APPENDIX II

84

_...-...

C'J I E

(.)

<I:

10- 2

T = 293°K

o DC-dark

-+

DC- white illumination o Initial current-dark

4- Initial current-white illumination

Device P8 W = 28p.m

A

=

8.2 x I0-3 cm2

\

I0-4L_ __ ~_LLL~WWUL----L-~~LLLLU-_ _ _L~~LL~~

102 10-¹ 10° 10 1

V (Volts)

Figure 25 J(V) charactcrjstics (initial current and DC) at low voltages of an unirradjated device at 293°K, made from the loH resistivity portion of the original crystal, showing no trapring. The subtractjon procedure is applied to the data obtained, yielcUng a \ve]J·-defin'"cl square lavi .:1symptote.

86

T = 77°K

• DC-dark

+

^{DC- Si filtered} illumination .._ DC -Ge filtered illumination Jsc

-+-

DC-white illumination

o Initial current-dark

100

9

Initial current- Si filtered ilium. -o- Initial current-Ge filtered

4-

Initial current- white ilium.

V (Volts)

Device P8 W

=

28fLm

A = 8.2 x 10-3 cm2

\

Figure 26 J(V) characteristics (ini tial current and DC) at low voltages of an unirradiated device at 77°K, made from the low

resistjvity portion of the originol crystoJ, siH.Hving negl igible trapping.

Trapping at 77°K (observed as the difference between DC and initial current measurements in the transition and sclc ranges) is less than 5%

and thus negligible.

Figs. 27 and 28 show the J(V) characteristics of a device with noticeable trapping (~17% at 293°K and ~so% at 77°K). For this device, the free carrier concentrations at thermal equilibrium differ by about

0 0

a factor of 10 b~tween T = 293 K and T = 77 K. Hence we note the correlation between trapping in the transition and scl ranges and the strong decrease in the free carrier concentration at thermal equilibrium as the temperature is lowered. In all cases, tiapping before irradia- tion reduces the current in the scl range from its (trap-free) initi~l current value by at most 50%. Compared to trapping after irradiation which is orders of magnitude greater, we feel justified in neglecting whatever traps existed in the crystal before irradiation.

After Irradiation:

After irradiation, the initial (trap-free) current vs voltage

\

at

low temperatures exhibit a "threshold voltage" (Fig. 12). To study

this feature at 77°K, the following ambient conditions have been applied. a) dark \vi.th lit "' 10 min. , where L'lt is the time interval bct\·7een the instant at which the white light is turned off (see Section 2.3) while the device remains short-circuited and the time at \vhich the voltage pulse is applied.

b) dark with lit "' 1 s

c) constant i] lumination \..rith Ge light

1o-2

,--...

C\J I

E

(.)

<r

--:>

I0-³

88

T

=

^293°K

e~ DC- dork

+

^DC-white illumination

o Initial current-dark

-9-

Initial current- white illumination

• Steady state -dark

Jsc trap (oB-?-l

Jk white

0 •

0 •

traps(oB-tl

Device RB W = 48,u.m

A

=

7.15 x 10-3 cm2

V(Volts)

Figure 27 J(V) characteristics (initial current and DC) at low voltages of an unirradiatcd device at 29JOK, made from the high resistivity portion of the original crystal, shO\vjng ~;onw trapping. The suhtraction procedure is ap1)lied to the clara obt3ined from DC and initial current measurements, yielding well-defined squ~re law asymptotes.

T

=

77°K

o DC-dark

+

DC- Si filtered illumination -o- DC-Ge filtered illumination

-+-

DC- white illumination o Initial current- dark

100 9 Initial current- Si filtered illumination

<>- Initial current-Ge filtered illumination

4- Initial current- white illumination

Jsc trap free (o ¢ <>-4-}

Jsc with traps (~~·+l

Device R8 W = 48fLm

A= 7.15x lo-3cm2

V(Volts)

Figure 28 J(V) characteristics (initial current and DC) at low voltages of an unirradiated d~vic~ at 77°K, made from the high

resistivity r:'urtio:~ o~' rhe cni~:in~1l cryst<Jl, sho1ving some trapp-i.it3· ::otc: lh~· s:1ift in the v.:.rtical axis compared to

Figur~: 27.

90

d) constant illumination with Si light (see Fig. 21).

The results of these experiments indicate that the population at thermal equilibrium of free carriers in different levels is involved.

The voltage at

con~tant

current density (J

=

10-3 A/cm2

) in this

threshold range is plotted in function of device thickness in Fig. 29.

A dependence very close to linear is observed. As noted in section 3.3.2, the situation encountered here is one in which the thermal equilibrium fermi level is one or two kT below the 0.47 eV level, and in which the free carrier concentration near the edges is determined by the contacts. A similar situation is discussed by Tredgold(l2

). His computer calculations show that under such conditions, the current can increase faster \vith voltage than

v

² and that, in this range, the voltage at constant current is linear with thickness. As Tredgold expresses it, this situation involves a "current carried by injected space charge but in which the limiting process is confined to the contact

• II

reglons. Such a behavior is be~ieved to hold in S _r T_l

·o

(12)

3 . The similarity betweenTredgold's results and ours leads us to believe that the same mechanism is involved. More work will be required on this subject to uphold or ~efute this tentative explanation.

At room temperature, the Ohmic current in several devices has been measured before and after irradiation. From such measurements, the

thermal equilibrium concentration of free carriers can be obtained. The results are listed in table V.

The free carrier concentration is seen to decrease at the dose of

-

(/) ⁰

>

>

Figure 29