The photomagnetoelectric effect in heteroepitaxial silicon films

W. ECKE: The Photomagnetoelectric Effect in Heteroepitaxial Si Films 157

phys. stat. sol. (a) 42, 157 (1977)

Subject classification: 14.3.1; 16; 22.1.2

Zentralinstitut fCr Festkcrperphysik und Werkstofforsehuny der Akademie deer Wissenschaften der DDR, Instktutsteil fur magnetische Werkstoffe, Jena

The Photomagnetoelectric Effect in Heteroepitaxial Silicon Films

BY

W. ECKE

Minority and majority carrier lifetimes in evaporated heteroepitaxial silicon films are determined by observing the photomagnetoelectric effect and the photoconductivity. The majorit,y carrier lifetime is found to be about four orders of magnitude higher than the minority carrier lifetime. This indicates a strong trapping effect in these films. In order to value the measuring method, moreover, the minority carrier lifetime is inferred from the 1-77 data of p-n junctions and from the transient response of an MOS capacitor. The transient capacitance response indicates a too great lifetime in heteroepitaxial silicon films, which does not apply to the neutral material.

In aufgedampften heteroepitaktischen Siliziumschichten werden die Lebensdauern der Minori- tats- und Majoritatsladungstrlger mit Hilfe des photomagnetoelektrischen Effektes und der Photoleitfahigkeit bestimmt. Die Majoritiitsladungstragerlebensdauer ist bei Zimmertemperatur um vier GroBenordnungen hoher als die Lebensdauer der Minoritiitsladungstrkger, was auf einen starken Trapeffekt in den Schichten hinweist. Zur Einschiiteung der MeBmethode wird die Mino- ritiltsladungstrligerlebensdauer auBerdem an einem p-n-ubergang und an einem MOS-Konden- sator bestimmt. Dabei wird gefunden, da13 der Aufbau einer Inversionsschicht im MOS-Konden- sator eine zu groBe Lebensdauer in heteroepitaktischen Siliziumschichten anzeigt, die nicht fur das neutrale Material gilt.

1. Introduction

The minority carrier lifetime is a useful quantity to characterize the quality of an epitaxial silicon film. Actually introduced as a pure operand, under special experimental conditions (essentially at a low injection lesel of non-equilibrium carriers) it may be regarded as a material parameter reflecting the combination of concentration, energy, occupation state, and capture cross-section of one or several recombination centres in the semiconductor [ 11.

Previous measurements of the minority carrier lifetime in chemically deposited heteroepitaxial silicon films made use of the behaviour of special devices : current amplification in bipolar transistors [2], recombination current in diodes [3], generation of minority carriers in MOS capacitors [a]. In this paper the minority and majority carrier lifetimes determined by observing the photomagnetoelectric (PME) effect and the photoconductivity (PC) are compared wit8h the results from p-n junctions and MOS capacitors.

The most essential contributions to the interpretation of the PME data are given by Kurnick and Zitter [5] and by van Roosbroeck [GI. Zitter 171 extended the analysis in conjunction with the P C effect to the presence of defect states with trap behaviour. Lile [S] generalized the treatment of the P M E effect to include arbitrary sample thicknesses and surface recombination velocities.

158 W. ECKE

2. Sample Preparation

Thc iueasureinents were performed on p-type heteroepitaxial silicon films of d = 12 pui thickness. These filnis were deposited by electron beitni evaporation in an oil-free ultra-high vacuim onto (I 11) Mg-A1 spinel subst,rates. Further details of the deposition are given in [9].

I n order to enable t,he coinparison of the carrier lifet,imes determined by different methods, all films were oxidized under the conditions given in Table 1. The snbse-

Table 1 oxidizing agent temperature T = 1000 "C oxidation time t = 25min after-annealing agent N2, dry temperature T = 1000 "C annealing time t = 10min

O,, saturated with H,O at 85 "C

qucnt preparation steps differ for every measuring method. In order to iiieasure the PME effect, the oxide layer was removed by HF. Thereafter, these surfaces were weakly oxidized in dry oxygen ambient a t T = 900 "C. Contacts were forined by alloying of evaporated aluminium dots to two opposite oxide-free places of the silicon film.

I n order to produce n+-p junctions, phosphorus was diffused into the top layer of the silicon film (4 ruin a t 1200 "C) after removing the first oxide laycr which yields a surface electron concentration TL = 2 x lOI9 cm-5 and a junction depth about 1 pm. As described in [B], mesa diodes with 300 pm diameter are separated within the silicon fil ni.

MOS capacitors were completed by evaporated permalloy (80'$ Ni, 2Oy0 Fe) dots on the first oxide layer.

3. Measurements

The PMdE effect was measured in an arrangement similar to that described by Cunningharn [ 101. Howevcr, the alloyed contacts were of sufficiently low resistance and the four-contact configuration was not used. Practically monochromatic light with the wavelength A = 475 nni was obtained by collimating the light from an in- candescent lamp and passing i t through a metal film interference filter. The photon flux density I,, was measured by a calibrated silicon solar cell. The light beam was niechanically choppcd a t the frequency f = 30 s-l resulting in a sinnsoidal temporal response of the photon flux density. The magnetic induction U could be varied over the range B = -1.3 x

I n case of measurcnients on MOS structures, the transient response of the inversion layer capacitance was evaluated by a capacitance bridgc (Tesln BM 431 E) at the measuring frequency 3 MHz with a time resolution of 1 ins. A positive voltage step was applied to the metal electrode of the MOS capacitor to deplete the majority carriers from the semiconductor surface. I n Fig. 1 the detuning of the capacitance bridge is shown as a function of tinie. The bridge is balanced to the capacitance with formed inversion layer. This state is obtained in the course of about 40 ins after applying the positive voltage step. At the end of the oscilloscope display the MOS capacitor is switched back into the state of weak enhancement.

to f 1 . 3 x 10-4 Vs cni-2.

The Photomagnetoelectric Effect in Heteroepitaxial Silicon Films 159

Fig. 1. Transient response of the MOS capacitance during the formation of the inversion layer. X 10 mslcm, Y nonlinear, about 1 pF/cm

4. Results

4.1 Photomagnetoelectric effect

The determination of the minority and majority carrier lifetimes from PME and YC short-circuit currents follows the theoretical treatment given by Zitter [7] considering the influence of carrier traps. The determination is based on the following assumptions : - nieasurenient in the non-degenerate semiconductor ; - all material parameters are independent of magnetic field; - pzB2 < 1 ( p carrier mobility) ; - the injected carrier density is sniall compared to the thermal equilibrium concen-

- the optical absorption length is small compared to the sample thickness; - inobility and concentration of the thermal carriers are independent of position in

the photoelectrically affected upper part of the sufficiently thick film ; - no influence of the sample edge; - no influence of the surface recombination.

t ration ;

The validity range of the last assumption will be discussed by comparison with the general treatment given by Lile [8 ] in connection with the experimental results. In [71 the carrier lifetimes are given by

(1) Zl'ME(1 + c ) -ZF'C(1 + l / b )

c - l / b z = P

1 60 W. ECKE

and

Here, zP and Z, are the hole and electron lifetimes, respectively, Z ~ M E and zpc the SO-

called PME and YC lifetimes, respectively, i p M a and ipc the PME and PC short- circuit currents per unit sample width, respectively, b = pn/pp is the ratio of electron and hole Hall mobilities, c = no/po the ratio of thermal electron and hole concentra- tions, q the magnitude of the electronic charge, I the effective photon flux density, iy the optical absorption coefficient, E the electric field applied to the sample during the PC measurement) L z the ambipolar diffusion length, D, = p,, kT/q the electron diffusion constant, k the Boltzmann constant, and T the temperature.

The measurements were performed under the following conditions :

= 26 x 10-3 V , AB = 2.6 x Vs/cm2 ) po = 3 x cm-3, q

E = 4 V / c m , pp = 170 cm2/Vs, I = 1.4 x 1015 cm-2 s-l ,

c = 2.5 x 10-13 ) a = 3 x lo4 cm-1) b = 2.5.

The reflexion coefficient R = 360/, and the quantum efficiency rj = 1 [ll] are in- cluded in the value of I. The magnitude of M. results from measurements over the wavelength range X = 600 to 1000 nm where the films have exhibited 1.3 times the bulk absorption coefficient.

The value of c may be neglected a t the given doping level, also when c is compared to a mininiuni ratio zn/zp. With that, equations (l), (2), and (4) may be reduced to

ztp = zPC(1 + b ) - ZPIvIEb 7 (1')

z, = ZPME , and

Measurements on several films of the same kind yielded majority carrier lifetimes zD = 6 to 10 ps and minority carrier lifetimes z, = 0.06 to 0.17 ns. Corresponding to the possible errors in the measuring conditions, especially in I , a, and ,up, the accuracy of the minority carrier lifetime is limited to a factor 3.

I n order to investigate the influence of the finite sample thickness and of the surface recombination on the minority carrier lifetime the generalized relation ip3f.E(tpME) (given by Lile, equation (16) in [8]) has been calculated numerically with various values for the parameters a, d, 8,) and S,. Here, S, and S, are the recombination velocities a t the upper surface of the silicon film and a t the silicon-spinel interface, respectively. The results of curves a and b in Fig. 2 are identically with those of equation (4'). One obtains a great influence of a. on iP&fE when the minority carrier diffusion


Fig. 2. Dependence of the PME short-circuit current per unit sample width i p ~ ~ on the PME lifetime TPME calculated with the following values for the parameters a, d , Is,, and 8,:

curve

a b

d e f g

C

12 3 12 3 12 3 12

Is, (cm 8-I)

0 0 0 0

10s 104 105

4 (cm s-l)

0 0 0

106 0 0 0

length a t small zPME values is approximately equal to or smaller than the optical absorption length l/&. Considering 8, = 106 cm/s in curve d as the largest value of X at the silicon surfaces or S, = lo3 cm/s in curve e as an unusually large value a t a weakly oxidized silicon surface [12], the results of (4') are still correct over the range ~ p M g 5 10 ns at d 2 12 pm. Equation (4') still applies at higher a,, e.g. 8, = lo4 cm/s in curve f, up to ZPME 5 1 ns. With that, the above values ZPME < 0.2 ns cannot be falsified by the finite sample thickness and by the surface recombination. Only for 8, = lo6 cm/s in curve g the correct interpretation using (4') is not possible. As already seen from curves f and g, the correlationzpNE(ipME) is not unique under the considera- tion of a finite S,. However, at 8, 5 lo4 cm/s the difference between both possible z ~ M E values is so high that the right correlation of the smaller ZPME, representing the solution of (4'), is necessarily plausible.

4.2 Recombination of charge carriers in a p-n junction

Fig. 3 shows the current-voltage characteristic of a n+-p mesa diode a t low forward voltage. From U , = 0.1 V up to the current limitation by the series resistance of the diode the depenclence of the current J on the voltage U may be described by the relation

J = J , exp (g) with m = 1.8 and J,, = 2.3 x A in t,his special case. According to the Sah- Noyce-Shockley theory [13] the minority carrier lifetime z, in the p-type region of 11 physiea (a) 42/1

162 W. RCKE

Fig. 3. Current-voltage response of a mesa diode in forward direc- tion

\ z3

the n+-p junction can be determined from J,:

(5) A k T La z =----qn.

!:i-r' 703 ;'

J , ,' JO q Ua I '

'0"o u oz a3 04 where A is the junction area, n, the intrinsic density, L, the width of the space charge region, and Ua the diffusion voltage across the junction.

U,lLa = 4.5 x 105 V/cm was determined from the doping levels. Thus, equation

$[u-

(5) yields z,, = 0.4 ns.

4.3 Generation of m i n d t y c a d e r s within the MOS capacitor

The interpretation of the transient response of the MOS capacitance during the formation of the inversion layer follows the generally used analysis, e.g. given by Heiman [a]. After that, the minority carrier lifetime in the p-type layer depends as

1 ni 2 Po

z, = - -z,

on the time constant z, of the hansient. The measured z, = 16 ms yields z, = 4 ns.

5. Discussion and Conclusion

The measurenient of both PME effect and PC showed that the majority carrier lifetime zp surpasses z, by four orders of magnitude. This distinctly marked trapping behaviour of the defect states in the silicon films rules out many carrier lifetime measuring methods used in bulk silicon assuming the same electron and hole lifetimes (e.g. P C decay).

In this work three carrier lifetime determination methods are compared using the same recombination statistics [14] with neglects valid in each case. The density of the optically generated minority carriers is indeed relatively high, but the demand following from [ 131 for a concentration-independent minority carrier lifetime

zp(n + < zn(P + ~ 1 )

is still realized a t our doping level and at the position of the most effective recombination centre near the middle of the forbidden band. Here, n and p are the total con- centrations of electrons and holes, respectively, n, and pl ere the carrier concentra- tions when the Fermi level approaches the energy of the recombination level. The neglect of carrier density dependence and of direct optical trap recharging is supported by the measured independence of z, on I within the measuring accuracy. Also the comparison with the results of Huang and Li [15] shows that the used light intensities with their slight influence on the sample conductivity do not cause an intensity- dependent minority carrier lifetime. However, i t was found that the MOS minority carrier lifetime is about forty times higher than the corresponding PME result. This behaviour may be due to an inadmissible combination of minority and majority carrier lifetimes in the generally used analysis of the transient response, but was already deduced theoretically by Eernisse and Norris [ 161 from the particular arrangement of lattice defect induced trap levels. After [ 161 these traps are distributed inhomogene-


ously, for instance as clusters along stacking faults and dislocations. Filled with majority carriers these clusters may form around themselves a space charge region due to their dense agglomeration. With that, the effective capture cross-section is increased for minority carriers and reduced for majority carriers compared to the homogeneous distribution of the same trap density. Under high injection conditions or within a strong electric field this potential barrier around the defect cluster is reduced or unessential due to the superposition of the external field, respectively. Therefore, values with slightest falsification of the minority carrier lifetime as a material parameter of the undisturbed silicon film and a clear separation from the behaviour of majority carriers should result from the PME measurement.

As the above results show, the comparison of carrier lifetimes of different heteroepitaxial silicon films (as in 1171) cannot provide immediately differences of the film quality, when the carrier lifetime was determined from the MOS response and from other methods.

When the films have to be examined with respect to their qualification as devices with a defined recombination rate in regions of low injection level and low electric field (bipolar transistor, double-injection diode), the PME results should be taken into account.

Acknowledgements

The author thanks Dr. E. Steinbeiss, Dr. M. Manzel, and Dr. W. Brodkorb for stimulating discussions, Mr. P. Duda for preparing the silicon films, and Mrs. I. Uhlig for experimental assistance.

References [l] C. T. SAH and W. SHOCKLEY, Phys. Rev. 109,1103 (1958). 123 P. B. HART, D. J. ETTERS, B. W. JERVIS, and J. M. FLANDERS, Brit. J. appl. Phys. 18, 1389

[3] D. J. DUMIN and R. S. SILVER, Solid State Electronics 11, 353 (1968). [4] F. P. HEIMAN, IEEE Trans. Electron Devices 14, 781 (1967). [5] S. W. KURNICK and R. N. ZITTER, J. appl. Phys. 27, 278 (1956). [6] w. VAN ROOSBROECK, Phys. Rev. 101, 1713 (1956). [7] R. N. ZITTER, Phys. Rev. 112, 852 (1958). [8] D. L. LILE, Phys. Rev. R 8,4708 (1973). [9] W. ECKE and P. DUDA, Kristall und Technik 19, 447 (1977).

(1967).

[lo] R. W. CUNNINQHAM, Rev. sci. Instrum. 34, 83 (1963). [ll] P. S. KIREJEW, Physik der Halbleiter, Akademie-Verlag, Berlin 1974. [l2] A. MOSCEWITZEB, Halbleiterelektronik-Lhrbuch, Verlag Technik, Berlin 1973. [13] C. T. SAH, R. N. NOYCE, and W. SHOCKLEY, Proc. IRE 46,1228 (1957). [la] W. SHOCKLEY and W. T. READ, JR., Phys. Rev. 87,835 (1952). [15] C. I. HUANG and S. S. LI, J. appl. Phys. 44, 4214 (1973). [IS] E. P. EERNISSE and C. B. NORRIS, Solid State Electronics 16, 315 (1973). [17] K. H. ZAININGER and C. C. WANQ, Solid State Electronics 13, 943 (1970).

(Received April 28, 1977)

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The photomagnetoelectric effect in heteroepitaxial silicon films