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s p ut , F 0 /C n ct la a th he ta s. ~TEM!, in situ Auger electron spectroscopy ~AES! JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 9 1 MAY 2003 Dow sputtering,9–11 Rutherford backscattering spectrometry ~RBS!,6,12 x-ray diffraction ~XRD!,6 and secondary ion mass spectrometry.13,14 However, little work using scanning trans- mission electron microscopy ~STEM! was done. In our pre- vious work, we demonstrated that STEM combined with en- ergy dispersive spectroscopy can offer a good account of the elemental profile.9,15,16 In comparison with the various el- emental profiling techniques, STEM perhaps offers the most accurate information about the layer thickness and diffusion distance. For example, it is often difficult to determine the actual layer thickness in the AES profile since the width of each elemental profile depends upon its sputter rate. The dif- ferent sputter rate associated with different elements can lead to distortions in layer thickness. For the RBS technique, the profile is a result of all the superimposed elemental signals. For some cases, it is not as straightforward to interpret the a!Present address: Ortel, a Division of Emcore Corp., 2015 W. Chestnut St., Alhambra, CA 91803; electronic mail: jshuang@emcore.com FIG. 1. STEM cross-section images of ~a! as-deposited and ~b! alloyed Au/Zn/Au/Cr/Au contacts. There was a p-InGaAs contact layer between p-InP and metal. The sample in ~b! was alloyed at 430 °C for 30 s. Scanning transmission electron micro and AuÕTiÕPtÕAuÕCrÕAu contacts to p-ty J. S. Huanga) Agere Systems, Optical Access Division, 2015 West Chestn C. B. Vartuli Agere Systems, 9333 South John Young Parkway, Orlando ~Received 27 November 2002; accepted 7 February 2 We studied the interfacial reaction of Au/Zn/Au p-InGaAs/p-InP using scanning transmission electro morphology was distinctly different in the two conta interdiffusion between the metal and InGaAs contact formed: one was rich in Au and the other was rich in G that a significant amount of As has outdiffused into Au/Cr/Au, only interfacial layers were involved in t and Au-Ga were formed, and the Cr layer remained in are discussed. © 2003 American Institute of Physic I. INTRODUCTION Achieving low resistance is vital for fabrication of semi- conductor lasers and photodiodes. The issues associated with high resistance include device speed, device performance, and reliability. Metal contact is one of the typical sources of high resistance. Obtaining low contact resistance for p-type semiconductors has been a challenging issue due to the high barrier height and heavy effective hole mass. For long wave- length applications, indium phosphide ~InP! systems have been of particular interest.1 Various combinations, such as pure Au,2 AuZn/Au,3,4 Au/Ge,1 In/Zn/Au,1 and Ti/Pt,5 have been tried to reduce the contact resistance for p-InP. Most of the previous characterization work for the study of contact reactions was primarily based on scanning elec- tron microscopy,1,6–8 transmission electron microscopy 9,10 5190021-8979/2003/93(9)/5196/5/$20.00 nloaded 13 Sep 2010 to 210.72.148.51. Redistribution subject to AIP lic copy study of AuÕZnÕAuÕCrÕAu e InGaAsÕInP Street, Alhambra, California 91803 lorida 32819 03! r/Au and Au/Ti/Pt/Au/Cr/Au contacts to microscopy. We found that the alloying systems. For Au/Zn/Au/Cr/Au, significant yer occurred. Two types of compound were and As. Another interesting observation was e Cr layer after alloying. For the Au/Ti/Pt/ reaction. Compounds of Au-Ga-In, Ti-As, ct. The mechanisms of compound formation @DOI: 10.1063/1.1565187# results in the RBS profile compared to those in the STEM profile. In this article, we describe the use of STEM to study the interfacial reactions of Au/Zn/Au/Cr/Au and Au/Ti/Pt/Au/ Cr/Au contacts to p-InP. It is shown that the alloying mor- phology in the Au/Zn/Au/Cr/Au and Au/Ti/Pt/Au/Cr/Au con- tacts is drastically different. For Au/Zn/Au/Cr/Au, a significant amount of interdiffusion involving the majority of the semiconductor contact layer occurred. For Au/Ti/Pt/Au/ Cr/Au, only the interfacial layers were involved in the reac- 6 © 2003 American Institute of Physics ense or copyright; see http://jap.aip.org/about/rights_and_permissions Dow tion. In a comparison of prealloying and postalloying, we also studied the diffusing metal species during the reaction and identified the metallic compounds. II. EXPERIMENT Epitaxial p-type InP layers were grown on a n-type InP substrate by the metal-organic chemical vapor deposition technique. Prior to p-metal deposition, p-InGaAs was grown to form a contact layer. A metal stack of 50 Å Au/150 Å Zn/500 Å Au/150 Å Cr/2000 Å Au or 60 Å Au/500 Å Ti/500 Å Pt/150 Å Au/150 Å Cr/2000 Å Au was deposited on the p-InGaAs layer and alloyed at 430 °C for 30 s. For compari- son, some samples were not alloyed. Au/Zn/Au metalliza- tion was deposited by thermal evaporation while Au/Ti/Pt/Au was deposited by e-beam evaporation. The metal thickness reading was based on a crystal monitor calibrated with ref- erence to Au. To study the chemical reaction, samples were examined by scanning transmission electron microscopy using a Hita- chi HD-2000 dedicated STEM operating at 200 kV. Cross- sectional samples were prepared in a FEI 200 focused ion beam system using the lift-out technique.17 The chemical el- ement analysis was done by energy dispersive spectroscopy using an EDAX Pheonix Pro SiLi detector with a resolution of 130 eV. The data were taken in line scan mode to obtain the elemental depth concentration profile.15,16 A ten-point moving average was used to smooth the data. To increase the confidence level of the results, each sample was scanned in five different areas. FIG. 2. STEM spectra of the Au/Zn/Au/Cr/Au contact with no alloy. The sample was scanned from the top surface into the InP substrate. FIG. 3. STEM spectra of Au/Zn/Au/Cr/Au contact alloyed at 430 °C for 30 J. Appl. Phys., Vol. 93, No. 9, 1 May 2003 s. The sample was scanned from the top surface into the InP substrate along AA8. nloaded 13 Sep 2010 to 210.72.148.51. Redistribution subject to AIP lic III. RESULTS AND DISCUSSION Figures 1~a! and 1~b! show the STEM cross-section im- ages of the p-InP/p-InGaAs/Au~bottom!/Zu/Au~middle!/Cr/ Au~top! samples before and after alloying, respectively. The sample in ~b! was alloyed at 430 °C for 30 s. In Fig. 1~a!, the InGaAs layer was intact prior to alloying. The Au/Zn/Au layers appeared to be intermixed, not separated. In Fig. 1~b!, the most interesting feature of the alloyed sample is the for- mation of scallop-type compounds. The compound formation in the InGaAs layer ~0.36 mm thick! was nonuniform. There were two distinct regions in the compounds. One was a large and dark compound, and the other was a small and white compound. Figure 2 shows the STEM spectra of the unan- nealed sample in Fig. 1~a! scanning from top metal to InP. The Au-Zn intermixing was evident in the spectra where Au and Zn signals overlapped with each other. There was little overlapping between Au-Zn and InGaAs, indicating that little chemical interaction at the metal-semiconductor inter- face occurred prior to alloying. Figure 3 shows the STEM scan along the line AA8 in Fig. 1~b!. The line AA8 represents the concentration profile of the large and dark compound. Figure 4 shows the STEM spectra along the line BB8 which represents the profile of the small and white compound. There were several features observed in the spectra. First, considerable interdiffusion between AuZnAu and InGaAs occurred, leading to the formation of the scallop-type com- pounds shown in Fig. 1~b!. The large and dark compound FIG. 4. STEM spectra of Au/Zn/Au/Cr/Au contact alloyed at 430 °C for 30 s. The sample was scanned from the top surface into the InP substrate along BB8. 5197J. S. Huang and C. B. Vartuli FIG. 5. STEM cross-section image of Au/Zn/Au/Cr/Au contacts alloyed at 430 °C for 30 s. ense or copyright; see http://jap.aip.org/about/rights_and_permissions Dow was a Au-rich mixture consisting of Au, Zn, In, Ga, and As. The small and white compound was rich in Ga and As, but deficient in In. Second, some amount of Zn appeared to have outdiffused to the top Au layer. Third, a significant amount of As outdiffused into the Cr layer to form Cr-As compound. The Cr-As compound appeared as the white layer in Fig. 1~b!. Fourth, Cr outdiffused to the bottom portion of the top Au layer. The formation of the Au-Cr mixture in the top Au layer was particularly evident in some samples. Figure 5 shows an example of the Au-Cr formation where the Au-Cr layer appears as a dim white color. Figure 6 shows the cor- responding STEM spectra of Fig. 5. Again, significant reac- tion between Au/Zn/Au and InGaAs occurred and the Cr-As compound was formed. The thickness of the InGaAs layer in this case was about 0.28 mm. A high concentration of Cr was observed in the bottom portion of the top Au layer, indicative of the Au-Cr mixture. Figures 7~a! and 7~b! show the STEM images of the FIG. 6. STEM spectra of the alloyed Au/Zn/Au/Cr/Au corresponding to Fig. 5. Outdiffusion of Cr in the top Au layer was observed. 5198 J. Appl. Phys., Vol. 93, No. 9, 1 May 2003 FIG. 7. STEM cross-section image of ~a! as-deposited and ~b! alloyed Au/ Ti/Pt/Au/Cr/Au contacts. The sample in ~b! was annealed at 430 °C for 30 s. nloaded 13 Sep 2010 to 210.72.148.51. Redistribution subject to AIP lic p-InP/p-InGaAs/Au~bottom!/Ti/Pt/Au~middle!/Cr/Au~top! samples before and after anneal, respectively. In the as- deposited sample shown in Fig. 7~a!, separate Au, Cr, Au, Pt, Ti, Au, InGaAs, and InP layers were observed. The Ti layer appeared white, and the Cr layer appeared dim white. After anneal, some interfacial layers at metal-semiconductor inter- face showed up. The interfacial layers were rough and ap- peared dark in Fig. 7~b!. Figures 8 and 9 show the corre- sponding STEM spectra of Figs. 7~a! and 7~b!. The samples were scanned from the top to the InGaAs layer. In Fig. 8, distinct elemental signals ~Au, Cr, Au, Pt, Ti, and Au! were detected. The InGaAs layer was present beyond a distance of 0.31 mm. The Au signal appeared to overlap with In, Ga, and As signals, indicating that some interfacial reaction between Au and InGaAs might have occurred prior to anneal. Upon anneal, there were several interesting changes in elemental signals as shown in Fig. 9. First, a shoulder started to de- velop on the Cr signal. The Cr shoulder overlapped with the Au signal from the middle Au layer to form a Cr-Au mixture. Second, the Ti and bottom Au layers were transformed into three layers of compounds labeled A, B, and C. The A layer was rich in Au, Ga, and In; the B layer was rich in Ti and As; the C layer was rich in Au and Ga. The A, B, and C layers were likely to be Au-Ga-In, Ti-As, and Au-Ga compounds, respectively. To verify the formation of the three layers, five areas were checked and they all showed similar elemental profiles. Based on the STEM images and concentration profiles ~Table I!, we can envision the formation mechanisms of the compounds. In the following, we discuss the mechanisms of compound formation in Au/Zn/Au/Cr/Au and Au/Ti/Pt/Au/ Cr/Au separately. FIG. 8. STEM spectra of as-deposited Au/Ti/Pt/Au/Cr/Au contact. J. S. Huang and C. B. Vartuli FIG. 9. STEM spectra of annealed Au/Ti/Pt/Au/Cr/Au contact. The anneal condition was 430 °C for 30 s. ense or copyright; see http://jap.aip.org/about/rights_and_permissions Cr ant Dow Figure 10 illustrates the formation mechanism for p-InGaAs/Au/Zn/Au/Cr/Au system. Prior to alloying @Fig. 10~a!#, Au and Zn have intermixed with each other. During alloying in Fig. 10~b!, interdiffusion occurred. The large Au- rich compound shown in Fig. 10~c! might have been the result of interdiffusion of Au and InGaAs. The high Au con- centration in the large compound was indicative of signifi- cant Au diffusion. Near the interface of metal and the Au- rich compound, indium atoms might have been depleted during the formation of the Au-rich compound. The regions of In depletion eventually formed small compounds that were rich in Ga and As. The Cr-As compound was a result of excessive outdiffusion of As. In some cases, the top portion of the Cr layer has decomposed and reacted with the top Au layer to form a Au-Cr mixture. Figure 11 shows the schematics of compound formation in the p-InGaAs/Au/Ti/Pt/Au/Cr/Au system. Unlike the case of Au/Zu/Au, only layers adjacent to the metal- semiconductor interface were involved during the reaction. At a glance, the interfacial reaction appears to be simple FIG. 10. Schematics of compound formation in InGaAs/Au/Zn/Au/Cr/Au sample: ~a! as deposited, ~b! during alloy, and ~c! after alloy. TABLE I. Summary of morphology and compound formation for Au/Zn/Au/ Sample Au/Zn/Au/Cr/Au Illustrations Figs. 1–6 Fig. 10 Morphology Nonuniform compound formation involving signific Compound formation Large compound: Au ~Zn-In-Ga-As! Small compound: Ga-As ~In! nloaded 13 Sep 2010 to 210.72.148.51. Redistribution subject to AIP lic from the STEM image shown in Fig. 7~b!. However, the STEM concentration profile shown in Fig. 9 indicates that the compound formation may have involved complex chemi- cal reactions. We propose that Ti and Au atoms diffused downwards while Ga and As atoms diffused upwards. The Au diffused into the InGaAs layer and reacted with Ga and In to form a Au-Ga-In compound ~A layer!. The interdiffu- sion between Ti and As led to formation of a Ti-As com- pound ~B layer!. The original Au layer reacted with Ga to form a Au-Ga compound ~C layer!. IV. CONCLUSIONS We have studied the interfacial reaction of Au/Zn/Au/ Cr/Au and Au/Ti/Pt/Au/Cr/Au contacts to p-InGaAs/p-InP. The STEM data showed that Au/Zn/Au/Cr/Au and Au/Ti/Pt/ Au/Cr/Au contacts exhibited distinctly different alloying FIG. 11. Schematics of compound formation in InGaAs/Au/Ti/Pt/Au/Cr/Au sample: ~a! as deposited, ~b! during alloy, and ~c! after alloy. /Au and Au/Ti/Pt/Au/Cr/Au contacts to p-InGaAs/p-InP. Au/Ti/Pt/Au/Cr/Au Figs. 7–9 Fig. 11 diffusion Uniform compound formation involving only interfacial layers Au-Ga-In ~A layer! Ti-As ~B layer! 5199J. Appl. Phys., Vol. 93, No. 9, 1 May 2003 J. S. Huang and C. B. Vartuli Au-Ga ~C layer! ense or copyright; see http://jap.aip.org/about/rights_and_permissions morphology. The morphology in the former was more non- uniform. For Au/Zn/Au/Cr/Au, significant interdiffusion be- tween metal and InGaAs occurred upon alloying at 430 °C for 30 s, forming two types of compound. One was large and Au rich, and the other was small and GaAs rich. A significant amount of As has outdiffused to react with the Cr layer. For Au/Ti/Pt/Au/Cr/Au, only interfacial layers were involved in the chemical reaction. Upon annealing at 430 °C for 30 s, Ti and Au atoms diffused downwards while Ga and As atoms diffused upwards. Consequently, the interdiffusion between the metal and semiconductor components resulted in the for- mation of Au-Ga-In, Ti-As, and Au-Ga compounds. ACKNOWLEDGMENTS The authors would like to acknowledge Larry Cote for support of the work, P. C. Chen for review of the manuscript, P. Thai for assistance in lithography, A. Konkar for assistance in metal deposition, and Michele Jamison for sample prepa- ration. 1 E. Kupal, Solid-State Electron. 24, 69 ~1981!. 2 N. S. Fatemi and V. G. Weizer, J. Appl. Phys. 77, 5241 ~1995!. 3 C. L. Cheng, L. A. Goldren, B. I. Miller, J. A. Rentschler, and C. C. Shen, Electron. Lett. 18, 755 ~1982!. 4 K. Tabatabaie-Alavi, A. N. 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