THE concept of homojunction interfacial workfunction

Transcription

1 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 45, NO. 8, AUGUST Effect of Emitter Layer Concentration on the Performance of GaAs p -i Homojunction Far-Infrared Detectors: A Comparison of Theory and Experiment Wenzhong Shen, A. G. Unil Perera, Member, IEEE, M. H. Francombe, H. C. Liu, M. Buchanan, and William J. Schaff Abstract The performance of GaAs multilayer (p + -i-p + -i-...) homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR) detectors as a function of emitter layer (p + ) concentration is reported. The dark current characteristics have been investigated and compared with a model which includes the space charge, tunneling, and multiple-image-force effects. The experimentally determined detector cutoff wavelength is found to be in reasonable agreement with the high density (HD) theory. The detector responsivity follows well the quantum efficiency predicted by scaling the free carrier absorption coefficient linearly with the doping concentration. All these comparisons are necessary to design and optimize GaAs HIWIP FIR detectors. Index Terms FIR detectors, GaAs, homojunction, interfacial work function. I. INTRODUCTION THE concept of homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR) detectors was demonstrated on commercial Si, Ge, and InGaAs p-i-n diodes operated at 4.2 K, showing cutoff wavelengths up to 200 m [1]. The operation of HIWIP detectors is based on the internal photoemission occurring at the interface between a heavily doped absorber/emitter layer and an intrinsic layer, with the cutoff wavelength mainly determined by the interfacial work function : ( m) (ev). The detection mechanism of HIWIP detectors involves FIR absorption in the highly-doped thin emitter layers by free carrier absorption followed by the internal photoemission of photoexcited carriers across the junction barrier and then collection. Our modeling studies [2], [3] have shown that Si HIWIP FIR detectors could have a performance comparable to that of conventional Ge FIR photoconductors [4] or Ge blocked-impurity-band (BIB) FIR detectors [5], with unique material advantages. Therefore, this novel detector approach Manuscript received July 28, 1997; revised February 9, The review of this paper was arranged by Editor P. K. Bhattacharya. This work was supported in part by the National Aeronautics and Space Administration under Contract NAG W. Z. Shen, A. G. U. Perera, and M. H. Francombe are with the Department of Physics and Astronomy, Georgia State University, Atlanta, GA USA. H. C. Liu and M. Buchanan are with the Institute for Microstructural Sciences, National Research Council, Ottawa, Ont. K1A 0R6, Canada. W. J. Schaff is with the School of Electrical Engineering, Cornell University, Ithaca, NY USA. Publisher Item Identifier S (98) provides considerable promise for developing FIR imaging devices, by taking advantage of a mature Si or GaAs material technology and a tailorable which covers several tens to a few hundred micrometers. In addition to silicon, the recent rapid development of GaAs based long-wavelength quantum well focal plane array cameras makes GaAs another promising candidate for developing HIWIP FIR detectors. Although the progress of n-gaas FIR photoconductors has been slow due to difficulties in growing high purity materials [4], we have successfully demonstrated the first GaAs HIWIP FIR detectors using thin, highly doped p and undoped i multilayer structures [6]. The thin emitter layer offers high internal quantum efficiency due to the low inelastic hole-hole and hole-phonon scattering, and the high doping concentration facilitates strong free carrier absorption. Multilayer structures are used to further increase the quantum efficiency due to the increased photon absorption efficiency and possible photocurrent gain enhancement [2], also demonstrated in stacked SiGe/Si heterojunction internal photoemission detectors [7]. The theoretical analyzes for silicon [2], [3], [8] [10] can explain the performance of GaAs device structures with some modifications. In this paper, a comparison of modeling and experimental results on the performance of p-gaas HIWIP FIR detectors as a function of emitter layer concentration is reported, since, from the operation and detection mechanisms discussed above, the emitter layer concentration plays a key role in the detector performance. In addition to the cutoff wavelength and responsivity, which are very important parameters for detectors, the dark current is another factor influencing the noise equivalent power (NEP) or detectivity of detectors. The studies of dark current characteristics, cutoff wavelength, and the detector responsivity described here should lead to a clearer understanding of how the doping concentration affects the detector performance, which will be critical to detector design and optimization. II. EXPERIMENT Measurements were made on three p-gaas FIR HIWIP detector samples grown by molecular beam epitaxy (MBE) with the substrate temperature of 560 C. The MBE epilayers consist of a 4000 Åcm bottom contact layer, a 1500 Åcm /98$ IEEE

2 1672 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 45, NO. 8, AUGUST 1998 Fig. 1. (a) Schematic of the ten multilayer p-gaas HIWIP detectors after device processing. p ++,p +, and i are the contact layer, emitter layer, and undoped layer, respectively. A window is opened on the top side for frontside illumination. (b) Energy-band diagram of the detectors under forward bias. TABLE I DEVICE PARAMETERS FOR THREE DETECTOR STRUCTURES. HERE, W i, W e, AND W b ARE THE THICKNESSES OF INTRINSIC (i), EMITTER (p + ), AND BOTTOM CONTACT (p ++ )LAYERS, RESPECTIVELY. N e AND N b ARE THE DOPING CONCENTRATION OF EMITTER AND BOTTOM CONTACT LAYERS, RESPECTIVELY, N IS THE NUMBER OF MULTILAYERS. ALSO INCLUDED ARE THE MEASURED INTERFACIAL WORK-FUNCTION, (1), NEP, AND CALCULATED BAND-EDGE OFFSET 1E v undoped layer, ten periods of thin emitter (p ) layers (thickness of Å) and undoped intrinsic (i) layer (thickness of 1000 Å), and finally a 3000 Å top emitter layer and a 3000 Å top contact layer. The emitter layers were doped with Be which has an ionization energy of 28 mev in p-gaas. The doping concentration is between cm. The top and bottom contact layers were doped to cm, far above the Mott transition value to ensure an ohmic contact. The schematic of the detectors after device processing and their energy-band diagram are shown in Fig. 1 with the key nominal sample parameters given in Table I. Good control of MBE growth is indicated by secondary ion mass spectroscopy (SIMS) measurements. The contact was formed by deposition of Ti- Pt-Au. The GaAs HIWIP FIR detectors were characterized by current voltage ( ) measurements and photoresponse. The responsivity was obtained using a Perkin-Elmer, system 2000, Fourier transform infrared spectrometer (FTIR), and a Si composite bolometer as the reference. III. DARK CURRENT CHARACTERISTICS Unlike the case of GaAs/AlGaAs quantum well infrared photodetectors (QWIP s), where the characteristics show asymmetric behavior under positive and negative biases normally due to the asymmetrical growth of GaAs/AlGaAs and AlGaAs/GaAs interfaces [11], the present GaAs homojunction FIR detectors display symmetric characteristics. Fig. 2 shows the dark characteristics at 4.2 K for the three Fig. 2. Experimental (solid curves) and theoretical (dashed curves) dark current (I d ) characteristics at 4.2 K of p-gaas HIWIP FIR detector sample (a) no. 9404, (b) no. 9401, and (c) no The inset (i) shows the experimental (solid circles, at 0.1 mv bias) and calculated (open circles, at 0.5 mv bias) I d in the three HIWIP detectors at 4.2 K. The inset (ii) displays the log scale mesa area dependence of I d of two samples at 10 mv forward bias (with a constant shift for clarity) and its regression slope, where we have neglected the orders of the current and area since there is only constant shift. detectors under positive biases, together with the dark current value near zero bias shown in the inset. It is seen that the three detectors display almost same dark current at high biases, in contrast to the case of near zero bias where the dark current increases rapidly with the doping concentration. The dark current is modeled by assuming a uniform electric field in the multilayers, and thermodynamic carrier equilibrium. The properties of the multilayer detector are treated as repeat units, as in the case of QWIP s [11]. The energy band diagram for a HIWIP FIR detector should include both

3 SHEN et al.: EFFECT OF EMITTER LAYER CONCENTRATION ON PERFORMANCE OF GaAs p -i HOMOJUNCTION FIR DETECTORS 1673 the multiple-image-force (MIF) and space-charge (SC) effects. The latter is due to the free-carrier spillover from the heavily doped layer into the undoped i layer. The dark current is the sum of space-charge-limited (SCL) current, thermionic emission (TE) current, and tunneling current, and as a good approximation, the SCL current, TE current and tunneling current can be treated separately [10]. At low temperatures required for the HIWIP detector operation, the effect of SCL current is clearly negligible. The TE current density is given by the Richardson Dushmann equation where is the Richardson constant and is the Boltzmann constant. The tunneling current is a combination of thermionic field emission (TFE) and field emission (FE), given by where, the difference of Fermi Dirac distribution functions in emitter and collector layers, the total energy of the tunneling holes, the transverse energy, and the effective hole mass. is the tunneling probability and can be obtained in the Wentzel Kramers Brillouin (WKB) approximation where is the wave vector related with electrical field in the tunneling direction, and are the classical turning points. The detailed expressions have been given elsewhere [3], [9], [10]. The total dark current is with the detector area of cm. From the theory, it can be determined that the thermionic field emission (TFE) is the major source of dark current in our p-gaas HIWIP detectors at 4.2 K, so that small increase in dark current with increasing doping is expected as a result of a slight modification to the Fermi-Dirac distribution (see Section IV). The dark current calculated for the three detectors is also shown in Fig. 2. The parameters for calculation can be found in Table I of [8], except the hole scattering length and mobility, where 58 Å and 400 cm /Vs were used, respectively, for GaAs [12]. The dark characteristics are almost independent of the doping concentration, in good agreement with the experimental results. Although the general trends are correct we note that there is nearly one order of magnitude dark current difference between the experiment and theory. This fact is due to the existence of leakage current, as pointed out before [13], since no guard rings are used in the detector structures, which are widely used in Schottky-barrier and SiGe/Si detectors to suppress the edge leakage [14]. Further evidence of this leakage can be seen from the experimental fact that the dark current increases superlinearly with the mesa area [see inset (ii) in Fig. 2], in contrast with the linear relation of (3). The calculated dark current near zero bias is also independent of the doping concentration [see inset (i) in Fig. 2], (1) (2) (3) whereas the experimental data show a rapid increase with increasing concentration. We attribute the difference as due to the existence of surface or interface defect states in the higher doping concentration samples. The origin of these defect states can be the dangling bonds in the interfaces, Coulomb potential of charged ions, and impurities near interfaces. The existence of surface or interface defect states can result in recombination via the defect states and generate tunneling current (it increases with the defect states and applied bias), which dominates the dark current at low biases. The high experimental dark current in the high doping emitter layer samples near zero bias can be attributed to the high tunneling current and is associated with high surface or interface defect states. However, with increasing bias, the contribution of thermionic field emission rapidly increases and dominates the dark current, resulting in almost the same experimental dark current at high biases even in different doping samples. Further evidence for the identification of these defect states can be clearly seen from the detector response spectra, where a remarkably reproduciable spike response was observed in the spectra of highly doped emitter layer samples [6]. The sharp response was attributed to the localized nature of the defect states. Defects have been found to be the source of tunneling currents in HgCdTe detectors [15], and the elimination of these imperfections is a significant material challenge yet to be overcome, and the defect induced current has also been found in advanced MBE technology for GaAs/AlGaAs QWIP s [11], [16]. Recently, a successful approach reported in the literature described reduction of the dark current density due to the surface or interface defects by using thin films to passivate the detector surface [17], this also seems to be necessary for HIWIP detectors, especially for the highly doped emitter layer cases. IV. CUTOFF WAVELENGTH The responsivity curves of GaAs HIWIP FIR detectors show a strong bias dependence, increasing significantly with increasing bias [6]. However, the bias can not be increased indefinitely because of the increase in dark current. The cutoff wavelength for each bias, for a HIWIP detector is determined, shown in [6], as the wavelength at which the mean response (of at least 8 curves) first reaches the same level as the maximum standard deviation of the spectra (i.e., noise level). The also shows a strong bias dependence due to the MIF and SC effects. The longest s obtained from the responsivity spectra is m for no at 91.5 mv bias, m for no at mv bias, and m for no at mv bias. The lowering of the band edge due to high doping can be described using the high density (HD) theory. A simple expression for the shift of the majority band edge,, that can be used for all n- and p-type semiconductors [8], [18] was employed Here is the doping concentration, (ev) is the effective Rydberg energy, (Å) is the (4)

4 1674 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 45, NO. 8, AUGUST 1998 by employing the emitter layer concentration in the order of 10 cm, since, from the HD theory shown in Fig. 3, only a small increase in doping concentration can cause a large decrease in work function (large increase in ). Fig. 3. Calculated doping concentration dependence of the shift for the major band edge 1E maj and interfacial work function 1 at zero bias from the HD theory. The theoretical curve of 1 should have a deviation of 61 mev due to the modified Fermi level (see text). The experimental work function results [corrected to very low bias by (6)] are shown by solid circles. effective Bohr radius, is the effective density-of-state mass, is the relative dielectric constant, is the correction factor, and is the number of band minima. The parameters for p- GaAs can be found in [8]. The expression for the Fermi energy to take the multiplicity of the majority band into account is given by [19] However, the Fermi level determined by using the conventional density of state is considerably larger than the value determined directly from the observed luminescence spectra [18]. A comparison of the calculated results from (5) with the luminescence results [18] gave, for p-gaas, a difference of about mev for doping concentrations between to cm, and even smaller differences with higher concentrations. This difference is included here by using a modified Fermi level expression (mev), so the workfunction used is where the last term is the image force barrier lowering due to the electric field. Using the above equations, the doping concentration dependence of and at zero bias was calculated, as shown in Fig. 3, together with the experimental workfunction results [corrected to very low bias by (6)] by solid circles. It is seen that the experimental is in reasonable agreement with the calculations using the HD theory. This result demonstrates 1) the successful fabrication of GaAs HIWIP FIR detectors using the interfacial work function, and 2) the reliability and simplicity of HD theory in calculating the shift of the band edge in highly doped semiconductors. On the other hand, much longer GaAs HIWIP detectors could be obtained (5) (6) V. RESPONSIVITY From the HIWIP detection mechanism, it is seen that the free carrier absorption plays a very important role in the detector performance. It is important to understand the FIR free carrier absorption behavior in heavily doped GaAs thin films, due to both fundamental and device performance reasons. Although the free carrier absorption in p-gaas has been widely studied in the literature, this was limited to relatively short wavelengths ( 20 m) [20]. No free carrier absorption data were available for the wavelength range 50 m, where the HIWIP FIR detectors usually work. Shen et al. recently reported, both experimentally and theoretically, the strong free hole absorption in a p GaAs thin film in m FIR region, revealing the suitability for FIR detection [13]. Another important result, in agreement with theory, is that the free carrier absorption coefficient is almost independent of the FIR wavelength, in contrast with the wavelength squared dependence for shorter wavelengths. The same conclusion has been obtained in Si thin films [21]. Here, the free hole absorption in three highly doped (,, and cm ) reference p-gaas thin films (thickness of 1000 Å) grown by the same MBE system as the detectors is reported. In addition to the confirmation of above results, the relationship between the free hole absorption coefficient and the hole concentration, which is the most important result in connection with the HIWIP detectors, was also obtained. The strength of the free hole absorption at a wavelength of 80 m is shown in the inset of Fig. 4. The absorption can be well described by a linear relation between the absorption coefficient and the concentration of holes, just as in the case of Si thin films [21], [22]. The fitted regression formula as a function of hole concentration is found to be cm (7) A high absorption coefficient (10 10 cm ) in FIR range is an important advantage for HIWIP detectors. Also the absorption coefficient is almost independent of temperature due to the almost temperature invariant carrier concentration and mobility. The responsivity of a HIWIP detector is proportional to its total quantum efficiency, which is the product of photon absorption probability, internal quantum efficiency, and barrier collection efficiency [2]. Since the Fermi level, work function and hole mobility do not change very much with the hole concentration in the experimental region ( cm ) (see Section IV), it is assumed that the internal quantum efficiency and barrier collection efficiency do not change with the doping concentration. All three detector samples have ten multilayers, hence they should have the same photocurrent gain enhancement. Taking these into account, the variation of the detector responsivity with the doping concentration should follow that of the absorption probability.

5 SHEN et al.: EFFECT OF EMITTER LAYER CONCENTRATION ON PERFORMANCE OF GaAs p -i HOMOJUNCTION FIR DETECTORS 1675 Fig. 4. Theoretical (solid curve) photon absorption probability as a function of hole concentration by using the linear relationship [(7)] shown in the inset and the ten-layer detector structure parameters given in Table I. Solid circles are the experimental detector responsivity near 50 mv forward bias. The inset shows the experimental free hole absorption coefficient at 80 m (open circles) as a function of hole concentration at room temperature, together with its linear regression relation. The photon absorption probability for detectors can be calculated as multilayer HIWIP where is the front surface reflection and is the free carrier absorption coefficient in emitter layer (thickness ). By using the experimental relationship in (7) and, and the layer parameters of the detectors, the photon absorption probability was calculated for HIWIP detectors as a function of carrier concentration, and compared with the experimental results for detector responsivity near 50 mv forward bias, shown in Fig. 4. Due to the difference in the parameter of (see Table I) from the other two detectors, the experimental responsivity of detector no has been scaled to the same parameters as no and no by comparing with the different absorption probability in no The experimental responsivity of the detectors follows the absorption probability well. The small deviation at high concentration is due to the slight decrease in diffusion length, which affects the collection efficiency. Furthermore, it can be seen that the photon absorption probability increases rapidly when the hole concentration increases from to cm. Since the collection efficiency changes much less than the absorption probability (as seen above), this strong enhancement of the photon absorption probability with the carrier concentration shows that highly doped ( 10 cm order) emitter layers are more attractive for higher quantum efficiency (and responsivity) in p-gaas HIWIP FIR detectors. VI. DISCUSSION From Sections IV and V, it is clear that higher performance and longer p-gaas HIWIP FIR detectors can be obtained with the emitter layer concentration in the order of 10 cm. However, the surface or interface defect states can influence both the dark current and the responsivity spectrum. Some (8) passivation procedures may be necessary to overcome this problem. Further attempts to enhance the performance can be made by using of an optical cavity or an antireflection coating (especially for shorter wavelength structures) to increase the internal photoemission efficiency. Using the measured peak responsivity and the dark current data, the peak noise equivalent power (NEP) of these detectors was estimated at 4.2 K and 50 mv forward bias to be of the order of W/ Hz [ cm Hz/W]. Table I also gives the NEP value of each detector. The NEP value decreases with increasing doping concentration due to the increase in absorption efficiency (and also the responsivity). The response of detectors shows a broad spectrum (20 70 m) [6], the responsivity can further be enhanced by using filters to narrow the incident bandwidth [1]. Since there is near one order of dark current leakage, the NEP values of the p-gaas HIWIP FIR detectors could reach about (or even 10 ) W/ Hz by using optimum detector parameters and guard rings to suppress the edge leakage. The previous p -n-n samples showed extremely low dark current, approaching the measurement limits [2]. Embedding the HIWIP detectors in a p-n junction will give rise to a similar situation which could lead to much lower dark current and NEP. In this paper, the effect of emitter layer concentration on the performance of detectors was discussed. Three other detector parameters yet to be studied in connection with the detector performance are: 1) the number of multilayers, 2) the emitter layer thickness, and 3) the intrinsic layer thickness. The optimum number of multilayers should be decided by considering both the gain mechanisms, such as impact ionization and initiation of electron-phonon cascade, and the loss due to scattering and trapping etc. With a simple estimate [2] this is found to be. Reducing the emitter layer thickness enhances the internal quantum efficiency since photo-excited holes would suffer less inelastic scattering; however, it also reduces far-infrared absorption as well. Thus, the optimal layer thickness is determined by the tradeoff between absorption and internal quantum efficiency. Recently, we have presented theoretical results of the intrinsic region effect on the performance of silicon HIWIP FIR detectors [10]. It is shown that due to the SC effect, the optimum operating conditions of detectors also depend on the intrinsic layer thickness and compensating acceptor/donor concentration. VII. CONCLUSION In conclusion, a theoretical and experimental study of the performance of p-gaas HIWIP FIR detectors as a function of emitter layer concentration was carried out. Thermionic field emission was found to be a major source of dark current. By comparison with the theory, the leakage current and defect induced current have been identified. A reasonable agreement was found between the experimental and the HD theory. A linear regression relationship between the absorption coefficient and the hole concentration has been obtained and employed to calculate the photon absorption probability, which is found to follow the detector responsivity well. Higher

6 1676 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 45, NO. 8, AUGUST 1998 performance and longer p-gaas HIWIP FIR detectors are expected by using emitter layer concentrations of the order of 10 cm, and with some modifications on the devices. Further work is needed to find the optimum number of multilayers, emitter layer thickness and intrinsic region parameters to achieve the optimum HIWIP structures for FIR focal plane arrays. [20] M. L. Huberman, A. Ksendzov, A. Larsson, R. Terhune, and J. Maserjian, Optical absorption by free holes in heavily-doped GaAs, Phys. Rev. B, vol. 44, p. 1128, [21] A. G. U. Perera, W. Z. Shen, W. C. Mallard, M. O. Tanner, and K. L. Wang, Far-infrared free-hole absorption in epitaxial silicon films for homojunction detectors, Appl. Phys. Lett., vol. 71, p. 515, [22] D. K. Schroder, R. N. Thomas, and J. C. Swartz, Free carrier absorption in Silicon, IEEE Trans. Electron Devices, vol. ED-25, p. 254, Feb ACKNOWLEDGMENT The authors would like to acknowledge Dr. H. X. Yuan at EG & G Judson for help in early stages, P. Chow-Chong, P. Marshall, and S. J. Rolfe at NRC for sample fabrication and SIMS measurements, and S. G. Matsik and S. K. Gamage at Georgia State University for their technical help. REFERENCES [1] A. G. U. Perera, R. E. Sherriff, M. H. Francombe, and R. P. Devaty, Far infrared photoelectric thresholds of extrinsic semiconductor photocathodes, Appl. Phys. Lett., vol. 60, p. 3168, [2] A. G. U. Perera, Physics and novel device applications in semiconductor homojunctions, in Physics of Thin Films, M. H. Francombe and J. L. Vossen, Eds. New York: Academic, vol. 21, 1995, pp [3] H. X. Yuan and A. G. U. Perera, Dark current analysis of Si homojunction interfacial work function internal photoemission far-infrared detectors, Appl. Phys. Lett., vol. 66, p. 2262, [4] E. E. Haller, Advanced far-infrared detectors, Infrared Phys., vol. 35, p. 127, [5] D. M. Watson, M. T. Guptill, J. E. Huffman, T. N. Krabach, S. N. Raines, and S. Satyapal, Germanium block-impurity-band detector arrays: Unpassivated devices with bulk substrates, J. Appl. Phys., vol. 74, p. 4199, [6] A. G. U. Perera, H. X. Yuan, S. K. Gamage, W. Z. Shen, M. H. Francombe, H. C. Liu, M. Buchanan, and W. J. Schaff, GaAs multilayer p + -i homojunction far-infrared detectors, J. Appl. Phys., vol. 81, p. 3316, [7] J. S. Park, T. L. Lin, E. W. Jones, H. M. Del Castillo, and S. D. Gunapala, Long-wavelength stacked SiGe/Si heterojunction internal photoemission infrared detectors using multiple SiGe/Si layers, Appl. Phys. Lett., vol. 64, p. 2370, [8] A. G. U. Perera, H. X. Yuan, and M. H. Francombe, Homojunction internal photoemission far-infrared detectors: Photoresponse performance analysis, J. Appl. Phys., vol. 77, p. 915, [9] H. X. Yuan and A. G. U. Perera, Space-charge-limited conduction in Si n + -i-n + homojunction far-infrared detectors, J. Appl. Phys., vol. 79, p. 4418, [10], Effect of i-layer parameters on the performance of Si n + -in + homojunction far-infrared detectors, IEEE Trans. Electron Devices, vol. 44, p. 2180, Dec [11] B. F. Levine, Quantum-well infrared photodetectors, J. Appl. Phys., vol. 74, no. R1, 1993, and references therein. [12] S. M. Sze, Physics of Semiconductor Devices. New York: Wiley, [13] W. Z. Shen, A. G. U. Perera, S. K. Gamage, H. X. Yuan, H. C. Liu, M. Buchanan, and W. J. Schaff, A spectroscopic study of GaAs homojunction internal photoemission far infrared detectors, Infrared Phys. Technol., vol. 38, p. 133, [14] F. D. Shepherd, Infrared internal emission detectors, Proc. SPIE, vol. 1735, p. 250, [15] R. E. DeWames, G. M. Williams, J. G. Pasko, and A. H. B. Vanderwyck, J. Cryst. Growth, vol. 86, p. 849, [16] G. M. Williams, R. E. De Wames, C. W. Farley, and R. J. Anderson, Excess tunnel currents in AlGaAs/GaAs multiple quantum well infrared detectors, Appl. Phys. Lett., vol. 60, p. 1324, [17] K. Vaccaro, A. Davis, H. M. Dauplaise, S. M. Spaziani, E. A. Martin, and J. P. Lorenzo, Cadmium sulfide surface stabilization for InP-based optoelectronic devices, J. Electron. Mater., vol. 25, p. 603, [18] S. C. Jain and D. J. Roulston, A simple expression for band gap narrowing (BGN) in heavily-doped Si, Ge, GaAs, and Ge xsi10x strained layers, Solid State Electron., vol. 34, p. 453, [19] G. D. Mahan, Energy gap in Si and Ge: Impurity dependence, J. Appl. Phys., vol. 51, p. 2634, Wenzhong Shen received the B.S. and M.S. degrees in condensed matter physics from Suzhou University, China, in 1989 and 1992, respectively, and the Ph.D. degree in semiconductor physics and semiconductor devices from Shanghai Institute of Technical Physics (SITP), Chinese Academy of Sciences (CAS), in From 1995 to 1996, he was an Assistant Professor at SITP, where he worked on the spectroscopic study of semiconductor quantum well structures and their devices. Since 1996, he has been with Georgia State University, Atlanta, as a Post-doctoral Research Associate, where he is working on the development of HIWIP FIR detectors. He has published over 45 papers. A. G. Unil Perera (M 97) received the B.S. degree with First Class Honors in physics from the University of Colombo, Sri Lanka, in 1981, and the M.S. and Ph.D. degrees in physics from the University of Pittsburgh, Pittsburgh, PA, in 1983 and 1987, respectively. Until 1992, he was a Research Assistant Professor at the University of Pittsburgh. In 1992, he joined Georgia State University, Atlanta, where he is currently an Associate Professor. His research embraces development of semiconductor optoelectronic devices ranging from biology to electronics, as well as the search for fractional charge impurities (quarks) in semiconductors. He is the Director of Interaction of Radiation with Matter (IRML) Laboratory and is a member of the Center for Neural Communication and Computation and also the Graduate Director of Physics. He has published two book chapters and more than 50 research articles and has a patent on infrared detectors. Dr. Perera won the Outstanding Science Student of the Year Award at the University of Colombo, and was awarded the 1995 Junior Faculty Award at Georgia State University. He is a member of APS and SPIE. M. H. Francombe received the B.Sc., M.Sc., and physics degrees in physics from London University, U.K., in 1947, 1953, and 1958, respectively. He joined The General Electric Company Research Laboratories, London, in 1953, where he worked primarily on the research and development of new ferroelectric materials and devices. In 1958, he joined the Philco Research Laboratories, Philadelphia, PA, as Manager of a thin-film research group, to develop new materials and devices for dielectric and memory applications. In 1965, he was invited to join Westinghouse Research Laboratories, Pittsburgh, PA, first as Advisory Physicist, and in 1967 as Manager of the Device and Materials Research Department. His group pioneered in the development of new solid-state acoustic, magnetic, and semiconductor components, for application in emerging radar and infrared imaging systems. Since 1990, he has been an Adjunct Professor of Physics, first at the University of Pittsburgh, and since 1994, at Georgia State University, Atlanta. He has published more than 120 research articles, and has edited over 20 books in the thin-film research field. Dr. Francombe is a Fellow of the British Institute of Physics, and Fellow, Honorary Member, and Past President of AVS.

7 SHEN et al.: EFFECT OF EMITTER LAYER CONCENTRATION ON PERFORMANCE OF GaAs p -i HOMOJUNCTION FIR DETECTORS 1677 H. C. Liu received the Ph.D. degree in applied physics from the University of Pittsburgh, Pittsburgh, PA, in He is currently a Senior Research Officer and is the Advanced Devices Group Leader in the Institute for Microstructural Sciences at the National Research Council of Canada, Ottawa, Ont. He has authored and coauthored approximately 130 papers, has given presentations at approximately 50 conferences, and has been granted ten patents. William J. Schaff received the B.S. degree in electrical engineering in 1978 and the Ph.D. degree in 1984, both from Cornell University, Ithaca, NY. From 1978 to 1979, he was an Engineer in the Harris Corporation s RF Communications Division. In January 1984, he joined the staff at Cornell University, where he is now a Senior Research Associate responsible for the Molecular Beam Epitaxy (MBE) Facility. His research interests are in MBE growth and characterization of III V compound semiconductors for microwave electronic and photonic devices.