Investigation and Fabrication of the Semiconductor

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A metamorphic high electron mobility transistor (MHEMT) with “zig–zag” –like gate of a length of. 46 nm and cut-off frequencies for the current and power gain fT ...
International Journal of High Speed Electronics and Systems Vol. 24, Nos. 1 & 2 (2015) 1520001 (5 pages) © World Scientific Publishing Company DOI: 10.1142/S0129156415200013

Investigation and Fabrication of the Semiconductor Devices Based on Metamorphic InAlAs/InGaAs/InAlAs Nanoheterostructures for THz Applications Lavrukhin Denis Vladimirovich1, Yachmenev Aleksandr Eduardovich2, Galiev Rinat Radifovich3, Bugaev Aleksandr Sergeevich4, Fedorov Yurii Vladimirovich5, Khabibullin Rustam Anvarovich6, Ponomarev Dmitry Sergeevich7, and Maltsev Petr Pavlovich8 Institute of Ultra-High-Frequency Semiconductor Electronics of RAS, Russia, Moscow, 117105, Russia 1 [email protected] 2 [email protected] 3 [email protected] 4 [email protected] 5 [email protected] 6 [email protected] 7 [email protected] 8 [email protected] Received 9 June 2015 Accepted A metamorphic high electron mobility transistor (MHEMT) with “zig–zag” –like gate of a length of 46 nm and cut-off frequencies for the current and power gain fT = 0.13 THz and fmax = 0.63 THz respectively was fabricated on the base of InAlAs/InGaAs/InAlAs nanoheterostructure Keywords: Molecular-beam epitaxy; nanoheterostructure; MHEMT; electron-beam lithography; THz devices.

1. Introduction Development of THz devices can change the principles and approaches in the field of early medical diagnostics (especially tumors) and therapies of chronic diseases. The demand for the technologies is also due to counteraction to terrorism in public places, including transport. Besides, motor, aviation and space industries demand more and more sophisticated systems of nondestructive testing and location.1,2 Heterostructures with quantum well (QW) InAlAs/InGaAs/InAlAs grown on GaAs wafers are assumed to be one of the most perspective materials for contemporary microwave electronics in the range of 0.5–1.0 THz.3

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2. Metamorphic HEMT technology Via molecular-beam epitaxy (MBE) we grew and investigated several metamorphic InAlAs/InGaAs/InAlAs heterostructures based on GaAs wafers. Conductivity of such structures depends directly on the composition and growth technology of a metamorphic buffer (MB), which accommodates mechanical stresses in a nanoheterostructure during the epitaxial growth. A schematic diagram of proposed MHEMT-heterostructure is depicted on Fig. 1. n+ (Si) In0.42Ga0.58As In0.42Al0.58As δ-Si In0.42Al0.58As In0.42Ga0.58As In0.42Al0.58As In0.51Al0.49As In0.41Al0.59As In0.31Al0.69As In0.20Al0.80As In0.10Al0.90As GaAs GaAs (100)

15 nm 12 nm

cap layer barrier

3 nm 18 nm 400 nm 200 nm 200 nm 200 nm 200 nm 200 nm

spacer QW Barrier

Metamorphic buffer

Buffer Wafer

Fig. 1. Schematic diagram of the MHEMT-heterostructure.

The MHEMT-heterostructure included a step-like MB with a thickness of 1 µm and In0.42Ga0.58As channel. The growth temperatures for the MB and the channel were 380 and 470°C respectively. The concentrations of δ-doping in the contact n + and δ-layers were 6  1018 cm–3 and 7  1012 cm–2 respectively. In order to measure the high-frequency characteristics of the transistors: the cut-off frequencies for the current fT and power fmax, we fabricated MHEMTs with a gate width of 2  60 and 2  80 µm. The topology of MHEMTs extracted by the sequential use of the photolithography, etching and metallization methods is illustrated in Fig. 2.

Fig. 2. Topology of the fabricated MHEMTs. 1520001-2

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Small-length gates are mechanically unstable and prone to tumbling down. When such a gate is formed, it is possible to reach the gate length (Lg) less than 50 nm by means of a three-layer system of resists PММА950К/PМGI/PММА950К due to profiling of the central dose. Five-layer system of electron resists was developed in order to ensure the gate’s high mechanical stability. The gate themselves were formed by an electron-beam lithography (EBL) system Raith 150 TWO. In order to ensure control of the sizes and forms of the gate’s “leg” the area of its formation was divided in a number of sublayers of the system of resists, which were developed independently. This made it possible to increase the height of the “leg”, which should result in a reduction of the parasitic capacities. A “zig–zag” form of such a leg increases the mechanical rigidity of the gate.4 This is achieved due to enlargement of the effective area, supporting the wide “hat”, which in addition leads to reduction of the noise factor. The “zig–zag” itself has a size up to 200 nm with a period of 4000 nm. The total thickness of the “hat” is 800 nm. Figure 3 shows a schematic representation and a microphotograph (the view from above) of the developed “zig–zag”–shaped gate.

Fig. 3. Schematic representation and a microphotograph of “zig–zag” gate.

Fig. 4. SEM images of the “zig–zag”–shaped gate with Lg = 46 nm and gate width (a) 2  60 and (b) 2  80 μm.

The “zig–zag” shaped depression corresponding to the “leg” of the gate is clearly seen in the microphotograph. It is worth noting that the used system for the formation of such a gate differs from a direct gate only in terms of the geometry of the exposed “leg.” In addition, the “zig–zag”-like shape does not affect the microwave parameters of the transistors since the variation in the gate geometry is much smaller than the wavelength 1520001-3

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in the operating frequency range and is comparable to the effective region of field control over the channel under the transistor gate. Figure 4 shows scanning electron microscopy images of the developed gates. The S–parameters of MHEMTs were measured using a circuit with a common source in the frequency range 0.1–67 GHz. Using paired three contact probe heads, an Agilent E8361A vector analyzer was connected to the MHEMT directly on the wafer. The results of measurements of the frequency characteristics of the fabricated transistors are shown in Fig. 5. The measurements were performed for transistors located in the same working module on the wafer. The voltage between the source and drain UDS = +1.5 V, while the voltage between the source and gate was UGS = – 0.8 V.

Fig. 5. Dependences of |H21| and MasonsGain (MG) on frequency for fabricated MHEMT

Conclusion In this study, we have fabricated a MHEMT on the basis of a metamorphic In0.42Al0.58As/In0.43Ga0.58As/In0.42Al0.58As heterostructure grown by MBE on a GaAs (100) wafer. The use of a step-like metamorphic buffer made it possible to attain high structural quality of the sample surface (root square mean surface = 3.1 nm). Using EBL for a fivelayer system of resists, we obtained a “zig–zag”–shaped gate with a length of 46 nm. The highest cut-off frequencies for the current gain were obtained for a MHEMT with a gate width of Wg = 2  80 μm and amounted to fT = 0.13 THz and fmax = 0.63 THz respectively. Acknowledgments Additionally we would like to thank Dmitry Gnatyuk and Alexei Zuev for frequency characteristics measurements. This work was partially funded by RSF grant 14-29-00277. References 1. G. Ng, K. Radhakrishan, and H. Wang, Are We There Yet? – A Metamorphic HEMT and HBT Perspective, in the Proceedings of the 13th International GaAs Symposium, 13–20 (2005). 1520001-4

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2. A. Hülsmann, A. Leuther, I. Kallfass, R. Weber, A. Tessmann, M. Schlechtweg, and O. Ambacher, Advanced mHEMT technologies for space applications, in the Proceedings of the 20th International Symposium on Space Terahertz Technology, 178–182 (2009). 3. G. B. Galiev, I. S. Vasil'evskii, E. A. Klimov, S. S. Pushkarev, A. N. Klochkov, P. P. Maltsev, M. V. Presniakov, I. A. Trunkin, and A. L. Vasiliev, Effect of (100) GaAs substrate misorientation on electrophysical parameters, structural properties and surface morphology of metamorphic HEMT nanoheterostructures InGaAs/InAlAs, J. Cryst. Growth. 392, 11–19 (2014). 4. D. V. Lavrukhin, A. E. Yachmenev, R. R. Galiev, R. A. Khabibullin, D. S. Ponomarev, Yu. V. Fedorov, and P. P. Maltsev, MHEMT with a power gain cut-off frequency of fmax = 0.63 THz on the basis of a In0.42Al0.58As/In0.42Ga0.58As/In0.42Al0.58As/GaAs nanoheterostructure, Semicond. 48, 69–72 (2014).

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