Pergamon Solid-State Electronics Vol. 41, No. 1 I, pp. 1715-1719. 1997

© 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

PII: S0038-1101(97)00159-7 0038-1101/97 $17.00 + 0.00

IMPROVED PERFORMANCES OF InGaP SCHOTTKY CONTACT WITH Ti/Pt/Au METALS AND MSM PHOTODETECTORS BY (NH4)2Sx TREATMENT

CHING-TING LEEr, MOU-HSIEN LAN and CHANG-DA TSAIt Institute of Optical Sciences, National Central University, Chung-Li, Taiwan, People's Republic of China

(Received 17 March 1997)

Abstract To extend the high performances of the GaAs MSM photodetectors with InGaP buffer and capping layers, the (NH4)2S~ treatment of InGaP is investigated. The surface states are reduced by sulfur passivation. Whereupon, the performances of InGaP Schottky contact with Ti/Pt/Au metals are improved. The improved dark current and insensitive responsivity with incident optical power are demonstrated by suitable process control of sulfur passivation. © 1997 Elsevier Science Ltd

I. INTRODUCTION

For the compound semiconductor used in the electronic and optoelectronic circuits, the Schottky contact plays an important role in the performances of its resultant devices. Since the Schottky barrier height depends on the conduction band discontinuity, a wide bandgap semiconductor was used extensively for the Schottky contact. In addition to the lattice match between AIGaAs and GaAs, a wide bandgap AIGaAs capping layer can effectively passivate a GaAs surface because of the absence of the trap- induced effect[l]. However, the oxidation of AIGaAs will degrade the performance of the resultant devices and deteriorate their long term reliability. Recently, the aluminum-free, wide bandgap material In0sGa05P (referred to as InGaP) lattice matched to the GaAs was successfully grown by molecular beam epitaxy (MBE)[2,3] and organometallic vapor phase epitaxy (OMCVD)[4,5]. By using the InGaP/GaAs heterostructures, the high performances of the resultant metal-semiconductor field effect transis- tors (MESFETs)[6], high electron mobility transistors (HEMTs)[7], heterojunction bipolar transistor (HBTs)[8], P- I -N beterojunction photodiodes[9], metal-semiconductor-metal (MSM) photodetec- tors[10], and quantum well lasers[l l] were demon- strated. According to the reported results, the significant virtues of the InGaP include a low deep level concentration[l 2], a lower reactivity with carbon and oxygen, a processing insensitive surface recombi- nation on InGaP surface, and a very low recombina- tion velocity at the InGaP-GaAs interface[13]. Moreover, the high etching selectivity between InGaP and GaAs can offer a processing advantage

tTo whom correspondence should be addressed.

concerning yield and homogenity[14]. These advan- tages make InGaP a promising alternative for AIGaAs as a wide bandgap material.

Many useful Schottky contact materials on InGaP were developed[15]. Further, thermal reliability and characterization of the Ti/Pt/Au Schottky properties were reported[16]. However, the poor surface properties of compound semiconductors impede the performances of electronic and optical devices based on the metal-semiconductor and metal-insulator- semiconductor structures. Therefore, to improve performances of resultant devices and integrated circuits, the development of an appropriate surface passivation has received increasing interest for the application of these materials. Sulfur(S) treatment was reported to offer effective surface passivation of GaAs[17,18] and InP[19,20]. In addition to the studies of the structure and chemical bonds of improved GaAs and lnP surface treated with (NH4)2S~ solution, significant performances of the resultant devices caused by sulfur surface passivation were demonstrated[17,18,21 ]. For promising widegap material InGaP material, the surface recombination velocity and surface states can be further reduced by sulfur surface passivation[15,22,23].

2. SCHOTTKY CONTACT FABRICATION AND RESULTS

The epitaxial layers for Schottky contact studies were grown by a horizontal low-pressure organometallic vapor phase epitaxy (LP-OMCVD) on an (100)-oriented Si-doped GaAs substrate. The growth conditions and procedures were reported previously[16]. After Si-doped GaAs buffer layer (0.5/zm, 2 x 10'Scm -3) was first grown on the n+-GaAs substrate, a lattice-matched undoped InGaP epilayer with a thickness of 1/~m was grown

1715

1716 C.-T. Lee et al.

successively. The unintentional concentration of the lnGaP layer was measured as about 5 × 10 '~ cm -3.

Because of the demonstration of high perform- ances and reliability of Ti/Pt/Au on InGaP Schottky contact[10], the Ti /P t /Au metals are used to study the Schottky performance without and with sulfur treatment. The epitaxial materials were cleaned using acetone and methanol. After the back side of the grown samples was lapped down to about 80#m and etched away about 1/tm by chemical solution of H2SO4:H2Oz:H20= 1:8:80; Ohmic metals of AuGeNi/Au (100/300nm) were evaporated and thermally annealed at 400°C for 3 min. To remove the native oxide of InGaP layer, the samples were etched using chemical solution of NH4OH:H20 = 1:10 for 30 s. In order to understand and compare Schottky properties, various different surface treat- ments were used before Schottky metals of Ti/Pt/Au (50/100/300nm) were evaporated by electron-gun evaporator, successively. The diameter of the Schottky contact region is 200/~m. Those surface treatments were carried out by soaking in supersatu- rated ammonium polysulfide ((NH4)2Sx) solution for different time and at different temperature. Further, to study the function of the de-ionized (DI) water rinse, some of the surface-treated samples were rinsed and then blown dry with dry N2 gas while the others were evaporated by Ti/Pt/Au directly after blowing with dry N2 gas.

Figure 1 shows the I - V characteristics of the Au/Pt/Ti/InGaP Schottky contacts without and with sulfur treatments. The measurements of the I - V characteristics were performed using an HP4145B semiconductor parameter analyzer. The sulfur treat- ment processes include soaking in (NH4)2S, at 60°C for 30 min and then DI water rinse for 10 min, and only N2 blow without DI water rinse. It can be obtained that the Schottky barrier height and ideality factor are 0.98 ev and 1.18 for the one without sulfur treatment, respectively. While the two kinds of the

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Fig. l.Current-voltage characteristics of Au/Pt/Ti/InGaP Schottky contact with and without (NH+)2Sx treatment.

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sulfur-treated samples exhibit similar I - V character- istics, whose corresponding values are 1.06ev and 1.09. It can be seen that the Schottky barrier height and ideality factor can be improved by sulfur passivation caused by the reduction of surface states. The Schottky performances with and without DI water rinse exhibit similar results. However, the DI water rinsing process can move away residual particles to obtain uniform sulfur distribution on the surface, the rinsing process is suitable for the fabrication of devices. Further, to study the dependence of Schottky performance on the sulfur treatment time, the samples are first soaked in (NH4)2S~ solution at 60°C for 15, 30 and 60min, respectively, and then rinsed by DI water for 10 min. The measured I - V characteristics are shown in Fig. 2. Those corresponding Schottky barrier height and ideality factor are also listed in Fig. 2. It is seen that Schottky barrier height of 1.06 ev and ideality factor of 1.09 for the Au/Pt/Ti/InGaP Schottky contact with sulfur treatment at 60°C for 30 min can be achieved. Comparison the Schottky performance of Au/Pt/Ti/InGaP contact with and without sulfur treatment, both Schottky barrier height and ideality factor can be improved by suitable process control.

3. MSM PHOTDETECTORS FABRICATION AND RESULTS

The epitaxial structures designed as MSM photodetectors were grown on undoped semi- insulating (100) GaAs substrate by LP-OMCVD. The designed configuration is shown in Fig. 3. The growth conditions and processes are the same as those for Schottky contact studies mentioned above and were reported previously[16]. An undoped GaAs layer (l/.tm) was used as absorption layer. To improve carrier confinement, a wide bandgap undoped InGaP hetero-buffer layer (0.3 ~m) was grown on GaAs substrate. Further, an undoped InGaP capping

Improved performances of InGaP Schottky contact with Ti/Pt/Au metals

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Fig. 3. Schematic structure of MSM photodetector-grown epitaxial layers.

layer (50 nm) was grown on the GaAs absorption layer for making improved Schottky contacts. A Si3N4 film (150nm) was deposited on the grown samples. By using photoresister (PR) as mask, the photosensitive region (100 x 100 pm 2) was defined by standard photolithography technique. The Si3N4 layer without PR passivation was then etched using buffer oxide etchant (BOE). After etching with chemical solution of NH4OH:H20 = 1:10 to remove the native oxidation of the capping layer InGaP surface, the interdigital Ti/Pt/Au (50/100/300nm) Schottky metals were deposited on the unpassivated photosensitive region of the InGaP capping layer without and with (NH4)2S, treatment. The finger width and spacing of the interdigital structure are 1 and 3 #m, respectively. The (NH4)2S, treatment was carried out at 60°C for 30 min and then DI water rinse for 10 min.

Figure 4 shows the dark current as a function of the bias voltage without and with sulfur treatment for same configuration of MSM Photodetectors taken from one epitaxial wafer. The measured dark current is less than 1.8 nA because of the high performance of InGaP buffer and capping layers[10], when the bias voltage is smaller than 20 V. However, the dark

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Fig. 4. Dependence of dark current on bias voltage for MSM photodetector with and without (NH4hS,. treatment.

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current of the MSM photodetector without sulfur treatment increases gradually for higher bias voltages. While the dark current of the MSM photodetector with sulfur treatment is almost insensitive to the bias voltage. In general, the tunneling injection through metal-semiconductor barrier caused by the high electric field near the edge of the metal contact is a primarily origin of the soft I - V characteristics[24]. The electrons tunnel into the surface traps of semiconductor and are then accumulated, which in turn is likely to modify the electric field distribution near the contact edge. The induced electric field by trapped charges is reversed with the bias voltage. The more the surface traps are occupied, the higher the induced electric field is constructed to counteract the increasing field caused by bias voltage. Whereupon, the soft dark current increasing with bias voltage is observed in the MSM photodetector without sulfur treatment. In contrast, according to the report of interface properties of (NH4)2S:treated In05Ga05P Schottky contacts[15], the sulfur treatment can passivate the Phosphorus- vacancy-related interface deep traps and reduce the density of surface states[23]. In addition, the surface recombination velocity can be reduced by sulfur passivation[25]. Whereupon, the insensitive function of dark current to bias voltage can be deduced in the MSM photodetectors with sulfur treatment. There- fore, the soft breakdown phenomenon can be suppressed for the sulfur treated photodetector.

Figure 5 show the photocurrent as a function of the bias voltage for the MSM photodetectors with and without sulfur treatment under different incident optical power at wavelength of 830 rim. It is worth noting that the photocurrent of the MSM photode- tector with sulfur treatment is nearly independent on the bias voltage until 25 V as compared with that of the one without sulfur treatment. At lower bias voltages, the photocurrent of the both kinds of photodetector is almost the same. However, the

1718 C.-T. Lee et al.

photocurrent of the one without sulfur treatment increases rapidly at higher bias voltages. It can be seen that the onset bias voltage reduces slightly with the incident optical power. According to the measurement results of dark current and photocur- rent as a function of bias voltage shown in Fig. 4 and 5, the performance improvements of the MSM photodetector using sulfur passivation are verified.

The responsitivities of the MSM photodetectors with and without sulfur treatment as a function of the incident optical power with a wavelength of 830 nm at bias voltage of 25 V are shown in Fig. 6. The measured responsivity higher than the theoretical maximum responsivity of about 0 .32A/W for 0.83 nm light can be attributed to the photocurrent gain in MSM photodetectors[26,27]. It can be seen that the responsivity of the photodetector with sulfur treatment is nearly independent of the incident optical power when it is smaller than 100 #W. While the responsivity of the MSM photodetector without sulfur treatment decreases gradually with incident optical power caused by the photocurrent variation at bias voltage of 25 V. The very gradual decrease of responsivity caused by incident optical power can be attributed to the low surface recombination velocity and surface traps of the InGaP layer. Since the responsivity of the MSM photodetectors with sulfur treatment is independent on incident optical power, it can be concluded that the surface recombination velocity, surface traps and dangling bonds of the as-treated InGaP are significantly improved by the sulfur passivation.

4. C O N C L U S I O N

The InGaP Schottky contact with Ti/Pt/Au metal was demonstrated to posses superior performances and thermal reliability compared with those of GaAs Schottky contact[16]. High performances and ther-

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bias voltage.

mal reliability of GaAs MSM photodetectors with lnGaP buffer and capping layer were reported previously[10]. The high performances of InGaP Schottky contact with Ti/Pt/Au metals for MSM photodetectors applications can be attributed to the inherently excellent properties of InGaP itself compared with A1GaAs. However, since the surface states of the InGaP can be further reduced by using (NH4)2S, treatment, both dark current and responsi- tivity of the resultant MSM photodetectors are improved from suitable process control of sulfur passivation. Not only the higher bias voltage can be applied, but insensitive responsivity with incident optical power is obtained for the MSM photodetector with sulfur treatment. Therefore, the GaAs MSM photodetector with lnGaP buffer and capping layers by using sulfur passivation can be expected to be more suitable in practical electrooptical applications.

Acknowledgements--The epitaxial growth of the samples form the Telecommunication Laboratories of Chungwa Telecommunication Co. Ltd. is acknowledged. This work was supported by National Science Council of the Republic of China under Grant NSC 86- 2215-E008-017.

REFERENCES

1. Burroughes, J. H., IEEE Photon. Technol. Lett., 1991, 3, 660.

2. Chen, Y. K., Wu, M. C., Kuo, J. M., Chin, M. A. and Sergent, A. M., Appl. Phys. Lett., 1991, 59, 2929.

3. Zhang, G., N/ippi, J., V~inttinen, K., Asonen, H. and Pessa, M., Appl. Phys. Lett., 1992, 61, 96.

4. Takikawa, M., Ohori, T., Takechi, M., Suzuki, M." and Komeno, J., J. Cryst. Growth, 1991, 107, 942.

5. Shiao, H. P., Wang, C. Y., Tu, Y. K., Lin, W. and Lee, C. T., Solid-St. Electron., 1995, 38, 2001.

6. Hyuga, F., Aoki, T., Sugitani, S. and Asai, K., Apply. Phys. Left., 1992, 60, 1963.

7. Takikawa, M. and Joshin, K., IEEE Electron Device Lett., 1993, 14, 406.

8. Fresina, M. T., Ahmari, D. A., Mares, P. J., Harmann, Q. J., Feng, M. and Stillman, G. E., IEEE Electron Device Lett., 1995, 16, 540.

9. Hasse, M. A., Hafich, M. J. and Robinson, G. Y., Appl. Phys. Lett., 1991, 58, 616.

10. Tsai, C. D., Shiao, H. P., Lee, C. T. and Tu, Y. K., IEEE Photon. Technol. Left., 1997, 9, 660.

11. Ohkubo, M., ljichi, T., Iketani, A. and Kikuta, T., 1EEE J. Quantum Electron., 1994, 30, 408.

12. Chen, Y. J. and Pavlidis, D., IEEE Trans. Electron Devices, 1994, 41, 637.

13. Olson, J. M., Ahrenkiel, R. K., Dunlavy, D. J., Keyes, B. and Kibber, A. E., Appl. Phys. Left., 1990, 55, 1208.

14. Lothian, J. R., Kou, J. M., Pearton, S. J. and Ren, F., J. Electron Mater., 1992, 21, 441.

15. Kwon, S. D., Kim, C. H., Kwon, H. K., Choe, B. D. and Lim, H., J. Appl. Phys., 1995, 77, 2202.

16. Lee, C. T., Shiao, H. P., Yeh, N. T., Tsai, C. D., Lyu, Y. T. and Tu, Y. K., Solid-St. Electron., 1997, 41, 1.

17. Sandroff, C. J., Nottenburg, R. N., Bischoff, J. C. and Bhat, R., Appl. Phys. Lett., 1987, 51, 33.

18. Lee, J. L., Kim, D., Maeng, S. J., Park, H. H., Kang, J. Y. and Lee, Y. T., J. Appl. Phys., 1993, 73, 3539.

19. Iyer, R., Chang, R. R. and Lile, D. L., Appl. Phys. Lett., 1988, 53, 134.

20. Maeyama, S., Sugiyama, M., Heun, S. and Oshima, M., J. Electron. Mater., 1996, 25, 593.

Improved performances of InGaP Schottky contact with Ti/Pt/Au metals 1719

21. Schade, U., Koilakowski, St., B6ttcher, E. H. and Bimberg, D., Appl. Phys. Lett., 1994, 64, 1389.

22. Pearton, S. J., Ren, F., Hobson, W. S., Abernathy, C. R. and Chakrabarti, U. K., J. Vac. Sci. Technol., 1994, B12, 142.

23. Moon, C. R., Choe, B. D., Kwon, S. D. and Lim, H., J. Appl. Phys., 1997, 81, 2904.

24. Rhoderick, E. H. and Willams, R. H., Metal- Semiconductor Contacts in Monographs in Electrical and

Electronic Engineering, No. 19, Oxford Science, Oxford, 1988, 109-132.

25. Yablonovitch, E., Sandroff, C. J., Bhat, R. and Gmitter, T., Appl. Phys. Lett., 1987, 51, 439.

26. Kingenstein, M., Kuhl, J., Rosenzweig, J., Moglestue, C., Hulsmann, A., Schneider, J. and Kohler, K., Solid-St. Electron., 1994, 37, 333.

27. Burro, J. and Eastman, L. F., 1EEE Photon. Technol. Lett., 1996, 8, 113.