Properties of plasma oxyfluorides grown on GaAs

Thin Solid Films, 95 (1982) 327-331 PREPARATION AND CHARACTERIZATION

327

PROPERTIES OF PLASMA OXYFLUORIDES GROWN ON GaAs* R. K. AHRENKIEL, L. L. KAZMERSKI, O. JAMJOUM, P. E. RUSSELL AND P. J. IRELAND Solar Electric Conversion Research Division, Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 (U.S.A.) R. S. WAGNER Los Alamos National Laboratories, Los Alamos, N M 87545 (U.S.A.) (Received March 22, 1982; accepted April 6, 1982)

A glow discharge technique is used to grow native oxides and oxyfluorides on GaAs. The capacitance-voltage and conductance-voltage measurements of the resulting metal/insulator/semiconductor structures show vastly different properties. Specifically, the density of interface states in the d.c.-to-5 MHz range appears to be much lower at the oxyfluoride interface than at the oxide interface. Secondary ion mass spectroscopy analyses indicate that some fluorine ions are mobile and the fluorine concentration is reduced by about an order of magnitude after a thermal anneal at 450 °C. X-ray photoemission studies indicate that the glass is a mixture of GaF3 and AsOF3 molecules.

The rapid development of silicon technology has been aided by the surface passivation technology developed over the last 30 years. The thermal growth of SiO2 on silicon surfaces has been perfected such that only about one surface state per 1000 000 surface atoms remains after processing. Extensive efforts to passivate GaAs by growing native oxides on the surface have been unsuccessful. Films have been grown by thermal, anodic and plasma oxidation. In all cases, the interface state density is large 1 (greater than 1013 cm-2) and probably approaches the density of surface atoms. Surface states appear to be of two types: extrinsic and intrinsic. The extrinsic states arise from the decomposition2 of the binary glass (Ga2Oa-As203) at the interface to Ga20 3 and elemental arsenic. Lucovsky and Bauer 3 have shown that the As203 glass component has nonbonding occupied orbitals that act as hole traps at the interface. This effect produces a density of surface states equal to the density of A S 2 0 3 glass molecules at the interface. Oxidation of the arsenic glass molecule to a pentavalent state would remove these dangling bonds and might produce a chemically stable crystal-glass interface. Lucovsky4 has suggested growing an oxyfluoride glass of generic composition AsxOrF, to produce the desired (+ 5) valence state in the arsenic component of the dielectric. * Paper presented at the International Conference on Metallurgical Coatings and Process Technology, San Diego, CA, U.S.A., April 5-8, 1982. 0040-6090/82/0000-0000/$02.75

© Elsevier Sequoia/Printed in The Netherlands

328

R.K. AHRENKIELet al.

Oxyfluoride glasses were first grown on GaAs by Chang et al. 5 The working model at that time was the removal of excess arsenic at the interface by selective reaction with the fluorine. Chang et al. made metal/insulator/semiconductor (MIS) devices on n-type GaAs and presented capacitance-voltage (C-V) data indicative of low interface state densities. The removal of extrinsic surface states (produced by elemental arsenic) was believed to be the key mechanism in improving electrical properties. We believe that short-range order, which removes dangling bonds, is the most important interfacial effect. In other words, the complete removal of elemental arsenic will not eliminate states at the native oxide interface. Hence a conceptual model has emerged which is in accord with the earlier experimental work on oxyfluoride glasses 5. To compare interfacial electrical properties, we grew native glasses in plasmas containing pure oxygen and oxygen-fluorine mixtures. The fluorine was produced by the decomposition o f C F 4 in an r.f. glow discharge chamber. The glow discharge was produced between asymmetric electrodes which were driven at 75 MHz. The ptype GaAs wafer was introduced as a third electrode and was positively biased between 50 and 100 V. The plasma composition was continuously monitored with a residual gas analyzer. The sample stage temperature could be controlled from ambient to about 350°C. Pure oxide dielectrics were typically grown at partial oxygen pressures of 75 ~tmHg. At a substrate bias of 100 V, about 2000 ,~ of oxide grew on the substrate in several minutes. The growth rate increased with substrate temperature. Oxyfluoride glasses were typically grown at partial pressures of 75 ~tmHg (oxygen) and 15 p,mHg (CF4). Postgrowth annealing of the dielectric films especially affected the electrical properties. For most dielectrics, deep states are removed by a thermal anneal. Also, the d.c. conductivity of oxyfluoride glasses is greatly reduced by thermal annealing. Further studies are needed to optimize the d.c. electrical properties of the material, Figure 1 shows the megahertz conductance-voltage ( G - V ) spectrum of MIS structures made on a pure oxide and on oxyfluoride glasses. The loss peaks are caused by surface states and are similar to those observed at Si-SiO z interfaces. The intensity of the G-V peak is always greatly reduced in oxyfluoride structures relative

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PLASMA

OXYFLUORIDES

GaAs

ON

329

to that for pure oxide structures. In the data shown here, the peak intensity is reduced by a factor of about 50. A variety of C - V and G - V measurements indicate very weak surface state effects between d.c. and 10 MHz. We conclude that the oxyfluoride dielectric is effective in removing or passivating surface states on GaAs as compared with oxide dielectrics. In order to characterize the oxyfluoride film and to relate our results to bonding theory, we carried out a variety of compositional studies on these oxyfluorides. 10E7

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Fig. 3. The Ga 3p312XPS spectrum of an oxyfluorideglass on GaAs. Fig. 4. The F Is XPS spectrum of the same oxyfluorideas in Fig. 3. Secondary ion mass spectroscopy (SIMS) profiles using the Cameca ion microprobe are shown in Figs. 2(a) and 2(b). Figure 2(b) shows the sputter profile of an as-grown sample. A large fluorine build-up is evident near the interface. We believe the d.c. conductivity and frequency dispersion of the accumulation capacitance is related to excess fluorine in the dielectric. The profile of an annealed sample is shown in Fig. 2(a). The sample has been annealed in an H 2 - N 2 ( 5 0 ~ - 5 0 ~ ) atmosphere for 30 min at 450 °C. The fluorine concentration is reduced by a factor of about 10. The dispersion in accumulation capacitance is insignificant in this sample. There is still a fluorine gradient in the concentration profile.

330

R.K. AHRENKIELet al.

The molecular structure of the glass is crucial to understand the interfacial electrical properties. Extensive X-ray photoemission spectroscopy (XPS) was undertaken to identify the glass molecular structure. The G a 3p3/2 energy region of the XPS spectra is seen in Fig. 3. The peak at - 1 0 7 . 8 eV has previously been identified as corresponding to G a F 3 5. On the same graph, the G a 2 0 3 XPS peak position at - 105.5 eV has been identified. The signal here implies that there is very little G a 2 0 3. We note also that there is no peak at - 104.8 eV which is indicative of G a 3p3/2 in GaAs. This substantiates that we are sampling the dielectric and not the substrate and that there is no GaAs in the glass. Associated with the fuorine (F ls) energy window (Fig. 4) are two peaks; the peak at - 686.65 eV represents G a F 3 while the second peak does not correspond to compounds reported in the literature. A potential candidate for this peak is arsenic oxyfluoride (AsOF3).

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Fig. 5. The As 3d XPS spectrum of the oxyfluoride of Fig. 3.

The arsenic XPS signal, As 3d, is shown in Fig. 5. Peaks from A s 2 0 3 and A s 2 0 5 would occur at - 4 4 . 5 eV and - 4 5 . 9 eV respectively and are not seen. A peak at higher binding energy, - 46.45 eV, is present. The shift is appropriate for an arsenic oxyfluoride, AsOF3, and compares well with published data on sulfur oxides, fluorides and oxyfluorides. Therefore, without the resource of a reliable arsenic oxyfluoride reference standard, the chemical species at the glass-GaAs interface is tentatively identified as A s O F 3. This compound leaves arsenic in a + 5 valence state and apparently removes the dangling orbitals which appear to produce intrinsic surface states. In conclusion a large reduction in the density of surface states on p-type GaAs occurs upon plasma growth of oxyfluorides. The XPS data indicate a binary glass composition of G a F 3 and A s O F 3. The arsenic atom is oxidized to the pentavalent state which removes dangling bonds at the interface.

PLASMA OXYFLUORIDES ON G a A s

331

REFERENCES 1 H.H. Wieder, J. Vac. Sci. Technol., 18 (1981) 827. 2 G.P. Schwartz, C. D. Thurmond, G. W. Kammlett and B. Schwartz, J. Vac. Sci. Technol., 17 (1980) 958. 3 G. Lucovsky and R. S. Bauer, J. Vac. Sci. Technol., 17 (1980) 946. 4 G. Lucovsky, personal communication, 1981. 5 R . P . H . Chang, J. J. Coleman, A. J. Polak, L. C. Feldman and C. C. Chang, Appl. Phys. Lett., 34 (1979) 237.