Resonance-free optical response of a vertical cavity transistor laser

Resonances have been observed in masers and explained by Statz and deMars¹ via the transient solution of coupled carrier-photon rate equations describing the dynamics between population inversion and cavity electromagnetic energy. Resonance in semiconductor diode lasers can be explained as injected carrier “choking or accumulating” due to an overly slow electron-hole (e-h) recombination process. Laser-resonance peaks are clearly an undesirable limitation in high speed error-free optical data communications in a directly current-modulated oxide-confined vertical-cavity surface-emitting laser (VCSEL), resulting in bit-error rate degradation and requiring additional power consuming error-correction circuits to compensate these effects.² Thus, it is of great benefit for data communication and RF-photonics to reduce the laser-resonance peaks via fast radiative recombination processes in semiconductor lasers as first reported in the transistor laser (TL).³ 

In 2004, the observation of light emission from the base region of direct bandgap heterojunction bipolar transistors (HBTs) with quantum wells (QWs) in the heavily p-doped base regions has led to the demonstration of light-emitting transistors (LETs).^4,5 With fast radiative recombination lifetime, injected minority charge “tilted” in the base layer, and short-circuited base-collector (BC) by metallization in the LETs, the new light emitting diodes (LEDs) have demonstrated up to 7 GHz modulation bandwidth (corresponding to a fast recombination lifetime of τ_B ∼ 23 ps).^6,7 By incorporating a resonant cavity in QW-HBT, the stimulated recombination of the LET and the introduction of a transistor laser (TL) have been realized.⁸ The picosecond carrier recombination lifetimes and the unique dual electrical-optical outputs enable a TL for high-speed direct modulation and electrical “monitoring” of the optical parameters. Consequently, edge-emitting transistor lasers (EETLs) have demonstrated a 20 GHz resonant-free optical response, 40 Gb/s simultaneous electrical and optical outputs, a low laser relative intensity noise (RIN), and 22 Gb/s error-free data transmission.^3,9–11

Recently, vertical cavity transistor lasers (VCTLs) of small volume and high cavity Q have been reported.¹² By further improving the device processing and optimizing the device layout, lower threshold current and reduced power consumption VCTLs of a n-p-n structure have been demonstrated.^13,14 Subsequently, room temperature CW operation VCTL for a p-n-p structure has been demontrated.¹⁵ The TL or VCTL collector junction can be treated as a PIN diode for photon assisted tunneling (PAT) absorption under biases; however, the PAT in TL is inside the cavity which has enhanced absorption and resulted in high speed collector tunneling laser modulation. In earlier work, the concept of the voltage-driven switching of an edge-emitting TL employing the photon-assisted tunneling process to shift the operation from stimulated to spontaneous emission was realized.^16,17 With the base-collector tunnel junction, the intra-cavity photon-assisted tunneling can be enhanced due to the narrower bandgap.¹⁸ In previous work, the VCTL intensity switched by voltage-controlled photon-assisted tunneling has also been performed.¹⁹ In the present work, we demonstrate the microwave bandwidth of a selectively oxide confined VCTL in a lateral configuration employing current and voltage modulation. Following the development of oxide-confined EEDL and VCSEL,^20–24 the oxide-confined VCTL with a 4.7 $×$ 5.4 μm² aperture yields better electrical and optical confinement leading to a high-Q cavity. The gain compression evident on the collector I_C-V_CE indicates the base shifting its operation from spontaneous to stimulated. The emission spectra exhibit single-mode operation with the emission peak around 970 nm and a narrow linewidth of Δλ ∼ 0.23 Å (corresponding to Q = 42 216) at I_B = 4 mA and V_CE = 5 V. With a gradient in the base injected minority carrier stored charge in the transistor active mode and fast electron-hole recombination lifetimes to eliminate carrier “choking or accumulating,” a low power VCTL achieves a 3 dB bandwidth of 11 GHz resonance-free optical response.

The VCTL crystal epitaxial layers of the present work consist of a normal light-emitting transistor with two base quantum-wells interleaved between the top and bottom distributed Bragg reflectors (DBRs). It contains (from the semi-insulating GaAs substrate upward) a 3000 Å undoped GaAs buffer layer, followed by a high reflectivity bottom DBR mirror with 36 pairs of high/low index Al_0.12Ga_0.88As/Al_0.9Ga_0.1As layers. Next, an N-p-n light-emitting transistor structure is grown with a 540 Å n⁺-GaAs layer as a sub-collector, a 120 Å In_0.49Ga_0.51P etch stop layer, and a 1160 Å lightly doped n-type GaAs collector layer. The base layer includes a 800 Å heavily doped p-type GaAs layer with two 120 Å undoped In_0.2Ga_0.8As quantum wells. Above the 530 Å wide-bandgap In_0.49Ga_0.51P emitter, 595 Å Al_0.98Ga_0.02As for the oxidation layer is grown for aperture formation. Finally, another 24 pairs of Al_0.12Ga_0.88As/Al_0.9Ga_0.1As layers are grown for the top DBR mirror, followed by a 501 Å heavily doped n-type contact layer. The schematic device diagram is shown in Fig. 1(a); the fabrication is described in Ref. 13. To realize better current confinement and photon generation, the device is fabricated with selective oxidation in a lateral layout design. A shallow oxidation is performed to seal the sidewall first, followed by a trench opening achieved by dry etching for lateral deep-oxidation.

Figure 1(b) shows a scanning electron microscopy image of a fabricated VCTL with the 7 $×$ 5 μm² light emission area. To reduce the extrinsic base resistance, the base metal surrounds the emitter DBR cavity with 1 μm spacing. Due to selective oxidation, the electrons injected from the emitter do not go through the high composition AlGaAs layer underneath the emitter metal, avoiding undesired recombination. The effective radiative recombination occurs in the oxide-defined cavity region with the holes supplied by base current. The optical modes are also defined by the oxidation aperture because of the large index contrast between Al_xO_y and GaAs. The minority carriers of injected electrons that do not recombine in the base are swept to the collector. The gradient in the minority carrier concentration and the reverse-biased base-collector junction allow fast recombination at the base quantum well, which is determined by the picosecond base transit time. The quantum well material is designed for 980 nm emission. Due to the mismatch between the quantum well emission (λe) and the cavity mode (λc), the device is mounted on a copper stage equipped with a thermal-electrical controller and is set at 80 K to match λe = λc for lasing.

The common-emitter collector output (I_C-V_CE) characteristics and the corresponding optical output characteristics (L-V_CE) for a VCTL with a 4.7 $×$ 5.4 μm² aperture are shown in Figs. 2(a) and 2(b) with V_CE swept from 0 to 6 V and I_B varied in increments from 0.2 to 6 mA in 0.2 mA steps. The optical output in Fig. 2(b) is measured from the top of the device with a Si large-area photodetector. The large I_B dependent V_CE offset at I_C = 0 mA is attributed to the high emitter series resistance from the top DBR mirror which leads to an additional voltage drop between the emitter contact and InGaP emitter layer with increasing V_CE. From the Ebers-Moll circuit model extraction, we obtain an emitter resistance of 603 Ω. At low I_B bias, the collector I_C-V_CE curves shown in black indicate that the VCTL operates in the spontaneous emission region, while the total emission power in the L-V_CE curves is below 15 μW. When I_B > 2.4 mA above the laser threshold current (I_B,TH), the VCTL switches to stimulated emission, and the emission intensity exhibits a large jump (red curves) from 15 μW to 475 μW. The inset shows the I_B dependent current gain (β = ΔI_C/ΔI_B) at V_CE = 5 V. The unique current gain (beta) compression phenomenon of a transistor laser decreases from 0.67 to 0.44 also indicating the base shifting operation from spontaneous to stimulated recombination.²⁵ In contrast to an edge-emitting TL, the beta shows a gradual rather than a sharp decrease before stimulated emission due to the resonant-cavity effect.²⁶ From the L-V_CE characteristics, the collector voltage threshold (V_CE,TH) is observed for stimulated emission. As V_CE is increased above V_CE,TH, the base recombination is then sufficient to sustain the laser operation, while the base hole leakage to the collector is reduced as the collector junction is more reversed-biased.¹⁴ This is an important characteristic of the transistor laser that can be used in fast switching, e.g., in a new form of the photonics integrated circuit.

Figure 3 shows the optical spectra of the VCTL at I_B = 2, 3, and 4 mA and V_CE = 5 V. The optical output is collected by a fiber and detected with an Advantest Q8384 optical spectrum analyzer. At I_B = 2 mA, the magnified spectrum exhibits spontaneous emission with three distinct cavity modes near 968 nm. Since I_B > I_B,TH, the fundamental cavity mode becomes dominant with some heating red-shift in the wavelength of Δλ/ΔI_B = 1.21 nm/mA. At I_B = 4 mA, the stimulated recombination exhibits a peak at 970.96 nm with a full wave at half maximum (FWHM) of 0.23 Å which is extracted by a Gaussian fitting curve (not shown), corresponding to a cavity Q of 42 216 (Q $≈$ν/Δν). The inset shows the corresponding optical spectrum (log scale) with the VCTL exhibiting single-mode operation and side-mode suppression ratio (SMSR) of 31.76 dB. The measured mode-spacing between the fundamental and first order mode is 1.09 nm. For a rectangular aperture, the optical aperture can be accurately determined to be 4.7 $×$ 5.4 μm² from the measured mode-spacing.²⁷ 

Figure 4 shows the measured modulation characteristics at 80 K of the common-emitter VCTL at three different biases. In addition to the usual directly current-modulated (δI_B) at the base-emitter (BE) port, the VCTL can also be directly voltage-modulated (δV_CE) at the collector-emitter (CE) port. By changing the base-collector voltage (δV_BC), the cavity photon density can be modulated. The light emission is collected by a fiber and coupled into a high-speed photodetector. At V_CE = 3.5 V and I_B = 3.6 mA, the device is operated in the saturation mode with the base-emitter (BE) and base-collector (BC) in forward bias. In this mode of operation, the carriers injected from the junctions tend to pile up in the active region (similar to the diode laser) and the stored charge delays in recombining in the quantum wells, resulting in a delay of signal switching and a lower f_–3dB = 6 GHz. When the device is biased at V_CE = 5 V and I_B = 3.6 mA, the BC junction becomes reversed-biased and the device operates in the forward active mode. The gradient in the base carrier concentration (no carriers piled-up) reduces stored charge aided by the reverse-biased BC junction and allows the transistor to operate “faster” with a reduced laser-resonance peak. Thus, a higher bandwidth f_–3dB = 9 GHz is obtained. A further increase in the cavity photon density giving I_B = 6 mA enhances f_–3dB to 11.1 GHz. The device optical response with frequency for both base-current and collector-voltage modulation is absent for a laser-resonance peak up to 11 GHz despite the high emitter resistance of 603 Ω. The speed of this device is limited by the high input emitter resistance due to ∼1.5 μm thick DBR layers, which has resulted in the input RC time delay. With the further reduction of parasitics and better refined design and scaling, we expect that the VCTL will be an excellent low power laser for high speed and high signal integrity data communication.

In conclusion, we have reported the direct microwave base-current and collector-voltage bias and modulation of a vertical cavity transistor laser at 80 K. The 4.7 $×$ 5.4 μm² VCTL exhibits the simultaneous electrical and optical output characteristics at a low threshold current of 2.4 mA. The lasing peaks are around 970 nm with a cavity Q of 42 216 at V_CE = 5 V and I_B = 4 mA. The spectra exhibit single-mode operation with a SMSR of 31.76 dB. A 3 dB bandwidth of f_–3dB = 11.1 GHz is demonstrated. The “flat” optical responses of these high Q active region transistor laser devices are projected to provide higher signal integrity for improving eye-opening for better error-free data transmission. These results yield the capability of signal mixing and processing employing advantageously the enhanced voltage-modulation capability as well as the usual current-modulation.

This work has been supported in part by the Air Force Office of Scientific Research under Grant No. AF FA9550-15-1-012 for fundamental research in THz modulation of semiconductor lasers, Army Research Office under Grant No. W911NF-17-1-0112 for Vertical Cavity Transistor Laser Development and by the Ministry of Science and Technology of Taiwan (ROC) under Grant Nos. MOST 102-2221-E-002-192-MY3; MOST 106-2622-E-002-023-CC2; MOST 105-2628-E-002-007-MY3; MOST 104-2218-E-005-004; and MOST 106-2923-E-002-006-MY3.