66 IEEE TRANSACTIONS ON ELECTRON DEVICES, YOL. EC-27, NO. 1 , JANTJARY 1980

Double Injection in Extrinsic Silicon Infrared Detectors

Abstract-The current-voltage characteristics of p+-p-p+ and n+-p-p+ structures are investigated as their behavior impacts on the performance of extrinsic silicon monolithic focal plane arrays. In particular, the break- down voltage of n+-p-p+ devices, which exist as a result of the fabrica- tion process, is much lower than that of the photoconductors. This limits the voltage that can be applied to the MFPA. What this opera- tional limit is and how it can be overcome is discussed.

INTRODUCTION

I NFRARED extrinsic monolithic focal plane arrays (MFPA's) consist typically of a substrate, which is the detector, and

an epitaxial layer of opposite conductivity, which contains the signal-processing functions. In addition, there may be highly doped regions at the substrate/epi-layer interface, as shown in Fig. 1. The p + buried layer facilitates the photo- current zph collection, while the n + island shields the substrate from CCD clock feedthrough.

A positive substrate voltage drives the holes to the epi-layer. Such a voltage, however, forward biases the n (epi)/p (sub- strate) junction allowing electrons to be injected into the substrate. This can lead to dark currents Zd much higher than the photocurrents. As a result of the low operating tempera- ture, the bulk resistance of the substrate is very high and the diode behavior deviates considerably from the conventional exponential current-voltage characteristic. The Z-V behavior is primarily determined by double injection of electrons and holes and can exhibit negative resistance characteristics at voltages typical of the working voltages of MFPA's.

The phenomenon of double injection is, of course, well known and has been studied over the years. It is known to occur in extrinsic devices at low temperatures. We are con- cerned with it here as it affects MFPA's and possible problems it can pose in the operation of these devices.

SPACE-CHARGE LIMITED CURRENTS Space-charge limited (scl) currents can flow in both p+ -p-p+

and n+-p-p+ devices. In this section we give a brief qualitative discussion and then present quantitative, experimental results further on.

Consider the p+ -p-p+ photoconductor of Fig. 2(a). Under

Manuscript received June 12, 1979; revised August 28, 1979. D. K. Schroder was with the Fraunhofer Institut fur Angewandte

Festkorperphysik, D-7800 Freiburg, Germany, on leave from the Westinghouse Research and Development Center, Pittsburgh, PA 15235.

J . C. R. Hornung is with the Fraunhofer Institut fur Angewandte Festkorperphysik, D-7800 Freiburg, Germany.

'V, -v, t t

Fig. 1. Cross section of an extrinsic Si MFPA. On the left are shown the variously doped regions and currents. The potentials and electric fields are shown on the right, with surface potential cps and p + island potential cpp indicated by arrows.

Log v "80 VSCL

(b) Fig. 2. Band diagram and current-voltage characteristics for a p+-p-p+

photoconductor showing breakdown due to impact ionization (VBD) and space-charge limited current (Gc1).

normal operating conditions, where such a device would be used, p << N i << N j . Here p is the mobile hole concentra- tion given by p = pth t pop, with pth = thermal and pop = optical hole concentration, all in units of cmV3. N i is mainly determined by the compensating donors ND, so that N i "- ND. At low voltages, the current density is ohmic and given by

0018-9383/80/0100-0066$00.75 0 1980 IEEE

SCHRODER AND HORNUNG: DOUBLE INJECTION IN EXTRINSIC Si IR DETECTORS

where L is the device thickness. For the current to remain ohmic it is necessary for the p + anode contact to supply only the holes required by (1).

As the voltage is raised, it becomes possible for the anode contact to inject holes beyond those required by (l), i.e., the contact injects majority carriers. Consequently, the hole concentration increases from p to p + pinj, where pinj, is the injected hole density. Space-charge-limited current begins to flow when pinj y p , i.e., when as a result of injection, the hole concentration has doubled [l] . Such a doubling, how- ever, is not: easily achieved since initially almost all of the in- jected holes are captured by N i . In order to double p , a shift of the quasi-Fermi level downwards by 0.7 kT is required. It is easily shown with the aid of a Shockley graph [2] , that such a shift i s sufficient to fill the N i acceptors, leaving a positive bulk space charge of No. The voltage required to sus- tain this space charge is

= qND L2/2€. (2)

At voltages less than V& the current is ohmic since the in- jected carriers are captured and not available for a current increase. Beyond V& all injected holes contribute to the cur- rent and a rapid rise is expected at l&, indicated in Fig. 2(b).

Substitution of numerical values for Si in (2) gives

J&~ = 7.7 x io+ N~ L ~ .

For typical values of No = 1013-1014 cm-3 and L = 5 X cm, Kc! N 2 X 103-2 X lo4 V. Clearly Vscl is so high that the device would break down due to impact ionization at voltages much below indicated by the dashed line in Fig. 2(b). These considerations indicate that scl currents should not be observed in typical extrinsic Si photoconductors used in MFPA's. The rapid current rise has been seen in thin, boron-doped, extrinsic Si devices at 4.2 K with low No [3] , confirming (2). It has also been observed in neutron-irradiated Si [4] .

In contrast to p+-p-p+ elements, where only holes can be injected from the contacts, in n+-p-pt devices holes and electrons are injected. This has drastically different effects on the I-V characteristics [ S I , [6] and these manifest them- selves at voltages that fall well within the typical operating voltage range of MFPA's, because it is not necessary that N i be filed before a departure from ohmic conduction is observed. Consider Fig. 3(a). When the applied voltage ex- ceeds the built-in or diffusion potential V,, electrons are injected into the p substrate. The device behaves as a conven- tional diode and the current rises exponentially, indicated by the vertical current branch in Fig. 3(b). When the injected electron density is of the order of the hole density pth t pop, hole injection from the p' contact causes the current to be scl with I - V 2 . This can be clearly seen by subtracting the built- in voltage from the applied voltage. From Fig. 3(b) this gives ohmic behavior below V,, shown by the 01 = 1 line, indicating that hole injection has not o,ccurred up to that point, but does start beyond VD.

An exact analysis [6] reveals that at higher .voltages the current increases more steeply (CY > 2) as a result of the mobility and lifetime dependence on electric strength.

n* P P'

/ ---------I

"0D Log v (b)

Fig. 3. Band diagram and current-voltage characteristics for an n+-p-p+ diode showing the current rise at the diffusion potential (V') and double-injection breakdown (b,).

1 T 1

Extrinsic Substrate

Fig. 4. Schematic of the p+-pp+ and n+-pp+ devices used in this work.

Continued electron injection further raises N i , tending to re- duce the hole lifetime ( T ~ - I / N i ) in turn. As the voltage is further raised, more holes are also injected. Hole injection, however, reduces N i thereby raising T ~ . But a higher T~ allows hole transit at lower voltages. A result of this rather compli- cated double-injection phenomenon, in which both electrons and holes and their capture by N j and N i play the decisive role, is that the current can be sustained at lower voltages leading to a negative resistance at V,, indicated in Fig. 3(b). For a constant potential across the device, the current increases drastically leading to device destruction as a result of excess power dissipation in some cases. Such double-injection, negative-resistance behavior has been observed in Si n+-p-p+ devices doped with In [ 6 ] , T1 [7], Au, Zn, Cd, Co, and Mn [SI .

EXPERIMENTAL The structures of Fig. 4 were used for the measurements of

single and double injection. For simplicity, the devices were fabricated directly on the substrate without an epitaxial layer. Furthermore, the areas of both types of devices were equal at 2.5 X cm2 . Neither of these conditions are the same in an MFPA. '

The p + and .n+ regions of Fig. 4 were formed by ion implan- tation with B, 50 keV, 2 X IOl4 cm-2, and P, 100 keV, 5 X loz4 cm-*, respectively. The doses were chosen to give

6 8 IEEE TRANSACTIONS ON ELECTRON DEVICES, V3L. EC-27, PITO. 1 , JAMlJARY 1980

approximately equal infrared transmission through the doped layers [9] . The substrate material was Czochralski pulled with concentrations of 8 X 10l6 cm-3 for the Si:In and l O " ~ m - ~ for the Si:Ga. The former was phosphorus compensated with Np - N B E 7 X lOI3 cmm3, while the latter was slightly undercompensated. For this reason, we have chosen the Si:In curves as examples. The Si : Ga curves are very similar but show lower breakdown voltages than anticipated as a result of undercompensation. The doping concentrations were deter- mined from room temperature C-Vmeasurements [lo] on the n'-p-p+ devices, located immediately beside the p+-p-p+ photoconductors.

The current-voltage characteristics were measured both in the dark, with a cold shield surrounding the devices, and with 300 K background irradiation with a 90" field of view. The dc measurements were carried out so that heating was insignificant. This was not the case at voltages near breakdown, however, for either type of device, when the current was high and under these conditions rose with time, clearly the result of heating.

The breakdown of n+-p-p+ devices was measured by apply- ing a voltage ramp until breakdown occurred, observed by a voltage collapse across the device. The applied voltage was then immediately switched to zero to avoid overheating. This procedure could be repeated many times without device destruction. The ramp rate was varied over the 50-mV/s to lO-V/s range and the breakdown voltage was slightly higher for the higher ramp rates, probably due to less heating. Momentary current spikes were sometimes observed near breakdown as if a current filament had formed only to collapse again.

RESULTS

A. Current- Voltage Characteristics 1) ForwardBius: The current-voltage characteristicsof both

types of devices are shown in Figs. 5 (a) and 6(a). Fig. 5 is for dark current while Fig. 6 is the photocurrent. The p+-p-p +

curves are initially ohmic, followed by a superlinear rise and eventual breakdown. In order not to destroy the devices, the voltage was limited to 300 V, corresponding to an electric field of around 8.5 X lo3 Vlcm, and breakdown was not quite reached. For clarity, the curves are shown for only a few temperatures, but higher and lower temperature curves follow the trend shown, that is for the dark curves the current increases for higher and decreases for lower temperatures. The photo- current increases for the higher temperatures because the dominant current is thermally generated dark current. At lower temperatures, dark current can be ignored and the photo- current changes very little, determined only by temperature- dependent device parameters.

The n+-p-p+ curves rise rapidly around - 1 V, the voltage corresponding to the built-in potential of the n+-p junction. During the rapid rise, the I-V characteristic is determined by injection of electrons and a detailed measure, not shown here, reveals that the normal diode relation I = I , exp (qV/nkT) with n 1 at T = 50 K is followed. The current rise becomes less steep with increasing voltage and is then determined by the photoconductor bulk and double injection. Eventually, the curves become steeper again until the breakdown voltage is reached and the diode enters the negative resistance region.

10 - 2

- E 1 2 I 3 7-

6 10

t 67 0.1 1 10 100 1000 0.1 1 10 100 1000

-vd (VI +%(VI

(a) (b) Fig. 5. Dark current-voltage characteristic for negative and positive

bias on the top contacts of Fig. 4. These curves are for Si:In devices of 5 X 10-2-cm thickness. The slanted! arrows in (a) designate double- injection breakdown followed by a negative-resistance region in the device.

1

10 - 7 b - J 0.1 1 b 10 100 too0 0.1 t 10 100 1000

-v, (VI 'Vd IVI

(a) (b) Fig. 6. Photocurrent-voltage characteristics of the same devices used

in Fig. 5 . The temperature steps are higher here since the current changed less with temperature.

The voltage across the device drops to around -5 V and the current density reaches a value in the l-lO-Alcm' range, limited by the ammeter input resistance in these measurements.

2) Reverse Bias: Although in the usual mode of operation of MFPA's the top contact is) .negative with respect to the bottom, measurements on our devices were also carried out under positive bias. These are s.hown in Figs. 5 (b) and 6(b). For the photoconductor the results are similar to those of forward bias, as expected for a device with no asymmetry. They are usually not quite the same at the higher voltages where the current for positive bias on the top contact is frequently slightly higher. The cause is not understood, but may be due to a somewhat different field distribution.

For n+-p-p+ devices, the reverse-bias behavior is very differ- ent from the forward-bias case. The current is very low until the applied voltage exceeds the breakdown voltage of the n+-p junction, which is typically 15 V for extrinsic doping concentrations around 10'' ~ m - ~ . Beyond this voltage, the current rises rapidly and approaches that of the p+-p-p+ photoconductor. One can think of the n+-p-p+ structure as a reverse-biased space-charge region (scr) in series with a photo-

SCHRODER AND HORNUNG: DOUBLE INJECTION IN EXTRINSIC Si IR DETECTORS 69

o nap p*

100 _ _ _ _ _ _

10 30 100 10 30 100

T ( K ) T i K ) (a) (b)

Fig. 7. Double-injection and photoconductor breakdown voltages for Si :In and Si :Ga devices. For Si:In: curves (a), (c), ( d ) , and (e) are for L = 500 pm while ( b ) is for 320 pm; (a), (b) , (c), and ( d ) are for 300 K, 90° FOV, while (e) was taken in the dark. For Si:Ga: L = 350 pm, 300 K, 90' FOV; curves (a) and (c) are measured on devices slightly undercompensated, i.e., NB > ND, while ( b ) and ( d ) are ex- trapolated to properly compensated devices.

conductor. For voltages below reverse-bias breakdown, the I-V characteristics are governed by the scr, while for voltages above breakdown, the high resistance of the photoconductor is the current-limiting element. No carrier injection takes place under this bias polarity. By subtracting the breakdown voltage from the applied voltage, the I-V curves are very similar to those of the p+-p-p+ photoconductor, showing that the "p" photoconductor behaves similarly in both devices.

The reverse-biased scr supplies only as many holes as are required for the photoconductor current. In the dark, the series resistance is very high, the current low, and the scr has to supply only a small flow of holes. With irradiation, the resistance drops and a slightly larger fraction of the applied voltage shifts to the scr increasing the current just enough to supply that required by the photocurrent. Injection of holes beyond those required is not possible. This makes the n+-p-p+ device an interesting tool in the study of hole injection by comparing the current carefully with that of the p+-p-p+ photoconductor, where hole injection is more likely to occur as discussed in the introduction.

B. Breakdown Voltage The key results of this paper are contained in Fig. 7, where

the breakdown voltages are shown as a function of temper- ature for both types of devices. The main point to note about this figure is that breakdown due to impact ionization(p+-p-p+ photoconductors) is significantly higher than that due to double injection (n+-p-p +). Impact-ionization breakdown is discussed by Bratt [113 and the critical field is around 2 X lo3 V/cm for Si : Ga and lo4 V/cm for Si :In. The breakdown voltage of double injection according to existing theory [5] is

where L is the device thickness, El is the electric field beyond which the mobility is field dependant, cp is the capture co- efficient, and Y is defined by rP - E', Le., it expresses the

field dependence of the hole lifetime. It has been observed that Y "- 3 for Si : In [5] , giving

(4)

Equation (3) shows that VBD is determined by material parameters and device thickness and, to frrst order, one would expect Si : In and Si :Ga to behave similarly, as Fig. 7 indeed shows. The main difference between the two is the temper- ature at which V,, decreases sharply. This is determined by thermal activation and depends sensitively on the energy level of the extrinsic impurity.

Curves (a), (b), and (c) of Fig. 7(a) are for devices from the same wafer, while (c) was fabricated at another time. The dip in the curves at 40 K is not understood. The magnitude of the Si :In photoconductor breakdown voltage is in excess of 350 V for T < 50 K. For a diode breakdown of -20 to -40 V, there is a large ratio of these two voltages. For Si : Ga, the double- injection breakdown is in the same voltage range, but the photoconductor breaks down at a much lower voltage, giving a much lower ratio. The dashed curves of Fig. 7(b) are an ed- ucated guess of how properly compensated devices behave, based on the temperature dependence of photocurrent of Si :Ga photoconductors.

The thickness dependence of (4) is V'D - L By thinning the device from 500 to 320 ,urn, the breakdown voltage should decrease by (500/320)"5 = 1.95. From Fig. 7 (a) it is seen that VBD is reduced by roughly a factor of 2, from curve (a)-(e).

The double-injection breakdown voltage can be altered by changing material parameters and it is observed in MFPA's that it depends on processing. However, a reduction of ,up and T ~ ,

whch should lead to a VBD increase, also decreases the photo- current given by Jph = q@Bpp7p ? / L 2 . The only parameter left is the device thickness L , whch should be high for both VBD and Jph, where it is contained in the quantum efficiency Q for the latter. These considerations show that not much can be done in the device itself to alter the double-injection break- down voltage to any appreciable extent. However, there are some operational parameters that can be changed as discussed in the following section.

DISCUSSION

For an MFPA design similar to Fig. 1 there are n-p junctions in the device. If, in addition, the bias polarity is as shown in that figure, as it is under normal operating conditions, than this junction is forward biased. Such a forward-biased diode exhibits double-injection breakdown which, of course, means that V, cannot exceed this breakdown voltage.

The data presented above were obtained on devices of equal area. The designer of MFPA's, however, has the choice of making the p+ and n + areas quite different. This does not change the breakdown voltage, but does change the current magnitude prior to breakdown. This is important, because Fig. 6(a) shows that even at voltages below breakdown, the current density of the n+-p-p+ element is much higher than that of the p+-p-p+ photoconductor. Furthermore, it may be possible to eliminate the n + contacts altogether. Since the p + contacts must be separated, there will always be an n-p

70 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. E=l-27, KO. 1, JA.hTUARY 1980

junction, where in the absence of the n + layer, the doping of the n epi-layer is much lower. What effect this lower doping has was not investigated here. At the operating temperatures characteristic of Si:In, the electrons in the epi-layer are not frozen out and are, therefore, expected to behave similarly to what is observed in our case. For Si:Ga temperatures, the electron concentration is low due to freezeout and what effect this will have on the n+-p-p+ behavior is not known.

In addition to device layout, there is also a certain amount of flexibility in the operating voltages. For a grounded epi- layer, a positive potential on the lower p + contact drives the holes to the CCD. The electric field in the photoconductor under the n + contact is E = (V, - V*)/L 2: &/L, where VD is the diffusion potential of the n+-p diode, which is of the order of 1 V. The field under the p + contact is higher, how- ever, because the contact is held at a negative potential induced by the gate voltage - V,. The field is E = (V, - 4 p ) / L , where GP is the potential of the floating p + island. For the case shown in Fig. 1 , with top and bottom floating p + contacts, the top contact assumes a potential close to the surface potential under V,. The potential is considerably smaller because a good portion of the surface potential is dropped across the scr which connects the two p + islands. A through- diffused p + contact would be preferable since it would not require an scr through the epi-layer and the entire surface potential appears on the p + contact.

A sufficiently high resistor in series with MFPA prevents n+-p-p+ breakdown by limiting the current through the device. By shifting the voltage from the MFPA to the resistor, it, of course, reduces the photoconductor voltage as well, but pre- vents device destruction.

These considerations have shown that there are several means available to reduce the “double-injection problem” and by proper choice of layout and operating conditions it may be entirely eliminated. However, it is a phenomenon that exists and should be considered in the operation of ex- trinsic MFPA’s.

CONCLUSIONS In extrinsic Si infrared MFPA’s it is quite likely that

both p+-p-p+ and n+-p-p+ devices exist next to one another. While the former are conventional photoconductors with V,, 2 300-400 V for Si: In and 80-100 V for Si:Ga, the latter exhibit double injection with breakdown voltages around 30-50 V for both. These low voltages place a limit on the bias voltage that can be applied to the MFPA. The photo-

conductor breakdown is determined. b y device t:hickness, energy level, and doping concentration of the extrinsic im- purity. Double-injection breakdown also depends on device thickness, but depends on and is improved by reducing life- time and mobility in the material. If that can be achieved locally in the n+-p-p+ region of the device alone, then it could be used as a controlling variant. If, however, it cannot be done locally, then it will also degrade the photoconductor perfor- mance. Device design and proper choice of operating voltages can be used to reduce the effect of do-uble injection so that it may not be a significant limiting mechanism in extrinsic arrays.

ACKNOWLEDGMENT

The authors wish to thank Dr. J. Basm for encouragement and H. 0. Moessle, G. Bihlmann, and :E. Olander for expert experimental assistance, and Dr. R. N. Thomas and Dr. H. M. Hobgood of the Westinghouse Research and Development Center for the Si :In wafers,

[4 1

[71

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