GaAsBi light emitting diodes containing ∼6% Bi are grown on GaAs substrates. Good room-temperature electroluminescence spectra are obtained at current densities as low as 8 Acm − 2. Measurements of the integrated emitted luminescence suggest that there is a continuum of localised Bi states extending up to 75 meV into the bandgap, which is in good agreement with previous photoluminescence studies. X-ray diffraction analysis shows that strain relaxation has probably occurred in the thicker samples grown in this study

Abdul Rahman Mohmad

Batool Z.

Broderick C.A.

Buyanova I.A.

Christopher Jack Hunter

Faebian Bastiman

Feng G.

Gu Y.

Hasbullah N.F.

Hossain N.

John Paul R. David

Lewis R.B.

Ludewig P.

Mazur Y.I.

Mohmad A.R.

Robert Douglas Richards

Svensson S.P.

Richards, R.D.

Hunter, C.J.

Bastiman, F.

Mohmad, A.R.

David, J.P.R.

White Rose Research Online

IET OptoelectronicsSelected Papers from SIOE 2015Telecommunication wavelength GaAsBi lightemitting diodesIET Optoelectron., 201634 This is an open access article published by the IET under the Creative Commons Attribution License (htISSN 1751-8768Received on 2nd June 2015Revised on 20th July 2015Accepted on 30th July 2015doi: 10.1049/iet-opt.2015.0051www.ietdl.orgRobert Douglas Richards1 ✉, Christopher Jack Hunter1, Faebian Bastiman1,Abdul Rahman Mohmad1,2, John Paul R. David11Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield, UK2Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor, Malaysia✉ E-mail: r.richards@sheffield.ac.ukAbstract: GaAsBi light emitting diodes containing ∼6% Bi are grown on GaAs substrates. Good room-temperatureelectroluminescence spectra are obtained at current densities as low as 8 Acm−2. Measurements of the integratedemitted luminescence suggest that there is a continuum of localised Bi states extending up to 75 meV into thebandgap, which is in good agreement with previous photoluminescence studies. X-ray diffraction analysis shows thatstrain relaxation has probably occurred in the thicker samples grown in this study.1 IntroductionOver the past decade there has been interest in incorporating bismuthinto conventional III–V compounds. GaAsBi is the most common ofthese; however, more recently, other compounds such as InAsBi [1],InGaAsBi [2, 3], InPBi [4], and GaSbBi [5] have been reported.Incorporating bismuth leads to a reduction in the bandgap of thealloy and also increases the spin-orbit splitting energy of the alloy,which offers the potential to reduce Auger recombination in laserswhen ESO > Eg [6]. This has been shown to occur for Biconcentrations above 10% [7] in GaAsBi.Light emitting diodes (LEDs) containing GaAsBi active regionshave previously been grown by molecular beam epitaxy (MBE)[8]. The bismuth content was 1.8% and room-temperatureemission from the GaAsBi layer was observed at a wavelength of987 nm. Pressure-dependent measurements on similar samplesshowed evidence of carrier leakage into the GaAs barriers and alsosuggested that the GaAsBi/GaAs heterojunction is type I [9]. Morerecently, a single quantum well GaAsBi/(Al)GaAs laser operatingunder electrical injection has been grown by metal-organic vapourphase epitaxy (MOVPE) [10].For communications applications, lasers/LEDs operating at 1.3 or1.55 μm are desirable. To fully understand such devices, the bulkluminescence properties of GaAsBi (without the quantum effectsproduced by quantum wells or dots) must be investigated. In thispaper, we characterise p–i–n diode structures, containing bulk GaAsBilayers with ∼6% Bi, which act as LEDs operating at about 1.2 μm.2 Experimental methodsAll samples were grown on an Omicron MBE-scanning tunnellingmicroscopy (MBE-STM) system. The MBE chamber is fitted witheffusion cells for Al, Ga, In, and Bi, as well as a Be/Si dualdopant cell. The growth of the samples analysed during this workis described elsewhere [11]. Fig. 1 shows the structuresdiagrammatically. The thin GaAs spacer layers were included inan attempt to allow the Bi on the growing surface to desorb,preventing the formation of Bi droplets. For electricalcharacterisation, the samples were fabricated into 200 μm radiusmesa diodes using standard fabrication techniques.Small sections of each sample were mounted on TO5 headers inorder to carry out electroluminescence measurements. Theelectroluminescence (EL) from the samples was dispersed by amonochromator and detected using a liquid-nitrogen cooledgermanium detector. Low-temperature measurements were carriedout in a cryostat which was cooled by a closed-cycle heliumcompressor.The ω–2θ X-ray diffraction (XRD) measurements throughout thiswork were taken on a Bruker D8 Discover XRD machine usingCu kα1 radiation.3 Results and discussion3.1 Initial sample characterisationThe initial room-temperature EL spectra taken from all three diodesare shown in Fig. 1. As the GaAsBi region thickness is increasedfrom 100 to 200 nm, there is a reduction of 40% in the peak ELintensity. This is consistent with a doubling of the number ofdefects per unit area as the GaAsBi region thickness is doubled.However, when the GaAsBi region thickness is increased to350 nm (a further factor of 1.75), the peak EL intensity is reducedby 85%. This dramatic reduction in EL intensity suggests that R7has seen a significant increase in non-radiative processes and mayindicate that it has undergone significant strain relaxation.Further evidence for this is shown in the symmetric 004 XRDspectra, previously published in [11] and shown in Fig. 2. Thespectra show a tensile peak next to the substrate that becomesincreasingly prominent with increasing GaAsBi thickness. Thissuggests that as the GaAsBi layer thickness is increased, itbecomes increasingly strain relaxed; the upper GaAs claddinglayers are then grown in tensile strain on top of this layer.Owing to the weak EL signal from R7, the remaining discussionfocuses solely on R2 and R4.3.2 Room-temperature electroluminescenceRoom-temperature EL spectra taken from both samples as a functionof injection current are shown in Fig. 3. The peak emissionwavelength was about 1.2 μm in both cases, suggesting a Bicontent of ∼6% in both samples. Emission was observed atinjection current densities as low as 8 Acm−2, which is lower thanthe lowest injection current density used by Lewis et al. [8](50 Acm−2) or by Hossain et al. [9] (25 Acm−2) on similar diodes.In these spectra, a peak at ∼880 nm is observed, which is probablyemission from the GaAs cladding layers. Emission around this, Vol. 10, Iss. 2, pp. 34–38tp://creativecommons.org/licenses/by/3.0/)Fig. 1 Sample structures grown during this work (the structure pictured is that of R4). Also shown are the room-temperature EL spectra of the diodes, usinginjection currents of 80 Acm−2wavelength was also observed in the electroluminescence spectra ofthe LEDs in [8]; pressure-dependent measurements performed onsimilar samples [9] also showed the presence of carrier leakageinto the GaAs cladding.3.3 Low-temperature electroluminescenceEL measurements were also carried out at 20 K using a range ofinjection currents. The spectra from R2 and R4 are shown inFig. 4. The spectra have an asymmetric shape, and the peaksblue-shift slightly with increasing injection current (∼25 meV foran order of magnitude change in current). This behaviour can beexplained by the presence of disorder and Bi clustering, whichgive rise to localised states within the bandgap [12, 13]. At lowtemperatures, holes fall into these states and do not have sufficientthermal energy to escape, leading to the formation of localisedexcitons. The energy of the luminescence peak is thus less thanthe bandgap energy. As the carrier density (in this case, theinjection current) increases, the localised states become filled andthe luminescence peak shifts to higher energies [12–15].The spectra all show a low energy, exponential tail. Low-energytails in luminescence spectra have been observed for GaAsBi [16,17], and also for other disordered materials such as (In)GaAsNFig. 2 004 XRD spectra from the diodes grown in this work. Datapreviously published in [11]IET Optoelectron., 2016, Vol. 10, Iss. 2, pp. 34–38This is an open access article published by the IET under the Creativelicenses/by/3.0/)[18]. They are attributed to recombination of excitons trapped inlocalised states within the bandgap. In dilute nitrides these statesare close to the conduction band, whereas for bismides, they areclose to the valence band.The low-temperature spectra shown in Fig. 4 show a significantlyattenuated GaAs peak. This suggests that the GaAs peak is thermallyactivated; it is possibly due to carriers escaping from the GaAsBi andrecombining in the GaAs cladding layers. The small conductionband offsets in GaAsBi would suggest that the escaping carriersare electrons.3.4 Integrated emitted luminescenceThe dominant carrier recombination mechanisms in the GaAsBilayers were analysed by recording the integrated emittedluminescence (IEL) as a function of injection current, jinject.Following the analysis used in [19], when the IEL is plottedagainst jinject on a log–log plot, the gradient should vary between 1and 2, where a value of 1 shows that radiative recombination isdominant and a value of 2 shows that non-radiative recombinationis dominant. An intermediate value shows that both processes arecontributing.The results are shown in Fig. 5. The gradient is 1 at lowtemperature (20 K) and 2 at room temperature for both samples.This suggests that radiative recombination is dominant at lowtemperature, but non-radiative recombination is dominant at roomtemperature. Similar behaviour has been observed forphotoluminescence from GaAsBi layers with [Bi] = 3% [14]. InFig. 5a, there is a reduction in the gradient of both curves at highinjection current; this is probably due to sample heating. If thelow-temperature EL is dominated by radiative processes, then arough estimate of the radiative efficiency of the material at roomtemperature can be obtained by dividing the room-temperatureluminescence by the low-temperature luminescence. In this case, atroom temperature, the radiative efficiencies are 2.3 and 3.3% forR2 and R4, respectively.By plotting the log of the IEL against 1000/T, it is possible toextract the activation energy of the carriers in each sample. Theseplots are shown in Fig. 6.It is clear that there is more than one activation energy for eachdiode. The data in Fig. 6 have been fitted with several differentactivation energies, each one determined by using a least-squaresfit to the data in a given temperature range. However, the data35Commons Attribution License (http://creativecommons.org/Fig. 4 Low-temperature electroluminescencea Sample R2b Sample R4Fig. 5 Log–log plots of IEL as a function of injection current at room temperature and at 20 Ka Sample R2b Sample R4Fig. 3 Room-temperature electroluminescencea Sample R2b Sample R4could have been adequately fit using several different combinationsof activation energies at different temperatures. It seems likely,therefore, that there is no single, well-defined energy level in these36 This is an open access article published by the IET under the Crediodes. The largest activation energy used in this fitting is 75meV. It is probable that these activation energies arise from thepresence of localised energy levels in the bandgap caused by theIET Optoelectron., 2016, Vol. 10, Iss. 2, pp. 34–38ative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/)Fig. 7 Peak EL energy of sample R2 as a function of temperatureFig. 6 Activation energies extracted from the temperature dependence of the IEL of each samplea Sample R2b Sample R4clustering of Bi atoms. This result suggests that there is a continuousrange of Bi-induced localised states that extend up to 75 meV intothe bandgap, which is consistent with the findings of Yoshimotoet al. [20].3.5 Temperature-dependent luminescenceThe temperature dependence of the peak energy of the luminescencefrom GaAsBi samples has previously been observed to follow an‘S-shaped’ dependence [13–15]. This ‘S-shape’ can be explainedwith reference to localised states within the bandgap, as describedin [21]. The Varshni equation is shown in (1) [22]Eg(T ) = E0 −aT2(T + b) (1)where Eg is the bandgap at temperature T; E0 is the bandgap at 0 K; αand β are fitting parameters. For semiconductors that containlocalised states within the bandgap, the equation is modified, asshown in (2) [23]Eg(T ) = E0 −aT2(T + b) −s2kBT(2)where σ is a parameter that indicates the degree of localisation withinthe structure and kB is Boltzmann’s constant.IET Optoelectron., 2016, Vol. 10, Iss. 2, pp. 34–38This is an open access article published by the IET under the Creativelicenses/by/3.0/)The temperature dependence of the main EL peak from R2 isshown in Fig. 7, along with fits to the data using the standard andmodified Varshni equations ((1) and (2), respectively). Also shownare the fitting parameters for R2, compared with the values foundin [14] from a photoluminescence investigation of a GaAsBisample with ∼3% Bi.The main peak appears to follow an S-shaped temperaturedependence. This is in contrast to the EL results in [8] where theGaAsBi peak was observed to follow the conventional Varshniequation. The lower E0 value for R2 compared with that in [14] isexpected due to the higher Bi content of the samples in this work.The α values are similar and are both less than the value reportedfor a GaAs control sample in [14] (0.56 meV/K). This confirms thatincorporating Bi reduces the temperature dependence of the bandgap.4 ConclusionsGaAsBi LEDs were grown on GaAs substrates. The LEDs showedgood room-temperature EL at about 1.20 μm, indicating a Bicontent of about 6%. This is consistent with the previous work onthese devices [11]. Reasonable EL spectra were obtained fromthese devices at a current density of 8 Acm−2, which isconsiderably lower than the lowest current densities used inprevious studies by other groups [8]. The temperature dependenceof the IEL from these devices suggests that radiative recombinationmechanisms dominate at 20 K, and non-radiative recombinationdominates at room temperature. A temperature-dependent IEL37Commons Attribution License (http://creativecommons.org/investigation of these samples shows that there is probably acontinuous distribution of Bi-induced localised states that extendup to 75 meV into the bandgap.5 References1 Svensson, S.P., Hier, H., Sarney, W.L., et al.: ‘Molecular beam epitaxy control andphotoluminescence properties of InAsBi’, J. Vac. Sci. Technol. B, 2012, 30, (2),p. 02B1092 Feng, G., Oe, K., Yoshimoto, M.: ‘Bismuth containing III–V quaternary alloyInGaAsBi grown by MBE’, Phys. Status Solidi A, 2006, 203, (11), pp. 2670–26733 Zhong, Y., Dongmo, P.B., Petropoulos, J.P., et al.: ‘Effects of molecular beamepitaxy growth conditions on composition and optical properties of InxGa1−xBiyAs1−y’, Appl. Phys. Lett., 2012, 100, (11), p. 1121104 Gu, Y., Wang, K., Zhou, H., et al.: ‘Structural and optical characterizations ofInPBi thin films grown by molecular beam epitaxy’, Nanoscale Res. Lett., 2014,9, (1), p. 245 Rajpalke, M.K., Linhart, W.M., Birkett, M., et al.: ‘Growth and properties ofGaSbBi alloys’, Appl. Phys. Lett., 2013, 103, (14), p. 1421066 Broderick, C.A., Usman, M., Sweeney, S.J., et al.: ‘Band engineering in dilutenitride and bismide semiconductor lasers’, Semicond. Sci. Technol., 2012, 27,(9), p. 0940117 Batool, Z., Hild, K., Hosea, T.J.C., et al.: ‘The electronic band structure of GaBiAs/GaAs layers: influence of strain and band anti-crossing’, J. Appl. Phys., 2012, 111,(11), p. 1131088 Lewis, R.B., Beaton, D.A., Lu, X., et al.: ‘GaAs1−xBix light emitting diodes’,J. Cryst. Growth, 2009, 311, (7), pp. 1872–18759 Hossain, N., Marko, I.P., Jin, S.R., et al.: ‘Recombination mechanisms and bandalignment of GaAs1−xBix/GaAs light emitting diodes’, Appl. Phys. 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Quantum Electron., 2009, 45, (1), pp. 79–8520 Yoshimoto, M., Itoh, M., Tominaga, Y., et al.: ‘Quantitative estimation of densityof Bi-induced localized states in GaAs1−xBix grown by molecular beam epitaxy’,J. Cryst. Growth, 2013, 378, pp. 73–7621 Potter, R.J., Balkan, N.: ‘Optical properties of GaNAs and GaInAsN quantumwells’, J. Phys., Condens. Matter, 2004, 16, (31), p. S338722 Varshni, Y.P.: ‘Temperature dependence of the energy gap in semiconductors’,Physica, 1967, 34, (1), pp. 149–15423 Eliseev, P.G., Perlin, P., Lee, J., et al.: ‘‘Blue’ temperature-induced shift andband-tail emission in InGaN-based light sources’, Appl. Phys. Lett., 1997, 71,(5), pp. 569–571IET Optoelectron., 2016, Vol. 10, Iss. 2, pp. 34–38ative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/)

Telecommunication wavelength GaAsBi light emitting diodes

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Telecommunication wavelength GaAsBi light emitting diodes

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