Technical Insight
Bringing AlGaN from research to high-volume production
Recent AlGaN development work in the laboratory will help to shape the future market for a variety of devices, according to David Gotthold and Shiping Guo from Emcore.
With new applications rapidly emerging for AlGaN-based devices such as HEMTs and UV LEDs, the seamless move to growth on 3 inch (75 mm) and 4 inch (100 mm) substrates has become a key objective in the compound semiconductor industry. Current research and engineering for epitaxial structures requiring high aluminum content layers (25-60%) is laying the foundation for high-volume, low-cost growth of these advanced materials. To this end, the challenges associated with transferring such sensitive processes to a production platform are steadily being overcome through innovative equipment design combined with a flexible approach to deposition. As in the silicon industry, it is falling to equipment manufacturers to develop and transfer growth techniques that enable end users to smoothly ramp up to mass production.
Taking AlGaN HEMTs into production
Due to their promise as high-performance RF devices and their ability to operate at extreme temperatures, AlGaN/GaN HEMTs have attracted considerable attention from a number of industries. For high-frequency (10 GHz) applications, devices have been produced that can operate at power densities in excess of 11 W/mm (Eastman et al.). In addition, there has been a more immediate commercial interest in these devices from the telecommunication and automotive industries, for high-voltage and high-temperature applications. In order to successfully move these devices from the university laboratory to a full production environment, high-volume and cost-effective growth technologies must be developed.
Scaling the growth of AlGaN HEMTs to a true production-ready multiwafer growth platform requires focusing on a range of issues. These include the repeatability of the epitaxial structure, the process yield (especially given the high cost of semi-insulating SiC substrates), and the ability to transfer a laboratory-developed 2 inch process to 3 and 4 inch substrates. In particular, strict wafer temperature control during deposition is a major concern, as the thermal drift associated with high Al-content AlGaN growth can significantly affect run-to-run repeatability. Once a reliable and cost-effective production process is established that does not sacrifice the high performance levels indicated by earlier low-volume prototype devices, AlGaN/GaN HEMTs can be effectively commercialized.
Two different sizes of reactors are currently used for HEMT development in the Emcore laboratory: a research-scale D180GaN tool, used for basic structure and process development on 2 and 3 inch substrates; and a production scale E300GaN tool, for growth of up to five 4 inch substrates in a single run (figure 1). In both systems, a proprietary in situ temperature-monitoring instrument utilizing emissivity-compensated pyrometry and developed specifically for GaN growth is used to enhance process control. A range of HEMT structures can be grown, depending on the specific application and customer preferences. The structures consist of a 15-30 nm thick 25-35% AlGaN barrier layer on 1-3 µm of semi-insulating GaN, and a substrate-specific nucleation layer.
Approaches to different substrates
As advanced AlGaN-based structures move from the laboratory to production, reducing the overall cost of manufacturing these devices becomes necessary. In the case of AlGaN HEMTs, it is important to incorporate the use of larger, lower-cost substrates, both for process development and for final devices. When using low-cost substrates for process development, it is imperative to prove process transferability between sapphire and SiC. Through careful control of defect density, strain and surface morphology, it is possible to achieve essentially identical results on both sapphire and 4H-SiC substrates when using identical growth recipes for the semi-insulating GaN and the AlGaN barrier layer. On a test pair of 2 inch substrates, sheet charge and mobility results are found to be similar, with a carrier density (Ns) for both wafers of ~8 x 1012/cm2, and mobility (µ) of ~1550 cm2/Vs with equivalent uniformities. For low-cost 100 mm growth, silicon substrates are ideal - however, cracking of the epilayer can be a severe problem due to mismatches in the coefficients of thermal expansion between Si and GaN-based material. With adjustments to improve strain control, a crack-free surface was obtained, yielding an XRD (0002) reflection with a FWHM of 334 arcsec. Typical AlGaN HEMT characteristics were sheet resistance (Rs) = 360-380 Ω/sq, Rs uniformity 1500 cm2/Vs, Ns >8 x 1012/cm2.
Due to a build up of deposits on the platter, drift in the Al composition over time has presented a challenge for AlGaN HEMT repeatability in a production environment. Using the proprietary GaN RealTemp 200 to correct for drift in wafer temperature, it is possible to maintain a steady temperature over multiple runs. Using the research reactor over three consecutive runs (AlGaN HEMT on 2 inch SiC), an average Rs of 367.4 Ω/sq was achieved with a run-to-run standard deviation (σ) of 1.8% and an average thickness of 1.48 µm (σ = 0.46%). Over longer timeframes (~45 runs over 6 weeks), AlGaN composition for a set of basic HEMT runs was maintained at 30% with σ = 0.5%. In addition, the uniformity and repeatability of both the composition and thickness of the AlGaN barrier layer is critical for creating constant HEMT performance. Figure 2 shows composition and thickness maps of a reference, quasi-bulk AlGaN layer grown on a sapphire substrate.
To further the development of HEMTs, Emcore has entered a partnership with Cornell University, wherein Cornell processes submitted epiwafers into complete HEMT devices for evaluation. Through this collaboration, GaN HEMT device results on SiC (0.2 µm gates) show excellent DC and RF characteristics, with Imax >1 A/mm, Pout >7 W/mm at ~50% PAE (10 GHz), ft >80 GHz, fmax >110 GHz. Devices on silicon also show encouraging DC results, with early devices having Imax >600 mA/mm and a breakdown voltage >-50 V, with good pinch-off characteristics.
Taking AlGaN HEMTs into production
Due to their promise as high-performance RF devices and their ability to operate at extreme temperatures, AlGaN/GaN HEMTs have attracted considerable attention from a number of industries. For high-frequency (10 GHz) applications, devices have been produced that can operate at power densities in excess of 11 W/mm (Eastman et al.). In addition, there has been a more immediate commercial interest in these devices from the telecommunication and automotive industries, for high-voltage and high-temperature applications. In order to successfully move these devices from the university laboratory to a full production environment, high-volume and cost-effective growth technologies must be developed.
Scaling the growth of AlGaN HEMTs to a true production-ready multiwafer growth platform requires focusing on a range of issues. These include the repeatability of the epitaxial structure, the process yield (especially given the high cost of semi-insulating SiC substrates), and the ability to transfer a laboratory-developed 2 inch process to 3 and 4 inch substrates. In particular, strict wafer temperature control during deposition is a major concern, as the thermal drift associated with high Al-content AlGaN growth can significantly affect run-to-run repeatability. Once a reliable and cost-effective production process is established that does not sacrifice the high performance levels indicated by earlier low-volume prototype devices, AlGaN/GaN HEMTs can be effectively commercialized.
Two different sizes of reactors are currently used for HEMT development in the Emcore laboratory: a research-scale D180GaN tool, used for basic structure and process development on 2 and 3 inch substrates; and a production scale E300GaN tool, for growth of up to five 4 inch substrates in a single run (figure 1). In both systems, a proprietary in situ temperature-monitoring instrument utilizing emissivity-compensated pyrometry and developed specifically for GaN growth is used to enhance process control. A range of HEMT structures can be grown, depending on the specific application and customer preferences. The structures consist of a 15-30 nm thick 25-35% AlGaN barrier layer on 1-3 µm of semi-insulating GaN, and a substrate-specific nucleation layer.
Approaches to different substrates
As advanced AlGaN-based structures move from the laboratory to production, reducing the overall cost of manufacturing these devices becomes necessary. In the case of AlGaN HEMTs, it is important to incorporate the use of larger, lower-cost substrates, both for process development and for final devices. When using low-cost substrates for process development, it is imperative to prove process transferability between sapphire and SiC. Through careful control of defect density, strain and surface morphology, it is possible to achieve essentially identical results on both sapphire and 4H-SiC substrates when using identical growth recipes for the semi-insulating GaN and the AlGaN barrier layer. On a test pair of 2 inch substrates, sheet charge and mobility results are found to be similar, with a carrier density (Ns) for both wafers of ~8 x 1012/cm2, and mobility (µ) of ~1550 cm2/Vs with equivalent uniformities. For low-cost 100 mm growth, silicon substrates are ideal - however, cracking of the epilayer can be a severe problem due to mismatches in the coefficients of thermal expansion between Si and GaN-based material. With adjustments to improve strain control, a crack-free surface was obtained, yielding an XRD (0002) reflection with a FWHM of 334 arcsec. Typical AlGaN HEMT characteristics were sheet resistance (Rs) = 360-380 Ω/sq, Rs uniformity 1500 cm2/Vs, Ns >8 x 1012/cm2.
Due to a build up of deposits on the platter, drift in the Al composition over time has presented a challenge for AlGaN HEMT repeatability in a production environment. Using the proprietary GaN RealTemp 200 to correct for drift in wafer temperature, it is possible to maintain a steady temperature over multiple runs. Using the research reactor over three consecutive runs (AlGaN HEMT on 2 inch SiC), an average Rs of 367.4 Ω/sq was achieved with a run-to-run standard deviation (σ) of 1.8% and an average thickness of 1.48 µm (σ = 0.46%). Over longer timeframes (~45 runs over 6 weeks), AlGaN composition for a set of basic HEMT runs was maintained at 30% with σ = 0.5%. In addition, the uniformity and repeatability of both the composition and thickness of the AlGaN barrier layer is critical for creating constant HEMT performance. Figure 2 shows composition and thickness maps of a reference, quasi-bulk AlGaN layer grown on a sapphire substrate.
To further the development of HEMTs, Emcore has entered a partnership with Cornell University, wherein Cornell processes submitted epiwafers into complete HEMT devices for evaluation. Through this collaboration, GaN HEMT device results on SiC (0.2 µm gates) show excellent DC and RF characteristics, with Imax >1 A/mm, Pout >7 W/mm at ~50% PAE (10 GHz), ft >80 GHz, fmax >110 GHz. Devices on silicon also show encouraging DC results, with early devices having Imax >600 mA/mm and a breakdown voltage >-50 V, with good pinch-off characteristics.
