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STRUCTURAL AND ELECTRICAL PROPERTIES OF NOBLE METAL-HYDROGENATED
AMORPHOUS SILICON INTERFACES
C. Tsai, M. Thompson, R. Nemanich
To cite this version:
C. Tsai, M. Thompson, R. Nemanich. STRUCTURAL AND ELECTRICAL PROPERTIES OF
NOBLE METAL-HYDROGENATED AMORPHOUS SILICON INTERFACES. Journal de Physique
Colloques, 1981, 42 (C4), pp.C4-1077-C4-1080. �10.1051/jphyscol:19814236�. �jpa-00220867�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZe'ment au nOIO, Tome 42, octobre 1981 page C4-1077
STRUCTURAL AND ELECTRICAL PROPERTIES OF NOBLE METAL-HYDROGENATED AMORPHOUS S I L I C O N INTERFACES
C.C. Tsai, M . J . ~ h o m ~ s o n * and R . J . Nemanich
Xerox PaZo Alto Research Centers, PaZo Alto, Ca. 94304, U.S.A.
Abstract.
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Interfaces between hydrogenated amorphous Si and noble -- metals of Pd, Pt, and Au are probed by interference enhanced Raman spectroscopy and Schottky electrical measurements. Vacuum annealing of freshly prcpared Pd, Pt, and A u Schottky diodes to -200°C changes the structure of interfaces by forming crystalline Pd,Si, both crystalline P$Si and PtSi which grow simultaneously, and an intermixed Au-Si phase which lacks long-ranged order. Such struct~~ral changes are acconipaniecl by an improvcmcnt in diode ideality factors and modifications of barrier heights. With the growth of siiicidcs or intermixed phases at the interfaces, stable and almost ideal Schottky barr~ers can be formed.Introduction.
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Tne aim of a i s paper is ro in~esiigate the phenomena occurring ut the interface between hydrogenated amorphous Si (a-Si:H) and noble metals of Pd. Pt, and Au, as well as their implication on the Schottky barriers. From studies of metals on crystalline Si (c-Si), it has been established that various silicides can be formed as a result of solid state reactions taking place at temperatures well below the e~~tectic temperature of the corresponding binary system (1). The composition and structure of these compounds depend on the thermal history of the sample. Furthermore, many of these silicide-Si interfaces offer excellent Schottky barriers.Recently there has been much interest in making a-Si:H Schottky diodes (2). 'Thus, in this work the structure of the metal-a-Si:H interfaces are correlated with the electrical characteristics of the Schottky diodes as a function of thermal annealing. Reaction products in the three systems, Pd, Pt, and Au, are compared and contrasted. We hope to shed new light on the mechanism responsible for the formation of barriers on a-Si:H and learn how to make ideal and stable diodes.
The structure of interfaces are probed using interference enhanced Raman scattering (IERS) (3-8). Through optical interference effects, this technique permits Raman spectra to be obtained from very thin films with intensities enhanced -5-1000 times over that from the corresponding bulk samples (3). Raman scattering becomes an interface-sensitive tool since very thin films are i~sed. and interfaces yield a significant contribution to the spectrum.
Experimental.
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The IERS method (3-8) employs a multilayer sample configuration which consists of an A1 bottom reflector (>l00 nm), a middle SiO, dielectric spacer (40 nm), and a sample layer of a-Si:H (10 nm), topped by a very thin (2-6 nm) layer of Pd, Pt, or Au.Thicknesses of the layers are adjusted according to their optical constants so that most incident laser radiation is absorbed by the metal and a-Si:H. Furthennore, ~nost of the Raman scattered intensity is generated near the metal-a-Si:H interface, since the electrical field intensity is strongest there. Using this technique, spectral features due to thin film structures that are <2 nm in thickness can be observed (6-8). Backscatter Raman experiments were carried out with a 514.5 nm ArC laser operating at a relatively low power level of -40 mW to minimize sample heating.
*
Permanent address : Dept. of Electronic Engineering, University of Sheffield, England Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814236C4- 1078 JOUWAL UE PHYSIQUE
Schottky diodes were fabricated by evaporating 4-10 nm thick, 1 mm-diameter metal pads onto a 0.2-2 pm thick layer of undoped a-Si:H, which lies on top of 50 nm-thick n+ a-Si:H films and Ni, MO, or Cr bottom electrodes. While thermal evaporation was employed to prepare all the Pd and Au films, Pt films were obtained by electron-beam evaporation.
The a-Si:H films were deposited at the anode at a temperature of 230°C in a diode system (9) where a pure SiH, plasma was excited with 1-2W of rf power. This is a well characterized silicon-hydrogen alloy known to contain -8 at. O/o hydrogen and <3~10'~cm-~eV-~ defects. Before the metal deposition, all a-Si:H films suffered an exposure to air. The air exposure was minimized (<l hr.) or the a-SiH films were etched in a 10% HF solution to remove some surface oxide prior to the metal evaporation. All annealing studies were made in a vacuum better than 2x10" torr.
Results and Discussion.
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Consider first the structure of the interfaces as examined by the IERS method. The thin metallic films interact with a-Si:H at relatively low temperatures, and the resultant structures obtained from annealing at 200°C for 15 minutes are manifested in the Raman spectra shown in Fig. 1. The amorphous Si network vibrations give rise to broad features with the strongest mode at -480 cm-'. Since the polycrystalline metallic films do notApplied Voltage ( V )
0 0.1 0.2 0.3 0.4 0.5Frequency (cm-' Applied Voltage ( V )
Fig. 1. The Raman spectra of 2.
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6 . nm of Fig. 2. The J-V characteristics of -200°C (a) Pd. (b) J't, and (c) Au on 10. nm annealed Pd, Au, and Pt Schottky a-Si:H after 15 minute annealing at diodes on a-Si:II. Note that the -200°C compared to that of a-Si:H scales for the forward (top) and (d). All spectra were obtained using reversed (bottom) biases arethe IERS configuration. different.
exhibit first order Raman active modes, the fcatures due to the metal/a-Si:H interactions can easily be discerned. This is especially true for Pd on a-Si:H where the spectrum (Fig. l(a)) is dominated by sevcral sharp lines below 250 cm-' which are d ~ ~ e to the crysralline conlpound Pd,Si (6-8). These features exist shortly after deposition and are due to the consumption of -2 nm of Pd. Annealing at >150°C causes the Pd film to be totally reacted, but the remaining unconsumed a-Si:H contributes a broad background to the Kaman spectrum.
The spectrum of Pt on a-Si:H exhibits similarities and differences from that of Pd on a-Si:H.
As shown in Fig l(b), sharp Iiries at -80 and -140 cm-' indicate the presence of crystalline platinum silicide compounds, and these reat~~res only become evident after annealing at -200°C.
Additionally, a broad low freq~~ency component is observed in the spectrum which is not due to the a-Si network vibration. This feature is evident immcdiatcly after deposition of Pt on a-Si:H and indicates the presence of an intermixed Pt-Si phase which does not have long-ranged order.
Another aspect of the Pt on a-Si:H interactions which differs from that of Pd is that the two sharp features indicate the simultaneous presence of PGSi and PtSi (10). Even fi~rther annealing at 400°C will not cause either of the silicide compounds to become predom~nant. This is to be contrasted with Pt on crystalline Si where PtSi will be the remaining compound after annealing at 400°C (1).
In contrast, for Au on a-Si:H sharp spectral features are absent in Fig. 1 (c), except for a line near 520 cm-' indicative of the crystalline Si stl-ucture. Thus, ALI on a-Si:H produces no stoichiometric compounds with long-ranged order, but instead causes Si to crystallize at a surprisingly low temperature of -20OoC, much lower than the normal cl-ystallization temperature of >650°C for a-Si:H. Carefill examination of the spectral region below 200 cm-' reveals the appearance of broad low-frequency features with intensity increasing with temperature. These can be attributed to an intermixed Au-Si phase which lacks long-ranged order. Similar structures were observed on c-Si (11). Auger depth profiles and SEM micrographs indicate that Si diffuses into and through Au and appears near the top Au surface even at room temperature.
Moreover, the diffused Si rematns amorphous at room temperature and crystallizes upon annealing to 200°C. Then the Au-Si phase agglomerates to form regions surrounding the c-Si islands.
Since the a-Si:H films were exposed to air before the metal deposition, a thin oxide layer exists at the metal/a-Si:H interface. For Pd films, it has been shown that the silicide forms between the oxide and a-Si:H, producing an oxide free interface (7, 8, 12). If a s~tbstantial oxide is present, however, no silicide formation is observed. The effect of the oxide has not been determined for Pt or ALI interactions on a-Si:H.
But what happens to the Schottky barriers when such drastic changes occur at the interface?
Both the current-voltage characteristics and open-circuit photovoltage were measured before and after each heat treatment. The current density (J) o f a practical diode at a given temperature (T) depends on the applied voltage (V) nccorcling to the equation.
where J, is the saturation current density and &l) the diode ideality factor. An ideal diode has
q = l . Details of the electrical characteristics of the Pd-a-Si:H interface are given elsewhere (2, 12). Immediately following the preparation, all three types of diodes exhibit a high rectification ratio and an initial q of 1.1 to 1.25. Detailed measurements of the low voltage forward characteristics revealed that some were not linear on a semi-log J-V plot; this was particularly the case with Au Schottky junctions. The diodes are not stable at room temperature. The electrical characteristics are extremely sensitive to the condition of the interface, such as metallurgical reactions, oxides and contaminations. Shown in Fig. 2 are the J-V characteristics of the Pd, Pt and Au Sdxttky barriers after thermal annealing at -200°C. All iilree types O$ diodes now become almost ideal with q=1.03-1.06 and are stable. For heatings to 200°C. both Pd and Pt diodes exhibit increases in the barrier height (a,) by >0.06eV, while
a,,
stays unchanged for Au diodes. In contrast, Pd diodes made on aged a-Si:H remain non-ideal even after annealing and the Raman spectra indicated that no silicides were formed.C4-1080 JOURNAL DE PHYSIQUE
Since heat treatments below the deposition temperature of a-Si:H do not vary the material significantly, we believe the improvement in q is a direct result of metallurgical reactions at the interface. With the growth of either the silicides or intermixed phases, the interfaces are moved deeper into the b~llk a-Si:H, which presumably has fewer defects and impurities than the surface.
I h e barriers are then formed between a-Si:H and the silicides or intermixed phases.
Furthermore, Auger depth profiles reveal that the interface becomes essentially free of oxygen after annealing, and the native oxides originally present on the a-Si:H surface are now relocated above the inteface. Therefore, a more intimate contact is made, which also improves q. Thus, stable and almost ideal Schottky barriers can be obtained by low temperature (-200°C) vacuum annealing of freshly prepared diodes. It should be noted that after such heat treatments, one no longer has a simple interface between a noble metal and a-Si:H. Instead, crystalline silicides or intermixed phases are in contact with a-Si:H. This can a c c o ~ ~ n t for changes in 0, observed in both Pd and Pt diodes. While a single silicide phase is obtained in Pd diodes, two silicides and an intermixed Pt-Si phase coexist in Pt diodes. We have not determined the final positions of the Pt-silicides and the intermixed Pt-Si phase, or which constit~ltes the barrier. In the case of Au, the a-Si:H is in contact with c-Si islands surrounded by the intermixed Au-Si phase which lacks long-ranged order. Therefore, the interface is quite diffuse. Since both the intermixed phase and c-Si probably offer relatively low resistance, the Schottky barrier presumably lies between a-Si:H and the mixture of c-Si and the ALI-Si intenixed phase.
Acknowledgment. - The authors are indebted to N. M. Johnson for helpful discussions and R. I.
Johnson and R. A. Lujan for t h e ~ r assistance in sample preparation. This work was supported in part by the Solar Energy Research Institi~te under Contract No. XJ-0-9079-1.
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