GaSb-based Type-I Laser Diodes Operating at 3 μηι and Beyond

L. Shterengas, G. Kipshidze, T. Hosoda, and G. Belenky Department of Electrical and Computer Engineering, SUNY-Stony Brook Stony Brook, NY 11794-2350, U.S.A.

1. Introduction

Laser light sources operating in the spectral region near 3 μπι are required for variety of applications, ranging from aesthetic surgery to remote spectroscopic analysis. Semiconductor diode lasers operating at room temperature (RT) with watt-level continuous wave (cw) output power can benefit the design of the corresponding systems due to their inherent stability, compactness, and possibly low cost and high efficiency. Until recently, the very idea of cw diode lasers operating at RT with wavelength λ ~ 3 μιτι was thought to be questionable. This pessimism originated in the well-known fundamental increases of the nonradiative Auger recombination and of free carrier absorption with wavelength. The associated carrier and photon losses were considered likely to prevent the mid-infrared (mid-IR) diode lasers from reaching cw lasing at RT.

Today, cw RT operation of 3 μπι diode lasers with hundreds of mW of output power is a reality. Clearly, this experimental fact casts doubt on the validity of the old argument based on the destructive power of Auger recombination and free carrier absorption. Moreover, the narrow bandgap nature of the active region not only brings disadvantages but also can provide significant benefits. The advantage of using narrow bandgap material for generation of optical gain is rather straightforward, though rarely discussed in context of mid-IR laser development. The electron density of states in the Γ-valley is nearly proportional to the direct bandgap in III-V binary materials.1 This proportionality is more or less preserved in corresponding ternary and quaternary alloys used in quantum wells (QWs) of diode lasers. In QWs compressively strained above 1%, the top valence subband has an in-plane effective mass that is rather close to electron effective mass.2 The resulting reduced density of states facilitates separating the quasi-Fermi levels in narrow bandgap active QWs under injection. Since bulk transition matrix element does not show pronounced material bandgap dependence,1 this leads to improved differential gain in narrow bandgap QWs. Hence, it is expected that mid-IR diode lasers can have much lower transparency and, possibly, lower threshold carrier concentrations as compared to their near-IR counterparts. Auger recombination is a three-particle process and its net rate is superlinear in carrier concentration. Hence, the threshold current density of the mid-IR diode lasers can be rather low despite the increased probability of individual Auger events in narrow bandgap

Future Trends in Microelectronics. Edited by Serge Luryi, Jimmy Xu, and Alex Zaslavsky 65 Copyright © 2010 John Wiley & Sons, Inc.

66 Future Trends in Microelectronics

QWs. It is worth mentioning that the spontaneous recombination probability decreases dramatically with the bandgap, thus furnishing another advantage for narrow bandgap QWs.

The temperature stability of mid-IR lasing thresholds remains an open question because Auger rate can be highly temperature sensitive. Hence, even if the laser threshold current is relatively low, as long as it is determined by Auger processes it could increase dramatically at elevated temperatures. However, it has been argued that, under certain conditions, the Auger rate in QWs can have reduced temperature sensitivity. 3 Identification of the effects of Auger recombination on laser parameters is complicated by the experimental difficulties in distinguishing between penalties associated with Auger recombination and thermal carrier delocalization.

In this chapter, we demonstrate that GaSb-based diode lasers with improved carrier confinement in type-I InGaAsSb QWs can operate at room temperature up to λ = 3.5 μπι. In Section 2 we will describe the corresponding laser technology and review the material development for fabrication of the diode lasers operating near and above 3 μπι. Section 3 will present an example of design optimization aimed at improving the output power of the 3 μπι emitters. The conclusion will summarize our experimental observations and discuss future prospects.

2. Diode lasers operating at RT in the 3-3.5 μπι spectral range

Our laser heterostructures were grown by solid-source molecular beam epitaxy using VEECO GEN-930 reactor on tellurium doped GaSb substrates. The cladding layers were lattice-matched to GaSb quaternary AlGaAsSb alloys with Al content near 85%. Beryllium and tellurium were used for p- and «-type doping, respectively. The laser active region contained several compressively-strained InGaAsSb QWs with In content in the vicinity of 50%. The utilization of the compressive strain was previously identified as an effective way to improve hole confinement in QWs.2 The barrier and waveguide material was the quinary AlInGaAsSb alloy lattice-matched to GaSb. The use of the quinary In-containing material as QW barriers also improves hole confinement.4'5 The results presented in Refs. 2, 4, and 5 conclusively demonstrate that enhancement of the hole confinement barrier is crucial factor in reducing the threshold current density of GaSb-based diode lasers at room temperature.

Wafers were processed into 100 μπι wide index-guided ridge lasers by wet etching of the top-cladding layer outside of the current stripe. Laser mirrors were cleaved to form 1 and 2 mm-long cavities. For cw laser parameter characterization the devices were mirror-coated with reflectivity below 5% for anti-reflection (AR) and above 90% for high reflection (HR) coatings. Coated laser dies were soldered epi-side down with indium on polished gold-coated copper blocks.

Figure 1(a) shows the cw light-current characteristic of diode lasers emitting at 3 μιη. These devices demonstrated cw threshold current densities of about 280

GaSb-based Type-I Laser Diodes Operating at 3 μιη and Beyond 67

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A/cm2 and produced more than 150 mW of output power. The laser hetero-structure had two 12 nm wide InGaAsSb QWs spaced 40 nm apart, with compressive strain about 1.7%. The two-QW active region was placed in the center of waveguide region made of quinary Ino.2Alo.2Gao.6Aso.19Sbo.8i material. The total waveguide core width (from/»-cladding to «-cladding) was about 1070 nm.

The modal gain was measured by Hakki-Paoli method for 1 mm long uncoated devices - see Fig. 1(b). The estimated internal optical loss is ~5 cm"1 and transparency current density is about 100 A/cm2. The relatively low values of threshold current density and internal optical loss indicate that there is no dramatic increase of nonradiative recombination and free carrier absorption losses, at least for λ near 3 pm. The band offsets between QW and quinary barrier materials were estimated as about 300 meV and 160 meV in conduction and valence bands, respectively.1'6

Figure 2(a) plots the cw light-current characteristic of diode lasers emitting near 3.17 pm at room temperature. These devices demonstrated cw threshold current densities of about 480 A/cm2 and produced more than 50 mW of the output power. The laser heterostructure had four 14 nm wide InGaAsSb QWs spaced 40 nm apart, with compressive strain about 1.6%. The number of QWs in active region was twice that of 3 pm emitting devices with provision to decrease the threshold carrier concentration. The four-QW active region was placed in the center of waveguide region made of quinary Ino.2Alo.2Gao.6Aso.19Sbo.8i material. The total waveguide core width (from p-cladding to «-cladding) was about 1180 nm.

The current dependence of the modal gain spectrum measured by Hakki-Paoli method for 1 mm long uncoated devices is shown in Fig. 2(b). The estimated

68 Future Trends in Microelectronics

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Figure 2. (a) Light-current characteristics measured in cw regime at 15 C for 2 mm long, 100 μιτι wide, AR/HR coated diode lasers emitting near 3.17 μηι; (b) current dependence of the modal gain spectra for corresponding 1 mm long, 100 μιτι wide, uncoated lasers measured in pulsed regime (200 ns/2 MHz).

internal optical loss is 8-10 cm"1 and transparency current density is about 200 A/cm2. The twofold increase in the transparency current density compared to the 3 μιτι emitters of Fig. 1(b) is normal, given that the active region contains twice the number of QWs. The apparent reduction of laser efficiency and increase in the threshold of 3.17 μπι emitters as compared to 3 μπι emitters is partly due to weaker hole confinement. The valence band offsets between QWs and quinary barrier materials at the longer wavelength are smaller by -40 meV.

Increasing of the lasing wavelength above 3.17 μηι is expected to degrade the hole confinement even further. Devices operated in cw at RT with λ = 3.36 μπι produced only 15 mW of power.7 These laser heterostructure contained four 16 nm wide QWs in the active region of a waveguide of -1150 nm total width. Internal optical losses were estimated to be about 12-15 cm"1. Threshold current density was about 750 A/cm2 at 17 °C, for 2 mm long, 100 μπι wide, AR/HR coated etched lasers.

Figure 3(a) plots the pulsed light-current characteristics of diode lasers designed to operate above 3.5 μπι at RT. The laser heterostructure had four 15 nm wide InGaAsSb QWs spaced 40 nm apart, with compressive strain of -1.5%. The four-QW active region was placed in the center a quinary Ino.25Alo.2Gao.55 Aso.23Sbo.77 waveguide. The total waveguide core width (from p-cladding to n-cladding) was about 700 nm. The lasers were mirror-coated to reflect 30% (NR) and above 90% (HR).

A dramatic increase in the pulsed threshold current density up to above 1000 A/cm2 at 280 K prevented the lasers from operating in cw regime at room temperature - see Fig. 3(b).

GaSb-based Type-I Laser Diodes Operating at 3 μιτι and Beyond 69

Current (A) Current (A)

Figure 3. (a) Pulsed (200 ns/100 kHz) and (b) cw light-current characteristics measured in the 100-290 K temperature range for 2 mm long, 100 μηι wide, NR/HR coated lasers.

Clearly, the internal optical losses increase with laser wavelength from about 5 cm"1 at 3 μπι up to 12-15 cm"1 at 3.36 μπι. A part of this increase is due to the increased overlap of the optical field with doped cladding regions from about 3 -4% at 3 μπι to 4-5% at 3.4 μπι (for waveguide width of ~1100 nm). The rest of the internal loss increase can be accounted for by the rise of the free hole absorption coefficient with λ,8 as well as enhanced free hole absorption in the active region due to a greater number of QWs and less-localized holes. A substantial reduction of the internal optical losses can be achieved by either minimizing the overlap of the optical field with p-doped regions or by minimizing the doping level (or both). Strong carrier localization in the QWs is expected to reduce the concentration of free holes in waveguide region, also reducing the net internal optical losses.

The threefold increase in internal optical loss from 5 cm"1 at 3 μπι to 12-15 cm"1 at 3.36 μιτι corresponds to a less than twofold increase of the total loss for 2 mm long AR7HR coated devices (mirror loss is ~8 cm'1) and hence cannot explain the observed increase of the threshold current density. Indeed, the differential gain as a function of the current, as determined from Hakki-Paoli measurements shown in Figs. 1(b) and 2(b), decreases with wavelength by 40% and 55% when wavelength is changed from 3 to 3.17 μπι and from 3 to 3.36 μπι, respectively. The estimated reduction of the MQW optical confinement factor between 3 μπι emitters and 3.36 μπι emitters is below 10%; hence contribution of this effect to observed reduction of the differential gain is marginal.

The gradual decrease in the differential gain can be accounted for by the degradation of hole confinement, degradation of injection efficiency, and enhanced Auger recombination. It is likely that these effects are mutually reinforcing. For instance, reduced hole confinement leads to the thermal population of the waveguide valence band that, in turn, can reduce injection efficiency. Additionally, thermal population of the barrier regions in the valence band effectively "increases the hole density of states" in QWs, thus increasing the threshold carrier concentration and enhancing Auger recombination.

70 Future Trends in Microelectronics

3. Diode lasers operating at 3 μιη with 300 mW cw RT output power

Here we discuss the optimization of the diode laser heterostructure by modifying a single parameter - the waveguide width. The optimization of the waveguide width was performed for 3 μπι emitters and led to twofold increase of the cw RT output power level, i.e. from 160 mW of Fig. 1(a) to more than 300 mW.

Three laser heterostructures were grown with different waveguide widths: (a) 470 nm, (b) 1070 nm (Fig. 1), and (c) 1470 nm. The heterostructure parameters other than waveguide width were nominally kept the same for all the three lasers. Figure 4(a) shows pulsed (200 ns, 10 kHz) light-current characteristics measured at room temperature for 1 mm long uncoated lasers. The light-current characteristics demonstrate a rollover for all three structures that becomes more severe with increasing the waveguide width. Figure 4(b) shows the threshold current densities measured for three structures over a wide temperature range. At 150 K, all three devices have a very low threshold current density of-25 A/cm2 that increases with temperature at a rate that depends on waveguide width. At 300 K the threshold current density is more than two times higher for devices with the widest waveguide than for the narrowest one.

The earlier rollover of the light-current characteristics in devices with broader waveguides can be accounted for by the carrier transport constraints in waveguide region.9 Accumulation of free carriers in the waveguide between the cladding and the two-Q W active region can lead to reduction of the slope efficiency due to both free carrier absorption and recombination.

An analysis of the dependence of the threshold current density on waveguide width shown in Fig. 4(b) should take into account the optical field distribution in the heterostructure. The optical field distribution leads to 1.7 times higher MQW optical confinement factor in lasers with 470 nm waveguides then in devices with 1470 nm ones. Enhancement of the optical confinement factor should improve the differential gain in narrow waveguide lasers. Figure 5(a) shows the experimental

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GaSb-based Type-I Laser Diodes Operating at 3 μπι and Beyond 71

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dependence of peak modal gain on the current, as measured by Hakki-Paoli method for all lasers. The differential gain of the devices with the narrowest waveguide is about 2.5 times higher than in lasers with the widest waveguide, i.e. the experimental rate of the differential gain increase exceeds the improvement in optical confinement.

Thus, differential gain is enhanced by improved device injection efficiency, as shown in Fig. 5(b). Since the diffusion current through a narrower waveguide can be supported by a lower carrier concentration, we expect the waveguide recombination current to be smaller in narrower waveguides. Figure 5(b) shows that the broadening of the waveguide decreases internal losses, as expected.10

However, the combined effects of the carrier transport through the waveguide and poorer MQW optical confinement factor did not allow the 3 μπι lasers to benefit from associated internal optical loss reduction.

Based on the experimental studies of the effect of the waveguide width on laser parameters, we optimized the 3 μπι laser heterostructure. Diode lasers with 570 nm waveguide width and reduced p-cladding doping (1017cm3 instead of the 2x1017 cm"3 used previously) were grown to minimize optical loss while maximizing QW optical confinement and internal efficiency. Figure 6(a) shows cw RT light-current and power conversion characteristics of 2 mm long, 100 μπι wide, AR/HR coated devices. The maximum power of 310 mW was achieved at a current of 4.7 A at cooling water temperature of 17 °C. The power conversion efficiency was better than 5% for the whole range of operation with maximum value approaching 8% at -150 mW output power. The cw threshold current density was 200 A/cm2 (100 A/cm2 per QW). The voltage drop across laser heterostructure was below 1.4 V at the maximum output power level.

The current dependence of the modal gain spectrum measured by Hakki-Paoli method for 1 mm long uncoated devices is shown in Fig. 6(b). The estimated internal optical loss is 4-5 cm'1 and transparency current density is below 100 A/cm2. Differential gain as a function of current is better than in previous devices of Fig. 1(b). The external efficiency near threshold is 0.25 photons per

72 Future Trends in Microelectronics

electron, i.e. still only quarter of the cw current above threshold was being converted into light. This value should increase once the internal optical losses are minimized and carrier delivery to active QWs is further improved.

4. Conclusions

As of the time of writing, modern GaSb-based type-1 QW diode laser technology can produce diode lasers operating in cw regime at room temperature up to 3.36 μπι;7 similar designs can emit above 3.5 μπι in pulsed mode. The major breakthrough in mid-IR GaSb-based diode laser performance parameters was achieved when the hole confinement in active QWs was augmented. The optimization of the laser heterostructure design can effectively improve the device differential gain and internal efficiency. Diode lasers operating near 3 μπι can now produce above 300 mW in cw regime with power conversion efficiency near 8%.

High differential gain and relatively low internal optical losses have reduced the threshold current density of 3 μηι GaSb-based emitters (200 A/cm2) to values comparable with the best ~1 μιτι GaAs-based high power diode lasers (50 A/cm2)." It is worth noting that the internal efficiency of those GaAs-based state-of-the-art devices is approaching 100%, while GaSb-based 3 μιη emitters have internal efficiency near 50%. Nearly twofold difference in internal efficiencies implies even closer correspondence between actual QW threshold current densities of near-and mid-IR devices. This result implies that the "Auger limit" does not curtail the performance of mid-IR diode lasers, at least in spectral region near 3 μπι.

We speculate that the output power of the best current GaSb-based diode lasers is limited by the carrier transport constraints. Once the carrier delivery into active QWs is improved, the output power of lasers operating in the 3-3.5 μηι spectral region is expected to increase.

Current (A) Photon energy (eV)

Figure 6. (a) Light-current characteristics measured in cw regime at 17 °C for 2 mm long, 100 μιη wide, AR/HR coated optimized 3 μηι diode lasers; (b) modal gain spectra vs. current for corresponding 1 mm long, 100 μηι wide, uncoated lasers measured in pulsed regime (200 ns/2 MHz).

GaSb-based Type-I Laser Diodes Operating at 3 μηι and Beyond 73

Acknowledgment

This work was supported by the US Air Force Office of Scientific Research under contract FA95500810458 and US Army Research Office under contract W911NF0610399.

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