Traps in GaN-based Microwave Devices

P.B. Klein,¹ S.C. Binari,¹ K. Ikossi,¹ D.D. Koleske,² A.E. Wickenden,³ and R.L. Henry¹
¹Electronics Science and Technology Division
²Sandia National Laboratories
³Army Research Laboratory

Introduction: The Navy's requirement for a new generation of high-power, high-frequency, solid-state amplifiers for long-range detection cannot be met by current Si and GaAs device technologies because of limitations in the basic materials properties. A promising material system is the nitrogen-based widebandgap semiconductors (AlN, GaN, InN, and their alloys). In addition to excellent thermal and electron transport properties, these materials support the growth of a high-quality hetero-interface, such as AlGaN/GaN. Such heterostructures are necessary for the formation of a two-dimensional electron gas (2DEG)-a thin, high-mobility channel confining carriers to the AlGaN/GaN interface region. While excellent device characteristics have been reported for these high electron mobility transistors (HEMTs), they have not been incorporated into Navy systems because these characteristics are not always reproducible -a result of deep traps in the nitride material. A deep trap may be regarded as an impurity or crystal defect that captures a mobile charge carrier and keeps it strongly localized in the neighborhood of the trapping center. Deep traps can produce current collapse, a distortion of the device current-voltage (I-V) characteristic that is of particular concern because it ultimately limits the output power of the device. To eliminate the trapping centers that cause this phenomenon, the responsible defects must be detected, characterized, and identified. In this article, we describe a unique optical technique that has been developed through a collaboration of materials growth, device fabrication, and device characterization that now provides this capability.

FIGURE 4
Current collapse in an AlGaN/GaN HEMT structure due to trapping of hot carriers injected into the high resistivity GaN as the result of a high drain-source bias. The collapsed drain current is restored by light illumination through photoionization of the trapped carrier.

Current Collapse: When a large bias voltage is applied between the source and drain of a field effect transistor (FET) (such as the HEMT structure depicted in Fig. 4), the electrons in the conducting channel are rapidly accelerated. These "hot carriers" gain enough kinetic energy from the large electric field to be injected into an adjacent region of the device structure. If this region contains a significant concentration of traps, the injected carriers can become trapped. The resulting reduction in drain current, referred to as "current collapse," can severely compromise the performance of a microwave FET. In the case of the HEMT in Fig. 4, the hot carriers are trapped in the high-resistivity GaN buffer layer, which is known to contain a high trap concentration. The collapsed drain current can be restored by light illumination, which frees (photoionizes) the carriers from the traps. Since the trapped carriers represent a negative charge distribution in the GaN, the resulting transverse electric field causes the photoionized carriers to rapidly drift back to the conducting channel, thus restoring the drain current. This light-induced increase in the collapsed drain current is the basis for the photoionization spectroscopy technique that enables the detection and characterization of the responsible traps.¹

FIGURE 5
Spectral dependence of the normalized drain current increase S(hn), induced by light illumination of an AlGaN/GaN HEMT and a GaN MESFET.

Fig6 Image

FIGURE 6
Dependence of the concentration of traps responsible for current collapse in AlGaN/ GaN HEMT structures on the reactor pressure used during organometallic vapor phase epitaxial growth of the high-resistivity GaN layer. The variation of the Trap2 concentration tracks that of carbon impurities in the layer (as measured by SIMS), indicating that a carbonrelated defect introduced during growth is responsible for Trap2. (From Ref. 3.)

Photoionization Spectroscopy: Measurement of the wavelength dependence of this light-induced drain current increase, normalized by the amount of incident light, has been shown to reproduce the photoionization spectrum of the trap.2 This reflects the absorption spectrum associated with the ionization of the carrier from the trap and is a unique characteristic of a given trap. Consequently, this spectrum can be used as a signature of the trap. Figure 5 shows such spectra for an AlGaN/GaN HEMT structure and a GaN metal semiconductor FET (MESFET) that uses a thick n-type (electrons from impurities carry the current) GaN conducting channel in place of the 2DEG of the HEMT. In addition to the expected absorption at the GaN bandgap, two broad absorption bands are observed below the gap, associated with two distinct traps (labeled Trap1 and Trap2). The deduced absorption threshold energies reveal that Trap1 is roughly situated at midgap, while Trap2 is very deep (roughly 0.5 eV above the valence band). Similar absorptions observed in both types of devices indicate that the same traps (in the GaN buffer layer) are responsible for current collapse in both devices. The lack of any response at the bandgap of the AlGaN confirms the location of these traps in the GaN.

Modifying these measurements to investigate, at a fixed wavelength, the variation of the drain current increase with the amount of incident light has shown that the areal concentration of each trap can be determined. ² The technique was applied to devices fabricated on a set of four wafers, each prepared with a different trap concentration by growing the GaN buffer layer at a different growth pressure. Figure 6 shows the variations in the resulting trap concentrations with growth pressure.³ The concentration of Trap2 was found proportional to the concentration of carbon in the layers, as determined from secondary ion mass spectrometry (SIMS) measurements shown in the figure. This identifies Trap2 as a carbon- related defect.

Conclusions: Photoionization spectroscopy is a powerful tool for investigating traps that cause current collapse in electronic device structures. The technique provides a unique signature for each trap, as well as detailed trap characteristics. The technique is currently being expanded to investigate other traprelated phenomena occurring in nitride-based microwave devices.

[Sponsored by ONR]

References
¹ P.B. Klein, J.A. Freitas, Jr., S.C. Binari, and A.E. Wickenden, "Observation of Deep Traps Responsible for Current Collapse in GaN Metal Semiconductor Field Effect Transistors," Appl. Phys. Lett. 75, 4016 (1999).
² P.B. Klein, S.C. Binari, J.A. Freitas, Jr., and A.E. Wickenden, "Photoionization Spectroscopy of Traps in GaN Metal Semiconductor Field Effect Transistors," J. Appl. Phys. 88, 2843 (2000).
³ P.B. Klein, S.C. Binari, K. Ikossi, A.E. Wickenden, D.D.Koleske, and R.L. Henry, "Current Collapse and the Role of Carbon in AlGaN/GaN High Electron Mobility Transistors Grown by Metalorganic Vapor Phase Epitaxy," Appl. Phys. Lett. 79, 3527 (2001).