Unconventional thin film IV–VI photodiode structures

Thin Solid Films, 58 (1979) 73 78 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

73

U N C O N V E N T I O N A L T H I N FILM IV-VI P H O T O D I O D E S T R U C T U R E S * H. HOLLOWAY Ford Motor Company, P.O. Box 2053, Dearborn, Mich. 48121 (U.S.A.) (Received July 31, 1978 : accepted September 11, 1978)

An account is given of IV VI photodiode structures that are designed to exploit the unique properties of thin films. These devices include the low capacitance pinched-off photodiode and the photo field-effect transistor (which make use of the proximity of the depletion region to an insulating substrate), the lateral-collection photodiode (which uses confinement of photogenerated carriers) and a range of devices that use optical interference to modify the spectral quantum efficiency.

1. INTRODUCTION

The IV-VI semiconductors and their pseudo-binary alloys have become technologically important because of their applicability to the fabrication of IR photodiodes and IR injection lasers for a wide spectral range. Although most of the IR photodiode work witlT these materials has been with bulk crystals, the last seven years have yielded steady and significant progress towards a photodiode technology that is based on epitaxial IV-VI layers on single-crystal BaF 2 substrates. The first such devices were made with PbTe and were reported by Logothetis e t al. t Subsequent developments have been reviewed by Holloway and Walpole 2. These types of thin film devices have been shown to have performances that can compete well with those of bulk crystal devices, and the thin film techniques are particularly attractive because of their suitability for low cost planar processing of IR detector arrays. With the demonstration that useful performances may be obtained with thin film devices whose mode of operation is essentially bulk like, attention is turning to less conventional photodiode structures that exploit the unique properties of thin films. The present brief review describes the use of proximity effects in the pinchedoff photodiode (POP) and in the photo field-effect transistor (PFET) and the use of carrier confinement in the lateral-collection photodiode (LCP). Optical interference effects, whose influence has been evident since the earliest work with the thin film devices, are considered last because of their applicability to structures that are based on the LCP.

* Paper presented at the Fourth International Congress on Thin Films, Loughborough, Gt. Britain, September 11-15, 1978" Paper 3Bh

74

2.

H. HOLLOWAY

SUBSTRATE PROXIMITY AND THE PINCHED-OFF

PHOTODIODE

One significant disadvantage of the IV-VI semiconductors is the large junction capacitance (approximately 1 laF cm -2) that arises from their large dielectric constants when typical doping levels are employed. This limits the operating frequency and thereby reduces the range of applications of IV VI devices. One approach to this problem 3 uses the P O P geometry that is shown in Fig. l(a) where a photodiode is operated with its depletion region in contact with an insulating substrate. If the bias is changed, the change in depletion layer width is confined to the periphery of the device. This greatly reduces the change in the depletion region charge thatis the source of the dynamic capacitance dQ/d V. The POPs that have been demonstrated were lead-barrier PbTe devices on BaF 2 operating at 80 K. For maximum quantum efficiency near )~0 = 5 ~tm the PbTe thicknesses were chosen to be approximately 0.2 or 0.6 ~m, corresponding to a quarter-wave or a three-quarter-wave respectively. The quarter-wave structures were pinched off at zero bias but tended to be somewhat unstable. Stability was obtained with three-quarter-wave structures, which required back bias for pinch-off. In this case low noise operation required surface treatment to reduce the 1/f noise that is typical of IV-VI diodes that are not at zero bias 4. 1012

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Fig. 2. The bias-dependent properties of a three-quarter-wave PbTe POP at 80 K and 180° field of view. The black body and noise measurements were made at 1 kHz with 10 Hz bandwidth and the calculated noise current is derived fromthe measured value of the d.c. backgroundcurrent. Some properties of a typical PbTe P O P are shown in Fig. 2. The device of area 6 × 10 -4 cm 2 has a capacitance that decreases from the zero-bias value of 700 pF to about 70 pF for back biases greater than about 150 mV. The value of 70 pF is dominated by the stray capacitance from the metal lead-out. The 500 K black body

UNCONVENTIONAL THIN FILM

IV-VI

75

PHOTODIODE STRUCTURES

current responsivity .~1, which is proportional to the quantum efficiency, is unaffected by the back bias. The noise (measured at 1 kHz) is dominated by fluctuations in the background photon flux for back biases up to 300 mV. Beyond this bias 1/f noise becomes apparent. Thus the detectivity D*, which is a normalized signal-to-noise ratio, remains constant at the background-limited value through a bias range that permits low capacitance operation. 3. THE PHOTO FIELD-EFFECT TRANSISTOR

With the addition of a second ohmic contact to the p-type region the thin film metal barrier photodiode may be operated as a P F E T in the configuration that is shown in Fig. 3. With PbTe devices at 80 K and 180 ° field of view, backgroundlimited operation has been achieved 5. The photocurrent gain ~tis strongly frequency dependent because ~ ~ X j n g m where the junction impedance Xj. is much more dependent On frequency than the F E T transconductance gm is. Thus, although > 104 has been achieved at low frequencies, practical use of this device requires acceptance of smaller gains. With the addition of a shunt resistance R s (shown as a broken line in Fig. 3) a constant c( is obtained up to a cut-off frequency that is determined by R.Cj.. Typical data are shown in Fig. 4. With optimization we can expect useful gains (:t >~ 10) over the audio frequency range ( f ~< 100 kHz). IO S

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4. CARRIER CONFINEMENT AND THE LATERAL-COLLECTION PHOTODIODE

An alternative approach to capacitance reduction involves the replacement of the p-n j unction of a conventional photodiode by a matrix of smaller p-n junctions

76

H. H O L L O W A Y

that collect photogenerated minority carriers from the intervening non-junction region 6, v. To some extent this approach can be used with bulk crystal devices s but the insulating substrate of a thin film device gives the advantage that confinement of the photogenerated carriers prevents recombination losses in the region far from the surface. A schematic diagram of this arrangement is shown in Fig. l(b). Detailed analysis v shows that useful collection efficiencies are obtained with collector separations up to about two diffusion lengths, which is of the order of 20 ~tm for the IV VI semiconductors. To obtain a reduction injunction capacitance the collector diameters must be smaller than this dimension. With PbTe devices capacitance reduction by a factor of 20 has been demonstrated 6 and reduction by two orders of magnitude appears to be attainable. Under some circumstances the L C P may give an increased D*. Under Johnsonnoise-limited conditions we have D* ,~ q ( R o A ) 1/2 where q is the quantum efficiency, R 0 is the zero-bias junction resistance and A is the active area of the detector. The increase in D* depends on obtaining an increase in R o that outweighs the decrease in q. With collector dimensions of the order of the diffusion length we can no longer assume that the junction resistance is inversely proportional to the junction area. If the junction saturation current results from the diffusion of thermally generated carriers from outside the depletion region, these carriers may be collected laterally by the same process as the photocurrent. In this case the junction resistance of the LCP will not be as large as might be expected from its small junction area and the L C P will not give an increase in D*. However, if the saturation current results from the generation of electron hole pairs within the depletion region, the saturation current of the L C P will be significantly smaller than that of a conventional device and the consequent increase in resistance will give an increased value of D* via reduction of the Johnson noise. This condition has been observed with PbTe near 170 K where the L C P has given 6 an increase in the Johnson-noise-limited D* by about a factor of 3. 5. O P T I C A L INTERFERENCE

It has long been recognized that the quantum efficiencies of photon detectors may be influenced by optical interference but these effects have received little attention because they are usually insignificant in bulk crystal devices. In contrast, interference effects are prominent in the spectral responses of the thin film IV VI photodiodes and their use plays a significant part in the optimization of device performance. The spectral responses of metal barrier photodiodes with thicknesses of about 1 Jam show pronounced maxima at wavelengths for which the semiconductor thickness is an odd multiple of quarter-waves. A detailed analysis 9 of the structure BaF 2

substrate/IV-VI semiconductor/metal

shows that peak quantum efficiencies of 0.9 may be obtained. This is a significant increase over the value q ~ 0.6 that is obtained with thicker semiconductors for which the interference effects are small. The reflecting metal barrier is essential for the increased value of t/, which cannot be obtained with conventional p - n junctions in thin film structures.

UNCONVENTIONAL THIN FILM

(a)

IV-VI

PHOTODIODE STRUCTURES

77

(b)

(c) Fig. 5. Laser scans of a 320 pm square PbTe LCP at (a) 270 K, (b) 170 K and (c) 85 K. The collectors 5 ]am in diameter are on 20 pm centers. The vertical displacement is proportional to the current response and the instrumental resolution is about 10 lam.

With the L C P most of the response comes from the more complicated structure BaF 2 substrate/IV-VI semiconductor/BaF 2 insulator/metal In this case the spectral quantum efficiency is modulated to the same extent as in a simple metal barrier device but the positions and the widths of the peaks may differ. There are two extreme cases. (1) The semiconductor thickness is a multiple of half-waves and the optical thickness of the BaF 2 insulator is an odd multiple of quarter-waves. (2) This is the same as case (1) but with the optical thicknesses of the semiconductor and insulator interchanged. With case (1) the widths of the peaks are similar to those of the simple metal barrier device but in case (2) the peak widths are substantially reduced. Thus for thermal imaging applications, where interest lies in all the black body radiation that is transmitted by an atmospheric window, the optimum detector design is based on case (1). The L C P structure in case (2) may be used to obtain narrow (about 0.2 p,m) response peaks that are located to emphasize a particular spectral feature. Analysis

78

H. HOLLOWAY

of more complicated LCP structures shows that the presence of a rudimentary quarter-wave stack, such as BaF 2 substrate/IV-VI semiconductor

(2/4 )/BaF 2(2/4)/Ye( 2/4)/BaF 2(2/2 )/metal should give peaks with widths of about 0.02 pm. In this case the dependence of the peak position on the angle of incidence (as shown in Fig. 6) would permit use of the detector as a simple spectrometer. o

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Fig. 6. Quantum efficiencies of a narrow response detector at various angles of incidence. REFERENCES 1 E . M . Logothetis, H. Holloway, A. J. Varga and E. Wilkes, Appl. Phys. Lett., 19 ( 1971 ) 318. 2 H. Holloway and J. N. Walpole, Prog. Cryst. Growth Characterization, to be published. 3 H. Holloway and K. F. Yeung, Appl. Phys. Lett., 30 (1977) 210. 4 W . H . Rolls and D. V. Edolls, Infrared Phys., 13 (1973) 143. 5 H. Holloway and G. Jesion, unpublished, 1977. 6 H. Holloway, M. D. Hurley and E. B. Schermer, Appl. Phys. Lett., 32 (1978) 65. 7 H. Holloway, J. Appl. Phys., 49 (1978) 4264. 8 A . J . Noreika, M. H. Francombe, W. J. Takei, R. N. Ghoshtagore and J. L. Wentz, IRIS Detector 9

Speciality Group Meet., Colorado Springs, 1977. H. Holloway, J. Appl. Phys., March (1979).