HVPE-GROWN AlGaN/GaN HEMTs

 

M. A. Mastro(1), D. Tsvetkov(1), V. Soukhoveev(1), A. Usikov(1), V. Dmitriev(1), B. Luo(2), F. Ren(2), K. H. Baik(3) and S. J. Pearton(3)

 

(1)   TDI, Inc

Silver Spring, MD 20904

(2)   Department of Chemical Engineering

University of Florida, Gainesville, FL 32611

(3)   Department of Materials Science and Engineering

University of Florida, Gainesville, FL 32611

 

 

ABSTRACT

            High quality undoped AlGaN/GaN high electron mobility transistors (HEMTs) structures have been grown by Hydride Vapor Phase Epitaxy (HVPE), for the first time.  The morphology of the films grown on Al2O3 substrates is excellent, with a root-mean-square roughness of ~0.2 nm over 10´10 mm2 measurement area.  Capacitance-voltage measurements show the formation of a dense sheet of charge at the AlGaN/GaN interface.  This is the first ever report of the formation of a two-dimensional electron gas (2DEG) in a nitride structure grown by HVPE.  HEMTs with 1mm gate length fabricated on these structures show transconductances in excess of 110 mS/mm and drain-source current above 0.6A/mm. 


INTRODUCTION

            Hydride Vapor Phase Epitaxy (HVPE) was one of the first growth methods employed for GaN(1) and has been successfully used for growth of thick, high-purity quasi-bulk substrates(2-17).  The Ga is usually transported to the growth surface as a volatile chlorine compound, while NH3 is used as the N-precursor.  The use of carbon-free precursors is a major advantage relative to other chemical vapor deposition techniques.  It is generally considered that O and Si are not responsible for the often-observed n-type conductivity, but that nitrogen vacancies may play a dominant role(4).  This suggests that careful control of the growth process should produce high purity device-quality GaN.  Recently it has been reported that submicron multi-layer AlGaN/GaN structures can be grown by HVPE (18). 

            In this paper we report on the characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) grown by HVPE on sapphire substrates.  HEMTs have been by far the most heavily investigated III-N electronic devices, largely because of their promise in the radiofrequency semiconductor market(19-23).  The power handling capability of AlGaN/GaN HEMTs is already far superior to the best achieved with GaAs devices(22).  To create a real high-volume, manufacturable technology for AlGaN/GaN HEMTs, it is desirable to explore high growth rate techniques such as HVPE, which would reduce the cost relative to the more commonly used Molecular Beam Epitaxy (MBE) and Metal Organic Chemical Vapor Deposition (MOCVD).  In addition, a single growth method could then be used for other device platforms, including high voltage GaN rectifiers for utility switching and hybrid-electric vehicles(24).

 

EXPERIMENTAL

The HEMT structure consisted of a 2 mm undoped GaN layer grown on sapphire, followed by 30 nm of Al0.22Ga0.78N.  All of the layers were grown by HVPE at 1020°C, as described previously(18).  The HEMTs were fabricated by a 4 step process consisting of dry etch mesa isolation, ohmic metallization, ohmic anneal and gate metallization as described in detail previously(25).  In brief, the mesa formation was performed by Cl2/Ar Inductively Coupled Plasma etching at 1mTorr with 300W of 2MHz source power and 40W of rf(13.56MHz) chuck power.  Ohmic metallization of Ti/Al/Pt/Au was deposited by e-beam evaporation and patterned by lift-off.  A low specific contact resistance of
10-6 W-cm2 was achieved after 850°C, 30sec annealing.  Gate contacts (1´100mm2) of Ni/Au were patterned by lift-off.  A schematic of the completed device is shown in Figure 1.  The dc characteristics were measured in common-source mode with an HP 4145B parameter analyzer.

 

RESULTS AND DISCUSSIONS

Figure 2 shows Atomic Force Microscope (AFM) scans from various areas of an as-grown HEMT structure.  The best root-mean-square (RMS) roughness measured over a 10´10mm2 area was ~0.22 nm as shown in the scan at right, whereas the roughest area was still < 1nm, as shown in the scan at left.  This is a clear demonstration that HVPE is capable of growing high quality AlGaN/GaN heterostructures with excellent surface morphology, which is a necessary condition for achieving sharp heterointerfaces.

            To give an example of the ability of HVPE to form a HEMT structure, Figure 3 shows a typical result of capacitance-voltage (C-V) measurements performed on-wafer on large-area contact pads.  The data is shown in C-2 vs V form at top and the resulting carrier concentration as a function of voltage at bottom.  This is a clear indication of the presence of a dense sheet of charge at the AlGaN/GaN heterointerface.

            The donor profile measured by Hg-probe C-V measurements at 10KHz on an as-grown calibration sample is shown in Figure 4.  Once again, this data shows a sheet of carriers at the AlGaN/GaN heterointerface which we ascribe to a polarization-induced two dimensional electron gas, just as is observed in MBE or MOCVD grown structures(19).  This is the first observation of a 2DEG in a nitride structure grown by HVPE.

The maximum drain current density measured for fabricated HEMT devices was > 0.6A/mm before self-heating effects caused a decrease in the output current.  Drain I-V characteristics are shown at the top of Figure 5.  The devices showed excellent pinch-off behavior, with maximum transconductance > 100mS/mm as seen in the transfer characteristics at the bottom of the figure.  From a limited set of data, we found that the peak of the extrinsic transconductance curve shifted to more positive gate voltage with increasing gate length and the magnitude of gm scaled with inverse gate length.  The performance of the HVPE grown HEMTs in clearly very promising when compared with MBE or MOCVD HEMTs with similar gate lengths.

Gate I-V characteristics are shown in Figure 6.  Further optimization of the layer structure and growth conditions is expected to produce larger reverse breakdown voltages and these initial layer designs were not focused on power applications.  Our experience with the HVPE growth is that there is a relatively broad range of conditions that produce device-quality material.

One of the existing problems with AlGaN/GaN HEMT technology is the so-called “current-collapse” phenomenon that occurs at high drive current.  This is caused in most cases by intrinsic surface traps in the gate-drain region that can create an additional depletion region which pinches off the current flow(26).  Figure 7 shows a 40-50% decrease in drain-source current during pulsing of the gate terminal with the drain biased at a fixed dc voltage.  These results are consistent with results reported for MBE and MOCVD AlGaN/GaN HEMTs, indicating that the HVPE material has similar surface properties(27).  The current collapse problem can be effectively mitigated by use of high quality passivating dielectrics and oxides(27-29). 

 

SUMMARY AND CONCLUSIONS

The HVPE technique is shown to be a very promising approach for growth of AlGaN/GaN HEMT structures.  The surface morphology is excellent after growth of layers with total thicknesses similar those employed in MBE and MOCVD growth.  Electrical measurements show formation of an abrupt sheet of charge at the heterointerface.  HEMTs fabricated with a standard processing sequence display excellent dc characteristics and indicate that HVPE is an attractive, high throughput, low cost approach to growth of nitride-based devices.  Gate lag measurements show similar current collapse characteristics to HEMTs fabricated on MBE- or MOCVD grown material, indicating that the HVPE-grown material has similar surface quality.

 

 

 

ACKNOWLEDGMENTS

            The authors thank N. Kuznetsov for capacitance-voltage measurements by Hg-probe.  The work at both TDI and UF is partially supported by MDA and managed by the Office of Naval Research (STTR grant number N00014-02-M-0287) under the supervision of Dr. Colin E.C. Wood.

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Figure Captions

Figure 1.   Schematic of completed HEMT.

Figure 2.   AFM scans from 30nm Al0.22Ga0.78N / 2µm undoped GaN grown by HVPE on sapphire.

Figure 3.   C-2 vs V and resultant N vs V for HVPE-grown Al0.22Ga0.78N/GaN HEMT structure.

Figure 4.   ND-NA concentration profile measured by Hg probe at 10KHz for Al0.22Ga0.78N/GaN HEMT structure at different parts on the wafer.

Figure 5.   Drain I-V characteristics (top) and transfer characteristics (bottom) from HVPE-grown, 1´100mm2 HEMT.

Figure 6.   Gate I-V characteristics from 1´100mm2 HVPE-grown HEMT.

Figure 7.   Gate lag measurements on unpassivated HVPE-grown HEMT.