facet degradation of (al,in)gan laser diodes

phys. stat. sol. (a) 201, No. 12, 2635–2638 (2004) / DOI 10.1002/pssa.200404990

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Facet degradation of (Al,In)GaN laser diodes

Thomas Schoedl1, Ulrich T. Schwarz*, 1, Stephan Miller2, Andreas Leber2, Michael Furitsch2, Alfred Lell2, and Volker Härle2 1 Institut für Angewandte und Experimentelle Physik, Universität Regensburg, Universitätsstr. 31,

93040 Regensburg, Germany 2 OSRAM Opto Semiconductors GmbH, Wernerwerkstr. 2, 93049 Regensburg, Germany

Received 13 February 2004, accepted 5 July 2004 Published online 2 September 2004

PACS 42.55 Px, 78.67.De

We study the facet degradation behavior of (Al,In)GaN multiple quantum well laser diodes. Water vapor causes a fast degradation due to facet oxidation of the uncoated facet. Degradation in an inert nitrogen at-mosphere is slow and comparable to degradation of GaN laser diodes with coated facets. We also observe a reversible increase in the threshold current density due to a change in absorption caused by surface charges.


1 Introduction

Degradation of laser diode (LD) facets has been discussed in depth for LDs in the red and infrared spec-tral region. Ladany et al. coined the term “surface erosion” [1] for a chemical modification of the facet. For IR and red LDs this continuous degradation is known to be caused by facet oxidation [2]. In contrast to this the catastrophic optical damage (COD) causes a sudden degradation. COD has been linked to dark line formation at the cleaved LD surface [3]. For blue LDs based on GaN the contribution of the facet to the degradation process has hardly been studied [4]. We have demonstrated before that a facet coating dramatically increases the lifetime of GaN LDs [5]. For uncoated LDs the aging process can be slowed down if the LD is operated in an inert atmosphere [6]. These result are consistent with the facet degrada-tion observed for other III–V semiconductor systems [1, 2, 7–9]. In the present article we study the influence of the epitaxial process on the facet degradation. As second surface effect we discuss the influ-ence of a charged surface layer on threshold current density.

2 Laser diode characteristics

Our LDs are based on SiC substrate which allows a vertical current path, good heat dissipation into the substrate, and thus efficient cooling of the active region of the LD [10–12]. The facets are cleaved along the common (1100) plane of the SiC substrate and the GaN epilayer. The facets are atomically flat with a RMS roughness of 1< nm, as measured by atomic force microscopy. Stacks of SiO2 and TiO2 4λ/ layers, evaporated in high vacuum, act as high reflectivity ( 98%> ) mirror and 50% or 70% output coupler. We achieve high optical output power ( 100 mW> cw) using a single 2λ/ layer of SiO2. Figure 1 dis-plays the active region and the mirror stack in a secondary electron microscope image. The structure of the LD has been exposed by a focused ion beam (FIB) cut. One can clearly recognize the active layers and the mirror stack consisting of 11 layers.

* Corresponding author: e-mail: [email protected], Phone: +49 941 943 2113

2636 T. Schoedl et al.: Facet degradation of (Al, In)GaN laser diodes


SiC substrate

active layerSiO2 passivation

p - contact -metallization

n - epilayers

dielectricmirror coating

p - epilayers

5 µm

3 Degradation setup

For the degradation studies all LDs discussed in the present article were processed as gain guided LDs with the dimensions 5 µm × 600 µm of the p-contact. The laser factes were cleaved but left uncoated to study the surface effects. The LDs have a backside n-contact on the SiC-substrate. They are attached n-side down to a copper heat sink using silver conductive paint. The copper heat sink is held at 25 °C, the temperature is measured 2 mm below the LD. The LDs are operated under pulsed conditions. The power source generates voltage pulses with a duty cylce of 1% and a duration of τp = 500 ns. The current through the LD is calculated from the voltage drop over a known resistor. The LD is placed in an stainless steel container which can be evacuated and flushed with different gases. We use pure nitrogen, air (approximately 25% humidity), and nitrogen saturated with water vapor at T = 18 °C.

4 Degradation experiment and discussion

Figure 2a shows the threshold current thI over the effective operation time. The LD was degraded at a constant current which was chosen so that the LD was initially lasing. During the aging process the threshold current increases, and the LD stops lasing after approximately one minute. The optical intensity as function of time is plotted in Fig. 2b. During degradation the photon density changes over several orders of magnitudes, yet no pronounced change in the degradation rate can be seen. So for two reasons we exclude catastrophic optical damage (COD): the observed degradation is a continuous process and it is not strongly depending on the photon density. To see the influence of water vapor on the degradation process we have exposed uncoated LDs to different atmospheres. For a LD with uncoated facet in nitrogen atmosphere after a small initial increase

a) b)

Fig. 2 a) Threshold current thI over effective operation time (time/duty cycle) with constant aging current. b) Corresponding relative optical intensity over effective operation time.

Fig. 1 Section of the LD (sub-strate, epilayers, and p-contact) and the dielectric mirror stack. The cut was performed by a focused ion beam (FIB).

phys. stat. sol. (a) 201, No. 12 (2004) / www.pss-a.com 2637


N2

N + H O2 2

in thI only a negligible degradation was observed during 12 h operation (see Fig. 3a). With water vapor the threshold current density increases much faster and to a good approximation as 2

th ( ) .I t t∆ ∝ We propose that this facet degradation is due to oxidation in the presence of water. Water molecules can be cracked to H + and OH − and to further radicals by photolysis. Because there is no degradation during storage at ambient conditions, the process must be light, voltage, or current enhanced. The mechanism may be an acceleration of the oxidation process via transitions at nonradiative recombination centers, providing enough energy to the lattice to allow local transport. The necessary photon density correspond-ing to a few hundred µW is reached even at the level of spontaneous emission. Such a mechanism would be self–enhanced and would result in the observed 2t behaviour for thI .

5 Variation with epitaxy

While the fast degradation in the presence of water vapor was observed for all LDs, although to a differ-ent degree, the variation in dry air varied markedly for LDs with different epitaxial parameters. Figure 4 shows the degradation of two different LDs first in nitrogen atmosphere and then in dry air. Both LDs show the initial increase of thI of a few percent (please note the different thI scales of Fig. 3 and Fig. 4). The LD of Fig. 4a shows a decrease of thI in dry air while the LD of Fig. 4b is not affected by the differ-ent atmospheres.

N2 N2

dry air

dry air

a) b)

Fig. 4 Threshold current Ith over effective operation time for two different uncoated LDs from different epitaxial runs in nitrogen atmosphere and dry air.

Fig. 3 Threshold current thI over effective opera-tion time in nitrogen atmosphere and nitrogen with water vapor.

2638 T. Schoedl et al.: Facet degradation of (Al, In)GaN laser diodes


A decoration of the surface with charges changes the optical properties of the quantum well (QW). A induced variation of the surface barrier will modify the carrier density in the QW. This will increase or decrease the screening of the piezoelectric field inside the QW and thus result in a spectral shift of the gain spectrum. Any spectral shift of the gain spectrum in regions near the surface will reduce the overall gain at the wavelength maxλ of the maximum gain in the body of the LD. For an AlGaN heterostructure a modulation of the surface barrier was observed by Koley et al. [13]. Weidemann et al. and Steinhoff et al. study the effect of surface charges on GaN field–effect transistors [14, 15]. Even a monolayer of OH– causes a large effect [15]. A change of facet absorption due to a surface potential would be reversi-ble and saturate, as observed for the LD of Fig. 4a. This is essentially different from the non–reversible and probably several ten nanometer deep surface oxidation observed for degradation in the presence of water vapor. In the experiment shown in Fig. 4 the nitrogen atmosphere is likely to decrease the charged surface layer, while immersion in dry air will result in a surface decoration similar to the initial one after storage at ambient conditions. The InGaN QWs experience a biaxial compressive strain by the GaN barriers and the AlGaN waveguide. Near the surface the symmetry of the strain is changed from biaxial to uniaxial, because the lattice can relax in the direction towards the facet. This decreased strain causes a redshift of the bandgap, which results in an increased absoprtion of the QW near the facet at the lasing wavelength. The strength of this effect will depend on the strain present in the lattice and thus on growth variations. This mecha-nism will modify the degradation both because of different absorption near the LD facet and because of a different sensitivity of this absorption to charges present on the surface.

6 Conclusion

We have observed degradation of LDs with cleaved facets in different atmospheres. Uncoated LDs in water free atmospheres show a linear degradation after an initial fast degradation. Uncoated LDs in at-mospheres with enough water vapor show a accelerated degradation behavior proportional to 2t . We propose the buildup of an oxide layer on the LD facet as the origin of this enhanced degradation, as it is observed in other III–V semiconductor compounds. We also observed a reversible increase in the threshold current density which we attribute to a changed absorption near the LD facet due to a modifica-tion of the surface potential by surface charges.

Acknowledgements The authors would like to thank V. Kümmler for stimulating discussion. This work was supported in part by the German government (BMBF).

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facet degradation of (al,in)gan laser diodes

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