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AlGaAs quantum well structures
Journal of Alloys and Compounds 739 (2018) 987e996
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Optical properties of n- and p-type modulation doped GaAsBi/AlGaAs quantum well structures Caglar Cetinkaya a, Erman Cokduygulular a, Ferhat Nutku a, *, Omer Donmez a, Janne Puustinen b, Joonas Hilska b, Ayse Erol a, Mircea Guina b a b
Department of Physics, Faculty of Science, Istanbul University, Vezneciler, 34134, Istanbul, Turkey Optoelectronics Research Centre, Tampere University of Technology, Korkeakoulunkatu 33720, Tampere, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2017 Received in revised form 18 December 2017 Accepted 23 December 2017 Available online 28 December 2017
In this work, optical properties of n- and p-type modulation doped GaAsBi/AlGaAs single quantum well (QW) heterostructures are investigated via temperature- and excitation power-dependent photoluminescence (PL) and integrated photoluminescence (IPL) studies and the results are compared to the nand p-type GaAs/AlGaAs QW structures to determine the influence of Bi and doping type on the optical properties. The results of the temperature dependent PL peak energy reveal that, as an effect of doping type, the temperature dependence of the PL peak energy exhibit different characteristic for n- and p-type samples. The temperature dependence of the PL peak energy reveals S-shaped trend for the n-type GaAsBi/AlGaAs QW sample. On the other hand, the characteristic follows Varshni law for the p-type GaAsBi/AlGaAs QW sample. The observed S-shaped behaviour for the n-type sample is explained by considering the contribution of the Bi-induced states above valence band (VB) to the PL. As for p-type sample, the localised states-related contribution to the PL signal is drastically diminished, resulting in an almost S-shape free temperature dependence of the optical transition energy, which can be explained by the compensation of acceptor-like states. The observed PL spectra of n- and p-type samples successfully reconstructed by two Gaussian peaks, which are assigned to optical transitions due to recombination of free and localised excitons. The localised exciton-related peak is observed to be very weak in p-type sample compared to that in n-type one. The allowed transitions in GaAsBi QW is calculated by self€ dinger-Poisson equation to identify the origin of the observed transiconsistently solving the Schro tions. A comparison of the PL results of the Bi-containing samples with the results of the Bi-free ones is exhibited approximately 80 meV/Bi% decrease in the fundamental optical transition. Using the excitation-dependent IPL measurements, it is found that at low temperatures and under low excitations, recombination process is under the effect of Shockley-Read-Hall (SRH) non-radiative process. When the excitation power density increases, radiative recombination becomes dominant. At higher-temperatures and in the high-intensity regime, we observed Auger effect in recombination process for n- and p-type GaAsBi samples, but the effect of Auger loss is observed to be much less in the p-type GaAsBi sample due to enhanced spin orbit split-off energy as a result of incorporation of bismuth. Furthermore, when we compare the result for Bi-free and Bi-containing p-type samples, again, Auger recombination is found to be less effective for the Bi-containing sample. © 2017 Elsevier B.V. All rights reserved.
Keywords: GaAsBi Dilute bismide n-type GaAsBi p-type GaAsBi Recombination mechanisms in GaAsBi
1. Introduction Dilute bismides are the latest member of the so-called highly mismatched III-V semiconductor alloys. The bandgap of GaAsBi weakly depends on temperature and can be changed eminently by
* Corresponding author. E-mail address: [email protected] (F. Nutku). https://doi.org/10.1016/j.jallcom.2017.12.261 0925-8388/© 2017 Elsevier B.V. All rights reserved.
alloying GaAs with a few percent amount of Bi, as observed in their counterpart dilute nitrides. Furthermore, as a result of alloying GaAs above 10% Bi concentration, spin split-off band energy becomes higher than the bandgap energy, which leads to an efficient suppression of the non-radiative CHSH Auger recombination and intervalence band absorption processes [1]. For these reasons, dilute bismides are very promising for fabricating near-infrared photodetectors [2], solar cells [3,4], light emitting diodes [5] and
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lasers [6,7]. In this paper, we present a study of optical characterisation of nand p-type modulation doped GaAsBi/AlGaAs QWs together with n- and p-type modulation doped GaAs/AlGaAs QW structures. The samples were grown by MBE, Bi compositions were confirmed by €dinger-Poisson equation was solved to XRD measurements. Schro obtain band profile and subband energy levels of the QW samples. To reveal the influence of Bi on the optical properties, n- and p-type GaAs/AlGaAs reference samples with the same structure and doping amount were employed. Electrical and optical properties of bulk and epilayers dilute bismide alloys are widely studied in the literature [8e13] and there are some studies on GaAsBi QW structures [10,13e15], but to the best of our knowledge, there has not any reported research on optical properties of modulation doped n- and p-type GaAsBi/AlGaAs QW structures yet.
2. Experimental details In order to understand the effect of Bi on the optical properties of GaAs, n- and p-type GaAs reference samples were grown with molecular beam epitaxy (MBE) at 580 C with high As overpressure. In the growth process of Bi-containing samples, all layers up to the QW layer (GaAsBi layer) were grown at 580 C. After the completion of the first Al0:15 Ga0:85 As barrier layer, the growth process was stopped, and the substrate temperature was reduced to 370 C. After settling the temperature at 370 C, a 7 nm GaAsBi layer was grown. For the duration of the GaAsBi growth, the As/Ga flux ratio was adjusted to a near-stoichiometric value and a Bi-predeposition (with no Ga flux) had been performed before the actual GaAsBi QW growth. Once the growth of this layer has been completed, the growth process was interrupted, and the substrate temperature was set to 580 C. After performing HR-XRD measurements, the Bi concentration in the QW was determined to be 4% for the alloyed samples. All samples used in the study were doped with Si and Be atoms in order to obtain n- and p-type material, respectively. The structure of the investigated sample is given in Fig. 1. In order to determine the temperature dependence of the optical properties of the samples, temperature PL measurements were performed at a laser intensity of 2.5 W/cm2 at a temperature range of 30e300 K by using the 514.5 nm line of a continuous-wave Arion laser mechanically chopped at 130 Hz. Excitation power density-dependent PL and IPL were carried out in the range of 2.5e38 W/cm2. The IPL measurements were carried out as temperature- and intensity-dependent. A 0.5 m focal length monochromator was used to disperse the light, a GaInAs photomultiplier tube was used to detect the intensity of dispersed PL signal, and its output was read by a lock-in amplifier to measure the signal.
3. Theoretical background The valence band anticrossing (VBAC) model is used to define bandstructure of the samples. According to the VBAC model [16], the interaction between localised Bi-related states with the VB of GaAs splits heavy hole (hh) and light hole (lh) bands into two branches with high (þ) and low () energies given by
EVB ± ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 EVBmax þ EBi ± ðEVBmax þ EBi Þ2 þ 4xC 2Bi 2
(1)
where EVBmax is the energy of VB maximum of GaAs, EBi is the isolated Bi impurity level measured with respect to VB maximum of GaAs and taken as 0.183 eV [17], CBi is the coupling energy between the Bi level and the VB maximum of GaAs and taken as 1.65 eV [18], x is the Bi concentration. According to the recent studies [19], alloying with Bi not only affects the VB structure of the host material GaAs by increasing it ~53 meV/Bi%, but also affects the conduction band (CB) and decreases it by ~28 meV/Bi%, so in total, bandgap of GaAsBi is smaller than GaAs by an amount of ~81 meV/ Bi%. Decrease in the CB minimum of GaAsBi can be modelled with a linear function of Bi concentration by the following equation,
ECBmin ¼ Eg ðGaAsÞ DECBmin x
(2)
where DECBmin is the offset between the CB minima of GaAs and GaBi alloys and taken as 2.1 eV [16]. This value is found as 2.8 and 1.9 eV using k.p [17] and tight-binding [19] models, respectively. However, we have achieved the best fit in our calculations by using the value of 2.1 eV. Finally, bandgap of GaAsBi can be written as
Eg ¼ ECBmin EVBþ
(3)
Dilute bismide alloys are known to have localised states formed above the maximum of the VB due to Bi-related alloy fluctuations and Bi-related clusters, giving rise to DOS tail above VB [20]. Therefore, due to the presence of localised states, the temperature dependence of the PL spectrum cannot be simply modelled with the classical Varshni equation [18,21,22]. In the literature, in order to explain the abnormal temperature dependence of the PL peak energy, contribution of the PL from localised state ensemble has been modelled with including an extra linear temperature dependent term to the standard Varshni equation as the following [23],
EðTÞ ¼ E0
aT 2 cðTÞkB T bþT
(4)
where E0 is the theoretical bandgap of GaAs at zero temperature, a, b are called linear expansion coefficient and Debye temperature of
Fig. 1. The layer structure of the samples.
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GaAs, respectively. Here cðTÞ parameter depends on several physical parameters of system [23], which is overall governs the thermal distribution of carriers residing in the localised states. The bandgap of GaAsBi is determined by substituting EðTÞ calculated from Eq. (4) at a given T into Eq. (2) and by using Eq. (3). In the self-consistent €dinger-Poisson solutions, cðTÞ parameter is set by Schro comparing the e1-hh1 energy, which is obtained by Gaussian peak fitting procedure applied to PL spectrum. By using this method, other excitonic transitions with relatively higher energies can be predicted. Therefore, by comparing possible excitonic transition energies with the fitted peak energies, one can have information about whether a fitted peak is originated from a free or from a localised exciton. In GaAs reference samples Varshni equation is used without cðTÞ parameter. For GaAsBi samples near at room temperature cðTÞ can be approximated to zero, in which Eq. (3) gives a bandgap energy compatible with experimental observa€dinger-Poisson equation constructed for tions. In this work, Schro the structure is solved by Aestimo 1D code [24]. 4. Results and discussion By using the method described above, the calculated band diagrams, electron, hh/lh wavefunctions and Fermi levels are given in Fig. 2 and calculated optical transitions are tabulated in Table 1 for all samples at 30 K. As for GaAs samples, we used the classical Varshni equation (cðTÞ ¼ 0) to determine temperature dependence of the optical transition energy. It is worth noting that Fermi level in both n- and p-type samples lies above the first subband level in the quantum wells, indicating that all possible localised levels should be filled up by carriers originating from doped barriers. Therefore, it can be expected that not to observe an S-shaped temperature dependence of the PL signal for both n- and p-type samples, but in n-type GaAsBi sample, an S-shaped behaviour was observed. On € dinger-Poisson calculations we do not the other hand, in the Schro take into account of the localised states and trap levels. Therefore, it is expected that the Fermi level shifts down in the presence of the localised/trap levels because carriers occupy the localised levels as well as they can be captured by traps. Further discussion of the occupation of Bi-related localised states and the reason of S-shaped behaviour in n-type GaAsBi sample is discussed in the following sections. 4.1. Temperature dependence of PL Fig. 3 shows the PL spectra of all samples taken at 30 K under 3 W/cm2 laser power density. It is clear that incorporation of Bi increases the FWHM of the PL spectrum and shifts the optical transition energy to lower values as reported [18,25]. Both n- and ptype GaAsBi/AlGaAs QW samples have a low energy tail, which is an indication of the presence of localised/trap states due to the Birelated inhomogeneities above VB. It is obvious that the low energy tail of PL for n-type GaAsBi sample smears out lower energies more than that for p-type GaAsBi sample. Moreover, the PL spectrum has smaller FWHM and higher intensity for p-type samples, which can be explained by suppressed effect of localised/trap states as a result of p-type doping. For n-type GaAsBi sample, the characteristic of PL spectrum is far from excitonic-like, but rather localised-exciton-like at low temperature, which indicates that there is a strong effect of the localised states-related optical transition on observed PL. A comprehensive investigation of the temperature dependence of PL spectra and comparison of the experimental data with theoretical calculations give a better understanding of the results. PL spectra of n- and p-type GaAsBi samples measured between the
Fig. 2. Electron (blue solid line), hh (green solid line)/lh (red dashed line) wavefunctions, Fermi level (green dash-dotted line) obtained for n-type GaAsBi/AlGaAs (a) and p-type GaAsBi/AlGaAs (b) modulation doped QW structures. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
temperature range 30e300 K were given in Fig. 4a and b. PL spectra of n- and p-type samples show an asymmetric behaviour at low energies as seen in Fig. 4a and b. The observed asymmetry and steep fall of the PL peak in the high energy region, especially at low temperatures, indicates that there is a free excitonic transition occurring between subbands of the QWs. In order to understand the origin of PL, temperature dependence of PL has been investigated and to determine the type of optical transition energies, such as band-to-band involving e1-hh1 or band-tolocalised bismuth states located within the bandgap, a peak fit procedure was applied to the PL spectrum in the temperature range of 30e300 K by using the Fityk software [26]. For n- and p-type GaAsBi samples, the PL spectra obtained at 30 K are deconvoluted successfully by using two Gaussian peaks as seen in Fig. 5a and b accompanied with the experimental results. We assigned the first peak with higher energy to an e1-hh1 transition and the second peak to an e1-to-localised/trap state exciton transition. In the ntype sample, FWHM of localised exciton- and free exciton-related
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Table 1 Calculated optical transition energies between subbands and PL peaks at temperature 30 K. Sample
e1-hh1 free excitons (exp.) (eV)
e1-hh1 free excitons (theory) (eV)
Localised excitons (exp.) (eV)
e1-lh1 Exp. (eV)
e1-lh1 Theory (eV)
e1-hh3 Exp. (eV)
e1-hh3 Theory (eV)
n-GaAsBi p-GaAsBi n-GaAs p-GaAs
1.209 1.207 1.535 1.538
1.209 1.207 1.545 1.545
1.151 1.174 e e
e e 1.567 1.547
1.233 1.228 1.561 1.570
e e 1.587 e
1.322 1.238 1.582 1.571
Fig. 3. PL spectra of all the samples taken at 30 K. Black solid, dashed red, dotted green and blue dashed dot lines depict n-type GaAs, n-type GaAsBi, p-type GaAs and p-type GaAsBi samples results, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
peaks are ~110 nm and ~74 nm, respectively, whereas these values are ~60 nm and ~20 nm in the p-type sample. This is an indication of an optical transition originated by the recombination of the localised excitons at low temperatures, and the difference between the free exciton and localised states-related optical transition energies are 58 meV and 33 meV for n- and p-type GaAsBi samples, respectively. The smaller value of FWHM of localised excitonrelated PL and the smaller energy difference between free excitons and localised excitons-related optical transition are indications of the reduced density of the localised states in the p-type sample due to p-type doping. Since the localised excitonic transitions dominate the PL spectrum at low temperatures, it is also clear that, at 30 K, the free excitonic transition is more dominant on the PL for the p-type sample. Therefore, it can be concluded that thanks to the p-type doping, PL characteristic of the p-type sample rather exhibits an excitonic transition of e1-hh1 transition. On the other hand, the contribution of localised excitonic transition is quite effective on PL spectrum of the n-type GaAsBi sample. In PL spectra taken at 300 K, the PL peak shape of both n- and ptype samples are almost identical with their shapes and the PL peak energies. Since the high-temperature PL is dominated from e1-hh1, optical transition and the contribution from localised excitonrelated PL does no longer exist; the optical quality is slightly better for the n-type sample. It is seen from Fig. 4c that the PL intensity of the n-type GaAsBi sample is higher than that of the p-type GaAsBi sample at 300 K. The observation of almost the same PL characteristic for n- and p-type GaAsBi/AlGaAs samples as the temperature increases can be explained by the effect of traps. It is well-known that Bi incorporation into the GaAs lattice causes some deep levels, especially at around the mid-gap of GaAsBi. The nature of these traps can be electron or hole traps, and the density of the traps are quite high, and the mid-gap traps are active at high
temperatures with a quite high density [15], which is large enough to influence the radiative transition rate. Mooney et al. showed that n-type doping reduces the electron trap density [27], on the other hand, p-type doping does not affect the hole trap density [28]. In the light of these observations, at high temperatures, it can be expected that photo-generated carriers are captured by the traps and non-radiatively recombine in n- and p-type GaAsBi samples, therefore, the intensity of the PL peak approaches to each other. The observed higher PL intensity at 300 K for the n-type sample could be explained by reduced trap density in the n-type sample as observed in Ref. [27]. The observed high energy tail for both n- and p-type GaAsBi samples can be related to PL emission from the transitions of higher occupied states. Even the samples are excited by the same laser intensity at 30 K, and 300 K, some fraction of the photo-generated carriers are localised in the localised states and/or traps at 30 K. As temperature is increased, the photo-generated carriers gain thermal energy to occupy higher states, leading to PL from optical transition of higher states, therefore, at the same excitation power density PL spectrum at 300 K has a high energy tail for both samples and the characteristic of the PL is solely from e1-hh1 transition. In order to determine the effect of Bi on the bandgap of the samples, we also conducted PL measurements on the GaAs modulation doped reference samples. PL spectra of GaAs samples at 30 K are given in Fig. 6a and b. n-type GaAs reference sample has three distinct peaks, which are assigned to e1-hh1, e1-lh1 and e1hh3 free exciton transitions according to the calculations and they produce the shoulders of the spectrum at the high energy part. ptype GaAs reference sample has a sharp peak, which is assigned to e1-hh1 free exciton transition and the second peak at higher energy with much lower intensity can be due to e1-lh1 transition. For all samples, the calculated transition energies and PL peak energies separated by the fitting process at temperature 30 K are given in Table 1. For reference samples, measured optical transitions between higher subbands are in agreement with self-consistent so€dinger-Poisson equation. A similar contribution lutions of the Schro of high energetic excitons to a PL spectrum has also been observed in the literature for GaAs/AlGaAs heterostructures [29e31]. On the other hand, optical transitions involving excitons with higher energies have not been observed in reference samples at high temperatures due to the exponential decrease in the probability of optical transitions occurring across subbands with high energies. Although, we have observed higher subband transitions in the reference samples, only fundamental transitions are observed in GaAsBi samples, which can be explained by the presence of the Biinduced traps in GaAsBi samples. When some fraction of the photogenerated carriers are captured in GaAsBi samples, the higher subbands cannot be occupied, and no emission can be observed from higher subband transitions. Therefore, compared to GaAsBi samples, in GaAs samples, photo-generated carriers can occupy higher subbands, leading to the observation of radiative transitions involving higher levels at the same excitation power density. The peak corresponding to localised excitons in the n-type
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sample has 23 meV lower energy than p-type sample as seen in Table 1. This can be explained by the following; both alloy fluctuations and Bi-induced clusters form localised states. TEM images [14,32] have revealed that when the thickness of GaAsBi layer is thin enough, that is, in QW structures, the contribution of alloy fluctuations to the formation of localised states is negligible compared to the Bi-induced clusters. Since our samples are QW structures, we can consider that the more contribution comes from Bi-induced clusters. At 30 K, the energy difference between e1 and
Fig. 5. PL spectrum of n-type (a) and p-type (b) GaAsBi samples at 30 K. The black dashed curve corresponds to an individual peak obtained by fitting a Gaussian function; the red dashed curve indicates the theoretical spectrum obtained by superposing the separately obtained peaks and the blue curve is the experimental data. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Temperature-dependent PL spectra of n-type (a) and p-type (b) GaAsBi samples. Comparison of the PL spectra of n- and p-type GaAsBi samples measured at 30 K and 300 K (c).
VB maximum is ~1.190 eV and ~1.187 eV in n-type and p-type samples, respectively. When these values are compared with the fitted localised exciton-related peak energy values, it is found that localised levels are located at above 39 meV and 13 meV higher energy than the VB maximum of the n-type and p-type sample, respectively. So in the n-type sample, localised states are nearer to the CB compared to p-type, which causes a lower optical transition energy as depicted in Fig. 7b. It is seen from Table 1 that the optical transition energy is shifted by approximately ~80 meV/Bi% at 30 K by the incorporation of 4% Bi into GaAs. The change of the effective bandgap energy (e1-hh1 transition) with temperature was determined from the analysis of the PL spectra measured at different temperatures. The temperature dependence of the peak assigned to recombination of free e1-hh1 excitons are deduced from the fitting process, and the experimental data were given in Fig. 7a for n- and p-type samples. In the graphs given in Fig. 7a, it is seen that the temperature dependence of the PL peak energy of n-type sample has a prominent S-shaped change. The S-shape characteristic of temperature
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Fig. 6. PL spectrum of n-type (a) p-type (b) GaAs samples at T ¼ 30 K. The black dashed curve corresponds to an individual peak obtained by fitting a Gaussian function; the red dashed curve indicates the theoretical spectrum obtained by superposing the separately obtained peaks and the blue curve is the experimental data. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
dependent PL peak energy is a signature of the presence of localised states over VB, and it has been a well-known characteristic behaviour of highly-mismatched alloys [33e37]. Kini et al. [38] and Pettinari et al. [39,40] reported that Bi incorporation in GaAs introduces several acceptor-like traps above VB band edge and as Bi composition increases, some of these traps are suppressed. Furthermore, it has been reported that p-type doping of GaAsBi also has the similar effect [38]. Under the light of these experimental findings, we believe that p-type doping suppressed the some of the acceptor-like traps, resulting in the observation of e1-hh1 transition. Since the acceptor-like states cannot be suppressed in n-type samples, these states contribute to the observed lower energy PL and S-shape characteristic of PL peak energy in n-type GaAsBi sample. As temperature increases, in n-type GaAsBi, the captured electrons in acceptor-like traps are emitted and start to contribute to the e1-hh1 transition at around 160 K and above temperatures. Kini et al. [38] also reported the presence of a donor-like defect BiGa as a results of Bi incorporation, which causes to decrease in hole concentration. Even our samples are doped at the same amount of dopants, we have measured smaller hole density for p-type sample
Fig. 7. Temperature dependence of PL peak energies belonging to n-type (full symbols) and p-type (empty symbols) GaAsBi samples. Circle and up triangle, represents experimental data and 1st peak maxima determined by the fitting procedure, respectively (a). Band diagram of the GaAsBi QW structure, electron (blue solid line), hh (green solid line)/lh (red dashed line) levels, Fermi level (green dash-dotted line) €dingerand e1-hh1, e1-localised level transition energies found by the solution of Schro Poisson equation self-consistently at temperature 30 K. Bi-related localised level (black dash-dotted line) is 39 meV and 13 meV above the VB maximum in n- and p-type samples, respectively (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
by a factor of 10, which agrees with the results of Kini et al. and confirms the presence of a donor-like defect in the GaAsBi samples compensates acceptors. However, further experiments such as deep level spectroscopy are required to confirm the presence of acceptor-like and donor-like defects in the samples. In Fig. 8, we compare the ratio of the intensities of the peaks assigned to free and localised excitons obtained for n- and p-type samples. The temperature dependence of intensity ratio of the assigned free exciton peak (1st peak) to localised exciton peak (2nd peak) is given. For an n-type sample, at temperatures between 140 K and 160 K, the ratio becomes less than unity that is an indication of the localised excitons-related radiative recombination dominates over free exciton-related one. On the other hand, for ptype sample, the ratio is always higher than unity, which indicates that free exciton recombination is dominant in the range from 30 K
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Fig. 8. The intensity ratio of the fitted free and localised exciton peaks for n- and ptype GaAsBi samples.
to 300 K. Moreover, temperature dependence of FWHM for the ntype sample (Fig. 9) has a well-known characteristic of the PL spectrum containing localised exciton-related radiative recombination. On the other hand, as for the p-type sample, the temperature dependence of FWHM increases as expected. A similar phenomenon has also been observed in dilute nitride structures as can be referred to Refs. [33e37], which is explained by N-related disorder and localisation effects in the CB. In this study, similar localisation effect is observed in the VB, because Bi perturbates the VB edge prominently, which is in agreement with the VBAC model.
993
n-type samples, while in p-type samples, contribution of these states are suppressed by p-type doping. For this reason, the PL peak energies of the n-type sample were ~10 meV lower than that of the p-type sample. In order to investigate the excitation power density on the temperature dependence of PL peak energy of an n-type sample, we have taken PL spectrum at various excitation per intensities and we have observed that the S-shape characteristic can be suppressed at an excitation intensity of 38 W/cm2 (Fig. 10b). Similar behaviour is also observed in 4e5% Bi alloyed GaAsBi epilayer grown on GaAs [20]. The excitation intensity of 38 W/cm2 corresponds that a photon density of ~1 1019 cm2. Hence, localised state density for n-type GaAsBi might be approximated as 1 1019 cm2. Integrated PL (IPL) intensity gives information about the dominant recombination process. The carrier-density dependent total spontaneous recombination rate is radiative recombination rate plus non-radiative recombination rate (loss rate) and is expressed as in Eq. (5). The loss rate is composed of SRH recombination and Auger recombination rates. In order to determine the dominant recombination processes, IPL measurements were employed at various temperatures. The intensity of the IPL signal can be expressed in terms of photo-generated electron-hole pair density as [41,42].
4.2. Excitation power dependence of PL Excitation power-dependent PL measurements were carried out in order to have a better understanding the characteristic of optical transitions. The PL spectra of the n-type sample under various excitation intensities and the change of the PL peak energy for GaAsBi samples for the measurements taken at 30 K are given in Fig. 10a and b, respectively. The spectrum given in Fig. 10a has an asymmetric structure, which is also an indication of the presence localised states. As above-mentioned, the Bi-related acceptor-like states are empty in
Fig. 9. The FWHM of localised exciton peaks for n- and p-type GaAsBi samples.
Fig. 10. PL spectra of the n-type GaAsBi sample under various excitation intensities (a), the temperature dependence of the PL peak energy at different laser power densities (b).
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PIPL ¼ c An þ Bn2 þ Cn3
(5)
In this equation, n is the photo-generated carrier density, A is the SRH recombination coefficient, B is the radiative recombination coefficient, and C is the Auger recombination coefficient. If the excitation power is written as LIPL ¼ c1 Bn2 , then total intensity of the IPL signal can be written in terms of excitation power as the following, 1=2
shallow levels due to alloy fluctuations and Bi-related states such as Bi pair, triple and clusters. Therefore, dependence of carrier density of SRH in Eq. (6) can be in a more complex form, including additional possible transition-related terms, but modification of the SRH-related part of Eq. (6) is not a subject of this paper. Considering more complex nature of localised level-induced transitions in GaAsBi, in case of s < 1, we will also assume that the IPL is dominated by SRH recombination loss mechanism. Fig. 11 shows IPL as a function of excitation power density (photon density at top x-axis) at various temperatures. By using 1=2
3=2
PIPL ¼ APL LIPL þ BPL LIPL þ CPL LIPL
(6) 1=2
Eq. (6) can be written in a form of slogðLLp Þ and s refers to transition type. In the SRH recombination theory, non-radiative recombination is assisted by a single trap level located in the mid-gap of the semiconductor. It has been reported that GaAsBi has several kind of deep level traps such as AsGa, BiGa, VGa etc. as well as
slogðLILp Þ, we have found the slopes and determined the dominant recombination type. The slope values of 1, 2 and 3 corresponds to radiative, SRH and Auger recombination processes, respectively. At low temperatures under low excitation intensities, the slope is lower than 1, therefore, non-radiative SRH dominates recombination process for all samples. As the excitation power density increases, the slope is obtained to be ~2, which indicates radiative
Fig. 11. IPL versus excitation intensity (power density) plots of n-type GaAs (a), n-type GaAsBi (b), p-type GaAs (c), p-type GaAsBi (d) at various temperatures. The dominant transition type determined from the calculated slopes.
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recombination supresses the non-radiative recombination loss (SRH mechanism). At temperatures higher than 150 K and lower than 200 K, radiative recombination starts to dominate. For all samples except for n-type GaAs, at the temperatures above 200 K, the slope of the plots is higher than 2, which indicates that Auger effect also contributes to the recombination process. When p-type GaAs and p-type GaAsBi samples are compared, the effect of Auger recombination shows itself at higher temperatures for p-type GaAsBi, which is an indication of the suppression of Auger process due enhanced spin orbit split of energy with incorporation of bismuth. As for n-type GaAs, there is no contribution to recombination process from Auger process. On the other hand, for n-type GaAsBi, above 200 K under high excitation intensities, the slope of the curves in Fig. 11b becomes bigger than 2. The Auger process is more prominent at lower bandgap materials therefore the observation of Auger process in n-type GaAsBi, but not in n-type GaAs, can be related to reduced bandgap of n-type GaAsBi. Even the effective bandgap of the n- and p-type GaAsBi is almost the same, we obtained lower slope values for p-type GaAsBi due to larger spin-orbit split off energy of p-type GaAsBi. Since an S-shaped characteristic is observed for n-type GaAsBi sample (see Fig. 10b), in the S-shape region, IPL switches from SRH to radiative recombination, and the radiative recombination process becomes dominant at higher temperatures than that for p-type GaAsBi.
[3]
[4]
[5]
[6]
[7]
[8]
[9] [10]
[11]
5. Conclusion In this study, we have investigated optical properties of n- and p-type GaAsBi/AlGaAs single QW structures by using temperatureand excitation power-dependent PL and IPL measurements and compared the results with Bi-free n- and p-type QW structures to determine the effect of doping and bismuth. It has observed that ptype doping supresses the S-shaped temperature dependence. The bismuth incorporation cause about 80 meV/Bi% shift in the e1-hh1 transition. It has shown that by increasing optical excitation power density, S-shaped temperature dependence of the PL peak energy can be suppressed in the n-type sample. The recombination process has been observed to be dominated by non-radiative SRH mechanism at low temperatures under low excitation intensities for all Bifree and Bi-containing QWs. When excitation power density increases, radiative recombination becomes dominant over nonradiative recombination. Above 200 K, Auger effect is predominantly observed for n-type GaAsBi sample, because introducing Bi atoms into GaAs decreases bandgap of GaAsBi that increases Auger recombination rate for the n-type sample. Auger effect is also observed for p-type samples, but the effect of the Auger recombination is more effective for Bi-free sample. Due to the enhanced spin split-off band energy, with introducing Bi into GaAs, the probability of observing Auger effect for p-type GaAsBi sample decreases. Acknowledgments
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
This work is supported by The Scientific and Technical Research Council of Turkey (TUBITAK) under Grant No. 115F517 and supported by The Scientific Research Projects Coordination Unit of Istanbul University (Project Numbers FYL-2016-21993 and BEK2017-25763).
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optical properties of n- and p-type modulation doped GaAsBi/AlGaAs quantum well structures Caglar Cetinkaya a, Erman Cokduygulular a, Ferhat Nutku a, *, Omer Donmez a, Janne Puustinen b, Joonas Hilska b, Ayse Erol a, Mircea Guina b a b
Department of Physics, Faculty of Science, Istanbul University, Vezneciler, 34134, Istanbul, Turkey Optoelectronics Research Centre, Tampere University of Technology, Korkeakoulunkatu 33720, Tampere, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2017 Received in revised form 18 December 2017 Accepted 23 December 2017 Available online 28 December 2017
In this work, optical properties of n- and p-type modulation doped GaAsBi/AlGaAs single quantum well (QW) heterostructures are investigated via temperature- and excitation power-dependent photoluminescence (PL) and integrated photoluminescence (IPL) studies and the results are compared to the nand p-type GaAs/AlGaAs QW structures to determine the influence of Bi and doping type on the optical properties. The results of the temperature dependent PL peak energy reveal that, as an effect of doping type, the temperature dependence of the PL peak energy exhibit different characteristic for n- and p-type samples. The temperature dependence of the PL peak energy reveals S-shaped trend for the n-type GaAsBi/AlGaAs QW sample. On the other hand, the characteristic follows Varshni law for the p-type GaAsBi/AlGaAs QW sample. The observed S-shaped behaviour for the n-type sample is explained by considering the contribution of the Bi-induced states above valence band (VB) to the PL. As for p-type sample, the localised states-related contribution to the PL signal is drastically diminished, resulting in an almost S-shape free temperature dependence of the optical transition energy, which can be explained by the compensation of acceptor-like states. The observed PL spectra of n- and p-type samples successfully reconstructed by two Gaussian peaks, which are assigned to optical transitions due to recombination of free and localised excitons. The localised exciton-related peak is observed to be very weak in p-type sample compared to that in n-type one. The allowed transitions in GaAsBi QW is calculated by self€ dinger-Poisson equation to identify the origin of the observed transiconsistently solving the Schro tions. A comparison of the PL results of the Bi-containing samples with the results of the Bi-free ones is exhibited approximately 80 meV/Bi% decrease in the fundamental optical transition. Using the excitation-dependent IPL measurements, it is found that at low temperatures and under low excitations, recombination process is under the effect of Shockley-Read-Hall (SRH) non-radiative process. When the excitation power density increases, radiative recombination becomes dominant. At higher-temperatures and in the high-intensity regime, we observed Auger effect in recombination process for n- and p-type GaAsBi samples, but the effect of Auger loss is observed to be much less in the p-type GaAsBi sample due to enhanced spin orbit split-off energy as a result of incorporation of bismuth. Furthermore, when we compare the result for Bi-free and Bi-containing p-type samples, again, Auger recombination is found to be less effective for the Bi-containing sample. © 2017 Elsevier B.V. All rights reserved.
Keywords: GaAsBi Dilute bismide n-type GaAsBi p-type GaAsBi Recombination mechanisms in GaAsBi
1. Introduction Dilute bismides are the latest member of the so-called highly mismatched III-V semiconductor alloys. The bandgap of GaAsBi weakly depends on temperature and can be changed eminently by
* Corresponding author. E-mail address: [email protected] (F. Nutku). https://doi.org/10.1016/j.jallcom.2017.12.261 0925-8388/© 2017 Elsevier B.V. All rights reserved.
alloying GaAs with a few percent amount of Bi, as observed in their counterpart dilute nitrides. Furthermore, as a result of alloying GaAs above 10% Bi concentration, spin split-off band energy becomes higher than the bandgap energy, which leads to an efficient suppression of the non-radiative CHSH Auger recombination and intervalence band absorption processes [1]. For these reasons, dilute bismides are very promising for fabricating near-infrared photodetectors [2], solar cells [3,4], light emitting diodes [5] and
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lasers [6,7]. In this paper, we present a study of optical characterisation of nand p-type modulation doped GaAsBi/AlGaAs QWs together with n- and p-type modulation doped GaAs/AlGaAs QW structures. The samples were grown by MBE, Bi compositions were confirmed by €dinger-Poisson equation was solved to XRD measurements. Schro obtain band profile and subband energy levels of the QW samples. To reveal the influence of Bi on the optical properties, n- and p-type GaAs/AlGaAs reference samples with the same structure and doping amount were employed. Electrical and optical properties of bulk and epilayers dilute bismide alloys are widely studied in the literature [8e13] and there are some studies on GaAsBi QW structures [10,13e15], but to the best of our knowledge, there has not any reported research on optical properties of modulation doped n- and p-type GaAsBi/AlGaAs QW structures yet.
2. Experimental details In order to understand the effect of Bi on the optical properties of GaAs, n- and p-type GaAs reference samples were grown with molecular beam epitaxy (MBE) at 580 C with high As overpressure. In the growth process of Bi-containing samples, all layers up to the QW layer (GaAsBi layer) were grown at 580 C. After the completion of the first Al0:15 Ga0:85 As barrier layer, the growth process was stopped, and the substrate temperature was reduced to 370 C. After settling the temperature at 370 C, a 7 nm GaAsBi layer was grown. For the duration of the GaAsBi growth, the As/Ga flux ratio was adjusted to a near-stoichiometric value and a Bi-predeposition (with no Ga flux) had been performed before the actual GaAsBi QW growth. Once the growth of this layer has been completed, the growth process was interrupted, and the substrate temperature was set to 580 C. After performing HR-XRD measurements, the Bi concentration in the QW was determined to be 4% for the alloyed samples. All samples used in the study were doped with Si and Be atoms in order to obtain n- and p-type material, respectively. The structure of the investigated sample is given in Fig. 1. In order to determine the temperature dependence of the optical properties of the samples, temperature PL measurements were performed at a laser intensity of 2.5 W/cm2 at a temperature range of 30e300 K by using the 514.5 nm line of a continuous-wave Arion laser mechanically chopped at 130 Hz. Excitation power density-dependent PL and IPL were carried out in the range of 2.5e38 W/cm2. The IPL measurements were carried out as temperature- and intensity-dependent. A 0.5 m focal length monochromator was used to disperse the light, a GaInAs photomultiplier tube was used to detect the intensity of dispersed PL signal, and its output was read by a lock-in amplifier to measure the signal.
3. Theoretical background The valence band anticrossing (VBAC) model is used to define bandstructure of the samples. According to the VBAC model [16], the interaction between localised Bi-related states with the VB of GaAs splits heavy hole (hh) and light hole (lh) bands into two branches with high (þ) and low () energies given by
EVB ± ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 EVBmax þ EBi ± ðEVBmax þ EBi Þ2 þ 4xC 2Bi 2
(1)
where EVBmax is the energy of VB maximum of GaAs, EBi is the isolated Bi impurity level measured with respect to VB maximum of GaAs and taken as 0.183 eV [17], CBi is the coupling energy between the Bi level and the VB maximum of GaAs and taken as 1.65 eV [18], x is the Bi concentration. According to the recent studies [19], alloying with Bi not only affects the VB structure of the host material GaAs by increasing it ~53 meV/Bi%, but also affects the conduction band (CB) and decreases it by ~28 meV/Bi%, so in total, bandgap of GaAsBi is smaller than GaAs by an amount of ~81 meV/ Bi%. Decrease in the CB minimum of GaAsBi can be modelled with a linear function of Bi concentration by the following equation,
ECBmin ¼ Eg ðGaAsÞ DECBmin x
(2)
where DECBmin is the offset between the CB minima of GaAs and GaBi alloys and taken as 2.1 eV [16]. This value is found as 2.8 and 1.9 eV using k.p [17] and tight-binding [19] models, respectively. However, we have achieved the best fit in our calculations by using the value of 2.1 eV. Finally, bandgap of GaAsBi can be written as
Eg ¼ ECBmin EVBþ
(3)
Dilute bismide alloys are known to have localised states formed above the maximum of the VB due to Bi-related alloy fluctuations and Bi-related clusters, giving rise to DOS tail above VB [20]. Therefore, due to the presence of localised states, the temperature dependence of the PL spectrum cannot be simply modelled with the classical Varshni equation [18,21,22]. In the literature, in order to explain the abnormal temperature dependence of the PL peak energy, contribution of the PL from localised state ensemble has been modelled with including an extra linear temperature dependent term to the standard Varshni equation as the following [23],
EðTÞ ¼ E0
aT 2 cðTÞkB T bþT
(4)
where E0 is the theoretical bandgap of GaAs at zero temperature, a, b are called linear expansion coefficient and Debye temperature of
Fig. 1. The layer structure of the samples.
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GaAs, respectively. Here cðTÞ parameter depends on several physical parameters of system [23], which is overall governs the thermal distribution of carriers residing in the localised states. The bandgap of GaAsBi is determined by substituting EðTÞ calculated from Eq. (4) at a given T into Eq. (2) and by using Eq. (3). In the self-consistent €dinger-Poisson solutions, cðTÞ parameter is set by Schro comparing the e1-hh1 energy, which is obtained by Gaussian peak fitting procedure applied to PL spectrum. By using this method, other excitonic transitions with relatively higher energies can be predicted. Therefore, by comparing possible excitonic transition energies with the fitted peak energies, one can have information about whether a fitted peak is originated from a free or from a localised exciton. In GaAs reference samples Varshni equation is used without cðTÞ parameter. For GaAsBi samples near at room temperature cðTÞ can be approximated to zero, in which Eq. (3) gives a bandgap energy compatible with experimental observa€dinger-Poisson equation constructed for tions. In this work, Schro the structure is solved by Aestimo 1D code [24]. 4. Results and discussion By using the method described above, the calculated band diagrams, electron, hh/lh wavefunctions and Fermi levels are given in Fig. 2 and calculated optical transitions are tabulated in Table 1 for all samples at 30 K. As for GaAs samples, we used the classical Varshni equation (cðTÞ ¼ 0) to determine temperature dependence of the optical transition energy. It is worth noting that Fermi level in both n- and p-type samples lies above the first subband level in the quantum wells, indicating that all possible localised levels should be filled up by carriers originating from doped barriers. Therefore, it can be expected that not to observe an S-shaped temperature dependence of the PL signal for both n- and p-type samples, but in n-type GaAsBi sample, an S-shaped behaviour was observed. On € dinger-Poisson calculations we do not the other hand, in the Schro take into account of the localised states and trap levels. Therefore, it is expected that the Fermi level shifts down in the presence of the localised/trap levels because carriers occupy the localised levels as well as they can be captured by traps. Further discussion of the occupation of Bi-related localised states and the reason of S-shaped behaviour in n-type GaAsBi sample is discussed in the following sections. 4.1. Temperature dependence of PL Fig. 3 shows the PL spectra of all samples taken at 30 K under 3 W/cm2 laser power density. It is clear that incorporation of Bi increases the FWHM of the PL spectrum and shifts the optical transition energy to lower values as reported [18,25]. Both n- and ptype GaAsBi/AlGaAs QW samples have a low energy tail, which is an indication of the presence of localised/trap states due to the Birelated inhomogeneities above VB. It is obvious that the low energy tail of PL for n-type GaAsBi sample smears out lower energies more than that for p-type GaAsBi sample. Moreover, the PL spectrum has smaller FWHM and higher intensity for p-type samples, which can be explained by suppressed effect of localised/trap states as a result of p-type doping. For n-type GaAsBi sample, the characteristic of PL spectrum is far from excitonic-like, but rather localised-exciton-like at low temperature, which indicates that there is a strong effect of the localised states-related optical transition on observed PL. A comprehensive investigation of the temperature dependence of PL spectra and comparison of the experimental data with theoretical calculations give a better understanding of the results. PL spectra of n- and p-type GaAsBi samples measured between the
Fig. 2. Electron (blue solid line), hh (green solid line)/lh (red dashed line) wavefunctions, Fermi level (green dash-dotted line) obtained for n-type GaAsBi/AlGaAs (a) and p-type GaAsBi/AlGaAs (b) modulation doped QW structures. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
temperature range 30e300 K were given in Fig. 4a and b. PL spectra of n- and p-type samples show an asymmetric behaviour at low energies as seen in Fig. 4a and b. The observed asymmetry and steep fall of the PL peak in the high energy region, especially at low temperatures, indicates that there is a free excitonic transition occurring between subbands of the QWs. In order to understand the origin of PL, temperature dependence of PL has been investigated and to determine the type of optical transition energies, such as band-to-band involving e1-hh1 or band-tolocalised bismuth states located within the bandgap, a peak fit procedure was applied to the PL spectrum in the temperature range of 30e300 K by using the Fityk software [26]. For n- and p-type GaAsBi samples, the PL spectra obtained at 30 K are deconvoluted successfully by using two Gaussian peaks as seen in Fig. 5a and b accompanied with the experimental results. We assigned the first peak with higher energy to an e1-hh1 transition and the second peak to an e1-to-localised/trap state exciton transition. In the ntype sample, FWHM of localised exciton- and free exciton-related
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Table 1 Calculated optical transition energies between subbands and PL peaks at temperature 30 K. Sample
e1-hh1 free excitons (exp.) (eV)
e1-hh1 free excitons (theory) (eV)
Localised excitons (exp.) (eV)
e1-lh1 Exp. (eV)
e1-lh1 Theory (eV)
e1-hh3 Exp. (eV)
e1-hh3 Theory (eV)
n-GaAsBi p-GaAsBi n-GaAs p-GaAs
1.209 1.207 1.535 1.538
1.209 1.207 1.545 1.545
1.151 1.174 e e
e e 1.567 1.547
1.233 1.228 1.561 1.570
e e 1.587 e
1.322 1.238 1.582 1.571
Fig. 3. PL spectra of all the samples taken at 30 K. Black solid, dashed red, dotted green and blue dashed dot lines depict n-type GaAs, n-type GaAsBi, p-type GaAs and p-type GaAsBi samples results, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
peaks are ~110 nm and ~74 nm, respectively, whereas these values are ~60 nm and ~20 nm in the p-type sample. This is an indication of an optical transition originated by the recombination of the localised excitons at low temperatures, and the difference between the free exciton and localised states-related optical transition energies are 58 meV and 33 meV for n- and p-type GaAsBi samples, respectively. The smaller value of FWHM of localised excitonrelated PL and the smaller energy difference between free excitons and localised excitons-related optical transition are indications of the reduced density of the localised states in the p-type sample due to p-type doping. Since the localised excitonic transitions dominate the PL spectrum at low temperatures, it is also clear that, at 30 K, the free excitonic transition is more dominant on the PL for the p-type sample. Therefore, it can be concluded that thanks to the p-type doping, PL characteristic of the p-type sample rather exhibits an excitonic transition of e1-hh1 transition. On the other hand, the contribution of localised excitonic transition is quite effective on PL spectrum of the n-type GaAsBi sample. In PL spectra taken at 300 K, the PL peak shape of both n- and ptype samples are almost identical with their shapes and the PL peak energies. Since the high-temperature PL is dominated from e1-hh1, optical transition and the contribution from localised excitonrelated PL does no longer exist; the optical quality is slightly better for the n-type sample. It is seen from Fig. 4c that the PL intensity of the n-type GaAsBi sample is higher than that of the p-type GaAsBi sample at 300 K. The observation of almost the same PL characteristic for n- and p-type GaAsBi/AlGaAs samples as the temperature increases can be explained by the effect of traps. It is well-known that Bi incorporation into the GaAs lattice causes some deep levels, especially at around the mid-gap of GaAsBi. The nature of these traps can be electron or hole traps, and the density of the traps are quite high, and the mid-gap traps are active at high
temperatures with a quite high density [15], which is large enough to influence the radiative transition rate. Mooney et al. showed that n-type doping reduces the electron trap density [27], on the other hand, p-type doping does not affect the hole trap density [28]. In the light of these observations, at high temperatures, it can be expected that photo-generated carriers are captured by the traps and non-radiatively recombine in n- and p-type GaAsBi samples, therefore, the intensity of the PL peak approaches to each other. The observed higher PL intensity at 300 K for the n-type sample could be explained by reduced trap density in the n-type sample as observed in Ref. [27]. The observed high energy tail for both n- and p-type GaAsBi samples can be related to PL emission from the transitions of higher occupied states. Even the samples are excited by the same laser intensity at 30 K, and 300 K, some fraction of the photo-generated carriers are localised in the localised states and/or traps at 30 K. As temperature is increased, the photo-generated carriers gain thermal energy to occupy higher states, leading to PL from optical transition of higher states, therefore, at the same excitation power density PL spectrum at 300 K has a high energy tail for both samples and the characteristic of the PL is solely from e1-hh1 transition. In order to determine the effect of Bi on the bandgap of the samples, we also conducted PL measurements on the GaAs modulation doped reference samples. PL spectra of GaAs samples at 30 K are given in Fig. 6a and b. n-type GaAs reference sample has three distinct peaks, which are assigned to e1-hh1, e1-lh1 and e1hh3 free exciton transitions according to the calculations and they produce the shoulders of the spectrum at the high energy part. ptype GaAs reference sample has a sharp peak, which is assigned to e1-hh1 free exciton transition and the second peak at higher energy with much lower intensity can be due to e1-lh1 transition. For all samples, the calculated transition energies and PL peak energies separated by the fitting process at temperature 30 K are given in Table 1. For reference samples, measured optical transitions between higher subbands are in agreement with self-consistent so€dinger-Poisson equation. A similar contribution lutions of the Schro of high energetic excitons to a PL spectrum has also been observed in the literature for GaAs/AlGaAs heterostructures [29e31]. On the other hand, optical transitions involving excitons with higher energies have not been observed in reference samples at high temperatures due to the exponential decrease in the probability of optical transitions occurring across subbands with high energies. Although, we have observed higher subband transitions in the reference samples, only fundamental transitions are observed in GaAsBi samples, which can be explained by the presence of the Biinduced traps in GaAsBi samples. When some fraction of the photogenerated carriers are captured in GaAsBi samples, the higher subbands cannot be occupied, and no emission can be observed from higher subband transitions. Therefore, compared to GaAsBi samples, in GaAs samples, photo-generated carriers can occupy higher subbands, leading to the observation of radiative transitions involving higher levels at the same excitation power density. The peak corresponding to localised excitons in the n-type
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sample has 23 meV lower energy than p-type sample as seen in Table 1. This can be explained by the following; both alloy fluctuations and Bi-induced clusters form localised states. TEM images [14,32] have revealed that when the thickness of GaAsBi layer is thin enough, that is, in QW structures, the contribution of alloy fluctuations to the formation of localised states is negligible compared to the Bi-induced clusters. Since our samples are QW structures, we can consider that the more contribution comes from Bi-induced clusters. At 30 K, the energy difference between e1 and
Fig. 5. PL spectrum of n-type (a) and p-type (b) GaAsBi samples at 30 K. The black dashed curve corresponds to an individual peak obtained by fitting a Gaussian function; the red dashed curve indicates the theoretical spectrum obtained by superposing the separately obtained peaks and the blue curve is the experimental data. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Temperature-dependent PL spectra of n-type (a) and p-type (b) GaAsBi samples. Comparison of the PL spectra of n- and p-type GaAsBi samples measured at 30 K and 300 K (c).
VB maximum is ~1.190 eV and ~1.187 eV in n-type and p-type samples, respectively. When these values are compared with the fitted localised exciton-related peak energy values, it is found that localised levels are located at above 39 meV and 13 meV higher energy than the VB maximum of the n-type and p-type sample, respectively. So in the n-type sample, localised states are nearer to the CB compared to p-type, which causes a lower optical transition energy as depicted in Fig. 7b. It is seen from Table 1 that the optical transition energy is shifted by approximately ~80 meV/Bi% at 30 K by the incorporation of 4% Bi into GaAs. The change of the effective bandgap energy (e1-hh1 transition) with temperature was determined from the analysis of the PL spectra measured at different temperatures. The temperature dependence of the peak assigned to recombination of free e1-hh1 excitons are deduced from the fitting process, and the experimental data were given in Fig. 7a for n- and p-type samples. In the graphs given in Fig. 7a, it is seen that the temperature dependence of the PL peak energy of n-type sample has a prominent S-shaped change. The S-shape characteristic of temperature
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Fig. 6. PL spectrum of n-type (a) p-type (b) GaAs samples at T ¼ 30 K. The black dashed curve corresponds to an individual peak obtained by fitting a Gaussian function; the red dashed curve indicates the theoretical spectrum obtained by superposing the separately obtained peaks and the blue curve is the experimental data. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
dependent PL peak energy is a signature of the presence of localised states over VB, and it has been a well-known characteristic behaviour of highly-mismatched alloys [33e37]. Kini et al. [38] and Pettinari et al. [39,40] reported that Bi incorporation in GaAs introduces several acceptor-like traps above VB band edge and as Bi composition increases, some of these traps are suppressed. Furthermore, it has been reported that p-type doping of GaAsBi also has the similar effect [38]. Under the light of these experimental findings, we believe that p-type doping suppressed the some of the acceptor-like traps, resulting in the observation of e1-hh1 transition. Since the acceptor-like states cannot be suppressed in n-type samples, these states contribute to the observed lower energy PL and S-shape characteristic of PL peak energy in n-type GaAsBi sample. As temperature increases, in n-type GaAsBi, the captured electrons in acceptor-like traps are emitted and start to contribute to the e1-hh1 transition at around 160 K and above temperatures. Kini et al. [38] also reported the presence of a donor-like defect BiGa as a results of Bi incorporation, which causes to decrease in hole concentration. Even our samples are doped at the same amount of dopants, we have measured smaller hole density for p-type sample
Fig. 7. Temperature dependence of PL peak energies belonging to n-type (full symbols) and p-type (empty symbols) GaAsBi samples. Circle and up triangle, represents experimental data and 1st peak maxima determined by the fitting procedure, respectively (a). Band diagram of the GaAsBi QW structure, electron (blue solid line), hh (green solid line)/lh (red dashed line) levels, Fermi level (green dash-dotted line) €dingerand e1-hh1, e1-localised level transition energies found by the solution of Schro Poisson equation self-consistently at temperature 30 K. Bi-related localised level (black dash-dotted line) is 39 meV and 13 meV above the VB maximum in n- and p-type samples, respectively (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
by a factor of 10, which agrees with the results of Kini et al. and confirms the presence of a donor-like defect in the GaAsBi samples compensates acceptors. However, further experiments such as deep level spectroscopy are required to confirm the presence of acceptor-like and donor-like defects in the samples. In Fig. 8, we compare the ratio of the intensities of the peaks assigned to free and localised excitons obtained for n- and p-type samples. The temperature dependence of intensity ratio of the assigned free exciton peak (1st peak) to localised exciton peak (2nd peak) is given. For an n-type sample, at temperatures between 140 K and 160 K, the ratio becomes less than unity that is an indication of the localised excitons-related radiative recombination dominates over free exciton-related one. On the other hand, for ptype sample, the ratio is always higher than unity, which indicates that free exciton recombination is dominant in the range from 30 K
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Fig. 8. The intensity ratio of the fitted free and localised exciton peaks for n- and ptype GaAsBi samples.
to 300 K. Moreover, temperature dependence of FWHM for the ntype sample (Fig. 9) has a well-known characteristic of the PL spectrum containing localised exciton-related radiative recombination. On the other hand, as for the p-type sample, the temperature dependence of FWHM increases as expected. A similar phenomenon has also been observed in dilute nitride structures as can be referred to Refs. [33e37], which is explained by N-related disorder and localisation effects in the CB. In this study, similar localisation effect is observed in the VB, because Bi perturbates the VB edge prominently, which is in agreement with the VBAC model.
993
n-type samples, while in p-type samples, contribution of these states are suppressed by p-type doping. For this reason, the PL peak energies of the n-type sample were ~10 meV lower than that of the p-type sample. In order to investigate the excitation power density on the temperature dependence of PL peak energy of an n-type sample, we have taken PL spectrum at various excitation per intensities and we have observed that the S-shape characteristic can be suppressed at an excitation intensity of 38 W/cm2 (Fig. 10b). Similar behaviour is also observed in 4e5% Bi alloyed GaAsBi epilayer grown on GaAs [20]. The excitation intensity of 38 W/cm2 corresponds that a photon density of ~1 1019 cm2. Hence, localised state density for n-type GaAsBi might be approximated as 1 1019 cm2. Integrated PL (IPL) intensity gives information about the dominant recombination process. The carrier-density dependent total spontaneous recombination rate is radiative recombination rate plus non-radiative recombination rate (loss rate) and is expressed as in Eq. (5). The loss rate is composed of SRH recombination and Auger recombination rates. In order to determine the dominant recombination processes, IPL measurements were employed at various temperatures. The intensity of the IPL signal can be expressed in terms of photo-generated electron-hole pair density as [41,42].
4.2. Excitation power dependence of PL Excitation power-dependent PL measurements were carried out in order to have a better understanding the characteristic of optical transitions. The PL spectra of the n-type sample under various excitation intensities and the change of the PL peak energy for GaAsBi samples for the measurements taken at 30 K are given in Fig. 10a and b, respectively. The spectrum given in Fig. 10a has an asymmetric structure, which is also an indication of the presence localised states. As above-mentioned, the Bi-related acceptor-like states are empty in
Fig. 9. The FWHM of localised exciton peaks for n- and p-type GaAsBi samples.
Fig. 10. PL spectra of the n-type GaAsBi sample under various excitation intensities (a), the temperature dependence of the PL peak energy at different laser power densities (b).
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PIPL ¼ c An þ Bn2 þ Cn3
(5)
In this equation, n is the photo-generated carrier density, A is the SRH recombination coefficient, B is the radiative recombination coefficient, and C is the Auger recombination coefficient. If the excitation power is written as LIPL ¼ c1 Bn2 , then total intensity of the IPL signal can be written in terms of excitation power as the following, 1=2
shallow levels due to alloy fluctuations and Bi-related states such as Bi pair, triple and clusters. Therefore, dependence of carrier density of SRH in Eq. (6) can be in a more complex form, including additional possible transition-related terms, but modification of the SRH-related part of Eq. (6) is not a subject of this paper. Considering more complex nature of localised level-induced transitions in GaAsBi, in case of s < 1, we will also assume that the IPL is dominated by SRH recombination loss mechanism. Fig. 11 shows IPL as a function of excitation power density (photon density at top x-axis) at various temperatures. By using 1=2
3=2
PIPL ¼ APL LIPL þ BPL LIPL þ CPL LIPL
(6) 1=2
Eq. (6) can be written in a form of slogðLLp Þ and s refers to transition type. In the SRH recombination theory, non-radiative recombination is assisted by a single trap level located in the mid-gap of the semiconductor. It has been reported that GaAsBi has several kind of deep level traps such as AsGa, BiGa, VGa etc. as well as
slogðLILp Þ, we have found the slopes and determined the dominant recombination type. The slope values of 1, 2 and 3 corresponds to radiative, SRH and Auger recombination processes, respectively. At low temperatures under low excitation intensities, the slope is lower than 1, therefore, non-radiative SRH dominates recombination process for all samples. As the excitation power density increases, the slope is obtained to be ~2, which indicates radiative
Fig. 11. IPL versus excitation intensity (power density) plots of n-type GaAs (a), n-type GaAsBi (b), p-type GaAs (c), p-type GaAsBi (d) at various temperatures. The dominant transition type determined from the calculated slopes.
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recombination supresses the non-radiative recombination loss (SRH mechanism). At temperatures higher than 150 K and lower than 200 K, radiative recombination starts to dominate. For all samples except for n-type GaAs, at the temperatures above 200 K, the slope of the plots is higher than 2, which indicates that Auger effect also contributes to the recombination process. When p-type GaAs and p-type GaAsBi samples are compared, the effect of Auger recombination shows itself at higher temperatures for p-type GaAsBi, which is an indication of the suppression of Auger process due enhanced spin orbit split of energy with incorporation of bismuth. As for n-type GaAs, there is no contribution to recombination process from Auger process. On the other hand, for n-type GaAsBi, above 200 K under high excitation intensities, the slope of the curves in Fig. 11b becomes bigger than 2. The Auger process is more prominent at lower bandgap materials therefore the observation of Auger process in n-type GaAsBi, but not in n-type GaAs, can be related to reduced bandgap of n-type GaAsBi. Even the effective bandgap of the n- and p-type GaAsBi is almost the same, we obtained lower slope values for p-type GaAsBi due to larger spin-orbit split off energy of p-type GaAsBi. Since an S-shaped characteristic is observed for n-type GaAsBi sample (see Fig. 10b), in the S-shape region, IPL switches from SRH to radiative recombination, and the radiative recombination process becomes dominant at higher temperatures than that for p-type GaAsBi.
[3]
[4]
[5]
[6]
[7]
[8]
[9] [10]
[11]
5. Conclusion In this study, we have investigated optical properties of n- and p-type GaAsBi/AlGaAs single QW structures by using temperatureand excitation power-dependent PL and IPL measurements and compared the results with Bi-free n- and p-type QW structures to determine the effect of doping and bismuth. It has observed that ptype doping supresses the S-shaped temperature dependence. The bismuth incorporation cause about 80 meV/Bi% shift in the e1-hh1 transition. It has shown that by increasing optical excitation power density, S-shaped temperature dependence of the PL peak energy can be suppressed in the n-type sample. The recombination process has been observed to be dominated by non-radiative SRH mechanism at low temperatures under low excitation intensities for all Bifree and Bi-containing QWs. When excitation power density increases, radiative recombination becomes dominant over nonradiative recombination. Above 200 K, Auger effect is predominantly observed for n-type GaAsBi sample, because introducing Bi atoms into GaAs decreases bandgap of GaAsBi that increases Auger recombination rate for the n-type sample. Auger effect is also observed for p-type samples, but the effect of the Auger recombination is more effective for Bi-free sample. Due to the enhanced spin split-off band energy, with introducing Bi into GaAs, the probability of observing Auger effect for p-type GaAsBi sample decreases. Acknowledgments
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
This work is supported by The Scientific and Technical Research Council of Turkey (TUBITAK) under Grant No. 115F517 and supported by The Scientific Research Projects Coordination Unit of Istanbul University (Project Numbers FYL-2016-21993 and BEK2017-25763).
[23]
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