Solid-State Electronics Vol. 34, No. 10, pp. 1149-1156, 1991 0038-1101/91 $3.00+0.00 Printed in Great Britain. All rights reserved Copyright © 1991 Pergamon Press plc

QUANTUM-SIZE p - n JUNCTIONS IN SILICON

N. T. BAGRAEV, L. E. KLYACHKIN, A. M. MALYARENKO and V. L. SUKHANOV A. F. Ioffe Physico-Technical Institute, Academy of Science of the U.S.S.R., 19402 l, Leningrad, U.S.S.R.

(Received 20 July 1990; in revised form 3 March 1991)

Abstract--Kicking-out and dissociative vacancy substitution have been investigated as two alternate mechanisms of boron (B) diffusion in silicon (Si). The criteria for dominance of each mechanism, established by systematically varying the surface parameters, were used to identify the conditions leading to the parity between the two mechanisms. The slowing down of the diffusion process thus achieved has permitted the fractal growth of the developing p-n junctions, yielding quantum-size structures featuring the high external quantum efficiencies over a broad spectral range together with low values for dark leakage currents.

I. INTRODUCTION

Modification of the surface parameters in monocrys- talline Si has now been established to be an effective means of accomplishing the parity between the kick- out and the dissociative vacancy diffusion mechan- isms[I-4]. The supporting experimental data are as follows. Oxidation of the surface under high tempera- ture conditions has been demonstrated to result in the generation of excessive amounts of self-interstitials, which are known to stimulate the diffusion of do- pants (P, B) via the kick-out mechanism[l]. The use of low temperatures, in combination with prolonged oxidation treatment, was found, on the other hand, to favour the dissociative vacancy mode of diffusion for dopants, as has been so strikingly revealed in the doping experiments involving Sb[l,3].

The question as to whether there exists a paramet- ric range associated with complete balance between the two diffusion mechanisms, and the related ques- tion of what might be the shape of the corresponding diffusion profiles still remains to be answered. It would also be of interest to determine the relative diffusion rates of dopants (P, B) for the conditions that provide maximal stimulation for one of the competing mechanisms. The present work produces a positive answer to the first of the above questions by defining, for the first time, the set of conditions under which the kick-out and the dissociative vacancy mechanisms apparently operate on a par, leading to a complete annihilation of self-interstitials and vacancies in the near-surface region, as evidenced by a sharp drop in the dopant diffusion rate. Also presented are the quantum-size diffusion profiles and p - n junctions we have been able to produce, also for the first time, using the knowledge thus gained; the only additional requirement being the provision of effective gettering in the crystal bulk. As well as showing low values for dark leakage currents, the p - n

junctions obtained are characterized by high external quantum efficiency values that while being high also

allow for optimization--through adjustments of the junction depth or the impurity concentration in the profile--to suit the various wavelengths of the spec- tral range.

Use of thin oxide layers in combination with high diffusion temperatures has been found[I,3] to result predominantly in the generation of self-interstitials by the oxidized surface, and thus, in dramatically increased rates of diffusion for such impurities as B, P or As, which penetrate the Si lattice by the kick-out mechanism:

X,~ Xs + L (I)

where X~ and X s denote the dopant atom in its off- and on-site positions, respectively, and I represents the self-interstitial. In contrast, the main diffusion mechanism associated with thick oxide layers and low diffusion temperatures is one by which the dopant diffuses via the dissociative vacancy mechanism[l]:

x,+ v ~ L , (2)

where V stands for a vacancy. Excessive supplies of self-interstitials and vacancies produced play an im- portant role in the three-step gettering cycle as occurs in the crystal bulk[5,6]. They also appear to be the driving force for the process that leads to a drastic improvement in semiconductor device performance, the so-called combined gettering[7].

Considering the opportunities offered by the com- petition that can be maintained between the two diffusion mechanisms by varying the oxide layer thickness (dsio2) and diffusion temperature (Td), one may reasonably hope to obtain diffusion profiles and p - n junctions of the quantum-size dimensions. At the same time, success in determining the oxide thick- nesses and diffusion temperatures associated with the shallowest diffusion profiles should help define the conditions leading to the replacement of one diffusion mechanism by the other.

The present work, which studied B diffusion in monocrystalline Si for different oxide thicknesses and

1149

l 150 N.T. BAGRAEV et al.

diffusion temperatures, provides evidence that there is indeed a certain subset of parameters that favour both diffusion mechanisms in equal measure.

Experimental determination of the physical bound- aries within which occupation of the lattice sites is as likely to proceed via the kick-out mechanism as by the dissociative vacancy diffusion mechanism, has enabled us to achieve fractal growth of quantum-size diffusion profiles and p - n junctions for the first time. It appears that quantum-size diffusion profiles have characteristics that critically determine the way one selects to reduce the leakage currents in quantum-size p - n junctions to hyper-low values.

2. M E T H O D S

Under oxidation conditions for the monocrys- talline surface, the value of the dopant diffusion is given by addition of the contributions from the kick-out (Dr) and dissociative vacancy (Dr) processes[8]:

CI Cr D,,~ -~ O , ( ~ + D, c]~q, (3)

where Ct and C~. are thc concentrations of self- interstitials and vacancies due to the oxidized surface, and C~ q and C]q are their bulk equilibrium values.

Analysis of D~ and Dv contributions to D in terms of their respective diffusion coefficients (~bz = D f l D )

and (~b r = D r / D = 1 - ~/) using the corresponding ratios for excessive concentration, (st = ( G - c~q)/ C~ q) and (s~ = (C~ - C{9);C~9), it will show that the diffusion rate should increase sharply in the presence of a great excess of either contribution over the other:

Dox = D(I + Ao,), (4)

where

Oox - O Aox - - - = 0is1 + 4~,,s,.. (5)

D

Under the conditions of dynamic equilibrium, given by ( C I C v = C~qC]q).

$1 Ao~ = (2q~z + ~bls/-- 1 ) - - (6)

1 + s l

o r

S V - - . (7) A,,~ = (24~,- + ¢ , s , . - I) 1 +s , ,

For high temperatures, the diffusion process is dominated by the kick-out mechanism (qSt>0.5), whereas the dominant mechanism for low diffusion temperatures is the dissociative vacancy diffusion mechanism (~b~ > 0.5)[1].

In the case of ~bt = ~v = 0.5, the condition for the diffusion rate to increase is the excessive production of either self-interstitials (st > 0, sv < 0) or vacancies (s~ < O, Sv > 0). As was noted earlier, the relationship between st and sv is governed exclusively by the oxide

thickness parameter, so that if for instance, we have st = Sv = 0, which corresponds to Aox = 0, the diffu- sion rate remains unaffected by the changes in tem- perature. This allows one to keep Do~ at its lowest level of D at any diffusion temperature, provided the oxide thickness has been adjusted such that s~ = sv .

The minimization is especially easy to achieve for the range of parameters associated with the parity be- tween the two different mechanisms (qS~ = 05r). Obser- vations show that both the q5 t > qSv, s~ > 0 condition (corresponding to the use of thin oxide layers and high diffusion temperatures) and the q5 v > q5 t, sv > 0 condition (resulting from the combination of thick oxide layers and low diffusion temperature) lead to a sharp increase in the diffusion rate. Thus, the exper- imental realization of the parity condition for the two diffusion mechansisms (q~t~b~) or the complete annihilation for vacancies and self-interstitials (st = Sv = 0) near the working surface of the crystal would mean that the initial requirements for pro- duction of quantum-size diffusion profiles and p - n

junctions have been met. Boron diffusion was carried out from the gas phase

into monocrystalline n-type Si (100) wafers of 2 0 ~ c m resistivity and 350#m in thickness. The working and the back side of the wafers were pre- viously oxidized at 1150°C in dry oxygen containing CC14 vapours. In order to allow B diffusion, a number of windows were cut in the oxide layer. The variable parameters of the experiment were oxide layer thick- ness, diffusion temperature and CI levels in the gas phase during the diffusion.

Diffusion profiles were measured using precision layer-by-layer etching. Surface resistance measure- ments were taken on each exposed layer; the measure- ment technique was the four-point proof. It was the only method used to measure the depth of ultra-shallow diffusion profiles. The measurement data for the deeper profiles discussed herein were additionally verified after each etching by using C V characteristics taken in the semiconductor~electrolyte system. Each C - V measurement was routinely com- pared with the SIMS data. The two measurements were generally found to be in good agreement.

Dark leakage current values and external quantum efficiency spectra for diffusion-produced p - n junc- tions were taken on the structures shown in Fig. 1. The structure consists basically of two contrariwise switched-in p - n junctions, and is supplied with a ring-shaped AI electrode, thermally evaporated on its surface using a mask specially designed for the purpose.

The dark leakage current was determined from the reverse 1 - V characteristic yielded by one of the coupled p - n junctions as its counterpart was conducting d.c. The external quantum efficiency spectra for the coupled p ~ junctions were obtained over the 220-1200 nm wavelength range by compari- son with the spectra yielded by Si photodiode refer- ences.

Quantum-size p-n junctions in silicon I151

P+ J k P+ ~J i

Current VoLtage measuring sou rce system

2.

Fig. I. A schematic of the arrangement used for measuring photoelectric characteristics of coupled contrariwise

switched in p-n junctions.

3. EXPERIMENTAL RESULTS AND DISCUSSION

Figure 2a and b, shows the diffusion profiles of B in n-type Si, obtained for different oxide layer thick-

n e s t s and diffusion temperature. For all the tem- peratures used, the p -n junctions are seen to be anomalously shallow at dsio2/do = 1, which is evidence of a change of diffusion mechanism that apparently occurs as the oxide layer increases in thickness. While the oxide overlayers are still thin (curves 1, 4, 7 of Fig. 2), the diffusion proceeds via the kick-out mechanism, as the greatest depths of intrusion observed in this case, for which eqns (3-5) still hold, suggest. But as the overlayer thickness increases (dsio2/do~l), the conditions in the near-surface region change such that nearly all self-interstitials and vacancies present here undergo annihilation [Si = Sv = 0; eqns (3-5)], so that the diffusion front moves very little if at all toward the interior of the crystal (curves 3, 5, 8 of Fig. 2a and curve 5 of Fig. 2b). This is most clearly seen in curve 8 of Fig 2a which has been taken at low temperature when the competition between the two diffusion mechanisms reaches its point of balance ( ~ / = tPv=0.5). With overlayer thicknesses that are so large that dsio2/do> 1, the dissociative vacancy mechanism takes the upper hand, which occurs at diffusion temperatures of 850-900°C (~b v > 4h)[ 1,7], so that the excessive flux of vacancies enters the bulk [Sv> sl; eqns (3-5)], causing the p-n junct ion to increase in depth again (curves 6, 9 of Fig. 2a and curve 6 of Fig. 2b).

1021 (a)

1020

7 E

A ~ m

Z

1019

1018

I0zI

10 20

I 0 ~9

10 le

(b)

5 \ \

I \ \ I \ \

I \ \ I \ \ D

I \ \ I \ \ I \ \

i Ii J \ i J '

Fig. 2. Quantum-size diffusion profiles obtained for the B dopant of n-type at low (a) and high (b) CI concentrations and diffusion temperatures of 950°C (!, 2, 3), 900°C (4, 5, 6) and 850°C (7, 8, 9). dsio2/do- Values are 0.62 (A), 1 (O) and 1.28 (I-']), where d o is the critical thickness of the oxide overlayer, associated with the change of the type of intrinsic defects generated by the oxidized surface, which are self-interstitials at dsio2 < d o and vacancies at dslo: > do, produced in both cases in excessive amounts. The value of d o is determined by the general state of the surface, including the composition of the atmosphere and previous

treatment.

i i I ' I I

0 2 0 4 0 6 0 8 0 0 2 0 4 0 6 0 8 0 100 120 140

xt~) xl~)

1152 N.T. BAGRAEV et al.

As the diffusion temperature is further increased to 950°C, the situation reverts, however, to parity be- tween the two diffusion mechanisms in spite of the dsio/d o > I condition, and a new slowdown in the diffusion process is observed, see curve 3 of Fig. 2a.

The obvious conclusion is that by varying the diffusion temperature and the oxide overlayer thick- ness, it is possible to control the diffusion profile depth and the dopant concentration over a rather wide range.

The striking sharpness of the resultant concen- tration profiles, shown in Fig. 2a and b is clearly unusual for classical diffusion, and it argues for the fractal mode of the present diffusion process. What this implies is that the diffusing dopant atoms are, as it were, drawn along with the strong fluxes of vacancies and self-interstitials rushing into the bulk. As an observed phenomenon, therefore, the fractal diffusion is the inward movement of the dopant impurity front which propagates at a rate that is a function of the magnitude of the flux carrying exces- sive amounts of vacancies and self-interstitials from the surface, and which in doing so follows the pattern that is commonly associated with fractal for- mation[9,10]. There is dual evidence for the proposed fractal model of diffusion: Figs 2 and 3 display a striking timing between the minimal rate of diffusion profile propagation and the parity condition for the two diffusion mechanisms in question. Figure 2 also reveals a clear dependence of the rate of movement and the steepness of the diffusion profile on the C1 levels in the atmosphere at the surface, which have been shown[3,4] to be a controlling factor in the annihilation undergone by self-interstitials and vacancies in the near-surface region. Thus, curve 6 of Fig. 2b, which has been taken at ~bv> ~b~ and s v > O, shows an increase in the depth of diffusion profile

8O

'~ 60 X

40

100

x

o

2O

0 0.5

0 x

I I 1.0

d s i o / d o

1.5

+ Fig. 3. Depth ofp -n junctions obtained at low C1 concen- trations as a function of the dsio:/d o value. Diffusion tem-

peratures: 850°C (0), 900°C (O) and 950°C (×).

following the addition of CI to the ambient atmos- phere. The influence of atmospheric CI is not limited to vacancies, however. An important factor that promotes dopant diffusion at the kick-out stage of the process (q~/> ~bv; sl > 0), i.e. when we have a combi- nation of high diffusion temperatures and low oxide thicknesses, is gettering[3,4]; see curve 1 of Fig. 2b. As the zone of the annihilation extends into the bulk well beyond the working surface region, there is an en- hancement in the strength of fluxes of equilibrium self-interstitials flowing toward the rear surface, and a corresponding rise in the levels of drawn impurity atoms diffusing by the kick-out mechanism. DI thus appears to depend also on the percentage of C1 in the ambient atmosphere, which has an optimizing on gettering.

The parity condition for the two diffusion mechan- isms has been explored as a determinant on the magnitude of the dark leakage currents in reverse- biased p - n junctions, Fig. 4a and b. The relatively high leakage current values observed for 25 mm 2 junctions at low Ss~o2/do ratios are due to the shallow and deep defects that must arise in the region of the p - n junction from the presence of excessive amounts of self-interstitials when the gettering conditions are, as they were in the present case, nonoptimal[7]. That the defects are in fact responsible for the high leakage currents is clearly demonstrated by Fig. 4a where a reduction in the leakage current values is seen to occur with increasing thickness of the oxide over- layer, which condition brings about a decrease in the defect concentration as a result of the boosting effect it has on the gettering[7]. The p +-n-n + structures shown in Fig. 4a had a rather thick p*-region (~300A) and for this reason did not satisfy the quantum-size requirement[7]. Figure 4b presents the data obtained for quantum-size p +-n junctions under similar conditions. As can be seen, these junctions display a sharp rise in the dark leakage current values as the ds~o2/do ratio approaches unity. This is not unexpected given the associated minimization of the fractal profile (see curves 3, 5, 8 of Fig. 2a and Fig. 3) that results in a dramatic enhancement of the recombination-generation processes at the surface. This tendency to form ultra-shallow quantum-size p -n junctions is no longer present, however, once the ds~o,/do ratio is in excess of unity, since this condition (Fig. 2) promotes the predominant operation of the dissociative vacancy mechanism (~v > ~b~; sr > 0), on the one hand, while it meets the requirement for optimal gettering, on the other (Fig. 4a and b). Our determining the conditions in which both of these factors are present has allowed us to obtain the first low-noise quantum-size p - n junctions.

Figures 5 and 6a-c show the external quantum efficiency as a function of oxide layer thickness and diffusion temperature. With high-intensity absorp- tion of monochromatic light in the near-surface region (2 = 400 nm), the highest external quantum efficiencies were exhibited by the samples with p - n

Quantum-size p - n junctions in silicon !153

I(A)

10 -6

10 -7

lO-8

10 -9

~0-1o

10 -11 _

10 -~2 _

( a )

I(A)

• 3 10 - 6

10-7

10-8

2 10-9

10-1o

I0"1

I I 0.5 1.0

( b )

I 1 10-13 0.0 0.0 0.5 1,0

d sio2 / d o d s i o z / d o

Fig. 4. Dark leakage currents in (a) p +-n-n ÷ photodiodes[7] and (b) quantum-size p +-n junctions as a function of the ds~o2/do value at U,,v = 1 V. Diffusion temperatures: 950°C (1), 900°C (2) and 850°C (3).

junctions formed at low diffusion temperatures (Fig. 7). The maximum r/was observed at dsio2/do ~ 1, that is, in condit ions o f near-parity for the two

diffusion mechanisms, where the profile depths and the dopant concentration values are the lowest at Td = 850°C (Fig. 2). For this reason, any losses that

( a ) r/ ( b ) r/ ( c )

0.8 0.8 0.9

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0 0.0

x x x

• o , o o o

0 ~ •

[3 []

0.7

0.6

0.5

0.4

[] 0.3

0.2

0.1

• 8 Z~ x x

0.8

[]

0.7 o

0.6

Q~tO

x O

o 8 • s

I I o .o I I I I o.5 1 I 0.5 1.0 800 850 900 950 1000 0.0 0.5 1.0

dsioa/do T d (*C) ds ioz/do

Fig. 5. Quantum-size p - n junctions' external quantum efficiency as a function of the dsio=/d o ratio, for (a) 2 = 400 nm; (c) 2 = 800 nm and diffusion temperature (b) 2 = 400 nm. Data are shown for low (Fig. 5) and high (Fig. 6) C1 concentrations. The T a values for (a) and (c) are (O) 800°C, ( x ) 850°C, (©) 900°C, ([Z) 950°C, (A) 1000°C. The dsio:/do values for (b) are (O) 0.17, ( x ) 0.31, (©) 0.62, (f-I) 1.01, (A) 1.28.

1154

0 .8 --

( a )

0 . 8 -

N. T. BAGRAEV e t a l .

(b) 0.9-

(c)

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0 0.0

ex x x

o o 8. o

D Q 0 []

0.7

0 .6

ex 0.5

o

0.4

a 0 . 3

0 .2

0,1

o

k

0 . 8

0 . 7

0 . 6

,o• / [ 3 - , - - - o

• 2-~'. l e / T

/

I I - 0 . 0 I I I I 0 . 5 I I 0.5 1.0 800 850 900 950 1000 0.0 0.5 1.0

d s i o J d o T d (*C) dsioJd o

Fig. 6. Quantum-size p-n junctions' external quantum efficiency as a function of the dsio2/do ratio, for (a) 2 = 400 nm; (c) 2 = 800 nm and diffusion temperature (b) 2 = 400 nm. Data are shown for low (Fig. 5) and high (Fig. 6) CI concentrations. The T d values for (a) and (c) are ( 0 ) 800°C, ( x ) 850°C, (O) 900°C, (E3) 950°C, (A) 1000°C. The ds,o,./do values for (b) are ( 0 ) 0.17, ( x ) 0.31, (O) 0.62, (E]) 1.01, (A) 1.28.

r / m a y suffer in the heavily-doped region of the p - n junc t ion are insignificant. N o r would be impor t an t the efficiency of the gettering at 2 = 400 nm, as the light-sensitive region is now localized near the sur- face. The dark leakage currents, however, may in-

crease under these condit ions, because of the intense recombina t ion processes at the surface (Fig. 4b). While Td is kept at 850°C as the thickness of the oxide overlayer is allowed to increase so tha t the ds~o2/do rat io increases f rom < ! to > 1 ( that is, under t ran-

0 . 7

0.6

0.5

7? 0,4

0 3

0 . 2

0.1

0 . 8 -

2

0 . 0 I 1 I I I l I I 1 200 300 400 5o0 600 "too 800 9oo 1000 1100 1200

X ( n m )

Fig. 7. External efficiency quantum spectra for quantum-size p-n junctions obtained at dsio2/do ratios of (1) 0.31, (2) I and (3) 1.28.

Quantum-size p-n junctions in silicon 1155

sition from the dominance of the kick-out mechanism to that of the dissociative vacancy mechanism), the depth of the p - n junction increases, accompanied by a sharp rise in the dopant concentration value (Fig. 2a). This causes large losses of light energy in the near-surface region where the dopant concentration is the highest, and so, as curve ( x ) of Fig. 5a shows, the value of the external quantum efficiency drops. As the diffusion temperature is increased over 850°C, with a corresponding shift (Fig. 2a) of the parity domain toward the range of the dsioJdo values be- yond 1, the external quantum efficiency value contin- ues to lower (Fig. 5b) because of the continuing dissipation of light in the near-surface region, now caused by the rise of the dopant concentration inside the diffusion profile (Fig. 4a), which occurs under the dominance of the kick-out mechanism that takes place in this case. The persistent tendency for r/ to decrease reaches its climax, strange though it may seem, at Td = 950°C and at a ds~o2/do ratio (within the range extending beyond 1) that is very close to the one commanding parity between the two diffusion mechanisms. The explanation lies in the occurrence of an exceedingly high dopant concentration inside a narrow quantum-size profile (Fig. 2a). Supporting evidence for extensive losses of light energy in the near-surface region is provided by the spectral depen- dence R(2) of the coefficient of reflection, shown in Fig. 8. The severity of losses suffered by ~/ in the heavily-doped near-surface strata is seen in Fig. 5c to be less acute for the i.r. region of the absorption spectrum. This fact fits the general picture of ~/ behavior drawn here, indicating the weakening ~/-

0.6

I, ',\ \ I A

0 5 /

0.4 \ -

O. 3 1 I I 2O0 300 400 5 0 0

X ( n m )

Fig. 8. Coefficient of reflection vs wavelength plots for (I) Si samples that were not intentionally doped[11]; and for samples with quantum-size p÷-n junctions obtained at

diffusion temperatures of (2) 850°C and (3) 900°C.

dependence of T d, which is to be expected given that the quantum-size p - n junction's vertical dimension (Fig. 2a) is much less than the light absorption depth value for 2 = 800 nm. At this wavelength, ~/has been observed to peak under the dsio2/do = I condition, where, as the data of Fig. 5c demonstrate, the occur- rence of parity between the two diffusion mechanisms provides for the highest efficiency of gettering in the crystal bulk[7]. For the near-surface regions this means an almost complete annihilation of vacancies and self-intersititals, while the characteristic feature of the situation in the bulk is the transformation of equilibrium self-interstitials to butterfly defects which serve as effective getterers of residual impurities and defects[5,6]. The efficiency of gettering lowers some- what, however, as the thickness ratio decreases below unity; this is because the p -n junction region becomes populated with the getterers that arise from the self-interstitials supplied by the surface in excessive amounts. A reciprocal drop observed for t/ at dsio2/do > 1 is due, in contrast, to the reduction in the concentration of self-interstitials present in the bulk, brought about by a decline in the efficiency of gettering as a result of a change of diffusion mechan- ism and the consequent prevalence of vacancies in the bulk (Fig. 5c).

Figure 6 shows that the efficiency of gettering is also subject to changes introduced in the CI levels during the diffusion process. While at 2 = 400 nm this dependence can be disregarded if only because what- ever effect C1 may have is, as the preceding discussion suggests, overshadowed by the dissipation of light in the heavily-doped regions as the cause of losses suffered by r/, it becomes an influence to be reckoned with at 2 = 800 nm, in view of the heightened import- ance the excessive concentrations of vacancies in- duced by the presence of C1 near the surface assume in the bulk. At high diffusion temperatures and low dsioJd o ratios, the r /= f(dsiojdo) dependence dis- played a second maximum, indicating that the effec- tive gettering occurs in two regions rather than one. While the thickness ratios are still low, gettering of the bulk is performed predominantly by the microde- fects that are produced by self-interstitials supplied there in excessive amounts via the fluxes generated at the surface. However, as the comparison of Figs 5c and 6c will show, the microdefects fail to gain access to the p - n region, losing competition to the vacancies that arrive in the overwhelmingly large amounts as a result of the additional injections from the near- surface regions which are separated from the C1 atmosphere by the layer thin enough to allow the material to be activated accordingly. But the effect produced by the CI ceases as the thickness ratio reaches unity, in which conditions, as described ear- lier, a complete annihilation of both vacancies and self-interstitials takes place in the near-surface region, so that gettering can only proceed as a cluster for- mation process involving equilibrium self-intersti- tials.

1156 N.T. BAGRAEV et al.

4. SUMMARY

In conclusion by means of monitoring the oxide layer thickness and diffusion temperature, we have been able to define the conditions for mutual domina- tion of the kick-out vs the dissociative vacancy diffusion mechanisms. It has been established that the diffusion process experiences a considerable slow- down under the conditions of parity for the two diffusion pathways, with the associated drastic nar- rowing of the diffusion profile that takes place at low temperatures accompanied by a fall in the dopant concentration levels. It is noteworthy that the vel- ocities of movement of the diffusion front are virtu- ally identical for the two diffusion mechanisms. The ability, so acquired, to stimulate and control fractal diffusion has made it possible to obtain the first quantum-size p - n junctions with low dark leakage current values and high external quantum efficiencies over a broad spectral range. The values of dark leakage current and external quantum efficiency ap- pear to be determined by the processes of dopant diffusion and gettering that occur in the bulk. In the long wavelength region of the spectrum, the r/-value is a function of the gettering efficiency, which, as we have established, is typically optimal under the parity condition for the two diffusion mechanisms investi- gated. For the short wavelength radiation, r/ has also exhibited a dependence on the dissipation of

light in the heavily-doped near-surface regions; be- cause of this absorption, the role of gettering is minimized. The study of the electrophysical and optical properties of the quantum-size p - n junctions obtained has shown that those may vary over a wide range, depending on the conditions in which the fractal diffusion of the doping impurity is carried out.

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