An atomic force microscopy study of the surface morphology of InP/GaAs heteroepitaxial layers subjected to rapid thermal annealing Ferenc Riesza) Research Institute for Technical Physics of the Hungarian Academy of Sciences, P.O. Box 76, H-1325 Budapest, Hungary and MASPEC-CNR Institute, Via Chiavari 18/A, I-43100 Parma, Italy
C. Vignali Interfaculty Measurement Centre, University of Parma, Viale delle Scienze, I-43100 Parma, Italy
C. Pelosi MASPEC-CNR Institute, Via Chiavari 18/A, I-43100 Parma, Italy
K. Rakennus and T. Hakkarainenb) Department of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland
~Received 17 March 1997; accepted for publication 19 September 1997! The effect of proximity-cap rapid thermal annealing on the surface morphology of ~100! InP epitaxial layers grown on GaAs substrates is studied by atomic force microscopy. Only small roughening was found beside the macroscopic thermal etch pits up to annealing at 940 °C for 10 s. Artifacts in the image formation are identified. © 1998 American Institute of Physics. @S0021-8979~97!08024-9#
I. INTRODUCTION
Avoiding thermal decomposition of InP surfaces is an important issue in InP technology. Thermal decomposition has been studied by mass spectrometry1,2 and in situ optical microscopy.3 We have also reported4,5 the appearance of thermal etch pits in InP/GaAs heteroepitaxial layers upon 820 °C/10 s and 780 °C/30 s rapid thermal annealing ~RTA!; the aim of the annealing was to decrease the threading dislocation density. In this work, we apply atomic force microscopy ~AFM! to study the effects of RTA on the surface morphology of InP/GaAs heteroepitaxial layers. AFM is now an established tool for the study of surface morphology of semiconductors.6 The present study was prompted by the above-mentioned observation of thermal etch pits on the surface of InP/GaAs heteroepitaxial samples subjected to RTA. These pits, easily observable by optical microscopy and even with the naked eye, make any device application or subsequent epitaxial growth7 impossible. Therefore, InP processing techniques are usually optimized to avoid these pits. The aim of this work is to explore possible additional morphological effects of annealing on a more microscopic scale. II. EXPERIMENT
The epilayers were grown by gas-source molecular beam epitaxy8,9 at the Tampere University of Technology on ~100! GaAs substrates miscut by 2° towards the @010# direction. The use of miscut substrates was found to greatly improve the surface morphology.10 Samples from two wafers were studied. Wafer 2 had no buffer layer, while on wafer 7, a 17-nm-thick buffer was grown at 400 °C, followed by in situ a!
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thermal annealing at 520 °C for 2 min. ~The sample numbers conform to our previous studies on the same samples.9! We have used the buffer layer and the in situ annealing as tools in the search for layer quality improvement.8,9 The 2-mmthick final layers were deposited at 500 °C. About 5 35 mm2 or 838 mm2 samples were cut from these wafers for RTA. Table I summarizes the annealing parameters of the samples together with the relative surface area and distribution of the thermal etch pits as determined by optical microscopy. The anneals were carried out in an SHS100 system ~AST Elektronik GmbH!. The samples were placed face down on a polished Si wafer that served both as a susceptor and a proximity cap.4,5 We have tried InP proximity caps as well,5 but no improvement in the efficiency ~e.g., higher temperature without pit formation! was found; InP also has a disadvantage of contaminating the RTA chamber due to phosphorous evaporation. The susceptor temperature was measured by a thermocouple-calibrated pyrometer. We estimate that the real sample temperatures were about 20 °C below the temperature readings. The ramp-up time was 10 s and the cooling of the sample below 300 °C took about 15 s. During annealing, the chamber was purged with a nitrogen flow of 5 slm. The AFM studies were carried out at the University of Parma with a NanoScope IIIa instrument ~Digital Instruments, Inc.! operating in contact mode; 200 mm silicon nitride cantilevers ~force constant, 0.12 N/m! with Au coated integral tips were used. The square pyramidal tips had specified radii of curvature of r530– 40 nm. The scans were performed in air. The samples were cleaned using organic solvents prior to the measurements. Both normal height and friction-mode images were recorded simultaneously. In all ¯1# and @011#, respeccases, the X and Y axes were along @01 tively. The slow-scan axis was Y .
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© 1998 American Institute of Physics
TABLE I. The growth and annealing parameters and the relative area and distribution of the thermal etch pits of the InP/GaAs heterostructures studied. Samples denoted by the same numbers were cut from the same wafer, and differ only in the annealing parameters. Sample number
Annealing parameters
Relative area and distribution of etch pits
2a 2b 2c
No annealing 860 °C/5 s 900 °C/5 s
No pits Only a few pits over the whole sample area '2% close to one edge, a few pits in the rest of the area
7a 7b
820 °C/10 s 940 °C/10 s
0.2% ~uniform distribution! 4% ~uniform distribution!
III. RESULTS AND DISCUSSION
On a large scale ~1003100 m m2 scan area!, all samples exhibited the same morphology: elongated streaks aligned ¯1# direction ~see Fig. 1 for sample about 8° away from the @01 7b!, in accord with Nomarski microscopy and scanning electron microscopy ~SEM! studies.5 This morphology is attributed to the combined effect of the high In flux and the miscut substrate and was present in homoepitaxial InP layers grown using the same parameters as well.10 Figure 1 also shows some etch pits formed upon annealing. These pits are rectangular and bound by $111% faces with size in the mm range. Any differences between the samples could be observed only on a more microscopic scale ~scan area, 200 3200 nm2 or 4003400 nm2!. Typically five scans were recorded on various locations of the samples excluding the etch pits; as an example, Fig. 2 shows representative scans for samples cut from each wafer @Fig. 2~a!, sample 2c, Fig. 2~b!, sample 7a#. Based on the images alone, no differences were seen between samples from the same wafers. More quantitative information can be derived by evaluating the power spectral density @~PSD! the square of the Fourier transform# using the built-in routines of the AFM control software.11 Scans of near-periodic surfaces are affected by artifacts resulting from the finite tip radius.12 According to a simple
FIG. 1. Large-scale scan of the sample annealed at 940 °C for 10 s showing three thermal etch pits ~the dark elongated rectangles!. J. Appl. Phys., Vol. 83, No. 1, 1 January 1998
FIG. 2. Representative scan of two samples; ~a! sample annealed at 900 °C for 5 s, ~b! sample annealed at 940 °C for 10 s.
model,13 the tip tracks a sinusoidal surface faithfully if the amplitude A of the surface features of wavelength l obeys the following approximate law: A, @ l/ ~ 2 p !# 2 /r.
~1!
This relation yields a curve ~line in a log–log plot! in the Fourier ~or PSD! space. This line can be used to assess the presence and degree of scan artifacts. In general, if the scan’s PSD lies significantly below this line, the scan is expected to closely reflect the real surface morphology. If the PSD at some wavelength approaches the line, probably there are unrevealed surface features having that wavelength. PSD parts above the line indicate artifacts resulting from the errors of scan or postscan processing. Figure 3 shows the PSD for typical scans from all of our samples along both ^011& directions together with the corresponding lines of Eq. ~1! with r540 nm. As all scans’ PSDs lie well below the line except for the shortest wavelengths, we believe that the scans closely reflect the real surface. The only exception, the highfrequency tail along the @011# direction, is an artifact caused by the inevitable tip positioning errors along the slow-scan axis ~the image flattening process we applied could not entirely remove this effect!. The PSD data clearly show that annealing induces only negligible, however, observable, roughening beside the macRiesz et al.
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microprobe analysis on sample 7b using 15 keV electrons showed the surface to be stochiometric outside the etch pits,5 indicating that the loss of phosphorus induces mainly the etch pits. Second, we have found using in situ SEM combined with mass spectrometry that the major evaporation of phosphorous coincides with the formation of the etch pits.15 However, incongruent evaporation of InP already commences at 300 °C in vacuum,1 being a precursor to the pit formation and severe phosphorus loss above 510 °C ~see also Ref. 15!. It is therefore reasonable to assign the small roughening we observed to this precursor state of phosphorous evaporation, since the roughening is present in samples 2b and 2c, in which pitting is negligible. The friction-mode images reproduced the normal height scans and supplied no additional information.
IV. CONCLUSIONS
In summary, we have studied the effects of RTA on the surface morphology of InP/GaAs heteroepitaxial layers with AFM. Our results indicate that phosphorous loss causes only macroscopic etch pits, and only a negligible roughening occurs outside these pits. Such small roughening, however, does not affect device applications, since any processing step or subsequent epitaxial growth usually includes the removal of a thin surface layer. Therefore optimizing the annealing to avoid the macroscopic pit formation only seems to be sufficient.
ACKNOWLEDGMENTS
FIG. 3. Power spectral density of the samples along the orthogonal ^011& ¯1# direction, ~b! @011# direction. The legends also show the directions; ~a! @01 rms roughness values evaluated for a 2003200 nm2 scan area. The straight lines indicate the detection border due to finite tip radius.
roscopic etch pits. Figure 3 shows also the rms roughness evaluated from the PSD data for the 2003200 nm2 scan area. The rationale for the choice of the scan area was in fact that larger areas exhibited no significant difference in the rms values, indicating that the roughening occurs on a microscopic length scale only. ~Note also that the roughness values are affected by the high-frequency cutoff and the tail artifact in the @011# direction.! For sample 7b, scans were performed near the thermal etch pits as well; no difference was found. It is not clear, however, if the overall larger roughness of samples from wafer 7 is caused by the annealing alone or was present as grown. We note for comparison that RTA of implanted In0.51Ga0.49P induced a more pronounced roughening under similar conditions ~rms roughness increment from 4 nm to 24 and 68 nm upon 900 °C/15 s and 1000 °C/15 s annealing, respectively! beside the pit formation; the pit shape was also different ~irregular as opposed to the rectangular shape typical for InP!.14 Our results are reinforced by two findings. First, electron 248
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The authors thank T. Lepisto¨ and A. L. To´th for the microprobe analysis and for the SEM work. The work in Budapest was supported, in part, by the ~Hungarian! National Scientific Research Fund ~OTKA! through Grant No. F 016278; in Tampere, by the Technology Development Centre ~TEKES!, The Academy of Finland, Nokia Corporation, Wallac Ltd., DCA Instruments Ltd., Outokumpu Ltd., and Nippon Mining Ltd. The Italian–Hungarian cooperation is supported by the CNR and the Hungarian Academy of Sciences. NanoScope is a registered trademark of Digital Instruments, Inc.
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