Solid-State Electronics Vol. 42, No. 2, pp. 205±210, 1998 Copyright # Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1101/98 $19.00 + 0.00 S0038-1101(97)00226-8
IRIDIUM-BASED MULTILAYER CONTACTS TO n-GaAs T. LALINSKYÂ1, J. BREZA2, P. VOGRINCÏICÏ2, J. OSVALD1, ZÏ. MOZOLOVAÂ1 and J. SÏISÏOLAÂK1 Institute of Electrical Engineering, Slovak Academy of Sciences, DuÂbravskaÂ cesta 9, 842 39 Bratislava, Slovakia 2 Microelectronics Department, Slovak University of Technology, IlkovicÏova 3, 812 19 Bratislava, Slovakia
(Received 6 January 1997; in revised form 27 May 1997) AbstractÐWe report on the formation of Ir±Al/n-GaAs Schottky contacts via an Al±Ga exchange mechanism. Direct correlation between the electrical properties of the Schottky contacts and the shape of their Auger depth pro®le was observed. An Al±Ga exchange mechanism was found to be responsible for the Schottky barrier height enhancement. Based on the Ir±Al/n-GaAs contact formation, a multilayer Ir±In/n-GaAs contact system is proposed to form ohmic contacts through an In±Ga exchange mechanism. Ohmic contact behavior was observed when annealing the structure above 7508C for 10 s. Auger depth pro®les of as-deposited and annealed contacts are consistent with the proposed model of ohmic contact formation. # 1998 Elsevier Science Ltd. All rights reserved
Electrical and thermal stability of Schottky and ohmic contacts to n-GaAs constitute a challenging problem which motivates several groups of investigators to design novel contact systems and interface models. Recently Schottky gate contacts to n-GaAs with an enhanced barrier height and high thermal stability are required for devices based on heterostructures (heterostructure ®eld-eect transistors, HFET) and self-aligned gate FETs (SAG FET). Attention has been paid to M±Al/n-GaAs contact systems, where M stands for Ni, Co, Pt or Mo[4,5]. The attractivity of M±Al/n-GaAs contact systems follows from the eect of a signi®cant barrier height enhancement observed on high-temperature annealing[1,2,4]. The barrier height enhancement is attributed to the formation of a graded AlxGa1 ÿ xAs layer at the M±Al/n-GaAs interface due to an Al±Ga exchange reaction at elevated temperatures. Another interesting property of these contact systems is their high thermal stability, which is a consequence of higher heats of formation of M±Al compounds in comparison with those of M±Ga and M±As[3,5]. Thermal stability of ohmic contacts to n-GaAs is of critical concern for device applications. Recently ohmic contacts based on solid-state reactions, such as NiInW/n-GaAs, PdIn/n-GaAs, Pd±In±Ge/nGaAs and Si/Pd(Si, Ge)/n-GaAs, have attracted a great deal of interest because they form uniform interfaces with very low contact resistance values. At the same time, the conventional lift-o process can be utilized to de®ne the contact pattern, which is dicult to achieve by band-gap engineer205
ing techniques in which an InGaAs/GaAs heterostructure is grown by molecular beam epitaxy. Rather complicated solid-phase reaction mechanisms occurring at the interface of ohmic contacts in the course of thermal treatment may lead to serious problems with their thermal stability. Both diusion-controlled and reaction-controlled mechanisms can contribute to contact degradation at various temperatures. We have already reported on the advantages of the Ir±Al/n-GaAs contact system as a stable highbarrier Schottky contact[12,13]. Unlike in M±Al/nGaAs contact systems based on M±Al compounds, metallurgically stable Ir±Al amorphous-like multilayers were used. In addition the Ir±Al/n-GaAs multilayer contact systems allowed us to control both the barrier height and the thermal stability of the interface through the ®rst interfacial Schottky layer in contact with GaAs. A model of barrier height enhancement based on solid phase epitaxy of the graded AlxGa1 ÿ xAs layer at the interface at elevated temperatures via a simple Al±Ga exchange mechanism was adopted to explain the electrical properties of the contacts. In this paper, in order to support the proposed model of contact formation, the correlation between the electrical properties of Ir±Al/n-GaAs Schottky contacts and the shape of their Auger depth pro®le is investigated. Based on this model, a multilayer Ir±In/n-GaAs contact system is suggested to form a non-alloyed ohmic contact via an In±Ga exchange mechanism. Finally, Auger depth pro®les of asdeposited and annealed Ir±In/n-GaAs contacts are studied to con®rm ohmic contact formation.
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Fig. 1. Cross-sectional view of the studied structures.
The substrates were (100)-oriented, n-type wafers, with Si doping levels of 7 1016 cmÿ3. In the case of ohmic contacts the substrates with a concentration of 3 1017 cmÿ3 were also used. The samples were degreased in boiling trichloroethylene, acetone, and isopropylalcohol for 5 min in each. Prior to deposition the damaged surface layer was removed by etching in a sulfuric acid based solution. Nine circular contacts with various diameters (d = 40± 800 mm) were patterned using the lift-o technique. Before loading in a cryopumped evaporation system with a dual electron gun the samples were dipped into 1NH4OH:10H2O solution for 1 min in order to remove the native surface oxide. Sequential evaporation cycles were used to deposit Ir±Al and Ir±In multilayer ®lms. In addition, Ir±In(Te) multilayers with 15 wt% content of Te were included in the experiments. The multilayers consisted of six pairs of Ir±Al or Ir±In, the total thickness of metallization being about 60 nm. A 10 nm thick Ir layer was deposited as a cap layer in the case of the Ir±Al/n-GaAs contact. Three types of Ir±In/n-GaAs contacts (C1, C2, C3) with the same Ir/In ratio were deposited. These contacts diered from each other only in doping concentrations of GaAs wafers. Carrier concentrations were 7 1016 cmÿ3 and 3 1017 cmÿ3 for systems C1 and C2, respectively. In order to increase the doping concentration at the interface on annealing, an In(Te) alloy with 15 wt% content of Te was used instead of pure indium in contact C3. Schematic representation of the multilayer contact is shown in Fig. 1. The pressure in the vacuum chamber during evaporation was lower than 10ÿ5 Pa. Capless rapid thermal annealing (RTA) of the deposited multilayers was carried out within a Si susceptor placed face-to-face to a sacri®cial GaAs wafer at temperatures from 450 to 8008C for 10 s. A back side alloyed ohmic contact based on AuGe±Ni metallization was formed after RTA.
Fig. 2. The Schottky barrier heights j, and ideality factor n, of the contact system after RTA in dependence on the temperature of annealing. 3. RESULTS AND DISCUSSION
Electrical properties of the contact systems before and after annealing were evaluated by I±V measurements. The barrier height j, ideality factor n, series resistance RS, and leakage resistance RL, were extracted using a computer ®tting method. Thermionic emission was assumed to be the dominant mechanism of current transport in our model. The extracted barrier heights and ideality factors of the contact systems after RTA are plotted in Fig. 2 in dependence on the temperature of annealing. A steep rise of the barrier height with the temperature of annealing was observed for the Ir±Al/n-GaAs contact whose ®rst 1 nm thick Ir layer was in contact with GaAs. The contact is thermally stable up to 8008C. The contact annealed at 8008C has a barrier height as high as 0.95 eV and ideality factor about 1.1. The barrier height enhancement was evaluated to be 200 meV in comparison with the contact annealed at 4508C for 30 s (which is the alloying temperature of the back side ohmic contact). In order to explain the barrier height enhancement, Auger depth pro®ling of the circular contacts (d = 500 mm) was carried out immediately afterwards. In Fig. 3(a±c), Auger depth pro®les are shown of an as-deposited Ir±Al contact and of the contacts annealed at 600 and 8008C for 10 s. The multilayered composition with six pairs of Ir±Al is clearly resolved in the as-deposited contact. This is also indicated in both 600 and 8008C annealed contacts in spite of some Ir±Al interlayer diusion. The contact systems maintains the initial, as-deposited multilayer composition, which was con®rmed also by RBS analysis. As shown in Fig. 3(a), in the as-deposited initial contact the Ga curve overlaps with the As curve at the interface and no Ga outdiusion into the Ir±Al ®lm is observed. However, in the annealed contacts (Fig. 3(b) and (c)) the Ga
Iridium-based multilayer contacts to n-GaAs
Fig. 3. (a±c) (from top to bottom): Auger depth pro®les of the Ir±Al multilayer structure. (a) as deposited, (b), (c) annealed at 6008C and 8008C, respectively, for 10 s.
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Fig. 4. The total resistance of the contact in dependence on the reciprocal contact diameter.
curve lies above the As curve at the interface and Ga out-diused far into the Ir±Al ®lm. It is also obvious that Ga out-diusion becomes stronger with increasing temperature, hence the Al±Ga exchange reaction leading to AlxGa1 ÿ xAs formation is very likely. Auger data (peak shapes) similarly suggest that no new phases were formed in the contact upon annealing. This is exactly what one would have expected supposing that the exchange mechanism exists. Similar shapes of Auger depth pro®les were observed also in annealed Al/n-GaAs and Mo±Al/n-GaAs[4,5] contacts where an Al±Ga exchange reaction was found to be responsible for interfacial AlxGa1 ÿ xAs layer formation. The interfacial AlxGa1 ÿ xAs layer was clearly revealed in high-resolution cross-sectional transmission electron microscope images[4,5,14]. However, it was dicult to determine the composition of the layer because of its graded composition and relatively small thickness (about 5 nm).
Based on these results, the multilayer Ir±In/nGaAs contacts could lead to the formation of an InxGa1 ÿ xAs layer at the interface on high-temperature annealing via an In±Ga exchange mechanism, and hence an ohmic behavior should be observed. As shown in Fig. 2, the Ir±In/n-GaAs and Ir± In(Te)/n-GaAs structures behaved as Schottky contacts up to 7508C. Ohmic contact behavior, however, appeared on annealing above 7508C for 10 s. A slight increase of the barrier height was observed for all contacts, reaching a maximum at about 6508C. Furthermore, one can also see that the barrier height curves of the contact adhere to the increase of the interfacial doping concentration. The lowest barrier height values were found in contact C3. This is consistent with the formation of a narrow metal/semiconductor potential barrier which allows thermionic-®eld emission to contribute to the thermionic emission mechanism of the charge transport. The Cox and Strack method was employed to evaluate the contact resistance using the circular contacts of various diameters. The total resistance between the top contacts and the back side contact RT, was calculated directly from I±V characteristics. An excellent linear voltage dependence was observed at both polarities independently on the contact areas. Figure 4 shows the dependence of the total resistance on the reciprocal contact diameter for contact systems C2 and C3 annealed at 8008C for 10 s. The contact resistance can be extracted by curve ®tting if the substrate resistivity and thickness are known. A bulk resistivity of GaAs substrate was determined to be 5.39 10ÿ3 O cm using a Hall measurements. The thickness was about 355 mm. The contact resistivities of Ir±In(Te)/n-GaAs and Ir±In/n-GaAs contacts were about 9 10ÿ3 and 8 10ÿ4 O cm2, respectively. A positive role of Te impurity in
Fig. 5. Auger depth pro®les of the Ir±In multilayer structure: solid line ± as deposited, dotted line ± RTA processed at 7508C for 10 s.
Iridium-based multilayer contacts to n-GaAs
ohmic contact formation can be seen. It should be emphasized that neither additional optimization of the Ir±In or Ir±In(Te) composition nor of annealing conditions, with respect to contact resistance values, was performed. In order to support the suggested model of ohmic contact formation, Auger depth pro®ling of asdeposited and 7508C annealed C3 contacts was carried out. In Fig. 5, both depth pro®les are shown in order to illustrate more clearly the composition changes the Ir±In(Te)/n-GaAs interface. In the asdeposited contact, some In and Te seem to pile up at the contact surface and at the interface. One can also observe abrupt changes in Ga and As concentrations at the interface. However, in the annealed contact the piling-up of both In and Te at the contact surface is increased and remarkable in-diusion of the two elements into the GaAs substrate is observed. Moreover, a long tail of Ga extends into the Ir±In metallization while the pro®le of As decays at the interface. The concentration pro®le of Ir, which is not shown here, remains unchanged in comparison with that of the as-deposited contact. Finally, Auger peak shapes, similarly like in Ir±Al/ n-GaAs contacts, reveal that no new phases are formed in the Ir±In(Te)/n-GaAs contact upon annealing. The presented Auger depth pro®les strongly suggest that an exchange of In and Ga atoms occurs between the Ir±In multilayer and the GaAs substrate, and the formation of a low-barrier InxGa1 ÿ xAs layer at the interface could be responsible for ohmic contact behavior. Additionally, a highly doped surface semiconductor layer can be formed at the interface by substitution of Te atoms in As lattice sites, thus forming a shallow donor level. However, the role of Te in ohmic contact formation to n-GaAs is still a point of discussion[16,17]. It should be also noted that the Auger depth pro®les in Fig. 5 are fully consistent with those recently published for the PtIn2/n-GaAs contact. The In±Ga exchange mechanism was demonstrated to be responsible for ohmic contact formation. Under optimum conditions, speci®c contact resistances as low as 3 10ÿ6 O cm2 were obtained in the temperature range from 800 to 8508C. Since the contacts were formed at high temperatures, excellent thermal stability was also achieved. Relatively high contact resistance values of the Ir±In/n-GaAs ohmic contact could be further decreased by increasing the content of In in the Ir± In multilayer. The contact resistivity decreased from the order of 10ÿ4 O cm2 to 10ÿ6 O cm2 on raising In concentration from 5 to 20 at%. Likewise, rapid thermal annealing for 1 min seems to be better than that for 15 s in respect of lower contact resistivity values. Optimization of the Ir±In(Te) composition and annealing conditions will most likely lead to a further decrease in the contact resistivity.
209 4. CONCLUSION
Direct correlation between the electrical properties of Ir±Al/n-GaAs Schottky contacts and their depth pro®les was found. It was demonstrated that an Al±Ga exchange mechanism leading to the formation of a graded AlxGa1 ÿ xAs interfacial layer after high temperature annealing could be responsible for Schottky barrier height enhancement. Based on a model of Ir±Al/n-GaAs Schottky contact formation, a multilayer Ir±In/n-GaAs ohmic contact was proposed utilizing the formation of a graded low band gap InxGa1 ÿ xAs layer at the interface via an In±Ga exchange mechanism in the course of high temperature annealing. The predicted ohmic contact formation was observed after annealing at temperatures higher than 7508C for 10 s and con®rmed also by Auger depth pro®ling. A decrease of the relatively high contact resistivity from the order of 10ÿ3 O cm2 to 10ÿ4 O cm2 was observed on adding Te (as a shallow donor impurity in GaAs) into the Ir±In multilayer. A further decrease in the contact resistivity is expected on optimizing the Ir±In(Te) composition and annealing conditions. This allows to form hightemperature stable Ir±Al/n-GaAs Schottky contacts and Ir±In/n-GaAs ohmic contacts under the same annealing conditions via Al±Ga and In±Ga exchange mechanisms, respectively. Since the Ir based Schottky and ohmic contacts are formed at high temperatures (800±8508C) via a simple exchange mechanism without formation of any further new phases, very high thermal stability of the two kinds of metal/semiconductor interfaces can be expected. This predestinates the contacts to be potentially used in GaAs FET devices applied in high temperature working environment. However, further experimental veri®cation will be required to validate these expectations. Moreover, the presented Ir based GaAs contact technology makes also a promising device contact scheme for Al interconnecting metallization because there is no an underlying barrier layer required. AcknowledgementsÐThis work was supported, in part, by the Slovak Grant Agency for Science (Grants Nos. 2/ 4060/97 and 1753/94).
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