IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-33, NO. 9, SEPTEMBER 1986 1385

A Low-Voltage High-speed Silicon Photodiode with Buried Isolation Region for Short-Haul

Optical-Fiber Communications

G. K. CHANG, A. R. HARTMAN, M . ROBINSON, T. J. RILEY, AND K. Y. LEE

Absfruct-A new epitaxial silicon p-i-n photodiode has been devel- oped for short-haul optical-fiber communications that can be operated at biases as low as 4 V. The device has a heavily doped 5-gm-thick p++ isolation-region between the p+ substrate and the r-epitaxial layer. Fast rise and fall times (2 ns), and low leakage current (40 PA) result from the recombination and trapping of the minority-carrier electrons in the substrate. Experimental results on such an n+-n-p++-p+ device with l.l-mm2 photosensitive area and 2 5 - ~ m epi-layer thickness show quantum efficiency of 80 percent at 825-nm wavelength.

I. INTRODUCTION Silicon photodetectors are widely employed in optical-fiber

communication systems operating at wavelengths of 800-900 nm. The earlier AT&T-designed silicon p-i-n photodiode employs a planar structure with a high-resistivity (> 300 0 cm) epitaxial i- layer grown on top of a p + substrate. The structure is similar to the p+-T-p-n+ avalanche photodiode receiver detector described ear- lier [ l ] , [2]. The quantum efficiency of a p-i-n photodiode when fully depleted is determined by the light absorption and carrier col- lection in the depletion layer, which depend on the i-layer thick- ness. And, it is desirable to have a device that can be operated by standard CMOS or TTL level power supplies [3].

11. PERFORMANCE OF CONVENTIONAL p-i-n PHOTODIODE Double heterostructure lasers of 825- and 875-nm wavelength

were imaged onto the test detectors to measure the response of pho- todiodes. The rise and fall times of the lasers were measured to be 0.6 ns each. The detectors were biased at 4 , 8 . 5 , or 30 V. They had a relatively large area of 1.1 mm2 and their capacitance was approximately 1 1 pF at 4-V bias. Since the detector was loaded with a 50-0 circuit, the system response was dominated by the RC time constant, which exhibited a 10- to 90-percent response of 1.2 ns .

Fig. l(a) shows the transient response of a conventional device with 25-pm i-layer thickness illuminated with 825-nm radiation. Considering the 30- and 8.5-V biases, where the voltage is ade- quate for the depletion of the n-layer, the fall characteristics gen- erally exhibit two components. The first component is due to the instantaneous collection of photogenerated carriers in the depletion region [4]. The second component is due to photoabsorption in the substrate, the graded p + - ~ junction, and, when it occurs, the un- depleted epitaxial layer adjacent to the graded region. This com- ponent is slower since carriers generated in the substrate must dif- fuse to the p + - r junction. Additionally, these photoelectrons, as well as the ones generated in the graded region and any undepleted epitaxial region, must also traverse those regions before being drifted through the high-field depletion layer to the n+ junction. For 30-V bias, the rise and fall times total 2 .5 ns for both com- ponents. However, at 4-V bias the response degrades to longer than

Manuscript received November 14, 1985; revised February 21, 1986. G . K. Chang is with Bell Communications Research, Murray Hill, NJ

A. R. Hartman and K. Y. Lee are with AT&T Bell Laboratories, Whip-

M. Robinson is with Epsilon Technology, Tempe, AZ 85282. T. J. Riley is with AT&T Bell Laboratories, Reading, PA 19604. IEEE Log Number 8609 1 18.

01974.

pany, NJ 07981.

(b)

Fig. 1. (a) Response of a conventional p-i-n diode, illuminated with 18- ns laser pulses, 2 ns/div. (b) Transmission “eye” of a 45 Mbit/s optical- fiber receiver using a p-i-n photodiode biased at 4 V as the detector, 10 nsidiv.

18 ns for an 18-ns illumination pulse. The reduced photocurrent during the illumination indicates that charge is being stored in the undepleted epitaxial layer and the substrate. The charge, which is slowly collected after the illumination, produces long pulse decay times (so-called “diffusion tails”).

Longer wavelength radiation at 875 nm genlxates photoelectrons tens of micrometers into the substrate because of reduced absorp- tion coefficient. The minority-carrier lifetime in the substrate is suf- ficiently long that a major fraction of the carri’ers ultimately diffuse back to the epitaxial layer where they are collected. Slow diffusion tails, caused either by low bias or long wavelength excitation, in- crease the effective dark current of the detector and make that dark current dependent on the previous sequence of excitation pulses. The “eye” diagram of a 45-Mbit/s receiver using such a device is shown in Fig. l(b). The “eye” closure is caused by the dynami- cally generated dark current in the detector and it will reduce the sensitivity of the receiver.

111. NEW PHOTODETECTOR STRUCTURE The new detector has a very heavily doped p+ ’ buried layer that

serves to isolate the pc substrate and the photosensitive epitaxial layer. In addition to the p + + layer, the detector employs: 1) pc channel stop covering the whole perimeter o-F the device to mini- mize the lateral surface leakage current; 2) n’ guarding surround- ing the shallow photosensitive cathode to provide low-resistance ohmic contact and to increase the radius of curvature in this high electric field region; and 3) two annular metal plates-one con- tacting the cathode and also functioning as a field plate to shield the nc-T junction and the other contacting the. p+ channel stop and also serving as a field plate. The two-plate configuration results in low parasitic capacitance over the field oxide area.

The concentration profile of the n+-n-p++-pc diode is shown in Fig. 2. The p + + buried layer is formed by boron implantation into the p+ substrate, and, subsequently, a lightly doped epitaxial layer is grown on top of this composite two-layer substrate. The layer has a concentration of 3 X loi9 cm-3 and a thickness of 5 pm at the end of the fabrication process. The graded distribution of the

0018-9383/86/0900-1385$01.00 0 1986 IEEE

1386 IEEE ' 'ItANSACTIONS ON ELECTRON DEVICES, VOL. ED-33, NO. 9, SEPTEMBER 1986

1E+20

1E+19 +

E I E + 1 5 +

3 t

+ +

c

+ --+

+ + + +

Fig. 2. Concentration profile of the new n+-T-p++-p+ photodiode

p++-p+ junction occu ies approximately a 4-pm region, from X lOI9 to 6 X 10" cm-! The p++-epi-layer interface has rough y a 7-pm out-diffusion tail, 3 X lOI9 to 1 X lOI4 ~ m - ~ , which is s i l n - ilar to the out-diffusion tail of the conventional structure.

IV. PRINCIPLES OF OPERATION The electron carrier's lifetime is very short in the buried-layer

region, on the order of a few nanoseconds, which can be compared to a lifetime of a few hundred nanoseconds in the p + substrate ii]. In this way the buried layer serves as a minority-carrier reconhi- nation region. The electric field results from the doping gradiN!:llt, and can be estimated by the following equation:

E(x) = - - - kT 1 &,(x) 4 N&) dx

where Nu@) is the ionized impurity concentration at a distance x away from the p+ substrate as illustrated in Fig. 3. The elecxic field points from the epi-substrate interface to the p+ substrate. '"he repulsive built-in electric field at the p++-p+ interface also atien- uates the injected substrate current by reflecting the photoelect1,ms back into the substrate and away from the collection region. Eql. iv- alently, this interface serves as a potential barrier for photoelec- trons trapped in the p + substrate. The strong field at the p+ ' . T

graded junction increases the drift velocity of the diffusing carr u s generated inside the graded junction. Therefore, carriers collectl-d in both epi and graded p + + - r junction regions contributed to the fast rise and fall transient responses [6].

Measurement of reverse bias leakage, as shown in Fig. 4, irtli- cates that the leakage of the new device is a factor of 40 sma ler than in the conventional structure (40 pA versus 1600 pA at rclclm temperature and 4-V bias). In the new structure, the electron (: ar- riers in the p + + layer have very short diffusion length so that tlhe volume of the substrate that contributes leakage current to the F IO- todiode is small. In the conventional device, thermally generat1:d electrons in the longer lifetime p+ substrate diffused over ten: of micrometers allowing a much greater collection volume [7]. Willie the rate of generation of electrons in the heavily dopedlayers could be expected to be greater than in the p + substrate, the volume eflrct apparently dominates as evidenced by the experimental results. '.%e new device shows the same activation energy E,/2 for leakage in- crease with temperature as the conventional device (0.6 eV). This can be attributed to the mid-bandgap recombination centers in tlle substrate, epitaxial, and p + + regions.

V. ANALYSIS A N D DISCUSSION The response time measurements for the new device have b l m

obtained for bias voltages of 4, 8.5, and 30 V as shown on Fig.

GRADED REGION

ELECTRIC FIELD PROFILE OF NEW PHOTODIODE 10 20 30 40

-200 -

-400 -

Fig. 3. Electric field profile of the new n+-n-p++-p+ photodiode.

1000. x 10-5

100. x10"

- W

w a x~o.' E 10.

5 - + W

E 3 1. g x10-5 U 0

100 x10-'2

x10-'2 10

100 80 60 50 40 30 20 10 0"'

\ I I I I I I I I I I I I I I

2.7 2.8 2.9 3 0 3 1 3.2 3.3 3.4 3.5 3.6 3.7 x I O - ~ / T ( ' K )

Fig. 4. Temperature dependence of photodiode leakage currents.

5(a). For biases as low as 4 V, the rise and fall times are very fast, about 2 to 3 ns. No evidence of charge storage can be seen in these results. The heavy doping of the p + + layer with its short lifetime reduces the substrate photocurrent injected into the epitaxial re- gion. Adequate carrier transport across the thin undepleted region at 4-V bias is observed.

The heavily doped layer does reduce the quantum efficiency of the new photodetector from 80 to 60 percent at the longer wave- length of 875 nm. This is due to reduced collection near the epi- substrate interface. In effect, carriers injected into the substrate re- combine there rather than contribute to the total photoresponse. We

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL.. ED-33, NO. 9, SEPTEMBER 1986 1387

(b)

Fig. 5. Response of the new n + - r - p + + - p + photodiode illuminated with 18-ns laser pulses, 2 ns/div. (b) Transmission “eye” of a 180 Mbitis optical-fiber receiver using the new photodiode biased at 4 V as the de- tector, 2 ns/div.

have measured the “eye” diagram at 180 Mbitis NRZ data rates. Clear “eyes” are observed using this new device as the photode- tector as illustrated in Fig. 5(b). Two-nanosecond rise and fall times can also be observed here.

VI. CONCLUSION We have designed and fabricated a high-speed p-i-n photodiode

with a heavily doped minority-carrier isolation region. The pho- todiode achieves high-frequency signal detection and can be op- erated with as low as 4-V bias. The device also demonstrates sig- nificantly reduced dark current. All the new device properties make it suitable for high-bit-rate optical communication systems. Detec- tors of this design are now being deployed as receivers in the fiber- optic data links for Information System Network, and Systems 85TM PBX [8].

ACKNOWLEDGMENT The authors would like to recognize the technical contributions

of R. E. Carey, R. F. Yock, and A. Chen and the guidance of J . Godfrey, A. U. MacRae, and P. W. Shumate.

REFERENCES

[ l ] H . Melchior, A. R. Hartman, D. P. Schinke, and T. E. Seidel, Bell Syst. Tech. J . , vol. 57, no. 6, pp. 1791-1807, July-Aug. 1978.

[2] H . Melchior and A. R. Hartman, in IEDM Tech. Dig., p. 204, 1976. [3] G. K . Chang, A. R. Hartman, M. Robinson, T . J . Riley, and

[4] A . G. Jordan and A. 6. Milnes, IRE Trans. Electron Devices, vol.

[ 5 ] J . Dziewior and W. Schmid, Appl. Phys. Leu . , vol. 31, pp. 346-348,

[6] M . Conti, G . Conda, and M. De Padova, Solid-Srate Electron., vol.

[7] J . Y. Lee, IEEE Trans. Electron Devices, vol. ED-28, no. 4, pp. 412-

[8] D. P. Morrison, presented at FOCILAN’84, Las Vegas, NV, Sept.

K . Y . Lee, in IEDM Tech. Dig., pp. 733-736, 1984.

ED-7, pp. 242-251, 1960.

1977.

22, pp. 151-155, 1979.

416, 1981.

1984.

A Poisson C-V Profiler

JEFFREY M. BLACKSIN

Abstract-The determination of doping profiles from MOS C-V curves has previously been calculated by the use of the depletion ap- proximation and other correction techniques. A, method is presented based upon solving Poisson’s equation for the charge distribution of an assumed doping profile, calculating the capacitance from that dop- ing profile, and allowing an iterative calculation of the doping profile to produce the desired C-V curve.

I. INTRODUCTION The simplest technique used today to determine doping profiles

from MOS C-V curves is to use the depletion approximation and some numerical corrections to account for the {distribution of charge near the surface [1]-[lo]. The C-V curve is numerically differen- tiated to determine the doping profile. The numerical error that re- sults in performing the differentiation is a function of the numerical method used and the noise in the measured C-V curve. This tech- nique typically results in a large numerical error. In order to reduce the numerical error and to produce more accurate doping profiles, a method of solving Poisson’s equation has been employed, which helps reduce the numerical noise present in the data. In the original version of this program [ 131, the user constructed a doping profile manually and the profiler iterated to a solution based upon that user- defined profile. In this program, the profiler is then allowed to it- erate until the user, who can view both the given C-V curve and the current theoretical curve, decides the match is close enough. The technique described here provides a more accurate doping pro- file than any previously described methods.

11. MEASUREMENT TECHNIQUE The devices measured were boron ion-implanted MOS capaci-

tors with gate oxides in the range of 30 to 45 rim. The devices were measured with an HP 4192 LCR meter so as to eliminate any series resistances that might be present due to the leads and the bulk ma- terial. The bulk material used was p-type 40-60 D * ctn. The device under test was pulsed into heavy accumulation for a period of 300 ms and then pulsed to each of the bias points in the C-V curve to force the device into deep depletion at all bias points. An example of the resulting C-V curve can be seen in Fig,. 1.

111. How THE PROFILER WORKS Inputs to the profiler include the numerical data corresponding

to the C-Vcurve, a background concentration, an acceleration fac- tor (which is used to enhance the rate of convergence), and a ca- pacitor area. In the current version of the profiler, oxide charges are neglected. The program starts out assuming a constant back- ground doping as in the first iteration, and calculates the C-Vcurve from this profile. This is done by numerically solving Poisson’s equation in one dimension for each bias point. A grid is set so that the doping is represented as a discrete function of distance and the program numerically solves the differential equations to determine potential versus distance at each bias point. The boundary condi- tion is the applied voltage across the capacitor between the vertical bulk and the gate electrode. With this, the total MOS capacitance is determined as the difference in charge divided by the difference in voltage. This methodology of solutions is based upon that used in a modified version of the device simulator SEDAN [ 121.

The methodology used to converge on the correct profile pro-

Manuscript received November 13, 1985; revised February 24, 1986.

IEEE Log Number 8609120. The author is with Digital Equipment Corporation, Hudson, MA 01749.

0018-938318610900-1387$01.00 0 1986 IEEE