Journal of Crystal Growth 230 (2001) 517–521 Spatially and spectrally resolved measurement of optical loss in InGaN laser structures H.D. Summersa,*, P.M. Smowtona, P. Blooda, M. Dineenb, R.M. Perksb, D.P. Bourc, M. Kneisseld b a Department of Physics and Astronomy, Cardiﬀ University, P.O. Box 913, Cardiﬀ, Wales, CF24 3YB, UK Cardiﬀ Semiconductor and Microelectronics Centre, Cardiﬀ University, P.O. Box 913, Cardiﬀ, Wales, CF24 3YB, UK c Agilent Laboratories, 3500 Deer Creek Road, Palo Alto, California, USA d Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California, USA Abstract The optical loss co-eﬃcient in InGaN laser diodes, emitting at $410 nm, has been measured. The measurement technique is based on the transmission of internally generated spontaneous emission through varying lengths of the laser waveguide. It is unique in that it provides spectral and spatial information on the optical loss. The lasers studied are typical of InGaN structures showing a high degree of waveguide loss, ai ¼ 40 cmÀ1 . The measurements also show clear evidence of higher order transverse modes in the direction perpendicular to the growth plane with resonant leakage of the optical ﬁeld into the outer layers of the structure. This produces a modulation in the loss of these modes. # 2001 Elsevier Science B.V. All rights reserved. PACS: 42.55.P; 78.40.F; 42.25.B Keywords: A3. Quantum wells; B1. Nitrides; B3. Laser diodes 1. Introduction The accurate determination of optical loss within laser diode structures is essential for the optimisation of semiconductor laser performance. The need to minimise the waveguide loss to ensure a low threshold gain and hence a low threshold current is self-evident. Within quantum well systems this requirement has added importance because there is strong saturation of the optical gain at high carrier densities due to the constant *Corresponding author. Tel.: +44-0-2920-874458; fax: +440-2920-874056. E-mail address: [email protected]ﬀ.ac.uk (H.D. Summers). density of states within a single sub-band. Thus operation at high threshold gain can produce severe increases in current. Other aspects of the laser performance are also compromised; the diﬀerential gain is reduced thus aﬀecting the dynamic response and thermally activated carrier leakage from the quantum well is increased due to the high values of the quasi-Fermi level energies. The optical loss within semiconductor lasers is traditionally measured on devices of diﬀerent length. This provides a systematic variation in the distributed mirror loss, am whilst the intrinsic loss, ai remains ﬁxed. Measurements of the slope eﬃciency of the laser can be used to directly compare am and ai  or the threshold current with 0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 2 8 4 - 2 518 H.D. Summers et al. / Journal of Crystal Growth 230 (2001) 517–521 length can be used as this reﬂects the changing threshold gain due to the variation in the mirror loss . Both techniques are diﬃcult to implement accurately. Comparison of slope eﬃciencies requires measurements on large numbers of devices to reduce uncertainties introduced by variations in the facet quality and is only valid in the ‘ideal’ limit where complete pinning of the quasi-Fermi levels occurs at the onset of lasing. In many laser diode systems this is not the case due to the presence of current spreading or thermally activated leakage currents . Analysis of the threshold current is reliant upon a knowledge of the gain–current relationship, this is not always accurately known especially in new materials such as InGaN. A further aspect of these techniques is that they are based on an analysis of the lasing mode and thus provide only limited information on waveguide structures in which there are multiple modes. The loss is also determined solely at the lasing wavelength. Within InGaN/GaN/AlGaN laser structures the measurement of the optical loss is particularly important as it is diﬃcult to produce well controlled waveguiding of the optical emission within this material system [4–7]. Growth limitations on the thickness of the AlxGa1ÀxN cladding layers and the presence of high refractive index contact layers lead to structures which are multi-moded in the direction perpendicular to the epitaxial planes. They can also suﬀer from high optical loss due to the presence of large optical ﬁeld intensities within the substrate and outer buﬀer or capping layers of the laser. In this paper we present details of a loss measurement technique which allows us to determine the modal loss within InGaN laser diode structures as a function of wavelength. The technique also allows spatially distinct propagation modes to be distinguished. 2. Loss measurement technique The measurement of the optical loss is based upon a multi-section device conﬁguration as depicted in Fig. 1. A 4 mm wide trench is etched through the top metallic contact and upper Fig. 1. Schematic of the multi-section device conﬁguration used for the loss measurements. capping layer in order to electrically isolate 300 mm length sections . The electro-lumninescence generated within each section is collected from the front of the sample. The emission from the ﬁrst section is taken as the source light, I0 . The intensity collected when pumping subsequent sections, distance La away from the facet, is then related to the source by the expression IðLa Þ ¼ I0 expðÀaLa Þ; ð1Þ where a is the optical loss co-eﬃcient of the waveguide. At the long wavelength limit reabsorption of the luminescence within the unpumped sections is avoided because of bandgap renormalisation within the pumped section which red shifts the emission spectrum relative to the passive material. Thus, the loss co-eﬃcient correlates directly to the waveguide loss, ai . As the source light is generated by spontaneous emission within the semiconductor it is broadband and so the technique provides spectral information on the loss co-eﬃcient. To obtain spatial discrimination the collected light is passed through a monochromator and then focused onto a two-dimensional H.D. Summers et al. / Journal of Crystal Growth 230 (2001) 517–521 519 CCD array. This gives the intensity proﬁle at the device facet in the vertical direction (perpendicular to the growth plane). In the case of multi-mode devices the emission from the diﬀerent modes can be isolated by choosing the relevant detector pixels. 3. Experimental results The laser wafers studied were grown by metalorganic chemical vapour deposition (MOCVD) on a (0 0 0 1)-oriented sapphire substrate. The device consists of a 4 mm, n-doped, GaN buﬀer layer, a 0.6 mm, n-doped Al0.08Ga0.92N cladding layer, a 0.2 mm GaN waveguide region with ﬁve In0.15Ga0.85N quantum wells, a 0.5 mm p-doped Al0.08Ga0.92N cladding layer and a 0.1 mm, p-doped GaN capping layer. The wafer was processed into 50 mm wide, oxide isolated, stripe devices with Ti/Al/Ti/Au contacts on the n-side and Ti/Au metallisation on the p-side. As described above the p-type metallisation was separated into 300 mm length sections. The spontaneous emission spectrum, collected from the device facet, is shown in Fig. 2a. The luminescence is centred around 400 nm and has a 15 nm full-width at half-maximum. The spectrum from section 2 shows a reduction in intensity due to optical loss within the foremost section. This is particularly apparent at shorter wavelengths (l5420 nm) due to absorption within the InGaN quantum wells. The spectra were collected in the transverse electric (TE) polarisation as this is the lasing direction. The optical loss, derived from these spectra, is shown in Fig. 2b. At longer wavelengths, beyond the absorption edge of the wells, the loss tends to a constant value of $40 cmÀ1 which is typical of waveguide loss values within InGaN lasers and in agreement with previous measurements on these samples . The waveguide structure of the devices produces higher order modes in the vertical direction in which the optical ﬁeld leaks out into n-doped buﬀer layer and substrate. The spatial extent of this light is depicted in Fig. 3 which shows an image from the CCD array. The uppermost emission maximum corresponds to a mode which is well conﬁned Fig. 2. (a) Spontaneous emission spectrum collected from the device facet for electrical injection into sections 1 and 2, respectively; (b) loss co-eﬃcient calculated from the emission spectra. Fig. 3. Detector image of the device facet showing wavelength dependence in the x-direction and spatial variation on the yaxis. The dashed rectangle is a guide to the position of the facet. 520 H.D. Summers et al. / Journal of Crystal Growth 230 (2001) 517–521 within the waveguide region. It is this mode (labelled A) which is shown in the earlier ﬁgures. A second mode can be seen in the vertical direction which has a high intensity within the GaN buﬀer layer and the substrate (labelled B). In the case of mode B there are a number of distinct maxima in the horizontal direction indicating a spectral modulation in the optical intensity. The optical loss in modes A and B is shown in Fig. 4. At shorter wavelength both modes show an increasing loss due to absorption within the InGaN quantum wells. This indicates that the higher order mode B has some overlap with the waveguide region. There is a clear modulation in the absorption loss within mode B with a periodicity of 3.8 nm. Such resonance eﬀects have been observed within the longitudinal mode spectra of InGaN lasers  where they were attributed to scattering within the laser producing coupled cavity eﬀects. Similar eﬀects have also been observed in InGaAs quantum dot lasers  where they have been shown to be caused by resonant coupling of the modes within the diﬀerent layers of the structure . Theoretical studies on waveguiding in InGaN lasers have also concentrated on the eﬀects of such mode coupling in the vertical direction . This produces energy transfer into the highly doped GaN contact layers which increases the optical loss. These experimental results support this hypothesis as there is clearly a spectral modulation Fig. 4. Optical loss spectra for the modes A and B indicated in Fig. 3. in the loss co-eﬃcient. The modulation period of 3.8 nm corresponds to an interaction length, in the vertical direction, of $8 nm which agrees with the observation from Fig. 3 that the higher order mode is guided within the GaN buﬀer layer and sapphire substrate. At long wavelength the waveguide loss, ai is the same for both modes despite their diﬀering spatial proﬁles. We believe that this loss is due to optical scattering at dislocation sites as the dislocation density is constant through the structure and hence such losses will be independent of the mode proﬁle. 4. Summary In summary, a multi-section device structure has been used to measure the optical loss co-eﬃcient in InGaN laser structures. By implementing the technique using a CCD detector array the diﬀerent waveguide modes perpendicular to the epi-layers have been isolated. The results show a waveguide loss of 40 cmÀ1. 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