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Planar InSb photodiodes fabricated by Be and Mg ion implantation
Solid-Slate
Electronics, 1975, Vol. 18. pp. 753-756.
Pergamon Press. Printed inGreatBritain
PLANAR InSb PHOTODIODES FABRICATED BY Be AND Mg ION IMPLANTATION* C. E. HUR~ITZ and J. P.
DONNELLY
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173,U.S.A. (Received 13 November 1974;in revisedform 7 December 1974)
Abstract-Planar p-n junction photodiodes in InSb have been made by ion implantation of Be and Mg acceptors. Both ions were incorporated with a doping efficiency of about 50 per cent and each produced excellent photodiodes. At 77”K,20-mil-dia.diodes have typical zero-bias resistances of 6 Ma. At the wavelength peak of 53 pm, quantum efficiencies of 60-70 per cent and detectivities with a 77°Kbackground of 2 x 10’2cmfi/W were measured. Field plate guard rings were used to adjust the surface potential at the diode perimeters for optimum performance.
Planar photodiodes, which by their nature can be fabricated in the form of closely-spaced arrays, are much more easily made by ion implantation than by diffusion. Implantation into InSb of sulfur[l] and protons[2-51 to form n-p diodes and of zinc[l, 6,7] and cadmium@] to form p-n diodes have been reported previously. It was found, however, by several workers [9-l l] that Zn and Cd even at moderate doses (< 10” cm-‘) as well as Bi, Hg and Tl at somewhat higher doses (2 lOI cme3) produced a troublesome porous surface layer which did not anneal out, even at the melting temperature of InSb, and which was difficult to remove by dissolution. In an attempt to circumvent this surface problem and to make high quality p-n implanted diodes, a study of the implantation of the much lighter column IIa acceptors Be and Mg was undertaken. The results, reported here, indicate that the surface damage effects are greatly reduced and that excellent infrared photodetectors with zero-bias resistances of several megohms for 20-mil-dia. devices, quantum efficiencies in excess of 60 per cent and Johnson noise-limited peak detectivities as high as 2 X 10” cmfi/W can be produced using this technique. As in earlier work [3-51field plates were used to control the surface potential at the perimeters of the diodes in order to optimize their performance. FABRICATION PROCEDURE
Diodes were fabricated on (lOO)-oriented slices of n-type InSb with net donor concentrations in the range of 1 x 1014-3X 10” cm-‘. The wafers, which had been polished to a Linde B (0.05 pm) finish by the supplier (Cominco American), were electropolished in a 4: 1 mixture of acetic acid and perchloric acid[l21 removing about 20 wrn of material from one face. The resulting surfaces were smooth, highly polished and free from mechanical damage. Immediately after electropolishing, each wafer was anodized in a 0.1 N solution of NROH at 30 V for 30 sec. The resulting anodic oxide, 300-400 A thick, helped to protect the surface during subsequent processing. Shipley AZ 13505 photoresist was then applied by *This work was sponsored by the Department of the Air Force.
tTransene Corporation,Rowley,Massachusetts.
spraying to a thickness of 5 pm and 20-mil-dia. holes opened through the resist to the anodized InSb surface. Implantation was carried out with Mg’ or Be’ ions at energies ranging from 100 to 400 keV and doses from 5 x 10” cm-’ to 1 x 10” cmm2.Samples were tilted 7” from the normal to reduce channelling. Ions were stopped by the thick photoresist, and 20-mil-dia. diodes were thus formed. After removal of the photoresist, samples were annealed at temperatures of 300-400°C for times of 5-60min. Anneals were performed alternatively with anodized surfaces, bare surfaces, and surfaces covered with pyrolytically deposited SiO*.Results of these various procedures will be discussed later in this paper. After annealing and removal of any anodic oxide or Si02 in buffered HF, it was found that the implanted and unimplanted areas were indistinguishable. In order to delineate the diodes for subsequent photolithographic alignment, the wafer was masked with wax except for small areas at the edges which were alternately anodized and cleaned in buffered HF several times. This procedure etched the InSb preferentially at the exposed diode perimeters sufficiently to distinguish the implanted areas. Approximately 4000 .&of InSb was then removed from the entire wafer surface by etching in a freshly-mixed 1% solution by volume of Brz in isopropanol for about 20 sec. This etching step after annealing was found to be vital to the fabrication of good diodes. Unetched diodes had a very low reverse resistance, as did diodes etched only before annealing. It was concluded from this observation and from MOS capacitor measurements that, irrespective of whether or not there remained any unannealed surface damage due to ion bombardment, a surface inversion layer was formed during the anneal cycle. The gross physical surface change observed previously[9-111 with Zn and Cd implants was not present with either Be or Mg. Immediately after etching and thorough rinsing in isopropanol and Transene 1OOtthe wafers were coated with 1500A of silicon oxynitride, deposited pyrolytically at 200°C by the reaction of SiH+ 02, and NH3 in a N,-rich had been shown atmosphere. This insulator previously[f51 to produce surfaces with low densities of surface states on InSb. Since it is, however, somewhat permeable, it was overcoated with 1OOOAof sputtered 753
154
C.
E.
HURWITZand
SiOZ to provide a stable surface. The silicon oxynitride, while giving a surface with a low surface state density, occasionally leaves the surface in a somewhat inverted state, resulting in leaky diodes. Consequently, as in earlier work[3-51 a field plate guard ring was employed to permit adjustment of the surface potential for optimum diode performance. For these field plates, 750A of Ti and 2000 A of Au were sputtered onto the SiOZand patterned in alignment with and slightly overlapping the perimeters of the diodes. All sputtering operations were done at low power levels (
As described above, several combinations of implanted ion energy and dose, and annealing conditions were tried. These experiments were by no means exhaustive or even definitive. Within the range of this limited study, it was found that both Be and Mg implantations gave similar
p-
InSb
SiON
l-2pm 0.15 pm
m
SiO,
O.lOpm
m
TitAu
0.30pm
m
Au+In
2pm
Fig. 1. Artist’s representation of a cross-sectioned wafer of ion-implanted, field-plate-guarded InSb photodiodes. The implanted region of each diode is 20 mils dia. and the contacts 5 mils dia. Each field plate is 29.8 mils square with an Id-mil-dia. opening and 0.2 mil between adjacent field plates.
*For InSb at 77°K with a hole mobility of IO4cm*/V-set and a hole lifetime in lO”cm’ n-type material of 10m6sec[see R. A. Laff and H. Y. Fan, Phys. Rev. 121,53 (1961).the calculated hole diffusion length is about 80 pm]. tBy using the Schwarz inequality it can be shown that the van der Pauw measurement will a!ways underestimate the carrier concentration if the mobility varies within the implanted layer.
J. P.
DONNELL)
results. The best diodes, defined here as those with high zero-bias impedance and low reverse leakage, were made with ion energies of 100-200 keV, doses of (2-5)X lOI cm-‘, and were annealed at 350°C for 15 min after having been coated with 1000A of pyrolytic SiOZ deposited at 350°C for 2-3 min. At 77”K, Hall measurements of the van der Pauw type[l5] on layers implanted with either Be‘ or Mg’ at 200 keV and a dose of 5 x lOI cm-’ gave measured sheet hole concentrations of about 2.5 x lOI cm-’ for a measured doping efficiency of 50 per cent. It should be noted that this sheet measurement underestimates the doping efficiency and should be considered a lower limit[lS].t According to LSS range theory[l6], the projected range and standard deviation of 200 keV Be’ in InSb are 069 and 0.25 +m, respectively. For 200 keV Mg’, the corresponding values are 0.33 and 0, I I pm. Step etching experiments gave junction depths (in n-type substrates with (2-3) x 10” cm-’ net donors) of approximately 1.9 ym for Be’ and 0.7 pm for Mg’. These measurements agree well with theoretical values of I.7 and 0.69 pm, respectively, calculated from LSS theory assuming a Gaussian distribution. The average carrier concentrations of the implanted layers are therefore about 1.3x IO’*cm-’ and 3.6~ lO’“cm- for the respective ions. Hall mobilities ranged from 410 to 470cm’/V-set for both ions, in agreement with published values for hole mobilities in heavily doped bulk p-type InSb[l7]. As noted earlier, the differences between diodes made with Be and Mg ions in the range 100-200 keV were small, consisting mainly of variations in the short-wavelength spectral response resulting from differences in junction depths. We will therefore present the results for a typical diode made by implanting 5 x 1014cm-* ions of Be at 100keV into an n-type substrate with (2-3) x lO’5cm-’ net donors. This diode had optimum performance with zero field plate voltage. The forward and reverse I-V characteristics of the diode at 77°K in a 77°K background are shown in Figs. 7_ and 3. In the forward direction the diode current is given by If = I,, exp (qV(/vkT - 1) with I0 = I.2 x 10m9Aand n = 1.73. This yields a calculated zero-bias resistance R,)= qkT/q& of 9.6 x lo6 R in moderately good agreement with the measured value of 6~ lOhO. In the reverse direction, the diode exhibits a relatively soft breakdown at about 0.7 V. This breakdown was typical of diodes made from (2-3) x lOI cm-’ substrates. Breakdown voltages of 6-8 V were observed in 1 x IO’”cm-’ substrates. Capacitance-voltage measurements gave a functional dependence of C vs V between C-* and C-?, indicating that the junction was somewhat graded. The grading could be due to diffusion of the implanted ions during annealing or to a tail on the ion distribution profile. No published data on the diffusion of Mg or Be in InSb is known; however, since Mg diffuses very slowly in InAs[l8], it might be inferred that it diffuses slowly in InSb as well. The zero-bias capacitance of this 20-mil-dia. diode is 69 pF, giving a zero-bias RC time constant of 414 psec. Blackbody responsivity, relative spectral response and noise measurements were carried out with the InSb photodiode at 77°K. The blackbody responsivity was
InSb photodiodes by ion implantation
0.04
0
0.08
FORWARD
0.12
0.16
VOLTAGE
0.20
(VI
Fig. 2. Forward I-V characteristic of a typical diode at 77°K.
10-S
Diode Be 9a 77°K
755
sivity and relative response was that of a 20”field of view, 300°K background. Since there was .no indication that the responsivity or relative response were changing with background level, the same values were assumed for smaller background fluxes. The measured noise continued to decrease as the background flux was reduced below this level. The peak responsivity occurs at 5.3 pm. With a 20”field of view, 300°K background, the detectivity is background limited and the peak detectivity is 7.1 x IO” cmfi/W. With a 77°K background (diode completely shielded), the peak detectivity increases to 2 x lOI cm-/W due to the decrease in background noise. This value is equal to the value calculated assuming that the noise is the Johnson noise of a resistor equal to the zero-bias resistance of 6 megohms. The detectivity vs wavelength for this diode in the 77°K background is shown in Fig. 4. The peak quantum efficiency of 67 per cent is higher than expected since the 36 per cent reflectivity at an In&air interface should limit the value to a maximum of 64 per cent. In various diodes, QE-values ranging from 56 to 70 per cent have been measured. A rough estimate indicates that these higher-than-expected values of quantum efficiency probably result from some antireflection effect of the 2500 A of SiON-SiOZ layer covering the diode surfaces. Indeed, if the shape of the detectivity curve is examined closely, there is evidence of a periodic variation which could also be the result of the same interference effect. CONCLUSION
The results reported here indicate that Be and Mg can be used effectively as acceptor ions for the creation of p-type layers in InSb by ion implantation. Infrared
0.01 REVERSE
I
0.1
I
III/
VOLTAGE
1.0
I
III
IO
(VI
Fig. 3. Reverse I-V characteristic of same diode as in Fig. 2.
measured using a 500°K blackbody and a 500 Hz chopping frequency. The relative spectral response was measured using a prism spectrometer and the system noise was measured using a low-noise preamplifier and a wave analyzer. For these measurements, the background level was reduced by means of a cooled aperture. The smallest background flux used to measure the blackbody responSSE Vol. IS. No. 9-C
1
1
I
2 WAVELENGTH
I
,,,!I
4
6
8 10
(pm)
Fig. 4. Detectivity vs wavelength for a typical diode with a 77°K background.
C. E. HURWITZand J. P. DONNELLY
756
photodiodes fabricated using this technique have high
6. Solid State Research Report, Lincoln Laboratory, MIT.
detectivities and quantum efficiencies and their planar nature makes them especially suitable for fabrication of
(1!?72:), p. 1, DDC AD-740874. 7. A. G. Foyt, G. E. Hurwitz, J. P. Donnelly and W. T. Lindley, Meeting of IRIS Specialty Group on Infrared Detectors, Orlando,Florida, 17-18 March (1972). 8. D. M. Randall and D. G. Skvarna, Meeting of IRIS Specialty Group on Infrared Detectors, Washington, D.C., 13-15March (1973). 9. G. Langguth, E. Lang and 0. Meyer, Ion Implantation in Semiconductors, (Edited by I. Ruge and J. Graul), p. 228. Springer, Berlin (1971). IO. C. E. Hurwitz, A. G. Foyt and J. P. Donnelly, unpublished work. 1I. P. J. McNally, private communication. 12. J. W. Faust, Jr., Compound Semiconductors, Vol. I, (Edited by R. K. Willardson and H. L. Goering) Reinhold, New York. (1%2). 13. E. B. Korob, M. S. Mirgalovskayaand M. R. Rankhman, Sooiet Bull. Acad. Sci. (Phys. Series) 36, 1670(1972). 14. L. J. van der Pauw, Phil. Res. Rept. 13, 1 (1958). 15. P. L. Petritz, Phys. Rev. 110, 1254(1958). 16. J. Lindhard, M. Scharl?and H. Schiott, Kgl. Dan&e Videnskab. Selskab, Mat. Fys. Medd. 33, I (1963). 17. A. J. Strauss, J. Appl. Phys. 30, 559 (1959). 18. E. Schillman, Z. Naturforsch. lla, 472 (1956).
closely-spaced arrays. Acknowledgements-We wish to thank F. J. O’Donnell and R. L. Payer for technical assistance in the fabrication and evaluation of these detectors, and R. C. Brooks for technical assistance in performing the ion implantations. We also thank Mary C. Lavine for several helpful suggestions relative to etching of InSb. REFERENCES
P. J. McNally, Radiat. Efects 6, 149 (1970). A. G. Foyt, W. T. Lindley and J. P. Donnelly, Appl. Phys. Lefts. 16, 335 (1970). Solid State Research Report, Lincoln Laboratory, M.I.T. (1971:4), p. 1, DDC AD-736501; (1972:1), p. 1, DDC AD-740874;(1972:4), p. I, DDC AD-757565. C. E. Hurwitz, F. J. Leonberger, A. G. Foyt, W. T. Lindley and J. P. Donnelly, Meeting of the IRIS Specialty Group on Infrared Detectors, Orlando, Florida, 17-18 March (1972). C. E. Hurwitz, A. G. Foyt, and W. T. Lindley, Meeting of the IRIS Specialty Group on Infrared Defectors, Washington, D.C., 13-15 March (1973).
Electronics, 1975, Vol. 18. pp. 753-756.
Pergamon Press. Printed inGreatBritain
PLANAR InSb PHOTODIODES FABRICATED BY Be AND Mg ION IMPLANTATION* C. E. HUR~ITZ and J. P.
DONNELLY
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173,U.S.A. (Received 13 November 1974;in revisedform 7 December 1974)
Abstract-Planar p-n junction photodiodes in InSb have been made by ion implantation of Be and Mg acceptors. Both ions were incorporated with a doping efficiency of about 50 per cent and each produced excellent photodiodes. At 77”K,20-mil-dia.diodes have typical zero-bias resistances of 6 Ma. At the wavelength peak of 53 pm, quantum efficiencies of 60-70 per cent and detectivities with a 77°Kbackground of 2 x 10’2cmfi/W were measured. Field plate guard rings were used to adjust the surface potential at the diode perimeters for optimum performance.
Planar photodiodes, which by their nature can be fabricated in the form of closely-spaced arrays, are much more easily made by ion implantation than by diffusion. Implantation into InSb of sulfur[l] and protons[2-51 to form n-p diodes and of zinc[l, 6,7] and cadmium@] to form p-n diodes have been reported previously. It was found, however, by several workers [9-l l] that Zn and Cd even at moderate doses (< 10” cm-‘) as well as Bi, Hg and Tl at somewhat higher doses (2 lOI cme3) produced a troublesome porous surface layer which did not anneal out, even at the melting temperature of InSb, and which was difficult to remove by dissolution. In an attempt to circumvent this surface problem and to make high quality p-n implanted diodes, a study of the implantation of the much lighter column IIa acceptors Be and Mg was undertaken. The results, reported here, indicate that the surface damage effects are greatly reduced and that excellent infrared photodetectors with zero-bias resistances of several megohms for 20-mil-dia. devices, quantum efficiencies in excess of 60 per cent and Johnson noise-limited peak detectivities as high as 2 X 10” cmfi/W can be produced using this technique. As in earlier work [3-51field plates were used to control the surface potential at the perimeters of the diodes in order to optimize their performance. FABRICATION PROCEDURE
Diodes were fabricated on (lOO)-oriented slices of n-type InSb with net donor concentrations in the range of 1 x 1014-3X 10” cm-‘. The wafers, which had been polished to a Linde B (0.05 pm) finish by the supplier (Cominco American), were electropolished in a 4: 1 mixture of acetic acid and perchloric acid[l21 removing about 20 wrn of material from one face. The resulting surfaces were smooth, highly polished and free from mechanical damage. Immediately after electropolishing, each wafer was anodized in a 0.1 N solution of NROH at 30 V for 30 sec. The resulting anodic oxide, 300-400 A thick, helped to protect the surface during subsequent processing. Shipley AZ 13505 photoresist was then applied by *This work was sponsored by the Department of the Air Force.
tTransene Corporation,Rowley,Massachusetts.
spraying to a thickness of 5 pm and 20-mil-dia. holes opened through the resist to the anodized InSb surface. Implantation was carried out with Mg’ or Be’ ions at energies ranging from 100 to 400 keV and doses from 5 x 10” cm-’ to 1 x 10” cmm2.Samples were tilted 7” from the normal to reduce channelling. Ions were stopped by the thick photoresist, and 20-mil-dia. diodes were thus formed. After removal of the photoresist, samples were annealed at temperatures of 300-400°C for times of 5-60min. Anneals were performed alternatively with anodized surfaces, bare surfaces, and surfaces covered with pyrolytically deposited SiO*.Results of these various procedures will be discussed later in this paper. After annealing and removal of any anodic oxide or Si02 in buffered HF, it was found that the implanted and unimplanted areas were indistinguishable. In order to delineate the diodes for subsequent photolithographic alignment, the wafer was masked with wax except for small areas at the edges which were alternately anodized and cleaned in buffered HF several times. This procedure etched the InSb preferentially at the exposed diode perimeters sufficiently to distinguish the implanted areas. Approximately 4000 .&of InSb was then removed from the entire wafer surface by etching in a freshly-mixed 1% solution by volume of Brz in isopropanol for about 20 sec. This etching step after annealing was found to be vital to the fabrication of good diodes. Unetched diodes had a very low reverse resistance, as did diodes etched only before annealing. It was concluded from this observation and from MOS capacitor measurements that, irrespective of whether or not there remained any unannealed surface damage due to ion bombardment, a surface inversion layer was formed during the anneal cycle. The gross physical surface change observed previously[9-111 with Zn and Cd implants was not present with either Be or Mg. Immediately after etching and thorough rinsing in isopropanol and Transene 1OOtthe wafers were coated with 1500A of silicon oxynitride, deposited pyrolytically at 200°C by the reaction of SiH+ 02, and NH3 in a N,-rich had been shown atmosphere. This insulator previously[f51 to produce surfaces with low densities of surface states on InSb. Since it is, however, somewhat permeable, it was overcoated with 1OOOAof sputtered 753
154
C.
E.
HURWITZand
SiOZ to provide a stable surface. The silicon oxynitride, while giving a surface with a low surface state density, occasionally leaves the surface in a somewhat inverted state, resulting in leaky diodes. Consequently, as in earlier work[3-51 a field plate guard ring was employed to permit adjustment of the surface potential for optimum diode performance. For these field plates, 750A of Ti and 2000 A of Au were sputtered onto the SiOZand patterned in alignment with and slightly overlapping the perimeters of the diodes. All sputtering operations were done at low power levels (
As described above, several combinations of implanted ion energy and dose, and annealing conditions were tried. These experiments were by no means exhaustive or even definitive. Within the range of this limited study, it was found that both Be and Mg implantations gave similar
p-
InSb
SiON
l-2pm 0.15 pm
m
SiO,
O.lOpm
m
TitAu
0.30pm
m
Au+In
2pm
Fig. 1. Artist’s representation of a cross-sectioned wafer of ion-implanted, field-plate-guarded InSb photodiodes. The implanted region of each diode is 20 mils dia. and the contacts 5 mils dia. Each field plate is 29.8 mils square with an Id-mil-dia. opening and 0.2 mil between adjacent field plates.
*For InSb at 77°K with a hole mobility of IO4cm*/V-set and a hole lifetime in lO”cm’ n-type material of 10m6sec[see R. A. Laff and H. Y. Fan, Phys. Rev. 121,53 (1961).the calculated hole diffusion length is about 80 pm]. tBy using the Schwarz inequality it can be shown that the van der Pauw measurement will a!ways underestimate the carrier concentration if the mobility varies within the implanted layer.
J. P.
DONNELL)
results. The best diodes, defined here as those with high zero-bias impedance and low reverse leakage, were made with ion energies of 100-200 keV, doses of (2-5)X lOI cm-‘, and were annealed at 350°C for 15 min after having been coated with 1000A of pyrolytic SiOZ deposited at 350°C for 2-3 min. At 77”K, Hall measurements of the van der Pauw type[l5] on layers implanted with either Be‘ or Mg’ at 200 keV and a dose of 5 x lOI cm-’ gave measured sheet hole concentrations of about 2.5 x lOI cm-’ for a measured doping efficiency of 50 per cent. It should be noted that this sheet measurement underestimates the doping efficiency and should be considered a lower limit[lS].t According to LSS range theory[l6], the projected range and standard deviation of 200 keV Be’ in InSb are 069 and 0.25 +m, respectively. For 200 keV Mg’, the corresponding values are 0.33 and 0, I I pm. Step etching experiments gave junction depths (in n-type substrates with (2-3) x 10” cm-’ net donors) of approximately 1.9 ym for Be’ and 0.7 pm for Mg’. These measurements agree well with theoretical values of I.7 and 0.69 pm, respectively, calculated from LSS theory assuming a Gaussian distribution. The average carrier concentrations of the implanted layers are therefore about 1.3x IO’*cm-’ and 3.6~ lO’“cm- for the respective ions. Hall mobilities ranged from 410 to 470cm’/V-set for both ions, in agreement with published values for hole mobilities in heavily doped bulk p-type InSb[l7]. As noted earlier, the differences between diodes made with Be and Mg ions in the range 100-200 keV were small, consisting mainly of variations in the short-wavelength spectral response resulting from differences in junction depths. We will therefore present the results for a typical diode made by implanting 5 x 1014cm-* ions of Be at 100keV into an n-type substrate with (2-3) x lO’5cm-’ net donors. This diode had optimum performance with zero field plate voltage. The forward and reverse I-V characteristics of the diode at 77°K in a 77°K background are shown in Figs. 7_ and 3. In the forward direction the diode current is given by If = I,, exp (qV(/vkT - 1) with I0 = I.2 x 10m9Aand n = 1.73. This yields a calculated zero-bias resistance R,)= qkT/q& of 9.6 x lo6 R in moderately good agreement with the measured value of 6~ lOhO. In the reverse direction, the diode exhibits a relatively soft breakdown at about 0.7 V. This breakdown was typical of diodes made from (2-3) x lOI cm-’ substrates. Breakdown voltages of 6-8 V were observed in 1 x IO’”cm-’ substrates. Capacitance-voltage measurements gave a functional dependence of C vs V between C-* and C-?, indicating that the junction was somewhat graded. The grading could be due to diffusion of the implanted ions during annealing or to a tail on the ion distribution profile. No published data on the diffusion of Mg or Be in InSb is known; however, since Mg diffuses very slowly in InAs[l8], it might be inferred that it diffuses slowly in InSb as well. The zero-bias capacitance of this 20-mil-dia. diode is 69 pF, giving a zero-bias RC time constant of 414 psec. Blackbody responsivity, relative spectral response and noise measurements were carried out with the InSb photodiode at 77°K. The blackbody responsivity was
InSb photodiodes by ion implantation
0.04
0
0.08
FORWARD
0.12
0.16
VOLTAGE
0.20
(VI
Fig. 2. Forward I-V characteristic of a typical diode at 77°K.
10-S
Diode Be 9a 77°K
755
sivity and relative response was that of a 20”field of view, 300°K background. Since there was .no indication that the responsivity or relative response were changing with background level, the same values were assumed for smaller background fluxes. The measured noise continued to decrease as the background flux was reduced below this level. The peak responsivity occurs at 5.3 pm. With a 20”field of view, 300°K background, the detectivity is background limited and the peak detectivity is 7.1 x IO” cmfi/W. With a 77°K background (diode completely shielded), the peak detectivity increases to 2 x lOI cm-/W due to the decrease in background noise. This value is equal to the value calculated assuming that the noise is the Johnson noise of a resistor equal to the zero-bias resistance of 6 megohms. The detectivity vs wavelength for this diode in the 77°K background is shown in Fig. 4. The peak quantum efficiency of 67 per cent is higher than expected since the 36 per cent reflectivity at an In&air interface should limit the value to a maximum of 64 per cent. In various diodes, QE-values ranging from 56 to 70 per cent have been measured. A rough estimate indicates that these higher-than-expected values of quantum efficiency probably result from some antireflection effect of the 2500 A of SiON-SiOZ layer covering the diode surfaces. Indeed, if the shape of the detectivity curve is examined closely, there is evidence of a periodic variation which could also be the result of the same interference effect. CONCLUSION
The results reported here indicate that Be and Mg can be used effectively as acceptor ions for the creation of p-type layers in InSb by ion implantation. Infrared
0.01 REVERSE
I
0.1
I
III/
VOLTAGE
1.0
I
III
IO
(VI
Fig. 3. Reverse I-V characteristic of same diode as in Fig. 2.
measured using a 500°K blackbody and a 500 Hz chopping frequency. The relative spectral response was measured using a prism spectrometer and the system noise was measured using a low-noise preamplifier and a wave analyzer. For these measurements, the background level was reduced by means of a cooled aperture. The smallest background flux used to measure the blackbody responSSE Vol. IS. No. 9-C
1
1
I
2 WAVELENGTH
I
,,,!I
4
6
8 10
(pm)
Fig. 4. Detectivity vs wavelength for a typical diode with a 77°K background.
C. E. HURWITZand J. P. DONNELLY
756
photodiodes fabricated using this technique have high
6. Solid State Research Report, Lincoln Laboratory, MIT.
detectivities and quantum efficiencies and their planar nature makes them especially suitable for fabrication of
(1!?72:), p. 1, DDC AD-740874. 7. A. G. Foyt, G. E. Hurwitz, J. P. Donnelly and W. T. Lindley, Meeting of IRIS Specialty Group on Infrared Detectors, Orlando,Florida, 17-18 March (1972). 8. D. M. Randall and D. G. Skvarna, Meeting of IRIS Specialty Group on Infrared Detectors, Washington, D.C., 13-15March (1973). 9. G. Langguth, E. Lang and 0. Meyer, Ion Implantation in Semiconductors, (Edited by I. Ruge and J. Graul), p. 228. Springer, Berlin (1971). IO. C. E. Hurwitz, A. G. Foyt and J. P. Donnelly, unpublished work. 1I. P. J. McNally, private communication. 12. J. W. Faust, Jr., Compound Semiconductors, Vol. I, (Edited by R. K. Willardson and H. L. Goering) Reinhold, New York. (1%2). 13. E. B. Korob, M. S. Mirgalovskayaand M. R. Rankhman, Sooiet Bull. Acad. Sci. (Phys. Series) 36, 1670(1972). 14. L. J. van der Pauw, Phil. Res. Rept. 13, 1 (1958). 15. P. L. Petritz, Phys. Rev. 110, 1254(1958). 16. J. Lindhard, M. Scharl?and H. Schiott, Kgl. Dan&e Videnskab. Selskab, Mat. Fys. Medd. 33, I (1963). 17. A. J. Strauss, J. Appl. Phys. 30, 559 (1959). 18. E. Schillman, Z. Naturforsch. lla, 472 (1956).
closely-spaced arrays. Acknowledgements-We wish to thank F. J. O’Donnell and R. L. Payer for technical assistance in the fabrication and evaluation of these detectors, and R. C. Brooks for technical assistance in performing the ion implantations. We also thank Mary C. Lavine for several helpful suggestions relative to etching of InSb. REFERENCES
P. J. McNally, Radiat. Efects 6, 149 (1970). A. G. Foyt, W. T. Lindley and J. P. Donnelly, Appl. Phys. Lefts. 16, 335 (1970). Solid State Research Report, Lincoln Laboratory, M.I.T. (1971:4), p. 1, DDC AD-736501; (1972:1), p. 1, DDC AD-740874;(1972:4), p. I, DDC AD-757565. C. E. Hurwitz, F. J. Leonberger, A. G. Foyt, W. T. Lindley and J. P. Donnelly, Meeting of the IRIS Specialty Group on Infrared Detectors, Orlando, Florida, 17-18 March (1972). C. E. Hurwitz, A. G. Foyt, and W. T. Lindley, Meeting of the IRIS Specialty Group on Infrared Defectors, Washington, D.C., 13-15 March (1973).