The Impact of AlxGa1−xN Back Barrier in AlGaN/GaN High Electron Mobility Transistors (HEMTs) on Six-Inch MCZ Si Substrate

In this study, AlGaN/GaN high electron mobility transistors (HEMTs) with AlGaN back barriers (B.B.) were comprehensively investigated based on the different Al mole fractions and thicknesses in the design of the experiments. It was shown that the off-state leakage current can be suppressed following an increase of the Al mole fraction due to the enhancement of the back barrier height. Increasing the AlGaN thickness deteriorated device performance because of the generation of lattice mismatch induced surface defects. The dynamic on-resistance (RON) measurements indicated that the Al mole fraction and thickness of the B.B. both affected the buffer trapping phenomenon. In addition, the thickness of B.B. also influenced the substrate heat dissipation ability which is also a key index for high power RF device applications.


Introduction
Due to the numerous advancements of the IMT-2020 standard, the global competition to launch fifth-generation (5G) sub-six and mmWave frequencies could provide a distinct base-station market opportunity for the gallium nitride (GaN) high electron mobility transistor (HEMT) as an alternative to the silicon laterally diffused metal oxide semiconductor (LDMOS) [1][2][3][4][5]. For high efficiency and high linearity consideration, conventional Si CMOS processes are not suitable for emerging 5G applications due to their low operation drain voltage, where GaN HEMT can be operated under 50V. Traditionally, the GaN heterostructure devices have been grown on semi-insulating silicon carbide (SiC) due to its excellent thermal conductivity and low RF signal loss [6][7][8]. However, semi-insulating SiC substrate is still expensive but produced in high volume. A different approach to obtain a GaN RF HEMT is the adoption of 6-inch or 8-inch high resistivity float zone (FZ) Si substrate. The performance and reliability benefits of GaN-on-Si HEMTs for RF applications have been well demonstrated since the 2006 qualification of GaN-on-Si technology and this technology was implemented in 4G LTE and the 5G wireless base-station infrastructure successfully [4]. However, the cost and the crystal formation duration of high resistivity FZ silicon obtained by the vertical zone melting method are still two disadvantages for industrial applications.
Coatings 2020, 10 In addition, the substrate mechanical strength of FZ Si is also lower than conventional Czochralski (CZ) Si thus the substrate bowing phenomena of GaN heterostructure on FZ Si also limits its potential for high volume production [9][10][11]. Recently, high resistivity magnetic Czochralski (MCz) silicon (>2000 Ω·cm) has been found to be of high interest for the development of radio-frequency RFCMOS and SOI technologies, since this material significantly suppresses the RF noise coupling from the substrate and transmission line intermodulation parasitic, together with a high substrate mechanical strength performance. In this work, we comprehensively studied the GaN RF HEMT on high resistivity MCZ with the AlGaN back barrier (B.B.) optimization. The usage of a B.B. structure is an alternative solution for reducing the short-channel effects and improving the carrier confinement in two-dimensional electron gas (2DEG) for AlGaN/GaN HEMT [12][13][14][15][16]. In addition, the buffer related electron trapping effect can also be minimized. Moreover, AlGaN back barrier layer also forms a thermal barrier layer which influences the device linearity and thermal induced current reduction at high power operation. The Al mole fraction and the thickness of the AlGaN back layer were two key parameters studied and investigated in this work.

Device Fabrication
The AlGaN/GaN RF HEMT with AlGaN B.B. structures were grown using the metal organic chemical vapor deposition (MOCVD) method on MCz-Si substrate. After an AlN nucleation layer, a 1.5 µm iron (Fe) doping GaN buffer layer was grown followed by the AlGaN back barrier. To study the impact of the AlGaN B.B., the design of experiments (DOE) of AlGaN B.B. were of different Al mole fractions (3%, 5%, and 8%) and thicknesses (40, 50, and 60 nm). As to the active layers of these samples, a 1-nm-thick AlN layer was grown in all the samples on top of the 300 nm GaN channel, followed by a 18 nm-thick Al 0.24 Ga 0.76 N barrier layer and a 2 nm GaN cap layer. The thin AlN interlayer between the Al 0.24 Ga 0.76 N barrier layer and the GaN channel contributes to reduce interface roughness and enhance the carrier mobility in 2-DEG. The Hall measurement result of these back barrier conditions was quite similar. The mobility range of devices were about 1628 to 1756 cm 2 /V·s and the concentration of devices was about 1.14 × 10 13 /cm 2 to 1.37 × 10 13 /cm 2 . Figure 1a illustrates the cross-section of the fabricated device and the proposed epitaxial structures. Figure 1b illustrates the one-dimensional band diagrams of Schrödinger-Poisson simulation for the devices with various AlGaN B.B. designs. The higher potential of the Al x Ga 1-x N back barrier is expected to improve the 2DEG carrier confinement [10]. As to the device fabrication, the active region was protected by a photoresist, and the mesa isolation region was removed in a reactive ion etching chamber using BCl 3 + Cl 2 mixed-gas plasma. Ohmic contacts were prepared using electron beam evaporation involving a multilayered Ti/Al/Ni/Au (30 nm/125 nm/50 nm/200 nm) sequence, which was followed by rapid thermal annealing at 850 • C for 30 s in a nitrogen-rich environment. After ohmic formation, the Ni/Au (15/330 nm) gate metal was evaporated.
Although, the AlGaN back barrier can improve the carrier confinement in two-dimensional electron gas (2DEG), it also causes a lattice mismatch phenomenon between channel and buffer. To investigate the dislocation with various B.B. layer designs, the structure lattice dislocation was calculated by the XRD FWHM results as following [17,18]: (1) where N SCREW , N edge is the screw and edge dislocation density and b is the burger vector. Figure  substrate total dislocation results. The 60 nm AlGaN B.B. shows a 5.87 × 10 9 /cm 2 total dislocation due to the increasing thickness of the AlGaN which was due to the influence of the epitaxy quality of the structure. Figure 1a illustrates the cross-section of the fabricated device and the proposed epitaxial structures. Figure 1b illustrates the one-dimensional band diagrams of Schrödinger-Poisson simulation for the devices with various AlGaN B.B. designs. The higher potential of the AlxGa1-xN back barrier is expected to improve the 2DEG carrier confinement [10]. As to the device fabrication, the active region was protected by a photoresist, and the mesa isolation region was removed in a reactive ion etching chamber using BCl3 + Cl2 mixed-gas plasma. Ohmic contacts were prepared using electron beam evaporation involving a multilayered Ti/Al/Ni/Au (30 nm/125 nm/50 nm/200 nm) sequence, which was followed by rapid thermal annealing at 850 °C for 30 s in a nitrogen-rich environment. After ohmic formation, the Ni/Au (15/330 nm) gate metal was evaporated.  Although, the AlGaN back barrier can improve the carrier confinement in two-dimensional electron gas (2DEG), it also causes a lattice mismatch phenomenon between channel and buffer. To investigate the dislocation with various B.B. layer designs, the structure lattice dislocation was calculated by the XRD FWHM results as following [17,18]: where NSCREW, Nedge is the screw and edge dislocation density and b is the burger vector. Figure Figure 3 shows the characteristics of AlGaN B.B. by varying its thickness and Al mole fraction on the IDS-VGS transfer characteristics of the AlGaN/GaN HEMTs with gate voltage (Vg) from −6 to 2 V. In Figure  3a and the table, the off-state leakage current was suppressed following the increase of Al mole fraction due to the enhancement of back barrier height. However, the saturation current showed slight degradation with high Al mole fraction B.B. design because the rising of the barrier height also narrowed down the 2DEG channel. The Ion/Ioff ratios were 6.6 × 10 4 , 3.2 × 10 5 , 7.5 × 10 5, and the subthreshold swing slopes (S.S.) were 0.28, 0.27, 0.29 V/dec of 3%, 5%, and 8% Al mole fraction devices, respectively. It also affected the transconductance of the device, the gmmax value was decreased from 164 to 156 mS/mm with increasing Al mole fraction. As to the B.B. thickness  down the 2DEG channel. The I on /I off ratios were 6.6 × 10 4 , 3.2 × 10 5 , 7.5 × 10 5, and the subthreshold swing slopes (S.S.) were 0.28, 0.27, 0.29 V/dec of 3%, 5%, and 8% Al mole fraction devices, respectively. It also affected the transconductance of the device, the g mmax value was decreased from 164 to 156 mS/mm with increasing Al mole fraction. As to the B.B. thickness dependency of device characteristics shown in Figure 3b and the table, the saturation drain current for three samples (40, 50, and 60 nm) were compared. The off-state leakage current of the HEMTs can be improved by increase the B.B. thickness from 40 to 50 nm. It was noticed that the leakage current performances of the device with 60 nm B.B. was worse than 50 nm B.B design and the g mmax value was decreased from 162 to 158 mS/mm with increasing B.B. thickness. As to the results, the epitaxy quality was influenced by thick AlGaN B.B. and the optimized thickness in this study was 50 nm. The devices with 40, 50 and 60 nm AlGaN B.B. showed the I on /I off ratios were approximately 4.5 × 10 4 , 3.2 × 10 5, and 5.2 × 10 4 and their corresponding S.S. 0.32, 0.27, and 0.33 V/dec, respectively [19,20].      Figure 4, the table shows that the thickness and Al mole fraction were both negative factors for devices R ON and I DS because AlGaN B.B. played a thermal barrier role between channel and substrate. Thicker B.B. and higher Al mole fraction B.B. thus prevented the junction heat dissipation through the substrate and the devices R ON and I DS were degraded simultaneously. Figure 5 shows the horizontal breakdown characteristics of the AlGaN B.B. devices. The results were obtained using Agilent B1505. The measurement ohmic contact pattern distance was 40 µm mesa isolation region. Since the increasing Al composition influenced the crystal quality, the breakdown voltage of Al mole fraction 3%, 5%, and 8% AlGaN B.B. device was 740, 729, and 716 V, respectively. The 60 nm AlGaN B.B. device shows that from the highest breakdown voltage it was concluded that B.B. thickness is a positive factor to increase back barrier height and suppress the horizontal leakage current at high   Figure 4, the table shows that the thickness and Al mole fraction were both negative factors for devices RON and IDS because AlGaN B.B. played a thermal barrier role between channel and substrate. Thicker B.B. and higher Al mole fraction B.B. thus prevented the junction heat dissipation through the substrate and the devices RON and IDS were degraded simultaneously.  Al 0.03 GaN B.B. Al 0.08 GaN B.B. Al 0.05 GaN 50nm B.B.      To analyze the trapping and detrapping phenomena in GaN HEMTs with AlGaN B.B., lowfrequency noise spectra were measured at various gate bias voltages. The drain current power spectral density, SID, was measured and normalized to the square of the drain current versus frequency in the frequency range 1-10,000 Hz. To locate the source of noise in the channel area, the normalized current spectral density SI/I 2 at 100 Hz was plotted against the effective gate-to-source voltage (VGN) of AlGaN B.B. HEMTs, where VGN was set as VGS-Vth [21][22][23].

Results and Discussion
When SI/I 2 is proportional to VG −1 , there is noise from the heterostructure interface region. In the transition region, where SI/I 2 approaches VG −3 , the channel and buffer traps contribute to the noise, the drain-source resistance is dominated by the ungated channel region resistance, RU, which is consistent with a previous study [24][25][26]. The results ( Figure 6) indicated that increasing the Al mole fraction and thickness for the AlGaN B.B. layer induced more lattice dislocation or defects. It results To analyze the trapping and detrapping phenomena in GaN HEMTs with AlGaN B.B., low-frequency noise spectra were measured at various gate bias voltages. The drain current power spectral density, S ID , was measured and normalized to the square of the drain current versus frequency in the frequency range 1-10,000 Hz. To locate the source of noise in the channel area, the normalized current spectral density S I /I 2 at 100 Hz was plotted against the effective gate-to-source voltage (V GN ) of AlGaN B.B. HEMTs, where V GN was set as V GS -V th [21][22][23]. When S I /I 2 is proportional to V G −1 , there is noise from the heterostructure interface region. In the transition region, where S I /I 2 approaches V G −3 , the channel and buffer traps contribute to the noise, the drain-source resistance is dominated by the ungated channel region resistance, R U , which is consistent with a previous study [24][25][26]. The results ( Figure 6)  The dynamic RON test was performed using an AMCAD AM241 pulsed system. The dynamic RON characteristics were measured from different quiescent bias points at VGS = 1 V to investigate the effect of off-state drain bias stress on dynamic RON and current (Figure 7). The reference bias was set at (VGS,Q, VDS,Q) = (0 V, 0 V)-VDSQ is the quiescent drain bias-and did not induce any relevant trapping. The devices were switched with a 5 μs pulse width and 500 μs pulse period. VDS,Q was swept from 0 to 30 V in 10 V increments, and the quiescent gate bias was swept to −6 V. The dynamic R ON test was performed using an AMCAD AM241 pulsed system. The dynamic R ON characteristics were measured from different quiescent bias points at V GS = 1 V to investigate the effect of off-state drain bias stress on dynamic R ON and current (Figure 7). The reference bias was set at (V GS,Q , V DS,Q ) = (0 V, 0 V)-V DSQ is the quiescent drain bias-and did not induce any relevant trapping. The devices were switched with a 5 µs pulse width and 500 µs pulse period. V DS,Q was swept from 0 to 30 V in 10 V increments, and the quiescent gate bias was swept to −6 V.
The Figure 7a shows that the dynamic R ON ratio (R ON, D /R ON, Q ) of the device with 3% Al mole fraction B.B. under high-field pulse is about 1.26. With Al content increasing, the normalized dynamic R ON of the device with Al 0.08 GaN B.B. increased to 1.31. The increasing number of Al atoms introduce more lattice defects to deteriorate the crystalline quality. This was also affected by the hot electrons in the 2DEG channel which were injected into the buffer traps during the pulse test and caused the on state resistance. The dynamic degradation resulted from insufficient time to release the carriers from the ionized traps [27][28][29][30]. Figure 7b shows the ratio also increased when the AlGaN B.B. thickness increased. The thicker AlGaN B.B. devices' dynamic R ON ratio increased from 1.22 to 1.37 at V DS,Q = 30 V.  The dynamic RON test was performed using an AMCAD AM241 pulsed system. The dynamic RON characteristics were measured from different quiescent bias points at VGS = 1 V to investigate the effect of off-state drain bias stress on dynamic RON and current (Figure 7). The reference bias was set at (VGS,Q, VDS,Q) = (0 V, 0 V)-VDSQ is the quiescent drain bias-and did not induce any relevant trapping. The devices were switched with a 5 μs pulse width and 500 μs pulse period. VDS,Q was swept from 0 to 30 V in 10 V increments, and the quiescent gate bias was swept to −6 V.  The AlGaN back barrier layer also forms a thermal barrier layer which influences the device linearity and the thermal induced current reduction at high power operation. To experimentally measure the surface temperature distribution in the devices, an infrared (IR) thermographic system with micro-Raman spectroscopy was adopted, and the IR radiation of the device was detected using an IRM P384G detector. The surface temperature map values were obtained from the IR radiation intensity, which was determined following emissivity calibration performed for the B.B. devices at a current of 60 mA for 30 s.
As presented in Figure 8, the surface temperature of the increasing Al mole fraction AlGaN B.B. device was from 48.1 to 50.4 • C. The Al 0.08 GaN B.B. device had the highest thermal resistance of 30.6 • C/W. The temperature sensitivity is likely related to differences in impurity ionization and carrier hopping between the AlGaN back barrier and buffer structures. Because of increasing the Al mole fraction, the back barrier height also increased as a thermal barrier layer. The results indicated that the Al compositions caused thermal issues. The surface temperature of the thickness of AlGaN B.B. device was from 47.2 to 50.1 • C. Compared to the Al 0.08 GaN B.B. device, the 60 nm B.B. device had a lower thermal resistance of approximately 29.8 • C/W. The results are related to the device surface temperatures. In addition, the surface temperature distribution was not uniform, which decreased reliability due to the presence of hot spots.
To determine the effect of a B.B. on RF performance, the microwave S-parameters of both devices were evaluated using a common source configuration and a PNA network analyzer in conjunction with Cascade direct probes. The measurement frequency range of the S-parameters was 100 MHz to 50 GHz at the operating condition of V DS = 10 V. The gate length and gate width of devices was 1 and 100 µm. Based on the optimal RF operation of both devices, the V GS values were defined at the appearance of their g m peak.
As presented in Figure 9, the 8% Al mole fraction of AlGaN B.B. device exhibited a higher current gain cut-off frequency (f T = 6 GHz). The results indicated that increasing the Al mole fraction can improve Coatings 2020, 10, 570 8 of 11 2DEG confinement. This increase affected carrier concentration and the current gain cut-off frequency. However, the maximum stable gain cut-off frequency (f max = 10.5 GHz) was affected by the thermal issues.
device was from 48.1 to 50.4 °C. The Al0.08GaN B.B. device had the highest thermal resistance of 30.6 °C/W. The temperature sensitivity is likely related to differences in impurity ionization and carrier hopping between the AlGaN back barrier and buffer structures. Because of increasing the Al mole fraction, the back barrier height also increased as a thermal barrier layer. The results indicated that the Al compositions caused thermal issues. The surface temperature of the thickness of AlGaN B.B. device was from 47.2 to 50.1 °C. Compared to the Al0.08GaN B.B. device, the 60 nm B.B. device had a lower thermal resistance of approximately 29.8 °C/W. The results are related to the device surface temperatures. In addition, the surface temperature distribution was not uniform, which decreased reliability due to the presence of hot spots. To determine the effect of a B.B. on RF performance, the microwave S-parameters of both devices were evaluated using a common source configuration and a PNA network analyzer in conjunction with Cascade direct probes. The measurement frequency range of the S-parameters was 100 MHz to 50 GHz at the operating condition of VDS = 10 V. The gate length and gate width of devices was 1 and  100 μm. Based on the optimal RF operation of both devices, the VGS values were defined at the appearance of their gm peak. As presented in Figure 9, the 8% Al mole fraction of AlGaN B.B. device exhibited a higher current gain cut-off frequency (fT = 6 GHz). The results indicated that increasing the Al mole fraction can improve 2DEG confinement. This increase affected carrier concentration and the current gain cut-off frequency. However, the maximum stable gain cut-off frequency (fmax = 10.5 GHz) was affected by the thermal issues.