Infrared Detectors Based on III-V Materials

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1 PARTI Infrared Detectors Based on III-V Materials

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3 PHOTOCONDUCTIVE (PC) AND PHOTOVOLTAIC (PV) DUAL-MODE OPERATION III-V QUANTUM WELL INFRARED PHOTODETECTORS FOR 2-14 fim IR DETECTION SHENG S. LI*. Y. H. WANG*, M. Y. CHUANG*, AND P. HO** *Dept. of Electrical Engineering, University of Florida, Gainesville, FL **Electronics Lab., General Electric Co., Syracuse, NY ABSTRACT We present four new types of III-V quantum well infrared photodetectors (QWIPs) operating in photoconductive (PC) and photovoltaic (PV) modes for the wavelength range from 2 to 14 //m. These dual-mode (DM) operation QWIPs were grown by the MBE technique using GaAs/AlGaAs, AlAs/AlGaAs, and InGaAs/InAlAs material systems. Based on the bound-to-miniband (BTM) and the enhanced bound-to-continuum (BTC) intersubband transition schemes, these detectors provide the features of large absorption coefficient, low dark current, and high detectivity in the wavelength of interest, and show promising for use in large area IR focal plane array image sensor applications. I. Introduction The III-V quantum well infrared photodetectors (QWIPs) based on intersubband transition schemes for detection in both the mid-wavelength infrared (3-5 //m) and longwavelength infrared (8-14 /im) atmospheric windows have been extensively investigated in recent years 1 " 9. Much of the work has been reported on the GaAs/AlGaAs (GaAsbased) and InGaAs/InAlAs (InP based) multiple quantum well and superlattice systems using bound-to-bound (BTB) 1, bound-to-miniband (BTM) 2, bound-to-quasi-continuum (BTQC) 3 and bound-to-continuum (BTC) 4 ' 5 transition mechanisms. Most of the QWIPs are operated in the photoconductive (PC) mode, while a few of them are operated in the photovoltaic (PV) mode 2 ' 5 ' 6 " 8. Due to the inherent low dark current, low Johnson noise, and low power dissipation in the PV detection mode, it is highly desirable to develop high performance QWIPs using the PV mode in the temperature range of 65 to 85 K for infrared image sensor applications. II. QWIP structures We report here four types of dual-mode (PC and PV) operation III-V QWIPs fabricated on GaAs/AlGaAs, AlAs/AlGaAs, and InGaAs/InAlAs material systems. These QWIPs are based on the bound-to-miniband (BTM) and bound-to-continuum (BTC) intersubband transition mechanisms. Figure 1 shows the energy band diagrams for the four QWIPs (A, B, C, and D) studied. The photoresponse spectra of these QWIPs extend from the short-wavelength infrared (SWIR) and mid-wavelength infrared (MWIR) to the long-wavelength infrared (LWIR) bands. The device parameters for these QWIPs are summarized in Table I. QWIP-A and C samples were grown on the semi-insulating (SI) (100) GaAs substrates by using molecular beam epitaxy (MBE) technique, while QWIP-B and D were grown on SI (100) InP substrate and SI (110) GaAs substrate, respectively. Each structure has a l-/xm-thick buffer layer (GaAs or In.53Ga.47As) with a doping density of 2 x cm" 3, which was first grown on the SI substrate (GaAs or InP) for the ohmic contact. The top ohmic contact of the QWIPs was formed on a 0.3 fim cap layer (GaAs or In.53Ga.47As) with a doping density of 2 x cm" 3. For QWIP-B and C (i.e. BTM QWIPs), the barrier layer was formed by using a 6-period of undoped InGaAs/InAlAs or GaAs/AlGaAs superlattice barrier layer. The enlarged quantum wells of the QWIP-B and C with properly selected doping densities were sandwiched between the undoped superlattice barrier layers. Mat. Res. Soc. Symp. Proc. Vol Materials Research Society

4 Table I. Device parameters for the four types of III-V QWIPs QWIP (Type) A (I) B (I) C (I) D (II) W (^ GaAs 110 In.53Ga.47As 110 GaAs 88 AlAs 30 LB (A) Al.25Ga.75As 875 InGaAs/InAlAs 46/30 GaAs/AlGaAs 29/58 Al.5Ga.5As 500 Nz, (cm" 3 ) QW periods Grating coupler 1-D 5 //m 2-D 10 fim 1-D 10 fim Intersubband transition BTC BTM BTM BTC InGaAs InAIAs/lnGaAs SL -W2 E SL1 GaAs (QWIP A) (QWIP B) "W1 GaAs AIGaAs/GaAs SL AlAs Al s Ga s As AE c (QWIP C) (QWIP D) Figure 1. Schematic energy band diagrams for the four QWIPs. The QWIP-A structure consists of 40 periods of a very thick undoped barrier layer (875 A) of Al.25Ga.75As and a highly doped (5 x cm" 3 ) enlarged quantum well (110A). Due to heavy doping in the well, both the ground state and first excited state in the well are populated by the electrons at 77K. As a result, each populated level in the well will make intersubband transition to the continuum states under IR photoexcitation. Due to a larger absorption strength from excited state to continuum states compared with absorption strength from ground state to the continuum states, a large enhanced absorption

5 of the infrared radiation is expected in the excited state to the continuum states transition. Meanwhile the thicker barrier can eliminate the undesirable tunneling current. The layer structure for the QWIP-D sample is composed of AlAs/Al. 5 Ga. 5 As indirect bandgap material, which was grown along the [110] direction of the GaAs substrate. Since conduction band minima for the AlAs are located at the X-points of the Brillouin zone (BZ), the quantum well is formed by indirect bandgap AlAs while indirect bandgap Al.5Ga.5As becomes the barrier. As a result, a type-ii energy band alignment is formed 9. Due to the anisotropic band structure and the tilted growth direction with respect to the principal axes of the ellipsoidal valleys, it is possible to realize a normal incidence IR detection in a type-ii AlAs/AlGaAs QWIP without using the grating coupler. III. Dark currents To analyze the intersubband transition schemes in the QWIPs, we performed theoretical calculations of the energy states Ejy n, EsLn and Ec n (n = 1,2,...) in the quantum well (W), superlattice (SL), and continuum (C) states, and the transmission probability T*T for these QWIPs using a multi-layer transfer matrix method (TMM) 2. It is noted that the wide and highly degenerated global miniband is formed by using the superlattice barrier structure in the QWIP-B and C. In the type-ii QWIP-D, there are two bound states, Ev^i,2» in the AlAs X-band well and four continuum states, Ec2-5? in the X-band, which can find their resonant pair levels in the T-band, while the continuum states E C \ is located below the T-band minima of the Al. 5 Ga. 5 As layers (see Fig. 1 (QWIP-D)). Measurements of the dark current for all the QWIP samples were carried out between 30K and 85K by using a HP 4145 Parameter Analyzer. Figure 2 shows the current-voltage (I-V) curves for these QWIPs. The asymmetrical dark I-V characteristics with l<t being larger for positive bias than negative bias were observed in these QWIPs. The asymmetry is mainly due to the miniband conduction and the dopant migration effects. IV. The PV and PC mode responsivities and detectivities The responsivities of the QWIPs were measured as a function of temperature, bias voltage Vfc, and wavelength using a globar and automatic PC controlled single-grating monochrometer spectral measurement system with a normal IR illumination. The normalized responsivity versus wavelength for the QWIPs (A - D) are shown in Fig. 3. The absolute peak responsivities R^ (or Ry) for these QWIPs were measured and calibrated using a pyroelectric detector. Table II summarizes the measured and calculated peak wavelengths, cutoff wavelengths, responsivities, and detectivities for the four QWIPs. Table II. Summary of the peak, cutoff wavelengths, responsivities, detectivities for the four QWIPs. QWIP A B C D A p (/xm) 12 (PC) 7.7 (PV) 10.3 (PC) 10 (PV) 8.9 (PC) 8.9 (PV) 2.2 (PC) 2.2 (PV) 3.5 (PC) 3.5 (PV) 12.5 (PC) R (A/W) ,000 (V/W) ,000 (V/W) ,000 (V/W) ,000 (V/W) A c (//m) D* cm-hz^/w l.oxlo xlO y 5.8x10* 5.7x10* 1.2xlO x10* l.lxlo xlO lu 3.0x10" 1.2xlO lu 1.1x10*

6 I t3 Oa 52. <O Normalized Responsivity <o a «*- CD = 3 Dark Current Id (A) Dark Current 14 (A) i i \ o Cambridge University Press

7 For QWIP-A, two response peaks were detected: one at A p = 7.7 /zm using PV mode detection and the other at A p = 12 /xm using PC mode detection (V& > 1 V). In the PV mode, a peak voltage photoresponsivity, Ry = 11,000 V/W, at A p = 7.7 /zm with a spectral bandwidth of AA/A P = 18 % was obtained, which is attributed to the transition from the ground state E^i to the first continuum states Eci above the barrier. This internal photovoltage results from the spatial charge separation when the asymmetrical energy band bending occurs due to dopant migration and heavy doping effect. When a bias voltage is applied to the detector, the PV response at the A p = 7.7 /zm disappears, and the PC mode conduction becomes the dominant detection mechanism. In the PC detection mode, the peak wavelength shifted from 7.7 /zm to 12 /im. A maximum responsivity, R^, of 0.48 A/W was measured for this QWIP at V 6 = 2 V. The cutoff wavelength for the QWIP-A sample was found to be A c = 13.2 /zm with a spectral bandwidth AA/A = 18.3 %. For QWIP-B, it has a peak wavelength at A p = 10 /im for the PV mode, a peak wavelength at A p = 10.3 /zm for the PC mode at T = 67 K. The intersubband transitions for both the PC and PV modes are attributed to the resonant transition from the ground state Ewi to the global miniband states ESLI which are aligned with the first excited state Ew2 in the quantum well. The resonant transition depends strongly on the location of the first excited state Ew2 relative to the miniband ESLI- In the QWIP-B, the Ew2 lies near the top of the miniband ESLI which will result in a strong, narrow-band spectral response with AA/A P = 7 % and a 0.3 /zm short wavelength shift in the PV mode. Note that the BTM QWIP operating in the PV mode offers a unique feature of ultra-narrow bandwidth detection, which is not attainable in a conventional QWIP. As the bias voltage increases (in the PC mode), the relative position between the "embedding" localized state E^2 and the "framing"miniband state ESLI ca n be adjusted by a "controlling bias" due to the different dependence of Ejy2 and ESLI on the bias voltage. AtV& = 0.5 V and T = 67 K, a broad-band spectral linewidth of AA/A P = 24 % was obtained, which is about a factor of 3 increase in spectral bandwidth compared to the PV mode. The QWIP-C and B are both using the BTM intersubband transition mechanism, and both show similar characteristics under the PV and PC modes. For QWIP-C, a peak wavelength at A p = 8.9 /zm with a cutoff wavelength at A c = 9.3 /zm was observed for both the PV and PC mode detection, which is attributed to the transition from the ground state E#x to the miniband state ESLI- In contrast to QWIP-B, the resonant state Ew2 of the QWIP-C lies near the bottom of the miniband states, ESLI, which gives rise to a narrow bandwidth absorption peak with AA/A P = 8.5 % for the PV mode, and has a resonant peak wavelength coincidence at A p = 8.9 /zm for the PV and PC mode detection. The QWIP-A, B, and C discussed above offer a dual-mode detection in the LWIR (8-14 /zm) band, which can be realized by using either the BTM conduction scheme or by adjusting QWiP structure parameter. However, due to the energy band offset limitation and a very fine geometrical grating coupler requirement, the dual-mode operation in the MWIR and SWIR detection bands discussed above becomes very difficult using the intersubband transition scheme. Some researchers 3 ' 7 reported the PV mode detection at MWIR band using an ultra-thin ballistic transport and resonant barrier or F-band and an energy band coupling technique by using the grating coupling method. For QWIP-D, we used an AlAs/AlGaAs type-ii QWIP structure in the intersubband transition scheme with a direct normal incidence and no grating coupler. In this QWIP, the two main peak wavelengths were detected for both the PV mode and PC mode operation at A p = 2.2 and 3.5 /zm which fall in the SWIR and MWIR bands. The SWIR band has two peak wavelengths at A p = 2.2 /zm and A p ~ 2.7 /zm, while the MWIR spectral band has a peak wavelength at A p = 3.5 /zm and a long tail attributed to the A p ~ 4.8 /zm. The positions

8 for all four peak wavelengths observed in this QWIP are in excellent agreement with the values deduced from the FTIR measurements and theoretical calculations. In addition to the SWIR and MWIR band responses, a broad LWIR absorption band (i.e., 9 to 18/xm) was also observed in the PC mode detection for this QWIP with a peak wavelength at A p = 12.5 pm. V. Conclusions In conclusion, we have demonstrated four new dual-mode detection QWIPs using BTM and BTC intersubband transition mechanisms and operating between 65 K and 85 K. In the LWIR detection band, QWIP-A to C offer good detectivities (between 10 9 and 1.2 x 1O 10 cmy/wz/w) for the PV and PC detection modes. By properly optimizing the QWIP structures, the performance of a BTM QWIP in the PV mode operation can be further improved. The dual-mode detection in the SWIR and MWIR bands was also observed in the AlAs/GaAlAs type-ii QWIP. Therefore, III-V dual-mode QWIPs show a great potential for multi-color and multi-band infrared image sensor applications. Acknowledgement This work was supported by Defense Advanced Research Project Agency under a Navy grant No.N J References 1. B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker, and R. J. Malik, Appl. Phys. Lett. 50, 1092 (1987). 2. L. S. Yu and S. S. Li, Appl. Phys. Lett. 59, 1332 (1991); Y. H. Wang, S. S. Li, and Pin Ho, Appl. Phys. Lett. 62, 621 (1993). 3. J. D. Ralston, H. Schneider, D. F. Gallagher, K. K. Fuchs, P. Bittner, B. Dischler, and P. Koidl, J. Vac. Sci. Technol. B, 998 (1992). 4. B. F. Levine, C. G. Bethea, G. Hasnain, V. O. Shen, E. Pelve, R. R. Abbott, and S. J. Hseih, Appl. Phys. Lett. 56, 851 (1990). 5. Y. H. Wang,S. S. Li, and Pin Ho, Appl. Phys. Lett. 62, 93 (1993). 6. K. W. Goossen, S. A. Lyon, and K. Alavi, Appl. Phys. Lett. 52, 1701 (1988). 7. B. F. Levine, S. D. Gunapala, and R. F. Kopf, Appl. Phys. Lett. 58, 1551 (1991). 8. H. Schneider, K. Kheng, M. Ramsteiner, J. D. Ralston, F. Fuchs, and P. Koidl, Appl. Phys. Lett. 60, 1471 (1992). 9. L. S. Yu, Sheng S. Li, and Pin Ho, Electron. Lett. 28, 1468 (1992).

9 MODELING OF MBE GROWTH WITH INTERACTING FLUXES DAVID H. TOMICH* K. G. EYINK* T. W. HAAS* M. A. CAPANO* R. KASPI** AND W. T. COOLEY** *Wright Laboratory, WL/MLBM, Wright-Patterson AFB, OH **Wright Laboratory, WIVELRA, Wright-Patterson AFB, OH ABSTRACT Ternary and quaternary III-V alloys are important for many optical device applications, and a precise control of the composition is required. Molecular beam epitaxy (MBE) is generally considered a non-equilibrium or kinetically controlled process but most of these models are too computationally intensive for real time control. We report on using a precursor state growth model *' 2 for the growth of GaAsSb to control the growth conditions and hence the film composition. The activation energies and the parameters appearing in the relationship are determined by fitting the calculated compositions to experimental ones as determined by x-ray diffraction. The effect of substrate temperature, growth rate and flux intensities on composition is discussed. Introduction The quest for infrared (IR) devices in the wavelength range above 8 Jim has generated a substantial amount of effort in the development of semiconductor materials and structures. No conventional bulk III-V alloys have sufficiently small band gaps (E«) or operation in this range. Hence, most of the work has been centered around the II-VI alloy system HgCdTe. One technique to directly shift band gaps into the IR spectral range is through the use of superlattice strain-induced band-gap reduction. For IR sensing applications, the artificially structured superlattices offer a significant advantage over the conventional semiconductors; to a first approximation, the bandgap and the effective mass are decoupled. This means that it is possible to design a superlattice having a small bandgap and a large effective mass. The general principle involved with superlattice strain-induced band-gap reduction is to take a material with small band gap and small lattice constant (e.g. InAsQ 39Sb() ^l) and layer it coherently with a material having a larger gap and larger lattice constant, (e.g. InAs^xSb x, x>0.61) forming a strained-layer superlattice. Both In and Ga have been used to grow III-V compounds with mixed group V compositions (III-VA.-Vg) utilizing MBE over the whole range of compositions between the binary compounds (III-V^ to III- Vg)23. The compositional control in these systems has been found to be much more difficult than with mixed group III compounds such as AlGaAs 4. Most of the difficulty in growing the mixed group V compounds comes from the complicated incorporation behavior of the arsenic and antimony atoms. Experimental results have shown that antimony is more easily incorporated than arsenic and that this tendency is reduced with increasing growth temperature^. To effectively engineer a strained layer superlattice (SLS) using different compositions of mixed group V compounds one needs to be able to precisely control the composition. Some preliminary experimental results are reported here on the growth of GaAsSb layers and the prediction of the necessary growth conditions to obtain the desired antimony fraction in the Mat. Res. Soc. Symp. Proc. Vol Materials Research Society

10 10 Experiment Substrates used were semi-insulating (001) GaAs. They were etched using a standard H2SO4.:H2O2:H2O=3:1:1 solution for 1 minute. The MBE system used was a Varian 360 with 2" indium bonded wafers. The GaAsSb layers were grown on a GaAs buffer layer after oxide desorption under an As2 flux. A valved arsenic cracking cell was used to obtain As2, while an uncracked source was used for Sb4. All growth sequences were done at a substrate temperature of 475 C and under constant gallium and antimony fluxes while the arsenic flux was varied by adjusting the valve on the As cracker. Reflection high-energy electron diffraction (RHEED) was used to measure the growth rate of GaAs which was determined to be 1 monolayer per second (1 im/hr). The GaAsSb layers were examined with double crystal x-ray diffraction (DXRD) to obtain compositions and relaxation. DXRD data were collected using a Bede Model 150 diffractometer equipped with a scintillation detector and mounted on a Rigaku x-ray generator. The incident beam from a Cu target operated at 1.2 kw was conditioned with the (004) reflection from a GaAs crystal. Surface symmetric (004) reflections were used to measure the strain normal to the interface. Asymmetric (115) reflections we used to precisely determine the composition of the layers and the degree of strain relaxation. Peak splitting values were obtained by mathematically fitting the substrate and layer peaks to determine their angular positions. This procedure allowed the peak splitting to be measured to within 1 arcsec. Experimental rocking curves were compared with computer-simulated data to verify the calculated composition values. MBE Kinetic Model The growth model considers four principle processes for MBE growth^: the incidence of an atom to the precursor state or physisorped (Jf), chemisorption to the film from the precursor state (C j), sublimation to the precursor state by dissociation of the film (S^ and desorption of an atom from the precursor state (Dj). See Fig. 1. All of the associated behavior, dissociation of molecules to atoms, association of atoms to molecules and group V migration at the growing surface are considered a precursor state. These processes determine dn the concentration of the precursor state i\[ such that '- = J i -C t + S t - D o where in the at steady state, L = 0. The net growth of the film is given by Cj - Sj. dt All growths were performed under growth conditions where the substrate temperature is low enough for negligible Ga loss and the group V fluxes are larger than the Ga flux. Under these conditions the growth rate of the GaAsSb is determined by the arrival rate of the Ga. It is assumed that the chemisorption rates of group V elements are proportional to the product of the precursor state and the probability of their reaction with Ga. The interaction between As and Sb is assumed to negligible. The sum of the growth rates of As and Sb is equal to the growth rate of Ga since the GaAsSb growth rate is determined by the Ga flux intensity. The net growth rate of As and Sb can then be written as: