The interaction of gold thin films with InP

Vacuum/volume40/numbers 1/2/pages 189 to 191/1990

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The i n t e r a c t i o n of gold thin films w i t h InP R V e r e s e g y h f i z y , B P6cz, I M o j z e s a n d G G o m b o s , Research Institute for Technical Physics of the Hungarian Academy of Sciences, H- 1325 Budapest, Ujpest 1, PO Box 76, Hungary

The basic component of the most popular contacts to InP is gold. The interaction of gold with InP begins at low temperatures and is accompanied by the strong evaporation of phosphorus. The evaporation vs temperature curve is characterized by a multiple peak structure of the phosphorus loss. The peak structure is due to the formation of Au-ln alloys with growing In content at higher temperatures, the formation and decomposition of a Au-P compound and the melting of the metallization. Several samples were quenched at characteristic points of the evaporation vs temperature curve for further investigations. Scanning electron microscopy and Auger electron spectroscopic depth profiling of these samples showed changing surface morphology and increasing In content in the gold film in samples which were quenched subsequently at higher temperatures.

1. Introduction

InP is an important material for high frequency and optoelectronic applications. Gold is the basic component of many contact metallizations to InP. This metal strongly enhances the thermal decomposition of InP at moderate temperatures ~, which largely contributes to the undesirable effect of'bailing up' of the contacts, so, from the point of view of device technology, it is particularly important to know the thermal behaviour of A u InP contacts. In previous work 2'3 numerous phases were found which were formed as a result of reaction between gold and InP. e - A u In, Au4In, AugIn4, and AuInz phases were identified by X-ray diffraction (XRD) 2. Piotrowska et al found Au3In and Au2P3 phases in samples which were heat-treated in forming gas in the temperature range 320-360°C (ref 3). Tsai e t a l 4 also observed the formation of several intermetallic A u - I n compounds and the formation of Au2P 3 in samples annealed to various temperatures in flowing N2 gas. In this study Au (55 n m ) - I n P samples were annealed in vacuum and the evolution of the volatile component (phosphorus) was monitored as the function of temperature with a quadrupole mass spectrometer. This method is called mass spectroscopic Evolved Gas Analysis (EGA). A few samples were quenched at characteristic points of the temperature vs evaporation (EGA) curve. Scanning electron microscopic (SEM) pictures and Auger electron spectroscopic (AES) depth profiles were also recorded on these samples. 2. Experimental

Gold (55 nm) was evaporated onto (100) oriented n-InP substrate in a liquid-nitrogen-trapped oil-diffusion-pumped high vacuum evaporation unit with a normal base pressure of about 10-5 Pa. A substrate temperature of about 100°C and a pressure of about 10-4 Pa were maintained during the evaporation. The samples were annealed in vacuum in the EGA apparatus. Tile evolution of volatile component (phosphorus) was monitored using a Q 300 C quadrupole mass spectrometer (made by ATOMKI, Hungary) which was coupled to a microcomputer through an interface unit 5. The temperature was measured with NiCr-Ni thermocouples connected to the tantalum plates hold-

ing the samples. The surface temperature of the samples was estimated to be about 30°C higher than the measured temperature at around 500°C. The heating was performed by using a tungsten filament from the back side. The applied heating rate was 30°C m i n - ~. The vacuum chamber was cooled with liquid nitrogen to allow the registration of fast evaporation processes and to reduce the noise level. Auger electron spectra were obtained using a RIBER OPC 103 analyzer. The incident electron energy was 3 keV and the diameter of the beam was about 50 l~m. The ion-etching was performed by 1 keV Ar + ions. The pressure in the chamber was 10 -8 Pa before sputtering. During the continuous sputtering the Ar pressure was about 5-10 -3 Pa and the Ar was pumped with a titanium sublimation pump. During the sputtering a spectrum was recorded every 2 min. Sensitivity factors for the evaluation of Auger spectra were obtained through calibration on a nonmetallized InP substrate and on an evaporated, non-annealed gold thin film. Comparing values measured on the same and similar samples the relative error was found to be only a few %. Because of the lack of Au~Iny standards, the absolute values of In concentration are less accurate implying the possibility of at least a 20-50% systematic error. The concentration of oxygen was usually below l%--except on the surface--so it is not depicted in the figures. 3. Results and discussion

In Figure 1 the yield of P~- species is shown from a Au (55 n m ) InP sample applying 30°C min-~ heating rate. P~-, P~ and P~species were also monitored. The dominant species are P~" below 500°C and P+over 500°C which corresponds well with the calculations in the literature 6. The evaporation vs temperature curves have a multiple peak structure. Two smaller evaporation peaks are followed by a large and sharp evaporation peak, which itself usually splits into two sharp peaks. At higher temperatures the further decomposition of InP can be seen, which was observed for non-metallized lnP samples as well. Before this "exponential' part of the EGA curve an additional shoulder can also be seen. Several Au (55 n m ) - I n P samples were annealed in vacuum and were quenched at temperatures which correspond to certain characteristic points of the EGA curve, These points are marked

189

R Veresegyh#zy et al. The interaction of gold thin films with InP

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with letters in Figure 1. The colour of the samples was examined visually, while the surface morphology was studied by SEM. The distribution of elements (as a.function of distance from the surface) was measured using AES depth profiling.

No In was found in the gold film in the case of the as-deposited sample. The broadening of the interface was in the range typical for the roughness caused by ion sputtering. The colour of sample A was still golden, similarly to the case of the as-deposited sample, the surface was mirror-like (Figure 2). Uniform distribution of In appeared in the gold film showing that the interaction has already begun (Figure 3). After the first evaporation peak the colour of sample B changed to pink, the surface was mirror-like, but some change in the surface morphology could also be observed. The In content increased slightly above 6%. Here we should consider the possible reasons of the temporary decrease of the evaporation of phosphorus. We suggest that the reason is that the formation of the first phase of A u - l n in the phase diagram has been completed (this phase is the c~-Au-In with maximal In content (9%)) and the formation of other phases with higher In concentration has just begun. Considering the usual accuracy of AES depth profiling, this 9%o is close to the former value. However there are two reasons that the actual In concentration is even higher. One reason is that the colour of ~-Au-In phase itself is yellow 4. The other reason is that the In concentration was estimated from RBS spectra measured on similar samples to be higher than 10%o. In a sample annealed and quenched similarly but with a thicker gold layer (160 nm) the existence of a phase which is rich in phosphorus was evident.

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I Figure 2. Scanning electron micrographs of the samples marked with letters according to Figure 1. 190

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The colour of sample C is silver, the surface is still mirror-like and the In content is slightly higher than in the case of the previous sample. The increase of In content in gold can be explained by the formation of a new A u - I n phase or phases. Two or more phases may co-exist in the contact, which affects the average I n - A u ratio. The temperature of the second evaporation peak corresponds well with the temperature of formation of the Au4In phase which was identified by in situ X-ray diffraction 2. According to Piotrowska et al 3 this phase is probably Au31n. However, because of the small increase of In content, it is probable that in our case the ratio of ~-Au-In to other phases is still high. It is difficult to understand why the colour of this sample is silver instead of pink. We suppose that it is connected with the very different annealing conditions. We used short-time annealing in vacuum while Piotrowska et al 3 and Tsai et al 4 used longtime annealing at atmospheric pressure. These different conditions could be responsible for the different surface morphology, as well. In the case of sample D a melted, silver colour surface can be observed. Bailing up of the metallization and the formation of etch pits in the InP substrate has already begun. These pits are filled mostly with A u - I n alloy. The AES measures therefore an average of different areas, showing in the depth profile an average In content of about 20%. The SEM image taken on the sample quenched after the sharp double peak (E) shows further pitting and bailing up of the contact. The average phosphorus content is more than 20%, as measured with AES. The reason is the contribution of large uncovered areas of the substrate.

4. Conclusions The A u - l n P contact is a very reactive system especially at higher temperatures (above 350°C). These reactions are accompanied

by the evolution of a considerable amount of phosphorus. Some phosphorus is temporarily accumulated at the interface in a goldphosphorus phase, which was previously identified as Au2P3 (ref 3). The reactions, which cause the first and second evaporation peak, proceed in the solid phase. At somewhat higher temperatures the In content in the gold film will be high enough-at least at the interface--to initiate the melting of the contact. We suggest that the accelerating reaction in the liquid phase and the decomposition of the phosphorus rich phase lead to the sharp double evaporation peak of phosphorus. The two main driving forces of the reactions are the formation of A u - I n compounds and the entropy of vaporization of phosphorus 6. The contribution of entropy to decrease the Gibbs free energy of the system at higher temperatures is essential and it is the most important reason that endothermic reactions take place.

Acknowledgements The authors wish to thank A Sulyok and Cs Farkas for the AES measurements and L Dobos for the SEM pictures.

References 'I Mojzes, R Veresegyh~.zy and V Malina, Thin Solid Films, 144, 29 (1986). 2j Van den Berg, H Temkin, R A Hamm and M A DiGuiseppe, Thin Solid Films, 104, 419 (1983). 3A Piotrowska, P Auvray, A Guivarc'h, G Pelous and P Henoc, J Appl Phys, 52, 5112 (1981). 4C T Tsai and R S Williams, J Mater Res, l, 820 (1986). 5R Veresegyhfizy, I Mojzes, B Kovfics and B P6cz, Proc l l t h lnt Mass Spectrometry Con./', 29 August-2 September, 1988, Bordeaux, France. Advances in Mass Spectrometry, 11 A&B, (To appear). 6j H Pugh and R S Williams, J Mater Res, I, 343 (1986). 191