Resonant Cavity SiGe/Si MQW Heterojunction Phototransistor Grown on the SIMOX Substrate for 1.3pm Operation

Yuqing Zhu, Qinqing Yang, Qiming Wang

National Integrated Optoelectronics Laboritory Institute of Semiconductors, Chinese Academy of Sciences

Beijing 100083, China 86-14255813 1 X-5 1 l(TEL), 86-1-62562389(FAX), zhuyq@red.semi.ac.cn(Email)

Abstract We propose a method for fabricating a novel SiGe/Si

MQW resonant microcavity phototransistor. The device is to be grown on a SIMOX substrate. A window is formed by etch-stop technique on the back of the SIMOX substrate. A SiO2/Si quarter wave stack is grown on the etched window and acts as the low reflectivity mirror. The upper mirror is a low reflectivity quarter wave stack formed on the top of the designed device. Simulation of the device indicates that improvement of quantum efficiency of the device can be achieved. The characteristics of the phototransistor is calculated following the thermionic-fielddiffusion approach. It is expected that this device have an improved optical gain and bandwidth.

L Introduction High sensitivity photodetectors are key components for

optical communication, information and sensing systems [1*2*31. Researches of the SiGe photodetectors are of interest because of its high reliability and advantages for optical integration on the silicon However, due to the low absorption coefficient of Sil-xGex alloy at wavelength of 1.3pm, a thick absorption layer is needed to achieve high quantum efficiency, which results in a decreased response speed. The quantum efficiency and bandwidth tradeoff is required. Edgecoupled Si 1 -xGex/Si MQW waveguide photodetector has been proposed to improve the response speed and quantum efficiency ~imultanousel$’~~~. A 40% internal quantum efficiency at 1.3pm was obtained with the Ge fraction x=O.~[~] . But the large coupling loss and large junction area limit the further improvement of the device.

It was recently established that a resonant cavity structure provides an effective technique for circumventing the quantum efficiency-bandwidth trade-of$21. Inserting the photodetector into a Fabry-Perot cavity, it significantly enhances the quantum efficiency of the detector at resonant mode by multiple reflections between two mirrors. Usually Fabry-Perot cavity is formed by two mirrors, the bottom mirror is a high reflectivity mirror, and the top mirror is the native semiconductor to air interface providing a low reflectivity of approximately 30%. High-speed detection, high sensitivity and detected wavelength tunableness are the advantages of using resonant cavity and have been demonstrated by the AlxGal-xAs/GaAs resonant cavity

photo detector^[^'^^. In this paper, a SiGe/Si MQW resonant cavity

phototransistor grown on the SIMOX substrate is proposed. The reasons of using the SIMOX substrate lie in that: first, the Si02 layer in the substrate has the properly of etch-stop. Second, it has been demonstrated that high quality SiGe/Si MQW can be grown on the SIMOX substrate. These make it possible to fabricate a high Q microcavity for the SiGe/Si resonant cavity phototransistor. The structure of the proposed device is given in section 11. The calculated quantum efficiency and characteristics of the device based on the thermionic-fielddiion approach are given in section I11 and IV, respectively.

IL Structure Fig.1 is a schematic diagrams of the proposed SiGe/Si

MQW resonant cavity phototransistor. It’s a double heterojunction phototransistor. The SiGe/Si MQW acts as the base and the absorption layer. A window is opened by

hv

1

Fig. 1. Schematic cross section of SiWSi MQW resonant cavity phototransistor

etching deeply into the substrate from the back side of the substrate. The etching will stop automatically at the Si02 layer in the SIMOX substrate. A four periods Si02/Si

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quarter wave stack used as the bottom mirror will be deposited on the window and can provide a reflectivity of approximately 98%. Another one period SiOdSi quarter wave stack will be deposited on the top of the proposed device and can provide a reflectivity of approximately 65%. The cavity is defined by the lower SIMOX substrate and the upper SiO2/Si mirror. The resonant wavelength depends on the cavity length, epitaxial structure and phase changes at the top and bottom mirror. By fine tuning the epitaxial structure, an essentially identical cavity operating at a certain wavelength can be formed. The calculated wavelength dependency of the mirror reflectivity and phase are given in Fig.2 for the top and bottom mirror.

0.5 1.0 1.5 2.0 2.5

Wavelength (pn)

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

Wavelength (pm)

(b)

Fig. 2. (a). Mirror reflectivity for a SiOdSi (216nd92.8nm) quarter wave stack mirror for 2 (dotted) and 4 (solid) periods. (b). Phase of the same mirrors as a function of wavelength.

IIL Quantum Efficiency The origin of the drastic enhancement in quantum

efficiency is the greatly increased amplitude of the electric field inside a high Q resonant cavity which causes more energy to be absorbed in the active region. According to the Kishino's theod2], the internal optical power enhancement factor at resonance as a function of a d (a is the absorption coefficient and d is the active layer thickness) is shown in figure 3, where a d is a small value because of the low absorption coefficient (a=lOOcm-' for Sio 4Ge0 6 alloy) and the small critical thickness of SiGe/Si MQW. It is shown in the figure when the reflectivity of the bottom mirror R2 is fixed to be 0.98, a higher top mirror reflectivity R, can result in a higher enhancement factor. In the case of SIMOX substrate with R1=0.65 and R2=0.99, the factor is close to 20.

loo , . , . , . , . , . , . , . ,

R,=0.98

0.M)Z 0004 0.006 0.008 0.010 0012 0.014 0.016

Ud

Fig. 3. Internal power enhancement factor in resonant cavity with various mirror reflectivity as a function of ad.

0.6 -

0.4 -

1 R,=0.3, R,=O

-0.2 I 0.001 0.01

ad

Fig. 4. Calculated quantum efficiency as a function of ad. Solid lines show q at the resonant mode and dotted line the conventional detector case.

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Figure 4 shows quantum efficiency at resonance as a function of a d for varying RI, while R2 is fixed to be 0.98. The conventional case with R14.3 and R2=0 is given by a dotted line. It can be seen that a detector with a high Q cavity can yield a high quantum efficiency. For a SIMOX substrate based resonant cavity photodetector with 1 pm absorption layer, a quantum efficiency of 18% can be obtained.

IV. Optical Gain For a abrupt emitter-base junction phototransistor, a

planar model is given in Fig5 to illustrate the key features of the gain parameters.

B

C . . I E : n

Fig.5. Schematic illustration of the phototransistor structure

Following the thermionic-fielddiffusion approach developed by Grinberg et a1[*], the electron current density Jn across the hetero-interface (x=O) is the difference between two opposing fluxes:

A E C -_ V Jn(0) = -q-?-[n(O-) - n(O+)e kT ] (1)

4 where AEc is the conduction band discontinuity (spike), vn is the electron thermal velocity, n is the electron concentration. In (1):

n(O-) = N , exp(-YB1 / kf) n(O+) = n(X,)exp(V,, / kt)

(2)

and

(3)

where and ~2 are the dielectric constants and N1 and N2 are the doping levels in the base and emitter regions, respectively, VBI is the built-in potential and VEB is the

applied forward bias voltage. The electron current density given by (1) should be equal to the electron current density due to diffusion Jn(X2) at the boundary of the space-charge region in the base. For a resonant cavity HPT, the electron diffusion equation can be written as:

d2An, An, + ( 1 - R l ) D * F - -

znb 1 - 24-e-& + RlR2e-2nd (4)

x qaPo(e-" + R,e-'&e") = 0

Let:

It can be derived from (4):

where nl, Dob. and L,b are the equilibrium concentration, diffusion coefficient and diffusion length of the electrons in the base, respectively, "1 is the change of electron concentration caused by the optical absorption, and Wb is the base width.

Y, = cpA[(e-"Y1 + R2e-2ade"Y1)ch(3) Lnb

- + R2e-2aded3)

+ L n b s h ( y ) c x ( - e - d l wb + R,e -2ud e ox, )] (6) Lnb

' n b q a P O

cp = 1 -cx2LnbZ

Po is the input optical power. Since the current density given by (1) and (5) must be

equal, the two expression may be equated at the edge of the emitter depletion region, namely n(Xi)-nl:

where

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4 w -- R,, = I + q,,ch(+)e

Substituting (7) into (5 ) and using 'the following relationships:

we find

W e JpE 4y,,

J p (X , ) = -(e kT - l)ch(-) RP LPE

where 9DpeP2

LP

J , = *

Lpesh( 2)

- w _ _ B P

R, = 1 + q,ch(+)e

4vcn a, = 4VB2 --

kT - I)--] y l The expression for the hole component of the collector current density is the same as the equation given by the n,

(9)

The electron component of the collector current is determined by substituting (7) into the following equation:

The substituting yields

9vKn JnE - Rn

J,,(X3) = --{(e kT - 1)

wb 4

wb 4vcn --

- (e - l)[ch(-) + q,e-=sh2 (-)I Lnb Lnb 2% y, -- Rny2 +q,e k~ -->

n, n,

conventional diffusion model of a BJT:

Jpc = J , cosh(-)(exp(- w, -) 4 V C B - 1) LPC kT

where W, is the width of the neutral part of the collector region and Lpc is the hole diffusion length in the collector.

gDpCP3 J , =

Having solved for the current components at each junction, the equation for the emitter and collector currents can be obtained:

L, sinh(Wc / Lpc)

J E = J n ( X l ) + J p ( X 2 ) + JREG

J c = J " ( X 3 ) + J,, (14)

is the recombination current density in the where JRO' spacecharge region of the emitter-base junction. common emitter current gain may be found to be:

The

(15)

Y2 = cpA[(e-"rl + R,e-2*dea-l ) For a floating base phototransistor, the optically generated holes are trapped in the base region and the accumulation of excess holes causes an increase in the foward bias of the injection of electrons from the emitter to the base. Therefore, VBE contains two parts: one is the applied voltage and the other is caused by illumination. Since the emitter current JB must .be equal to the collector current Jc in a floating base Photo+JansistOr~

The value of VBE can be find out by solving the equation.

- (e-&3 + 4e-2"ea3)ch(3) Lnb

+ 4e-2adea3 )I + L,a ~h(-)(-e-&~ wb

Lnb

A similar derivation yields the following equation for the hole component of the emitter current density J " ( X , ) + J P ( & ) + J L - J J & ) - J p c = o (16)

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Table I. Symbol Value

Emitter Doping N. LOX IO'* cm" Emitter Width W, 0.5pm Base Doping Nb ~ . o x ~ o ' * cm-' Base Width wb 20 periods

Collector Doping No 1 . 0 ~ 1 0 ~ ~ cm-' Collector Width WO 0.52pm

Si~.&e,, 6(80A)/Si (250A)

C .- d TI 0

3oa ........, ........, ........, . ......., . . . . 280 - 260-

240 - 220-

200-

180 - 160 - 140 - 120 ........ ' ........ ' ........ ' ........ ' . . . .

1 o-O 1 0-8 1 IT' lad 10"s 10-4

Input Optical Power (W)

Fig. 6. Optical gain versus incident optical power. Solid line shows the case of resonant cavity phototransistor and dotted lines the case of conventional detector.

Calculated characteristics of the device are shown in Fig.6 and Fig.7, respectively. The parameters used in the calculation are given in Table I. Figure 6 shows the dependence of optical gain on the incident optical power, where the solid line represents the optical gain of phototransistor with resonant cavity and the dotted line the conventional phototransistor without resonant cavity. Figure 7 shows optical gain versus collector current for various doping level in the base. It has been shown experimentally that the optical gain of a phototransistoh depends on the magnitude of the input signal and on the current flowing through the device. When the quantum efficiency improved, the optically generated carriers in the base will increase. This will cause an increase of collector current density, and is valuable for the improvement of optical gain and bandwidth of the device. It is expected that the optical gain and bandwidth of a SiWSi phototransistor with resonant cavity phototransistor should be higher than that of SiGe/Si phototransistor without resonant cavity.

..................... ....., . . . . . . . . . . . ....

i" 200

0 180

160

140 10-2 lo-' 100 10' Id 10'

Collector Current Density (Ncm2)

Fig. 7. Optical gain versus collector current for various base doping level.

From figure 6 it can be seen that the optical gain of resonant cavity phototransistor is improved, but the ratio of optical gain of resonant cavity phototransistor to that of the phototransistor without resonant cavity is higher at the low incident optical power level than at the high incident power level. This is because at high incident optical power level, the injection of electrons from the emitter to the base becomes saturated and the optical gain are near the same for both phototransistors with and without resonant cavity. It suggests that the resonant cavity phototransistor will work more effectively at IOW incident optical power, for the responsitivity of resonant cavity phototransistor is much higher than that of the phototransistor without resonant cavity at low incident optical power level.

V. Speed of Response The small-signal gain-bandwidth product of a bipolar

transistor is: 1

2R(7B + 2, + 2, + ZBc) f T =

where ZE, ZB, ZC, ZW are the emitter charging time, base transit time, collector charging time, and collector transit time, respectively. The emitter charging time is: .

+ r,, , and r, is kT

where re is the emitter resistance (re = - the emitter contact and c e b and c o b are the emitter-base and basecollector junction capacitance, repectively. The calculated dependence of cut-off fiequency on the incident optical power level is shown in Fig.8.

9 J c

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l0l0 c RC-HPT /

.... ._.. /-

.__...... conventional HPT

1 o5 1 o - ~ 1 0-8 10.’ 1 0-6 1 o - ~ 1 0-4

Input Optical Power (W)

Fig. 8. Bandwidth versus incident power level.

It is shown in the calculation that the main limitation to the cut-off frequency is the emitter charging time zE. Since the high gain can result in a large collector current, an improvement to the current gain will reduce the emitter charging time and lead to an enhancement of the cut-off frequency. Thick active layer is another limitation to the improvement of the cut-off frequency of the device. Even though the resonant cavity structure provides an effective technique for circumventing the quantum efficiency- bandwidth trade-off, the trade-off is still have to be made for SiGe/Si MQW phototransistor. However, by optimizing the device structure, for example, using the grade SiGe base and sandwich the active layer between the base and the collector, the cut-off frequency and quantum efficiency can be improved simultaniously. This structure will be considered in a later paper.

VL Conclusion We have described a new method for fabricating SiGe/Si

MQW resonant microcavity phototransistor. Calculations indicate some degree of improvement in quantum efficiency, optical gain and bandwidth of the device considered. However, the double heterojunction is not an optimized structure. The trade-off between the quantum efficiency and bandwidth can not be circumvented. To further improve the performances of the device, the structure of the device should be optimized,

Acknowledgment The authors would like to acknowledgment helpful discussions with Dr. Da Teng.

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