II - AD-A Fifth Quarterly Progress Report

Transcription

1 - AD-A DARPA/ONR Grant #NO0014-9l-J-1976 Fifth Quarterly Progress Report (covering the period of August 1 - October 31, 1992) Project Title: Development of Ultra-Low Noise, High Sensitivity Planar Metal Grating Coupled AlGaAs/GaAs Multiquantum Well R Detectors for Focal Plane Array Staring R Sensor Systems Submitted to Max N. Yoder Office of Naval Research Code 1114 SS 800 North Quincy Street Arlington, VA DT 0n smil Dt Prepared by Prof. Sheng S. Li Dept. of Electrical Engineering University of Florida Gainesville, Florida Tel.(904) Fax(904) i ýnovember 10, U 92 1 iv i o jeme# -NJ~

2 Fifth Quarterly Progress Report: August 1 - October 31, 1992 Project Title: Development of ultra-low noise high-detectivity planar metal grating coupled -V multiquantum-well/superlattice barrier infrared photodetectors for focal plane array (FPA) staring infrared sensor systems. Program Manager: Max N. Yoder, Office of Naval Research, Code l114ss,arlington, VA Principal nvestigator: Sheng S. Li, Professor, University of Florida, Gainesville, FL Project Objectives: 1. To develop ultra-low dark current and high detectivity planar metal grating coupled boundto-miniband (BTM) -V quantum well infrared photodetectors (QWPs) for 8 to 12 pm focal plane array (FPA) staring R sensor systems. 2. To develop novel type- and tyep- -V QWPs with multicolor, broad and narrow band spectral response in the 8 to 14pm wavelength range. The material systems to be studied include GaAs/AlGaAs, AlAs/AlGaAs, and ngap/gaas grown on GaAs substrates, and nalas/ngaas grown on np substrate. 3. To conduct theoretical and experimental studies of the planar metal grating-coupled structures for normal incident illumination on the QWPs. Different metal grating coupling structures using -D (line) and 2-D (square) metal gratings for the QWPs will be studied in order to achieve high coupling efficiency under normal front- and back-illuminations. 4. To perform theoretical and experimental studies of dark current, photocurrent, optical absorption, spectral responsivity, noise, and detectivity in different types of QWPs developed under this program. TO QUALM MmM 4. ntroduction Q"0 During this reporting period ( to ) we have continued to make significant progress towards the program goals. We have designed, fabricated, and characterized several new Dist as~ 7 cedes Avail an4/oi SPOOL&& 1 n 1 nnm

3 types of metal grating coupled GaAs/AlGaAs, AlAs/AlGaAs, and nalas/ngaas QWPs for 8-12 pm focal plane array (FPA) staring infrared sensor applications. Specific tasks performed during this period include: (i) designed, fabricated, and characterized new bound-to-miniband (BTM) transition GaAs/AlGaAs QWPs with two different well doping densities, (ii) completed characterization of the normal incidence type- indirect gap AAs/AlGaAs QWPs grown on the (110) GaAs substrate, (iii) developed a new dual-mode (photovoltaic (PV) and photoconductive (PC)) two-color GaAs/AGaAs QWPs with two-band spectral response (6 to 9 pm and 9-16 pm), (iv) developed a new voltage-tunable dual-mode (PV and PC) detection nalas/ngaas BTM QWP with narrow- and broad-band spectral response around,p = 10 pm, and (v) completed the numerical simulation of quantum efficiency versus grating periodicity and wavelength for a twodimensional (2-D) double-period square metal grating couplers for normal incident transmission and reflection mode illumination on the BTM QWPs. Detailed results are discussed in Section 3.. Research Accomplishments and Publications Specific accomplishments and publications during this period are summarized as follows: 2.1 Research Accomplishments: 1. Developed a new dual-mode (PV and PC) detection GaAs/AGaAs QWP using an enlarged- GaAs quantum well (110 X) and AGaAs barrier (875 A) layer for the 6-9 and 9-16 pm wavelength R detection. For the 6-9 pm wavelength detection, the detector operated in the PV mode detection by using the ground-state to continuum states transition. For the 9-16 pm detection, the QWP operated in PC mode detection by using the first-excited state to continuum states transition. A responsivity of RA = 2 ma/w and a detectivity of D* = 1.48x 10 9 cmv/1h-z/w were obtained at A = 7.7pm and T = 77 K for the PV mode operation. For the PC mode operation, RA = 0.49 A/W and D* = X 1010 at Vb = 2 V were obtained at 2 = 12 pm and T = 77 K. Detailed results have been discussed in the previous quarterly report and submitted for publication. 2. Developed a new voltage-tunable dual-mode (PV and PC) operation nalas/ngaas BTM QWP with narrow- and broad-band spectral response around AP = 10 pm. This BTM QWP uses an no. 53 Gao. 47 As quantum well (110 A) and a short-period no. 52 Alo. 48 As/no.s 3 Gao. 47 As (35/50 A) superlattice barrier layer grown on np substrate. The detectivity D* for this QWP operating in PV and "C mode was found to be 5.8x 109 cm - V1z/W at Vb = 0, and 0.5 V, and A =10, and 10.3 pm, and T = 67 K. Detailed results are described in Section 3.1.! 2

4 1 3. Completed detailed characterization of a normal incidence type-l QWP using an indirect Alo. 5 GaO. 5 As/AlAs system grown on [110] GaAs substrate. A broad spectral response covering four different wavelength windows from pm was observed in this new QWPs. The dark current, optical absorption coefficient, responsivity, detectivity, and spectral response of this detector have been characterized. An ultra-large photocurrent gain (g = 100) was obtained in this QWP at A = 2.2pm. Detailed results are discussed in Section Completed numerical simulation of the 2-D square reflection and transmission metal grating structures formed on the GaAs/AGaAs BTM QWPs using the method of moment. The advantages of these grating structures are for normal incident illumination and that the coupling of incident radiation is independent of the polarization direction. The results showed that a significant enhancement of light coupling efficiency by using the 2-D square grating structure operated in the transmission as well as the reflection modes. Based on the simulation results, it is possible to design and fabricate a variety of square grating couplers with excellent coupling efficiencies (15 to 25 %) in the pm wavelength range. We will be fabricating the 2-D square grating coupler structures with different grating periods on the BTM QWPs during the next reporting period to compare the coupling quantum efficiency versus grating period in these new structures. Detailed theoretical description of the method of moment and numerical simulation results for the 2-D square grating coupled BTM QWPs are given in i Section Refereed Journal Papers: 1. L. S. Yu and S. S. Li,"A Low Dark Current, High Detectivity Grating Coupled AGaAs/GaAs Mulitple Quantum Well R Detector Using Bound-to-Miniband Transition for 10 pm Detection,"Appl. Phys. Letts., 59 (11), p.1332, Sept.9, L. S. Yu, S. S. Li, and Pin Ho "Largely Enhanced Bound-to-Miniband Absorption in an ngaas Multiple Qua'ntum Well with a Short-Period Superlattice nalas/ngaas Barrier" Applied Phys. Letts., 59 (21), p.2712, Nov. 18, L. S. Yu, Y. H. Wang, S. S. Li and Pin Ho,"A Low Dark Current Step-Bound-to-Miniband Transition ngaas/gaas/agaas Multiquantum Well nfrared Detector," Appl. Phys. Letts., 60(8), p.992, Feb.24,

5 L. S. Yu, S. S. Li,and P. Ho,"A Normal ncident Type-l Quantum Well nfrared Detector Using an ndirect AlAs/Alo. 5 Ga0. 5 As System Grown on [110] GaAs," Electronics Letts 28(15) p.1468, July,16, L. S. Yu, S. S. Li, Y. H. Wang, and Y. C. Kao,"A Study of Coupling Efficiency versus Grating Periodicity in A Normal ncident Grating-Coupled GaAs/AGaAs Quantum Well nfrared Detector," J. Appl. Phys., 72(6), pp.2105, Sept. 15, Y. C. Wang and S. S. Li,"A Numerical Analysis of Double Periodic Reflection Metal Grating Coupler for Multiquantum Well nfrared Photodetectors," submitted to Journal Appl. Phys., Sept., Y. H. Wang, S, S. Li, and P. Ho,"A Photovoltaic and Photoconductive Dual Mode Operation GaAs/AlGaAs Quantum Well nfrared Detector for Two Band Detection," accepted, Appl.Phys. Lett.,Jan. issue, Y. H. Wang, S. S. Li, and P. Ho,"A Voltage-Tunable Dual Mode Operation nalas/ngaas Bound-to-Miniband Transition QWP for Narrow and Broad Band Detection at 10 pm," submitted to Appl. Phys. Lett., Sept., P. Ho, P. A. Martin, L. S. Yu, and S. S. Li,"Growth of GaAs and AGaAs on Misoriented (110) GaAs and a Normal ncidnece Type- Quantum Well nfrared Detector," accepted, J. Vacuum Science and Technology, Oct., S. S. Li, M. Y. Chuang and L. S. Yu,"Current Conduction Mechanisms in Bound-to-Miniband Transition -V Quantum Well nfrared Photodetectors," accepted, Semiconductor Science and Technology, nstituie of Physics Publishing, Bristol, England, Oct., Workshop and Conference Presentations 1. L. S. Yu, S. S. Li, and Pin Ho, " Largely Enhanced ntra-subband Absorption in a Wide naas/ngaas Quantum Well with a Short-Period Superlattice Barrier Structure,"paper presented at the SPE's Symposium on Quantum Wells and Superlattices, Somerset, NJ, March,1992. Paper published in the SPE Conference Proceeding. 2. S. S. Li and L. S. Yu, "Grating Coupled Bound-to-Miniband Transition -V Quantum Well nfrared Detectors," invited paper, presented at the nnovative Long Wavelength nfrared Photodetector Workshop, Jet Propulsion Lab., Pasadena, CA, April 7-9,

6 ' 1 3. L. S. Yu and S. S. Li,"A Normal ncident Type- Quantum Well nfrared Detector Using an ndirect AlAs/Al 0. 5 Gao. 5 As System Grown on [110] GaAs, paper presented at the nnovatsve Long Wavelength nfrared Photodetector Workshop, Jet Propulsion Lab., Pasadena, CA, April 7-9, L. S. Yu, S. S. Li, Y. H. Wang, and P. Ho," Grating Coupled -V Quantum Well nfrared Detectors Using Bound-to-Miniband Transition," paper presented at the SPE Conference on nfrared Detectors and Focal Plane Arrays at OE/Aerospace Sensing 92," Orlando, FL, April 20-24, Full paper published in the SPE conference proceeding. 5. S. S. Li,"Grating Coupled Bound-to-Miniband Transition -V Multiquantum Well nfrared Photodetectors," presented at the DARPA R Detector Workshop, Washington D.C., June 12, S. S. Li, M. Y. Chuang and L. S. Yu,"Current Conduction Mechanisms in Bound-to-Miniband Transition -V Quantum Well nfrared Photodetectors," paper presented at the nternational Conference on Narrow Gap Semiconductors, University of Southampton, Southampton, UK, July 19-23, P. Ho, P. A. Martin, L. S. Yu, and S. S. Li, "Growth of GaAs and AlGaAs on Misoriented (110) GaAs and a Normal ncidnece Type-l Quantum Well nfrared Detector," paper presented at the 12th North American Conference on Molecular Beam Epitaxy," Oct.12-14, S. S. Li, Y. H. Wang, M. Y. Chuang, P. Ho, and 0. Manasreh, "A Normal ncidence Type-l AAs/AlGaAs QWP for Multicolor nfrared Detection," submitted to Materials Research Society, Symposium C2, nfrared Detectors, April 12-16, San Francisco, nteractions with Government and ndustrial Laboratories 1. Continued to collaborate with Dr. Pin Ho of General Electric Co.,in Syracuse, NY, on the growth of -V QWP's structures by using the molecular beam epitaxy (MBE) technique. 2. Continued to collaborate and exchange technical information on QWP's research with Dr. Barry Levine of A T & T Bell Laboratories. Dr. Swami Swaminathan of A T & T Bell lab., in Breinigsville, PA, has expressed interest in collaboration with Dr. Li on the planar metal grating coupler structure developed at the University of Florida for possible applications in their QWP focal plane arrays. 5

7 3. Dr. Li gave an invited talk at the Electronics Technology Laboratory, WRDC/ELRA, WPAFB, Ohio, on The bound-to-minihand transition lll-v QWPs on August 21, and discussed with Dr.Omar Manasreh and his colleagues in the Electronics Technology Laboratory at the Wright Patterson Air Force Base. Dr. Manasreh had performed optical absorption measurements on Dr. Li's QWP samples, while Dr. Li has fabricated and characterized the QWP samples provided by Dr. Manasreh. 4. Dr. Li was invited by American Engineering Education Association to serve on the review panel for Navel Postdoctoral Fellowship program in Washington D.C. August 7, to review a dozen proposals with other pannel members submitted by various applicants. Dr. Li was responsible for recommending one of the applicants whose proposal was on quantum well R detector research.. Technical Results and Discussion 3.1 A Voltage-Tunable Dual-Mode Operation nalas/ngaas BTM QWP for 10 pm Detection Sununary: A new photoconductive (PC) and photovoltaic (PV) dual-mode operation quantum well infrared photodetector (QWP) using a n-type no.s 2 Alo. 4 sas/no.s3gao. 4 7As system grown on np substrate has been developed for both narrow-band (AA/Ap = 7 %) and broad-band (AA/Ap = 24 %) detection with a peak spectral response around,p = 10 pm. The detection scheme utilizes a voltage-tuned bound-to-miniband transition from the ground-state in the n0. 53 Ga0. 4 7As(110 1) quantum well to the global miniband states in the nalas/ngaas superlattice barrier layers. The detectivity Dý for the PV mode operation was found to be 5.7x10 9 cm V-fl'Hz/W at )p = 10 pm and T = 67 K, while D for the PC mode operation was found to be 5.8x 10 9 v/-h-'z/w at Vb = 0.5 V, A = 10.3 pm and T = 67 K ntroduction The long-wavelength quantum well infrared photodetectors (QWPs) based on intersubband transitions for detection in the 8-14 pm atmospheric window have been extensively investigated in recent years' A great deal of the works have been reported on the GaAs/AGaAs quantum well/superlattice infrared photodetectors using bound-to-bound 1-2, bound-to-miniband3-5 and bound-to-continuum6-7 conduction intersubband transitions. Studies of the intersubband absorption have also been conducted in the ngaas/nalas system in the 3 to 5 pm wavelength range 6

8 8-10. Since the nalas/ngaas heterostructure has a larger conduction band offset (AE,, 500 mev) compared to the GaAs/AlGaAs system, it is a promising candidate for both mid-wavelength infrared (MWR) and long-wavelength infrared (LWR) applications. Recently, we have reported the observation of a largely enhanced intersubband absorption in the nalas/ngaas system using bound-to-miniband transition in the 8-14 pm"l wavelength range. The results showed that multicolor (in the 3-5pm, 8-14 pm or even longer wavelength (, 100 pm)) infrared detectors can be realized in the nalas/ngaas system due to a much larger potential barrier (AEC = 500 mev) created by using a short period superlattice barrier structure and resonant miniband conduction mechanism". n addition, photovoltaic (PV) mode operation QWPs using bound-to-miniband conduction3,12,1 3 have been reported recently. n this section we report a dual-mode (PV and PC) operation nalas/ngaas multiple quantum well with superlattice barrier infrared photodetector (QWP) using voltage-tuned boundto-miniband transition mechanism. This new QWP structure was grown on a lattice-matched semi-insulating (S) np substrate by using the molecular beam epitaxy (MBE) technique. A 1-pm no. 5 3 Gao. 47 As buffer layer with doping density of 2x 1018 cm- 3 was first grown on the S np substrate, followed by the growth of 20 periods of enlarged no.53gao. 47 As quantum wells with a well width of 110 A and a dopant density of 5x cm-3. The barrier layers on each side of the quantum well consist of 6 periods of undoped no. 5 2 A As (35.)/no.5 3 Gao. 47 As (50 A) superlattice barrier layers. A 0.3 pm thick n+-no.sagao. 47 As cap layer with a dopant density of 2x10' 8 cm-3 was grown on top of this QWP structure for ohmic contacts. Figure 1 shows the energy band diagram for this BTM QWP. The transition scheme for this QWP is from the localized ground state level EEW1 in the enlarged well (EW) to the global resonant-coupled miniband ESLM in the superlattice (SL) barrier layers. The physical parameters of the quantum well and superlattice barrier are chosen so that the first excited level EEW2 of the EW is merged and lined up with the ground level of the minibapd ESL on both sides of the EW to obtain a maximum intersubband absorption strength Results and Discussion The mesa structure for the ngaas BTM QWP was formed by chemical etching through the active layers and stopped at the n+ GaAs buffer layer for ohmic contacts. The active area of the detector is 200x200pm 2. To enhance coupling efficiency for normal illumination and angular independent radiation polarization, a planar 2-D cross metal grating coupler was deposited on the 7

9 QWP by using electron beam (E-beam) evaporation with 0.2pm AuGe/Ni/Au films. The 2-D cross metal grating coupler consists of equally spaced square shape metal gratings with a grating period A = 10 pm and a geometrical ratio factor g = d/a = 0.5, where d is the width of the square metal grating. Numerical simulations of the energy states EEWn, ESLn (n = 1,2,...) and the transmission coefficient T * T for the QWP were carried out by using a multi-layer transfer matrix method (TMM) 3. The results are shown in Fig.2 for Vb = 0, 0.5 V. n our design, a broad and highly degenerated miniband was formed by using the superlattice barrier structure. The center energy position of the first miniband is located at 163 mev above the conduction band edge of ngaas EW with a bandwidth of 21,- 60 mev. n order to determine accurately the intersubband transition levels, detailed calculations of the energy differences between the subband levels by taking into account the effects of band nonparabolicityl 4, electron-electron interaction 15, and electron plasma16 are needed. n analysing our QWP, we have considered both electron-electron interaction (exchange energy) E.,h and depolarization Ede effects. The results showed a lowering of "'5 mev for the heavily populated bound states EEW1 in the quantum well. The peak absorption wavelength can be found from the relation 1.24 "AP - ESL1 - EEW + Exch - Ede (pm) Now, substituting values of ESL = 163 mev, EEwl = 51 mev, and Exch-Ed, - 5 mev into the above equation, we obtain AP = 10.6 pm. The infrared intersubband absorption versus wavelength for this QWP was measured at Brewster's angle1 4 (OB = - 730) by using a Perkin-Elmer Fourier transform interferometer (FTR) at room temperature" 1. The results showed a main absorption peak centered at AP = 10.7 pm with a spectral linewidth of Av = 500 cm- 1. Figure 3 shows the measured dark current as a function of the bias voltage for ngaas/nalas BTM QWP at different temperatures. Figure 4 shows the dark current and differential resistance (rd)versus bias voltage for this QWP measured at T = 67 K. The asymmetry property of the dark current under positive and negative bias conditions was clearly shown in both figures, which can be attributed to the doping migration effect created during the growth of the quantum well/superlattice layers. The doping impurities in the quantum well spill over to the superlattice barrier layer along the direction of epilayer growth which leads to the asymmetry of the potential barrier on either side of the quantum well. Since the dark current in the BTM QWP is controlled by the thermionically assisted tunneling current through the miniband, asymmetry in potential barrier on either side of 8

10 the quantum well will result in asymmetrical dark current flow under positive and negative bias conditions. The photocurrent was measured as a function of temperature, bias voltage (Vb), polarization direction, and wavelength using an OREL single grating monochromator and ceramic element infrared source. Figure 5 shows the normalized responsivity versus wavelength measured at Vb = 0, 0.5 V and T = 67 K. As shown in the figure, under the photovoltaic (PV) mode operation (Vb = 0), the detector has a peak response wavelength at AP = 10 pm. When a voltage is applied to this QWP, the photoconductive (PC) mode detection becomes the predominant conduction mechanism for Vb > 0.5V. The peak wavelength AP for the PC mode detection was found to be at 10.3 pm. A full width at half maximum (FWHM) of Am, = 232 cm- 1 (,-, 29 mev) was obtained from Fig.5. The band width A,/, = 24 % from the PC response curve was found to be much narrower than the room-temperature FTR absorption 11 curve. The intersubband transitions for both the PC and PV modes may be attributed to the resonant transition from the ground state EEw1 to the global miniband ESL1 states which are aligned with the first excited state EEW2 in the quantum well. The intersubband resonant transition (i.e., maximum absorption strength) depends strongly on the location of the first excited state EW2 of the quantum well relative to the miniband edge, ESL 5. n this BTM QWP structure, the EEW2 lies near the top of the miniband edge ESL1, which results in a strong, narrow-band spectral response in the PV mode detection with a linewidth of AAX = 0.7 pm at a half maximum. The bound-to-miniband (BTM) transition QWP operating in the PV mode offers a unique feature of ultra-narrow bandwidth (AA/Ap = 7 %) infrared detection, which is not attainable in a conventional bound-to continuum transition QWP. As the bias voltage increases, relative position between the "embedding" localized state EEW2 and the"framing" miniband states ESL can be adjusted by a "controlling bias" due to the different dependence of EEW2 and Es, on the bias voltage. An energy redshift of about 3.6 mev between the resonant states was observed at Vb = 0.5 V and T = 67 K. As expected, a broad-band spectral linewidth of 24 % at Vb = 0.5 V was obtained in the PC mode operation as shown in Fig.5. n the voltage-tunable BTM QWP structure, not only can the spectral bandwidth be tailored to the desired width (from 7 % to 24 %), but the spectral response peak can also be tuned as well. The photocurrent responsivity RA and the detectivity D* for the PC and PV modes were measured at T = 67 K, A = 10.3 pm and 10 pm, respectively, and the results are shown in Fig. 6. The peak responsivity for the PV mode was found to be 27 ma/w at A = 10 pm. The pbotocurrent responsivity RA, measured at Vb = 0.5, 1.5 V, was found to be 38 ma/w and 145 ma/w, respectively. The detectivity D* was calculated from the measured responsivity and dark current, using the formula Dý = RA(AA)/'/ 2 /n, 9

11 where A is the effective area of the detector, Af is the noise bandwidth. The dark current shot noise in, is given by in = V14qldgAf, where g is the optical gain, and may be evaluated from the measured responsivity RA = Y7(A/1.24)g and the unpolarized quantum efficiency expression Y7 = (1/2)(1-e-2a'). Optical gain can also be derived from the noise measurement. The results yielded a peak detectivity D,\ = 5.8x109 cm -/H-/W at A = 10.3 pm, Vb = 0.5 V, and T = 67 K for the PC mode operation. As shown in Fig. 6, the value of D decreases with increasing Vb due to the increase of dark current with increasing bias voltage. The zero bias differential resistance rd was found to be about 450 KQ at 67 K. Since the detector operating in the PV mode is limited by Johnson noise (i.e., ij = :/1K T/rd), the detectivity D1 for the PV mode was found to be 5.7 x 109 cm V'-17z/W. n order to verify the zero bias noise, we also measured the noise current by using a lock-in amplifier, which yielded a value of ij = 9.OX10-14 A/V'-H-z, in good agreement with the calculated value from the Johnson noise expression. Figure 7 shows the measured noise current and differential resistance as a function of the temperature for the QWP operating at zero bias condition (PV mode), for 40 < T < 65K. A linear relation was obtained between the semilog plot of noise current (or differential resistance) and temperature. For Vb < 0.15 V, the photoresponse at AP = 10.3 pm decreases with increasing bias voltage, indicating that the internal photovoltage is offset by the applied bias voltage in this bias range. For Vb > 0.15 V, the photoresponse starts to increase again, which implies that the PC mode conduction becomes dominant when the applied bias exceeds the built-in potential in the QWP. However, when the polarity of the bias voltage was reversed, the responsivity of the QWP was found to decrease rapidly and the internal photovoltage disappeared. To further understand the PV mode detection in the ngaas/nalas BTM QWP, the shortcircuit current (,,) and open-circuit voltage (V,,) were measured as a function of temperature using a HP4145 at Vb = 0, and the results are shown in Fig.8. t is interesting to note that V1C increases monotonically with decreasing temperature for T < 65K, and,, increases with decreasing temperature for T < 77K and reaches a saturation value of 0.1 V at T = 45 K. The large V,, observed in this BTM QWP may be attributed to the doping impurities migration in the direction of growth layers: Part of the doping impurities migrate from doped quantum well to the adjacent superlattice barrier layers. At low temperatures, some electrons from the dopant impurities in the superlattice are trapped in the quantum well due to the lower thermal energy. The separated electrons and positive ionized impurities form a build-in electric field. As temperature decreases, more electrons are trapped in the doped quantum well, which in turn leads to a larger build-in field in the QWP. For T > 68K, electrons are excited out of the quantum well due to increasing 10

12 thermal energy and as a result the build-in potential vanishes. The asymmetric distribution of the impurities can be verified by the observed large asymmetrical dark current under positive and negative bias conditions, as can be seen in Figs.3 and 4. Figure 9 shows the PV mode responsivity R and detectivity D* as a function of temperature measured at AP = 10pm for the BTM QWP. The responsivity was found to decrease with decreasing temperature, while detectivity increases with decreasing temperature. The measured peak current response was 63 ma/w at A = 10pm and T = 65 K. The large responsivity is due to the enhanced absorption in the enlarged quantum well (110A) and the smaller electron effective mass m* (m* = 0.042mr,) 6. Although the absorption spectral linewidth (full width at half maximum, FWHM) of the transition is equal to 322 cm-'(- 40meV), the measured photovoltaic response FWHM ( - 68crn-') is much narrower than that of the room temperature absorption spectrum and the photoconduction response (FWHM=232 cn-'). Calculation of the transition energy shows that the major transport route of the photovoltaic response is via the excited state of the doped quantum well and the miniband nearby the excited state. For temperature exceeds 67 K, the signal tend to become comparable with noise. This temperature coincides with that of the vanishing Voc, implying that the photovoltaic phenomenon vanishes for T > 67 K. The predominant noise current of a BTM QWP operating in PV mode is due to Johnson noise current ij = J/P7/7. From Fig.7, the noise current for the ngaas BTM QWP was estimated to be 0.17 pa/v77h", which is about one order of magnitude lower than the photoconductor detectors. From the responsivity and the noise current, the detectivity D* = Rv/A/&n was calculated and the results were shown in Fig.9. As shown in Fig.9, increasing the temperature, D* will decrease due to the exponential increase in noise current with temperature, whereas the responsivity increases with increasing temperature. The D* was found to be 7.06 x 109cm'V/7z /W at Ap = 10prm and T= 65 K, and the dark current was found to be 21 na. t is noted that D* was measured using normal incident illumination with metal grating to couple the effective electric field E± into the quantum wells. The coupling quantum efficiency is a strong function of the grating periodicity for a given wavelength. However, the equally spaced square metal grating with a periodicity length of A = 10pm and the width d = 5pm of the square metal grating used in this QWP was not optimized. The geometrical ratio factor g = d/a = 0.5 of the grating yields about 8.3% quantum efficiency 4. A higher quantum efficiency square metal grating is presented in Section 3.3. The use of this grating can significant increase the value of D*. 11

13 n conclusion, we have demonstrated a new high performance PV and PC dual-mode operation ngaas/nalas QWP using the voltage-tunable bound-to-miniband (BTM) transition mechanism. Both narrow-band PV mode and broad-band PC mode detections at A - 10 pm peak wavelength have been achieved. Using the dual-mode operation and bound-to-miniband transition 3 nalas/ngaas QWP structure grown on np substrate, it is possible to design high performance two-color staring focal plane arrays and infrared image sensors for use in the 3-5 pm and 8-14 pm wavelength applications. A significant improvement in detectivity and responsivitycan be predicted by using a high quantum efficiency 2-D metal grating coupler on the QWP. 1 1

14 References 1. J. S. Smith, L. C. Chiu, S. Margalit, A. Yariv, and A. Y. Cho, J. Vac. Sci. Technol. B1, 376 (1983). 2. B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker, and R. J. Malik, Appl. Phys. Lett.50, 1092 (1987). 3. L. S. Yu and S. S. Li, Appl. Phys. Lett. 59, 1332 (1991). 4. L. S. Yu, Y. H. Wang, and S. S. Li, Appl. Phys. Lett. 60, 992 (1992). 5. T. S. Faska, J. W. Little, W. A. Beck, K. J. Ritter, A. C. Goldberg, and R. LeBlanc., nnovative nfrared Photodetector Workshop, JPL, Pasadena, April 7-9, B. F. Levine, G. Hasnain, C. G. Bethea, N. Chand, Appl. Phys. Lett. 54, 2704 (1989). 7. B. F. Levine, C. G. Bethea, G. Hasnain, V. 0. Shen, E. Pelve, R. R. Abbott, and S. J. Hseih, Appl. Phys. Lett. 56, 851 (1990). 8. H. Asai and Y. Kawamura, Appl. Phys. Lett. 56, 746 (1990). 9. B. F. Levine, A. Y. Cho, J. Walker, R. J. Malik, D. A. Kleinman, and D. L. Sivco, Appl. Phys. Lett. 52, 1481 (1988). 10. G. Hasnain, B. F. Levine, D. L. Sivco, and A. Y. Cho, Appl. Phys. Lett. 56, 770 (1990). 11. L. S. Yu and S. S. Li, Appl. Phys. Lett. 59, 2712 (1991). 12. M. 0. Manasreh, F. Szmulowicz, D. W. Fisher, K. R. Evans, and C. E. Stutz, Appl. Phys. Lett. 57, 1790 (1990). 13. S. D. Gunapala, B. F. Levine, and N. Chand, J. Appl. Phys. 70, 305 (1991). 14. A. Raymond, J. L. Robert, and C. Bernard, J. Phys. C 12, 2289 (1979). 15. K. M. S. V. Bandara, D. D. Coon, and Byungsung 0, Appl. Phys. Lett. 53, 1931 (1988). 16. M. Ramsteiner, J. D. Ralston, P. Koidl, B. Dischler, H. Biebl, J Wagner, and H. Ennen, J. Appl. Phys. 67, 3900 (1990). 17. A. Kastalsky, T. Duffield, S. J. Allen, and J. Harbison, Appl. Phys. Lett. 52, 1320 (1988). 13

15 3A Ec =500 mev naas/ngaas S ESL fi1e [L EEWl ngaas Figure 1. Energy band diagram for an ngaas/nalas BTM QWP. 14

16 * 0 ~-4 5 ~-8/.., -16 * o -20- V= 0V -24 V_0. V Electron Energy (mev) Figure 2. Energy states and transmission probability for an ngaas/nalas BTM QWP. 15

17 ' / -- 15K! cc # * i K,' VOLTAGE Figure 3 Dark current vs. bias voltage for an ngaas/nalas BTM QWP for 45 < T < 75 K. 16

18 T =67K io "104 i , ', i,,,,,, Bias Voltage Vb (V) Figure 4 Dark current and differential resistance vs. bias voltage for an ngaas/nalas BTM QWP at T = 67 K. 17

19 i S0p 6 S0.6 a. PC Mode: 0.8p = 10.3 pm T =67 K Ac = 11.7 pm PV Mode: =lopm S0.4 Vb= 00 V 0,.2 02Vb 0V Wavelength (pm) Figure 5 Relative responsivity vs. wavelength for Vb = 0, 0.5V for an ngaas/na1as BTM QWP at T = 67 K 18

20 T=67K 120 Ap =10.3 pm 5.5 S5.0 E E a 190 0t c 60 0 *. 4.0 i3.5 * x Bias Voltage Vb M Figure 6 Responsivity and detectivity vs. bias voltage for an ngaas/nalas BTM QWP. 19

21 ,o, ngaas/naas BTM QWP a w + 0 z CO c/_)..- z +,, Oc TEMPERATURE (K) Figure 7 Noise current and differential resistance vs. temperature for an ngaas/na1as BTM QWP in PV mode detection. 20

22 x 6-2 ngaas/nalas BTM QWP w o Z 0J - * cc cc > o q:" z w cc 0~ PV x () TEMPERATURE (K) C Figure 8 Open-circuit voltage and short-circuit current vs. temperature for an ngaas/nalas BTM QWP. 21

23 E 50 i o) E 8 ~30 a 20 Lj4 1nGaAs/nAAs BTM QWP TEMPERATURE (K) Figure 9 Detectivity and responsivity vs. temperature measured at 10 um wavelength for an ngaas/naas BTM QWP. 22

24 3.2 Characterization of a normal incidence type- AlAs/AGaAs QWP grown on [110] GaAs Summary: A detailed characterization of the normal incidence type- AlAs/Alo.sGao. 5 As QWP grown on [110] GaAs by MBE technique has been carried out. The normal incidence radiation of intersubband transition is achieved in the [110] X-band confined AlAs quantum wells. Two main absorption bands were observed which covers the mid-wavelength infrared (MWR) of 2-8 pm and the long wavelength infrared (LWR) of 9-17 pm. The peak photoresponse in the LWR band occurs at A =12.5pm with a peak responsivity R = 24 ma/w and a detectivity D* = 5 x 10 9 cm - sqrihz/w at Vb = 2 V and T = 77 K. n the MWR region, an ultrahigh photoconductive gain was observed at A = 2.2 pm with a responsivity R =110 A/W and a detectivity D* = 1.1 x 10' 2 cm - sqrlhz/w, at Vb = 6 V and T = 77 K ntroduction Many interesting characteristics of intersubband absorption and infrared detection using type- and type- quantum well infrared photodetectors (QWPs) have been investigated extensively in recent years'- 9. n type- quantum well structure, the direct bandgap material system is used and hence the shape of constant energy surface is usually spherical. As a result, only the component of infrared (R) radiation with electric field perpendicular to the quantum well layers will give rise to intersubband transition in the quantum well. This leads to forbidden intersubband transition at normal incidence in type- QWPs. n order to achieve strong absorption of normal incidence R radiation in the quantum wells, indirect bandgap semiconductors are highly desirable for QWP applications. n the indirect bandgap materials, conduction electrons occupy indirect valleys with ellipsoidal constant energy surfaces. The effective-mass anisotropy (mass tensor) of electrons in the ellipsoidal valleys can provide coupling between the parallel and perpendicular motions of the electrons when the principal axes of one of the ellipsoids are tilted with respect to the growth direction. As a result of the coupling, intersubband transitions at normal incidence in a indirect bandgap QWP structure are allowed. Since the AAs/Alo. 5 GaO. 5 As material systems are indirect bandgap materials, the conduction band minima for the AlAs quantum wells are located at the X point in the Brillouin zone (BZ). The constant energy surface will also undergo change from a typical sphere at the zone center for the direct gap material (i.e. GaAs) to off-center ellipsoids of an indirect gap material (i.e. AlAs). For AlAs, there are six ellipsoids along [100] axes with the centers of the ellipsoids located at about 23

25 three-fourth of the distance from the BZ center. By choosing a proper growth direction such as [110], [111], [113], [115] directions 7 ' 8, due to the anisotropic band structure, it is possible to realize large area normal incidence R radiation in a type- AAs/AlGaAs QWP. Recently we demonstrated for the first time a normal incidence type-t QWP using an indirect gap AlAs/Alo. 5 Gao. 5 As system 7 ' 9 grown on [110] GaAs substrate. We use the indirect bandgap AlAs for the quantum well layer and Ao.s 5 Gao. 5 As as the barrier layer. Since the compound of AlxGal-,As becomes an indirect bandgap material for x > 0.45, the conduction-band minimum shifts from the P-band to the X-band. The conduction band offset of Ao. 5 Gao. 5 As over the AlAs is about 180 mev. Figure 1 shows a schematic conduction-band diagram for the type- indirect AlAs/Alo. 5 Gao. 5 As quantum well structure, in which the electrons are confined inside the AlAs layer. The intersubband transition is between the ground bound state in the AlAs quantum well and the first excited state in the well or the quasi-continuum states above the AlAs/Alo. 5 Gao. 5 As barrier layers Theory and Experiment The indirect gap AAs/AGaAs QWP structure was grown on a [110] GaAs substrate by using the molecular beam epitaxy (MBE) technique. A 1.0 pm thick n-gaas buffer layer with Nd = 2 x 1018 CM-3 was first grown on the [110] oriented semi-insulating GaAs substrate, followed by the growth of 20 periods of AlAs quantum wells with a well width of 30kA and doping concentration of 2 x 1018 Crn- 3. The barrier layers on either side of the quantum well consist of an undoped Alo. 5 Gao. 5 As (500 A) barrier layer. Finally, a 0.3 pm thick n+-gaas cap layer with a dopant density of 2 x 108 cm-3 was grown on top of the quantum well layers for ohmic contacts. To derive the basic equations for the normal induced intersubband transitions and the corresponding indirect type- quantum well infrared photodetectors, we start with the Hamiltonian description of quantum mechanics for an electron' h Ho = + V(r) + A a (VV(r) x p), (1) p 2 where m, p, and a are the mass, momentum, and spin operators on an electron, respectively. V(r) is a periodic potential function. The system under consideration consists of an assembly of electrons and the infrared radiation field. The Hamiltonian of this system, H, may be written as the sum of the unperturbed Hamiltonian H 0 and the perturbing Hamiltonian Hrd' which represents the 24

26 interaction between the electrons and the incident infrared photon written as 11 H'rld A [P + (Tx-O). X 'v7v, (2) MC where A is the vector potential of the R radiation field and P is the canonical momentum. The matrix element of intersubband transition in the quantum well is given by12"13 Mfi OkH,~addikidr = --q( w)1/2ew. Vk~k (3) m where the parameters i and f denote the initial and final states, e, is the unit polarization vector of the incident photon, w the light frequency, q the electronic charge, V the volume of the crystal, n the refractive index, and El is conduction band energy of the X-valley material in the well. t can be shown that the intersubband transition probability i1k may be expressed as 12 ' 14 17k : 2-K Mfi 28(Ef - Ei - h1w) 2gk k 2o j92 k 12 B 0 k 29k k (e -- xo) + -- (ew. Yo) + kk (e. z0) S(E~ f - E - hw) (4) -- a kzakx Okz8k kk 2 2 where Bo is a constant equal to c-l7; xo, yo, and zo are the directional unit vectors. For an indirect type-l AlAs quantum well layers grown along [110] direction of GaAs substrate, due to the tilted anisotropic energy band with minimum point away from the center, the second 8 2 C partial derivatives, (i = x, y) can be different from zero. Therefore, it is possible to excite long wavelength intersubband transitions in the quantum well under normal incidence R radiation. However, for a direct type- system (e.g., GaAs) due to the isotropic spherical energy surface and the axis symmetrical parabolic band E = E, + h 2 (k2 + k)/2m*, it always has =0, (where t $ z). The corresponding transition probability becomes PAk- = B ak (e. zo) ](Ef - E-1,) (-) Equation (5) reveals that the optical transition would become zero for the type- structures under normal incidence radiation Results and Discussion A BOMEM interferometer was used to measure the infrared absorbance of the AAs/AGaAs QWP sample. n order to eliminate substrate absorption, we performed absorbance measurements 25

27 on the QWP sample with and without the quantum well layers. The absorbance data were taken using normal incidence radiation at 77 K and room temperature. The absorption coefficients deduced from the absorbance data are shown in Fig. 2. Two absorption peaks at A = 6.8 pm and 14 pm were detected, while two additional absorption peaks at 2.3 pm and 3.5 pm were also 3 observed. All the absorption coefficients measured at 77 K were found to be about a factor of 1.2 higher than the room temperature values. From our theoretical analysis, the 14 pm peak with 3 absorption coefficient of about 2000 cm 1 is the transition between the ground state and the first excited state in the quantum well, while the 6.8 pm peak with absorption coefficient of about 1600 cm- is due to transition between the ground state and the continuum states. The absorption strengths at 2.3 pm and 3.5 pm are very strong, with a narrower linewidth than that of the 14 pm peak. The exact physical origins of these absorption peaks are not clear, and require further study. To facilitate the normal incidence infrared illumination, an array of QWPs with a 210 x pm 2 mesa structure were chemically etched down to n+-gaas buffer contact layer on the GaAs substrate. Finally, AuGe/Ni/Au ohmic contact was formed on top of the QWP structures, leaving 3 a central sensing area of 190 x 190 pm 2 for normal incidence illumination. Device characterization was performed in a liquid-helium cryogenic dewer. A HP41 l- ý,eicoiiductor parameter analyzer Swas used to measure the dark current versus bia- you age.. Figure 3 shows the measured dark current as a function of the bias voltage for temperatures between 68 and 89 K. Substantial reduction in 5 device dark current was achieved in the present type-l structu,:-. The photocurrent was measured using a CV Laser Digikrom 240 monochromator and an OREL ceramic element infrared source. A pyroelectric detector was used to calibrate the radiation intensity from the source. Figure 4 shows the LWR photoresponse and absorption coeffic:ent for wavelengths from 9 to 18 pm. The peak response wavelength is at,,p = 12.5 pm with a cutoff wavelength of 14.5 pm and a peak responsivity of Rp = 24 ma/w at T = 77 K and Vb = 2 V. A broader linewidth of AA/A = 30% than the type- QWP and a 1.5 pm shorter wavelength shift was found in this peak wavelength at 2p =12.5 Om. Detectivity was found to Li' about 1.1 X10 9 cm (Hz)1/ 2 /W under above condition. 3_ Figure 5 shows the photovoltaic (PV) spectral response bands in the visible to mid-r wavelengths.the main response band covers a relatively narrow band width from A = 2 pm to 3.25 Pm, 3i with AA/A = 15.9 %. The peak response occured at AP = 2.2 pm with a responsivity R = 32 ma/w at T = 77 K. The responsivity for this detector remains nearly constant for Vb < 2 V. However, for Vb > 2 V, an exponential increase in the responsivity with the applied bias voltage was observed, as is shown in Fig. 6. A huge photoconductivity gain of 3,200 (as compared to the value at Vb = 0) 26

28 , was obtained at Vb = 6 V. Although the exact mechanism for this large photocurrent gain is not clear, possible explanations are given as follows. n the type- indirect AAs/AGaAs QWP, free carriers are confined in the AlAs quantum well formed in the X-conduction band minima, which has a larger electron effective mass than that in the P-band valley. When a resonant radiation impinges on this QWP, electrons in the ground-state of the X-valley quantum well are excited resonantly to the excited states in the P-band of AlAs well. Due to the effective mass difference between the X-band and the P-band, electron velocity and mobility in the P-valley will be much higher compared to the electrons in the X-band valley. Since the photocurrent is proportional to the electron velocity and the mobility (i.e., ph = AqvdgT, where A is the effective area of the detector, vd is the drift velocity, g is the photogeneration rate, l/t is the recombination rate of electrons in the P-band), a large increase in the photocurrent is expected when bias induced transition from the X-band to P-band takes place under illumination. Since this process is the inverse of negative differential resistance (NDR), a super differential resistance (SDR) can be expected from the X-band to the P-band transition, which is highly desirable for the MWR and LWR QWP applications. The photoresponse at A = 3.5 and 6.8 pm were not measured due to light source and monochromator limitation and will be performed in the near future. n conclusion, we have demonstrated the multispectral responses in normal incidence type- AlAs/AGaAs QWPs using an indirect X-band A1As/Alo. 5 Gao. 5 As system. The desirable normal 5 incidence radiation is allowed due to the tilted and anisotropic energy band structure grown on a [110] GaAs substrate. The detector was found to have two peak wavelength responses: one at AP = 2.2 and the other at 12.5 pm, and spectral response ranges from 2 to 3.25 and 9 to 18 pm, respectively. The capabilities of normal incidence, large spectral sensing range, and low device dark current make the present type-l AAs/AGaAs QWPs highly desirable for many infrared applications. Further studies on the interaction effects between the X- and P-bands, transition coupling, bandgap engineering, and hot electron transport mechanisms in the type indirect - V multiquantum well structures may lead to new and improved infrared detectors, lasers, and modulators. 27

29 . References 11. P. Dawson, B. A. Wilson, C. W. Tu, and R. C. Miller, Appl. Phys. Lett. 48, 541 (1986) B. F. Levine, R. J. Malik, J. Walker, K. K. Choi, C. G. Bethea, D. A. Kleinman, and J. M. Vandenberg, Appl. Phys. Lett., 50, 273 (1987). 33. K. K. Choi, B. F. Levine, C. G. Bethea, J. Walker, and R. J. Malik, Appl. Phys. Lett., 50, 1814; (1988). 4. E. R. Brown and S. J. Eglash, Phys. Rev. 41, 7559 (1990) P. Lefebvre, B. Gil, H. Mathieu, and R. Planel, Phys. Rev.39, 5550 (1989). 6. M. Dabbicco, R. Cingolani, M. Ferrara, K. Ploog, and A. Fisher, Appl. Phys. Lett.59, (1991). 7. L. S. Yu, S. S. Li, and P. H1o, Electron. Lett. 28, 1468 (1992). 8. J. Katz, Y. Zhang, and W.. Wang, Appl. Phys. Lett., Oct., (1992) P. Ho, P. A. Martin, i. S. Yu, Y. H. Wang, and S. S. Li, accepted, J. Vacuum Science Technology, Oct. (1992) E. 0. Kane, J. Phys. Chem. Solid 1, 249 (1957) L.. Schiff, Quantum Mechanics McGraw-Hill, New York, (1968). 12. E. Haga and H. Kimura, J. Phys. Soc. Jpn. 18, 777 (1963). 13. F. Stern and W. E. Howard, Phys. Rev. 163, 816 (1967) C. L. Yang, D. S. Pan, and R. Somoano, J. Appl. Phys. 65, 3253 (1989). 2

30 U (a) [110] AEcr L----~~--W X valozsgaozsas AEX 190 mey '-"X 50 nm 2E AlAs 3 nm [0011 (b) - [010] [100] [110] Figure 1 The conduction band diagram for the normal incidence type- A1As/A1O.5Gao.sAs QWP. Solid line is for the X-band and dashed line denotes the F-band. 29

31 i A T=77K T = 295 K AAs/AGaAs QWP C 3000 (Type-l) C Wavelength (pmn) Figure 2 ntersubband absorbance versus wavelength by BOMEM interferometer at normal incidence for the type- QWP "measured at 77 K and room temperature. 30

32 1i i10.4 T" T 98K K S a 10-7 mo * Bias Voltage Vb (V) Figure 3 Dark current versus applied bias voltage for T = 68, 77, 89 K. 31

33 F E B E 0C cd ( o * O * 0 Normal ncidence,0 5 T=77K 400 Vb= 2V e Wavelength (pm) Figure 4 Responsivity and absorption coefficient versus wavelength atvb=2v, T=77 K, andfor 9< X< 18 pm. 32

34 T =77K " 25 Vb=OV,p =2.2 pm U Wavelength (pm) Figure 5 Photovoltaic responsivity versus wavelength for the type-l1 AlAs/AGaAs QWP measured at Vb =0, T =77 K, and for 0.5 < X < 3.3 rm. 33

35 i T = 77K Ap = 2.2 pm 80 Ac =2.45 pm ' ~40 # ~'40 20 * 0,ai.F Bias Voltage Vb (V) Figure 6 Responsivity versus bias voltage for the type- QWP shown in Fig.5 at a peak response wavelength X = 2.2 gim and T = 77 K. 34

36 3.3 Design of 2-D Square Metal Grating Couplers for a GaAs BTM QWTP Sumnmary: A numerical simulation of 2-D (two-dimensional) transmission square mesh metal grating and reflection square cross metal grating coupled structures formed on top of a miniband transport (BTM) GaAs/AGaAs quantum well infrared photodetector (QWP) has been carried out. The main advantage of these grating structures is that the coupling of normal incident infrared radiation into the doped quantum wells of a BTM QWP is independent of its polarization direction. Two normalized parameters (i.e., s = A/g and h = a/g, where g is the grating period and a is width of the square mesh or cross grating) are introduced to characterize the 2-D metal grating structures. Using the universal charts plotting from the normalized parameters, the total power and angle of the first order transmitted or refracted light are determined for different grating periods and sizes in the 6 to 14 pm wavelength range. Moreover, the absorption constant and coupling quantum efficiency for the BTM QWP are also be calculated from these results ntroduction Recently, there has been a considerable interest in the long wavelength intersubband quantum well infrared photodetectors (QWPs) for operating in the 8pm to 12 pm atmospheric window region at 77K'- 15. Most of the QWPs reported in the literature are fabricated using a larger bandgap ll-v semiconductor material system such as GaAs/AGaAs instead of the more difficult narrow bandgap material such as HgCdTe. The GaAs QWP offers a very promising approach for the low background, long wavelength infrared detection due to the matured GaAs growth and processing technology. Thus, low cost and extremely uniform large area focal plane arrays (FPAs) can be fabricated using GaAs QWPs for staring R image sensor applications. However, the intersubband transition QWPs using GaAs/AGaAs material system do not absorb normal incident R radiation on the surface of QWP, since the electric field vector of incident light must be polarized perpendicular to the quantum well in order to induce intersubband transitions As a result, the angle of incidence with respect to the QW layers must be different from zero in order to induce intersubband absorption in the quantum well. However, for FPAs applications, a response in normal incident light is required. This may be solved by incorporating a lamellar grating 2 '7 ' 1, which has a low coupling efficiency due to its polarization sensitivity. A double periodic metal grating formed on the top of a QWP can be employed to deflect the normal incident light into an absorbable angle independent of light polarization. There are two approaches to form a double periodic grating on the QWP: one approach is to deposit a metal grating directly onto the 35