We report on a novel triple transit region (TTR) layer structure for 1.55 μm waveguide photodiodes (PDs) providing high output power in the millimeter wave (mmW) regime. Basically, the TTR-PD layer structure consists of three transit layers, in which electrons drift at saturation velocity or even at overshoot velocity. Sufficiently strong electric fields (>3000 V/cm) are achieved in all three transit layers even in the undepleted absorber layer and even at very high optical input power levels. This is achieved by incorporating three 10 nm thick p-doped electric field clamp layers. Numerical simulations using the drift-diffusion model (DDM) indicate that for optical intensities up to ~500 kW/cm2, no saturation effects occur, i.e. the electric field exceeds the critical electric field in all three transit layers. This fact in conjunction with a high-frequency double-mushroom cross-section of the waveguide TTR-PD ensures high output power levels at mmW frequencies. Fabricated 1.55 µm InGaAs(P)/InP waveguide TTR-PDs exhibit output power levels exceeding 0 dBm (1 mW) and a return loss (RL) up to ~24 dB. Broadband operation with a 3 dB bandwidth beyond 110 GHz is achieved.
© 2014 Optical Society of America
Research exploring the millimeter wave (mmW) frequency range from 30 to 300 GHz has recently been in a revival phase, which is mainly application driven. Many modern and future applications in sensing , radar  and especially in wireless communications  tend to operate at higher frequencies in the mmW range. This trend has resulted in a dynamic demand for solid-state components and it has had a strong impact on the development of high-performance devices for the generation and detection of high-power mmW signals . Besides the use of purely electrical components, the generation of electromagnetic waves in the mmW regime can also be accomplished using ultra-compact and high-frequency photonic devices. Especially for wireless communications, the utilization of photonic technologies and techniques such as radio-over-fiber (RoF) has resulted in a number of impressive demonstrations, e.g. high-data rate wireless transmission with speeds up to 100 Gbit/s .
High-frequency and high-output-power photodiodes (PDs) are one of the key elements in such ultra-broadband RoF wireless links and because of that reason, different approaches for developing high-performance PDs are currently being investigated by various groups [1,4,6–20]. Key objective of the actual research in high-performance PD development is to enhance the operational frequency limitations and simultaneously to achieve high radio frequency (RF) power saturation levels without affecting the PD bandwidth . High-frequency operation requires an appropriate PD layer structure design, which comprises relatively thin absorption layers to reduce transit time limitations but thus handicaps the overall responsivity especially for top-illuminated PDs. In return, the resistance-capacitance (RC) limitations overweight due to the thin absorption layers requiring a dedicated layer structure design for balancing those limitations. Actually, the clear trend to simultaneously obtain a high responsivity, a high bandwidth (or operational frequency for narrow-band applications) and a high output RF power is to use waveguide-type PDs rather than top-illuminated ones [7,8] as waveguide PDs provide more freedom in the designs of the PD layer structure and topology.
Among the different types of high-performance PDs, the uni-traveling-carrier PD (UTC-PD), which was first introduced in 1997 [1,9], has already been demonstrated with record-breaking performances within the terahertz (THz) frequency range . In contrast to the conventional pin-PD, where both types of the photo-generated carriers, i.e. electrons and the relatively slow holes contribute to the overall photocurrent, there is only electron drift in the UTC-PD, which is key for high-frequency operation and for overcoming saturation effects such as the space charge effect . In a conventional UTC-PD, this is achieved by using an undepleted p-type absorber instead of a depleted non-intentionally doped (n.i.d.) absorber as it is the case in the pin-PD. Thus, one can assume hole relaxation, i.e. there is “no” hole drift or hole diffusion in the absorption region of the UTC-PD. However, the transit time limited bandwidth of the UTC-PD still suffers from the slow electron diffusion in the highly p-doped (InGaAs) absorber layer. This drawback has been tackled in the recent past with the invention of the so-called modified UTC-PDs (MUTC-PDs), which also greatly overcome the saturation effects [10,11]. Compared with the original UTC-PD structure, the MUTC-PD structure consists not only of an undepleted p-doped absorption region but also exploits absorption in an additional depleted InGaAs absorption layer. This, however, implicates that there is a substantial hole drift in the depleted absorber of MUTC-PDs, which is somewhat in contradiction to the expression “uni-travelling-carrier”.
In this work, we report on 1.55 µm InGaAs(P)/InP waveguide PDs utilizing a novel triple transit region (TTR) layer structure, which provides high output power levels in the mmW range. In contrast to MUTC-PD or UTC-PD, in which electron diffusion in the undepleted absorber occurs, the TTR-PD ensures carrier drift also in the undepleted absorber. Thus, in the entire transit region of the TTR-PD, which consists of the undepleted absorber layer, the depleted absorber layer and the collector layer, carriers drift at saturation velocity or faster to enhance both, the maximum output RF power and the transit time limited bandwidth of the TTR-PD. The TTR-PD has also been designed for suppressing space charge saturation effects; corresponding numerical simulations using a drift-diffusion model (DDM) show that even for high optical intensities of up to 500 kW/cm2, no saturation effects occur in the TTR-PD structure. For achieving an optimum balance between the series resistance and the junction capacitance, the core of the active waveguide is intentionally underetched, forming a mushroom-type structure . For achieving efficient fiber-to-chip coupling, the TTR-PDs are monolithically integrated with a passive optical waveguide (POW) structure, which is located underneath the active waveguide. In addition, to keep optical losses in the POW section low, also the cross-section of the POW is fabricated with a mushroom-type shape. This approach allows keeping the optical mode field in the POW away from the etched surfaces and thus minimizes optical radiation losses. Experimentally, we report fabricated TTR-PDs exhibiting a 3 dB bandwidth beyond 110 GHz and an output power level exceeding 0 dBm (1mW).
2. Operation principle of the double mushroom-type waveguide TTR-PD
For use in broadband mmW RoF wireless systems, high-output-power and high-3-dB-bandwidth 1.55 µm waveguide TTR-PDs are desired. A sketch of the designed TTR-PD is shown in Fig. 1, which also explains the operation principle of the waveguide TTR-PD. Basically, the waveguide TTR-PD structure is composed of a passive, i.e. non-absorbing, and an active optical waveguide. The core of the active waveguide consists of an undepleted absorber and a depleted absorber both made of ternary composite InGaAs layers. Light, which is coupled into the POW, is vertically coupled into the active waveguide as illustrated in Fig. 1.
The core of the POW is designed for efficient optical coupling from a lensed SMF. Other than in a conventional shallow ridge POW, the mode-guiding stripe is located below the InGaAsP core layer forming also a mushroom-type topology  with a thin but wider InP upper cladding layer. Consequently, the optical mode in the POW is mostly confined to the core layer and the lower InP cladding. Thus, the optical mode center is farther away from the etched top surface. This significantly reduces the optical waveguide losses of the POW. Light coupled into the mushroom-type POW section propagates towards the active TTR-PD section, where it is first coupled into a shallow ridge POW section, which is located underneath the active TTR-PD section. Finally, the light couples from the shallow ridge POW vertically up into the active waveguide featuring the TTR-PD section, where it gets absorbed and converted into an electrical signal.
The generated electrical signal is coupled out of the active section by a linearly tapered grounded coplanar waveguide (GCPW) transmission line. The GCPW is designed for matching the impedance of the active TTR-PD section to 50 Ω output pads. For on-chip characterization, conventional 100 µm or even 150 µm pitch ground-signal-ground (GSG) coplanar RF probes can be applied.
3. Modeling and simulation of the TTR-PD layer structure
The band diagram and the layer structure of the TTR-PD are presented in Fig. 2. Generally, the TTR-PD layer structure consists of three individual transit regions, the undepleted absorption layer, the depleted absorption layer and the non-absorbing collector layer. The TTR-PD differs from MUTC-, UTC- and pin-PDs mainly because special clamp layers and the direct current (DC) driving conditions ensure that in the TTR-PD all carriers are drifting at saturation velocity (or faster) in all three transit layers even for high optical intensity levels.
Describing the TTR-PD layer structure top down, below the contact layer there is a highly p-doped InP layer (800 nm), which serves as the upper cladding layer of the active optical waveguide, as well as a diffusion blocker for electrons. This is followed by three 10 nm thick highly p-doped quaternary (InGaAsP) electric field clamp layers with band-gap wavelengths of 1.15 µm (Q1.15), 1.26 µm (Q1.26) and 1.46 µm (Q1.46), which are introduced between the InP cladding layer and the undepleted InGaAs absorber (40 nm). Next, there is a 70 nm thick depleted InGaAs absorber that acts as electric field booster for the graded p-doped absorber. This is to achieve sufficiently strong electric fields in the graded p-doped absorption layer beyond the critical electric field so that electrons drift at overshoot velocity in the p-doped absorber and thus, it is also called the overshoot launcher. Thanks to the clamp layers, the overshoot velocity can be maintained even at high optical intensity levels. Additionally, the clamp layers manage to overcome carrier trapping at the InP/InGaAs interface , which further enhances the overall transit time. Moreover, in the TTR-PD structure three 10 nm thick slightly n-doped InGaAsP layers (Q1.46, Q1.26 and Q1.15) function as electric field balancing layers between the field booster layer and the slightly n-doped InP collector layer (280 nm), as well as bridging layers to handle the band discontinuity . The entire TTR-PD heterostructure including the monolithically integrated InGaAsP/InP POW is grown on a semi-insulating (s.i.) InP substrate (125 µm).
As discussed above, the TTR-PD structure comprises three transit sections contributing to the drift motion of electrons to enhance the transit time limited bandwidth. In contrast to MUTC-PDs  or rather UTC-PDs , there is a sufficiently strong electric field even in the undepleted absorption layer (overshoot launcher) of the TTR-PD, which slightly exceeds the critical electric field in InGaAs of 3 kV/cm . For this reason, electrons drift at overshoot velocity (>6.1x106 cm/s). Thus, drift motion at overshoot velocity instead of diffusion of electrons distinguishes the overshoot launcher of the TTR-PD from the doped absorber in MUTC-PDs or UTC-PDs [11,18].
For investigating the electric fields and velocities in the different layers of the TTR-PD structure, numerical DDM simulations have been performed using the technology computer-aided design (TCAD) software Atlas. Figure 3 shows the simulated electric field for different DC drive voltages versus the layer depth without illumination.
As can be seen from the figure, there is still a sufficiently strong electric field in the doped absorber, which is due to the high electric fields in the adjacent clamp layers and the n.i.d. field booster. The balance layers function as interlayers between the field booster and the collector layer to achieve an almost equilibrated electric field distribution within the whole intrinsic and quasi-intrinsic region. This electric field is way beyond the critical electric fields in InGaAs and InP of 3 kV/cm and 11 kV/cm, respectively. Thus electrons can be considered to drift at saturation velocity throughout the whole intrinsic and quasi-intrinsic region of the TTR-PD.
It should be further noted that there are no excessive electrical fields at the interface between the doped and undoped absorber for reverse voltages above 6 V.
The impact of the clamp layers, the field boosting layer and the balance layers on the electric field distribution and the resulting electron velocity at different optical input intensity levels can be seen from Fig. 4 and Fig. 5, respectively.
As can be seen from Fig. 4, the clamp layers ensure that the remaining electric field strength in the absorber still exceeds the critical electric field even at high optical excitation. Thus, although an increase of the optical intensity results in a significantly reduced electric field (see Fig. 4), the electron velocity remains being in the overshoot regime (>6.1x106 cm/s) even at extremely high optical intensity levels of up to 500 kW/cm2. Furthermore, in the intrinsic and quasi-intrinsic InGaAs and InP region, electrons still drift at saturation velocities of 5.4x106 cm/s and 7.5x106 cm/s, respectively .
As regards the holes, relaxation can be assumed for the undepleted absorption layer, whereas in the depleted absorber, holes will drift at a somewhat slower velocity of ~4.8x106 cm/s.
Only at optical intensity levels exceeding 500 kW/cm2, the accumulation of holes and consequently of electrons in the depleted absorber rises significantly, which finally causes the electric field to fall below the critical electric field level, i.e. saturation will occur. At intensity levels of about 600 kW/cm2 or higher, the electrical fields and carrier velocities are both close to zero according to DDM simulations.
4. Fabrication of waveguide TTR-PD chips
The epitaxial growth of the TTR heterostructure shown in Fig. 2 is accomplished on s.i.-InP substrate. Metal organic chemical vapor phase epitaxy (MOVPE) has been used, with Zn and Si as dopants for p-type and n-type layers, respectively. As shown in Fig. 2, the POW section is monolithically integrated in the layer design. In general, the fabrication process is divided in four key fabrication steps: wet etching of the POW mesa, dry etching of the active PD mesa, passivation of PD layers and vapor deposit of the p-type and n-type metal contacts. A light-optical microscope photograph and further two scanning electron microscope (SEM) photos of a fabricated TTR-PD chip (~450x200 µm2) are shown in Fig. 6.
The mushroom-type POW with the underetched shape is realized by a series of wet chemical etching steps. For accurately defining the active TTR-PD section, inductively coupled plasma (ICP) dry chemical etching has been applied. In addition, to create the mushroom-type active mesa of the active PD section after the ICP dry etch process, a supportive wet chemical etching step has been introduced. Benzocyclobutene (BCB) was used for the passivation of the semiconductor layers below the metal conductors. Simultaneously, the BCB cushion is also necessary to accomplish a planarization of metal crossovers at critical intersections. Finally, the p-type and n-type metallic contacts as well as the backside shielding of the GCPW transmission line were formed during various electron beam (e-beam) evaporation processes.
5. Experimental results up to 110 GHz
Discrete TTR-PD chips comprising a 3x20 µm2 active PD area were employed for experimental characterization. On-chip measurements were performed without additional cooling or heat sink arrangements to characterize the performance of the developed TTR-PDs. The series resistance and the junction capacitance were found to be about 26 Ω and 14 fF, respectively. The corresponding RC-limited bandwidth was around 150 GHz. Along with the simulated transit-time limited bandwidth in excess of 130 GHz, a 3 dB bandwidth of about 100 GHz was expected for the fabricated TTR-PDs. A lensed SMF with a 1.7 µm spot diameter was used for optical fiber-chip coupling. At the applied optical wavelength of 1.55 µm, a DC responsivity of up to 0.5 A/W was measured without anti-reflection coated POW facets.
Calibrated measurements of the frequency response up to 110 GHz were performed using a scalar network analyzer setup consisting of an external cavity laser, an external Mach-Zehnder modulator, which was operated at minimum transmission point for double-sideband carrier-suppressed modulation. A 100 µm pitch coplanar 110 GHz RF probe (~50 Ω) and an external 110 GHz bias-T, both featuring a 1 mm W1-connector, as well as an electrical spectrum analyzer with external harmonic mixers were used for on-chip power measurements. The measured frequency response for a reverse bias of 8 V is plotted in Fig. 7. For the frequency response measurement, the photocurrent was fixed at 1 mA. As can be seen from Fig. 7, the 3 dB bandwidth is exceeding 110 GHz. The RF responsivity was found to be ~0.17 A/W. An output return loss (RL) of up to ~24 dB was measured and already reported in .
For investigating saturation effects, the detected output RF power at 110 GHz was measured as function of the photocurrent for different drive voltages and is plotted in Fig. 8. As can be seen, the maximum output power that was achieved with the developed TTR-PD is 0.2 dBm at 110 GHz. No saturation effects were observed up to this level, which is solely limited by the maximum optical input power available in the measurement setup. The photocurrent level at the highest measured output power is about 16 mA. It is worth mentioning that even higher RF output power levels are likely to be expected as there is no 1 dB compression point observed.
In this paper, we presented a novel InP-based triple transit region (TTR) layer structure for high-performance 1.55 μm waveguide photodiodes (PDs) providing high output power levels at millimeter wave (mmW) frequencies. The active section of the TTR-PD consists of three transit layers, in which the photo-generated carriers travel at saturation velocity or even at velocity overshoot that enhances both, the maximum output radio frequency (RF) power as well as the transit time limited bandwidth of the TTR-PD. The cross-section of the active waveguide section of the TTR-PD is formed as a mushroom-type shape to reduce the resistance-capacitance (RC) time limitation. Additionally, the cross-section of the passive optical waveguide (POW) is of mushroom-type, too, for reducing the optical losses in the POW. In contrast to uni-traveling-carrier PDs (UTC-PDs) and modified UTC-PDs (MUTC-PDs), there exists a strong electric field beyond the critical electric field in the doped absorbing layer of the TTR-PD (i.e. in the overshoot launcher) even at very high optical input power levels. This is achieved by employing three 10 nm thick highly p-doped electric field clamp layers. As a result, electron drift at overshoot velocity is achieved instead of electron diffusion, which drastically reduces saturation effects (space charge effects). Numerical simulations using the drift-diffusion model (DDM) reveal that even at high optical intensity levels of around 500 kW/cm2, no saturation effects occur. Experimentally, processed 1.55 µm InGaAs(P)/InP waveguide TTR-PDs exhibit output power levels exceeding 0 dBm (1 mW) at 110 GHz. Furthermore, we demonstrated broadband operation with a 3 dB bandwidth beyond 110 GHz. An output return loss (RL) of up to ~24 dB was achieved.
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