Electron tunneling spectroscopy study of electrically active traps in AlGaN/GaN highelectron mobility transistorsJie Yang, Sharon Cui, T. P. Ma, Ting-Hsiang Hung, Digbijoy Nath, Sriram Krishnamoorthy, and Siddharth Rajan Citation: Applied Physics Letters 103, 223507 (2013); doi: 10.1063/1.4834698 View online: http://dx.doi.org/10.1063/1.4834698 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/22?ver=pdfcov Published by the AIP Publishing

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Electron tunneling spectroscopy study of electrically active traps inAlGaN/GaN high electron mobility transistors

Jie Yang,1,a) Sharon Cui,1 T. P. Ma,1 Ting-Hsiang Hung,2 Digbijoy Nath,2

Sriram Krishnamoorthy,2 and Siddharth Rajan2

1Department of Electrical Engineering, Yale University, New Haven, Connecticut 06520, USA2Department of Electrical and Computer Engineering, Ohio State University, Columbus, Ohio 43210, USA

(Received 16 July 2013; accepted 12 November 2013; published online 25 November 2013)

We investigate the energy levels of electron traps in AlGaN/GaN high electron mobility transistors

by the use of electron tunneling spectroscopy. Detailed analysis of a typical spectrum, obtained in a

wide gate bias range and with both bias polarities, suggests the existence of electron traps both in

the bulk of AlGaN and at the AlGaN/GaN interface. The energy levels of the electron traps have

been determined to lie within a 0.5 eV band below the conduction band minimum of AlGaN, and

there is strong evidence suggesting that these traps contribute to Frenkel-Poole conduction through

the AlGaN barrier. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4834698]

AlGaN/GaN based High Electron Mobility Transistors

(HEMTs) have been extensively studied because of their ad-

vantageous properties for high frequency1–4 and high power5–7

applications, including high densities of two-dimensional elec-

tron gas (2DEG), high electron mobility, high break-down volt-

age, and wide band gap.8 However, the density of the 2DEG in

the channel of an AlGaN/GaN HEMT is often altered by traps

on the surface and in the bulk of the heterostructure, causing

severe degradation of device performance.9,10 Attempts have

been made to probe and characterize these traps. Among the

most popular methods used in this research field are the photo-

ionization spectroscopy11 and the deep level transient spectros-

copy (DLTS).12 However, these methods appear to mainly

focus on the deep level traps located in the GaN substrate, nei-

ther in the AlGaN layer nor at the AlGaN/GaN interface.

In this work, we use Electron Tunneling Spectroscopy

(ETS)13–20 to probe the electrically active traps in an

AlGaN/GaN HEMT structure. By identifying the ETS fea-

tures that are associated with traps (as opposed to phonon or

impurity features), aided by the energy band diagram of the

AlGaN/GaN HEMT, we deduce the energy distribution of

electrically active traps.

Fig. 1 shows (a) the schematic cross-section of the device

under study, (b) the corresponding energy band-diagram at

zero bias, and (c) the AlGaN surface morphology as revealed

by AFM imaging. The device fabrication process has been

reported elsewhere.21,22 Basically, a 70 nm AlN nucleation

layer is grown on top of the Si-face of a SiC substrate, fol-

lowed by the deposition of a 150 nm-thick rough GaN layer.

Then, a 300 nm-thick smooth GaN layer and a 23 nm-thick

AlGaN layer are sequentially grown in a Veeco rf-plasma mo-

lecular beam epitaxy system. By the use of an e-beam evapo-

rator, Ti/Al/Ni/Au alloy is made to form the Ohmic contact,

and Ni/Au is deposited as the gate metal. The I-V characteris-

tics for the HEMT device made under the same processing

condition is shown in Fig. 2. A relatively large circular gate

electrode with a radius of 150 lm is used for the ETS

measurements. An ETS spectrum, such as the one shown in

Fig. 3, is a plot of the second derivative of the I-V curve aris-

ing from electron tunneling through a barrier. In general, an

ETS spectrum carries a wealth of information about the bar-

rier, including phonons, microstructures, impurities, and elec-

trically active traps.13–20 This work focuses on the traps.

When tunneling electrons interact with traps, two types of trap

features may be observed13 as shown in the inset of Fig. 3:

one has the feature of “peak-followed-by-valley,” arising

from trap-assisted conduction; the other has the feature of

“valley-followed-by-peak,” arising from charge trapping. The

key instrument for the measurement setup is an SR830 DSP

lock-in amplifier, and the voltage error along the X-axis for

the ETS measurement is 0.1 mV.

It should be noted that the ETS spectrum shown in

Fig. 3 is obtained on a representative AlGaN/GaN HEMT

device as the gate voltage is swept from �1 V to þ1 V.

Among the three samples measured, the variations of the

voltage locations for the major features are less than 5 mV.

In a close-up view, the spectrum in Fig. 3 may be divided

into three characteristic regions. Though containing some

weak trap features, the narrow region B (from 0 V to

þ0.1 V) is basically dominated by phonon vibration modes,

FIG. 1. (a) Schematic cross-section of the AlGaN/GaN HEMT; (b) band dia-

gram at zero bias; and (c) surface morphology.

a)Author to whom correspondence should be addressed. Electronic mail:

jie.yang@yale.edu. Telephone: þ1 203-843-4211

0003-6951/2013/103(22)/223507/4/$30.00 VC 2013 AIP Publishing LLC103, 223507-1

APPLIED PHYSICS LETTERS 103, 223507 (2013)

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and therefore region B is not the focus of this work.

Meanwhile, region C (Vg¼ 0 V to �1 V) and region A

(Vg¼þ0.1 V to þ1.0 V) appear to be in sharp contrast: the

former shows strong trap features that primarily exhibit

“peak-followed-by-valley” behavior, while the latter is basi-

cally flat with hardly any discernible trap features. In the

next paragraph, we analyze each characteristic region by the

use of the energy band diagram of the AlGaN/GaN HEMT.

Shown in Figs. 4(a) and 4(b) are the schematic band dia-

grams at gate biases of þ0.1 V and þ1.0 V, respectively, cor-

responding to the beginning and the end of region A as

shown on the right. At a positive gate bias, electrons are

injected from the 2DEG into the AlGaN, moving towards the

gate electrode. If we use an arrow in Fig. 4(a) to indicate the

electron-tunneling path when the gate is biased at þ0.1 V,

then the grey area in Fig. 4(b) covers all the electron tunnel-

ing trajectories as the gate bias sweeps from þ0.1 V to

þ1.0 V. Were there electrically active traps in the tunneling

paths, interactions between the passing electrons, and the

traps would produce trap features in the ETS spectrum. The

lack of trap features in region A suggests that the grey area

in Fig. 4(b) represents a trap free energy zone, whose upper

bound lies around 0.5 eV below the conduction band mini-

mum (Ec) of the AlGaN. In other words, if there are electri-

cally active traps in the AlGaN barrier, their energy levels

should only be within this 0.5 eV band above the grey zone.

Figure 5 shows the energy band diagrams as the gate

electrode is biased from 0 V to �1 V (corresponding to

region C). In the negative bias polarity, electrons are injected

from the gate electrode toward the 2DEG. Fig. 5(a) shows an

example as the gate is biased at �0.35 V. As seen, once

injected from the gate, electrons enter the grey zone immedi-

ately, where they interact with no traps until they reach the

AlGaN/GaN interface. The fact that there is a 0.5 eV

FIG. 2. Representative (a) family of

Ids-Vds curves, (b) corresponding

Ids-Vgs transfer characteristics and

transconductance, for a HEMT device

with Lg¼ 1 lm, Lgs¼Lgd¼ 3 lm, and

W¼ 50 lm.

FIG. 4. Schematic band diagrams for

(a) Vg¼þ0.1 V; (b) Vg¼þ1 V, along

with the ETS spectrum shown on the

right. The solid arrow across the bar-

rier represents elastic tunneling path.

The grey area in 3(b) covers elastic

tunneling paths as Vg sweeps from

þ0.1 V to þ1.0 V.

FIG. 3. An ETS spectrum obtained on an AlGaN/GaN HEMT for gate volt-

age sweep from þ1 V to �1 V. The inset shows two types of the trap-

induced features: “peak-followed-by-valley” due to trap assisted conduction

and “valley-followed-by-peak” due to charge trapping.

223507-2 Yang et al. Appl. Phys. Lett. 103, 223507 (2013)

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conduction band offset between GaN and AlGaN (refer to

Fig. 1(b)) makes the Fermi level at the interface fall into the

aforementioned 0.5 eV energy band, and the interactions

between electrons and the interface traps generate trap fea-

tures in the corresponding ETS spectrum. In Fig. 5(a), we

use the symbol X to highlight the interaction at 0.35 eV

above the Ec of GaN, which is associated with the trap fea-

ture at Vg¼�0.35 V in the ETS displayed on the right.

As the gate voltage sweeps further negative to a level

higher than the AlGaN/GaN conduction band offset (0.5 eV),

as exemplified in Fig. 5(b), electrons getting out of the grey

zone would interact with those traps in AlGaN with energy

levels in the 0.5 eV band before reaching the AlGaN/GaN

interface. Similarly, in Fig. 5(b), we use heavy (X) symbols

to depict possible electron-trap interactions that produce

those pronounced ETS trap features in the region between

Vg¼�0.7 V and �0.9 V, displayed on the right.

Based on the analysis described above, we summarize in

Fig. 6, the trap energy distribution in the AlGaN/GaN

HEMT device under study. It is consistent with the results in

Refs. 23 and 24. Further studies of the temperature depend-

ence of the I-V characteristics through the AlGaN barrier

indicate primarily Frenkel-Poole (F-P) conduction mecha-

nism, as evidenced by the excellent fit of the data on the

ln(J/EAlGaN) vs. EAlGaN1/2 plots in Ref. 24. The trap energies

have been extracted from the aforementioned plots, as also

shown in Ref. 24.

In summary, by the use of the Electron Tunneling

Spectroscopy, in conjunction with the AlGaN/GaN energy

band diagrams, we have determined that the majority of elec-

trically active traps are energetically located within a 0.5 eV

band below the Ec of the AlGaN barrier. Spatially, these

traps are located both in the bulk of AlGaN and at the

AlGaN/GaN interface. The temperature dependence of the

I-V characteristics suggests that the current flowing through

the AlGaN barrier follows the F-P conduction mechanism.

This work was supported partially by the Office of

Naval Research (ONR) under the MURI DEFINE program

and partially by the National Science Foundation (NSF)

under Contract No. MRSEC DMR 1119826.

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FIG. 5. Schematic band diagrams for

region C (0 V to �1 V): (a) an example

at Vg¼�0.35 V; (b) an example when

Vg surpasses the band offset (0.5 eV).

Heavy symbols depict possible

electron-trap interactions that produce

those pronounced ETS trap features

shown on the right of each diagram.

The traps in (a) are near the interface

of AlGaN/GaN, while the traps probed

beyond 0.5 V in (b) are predominantly

in the bulk of the AlGaN epilayer.

FIG. 6. The energy distribution deduced by ETS analysis of the electrically

active traps in the AlGaN/GaN sample. The grey area represents the trap

free energy zone. The red shaded region represents the 0.5 eV trap energy

band below Ec of AlGaN.

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