The dark current and noise spectra were investigated in Hg₁₋xCdxTe (x≅0.22) photodiodes at zero and low reverse bias voltages. The photodiodes were prepared by boron implantation into LPE films. The 1/f noise is proved to be correlated with tunneling current via the deep defect states in the gap at low reverse biases U≤0.1 V. In the photodiodes, where the tunneling current is found to be dominating, the 1/f noise is observed up to frequencies 10⁴ Hz. The decrease of tunneling current results in the decrease of 1/f noise

Ivasiv, Z.F.

Sizov, F.F.

Tetyorkin, V.V.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

21' 1999, Institute of Semiconductor Physics, National Academy of Sciences of UkraineSemiconductor  Physics, Quantum  Electronics & Optoelectronics. 1999. V. 2, N 3. P. 21-25.PACS: 85.60.D, 07.57.K, 85.60.G, 73.40Noise spectra and dark current investigationsin n+-p-type Hg1-xCdxTe (x @@ @@ @  0.22) photodiodesZ.F. Ivasiv, F.F. Sizov, V.V. TetyorkinInstitute of Semiconductor Physics, NAS of Ukraine45, prospect Nauki, 252650 Kyiv, UkraineAbstract. The dark current and noise spectra were investigated in Hg1-xCdxTe (x@ 0.22) photodiodes atzero and low reverse bias voltages. The photodiodes were prepared by boron implantation into LPEfilms. The 1/f noise is proved to be correlated with tunneling current via the deep defect states in the gapat low reverse biases U£ 0.1 V. In the photodiodes, where the tunneling current is found to be dominat-ing, the 1/f noise is observed up to frequencies 104 Hz. The decrease of tunneling current results in thedecrease of 1/f noise.Keywords: long wavelength intra-red photodiode, trap assisted tunneling, 1/f noise.Paper received 07.09.99; revised manuscript received 30.09.99; accepted for publication 11.10.99.1. IntroductionHg1-xCdxTe ternary compounds are recognized to be themost important materials for fabrication of long wavelengthinfrared (LWIR) photodiodes for 3-5 and 8-12 m m wave-length regions [1]. Ion implantation seems to be the mostimportant method for manufacturing single n+-p-typephotodiodes as well as multi-element focal plane arrays [1].Carrier transport mechanisms in implanted LWIR photo-diodes has been investigated both theoretically and experi-mentally by many authors [1-9]. It has been found that ingeneral case at temperatures close to 77 K total dark cur-rent are composed of several components: bulk diffusion,generation-recombination (GR) current in the depletion re-gion, trap assisted tunneling (TAT) current, band-to-band(BTB) tunneling current and surface leakage current. GRand TAT currents dominate the total dark current at zeroand low reverse bias voltage as well as BTB current is domi-nant at large reverse bias voltage. In the photodiodes withimproved characteristics dark current is dominated by thediffusion current. Due to the fact that carrier transportmechanisms listed above are influenced by deep defect statesin the gap, fitting procedure can be used for estimation ofseveral important parameters of these states such as con-centration Nt, energy Et and capture rates [4-9]. Because ofsome discrepancies exist in the models used for the fittingcalculation the values of above mentioned parameters arechanged in a rather wide range (e.g., Nt is changed from~1012 cm-3 up to ~1017 cm-3 [6,7,8,14]).As to investigations of noise mechanisms in HgCdTephotodiodes the origin of the 1/f noise still seems to be themost important and unsolved problem. Several sources ofthe 1/f noise have been proposed in application to thesephotodiodes [10-15]. One of the possible source is latticedefects of different types unintentionally or intentionallyintroduced during growth of starting material. Fluctuationof deep point defects, shallow etch pits, dislocation multi-plication and clustering are recognized to be possible for1/f noise source. As shown in [12], 1/f noise in LWIR photo-diodes is a bulk phenomenon associated with defects in thedepletion region. However, the states at the interface ofHgCdTe and passivating layer can also contribute to 1/f noise[11,13,14]. Finally, the dark current is suspected to be apossible source of 1/f noise, too. In several papers [4, 9] thecorrelation between 1/f noise and bulk BTB tunneling cur-rent was observed. Quite the contrary, the correlation be-tween 1/f noise and surface BTB tunneling current has beenfound in the papers published earlier [15]. The role of trapassisted tunneling current at zero and low reverse biasvoltages seems to be not investigated yet.The aim of this work is to investigate dark current and1/f noise in LWIR HgCdTe photodiodes in order to findpossible correlation between these phenomena. More thanthirty diodes were investigated to obtain statistical results.2. Experimental results and discussion.The starting materials were undoped Hg1-xCdxTe (x@ 0.22)films of p - type conductivity. The films were used for p-n-junction fabrication by boron ion implantation. The holeconcentration in the films was close to 1 × 1016 cm-3 at tem-perature 77 K.Z.F. Ivasiv et al.: Noise spectra and dark current investigations...22 SQO, 2(3), 1999The dark current in the photodiodes has been measuredat 77 K. By differentiation of I-U curves the dynamic re-sistance R characteristics versus voltage bias U were ob-tained. To estimate the cut-off wavelength l c values thephotoresponse spectra were measured at 82 K. The noisespectra were investigated in the frequency range 0.07 ‚104 Hz at 77 K using CK4-73 spectrum analyzer.Typical normalized photoresponse spectra of thephotodiodes are shown in Fig.1. The cutoff wavelength l cfor the photodiodes investigated are shown in Table 1 to-gether with intrinsic carrier concentration, ni, and energyband gap, Eg, values. The last two parameters were calcu-lated using appropriate empirical expressions from [2].Table 1.Fitting parameters used for modeling calculations1. Band gap, eV, T=77 K       0.1302. Cutoff wavelength, l æ, m m, T=82K             10.3–0.23. Intrinsic carrier concentration,    ni, cm-3, T =77 K                                           4.87× 10124. Trap density, Nt, cm-3                                    1013-10165. Capture rate, Cp, cm3/s                                  10-7-10-66. Trap energy, Et, eV                                        0.72 EgMeasured and calculated current-voltage characteristicsare shown in Fig.2 (a,b). In calculations several componentswere taken into account: bulk diffusion current, generationcurrent in the depletion region and TAT current. The for-mulae used for calculations of each current component arebriefly discussed in Appendix. In calculation of TAT cur-rent thermal and tunnel transitions from the valence bandto deep defect states in the gap followed by tunnel transi-tions to the conduction band were taken into account. Thedark current caused by band-to-band tunneling was not takeninto account because of investigations were mainly per-formed at zero and low reverse bias voltage (U £  0.1 V).Surface leakage current seems to be negligibly small in thesephotodiodes (see Appendix).It is seen from Fig.2 that the photodiodes with differenttype of current-voltage I-U characteristics were investigated.In the photodiode with the lowest values of the reverse cur-rent (curve 1in Fig.2(a)) the tendency to its saturation atreverse bias is clearly seen. With increasing the reverse cur-rent this tendency diminishes (curve 2). Further increasingof the current is accompanied by appearance of the softbreakdown characteristic (curve 3). Sharp soft breakdowncharacteristics were observed in several photodiodes, Fig.2(b).Typical dependencies of the R0A vs. voltage bias U (R0is the differential resistance at zero bias, A is the junctionarea) for the photodiodes investigated are shown in2 4 6 8 10 121.00.80.60.40.20.0W ave le ng th , m icro n sSensitivity, arb.un.I, x10  A-6Fig.1. Normalized photoresponse spectra of implanted n+-p photodiodesHg1-xCdxTe (x @  0.22) at 82 K.I, x10  A-6Fig.2. Measured and calculated current-voltage characteristics of n+-pphotodiodes at 77 K. The dots represent measured data and the solidlines represent theory. The fit was obtained for Nt = 1.2 × 1013; 2 × 1014;4 × 1014 for curves 1-3 (a), respectively. The values Et = 0.72Eg,Cp = 8× 10-7 and Cn = 0.01Cp were the same for all three curves. To fitexperimental and calculated curves in (b) trap density was taken tobe  Nt =3.2 × 1015; 4.8 × 1015 cm-3, for curve 1 and 2, respectively. Also, thevalues Et = 0.72Eg, Cp = 1× 10-7 and Cn = 0.01Cp were the same for thesecurves.baZ.F. Ivasiv et al.: Noise spectra and dark current investigations...23SQO, 2(3), 1999Fig. 3 (a,b). At zero bias the R0A value less than 1.0 W· cm2has been found to be typical for the photodiodes in whichsoft breakdown was observed. Note that maximum in R0Avs. U curves is shifted towards zero bias in thesephotodiodes. The parameters used in calculations I-U andR0A-U characteristics are listed in Table 1.Noise power spectral density measurements were per-formed in the frequency range of 0.7‚ 104 Hz for the samephotodiodes in which the  dark current  was  measured. Typi-cal noise spectra measured at 77 K are shown in Fig. 4 to-gether with the calculated curves. As one can see, each noisecurve consist of frequency-dependent as well as frequency-independent parts which can be attributed to 1/f noise and togeneration-recombination noise, respectively. The transitionregion between them is rather narrow and it shifts towardsthe higher frequency as the dark current increases.From the measured and calculated data shown inFig.2(a,b) one can conclude the following. The best fit wasobtained for energy of deep defect states Et @  0.72Eg (Et ismeasured from the top of the valence band). This valuecorrelate well with previously observed one Et @  0.75Eg[7] in photodiodes prepared by boron implantation to bulkmaterial. Also this value is in agreement with the results ofDLTS measurements in undoped HgCdTe crystals [16]. Itseems that these centers are introduced into the bulk ofHgCdTe materials during their preparation and can be at-tributed to different states of the Hg vacancies [7].Some contradiction exists in the concentration ofdeep defect centers with previously published values Nt== 0.1-10 NA [7], where NA is the shallow acceptor concentra-tion (NA is approximately equal to the hole concentration).The values of Nt obtained in present work range  from ~1013cm-3  to  ~1016 cm-3  in  photodiodes  with  NA @  1016 cm-3.This contradiction may be explained as follows. The con-centration of shallow acceptors as well as deep defects inHgCdTe implanted photodiodes is not uniform in the re-gion adjacent to n+-p-junction. Nemirovsky et al. [4] havefound that n+-p-junctions can be formed with low concen-tration of shallow acceptors near the junction. Since themeasurements were performed at zero and low reverse biasvoltage the concentration Nt derived from the fitting proce-dure  can   be  smaller than in the bulk (e.g., in implantedHg1-xCdxTe (x@ 0.22) photodiodes investigated previously[7] Nt reached the bulk value at reverse bias U@ 0.6V).The capture rates found at this study Cp » 10-7‚ 10-6 cm3/sand Cn = (0.1‚ 0.01)Cp differ much from the result previ-ously obtained for Cp= (10-10‚ 10-9) and Cn = (10 ‚ 100) Cp[8,14]. It should be pointed out that in these studies deepdefect centers in p-HgCdTe material were assumed to bedonor-like type. However, in present investigation the bestfit was obtained for acceptor-like centers in the gap. Thisresult are in accordance with the results of comprehensivestudy performed by Rosenfeld and Bahir [7].The correlation between 1/f noise and tunneling darkcurrent was previously observed by Nemirovsky et. al. [4]at rather high reverse bias voltages U ‡ 200 mV. In presentinvestigation the correlation is revealed at low reverse biasU £  100 mV. The result obtained seems to be importantbecause of HgCdTe photodiodes are mainly used at zeroand low reverse bias voltage operating conditions [1]. Thecalculation of the 1/f noise was performed by the formula(A6) [4]. To fit experimental and calculated data the con-stant a  was assumed to lie in the range from 10-8 to 10-7 .These values agree with the value a  = 10-7 found in [4].Because of the increase of the dark current in the investi-gated photodiodes is related with the tunneling through deepdefect states in the gap, one can assume the origin of bothphenomena to be the same. Also it should be pointed outthat no correlation has been observed between GR currentin the depletion region and 1/f noise.The mechanism of 1/f noise in diodes with dominanttrap-assisted tunneling current is unknown yet. One possi-bility of its origin has been pointed out in [14]. The funda-mental fluctuations in the process rates can exist due to theinfrared divergent coupling of free carriers to low-frequencyphotons and other infraquanta.R  A, cmW02Fig.3. Typical resistance-area versus bias voltage characteristics for several photodiodes at 77 K. The designations are the same as in Fig.2a (curve 1)and Fig.2b (curves 1 and 2).a bR  A, cmW02Z.F. Ivasiv et al.: Noise spectra and dark current investigations...24 SQO, 2(3), 1999ConclusionDark current have been measured and calculated in n+-p-type Hg1-xCdxTe (x @  0.22) photodiodes at 77 K. It has beenfound that generation in the depletion region and tunnelingthrough deep defect states in the gap are dominant carriertransport mechanisms at low reverse bias voltage. From thefitting procedure the density of the tunneling centersNt  = 1013 –1015 cm-3 and their energy E t @  0.72Eg has beenestimated. The 1/f noise measured at low reverse bias canbe attributed to the tunneling dark current through deepdefect states.References1. C.T.Elliot and N.T.Gordon, Infrared Detectors, in Handbook on Semi-conductors, Vol.4, pp.841-936, Ed. by C.Hilsum, North-Holland,Amsterdam (1993).2. J.R.Lowney, D.G.Seiler, C.L.Littler and I.T.Yoon, Intrinsic carrierconcentration of narrow gap mercury cadmium telluride// J.Appl.Phys.71, pp.1253-1258 (1992).3. J.Y.Wang, Effect of trap tunneling on the performance of long-wave-length Hg1-xCdxTe photodiodes // IEEE Trans. on Electron DevicesED - 27 (1), pp.48-57 (1980).4. Y.Nemirovsky, D.Rosenfeld, R.Adar, and A.Kornfeld, Tunneling anddark currents in HgCdTe photodiodes // J.Vac.Sci.Technology A7 (2),pp.528-535 (1989).5. N.L.Bazhenov, S.I.Gasanov and V.I.Ivanov-Omskii, Excess currentsin narrow-gap Hg1-xCdxTe p-n junctions // Infrared Phys. 34 (1),pp.37-41 (1993).6. Akira Ajisawa and Naoki Oda, Improvement in HgCdTe diode char-acteristics by low temperature post-implantation annealing //J.Electron. Materials 24 (9), pp.1105-1111 (1995).7. D.Rosenfeld and G.Bahir, A model for the trap-assisted tunnelingmechanism in diffused n-p and implanted n+-p HgCdTe photodiodes// IEEE Trans. on Electron Dev. 39 (7), pp.1638-1645 (1992).8. Y.Nemirovsky, R.Fastow, M.Meyassed, and A.Unikovsky, Trappingeffects in HgCdTe //J.Vac.Sci.Technol. B.9 (3), pp.1829-1839 (1991).9. R.E.Dewames, J.G.Pasko, E.S.Yao, A.H.B.Vanderwyck, andG.M.Williams, Dark current generation mechanisms and spectral noisecarrent in long-wavelength infrared photodiodes // J.Vac.Sci.Technol.A6, p.2655 (1988).10. Tian, Ji Zhongyun, Lei Ruoxi, Gu Shengqiong, Boqi, Measurementsof noise of photovoltaic HgCdTe and InSb infrared detector //Proc.SPIE 2894, pp.174-179 (1996).11. Wenmu He and Zeynep Celik-Butler, Temperature dependence of 1/fnoise in Hg1-xCdxTe MIS infrared detectors // Trans. Electron Dev.42 (1), pp.160-165 (1995).12. R.Schiebel, D.Bartolomew, M.Bevan, R.S.List, and M.Ohlson, Noiseand material defects in HgCdTe diodes // Journal of Elecrtonic Ma-terials 24 (9), pp.1299-1203 (1995).13. S.R.Babu, K.Hu, Manthripragada et al., Improved HgCdTe detectorswith novel anti-reflection coating //Proc. SPIE 2816, pp.84-89 (1995).14. Wenmu He and Zeynep Celik-Butler, 1/f noise and dark current com-ponents in HgCdTe MIS infrared detectors // Solid - State Electron-ics  1 (1), pp.127-132 (1996).15. W.A.Radford, and C.E.Jones, 1/f noise in ion-implanted and double-layer epitaxial HgCdTe photodiodes // J.Vac.Sci.Technol. A3 (1),pp.183-188 (1985).16. C.E.Jones, V.Nair, and D.L.Polla, Generation-recombination centersin p-type HgCdTe // Appl. Phys. Lett. 39 (3), pp. 248-250 (1981).AppendixThe dark current in the photodiodes investigated can begiven by the sum of several components, namelysurfBTBTATGRD JJJJJJ ++++= ,                        (A1)Fig. 4. Measured (dots) and calculated (solid lines) noise spectra for the photodiodes with R0A = 1.0 W× cm2 (1) and R0A = 0.3 W× cm2 (2) at 77 K. Thereference shot noise level (2qJ)1/2  at reverse bias 10 mV is 1.7× 10-13 and 7.92× 10-13 A/Hz-1/2, respectively.U  ,V/Hz01000 10000100 f , H z10U  ,V/Hzc1000100f , H z1011 2Z.F. Ivasiv et al.: Noise spectra and dark current investigations...25SQO, 2(3), 1999were JD is diffusion current, JGR is the depletion genera-tion-recombination current, JTAT  is the trap-assisted cur-rent, JBTB is the band-to-band tunneling current and Jsurf  isthe surface leakage current.For asymmetric junctions the first two components aregiven by expressions:2102œßøŒºØ××=nniDqTkpnqJtm,                                              (A2)10102npWqnJpniRGtt += ,                                                   (A3)where p0 is the hole concentration, m n and t n are the elec-tron mobility and the minority carrier lifetime in the bulk,where ( ) 10-= tnn Nct , ( ) 10 -= tpp Nct , cn and cp are thecapture coefficients for electrons and holes, respectively,Nt is the trap density, p1 = Nv × exp(-Et/kT), n1 = Nc× exp(-Eg++Et/kT), Nv and Nc are the effective density of states in thevalence and conduction bands, W is the depletion regionwidth, Et is the trap energy measured from the valence bandedge. The second and third terms in (A1) were intensivelydiscussed earlier [1]. In this work, the appropriate equa-tions for JTAT current were taken from [7]. The trap-assistedtunneling current is given by÷÷łöççŁæ++=ccvvptTAT nnpcWqNJww111,                     (A4)were w cNc and w vNv are the tunneling rates .The surface leakage current caused by the fast surfacestates is given by [14]2SnqJ isurf = ,                                                              (A5)where S is the surface recombination velocity. In properly pre-pared photodiodes the value of S does not exceed 100 cm/s[14]. As one can see from the formula (5), the contributionof the surface leakage current to the dark current can beneglected in our case. In order this current to be dominant,the value S should be higher than 103 cm/s. However, sohigh values of S were not observed in the diodes investi-gated.The calculation of the 1/f noise current was carried outwithJn = ÷÷łöççŁæ=fJTATa ,                                                                                 (A6)where JTAT current was calculated using (A4).

Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes

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Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)