Process development of AlN/GaN MOS-HEMTs is presented, along with issues and problems concerning the fabrication processes. The developed technology uses thermally grown Al2O3 as a gate dielectric and surface passivation for devices. Significant improvement in device performance was observed using the following techniques: (1) Ohmic contact optimisation using Al wet etch prior to Ohmic metal deposition and (2) mesa sidewall passivation. DC and RF performance of the fabricated devices will be presented and discussed in this paper.
The search for improved high power and high frequency performance has called attention to the most recent development in aluminium nitride/gallium nitride- (AlN/GaN-) based high electron mobility transistors (HEMTs) which target future microwave power devices. Key properties of this material system are high 2DEG sheet carrier concentration at the heterojunction interface, high carrier electron velocity, and large electric breakdown field, and so superior performance compared to conventional AlGaN/GaN devices could be achieved. With improvements in material growth and processing techniques, record performances made in this material system include 2DEG sheet carrier concentration over 3 × 1013 cm−2 with very low sheet resistance,
Despite the demonstrated potential, problems such as surface sensitivity [
In this paper, the process development of AlN/GaN MOS-HEMT technology will be reviewed and discussed. The devices discussed here employ thermally grown Al2O3 as a gate dielectric and surface passivation [
Optimal performance of AlN/GaN-based HEMT devices requires the use of low-resistance, thermally stable Ohmic contacts with good surface morphology. This is required for the following reasons: (1) to obtain the maximum value of drain current,
Ohmic contacts on both protected (with 2 nm evaporated Al which is later oxidised to form Al2O3) and unprotected (as grown) AlN/GaN samples were fabricated and characterised. Figure
Summary of results for the optimised
Sample | Description | ||
---|---|---|---|
A | Unprotected and unpassivated (HEMT) | 0.31 | 480 |
B2 | Protected and passivated (MOS-HEMT) | 0.49 | 159 |
Optimised TLM processing summary for unprotected and unpassivated AlN/GaN HEMT samples. Processing includes (a) sample cleaning with acetone, isopropanol, and deionised water, (b) deoxidation, (c) Ohmic metallisation, (d) Ohmic annealing, and (e) TLM measurements.
Optimised TLM processing summary for protected and passivated AlN/GaN MOS-HEMT samples. Processing includes (a) sample cleaning with only deionised water and deoxidation, (b) 2 nm Al deposition, (c) etching Al from Ohmic contact regions, (d) thermal oxidation of Al, (e) Ohmic metallisation, (f) Ohmic annealing, and (g) TLM measurements.
By employing the structure in Figure
Current-voltage (
Comparison of Ohmic contact resistance,
A gate wrap-around layout technique [
SEM micrograph of completed gate wrap-around MOS-HEMT layout. Inset: Device with
AlN/GaN structures are known to be very sensitive to processing liquids, and so unprotected and unpassivated AlGaN/GaN HEMTs (from same/similar growth conditions) were also processed and fabricated to provide comparative data. DC measurements were done by contacting the probe needles directly on top of the source (S), drain (D), and gate (G) structures. All measurements were made at room temperature using Agilent’s B1500A Semiconductor Parameter Analyzer. Figure
These results, together with the TLM results described in the previous subsection, showed that there were some issues with processing of AlN/GaN HEMT structure which are not seen in AlGaN/GaN HEMTs. Exposure to different processing chemicals such as resist developer and solvents solutions could help reduce the Ohmic contact resistance but at the same time this may have led to the degradation of the quality of the AlN/GaN epilayer structures. Similar observations were made by Fan et al. [
A new process for the fabrication AlN/GaN-based devices was therefore developed. It involved employing thermally grown Al2O3 for protection of the very sensitive AlN epilayer from exposure to liquid chemicals during processing [
Process flow for fabrication of protected and passivated AlN/GaN MOS-HEMTs using the gate wrap-around technique. Processing includes (a) sample cleaning and deoxidation, (b) 2 nm Al deposition, (c) etching Ohmic regions and thermal oxidation of Al, (d) Ohmic metallisation and annealing, and (e) gate metallisation and device measurements.
To further directly explore the impact of Ohmic contacts optimisation on device performance, devices were fabricated in which the etching time of the Al in Ohmic contact region was varied. Figure
The developed process technology was extended to realise AlN/GaN MOS-HEMTs using the conventional mesa isolation technique for devices. The process flow is similar to that for the gate wrap-around devices (Figure
Schematics cross-section of fabricated MOS-HEMT (a) without mesa sidewalls edge passivation, (b) with mesa sidewalls edge passivation, and (c) top-view SEM micrograph of completed two-finger 2.5
Initially, mesa devices were fabricated without mesa sidewall passivation and with an unoptimised Ohmic contact process. Figure
(a)
The small signal RF performance of this device was also measured (not shown here). A unity current gain cutoff frequency,
(a)
The processing of AlN/GaN-based HEMTs has been described and discussed. The sensitivity of the AlN/GaN epitaxial layer structure necessitated the introduction of special processing requirements and the use of thermally grown Al2O3 as a gate dielectric and device passivation. Excellent DC and RF characteristics on AlN/GaN MOS-HEMTs were achieved but further reduction in the Ohmic contact resistance is still required before the full potential of this material system can be realised. The achieved results indicate the potential of AlN/GaN MOS-HEMT technology for high frequency and high power applications.
The authors would like to thank staff at the James Watt Nanofabrication Centre (JWNC), University of Glasgow, for supporting and assisting this work.