GaAs(100) growth by metalorganic vapor phase epitaxy

,. . . . . . . .

CRYSTAL G R O W T H

ELSEVIER

Journal of Crystal Growth 165 (1996) 227-232

VI/II ratio dependence of surface macrodefects in CdTe/ZnTe/GaAs(100) growth by metalorganic vapor phase epitaxy H. Nishino *, T. Saito, Y. Nishijima Fujitsu Laboratories Ltd., 10-I Morinosato- Wakamiya, Atsugi 243-01, Jupan

Received 6 November 1995; accepted 5 December 1995

Abstract

We studied the surface morphology of CdTe(100) layers on GaAs(100) by metalorganic vapor phase epitaxy (MOVPE). When CdTe(100) layers were obtained using thin ZnTe nucleation layers, we observed high-density pyramidal macrodefects, known as hillocks, in the epilayer surface. We found the density of hillocks to be strongly dependent on the VI/II ratio during both ZnTe and CdTe growth processes. We optimized both the Te/Zn and Te/Cd ratios to obtain a minimum hillock density of 1 × 102 c m -2. These results show that hillocks were nucleated at both the epilayer/substrate and CdTe/ZnTe interfaces. By X-ray diffraction measurement, we confirmed that the quality of the crystal structural was also good under this condition. We also found the initial nucleation conditions to be more important for the structural quality. In (100)HgCdTe/CdTe/GaAs growth, pyramidal hillocks on the CdTe buffer surface caused substantial (> 100 txm) macrodefects in HgCdTe layers, which were fatal for infrared devices. Their shape was enlarged especially in one direction. To achieve a low density of surface macrodefects in HgCdTe(100) or CdTe(100) layers on GaAs(100) substrates, we need to precisely control the VI/II ratio.

1. I n t r o d u c t i o n

We need large high-quality HgCdTe wafers to realize large-area infrared focal plane arrays (IRFPA). The growth of HgCdTe layers on large alternative substrates such as GaAs and Si have been studied for this purpose [1,2]. M O V P E is one promising technique for growing HgCdTe because of its high throughput and suitability for large substrates. The quality of HgCdTe layers is limited by that of CdTe buffer layers grown on the alternative substrates. In CdTe growth on GaAs(100) substrates, both Corresponding author. Fax: + 81 462 48 5193.

CdTe(100) and CdTe(1 1 I)B epitaxial layers are obtained because of the large lattice mismatch (14.6%) between GaAs and CdTe buffer layers [3,4]. HgCdTe(100) is an attractive plane for constructing grown p - n junctions for infrared photodiodes. This is because its doping efficiency of arsenic, which is a commonly used acceptor, is higher than that of a H g C d T e ( I l l ) B plane [5]. Lamella twins, which are frequently observed in (11 I)B planes, are easily suppressed in (100) planes [6]. However, the high density of substantial hillocks which degrade the IRFPA performance is a serious problem in using HgCdTe(100) layers [7]. We think that this problem is related to the CdTe buffer surface, so we studied

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14. Nishino et al. / Journal of Co,stal Growth 165 (1996) 227-232

the surface macrodefects of CdTe(100) layers. We especially considered its dependence on the V I / I I ratio.

2. Experiment procedure We carried out epitaxial growth in a horizontal reactor with multiple nozzles and a rotating graphite susceptor heated by RF induction [8]. We used 3 inch GaAs substrates (100) misoriented 2 ° toward the nearest (110). We preheated the GaAs substrates at 580°C for 20 min in hydrogen, and grew the CdTe layers at 410°C under low pressure (150 Torr) to improve the thickness uniformity. We kept the total gas flow at 6 LM. Although the growth orientation of CdTe layers can be controlled by changing several growth conditions, we utilized thin ZnTe nucleation layers to obtain CdTe(100) layers [9]. This layer will ensure (100)/(100) epitaxy because of the reduction of the lattice mismatch between CdTe and GaAs. ZnTe thickness was typically 15 nm. We used dimethylcadmium (DMCd), diethyl-zinc (DEZn) and diisopropyl-tellurium (DIPTe) as precursors. To study the relation between CdTe and HgCdTe surface macrodefects, we also grew HgCdTe(100) layers on CdTe buffer layers. We grew the HgCdTe layers at 350°C under atmospheric pressure by direct alloy growth (DAG), using DMCd, DIPTe and elemental mercury (Hg). We observed surface macrodefects by using optical microscope. To evaluate the epitaxial layer crystallinity, we measured the double-crystal X-ray diffraction rocking curve (DCRC) of the (400) reflection.

3. Results and discussion 3.1. Dependence of surface macrodefects on V I / I I ratio In preliminary experiments, we grew only ZnTe(100) layers on GaAs(100) substrates and found their surface morphology relied on the V I / I I ratio (Fig. 1). The V I / I I ratio was controlled by changing the DIPTe flows and DEZn flows were kept con-

(a)

20/1m (b) Fig. 1. Optical micrographs of ZnTe(100) surface grown on GaAs(100) with different V I / I I ( T e / Z n ) ratio and thickness. (a) V I / I I = 2.0 and t = 2.5 p.m and (b) V I / I I = 0.5 and t = 1.5 ~zm.

stant. We obtained a smooth morphology at the ZnTe layers surface with 1-3 p~m thickness around the V I / I I ( T e / Z n ) = 0.5. We grew CdTe(100) layers on GaAs(100) under various V I / I I (Te/Cd) ratio conditions. The DMCd supply was kept constant. In this experiment, we first grew 15 nm ZnTe nucleation layers at V I / I I ( T e / Z n ) = 0.6. In the ZnTe nucleation growth process, the ZnTe growth rate was reduced by decreasing both the DEZn and DIPTe flows to 1/10 of those of the preliminary experiments. We found that the CdTe surface morphology was strongly dependent on the V I / I I ( T e / C d ) ratio (Fig. 2). We obtained an almost specular surface at VI,/II = 1.0. Slight V I / I I changes caused many pyramidal hillocks and their density increased up t o 107 c m - 2 (Table 1). In order to consider the relationship between surface morphology and growth kinetics, we studied the CdTe growth rate (Fig. 3). The growth rate was increased by increasing the V I / I I ratio up to 2. Then the density of hillocks was increased at the low

H. Nishino et a l . / Journal of Co'stal Growth 165 (1996) 227-232

V I / I I ratio condition, in spite of the slow growth rate. Although a high growth rate may have little effect on surface hillocks, we think the V I / I I ratio plays an important role in the hillock formation. The optimum condition for suppressing hillocks ( V I / I I = 1.0) seems a slight Cd-rich condition, as in Fig. 3. We think that most of high density hillocks shown in Fig. 2 were nucleated at the C d T e / Z n T e interface, not at the epilayer/substrate (ZnTe/GaAs) inter-

229

Table l Dependence of CdTe(100) surface hillock density on VI/II (Te/Cd) ratio T e / C d ratio

Hillock density

0.5

3X 106 l X l02 4X 106

1.0

1.5

face, since all CdTe layers were on the same ZnTe layers. Ferid et al. [10] and Irvine et al. [11] also reported similar V I / I I ( T e / C d ) ratio effects on the CdTe surface morphology in CdTe(100)/GaAs(100) growth. The relationship between the growth conditions of ZnTe nucleation layers and CdTe surface hillocks has not yet been examined, however. To confirm the effect of ZnTe nucleation layers, we grew CdTe with ZnTe nucleation layers using various T e / Z n ratios. We carried out CdTe growth under the optimum condition, V I / I I ( T e / C d ) = 1.0. We found that the hillock density of CdTe surface was also significantly dependent on the V I / I I ratio of nucleation layers (Fig. 4). Its dependence on the V I / I I ( T e / Z n ) ratio was similar to that on the T e / C d ratio. In these cases, we think that this high density of hillocks was caused at the epilayer/substrate (ZnTe/GaAs) interface. From studying the ZnTe growth rate, the condition of fewer hillocks ( T e / Z n = 0.6) seems to be a Zn-rich condition. Then we think slight excess of group II atoms play a key role in suppressing hillocks. We also clarified that hillocks could be caused at all interfaces in a C d T e / Z n T e / G a A s system by a small deviation of the source supply valance.

(a)

(b)

2.0 -

DMCd mol ratio : lx10 4

~1.5 m

20/~m (c) Fig. 2. Optical micrographs of CdTe(100) surface grown on GaAs(100) with different VI/II (Te/Cd) ratio and thickness. (a) VI/II = 0.5 and t = 1.3 p~m, (b) VI/II = 1.0 and t = 1.6 ~tm and (c) VI/II = 1.5 and t = 2.3 p~m.

~

1.0

~

0.5 t

I 1

I 2

Te/Cd ratio Fig. 3. Dependence of CdTe(100) growth rate on V I / l I (Te/Cd) ratio. DMCd tool fraction was fixed to 1 x 10 4.

230

H, Nishino et al. /Journal of Crystal Growth 165 (1996) 227-232

108

1000

107

u~

/ •

P

04

0~106 e~

.~105 C

O

CdTe g r o w t h

"~ 10 4

103

LL 100 0.1

10 2 _ CdTe

101 0.1

growth

I

: VI/II ( T e / C d )

I 1.0

I

I

I I1111

: VI/II ( T e / C d ) t=l,5~m

I

1.0 Te/Zn ratio

I

I

= 1.0

I Ill

10

= 1.0

I 10

Fig. 6. Dependence of DCRC-FWHM of CdTe(100) layers on ZnTe nucleation conditions.

Te/Zn ratio

Fig. 4. Hillock density of CdTe(100) surface using various ZnTe nucleation layers grown with different V I / I I ( T e / Z n ) ratio.

3.2. Crystallinity eualuation by X-ray diffraction The crystal structural quality of the CdTe(100)/GaAs(100) is as important as the surface quality. For this viewpoint, we evaluated the crystallinity of CdTe layers by the DCRC measurements. We used the full width at half maximum (FWHM) of DCRC as an indicator of crystal quality. We studied the DCRC-FWHM dependence on CdTe thickness (Fig. 5). Most samples in Fig. 5 were grown using the optimum condition of T e / C d = 1.0 and T e / Z n = 0.6. The DCRC-FWHM monotonically decreased as CdTe thickness increased and we obtained the best result of FWHM = 70 arcsec for a thick (18 txm) layer. The DCRC-FWHM of CdTe

0 1000

layers grown under other T e / C d ratios were also shown in Fig. 5, but we could not distinguish its dependence on the T e / C d ratio from that on CdTe thickness. We think that the T e / C d ratio had a weak effect on the structural quality. In contrast, the dependence of the T e / Z n ratio on the DCRC-FWHM was clear, as shown in Fig. 6. When the CdTe layer with a lower hillock density ( T e / Z n = 0.6) was grown, we obtained a smaller DCRC-FWHM than with other T e / Z n conditions, at the same CdTe thickness. Thus, we believe that the best growth conditions, determined from surface quality, brought about the best crystal structure quality. The dependence of the DCRC-FWHM on each V I / I I ratio indicates that initial growth conditions at the heterointerface, including the material conversion from the I I I - V compounds to the I I - V I compounds, governed the whole CdTe structural quality rather than the conditions for later growth in homoepitaxial mode.

A

Te/Cd ratio

Z~ :0.5 • :1.0

u3

a

3.3. HgCdTe(lO0) growth on GaAs( l O0)

100

l! -r

10

I

I

I

I III1[

I

I

I

I III

10 100 CdTe thickness ( ~ m) Fig. 5. Dependence of DCRC-FWHM of CdTe(100) layers on CdTe thickness.

CdTe / ZnTe /

We studied the relationship between the surface quality of CdTe buffer layers and that of HgCdTe layers. A typical surface m o r p h o l o g y of HgCdTe(100) layers using the optimum buffer growth conditions is shown in Fig. 7. We observed very large macrodefects in the HgCdTe surface and the density of these ((1-2) × 102 cm -2) was similar to the CdTe hillock density. We believed these macrodefects were caused by the remaining CdTe

H. Nishino et al. / Journal qf Co'stal Growth 165 (1996) 227-232

<01 1> <100; <[31i> m

Fig. 7. Optical micrograph of typical macrodefects in HgCdTe(100) surface grown on CdTe/ZnTe/GaAs(100).

hillocks. The size of the substantial hillocks exceeded 100 0,m for 4 p~m thick HgCdTe layers and their shapes were extended more along the <01~> direction than the <011) direction. Side planes of each macrodefect consisted of ( l l n ) facets and the difference in their polarity probably caused the long shape. The structural quality such as the dislocation density of HgCdTe(100) layers has been discussed elsewhere [12]. We needed to further reduce of the macrodefects for the application of large-area IRFPA [7]. To investigate the origin of the remaining macrodefects, we compared the composition of hillocks and other parts by electron probe microanalysis (EPMA). We detected higher amounts of C and Hg in parts just beneath the hillocks than in other parts. We think that one of the main origin of the remaining hillocks was the surface contamination during the substrate loading process.

231

tions from both ratios increased the hillock density up to 1 0 7 c m - 2 From the growth rate dependence on the V I / I I ratio, we think that the slight excess of group II atoms plays a key role in suppressing hillocks. We confirmed that the crystal structural quality was also good under this optimum condition by double crystal X-ray diffraction measurement. The structural quality improved as the CdTe thickness was increased, and we obtained a minimum D C R C - F W H M less than 100 arcsec for thick layers. We also clarified that the ZnTe nucleation conditions had a more significant effect on the structural quality than the later CdTe growth conditions. In ( 1 0 0 ) H g C d T e / C d T e / G a A s growth, pyramidal hillocks on the CdTe buffer surface caused gigantic hillocks on HgCdTe layers, destroying the IR photodiodes. The size of these hillocks exceeded 100 p~m and their shape was considerably extended along the (011) direction. To achieve a low density of surface macrodefects in HgCdTe(100) or CdTe(100) layers on GaAs(100) substrates, we require precise control over the V I / I I ratio.

Acknowledgements We thank Mr. T. Okamoto and Mr. S. Murakami for discussions, and Mr. K. Maruyama for providing and explaining the EPMA data. We also thank Dr. H. Takigawa and Dr. S. Yamakoshi for their encouragement.

4. Summary References We studied the surface and structural quality of CdTe(100) layers grown on GaAs(100) by MOVPE. We frequently observed a high density of pyramidal macrodefects (hillocks) in the epilayer surface in CdTe(100) layers obtained using thin ZnTe nucleation layers. We found the density of hillocks to be strongly dependent on the V I / I I ratio during both ZnTe and CdTe growth processes. Thus hillocks were nucleated at both the epilayer/substrate and C d T e / Z n T e interfaces. We optimized the growth conditions to reduce hillocks and obtained a minimum hillock density of 1 X 1 0 2 cm -2 at the V I / I I ratios T e / C d = 1.0 and T e / Z n = 0.6. Slight devia-

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