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Growth of dilute quaternary alloy InPNBi and its′ characterization
Journal Pre-proofs Growth of dilute quaternary alloy InPNBi and its‘ characterization Subhasis Das, Akant Sagar Sharma, Sanowar Alam Gazi, S. Dhar PII: DOI: Reference:
S0022-0248(20)30055-5 https://doi.org/10.1016/j.jcrysgro.2020.125532 CRYS 125532
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Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
25 November 2019 25 January 2020 27 January 2020
Please cite this article as: S. Das, A. Sagar Sharma, S. Alam Gazi, S. Dhar, Growth of dilute quaternary alloy InPNBi and its‘ characterization, Journal of Crystal Growth (2020), doi: https://doi.org/10.1016/j.jcrysgro. 2020.125532
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Growth of dilute quaternary alloy InPNBi and its` characterization Subhasis Das1,2*, Akant Sagar Sharma1,2, Sanowar Alam Gazi3 and S. Dhar2 1Centre
for Research in Nanoscience and Nanotechnology, University of Calcutta, JD Sector-2, Salt Lake City, Kolkata 700098, India 2Department
of Electronic Science, University of Calcutta, 92, APC Road, Kolkata 700009, India
3Department
of Electrical Engineering, Indian Institute of Technology-Bombay, Mumbai 400076, India
*Corresponding
Author Email Address: [email protected]
ABSTRACT InPNBi alloy semiconductor is grown for the first time on InP substrates by liquid phase epitaxy (LPE) technique by adding minute amounts of polycrystalline InN and Bi to the growth melt (upto 3 wt% of InN and 3 wt% of Bi in the melt). Energy dispersive X-ray (EDX) spectroscopy shows the presence of Bi and N in the material. Crystalline quality of the layer and the lattice contraction and dilation with respect to the substrate are demonstrated by high resolution X-Ray diffraction (HRXRD) measurements on layers containing N and Bi in different ratios. This result is qualitatively substantiated from the values of N and Bi contents obtained from the analysis of the X-ray photoelectron spectroscopy (XPS) measurements performed on each layer. XPS further provides details of the N and Bi bonding with In in the lattice. Raman spectroscopy measurements are done on the layers to investigate the different vibrational modes associated with the constituent elements and their bonds. 10K photoluminescence (PL) indicate band gap reduction in the alloy upto 63 meV due to the incorporation of N and Bi atoms at the P sites of the InP lattice.
Keywords: III-V semiconductor, Liquid Phase Epitaxy, InP, Dilute nitride-bismide.
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1. INTRODUCTION InP is an important member of the family of Group III-V compound semiconductors and the devices made from InP based materials like InGaAs, InGaP, InGaPN, InGaAsP play a major role in fiber optic communications, photovoltaics, high electron mobility transistors and other optoelectronic and electronic device applications [1-5]. In semiconductor device technology, there is always a continuous demand for novel semiconductor materials with properties suited for devices with enhanced performance in the required areas. Over the last two decades, new kinds of alloys, based on InP, have been introduced, such as, InPN [6,7] and InPBi [8,9]which are epitaxially grown by incorporating dilute amounts of isoelectronic impurity atoms (N or Bi) at the P sites in the host InP. Both N and Bi cause significant band gap reduction of the host material, but in a different way. While incorporation of N produces an isoelectronic state 1.79 eV above the conduction band edge of InP [10], Bi related isoelectronic states are formed 0.23 and 1.73 eV below the valence band edge of InP [11]. These isoelectronic states interact with the band states of the host InP lattice through a band anticrossing (BAC) [12] mechanism and results in a restructuring of the band, followed by a large band gap reduction in both InPN and InPBi [10, 11]. Although the band gap reduction in InPBi is relatively smaller as compared to that observed in InPN containing the same amount of N, the presence of Bi in InP makes the band gap less dependent on temperature which makes the material technologically attractive [13]. Incorporation of N or Bi causes a lattice contraction for InPN and a lattice dilation for InPBi. Hence codoping with N and Bi in right proportions will make it possible to grow the resultant InPNBi under exact lattice matched condition while achieving a large band gap reduction by the combined effects of N and Bi. A stress free InPNBi may find applications as photodetector material for the 1.55 μm optical communication and the 1 eV window layer for multilayer solar cells. Mascarenhas and co-workers [14] discussed the effect of codoping of III-V semiconductors with N and Bi on the physical properties of the resultant quaternary material. GaAsNBi is the mostly studied candidate in this field with increasing research interest [15-18]. Very recently theoretical studies of the band structure and the related properties of InPNBi alloy have been reported [19, 20] but as per our knowledge, till date there is no report of the growth of this material. In this paper, we report successful growth of InPNBi layers by liquid phase epitaxy (LPE) technique and discuss the actually observed properties of the same in details.
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2. MATERIALS GROWTH Growth of the InPNBi layers was done in a conventional horizontal sliding boat LPE reactor using high purity quartz tube and a semi-transparent gold coated furnace. 6N pure In metal (Alfa azile, France) was used as the solvent which was first baked at 750 0C for 15 hours under flowing Pd-diffused ultrapure hydrogen gas and then saturated with P by placing the same over a polycrystalline InP wafer at 650 0C for 1hr to form the growth melt. In the next step, required amount of 6N pure Bi (Alfa azile, France) was added to the melt and a further baking was done for 4 hr at 6800C with the melt covered by a tight fitting graphite cap in order to reduce the loss of P from the melt. Prior to the growth, polycrystalline InN (Sigma Aldrich) powder was added to the melt along with a polished and etched Fe-doped <001> semi-insulating InP substrate, placed in the substrate groove. The liquidus temperature of the melt was noted at around 6400C by visual observation through the transparent shield of the gold coated furnace. Three epitaxial layers of InPNBi with different N and Bi concentrations in the melt were grown under a melt super cooling of 8-100 C and a constant cooling ramp of 0.30C/min. Growth details for each layer are given in Table I.
Table 1. Details of the growth parameters for InPNBi samples. Sample name
Bi content in the melt
InN content in the melt
Growth temperature
(wt%)
(wt%)
(0C)
NB1
1
3
637
NB2
2
1
639
NB3
3
1
636
3
3. CHARACTERIZATION Thickness of the layers were determined from the cross sectional image of the layer-substrate interface in a scanning electron microscope (SEM). Crystalline quality of the layers were observed from high resolution X-ray diffraction (HRXRD) rocking curves which were produced by a RIGAKU Smartlab X-ray diffractometer using Cu kα1 radiation, generated under an accelerating voltage of 45 keV and a filament current of 200mA at 9 kW power. In order to get a highly monochromatic beam, a Ge (200) 4-bounce monochromator and a 0.114o parallel slit analyzer were used. The thickness of the InPNBi layers, used in these experiments were in the range 4-5 μm which is much larger than the critical thickness to have a pseudomorphic strained layer on InP substrate. So, the layers are completely relaxed. For relaxed layer asymmetric reflections result in a similar manner as that of symmetric reflections showing both substrate and layer peaks and hence, only symmetric reflections were measured. Composition of the layers were obtained from energy dispersive X-ray (EDX) spectroscopy (ZEISS EVO-MA 10). X ray photoemission spectroscopy (XPS) measurements on the samples were done in an AXIS Supra instrument (Kratos Analytical, UK), which uses a monochromatic Al k-alpha excitation source of 75W under a chamber pressure of 2.0×10-7 Pa. The XPS data were processed using CASAXPS software. Room temperature Raman spectroscopy was done in a Horiba Jobin Yvon instrument, using a HR800 - UV confocal micro-Raman spectrometer, with excitation source of 532 nm at 50mW power. Band gap and the defect related energy levels were investigated using photoluminescence (PL) measurements at 10 K. In PL measurements, the sample was mounted inside an APD cryogenics closed cycle helium cryostat and a diode pumped solid state laser (RGBLase LLC, USA) emitting at 532 nm wavelength with a maximum optical power of 50mW was used as the excitation source. Luminescence from the layer was dispersed in a 0.5m spectrometer (Acton Research Corporation, USA) and detected in a cooled InGaAs photodetector and a lock-in amplifier (Stanford Research System) combination.
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4. RESULTS AND DISCUSSIONS
Fig.1.EDX spectrum of InPNBi sample NB2 showing peaks due to the constituent elements.
The surface of each layer was mirror-smooth and shiny in appearance. Cross sectional SEM image exhibited abrupt planar interfaces between the epilayers and the substrate with thickness of the layers in the range of 4-5 μm. Typical EDX spectrum for a sample, in Fig. 1, shows distinct peaks representing the constituting elements and confirms the presence of N and Bi in the layer. HRXRD spectra for the three samples are shown in Fig.2. Each spectrum consisted of two sharp peaks. The peak at the origin on the ω-2θ axis is due to the reflection from the InP substrate, The position of the second peak is found to differ from sample to sample and is attributed to the InPNBi layer. For sample NB1, the layer peak is at a higher angle with respect to that for the substrate, indicating lattice contraction whereas the XRD peaks due to the samples NB2 and NB3 appear at angles less than that for the substrate peak and correspond to a lattice expansion. We will later show that such lattice contraction and expansion is a consequence of the N/Bi ratio in each layer which will determine whether the resultant strain on the material will be compressive or tensile.
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Fig.2. HRXRD curves for three InPNBi samples grown from melts containing different amounts of InN and Bi as indicated in Table 1. The N/Bi ratio for the samples, measured by XPS, are also indicated in the figure.
For a completely relaxed InPNBi layer on InP substrate, the lattice mismatch between the layer and the substrate were calculated using the relation, Mismatch =
aInPNBi ― aInP aInP
(1)
× 100%
For cubic crystals, like, InP and for reflection from the (004) plane, the lattice constant ‘a’ is related to the spacing between the reflecting planes ‘d’
dhkl =
a h2 + k2 + l2
(2) It is obtained from the corresponding Bragg relation for first order reflection as, (3)
2d Sinθ = λ
Using the values of θ from the XRD curve and the known value of λ = 1.5406, the values of aInPNBi were calculated and then using eq.(1), the lattice mismatch values for each layer were
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obtained which are put in Table 2 for comparison with similar data obtained from XPS, being discussed below. From an analysis of the XPS data, we obtained the atomic concentrations of the constituting elements in InP1-x-yNxBiy and from there the N mole fraction (x) and Bi mole fraction (y) were found and listed in Table II. The detailed analysis was done as follows. We consider that in InPNBi epitaxial layers, both N and Bi are isoelectronic with P and thus incorporate into the P site by forming In-N and In-Bi bonds. However there are also possibilities that both N and Bi are incorporated as antisites at the In sites as NIn and BiIn. In order to calculate the atomic concentrations of N and Bi in InPNBi, we need to extract the contribution of In-N bond from the N-1s spectra as well as that of the In-Bi bond from the Bi-4f spectra since only the substitutional N and Bi at the group V sublattice (P) are responsible for the modification of the band structure and the lattice constant [20]. In Fig.3, XPS core-line spectra of N-1s and Bi – 4f for the InPNBi sample NB2 are shown. The fitting of the N-1s spectrum involved three components, one In-N and two P-N bonds with different binding energies (BE) by considering the fact that the N bonding with an element, having higher electronegativity, has larger binding energy. The Bi-4f spectrum contains two spin-orbit (SO) components 7/2 and 5/2 for each of the In-Bi and the P-Bi bonds. Extraction of the In-N and In-Bi parts from the N-1s and the Bi-4f spectra were done by deconvolution of each spectrum by peak fitting using a Tougaard background and a series of Gaussian functions. The composition of the layers were determined from the peak areas of the XPS core spectra of the constituting elements, In (3d), P (2p), N (1s: In-N) and Bi (4f 7/2: In-Bi) and taking into account the atomic sensitivity factors.
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Fig.3. (a) N-1s spectrum with three peaks, related to In-N (BE: 396.76 eV) and P-N (BE: 398.4 eV and 400.1 eV). (b) Bi-4f 7/2 spectrum with peaks due to In-Bi (BE: 157.8 eV) and P-Bi (BE: 159.6 eV) of InPNBi epitaxial layer No. NB2.
In general, the lattice constant of a quaternary alloy like InP1-x-yNxBiy with dilute amount of N and Bi can be expressed as a linear relation using Vegard’s law [23] as,
aInP1 ― x ― yNxBiy = (1 ― x ― y)aInP +x aInN +y aInBi
(3)
The lattice constants of InN ( aInN) = 4.98 and InBi( aInBi) = 7.024 are taken from references [24, 25] and that of InP( aInP) = 5.867. By substituting the values of x and y, obtained from XPS (Table 2) in eqn. (3), we get the values of the lattice constants of NB1, NB2 and NB3 and use them to calculate the lattice mismatch of the layer with the InP substrate which are listed in Table 2. Table 2 also shows the lattice mismatch for the InPNBi/InP system, obtained from the HRXRD data of Fig.2 and the nice agreement between the values of lattice mismatch, obtained from the analysis of the XPS data and that from the XRD measurements is quite evident. The calculations further indicated that InPNBi layers can be grown lattice matched with the InP substrates for x/y ~ 1.27.
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Table 2. N and Bi mole fractions in LPE InP1-x-yNxBiy and the corresponding lattice mismatch with the substrate InP.
Sample
N mole
Bi mole
Lattice mismatch
Lattice mismatch
fraction (x)
fraction (y)
calculated from
calculated from
Vegard’s rule (%)
HRXRD(%)
NB1
0.0044
0.0010
-0.047
-0.052 (tensile)
NB2
0.0007
0.0029
0.046
0.049 (compressive)
NB3
0.0011
0.0039
0.060
0.061 (compressive)
From the data of Table 2, we find that in sample NB1, due to a larger N/Bi ratio, the lattice is expanded whereas for the other two samples, N/Bi ratio is much smaller and the effect of Bi is more prominent, giving rise to a compression of the lattice. This is in agreement with the XRD results of Fig.2, where the Bragg reflection peak for NB1 appears at a higher angle than the substrate peak in contrast to the XRD peaks due to samples NB2 and NB3 which are found at angles smaller than that for the substrate peak. We have further measured the XPS depth profile for the sample NB3 down to 0.4 μm, in order to investigate the distribution of the constituent elements below the surface of the epitaxial layer. The result is plotted in Fig.4 which shows that both N and Bi have uniform concentration along the depth of the layer.
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Fig. 4. XPS depth profile of the constituents of the InPNBi sample NB3.
Fig. 5(a) shows the Raman spectrum of sample NB3. Due to a simultaneous incorporation of N and Bi in the host InP, the spectrum is expected to have three different phonon modes (InP, InBi and InN like). Hence, for a better interpretation of the obtained results
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Fig. 5. (a) Raman shift observed in sample NB3, (b) Zone A (130-230 cm-1), (c) Zone B (250-400 cm-1), (d) Zone-C (400-600 cm-1). The peaks P1 to P10 are related to different vibrational modes in the material, discussed in the text.
we have subdivided the spectrum into three different zones namely A, B and C. Zone A (130 to 230 cm-1) of Fig. 5.(b) contains four different peaks named P1 (~148.7 cm-1), P2 (~162 cm-1), 11
P3 (~170 cm-1) and P4(~190 cm-1) . Peaks P1 and P3 are respectively attributed to the InBi like TO and LO phonon modes [26, 27]. The high resolution technique allowed us to identify the peaks P2 and P4 with InP like LA (L) and 2TA (W) modes, obtained at very low energy [27, 28, 29]. Zone B (250 cm-1 to 400 cm-1) in Fig. 5(c), shows three different peaks P5 (~304cm-1), P6 (~331.9 cm-1) and P7 (~365 cm-1). In polar semiconductors with high carrier concentrations (~1018/cm3), due to the relative displacement of the charged ions with respect to the electric field associated with the free carriers, the electric dipole moment between the LO phonon and the free carrier Plasmon interact and couple to form LO-phonon-Plasmon coupled mode (LOPC) [30]. The electron concentration in our InPNBi epitaxial layer is of the order of 1018cm-3, as obtained from Hall measurements, and hence we can expect to observe coupled Plasmon - phonon mode in the Raman Spectra of InPNBi. Peak P5, observed at ~ 304cm-1, may be related to a LOPC mode in conformity with the earlier results of Wei et.al [27]. Peak P6 and P7 are associated with InP like LO phonon modes [26, 27]. Finally, Zone C (400 cm-1 to 550 cm-1) indicates two strong peaks at 438 cm-1 and 489 cm-1, as shown in Fig. 5(d). In InPNBi quaternary systems, the bond length of In-Bi, In-P and In-N are 2.880Å, 2.541 Å and 2.14 Å respectively and therefore the bond strength of In-N is stronger than those of In-Bi and In-P, making the formation of the In-N bonds more likely. This is the reason for the higher intensity of the Raman peaks in Zone C which are due to InN like phonon modes. The peak P8 is identified with InN like vibrational mode which was earlier observed in LPE grown InGaAsN [31]. The second peak is deconvoluted into two peaks P9 (~ 488cm-1) and P10 (~ 495cm-1). Peak P9 is attributed to InN like E2 (high) [32] and peak P10 to the local vibrational mode (LVM) of N substituting for P (NP). Ulrici et al. [33] and Thompson et al.[34] observed this LVM of Np at ~ 496cm-1 in the Fourier transform infrared (FTIR) spectrum at 77K.
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Fig.6. 10 K PL spectrum of sample NB3 showing different peaks revealed by Gaussian deconvolution. Inset shows the energy positions of the peaks at different temperatures.
Fig.6 depicts the PL spectrum of the sample NB3, obtained at 10 K. The experimental PL spectrum is deconvoluted by using Gaussian peak fitting to get five distinct peaks A, B, C, D and E. Peak A at 1.426 eV represents emission from the InP substrate whereas the peak B at 1.393eV is due to the band edge emission from the InPNBi epilayer indicating reduction in band gap by 33 meV with respect to that of InP. Both peaks A and B follow the usual decrease in energy as the temperature increases above 10 K, shown in the inset of Fig.6. Peak C at 1.334 eV rapidly quenches above 50 K and might be due to Bi related defect complexes with P vacancy which has been reported recently and discussed in details [35]. The peak D at energy 1.26 eV represents electronic transitions from a deep level, situated 130 meV below the band edge, and may be a consequence of defects related to N. Peak E at 1.16 eV originates due to the P vacancy in the layer and was earlier designated as the V-band in InP [36]. We have also studied the PL for the sample NB1 which showed similar features though the band gap reduction for this layer was obtained as 63 meV which is substantially larger than 33 meV, found for sample NB3. This is explained considering the relatively larger incorporation of N in sample NB1 compared to that in sample NB3 and that N is known to produce a much larger band gap reduction in InP than that obtained for Bi incorporation.
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SUMMARY AND CONCLUSIONS In summary, we have shown that quaternary InPNBi epilayers can be grown by LPE with varying amounts of N and Bi in the material. The presences of N and Bi in the layers are confirmed by EDX analysis. Crystalline quality of the layers are investigated using HRXRD which further shows that either lattice compression or lattice expansion may occur in the layer, with respect to the InP substrate, depending on the relative proportions of N and Bi in the solid. XPS analysis is done in order to determine the amount of N and Bi in the material and also to investigate the occupancy of these two atoms at different sub-lattice sites. Raman spectroscopy indicated InBi like LO and TO phonon modes and InN like vibrational modes. PL spectroscopy indicated that the net band gap reduction produced in the material is a function of the relative proportions of N and Bi contents.
ACKNOWLEDGEMENTS The authors are thankful to the Centre For Research in Nanoscience and Nanotechnology (CRNN), University of Calcutta for SEM and HRXRD measurements and also to the Central Surface Analytical Facility and the Centre for Research in Nanotechnology and Science (CRNTS) of Indian Institute of Technology Bombay for the XPS and the Raman spectroscopy measurements.
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CrediT author statement Subhasis Das: Conceptualization, Methodology, Investigation, Formal analysis, Writing- Original draft preparation. Akant Sagar Sharma: Investigation, Validation. Sanowar Alam Gazi: Resources. Sunanda Dhar: Supervision,
Writing- Reviewing and Editing.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Subhasis Das Akant Sagar Sharma Sanowar Alam Gazi S. Dhar
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Highlights:
Reports the growth of quaternary InPNBi on InP by liquid phase epitaxy
Incorporation of both N and Bi shows large band gap reduction in InPNBi
Lattice mismatch of layer-substrate can be removed by combination of N and Bi
20
S0022-0248(20)30055-5 https://doi.org/10.1016/j.jcrysgro.2020.125532 CRYS 125532
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
25 November 2019 25 January 2020 27 January 2020
Please cite this article as: S. Das, A. Sagar Sharma, S. Alam Gazi, S. Dhar, Growth of dilute quaternary alloy InPNBi and its‘ characterization, Journal of Crystal Growth (2020), doi: https://doi.org/10.1016/j.jcrysgro. 2020.125532
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Growth of dilute quaternary alloy InPNBi and its` characterization Subhasis Das1,2*, Akant Sagar Sharma1,2, Sanowar Alam Gazi3 and S. Dhar2 1Centre
for Research in Nanoscience and Nanotechnology, University of Calcutta, JD Sector-2, Salt Lake City, Kolkata 700098, India 2Department
of Electronic Science, University of Calcutta, 92, APC Road, Kolkata 700009, India
3Department
of Electrical Engineering, Indian Institute of Technology-Bombay, Mumbai 400076, India
*Corresponding
Author Email Address: [email protected]
ABSTRACT InPNBi alloy semiconductor is grown for the first time on InP substrates by liquid phase epitaxy (LPE) technique by adding minute amounts of polycrystalline InN and Bi to the growth melt (upto 3 wt% of InN and 3 wt% of Bi in the melt). Energy dispersive X-ray (EDX) spectroscopy shows the presence of Bi and N in the material. Crystalline quality of the layer and the lattice contraction and dilation with respect to the substrate are demonstrated by high resolution X-Ray diffraction (HRXRD) measurements on layers containing N and Bi in different ratios. This result is qualitatively substantiated from the values of N and Bi contents obtained from the analysis of the X-ray photoelectron spectroscopy (XPS) measurements performed on each layer. XPS further provides details of the N and Bi bonding with In in the lattice. Raman spectroscopy measurements are done on the layers to investigate the different vibrational modes associated with the constituent elements and their bonds. 10K photoluminescence (PL) indicate band gap reduction in the alloy upto 63 meV due to the incorporation of N and Bi atoms at the P sites of the InP lattice.
Keywords: III-V semiconductor, Liquid Phase Epitaxy, InP, Dilute nitride-bismide.
1
1. INTRODUCTION InP is an important member of the family of Group III-V compound semiconductors and the devices made from InP based materials like InGaAs, InGaP, InGaPN, InGaAsP play a major role in fiber optic communications, photovoltaics, high electron mobility transistors and other optoelectronic and electronic device applications [1-5]. In semiconductor device technology, there is always a continuous demand for novel semiconductor materials with properties suited for devices with enhanced performance in the required areas. Over the last two decades, new kinds of alloys, based on InP, have been introduced, such as, InPN [6,7] and InPBi [8,9]which are epitaxially grown by incorporating dilute amounts of isoelectronic impurity atoms (N or Bi) at the P sites in the host InP. Both N and Bi cause significant band gap reduction of the host material, but in a different way. While incorporation of N produces an isoelectronic state 1.79 eV above the conduction band edge of InP [10], Bi related isoelectronic states are formed 0.23 and 1.73 eV below the valence band edge of InP [11]. These isoelectronic states interact with the band states of the host InP lattice through a band anticrossing (BAC) [12] mechanism and results in a restructuring of the band, followed by a large band gap reduction in both InPN and InPBi [10, 11]. Although the band gap reduction in InPBi is relatively smaller as compared to that observed in InPN containing the same amount of N, the presence of Bi in InP makes the band gap less dependent on temperature which makes the material technologically attractive [13]. Incorporation of N or Bi causes a lattice contraction for InPN and a lattice dilation for InPBi. Hence codoping with N and Bi in right proportions will make it possible to grow the resultant InPNBi under exact lattice matched condition while achieving a large band gap reduction by the combined effects of N and Bi. A stress free InPNBi may find applications as photodetector material for the 1.55 μm optical communication and the 1 eV window layer for multilayer solar cells. Mascarenhas and co-workers [14] discussed the effect of codoping of III-V semiconductors with N and Bi on the physical properties of the resultant quaternary material. GaAsNBi is the mostly studied candidate in this field with increasing research interest [15-18]. Very recently theoretical studies of the band structure and the related properties of InPNBi alloy have been reported [19, 20] but as per our knowledge, till date there is no report of the growth of this material. In this paper, we report successful growth of InPNBi layers by liquid phase epitaxy (LPE) technique and discuss the actually observed properties of the same in details.
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2. MATERIALS GROWTH Growth of the InPNBi layers was done in a conventional horizontal sliding boat LPE reactor using high purity quartz tube and a semi-transparent gold coated furnace. 6N pure In metal (Alfa azile, France) was used as the solvent which was first baked at 750 0C for 15 hours under flowing Pd-diffused ultrapure hydrogen gas and then saturated with P by placing the same over a polycrystalline InP wafer at 650 0C for 1hr to form the growth melt. In the next step, required amount of 6N pure Bi (Alfa azile, France) was added to the melt and a further baking was done for 4 hr at 6800C with the melt covered by a tight fitting graphite cap in order to reduce the loss of P from the melt. Prior to the growth, polycrystalline InN (Sigma Aldrich) powder was added to the melt along with a polished and etched Fe-doped <001> semi-insulating InP substrate, placed in the substrate groove. The liquidus temperature of the melt was noted at around 6400C by visual observation through the transparent shield of the gold coated furnace. Three epitaxial layers of InPNBi with different N and Bi concentrations in the melt were grown under a melt super cooling of 8-100 C and a constant cooling ramp of 0.30C/min. Growth details for each layer are given in Table I.
Table 1. Details of the growth parameters for InPNBi samples. Sample name
Bi content in the melt
InN content in the melt
Growth temperature
(wt%)
(wt%)
(0C)
NB1
1
3
637
NB2
2
1
639
NB3
3
1
636
3
3. CHARACTERIZATION Thickness of the layers were determined from the cross sectional image of the layer-substrate interface in a scanning electron microscope (SEM). Crystalline quality of the layers were observed from high resolution X-ray diffraction (HRXRD) rocking curves which were produced by a RIGAKU Smartlab X-ray diffractometer using Cu kα1 radiation, generated under an accelerating voltage of 45 keV and a filament current of 200mA at 9 kW power. In order to get a highly monochromatic beam, a Ge (200) 4-bounce monochromator and a 0.114o parallel slit analyzer were used. The thickness of the InPNBi layers, used in these experiments were in the range 4-5 μm which is much larger than the critical thickness to have a pseudomorphic strained layer on InP substrate. So, the layers are completely relaxed. For relaxed layer asymmetric reflections result in a similar manner as that of symmetric reflections showing both substrate and layer peaks and hence, only symmetric reflections were measured. Composition of the layers were obtained from energy dispersive X-ray (EDX) spectroscopy (ZEISS EVO-MA 10). X ray photoemission spectroscopy (XPS) measurements on the samples were done in an AXIS Supra instrument (Kratos Analytical, UK), which uses a monochromatic Al k-alpha excitation source of 75W under a chamber pressure of 2.0×10-7 Pa. The XPS data were processed using CASAXPS software. Room temperature Raman spectroscopy was done in a Horiba Jobin Yvon instrument, using a HR800 - UV confocal micro-Raman spectrometer, with excitation source of 532 nm at 50mW power. Band gap and the defect related energy levels were investigated using photoluminescence (PL) measurements at 10 K. In PL measurements, the sample was mounted inside an APD cryogenics closed cycle helium cryostat and a diode pumped solid state laser (RGBLase LLC, USA) emitting at 532 nm wavelength with a maximum optical power of 50mW was used as the excitation source. Luminescence from the layer was dispersed in a 0.5m spectrometer (Acton Research Corporation, USA) and detected in a cooled InGaAs photodetector and a lock-in amplifier (Stanford Research System) combination.
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4. RESULTS AND DISCUSSIONS
Fig.1.EDX spectrum of InPNBi sample NB2 showing peaks due to the constituent elements.
The surface of each layer was mirror-smooth and shiny in appearance. Cross sectional SEM image exhibited abrupt planar interfaces between the epilayers and the substrate with thickness of the layers in the range of 4-5 μm. Typical EDX spectrum for a sample, in Fig. 1, shows distinct peaks representing the constituting elements and confirms the presence of N and Bi in the layer. HRXRD spectra for the three samples are shown in Fig.2. Each spectrum consisted of two sharp peaks. The peak at the origin on the ω-2θ axis is due to the reflection from the InP substrate, The position of the second peak is found to differ from sample to sample and is attributed to the InPNBi layer. For sample NB1, the layer peak is at a higher angle with respect to that for the substrate, indicating lattice contraction whereas the XRD peaks due to the samples NB2 and NB3 appear at angles less than that for the substrate peak and correspond to a lattice expansion. We will later show that such lattice contraction and expansion is a consequence of the N/Bi ratio in each layer which will determine whether the resultant strain on the material will be compressive or tensile.
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Fig.2. HRXRD curves for three InPNBi samples grown from melts containing different amounts of InN and Bi as indicated in Table 1. The N/Bi ratio for the samples, measured by XPS, are also indicated in the figure.
For a completely relaxed InPNBi layer on InP substrate, the lattice mismatch between the layer and the substrate were calculated using the relation, Mismatch =
aInPNBi ― aInP aInP
(1)
× 100%
For cubic crystals, like, InP and for reflection from the (004) plane, the lattice constant ‘a’ is related to the spacing between the reflecting planes ‘d’
dhkl =
a h2 + k2 + l2
(2) It is obtained from the corresponding Bragg relation for first order reflection as, (3)
2d Sinθ = λ
Using the values of θ from the XRD curve and the known value of λ = 1.5406, the values of aInPNBi were calculated and then using eq.(1), the lattice mismatch values for each layer were
6
obtained which are put in Table 2 for comparison with similar data obtained from XPS, being discussed below. From an analysis of the XPS data, we obtained the atomic concentrations of the constituting elements in InP1-x-yNxBiy and from there the N mole fraction (x) and Bi mole fraction (y) were found and listed in Table II. The detailed analysis was done as follows. We consider that in InPNBi epitaxial layers, both N and Bi are isoelectronic with P and thus incorporate into the P site by forming In-N and In-Bi bonds. However there are also possibilities that both N and Bi are incorporated as antisites at the In sites as NIn and BiIn. In order to calculate the atomic concentrations of N and Bi in InPNBi, we need to extract the contribution of In-N bond from the N-1s spectra as well as that of the In-Bi bond from the Bi-4f spectra since only the substitutional N and Bi at the group V sublattice (P) are responsible for the modification of the band structure and the lattice constant [20]. In Fig.3, XPS core-line spectra of N-1s and Bi – 4f for the InPNBi sample NB2 are shown. The fitting of the N-1s spectrum involved three components, one In-N and two P-N bonds with different binding energies (BE) by considering the fact that the N bonding with an element, having higher electronegativity, has larger binding energy. The Bi-4f spectrum contains two spin-orbit (SO) components 7/2 and 5/2 for each of the In-Bi and the P-Bi bonds. Extraction of the In-N and In-Bi parts from the N-1s and the Bi-4f spectra were done by deconvolution of each spectrum by peak fitting using a Tougaard background and a series of Gaussian functions. The composition of the layers were determined from the peak areas of the XPS core spectra of the constituting elements, In (3d), P (2p), N (1s: In-N) and Bi (4f 7/2: In-Bi) and taking into account the atomic sensitivity factors.
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Fig.3. (a) N-1s spectrum with three peaks, related to In-N (BE: 396.76 eV) and P-N (BE: 398.4 eV and 400.1 eV). (b) Bi-4f 7/2 spectrum with peaks due to In-Bi (BE: 157.8 eV) and P-Bi (BE: 159.6 eV) of InPNBi epitaxial layer No. NB2.
In general, the lattice constant of a quaternary alloy like InP1-x-yNxBiy with dilute amount of N and Bi can be expressed as a linear relation using Vegard’s law [23] as,
aInP1 ― x ― yNxBiy = (1 ― x ― y)aInP +x aInN +y aInBi
(3)
The lattice constants of InN ( aInN) = 4.98 and InBi( aInBi) = 7.024 are taken from references [24, 25] and that of InP( aInP) = 5.867. By substituting the values of x and y, obtained from XPS (Table 2) in eqn. (3), we get the values of the lattice constants of NB1, NB2 and NB3 and use them to calculate the lattice mismatch of the layer with the InP substrate which are listed in Table 2. Table 2 also shows the lattice mismatch for the InPNBi/InP system, obtained from the HRXRD data of Fig.2 and the nice agreement between the values of lattice mismatch, obtained from the analysis of the XPS data and that from the XRD measurements is quite evident. The calculations further indicated that InPNBi layers can be grown lattice matched with the InP substrates for x/y ~ 1.27.
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Table 2. N and Bi mole fractions in LPE InP1-x-yNxBiy and the corresponding lattice mismatch with the substrate InP.
Sample
N mole
Bi mole
Lattice mismatch
Lattice mismatch
fraction (x)
fraction (y)
calculated from
calculated from
Vegard’s rule (%)
HRXRD(%)
NB1
0.0044
0.0010
-0.047
-0.052 (tensile)
NB2
0.0007
0.0029
0.046
0.049 (compressive)
NB3
0.0011
0.0039
0.060
0.061 (compressive)
From the data of Table 2, we find that in sample NB1, due to a larger N/Bi ratio, the lattice is expanded whereas for the other two samples, N/Bi ratio is much smaller and the effect of Bi is more prominent, giving rise to a compression of the lattice. This is in agreement with the XRD results of Fig.2, where the Bragg reflection peak for NB1 appears at a higher angle than the substrate peak in contrast to the XRD peaks due to samples NB2 and NB3 which are found at angles smaller than that for the substrate peak. We have further measured the XPS depth profile for the sample NB3 down to 0.4 μm, in order to investigate the distribution of the constituent elements below the surface of the epitaxial layer. The result is plotted in Fig.4 which shows that both N and Bi have uniform concentration along the depth of the layer.
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Fig. 4. XPS depth profile of the constituents of the InPNBi sample NB3.
Fig. 5(a) shows the Raman spectrum of sample NB3. Due to a simultaneous incorporation of N and Bi in the host InP, the spectrum is expected to have three different phonon modes (InP, InBi and InN like). Hence, for a better interpretation of the obtained results
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Fig. 5. (a) Raman shift observed in sample NB3, (b) Zone A (130-230 cm-1), (c) Zone B (250-400 cm-1), (d) Zone-C (400-600 cm-1). The peaks P1 to P10 are related to different vibrational modes in the material, discussed in the text.
we have subdivided the spectrum into three different zones namely A, B and C. Zone A (130 to 230 cm-1) of Fig. 5.(b) contains four different peaks named P1 (~148.7 cm-1), P2 (~162 cm-1), 11
P3 (~170 cm-1) and P4(~190 cm-1) . Peaks P1 and P3 are respectively attributed to the InBi like TO and LO phonon modes [26, 27]. The high resolution technique allowed us to identify the peaks P2 and P4 with InP like LA (L) and 2TA (W) modes, obtained at very low energy [27, 28, 29]. Zone B (250 cm-1 to 400 cm-1) in Fig. 5(c), shows three different peaks P5 (~304cm-1), P6 (~331.9 cm-1) and P7 (~365 cm-1). In polar semiconductors with high carrier concentrations (~1018/cm3), due to the relative displacement of the charged ions with respect to the electric field associated with the free carriers, the electric dipole moment between the LO phonon and the free carrier Plasmon interact and couple to form LO-phonon-Plasmon coupled mode (LOPC) [30]. The electron concentration in our InPNBi epitaxial layer is of the order of 1018cm-3, as obtained from Hall measurements, and hence we can expect to observe coupled Plasmon - phonon mode in the Raman Spectra of InPNBi. Peak P5, observed at ~ 304cm-1, may be related to a LOPC mode in conformity with the earlier results of Wei et.al [27]. Peak P6 and P7 are associated with InP like LO phonon modes [26, 27]. Finally, Zone C (400 cm-1 to 550 cm-1) indicates two strong peaks at 438 cm-1 and 489 cm-1, as shown in Fig. 5(d). In InPNBi quaternary systems, the bond length of In-Bi, In-P and In-N are 2.880Å, 2.541 Å and 2.14 Å respectively and therefore the bond strength of In-N is stronger than those of In-Bi and In-P, making the formation of the In-N bonds more likely. This is the reason for the higher intensity of the Raman peaks in Zone C which are due to InN like phonon modes. The peak P8 is identified with InN like vibrational mode which was earlier observed in LPE grown InGaAsN [31]. The second peak is deconvoluted into two peaks P9 (~ 488cm-1) and P10 (~ 495cm-1). Peak P9 is attributed to InN like E2 (high) [32] and peak P10 to the local vibrational mode (LVM) of N substituting for P (NP). Ulrici et al. [33] and Thompson et al.[34] observed this LVM of Np at ~ 496cm-1 in the Fourier transform infrared (FTIR) spectrum at 77K.
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Fig.6. 10 K PL spectrum of sample NB3 showing different peaks revealed by Gaussian deconvolution. Inset shows the energy positions of the peaks at different temperatures.
Fig.6 depicts the PL spectrum of the sample NB3, obtained at 10 K. The experimental PL spectrum is deconvoluted by using Gaussian peak fitting to get five distinct peaks A, B, C, D and E. Peak A at 1.426 eV represents emission from the InP substrate whereas the peak B at 1.393eV is due to the band edge emission from the InPNBi epilayer indicating reduction in band gap by 33 meV with respect to that of InP. Both peaks A and B follow the usual decrease in energy as the temperature increases above 10 K, shown in the inset of Fig.6. Peak C at 1.334 eV rapidly quenches above 50 K and might be due to Bi related defect complexes with P vacancy which has been reported recently and discussed in details [35]. The peak D at energy 1.26 eV represents electronic transitions from a deep level, situated 130 meV below the band edge, and may be a consequence of defects related to N. Peak E at 1.16 eV originates due to the P vacancy in the layer and was earlier designated as the V-band in InP [36]. We have also studied the PL for the sample NB1 which showed similar features though the band gap reduction for this layer was obtained as 63 meV which is substantially larger than 33 meV, found for sample NB3. This is explained considering the relatively larger incorporation of N in sample NB1 compared to that in sample NB3 and that N is known to produce a much larger band gap reduction in InP than that obtained for Bi incorporation.
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SUMMARY AND CONCLUSIONS In summary, we have shown that quaternary InPNBi epilayers can be grown by LPE with varying amounts of N and Bi in the material. The presences of N and Bi in the layers are confirmed by EDX analysis. Crystalline quality of the layers are investigated using HRXRD which further shows that either lattice compression or lattice expansion may occur in the layer, with respect to the InP substrate, depending on the relative proportions of N and Bi in the solid. XPS analysis is done in order to determine the amount of N and Bi in the material and also to investigate the occupancy of these two atoms at different sub-lattice sites. Raman spectroscopy indicated InBi like LO and TO phonon modes and InN like vibrational modes. PL spectroscopy indicated that the net band gap reduction produced in the material is a function of the relative proportions of N and Bi contents.
ACKNOWLEDGEMENTS The authors are thankful to the Centre For Research in Nanoscience and Nanotechnology (CRNN), University of Calcutta for SEM and HRXRD measurements and also to the Central Surface Analytical Facility and the Centre for Research in Nanotechnology and Science (CRNTS) of Indian Institute of Technology Bombay for the XPS and the Raman spectroscopy measurements.
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CrediT author statement Subhasis Das: Conceptualization, Methodology, Investigation, Formal analysis, Writing- Original draft preparation. Akant Sagar Sharma: Investigation, Validation. Sanowar Alam Gazi: Resources. Sunanda Dhar: Supervision,
Writing- Reviewing and Editing.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Subhasis Das Akant Sagar Sharma Sanowar Alam Gazi S. Dhar
19
Highlights:
Reports the growth of quaternary InPNBi on InP by liquid phase epitaxy
Incorporation of both N and Bi shows large band gap reduction in InPNBi
Lattice mismatch of layer-substrate can be removed by combination of N and Bi
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