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GaAs epilayers grown by MOCVD
Optical Materials 60 (2016) 487e494
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Impact of photoluminescence temperature and growth parameter on the exciton localized in BxGa1-xAs/GaAs epilayers grown by MOCVD Tarek Hidouri a, *, Faouzi Saidi a, Hassen Maaref a, Philippe Rodriguez b, Laurent Auvray b Laboratoire des Micro-Opto electroniques et Nanostructures, Universit e de Monastir, Facult e des Sciences de Monastir, Avenue de l’Environnement, 5000 Monastir, Tunisia b Laboratoire Multimat eriaux et Interfaces, Universit e Claude Bernard Lyon 143, Boulevard du 11 Novembre 1918, France a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2016 Received in revised form 23 August 2016 Accepted 25 August 2016
In this work, BxGa1-xAs/GaAs epilayers with three different boron compositions were elaborated by metal organic chemical vapor deposition (MOCVD) on GaAs (001) substrate. Structural study using High resolution X-ray diffraction (HRXRD) spectroscopy and Atomic Force Microscopy (AFM) have been used to estimate the boron fraction. The luminescence keys were carried out as functions of temperature in the range 10e300 K, by the techniques of photoluminescence (PL). The low PL temperature has shown an abnormal emission appeared at low energy side witch attributed to the recombination through the deep levels. In all samples, the PL peak energy and the full width at half maximum (FWHM), present an anomalous behavior as a result of the competition process between localized and delocalized carriers. We propose the Localized-state Ensemble model to explain the unusual photoluminescence behaviors. Electrical carriers generation, thermal escape, recapture, radiative and non-radiative lifetime are taken into account. The temperature-dependent photoluminescence measurements were found to be in reasonable agreement with the model of localized states. We controlled the evolution of such parameters versus composition by varying the V/III ratio to have a quantitative and qualitative understanding of the recombination mechanisms. At high temperature, the model can be approximated to the band-tail-state emission. © 2016 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence HRXRD AFM BGaAs/GaAs LSE V/III ratio
1. Introduction BGaAs alloy has attracted considerable attention in the recent few years for both theoretical and experimental investigations due to its large possible optoelectronic application based on GaAs. Boron incorporation into conventional IIIeV compounds such as GaAs to synthesize new boron-incorporated materials has received considerable attention, due to their potential applications in the field of band gap engineering, strain compensation and optoelectronic integration. Therefore, it is a promising way to develop solar cell devices [1,2] and references their] or active materials for 1.3 mm laser diodes [2e4]. Many theoretical and experimental investigations have been carried out to grow theses new materials. The zinc-blend BGaAs alloys have been grown by many techniques such as metal organic chemical vapor deposition (MOCVD) [1,2], metal organic vapor
* Corresponding author. E-mail address: [email protected] (T. Hidouri). http://dx.doi.org/10.1016/j.optmat.2016.08.029 0925-3467/© 2016 Elsevier B.V. All rights reserved.
phase epitaxy (MOVPE) [5] and molecular beam epitaxy [6]. The two binary compounds have an almost similar band gap, so, little is known about the BGaAs/GaAs gap [1,7]. Gupta et al. reported a red shift of about 50 meV compared to the GaAs gap. However, Geisz et al. reported a blue shift of about 4e5 meV for 1% of boron incorporated [8,9]. The photoluminescence study versus temperature provides useful information about the material which is of considerable practical and theoretical interest. Classically, the band gap of a semiconductor material reduces monotonically with increasing temperature. Special materials, such as B-III-V, have shown an anomaly at low PL temperature. In order to understand this behavior, many models have been proposed during the last ^ssler, Vina and Varshni one. This last is the decades, such as the Pa very popular among researchers because of its simplicity. Nevertheless, in some cases, none of the existing empirical laws are able to reveal the temperature dependence of the PL energy. In order to produce reliable devices, temperature behavior of such kind of samples must be well understood. We hereby use a correction given by the thermal coefficient, of the Varshni classical model, to understand the observed S-shaped temperature dependence of the
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excitonic band gap. The modified Varshni formula can fit well all the temperature dependence of the luminescence properties such as the PL intensity, the Full Width at Half Maximum (FWHM) and the band gap energy peak emission [10]. 2. Experimental details The samples were grown by metal organic chemical vapor deposition (MOCVD). Triethylgallium (TEG) and diborane (B2H6) were used as group III precursors. Arsine (AsH3) was used for the
arsenic source as group V precursor. Prior to BGaAs growth, a GaAs buffer layer of approximately 450 mm thick was grown at the same temperature as the epilayer (580 C) http://www.sciencedirect. com/science/article/pii/S0022231309001987 [11]. The GaAs substrates, weakly doped silicon Si (n~3e4 1017 cm3). These substrates are considered “ready to use”, that is to say, they do not require a prior chemical treatment of the surface. Thereof is covered by a superficial layer of native oxides obtained chemically. It is eliminated by an in-situ annealing. The growth was done in a Tshape horizontal reactor under arsenic rich conditions onto vicinal GaAs (0 0 1) substrates 1 off (±0.05 ) towards [1 1 0]. The boron composition was varied by varying V/III flux ratio. The diborane flow-rate was kept constant. The boron gas-phase concentration was quantified by the initial molar flow-rate ration:
wx ¼
2½B2 H6 2½B2 H6 þ ½TEG
wx corresponds to the percentage of boron introduced in the gas phase relative to the total of the element III. This magnitude is defined as the ratio of the molar concentration of diborane on the total molar concentration of the group III elements. The duration of the growth of GaAs was 60 min for all layers, which corresponds to an estimated thickness between 150 and 250 nm. The structural study was performed using the atomic force microscopy (AFM) and X-ray u/2q measurements of (0 0 4) plane reflection. We use a copper target (lCuKa1 ¼ 1.54056 Å) radiation from a Discover D8 (40 Kv 55 mA) high-power X-ray generator. The boron composition was determined using a Vegard's law from separation angle between the epilayers and substrate (4 0 0) diffraction peaks, assuming coherent tensile strain and a Poisson ratio of 0.313 http://www.sciencedirect.com/science/article/pii/ S0022231309001987 [12]. The composition calculation was done numerically. Photoluminescence (PL) measurements were carried out between 10 and 300 K while keeping the samples in a closed-cycle helium circulation cryostat. The excitation wavelength used is the 514.5 nm line of the cw Arþ laser with an excitation density of 80 W/cm2. The emission was dispersed by a high-resolution spectrometer and detected by a thermoelectrically cooled InGaAs photodetector with a built-in amplifier.
Fig. 1. Influence of the V/III ratio on the surface morphology of BGaAs/GaAs epitaxially grown at 580 C.
Fig. 2. The ue2q scan profiles of (004) reflection for BGaAs/GaAs epilayers: B48 (solid black line), B65 (red solid line) and B88 (blue solid line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
T. Hidouri et al. / Optical Materials 60 (2016) 487e494
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3. Results and discussion 3.1. Morphological and structural study
Fig. 3. 10K PL spectra of three BGaAs/GaAs epilayers: B48 (black solid squares), B65 (red solid squares) and B88 (blue solid squares). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Many specifics related to the incorporation of boron into several matrixes. The very low theoretical solubility of boron at thermodynamic equilibrium, strong interactions between the various precursors in the gas phase and the need to stabilize the cubic form of BAs make the incorporation of boron very delicate. Various growth parameters should be studied and optimized to control the composition of the BGaAs layers and its effects on the optical response. The boron composition of the gaseous phase is a major parameter that will control the composition of the layers. To change this parameter, you can of course change the flow of diborane (B2H6) and/or Triethylgallium flows. The growth temperature, the position of the sample along the inclined plane and the V/III ratio are also key parameters for controlling the incorporation of boron and composition of layers. Fig. 1(a, cee) illustrates the influence of V/III ratio on the layer surface morphology observed by AFM. Two trends are highlighted: for a high V/III ratio (e.g. Fig. 1(d) and (e)), the increase of this last has no significant influence on the surface
Fig. 4. Deconvolution of PL spectra of three BGaAs/GaAs epilayers at (a) 10 K-PL temperature and the selected high temperature (b). The experimental evolution (solid black squares) were fitted using two Gaussian peaks (solid green line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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morphology. The layers also have very similar surface roughness accompanied by a small decrease in the boron composition. Though, in the other case (low ratio; e.g. Fig. 1(b)), a sharp deterioration of the surface morphology was observed, with the development of a three dimensional growth with a relatively high roughness. This degradation is specific to the growth of the BGaAs layers. Parallel to the strong morphological degradation of the layers, the boron incorporation decreases marginally. In fact, a GaAs layer (Fig. 1(a)) deposited under the same conditions of temperature and flow, has a good surface morphology (RMS less than 3 Å on an area of 5 5 mm2). In order to estimate the boron composition, X-ray u/2Ɵ measurements of (0 0 4) plane reflection were performed. The results are shown in Fig. 2. The compositions are estimated numerically using Veguard's law to be 0.04851, 0.06555 and 0.08803; respectively, for samples referred as B48, B65 and B88. 3.2. Optical study In order to compare the optical properties of the BGaAs epilayers, firstly, we show the low temperature (10 K) photoluminescence spectra in Fig. 3. The PL emission from the GaAs substrates was dominated by the transition (e-CAs) carbon impurity and its optical LO-phonon replica. It is originated from the precursors under the growth condition [13,14]. This transition shows a few redshift with increasing boron concentration. This shift may be due to the experimental conditions. The intensity increases firstly
and then drops. Its follows the same behavior as the BGaAs epilayers emissions. We associate this behavior to the boron concentration and the growth condition. Indeed, the increase of boron content in the III/V flux during the MOCVD process increases the eCAs impurity and its replica. Moreover, an asymmetric PL band (designed as HE) is centered at 1.3260 eV with FWHM 95 meV, 1.3407 eV with FWHM 85 meV and 1.3610 eV with FWHM 92 meV, respectively for B48, B65 and B88. This emission band is associated to the exciton recombination in the BGaAs epilayers. Next, we will focus only on this emission energy. The principal PL peak intensity increases with increasing boron composition firstly, and then drops for high boron concentration (B88). This is in agreement with the results founded by Hamila et al. [4]. It could be explained by the presence of the crystal imperfections (surface roughness), defects and composition fluctuations in BGaAs epilayer [14]. Furthermore, for high boron composition, the PL quenching can be explained by the fact that the increase of the interaction between the localized states reduces the localization effects [16]. Nevertheless, the small blue shift observed is due to the weak bowing of the band gap and the localized states extended their. To more understand the nature of recombination mechanisms, the PL spectra were de-convoluted with the Gaussian function fitting procedure as shown in Fig. 4(a). At 10K, the low energy side (LE) and GaAs peaks emission are well fitted by one Gaussian. However, the main PL peak (HE) is fitted by two Gaussian peaks. Viewing that the localization phenomenon is a common phenomenon in many materials from point of view luminescence mechanisms, we suppose that the additional
Fig. 5. Photoluminescence spectra at selected temperature showed the evolution of the LE peak emission for the designed structures.
T. Hidouri et al. / Optical Materials 60 (2016) 487e494
emission at the low energy side of the BGaAs peak maybe appears as a signature of band-tail states [17, 18 and references their]. It represents the distribution of the carriers into band-tail state of the DOS [17]. Furthermore, it is the result of the potential fluctuation in the confinement potential energy of the heterostructure. To control its evolution, we isolated the BGaAs emission. At high PL temperature, once again the PL spectrum can be fitted by using two Gaussian peaks (Fig. 4(b)). The localized excitons feature disappears, whereas the band-tail states luminescence peak is replaced by one large peak starts to dominate the luminescence. It is may be explained by the fact that the excitons are delocalized and the defects are activated under high temperature. Compared to those published elsewhere on B-III-V's structures [11,12,4], an abnormal third broad emission peak at low energy side (designed as LE) appears. It is centered at 0.94, 0.99 and 1.05 eV respectively for B48, B65 and B88. This blue shift as function of boron fraction is not well understood until now and will be the aim of a next work. For each sample, and as function of PL temperature, there is no detectable shift of the PL peak position (Fig. 5). But its relative intensity is increased for high temperature values (higher than: 100K for B48; 180K for B65 and 200K for B88). These observations indicate that this behavior cannot be related to the energy potential modulation associated to the boron-clusters. It's well known that clusters will act as quantum dot-like versus temperature. As a result, we should observe an inverted S-shape variation [15,16]. In our case, this emission might be associated to the recombination through the deep energy level. To get insight into the PL recombination mechanisms, temperature-dependent photoluminescence measurements. Those lasts are performed using a data acquisition interface during the photoluminescence measurements for each temperature. Fig. 6 shows the PL peak position (a), the PL intensity (b) and the
491
FWHM (c) of the principal peak (associated to BGaAs). This behavior is an abnormality with respect to those usually observed in other III-V ternary alloys such as conventional GaAlAs [19]. It exhibits an anomalous behavior between 10 and ~100 K. Firstly, from 10 to 60 K (region (i)), the emission energy peak shows a red shift around 27.7 meV, 36.9 meV and 29.4 meV respectively for B48, B65 and B88. Then, between 60K and 100K (region (ii)), the peak energy increases. The energy separation between the minimum and maximum positions of the S-shaped was defined as the exciton localization energy [20]. The characteristic temperature in which the two recombination systems (localized and delocalized carriers) participate equally in the PL signal is denoted as Tloc/deloc. In the high PL temperature (region (iii)), the PL band is similar to the band edge evolution. The characteristic temperature is denoted Tdeloc witch decreases as well as the composition increases. On the other hand, the PL linewidth increases till the high PL temperature and then drops above. This evolution could be explained by the transfer and the thermalization of localized states. Indeed, when the temperature increases up to the thermalization point, some of localized carriers occupy shallower localized states. This leads to an increase of the linewidth. When approaching the high temperature, most localized carriers become progressively mobile. Finally, the temperature dependence of the normalized intensity of PL was investigated. Thus, at low temperature, excitons trapped into localized states. They perform energy-loss hopping transitions between the traps. Concurrently, the energy distribution of excitons shifts towards deeper states in BGaAs band gap with respect to the mobility edge. Therefore, the thermal quenching of the PL signal is a result of the interplay between two processes that can be described as follows: radiative
Fig. 6. The experimental luminescence keys versus temperature of BGaAs/GaAs epilayers: B48 (black solid squares), B65 (red solid squares) and B88 (blue solid squares). The Sshaped and N-shaped behaviors, respectively for PL peak energy and FWHM, are clearly shown. The FWHM was smoothed by a solid line to clarify the evolution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
T. Hidouri et al. / Optical Materials 60 (2016) 487e494
recombination from the localized states and their thermal activation to the delocalized states and subsequent non-radiative recombination [4]. By increasing temperature, the overall emission intensity gradually decreases. It indicates the presence of non-radiative recombination centers. The quenching behavior should correspond to the thermal activated non-radiative recombination mechanism. A theoretical treatment using the Localized-state Ensemble model is performed.
3.3. Localized-state ensemble model (LSE) The literature presented a lot of approaches to describe the abnormal behaviors of the luminescence process of such materials [21e24]. But, due to the nature of the luminescence process in our material (excitons in localized states) and because of its evidence to achieve all the luminescence keys, the LSE model has been chosen as the promote one to describe quantitatively our material. Furthermore, the equations are simple to treat numerically compared to others. The luminescence keys dependence can be further quantitatively approximated and reinterpreted by a localized exciton state model proposed by Li et al. [10 and references their]. This model treats the system of localized states where the states distribution function is given by a Gaussian distribution [25]. Carriers generation, thermal escape, recapture, radiative and non-radiative lifetime are taken into account. As the temperature increases, excitons will redistribute over the BGaAs epilayer through thermal escape and re-capture processes. It is known that the band gap of an idealized semiconductor material is usually described by the wellknown Varshni empirical formula. After taking into account the correction due to the thermal redistribution coefficient, the variation of the peak position of luminescence from LSE is described by the following expression:
EðTÞ ¼ E0
aT 2 xðTÞKb T qþT
(1)
where a and q are the Varshni parameter and Debye temperature, respectively. E0 is the bandgap energy at 10K. The quantity x(T). KbT represents the extra red shift induced by the thermal redistribution [10]. In the high-temperature region, Equation (4) becomes:
EðTÞ ¼ E0
aT 2 s2 q þ T Kb T
(2)
The full width at half-maximum (FWHM) is another parameter for a luminescence spectrum. The shape of the luminescence spectrum resulting from the inhomogeneous distribution of localized carriers and broadening duo to the impurity and phonon scattering is the origin of FWHM. Based on this model, the broadening can be described by the following equation [10]:
GðTÞ ¼ G0 þ hA T þ
h
LO LO exp ħw 1 Kb T
(3)
where G0 is due to impurity/imperfection scattering. The second term due to phonon scattering, there, hA and hLO are the acousticphonon and longitudinal optical (LO)-phonon scattering coefficients, respectively. ħwLO is the LO-phonon energy. The integrated intensity of the luminescence spectrum is described by the following equation:
3 1 ðE0 Ea Þ þ Kb Tln tttrr 6 7 IðTÞ ¼ 1 þ ð1 gc Þexp4rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi5 2
ðKb TÞ2 þ 2 s 2:41 =
492
2
(4)
Thus, the temperature dependence of peak position, FWHM and normalized PL intensity change by inserting small composition of boron to the GaAs matrix that they are investigated and reinterpreted by LSE model in the following. The modeling results were represented in Fig. 7(aec), respectively for B48, B65 and B88. The fitting parameters were summarized in Table 1a and b. A good agreement between theory and experiment is achieved by the best-chosen of LSE parameters; E0, Ea-E0, s, the Varshni parameters (Ɵ and a) and the ration tr/ttr. The magnitude Ea-E0has a decisive influence on the temperature dependence of the PL peak position. For all samples, it is positive and it decreases by increasing the boron composition. It takes the values 13 meV, 8 meV, and 4 meV for samples B48, B65 and B88, respectively. By comparing the calculated parameters of different samples, the increasing of boron composition increases s and then reduces it for high boron concentration. Although, it shows a reduction of the occupied localized states and therefore, a weakness in S-shape behaviors is created. The ratio tttrr and gc are two key parameters that determine the thermal quenching of the luminescence band. It increases versus composition and then drops for B88. So, the thermal escape of excitons and therefore the thermal quenching rate follows this evolution versus boron composition. This explains the PL intensity behavior already represented. The lower recapture coefficient also leads to the higher rate of thermal quenching. But we noted that the high boron composition is weakly affected gc (0.9935 and 0.9900, respectively, for B65 and B88). A good agreement between the theoretical FWHM and the experimental data at low temperature in all samples. By increasing temperature, the recombination due to the delocalized states starts, and the theoretical results deviate from experimental evolution. This deviation is bigger for samples having a stronger S-shaped. Given that the variation of FWHM at high temperature is due to the phonon scattering that's follows from Equation (3). The impurity/defect coefficient G0 increases versus boron composition. This can be explained by the fact that the increase of boron composition increases the defects and impurity originated from the precursors under the growth condition. The LO-phonon scattering coefficient hLO and the phonon energy increases firstly by increasing the composition and then drops for high boron composition. We can explain this decrease by the reduction of the localization effects. 4. Conclusion To sum up, we have studied the thermal and boron composition effects on the morphological, structural and optical proprieties of BGaAs/GaAs epilayers. We have shown that the boron incorporation into GaAs matrix causes a surface modification, a blue shift and PL signal degradation with increasing boron concentration. A distribution of the carriers into band-tail state of the DOS and a presence of deep level associated to the B-atoms has been demonstrated. From the temperature dependence on the photoluminescence properties, we have shown that the broadening of the BGaAs PL band results from the localization effects induced by the potential modulation caused by the fluctuation of the composition. The luminescence measurements were successfully modeled and re-interpreted using the developed LSE model. The theoretical
T. Hidouri et al. / Optical Materials 60 (2016) 487e494
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Fig. 7. PL peak position, FWHM and normalized PL intensity simulated using the LSE model, respectively, for the investigated samples: (a) B48, B65 (b) and B88 (c). The solid symbols are the experimental data, while the solid lines represent the calculated results using theoretical model.
Table 1 (Parameters used to fit the experimental results: (a) by the LSE model and (b) using empirical Varshni relation. (a) B48 B65 B88
E0 (eV) 1.33 1.377 1.363
Ea- E0 (eV) 0.01 0.008 0.004
s (eV) 3
10 10 20 103 9.5 103
hLO (meV/K)
G0 (meV)
ħWLO (meV)
gc
tr ttr
hA (meV/K)
320 770 200
64 87 98
33.606 40 20.68
0.9398 0.9935 0.99
12500 95000 3500
350 103 150 103 900 104
(b)
E0 (eV)
a (eV/K)
Ɵ (K)
B48 B65 B88
1.325 1.355 1.360
6 10 8 104 9 104
140 150 170
study has quantitatively interpreted the observed temperaturedependent spectral, to better understand the complicated spontaneous emission mechanisms in real ternary material, based on the fitting parameters. This paper gives a recommended estimation for
the study of B-III-V materials for optoelectronic applications. In order to understand such an unusual PL emission, we propose to provide more optical study of the observed low energy side emission and a thorough theoretical analysis.
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248 (2003) 463. [14] H. Dumont, D. Rutzinger, C. Vincent, J. Dazord, Y. Monteil, F. Alexander, J.L. Gontner, Appl. Phys. Lett. 82 (2003) 1830. [15] Y.S. Lin, K.J. Ma, Ch. Hsu, Y.Y. Chung, Ch.W. Liu, Sh.-W. Feng, Y.Ch. Cheng, C.C. Yang, M.H. Mao, H.W. Chuang, Ch.T. Kuo, J.Sh. Tsang, Th. E. Weirich, Appl. Phys. Lett. 80 (2002) 2571e2573. [16] B. Fluegel, K. Alberi, D.A. Beaton, S.A. Crooker, A.J. Ptak, A. Mascarenhas, Phys. Rev. B 86 (2012) 205203e205206 and references in. [17] Kalyan Nunna, S. Iyer, L. Wu, J. Li, S. Bharatan, et al., J. Appl. Phys. 102 (2007) 1e8. [18] S.A. Lourenço, I.F.L. Dias, J.L. Duarte, E. Laureto, V.M. Aquino, J.C. Harmand Brazilian, J. Phys. 37 (2007) 4. [19] M.E. Groenert, R. Averbeck, W. Hosler, M. Schuster, H. Riechert, J. Cryst. Growth 264 (2004) 123e127. [20] S. Shirakata, M. Kondow, T. Kitatani, Appl. Phys. Lett. 79 (2001) 54. [21] M.K. Shakfa, M. Wiemer, P. Ludewig, K. Jandieri, K. Volz, W. Stolz, S.D. Baranovskii, M. Koch, J. Appl. Phys. 117 (2015) 025709. [22] D. Ghodsi Nahri, C.H. Raymond Ooi, J. Opt. Soc. Am. B 31 (2014) 1182e1191. [23] M. Oueslati, M. Zouaghi, Phys. Rev. B 32 (1985) 8220e8227. [24] S. Sanguinetti, Phys. Rev. B 60 (1999) 8276e8283. [25] M.H. Hadj Alouane, A. Helali, D. Morris, H. Maaref, V. Aimez, B. Salem, M. Gendry, B.Ilahi, J. Luminescence 145 (2014) 595e599.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Impact of photoluminescence temperature and growth parameter on the exciton localized in BxGa1-xAs/GaAs epilayers grown by MOCVD Tarek Hidouri a, *, Faouzi Saidi a, Hassen Maaref a, Philippe Rodriguez b, Laurent Auvray b Laboratoire des Micro-Opto electroniques et Nanostructures, Universit e de Monastir, Facult e des Sciences de Monastir, Avenue de l’Environnement, 5000 Monastir, Tunisia b Laboratoire Multimat eriaux et Interfaces, Universit e Claude Bernard Lyon 143, Boulevard du 11 Novembre 1918, France a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2016 Received in revised form 23 August 2016 Accepted 25 August 2016
In this work, BxGa1-xAs/GaAs epilayers with three different boron compositions were elaborated by metal organic chemical vapor deposition (MOCVD) on GaAs (001) substrate. Structural study using High resolution X-ray diffraction (HRXRD) spectroscopy and Atomic Force Microscopy (AFM) have been used to estimate the boron fraction. The luminescence keys were carried out as functions of temperature in the range 10e300 K, by the techniques of photoluminescence (PL). The low PL temperature has shown an abnormal emission appeared at low energy side witch attributed to the recombination through the deep levels. In all samples, the PL peak energy and the full width at half maximum (FWHM), present an anomalous behavior as a result of the competition process between localized and delocalized carriers. We propose the Localized-state Ensemble model to explain the unusual photoluminescence behaviors. Electrical carriers generation, thermal escape, recapture, radiative and non-radiative lifetime are taken into account. The temperature-dependent photoluminescence measurements were found to be in reasonable agreement with the model of localized states. We controlled the evolution of such parameters versus composition by varying the V/III ratio to have a quantitative and qualitative understanding of the recombination mechanisms. At high temperature, the model can be approximated to the band-tail-state emission. © 2016 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence HRXRD AFM BGaAs/GaAs LSE V/III ratio
1. Introduction BGaAs alloy has attracted considerable attention in the recent few years for both theoretical and experimental investigations due to its large possible optoelectronic application based on GaAs. Boron incorporation into conventional IIIeV compounds such as GaAs to synthesize new boron-incorporated materials has received considerable attention, due to their potential applications in the field of band gap engineering, strain compensation and optoelectronic integration. Therefore, it is a promising way to develop solar cell devices [1,2] and references their] or active materials for 1.3 mm laser diodes [2e4]. Many theoretical and experimental investigations have been carried out to grow theses new materials. The zinc-blend BGaAs alloys have been grown by many techniques such as metal organic chemical vapor deposition (MOCVD) [1,2], metal organic vapor
* Corresponding author. E-mail address: [email protected] (T. Hidouri). http://dx.doi.org/10.1016/j.optmat.2016.08.029 0925-3467/© 2016 Elsevier B.V. All rights reserved.
phase epitaxy (MOVPE) [5] and molecular beam epitaxy [6]. The two binary compounds have an almost similar band gap, so, little is known about the BGaAs/GaAs gap [1,7]. Gupta et al. reported a red shift of about 50 meV compared to the GaAs gap. However, Geisz et al. reported a blue shift of about 4e5 meV for 1% of boron incorporated [8,9]. The photoluminescence study versus temperature provides useful information about the material which is of considerable practical and theoretical interest. Classically, the band gap of a semiconductor material reduces monotonically with increasing temperature. Special materials, such as B-III-V, have shown an anomaly at low PL temperature. In order to understand this behavior, many models have been proposed during the last ^ssler, Vina and Varshni one. This last is the decades, such as the Pa very popular among researchers because of its simplicity. Nevertheless, in some cases, none of the existing empirical laws are able to reveal the temperature dependence of the PL energy. In order to produce reliable devices, temperature behavior of such kind of samples must be well understood. We hereby use a correction given by the thermal coefficient, of the Varshni classical model, to understand the observed S-shaped temperature dependence of the
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excitonic band gap. The modified Varshni formula can fit well all the temperature dependence of the luminescence properties such as the PL intensity, the Full Width at Half Maximum (FWHM) and the band gap energy peak emission [10]. 2. Experimental details The samples were grown by metal organic chemical vapor deposition (MOCVD). Triethylgallium (TEG) and diborane (B2H6) were used as group III precursors. Arsine (AsH3) was used for the
arsenic source as group V precursor. Prior to BGaAs growth, a GaAs buffer layer of approximately 450 mm thick was grown at the same temperature as the epilayer (580 C) http://www.sciencedirect. com/science/article/pii/S0022231309001987 [11]. The GaAs substrates, weakly doped silicon Si (n~3e4 1017 cm3). These substrates are considered “ready to use”, that is to say, they do not require a prior chemical treatment of the surface. Thereof is covered by a superficial layer of native oxides obtained chemically. It is eliminated by an in-situ annealing. The growth was done in a Tshape horizontal reactor under arsenic rich conditions onto vicinal GaAs (0 0 1) substrates 1 off (±0.05 ) towards [1 1 0]. The boron composition was varied by varying V/III flux ratio. The diborane flow-rate was kept constant. The boron gas-phase concentration was quantified by the initial molar flow-rate ration:
wx ¼
2½B2 H6 2½B2 H6 þ ½TEG
wx corresponds to the percentage of boron introduced in the gas phase relative to the total of the element III. This magnitude is defined as the ratio of the molar concentration of diborane on the total molar concentration of the group III elements. The duration of the growth of GaAs was 60 min for all layers, which corresponds to an estimated thickness between 150 and 250 nm. The structural study was performed using the atomic force microscopy (AFM) and X-ray u/2q measurements of (0 0 4) plane reflection. We use a copper target (lCuKa1 ¼ 1.54056 Å) radiation from a Discover D8 (40 Kv 55 mA) high-power X-ray generator. The boron composition was determined using a Vegard's law from separation angle between the epilayers and substrate (4 0 0) diffraction peaks, assuming coherent tensile strain and a Poisson ratio of 0.313 http://www.sciencedirect.com/science/article/pii/ S0022231309001987 [12]. The composition calculation was done numerically. Photoluminescence (PL) measurements were carried out between 10 and 300 K while keeping the samples in a closed-cycle helium circulation cryostat. The excitation wavelength used is the 514.5 nm line of the cw Arþ laser with an excitation density of 80 W/cm2. The emission was dispersed by a high-resolution spectrometer and detected by a thermoelectrically cooled InGaAs photodetector with a built-in amplifier.
Fig. 1. Influence of the V/III ratio on the surface morphology of BGaAs/GaAs epitaxially grown at 580 C.
Fig. 2. The ue2q scan profiles of (004) reflection for BGaAs/GaAs epilayers: B48 (solid black line), B65 (red solid line) and B88 (blue solid line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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3. Results and discussion 3.1. Morphological and structural study
Fig. 3. 10K PL spectra of three BGaAs/GaAs epilayers: B48 (black solid squares), B65 (red solid squares) and B88 (blue solid squares). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Many specifics related to the incorporation of boron into several matrixes. The very low theoretical solubility of boron at thermodynamic equilibrium, strong interactions between the various precursors in the gas phase and the need to stabilize the cubic form of BAs make the incorporation of boron very delicate. Various growth parameters should be studied and optimized to control the composition of the BGaAs layers and its effects on the optical response. The boron composition of the gaseous phase is a major parameter that will control the composition of the layers. To change this parameter, you can of course change the flow of diborane (B2H6) and/or Triethylgallium flows. The growth temperature, the position of the sample along the inclined plane and the V/III ratio are also key parameters for controlling the incorporation of boron and composition of layers. Fig. 1(a, cee) illustrates the influence of V/III ratio on the layer surface morphology observed by AFM. Two trends are highlighted: for a high V/III ratio (e.g. Fig. 1(d) and (e)), the increase of this last has no significant influence on the surface
Fig. 4. Deconvolution of PL spectra of three BGaAs/GaAs epilayers at (a) 10 K-PL temperature and the selected high temperature (b). The experimental evolution (solid black squares) were fitted using two Gaussian peaks (solid green line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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morphology. The layers also have very similar surface roughness accompanied by a small decrease in the boron composition. Though, in the other case (low ratio; e.g. Fig. 1(b)), a sharp deterioration of the surface morphology was observed, with the development of a three dimensional growth with a relatively high roughness. This degradation is specific to the growth of the BGaAs layers. Parallel to the strong morphological degradation of the layers, the boron incorporation decreases marginally. In fact, a GaAs layer (Fig. 1(a)) deposited under the same conditions of temperature and flow, has a good surface morphology (RMS less than 3 Å on an area of 5 5 mm2). In order to estimate the boron composition, X-ray u/2Ɵ measurements of (0 0 4) plane reflection were performed. The results are shown in Fig. 2. The compositions are estimated numerically using Veguard's law to be 0.04851, 0.06555 and 0.08803; respectively, for samples referred as B48, B65 and B88. 3.2. Optical study In order to compare the optical properties of the BGaAs epilayers, firstly, we show the low temperature (10 K) photoluminescence spectra in Fig. 3. The PL emission from the GaAs substrates was dominated by the transition (e-CAs) carbon impurity and its optical LO-phonon replica. It is originated from the precursors under the growth condition [13,14]. This transition shows a few redshift with increasing boron concentration. This shift may be due to the experimental conditions. The intensity increases firstly
and then drops. Its follows the same behavior as the BGaAs epilayers emissions. We associate this behavior to the boron concentration and the growth condition. Indeed, the increase of boron content in the III/V flux during the MOCVD process increases the eCAs impurity and its replica. Moreover, an asymmetric PL band (designed as HE) is centered at 1.3260 eV with FWHM 95 meV, 1.3407 eV with FWHM 85 meV and 1.3610 eV with FWHM 92 meV, respectively for B48, B65 and B88. This emission band is associated to the exciton recombination in the BGaAs epilayers. Next, we will focus only on this emission energy. The principal PL peak intensity increases with increasing boron composition firstly, and then drops for high boron concentration (B88). This is in agreement with the results founded by Hamila et al. [4]. It could be explained by the presence of the crystal imperfections (surface roughness), defects and composition fluctuations in BGaAs epilayer [14]. Furthermore, for high boron composition, the PL quenching can be explained by the fact that the increase of the interaction between the localized states reduces the localization effects [16]. Nevertheless, the small blue shift observed is due to the weak bowing of the band gap and the localized states extended their. To more understand the nature of recombination mechanisms, the PL spectra were de-convoluted with the Gaussian function fitting procedure as shown in Fig. 4(a). At 10K, the low energy side (LE) and GaAs peaks emission are well fitted by one Gaussian. However, the main PL peak (HE) is fitted by two Gaussian peaks. Viewing that the localization phenomenon is a common phenomenon in many materials from point of view luminescence mechanisms, we suppose that the additional
Fig. 5. Photoluminescence spectra at selected temperature showed the evolution of the LE peak emission for the designed structures.
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emission at the low energy side of the BGaAs peak maybe appears as a signature of band-tail states [17, 18 and references their]. It represents the distribution of the carriers into band-tail state of the DOS [17]. Furthermore, it is the result of the potential fluctuation in the confinement potential energy of the heterostructure. To control its evolution, we isolated the BGaAs emission. At high PL temperature, once again the PL spectrum can be fitted by using two Gaussian peaks (Fig. 4(b)). The localized excitons feature disappears, whereas the band-tail states luminescence peak is replaced by one large peak starts to dominate the luminescence. It is may be explained by the fact that the excitons are delocalized and the defects are activated under high temperature. Compared to those published elsewhere on B-III-V's structures [11,12,4], an abnormal third broad emission peak at low energy side (designed as LE) appears. It is centered at 0.94, 0.99 and 1.05 eV respectively for B48, B65 and B88. This blue shift as function of boron fraction is not well understood until now and will be the aim of a next work. For each sample, and as function of PL temperature, there is no detectable shift of the PL peak position (Fig. 5). But its relative intensity is increased for high temperature values (higher than: 100K for B48; 180K for B65 and 200K for B88). These observations indicate that this behavior cannot be related to the energy potential modulation associated to the boron-clusters. It's well known that clusters will act as quantum dot-like versus temperature. As a result, we should observe an inverted S-shape variation [15,16]. In our case, this emission might be associated to the recombination through the deep energy level. To get insight into the PL recombination mechanisms, temperature-dependent photoluminescence measurements. Those lasts are performed using a data acquisition interface during the photoluminescence measurements for each temperature. Fig. 6 shows the PL peak position (a), the PL intensity (b) and the
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FWHM (c) of the principal peak (associated to BGaAs). This behavior is an abnormality with respect to those usually observed in other III-V ternary alloys such as conventional GaAlAs [19]. It exhibits an anomalous behavior between 10 and ~100 K. Firstly, from 10 to 60 K (region (i)), the emission energy peak shows a red shift around 27.7 meV, 36.9 meV and 29.4 meV respectively for B48, B65 and B88. Then, between 60K and 100K (region (ii)), the peak energy increases. The energy separation between the minimum and maximum positions of the S-shaped was defined as the exciton localization energy [20]. The characteristic temperature in which the two recombination systems (localized and delocalized carriers) participate equally in the PL signal is denoted as Tloc/deloc. In the high PL temperature (region (iii)), the PL band is similar to the band edge evolution. The characteristic temperature is denoted Tdeloc witch decreases as well as the composition increases. On the other hand, the PL linewidth increases till the high PL temperature and then drops above. This evolution could be explained by the transfer and the thermalization of localized states. Indeed, when the temperature increases up to the thermalization point, some of localized carriers occupy shallower localized states. This leads to an increase of the linewidth. When approaching the high temperature, most localized carriers become progressively mobile. Finally, the temperature dependence of the normalized intensity of PL was investigated. Thus, at low temperature, excitons trapped into localized states. They perform energy-loss hopping transitions between the traps. Concurrently, the energy distribution of excitons shifts towards deeper states in BGaAs band gap with respect to the mobility edge. Therefore, the thermal quenching of the PL signal is a result of the interplay between two processes that can be described as follows: radiative
Fig. 6. The experimental luminescence keys versus temperature of BGaAs/GaAs epilayers: B48 (black solid squares), B65 (red solid squares) and B88 (blue solid squares). The Sshaped and N-shaped behaviors, respectively for PL peak energy and FWHM, are clearly shown. The FWHM was smoothed by a solid line to clarify the evolution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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recombination from the localized states and their thermal activation to the delocalized states and subsequent non-radiative recombination [4]. By increasing temperature, the overall emission intensity gradually decreases. It indicates the presence of non-radiative recombination centers. The quenching behavior should correspond to the thermal activated non-radiative recombination mechanism. A theoretical treatment using the Localized-state Ensemble model is performed.
3.3. Localized-state ensemble model (LSE) The literature presented a lot of approaches to describe the abnormal behaviors of the luminescence process of such materials [21e24]. But, due to the nature of the luminescence process in our material (excitons in localized states) and because of its evidence to achieve all the luminescence keys, the LSE model has been chosen as the promote one to describe quantitatively our material. Furthermore, the equations are simple to treat numerically compared to others. The luminescence keys dependence can be further quantitatively approximated and reinterpreted by a localized exciton state model proposed by Li et al. [10 and references their]. This model treats the system of localized states where the states distribution function is given by a Gaussian distribution [25]. Carriers generation, thermal escape, recapture, radiative and non-radiative lifetime are taken into account. As the temperature increases, excitons will redistribute over the BGaAs epilayer through thermal escape and re-capture processes. It is known that the band gap of an idealized semiconductor material is usually described by the wellknown Varshni empirical formula. After taking into account the correction due to the thermal redistribution coefficient, the variation of the peak position of luminescence from LSE is described by the following expression:
EðTÞ ¼ E0
aT 2 xðTÞKb T qþT
(1)
where a and q are the Varshni parameter and Debye temperature, respectively. E0 is the bandgap energy at 10K. The quantity x(T). KbT represents the extra red shift induced by the thermal redistribution [10]. In the high-temperature region, Equation (4) becomes:
EðTÞ ¼ E0
aT 2 s2 q þ T Kb T
(2)
The full width at half-maximum (FWHM) is another parameter for a luminescence spectrum. The shape of the luminescence spectrum resulting from the inhomogeneous distribution of localized carriers and broadening duo to the impurity and phonon scattering is the origin of FWHM. Based on this model, the broadening can be described by the following equation [10]:
GðTÞ ¼ G0 þ hA T þ
h
LO LO exp ħw 1 Kb T
(3)
where G0 is due to impurity/imperfection scattering. The second term due to phonon scattering, there, hA and hLO are the acousticphonon and longitudinal optical (LO)-phonon scattering coefficients, respectively. ħwLO is the LO-phonon energy. The integrated intensity of the luminescence spectrum is described by the following equation:
3 1 ðE0 Ea Þ þ Kb Tln tttrr 6 7 IðTÞ ¼ 1 þ ð1 gc Þexp4rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi5 2
ðKb TÞ2 þ 2 s 2:41 =
492
2
(4)
Thus, the temperature dependence of peak position, FWHM and normalized PL intensity change by inserting small composition of boron to the GaAs matrix that they are investigated and reinterpreted by LSE model in the following. The modeling results were represented in Fig. 7(aec), respectively for B48, B65 and B88. The fitting parameters were summarized in Table 1a and b. A good agreement between theory and experiment is achieved by the best-chosen of LSE parameters; E0, Ea-E0, s, the Varshni parameters (Ɵ and a) and the ration tr/ttr. The magnitude Ea-E0has a decisive influence on the temperature dependence of the PL peak position. For all samples, it is positive and it decreases by increasing the boron composition. It takes the values 13 meV, 8 meV, and 4 meV for samples B48, B65 and B88, respectively. By comparing the calculated parameters of different samples, the increasing of boron composition increases s and then reduces it for high boron concentration. Although, it shows a reduction of the occupied localized states and therefore, a weakness in S-shape behaviors is created. The ratio tttrr and gc are two key parameters that determine the thermal quenching of the luminescence band. It increases versus composition and then drops for B88. So, the thermal escape of excitons and therefore the thermal quenching rate follows this evolution versus boron composition. This explains the PL intensity behavior already represented. The lower recapture coefficient also leads to the higher rate of thermal quenching. But we noted that the high boron composition is weakly affected gc (0.9935 and 0.9900, respectively, for B65 and B88). A good agreement between the theoretical FWHM and the experimental data at low temperature in all samples. By increasing temperature, the recombination due to the delocalized states starts, and the theoretical results deviate from experimental evolution. This deviation is bigger for samples having a stronger S-shaped. Given that the variation of FWHM at high temperature is due to the phonon scattering that's follows from Equation (3). The impurity/defect coefficient G0 increases versus boron composition. This can be explained by the fact that the increase of boron composition increases the defects and impurity originated from the precursors under the growth condition. The LO-phonon scattering coefficient hLO and the phonon energy increases firstly by increasing the composition and then drops for high boron composition. We can explain this decrease by the reduction of the localization effects. 4. Conclusion To sum up, we have studied the thermal and boron composition effects on the morphological, structural and optical proprieties of BGaAs/GaAs epilayers. We have shown that the boron incorporation into GaAs matrix causes a surface modification, a blue shift and PL signal degradation with increasing boron concentration. A distribution of the carriers into band-tail state of the DOS and a presence of deep level associated to the B-atoms has been demonstrated. From the temperature dependence on the photoluminescence properties, we have shown that the broadening of the BGaAs PL band results from the localization effects induced by the potential modulation caused by the fluctuation of the composition. The luminescence measurements were successfully modeled and re-interpreted using the developed LSE model. The theoretical
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Fig. 7. PL peak position, FWHM and normalized PL intensity simulated using the LSE model, respectively, for the investigated samples: (a) B48, B65 (b) and B88 (c). The solid symbols are the experimental data, while the solid lines represent the calculated results using theoretical model.
Table 1 (Parameters used to fit the experimental results: (a) by the LSE model and (b) using empirical Varshni relation. (a) B48 B65 B88
E0 (eV) 1.33 1.377 1.363
Ea- E0 (eV) 0.01 0.008 0.004
s (eV) 3
10 10 20 103 9.5 103
hLO (meV/K)
G0 (meV)
ħWLO (meV)
gc
tr ttr
hA (meV/K)
320 770 200
64 87 98
33.606 40 20.68
0.9398 0.9935 0.99
12500 95000 3500
350 103 150 103 900 104
(b)
E0 (eV)
a (eV/K)
Ɵ (K)
B48 B65 B88
1.325 1.355 1.360
6 10 8 104 9 104
140 150 170
study has quantitatively interpreted the observed temperaturedependent spectral, to better understand the complicated spontaneous emission mechanisms in real ternary material, based on the fitting parameters. This paper gives a recommended estimation for
the study of B-III-V materials for optoelectronic applications. In order to understand such an unusual PL emission, we propose to provide more optical study of the observed low energy side emission and a thorough theoretical analysis.
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