Plasma deposition of amorphous hydrogenated carbon films on III-V semiconductors

Thin Sohd Ftlm~, 157 (1988) 97 104

97

PREPARATION AND CHARACTERIZATION

P L A S M A D E P O S I T I O N OF A M O R P H O U S H Y D R O G E N A T E D CARBON FILMS ON III-V SEMICONDUCTORS JOHN J. POUCH, JOSEPH D. WARNER, DAVID C. LIU AND SAMUEL A. ALTEROVITZ Lewis Research Center, Natzonal Aeronautics and Space Admmtstratlon. Cleveland, OH 44135 ( L1S A )

(Received October 23, 1986,revised June 2, 1987, accepted September 16, 1987)

Amorphous hydrogenated carbon films were grown on GaAs, InP and fused silica substrates using plasmas generated from hydrocarbon gases. Methane and nbutane sources were utilized. The effects of flow rate and power density on film growth were mvestigated. Carbon was the major constituent in the films. The degree of asymmetry at the carbon-semiconductor interface was approximately independent of the power density. Different H - C bonding configurations were detected by the technique of secondary ion mass spectrometry Band gaps up to 3 eV were obtained from optical absorption studies. Breakdown strengths as high as 6 x 108 V m - ~ were measured.

1. INTRODUCTION Amorphous carbon films 1 39 can be prepared on different substrates using several techmques, e.g. 1on beam deposition, ion beam sputtering and plasma decomposition of hydrocarbon gases. These resulting films can exhibit a high electrical resistivity and breakdown strength, semitransparency, mechanical hardness and chemical inertness. The properties are sensmve to the deposition conditions. The films are useful as wear-resistant coatings for optical components. In addition, carbon films show promise as gate dielectrics 9'12 and passivating layers ~3 in semiconductor device processmg, insulators for metal-insulator-metal fabrication and masks for nanometer lithography ~5. In this investigation, carbon films were grown on GaAs, I n P and fused sdica substrates by means of plasma chemical vapor deposition at 30 kHz. The influence of growth conditions on film characteristics were studied by Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS) and UV-visible-near IR spectrophotometry. Breakdown strengths were obtained from electrical measurements performed on metal-insulator-semiconductor (MIS) structures using carbon as the insulator

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2. EXPERIMENTAL DETAILS

A planar plasma reactor operating at 3 0 k H z was utlhzed for the carbon depositions. The diameter of each circular electrode was 0 3 m, and the electrode spacing was 0.03 m. The power was capacltlvely coupled to the upper electrode, the lower electrode and chamber wall were grounded. The chamber pressure was controlled by the input gas flow rate and pumping speed. The gas sources were CH4 and C4H lO. The samples were n and p type (approximately 1023 m a I GaAs < 100 > and InP < 1 0 0 > crystals ( 1 0 m m x l 0 m m x 0 . 3 m m ) grown by the hquldencapsulated Czochralski method, and fused slhca substrates (25.4minx 25.4mm x lmm). After standard cleaning 2°, the samples were placed on the grounded anode inside the chamber. The background pressure was approximately 2 7 Pa The deposition gas ( C H 4 o r C4H10 ) was used to flush the system three times prior to each run Amorphous carbon films were produced by subjecting the substrates to the a.c. glow discharge The power density and flow rate settings covered the ranges 0.4-5 kW m -' and 3-9 x 10 5 m 3 mln 1 Auger compositional profiles of the films were obtained using a PHI AES Xray photoelectron spectroscopy-SIMS system interfaced with a multiple-technique analytical computer system. Depth profiling studies were conducted with 3 keV Ar + ions. Measurements of optical absorption as a function of photon energy for carbon films deposited onto fused silica substrates were made with a UV visible-near IR spectrophotometer. Prior to these measurements, the sample compartment was continuously purged with dry N 2, and a scan of the background for data correction was made over the photon range investigated. A slit width of 2 nm was chosen for the measurements. In addition, a blank fused silica substrate was placed in the reference beam path The differences between the film and sihca substrate reflectlvltles were not taken into account. In order to study the electrical properties of the carbon films and interfaces, MIS structures were fabricated. An ohmic contact was made to the rear surface of an n-type GaAs wafer by thermal evaporation of Au-Ge, nickel and gold, followed by annealing in a n N 2 ambient at 723 K for 5 mln. Aluminum dots were evaporated thermally onto the carbon film surface 3.

RESULTS AND DISCUSSION

The AES measurements show carbon to be the major constituent in the film, with less than 10at.°o O at the film-semiconductor interface These results are independent of the hydrocarbon source used for the depositions. A representative AES profile of a carbon film deposited from C4Hlo onto InP is shown in Fig 1. The flow rate is 3 x l 0 - ~ m a m l n 1 (corresponding to a steady state pressure of approximately 23.3 Pa), and the power density IS 0.41 kW m 2 We denote the AES profiling (or sputtering) time for the carbon film by tp, and the deposition time by t d The parameter G ( = tp/td) IS plotted t,s. power density divided by pressure for two C 4 H l o pressures: 23.3 and 32.7 Pa (Fig. 2). The Ar + ion energy is 3 keV. We note that G becomes sensitive to the chamber pressure for a

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power-density-to-pressure ratio of about 100 W m - 2 P a - 1 or higher. The parameter G ts influenced by several sputtering and deposition processes and is proportional to the ratio of deposition rate to sputtering rate; thus, it is a complex problem to separate out the relative contribution of each mechanism to G. The degree of symmetry associated with the carbon-semiconductor interface is proportional to 6 = fl(TR-- 2TM+ TL) where fl is a constant, and TR, T~ and TLare the AES sputtering times corresponding to the carbon peak-to-peak signals at 10~o, 50~o and 90~o respectively of the maximum attained in a depth profile. The influence of CH4 flow rate on 6 can be seen in Fig. 3 for several power densities. The results suggest that 6, at a fixed pressure, ~s approximately independent of power densities in

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the 0.82 4.89 k W m 2 range Additional studies of the interracial structure and the adhesion of carbon on G a A s and InP are in progress Relative counts of hydrocarbon ions sputtered from carbon ( s u b s t r a t e - l n P ) were determined by means of S I M S ~' depth-profihng studies performed with 3 keV Ar + ions. In Fig. 4(a) the distribution of ion counts is plotted as a function of mass-to-charge ratio for various deposition conditions using a C 4 H l o plasma The predominant ton ~s CH +, it is interesting to note that a higher C H + level is obtained from films produced at the higher power densities Ad&tional Ions are presented in Fig. 4(a) CH2 +, CH3 +, C2H +, C z H 2 + and C2H 3 +

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PLASMA

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The ion distributions extracted from carbon films prepared by a CH4 discharge are shown in Fig. 4(b). Evtdently, C H ÷ has a higher probability of being sputtered from each film. At 5 × 10 -5 m 3 min - t (32.7 Pa), more C H ÷ is generated from the carbon deposit made at 2.45 kW m - 2. In addition, some of the films obtained from C4H ~o discharges (Fig. 4(a)) have higher amounts of incorporated C2I-I3 relative to the CH4-derlved films (Fig. 4(b)). Figures 4(a) and 4(b) indicate that the lowest populations are associated with C H 3 +. The ion distributions thus reflect some of the bonding arrangements 4° that result from the interaction of the plasma radicals with the growing film ~7 , 3 9 , A SIMS depth profile (3 k e V A r + ions) of carbon depostted onto GaAs is presented in Fig. 5. The CHx + (x = 0, 1, 2, 3) distributions are at lower levels in the substrate region (relative to the bulk of the film). Moreover, Fig. 5 shows oxygen to be present throughout the film. This determination can not be made with the lesssensitive AES technique. It ~s apparent that G a ÷ and As ÷ are readily detected as the carbon film mssputtered. IM. P~OFILE

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Fig 5 SIMSdepth profile of carbon on Gaks using 3 keV Ar ÷ ion bombardment Figure 6 shows a SIMS depth profile of carbon on InP. We note that the O + and C H 3 + are "noisy". Discernible levels of In + and P + are evident during the film sputtering process. The band gap E o is determined by fitting the absorption data to a theoretical model describing mterband optical absorption in a non-crystalline sohd 41. A linear relationship between (AE) 1/2 and E is characteristic of an amorphous material. The absorbance is A and the photon energy is E. Eo is estimated to be 3 eV for a carbon film deposited onto a silica substrate (deposition parameters: C4H~o; 5 x 10 5 m 3 m i n - x; 2.45 kW m 2 ) . At low power densities (low ion energies), the production of hydrocarbon species may be incomplete. Therefore, by using a heavier molecule in the working gas (e.g. C4Hxo in place of CH4), it may be possible to generate polymer-like films 23 containing higher levels of hydrogen relative to films

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made at the higher power densities. These films would exhibit higher band gaps. Band gap values for carbon on InP obtained from CH4 discharges are typically 34 about 2.35 eV or less. The capacitance-voltage (C-V)characteristics of the MIS structures on GaAs yielded a flat C - V curve This result suggests Fermi level pinning of the c a r b o n - G a A s interface The phenomenon Is attributable to the existence of a large density of interface states. Voltages of more than 100V were applied to the MIS structures The "avalanche" breakdown voltage VBrwas determined by means of a current-voltage ( I - V ) curve tracer. After the breakdown condinon was reached (fixed carbon film thickness), a subsequent measurement showed VBr to be approximately 30°o lower than the first value measured, indicating an irreversible process. The variation in VB, over the entire area of the carbon deposit (approximately 10 mm x 10 ram) was 20°0 The deposmon con&tions, film thicknesses and breakdown fields are summarized in Table I. It is apparent that higher breakdown fields can be obtained using C4Hlo rather than CH,~. Carbon films (greater than 0 2 ~tm thick) can thus be made to withstand applied voltages (higher than 150 V) that are relevant to the area of semiconductor device fabncanon A thin (approximately 10-50 nm) film reduces the voltage level that can be utihzed 14. Internal stress in a thlck (greater than 1 p.m) film TABLE 1 B R E A K D O W N FIELDS FOR ( A R B O N FILMS DEPOSITED O N T O r l - G a A s

Deposmon parameters

CH~.,163kWm 2,3×10 5m3mm i CH,~,163kWm 2,5×10 Sm3mm-~ C4Hlo, 163kWm 2 3xl 0 5m3mm i CgHlo, 163kWr n 2 5xl0-Sm3ml n i

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is related to h y d r o g e n i n c o r p o r a t i o n d u r i n g growth 21"22 a n d this can also c o n t r i b u t e to a lowering of VBr. 4. SUMMARY We have d e m o n s t r a t e d that a m o r p h o u s h y d r o g e n a t e d c a r b o n films can be deposited onto GaAs, I n P a n d fused silica substrates by m e a n s of plasma d e c o m p o s i t i o n of the gaseous h y d r o c a r b o n s CH4 a n d C4H~0. AES m e a s u r e m e n t s (up to 0.1 a t . ~ ) indicate c a r b o n to be the m a j o r constituent in the films, whereas data on H - C b o n d i n g configurations are o b t a i n e d from the SIMS analysis. Heavier h y d r o c a r b o n ions (e.g. C2H3 +) are extracted from films m a d e with C4Hlo discharges. Moreover, at a fixed pressure, the C H ÷ level seen in the S I M S spectra correlates with the apphed power density, This result is i n d e p e n d e n t of the w o r k i n g gas. The degree of a s y m m e t r y at the c a r b o n - s e m i c o n d u c t o r interface is approximately i n d e p e n d e n t of the power density. The p a r a m e t e r G is a function of the power density divided by pressure. Films prepared with C4H~o discharges exhibit b a n d gaps 19 up to 3 eV. B r e a k d o w n fields as high as 6 x 108 V m 1 have been measured.

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11 12 13 14 15 16 17 18 19 20 21 22 23 24

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