AlN/GaN insulated gate HEMTs with HfO2
gate dielectric
AlN/GaN single heterojunction MOS-HEMTs grown by molecular beam epitaxy have been fabricated utilising HfO2 high-K dielectrics deposited by atomic layer deposition. Typical DC transfer characteristics of 1.3 mm gate length devices show a maximum drain current of 950 mA/mm and a transconductance of 210 mS/mm with gate currents of 5 mA/mm in pinch-off. Unity gain cutoff frequencies, ft and fmax, were measured to be 9 and 32 GHz, respectively.
Introduction: The AlN/GaN high electron mobility transistor (HEMT) is an attractive structure for extending the frequency performance of IIIN based transistors. This stems from the very thin AlN barrier layer that is utilised, which permits a significant reduction in gate length while maintaining an appropriate gate-to-channel aspect ratio to mitigate short channel effects. Recently, the AlN/GaN HEMT has shown its potential by demonstrating some of the highest current densities and
transconductances in a single heterostructure with appreciable cutoff frequencies. However, a major issue in such structures is high Schottky gate currents due to tunnelling through the thin AlN cap, which necessitates the insertion of a gate insulator. High-K dielectrics have matured to where they are now of interest in application to MOS-gated devices. In this Letter, the first demonstration of an ALD deposited HfO2 gate-dielectric AlN/GaN MOS-HEMT is presented. Owing to the insertion of a 3.6 nm HfO2 layer, the gate current in these devices was extremely low, typically 5 mA/mm. This is particularly noteworthy since this result was achieved in a structure with a thin 4.5 nm AlN barrier layer. In this Letter, we report on DC, RF and power performance of the AlN/GaN HEMT.
Device growth and fabrication: The AlN/GaN heterojunctions were grown by plasma assisted MBE on 2 inch diameter semi-insulating 4H-SiC substrates. The procedures for SiC substrate preparation are similar to those we have described elsewhere. The 60 nm-thick AlN nucleation layer was grown, followed by subsequent unintentionally doped (UID) GaN buffer and AlN barrier layers with thicknesses of 1 mm and 4.5 nm, respectively. The AlN barrier thickness was chosen on the basis of work by Cao et al. who showed a minimum sheet resistance in single heterojunction AlN/GaN HEMTs with AlN thicknesses between 3–5 nm [7]. The GaN growth rate was determined to be 1.2A˚ /s from optical reflectance measurements. All epitaxial layers were grown without interruptions. Contactless resistance measurements on the unprocessed sample showed sheet resistances in the range 600–800 V/A, which we speculate is due to an AlN cap layer thickness variation across the wafer surface. Device fabrication was initiated by e-beam evaporation of Ti/Al/Ni/Au contact metallisation, which were annealed at 8008C for 30 s to form ohmic contacts. Contact resistances were found to be in the range 0.8–1.1 V mm, determined by circular transfer length method measurements. Mesa isolation was achieved via a BCl3/Cl2 plasma etch. HfO2 films were deposited from tetrakis (ethylmethyl) amino hafnium (TEM AHf) and water at 2508C using a hot wall stainless steel flow tube atomic layer deposition (ALD) reactor. The fixed volume approach as described elsewhere was used for the delivery of both reagents. The Ni/Au gate metallisation was then defined and deposited through standard photolithography and e-beam evaporation. A BCl3/Cl2 plasma etch was applied to open windows in the HfO2 to access the ohmic metal for the final overlay metallisation. This was followed by the deposition of a 100 nm-thick PECVD SiN film and a subsequent photolithographic patterning and SF6 plasma etch to open windows to the overlay metal contacts for probing. Device characterisation: Hall effect measurements were performed both prior to HfO2 deposition and after gate metallisation. Rsh decreased on average by 12% through an increase in sheet density, suggesting the partial passivation of surface states. Room temperature 2DEG density and mobility after HfO2 deposition was found to be 1.48 _ 1013 cm22 and 620 cm2/Vs, respectively. No change in mobility was observed, which indicates the ALD HfO2 is a low damage process. Fig. 1 shows two-terminal diode current density for both Ni/Au-AlN and Ni/Au- HfO2 Schottky diodes with 150, 100 and 50 mm diameter diode dot sizes. A 7 to 8 order of magnitude reduction in diode current down to approximately 1 mA/cm2 was observed in the sample with the 3.6 nm HfO2 dielectric compared to the sample without the dielectric layer, indicating excellent gate-current suppression due to the dense ALD dielectric. It should be noted that, owing to high reverse bias Schottky diode current for the uninsulated Ni/Au-AlN diodes, output characteristics of HEMTs with such gates could not be taken and therefore are not compared. This is consistent with other reports in literature.
Representative drain characteristics for 1.3 mm gate length (LG), 150 mm gate width (WG), and 5 mm source–drain spacing MOSHEMT are shown in Fig. 2. As seen in the Figure, a current density of _800 mA/mm at VGS ¼ 0 V was measured. A maximum drain– source current of 950 mA/mm at VGS ¼ þ4 V and transconductance of 210 mS/mm at VDS ¼ þ10 V was measured despite the high sheet and contact resistances (not shown). Analysis of capacitance–voltage characteristics of the HfO2/AlN stacked capacitor confirmed the dielectric constant of the HfO2 to be _20, which is typical of ALD HfO2. As shown in Fig. 2a, the MOS-HEMTs have excellent pinch-off characteristics, demonstrating that leakage current through the AlN barrier is dramatically suppressed by the ALD HfO2 gate insulator. The threshold voltage is 24.1 V. In deep pinch-off (VGS ¼ 27 V, VDS ¼ 10 V, IDS ¼ 7 mA/mm) the gate leakage current was measured to be 5 mA/ mm (not shown). The off-state breakdown voltage was .25 V with the criterion of IDS ¼ 1 mA/mm under pinch-off conditions. Sparameter measurements at a quiescent bias of VDS ¼ 15 V and VGS ¼ 21.5 V yield a current gain cutoff frequency, ft, of 9 GHz and a power gain cutoff frequency, fmax, of 32 GHz, as shown in Fig. 2b. A Focus Microwaves load-pull system was used to measure the output power at 2 GHz. An output power of 2.6 W/mm and a PAE of 33% was measured.
Conclusions: An AlN/GaN MOS-HEMT employing a 3.6 nm ALD HfO2 gate dielectric has been demonstrated. Despite higher sheet and contact resistances, 1.3 mm gate lengths and a non-optimised growth, these devices showed maximum drain currents of 950 mA/mm and 210 mS/mm transconductance. Unity gain cutoff frequencies, ft and fmax were measured to be 9 and 32 GHz for 1.3 mm long gates, respectively. Most notable was the reduction in gate current to 5 mA/mm in deep pinch off conditions (VGS ¼ 27 V, VDS ¼ 10 V) in a structure with an overall barrier thickness of 8 nm. Such performance exemplifies the potential for ultrathin AlN/GaN heterostructures in conjunction with ALD HfO2 dielectric layers for the application to high-speed, highpower III-N technology.
Luiggi Marquez C.I 17677911 CRF
http://userpages.umbc.edu/~gougousi/papers/HfO2_HEMT.pdf
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