PBSE structures for IR detection

ln]kared Phys. Vol. 25, No. 1~2, pp. 333 336, 1985

0020 0891/85 $3.00 + 0.00 Copyright ,c 1985 Pergamon Press Ltd

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MIS CAPACITORS ON BaF2/PbSe LAYERS AND EPITAXIAL Si/BaF2/PbSe STRUCTURES FOR IR DETECTION H.

ZOGG,W, VOGI and H. MELCHIOR

Institute of Applied Physics and AFIF, Swiss Federal Institute of Technology, 8093 Zurich, Switzerland

(Receit:ed 27 July 1984) Abstract E p i t a x i a l BaF2 g r o w n by v a c u u m d e p o s i t i o n has been investigated as: ( a ) a n i n s u l a t i n g layer in M IS c a p a c i t o r s on n a r r o w - g a p s e m i c o n d u c t i n g PbSe (111)-surfaces; and ( b ) a buffer layer for g r o w t h of epitaxial PbSe o n t o (111 )-Si substrates. The M I S c a p a c i t o r s revealed b r e a k d o w n strengths up to 4 M V cm 1 at 77 K and capacitance voltage curves followed theoretical predictions from a c c u m u l a t i o n to inversion. E s t i m a t e d interface state densities were ~ 10 ~2 cm - 2 V - l and carrier lifetimes were in the picosecond range. In p r e l i m i n a r y experiments, S c h o t t k y diodes were fabricated on the t o p PbSe layer of P b S e / B a F j S i stacks. These diodes revealed resistance area p r o d u c t s up to 0 . 1 4 f l c m z at q u a n t u m efficiencies of ~40°,o, c o r r e s p o n d i n g to detectivities of u p to D* (2p) ~ 1.5 - 1010 cm Hz ~ W - 1 at the peak wavelengths .~p ~ 7 ~tm at 77K.

INTRODUCTION Group IIa fluorides (CaF2, SrF2 and BaF2) are insulators that have been grown by molecular beam epitaxy (MBE) on a variety of semiconductors including Si, Ge, InP and GaAs I~ ol with a view to possible applications for 3-D integrated circuits. These fluorides sublime undissociated and their epitaxial growth has been found to be rather insensitive to lattice mismatch between substrate and overlayer. On the other hand, bulk BaF2 single crystals are well known as substrate material for the epitaxial growth of narrow-gap Pb chalcogenides--(Pb, Sn)Te, (Pb, Sn)Se and Pb(S, Se) which are suitable for fabrication of photovoltaic IR detectors in the thermal 3-5 and 8-14/~m rangeJ :'8~ These IV VI semiconductors and BaF2 exhibit almost equal thermal expansion coefficients over a wide temperature range (~ ~ 2 • 10 -5 K 1 at 300K) and lattice mismatch of less than 5 0Jo. In the present work, we investigated two different applications of epitaxial BaF2 in conjunction with an example of a narrow-gap Pb chalcogenide, PbSe. (a) Epitaxially-deposited BaF2 was used as insulator in metal insulator semiconductor (MIS) capacitors on PbSe. It may be expected that such thermal expansion and lattice-matched MIS structures exhibit reliable properties and low interface state densities. Only a few reports on MIS capacitors with IV-VI semiconductors, PbS (91 and PbTe ~1°' 111 (where oxides as insulating materials were used) have been published up to now. Such MIS capacitors are of interest for the development of IR focal-plane arrays (FPA) with MIS cells as photon flux integrating sensitive elements. Pb chalcogenides would be advantageous with respect to their high charge-storage capacity due to their high static dielectric constants, es > 200. A high charge-storage capacity is favorable in IR FPAs because of the large background flux in the thermal region. (b) We have grown epitaxial PbSe layers onto (111 )-Si wafers using epitaxial BaF2 as a buffer film. Such IV-VI narrow-gap semiconductor/IIa-fluoride/Si stacks could be used to develop monolithic FPAs with the array of intrinsic photovoltaic IR sensors integrated in the narrow-gap semiconductor layer and the Si substrate serving for preamplification and/or multiplexing of the photogenerated signals. Such FPAs would be considerably less complicated than hybrid realizations described previously jl 2~ EXPERIMENTAL

(a) MIS capacitors with epitaxial BaF2/PbSe Single-crystal layers of PbSe, ~ 5 #m thick, were grown on polished (111)-faces of B a F / b u l k single crystals using a normal hot-wall epitaxy (HWE) methodJ 13) After terminating growth, ~ 100nm BaF2 was evaporated from a Knudsen cell at rates <0.1 n m s - 1 and sample temperatures up to 450°C. The H W E furnace and BaF2 effusion cell were mounted in the same vacuum system, so that 333

334

H. Zo(;(; et al.

both operations could be performed in situ at < 10 7 mb. Epitaxy of the BaF2 films grown was verified by Rutherford backscattering spectroscopy (R BS)~14~ and by channeling patterns recorded in a secondary electron microscope (SEM). Figure 1 shows two such channeling patterns for tile B a F , film (grown at ~250°C), and for the underlying PbSe layer recorded at an area not covered with BaF2. The orientation of the sample was the same in both tigures. The < 111)-direction lies at the center of these patterns. The "equilateral triangles" formed from the Koessel cones of the set of three {1131-planes, as indicated in Fig. 1, may be used for determining the complete orientation of the crystal lattice. The orientation of the "'equilateral triangles" is identical in both patterns which means that the BaF2 layer had grown with identical orientation to that of the PbSe underlayer lType A orientation). O n the contrary, it is well k n o w n that Pb chalcogenides grow on bulk t l 1 I I-BaF, with the lattice rotated by ~z about the (111)-direction (Type B orientation)J ;~ The same rcsult was obtained using RBSJ 1,~ The small ratio Zm~, < 0.1 (backscattering yield in the oriented, channeling < 111 )- to random-direction) obtained for the same sample from RBS is indicative of the considerable quality of the BaF2 layer. To fabricate MIS structures from such samples, Pb or Au metal electrodes (area - 10 ~ cm 2) were evaporated in a separate vacuum c h a m b e r through hard masks. Capacitance C and conductance G vs gate voltage V measurements of the completed MIS capacitors were performed mostly at 77 K and after further annealing (200 300°C) in a vacuum. (b) Epitaxial PbSe/BaF2/Si stacks

BaF2 was v a c u u m deposited onto ( 111 )-Si surfaces by partly following the procedure described by Asano et alJ 3~ Typically, growth rates in our work were 0.1 0.3 n m s l, final film thicknesses were 100 300 nm, sample temperatures were 450 600°C and b a c k g r o u n d pressures were 5 • 10 7 rob. PbSe films of 2 5/~m thickness were then deposited in a separate vacuum c h a m b e r using the H W E apparatus described above. Because of the different thermal expansions of Si and BaF2 and the

Fig. 1. SEM channeling patterns of a ~ 110nm thick epitaxial BaF2 layer and of the underlaying PbSc substrate. The orientation of the set of three ~311I-planes is indicated in the corresponding insets.

Two applications of epitaxial BaF2

335

probably incommensurate BaF2/Si interface, t2} it was found that adhesion and quality of the BaF2 and PbSe layers depend critically on details of the deposition conditions and are not fully understood at present. Furthermore, it was found that a very thin layer of CaF2 (lattice mismatch to Si 0.6 ~0, in contrast to BaF2/Si with 14 % mismatch) may be sufficient to obtain more reliable structures. The best stacks grown up to now exhibited channeling patterns of comparable quality to those shown in Fig. 1. According to these patterns, BaF2 grows onto Si with predominantly Type B orientation. This is in contrast to the work of Asano et al., {3} where a mixture of Type A and Type B orientation was observed. In a preliminary experiment, Schottky diodes were fabricated on a p-type top PbSe layer of nonoptimized quality. Vacuum-deposited Pb and Pt were used as blocking and ohmic contacts, respectively. IR illumination was performed from the backside through the Si substrate. RESULTS AND DISCUSSION (a} B a F z / P b S e M I S capacitors

The fabricated samples could be biased from inversion to accumulation and no hysteresis was observed. Mean breakdown fields were as high as 4 M V c m - ~ at T = 77 K. With a measuring frequency of 10kHz, the C Vcurves were of low-frequency (LF) type, whereas at > 300kHz highfrequency (H F) dependence occurred (Fig. 2). The flatband voltage Vfb = 6 V in this particular example corresponds to negative fixed charges at the interface of the order of 1012 cm - 2. With further annealing of the samples, Vrb could be shifted considerably. The theoretical curves in Fig. 2 are calculated taking account of degeneracy and with a Kane band model. 115} Doping concentrations N in the depletion region and Vfb were used as fitting parameters in these calculations, the former being considerably different from their mean values determined by Hall measurements on the same wafers. Changes in C of a few percent only occur in these C - Vcurves due to the series combination of the insulator capacitance C~ (BaF2:c~ = 7.3) with the semiconductor capacitance of high es = 220 and N = 3.5.1017 cm-3. With smaller N (values down to ~ 1016 c m - 3 are obtainable) and thinner insulator thickness, changes up to 20 °~i would result. Interface state densities Dit ~ 1012 c m - 2 V 1 were estimated from the difference of the LF and H F C V curves in depletion. From the measured conductance G in inversion as a function of frequency, the conductance Gp of the semiconductor in inversion is calculated and, from Gp, carrier lifetimes ~ can be estimated, t~°l Values in the picosecond range are obtained in this way which would lead to -

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Fig. 2. C V curves of epitaxial BaF2/PBSe MIS capacitors at 77 K. . , Experimental values at different frequencies; , calculated with the Kane band model and degenerate statistics.

336

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theoretical charge-storage times in the MIS structure in the microsecond range. These lifetimes are rather short even if one takes the narrow bandgap of PbSe into account. However, as the temperature dependence of the conductance Gp did not show the expected variation caused by g r or diffusion mechanisms, it is suspected that the small values of the lifetimes z are due to unoptimized device processing, possibly to trap assisted tunneling or lateral depletion beyond the gate electrode, and arc considerably improw~ble towards the theoretical values (10 : l0 ~s). '~<+'

(b) Epitaxial PhSe/:Ba~/'Si stacks Hall mobilities up to 7- 104 c m 2 V I s I at T < 30 K were obtained in n-type PbSe top layers. The Hall mobilities showed the essential features ofepitaxial PbSe layers on bulk BaF2 single crystals, or bulk PbSe a strong, approx. T s.,2increase with decreasing temperatures up to saturation values m the low 105cm xV is 1 range.i:i Quite encouraging parameters of the first Schottky diodes fabricated on a p-type top PbSe layer were measured (at 77 K): differential resistance at 1/= 0 V" Ro ~ 200 f~ resistance area product (~t 0.0007cm2): RoA A 0 . 1 4 ~ c m 2 peak wavelength (from spectral responsivity); 2p _-- 7 l~m quantum efficiency (from 500 K blackbody radiator): ~1 x 40 '!o. These values correspond to a detectivity at the peak wavelength of D*(2p)~ 1 . 5 . 1 0 1 ° c m H z ~ W

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whereas the detectivity for the background noise limit (BNL) for a 295 K background and 180 ° FOV is D* (BNL, 2p, ~1) ~- 5" 10~°cm Hz ~ W CONCLUSIONS

(a) MIS structures on PbSe with lattice and thermal expansion match BaF2 as an insulator, follow theoretical C ~: curves without hysteresis, withstand breakdown fields > 4 M V c m 1 and small flatband voltages are obtainable. The rather short charge-storage times obtained so far must be improved for possible device applications. (b) Epitaxial Pb chalcogenide/lla-fluoride/Si stacks as demonstrated for the example PbSe/BaF2/Si can be grown with suitable quality for fabrication of photovoltaic I R sensors in the IV VI semiconducting layer. Such stacks may be useful for application in FPAs for thermal imaging in the 3 5 and 8 141zm range, the narrow-gap semiconductor being used for the integration of a matrix of photovoltaic I R sensors and the Si wafer for integration of preamplifiers and:or multiplexers of the photogenerated charges. .4 cknowledgements The authors greatly, acknowledge the help of P. Maier and M. Ospelt, in preparing the Si.' BaF2 struct u rcs, and P. Waegli and R. Wessicken for recording numerous SEM patterns.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. ll. 12. t3. 14. 15. 16.

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