InAlAs metamorphic HEMTs on GaAs substrate

Journal of Crystal Growth 323 (2011) 522–524

Contents lists available at ScienceDirect

Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

Effects of AlGaAsSb electron supply layer for InGaAs/InAlAs metamorphic HEMTs on GaAs substrate Hirotaka Geka n, Satoshi Yamada, Masato Toita, Kazuhiro Nagase, Naohiro Kuze Asahi Kasei Microdevices Corporation, 2-1 Samejima, Fuji, Shizuoka 416-8501, Japan

a r t i c l e i n f o

a b s t r a c t

Available online 20 January 2011

We studied the effects of an AlGaAsSb electron supply layer for InGaAs/InAlAs metamorphic high electron mobility transistors (mHEMTs) on GaAs substrate. By implementing an AlGaAsSb electron supply layer, we drastically improved the electron mobility of InGaAs/InAlAs heterostructures for mHEMTs. An AlGaAsSb electron supply layer for InGaAs/InAlAs heterostructures on GaAs substrates promises high-performance mHEMTs with low production cost. & 2011 Elsevier B.V. All rights reserved.

Keywords: A1. Doping A3. Molecular beam epitaxy A3. Quantum wells B1. Antimonides B2. Semiconducting III–V materials B3. High electron mobility transistors

1. Introduction InGaAs/InAlAs metamorphic high electron mobility transistors (mHEMTs) on GaAs substrates are attractive because they are suitable for high-performance monolithic millimeter wave integrated circuits (MMICs) with low production cost to high electron mobility and high electron density [1]. Since there are large lattice mismatch between the InGaAs channel layer and GaAs substrate, InGaAs/InAlAs mHEMTs on GaAs substrates require a suitable buffer layer to relax the lattice mismatch and to obtain high electron mobility. Therefore, various buffer layers have been reported [2–5]. Dependence of indium content of the InGaAs channel layer on transport properties has been also studied for obtaining high electron mobility [6]. However, the effects of the electron supply layer have not been extensively researched. The conduction band offset between a channel layer and an electron supply layer determines the electron distribution and transport properties in the channel layer. The maximum value of the electron density is limited by the value of the conduction band offset. Therefore, material design for the electron supply layer is important for InGaAs/InAlAs mHEMTs on GaAs substrates. Due to a smaller electron affinity on AlGaAsSb than InAlAs, InGaAs/AlGaAsSb heterostructures have a larger conduction band offset than that of InGaAs/InAlAs. By using an AlGaAsSb electron supply layer, we can, therefore, change the electron distribution in the InGaAs channel layer improving transport properties of InGaAs/InAlAs heterostructures on GaAs substrate.

We studied the effect of AlGaAsSb electron supply layer on transport properties of InGaAs/InAlAs heterostructures on GaAs substrates.

2. Experimental procedure InGaAs/InAlAs heterostructures, as schematically shown in Fig. 1, were grown by molecular beam epitaxy. A metamorphic buffer layer consisting of a 1000-nm InxAl0.45Ga0.55 xAs(x¼0-0.55) graded layer and a 250-nm In0.55Al0.45As layer was grown at 400 1C. A HEMT structure was grown on the metamorphic buffer layer at 450 1C. The HEMT structure consists of 100-nm In0.55Al0.45As, 20-nm In0.56Ga0.44As channel, In0.55Al0.45As spacer (thickness: dInAlAs), Al0.5Ga0.5As0.52Sb0.48 (thickness: dAlGaAsSb), In0.55Al0.45As (thickness: 16-nm dInAlAs dAlGaAsSb), and 10-nm non-doped In0.56Ga0.44As cap layers. A Sn delta doped layer is consistently placed 4 nm above the upper edge of the In0.56Ga0.44As channel layer. The Al0.5Ga0.5As0.52Sb0.48 layer is lattice matched to the In0.55Al0.45As layer as well as the In0.56Ga0.44As channel layer. We compared two types of HEMT structures: a conventional structure without an Al0.5Ga0.5As0.52Sb0.48 layer (dAlGaAsSb ¼ 0 nm), denoted as ‘‘Type-A’’, and the other with an Al0.5Ga0.5As0.52Sb0.48 layer (dAlGaAsSb ¼10 nm), denoted as ‘‘Type-B’’. Electron mobility and electron density were determined by Hall measurement and electron distribution in the InGaAs channel layer was calculated using silvaco’s TCAD.

3. Results and discussion n

Corresponding author. Tel.: +81 545 62 3365; fax: + 81 545 62 3398. E-mail address: [email protected] (H.R. Geka).

0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.12.078

Fig. 2 shows the relationship between dInAlAs and electron mobility for the Type-B structure. dInAlAs represents the distance between the

H. Geka et al. / Journal of Crystal Growth 323 (2011) 522–524

Type -A

Type -B

InGaAs -cap

10 nm

523

Sn delta doping layer 12 nm

InAlAs

4 nm

InAlAs

20 nm

InGaAs -ch

InGaAs -cap InAlAs AlGaAsSb

10 nm (6nm -d InAlAs ) 10 nm

4 nm (fixed) InAlAs InGaAs -ch

(d InAlAs )

100 nm

InAlAs

InAlAs

20 nm 100 nm

250 nm

InAlAs buffer

InAlAs buffer

250 nm

1000 nm

InAlGaAs Graded buffer

InAlGaAs Graded buffer GaAs substrate

GaAs substrate

1000 nm

Fig. 1. Layer structures of Type-A and Type-B HEMT structures.

11000 Sn delta doping concentration: 4 1012/cm2 (fixed)

Electron mobility (cm2/Vs)

10500 10000 9500 9000 8500 8000 7500 7000 0

1

2

3

4

5

6

7

dInAlAs (nm) Fig. 2. Relationship between dInAlAs and electron mobility for Type-B structures.

AlGaAsSb electron supply layer and the upper edge of the In0.56Ga0.44As cannel layer. It is clear that smaller dInAlAs provides higher electron mobility in the channel. This is because the large band offset introduced by the AlGaAsSb layer takes more pronounced effects when the upper edge of the channel is closer to the AlGaAsSb electron supply layer. When a AlGaAsSb layer was grown directly on InGaAs channel layer, i.e., dInAlAs ¼0, electron mobility took the highest value, i.e., 10,500 cm2/Vs, at a sheet electron density of 3.2  1012/cm  2. This mobility is much higher than that for the Type-A structure, i.e., 8600 cm2/Vs, at an almost identical sheet electron density of 3.3  1012/cm  2. Fig. 3 shows relationships between sheet electron density and the electron mobility in the In0.56Ga0.44As channel layer. The electron mobility of the Type-A structure was close to the value found in the work Benkhelifa et al. [1]. The electron mobility of the Type-A structure decreased with increase in sheet electron density. On the other hand, the electron mobility of the Type-B structure was almost constant in the region where sheet electron density was less than 4  1012 cm  2. The electron mobility for the Type-B structure is the highest in a wide range of electron densities. The effect of AlGaAsSb is particularly noticeable in higher electron density regions. This means that by implementing an AlGaAsSb electron supply layer, we can achieve high transconductance and high electron mobility transistors. Fig. 4 shows simulated electron distribution in the In0.56Ga0.44As channel layer of the Type-A and Type-B structures. We solved the

Fig. 3. Relationships between sheet electron density and electron mobility in In0.56Ga0.44As channel layer.

self-consistent coupled Shrodinger–Poisson equation by using silvaco’s TCAD to simulate electron distribution in the In0.56Ga0.44As channel layer. Since the Type-B structure has a larger conduction band offset than that of the Type-A structure, the electron density near the hetero-interface decreases. The electron mobility near the hetero-interface is very low because of inherent hetero-interface scattering. Therefore, the improvement in electron mobility in the Type-B structure is due to the reduction in hetero-interface scattering. By implementing an AlGaAsSb electron supply layer for the InGaAs/ InAlAs heterostructure on GaAs substrates, we can achieve very high electron mobility and electron density. We also studied the effect of the AlGaAsSb electron supply layer for structures with higher indium content in the channel layer. Type-B structures with an In0.7Ga0.3As channel marked an electron mobility of 12,500 cm2/Vs at a sheet electron density of 3.5  1012 cm  2. This value is much higher than that for Type-A structures with the equivalent indium content, i.e., In0.7Ga0.3As channel, 9500 cm2/Vs at almost identical sheet electron density of 3.4  1012 cm  2. We also studied the effect of the AlGaAsSb electron supply layer for structures with even higher indium content, In0.8Ga0.2 As channel layer. A metamorphic buffer layer consisting of a 1300-nm InxAl0.3Ga0.7-xAs(x ¼0-0.7) graded layer and a 250-nm In0.7Al0.3As layer was used. The Al0.5Ga0.5As0.39Sb0.61 layer, which is lattice matched to the In0.7Al0.3As layer, was used for the electron supply layer. Type-B structures with an In0.8Ga0.2As channel showed an electron mobility of 15,100 cm2/Vs at a sheet

H. Geka et al. / Journal of Crystal Growth 323 (2011) 522–524

InAlAs

AlGaAsSb

InAlAs

InGaAs

0.6

1.6×1018 Type-A Type-B

Electron of condution band edge (eV)

0.5 0.4

1.4×1018 1.2×1018

0.3

1.0×1018

0.2 8×1017

0.1

6×1017

0 -0.1

4×1017

-0.2

2×1017

-0.3 20

30

40 Distance (nm)

50

Electron density (cm-3)

524

0 60

Fig. 4. Simulated electron distribution in In0.56Ga0.44As channel layer of Type-A and Type-B structures.

If we compare Type-A and Type-B structures with a given indium content, Type-B structures always have higher electron mobility than that of Type-A structures. For instance, as shown in Fig. 5, the mobility in Type-B structures with an In0.8Ga0.2As channel is much higher than that of Type-A structures with an In0.8Ga0.2As channel. In terms of electron density, the AlGaAsSb electron supply layer is useful for achieving a very high electron density particularly in a higher indium content channel.

18000

Electron mobility (cm2/Vs)

17000 16000 15000 14000 13000 12000 11000

Type–A (In80%)

10000

Type–B (In70%)

9000

Type–B (In80%)

8000

0

1×1012

2×1012

3×1012

Sheet electron density

4. Conclusion

4×1012

5×1012

(cm–2)

Fig. 5. Relationships between sheet electron density and electron mobility in In0.7Ga0.3As and In0.8Ga0.2As channel layers.

electron density of 3.6  1012 cm  2. This value is much higher than that of Type-B structure with an In0.7Ga0.3As channel. Fig. 5 shows relationships between sheet electron density and electron mobility in the In0.7Ga0.3As and In0.8Ga0.2As channel layers. The electron mobility of Type-B structures with an In0.7Ga0.3As channel layer was comparative to that of the Type-A structures with an In0.8Ga0.2As channel layer. By using the AlGaAsSb electron supply layer on a given indium content channel, we can practically obtain the electron mobility of much higher channel indium content. If a higher indium content causes any drawbacks, such as a pronounced short-channel effect, or a higher gate to source/drain leakage current in a particular mHEMT structure, the AlGaAsSb layer can be ideal for achieving a higher electron mobility even for a relatively lower indium content channel.

We studied the benefits of implementing an AlGaAsSb electron supply layer for InGaAs/InAlAs mHEMTs on a GaAs substrate. Since InGaAs/AlGaAsSb heterostructures have a larger conduction band offset than that of InGaAs/InAlAs, we can shift the vertical distribution of the electron density in the InGaAs channel layer. Therefore, we drastically improved the electron mobility of InGaAs/InAlAs heterostructure for mHEMTs by implementing the AlGaAsSb electron supply layer. An AlGaAsSb electron supply layer for InGaAs/InAlAs heterostructures on GaAs substrates promises high-performance mHEMTs with low production cost.

References [1] F. Benkhelifa, et al., Conference Proceedings, International Conference on IPRM, 2000, pp. 290–293. [2] D. Lubyshev, et al., Conference Proceedings, International Conference on GaAs Manufacturing Technology, 2000, pp. 85–88. [3] S. Miya, et al., Conference Proceedings, International Conference on IPRM, 1995, pp. 440–443. [4] S. Miya, et al., Journal of Electronic Materials 25 (3) (1996) 415–420. [5] R.T. Webster, et al., Conference Proceedings, IEEE Lester Eastman Conference on High Performance Devices, 2002, pp. 315–323. [6] H. Ono, et al., Japanese Journal of Applied Physics 43 (4B) (2004) 2259–2263.