Microelectronics Journal 34 (2003) 575–577 www.elsevier.com/locate/mejo
The investigation of properties of electron transport in AlGaN/GaN heterostructures S.V. Danylyuka,*, S.A. Vitusevicha, B. Podorb, A.E. Belyaevc, A.Yu. Avksentyevc, V. Tilakd, J. Smartd, A. Vertiatchikhd, L.F. Eastmand a
b
Institut fu¨r Schichten und Grenzfla¨chen, Forschungszentrum Ju¨lich, Ju¨lich D-52425, Germany Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science, Budapest, Hungary c Institute of Semiconductor Physics, NASU, Kiev 03028, Ukraine d School of Electrical Engineering, Cornell University, Ithaca, NY 14853, USA Accepted 9 December 2002
Abstract Magnetoresistance of two-dimensional electron gas in the triangular quantum well created as a result of polarization effects at AlGaN/GaN heterointerface has been studied. Magnetotransport measurements clearly revealed Shubnikov-de Haas oscillations in both two terminal transmission line model samples and three terminal high electron mobility transistor structures at low temperature. Low value of quantum scattering time was obtained for different structures. Region with negative magnetoresistance was observed at low magnetic field. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Shubnikov-de Haas oscillations; Piezoelectric polarization; Transmission line model
1. Introduction
2. Experimental details
During long time, Shubnikov-de Haas (SdH) oscillations were not observed in GaN-based films due to poor material quality. Significant progress in the growth technology revived magnetotransport measurements as powerful method in the investigation of electronic properties of two-dimensional electron gas (2DEG) [1,2]. Unique property of AlGaN/GaN heterojunction is spontaneous and piezoelectric polarization have opened new ways of high density 2DEG formation at the interface between two materials without any modulation doping [3]. Magnetotransport measurements allow to investigate intrinsic transport scattering mechanisms that result into definite mobility of AlGaN/GaN heterostructures and give useful information for optimization of their properties. In this contribution, we present results of magnetotransport investigation of high quality undoped AlGaN/GaN heterostructures.
The investigated heterostructures were grown by metal organic chemical vapor deposition. On the (0001) surface of sapphire substrate, a nucleation 40 nm AlGaN (16% Al) layer was grown, followed by a 1.1 mm GaN, a 23 nm AlGaN (33%) covered with 320 nm Si3N4 passivation layer. The transmission line model (TLM) structures and high electron mobility transistor (HEMT) device were patterned on the same wafer. The conducting channel of TLM devices was of 100 mm width with the length L scaled to varied distance as 1, 5, 15, 20, and 25 mm. The ohmic contacts were processed by Ti/Al/Ti/Au metallization annealed for 40 s at 800 8C. Ni/Au metallization deposited using electronic beam lithography was used for the gate contact in HEMT structures. The HEMT devices had the gate length of 0.15, 0.25, 0.30, 0.35 mm, gate width of 100– 400 mm, and drain-gate spacing of 2 mm. The source-drain spacing was of 3 mm. A room temperature Hall mobility and 2DEG sheet electron density were measured as 1250 cm2/V s and 1.05 £ 1013 cm22, respectively. Magnetoresistance measurements were performed on TLM and HEMT processed samples at 30 mK and 1.4 K
* Corresponding author. Tel.: þ 49-2461-612464; fax: þ 49-2461612470. E-mail address:
[email protected] (S.V. Danylyuk).
0026-2692/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0026-2692(03)00052-1
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S.V. Danylyuk et al. / Microelectronics Journal 34 (2003) 575–577
and high magnetic field up to 14 T. Low current of about 10 mA was used to avoid heating of the samples.
3. Results and discussion Measured magnetic field dependence of the longitudinal resistance of 2DEG for TLM sample with length of 25 mm is shown in Fig. 1. The strong SdH oscillations are observed. The SdH oscillations appear at relatively low magnetic fields that is evidence of good quality of the heterointerface. The carrier concentration of the 2DEG determined from SdH oscillations was nearly the same as value of 1013 cm22 measured using Van der Pauw contact configuration method. In the magnetic field range up to 1 T, a negative magnetoresistance with parabolic magnetic field dependence was clearly observed. It demonstrates strong dependence on applied gate voltage for HEMT sample configuration. The nature of the negative component is not quite clear and need further investigation. The quantum scattering time, tq ; is related to the Landau level broadening and characterizes the total carrier scattering mechanisms. A simple equation can be used [2]: ln
Drxx r0 DT
¼C2
pmp 1 ; etq B
where DT ¼
4X ; sinh X
X¼
2p2 kT ; "vc
Drxx is the amplitude of the oscillations, r0 is the zero-field resistivity, vc is the cyclotron frequency, e is the electron charge, mp is effective mass, B is magnetic field, C is constant at a given temperature, " is reduced Plank’s constant, k is Boltzmann’s constant and T is temperature. By fitting the data plotted in Fig. 2 and taking the electron effective mass mp as 0:23me ; we obtain that the quantum
Fig. 2. Dingle plot of SdH oscillations amplitude (data from Fig. 1) vs the reciprocal magnetic field.
scattering time is 61.8 fs. The latter corresponds to broadening value of the Landau level of 2.86 meV. Classical transport time, tt ; was calculated from measured mobility of 4000 cm2/V s, that is relatively large and corresponds to high quality heterointerface. Estimated value of the time is 0.53 ps. The relation between classical transport time and quantum scattering time is a characteristic of the dominant angle of scattering events in the 2DEG. For short-range scattering process on roughness and imperfections of the heterointerface, large angle scattering events will be dominant and the ratio is expected to be unity. It will be several times higher then unity in the case of longrange/small angle scattering processes. Table 1 summarises the averages of the numerical data obtained from the SdH oscillations measurement for three different samples. It should be noted that period of the SdH oscillations in the HEMT samples does not strongly depend on applied gate voltages. At the same time, small source-drain voltage, USD ; results in considerable damping of the oscillations and their disappearance already at USD ¼ 30 mV. The average ratio between classical and quantum scattering time for our samples was found to be around 8. The value is consistent with reduced defect scattering of 2DEG formed at interface between undoped layers and demonstrates the interface good quality. For comparison, the highest value observed in high-mobility modulationdoped AlGaAs/GaAs 2DEG, where small angle scattering processes are dominant, is 30 [4]. High quality of AlGaN/ GaN heterostructures additionally has been confirmed by our low frequency noise measurements. Obtained values of Table 1 Characteristic times determined from Dingle plot for three samples
Fig. 1. SdH oscillations measured at 1.4 K for sample with length between contacts of 25 mm.
Sample
tq (fs)
tt /tq
TLM, 5 mm TLM, 25 mm HEMT, 3 mm
75.2 61.8 57.5
7.0 8.5 9.1
S.V. Danylyuk et al. / Microelectronics Journal 34 (2003) 575–577
Hooge parameters are comparative with such reported for GaAs structures [5]. The low value of magnetic field at which the SdH oscillations are resolved is also characteristic of small disorder at AlGaN/GaN interface. 4. Conclusion In summary, high quality of 2DEG is obtained between undoped MOCVD grown AlGaN/GaN heterojunction due to piezoelectric and spontaneous polarization effects. The transport-to-quantum lifetime ratio (. 7) demonstrate reduced scattering in 2DEG obtained without any modulation doping of the heterostructure. The results can be used for design GaN-based heterostructure devices with increased performance. Acknowledgements Financial support was provided by the Office of Naval Research under contract no. N00014-01-1-0828 (monitored by Dr Colin E.C. Wood).
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