- Email: [email protected]
GaAs heterostructures down to submicron length scale
ARTICLE IN PRESS
Physica E 40 (2008) 1579–1581 www.elsevier.com/locate/physe
Alloyed ohmic contacts to two-dimensional electron system in AlGaAs/GaAs heterostructures down to submicron length scale Oktay Go¨ktas-, Jochen Weber, Ju¨rgen Weis, Klaus von Klitzing Max-Planck-Institut fu¨r Festko¨rperforschung, HeisenbergstraX e 1, 70569 Stuttgart, Germany Available online 29 September 2007
Abstract We have realized and investigated Au/Ge/Ni alloyed ohmic contacts to two-dimensional electron systems (2DES) embedded in various Al0:67 Ga0:33 As=GaAs heterostructures. Transport and geometrical investigations have been carried out to understand fundamental properties of these contacts. A recipe for creating low resistive and reliable alloyed ohmic contacts to 2DES is given, which is successful even for contacts to deep lying 2DES and contact sizes down to submicron length scale (0:2 mm). The contact resistance between 2DES and alloyed metal depends on the length and orientation of the interface line between 2DES and alloyed metal relative to the crystal orientation of the heterostructure. r 2007 Elsevier B.V. All rights reserved. PACS: 73.43.F; 73.40.Cg Keywords: Ohmic contacts; Two-dimensional electron system; Quantum Hall effect
Reproducible and reliable low resistive ohmic contacts are crucial for device applications. Au/Ge/Ni alloyed ohmic contacts are well established for GaAs based electronic and optoelectronic devices. They are also used to contact two-dimensional electron systems (2DES) embedded in Al1x Gax As=GaAs heterostructures. However, in case of 2DES, systematic investigations are rarely reported in literature [1–3], especially for small contact width. Understanding of ohmic contacts to 2DES is not only of technical but rather fundamental interest: The Bu¨ttiker edge state picture of the integer quantum Hall effect has attributed an extraordinary role to the contacts [4]. Recent interference experiments of edge states count on the dephasing properties of such contacts [5]. Here we report on our experience of making good ohmic contacts even down to the submicron length scale, highlighting some properties which one should be aware of when interpreting experimental results. About one decade ago, our group realized and investigated Au/Ge/Ni alloyed ohmic contacts to 2DES embedded Corresponding author. Tel.: +49 7116891520; fax: +49 7116891572.
E-mail address: [email protected] (O. Go¨ktas-). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.09.115
in various AlGaAs/GaAs heterostructures [6]. For this purpose, heterostructure mesas were formed by optical lithography and wet etching. Contact areas were defined by optical lithography, and finally cleaned. Au, Ge and Ni were evaporated in different layer sequences and in various compositions on top of the heterostructure. The metal layers were alloyed under N2 =H2 gas atmosphere at various temperatures between 380 C and 470 C. For characterization, the transmission line method (TLM) [1,2] was used to determine the contact resistance between the 2DES and the alloyed metal. To apply this method, mesa stripes were etched out of the heterostructure, and a series of contacts of varying distance d were alloyed along such a mesa (see Fig. 1). The two-terminal resistance between two contacts was determined by measuring the current-voltage characteristics at liquid Helium temperature or at about 30 mK. When plotting the two-terminal resistance, corrected by the respective series resistance due to the wiring in the measurement setup, as a function of the distance d between the contacts, a linear relation is usually obtained (see Fig. 1). The extrapolated value of the best fit line to the data at d ¼ 0 gives twice the contact resistance Rc of the 2DES/metal interface.
ARTICLE IN PRESS O. Go¨ktas- et al. / Physica E 40 (2008) 1579–1581
1k d [011]
W
R (Ω)
− [011]
2Rc 0
50 d (μm)
100
Fig. 1. TLM measurement on a mesa with a width w of 10 mm (heterostructure with the AlGaAs/GaAs junction at 50 nm). Triangular (rectangular) data points correspond to contacts defined on the mesa ¯ direction. Inset: Sketch of two mesa stripes along the [0 1 1] ð½0 1 1Þ (contacts in dark, mesa in light) with the respective crystal orientations.
Au and Ge were thermally evaporated together or in separate layers so that both were finally found in the weight ratio of 88% Au : 12% Ge on the heterostructure surface. In this ratio Au and Ge behaves as an eutectic and melts at 361 C. It was observed that both contact resistance and reproducibility strongly depend on the Ni content (see also Ref. [2]). The optimum Ni thickness was found to be one quarter of the total Au/Ge layer thickness. Best results were obtained when Ni was evaporated as the last layer. For contacting a 2DES up to a depth of D ¼ 80 nm, we have found that evaporating 107.2 nm of Au, 52.8 nm of Ge ð¼ 160 nm Au/Ge), followed by 40 nm of Ni gives reliable contacts. The alloying procedure consists of the following steps: (1) rapidly heating to 370 C and holding at 370 C for 120 s, (2) then for 50 s at 440 C, and (3) rapidly cooling down to room temperature. The whole annealing process is carried out under a N2 =H2 gas atmosphere (volume ratio 80% : 20%; pressure 300 mbar). The last step is under a gas flow with constant pressure whereas the first two steps are performed under a static gas overpressure. For a deeper lying 2DES of depth D, we found that the metal layer thicknesses have to be scaled by a factor s, D þ 30 nm , s¼ 110 nm
(1)
assuring that enough metal is available to penetrate deep enough into the heterostructure. The cleaning procedure after the lithography step for defining the contact areas is crucial for obtaining reliably low contact resistances. Before depositing the metal under high-vacuum condition; the sample is: (1) cleaned in an oxygen plasma to remove any remaining resist on the
heterostructure surface in the contact areas after developing, (2) etched for 2 min in Semico Clean 23 [7] and (3) etched for 5 s in HCl (30%), with a 5 s rinsing by DI water after each etching step. After drying with nitrogen gas, the sample is immediately transferred into the evaporator and pumped down to high vacuum. We emphasize that without careful performance of these cleaning steps the contacts are not reproducible. The TLM measurements of different mesa stripe and contact geometries clearly revealed that the contact resistance does not depend on the area below the contact but rather on the length of the border line between contact and 2DES. Furthermore we observe an anisotropy for the contact resistance, also reported in literature [1,8]. When the border between 2DES and alloyed metal is orientated perpendicular to the [0 1 1] direction of the heterocrystal, the contact resistance is lowest, whereas when the border is ¯ direction the contact resistances perpendicular to the ½0 1 1 are high—sometimes even not measurable, show a large spread and are not reproducible. A typical set of data is presented in Fig. 1. In Fig. 2, the normalized (unity length) contact resistance rc in the [0 1 1] direction for different mesa stripe width w—from 50 mm down to 1 mm—are shown. The normalization is done by multiplying the contact resistance value Rc with the respective mesa stripe width w. The error bars for rc are due to the error in determining the respective contact resistance Rc via the TLM method and the error in determining the mesa width w due to depletion at the edges of the mesa. The contact resistance per mm is around 250 O, and this value is almost constant for all mesa widths. This value is comparable to rc ¼ 0:2 Omm given in Ref. [2] where however no orientation for the 2DES/alloyed metal border is given. This observation implies that the interface between alloyed metal and 2DES is quite homogenous down to 1 mm. On ¯ direction the other hand, contacts defined in the ½0 1 1 show a large spread and the percentage of working contacts drastically decrease as the mesa width gets smaller. Although reported in literature [1], the origin of the contact resistance anisotropy—for instance, due to an
500
rc (Ω μm)
1580
250
0
1
10 w (μm)
100
Fig. 2. Unity length contact resistance rc ¼ Rc w as a function of mesa width w.
ARTICLE IN PRESS O. Go¨ktas- et al. / Physica E 40 (2008) 1579–1581
anisotropy in diffusion, strain or piezoelectricity—has not been clarified. To ensure low resistive contacts on Hall bars, we usually design meander-like borderlines between 2DES and alloyed metal with long sections perpendicular to the [0 1 1] direction. From low-temperature scanning probe experiments on quantum Hall samples we know that a partial depletion of the 2DES along the border to the alloyed metal exists [9]. This means that potential probes do not always enforce an equilibrium between edge channels in the Landauer–Bu¨ttiker formalism since some compressible stripes at the edge remain separated from the contacts by an incompressible region. Recent scanning probe measurements demonstrate adiabatic transport behavior in quantum Hall samples with working contacts when orientated in the worse direction, ¯ i.e., with the 2DES/metal border perpendicular to the ½0 1 1 direction [10]. Recently we have defined contacts of submicron size by using electron beam lithography. An SEM image of such a contact is shown in Fig. 3. The smallest attainable diameter successfully contacting a 2DES has been 0:2 mm. About 50% of these contacts are working and have a linear I–V characteristics with a contact resistance between 2 and 10 kO (in Ref. [3] 0:5 mm contacts with 5 kO were reported). Taking the perimeter p 0:2 mm as the border length between 2DES and metal, the normalized contact resistance lies between 1 and 6 kO for 1 mm, significantly higher than seen in Fig. 2. However this becomes less dramatic if the anisotropy is taken into account, reducing the effective border length. Recipe and rules presented here have been transferred to several groups in the last few years and have been successfully applied to contact 2DES in a variety of heterostructures. Even deep-lying low-density 2DES (D ¼ 220 nm, n ¼ 8 1010 cm2 ) have been easily contacted (5 O per 100 mm in the [0 1 1] direction) by following the scaling rule (1).
1581
200 nm Fig. 3. A cross sectional SEM image of a contact of about 0:2 mm diameter defined on a heterostructure with the AlGaAs/GaAs junction at 40 nm. The ohmic contact is encircled in white. The back going metal is a Cr/Au lead.
We thank U. Graumann and J. Schmid starting this work in 1998. M. Hauser, K. Eberl and W. Dietsche have supplied the heterostructures. We also acknowledge the financial support by the BMBF, the DFG and the Landesstiftung BW. References [1] M. Kamada, T. Suzuki, F. Nakamura, Y. Mori, M. Arai, Appl. Phys. Lett. 49 (1986) 1263. [2] H.J. Bu¨hlmann, M. Ilegems, J. Electrochem. Soc. 138 (1991) 2795. [3] R.P. Taylor, P.T. Coleridge, M. Davies, Y. Feng, J.P. McCaffrey, P.A. Marshall, J. Appl. Phys. 76 (1994) 7966. [4] M. Bu¨ttiker, Phys. Rev. B 38 (1988) 9375. [5] Y. Ji, Y. Chung, D. Sprinzak, M. Heiblum, D. Mahalu, H. Strikman, Nature 422 (2003) 415. [6] U. Graumann, Praktikumsarbeit, Max-Planck-Institut fu¨r festko¨rperforschung, Stuttgart, 1998. [7] Furuuchi Chemical Corporation, Fine Trading Division 6-17-17 Minamioi, Shinagawa-ku Tokyo 140-0013, Japan. [8] B.E. Kane, L.N. Pfeiffer, K.W. West, Appl. Phys. Lett. 67 (1995) 1262. [9] E. Ahlswede, J. Weis, K. von Klitzing, K. Eberl, Physica E 12 (2002) 165. [10] F. Dahlem, J. Weis, K. von Klitzing, to be published.
Physica E 40 (2008) 1579–1581 www.elsevier.com/locate/physe
Alloyed ohmic contacts to two-dimensional electron system in AlGaAs/GaAs heterostructures down to submicron length scale Oktay Go¨ktas-, Jochen Weber, Ju¨rgen Weis, Klaus von Klitzing Max-Planck-Institut fu¨r Festko¨rperforschung, HeisenbergstraX e 1, 70569 Stuttgart, Germany Available online 29 September 2007
Abstract We have realized and investigated Au/Ge/Ni alloyed ohmic contacts to two-dimensional electron systems (2DES) embedded in various Al0:67 Ga0:33 As=GaAs heterostructures. Transport and geometrical investigations have been carried out to understand fundamental properties of these contacts. A recipe for creating low resistive and reliable alloyed ohmic contacts to 2DES is given, which is successful even for contacts to deep lying 2DES and contact sizes down to submicron length scale (0:2 mm). The contact resistance between 2DES and alloyed metal depends on the length and orientation of the interface line between 2DES and alloyed metal relative to the crystal orientation of the heterostructure. r 2007 Elsevier B.V. All rights reserved. PACS: 73.43.F; 73.40.Cg Keywords: Ohmic contacts; Two-dimensional electron system; Quantum Hall effect
Reproducible and reliable low resistive ohmic contacts are crucial for device applications. Au/Ge/Ni alloyed ohmic contacts are well established for GaAs based electronic and optoelectronic devices. They are also used to contact two-dimensional electron systems (2DES) embedded in Al1x Gax As=GaAs heterostructures. However, in case of 2DES, systematic investigations are rarely reported in literature [1–3], especially for small contact width. Understanding of ohmic contacts to 2DES is not only of technical but rather fundamental interest: The Bu¨ttiker edge state picture of the integer quantum Hall effect has attributed an extraordinary role to the contacts [4]. Recent interference experiments of edge states count on the dephasing properties of such contacts [5]. Here we report on our experience of making good ohmic contacts even down to the submicron length scale, highlighting some properties which one should be aware of when interpreting experimental results. About one decade ago, our group realized and investigated Au/Ge/Ni alloyed ohmic contacts to 2DES embedded Corresponding author. Tel.: +49 7116891520; fax: +49 7116891572.
E-mail address: [email protected] (O. Go¨ktas-). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.09.115
in various AlGaAs/GaAs heterostructures [6]. For this purpose, heterostructure mesas were formed by optical lithography and wet etching. Contact areas were defined by optical lithography, and finally cleaned. Au, Ge and Ni were evaporated in different layer sequences and in various compositions on top of the heterostructure. The metal layers were alloyed under N2 =H2 gas atmosphere at various temperatures between 380 C and 470 C. For characterization, the transmission line method (TLM) [1,2] was used to determine the contact resistance between the 2DES and the alloyed metal. To apply this method, mesa stripes were etched out of the heterostructure, and a series of contacts of varying distance d were alloyed along such a mesa (see Fig. 1). The two-terminal resistance between two contacts was determined by measuring the current-voltage characteristics at liquid Helium temperature or at about 30 mK. When plotting the two-terminal resistance, corrected by the respective series resistance due to the wiring in the measurement setup, as a function of the distance d between the contacts, a linear relation is usually obtained (see Fig. 1). The extrapolated value of the best fit line to the data at d ¼ 0 gives twice the contact resistance Rc of the 2DES/metal interface.
ARTICLE IN PRESS O. Go¨ktas- et al. / Physica E 40 (2008) 1579–1581
1k d [011]
W
R (Ω)
− [011]
2Rc 0
50 d (μm)
100
Fig. 1. TLM measurement on a mesa with a width w of 10 mm (heterostructure with the AlGaAs/GaAs junction at 50 nm). Triangular (rectangular) data points correspond to contacts defined on the mesa ¯ direction. Inset: Sketch of two mesa stripes along the [0 1 1] ð½0 1 1Þ (contacts in dark, mesa in light) with the respective crystal orientations.
Au and Ge were thermally evaporated together or in separate layers so that both were finally found in the weight ratio of 88% Au : 12% Ge on the heterostructure surface. In this ratio Au and Ge behaves as an eutectic and melts at 361 C. It was observed that both contact resistance and reproducibility strongly depend on the Ni content (see also Ref. [2]). The optimum Ni thickness was found to be one quarter of the total Au/Ge layer thickness. Best results were obtained when Ni was evaporated as the last layer. For contacting a 2DES up to a depth of D ¼ 80 nm, we have found that evaporating 107.2 nm of Au, 52.8 nm of Ge ð¼ 160 nm Au/Ge), followed by 40 nm of Ni gives reliable contacts. The alloying procedure consists of the following steps: (1) rapidly heating to 370 C and holding at 370 C for 120 s, (2) then for 50 s at 440 C, and (3) rapidly cooling down to room temperature. The whole annealing process is carried out under a N2 =H2 gas atmosphere (volume ratio 80% : 20%; pressure 300 mbar). The last step is under a gas flow with constant pressure whereas the first two steps are performed under a static gas overpressure. For a deeper lying 2DES of depth D, we found that the metal layer thicknesses have to be scaled by a factor s, D þ 30 nm , s¼ 110 nm
(1)
assuring that enough metal is available to penetrate deep enough into the heterostructure. The cleaning procedure after the lithography step for defining the contact areas is crucial for obtaining reliably low contact resistances. Before depositing the metal under high-vacuum condition; the sample is: (1) cleaned in an oxygen plasma to remove any remaining resist on the
heterostructure surface in the contact areas after developing, (2) etched for 2 min in Semico Clean 23 [7] and (3) etched for 5 s in HCl (30%), with a 5 s rinsing by DI water after each etching step. After drying with nitrogen gas, the sample is immediately transferred into the evaporator and pumped down to high vacuum. We emphasize that without careful performance of these cleaning steps the contacts are not reproducible. The TLM measurements of different mesa stripe and contact geometries clearly revealed that the contact resistance does not depend on the area below the contact but rather on the length of the border line between contact and 2DES. Furthermore we observe an anisotropy for the contact resistance, also reported in literature [1,8]. When the border between 2DES and alloyed metal is orientated perpendicular to the [0 1 1] direction of the heterocrystal, the contact resistance is lowest, whereas when the border is ¯ direction the contact resistances perpendicular to the ½0 1 1 are high—sometimes even not measurable, show a large spread and are not reproducible. A typical set of data is presented in Fig. 1. In Fig. 2, the normalized (unity length) contact resistance rc in the [0 1 1] direction for different mesa stripe width w—from 50 mm down to 1 mm—are shown. The normalization is done by multiplying the contact resistance value Rc with the respective mesa stripe width w. The error bars for rc are due to the error in determining the respective contact resistance Rc via the TLM method and the error in determining the mesa width w due to depletion at the edges of the mesa. The contact resistance per mm is around 250 O, and this value is almost constant for all mesa widths. This value is comparable to rc ¼ 0:2 Omm given in Ref. [2] where however no orientation for the 2DES/alloyed metal border is given. This observation implies that the interface between alloyed metal and 2DES is quite homogenous down to 1 mm. On ¯ direction the other hand, contacts defined in the ½0 1 1 show a large spread and the percentage of working contacts drastically decrease as the mesa width gets smaller. Although reported in literature [1], the origin of the contact resistance anisotropy—for instance, due to an
500
rc (Ω μm)
1580
250
0
1
10 w (μm)
100
Fig. 2. Unity length contact resistance rc ¼ Rc w as a function of mesa width w.
ARTICLE IN PRESS O. Go¨ktas- et al. / Physica E 40 (2008) 1579–1581
anisotropy in diffusion, strain or piezoelectricity—has not been clarified. To ensure low resistive contacts on Hall bars, we usually design meander-like borderlines between 2DES and alloyed metal with long sections perpendicular to the [0 1 1] direction. From low-temperature scanning probe experiments on quantum Hall samples we know that a partial depletion of the 2DES along the border to the alloyed metal exists [9]. This means that potential probes do not always enforce an equilibrium between edge channels in the Landauer–Bu¨ttiker formalism since some compressible stripes at the edge remain separated from the contacts by an incompressible region. Recent scanning probe measurements demonstrate adiabatic transport behavior in quantum Hall samples with working contacts when orientated in the worse direction, ¯ i.e., with the 2DES/metal border perpendicular to the ½0 1 1 direction [10]. Recently we have defined contacts of submicron size by using electron beam lithography. An SEM image of such a contact is shown in Fig. 3. The smallest attainable diameter successfully contacting a 2DES has been 0:2 mm. About 50% of these contacts are working and have a linear I–V characteristics with a contact resistance between 2 and 10 kO (in Ref. [3] 0:5 mm contacts with 5 kO were reported). Taking the perimeter p 0:2 mm as the border length between 2DES and metal, the normalized contact resistance lies between 1 and 6 kO for 1 mm, significantly higher than seen in Fig. 2. However this becomes less dramatic if the anisotropy is taken into account, reducing the effective border length. Recipe and rules presented here have been transferred to several groups in the last few years and have been successfully applied to contact 2DES in a variety of heterostructures. Even deep-lying low-density 2DES (D ¼ 220 nm, n ¼ 8 1010 cm2 ) have been easily contacted (5 O per 100 mm in the [0 1 1] direction) by following the scaling rule (1).
1581
200 nm Fig. 3. A cross sectional SEM image of a contact of about 0:2 mm diameter defined on a heterostructure with the AlGaAs/GaAs junction at 40 nm. The ohmic contact is encircled in white. The back going metal is a Cr/Au lead.
We thank U. Graumann and J. Schmid starting this work in 1998. M. Hauser, K. Eberl and W. Dietsche have supplied the heterostructures. We also acknowledge the financial support by the BMBF, the DFG and the Landesstiftung BW. References [1] M. Kamada, T. Suzuki, F. Nakamura, Y. Mori, M. Arai, Appl. Phys. Lett. 49 (1986) 1263. [2] H.J. Bu¨hlmann, M. Ilegems, J. Electrochem. Soc. 138 (1991) 2795. [3] R.P. Taylor, P.T. Coleridge, M. Davies, Y. Feng, J.P. McCaffrey, P.A. Marshall, J. Appl. Phys. 76 (1994) 7966. [4] M. Bu¨ttiker, Phys. Rev. B 38 (1988) 9375. [5] Y. Ji, Y. Chung, D. Sprinzak, M. Heiblum, D. Mahalu, H. Strikman, Nature 422 (2003) 415. [6] U. Graumann, Praktikumsarbeit, Max-Planck-Institut fu¨r festko¨rperforschung, Stuttgart, 1998. [7] Furuuchi Chemical Corporation, Fine Trading Division 6-17-17 Minamioi, Shinagawa-ku Tokyo 140-0013, Japan. [8] B.E. Kane, L.N. Pfeiffer, K.W. West, Appl. Phys. Lett. 67 (1995) 1262. [9] E. Ahlswede, J. Weis, K. von Klitzing, K. Eberl, Physica E 12 (2002) 165. [10] F. Dahlem, J. Weis, K. von Klitzing, to be published.