Photoconductivity of graded-gap CdxHg1−xTe∗

,x,2u-wi9 I 19 030, -01 7VSO?.W~l

PHOTOCONDUCTIVITY

OF

GRADED-GAP

Cd,Hg,

_,Te*

JANUSZ M. PAWLIKOWSKI~ Institute

of Physics. Technical (Rewired

University

30 Srptrmher

of Wroclaw

1978)

Abstract-Photoconductivity measurements have been performed at 77 and 300 K in the 0.5-6p-n wave-range on samples cut from graded-gap epitaxial Cd,Hg,_,Te layers and compared with a simple theoretical model. The possible application of epitaxial graded-gap semiconductor layers as broad range photoconductive detectors is discussed.

1. INTRODUCTION Over the past 30 years, intense effort has been devoted to the development of semiconductor photon detectors that exploit internal photo-effects in the photoconductive (PC), photovoltaic (PV) and photoelectromagnetic (PEM) modes. For a homogeneous semiconductor the photoresponse in these modes attains high values over a relatively narrow range of wavelengths (for which, to a first approximation, the photon energy is equal to the value of the energy gap of the material). In the last few years many theoretical and experimental papers have been devoted to the properties of mixed semiconductor crystals with position-dependent molar composition. The physical properties of such alloys with a slowly varying composition are of a great interest, because the band parameters (energy gap, effective mass and lifetime of carriers, etc.) are also position dependent. A typical example is provided by epitaxial Cd,Hg, _,Te graded-gap layers, whose basic characteristics are reviewed in Ref. 1. The variable energy gap leads to several effects in this semiconductor, of which those due to the position dependence of the energy gap and those produced by the effective-mass gradient are of special interest. The band-gap gradient results in a difference between the electric field acting on the carrierst2’ and influences, for example, the PEM effect.‘3’ The effective-mass gradient influences, among others, the PV effect.‘*’ The PC effect in graded-gap semiconductor samples were discussed in Ref. 5, in which the carrier distributions influenced by band-edge gradients and different front- and backsurface recombination were obtained (using the Gora-Williams conductivity equation and the usual boundary conditions but ignoring the position dependence of the absorption coefficient a). In Fig. 1 the relative photoconductivity a/a0 has been plotted after Ref. 5 as a function of the absorption coefficient of the semiconductor for the case of large front-surface recombination (S, = 10’ cm/set) and small back-surface recombination (S, = 10m3 cm/ set) for a graded-gap sample with a thickness of 1 pm and with different gradients of band gap. The usual band-edge configuration has been taken (see inset of Fig. 1). These curves show how the photoconductivity (i.e. the total excess minority-carriers concentration) would vary with the absorption coefficient in the graded-gap sample. One finds that for large dEg/dx, the photoconductivity is large, as the band-edge gradient assists the diffusion on minority carriers away from the front surface (where recombination is high). The decrease in photoconductivity with increase in the absorption coefficient as seen in Fig. 1 can readily be explained as for a homogeneous semiconductor; i.e. with a larger a, a greater number of carriers are created at the front surface where * Work under contract “Materials Engineering”, t Address for correspondence: 50-370 Wroclaw,

7/78 (IM-116). St. Wyspianskiego 179

27. Poland.

IX0

JANUSZ M.

PAWLIKOWSKI

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I

0

500

‘\

‘\

‘\

‘- c

‘-.

‘.

b

a

1000 oC(arb. units&--

Fig. 1. Variation of relative photoconductivity with absorption to zero (curve a). with moderate value of dEg/dx (curve b) and c).

coefficient for dEg/dx large value of dEg/dx

equal (curve

the recombination is large. The spectral characteristics of photoconductivity are not, however, shown in this paper. One approach to the problems of carrier distributions and internal photo-effects in graded-gap semiconductor has been proposed in Ref. 6 which treated the graded-gap sample as a series of slices, each with uniform band parameters which change progressively from slice to slice. The spectral characteristics were obtained and will be discussed below with the experimental results. 2. EXPERIMENTS

AND

DISCUSSION

Epitaxial graded-gap layers were produced by epitaxy from the gaseous phase in isothermal conditions (details-see Ref. 1). Typical parameters of the technological process were as follows: temperature 833 K, epitaxy duration 100 hr, and layer thickness about 500pm. The layers were p-type and usually strongly compensated.“’ The preparation of samples with the required difference in molar composition and energy gap gradient were made by cutting specimens out of the,appropriate part of the epitaxial layer. After suitable grinding, polishing and etching the samples obtained were about 3 x 1 x 0.1 mm and the energy gap gradient _ lo4 eV/m, usually within the region

I Fig. 2. Geometrical

configuration

hi of sample

\B measured

(A-irradiated

space,

B-air

hole).

Photoconductivity

74 I I

1

of graded-gap Cd, Hg, _,Te

2 I

1 I

181

0.7 I

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1

2 h3 (eV)------

nu I

I

A (ym) 74 2 I

I

1

I

0.7

I

I

l-

I

I

2

1

h3 leV) ----Fig. 3. Photoresponses of PC-8 sample at 300 K (a) and 77 K (b) illuminated on the front (dashed line) and back (solid line) surfaces, respectively. Thickness of sample is equal to 50pm and x1 = 1, x2 2 0.27

of molar compositions 0.2 < x < 1 (first group) and 0.2 < x < 0.8 (second group). In order to improve stoichiometry and to obtain n-type samples, heating in mercury vapour at 500 K was used. The contacts for both p- and n-type samples were made by evaporation of gold and indium, respectively, according to Refs 8 and 9. Finally, the samples were etched from the side of small cadmium content and mounted on the special mounting plate with a window in the center (see Fig. 2). The PC measurements were made at temperatures of 77 K and 300 K in a conventional measurement set-up using the SPM2 with LiF prism and SPMl with NaCl prism monochromators, with a chopper at 12.3 Hz frequency, and a thermocouple Vth-1 as a reference detector. Typical results for the first-group samples are presented in Fig. 3a,b for sample PC-8 illuminated both from the front surface (denoted by index 1) and the back (denoted by index 2), with Eg, > Eg,. Two different parts of the curves are Clearly seen: a narrow maximum at higher photon energies and then a slowly increasing photoresponse (being about two orders smaller) with increasing wavelength. The first maximum we connect with the long lifetime of carriers generated in the CdTe and, especially, in the region

1x2

JANUW M. PAWLKOWSKI

to CdTe in which the Cd,Hg,_,Te (with x close to I) is near intrinsic and has relatively good stoichiometry. ‘lo’ The long wavelength cut-off for all curves are in a good agreement with the measured absorption edges and also with the energy gap estimated from the known molar composition. The long-wave cut-offs are shifted at 77 K to longer wavelengths as a result of the positive temperature coefficient of the energy gap in this molar composition region (X < 0.5) It is easy to explain the small difference between photoresponses with front- and back-surface illuminations. It results from the effects of free-carrier absorption (especially in the narrow-gap region) with back-surface illumination. At 77 K the free-carrier concentration is, of course, smaller than at 300 K and the difference in photoresponses is also smaller. The normalized detectivity D* is large in these samples only in the short-wavelength maximum region, reaching 5 x 10’” at 300 K and 5 x 10” at 77 K (all values in W ’ cm Hz”‘). In the long wavelength region D* is about two orders of magnitude smaller. mostly due to shorter carrier lifetimes but partly due to processing effects. When the CdTe regions in the samples were ground and etched, and if the Cd-rich surface had a molar content in the range 0.8 < .X < 0.9 (after heating in mercury vapour). the stoichiometry distinctly improved-at the same time the carrier lifetime increased. In the samples obtained by this method (i.e. the second group) the detectivities in the long-wave region were greater. The comparison between the PC-8 and second-group PC-315 detectivities is shown in Fig. 4. As shown, the photoresponse of the latter covered a relatively broad wavelength region with useful detectivity. This is an important advantage of graded-gap structures in comparison with homogeneous semiconductor. It is clearly shown in Fig. 5. which compares spectral plots Cd,XHg, _ .Te sample and a graded-gap computed theoretically (’ ‘) for a homogeneous one with the molar composition on the front side the same as in the homogeneous specimen. The solid-line theoretical plot for the graded-gap structure was c~)mputed for slowly-varying lifetime of carriers and small surface recombination, whereas the dashed-line one was computed for position dependence of lifetimes as observed in the and for great surface recombination. Plot for the homoexperiments of Cohen-Solal”’ geneous sample was computed for moderate surface recombination using literature data used has been described in detail of Cd,Hg, _,Te. (12) The model of carrier distributions in Ref. 6. We have allowed in the calculations for the linear position dependence of conductionand valence-band edges, absorption coefficients and effective masses of carriers. and the exponential position dependences of electron mobility and carrier concentration and lifetime. The comparison between theoretical and experimental plots for dose

1

i PC-315

h(ym)-----Fig. 4, Detectivities of PC-S (lirst group) and PC-315 (second tion on the front (Cd-rich) side. Thickness of PC-315 sample Yz 1 0.32

group) samples at 77 K. Illuminais equal to lOO!lrn and x, 2 0.8.

Photoconductivity

of graded-gap

Cd, Hg, _,Te

183

I

I

I

-

l-

Cd,Hgl_,Je

‘\, x,=0.35

i!!

Cd0.35Hg0.65Te homogeneous

: IO3

‘\\ \ ‘\

x2=Oj5 1

6

5

I

h(ymI

-%

Fig. 5. Comparison Cd,Hg,

of theoretically predicted PC-responses for homogeneous and graded-gap _,Te samples with the same molar composition on the side illuminated.

graded-gap Cd,Hgl_XTe sample with x1 z 0.8 and x2 z 0.32 is shown in Fig. 6. The curves are matched at the maximum of the experimental response. The shape of these curves is not the same, especially in the long-wave region, near the long-wave cut-off of the PC-response. Nevertheless, Figs 5 and 6 show that the PC-response of graded-gap Cd,Hg, _,Te structure can be useful in broad-range infrared detectors. An improved theoretical model of PC and PEM effects in graded-gap semiconductor is still needed. 3.

CONCLUSIONS

(i) For a homogeneous semiconductor the PC-response attains high values only over a relatively narrow range of wavelengths (depending also on surface conditions) for photon energies which are equal to, or a little greater than, Ey (see also Ref. 13). (ii) It is generally expected that the concentration of opticaly excited carriers should decrease rapidly with the distance from the generation region. In the case of graded-gap samples, however, the carriers generated in the whole of the specimen are subjected to the relatively large internal fields which move the carriers to the back-wall. This displacement is proportional to the lifetime of the carriers. For this reason a relatively small number of photo-carriers diffuse to the front surface which has high surface recombination.

Fig.

6. Comparison

between

theoretical Cd,Hg,

and experimental _,Te structure.

plots

of

PC

for

graded-gap

184

JANUSZ M. PAWLIKOWSKI

(iii) To a first approximation, the limits of the detectable wavelength range are determined by the values of the energy gap at the front and back faces of the graded-gap sample, although in practice the internal electric quasi-field and the position-dependence of the carrier lifetime and the absorption coefficient are also important. (iv) It has been shown that graded-gap epitaxial Cd,Hg, _,Te layers can be used effectively as a relatively broad range infrared detector (see also Ref. 14). These conclusions may be generalized to the other graded-gap semiconductor structure with energy gaps that vary over an adequate range. Ac,l,no~~/rt/yc~rllc~rtrs--The author is indebted IO J. F. Kasprzak for help in calculations and to A. Baran P. Becla for measurements provided. Special thanks are due to P. Becla for helpful discussions.

and

REFERENCES 1. PAWLIKOWSKI, J. M., Thin Solicl Films 44, 241 (1977). KROEMER. H.. RCA Rev. 18, 332 (1957). COHEN-SOLAL G. & Y. MARFAING. So/it/ Sfc~fe Elecrron. II. 1131 (1968). MARFAING. Y. & J. CHEVALLIER, IEEE Trtrns. ED 18, 465 (1971). CHATTOPADHYAYA. S. K. & V. K. MATHUR. Phys. Rec. B9, 3517 (1974). PAWLIKOWSKI. J. M. & J. F. KASPRZAK. Solid Srtrre Commun. 25, 645 (1978). PAWLIK~WSKI J. M. & P. BETLA. Phys. Sttr~. Sol. (a) 32, 639 (1975). PAWLIKOWSKI J. M.. P. BECLA. K. LUBOWSKI & K. ROSZKIEWITZ. Acttr Ph,rs. Pd. A49, 563 (1976). PAWLIKOWSKI, J. M., A<,rcr Phrs. PO/. A49, 139 (1976). BECLA P. & J. M. MELANIUK. Mater. Sci. 4 (1978) In press. KASPRZAK. J. F.. E. MAJ~HROWSKA & J. M. PAWLIKOWSKI, unpublished data. DORHAUS, R. & G. NIMTZ. Solid Srore Physics (edited by G. H~~HLER), Vol. 78. Springer-Verlag. Berlin (1976). 13. Moss. T. S.. J. Lwnirtcsc. 7. 359 (I 973). 14. PAWLIKOWSKI. J. M.. Thin So/it/ Film 50, 269 (1978). 2. 3. 4. 5. 6. 7. 8. 9. IO. Il. 12.