Observation of below band gap photoconductivity in mercury cadmium telluride

Per~amon

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OBSERVATION OF BELOW BAND GAP PHOTOCONDUCTIVITY IN MERCURY CADMIUM TELLURIDE R. K. SHARMA, D. VERMA and B. B. SHARMA Solid State Physics

Laboratory, (Received

Lucknow

Road,

7 January

1994)

Delhi-l 10054, India

Abstract-Significant photoresponse beyond the expected cutoff wavelengthshas been observed

in mercury cadmium telluride (MCT) photoconductive detectors fabricated for ambient temperature operations for applications in 3-5 pm wavelength range. The results have been explained in light of the intervalence excitations from heavy to light hole bands resulting in enhanced conductivity due to a large difference in the light and heavy hole mobilities in MCT. The overall relative photoconductive response has been modelled in terms of both band-to-band and intervalence contributions to the photoconductivity.

INTRODUCTION

(PC) detectors Hg, .YCd,Te [mercury cadmium telluride (MCT)] based uncooled photoconductive are suitable for a variety of applications in the mid as well as far-infrared ranges. Recent developments in their fabrication technology have made it possible to achieve a fairly high performance level. Particularly in the MWIR (Mid Wavelength Infra-Red) detection range their performance has been very close to the ultimate limit for this class of detectors.“-“’ However, detectors fabricated for detection in the LWIR (Long Wavelength Infra-Red) range have serious limitations to their performance due to a very low excess carrier lifetime and high thermal noise arising as a consequence of a very low (- 0.1 eV) band gap of the material used.(‘,2) In the present communication we report on our observation of significant LWIR photoresponse beyond the expected cutoff wavelength in some uncooled MCT photoconductive detectors fabricated for MWIR applications. This below band gap photoresponse is proposed to be due to p-type photoconductivity,(4) that is, the enhancement in the conductivity of holes excited to the tight hole band as a consequence of pronounced intervalence absorption in MCT.“’ The observed photoconductive spectral response has been modelled in terms of the contributions from band-to-band as well as intervalence excitations.

EXPERIMENTAL

DETAILS

MCT crystals used for the fabrication of devices for this work were grown by the Solid State Recrystallization Technique. (6) The grown ingots were annealed at 300°C under vacuum(7’8) for adjusting the concentration of Hg-vacancies to obtain the electrical characteristics suitable for fabricating high response uncooled pc detectors. These ingots were then sliced and the wafers polished mechanically and chemomechanically to get the surface finish. The crystals so obtained normally show very uniform and reproducible electrica characteristics.~*~ The composition of crystals used for the fabrication of devices reported in this work was x - 0.27 as determined by FTIR spectrometry. The crystals showed mixed conduction at room temperature. However, with the aid of low temperature and variable magnetic field Hall datac9’ the room temperature hole and electron concentrations were determined to be - 5(1016) cm-3 and - 7( 1015) cme3, respectively. The structure of pc detectors fabricated for the present work is shown in Fig. 1. Elements having 2 x 5 mm* area were initially cut from the wafers obtained through the above process. After giving 673

R. K. SHARMA ef rd.

CONTACTING 8ONDiN~

PAD

WIRE

SUBSTRATE Fig.

I Structure

ACTIVE

M
elements

AREA

EPOXY

fabricated

for the present

work

a deep etch with a 2% Brz in CH,OH solution, one surface was mounted on a sapphire/silicon substrate with epoxy. These elements were then lapped/polished down to a thickness of - 60 pm. After protecting the contacting pads with photoresist, the active area of the detectors was thinned down to -25-30 jlrn through etching with 2% Br, in CH,OH solution. The electrical connections from the contacting pads were taken out at this stage. The spectral photoresponse of these detectors in the 3-12 pm range was obtained with the help of an IR prism monochromator and a globar source. The measured spectral response was normalized against the response of a linear thermal detector to the monochromated radiation. The detectors were also tested for response to a 500 K black body and a COz laser source. EXPERIMENTAL

RESULTS

Figure 2 shows the pc responsivity spectra of two of the typical detectors (viz. A and B) showing significant response in the LWTR range. The peak responsivity is observed around 4.5 i&m as expected but instead of a steep fali in responsivity for higher wavelengths a plateau is observed

G-l

2 g

40

a id Lx 20 i 4

6 WAVELENGTH

8

10

12

t MiCRONS I

Fig. 2. The relative photoconductive response of detectors .4 and 8. # and 0 are experimental for A and B, respectively. The lines correspond to the theoretical fit.

points

Below band gap photoconductivity

675

Table I. Black body (500 K) response characteristics of MCT photoconductive detectors, for which the relative response data is presented in Fig. 2 Detector svmbol

Thickness

ffim)

(V/W)

D* (sOOK, I kHz, 1Hz) (cm W - ’ Hz&‘~)

A B

30 25

16.1 17.5

3.8 (108) I.1 (109)

Responsivityt

tMeasured in constant current mode but for bias currents corresponding to an applied electric field - 8 V/cm.

in 5-12 pm wavelength range. The responsivity in this range is about 20% of the peak value. The characteristics of the black body response of these detectors is summarized in Table 1. The black body responsivity values indicate a fairly good response in the LWIR range as well. The response of one of these detectors to a CO, laser source is shown in Fig. 3 as responsivity vs bias current plot. The observed responsivity value is of the order of a few volts/watt and is comparable with the values obtained for the uncooled LWIR MCT detectors.@’ PHOTOCONDUCTIVE

RESPONSE

MODEL

The intervalence absorption is a dominant mechanism near and below the absorption edge in MCT (x - 0.2-0.3).“) The transition of holes from heavy hole band to the light hole band as a consequence of this effect is expected to give rise to significant photoconductivity as mobility of light holes is about two orders of magnitude higher than that of heavy holes in MCT.c9’ Our modelling of the observed anomolous LWTR photoresponse in MCT(x - 0.27) material is based on the same effect and as can be seen in Fig. 2 the fit obtained for the experimental data is quite good. Before presenting the expression for the photoresponse, it would, however, be useful to examine the IR absorption characteristics of the MCT material used in this work in the wavelength range near and below its absorption edge. Figure 4 shows the absorption coefficient (a,.P) of this material in 5-l 5 pm range as deduced from the FTIR absorption spectrum using the standard expression.“) The contributions of three main absorption mechanisms viz. the band-to-band, the intervalence and

01

0

1

I

55

t0

I

15

BIAS (URRENT

I

t

t

20

25

30

(mAI-

Fig. 3. Responsivity of detector B to a CO, laser source at 10.6pm wavelength vs bias current

R. K. SHARMA ei 01.

5

6

7

8

9

10

WAVELENGTH

11

Q.

13

14

1s

(vml-

Fig. 4. IR absorption characteristics of MCT material used in this work m 5 IS Elm wavelength range. z,,~ is the absorption coefficient deduced from FTiR spectrum, and xHB. q, and a,, arc the absorption coeficients, calculated as per Appendix A. corresponding to bated-to-end, intervalence and free carrier mechanisms, respectively.

the free carrier absorption, i.e. xBB, c(,v and xFc, as obtained from the theoreticaljsemiempirical expressions summarized in Appendix A, are also shown there. The extinction of IR radiation, due signi~cantl~ to the to scattering from Te precipitates or HgTe pockets”01 also contributes absorption spectrum. But, as there is no absorption through this mechanism its contribution is excluded. It is apparent, from Fig. 4, that the intervalence absorption contributes significantly in the wavelength range shown and one can qualitatively explain the observed photoresponse characteristics on its basis. In the shorter wavelength range the band-to-band absorption is predomit~ant and normal absorption edge is expected. The inter~falel~ce absorption is quite uniform and predominant in the longer wavelength range and a some-what uniform photoresponse is also expected. However, as the free carrier absorption also takes over slowly for higher wavelengths a slow decrease in the relative response is also expected. The responsivity improves with increasing number of photons per unit incident energy with increasing wavelength too. The model presented below takes into account all these factors discussed above. The current signal from a photoconducting detector element can be expressed as:“”

where rl is the quantum efficiency, P is the electronic charge, P is the optical power incident on the detector, /IV is the energy of incident photons, I’ is the drift velocity of charge carriers. r is the excess carrier lifetime and I is the length of detector element. In case of more than one charge carriers contributing to the phot~~cond~lcti\;~t~ the above equation can be written as:

Below band gap photoconductivity

where the subscripted

quantities

correspond

to the charge carriers

power absorbed to generate the jth type of charge carriers absorbed in the detector element, VP, through relation”*)

or simply

q, = (ol,/Ccr)~, where Ccl = aBB+ c(,v + +c,

611

of type j in general.

and can be related

is the total absorption

qiP is the

to the total power

coefficient.

Assuming that only the band-to-band (BB) and intervalence (IV) absorptions photoconductivity the expression for the current signal can be written as:

contribute

to the

i, = iBB+ i,, , =

&

(%3Pe7c

+

%/-4h7lhE

where E is the electric field applied to the photoconductor and, p, and ,u,~ are the mobilities of electrons and light holes, respectively. In the above expressions the heavy hole contributions (additive for BB and subtractive for IV mechanism) have been neglected due to their very low mobility as compared to that of electrons and light holes (,u~//J,,~, pLlh/pLhh - 50).‘9’ Now the voltage responsivity for a detector of resistance R, can be expressed as: R,

iSaMD eE~,Gh /. -= P A(a), hcl =

where

with i = (plh 71h )/(kz Also the term ~(2) for a detector

element

q(n) = (1 - r,)[l

of thickness

7e ).

t can be expressed

as:(*)

+ r2exp(-~~t)l[l -exp(-~~t)l [1 - Y,r2 exp( - 2Cat)]

here r, and r2 are the reflectances at the two surfaces The relative response could be obtained as:

of the detector

element.

The quantities in the denominator above represent the maximum value of the respective quantities. For fitting the experimental relative response data shown in Fig. 2, the parameters [ appearing in the above expression and x appearing in the expressions for GL’Swere used as fitting variables. The values of x for a reasonable fit were found to be -0.27 and -0.275 for the two detectors A and B, respectively. These values are almost within the spread of x observed in the starting MCT wafers. However, the value of parameter [ was -0.6 for both the detectors. This result indicates that, since pLlhand ~1, have the same order of magnitude, the two time-constants s,~ and 7, should also be comparable. The value of 7, has been found to be of - 300 ns.“) It is also possible to estimate the factor ,ulhrlh from the value of R, at a particular wavelength in the plateau region of photoresponse. Since the contribution of band-to-band photoconductivity is negligible in this range the above expression for responsivity simplifies to R, where I,, is the detector

bias current.

=

678

R.

K.

SHARMA

et al

Estimating from the value of Rj,for E. = 10.6 pm shown obtains plh t,,, - (I .3) 10 ~’ cm* V ‘. Taking p, - 6000 cm’ V’ is very close to the value of [ obtained in the fit.

in Fig. 3, at 10 mA bias current one SK’ one estimates that [ - 0.7 which

DISCUSSION

Apart from the intervalence absorption discussed in the previous section, there may be some other possible mechanisms responsible for below gap photoconductivity. They could mainly be related to the band tailing or the Anderson states expected in MCT due to high defect density.“3’ However they should generally result in an exponential absorption behavior’li’ and probably are the cause of exponential absorption edge (14’observed in this material. The other important mechanism could be electronic excitations from the impurity/defect states in the band gap to the conduction/ valence band. In MCT due to very low electron effective mass the donor states are expected to form a band which is well merged with the conduction band even at concentrations as low as - 10” cm 3(“) and are expected to contribute to the exponential edge only.“3’ The acceptor states, on the other hand, would remain unaffected even upto the concentrations of - IO” cm 3 due to high heavy hole effective mass (-0.5m,). The photoresponse due to these states is expected to have a well-defined wavelength cutoff”’ and not the type of flat response observed in the present study. Moreover, such transitions, if dominant, should show definite absorption peaks, which are not seen in the observed FTIR transmittance spectrum. The intervalence mechanism, on the other hand, gives a good account of the observed absorption behavior and also explains observed photoresponse. The time constant T,~is linked with the mechanism of restoration of thermal equilibrium between light and heavy holes, which gets disturbed by intervalence excitations. However, a normal band relaxation process involving phonon or ionized impurity scattering, as considered by Hattori et ~11.‘~’ in modelling ,u-type photoconductivity observed in case of germanium, does not explain our results. The corresponding relaxation time (T) can be estimated from the drift mobility (p) and the effective mass (m*) using relation p = ez/tTl*. Assuming that the band parameters for the conduction and light hole bands are not very different one finds T - lo- “-10~” s. while the estimated value of T,~ is - 10 ’ s as discussed earlier. The process controlling the time constant T,,, is, therefore, quite slow as compared to the band relaxation and may involve a sort of direct back transition from light to heavy hole band.@’ Although it is difficult to suggest an exhaustive model on the basis of limited data at present, it is only proposed that a back transition may be necessary because the electronic wavefunctions associated with the states in the two valence bands do not have identical symmetries. The two bands are degenerate at k = 0 (the band edge), but the states near the band edge in the light hole band would be heavily populated as the acceptor ionization energies in vacancy doped MCT crystals, the type used in the present work, have been found to be in the range of lo-15 meV”“’ and are much less than the thermal energy at ambient temperatures. Therefore. relaxation of light holes to the heavy hole band at the band edge may also not be favorable and they are held in the light hole band until a direct transition process. with a characteristic time constant -T,,,,switches them back. The back transition process may be phonon assisted and the dependence of T,,, on temperature, as well as on hole concentration, may throw some more light on the nature of this process. Further relevant studies are currently underway and the results thereof would be communicated later. An important aspect of the model discussed here is the possibility of fabricating thick detectors for use in LWIR range, since the intervalence absorption coefficient is generally quite low ( -20 cm ’ in the present case, giving, I/r value as - 500 pm). This will improve the detector performance in cases where surface recombination is a dominant factor in determining the detector performance, because the majority of excess carriers will be generated in the bulk of detectors. The optimum thickness could be calculated from a modified expression for responsivity involving surface recombination terms.

Below band gap photoconductivity

679

CONCLUSIONS Significant photoresponse in the LWIR range has been observed in uncooled MCT photoconductive detectors fabricated for MWIR applications. The phenomenon has been attributed to the intervalence excitation resulting in the increased conductivity due to the enhancement in the hole mobility (p-type photoconductivity). The work indicates the possibility of developing a new class of MCT detectors which could be useful in both MWIR and LWIR ranges. ~c~~o~,~e~g~~~f~-The authors are highly grateful to Professor G. C. Bhar (Bardwan University, India) for providing the photoresponse data of our detectors for CO? laser source. They are thankful to Mr V. K. Singh and Mr 1. P. Singh for their active help in crystal growth and device characterization, respectively, and to Dr R. D. S. Yadava for helpful discussion. They are also thankful to the Director SSPL for his permission to publish this work.

REFERENCES I. E. H. Putiey, Phys. Technol. 4, 202 (1973). Solid-State Eiectron. 33, 351 (1990). 2. 1. Niedziela and J. Piotrowski, 3. D. Verma, J. P. Singh, R. K. Sharma and B. B. Sharma, Proc. VI In!. ~ur~~hop on the Physics of Semiconductor Devices, p. 649, New Delhi, 2-6 December (1991). 4. H. Hattori, 0. Fujitani, and M. Umeno, J. P/tys. Sot. &I. 36, 485 (1974). and D. A. Nelson, J. appl. Phys. 54, 2041 (1983). 5. J. A. Mrockzkowski 6. B. B. Sharma, R. K. Sharma, V. K. Singh and N. K. Nayyar, Proc. nom1 Seminar on Recent Trends in IR Decices and Their Dszfence Applications, p. p-4, S.S.P.L., Delhi (1989). 7. G. L. Destefanis, J. Crystal Growth 86, 700 (1988). work. 8. R. K. Sharma, V. K. Singh and B. B. Sharma, Unpublished 9. M. C. Gold and D. A. Nelson, J. Vat. Sci. Technol. A4, 2040 (1986). Phys. Status Sotidi (a) 94, 573 (1986). 10. Qian Dingrong, 11. R. H. Kingston. Detection of OpricaL and Infrared Radiation, p. 54. Springer, Berlin (1978). 12. J. Djuric, B. Livada, V. Jovic, M. Sniljanic, M. Matic and Z. Lazic, Infrared Phys. 29, 1 (1989). 13. B. 1. Shklowskii and E. L. Efros, Electronic Properties of Doped Semicondu~lors, Chap. 2. Springer, Berlin (1984). 14. E. Finkman and S. E. Schacham, 1. appl. Phys. 56, 2896 (1984). 15. R. D. S. Yadava and A. V. R. Warrier, f. Electron. Mater 18, 537 (1989), P. Capper, C. L. Jones, J. J. Gosney and W. G. Coates, Semiconducror Sci. Technol. 5, 854 (1990). 16. 1. Kenworthy, 17. Juh-hao Chu. Zheng-yu Mi and Ding-yuan jang, J. appl. Phys. 71, 3955 (1992). 18. C. A. Hougen. J. appl. Phyx. 66, 3763 (1989).

APPENDIX

A

Oplical Absorption Coeflicients Near and Below the Absorption Edge in MCT(.x 5 0.2-0.3) in frinsic band-to -band absorption fij E.~ponelz~~al ab.~orprjonedge. It has been experimentally observed that the absorption edge in MCT follows the Urbach exponential rule. I’+ Chu et al.“” have recently obtained the following expression for the exponential tail. The absorption coefficient in the edge region is expressed as

c(BB= 010(ag,G(U)(6-E,I),IE,-b,, where,

the parameters

x0 and E0 are expressed

as functions

of composition

(A.1) (x) as:

In CI= - 18.5 + 45.68~. &(eV) = -0.335 and, 4 is the optical band gap and clg is the intrinsic of s and T (the absolute temperature) as: E,(eV) = -0.295 zg = -65

absorption

+ 1.87.~ -0.28.x’+

+ 1.77s coefficient

at E = Er. They are expressed

(6 - 14.x -I+3s’)flO-“)T

as functions

+ 0.35.~’

+ 1.8837. + (8694 - 10.314T)~.

(ii) ‘T&r k&e region. The simplified Kane equation”” could be used to represent tail region. The corresponding absorption coefficient can be expressed as: ilgs = fi(E -E;)’

2.

the region just above the exponential

(A.2)

here fl is a fitting parameter and E; is the energy band gap with no band tailing distortions. The band gap, E,, defined by Chu el al.,‘” as expressed above. however, corresponds to the turning points on experimentally obtained absorption

K. K.

680 coefficient continuous

vs energy plots, where at E = E; one obtains

the exponential from equations

%iARMA

et a/.

absorption edge and the Kane (A.1) and (A.2):

plateau

region

meet.

For lxBBto be

2, = B(E, - E;,“z

After simplification

one gets:

and E;! = E, - (Er - E,)/ln(cc, 12,)‘. (b) The intervalence absorption The intervalence absorption in MCT has been discussed relation for the intervalence absorption cross section:

in detail by Mroczkowski

and Nelson.‘s’ They give the following

where a, is the fine structure constant, P is the Kane matrix element, E is the Photon E,,(k) = average valence band energy, and

energy. k is the wave vector amplitude,

Y/= (Ep + WkL’3) fl= h’(l/m, here m, and mhh are, respectively, the free electron be given by the empirical expression of Finkman

+ l;m,,).

mass and effective mass of heavy holes. Also the refractive and Schacham:“4’

nz = A + B;[l - (c;i)‘] here the interpolated

parameters(m

2%

are given by the following A = 16.4135 - 22.1914~

index n can

+ DA’+ Ei.4. expressions:

+ Il.081.~’

B = 0.037514 + 1.060482.~ + 0.876032.~’ C zz 0.5694.x

’ ‘W

D=(-1.4917+2.1144x

- 1.0415.~‘)(10~‘)

E = - 1.63(10-‘). for the wavelength The intervalence

(2) in pm. absorption coefficient

where p,, is the density

is defined

as

of free holes.

(c) The free carrier absorption The free carrier absorption due to heavy holes in MCT follows the classical I.’ dependence expression could be used to calculate the corresponding absorption coefficient:‘“’ T,, ~

and, therefore.

the following

&“L 4n’c’nf,m~,~l,,

where P is the electronic charge. c is the speed of light. 6” is the dielectric constant. p,) 1s the free hole density and plhh IS the heavy hole mobility. The following semiempirical expression for free carrier absorption coeficient for electrons could be used:“’ r,, = 3(1O~“)i?“n,,. here n,, is the density

of free electrons.

The total free carrier

absorption

could

now be expressed

as:

Yr< = x\1 + 2,. The contributton

of light holes to the free carrier

absorption

may be neglected

due to thetr IGW denstty

and high mobility.