High temperature annealing behaviour of Schottky barriers on GaAs with gold and gold-gallium contacts

High temperature annealing behaviour of Schottky barriers on GaAs with gold and gold-gallium contacts

Solid-Slnfe Electronics, 1977, Vol. 20. pp. 431-+32. Pergamon Press. Printed in Great Britain HIGH TEMPERATURE ANNEALING BEHAVIOUR OF SCHOTTKY BARR...

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Solid-Slnfe Electronics, 1977, Vol. 20. pp. 431-+32.

Pergamon Press.

Printed in Great Britain



B. M. ARORAand V. P. SALvIt

Tata Institute of Fundamental

Research, Bombay 400005, India

(Receioed12 August 1976;in revised form 11 November 1976) Abstract-It is shown that unlike Au-GaAs Schottky diodes, a Schottky barrier made on GaAs using an evaporated film of Au-Ga eutectic alloy does not degrade on heat treatment. It is shown that the presence of Ga in the top contact prevents micro-alloying to take place during heat treatment and thus prevents degradation of the diode behaviour.

Heat treatment of Au-GaAs Schottky diodes is known[ l51 to cause an increase in the ideality factor and a decrease in the barrier height of these devices. While SEM pictures [2] have revealed that considerable alloying takes place during the heat treatment, back scattering[3] and secondary ion mass spectroscopy[4] results have shown that in addition to the penetration of gold into GaAs, there is considerable out-diffusion of Ga during the annealing. The exact mechanism that causes the degradation of the devices, however, is not very clear. It has been suggested that during the heat treatment a thin highly doped n-type region is created just below the interface due to either the in-diffusion of Au[l] of out-diffusion of Ga[5]. This would result in considerable field emission across the barrier and would increase the current flowing across it. The creation of the n+ region due to the above mechanisms is possible only if Au or Ga-vacancies behave as donor in n-GaAs. Experimental results however indicate that both Au[6,7] and Ga-vacancies [8] give rise to acceptor-like states in GaAs. In order to find out the mechanism that causes the degradation of the diodes, we have studied the effect of heat treatment on Schottky barriers formed by flash evaporating an eutectic alloy of Au-Ga on GaAs. For the sake of comparison, studies were also made on Au-GaAs diodes which were subjected to the same heat treatments. The results are reported in this paper. The samples used were bulk n-type GaAs having a carrier concentration of 2 x 10’6/cc and vapour phase epitaxial n on n+ GaAs with a carrier concentration of 3 x lO”/cc. Ohmic contacts were made on the back side by evaporation and alloying of Au-Ge-Ni. A flash evaporation technique was used for the Au-Ga evaporation where fine grains of the alloy (85% Au 15% Ga) were dropped from a magnetically operated hopper on to a tungsten boat kept at around 1400°C. This ensured the homogeneity of the contact material. The evaporations were carried out at a vacuum of lo-’ torr and in order to minimise the effect of an interface layer the substrates were kept at 150°C during deposition. After measuring the I-V charactPresent address: harashtra, India.







teristics, the diodes were heat treated in flowing hydrogen atmospheres and measurements were carried out again. The ideality factor and the barrier height as obtained from the forward current-voltage characteristics of the diodes for various heat treatments are shown in Table 1. The barrier height of the as-deposited Au-GaAs diodes is found to be 0.86 eV. This is in agreement with the values reported in the literature which range from 0.80[9] to 1.03eV [lo] and is close to the value obtained for a clean Au-GaAs interface[ll]. The ideality factor for the as-deposited diodes is 1.04. The barrier height for the interface between Au-Ga and GaAs is found to be 0.75 eV. This is slightly lower than the Au-GaAs barrier height and may be attributed to the fact that the work function for Ga is lower than that for Au[12]. In agreement with earlier reports, the Au-GaAs diodes are seen to degrade with heat treatment. The characteristics of the Au-Ga Schottky diodes on the other hand change very little as a result of the heat treatment. The ideality factor remains almost unchanged even after heat treatment at 400°C for 30 min where as the barrier height changes from 0.75 to 0.71 eV for the same heat treatment. This is in sharp contrast to the drastic deterioration of the characteristics that one finds for the heat treated Au-GaAs contacts. SEM photographs taken for the two types of contacts for a heat treatment of 300°C 1 hr are shown in Fig. 1. Considerable alloying is observed at the Au-GaAs interface whereas the interface between the Au-Ga and the semiconductor is remarkably clean. Since the gold diodes degrade on heat treatment and the gold-gallium diodes do not, it is clear that the out-diffusion of Ga does play a role in the degradation of the devices. The conjecture that an n+ layer is created which causes enhanced field emission is questionable since the experiments of Munoz et al. [8] show that Ga vacancies give rise to acceptor-like states in GaAs. One may argue that the region below the contact in the heat treated Au-GaAs diodes may be disordered and Ga-vancies may behave donor-like in such materials. Experimental results[l3] however give evidence to the contrary. The incorporation of Au in the lattice during the heat treatment cannot also give rise to the n+ layer since diffusion studies[8] show that Au goes as an acceptor into GaAs.

S. GUHA el al.


Table 1. Variation of dB and n of the Schottky barriers with heat treatment Au-contact Anneal temperature and time Unannealed 250°C0.5 hr 300°C 1hr 400°C 0.5 hr

& (eV)


0.86 0.60 0.51

1.04 1.4 I.8

Au-Ga contact &(eV) 0.75 0.73 0.72 0.71

II 1.08

I .08 1.06 I.1

An explanation for the deterioration of the contact due to the heat treatment may come from metallurgical considerations. As we find from the SEM photographs, the surface below the metal in the heat treated Au diodes is heavily pitted. The pitting would result in the creation of a large number of recombination centres at the interface and would deteriorate the performance of the Schottky barrier. An irregular interface resulting from the pitting can also cause high local fields within the barrier leading to its degradation. Moreover, the micro-alloying may cause the formation of tiny metallic channels across the barrier giving rise to an increase in current. The alloying process that is responsible for the pitting essentially takes place because of the large solid solubility of Ga in Au. When the top contact is made of the Au-Ga eutectic alloy, it cannot take up any more Ga from the GaAs sample and no alloying therefore can take place. The performance of the Schottky barrier, hence, does not deteriorate on annealing. In conclusion, we have shown that a Schottky barrier made on GaAs using an evaporated film of Au-Ga

eutectic alloy does not degrade on heat treatment. The deterioration of Au-GaAs Schottky diodes on heat treatment is ascribed to the micro-alloying that takes place on heat treatment due to the large solid solubility of gallium in gold. Acknowledgements-The authors are grateful to V. T. Karulkar for experimental help and discussions. The SEM photographs were taken at BARC, Bombay and the authors would like to thank the members of the SEM laboratory for their help.


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