007) 4412–4418www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Raman and luminescence probes for the study of compound semiconductors

J. Jiménez ⁎, M. Avella, O. Martínez

Física de la Materia Condensada, ETS Ingenieros Industriales, 47011 Valladolid, Spain

Available online 14 September 2006

Abstract

Optical characterization using local probes with submicrometric spatial resolution is very useful for many problems concerning compoundsemiconductors and devices. In particular, micro-Raman spectroscopy and cathodoluminescence spectrum imaging are very powerful analyticaltechniques that manage good signal/noise ratios allowing to acquire images including spectral information and slightly submicrometric spatialresolution in a short time. The main analytical and spatial resolution aspects concerning both techniques are addressed. Several examples arepresented in order to illustrate the capabilities of micro-Raman and cathodoluminescence spectrum imaging for the characterization of compoundsemiconductors and the devices based on them.© 2006 Elsevier B.V. All rights reserved.

Keywords: Cathodoluminescence; Micro-Raman; Compound semiconductors

1. Introduction

Compound semiconductors are the base of advancedoptoelectronic and microelectronic devices. High-quality mate-rials are necessary to improve the performance and reliability ofthe devices. Also, new structures with well-defined propertieswill help to make new devices. Because of the reduced dimen-sion of the devices and the importance that local properties haveon both their performance and reliability local probes are be-coming very important tools to characterize these semicon-ductors. The reliability of the devices is tightly related to thepresence of defects, which can be created during the differenttechnological steps followed to build up the device [1,2]. Also,one should consider external factors that can induce defectgeneration; for example, local stresses can be generated at thedifferent stages of the device processing and packaging, beingwell known that stress accelerates the degradation [3–5].Probing techniques with high analytical capabilities are nec-essary to characterize compound semiconductors at differenttechnological steps. Usually, the relevant information arises

⁎ Corresponding author. Tel.: +34 983 423191, 3379; fax: +34 983 423192.E-mail address: jiménez@fmc.uva.es (J. Jiménez).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.07.113

from the local distributions of the different physical parameters.For example, device degradation, from the point of view of thematerial properties, appears as a local event where a stronggeneration of defects is responsible to kill the device perfor-mance [1,2]. On the other hand, the continuous size reductionof the structures demands techniques with very high spatialresolution [6]. However, slightly submicrometric resolutionusing optical and scanning electron microscopes (SEM) is veryuseful for many problems regarding optoelectronic materialsand devices where the relevant dimensions are in the slightlysubmicrometric scale.

Optical techniques are very useful because they are con-tactless, which allows to use them as screening methods at thedifferent stages of the device processing. In particular, Ramanspectroscopy and luminescence techniques are analytically verypowerful, because they provide information on many materialsproperties that are essential to the device performance. Manyproblems concerning III–V compounds in relation to conven-tional devices as laser diodes, LEDs, HBTs, non-linear opticsfilters and others can be afforded using characterization tech-niques with submicrometric spatial resolution. Raman andluminescence techniques have a good signal-to-noise ratio,which makes them very suitable for microscopic analysis andmapping [7–9]. We present herein a detailed analysis of the useof Raman and luminescence probes at a submicrometric scale

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for the characterization of different III–V materials from thesubstrate to the degraded device. One should keep in mind thatthe experimental techniques must be adequate to the specificproblem; therefore, for many purposes conventional techniquesproviding submicrometric spatial resolution are more powerfulthan scanning probe microscopy (SPM) techniques that givebetter spatial resolution, but much poorer signals and higherhandling difficulties without adding a better insight into theproblem. In that sense, submicrometric resolution techniquesremain to be basic tools for the characterization of compoundsemiconductors and the devices based on them, because theycombine easy handling, good signal-to-noise ratio, acceptableacquisition times and spatial resolution very useful for manycommon problems concerning compound semiconductors.

2. Experimental techniques

2.1. Raman scattering

The Raman effect in solids consists of the inelastic scatteringof light by elementary excitations. In general, the incident lightinteracts with the lattice vibrations; therefore, the Raman spec-trum is very sensitive to the local environment, providing in-formation about the material structure at the scale of a fewlattice parameters [8,10]. Thus, it can estimate parameters asimportant for the device performance as stress, composition,crystal disorder, etc. [11].

When the Raman spectrometer is attached to an opticalmicroscope, the lateral spatial resolution can be better than1 μm, which makes it very useful as a local probe [8]. Contrarilyto luminescence, that is strongly enhanced in the presence ofconfinement due to the spatial localization of excitons, e.g., inQWs, the Raman signal is weaker, it cannot be detected in QWsunder standard experimental conditions, due to the very smallvolume probed by the excitation beam; however, it can be usedfor these purposes making use of resonant excitation [12].

III–Vs are non-polar semiconductors, which allows toestimate the concentration and mobility of free carriers usingthe LO-phonon plasmon coupled (LOPC) modes. This is a verypowerful application since it provides information on the trans-port properties using a contact less optical probe of a very smalldimension. Comparisons of Raman data and electric transportdata have demonstrated a good accuracy for the transport para-meters deduced from the Raman data [13,14].

2.2. Luminescence

The luminescence consists of the emission of light resultingfrom the recombination of excess carriers. The basic mechanismis the generation of an e–h pair by an excitation source, which,when recombined, emits a photon. Depending on the excitationsource, one can distinguish photoluminescence (PL), cathodo-luminescence (CL) and electroluminescence (EL). One of themain advantages of these techniques in relation to the problemof optoelectronic devices, such as light emitting devices (LEDs)or laser diodes (LDs) is the possibility of mapping the lightgeneration efficiency. For example, device degradation is a local

event; normally, it does not take place over the full extension ofthe device, but at specific regions of it appearing as regions ofreduced luminescence emission. Therefore, luminescenceprobes are very useful tools for the characterization of opto-electronic materials and devices, since they probe their ability toproduce light.

The main luminescence spectral parameters are tightlyrelated to the properties of the materials. Thus, the luminescenceintensity allows to get information on the radiative recombina-tion efficiency, which is crucial to the device degradation; inparticular, degraded regions appear dark. On the other hand,non-radiative recombination plays a major role in the degra-dation processes. The degradation is usually assisted by amechanism known as recombination enhanced defect reaction(REDR) [15,16], in which the energy released by non-radiativerecombination at either crystal defects or deep levels is suppliedto the lattice to lower the activation energy for defect generationand/or diffusion, which otherwise would have a very lowprobability to occur at the usual temperature of device oper-ation. The competition between radiative and non-radiativerecombination paths is at the origin of the contrast in lumi-nescence topography [7] and CL imaging [9].

Additional information on the mere observation of bright-ness distribution can be obtained when the full spectrum isobtained; in this case, one can map other luminescence para-meters as the peak wavelength and the FWHM, that can becorrelated with each other and also correlated to the intensitydistribution. This allows us to get a complete spatial image ofthe properties of the studied region in terms of its luminescenceperformance. For this application sensitive multichannel de-tectors are necessary. CL is the adequate technique for such anapplication because it allows to image regions several squaremicrometers large in a short time using the scanning control ofthe scanning electron microscope (SEM). Once the full spectralinformation is recorded, one should manage the data in order toextract information about the spatial distribution of the differentphysical properties. Stress distribution, distribution of impuri-ties or point defects, concentration of free carriers and otherphysical parameters can be mapped using spectral imaging CL.

2.3. Spatial resolution and probe depths

Now, the question is what are we probing with the differentexcitation sources? One should distinguish between the lateraland the depth probing dimensions. When using a laser beamfocused throughout an optical microscope, the lateral resolutionis determined by the size of the diffraction spot, whichthe diameter is given by the Rayleigh formula: D=1.22λ/NA,where λ is the excitation wavelength and NA is the numericalaperture of the objective.

Therefore, using UV light, e.g., a He–Cd laser (λ=325 nm),a typical resolution of 0.7 μm can be achieved. The UVobjectives have numerical apertures around 0.5. A similar spotsize can be achieved using light from an Ar+ laser (488 nm or514.5 nm), because the NAs are around 0.95. However, whenworking with cryostats, one needs long-distance work objec-tives and this reduces the numerical apertures down to 0.5, with

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the corresponding enlargement of the spot size at the focalplane. The use of confocal systems allows us to reject out-of-focus light, which is very convenient when the studied materialis transparent to the incident light, allowing in-depth studies.

The lateral resolution in Raman spectroscopy coincideswith the spot size. However, this is not true for luminescence,because in this case, one collects also the light produced bythe recombination of carriers that diffuse out of the generationvolume. For example, the diffusion length is very long in QWs,an e–h pair can recombine very far from the region where it wasgenerated [17], which can significantly fuzzy the image. How-ever, we are interested to map inhomogeneities, and in that case,the presence of potential barriers due to QW thickness fluc-tuations, compositional changes, etc., or the recombination atcrystal defects, e.g., dislocations, interface defects, etc., reducesdrastically the diffusion length making it possible for us to mapthe defective regions.

The depth analysis is determined by the absorption coef-ficient corresponding to both the incoming and the outgoinglights. This changes substantially the penetration depth in pho-toluminescence and Raman spectroscopy. Because the Ramaneffect is an inelastic light scattering, both the incoming andoutgoing lights are very close to each other in energy and theeffective penetration depth is 1/2α. In the case of photolumi-nescence, the incoming light is heavily absorbed; however, theoutgoing light is very lightly absorbed, which gives an effectivepenetration depth of 1/αincoming, plus the effect of carrierdiffusion.

In the case of excitation by the electron beam of the SEM, thespatial resolution is governed by the characteristics of the e-beam, acceleration voltage and beam current, being determinedby the attenuation of the electron beam inside the target; seeRef. [18] for more details. The range of depth penetrationscan be tuned by the acceleration voltage. An estimation of thepenetration depth can be obtained from the Kanaya–Okayamaformula [18]. By tuning the acceleration voltage of the e-beam,the range of depth penetrations is more extended than the onereached using optical beams for PL measurements, for whichthe excitation light must have the energy above the band gapone, limiting the penetration depth. In this sense, the deeperprobing depth of the e-beam in CL is suitable for us to study the

Fig. 1. Panchromatic CL image of a GaAs/Si conformal layer showing quasi-periodiband. The dashed line is the total stress calculated from the Raman shift.

QWs of laser diodes, which are buried by confinement, claddingand contacting layers avoiding the usual optical probe beams toreach to top-view measurements. The lateral extension of theexcitation volume is controlled by both the acceleration voltageand the beam current [17,18]. In materials with high quantumefficiency and short minority carrier diffusion length spatialresolutions below a few hundred nanometres can be achievedusing low acceleration voltages and small beam currents.

3. Applications

Some applications of CL and Raman probes are presentedthereafter. For this, we have selected heteroepitaxial GaAslayers grown on Si substrates by the conformal method anddegraded high-power laser diodes. Both constitute excellentexamples of the analytical capabilities of CL and μ-R.

3.1. Heteroepitaxial GaAs/Si layers grown by the conformalmethod

3.1.1. Stress distributionThere is a huge interest on the deposition of GaAs on Si to be

used in high-speed and opto-electronic devices [19]. However,the large lattice mismatch between GaAs and Si (4%) and thedifference in the thermal expansion coefficients (56%) result ina very high crystal defect density that needs to be drasticallyreduced to obtain materials suitable for optoelectronic devices.Among the different approaches tried to grow high-qualityheteroepitaxies, conformal growth has demonstrated to reducethe crystal defect density by several orders of magnitude in theGaAs layer with significant enhancement of the luminescenceefficiency with respect to the conventional GaAs/Si layers.Conformal growth is a confined lateral growth technique de-scribed in previous works [20]. Using this technique, differentstructures with improved crystalline quality can be prepared inview to make optoelectronic devices with heteroepitaxial GaAs/Si, e.g., heterojunction formed by GaAs and ternary alloys, pnjunctions, etc.

A typical CL image of an undoped conformal GaAs layer isshown in Fig. 1. The layer thickness is submicrometric and thewidth of the layer grown laterally from the seeds can get 30–

c intensity modulations. The black line is the peak frequency of the LO Raman

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40 μm. The CL image shows a quasi-periodic modulation of theintensity parallel to the growth front, this modulation has beendiscussed elsewhere [21,22] and is related to the in-plane biaxialstress distribution. There is a spatial anticorrelation between theluminescence intensity and the peak energy. Scanning theRaman microprobe parallel to the growth front one can re-produce the stress distribution, see the line representing the LOphonon frequency shift along the scanning line, which followsboth the peak energy fluctuations and the intensity fluctuationof the CL peak, evidencing the dominant role played by stress inthe luminescence contrast of the conformal layers [22]. Theregions with lower luminescence efficiency are tensile strainedwith respect to the regions with higher luminescence emission.Since the crystal defect density is very low in these layers, onecan argue that the regions under tensile stress accumulate pointdefects, probably excess arsenic-forming arsenic antisitesdefects, AsGa, which are the main non-radiative recombinationcenters in dislocation-free GaAs [23].

Also, one observes a bright stripe parallel to the seed, Fig. 2.This bright stripe cannot be related to stress relaxation, becausethe luminescence peak energy is not shifted. The Raman mic-roprobe was scanned across it. The first-order Raman spectrumof GaAs consists of two optical phonon modes: a longitudinaloptic (LO) at 292 cm−1 and a transverse optic (TO) at 268 cm−1.The observation of these modes depends on the scatteringgeometry. In backscattering on (100) planes, only the LO modeis allowed, the TO mode being forbidden. Deviations from thisoccur in the presence of disorder. In general, the Raman spectraof the conformal layers match the symmetry selection rules, i.e.,an intense LO and a weak TO modes are observed over theconformal layer. However, the selection rules were broken downat the bright stripe next to the seed where forbidden TO-likemodes are locally activated, see Fig. 2. This behavior can be dueto either symmetry breakdown or doping in concentration highenough to activate LOPC modes, with phonon-like character[10]. To elucidate the origin of the forbidden band the Ramanparameters were obtained as a function of the position across thelayer. We observe that when the TO-like band is activated,

Fig. 2. Panchromatic CL image of a GaAs/Si conformal layer showing a brightstripe parallel to the seed. The Raman spectra in the bright stripe and the layerare shown.

its FWHM (full width at half maximum) scales up from 2 to11 cm−1, while the LO band decreases its intensity. Thisbehavior illustrates that the TO-like band is not activated bysymmetry breakdown, but it is likely the L-branch of the LOPCmode, due to the presence of free carriers in the region of thelayer close to the seed. Because the layers were not intentionallydoped, one can assume a self-doping effect on the part of thelayer close to the seed, which is allowed by Si diffusion [21].

3.1.2. Alloy compositionContrarily to hydride vapor phase epitaxy (HVPE) GaAs

conformal layers, Metal organic vapor phase epitaxy (MOVPE)AlxGa1−xAs conformal layers present an irregular growth front,which is due to the fact that the mean surface migration lengthfor Al atoms is much shorter than that of Ga atoms [24]. Thisresults in a granular-like growth as observed in CL images,Fig. 3. The quality of the layers was dependent on the seedquality and the homogeneity of the AlGaAs layers increasedlowering the Al flow, see also Fig. 3. The estimation of theAl incorporation can be carried out by means of the phononfrequencies from the Raman data, since the Raman spectrum issensitive to the alloy composition. The Al content was deducedfrom the shift of the GaAs-like Longitudinal Optic phonon(labelled LO1) according to [25,26]:

x ¼ 7:309−0:0254xLO−1 ð2ÞComposition profiles were obtained both parallel and trans-

versal to the conformal layer. A maximum value of x=0.2 wasobtained far away from the seed, with fluctuations along thelayer of about 2%, while the lateral homogeneity was better. Forthe square-shaped layers, it was observed that the incorporationof Al is better in the corners, corresponding to ⟨100⟩ crystaldirection. It should be noted that the estimated compositions canpresent errors due to the existence of non-homogeneous stress,which can induce additional frequency shifts of the phononmodes [27]. Depending on the growth conditions Al fluctua-tions range from 2% to 20% over the average value, whichshows that not only the morphologic quality of the layer im-proves when Al flow decreases, but also the distribution of Al ismore homogeneous.

3.1.3. Free carriersThe measure of the free carrier concentration is usually

achieved by Hall effect; however, the low spatial resolution ofthe Hall probes as well as the need to prepare electric contactsdemands alternative ways to estimate the transport parametersthat can present significant spatial fluctuations. Informationabout the doping level can be achieved by both Raman andluminescence spectroscopies at a submicrometric scale. Wepresent herein the example of a selectively doped conformallayer, where we have studied the influence of free charge onboth Raman and luminescence spectra.

In the presence of charge, the LO phonons couple with theplasmons resulting in the formation of LOPC modes, fromwhich the transport parameters can be extracted. There areseveral approaches that consider different dielectric functions.

Fig. 3. Growth front in AlGaAs conformal layers showing the dependence withthe Al flow (a) for a DMAlCl flow of 5 cm3/min; (b) for a DMAlCl flow of4 cm3/min.

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We used a hydrodynamic approach [28]. In that case the electricsusceptibility is simplified to the expression:

ve gð Þ ¼ −U2

hm2iq2−x2 þ ixCð3Þ

where η, ν2, ω and Γ are the dimensionless Fermi energy (η≡[(EF−EC)/kT]), the electron mean square velocity, the Raman

Fig. 4. Calculated free-carrier concentration as a function of the position,superposed to the panchromatic CLmap of the conformal sample region studied.

frequency and the plasmon damping parameter (full width athalf maximum of the plasmon mode), respectively.

U2 ¼ e2N0

m*0F1=2 gð Þ 1þ 5

12am*0E0

hm2i" #

ð4Þ

where N0, F1/2 (η), α, m0⁎, and E0 are the effective density ofconduction band states, a member of the Fermi–Dirac integralfamily, the non-parabolicity coefficient, the electron band-edgeeffective mass and the band gap energy, respectively.

Plasmon modes are fitted with a Raman differential cross-section expression that includes the contribution of the defor-mation potential mechanism and the electro-optic mechanismgiven by Hon and Faust [29]. Using the value of the Fermienergy determined from the fitting of the Raman spectrum onecan determine the free-carrier concentration from

Ne ¼ N0F1=2 gð Þ 1þ 154akBTE0

F3=2ðgÞF1=2ðgÞ

� �ð5Þ

Fig. 4 represents the calculated values of the free electronconcentration with respect to the position in the conformal layer,superposed to the panchromatic CL image. The points shown inthe non-doped regions are a guide to the eye; since the carrierconcentration limit for LOPC mode observation is below∼5×1016 cm−3 it shouldmean that the free carrier concentrationin undoped regions is below that value. The bright stripebetween 0 and 3 μm corresponds to an undoped GaAs layer; thenext stripe 8 μmwide is Si-doped. The undoped GaAs seed is thedark stripe located in the right side of the CL image. It has to bestated that the thin bright stripe in the doped region, near theseed, corresponds to a self-doped stripe as discussed above. Thecentral part of the doped stripe shows a little fluctuation ofthe carrier concentration that varies between 2.94×1018 and2.00×1018 cm−3. Also one observes the sharp interface betweenthe undoped and doped regions. The profile is not so much sharpin CL, which is the consequence of the carrier diffusion length.

In doped semiconductors the band filling with free carriersshifts the luminescence peak energy to the blue, Burstein–Mosseffect [30]; therefore, the density of free electrons can be

Fig. 5. Distribution of the carrier concentration in a GaAs/Si conformal layer asobtained from the spectral CL image, once the peak energy has been convertedto free electron concentration using Eq. (6), see the text. The free electronconcentration scale is given in 1018 cm−3.

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obtained from the peak energy of the band-to-band lumines-cence. The magnitude of this displacement with respect to thedoping concentration in n and p type materials has been studiedby different authors [31,32] who have proposed empiricalexpressions to determine de doping concentration of the freecarriers in GaAs layers. Here, we have used the expressionproposed by Lee et al. [31] for the peak energy dependence withthe concentration of free electrons at room temperature:

Emax ¼ 1:426þ 2:4� 10−14N3=2e ðeVÞ ð6Þ

where the value 1.426 eV corresponds to the peak energy emit-ted by undoped GaAs.

Fig. 5 shows the distribution of the carrier concentration asobtained from the spectral CL image, once the peak energy hasbeen converted to free electron concentration using Eq. (6). Theconcentrations obtained by luminescence are lower than theones obtained by micro-Raman spectroscopy. This could beexplained by the presence of tensile strain in the conformallayers (∼1.0 kbar), which have a higher influence on the lumi-nescence spectrum that in the LOPC modes.

3.2. Degraded high-power laser diodes

The laser degradation consists of the decrease of the outputpowerwith the operation time. Basically degradationmechanisms

Fig. 6. Monochromatic CL images of a degraded broad emitter of a high powerlaser bar. (a) λ=850 nm, corresponding to the emission from the p-type contactlayer; b) λ=752 nm, corresponding to QW emission.

Fig. 7. Spectral images of a degraded region in a high power laser bar: (a) amplitude,(b) wavelength and (c) FWHM of the QWemission. The encircled areas show twotypes of behaviour: (ii) spatial correlation, (iii) spatial anticorrelation.

in high-power lasers may be classified as process-, material- orfacet-related mechanisms [1,2,33]. The degradation occurs indifferent time scales, being classified as rapid, gradual andcatastrophic [1,2,33,34]. The result of the degradation is areduction of the capability to emit light. The understanding of thedegradation mechanism is crucial to make reliable devices. Thedegradation is the consequence of the generation of defects in theactive layers of the laser structure. Usually, facet degradation isthe main cause of catastrophic degradation [33]; however, defectscan also be generated inside the cavity, which is more difficult tostudy. Removing the overlay metal contact layer, one can imagethe cavity using CL. Because the penetration depth of the e-beamcan be controlled by the acceleration voltage one can probe theluminescence emission across the full structure of the laser.Monochromatic images allow us to determine where the damage

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is localized. The monochromatic CL images of a degraded broademitter of a high-power laser bar (QWof AlxGa1−xAswith x closeto 0.1, cladding layers Al0.6Ga0.4As and GaAs contact layer) areshown in Fig. 6. One observes that the typical damage pattern isonly visible in the QW, with the other layers being free of defects.

The spectral analysis of the damage allows getting insighton the physical mechanisms responsible for the degradation.Spectral images of a degraded region are shown in Fig. 7. Oneobserves that the dark regions exhibit energy shift to the red andline broadening. However, in the most heavily degradedregions, those that appear darkest in the intensity map, thecorrelation with the other spectral parameters does not followthe same trends, and one observes a strong broadening of theluminescence band and a blue shift, which can be understood interms of cation intermixing between the QW and the con-finement layer [35]. The possibility to open the laser cavity ofdegraded lasers allows us to use spectroscopic techniques inorder to understand the physics underlying the degradationmechanisms of QW high-power laser diodes [36].

4. Conclusions

The validity of optical characterization techniques with aspatial resolution ranging from a few hundred nanometers to afew micrometers to study many problems concerning com-pound semiconductors has been discussed. These techniquespresent a relative handling simplicity and good signal/noiseratios, which make them suitable for many applications, forwhich the application of other techniques with higher resolutionnot only introduces experimental complexity, but also do notreport additional benefits to the comprehension of the problem.Several examples showing different applications and thedifferent physical issues that can be investigated are presentedand discussed.

References

[1] P.M. Petroff, Semicond. Semimet. 22 (1985) 379 (part A, Chapter 6).[2] R.G. Waters, Prog. Quantum Electron. 15 (1992) 153.[3] P. Martin, J.P. Landesman, J.P. Hirtz, A. Fily, Appl. Phys. Lett. 75 (1999)

2521.[4] J.W. Tomm, A. Gerhardt, T. Elsaesser, D. Lorenzen, P. Hennig, Appl. Phys.

Lett. 81 (2002) 3269.

[5] J.W. Tomm, A. Gerhardt, R. Müller, V. Malyarchuk, Y. Sainte Marie, P.Galtier, J. Nagle, J.P. Landesman, J. Appl. Phys. 93 (2003) 1354.

[6] C. Lienau, in: J. Jiménez (Ed.), Microprobe characterization of semi-conductors, Taylor and Francis, N.York 2002, ch.3; N.Jager, M.Salmerón,E.R.Weber; ibid ch.4.

[7] M. Baeumler, W. Jantz; ibid ch.1.[8] J. Jiménez, I. De Wolf, J.P. Landesman; ibid ch. 2.[9] A. Gustaffson, L. Samuelson; ibid ch.6.[10] F.H. Pollak, in: J. Grasseli, B.J. Bulkin (Eds.), Analytical Raman Spec-

troscopy, Chem Anal. Ser., vol. 114, J. Wiley, N.York, 1991.[11] P.W. Epperlein, SPIE-International Society for Optical Engineering

Symposium Proceedings, vol. 3001, 1997, p. 13.[12] J. Geurts, Prog. Cryst. Growth Charact. 32 (1996) 185.[13] H. Harima, S. Nakashima, T. Uemura, J. Appl. Phys. 78 (1995) 1996.[14] M. Chafai, J. Jiménez, E. Martin, W.C. Mitchel, A. Saxler, R. Perrin,

Mater. Sci. Forum 338–342 (2000) 707.[15] L.C. Kimerling, Solid-State Electron. 21 (1978) 1391.[16] A.A. Hopgood, J. Appl. Phys. 76 (1994) 4068.[17] A. Gustaffson, L. Samuelson, J. Appl. Phys. 84 (1998) 1715.[18] K. Kanaya, S. Okayama, J. Phys. D: Appl. Phys. 5 (1972) 43.[19] S.F. Fang, K. Adomi, S. Iyer, H. Morkoc, H. Zabel, C. Choi, N. Otsuka,

J. Appl. Phys. 68 (1990) R31.[20] B. Gerard, D. Pribat, Inst. Phys. Conf. Ser. 166 (2000) 23.[21] A.M. Ardila, O. Martínez, M. Avella, J. Jiménez, B. Gérard, J. Napierala,

E. Gil-Lafon, Appl. Phys. Lett. 79 (2001) 1270.[22] A.M. Ardila, O. Martínez, M. Avella, J. Jiménez, B. Gérard, J. Napierala,

E. Gil-Lafon, J. Mater. Res. 17 (2002) 1341.[23] G.M. Martin, Makram-Ebeid, in: S.T. Pantelides (Ed.), Deep Centers in

Semiconductors, Gordon, Breach Science Publishers, 1986, Chapter 6.[24] K. Hiruma, T. Haga, M. Miyazaki, J. Cryst. Growth 102 (1990) 71.[25] Z.C. Feng, S. Perkowitz, D.K. Kinell, R.L. Whitney, D.N. Talwar, Phys.

Rev., B 47 (1993) 13466.[26] T. Yuasa, S.Naritsuka,M.Mannoh,K. Shinozaki, K.Yamanaka, Y. Nomura,

M. Mihara, M. Ishii, Phys. Rev., B 33 (1986) 1222.[27] S. Adachi, J. Appl. Phys. 58 (1985) R1.[28] J. Ibáñez, R. Cuscó, L. Artús, Phys. Status Solidi, B Basic Res. 223 (2001)

715.[29] D.T. Hon, W.L. Faust, Appl. Phys. 1 (1973) 241.[30] E. Burstein, Phys. Rev. 83 (1954) 632;

T.S. Moss, Proc. Phys. Soc. 123 (1954) 1232.[31] N.Y. Lee, K.J. Lee, C. Lee, J.E. Kim, H.Y. Park, D.H. Kwak, H.C. Lee,

H. Lim, J. Appl. Phys. 78 (1995) 3367.[32] G. Borghs, K. Bhattacharyya, K. Deneffe, P. Van Mieghem, R. Mertens,

J. Appl. Phys. 66 (1989) 4381.[33] M. Fukuda, Reliability and Degradation of Semiconductor Lasers and

LEDs, Artech House, Boston, 1991.[34] J. Jiménez, C. R. Phys. 4 (2003) 663.[35] M. Pommiès, M. Avella, E. Cánovas, J. Jiménez, T. Fillardet, M. Oudart,

J. Nagle, Appl. Phys. Lett. 86 (2005) 131103.[36] J. Jiménez, E. Cánovas, M. Avella, Europhys. J. 27 (2004) 67.