Characterization of deep levels in LEC GaAs crystals by the photoluminescence technique

Paper presented at ICDS-12 Amsterdam, August 31 - September 3, 1982

Physica 116B (1983) 404-408 North-Holland Publishing Company

CHARACTERIZATION OF DEEP LEVELS IN LEC GaAs CRYSTALS BY THE PHOTOLUMINESCENCE TECHNIQUE

M. TAJIMA* and Y. OKADA

Electrotechnieal Laboratory, Umezono, Sakura-mura, Ibaraki 305, Japan The d i s t r i b u t i o n s of the deep l e v e l s in undoped LEC GaAs c r y s t a l s have been i n v e s t i gated by photoluminescence (PL) spectroscopy together with e t c h i n g / o p t i c a l microscopy. The i n t e n s i t y of the O.65-eV PL band shows the same p r o f i l e along a wafer diameter as the d i s l o c a t i o n density, while that of the O.80-eV PL band shows an inverse p r o f i l e . The etching observation has revealed that the two types of etching f e a t u r e s , grooves and ridges, are present in the c r y s t a l s , and both of them o r i g i n a t e in d i s l o c a t i o n s . The d i s t r i b u t i o n s of the grooves and ridges correspond c l o s e l y with the i n t e n s i t y p r o f i l e s of the 0.65- and O.80-eV bands, r e s p e c t i v e l y . The o r i g i n s of the microdefects responsible f o r the 0.65- and O.80-eV bands are discussed with emphasis on t h e i r r e l a t i o n s h i p to the EL2 l e v e l . i.

INTRODUCTION

Undoped s e m i - i n s u l a t i n g ( S . I . ) GaAs c r y s t a l s grown by the l i q u i d encapsulated Czochralski (LEC) method are the key m a t e r i a l s used f o r the f a b r i cation of the high-speed i n t e g r a t e d c i r c u i t s as well as the o p t o e l e c t r o n i c devices. The clear understanding of the deep levels in these mater i a l s is of q u i t e importance, since the semii n s u l a t i n g p r o p e r t i e s are r e a l i z e d by the compensation of the residual shallow i m p u r i t i e s by the deep l e v e l s . I t has been reported that the dominant deep level in undoped m a t e r i a l s is the deep donor EL2 at 0.82 eV below the conduction band.(1) Therefore, many e f f o r t s have been made to c l a r i f y the o r i g i n of the EL2 l e v e l . ( 2 ) The photoluminescence (PL) technique is known as one of the promissing techniques to c h a r a c t e r i z e the deep l e v e l s . Mircea-Roussel and MakramEbeid have reported that the broad band with a peak at 0.65 eV, which appears always in undoped c r y s t a l s , is due to the EL2 l e v e l . ( 3 ) However, we have found that the O.80-eV band as well as the O.65-eV band is commonly observed in undoped c r y s t a l s and that the i n t e n s i t i e s of the two bands have nonuniform d i s t r i b u t i o n s in a wafer.

(4)

The purposes of t h i s work are to i n v e s t i g a t e the nonuniform d i s t r i b u t i o n s of the deep levels responsible f o r the 0.65- and O.80-eV band, and to c l a r i f y the o r i g i n of these deep l e v e l s and t h e i r r e l a t i o n s h i p to the EL2 l e v e l . 2.

EXPERIMENTAL

The samples used in t h i s study were commercially a v a i l a b l e LEC GaAs wafers obtained from three Currently on leave at M a x - P l a n c k - l n s t i t u t fur Festk~rperforschung, HeisenbergstraBe I , 7000 S t u t t g a r t 80, West Germany.

0378-4363/83/0000-0000/$03.00©1983 N o r t ~ H o D a n d

vendors. They were undoped or l i g h t l y Cr-doped ( ~ 0.4 wt. ppm ), and grown in the d i r e c t i o n from quartz (Si02) or p y r o l y t i c boron n i t r i d e (pBN) c r u c i b l e s . The m i r r o r - p o l i s h e d surface of the sample was f i r s t i n v e s t i g a t e d by PL spectroscopy and then by e t c h i n g / o p t i c a l microscopy. The sample immersed in l i q u i d helium was excited by the 514.5 nm l i n e of an Ar ion l a s e r . The i n c i d e n t power was 300 mW with a beam diamet e r of 2.5 mm ( i / e 2 - i n t e n s i t y - p o i n t ) . The PL from the sample was analyzed by a Spex 1701 g r a t i n g monochromator ( f / 7 ; 0.75 m) and detected by an SBRC PbS d e t e c t o r cooled with dry ice. The spectra were corrected f o r the spectral response of the measurement apparatus using an Eppley standard lamp. The sample surface was etched with the AB s o l u t i o n (5) at room temperature f o r 5 min and with molten KOH at 390°C f o r 5 min, and then examined by Nomarski-contrast o p t i c a l microscopy.(6) 3. 3.1

RESULTS Etching/Optical Microscopy

When the LEC samples were treated with the AB s o l u t i o n , two types of c h a r a c t e r i s t i c l i n e features appeared as shown in Fig. I . One type of the features, labeled G in the f i g u r e , has an inverse c o n t r a s t with respect to the other type, labeled R. I t turns out t h a t the G-features are grooves and the R-features are ridges. The Gfeatures are mainly observed in the edge part of a wafer, while the R-features in the center part. I t should be pointed out that many small p i t s l i e on the ridges. The s i m i l a r etching features have been reported by C u l l i s et a l . ( 7 ) They have concluded that these p i t s o r i g i n a t e in the As p r e c i p i t a t e s . I t is d i f f i c u l t to count c o r r e c t l y the d e n s i t i e s of the G- and R-features. The molten KOH is widely used to estimate the d i s l o c a t i o n density by counting e t c h - p i t s . I f the sample was etched

M. Ta]ima, Y. Okada / Characterization o f deep levels in LEC GaAs crystals

405

Figure 2 shows a typical example of a PL spectrum from an undoped LEC GaAs crystal at 4.2 K. (4) Three emission bands appear at 1.49, 0.80 and 0.65 eV. The 1.49-eV band is due to the r a d i a t i v e recombinations involving carbon accept o r s . ( 8 ) The o r i g i n of the O.80-eV band has been t e n t a t i v e l y interpreted as microdefects. (4) Although the O.80-eV band l i e s in the same wavelength region as the Cr band, there is a clear d i s t i n c t i o n between the two bands.(4) The O.65-eV band has been reported to be associated with oxygen (9) or the EL2 l e v e l . ( 3 ) We have already reported that the i n t e n s i t i e s of these three bands change g r e a t l y depending on the measurement p o s i t i o n on a wafer.(4) Figure 3 represents an example of the v a r i a t i o n of the PL spectra in a wafer. The O.65-eV band is strong at the wafer edge, but looses i t s i n t e n s i ty as the measurement points approaches the wafer center. In contrast with t h i s , the 0.80eV band is weak at the wafer edge and becomes strong at the wafer center.

Figure 1: Optical micrograph of undoped LEC GaAs crystal a f t e r AB etching. G and R mean groove and ridge, respectively.

3.3

In this section, we compare the results of etching and PL observations on the two pieces of wafers; one is sliced from the seed side and the other from the t a i l side of the undoped LEC ingot grown from a pBN crucible. Figures 4 and 5 show the v a r i a t i o n s of the PL and etching results across a wafer diameter for the two wafers. In these figures, (a) shows the schematic i l l u s t r a t i o n of the density p r o f i l e for the G- and R-features revealed by the AB s o l u t i o n ; (b) represents the e t c h - p i t - d e n s i t y (EPD) p r o f i l e obtained by molten KOH etching; (c) is the PL in-

with molten KOH a f t e r the AB etching, a pair of pits were observed at both ends of the respect i v e G- and R-features. This shows that both of the G- and R-features o r i g i n a t e in dislocations. The d i s t r i b u t i o n s of the G- and Rfeatures w i l l be described in 3.3. The d e t a i l e d discussion on the etching characterization w i l l appear in a separate paper.(6) 3.2

PL Spectroscopy

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M. Tajima, Y. Okada / Characterization of deep levels in LEC GaAs crystals

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t e n s i t y p r o f i l e s for the 1.49-, 0.80- and 0.65eV bands. The density of the G-features takes a maximum value at the wafer edge. As the measurement position approaches the wafer center, the density is f i r s t decreased and then almost disappears in the intermediate region ( 2 ~ r ~ I0 f o r the seed-side wafer and i0 ~ r ~ 12 for the t a i l side wafer; r is a radial distance in mm). Then, the density is increased again, as the measurement p o s i t i o n f u r t h e r approaches the wafer center. The R-features show an inverse p r o f i l e with respect to the above-mentioned profile. Since the maximum density of the G-features is much higher than that of the R-features, the p r o f i l e of the overall d i s l o c a t i o n density, shown in figure (b), is s i m i l a r to that of the G-features. The PL i n t e n s i t y p r o f i l e s for the 1.49- and 0.65eV bands correspond to the density p r o f i l e for the G-features or the EPD p r o f i l e , while the p r o f i l e for the O.80-eV band corresponds to the density p r o f i l e for the R-features. The comparison between the seed-side and the t a i l - s i d e wafers shows that the average EPD is lower and the f r a c t i o n of the region where the R-features are dominant is higher in the seedside than in the t a i l - s i d e . Corresponding with these, the average i n t e n s i t y of the O.65-eV band is lower and that of the O.80-eV band is higher

DISCUSSION

We have concluded in the previous paper that the 0.65-eV band is associated with the EL2 l e v e l , since the i n t e n s i t y p r o f i l e for t h i s band corresponds to the EPD p r o f i l e . ( 4 ) This is based on the fact that the density p r o f i l e for the EL2 level coincides with the EPD p r o f i l e . ( l O ) We have also interpreted t e n t a t i v e l y that the 0.80eV band is due to microdefects.(4) Here we discuss f u r t h e r the o r i g i n of the O.80-eV band. The O.80-eV band becomes strong in the region where the R-features are dominant. This region is considered to be grown under the As-rich condition, since the small pits on the R-features have been reported to be As p r e c i p i t a t e s . (7) I t is therefore speculated that the radiat i v e recombination centers responsible for the O.80-eV band are formed p r e f e r e n t i a l l y in the As-rich condition during the crystal growth. This is supported by the other fact that the O.80-eV band is stronger in the seed-side than in the t a i l - s i d e of the ingot, since the seedside is considered to be grown under the As-rich condition in comparison with the t a i l side because of the strong evaporation e f f e c t of As. (11) Our speculation contradicts to the conclusion given by Yu et a l . ( 1 2 ) They concluded that the O.77-eV band, which seems to be i d e n t i c a l with our O.80-eV band, is typical in Ga-rich grown crystals. A possible reason for this discrepancy is the v a r i a t i o n of the PL spectra in the wafer as shown in Figs. 3, 4(c) and 5(c), since they did not state the measurement positions in the wafers. Recently several workers have reported that the EL2 level is related to the ASGa a n t i s i t e d e f e c t (2) The formation of the ASGa defect is enhanced by the As-rich condition. The EL2 level is reported to be present commonly in undoped S . I . GaAs c r y s t a l s . ( 1 ) The present results show that the O.80-eV band is also enhanced by the As-rich condition and commonly observed in undoped S.I. GaAs crystals. Therefore, i t is inferred that the O.80-eV band is related to the ASGa defect. Quite recently, Windscheif et a l . have reported that the annealing behavior of the O.80-eV band is analogous to that of the EPR signal from the ASGa defect.(13) This is the strong evidence for the idea that the O.80-eV band is related to the ASGa defect. In our conclusion, both the 0.65- and O.80-eV bands are related to the EL2 l e v e l . This may be explained e i t h e r by the double-donor a c t i v i t y of the EL2 level (2,13) or by the existence of two kinds of "EL2" l e v e l s . ( 1 4 ) The contribution of oxygen to the appearance of the O.65-eV band should also be taken into account.

M. Ta/ima, Y. Okada / Characterization of deep levels in LEC GaAs crystals

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Figure 5: Variation of etch-pit and PL intensity along wafer diameter for the undoped LEC GaAs wafer sliced from the t a i l side. (a) Schematic i l l u s t r a t i o n of densities of ridges and grooves revealed by AB etching. (b) EPD revealed by molten KOH etching. (c) PL intensity profile.

M. Ta]ima, Y. Okada / Characterization o f deep levels in LEC GaAs crystals

408

5.

CONCLUSION

The PL analysis has shown that the two emission bands at 0.65 and 0.80 eV are always observed in undoped LEC GaAs crystals. The etching observation using the AB solution has revealed that the dislocations appear as grooves and/or ridges depending on the dislocation density. The dist r i b u t i o n s of the grooves and ridges correspond closely with the i n t e n s i t y profiles of the 0.65and O.80-eV bands, respectively. I t is speculated that the appearance of the 0.80-eV band is related to the As-rich condition in the melt during the crystal growth. This supports the idea that the microdefects responsible for the O.80-eV band is due to the ASGa a n t i s i t e defect. Acknowledgement We would like to thank J. Windscheif, U. Kaufmann and J. Schneider for helpful discussions and suggestions on the o r i g i n of the O.80-eV band.

REFERENCES ( i ) Martin, G.M., Farges, J.P., Jacob, G and Hallais, J.P., J. Appl. Phys. 51 (1980) 2840 (2) Schneider, J, to be published in Proc. 2nd Conf. Semi-lnsulating I I I - V Materials, Evian, 1982. (3) Mircea-Roussel, A. and Makram-Ebeid, S., Appl. Phys. Lett. 38 (1981) 1007. (4) Tajima, M., Jpn. J. Appl. Phys. 2__~i(1982) L227. (5) Abrahams, M.S. and Buiocchi, C.J., J. Appl. Phys. 3__66(1965) 2855. (6) Okada, Y., in preparation. (7) C u l l i s , A.G., Augustus, P.D. and Stirland, D.J., J. Appl. Phys. 5__!1(1980) 2556. (8) Ozeki, M., Nakai, K., Dazai, K. and Ryuzan, 0., Jpn. J. Appl. Phys. 13 (1974) 1121. (9) Yu, P.W., Solid State Commun. 3__22(1979) i i i i (i0) Martin, G.M., Jacob, G., Poiblaud, G., Goltzene, A. and Schwab, C., Defects and Radiation Effects in Semiconductors 1980, ed Hasiguti (Inst. Phys., B r i s t o l , 1981) p. 281 ( I i ) Holmes, D.E., Chen, R.T., E l l i o t , K.R. and Kirkpatrick, C.G., Appl. Phys. Lett. 4(] (1982) 46. (12) Yu, P.W., Holmes, D.E. and Chen, R.T., Gallium Arsenide and Related Compounds 1981 (Inst. Phys., B r i s t o l , 1982) p. 209. (13) Windscheif, J., Ennen, H., Kaufmann, U., Schneider, J. and Kimura, T., to be published in Appl. Phys. A. (14) Taniguchi, M. and Ikoma, T., to be published in Proc. 2nd Conf. Semi-lnsulating I I I - V Materials, Evian, 1982.