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GaP heterostructures
Solid-State Electronics 114 (2015) 135–140
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Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Electrically tunable spectral responsivity in metal–semiconductor–metal photodetectors based on low-dimensional ZnCdS/ZnMgS/GaP, ZnCdS/ZnS/GaP heterostructures S.V. Averin ⇑, P.I. Kuznetsov, V.A. Zhitov, L.Yu. Zakharov, V.M. Kotov, N.V. Alkeev Fryazino Branch of the Kotel’nikov Institute of Radioengineering and Electronics of Russian Academy of Sciences, 141190, Square of Academician Vvedenski 1, Fryazino, Russia
a r t i c l e
i n f o
Article history: Received 22 May 2015 Received in revised form 3 August 2015 Accepted 8 September 2015 Available online 30 September 2015 Keywords: Metal–semiconductor–metal (MSM) diode Heterostructure Dark current Spectral response
a b s t r a c t We report on growth, fabrication and characterization of metal–semiconductor–metal (MSM) photodetectors based on periodic heterostructures with ZnCdS quantum wells separated by ZnMgS and ZnS barrier layers. Heterostructures were grown on semi-insulating GaP substrates by MOVPE. MSM-diodes with width of Ni–Au interdigitated Schottky barrier contacts and gap between them of 3 lm and total detector area of 100 100 lm2 have been fabricated. Detecting properties of MSM-heterophotodiodes have been investigated. We observe electrically tunable spectral response of these detectors. At low bias detectors provide narrowband response (FWHM = 18 nm at the wavelength 350 nm) determined by a composition of ZnCdS quantum well. Increasing bias up to 70 V shifts maximum detector sensitivity at the wavelength 450 nm while narrowband response at 350 nm remains. Thus, a two-color detection of light emission is provided. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Active interest to ultraviolet (UV) photonics in the past few years gives rise to intense research on wide band gap semiconductor materials and photodetectors [1,2]. Various types of UV semiconductor detectors have been proposed and investigated including p–n junction diodes [3], p–i–n diodes [4,5], Schottky barrier diodes [6], metal–semiconductor–metal (MSM) photodetectors [7–9] and photoconductors [2]. Gallium nitride and related ternary compounds are the basis for the contemporary photoelectronics [1,2]. By changing Al content the bandgap energy of AlGaN can be adjusted to match up required detector cut off wavelength in the UV-region of spectrum. Usually the active GaN, AlGaN layers are grown on sapphire substrates [1,2,8,10]. Large lattice mismatch of the epi-layer and substrate material results in high level of dislocations. They adversely influence on the detector parameters increasing dark current and lowering quantum efficiency [11]. To solve this problem UV-detectors on wide band gap ZnSe have been investigated [12,13]. ZnSe epitaxial layers were grown on closely lattice-matched GaAs substrates [7,14]. However, even slight lattice mismatch between ZnSe and GaAs (0.27% at RT) generates a huge amount of defects at the ZnSe–GaAs interface thus degrading
⇑ Corresponding author. Tel.: +7 495 5269273; fax: +7 495 7029572. E-mail address: [email protected] (S.V. Averin). http://dx.doi.org/10.1016/j.sse.2015.09.008 0038-1101/Ó 2015 Elsevier Ltd. All rights reserved.
the efficiency and sensitivity of the ZnSe-based photodetectors [7,14,15]. ZnCdS/GaP heterostructures were grown by MOCVD for MSM-detectors operating at 300–450 nm [8]. The significant advantage of ZnCdS compound is that it can be isoperiodically grown on s.i. GaP substrates with low level of dislocations (2 105 cm 2) thus reducing dark current of detector [8]. Another challenge is the development of detectors of separate parts of the UV spectrum, i.e. selectively sensitive narrow-band photodetectors [1]. These detectors allow band pass filtering of the incoming optical signal just at the input of an optical registration system. This greatly simplifies the registration system and increases its potential. Spectral response of traditionally used silicon and gallium arsenide detectors is observed in a sufficiently wide range of UV and visible light and does not satisfy the requirements of selectivity [2,3,16]. Selective reception with these detectors requires the use of external filters [2]. Transmission visible wavelength filters based on strong interference in an ultrathin semiconductor material between two metal layers have been proposed recently [17]. At normal incident angles the filters offer band pass of 150 and 160 nm (at the level of 30% from maximum) on the central wavelength of 488 and 514 nm accordingly. By changing the thickness of semiconductor layer the filter could be adjusted to a certain wavelength. However, a band pass of the filters is large enough (30% of central wavelength) and cannot provide effective filtering and narrow-band reception of the optical signals with conventional Si or GaAs detectors. Another method of filtering
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the optical radiation is using chemical colorant pigments [18]. This approach yet has disadvantage of susceptibility to the harmful effects of both heat and continual UV radiation [17]. Finally, the newly emerged filters based on surface plasmon polaritons have a strong dependence of optical transmission coefficient to the angle of incidence of the optical radiation [17]. Thus, the modern methods of filtering optical radiation in UV and visible spectra are very diverse but their common weakness is complication of the receiving system and reduced sensitivity [2,16,18]. Therefore a photodetector with high spectral selectivity is highly desirable. In this work we have explored a new type of photodiode structures – selective photodetectors the maximum spectral sensitivity of which can be adjusted to a specific wavelength of UV portion of spectrum. A filtration efficiency of the incoming informational signal is provided by a narrow spectral response of the detectors. The detectors are made on a base of low-dimensional heterostructures ZnCdS/ZnMgS/GaP and ZnCdS/ZnS/GaP, have low dark currents and high quantum efficiencies. We observe electrically tunable spectral response of these detectors resulting in a two-color detection of light emission.
2. Experimental results 2.1. MSM-PD structural arrangement, preparation of the heterostructures The metal–semiconductor–metal (MSM) diode structure has been chosen as a basic structure for investigation. MSM-diode is widely used last years because of its fabrication simplicity, fast response, low dark current and direct compatibility with modern high-speed integrated MISFET’s circuitry [8–10]. Our MSMdetectors (Fig. 1) have been fabricated on the base of periodical heterostructures with ZnCdS multiquantum wells (MQW’s) which are separated by ZnMgS or ZnS barrier layers (Table 1). The heteroepitaxial structures were grown by MOVPE on semiinsulated (100) GaP substrates doped with Cr up to 1015 cm 3 (q = 106 Ohm cm). The deposition was carried out in a horizontal quartz reactor with a slit-shaped optical window for in-situ reflectometry at a hydrogen pressure close to atmospheric. The source materials were diethylzinc, dimethylcadmium, bis(ethylcyclopentadienyl) magnesium and di-tertbutylsulfide. The substrate was heated resistively to 465 °C. The thicknesses of individual epilayers were monitored by in-situ reflectometry and the total thickness of the heterostructures was also calculated from the reflectance spectra. A quality of growth surface was assessed on atomic force microscope Smart SPM (AIST-NT). The AFM-images (2 2 lm) had root mean square heights below 10 nm. Thick ZnCdS and ZnMgS control samples were grown under similar growth conditions to determine
Fig. 1. Heterostructure (the parameters are in the Table 1) and interdigitated contacts of the MSM-photodiode on their base.
a composition of the solid solutions by the X-ray diffraction technique. The physical and geometrical parameters of the samples UV1199, A7, A8 used in this study are listed in Table 1. 2.2. Photoluminescence The fourth harmonic (266 nm) of solid-state pulsed laser (YAG: Nd + 3) was used to excite the investigated heterostructures. Fig. 2 shows photoluminescence spectra (PL) of the samples UV 1199, A7, A8 at 300 K. There are two peaks in the PL spectra of the sample UV 1199 (curve 1) which are due to band-to-band transitions in ZnCdS QW’s (351.5 nm) and ZnMgS barrier layers (322 nm). In spite of the relatively large thickness of the barrier layers 119.6 nm, there is an effective drain of nonequilibrium carriers in ZnCdS QW’s and their emission dominates in the spectrum of the PL signal. The QW’s peak emission of the sample UV 1199 has FWHM = 8 nm which indicates good quality of heterostructure under investigation. In samples A7 and A8 we used a combination of ZnS barrier layers and Zn0.65Cd0.35S QW’s, the lattice parameters 5.41 Å and 5.53 Å of which are on either side of GaP substrate with lattice parameter 5.4505 Å. Therefore, to prevent relaxation, a total thickness, period and width of the quantum well for the samples A7 and A8 have been substantially reduced compared with sample UV 1199 (see Table 1). PL spectra of these samples are represented by curves 2 and 3 in Fig. 2. Emission from ZnS barriers is completely absent, effective drain of nonequilibrium carriers in the QW’s indicates good structural perfection of grown heterostructures. It should be noted that an increase of QW thickness from 3.7 nm (sample A7) to 6.1 nm (A8) has resulted in a red shift of the maximum emission from 366 nm to 382 nm and in PL peak broadening from 17 nm to 22 nm. 2.3. MSM-diodes: I–V characteristics and dark currents MSM-detector (Fig. 1) is a planar device consisting of two fork-shaped interdigitated contacts oppositely laying on the active semiconductor surface [7–10]. These contacts function as back-to-back Schottky diodes. In this study we have fabricated MSM-diodes with finger width and length of multi-finger metal electrodes equal to 3 and 100 lm with 3 lm spacing between the fingers. The Schottky contact fingers were defined by optical lithography and then formed by e-beam evaporation of Ni–Au bilayers and subsequent etching processes of Ni–Au. Fig. 3 shows a surface microphotograph of UV 1199 structure with interdigital contacts of MSM-diode as well as cross sectional view through the line 1 on the atomic force microscope SMART SPM. The I–V characteristics of fabricated MSM-diodes have been measured at room temperature on the parameter analyzer of semiconductor devices Agilent B-1500A and show the ability of the diodes to work at high bias voltages with very low leakage currents, Fig. 4. The dark current of MSM-diode UV 1199 is 2 10 12 A at a bias 45 V and 8 10 12 A at 70 V. Dark current of MSM-diode A7 is even smaller, 8 10 13 A at 45V and 7 10 12 A at 70 V. These values are more than two orders of magnitude lower than that of reported for AlGaN MSM-diodes with the same geometry of interdigital contacts [8] and much less than dark current of MSM-diode based on MgZnO: 7 10 11 A at bias 4 V [9]. Dark current of homoepitaxial ZnSe photodetectors was also much higher, 5 10 9 A, and at lower bias of 4 V [15]. The dark current is largely determines the sensitivity of the photodetector and should be, if possible, reduced [19]. In our detectors a dark current could be further reduced by isolation of MSM-diode contact pads from the active layers of underlying semiconductor material. This, however, complicates the technology of detector. We also have to note a slight asymmetry of MSM-diode I–V curves under positive and negative bias due to discrepancy between the barrier heights of
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S.V. Averin et al. / Solid-State Electronics 114 (2015) 135–140 Table 1 Parameters of heterostructures for the MSM-detectors. MSM-detector/ hetero-structure
Composition of QW’s
Composition of the barriers
Period/total thickness of a sample (nm)
Number of periods
Thickness of QW (nm)
QW photo-luminescence maximum (nm)
rms of surface (2 2 lm) (nm)
UV 1199 A7 A8
Zn0.92Cd0.08S Zn0.65Cd0.35S Zn0.65Cd0.35S
Zn0.8Mg0.2S ZnS ZnS
133/800 40/360 45/405
5 8 8
13.4 3.7 6.1
351.5 366.1 382
10 3.2 3.5
two serially connected Schottky barrier interdigitated contacts owing to differences of density of states at the Me–Se interface and defects of growth resulting in uneven surfaces of grown semiconductor structure (Table 1) and, as a result, to unequal effective areas of the contacts. The breakdown voltages in the MSM-diode array slightly differ with transition from one diode to another and were 90–100 V in MSM-diode A7, 80–90 V in diode A8 and 100 V or higher in MSM-diode UV 1199. High breakdown voltages and low dark currents of the MSM-diodes under investigation are an indication of high perfection of low-dimensional ZnCdS/ZnMgS/GaP and ZnCdS/ZnS/GaP heterostructures. 2.4. Spectral sensitivity of the MSM-detectors
Fig. 2. RT photoluminescence spectra of heterostructures UV 1199 (curve 1, FWHM = 8 nm), A7 (curve 2, FWHM = 17 nm), A8 (curve 3, FWHM = 22 nm).
The measurements of photocurrent response were performed with Xe arc lamp, a calibrated monochromator, modulator and selective voltmeter with synchronous detection of electrical signal from MSM-diode under investigation. The optical power output was calibrated against silicon photodiode. 2.4.1. Spectral response of the MSM-heterophotodiodes at low bias conditions Figs. 5 and 6 show photoresponse of MSM-heterophotodiodes on the base of ZnCdS/ZnMgS/GaP and ZnCdS/ZnS/GaP as a function of wavelength at bias 20 and 40 V. Different behavior is clearly seen for the MSM-diodes with a changing bias. Spectral response of MSM-diode UV 1199 at both 20 and 40 V has two obvious spikes at wavelengths of 320 nm and 350 nm, they well agree with the peak positions of the photoluminescence signal of ZnCdS quantum wells and ZnMgS barrier layers of this sample (Fig. 2, Table 1). Photoresponse spectra of detectors A7 and A8 are of different character and greatly transformed with bias changing. Even at 20 V bias the spectra contain bursts at 440 nm, which are repeatedly increasing at 40 V bias (more than an order of magnitude!) and dominate the response of detectors A7 and A8 under this condition. Photoresponse of the MSM-diodes A7 and A8 at a wavelength of 440 nm is in good agreement with a threshold energy of direct optical
Fig. 3. Interdigitated Schottky barrier contacts of MSM-diode UV 1199 and cross sectional view of the structure through the line 1.
Fig. 4. I–V characteristics of the MSM-heterostructures: 1 – A7, 2 – UV 1199.
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Fig. 5. Spectral response of the MSM-diodes at bias 20 V: 1 – UV 1199, 2 – A7, 3 – A8.
transitions in GaP, E0 = 2.8 eV (k = 443 nm) [2]. It should also be noted that in the response spectra of detectors A7 and A8 there are spikes at the wavelengths of ZnS barrier layers and Zn0.65Cd0.35S QW’s. At low bias of detector UV 1199 photogenerated carriers are mainly confined in Zn0.65Cd0.35S quantum wells and partly in ZnMgS barrier layers. Under these conditions detector has UV response with a maximum spectral sensitivity at the wavelength 350 nm, Figs. 5 and 6. A lack of response for the wavelengths longer than 360 nm in bias range 20–40 V allows to characterize detector UV 1199 as blind to visible UV-detector. Detector provides sufficiently narrowband response, at the wavelength 350 nm the full width at half maximum (FWHM) is 18 nm, Fig. 5. This value is lower than reported for selectively sensitive MSM-detector based on GaN/AlGaN MQW structure (FWHM60 nm) at central wavelength of 325 nm [20] and for AlGaN/AlN MSM-detector with illumination from the substrate side at 230 nm (FWHM = 27 nm) [8]. Narrowband response with FWHM14 nm at 361 nm was reported for AlGaN/GaN MSM photodetectors and explained by contribution of photoelectrons generated in the two-dimensional electron gas channel [20]. Even more narrow peak of response with FWHM = 7 nm at 370 nm is reported for the MSM detectors based on MgZnO heterostructure [9]. However, as it was already mentioned, the dark currents of these diodes exceed substantially dark currents realized in our study. We have experimentally established that a peak position of detector narrowband response is determined by the composition and the width of the ZnCdS QW’s. With increasing Cd content in the quantum wells a shift of the peak position of the narrowband detector response to longer wavelengths is observed. A shift of narrowband response of the MSM detector has been experimentally realized within 345–395 nm. 2.4.2. Spectral response of the MSM-heterophotodiodes at high bias conditions As it was already noted, the detectors exhibit a strong response dependence on a bias voltage. Measurements have shown that increasing the bias from 20 to 50 V gives rise to triple raise of photoresponse signal of detector UV 1199 without substantial change in a shape of its spectral response curve, but starting from 60 V there is an increase of detector broadband response along with a shift of its maximum sensitivity at the wavelength 450 nm followed by sharp slump, Fig. 7. Curve 2 in Fig. 7 shows spectral response of MSM-detector UV 1199 at bias voltage of 70 V. For this bias detector UV 1199 shows two-color response with peak sensitivities at 350 and 450 nm. Photoresponse in a region of direct
Fig. 6. Photoresponse of the MSM-diodes at bias 40 V: 1 – UV 1199, 2 – A7, 3 – A8.
Fig. 7. RT surface photoreflection of the heterostructure UV 1199 (curve 1) and spectral responsivity of the MSM-detector UV 1199 at bias 70 V (curve 2).
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optical transitions in GaP is dominated which indicates localization of strong electric field in the GaP substrate at this high bias voltage. In this case detector spans a whole violet portion of visible light and its spectral sensitivity is close to the region of maximum pigmentation effect of solar radiation (360–440 nm). We explain this shift of the maximum detector sensitivity by efficient collection of the photogenerated charge carriers from the underlying GaP layer due to insufficient thickness of the ZnCdS/ZnMgS QW’s layers for complete absorption of the charge carriers at high bias voltages when electric field effectively penetrates through a whole thickness of a sample. This is well illustrated by Fig. 6, when the total heterostructure thickness of the MSM-detectors A7 and A8 is respectively 360 and 405 nm, i.e. 2 times less than that of detector UV 1199. For these detectors long wavelength response has been observed for much smaller bias voltages of 20–40 V. Thus, the total thickness of heterostructure defines a bias voltage of detector long wavelength response associated with substrate. The long-wave response of detector UV 1199 is also in good agreement with threshold energy of direct optical transitions in GaP. The maximum photoresponse of the MSM-detector UV 1199 at the wavelength 440 nm corresponds to current sensitivity S = 0.11 A/W and the external quantum efficiency EQE = 28%. These values are in good agreement with the homoepitaxial ZnSe detectors at 448 nm: S = 0.128 A/W and EQE = 36% [15]. Given the 50% loss of light radiation due to its reflection from interdigital metal contacts of the diode and assuming 30% reflection from the semiconductor surface, the detector efficiencies are close to theoretical detection limit. The current sensitivity of MSM-detector UV 1199 at the wavelength 350 nm is 0.033 A/W. This corresponds to EQE = 12%. For comparison current sensitivity of GaN/AlGaN MQW photodiode at this wavelength is 0.03 A/W and EQE = 10.6% [21], current sensitivity of AlGaN PIN photodetector at 355 nm is 0.064 A/W and EQE = 22% [22], current sensitivity of MgZnO MSM-detector at 270 nm is 0.022 A/W and EQE = 10.5% [23]. It should be noted a strong modulation of the MSM-detector UV 1199 spectral response (curve 2 in Fig. 7), which is due to interference of the incident light and reflected from the heterostructure/substrate interface. This statement is made on the basis of coincidence of extrema of detector spectral response (curve 2) and signal of photoreflection from the surface of the heterostructure located next to the active area of the photodetector under investigation (curve 1). This effect is indistinguishable for MSM diode UV1199 at low bias when photoresponse of detector is quite weak in the region 355–500 nm, but it may be well seen as a notch in detector response at 340 nm for 20 and 40 V in Figs. 5 and 6. A similar effect of modulation of photodetector response due to interference of light in ZnSSeT/GaAs epitaxial layers was observed by the authors of [14]. This detector behavior of the MSMheterobarrier diodes needs further detail investigation. Hereby, our experiments show that in the case of small thickness of diode active layers (detectors A7, A8) it is difficult to eliminate the influence of GaP substrate on the MSM-detector response in the visible. However, it is well known that a depth of electric field effective penetration in the active volume of MSM-diode sharply decreases with decreasing gap between the interdigital contacts [24,25]. Two-dimensional modeling demonstrates that if the gap between the contact fingers is reduced from 3 to 0.5 lm, the electric field is concentrated quite near to surface of MSMdiode [24] and thus the influence of the substrate on the MSMdetector response will be possible to eliminate even for thin active heterobarrier layers. 3. Conclusion We have designed, fabricated and characterized the metal–semiconductor–metal photodetectors based on periodic
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heterostructures with ZnCdS quantum wells separated by ZnMgS and ZnS barrier layers. Heterostructures were grown on semiinsulating GaP substrates by MOVPE. Detectors exhibit very low dark currents and electrically tunable spectral response. At low bias detectors provide narrowband UV-response determined by a composition of ZnCdS quantum well. A shift of the peak position of the narrowband detector response to longer wavelengths is observed with increasing Cd content. Higher operating bias shifts maximum detector sensitivity in the visible part of spectrum due to the penetration of external electric field down to the semiinsulating GaP substrate while a narrowband UV-response remains. It is shown that the total thickness of heterostructure defines a bias voltage of detector long wavelength response associated with substrate. Highly selective two-color photodetector allows discriminating the optical channels and increasing the dynamic range and noise immunity of optical informational and measuring systems. Acknowledgements This work was supported by RFFI (Russian Fund of Fundamental Investigations, Grants Nos. 14-07-00014 and 15-07-02312). References [1] Ambacher O. Growth and application of Group III-nitrides. J Phys D Appl Phys 1998;31:2653–710. and references therein. [2] Monroy, Omnes F, Calle F. Wide-bandgap semiconductor ultraviolet photodetectors. Semicond Sci Technol 2003;18:R33–51. and references therein. [3] Monroy E, Munoz E, Sanchez FJ, Calle F, Calleja E, et al. High-performance GaN p–n junction photodetectors for solar ultraviolet applications. Semicond Sci Technol 1998;13(9):1042–6. [4] Vigue F, Tournie E, Faurie J-P. Evaluation of the potential of ZnSe and Zn(Mg) BeSe compounds for ultraviolet photodetection. IEEE J Quantum Electron 2001;37:1146–52. [5] Xie XH, Zhang ZZ, Shan CX, Chen HY, Shen DZ. Dual-color ultraviolet photodetector based on mixed-phase-MgZnO/i-MgO/p-Si double heterojunction. Appl Phys Lett 2012;101:081104-1-3. [6] Monroy E, Calle F, Pau JL, Sanche FJ, Munos E, Omnes F. Analysis and modeling of AlxGa1 xN-based Schottky barrier photodiodes. J Appl Phys 2000;88: 2081–91. [7] Chang SJ, Su YK, Chen WR, Chen JF, Lan WH, Lin WJ. ZnSTeSe metal– semiconductor–metal photodetectors. IEEE Photonics Technol Lett 2002;14: 188–90. [8] Averin S, Kuznetzov P, Zhitov V, Zakharov L, Alkeev N, Gladisheva N. Selectively-sensitive-metal–semiconductor–metal photodetectors based on AlGaN/AlN and ZnCdS/GaP heterostructures. Phys Status Solidi 2013;C 10:298–301. http://dx.doi.org/10.1002/pssc.20120063. [9] Zhang Z, Wenckstern H, Schmidt M, Grundmann M. Wavelength selective metal–semiconductor–metal photodetectors based on (Mg, Zn)O-heterostructures. Appl Phys Lett 2011;99:083502-1-3. http://dx.doi.org/10.1063/ 1.362833. [10] Averin SV, Kuznetzov PI, Zhitov VA, Alkeev NV. Solar-blind MSMphotodetectors based on AlxGa1 xN/GaN heterostructures grown by MOCVD. Solid State Electron 2008;52:618–24. http://dx.doi.org/10.1016/sse.2007.10. 037. [11] Sou IK, Wu MCW, Sun T, Wong KC, Wong GKL. MBE-grown ZnMgS ultra-violet photodetectors. J Electron Mater 2001;30(6):673–6. [12] Monroy E, Vigue F, Calle F, Izpura I, Munoz E, Faurie J-P. Time response analysis of ZnSe and ZnMgBeSe based Schottky barrier photodetectors. Appl Phys Lett 2000;77:2761–3. [13] Vigue F, Tournie E, Faurie J-P. ZnSe-based Schottky barrier photodetectors. Electron Lett 2000;36(4):352–4. [14] Chen WR, Meen TH, Cheng YC, Lin WJ. P-Down ZnSTeSe/ZnSe/GaAs heterostructure photodiodes. IEEE Electron Device Lett 2006;27(5):347–9. [15] Lin TK, Chang SJ, Su YK, Chiou YZ, Wang CK, Chang SP, et al. ZnSe MSM photodetectors prepared on GaAs and ZnSe substrates. Mater Sci Eng, B 2005;119:202–5. [16] Metzger RA. The state of SOI Technology. Compound Semiconductor Magazine; 1996; May/June: 29. [17] Lee KT, Seo S, Lee JY, Guo LJ. Ultrathin metal–semiconductor–metal resonator for angle invariant visible band transmission filters. Appl Phys Lett 2014;104: 231112-1-4. [18] Sabnis RW. Color filter technology for liquid crystal displays. Displays 1999;20:119–29. [19] Pernot C, Hirano A, Iwaya M, Detchprohm T, Amano H. Low-intensity ultraviolet photodetectors based on AlGaN. Jpn J Appl Phys 1999;38(Pt.2 (5A)):L487–9.
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Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Electrically tunable spectral responsivity in metal–semiconductor–metal photodetectors based on low-dimensional ZnCdS/ZnMgS/GaP, ZnCdS/ZnS/GaP heterostructures S.V. Averin ⇑, P.I. Kuznetsov, V.A. Zhitov, L.Yu. Zakharov, V.M. Kotov, N.V. Alkeev Fryazino Branch of the Kotel’nikov Institute of Radioengineering and Electronics of Russian Academy of Sciences, 141190, Square of Academician Vvedenski 1, Fryazino, Russia
a r t i c l e
i n f o
Article history: Received 22 May 2015 Received in revised form 3 August 2015 Accepted 8 September 2015 Available online 30 September 2015 Keywords: Metal–semiconductor–metal (MSM) diode Heterostructure Dark current Spectral response
a b s t r a c t We report on growth, fabrication and characterization of metal–semiconductor–metal (MSM) photodetectors based on periodic heterostructures with ZnCdS quantum wells separated by ZnMgS and ZnS barrier layers. Heterostructures were grown on semi-insulating GaP substrates by MOVPE. MSM-diodes with width of Ni–Au interdigitated Schottky barrier contacts and gap between them of 3 lm and total detector area of 100 100 lm2 have been fabricated. Detecting properties of MSM-heterophotodiodes have been investigated. We observe electrically tunable spectral response of these detectors. At low bias detectors provide narrowband response (FWHM = 18 nm at the wavelength 350 nm) determined by a composition of ZnCdS quantum well. Increasing bias up to 70 V shifts maximum detector sensitivity at the wavelength 450 nm while narrowband response at 350 nm remains. Thus, a two-color detection of light emission is provided. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Active interest to ultraviolet (UV) photonics in the past few years gives rise to intense research on wide band gap semiconductor materials and photodetectors [1,2]. Various types of UV semiconductor detectors have been proposed and investigated including p–n junction diodes [3], p–i–n diodes [4,5], Schottky barrier diodes [6], metal–semiconductor–metal (MSM) photodetectors [7–9] and photoconductors [2]. Gallium nitride and related ternary compounds are the basis for the contemporary photoelectronics [1,2]. By changing Al content the bandgap energy of AlGaN can be adjusted to match up required detector cut off wavelength in the UV-region of spectrum. Usually the active GaN, AlGaN layers are grown on sapphire substrates [1,2,8,10]. Large lattice mismatch of the epi-layer and substrate material results in high level of dislocations. They adversely influence on the detector parameters increasing dark current and lowering quantum efficiency [11]. To solve this problem UV-detectors on wide band gap ZnSe have been investigated [12,13]. ZnSe epitaxial layers were grown on closely lattice-matched GaAs substrates [7,14]. However, even slight lattice mismatch between ZnSe and GaAs (0.27% at RT) generates a huge amount of defects at the ZnSe–GaAs interface thus degrading
⇑ Corresponding author. Tel.: +7 495 5269273; fax: +7 495 7029572. E-mail address: [email protected] (S.V. Averin). http://dx.doi.org/10.1016/j.sse.2015.09.008 0038-1101/Ó 2015 Elsevier Ltd. All rights reserved.
the efficiency and sensitivity of the ZnSe-based photodetectors [7,14,15]. ZnCdS/GaP heterostructures were grown by MOCVD for MSM-detectors operating at 300–450 nm [8]. The significant advantage of ZnCdS compound is that it can be isoperiodically grown on s.i. GaP substrates with low level of dislocations (2 105 cm 2) thus reducing dark current of detector [8]. Another challenge is the development of detectors of separate parts of the UV spectrum, i.e. selectively sensitive narrow-band photodetectors [1]. These detectors allow band pass filtering of the incoming optical signal just at the input of an optical registration system. This greatly simplifies the registration system and increases its potential. Spectral response of traditionally used silicon and gallium arsenide detectors is observed in a sufficiently wide range of UV and visible light and does not satisfy the requirements of selectivity [2,3,16]. Selective reception with these detectors requires the use of external filters [2]. Transmission visible wavelength filters based on strong interference in an ultrathin semiconductor material between two metal layers have been proposed recently [17]. At normal incident angles the filters offer band pass of 150 and 160 nm (at the level of 30% from maximum) on the central wavelength of 488 and 514 nm accordingly. By changing the thickness of semiconductor layer the filter could be adjusted to a certain wavelength. However, a band pass of the filters is large enough (30% of central wavelength) and cannot provide effective filtering and narrow-band reception of the optical signals with conventional Si or GaAs detectors. Another method of filtering
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the optical radiation is using chemical colorant pigments [18]. This approach yet has disadvantage of susceptibility to the harmful effects of both heat and continual UV radiation [17]. Finally, the newly emerged filters based on surface plasmon polaritons have a strong dependence of optical transmission coefficient to the angle of incidence of the optical radiation [17]. Thus, the modern methods of filtering optical radiation in UV and visible spectra are very diverse but their common weakness is complication of the receiving system and reduced sensitivity [2,16,18]. Therefore a photodetector with high spectral selectivity is highly desirable. In this work we have explored a new type of photodiode structures – selective photodetectors the maximum spectral sensitivity of which can be adjusted to a specific wavelength of UV portion of spectrum. A filtration efficiency of the incoming informational signal is provided by a narrow spectral response of the detectors. The detectors are made on a base of low-dimensional heterostructures ZnCdS/ZnMgS/GaP and ZnCdS/ZnS/GaP, have low dark currents and high quantum efficiencies. We observe electrically tunable spectral response of these detectors resulting in a two-color detection of light emission.
2. Experimental results 2.1. MSM-PD structural arrangement, preparation of the heterostructures The metal–semiconductor–metal (MSM) diode structure has been chosen as a basic structure for investigation. MSM-diode is widely used last years because of its fabrication simplicity, fast response, low dark current and direct compatibility with modern high-speed integrated MISFET’s circuitry [8–10]. Our MSMdetectors (Fig. 1) have been fabricated on the base of periodical heterostructures with ZnCdS multiquantum wells (MQW’s) which are separated by ZnMgS or ZnS barrier layers (Table 1). The heteroepitaxial structures were grown by MOVPE on semiinsulated (100) GaP substrates doped with Cr up to 1015 cm 3 (q = 106 Ohm cm). The deposition was carried out in a horizontal quartz reactor with a slit-shaped optical window for in-situ reflectometry at a hydrogen pressure close to atmospheric. The source materials were diethylzinc, dimethylcadmium, bis(ethylcyclopentadienyl) magnesium and di-tertbutylsulfide. The substrate was heated resistively to 465 °C. The thicknesses of individual epilayers were monitored by in-situ reflectometry and the total thickness of the heterostructures was also calculated from the reflectance spectra. A quality of growth surface was assessed on atomic force microscope Smart SPM (AIST-NT). The AFM-images (2 2 lm) had root mean square heights below 10 nm. Thick ZnCdS and ZnMgS control samples were grown under similar growth conditions to determine
Fig. 1. Heterostructure (the parameters are in the Table 1) and interdigitated contacts of the MSM-photodiode on their base.
a composition of the solid solutions by the X-ray diffraction technique. The physical and geometrical parameters of the samples UV1199, A7, A8 used in this study are listed in Table 1. 2.2. Photoluminescence The fourth harmonic (266 nm) of solid-state pulsed laser (YAG: Nd + 3) was used to excite the investigated heterostructures. Fig. 2 shows photoluminescence spectra (PL) of the samples UV 1199, A7, A8 at 300 K. There are two peaks in the PL spectra of the sample UV 1199 (curve 1) which are due to band-to-band transitions in ZnCdS QW’s (351.5 nm) and ZnMgS barrier layers (322 nm). In spite of the relatively large thickness of the barrier layers 119.6 nm, there is an effective drain of nonequilibrium carriers in ZnCdS QW’s and their emission dominates in the spectrum of the PL signal. The QW’s peak emission of the sample UV 1199 has FWHM = 8 nm which indicates good quality of heterostructure under investigation. In samples A7 and A8 we used a combination of ZnS barrier layers and Zn0.65Cd0.35S QW’s, the lattice parameters 5.41 Å and 5.53 Å of which are on either side of GaP substrate with lattice parameter 5.4505 Å. Therefore, to prevent relaxation, a total thickness, period and width of the quantum well for the samples A7 and A8 have been substantially reduced compared with sample UV 1199 (see Table 1). PL spectra of these samples are represented by curves 2 and 3 in Fig. 2. Emission from ZnS barriers is completely absent, effective drain of nonequilibrium carriers in the QW’s indicates good structural perfection of grown heterostructures. It should be noted that an increase of QW thickness from 3.7 nm (sample A7) to 6.1 nm (A8) has resulted in a red shift of the maximum emission from 366 nm to 382 nm and in PL peak broadening from 17 nm to 22 nm. 2.3. MSM-diodes: I–V characteristics and dark currents MSM-detector (Fig. 1) is a planar device consisting of two fork-shaped interdigitated contacts oppositely laying on the active semiconductor surface [7–10]. These contacts function as back-to-back Schottky diodes. In this study we have fabricated MSM-diodes with finger width and length of multi-finger metal electrodes equal to 3 and 100 lm with 3 lm spacing between the fingers. The Schottky contact fingers were defined by optical lithography and then formed by e-beam evaporation of Ni–Au bilayers and subsequent etching processes of Ni–Au. Fig. 3 shows a surface microphotograph of UV 1199 structure with interdigital contacts of MSM-diode as well as cross sectional view through the line 1 on the atomic force microscope SMART SPM. The I–V characteristics of fabricated MSM-diodes have been measured at room temperature on the parameter analyzer of semiconductor devices Agilent B-1500A and show the ability of the diodes to work at high bias voltages with very low leakage currents, Fig. 4. The dark current of MSM-diode UV 1199 is 2 10 12 A at a bias 45 V and 8 10 12 A at 70 V. Dark current of MSM-diode A7 is even smaller, 8 10 13 A at 45V and 7 10 12 A at 70 V. These values are more than two orders of magnitude lower than that of reported for AlGaN MSM-diodes with the same geometry of interdigital contacts [8] and much less than dark current of MSM-diode based on MgZnO: 7 10 11 A at bias 4 V [9]. Dark current of homoepitaxial ZnSe photodetectors was also much higher, 5 10 9 A, and at lower bias of 4 V [15]. The dark current is largely determines the sensitivity of the photodetector and should be, if possible, reduced [19]. In our detectors a dark current could be further reduced by isolation of MSM-diode contact pads from the active layers of underlying semiconductor material. This, however, complicates the technology of detector. We also have to note a slight asymmetry of MSM-diode I–V curves under positive and negative bias due to discrepancy between the barrier heights of
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S.V. Averin et al. / Solid-State Electronics 114 (2015) 135–140 Table 1 Parameters of heterostructures for the MSM-detectors. MSM-detector/ hetero-structure
Composition of QW’s
Composition of the barriers
Period/total thickness of a sample (nm)
Number of periods
Thickness of QW (nm)
QW photo-luminescence maximum (nm)
rms of surface (2 2 lm) (nm)
UV 1199 A7 A8
Zn0.92Cd0.08S Zn0.65Cd0.35S Zn0.65Cd0.35S
Zn0.8Mg0.2S ZnS ZnS
133/800 40/360 45/405
5 8 8
13.4 3.7 6.1
351.5 366.1 382
10 3.2 3.5
two serially connected Schottky barrier interdigitated contacts owing to differences of density of states at the Me–Se interface and defects of growth resulting in uneven surfaces of grown semiconductor structure (Table 1) and, as a result, to unequal effective areas of the contacts. The breakdown voltages in the MSM-diode array slightly differ with transition from one diode to another and were 90–100 V in MSM-diode A7, 80–90 V in diode A8 and 100 V or higher in MSM-diode UV 1199. High breakdown voltages and low dark currents of the MSM-diodes under investigation are an indication of high perfection of low-dimensional ZnCdS/ZnMgS/GaP and ZnCdS/ZnS/GaP heterostructures. 2.4. Spectral sensitivity of the MSM-detectors
Fig. 2. RT photoluminescence spectra of heterostructures UV 1199 (curve 1, FWHM = 8 nm), A7 (curve 2, FWHM = 17 nm), A8 (curve 3, FWHM = 22 nm).
The measurements of photocurrent response were performed with Xe arc lamp, a calibrated monochromator, modulator and selective voltmeter with synchronous detection of electrical signal from MSM-diode under investigation. The optical power output was calibrated against silicon photodiode. 2.4.1. Spectral response of the MSM-heterophotodiodes at low bias conditions Figs. 5 and 6 show photoresponse of MSM-heterophotodiodes on the base of ZnCdS/ZnMgS/GaP and ZnCdS/ZnS/GaP as a function of wavelength at bias 20 and 40 V. Different behavior is clearly seen for the MSM-diodes with a changing bias. Spectral response of MSM-diode UV 1199 at both 20 and 40 V has two obvious spikes at wavelengths of 320 nm and 350 nm, they well agree with the peak positions of the photoluminescence signal of ZnCdS quantum wells and ZnMgS barrier layers of this sample (Fig. 2, Table 1). Photoresponse spectra of detectors A7 and A8 are of different character and greatly transformed with bias changing. Even at 20 V bias the spectra contain bursts at 440 nm, which are repeatedly increasing at 40 V bias (more than an order of magnitude!) and dominate the response of detectors A7 and A8 under this condition. Photoresponse of the MSM-diodes A7 and A8 at a wavelength of 440 nm is in good agreement with a threshold energy of direct optical
Fig. 3. Interdigitated Schottky barrier contacts of MSM-diode UV 1199 and cross sectional view of the structure through the line 1.
Fig. 4. I–V characteristics of the MSM-heterostructures: 1 – A7, 2 – UV 1199.
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Fig. 5. Spectral response of the MSM-diodes at bias 20 V: 1 – UV 1199, 2 – A7, 3 – A8.
transitions in GaP, E0 = 2.8 eV (k = 443 nm) [2]. It should also be noted that in the response spectra of detectors A7 and A8 there are spikes at the wavelengths of ZnS barrier layers and Zn0.65Cd0.35S QW’s. At low bias of detector UV 1199 photogenerated carriers are mainly confined in Zn0.65Cd0.35S quantum wells and partly in ZnMgS barrier layers. Under these conditions detector has UV response with a maximum spectral sensitivity at the wavelength 350 nm, Figs. 5 and 6. A lack of response for the wavelengths longer than 360 nm in bias range 20–40 V allows to characterize detector UV 1199 as blind to visible UV-detector. Detector provides sufficiently narrowband response, at the wavelength 350 nm the full width at half maximum (FWHM) is 18 nm, Fig. 5. This value is lower than reported for selectively sensitive MSM-detector based on GaN/AlGaN MQW structure (FWHM60 nm) at central wavelength of 325 nm [20] and for AlGaN/AlN MSM-detector with illumination from the substrate side at 230 nm (FWHM = 27 nm) [8]. Narrowband response with FWHM14 nm at 361 nm was reported for AlGaN/GaN MSM photodetectors and explained by contribution of photoelectrons generated in the two-dimensional electron gas channel [20]. Even more narrow peak of response with FWHM = 7 nm at 370 nm is reported for the MSM detectors based on MgZnO heterostructure [9]. However, as it was already mentioned, the dark currents of these diodes exceed substantially dark currents realized in our study. We have experimentally established that a peak position of detector narrowband response is determined by the composition and the width of the ZnCdS QW’s. With increasing Cd content in the quantum wells a shift of the peak position of the narrowband detector response to longer wavelengths is observed. A shift of narrowband response of the MSM detector has been experimentally realized within 345–395 nm. 2.4.2. Spectral response of the MSM-heterophotodiodes at high bias conditions As it was already noted, the detectors exhibit a strong response dependence on a bias voltage. Measurements have shown that increasing the bias from 20 to 50 V gives rise to triple raise of photoresponse signal of detector UV 1199 without substantial change in a shape of its spectral response curve, but starting from 60 V there is an increase of detector broadband response along with a shift of its maximum sensitivity at the wavelength 450 nm followed by sharp slump, Fig. 7. Curve 2 in Fig. 7 shows spectral response of MSM-detector UV 1199 at bias voltage of 70 V. For this bias detector UV 1199 shows two-color response with peak sensitivities at 350 and 450 nm. Photoresponse in a region of direct
Fig. 6. Photoresponse of the MSM-diodes at bias 40 V: 1 – UV 1199, 2 – A7, 3 – A8.
Fig. 7. RT surface photoreflection of the heterostructure UV 1199 (curve 1) and spectral responsivity of the MSM-detector UV 1199 at bias 70 V (curve 2).
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optical transitions in GaP is dominated which indicates localization of strong electric field in the GaP substrate at this high bias voltage. In this case detector spans a whole violet portion of visible light and its spectral sensitivity is close to the region of maximum pigmentation effect of solar radiation (360–440 nm). We explain this shift of the maximum detector sensitivity by efficient collection of the photogenerated charge carriers from the underlying GaP layer due to insufficient thickness of the ZnCdS/ZnMgS QW’s layers for complete absorption of the charge carriers at high bias voltages when electric field effectively penetrates through a whole thickness of a sample. This is well illustrated by Fig. 6, when the total heterostructure thickness of the MSM-detectors A7 and A8 is respectively 360 and 405 nm, i.e. 2 times less than that of detector UV 1199. For these detectors long wavelength response has been observed for much smaller bias voltages of 20–40 V. Thus, the total thickness of heterostructure defines a bias voltage of detector long wavelength response associated with substrate. The long-wave response of detector UV 1199 is also in good agreement with threshold energy of direct optical transitions in GaP. The maximum photoresponse of the MSM-detector UV 1199 at the wavelength 440 nm corresponds to current sensitivity S = 0.11 A/W and the external quantum efficiency EQE = 28%. These values are in good agreement with the homoepitaxial ZnSe detectors at 448 nm: S = 0.128 A/W and EQE = 36% [15]. Given the 50% loss of light radiation due to its reflection from interdigital metal contacts of the diode and assuming 30% reflection from the semiconductor surface, the detector efficiencies are close to theoretical detection limit. The current sensitivity of MSM-detector UV 1199 at the wavelength 350 nm is 0.033 A/W. This corresponds to EQE = 12%. For comparison current sensitivity of GaN/AlGaN MQW photodiode at this wavelength is 0.03 A/W and EQE = 10.6% [21], current sensitivity of AlGaN PIN photodetector at 355 nm is 0.064 A/W and EQE = 22% [22], current sensitivity of MgZnO MSM-detector at 270 nm is 0.022 A/W and EQE = 10.5% [23]. It should be noted a strong modulation of the MSM-detector UV 1199 spectral response (curve 2 in Fig. 7), which is due to interference of the incident light and reflected from the heterostructure/substrate interface. This statement is made on the basis of coincidence of extrema of detector spectral response (curve 2) and signal of photoreflection from the surface of the heterostructure located next to the active area of the photodetector under investigation (curve 1). This effect is indistinguishable for MSM diode UV1199 at low bias when photoresponse of detector is quite weak in the region 355–500 nm, but it may be well seen as a notch in detector response at 340 nm for 20 and 40 V in Figs. 5 and 6. A similar effect of modulation of photodetector response due to interference of light in ZnSSeT/GaAs epitaxial layers was observed by the authors of [14]. This detector behavior of the MSMheterobarrier diodes needs further detail investigation. Hereby, our experiments show that in the case of small thickness of diode active layers (detectors A7, A8) it is difficult to eliminate the influence of GaP substrate on the MSM-detector response in the visible. However, it is well known that a depth of electric field effective penetration in the active volume of MSM-diode sharply decreases with decreasing gap between the interdigital contacts [24,25]. Two-dimensional modeling demonstrates that if the gap between the contact fingers is reduced from 3 to 0.5 lm, the electric field is concentrated quite near to surface of MSMdiode [24] and thus the influence of the substrate on the MSMdetector response will be possible to eliminate even for thin active heterobarrier layers. 3. Conclusion We have designed, fabricated and characterized the metal–semiconductor–metal photodetectors based on periodic
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heterostructures with ZnCdS quantum wells separated by ZnMgS and ZnS barrier layers. Heterostructures were grown on semiinsulating GaP substrates by MOVPE. Detectors exhibit very low dark currents and electrically tunable spectral response. At low bias detectors provide narrowband UV-response determined by a composition of ZnCdS quantum well. A shift of the peak position of the narrowband detector response to longer wavelengths is observed with increasing Cd content. Higher operating bias shifts maximum detector sensitivity in the visible part of spectrum due to the penetration of external electric field down to the semiinsulating GaP substrate while a narrowband UV-response remains. It is shown that the total thickness of heterostructure defines a bias voltage of detector long wavelength response associated with substrate. Highly selective two-color photodetector allows discriminating the optical channels and increasing the dynamic range and noise immunity of optical informational and measuring systems. Acknowledgements This work was supported by RFFI (Russian Fund of Fundamental Investigations, Grants Nos. 14-07-00014 and 15-07-02312). References [1] Ambacher O. Growth and application of Group III-nitrides. J Phys D Appl Phys 1998;31:2653–710. and references therein. [2] Monroy, Omnes F, Calle F. Wide-bandgap semiconductor ultraviolet photodetectors. Semicond Sci Technol 2003;18:R33–51. and references therein. [3] Monroy E, Munoz E, Sanchez FJ, Calle F, Calleja E, et al. High-performance GaN p–n junction photodetectors for solar ultraviolet applications. Semicond Sci Technol 1998;13(9):1042–6. [4] Vigue F, Tournie E, Faurie J-P. Evaluation of the potential of ZnSe and Zn(Mg) BeSe compounds for ultraviolet photodetection. IEEE J Quantum Electron 2001;37:1146–52. [5] Xie XH, Zhang ZZ, Shan CX, Chen HY, Shen DZ. Dual-color ultraviolet photodetector based on mixed-phase-MgZnO/i-MgO/p-Si double heterojunction. Appl Phys Lett 2012;101:081104-1-3. [6] Monroy E, Calle F, Pau JL, Sanche FJ, Munos E, Omnes F. Analysis and modeling of AlxGa1 xN-based Schottky barrier photodiodes. J Appl Phys 2000;88: 2081–91. [7] Chang SJ, Su YK, Chen WR, Chen JF, Lan WH, Lin WJ. ZnSTeSe metal– semiconductor–metal photodetectors. IEEE Photonics Technol Lett 2002;14: 188–90. [8] Averin S, Kuznetzov P, Zhitov V, Zakharov L, Alkeev N, Gladisheva N. Selectively-sensitive-metal–semiconductor–metal photodetectors based on AlGaN/AlN and ZnCdS/GaP heterostructures. Phys Status Solidi 2013;C 10:298–301. http://dx.doi.org/10.1002/pssc.20120063. [9] Zhang Z, Wenckstern H, Schmidt M, Grundmann M. Wavelength selective metal–semiconductor–metal photodetectors based on (Mg, Zn)O-heterostructures. Appl Phys Lett 2011;99:083502-1-3. http://dx.doi.org/10.1063/ 1.362833. [10] Averin SV, Kuznetzov PI, Zhitov VA, Alkeev NV. Solar-blind MSMphotodetectors based on AlxGa1 xN/GaN heterostructures grown by MOCVD. Solid State Electron 2008;52:618–24. http://dx.doi.org/10.1016/sse.2007.10. 037. [11] Sou IK, Wu MCW, Sun T, Wong KC, Wong GKL. MBE-grown ZnMgS ultra-violet photodetectors. J Electron Mater 2001;30(6):673–6. [12] Monroy E, Vigue F, Calle F, Izpura I, Munoz E, Faurie J-P. Time response analysis of ZnSe and ZnMgBeSe based Schottky barrier photodetectors. Appl Phys Lett 2000;77:2761–3. [13] Vigue F, Tournie E, Faurie J-P. ZnSe-based Schottky barrier photodetectors. Electron Lett 2000;36(4):352–4. [14] Chen WR, Meen TH, Cheng YC, Lin WJ. P-Down ZnSTeSe/ZnSe/GaAs heterostructure photodiodes. IEEE Electron Device Lett 2006;27(5):347–9. [15] Lin TK, Chang SJ, Su YK, Chiou YZ, Wang CK, Chang SP, et al. ZnSe MSM photodetectors prepared on GaAs and ZnSe substrates. Mater Sci Eng, B 2005;119:202–5. [16] Metzger RA. The state of SOI Technology. Compound Semiconductor Magazine; 1996; May/June: 29. [17] Lee KT, Seo S, Lee JY, Guo LJ. Ultrathin metal–semiconductor–metal resonator for angle invariant visible band transmission filters. Appl Phys Lett 2014;104: 231112-1-4. [18] Sabnis RW. Color filter technology for liquid crystal displays. Displays 1999;20:119–29. [19] Pernot C, Hirano A, Iwaya M, Detchprohm T, Amano H. Low-intensity ultraviolet photodetectors based on AlGaN. Jpn J Appl Phys 1999;38(Pt.2 (5A)):L487–9.
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