GaAs strained-layer quantum wells

Surface Science 267 (1992) 107-109 North-Holland

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Anomalies in photoluminescence linewidth of InGaAs/GaAs strained-layer quantum wells K. Muraki, S. Fukatsu, Y. Shiraki and R. Ito Research Center for Adcanced Science and Technolo.w (RCAST), The UnicersiO, of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153, Japan Received 2 June 1991- accepted for publication 27 June 1991

Anomalous temperature dependences of photoluminescence (PL) linewidth were found in In,Ga I _xAs/GaAs strained-layer quantum wells (QW's). The PL linewidth exhibited two different types of anomalous behavior depending on the well width L. and the indium composition x. With increasing temperature, (i) QW's with L z exceeding the critical layer thickness exhibited a linear increase followed by an abrupt decrease of the linewidth, whereas (ii) QW's with x > 0.144 and L. -'- 50-70 ,~, showed linewidth nar"owing. We explain these phenomena as due to (i) competition between free excitonic and dislocation-related emissions and (ii) exciton localization due to alloy disorder, respectively. By taking account of these anomalous behaviors, thc dependence of the PL linewidth on L. was satisfactorily explained in terms of alloy disorder.

I n G a A s / G a A s strained-layer heterostructures have received much attention in recent years because of their potential application for novel optoelectronic devices. In this lattice-mismatched system, critical layer thickness as well as interface quality are crucial issues. However, despi~e great efforts to determine the cirtical layer thickness, little is known about the influence of misfit dislocations on photoluminescence (PL) properties which significantly affects the performance of light-emitting devices. As for the interface quality, PL linewidth has been believed to provide a good measure. However, it has been pointed out that the PL linewidth of an I n G a A s / G a A s quantum well (QW) is limited not only by interface roughness but also by alloy disorder [1-4]. Which dominates the linewidth in this system is, there" "-" im.,',,,rtant issue to be clarified.

GaAs(100) substrates. Each sample consists of several I n G a A s / G a A s QW's with various well widths separated bv 500 A GaAs barriers. The growth temperature was 520 ° C and the As a t]ux was minimized to maintain a 2 × 4 As-stabi}ized surface during the growth of InGaAs and GaAs. Photoluminescence was measured in the temperature range from 15 :o 180 K vdth an A r ' ion laser as an excitation source. The excitation power deasity was set low so that the band filling effect would not affect the PL linewidth. Fig. 1 depicts the temperature dependence of the PL linewidth of I n x G a ~ _ x A s / G a A s ( x = 0.144) QW's with various wee widths L.. As seen in this f!gure, there were three types of temperature dependence. The linewidth of the 89 and 117 A wells exhibited a linear increase with temperature followed by an a b r u w decrease at 40 K ./1t~ . . . . . ~2~. . . . . ,9 . . . . !n~iK;dlll~l lglKSll~dk.l

the PL linewidth of I n G a A s / G a A s QW's with a special emphasis on its temperature dependence. The observed behaviors are discussed in terms of the misfit dilocations and the alloy disorder. All the samples used in this study were grown by molecular beam epitaxj on semi-insulating

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decreased with increasing terng~crature (typc-Bi. while those of the 20 and 31 A wells remained almost unchanged. These behaviors of the linewidth with temperature were strongly dependent on L. and x , as

0039-6()28 /92 /$05.1)0 ~" 1992 -- Elsevier Science Publishers B.V. and Yamada Science Foundation. All rights reserxed

K. Muraki et al. / PL linewidth of InGaAs /GaAs strained-layer QW~"

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ln~.Gat_,As/GaAs (x=0.144) QW's with various well widths. Broken lines are the guides to the cyc.

shown in fig. 2. The lower and the upper b r o k e n lines ~n the figure represent the critical layer thicknesses calculated by Matthews' theory assuming that the misfit dislocations are introduced before and after the overlayer is formed, respectively. The figure clearly shows that the lower

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curve well characterizes the region where type A behavior is found. The anomalous behavior of the linewidth may be caused by some additional effects other than interface roughness or alloy disorder. In order to clarify the origin of the linewidth, therefore, the minimum Fnewidth in all of the temperature range studied, in which the cxtfinsic effects are thought to almost disappear, was considered as the intrinsic one and plotted in fig. 3 as a function of the well width. Surprisingly, the linewidth shows a simple and universal d e p e n d e n c e on the well width as seen in the figure. Singh and Bajaj have developed a theory on the PL linewidth due to alloy disorder in Q W ' s consisting of ternary alloy wells anad binary barriers [5]. Here, wc modified their thcory and obtained the linewidth as follows:

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PL linewidth of

l n , G a l ,As/GaAs OW's. Broken lines are the calculated linewidth due to alloy disorder.

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in Composition Fig. 2. Mapping of the temperature dependence of the PL lincwidth of In,Gal _~As/GaAs QW's. Lower and upper broken lines represent the critical layer thicknesses calculated by Matthcws" theory assuming that the misfit dislocations arc introduced hotore and after the ovcrlaycr is formed, respectively.

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where K(c]] .... ]1111) is the '¢ransilion energy of the n = 1 e l e c t r o n - h e a v y hole exciton, x , is the mean composition cf indium, and r c is thc radius of the sma;lesl size over which the fluctuation can occur. The p a r a m c t e r r,. represents the dcgrec of clustering prcscnt in the alloy and reaches its

K. Muraki et al. / PL linewidth 01" lnGaAs / GaAs strained-laver QW',

minimum value r() ( = 2.2 ,A) when the atoms are distributed randomly. Singh and Bajaj took 4 :~,';rR~,(1 - P ) as the volume over which the fluctuation has to be calculated, where Re~ is the Bohr radius of the exciton and P is the probability of finding the exciton outside the well [5]. We modified their theory by taking 7rLZ~xL: as the relevant volume. For excitons bound to impurities or defects, L ~ can be replaced by the Bohr radius of the excitons. However, L~x should be larger for free excitons, since they can move in the well plane. Therefore, L~x was trt.ated as a fitting parameter here and was chosen in such a way that the best fitting was obtained for x = 0.087 with r~ - r 0. For x > 0.120, r~ was varied to fit the data with L ~ fixed at the same value, 2511 A. The broken lines in fig. 3 are the calculated linewidths. As seen in the figure, the agreement is excellent. This implies that the PL linewidth of I n G a A s / G a A s strained-layer QW's is not determin~d by the interface roughness but by the alloy disorder. Interface roughness, if present, was found to give a significantly different dependencc on the well width. Hence, it can be said that the interfaces are very smooth. Nex" we discuss the mechanism of the extrinsic linewidth broadening. At low tempera lures, excitons are thought to be localized or trapped by the potential irregularities caused by alhw disorder or dcfccts [¢~]. This localization of the cx~.itkm reduces L~x, which would increase the liaewidth. At higher temperatures, on the other hand, the excitons become mobile and the potential fluctuation is averaged over a wider range, leading to the motional narrowing of the excitons. This is a plausible explanation for the type-B temperature dependence. Since this type of bch:-,dor is observed in QW's with larger x and L_ = 50-70 where the alloy broadening becomes maximum, the ioc,_lization is likely to '.~e caused by the alloy dis,,:rdcr. [:inally, we discuss the type-A behavior. As seen in fig. _. "~ this behavior is found Ohio' in OW's with L, exceeding the critical layer thickness. Hence, the origin of the type-A behavior is con-

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sidered to be related to tho misfit disk)cations. Moreover, around the temperatures ~hcrc lhc linewidth was drastically reduced, the P l '<[)cctrum was found to consist of two components separted by 1.5-2 meV. The lower energy component, which dominates the spectrum at lower temperatures, disappears at higher temperatures, resulting in the drastic reduction of the apparent linewidth. These observations suggest that the higher and the lower energy components are free excitonic and defect-related emissions, respectively. According to the above discussion, this :tefect state is likely to be associated with m~sfit dislocations. This is plausible, since some dislocations are known to act as luminescence centers. However, further investigation is required to ciarify the carrier trapping mechanism at the misfit dislocations in I n G a A s / G a A s system. In summary, we found two types of anomalous temperature dependence of the PL linewidth in l n G a A s / G a A s QW's. These behaviors were ascribed to the exciton localization due to the allo~ disorder and the competition between free cxcitonic and defect-related emissions. By taking account of these anomalous behaviors, the dependence of the linewidth on th< ~cll ~idth x~a• satisfactorily explained in terms of the alloy disorder. This indicates that the hereto-interfaces arc atomicalh, fiat and. hence, that lnGaA/(~,~..\s provides

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References [1] R.I,.S. Dc~inc and W.T. Moore, J. Appl. Ph~. ¢)2 I I~ST~ 3900. [2] D.C. Bcrtolct, J. ]tsu, S.It. Jones and K.M. l,,m, , \ p p l Phys. Loll. 52 (10SS) 293. 131 P.B. Kirby. J.A. ('onstablc ant] R.N. N~llith. Ph\- !,' 41) (1OSC)) 31113. [4i .I.-P. ~,cMhlnalcl. 43 ( IL,m I ) 4~,)33.

i<. lh~gcr a'~.d i l. R~cct:c~l, i't~,~, I<~'. h;

[5] J. Singh and K.K. Bajaj, J. Appl. Phys. 57 (It)S5) ~433. In] J. llegarty and M.D. Sturg¢. S,urf. Sci. log (lUSSl S55,