Bonn, M.

Hangleiter, A.

Jin, Z.

Lahmann, S.

Rossow, U.

Turchinovich, Dmitry

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 19, 2017Transient THz photoconductivity in dynamically screened InGaN/GaN quantum wellsJin, Z.; Lahmann, S.; Rossow, U.; Hangleiter, A.; Bonn, M.; Turchinovich, DmitryPublished in:Proceedings of the 38th International Conference on Infrared, Millimeter and Terahertz Waves IRMMW-THz2013Link to article, DOI:10.1109/IRMMW-THz.2013.6665920Publication date:2013Link back to DTU OrbitCitation (APA):Jin, Z., Lahmann, S., Rossow, U., Hangleiter, A., Bonn, M., & Turchinovich, D. (2013). Transient THzphotoconductivity in dynamically screened InGaN/GaN quantum wells. In Proceedings of the 38th InternationalConference on Infrared, Millimeter and Terahertz Waves IRMMW-THz 2013 IEEE. DOI: 10.1109/IRMMW-THz.2013.6665920Abstract—Using optical pump – THz probereveal complex ultrafast photoconductivitInGaN/GaN quantum wells under dynaconditions, where, at sufficiently high excitatphoto-generated carriers fully screen the initialMV/cm. The THz photoconductivity spectra cboth localized and free charges. I. INTRODUCTION AND BACKGROUANTUM wells (QWs) in the presence ofwell described within the quantum-conf(QCSE) formalism. The electric field breakdecreasing the optical transition energy and rematrix element (and hence absorption and recvia spatial separation of electron and holeHowever, strong and ultrafast (i.e. on the timethan the recombination time) optical excitatiquantum well will lead to a coherent dynamica(CDS), as a significant photogenerated populaelectrons and holes will decrease, or even cominitial bias field in the QWs [1-4]. In the latteQCSE can be completely removed on timescthe duration of the excitation pulse, leavingscreened “rectangular-shape” state. As a resremoved, and the optical transition erecombination rate increase. However, recombine, the bias field, and hence the Qrestore. The complete process is depicted in FII. RESULTS Rigorous optical pump–THz probe (OPreveal the photoconductivity dynamics of the Z. Jina,b, S. Lahmannc, U.aMax Planck InstbShangcTechnical UniversdTechnical UTransient THz PhotoconQFig. 1. Principle of dynamical screening effect in a optical excitation produces a polarized screening red and green “clouds”) of sufficient density to combias field, removing the QCSE. As the screening cthe bias field and the QCSE are restored.  spectroscopy we y dynamics in mical screening ion densities, the  internal field of 3 ontain features of UND  electric field are ined Stark effect s the symmetry, ducing interband ombination rates)  wavefunctions. scale much faster on of the biased l screening effect tion of polarized pletely screen the r case, the initial ales shorter than  the QW in the ult, the QCSE is nergy and the as the carriers CSE, gradually ig. 1.  TP) experiments screening charge in internally-biased InGaN/GaN Qfluence at 400 nm. Our sample cIn0.2Ga0.8N QWs with a well thickneGaN barriers with the width of 7built-in field in the QWs, causemismatch, was 3.1 MV/cm [1]. Thsandwiched between a 180 nm thick μm GaN buffer layer. The sample substrate by metalorganic vapor-phexcitation wavelength assures absorption, electron-hole pairs canInGaN QWs and not in the GaN barpresence of the QCSE, time-integratmeasurements were performed wfluences. The photoexcitation-isubsequent de-screening of the buusing the time-integrated photolumshown). The saturation of the blue-sfor highest fluences indicates the rethus the complete screening of the [1,2]. As carriers recombine, a decrearesults in a red-shift of the PL specComplete screening is achieved alr~0.5 mJ/cm2, leading to a non-triviacharge decay, as described below.   Fig. 2 shows the THz frequphotoconductivity in InGaN/GaN QOPTP experiment for two pump fluemJ/cm2. A non-exponential decay o Rossowc, A. Hangleiterc, M. Bonna, and D. Turchinitute for Polymer Research, 55128 Mainz, Germanyhai University, 200444 Shanghai, China ity of Braunschweig, 38106 Braunschweig, Germanniversity of Denmark, 2800 Lyngby, Denmark ductivity in Dynamically Screened Quantum Wells Fig. 2 Frequency-integrated THz trasample photoexcited by 400 nm, sub-mJ/cm2, insignificant screening) and sscreening) pump fluence.  biased QW. Strongcharges (shown aspensate the initialharges recombine,Ws excited with variable onsists of 10 periods of ss of 1.8 nm, separated by  nm. The strain-induced d by QW-barrier lattice e multi-QWs structure is GaN capping layer and a 2 was grown on a sapphire ase epitaxy. The 400 nm that with single-photon  only be excited in the riers. In order to assess the ed photoluminescence (PL) ith different excitation nduced screening and ilt-in field was monitored inescence (PL, data not hift of the main PL feature moval of Stark shift, and built-in piezoelectric field se of the transition energy tra, as the QCSE returns. eady at pump fluences of l speed-up of the screening  ency-integrated transient Ws as measured in a 1D nces, 0.065 mJ/cm2 and 0.5 f the photoconductivity is ovicha,d   y InGaN/GaN nsmission through the QW 50 fs pulses at weak (0.065 trong (0.5 mJ/cm2, complete clearly evident (with an accelerated initial pump fluences. At high pump fluences and edelay times, the complete screening of the buifield results in a large overlap of the electrofunctions and hence a large recombinatiopump-probe delay times, the carrier density electron-hole recombination, resulting in the built-in piezoelectric field, which in turnrecombination rate.  Fig.3 Real (s1) and imaginary (s2) THz conduscreening photoexcitation of 0.5 mJ/cm2, me600 ps after pump. The QW thickness is 1.8 n Spectral information about THz photoconducan be obtained by time-resolving the probvarious times after optical excitation, andspectral analysis in the Fourier domain. TheTHz frequency-resolved transmission functionthe photoinduced complex THz conductivity, delay after the photoexcitation, given that σ෥(relative to the duration of the THz pulse (~1 pFig. 3 shows such complex conductivity sptwo pump-probe delays. To the best of our kthe first spectrally-resolved observatiophotoconductivity dynamics in a GaN-based found that the shape of the spectra stays unchwhole range of our pump-probe delays 13 – 8decreasing in magnitude with time. Reduceconductivity and the zero-crossing of the imaconductivity at about 0.6 THz indicatconductivity response. The measured spectra cby any simple analytical conductivity model Drude-Lorentz], however their features cqualitative conclusions regarding thphotoconductivity in InGaN/GaN QWs systemAt lowest THz frequencies the impconductivity, its growth with frequencyimaginary conductivity suggest the “presponse of bound or localized charge. At hispectrally-flat real conductivity and slowly-imaginary conductivity suggest free carrierhigh momentum relaxation rate. These featurof carrier localization on the long-range corenergy surface in InGaN/GaN QWs [5], whidecay) at larger arly pump-probe lt-in piezoelectric n and hole wave n rate. At later decreases due to restoration of the  decreases the  ctivity spectra at asured 13 ps and m. ctivity dynamics e THz pulses at  performing the  complex-valued  can be related to σ෥(ω), at any time ω) varies slowly s). ectra measured at nowledge, this is n of ultrafast system. We have anged within the 00 ps, while only d low-frequency ginary part of the e a non-Drude ould not be fitted [Drude (-Smith), an still lead to e nature of .  eded DC real , and negative olarization-type” gher frequencies, growing positive  conductivity at es are indicative rugated potential ch may have an impact on the performance of GaNdiodes and lasers in the blue-greecurrently the subject of further invesIn summary, we have observed a sphotoexcited screening charge at incshowing an increase in recombinatioscreened QWs. The time-resolved Tvarious time delays after onon-Drude-like behavior, which welocalization of free charge on long-raREFERENCE[1] D. Turchinovich, P. Uhd Jepsen, B. S. MU. Rossow, and A. Hangleiter, “Ultrafaquantum wells under strong femtosecon68, 241307 (R) (2003). [2] D. Turchinovich, B. S. Monozon, and Pscreening in excitation kinetics of biabsorption and ultrabroadband teraher013510 (2006). [3] P. J. S. van Capel, D. Turchinovich, H. PA. Hangleiter, and J. I. 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Transient THz photoconductivity in dynamically screened InGaN/GaN quantum wells

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