Mn contacts to p-type InP

Thin Solid Films, 190 (1990) 217-226

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ELECTRONICS AND OPTICS

AN I N V E S T I G A T I O N OF Au/Mn C O N T A C T S TO p-TYPE lnP D. G. IVEY AND PING JIAN Department of Mining, Metallurgical and Petroleum Engineering, University of Alberta, Edmonton, Alberta T6G 2G6 (Canada) R. BRUCE Bell Northern Research, P.O. Box 3511, Station C, Ottawa, Ontario K174H (Canada) (Received June 22, 1989; revised February 19, 1990; accepted March 12, 1990)

The performance of a Au/Mn contact metallization to p-type InP has been reported. Electrical resistance measurements done on annealed contacts have been correlated to the accompanying microstructural changes, by means of electron microscopy and X-ray diffraction techniques. Manganese was found to react readily with the underlying InP, leading to the formation of Mn2P followed by MnP. Subsequent outward diffusion of indium towards the gold layer led to the formation of Au3In, which replaced the original gold layer. Inward diffusion of gold resulted in the formation of an A u - I n - M n ternary phase at the M n P - I n P interface. This phase may have supplied the necessary manganese for InP doping required to lower the contact resistance. A minimum resistance of 6 x 10-4f~ cm z was obtained.

1. INTRODUCTION

There is considerable interest in developing a low resistance ohmic contact to p-type lnP. Fabricating a suitable ohmic contact for InP can prove troublesome, particularly for the p-type material. Metallizations to p-type semiconductors generally have higher barrier heights and the charge carriers have a larger effective mass than those in corresponding n-type materials 1. Both of these effects contribute to a higher contact resistance for p-type semiconductors. In a recent paper, Dubon-Chevaillier et al. 2 have reported the formation of a successful contact to p-type GaAs using an Au/Mn metallization. A minimum contact resistance of 2 × 10- v f~ cm 2 was reported for a fairly high initial (beryllium) doping level of 1019cm -3. For more moderate doping levels of 10~Scm 3, the minimum resistance was somewhat higher, i.e. 10 5 f~cm 2. This, however, still compares quite favourably with results obtained for Au/Zn and Au/Be metallizations to p-type InP. Minimum resistances of 3.7× 10-Sf~cm 2 and 7 . 8 × 1 0 - S f ~ c m 2 have been reported for Au/Zn (1018cm-3) 3 and Au/Be (8 X 1018 cm-3)4 respectively. For all these contact structures the metal, other than gold, presumably degeneratively dopes the surface of the semiconductor. This reduces the barrier width and barrier height somewhat, allowing tunnelling of charged carriers. 0040-6090/90/$3.50

© ElsevierSequoia/Printedin The Netherlands

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The viability of employing an A u / M n metallization as a low resistance contact to p-type I n P was investigated. Metal contacts were deposited by electron beam evaporation. Contact resistances were measured from annealed metallizations: these were correlated to corresponding microstructura[ changes and monitored using both transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) of plan view and cross-sectional T E M specimens. 2. EXPERIMENTALPROCEDURE The starting material was a zinc-doped (2 x 101Scm 3) (100>-oriented I n P substrate. Two types of specimens were studied, i.e. the actual contact structure ( ( A u / M n l - I n P ) and M n - I n P . The purpose of the single-layer structure was to facilitate analysis of the interface reactions in the contact structure. The metallization process has been reported previously 5'6 and is outlined in Table I, For the contact structure, manganese layer thicknesses of either 20 or 40 nm were deposited. These were then covered with a gold layer 150 nm thick. The metal layers were deposited by electron beam evaporation in the same evaporator. F o r the single-layer metallization (Mn/lnP), a 6 0 n m layer of manganese was deposited. Blanket metallizations were prepared for electron microscopy, while dot contacts were prepared for electrical characterization. Specimens were annealed in nitrogen gas at temperatures ranging from 250 to 450 ~C and for times of up to 4 rain. TABLE 1 AH/Mn METALLIZATIONPRO('ESSIN(ISTEPS (1) (2) {3) {4) (5) (6) (7) t8) (9) (10) (l 1) (12) (13)

Polish,with 5% Br in CH 3OH Rinsein methanol Degrease, with 1:1 HCl:H20:rinse Etch and clean in 1°,iBr in CH3OH SiO2deposition Photoengrave Bakeat 120 Cfor60min Etch SiO2 Strip photoresisl Degrease Photoengrave Metallize Lift-offofphotoresist

A standard four-point probe technique was used to measure the contact resistance. Dots, 35 I~m in diameter, and spaced 0.5 m m apart were utilized. At least 20 measurements were made for each anneal. Electrical measurements were m a d e only on the ( A u / M n ) - I n P contacts. Both plan view and cross-sectional specimens were prepared for electron microscopy using chemical and ion milling techniques, as described in refs. 5-7. D u r i n g ion milling of cross-sections, preferential sputtering problems were encountered, as I n P ions milled at a much faster rate than the metal layers. This was especially p r o n o u n c e d in the case o f the gold layers, resulting in electron-opaque

Au/Mn CONTACTSTO p-InP

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gold regions. The problem was alleviated somewhat by removing most of the gold layer prior to specimen preparation. Thin foils were examined in a Hitachi H-600 T E M - S T E M set-up equipped with a Kevex beryllium window X-ray detector and a VG HB5 dedicated scanning transmission electron measure with a Links System windowless X-ray detector. Approximate compositions were determined from the peak intensity ratios along with appropriate calculated Cliff-Lorimar k factors 8. Microstructural results were corroborated using X-ray diffraction, with monochromatic Cu K s X-rays. 3. RESULTSAND DISCUSSION 3.1. ( A u / M n ) - I n P The electrical results were very similar for Au/Mn contacts with either 20 or 40 nm manganese layers. Consequently, only the 20 nm manganese layer contacts were examined further. Plots of contact resistance vs. annealing temperature for various annealing times were obtained. A minimum contact resistance of 6× 10-4f~cm 2 was achieved. A representative plot is shown in Fig. 1, for 15s anneals. The results for longer times were quite similar, except that corresponding changes occurred at lower temperatures. The points on this graph correspond to specimens that were examined in detail by electron microscopy. It should be noted that there is only a small decrease in resistance up to about 300 °C, followed by a marked reduction in the 300-350°C range. In the 3 5 ~ 4 5 0 ° C range there is a continued, but more gradual, decrease in contact resistance. It is possible that a further reduction in resistance may have occurred at temperatures higher than

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D. G. IVEY, P. JIAN, R. BRUCE

450 cC. H o w e v e r , we did not a n n e a l at higher t e m p e r a t u r e s because, by 400 ')C, the surface of the c o n t a c t h a d c h a n g e d from a gold c o l o u r to a pink colour. This i n d i c a t e d that the gold had u n d e r g o n e s u b s t a n t i a l i n t e r a c t i o n with the u n d e r l y i n g material, t h e r e b y c o m p r o m i s i n g the electrical integrity of the gold layer. Bright field images of cross-sectional specimens e x a m i n e d by T E M are shown in Fig. 2. It s h o u l d be n o t e d that in all these m i c r o g r a p h s the gold layer o r the reacted

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(e) Fig. 2. Bright field micrographs from TEM cross-sections oflAu/Mn)-lnP metallizations, for annealing times corresponding to Fig. 1;(a) as deposited; (bj annealed at 250 'C: (ci annealed at 340' C: (d) annealed at 400 C" (e) annealed at 450 C.

Au/Mn CONTACTSTO p-lnP

221

gold layer, in the higher temperature anneals, is not electron transparent owing to the preferential sputtering problems discussed earlier. Examination of the asdeposited specimen (Fig. 2(a)) revealed the presence of a third layer (about 5 nm thick), at the Mn-InP interface, in addition to the deposited gold and manganese layers. This layer was amorphous, as indicated by the selected-area diffraction (SAD) pattern in Fig. 3, with a composition approximating MnP 3. A small amount of indium was also detected within this phase. Clearly, the manganese had reacted with the InP at the Mn-InP interface during deposition. Heating during deposition, with temperatures possibly exceeding 100 °C, probably aided this decomposition. Similar behaviour has been reported previously for Au/Ni and nickel metallizations6'9, where an amorphous Ni-In-P-rich region was detected at the Ni-InP interface. Indium was also detected in the overlying manganese layer, indicating that indium was already moving towards the surface. X-ray diffraction results demonstrated that the gold layer was (111) preferentially oriented in a direction perpendicular to the InP surface. Examination of the other micrographs in Fig. 2 revealed several trends. First of all, upon annealing of the metallizations, there was continued reaction of manganese with InP, resulting in further decomposition of InP, the liberation of metallic indium and the formation of manganese phosphides. Mn2P formed first at or below 250 °C, followed by MnP formation by 350 °C. These phases were identified by SAD and X-ray spectroscopy of plan view specimens. Thermodynamically, the decomposition of InP, with the resultant formation of MnP, is favourable, since MnP has a more highly negative heat of formation than InP. The heat of formation for MnP t° is - 9 6 k J m o l ~compared to - 8 9 k J m o l 1 for InP ~. Both gold and indium were detected in the phosphide layers, indicating diffusion of these elements in opposite directions, i.e. gold towards the InP surface and indium towards the outer gold layer. The sequence of formation from metal-rich to phosphorus-rich phosphides is expected, since the InP substrate represents essentially an unlimited supply of phosphorus relative to manganese. Also, there is complete formation of Mn2P, consuming all the manganese layer, before the formation of MnP. This too is expected and has been observed for other thin film systems6' 12. The outward diffusion of indium as well as the inward diffusion of gold, alluded to above, were the other effects of note. Indium was incorporated in the gold layer, initially in solid solution (Fig. 2(c)). Indium is highly soluble in gold (up to 10 wt.% In). By 400 °C the solubility limit of gold for indium had been exceeded and the gold layer had transformed to Au3In (Fig. 2(d)). This phase transformation coincided with a colour change on the contact surface, from gold to pink, and was verified through both X-ray and electron diffraction (Fig. 4). No further transformation was detected at higher temperatures, although the Au3In-MnP interface became quite irregular at 450 °C (Fig. 2(e)). A diffusion barrier between the manganese and gold layers may have been beneficial, to restrict indium diffusion to the gold layer. Some indium out-diffusion is, of couse, necessary to provide vacant sites for manganese incorporation in the InP lattice. However, the loss of too much indium to the gold layer substantially reduces the electrical conductivity of the gold. Inward diffusion of gold to the InP surface resulted in the formation of a goldbased ternary compound at the MnP-InP interface (Figs. 2(c)-2(e)). This, along

222

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I). G. IVEY, P. MAN, R. BRUCE

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Au/Mn CONTACTSTO p-InP

la) Fig. 4. (a) Bright field image and (b) S A D b = 0.474 n m and c = 0.515 nm).

223

Ib) pattern from the orthorhombic Au31n phase (o = 0.586 nm,

with gold acting as a sink for indium, helped to drive the decomposition of InP. The composition of the ternary phase was variable, decreasing in manganese content and increasing in indium content as the annealing temperature was increased. The crystal structure was very similar to the orthorhombic Au3In-type structure, implying an approximate composition of Au3(In, Mn). By the 450 °C anneal, there was almost no manganese present, leaving essentially Au3In. It should also be noted, from Figs. 2(c)-2(e), that this ternary phase grew into a continuous layer by 400 °C but became irregular by 450 °C. The appearance and growth of this gold-based ternary phase appears to correlate with the largest decrease and, in fact, the first significant decrease in contact resistance, i.e. in the 300-350°C range (Fig. 1). It is possible that Au3(In, Mn) supplies the manganese necessary for doping of the InP surface region. The further reduction in contact resistance, in the 350-450 °C range, corresponds to a decrease in the manganese concentration of the ternary phase, suggesting further doping at the higher temperatures. Accordingly, one would expect little or no decrease in contact resistance above 450 °C, as virtually all the manganese has been removed from the ternary phase at this point. The limited decrease in contact resistance, compared with Au/Zn or Au/Be, may be attributed to the activation energy of manganese in InP, i.e. about 220meV 13. This is a fairly moderate level, but not as low as for either zinc or beryllium in InP, with activation energies of about 46 and 41 meV respectively ~4. Consequently, the number of electrically active charge carriers would be higher for zinc and beryllium than for manganese, which would account for the approximately order-of-magnitude difference between the contact resistance of Au/Mn and those of Au/Zn and Au/Be. 3.2. M n - I n P

There were many similarities between the annealed M n - I n P metallizations and the Au/Mn contacts, as well as a few differences due primarily to the absence of the protective gold layer. One obvious consequence of the absent gold was the oxidation

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of the manganese layer resulting in the formation of an outer oxide layer (Mn30,,), which increased in thickness with increasing annealing temperature (Fig. 5). The asdeposited structure was very similar to the Au/Mn structure, with an amorphous layer of virtually the same composition at the M n - I n P interface. Phosphides formed at the M n - I n P interface as well, in the same sequence as the Au/Mn contact, i.e. Mn2P formed initially followed by MnP formation (Fig. 5). These phosphides were identified from plan view specimens (Fig. 6). MnP, was, in fact, quite stable even after annealing the metallization for 1 h at 400~C. The relative inactivity of MnP is important when considering the long-term reliability of any Au/Mn contact structure. Indium liberated through phosphide formation diffused towards the surface, where, in the absence of gold, it oxidized to form In203 . Regions of finegrained (less than 20nm) In203 are also observable in the bright field image in Fig. 6(a), overlapping the MnP.

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Fig. 5. Bright field micrographs from TEM cross-sections of Mn lnP metallizations annealed at laJ 350' C and tbJ 400 C for 60 s.

4. CONCLUSIONS (1) Manganese reacts readily with lnP. Decomposition results in the formation of manganese phosphides and the liberation of metallic indium. (2) This indium can then diffuse to the surface, where it either is oxidized (Mn-InP) or is incorporated in the outer gold layer ((Au/Mn) InP), eventually forming Au3In. (3) Gold, in the contact structure, diffuses to the InP surface and aids in the decomposition process through the formation ofAu3(In, Mn) and Au3In. (4) The major decrease in contact resistance occurs between 300 and 350 C , which corresponds to the formation of Aua(ln, Mn). This compound may supply the managanese necessary for doping purposes. (5) The minimum resistance obtained was 6 x 10 4~C1T12. The limited de-

A u / M n CONTACTSTO p-InP

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(a)

(b) Fig. 6. Bright field micrographs and SAD patterns from plan view sections of the metallizations shown in Fig. 5: (a) MnzP (hexagonal; a - 0.6081nm and c =0.346nm) and (bt MnP (orthorhombic; a = 0.5917nm, b = 0.5259 nm and c = 0.3173 nm).

crease in resistance may be attributed to the relatively high activation energy of manganese in InP, i.e. about 220 meV. ACKNOWLEDGMENTS

The authors are grateful to the Natural Science and Engineering Research Council of Canada and Bell Northern Research Ltd. for providing financial assistance. REFERENCES l

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