© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

p s scurrent topics in solid state physics

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caphys. stat. sol. (c) 5, No. 6, 1929–1931 (2008) / DOI 10.1002/pssc.200778451

Enhancement-mode AlGaN/GaN HEMTs with thin InGaN cap layer

M. Ito1, S. Kishimoto1, F. Nakamura2, and T. Mizutani*,1

1 Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 2 POWDEC K.K. 500 Hagizono, Chigasaki 253-8543, Japan

Received 4 September 2007, accepted 26 November 2007

Published online 27 March 2008

PACS 72.80.Ey, 73.40.Kp, 73.61.Ey, 85.30.De, 85.30.Tv

* Corresponding author: e-mail tmizu@nuee.nagoya-u.ac.jp, Phone: +81-52-789-5230, Fax: +81-52-789-5232

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction AlGaN/GaN HEMTs receive consid-erable attention for high-power and high-frequency appli-cations because of the advantages of a high breakdown voltage and a high electron velocity. In order to apply them to the high-power switching system, normally-off HEMTs are indispensable to realize fail-safe system to avoid the problem of the circuit burn out when the gate signal be-comes ground voltage [1]. The conventional normally-off HEMTs require a precise etching control for recess gate [2] or heavy p+-doping for the junction gate [3]. In addition to these process requirements, the normally-off HEMTs sometimes show a problem of transconductance (gm) de-crease due to a large parasitic resistance. Recently, the normally-off HEMTs fabricated by fluoride-based plasma treatment have been reported [4-6]. This process seems promising because it does not require such processes. However, understandings of the mechanism are not suffi-cient. In addition to that, it is not clear at present whether the fluorine atoms incorporated in the AlGaN barrier layer are stable or not even though the threshold voltage of the device is stable for 80 days at 200 °C [7]. Further study is required for these devices to be applied to the real systems. Another approach to implement the normally-off HEMTs

is to take the advantage of the polarization of the III-nitride material system. In this paper, we propose to introduce a thin InGaN cap layer on a conventional AlGaN/GaN HEMT structure

Figure 1 Schematic cross section of the AlGaN/GaN HEMTs

with InGaN cap layer.

to implement the normally-off HEMTs. Polarization- in-duced field in the InGaN cap layer is expected to raise the conduction band of the AlGaN/GaN interface leading to the threshold voltage shift to the positive direction. It will be possible to decrease the parasitic source resistance by removing the InGaN cap layer at the access region.

AlGaN/GaN HEMTs with a thin InGaN cap layer has been

proposed to implement the normally-off HEMTs. The key

idea is to employ the polarization-induced field in the InGaN

cap layer, by which the conduction band is raised leading to

the normally- off operation. Fabricated HEMT with an

In0.2Ga0.8N cap layer with a thickness of 5 nm showed nor-

mally-off operation with a threshold voltage of 0.4 V and a

maximum transconductance of 85 mS/mm forthe device with

a 1.9-µm-long gate. By etching-off the In0.2Ga0.8N cap layer

at the access region using gate electrode as an etching mask,

the maximum transconductance has increased from 85 to 130

mS/mm due to a reduction of the parasitic source resistance.

The transconductance was increased from 130 to 145 mS/mm

by annealing the devices at 250 °C for 20 minutes in a N2 at-

mosphere.

i-GaN

Sapphire

n-AlGaN

InGaN Source Drain

Gate

PSP

PPE

PSP

PPE

PSP

1930 M. Ito et al.: AlGaN/GaN HEMTs with thin InGaN cap layer

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

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2 Device structure A schematic cross section of the proposed HEMTs is shown in Fig. 1. Thin layer of InGaN is grown on a conventional n-Al0.25Ga0.75N/i-GaN HEMT structure to induce the polarization-induced field in the InGaN cap layer. The directions of the polarization are shown by arrows. Even though the direction of spontane-ous polarization (PSP) is same for all layers, the directions of piezo-electric polarization (PPE) are opposite

Figure 2 Magnitude of polarization in InGaN cap layer.

Figure 3 Energy band diagrams of the HEMTs with (a) and

without (b) InGaN cap layer.

between InGaN cap layer and AlGaN barrier layer. Figure 2 shows the magnitude of polarization in InGaN cap layer calculated using parameters reported in ref. [8]. The piezo-electric polarization is much larger than the spontaneous polarization. Then overall polarization in the InGaN cap layer is opposite to that in the AlGaN barrier layer. Figures 3(a) and 3(b) show the energy band diagrams of the device with and without the InGaN cap layer. The conduction band is raised by the polarization-induced field in the InGaN cap layer, as shown in Fig. 3(a), leading to normally-off operation. In the case of the HEMTs without the InGaN cap layer, two-dimensional electron gas (2DEG) is formed in the AlGaN/GaN interface, which leads to the normally-on operation. Then, if the InGaN cap layer is introduced only in the gate region, it is possible to decrease the parasitic source resistance even in the nor-mally-off HEMTs because the carrier in the access region is not depleted. The gate recess etching which affects the threshold voltage is not required in the present device.

3 Results and discussion The devices were fabri-cated on an i-In0.2Ga0.8N(5 nm)/i-Al0.25Ga0.75N (5 nm)/ n-Al0.25Ga0.75N (2 x 1018 cm–3, 10 nm)/i-Al0.25Ga0.75N (3 nm)/i-GaN (3 µm) heterostructure grown by MOCVD on a sapphire substrate. Control device without the In0.2Ga0.8N

cap layer was also fabricated for comparison. AlGaN/GaN layered structure of the control device was same as that with the In0.2Ga0.8N cap layer. The sheet resistances were 33 kΩ and 513 Ω for the wafers with and without the In0.2Ga0.8N cap layer, respectively. The larger sheet resis-tance of the wafer with the In0.2Ga0.8N cap layer is thought to be due to a reduction of the 2DEG density caused by the polarization-induced field in the In0.2Ga0.8N cap layer. Ohmic contacts were formed by alloying Ti/Al/Ni/Au (20/120/30/50 nm) at 830 °C for 30 s in N2 ambient. The gate electrodes were formed using Pd/Ni/Pt/Au (3/3/30/300 nm). The gate length was 1.9 µm. Figure 4(a) shows ID-VDS characteristics and Fig. 4(b) shows ID-VGS and gm-VGS transfer characteristics of the HEMTs with the In0.2Ga0.8N cap layer. Normally-off op-eration with a threshold voltage of 0.4 V which was evalu-ated by extrapolating ID-VGS curve to zero ID was con-firmed, as shown in Fig. 4(b). The maximum transconduc-tance (gmmax) was 85 mS/mm. Figure 5 shows the results of the device without the In0.2Ga0.8N cap layer. In this case, normally-on operation with a threshold voltage of -1.5 V and gmmax of 145 mS/mm was obtained. This means that the In0.2Ga0.8N cap layer caused a threshold voltage shift of about 1.9 V. The smaller gm of the device with the In0.2Ga0.8N cap layer than that of the device without the In0.2Ga0.8N cap layer is probably due to a large parasitic source resistance which results from the reduced 2DEG density caused by the In0.2Ga0.8N cap layer in the access region.

(a)

(b)

Figure 4 ID-VDS (a) and transfer (b) characteristics of the

AlGaN/GaN HEMT with In0.2Ga0.8N cap layer. Gate voltage step

is 0.5 V for (a) and VDS = 5 V for (b).

EC

EF

AlGaN GaNMetal

EC

EF

AlGaN GaNMetalGaN

EC

EF

InGaNAlGaNMetal

+－

GaN

EC

EF

InGaNAlGaNMetal

+－

(a) (b)

-0.05

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= 0~2 V, 0.5 V/step VGS

= 0 V

Dra

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urr

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VGS

= 2 V

Drain voltage (V)

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25

50

75

100

125

150

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-0.08

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0

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0 0.1 0.2 0.3 0.4 0.5

Po

lari

za

tio

n (

C/m

2)

In-composition

InxGa

1-xN/GaN

PSP

PPE

-0.08

-0.06

-0.04

-0.02

0

0.02

0 0.1 0.2 0.3 0.4 0.5

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InxGa

1-xN/GaN

PSP

PPE

phys. stat. sol. (c) 5, No. 6 (2008) 1931

www.pss-c.com © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Contributed

Article

(a)

(b)

Figure 5 ID-VDS (a) and transfer (b) characteristics of the

AlGaN/GaN HEMT without In0.2Ga0.8N cap layer. Gate voltage

step is 0.5 V for (a) and VDS = 5 V for (b).

Figure 6 Transfer characteristics of the AlGaN/GaN HEMT

with In0.2Ga0.8N cap layer after etching the In0.2Ga0.8N cap layer

at the access region. VD S= 5 V.

The In0.2Ga0.8N cap layer was removed by Cl2 reactive ion etching using gate electrode as an etching mask to de-crease the parasitic source resistance. The measured trans-fer characteristics after etching-off the In0.2Ga0.8N cap layer at the access region are shown in Fig. 6. The maximum transconductance increased from 85 mS/mm to 130 mS/mm. These results indicate the validity of the HEMTs with InGaN cap layer for realizing normally-off HEMTs with small parasitic source resistance. A threshold voltage shift of -0.3 V toward negative direction was observed af-ter etching-off the In0.2Ga0.8N cap layer at the access region. Even though the origin of the threshold voltage shift is not

clear at present, it may have some relation with the local strain relaxation by etching-off the In0.2Ga0.8N cap layer at the access region. The transconductance was increased from 130 to 145 mS/mm by annealing the devices at 250 °C for 20 minutes in a N2 atmosphere. This was probably due to the improvement of the Schottky contact.

4 Conclusion AlGaN/GaN HEMTs with a thin In-GaN cap layer have been proposed to implement the nor-mally-off HEMTs. The polarization-induced field in the InGaN cap layer was employed to raise the conduction band resulting in a normally-off operation. Fabricated HEMT with an In0.2Ga0.8N cap layer with a thickness of 5 nm showed normally-off operation with a threshold volt-age of 0.4 V and a maximum transconductance of 85 mS/mm for the device with a 1.9-µm-long gate. By etch-ing-off the In0.2Ga0.8N cap layer at the access region using gate electrode as an etching mask, the maximum transcon-ductance has increased from 85 to 130 mS/mm due to a re-duction of the parasitic source resistance. The transconduc-tance was increased from 130 to 145 mS/mm by annealing the devices at 250 °C for 20 minutes in a N2 atmosphere. These results indicate the validity of introducing an In0.2Ga0.8N cap layer to implement the normally-off HEMTs.

Acknowledgements This work was supported in part by

the Grant-in-Aid (No. 18206041) from MEXT, Japan.

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-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 1 2 3 4 5 6 7 8

VGS

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Dra

in c

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