Substrate-transfer Technique Using h-BN for
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2. Transfer technique for AlGaN/GaN HEMTs using h-BN ELO
First, we present our ELO technique based on the h-BN release layer. The crystal structure of h-BN is shown in Fig. 1. The h-BN crystal has a layered structure like graphite, and each layer is weakly bonded by van der Waals forces. Thus, the layers can be easily exfoliated. Moreover, single-crystal GaN layers can be grown on an AlN or AlGaN nucleation layer (NL) deposited on h-BN. Thus, the GaN layers can be released from sapphire substrates by mechanical forces.
Fig. 1. Crystal structure of h-BN.
The transfer process for AlGaN/GaN HEMT is illustrated in Fig. 2. First, the h-BN layer is grown on a C-plane sapphire substrate by metal organic vapor phase epitaxy (MOVPE). Next, a 100-nm-thick AlN NL is grown on the h-BN layer. Subsequently, an AlGaN/GaN heterostructure is grown by MOVPE at a temperature of 1000°C at a pressure of 300 Torr. These are the same as the conventional conditions for the direct growth on sapphire substrate. The heterostructure consists of a 3-μm-thick GaN buffer layer and 27-nm-thick AlGaN barrier layer. Two-dimensional electron gas (2DEG) with a 1 × 1013 cm−2 density is induced by a polarization charge at the AlGaN/GaN heterointerfaces, which forms a conductive path from the source to the drain.
Fig. 2. Schematic views of h-BN ELO and transfer processes of AlGaN/GaN HEMT. (a) Growth of AlGaN/GaN heterostructures on AlN NL deposited on h-N/sapphire. (b) Fabrication of AlGaN/GaN HEMTs by conventional photolithography and lift-off technique. The labels S, G, and D indicate source, gate, and drain electrodes, respectively. (c) Mechanical ELO of the HEMTs from sapphire substrate. (d) Transfer of the HEMTs to a copper plate by Au-Au thermocompression bonding.
AlGaN/GaN HEMTs were fabricated using conventional photolithography and lift-off techniques. The AlGaN barrier outside the active region was etched away by inductively coupled plasma etching to remove the 2DEG conductive channel for electrical isolation between devices (mesa isolation). We used electron-beam evaporation for metal deposition and rapid thermal annealing to form the ohmic contacts. The source/drain ohmic electrodes consisted of titanium (Ti; 15 nm)/ aluminum (Al; 80 nm)/ nickel (Ni; 30 nm)/gold (Au; 50 nm) annealed at 850°C for 30 s. The Schottky gate electrode was Ni (50 nm)/Au (100 nm). The gate length, source-drain spacing, and gate width were 1.5, 6, and 100 μm, respectively.
The next step in the process is to release AlGaN/GaN HEMTs from the sapphire substrate and transfer them to a copper plate. For that purpose, we first coated the sample surface with PMMA (polymethyl methacrylate) and adhered the sample to a glass plate. Then we mechanically released it from the sapphire substrate. For transfer to a copper plate, we used Au-Au thermocompression bonding . We deposited Ti/Au on the back side surface of the HEMT, and we deposited Au on the copper plate. The two Au surfaces were brought into contact with each other, and then a pressure of approximately 20 MPa was applied at about 200°C. The cross-sectional transmission electron microscopy (TEM) image of the transferred HEMT is shown in Fig. 3. The TEM image reveals that the GaN layer strongly adhered to the copper layer at the Au-Au thermocompression interface.
Fig. 3. Cross-sectional TEM image of an AlGaN/GaN HEMT transferred to a copper plate by Au-Au thermocompression bonding. White and grey layers on the sample surface consist of carbon, which was coated for surface protection in TEM measurements. The difference in the colors is caused by the purity of carbon.
3. Enhancement of heat dissipation of AlGaN/GaN HEMTs using substrate-transfer technique
Typical drain current (Id)–drain bias (Vds) characteristics of the AlGaN/GaN HEMT before release from the substrate and after transfer to the copper plate are shown in Fig. 4. Good pinch-off and saturation characteristics were obtained even after the transfer, indicating that no degradation took place in the transfer process. The device performance significantly improved after transfer. The maximum Id at a gate bias (Vgs) of 1 V significantly increased from 0.55 A/mm to 0.68 A/mm. Before release, a large reduction in Id vs Vds was observed in the saturation region at a large positive gate bias (Vgs). At Vgs = 1 V, the Id decreased by 25% as the Vds increased from 5 V to 20 V. In contrast, the decrement in Id vs Vds was as small as 3% for the transferred HEMT. The negative slope is commonly attributed to self-heating in the device operation . The transfer from the sapphire to the copper plate improves the heat dissipation efficiency and thereby suppresses the decrease in drain current under high-bias operation.
Fig. 4. Id–Vds characteristics of AlGaN/GaN HEMT before release and after transfer.
We also estimated the temperatures in the active region of HEMTs using micro-Raman spectroscopy. The temperatures of the devices can be estimated from the peak shift in the A1 phonon mode [9, 10]. The temperature maps of an HEMT on a sapphire substrate and another HEMT transferred to copper are shown in Fig. 5. The highest temperature was observed at the center of the channel at the gate edge on the drain side. For the HEMT on sapphire (before its release), the temperature exceeded 200°C at a power dissipation (P) level of 0.68 W. In contrast, for the transferred HEMT, the temperature was only 110°C at P = 1.54 W. The channel temperature as a function of P in the HEMTs is plotted in Fig. 6 before their release and after their transfer. The channel temperature is the average one in the center region of the channel at the gate edge on the drain side where the highest temperature was observed. A linear approximation revealed that the thermal resistances of the HEMTs before their release and after their transfer were 28 and 6.5°C mm/W, respectively, indicating a one-fourth reduction. The transfer to copper significantly improved the heat dissipation in the HEMTs.
Fig. 5. (a) Schematic view of AlGaN/GaN HEMT. The dotted rectangle corresponds to the measurement area by micro-Raman spectroscopy. Temperature maps of (b) an HEMT on sapphire (before release) at a power dissipation of 0.68 W and (c) another HEMT transferred to a Cu plate at a power dissipation of 1.54 W. The white regions in each map represent the source, gate, and drain electrodes from left to right.
Fig. 6. Temperature at the gate edge on drain side as a function of power dissipation in HEMTs before release and after transfer.
We transferred AlGaN/GaN HEMTs from a sapphire substrate to a copper plate using the ELO technique with an h-BN release layer. The negative slope of Id caused by the self-heating effect was reduced after the transfer. We also observed the temperatures in the active HEMTs using micro-Raman spectroscopy and found that the temperature rise was suppressed after the transfer to the copper plate. The temperature at P = 1.54 W was 110°C for the HEMT after transfer, whereas it exceeded 200°C at P = 0.68 W before the release. These results indicate that the transfer technique is effective for enhancing the heat dissipation in GaN-based devices.
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