Raman and luminescence probes for the study of compound semiconductors

Thin Solid Films 515 (2007) 4412 – 4418 www.elsevier.com/locate/tsf

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 compound semiconductors and devices. In particular, micro-Raman spectroscopy and cathodoluminescence spectrum imaging are very powerful analytical techniques that manage good signal/noise ratios allowing to acquire images including spectral information and slightly submicrometric spatial resolution in a short time. The main analytical and spatial resolution aspects concerning both techniques are addressed. Several examples are presented in order to illustrate the capabilities of micro-Raman and cathodoluminescence spectrum imaging for the characterization of compound semiconductors 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 advanced optoelectronic and microelectronic devices. High-quality materials are necessary to improve the performance and reliability of the devices. Also, new structures with well-defined properties will help to make new devices. Because of the reduced dimension of the devices and the importance that local properties have on both their performance and reliability local probes are becoming very important tools to characterize these semiconductors. The reliability of the devices is tightly related to the presence of defects, which can be created during the different technological steps followed to build up the device [1,2]. Also, one should consider external factors that can induce defect generation; for example, local stresses can be generated at the different stages of the device processing and packaging, being well known that stress accelerates the degradation [3–5]. Probing techniques with high analytical capabilities are necessary to characterize compound semiconductors at different technological steps. Usually, the relevant information arises

⁎ Corresponding author. Tel.: +34 983 423191, 3379; fax: +34 983 423192. E-mail address: jimé[email protected] (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 the material properties, appears as a local event where a strong generation of defects is responsible to kill the device performance [1,2]. On the other hand, the continuous size reduction of the structures demands techniques with very high spatial resolution [6]. However, slightly submicrometric resolution using optical and scanning electron microscopes (SEM) is very useful for many problems regarding optoelectronic materials and devices where the relevant dimensions are in the slightly submicrometric scale. Optical techniques are very useful because they are contactless, which allows to use them as screening methods at the different stages of the device processing. In particular, Raman spectroscopy and luminescence techniques are analytically very powerful, because they provide information on many materials properties that are essential to the device performance. Many problems concerning III–V compounds in relation to conventional devices as laser diodes, LEDs, HBTs, non-linear optics filters and others can be afforded using characterization techniques with submicrometric spatial resolution. Raman and luminescence techniques have a good signal-to-noise ratio, which makes them very suitable for microscopic analysis and mapping [7–9]. We present herein a detailed analysis of the use of Raman and luminescence probes at a submicrometric scale

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for the characterization of different III–V materials from the substrate to the degraded device. One should keep in mind that the experimental techniques must be adequate to the specific problem; therefore, for many purposes conventional techniques providing submicrometric spatial resolution are more powerful than scanning probe microscopy (SPM) techniques that give better spatial resolution, but much poorer signals and higher handling difficulties without adding a better insight into the problem. In that sense, submicrometric resolution techniques remain to be basic tools for the characterization of compound semiconductors and the devices based on them, because they combine easy handling, good signal-to-noise ratio, acceptable acquisition times and spatial resolution very useful for many common problems concerning compound semiconductors. 2. Experimental techniques 2.1. Raman scattering The Raman effect in solids consists of the inelastic scattering of light by elementary excitations. In general, the incident light interacts with the lattice vibrations; therefore, the Raman spectrum is very sensitive to the local environment, providing information about the material structure at the scale of a few lattice parameters [8,10]. Thus, it can estimate parameters as important for the device performance as stress, composition, crystal disorder, etc. [11]. When the Raman spectrometer is attached to an optical microscope, the lateral spatial resolution can be better than 1 μm, which makes it very useful as a local probe [8]. Contrarily to luminescence, that is strongly enhanced in the presence of confinement due to the spatial localization of excitons, e.g., in QWs, the Raman signal is weaker, it cannot be detected in QWs under standard experimental conditions, due to the very small volume probed by the excitation beam; however, it can be used for these purposes making use of resonant excitation [12]. III–Vs are non-polar semiconductors, which allows to estimate the concentration and mobility of free carriers using the LO-phonon plasmon coupled (LOPC) modes. This is a very powerful application since it provides information on the transport properties using a contact less optical probe of a very small dimension. Comparisons of Raman data and electric transport data have demonstrated a good accuracy for the transport parameters deduced from the Raman data [13,14]. 2.2. Luminescence The luminescence consists of the emission of light resulting from the recombination of excess carriers. The basic mechanism is the generation of an e–h pair by an excitation source, which, when recombined, emits a photon. Depending on the excitation source, one can distinguish photoluminescence (PL), cathodoluminescence (CL) and electroluminescence (EL). One of the main advantages of these techniques in relation to the problem of optoelectronic devices, such as light emitting devices (LEDs) or laser diodes (LDs) is the possibility of mapping the light generation efficiency. For example, device degradation is a local

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event; normally, it does not take place over the full extension of the device, but at specific regions of it appearing as regions of reduced luminescence emission. Therefore, luminescence probes are very useful tools for the characterization of optoelectronic materials and devices, since they probe their ability to produce light. The main luminescence spectral parameters are tightly related to the properties of the materials. Thus, the luminescence intensity allows to get information on the radiative recombination efficiency, which is crucial to the device degradation; in particular, degraded regions appear dark. On the other hand, non-radiative recombination plays a major role in the degradation processes. The degradation is usually assisted by a mechanism known as recombination enhanced defect reaction (REDR) [15,16], in which the energy released by non-radiative recombination at either crystal defects or deep levels is supplied to the lattice to lower the activation energy for defect generation and/or diffusion, which otherwise would have a very low probability to occur at the usual temperature of device operation. The competition between radiative and non-radiative recombination paths is at the origin of the contrast in luminescence topography [7] and CL imaging [9]. Additional information on the mere observation of brightness distribution can be obtained when the full spectrum is obtained; in this case, one can map other luminescence parameters as the peak wavelength and the FWHM, that can be correlated with each other and also correlated to the intensity distribution. This allows us to get a complete spatial image of the properties of the studied region in terms of its luminescence performance. For this application sensitive multichannel detectors are necessary. CL is the adequate technique for such an application because it allows to image regions several square micrometers large in a short time using the scanning control of the scanning electron microscope (SEM). Once the full spectral information is recorded, one should manage the data in order to extract information about the spatial distribution of the different physical properties. Stress distribution, distribution of impurities or point defects, concentration of free carriers and other physical 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 different excitation sources? One should distinguish between the lateral and the depth probing dimensions. When using a laser beam focused throughout an optical microscope, the lateral resolution is determined by the size of the diffraction spot, which the diameter is given by the Rayleigh formula: D = 1.22λ/NA, where λ is the excitation wavelength and NA is the numerical aperture 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 UV objectives have numerical apertures around 0.5. A similar spot size can be achieved using light from an Ar+ laser (488 nm or 514.5 nm), because the NAs are around 0.95. However, when working with cryostats, one needs long-distance work objectives and this reduces the numerical apertures down to 0.5, with

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the corresponding enlargement of the spot size at the focal plane. The use of confocal systems allows us to reject out-offocus light, which is very convenient when the studied material is transparent to the incident light, allowing in-depth studies. The lateral resolution in Raman spectroscopy coincides with the spot size. However, this is not true for luminescence, because in this case, one collects also the light produced by the recombination of carriers that diffuse out of the generation volume. For example, the diffusion length is very long in QWs, an e–h pair can recombine very far from the region where it was generated [17], which can significantly fuzzy the image. However, we are interested to map inhomogeneities, and in that case, the presence of potential barriers due to QW thickness fluctuations, compositional changes, etc., or the recombination at crystal defects, e.g., dislocations, interface defects, etc., reduces drastically the diffusion length making it possible for us to map the defective regions. The depth analysis is determined by the absorption coefficient corresponding to both the incoming and the outgoing lights. This changes substantially the penetration depth in photoluminescence and Raman spectroscopy. Because the Raman effect is an inelastic light scattering, both the incoming and outgoing lights are very close to each other in energy and the effective penetration depth is 1/2α. In the case of photoluminescence, the incoming light is heavily absorbed; however, the outgoing light is very lightly absorbed, which gives an effective penetration depth of 1/αincoming, plus the effect of carrier diffusion. In the case of excitation by the electron beam of the SEM, the spatial resolution is governed by the characteristics of the ebeam, acceleration voltage and beam current, being determined by the attenuation of the electron beam inside the target; see Ref. [18] for more details. The range of depth penetrations can be tuned by the acceleration voltage. An estimation of the penetration depth can be obtained from the Kanaya–Okayama formula [18]. By tuning the acceleration voltage of the e-beam, the range of depth penetrations is more extended than the one reached using optical beams for PL measurements, for which the excitation light must have the energy above the band gap one, limiting the penetration depth. In this sense, the deeper probing depth of the e-beam in CL is suitable for us to study the

QWs of laser diodes, which are buried by confinement, cladding and contacting layers avoiding the usual optical probe beams to reach to top-view measurements. The lateral extension of the excitation volume is controlled by both the acceleration voltage and the beam current [17,18]. In materials with high quantum efficiency and short minority carrier diffusion length spatial resolutions below a few hundred nanometres can be achieved using low acceleration voltages and small beam currents. 3. Applications Some applications of CL and Raman probes are presented thereafter. For this, we have selected heteroepitaxial GaAs layers grown on Si substrates by the conformal method and degraded high-power laser diodes. Both constitute excellent examples of the analytical capabilities of CL and μ-R. 3.1. Heteroepitaxial GaAs/Si layers grown by the conformal method 3.1.1. Stress distribution There 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 the difference in the thermal expansion coefficients (56%) result in a very high crystal defect density that needs to be drastically reduced to obtain materials suitable for optoelectronic devices. Among the different approaches tried to grow high-quality heteroepitaxies, conformal growth has demonstrated to reduce the crystal defect density by several orders of magnitude in the GaAs layer with significant enhancement of the luminescence efficiency with respect to the conventional GaAs/Si layers. Conformal growth is a confined lateral growth technique described in previous works [20]. Using this technique, different structures with improved crystalline quality can be prepared in view to make optoelectronic devices with heteroepitaxial GaAs/ Si, e.g., heterojunction formed by GaAs and ternary alloys, pn junctions, etc. A typical CL image of an undoped conformal GaAs layer is shown in Fig. 1. The layer thickness is submicrometric and the width of the layer grown laterally from the seeds can get 30–

Fig. 1. Panchromatic CL image of a GaAs/Si conformal layer showing quasi-periodic intensity modulations. The black line is the peak frequency of the LO Raman band. The dashed line is the total stress calculated from the Raman shift.

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40 μm. The CL image shows a quasi-periodic modulation of the intensity parallel to the growth front, this modulation has been discussed elsewhere [21,22] and is related to the in-plane biaxial stress distribution. There is a spatial anticorrelation between the luminescence intensity and the peak energy. Scanning the Raman microprobe parallel to the growth front one can reproduce the stress distribution, see the line representing the LO phonon frequency shift along the scanning line, which follows both the peak energy fluctuations and the intensity fluctuation of the CL peak, evidencing the dominant role played by stress in the luminescence contrast of the conformal layers [22]. The regions with lower luminescence efficiency are tensile strained with respect to the regions with higher luminescence emission. Since the crystal defect density is very low in these layers, one can argue that the regions under tensile stress accumulate point defects, probably excess arsenic-forming arsenic antisites defects, AsGa, which are the main non-radiative recombination centers 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, because the luminescence peak energy is not shifted. The Raman microprobe was scanned across it. The first-order Raman spectrum of GaAs consists of two optical phonon modes: a longitudinal optic (LO) at 292 cm− 1 and a transverse optic (TO) at 268 cm− 1. The observation of these modes depends on the scattering geometry. In backscattering on (100) planes, only the LO mode is allowed, the TO mode being forbidden. Deviations from this occur in the presence of disorder. In general, the Raman spectra of the conformal layers match the symmetry selection rules, i.e., an intense LO and a weak TO modes are observed over the conformal layer. However, the selection rules were broken down at the bright stripe next to the seed where forbidden TO-like modes are locally activated, see Fig. 2. This behavior can be due to either symmetry breakdown or doping in concentration high enough to activate LOPC modes, with phonon-like character [10]. To elucidate the origin of the forbidden band the Raman parameters were obtained as a function of the position across the layer. We observe that when the TO-like band is activated,

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

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its FWHM (full width at half maximum) scales up from 2 to 11 cm− 1, while the LO band decreases its intensity. This behavior illustrates that the TO-like band is not activated by symmetry breakdown, but it is likely the L-branch of the LOPC mode, due to the presence of free carriers in the region of the layer close to the seed. Because the layers were not intentionally doped, one can assume a self-doping effect on the part of the layer close to the seed, which is allowed by Si diffusion [21]. 3.1.2. Alloy composition Contrarily 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 length for Al atoms is much shorter than that of Ga atoms [24]. This results in a granular-like growth as observed in CL images, Fig. 3. The quality of the layers was dependent on the seed quality and the homogeneity of the AlGaAs layers increased lowering the Al flow, see also Fig. 3. The estimation of the Al incorporation can be carried out by means of the phonon frequencies from the Raman data, since the Raman spectrum is sensitive to the alloy composition. The Al content was deduced from the shift of the GaAs-like Longitudinal Optic phonon (labelled LO1) according to [25,26]: x ¼ 7:309−0:0254xLO−1

ð2Þ

Composition profiles were obtained both parallel and transversal to the conformal layer. A maximum value of x = 0.2 was obtained far away from the seed, with fluctuations along the layer of about 2%, while the lateral homogeneity was better. For the square-shaped layers, it was observed that the incorporation of Al is better in the corners, corresponding to 〈100〉 crystal direction. It should be noted that the estimated compositions can present errors due to the existence of non-homogeneous stress, which can induce additional frequency shifts of the phonon modes [27]. Depending on the growth conditions Al fluctuations range from 2% to 20% over the average value, which shows that not only the morphologic quality of the layer improves when Al flow decreases, but also the distribution of Al is more homogeneous. 3.1.3. Free carriers The measure of the free carrier concentration is usually achieved by Hall effect; however, the low spatial resolution of the Hall probes as well as the need to prepare electric contacts demands alternative ways to estimate the transport parameters that can present significant spatial fluctuations. Information about the doping level can be achieved by both Raman and luminescence spectroscopies at a submicrometric scale. We present herein the example of a selectively doped conformal layer, where we have studied the influence of free charge on both Raman and luminescence spectra. In the presence of charge, the LO phonons couple with the plasmons resulting in the formation of LOPC modes, from which the transport parameters can be extracted. There are several approaches that consider different dielectric functions.

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frequency and the plasmon damping parameter (full width at half maximum of the plasmon mode), respectively. " # e2 N 0 5 am0* 2 2 F1=2 ðgÞ 1 þ hm i ð4Þ U ¼ 12 E0 m* 0

where N0, F1/2 (η), α, m0⁎, and E0 are the effective density of conduction band states, a member of the Fermi–Dirac integral family, the non-parabolicity coefficient, the electron band-edge effective mass and the band gap energy, respectively. Plasmon modes are fitted with a Raman differential crosssection expression that includes the contribution of the deformation potential mechanism and the electro-optic mechanism given by Hon and Faust [29]. Using the value of the Fermi energy determined from the fitting of the Raman spectrum one can determine the free-carrier concentration from   15 kB T F3=2 ðgÞ Ne ¼ N0 F1=2 ðgÞ 1 þ a ð5Þ 4 E0 F1=2 ðgÞ

Fig. 3. Growth front in AlGaAs conformal layers showing the dependence with the Al flow (a) for a DMAlCl flow of 5 cm3/min; (b) for a DMAlCl flow of 4 cm3/min.

We used a hydrodynamic approach [28]. In that case the electric susceptibility is simplified to the expression: ve ð gÞ ¼ −

U2 hm2 iq2 −x2 þ ixC

ð3Þ

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 CL map of the conformal sample region studied.

Fig. 4 represents the calculated values of the free electron concentration with respect to the position in the conformal layer, superposed to the panchromatic CL image. The points shown in the non-doped regions are a guide to the eye; since the carrier concentration limit for LOPC mode observation is below ∼5 × 1016 cm− 3 it should mean that the free carrier concentration in undoped regions is below that value. The bright stripe between 0 and 3 μm corresponds to an undoped GaAs layer; the next stripe 8 μm wide is Si-doped. The undoped GaAs seed is the dark stripe located in the right side of the CL image. It has to be stated that the thin bright stripe in the doped region, near the seed, corresponds to a self-doped stripe as discussed above. The central part of the doped stripe shows a little fluctuation of the carrier concentration that varies between 2.94 × 1018 and 2.00 × 1018 cm− 3. Also one observes the sharp interface between the undoped and doped regions. The profile is not so much sharp in CL, which is the consequence of the carrier diffusion length. In doped semiconductors the band filling with free carriers shifts the luminescence peak energy to the blue, Burstein–Moss effect [30]; therefore, the density of free electrons can be

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

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obtained from the peak energy of the band-to-band luminescence. The magnitude of this displacement with respect to the doping concentration in n and p type materials has been studied by different authors [31,32] who have proposed empirical expressions to determine de doping concentration of the free carriers in GaAs layers. Here, we have used the expression proposed by Lee et al. [31] for the peak energy dependence with the concentration of free electrons at room temperature: Emax ¼ 1:426 þ 2:4  10−14 Ne3=2

ðeVÞ

ð6Þ

where the value 1.426 eV corresponds to the peak energy emitted by undoped GaAs. Fig. 5 shows the distribution of the carrier concentration as obtained from the spectral CL image, once the peak energy has been converted to free electron concentration using Eq. (6). The concentrations obtained by luminescence are lower than the ones obtained by micro-Raman spectroscopy. This could be explained by the presence of tensile strain in the conformal layers (∼ 1.0 kbar), which have a higher influence on the luminescence spectrum that in the LOPC modes. 3.2. Degraded high-power laser diodes The laser degradation consists of the decrease of the output power with the operation time. Basically degradation mechanisms

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

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

in high-power lasers may be classified as process-, material- or facet-related mechanisms [1,2,33]. The degradation occurs in different time scales, being classified as rapid, gradual and catastrophic [1,2,33,34]. The result of the degradation is a reduction of the capability to emit light. The understanding of the degradation mechanism is crucial to make reliable devices. The degradation is the consequence of the generation of defects in the active layers of the laser structure. Usually, facet degradation is the main cause of catastrophic degradation [33]; however, defects can also be generated inside the cavity, which is more difficult to study. Removing the overlay metal contact layer, one can image the cavity using CL. Because the penetration depth of the e-beam can be controlled by the acceleration voltage one can probe the luminescence 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 broad emitter of a high-power laser bar (QW of AlxGa1−xAs with x close to 0.1, cladding layers Al0.6Ga0.4As and GaAs contact layer) are shown in Fig. 6. One observes that the typical damage pattern is only visible in the QW, with the other layers being free of defects. The spectral analysis of the damage allows getting insight on the physical mechanisms responsible for the degradation. Spectral images of a degraded region are shown in Fig. 7. One observes that the dark regions exhibit energy shift to the red and line broadening. However, in the most heavily degraded regions, those that appear darkest in the intensity map, the correlation with the other spectral parameters does not follow the same trends, and one observes a strong broadening of the luminescence band and a blue shift, which can be understood in terms of cation intermixing between the QW and the confinement layer [35]. The possibility to open the laser cavity of degraded lasers allows us to use spectroscopic techniques in order to understand the physics underlying the degradation mechanisms of QW high-power laser diodes [36]. 4. Conclusions The validity of optical characterization techniques with a spatial resolution ranging from a few hundred nanometers to a few micrometers to study many problems concerning compound semiconductors has been discussed. These techniques present a relative handling simplicity and good signal/noise ratios, which make them suitable for many applications, for which the application of other techniques with higher resolution not only introduces experimental complexity, but also do not report additional benefits to the comprehension of the problem. Several examples showing different applications and the different physical issues that can be investigated are presented and 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.

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