Uncooled Detection. Chapter Thermal Detection

Transcription

1 Chapter 6 Uncled Detectin The ultimate IRFPA will perate at rm temperature and exhibit backgrund limited perfrmance (BLIP), regardless f the perating cutff wavelength. Currently the primary vehicle fr rm-temperature IR detectin is the thermal detectr, but it is essentially limited t the detectin f LWIR radiatin, at relatively slw frame rates, r integratin times. These limitatins were discussed briefly in Sec. 2.2 fr the ptimized perfect thermal detectr. The successful rm-temperature detectin f the cmplete IR spectrum, with BLIP perfrmance, will require the utilizatin f ther detectr cncepts. The HOT detectr f Ellitt and Ashley [80] is just such a cncept, the ptential f which is limited entirely by the quality f the direct bandgap semicnductr utilized t detect the IR radiatin, and the cntacts and passivatin applied t that material. The HgCdTe HOT detectr cncept is discussed here, tgether with a cnsideratin f relevant material prperties within the active vlume f the device fr successful rm-temperature peratin, taking int accunt the all-imprtant issue f Shckley Read (S-R) centers in HgCdTe. The HOT cncept is nt limited t the HgCdTe materials system alne, but can be implemented in any direct bandgap semicnductr material, r its bandgap engineered equivalent. 6.1 Thermal Detectin The principle f the thermal detectr is illustrated in Fig The detectr element, f heat capacity C th, is thermally cupled t a heat sink by a cnductance G th. The thermal detectr element is micrmachined nt the surface f a silicn readut integrated circuit (ROIC) and the resulting pedestal is cnnected t the ROIC by thin metal buss leads. The pedestal can be fashined frm any ne f a number f temperature-sensitive materials that are prcess cmpatible with silicn ROIC prcessing. We will assume here that the temperature sensitivity f the element is sufficiently large that all nise surces ther than temperature fluctuatins f the detectr element can be ignred. As discussed in Sec. 2.2, the pwer fluctuatin f the element is then given by ΔW n =[4kT 2 G th Δf ] 1/2, where Δf is the nise bandwidth, which translates int a nise temperature fluctuatin, ΔT n = [4kT 2 R th Δf ] 1/2. The detectr signal is 93

2 94 M. A. Kinch C th Heat sink T G th = 1 R th T + ΔT Signal phtns Figure 6.1 IR thermal detectr. given by ΔT s = ηr th (dp/dt) λ ΔTA/(1 + 4F 2 ), where η is the quantum efficiency, (dp/dt) λ the differential change in radiated pwer per scene temperature change in the spectral regin f interest, A the detectr area, and F the F/#f the ptical system. Thus the detectivity f the detectr, defined as [signal/flux pwer][a 1/2 /(nise/δf 1/2 )], is given by D =[η 2 A/(4kT 2 G th )] 1/2. (6.1) If G th is radiatively limited, then G th = 4PA/T, where P = σ B T 4. Thus fr η = 1,D = cm Hz 1/2 /W, and is independent f area. This value f D is cmparable t the theretical limit f a LWIR phtn detectr, but prvides nly a limited insight int the suitability f the thermal detectr fr specific IR systems. As pinted ut in Sec. 2.2, signal and spectral bandwidth requirements are als imprtant, and t this end it is meaningful t cnsider the available nise equivalent temperature difference, NEΔT. Crrelated duble sampling [4] f the bandwidth-limited temperature fluctuatin, with an integratin time τ int, gives ΔT n =[2kT 2 (1 exp( τ int /R th C th ))/C th ] 1/2, where we have assumed Δf = 1/4R th C th. Equating this t a signal temperature change given by ΔT s = (dp/dt) λ ΔTA(1 exp( τ int /R th C th ))(1 + 4F 2 )R th,gives NEΔT =[2kT 2 /C th /(1 exp( τ int /R th C th ))] 1/2 (1 + 4F 2 )/(dp/dt) λ /AR th. (6.2) Image smearing cnsideratins require τ int 2R th C th, resulting in NEΔT =[2kT 2 /C th ] 1/2 (1 + 4F 2 )/(dp/dt) λ /AR th. Fr R th <R rad NEΔT [2kT 2 C th /τ 2 int ]1/2 (1 + 4F 2 )/(dp/dt) λ /A, (6.3) and fr R th = R rad NEΔT [32k/C th ] 1/2 (1 + 4F 2 )/[(dp/dt) λ /P ]. (6.4) This predicted dependence f NEΔT n integratin time, τ int, fr LWIR, and MWIR detectin is shwn in Fig. 6.2, fr a 25-μm pixel, perating with F/1 ptics, and η = 1.

3 Uncled Detectin LWIR 10 1 MWIR 300 Å 3000 Å thick NEΔT (K) Å 3000 Å thick C th = τ ιnt R rad NEΔT (K) C th = τ ιnt R rad Integratin time (s) Integratin time (s) Figure 6.2 NEΔT vs. integratin time fr perfect LWIR, MWIR thermal detectrs, fr 25 μm pixels, F/1 ptics, and η = 1. The curves labeled C th = τ int /R rad represent the ptimum mde f detectr peratin, assuming that (1) the detectr thermal cnductance is limited by radiatin t its surrundings, and (2) the thermal capacitance can always be made small enugh t satisfy the requirement f τ int R rad C th. In practice, the available thermal capacitance is limited by the thickness f the blmeter pedestal and material specific heat. Realistic values fr blmeter thickness are in the range Å, and the vlumetric specific heat is 1 2 J/cm 3 /K. Examples f fixed blmeter thickness values f 300Å, and 3000Å are shwn in Fig. 6.2, encmpassing bth ranges f R th described by Eqs. (3) and (4). Blmeters tday are typically 3000Å thick, and their perfrmance appraches the values shwn in Fig. 6.2 fr LWIR at relatively lng integratin times, such as ms. Hwever, it is apparent that high perfrmance in a snapsht mde (τ int 2 ms) will nt be realized. At best, NEΔT values can apprach 0.1 K fr LWIR under these cnditins. The NEΔT achieved by thermal detectrs in the MWIR is medicre at best. This is due t the lw value f (dp/dt) λ available fr signal in this spectral band, relative t the temperature nise, which is determined by all f the black-bdy flux absrbed by the blmeter. Theretically, this situatin culd be imprved by resnant tuning f the pedestal t its surrundings fr the same IR spectral band as the desired signal; hwever, this wuld nt nly be difficult but, if successful, wuld als increase R rad significantly, and result in cnsiderably lnger thermal time cnstants. Fr slwer ptics the situatin is exacerbated, varying as (F/#) 2. Thinner detectr elements imprve the NEΔT as t 1/ Phtn Detectin HOT detectr thery The high perating temperature (HOT) phtn detectr cncept, first prpsed by Ellitt and Ashley, is depicted in Fig. 6.3 fr an n + /π/p + architecture, where π

4 96 M. A. Kinch J Φ P + J p+ Π J SR J n+ J A7 N + n i /τ SR J = J Φ + J p+ + J A7 + J SR + J n+ p/τ A7 p N r n i /τ SR p/τ A7 p n n/τ A1 n i /τ SR t << L d, diffusin length Figure 6.3 HOT detectr cncept with dark current surces. designates an intrinsic regin cntaining a p-type backgrund dpant. The intrinsic IR absrbing vlume is cnnected t bth a minrity carrier cntact and a majrity carrier cntact. The gemetry f the active vlume is small relative t a minrity carrier diffusin length. It is perated in strng nn-equilibrium by reverse biasing the minrity carrier cntact t cmpletely extract all f the intrinsically generated minrity carriers. Charge neutrality in the active vlume is vilated, creating an electric field t sweep ut the intrinsically generated majrity carriers until the majrity carrier cncentratin equals the backgrund dpant cncentratin. The active vlume thus cnsists f a depletin regin, with a width determined by the dping cncentratin and applied bias, and a field-free regin with a majrity carrier cncentratin determined by backgrund dping. The eliminatin f minrity carriers thrughut the device means that minrity carrier recmbinatin is a nn-issue, and dark current is determined entirely by generatin mechanisms within the active vlume f the device, plus pssible cmpnents assciated with injectin frm the cntact and surface regins. The relevant dark current mechanisms within the active vlume f the HOT detectr are (1) Auger generatin assciated with majrity carriers in the field-free regin, and (2) thermal generatin thrugh S-R centers thrughut the whle nn-equilibrium vlume. As discussed earlier, the arguments f Humphreys suggest that radiative mechanisms are nt imprtant in this device. Auger generatin recmbinatin in direct-gap semicnductrs is a wellunderstd phenmenn invlving the interactin f three carriers. The Auger1

5 Uncled Detectin 97 lifetime in n-type material is due t the interactin f tw electrns and a heavy hle and is given by τ A1 = 2τ Ai1 n 2 i /n(n + p), (6.5) τ Ai1 = ε 2 (1 + μ) 1/2 (1 + 2μ)/((m e /m ) F 1 F 2 2 (kt/e g ) 3/2 ) [exp((1 + 2μ)E g /(1 + μ)kt)], (6.6) and μ = (m e /m h ). τ Ai1 is defined as the intrinsic Auger1 lifetime. The largest uncertainty in the Auger mdel lies in the calculatin f the verlap integral F 1 F 2, and empirical values, prvided by a cmparisn t experimental data [81], are typically used. Fr HgCdTe, the empirical value fr F 1 F 2 is In p-type material Auger recmbinatin invlves tw hles and an electrn and is referred t as Auger7. Calculatins by Casselman [82] fr HgCdTe give τ Ai7 6τ Ai1, althugh experimental data wuld indicate that it may be smewhat higher, namely 10τ Ai1. This lnger lifetime suggests that p-hgcdte is the material f chice fr the HOT detectr active vlume with regard t ultimate dark current perfrmance. The Auger generatin rate/unit vlume in the field-free regin f the HOT detectr, under nn-equilibrium cnditins, is prprtinal t the majrity carrier cncentratin, and is given by G A7 = n a /2τ Ai7. (6.7) In p-hgcdte under nn-equilibrium cnditins, the minrity carrier generatin rate/unit vlume assciated with an S-R center f density N r, lcated E r frm the cnductin band, is given by G SR = n 2 i /[τ prn 1 + τ nr (p + p 1 )], n 1r = N c exp( E r q/kt), (6.8) p 1r = N v exp( (E g E r )q/kt), where τ nr = 1/γ n N r, τ pr = 1/γ p N r. γ n and γ v are the capture cefficients fr electrns and hles int N r, and N c, and N v are the cnductin-band and valence-band density f states respectively. In the field-free regin f the detectr p = n a, whereas in the depletin regin p =0. The dark current density in a simple planar gemetry HOT detectr f thickness t, with a depletin regin width W, is thus given by J = qn a (t W)/2τ Ai7 + r qn 2 i {W/(τ prn 1r + τ nr p 1r ) + (t W) /[τ pr n 1r + τ nr (n a + p 1r )]}, (6.9) where the summatin is carried ut ver all the S-R centers in the device. The materials technlgy develpment required fr HOT detectr ptimizatin is clear: Reduce the majrity carrier cncentratin in the active vlume f the device, and Reduce the density f S-R centers in the active vlume t a minimum.

6 98 M. A. Kinch Reducing the majrity carrier cncentratin is beneficial in tw respects. It nt nly results in a reductin in Auger generatin rate, but als results in a larger depletin regin fr the dide at a cnstant bias, and hence a smaller required field-free vlume. Fr an applied bias f 1V, a depletin regin width f 5 μm is achieved fr dping cncentratins f cm 3. Such a depletin regin width will prvide ample absrptin f the IR radiatin, with gd quantum efficiency, and cmpletely eliminates Auger generatin due t the ttal absence f majrity carriers. The device is then essentially a P-I-N dide, and either p-type r n-type material can be emplyed fr the intrinsic regin. Interestingly enugh, dping cncentratins f cm 3 are already available using indium backgrund dping in LPE layers grwn in Te-rich melts. A P-I-N dide is shwn in Fig. 6.4, where the p + cntact is shwn as wide-bandgap HgCdTe, t avid the pssibility f minrity carrier injectin int the intrinsic active vlume f the HOT device. The band-filling in the n + cntact is assumed deep enugh t inhibit Auger transitins in that regin, and hence minrity carrier generatin and injectin. A wide-bandgap n + regin culd als be utilized fr this purpse. The dark current in such a device is given by J d = r qn 2 i [W/(τ prn 1r + τ nr p 1r ], (6.10) where the summatin is ver all the S-R centers, and the depletin width is equal t the device width. The mst efficient S-R centers fr thermal generatin are nrmally lcated clse t the intrinsic energy level in the bandgap, fr τ pr τ nr. In HgCdTe, the intrinsic level is lcated far abve mid-gap due t the fact that the cnductin band effective mass is much less than the heavy hle mass. Thus, fr the well-designed P-I-N device, perating under reverse bias, the dark current is determined slely by the density and distributin f S-R centers in the bandgap f the active vlume f the dide, and the success r failure f the HOT detectr depends entirely upn the management f these bandgap states. It is f interest t ascertain the magnitude f N r required t enable backgrund-limited perfrmance at rm temperature fr a particular cutff wavelength. Wide gap p + p V J d = Σn i 2 /[n 1r τ pr + p 1r τ nr ] n n + E r Figure 6.4 A reverse-biased P-I-N Dide.

7 Uncled Detectin 99 Fr neutral S-R centers this requires that G = Wn i γn r < Φ B, where we have assumed unit quantum efficiency. Further assuming that γ 10 9 cm 3 /s, which is typical f neutral S-R centers, W = cm, then, fr LWIR HgCdTe at 295 K, n i cm 3, Φ B = /cm 2 /s, and the requirement n N r is < cm 3. This refers t the sum ttal f S-R centers in the material. The maximum allwed density f S-R centers fr BLIP peratin f a P-I-N dide at 295 K, as a functin f cutff wavelength, is shwn in Fig Under these cnditins, in the imprtant MWIR/LWIR spectral regins, it is apparent that N r needs t be <10 12 cm 3, which represents a significant materials challenge, but with enrmus ptential payff. Fr a recmbinatin cefficient f cm 3 /s the requirement n N r is relaxed by an rder f magnitude. The presence f S-R centers away frm the intrinsic energy level will mdify the predictins f Fig. 6.5, but nt t significantly, due t the magnitude f kt relative t the bandgaps invlved. This requirement n N r is at the limit f sensitivity fr mst impurity detectin techniques such as glw discharge mass spectrscpy (GDMS), and secndary in mass spectrscpy (SIMS). Knwledge f S-R centers in HgCdTe is limited, but imprving. As discussed in Sec , metal vacancies and extrinsic p-type dpants have been characterized by Hall and minrity carrier lifetime data ver a limited range f Hg 1 x Cd x Te x-values, primarily in the range f 0.2 <x <0.3 (LWIR t MWIR). Metal vacancies are characterized by a dnr-like S-R center lcated 30mV frm the cnductin band fr cmpsitins in the 0.2 <x<0.3 range. Lifetime measurements indicate an electrn lifetime fr the vacancy given by τ vac = [(n + n 1 )/p]/n vac. (6.11) S-R center density (cm 3 ) γ = 1e-9 W = 5 μm Cut ff wavelength (μm) Figure 6.5 Maximum density f allwed S-R centers in a P-I-N dide fr BLIP at 295 K.