A Comparison of the Optical Characteristics of AlGaN, GaN, and InGaN Thin Films

Yong-Hoon Cho et al.:Optical Characteristics of AlGaN,GaN,and InGaN227

phys.stat.sol.(b)216,227(1999)

Subject classification:78.45.+h;78.47.+p;78.55.Cr;S7.14

A Comparison of the Optical Characteristics

of AlGaN,GaN,and InGaN Thin Films

Yong-Hoon Cho1)(a,d),T.J.Schmidt(a,e),G.H.Gainer(a),https://www.360docs.net/doc/e2667403.html,m(a),

J.J.Song(a),S.Keller(b),U.K.Mishra(b),S.P.DenBaars(b),

W.Yang(c),D.S.Kim(d),and W.Jhe(d)

(a)Center for Laser and Photonics Research and Department of Physics,

Oklahoma State University,Stillwater,Oklahoma74078,USA

(b)Electrical and Computer Engineering and Materials Departments,

University of California,Santa Barbara,California93106,USA

(c)Honeywell Technology Center,Plymouth,Minnesota55441,USA

(d)Center for Near-Field Atom-Photon Technology and Department of Physics,

Seoul National University,Seoul151-742,Korea

(e)Technology Development,Williams Communications,Tulsa,Oklahoma74172,USA

(Received July4,1999)

We have investigated the optical properties of InGaN,GaN,and AlGaN epilayers using photolu-minescence(PL),PL excitation,time-resolved PL,and optically pumped stimulated emission(SE) spectroscopy.The InGaN layers had a larger(i)Stokes shift,(ii)spectral broadening,(iii)decay time,and(iv)PL redshift with time than AlGaN layers of comparable alloy composition.The optically pumped SE behavior of InGaN layers is significantly different than that of GaN and AlGaN layers.This is attributed to the suppression of nonradiative recombination and the elimina-tion of below-gap induced absorption in highly excited InGaN alloys,effects that accompany the incorporation of indium into GaN.In contrast,the SE behavior of AlGaN layers is similar to that of highly excited GaN.

Group III nitride semiconductors have recently attracted much attention for their versa-tile applications such as short-wavelength light emitting devices,solar-blind ultraviolet (UV)detectors,and high power and high temperature devices[1].Recently,room tem-perature cw violet laser diodes with lifetimes in excess of10,000hours have been com-mercialized by the Nichia group[2].The(Al,In)GaN alloys,consisting of AlN,GaN, and InN cover the entire visible to deep UV region of the spectrum(1.9to6.2eV).To date,only InGaN active layers have been used in commercialized group III-nitride based light emitting devices,due to difficulties in fabricating highly efficient light emit-ting devices using(Al)GaN layers.The recombination mechanism for InGaN-based structures has been widely investigated by several authors and detailed studies of the emission properties have demonstrated localized state recombination to account for both spontaneous and stimulated emission from InGaN/GaN quantum structures[3to 7].According to thermodynamical calculations,ternary AlGaN alloys are predicted not 1)Corresponding author;Tel.:82-11-289-6407;Fax:82-2-882-0246;

e-mail:lionsun@phya.snu.ac.kr

228Yong-Hoon Cho et al. to have an unstable mixing region,in contrast to InGaN and InAlN alloys,so no phase separation is expected in AlGaN alloys[8].Understanding the emission mechanisms in (Al,In)GaN alloys is important in order to extend the spectral range of light emitting devices into the near and deep UV,but the emission properties of(Al,In)GaN-based structures and comparisons between the alloys are not yet fully understood.

In this work,we report the spontaneous and stimulated emission behavior of GaN, InGaN,and AlGaN epilayers by means of photoluminescence(PL),PL excitation (PLE),time-resolved PL(TRPL),and optically pumped stimulated emission(SE)spec-troscopy.By studying alloy epilayers,we can avoid(or minimize)ambiguous effects usually involved in quantum structures,such as strain-induced piezoelectric polarization, quantum confinement,layer thickness variations,and interface-related defects.The InGaN,GaN,and AlGaN epilayers used in this work were grown by metalorganic chem-ical vapor deposition(MOCVD)on(0001)sapphire substrates.The layer thickness was on the order of1m m.In order to evaluate the In or Al alloy composition,the samples were analyzed with a high-resolution X-ray diffractometer using Cu-K a1radiation.PL experiments were performed using the244nm line of an intracavity doubled cw Ar+ laser as an excitation source.PL and PLE experiments were also carried out using the quasimonochromatic light emission from a Xe lamp dispersed by aYm monochroma-tor as the excitation source.Both the PL and PLE experiments used a photomultiplier tube in conjunction with a1m double spectrometer as a detector.TRPL measurements were carried out using a picosecond pulsed laser system consisting of a cavity-dumped dye laser synchronously pumped by a frequency-doubled modelocked Nd:YAG laser for sample excitation and a streak camera for detection.SE experiments were carried out in an edge-emission geometry using frequency doubled radiation from an Nd:YAG pumped dye laser,or the third harmonic of the Nd:YAG laser itself,as a nanosecond optical pumping source(%4ns pulse width and10Hz repetition rate).The emission was spectrally analyzed using a1m spectrometer in conjunction with an UV enhanced CCD.

Figure1shows TRPL and time-integrated PL(TIPL)results measured at10K for In0.18Ga0.82N,GaN,Al0.17Ga0.83N,and Al0.33Ga0.67N epilayers.In the case of the GaN epilayer,the lifetime of the free exciton(FX)is about30ps while that of the bound exciton(BX)is about40ps.We observed that the full width at half maximum (FWHM)is about50,4,17,and48meV and the overall lifetime is about600,35,230, and450ps for the In0.18Ga0.82N,GaN,Al0.17Ga0.83N,and Al0.33Ga0.67N epilayers, respectively.For the Al0.17Ga0.83N and Al0.33Ga0.67N epilayers,as well as the In0.18Ga0.82N layer,the measured lifetime increases with decreasing emission energy, and hence,the peak energy of the emission shifts to lower energy as time proceeds,as is typically observed for InGaN-based structures[5,6].This behavior has been attribu-ted to inhomogeneous alloy potential fluctuations.However,we observed that the In0.18Ga0.82N layer exhibits a larger(i)Stokes shift(between the PL emission peak energy and the absorption edge measured by PLE),(ii)PL peak FWHM,(iii)decay time in TRPL spectra,and(iv)redshift with time than the Al0.17Ga0.83N layer of com-parable alloy composition.We also note that the Stokes shift,FWHM,and decay time of the Al0.33Ga0.67N epilayer are larger than those of the Al0.17Ga0.83N epilayer,indicat-ing that Al alloy potential fluctuations increase with increasing Al composition.

To compare the spontaneous and stimulated emission behavior of the In0.18Ga0.82N and Al0.17Ga0.83N layers,PL,PLE,and SE spectra of both samples are illustrated in

Figs.2a and b.For both samples,the low temperature SE spectrum is superimposed on the low power cw PL peak (dashed line)and the band edge as measured by PLE (solid line).The spontaneous emission of the In 0.18Ga 0.82N (Al 0.17Ga 0.83N)layer is Stokes shifted by %50(20)meV with respect to the absorption band edge.The SE peak from the In 0.18Ga 0.82N layer is situated at the end of the absorption tail and is almost at the same energy as the low-power spontaneous emission peak (Fig.2a).In contrast,the incorporation of Al into GaN results in AlGaN layers with high excitation-density emis-sion behavior similar to GaN but with a SE threshold I th greater than that of GaN.The SE from the Al 0.17Ga 0.83N layer originates at %340nm at threshold and redshifts with increasing excitation density I exe (Fig.2b).The redshift of %7nm of the Al 0.17Ga 0.83N SE with respect to the absorption edge measured at 10K is consistent with the behav-ior of GaN thin films at elevated temperatures and/or high I exe ,and is attributed to

Optical Characteristics of AlGaN,GaN,and InGaN Thin Films

229Fig.1.Time-integrated (solid curves)and time-resolved (left scale)PL results measured at 10K for a)In 0.18Ga 0.82N,b)GaN,c)Al 0.17Ga 0.83N,and d)Al 0.33Ga 0.67N epilayers.The FWHM of the In 0.18Ga 0.82N epilayer is about three times larger than that of the Al 0.17Ga 0.83N epilayer and simi-lar to that of the Al 0.33Ga 0.67N epilayer.The overall lifetimes are about 600,35,230,and 450ps for the In 0.18Ga 0.82N,GaN,Al 0.17Ga 0.83N,and Al 0.33Ga 0.67N epilayers,respectively

stimulated recombination of an elec-tron±hole plasma (EHP)as follows.At low temperatures and I exe close to I th ,GaN stimulates about 2.1nm below the 1s A FX position,consis-tent with an exciton±exciton scatter-ing related gain mechanism [9,10].The fact that the AlGaN thin films do not exhibit this behavior is not surpris-ing,since no excitonic features were observed in the PLE spectra of these samples.At elevated temperatures and/or I exe several times greater than I th ,GaN exhibits a large Stokes shift of the SE compared to the band edge,exceeding %10.4nm near I th at room temperature [11].In this regime,the SE intensity I SE is proportional to (I exe )n ,where n %4,and the SE is attributed to stimulated recombination of an EHP .We observed that the AlGaN thin films show a large Stokes shift of the SE with respect to the spontaneous emission (>8nm)and I SE G (I exe )n ,where n %3.5,consistent with the behavior of GaN in the EHP recombination regime [11].On the other hand,the SE behavior of InGaN layers was observed to be significantly different than that of GaN and AlGaN layers,where an order of magnitude reduction in I th relative to GaN was typically observed.We note that a spatial increase in In content in an InGaN layer will lead to a potential minimum,whereas a spatial increase in Al content in an AlGaN layer will result in a potential maximum.InGaN layers typically exhibit a large Stokes shift of the SE with respect to the band edge but only a slight redshift (or even a blueshift)with respect to the spontaneous emission.(We note that results for InGaN layers can vary strongly from sample to sample and with differ-ent experimental conditions [7].)The differences in SE behavior,most notably the sig-nificant reduction in I th ,of InGaN thin films relative to GaN are attributed to the sup-pression of the nonradiative recombination that plagues GaN films [12].This is most 230Yong-Hoon Cho et

al.

Fig.2.PL,PLE,and SE spectra taken at 10K for a)In 0.18Ga 0.82N and b)Al 0.17Ga 0.83N epilayers.The SE spectrum was taken with an excitation density I exe twice that of the SE threshold I th .The de-crease in PLE signal above the PLE peak position of Al 0.17Ga 0.83N with increasing excitation energy is due to the decrease in the excitation intensity of a Xe lamp source

Optical Characteristics of AlGaN,GaN,and InGaN Thin Films231

likely a result of carrier confinement at(large)potential fluctuations in the InGaN layers,a result of difficulties in uniform indium incorporation[3to6].It has also been shown that the below-gap induced absorption present in the gain region of highly ex-cited GaN films is absent in high quality InGaN films[13].This effect,a result of the incorporation of indium into GaN,also aids in the reduction of I th.The absorption changes in highly excited AlGaN films were found to be similar to those exhibited by GaN,which is consistent with the observed increase in I th relative to(In)GaN.

In summary,the optical properties of InGaN,GaN,and AlGaN epilayers have been studied using PL,PL excitation,time-resolved PL,and optically pumped SE spectro-scopy.The InGaN layers had a larger(i)Stokes shift,(ii)spectral broadening,(iii) decay time,and(iv)PL redshift with time than AlGaN layers of comparable alloy composition.The optically pumped SE behavior of AlGaN layers is similar to the high excitation density emission behavior of GaN.However,the SE from InGaN layers is significantly different than that of GaN layers.This is attributed to the suppression of nonradiative recombination caused by carrier localization at large potential fluctuations in the InGaN layers and the absence of below-gap induced absorption that plagues highly excited GaN and AlGaN films.

Acknowledgements This work was supported by AFOSR,BMDO,ONR,DARPA, and CRI of MOST.

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