivation [39]. They also suggested source of increment in sheet carrier density is increase in tensile strain and decrease in surface states. Hence, we calculated tensile strain of each sample using expressions in Refs. [39,40]. We also used sheet carrier density values measured from Hall-effect measurements. Consequently, tensile strain in undoped heterostructures was increased from 0.0079 to 0.0105 after passivation, while it was increased from 0.0064 to 0.0136 in doped heterostructures. This additional strain on the interface may lead to an increase in interface roughness and in narrowing quantum well width [41]. Increase in interface roughness scattering also can occur due to wave function of 2DEG carriers is pushed closer to interface by increased the tensile strain. The correlation length of the interface roughness parameters obtained from the scattering analysis by using the single-field Hall measurements is comparable with previous D–K estimation calculations [42,43]. Also the quantum well width parameter values are acceptable when compared with the Fermi wavelength well-width approximation [43]. Consequently, in especially doped heterostructures, we evaluated several parameters such as n2D, K and D to explain decrement in the low temperature 2DEG mobility. We concluded that decrement in 2DEG mobility after passivation arises from interface roughness scattering mechanism because of increase in sheet carrier density.
(a)
4. Conclusion
(b)
Fig. 5. The scattering mechanisms limiting 2DEG mobility of (a) 952 and (b) 953 samples.
density, the increased electrical field at the interface can result in the narrowing of the effective quantum well width. As quantum well width is narrow and sheet carrier density is increase after passivation, effect of background impurity scattering mechanism on 2DEG mobility was reduced in passivated samples. On the other hand, it well known that effect of interface roughness on the 2DEG mobility is more important in narrower quantum wells [37]. In doped heterostructures, after passivation correlation length of interface roughness was decreased from 3.22 to 2.96 nm. This decrement is an acceptable variation due to interface roughness related to geometrical irregularities at the interface of samples [38]. This reduction trend in the correlation length may be also associated with increase in tensile strain after passivation. Dinara et al., reported tensile strain of Al0.3Ga0.7N barrier layer on
Table 3 The effect of in situ Si3N4 surface passivation on 2DEG interface-related parameters for the investigated samples. 951N and 952 are unpassivated undoped and doped AlGaN/AlN/GaN heterostructures, respectively. 951Y and 953 are passivated undoped and doped AlGaN/AlNGaN heterostructures, respectively. Interface related parameters
951N
951Y
952
953
Z0 (nm) N (eV) K (nm)
4.81 5.00 3.39
4.79 5.00 3.35
4.28 5.60 3.22
4.24 5.60 2.96
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