Chapter 1 Introduction
1.1 General Background and Motivation
The wide spread communication systems such as third-generation (3G) mobile systems, wireless LAN, electronic toll collection system (ETC), and global positioning system (GPS). Recently, wide band-gap semiconductors have attracted considerable attentions as the next generation materials for RF power electronic applications such as mobile, satellites, and cable TV systems[1] for power transmitter applications. In the mobile communication applications, the next generation cell phone need, widen bandwidth and higher efficiencies; also, the development of satellites communication and TV broadcasting systems also require amplifiers which can operate at higher frequencies and higher power.
Because of these demands, the outstanding properties of AlGaN/GaN HEMTs make them most promising candidate for microwave power applications in the wireless communication.Some of the commercial and military markets that are targeted by GaN devices are shown in Fig. 1.1.
However, for the microwave monolithic integrated circuits (MMICs) for power applications, a major problem is to increase the operating frequency. One of the major factors limited the performance of the GaN HEMT devices is the high gate leakage current due to the surface defects of the devices and finite barrier height of the Schottky gate. This gate leakage problem becomes more serious when dealing with high-power and high-temperature RF applications.
For this reason, to improve the device high frequency performance, the lowest
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possible gate leakage current level must be achieved. The use of high-k gate insulator for AlGaN/GaN MOS-HEMT can significantly suppress the direct-tunneling gate current.
In the past few years, many studies regarding of about the III-V/high-k interface issues have been published. It is well known that the surface pretreatment including sulfide and ammonia solution treatments could eliminate the undesired particles and native oxides. Furthermore, the surface of III-V material could be passivated by the treatment to prevent the surface exposed to air. With the progress of advanced deposition technologies, many passivation methods had been reported including Gd2O3/Ga2O3 or SiNx as gate insulators, the Al2O3 growth by atomic layer deposition (ALD) [2-4]. Among all the deposition technologies, the ALD shows superior characteristics for oxides deposited. ALD has several advantages over other techniques due to the actual mechanism used to deposit the films. ALD is especially advantageous when film quality or thickness is critical. ALD is also quite effective to be deposited at coating ultra high aspect ratio substrates or substrates that would be difficult to coat with other thin film techniques. It can achieve the high purity level than any other deposition technologies. In this study, the ALD system was used for the high-k oxide deposition.
The main purpose of this study is to establish the AlGaN/GaN MOS-HEMT devices technology with low leakage current for high power, high frequency applications. The technology include the applications of ALD Al2O3 with a high insulator constant (8.6-10) and a high breakdown field (5~10 MV/cm) for the gate insulators for AlGaN/GaN MOS-HEMTs. The RF and DC performances of the AlGaN/GaN HEMTs with gate oxide in this study will be evaluated in this thesis.
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1.2 Organization of the Thesis
This thesis is comprised of six chapters including conclusions. After background introduction, the GaN material properties and operation principle of HEMTs will be introduced in chapter 2. The AlGaN/GaN MOS-HEMTs device fabrication process is introduced in chapter 3. In Chapter 4, the electrical characterization methods for AlGaN/GaN MOS-HEMTs are described. Chapter 5 is the experiment results and discussion of the performance AlGaN/GaN MOS-HEMTs on Si substrate compared to regular HEMT. Finally, the conclusions will be given in chapter 6.
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Fig. 1.1 Commercial and military markets targeted by GaN.
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Chapter 2
AlGaN/GaN Metal-Oxide-Semiconductor HEMTs
An overview on GaN material and AlGaN/GaN HEMTs will be presented in this chapter. Material properties of GaN, especially the unique characteristics for electronic applications are introduced first. This is followed by a brief description of the polarization effect of GaN, along with AlGaN/GaN HEMTs.
Thereafter, the AlGaN/GaN MOS HEMT, which is the focus of this study, is discussed. The historical approaches for the gate insulator of GaN MOS devices are reviewed and the device advantages of MOS-HEMTs over Schottky-gate HEMTs are highlighted. Finally, the basic operation and the non-ideal phenomena of MOS HEMTs are introduced.
2.1. Material properties of GaN
GaN-based materials, GaN, indium nitride (InN), and aluminum nitride (AlN), are wide bandgap semiconductors. They are the candidates for next generation high power devices. These materials have several advantages, such as high band gap, high electron velocity and high breakdown electric field. The bandgaps of GaN, InN, and AlN are respectively 3.4 eV, 1.89 eV and 6.2 eV, as shown in Fig 2.1. The figure not only shows the wide range of energies of III-V nitride materials, but also shows the wavelengths from the visible-light to the ultraviolet (UV) regions if they are used for optical devices. Thus, three-nitrides are good candidates for optoelectronic devices, such as light emission diodes (LEDs), laser diodes (LDs), detectors, and so on.
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Compared with other III-V semiconductor materials, GaN gains a considerable attention for the RF power application. Fig. 2.2 compared the power vs. frequency performance of GaN in comparison with other semiconductors, and it indicates that III-V nitrides materials. GaN are promising candidates for the RF power application. Besides, due to the strong bonding energy between the Ga and N, GaN has a high breakdown field at about 3.3 MV/cm, which means the GaN devices can withstand high operating voltage.
Although GaN has the lower room temperature electron mobility around 1500cm2/Vs than that of GaAs, GaN has very high electron saturation velocity about 3×107 cm2/s. This suggests the high frequency applications of the GaN-based devices. Moreover, GaN has a high thermal conductivity around 1.3 W/cmk, this makes it possible to operate at high temperatures. The benefits of GaN for electronic applications are listed in Table 2.1. Table 2.2 and Fig 2.3 show the material properties and figures of merit of GaN compared with the competing material such as 4H-SiC, GaAs and Si. According to the outstanding material properties of GaN, GaN-based devices are the good candidates in high-power, high-frequency, high speed, and high-temperature applications.
2.2 Polarization effect of GaN
The polarization effects in GaN are due to two types, one is strain-induced piezoelectric polarization, and the other is spontaneous polarization. The strain-induced piezoelectric polarization is resulted from the lattice mismatch, and the spontaneous polarization is due to the noncentrosymmetry of the wurtzite GaN and large iconicity of the covalent GaN bonds. The crystal
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structure induced polarization of GaN, then the strain-induced piezoelectric and spontaneous polarization, will be described in next section.
2.2.1 Crystal structure and piezoelectric polarization [5]
GaN-based materials have two crystal structure, hexagonal wurtzite structure and cubic zinc blends, as shown in Fig 2.4. Since there is no native GaN substrate, GaN-based materials are grown on the other substrate, such as sapphire and silicon carbide. Recently, GaN growth on Si substrate is well-explored. SiC and sapphire are hexagonal structure, and Si is diamond cubic structure. In general, if noncentrosymmetric compound crystals have two different sequences of the atomic layering in the two opposing directions parallel a certain crystallographic axes; crystallographic polarity along these axes can be observed. In early 90’s, the role of nucleation layer was discovered, and (0001) GaN was growth on (0001) sapphire. After that, hexagonal GaN-based materials are widely used in LED and HEMT structure. These are two different growth directions lead GaN to nonequivalent surfaces of Ga- or N- faced. In Ga-face, the Ga atoms are on the top position of bilayers, corresponding to the [0001] polarity. On the other hand, in N-face, the N atoms are located on the surface of {0001}, corresponding to the [0001] polarity. Fig 2.5 shows two different crystal planes of hexagonal GaN lattice structure.
According to the specific crystallographic polarities, GaN exhibits different chemical and physical properties.
2.2.2 Strain-induced piezoelectric and spontaneous polarization [5]
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In case of Ga-face of GaN shown in Fig. 2.5, the spontaneous polarization PSP direction is downward to the substrate. In the other word, the polarization in N-face of GaN in Fig. 2.5 is in opposite direction to Ga-face. In addition, GaN has piezoelectric spontaneous polarization PPE result from the lattice mismatch between AlGaN and GaN. The PPE can be calculated by piezoelectric constants e33 and e31, elastic constants c13 and c 33, and the lattice parameters a0, is given by equation.
(2-1) where a is the lattice constant of GaN along a-axis, and a0 is equilibrium value of lattice constant. (a- a0)/a represents the in-plan strain along a-axis. Since [ ] is less than zero. For AlGaN, over the whole range of compositions, piezoelectric polarization is positive for compressive and negative for tensile barriers. On the other hand, the spontaneous polarization of GaN and AlN is negative. Fig 2.6 shows the directions of the spontaneous and piezoelectric polarization in Ga- and N- face strained and relaxed AlGaN/GaN heterostrucutre.
2.3 AlGaN/GaN HEMTs
2.3.1 Hetero-epitaxial growth of AlGaN/GaN HEMTs
Due to the lack of large-size and low-cost commercial-grade substrate, GaN materials are usually grown on the foreign substrates such as sapphire, Si or SiC.
Table 2.2 shows some of the material properties of these substrates as compared
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to the GaN and AlN layers. Generally, the lattice constants and thermal expansion coefficients of these substrates differ significantly (except SiC) from that of GaN. The first successful epitaxial layer layers of GaN were grown on Sapphire. However, the very large lattice mismatch (14.8%) and the difference in the thermal expansion coefficient between GaN and sapphire substrate cause the huge challenges in the grown of nitrides. As a result of these mismatches, large amount of dislocations are generated in the GaN film. The quality of the GaN film is therefore critically dependent on the ability of the transition layer (buffer layer) used to accommodate the stress generated from these mismatches.
The commonly used buffer layers include low temperature GaN [6-7], AlN [8-10] or their variations [11-13]. Dislocations generated in GaN are mainly screw, edge and mixed TDs. In addition to the buffer layers, other approaches are also used to improve the crystal quality of GaN film such as the insertion of AlN interlayers [14] or Si delta-doping layer [15].
High crystalline quality GaN materials are usually grown by metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) methods. MOCVD is famous for growing the LED-quality GaN and is also used to grow GaN materials for HEMT applications lately. The main advantages of MOCVD, as compared to MBE, are the high growth rate and high crystal quality even for the direct growth of GaN layers on the foreign substrates.
Besides, MBE has also proven to be a promising technique to grow GaN materials for HEMT devices application [16-18]. The benefits of growing GaN by MBE include real-time monitoring of crystal growth with reflection high-energy electron deflection (RHEED), a carbon-free and hydrogen-free growth environment, a smooth surface, sharp interfaces and low point defect density. These attributes are important for achieving high quality materials for
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HEMT devices. Table 2.3 lists some important developments on the electrical properties of AlGaN/GaN structure grown by MOCVD and MBE techniques.
Although these difficulties have been solved, the low thermal conductivity is still an unneglectable problem. Compared with sapphire, SiC has less lattice mismatch (4%) with GaN and very good thermal properties, which is nearly 10 times more than that of sapphire. Therefore, SiC is rather popular substrate. Yet, SiC is too expensive for commercialization. Recently, GaN HEMTs grown on Si substrate was been widely investigated due to lower material cost and compatible with Si technology for circuit integration.
2.3.2 The Basic Structure and Operation of AlGaN/GaN HEMTs
GaN materials for HEMT fabrication consists of a higher bandgap material, such as AlGaN [19] or AlInN [20], grown on the top of the GaN film as the barrier layer. The discontinuity in conduction bands between the two materials forms a 2-dimentional electron gas (2DEG) channel at the hetero-interface.
Basic GaN HEMT structure and band diagrams are shown in Fig. 2.7.
AlGaN/GaN HEMT 2DEG formation is totally different from GaAs HEMT. In AlGaAs/GaAs HEMT, the channel electrons come from the surface states in the AlGaAs. The electrons in the AlGaAs where driven into the GaAs layer, because the hetero-junction created by different band-gap materials. The formation mechanism of GaN HEMT 2DEG is due to the strong polarization effect and large amount of surface states. High electron density (~1.5x1013 cm-2) can be induced at the 2DEG by AlGaN barrier layer with Al~25%, and high electron mobility (~2000 cm2/V*s) can be achieved on an AlGaN/GaN heterostructure. Therefore, AlGaN/GaN HEMT does not require intentional
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doping in the barrier to provide carrier in the 2DEG channel. The 2DEG enables better electron confinement and less carrier scattering. Due to both high mobility and high carrier density, AlGaN/GaN HEMT device of high current density (>2 A/mm) has been demonstrated [21].
2.3.3 Issues of High Gate Leakage Current AlGaN/GaN HEMTs
Despite the impressive device performance, the potential of AlGaN/GaN HEMTs for commercial application have not been fully realized as yet. The RF power expected from fundamental nitride material properties significantly exceeds the experimental data. One of the key problems limiting the HEMT RF power is the high Schottky-gate leakage current, which results in the degradation of DC/RF parameters. At positive gate bias, high forward gate current can shunt the gate-channel capacitance, thus limiting the maximum drain current. At negative gate bias, high voltage drop between the gate and drain results in premature breakdown and the maximum applied drain voltage is restricted [22].
In addition, gate leakage currents increase the device sub-threshold currents, which decrease the achievable amplitude of the RF output. All these limitations become even more severe at high ambient temperatures. Mechanisms of the high gate leakage current in AlGaN/GaN HEMTs have been investigated and possible solutions to suppress the leakage have been explored in the past few years. Through numerical simulations and DC electrical measurements, Miller et al. reported, found that vertical tunneling through the gate area is the dominant mechanism for gate leakage in AlGaN-barrier HEMTs, while additional leakage current mechanisms such as lateral tunneling and defect-assisted tunneling also contributed to the total gate leakage [23]. To suppress the high gate current,
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Miller et al. proposed an enhanced-barrier HEMT structure, in which a GaN cap layer was grown on the top of the standard AlGaN barrier. Owing to the strong polarization effects in the nitrides, the peak barrier height in the new GaN/AlGaN/GaN HEMT was increased, thus decreasing the tunneling gate leakage current. Mizuno et al. compared the gate leakage current of a GaN-based HEMT with a GaAs-based HEMT [24]. They observed both a two to three orders of magnitude larger gate leakage of the GaN-based HEMTs as compared to that of the GaAs-based HEMTs, and the temperature-independence for the gate leakage current in GaN-based HEMTs. Considering that AlGaN has a larger Schottky barrier height (1.4 eV) than GaAs HEMTs (~1.0 eV), the authors attributed tunneling to be the main leakage mechanism instead of the thermionic emission. They also found that surface treatment with CF4 plasma prior to the gate metal deposition was able to reduce the gate leakage current by two to three orders of magnitude. A possible explanation of such leakage suppression is that the plasma treatment introduces deep acceptors to compensate the high-density positive charge on the AlGaN surface. Thus, the depletion layer thickness under the gate increases, and gate leakage current due to electron tunneling becomes small.
2.4 AlGaN/GaN MOS HEMTs
2.4.1 Introduction
As described above, device performance of conventional Schottky gate AlGaN/GaN HEMT device suffers from high gate leakage current. As a result,
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the drain current collapse when operating at high-frequency and poor long-term reliability of Schottky gate. In order to reduce the gate leakage current, a concept of high-k insulators layers between gate metal and semiconductor were investigated in the past years. A schematic comparison between HEMT and MOS-HEMTs for AlGaN/GaN is illustrated in Fig. 2.8.
2.4.2 The requirements of high-k insulators oxide
(A) Insulator constant
Insulator constant is the most important parameter for oxide material used in the MOS structure. Due to the reduction of chip’s size in the future, the horizontal electrical field is increased and the gate modulation ability is decreased. In order to solve these problems, the capacitance per unit area must be improved to decrease the effect of undesired electrical field.
(2-2) where C is capacitance, Q is charges, and V is turned on voltage.
, ε ן C (2-3) where ε is the insulator constant of oxide, A is cross section area, and d is the distance between the two plates. According to Eq. (2-2), the devices with larger accumulation capacitance can be turn on more easily by a smaller voltage. Using smaller operating voltage will result in higher device efficiency and cost saving.
According to the Eq. (2-3), the MOS device which using oxide material with larger insulator constant as its gate insulator will have larger accumulation capacitance. So, the high-k oxide is desired for III-V MOS devices technology.
The energy band gap versus insulator constants of different oxide materials is
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plotted depicted in Fig. 2.9.
(B) Energy band gap
The energy band gap of oxide materials is an important factor which influences the leakage current of the MOS devices. The oxide with smaller energy band gap causes the carrier tunneling more easily; it will induce undesired leakage current and influence the devices performance. The oxide with larger energy band gap can prevent the carriers tunneling. But, the oxide with higher insulator constant will have the smaller energy bandgap. So, it is important to find the suitable oxide to improve the MOS devices performance.
Several gate oxide candidates are listed in Table 2.4. Besides, the band offset of oxide on semiconductor material is also needed to be considered, the value must exceed 1 eV so that the oxide can serve an effective insulator [18].
ALD Al2O3 is introduced in this study due to its relatively high band gap (about 8.7 eV) and remains amorphous under typical processing conditions. In addition, Al2O3 also possesses high breakdown electric field (5~20 MV/cm), high thermal stability (up to 1000℃) and strong adhesion with dissimilar materials [25]. With well-controlled thickness and uniformity for the Al2O3 layer deposited by ALD technology by the good insularity of Al2O3 layer, ALD Al2O3 is the leading candidate for the gate insulators in MOS-HEMT device.
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Table 2.1 Advantages of GaN material for electronic applications.
Material property Advantages Wide bandgap
3.42 eV
Great endurance for high device operating temperature
Suitable for high power applications.
Working under high temperature environment +High breakdown field
4×106 V/cm
Larger power density
High thermal conductivity
~1.3W/cm* K
Better heat dissipation, enhanced device performance
Easier device packaging High saturate electron velocity
~2.7×107 cm/sec
Suitable for high frequency applications
Table 2.2 Material properties and figure of merit (FOM) of GaN, 4H-SiC, GaAs
Table 2.2 Material properties and figure of merit (FOM) of GaN, 4H-SiC, GaAs