Origin of oval defects in GaAs layers grown by molecular beam epitaxy

Journal of Crystal Growth 73 (1985) 117—122 North-Holland, Amsterdam

117

ORIGIN OF OVAL DEFECTS IN GaAs LAYERS GROWN BY MOLECULAR BEAM EPITAXY K. AKIMOTO, M. DOHSEN, M. ARAI and N. WATANABE Sony Corporation Research Center, Hodogaya, Yokohama 240, Japan

Received 19 April 1985; manuscript received in final form 6 June 1985

The oval-defect density of GaAs layers grown by molecular beam epitaxy under various growth conditions was investigated, and the origin of the defect was proposed. Ga 20 and gallium complex are the probable origins of oval-defect formation. The oval-defect density could be decreased by introducing H2, and the reduction reaction of the oxide seems to take place in the Ga cell. We have found no differences in chemical composition between the oval defect and the smooth surface by either Auger, secondary ion mass or X-ray microanalysis.

1. Introduction Molecular beam epitaxy (MBE) is potentially superior to conventional epitaxial technologies for the growth of very thin layers and for the formation of abrupt and functional doping profiles, However, the presence of macroscopic surface defects called oval defects causes significant degradation of the electrical [1] and optical [2] properties. Several studies have been made on the origin of the oval defect. Substrate surface contaminants [3,4], Ga spitting from the Ga effusion cell [5,6], and the presence of Ga20 in the Ga melt [7,8] have been proposed as flossible origins but the matter is still open to question. In this paper, the relation between the oval-defect density and the growth conditions are examined and the origin of the oval defect is proposed. In addition, a method for eliminating oval defects is presented.

2. Experimental All the epitaxial GaAs layers were grown in a Riber MBE2300 R/D machine. The ultimate pressure of the system was 3 X lO~ Torr. The substrate was (100) oriented Cr-doped GaAs grown by the liquid encapsulation Czochralski process. The substrates were prepared by etching in a 2%

KOH: H202 solution (20: 1) for 2 mm, then mounted on a molybdenum block using indium as a solder. The substrates were then heat-cleaned at 150°Cin a preparation chamber and then placed at a growth temperature in the growth chamber until the peak height ratio of CO/As [m/e(CO) 28, m/e(As) 75] in the mass spectra was reduced to less than 1/100. The substrate temperature was monitored with an infrared pyrometer. The electrical and the optical properties of the GaAs grown by our MBE system have been described elsewhere [9]. The oval defects on as-grown epi-layers were counted under a Nomarski optical microscope. =

=

3. Results and discussion On the epitaxial layer, defects of various sizes from 1 to 8 ~tm in larger diameter could be observed. They all had a similar oval shape whose long axis was in a <110) direction. An SEM (scanfling electron microscopy) micrograph of a typical oval defect is shown in fig. 1. In order to study the origin of the oval defects, the following parameters were varied over an cxtended period: the Ga cell temperature range was from 865 to 1140°C, the substrate temperature range was from 520 to 660°C.Three different As cells were used together in other to obtain a high

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_______

The difference between cells A and B is in the baking condition of the growth chamber after the sources are charged. For cell A. the growth chamber was baked at 180°Cfor 2 days and the final pressure obtained in the baking process was 4 X l0~ Torr. For cell B, on the other hand, after having been baked at 90°C for 7 days, the final pressure in the baking process was 3 x 10° Torr. Note in the figure that the minimum oval-defect density for cell A at a thickness of 0.5 gm was 200 cm The oval-defect density is proportional to the growth thickness. This result indicates that the oval defect is generated during the whole growth process and that the substrate is not a particular source of the oval defect. The oval-defect density on the sample grown using cell B is much higher than that grown using cell A. Since the purity of the source material and the whole epitaxial procedure were the same except for the baking conditions. it appears that the origin of the defect has some relation with the baking procedure, possibly with the presence of oxidized complexes. This result supports the view that the main origin of the oval defects does not stem from the substrate surface. Fig. 3 shows the variation in the oval-defect density as a function of the number of epitaxy counted from finishing the baking of the growth -

Fig. I. SEM photograph i’f

.i

i\pIc~tl ~,iI defect,

As flux at low As cell temperatures. Fig. 2 shows the oval-defect density as a function of the thicknes of the epitaxial GaAs layer.

.

.

.

.

0 Ceil A * Ceii B

4000

4

i0

I

__

Thickness (pm) Fig. 2. Average density of oval defects versus growth thickness for GaAs films grown by MBE.

Number of Epitaxy Aftel’ Baking Fig. 3. Variation of the oval-defect density with the number of epitaxy counted from finishing the baking of the growth chamher.

K. A kimoto et al.

/

Origin of oval defects in GaAs layers grown by MBE

119

chamber. The thickness of the grown GaAs and the growth rate were kept at 0.9 ~smand 0.9 p.m/h, respectively. The oval-defect density obviously decreased as the number of epitaxy increased, which indicates that the additional purification achieved by outgassing the cells was effective to reduce the oval-defect density. In fig. 4 the oval-defect density in GaAs grown using cell A and B is plotted against the reciprocal temperature of the Ga cell. The growth thickness

relation between the density of the defect (D) and the heat of evaporation (E) can he expressed as D exp( E/kT) (1)

was also kept at 0.9 p.m for all samples, so that the oval-defect density shown in fig. 4 was multiplied by each growth rate in order to obtain the oval-dcfect density at the same growth time. The oval-dcfect density is strongly correlated with the Ga cell temperature. and this is in agreement with the results reported by Metze et al. [10] and Ito et al. [6]. Note that in both samples grown using cell A and B, the rate of increase of the oval-defect density with the cell temperature is much steeper in the higher than in the lower temperature region of the Ga cell. Impurities which evaporate from the that Ga cell possible density origins should of the oval defects so the are oval-defect be propor-

heat of evaporation of 3 eV. The origin of the oval defect in the lower temperature region of the Ga cell can be assumed to be Ga2O since the observed heat of evaporation is close to that of Ga2O. The observed energies were 5.7 eV in the higher ternperature region of the Ga cell. We could not, however, find a gallium compound which has a corresponding heat of evaporation or activation energy of dissociation reaction of the order of 5.7 eV. Antkiw and Dibeler [12] reported the mass spectrum of Ga vapor at temperatures in the range 4, 865°to 1025°C.The spectra showed the ions Ga Ga 20~ and Ga~ with relative intensities of 100: 10: 1, respectively, varying with temperature. The existence of polyatomic molecules of Ga is plausible, so one of the possible origins of the oval defect in the higher temperature region of the Ga cell may be a Ga cluster. In the next experiment, the effect of the As pressure was also studied. Fig. 5 shows the change

tional to the vapor pressure of the impurity. The _____________________________

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r E

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5 C

—

where T is the cell temperature. From fig. 4 two kinds of heat of evaporation, one of 5.7 eV for the steeper inclination and the other of 2.9 eV for the lower inclination, were evaluated using eq. (1). The solid line in fig. 4 indciates the vapor pressure of Ga20 over a Ga—Ga 201 mixture [11] which has a

in the oval-defect density with increasing As pres‘~

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Fig. 4. Oval-defeci density as a function of the reciprocal temperature of the Ga cell. The solid line represents the vapor pressure of Ga 20 over a Ga—Ga 203 mixture.

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2

4

6 810

20

~ ~As C torr

Fig. 5. Oval-defect density as a function of As pressure.

120

K. Akimoto et of.

/

Origin of oval defects in GaAs laden grown hi’ MBE

sure using a single As cell (upper curve) and three As cells (lower curve). The thickness and growth rate were 0.9 p.m and 0.9 p.m/h. respectively, for all samples shown in fig. 5. The oval-defect density increases with As pressure when a single cell is used. It indicates that the As cell also contains some impurities generating the oval defects. The defect density is decreased by sufficient outgassing of the As cell, so that the impurities which induce the defects evaporated from the As cell do not play a serious role. Using three cells, a high As pressure can he obtained at a low temperature. The oval-defect density does not significantly depend on the As pressure when the three cells were used. This result suggests that the rate of defect formation has no relation to the As4/Ga flux ratio. Kirchner et al. [13] reported that a cause of the oval defects is Ga203 which is formed by the reaction between Ga20 and As4 on the substrate surface: 4 GaAs

+

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*-s

3 Ga.,0 + As4.

(2)

-

This reaction suggests that the formation of Ga .03 is enhanced under a high As pressure. As noted above, the oval defect density stays essentially constant with the increase of the As pressure. Therefore we can conclude that the origin of oval defects is Ga20 and not Ga2O3. Fig. 6 shows the oval-defect density plotted against the reciprocal temperature of the substrate, The oval-defect density decreases as the substrate temperature increases for both cell A and B, and the inclination is almost the same. If the cause of the decrease of the oval-defect density with the substrate temperature is due to the evaporation of impurities from the substrate surface, then relation (1) can he used to discuss the origin of the oval defects, by replacing the cell temperature with the substrate temperature. The calculated energy from fig. 6 is 0.64 eV. This energy is much lower than that observed in fig. 4. The possibility of evaporation of volatile arsenic oxide which is formed on the substrate surface by the reaction of As with Ga,0 can be excluded since the oval-defect density does not depend on the arsenic pressure as shown in fig. 5. Therefore the decrease in the oval-defect density with the increased substrate

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Fig. 6. Oval-defect density as a function of the reciprocal

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temperature may he caused by the dissociation or change of bonding configuration of Ga,O or Ga cluster. As has been noted above, it was found that the .

origin of the oval defect is Ga20 in the lower temperature region of the Ga cell, so that the elimination of oxide from the Ga cell is very important. Kirchner et al. [13] suggested two methods to eliminate the oxide: by adding a small amounts of Al or Mg to the Ga cell. Neither of these methods are ideal, however, since the GaAs which contains some amounts of Al will have a degradation of the carrier mobility and in the case of additions of Mg. highly Mg-doped AIGaAs is grown due to the high doping efficiency of Mg for AIGaAs [14.15]. We tried, therefore, to grow GaAs under a reducing atmosphere 2 to 5 ~ iü~ Torr of H2. Fig. 7 shows the oval defect density with respect to the reciprocal temperature of the Ga cell for samples grown under the presence and absence of a H2 pressure. The oval defect density in the presence of H2 is lower than for the samples grown without H2, especially in the lower temperature region of the Ga cell. This result supports the model that the origin of the defects is Ga20 in the lower temperature region of the Ga cell. Fig. 8 shwos the change in the oval-defect density with the increasing substrate temperature for samples grown in the presence and absence of H2.

K. Akimoto ci al.

/

Origin of oval defects in GaAs layers grown by MBE

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Fig. 7. Oval-defect density of GaAs grown under a H 2 pressure of 2 to 5>< i0~ Torr and without H,. as a function of the reciprocal temperature of the Ga cell.

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1.0

Sputtering

As can be seen in the figure, the inclinations are almost the same for both growth conditions, suggesting that the decrease of the oval-defect density with the increased substrate temperature has no relation with H2, namely, that the reduction reaction by H2 does not take place on the substrate surface but on the Ga melt surface in the Ga cell. We found that the reduction of the oval-defect density is effective by introducing H2 gas in the growth chamber before rather than during growth. If the origin of the oval defect in the higher Ga cell temperature region is Ga2, then visible light irradiation may be effective to eliminate 2 since

Fig. 9. Depth profiling of Ga. As. 0 and C Auger signals at (a) a site of an oval defect and (h) a smooth defectless surface.

the bond energy of Ga2 is estimated to be 1.4 eV [16] and the light could induces the dissociation to monoatomic Ga. However, further study is needed _________________________ (a)

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Fig. 8. Oval-defect density of GaAs grown under a H 2 pressure Torr and without H2, as a function of the reciprocal temperature of the substrate. of 2 to 5 X ~

Fig. 10. SIMS depth profiling at (a) a Site of an oval defect and (b) a smooth defectless surface.

122

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surface hut in the Ga cell. Another molecular gal liii m complex or Ga cluster is thought to he another origin of the defect ui the higher tempera— lure region of the Ga cell. The chemical compost(ions in the vicinity of and at a distance from the oval defect were found to he unchanged as de— termu’ied h’s Auger. SIMS and \-ra\ mici’oprohe

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I he aLithors wish to thank I ora~ Research (‘euler for measLiring SIMS and F.. l’atsuki and N.

As L

Ktmura for measuring Auger and X-ray micro~ ould like to thank Dr. K. Kaneko for useful discussions and Dr. M. KikLichi for encouragement during the

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References to identify the origin of the defects ui the higher Ga cell temperature region. .Auoer. SIMS (secondar’s ion mass spectroscopv) and X-ray microprohe analysis were pertormed to detect the chemical composition in the icinity of and at a distance from the defects. ‘l’he results are shown in figs. 9 II. respectively. No coiliposition difference was observed at the s I detect regions. ‘1 hese results suggest that the seed of the oval—defect formation is of microscopic sue. less than the detection limit of all three chemical analytical detection techniques.

II] \1.

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4. ( onclusion ‘I he change of the oval—defect densit\ ss ith the growth conditions of GaAs layers deposited h’s M BF was studied, and the origin ol the os a! defects was investigated. The origin of the defect iS not on the substrate surface hut in the Ga cell. (ia~0 is proposed to he the origIn ot the osal defect in the lower temperature region ol the Ga cell. ‘The defect density could he decreased h’s growIng under a low pressure ot H 2. and it seems that I-I reacts with the oxide not on the sLibstrate

8] (.1). t’ctai, J.M. Wusudall. SI.. Wrighi. 0.1). Kirchiii~rand .1 L i’rcu.’uuuf. .1 \‘,uuuunl Su,s. ‘l’c,.’hnuil. B2 (1984) 241. 1~1 K. Aki mat’,. M. 1)uliscii. M. Ar:ii and ~. Waian.ihc. .‘\ppl. l’livi. Letters 451984) 921: 43(1983) 1062 11111 ( .M. Meii.e. A.R ( ula’,s a and .J.( ,. M:uvnuiide’.. ,1. ‘, Su ‘I eu.linuil. hi (1953) 1ô(~. [ii] (‘.1. I’nasuh and (‘.1) ‘l’hur’inuiiid. .J. I’hi\.. ( hem. (iS I 1962) 1111 5

Antkiw and V II

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1953)

59)). 1131 l’.l). Kirchner. .I.M. \\uiuid:ilI. .1.1 , t’reeuiul :uiul GE). l’eiA ‘h ~5 l9Ci 4~7 ill. PP . S’s. rift , . I 1— i’biss. 43 1372) 5115. 1141 A.’i’, (‘ha and M.B. i’anish ‘\pp 1151 lii’s. Joyce arid ( I. I’uixuin. Japan. .1. AppI l’h’,s. 16 Suppl ((1977) 17 (IS] W A. (.‘hupku. .1

Herkuiv, Ii,. (‘. F’. ( ,iese and \1.( ,. gliram. 3 l’hss. ( hem (i2 1958) (ill

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