Journal ArticleWe have investigated the modulated structures and its associated diffracted diff_x000B_use intensity, of organometallic vapor phase epitaxially grown GaPSb (001) layers by using transmission electron microscopy (TEM) and transmission electron diff_x000B_raction (TED). The TEM results reveal the co-existence of a _x000C_fine-scale modulated contrast and a _x000C_fine-scale speckled contrast. In addition, a _x000C_fine needle-like contrast is observed. The [001] TED results show lines of [110]-oriented diff_x000B_use intensity diffuse streaks passing through the fundamental reflections, satellite spots at 1/4g[220] positions, and a [010]-oriented diff_x000B_use intensity with spacing of 1/6g[040]. Simulations using the  Valence Force Field model were performed to understand the origin of the di_x000B_ffracted features. The observed distributions of di_x000B_ffuse intensity are shown to be partially consistent with random disorder. Furthermore, the [110]-oriented diff_x000B_use lines are attributed to a static displacement of the sites of the mixed sublattices

Seong, Tae-Yeon

Stringfellow, Gerald B.

English

The University of Utah: J. Willard Marriott Digital Library

Journal of the Korean Physical Society, Vol. 52, No. 2, February 2008, pp. 471~475Modulated Contrast and Associated Diffracted Intensity of GaPySbi^y Layers Grown Using Organometallic Vapor Phase EpitaxyTae-Y eon S e o n g *Department of Materials Science and Engineering, Korea University, Seoul 136-713G. R . B o o k e rDepartment o f Materials, University of Oxford, Parks Road, Oxford 0X 1 3PH, U.K.A. G. N o r m a nNational Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401-3393, U.S.A.F. G lasFrance Telecom,, Centre National d ’Etudes des Telecommunications, Paris, B,Lab de Bagneux, 196 Ave Henri Ravera, B P  107, 92225 Bagneux, FranceG. B. St r in g f e l l o wDepartment o f Materials Science and Engineering,University of Utah, Salt Lake City, Utah 84112, U.S.A.(Received 20 November 2007)We have investigated the modulated structures and its associated diffracted diffuse intensity, of organometallic vapor phase epitaxially grown GaPSb (001) layers by using transmission electron microscopy (TEM) and transmission electron diffraction (TED). The TEM results reveal the co­existence of a fine-scale modulated contrast and a fine-scale speckled contrast. In addition, a fine needle-like contrast is observed. The [001] TED results show lines of [110]-oriented diffuse intensity diffuse streaks passing through the fundamental reflections, satellite spots at l/4g[220] positions, and a [010]-oriented diffuse intensity with spacing of l/6g[040]. Simulations using the Valence Force Field model were performed to understand the origin of the diffracted features. The observed distributions of diffuse intensity are shown to be partially consistent with random disorder. Furthermore, the [110]-oriented diffuse lines are attributed to a static displacement of the sites of the mixed sublattices.PACS num bers: 81.05.EaKeywords: M odulated  stru c tu res , D iffracted intensityI. IN T R O D U C T IO NIII-V  com pound sem iconductor epitaxial layers are of im portance for various applications in optoelectronic de­vices, such as laser diodes and photo-detectors [1, 2]. Growth of uniform epitaxial layers of te rn ary  and quater­nary  III-V  sem iconductors is sometimes difficult for cer­ta in  alloy com position and tem peratu res due to  the  pres­ence of miscibility gaps [3-6]. However, non-equilibrium  growth techniques, such as organom etallic vapor phase epitaxy (OM VPE) and molecular beam  epitaxy (MBE), have been used to  grow good quality  epitaxial layers of such semiconductors.*E-mail: tyseong@ korea.ac.krElectron microscopy results showed the  presence of a  fine m odulated contrast a ttrib u ted  to  alloy clustering possibly occurring as a  result of spinodal decomposition in epitaxial III-V  com pound sem iconductor layers [7-10, 12]. I t was shown th a t  there are two different kinds of m odulated structures, one giving a  coarse-scale tweed­like contrast w ith a  periodicity of 100 -  200 nm , and the  o ther a  quasi-periodic fine-scale contrast w ith a  pe­riodicity of ~10  nm. For example, Blood and Grassie [13] reported  calculations based on previously published structu ra l da ta , showing th a t  the  conduction band in G alnA sP  alloys had a  spatial variation w ith an am pli­tude  of ~0.08 eV and th a t  such a  variation could re­sult from partia l segregation into clusters. The effects of alloy clustering on the  photolum inescence properties-471--472- Journal of the Korean Physical Society, Vol. 52. No. 2. February 2008Table 1. Growth conditions for epitaxial G aP „S bi_B OM VPE layers.Sample No.G aP ,, Sbi — jj (y)Growth temp. (°C)Growth rate  (nm /s)3-38 0.58 500 0.243-37 0.59 530 0.763-23 0.51 560 0.283-24 0.75 560 0.563-44 0.77 600 1.46of G alnP  was also described, and it was argued th a t alloy clustering due to  spinodal decomposition during the growth process might have an adverse effect on both  the electrical and optical properties of the layers [13,14]. So far. detailed structu ra l investigations have not been widely perform ed on G aPSb layers to  date. In th is work, we present transm ission electron microscopy (TEM ) and transm ission electron diffraction (TED ) exam inations of the m odulated structu re  and diffracted diffuse intensity of G aPSb layers grown by OM VPE.II. EXPERIM ENTThe O M V PE growth was carried out in an atm osplieric-prcssure. horizontal, infrared-heated reac­to r [15]. The reactan ts were triniethylgallium  (TM Ga) and trinietliylantim ony (TM Sb) held in tem peratu re  controlled bubblers a t 12 and 11 °C. respectively, and phosphine consisting of a 10 % m ixture in Pd-diffused H2. The m olar flow rates were varied in the  following ranges: f tm g < i  =  1-92 2.88 x 10-5  m ol/m in . f TM.st =  3.37 7.02 x 10-5  m ol/m in. and fp h i , =  0.83 4.16 x 10-5  m ol/m in. The layer com position G aP ^S b i-j, was varied from y  =  0.51 to  0.77. the growth tem peratu re  between 500 and 600 °C. and growth ra te  between 0.24 and 1.46 nm /s. The layers were grown on (001) GaAs substrates. The details of the layers examined are listed in Table 1.(001) plan-view and {110} cross-sectional TEM  spec­imens were prepared using standard  techniques and the th inned m aterials were exam ined using a JE O L  4000EX electron microscope operated  a t 400 kV. The conver­gent beam  electron diffraction (CBED) technique [16] was used to  determ ine the polarity between the [110] and [-110] directions. The polarity  of the  G aPSb layers was assum ed to  be the same as th a t of the  underlying GaAs substrate.III. RESULTS AN D DISCUSSIONTEM  dark-field (DF) investigations using g< 22 0 >  and g< 4 0 0 >  reflections were carried out on [001] plan-viewFig. 1. (a) [110] cross-sectional (2-20) and (b) [-110] cross­sectional (220) TEM  DF images from an InPo.r,»Sbo.4 i layer grown a t 470 °C and 0.55 nm /s.and < 110>  cross-sectional specimens to  investigate the m icrostructure of the  G aP ^S b i-j, layers. The exam ina­tions showed a quasi-periodic fine-scale m odulated con­tra s t sim ilar to  th a t previously observed in In P ^ S b i-j, layer [17] and a ttrib u ted  to  alloy clustering due to  spin­odal decomposition occurring a t the growing surface during layer growth. A quasi-periodic fine needle-like contrast also occurred, which was also observed in the In P ^S b i-j, layers and was a ttrib u ted  to  segregation of atom s associated w ith rows of missing dim m ers present in the  reconstructed surface during growth. For exam ­ple. < 220>  cross-sectional DF images of an InPo.59Sbo.41 layer grown at 470 °C and 0.56 n m /s  are shown in Fig­ure 1 [17]. The D F images exhibit the  co-existence of a fine-scale m odulated contrast (15 20 nm) and a fine- scale speckled contrast (~ 5  nm) in bo th  the [110] and the [-110] cross-sectional samples. For the [-110] cross­sectional sample, however, the  D F image taken using g[220] reflection additionally reveals a fine needle-like contrast (~1 .5  2.1 nm) (Figure 1(b)).In order to  investigate the  diffracted intensity d istri­bution in reciprocal space, we perform ed detailed TED exam inations. It should be stressed th a t the  m ain char­acteristic features observed in the plan-view and the cross-sectional TED  p a tte rns are very sim ilar for all G aP yS bi-j, layers grown a t different tem peratures and growth rates, although there are small differences, de­pending on the growth tem perature, the growth ra te  and layer composition. The intensity of diffuse streaks pass­Modulated Contrast, and Associated Diffracted- • • Tae-Yeon S eo n c j  et al. -473-Fig. 2. (a) [001] and (b) [112] TED patterns obtained from FiS' 3' (a ) [1101 aud (b ) I"110! TED patterns obtaineda G aP 0 .7 3Sbu.23 layer grown a t 560 °C and 0.56 rnn/s. irom a GaPu.7 3Sbo.23 layer grown a t 560 °C and 0.56 rnn/s.ing through the main spots depends on the growth tem ­pera tu re  and the  composition. Typical results obtained from a G a P o .7 5 S b o .2 5  layer grown a t 560 °C and 0.56 rnn /s are as follows:The [001] TED  p a tte rn  is com plicated, as shown in Figure 2(a). The standard  array of main spots associ­ated  w ith the zinc-blende-type structu re  is present. The {200} spots are weak, which is consistent w ith the calcu­lated  difference of atom ic scattering factors between G aP and GaSb. The other m ain features for the [001] pole TED  p a tte rn  are i) lines of the [110]-oriented diffuse in­tensity. ii) diffuse streaks passing through the fundam en­ta l reflections and elongated along the [110] direction, iii) extra spots a t ± l/4g[220] positions, and iv) a [010]- oriented diffuse intensity w ith a spacing of ± l/6g[040]. The lines of the  [110]-oriented diffuse intensity pass close to . but not through, the main spots. These [110]-oriented lines of diffuse intensity correspond to  the intersection of the Ewald sphere w ith planes of diffuse intensity perpen­dicular to  all <  110> directions of the  crystal. The in­tensity  varies w ith position and has a maxim um  intensity near the  m ain diffraction spots. The extra spots a t the ± l/4g[220] positions seem to  be due to  the intersectionof the Ewald sphere w ith the [110]-oriented diffuse inten­sity along the [001] direction. The TEM  investigation of the InPSb layers [17] showed th a t such extra spots associ­ated  w ith [110]-diffuse streaks are related to  disc-shaped aggregations of atom s, namely, the fine needle-like TEM  contrast. Figure 1(b). The [112] TED  p a tte rn  is illus­tra ted  in Figure 2(b) and exhibits hexagonally-shaped diffuse intensity aligned long the [1-11] direction. The m axim a in intensity are located a t the ± l/4g[220] and ± l / 4 g [ l l l ]  positions.For the  [110] TED  p a tte rn . Figure 3(a). the higher order spots are elongated along the [001] and the [-110] directions, and a hexagonally-shaped diffuse intensity oc­curs along the [001] direction. For the [-110] TED  p a t­tern . Figure 3(b). a diffuse streak through the funda­m ental spots is present and elongated along the [110] direction; its length corresponds to  the w idth of the fine needle-like structu re  visible in TEM  images observed from the [-110] cross-sectional specimen, (e .g .. see Figure 1(b)). Higher-order spots are also elongated along the [001] direction and fine diffuse lines are present running through the m ain spots along the [111] and the [11-1] directions.-474- Journal of the Korean Physical Society, Vol. 52, No. 2, February 2008*.?r-V.vV ***.T. ,  ■ €x \ % ’ {v- >1 ., . . ' v»*A . *T >■ +■ ■ + : . '.. »  -• i f  . tJrf*v . > v 4 : ;+3-3 3 +  2-2 2 +  1-1 1 +  ooo+5-1 3 +4 0 2+3 1 1 +220rW T -' ‘>T , v 'yoiiABOVE4.002.00 - 4.00 1.00- 2.00 0 .5 0 - 1.00BELOW 0.508.00e.oo$•fT': - - -*~<S + ■ \*:V-"’"‘SL »■ * ». ,t  ^, /  t*>:,v.■ '£ $V ,*+00B+2 2 6 4 4 6 ABOVE 8.00+0 04+2 2 4+44  4 [^W*l4.00 -2.00 -8.004.00+0 02+2 2 2+44  21 .0 0 ­0.50 -2.001 .00+0004­2 2 0 +4 4 0BELOW 0.50Fig. 4. Calculated distribution of diffuse intensity in a quadrant, of a [1- 1 2 ] reciprocal plane for the GaPo.7rjSbo.25 layer grown a t 560 °C and 0.56 um /s.Fig. 5. Calculated d istribution of diffuse intensity in a quadrant, of a [1 -1 0 ] reciprocal plane for the GaPo.7rjSbo.2rj layer grown a t 560 °C and 0.56 um /s.The characteristic features in the TED  pa tte rns of the G aP yS bi-j, layers are different from those reported  by Glas e t al. [18] for InGaAs. They reported  only the presence of {110}-oriented diffuse lines. As discussed by Glas et al. [18], one of the m ain features revealed by TED  exam inations is th a t the m axim a in the intensity of the [110] diffuse lines are located away from the main diffraction spots. This indicates th a t these characteris­tic features are not associated w ith phonon scattering, (i.e ., similar [110]-oriented diffuse lines can occur as a result of phonon scattering), bu t in th is case, pass ex­actly through the m ain diffraction spots [19]. In addi­tion to  the  [110]-oriented diffuse lines, o ther com plicated features are revealed by the TED  investigations. Diffuse streaks, elongated along the  [110] direction, are present in the [001] and the [-110] TED  patterns, Figures 2(a) and Figure 3(b), respectively, which are thought to  be sim ilar to  those for In P ^S b i-t, layers [17]. These may also arise from segregation, which may be due to  stresses associated w ith surface reconstruction.In order to  understand the origin of the  diffuse lines, we calculated the structu re  of G aPo.75Sbo.25 (grown at 560 °C and 0.56 rirn/s) by using the  Valence Force Field (V FF) model of K eating [20] and M artin  [21], which is known to  yield distributions of interatom ic dis­tances com patible w ith the extended X-ray absorption fine structu re  (EXAFS) results for disordered ternary  al­loys [22]. The design of the model took into consideration the ease in which the excess energy of a given d istribu­tion of atom s was displaced from their v irtual crystal(VC) sites in a ternary  alloy. The details of the crys­ta l model and com puter sim ulations have been reported elsewhere [23]. Figure 4 shows the calculated d istribution of the  diffuse intensity in a quadran t of a [1-12] recipro­cal plane for the  GaPo.75Sbo.25 layer (grown at 560 °C and 0.56 rirn/s). The grey scales have been adjusted  so th a t the  sm oothly decreasing background arising from random  disorder is alm ost entirely included in the  first grey level. A p art of the  main features is reproduced. A hexagonally-shaped diffuse intensity is present along the [1-11] direction.Figure 5 exhibits the calculated distribution  of the dif­fuse intensity in a quadran t of a [1-10] reciprocal plane for the GaPo.75Sbo.25 layer (grown at 560 °C and 0.56 rirn/s). The elongation of high-order spots observed in the [1-10] experim ental diffraction p a tte rn  is reproduced: high order spots are elongated in the [001] and the [110] directions. However, the diffuse streaks passing through the fundam ental spots, parallel to  the  [110] direction, are not reproduced, indicating th a t they are not associated w ith a statistically  random  distribution  of atom s on the mixed G roup V sublattice, bu t are due to  another source, such as th in  disc-type segregation of atom s, as described for the In P ^ S b i-j, layers [17]. B etter agreem ent between the experim ental and the calculated diffraction patterns would undoubtedly have occurred if in the sim ulations the effects of the  fine needle-like contrast visible in the (220) and the (002) TEM  D F images had been included. In addition, there is relatively strong intensity a t theModulated Contrast and Associated Diffracted- • • -  Tae-Yeon Se o n g  et al. -475-{111} position. However, the  {111} intensity cannot be distinguished from the  {111} m ain spots of the  experi­m ental p a tte rn  because they  overlap each other.The results obtained by TED  exam inations and sim­ulations show th a t  the  observed distributions of diffuse intensity are partially  consistent w ith random  disorder. Consideration of random  disorder leads to  possible points th a t  may be related to  atom ic configuration behavior,i.e., deviation from mean virtual crystal sites even in a perfectly disordered alloy and atom ic random  distribu­tions of atom s in mixed sublattices. I t is notew orthy th a t the  observed [110]-oriented diffuse lines could arise from a  sta tic  displacem ent of the  sites of the  mixed sublattices. Glas [23] dem onstrated  th a t  in InGaAs, the  result of sim­ulations of the  displacem ents associated w ith chains of 2 or 3 In atom s in a  GaAs m atrix  is consistent w ith those of the  experim ental observations. Likewise, in G aPSb, a num ber of short random  chains of P  or Sb atom s might be predicted. The random  chains and the  atom ic dis­placem ents induced by atom s along th e  < 110>  directions could result in the  <110>-oriented diffuse lines.IV . SU M M A R YTEM  and TED  exam inations were used to  investigate the  characteristic features observed in the  plan-view and the  cross-sectional TED  p a tte rns from O M V PE G aPSb layers grown in th e  range 460 to  500 °C a t growth rates of0.56 and 0.83 nm /s . TEM  results showed the  th a t  a  fine- scale m odulated contrast (15 -  20 nm) and a  fine-scale speckled contrast (~ 5  nm) were present simultaneously. In addition, a  fine needle-like contrast (~ 1 .5  -  2.1 nm) was observed. The TED  results revealed com plicated diffracted diffuse intensities. Sim ulations using the  Va­lence Force Field model indicated th a t  the  observed dis­tribu tions of diffuse intensity  were partia lly  consistent w ith random  disorder and th a t  a  sta tic  displacem ent of the  sites of the  mixed sublattices could be responsible for the  [110]-oriented diffuse lines.R E F E R E N C E S[1] B. H. Koo, G. B. Chon, H. S. Lim, C. G. Lee and T. Yao, J. Korean Phys. Soc. 49, 873 (2006).[2] S. W. Chae, J. S. Kwak, S. K. Yoon, M. Y. Kim, J. O. Song, T. Y. Seong, and T. G. Kim, J. Korean Phys. Soc. 49, 899 (2006).[3] B. de Cremoux, J. de Physique C 5, suppl. No 12, C5-19 (1982).[4] K. Onabe, Jpn. J. Appl. Phys. 21, 964 (1982).[5] G. B. Stringfellow, J. Cryst. Growth 54, 404 (1983).[6] K. Ishida, T. Shumiya, T. Nomura, H. O htani and T. Nishizawa, J. Less-Common Metals 142, 135 (1988).[7] A. G. Norm an and G. R. Booker, J. Appl. Phys. 57, 4715(1985).[8] A. G. Norm an and G. R. Booker, Inst. Phys. Conf. Ser. 76, 257 (1985).[9] S. N. G. Chu, S. 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Modulated contrast and associated diffracted intensity of GaPySb1-y layers grown using organometallic vapor phase epitaxy

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Modulated contrast and associated diffracted intensity of GaPySb1-y layers grown using organometallic vapor phase epitaxy

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