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A high-sensitivity sampling IR detector
In[rated Phys. Vol. 25, No. 1/2. pp. 323 325, 1985
0020 0891.85 S3.00 + 0.00 Copyright C 1985 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
A HIGH-SENSITIVITY S A M P L I N G IR DETECTOR D. D. Coos, S. D. GUNAPALA, R. P. (3. KARUNASIRIand H.-M. MUEHCHOVV Applied Technology Laboratory, Department of Physics, University of Pittsburgh, Pittsburgh, PA 15260, U.S.A.
(Received 27 July 1984) Photoionization of carriers trapped in shallow levels in semiconductors is used to realize an integrating IR detector. Low dark currents and extremely long integration times permit detection of IR photons with very high sensitivity. Lowering the photoionization barrier with an applied electric field permits signal sampling at subthreshold wavelengths within the detector, greatly reducing noise in sampling applications. Abstract
INTRODUCTION We are presently developing FIR detectors, which are based on extrinsic Si. Unlike conventional photoconductors, however, these devices utilize the integrated depletion of charge trapped in shallow impurities as the mode of detection. The device has the structure of a p - v - n diode, which is briefly forward biased to inject charge into impurity traps prior to IR integration. Maintaining an electric field during IR integration by reverse biasing the p v n diode assures complete removal of all photoionized carriers. Monitoring the remaining stored charge after IR integration via high-field impurity-field ionization ( > 1 V/#m) provides a convenient way to readout the device. It has been shown that shallow impurities in semiconductors can be field ionized by a quantum-mechanical tunneling process, m Presently, attention is focused on Si: P but other extrinsic semiconductors can be used. Si: P requires cooling to less than 12 K to prevent appreciable thermal ionization of P impurities.
E X P E R I M E N T A L AND R E S U L T S The test system for these detectors has been described before ~21 including performance data for detectors with a 16 #m thick photosensitive region. A newly fabricated version has a thicker v-region (110/~m). This detector has a quantum efficiency of about 4.5 oj;. Noise equivalent power (NEP) has been reduced to about 4.2 x 10-15 W/(Hz)I/2. The area of the detector is 8.5 x 10 3 cm: and the P concentration in the v-region is 3 x 1014/cm3. Using an integration time of 4.2 s, this detector has been able to detect radiation from a blackbody source at 25K, which produced a photon flux of 3 x 101°/cm2 s incident on the detector. Further increases in impurity concentration should raise the quantum efficiency considerably. We expect that an increase by a factor of 10 will not seriously degrade the ultralow dark currents ( < 1.2 x 10-15 A/cm:) that are currently being achieved. These low dark currents permit signal integration of up to 12 h inside the device before a fast-read pulse ejects the remaining stored charge. Applications in FIR-staring arrays have been considered and a 5 x 5 element array will be tested in the near future. The wavelength response of the detector for subthreshold wavelengths can be modulated by the applied electric field. Figure 1 shows the impurity potential under the influence of the applied field and possible transitions above and below threshold. Depending on the size of the electric field, photons with wavelengths beyond the long wavelength, classical photoionization cutoff can excite carriers from the ground state to higher excited states, from which they may tunnel through or pass over the remaining barrier. ~3) The excited states 2p+ and 2po give rise to resonance effects resulting in absorption maxima. The occurrence of these maxima is strongly field dependent. Figure 2 shows the dependence of the relative response on the applied electric field and wavelength at subthreshold wavelengths. The higher excited state 2p+ contributes to photoionization already at a lower electric field than the lower 2po state because of the different heights of remaining barriers. At zero field there is no subthreshold response. Gating the detector with bias pulses permits a sampling mode of detection for subthreshold wavelengths, in which signal integration can be performed inside 323
324
D . D . Coot-, et al.
POTENTIAL
2p._~
®
2Po
PHOTON
GROUND
STATE
Fig. 1. (AjClassical photoionization; (B, C) resonance photoionization via tunneling out of excited states: (D) photoionization via tunneling.
t.lJ Z
A
o~'
w ~ o I.-./ v
hi Or"
,/.zrn) Fig. 2. 3-D plot of the relative response of the detector vs wavelength and electric field. There is nt~ subthreshold response at zero electric tield.
A high-sensitivity sampling IR detector
E-L'¢~Cq /pm
02
325
RELATIVE RESi2ONSE (arbitrary units}
1> < m rm Z
I
3
Fig. 3. 3-D plot of the relative response at low fields. When the field is raised, subthreshold response resonant with the 2p± and 3p0 states can be seen. At higher fields ( ~ 0.6 V//~m) these states are in the c o n t i n u u m and can no longer be resolved.
the device during multiple sampling cycles before a readout pulse is applied to determine the integrated signal. The detector described here can therefore act as an optical sampling unit. ~4~ Figure 3 shows the responsivity at lower electric fields. When the field is turned on, subthreshold response becomes possible. The contributions of the 2p+ and 3p0 states have been resolved. However, at higher fields ( ~ 0.6 V//2m) both states are in the continuum where their contributions seem to merge into one peak. Both peaks seem to have about the same height. This observation differs from standard absorption experimentsj 5~ where the 2p± contribution is generally much stronger than the one for the 3p0 state. Acknowledgement
This work was supported in part by the U.S. National Science Foundation under Grant No. ECS-8202473.
REFERENCES l. 2. 3. 4. 5.
Banavar J. R., Coon D. D. and Derkits G. E., Phys. Rer. Lett. 41,576 {1978). Coon D. D., Gunapala S., Karunasiri R. P. G.. Muehlhoff H.-M. and Derkits G., Proc. SPIE 395, 198 (1983). Coon D. D. and Karunasiri R. P. G., Electron. Lett. 19, 284 (1983). Coon D. D., Gunapala S., Karunasiri R. P. G. and Muehlhoff H.-M.. Electron. Lett. 19, 1070 (1983). Aggarwal R. L. and Ramdas A. K., Phys. Rel. 137, A602 (1965).
0020 0891.85 S3.00 + 0.00 Copyright C 1985 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
A HIGH-SENSITIVITY S A M P L I N G IR DETECTOR D. D. Coos, S. D. GUNAPALA, R. P. (3. KARUNASIRIand H.-M. MUEHCHOVV Applied Technology Laboratory, Department of Physics, University of Pittsburgh, Pittsburgh, PA 15260, U.S.A.
(Received 27 July 1984) Photoionization of carriers trapped in shallow levels in semiconductors is used to realize an integrating IR detector. Low dark currents and extremely long integration times permit detection of IR photons with very high sensitivity. Lowering the photoionization barrier with an applied electric field permits signal sampling at subthreshold wavelengths within the detector, greatly reducing noise in sampling applications. Abstract
INTRODUCTION We are presently developing FIR detectors, which are based on extrinsic Si. Unlike conventional photoconductors, however, these devices utilize the integrated depletion of charge trapped in shallow impurities as the mode of detection. The device has the structure of a p - v - n diode, which is briefly forward biased to inject charge into impurity traps prior to IR integration. Maintaining an electric field during IR integration by reverse biasing the p v n diode assures complete removal of all photoionized carriers. Monitoring the remaining stored charge after IR integration via high-field impurity-field ionization ( > 1 V/#m) provides a convenient way to readout the device. It has been shown that shallow impurities in semiconductors can be field ionized by a quantum-mechanical tunneling process, m Presently, attention is focused on Si: P but other extrinsic semiconductors can be used. Si: P requires cooling to less than 12 K to prevent appreciable thermal ionization of P impurities.
E X P E R I M E N T A L AND R E S U L T S The test system for these detectors has been described before ~21 including performance data for detectors with a 16 #m thick photosensitive region. A newly fabricated version has a thicker v-region (110/~m). This detector has a quantum efficiency of about 4.5 oj;. Noise equivalent power (NEP) has been reduced to about 4.2 x 10-15 W/(Hz)I/2. The area of the detector is 8.5 x 10 3 cm: and the P concentration in the v-region is 3 x 1014/cm3. Using an integration time of 4.2 s, this detector has been able to detect radiation from a blackbody source at 25K, which produced a photon flux of 3 x 101°/cm2 s incident on the detector. Further increases in impurity concentration should raise the quantum efficiency considerably. We expect that an increase by a factor of 10 will not seriously degrade the ultralow dark currents ( < 1.2 x 10-15 A/cm:) that are currently being achieved. These low dark currents permit signal integration of up to 12 h inside the device before a fast-read pulse ejects the remaining stored charge. Applications in FIR-staring arrays have been considered and a 5 x 5 element array will be tested in the near future. The wavelength response of the detector for subthreshold wavelengths can be modulated by the applied electric field. Figure 1 shows the impurity potential under the influence of the applied field and possible transitions above and below threshold. Depending on the size of the electric field, photons with wavelengths beyond the long wavelength, classical photoionization cutoff can excite carriers from the ground state to higher excited states, from which they may tunnel through or pass over the remaining barrier. ~3) The excited states 2p+ and 2po give rise to resonance effects resulting in absorption maxima. The occurrence of these maxima is strongly field dependent. Figure 2 shows the dependence of the relative response on the applied electric field and wavelength at subthreshold wavelengths. The higher excited state 2p+ contributes to photoionization already at a lower electric field than the lower 2po state because of the different heights of remaining barriers. At zero field there is no subthreshold response. Gating the detector with bias pulses permits a sampling mode of detection for subthreshold wavelengths, in which signal integration can be performed inside 323
324
D . D . Coot-, et al.
POTENTIAL
2p._~
®
2Po
PHOTON
GROUND
STATE
Fig. 1. (AjClassical photoionization; (B, C) resonance photoionization via tunneling out of excited states: (D) photoionization via tunneling.
t.lJ Z
A
o~'
w ~ o I.-./ v
hi Or"
,/.zrn) Fig. 2. 3-D plot of the relative response of the detector vs wavelength and electric field. There is nt~ subthreshold response at zero electric tield.
A high-sensitivity sampling IR detector
E-L'¢~Cq /pm
02
325
RELATIVE RESi2ONSE (arbitrary units}
1> < m rm Z
I
3
Fig. 3. 3-D plot of the relative response at low fields. When the field is raised, subthreshold response resonant with the 2p± and 3p0 states can be seen. At higher fields ( ~ 0.6 V//~m) these states are in the c o n t i n u u m and can no longer be resolved.
the device during multiple sampling cycles before a readout pulse is applied to determine the integrated signal. The detector described here can therefore act as an optical sampling unit. ~4~ Figure 3 shows the responsivity at lower electric fields. When the field is turned on, subthreshold response becomes possible. The contributions of the 2p+ and 3p0 states have been resolved. However, at higher fields ( ~ 0.6 V//2m) both states are in the continuum where their contributions seem to merge into one peak. Both peaks seem to have about the same height. This observation differs from standard absorption experimentsj 5~ where the 2p± contribution is generally much stronger than the one for the 3p0 state. Acknowledgement
This work was supported in part by the U.S. National Science Foundation under Grant No. ECS-8202473.
REFERENCES l. 2. 3. 4. 5.
Banavar J. R., Coon D. D. and Derkits G. E., Phys. Rer. Lett. 41,576 {1978). Coon D. D., Gunapala S., Karunasiri R. P. G.. Muehlhoff H.-M. and Derkits G., Proc. SPIE 395, 198 (1983). Coon D. D. and Karunasiri R. P. G., Electron. Lett. 19, 284 (1983). Coon D. D., Gunapala S., Karunasiri R. P. G. and Muehlhoff H.-M.. Electron. Lett. 19, 1070 (1983). Aggarwal R. L. and Ramdas A. K., Phys. Rel. 137, A602 (1965).