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GaN MOSHFETs with HfO2 gate oxide: A simulation study
Solid-State Electronics 54 (2010) 1367–1371
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
AlGaN/GaN MOSHFETs with HfO2 gate oxide: A simulation study Y. Hayashi, S. Sugiura, S. Kishimoto, T. Mizutani * Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 27 January 2010 Received in revised form 30 March 2010 Accepted 30 March 2010 Available online 3 June 2010 The review of this paper was arranged by E. Calleja Keywords: MOSFET GaN HfO2 Device simulation Interface trap
a b s t r a c t Two-dimensional device simulations of a HfO2/AlGaN/GaN metal–oxide–semiconductor–heterostructure FET (MOSHFET) have been carried out based on the drift–diffusion model focusing on the effects of HfO2/ AlGaN interface properties. In the case of MOSHFETs with no trap at the HfO2/AlGaN interface, the transconductance was found to decrease at large VGS due to channel formation at the HfO2/AlGaN interface, resulting in a plateau structure of gm. When the interface states were incorporated at the HfO2/AlGaN interface, gm decreased due to electron capture by the trap at a smaller gate voltage than the onset of gm decrease for the case with no trap at the HfO2/AlGaN interface. This is because the trap level reached EF earlier than the channel formation at the HfO2/AlGaN interface. This resulted in a peak structure of the gm when the interface states were deep, which is consistent with experimental results. It was pointed out that if the trap concentration was less than 4 1011 cm2, the threshold voltage shift was less than 0.3 V and the gm decrease was less than 10%. Ó 2010 Published by Elsevier Ltd.
1. Introduction GaN is attracting considerable attention for high-frequency and high-power devices due to its remarkable material properties such as a high breakdown voltage and a high electron velocity [1]. In order to apply these advantages to high-power switching systems, it is important to design for normally-off operation to ensure a failsafe system, that is, one that avoids the problem of circuit burn out when the gate signal drops to ground voltage [2]. The conventional normally-off HEMTs require a precise etching control for the gate recess [3,4] or heavy p+-doping for the junction gate [5]. In addition to these process requirements, the normally-off HEMTs sometimes show a problem of small transconductance (gm) due to a large parasitic resistance [4]. Recently, the normally-off HEMTs fabricated by fluoride-based plasma treatment, which introduced fluorine atoms into the AlGaN barrier layer, have been reported [6,7]. This process seems promising because it does not require the etching precision or p+-doping of previous designs for normally-off operation. However, our present understanding of the role played by the fluorine atoms is not sufficient. In addition, it is not clear at present whether the fluorine atoms incorporated in the AlGaN barrier layer are stable, even though the threshold voltage of the device has been found to be stable for 80 days at 200 °C [8]. In most of the normally-off devices published in the literature, there is an issue that the maximum drain current is small because * Corresponding author. Tel./fax: +81 052 789 5455. E-mail address: [email protected] (T. Mizutani). 0038-1101/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.sse.2010.03.022
of the small available forward gate voltage. It is expected that GaN MOSFETs will be suitable for solving these problems because it is possible to apply a large forward voltage to the MOS gate. Even though there are many reports of normally-on GaN MOSFETs [9– 12], there are not so many reports of normally-off GaN MOSFETs [13–15]. In previous work, GaN MOSFETs using SiO2 as a gate oxide showed a small drain current and a small transconductance (gm) probably due to their small gate input capacitance and insufficient interface property. To improve the performance we have fabricated GaN MOSFETs using HfO2 with a large dielectric constant and achieved a maximum transconductance (gmmax) of 45 mS/mm and maximum drain current of 400 mA/mm [16]. Even though these values are several times larger than those of conventional GaN MOSFETs, they are still smaller than those of AlGaN/GaN HEMTs. This is probably because the quality of the HfO2/GaN interface where the channel is formed is lower than that of the AlGaN/ GaN hetero interface, and also because polarization-induced charge is not available for the GaN MOSFETs. To improve the device performance we have proposed a HfO2/AlGaN/GaN metal–oxide–semiconductor-heterostructure FET (MOSHFET) in which a high-quality interface was used as a channel [17]. The maximum drain current (ID) of 780 mA/mm and the maximum gm of 185 mS/mm were realized at VGS = 10 V with a gate leakage current of 1 mA/mm. These results were fairly good. However, the gm decreased at large gate voltage. It has been pointed out that a 2-dimensional electron gas (2DEG) is formed at the oxide/semiconductor interface at high positive gate bias [18]. If the interface quality is not good, the gm degradation is
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Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371
expected. Another possibility is electron trapping at the interface traps. Until now, however, there has been little research published on these phenomena. In this paper, two-dimensional device simulations of HfO2/AlGaN/GaN MOSHFETs have been carried out to study the electrical characteristics of the AlGaN/GaN MOSHFETs and the effects of interface traps on the device characteristics.
Table 1 Polarization charge density at each interface.
2. Simulation setup
Table 2 Values of m–E curve parameters in GaN and AlGaN used in the simulations.
Two-dimensional numerical device simulations based on a drift–diffusion model were carried out using the commercial simulation program DESSIS. Fig. 1a and b shows a schematic cross section of the device and the corresponding energy band diagram perpendicular to the device surface used in the simulation. The channel length was 1.5 lm. Aluminum mole fraction in the AlGaN barrier was 0.25 and the thickness was 12 nm. The thickness was chosen to realize normally-off operation for the HEMTs. The GaN and AlGaN were assumed to be lightly p-doped, with a concentration of 1 1014 cm3. The AlGaN/GaN heterostructure has spontaneous and piezoelectric polarizations (PSP and PPE). To take these effects into account, fixed charges (nSP and nPE) shown in Table 1 were introduced at each interface [19]. Spontaneous polarization charges at both sides of the GaN were also included in the model
(a)
nSP(GaN) (cm2) HfO2/AlGaN AlGaN/GaN Bottom of GaN
1.81 1013 1.81 1013
nSP(AlGaN) (cm2)
nPE(AlGaN) (cm2)
Total (cm2)
2.62 1013 2.62 1013
5.74 1012 5.74 1012
3.20 1013 1.39 1013 1.81 1013
llow (cm2/Vs) msat (cm/s) b
GaN
AlGaN
800 3.3 107 1
180 1.8 107 2
to align the modeled system closer to the real device. If no polarization charges are assumed as employed in the conventional simulation, threshold voltage shifts to the negative direction. In addition to the polarization charge, fixed positive charge with a concentration of 1.5 1013 cm2 was introduced at the HfO2/AlGaN interface in order to match the simulated threshold voltages with those observed in experiments with the HEMTs. The introduction of the positive charge and its concentration are consistent with Ref. [20]. Even though we do not know the origin of the positive charge at the surface, if the Fermi level of the HfO2/AlGaN interface is below the charge neutrality level, positive charge will be formed at the HfO2/AlGaN interface. Another possibility is the nitrogen vacancy which is believed to be donor-type defect [21]. Further study is necessary to clarify the origin of the positive charge. Mobility and velocity of electrons used in the simulation are summarized in Table 2 assuming the electron velocity–electric field (m–E) curve of
mðEÞ ¼ h
llow E l E 1þð Vlow Þb
i1=b . The parameters for GaN
sat
(b)
and AlGaN listed in Table 2 were chosen to fit the m–E curve with the results of Monte Carlo simulation [22]. Traps introduced at the HfO2/AlGaN interface to evaluate the effects of the interface states were assumed to be donor-type based on reports from the Mishra [20] and Hashizume [21] groups. The energy of the trap level was defined as a depth from the conduction band minimum EC of AlGaN, ECET, as shown in Fig. 1b. Simulations were carried out with various energy levels and trap concentrations, NT. As a typical value, ECET = 1.0 eV was used based on Refs. [20] and [23], and a trap density NT of 6 1012 cm2 was used considering the threshold voltage shift of about 5 V during the measurement of gate voltage sweep. Even though the device simulation was performed for the device with uniform distribution of the deep level at the interface, the effects of non-uniform distribution of the trapped electrons in a direction parallel to the interface was taken into account in the device simulation because the current continuity equation and Poisson equation are solved self-consistently. In the present device simulation, no gate leakage current throughout the MOS system was assumed. This assumption is widely used in the standard device simulation of the MOSFETs. It is expected that the assumption will not cause serious error because a small gate leakage current of 1010–104 A/mm was confirmed in the experiment [24], even though some modification might become necessary at large forward gate voltage. 3. Simulation results
Fig. 1. Schematic cross section of the device and the corresponding energy band diagram used in the simulations. Donor-type traps at an energy level ET below the conduction band minimum EC of AlGaN were assumed.
Fig. 2 shows the drain current–gate voltage (ID–VGS) (dotted line) and transconductance gm–VGS (solid line) characteristics at a drain voltage VDS of 4 V. The densities of the 2DEG at the AlGaN/
Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371
Fig. 2. ID, gm, nGaN, and nAlGaN as functions of VGS for the device without traps. VDS = 4 V.
GaN heterointerface (nGaN) and the HfO2/AlGaN interface (nAlGaN) are also shown by a long-dash line and short-dash line, respectively. The device exhibited normally-off operation with a threshold voltage Vth of 0.1 V and maximum gm of 191 mS/mm. The gm increased rapidly at first, saturated at a VGS of about 5 V, and decreased at VGS = 15 V, resulting in a plateau structure. In order to examine the reason for the gm decrease, nGaN and nAlGaN were calculated as a function of VGS. The value of nGaN was found to build up at VGS = 0 V and linearly increased as shown by the long-dash line in Fig. 2. For VGS larger than 15 V, the nGaN saturated and an increase in nAlGaN was induced at the HfO2/AlGaN interface, as shown by short-dash line in Fig. 2. The nGaN saturation at VGS = 15 V is explained by the shielding effect of VGS by the nAlGaN induced at the HfO2/AlGaN interface. This is consistent with Ref. [18] in which electron accumulation was observed at the oxide/AlGaN interface. Since the electron velocity in AlGaN is lower than that in GaN, gm decreases rapidly when nAlGaN builds up. In the experiments, however, gate voltage dependence of gm showed a peak structure with no plateau [17], which will be discussed later. We next studied the effects of traps at HfO2/AlGaN interface. Fig. 3 shows ID–VGS (dotted line) and gm–VGS (solid line) characteristics when the trap (ECET = 1 eV, NT = 6 1012 cm2) is introduced at the HfO2/AlGaN interface. In addition to the threshold voltage shift in the negative direction (Vth = 4.5 V) compared to the case without the trap, another gm decrease and its recovery were observed at VGS between 6 and 15 V prior to the onset of
Fig. 3. ID, gm, nGaN, nAlGaN, and nT as functions of VGS for the device with traps (ECET = 1 eV, NT = 6 1012 cm2). VDS = 4 V.
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the previously-mentioned gm decrease due to the channel formation at the HfO2/AlGaN interface. In order to understand these behaviors, electron density was calculated as a function of VGS. Electrons captured by the traps, nT, were found to build up at almost the same voltage of 6 V where gm (solid line) decreases and nGaN (long-dash line) saturates as shown by the dot-dash line in Fig. 3. The above results are explained as follows. Fig. 4 shows the EC distribution at the center of the gate in a direction perpendicular to the device surface for VGS from 6 V (top) to 20 V (bottom) in 1 V steps at VDS = 0 V for NT = 6 1012 cm2. In the off-state between VGS = 6 V and 4 V, EC shifted downward in the whole region under the gate with increase in VGS. For VGS > 4 V on the other hand, the channel was formed at the AlGaN/GaN interface and nGaN increased as shown in Fig. 3. Then, EC of AlGaN/GaN interface was almost pinned at EF and the VGS increase was consumed by the EC down shift of HfO2 and AlGaN. For VGS > 6 V, ET reached EF and electrons were captured by the trap. Then the increase of VGS was consumed by the increase of the trapped electrons nT and ET was almost pinned at EF. Consequently the electron density at the AlGaN/GaN interface, nGaN, increased very little, resulting in a decrease in gm. For VGS > 11 V, most of the traps were filled with electrons and then EC of HfO2 and AlGaN shifted downward again with increasing VGS, so that nGaN increased again resulting in a recovery of the gm as shown in Fig. 3. For VGS > 15 V, another channel was formed at the HfO2/AlGaN interface and EC of HfO2/AlGaN interface was pinned at EF. Then, the gm decreased as discussed for the device without interface traps. Fig. 5 shows gm–VGS characteristics at VDS = 4 V for different ET values (ECET = 0.5, 1.0, 1.5 eV) with a trap concentration of NT = 6 1012 cm2. All devices showed the same threshold voltage of about 5 V since the traps were completely ionized (ET > EF) independent of the energy level of the trap. On the contrary, the VGS at which the gm started to decrease was small for the deep trap resulting in a decrease of ID. This is because the deeper the ET is, the earlier it reaches EF. It is notable that the plateau structure of gm obtained for the shallow interface trap disappeared when the interface trap was deep, resulting in a peak structure of gm. This is consistent with the experimental results with the gm peak, as shown in Fig. 6. This suggests that the gm decrease obtained in Ref. [17] was not due to the channel formation at the HfO2/AlGaN interface but to electron capture at the interface states. If the AlGaN/GaN MOSHFETs have interface states, the drain current does not increase so much as that in Fig. 2 in which no interface trap
Fig. 4. EC profiles for various VGS from 6 to 20 V (in 1 V steps) at the center of the gate. VDS = 0 V. NT = 6 1012 cm2.
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Fig. 5. gm–VGS characteristic at VDS = 4 V for different ECET values of 0.5, 1.0, and 1.5 eV. NT = 6 1012 cm2. Fig. 8. Amounts of the threshold voltage shift and the gm decrease as functions of NT. VDS = 4 V.
800
200
700
DS
600
150
500 400
100
300 200
50
Drain current (mA/mm)
Transconductance (mS/mm)
V = 8V
100 0
-2
0
2
4
6
8
0 10
Gate voltage (V) Fig. 6. ID–VGS and gm–VGS characteristics of the fabricated AlGaN/GaN MOSHFETs with HfO2 gate oxide. VDS = 8 V.
Fig. 7. gm–VGS characteristics for different NT values of 1, 6, and 10 1012 cm2. ECET = 1 eV. VDS = 4 V.
was assumed. The gm in the experiments showed monotonic decrease in contrast with the double peak structure in the device simulation. One possibility of this difference is that the VGS is not sufficiently large in the experiment and the other possibility is the continuum distribution of the interface states. It is difficult at present to determine which is dominant. Drain current deep level transient spectroscopy [25] is effective in clarifying this point
because it is possible to determine the energy distribution of the deep level by measuring the temperature dependence of the drain current transient. This is now under study. Fig. 7 shows simulated gm–VGS characteristics for a different trap concentration (NT = 1, 6, 10 1012 cm2). Here, ECET was assumed to be 1 eV. The larger concentration led to a larger threshold voltage shift and a greater gm decrease. Fig. 8 shows NT dependences of the threshold voltage shift and the gm decrease. The threshold voltage shifted in proportion to the NT and gm decreased linearly at small NT, saturating at large NT. If the trap concentration was less than 4 1011 cm2, the threshold voltage shift was less than 0.3 V and the gm decrease was less than 10%.
4. Conclusion Two-dimensional device simulations of HfO2/AlGaN/GaN MOSHFET have been carried out to investigate the operation mechanism and the effects of interface traps on the device characteristics. First, devices without the traps were studied. Although the transconductance increased with the gate voltage for small VGS, it decreased for larger VGS, resulting in a plateau structure. This is explained by the channel formation at the HfO2/AlGaN interface where the electron velocity was rather low. Next, simulations for devices with donor-type traps at HfO2/AlGaN interface were performed. Two effects were observed: the shift of the threshold voltage in the negative direction, and the decrease of gm at large VGS which was different from the previous gm decrease. The gm decrease was due to the increase of VGS being expended to increase the electrons captured by the traps at the HfO2/AlGaN interface, and was therefore unavailable to increase the density of the 2DEG at the AlGaN/GaN interface. The gm decrease occurred at a smaller gate voltage than the onset of the channel formation at the HfO2/AlGaN interface, resulting in a peak structure of the gm. The gm peak structure obtained in the simulation was consistent with the experimental results. The magnitude of the threshold voltage shift and the gm decrease were dependent on the trap concentration. If the trap concentration was less than 4 1011 cm2, the threshold voltage shift was less than 0.3 V and the gm decrease was less than 10%. References [1] Levinshtein ME, Rumyantsev SL, Shur MS. Properties of advanced semiconductor materials: GaN, AIN, InN, BN, SiC, SiGe. Wiley-Interscience; 2001.
Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371 [2] Ikeda N, Li J, Yoshida S. Normally-off operation power AlGaN/GaN HFET. Proc Int Symp Power Semicond Dev ICs 2004:369–72. [3] Lanford WB, Tanaka T, Otoki Y, Adesida I. Recessed-gate enhancement-mode GaN HEMT with high threshold voltage. Electron Lett 2005;41:449–50. [4] Saito W, Takada Y, Kuraguchi M, Tsuda K, Omura I. Recessed-gate structure approach toward normally off high-voltage AlGaN/GaN HEMT for power electronics applications. IEEE Trans Electron Dev 2006;53:356–62. [5] Hu X, Simin G, Yang J, Asif Khan M, Gaska R, Shur MS. Enhancement mode AlGaN/GaN HFET with selectively grown pn junction gate. Electron Lett 2000;36:753–4. [6] Cai Y, Zhou Y, Chen KJ, Lau KM. High-performance enhancement-mode AlGaN/ GaN HEMTs using fluoride-based plasma treatment. IEEE Electron Dev Lett 2005;26:435–7. [7] Cai Y, Cheng Z, Tang WCW, Lau KM, Chen KJ. Monolithically integrated enhancement/depletion-mode AlGaN/GaN HEMT inverters and ring oscillators using CF4_plasma treatment. IEEE Trans Electron Dev 2006;53:2223–30. [8] Mizuno H, Kishimoto S, Maezawa K, Mizutani T. Quasi-normally-off AlGaN/ GaN HEMTs fabricated by fluoride-based plasma treatment. Phys Stat Sol (c) 2007;4:2732–5. [9] Simin G, Hu X, Ilinskaya N, Kumar A, Koudymov A, Zhang J, et al. 7.5 kW/mm2 current switch using AlGaN/GaN metal–oxide–semiconductor heterostructure field effect transistors on SiC substrates.. Electron Lett 2000;36:2043–4. [10] Hu X, Koudymov A, Simin G, Yang J, Asif Khan M, Tarakji A, et al. Si3N4/AlGaN/ GaN–metal–insulator–semiconductor heterostructure field-effect transistors. Appl Phys Lett 2001;79:2832–4. [11] Ochiai M, Akita M, Ohno Y, Kishimoto S, Maezawa K, Mizutani T. AlGaN/GaN heterostructure metal–insulator–semiconductor high-electron-mobility transistors with Si3N4 gate insulator. Jpn J Appl Phys 2003;42:2278–80. [12] Hashizume T, Anantathanasarn S, Negoro N, Sano E, Hasegawa H, Kumakura K, et al. Al2 O3 insulated-gate structure for AlGaN/GaN heterostructure field effect transistors having thin AlGaN barrier layers. Jpn J Appl Phys 2004;43:L777–9. [13] Matocha K, Chow TP, Gutmann RJ. High-voltage normally off GaN MOSFETs on sapphire substrates. IEEE Trans Electron Dev 2005;52:6–10.
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[14] Irokawa Y, Nakano Y, Ishiko M, Kachi T, Kim J, Ren F, et al. MgO/p-GaN enhancement mode metal–oxide semiconductor field-effect transistors. Appl Phys Lett 2004;84:2919–21. [15] Huang W, Khan T, Chow TP. Enhancement-mode n-channel GaN MOSFETs on p and n-GaN/sapphire substrates. IEEE Electron Dev Lett 2006;27:796–8. [16] Sugiura S, Kishimoto S, Mizutani T, Kuroda M, Ueda T, Tanaka T. Enhancementmode n-channel GaN MOSFETs fabricated on p-GaN using HfO2 as gate oxide. Electron Lett 2007;43:952–3. [17] Sugiura S, Kishimoto S, Mizutani T, Kuroda M, Ueda T, Tanaka T. Normally-off AlGaN/GaN MOSHFETs with HfO2 gate oxide. Phys Stat Sol (c) 2008;5:1923–5. [18] Saygi S, Koudymov A, Adivarahan V, Yang J, Simin G, Asif Khan M, et al. Realspace electron transfer in III-nitride metal–oxide–semiconductorheterojunction structures. Appl Phys Lett 2005;87:043505. [19] Ambacher O, Smart J, Shealy JR, Weimann NG, Chu K, Murphy M, et al. Twodimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures. J Appl Phys 1999;85:3222–33. [20] Ibbetson JP, Fini PT, Ness KD, DenBaars SP, Speck JS, Mishra UK. Polarization effects surface states and the source of electrons in AlGaN/GaN heterostructure field effect transistors. Appl Phys Lett 2000;77:250–2. [21] Hashizume T, Nakasaki R. Discrete surface state related to nitrogen-vacancy defect on plasma-treated GaN surfaces. Appl Phys Lett 2002;80:4564–6. [22] Albrecht JD, Wang RP, Ruden PP, Farahmand M, Brennan KF. Electron transport characteristics of GaN for high temperature device modeling. J Appl Phys 1998;83:4777–81. [23] Sugawara K, Kotani J, Hashizume T. Near-midgap deep levels in Al0.26Ga0.74N grown by metal–organic chemical vapor deposition.. Appl Phys Lett 2009;94:152106. [24] Sugiura S, Hayashi Y, Kishimoto S, Mizutani T, Kuroda M, Ueda T, et al. Fabrication of normally-off mode GaN and AlGaN/GaN MOSFETs with HfO2 gate insulator. Solid State Electron 2010;54:79–83. [25] Mizutani T, Okino T, Kawada K, Ohno Y, Kishimoto S, Maezawa K. Drain current DLTS of AlGaN/GaN HEMTs. Phys Stat Sol (a) 2003;200:195–8.
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
AlGaN/GaN MOSHFETs with HfO2 gate oxide: A simulation study Y. Hayashi, S. Sugiura, S. Kishimoto, T. Mizutani * Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 27 January 2010 Received in revised form 30 March 2010 Accepted 30 March 2010 Available online 3 June 2010 The review of this paper was arranged by E. Calleja Keywords: MOSFET GaN HfO2 Device simulation Interface trap
a b s t r a c t Two-dimensional device simulations of a HfO2/AlGaN/GaN metal–oxide–semiconductor–heterostructure FET (MOSHFET) have been carried out based on the drift–diffusion model focusing on the effects of HfO2/ AlGaN interface properties. In the case of MOSHFETs with no trap at the HfO2/AlGaN interface, the transconductance was found to decrease at large VGS due to channel formation at the HfO2/AlGaN interface, resulting in a plateau structure of gm. When the interface states were incorporated at the HfO2/AlGaN interface, gm decreased due to electron capture by the trap at a smaller gate voltage than the onset of gm decrease for the case with no trap at the HfO2/AlGaN interface. This is because the trap level reached EF earlier than the channel formation at the HfO2/AlGaN interface. This resulted in a peak structure of the gm when the interface states were deep, which is consistent with experimental results. It was pointed out that if the trap concentration was less than 4 1011 cm2, the threshold voltage shift was less than 0.3 V and the gm decrease was less than 10%. Ó 2010 Published by Elsevier Ltd.
1. Introduction GaN is attracting considerable attention for high-frequency and high-power devices due to its remarkable material properties such as a high breakdown voltage and a high electron velocity [1]. In order to apply these advantages to high-power switching systems, it is important to design for normally-off operation to ensure a failsafe system, that is, one that avoids the problem of circuit burn out when the gate signal drops to ground voltage [2]. The conventional normally-off HEMTs require a precise etching control for the gate recess [3,4] or heavy p+-doping for the junction gate [5]. In addition to these process requirements, the normally-off HEMTs sometimes show a problem of small transconductance (gm) due to a large parasitic resistance [4]. Recently, the normally-off HEMTs fabricated by fluoride-based plasma treatment, which introduced fluorine atoms into the AlGaN barrier layer, have been reported [6,7]. This process seems promising because it does not require the etching precision or p+-doping of previous designs for normally-off operation. However, our present understanding of the role played by the fluorine atoms is not sufficient. In addition, it is not clear at present whether the fluorine atoms incorporated in the AlGaN barrier layer are stable, even though the threshold voltage of the device has been found to be stable for 80 days at 200 °C [8]. In most of the normally-off devices published in the literature, there is an issue that the maximum drain current is small because * Corresponding author. Tel./fax: +81 052 789 5455. E-mail address: [email protected] (T. Mizutani). 0038-1101/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.sse.2010.03.022
of the small available forward gate voltage. It is expected that GaN MOSFETs will be suitable for solving these problems because it is possible to apply a large forward voltage to the MOS gate. Even though there are many reports of normally-on GaN MOSFETs [9– 12], there are not so many reports of normally-off GaN MOSFETs [13–15]. In previous work, GaN MOSFETs using SiO2 as a gate oxide showed a small drain current and a small transconductance (gm) probably due to their small gate input capacitance and insufficient interface property. To improve the performance we have fabricated GaN MOSFETs using HfO2 with a large dielectric constant and achieved a maximum transconductance (gmmax) of 45 mS/mm and maximum drain current of 400 mA/mm [16]. Even though these values are several times larger than those of conventional GaN MOSFETs, they are still smaller than those of AlGaN/GaN HEMTs. This is probably because the quality of the HfO2/GaN interface where the channel is formed is lower than that of the AlGaN/ GaN hetero interface, and also because polarization-induced charge is not available for the GaN MOSFETs. To improve the device performance we have proposed a HfO2/AlGaN/GaN metal–oxide–semiconductor-heterostructure FET (MOSHFET) in which a high-quality interface was used as a channel [17]. The maximum drain current (ID) of 780 mA/mm and the maximum gm of 185 mS/mm were realized at VGS = 10 V with a gate leakage current of 1 mA/mm. These results were fairly good. However, the gm decreased at large gate voltage. It has been pointed out that a 2-dimensional electron gas (2DEG) is formed at the oxide/semiconductor interface at high positive gate bias [18]. If the interface quality is not good, the gm degradation is
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Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371
expected. Another possibility is electron trapping at the interface traps. Until now, however, there has been little research published on these phenomena. In this paper, two-dimensional device simulations of HfO2/AlGaN/GaN MOSHFETs have been carried out to study the electrical characteristics of the AlGaN/GaN MOSHFETs and the effects of interface traps on the device characteristics.
Table 1 Polarization charge density at each interface.
2. Simulation setup
Table 2 Values of m–E curve parameters in GaN and AlGaN used in the simulations.
Two-dimensional numerical device simulations based on a drift–diffusion model were carried out using the commercial simulation program DESSIS. Fig. 1a and b shows a schematic cross section of the device and the corresponding energy band diagram perpendicular to the device surface used in the simulation. The channel length was 1.5 lm. Aluminum mole fraction in the AlGaN barrier was 0.25 and the thickness was 12 nm. The thickness was chosen to realize normally-off operation for the HEMTs. The GaN and AlGaN were assumed to be lightly p-doped, with a concentration of 1 1014 cm3. The AlGaN/GaN heterostructure has spontaneous and piezoelectric polarizations (PSP and PPE). To take these effects into account, fixed charges (nSP and nPE) shown in Table 1 were introduced at each interface [19]. Spontaneous polarization charges at both sides of the GaN were also included in the model
(a)
nSP(GaN) (cm2) HfO2/AlGaN AlGaN/GaN Bottom of GaN
1.81 1013 1.81 1013
nSP(AlGaN) (cm2)
nPE(AlGaN) (cm2)
Total (cm2)
2.62 1013 2.62 1013
5.74 1012 5.74 1012
3.20 1013 1.39 1013 1.81 1013
llow (cm2/Vs) msat (cm/s) b
GaN
AlGaN
800 3.3 107 1
180 1.8 107 2
to align the modeled system closer to the real device. If no polarization charges are assumed as employed in the conventional simulation, threshold voltage shifts to the negative direction. In addition to the polarization charge, fixed positive charge with a concentration of 1.5 1013 cm2 was introduced at the HfO2/AlGaN interface in order to match the simulated threshold voltages with those observed in experiments with the HEMTs. The introduction of the positive charge and its concentration are consistent with Ref. [20]. Even though we do not know the origin of the positive charge at the surface, if the Fermi level of the HfO2/AlGaN interface is below the charge neutrality level, positive charge will be formed at the HfO2/AlGaN interface. Another possibility is the nitrogen vacancy which is believed to be donor-type defect [21]. Further study is necessary to clarify the origin of the positive charge. Mobility and velocity of electrons used in the simulation are summarized in Table 2 assuming the electron velocity–electric field (m–E) curve of
mðEÞ ¼ h
llow E l E 1þð Vlow Þb
i1=b . The parameters for GaN
sat
(b)
and AlGaN listed in Table 2 were chosen to fit the m–E curve with the results of Monte Carlo simulation [22]. Traps introduced at the HfO2/AlGaN interface to evaluate the effects of the interface states were assumed to be donor-type based on reports from the Mishra [20] and Hashizume [21] groups. The energy of the trap level was defined as a depth from the conduction band minimum EC of AlGaN, ECET, as shown in Fig. 1b. Simulations were carried out with various energy levels and trap concentrations, NT. As a typical value, ECET = 1.0 eV was used based on Refs. [20] and [23], and a trap density NT of 6 1012 cm2 was used considering the threshold voltage shift of about 5 V during the measurement of gate voltage sweep. Even though the device simulation was performed for the device with uniform distribution of the deep level at the interface, the effects of non-uniform distribution of the trapped electrons in a direction parallel to the interface was taken into account in the device simulation because the current continuity equation and Poisson equation are solved self-consistently. In the present device simulation, no gate leakage current throughout the MOS system was assumed. This assumption is widely used in the standard device simulation of the MOSFETs. It is expected that the assumption will not cause serious error because a small gate leakage current of 1010–104 A/mm was confirmed in the experiment [24], even though some modification might become necessary at large forward gate voltage. 3. Simulation results
Fig. 1. Schematic cross section of the device and the corresponding energy band diagram used in the simulations. Donor-type traps at an energy level ET below the conduction band minimum EC of AlGaN were assumed.
Fig. 2 shows the drain current–gate voltage (ID–VGS) (dotted line) and transconductance gm–VGS (solid line) characteristics at a drain voltage VDS of 4 V. The densities of the 2DEG at the AlGaN/
Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371
Fig. 2. ID, gm, nGaN, and nAlGaN as functions of VGS for the device without traps. VDS = 4 V.
GaN heterointerface (nGaN) and the HfO2/AlGaN interface (nAlGaN) are also shown by a long-dash line and short-dash line, respectively. The device exhibited normally-off operation with a threshold voltage Vth of 0.1 V and maximum gm of 191 mS/mm. The gm increased rapidly at first, saturated at a VGS of about 5 V, and decreased at VGS = 15 V, resulting in a plateau structure. In order to examine the reason for the gm decrease, nGaN and nAlGaN were calculated as a function of VGS. The value of nGaN was found to build up at VGS = 0 V and linearly increased as shown by the long-dash line in Fig. 2. For VGS larger than 15 V, the nGaN saturated and an increase in nAlGaN was induced at the HfO2/AlGaN interface, as shown by short-dash line in Fig. 2. The nGaN saturation at VGS = 15 V is explained by the shielding effect of VGS by the nAlGaN induced at the HfO2/AlGaN interface. This is consistent with Ref. [18] in which electron accumulation was observed at the oxide/AlGaN interface. Since the electron velocity in AlGaN is lower than that in GaN, gm decreases rapidly when nAlGaN builds up. In the experiments, however, gate voltage dependence of gm showed a peak structure with no plateau [17], which will be discussed later. We next studied the effects of traps at HfO2/AlGaN interface. Fig. 3 shows ID–VGS (dotted line) and gm–VGS (solid line) characteristics when the trap (ECET = 1 eV, NT = 6 1012 cm2) is introduced at the HfO2/AlGaN interface. In addition to the threshold voltage shift in the negative direction (Vth = 4.5 V) compared to the case without the trap, another gm decrease and its recovery were observed at VGS between 6 and 15 V prior to the onset of
Fig. 3. ID, gm, nGaN, nAlGaN, and nT as functions of VGS for the device with traps (ECET = 1 eV, NT = 6 1012 cm2). VDS = 4 V.
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the previously-mentioned gm decrease due to the channel formation at the HfO2/AlGaN interface. In order to understand these behaviors, electron density was calculated as a function of VGS. Electrons captured by the traps, nT, were found to build up at almost the same voltage of 6 V where gm (solid line) decreases and nGaN (long-dash line) saturates as shown by the dot-dash line in Fig. 3. The above results are explained as follows. Fig. 4 shows the EC distribution at the center of the gate in a direction perpendicular to the device surface for VGS from 6 V (top) to 20 V (bottom) in 1 V steps at VDS = 0 V for NT = 6 1012 cm2. In the off-state between VGS = 6 V and 4 V, EC shifted downward in the whole region under the gate with increase in VGS. For VGS > 4 V on the other hand, the channel was formed at the AlGaN/GaN interface and nGaN increased as shown in Fig. 3. Then, EC of AlGaN/GaN interface was almost pinned at EF and the VGS increase was consumed by the EC down shift of HfO2 and AlGaN. For VGS > 6 V, ET reached EF and electrons were captured by the trap. Then the increase of VGS was consumed by the increase of the trapped electrons nT and ET was almost pinned at EF. Consequently the electron density at the AlGaN/GaN interface, nGaN, increased very little, resulting in a decrease in gm. For VGS > 11 V, most of the traps were filled with electrons and then EC of HfO2 and AlGaN shifted downward again with increasing VGS, so that nGaN increased again resulting in a recovery of the gm as shown in Fig. 3. For VGS > 15 V, another channel was formed at the HfO2/AlGaN interface and EC of HfO2/AlGaN interface was pinned at EF. Then, the gm decreased as discussed for the device without interface traps. Fig. 5 shows gm–VGS characteristics at VDS = 4 V for different ET values (ECET = 0.5, 1.0, 1.5 eV) with a trap concentration of NT = 6 1012 cm2. All devices showed the same threshold voltage of about 5 V since the traps were completely ionized (ET > EF) independent of the energy level of the trap. On the contrary, the VGS at which the gm started to decrease was small for the deep trap resulting in a decrease of ID. This is because the deeper the ET is, the earlier it reaches EF. It is notable that the plateau structure of gm obtained for the shallow interface trap disappeared when the interface trap was deep, resulting in a peak structure of gm. This is consistent with the experimental results with the gm peak, as shown in Fig. 6. This suggests that the gm decrease obtained in Ref. [17] was not due to the channel formation at the HfO2/AlGaN interface but to electron capture at the interface states. If the AlGaN/GaN MOSHFETs have interface states, the drain current does not increase so much as that in Fig. 2 in which no interface trap
Fig. 4. EC profiles for various VGS from 6 to 20 V (in 1 V steps) at the center of the gate. VDS = 0 V. NT = 6 1012 cm2.
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Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371
Fig. 5. gm–VGS characteristic at VDS = 4 V for different ECET values of 0.5, 1.0, and 1.5 eV. NT = 6 1012 cm2. Fig. 8. Amounts of the threshold voltage shift and the gm decrease as functions of NT. VDS = 4 V.
800
200
700
DS
600
150
500 400
100
300 200
50
Drain current (mA/mm)
Transconductance (mS/mm)
V = 8V
100 0
-2
0
2
4
6
8
0 10
Gate voltage (V) Fig. 6. ID–VGS and gm–VGS characteristics of the fabricated AlGaN/GaN MOSHFETs with HfO2 gate oxide. VDS = 8 V.
Fig. 7. gm–VGS characteristics for different NT values of 1, 6, and 10 1012 cm2. ECET = 1 eV. VDS = 4 V.
was assumed. The gm in the experiments showed monotonic decrease in contrast with the double peak structure in the device simulation. One possibility of this difference is that the VGS is not sufficiently large in the experiment and the other possibility is the continuum distribution of the interface states. It is difficult at present to determine which is dominant. Drain current deep level transient spectroscopy [25] is effective in clarifying this point
because it is possible to determine the energy distribution of the deep level by measuring the temperature dependence of the drain current transient. This is now under study. Fig. 7 shows simulated gm–VGS characteristics for a different trap concentration (NT = 1, 6, 10 1012 cm2). Here, ECET was assumed to be 1 eV. The larger concentration led to a larger threshold voltage shift and a greater gm decrease. Fig. 8 shows NT dependences of the threshold voltage shift and the gm decrease. The threshold voltage shifted in proportion to the NT and gm decreased linearly at small NT, saturating at large NT. If the trap concentration was less than 4 1011 cm2, the threshold voltage shift was less than 0.3 V and the gm decrease was less than 10%.
4. Conclusion Two-dimensional device simulations of HfO2/AlGaN/GaN MOSHFET have been carried out to investigate the operation mechanism and the effects of interface traps on the device characteristics. First, devices without the traps were studied. Although the transconductance increased with the gate voltage for small VGS, it decreased for larger VGS, resulting in a plateau structure. This is explained by the channel formation at the HfO2/AlGaN interface where the electron velocity was rather low. Next, simulations for devices with donor-type traps at HfO2/AlGaN interface were performed. Two effects were observed: the shift of the threshold voltage in the negative direction, and the decrease of gm at large VGS which was different from the previous gm decrease. The gm decrease was due to the increase of VGS being expended to increase the electrons captured by the traps at the HfO2/AlGaN interface, and was therefore unavailable to increase the density of the 2DEG at the AlGaN/GaN interface. The gm decrease occurred at a smaller gate voltage than the onset of the channel formation at the HfO2/AlGaN interface, resulting in a peak structure of the gm. The gm peak structure obtained in the simulation was consistent with the experimental results. The magnitude of the threshold voltage shift and the gm decrease were dependent on the trap concentration. If the trap concentration was less than 4 1011 cm2, the threshold voltage shift was less than 0.3 V and the gm decrease was less than 10%. References [1] Levinshtein ME, Rumyantsev SL, Shur MS. Properties of advanced semiconductor materials: GaN, AIN, InN, BN, SiC, SiGe. Wiley-Interscience; 2001.
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