INFLUENCE OF MgO AND Sc2O3 PASSIVATION ON AlGaN/GaN HIGH ELECTRON MOBILITY TRANSISTORS

 

B. Luo, J. W. Johnson, J. Kim, R. M. Mehandru, and F. Ren

Department of Chemical Engineering

University of Florida, Gainesville, FL 32611

 

B. P. Gila, A. H. Onstine, C. R. Abernathy and S. J. Pearton

Department of Materials Science and Engineering

University of Florida, Gainesville, FL 32611

 

A. G. Baca, R. D. Briggs, R. J. Shul and C. Monier

Sandia National Laboratories, Albuquerque, NM 87185

 

J. Han

Yale University, New Haven, CT 06520

 

ABSTRACT

            Unpassivated AlGaN/GaN high-electron-mobility transistors show significant gate lag effects due to the presence of surface states in the region between the gate and drain contact.  Low -temperature (100°C) layers of MgO or Sc2O3 deposited by plasma-assisted Molecular -Beam Epitaxy are shown to effectively mitigate the collapse in drain current through passivation of the surface traps.  These dielectrics may have advantages over the more conventional SiNX passivation in terms of long-term device stability.

 

Remarkable progress has been made in recent years in high performance AlGaN/GaN high electron mobility transistors (HEMTs) grown a variety of substrates, including sapphire, SiC and Si[1-18].  The extremely high current density achievable with AlGaN/GaN heterostructures (up to 1.6A×cm-1 has been reported) is a result of the large polarization-induced field and large conduction band offset in the AlGaN/GaN system[9, 13].  This polarization field has both a piezoelectric strain-induced component in addition to a spontaneous polarization component.  For the AlGaN/GaN HEMTs most widely studied to date, the spontaneous polarization field dominates.  Even in the absence of modulation doping, this built-in field induces a two-dimensional electron gas whose density (nS) linearly proportional to the Al mole fraction, X, as nS = X (5´1013 cm-2)[14].  In the ~30% Al concentration most widely studied, a channel sheet electron density of ~1.5´1013 cm-2 can be realized that is a factor of 5-10 times higher than typical GaAs or InP HEMTs[17].  Over the past few years, dramatic progress has been made in understanding the AlGaN/GaN HEMT device physics and in demonstrating excellent microwave power performance[14].

One frequently reported problem for these devices is that the rf power obtained is still much lower than expected from the dc characteristics[5, 7, 19-23].  This problem is manifested by a collapse in drain current or frequency dispersions in transconductance and output resistance, leading to severely reduced output power and power-added efficiency.  Several mechanisms have been identified, including the presence of surface states between the gate and drain which deplete the channel in this region with time constant long enough to disrupt modulation of the channel charge during large signal operation[19] or of trap states in the buffer layer[8].  Several studies have shown that the use of SiNX passivation layers can be effective in reducing the effects of surface states[8, 19, 22].  One drawback of typical plasma enhanced chemical vapor deposited SiNX is the high hydrogen content which could migrate into the GaN or the gate metallization.  Two alternative candidates for HEMT passivation are MgO and Sc2O3, which are under development as gate dielectrics for GaN[24].  These materials have larger bandgaps (8eV for MgO, 6.3eV for Sc2O3) than the previously reported Gd2O3(5.3eV) and smaller lattice mismatches to GaN (6.5% for MgO, 9.2% for Sc2O3 versus 20% for Gd2O3)[25-27].  We find that layers of these materials deposited by Molecular Beam Epitaxy (MBE) on AlGaN/GaN HEMTs prevent much of the gate lag response found in unpassivated devices.

The HEMT structures were grown on c-plane sapphire substrates by Metal Organic Chemical Vapor Deposition.  The layer structures and processing and dc performance of similar devices has been described in detail previously[28, 29].  The 100Å thick MgO and Sc2O3 layers were deposited on completed 1.2´100mm2 HEMTs, also using plasma-assisted MBE.  Oxygen was supplied from an Electron Cyclotron Resonance (ECR) source operating at 2.45GHz, 200W forward power and 10-4 Torr pressure.  Effusion cells operating at ~1150°C for Sc and 380°C for Mg provided the metal flux.  We found the optimum surface cleaning was a 25 min exposure to UV/O3, heating to 300°C and then deposition of the MgO or Sc2O3 at 100°C[30].  In separate experiments we also measured the interface trap density using both the Terman and conductance methods on MgO/n-GaN and Sc2O3/n-GaN diodes.  These measurements were carried out from 25-300°C for the MgO/GaN and at 25°C for the Sc2O3/GaN.  These capacitance and conductance measurements were performed with a precision HP 4284 LCR meter, while the HEMT dc characteristics were measured in both dc and pulsed mode using a HP 4145B parameter analyzer for the dc and a pulse generator, dc power supply and oscilloscope for the pulsed measurements.  The rf performance of the HEMTs showed typical fT values of 12GHz and fMAX of 22GHz prior to passivation.

Figure 1 shows the interface state densities obtained from the ac conductance measurements on diodes.  The MOS diodes showed breakdown fields of 1.2MV×cm-1 (MgO) and 1.5MV×cm-1 (Sc2O3) at a current density of 1mA×cm-2.  The capacitance-voltage measurements showed clear charge modulation from accumulation to depletion and deep depletion at 25°C for both types of diodes.  The dielectric constants were calculated to be 10.5 (MgO) and 14.2(Sc2O3), in close agreement with tabulated values (9.8 for MgO, 14.5 for Sc2O3).  The Terman method yielded slightly lower (10-15%) values for interface state densities than the values shown in Figure 1.  The data in this figure give us confidence that both oxides are well-suited for GaN device passivation.

We have employed gate lag measurements on the HEMTs as a metric for establishing the effectiveness of the oxide passivation[8].  In this method, the drain current (IDS) response to a pulsed gate-source voltage (VG) is measured.  Figure 2(top) shows the normalized IDS as a function of drain-source voltage (VDS) for both dc and pulsed measurements.  In the data, VG was pulsed from –5V to 0 at 0.1MHz and 10% duty cycle.  The bottom of Figure 2 shows the normalized IDS for gate-source voltage, VG, switched from –5V to 0V with VDS held constant at a low value (3V) to avoid the complications of device heating.  The large differences between dc and pulsed drain currents is consistent with the presence of surface traps that deplete the channel in the access regions between the gate and drain contacts[19, 21].

After MgO deposition, the HEMTs showed an increase in drain-source current of 20% in the dc mode which is consistent with passivation of surface states[30]. Figure 3 shows the corresponding gate-lag measurements, with a marked improvement in drain current response relative to the unpassivated device.  This is clear evidence for the assumption that surface states are the cause of the gate-lag phenomena and also that MgO passivation mitigates this problem.

Similar results are shown in Figure 4 for Sc2O3 passivated HEMTs.  Once again there is a dramatic improvement on the drain current response.  A comparison of the data from MgO passivation shows the latter was somewhat more effective in increasing IDS, which we suggest is due to an increase in positive charge at the MgO/GaN (or MgO/AlGaN) interface resulting in an increase in effective sheet carrier density in the channel.  Future work should focus on large signal measurements in which we would expect to observe large increases in saturated power and power-added efficiency.

In summary, MBE-deposited MgO and Sc2O3 are found to dramatically mitigate gate-lag problems due to surface states on AlGaN/GaN HEMTs.  The absence of hydrogen in these dielectrics makes them attractive candidates for long-term stable passivation of the HEMTs.

ACKNOWLEDGEMENTS

            The work at UF is partially supported by ONR (N00014-98-1-02-04, J. C. Zolper) and NSF (DMR 0101438, CTS 9901173).  Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin company, for the US Department of Energy under contract number DEAC04-94AL85000.

 

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Figure Captions

Figure 1. Interface state density obtained from conductance measurements as a function of temperature on MgO/n-GaN and Sc2O3/n-GaN MOS diodes.

Figure 2. Gate lag measurements on unpassivated AlGaN/GaN HEMTs.  At top, VG is switched from –5V to 0V, while at bottom it is switched from –5V to the value shown on the X-axis.

Figure 3. Gate lag measurements on MgO passivated AlGaN/GaN HEMTs.  At top, VG is switched from –5V to 0V, while at bottom it is switched from –5V to the value shown on the X-axis.

Figure 4. Gate lag measurements on Sc2O3 passivated AlGaN/GaN HEMTs.  At top, VG is switched from –5V to 0V, while at bottom it is switched from –5V to the value shown on the X-axis.