2001 Intemational Conference on Indium Phosphide and Related Materials Conference Proceedings 13th IPRM 14-18, May 2001 Nara, Japan

17:OO - 17:15 WA3-4

Integration of Heterostructure Bipolar Transistor and Electroabsorption Waveguide Modulator Based on a Multifunctional Layer Design for 1 S5pm

T. Reimann*, M. Schneider', P.Vellin<, S. Neumann*, M. Agethen*, R.M. Bedenburg*, R. Heinzelmann', A. Stohr', D. Jager', F.-J. Tegude*

' Solid-state Electronics Dept., Fachgebiet Optoelektronik Zentrum fur Halbleitertechnik und Optoelektronik (ZHO), Gerhard-Mercator-

Universitat Duisburg, Lotharstr. 55, 47057 Duisburg, Germany, reimann@hlt.uni-duisburg.de

Abstract

An electroabsorption waveguide modulator (EAM) for 1.55pm is embedded into the layer-stack of a.heterostructure bipolar transistor (HBT). The collector consists of a complete optical waveguide and enables the monolithic integration of modulators, transistors and also a new merged device to result in a modulator with integrated amplifier. At present this device offers a 3dB cut-off frequency for optical modulation of 7GHz and for pure electrical operation cut-off frequencies of fT and fmax

both of about 25GHz.

I. Introduction

There is a continuous development in the monolithic integration of electronic and optical devices to add functionality and improve performance. Opto-electronic components combining HBT and e.g. lasers and photo- transistors have been shown (for a review see (')). Other circuits make use of HBTs and optical modulators. To combine both devices a straight-forward way is to grow the two layer structures for each device on top of the other.'" However this requires complex processing as both devices have to be fabricated separately. A different approach is to place the modulator inside the layer stack of a HBT. For GaAs-based HBTs Okada et al. (') used the base region as a carrier-injected optical intensity modulator. The base has the disadvantage of higher intrinsic absorption, due to the high p-doping. Integrating the modulator inside a HFET and also using the current through the device for modulation (band-filling effect) has been reported,

Other groups place the waveguide inside the collector r eg i~n . (~ ) - (~ ) Shaw et al. (" developed switches, where the HBT controls the reverse bias at the switch for changing

I 8

0 . L I

Fig. I . HBT-EAM in common-emitter configuration. The EAM is embedded inside the HBT-collector and is driven according to VcD and I,-

the refractive index. Shamir et al. (6) investigated the influence of the current flow through a modulator and use the transistor as a current source for modulation.

Our approach employs mainly the voltage changes between the base and collector and use the electric field, which changes the band edge due to the Franz-Keldysh effect (FKE). Additionally the collector current influences the band gap due to the band filling effect. The final layer stack enables the hnction of single modulators and HBTs, and also of a new type of merged device (HBT-EAM). The advantage of this concept is the ability to control the electric field inside the collector region by small base currents.

The principle of operation is as follows: The HBT of the merged device is operated in the common emitter configuration with load resistance (Fig. 1). In this mode the base-collector diode is reverse biased and a variation of the base-collector voltage V,, results in an absorption change. If the transistor is in the off-state V,, is high and this results in a band-gap shrinkage of the waveguide material, thus in a red-shift of the absorption edge. On the other hand, if the transistor is switched to the on- state, the electric field and so the absorption inside the waveguide is reduced. This process is supported by the high collector-current which further increases the band gap by band-filling.

11. Epitaxial Growth

As the focus is on the merged device, the operation of the individual devices, HBT and EAM, should be reasonable. To enable high voltage swings, needed for modulation of the EAM, the layer stack is based on a double-HBT (Fig. 2). The choice of the high band gap

0-7803-6700-6/01/$10.00 02001 IEEE 440

Fig. 2. InP-based layer-stack grown by LP-MOVPE. The EAM is integrated into the collector region of the HBT.

5 -25

$, -50

-75 E .z -100

5 -125

.

U-

W - a, -150

-175

material InP in the collector accomplishes waveguiding in the quaternary material and also improves the breakdown voltage BV,,. To optimize device operation the design includes different trade-offs. The light has to be confined inside the waveguide and should not leak into the base with its low band-gap material. On the other hand the collector region has to be thin to achieve large electric fields and low carrier transit-times. The thickness of the upper cladding depends on the thickness of the guide material. A commercial tool is used to simulate the light propagation inside the waveguide. We found, that a thickness of 420nm for the guide and 450nm for the upper cladding provide optical losses of only 20dB/mm and a small overall thickness (at h = 1 S5pm). The lower cladding can be combined with the sub-collector and substrate. To reduce current blocking due to the conduction band spike at the base-collector transition, an undoped InGaAs spacer is introduced. Nevertheless low collector, i.e. low waveguide doping, which is preferable for high electric fields and thus efficient modulation, results in degraded transistor performance in comparison to higher doping. So the layer structure shown in Fig. 2 results. The layer stack is grown on si.-InP by LP- MOVPE. First the n-doped sub-collector (n’-1nP:Si) was deposited, which behaves together with the substrate as the lower cladding. The focus on the 1 S5pm wavelength range for communication networks suggests the use of an InGaAsP (hg=l .48pm) collector (intrinsic) as the core and active region of the optical waveguide modulator. A further InP layer acts as the upper cladding. The stack is finished by the InGaAs spacer, the base (p’-1nGaAs:C) and emitter (n-InP).

After growth the upper cladding has a low n-type background doping resulting in a pn-junction at the base- collector transition. A one-dimensional simulation, based

.-v

. Base Cladding Guide Cladding .

-

-

- r

-

- -

-

- I I

(Reverse bias V,..=5V)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

x (vm)

Electric field simulation of the basekollector Fig. 3. region from the HBT-EAM in reverse bias with different dopings in the upper cladding.

Fig. 4. SEM viewgraph of a HBT-EAM. The symbol “ W t denotes the passive wavegide.

on Poisson- and continuity-equation and Fermi-Dirac statistics shows, that the electric field is highest at this transition (Fig. 3). To study the waveguide doping influence two wafers were grown with different n-doping in the upper cladding: M1993 (n = l . lO”~m.~) and M2038 (n = 5.10”cm”).

111. Device Fabrication

From the resulting layer stacks single HBTs and merged devices (HBT-EAMs) were processed in the same run. A four mesa process enables fabrication of the different devices. For comparison EAMs were made from the same layers in a different run. We used optical lithography with conventional wet-chemical etching and metalization layers. As an example Fig. 4 shows a SEM- picture of one HBT-EAM. The electrical contacts are provided via air-bridges and are denoted with E: emitter, B: base, and C: collector. Different contact schemes for the active region have been employed. Optical connections on chip between the device and cleaved

441

. .... M2038A not self-aligned, HMO3 12 - 10 -

z 8 - E . - o 6 -

4 -

2 .

0 -

Y

0 1 2 3 4 5 6 v,, (V)

Fig. 5. HBT-EAM common-emitter characteristics for two devices with different collector dopings. The devices dqfer strongly in turn-on voltages.

facets are realized by passive waveguides (W), consisting of the collector and sub-collector layers.

IV. Measurements

Different kinds of measurements were performed on the finished devices.

I ) Electrical Measurements

The merged devices (HBT-EAMs) can be used also as pure HBTs. For two types of devices the common- emitter characteristics are recorded (Fig. 5). They differ in the background doping of the upper cladding as mentioned. The HBT-EAM with the higher doping in the collector (Fig. 5, wafer M1993) has a lower tum-on voltage (less current blocking), but the corresponding EAMs need higher voltage swings for the same optical extinction, as expected (Fig. 3). RF-measurements taken from the HBT-EAM (M2038) are shown in Fig. 6 with f , = 23GHz andL,,, = 20GHz. For M1993 the values are similar& = 24GHz andx,,x = 25GHz.

In comparison to the merged HBT-EAM devices, HBTs made from M1993 show 5mA (A, = 3.10pmZ, not self-aligned) and maximum current gain B,,,A, = 110. RF-measurements give f, = 22GHz and f,,, = 23GHz.

2) Optical Measurements

DC and RF characteristics of the HBT-EAM were investigated in an optical link as shown in Fig. 7. A tunable laser source is connected to the HBT-EAM by a tapered single-mode fiber.(7) A cleaved single-mode fiber is used to pick up the modulated mode. The absorption of the optical waveguide and the mismatch of the optical modes of the waveguide and the single-mode fiber cause high losses. These can be compensated by post- amplification with an EDFA. An optical bandpass filter is used to reduce spontaneous emission of the EDFA whose output is monitored by a powermeter connected to the 5%-branch of a 95:5-coupler. The 95%-branch is

M2038A (HM03) bias point: VCE=6V, /,=40pA

short circuit current gain h,,

1 f,=23 GHz

0.1 1 10 100 f(GHz)

Fig. 6. RF characteristics of a HBT-EAM.

Fig. 7 . Optical Setup for optoelectronic measurements. DUT: device under test, EDFA: erbium doped fiber amplifier, PM: optical powermeter, PD: photodetector, SA: spectrum analyzer.

linked to a fast photodetector whose signal is detected by an electrical spectrum analyzer.

Electrical connections of the HBT-EAM are performed by microwave coplanar probes. Bias-tees are connected to both the base and collector with emitter as ground. The base is biased by a DC-current ZB and the collector by a DC-voltage Vcc and an external series resistor RL in the DC-branch of the collector bias-tee. For DC-measurements RL acts as load resistor in the common emitter configuration.

DC-measurements on the HBT-EAM were made with optical excitation to determine the optical contrast when switching the base current between two values. Results of this measurement are shown in Fig. 8. As can be seen, a modulation contrast of 7.3dB is achieved for a current change of only 60pA. When Vcc is decreased to 6.0V the modulation contrast is reduced to 3.2dB. The total base current is the sum of the photo current, induced by absorption in the base layer, and the externally injected base current, which must be extended to negative values and is given on the x-axis (Fig. 8).

For optical RF-measurements the HBT-EAM was biased with Vc- = 6.0V and IB = -20.OpA. Furthermore a 50Q external load resistance was applied to the FW port of the collector bias-tee. The RF-input power was - 1 OdBm for small-signal modulation with a linear range up to lOdBm input power. As shown in Fig. 9 the HBT-

442

. M1993E HMO4 A=8x50 pm2

. Laser: 1570 nm. 0.5 m W 0 .

. R,=10.0 kR 0

- v,,=12.0 v 0

0

0

0

0 0

-60 -50 -40 -30 -20 -10 0

Base current /B (PA)

Fig. 8. Optical modulation contrast of the HBT-EAM.

EAM achieves a 3dB cut-off frequency of about 7GHz. If the RF load resistance is increased to 330Q an RF gain of 3dB is achieved at lower frequencies cf= 10MHz) with an optical power of 2mW incident on the photodetector. The RF link performance of the integrated HBT-EAM was compared to a pure EAM (9x200pm) processed separately from the same layer stack without the emitter layers reaches a 3dB cut-off frequency of 1 OGHz.

The multifimctionality of the device is shown by the use of the HBT-EAM as a photodetector. A modulated optical mode generates free carriers in the base modulating the collector-emitter current. The modulated light is generated by heterodyne mixing of two laser sources around 1570nm amplified by an EDFA to 19mW. The HBT-EAM is biased by a collector-emitter voltage of VCE = 2.5V and the RF-branch of the bias-tee is terminated by 5OQ while the base of the HBT-EAM is left open. Measurements of the detector efficiency were made for frequencies from 90MHz to 30GHz. At 90MHz an efficiency of 0.26NW is achieved. This value drops for increasing frequencies with a rate of lOdB per decade. An improvement of the efficiency is to be expected by use of a higher quality fiber taper for a better coupling and improvements in the device design.

V. Conclusions

In summary we have developed a niultifunctional layer design to enable the fabrication of HBT, EAMs and a new merged device (HBT-EAM). This will offer the opportunity for monolithic integration of the different devices to result in a variety of applications. To improve device operation our composite collector design has to be optimized to decrease turn-on voltage. Furthermore, the influence of voltage- versus current- effects on the optical modulation have to be clarified.

0.01 0.1 1 10

Frequency f (GHz)

Fig. 9. Relative optical modulation of the HBT-EAM vs. RF-frequency.

Acknowledgement

The authors acknowledge the Deutsche Forschungs- gemeinschaft (DFG) for funding.

References

S. Chandrasekar, "Optoelectronic system integration using InP-based HBTs for lightwave communication", Solid-state Electron., vol. 41, p. 1413, 1997. M.T. Camargo Silva, J.E. Zucker, L.R. Carrion, C.H. Joyner, A.G. Dentai, "Growth Optimization for p-n Junction Placement in the Integration of Heterojunction Bipolar Transistors and Quantum Well Modulators on InP", IEEE J. Selected Topics in Quantum Electron., vol. 6, p. 26, 2000. Y. Okada, K. Tada, "Application of bipolar transistor structures to optical waveguide modulators and switches", J. Appl. Phys., vol. 69, p. 73, 1991. G.W. Taylor, T. Vang, S.K. Sargood, P. Cooke, P. Claisse, "Demonstration of the heterostructure field- effect transistor as an optical modulator", Appl. Phys. Lett., vol. 59, p. 1031, 1991. N. Shaw, P.J. Topham, M.J. Wale, "Optical switches and heterojunction bipolar transistors in InP for monolithic integration", ESSDERC'90, p. 55 1, 1990. N. Shamir, D. Ritter, D. Gershoni, "Current Induced Absorption Modulation Using Quantum Structures in the Collector of Heterojunction Bipolar Transistors", IEEE LEOSP9, vol. 2, p. 842, 1999. T. Alder, A. Stohr, R. Heinzelmann, and D. Jager, "High-efficiency fiber-to-chip coupling using low- loss tapered single-mode fiber", IEEE Photon. Technol. Lett., vol. 12, p. 1016, August 2000.

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