DOI: 10.1002/adma.200501691

Pyramid-Shaped Si/Ge Superlattice Quantum Dots with EnhancedPhotoluminescence Properties**

By Huai-Chung Chen, Chun-Wen Wang, Sheng-Wei Lee, and Lih-Juann Chen*

Recently, intensive research efforts have been devoted tothe synthesis and characterization of nanoscale materials thatare endowed with unique properties due to their special ge-ometry. Many physical phenomena have been predicted andconfirmed in such nanomaterials.[1–3] Photoluminescence (PL)properties are of critical importance for applications of nano-structured materials in optoelectronic devices.

Quantum dots (QDs) in multidimensional quantum con-finement structures have been extensively studied.[4–6] Thesequantum structures possess numerous advantages over con-ventional one-dimensional (1D) confinement structures, suchas increased differential gain and improved temperature sta-bility of semiconductor lasers. Semiconductor superlattices(SLs) are attracting increasing attention due to their potentialapplications in thermoelectric and optoelectronic devices.While two-dimensional (2D) semiconductor structures exhibitpromising properties for optoelectronic devices such as light-emitting diodes, zero-dimensional (0D) nanometer-sized het-erostructures may provide more appropriate characteristicsand improve device performance.

Si1–xGex/Si heterostructures are used to fabricate high-speed transistors that extend the range of applications of Sitechnology.[7] In addition to Ge QDs, various nanostructures,such as silicide nanodots and SiGe nanorings, have also beengrown on SiGe.[8,9] In the present study, taking advantage ofthe relatively low etching rate and uniform size of Ge QDs ona Si/Ge SL, a method has been developed to fabricate pyra-mid-shaped Si/Ge SL nanodots with excellent uniformity overlarge areas. The fabrication process is compatible with Si/SiGe-based integrated circuit technology.[10,11] The pyramid-shaped nanodots exhibit an approximately tenfold increase inPL over conventional Si/Ge SL heterostructures. The en-hancement of PL is attributed to quantum size effects.

Figure 1a shows a cross-sectional transmission electron mi-croscopy (XTEM) image depicting the as-prepared SL struc-ture on Si(001). The as-grown sample consists of 20 periods of

3.5 nm thick Si/Ge bilayers [Si (3 nm)/Ge (0.5 nm)] with a30 nm thick Si spacer layer and a top layer of Ge QDs. No de-fects, such as dislocations, have been detected in the Si/Ge SLlayer, leaving the top Ge QD layer free from threading dislo-cations. This indicates that the Si/Ge SL layer is still in thepseudomorphic state after the growth of the Ge QD layer.These Ge QDs have an average diameter of 70 ± 5 nm with ameasured density of about 1 × 1010 cm–2. A field-emissionscanning electron microscopy (FESEM) image of the QDs isshown in Figure 1b.

The self-assembled Ge QDs deposited on the substrate actas a virtual nanomask against etching because of their rela-tively low etching rate and good size uniformity.[10] Figure 2presents a schematic depiction of the formation of pyramid-shaped nanodots. After deposition in the ultrahigh vacuumchemical-vapor-deposition (UHV-CVD) apparatus, the sam-ple with a top layer of Ge QDs, as seen in Figure 2a, is etchedby a wet-etching process. The top layer of monodisperse GeQDs prevents the underlying Si spacer layer from beingetched. In the meantime, etching proceeds on the uncapped Sisurface. As a result, V-groove trenches are produced by theanisotropic wet-etching process, as shown in Figure 2b. Dur-ing the etching process, the Ge QDs are gradually reduced insize, although this happens with a relatively low etching rate.Eventually, pyramid-shaped nanodots containing a Si/Ge SLare formed, as seen in Figure 2c.

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–[*] Prof. L.-J. Chen, H.-C. Chen, C.-W. Wang, Dr. S.-W. Lee

Department of Materials Science and EngineeringNational Tsing Hua UniversityHsinchu 300, Taiwan (Republic of China)E-mail: ljchen@mx.nthu.edu.tw

[**] This research was supported by the Republic of China NationalScience Council grant No. NSC 94-2215-E-007-003 and Ministry ofEducation grant No. 91-E-FA04-1-4.

Figure 1. a) XTEM image of an as-grown sample consisting of 20 periodsof 3.5 nm thick Si/Ge bilayers, a 30 nm thick Si spacer, and a top layer ofGe QDs. b) A FESEM image showing the top view of Ge QDs.

The SL-QD samples are etched with different concentra-tions of aqueous tetramethylammonium hydroxide (TMAH)solutions for various periods of time.Based on both XTEM and top-viewSEM observations, etching with 50 %and 75 % TMAH solutions (with 20 and30 s immersion, respectively) appears toproduce the most distinct pyramid-shaped SL QDs. These distinctive nano-dots are also found to contain the high-est numbers of SL layers. For conveni-ence, the two samples are designated assamples A and B, respectively. For thefirst set of conditions, the apex radii,size, and number of SL layers is 5 nm,60 ± 7 nm, and 5 ± 1, respectively. Onthe other hand, for the latter method,the corresponding parameters for thenanodots are 15 nm, 80 ± 10 nm, and10 ± 2. To the best of our knowledge,there have been no previous reports ofnanodots with a SL structure formedwithout lithography. Our observationsindicate that after etching, the initialupper layer of Ge QDs transfers the un-derlying 2D Si/Ge layer structure into a0D dot structure with a Si/Ge SL. Ex-amples of etched structures are shown

in Figures 3a–d. High-resolution transmission electron mi-croscopy (HRTEM) images reveal that, although the surfacesof the pyramids are {111} planes locally, the overall planarcontours are inclined to the Si(001) plane with varied obliqueangles. An example of this is shown in Figure 3e.

Figure 4a is an atomic force microscopy (AFM) image ofthe as-grown Ge–SL QDs showing the uniform distribution ofGe islands. The image reveals well-known bimodal islands,pyramids, and domes, which are commonly observed forsamples deposited at high temperatures. The density of theGe–SL QDs is 1.2 × 1010 cm–2. After etching in 50 % TMAHsolution for 20 s, most nanodots are transformed from domesto multifaceted structures in sample A. The nanodot densityof sample A is about 1.0 × 1010 cm–2, as shown in Figure 4b.Figure 4c shows clear pyramid-shaped structures and a broadsize distribution for sample B (etched with 75 % TMAH solu-tion for 30 s). The density of pyramids is measured to be8.9 × 109 cm–2. Figure 4d shows the distribution of the heightsof the QDs, as derived from the AFM images. From analysisof Gaussian fits to the nanodot height histograms, the centerheights for samples A and B are located at 17.4 nm (full widthat half maximum (FWHM): 4.4 nm) and 33.1 nm (FWHM:9.6 nm), respectively. Therefore, the average number of Si/Gebilayers in the etched samples is about five (≈ 17.4/3.5) in sam-ple A and ten (≈ 33.1/3.5) in sample B.

It has been recognized that carrier confinement in two orthree dimensions can improve the performance of semicon-ductor laser devices. In addition to the confinement in the ver-tical (growth) direction, various attempts have been made toinduce lateral confinement, based on lithography with chemi-

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Figure 2. Schematic depiction of the formation of pyramidal Si/Ge SLnanodots: a) as-grown layered structure, b) the initial stage of etchingwith the top layer of Ge QDs acting as a nanomask to introduce V-groovetrenches, and c) final structure with pyramidal Si/Ge-SL QDs.

Figure 3. a,b) XTEM images of an SL-QDs sample selectively etched with 50 and 75 % TMAH solu-tion for 20 and 30 s, respectively. c,d) Corresponding top-view SEM images of the nanodots in (a)and (b). e) HRTEM image showing the planar contours of the pyramid surface.

cal etching, lateral strain modulation with stressors, and self-organized QD growth in Stranski–Krastanov (SK) mode forhighly lattice-mismatched materials.[12–14] The strain relaxa-tion at the free surface of the stressor layer results in a lateralstrain redistribution, and subsequently a lateral bandgap mod-ulation within the buried quantum well. The formation of SLnanodots in the present study is expected to lead to furtherimprovement in the PL properties. Figure 5 shows the PLspectra of the nanodots at 10 K, with and without the selec-tive etching treatment. The nanodot samples etched with50 % TMAH solution for 20 s and 75 % TMAH solution for30 s exhibit a considerably stronger PL intensity for the SLpeak located at about 1.3 lm, as compared to the unetchedSL-QDs sample. The intensities of the non-phonon (NP)peaks from samples A and B, normalized to the PL intensityof the as-grown SL-QDs sample, are about twelve and thir-teen times greater, respectively, compared to the as-grownsample. For the transverse-optical (TO) phonon peak, PL en-hancement of seven and nine times is found for samples Aand B, respectively. It is worthwhile mentioning that the dif-ference in PL intensities for samples A and B can be corre-lated to the average number of Si/Ge SL layers confined inthe nanodots. The narrow peak at 1.6 lm (called the P line) isattributed to C–O complexes in silicon, as reported for a num-

ber of different SiGe structures.[15–17] The observedincrease in the luminescence efficiency, as seen inthe PL spectra, can be attributed to quantum car-rier confinement in all three directions.[18,19] It isthought that utilizing the mask of Ge nanodots totransfer the structure onto the Si/Ge SL layersleads to the lateral confinement of electrons andholes. Owing to the lateral energy barrier, elec-trons and holes in the Si/Ge SL are restrained inplanar directions. The confinement in planar direc-tions leads to an increase in the possibility of elec-tron–hole recombination, and thus enhances theluminescence efficiency. Compared to the excitonBohr radii of Ge (24.3 nm) and Si (4.9 nm), theaverage apex sizes of the SL pyramids are 5 and15 nm in samples A and B, respectively.[20–22] Sincethe PL enhancement is about the same for bothsamples A and B relative to the as-prepared SL-QDs sample, the quantum confinement effect is re-lated mainly to the Ge layer near the apex.

For device applications, it is important to knowthe quantum yield of the nanostructured materials.The quantum yield Q is calculated using

Q = QR(I/IR)(ODR/OD)(n/nR)2 (1)

where I is the integrated intensity, OD is the opti-cal density, and n is the refractive index.[23] Thesubscript R refers to the reference sample with aknown quantum yield. In the expression, it is as-sumed that the sample and reference are excited atthe same wavelength, which is 514.5 nm in the pre-

sent case. The optical densities have been measured to be0.0018, 0.0058, and 0.0029 for the unetched SL-QDs sampleand samples A and B, respectively. Using the SL-QDs sampleas a standard, the integrated intensity ratios, I/IR, are 7 and 18for samples A and B, respectively. As a first-order approxima-

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Figure 4. AFM images (1.0 lm × 1.0 lm) of nanodots of a) the SL-QDs sample,b) after etching in 50 % TMAH solution for 20 s, and c) after etching in 75 % TMAHsolution for 30 s. d) The corresponding size distributions of the nanodot heights ex-tracted from the images. The dotted lines represent Gaussian fits to the dot heighthistograms.

Figure 5. PL spectra (NP and TO indicate non-phonon and transverse-optical peaks, respectively) of the SL-QDs sample, nanodots etched in50 % TMAH solution for 20 s, and nanodots etched in 75 % TMAH solu-tion for 30 s. All the spectra have been obtained at 10 K.

tion, (n/nR)2 is assumed to be 1, because the refractive indicesfor bulk Si (3.42) and Ge (4.00) are rather close, and the re-fractive index for the SiGe alloy exhibits a roughly linear rela-tionship with the fraction of Ge.[24] Therefore, the quantumyield ratios, QA/QR and QB/QR, can be calculated to be 2.2and 11.2, respectively. The quantum yields for bulk group IVsemiconductors are known to be rather low. However, with Sinanodots dispersed in a silicon dioxide matrix, optical gainsof the same order of magnitude as direct-bandgap quantumdots have been achieved.[25] Furthermore, efficient siliconlight-emitting diodes have been fabricated using invertedpyramids on the top surface of Si.[26] In addition, the PL inshort-period Si/Ge SLs has been shown to be influenced bythe period, layer thickness ratio, and strain distribution.[27]

Many SiGe/Si devices, including photodetectors, light-emit-ting diodes, and heterojunction phototransistors, operating inthe 1.3–1.5 lm range have already been demonstrated.[7] Thesignificant increase in quantum yield for samples with pyrami-dal SL QDs should encourage further investigations into pos-sible optoelectronic device applications for these materials.

In summary, uniformly sized Ge QDs can be used as amask, due to their relatively low etching rates, to fabricatepyramidal nanodots that possess a Si/Ge SL structure with ex-cellent uniformity over large areas. PL measurements revealthat the Si/Ge SL nanodots exhibit an approximately tenfoldenhancement in intensity over conventional Si/Ge SL hetero-structures, which may lead to applications in optoelectronicdevices. The enhancement in PL is attributed to quantum con-finement effects. The Ge QD–Si/Ge SL heterostructure, con-taining Ge QDs with good size uniformity and low etchingrate, is used to obtain pyramidal Si/Ge SL structures. An addi-tional incentive is that the method described here is compati-ble with existing Si/SiGe-based integrated circuit technology.

Experimental

The Ge QDs with Si/Ge SL samples were grown in a UHV-CVDsystem. Pure SiH4 and 5 % GeH4 diluted in He were used as precur-sors. The SL layer stack was grown on a Si(001) substrate and con-sisted of 20 periods of a 3.5 nm thick Si/Ge bilayer [Si (3 nm)/Ge(0.5 nm)] grown at 550 °C. A 30 nm thick Si spacer layer was thengrown, followed by 13.1 eq-ML (eq-ML: equivalent monolayer) Gelayer deposition, both at 600 °C for the top layer to form Ge quantumdots. The selective etching process was performed by dipping samplesin a dilute TMAH solution.

For phase identification and examination of the structure quality,TEM was carried out using a JEOL 3010F field-emission gun electronmicroscope, operating at 300 kV. XTEM images were used to deter-mine the size and facets of the etched samples. A JEOL 6500FFESEM instrument was utilized to examine the surface morphology.The evolution of surface topography was analyzed by AFM using aDimension 3000 system with a Nanoscope controller from Digital In-struments, operating in tapping mode. The measurements were per-formed in air using silicon cantilevers with a radius less than 10 nm.PL measurements were employed to study the enhancement of the

optical characteristics of the nanodots using the 514.5 nm line from anAr+ laser. The PL spectra were recorded by a liquid-nitrogen-cooledGe photodetector with the standard lock-in technique.

Received: August 13, 2005Final version: October 25, 2005

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