High-resolution X-ray Microdiffraction System for Characterizing Selectively Grown Layers Using a Zone Plate Combined with a Narrow Slit A microdiffraction system using a zone plate (ZP) is suitable for characterizing selectively grown semiconductor epitaxial layers and multi-quantum-well (MQW) structures [1,2]. This is because the hard X- ray microbeam produced by a ZP has both a sub- micrometer beam size and a relatively low angular divergence of the sub-mrad order. Using such a system, we were able to measure the lattice strain distribution in InGaAsP metal-organic vapor-phase epitaxy (MOVPE) layers selectively grown on 3- and 4- μ m-wide InP stripe regions with a strain sensitivity of about 1 10 -4 [2]. Although this strain sensitivity was fairly good, further improvement was desired to enable more precise strain measurements. We have therefore developed a new high- resolution microdiffraction system [3], which enabled high-resolution X-ray microdiffraction experiments. The system (Fig. 1) was set up at beamline BL24XU using a newly developed apparatus for hard X-ray m i c r o i m a g i n g a n d m i c r o d i f f r a c t i o n [ 4 ] . T h e monochromatized beam (15 keV) was focused using a phase ZP, which was fabricated by NTT Advanced Technology. The diameter, outermost zone width, tantalum absorber thickness, and focal length of the ZP were, respectively, 180 μ m, 50 nm, 800 nm, and 108.9 mm at a photon energy of 15 keV. A platinum 20- μ m-diameter pinhole was used as an order-sorting aperture (OSA). To suppress the beam divergence, a 5- μ m-wide slit was placed in front of the ZP. Since only one part of the ZP was used, the beam divergence became smaller as shown in Fig. 1. The center of the Fig.1. Schematic figure of the experimental arrangement. The monochromatized beam was focused by using a phase zone plate (ZP). Phase ZP OSA Sample X-ray detector Slit Monochromatized X-ray beam (15keV) op view To op To p op v vie vi ew ie w ew slit was off-placed from the ZP center so that the OSA would eliminate unfocused and going straight beams. The focused beam that passed through the OSA enabled us to perform high-resolution X-ray microdiffraction experiments using a precise θ – 2 θ goniometer with sub-100-nm-resolved XYZ sample- positioning stages. We measured the focused beam size using the knife-edge scan method. The result was 0.32 μ m horizontally and 1.3 μ m vertically at the full width at half maximum. The measured photon fl ux at the sample position was about 1 10 7 photons / s at 100 mA. The horizontal angular divergence of the focused beam, estimated by measuring a Si 400 rocking curve, was about 7 0 μ rad. To confirm the capabilities of this system, we measured the spatial non-uniformity of the strain and the period in as-grown InGaAsP multi-quantum-well (MQW) structures selectively grown by MOVPE (see the inset of Fig. 2). Three rocking curves were taken at the center position and positions 0.5 μ m from the center position of the 1.8- μ m-wide stripe region. The measured rocking curves are shown in Fig. 2. The log of the diffraction intensity was plotted versus ∆ q / q InP = ∆θ cot θ B in percentages ( 100), where ∆θ = θ – θ B is the goniometer angle relative to the InP 400 B ragg peak angle. Also, q InP = 2 π / d InP is the momentum transfer, where d InP is the lattice spacing of the InP 400 diffraction. In all rocking curves, the 0th- and 1st-order satellite peaks of the MQW superlattice were clearly observed in addition to a 123 S higeru Kimura SPring-8 / JASRI E-mail: kimuras@spring8.or.jp References [ 1] Z. Cai e t al. : Appl. Ph y s. L e tt ers 75 ( 1 999) 1 00 . [ 2 ] S . Kimura et al. : J . Jpn. J. Appl. Ph y s. 41 ( 2002) L 1 0 1 3 . [ 3 ] S . Kimura , Y . Kagoshima , T . K o y ama , I. W a d a , T . N ii mi , Y . T susaka , J. M a t sui an d K. I z u mi: Pro c . o f t he SRI 2003, p.1 275 . [ 4 ] Y . Kagoshima e t al. : Pro c . o f t he SRI 2003, p. 1 263 . [ 5 ] Y . Saka t a e t al. : J. Cr y s t . G ro wt h 208 ( 2000) 1 30 . Fig. 2. A series of rocking curves taken at 0.5 μ m intervals across the stripe region. The log of the diffraction intensity was plotted against ∆ q / q InP . Note that the curves are vertically displaced for clarity. 1.8 μ m I ntensity (cps) L = +0.5 μ m L = 0.0 μ m L = –0.5 μ m -1 -2 0 1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 ∆ ∆ q / q / InP (%) s t rong InP 400 p eak a t ∆ q / q q / / InP = 0 . Sin c e ∆ q / q q InP = – ∆ d / d I n P f r o m t h e B r a g g s ’ l a w , t h e s t r a i n perpen d i c ular t o t he su r f a c e w as d i re ct l y d e t e rmine d f rom t he 0 t h -or d e r peak posi t i on. Also , t he MQW p erio d is p ropor t i onal t o t he i n v e rse o f t he separa t i on b e tw ee n a d j a c en t sa t elli t e peaks. W e f ou n d t ha t t he spa t i al non-uni f ormi ty o f t he s t rain an d t he p erio d in t he MQW s t ru ct ure , i.e. , t he magni t u d e o f t he s t rain near t he s t ripe e d g es , w as g rea t er t han t ha t a t t h e s t ripe c en t er , an d t he perio d near t he s t ripe e d g es w as l onger t ha n t ha t a t t he s t ripe c en t e r. T his i n d i c a t es t ha t t he su r f a c e m ig ra t i on f rom t he s t ripe e d g e t o t he s t ripe c en t e r is insu ff i c ie n t u n d er t hese gro wt h c on d i t i ons [ 5 ]. 4 4 4 4 4 4 4 4 4 4 4 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 124 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12
home
- JP
- EN